OF FORMING A THIN-FILM COATING

BACKGROUND

[0001] Organic solar cells including small molecule photovoltaics and bulk heteroj unction solar cells, for example, utilize thin-film active layers with an electron donor and acceptor blend. For example, bulk heteroj unction solar cells may include a blend of donor and acceptor materials that are casted and then allowed to phase separate and self- assemble into an interpenetrating network between two electrodes. While scalable processing techniques generally suffice with respect to forming thin-film active layers on par with laboratory-based processing techniques (such as spin coating) for solar cell applications, the success of those techniques is generally limited to polymer-based solar cells. In context of small molecule organic photovoltaics, those techniques suffer from various shortcomings, particularly with respect to scalability, among other things. For example, spin coating is a conventional laboratory-based technique with losses of greater than 95% of material during coating. Further, spin coating is not compatible with continuous manufacturing processes (e.g., roll-to-roll manufacturing). In addition, drying profiles cause non-uniformity on large area substrates. On the other hand, conventional scalable manufacturing techniques (e.g., blade coating, wire-bar coating, slot die coating, etc.) fail to provide adequate control over the microstructure and morphology of the thin-film active layers necessary for solar cell applications. In particular, conventional techniques cannot provide rapid solvent evaporation at low temperatures, leading to pinholes, cracks, and other defects, as well as over- crystallization and non-uniform distribution.

[0002] It therefore would be desirable to provide a scalable manufacturing technique that provides sufficient control over the morphology, microstructure, and performance of the thin film coating, but without any significant material losses.

SUMMARY

[0003] In general, embodiments of the present disclosure describe a thin-film coating apparatus and methods of thin-film coating.

[0004] Accordingly, embodiments of the present disclosure describe an apparatus for thin film coating comprising a mount configured to secure a substrate, a coating device proximate to the mount and configured to deposit a solution on the substrate, and a dryer component fluidly coupled to the mount and configured to spin a gas proximate to deposited solution to dry the deposited solution and form a thin film coating.

[0005] Embodiments of the present disclosure also describe an apparatus for thin film coating comprising a coating device configured to deposit a solution on a substrate; and a dryer component configured to spin gas proximate to the deposited solution to dry the deposited solution and form a thin film coating.

[0006] Embodiments of the present disclosure further describe a method of forming a thin-film coating comprising depositing a solution on a substrate and spinning a gas proximate to the deposited solution to dry the deposited solution and form a thin film coating.

[0007] The details of one or more examples are set forth in the description below. Other features, objects, and advantages will be apparent from the description and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0008] This written disclosure describes illustrative embodiments that are non-limiting and non-exhaustive. In the drawings, which are not necessarily drawn to scale, like numerals describe substantially similar components throughout the several views. Like numerals having different letter suffixes represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.

[0009] Reference is made to illustrative embodiments that are depicted in the figures, in which:

[0010] FIG. 1 is a schematic diagram of a thin-film coating apparatus, according to one or more embodiments of the present disclosure.

[0011] FIG. 2 is a flowchart of a method of forming a thin-film coating, according to one or more embodiments of the present disclosure.

[0012] FIGS. 3a-3c are microscopy images including a polarized optical microscopy (POM) image of p-DTS(FBTTh ₂ )2:PC7iBM films coated by wire-bar coating from CB with 21% w/w (DIO/solutes) (3a); a POM image of a region near an edge of the film in (a) showing coffee -ring effect (3b); and a transmission electron microscopy (TEM) image of p- DTS(FBTTh ₂ ) ₂ :PC7iBM films coated from CB with 21% w/w (DIO/solutes) (3c), according to one or more embodiments of the present disclosure.

[0013] FIG. 4a is a 2D GIWAXS image of p-DTS(FBTTh ₂ ) ₂ :PC7iBM wire-bar coated film at room temperature, according to one or more embodiments of the present disclosure. [0014] FIG. 4b is a graphical view of line cuts of integrated scattering intensity versus q (nm ^_1 ) of wire-bar coated samples processed with identical DIO content (21% 2/2 DIO/solutes) at room temperature (black) and at 45 °C (red), with a spin-coated reference sample (blue) (inset highlights scattering from p-DTS(FBTTh2)2 alkyl-stacking peak (001)), according to one or more embodiments of the present disclosure.

[0015] FIG. 4c is a 2D GIWAXS color plot showing a logarithmic plot of integrated scattering intensity as a function of time (s) measured in situ during wire-bar coating of p- DTS(FBTTh ₂ )2:PC ₇ iBM from CB with 21% w/w (DIO/solutes) at room temperature, according to one or more embodiments of the present disclosure.

[0016] FIG. 4d is a graphical view of an integrated scattering intensity of solvent scattering (black) and p-DTS(FBTTh2)2 alkyl-staking scattering (Xooi) (red) calculated from in situ GIWAXS in (c) (thickness of films was 120 +/- 10 nm), according to one or more embodiments of the present disclosure.

[0017] FIG. 4e is a 2D GIWAXS color plot showing a logarithmic plot of integrated scattering intensity as a function of time (s) measured in situ during wire-bar coating of p- DTS(FBTTh2) ₂ :PC ₇ iBM from CB with 21% w/w (DIO/solutes) at 45°C, according to one or more embodiments of the present disclosure.

[0018] FIG. 4f is a graphical view of an integrated scattering intensity of solvent scattering (black) and p-DTS(FBTTh2)2 alkyl-staking scattering (Xooi) (red) calculated from in situ GIWAXS in (e) (thickness of films was 120 +/- 10 nm), according to one or more embodiments of the present disclosure.

[0019] FIG. 5a is a 2D GIWAXS image of p-DTS(FBTTh ₂ )2:PC ₇ iBM film wire-bar coated from pure CB at room temperature, according to one or more embodiments of the present disclosure.

[0020] FIG. 5b is a 2D GIWAXS color plot showing a logarithmic plot of integrated scattering intensity as a function of time(s) measured in situ during wire-bar coating of p- DTS(FBTTh2)2:PC7iBM from pure CB at room temperature, according to one or more embodiments of the present disclosure.

[0021] FIGS. 6a-6d are schematic diagrams of stages of a spin-coating processing including deposition (6a), spin-up (6b), spin-off (6c), and evaporation (6d), according to one or more embodiments of the present disclosure.

[0022] FIGS. 7a-7d are POM images of p-DTS(FBTTh ₂ )2:PC ₇ iBM films coated by: spin coting (total solutes concentration = 35 mg/mL) (7a), conventional wire-bar coating (total solutes concentration = 35 mg/mL) (7b), modified wire-bar coating (total solutes concentration = 35 mg/mL) (7c), and modified wire-bar coating (total solutes concentration = 12.5 mg/mL) (7d), according to one or more embodiments of the present disclosure. All samples were coated from CB with 21% w/w (DIO/solutes). The thickness of the films in 7a, 7b, 7c, and 7d is 100+/-5 nm, 290 +/- 10 nm, 275 +/- 10 nm, and 115+/- 5 nm, respectively.

[0023] FIGS. 8a-8c are 2D GIWAXS images of p-DTS(FBTTh ₂ ) ₂ :PC7iBM films coated by spin coating (8a) and modified wire-bar coating (8b), with a graphical view of line cuts of integrated scattering intensity versus q (nm ^_1 ) of p-DTS(FBTTh2)2:PC7iBM samples coated by modified wire -bar coating (black curve) and spin coated (red curve) (8c), according to one or more embodiments of the present disclosure. Both samples were processed with identical DIO content; 21% w/w (DIO/solutes).

[0024] FIG. 9 is a current density-voltage (J-V) curve of solar cell devices fabricated from p-DTS(FBTTh2)2:PC7iBM blends coated by spin coating with total solute concentration 35 mg/mL (black) and new wire-bar coating at different concentrations, including 35 mg/mL (red), 25 mg/mL (blue), 17 mg/mL (pink),and 12.5 mg/mL (green), according to one or more embodiments of the present disclosure. All sample were coated in air from CB with 21% w/w (DIO/solutes). Notably, fabricated solar cell devices in glove-box by spin coating observed higher performance (PCE > 7%).

DETAILED DESCRIPTION

[0025] Embodiments of the present disclosure describe a thin-film coating apparatus and methods of forming thin-film coatings. The thin-film coating apparatus and methods can achieve unprecedented control over the kinetics of drying and uniformity of drying, without substrate rotation. In addition, the thin-film coating apparatus and methods overcome the challenges of over-crystallization and non-uniform drying to produce high quality, uniform thin films suitable for small molecule solar cells (e.g., organic solar cells), among other applications. It can accelerate drying to achieve rapid solvent evaporation at low temperatures sufficient to prevent significant crystallization and/or over-crystallization in a solvent- saturated environment and/or prevent increased diffusion favoring crystallization. It also achieves uniformity of drying sufficient to prevent and/or minimize the formation of defects such as pinholes, cracks, and coffee-ring effect. The thin-film apparatus and methods achieve these and other advantages by spinning a gas proximate to coating material, as opposed to spinning the coating material, to dry it. In the absence of substrate rotation, the invention avoids the loss of significant amounts of coating material as waste during the coating process. Furthermore, the thin-film apparatus is production compatible and suitable for scalable manufacturing as it is capable of high throughput and high-speed thin-film coating. In this way, the invention of the present disclosure minimizes defects and improves thin film quality, operational efficiency, and scalability over convention methods and devices to produce uniform and/or substantially uniform thin film coatings.

[0026] The terms recited below have been defined as described below. All other terms and phrases in this disclosure shall be construed according to their ordinary meaning as understood by one of skill in the art.

[0027] As used herein, "depositing" refers to etching, doping, epitaxy, thermal oxidation, sputtering, casting, depositing, spin-coating, evaporating, applying, treating, and any other technique and/or method known in the art.

[0028] As used herein, "spinning" refers to rotating, circulating, agitating, flowing, and streaming.

[0029] FIG. 1 is a schematic diagram of a thin-film coating apparatus, according to one or more embodiments of the present disclosure. As shown in FIG. 1, the thin-film coating apparatus may include a mount 101 configured to secure a substrate 102. A coating device 103 (e.g., a wire-bar coater is shown) may be provided proximate to the mount 101 and configured to deposit a solution 104 on the substrate. A dryer component 105 may be fluidly coupled to the mount and configured to spin a gas proximate to the deposited solution to dry the deposited solution and form a thin film coating. Although the apparatus shown in FIG. 1 includes a mount 101 combined with a temperature controller 106, the mount 101 and temperature controller 106 are each independently optional. Accordingly, in other embodiments, the apparatus for thin film coating comprises a coating device 103 configured to deposit a solution 104 on a substrate 102 and a dryer component 105 configured to spin gas proximate to the deposited solution to dry the deposited solution and form a thin film coating.

[0030] In general, any substrate 102 known in the art may be used. In an embodiment, the mount 101 may be utilized to secure the substrate 102. As shown in FIG. 1, the mount 101 may secure the substrate 102 by supporting only a single surface of the substrate (e.g., the bottom surface) without any lateral or horizontal support. In other embodiments (not shown), the mount 101 may secure the substrate 102 by constraining the substrate 102 and/or preventing the substrate 102 from moving in any lateral direction (e.g., pressure is applied to at least two opposing side surfaces of the substrate to constrain the substrate from moving in a lateral direction). These examples shall not be limiting as the mount may utilize any method and/or apparatus known in the art capable of securing a substrate. As described above, a mount is optional.

[0031] The coating device 103 may include any coating device suitable to deposit the solution on the substrate. In embodiments including the optional mount 101, the coating device may be proximate to and/or include the mount. In many embodiments, the coating device includes scalable and/or high speed coating devices. For example, the coating device may include, but is not limited to, one or more of a bar-coater, slot-die-coater, blade-coater, knife-coater, roll-coater, wire-bar coater, dip-coater, and spray-coater.

[0032] The coating devices 103 may be low-speed coating devices, high-speed coating devices, or combinations thereof. In general, the coating speed of a low-speed coating device ranges from about 5 mm/s to about 10 mm/s; and the coating speed of a high-speed coating device is about 100 mm/s and/or greater than about 100 mm/s. The coating speed of the coating device 103 may range from about 5 mm/s to about 100 mm/s. In many embodiments, the coating speed of the coating device is about 100 mm/s or is greater than about 100 mm/s. In other embodiments, the coating speed of the coating device is less than 100 mm/s. For example, the coating speed of the coating device may range from about 5 mm/s to about 10 mm/s. These ranges shall not be limiting as any other range is possible depending on the coating device used.

[0033] The solution to be deposited may include any solution known in the art. In many embodiments, the solution to be deposited is a small molecule material. The small molecule material may be any material suitable for organic photovoltaic applications, including, but not limited to solar cells. The small molecule material may include, for example, small molecule OPV formulations. In many embodiments, the small molecule material includes p- DTS(FBTTh2):PC7iBM and one or more of a bulk solvent and an additive solvent. The bulk solvent may include chlorobenzene (CB). The additive solvent may include 1,8-diiodooctane (DIO). In many embodiments, concentration of the additive solvent is about 21% w/w (DIO/solutes). In some embodiments, the small molecule material may be characterized as a material other than a polymer or polymer-based material.

[0034] The solution (e.g., small molecule material) may be deposited as a layer. A thickness of the deposited solution may range from about 50 to about 300 nm. For example, the thickness may range from about 100 nm to about 120 nm or about 100 nm to about 105 nm. In some embodiments, the thickness of the solution is at least about 50 nm, at least about 100 nm, at least about 150 nm, at least about 200 nm, at least about 250 nm, or at least about 300 nm. [0035] The dryer component 105 may include any component suitable for providing the uniformity, morphology, and microstructure necessary for high performance solar cells (e.g., solar cells fabricated from p-DTS(FBTTh2)2:PC7iBM blends). The dryer component 105 may be fluidly coupled to the optional mount, if present, and/or configured to spin a gas proximate to (e.g., above) the layer of deposited solution to dry the deposited solution and form the thin film coating. The dryer component may form a vortex of gas proximate to and/or above the deposited solution in a scalable process that emulates the drying kinetics of spin coating. The dryer component may be configured to dry the deposited solution at a substantially uniform rate without substrate rotation to achieve effective and efficient usage of the deposited solution (e.g., without significant losses of solution and/or deposited solution), while aiding in film uniformity. The dryer component may further accelerate the drying and/or evaporation rates of the solvent from the deposited solution at low enough temperatures (e.g., room temperature) sufficient to prevent significant crystallization and/or overcrystallization. The dryer component may also provide a substantially uniform rate of drying/evaporation to prevent the formation of concentration gradients in the deposited material, as well as the formation of defects, such as pinholes, cracks, coffee-ring effect, etc.

[0036] In many embodiments, the dryer component may include a supply conduit for supplying a gas or a vacuum (e.g., to spin gas proximate to the deposited solution and/or to dry the deposited solution and/or layer of deposited solution), a rotating head fluidly coupled to the supply conduit, and a motor component coupled to the rotating head and configured to rotate the rotating head. In many embodiments, the dryer component may be adjusted to modify an air-substrate distance (e.g., to facilitate drying, uniformity of drying, etc.). In some embodiments, the gas is supplied through the supply conduit and spun via the rotating head as it exits an outlet of the rotating head. In other embodiments, a vacuum is applied through the supply conduit and rotating head to spin gas proximate to the deposited solution. The spinning gas may create a vortex of gas. In some embodiments, the motor component permits adjustment of the spinning rate (e.g., an adjustable rate) in revolutions per minute, for example. In many embodiments, the outlet of the rotating head may include a slit. The gas may include air and/or any inert gas, such as nitrogen, including mixtures thereof. In some embodiments, the inert gas may include nitrogen.

[0037] In many embodiments, the thin-film apparatus is used at temperatures sufficient (e.g., sufficiently low temperatures) to prevent significant crystallization while drying. A suitable temperature includes, but is not limited to, about room temperature. Accordingly, although shown in FIG. 1, the temperature controller 106 is optional and not required. However, in some embodiments, as shown in FIG. 1, the thin-film apparatus may include a temperature controller. In these embodiments, the temperature controller may be utilized to accelerate the drying kinetics of the coating material (e.g., drying rate and/or evaporation rate of the deposited solution) by raising the coating temperature to between about 25 °C to about 90°C. However, use of the temperature controller to accelerate drying must be carefully controlled because it may result in over-crystallization and non-uniform drying. Drying the deposited solution at an elevated temperature may increase diffusion of molecules and result in over-crystallization of the thin film coating. In addition, non-uniform drying may form a solute concentration gradient and lead to the formation of a coffee -ring effect, pinholes and/or cracks in areas with relatively low solute concentration. In addition, the dryer component outperforms use of a temperature controller with respect to accelerated and substantially uniform drying. Consequently, a temperature controller is optional and generally not required.

[0038] The thin-film coating apparatus may reproducibly provide high quality, uniform thin films suitable for, for example, bulk heterojunction coatings for organic photovoltaics applications (e.g., small molecule solar cells). The thin-film coating apparatus minimizes the amount of coating material lost or consumed during coating by eliminating substrate rotation. The coating material loss may be less than about 95%. For example, the coating material loss may be less than about 20%, less than about 15%, less than about 10%, less than about 5%, or less than about 1% (with a lower boundary including no coating material loss). Coating material consumption for thin-films may be about 25 times less than conventional apparatuses and techniques for coating speeds on the order of several meters per minute. In addition, the thin-film coating apparatus may be compatible with roll-to-roll manufacturing and may be used for high-speed coating in scalable manufacturing.

[0039] FIG. 2 is a flowchart of a method of forming a thin-film coating, according to an embodiment of the present disclosure. Any of the embodiments previously discussed may be utilized in connection with this embodiment. The method may comprise depositing 201 a solution on a substrate and spinning 202 a gas proximate to the deposited solution to dry the deposited solution and form a thin film coating.

[0040] At step 201, a solution is deposited on a substrate. Depositing may generally refer to placing, coating, transferring, adding, providing, dipping, rolling, casting, and any other device or process for bringing a solution into contact with a substrate. The solution may be deposited as a layer and/or coating. The solution may include any of the solutions described herein. The deposited solution may be uniform, substantially uniform, and/or non- uniform. Depositing may occur at any of the speeds described herein and any of the temperatures described herein. In many embodiments, a bar-coater may be utilized to deposit the solution on the substrate. In other embodiments, any coating device may be utilized to deposit the solution. For example, the coating device may include one or more of casting, bar-coating, slot-die-coating, blade-coating, knife-coating, roll-coating, wire-bar coating, dip- coating, and spray-coating.

[0041] At step 202, a spinning gas is provided proximate to and/or above the deposited solution to dry the deposited solution and form a thin film coating. The spinning may occur at any of the temperatures described herein. In many embodiments, the spinning occurs at about room temperature. In many embodiments, spinning substantially uniformly dries the deposited solution. For example, the deposited solution may be uniformly dried or substantially uniformly dried to produce a uniform (e.g., substantially uniform) and/or defect- free (e.g., substantially defect-free) thin film coating. To spin gas proximate to the deposited solution, a vacuum and/or gas stream may be supplied and/or applied through the dryer components described herein. For example, spinning may include supplying a gas stream proximate to the deposited substrate via a rotating head of the dryer component. In other embodiments, spinning may include applying a vacuum proximate to the deposited solution via a rotating head of the dryer component. Spinning may form a vortex of gas. Drying of the wet solution is controlled by spinning the surrounding gas (e.g., as in the case of a vacuum), or supplying spinning gas (e.g., as in the case of a gas stream). In this way, the deposited solution may be evenly and/or uniformly dried without substrate rotation, thereby increasing efficiency and minimizing coating material losses. In some embodiments, the control over the spinning gas may be achieved by adjusting the distance between the dryer and/or spinning gas and the substrate.

[0042] The following Examples are intended to illustrate the above invention and should not be construed as to narrow its scope. One skilled in the art will readily recognize that the Examiners suggest many other ways in which the invention could be practiced. It should be understand that numerous variations and modifications may be made while remaining within the scope of the invention.

EXAMPLE 1

Wire-bar coating and blade-coating of p-DTS(FBTTh ₂ )2:PC7iBM films

[0043] The need for high throughput production-compatible processing has attracted attention for decades. Several efforts have attempted to bridge the performance gap between lab-based processing techniques, such as spin coating, and roll-to-roll compatible manufacturing techniques, such as blade -coating, slot-die coating and wire-bar coating. In the context of polymer-based solar cells, there have been reports of achieving performance parity between spin coating and scalable techniques. On the other hand, for small molecule solar cells, the aggregation of small molecules, unlike polymers, is much more sensitive to process kinetics and little progress has been made to bridge the performance gap between spin coated and scalably-coated small molecule organic photovoltaics (OPV). Here, it is shown that scalable solution-coating of small molecule BHJs, using the conventional techniques, led to very significant performance gaps with the optimum device performance obtained by spin coating. The origin of this difference was investigated and it was determined that it was indeed closely linked to drying kinetics. A new coating method was devised which combines scalability with a vortex of air to emulate the drying kinetics of spin coating to demonstrate microstructural and morphological parities which result in performance parity for small molecular organic solar cells.

[0044] Spin coating has been known to produce high quality, uniform thin films reproducibly and to be the backbone of lab-based BHJ coating experiments in the organic photovoltaic community. However, the technique involved the loss of > 95% of the ink during coating, was characterized by a radial drying profile that caused non-uniformity on large area substrates, very challenging temperature control, and was not compatible with continuous roll-to-roll manufacturing. Instead, techniques such as blade-coating, bar-coating and slot-die coating were slated to be used for manufacturing of emerging photovoltaic materials, such as organic semiconductors, as these methods produced uniform films with minimal material wastage and improved control over the process temperature. Despite these benefits, the rapid solvent evaporation at low temperature in spin-coating due to the fast rotating sample cannot be easily recreated in processes such as blade-coating and wire-bar coating, limiting the ability of optimal process parameters to be successfully transferred from spin-coating.

[0045] The effects of drying speed on small-molecule OPV formulations were investigated. The small-molecule OPV formulations were based on p- DTS(FBTTh2)2:PC7iBM without modifying the process temperature. Initial efforts in making working solar cells without recreating similar drying kinetics when transferring the formulation to blade- and wire-bar coating processes from spin-coating were unsuccessful. This led to a new coating apparatus that combined the benefit of an air vortex in terms of accelerated thin film drying with scalable and high speed coating methods, such as blade- coating and wire-bar-coating. In situ investigations performed in these processes demonstrated that process kinetics were crucial to achieving the same small-molecule BHJ formation behavior, microstructure, morphology and ultimately solar cell performance. Slow drying conditions yielded over-crystallization of the donor, whereas accelerated drying helped attain thin film uniformity, microstructure, morphology and performance comparable to spin coated films but with 25 times less material consumption for coating speeds on the order of several meters per minute.

[0046] The fabrication of solar cells devices by the conventional wire-bar coating and blade -coating techniques were first investigated. A stock solution with the optimized spin coated solution formulation was prepared: about 35 mg/mL total solutes of p- DTS(FBTTh ₂ ) ₂ :PC7iBM (3:2) in CB with about 0.4% (v/v) 1.8 diiodooctane (DIO) (which is equivalent to about 21% w/w (DIO/solutes)). The content of the additive DIO (DIO to solute ratio) is a key factor in tuning the extent of crystallization and phase separation. Thus, for additive-processed samples, the DIO to solute ratio was identical to spin coating condition (21% w/w (DIO/solutes)).

[0047] The initial trials to fabricate solar cells were unsuccessful leading to either non- working (shorted) devices or extremely poor performance (PCE < 1 %) with a few exceptions that remained non-reproducible. This observation remained the same across the board for various coating conditions, including coating with or without solvent additives, different coated thicknesses (50 to 300 nm), and varying coating temperatures (25 - 90°C).

[0048] A closer look at the films revealed poor uniformity and the formation of over- crystallized crystals as revealed by polarized optical microscopy (POM) images (Error! Reference source not found.a, 3b). Coffee -ring effect was observed as shown in Error! Reference source not found.b indicating non-uniform drying. Further observations of the nano-scale morphology by transmission electron microscopy (TEM) revealed the formation of wire-like structures that extended to the length of microns (Error! Reference source not found.c). These structures were typically from p-DTS(FBTTh2)2 crystals, confirming over- crystallization in the film. It was not surprising that most of the fabricated devices with conventional wire-bar coating and blade-coating failed due to shorting, which was likely due to the presence of pinholes. To understand the reason behind these observations, in situ investigation of the crystallization during conventional wire-bar coating was carried out.

[0049] The blade -coating and wire-bar coating speeds were varied from low-speed (0.5 mm/s to 10 mm/s) to high-speed (> 100 mm/s) in the initial trials. Afterwards, due to importance of high-speed coating in scalable manufacturing, the coating speed was held constant at 100 mm s for all studied conditions below. EXAMPLE 2

Crystallization dynamics of p-DTS(FBTTh2)2 during wire-bar coating of p- DTS(FBTTh ₂ )2:PC7iBM films

[0050] In order to reveal the reason behind the over-crystallization of p-DTS(FBTTh2)2 in conventional wire-bar coated samples, film formation was investigated by in situ grazing incidence wide angle X-ray scattering (GIWAXS) during wire-bar coating. In Error! Reference source not found., a 2D GIWAXS color plot of integrated scattering intensity (shown as logarithmic color scale) versus q (nm ^_1 ) versus time is shown, measured in situ during wire-bar coating of p-DTS(FBTTh ₂ )2:PC ₇ iBM from CB with 21% w/w (DIO/solutes) at room temperature (RT) (Error! Reference source not found.c) and at 45DC (Error! Reference source not found, e). Line cuts of integrated scattering intensity are shown in Error! Reference source not found.d, 4f. For samples coated at RT, broad scattering from the bulk solvent was observed at a q range from about 11 nm ^"1 to about 18 nm ^"1 that lasted up to about 35 seconds after coating. This revealed significant slow evaporation for wire-bar coating at room temperature as compared to spin coating, which lasted for only about 6 seconds. The formation of p-DTS(FBTTh2)2 crystals in an early stage (after about 10 seconds) revealed by the scattering at q (2.8 nm ¹ ) corresponded to alkyl-stacking of p- DTS(FBTTh2)2. However, the scattering intensity remained low up to about 25 seconds, indicating that most of p-DTS(FBTTh2)2 remained solvated by the bulk solvent at this early stage. This was followed by a sharp increase in p-DTS(FBTTh2)2 crystallization revealed by the sharp increase in the alkyl-stacking scattering intensity (Error! Reference source not found.d) and the appearance of higher order reflections (namely X002 and X003) (Error! Reference source not found.c). After bulk solvent evaporation, saturation of p-DTS(FBTTh2)2 crystallization around 40 seconds (Error! Reference source not found.c, 4d) was observed. Notably, the formation of LC phase was not observed, unlike spin coating in which p- DTS(FBTTh2)2 experienced a vitrified LC phase before crystallization (Error! Reference source not found.c). Furthermore, significant p-DTS(FBTTh2)2 crystallization occurred in an environment saturated with the bulk solvent: at the moment of bulk solvent removal, the scattering intensity of p-DTS(FBTTh2)2 alkyl-stacking was 75% of its value at the final solid- state (Error! Reference source not found.d). Due to slow drying kinetics, p-DTS(FBTTli2)2 must have enough time for nucleation and growth to occur without going through the vitrified state. Moreover, the presence of solvent (CB) in the wet film provided the kinetics for p- DTS(FBTTh2)2 crystallization which also led also over-crystallization of p-DTS(FBTTh2)2 (Error! Reference source not found.b). In Error! Reference source not found.b, line cuts of integrated scattering intensity versus q (nm ^-1 ) of wire-bar coated samples versus spin-coated reference sample are shown. The crystalline correlation length (CCL) and the relative crystallinity for the samples were calculated (Table 1). Wire-bar coated sample at RT exhibited a much higher CCL (about 22.4 nm) than the spin coated sample (CCL about 13.6 nm), indicating larger crystallites for RT wire-bar coated sample. Moreover, the relative crystallinity for RT wire-bar coated sample was more than three time higher than the spin coated samples. Over-crystallization of p-DTS(FBTTli2)2 occurred for spin coating samples when an excessive amount of the additive DIO was used. Over-crystallization of p- DTS(FBTTli2)2 occurs when p-DTS(FBTTh2)2 crystallization occurs in an environment saturated with the solvents (either the bulk solvent or the additive solvent).

[0051] In order to accelerate the drying kinetics, wire-bar coating at an elevated temperature (ca. 45 DC) was performed. Indeed, the drying time of the bulk solvent was significantly reduced to about 6.6 s, which was comparable to the drying time obtained by spin coating. However, this strategy failed to hinder the over-crystallization of p- DTS(FBTTh2)2. LC phase on the route before crystallization, again, was not observed. Similar to wire-bar coating at RT, significant crystallization occurred in the presence of the bulk solvent; crystallization period lasted from about 4.4 seconds to about 8.8 seconds, whereas solvent scattering was observable up to about 6.6 seconds. Wire-bar coating at 45DC resulted in a slight reduction in CCL to about 19 nm and relative crystallinity to about 3.1, as compared to wire-bar coating at RT. However, these values still indicated greater crystallinity for wire-bar coating samples than spin coating samples. The failure of wire-bar coating at 45 DC to hinder p-DTS(FBTTh2)2 over-crystallization was attributed to the increase in the diffusion of p-DTS(FBTTli2)2 molecules at elevated temperatures that was expected to favor the crystal growth - specifically if the growth occurred in an environment saturated with a plasticizer (ca. CB).

Table 1: Summary of the relative crystallinity and crystalline correlation length of p-DTS(FBTTh2)2 , calculated from the alkyl-stacking peak (001)

Spin coating 1 0.46 13.6

Wire-bar coating ; at RT 3.4 +/- 0.2 0.28 22.4

Wire-bar coating ; at 45 DC 3.1 +/- 0.2 0.33 19 [0052] The observed crystallization behavior was mediated by crystal growth in an environment saturated with CB. However, DIO was used in the conditions discussed above. Therefore, it was investigated whether the same behavior was observable in the absence of DIO. Indeed, similar behavior was observed for additive-free wire-bar coated sample (Error! Reference source not found.) confirming that this crystallization behavior was dominated by the presences of the CB during crystal growth.

EXAMPLE 3

Introducing modified wire-bar coating:

Lessons to scalable fabrication of small-molecule solar cells:

[0053] Generally small-molecule BHJ systems were more sensitive to coating conditions than their polymer counterparts. For instance, for additive processed systems, the optimal content of additive was quite sharp. Minute increases in additive content over the optimum content led to extreme deterioration of device performance which was also associated with significant differences in the microstructure. On the other hand, polymer- based solar cells, either polymer-molecule blends or all-polymer blends, can perform quite efficiently for a wide-range of additive content, and usually, much higher additive content is used as compared to all-small-molecule solar cells. The BHJ microstructure in polymer- based solar cells exhibited more resistance to change with additive content. This behaviour was attributed to the high diffusion of small molecules, unlike polymers, which led to high sensitivity to the kinetics of film formation. On the other hand, polymers were much less mobile and, in many systems, polymer long chains created frameworks that template the microstructure.

[0054] To achieve high performance solar cells with scalable manufacturing technique, the challenge of over-crystallization was overcome. This was achieved by preventing the crystallization from happening in solvent-saturated environment, either the main solvent or the additive. For the bulk solvent CB, it was quickly evaporated at a temperature (RT) low enough to prevent significant crystallization. For the additive DIO, its content was maintained similar to the one used in the optimized spin coated condition (21 % w/w DIO/solutes). Another challenge was attaining uniformity of drying sufficiently for avoiding the formation of pinholes/cracks and coffee -ring effect. Non-uniform drying gave rise to a gradient in solvent evaporation rate and consequently gradient in solute concentration. Under these circumstances, capillary forces arose in the wet film leading to random diffusion of the solutes across the wet film. Thus, pinholes or cracks formed where there was lack of the solute supply. Over-crystallization also led to the formation of pinholes or cracks when most of the solutes were consumed in crystal growth leading to run out of the solutes nearby. Therefore, the solution provided uniformity of drying without over-crystallization. This was achieved by designing and implementing a new apparatus for controlling the kinetics of drying as well as achieving sufficient uniformity of drying.

New perspective on thin film coating:

From spin coating towards modified wire-bar coating

[0055] Spin coating is currently the most common lab-based technique used for fabrication of solution processed thin films. However, spin coating is a wasteful process and non-compatible with high throughput fabrication. Spin coating and its drawbacks may be partitioned into four different steps as summarized below (Error! Reference source not found.):

[0056] Deposition: During this stage, solution is deposited onto static or rotating substrates from micro-syringes. Then, the substrate is accelerated to the desired speed while spreading of the solution occurs due to centrifugal force.

[0057] Spin-up: The second stage involves accelerating up the substrate to its final desired rotation speed and is characterized by aggressive fluid expulsion from the substrate due to rotation. During this early stage, inertia of the fluid driven by the accelerating substrate results in twisting motion of the fluid and the formation of spiral vortices leading to significant ejection of the ink out of the substrate.

[0058] Spin-off (also called: stable fluid outflow): This stage starts when the substrate is spinning at a constant rate and eventually the fluid is thin enough to be completely co- rotating with the substrate. This stage is characterized by gradual quite uniform fluid thinning while the fluid thinning behavior is mainly modulated by fluid viscous forces. The fluid continues to flow uniformly outward, while continuous loss of the ink occurs through spinning off the ink from the edges of the substrate. Solvent evaporation occurs also during this step; however the thinning behavior is dominated by viscous flow.

[0059] Evaporation: The fourth stage starts when the centrifugal out flow stops and consequently no further loss of the ink occurs. During this stage, the thinning behavior is dominated by solvent evaporation only. The evaporation rate is mediated by the difference in partial pressure of the ink species (volatiles) between the free surface of the ink layer and the bulk gas surrounding the ink at the liquid/vapor interface.

[0060] The primary drawback of spin coating arises from the initial steps, namely spin- up and spin-off, which involve ejection of the ink. The last step involves drying of the solution via an interaction of the ink with the surrounding environment (ca. air, dry N ₂ ). Airflow dynamics at the liquid/air interface during the evaporation step play a decisive role on the film uniformity, one of the main benefits of spin coating. The evaporation mechanism during this step can be summarized as follows. Thin layer of the surrounding gas is spun with the substrate whereas the surrounding gas far from the substrate is much less mobile. Consequently, a gradient of the air speed further from the rotating substrate emerges leading to gradient in the partial pressure of the surrounding gas. This gradient in the partial pressure drives the evaporation of the voiatiles,

[0061] Due to the drawback of the poor material usage in spin coating techniques, several alternative techniques, such as wire-bar / knife coating, slot-die coating, were developed and used as replacement for spin coating due to their effective materials usage approaching 0% ink wastage. However, control of film drying after ink casting was challenging. Since spin coating is known for delivering highly uniform films, a modification of spin coating's unique drying mechanism was implemented into scalable processing. Regardless of the benefits gained from substrate rotation in spin coating technique, such as uniform distribution of the ink, a primary drawback of spin coating was significant material losses during the coating steps, as discussed previously. Therefore, in order to attain effective material usage, it is essential to manage the uniformity of drying without substrate rotation. An invention of the present disclosure includes a new apparatus designed with a drying mechanism that does not require substrate rotation. The apparatus may mimic the fluid dynamics on top of the substrate but avoids rotating the substrate thereby providing the advantage of curbing ink-loss and aiding film uniformity.

[0062] The invention of the present disclosure may be composed of conventional wire- bar coating combined with an apparatus for film drying control, illustrated in Error! Reference source not found.. The uniform distribution of the ink that was achieved at the initial steps of spin coating was managed by the alternative scalable techniques, such as wire- bar in the present invention. After the ink was screened via a high speed wire-bar coating, air was spun on the sample surface (adjustable air-substrate distance). Simply stated, instead of spinning the sample that involved ejection of the ink, the air (or inert gas) on top of the sample was spun after wet film was cast. This was done via applying vacuum or supplying an inert gas through a rotating slit on top of the sample.

Film uniformity obtained by modified wire-bar coating versus spin coating

[0063] To elucidate the effectiveness of the new apparatus in attaining film uniformity comparable with spin-cast samples, film uniformity of samples coated by the new apparatus was investigated using polarized optical microscopy. POM images of spin coated and conventional wire-bar coated samples are shown in Error! Reference source not found.a, 7b. Samples coated by conventional wire-bar coating appear non-uniform under POM (Error! Reference source not found.b). Throughout the whole film, the formation of randomly distributed large crystallites with size as high as hundreds of microns was observed. Using the modified wire-bar coating setup, the formation of these large crystallites was overcome as shown in Error! Reference source not found.c,d. Notably, for samples coated by modified wire-bar coating at identical solute concentration as used for spin coating provided a much thicker film (about 2.5-3 times thicker films). Large crystallites in thick films were coated by modified wire-bar coating but their size and distribution were much less than the ones observed in conventional wire-bar coated samples (Error! Reference source not found.d). However, apart from these crystallites, the film appeared uniform elsewhere (Error! Reference source not found.c). To attain thinner films with thickness comparable to spin coating sample, the solute concentration was diluted about 2.8 times (Error! Reference source not found.d). For thinner films obtained by modified wire-bar coating, the formation of the large crystallites was completely hindered and the films appeared as uniform as the spin coated sample (Error! Reference source not found.d).

Relative crystallinity of the modified wire-bar coated samples

[0064] To check the capability of the new apparatus in overcoming the over- crystallization of p-DTS(FBTTh2)2 in the blend films, GIWAXS measurements were performed on samples coated by the new apparatus and spin-cast reference samples. 2D GIWAXS plots of the dry films coated by the new apparatus and spin coating are shown in Error! Reference source not found.a, 8b; line-cuts of the integrated scattering intensity are shown in Error! Reference source not found.c. Both samples exhibited high order in the alkyl-stacking direction revealed by the appearance of alkyl-stacking reflections (001, 002 and 003) that appeared as partial arc scattering but with more dominating scattering in the out-of-plane direction. The π-π stacking scattering revealed preferred edge-on preferential texture for both samples. The relative crystallinity and CCL were calculated for both conditions; the calculation was done for three measurements per each condition, and reported in Table 2. The samples had an average thicknesses of about 100-120 nm and about 100-105 nm for modified wire-bar coating and spin coating, respectively. Samples coated by both coating methods exhibited similar relative crystallinity and average crystal size (CCL about 13-14 nm) of p-DTS(FBTTh2)2 crystallites. These results demonstrated that the new apparatus was effective in overcoming p-DTS(FBTTh2)2 over-crystallization. Table 2: Summary of the relative crystallinity and crystalline correlation length of p-DTS(FBTTh2)2, calculated from the alkyl-stacking ; peak (001)

Coating method Relative crystallinity FWHM(ooi) CCL(ooi)

(nm ¹ ) (nm)

Spin coating 1 0.45+/-0.02 13.9+/-0.6

Modified wire-bar coating 1.12+/-0.08 0.47+/-0.02 13.4+/-0.6

The errors include standard deviation for three measurements.

Solar cell devices fabrication by the modified wire-bar coating apparatus

[0065] Solar cell devices using the modified wire-bar coating appartus were fabricated to test its applicability in attaining high performace solar cells. Solar cell device characteristics were shown in Error! Reference source not found, and Table 3. For all the devices, the active layer was coated in air. The reference spin coated samples exhibited slightly lower PCE (ca. about 6%) than samples coated in a nitrogen glove-box (PCE about 7.6%). Solar cells fabricated by modified wire-bar coating with identical solution formulation used for optimized spin coated condition showed much lower PCE (about 2 times less) than the spin-cast reference device. The samples suffered from much lower FF and also from a reduction in Jsc. This was attributed to lower FF to the high thickness of the film (about 2.7 times thicker than the spin-cast samples). Generally, in too thick organic photovoltaics, the photogenerated charges had to migrate longer distances before reaching the electrodes, increasing the probabilty of the recombination losses on their way. Therefore, the thickness of the active layer was systematically reduced by reducing the solute concentration. With lower thickness of the active layer, FF and Jsc improved gradually leading to an increase in PCE. The best performance of the modified wire-bar coated devices was attained at a total solute concentration of 12.5 mg/mL which resulted in a slightly thicker film (about 118 nm) than the spin coated samples (about 105 nm). The new wire-bar coated devices had a slightly higher Jsc but lower FF, resulting in slight improvement in the overall device performance. These results were promising and further improvements were expected if the devices were fabricated in controlled nitrogen atmosphere. Overall, using the new appartus, similar, if not higher, performance than the reference spin-coated samples was successfully obtained, but with a scalable technique and with much lower materials consumption.

Table 3 Device performance parameters of the solar cells fabricated from p-DTS(FBTTh2)2 : PC71BM by spin coating versus the new wire-bar coating apparatus.

Cone Thickness Jsc Voc EF PCE

[mg/mL] nm [mA/cm ² ] [mV] [%] [%]

Spin coating 35 105 14.4 755 56 6.0 New wire-bar-coating 35 275 12.5 763 33 3.1

New wire-bar-coating 25 215 12.9 754 36 3.5

New wire-bar-coating 17 137 14.8 766 38 4.3

New wire-bar-coating 12.5 118 15.3 784 52 6.3

[0066] In conclusion, a new capability of thin film coating was provided that combines scalability of solar cell active layer coating with obtaining high performance solar cells comparable to the common lab-based coating method (ca. spin coating). Conventional wire- bar coating failed to deliver sufficient film uniformity and microstructure for obtaining high performance solar cell devices fabricated from p-DTS(FBTTh2)2:PC7iBM blends. Through investigation of the crystallization behavior of p-DTS(FBTTh2)2 in situ during conventional wire-bar coating, p-DTS(FBTTh2)2 crystallization occured in an environment saturated with the main solvent, leading to p-DTS(FBTTh2)2 over-crystallization in conventional wire-bar coated films. The latter was responsible for the formation of cracks/pin holes in the conventional wire-bar coated films by the depletion of the molecules that were consumed by crystal growth. To overcome the problems of film uniformity and donor over-crystallization, a new apparatus was introduced that combines a drying mechanism, similar to spin coating, with a wire-bar coating. The new apparatus overcomes problems with over-crystallization in p-DTS(FBTTh2)2:PC7iBM films and obtained film uniformity comparable to spin coated films. Using this new capability, efficient solar cell devices on par with the ones fabricated by spin coating were successfully fabricated, but with negligible material waste and scalable technique at high-speed coating compatible with industrial applications.

[0067] Other embodiments of the present disclosure are possible. Although the description above contains much specificity, these should not be construed as limiting the scope of the disclosure, but as merely providing illustrations of some of the presently preferred embodiments of this disclosure. It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the embodiments may be made and still fall within the scope of this disclosure. It should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form various embodiments. Thus, it is intended that the scope of at least some of the present disclosure should not be limited by the particular disclosed embodiments described above.

[0068] Thus the scope of this disclosure should be determined by the appended claims and their legal equivalents. Therefore, it will be appreciated that the scope of the present disclosure fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present disclosure is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present disclosure, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims.

[0069] The foregoing description of various preferred embodiments of the disclosure have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise embodiments, and obviously many modifications and variations are possible in light of the above teaching. The example embodiments, as described above, were chosen and described in order to best explain the principles of the disclosure and its practical application to thereby enable others skilled in the art to best utilize the disclosure in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the claims appended hereto

[0070] Various examples have been described. These and other examples are within the scope of the following claims.