Efficient Characterization of Bulk Heterojunction Films ... - Darren Lipomi

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Efficient Characterization of Bulk Heterojunction Films by Mapping Gradients by Reversible Contact with Liquid Metal Top Electrodes Suchol Savagatrup, Adam D Printz, Timothy F. O'Connor, Insik Kim, and Darren J. Lipomi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04192 • Publication Date (Web): 03 Nov 2016 Downloaded from http://pubs.acs.org on November 8, 2016

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Chemistry of Materials

Efficient Characterization of Bulk Heterojunction Films by Mapping Gradients by Reversible Contact with Liquid Metal Top Electrodes Suchol Savagatrup, Adam D. Printz, Timothy F. O’Connor, Insik Kim, and Darren J. Lipomi* Department of NanoEngineering, University of California, San Diego 9500 Gilman Drive, Mail Code 0448, La Jolla, CA 92093-0448 *Author to whom correspondence should be addressed: [email protected]

Graphical abstract

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Abstract The ways in which organic solar cells (OSCs) are measured and characterized are inefficient: many substrates must be coated with expensive or otherwise precious materials to test the effects of a single variable in processing. This serial, sample-by-sample approach also takes significant amounts of time on the part of the researcher. Combinatorial approaches to research on OSCs generally do not permit microstructural characterization on the actual films from which photovoltaic measurements were made, or require specialized equipment that is not widely available. This paper describes the formation of one- and two-dimensional gradients in morphology and thickness. Gradients in morphology are formed using gradient annealing, and gradients in thickness are formed using asymmetric spin coating. Use of a liquid metal top electrode, eutectic gallium-indium (EGaIn), allows reversible contact with the organic semiconductor film. Reversibility of contact permits subsequent characterization of the specific areas of the semiconductor film from which the photovoltaic parameters are obtained. Microstructural data from UV-vis experiments extracted using the weakly interacting Haggregate model, along with atomic force microscopy, are correlated to the photovoltaic performance. The technique is used first on the model bulk heterojunction system comprising regioregular poly(3-hexylthiophene) (P3HT) and the soluble fullerene derivative, [6,6]-phenyl C61 butyric acid methyl ester (PCBM). To show that the process can be used to optimize the thickness and annealing temperature using only small (≤10 mg) amounts of polymer, the technique was then applied to a bulk heterojunction blend comprising a difficult-to-obtain lowbandgap polymer. The combination of the use of gradients and a non-damaging top electrode allows for significant reduction in the amount of materials and time required to understand the effects of processing parameters and morphology on the performance of OSCs.

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Introduction The two most precious resources of a researcher are (1) time and (2) materials produced by time-consuming, multistep syntheses. For researchers in the field of organic solar cells (OSCs), several devices must be fabricated and tested in serial to determine the effects of a single variable in processing conditions. Serial testing consumes significant amounts of time and precious—i.e., expensive or non-commercial—π-conjugated polymers and other materials. Sample-by-sample testing also paints an incomplete picture of the parameters that determine the photovoltaic performance.1 True optimization of devices is essentially impossible, because only a small number of variables for a new system of materials can be tested in practice. Given the importance of thickness, processing conditions, and additives in determining the performance of new materials, it is very likely that excellent materials that collectively have taken thousands of researcher-hours to produce have been passed over, simply because there was not enough material available. Despite this inefficiency in testing devices and screening materials, OSCs now have efficiencies greater than 10 percent in the laboratory,2 along with long projected lifetimes.3 While there has been some effort to standardize conditions of fabrication in order to compare results between laboratories,4 one area that has not been changed fundamentally is the sub-optimal experimental methodology by which photovoltaic properties are measured. This paper describes a technique for measuring the photovoltaic properties of OSCs based on mapping of one- and two-dimensional gradients. The gradients generated in this paper were in morphology and thickness (though in principle the technique could be used for gradients in composition, strain, degradation, or other parameters). The key enabling aspect of this technique is the use of a non-damaging liquid metal as the top electrode. This technique— photovoltaic mapping of gradients (PVMAP)—provides a tool for researchers to understand the

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effects of processing parameters on the photovoltaic properties without evaporating electrodes. The reversibility of contact between the liquid metal and the semiconductor film permits interrogation over areas on the order of square millimeters and thus the generation of realistic photovoltaic data. (The process is contrasted to scanning-probe methods, which interrogate only nanometer-sized areas.) Each spot measured can be treated as individual device with the active area equal to the footprint of the liquid metal droplet. The semiconducting films can then be reused to obtain morphological information using UV-vis absorption spectroscopy and atomic force microscopy. We began our investigation by performing PVMAP on films of the standard materials used for bulk heterojunction (BHJ) solar cells, poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). The P3HT:PCBM films had gradients in morphology (formed by a gradients in annealing temperature) and thickness (formed by a new method based on asymmetric spin coating). The processes could be used serially to generate gradients in two dimensions. The results matched those obtained using serial testing, but the PVMAP procedure saved time and materials by approximately a factor of ten. We then used PVMAP to optimize the annealing temperature and the thickness of a polymer synthesized in our laboratory and available in only small amounts. We were able to find the optimal points for both parameters of this material in a BHJ film using a total of only ~10 mg.

Background Expense of Organic Solar Cell Research. When a newly designed organic semiconductor is synthesized, or a new processing technique is developed, the details of morphology, charge transport, and composition (including, at minimum, optimal thickness, ratio

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of donor-to-acceptor, and processing temperature) must be determined. A lab-scale synthesis of a new conjugated polymer might yield on the order of 100 mg of purified material, which is enough to spin coat approximately 10 – 20 samples (the exact number depends on the concentration needed, which is typically near saturation, ≥10 mg mL–1). Commercial materials such P3HT and PCBM are ≥$600 g–1 for research quantities (although this cost will be lowered when produced at the scale required for eventual manufacturing). In a device with a conventional geometry, a low-work-function metal such as silver or aluminum is evaporated directly on top of the semiconductor for electrical characterization. The morphology of the active layer—and thus the details of charge transport—is also notoriously sensitive to the conditions of fabrication. For example, temperature gradients5–7 on the surface of a hotplate can produce different thermal histories for substrates annealed nominally at the same temperature, just as the gradient in velocity across the surface of a film during spin coating produces gradients in morphology or thickness8–10 that affect the photovoltaic properties of a semiconductor film.11 These idiosyncrasies, as well as the time in preparing new materials and the cost in purchasing commercial materials, not only makes experimentation in this field prohibitively expensive to some researchers, but also reduces the completeness, quality, and reproducibility of the data that can be collected.4 Furthermore, evaporation of top contacts can change the characteristics of the films in a way that differs from evaporator to evaporator and from experiment to experiment. Gradients in Combinatorial Science. Combinatorial science involves the preparation of a large number of samples in a single process or many related processes executed in parallel. It plays a prominent role in chemistry and materials science, from screening libraries of compounds for biological activity12 to measuring the activity of heterogeneous catalysts.13 In the science of organic thin films, combinatorial methods have been used to measure mechanical properties14–16

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and the morphologies of block copolymers.17 In thin-film materials science, combinatorial methods are facilitated by the use of gradients. Gradients in thickness have been prepared, for example, by dip coating9 and the coffee-ring effect.18 Gradients in annealing temperature can be prepared using a one-dimensional fin, in which one end of the substrate is in contact with a heat source. For example, Verploegen et al. prepared morphological gradients in P3HT:PCBM by annealing along a thermal gradient.19 The authors observed reorientation and enlargement of the crystalline phases of P3HT in grazing-incidence X-ray diffraction (GIXD) data as a function of position along the gradient.19 Photovoltaic properties were not measured in these gradients because evaporating metal contacts precluded characterization of the underlying active material by GIXD. Bettinger et al. used an approach to deposit a two-dimensional grid of organic fieldeffect transistors that combined a gradient of morphology produced by a gradient of solvent composition along one axis and by a gradient of thermal annealing along the other.1 While the authors were able to measure differences in charge-carrier mobility across the substrate, evaporation of electrodes again precluded morphological analysis by other techniques.1 What is needed is a method that permits characterization of the electronic properties and the morphology in a single sample. We believed that the missing ingredient was a top contact that could be put in place and removed without damaging the underlying film.

Experimental Design Eutectic Gallium-Indium (EGaIn). Whitesides, Rampi, and coworkers20 pioneered the use of liquid metal probes to measure the charge-transport properties through ultrathin insulating and semiconductor films, especially films of self-assembled monolayers (SAMs) of alkane thiolates on noble metal surfaces21 and of semiconductor quantum dots.22 While initial studies

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were carried out using mercury,23 concerns about the toxicity and spherical shape of droplets of the metal made its use as a small-area probe intended for use in the ambient laboratory atmosphere undesirable.24 Eutectic gallium-indium (EGaIn) is a non-volatile, non-toxic alternative to mercury.24 (When used in air, it forms a ~1-nm skin of gallium oxide that resists flow and allows the material to be drawn into tips.24) Moreover, EGaIn does not permanently wet hydrophobic surfaces with which it comes into contact,25 and thus the same EGaIn tip, extruded from a syringe, can be used for hundreds of measurements in different locations across different substrates.26 While the Whitesides Laboratory has used EGaIn in devices such as molecular rectifiers based on ferrocene-terminated SAMs,27 the technique has not been used to generate spatially resolved maps of charge-transport through organic thin films, as would be necessary to use EGaIn as a reversible top contact for OSCs. EGaIn has been used widely in organic electronics. Its work function (4.3 eV) permits it to be used in place of aluminum (4.2 eV), which is the most common low-work-function electrode used in OSCs28–30 and perovskite solar cells.31 Moreover, devices with sizes of a few square mm can be addressed by extrusion of EGaIn from a syringe without the time and expense of vacuum deposition. By mounting an EGaIn-filled syringe on a micromanipulator, we reasoned that we could generate spatially resolved maps of realistic device parameters (i.e., full currentvoltage curves), as opposed to alternative methods that use scanning probe tips.32 Selection of Organic Semiconductors. We selected two polymers and their blends with PCBM: a well-known polymer (P3HT) and a novel polymer synthesized in our group33 (PDPP2FT-seg-2T, Figure 1). The blend of P3HT and PCBM was used to validate the method because it is the most widely studied material in the literature for BHJ devices.34–36 Its photovoltaic power conversion efficiency (PCE), photochemical stability, and mechanical

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properties have been used as the benchmark to which the properties of novel materials are compared.37,38 For the novel system, we chose the polymer PDPP2FT-seg-2T. We selected this unusual material based on the following realistic real-world scenario: the sample of polymer was synthesized in our laboratory in 2013 by a researcher who is no longer a member of our laboratory,33 and we had less than 100 mg available. We deemed this material scarce because it could have taken a new researcher several days or more to produce more at the expense of other projects.

Figure 1. Chemical structure of regioregular poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and a low-bandgap polymer based on N-alkylated diketopyrrolopyrrole (PDPP2FT-seg-2T). The full synthesis of PDPP2FT-seg-2T was reported previously by our group.33

Selection of Gradients. We used gradients to explore the effects of two processing parameters critical to the performance of OSCs: post-deposition temperature (i.e., thermal annealing) and film thickness. The time and temperature of thermal annealing strongly affects the morphology of organic semiconductor films, which affects the photovoltaic performance. For example, GIXD measurements of P3HT:PCBM films treated with thermal gradients revealed re8


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orientation of P3HT lamella parallel to the substrate and partial crystallization of PCBM.19 Furthermore, the optimal annealing temperature differs between materials, even those which are structurally similar. For example, the optimal annealing temperatures for poly(3-alkylthiophene)s are different for each length of the alkyl side chains.7 Similarly, the thickness of the active layer has a strong influence on the electrical and optical properties of OSCs.8–10 Predicting the optimal film thickness for a novel system is challenging due to the differences in the absorption coefficients, charge transport properties, and thin-film interference.9 The architecture of the device, the refractive indices, and the extinction coefficients of the layers (substrate, transparent electrode, and active layer) contributed to the absorption profiles of the device that is therefore not a trivial function of the layer thicknesses.8,39 Device Architecture and PVMAP Apparatus. The architecture of the solar cells for this study was chosen to facilitate application and removal of the top electrode. We started by depositing a layer of PEDOT:PSS containing 7% DMSO and 0.1% Zonyl fluorosurfactant (by weight) as the transparent electrode on a glass substrate.40 The active layers were deposited by spin coating from a solution of 1:1 P3HT:PCBM or 1:2 PDPP2FT-seg-2T:PCBM. The detailed experimental procedures are outlined in the Experimental Methods. EGaIn suspended on a movable probe was used as the top electrode to complete the circuit (Figure 2a). A micromanipulator with degrees of freedom along the x, y, and z axes (Figure 2b and 2d) was used to position the top electrode to obtain spatially resolved photovoltaic figures of merits— short circuit current density (JSC), open circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE)—of the OSCs. Photographs, taken from below the samples in the same direction as the incident light (Figure 2c), were used to determine the contact areas between the EGaIn probe and the sample surface. Contact areas were calculated by an image

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processing software. We chose EGaIn because it facilitated the rapid characterization of the films. In an inert atmosphere, e.g. nitrogen-filled glovebox, it can be placed and displaced without fouling of the device surface. In a nitrogen atmosphere, EGaIn does not form an oxide skin and has an appearance and behavior similar to mercury, but without the toxicity of mercury. Using these removable EGaIn top contacts, the thin films were reused for further characterizations by UV-vis spectroscopy using the weakly interacting H-aggregate analysis, along with atomic force microscopy (AFM).

Figure 2. Schematic diagrams and photographs of the purpose-built device for photovoltaic mapping of gradients (PVMAP) used in this work. (a) Movable probe with liquid metal, eutectic gallium-indium (EGaIn), was used as the removable and non-permanent top electrode. (b) Micromanipulator with x, y, and z degrees of freedom was used to position the top electrode to obtain spatially resolved photovoltaic measurements of the OSCs. The architecture of the OSCs was then glass/PEDOT:PSS/active

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layer/EGaIn. Photographs of the contact area between the EGaIn probe and the surface of the OSC (c) and the micromanipulator with the EGaIn probe (d).

Results and Discussion Gradients Prepared by Thermal Annealing. We began by examining the effects of temperature gradients to mimic thermal annealing. Figure 3a shows a schematic diagram of the process used to generate thermal gradients. The detailed procedure is outlined in the experimental methods. Briefly, the photoactive layer was uniformly spin coated onto a layer of PEDOT:PSS on a glass substrate (3 in × 1 in). The sample was then placed on an aluminum fin and positioned perpendicular to the surface of a hot plate to create a thermal fin. By setting the nominal temperature of the hot plate at either 150 °C or 250 °C, we obtained two different temperature gradient profiles with the temperature ranges of 45°C to 105°C and 120°C to 175°C, as shown in Figure 3b. The temperature at each location on the samples was measured using a non-contact infrared digital thermometer. To verify the uniformity of the temperature across the 1 inch width of the samples, we measured the temperature along three lines perpendicular to the heat source (at 0.25, 0.5, and 0.75 inches away from the edge of the sample). We observed no significant changes in the temperature profile along these lines. The surface temperature profiles were then fitted by modeling the aluminum substrate as a one-dimensional fin with the boundary conditions of a fixed temperature at the hot plate end and a convective heat flux at the tip. Using this model, the temperature profiles, T(x), along the distance from the hot plate, x, can be expressed by the Equation 1. ℎ − cosh − + sinh − ℎ = , = . ! ℎ − cosh + sinh

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Where TC is the ambient temperature in the nitrogen-filled glovebox, TH is the temperature of the hot plate, L is the total length of the aluminum substrate, h is the heat transfer coefficient, k is the heat conductivity of aluminum, Ac is the cross-sectional area of the aluminum substrate, and P is the perimeter of Ac.

Figure 3. Temperature gradients for thermal annealing of organic solar cells. (a) Schematic diagram of the one-dimensional fin used for generating temperature gradients. (b) Temperature profiles of two separate fins with different hot plate temperature, TH. (c) Power conversion efficiency of P3HT:PCBM (1:1 mass ratio) as a function of annealing temperature using the two temperature gradient profiles (black circles and squares) and the values obtained from sample-by-sample measurements (blue stars). (d) Power conversion efficiency of PDPP2FT-seg-2T:PCBM (1:2 mass ratio) as a function of annealing temperature on temperature gradient profiles. The red and orange colored boxes in (b), (c), and (d) correspond to the two different temperatures for the hot plate.

To validate our method, we fabricated solar cell devices based on the blend of P3HT:PCBM and annealed them using the temperature gradient. Using PVMAP, we obtained

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the photovoltaic properties as a function of the annealing temperature. The liquid metal electrode, EGaIn, allowed us to generate spatially resolved map of the photovoltaic properties through the organic thin films without permanently depositing metal on or fouling the surface of the devices. We note here that, under inert atmosphere, the EGaIn probe leaves no residue on hydrophobic surfaces. However, defects on the surface such as large dust particles and significant surface roughness led to trace amount of gallium and indium deposition as observed from scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Using this method, the contact area of the EGaIn droplet, as measured by analyzing the photograph obtained at the time of each measurement, was used as the active area for the effective solar cell devices. The typical contact area of the EGaIn droplets were measured to be on the order of 3 mm2. The locally measured photovoltaic properties—short circuit current density (JSC), open circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE)—were averaged over four samples. As shown in Figure 3c, the PCE of the P3HT:PCBM devices annealed using the two temperature gradients was plotted as a function of annealing temperature. We observed the maximum PCE of P3HT:PCBM devices at the annealing temperature of 125°C to be 2.5 ± 0.03 %, which agreed well with the commonly found optimal annealing temperature in literature for the same device architecture (PEDOT:PSS/active layer/EGaIn)41 and typical architecture (ITO/PEDOT:PSS/active layer/Ca/Al).35 To compare these results with measurements taken using serially fabricated devices, we prepared individual solar cells from P3HT:PCBM and annealed them separately at the temperatures of 75, 100, 125, 150, 175, and 200 °C. The average values (N ≥ 4) of the PCE of these reference devices, along with the as-cast devices (shown in Figure 3c as blue stars) exhibited the same trends as the sample with thermal gradients. The serial, sample-by-sample measurements even reproduced the

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apparent discontinuity observed between 100 and 125 °C. This finding indicates that the large increase in PCE with temperature observed here was a real effect, rather than an artifact of the stitching together of two separate gradients. We applied the same experimental procedure to a novel low-bandgap polymer, PDPP2FT-seg-2T, to determine the optimal annealing temperature. Figure 3d shows the PCE of the 1:2 PDPP2FT-seg-2T:PCBM devices as a function of annealing temperature. Similar to the P3HT:PCBM, we observed a single maximum value of PCE at approximately 75°C. Gradients in Thickness Prepared by Spin Coating We then fabricated organic solar cells comprising thickness gradients for both systems using a modified flow-restricted spin coating method, Figure 4a. Briefly, before the photoactive layer was deposited onto the glass substrate (3 in × 1 in) with a PEDOT:PSS layer spin coated uniformly, a rectangular block of polydimethylsiloxane (PDMS) was placed across the width of the substrate. The exposed area of the substrate was thus reduced to an effective area of 2 in × 1 in. The substrate with the PDMS block was then placed onto the spin coater off-center such that the center of rotation was located at the edge of the PDMS block, inset of Figure 4b. For each position, we used a stylus profilometer to obtain the thickness profile along three lines perpendicular to the PDMS block. We observed no significant radial profile using the spin speeds and the solutions with the concentration reported in this work. The characteristic thickness gradient is shown in Figure 4b, where the film of P3HT:PCBM (40 mg mL–1) was spin coated using this modified method is compared to the film thickness if the sample was spin coated without the PDMS block. The active layer thickness varied from 40 nm to 200 nm over the span of 5 cm, for P3HT:PCBM devices using the spin speed of 1000 rpm.

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Figure 4. Thickness gradients. (a) Schematic diagram depicting the modified process of restricted-flow spin coating to generate the thickness gradient profile. (b) Comparison between the film thickness profiles generated from typical spin coating and restricted-flow spin coating at 1000 rpm of the solution of P3HT:PCBM (40 mg mL–1, ODCB). The inset shows the photograph of the same sample. The thickness gradient profiles are heavily dependent on the spinspeed (c) and the properties of the solution (d). Power conversion efficiency as a function of active layer thickness for P3HT:PCBM (e) and PDPP2FT-seg-2T (f).

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The mechanism by which the gradient forms using the PDMS block is currently under detailed investigation. Increasing thickness that extends radially outward from the edge of the PDMS block to the edge of the substrate is provisionally attributed to two factors: (1) the low surface energy of PDMS, which discourages wetting at the edge of the PDMS block, and (2) reduced airflow near the edge of the PDMS block, and thus slower drying (thinner film). Offcentered spin coating has been used previously to study the effect of drying rate on the morphology of BHJ;11,42,43 however, the method described here has not previously been used to generate gradients in thickness in films for OSCs. We investigated the effect of varying the spin speed using the spin rate of 500, 1000, and 1500 rpm. As shown in Figure 4c, the thickness profile of the P3HT:PCBM on a single device depended strongly on the spin-speed.44 As expected from unmodified spin coating, faster spin rates produced gradients with smaller average thicknesses, along with smaller overall gradients (the largest range in thicknesses was produced with the slowest spin speed tested, 500 rpm). We observed that the film thicknesses near the PDMS block—at the distance less than 0.635 cm (0.25 inch)—for all three spin speeds are within error of each other. We hypothesized that the film thickness in this region is dominated by the low surface energy of PDMS, and thus the contribution from spin speed may not be significant near the PDMS block. The concentration of the solution also had a significant effect on the thickness profile. Figure 4d demonstrates the differences in the thickness profile of P3HT:PCBM and PDPP2FT-seg-2T:PCBM films. The higher concentration of the P3HT:PCBM solution (1:1, 40 mg mL–1, ortho-dichlorobenzene, ODCB) when compared to the solution of PDPP2FT-seg-2T:PCBM (1:2, 10 mg mL–1, 1:4 ODCB:CHCl3) led to the gradients with thicker films at each position, despite the slower rate of evaporation of pure ODCB compared to 1:4 ODCB:CHCl3.

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Using the thickness gradients for both polymer solutions, we measured the spatially resolved photovoltaic properties as a function of the film thickness. As shown in Figure 4e, we observed a clear optimal thickness at 200 nm and a secondary optimal point near 100 nm for P3HT:PCBM, which agreed well with previously reported values.9,45 Similarly, the trend of the thickness dependent PCE correlated well with the previous studies8,9 (the double maximum is associated with the effects of optical interference in the active layer). For the PDPP2FT-seg2T:PCBM devices, Figure 4f, we observed an optimal thickness at approximately 100 nm, which is similar to the reported optimized thickness for system comprising PDPP2FT.46 Two-Dimensional Gradients. In order to extend our application of PVMAP, we fabricated samples of P3HT:PCBM comprising both gradients of thickness and annealing temperature on the same substrate. Figure 5a shows a sample with a gradient in thickness along the horizontal axis and a gradient annealing temperature along the vertical axis. The process used to create the thickness gradient in the two-dimensional sample was the same as for the onedimensional gradients described earlier, except that the size of the glass substrates was larger, 3 in × 2 in. The PDMS blocks were placed to create an effective area of 2 in × 2 in. The annealing temperature and the film thickness at each of the 16 locations were verified and summarized in Figure 5b. The rows—A, B, C, and D—represent the different annealing temperature (Figure 5b top); and the columns—1, 2, 3, and 4—represent the different film thickness (Figure 5b bottom).

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Figure 5. Two-dimensional gradients of both thermal annealing and thickness on a single substrate. (a) Photograph of a two-dimensional gradient of P3HT:PCBM. The positions A, B, C, and D represent different annealing temperatures (b, top) and the positions 1, 2, 3, and 4 represent different film thicknesses (b, bottom). Photovoltaic measurements were taken at 16 positions, corresponding to the different thicknesses and annealing temperatures.

We obtained the photovoltaic measurements at 16 positions, the combination of the four rows and four columns. The PCE at each position is plotted on a contour plot in Figure 6a. Each value shown is an average of four samples. The optimal combination of annealing temperature and film thickness were found to be at the positions B2 and B3, corresponding to an extrapolated annealing temperature of 130 °C and film thickness of between 150 and 180 nm. The trends agreed those obtained for one-dimensional gradients. We attributed the slight discrepancy to the small sample size of only 16 locations tested on samples with 2D gradient (only four annealing temperature per each film thickness, instead of at least eight temperatures for samples with 1D gradient).

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Figure 6. Contour plots of the photovoltaic performance and the morphology of the P3HT:PCBM obtained from two-dimensional gradient on a single substrate.(a) Contour plot of the power conversion efficiency for the 16 positions. (b) Contour plot for the fraction of P3HT aggregate measured from applying the weakly H-aggregate model to the UV-vis absorption spectra obtained from the 16 locations after the photovoltaic properties were taken. (c) Root mean squared roughness (Rq) of the height images of the films obtained from tapping mode atomic force microscopy.

After the photovoltaic measurements, the same samples were then used to obtain the UVvis absorption spectra at the 16 locations to compare the aggregation behavior with the photovoltaic performance. Order in P3HT can be determined using a widely practiced method based on the work of Spano and coworkers, who showed that the UV-vis spectra can be deconvoluted into contributions from the aggregated and amorphous phases using the weakly interacting H-aggregate model.47,48 The contribution of the PCBM was subtracted from the UVvis spectra obtained from the P3HT:PCBM films before the analysis. The ratio of these contributions, after taking into account the unequal absorption coefficients of the aggregate and the amorphous domains, can be used to determine the percent aggregated polymer. Using this method, we obtained values for the aggregate fraction of the P3HT as a function of both thermal annealing temperature and the film thickness, Figure 6b. Analysis of the spectra revealed that the range of polymer aggregate fraction ranged from 44% to 53%. The higher aggregate fractions were found at the locations B1 and C1, approximately corresponding to the annealing temperature of 130 °C and 155 °C and film thicknesses of 95 nm.

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In addition to UV-Vis absorption spectra, we obtained the surface morphologies of the same P3HT:PCBM sample by atomic force microscopy (AFM). Figure 6c shows the contour plot of the root mean square roughness (Rq) of the height images from the 16 locations. We observed that the surface roughness is affected by the temperature gradient more than the thickness gradient, with higher roughness occurring at the lower annealing temperature. Conversely, from the phase images shown in Figure 7, we observed more phase contrast in the sample annealed at higher temperatures. The decrease in surface roughness and increase in phase contrast are consistent with previously reported trends.41,49

Figure 7. Atomic force micrographs of the phase images of the P3HT:PCBM obtained from the tapping mode from the two-dimensional gradient on a single substrate. The dimensions of each image are 1.5 µm × 1.5 µm.

Conclusion

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In order for organic solar cells to contribute to the global production of carbon-free energy, the efficiency with which new materials are evaluated must be improved.4 The technique described in this paper—PVMAP—represents one possible way to address this challenge. In particular, PVMAP has three unique features: (1) the efficient use of precious materials and the experimenter’s time; (2) the generation of a single substrate comprising many devices that share the same processing history; and (3) the ability to reuse the same gradient for subsequent characterization. PVMAP has the potential to bridge established methods for combinatorial materials processing and measurement with optoelectronic measurements. This technique could find use not only in organic solar cells, but also in photovoltaic devices based on semiconductor nanocrystals and other solution-processed “solar paints,” light-emitting devices, and as a tool for studying charge-transport through delicate, ultrathin films with spatial resolution.

Experimental Methods Materials. Regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were obtained from Sigma-Aldrich and used as received. PEDOT:PSS (Clevios PH1000) was purchased from Heraeus. DMSO was purchased from BDH with purity of 99.9%, and Zonyl (FS-300) fluorosurfactant was purchased from Sigma-Aldrich. All reagents were obtained from commercial suppliers and used without purification. Chloroform (CHCl3), ortho-dichlorobenzene (ODCB), acetone, isopropyl alcohol (IPA) were obtained from SigmaAldrich. Eutectic gallium-indium was obtained from Alfa Aesar. Conductive silver paint was obtained from Ted Pella Inc. PDMS, Sylgard 184 (Dow Corning), was prepared according to the manufacturer’s instructions at a ratio of 10:1 (base:cross-linker) and cured at room temperature

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for 36 h before it was used. PDPP2FT-seg-2T was synthesized and fully characterized by our group previously.33 Preparation of Solutions and Substrates. The solution of P3HT:PCBM was prepared by dissolving a 1:1 mixture by weight of P3HT and PCBM in ODCB at the concentration of 40 mg mL–1. The solution of PDPP2FT-seg-2T:PCBM was prepared at a 1:2 ratio of polymer:PCBM at 10 mg mL–1 in 1:4 ODCB:chloroform. The solutions were stirred using a magnetic stirrer for 36 h and filtered through a 1-µm glass microfiber filter. The solution of PEDOT:PSS contained 7% DMSO and 0.1% Zonyl by weight and was filtered before spin coating. The glass substrates were cleaned by bath sonication of Alconox solution, deionized water, acetone, and isopropanol for 10 min each and dried under compressed air before they were plasma treated for 3 min (30 W, 200 mTorr ambient air). The glass substrates were then used immediately for spin coating. Generation of Temperature Gradients for Thermal Annealing. To probe the effect of temperature gradients for thermal annealing, we began by preparing the organic solar cells with uniform thickness. On clean glass substrates (7.62 cm × 2.54 cm, 3 in × 1 in), a layer of PEDOT:PSS was spin coated at 500 rpm for 240 s, followed by 2000 rpm for 30 s. The films were then dried at 150 °C for 30 min on a hot plate, resulting in films with thickness of approximately 150 nm. The photoactive layers were subsequently spin coated on top of the PEDOT:PSS layer. For P3HT:PCBM, the spin parameters were 500 rpm for 240 s, followed by 2000 rpm for 30 s. Films of PDPP2FT-seg-2T:PCBM were spin coated at 1000 rpm for 240 s and 2000 rpm for 30 s. The resulting film thicknesses were 225 ± 3.8 nm and 133 ± 5.7 nm for P3HT:PCBM and PDPP2FT-seg-2T:PCBM, respectively. The devices were moved into the N2filled glovebox immediately after deposition of the photoactive layers. To create the gradient in

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annealing temperature, the samples were placed on an aluminum fin (length = 15.24 cm, width = 3.175 cm, depth = 0.1588 cm) and then placed on a hotplate perpendicular to the heated surface, Figure 3a. This configuration created a heating fin with an effective length of 15.24 cm. The nominal surface temperature of the hotplate was set at either 150 °C or 250 °C; the ambient temperature in the N2 filled glovebox was constant at 23.2 °C. The samples were annealed for 30 min on the aluminum fin and cooled to room temperature for 20 min. Surface temperatures of the samples were measured at specific points using a digital thermometer. The surface temperature profiles were fitted by modeling the aluminum substrate as a one-dimensional fin with the boundary conditions of a fixed temperature at the hot plate end and a convective heat flux at the tip. The heat conductivity of aluminum was set at a constant value of 250 W m-1 K-1. The heat transfer coefficient h was fitted using a least-squares method. Detailed photographs of the procedure are shown in S1 of the Supporting Information. The photovoltaic performance of the annealed sample at each specific point was measured using PVMAP as described in the section below. Generation of Thickness Gradients of the Active Layer. For samples comprising thickness gradients in the active layer, the PEDOT:PSS layers were prepared using the same conditions as the samples with thermal annealing gradients on clean glass substrate with dimension 7.62 cm × 2.54 cm (3 in × 1 in). Before spin coating of the photoactive layer, a PDMS rectangle (l = 3 cm, w = 1 cm, d = 3 mm) was placed on top of the PEDOT:PSS film perpendicular to the length of the glass substrate to create an effective area of 2 in × 1 in, the inset of Figure 4b. PDMS was chosen because of its hydrophobicity and simplicity in handling; hydrophobic glass slides can be used instead. The glass substrate with a layer of PEDOT:PSS and the PDMS rectangle was then placed on the spin coater with the center of rotation on the

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edge of the PDMS rectangle as shown in the inset of Figure 4b. For P3HT:PCBM, the parameters were 1000 rpm for 4 min and 2000 rpm for 30 s, and for PDPP2FT-seg-2T, 500 rpm for 4 min and 2000 rpm for 30 s. The film thickness at each position was measured using a stylus profilometer. P3HT:PCBM substrates were annealed uniformly at 125°C; PDPP2FT-seg2T:PCBM substrates were not annealed. Detailed photographs of the procedure are shown in S2 of the Supporting Information. Generation of Two-Dimensional Gradients. Samples comprising 2D gradients were fabricated on glass substrates with the dimensions of 7.62 cm × 5.08 cm (3 in × 2 in). The PEDOT:PSS layer was spin coated over the whole glass substrate using the same parameters as above. Before depositing the photoactive layer, a PDMS rectangle (l = 6 cm, w = 1 cm, d = 3 mm) was placed to create an effective area of 2 in × 2 in, Figure 5a. The active layers of P3HT:PCBM were then spin coated at 1000 rpm for 240 s and 2000 rpm for 30 s to generate the thickness gradient. The samples were then placed on an aluminum fin (length = 15.24 cm, width = 6.350 cm, depth = 0.1588 cm) and placed perpendicular on top of the hot plate. The samples were placed on the aluminum fin such that the thickness gradient is perpendicular to the thermal annealing gradient, Figure 5a. The nominal temperature of the hot plate was set at 250°C; the samples were annealed for 30 min and cooled to room temperature for 20 min. Measurement of Photovoltaic Properties Using PVMAP. The photovoltaic properties were measured in a nitrogen-filled glovebox using a solar simulator with 100 mW cm–2 flux under AM 1.5G condition (ABET Technologies 11016-U up-facing calibrated using a reference cell with a KG5 filter. The current density versus voltage was measured using a Keithley 2400 SourceMeter. The movable probes were fabricated by mounting 3 mL Luer-Lock syringes onto the XYZ micropositioner. The tips of the syringes were made conductive by multiple coatings of

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conductive silver paint. Copper wires were used to complete the circuit. EGaIn was then extruded slowly from the syringe to create a stable suspension. The syringe with the EGaIn tip was then lowered onto the surface of the sample, via a micromanipulator with degrees of freedom in x, y, and z axes, to create an electrical contact. Photographs taken from below, above the light source, of the contact between EGaIn and the samples were used to determine the active area of the solar cells. After each measurement, the syringe was raised slowly and moved to a new location of measurement. Detailed photographs of the procedure are shown in S3 of the Supporting Information. UV-vis Spectroscopy, Weakly Interacting H-aggregate Model, and Atomic Force Microscopy. The absorbance of the materials was measured using an Agilent 8453 UV-vis spectrophotometer. The wavelength range measured was 300–850 nm with a step size of 1 nm. A film of PEDOT:PSS on glass substrate was used as a baseline for the absorption to account for the contribution to the absorption spectra. The weakly interacting H-aggregate model was then used to perform a least squares fit to the absorption spectra between 550 and 620 nm (2.25 and 2.00 eV) using a Matlab program. The initial spectra of P3HT:PCBM were initially normalized by setting the lowest point between 670 and 750 nm to zero and then normalizing to the peak between 480 and 560 nm. The contributions from PCBM were subtracted out using the absorption spectra of the pure PCBM annealed at corresponding temperature. The window for each of the measurement were 0.635 cm × 0.635 cm. Atomic force microscopy (AFM) micrographs were taken using a Veeco Scanning Probe Microscope in tapping mode. Data was analyzed with NanoScope Analysis v1.40 software (Bruker Corp.). The samples used for UVVis experiment were used for AFM experiments.

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Photographs of detailed experimental procedures for generation of temperature gradients for thermal annealing, generation of thickness gradients of the active layer, and measurement of photovoltaic properties using PVMAP.

Acknowledgements This work was supported by the National Science Foundation, grant number EEC1341973. Additional support was provided by the National Science Foundation Graduate Research Fellowship under grant number DGE-1144086 and the Kaplan Dissertation Year Fellowship, awarded to S.S., and by laboratory startup funds from the University of California, San Diego. This work was performed in part at the San Diego Nanotechnology Infrastructure (SDNI), a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542148).

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Efficient Characterization of Bulk Heterojunction Films ... - Darren Lipomi

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