Imaging the Evolution of Nanoscale Photocurrent Collection and

NANO LETTERS

Imaging the Evolution of Nanoscale Photocurrent Collection and Transport Networks during Annealing of Polythiophene/Fullerene Solar Cells

2009 Vol. 9, No. 8 2946-2952

Liam S. C. Pingree,† Obadiah G. Reid, and David S. Ginger* Department of Chemistry, UniVersity of Washington, Box 351700, Seattle, Washington 98195-1700 Received April 28, 2009; Revised Manuscript Received June 12, 2009

ABSTRACT We use photoconductive atomic force microscopy to image nanoscale spatial variations in photocurrent across the surfaces of photovoltaic cells made from blends of the conjugated polymer regioregular poly(3-hexylthiopene) (P3HT) with phenyl-C61-butyric acid methyl ester (PCBM). We study how the spatial variations in photocurrent evolve with thermal annealing, and we correlate these changes with the evolution of macroscopic film and device properties such as external quantum efficiency and carrier mobility. We use conductive atomic force microscopy to examine the development of injection and transport networks for both electrons and holes as a function of annealing. We find that the hole transport, electron transport, and photocurrent collection networks become increasingly heterogeneous with thermal annealing and remain heterogeneous on the 10-100 nm length scale even in the most efficient P3HT/PCBM devices. After annealing, the regions of the greatest dark hole currents, greatest dark electron currents, and greatest photocurrents are each associated with different regions of the nanostructured films. These results suggest spatial heterogeneity can contribute to the imperfect internal quantum efficiency even in relatively efficient organic photovoltaics and that fully 3D modeling is needed to describe the devices physics of polymer blend solar cells.

Nanostructured organic solar cells have been proposed as potential low-cost alternatives to conventional photovoltaics.1-4 In a typical organic solar cell, light absorption creates strongly bound excitons that must be dissociated into free charges at a donor/acceptor interface. The need for exciton dissociation complicates the design and fabrication of efficient organic solar cells because the light absorption depth is roughly 10 times larger than the exciton diffusion length in most conjugated polymers. Blended films with nanostructured donor/acceptor interfaces (bulk heterojunction cells) are thus commonly used to provide large internal surface areas for charge separation in optically thick films.1,2,5,6 However, the complicated morphologies that arise in such blended films can lead to dramatic performance variations, as is readily observed when preparing films under different processing conditions.7-15 Morphology control lies at the heart of recent efforts to scale up manufacturing of early polymer photovoltaic devices,16,17 and the understanding and control of film morphology has become a central challenge in the field of organic photovoltaics. The most widely studied model system in which the effects of morphology on polymer solar cell performance have been examined is the blend of regioregular poly(3-hexythiophene) * Corresponding author, [email protected]. † Present address: Boeing Research. 10.1021/nl901358v CCC: $40.75 Published on Web 07/09/2009

 2009 American Chemical Society

with the fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM). The photovoltaic performance of spin-coated P3HT/PCBM blends is known to improve dramatically upon either thermal or solvent-vapor annealing,3,4,18 and the structural changes that accompany annealing have been widely studied with techniques ranging from X-ray diffraction, to 3D electron tomography.11,19-22 Although the specific details depend on the molecular weight and regioregularity of the polymer chosen, annealing generally leads to nanoscale phase separation and improved ordering or crystallization of the P3HT and PCBM components. While these techniques provide detailed structural data, they do not provide direct information about how the specific nanoscale morphological changes are related to nanoscale changes in optical and electrical properties. Various scanning-probe microscopy methods offer the ability to correlate local optoelectronic properties with local film structure in organic electronic devices.23 Previous studies have used conductive atomic force microscopy (cAFM) to examine dark charge transport in blends24-26 and to distinguish between different component domains based on transport properties. However, we are not aware of any data which correlate local photocurrent measurements with the systematic changes in the film structure that take place during annealing of P3HT/ PCBM blends. Our group has previously shown that scan-

ning-probe microscopy combined with optical excitation can be used to make direct structural and electronic correlations and can map local currents and photocurrents in a variety of organic optoelectronic devices.23,27-31 In this Letter we apply both cAFM and photoconductive AFM (pcAFM) techniques to follow the evolution of the hole transport, electron transport, and photocurrent networks as we thermally anneal sets of P3HT/PCBM devices. We studied conventional bulk heterojunction devices made from 1.0:0.9 blends of P3HT (90-93% RR, Rieke Metals, Inc., Lincoln, NE) and PCBM (99.5% pure, Nano-C, Westwood, MA) as follows. First, a poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) (Baytron P VP 4083) layer was spin-coated onto prepatterned, ITOcoated glass substrates (Thin Film Devices Inc., Anaheim CA) within 60 s of plasma cleaning the ITO (Harrick Plasma, model PDC-32G, 18 W applied, 4 min cleaning time) and then annealed at 140 °C for 30 min under a nitrogen purge on a hot plate, yielding a ∼40 nm thick PEDOT:PSS layer. Next, a film of P3HT/PCBM was spin-coated from a warmed (45 °C) chlorobenzene solution to give a final device thickness of 160 ( 15 nm (for most measurements) or 90 ( 15 nm (to allow higher fields for imaging electron transport). Aluminum top contacts were deposited by thermal evaporation through a shadow mask. Annealing of the P3HT/PCBM active layers was performed after contact evaporation on a preheated Al block at a temperature of 110 °C in a N2 glovebox for the times indicated. Device measurements were carried out under vacuum, and the devices were illuminated through a mask smaller than the area of the aluminum contact to minimize edge effects. Hole mobility measurements were made using the SCLC method on separate devices with gold top contacts. The current density-voltage curves and Mott-Gurney law fits are shown in Figure S7 in the Supporting Information. All AFM measurements were made with an Asylum Research MFP3D AFM. In both cAFM and pcAFM, we use a cAFM tip as the top contact of the device to record current-voltage curves under dry nitrogen. In pcAFM we record the local photocurrent induced by a diffraction-limited laser spot that is aligned with the tip in an inverted optical microscope (Figure 1A). We used Aucoated (with a Cr adhesion layer) contact-mode AFM probes (Budget Sensors, ContE-GB, spring constant ) 0.2 N/m) for all measurements reported in this work. We used 532 nm illumination adjusted to an intensity of 218 W/cm2 prior to entering the microscope optics, unless specifically noted. In all cases, the tip-sample contact force was kept to a minimum consistent with topography tracking. Typical values were ∼10 nN. We have previously shown that the current observed in cAFM experiments on conjugated polymers is relatively insensitive to the contact force under similar conditions.31 Our devices exhibited performance characteristics within the range of those reported for P3HT/PCBM blend devices fabricated under similar conditions. Before annealing, the peak external quantum efficiencies (EQE) were typically 20-30%, open circuit voltages (Voc) were ∼ 570 mV, and fill factors (FF) were 0.36. As expected, these values change Nano Lett., Vol. 9, No. 8, 2009

Figure 1. (A) Schematic of the pcAFM experiment used to collect local photocurrent maps. (B) External quantum efficiency measured at 532 nm and average photocurrent measured via pcAFM for P3HT/PCBM blend devices as a function of annealing time.

upon annealingsreaching typical EQE values of ∼65%, Voc values of ∼450 mV, and fill factors of 0.46, again within the range values reported in the literature.11 Figure 1B plots the EQE values measured for our devices at 532 nm as a function of annealing time at 110 °C. Plotted on the same graph are the average photocurrents recorded over 2 µm × 2 µm images via pcAFM collected with Au-coated tips. We note that there is good qualitative agreement between the macroscopic device measurements and the local photocurrent measurements obtained via pcAFMsboth show significant increases in photocurrent after the first few minutes of thermal annealing and then plateau at longer times. We believe the quantitative variations between the device and pcAFM data are due to intrinsic fluctuations in performance from region to region, as well as the differences in illumination conditions and charge extraction barriers/current flow directions between the device and pcAFM measurements which we discuss in detail below. 2947

Figure 2. (A-C) AFM topography and (D-F) dark hole current images taken with a +5 V tip bias on P3HT/PCBM blend devices spincoated and annealed at 110 °C for 0, 2, and 30 min as indicated show an increase in the average and heterogeneity in the current with increasing annealing. Each image is associated with a current histogram that quantifies the change in average value and distribution of the current. (G) Hole mobility (blue squares) measured via the space charge limited current method and average cAFM dark current (red triangles) plotted as a function of device annealing time showing that the average hole current measured via cAFM closely tracks the hole mobility measured on the devices.

Before considering the photocurrent maps, we discuss the changes in the distribution of local electron and hole conduction that take place as the devices are thermally annealed. Parts A-F of Figure 2 show the evolution of both the film topography and the distribution of hole currents obtained with a +5 V bias on the tip following annealing at ∼110 °C for 0, 2, and 30 min. Figure 2G compares the average hole current obtained in the dark cAFM images with the hole mobilities measured via the space-charge-limited current method on a set of identically fabricated and annealed macroscopic P3HT/PCBM diodes. Although the spatial maps of the local hole mobility in cAFM are not quantitative without rescaling the local current density vs voltage curves for the microscopic geometry of the tip,31 we observe good qualitative agreement between the average local hole currents and the measured hole mobilities shown in Figure 2G. This correlation indicates that the dark cAFM measurements can provide useful information about how the relative efficiency of local hole transport changes as the devices are annealed. Importantly, the progression in Figure 2D-F shows that both the average value and the spatial variation of the local hole current increases significantly as the films are annealed. These results show that hole injection and transport are extremely heterogeneous on the 10-100 nm length scale in annealed P3HT/PCBM blends. In the opposite bias direction (tip negative relative to substrate), the dark current images are dramatically different as seen in Figure 3. Electron injection into the PCBM from the Au-coated cAFM tips at moderate negative bias is made possible by the high local field concentration (the lightning rod effect) at the apex of the sharp cAFM 2948

tip.32,33 Large currents can even be measured on pure PCBM films with Au tips in this bias direction (Figure S3 in Supporting Information). It is thus possible to inject electron currents through the PCBM network and image them as a function of annealing with the Au-coated tips at reverse bias. If we assume that the images in this bias direction are dominated by a combination of injection and bulk-limited electron currents, then Figure 3 shows the evolution of the distribution of electron currents obtained with a -5 V relative bias on the tip following annealing of the films at 110 °C for 0, 2, and 30 min. Similar to the case of hole transport, the local electron transport maps show a significant increase in both the average value of the electron current and the amount of spatial variation in the electron current with annealing. The increased currents and increased heterogeneity are both consistent with the many structural studies that have reported PCBM crystallization and domain formation upon annealing,4,21,22,34 as well as with the increased photoluminescence from PCBM aggregates that we observe upon annealing the films (Figure S8B in Supporting Information). While we have interpreted Figure 3 as showing electron only currents, we note that the true situation is likely to be more complex. When imaging an ambipolar matrix like a PCBM/ P3HT blend with the tip biased negative relative to the substrate, it is also possible to obtain hole currents injected from the substrate and collected by the tip. However, even if the dark current images in this bias direction contain a non-negligible contribution from hole currents, the qualitative conclusion that the PCBM network becomes more heterogeneous and better connected would remain unchanged. Thus, these results show that, like hole transport, electron Nano Lett., Vol. 9, No. 8, 2009

Figure 3. Dark current images taken with a -5 V tip bias on P3HT/PCBM blend devices show the evolution with annealing at 110 °C for 0, 2, and 30 min as indicated.

injection and transport pathways are extremely spatially heterogeneous in P3HT/PCBM films and that these local variations increase during thermal annealing. Having established that carrier transport in the dark shows increasing degrees of spatial heterogeneity as P3HT/PCBM films are annealed no matter what the tip bias direction, we now turn to the evolution of the photocurrent with annealing. Figure 4 shows high-resolution photocurrent maps obtained using pcAFM28,35 at short-circuit conditions (zero tip-sample bias) for a series of 1.0/0.9 P3HT/PCBM blends annealed at 110 °C for times from 0 to 30 min. These images provide several insights. First, consistent with Figure 1B, the maximum and average local photocurrent both increase significantly as the films are annealedseven after annealing for only 1 min at 110 °C, the photocurrent has already doubled in some locations (Figure 4C). Second, the images show an increase in the fine wormlike structures with ∼10 nm pitch that are most visible at intermediate annealing times (Figure 4B-D). These regions of the current images strongly resemble the wormlike surface morphologies that have been reported in other studies of P3HT/PCBM blends.20 The pcAFM images also show a general increase in the area of relatively featureless high-photocurrent regions with annealing time (Figure 4E,F). As we can see from the local current-voltage curves in Figure 5, our use of a high-work function tip for pcAFM results in smaller open-circuit voltages and short-circuit currents in the reverse direction compared to devices with thermally evaporated top contacts. We thus attribute the featureless regions of high photocurrent in Figure 4E,F to regions of efficient hole extraction resulting from the growth of a contiguous P3HT layer at the film-air interface. Similar polymer wetting layers at the film-air interface have also been observed for copolymer/fullerene blends following solvent annealing.35 Although contact effects prevent quantitative comparisons between the scanning probe and device measurements, the Nano Lett., Vol. 9, No. 8, 2009

agreement between overall qualitative trends is good. We have already highlighted the agreement in the trends between the average photocurrents measured via pcAFM and the EQEs of the conventional devices as the films are annealed (Figure 1B). It is thus interesting to compare the images of the local electron and hole transport networks collected in the dark, with the local photocurrents measured under illumination. Figure 6 shows the topography (Figure 6A), dark hole current measured at +5 V tip bias (Figure 6B), dark electron current collected at -5 V tip bias (Figure 6C), and photocurrent collected at short circuit conditions (Figure 6D) for the exact same area of a P3HT/PCBM blend film that was annealed for 2 min at 110 °C. Given the expected correlation between carrier transport and photocurrent collection (which should hold regardless of collection direction), what is most striking about the images in Figure 6C,D is the apparent lack of correlation between the local photocurrent distribution and the local dark electron and hole currents. Indeed, when the images are overlaid (Figure S5 in Supporting Information), it appears that the photocurrent is to some degree even anticorrelated with the transport networks. This is a significant observation: when measured with cAFM, the regions of maximum dark hole current, maximum reverse bias (electron) current, and maximum photocurrent that develop upon annealing are not spatially correlated. Given the expected correlation between charge transport and photocurrent, this lack of correlation is surprising at first glance but makes sense upon further reflection. The regions of the film that give the largest local dark hole currents will be those that support facile hole injection and well-connected pathways of P3HT almost completely from one film surface to the other. Likewise, the regions of largest local electron currents will be those where lower barriers to injection meet well-connected pathways of PCBM through the bulk of the film. On the other hand, the largest photocurrents would not necessarily be associated with such large, electrode spanning 2949

Figure 4. Short circuit pcAFM maps show the evolution of the photocurrent distribution when excited with 532 nm laser light as the P3HT/PCBM blend films are annealed for (A) 0 s, (B) 30 s, (C) 60 s, (D) 2 min, (E) 10 min, and (F) 30 min. Each image is associated with a current histogram. The white letters (a, b, and c) in (D) refer to the location where I-V measurements were made, shown in Figure 5.

domains but rather would be associated with the interfacial regions between them or with regions that show some preferential vertical as well as lateral orientation. While straightforward, this conclusion nevertheless has important implications for how we interpret device-scale transport measurements on bulk-heterojunction solar cells. For instance, currents and carrier mobilities that are measured on a phase-separated device will be preferentially weighted toward those subsets of conduction pathways that penetrate the entire film thickness. While the structures and mobilities in these pathways may be correlated with those regions which are responsible for the photocurrent, we caution that such a correlation might not always exist. This conclusion could also explain why the EQE and power conversion efficiency can begin to level off at 2950

annealing times before the hole mobility and electron mobility have reached their maximum values (Figure S8C in Supporting Information). Although such a result is surprising given the expected positive relationship between carrier mobility and quantum efficiency,36 this lack of perfect correlation can be explained because the increases in electron and hole mobilities that are measured in devices at longer annealing times are occurring with an increase in the number of conductive pathways that substantially penetrate through the entire thickness of the device as the morphology ripens. We have used cAFM and pcAFM to image the spatial evolution of the local dark current distribution under forward and reverse bias and the local short-circuit photocurrent distribution in P3HT/PCBM blends as a function of annealing. The local currents show good Nano Lett., Vol. 9, No. 8, 2009

Supporting Information Available: Power dependence of pcAFM photocurrent, large scale pcAFM images, electron current images on pure PCBM films, overlay of dark cAFM with pcAFM images for Figure 6, additional images and I-V curves for films at different annealing times, absorption spectra of the films as a function of annealing time, SCLC data and fits, quantum efficiency, and power conversion efficiency as a function of annealing time. This material is available free of charge via the Internet at http://pubs.acs.org. References Figure 5. Current-voltage curves taken with the cAFM in the dark at the locations indicated by the letters in Figure 4D. The inset shows current-voltages curves taken at the same locations under 532 nm illumination.

Figure 6. (A) AFM topography, (B) pcAFM short circuit photocurrent, (C) dark (hole) current at +5 V tip bias, and (D) dark current at -5 V bias for the same region of a P3HT/PCBM blend annealed for 2 min at 110 °C.

qualitative agreement with corresponding values measured on macroscopic photodiodes. We find that both the total currents and their spatial variation increase with annealing. Under optimal annealing conditions, charge transport is extremely heterogeneous on the 10-100 nm length scale, and there is little direct correlation between the electron transport network, hole transport network, and photocurrent collection network. These results indicate that the imperfect internal quantum efficiency of some blends may be linked to morphological heterogeneity and suggest that truly 3D models of charge transport and recombination34,37,38 are needed to accurately capture the device physics of bulk-heterojunction solar cells. Acknowledgment. The authors acknowledge the support of the NSF (DMR 0449422 and 0120967) for supporting development of the pcAFM instrument and DOE BES for supporting these specific experiments. D.S.G. thanks the Camille Dreyfus Teacher-Scholar Awards Program for support. D.S.G. is a Cottrell Scholar of Research Corporation and an Alfred P. Sloan Foundation Research Fellow. O.G.R. acknowledges support from an IGERT Fellowship Award NSF #DGE0504573. L.S.C.P. acknowledges the support of the NSF Discovery Corps Fellowship program (CHE 0725139). Nano Lett., Vol. 9, No. 8, 2009

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