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Mapping the Competition between Exciton Dissociation and Charge Transport in Organic Solar Cells Soong Ju Oh, Jong Bok Kim, Jeffrey M. Mativetsky, Yueh-Lin Loo, and Cherie R. Kagan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07810 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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ACS Applied Materials & Interfaces

Mapping the Competition between Exciton Dissociation and Charge Transport in Organic Solar Cells Soong Ju Oh∥1,6, Jongbok Kim∥4,7, Jeffrey M. Mativetsky4,8, Yueh-Lin Loo4,5, Cherie R. Kagan1,2,3* 1

Department of Materials Science and Engineering, 2Department of Electrical and System

Engineering, 3Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA. 4

Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ

08544, USA, 5

Andlinger Center for Energy and the Environment, Princeton University, Princeton NJ 08544,

USA 6

Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea

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Department of Materials Science and Engineering, Kumoh National Institute of Technology,

Gyeongbuk 39177, Korea 8

Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton,

NY 13902, USA

ACS Paragon Plus Environment

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∥These authors equally contribute to the work.

KEYWORDS. Organic Solar Cells, Charge transport, Exciton Dissociation, Scanning Confocal Photoluminescence Microscopy, Scanning Photocurrent Microscopy

ABSTRACT.

The competition between exciton dissociation and charge transport in organic solar cells comprising poly(3-hexylthiophene) [P3HT] and phenyl-C61-butyric acid methyl ester [PCBM] is investigated by correlated scanning confocal photoluminescence and photocurrent microscopies. Contrary to the general expectation that higher photoluminescence quenching is indicative of higher photocurrent, microscale mapping of bulk-heterojunction solar-cell devices shows that photoluminescence quenching and photocurrent can be inversely proportional to one another. To understand this phenomenon, we construct a model system by selectively laminating a PCBM layer onto a P3HT film to form a PCBM/P3HT planar junction on half of the device and a P3HT single junction on the other half. Upon thermal annealing to allow for interdiffusion of PCBM into P3HT, an inverse relationship between photoluminescence quenching and photocurrent is observed at the boundary between the PCBM/P3HT junction and P3HT layer. Incorporation of PCBM in P3HT works to increase photoluminescence quenching, consistent with efficient charge separation, but conductive atomic force microscopy measurements reveal that PCBM acts to decrease P3HT hole mobility, limiting the efficiency of charge transport. This suggests that


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photoluminescence-quenching measurements should be used with caution in evaluating new organic materials for organic solar cells.

INTRODUCTION Thin-film organic solar cells (OSCs) have attracted much attention due to their potential application as low-cost, high efficiency, and flexible solar energy-conversion devices.1,2 Early OSCs were fabricated via sequential stacking of thin films of a polymeric electron donor and a derivatized fullerene electron acceptor, forming a planar-heterojunction (PHJ) active layer. However, PHJ OSCs have low interfacial areas between the electron donor and acceptor, limiting exciton-dissociation and therefore power-conversion efficiency. To enhance exciton-dissociation and power-conversion efficiencies in OSCs, bulk-heterojunction (BHJ) active layers were introduced. In the BHJ architecture, the photoactive layer comprises an electron donor and an electron acceptor that is intermixed at the nanoscale, creating a more extensive donor-acceptor interface that increases the exciton-dissociation efficiency in comparison to the active layers in PHJ devices.3,4 To evaluate the exciton-dissociation efficiency in OSCs, which operate far below the radiative limit as exciton-dissociation is followed by rapid, non-radiative recombination through a manifold of charge transfer states, photoluminescence (PL) quenching measurements are commonly used.5– 8

PL quenching measurements indirectly infer that a reduction in the radiative recombination of

electrons and holes arises from an increase in charge transfer and therefore macroscopic photoconductivity. Indeed, researchers have observed that higher PL quenching generally induces higher photocurrent (PC) in thin films having various donor and acceptor concentrations in


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photoactive layers of solar cells.9,10 However, the relationship between PL quenching and PC has not been precisely established as they are typically probed in different devices. It is therefore questionable whether PL quenching is a suitable predictor for the performance of OSCs comprising new materials. Here, we study the relationship between PL quenching and PC by simultaneously mapping and correlating their signals in the same device through a combination of scanning photocurrent microscopy (SPCM) and scanning confocal photoluminescence microscopy (SPLM). SPCM and SPLM have been utilized to understand the structure, phase, and composition of organic solar cells.11–14 Using these techniques, we first probed a BHJ OSC of poly(3-hexylthiophene) [P3HT] and phenyl-C61-butyric acid methyl ester [PCBM]. In contrast to the generally accepted relationship that a reduction in PL is reflective of higher PC,8 we observe PL quenching to be inversely proportional to PC. To elucidate the origin of this unexpected observation, we designed a model structure in which we laminated a PCBM thin film atop a portion of a uniform P3HT thin film, leaving the remaining portion of P3HT exposed. SPCM and SPLM were conducted across this same laminated interface. We find PL quenching and PC to be inversely proportional at the boundary between the P3HT-only region and the P3HT/PCBM stack, where PCBM has diffused into P3HT upon thermal annealing such that this stack resembles a BHJ.15–18 We carried out conductive atomic force microscopy (C-AFM) to map the hole mobility.19 These experiments reveal that the competition between charge transport and charge separation is responsible for this inverse relationship between PC quenching and PL and implicates PL quenching to not necessarily be an accurate indicator of OSC performance.

EXPERIMENTAL SECTION


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To study the relationship between PL and PC, we first fabricated a P3HT:PCBM BHJ inverted solar cell with an efficiency of 2.0%.13 We have opted to build our devices in the inverted architecture because the gold contacts employed in this architecture are more stable than the aluminum contacts that would have been used in the conventional architecture. In our hands, P3HT:PCBM bulk-heterojunction devices of both the conventional and inverted architectures yield comparable power-conversion efficiencies of 2-3%.20–22 To construct OSCs with a BHJ architecture, pre-patterned ITO substrates (15 Ω/sq.; Colorado Concept Coatings) were cleaned with acetone, isopropyl alcohol and deionized water by sonication. After drying in a nitrogen stream and further cleaning by UV/Ozone for 10 min, we spin-coated a solution of 1 wt% titanium isopropoxide in isopropyl alcohol on the ITO substrates at 3000 rpm for 30 s. The precursor films were allowed to hydrolyze in air for 1 hr at room temperature followed by 10 min at 170 oC to yield 40-nm thick titania electron transport layers.21 A co-solution was prepared by dissolving P3HT and PCBM at an equimass ratio to form a 2.4 wt% solution in chlorobenzene. Then, the cosolution of P3HT and PCBM was deposited by spin-coating on the titania-coated ITO substrates at 500 rpm for 60 s and thermally annealed at 150 oC for 1 min.23 Thermal evaporation of gold through a stencil mask completed the construction of P3HT:PCBM BHJ OSCs with active areas of 0.18 cm2. The fabricated devices were subsequently sealed by epoxy under a cover glass slip in a nitrogen glove box to limit possible oxidation upon air exposure during measurements. SPCM and SPLM measurements were carried out by illuminating the OSC through the transparent ITO top window of the device with 488-nm light from an Innova 70C spectrum Ar:Kr laser focused to a 1 µm diameter spot using a 0.3 NA objective lens. Figure 1 (A) depicts the experimental set-up used to simultaneously measure and map PC and PL. The device was mounted on a stage with stepper motors (Thorlabs BSC102) and, at zero applied voltage and under focused


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illumination, was scanned across a 100 µm x 100 µm area with a step size of 0.5 µm while simultanesouly measuring PC and PL. The short-circuit photocurrent (PC) was recorded by a source measure unit (Keithley 2400). PL was collected through the same objective lens as the excitation, filtered by a 488 nm super notch filter, captured confocally by a fiber, relayed to a monochrometer, and recorded by a CCD camera. Forward and reverse scans were conducted to rule out possible aging or bias stress effects.

RESULTS AND DISCUSSION Figure 1 (B) is a schematic of the OSC device and Figure 1 (C) shows the PC and PL intensity as a function of position as the devices is scanned along the x-axis, as illustrated in Figure 1 (B). As seen in Figure 1 (C), the region that yields higher PC generally shows higher PL. This is in direct contrast to the expectation that higher PL quenching arises from more efficient charge separation and should therefore yield higher PC.5 Because it is possible for this unexpected commensurability between PC and PL in the PCBM/P3HT BHJ solar cell to result from local differences in light absorption and exciton generation, we probed a model system22,24. The model system comprises a PCBM/P3HT PHJ on half of the device and a P3HT single junction on the other half, shown in Figures 2 (A) and (B). In the model system, we expected comparable light absorption and exciton generation across the device because the amount of P3HT, which is the main light absorber in this materials pair, is the same over the entire sample. To fabricate the model system comprising a P3HT/PCBM PHJ and a P3HT single junction, we spin-coated PEDOT:PSS (Clevios P) at 2000 rpm for 120 s on prepatterned ITO substrates and annealed the samples at 150 oC for 20 min. Then, 1 wt% P3HT in chlorobenzene was deposited by spin-coating at 1000 rpm for 1 min. We laminated and transferred


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a thin layer of PCBM (45 nm) partially onto the P3HT film to create a bilayer step junction. 17 P3HT was chosen as the underlayer to provide film uniformity across large areas. The PCBM thin film was prepared by spin-coating a 0.5 wt% PCBM solution in chloroform at 3000 rpm for 60 s onto a UV/Ozone-treated Si wafer. A PDMS stamp was prepared, by mixing the PDMS precursor (Sylgard 184A) and curing agent (Sylgard 184B) at a 10:1 weight ratio, and curing it at 80 oC for 4 h. The cured PDMS stamp was placed into direct contact with the PCBM/Si assembly. Then the assembly was immersed in water. The water penetrated the interface between the hydrophobic PCBM and the hydrophilic UV/Ozone-treated Si wafer, allowing the Si wafer to be removed from the PCBM/PDMS stack and the PCBM layer to be transferred onto the PDMS stamp. 25 We then selectively laminated PCBM on half of the P3HT layer on the PEDOT:PSS-coated ITO substrate, forming a P3HT/PCBM planar-heterojunction on the left (L)-side and leaving P3HT exposed on the right (R)-side of the step junction. After removing the PDMS stamp, we evaporated 80-nm thick aluminum through a stencil mask to complete the fabrication of the patterned OSC device. We studied this model system before and after annealing at 130 ºC for 3 min. The device performance of PCBM/P3HT PHJ and P3HT-only Schottky device are shown in Supporting Information Figure S1. We denote the PCBM/P3HT PHJ side by the L-side, the P3HT-only layer by the R-side, and the interfacial region (in which interdiffusion of P3HT and PCBM occurs during thermal annealing) by the I-zone.15,26 To confirm the model system is constructed as expected, we examined it using energy dispersive x-ray (EDX) elemental analysis. EDX mapping (Figure 2) is shown (C) before and (D) after annealing. Before annealing, a dramatic change in carbon content is seen across the interface because PCBM is substantially more carbon-dense than P3HT. The interfacial width over which the carbon content decreases is approximately 5 μm. After annealing, interdiffusion of PCBM and


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P3HT occurs over ~10 μm and an extended I-zone15,26 forms across the mapped region from -5 μm to 5 μm. Interdiffusion of PCBM occurs over very long micron-scale distances within a few minutes of annealing at 100 - 150 Cº,15 resulting in the carbon concentration gradient seen in the EDX measurements. Therefore, the I-zone structurally resembles a BHJ, though with a gradient of donor:acceptor composition across the zone. We mapped the PC and PL across the junction along the x axis, as depicted in Figure 2 (A). Figure 3 shows representative line scans (A) before and (B) after thermal annealing tracking the PC (black) and PL (red) across the interface. The PL spectra of the samples are shown in Supporting Information Figure S2. Before thermal annealing, the R-side shows lower PC and higher PL than the L-side, consistent with photoexcitons recombining radiatively rather than undergoing dissociation in the absence of PCBM. In the presence of PCBM on the L-side, PL quenching and charge separation are, in comparison, more efficient. After annealing, a similar phenomenon is observed on the L-side and on the R-side away from the junction, i.e., higher PL is correlated with lower PC. However, the behavior of PC and PL is interesting in the I-zone. Before thermal annealing [Figure 3 (A)], the amplitudes of the PC and PL signals are inversely proportional to one another in the I-zone. In Figure 3 (B), we observe that the annealing extends the I-zone over a larger region across the junction and in contrast, the PC and PL signals are now directly proportional to one another. As the device is translated from -5 µm to 5 µm [Figure 3 (B)], the PCBM content decreases gradually [Figure 2 (D)], and the PL increases as the probability of charge separation is reduced. However, in this region the PC also increases even though more excitons decay. Although PL quenching is commonly accepted as a measure of charge separation and therefore one would expect a concomitant increase in PC in organic heterojunction thin films, here we observe that


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increases in PC need not come at the expense of PL; the signal amplitudes of both can increase simultaneously. To understand why the left side of the I-zone with higher PCBM content and PL quenching shows relatively low PC compared to right side of the I-zone, we turn to the contributing processes of photon-to-electron conversion efficiency (eqe): 1) light absorption (abs), 2) exciton dissociation (diss), 3) charge transport (ct) and 4) charge collection at the electrodes (cc).27,28 ηeqe = ηabs · ηdiss · ηct· ηcc Since Ohmic contacts are formed at both the ITO anode and Al cathode,29 we exclude effects of charge-collection efficiency at the electrodes and assume cc is the same. We next compare light absorption in both the L-side and R-side. The absorption on both sides of the junction should be comparable since P3HT is the dominant light absorber of the materials pair and PCBM contributes negligibly to light absorption, as seen in absorption spectra (Supporting Information Figure S3). Therefore, we can also exclude the effect of absorption and assume abs to be constant across the interface. We are left to focus on the contributions of exciton dissociation and charge transport to the overall efficiency of the OSCs. To quantify the transport properties, we carried out C-AFM, as seen in the inset in Figure 4(A). For this measurement, we prepared the same patterned device comprising a PCBM/P3HT PHJ on half of the device and P3HT only layer on the other half as described above, but without depositing the cathode. We used a Pt-Ir tip in the C-AFM experiments as the top electrode and measured the current passing through the depth of the active layer as we recorded current-voltage characteristics at multiple positions along the sample. The high work function of the electrical contacts ensures hole-only transport. Using the space-charge limited current model with a correction to account for the tip-sample geometry,30 we extracted the hole mobility as a function of position. As noted in


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Figure 4, the position at 0 m denotes the interface between P3HT only and P3HT:PCBM regions, with positive values indicating the P3HT-only region, as seen in the inset of Figure 4 (A). We are unable to extract the hole mobility in the x < 0 region of P3HT:PCBM region, as PCBM blocks hole transport. Before annealing [Figure 4 (A)], the hole mobility in the P3HT region is low, around 1 x 10-4 cm2/Vs, and is invariant with position. After annealing [Figure 4 (B)], the mobility increases to 5 x 10-4 cm2/Vs on the R-side (low PCBM:P3HT ratio), and then decreases as we move towards the interface, down to a value of 1 x 10-4 cm2/Vs at the interface (high PCBM: P3HT ratio). Thus, we expect hole mobilities lower than 1 x 10-4 cm2/Vs on the L-side where the PCBM:P3HT ratio is even higher. This observation is consistent with a previous report31 that as PCBM content decreases, the hole mobility increases, due to a less distortion of crystallinity of P3HT. As PC is defined as σph = e(nμe + pμh),32 the mobility33 is an important factor in determining the overall photovoltaic performance, where σph is photoconductivity, e is electric charge, μe and μe are the electron and hole mobilities, n and p are electron and hole concentrations, respectively. In the unannealed sample, the hole mobility is constant and therefore charge separation governs device efficiency. However after annealing, we hypothesize that while on the left side of the Izone (higher PCBM:P3HT ratio) the efficiency of charge separation is higher, the lower mobility for hole transport limits charge transport and therefore the overall photon-to-electron conversion efficiency. On the right side of the I-zone (lower PCBM:P3HT ratio), charge separation is less efficient, but the P3HT mobility is at least five times higher than the mobility in the left side. This observation is consistent with the observation that the PL and PC signals are high relative to their signals on the left side. The spatial variation of charge separation and hole mobility thus shows that donor and acceptor intermixing has competing effects on the charge separation and transport


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processes, limiting the photon-to-electron conversion efficiency. It should be noted that the proportionality between PC and PL does not stem from variations in film thickness as both signals are continuous over the scanned area and are inversely proportional prior to annealing. This conclusion is further supported by previous work showing that, among samples with different ratios of P3HT and PCBM34 or other similar blends,35 the greatest PL quenching does not always show the highest device efficiency. This assertion also agrees well with our observation that the PCBM:P3HT gradient in the I-zone creates a gradient in carrier mobility, resulting in a change in PC that is proportional to PL. Therefore, in BHJs, the correlation between PC and PL is attributed to the distribution of local mobility that results from the local variations in the PCBM:P3HT ratio. This idea of local heterogeneities affecting local charge-collection efficiencies agrees with literature reports that suggest charge separation may not be the limiting factor of the photovoltaic performance.36 While we have simplified the model and further study is needed to completely understand the quantitative contributions of exciton dissociation and charge transfer with respect to structural development that takes place on annealing, we believe this study explains the uncommon correlation between PC and PL observed in our experiments. It should be noted that the proportional behavior between PC and PL quenching is macroscopically observed in many donor-acceptor systems.37–43 More detailed analysis on other OSC systems having different charge transfer states has to be conducted to extend our findings.

CONCLUSIONS In conclusion, we studied the behavior of photoexcited carriers using a correlated spatially resolved photoconductivity and photoluminescence mapping technique and further investigated charge transport using conductive atomic force microscopy. We prepared bulk-heterojunction


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solar cells and patterned devices to study the fate of photoexcited carriers in OSCs. Unlike the conventional expectation where PC and PL are inversely proportional, we observe that PC and PL may be proportional to one another in BHJs. That is, regions with higher PCBM show more PL quenching due to efficient charge separation, but lower PC arising from the lower hole mobility in P3HT. We found this phenomenon to be consistent with intermixing of the donor and acceptor leading to competition between charge separation and transport. We conclude that in well-blended bulk-heterojnuction active layers, PL quenching cannot be directly used as a measure of PC or OSC efficiency, and that for efficient photon-to-electron conversion, all factors including charge separation and mobility need to be carefully considered.


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FIGURES

Figure 1. (A) Schematic of the correlated, scanning photocurrent microscopy and scanning confocal photoluminescence microscopy setup. (B) BHJ solar cell structure and (C) line scan (along x direction in Figure 1 (B)) mapping the microscopic PC and PL across a BHJ solar cell.


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Figure 2. (A) Schematic and (B) photograph of the PCBM/P3HT bilayer (left) and P3HT only step junction (right). EDX mapping the carbon content (C) before and (D) after annealing at 130 ºC for 3 min.


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Figure 3. Line scan of PC and PL on a patterned device (A) before and (B) after annealing at 130 ºC for 3 min.


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Figure 4. Hole mobility in P3HT only region as a function of distance from the interface between P3HT/PCBM and P3HT only regions (A) before and (B) after annealing. Inset (A) a schematic of conductive atomic force microscopy characterization of the junction.


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ASSOCIATED CONTENT Supporting Information. Device performance of solar cells and photoluminescence and absorption spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Corresponding to : [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT CRK and SJO are grateful for support of this work from the NSF under Award DMR-1309053 for correlated scanning photoluminescence and photocurrent microscopies of PCBM/P3HT junctions. YLL and JBK are grateful for funding from the Princeton Center for Complex Materials (DMR-1420541) and an EAGER grant (ECCS-1549619) through the National Science Foundation. JMM gratefully acknowledges support from the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry for conductive atomic force microscopy.


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(22) Kim, J. B.; Guan, Z.-L.; Shu, A. L.; Kahn, A.; Loo, Y.-L. Annealing Sequence Dependent Open-Circuit Voltage of Inverted Polymer Solar Cells Attributable to Interfacial Chemical Reaction between Top Electrodes and Photoactive Layers. Langmuir 2011, 27, 11265–11271. (23) Kim, C. S.; Lee, S. S.;Gomez, E. D.; Kim, J. B.; Loo, Y. -L. Transient Photovoltaic Behavior of Air-Stable, Inverted Organic Solar Cells with Solution-Processed Electron Transport Layer. Appl. Phys. Lett. 2009, 94, 113302 (24) Kim, J. B.; Guan, Z.-L.; Lee, S.; Pavlopoulou; Toney, M. F.; Kahn, A.; Loo, Y.-L, E. Modular Construction of P3HT/PCBM Planar-Heterojunction Solar Cells by Lamination Allows Elucidation of Processing–structure–function Relationships. Org. Electron. 2011, 12, 1963-1972. (25) Wang, H.; Gomez, E. D.; Kim, J.; Guan, Z.; Jaye, C.; Fischer, D. A.; Kahn, A.; Loo, Y. – L. Device Characteristics of Bulk-Heterojunction Polymer Solar Cells Are Independent of Interfacial Segregation of Active Layers. Chem. Mater. 2011, 23, 2020–2023. (26) Treat, N. D.; Mates, T. E.; Hawker, C. J.; Kramer, E. J.; Chabinyc, M. L. Temperature Dependence of the Diffusion Coefficient of PCBM in Poly(3-Hexylthiophene). Macromolecules 2012, 46, 1002–1007. (27) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736–6767. (28) Guo, J.; Ohkita, H.; Benten, H.; Ito, S. Charge Generation and Recombination Dynamics in Poly (3-Hexylthiophene)/fullerene Blend Films with Different Regioregularities and Morphologies J. Am. Chem. Soc. 2010, 132, 6154-6164.


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(43) Jamieson, F. C.; Domingo, E. B.; McCarthy-Ward, T.; Heeney, M.; Stingelin, N.; Durrant, J. R. Fullerene Crystallisation as a Key Driver of Charge Separation in Polymer/fullerene Bulk Heterojunction Solar Cells. Chem. Sci. 2012, 3, 485-492.


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Table of Contents Graphic


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Figure 1. (A) Schematic of the correlated, scanning photocurrent microscopy and scanning confocal photoluminescence microscopy setup. (B) BHJ solar cell structure and (C) line scan (along x direction in Figure 1 (B)) mapping the microscopic PC and PL across a BHJ solar cell. 229x204mm (150 x 150 DPI)


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Figure 2. (A) Schematic and (B) photograph of the PCBM/P3HT bilayer (left) and P3HT only step junction (right). EDX mapping the carbon content (C) before and (D) after annealing at 130 ºC for 3 min. 233x172mm (150 x 150 DPI)



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Figure 3. Line scan of PC and PL on a patterned device (A) before and (B) after annealing at 130 ºC for 3 min. 263x104mm (150 x 150 DPI)


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Figure 4. Hole mobility in P3HT only region as a function of distance from the interface between P3HT/PCBM and P3HT only regions (A) before and (B) after annealing. Inset (A) a schematic of conductive atomic force microscopy characterization of the junction. 250x112mm (150 x 150 DPI)



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TOC Graphics 237x108mm (150 x 150 DPI)


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