ARTICLE pubs.acs.org/Langmuir
Solvent-Assisted Soft Nanoimprint Lithography for Structured Bilayer Heterojunction Organic Solar Cells Jin Young Park, Nicholas R. Hendricks, and Kenneth R. Carter* Polymer Science and Engineering, University of Massachusetts, 120 Governors Drive, Amherst, Massachusetts 01003, United States
bS Supporting Information ABSTRACT: We introduce a novel method to easily fabricate nanopatterns at ambient conditions using solvent-assisted soft nanolithography. For this purpose, a P3HT/PCBM bilayer, one of well-known standard models of solar cell systems, was chosen to optimize bilayer solar cells using the new lithographic technique. The nanopatterns of P3HT made using this method have improved device efficiency compared to planar bilayer heterojunction of the solar cell. The new patterning process creates solar cell devices with a greater than 2-fold increase in power conversion efficiency (PCE) compared to an otherwise equivalent, flat device. This improvement in efficiency is due to the increased interfacial area created by the patterning process. This result demonstrates the feasibility of extensive applications toward nanolithography, relevant to device fabrication, such as electronic devices.
’ INTRODUCTION Organic heterojunction solar cells are of attractive interest in achieving lower cost and simpler industrial processability (e.g., rollto-roll processing) with improved power conversion efficiency (PCE).1 For this purpose, research on efficient device structures is currently being performed to obtain improved device performance. However, the PCE of organic photovoltaic (OPV or simply PV) cells is still low compared to silicon-based inorganic solar cells. In general, primarily two device structures have been studied to develop optimized device performance on the organic solar cells: (1) electron donor and acceptor (D/A) bilayer heterojunction (BLHJ) and (2) bulk heterojunction (BHJ) PV cells. Recently, as intermediate nanostructured systems, interpenetrated patterns of the donor have been developed to generate larger interfacial area with the acceptor in the active layer. When considering the exciton diffusion limit (typically ∼5 10 nm),2 this system provides effective charge carrier pathways on large interfacial areas, resulting in higher efficiency of the heterojunction solar cells. In general, nanoimprint lithography (NIL) has been employed with a variety of templates such as microsphere colloid array,3 porous anodic aluminum oxide,4 and patterned silicon.5 Hard molds (e.g., Si master molds) in particular have been widely used to imprint nanopatterns onto a donor layer (or acceptor)6,7 or blend film8 at high temperatures and pressures. The advantage of nanoimprint lithography is attributed to the desirable molecular ordering of nanostructural conjugated polymer, i.e., chain orientation (crystallinity) and polymer morphology, resulting in positive effects in charge transport and mobility on device performance.9 In soft lithography, flexible, conformal stamps based upon materials like cross-linked poly(dimethylsiloxane) (PDMS) polymers have widely been used as a microcontact printing tool in the application of electronic devices, such as organic field effect transistors (OFET) and organic light-emitting diodes r 2011 American Chemical Society
(OLED).10 12 Recently, the Guo research group has demonstrated a route to bulk heterojunction solar cells using a flat PDMS mold (so-called “ESSENCIAL” method).13 They were able to demonstrate that the use of a flat PDMS sheet in contact with a semiconducting polymer blend layer during drying led to an optimized surface morphology and that fabrication of large-area devices by roll-to-roll processing could be accomplished with these blend systems. The efficiency they reported (η = 3.2%) is similar to the value observed with similar blend systems made by the spin-coating method. In fact, any contact lithographic method for PVs using hard molds14 has to meet a number of stringent process conditions (e.g., long-term process or high temperature and pressure). To date, it has proved difficult for hard mold-based NIL methods to be used in industrial fabrication processes due to the high cost of hard molds, short mold life, and general process unsuitability. In this paper we introduce a new simple method to fabricate PV devices with an interpenetrated donor layer at ambient conditions using solventassisted soft stamp-based nanoimprint lithography (SASSNIL). In this process a confined solution containing a semiconducting polymer is processed into a patterned thin film. This is accomplished by solvent evaporation and solvent absorption toward a patterned PDMS template which occurs by injection of solvent onto the template, followed by placing appropriate transparent anode substrates on top of the template under an applied pressure. The resulting structure is coated with an electron acceptor layer and top cathode, completing the assembly of an interdigitated organic PV device. We are especially excited about this new approach as it offers a simple, potentially of a low-cost fabrication route. More importantly, this new process is amenable to Received: May 13, 2011 Revised: June 29, 2011 Published: July 12, 2011 11251
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Table 1. Dimensions of Poly(dimethoxylsiloxane) (PDMS) Replica Molds Used in This Experiment and the Resulting P3HT Patterns Nanoimprinted by the SASSNIL Method PDMS molds
imprinted P3HT patterns
nanoimprinting molds
period (nm)
height (nm)
width (nm)
height (nm)
width (nm)
#1
1600
220
400
123 ( 7
827 ( 20
#2
810
180
410
107 ( 7
384 ( 19
#3 #4
420 360
70 20
220 180
44 ( 7 16 ( 2
148 ( 10 157 ( 10
adaptation to a continuous roll-to-roll manufacturing processes. For the purpose of this study, a well-known standard model of solar cell systems (e.g., P3HT as a donor and PCBM as an acceptor) was chosen to optimize BLHJ solar cells using this new lithographic technique.
’ EXPERIMENTAL SECTION Materials. Poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), phenyl-C61-butyric acid methyl ester (PCBM), chlorobenzene, and methylene choride were purchased from Aldrich and used without further purification. Poly(3-hexylthiophene-2,5-diyl) [P3HT, highly regioregular electronic grade, Mw: 50 000] was obtained from Rieke Metals, Inc., and used without further purification. Customized indium tin oxide (ITO) coated glasses (thickness: 1450 ( 100 Å; resistivity: 20 ( 2 ohms/sq) as the anode electrode for solar cells were purchased from Thin Film Devices, Inc. Preparation of PDMS Replica Molds. Three PDMS replica molds used for solvent-assisted nanoimprinting were prepared as follows. Silicone elastomer base and curing agent (Sylgard 184, Dow Corning Co.) with a ratio of 10:1 were well mixed, degassed under vacuum, and poured onto the master molds in a plastic Petri dish. Thermal curing was performed at a temperature of 70 °C for 4 h in an oven. After demolding, the PDMS stamps were stored in a clean Petri dish prior to usage. Three master molds were used to replicate the PDMS stamps, and Table 1 is summarized for the dimensions of the PDMS replica molds. For 200 nm period grating mold, a “hard-PDMS” mold was prepared as previously reported in the literature.15,16 1.7 g of 7 8% vinylmethylsiloxane dimethylsiloxane copolymer (Gelest, Inc.) and 5 μL of 2,4,6,8-tetramethylvinylcyclotetrasiloxane (Fluka) were mixed by stirring. 9 μL of platinum divinyltetramethyldisiloxane complex in xylene (Gelest, Inc.) was added to the magnetically stirred mixture. After degassing, 0.5 mL of 25 30% methylhydrosiloxane dimethylsiloxane copolymer was then added. A thick layer of h-PDMS mixture was spin-coated on a 200 nm grating pitch mold at 1000 rpm for 40 s. The coated master was precured in an oven at 60 °C for 2 min. The master mold was then placed on the Petri dish, and the as-prepared PDMS mixture was poured onto the master mold. After curing at 60 °C for 4 h, the h-PDMS replica was carefully peeled off from the mold. Preparation for Solvent-Assisted Soft Nanoimprint Lithography. ITO coated glass substrates were cleaned by sonication in deionized water, isopropanol, and acetone several times and sequentially ozone-cleaned by an UVO cleaner (Jelight Co. Inc.) to remove contaminants. Using a spin-coater (Specialty Coating Systems, Spincoat G3P-8, 4000 rpm for 60 s), a ∼75 nm thick layer of PEDOT:PSS was spin-coated on the ITO and annealed at 150 °C for 30 min on a precision hot plate (Electonic Micro Systems Ltd. Model 1000-1) to remove water. A gentle stream of nitrogen was blown onto the sample surface (PEDOT:PSS coated ITO) prior to the nanoimprinting process. For the donor layer, P3HT was dissolved in chlorobenzene at a concentration of 11 mg/mL (1.0 wt %). To do solution-based processing subsequently on the underlayer of nanoimprinted P3HT film using
the insolubility of P3HT in dichloromethane, 1 wt % concentration of PCBM solution was prepared. All the solutions were filtered using 0.45 μm PTFE syringe filters. The film thickness was measured by a surface profiler (Alpha Step-IQ).
Atomic Force Microscopy and Scanning Electron Microscopy. Atomic force microscopy (AFM, Digital Instruments, Dimension 3000) was used to investigate surface morphologies and surface analysis. The AFM measurements were carried out with a piezoscanner at room temperature. Commercially available tapping mode tips (Veeco, phosphorus n-doped Si, f0: 312 342 kHz) were used on cantilevers with a resonance frequency in the range of 290 410 kHz. All images (AFM topography, tapping mode) were flattened, filtered, and analyzed using SPIP software (Scanning Probe Image Processor, www.Imagemet.com). All data in the dimensions (height) of imprinted patterns were collected and averaged by at least 20 measurements from the line profile of AFM images. For SEM images, a Carl Zeiss EVO 50 SEM and JEOL JSM7001F SEM were used to investigate the surface and cross-sectional features of the PV devices. Device Fabrication and Photovoltaic Testing. All device fabrication and PV testing were performed in a glovebox workstation (MBraun Inc.). To complete the device fabrication, 100 nm of aluminum (Al) as cathode was evaporated onto the organic active layer under high vacuum (below 3 10 6 mbar) through a shadow mask. The metal thickness was controlled by a calibrated crystal oscillator. After the metal cathode evaporation, postannealing was performed at 150 °C for 30 min under nitrogen. The active device area was measured to be 0.073 cm2. The PV characteristics were investigated with a testing system consisting of a solar simulator with an Oriel 300 W Xe lamp and a digital exposure controller (Newport Co.) The simulator lamp intensity was set to air mass 1.5 solar simulated light at 100 mW/cm2 as determined by a universal power meter (Nova-Oriel, Ophir Optronics Ltd.) and a standard silicon photodiode (Bunko Keiki, BS-520). The current voltage characteristics were measured using a source meter (Keithley 2400) under irradiance.
’ RESULTS AND DISCUSSION Using four PDMS replica molds with different grating pitches (Table 1), the SASSNIL method was performed as follows (Scheme 1): (a) A droplet of 5 10 μL of P3HT solution was carefully dropped on the patterned PDMS template surface. (b) A ∼55 nm thick PEDOT:PSS coated indium tin oxide (ITO) substrate was placed on the mold under an applied pressure (a 109 g weight stainless steel) such that the solution spread out over the contact area between the two solid substrates. During this contact process, the solution is uniformly trapped into the trenches of the PDMS mold by induced shear flow, and then, while the organic solvent vaporizes and absorbs into the PDMS mold, the P3HT is solidified and accumulated on the PEDOT: PSS film in 3 5 min. (c, d) After demolding, the transferred P3HT pattern is uniformly distributed over the entire surface. (e, f) After drying the patterned P3HT film, PCBM was spin-coated on 11252
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Scheme 1. Schematic Diagram of Bilayer Heterojunction Solar Cells Fabricated Using the SASSNIL Method
Figure 1. Atomic force microscopy (AFM) images (topography) of the P3HT films imprinted with 1600, 810, 420, and 360 nm period grating molds (a d), respectively. The scale bars are 2 μm.
the donor layer and Al was finally evaporated onto the active layer as the cathode electrode. Figures 1 and 2 show the surface topology (AFM and SEM) of the nanoimprinted patterns obtained by the SASSNIL method, confirming the effectiveness of this processing. Each P3HT patterned line was precisely transferred with no discernible distortion and P3HT solidification occurred confined within the trenches of the PDMS mold. The imprinted patterns (#1 and #2) show straight broad lines with rough sidewalls, while narrow patterns (#3 and #4) formed from 420 and 360 nm period grating molds feature wave-shaped surface roughness and partially discontinuous lines. In particular, the 360 nm period grating shows the significant features of line discontinuity (defects) over the large casting area. In general, the size of the pattern imprinted
on the active donor layer is mainly determined by the mold features, but it may also be manipulated by varying the solution concentration and the applied pressure. The surfaces of the resulting patterns are relatively rougher than when nanoimprinting with the hard molds (see Supporting Information Figure S1). Our extensive experience with nanoimprint lithography and its observed ability to accurately replicate nanometer sized defects in molds leads us to conclude that the surface of the molded features accurately tracks the morphology of the PDMS mold surface. The surface roughness of the molded P3HT layer likely reflects the rough surface of the solvent-swollen PDMS mold. It is difficult to estimate the roughness of the swollen PDMS layer as we were unable to record AFM of this transient, low modulus PDMS surface. Nevertheless, the roughness causes an 11253
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Figure 2. Scanning electron microscopy (SEM) images of the P3HT films imprinted with 1600, 810, 420, and 360 nm period grating molds (a d), respectively. The scale bars are 1 μm.
advantageous increase in the interfacial area (as will be discussed later). All dimensions of the patterns are summarized in Table 1. With line continuity, the height of the imprinted patterns (#1 and #2), as measured by AFM, is slightly over 100 nm. This height on the pattern leads to a relative increase in the interfacial area, resulting in the transport of more charge carriers. The width of the pattern (#1) (measured from AFM image) is 2-fold larger than that of imprint #2 (810 nm grating mold). In contrast, the height (dh = ∼44 and 16 nm) and width (dw = ∼128 and 176 nm) of the imprinted patterns #3 and #4 (respectively) are relatively smaller. While the periodicity of each P3HT pattern is the same as that of the PDMS mold, the observed width is smaller because solvent swelling causes a reduction in size of the mold patterns during the solvent absorption process. In general, degree of PDMS swelling in solvents with different swelling factors can be defined by the factor S (= D/D0), which is the ratio of the length (D) of PDMS in the solvent to the length (D0) of the PDMS in air.17 Although the same solvent, chlorobenzene (S = 1.22), was used in all of the SASSSNIL experiments, the reduction ratio of the molded feature size differs among the various molds used. For example, in SASSNIL using a 1600 nm grating pitch results in a reduction in width of 32% (d = 0.68 d0 = 821 nm) with a corresponding fill factor reduction of 62%. The 810, 420, and 360 nm grating pitches showed a reduction of 45%, 60%, and 22% fill factor in the trenches, respectively. The reduction in pattern size is a function of various factors, such as mold features (solvent volume and solid state P3HT occupied in the trench), different forces applied on the mold from external pressure, and swelling behaviors during solvent absorption before reaching equilibrium swelling. To complete PV device fabrication, a 1 wt % concentration of PCBM solution in dichloromethane was spin-coated on top of the dried P3HT layer (700 rpm for 1 min). This solution-based casting of PCBM atop of the nanoimprinted P3HT layer is facilitated by the insolubility of P3HT in dichloromethane. A cathode metal layer was evaporated on the interdigitated P3HT/ PCBM layer. We required a control PV cell so a simple bilayer device system was prepared by the sequential spin-coating of 177 nm PCBM on top of a flat 53 nm P3HT donor layer. In our study, we needed to consider the possible solubility of P3HT in dichloromethane during the spin-coating of the PCBM layer. If the application of PCBM solution to the surface of the
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patterned P3HT surface obliterates the pattern, we would have essentially made a blend or flat bilayer or blend device. To ascertain if this was the case, we performed a simple test. In one sample the PCBM spin-coating process was immediately performed after making the P3HT pattern using the SASSNIL method. When this procedure was followed, the patterned P3HT film was completely dissolved, leaving no patterns. This pattern obliteration was due to the residual solvent (chlorobenzene) in the P3HT film. However, in cases where the patterned P3HT layer was completely dried overnight, and the PCBM spincoating was performed at same spin-coating conditions with pure solvent (dichloromethane), the P3HT patterns had no damage as determined by AFM and cross-sectional SEM (see Supporting Information, Figures S2 and S4). Therefore, the redissolution of the P3HT underlayer is negligible during spincoating of the PCBM overlayer. It bears mentioning that the precise morphology of these so-called “bilayer” structures has recently been the subject of intense research. It has been reported that the thermal annealing of bilayer PV devices induces significant intermixing of the PCBM into the P3HT layer.18 24 We would expect our patterned devices to undergo a similar level of intermixing as these flat PV devices. All devices were measured using an air mass 1.5 solar simulated light at 100 mW/cm2. The device parameters [open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and PCE (η)] were measured before and after thermal annealing and were measured for all of the imprinted cells. Device postannealing (at 150 °C for 30 min) after Al evaporation increases the crystallinity of the P3HT, resulting in the higher degree of exciton dissociation and enhanced charge carrier transfer.5 As expected, all the device parameters of the imprinted PVs were significantly improved after postannealing (see Supporting Information Figure S3). However, with the exception of device ISC #1, the imprinted cells have a higher FF than those before annealing, due to the increase in the shunt resistance (Rsh) and the decrease in the series resistance (Rs). The device ISC #1 shows an almost identical value of FF of 52% before and after thermal annealing. This may be related to the undesirable physical contact between the acceptor and the cathode electrode, resulting in no change in resistance after thermal treatment. The I V curves of nanoimprinted and nonpatterned devices, measured after postannealing, are shown in Figure 3a. The resulting performance parameters of these devices are listed in Table 2. The solar cells built on the imprinted P3HT layers show improved performance: a 2.1 2.4 times increase in PCE, 18 32% increase in FF, and 66 85% increase in Jsc compared to the flat reference device. The major reason for the increase in the device properties is more efficient exciton dissociation and charge transport due to increased interfacial area.6,7,25,26 All the imprinted cells have Voc in a range of 0.56 0.58 V, indicating the energy gap between the HOMO of donor and the LUMO of acceptor. Increasing the interfacial area gives a higher Jsc and maximum power output (Pmax = Vmax Imax), resulting in an increased PCE. The increase in FF was shown with the decrease in the size of patterns. The device ISC #3 shows the highest Jsc of 8.5 mA/cm2 and Pmax of 0.203 mW. However, the FF of 56% is slightly lower than device ISC #2 and ISC #4, due to a decrease in Rsh, arising from the relatively high aspect ratio of the imprinted pattern. The PCE of the imprinted cells exhibit a 2 4-fold increase upon post-thermal annealing due to the increased crystallinity of P3HT patterns, although the interfacial area was relatively small (A/A0 = 1.09 1.28). Recently, Aryal et al. have 11254
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Figure 3. Current density voltage response of nanoimprinted organic solar cells. The reference cell is a planar device with 55 nm PEDOT:PSS/53 nm P3HT/69 nm PCBM/100 nm Al, with an interface area, A0. The relative interface areas are calculated assuming a residual layer of 2 3 nm and smooth imprinted patterns.
Table 2. Comparison of the Device Performance in Configuration of ITO/PEDOT:PSS/Active Layer (P3HT/PCBM)/Al with Different Nanoimprinted Layers device
A/A0a
Voc (V)
Jsc (mA/cm2)
Pmax (mW)
FFb (%)
PCE (%)
reference cell
1
0.571
4.607
0.070
44.13
1.16
imprinted cell (#1)
1.13
0.563
8.357
0.178
51.84
2.44
imprinted cell (#2) imprinted cell (#3)
1.28 1.23
0.570 0.582
8.280 8.513
0.196 0.203
56.95 56.03
2.69 2.77
imprinted cell (#4)
1.09
0.582
7.638
0.189
58.19
2.59
a
A/A0 is the ratio of the interface area of the imprinted cell to the interface of the reference cell. b Fill factor (ImaxVmax/IscVoc, where Imax and Vmax are the current and voltage for the maximum power output).
Figure 4. Scanning electron microscopy (SEM) images of imprinted solar cells [(a) ISC #1, (b) ISC #2, (c) ISC #4, and (d) ISC #3]. (a) and (c) are cross-sectional and 40° tilted angle, respectively.
demonstrated the device properties of imprinted bilayer solar cells, consisting of the same materials (P3HT and PCBM).4 Nanoimprinted P3HT nanopillars (80 nm in width and 80 nm in height) fabricated using an anodized aluminum oxide (AAO) mold at high pressure and temperature showed an efficiency of 2.57% at interfacial area (A/A0) ratio of 2. Although the interfacial area of our imprinted cells is relative smaller than the devices prepared by Aryal (due to larger mold sizes we utilized), our devices showed better efficiency (2.44 2.77%) due to an improved device fabrication process.
The trends in observed PCE of our devices were not proportional to the calculated interfacial area, as shown in Figure 3b. There are two possible reasons: First, the ratio of the patterned P3HT interfacial area to flat film on the reference cell was calculated using the assumption that the patterned surface is smooth. This introduces error as we know from AFM measurements that the imprinted cells #3 and #4 had considerable surface roughness, meaning the true interfacial area for these samples must be higher than our crude estimate. Second, the topography from the P3HT patterns makes it difficult to create a smooth surface of PCBM after spin-coating. As shown in Figure 4a,b and Supporting Information (Figure S4), rough metal lines remain due to the rough PCBM layer—in effect poor planarization of the PCBM film on top of the imprinted P3HT layer. This is in contrast to the much smoother surface observed in the imprinted device with
