Patent Publication Number: US-2023150277-A1

Title: Inkjet printing apparatus and printing method of bipolar element using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a national entry of International Application No. PCT/KR2021/004073, filed on Apr. 1, 2021, which claims under 35 U.S.C. §§ 119(a) and 365(b) priority to and benefits of Korean Patent Application No. 10-2020-0048774, filed on Apr. 22, 2020, in the Korean Intellectual Property Office (KIPO), the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     Embodiments relate to an inkjet printing apparatus and a printing method of a bipolar element by using the inkjet printing apparatus. 
     2. Description of the Related Art 
     The importance of display devices has steadily increased with the development of multimedia technology. In response thereto, various types of display devices such as an organic light emitting display (OLED), a liquid crystal display (LCD) and the like have been used. 
     A display device is a device for displaying an image, and includes a display panel, such as an organic light emitting display panel or a liquid crystal display panel. The light emitting display panel may include light emitting elements, e.g., light emitting diodes (LED). The examples of the light emitting diode include an organic light emitting diode (OLED) by using an organic material as a fluorescent material and an inorganic light emitting diode by using an inorganic material as a fluorescent material. 
     SUMMARY 
     Embodiments provide an inkjet printing apparatus capable of continuously performing different processes by disposing a plurality of devices performing a printing process in a process line. 
     Embodiments also provide a printing method of bipolar elements capable of improving a degree of alignment of the bipolar elements. 
     It should be noted that aspects of the disclosure are not limited thereto and other aspects, which are not mentioned herein, will be apparent to those of ordinary skill in the art from the following description. 
     According to an embodiment, an inkjet printing apparatus may include a stage that moves in a first direction, an inkjet device that sprays ink onto the stage, a plurality of electric field generating devices that generate an electric field on the stage, are spaced apart from the stage, and are movable in the first direction independently from the stage, and, a light irradiation device that irradiates the stage with light, and a drying device that dries the ink jetted onto the stage, wherein the inkjet device, the light irradiation device, and the drying device may be disposed along the first direction. 
     The plurality of electric field generating devices may be configured to generate the electric field on the stage with moving along the stage. 
     The plurality of electric field generating devices may include a first electric field generating device disposed on a side of the stage and a second electric field generating device disposed on another side of the stage, and the first electric field generating device and the second electric field generating device may be spaced apart from each other and may be movable in the first direction independently from each other. 
     At least one of the first electric field generating device and the second electric field generating device may be configured to move in a direction opposite to a moving direction of the stage in case that the stage moves to the drying device. 
     The inkjet device may be configured to spray the ink onto the stage on which the electric field may be generated by the plurality of electric field generating devices. 
     The ink may include a solvent and a plurality of bipolar elements dispersed in the solvent, and end portions of the bipolar elements may be oriented to have initial orientation directions by the electric field. 
     The light irradiation device may be configured to irradiate the ink disposed in the electric field with the light. 
     In case that the ink is irradiated with the light, the initial orientation directions of end portions of some of the bipolar elements may be changed by the electric field and the light. 
     The inkjet printing apparatus may further comprise a plurality of rails including a first rail and a second rail extending in the first direction, and a plurality of frames including a first frame and a second frame disposed above the first rail and the second rail, wherein the stage may be disposed on the first rail, the plurality of electric field generating devices may be disposed on the second rail, and the stage and the plurality of electric field generating devices may be configured to pass below the plurality of frames with moving in the first direction. 
     The inkjet device may be disposed on the first frame, and the light irradiation device may include a first light irradiation device disposed on the first frame and a second light irradiation device disposed on the second frame spaced apart from the first frame in the first direction. 
     The ink may be sprayed in case that the stage moves to the first light irradiation device, and the first light irradiation device may be configured to irradiate the stage with the light while the ink is sprayed onto the stage. 
     The second light irradiation device may be configured to irradiate the stage with the light after the ink is sprayed onto the stage. 
     The drying device may include a first drying device to which the plurality of electric field generating devices and the stage move, and the stage may be configured to move to the first drying device in a state in which the electric field is generated. 
     The drying device may further include a second drying device including an electric field generating unit different from the plurality of electric field generating devices, and the electric field generating unit may be configured to generate an electric field on the stage in case that the stage moves to the second drying device. 
     The inkjet printing apparatus may further comprise a sub-stage which is disposed below the second drying device and on which the electric field generating unit is disposed, wherein the stage and the plurality of electric field generating devices may be configured not to move to the second drying device. 
     According to an embodiment, a printing method of a bipolar element, may include providing a target substrate, generating an electric field on the target substrate, and spraying ink onto the target substrate, the ink including a solvent and bipolar elements dispersed in the solvent, arranging the bipolar elements on the target substrate by irradiating the ink disposed in the electric field with light, and seating the bipolar elements on the target substrate by removing the solvent of the ink. 
     In the spraying of the ink onto the target substrate, end portions of the bipolar elements may be oriented to have initial orientation orientation directions by the electric field. 
     In the arranging of the bipolar elements, the initial directions of end portions of some of the bipolar elements may be changed by by the electric field and the light. 
     The target substrate may be irradiated with the light in case that the ink is sprayed. 
     The seating of the bipolar elements may include removing the solvent in a state in which the electric field is generated on the target substrate. 
     The target substrate may include a first electrode and a second electrode spaced apart from each other, and the end portions of the bipolar elements may be disposed on the first electrode and another end portion disposed on the second electrode. 
     In an inkjet printing apparatus according to an embodiment, devices for printing processes of bipolar elements may be disposed in a process line, and a stage may pass through the devices with moving in a direction. The printing processes of bipolar elements may be continuously performed according to the movement of the stage, such that a process time of the printing processes may be shortened or reduced. 
     For example, a stage and an electric field generating device may be spaced apart from each other and moved individually, such that the electric field generating device may prepare for the next printing process before a printing process is completed. Accordingly, an unnecessary preparation time between the printing processes repeated several times may be minimized, such that the overall process time may be further shortened or reduced. 
     The effects according to the embodiments are not limited by the contents described above, and more various effects are included in this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view of an inkjet printing apparatus according to an embodiment; 
         FIG.  2    is a schematic perspective view illustrating an arrangement of an inkjet device, an electric field generating device, and a light irradiation device according to an embodiment; 
         FIG.  3    is a schematic perspective view illustrating an arrangement of a drying device and an inspection device according to an embodiment; 
         FIG.  4    is a schematic plan view illustrating the inkjet device, the electric field generating device, and the light irradiation device according to an embodiment; 
         FIG.  5    is a schematic cross-sectional view illustrating that ink is discharged from the inkjet device according to an embodiment; 
         FIG.  6    is a schematic cross-sectional view illustrating the ink discharged from the inkjet device according to an embodiment; 
         FIG.  7    is a schematic plan view illustrating a stage and the electric field generating device according to an embodiment; 
         FIGS.  8  and  9    are schematic views illustrating an operation of e electric field generating device according to an embodiment; 
         FIG.  10    is a schematic view illustrating that an electric field is generated on a target substrate by the electric field generating device according to an embodiment; 
         FIG.  11    is a schematic view illustrating that discharged bipolar elements are arranged on the target substrate according to an embodiment; 
         FIG.  12    is a schematic side view illustrating the inkjet device and the light irradiation device according to an embodiment; 
         FIG.  13    is a schematic cross-sectional view illustrating the light irradiation device according to an embodiment; 
         FIG.  14    is a schematic view illustrating hat bipolar elements arranged on the target substrate are irradiated with light according to an embodiment; 
         FIG.  15    is a schematic front view illustrating the drying device according to an embodiment; 
         FIG.  16    is a schematic view illustrating that the ink discharged onto the target substrate is dried and the bipolar elements are seated according to an embodiment; 
         FIG.  17    is a schematic view illustrating that a solvent of the ink is dried according to an embodiment, 
         FIG.  18    is a schematic view illustrating movement of the electric field generating device according to an embodiment; 
         FIG.  19    is a schematic front view illustrating the inspection device according to an embodiment; 
         FIG.  20    is a schematic plan view of an inkjet printing apparatus according to an embodiment; 
         FIG.  21    is a schematic front view illustrating a drying device according to an embodiment; 
         FIG.  22    is a schematic front view illustrating a drying device according to an embodiment; 
         FIG.  23    is a schematic view illustrating an electric field generating device according to an embodiment; 
         FIG.  24    is a flowchart illustrating a printing method of a bipolar element according to an embodiment; 
         FIGS.  25  to  28    are schematic cross-sectional views illustrating the printing method of a bipolar element according to an embodiment; 
         FIGS.  29  and  30    are schematic views illustrating inspecting bipolar elements printed on a target substrate according to an embodiment; 
         FIG.  31    is a schematic view of a light emitting element according to an embodiment; 
         FIG.  32    is a schematic plan view of a display device according to an embodiment; 
         FIG.  33    is a schematic plan view illustrating a pixel of the display device according to an embodiment; and 
         FIG.  34    is a schematic cross-sectional view taken along line IIIa-IIIa′, line IIIb-IIIb′, and line IIIc-IIIc′ of  FIG.  33   . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     It will also be understood that in case that a layer is referred to as being “on” another layer or substrate, it can be directly on another layer or substrate, or intervening layers may also be present. The same reference numbers indicate the same components throughout the specification. When an element, such as a layer, is referred to as being “connected to,” or “coupled to” another element or layer, it may be directly connected to, or coupled to the other element or layer or intervening elements or layers may be present. When, however, an element or layer is referred to as being “directly connected to,” or “directly coupled to” another element or layer, there are no intervening elements or layers present. To this end, the term “connected” may refer to physical, electrical, and/or fluid connection, with or without intervening elements. 
     It will be understood that, although the terms “first,” “second,” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. For instance, a first element discussed below could be termed a second element without departing from the teachings of the invention. Similarly, the second element could also be termed the first element. 
     Spatially relative terms, such as “beneath,” “below,” “under,” “lower,” “above,” “upper,” “over,” “higher,” “side” (e.g., as in “sidewall”), and the like, may be used herein for descriptive purposes, and, thereby, to describe one elements relationship to another element(s) as illustrated in the drawings. Spatially relative terms are intended to encompass different orientations of an apparatus in use, operation, and/or manufacture in addition to the orientation depicted in the drawings. For example, if the apparatus in the drawings is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” can encompass both an orientation of above and below. Furthermore, the apparatus may be otherwise oriented (e.g., rotated  90  degrees or at other orientations), and, as such, the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. As used herein, the singular forms, “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Moreover, the terms “comprises,” “comprising,” “includes,” and/or “including,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It is also noted that, as used herein, the terms “substantially,” “about,” and other similar terms, are used as terms of approximation and not as terms of degree, and, as such, are utilized to account for inherent deviations in measured, calculated, and/or provided values that would be recognized by one of ordinary skill in the art. 
     Hereinafter, embodiments will be described with reference to the accompanying drawings. 
       FIG.  1    is a schematic view of an inkjet printing apparatus  1000  according to an embodiment.  FIG.  2    is a perspective view illustrating an arrangement of an inkjet device  300 , an electric field generating device  700 , and a light irradiation device  500  according to an embodiment.  FIG.  3    is a schematic perspective view illustrating an arrangement of a drying device  800  and an inspection device  900  according to an embodiment. 
       FIG.  1    schematically illustrates an arrangement of respective components included in an inkjet printing apparatus  1000 ,  FIG.  2    illustrates an inkjet device  300 , a light irradiation device  500  (e.g.,  510  and  520 ), and an electric field generating device  700  (e.g.,  710  and  720 ) of the inkjet printing apparatus  1000 , and  FIG.  3    illustrates a drying device  800  and an inspection device  900  of the inkjet printing apparatus  1000 .  FIG.  1    illustrates the inkjet printing apparatus  1000  when viewed from above. 
     Referring to  FIGS.  1  to  3   , the inkjet printing apparatus  1000  according to an embodiment may include a stage STA, an inkjet device  300 , light irradiation devices  500 , electric field generating devices  700 , and a drying device  800 . For example, the inkjet printing apparatus  1000  may further include an inspection device  900 . 
     In  FIGS.  1  to  3   , a first direction DR 1 , a second direction DR 2 , and a third direction DR 3  are defined. The first direction DR 1  and the second direction DR 2  are directions disposed on the same plane and perpendicular to each other, and the third direction DR 3  is a direction perpendicular to each of the first direction DR 1  and the second direction DR 2 . The first direction DR 1  refers to a transverse direction in the drawing, the second direction DR 2  refers to a longitudinal direction in the drawing, and the third direction DR 3  refers to an upward or downward direction in the drawing. 
     The inkjet printing apparatus  1000  may jet (or spray) ink onto the stage STA or a target substrate SUB disposed on the stage STA by using inkjet heads  330 . In case that the ink is jetted onto the target substrate SUB, the electric field generating device  700  may generate an electric field on the target substrate SUB. Particles, for example, bipolar elements, included in the ink may be aligned in case that their orientation directions are changed by the electric field. The bipolar elements may be printed on the target substrate SUB in case that the stage STA moves via the light irradiation device  500  and the drying device  800 . ‘Printing’ of the bipolar elements as used herein means discharging, spraying, or jetting the bipolar elements from the inkjet printing apparatus  1000  to an object. For example, printing the bipolar elements means seating the bipolar elements or the ink on the target substrate SUB, in addition to jetting the bipolar elements on the target substrate SUB by using the inkjet device  300 . Hereinafter, components of the inkjet printing apparatus  1000  and a process of printing bipolar elements by using the inkjet printing apparatus  1000  will be described below. 
     In order to print the bipolar elements on the target substrate SUB, processes by using the inkjet device  300  that jets the ink including the bipolar elements, the electric field generating device  700  and the light irradiation device  500  that align the bipolar elements, and the drying device  800  that seats the bipolar elements on the target substrate SUB may be performed. In the inkjet printing apparatus  1000  according to an embodiment, processes of printing the bipolar elements may be continuously performed in case that the target substrate SUB moves via the inkjet device  300 , the light irradiation device  500 , and the drying device  800 . For example, the electric field generating device  700  generating the electric field in order to align the bipolar elements may move along the target substrate SUB in a state which the electric field generating device  700  is disconnected (or spaced apart) from the stage STA on which the target substrate SUB is disposed. For example, the electric field generating device  700  generating the electric field may be movable in a direction (e.g., in the second direction DR 2 ) independently from the stage STA on which the target substrate SUB. For example, the electric field generating device  700  generating the electric field may be separately movable from the stage STA or may be simultaneously movable with the stage STA. In the inkjet printing apparatus  1000 , devices may be sequentially disposed in a direction (e.g., in the second direction DR 2 ) so that different processes may be sequentially performed. For example, in the inkjet printing apparatus  1000 , the inkjet device  300 , the light irradiation device  500 , and the drying device  800  may be disposed in a line along a direction (e.g., in the second direction DR 2 ) in which the stage STA moves. For example, in the inkjet printing apparatus  1000 , the electric field generating device  700  and the stage STA may be disconnected (or spaced apart) from each other, such that a time required for detachment between the electric field generating device  700  and the target substrate SUB may be saved, and a period between the preceding process and the subsequent process may be shortened, and continuity of the processes may thus be improved. 
       FIG.  4    is a schematic plan view illustrating the inkjet device  300 , the electric field generating device  700 , and the light irradiation device  500  according to an embodiment.  FIG.  4    schematically illustrates an arrangement of the stage STA, the inkjet device  300 , the light irradiation device  500 , and the electric field generating device  700  of the inkjet printing apparatus  1000 . 
     Referring to  FIG.  4    in addition to  FIGS.  1  to  3   , the stage STA may provide a region in which the target substrate SUB is disposed. A shape of the stage STA is not limited, and as an example, the stage STA may have a rectangular shape with sides extending in the first direction DR 1  and the second direction DR 2 . The stage STA may include long sides extending in the first direction DR 1  and short sides extending in the second direction DR 2 . However, an overall shape of the stage STA in a plan view may change according to a shape of the target substrate SUB in a plan view. For example, in case that the target substrate SUB has a rectangular shape in a plan view, the stage STG may have a rectangular shape, and in case that the target substrate SUB has a circular shape in a plan view, the stage STA may also have a circular shape in a plan view. However, embodiments are not limited thereto, and the stage STA and the target substrate SUB may also have different shapes. 
     The inkjet printing apparatus  1000  may include first rails RL 1  and second rails RL 2  extending in the second direction DR 2 , and the stage STA may be disposed on the first rails RL 1 . The first rails RL 1  and the second rails may extend in the second direction DR 2 , respectively, and the first rails RL 1  may be disposed in a space between the second rails RL 2  spaced apart from each other. The stage STA may move in the second direction DR 2  on the first rails RL 1  through a moving member. In case that the target substrate SUB is disposed on the stage STA, the stage STA may reciprocate in the second direction DR 2  along the first rails RL 1 , and the particles may be printed on the target substrate SUB. An electric field generating device  700  to be described below may be disposed on the second rails RL 2 . The stage STA and the electric field generating device  700  may move in the second direction DR 2  on the first rails RL 1  or the second rails RL 2 . 
     Aligners AL may be disposed on the stage STA. The aligners AL may be disposed on each side of the stage STA, and a region surrounded by the aligners AL may be a region in which the target substrate SUB is disposed. For example, two aligners AL may be disposed to be spaced apart from each other on each side of the stage STA, and a total of eight aligners AL may be disposed on the stage STA. However, embodiments are not limited thereto, and the number, dispositions, and the like, of aligners AL may change according to a shape or a type of the target substrate SUB. For example, in some cases, the aligners AL may be omitted. 
     The target substrate SUB may be prepared on the stage STA. The target substrate SUB may provide a target space in which the particles printed by the inkjet printing apparatus  1000  are seated. As described below, specific members may be disposed on the target substrate SUB, and the particles may be seated or printed on the specific members. The target substrate SUB may be positioned on the stage STA in consideration of positions where the particles are printed together with the aligners AL. 
     The inkjet device  300  may include inkjet heads  330  (see  FIG.  5   ) and may be disposed on a first frame FM 1 . The inkjet device  300  may jet inks  90  (see  FIG.  5   ) onto the target substrate SUB by using the inkjet heads  330  connected to an ink circulation unit  600 . 
     The inkjet printing apparatus  1000  may include frames FM 1  to FM 6 . The frames FM 1  to FM 6  may be disposed above the first rails RL 1  and the second rails RL 2 , and devices performing a printing process of the bipolar elements  95  may be disposed on the frames FM 1  to FM 6 . In an embodiment, the stage STA and the electric field generating device  700  may pass below the frames FM 1  to FM 6  with moving in the second direction DR 2  on the rails RL 1  and RL 2 . 
     The first frame FM 1  may include support parts FM_C and FM_ R. The support parts FM_C and FM_R may include a first support part FM_C extending in the first direction DR 1 , which is a horizontal direction, and second support parts FM_R connected to the first support part FM_C and extending in the third direction DR 3 , which is a vertical direction. An extension direction of the first support part FM_C may be substantially the same as the first direction DR 1 , which is a long side direction of the stage STA. The inkjet device  300  may be mounted on the first support part FM_C. 
     The inkjet device  300  may be spaced apart from the stage STA passing below the first frame FM 1  by a distance. The distance by which the inkjet device  300  is spaced apart from the stage STA may be adjusted by a height of the second support part FM_R of the first frame FM 1 . A distance between the inkjet device  300  and the stage STA spaced apart from each other may be adjusted within a range in which the inkjet device  300  has a certain distance from the target substrate SUB in case that the target substrate SUB is disposed on the stage STA, such that a space for a printing process may be secured. 
       FIG.  5    is a schematic cross-sectional view illustrating that ink is discharged from the inkjet device  300  according to an embodiment.  FIG.  6    is a schematic cross-sectional view illustrating the ink discharged from the inkjet device according to an embodiment. 
     Referring to  FIGS.  5  and  6   , the inkjet device  300  may include a first base part  310  and inkjet heads  330  disposed on a bottom surface of the first base part  310 . The inkjet head  330  may include nozzles  335 , and ink provided from the ink circulation unit  600  may be discharged (or sprayed) through the nozzles  335  of the inkjet head  330 . 
     The inkjet heads  330  may be spaced apart from each other in a direction, and may be arranged in one row or a plurality of rows. For example, the inkjet heads  330  may be arranged in one row, but embodiments are not limited thereto. The inkjet heads  330  may be arranged in a greater number of rows, and may be misaligned with each other or be disposed to neighbor to each other. A shape of the inkjet head  330  is not limited, but as an example, the inkjet head  330  may have a rectangular shape. 
     In some embodiments, at least one inkjet head  330 , for example, two inkjet heads  330  may form a pack (or a single pack) to be disposed adjacent to each other. However, the number of inkjet heads  330  included in the pack is not limited thereto, and for example, the number of inkjet heads  330  included in the pack may be about  1  to about  5 . For example, only five inkjet heads  330  may be disposed in the inkjet device  300 , but this is for schematically illustrating the inkjet device  300  and the number of inkjet heads  330  is not limited thereto. 
     In some embodiments, a width of the target substrate SUB measured in the first direction DR 1  may be greater than a width of the inkjet device  300 . For example, the inkjet device  300  may move in the first direction DR 1  and jet (e.g., entirely jet) the ink  90  onto the target substrate SUB. For example, in case that target substrates SUB are provided on the stage STA, the inkjet device  300  may jet (or spray) the ink  90  onto each of the target substrates SUB with moving in the first direction DR 1 . 
     However, embodiments are not limited thereto, and the inkjet device  300  may be positioned outside the first rails RL 1  and the second rails RL 2 , and may then move in the first direction DR 1  and may jet the ink  90  onto the target substrate SUB. In case that the stage STA moves in the second direction DR 2  to be positioned below the first frame FM 1 , the inkjet device  300  may move between the first rails RL 1  and may jet the ink  90  through the inkjet head  330 . An operation of such an inkjet head  330  is not limited thereto, and may be variously modified as long as the inkjet head  330  performs a similar process. 
     The inkjet head  330  disposed in the inkjet device  300  may jet the ink  90  onto the target substrate SUB disposed on the stage STA. 
     In an embodiment, the ink  90  may include a solvent  91  and bipolar elements  95  included in the solvent  91 . In an embodiment, the ink  90  may be provided in a solution or colloidal state. For example, the solvent  91  may be acetone, water, alcohol, toluene, propylene glycol (PG) or propylene glycol methyl acetate (PGMA), triethylene glycol monobutyl ether (TGBE), diethylene glycol monophenyl ether (DGPE), an amide-based solvent, a dicarbonyl-based solvent, diethylene glycol dibenzoate, a tricarbonyl-based solvent, triethly citrate, a phthalate-based solvent, benzyl butyl phthalate, bis(2-ethlyhexyl) phthalate, bis(2-ethylhexyl) isophthalate, ethyl phthalyl ethyl glycolate, or the like, but embodiments are not limited thereto. The bipolar elements  95  may be included in a state in which the bipolar elements  95  are dispersed in the solvent  91 , and be supplied to and discharged from the inkjet device  300 . 
     The inkjet printing apparatus  1000  may further include the ink circulation unit  600 . The ink circulation unit  600  may supply the ink  90  to the inkjet device  300 , and the inkjet head  330  may discharge (or spray) the supplied ink  90 . The ink  90  may be circulated between the ink circulation unit  600  and the inkjet head  330 , and some of the ink  90  supplied to the inkjet head  330  may be discharged from the inkjet head  330 , and the remainder of the ink  90  may be supplied to the ink circulation unit  600  again. In some embodiments, the ink circulation unit  600  may include ink storage parts, a pressure pump, a compressor, and a flow meter. In the ink circulation unit  600 , the ink storage part may be connected to the inkjet head  330 , and the ink storage part and the inkjet head may form an ink circulation system. A detailed description thereof will be omitted for descriptive convenience. 
     The ink circulation unit  600  may be connected to the inkjet head  330  through a first connection tube IL 1  and a second connection tube IL 2 . For example, the ink circulation unit  600  may supply the ink  90  to the inkjet head  330  through the first connection tube IL 1  and a flow rate of the supplied ink  90  may be adjusted through a first valve VA 1 . For example, the ink circulation unit  600  may be supplied with the remainder of the ink  90  remaining after being discharged from the inkjet head  330 , through the second connection tube IL 2 . A flow rate of the ink  90  supplied to the ink circulation unit  600  through the second connection tube IL 2  may be adjusted through a second valve VA 2 . As the ink  90  is circulated through the ink circulation unit  600 , a deviation in the number of bipolar elements  95  included in the ink  90  discharged from the inkjet head  330  may be minimized. 
     The ink circulation unit  600  may be mounted on the first frame FM 1 , but embodiments are not limited thereto. The ink circulation unit  600  may be formed in the inkjet printing apparatus  1000 , but a position or a shape of the ink circulation unit  600  is not limited. For example, the ink circulation unit  600  may be disposed through a separate device, and may be variously disposed as long as the ink circulation unit  600  is connected to the inkjet head  330 . 
     The inkjet head  330  may include an inner tube  331  and nozzles  335  and may discharge the ink  90  through the nozzles  335 . The ink  90  discharged from the nozzles  335  may be jetted onto the target substrate SUB provided on the stage STA. The nozzles  335  may be disposed on a bottom surface of the inkjet head  330  and may be arranged along a direction in which the inkjet head  330  extends. 
     The inner tube  331  may be connected to an inner flow path of the first base part  310 , and may be supplied with the ink  90  from the ink circulation unit  600 . The inner tube  331  may be supplied with the ink  90  through the first connection tube IL 1  connected to the ink circulation unit  600 , and the ink  90  remaining after being discharged from the nozzles  335  may be supplied to the ink circulation unit  600  through the second connection tube IL 2 . The inner tube  331  may be formed along an extension direction of the inkjet head  330 . The ink  90  supplied through the inkjet device  300  may flow through the inner tube  331  and may be then discharged through the nozzles  335  of the inkjet head  330 . 
     The nozzles  335  may be positioned on a lower surface of the inkjet head  330 . The nozzles  335  may be spaced apart from each other and arranged along the extension direction of the inkjet head  330 , and may be connected to the inner tube  331  to discharge the ink  90 . For example, the nozzles  335  may be arranged in one row or a plurality of rows. In some embodiments, the number of nozzles  335  included in the inkjet head  330  may be about  128  to about  1800 . An amount of the ink  90  jetted through the nozzles  335  may be adjusted according to a voltage applied to each nozzle  335 . In an embodiment, an amount of the ink  90  discharged once from each nozzle  335  may be 1 to 50 pl (pico-litter), but embodiments are not limited thereto. 
     The ink  90  discharged through the nozzle  335  may include the solvent  91  and the bipolar elements  95  dispersed in the solvent  91 . According to an embodiment, the bipolar element  95  may have a shape in which the bipolar element  95  extends in a direction. The bipolar elements  95  may be randomly dispersed in the ink  90 , flow along the inner tube  331 , and then be supplied to the nozzle  335 . As the bipolar element  95  has a shape in which the bipolar element  95  extends in a direction, the bipolar element  95  may have an orientation direction, which is a direction toward which a major axis is directed. For example, the bipolar element  95  may include a first end portion having a first polarity and a second end portion having a second polarity, and the first end portion and the second end portion may be end portions (e.g., opposite end portions) of the bipolar element  95  in a major axis direction. The orientation direction of the bipolar element  95  extending in a direction may be defined on the basis of a direction toward which the first end portion is directed. The bipolar elements  95  flowing in the inner tube  331  and the nozzle  335  of the inkjet head  330  may be dispersed in random orientation directions rather than a constant orientation direction. However, embodiments are not limited thereto, and the bipolar elements  95  may flow in the inner tube  331  and the nozzle  335  in a state in which they have specific orientation directions. 
     The ink  90  discharged from the inkjet head  330  may be jetted onto the target substrate SUB. The bipolar elements  95  may be jetted onto the target substrate SUB with having specific orientation directions, and be then arranged on the target substrate SUB with having a constant orientation direction by the electric field generated by the electric field generating device  700 . For example, the bipolar elements  95  may be aligned in a direction on the target substrate SUB by the electric field. 
       FIG.  7    is a schematic plan view illustrating a stage STA and the electric field generating device  700  according to an embodiment.  FIG.  7    illustrates an arrangement of the stage STA, the target substrate SUB, and the electric field generating device  700 . 
     Referring to  FIG.  7    in addition to  FIGS.  2  and  4   , the inkjet printing apparatus  1000  may include electric field generating devices  700  disposed on the second rails RL 2 . The electric field generating device  700  may reciprocate (or move back and forth) in the second direction DR 2  on the second rails RL 2 , similar to the stage STA. The electric field generating device  700  may be connected (e.g., electrically connected) to the target substrate SUB in order to generate the electric field on the target substrate SUB disposed on the stage STA. In case that the electric field generating device  700  and the target substrate SUB are connected (e.g., electrically connected) to each other, the electric field may be generated on the target substrate SUB by an electrical signal applied from the electric field generating device  700 . 
     In an embodiment, the electric field generating device  700  may include a first electric field generating device  710  disposed on a side of the stage STA and a second electric field generating device  720  disposed on another side of the stage STA. The first electric field generating device  710  and the second electric field generating device  720  may be disposed on the second rails RL 2 , may be connected (e.g., electrically connected) to the target substrate SUB on a side and another side of the stage STA, respectively, and may generate an electric field of uniform strength regardless of a position even though an area of the target substrate SUB is great. 
     The first electric field generating device  710  and the second electric field generating device  720  may be driven individually or be driven simultaneously. For example, in case that the target substrate SUB is prepared on the stage STA and the ink  90  is jetted onto the target substrate SUB, the first electric field generating device  710  may form an electric field on the target substrate SUB, and the second electric field generating device  720  may not be connected to the target substrate SUB. Thereafter, the first electric field generating device  710  may be disconnected (e.g., electrically disconnected) from the target substrate SUB, and the second electric field generating device  720  may be connected to the target substrate SUB to form an electric field. For example, the electric field generating devices  700  may be simultaneously driven to form the electric fields, or may be sequentially driven to form the electric fields. 
     According to an embodiment, the electric field generating device  700  may move on the second rail RL 2  in a state in which the electric field generating device  700  is disconnected from the stage STA. In case that the stage STA moves according to the printing process of the bipolar elements  95 , the electric field generating device  700  may generate an electric field on the target substrate SUB in the printing process while (or with) moving together with the stage STA. For example, the electric field generating device  700  may be disconnected from the stage STA and moved before carrying the target substrate SUB on which the printing of the bipolar elements  95  is completed, and may be prepared in a state in which the electric field generating device  700  may be connected to another target substrate SUB. 
     For example, the first electric field generating device  710  and the second electric field generating device  720  may also be disconnected (or spaced apart) from each other and may be movable individually or independently from each other. In the printing process of the bipolar elements  95 , the first electric field generating device  710  and the second electric field generating device  720  may move along the stage STA. However, for the subsequent printing process, any one of the first electric field generating device  710  and the second electric field generating device  720  may move in a direction opposite to a moving direction of the stage STA. A more detailed description thereof will be provided below. 
     The electric field generating device  700  (e.g., each of the first and second electric field generating devices  710  and  720 ) may include a probe support  701 , a probe driver  703 , a probe jig  705 , and a probe pad  708 . In the electric field generating device  700 , the probe driver  703  and the probe jig  705  move, such that the probe pad  708  may be connected (e.g., electrically connected) to the target substrate SUB. 
     The probe support  701  may provide a space in which the probe driver  703 , the probe jig  705 , and the like, are disposed. The probe support  701  may be connected to the second rail RL 2  and may move in the second direction DR 2 . The probe support  701  may be disposed on a side of the stage STA and have a shape in which the probe support  701  extends in a direction. For example, the probe support  701  may have a shape in which the probe support  701  extends in the second direction DR 2  along the second rail RL 2 , and may have a length corresponding to short sides of the stage STA or the target substrate SUB extending in the second direction DR. However, embodiments are not limited thereto, and a shape of the probe support  701  may change according to a shape, a structure, or the like, of the electric field generating device  700 , the target substrate SUB, or the stage STA. 
     The probe driver  703 , the probe jig  705  connected to the probe driver  703  to receive an electrical signal, and the probe pad  708  connected to the probe jig  705  to transfer the electrical signal onto the target substrate SUB may be disposed on the probe support  701 . 
     The probe driver  703  may be disposed on the probe support  701  and move the probe jig  705  and the probe pad  708 . In an embodiment, the probe driver  703  may move the probe jig  705  in a horizontal direction and a vertical direction, for example, the first direction DR 1  which is the horizontal direction and the third direction DR 3  which is the vertical direction. The probe pad  708  may be connected to or disconnected from the target substrate SUB by driving of the probe driver  703 . Among processes of the inkjet printing apparatus  1000 , in a step of forming an electric field in the target substrate SUB, the probe driver  703  may be driven to connect the probe pad  708  to the target substrate SUB, and in other steps, the probe driver  703  may be driven again to disconnect the probe pad  708  from the target substrate SUB. This will be described in detail below with reference to other drawings. 
     The probe jig  705  may be connected to the probe pad  708  and may be connected to a separate voltage applying device. The probe jig  705  may transfer an electrical signal transferred from the voltage applying device to the probe pad  708  to form the electric field on the target substrate SUB. The electrical signal transferred to the probe jig  705  may be a voltage for forming the electric field, for example, an alternating current (AC) voltage. 
     The electric field generating device  700  may include probe jigs  705 , and the number of probe jigs  751  is not limited. For example, two probe jigs  705  and two probe drivers  703  may be disposed in each of each of the first and second electric field generating devices  710  and  720 , but the probe unit  750  may include a larger number of probe jigs  705  and a larger number of probe drivers  703  to form an electric field having a higher density on the target substrate SUB. 
     The probe pad  708  may generate an electric field on the target substrate SUB through the electrical signal transferred from the probe jig  705 . The probe pad  708  may be connected to the target substrate SUB and may transfer the electrical signal to the target substrate SUB to generate the electric field on the target substrate SUB. As an example, the probe pad  708  may be in contact with an electrode, a power source pad, or the like, of the target substrate SUB, and the electrical signal of the probe jig  705  may be transferred to the electrode or the power source pad. The electrical signal transferred to the target substrate SUB may generate the electric field on the target substrate SUB. However, embodiments are not limited thereto, and the probe pad  708  may also be connected (e.g., electrically connected) to the target substrate SUB and may generate the electric field on the target substrate SUB, in a state in which the probe pad  708  is not in contact with the target substrate SUB. 
     A shape of the probe pad  708  is not limited, but in an embodiment, the probe pad  708  may have a shape in which the probe pad  708  extends in a direction so as to cover a side of the target substrate SUB, for example, a short side of the target substrate SUB extending in the second direction DR 2 . 
     In case that the target substrate SUB is prepared on the stage STA, the electric field generating device  700  may be connected (e.g., electrically connected) to the target substrate SUB by the movement of the probe driver  703 . The electric field generating device  700  may generate the electric field on the target substrate SUB before, while, or after the ink  90  is jetted onto the target substrate SUB. 
       FIGS.  8  and  9    are schematic views illustrating an operation of the electric field generating device  700  according to an embodiment. 
     Referring to  FIGS.  8  and  9   , in a first state in which the electric field is not formed on the target substrate SUB, the probe pad  708  of the electric field generating device  700  may be in a state in which the probe pad  708  is spaced apart from the target substrate SUB. The probe driver  703  may be driven in the second direction DR 2 , which is the horizontal direction, and the third direction DR 3 , which is the vertical direction, such that the probe pad  708  may be spaced apart from the target substrate SUB. 
     In a second state in which the electric field is formed on the target substrate SUB, the probe driver  703  may be driven to connect (e.g., electrically connect) the probe pad  708  to the target substrate SUB. In an embodiment, the probe driver  703  may be driven in the third direction DR 3 , which is the vertical direction, and the first direction DR 1 , which is the horizontal direction, such that the probe pad  708  may be in contact with the target substrate SUB. Pad parts to which the electrical signal may be applied may be disposed on the target substrate SUB, and the probe pad  708  may be in contact with the pad part of the target substrate SUB to transfer the electrical signal. The probe jig  705  may transfer the electrical signal to the probe pad  708 , and the electric field may be formed on the target substrate SUB. 
     For example, a configuration (or a structure) of the electric field generating device  700  is not limited thereto. In an embodiment, the electric field generating device  700  may be an antenna unit, a device including electrodes, or the like. 
       FIG.  10    is a schematic view illustrating that an electric field is generated on a target substrate SUB by the electric field generating device  700  according to an embodiment.  FIG.  11    is a schematic view illustrating that discharged bipolar elements  95  are arranged on the target substrate SUB according to an embodiment. 
     Referring to  FIGS.  10  and  11   , the inkjet device  300  may jet (or spray) the ink  90  onto the target substrate in case that an electric field EL is generated on the stage STA or the target substrate SUB. As described above, the bipolar element  95  may include the first end portion and the second end portion that have the polarities, and in case that the bipolar element  95  is disposed in an electric field, a dielectrophoretic force may be transferred to the bipolar element  95 , such that a position or an orientation direction of the bipolar element  95  may change. Positions and orientation directions of the bipolar elements  95  in the ink  90  jetted onto the target substrate SUB may change by the electric field EL generated by the electric field generating device  700 . In the printing process of the bipolar elements  95  by using the inkjet printing apparatus  1000 , in case that the ink  90  is jetted onto the target substrate SUB, a first alignment step of orienting the bipolar elements  95  in a direction may be performed. 
     In case that the inkjet device  300  discharges the ink  90  in a state in which the electric field generating device  700  generates the electric field EL on the target substrate SUB, the ink  90  discharged from the inkjet head  330  may pass through the electric field EL and be jetted onto the target substrate SUB. The bipolar elements  95  may receive a dielectrophoretic force by the electric field EL until the ink  90  reaches the target substrate SUB or even after the ink  90  reaches the target substrate SUB. The bipolar elements  95  may be dispersed in random orientation directions within the ink  90 , and orientation directions and positions of the bipolar elements  95  may change by the electric field EL generated by the electric field generating device  700  after the bipolar elements  95  are discharged from the inkjet head  330 . 
     In some embodiments, the electric field EL generated by the electric field generating device  700  may be formed in a direction parallel to an upper surface of the target substrate SUB. The bipolar elements  95  jetted onto the target substrate SUB may be oriented so that an extension direction of major axes of the bipolar elements  95  is the direction horizontal to the upper surface of the target substrate SUB by the electric field EL. For example, the bipolar elements  95  may be seated (or disposed) on the target substrate SUB in a state in which first end portions of the bipolar elements  95  having the polarity are oriented in a specific direction. 
     In case that the bipolar elements  95  are seated on the target substrate SUB, a degree of alignment may be measured in consideration of a deviation in orientation directions of the bipolar elements  95  or a deviation in positions of the bipolar elements  95  seated on the target substrate SUB. For the bipolar elements  95  seated on the target substrate SUB, a deviation in orientation directions and a deviation in seated positions of another bipolar elements  95  with respect to any one bipolar element  95  may be measured, and the degree of alignment of the bipolar elements  95  may be measured through these deviations. The ‘degree of alignment’ of the bipolar elements  95  may refer to deviations in orientation directions and seated positions of the bipolar elements  95  aligned on the target substrate SUB. For example, in case that the deviations in the orientation directions and the seated positions of the bipolar elements  95  are great, the degree of alignment of the bipolar elements  95  may be low. For example, in case that the deviations in the orientation directions and the seated positions of the bipolar elements  95  are small, the degree of alignment of the bipolar elements  95  may be high or improved. 
     A point in time at which the electric field generating device  700  generates the electric field EL on the target substrate SUB is not limited thereto. For example, the electric field generating device  700  may generate the electric field EL in case that the ink  90  is discharged from the nozzle  335  and reaches the target substrate SUB. Accordingly, the bipolar elements  95  may receive a dielectrophoretic force by the electric field EL until the bipolar elements  95  are discharged from the nozzle  335  and reach the target substrate SUB. Accordingly, a time for which the bipolar elements  95  are disposed in the electric field EL may increase, and may be jetted onto the target substrate SUB in case that their positions and directions change within the ink  90 . However, embodiments are not limited thereto, and in some cases, the electric field generating device  700  may also generate the electric field EL after the ink  90  is seated on the target substrate SUB. For example, the electric field generating device  700  may generate the electric field EL when or after the ink  90  is jetted from the inkjet head  330 . 
     For example, the bipolar element  95  jetted onto the target substrate SUB may be oriented in a direction by the electric field EL formed by the electric field generating device  700 . However, in some embodiments, the bipolar elements  95  may include a semiconductor material having a high specific gravity, and the solvent  91  of the ink  90  may be a solution having a high viscosity so that the bipolar elements  95  having the high specific gravity may be dispersed in the solution for a long time. For example, positions and directions of the bipolar elements  95  may not be smoothly changed by the electric field EL generated by the electric field generating device  700 . For example, the bipolar elements  95  may include first end portions and second end portions having different polarities, and any one of the first end portions and the second end portions of the bipolar elements  95  may be oriented in a direction toward which the electric field EL is directed (or oriented). Referring to  FIG.  11   , even though the bipolar elements  95  are oriented by the electric field EL to have the initial orientation directions, in case that a viscosity of the solvent  91  is high or alignment reactivity of the bipolar elements  95  by the electric field EL is low, directions of specific end portions of the bipolar elements  95  may not be constant. 
     The inkjet printing apparatus  1000  according to an embodiment may include a light irradiation device  500  irradiating the ink with light in order to improve a degree to which the bipolar elements  95  are oriented by the electric field EL. In case that the ink  90  is irradiated with the light when or before the electric field generating device  700  generates the electric field EL, dipole moments of the bipolar elements  95  may become great, and the bipolar elements  95  may receive a stronger force even with the electric field EL of the same strength. For example, the alignment reactivity of the bipolar elements  95  by the electric field EL may increase. Thus, the initial orientation directions of the bipolar elements  95  may be further changed by the electric field EL and the light. Accordingly, the final orientation directions of the bipolar elements  95  may be aligned more uniformly. 
       FIG.  12    is a schematic side view illustrating the inkjet device  300  and the light irradiation device  500  according to an embodiment.  FIG.  13    is a schematic cross-sectional view illustrating the light irradiation device  500  according to an embodiment.  FIG.  12    illustrates side surfaces of the inkjet device  300  and a first light irradiation device  510  disposed on the first frame FM 1  together, and  FIG.  13    is a schematic front view illustrating that a second light irradiation device  520  irradiates the target substrate SUB with light hv. 
     Referring to  FIGS.  12  and  13   , the inkjet printing apparatus  1000  may include at least one light irradiation device  500  (e.g.,  510  and  520 ). According to an embodiment, the light irradiation device  500  may include a second light irradiation device  520  disposed between a second frame FM 2  and a third frame FM 3  in addition to a first light irradiation device  510  disposed on the first frame FM 1  like the inkjet device  300 . 
     The inkjet printing apparatus  1000  may include a larger number of frames FM 2  to FM 6  in addition to the first frame FM 1  on which the inkjet device  300  is disposed. Frames FM 2  to FM 6  may be spaced apart from each other along a direction in which the first rails RL 1  and the second rails RL 2  extend. The inkjet printing apparatus  1000  may include the frames FM 2  to FM 6  so that devices for the printing process and an inspection process of the bipolar elements  95  may be disposed. Each of the frames FM 2  to FM 6  may include a first support part FM_C and a second support part FM_R, similar to the first frame FM 1 , and necessary devices may be disposed on the frames FM 2  to FM 6 . A shape and an arrangement of each of the frames FM 2  to FM 6  may be substantially the same as those of the first frame FM 1  as described above by way of example, and a detailed description thereof will thus be omitted for descriptive convenience. 
     Each light irradiation device  500  (e.g.,  510  and  520 ) may include a second base part  501  and a light irradiation unit  503 . 
     The second base part  501  may have a shape in which the second base part  501  extends in a direction, similar to the first base part  310  of the inkjet device  300 . The second base part  501  may have a shape in which the second base part  501  extends in the first direction DR 1  so as to correspond to the long sides of the stage STA or the target substrate SUB, for example, sides extending in the first direction DR 1 . A schematic shape of the second base part  501  of the light irradiation device  500  is illustrated in the drawings, but embodiments are not limited thereto. The second base part  501  of the light irradiation device  500  may also have a shape independent of shapes of the stage STA and the target substrate SUB. Embodiments are not limited thereto. 
     The light irradiation unit  503  may be disposed on the second base part  501 . The light irradiation unit  503  may irradiate the target substrate SUB disposed on the stage STA with light hv. A manner in which the light irradiation unit  503  is disposed on the second base part  501  is not limited. For example, the light irradiation unit  503  may be fastened (e.g., directly fastened) to a lower surface of the second base part  501 , but the light irradiation unit  503  may be coupled to or mounted on the second base part  501  through a separate member. 
     A type of the light irradiation unit  503  is not limited. In some embodiments, the light irradiation unit  503  may include mercury light, Fe-based metal halide-based, Ga-based metal halide-based, semiconductor light emitting elements, and the like. However, embodiments are not limited thereto. 
     In an embodiment, the first light irradiation device  510  may be mounted on the first frame FM 1  together with the inkjet device  300 , and may irradiate the target substrate SUB with the light hv simultaneously with a process of jetting the ink  90  in the printing process of the bipolar elements  95 . Referring to  FIG.  11   , the ink  90  may be jetted onto the electric field EL generated by the electric field generating device  700 , on the target substrate SUB disposed on the stage STA passing below the first frame FM 1 . In case that the stage STA passes through the inkjet device  300 , a partial region of the target substrate SUB may be irradiated with the light hv emitted from the first light irradiation device  510  mounted on the first frame FM 1 . Since the first light irradiation device  510  irradiates only a region with the light hv in case that the stage STA moves, a primary light irradiation process performed by the first light irradiation device  510  may be performed in a scan manner according to the movement of the stage STA. Since the first light irradiation device  510  is mounted on the first frame FM 1  together with the inkjet device  300 , an area irradiated with the light hv from the first light irradiation device  510  may be small, and the target substrate SUB may not be sufficiently irradiated with the light hv. The inkjet printing apparatus  1000  according to an embodiment may further include the second light irradiation device  520  capable of irradiating an area greater than the area irradiated with the light from the first light irradiation device  510  with light, and in the printing process of the bipolar elements  95 , a secondary light irradiation process following the primary light irradiation process may be performed. 
     The second base part  501  of the second light irradiation device  520  may be mounted on the second frame FM 2  and the third frame FM 3 . The stage STA passing through the first frame FM 1  may pass below the second light irradiation device  520  with passing through the second frame FM 2  and the third frame FM 3 . The light irradiation unit  503  of the second light irradiation device  520  may have a greater area than the light irradiation unit  503  of the first light irradiation device  510  so as to cover the entirety of the target substrate SUB. The second light irradiation device  520  may also irradiate the target substrate SUB with the light hv in case that the stage STA passes through the second light irradiation device  520 , but the second light irradiation device  520  may have a greater area than the first light irradiation device  510 , and thus, a time for irradiating the target substrate SUB with the light hv by the second light irradiation device  520  may be longer than a time for irradiating the target substrate SUB with the light hv by the first light irradiation device  510 . The second light irradiation device  520  may have a greater area than the target substrate SUB, such that the target substrate SUB may be irradiated (e.g., entirely irradiated) with the light in the secondary light irradiation process. 
     According to an embodiment, the second light irradiation device  520  may irradiate the target substrate with the light hv after the ink  90  is jetted from the inkjet device  300 , unlike the first light irradiation device  510 . The inkjet printing apparatus  1000  may include light irradiation devices  500  (e.g.,  510  and  520 ), and thus perform the light irradiation process for improving a degree of alignment of the bipolar elements  95  twice. 
     For example, the second light irradiation device  520  may irradiate the target substrate with the light hv in case that the stage STA passes through the second light irradiation device  520 , but embodiments are not limited thereto. In some embodiments, the stage STA may be subjected to the secondary light irradiation process in a state where the stage STA is stopped for a time below the second light irradiation device  520 , and then move again. This may be adjusted according to a light irradiation degree for alignment of the bipolar elements  95 . 
     The light irradiation device  500  may irradiate the ink  90  jetted (or sprayed) onto the target substrate SUB with the light hv to improve alignment reactivity of the bipolar elements  95  by the electric field EL. The bipolar elements  95  may include first end portions having a first polarity and second end portions having a second polarity different from the first polarity to have dipole moments. The bipolar elements  95  having the dipole moments may be oriented in a direction by receiving an electrical force by the electric field EL generated by the electric field generating device  700 . In case that the light irradiation device  500  irradiates the bipolar elements  95  with the light hv, a partial polarity is further formed in the bipolar elements  95 , such that the dipole moments may become greater, and the bipolar elements  95  may receive a greater electrical force by the electric field EL. Accordingly, the bipolar elements  95  dispersed in the ink  90  may have increased alignment reactivity, and may be oriented with a high degree of alignment on the target substrate SUB. 
       FIG.  14    is a schematic view illustrating that bipolar elements  95  arranged on the target substrate SUB are irradiated with light hv according to an embodiment. 
     Referring to  FIG.  14   , bipolar elements  95  may be jetted onto the target substrate SUB prepared on the electric field generating device  700 , and the light irradiation device  500  may irradiate the ink  90  jetted onto the target substrate SUB with the light hv. In the printing process of the bipolar elements  95  by using the inkjet printing apparatus  1000 , after the ink  90  is jetted onto the target substrate SUB, a second alignment step of orienting the bipolar elements  95  with irradiating the target substrate SUB with the light hv may be performed. 
     For example, as in the primary light irradiation process, a first region AA 1  of the target substrate SUB may not be irradiated with the light hv, a second region AA 2  of the target substrate SUB may be irradiated with the light hv, and there may be first bipolar elements  95 A positioned in the first region AA 1  and not irradiated with the light hv and second bipolar elements  95 B positioned in the second region AA 2  and irradiated with the light hv among the bipolar elements  95  jetted onto the target substrate SUB. 
     In the second bipolar elements  95 B irradiated with the light hv, electrons of portions having polarities may react with or may be excited (or activated) by the irradiated light hv, such that the dipole moments between the first end portions having the first polarity and the second end portions having the second polarity may become greater. In case that the bipolar element  95  has a great bipolar moment, a magnitude of the dielectrophoretic force caused by the electric field EL generated on the target substrate SUB may be increased. As described above, orientation directions of the bipolar elements  95  may be determined on the basis of directions toward which the first end portions having the first polarity are directed (or oriented) in case that positions and directions of the bipolar elements  95  are changed by the electric field EL. The bipolar elements  95  having the greater dipole moments may have increased alignment reactivity with respect to the electric field EL, and the bipolar elements  95  may be aligned so that orientation directions thereof may be substantially uniform. 
     The first bipolar elements  95 A jetted onto the first region AA 1  may be oriented so that extension directions thereof are a specific direction by the electric field EL, but orientation directions toward which the first end portions of the first bipolar elements  95 A are directed (or oriented) may not be uniform. The second bipolar elements  95 B jetted onto the second region AA 2  may be irradiated with the light hv, and may thus have increased alignment reactivity with respect to the electric field EL, and may be re-oriented with rotating or moving from an initial position (e.g., dotted line portion) so that the orientation directions toward which the first end portions of the second bipolar elements  95 B are directed (or oriented) may be substantially uniform. 
     For example, the inkjet printing apparatus  1000  may include the electric field generating device  700  disconnected from the stage STA but capable of moving simultaneously with the stage STA. The ink  90  may be jetted onto the target substrate SUB or the target substrate SUB may be irradiated with the light hv according to the movement of the stage STA, and the electric field generating device  700  may continuously generate the electric field EL on the target substrate SUB regardless of a process step in the printing process of the bipolar elements  95 . Accordingly, the electric field EL may be generated before or simultaneously with the jetting of the ink  90 , such that a time for which the bipolar elements  95  may be disposed in the electric field EL may increase, and the generation of the electric field EL may be maintained even during the light irradiation process, such that the orientation directions of the bipolar elements  95  may be substantially uniform and a degree of alignment of the bipolar elements  95  may be improved. 
     In some embodiments, a central wavelength band of the light hv irradiated from the light irradiation device  500  is not limited. The light hv may change according to a type of the bipolar element  95  as described below, the bipolar element  95  may include a semiconductor material, and the central wavelength band of the light hv irradiated from the light irradiation device  500  may change according to a material of the bipolar element  95 . In an embodiment, the central wavelength band of the light irradiated from the light irradiation device  500  may be in the range of about 300 nm to about 700 nm or in the range of about 350 nm to about 500 nm, but embodiments are not limited thereto. 
     In case that the bipolar elements  95  jetted onto the target substrate SUB are oriented or aligned in a direction, a drying process for removing the solvent  91  of the ink  90  may be performed. The inkjet printing apparatus  1000  according to an embodiment may further include the drying device  800  behind the light irradiation device  500 . 
       FIG.  15    is a schematic front view illustrating the drying device  800  according to an embodiment.  FIG.  15    illustrates the drying device  800  irradiating the stage STA with heat when viewed from the front. 
     Referring to  FIG.  15   , the drying device  800  of the inkjet printing apparatus  1000  may include a third base part  801  and a heat treatment unit  805 . According to an embodiment, the inkjet printing apparatus  1000  may include the drying device  800  disposed between a fourth frame FM 4  and a fifth frame FM 5 . 
     As described above, the inkjet printing apparatus  1000  may include the frames FM 1  to FM 6 . The frames FM 1  to FM 6  may be spaced apart from each other along a direction in which the first rails RL 1  and the second rails RL 2  extend. The fourth frame FM 4  and the fifth frame FM 5  may be further disposed behind the second frame FM 2  and the third frame FM 3  between which the second light irradiation device  520  is disposed, and the drying device  800  may be disposed between the fourth frame FM 4  and the fifth frame FM 5 . 
     The third base part  801  may have a shape similar to that of the first base part  310  of the inkjet device  300  and the second base part  502  of the light irradiation device  500 . A detailed description thereof will be omitted for descriptive convenience. 
     The heat treatment unit  805  may be disposed on the third base part  801 . The heat treatment unit  805  may irradiate an upper portion of the target substrate SUB disposed on the stage STA with heat. A drying device  800  that dries the solvent  91  through the heat by including the heat treatment unit  805  is described as an example of the drying device  800  in the specification, but embodiments are not limited thereto. The drying device  800  may be a device for drying the solvent  91  of the ink  90 , and may include various units. For example, the drying device  800  may include an infrared radiation (IR) irradiation unit irradiating the target substrate with infrared. However, embodiments are not limited thereto. 
     A manner in which the heat treatment unit  805  is disposed on the third base part  801  is not limited. For example, the heat treatment unit  805  may be fastened (e.g., directly fastened) to the third base part  801 , but the heat treatment unit  805  may be coupled to or mounted on the third base part  801  through a separate member. The heat treatment units  805  of the drying device  800  may be spaced apart from other members disposed on the target substrate SUB enough not for the other members to be damaged by the irradiated heat. For example, in some embodiments, a shielding device may be further disposed on a lower surface of the heat treatment unit  805 . The shielding device may block (e.g., partially block) the heat irradiated from the heat treatment unit  805  so that the target substrate SUB may not be damaged. 
     The drying device  800  may irradiate the target substrate SUB with the heat in case that the stage STA passes through the drying device  800  behind the second light irradiation device  520 . However, embodiments are not limited thereto, and the stage STA may be subjected to a drying process in a state in which is stopped for a time below the drying device  800 . 
     The ink  90  jetted onto the target substrate SUB may include the solvent  91  in which the bipolar elements  95  are dispersed, in addition to the bipolar elements  95  oriented in a direction. The drying device  800  may remove the solvent  91  of the ink  90 , and the bipolar elements  95  may be seated on the target substrate SUB so that positions thereof may be fixed. According to an embodiment, in order to prevent orientation directions and positions of the bipolar elements  95  from changing in case that the solvent  91  is removed, the inkjet printing apparatus  1000  may perform the drying process of the solvent  91  in a state in which the electric field generating device  700  generates the electric field EL on the target substrate SUB. 
       FIG.  16    is a schematic view illustrating that aligned bipolar elements  95  are seated on the target substrate SUB according to an embodiment  FIG.  17    is a schematic view illustrating that a solvent of the ink is dried according to an embodiment. 
     Referring to  FIGS.  16  and  17   , bipolar elements  95  may be aligned in the first region AA 1  and the second region AA 2  of the target substrate SUB in a state in which the bipolar elements  95  are oriented in a direction. In case that the stage STA passes through the drying device  800 , the target substrate SUB may be irradiated with the heat, and the bipolar elements  95  may be seated on the target substrate SUB in case that the solvent  91  is removed. However, as described above, the solvent  91  of the ink  90  may be a solvent having a high viscosity in order to maintain a state in which the bipolar elements  95  are dispersed for a long time. An initial alignment state of the bipolar elements  95  may change by an attractive force by a flow of a fluid or an attractive force between the solvent  91  and the bipolar elements  95  in a process in which the solvent  91  is dried or volatilized and removed by the heat. The electric field generating device  700  of the inkjet printing apparatus  1000  according to an embodiment may generate the electric field EL on the target substrate SUB even during the drying process of the solvent  91 , and may prevent a misalignment problem that the orientation directions and positions of the bipolar elements  95  change. 
     For example, the drying device  800  of the inkjet printing apparatus  1000  may irradiate the upper portion of the target substrate SUB with the heat, and thus, the solvent  91  may be dried from a surface of the target substrate SUB, such that the occurrence of internal convection due to the heat may be minimized. In case that the convection occurs in the solvent  91  by heat treatment in an initial drying process after the secondary alignment step performed in the light irradiation process, the bipolar elements  95  may be misaligned. In the inkjet printing apparatus  1000  according to an embodiment, the drying device  800  may irradiate the upper portion of the stage STA or the target substrate SUB with the heat, and the solvent  91  may be dried from the surface of the target substrate SUB, such that a misalignment phenomenon of the bipolar elements  95  may be minimized. 
     In order to prevent the misalignment phenomenon of the bipolar elements  95 , a strength of the electric field EL required in the drying process may be lower than that of the electric field EL required in an alignment process of the bipolar elements  95 . The electric field generating device  700  may connect the target substrate SUB and the probe pad  708  to each other through the movement of the probe driver  703 , and a time may be required in this process. In case that a lot of time is required in a process of connecting and disconnecting (e.g., electrically connecting and electrically disconnecting) the electric field generating device  700  and the target substrate SUB to and from each other, even though a process time is shortened by performing continuously the printing processes of the bipolar elements  95 , it may take a lot of time to prepare for the next process. 
     In the inkjet printing apparatus  1000  according to an embodiment, the stage STA and the electric field generating device  700  may be disconnected from each other and moved, respectively, and before a printing process is completely completed, at least one electric field generating device  700  may be disconnected (e.g., electrically disconnected) from the target substrate SUB and moved. 
       FIG.  18    is a schematic view illustrating movement of the electric field generating device  700  according to an embodiment. 
     Referring to  FIG.  18   , in case that the stage STA is subjected to the drying process in the drying device  800 , the electric field generating device  700  may generate the electric field EL on the target substrate SUB. However, as described above, the strength of the electric field EL required in the drying process may be weaker than that of the electric field required in the alignment process, and thus, both the first electric field generating device  710  and the second electric field generating device  720  may not be connected to the target substrate SUB. In an embodiment, the first electric field generating device  710  and the second electric field generating device  720  may be disconnected (or spaced apart) from the stage STA and moved, and in case that the stage STA moves to a specific process device, at least one of the first electric field generating device  710  and the second electric field generating device  720  may move in a direction opposite to a moving direction of the stage STA. For example, in case that the stage STA moves to the drying device  800 , the first electric field generating device  710  may be disconnected (e.g., electrically disconnecting) from the target substrate SUB and moved to a position before the first frame FM 1 , which is an initial position. The second electric field generating device  720  may be connected (e.g., electrically connected) to the target substrate SUB to generate the electric field EL during the drying process. The first electric field generating device  710  may move in the direction opposite to the moving direction of the stage STA to prepare for the next printing process, and the second electric field generating device  720  may move together with the stage STA to prevent the bipolar elements  95  from being misaligned in the drying process and be then disconnected (e.g., electrically disconnected) from the target substrate SUB. 
     For example, the first electric field generating device  710  may be disconnected from the stage STA in case that the stage STA moves to the drying device  800  to be subjected to the drying process, but embodiments are not limited thereto. In some embodiments, any one electric field generating device  700  may be disconnected from the stage STA in case that the stage STA moves to the second light irradiation device  520  and the secondary light irradiation process is performed. The electric field generating device  700  may be connected (e.g., electrically connected) to the target substrate SUB so as to generate the electric field EL in at least the secondary light irradiation process, and may be disconnected (e.g., electrically disconnected) from the target substrate SUB in the subsequent process and moved in order to prepare for the next process. 
     According to an embodiment, the electric field generating devices  700  (e.g.,  710  and  720 ) or the electric field generating device  700  and the stage STA may move individually. Accordingly, a time for connecting and disconnecting (e.g., electrically connecting and electrically disconnecting) the electric field generating device  700  and the target substrate SUB to and from each other, which requires a lot of time in the printing process, may be shortened. In the inkjet printing apparatus  1000 , devices for the printing process may be disposed in a line, such that the respective processes may be continuously performed, and thus, an unnecessary time between the processes may be minimized and a time required for preparing for the next process may be shortened. 
       FIG.  19    is a schematic front view illustrating the inspection device  900  according to an embodiment. 
     Referring to  FIG.  19   , the inkjet printing apparatus  1000  may further include the inspection device  900  to inspect a degree of alignment of the bipolar elements  95  aligned on the target substrate SUB. In case that all of the solvents  91  are removed after the drying process, the electric field generating devices  700  may be disconnected (e.g., electrically disconnected) from the target substrate SUB and moved in order to prepare for the next process. For example, the stage STA may pass through the drying device  800  and moves to the inspection device  900 , such that an inspection process of the degree of alignment of the bipolar elements  95  may be further performed. 
     The inspection device  900  of the inkjet printing apparatus  1000  may include a fourth base part  910  and sensing units  950 . According to an embodiment, the inkjet printing apparatus  1000  may include the inspection device  900  disposed on a sixth frame FM 6 . 
     The fourth base part  910  may have a shape similar to that of the first base part  310  of the inkjet device  300  and the second base part  502  of the light irradiation device  500 . A detailed description thereof will be omitted for descriptive convenience. 
     The sensing units  950  may be disposed on the fourth base part  910 . The sensing unit  950  may measure the positions or the orientation directions of the bipolar elements  95  seated or aligned on the target substrate SUB, and may measure the degree of alignment of the bipolar elements  95  through deviations in the positions and the orientation directions of the bipolar elements  95 . 
     For example, the sensing unit  950  may measure positions where the bipolar elements  95  are seated on the target substrate SUB, a distance between neighboring bipolar elements  95 , the number of bipolar elements  95  seated in a region, or the like. In case that regions are defined on the target substrate SUB, the inkjet printing apparatus  1000  may print the number of the bipolar elements  95  on the regions defined on the target substrate SUB. The inspection device  900  may inspect whether or not the bipolar elements are seated in a state in which they are agglomerated with the other bipolar elements  95  in addition to how many bipolar elements  95  are accurately seated in the regions. 
     For example, since the bipolar elements  95  have a shape in which they extend in a direction and end portions (e.g., opposite end portions) of the bipolar elements  95  have different polarities, the orientation directions toward which the first end portions of the bipolar elements  95  having the first polarity are directed (or oriented) may be determined. The inspection device  900  may measure the degree of alignment of the bipolar elements  95  by measuring the orientation directions of the bipolar elements  95  with measuring the positions of the bipolar elements  95 . The inspection device  900  may measure directions toward which the first end portions of the bipolar elements  95  are directed, angles between any line and the directions toward which the first end portions of the bipolar elements  95  are directed, e.g., orientation angles, and the like. In case that portions where the bipolar elements  95  are disposed on the target substrate SUB are specified, the inspection device  900  may inspect whether or not the bipolar elements  95  are accurately disposed on these portions. The inkjet printing apparatus  1000  may improve reliability of the printing process by confirming completeness of the printing process through seating position deviations, the degree of alignment, and the like, of the bipolar elements  95  measured by the inspection device  900 , and at the same time, providing feedback to the respective devices based on information obtained through the confirmation of the completeness. 
     In case that the solvents  91  of the inks  90  are solvents having a high viscosity, the solvents  91  may not be completely removed in the drying process, and may remain as foreign materials on the target substrate SUB in a subsequent process. In order to completely remove the solvents  91  remaining on the target substrate SUB, the inkjet printing apparatus  1000  according to an embodiment may include a larger number of drying devices  800  to perform one or more drying processes in the printing process of the bipolar elements  95 . 
       FIG.  20    is a schematic plan view of an inkjet printing apparatus  1000 _ 1  according to an embodiment.  FIG.  21    is a schematic front view illustrating a drying device  800  according to an embodiment. 
     Referring to  FIGS.  20  and  21   , an inkjet printing apparatus  1000 _ 1  according to an embodiment may include a larger number of drying devices  800  (e.g.,  810  and  820 ). The drying device  800  may include a first drying device  810  and a second drying device  820  disposed behind the second light irradiation device  520 . The stage STA may be subjected to a primary drying process in the first drying device  810 , and may then move to the second drying device  820  to be subjected to a secondary drying process. The inkjet printing apparatus  1000 _ 1  according to an embodiment is different from the inkjet printing apparatus  1000  according to the above-described embodiment in that the inkjet printing apparatus  1000 _ 1  further includes the second drying device  820  to perform drying processes in the printing process of the bipolar elements  95 . A description of the first drying device  810  is substantially the same as that described above, and thus, the second drying device  820  will hereinafter be described in detail. 
     The inkjet printing apparatus  1000 _ 1  may further include a seventh frame FM 7  and an eighth frame FM 8 , and the second drying device  820  may be disposed between the seventh frame FM 7  and the eighth frame FM 8 . The second drying device  820  may also include a third base part  801  and a heat treatment unit  805 , and may irradiate the stage STA or the target substrate SUB moved below the second drying device  820  with heat. 
     Even though the primary drying process is performed in the first drying device  810 , the solvents  91  disposed on the target substrate SUB may not be completely removed, and some of the solvents  91  may remain. As described above, the solvents  91  may be solvent materials having a high viscosity, and the solvents  91  may be removed from the surfaces of the target substrate SUB in the primary drying process in order to prevent the misalignment of the bipolar elements  95 , and thus, some solvents  91  may remain on the target substrate SUB. The solvents  91  remaining on the target substrate SUB may remain as foreign materials in a subsequent process for manufacturing a product including the bipolar elements  95 . 
     The inkjet printing apparatus  1000 _ 1  may perform the drying process twice by including the drying devices  800  (e.g.,  810  and  820 ) in order to completely remove the solvents  91  of the inks  90 . After the primary drying process is performed through the first drying device  810 , the bipolar elements  95  may be safely seated on the target substrate SUB, and a misalignment problem may not occur. Accordingly, in the secondary drying process by using the second drying device  820 , a heat treatment process may be performed at a higher temperature than the primary drying process. 
     For example, in an embodiment, the inkjet printing apparatus  1000 _ 1  may further include electric field generating units  730 , which is different from the electric field generating device  700 ) and disposed in the second drying device  820 . The electric field generating unit  730  may generate an electric field EL on the target substrate SUB by including a probe driver  703 , a probe jig  705 , and a probe pad  708 , similar to the electric field generating device  700  (e.g.,  710  and  720 ). However, the electric field generating units  730  may not move in the second direction DR 2  along the stage STA unlike the first electric field generating device  710  and the second electric field generating device  720 . The electric field generating units  730  may be disposed between the seventh frame FM 7  and the eighth frame FM 8 , and may be connected (e.g., electrically connected) to the target substrate SUB to generate the electric field EL on the target substrate SUB in case that the stage STA moves to the second drying device  820 . In case that the secondary drying process is performed at a high temperature in the second drying device  820 , the electric field generating units  730  may prevent the bipolar elements  95  on the target substrate SUB from being misaligned. 
     Some electric field generating units  730  may be disposed on the second rails RL 2  and be disposed on the sides (e.g., opposite sides) of the stage STA in the first direction DR 1 . Embodiments are not limited thereto, and in case that the stage STA moves to the second drying device  820 , some electric field generating units  730  may move to the sides (e.g., opposite sides) of the stage STA in the second direction DR 2  to be connected (e.g., electrically connect) to the target substrate SUB. For example, after some electric field generating units  730  may be mounted on the seventh frame FM 7  and the eighth frame FM 8 , in case that the stage STA moves, the probe drivers  703  of some electric field generating units  730  move, such that some electric field generating units  730  may be connected to the target substrate SUB. For example, two electric field generating units  730  may be disposed on the sides of the stage STA in the first direction DR 1  and two electric field generating units  730  may be disposed on the other sides of the stage STA in the second direction DR 2 , such that a total of four electric field generating units  730  may be disposed. However, embodiments are not limited thereto. In some cases, the two electric field generating units  730  disposed on the sides of the stage STA in the first direction DR 1  may also be omitted, and the electric field generating devices  700  (e.g.,  710  and  720 ) may also move to the second drying device  820  together with the stage STA. 
     Since the inkjet printing apparatus  1000 _ 1  includes electric field generating units  730  disposed together with the second drying device  820 , the electric field generating devices  700  (e.g.,  710  and  720 ) may be disconnected from the stage STA after the first drying process performed in the first drying device  810 . After the primary drying process, the bipolar elements  95  may remain on the stage STA in a state in which some of the solvents  91  are removed, and in case that the stage STA moves to the second drying device  820 , misalignment of the bipolar elements  95  may be prevented by the electric field EL generated by the electric field generating units  730 . Accordingly, the electric field generating devices  710  and  720  may not move to the second drying device  820 , and may be disconnected from the stage STA to prepare for a subsequent printing process. In case that the inkjet printing apparatus  1000 _ 1  includes stages STA, a target substrate SUB may be prepared on a second stage and the electric field generating devices  700  may move together with the second stage, in case that a first stage is subjected to the secondary drying process in the second drying device  820 . Accordingly, even though a process time of the printing process is increased because the inkjet printing apparatus  1000 _ 1  further includes the second drying device  820 , a preparation time between printing processes may be shortened, such that an overall process time may be shortened. 
     For example, the stage STA may not move to the second drying device  820 , and only the target substrate SUB may move to the second drying device  820 . In some embodiments, the inkjet printing apparatus  1000 _ 1  may further include a stage and electric field generating units  730  together with the second drying device  820 . For example, only the target substrate SUB may move for the secondary drying process, and the stage STA and the electric field generating devices  700  may move to initial positions for a subsequent printing process. 
       FIG.  22    is a schematic front view illustrating a drying device  800  according to an embodiment. 
     Referring to  FIG.  22   , an inkjet printing apparatus  1000 _ 2  may further include a sub-stage STA 2  disposed together with the second drying device  820 . The electric field generating units  730  may be disposed on the sub-stage STA 2 , and in case that the target substrate SUB is prepared on the sub-stage STA 2 , the electric field generating units  730  may be connected to the target substrate SUB to generate an electric field EL. The inkjet printing apparatus  1000 _ 2  according to an embodiment is different from the inkjet printing apparatus  1000  (or  1000 _ 1 ) according to the above-described embodiment in that the inkjet printing apparatus  1000 _ 2  further includes the sub-stage STA 2  on which a secondary drying process is performed. Hereinafter, a redundant description will be omitted, and contents different from those described above will be described for descriptive convenience. 
     The sub-stage STA 2  may be disposed below the second drying device  820  between the seventh frame FM 7  and the eighth frame FM 8 . The sub-stage STA 2  and the stage STA may have substantially the same shape. However, the sub-stage STA 2  may not move in a direction, and may be fixedly disposed below the second drying device  820 . However, embodiments are not limited thereto. For example, the sub-stage STA 2  may not be disposed on the rails RL 1  and RL 2 , but embodiments are not limited thereto, and the sub-stage STA 2  may also be disposed on the first rails RL 1  to move between the second drying device  820  and the inspection device  900 . 
     In the secondary drying process, a drying process may be performed at a higher temperature than the primary drying process in order to completely remove the solvents  91 . Since the secondary drying process is performed in a state in which the solvents  91  are removed to some extent unlike the primary drying process, the possibility of the occurrence of internal convection of the inks  90  is low. According to an embodiment, in order to completely remove the solvents  91  on the target substrate SUB, the sub-stage STA 2  may include a heat sink STA_H capable of transferring heat below the target substrate SUB. The heat sink STA_H may be disposed inside the sub-stage STA 2  and may irradiate a lower portion of the target substrate SUB disposed above the heat sink STA_H with heat. In the secondary drying process, the solvents  91  may be completely removed through the second drying device  820  disposed above the target substrate SUB and the heat sink STA_H transferring the heat below the target substrate SUB. For example, since the electric field generating units  730  disposed on the sub-stage STA 2  generate the electric field EL on the target substrate SUB, misalignment of the bipolar elements  95  that may occur at the time of removal of the solvents  91  may also be prevented. 
     The target substrate SUB that is subjected to the primary drying process with passing through the first drying device  810  may move from the stage STA to the sub-stage STA 2  through a separate transport device. In case that the target substrate SUB moves to the sub-stage STA 2 , the stage STA and the electric field generating device  700  may move to initial positions for a subsequent printing process. The inkjet printing apparatus  1000 _ 2  according to an embodiment may further include the sub-stage STA 2  on which the secondary drying process is performed, and thus, the stage STA may move in order to prepare for a subsequent process before the printing process ends, such that a total process time may be shortened. 
     In the electric field generating device  700 , the probe pad  708  may be required to be accurate contact with the pad part disposed on the target substrate SUB in an aligned state in order for the probe pad  708  to be connected to the target substrate SUB in case that the probe driver  703  moves. In this process, it may take a lot of time to align the probe pad  708  and the target substrate SUB with and bring the probe pad  708  and the pad part into contact with each other, and a total process time of the printing process may increase. In an embodiment, the electric field generating device  700  may not be in direct contact with the target substrate SUB, and may be wirelessly connected to the target substrate SUB to generate the electric field EL on the target substrate SUB. Accordingly, since a contact process between the probe pad  708  of the electric field generating device  700  and the target substrate SUB is omitted, a preparation time of the printing process may be shortened. 
       FIG.  23    is a schematic view illustrating an electric field generating device  700 _ 1  according to an embodiment. 
     Referring to  FIG.  23   , in an electric field generating device  700 _ 1  according to an embodiment, a probe pad  708  may include electrode pads PAD_E capable of forming electrical connections wirelessly. The electrode pads PAD_E may be connected (e.g., electrically connected) to pad parts PAD_S disposed on the target substrate SUB in a state in which they are not in direct contact with the pad parts PAD_S. 
     In case that the target substrate SUB is prepared on the stage STA, the electrode pads PAD_E of the probe pad  708  of the electric field generating device  700 _ 1  may be aligned with the pad parts PAD_S of the target substrate SUB on the basis of alignment marks AM disposed on the target substrate SUB. In case that the electrode pads PAD_E and the pad parts PAD_S are wirelessly connected to each other by adjusting distances between the electrode pads PAD_E and the pad parts PAD_S, the electric field generating device  700 _ 1  may generate an electric field EL on the target substrate SUB. For example, the target substrate SUB and the probe pads  708  of the electric field generating device  700 _ 1  may be connected to each other in a state in which they are spaced apart from each other by a distance, but embodiments are not limited thereto. In some embodiments, the electrode pads PAD_E of the probe pad  708  of the electric field generating device  700 _ 1  and the pad parts PAD_S of the target substrate SUB may be connected (e.g., electrically connected) to each other in a state in which they overlap each other in a thickness direction or the third direction DR 3  and are aligned with each other. The electric field generating device  700 _ 1  according to an embodiment may be different from the electric field generating device according to the above-described embodiment in that the electric field generating device  700 _ 1  may wirelessly generate the electric field EL on the target substrate SUB. Other portions are substantially the same as those described above, and a detailed description thereof will thus be omitted for descriptive convenience. 
     Hereinafter, a printing method of a bipolar element  95  by using the inkjet printing apparatus  1000  according to an embodiment will be described in detail. 
       FIG.  24    is a flowchart illustrating a printing method of a bipolar element according to an embodiment.  FIGS.  25  to  28    are schematic cross-sectional views illustrating the printing method of a bipolar element  95  according to an embodiment. 
     Referring to  FIGS.  1  and  24  to  28   , the printing method of a bipolar element  95  according to an embodiment may include setting the inkjet printing apparatus  1000  (S 100 ), jetting the bipolar elements  95  onto the target substrate SUB (S 200 ), and seating the bipolar elements  95  on the target substrate SUB by generating an electric field on the target substrate SUB and irradiating the target substrate SUB with light (S 300 ). 
     The printing method of a bipolar element  95  according to an embodiment may be performed by using the inkjet printing apparatus  1000  described above with reference to  FIG.  1   , and in the seating of the bipolar elements  95  on the target substrate SUB, the electric field generating device  700  may generate the electric field EL on the target substrate SUB. The electric field EL may be generated when or after the ink  90  is jetted from the inkjet device  300  and may be continuously generated in the light irradiation process and the drying process. 
     For example, the inkjet printing apparatus  1000  may be set (S 100 ). The setting (S 100 ) of the inkjet printing apparatus  1000  may be tuning the inkjet printing apparatus  1000  according to a target process. For precise tuning, an inkjet print test process may be performed on an inspection substrate, and a set value of the inkjet printing apparatus  1000  may be adjusted according to a test result. 
     For example, the inspection substrate may be first prepared. The inspection substrate and the target substrate SUB may have the substantially same structure, but a bare substrate such as a glass substrate may be used as the inspection substrate. 
     For example, a water repellent treatment may be performed on an upper surface of the inspection substrate. The water repellent treatment may be performed by fluorine coating, a plasma surface treatment, or the like. 
     For example, the ink  90  including the bipolar elements  95  may be jetted (or sprayed) onto the upper surface of the inspection substrate by using the inkjet printing apparatus  1000 , and droplets for each inkjet head  330  may be measured. The measurement of the droplets for each inkjet head  330  may be performed in a manner of confirming a size of a droplet at the moment of jetting the ink and a size of a droplet applied to the substrate by using a camera. In case that the measured droplets are different from reference droplets, a voltage for each corresponding inkjet head  330  may be adjusted so that the reference droplets may be discharged. Such an inspection method may be repeated several times until each inkjet head  330  discharges accurate droplets. 
     For example, in the setting of the inkjet printing apparatus  1000 , in case that the setting of the reference set value is completed, the ink  90  in which the bipolar elements  95  are dispersed may be prepared in the ink circulation unit  600 , and may be supplied to the inkjet head  330 . The ink circulation unit  600  and the inkjet head  330  may be maintained so that the bipolar elements  95  in the ink  90  have a uniform dispersion degree by the ink circulation system. 
     However, embodiments are not limited thereto, and the setting (S 100 ) of the inkjet printing apparatus described above may also be omitted. 
     In case that the setting of the inkjet printing apparatus  1000  is completed, the target substrate SUB may be prepared, referring to  FIG.  25   . In an embodiment, a first electrode  21  and a second electrode  22  may be disposed on the target substrate SUB. For example, a pair of electrodes may be disposed, but larger pairs of electrodes may be formed on the target substrate SUB, and inkjet heads  330  may jet (or spray) the ink  90  onto each pair of electrodes in the same manner. 
     Referring to  FIG.  26   , the ink  90  including the solvent  91  in which the bipolar elements  95  are dispersed is jetted (or sprayed) onto the target substrate SUB. The ink  90  may be discharged (or sprayed) from the inkjet head  330 , and may be jetted onto the first electrode  21  and the second electrode  22  disposed on the target substrate SUB. The ink  90  may be jetted onto the first electrode  21  and the second electrode  22  disposed on the target substrate SUB, and the bipolar elements  95  dispersed in the ink  90  may be jetted onto the target substrate SUB in a state in which the bipolar elements  95  extend in a direction. 
     In an embodiment, before the jetting of the ink  90  onto the electrodes  21  and  22 , the electric field generating device  700  may be electrically connected to the electrodes  21  and  22  of the target substrate SUB and may generate the electric field EL on the electrodes  21  and  22  of the target substrate SUB. Accordingly, the ink  90  may be jetted onto the target substrate SUB on which the electric field EL is generated. In case that the target substrate SUB is prepared (or provided) on the stage STA, the electric field generating device  700  may be electrically connected to the electrodes  21  and  22  on the target substrate SUB. Pad parts connected to the electrodes  21  and  22  are disposed on the target substrate SUB, and the probe driver  703  of the electric field generating device  700  moves, such that the probe pad  708  and the pad parts may be in contact with each other. Before the stage STA moves to the inkjet device  300  and the ink  90  is jetted onto the target substrate SUB, the electric field generating device  700  may generate the electric field EL on the target substrate SUB, and the ink  90  may pass through the electric field EL and be then jetted onto the electrodes  21  and  22 . 
     However, embodiments are not limited thereto, and the electric field generating device  700  may also be connected (e.g., electrically connected) to the target substrate SUB and may generate the electric field EL on the target substrate SUB, after the inkjet device  300  discharges the ink  90 . 
     The bipolar elements  95  included in the ink  90  may be oriented on the target substrate SUB by the electric field EL to have the initial positions and the initial orientation directions. In some embodiments, the bipolar elements  95  may be disposed on the first electrode  21  and the second electrode  22  by receiving a dielectrophoretic force transferred by the electric field EL generated on the target substrate SUB. As described above, the bipolar elements  95  may have the initial positions and the initial orientation directions by the electric field EL. The initial positions and the initial orientation directions of the bipolar elements  95  may be changed by the light irradiation process of irradiating the target substrate SUB with the light hv to have the final positions and the final orientation directions. Thus, the bipolar elements  95  may be more effectively and accurately aligned on the first electrode  21  and the second electrode  22 . 
     Referring to  FIG.  27   , in case that the light irradiation device  500  irradiates the target substrate SUB with the light hv, dipole moments of the bipolar elements  95  may increase in response to the light hv. In an embodiment, in case that the light irradiation device  500  irradiates the target substrate SUB with the light hv, directions toward which the first end portions of at least some of the bipolar elements  95  are directed (or oriented) may change by the electric field EL. The bipolar elements  95  having the increased dipole moments may be oriented so that the first end portions may be directed (or oriented) toward a constant direction, in response to the electric field EL generated on the electrodes  21  and  22 . At the same time, at least one end portion of the bipolar elements  95  may be disposed on the first electrode  21  or the second electrode  22 . For example, the first end portions of the bipolar elements  95  may be disposed on the first electrode  21 , and the second end portions of the bipolar elements  95  may be disposed on the second electrode  22 . However, embodiments are not limited thereto, and some bipolar elements  95  may be disposed (e.g., directly disposed) on the target substrate SUB between the first electrode  21  and the second electrode  22 . 
     Referring to  FIG.  28   , the solvent  91  of the ink  90  jetted onto the target substrate SUB may be removed. The removing of the solvent  91  may be performed through the drying device  800 , and as described above, in order to prevent the misalignment of the bipolar elements  95 , the electric field generating device  700  may generate the electric field EL on the target substrate SUB even during the drying process. The solvent  91  may be removed from the ink  90  jetted onto the target substrate SUB, such that positions of the bipolar elements  95  may be fixed, and the bipolar elements  95  may be seated on the electrodes  21  and  22 . 
     In the printing method of a bipolar element  95  according to an embodiment, the bipolar elements  95  may be seated on the electrodes  21  and  22  disposed on the target substrate SUB by using the inkjet printing apparatus  1000  of  FIG.  1   . 
     For example, the inkjet printing apparatus  1000  may include the inspection device  900 , and the printing method of a bipolar element  95  may further include measuring a degree of alignment of the bipolar elements  95  disposed on the electrodes  21  and  22 . 
       FIGS.  29  and  30    are schematic views illustrating inspecting bipolar elements  95  printed on a target substrate SUB according to an embodiment. 
     Referring to  FIGS.  29  and  30   , the printing method of a bipolar elements  95  may include measuring the number and positions of bipolar elements  95  disposed on the target substrate SUB by using the inspection device  900 . The sensing unit  950  of the inspection device  900  may measure the number of bipolar elements  95  disposed in unit regions AA 1 , AA 2 , and AA 3  (see  FIG.  30   ) defined on the target substrate SUB or measure orientation directions of the bipolar elements  95  disposed on the electrodes  21  and  22 . 
     First, the sensing unit  950  may measure the number of bipolar elements  95  disposed in unit regions AA 1 , AA 2 , and AA 3 . In the drawing, a first region AA 1 , a second region AA 2 , and a third region AA 3  defined as arbitrary regions are illustrated. The sensing unit  950  may measure the number of the bipolar elements  95  disposed in each of the unit regions AA 1 , AA 2 , and AA 3  and compare the number of the bipolar elements  95  with a reference set value. In case that an error occurs in the number of bipolar elements  95  disposed in each of the unit regions AA 1 , AA 2 , and AA 3  as compared with the reference set value, the number of bipolar elements  95  may be adjusted by feeding back the error. For example, the inkjet printing apparatus  1000  may adjust the number of bipolar elements  95  disposed in each of the unit regions AA 1 , AA 2 , and AA 3  by adjusting a dispersion degree of the bipolar elements  95  in the ink  90  discharged from the inkjet head  330  of the inkjet device  300 . 
     For example, the sensing unit  950  may measure the degree of alignment of the bipolar elements  95  by measuring the positions and the orientation directions of the bipolar elements  95  disposed on the first electrode  21  and the second electrode  22 . For example, in case that the electrodes  21  and  22  disposed on the target substrate SUB have a shape in which they extend in a direction and the bipolar elements  95  are disposed between the electrodes  21  and  22 , acute angles θ 1 , θ 2 , and θ 3  formed between a direction in which the bipolar elements  95  extend and a direction perpendicular to the direction in which the electrodes  21  and  22  extend may be measured. In some cases, the sensing unit  950  may measure positions of end portions (e.g., opposite end portions) of the bipolar elements  95  to confirm whether or not the end portions are disposed on the electrodes  21  and  22 . The inkjet printing apparatus  1000  may measure the degree of alignment of the bipolar elements  95  by comparing the measured acute angles and the positions of the end portions of the bipolar elements  95  with reference set values. In case that an error occurs in the degree of alignment of the bipolar elements  95  as compared with the reference set value, the degree of alignment of the bipolar elements  95  may be adjusted by feeding back the error. For example, the inkjet printing apparatus  1000  may adjust the degree of adjustment of the bipolar elements  95  by adjusting a strength of the electric field EL generated by the electric field generating device  700 , an amount of light hv irradiated from the light irradiation device  500 , or the like. 
     In the printing method of a bipolar element  95  according to an embodiment, the bipolar elements  95  may be disposed and aligned at desired positions on the target substrate SUB by by using the inkjet printing apparatus  1000 . During the printing process, the electric field generating device  700  may continuously generate the electric field EL during an ink jetting process, the light irradiation process, and the drying process. According to an embodiment, the bipolar elements  95  may be printed with a high degree of alignment on the target substrate SUB by using the inkjet printing apparatus  1000 . 
     For example, the above-described bipolar element  95  may be a light emitting element including semiconductor layers, and according to an embodiment, a display device  10  including the light emitting elements may be manufactured by using the inkjet printing apparatus  1000 . 
       FIG.  31    is a schematic view of a light emitting element  30  according to an embodiment. 
     The light emitting element  30  may be a light emitting diode. For example, the light emitting element  30  may be an inorganic light emitting diode having a size of a micrometer or nanometer scale and made of an inorganic material. The inorganic light emitting diodes may be aligned between two electrodes in which polarities are formed in case that an electric field is formed in a specific direction between the two electrodes facing each other. The light emitting elements  30  may be aligned between the two electrodes by the electric field formed on the two electrodes. 
     The light emitting element  30  according to an embodiment may have a shape in which the light emitting element  30  extends in a direction. The light emitting element  30  may have a shape such as a rod shape, a wire shape, or a tube shape. In an embodiment, the light emitting element  30  may have a cylindrical shape or a rod shape. However, the light emitting element  30  is not limited to having the shape described above, and may have various shapes. For example, the light emitting element  30  may have a polygonal prismatic shape such as a cubic shape, a rectangular parallelepiped shape, or a hexagonal prismatic shape or have a shape in which the light emitting element  30  extends in a direction, but outer surfaces of the light emitting element  30  may be inclined (e.g., partially inclined). Semiconductors included in a light emitting element  30  to be described below may have a structure in which they are sequentially disposed or stacked along the direction. 
     The light emitting element  30  may include a semiconductor layer doped with any conductivity-type impurities (e.g., a p-type dopant or an n-type dopant). The semiconductor layer may receive an electrical signal applied from an external power source to emit light of a specific wavelength band. 
     Referring to  FIG.  31   , the light emitting element  30  may include a first semiconductor layer  31 , a second semiconductor layer  32 , an active layer  36 , an electrode layer  37 , and an insulating film  38 . 
     The first semiconductor layer  31  may be an n-type semiconductor. As an example, in case that the light emitting element  30  emits light of a blue wavelength band, the first semiconductor layer  31  may include a semiconductor material having a chemical formula of Al x GayIn 1-x-y N (0≤x≤1, 0≤y≤1, and 0≤x+y≤1). For example, the semiconductor material may be one or more of AlGaInN, GaN, AlGaN, InGaN, AlN, and InN doped with an n-type dopant. The first semiconductor layer  31  may be doped with an n-type dopant, which may be Si, Ge, Sn, or the like, as an example. In an embodiment, the first semiconductor layer  31  may be made of n-GaN doped with n-type Si. A length of the first semiconductor layer  31  may be in the range of about 1.5 μm to about 5 μm, but embodiments are not limited thereto. 
     The second semiconductor layer  32  may be disposed on an active layer  36  to be described below. The second semiconductor layer  32  may be a p-type semiconductor, and as an example, in case that the light emitting element  30  emits light of a blue or green wavelength band, the second semiconductor layer  32  may include a semiconductor material having a chemical formula of Al x GayIn 1-x-y N (0≤x≤1, 0≤y≤1, and 0≤x+y≤1). For example, the semiconductor material may be one or more of AlGaInN, GaN, AlGaN, InGaN, AlN, and InN doped with a p-type dopant. The second semiconductor layer  32  may be doped with a p-type dopant, which may be Mg, Zn, Ca, Se, Ba, or the like, as an example. In an embodiment, the second semiconductor layer  32  may be made of p-GaN doped with p-type Mg. A length of the second semiconductor layer  32  may be in the range of about 0.05 μm to about 0.10 μm, but embodiments are not limited thereto. 
     For example, each of the first semiconductor layer  31  and the second semiconductor layer  32  may be formed as a layer, but embodiments are not limited thereto. According to some embodiments, each of the first semiconductor layer  31  and the second semiconductor layer  32  may further include a larger number of layers (e.g., a clad layer or a tensile strain barrier reducing (TSBR) layer) according to a material of the active layer  36 . 
     The active layer  36  may be disposed between the first semiconductor layer  31  and the second semiconductor layer  32 . The active layer  36  may include a material having a single quantum well structure or a multiple quantum well structure. In case that the active layer  36  includes the material having the multiple quantum well structure, the active layer  36  may have a structure in which quantum layers and well layers are alternately stacked. The active layer  36  may emit light by a combination of electron-hole pairs according to electrical signals applied through the first semiconductor layer  31  and the second semiconductor layer  32 . As an example, in case that the active layer  36  emits light of a blue wavelength band, the active layer  36  may include a material such as AlGaN or AlGaInN. In case that the active layer  36  has the multiple quantum well structure (e.g., the structure in which the quantum layers and the well layers) may be alternately stacked, the quantum layers may include a material such as AlGaN or AlGaInN, and the well layers may include a material such as GaN or AlInN. In an embodiment, the active layer  36  may include AlGaInN as a material of the quantum layers and AlInN as a material of the well layers to emit blue light having a central wavelength band of about 450 nm to about 495 nm, as described above. 
     However, embodiments are not limited thereto, and the active layer  36  may have a structure in which semiconductor materials having large band gap energy and semiconductor materials having small band gap energy are alternately stacked, and may include other Group III to Group V semiconductor materials according to a wavelength band of emitted light. The light emitted by the active layer  36  is not limited to the light of the blue wavelength band, and in some case, the active layer  36  may emit light of red and green wavelength bands. A length of the active layer  36  may be in the range of about 0.05 μm to about 0.10 μm, but embodiments are not limited thereto. 
     For example, the light emitted from the active layer  36  may be emitted not only to outer surfaces of the light emitting element  30  in a length direction, but also to side surfaces (e.g., opposite side surfaces) of the light emitting element  30 . The transmission direction of the light emitted from the active layer  36  is not limited thereto. 
     The electrode layer  37  may be an ohmic connection electrode. However, embodiments are not limited thereto, and the electrode layer  37  may also be a Schottky connection electrode. The light emitting element  30  may include at least one electrode layer  37 . Referring to  FIG.  31   , the light emitting element  30  may include an electrode layer  37 , but embodiments are not limited thereto. In some cases, the light emitting element  30  may also include a larger number of electrode layers  37  or the electrode layer  37  may also be omitted. A description of a light emitting element  30  to be provided below may be applied even though the number of electrode layers  37  is changed or the light emitting element  30  may further include another structure. 
     The electrode layer  37  may decrease resistance between the light emitting element  30  and the electrode or the connection electrode in case that the light emitting element  30  is connected (e.g., electrically connected) to the electrode or the connection electrode in a display device  10  according to an embodiment. The electrode layer  37  may include a metal having conductivity. The electrode layer  37  may include at least one of aluminum (Al), titanium (Ti), indium (In), gold (Au), silver (Ag), indium tin oxide (ITO), indium zinc oxide (IZO), and indium tin zinc oxide (ITZO). The electrode layer  37  may include a semiconductor material doped with an n-type or p-type dopant. 
     The insulating film  38  may be disposed to surround outer surfaces of the semiconductor layers and the electrode layers described above. In an embodiment, the insulating film  38  may surround at least an outer surface of the active layer  36 , and may extend in a direction in which the light emitting element  30  extends. The insulating film  38  may protect these members. As an example, the insulating film  38  may surround side surface portions of these members, but may expose end portions of the light emitting element  30  in the length direction. 
     For example, the insulating film  38  may extend in the length direction of the light emitting element  30  to cover side surfaces of the first semiconductor layer  31  to the electrode layer  37 , but embodiments are not limited thereto. The insulating film  38  may cover only outer surfaces of some of the semiconductor layers as well as the active layer  36  or may cover only a portion of an outer surface of the electrode layer  37 , such that the outer surface of each electrode layer  37  may be exposed (e.g., partially exposed). For example, the insulating film  38  may also be formed so that an upper surface of the insulating film  38  may be rounded in cross section in an area adjacent to at least one end portion of the light emitting element  30 . 
     A thickness of the insulating film  38  may be in the range of about 10 nm to about 1.0 μm, but embodiments are not limited thereto. For example, the thickness of the insulating film  38  may be about 40 nm. 
     The insulating film  38  may include materials having insulating properties, such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y ), aluminum nitride (AlN x ), and aluminum oxide (AlO x ). Accordingly, an electrical short circuit that may occur in case that the active layer  36  is in direct contact with an electrode through which an electrical signal is transferred to the light emitting element  30  may be prevented. For example, the insulating film  38  may protect an outer surface of the light emitting element  30  as well as the active layer  36 , and may thus prevent a decrease in luminous efficiency. 
     For example, in some embodiments, an outer surface of the insulating film  38  may be surface-treated. The light emitting elements  30  may be jetted onto electrodes in a state in which they are dispersed in ink and be aligned. In order to maintain the light emitting elements  30  in a state in which the light emitting elements  30  are dispersed without being agglomerated with other adjacent light emitting elements  30  in the ink, a hydrophobic or hydrophilic treatment may be performed on a surface of the insulating film  38 . 
     The light emitting element  30  may have a length h in the range of about 1 μm to about 10 μm or in the range of about 2 μm to about 6 μm. For example, the light emitting element  30  may have the length h in the range of about 3 μm to about 5 μm. For example, a diameter of the light emitting element  30  may be in the range of about 30 nm to about 700 nm, and an aspect ratio of the light emitting element  30  may be about 1.2 to about 100. However, embodiments are not limited thereto, and light emitting elements  30  included in the display device  10  may also have different diameters according to a difference in composition between the active layers  36 . For example, the diameter of the light emitting element  30  may be about 500 nm. 
     According to an embodiment, the inkjet printing apparatus  1000  may disperse the light emitting elements  30  of  FIG.  31    in the ink  90  and may jet or discharge the ink  90  in which the light emitting elements  30  are dispersed onto the target substrate SUB to manufacture the display device  10  including the light emitting elements  30 . 
       FIG.  32    is a schematic plan view of a display device  10  according to an embodiment. 
     Referring to  FIG.  32   , the display device  10  may display a moving image or a still image. The display device  10  may refer to all electronic devices that provide display screens. For example, televisions, laptop computers, monitors, billboards, the Internet of Things (IoT), mobile phones, smartphones, tablet personal computers (PCs), electronic watches, smart watches, watch phones, head mounted displays, mobile communication terminals, electronic notebooks, electronic books, portable multimedia players (PMPs), navigation devices, game machines, digital cameras, camcorders, and the like, which provide display screens, may be included in the display device  10 . 
     The display device  10  may include a display panel providing the display screen. Examples of the display panel may include an inorganic light emitting diode display panel, an organic light emitting display panel, a quantum dot light emitting display panel, a plasma display panel, a field emission display panel, and the like. Hereinafter, a case where an inorganic light emitting diode display panel is applied as an example of the display panel will be described by way of example, but embodiments are not limited thereto, and other display panels may be applied thereto. 
     A shape of the display device  10  may be variously modified. For example, the display device  10  may have a shape such as a rectangular shape with a width greater than a length, a rectangular shape with a length greater than a width, a square shape, a rectangular shape with rounded corners (e.g., vertices), other polygonal shapes, or a circular shape. A shape of a display area DPA of the display device  10  may also be similar to an overall shape of the display device  10 . Referring to  FIG.  1   , the display device  10  and the display area DPA may have the rectangular shape with the width greater than the length. 
     The display device  10  may include a display area DPA and non-display areas NDA. The display area DPA may be an area in which an image is displayed, and the non-display area NDA may be an area in which any image is not displayed. The display area DPA may also be referred to as an active area, and the non-display area NDA may also be referred to as a non-active area. The display area DPA may occupy substantially the center of the display device  10 . 
     The display area DPA may include pixels PX. The pixels PX may be arranged in a matrix direction. A shape of each pixel PX may be a rectangular shape or a square shape in a plan view, but embodiments are not limited thereto, and may also be a rhombic shape of which each side is inclined with respect to a direction. The respective pixels PX may be alternately arranged in a stripe type or a PenTile® type. For example, each of the pixels PX may include one or more light emitting elements  30  emitting light of a specific wavelength band to display a specific color. 
     The non-display areas NDA may be disposed around the display area DPA. The non-display areas NDA may entirely or partially surround the display area DPA. The display area DPA may have a rectangular shape, and the non-display areas NDA may be disposed adjacent to four sides of the display area DPA. The non-display areas NDA may form a bezel of the display device  10 . Lines or circuit drivers included in the display device  10  may be disposed or external devices may be mounted, in each of the non-display areas NDA. 
       FIG.  33    is a schematic plan view illustrating a pixel PX of the display device  10  according to an embodiment. 
     Referring to  FIG.  33   , each of the pixels PX may include sub-pixels PXn (where n is an integer of 1 to 3). For example, each pixel PX may include a first sub-pixel PX 1 , a second sub-pixel PX 2 , and a third sub-pixel PX 3 . The first sub-pixel PX 1  may emit light of a first color, the second sub-pixel PX 2  may emit light of a second color, and the third sub-pixel PX 3  may emit light of a third color. The first color may be blue, the second color may be green, and the third color may be red. However, embodiments are not limited thereto, and the respective sub-pixels PXn may also emit light of the same color. For example, referring to  FIG.  2   , the pixel PX may include three sub-pixels PXn, but embodiments are not limited thereto, and the pixel PX may include a larger number of sub-pixels PXn. 
     Each of the sub-pixels PXn of the display device  10  may include an area defined as an emission area EMA. The first sub-pixel PX 1  may include a first emission area EMA 1 , the second sub-pixel PX 2  may include a second emission area EMA 2 , and the third sub-pixel PX 3  may include a third emission area EMA 3 . The emission area EMA may be defined as an area in which the light emitting elements  30  included in the display device  10  are disposed to emit light of a specific wavelength band. The active layer  36  of the light emitting element  30  may emit light of a specific wavelength band without any specific transmission direction, and the light may be emitted toward side surfaces (e.g., opposite side surfaces) of the light emitting element  30 . The emission area EMA may include an area in which the light emitting elements  30  are disposed, and may include an area in which the light emitted from the light emitting elements  30  is emitted, as an area adjacent to the light emitting elements  30 . 
     Embodiments are not limited thereto, and the emission area EMA may also include an area in which the light emitted from the light emitting elements  30  is reflected or refracted by other members and then emitted. Light emitting elements  30  may be disposed in each sub-pixel PXn, and the emission area EMA including an area in which the light emitting elements  30  are disposed and an area adjacent to the light emitting elements  30  may be formed. 
     For example, each of the sub-pixels PXn of the display device  10  may include a non-emission area defined as an area other than the emission area EMA. The non-emission area may be an area in which the light emitting elements  30  are not disposed and the light emitted from the light emitting elements  30  may not be transmitted, and thus, the light may not be emitted. 
       FIG.  34    is a schematic cross-sectional view taken along line line and line IIIc-IIIc′ of  FIG.  33   .  FIG.  34    illustrates only a cross section of the first sub-pixel PX 1  of  FIG.  3   , but may be applied to other pixels PX or sub-pixels PXn.  FIG.  34    illustrates a cross section crossing an end portion and another end portion of the light emitting element  30  disposed in the first sub-pixel PX 1 . 
     Referring to  FIG.  34    in conjunction with  FIG.  33   , the display device  10  may include a first substrate  11 , and a semiconductor layer, conductive layers, and insulating layers disposed on the first substrate  11 . 
     For example, the first substrate  11  may be an insulating substrate. The first substrate  11  may be made of an insulating material such as glass, quartz, or a polymer resin. For example, the first substrate  11  may be a rigid substrate, but may also be a flexible substrate that may be bent, folded, or rolled. 
     A first conductive layer may be disposed on the first substrate  11 . The first conductive layer may include first and second lower metal layers BML 1  and BML 2 . For example, the first lower metal layer BML 1  and the second lower metal layer BML 2  of the first and second lower metal layers BML 1  and BML 2  may overlap at least active material layers DT_ACT and ST_ACT of a driving transistor DT and a switching transistor ST, respectively. The first and second lower metal layers BML 1  and BML 2  may include a light blocking material to prevent light from being incident on the active material layers DT_ACT and ST_ACT of the respective transistors as an example, the first and second lower metal layers BML 1  and BML 2  may be made of an opaque metal material blocking transmission of the light. However, embodiments are not limited thereto, and in some cases, the first and second lower metal layers BML 1  and BML 2  may be omitted or only the first lower metal layer BML 1  may be included. 
     A buffer layer  12  may be disposed (e.g., entirely disposed) on the first conductive layer and the first substrate  11 . The buffer layer  12  may be formed on the first substrate  11  in order to protect the transistors DT and ST of the pixel PX from moisture permeating through the first substrate  11  vulnerable to moisture permeation, and may perform a surface planarization function. The buffer layer  12  may include inorganic layers that are alternately stacked. For example, the buffer layer  12  may be formed as a double layer in which inorganic layers including at least one of silicon oxide (SiO x ), silicon nitride (SiN x ), and silicon oxynitride (SiO x N y ) are stacked or multiple layers in which these layers are alternately stacked. 
     The semiconductor layer may be disposed on the buffer layer  12 . The semiconductor layer may include a first active material layer DT_ACT of the driving transistor DT and a second active material layer ST_ACT of the switching transistor ST. The first active material layer DT_ACT and the second active material layer ST_ACT may overlap (e.g., partially overlap) gate electrodes DT_G and ST_G or the like of a second conductive layer to be described below. 
     In an embodiment, the semiconductor layer may include polycrystalline silicon, single crystal silicon, an oxide semiconductor, or the like. The polycrystalline silicon may be formed by crystallizing amorphous silicon. In case that the semiconductor layer includes the polycrystalline silicon, the first active material layer DT_ACT may include doped regions DT_ACTa and DT_ACTb doped with impurities and a channel region DT_ACTc disposed between the doped regions DT_ACTa and DT_ACTb. The second active material layer ST_ACT may also include doped regions ST_CTa and ST_CTb and a channel region ST_ACTc disposed between the doped regions ST_ACTa and ST_ACTb. 
     In an embodiment, the semiconductor layer may include an oxide semiconductor. For example, the doped regions of the respective active material layers DT_ACT and ST_ACT may be conductive regions. The oxide semiconductor may be an oxide semiconductor containing indium (In). In some embodiments, the oxide semiconductor may be indium tin oxide (ITO), indium zinc oxide (IZO), indium gallium oxide (IGO), indium zinc tin oxide (IZTO), indium gallium tin oxide (IGTO), indium gallium zinc oxide (IGZO), indium gallium zinc tin oxide (IGZTO), or the like. However, embodiments are not limited thereto. 
     A first gate insulating layer  13  may be disposed on the semiconductor layer and the buffer layer  12 . The first gate insulating layer  13  may function as a gate insulating film of each of transistors DT and ST. The first gate insulating layer  13  may be formed as a double layer in which inorganic layers including an inorganic material, for example, at least one of silicon oxide (SiO x ), silicon nitride (SiN x ), and silicon oxynitride (SiO x N y ) may be stacked or multiple layers in which these layers are alternately stacked. 
     A second conductive layer may be disposed on the first gate insulating layer  13 . The second conductive layer may include a first gate electrode DT_G of the driving transistor DT and a second gate electrode ST_G of the switching transistor ST. The first gate electrode DT_G may overlap a first channel region DT_ACTc of the first active material layer DT_ACT in a thickness direction, and the second gate electrode ST_G may overlap a second channel region ST_ACTc of the second active material layer ST_ACT in the thickness direction. The second conductive layer may be formed as a single layer or multiple layers made of any one of molybdenum (Mo), aluminum (Al), chromium (Cr), gold (Au), titanium (Ti), nickel (Ni), neodymium (Nd), and copper (Cu), or alloys thereof. However, embodiments are not limited thereto. 
     A first passivation layer  15  may be disposed on the second conductive layer. The first passivation layer  15  may cover the second conductive layer to protect the second conductive layer. The first passivation layer  15  may be formed as a double layer in which inorganic layers including an inorganic material, for example, at least one of silicon oxide (SiO x ), silicon nitride (SiN x ), and silicon oxynitride (SiO x N y ) may be stacked or multiple layers in which these layers are alternately stacked. 
     A third conductive layer may be disposed on the first passivation layer  15 . The third conductive layer may include a first capacitor electrode CE 1  of a storage capacitor of which at least a partial area is disposed to overlap the first gate electrode DT_G in the thickness direction. The first capacitor electrode CE 1  may overlap the first gate electrode DT_G in the thickness direction with the first passivation layer  15  interposed therebetween, and the storage capacitor may be formed between the first capacitor electrode CE 1  and the first gate electrode DT_G. The third conductive layer may be formed as a single layer or multiple layers made of any one of molybdenum (Mo), aluminum (Al), chromium (Cr), gold (Au), titanium (Ti), nickel (Ni), neodymium (Nd), and copper (Cu), or alloys thereof. However, embodiments are not limited thereto. 
     A first interlayer insulating layer  17  may be disposed on the third conductive layer. The first interlayer insulating layer  17  may function as an insulating film between the third conductive layer and other layers disposed above the third conductive layer. The first interlayer insulating layer  17  may be formed as a double layer in which inorganic layers including an inorganic material, for example, at least one of silicon oxide (SiO x ), silicon nitride (SiN x ), and silicon oxynitride (SiO x N y ) are stacked or multiple layers in which these layers are alternately stacked. 
     A fourth conductive layer may be disposed on the first interlayer insulating layer  17 . The fourth conductive layer may include a first source/drain electrode DT_SD 1  and a second source/drain electrode DT_SD 2  of the driving transistor DT and a first source/drain electrode ST_SD 1  and a second source/drain electrode ST_SD 2  of the switching transistor ST. 
     The source/drain electrodes DT_SD 1  and DT_SD 2  of the driving transistor DT may be in contact with the doped regions DT_ACTa and DT_ACTb of the first active material layer DT_ACT through contact holes penetrating through the first interlayer insulating layer  17  and the first gate insulating layer  13 , respectively. The source/drain electrodes ST_SD 1  and ST_SD 2  of the switching transistor ST may be in contact with the doped regions ST_ACTa and ST_ACTb of the second active material layer ST_ACT through contact holes penetrating through the first interlayer insulating layer  17  and the first gate insulating layer  13 , respectively. For example, the first source/drain electrode DT_SD 1  of the driving transistor DT and the first source/drain electrode ST_SD 1  of the switching transistor ST may be electrically connected to the first lower metal layer BML 1  and the second lower metal layer BML 2  through other contact holes, respectively. 
     The fourth conductive layer may be formed as a single layer or multiple layers made of any one of molybdenum (Mo), aluminum (Al), chromium (Cr), gold (Au), titanium (Ti), nickel (Ni), neodymium (Nd), and copper (Cu), or alloys thereof. However, embodiments are not limited thereto. 
     A second interlayer insulating layer  18  may be disposed on the fourth conductive layer. The second interlayer insulating layer  18  may be disposed (e.g., entirely disposed) on the first interlayer insulating layer  17  with covering the fourth conductive layer, and may protect the fourth conductive layer. The second interlayer insulating layer  18  may be formed as a double layer in which inorganic layers including an inorganic material, for example, at least one of silicon oxide (SiO x ), silicon nitride (SiN x ), and silicon oxynitride (SiO x N y ) are stacked or multiple layers in which these layers are alternately stacked. 
     A fifth conductive layer may be disposed on the second interlayer insulating layer  18 . The fifth conductive layer may include a first voltage line VL 1 , a second voltage line VL 2 , and a first conductive pattern CDP. A high potential voltage (or a first source voltage) supplied to the driving transistor DT may be applied to the first voltage line VL 1 , and a low potential voltage (or a second source voltage) supplied to a second electrode  22  may be applied to the second voltage line VL 2 . For example, an alignment signal for aligning light emitting elements  30  may be applied to the second voltage line VL 2  in processes of manufacturing the display device  10 . 
     The first conductive pattern CDP may be electrically connected to the first source/drain electrode DT_SD 1  of the driving transistor DT through a contact hole formed in the second interlayer insulating layer  18 . The first conductive pattern CDP may also be in contact with a first electrode  21  to be described below, and the driving transistor DT may transfer the first source voltage applied from the first voltage line VL 1  to the first electrode  21  through the first conductive pattern CDP. For example, the fifth conductive layer may include a second voltage line VL 2  and a first voltage line VL 1 , but embodiments are not limited thereto. The fifth conductive layer may include larger numbers of first voltage lines VL 1  and second voltage lines VL 2 . 
     The fifth conductive layer may be formed as a single layer or multiple layers made of any one of molybdenum (Mo), aluminum (Al), chromium (Cr), gold (Au), titanium (Ti), nickel (Ni), neodymium (Nd), and copper (Cu), or alloys thereof. However, embodiments are not limited thereto. 
     A first planarization layer  19  may be disposed on the fifth conductive layer. The first planarization layer  19  may include an organic insulating material, for example, an organic material such as polyimide (PI), and perform a surface planarization function. 
     First banks  40 , electrodes  21  and  22 , light emitting elements  30 , a second bank  45 , and connection electrodes  26  and  27  may be disposed on the first planarization layer  19 . For example, insulating layers  51 ,  52 ,  53 , and  54  may be further disposed on the first planarization layer  19 . 
     The first banks  40  may be disposed (e.g., directly disposed) on the first planarization layer  19 . The first banks  40  may extend in the second direction DR 2  within each sub-pixel PXn, but may be spaced apart from each other and terminated (or ended) at boundary areas between the sub-pixels PXn so as not to extend to other sub-pixels PXn neighboring in the second direction DR 2 . For example, the first banks  40  may be spaced apart from and face each other in the first direction DR 1 . The first banks  40  may be disposed to be spaced apart from each other, such that an area in which the light emitting elements  30  is disposed may be formed between the first banks  40 . The first banks  40  may be disposed for each sub-pixel PXn to form a linear pattern in the display area DPA of the display device  10 . Two first banks  40  are illustrated in drawing, but embodiments are not limited thereto. A larger number of first banks  40  may also be further disposed according to the number of electrodes  21  and  22  to be described below. 
     The first banks  40  may have a structure in which at least portions thereof protrude from an upper surface of the first planarization layer  19 . Protruding portions of the first banks  40  may have inclined side surfaces, and light emitted from the light emitting elements  30  may transmit toward the inclined side surfaces of the first banks  40 . The electrodes  21  and  22  disposed on the first banks  40  may include a material having high reflectivity, and the light emitted from the light emitting elements  30  may be reflected by the electrodes  21  and  22  disposed on the side surfaces of the first banks  40  and be emitted in an upward direction of the first planarization layer  19 . For example, the first banks  40  may function as reflective partition walls reflecting the light emitted from the light emitting elements  30  toward the upward direction with providing the area in which the light emitting elements  30  are disposed. The side surfaces of the first banks  40  may be inclined in a linear shape, but embodiments are not limited thereto, and the first banks  40  may also have a semi-circular shape or a semi-elliptical shape with curved outer surfaces. In an embodiment, the first banks  40  may include an organic insulating material such as polyimide (PI), but embodiments are not limited thereto. 
     The electrodes  21  and  22  may be disposed on the first banks  40  and the first planarization layer  19 . The electrodes  21  and  22  may include a first electrode  21  and a second electrode  22 . The first electrode  21  and the second electrode  22  may extend in the second direction DR 2 , and may be spaced apart from and face each other in the first direction DR 1 . The first electrode  21  and the second electrode  22  may have a shape substantially similar to that of the first banks  40 , but may have a shape in which a length each of the first and second electrodes  21  and  22  measured in the second direction DR 2  is greater than that of the first banks  40 . 
     The first electrode  21  and the second electrode  22  may extend in the second direction DR 2  within the sub-pixel PXn, respectively, but may be spaced apart from the other electrodes  21  and  22  at boundary areas with other sub-pixels PXn neighboring in the second direction DR 2 . In some embodiments, the second bank  45  may be disposed at the boundary areas between the respective sub-pixels PXn, and the electrodes  21  and  22  disposed in the respective sub-pixels PXn neighboring in the second direction DR 2  may be spaced apart from each other at portions overlapping the second bank  45 . However, embodiments are not limited thereto, and some electrodes  21  and  22  may not be separated from each other for each sub-pixel PXn, and may extend beyond the sub-pixels PXn neighboring in the second direction DR 2 . 
     The first electrode  21  may be electrically connected to the driving transistor DT through a first contact hole CT 1  at a boundary with the sub-pixel PXn neighboring in the second direction DR 2 . For example, the first electrode  21  may be disposed to at least partially overlap a portion of the second bank  45  extending in the first direction DR 1 , and may be in contact with the first conductive pattern CDP through the first contact hole CT 1  penetrating through the first planarization layer  19 . The second electrode  22  may be electrically connected to the second voltage line VL 2  through a second contact hole CT 2  at a boundary with the sub-pixel PXn neighboring in the second direction DR 2 . For example, the second electrode  22  may overlap a portion of the second bank  45  extending in the first direction DR 1 , and may be in contact with the second voltage line VL 2  through the second contact hole CT 2  penetrating through the first planarization layer  19 . However, embodiments are not limited thereto. In some embodiments, the first contact hole CT 1  and the second contact hole CT 2  may also be disposed in an area surrounded by the second bank  45  so as not to overlap the second bank  45 . 
     For example, a first electrode  21  and a second electrode  22  may be disposed for each sub-pixel PXn, but embodiments are not limited thereto. In some embodiments, the numbers of first electrodes  21  and second electrodes  22  disposed for each sub-pixel PXn may be greater than those illustrated in the drawing. For example, the first electrode  21  and the second electrode  22  disposed in each sub-pixel PXn may not have a shape in which they extend in a direction, and the first electrode  21  and the second electrode  22  may be disposed in various structures. For example, the first electrode  21  and the second electrode  22  may have a partially curved or bent shape, and any one of the first electrode  21  and the second electrode  22  may surround the other of the first electrode  21  and the second electrode  22 . The first electrode  21  and the second electrode  22  are not limited in arrangement structures and shapes thereof as long as at least partial areas thereof are spaced apart from and face each other and accordingly, an area in which the light emitting elements  30  are to be disposed is formed between the first electrode  21  and the second electrode  22 . 
     The electrodes  21  and  22  may be electrically connected to the light emitting elements  30 , and may receive a voltage applied thereto so that the light emitting elements  30  emits light. For example, the electrodes  21  and  22  may be electrically connected to the light emitting elements  30  through connection electrodes  26  and  27  to be described below, and electrical signals applied to the electrodes  21  and  22  may be transferred to the light emitting elements  30  through the connection electrodes  26  and  27 . 
     Each of the electrodes  21  and  22  may be utilized to generate an electric field in the sub-pixel PXn in order to align the light emitting elements  30 . The light emitting elements  30  may be disposed between the first electrode  21  and the second electrode  22  by an electric field formed on the first electrode  21  and the second electrode  22 . In a case of using the above-described inkjet printing apparatus  1000 , ink including the light emitting elements  30  may be jetted onto each of the electrodes  21  and  22 , and the electric field generating device  700  may be electrically connected to each of the electrodes  21  and  22  to generate the electric field EL on each of the electrodes  21  and  22 . The light emitting elements  30  dispersed in the ink may be aligned on the electrodes  21  and  22  by receiving a dielectrophoretic force by the electric field EL generated on the electrodes  21  and  22 . 
     The first electrode  21  and the second electrode  22  may be disposed on the first banks  40 , respectively. The first electrode  21  and the second electrode  22  may be spaced apart from and face each other in the first direction DR 1 , and light emitting elements  30  may be disposed between the first electrode  21  and the second electrode  22 . The light emitting elements  30  may be disposed between the first electrode  21  and the second electrode  22 , and at least one end portion of the light emitting elements  30  may be electrically connected to the first electrode  21  and the second electrode  22 . 
     In some embodiments, the first electrode  21  and the second electrode  22  may have a width greater than that of the first banks  40 , respectively. For example, the first electrode  21  and the second electrode  22  may cover outer surfaces of the first banks  40 , respectively. The first electrode  21  and the second electrode  22  may be disposed on the side surfaces of the first banks  40 , respectively, and a distance between the first electrode  21  and the second electrode  22  may be smaller than a distance between the first banks  40 . For example, at least partial areas of the first electrode  21  and the second electrode  22  may be disposed (e.g., directly disposed) on the first planarization layer  19 . 
     Each of the electrodes  21  and  22  may include a transparent conductive material. As an example, each of the electrodes  21  and  22  may include a material such as indium tin oxide (ITO), indium zinc oxide (IZO), or indium tin zinc oxide (ITZO), but embodiments are not limited thereto. In some embodiments, each of the electrodes  21  and  22  may include a conductive material having high reflectivity. For example, each of the electrodes  21  and  22  may include a metal such as silver (Ag), copper (Cu), or aluminum (Al) as the material having the high reflectivity. For example, each of the electrodes  21  and  22  may reflect the light emitted from the light emitting elements  30  and transmitting toward the side surfaces of the first banks  40  in an upward direction of each sub-pixel PXn. 
     Embodiments are not limited thereto, and the respective electrodes  21  and  22  may have a structure in which one or more layers made of the transparent conductive material and one or more layers made of the metal having the high reflectivity are stacked or may be formed as a layer including the transparent conductive material and the metal having the high reflectivity. In an embodiment, each of the electrodes  21  and  22  may have a stacked structure of ITO/silver (Ag)/ITO/, ITO/Ag/IZO, or ITO/Ag/ITZO/IZO or be made of an alloy including aluminum (Al), nickel (Ni), lanthanum (La), and the like. In another example, each of the electrodes  21  and  22  may have a structure in which a metal layer made of titanium (Ti) and molybdenum (Mo) and the alloy are stacked. In some embodiments, each of the electrodes  21  and  22  may be formed as a double layer or multiple layers in which an alloy including aluminum (Al) and one or more metal layers made of titanium (Ti) or molybdenum (Mo) are stacked. 
     A first insulating layer  51  may be disposed on the first planarization layer  19 , the first electrode  21 , and the second electrode  22 . The first insulating layer  51  may be disposed to partially cover the first electrode  21  and the second electrode  22  as well as an area between the first electrode  21  and the second electrode  22 . For example, the first insulating layer  51  may cover most of upper surfaces of the first electrode  21  and the second electrode  22 , but expose portions of the first electrode  21  and the second electrode  22 . In other words, the first insulating layer  51  may be formed (e.g., substantially entirely formed) on the first planarization layer  19 , but may include openings partially exposing the first electrode  21  and the second electrode  22 . 
     In an embodiment, the first insulating layer  51  may have a step formed so that a portion of an upper surface thereof may be recessed between the first electrode  21  and the second electrode  22 . However, embodiments are not limited thereto. The first insulating layer  51  may have a flat upper surface formed so that the light emitting elements  30  may be disposed thereon. 
     The first insulating layer  51  may insulate the first electrode  21  and the second electrode  22  from each other with protecting the first electrode  210  and the second electrode  220 . For example, the first insulating layer  51  may prevent the light emitting elements  30  disposed on the first insulating layer  51  from being in direct contact with and being damaged by other members. However, a shape and a structure of the first insulating layer  51  are not limited thereto. 
     The second bank  45  may be disposed on the first insulating layer  51 . The second bank  45  may surround an area in which the first banks  40  are disposed, on the first insulating layer  51 , and may be disposed at boundary areas between the respective sub-pixels PXn. The second bank  45  may have a shape in which the second bank  45  extends in the first direction DR 1  and the second direction DR 2  to form a lattice-shaped pattern over the entire display area DPA. A portion of the second bank  45  extending in the first direction DR 1  may overlap (e.g., partially overlap) the first electrode  21  and the second electrode  22 , in case that a portion of the second bank  45  extending in the second direction DR 2  may be spaced apart from the first banks  40 , the first electrode  21 , and the second electrode  22 . 
     According to an embodiment, a height of the second bank  45  may be greater than a height of the first bank  40 . Unlike the first bank  40 , the second bank  45  may prevent ink from overflowing into adjacent sub-pixels PXn in an inkjet printing process of processes of manufacturing the display device  10  with dividing neighboring sub-pixels PXn. The second bank  45  may separate inks in which different light emitting elements  30  are dispersed for each of different sub-pixels PXn from each other so that these inks may not be mixed with each other. The second bank  45  may include polyimide (PI) like the first bank  40 , but embodiments are not limited thereto. 
     The light emitting elements  30  may be disposed on the respective electrodes  21  and  22 . The light emitting elements  30  may be spaced apart from each other, and may be aligned substantially parallel to each other. A distance between the light emitting elements  30  spaced apart from each other is not limited. In some cases, light emitting elements  30  may be disposed adjacent to each other and be grouped, and other light emitting elements  30  may be grouped in a state in which they are spaced apart from each other by a distance and may be disposed with a non-uniform density. For example, a direction in which the respective electrodes  21  and  22  extend and a direction in which the light emitting elements  30  extend may be substantially perpendicular to each other. However, embodiments are not limited thereto, and the light emitting elements  30  may not be perpendicular to the direction in which the respective electrodes  21  and  22  extend, and may also be oblique with respect to the direction in which the respective electrodes  21  and  22  extend. 
     The light emitting elements  30  may include active layers  36  including different materials to emit light of different wavelength bands to the outside. The display device  10  may include the light emitting elements  30  emitting light of different wavelength bands. For example, the light emitting elements  30  of the first sub-pixel PX 1  may include active layers  36  emitting light of a first color of which a central wavelength band is a first wavelength, the light emitting elements  30  of the second sub-pixel PX 2  may include active layers  36  emitting light of a second color of which a central wavelength band is a second wavelength, and the light emitting elements  30  of the third sub-pixel PX 3  may include active layers  36  emitting light of a third color of which a central wavelength band is a third wavelength. Accordingly, the light of the first color, the light of the second color, and the light of the third color may be emitted from the first sub-pixel PX 1 , the second sub-pixel PX 2 , and the third sub-pixel PX 3 , respectively. However, embodiments are not limited thereto. In some case, the respective sub-pixels PXn may also include the same type of light emitting elements  30  to emit light of substantially the same color. 
     The light emitting elements  30  may be disposed on the first insulating layer  51  between the first banks  40  or between the respective electrodes  21  and  22 . For example, the light emitting elements  30  may be disposed so that at least one end portion of the light emitting elements  30  may be disposed on the first electrode  21  or the second electrode  22 . An extension length of the light emitting elements  30  may be greater than the distance between the first electrode  21  and the second electrode  22 , and end portions of the light emitting elements  30  may be disposed on the first electrode  21  and the second electrode  22 , respectively. However, embodiments are not limited thereto, and only any one end portion of the light emitting elements  30  may be disposed on the electrodes  21  and  22 , or end portions of the light emitting elements  30  may not be disposed on the electrodes  21  and  22 , respectively. Even though the light emitting elements  30  are not disposed on the electrodes  21  and  22 , end portions of the light emitting elements  30  may be connected (e.g., electrically connected) to the electrodes  21  and  22  through connection electrodes  26  and  27  to be described below, respectively. 
     The light emitting element  30  may include layers disposed in a direction parallel to an upper surface of the first substrate  11  or the first planarization layer  19 . The light emitting element  30  of the display device  10  may be disposed so that a direction in which the light emitting element  30  extends is parallel to the first planarization layer  19 , and semiconductor layers included in the light emitting element  30  may be sequentially disposed along a direction parallel to the upper surface of the first planarization layer  19 . However, embodiments are not limited thereto. In some cases, in case that the light emitting element  30  has another structure, the layers may also be disposed in a direction perpendicular to the first planarization layer  19 . 
     For example, end portions of the light emitting element  30  may be in contact with the connection electrodes  26  and  27 , respectively. According to an embodiment, since the insulating film  38  is not formed on end surfaces of the light emitting element  30  in a direction in which the light emitting element  30  extends and portions of the semiconductor layers are exposed, the exposed semiconductor layers may be in contact with the connecting electrodes  26  and  27 . However, embodiments are not limited thereto. In some cases, at least partial areas of the insulating film  38  of the light emitting element  30  may be removed and the insulating film  38  may be removed, such that side surfaces of end portions of the semiconductor layers may be exposed (e.g., partially exposed). The exposed side surfaces of the semiconductor layers may also be in direct contact with the connection electrodes  26  and  27 . 
     A second insulating layer  52  may be disposed (e.g., partially disposed) on the light emitting element  30  disposed between the first electrode  21  and the second electrode  22 . The second insulating layer  52  may surround (e.g., partially surround) an outer surface of the light emitting element  30 . A portion of the second insulating layer  52  disposed on the light emitting element  30  may have a shape in which the second insulating layer  52  extends in the second direction DR 2  between the first electrode  21  and the second electrode  22 , in a plan view. As an example, the second insulating layer  52  may form a linear or island-shaped pattern within each sub-pixel PXn. 
     The second insulating layer  52  may be disposed on the light emitting element  30 , but may expose an end portion and another end portion of the light emitting element  30 . The second insulating layer  52  may fix the light emitting element  30  in a process of manufacturing the display device  10  with protecting the light emitting element  30 . For example, in an embodiment, a portion of a material of the second insulating layer  52  may also be disposed between a lower surface of the light emitting element  30  and the first insulating layer  51 . As described above, the second insulating layer  52  may also fill a space between the first insulating layer  51  and the light emitting element  30  formed during the process of manufacturing the display device  10 . Accordingly, the second insulating layer  52  may surround the outer surface of the light emitting element  30  to fix the light emitting element  30  during the process of manufacturing the display device  10  with protecting the light emitting element  30 . 
     The connection electrodes  26  and  27  and a third insulating layer  53  may be disposed on the second insulating layer  52 . 
     The connection electrodes  26  and  27  may have a shape in which they extend in a direction. The connection electrodes  26  and  27  may be in contact with the light emitting element  30  and the electrodes  21  and  22 , respectively. A first connection electrode  26  and a second connection electrode  27  of the connection electrodes  26  and  27  may be disposed on portions of the first electrode  21  and the second electrode  22 , respectively. The first connection electrode  26  may be disposed on the first electrode  21 , the second connection electrode  27  may be disposed on the second electrode  22 , and each of the first connection electrode  26  and the second connection electrode  27  may have a shape in which each of the first connection electrode  26  and the second connection electrode  27  extends in the second direction DR 2 . The first connection electrode  26  and the second connection electrode  27  may be spaced apart from and face each other in the first direction DR 1 , and may form a stripe-shaped pattern in the emission area EMA of each sub-pixel PXn. 
     In some embodiments, widths of the first connection electrode  26  and the second connection electrode  27  measured in a direction may be equal to or smaller than widths of the first electrode  21  and the second electrode  22  measured in the direction, respectively. The first connection electrode  26  and the second connection electrode  27  may cover portions of the exposed upper surfaces of the first electrode  21  and the second electrode  22  with being in contact with an end portion and another end portion of the light emitting element  30 , respectively. As described above, portions of the upper surfaces of the first electrode  21  and the second electrode  22  may be exposed, and the exposed upper surfaces of the first electrode  21  and the second electrode  22  may be in contact with the connection electrodes  26  and  27 , respectively. 
     As described above, the light emitting element  30  may have the semiconductor layers exposed on end surfaces (e.g., opposite end surfaces) thereof in the direction in which the light emitting element  30  extends, and the first connection electrode  26  and the second connection electrode  27  may be in contact with the light emitting element  30  on the end surfaces on which the semiconductor layers are exposed. An end portion of the light emitting element  30  may be connected (e.g., electrically connected) to the first electrode  21  through the first connection electrode  26 , and another end portion of the light emitting element  30  may be connected (e.g., electrically connected) to the second electrode  22  through the second connection electrode  27 . 
     For example, a single first connection electrode  26  and a single second connection electrode  27  may be disposed in a single sub-pixel PXn, but embodiments are not limited thereto. The numbers of first connection electrodes  26  and second connection electrodes  27  may change according to the numbers of first electrodes  21  and second electrodes  22  disposed in each sub-pixel PXn. 
     The third insulating layer  53  may be disposed on the first connection electrode  26 . The third insulating layer  53  may electrically insulate the first connection electrode  26  and the second connection electrode  27  from each other. The third insulating layer  53  may cover the first connection electrode  26 , but may not be disposed on another end portion of the light emitting element  30  so that the light emitting element  30  may be in contact with the second connection electrode  27 . The third insulating layer  53  may be in partial contact with the first connection electrode  26  and the second insulating layer  52  on an upper surface of the second insulating layer  52 . A side surface of the third insulating layer  53  in a direction in which the second electrode  22  is disposed may be aligned with a side surface of the second insulating layer  52 . For example, the third insulating layer  53  may also be disposed on the non-emission area, for example, on the first insulating layer  51  disposed on the first planarization layer  19 . However, embodiments are not limited thereto. 
     The second connection electrode  27  may be disposed on the second electrode  22 , the second insulating layer  52 , and the third insulating layer  53 . The second connection electrode  27  may be in contact with another end portion of the light emitting element  30  and the exposed upper surface of the second electrode  22 . The another end portion of the light emitting element  30  may be connected (e.g., electrically connected) to the second electrode  22  through the second connection electrode  27 . 
     For example, the first connection electrode  26  may be disposed between the first electrode  21  and the third insulating layer  53 , and the second connection electrode  27  may be disposed on the third insulating layer  53 . The second connection electrode  27  may be in partial contact with the second insulating layer  52 , the third insulating layer  53 , the second electrode  22 , and the light emitting element  30 . An end portion of the second connection electrode  27  may be disposed on the third insulating layer  53 . The first connection electrode  26  and the second connection electrode  27  may not be in contact with each other by the second insulating layer  52  and the third insulating layer  53 . However, embodiments are not limited thereto, and in some cases, the third insulating layer  53  may be omitted. 
     The connection electrodes  26  and  27  may include a conductive material. For example, the connection electrodes  26  and  27  may include ITO, IZO, ITZO, aluminum (Al), or the like. As an example, the connection electrodes  26  and  27  may include a transparent conductive material, and the light emitted from the light emitting elements  30  may be transmitted through the connection electrodes  26  and  27  and may transmit toward the electrodes  21  and  22 . Each of the electrodes  21  and  22  may include a material having high reflectivity, and the electrodes  21  and  22  disposed on the inclined side surfaces of the first banks  40  may reflect the light incident thereon in an upward direction of the first substrate  11 . However, embodiments are not limited thereto. 
     A fourth insulating layer  54  may be disposed (e.g., entirely disposed) on the first substrate  11 . The fourth insulating layer  54  may protect members disposed on the first substrate  11  from an external environment. 
     Each of the first insulating layer  51 , the second insulating layer  52 , the third insulating layer  53 , and the fourth insulating layer  54  described above may include an inorganic insulating material or an organic insulating material. In an embodiment, the first insulating layer  51 , the second insulating layer  52 , the third insulating layer  53 , and the fourth insulating layer  54  may include an inorganic insulating material such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon joxynitride (SiO x N y ), aluminum oxide (AlO x ), or aluminum nitride (AlN x ). In another example, the first insulating layer  51 , the second insulating layer  52 , the third insulating layer  53 , and the fourth insulating layer  54  may include an organic insulating material such as an acrylic resin, an epoxy resin, a phenolic resin, a polyamide resin, a polyimide resin, an unsaturated polyester resin, a polyphenylene resin, a polyphenylene sulfide resin, a benzocyclobutene, a cardo resin, a siloxane resin, a silsesquioxane resin, polymethyl methacrylate, polycarbonate, or polymethyl methacrylate-polycarbonate synthetic resin. However, embodiments are not limited thereto. 
     The inkjet printing apparatus  1000  may jet the light emitting elements  30  onto the electrodes  21  and  22  of the display device  10  through the inkjet device  300 . For example, the electric field generating device  700  may be electrically connected to the respective electrodes  21  and  22  to generate the electric field EL on the respective electrodes  21  and  22 , and the light emitting elements  30  may be aligned on the electrodes  21  and  22  by the electric field EL. According to an embodiment, the display device  10  may be manufactured by printing the light emitting element  30  disposed on the electrodes  21  and  22  by using the inkjet printing apparatus  1000 . 
     In concluding the detailed description, those skilled in the art will appreciate that many variations and modifications can be made to the embodiments without substantially departing from the principles of the invention. Therefore, the disclosed embodiments of the invention are used in a generic and descriptive sense only and not for purposes of limitation.