Patent Publication Number: US-2023145433-A1

Title: Inkjet printing device, method for printing bipolar elements, and method for manufacturing display device

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application is a national entry of International Application No. PCT/KR2020/007391, filed on Jun. 8, 2020, which claims under 35 U.S.C. §§ 119(a) and 365(b) priority to and benefits of Korean Patent Application No. 10-2020-0023612, filed on Feb. 26, 2020, in the Korean Intellectual Property Office (KIPO), the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to an inkjet printing device, a method for printing bipolar elements, and a method for manufacturing a display device. 
     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), and examples of the light emitting diode include an organic light emitting diode (OLED) using an organic material as a fluorescent material and an inorganic light emitting diode using an inorganic material as a fluorescent material. 
     Aspects of the disclosure provide an inkjet printing device including a uniform number of bipolar elements per unit droplet of a discharged ink. 
     Aspects of the disclosure also provide a method for printing bipolar elements capable of keeping the number of bipolar elements dispersed in a unit droplet constant using an inkjet printing device, and a method for manufacturing a display device including light emitting 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. 
     SUMMARY 
     According to an embodiment of the disclosure, an inkjet printing device may include an inkjet head positioned above a stage and including a plurality of nozzles through which ink including bipolar elements having areas doped with partially different polarities is discharged, an actuator disposed on the inkjet head and adjusting an amount of droplet of the ink discharged through the plurality of nozzles; and at least one sensing part disposed on the inkjet head and measuring a number of bipolar elements discharged through the plurality of nozzles. 
     The inkjet head may include a base part, a discharge part disposed on the base part, and an inner pipe to which the ink is supplied. The plurality of nozzles may be disposed on the discharge part, the ink may be supplied to and may flow in the inner pipe, and the ink may be discharged through the plurality of nozzles. 
     The at least one sensing part may include a first sensor measuring a magnetic force in the ink generated due to movement of the bipolar elements, and a second sensor transferring a signal based on the number of the bipolar elements calculated from the magnetic force measured by the first sensor to the actuator. 
     The actuator may receive the signal based on the number of bipolar elements measured by the at least one sensing part and adjust the amount of droplet of the ink discharged through the plurality of nozzles. 
     The at least one sensing part may be disposed at an inlet of the inner pipe to which the ink is supplied. 
     The inlet of the inner pipe may have a first portion and a second portion, a diameter of the first portion may be smaller than a diameter of the second portion, and the at least one sensing part may be disposed on the first portion of the inlet of the inner pipe. 
     The inkjet printing device may further include a sub-actuator disposed on the inkjet head and adjusting a diameter of the inner pipe. The sub-actuator may increase a flow velocity of the ink in the inner pipe. 
     The at least one sensing part may be disposed between an inlet of the inner pipe and the plurality of nozzles. 
     The at least one sensing part may be disposed on the discharge part. 
     The at least one sensing part may further include a light emitting part irradiating the bipolar elements with light. 
     The base part of the inkjet head may include a transparent material. 
     According to an embodiment of the disclosure, a method for printing bipolar elements, may include preparing ink in which a plurality of bipolar elements are dispersed, supplying the ink to an inkjet head, discharging the ink from the inkjet head, measuring a number of the plurality of bipolar elements in the discharged ink, and adjusting an amount of droplet of the discharged ink from the inkjet head in case that the number of the plurality of bipolar elements in the discharged ink exceeds a reference value. 
     The measuring of the number of the plurality of bipolar elements in the discharged ink may be performed by at least one sensing part disposed on the inkjet head, and the at least one sensing part may measure a magnetic force in the ink generated due to movement of the plurality of bipolar elements. 
     The adjusting of the amount of droplet of the discharged ink may include sensing a change in the number of the plurality of bipolar elements by the at least one sensing part, and adjusting the amount of droplet of the discharged ink per unit process based on the change in the number of the plurality of bipolar elements. 
     The ink discharged from the inkjet head may be provided onto a target substrate, and the method for printing bipolar elements may further include seating the plurality of bipolar elements on the target substrate. 
     A plurality of areas may be defined on the target substrate, and the amount of droplet of the discharged ink may be adjusted so that the numbers of the plurality of bipolar elements provided onto the areas may be uniform based on the sensed change in the number of the plurality of bipolar elements. 
     The measuring of the number of the plurality of bipolar elements in the discharged ink may be performed using an inkjet printing device. The inkjet printing device may include the inkjet head positioned above a stage and including a plurality of nozzles through which the ink including the plurality of bipolar elements having areas doped with partially different polarities is discharged, an actuator disposed on the inkjet head and adjusting the amount of droplet of the discharged ink through the plurality of nozzle. The at least one sensing part may measure the number of the plurality of bipolar elements discharged through the plurality of nozzles. 
     The actuator may adjust the amount of droplet of the ink discharged through the plurality of nozzles based on a change in the number of the plurality of bipolar elements measured by the at least one sensing part. 
     According to an embodiment of the disclosure, a method for manufacturing a display device may include preparing a target substrate including a plurality of areas divided from each other and having a first electrode and a second electrode, each of the first electrode and the second electrode disposed in one of the plurality of areas, discharging ink onto the plurality of areas while controlling an amount of droplet of the discharged ink based on a number of light emitting elements dispersed in the ink, and seating the light emitting elements on one of the first electrode and the second electrode. 
     The method may further include measuring the number of light emitting elements in the discharged ink, and controlling the number of light emitting elements in the discharged ink by adjusting the amount of droplet of the discharged ink in case that the number of the light emitting elements in the discharged ink exceeds a reference value. 
     The light emitting element may include a first semiconductor layer doped with a first polarity, a second semiconductor layer doped with a second polarity different from the first polarity, and an active layer disposed between the first semiconductor layer and the second semiconductor layer. 
     The measuring of the number of light emitting elements in the discharged ink may include measuring a magnetic force in the ink due to movement of the light emitting elements. 
     The details of other embodiments are included in the detailed description and the accompanying drawings. 
     The inkjet printing device according to an embodiment includes the sensing part capable of measuring the number of bipolar elements in the discharged ink. The sensing part may measure the number of bipolar elements included per unit volume of the ink flowing in the inkjet head through various methods. The sensing part may also sense a change in the number of bipolar elements while discharging the ink, and may feed the sensed change back to adjust the droplets of the ink discharged through the nozzle, thereby controlling the number of bipolar elements jetted onto the target substrate. 
     Accordingly, in the method for printing bipolar elements using the inkjet printing device according to an embodiment, the number of bipolar elements discharged to a unit space may be uniformly maintained, and in the display device including the light emitting elements manufactured using the inkjet printing device, light emission reliability for each pixel may be improved. 
     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 perspective view of an inkjet printing device according to an embodiment; 
         FIG.  2    is a schematic bottom view of a print head unit according to an embodiment; 
         FIG.  3    is a schematic view illustrating an operation of the print head unit according to an embodiment; 
         FIG.  4    is a schematic view illustrating an ink circulation unit and the print head unit according to an embodiment; 
         FIG.  5    is a schematic cross-sectional view of an inkjet head according to an embodiment; 
         FIG.  6    is an enlarged view of part A of  FIG.  5   ; 
         FIG.  7    is a schematic view illustrating that ink discharged from the inkjet head is jetted onto a target substrate according to an embodiment; 
         FIG.  8    is a schematic view illustrating that the number of bipolar elements flowing in the inkjet head changes; 
         FIG.  9    is a schematic view illustrating that an amount of ink discharged from the inkjet head changes according to an embodiment; 
         FIG.  10    is a schematic plan view of a probe device according to an embodiment; 
         FIGS.  11  and  12    are schematic cross-sectional views illustrating an operation of a probe unit according to an embodiment; 
         FIG.  13    is a schematic view illustrating that an electric field is generated on a target substrate by a probe device according to an embodiment; 
         FIG.  14    is a flowchart illustrating a method for printing the bipolar elements according to an embodiment; 
         FIGS.  15  to  22    are schematic cross-sectional views illustrating a method for printing the bipolar elements using the inkjet printing device according to an embodiment; 
         FIGS.  23  and  24    are schematic cross-sectional views of inkjet heads according to other embodiments; 
         FIG.  25    is a schematic cross-sectional view illustrating that a sensing part detects the number of bipolar elements according to another embodiment; 
         FIG.  26    is a schematic cross-sectional view illustrating that a sensing part detects the number of bipolar elements according to another embodiment; 
         FIGS.  27  and  28    are schematic cross-sectional views illustrating portions of inkjet heads according to another embodiment; 
         FIG.  29    is a schematic cross-sectional view illustrating that a sensing part detects the number of bipolar elements according to another embodiment; 
         FIG.  30    is a schematic view of a light emitting element according to an embodiment; 
         FIG.  31    is a schematic plan view of a display device according to an embodiment; 
         FIG.  32    is a plan view illustrating a pixel of the display device according to an embodiment; 
         FIG.  33    is a schematic cross-sectional view taken along line Xa-Xa′, line Xb-Xb′, and line Xc-Xc′ of  FIG.  32   ; 
         FIGS.  34  to  36    are schematic cross-sectional views illustrating some processes of a method for manufacturing the display device according to an embodiment; and 
         FIGS.  37  and  38    are schematic views of light emitting elements according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the disclosure are shown. The disclosure 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 disclosure to those skilled in the art. 
     When an element, such as a layer, is referred to as being “on,” “connected to,” or “coupled to” another element or layer, it may be directly on, 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 on,” “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. The same reference numbers indicate the same components throughout the specification. 
     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 disclosure. Similarly, the second element could also be termed the first element. 
     For the purposes of this disclosure, “at least one of X, Y, and Z” and “at least one selected from the group consisting of X, Y, and Z” may be construed as X only, Y only, Z only, or any combination of two or more of X, Y, and Z, such as, for instance, XYZ, XYY, YZ, and ZZ. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     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”, for example, 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. 
     Unless otherwise defined or implied herein, all terms (including technical and scientific terms) used have the same meaning as commonly understood by those skilled in the art to which this disclosure pertains. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and should not be interpreted in an ideal or excessively formal sense unless clearly defined in the specification. 
     Hereinafter, embodiments will be described with reference to the accompanying drawings. 
       FIG.  1    is a perspective view of an inkjet printing device according to an embodiment.  FIG.  2    is a schematic bottom view of a print head unit according to an embodiment.  FIG.  3    is a schematic view illustrating an operation of the print head unit according to an embodiment.  FIG.  4    is a schematic view illustrating an ink circulation unit and the print head unit according to an embodiment.  FIG.  3    illustrates shapes of a print head unit  100  and the probe device  700  disposed on a stage STA in a front view according to an embodiment. 
     Referring to  FIGS.  1  to  4   , an inkjet printing device  1000  according to an embodiment may include a print head unit  100  including multiple inkjet heads  300 . The inkjet printing device  1000  may further include a stage STA, an ink circulation unit  500 , a probe device  700 , and a base frame  600 . 
     In  FIG.  1   , 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  may be directions disposed on the same plane and may be perpendicular to each other, and the third direction DR 3  may be a direction perpendicular to each of the first direction DR 1  and the second direction DR 2 . It may be understood that the first direction DR 1  may refer to a transverse direction in the drawing, the second direction DR 2  may refer to a longitudinal direction in the drawing, and the third direction DR 3  may refer to an upward or downward direction in the drawing. 
     The inkjet printing device  1000  may jet predetermined (or selectable) ink  90  onto a target substrate SUB using the print head unit  100 . An electric field may be generated on the target substrate SUB onto which the ink  90  is jetted, by the probe device  700 , and particles such as bipolar elements included in the ink  90  may be aligned on the target substrate SUB. 
     The target substrate SUB may be provided on the probe device  700 , the probe device  700  may form the electric field on the target substrate SUB, and the electric field may be transferred to the ink  90  jetted onto the target substrate SUB. Particles such as bipolar elements  95  included in the ink  90  may have a shape extending in a direction, and may be aligned so that the direction in which they extend is directed to one direction by the electric field. 
     The inkjet printing device  1000  according to an embodiment may include an inkjet head  300  and a sensing part  400  (see  FIG.  5   ) disposed in the inkjet head  300 . The inkjet head  300  may jet, discharge, or print the ink  90  including the bipolar elements  95  on the target substrate SUB, and the sensing part  400  may detect the number of bipolar elements  95  in the ink  90  jetted or discharged from (or through) the inkjet head  300 . The inkjet printing device  1000  may sense a change in the number of bipolar elements  95  in the discharged ink  90 , and may feed the sensed change back to the inkjet head  300  to adjust droplets discharged from the inkjet head  300  so that the number of bipolar elements  95  discharged in a unit space may be uniformly maintained. Hereinafter, the inkjet printing device  1000  will be described in detail with reference to the drawings. 
     The stage STA may provide an area in which the probe device  700  is disposed. The inkjet printing device  1000  may include a first rail RL 1  and a second rail RL 2  extending in the second direction DR 2 , and the stage STA may be disposed on the first rail RL 1  and the second rail RL 2 . The stage STA may move in the second direction DR 2  on the first rail RL 1  and the second rail RL 2  through a separate moving member. The probe device  700  may move in the second direction DR 2  together with the stage STA, and the ink  90  may be jetted onto the probe device  700  while the probe device  700  passes through the print head unit  100 . However, the disclosure is not limited thereto. A structure in which the stage STA moves has been illustrated in the drawing, but in some embodiments, the stage STA may be fixed and the print head unit  100  may move. The print head unit  100  may be mounted on a frame disposed above the first rail RL 1  and the second rail RL 2 . 
     The print head unit  100  may include multiple inkjet heads  300  and may be disposed on the base frame  600 . The print head unit  100  may jet the predetermined (or selectable) ink  90  onto the target substrate SUB provided on the probe device  700  using the inkjet heads  300  connected to a separate ink storage part. 
     The base frame  600  may include a support part  610  and a moving unit  630 . The support part  610  may include a first support part  611  extending in the first direction DR 1 , which is a horizontal direction, and a second support part  612  connected to the first support part  611  and extending in the third direction DR 3 , which is a vertical direction. An extension direction of the first support part  611  may be the first direction DR 1 , which may be a long side direction of the probe device  700 . The print head unit  100  may be disposed on the moving unit  630  mounted on the first support part  611 . 
     The moving unit  630  may include a moving part  631  which is mounted on the first support part  611  and may move in one direction and a fixing part  632  which is disposed on a lower surface of the moving part  631  and on which the print head unit  100  is disposed. The moving part  631  may move in the first direction DR 1  on the first support part  611 , and the print head unit  100  may be fixed to the fixing part  632  and may move in the first direction DR 1  together with the moving part  631 . 
     The print head unit  100  may be disposed on the base frame  600 , and may jet the ink  90  provided from an ink reservoir onto the target substrate SUB through the inkjet head  300 . The print head unit  100  may be spaced apart from the stage STA passing through the base frame  600  by a distance. A distance by which the print head unit  100  is spaced apart from the stage STA may be adjusted by adjusting a height of the second support part  612  of the base frame  600 . The distance between the print head unit  100  and the stage STA spaced apart from each other may be adjusted in a range in which the print head unit  100  has a certain distance from the target substrate SUB in case that the probe device  700  and the target substrate SUB are disposed on the stage STA, such that a space required for a printing process may be secured. 
     According to an embodiment, the print head unit  100  may include an inkjet head  300  including multiple nozzles  350 . The inkjet head  300  may be disposed on a lower surface of the print head unit  100 . 
     The inkjet heads  300  may be disposed to be spaced apart from each other in a direction, and may be arranged in q row or multiple rows. It has been illustrated in the drawing that the inkjet heads  300  are arranged in two rows and the inkjet heads  300  of the respective rows are arranged to be misaligned with each other. However, the disclosure is not limited thereto, and the inkjet heads  300  may be arranged in more than two rows and may be arranged to overlap each other without being misaligned with each other. In an embodiment, the inkjet head  300  may have a rectangular shape, but a shape of the inkjet head  300  is not particularly limited. 
     At least one inkjet head  300 , for example, two inkjet heads  300  may form one pack to be disposed adjacent to each other. However, the number of inkjet heads  300  included in one pack is not limited thereto, and for example, the number of inkjet heads  300  included in one pack may be 1 to 5. It has been illustrated in the drawing that only six inkjet heads  300  are arranged in the print head unit  100 , but this is for schematically illustrating the print head unit  100 , and the number of inkjet heads  300  is not limited thereto. 
     The inkjet head  300  disposed in the print head unit  100  may jet the ink  90  onto the target substrate SUB disposed on the stage STA. According to an embodiment, the print head unit  100  may move along the first support part  611  in a direction, and the inkjet head  300  may move in the direction to jet the ink  90  onto the target substrate SUB. 
     The print head unit  100  may move in the first direction DR 1  in which the first support part  611  extends, and the inkjet head  300  may jet the ink  90  onto the target substrate SUB while moving in the first direction DR 1 . 
     In an embodiment, the ink  90  may include a solvent  91  and multiple 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 include 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 is not limited thereto. The bipolar elements  95  may be included in a state in which they are dispersed in the solvent  91 , and may be supplied to and discharged from the print head unit  100 . 
     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 print head unit  100 . The print head unit  100  may jet the ink  90  onto the target substrate SUB while moving in the first direction DR 1 . In case that multiple target substrates SUB are provided on the probe device  700 , the print head unit  100  may jet the ink  90  onto each of the target substrates SUB while moving in the first direction DR 1 . 
     However, the disclosure is not limited thereto, and the print head unit  100  may be positioned outside the first rail RL 1  and the second rail RL 2 , and may move in the first direction DR 1  and jet the ink  90  onto the target substrate SUB. While the stage STA moves in the second direction DR 2  to be positioned below the base frame  600 , the print head unit  100  may move between the first rail RL 1  and the second rail RL 2  and jet the ink  90  through the inkjet head  300 . An operation of such an inkjet head  300  is not limited thereto, and may be variously modified as long as it may implement a similar process. 
     The inkjet printing device  1000  may further include the ink circulation unit  500 . The ink circulation unit  500  may supply the ink  90  to the print head unit  100 , and the inkjet head  300  may discharge the supplied ink  90 . The ink  90  may be circulated between the ink circulation unit  500  and the inkjet head  300 , and some of the ink  90  supplied to the inkjet head  300  may be discharged from the inkjet head  300 , and the remainder of the ink  90  may be supplied back to the ink circulation unit  500 . 
     The ink circulation unit  500  may be connected to the inkjet head  300  through a first connection pipe IL 1  and a second connection pipe IL 2 . For example, the ink circulation unit  500  may supply the ink  90  to the inkjet head  300  through the first connection pipe ILL and a flow rate of the supplied ink  90  may be adjusted by a first valve VA 1 . The ink circulation unit  500  may be supplied with the remainder of the ink  90  remaining after being discharged from the inkjet head  300 , through the second connection pipe IL 2 . A flow rate of the ink  90  supplied to the ink circulation unit  500  through the second connection pipe IL 2  may be adjusted by a second valve VA 2 . As the ink  90  circulates through the ink circulation unit  500 , a deviation in the number of bipolar elements  95  included in the ink  90  discharged from the inkjet head  300  may be minimized. 
     The ink circulation unit  500  may be mounted on the base frame  600 , but is not limited thereto. The ink circulation unit  500  may be provided in the inkjet printing device  1000 , but a position or a shape thereof is not particularly limited. For example, the ink circulation unit  500  may be disposed on a separate device, and may be variously disposed as long as it is connected to the inkjet head  300 . 
     In some embodiments, the ink circulation unit  500  may include a first ink storage part  510 , a second ink storage part  520 , a third ink storage part  530 , a pressure pump  550 , a compressor  560 , and a flow meter  580  In the ink circulation unit  500 , the second ink storage part  520 , the pressure pump  550 , and the third ink storage part  530  may be connected to the inkjet head  300 , and may form one ink circulation system. 
     The first ink storage part  510  may be a storage part in which the manufactured ink  90  is prepared. The ink  90  including the solvent  91  and the bipolar elements  95  may be prepared in the first ink storage part  510  of the ink circulation unit  500 , and may be supplied to the ink circulation system. 
     The second ink storage part  520  may be connected to the first ink storage part  510  to be supplied with the prepared ink  90 . The second ink storage part  520  may be supplied with the ink  90  remaining after being discharged from the inkjet head  300  through the second connection pipe IL 12 . The second ink storage part  520  may be connected between the third ink storage part  530 , the inkjet head  300 , and the first ink storage part  510  to form the ink circulation system. In case that the second ink storage part  520  is omitted, an excessive amount of ink  90  may be supplied to the third ink storage part  530 , such that the bipolar elements  95  may not be uniformly dispersed. The ink circulation unit  500  may further include the second ink storage part  520  to prevent the excessive amount of ink  90  from being supplied to the third ink storage part  530 . In an embodiment, the second ink storage part  520  may serve as a buffer storage part in which some of the ink  90  circulated in the ink circulation system is stored. 
     The ink  90  supplied to the second ink storage part  520  may be supplied to the third ink storage part  530  by the pressure pump  550 . The pressure pump  550  may be a pump transferring power to a fluid so that the ink  90  in the ink circulation system may be circulated. The ink  90  supplied to the second ink storage part  520  may be supplied to the third ink storage part  530  by the pressure pump  550 . The flow meter  580  may be provided between the pressure pump  550  and the third ink storage part  530 , and may measure a flow rate of the ink  90  supplied to the third ink storage part  530 . The pressure pump  550  may adjust a flow rate of the ink  90  supplied to the third ink storage part  530  according to the flow rate of the ink  90  measured by the flow meter  580 . 
     The ink circulation unit  500  may further include the compressor  560 , and the compressor  560  may adjust a pressure in the third ink storage part  530 . The compressor  560  may remove gas from the third ink storage part  530  so that an inner portion of the third ink storage part  530  becomes a vacuum state, or may introduce an external inert gas into the third ink storage part  530  so that the third ink storage part  530  has a predetermined (or selectable) pressure. However, the disclosure is not limited thereto, and the compressor  560  of the ink circulation unit  500  may also be omitted. 
     The third ink storage part  530  may be connected to the second ink storage part  520  through the pressure pump  550  to be supplied with the ink  90 . The third ink storage part  530  may supply the ink  90  to the inkjet head  300  through the first connection pipe IL 1 . In an embodiment, the third ink storage part  530  may include a stirrer ST, and the stirrer ST may disperse the bipolar elements  95  in the ink  90 . The ink  90  supplied to the third ink storage part  530  may maintain a state in which the bipolar elements  95  do not sink and are dispersed as the stirrer ST rotates. For example, the stirrer ST of the third ink storage part  530  may prevent a phenomenon in which the bipolar elements  95  sink to a lower portion of the third ink storage part  530 , such that the number of bipolar elements  95  in the ink  90  discharged through the inkjet head  300  decreases. The third ink storage part  530  may supply the ink  90  in which the bipolar elements  95  are uniformly dispersed to the inkjet head  300 , and the inkjet head  300  may discharge the ink  90  including a predetermined (or selectable) number or more of the bipolar element  95 . 
     In the inkjet printing device  1000 , unit droplets of the ink  90  discharged from the inkjet head  300  may be required to be constant, and the number of bipolar elements  95  dispersed in the unit droplets may need to be uniformly controlled. While the ink  90  is discharged from the inkjet head  300  by the ink circulation system, in case that the number of bipolar elements  95  per unit droplet of the ink  90  is not uniform, reliability of the inkjet printing device  1000  may decrease. According to an embodiment, the inkjet printing device  1000  may include at least one sensing part  400  (see  FIG.  5   ) disposed in the inkjet head  300 , and may measure the number of bipolar elements  95  in the ink  90  discharged from the inkjet head  300 . The inkjet printing device  1000  may sense a change in the number of bipolar elements  95  in the ink  90 , and may feed the sensed change back to the inkjet head  300  to uniformly maintain the number of bipolar elements  95  discharged in a unit space. Hereinafter, the inkjet head  300  and the sensing part  400  will be described in more detail. 
       FIG.  5    is a schematic cross-sectional view of an inkjet head according to an embodiment. 
     Referring to  FIG.  5   , the inkjet head  300  may include multiple nozzles  350  and may discharge the ink  90  through the nozzles  350 . The ink  90  discharged from (or through) the nozzles  350  may be jetted onto the target substrate SUB provided on the stage STA or the probe device  700 . The nozzles  350  may be disposed on a bottom surface of the inkjet head  300  and may be arranged in a direction in which the inkjet head  300  extends. 
     The inkjet head  300  may include a base part  310 , an inner pipe  330 , and multiple nozzles  350 . The inkjet head  300  may further include a discharge part  370  and an actuator  390 . In some embodiments, the sensing part  400  may be disposed on the inkjet head  300 . 
     The base part  310  may constitute a body of the inkjet head  300 . The base part  310  may be attached to the print head unit  100 . The base part  310  may have a shape extending in the first direction DR 1  and the second direction DR 2 , as described above with reference to  FIG.  2   . However, the disclosure is not limited thereto, and the base part  310  may also have other shapes, such as a circular or polygonal shape. 
     The discharge part  370  may be a portion of the base part  310  of the inkjet head  300  in which the nozzles  350  are disposed. It has been illustrated in the drawing that a discharge part  370  connected to the base part  310  and another discharge part  370  spaced apart from the discharge part  370  are disposed, and the nozzles  350  are formed between these discharge parts  370 . However, substantially, the discharge parts  370  may be not spaced apart from each other and may be an integrated member, and the nozzles  350  may be formed in the shape of holes penetrating through the discharge part  370 . For example, the discharge parts  370  may be not disposed to be spaced apart from each other, and may be formed as a single member. However, the disclosure is not limited thereto, and in some embodiments, the inkjet head  300  may also include multiple units including the discharge part  370  in which the nozzle  350  is formed. The multiple discharge parts  370  may be disposed to be spaced apart from each other and may be connected to the base part  310 . 
     The inner pipe  330  may be disposed in the base part  310  and may be connected to an inner flow path of the print head unit  100 , and may be supplied with the ink  90  from the ink circulation unit  500 . The print head unit  100  may be supplied with the ink  90  through the first connection pipe IL 1  connected to the ink circulation unit  500 , and the ink  90  remaining after being discharged from the nozzles  350  may be supplied to the ink circulation unit  500  through the second connection pipe IL 12 . The inner pipe  330  of the inkjet head  300  may be supplied with the ink  90  at an inlet  331  connected to the inner flow path of the print head unit  100 , and the ink  90  remaining after being discharged may exit to the inner flow path through an outlet  333 . 
     The inkjet head  300  may include a filter F disposed in the inner pipe  330 . The filter F may prevent materials other than the bipolar element  95  from being introduced into the nozzles  350  when the ink  90  flowing along the inner pipe  330  is introduced into the nozzles  350 . Accordingly, it is possible to prevent the nozzles  350  from being clogged by foreign materials or foreign materials from being mixed in the ink  90  discharged from the nozzles  350 . 
     The base part  310  may have a shape in which it extends in a direction, and the inner pipe  330  may be formed along the extension direction of the base part  310 . The ink  90  supplied from the print head unit  100  may flow through the inner pipe  330  and may then be discharged through the nozzles  350  of the inkjet head  300 . 
     The multiple nozzles  350  may be disposed on the discharge part  370  positioned on a surface, for example, a lower surface, of the base part  310 . The nozzles  350  may be spaced apart from each other, may be arranged along the extension direction of the base part  310 , may penetrate through the discharge part  370  of the base part  310 , and may be connected to the inner pipe  330  to discharge the ink  90 . Although not illustrated in the drawing, the nozzles  350  may be arranged in a row or multiple rows. It has been illustrated in the drawing that four nozzles  350  are formed in the inkjet head  300 , but the disclosure is not limited thereto. In some embodiments, the number of nozzles  350  included in the inkjet head  300  may be  128  to  1800 . The nozzles  350  may discharge the ink  90  introduced from the inner pipe  330 . An amount of the ink  90  jetted through the nozzles  350  may be adjusted based on a voltage applied to each nozzle  350 . In an embodiment, an amount of the ink  90  discharged in one spray from each nozzle  350  may be 1 to 50 pl (pico-litter), but is not limited thereto. 
     According to an embodiment, the nozzle  350  may include an inlet  351  and an outlet  353 . The inlet  351  may be a portion which is directly connected to the inner pipe  330  and through which the ink  90  flowing from the inner pipe  330  is supplied to the nozzle  350 . The outlet  353  may be a portion which is connected to the inlet  351  and through which the ink  90  supplied from the inlet  351  is discharged from the nozzle  350 . The inlet  351  and the outlet  353  of the nozzle  350  may have the same diameter, but are not limited thereto. Diameters of the inlet  351  and the outlet  353  of the nozzle  350  may be different from each other depending on a shape of the discharge part  370 . The discharge part  370  may have different shapes depending on a diameter of the nozzle  350  by including portions divided from each other. 
     The ink  90  discharged through the nozzle  350  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 it extends in a direction. The bipolar elements  95  may be randomly dispersed in the ink  90 , flow along the inner pipe  330 , and then be supplied to the nozzle  350 . As the bipolar element  95  has a shape in which it extends in a direction, the bipolar element  95  may have an orientation direction, which is a direction to which a longitudinal axis is directed. The bipolar element  95  may include portions having partially different polarities. For example, the bipolar element  95  may include a first end having a first polarity and a second end having a second polarity, and the first end and the second end may be both ends of the bipolar element  95  in the longitudinal axis direction. The orientation direction of the bipolar element  95  extending in a direction may be defined based on a direction to which the first end is directed. The bipolar elements  95  flowing in the inner pipe  330  and the nozzle  350  of the inkjet head  300  may be dispersed in random orientation directions rather than a constant orientation direction. However, the disclosure is not limited thereto, and the bipolar elements  95  may flow in the inner pipe  330  and the nozzle  350  in a state in which they have specific orientation directions. 
     The actuator  390  may be disposed on the discharge part  370  of the base part  310 . The actuator  390  may be disposed to surround the nozzle  350 . The actuator  390  may apply a hydraulic pressure to the ink  90  introduced into the nozzle  350  so that the ink  90  may be uniformly discharged through the nozzle  350 . The actuator  390  may have substantially the same length as the discharge part  370 , but is not limited thereto. The actuator  390  may be disposed to correspond to the nozzle  350 , may surround the nozzle  350 , and may be disposed to be spaced apart from other actuators  390  by a distance by which the nozzles  350  are spaced apart from each other. 
     According to an embodiment, the actuator  390  may control an amount of the ink  90  discharged through the nozzle  350 . The actuator  390  may adjust the hydraulic pressure applied to the ink  90 , and may adjust droplets of the ink  90  discharged to a unit space during a printing process of the inkjet printing device  1000 . For example, an amount of the ink  90  discharged once from the nozzle  350  may be 1 to 50 pl (Pico-litter), and a discharge amount of the ink  90  required for a unit space in one printing process may be 50 pl or more. The actuator  390  may control droplets of the ink  90  discharged from the nozzle  350  in one printing process by adjusting strength, a frequency, or the like, of the hydraulic pressure. 
     Multiple bipolar elements  95  may be dispersed in the ink  90  discharged from the inkjet head  300 . The ink  90  discharged once from the nozzle  350  may include a specific number of bipolar elements  95  according to a dispersion degree of the bipolar elements  95 . However, the bipolar elements  95  may include a material having a relatively high specific gravity, and in a circulation system of the inkjet head  300  and the ink circulation unit  500 , the bipolar elements  95  may sink in the ink  90 . Accordingly, the dispersion degree of the bipolar elements  95  in the ink  90  may be not kept constant, and the number of bipolar elements  95  in the ink  90  discharged once may change. 
     According to an embodiment, the inkjet printing device  1000  may include the sensing part  400  capable of measuring the number of bipolar elements  95  included in the ink  90  discharged from the nozzle  350 . The sensing part  400  may sense the number of bipolar elements  95  discharged from the nozzle  350  per unit time or a change in the number of bipolar elements  95 , and may feedback the sensed number or the sensed change to the actuator  390 . The actuator  390  may sense the feedback signal received from the sensing part  400  to adjust droplets of the ink  90  discharged from the nozzle  350  per unit time or during a unit process. The inkjet printing device  1000  may include the sensing part  400  and the actuator  390  to uniformly maintain the number of bipolar elements  95  discharged from the inkjet head  300  during a printing process. 
     The sensing part  400  may be disposed in the inkjet head  300 . The sensing part  400  may measure the number of bipolar elements  95  in the ink  90  discharged from the nozzle  350 . For example, the sensing part  400  may be disposed on the base part  310  of the inkjet head  300 , and may be disposed on the inlet  331  of the inner pipe  330  through which the ink  90  is introduced. In some embodiments, the sensing part  400  may be disposed in a form in which it is inserted into the base part  310  of the inkjet head  300 . However, the disclosure is not limited thereto, and the sensing part  400  may be disposed on another position within the inkjet head  300 . 
     The sensing part  400  may include a first sensor  410  and a second sensor  420 . The first sensor  410  and the second sensor  420  may be disposed to be spaced apart from each other with a space in which the ink  90  flows, for example, the inlet  331  of the inner pipe  330  therebetween. The first sensor  410  and the second sensor  420  may be inserted into the base part  310  and may be disposed in contact with an outer wall of the inner pipe  330 , but are not limited thereto. The first sensor  410  and the second sensor  420  may be disposed to be spaced apart from an outer wall of the space through which the ink  90  flows. 
     The first sensor  410  and the second sensor  420  may measure the number of bipolar elements  95  in a unit space or a unit volume of the ink  90 . The first sensor  410  and the second sensor  420  may generate or receive a specific signal for measuring the number of bipolar elements  95  in the ink  90 , and accordingly, may measure the number of bipolar elements  95 . 
     A method for the sensing part  400  to measure the number of bipolar elements  95  may be variously modified. For example, the sensing part  400  may measure the number of bipolar elements  95  included in a unit volume by capturing an image of the ink  90  flowing in the inner pipe  330 . The first sensor  410  may be an image capturing part for capturing an image of the ink  90 , and the second sensor  420  may be a processing part measuring the number of bipolar elements  95  per unit volume through the image captured by the first sensor  410 . 
       FIG.  6    is an enlarged view of part A of  FIG.  5   .  FIG.  7    is a schematic view illustrating that ink discharged from the inkjet head is jetted onto a target substrate according to an embodiment. 
     Referring to  FIGS.  6  and  7    in addition to  FIG.  5   , the first sensor  410  of the sensing part  400  may capture an image of the ink  90  flowing in the inlet  331  of the inner pipe  330 , and may transfer the captured image to the second sensor  420 . The second sensor  420  may calculate or count the number of bipolar elements  95  in the ink  90  from the image. The sensing part  400  disposed adjacent to the inlet  331  of the inner pipe  330  may measure the number of bipolar elements  95  per unit droplet of the ink  90  discharged from the nozzle  350  of the inkjet head  300  from the number of bipolar elements  95  measured at the inlet  331 . 
     The ink  90  discharged from the inkjet head  300  may be jetted onto the target substrate SUB. The target substrate SUB may be a substrate providing a space on which the bipolar elements  95  are printed using the inkjet printing device  1000 . The ink  90  and the bipolar element  95  discharged from the inkjet head  300  may be seated on the target substrate SUB. The sensing part  400  may measure the number of bipolar elements  95  included in the ink  90  flowing in the inkjet head  300 , and may calculate the number of bipolar elements  95  seated on the target substrate SUB from droplets of the ink  90  discharged from the nozzle  350 . For example, the ink  90  discharged from any one nozzle  350  of the inkjet head  300  in the printing process may be positioned in a predetermined (or selectable) area partitioned on the target substrate SUB. The inkjet printing device  1000  may measure the number of bipolar elements  95  positioned in the predetermined (or selectable) area on the target substrate SUB from the number of bipolar elements  95  in the ink  90  measured by the sensing part  400  and the droplets of the ink  90  discharged from the nozzle  350  per unit process time from the actuator  390 . 
     During the printing process in which the inkjet head  300  discharges the ink  90 , the ink  90  may be continuously circulated between the inkjet head  300  and the ink circulation unit  500 . In this process, the bipolar elements  95  having a large specific gravity may sink in the ink  90 , and the number of bipolar elements  95  per unit volume or unit flow rate of the ink  90  introduced into the inner pipe  330  of the inkjet head  300  may decrease. In case that the droplets of the ink  90  discharged from the actuator  390  per unit time or per unit process are constant, the number of bipolar elements  95  positioned in the predetermined (or selectable) area on the target substrate SUB may decrease. 
       FIG.  8    is a schematic view illustrating that the number of bipolar elements flowing in the inkjet head changes. 
     Referring to  FIG.  8   , the sensing part  400  may sense a change in the number of bipolar elements  95  in the ink  90  introduced into the inkjet head  300 . In case that an image is captured by the first sensor  410 , a second sensor  420 ′ may sense a change in the number of bipolar elements  95  in the ink  90  from a change in the captured image. In case that the number of bipolar elements  95  per unit volume or unit flow rate in the ink  90  introduced into the inner pipe  330  decreases, the number of bipolar elements  95  discharged from the nozzle  350  may also decrease. For example, in case that the bipolar elements  95  sink in the circulation system between the ink circulation unit  500  and the inkjet head  300 , the number of bipolar elements  95  in the ink  90  introduced from the ink circulation unit  500  to the inkjet head  300  may decrease. However, the disclosure is not limited thereto, and various causes may exist for the change in the number of bipolar elements  95 . The inkjet printing device  1000  may include a larger number of sensing parts  400  to provide accurate feedback with respect to the change in the number of bipolar elements  95 . The inkjet printing device  1000  may uniformly maintain the number of bipolar elements  95  in the ink  90  discharged from the inkjet head  300  from the feedback provided from the sensing part  400 . 
     The sensing part  400  may measure the number of bipolar elements  95  in the discharged ink  90  at the same time the inkjet printing device  1000  discharges the ink  90 , and may sense a dispersion degree and a change in the number of bipolar elements  95  per unit droplet of the ink  90  during the printing process. In case that the change in the number of bipolar elements  95  exceeds a reference set value, the sensing part  400  may feedback the change in the number of bipolar elements  95  to the actuator  390  in real time. 
     According to an embodiment, in the inkjet printing device  1000 , the sensing part  400  may sense the change in the number of bipolar elements  95  in the ink  90  introduced into the inkjet head  300  during the printing process, and may transfer a feedback signal of the sensed change to the actuator  390 . The actuator  390  may adjust the droplets of the ink  90  discharged from the nozzle  350  so that the number of bipolar elements  95  discharged per unit time or per unit process is constant based on the feedback signal. 
       FIG.  9    is a schematic view illustrating that an amount of ink discharged from the inkjet head changes according to an embodiment. 
     Referring to  FIG.  9    in conjunction with  FIG.  7   , the actuator  390  of the inkjet head  300  may control the droplets of the ink  90  discharged per unit time or per unit process based on the feedback signal transferred from the sensing part  400 . For example, in case that the sensing part  400  senses the change in the number of bipolar elements  95 , the actuator  390  may adjust a hydraulic pressure or the like to the ink  90  to adjust droplets of the ink  90  discharged once from the nozzle  350  or adjust droplets or the number of drops discharged from the nozzle  350  per unit time. Accordingly, even though the number of bipolar elements  95  introduced into the inkjet head  300  during the printing process decreases, the number of bipolar elements  95  seated on the target substrate SUB during the unit process may be uniformly maintained. In particular, in case that the target substrate SUB includes multiple areas that are uniformly partitioned, the inkjet printing device  1000  may discharge a constant number of bipolar elements  95  for each area. 
     The inkjet printing device  1000  according to an embodiment may include the sensing part  400  and the actuator  390  to sense the change in the number of bipolar elements  95 , and may adjust the droplets of the ink  90  discharged from the nozzle  350  per unit process in response to the sensed change. Accordingly, the inkjet printing device  1000  may print or jet a uniform number of bipolar elements  95  in a predetermined (or selectable) area. As described later, the inkjet printing device  1000  may minimize an error in the number of bipolar elements  95  for each area of a device including the bipolar elements  95 , and may improve reliability of a product. 
     The bipolar elements  95  may be jetted onto the target substrate SUB while having orientation directions, and may be then seated on the target substrate SUB while having a constant orientation direction by the electric field generated by the probe device  700 . For example, the bipolar elements  95  may be aligned in a direction on the target substrate SUB by the electric field generated by the probe device  700 . Hereinafter, the probe device  700  will be described with reference to other drawings. 
       FIG.  10    is a schematic plan view of a probe device according to an embodiment. 
     Referring to  FIGS.  1  and  10   , the probe device  700  may include a sub-stage  710 , probe supports  730 , probe units  750 , and aligners  780 . 
     The probe device  700  may be disposed on the stage STA, and may move together with the stage STA in the second direction DR 2 . The ink  90  may be jetted onto the probe device  700  while the probe device  700  on which the target substrate SUB is disposed moves with the stage STA. While the ink  90  is jetted, the probe device  700  may generate an electric field on the target substrate SUB. However, the disclosure is not limited thereto. In some embodiments, the stage STA may not move, and the print head unit  100  may jet the ink  90  onto the stage STA while moving along the second direction DR 2 . 
     The sub-stage  710  may provide a space in which the target substrate SUB is disposed. The probe supports  730 , the probe units  750 , and the aligners  780  may be disposed on the sub-stage  710 . A shape of the sub-stage  710  is not particularly limited, and in an embodiment, the sub-stage  710  may have a rectangular shape with both sides extending in the first direction DR 1  and the second direction DR 2  as illustrated in the drawings. The sub-stage  710  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 sub-stage  710  in a plan view may change depending on 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 sub-stage  710  may have a rectangular shape as illustrated in the drawings, and in case that the target substrate SUB has a circular shape in a plan view, the sub-stage  710  may have a circular shape in a plan view. However, the disclosure is not limited thereto, and the sub-stage  710  and the target substrate SUB may have different shapes. 
     At least one aligner  780  may be disposed on the sub-stage  710 . The aligners  780  may be disposed on each side of the sub-stage  710 , and an area surrounded by multiple aligners  780  may be an area in which the target substrate SUB is disposed. It has been illustrated in the drawings that two aligners  780  are disposed to be spaced apart from each other on each side of the sub-stage  710 , and a total of eight aligners  780  are disposed on the sub-stage  710 . However, the disclosure is not limited thereto, and the number, positions, and the like, of aligners  780  may change depending on a shape or a type of the target substrate SUB. 
     The probe supports  730  and the probe units  750  may be disposed on the sub-stage  710 . The probe supports  730  may provide a space in which the probe units  750  are disposed on the sub-stage  710 . For example, the probe support  730  may be disposed on at least one side of the sub-stage  710 , and may extend in a direction in which a side portion extends. In an embodiment, as illustrated in the drawings, the probe supports  730  may be disposed to extend in the second direction DR 2  on left and right side portions of the sub-stage  710 . However, the disclosure is not limited thereto, and a larger number of probe supports  730  may be included, and in some cases, the probe supports  730  may also be disposed on the upper and lower sides of the sub-stage  710 . For example, structures of the probe supports  730  may change depending on the number, positions, structures, or the like, of probe units  750  included in the probe device  700 . 
     The probe units  750  may be disposed on the probe supports  730  to form electric fields on the target substrate SUB prepared on the sub-stage  710 . The probe units  750  may extend in a direction, for example, in the second direction DR 2 , like the probe supports  730 , and an extension length of the probe units  750  may cover an entire area of the target substrate SUB. For example, sizes and shapes of the probe supports  730  and the probe units  750  may change depending on the target substrate SUB. 
     In an embodiment, the probe unit  750  may include probe drivers  753  disposed on the probe support  730 , probe jigs  751  disposed on the probe drivers  753  and receiving an electrical signal, and probe pads  758  connected to the probe jigs  751  and transferring the electrical signal to the target substrate SUB. 
     The probe driver  753  may be disposed on the probe support  730  to move the probe jig  751  and the probe pad  758 . In an embodiment, the probe driver  753  may move the probe jig  751  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  758  may be connected to or disconnected from the target substrate SUB by driving the probe driver  753 . Among processes of the inkjet printing device  1000 , in a step of forming an electric field in the target substrate SUB, the probe driver  753  may be driven to connect the probe pad  758  to the target substrate SUB, and in other steps, the probe driver  753  may be driven again to disconnect the probe pad  758  from the target substrate SUB. This will be described in detail below with reference to other drawings. 
     The probe pad  758  may form an electric field on the target substrate SUB through the electrical signal transferred from the probe jig  751 . The probe pad  758  may be connected to the target substrate SUB and may transfer the electrical signal to the target substrate SUB to form the electric field on the target substrate SUB. For example, the probe pad  758  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  751  may be transferred to the electrode or the power source pad. The electrical signal transferred to the target substrate SUB may form the electric field on the target substrate SUB. 
     However, the disclosure is not limited thereto, and the probe pad  758  may be a member forming the electric field through the electrical signal transferred from the probe jig  751 . For example, in case that the probe pad  758  forms the electric field by receiving the electrical signal, the probe pad  758  may not be connected to the target substrate SUB. 
     A shape of the probe pad  758  is not particularly limited, but in an embodiment, the probe pad  758  may have a shape in which it extends in a direction to cover the entire area of the target substrate SUB. 
     The probe jig  751  may be connected to the probe pad  758  and may be connected to a separate voltage applying device. The probe jig  751  may transfer an electrical signal transferred from the voltage applying device to the probe pad  758  to form the electric field on the target substrate SUB. The electrical signal transferred to the probe jig  751  may be a voltage for forming the electric field, for example, an alternating current (AC) voltage. 
     The probe unit  750  may include multiple probe jigs  751 , and the number of probe jigs  751  is not particularly limited. It has been illustrated in the drawings that three probe jigs  751  and three probe drivers  753  are disposed, but the probe unit  750  may include a larger number of probe jigs  751  and a larger number of probe drivers  753  to form an electric field having a higher density on the target substrate SUB. 
     The probe unit  750  according to an embodiment is not limited thereto. It has been illustrated in the drawings that the probe unit  750  is disposed on the probe support  730 , that is, the probe device  700 , but in some cases, the probe unit  750  may also be disposed as a separate device. As long as the probe device  700  forms an electric field on the target substrate SUB by including a device capable of forming the electric field, a structure or a disposition of the probe device  700  is not limited. 
       FIGS.  11  and  12    are schematic cross-sectional views illustrating an operation of a probe unit according to an embodiment. 
     As described above, the probe driver  753  of the probe unit  750  may operate according to a process step of the inkjet printing device  1000 . Referring to  FIGS.  11  and  12   , in a first state in which the electric field is not formed in the probe device  700 , the probe unit  750  may be disposed on the probe support  730  to be spaced apart from the target substrate SUB. The probe driver  753  of the probe unit  750  may drive in the second direction DR 2 , which is the horizontal direction, and the third direction DR 3 , which is the vertical direction, to place the probe pad  758  to 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  753  of the probe unit  750  may drive to connect the probe pad  758  to the target substrate SUB. The probe driver  753  may drive in the third direction DR 3 , which is the vertical direction, and the first direction DR 1 , which is the horizontal direction, so that the probe pad  758  may come into contact with the target substrate SUB. The probe jig  751  of the probe unit  750  may transfer the electrical signal to the probe pad  758 , and the electric field may be formed on the target substrate SUB. 
     It has been illustrated in the drawings that one probe unit  750  is disposed on each of both sides of the probe device  700 , and two probe units  750  are simultaneously connected to the target substrate SUB. However, the disclosure is not limited thereto, and multiple probe units  750  may drive separately. For example, in case that the target substrate SUB is prepared on the sub-stage  710  and the ink  90  is jetted onto the target substrate SUB, a first probe unit  750  may first form an electric field on the target substrate SUB, and a second probe unit  750  may not be connected to the target substrate SUB. Thereafter, the first probe unit  750  may be disconnected from the target substrate SUB, and the second probe unit  750  may be connected to the target substrate SUB to form an electric field. For example, the probe units  750  may simultaneously drive to form the electric fields, or may sequentially drive to sequentially form the electric fields. 
       FIG.  13    is a schematic view illustrating that an electric field is generated on a target substrate by a probe device according to an embodiment. 
     Referring to  FIG.  13   , as described above, the bipolar element  95  may include a first end and a second end that have the polarities, and in case that the bipolar element  95  is put in a predetermined (or selectable) electric field, a dielectrophoretic force may be applied to the bipolar element  95 , such that a position or an orientation direction of the bipolar element  95  may change. The bipolar elements  95  in the ink  90  jetted onto the target substrate SUB may be seated on the target substrate SUB while their positions and orientation directions change by an electric field IEL generated by the probe device  700 . 
     The probe device  700  may generate the electric field IEL on the target substrate SUB, and the ink  90  discharged from the nozzle  350  of the inkjet head  300  may pass through the electric field IEL and be jetted onto the target substrate SUB. The bipolar elements  95  may receive a dielectrophoretic force by the electric field IEL until the ink  90  reaches the target substrate SUB or even after the ink  90  reaches the target substrate SUB. According to an embodiment, after the bipolar elements  95  are discharged from the inkjet head  300 , orientation directions and positions of the bipolar elements  95  may change by the electric field IEL generated by the probe device  700 . 
     The electric field IEL generated by the probe 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 a direction in which longitudinal axes of the bipolar elements  95  is directed to a direction horizontal to the upper surface of the target substrate SUB by the electric field IEL. The bipolar elements  95  may be seated on the target substrate SUB in a state in which first ends thereof 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 the other 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 by 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, it may be understood that in case that the deviations in the orientation directions and the seated positions of the bipolar elements  95  are large, the degree of alignment of the bipolar elements  95  may be low, and 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 probe device  700  generates the electric field IEL on the target substrate SUB is not particularly limited. It has been illustrated in the drawing that the probe unit  750  generates the electric field IEL while the ink  90  is discharged from the nozzle  350  and reaches the target substrate SUB. Accordingly, the bipolar elements  95  may receive a dielectrophoretic force by the electric field IEL until they are discharged from the nozzle  350  and reach the target substrate SUB. However, the disclosure is not limited thereto, and in other embodiment, the probe unit  750  may also generate the electric field IEL after the ink  90  is seated on the target substrate SUB. For example, the probe device  700  may generate the electric field IEL when or after the ink  90  is jetted from the inkjet head  300 . 
     Although not illustrated in the drawings, an electric field generating member may be further disposed on the sub-stage  710  in some embodiments. The electric field generating member may form an electric field in an upward direction (i.e., the third direction DR 3 ) or on the target substrate SUB like a probe unit  750  to be described later. In an embodiment, the electric field generating member may be an antenna unit, a device including multiple electrodes, or the like. 
     Although not illustrated in the drawings, the inkjet printing device  1000  according to an embodiment may further include a heat treatment unit which performs a process of volatilizing the ink  90  jetted onto the target substrate SUB. The heat treatment unit may irradiate the ink  90  jetted onto the target substrate SUB with heat, such that the solvent  91  of the ink  90  may be evaporated and removed, and the bipolar elements  95  may be disposed on the target substrate SUB. A process of removing the solvent  91  by irradiating the ink  90  with the heat may be performed using a general heat treatment unit. A detailed description thereof will be omitted. 
     Hereinafter, a method for printing the bipolar elements  95  using the inkjet printing device  1000  according to an embodiment will be described in detail. 
       FIG.  14    is a flowchart illustrating a method for printing the bipolar elements according to an embodiment.  FIGS.  15  to  22    are schematic cross-sectional views illustrating a method for printing the bipolar elements using the inkjet printing device according to an embodiment. 
     Referring to  FIGS.  1  and  14  to  22   , a method for aligning the bipolar elements  95  according to an embodiment may include setting the inkjet printing device  1000  (S 100 ), discharging the bipolar elements  95  through the nozzle  350  and measuring the number of bipolar elements  95  discharged from the nozzle  350  (S 200 ), determining whether or not the number of bipolar elements  95  exceeds a reference set value (S 300 ), and controlling droplets of the ink  90  discharged from the nozzle  350  per unit process based on the determination (S 400 ). 
     The method for printing the bipolar elements  95  according to an embodiment may be performed using the inkjet printing device  1000  described above with reference to  FIG.  1   , the number of bipolar elements  95  per unit droplet of the ink  90  discharged from the inkjet head  300  may be measured, and the bipolar elements  95  may be discharged. In the specification, ‘printing’ of the bipolar elements  95  may mean discharging or jetting the bipolar elements  95  to a predetermined (or selectable) object from the inkjet printing device  1000 . For example, printing the bipolar elements  95  may mean directly discharging the bipolar elements  95  through the nozzle  350  of the inkjet head  300  or discharging the bipolar elements  95  in a state that the bipolar elements  95  are dispersed in the ink  90 . The disclosure is not limited thereto, and printing the bipolar elements  95  may mean jetting the bipolar elements  95  or the ink  90  in which the bipolar elements  95  are dispersed onto the target substrate SUB (see  FIG.  7   ) to seat the bipolar elements  95  or the ink  90  on the target substrate SUB. 
     The method for printing the bipolar elements  95  using the inkjet printing device  1000  may include measuring the number of bipolar elements  95  flowing in the inkjet head  300  and determining whether the number of bipolar elements  95  exceeds the reference set value. A change in the number of bipolar elements  95  discharged from the nozzle  350  may be sensed based on the determination, and the sensed change may be fed back to the actuator  390 . In case that the number of bipolar elements  95  discharged from the nozzle  350  per unit process needs to be changed, the actuator  390  may control droplets of ink  90  discharged from the nozzle  350  per unit process based on the feedback signal. 
     First, the inkjet printing device  1000  may be set (S 100 ). The step (S 100 ) of setting the inkjet printing device  1000  may be a step of tuning the inkjet printing device  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 device  1000  may be adjusted according to a test result. 
     The inspection substrate may be first prepared. The inspection substrate may have the same structure as the target substrate SUB, but a bare substrate such as a glass substrate may be used as the inspection substrate. 
     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. 
     The ink  90  including the bipolar elements  95  may be jetted onto the upper surface of the inspection substrate using the inkjet printing device  1000 , and droplets for each inkjet head  300  may be measured. The measurement of the droplets for each inkjet head  300  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 using a camera. In case that the measured droplets are different from reference droplets, a voltage for each corresponding inkjet head  300  may be adjusted so that the reference droplets may be discharged. Such an inspection method may be repeated several times until each inkjet head  300  discharges accurate droplets. 
     Here, according to an embodiment, the step of setting the inkjet printing device  1000  may include a step of measuring the number of bipolar elements  95  in the droplet jetted onto the inspection substrate. The number of bipolar elements  95  included in the reference droplets jetted onto the inspection substrate may mean a reference set value of the number of bipolar elements  95  per unit droplet of the ink  90  in the inkjet printing device  1000 . Based on the reference set value set in the inkjet printing device  1000 , a change in the number of bipolar elements  95  may be sensed, and the droplets of ink  90  discharged from the nozzle  350  per unit process or the number of bipolar elements  95  discharged through the nozzle  350  may be controlled. 
     In the step of setting the inkjet printing device  1000 , after 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  500 , and may be supplied to the inkjet head  300 . The ink circulation unit  500  and the inkjet head  300  may be maintained so that the bipolar elements  95  in the ink  90  have a uniform dispersion degree by the ink circulation system. 
     However, the disclosure is not limited thereto, and the step (S 100 ) of setting of the inkjet printing device described above may be omitted. 
     After the setting of the inkjet printing device  1000  is completed, the target substrate SUB may be prepared, as illustrated in  FIG.  15   . In an embodiment, a first electrode  21  and a second electrode  22  may be disposed on the target substrate SUB. It has been illustrated in the drawing that a pair of electrodes are disposed, but more than two electrodes may be formed on the target substrate SUB, and multiple inkjet heads  300  may jet the ink  90  onto each pair of electrodes in the same manner. 
     As illustrated in  FIGS.  16  and  17   , the ink  90  including the solvent  91  in which the bipolar elements  95  are dispersed may be jetted onto the target substrate SUB. The ink  90  may be discharged from the inkjet head  300 , 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 they extend in a direction. In some embodiments, which the bipolar elements  95  dispersed in the ink  90  may be oriented so that a direction in which they extend may be directed to a direction perpendicular to an upper surface of the target substrate SUB. In some embodiments, the respective bipolar elements  95  may be jetted in a state in which they are aligned so that first ends thereof having a first polarity or second ends thereof having a second polarity have the same direction. However, the disclosure is not limited thereto. 
     As illustrated in  FIG.  17   , the ink  90  may be discharged to a first area AA 1  defined on the target substrate SUB during a first printing process. The ink  90  may be seated in the first area AA 1  together with the bipolar elements  95 . The bipolar elements  95  corresponding to the droplets of the ink  90  discharged from the inkjet head  300  and the number of bipolar elements  95  per unit droplet in the ink  90  may be jetted into the first area AA 1 . 
     The inkjet printing device  1000  according to an embodiment may include the sensing part  400  to discharge or jet the ink  90  through the nozzle  350  and measure the number of bipolar element  95  discharged from the nozzle  350 . The sensing part  400  may measure at least the number of bipolar elements  95  introduced into the inkjet head  300  to measure the number of bipolar elements  95  per unit droplet of the ink  90  discharged from the nozzle  350 . However, the disclosure is not limited thereto, and in some embodiments, the sensing part  400  may also measure the number of bipolar elements  95  flowing in the inner pipe  330  and the number of bipolar elements  95  introduced into the nozzle  350 , in addition to the bipolar elements  95  introduced into the inkjet head  300 , in order to measure the number of bipolar elements  95  flowing in the inkjet head  300 . Information on the measured number of bipolar elements  95  may be collected by each sensing part  400  and may be utilized to measure the number of bipolar elements  95  per unit droplet of the ink  90  discharged through the nozzle  350 . 
     The sensing part  400  may measure the number of bipolar elements  95  discharged from the nozzle  350  and sense a change in the number of bipolar elements  95 . For example, as illustrated in  FIG.  18   , in case that the number of bipolar elements  95  per unit droplet of the ink  90  discharged through the nozzle  350  decreases, the sensing part  400  may determine whether a decrease amount in the number of bipolar elements  95  exceeds a reference set value (S 300 ). In case that the number of bipolar elements  95  per unit droplet of the ink  90  discharged through the nozzle  350  increases, the sensing part  400  may determine whether an increase amount in the number of bipolar elements  95  exceeds another reference set value (S 300 ). As described above, the sensing part  400  may sense the change in the number of bipolar elements  95 , determine whether number of bipolar elements  95  exceeds the reference set value based on the change in the number of bipolar elements  95 , and feedback a determination result to the actuator  390 . The actuator  390  may receive a feedback signal transferred from the sensing part  400  and adjust droplets of the ink  90  discharged from the nozzle  350  of the inkjet head  300 . 
     For example, as illustrated in  FIGS.  19  and  20   , the actuator  390  may receive the change in the number of bipolar elements  95  provided from the sensing part  400  and adjust droplets or the number of drops of the ink  90  discharged from the nozzle  350  per unit process. In case that the number of bipolar elements  95  in the ink  90  decreases, the actuator  390  may increase the number of drops of the ink  90  discharged from the nozzle  350  per unit process. 
     For example, the ink  90  may be discharged to a second area AA 2  defined on the target substrate SUB during a second printing process. The ink  90  may be seated in the second area AA 2  together with the bipolar elements  95 . In a case where the number of bipolar elements  95  in the ink  90  decreases, in case that the same droplets of ink  90  as that in the first printing process is discharged from the nozzle  350 , the number of bipolar elements  95  jetted into the second area AA 2  may be smaller than the number of bipolar elements  95  jetted into the first area AA 1 . The actuator  390  may allow more droplets of ink  90  to be discharged so that the numbers of the bipolar elements  95  jetted into the first area AA 1  and the second area AA 2  may be uniform even though there is a change in the number of bipolar elements  95  in the ink  90 . In the first printing process and the second printing process, the droplets of the discharged ink  90  may be different from each other, but the numbers of jetted bipolar elements  95  may be the same as each other. Therefore, the inkjet printing device  1000  according to an embodiment may uniformly maintain the number of bipolar elements  95  discharged onto the target substrate SUB per unit process, and improve reliability of a manufactured product. 
     As described above, the bipolar elements  95  may have a shape in which they extend in a direction, and the respective ends of the bipolar elements  95  in a longitudinal direction may have different polarities. The method for printing the bipolar element  95  according to an embodiment may further include seating the bipolar elements  95  so that the direction in which the bipolar elements  95  extend is directed to a direction. 
     Referring to  FIG.  21   , during or after the ink  90  in which the bipolar elements  95  are dispersed is jetted onto the target substrate SUB, an electric field IEL may be generated on the target substrate SUB. The bipolar elements  95  may be seated on the target substrate SUB while being oriented in a direction by the electric field IEL. In some embodiments, the bipolar elements  95  may be disposed between the first electrode  21  and the second electrode  22  by receiving a dielectrophoretic force by the electric field IEL generated on the target substrate SUB. 
     An electrical signal may be applied to the first electrode  21  and the second electrode  22  using the probe units  750 . The probe units  750  may be connected to predetermined (or selectable) pads provided on the target substrate SUB, and may apply the electrical signal to the first electrode  21  and the second electrode  22  connected to the pads. In an embodiment, the electrical signal may be an alternating current (AC) voltage or a direct current (DC) voltage. After the AC voltage is applied to the first electrode  21  and the second electrode  22 , the electric field IEL may be formed between the first electrode  21  and the second electrode  22 , and the bipolar elements  95  may receive a dielectrophoretic force by the electric field IEL 2 . The bipolar elements  95  to which the dielectrophoretic force is applied may be disposed between the first electrode  21  and the second electrode  22  while their orientation directions and positions change. 
     As illustrated in the drawing, orientation directions of the bipolar elements  95  extending in a direction in the ink  90  may change depending on a direction of the electric field IEL. According to an embodiment, the bipolar elements  95  may be aligned so that a direction in which they extend is directed to a direction to which the electric field IEL is directed. In case that the electric field IEL generated on the target substrate SUB is generated in parallel with the upper surface of the target substrate SUB, the bipolar element  95  may be aligned so that the direction in which they extend is parallel to the target substrate SUB and be disposed between the first electrode  21  and the second electrode  22 . In some embodiments, a step of orienting the bipolar elements  95  may include a step of seating the bipolar elements  95  between the first electrode  21  and the second electrode  22 , and at least one ends of the bipolar elements  95  may be disposed on at least one of the first electrode  21  or the second electrode  22 . However, the disclosure is not limited thereto, and the bipolar elements  95  may be directly disposed on the target substrate SUB between the first electrode  21  and the second electrode  22 . 
     As illustrated in  FIG.  22   , the solvent  91  of the ink  90  jetted onto the target substrate SUB may be removed. A step of removing the solvent  91  may be performed by a heat treatment device, which may irradiate the target substrate SUB with heat or infrared rays. Since the solvent  91  is removed from the ink  90  jetted onto the target substrate SUB, a flow of the bipolar elements  95  may be prevented, and the bipolar elements  95  may be seated on the electrodes  21  and  22 . 
     Through the method described above, the inkjet printing device  1000  according to an embodiment may print the bipolar elements  95  on the target substrate SUB. The method for printing the bipolar elements  95  according to an embodiment may uniformly maintain the number of bipolar elements  95  discharged per unit process using the inkjet printing device  1000  of  FIG.  1   . The method for printing the bipolar elements  95  may further include seating the bipolar elements  95  so that the bipolar elements  95  are oriented in a direction. The inkjet printing device  1000  may manufacture a device including bipolar elements  95  oriented in a direction, and the device may include a uniform number of bipolar elements  95  per unit area, such that reliability of a product may be improved. 
     Hereinafter, various embodiments of the inkjet printing device  1000  will be described. 
     As described above, the inkjet printing device  1000  may include the sensing part  400  disposed in the inkjet head  300 . The sensing part  400  may measure the number of bipolar elements  95  per unit droplet of the ink  90  discharged from the nozzle  350  by the number of bipolar elements  95  per unit volume of the ink  90  flowing in the inkjet head  300 . According to another embodiment, the sensing part  400  of the inkjet printing device  1000  may be disposed at another position within the inkjet head  300  and may precisely measure the number of bipolar elements  95 . 
       FIGS.  23  and  24    are schematic cross-sectional views of inkjet heads according to another embodiments. 
     Referring to  FIG.  23   , the sensing part  400  may be disposed between an inlet  331  of an inner pipe  330  and a nozzle  350  of an inkjet head  300 _ 1 . The sensing part  400  may be disposed in the inner pipe  330  in which the ink  90  flows, and may measure the number of bipolar elements  95  before the ink  90  supplied to the inlet  331  of the inner pipe  330  is introduced into the nozzle  350  through a filter F. An embodiment of  FIG.  23    is different from an embodiment of  FIG.  5    in a position of the sensing part  400 . Hereinafter, an overlapping description will be omitted, and contents different from those described above will be described. 
     The sensing part  400  may be disposed in the inner pipe  330  in which the ink  90  flows before being introduced into the nozzle  350 . The sensing part  400  may be disposed more adjacent to the nozzle  350 , and may measure at least the number of bipolar elements  95  in the ink  90  introduced into the inlet  351  of the nozzle  350 . In some embodiments, the inkjet head  300 _ 1  may further include a sensing part  400  disposed at the inlet  331  of the inner pipe  330  as in an embodiment of  FIG.  5   , in addition to the sensing part  400  disposed on the inner pipe  330 . 
     In the inkjet printing device  1000  according to an embodiment, the sensing part  400  may be disposed more adjacent to the nozzle  350  from which the ink  90  is discharged, and may more accurately measure a change in the number of bipolar elements  95  discharged from the inkjet head  300 _ 1 . 
     Similarly, in some embodiments, the sensing part  400  of the inkjet printing device  1000  may be disposed in the nozzle  350  from which the ink  90  is discharged. 
     Referring to  FIG.  24   , the sensing parts  400  of the inkjet printing device  1000  may be disposed on discharge parts  370  of an inkjet head  300 _ 2 . The sensing parts  400  may be provided on the discharge parts  370  and be disposed to correspond to the respective nozzles  350 . The first sensor  410  and the second sensor  420  of the sensing part  400  may be disposed to be spaced apart from each other with the nozzle  350  interposed therebetween. The first sensor  410  may measure the number of bipolar elements  95  in the ink  90  introduced into the nozzle  350 . 
     In some embodiments, the sensing parts  400  may be disposed in a form in which they are inserted into the discharge parts  370  so as to correspond to the respective nozzles  350  of the inkjet head  300 _ 2 , and may be arranged in the first direction DR 1  along the nozzles  350  arranged in the first direction DR 1 . Multiple sensing parts  400  may also be arranged in the second direction DR 2  along the nozzles  350  arranged in the second direction DR 2 . The sensing parts  400  may be disposed to be spaced apart from other neighboring sensing parts  400 , and may be disposed to correspond to the respective nozzles  350 . 
     According to an embodiment, the sensing part  400  may be disposed above the actuator  390  disposed in the discharge part  370 . The sensing part  400  may be disposed adjacent to the inlet  351  of the nozzle  350  between the actuator  390  and the inner pipe  330 , and may measure the number of bipolar elements  95  before the ink  90  passes through the actuator  390  and is discharged. 
     The bipolar elements  95  may have a shape in which they extend in a direction, and thus may be agglomerated at the inlet  351  of the nozzle  350 , such that the number of bipolar elements  95  introduced into the nozzle  350  may decrease. The number of bipolar elements  95  introduced into the inner pipe  330  may not change, but the number of bipolar elements  95  discharged through the nozzle  350  may decrease. In case that the sensing part  400  is disposed only at the inlet  331  of the inner pipe  330 , a decrease in the number of bipolar elements  95  at the inlet  351  of the nozzle  350  may not be sensed. In order to prevent such a problem, the sensing part  400  may be disposed on the discharge part  370  of the inkjet head  300 , and may measure the number of bipolar elements  95  in the ink  90  immediately before being discharged from the nozzle  350 . The sensing parts  400  may be disposed at the inlet  331  of the inner pipe  330  and the discharge part  370  in which the nozzle  350  is disposed, respectively, and may more accurately measure the change in the number of bipolar elements  95 . 
     A method for the sensing part  400  to measure the number of bipolar elements  95  is not limited to an embodiment of  FIG.  6   . As described above, the bipolar elements  95  may have a small size and may have partially different polarities. The sensing part  400  may measure the number of bipolar elements  95  by a method other than a method for directly capturing an image in consideration of characteristics of the bipolar element  95 . 
       FIG.  25    is a schematic cross-sectional view illustrating that a sensing part detects the number of bipolar elements according to another embodiment. 
     Referring to  FIG.  25   , in a sensing part  400 _ 3 , a first sensor  410 _ 3  may emit light of a specific wavelength band, and the second sensor  420 _ 3  may sense the light emitted from the first sensor  410 _ 3  to measure the number of bipolar elements  95  in the ink  90 . In the embodiment, the first sensor  410 _ 3  may be a light emitting part, and the second sensor  420 _ 3  may be a light receiving part. An embodiment of  FIG.  25    is different from the embodiment of  FIG.  6    in a method for the sensing part  400 _ 3  to measure the number of bipolar elements  95 . Hereinafter, an overlapping description will be omitted, and contents different from those described above will be described. 
     The first sensor  410 _ 3  and the second sensor  420 _ 3  of the sensing part  400 _ 3  may be disposed to be spaced apart from each other with the inner pipe  330  or the nozzle  350  through which the ink  90  flows, interposed therebetween. The first sensor  410 _ 3  and the second sensor  420 _ 3  may be disposed in contact with an outer wall of the inner pipe  330  or the nozzle  350 , but the disclosure is not limited thereto, and the first sensor  410 _ 3  and the second sensor  420 _ 3  may also be spaced apart from the outer wall. 
     The first sensor  410 _ 3  may emit light of a specific wavelength band, and the light emitted from the first sensor  410 _ 3  may be incident on the second sensor  420 _ 3 . A type of the first sensor  410 _ 3  is not particularly limited. In some embodiments, the first sensor  410 _ 3  may be an ultraviolet (UV) laser irradiation device or lamp capable of emitting ultraviolet light or may be a light irradiation device or lamp capable of emitting visible light or white light. However, the embodiment is not limited thereto, and the first sensor  410 _ 3  may be a device capable of irradiating the ink  90  with light and may be variously modified as long as it may be adopted in the technical field. The first sensor  410 _ 3  may irradiate the ink  90  with the light, and the light may pass through the ink  90  and may be incident on the second sensor  420 _ 3 . 
     As the ink  90  is introduced into the inkjet head  300 , the light emitted from the first sensor  410 _ 3  may be incident on the bipolar elements  95 . The light may be partially scattered by interference of the bipolar elements  95 , and the second sensor  420 _ 3  may receive the light emitted from the first sensor  410 _ 3  and the scattered light to measure the number of bipolar elements  95  per unit volume or unit droplet of the ink  90  flowing through the inner pipe  330 . 
     The first sensor  410 _ 3  of the sensing part  400 _ 3  may radiate light L (see  FIG.  25   ) toward the second sensor  420 _ 3 . The light L may be at least partially irradiated to the bipolar elements  95  while passing through the ink  90  and being incident on the second sensor  420 _ 3 . The bipolar elements  95  may have a small size, and as the light L is irradiated to the bipolar elements  95  dispersed in the ink  90 , some of the light L may be scattered (L′ of  FIG.  25   ). In addition to the light L emitted from the first sensor  410 _ 3 , the light L′ scattered by the bipolar elements  95  may be incident on the second sensor  420 _ 3 . The light L′ scattered by the bipolar elements  95  may have a specific waveform according to a Brownian motion of the bipolar elements  95 . The second sensor  420 _ 3  may measure sizes of the bipolar element  95  by analyzing the waveform of the light L′ scattered by the bipolar elements  95 , and may measure the number of bipolar elements  95  per unit volume or unit droplet of the ink  90  through the measured sizes. The sensing part  400 _ 3  may measure the number or a dispersion degree of bipolar elements  95  in the discharged ink  90  by the measured number of bipolar elements  95  and droplets of the ink  90  discharged from the nozzle  350 . 
     According to an embodiment, the base part  310  of the inkjet head  300  may have an outer wall made of a transparent material so that the light emitted from the first sensor  410 _ 3  of the sensing part  400 _ 3  may be irradiated to the bipolar elements  95  in the ink  90 . In another embodiment, the inner pipe  330  or the nozzle  350  of the inkjet head  300  may have an outer wall made of a transparent material. The first sensor  410 _ 3  and the second sensor  420 _ 3  may be positioned in contact with at least the outer wall of the inner pipe  330  or the nozzle  350 , and due to the outer wall made of the transparent material, the light emitted from the first sensor  410 _ 3  may pass through the ink  90  and be incident on the second sensor  420 _ 3 . 
     The bipolar element  95  may include portions having different polarities, and the bipolar element  95  itself may have a dipole moment directed to a direction. The bipolar elements  95  may form an electric field (E-field) due to the dipole moment, and as represented in the following Equation 1, a magnetic field may be generated by a change in the electric field while the bipolar elements  95  flow in the inner pipe  330  in a state in which they are dispersed in the ink  90 . 
     
       
         
           
             
               
                 
                   
                     ∇ 
                     × 
                     B 
                   
                   = 
                   
                     
                       μ 
                       0 
                     
                     ( 
                     
                       J 
                       + 
                       
                         
                           ε 
                           0 
                         
                         ⁢ 
                         
                           
                             ∂ 
                             E 
                           
                           
                             ∂ 
                             t 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                         
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 1, ‘B’ is strength (or force) of the magnetic field, ‘μ’ is permeability, ‘J’ is a current density, ‘ε’ is permittivity, ‘E’ is strength of the electric field, and ‘t’ is a time. 
     Referring to Equation 1, the strength of the magnetic field may have a value based on a change in the strength of the electric field per unit time. A change in strength of an electric field corresponding to the number of bipolar elements  95  per unit time may occur within a section while the bipolar elements  95  move. In case that the number of bipolar elements  95  in the ink  90  introduced into the inkjet head  300  changes, the change in the strength of the electric field per unit time within the section may change, and accordingly, strength of the magnetic field may change. According to an embodiment, the sensing part  400  may measure the magnetic field by the movement of the bipolar elements  95  to calculate the number of bipolar elements  95  per unit droplet of the ink  90 . 
       FIG.  26    is a schematic cross-sectional view illustrating that a sensing part detects the number of bipolar elements according to another embodiment. 
     Referring to  FIG.  26   , as the bipolar elements  95  are supplied to the inlet  331  of the inner pipe  330 , a magnetic field B due to the movement of the bipolar elements  95  may be generated at the inlet  331  of the inner pipe  330 . According to an embodiment, a sensing part  400 _ 4  may measure strength of the magnetic field B due to the movement of the bipolar elements  95 . A first sensor  410 _ 4  may measure strength of the magnetic field B generated in a predetermined (or selectable) section within the inner pipe  330 , and a second sensor  420 _ 4  may calculate the number of bipolar elements  95  based on the strength of the magnetic field B measured by the first sensor  410 _ 4  and generate a feedback signal according to the calculated number of bipolar elements  95 . However, the disclosure is not limited thereto. 
     The sensing part  400 _ 4  may be disposed at the inlet  331  of the inner pipe  330  to which the ink  90  is supplied, and may measure the strength of the magnetic field B formed in a predetermined (or selectable) section of the inlet  331 . The strength of the magnetic field B may change depending on a change in the strength of the electric field, for example, the number and the movement of bipolar elements  95 . In case that the number of bipolar elements  95  in the ink  90  supplied to the inkjet head  300  changes, the change in the strength of the electric field by the bipolar elements  95  may change, and the strength of the magnetic field B formed in the predetermined (or selectable) section may change. The first sensor  410 _ 4  of the sensing part  400 _ 4  may sense such a change in the strength of the magnetic field, and the second sensor  420 _ 4  of the sensing part  400 _ 4  may measure a change in the number of bipolar elements  95  flowing in the predetermined (or selectable) section. Therefore, the sensing part  400 _ 4  may transfer a feedback signal to the actuator  390 , and the actuator  390  may control droplets of the ink  90  discharged per unit process. 
     The change in the strength of the magnetic field B formed in the predetermined (or selectable) section by the bipolar elements  95  may be due to the movement of the bipolar elements  95 , and may change depending on a flow velocity of the ink  90 . According to an embodiment, in order to more effectively measure the change in the strength of the magnetic field, the inkjet head  300  may partially include a portion in which the flow velocity of the ink  90  changes. The sensing part  400  may be disposed to correspond to a portion in which the flow velocity of the ink  90  changes, and may more precisely measure the change in the strength of the magnetic field by the bipolar elements  95 . 
       FIGS.  27  and  28    are schematic cross-sectional views illustrating portions of inkjet heads according to another embodiment. 
     First, referring to  FIG.  27   , in an inkjet head  300 _ 5  according to the embodiment, the inner pipe  330  or the inlet  331  of the inner pipe  330  may have a smaller diameter DH at a portion where the sensing part  400  is disposed than other portions. As the ink  90  is supplied to the inlet  331  of the inner pipe  330  or flows in the inner pipe  330  and then passes through a section having a narrow diameter, a flow velocity of the ink  90  may increase. In case that the flow velocity of the ink  90  including the bipolar elements  95  increases, a change in strength of an electric field per unit time by the bipolar elements  95  within a predetermined (or selectable) section may increase, and strength of a magnetic field measured by the sensing part  400  may increase. In case that the strength of the magnetic field by the bipolar elements  95  increases, the change in the strength of the magnetic field according to the change in the number of bipolar elements  95  may be sensed. 
     According to the embodiment, in the inkjet head  300 _ 5 , the inner pipe  330  in which the ink  90  flows may include a portion of which a diameter is narrower, and the sensing part  400  may be disposed in the portion in which the diameter decreases. Even though the strength of the magnetic field generated by the bipolar elements  95  is weak, a flow velocity of the ink  90  flowing in an area having the narrow diameter increases, such that the strength of the magnetic field may increase, and the sensing part  400  may precisely measure a change in the strength of the magnetic field. 
     Similarly, the inkjet head  300  may further include a sub-actuator PZT changing the diameter of the inner pipe  330 . Referring to  FIG.  28   , an inkjet head  300 _ 6  according to the embodiment may further include a sub-actuator PZT disposed at the inner pipe  330  or the inlet  331  of the inner pipe  330  and decreasing a diameter of the inner pipe  330 . The sub-actuator PZT may apply a hydraulic pressure to the inner pipe  330  in which the ink  90  flows, and a diameter of the inner pipe  330  may decrease. A flow velocity of the ink  90  may increase as the diameter of the inner pipe  330  decreases. 
     The sensing part  400  may be disposed adjacent to the sub-actuator PZT and may be disposed in a section where the flow velocity of the ink  90  increases. As the flow velocity of the ink  90  instantaneously increases by the sub-actuator PZT, a moving speed of the bipolar elements  95  also increases, and strength of the magnetic field B generated in a predetermined (or selectable) section may increase. 
       FIG.  29    is a schematic cross-sectional view illustrating that a sensing part detects the number of bipolar elements according to another embodiment. 
     In some embodiments, strengths of dipole moments of the bipolar elements  95  may change by light irradiated to the bipolar elements  95 . In case that the strengths of the dipole moments of the bipolar elements  95  change, strength of the electric field by the bipolar elements  95  may change, which means that strength of the magnetic field generated by the bipolar elements  95  within a predetermined (or selectable) section increases. 
     Referring to  FIG.  29   , a sensing part  400 _ 7  according to an embodiment may further include a light emitting part  430 _ 7  capable of irradiating the ink  90  with light. The light emitting part  430 _ 7  may be disposed in the base part  310  together with a first sensor  410 _ 7 , and may irradiate the ink  90  and the bipolar elements  95  flowing in the inner pipe  330  or the nozzle  350  with light of a specific wavelength band. In some embodiments, in the sensing part  400 _ 7 , a first sensor  410 _ 7  and the second sensor  420 _ 7  may be disposed to be inserted into the base part  310 , and the light emitting part  430 _ 7  may be disposed on an outer surface of the base part  310 . As described above, in some embodiments, the base part  310  of the inkjet head  300  may be made of a transparent material, and the light emitted from the light emitting part  430 _ 7  may reach the ink  90  flowing in the inner pipe  330 . 
     The dipole moments of the bipolar elements  95  may increase in response to the light irradiated from the light emitting part  430 _ 7 , and strength of an electric field by the bipolar elements  95  may increase. Accordingly, strength of a magnetic field generated by the bipolar elements  95  within the predetermined (or selectable) section may also increase, and the sensing part  400 _ 7  may effectively sense a change in the strength of the magnetic field. 
     However, the method for measuring the number of bipolar elements  95  is not limited thereto. The bipolar elements  95  may have dipole moments, and accordingly, a capacity or a current may be formed within the predetermined (or selectable) section while the bipolar elements  95  move. The sensing part  400  may measure the capacity or the current of the bipolar elements  95 , and may measure the number of bipolar elements  95  from the measured capacity or current. 
     The inkjet printing device  1000  according to an embodiment may include the sensing part  400  disposed in the inkjet head  300  and capable of measuring the number of bipolar elements  95 . The sensing part  400  may directly or indirectly measure the number of bipolar elements  95  in the ink  90  discharged from the inkjet head  300  and sense a change in the number of bipolar elements  95 . The change in the number of bipolar elements  95  may be transferred to the actuator  390  of the inkjet head  300 , and the actuator  390  may adjust the droplets of the discharged ink  90  The inkjet printing device  1000  may uniformly maintain the number of bipolar elements  95  discharged per unit process by adjusting the droplets of the discharged ink  90 . Accordingly, the inkjet printing device  1000  may improve reliability of a product manufactured to include the bipolar elements  95 . 
     The above-described bipolar element  95  may be a light emitting element including multiple semiconductor layers, and according to an embodiment, a display device including the light emitting elements may be manufactured using the inkjet printing device  1000 . 
       FIG.  30    is a schematic view of a light emitting element 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. 
     Referring to  FIG.  30   , the light emitting element  30  according to an embodiment may have a shape in which it 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 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 it extends in a direction, but outer surfaces thereof are partially inclined. 
     The light emitting element  30  may include a semiconductor layer doped with any conductivity-type (e.g., p-type or n-type) impurities. The semiconductor layer may receive an electrical signal applied from an external power source to emit light of a specific wavelength band. Multiple semiconductors included in the light emitting element  30  may have a structure in which they are sequentially disposed or stacked each other along the direction. 
     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 .  FIG.  30    illustrates a state in which the insulating film  38  is partially removed to expose the semiconductor layers  31 ,  32 , and  36  in order to visually illustrate the respective components of the light emitting element  30 . However, as described later, the insulating film  38  may be disposed to surround outer surfaces of the semiconductor layers  31 ,  32 , and  36 . 
     The first semiconductor layer  31  may be an n-type semiconductor. For 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 Ga y In 1-x-y N (0≤x≤1, 0≤y≤1, and 0≤x+y≤1). For example, the semiconductor material may include 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 include Si, Ge, Sn, or the like. In an embodiment, the first semiconductor layer  31  may be made of 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 is not limited thereto. 
     The second semiconductor layer  32  may be disposed on an active layer  36  to be described later. The second semiconductor layer  32  may be a p-type semiconductor, and for 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 Ga y In 1-x-y N (0≤x≤1, 0≤y≤1, and 0≤x+y≤1). For example, the semiconductor material may include one or more of AlGaInN, GaN, AlGaN, InGaN, A 1 N, and InN doped with a p-type dopant. The second semiconductor layer  32  may be doped with a p-type dopant, which may include Mg, Zn, Ca, Se, Ba, or the like. In an embodiment, the second semiconductor layer  32  may be made of 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 is not limited thereto. 
     It has been illustrated in the drawing that each of the first semiconductor layer  31  and the second semiconductor layer  32  is configured as one layer, the disclosure is not limited thereto. According to some embodiments, each of the first semiconductor layer  31  and the second semiconductor layer  32  may further include more layers, for example, 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 or 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 multiple quantum layers and well layers are alternately stacked each other. The active layer  36  may emit light by a combination of electron-hole pairs in response to electrical signals applied through the first semiconductor layer  31  and the second semiconductor layer  32 . For 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 particular, in case that the active layer  36  has the multiple quantum well structure, for example, the structure in which the quantum layers and the well layers are alternately stacked each other, 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. 
     However, the disclosure is 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 each other, and may include Group III to Group V semiconductor materials depending on 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 embodiments, the active layer  36  may emit light of red or 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 is not limited thereto. 
     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 both side surfaces of the light emitting element  30 . Directivity of the light emitted from the active layer  36  is not limited to one direction. 
     The electrode layer  37  may be an ohmic contact electrode. However, the disclosure is not limited thereto, and the electrode layer  37  may also be a Schottky contact electrode. The light emitting element  30  may include at least one electrode layer  37 . It has been illustrated in  FIG.  30    that the light emitting element  30  includes one electrode layer  37 , but the disclosure is not limited thereto. In some embodiments, the light emitting element  30  may also include more electrode layers  37  or the electrode layer  37  may also be omitted. A description of a light emitting element  30  to be provided later may be equally applied even though the number of electrode layers  37  is changed or the light emitting element  30  further includes another structure. 
     The electrode layer  37  may decrease resistance between the light emitting element  30  and an electrode or a contact electrode in case that the light emitting element  30  is electrically connected to the electrode or the contact 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 electrode layer  37  may include the same material or include different materials. A length of the electrode layer  37  may be in the range of about 0.05 μm to about 0.10 μm, but is not limited thereto. 
     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 be disposed to 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 serve to protect these members. For example, the insulating film  38  may be formed to surround side surface portions of these members, but may be formed to expose both ends of the light emitting element  30  in a longitudinal direction. 
     It has been illustrated in the drawing that the insulating film  38  is formed to extend in the longitudinal direction of the light emitting element  30  to cover side surfaces of the first semiconductor layer  31  to the electrode layer  37 , but the disclosure is 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 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 partially exposed. The insulating film  38  may also be formed so that an upper surface thereof is rounded in cross-sectional view in an area adjacent to at least one end 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 is not limited thereto. 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), and aluminum oxide (Al 2 O 3 ). 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. 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. 
     In some embodiments, an outer surface of the insulating film  38  may be surface-treated. The light emitting elements  30  may be jetted and aligned onto electrodes in a state in which they are dispersed in a predetermined (or selectable) ink when the display device  10  is manufactured. Here, 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 a range of about 1 μm to about 10 μm, about 2 μm to about 6 μm, or about 3 μm to about 5 μm. 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, the disclosure is not limited thereto, and multiple light emitting elements  30  included in the display device  10  may have different diameters according to a difference in composition between the active layers  36 . The diameter of the light emitting element  30  may be about 500 nm. 
     According to an embodiment, the inkjet printing device  1000  may disperse the light emitting elements  30  of  FIG.  30    in the ink  90  and 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.  31    is a schematic plan view of a display device according to an embodiment. 
     Referring to  FIG.  31   , a display device  10  may display a moving image or a still image. The display device  10  may include 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 (P 1 VIPs), navigation devices, game machines, digital cameras, camcorders, and the like, which provide display screens, may be included in the display device  10 . 
     A shape of the display device  10  may vary. 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 (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 . In  FIG.  31   , the display device  10  and the display area DPA having the rectangular shape with the width greater than the length are illustrated. 
     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 a screen may be displayed, and the non-display areas NDA may be areas in which the screen 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 multiple 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 is not limited thereto, and may also be a rhombic shape of which each side is inclined with respect to one direction. 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. 
       FIG.  32    is a schematic plan view of a pixel of the display device according to an embodiment. 
     Referring to  FIG.  32   , each of the pixels 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, the disclosure is not limited thereto, and the respective sub-pixels PXn may emit light of the same color. It has been illustrated in  FIG.  32    that the pixel PX includes three sub-pixels PXn, but the disclosure is not limited thereto, and the pixel PX may include more than three 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. 
     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  does not arrive, and thus, the light is not emitted. 
     Each sub-pixel PXn of the display device  10  may include multiple electrodes  21  and  22 , light emitting elements  30 , multiple contact electrodes  26 , and multiple internal banks  41  and  42  (see  FIG.  33   ), an external bank  43 , and at least one insulating layers  51 ,  52 ,  53 , and  55  (see  FIG.  33   ). 
     The electrodes  21  and  22  may be electrically connected to the light emitting elements  30 , and may receive a predetermined (or selectable) voltage applied thereto so that the light emitting elements  30  emits light of a specific wavelength band. At least a portion of each of the electrodes  21  and  22  may be utilized to form an electric field in the sub-pixel PXn in order to align the light emitting elements  30 . 
     The electrodes  21  and  22  may include a first electrode  21  and a second electrode  22 . In an embodiment, the first electrode  21  may be a pixel electrode separated for each sub-pixel PXn, and the second electrode  22  may be a common electrode commonly connected to each sub-pixel PXn. One of the first electrode  21  and the second electrode  22  may be an anode electrode of the light emitting element  30 , and the other of the first electrode  210  and the second electrode  220  may be a cathode electrode of the light emitting element  30 . However, the first electrode  21  and the second electrode  22  are not limited thereto, and vice versa. 
     The first electrode  21  and the second electrode  22  may include, respectively, electrode stem parts  21 S and  22 S disposed to extend in a fourth direction DR 4  and at least one electrode branch parts  21 B and  22 B extending and branching from the electrode stem parts  21 S and  22 S in a fifth direction DR 5 , which is a direction crossing the fourth direction DR 4 . 
     The first electrode  21  may include a first electrode stem part  21 S disposed to extend in the fourth direction DR 4  and at least one first electrode branch parts S 21 B branching from the first electrode stem part  210 S and extending in the fifth direction DR 5 . 
     The first electrode stem part  21 S of any one pixel may have both ends spaced apart from each other and terminated between the respective sub-pixels PXn, but the first electrode stem parts  21 S may be disposed on substantially the same straight line as a first electrode stem part  21 S in the same row (e.g., in the fourth direction DR 4 ). The first electrode stem parts  21 S disposed in the respective sub-pixels PXn may have both ends spaced apart from each other to apply different electrical signals to the respective first electrode branch parts  21 B, and the first electrode branch parts  21 B may be separately driven. 
     The first electrode branch parts  21 B may branch from at least portions of the first electrode stem part  21 S and may be disposed to extend in the fifth direction DR 5 , but may be terminated in a state in which they are spaced apart from the second electrode stem part  22 S disposed to face the first electrode stem part  21 S. 
     The second electrode  22  may include a second electrode stem part  22 S extending in the fourth direction DR 4  and spaced apart from and facing the first electrode stem part  21 S in the fifth direction DR 5  and a second electrode branch part  22 B branching from the second electrode stem part  22 S and extending in the fifth direction DR 5 . The second electrode stem part  22 S may have an end extended to a second electrode stem part  22 S of adjacent sub-pixel PXn in the fourth direction DR 4 . For example, the second electrode stem part  22 S may be disposed to extend in the fourth direction DR 4  to cross the respective sub-pixels PXn, unlike the first electrode stem part  21 S. The second electrode stem part  22 S crossing the respective sub-pixels PXn may be connected to a portion extending in a direction in an outer portion of the display area DPA in which the respective pixels PX or sub-pixels PXn are disposed or the non-display area NDA. 
     The second electrode branch part  22 B may be spaced apart from and face the first electrode branch part  21 B, and may be terminated in a state in which it is spaced apart from the first electrode stem part  21 S. The second electrode branch part  22 B may be connected to the second electrode stem part  22 S, and an end of the second electrode branch part  22 B in an extension direction may be disposed in the sub-pixel PXn in a state in which it is spaced apart from the first electrode stem part  21 S. 
     The first electrode  21  and the second electrode  22  may be electrically connected to a circuit element layer of the display device  10  through contact holes, for example, a first electrode contact hole CNTD and a second electrode contact hole CNTS, respectively. It has been illustrated in the drawing that the first electrode contact hole CNTD is formed for each of the first electrode stem parts  21 S of the respective sub-pixels PXn and only one second electrode contact hole CNTS is formed in one second electrode stem part  22 S crossing the respective sub-pixels PXn. However, the disclosure is not limited thereto, and in some embodiments, the second electrode contact hole CNTS may also be formed for each sub-pixel PXn. 
     The external bank  43  may be disposed at boundaries between the respective sub-pixels PXn, and the internal banks  41  and  42  may be adjacent to a central portion of each sub-pixel PXn and may be disposed below the respective electrodes  21  and  22 . The internal banks  41  and  42  have not been illustrated in the drawings, but a first internal bank  41  and a second internal bank  42  may be disposed below the first electrode branch part  21 B and the second electrode branch part  22 B, respectively. 
     The external bank  43  may be disposed at the boundaries between the respective sub-pixels PXn. Respective ends of multiple first electrode stem parts  21 S may be spaced apart from each other and terminated by the external banks  43 . The external bank  43  may extend in the fifth direction DR 5  and be disposed at the boundaries between the sub-pixels PXn arranged in the fourth direction DR 4 . However, the disclosure is not limited thereto, and the external bank  43  may also extend in the fourth direction DR 4  and may also be disposed at boundaries between sub-pixels PXn arranged in the fifth direction DR 5 . The external bank  43  and the internal banks  41  and  42  may include the same material and may be formed simultaneously in one process. 
     The light emitting elements  30  may be disposed between the first electrode  21  and the second electrode  22 . One ends of the light emitting elements  30  may be electrically connected to the first electrode  21 , and the other ends of the light emitting elements  30  may be electrically connected to the second electrode  22 . The light emitting elements  30  may be electrically connected to the first electrode  21  and the second electrode  22  through contact electrodes  26  to be described later, respectively. 
     The light emitting elements  30  may be disposed to 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 particularly limited. In some cases, multiple light emitting elements  30  may be disposed adjacent to each other and may be grouped, and other multiple light emitting elements  30  may be grouped in a state in which they are spaced apart from the light emitting elements  30  by a predetermined (or selectable) distance or multiple light emitting elements  30  may have a non-uniform density, but may be oriented and aligned in a direction. In an embodiment, the light emitting elements  30  may have a shape in which they extend in a direction, and a direction in which the respective electrodes, for example, the first electrode branch part  21 B and the second electrode branch part  22 B extend, and the direction in which the light emitting elements  30  extend may be substantially perpendicular to each other. However, the disclosure is not limited thereto, and the light emitting elements  30  may be not perpendicular to the direction in which the first electrode branch part  21 B and the second electrode branch part  22 B extend, and may be disposed to be oblique with respect to the direction in which the first electrode branch part  210 B and the second electrode branch part  220 B extend. 
     The light emitting elements  30  according to an embodiment may include active layers  36  including different materials to emit light of different wavelength bands to the outside. In the display device  10 , the light emitting elements  30  of the first sub-pixel PX 1  may emit first light of which a central wavelength band is a first wavelength, the light emitting elements  30  of the second sub-pixel PX 2  may emit second light of which a central wavelength band is a second wavelength, and the light emitting elements  30  of the third sub-pixel PX 3  may emit third light of which a central wavelength band is a third wavelength. Accordingly, the first light may be emitted from the first sub-pixel PX 1 , the second light may be emitted from the second sub-pixel PX 2 , and the third light may be emitted from the third sub-pixel PX 3 . In some embodiments, the first light may be blue light having a central wavelength band in the range of about 450 nm to about 495 nm, the second light may be green light having a central wavelength band in the range of about 495 nm to about 570 nm, and the third light may be red light having a central wavelength band in the range of about 620 nm to about 750 nm. However, the disclosure is not limited thereto. 
     Although not illustrated in  FIG.  30   , the display device  10  may include a second insulating layer  52  covering at least portions of the first electrode  21  and the second electrode  22 . 
     The second insulating layer  52  may be disposed in each sub-pixel PXn of the display device  10 . The second insulating layer  52  may be disposed to substantially entirely cover each sub-pixel PXn, and may also be disposed to extend to adjacent sub-pixels PXn. The second insulating layer  52  may be disposed to cover at least portions of the first electrode  21  and the second electrode  22 . The second insulating layer  52  may be disposed to expose portions of the first electrode  21  and the second electrode  22 , for example, partial areas of the first electrode branch part  21 B and the second electrode branch part  22 B. 
     The contact electrodes  26  may have a shape in which at least partial areas thereof extend in a direction. Each of the contact electrodes  26  may be in contact with the light emitting elements  30  and the electrodes  21  and  22 , and the light emitting elements  30  may receive electrical signals from the first electrode  21  and the second electrode  22  through the contact electrodes  26 . 
     The contact electrode  26  may include first contact electrodes  26   a  and a second contact electrode  26   b.  The first contact electrodes  26   a  and the second contact electrode  26   b  may be disposed on the first electrode branch parts  21 B and the second electrode branch part  22 B, respectively. 
     The first contact electrode  26   a  may be disposed on the first electrode  21  or the first electrode branch part  21 B and may extend in the fifth direction DR 5 . The first contact electrode  26   a  may be in contact with one ends of the light emitting elements  30 . The first contact electrode  26   a  may be in contact with the first electrode  21  exposed without the second insulating layer  52  disposed thereon. Accordingly, the light emitting elements  30  may be electrically connected to the first electrode  21  through the first contact electrode  26   a.    
     The second contact electrode  26   b  may be disposed on the second electrode  22  or the second electrode branch part  22 B and may extend in the fifth direction DR 5 . The second contact electrode  26   b  may be spaced apart from the first contact electrode  26   a  in the fourth direction DR 4 . The second contact electrode  26   b  may be in contact with the other ends of the light emitting elements  30 . The second contact electrode  26   b  may be in contact with the second electrode  22  exposed without the second insulating layer  52  disposed thereon. Accordingly, the light emitting elements  30  may be electrically connected to the second electrode  22  through the second contact electrode  26   b.  It has been illustrated in the drawings that two first contact electrodes  26   a  and one second contact electrode  26   b  are disposed in one sub-pixel PXn, but the disclosure is not limited thereto. The numbers of first contact electrodes  26   a  and second contact electrodes  26   b  may change depending on the numbers of first electrodes  21  and second electrodes  22  disposed in each sub-pixel PXn or first electrode branch parts  21 B and second electrode branch parts  22 B. 
     In some embodiments, widths of the first contact electrode  26   a  and the second contact electrode  26   b  measured in a direction may be greater than widths of the first electrode  21  and the second electrode  22  or the first electrode branch part  21 B and the second electrode branch part  22 B measured in the direction, respectively. However, the disclosure is not limited thereto, and in some embodiments, the first contact electrode  26   a  and the second contact electrode  26   b  may be disposed to cover only one side portions of the first electrode branch part  21 B and the second electrode branch part  22 B, respectively. 
     The display device  10  may include a circuit element layer positioned below the respective electrodes  21  and  22 , and a third insulating layer  53  (see  FIG.  33   ) and a passivation layer  55  (see  FIG.  33   ) disposed to cover at least portions of the respective electrodes  21  and  22  and the light emitting element  30 , in addition to the second insulating layer  52 . Hereinafter, a structure of the display device  10  will be described in detail with reference to  FIG.  33   . 
       FIG.  33    is a schematic cross-sectional view taken along line Xa-Xa′, line Xb-Xb′, and line Xc-Xc′ of  FIG.  32   . 
       FIG.  33    illustrates only a cross section of the first sub-pixel PX 1 , which may be equally applied to other pixels PX or sub-pixels PXn.  FIG.  33    illustrates a schematic cross section crossing an end and another end of the light emitting element  30  disposed in the first sub-pixel PX 1 . 
     Although not illustrated in  FIG.  33   , the display device  10  may further include a circuit element layer positioned below the respective electrodes  21  and  22 . The circuit element layer may include multiple semiconductor layers and multiple conductive patterns, and may include at least one transistor and a power line. 
     Referring to  FIGS.  32  and  33   , the display device  10  may include a first insulating layer  51 , the electrodes  21  and  22 , the light emitting elements  30 , and the like, disposed on the first insulating layer  51 . A circuit element layer (not illustrated) may be further disposed below the first insulating layer  51 . The first insulating layer  51  may include an organic insulating material to perform a surface planarization function. 
     The internal banks  41  and  42 , the external bank  43 , the electrodes  21  and  22 , and the light emitting elements  30  may be disposed on the first insulating layer  51 . 
     The external bank  43  may serve to prevent the ink in which the light emitting elements  30  are dispersed from crossing the boundaries between the sub-pixels PXn when the ink is jetted using the inkjet printing device of  FIG.  1    described above, at the time of manufacturing the display device  10 . The external bank  43  may separate inks in which different light emitting elements  30  are dispersed for each of the different sub-pixels PXn from each other so that these inks are not mixed with each other. However, the disclosure is not limited thereto. 
     The internal banks  41  and  42  may include a first internal bank  41  and a second internal bank  42  disposed adjacent to a central portion of each sub-pixel PXn. 
     The first internal bank  41  and the second internal bank  42  may be disposed to be spaced apart from and face each other. The first electrode  21  may be disposed on the first internal bank  41 , and the second electrode  22  may be disposed on the second internal bank  42 . Referring to  FIGS.  32  and  33   , it may be understood that the first electrode branch part  21 B may be disposed on the first internal bank  41  and the second electrode branch part  22 B may be disposed on the second internal bank  42 . 
     The first internal bank  41  and the second internal bank  42  may be disposed to extend in the fifth direction DR 5  in each sub-pixel PXn. However, the disclosure is not limited thereto, and the first internal bank  41  and the second internal bank  42  may be disposed for each sub-pixel PXn to form a pattern in the entirety of the display device  10 . The internal banks  41  and  42  and the external banks  43  may include polyimide (PI), but are not limited thereto. 
     The first internal bank  41  and the second internal bank  42  may have a structure in which at least portions thereof protrude with respect to the first insulating layer  51 . The first internal bank  41  and the second internal bank  42  may protrude upward with respect to a plane on which the light emitting elements  30  are disposed, and at least portions of the protruding portions may have an inclination. Since the internal banks  41  and  42  protrude with respect to the first insulating layer  51  and have inclined side surfaces, light emitted from the light emitting elements  30  may be reflected from the inclined side surfaces of the internal banks  41  and  42 . As described later, in case that the electrodes  21  and  22  disposed on the internal banks  41  and  42  include a material having high reflectivity, the light emitted from the light emitting elements  30  may be reflected by the electrodes  21  and  22  and travel in an upward direction of the first insulating layer  51 . 
     The external bank  43  may be disposed at the boundaries between the adjacent sub-pixels PXn to form a lattice pattern, but the internal banks  41  and  42  may be disposed within each sub-pixel PXn and have a shape in which they extend in a direction. 
     The electrodes  21  and  22  may be disposed on the first insulating layer  51  and the internal banks  41  and  42 , respectively. As described above, the electrodes  21  and  22  may include the electrode stem parts  21 S and  22 S and the electrode branch parts  21 B and  22 B, respectively. 
     Partial areas of the first electrode  21  and the second electrode  22  may be disposed on the first insulating layer  51 , and the other partial areas of the first electrode  21  and the second electrode  22  may be disposed on the first internal bank  41  and the second internal bank  42 , respectively. As described above, the first electrode stem part  21 S of the first electrode  21  and the second electrode stem part  22 S of the second electrode  22  may extend in the fourth direction DR 4 , and the first internal bank  41  and the second internal bank  42  may extend in the fifth direction DR 5  and may also be disposed in adjacent sub-pixels PXn in the fifth direction DR 5 . 
     The first electrode contact hole CNTD penetrating through the first insulating layer  51  and exposing a portion of the circuit element layer may be formed in the first electrode stem part  21 S of the first electrode  21 . The first electrode  21  may be electrically connected to a transistor of the circuit element layer through the first electrode contact hole CNTD. A predetermined (or selectable) electrical signal may be transferred from the transistor to the first electrode  21 . 
     The second electrode stem part  22 S of the second electrode  22  may extend in a direction and may also be disposed in the non-emission area in which the light emitting elements  30  are not disposed. The second electrode contact hole CNTS penetrating through the first insulating layer  51  and exposing a portion of the circuit element layer may be formed in the second electrode stem part  22 S. The second electrode  22  may be electrically connected to a power source electrode through the second electrode contact hole CNTS. A predetermined (or selectable) electrical signal may be transferred from the power source electrode to the second electrode  22 . 
     Partial areas of the first electrode  21  and the second electrode  22 , for example, the first electrode branch part  21 B and the second electrode branch part  22 B may be disposed on the first internal bank  41  and the second internal bank  42 , respectively. The light emitting elements  30  may be disposed in an area between the first electrode  21  and the second electrode  22 , for example, in a space where the first electrode branch part  21 B and the second electrode branch part  22 B are spaced apart from and face each other. 
     Each of the electrodes  21  and  22  may include a transparent conductive material. For 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 is 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. Light incident on each of the electrodes  21  and  22  may be reflected to be emitted in an upward direction of each sub-pixel PXn. 
     The 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 each other or may be formed as one 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/IZO or be made of an alloy including aluminum (Al), nickel (Ni), lanthanum (La), and the like. However, the disclosure is not limited thereto. 
     The second insulating layer  52  may be disposed on the first insulating layer  51 , the first electrode  21 , and the second electrode  22 . The second insulating layer  52  may be disposed to partially cover the first electrode  21  and the second electrode  22 . The second insulating layer  52  may be disposed to cover most of upper surfaces of the first electrode  21  and the second electrode  22 , but may expose portions of the first electrode  21  and the second electrode  22 . The second insulating layer  52  may be disposed so that portions of the upper surfaces of the first electrode  21  and the second electrode  22 , for example, portions of an upper surface of the first electrode branch part  21 B disposed on the first internal bank  41  and an upper surface of the second electrode branch part  22 B disposed on the second internal bank  42  are exposed. For example, the second insulating layer  52  may be substantially entirely formed on the first insulating layer  51 , but may include openings partially exposing the first electrode  21  and the second electrode  22 . 
     In an embodiment, the second insulating layer  52  may have a step formed so that a portion of an upper surface thereof is recessed between the first electrode  21  and the second electrode  22 . In some embodiments, the second insulating layer  52  may include an inorganic insulating material, and a portion of the upper surface of the second insulating layer  52  disposed to cover the first electrode  21  and the second electrode  22  may be recessed due to a step of a member disposed below the second insulating layer  52 . The light emitting element  30  disposed on the second insulating layer  52  between the first electrode  21  and the second electrode  22  may form an empty space between the light emitting element  30  and the recessed upper surface of the second insulating layer  52 . The light emitting element  30  may be disposed to be partially spaced apart from the upper surface of the second insulating layer  52 , and a material constituting a third insulating layer  53  to be described later may be filled in the space. However, the disclosure is not limited thereto. The second insulating layer  52  may form a flat upper surface on which the light emitting element  30  is disposed thereon. 
     The second insulating layer  52  may insulate the first electrode  21  and the second electrode  22  from each other while protecting the first electrode  21  and the second electrode  22 . The second insulating layer  52  may prevent the light emitting elements  30  disposed on the second insulating layer  52  from being in direct contact with and being damaged by other members. However, a shape and a structure of the second insulating layer  52  are not limited thereto. 
     The light emitting elements  30  may be disposed on the second insulating layer  52  between the respective electrodes  21  and  22 . For example, at least one light emitting element  30  may be disposed on the second insulating layer  52  disposed between the respective electrode branch parts  21 B and  22 B. However, the disclosure is not limited thereto, and although not illustrated in the drawings, at least some of the light emitting elements  30  disposed in each sub-pixel PXn may also be disposed in an area other than an area between the electrode branch parts  21 B and  22 B. The light emitting elements  30  may be disposed on respective ends of the first electrode branch part  21 B and the second electrode branch part  22 B facing each other, and may be electrically connected to the respective electrodes  21  and  22  through the contact electrodes  26 . 
     The light emitting element  30  may include multiple layers disposed in a direction parallel to the first insulating layer  51 . The light emitting element  30  of the display device  10  according to an embodiment may have a shape in which it extends in a direction, and may have a structure in which multiple semiconductor layers are sequentially disposed in the direction. As described above, in the light emitting element  30 , the first semiconductor layer  31 , the active layer  36 , the second semiconductor layer  32 , and the electrode layer  37  may be sequentially disposed along the direction, and the insulating film  38  may surround outer surfaces of the first semiconductor layer  31 , the active layer  36 , the second semiconductor layer  32 , and the electrode layer  37 . The light emitting element  30  disposed in the display device  10  may be disposed so that the direction in which the light emitting element  30  extends is parallel to the first insulating layer  51 , and the semiconductor layers included in the light emitting element  30  may be sequentially disposed along the direction parallel to an upper surface of the first insulating layer  51 . However, the disclosure is not limited thereto. In some embodiments, in case that the light emitting element  30  has another structure, the layers may also be disposed in a direction perpendicular to the first insulating layer  51 . 
     An end of the light emitting element  30  may be in contact with the first contact electrode  26   a,  and another end of the light emitting element  30  may be in contact with the second contact electrode  26   b.  According to an embodiment, end surfaces of the light emitting element  30  extending in the direction are exposed without the insulating film  38 , and thus, the light emitting element  30  may be in contact with the first contact electrode  26   a  and the second contact electrode  26   b  to be described later in the exposed areas. However, the disclosure is not limited thereto. In some embodiments, at least partial areas of the insulating film  38  of the light emitting element  30  are removed, such that both end surfaces of the light emitting element  30  may be partially exposed. 
     The third insulating layer  53  may be partially disposed on the light emitting element  30  disposed between the first electrode  21  and the second electrode  22 . The third insulating layer  53  may be disposed to partially surround an outer surface of the light emitting element  30 . The third insulating layer  53  may protect the light emitting element  30  and may serve to fix the light emitting element  30  in a process of manufacturing the display device  10 . In an embodiment, a portion of the third insulating layer  53  may be disposed between a lower surface of the light emitting element  30  and an upper surface of the second insulating layer  52 . As described above, the third insulating layer  53  may be formed to fill a space between the second insulating layer  52  and the light emitting element  30  formed during the process of manufacturing the display device  10 . Accordingly, the third insulating layer  53  may be formed to surround the outer surface of the light emitting element  30 . However, the disclosure is not limited thereto. 
     The third insulating layer  53  may be disposed to extend in the fifth direction DR 5  between the first electrode branch part  21 B and the second electrode branch part  22 B in a plan view. For example, the third insulating layer  53  may have an island shape or a linear shape in a plan view on the first insulating layer  51 . According to an embodiment, the third insulating layer  53  may be disposed on the light emitting element  30 . 
     The first contact electrode  26   a  and the second contact electrode  26   b  may be disposed on the electrodes  21  and  22 , respectively, and may be disposed on the third insulating layer  53 . The first contact electrode  26   a  and the second contact electrode  26   b  may be disposed to be spaced apart from each other on the third insulating layer  53 . The third insulating layer  53  may insulate the first contact electrode  26   a  and the second contact electrode  26   b  from each other so that the first contact electrode  26   a  and the second contact electrode  26   b  are not in direct contact with each other. 
     The first contact electrode  26   a  may be in contact with the exposed partial area of the first electrode  21  on the first internal bank  41 , and the second contact electrode  26   b  may be in contact with the exposed partial area of the second electrode  22  on the second internal bank  42 . The first contact electrode  26   a  and the second contact electrode  26   b  may transfer the electrical signals transferred from the respective electrodes  21  and  22  to the light emitting element  30 . 
     The contact electrode  26  may include a conductive material. For example, the contact electrode  26  may include ITO, IZO, ITZO, aluminum (Al), or the like. However, the disclosure is not limited thereto. 
     The passivation layer  55  may be disposed on the contact electrode  26  and the third insulating layer  53 . The passivation layer  55  may serve to protect members disposed on the first insulating layer  51  from an external environment. 
     Each of the second insulating layer  52 , the third insulating layer  53 , and the passivation layer  55  described above may include an inorganic insulating material or an organic insulating material. In an embodiment, the second insulating layer  52 , the third insulating layer  53 , and the passivation layer  55  may include an inorganic insulating material such as silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y ), aluminum oxide (Al 2 O 3 ), or aluminum nitride (AlN). The second insulating layer  52 , the third insulating layer  53 , and the passivation layer  55  may include 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, benzocyclobutene, a cardo resin, a siloxane resin, a silsesquioxane resin, polymethylmethacrylate, polycarbonate, a polymethylmethacrylate-polycarbonate synthetic resin, and the like, as the organic insulating material. However, the disclosure is not limited thereto. 
       FIGS.  34  to  36    are schematic cross-sectional views illustrating some processes of a method for manufacturing the display device according to an embodiment. 
     Referring to  FIGS.  34  to  36   , the display device  10  according to an embodiment may be manufactured using the inkjet printing device  1000  described above with reference to  FIG.  1   . The inkjet printing device  1000  may jet the ink  90  in which the light emitting elements  30  are dispersed, and the light emitting elements  30  may be disposed between the first electrode  21  and the second electrode  22  of the display device  10 . 
     First, as illustrated in  FIG.  34   , the first insulating layer  51 , the first internal bank  41  and the second internal bank  42  disposed spaced apart from each other on the first insulating layer  51 , the first electrode  21  and the second electrode  22  disposed on the first internal bank  41  and the second internal bank  42 , respectively, and a second insulating material layer  52 ′ covering the first electrode  21  and the second electrode  22  may be prepared. The second insulating material layer  52 ′ may be partially patterned in a subsequent process to form the second insulating layer  52  of the display device  10 . The members described above may be formed by patterning a metal, an inorganic material, an organic material, or the like by a general mask process. 
     The ink  90  in which the light emitting elements  30  are dispersed may be jetted onto the first electrode  21  and the second electrode  22 . The light emitting element  30  may be a type of bipolar element, and the ink  90  in which the light emitting elements  30  are dispersed may be jetted using the inkjet printing device  1000  and the method for printing the bipolar elements described above. As illustrated in the drawing, the inkjet printing device  1000  according to an embodiment may discharge the ink  90  while uniformly maintaining the number of light emitting elements  30  in the ink  90 . A description thereof is the same as that described above, and a detailed description will thus be omitted. 
     As illustrated in  FIG.  35   , an electrical signal may be applied to the first electrode  21  and the second electrode  22  to generate an electric field IEL in the ink  90  in which the light emitting elements  30  are dispersed. The light emitting elements  30  may receive a dielectrophoretic force transferred by the electric field IEL, and may be seated on the first electrode  21  and the second electrode  22  while their orientation directions and positions change. 
     As illustrated in  FIG.  36   , the solvent  91  of the ink  90  may be removed. The light emitting elements  30  may be disposed between the first electrode  21  and the second electrode  22  through the processes described above. Thereafter, although not illustrated in the drawings, the second insulating material layer  52 ′ may be patterned to form the second insulating layer  52 , and the third insulating layer  53 , the first contact electrode  26   a,  the second contact electrode  26   b,  and the passivation layer  55  may be formed to manufacture the display device  10 . 
     A shape and a material of the light emitting element  30  are not limited to those of  FIG.  30   . In some embodiments, the light emitting element  30  may include more layers or may have other shapes. 
       FIGS.  37  and  38    are schematic views of light emitting elements according to another embodiment. 
     Referring to  FIG.  37   , a light emitting element  30 ′ according to the embodiment may further include a third semiconductor layer  33 ′ disposed between a first semiconductor layer  31 ′ and an active layer  36 ′, and a fourth semiconductor layer  34 ′ and a fifth semiconductor layer  35 ′ disposed between the active layer  36 ′ and a second semiconductor layer  32 ′. The light emitting element  30 ′ is different from the light emitting element according to the embodiment of  FIG.  30    in that multiple semiconductor layers  33 ′,  34 ′, and  35 ′ and electrode layers  37   a ′ and  37   b ′ are further disposed and the active layer  36 ′ contains other elements. A disposition and a structure of an insulating film  38 ′ is substantially the same as those in  FIG.  30   . 
     As described above, in the light emitting element  30  of  FIG.  30   , the active layer  36  may include nitrogen (N) to emit the blue or green light. On the other hand, in the light emitting element  30 ′ of  FIG.  37   , the active layer  36 ′ and other semiconductor layers may be semiconductors including at least phosphorus (P), respectively. For example, the light emitting element  30 ′ according to the embodiment may emit red light having a central wavelength band in the range of about 620 nm to about 750 nm. However, it is to be understood that the central wavelength band of the red light is not limited to the above-described range and may include all wavelength ranges that may be recognized as red in the technical field. 
     The first semiconductor layer  31 ′ may be an n-type semiconductor layer, and in case that the light emitting element  30 ′ emits red light, the first semiconductor layer  31 ′ may include a semiconductor material having a chemical formula of In x Al y Ga 1-x-y P (0≤x≤1, 0≤y≤1, and 0≤x+y≤1). For example, the first semiconductor layer  31 ′ may be made of one or more of InAlGaP, GaP, AlGaP, InGaP, AlP, and InP doped with an n-type dopant. The first semiconductor layer  31 ′ may be doped with an n-type dopant, which may include Si, Ge, Sn, or the like. In an embodiment, the first semiconductor layer  31 ′ may be made of AlGaInP 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 is not limited thereto. 
     The second semiconductor layer  32 ′ may be a p-type semiconductor layer, and in case that the light emitting element  30 ′ emits red light, the second semiconductor layer  32 ′ may include a semiconductor material having a chemical formula of In x Al y Ga 1-x-y P (0≤x≤1, 0≤y≤1, and 0≤x+y≤1). For example, the second semiconductor layer  32 ′ may be made of one or more of InAlGaP, GaP, AlGaNP, InGaP, AlP, and InP doped with a p-type dopant. The second semiconductor layer  32 ′ may be doped with a p-type dopant, which may include Mg, Zn, Ca, Se, Ba, or the like. In the embodiment, the second semiconductor layer  32 ′ may be made of GaN doped with p-type Mg. A length of the second semiconductor layer  32 ′ may be in the range of about 0.08 μm to about 0.25 μm, but is not limited thereto. 
     The active layer  36 ′ may be disposed between the first semiconductor layer  31 ′ and the second semiconductor layer  32 ′. Like the active layer  36  of  FIG.  30   , the active layer  36 ′ of  FIG.  37    may include a material having a single or multiple quantum well structure to emit light of a specific wavelength band. For example, in case that the active layer  36 ′ emits light of a red wavelength band, the active layer  36 ′ may include a material such as AlGaP or AlInGaP. In particular, in case that the active layer  36 ′ has the multiple quantum well structure, for example, a structure in which quantum layers and well layers are alternately stacked each other, the quantum layers may include a material such as AlGaP or AlInGaP, and the well layers may include a material such as GaP or AlInP. In the embodiment, the active layer  36 ′ may include AlGaInP as a material of the quantum layers and AlInP as a material of the well layers to emit red light having a central wavelength band of about 620 nm to about 750 nm. 
     The light emitting element  30 ′ may include clad layers disposed adjacent to the active layer  36 ′. As illustrated in drawing, the third semiconductor layer  33 ′ and the fourth semiconductor layer  34 ′ disposed between the first semiconductor layer  31 ′ and the second semiconductor layer  32 ′ below and above the active layer  36 ′, respectively, may be the clad layers. 
     The third semiconductor layer  33 ′ may be disposed between the first semiconductor layer  31 ′ and the active layer  36 ′. The third semiconductor layer  33 ′ may be an n-type semiconductor like the first semiconductor layer  31 ′, and may include a semiconductor material having a chemical formula of In x Al y Ga 1-x-y P (0≤x≤1, 0≤y≤1, and 0≤x+y≤1). In an embodiment, the first semiconductor layer  31 ′ may be made of n-AlGaInP, and the third semiconductor layer  33 ′ may be made of n-AlInP. However, the disclosure is not limited thereto. 
     The fourth semiconductor layer  34 ′ may be disposed between the active layer  36 ′ and the second semiconductor layer  32 ′. The fourth semiconductor layer  34 ′ may be an n-type semiconductor like the second semiconductor layer  32 ′, and may include a semiconductor material having a chemical formula of In x Al y Ga 1-x-y P (0≤x≤2, 0≤y≤1, and 0≤x+y≤1). In an embodiment, the second semiconductor layer  32 ′ may be made of p-GaP, and the fourth semiconductor layer  34 ′ may be made of p-AlInP. 
     The fifth semiconductor layer  35 ′ may be disposed between the fourth semiconductor layer  34 ′ and the second semiconductor layer  32 ′. The fifth semiconductor layer  35 ′ may be a semiconductor doped with a p-type dopant like the second semiconductor layer  32 ′ and the fourth semiconductor layer  34 ′. In some embodiments, the fifth semiconductor layer  35 ′ may serve to reduce a difference in lattice constant between the fourth semiconductor layer  34 ′ and the second semiconductor layer  32 ′. For example, the fifth semiconductor layer  35 ′ may be a tensile strain barrier reducing (TSBR) layer. For example, the fifth semiconductor layer  35 ′ may include p-GaInP, p-AlInP, p-AlGaInP, or the like, but is not limited thereto. Lengths of the third semiconductor layer  33 ′, the fourth semiconductor layer  34 ′, and the fifth semiconductor layer  35 ′ may be in the range of about 0.08 μm to about 0.25 μm, but are not limited thereto. 
     A first electrode layer  37   a ′ and a second electrode layer  37   b ′ may be disposed on the first semiconductor layer  31 ′ and the second semiconductor layer  32 ′, respectively. The first electrode layer  37   a ′ may be disposed on a lower surface of the first semiconductor layer  31 ′, and the second electrode layer  37   b ′ may be disposed on an upper surface of the second semiconductor layer  32 ′. However, the disclosure is not limited thereto, and at least one of the first electrode layer  37   a ′ and the second electrode layer  37   b ′ may be omitted. For example, in the light emitting element  30 ′, the first electrode layer  37   a ′ may not be disposed on the lower surface of the first semiconductor layer  31 ′, and only one second electrode layer  37   b ′ may be disposed on the upper surface of the second semiconductor layer  32 ′. 
     Referring to  FIG.  38   , a light emitting element  30 ″ may have a shape in which it extends in a direction, but may have a shape in which side surfaces thereof are partially inclined. For example, the light emitting element  30 ″ according to the embodiment may have a partially conical shape. 
     The light emitting element  30 ″ may be formed so that multiple layers are not stacked each other in a direction and the respective layers surround an outer surface of another layer. The light emitting element  30 ″ may be formed so that the semiconductor layers surround at least a portion of an outer surface of another layer. The light emitting element  30 ″ may include a semiconductor core of which at least a partial area extends in a direction and an insulating film  38 ″ formed to surround the semiconductor core. The semiconductor core may include a first semiconductor layer  31 ″, an active layer  36 ″, a second semiconductor layer  32 ″, and an electrode layer  37 ″. The light emitting element  30 ″ is the same as the light emitting element  30  of  FIG.  30    except that shapes of the respective layers are partially different. Hereinafter, a description of the same content will be omitted, and contents different from those described above will be described. 
     According to the embodiment, the first semiconductor layer  31 ″ may be formed to extend in a direction and to have both ends inclined toward a central portion. The first semiconductor layer  31 ″ may have a body portion having a rod shape or a cylindrical shape and ends formed at upper and lower portions of the body portion, respectively, and formed to have inclined side surfaces. An upper end of the body portion may have a steeper inclination than a lower end of the body portion. 
     The active layer  36 ″ may be disposed to surround an outer surface of the body portion of the first semiconductor layer  31 ″. The active layer  36 ″ may have a ring shape in which it extends in a direction. The active layer  36 ″ may not be formed on an upper end and a lower end of the first semiconductor layer  31 ″. The active layer  36 ″ may be formed only on non-inclined side surfaces of the first semiconductor layer  31 ″. However, the disclosure is not limited thereto. Accordingly, light emitted from the active layer  36 ″ may be emitted not only to both ends of the light emitting element  30 ″ in a length direction, but also to both side surfaces of the light emitting element  30 ″ with respect to the length direction. The active layer  36 ″ of the light emitting element  30 ″ of  FIG.  38    may have a greater area than that of the light emitting element  30  of  FIG.  30    to emit a greater amount of light than that of the light emitting element  30  of  FIG.  28   . 
     The second semiconductor layer  32 ″ may be disposed to surround an outer surface of the active layer  36 ″ and the upper end of the first semiconductor layer  31 ″. The second semiconductor layer  32 ″ may include a body portion extending in a direction and having a ring shape and an upper end formed to have inclined side surfaces. For example, the second semiconductor layer  32 ″ may be in direct contact with parallel side surfaces of the active layer  36 ″ and the inclined upper end of the first semiconductor layer  31 ″. However, the second semiconductor layer  32 ″ may be not formed on the lower end of the first semiconductor layer  31 ″. 
     The electrode layer  37 ″ may be disposed to surround an outer surface of the second semiconductor layer  32 ″. For example, a shape of the electrode layer  37 ″ may be substantially the same as that of the second semiconductor layer  32 ″. For example, the electrode layer  37 ″ may be in entirely contact with the outer surface of the second semiconductor layer  32 ″. 
     The insulating film  38 ″ may be disposed to surround outer surfaces of the electrode layer  37 ″ and the first semiconductor layer  31 ″. The insulating film  38 ″ may be in direct contact with the lower end of the first semiconductor layer  31 ″ and exposed lower ends of the active layer  36 ″ and the second semiconductor layer  32 ″ as well as the electrode layer  37 ″. 
     The display device  10  in which the number of light emitting elements  30  disposed per pixel PX and sub-pixel PXn is uniform may be manufactured using the inkjet printing device  1000  according to an embodiment. In the display device  10 , a deviation in the number of light emitting elements  30  per pixel PX and sub-pixel PXn may be minimized, and light emission reliability for each pixel PX may be improved. 
     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 disclosure. Therefore, the disclosed embodiments of the disclosure are used in a generic and descriptive sense only and not for purposes of limitation.