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Dielectric Screening to Reduce Charge Transfer State Binding Energy in Organic Bulk Heterojunction Photovoltaics Sibel Y. Leblebici, Jiye Lee, Alexander Weber-Bargioni, and Biwu Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12463 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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The Journal of Physical Chemistry

Dielectric Screening to Reduce Charge Transfer State Binding Energy in Organic Bulk Heterojunction Photovoltaics Sibel, Leblebici†‡* ; Jiye, Lee†; Alexander, Weber-Bargioni†; Biwu, Ma§ †

The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720,

United States. ‡

Department of Materials Science and Engineering, University of California, Berkeley, Berkeley,

California 94720, United States. §

Department of Chemical & Biomedical Engineering, FAMU-FSU College of Engineering,

Florida State University, Tallahassee, Florida 32310, United States. *[email protected] Lumiode, Inc., 1361 Amsterdam Ave. Suite 340, New York, NY, 10027, United States

ACS Paragon Plus Environment

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Abstract

Reaching high efficiencies in organic photovoltaics is often limited by dissociating charge transfer states at electron donor-acceptor interfaces due to their large binding energies and rapid geminate recombination. By adding a high permittivity small molecule, camphoric anhydride, at 20 weight percent to a polymer-fullerene bulk heterojunction photovoltaic devices, we increased the film permittivity, reduced the charge transfer state energy, and increased the power conversion efficiency by 75%. At higher concentrations, the camphoric anhydride begins to crystallize and phase separate from the bulk heterojunction, causing the permittivity and power conversion efficiency to decrease. Adding camphoric anhydride in low concentrations is an effective strategy to increase permittivity and reduce recombination at the donor-acceptor interface, and as a result increase organic photovoltaic efficiency in bulk heterojunction devices.


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Organic photovoltaics (OPVs) are suitable for flexible1,2, lightweight3,4, and inexpensive5–9 applications and have steadily improved the power conversion efficiency (PCE) to record efficiencies over 11%.10 However, the efficiency of OPVs is still limited by the high exciton binding energy (Eeb) in organic semiconductors that results from the low permittivity in these materials.11–15 Multiple excitonic stages are involved in the OPV photocurrent generation process: incoming light excites the donor or acceptor molecule to generate a Frenkel exciton; the exciton diffuses to the interface between the donor and acceptor materials; at the interface, the exciton dissociates to form a charge transfer (CT) exciton across the donor-acceptor interface. In order to generate free carriers that can be collected by the electrodes, the CT exciton, or CT state, must dissociate and the resulting electron and hole must have a low geminate recombination rate. The efficiency of this dissociation strongly impacts the short circuit current and is dependent on the CT state binding energy.16,17 Also, the open circuit voltage (VOC) is impacted by the CT binding energy because the energetic driving force for exciton dissociation results in an energy loss.18 Here we demonstrate that increasing the permittivity by incorporating camphoric anhydride (CA) into a polymer:fullerene bulk heterojunction (BHJ) OPV reduces the CT state binding energy and effectively increases the PCE by 75%. Exciton dissociation, the reciprocal of geminate recombination, as described by the OnsagerBraun model, is dependent on exciton lifetime, permittivity, distance between the electron and hole, local electric field, and Eeb.19 The Eeb itself is based on the Coulombic attraction of the electron and hole: ⁄4 , where e is the charge of an electron, is the relative permittivity, is the permittivity of vacuum, and a is the distance between the charges.17 There are two main approaches to manipulate the CT state dissociation efficiency: alter the main parameters in the Onsager-Braun model or directly reduce the CT state binding energy. For the


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former, existing approaches include doping the active layer with ferroelectric polymers or magnetic nanoparticles to create localized electric fields20–22. These approaches do result in improvements to solar cell performance, but the polling step may need to be repeated to maintain high CT state separation efficiencies. An alternative is to directly reduce the CT state binding energy. One method is to synthetically increase the dipole change between ground and excited states.23,24 Another prevalent method to decrease the CT state binding energy is to increase the permittivity of the active layer. Approaches to increasing the permittivity include doping with salts25, synthetically incorporating fluorine and high permittivity side chains in semiconducting polymers26–29, and increasing the concentration of fullerenes in the BHJ.30,31 Synthetic methods to reduce CT state binding energy often result in significant changes to semiconducting properties and the morphology of the BHJ film. Additionally, many of these approaches do not result in improved solar cell efficiency or do not differentiate between improvements due to changes in the CT state binding energy and BHJ morphology. Our strategy is to reduce the CT state binding energy by incorporating a high permittivity small molecule, CA, into the BHJ active layer while also monitoring the morphology. We have previously demonstrated this approach by reducing Eeb of Frenkel excitons in small molecule layers for planar heterojunction devices by blending a solution-processable small molecule with CA.32–34 We observed an increase in permittivity and internal quantum efficiency (IQE) when CA was added to the donor film, and we concluded that this enhancement is due to a decrease in Eeb of the donor Frenkel exciton. This approach to increase the permittivity should also impact the CT state binding energy29, which we were not able to measure in the previous work because the planar heterojunction device geometry induces low signal-to-noise ratio to detect the CT state energy.


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The active layers for OPV devices, permittivity measurements, and electroluminescence (EL) devices were made using the following process. poly[2- methoxy-5-(3′,7′-dimethyloctyloxy)-1,4phenylene vinylene) (MDMO-PPV), 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]methanofullerene (PCBM), and CA were purchased from H.W. Sands Corp., nano-c, and Alfa Aesar respectively. MDMO-PPV was dissolved in chlorobenzene by stirring overnight. PCBM and CA were dissolved in chlorobenzene and passed through a 0.2 µm filter. The MDMOPPV:PCBM:CA solutions were formed at a 1:1 weight ratio of MDMO-PPV:PCBM with CA added at the appropriate ratios to form solutions with various weight percent (wt%) CA. For all devices, pre-patterned indium doped tin oxide (ITO)-coated glass substrates, from Thin Film Devices Inc., were cleaned by successive sonication in soap solution, deionized water, acetone, and isopropanol for 15 min at 40 °C and UV ozone cleaned for 10 min. Poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) (Baytron PH 500) was spin cast onto the substrates at 4000 rpm and subsequently baked in air at 140 °C for 20 minutes. The MDMO-PPV:PCBM:CA solutions were spin coated onto the PEDOT:PSS layer to form the active layer. The OPV devices were fabricated in a BHJ structure of ITO/PEDOT:PSS (30 nm)/MDMOPPV:PCBM:CA (90 nm) / lithium fluoride (LiF) (7 Å)/Al (100 nm). To maintain a thickness of 90 nm for all concentrations of CA, the MDMO-PPV:PCBM:CA solutions (15 mg/mL) were spin coated at rates ranging from 4000 to 1400 rpm for 40 s. Subsequently, LiF/Al electrodes, which define the device area with a shadow mask of 0.03 cm2, were thermally evaporated under high vacuum (~ 2×10-6 mbar) at a rate of 0.5 and 2 Å/s. Parallel plate capacitors used to measure permittivity had the same structure as the OPV devices except the MDMO-PPV:PCBM:CA solutions (20 mg/mL) were all spin coated at 1000 rpm for 40 s to form thicker films.


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Devices to measure EL had the structure ITO/PEDOT:PSS (30 nm)/MDMOPPV:PCBM:CA/LiF (7 Å)/Al (70 nm). MDMO-PPV:PCBM:CA solutions (20 mg/mL) were spin cast at 1000 rpm for 60 s. The top electrode of LiF/Al was defined using a 1.4 cm2 shadow mask. LiF and Al were evaporated under high vacuum (~ 2×10-6 mbar) at rates of 0.5 and 2 Å/s. To minimize degradation during the EL measurements under high bias, the devices were encapsulated in the glovebox by capping with a second glass substrate and securing with UV curable epoxy (Epo-tek OG159-2). OPV devices were characterized using a Thermal-Oriel 300 W solar simulator with AM 1.5G solar illumination at 100 mW/cm2, and current density-voltage (J–V) curves were measured with a Keithley 236 source-measure unit. External quantum efficiency (EQE) spectra were measured with a monochromator and calibrated with a silicon photodiode. All devices were measured without exposure to air. To calculate IQE spectra (IQE = EQE/(1-R)), spectral reflectance (R) of the full devices was measured using a CARY 5000 UV-Vis-NIR spectrophotometer with the CARY 5000 Internal Diffuse Reflectance accessory that has an integrating sphere and a polytetrafluoroethylene reference. Permittivity was measured via impedance spectroscopy of parallel plate capacitors using a VMP3 potentiostat from BioLogic without exposure to air and in the dark. The real and imaginary impedance were measured by sweeping the AC frequency from 106 to 10 Hz and at an AC level of 100 mV. The permittivity was calculated from the impedance spectroscopy measurement using the methods and models described by Carr et al.35 To accurately measure the geometric capacitance to extract the permittivity, the DC voltage must be sufficient to fully deplete the active layer. DC biases of 0, 1, and 2 V all showed similar capacitive behavior suggesting that there is a low doping level in the BHJ.35


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EL was measured using a Horiba Nanolog Spectrofluorometer with a R5509PMT visible/NIR detector and a 1200, 500 grating. An extech instruments DC power supply was used to excite the films in a nitrogen protecting environment. All samples were measured under an electric field of 1.8x105 V/cm. All film thicknesses were measured using a Dektak 150 profilometer. Atomic force microscopy (AFM) images were measured with a Park Systems NX10 AFM in tapping mode to acquire phase information. A single silicon NCHR AFM tip purchased from NanoWorld was used for all the images. Here, we apply our strategy of adding high permittivity CA to MDMO-PPV:PCBM BHJ devices to improve OPV performance. The MDMO-PPV:PCBM BHJ is used as a model system because the CT state energy can conveniently be observed via EL.36 See figure S1 for molecular structures and energy level alignment of MDMO-PPV and PCBM. CA is used because it is a small organic molecule with a large permanent dipole that allows for a permittivity of 24.8 in the amorphous phase when the dipole can rotate but a permittivity of 3.2 when crystalline.34 By increasing the permittivity of the active layer, we expect improved CT state dissociation and reduced geminate recombination, leading to improved PCE. The effect of enhancing the local permittivity on the CT state energy and device efficiency was studied by varying the concentration of CA in the BHJ active layer. The device structure is ITO/PEDOT:PSS/MDMO-PPV:PCBM:CA/LiF/Al. All OPVs were fabricated using a 1:1 weight ratio of MDMO-PPV to PCBM and CA was incorporated at different wt% of the BHJ mixture. We did not use the higher efficiency 1:4 MDMO-PPV:PCBM ratio37, because CA forms large domains rather than mixing into the donor and acceptor phases at that ratio. OPV devices were


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tested under nitrogen with standard AM1.5G illumination. The permittivity of the MDMOPPV:PCBM:CA films was measured with impedance spectroscopy of parallel plate capacitors and the CT state energy was measured using EL. Figure 1a shows the change in MDMO-PPV:PCBM PCE with varying concentrations of CA; the average PCE of 8 devices are plotted and the standard deviation is represented by the error bars. Because the maximum reported PCE for MDMO-PPV:PCBM OPVs is only 2.5% and the maximum EQE and IQE are 50%37 and 85%38 respectively, we assume that the geminate recombination rate in MDMO-PPV:PCBM OPV devices are high and that CT state will dissociate more efficiency by lowering the CT state binding energy. The PCE continually increases by 75% with increasing wt% of CA until CA reaches a concentration of 20 wt%. The short circuit current density (JSC) and fill factor (FF) follow the same trend as the PCE: both increase up to 20 wt% CA. For concentrations of greater than or equal to 30 wt%, the PCE rapidly decreases as well as JSC and FF. As the concentration of CA increases from 0 to 50 wt%, the VOC decreases linearly from 0.90 to 0.85 V. Because VOC is strongly correlated to the CT stage energy, the decrease in VOC suggests that the CT state energy is reduced with added CA.39– 41

The standard deviation, represented by the error bars in Fig. 1a, increases as more CA is added

to the BHJ, suggesting that the film uniformity decreases as more CA is added. The increase in film heterogeneity is also seen in JSC and FF standard deviations. Device performance details including J–V curves and average values of VOC, JSC, and FF are available in figures S2 and S3.


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Figure 1. Performance of MDMO-PPV:PCBM photovoltaics blended with CA. (a) Average PCE of 8 devices with increasing wt% of CA blended into the BHJ. The error bars represent PCE standard deviation. PCE initially increases to a 75% enhancement at 20 wt% CA. As the concentration of CA increases beyond 20 wt%, the efficiency decreases rapidly and the standard deviation increases substantially. (b) IQE spectra of BHJ devices with CA in solid lines and extinction coefficients for MDMO-PPV and PCBM in dashed lines. The IQE increases throughout the spectrum up to 20 wt% CA and then quickly decreases at higher concentrations of CA.


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Because the absorption is reduced as CA displaces the absorbing donor and acceptor materials, we measured IQE, the ratio of the number of carriers collected to the number of photons absorbed. De-convolving the influence of changing absorption provides a deeper insight into the photocurrent generation processes. IQE spectra of the best performing devices are plotted in figure 1b along with the absorption spectra of MDMO-PPV and PCBM films for reference. The corresponding EQE spectra are available in figure S4. As expected, photocurrent is generated strongly where the PCBM and MDMO-PPV absorption overlap spectrally.37 The IQE spectra reveal that the efficiency modification of the photocurrent generation due to different CA concentrations can be divided into three concentration regimes: 1) Rapid increase from 0 to 5 wt% CA, 2) slower increase from 5 to 20 wt% CA, and 3) rapid decrease for higher concentrations of CA. The change in IQE with increasing CA concentration is equivalent to the trend in PCE. However, IQE is the product of exciton diffusion, CT state dissociation, carrier transport, and carrier collection efficiencies. Hence, simply based on the IQE spectra, we cannot deduce which of these photocurrent generation steps is responsible for the increase in PCE for concentrations of CA up to 20 wt%. To test our hypothesis of modifying the CT state binding energy, the permittivity should increase as more CA is added to the BHJ film. Hence, we measured the real and imaginary impedance of parallel plate capacitors with the BHJ layer as the dielectric between the two electrodes to calculate the relative permittivity.35 Figure 2 shows the change in permittivity with added CA. Again three regimes of behavior are observed in the MDMO-PPV:PCBM films as increasing concentrations of CA are added: (1) monotonic increase in permittivity by weight addition of CA from 0 to 10 wt% CA, (2) slight decrease in permittivity from 10 to 20 wt% CA, and (3) rapid decrease in permittivity decreases 20 to 50 wt% CA. These three regimes are comparable and


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relate to the regions found in the behavior of the PCE and IQE. The dashed red line in figure 2 represents the weighted average of the permittivity of each material, where the relative permittivity of the BHJ and CA are 4.7 and 24.8 respectively.34 Up to 10 wt% CA, the measured permittivity follows the expected behavior33 before it begins to deviate. Beyond 20 wt% the permittivity drops rapidly towards the permittivity of crystalline CA of 3.2 at room temperature because the permanent dipoles cannot rotate.34 As we will show below, deviation from the expected trend is related to crystallization and phase separation of CA in the BHJ at high loading. However, within the first regime up to 10 wt% CA, the effect of permittivity increase should modify the CT state binding energy - the goal of this work.

Figure 2. Permittivity of MDMO-PPV:PCBM BHJs with increasing CA concentration. The dashed line indicates the expected permittivity based on the weighted average permittivity of each component. The permittivity unexpectedly decreases for CA concentrations above 10 wt% CA.


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Since the permittivity and PCE behavior are strongly correlated, the next step was to determine if the change in permittivity indeed reduced the CT binding energy. The Frenkel exciton binding energies in MDMO-PPV and PCBM are possibly also reduced by the addition of CA to the BHJ film; changes to the optical band gaps as a result of increasing the permittivity are unclear due to the overlap of MDMO-PPV and PCBM absorption spectra at low energies. The CT state binding energy is the difference between the charge separated (CS) state, meaning the energy difference between the LUMO of the acceptor and the HOMO of the donor, and the CT state energy. The CT state energy can provide insight into changes in the system energetics. We measured the CT state energy in MDMO-PPV:PCBM:CA films via electroluminescence (EL)36,39, as shown in figure 3. In figure 3a, the EL spectra of PCBM and MDMO-PPV peak at 730 and 590 nm respectively, which are distinct from the EL spectrum at 980 nm for the CT state in the MDMOPPV:PCBM blend. Our results are corroborated by those of Tvingstedt et al.36 The EL spectra for BHJ films incorporating CA are also shown in figure 3a. To determine the peak maxima, the spectra were fit with an exponentially modified Gaussian function because it best described the data (see SI).42 The peak maxima and the full width at half maximum (FWHM) are shown in figure 3b. The peak maxima and the FWHM behave very similarly: they increase significantly from 0 to 10 wt% CA and then begin to plateau beyond 20 wt% CA. This indicates that the CT state energy indeed decreases at least within the first wt% regime from 1.26 to 1.21 eV at 0 and 10 wt% CA respectively.


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Figure 3. CT state energies measured with electroluminescence. (a) Normalized EL spectra of PCBM and MDMO-PPV individually are distinct from the CT state spectra of the MDMOPPV:PCBM BHJ. Spectra are also shown for BHJs with varying concentrations of CA. (b) Peak maxima and FWHM of fits to EL spectra for each concentration of CA. As more CA is added to the BHJ, the CT state energy decreases and the FWHM increases. The CT state energy reduction might initially suggest that the CT exciton binding energy is increasing; however, an increase in permittivity will also affect the energy of the CS state.43 The change in the CS state is due to an increase in the polarization energy, which increases the stability of ions in the material.44 The CS state energy is expected to decrease more rapidly than that of the CT state with increasing permittivity, resulting in an overall reduction of the CT


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exciton binding energy.44–46 Unfortunately, accurately measuring the CS state in organic solar cells is challenging.47 Hence, several other studies have identified a decrease in the CT state binding energy by measuring a reduction in the CT state energy.30,31,48 The observed change of the CT state binding energy can follow two different models: As described earlier, the CT state binding energy can be modeled by the screened Coulomb law; as the permittivity of the film increases, the CT state binding energy should decrease. However, van Duijnen et al. argue that the Coulomb description of the exciton does not apply in this case because the charges are closer than 10 nm.49 Alternatively, the CT state binding energy dependence on permittivity can also be modeled by the solid state solvation effect (SSSE)33,43, which accounts for the interaction of a solute with an electric field produced by the solvent molecules. The SSSE model is based on the equation = − 21 − ⁄2 − 1, where ECT is the CT state energy, C and A are constants, and ε is the permittivity. Thus, this model also predicts that the CT state binding energy will decrease as the low frequency permittivity increases. In the films here, the solvent in the latter model would be the CA and the solute that is effected is the CT state. To determine if the change in CT state energy follows the coulombic or SSSE model, the EL peak maxima were compared to the measured permittivity values for the different concentrations of CA (see SI). But, we only considered the data up to 10 wt% CA since the behaviors of both the permittivity and EL spectral peak positions change significantly at higher concentrations. From the plots of permittivity versus CT state energy, both models appear to be valid. We show that increasing the concentration of CA up to 10 wt% indeed enhances the BHJ permittivity, reduces the CT state binding energy, causing the PCE to increase. The broadening of the EL spectra with higher concentrations of CA suggests that the films become less uniform over the 1.4 cm2 active area, similar to the observed increase in standard deviation of the PCE. To understand the increase in


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FWHM and why the CT state plateaus beyond 10 wt% of CA, we studied the morphology of the BHJ films at varying CA concentration using high resolution AFM.

Figure 4. AFM phase images of MDMO-PPV:PCBM blended with increasing concentrations of CA. 0 wt% (a), 5 wt% (b), 10 wt% (c), 20 wt% (d), and 50 wt% (e) CA in BHJ films spin cast on PEDOT:PSS/ITO/glass substrates. Scale bars for a-e are 50 nm. At 20 wt% CA (d), the morphology changes significantly and large domains of a new phase form. At 50 wt% CA (e), the new phase domains become much larger. (f) An enlarged image of a 50 wt% CA film. Scale bar is 500 nm. We employed AFM tapping mode imaging to differentiate material domains within the organic films.50–52 The phase images in figure 4a-c show that although the domain size increases slightly from 0 to 10 wt% CA, the morphology does not change significantly when CA is added. These results are representative for many measured regions over these three films. We conclude from these results that CA is effectively incorporated into the BHJ when increasing the concentration


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up to 10 wt%. This observation fits the behavior in the first regime (0-10 wt% CA) where the permittivity increases, the CT state energy decreases, and the photovoltaic performance is enhanced. The phase images in figure 4d,e show a drastic change in morphology with the formation of a new phase at 20 wt% CA, and the domains of the new phase become larger as more CA is added, as seen in the larger scale image in figure 4f. Thus, in the regime between 10 and 20 wt% CA, we believe CA is beginning to phase separate out of the MDMO-PPV:PCBM BHJ. As a result, CA does not interact as strongly with the excitons, which explains the stagnation in permittivity and decrease in CT state energy in this second regime. Above 20 wt% CA, the CA begins to form very large domains and likely forms crystals, which have a lower permittivity of 3.2 because the CA can no longer rotate in the solid.34 This behavior was not observed in the previously studied small molecule films because the amorphous nature of the donor molecule prevented phase separation32, whereas phase separation is a known behavior in the MDMO-PPV:PCBM BHJ system.53,54 The low permittivity of crystalline CA explains the decrease of the MDMO-PPV:PCMB:CA film permittivity at high CA loading (figure 2). Because of the lower permittivity, we initially expected the CT state energy to increase in the third regime. But, based on the recent results from Deotare et al., we suspect that the CT state excitons are diffusing to the low energy sites where CA remains mixed in the BHJ55, leading to a plateau in CT state energy. In conclusion, we have shown the effectiveness of increasing dielectric screening at donoracceptor interfaces to increase CT state dissociation efficiency, reduce geminate recombination, and increase PCE in polymer-fullerene BHJ devices. By doping the BHJ films with the high dielectric molecule CA at concentrations up to 20 wt%, we increased the thin film permittivity significantly without changing the film morphology, which leads to a decrease in CT state


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binding energy as confirmed by EL. Our work further confirms that increasing film permittivity represents a highly effective method to overcome the intrinsic limitations of excitons in organic semiconductors for photovoltaic devices to achieve enhanced PCEs. ACKNOWLEDGMENT Work at the Molecular Foundry, Lawrence Berkeley National Laboratory, was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This material is also based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. NSF DGE 1106400. Biwu Ma thanks the Florida State University for the financial support through the Energy and Materials Initiative, as well as the Beijing National Laboratory for Molecular Sciences (BNLMS) for the support through the Open Project Program. ASSOCIATED CONTENT Supporting Information. Exponentially modified Gaussian function, molecular structures, HOMO and LUMO energies, current density–voltage curves, photovoltaic efficiency parameters, external quantum efficiency spectra, plots of permittivity versus charge transfer state energy. REFERENCES (1)

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Extinction Coefficient (105 cm-1)

IQE (%)

Efficiency (%)

1 0.6 2 3 0.5 4 5 6 0.4 7 8 9 0.3 10 0 10 20 30 40 50 11 Concentration CA (%) 12 b13 1.5 11 PCBM 0CA 1410 MDMOPPV 5CA 15 9 10CA 16 20CA 8 30CA 17 1.0 50CA 7 18 19 6 20 5 21 4 0.5 22 3 23 2 24 1 25 0 ACS Paragon Plus Environment 0.0 400 500 600 700 26 300 Wavelength (nm) 27

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Permittivity

1 2 3 4 5 5 6 7 4 8 9 103 11 12 13

ACS Paragon Plus Environment 0

10

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Concentration CA (%)

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PCBM MDMO-PPV BHJ 5% CA 10% CA 20% CA 50% CA

1.0

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Peak Center (nm)

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Center FWHM

1040

160

FWHM (nm)

Norm EL

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Concentration CA (%) ACS Paragon Plus Environment

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Energy

S1 27 of 27 hePage Journal Physical Chemist CS Eeb CS 1 CT Eeb CT 2 3 4 o + 5 o o ACS 6 Paragon Plus Environmen 7S0 8 OCH3

O

O

n

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Dielectric Screening To Reduce Charge Transfer ... - ACS Publications

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