Critical Electron Transfer Rates for Exciton Dissociation Governed by

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Critical Electron Transfer Rates for Exciton Dissociation Governed by Extent of Crystallinity in Small Molecule Organic Photovoltaics Susan Spencer, Jeremy Alan Cody, Scott T. Misture, Brandon Cona, Patrick Heaphy, Garry Rumbles, John D Andersen, and Christopher J. Collison J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 17 Jun 2014 Downloaded from http://pubs.acs.org on June 24, 2014

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The Journal of Physical Chemistry

Critical Electron Transfer Rates for Exciton Dissociation Governed by Extent of Crystallinity in Small Molecule Organic Photovoltaics Susan Spencera, Jeremy Codya, Scott Mistureb , Brandon Conaa , Patrick Heaphya, Garry Rumblesc, John Andersena, Christopher Collison*a a

1 Lomb Memorial Drive Rochester Institute of Technology, Rochester, NY, 14623, USA

b

Inamori School of Engineering, Alfred University, Alfred, NY, 14802, USA

c

Renewable and Sustainable Energy Institute, National Renewable Energy Laboratory, Golden, CO, 80401, USA Corresponding author: Christopher Collison, [email protected], 1.585.475.6142

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Critical Electron Transfer Rates for Exciton Dissociation Governed by Extent of Crystallinity in Small Molecule Organic Photovoltaics Abstract Solution-processed bulk heterojunction organic solar cells fabricated with 1,3-Bis[4-(N,Ndiisopentylamino)-2,6-dihydroxyphenyl]squaraine and phenyl-C61-butyric acid methyl ester were found to exhibit unexpectedly low external quantum efficiency in the squaraine regions upon annealing. X-ray diffraction (XRD), spectral response, and time-resolved microwave absorption were all used to characterize the materials used and the devices prepared from them. An explanation for the drop in efficiency is proposed using Marcus-Hush Theory to tie together the changes in coherent crystal domain size found by XRD and the external quantum efficiency results. Exciton dissociation at the interface was determined to be the rate-limiting step in efficient current generation for these devices. Keywords: aggregation, thin films, nanocrystals, squaraines


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INTRODUCTION Organic photovoltaic solar cells [OPV] offer high potential as an inexpensive alternative energy source1–4 but present significant challenges for commercialization and development5,6. One such challenge is to better understand the process of charge photogeneration. As part of ongoing fundamental work to address charge photogeneration7–9, materials must be selected for device inclusion that will both increase the functionality and efficiency of the device while allowing for comprehensive study of the processes which successfully generate photocurrent in an OPV. Squaraines, including DiPSQ(OH)2, have a propensity to form crystalline domains with nanoscale aggregate character10–13 and are very strong absorbers in the near infra-red (NIR) when spin-cast in thin films12,14,15. These characteristics, combined with high solubility, photostability, and ease of synthesis, make squaraines promising candidates for single junction NIR devices and NIR layer components for tandem cells16–19. This work will further demonstrate the role that squaraines play in terms of developing a better mechanistic understanding of small-molecule OPV devices such that limiting charge photogeneration challenges can ultimately be overcome. The

materials

used

in

this

study

are

1,3-Bis[4-(N,N-diisopentylamino)-2,6-

dihydroxyphenyl]squaraine [DiPSQ(OH)2] for the absorbing electron donor material and phenyl-C61butyric acid methyl ester [PCBM] as the electron acceptor. Figure 1 shows the molecular structure of the materials. The detailed synthesis for the squaraine has been published elsewhere20.

Figure 1. Molecular structure of the donor (L) and acceptor (R). 3 ACS Paragon Plus Environment


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In this paper, we show that increasing the annealing temperature in our films results in enhanced absorption and crystallinity, quantified using solid-state absorbance, thin-film x-ray diffraction (TFXRD). Annealing should thus have a positive impact on exciton generation and diffusion efficiencies21–23 as well as free charge carrier generation and charge collection efficiencies24,25. Localized charge carrier generation is measured using transient microwave photoconductivity. Despite the expected positive impact, when the spectral response of devices made with and without annealing is examined, the results indicate that annealing has a deleterious effect on the external quantum efficiency (EQE) of the squaraine region. After examining each potential cause for the drop in efficiency, we hypothesize that poor EQE results, measured after annealing, result from a drop in the overall rate of electron transfer during the exciton dissociation. This could occur from either a decrease in the driving energy for electron transfer or a change in the electronic coupling matrix element between the reactant state and the product state as described by Marcus-Hush theory26–28. Both changes could also occur simultaneously and work in concert or opposition to each other; in this work we consider the origins of these changes. We find annealing causes no phase separation of the small molecules, but instead a subtle change in the interplanar spacing and size of the donor crystals, and propose a resulting displacement of the normal phonon mode oscillation and a change in the dipole-dipole interaction environment. This will change the energy level of the charge transfer state, resulting in a change in driving energy for electron transfer, as well as a change in the electronic coupling. Therefore, despite the positive effects of enhanced NIR absorption and crystallization, the limiting factor in our EQE measurement is electron transfer during exciton dissociation. This determination could help refocus research efforts onto materials which have high potential for larger electron transfer rates, given that it can play such a significant role as a bottleneck in bulk heterojunction organic photovoltaic cells.


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EXPERIMENTAL METHODS Materials and General Techniques. Reagent grade solvents were used as purchased from Sigma Aldrich without further purification. Absorbance measurements were performed on a Shimadzu High Resolution UV-2401 spectrophotometer with solutions measured in a quartz cell of path length 1 cm. Evaporative deposition was performed on an Angstrom Engineering COVAP II at a rate of 3.0 Å s1

. Thin-Film X-Ray diffraction (TFXRD) was performed on a Theta-Theta Siemens D500 with custom

diffraction furnace and gas controllers. TFXRD data are shown for specific peaks corresponding to squaraine crystallinity. This was determined by scans on controls [substrate/Indium tin oxide (ITO), substrate/ITO/PEDOT:PSS, substrate/ITO/PCBM and substrate/ITO/PEDOT:PSS/PCBM] to ensure that the peaks examined were those of the squaraine. Values of τ, the coherent crystal domain size, were calculated using the Scherrer equation29. For all absorbance and TFXRD measurements, films were prepared with identical processes. Multiple measurements for a given property were taken for each film, and the reported results are averaged over these films with error bars shown. In the case of TFXRD, the error bars are smaller than the symbol. External quantum efficiency data are shown both “raw” and analyzed. The raw data are averaged over multiple films. The analyzed data (Figure 5) are the EQE raw averaged data normalized for absorbance efficiency as measured by % transmittance of the device. Transient Microwave Photoconductivity. Experimental details of the technique are described in detail elsewhere8,30. Measurements were performed on the flash-photolysis time-resolved microwave conductivity experimental setup at the National Renewable Energy Laboratory. All transient data reported were fit with a bi-exponential function convoluted with the instrument response function (IRF). The IRF was modeled with a Gaussian laser pulse convoluted with the known single exponential decay time of the microwave cavity. All fitting was performed in Igor Pro 6 with a global analysis technique to fit all transients at varying light intensities for a given sample. The reported figure of 5 ACS Paragon Plus Environment


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merit, φΣµ, is calculated from the change in photoconductance at t=0 extracted from the global analysis fit. This allows direct comparison of signal magnitudes, without interference from differing decay dynamics. Device Preparation and Spectral Responsivity Measurement. DiPSQ(OH)2 and PCBM, whose structures are presented in Figure 1, were dissolved together in chloroform with sonication to achieve weight percent ratios of 3:7 DiPSQ(OH)2 to PCBM. ITO glass slides, (24 mm)2 in size, were cleaned with consecutive sonication in acetone and isopropanol. On top of the clean ITO surfaces, a buffer layer of PEDOT-PSS solution (Baytron PH, σ = 10-100 S cm-1, diluted 1:1 with deionized water) was spin-coated at 5000 rpm. The PEDOT-PSS films were then annealed at 160 ˚C for 30 minutes. All spin-coating of DiPSQ(OH)2:PCBM films was performed in an inert (N2) environment, with optimized spin speed/time of 700 min-1 for 18 s. The films were then allowed to dry in inert atmosphere at ambient temperature for 1 h before deposition in high vacuum of a 5-10 nm thick layer of tris-(8-hydroxyquinoline)aluminum (Alq3) followed by deposition of a 150 nm thick aluminum top electrode. Current density-Voltage (J-V) characteristics of the devices were obtained in the dark and under simulated 1 sun, 100 mW cm-2 power density, provided by a Newport 91159 Full Spectrum solar simulator with xenon lamp that had been calibrated with an international measurement round-robin InGaAs photovoltaic cell fabricated at the NASA Glenn Research Center, Photovoltaics Branch 5410, Cell #2X0104-0707 NASA that was independently calibrated prior to shipment. The devices were tested immediately after evaporation of the aluminum contacts. The devices retain approximately 20% of their original efficiency after two weeks in ambient conditions, but their potential for high stability has not been completely evaluated and will therefore not be reported in this work. Spectral responsivity was measured on an Optronic Laboratories 750 spectroradiometric measurement system with a high intensity source attachment using a quartz halogen bulb, and pre-calibrated against a Si standard. The system uses a chopped source and lock-in phase sensitive detection. Optical filters were modified 6 ACS Paragon Plus Environment

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slightly from the supplied system to eliminate small but non-negligible second order diffraction from leaking through the system at undesired wavelengths. RESULTS The absorbance spectrum of a DiPSQ[OH]2:PCBM film is presented in Figure 2 along with a corresponding absorbance spectrum of the squaraine in dilute chloroform solution. Three distinct features are observed for the film and are assigned to respectively, a J-mer peak centered at 748 nm, an H-mer peak centered at 564 nm, and a monomer peak centered at 679 nm, which is shifted from the corresponding monomer peak in solution at 648 nm. These assignments are described in more detail elsewhere10,12. The colored background highlights the distinctness of the states contributing to absorbance for each of these three regions.

Figure 2. Absorbance spectra of the DiPSQ[OH]2:PCBM blend, both pristine (solid line) and annealed (dashed line) as well as solution (dotted line). The regions illustrated within the plot correspond to the assigned H-mer region (blue diagonal lines), monomer region (green vertical lines), and J-mer region (red horizontal lines). The spectral properties of this and similar squaraine films have been shown to change as a function of heat treatments for blended films, as illustrated by the dashed line in Figure 2. Corresponding absorbance peak height data from a series of DiPSQ[OH]2:PCBM thin films are shown 7 ACS Paragon Plus Environment


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in figure 3, illustrating an increase in peak height for the J-mer upon annealing and the concurrent decrease in peak height for the monomer regions. The increasing J-mer/decreasing monomer trend has been shown to correlate with an increase in film crystallinity measured with thin film X-ray diffraction20,31. The increase in J-aggregate population was expected to improve device efficiency, given recent observations that suggest J-aggregation empirically leads to increased power conversion efficiencies10,32. In addition, increasing the NIR absorbance of the films is a known method for improving efficiency, given that larger currents can theoretically be obtained in spectral regions of high photon flux in the solar spectrum. Thus, both the increased absorbance and the enhanced crystallinity after annealing were expected to benefit the device efficiency.

Figure 3. Absorbance peak height values for the three spectral features of interest (J-mer, Monomer and H-mer) are shown as a function of increasing annealing temperature. A rigorous in-situ XRD measurement was made to study the changes taking place during the annealing process in a blended film, analogous to a device. The results are shown in Figure 4. All data were collected from the 7.5° 2θ peak generated specifically in the squaraine crystalline regions. Thus, only the changes in squaraine crystallinity are considered when the peak height, interplanar spacing, and coherent crystal domain size are calculated. The impact of annealing on the squaraine is illustrated through collective analysis of Figures 3 and 4. An increase in the J-mer absorbance and a decrease in 8 ACS Paragon Plus Environment

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the monomer absorbance are observed in figure 3, for an overall increase in absorbance of ~6% from pristine film to a film annealed at 180 °C. There is also a significant increase (~67%) in extent of crystallinity of the squaraine with increasing temperature, as seen in Figure 4, associated with the peak height increase. In addition, increasing annealing temperature leads to an increase in size for the crystalline regions in the squaraine blend as evidenced by the increase in τ, while the interplanar spacing of the squaraines decreased20. After cooling the annealed film the increased crystallinity, domain size and reduced interplanar spacing remains as demonstrated in the second set (open symbols) of data recorded at 30°C.

Figure 4. TFXRD data taken on a device analogue film of DiPSQ[OH]2:PCBM. The LHS red y-axis corresponds to the interplanar spacing (red circles), the inner RHS black y-axis corresponds to the peak height (black squares), and the outer RHS blue y-axis corresponds to the coherent crystal domain size, τ (blue triangles). The open symbols for each data set represent the measurement taken after the annealing, once the films had been cooled back to 30°C. As introduced above, this enhancement of crystallinity is expected to result in better exciton diffusion efficiency and increased free charge carrier mobility in the squaraine region. Both exciton diffusion efficiency and free charge carrier mobility will be enhanced because there will be fewer 9 ACS Paragon Plus Environment


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defect sites for recombination33. AFM results on similar films, published elsewhere20, demonstrate a decreasing RMS roughness of the organic active layer surface with increasing annealing. Since AFM is measuring the surface upon which the aluminum electrode will be deposited, a decreasing RMS roughness will correlate with a smoother deposition of the contact. A smoother interface between organic and electrode would reduce the amount of shunting thereby enhancing the charge collection efficiency34 and also contributing to an expected increase in power conversion efficiency. In summary, exciton generation and diffusion, charge transfer state dissociation, and charge collection should all be improved with enhanced crystallinity and aggregation and efficiency is expected to increase. External quantum efficiency (EQE) data for pristine and annealed devices were measured to test for the expected increase in efficiency with annealing, and are shown in Figure 5. A significant drop in EQE for the squaraine region (510-800 nm) is shown for the annealed film. It is also important to note that there is very little change in the PCBM region (300-450 nm). The simplest explanation for a loss of EQE with annealing and consequent crystallization would be phase separation, associated with a reduction of total surface area of heterojunction. However, since the PCBM EQE does not change significantly upon annealing, this explanation is inconsistent; if phase separation tended towards a bilayer geometry then charge generation resulting from photoexcited PCBM excitons would similarly be reduced because of the smaller heterojunction surface area. Instead, there is only a dramatic decrease in charge generation resulting from squaraine absorption but no change associated with PCBM absorption.


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Figure 5. The black line shows the as-cast DiPSQ[OH]2 device EQE. The red dashed line is an annealed device at 165°C, and the blue dotted line is an annealed device at 185°C. Given that the absorption of the film changes with the annealing process, it is instructive to look at an EQE plot that is corrected and normalized to the absorption and this data is shown in Figure 6. Despite correcting the EQE for the absorbance changes (a drop in the monomer region and an increase in the J-mer region), there is still a significant difference between the pristine films and the annealed films. This further indicates that the loss in efficiency in the squaraine region is not due to changes in absorbance and, again, enhancing crystallinity would be expected to produce favorable results for the other steps, as described above.

Figure 6. EQE data, normalized using the calculated absorbance efficiency of the squaraine. 11 ACS Paragon Plus Environment


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We conclude, therefore, that the decrease in performance is not due to changing populations of excited states and that it does not result from changes in charge collection efficiency or exciton diffusion efficiency since improved crystallinity would undoubtedly enhance the associated efficiency – not reduce it. The two remaining efficiency contributors to examine are exciton dissociation efficiency and the resultant charge carrier generation efficiency. Charge carrier generation efficiency was measured for a blended film with very low levels of PCBM (

Critical Electron Transfer Rates for Exciton Dissociation Governed by

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