Compact Roll-to-Roll Coater for in Situ X-ray ... - ACS Publications

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Compact Roll-to-Roll Coater for in Situ X‑ray Diffraction Characterization of Organic Electronics Printing Xiaodan Gu,†,‡ Julia Reinspach,† Brian J. Worfolk,† Ying Diao,†,‡,§ Yan Zhou,† Hongping Yan,‡ Kevin Gu,† Stefan Mannsfeld,*,‡,# Michael F. Toney,*,‡ and Zhenan Bao*,† †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States

‡

S Supporting Information *

ABSTRACT: We describe a compact roll-to-roll (R2R) coater that is capable of tracking the crystallization process of semiconducting polymers during solution printing using X-ray scattering at synchrotron beamlines. An improved understanding of the morphology evolution during the solutionprocessing of organic semiconductor materials during R2R coating processes is necessary to bridge the gap between “lab” and “fab”. The instrument consists of a vacuum chuck to hold the flexible plastic substrate uniformly flat for grazing incidence X-ray scattering. The time resolution of the drying process that is achievable can be tuned by controlling two independent motor speeds, namely, the speed of the moving flexible substrate and the speed of the printer head moving in the opposite direction. With this novel design, we are able to achieve a wide range of drying time resolutions, from tens of milliseconds to seconds. This allows examination of the crystallization process over either fast or slow drying processes depending on coating conditions. Using regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) inks based on two different solvents as a model system, we demonstrate the capability of our in situ R2R printing tool by observing two distinct crystallization processes for inks drying from the solvents with different boiling points (evaporation rates). We also observed delayed on-set point for the crystallization of P3HT polymer in the 1:1 P3HT/ PCBM BHJ blend, and the inhibited crystallization of the P3HT during the late stage of the drying process. KEYWORDS: roll-to-roll process, X-ray scattering, solar cell, organic electronics, synchrotron radiation

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INTRODUCTION Generating renewable energy while reducing green house gas CO2 emissions is becoming increasingly important for supplying the ever-growing energy demand. Among various renewable energy sources, solar cells are a highly attractive power source, which requires low maintenance and operates without generating noise. Remarkable achievements have been made in the past few decades in terms of increasing the power conversion efficiencies (PCEs) of solar cells.1 Organic solar cells are commonly solution processed and capable of being fabricated in large scale for roll-to-roll (R2R) coating processes. Their potential for low cost and high throughput production has attracted significant attention from both industry and academia.2−6 It has been estimated that the energy payback time for organic solar cells on indium tin oxide (ITO) free substrates with 3% PCE can be as short as 90 days.7 The PCEs of organic solar cells have rapidly improved in the past several years to currently more than ten percent, as results of material design and engineering, improved synthesis, advanced device structure, and careful morphology control.5,6,8−11 The active layer of the organic solar cells, so-called bulk heterojunction (BHJ) structure, consists of electron © 2015 American Chemical Society

donating semiconducting polymer (donor) and, typically, a molecular electron acceptor (acceptor), such as a fullerene derivative. When the donor material absorbs sunlight, an exciton is formed and needs to diffuse to the interface between the donor and acceptor for effective exciton splitting. Ideally the donor and acceptor should form a nanoscale phase separated blend due to the short exciton diffusion length of most organic materials.12 After exciton splitting, the separated charges are transported to the respective electrodes. One determining factor that affects the performance of organic solar cells is the nanoscale phase separation size scale for the active layer, that is, the average phase separation between polymer and fullerene. Current research efforts in organic solar cells largely focus on improving PCEs of solar cells. Most devices reported in the literature are processed by spin coating with active device area less than a few square millimeters on rigid glass substrates. Spin coating equipment is relatively inexpensive and readily available in most laboratories, and thus, is the dominant method for labReceived: September 29, 2015 Accepted: December 29, 2015 Published: December 29, 2015 1687

DOI: 10.1021/acsami.5b09174 ACS Appl. Mater. Interfaces 2016, 8, 1687−1694

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ACS Applied Materials & Interfaces scale organic solar cell device fabrication. While it has aided the development of the field, it is not suitable for high throughout mass production on flexible substrates. Moreover, solar cell performance is extremely sensitive to the morphology of the BHJ active layer, such as the phase separation length scale of the donor and acceptor, domain purity, and the crystallinity of the respective materials used.5,13 A bicontinuous structure with ∼10 nm phase separation is needed for efficient exciton splitting.14 Since the BHJ morphology is highly sensitive to processing conditions and the dramatically different solvent drying dynamic, it is not surprising that the optimized processing conditions for spin coating cannot be directly transferred to R2R coating. Therefore, detailed understanding of the morphology and crystallization evolution of organic semiconductor materials during R2R coating processes can help bridge the gap between “lab” and “fab”.3,15 X-ray scattering is a powerful nondestructive method for characterizing morphology of organic molecules either in thin film or bulk.16 Using high-flux synchrotron radiation facilities, X-ray scattering techniques have been widely used for understanding the molecular packing structures of organic semiconducting polymers, such as molecular orientation, relative crystalline size distribution, and the relative degree of crystallinity. Detailed reviews of X-ray scattering techniques for organic solar cell morphology characterization have recently been published.16−18 However, previous morphology characterization using X-ray scattering mostly focused on solid films, where the dynamics of the solvent drying process is missing. On the other hand, aggregation and crystallization of the semiconductor, and the phase separation between donor and acceptor are critical factors determining the final film morphology. Thus, understanding the process of the film formation is important. Previously, in situ X-ray scattering work on organic solar cells has been reported for spin coating and doctor blading experiments.1,19−25 These have provided valuable insights into the film formation process, but most are limited to small rigid substrates. To the best of our knowledge, there are no reports of in situ X-ray scattering during the roll-to-roll coating, in which the speed of coating may be much higher to mimic high throughput large-scale coating. Even though ex situ X-ray scattering characterization of R2R coated films have yielded some insights into the final coated films,26 the crystallization dynamics during the solvent evaporation process is still missing, which has limited our understanding of these processes and makes it difficult to design rational approaches to control morphology.7,26 Here we describe a R2R coater designed to be portable and adaptable. This has enabled in situ X-ray scattering experiments during the R2R coating process. The design of this mini roll-to-roll coater is discussed in detail. To demonstrate its capabilities, the drying process of P3HT inks coated from chloroform and chloroform/ dichlorobenzene solvents were investigated, and the diffraction results provide insight into the drying process. This setup will also benefit other solution-processed materials, such as, printing polymer membranes,27 and battery electrodes,28 and inorganic−organic hybrid solar cells.29

Figure 1. Schematic of the in situ X-ray scattering setup for direct monitoring of the drying dynamics and morphology evolution using a mini roll-to-roll coater. Grazing incidence X-ray scattering is performed during the printing process, and the diffraction images are collected with a fast area detector. The crystalline structure of polymer and polymer blends are extracted from diffraction data to understand the morphology evolution of the R2R-coated semiconductor film.

the polymer is directly probed by X-ray scattering, and thus provides important insights into potential approaches to optimize the morphology of the solar cells. The polymer chain packing order near the drying front and polymer chain orientation in the dried film can be directly probed in a single in situ printing experiment. A detailed schematic drawing of the mini R2R setup is shown in Figure 2. The setup contains two major parts, a low friction flat surface facilitate the unhindered, continuous transport of the plastic film across the coating stage (Figure 2a), and two rollers (labeled as part 1 in Figure 2b). In an industrial coating process, typically the polyethylene terephthalate (PET) sheet is transported on a round roller, above which the coating head is placed. Such a setup, however, is difficult to allow real time Xray monitoring of various drying stages in the coated film. Instead, we designed a vacuum chuck (part 7 in Figure 2a) to hold the flexible PET substrate flat on the stage via vacuum suction from hundreds of submm sized holes, uniformly distributed on the stage (part 6 in Figure 2a). Above the coating stage sits the printer head (part 5 in Figure 2a), for which there are two different options. The slot-die coating head is a miniature version of an industrial scale slot die coating head with a relatively small dead volume (∼50 μL). A computer programmed syringe pump is used for ink delivery. Another option for the printer head is to use the solution-shearing method developed in our lab.30 In this case, a hydrophobic octadecyltrichlorosilane (OTS) treated silicon wafer is secured by vacuum and placed above the PET substrate with a gap of tens of micrometers. The silicon wafer is tilted by 5 degrees in the shearing direction to create a reservoir for the ink. The ink is dispersed near the silicon wafer edge using a pipet with predefined volume (see Figure S-2). Capillary force drives the ink under the blade and this serves as ink reservoir during the coating process. Typically, ∼10 μL ink is sufficient to print a solar cell with device area of 1500 mm2 (15 mm × 100 mm). To achieve a uniform thickness for the final dried film, it is critical to align the printer head parallel to the PET substrate. There are two independent tilt stages, one mounted with the

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COATER DESIGN AND MEASUREMENT METHODS Figure 1 shows a schematic of the in situ X-ray scattering setup for data collection during R2R coating of organic solar cells. The ink is printed on a flexible substrate, and the measured Xray scattering at the different drying stage provides the corresponding morphology information. The crystallization of 1688


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The flexible PET substrate is driven by a roller that is connected to a stepper motor. The stepper motor is coupled with a three-inch outer diameter wind up roller to minimize the ITO crack when bent.12,31 The feed roller is attached to magnetic clutches with adjustable friction to supply sufficient tension to the PET tape to ensure that its movement is smooth and steady. Two grooves on both sides of the vacuum chuck are machined to guide the movement of the PET tape (part 7). To probe the morphology evolution during different drying stages using in situ X-ray scattering, the printer head is designed to move in a direction orthogonal to the incident X-ray beam. Figure 3 shows the schematic of X-ray diffraction for different

Figure 3. Schematic views (a, b, c) and top down photographs (d, e, f) for the different drying stages of organic solar cell ink by solution shearing coating. (a, d) X-ray passes through the wet-film, (b, e) X-ray passes through the drying film, and (c, f) X-ray passes through the dried film.

Figure 2. Schematic drawings of (a) the mini roll-to-roll coater stage and (b) the entire mini roll-to-roll setup. X-ray beam is illustrated here in red color. Part 1, roller for flexible substrate; part 2, printer head tilt stage; part 3, linear micrometer controller; part 4, linear piezoelectric motor; part 5, printer head; part 6, flat stage with vacuum holes; part 7, vacuum chuck; part 8 thermoelectric Peltier heater; part 9, tilt stage for vacuum chuck.

drying stage using the roll-to-roll coater. The Pilatus X-ray detector takes images continuously during the in situ experiment with a readout time of 10 ms. Meanwhile, the linear piezoelectric motor (part 4 in Figure 2a) moves the coating head in the direction perpendicular to the X-ray beam and opposite the direction of the moving web. At the initial stage of the experiment, the printer head is set to a position that is adjacent to the incident X-ray beam, with the X-ray illuminated volume at the ink immediately after it is dispensed from the printer head, as shown in Figure 3a and d. Any aggregation of the polymer in the wet state can thus be detected.32 As the experiment progresses, the printer head is moved away from the X-ray beam path, dragging the meniscus away from the beam. The distance between the printer head and the X-ray dictates the drying time probed by the X-ray beam. More time is given for the solvent to evaporate at a further separation distance between the printer head and X-ray beam path. Later, the X-ray impinges on the drying front of the sample and eventually hits the dried film (Figure 3b, 3c). The advantage of this setup is that the X-ray beam can be fixed in one location. Another added benefit is that beam damage is significantly reduced since a fresh area is exposed to the X-ray

printer head (part 2 in Figure 2a) and the other with coating stage (part 9 in Figure 2a). They enable parallel alignment between the coating head and the substrate. Two different tilt angles were controlled by the tilt stage in two independent directions, that is, parallel and orthogonal to the X-ray beam direction. A linear micrometer controller is used to adjust the gap between the printer head and the top surface of the stage (part 3 in Figure 2a). A detachable CCD camera with a highresolution macro lens is placed at the side of the sample stage to aid the alignment of the coating head to the sample surface. The typical distance between the coating head and the substrate to coat an ∼100 nm organic semiconducting thin film is about tens of micrometers. To equip the coater with capability of temperature control during film fabrication, a thermoelectric Peltier heater (part 8 in Figure 2a) is sandwiched between the vacuum chuck and tilt stage. The temperature can be varied between 10 to 200 °C, which allows the possibility to investigate film drying dynamics as a function of drying temperature. 1689


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ACS Applied Materials & Interfaces beam as the PET substrate is moved. This obviates beamdamage problems described previously.33 The design of the two independent motors used to control the web and printing head speed provides a wide range of accessible time resolutions. For measuring the drying process of a solution-shearing coated sample on a stationary substrate, the time resolution is determined by the time resolution of the Xray detector, or how fast the detector can take a single image frame. In our current setup, however, the time resolution of the in situ X-ray diffraction experiment (Tresolution) is defined by a combination of the PET substrate web speed (Vweb), printer head translation speed (Vhead), and the exposure time for a single frame (TX‑ray), which is given by the following equation: Tresolution =

Vhead *Tx‐ray Vweb

(1)

Here, TX‑ray is the exposure time for a single detector frame; Vhead is the speed of the printer head, and Vweb is the speed for the flexible PET substrate. For example, when the printer head speed is 0.2 mm/s, the flexible substrate moving speed is 10 mm/s, and together with the X-ray exposure time of 500 ms, the effective drying time resolution can easily go down to 10 ms (see SI Movie 2 for example). For semiconductor materials with high crystallinity, we expected the time resolution can go down to 0.6s drying time, stage II), P3HT started to nucleate and pack into ordered crystalline domains as indicated by the appearance of the outof-plane (100) diffraction peak and the increasing of peak intensity with time. The peak position initially appeared at a q of 0.365 Å−1 and slowly increased until it reached at a value of 0.39 Å−1 after 3s, corresponding to packing lamellar distance of 1.61 nm. Coupled with an increase in peak position, the FWHM of the diffraction peak dropped from 0.046 Å−1 to a final value of 0.040 Å−1 within 0.6s, which is much faster compared to the time taken for the peak position to reach its final value. Debye−Scherrer analysis can be used to estimate the coherence length of the P3HT crystals. In the initial stage, P3HT crystalline domain has a coherence length of 14 nm. In stage III (after 1s), the swollen P3HT vitrified when a majority of the solvent had evaporated, and the coherence length of the

Figure 6. Plot of peak position (red circle), FWHM (blue square), and normalized integrated peak intensity (black triangle) with the drying time. (a) Drying of P3HT from chloroform. (b) Drying of the P3HT from chloroform with 10% ODCB. (c) Drying of the P3HT/PCBM BHJ from chloroform with 10% ODCB.

film did not change further. As the residual solvent continues to evaporate from the film, the alkyl chains pack more closely as the lamellar distance reduced from 1.67 to 1.61 nm (peak position increased from 0.377 to 0.39 Å−1). The above observations suggest that the nucleation and growth of P3HT crystals occurred between 0.6s and 1s in stage II. With limited 1691


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P3HT.37−39 The glass transition temperature of the PCBM is around 150 °C,40 which is higher than that of P3HT. Consequently, the glass transition temperature of solvent swollen P3HT/PCBM mixture phase reached room temperature earlier than that of P3HT amorphous domain.33 As a result, the P3HT polymers in the BHJ system were unable to rearrange to further crystallize during from 90 to 110 s of drying process. The P3HT polymer in the P3HT/PCBM BHJ is ∼30% lower in relative degree of crystallinity because of the presence of PCBM. The above results indicated that with added PCBM, the crystallization process of P3HT was delayed and relative degree of crystallinity of P3HT is reduced.

time given for the chain to reorganize, the crystallization process is limited to a short time, resulting in low relative degree of crystallinity as compared to P3HT printed from mixture solvent as discussed below, and small crystallite size. Next, we observed the coating of P3HT from chloroform solution with 10% ODCB as shown Figure 6b. In this experiment, the time resolution used is 400 ms due to the slow evaporation of ODCB. The crystallization process (stage II) is now extended from 0.6s for chloroform to 65s with 10% added ODCB. The crystallization process of the P3HT did not begin until after 18s of chlorobenzene drying, as shown in Figure 6b, where the (100) diffraction peak was not observed until 18s. The subsequent nucleation and growth of P3HT continued to 85s as the peak intensity continued to increase and finally saturated throughout the crystallization process in stage II, similar to the drying process from chloroform, the peak position initially increased rapidly and then slowly and finally plateaued at a value of 0.392 Å−1, which is slightly higher than the sample dried from chloroform. The FWHM of the diffraction peak rapidly dropped from 0.056 Å−1 to 0.040 Å−1 during the nucleation stage II. After an additional 60s of drying time (at the end of growth stage II), the value of FWHM eventually dropped to 0.028 Å−1. Debye−Scherrer analysis indicated that the P3HT crystals have a larger coherence length of 20 nm, which is 50% higher compared to chloroform solution. The relative degree of crystallinity of the P3HT can be calculated based on pole figure construction with (100) diffraction peak.18 Here, we used the total integrated peak intensity (normalized by dry film thickness and exposure time) for comparing the relative degree of crystallinity. We argue that the FWHM in chi direction in the pole figure for both samples are similar, thus geometry correction for both samples is the same (Figure S-6). The relative degree of crystallinity of P3HT film printed from chloroform with ODCB was estimated to be 2.7 times higher than the P3HT film printed chloroform only. These results indicate that with added high boiling point solvent, the P3HT has a longer time to crystallize during the drying process, resulting in larger crystalline domains as well as higher relative degree of crystallinity compared to P3HT casted from the fast-drying chloroform. Last, we examined the coating of P3HT/PCBM blends from chloroform solution with 10% ODCB as shown in Figure 6c. The onset time for P3HT to crystallize in this blend was delayed compared to pure P3HT. The end of stage I was 40 s for BHJ sample compared to 18 s for pure P3HT. The delayed crystallization of the P3HT could be two reasons. First, in the BHJ blend, the concentration of the P3HT in 1:1 blends of P3HT/PCBM solution is half of that of the pure P3HT solutions to keep the total solids concentration fixed at 30 mg/ mL. As a result, the time for P3HT polymers to reach the critical concentration for its crystallization is delayed. The second reason could be that the PCBM inhibits the nucleation of the P3HT. Similar to pure P3HT, the peak position value increased sharply from 0.372 to 0.385 Å−1, from 40 to 60s, then followed by a modest increase from 0.385 to 0.392 Å−1 from 60 to 110 s in stage II. The FWHM value for the BHJ rapidly decreased from 0.055 to 0.033 Å−1 in stage II. the value of FWHM reached a value of 0.028 Å−1. The final dried BHJ film showed a similar P3HT (100) peak position, compared with pure donor film. Interestingly, the rate of P3HT crystallization is reduced near the end of stage II (90−110 s) as evidenced by the decrease in the slope of the integrated peak intensity. PCBM is known to be miscible in the amorphous region of the

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SUMMARY A compact roll-to-roll setup for in situ X-ray diffraction monitoring of solution coated polymer film drying process was designed, constructed, and implemented at the Stanford Synchrotron Radiation Lightsource. The mini roll-to-roll instrument enables in situ X-ray diffraction data acquisition during fast solvent evaporation throughout printing with a time resolution of 10 ms while maintaining a sufficient exposure time to reach satisfying signal-to-noise ratios. As a demonstration, the R2R system was used to observe the crystallization process of P3HT and P3HT/PCBM films from both a fast and a slow drying solvent. We observed that longer drying times during the printing process resulted in larger crystalline domains and higher crystallinity, though the lamellar packing distance remains the same. The onset for the crystallization of P3HT polymer in blends with PCBM is delayed relative to pure P3HT formulations. The presence of the PCBM in the BHJ blends also inhibited the crystallization process of the P3HT polymer during the late stage of drying, thus, reduced the relative degree of crystallinity of the P3HT. We anticipate that this setup will be broadly applicable for in situ studies of printing of not only similar small molecule−polymer or polymer−polymer blends, but general solution-processable materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09174. Flatness control of the thin film, shearing coater printer head, diffraction of ITO/PET substrate, data reduction, and pole figure (PDF) Movie of the printing process of organic electronic polymers (AVI) Movie of the GIXD images during the drying process (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Present Addresses §

Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana−Champaign, Urbana, IL 61801. # Center for Advancing Electronics Dresden, Dresden University of Technology, 01062 Dresden, Germany Notes

The authors declare no competing financial interest. 1692


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ACKNOWLEDGMENTS X.G., B.W., Y.D., S.M., M.T., Z.B. acknowledge support through the Bridging Research Interactions through collaborative the Development Grants in Energy (BRIDGE) program under the SunShot initiative of the Department of Energy program under contract DE-FOA-0000654-1588. Y.Z. and Z.B. acknowledge support from the Office of Naval Research N00014-14-1-0142. J.R. acknowledges postdoctoral support from the Swedish Knut and Alice Wallenberg Foundation. H.Y. acknowledges support from the National Science Foundation Materials Genome Program (Award no. 1434799). In situ measurements were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515. We thank Ron Marks and Bart Johnson for assistance with SSRL Beamline 7-2.

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