Patent Publication Number: US-2017361354-A1

Title: Organic vapor phase deposition system and methods of use for simultaneous deposition of low and high evaporation temperature materials, and devices produced therein

Description:
This application claims the benefit of priority of U.S. Provisional Patent Application No. 62/090,665, filed Dec. 11, 2014, which is incorporated herein by reference in its entirety. 
    
    
     The present disclosure is directed to organic vapor phase deposition system and methods of use for simultaneous deposition of low and high evaporation temperature materials, and devices produced with said systems. 
     Organic vapor phase deposition (OVPD) has been developed in organic thin-film growth as an analogous process to chemical vapor deposition. For reference, successful demonstrations of OVPD as an alternative technique to deposit organic films are described in “Organic Vapor Phase Deposition: A New Method for the Growth of Organic Thin Films with Large Optical Non-Linearities” by Burrows et al, and in “Organic Vapor Phase Deposition” by Baldo et al ([1] J. L. F. P. E. Burrows, S. R. Forrest, L. S. Sapochak, J. Schwartz, P. Fenter, T. Buma, V. S. Ban, J. Cryst. Growth 1995, 156, 91.; [2] M. Baldo, M. Deutsch, P. Burrows, H. Gossenberger, M. Gerstenberg, V. Ban, S. Forrest.  Adv. Mater.  1998, 10, 1505.) OVPD utilizes a stream of hot inert carrier gas to transport organic source vapor towards cooled substrate, and the material transport mechanism in OVPD systems was rigorously revealed by simulation and experimental data in the following references: [2] M. Baldo, M. Deutsch, P. Burrows, H. Gossenberger. M. Gerstenberg, V. Ban, S. Forrest.  Adv. Mater.  1998, 10, 1505. [3] M. Shtein, H. F. Gossenberger, J. B. Benziger. S. R. Forrest,  J. Appl. Phys.  2001, 89, 1470. [4] M. Shtein, P. Peumans, J. B. Benziger, S. R. Forrest,  J. Appl. Phys.  2003, 93, 4005. 
     The above references determined that a stagnant boundary layer formed near cooled substrate in OVPD, due to a relatively high growth pressure from the background carrier gas, which is where the molecular interaction of organic material occurs. 
     An initial step of an OVPD sequence is injection of organic vapor into the main reactor of an OVPD system that is heated to a temperature that is at least as hot as the evaporation temperature of all the organic materials used in the system. Additional descriptions of OVPD systems are described in the above cited references. The main reactor should maintain a higher temperature than evaporation temperature of all the organic materials used in the system in order to eliminate any possible condensation on the wall of the main reactor. This temperature requirement of main reactor prevents cross-contamination of organic materials during subsequent OVPD sequences, which in turn makes the maintenance of the OVPD system easier compared to vacuum thermal evaporation (VTE), which is an alternative technique. 
     However, this temperature restriction brings about limited material choices for situations where it would be desirable for more than one material to be evaporated simultaneously in the OVPD system. Low evaporation-temperature materials (LTM) experience thermal degradation or decomposition when LTM travel through a reactor with an elevated temperature required by high evaporation-temperature material (HTM). There is no such issue in VTE because each source material can be heated separately and VTE proceeds in a high vacuum (˜10-7 torr). For these reasons, OVPD has been used mostly in laboratory-scale even though it shows outstanding advantages over VTE in terms of device performance, scalability, and material utilization efficiency, through superior control over the morphology of deposited organic films, as described by B. Song er al. in Adv. Matter (2014, 26, 2914), R. R. Lunt et al. in Appl. Phys. Lett. (2009, 95, 233305), and by M. Schwambera et al. in SID Symp. Dig. Tech. Pap. (2003, 34, 1419). 
     In one aspect, the present disclosure may be directed to an organic vapor phase deposition system. The system may include a main reactor defined by a main reactor wall and at least two source barrels configured to introduce at least two organic vapors into the main reactor. The system may also include a substrate stage positioned in the main reactor and at least one carrier gas injection line configured to distribute a carrier gas along the main reactor wall, which reduces condensation of the organic vapors onto the main reactor wall as the organic vapors flow toward the substrate stage. 
     In some embodiments, the temperature of the at least two source barrels may be independently controlled. In some embodiments, the temperature of the at least two source barrels may be controlled by heating coils surrounding at least a portion of the source barrels. In some embodiments, the at least one carrier gas injection line is ring shaped and extends concentric to the main reactor wall. In some embodiments, the at least one carrier gas injection line may discharge a carrier gas, which includes at least one of Argon gas and Nitrogen gas. In some embodiments, one of the source barrels may be configured to be controlled to a temperature of about 200° C. and another one of the source barrels may be configured to be controlled to a temperature of about 500° C. while the main reactor may be configured to be controlled to a temperature of about 300° C. In some embodiments, the organic vapors may be introduced into the main reactor at about 20 sccm while the carrier gas is distributed into the main reactor at about 20 sccm. In some embodiments, the at least one carrier gas injection line may begin distributing the carrier gas along the main reactor wall above where the two organic vapors are introduced into the main reactor. In some embodiments, the system may be configured in a vertical orientation where the organic vapors are introduced into the main reactor above the substrate stage. 
     In another aspect, the present disclosure may be directed to an organic vapor phase deposition system that includes a main reactor and a first source barrel configured to introduce a first organic vapor into a first end of the main reactor. The system may also include a substrate stage positioned at a second end of the main reactor and a second source barrel configured to distribute a second organic vapor, which is at a lower temperature than the second organic vapor, into the main reactor through a second source barrel outlet. The second source barrel outlet may be positioned between the first end of the main reactor and the substrate stage a distance from the substrate, wherein the distance may be selected to minimize the exposure of the first organic vapor to the higher temperature of the second organic vapor as the first organic vapor travels toward the substrate stage. 
     In some embodiments, the at least one source barrel outlet may be shower-head ring shaped and configured to distribute the second organic vapor toward the substrate. In some embodiments, the system may include an insulating plate positioned on the opposite side of the second source barrel outlet than the substrate stage. In some embodiments, the first organic vapor may be configured to be introduced into the main reactor at a temperature of about 200° C. and the second organic vapor may be configured to be distributed into the main reactor at a temperature of about 500° C. while a substrate on the substrate stage may be at a temperature of about 30° C. 
     In another aspect, the present disclosure is directed to a method of fabricating an organic film using organic vapor phase deposition. The method may include introducing a first organic vapor at a first temperature into a main reactor defined by a perimeter wall and introducing a second organic vapor at a second temperature, which is greater than the first temperature, into the main reactor. The method may also include distributing a carrier gas into the main reactor along an inner surface of the perimeter wall. The carrier gas may be configured to reduce condensation of the second organic vapor on the perimeter wall as the first organic vapor and the second organic vapor flow through the main reactor toward a substrate. 
     In some embodiments, the first temperature may be about 200° C., the second temperature may be about 500° C., and a main reactor temperature may be about 300° C. In some embodiments, the method may also include independently controlling the first organic vapor to the first temperature and the second organic vapor to the second temperature using heating coils surrounding a first source barrel and a second source barrel. In some embodiments, the carrier gas includes at least one of Argon gas and Nitrogen gas. In some embodiments, the first organic vapor and the second organic vapor are each introduced into the main reactor at about 20 sccm while the carrier gas is distributed into the main reactor at about 20 sccm. In some embodiments, distributing the carrier gas begins above where the first organic vapor and the second organic vapor are introduced into the main reactor. In some embodiments, the carrier gas is distributed from a ring that extends around the main reactor. 
     In another aspect, the present disclosure is directed to a method of fabricating an organic film using organic vapor phase deposition. The method may include introducing a first organic vapor at a first temperature into a first end of a main reactor and directing the first organic vapor toward a substrate positioned at a second end of the main reactor and introducing a second organic vapor at a second temperature, which is greater than the first temperature, into the main reactor toward the substrate. The second organic vapor may be introduced into the main reactor between the first end of the main reactor and the substrate a distance from the substrate, wherein the distance is selected to minimize the exposure of the first organic vapor to the higher temperature of the second organic vapor as the first organic vapor travels toward the substrate. 
    
    
     
         FIG. 1A  is a plot of refractive indexes (n) vs. wavelength of DTDCPB and C 70  grown by VTE or OVPD. 
         FIG. 1B  is a plot of extinction coefficients (k) vs. wavelength of DTDCPB and C 70  grown by VTE or OVPD. 
         FIG. 2A  is a plot of current density vs. voltage of dark J-V characteristics of DTDCPB:C 70  mixed heterojunction device whose active layers were grown by either VTE or OVPD. 
         FIG. 2B  is a plot of current density vs. voltage of J-V characteristics of the same devices as  FIG. 2A  under simulated AM 1.5G illumination. 
         FIG. 3A  is a schematic of a horizontal OVPD system. 
         FIG. 3B  is a photograph of a horizontal OVPD system. 
         FIG. 3C  is a schematic of a vertical OVPD system, according to an exemplary embodiment. 
         FIG. 4A  shows a numerical simulation that displays the temperature distribution of the vertical OVPD system of  FIG. 3C . 
         FIG. 4B  shows a numerical simulation that displays the isosurface plot of temperature of the vertical OVPD system of  FIG. 3C . 
         FIG. 4C  shows a numerical simulation that displays the concentration of low evaporation temperature material (LTM) for the vertical OVPD system of  FIG. 3C . 
         FIG. 4D  shows a numerical simulation that displays the concentration of high evaporation temperature material (HTM) for the vertical OVPD system of  FIG. 3C . 
         FIG. 5A , is a schematic of a Gen-6 OVPD system used in large-area device fabrication. 
         FIG. 5B  shows a photograph of the system of  FIG. 5A . 
         FIG. 5C  shows a top view and section view of an OVPD system, according to an exemplary embodiment. 
         FIG. 6A  shows a numerical simulation of the temperature distribution inside the chamber of the OVPD system of  FIG. 5C . 
         FIG. 6C  shows a numerical simulation of the isosurface of temperature inside the chamber of the OVPD system of  FIG. 5C . 
         FIG. 6C  shows a numerical simulation of the concentration of LTM inside the chamber of the OVPD system of  FIG. 5C . 
         FIG. 6D  shows a numerical simulation of the concentration of HTM inside the chamber of the OVPD system of  FIG. 5C . 
     
    
    
     The following acronyms are utilized in the following description: OVPD: Organic vapor phase deposition; HTM: High-temperature evaporation material; LTM: Low-evaporation temperature material; DTDCPB: 2-[(7-{4-[N,N-Bis(4-methylphenyl)amino]phenyl}-2,1,3-benzothiadiazol-4-yl)methylene]propanedinitrile; Bphen: Bathophenanthroline; ITO: Indium Tin Oxide; VTE: Vacuum Thermal Evaporation. 
       FIGS. 1A, 1B, 2A and 2B  show plots related to devices fabricated without the principles of the present disclosure and demonstrate the deleterious effects of high temperature on LTM during fabrication in an OVPD reactor, as compared to equivalent devices fabricated in a VTE reactor. DTDCPB and C 70  were evaporated at 200±2° C., and 480±2° C., respectively, in a furnace configured with three zones, each zone was independently set at 420° C., 480° C., and 540° C.  FIGS. 1A and 1B  show optical constants determined with variable-angle spectroscopy ellipsometry of 20 nm thick organic thin films of each material. 
     The refractive index of  FIG. 1A  and extinction coefficient of  FIG. 1B  of the DTDCTB material evaporated in OVPD differ by a maximum of 6±2% compared to the same material evaporated in VTE. In contrast, C 70 , which is a HTM, shows identical optical properties for both growth techniques. 
     A mixed heterojunction device with DTDCPB:C 70  was fabricated with the following structure: ITO/MoO3 (10 nm)/1:1 (by vol.) DTDCTB:C 70  (80 nm)/1:1 (by vol.) Bphen:C60 (8 nm)/Bphen (5 nm)/Ag (100 nm). DTDCTB:C 70  active layers were grown by VTE and OVPD. DTDCPB:C 70  were co-evaporated at 200±2° C., and 480±2° C., respectively to reach deposition rates of 0.5 Å/s, which provides a 1:1 volume ratio. 10 sccm (standard cubic centimeters per minute) N 2  flow was used in each source barrel along with 6 sccm dilution flow, leading to 0.28 torr of chamber pressure using the system configuration described in the above reference by Baldo et al. The substrate was water-cooled to TTss=25° C. 
     An embodiment of the present disclosure includes an organic photovoltaic device comprising an organic film, wherein the organic film comprises low evaporation temperature material and high evaporation temperature material and the organic film is deposited by an organic vapor phase deposition system configured to separate LTM from high temperatures in the main reactor and separate HTM from low temperature surfaces aside from the substrate. The organic photovoltaic device may comprise the components and layers as described above. Other embodiments of the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the present disclosure herein. 
       FIGS. 2A and 2B  contrasts the dark and light J-V characteristics of the samples in  FIGS. 1A and 1B .  FIG. 2A  shows the dark J-V characteristics of DTDCPB:C 70  samples prepared by OVPD and VTE. Series resistance Rs, determined from  FIG. 2A , were 0.36±0.05, 1.29±0.09 Ω·cm 2  for VTE, and OVPD-grown devices, respectively. 
     The low conductivity (and high resistance) of the DTDCPB:C 70  sample grown with OVPD is speculated to be from the thermal degradation of the DTDCTB molecule after the molecule evaporated inside the hot reactor in OVPD. Measured device efficiency of the OVPD-grown sample under 1 sun illumination is 4.7±0.2%, which is significantly lower than the equivalent VTE-grown device, 8.0±0.2%. 
       FIGS. 3A and 3B  show a schematic and photograph of a horizontal OVPD system  300 . OVPD system  300  includes a quartz tube for the main reactor vessel  312 , source barrels  306  (e.g., a first source barrel and a second source barrel), a substrate or substrate stage  310 , a shutter  302 , and a mechanical pump  308 . OVPD system  300  is configured such that the main reactor  312  may be surrounded by a furnace  304  with three heating zones, each source barrel  306  may have an inert carrier gas injection line configured to control the flow of carrier gas through each source barrel  306 , and the substrate stage  310  may be attached to a water line to adjust the substrate temperature. The OVPD system  300  is configured such that the temperatures of each source barrel  306  may be controlled by adjusting the position of an organic source inside the furnace  304 . Based on this configuration, the temperature of furnace  304  should be at least as high as the evaporation temperature of the HTM to ensure that there is no condensation on the wall of the main reactor  312  tube. In such a configuration, the LTM vapors are exposed to the high temperature zone required by HTM, causing the degradation of the LTM. 
       FIG. 3C  shows a vertical OVPD system  400 , according to an exemplary embodiment. OVPD system  400  may include a main reactor  412  defined by a main reactor wall, at least two source barrels  406  configured to introduce at least two organic vapors into the main reactor  42 . The OVPD system  400  may also include a substrate  410  stage positioned in the main reactor  412  and at least one carrier gas injection line  414  (also referred to as “heavy gas line”) configured to distribute a carrier gas along the main reactor wall. The carrier gas may be configured to reduce condensation of the organic vapors onto the main reactor wall as the organic vapors flow toward the substrate stage  410  by directing all the flow paths of the organic vapors toward the substrate  410 . By reducing the likelihood of condensation on the main reactor, furnace  404  of OVPD system  400  may operate at a reduced temperature. For example, furnace  400  may operate at a temperature below the evaporation temperature of the high evaporation temperature vapor. OVPD system may also include heating coils surrounding at least a portion of the source barrels enabling separate independent temperature control over each source barrel  406 . 
       FIG. 3C  shows that the at least one carrier gas injection line  414  may be ring shaped and may extend concentrically around the wall of the main reactor  412  such that the additional carrier gas may flow along the wall of the main reactor thereby directing the organic vapors down the center region of the main reactor way from the wall of the main reactor  412 . In some embodiments Ar gas may be preferred to N 2  gas due to the heavier mass of Ar gas. As described herein, distributing the carrier gas from carrier gas injection line  414  may keep the organic vapors from the source barrels  406  from hitting the wall of the main reactor  412 , thereby prohibiting condensation from forming on the wall even at a relatively low temperature in the main reactor  412  when formation of condensation is usually a concern. 
       FIGS. 4A and 4B  show numerical simulations of operation for OVPD system  400  shown in  FIG. 3C . For the numerical simulation, the temperature of the LTM, and HTM source barrels were set at 200° C. (473 K), and 500° C. (773 K) respectively, with the main reactor temperature set at 300° C. (573 K). The flow rate for N 2  gas was set for 20 sccm for each source barrel  406 , and the additional background carrier gas injection line  414  flow rate was set to 20 sccm of Ar gas and was configured to flow around the perimeter of the main reactor  412 . A steady-state model for a compressible Newtonian fluid was used to solve the gas-fluid dynamics by the finite-difference time-domain (FDTD) method. In some embodiments the temperature of the LTM source barrel could be set in the range of 150° C. to 250° C. In some embodiments the temperature of the LTM source barrel could be set in the range of 100° C. to 300° C. In some embodiments the temperature of the HTM source barrel could be set in the range of 450° C. to 550° C. In some embodiments the temperature of the HTM source barrel could be set in the range of 400° C. to 600° C. In some embodiments the temperature of the main reactor could be set in the range of 250° C. to 350° C. In some embodiments the temperature of the main reactor could be set in the range of 200° C. to 400° C. In some embodiments the flow rate for the carrier gas could be set in the range of 15 sccm to 25 sccm for at least one source barrel  406 . In some embodiments the flow rate for the carrier gas could be set in the range of 10 sccm to 30 sccm for at least one source barrel  406 . 
     The numerical simulation of  FIG. 4A  shows the temperature distribution of OVPD system  400  operating at the above parameters. The numerical simulation of  FIG. 4B  shows the isosurface plot of temperature of OVPD system  400  operating at the above parameters. Arrows in the space indicate the total heat flux. As shown in  FIG. 4B , the total heat flux is directed inward towards the cooled-substrate. As shown in  FIGS. 4C, and 4D , the concentration of each material is the highest at the inlet to the main reactor  412  from the source barrels  406 , and is diluted as each material is transported through the N 2  carrier gas towards the substrate. The calculated ratio of total amount of each material deposited on the substrate is about 1.18, indicating a homogeneous mixture of both materials may be achieved by this exemplary design. 
       FIGS. 5A and 5B  show a Gen-6 OVPD system  500  that may be used in large-area device fabrication. The Gen-6 (1.5 m×1.8 m) OVPD system was demonstrated previously for large area device fabrication, and is described in  Organic Vapor Phase Deposition for the Growth of Large Area Organic Electronic Devices  by Lunt et al. (R. R. Lunt, B. E. Lassiter, J. B. Benziger, S. R. Forrest. “Organic Vapor Phase Deposition for the Growth of Large Area Organic Electronic Devices” Appl. Phys. Lett., 95, 233305, 2009). The Gen-6 OVPD system is configured to have automated source position inside a stainless-steel main reactor  512 , and the substrate stage  510  has an elevator to adjust the height of the substrate depending on the deposition sequence. As shown, the Gen-OVPD system  500  may also include source cells  506 , heated chamber  504 , and shutter  502 . 
       FIG. 5C  shows an OVPD system  600 , according to an exemplary embodiment, which may include a modified main reactor chamber. The OVPD system  600  shown in  FIG. 5C  may be similar to the Gen-6 OVPD system  500  shown in  FIGS. 5A and 5B  except OVPD system  600  may be modified at the area within the dotted circle shown in  FIG. 5A . The modification to OVPD system  600  may include a second source barrel  606  configured to distribute a second organic vapor, which is at a lower temperature than the second organic vapor, into the main reactor  512  through a second source barrel outlet  607 , wherein the second source barrel outlet  607  is positioned between the first end of the main reactor  512  and the substrate stage  510  a distance from the substrate, wherein the distance is selected to minimize the exposure of the first organic vapor to the higher temperature of the second organic vapor as the first organic vapor travels toward the substrate stage  510 . The source barrel outlet  607  may be shower-head ring shaped and configured to distribute the second organic vapor toward the substrate  510 . System  600  may also include an insulating plate  604  positioned on the opposite side of the second source barrel outlet  607  than the substrate stage  510 , in order to reduce the amount of first organic vapor (e.g., LTM vapor) colliding with the hot shower-head ring shaped outlet  607 . The hot shower-head ring shaped outlet  607  may be configured with multiple holes around its perimeter to blow HTM vapor on the substrate  510 . 
       FIGS. 6A   6 B,  6 C, and  6 D show numerical simulations of OVPD system  600  shown in  FIG. 5C . For the numerical simulation, the temperature of the HTM source barrel, which is configured like a shower-head, is set to 500° C. (773 K). In some embodiments the temperature of the HTM source barrel could be set in the range of 450° C. to 550° C. In some embodiments the temperature of the HTM source barrel could be set in the range of 400° C. to 600° C. For the numerical simulation, the temperature of the main tube was set at 200° C. (473K), which is appropriate for LTM flux from the source barrels  506 , and the substrate  510  temperature was set at 30° C. (303 K). In some embodiments the temperature of the main reactor could be set in the range of 150° C. to 250° C. In some embodiments the temperature of the main reactor could be set in the range of 100° C. to 300° C. In some embodiments the temperature of the LTM source barrel could be set in the range of 150° C. to 250° C. In some embodiments the temperature of the LTM source barrel could be set in the range of 100° C. to 300° C. In some embodiments the temperature of the substrate could be set in the range of 25° C. to 35° C. In some embodiments the temperature of the substrate could be set in the range of 20° C. to 30° C. For the numerical simulation, the flow rate of N 2  from the shower-head  607  was set to 20 sccm. In some embodiments the flow rate for the carrier gas could be set in the range of 15 sccm to 25 sccm for at least one source barrel. In some embodiments the flow rate for the carrier gas could be set in the range of 10 sccm to 30 sccm for at least one source barrel. For the numerical simulation, the total N 2  gas flow from the source cells (including dilution line in the chamber) was 80 sccm. In some embodiments the total N 2  gas flow from the source cells could be set in the range of 70 sccm to 90 sccm. In some embodiments the total N 2  gas flow from the source cells could be set in the range of 60 sccm to 100 sccm. In some embodiments the total N 2  gas flow from the source cells could not include a dilution line in the chamber and the total N 2  gas flow could be set in a range of 20 sccm to 60 sccm. 
     As shown in  FIG. 6B , a large temperature difference exists between the hot shower-head  607  and the cooled-substrate  510 , which induces a strong heat flux from the shower-head to the substrate. The LTM concentration plot in  FIG. 6C  shows the LTM flux from the top of the main reactor  512  approaches the entrance of the shower-head  607 , condensing on the substrate  510 . Similarly, injected HTM flux from the shower-head  607  is diluted as the HTM approaches the substrate  510 , as shown in  FIG. 6D . 
     An embodiment of the present disclosure may also include a method of utilizing OVPD system  600  to fabricate an organic film. The method may include introducing a first organic vapor at a first temperature into a first end of a main reactor and directing the first organic vapor toward a substrate positioned at a second end of the main reactor. The method may also include introducing a second organic vapor at a second temperature, which may be greater than the first temperature, into the main reactor toward the substrate, wherein the second organic vapor may be introduced into the main reactor between the first end of the main reactor and the substrate a distance from the substrate. The distance may be selected to minimize the exposure of the first organic vapor to the higher temperature of the second organic vapor as the first organic vapor travels toward the substrate. 
     Other embodiments of the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the present disclosure herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the present disclosure being indicated by the following claims.