Ternary Bulk Heterojunction Photovoltaic Cells Composed of Small

Article pubs.acs.org/JPCC

Ternary Bulk Heterojunction Photovoltaic Cells Composed of Small Molecule Donor Additive as Cascade Material Lei Ye,† Hai-Hua Xu,‡ Hui Yu,‡ Wang-Ying Xu,† Hao Li,† Han Wang,† Ni Zhao,*,‡ and Jian-Bin Xu*,† †

Department of Electronic Engineering, Materials Science and Technology Research Center and ‡Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China S Supporting Information *

ABSTRACT: To explore the potential of ternary blend bulk heterojunction (BHJ) solar cells as a general platform for improving the performance of organic photovoltaics, we studied a ternary BHJ system based on poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PC61BM), and DTDCTB. The optimized ternary structure containing a weight ratio of 20% DTDCTB as the cascade material demonstrates a ∼25% improvement of the power conversion efficiency (PCE) as compared to the binary P3HT/PC61BM solar cells. A systematic spectroscopic study is carried out to elucidate the underlying mechanism of charge transfer in the ternary system. Wavelength-dependent external quantum efficiency measurement confirms the contribution of DTDCTB to the enhanced photocurrent. Photoinduced absorption spectroscopy and transient photovoltage measurement reveal unambiguously that charges generated in DTDCTB are efficiently transferred to and subsequently transported in P3HT and PC61BM. The results also suggest that despite the realization of cascade charge transfer, the bimolecular charge recombination process in the ternary system is still dominated by the P3HT/PC61BM interface.

1. INTRODUCTION Polymer bulk heterojunction (BHJ) solar cells based on mixtures of conjugated polymers as the electron donor and fullerene derivatives as the electron acceptor can potentially be used as a low-cost, printable, and portable renewable energy device.1−9 Recently, the power conversion efficiency (PCE) of BHJ solar cells has been significantly improved to close to 10%, primarily due to the development of new photoactive materials.10,11 On the other hand, prototype BHJ solar cells formed by the blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) still attract research efforts for further PCE improvement, as this material system is easy to process and can be made relatively thick (∼300 nm) in a solar cell configuration, which are important for fabrication of large area and flexible solar modules. The efficiency of P3HT/PC61BM BHJ solar cell is mainly limited by two issues. First, P3HT/PC61BM solar cells exhibit small open-circuit voltage (VOC) because the unoptimized energy level alignment between P3HT and PC61BM. Second, P3HT absorbs light only up to a wavelength of ∼650 nm and is thereby unable to cover a significant portion of the solar radiation spectrum.12 One of the approaches to improve the performance of P3HT/PC61BM solar cells13−16 is developing ternary blended BHJ solar cells by employing a third component. In these ternary blend solar cells, the introduction of the third component can provide complementary absorption17−21 in the near-infrared region to enhance the photocurrent and/or increase the energy level difference © 2014 American Chemical Society

between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) levels of the acceptor22−24 to improve the open circuit voltage. Despite the early conceptual design of ternary BHJ solar cells, there are rather a limited number of material systems that have been shown to deliver PCE enhancement. Furthermore, the charge transfer and recombination processes in ternary systems are still not fully understood. In this work we introduced a low band gap, small molecule semiconductor, DTDCTB (shown in Figure 1), into the P3HT/PC61BM blend to form a ternary system. With the optimized weight ratio of P3HT, DTDCTB, and PC61BM (0.8:0.2:1), the ternary solar cell shows ∼25% improvement of PCE as compared to the binary P3HT/PC61BM devices, resulting in the enhancement in both JSC and VOC. The photophysical mechanism behind the sensitization effect of the cascade material DTDCTB in the ternary system is investigated by external quantum efficiency (EQE), photoinduced absorption (PIA) spectroscopy,25 and transient photovoltage (TPV) measurement.26,27 The results reveal efficient charge transfer from DTDCTB to P3HT and PC61BM as well as predominant charge recombination at the P3HT/PC61BM interface. Received: May 4, 2014 Revised: August 12, 2014 Published: August 13, 2014 20094

dx.doi.org/10.1021/jp504365y | J. Phys. Chem. C 2014, 118, 20094−20099

The Journal of Physical Chemistry C

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Figure 1. Molecular structures and energy levels of active components in the ternary bulk heterojunction solar cells.

2. EXPERIMENTAL SECTION Ternary organic blend solar cells were fabricated with the structure ITO/PEDOT:PSS/active layer/LiF/Al. A prepatterned ITO was ultrasonicated in detergent, deionized water, acetone, and isopropanol for 15 min sequentially. Afterward, the surface of ITO on the glass was modified by UV-ozone treatment for 1.5 min. The PEDOT:PSS thin film with a thickness of ∼30 nm was spin-coated on the ITO substrate to form a hole transporting layer. The mixed solutions of P3HT, DTDCTB, and PC61BM in dichlorobenzene (DCB) containing 3 vol % 1,8-diiodooctane (DIO) were spun-cast on the top of the PEDOT:PSS layer. Subsequently, a thin LiF layer was deposited by thermal evaporation under a high vacuum. Finally, an aluminum electrode with a thickness of 100 nm was thermally evaporated onto the top of the devices. Binary organic blend solar cells were fabricated with the same process of ternary devices. The J−V characteristics were measured using a Keithley 236 source measure unit in combination with a Newport Oriel 91160 solar simulator. The EQE was measured using a photomodulation spectroscopic setup (model Merlin, Oriel), a calibrated Si UV detector, and a SR570 low noise current amplifier. UV−vis absorption (Varian CARY 1E) and photoluminescence (FR 650, JASCO) measurements were performed to analyze the optical properties of the blend ternary photoactive layers. The AFM was operated in tapping mode to acquire images of the surfaces of blend ternary photoactive layers. PIA spectra at the VIS/NIR region were taken using two pump lasers with different wavelengths. The mechanically modulated pump beam and white light probe beam transmission (−ΔT) were detected after dispersion with a monochromator in the range from 750 to 1600 nm with a germanium detector and recorded with a lock-in amplifier. The wavelength resolution of this measurement is 4 nm. All measurements were carried out at a high vacuum at 78 K.

For the transient photovoltage (TPV) measurement, a red laser (λ = 635 nm) driven by a function generator was used as the triggering light source to provide square-wave modulated illumination, which resulted in a small perturbation voltage ΔV in addition to VOC. The transient decay of ΔV was then recorded by an oscilloscope, and the time resolution of overall TPV system is 40 ns.

3. RESULTS AND DISCUSSION The molecular structures of P3HT, DTDCTB, and PC61BM are shown in Figure 1. DTDCTB displays the energy levels28 properly positioned with respect to the P3HT/PC61BM blend as also shown in Figure 1; thus, the photoinduced excitons could be separated and collected at each electrode because the difference between the LUMO levels of DTDCTB and PC61BM provides a driving force that can funnel electrons out.29 Consequently, photoinduced charge transfer is energetically favorable in the blends comprising of P3HT, DTDCTB, and PC61BM. The UV−vis absorption spectra of DTDCTB thin film are revealed in Figure 2a, covering an absorption range from 500 to 800 nm. After blending P3HT with DTDCTB at the weight ratio of 0.8:0.2, the absorption of the composite film demonstrates wider absorption range as compared to P3HT, with a small peak around 690 nm. The absorption profiles of P3HT, DTDCTB, and the P3HT/DTDCTB blends can readily explain the enhanced photoresponse of P3HT/PC61BM solar cells with DTDCTB additive. To check whether the blend of P3HT, DTDCTB, and PC61BM can form a proper donor− acceptor energy cascade for OPVs, steady-state fluorescence quenchings of the blends were compared based on the photoluminescence spectra shown in Figure 2b. The remarkable reductions of photoluminescence are observed, indicating that efficient charge transfer occurs at the interface of donor−acceptor pairs.29 20095



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Figure 2. (a) Normalized UV−vis absorption of the films of P3HT, DTDCTB, and P3HT/DTDCTB with weight ratio of 0.8:0.2. (b) Photoluminescence spectra of 100 nm thick films on quartz for pure P3HT, DTDCTB, and a blend of P3HT/DTDCTB/PC61BM with a weight ratio of 0.8:0.2:1.

Figure 3. (a) J−V characteristics of bulk heterojunction solar cells fabricated with binary mixtures containing P3HT/PC61BM (1:1) and ternary mixtures containing P3HT/DTDCTB/PC61BM with four different weight ratios and illuminated under 1.5 conditions (100 mW/ cm2). (b) External quantum efficiency (EQE) as a function of wavelength for the binary device of P3HT/PC61BM (1:1) solar cell and the ternary devices of P3HT/DTDCTB/PC61BM with four different weight ratios.

The ternary blend BHJ solar cells based on the mixtures of P3HT/DTDCTB/PC61BM and the control device P3HT/ PC61BM BHJ solar cell were fabricated. The mixtures were dissolved in dichlorobenzene (DCB) solvent containing 3 vol % 1,8-diiodooctane (DIO) to improve the morphology and thus reduce the resistance.11,30 (The performance of the devices without DIO is presented in the Supporting Information.) According to the early studies on ternary blend solar cells,31,32 the weight ratio of the three active layer components is very important for the performances of devices. Here, in order to obtain the optimized performance, the devices with different weight ratios of P3HT/DTDCTB/PC61BM were fabricated and tested under an AM1.5G solar simulator. As shown in Figure 3a, the device with the weight ratio of P3HT/ DTDCTB/PC61BM as 0.8:0.2:1 displays the best performance. Table 1 shows the summary of the photovoltaic characteristics for ternary BHJ solar cells with different weight ratios, where the binary device of P3HT/PC61BM fabricated with the same method is presented as a reference. It can be seen from Table 1 that the PCE improvement in the P3HT/DTDCTB/PC61BM system results from enhancement in both JSC and VOC. On the other hand, the binary blend device of DTDCTB/PC61BM shows a low PCE of 0.85% (Supporting Information). The contribution of DTDCTB to the photocurrent can also be seen in the external quantum efficiency (EQE) measurements, as shown in Figure 3b. According to the EQE spectra, adding 10% DTDCTB into the P3HT/PC61BM system results in ∼10% EQE in the 650−800 nm range, corresponding to the

absorption of DTDCTB. Meanwhile, the EQE resulted from the P3HT absorption (400−600 nm) remains to be similar to the binary P3HT/PC61BM system. After the ratio of DTDCTB exceeds 20%, the EQE contributed by P3HT is quickly decreased with little enhancement of the EQE in 650−800 nm range. This is likely due to poor film morphology induced by aggregation of DTDCTB at high loading ratios.33 As shown by the atomic force microscopy (AFM) (Supporting Information), the P3HT/PC61BM binary blend film shows a smooth surface with root-mean-square roughness of Rrms about 3.0 nm. On the other hand, for the ternary composite films the roughness of the films is increased with the increase in content of DTDCTB. To understand the charge transfer process in the ternary system, we employed steady-state photoinduced absorption measurement to investigate the polaron population in P3HT and DTDCTB. Two different pump lasers with different wavelengths were used for the selective excitation of P3HT and DTDCTB. As shown in Figure 4, excitation of the DTDCTB/ PC61BM film by the pump laser with the wavelength of 780 nm (1.59 eV) results in the measurable PIA characteristic signal of DTDCTB at the wavelength of 1320 nm. The P3HT/PC61BM film cannot be excited by the pump laser with the wavelength of 780 nm, whereas light excitation by the pump laser with the wavelength of 532 nm (2.33 eV) yields the typical polaron peak of P3HT at the wavelength of 990 nm.34 Exciting the ternary P3HT/DTDCTB/PC61BM film by the pump laser with the 20096



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Table 1. Summary of Photovoltaic Characteristics for the Binary BHJ Solar Cell of P3HT/PC61BM and the Ternary BHJ Solar Cells of P3HT/DTDCTB/PC61BM with Varied Weight Ratios of the Three Componentsa P3HT/DTDCTB/PC61BM (wt %)

JSC (mA cm−2)

1:0:1 0.9:0.1:1 0.8:0.2:1 0.7:0.3:1 0.6:0.4:1 a

8.67 8.75 9.67 7.38 3.63

VOC (V)

(±0.3) (±0.3) (±0.4) (±0.5) (±0.5)

0.58 0.66 0.69 0.70 0.72

(±0.01) (±0.01) (±0.01) (±0.01) (±0.02)

FF 0.64 0.62 0.61 0.55 0.48

(±0.01) (±0.01) (±0.02) (±0.03) (±0.03)

η (%) 3.25 3.58 4.07 2.84 1.25

(±0.15) (±0.17) (±0.20) (±0.22) (±0.24)

All various blends are fabricated for eight devices.

Figure 4. Photoinduced absorption (PIA) spectra of thin films of P3HT/DTDCTB/PC 61 BM (0.8:0.2:1) (solid squares) and DTDCTB/PC61BM (1:1) (solid circle) which were all excited with the pump wavelength of 780 nm and P3HT/PC61BM (1:1) (solid up triangle) excited with the pump wavelength of 532 nm serving as a reference for the spectroscopic position of the P3HT polaron.

wavelength of 780 nm results in a pronounced absorption peak at 990 nm, which suggests that a significant density of polarons in P3HT is generated by photoexcitation of DTDCTB. In addition, the intensity of the DTDCTB polaron peak in the ternary blend is negligible compared to that of the P3HT polaron peak and the DTDCTB polaron peak in the binary DTDCTB/PC61BM film. These observations show unambiguously that the positive polarons are transferred from DTDCTB to P3HT in the ternary system. The PIA and EQE measurements suggest that the photoexcitation in the DTDCTB phase contributes to the photocurrent through efficient holes transfer from DTDCTB to P3HT. In order to further understand how the DTDCTB phase affects the subsequent charge transport and recombination processes in the P3HT/DTDCTB/PC61BM blend, we performed the transient photovoltage (TPV) measurement on both the binary and ternary solar cells. The characteristic decay time (τ) of transient photovoltage is associated with the charge recombination process in solar cells,35−37 which is often described by the Langevin equation R = knp, where k, the inverse of τ, is the recombination rate constant, and n and p are the concentrations of photoinduced electrons and holes, respectively.38 Here, we applied the TPV measurements to P3HT/PC61BM (1:1), P3HT/DTDCTB/PC61BM (0.8:0.2:1), and DTDCTB/PC61BM (1:1) solar cells under white light illumination (85 mW/cm2). As revealed in Figure 5a, the TPV decay time of the DTDCTB/PC61BM device is significantly longer than that of the of P3HT/PC61BM device, suggesting a slower charge recombination process at the DTDCTB/ PC61BM interface as compared to P3HT/PC61BM. Interestingly, the P3HT/DTDCTB/PC61BM device exhibits similar

Figure 5. Transient photovoltage curves of the solar cells of (a) P3HT/PC61BM (1:1), P3HT/DTDCTB/PC61BM (0.8:0.2:1), and DTDCTB/PC61BM (1:1) under the bias light intensity of 85 mW/ cm2. (b) Decay time as a function of three different bias light intensities derived from the devices of P3HT/PC61BM (1:1), P3HT/ DTDCTB/PC61BM (0.8:0.2:1), and DTDCTB/PC61BM (1:1). The error bars represent ∼10% deviation.

TPV decay dynamics as the P3HT/PC61BM system, and the decay time constants of the two material systems show almost identical light intensity dependence as illustrated in Figure 5b. These results suggest that the predominant charge recombination process in the ternary system occurs at the P3HT/PC61BM interface. Therefore, it is unlikely that DTDCTB forms an interface phase between P3HT and PC61BM, which would otherwise greatly decrease the recombination rate in the ternary system. Nevertheless, the existence of the DTDCTB phase still increases the decay time of TPV to some extent. This phenomenon might explain the improved VOC in the ternary solar cells. In summary, the TPV measurement well correlates with the PIA result to confirm that in the ternary system holes are transported in the P3HT phase while DTDCTB mainly contributes to the exciton generation and charge transfer processes. 20097



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(5) Ye, L.; Xiao, T.; Zhao, N.; Xu, H. H.; Xiao, Y. B.; Xiong, Y. Z.; Xu, W. J.; Xu, J. B. Derivitization of Pristine Graphene for Bulk Heterojunction Polymeric Photovoltaic Devices. J. Mater. Chem. 2012, 22, 16723−16727. (6) Andersen, T. R.; Dam, H. F.; Andreasen, B.; Hösel, M.; Madsen, M. V.; Gevorgyan, S. A.; Sondergaard, R. R.; Jorgensen, M.; Krebs, F. C. A Rational Method for Developing and Testing Stable Flexible Indium- and Vacuum-Free Multilayer Tandem Polymer Solar Cells Comprising up to Twelve Roll Processed Layers. Sol. Energy Mater. Sol. Cells 2014, 120, 735−743. (7) Fang, L.; Zhou, Y.; Yao, Y. X.; Diao, Y.; Lee, W. Y.; Appleton, A. L.; Allen, R.; Reinspach, J.; Mannsfeld, S. C. B.; Bao, Z. N. Side-Chain Engineering of Isoindigo-Containing Conjugated Polymers Using Polystyrene for High-Performance Bulk Heterojunction Solar Cells. Chem. Mater. 2013, 25, 4874−4880. (8) Pachoumi, O.; Li, C.; Vaynzof, Y.; Banger, K. K.; Sirringhaus, H. Improved Performance and Stability of Inverted Organic Solar Cells with Sol−Gel Processed, Amorphous Mixed Metal Oxide Electron Extraction Layers Comprising Alkaline Earth Metals. Adv. Energy Mater. 2013, 3, 1428−1436. (9) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C. S.; et al. A Strong Regioregularity Effect in Self-organizing Conjugated Polymer Films and High-Efficiency Polythiophene: Fullerene Solar Cells. Nat. Mater. 2006, 5, 197−203. (10) Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat. Photonics 2009, 3, 297−302. (11) Chu, T. Y.; Lu, J. P.; Beaupré, S.; Zhang, Y. G.; Pouliot, J. R.; Wakim, S.; Zhou, J. Y.; Leclerc, M.; Li, Z.; Ding, J. F.; et al. Bulk Heterojunction Solar Cells Using Thieno[3,4-c]pyrrole-4,6-dione and Dithieno[3,2-b:2′,3′-d]silole Copolymer with a Power Conversion Efficiency of 7.3%. J. Am. Chem. Soc. 2011, 133, 4250−4253. (12) Cha, H.; Chung, D. S.; Bae, S. Y.; Lee, M. J.; An, T. K.; Hwang, J.; Kim, K. H.; Kim, Y. H.; Choi, D. H.; Park, C. E. Complementary Absorbing Star-Shaped Small Molecules for the Preparation of Ternary Cascade Energy Structures in Organic Photovoltaic Cells. Adv. Funct. Mater. 2013, 23, 1556−1565. (13) Peet, J.; Soci, C.; Coffin, R. C.; Nguyen, T. Q.; Mihailovsky, A.; Moses, D.; Bazan, G. C. Method for Increasing the Photoconductive Response in Conjugated Polymer/fullerene Composites. Appl. Phys. Lett. 2006, 89, 252105−252108. (14) Wang, W.; Wu, H.; Yang, C.; Luo, C.; Zhang, Y.; Chen, J.; Cao, Y. High-efficiency Polymer Photovoltaic Devices from RegioregularPoly(3-hexylthiophene-2,5-diyl) and [6,6]-Phenyl-C61-butyric Acid Methyl Ester Processed with Oleic Acid Surfactant. Appl. Phys. Lett. 2007, 90, 183512−183515. (15) Ameri, T.; Khoram, P.; Min, J.; Brabec, C. J. Organic Ternary Solar Cells: A Review. Adv. Mater. 2013, 25, 4245−4266. (16) Lim, B.; Bloking, J. T.; Ponec, A.; McGehee, M. D.; Sellinger, A. Ternary Bulk Heterojucntion Solar Cells: Addition of Soluble NIR Dyes for Photocurrent Generation beyond 800 nm. ACS Appl. Mater. Interfaces 2014, 6, 6905−6913. (17) Andrew, T. L.; Bulović, V. Bulk Heterojuction Solar Cells Containing 6,6-Dicyanofulvenes as n-Type Additives. ACS Nano 2012, 6, 4671−4677. (18) Sharma, S. S.; Sharma, G. D.; Mikroyannidis, J. A. Improved Power Conversion Efficiency of Bulk Heterojunction Poly(3hexylthiophene): PCBM Photovoltaic Devices Using Small Molecule Additive. Sol. Energy Mater. Sol. Cells 2011, 95, 1219−1223. (19) Honda, S.; Nogami, T.; Ohkita, H.; Benten, H.; Ito, S. Improvement of the Light-Harvesting Efficiency in Polymer/Fullerene Bulk Heterojunction Solar Cells by Interfacial Dye Modification. ACS Appl. Mater. Interfaces 2009, 1, 804−810. (20) Ameri, T.; Min, J.; Li, N.; Machui, F.; Baran, D.; Forster, M.; Schottler, K. J.; Dolfen, D.; Scherf, U.; Brabec, C. J. Performance Enhancement of the P3HT/PCBM Solar Cells through NIR

4. CONCLUSIONS We have demonstrated an efficient ternary BHJ solar cell based on the P3HT/DTDCTB/PC61BM blend. The introduction of the DTDCTB component expands the absorption range of the solar cells and increases the PCE by ∼25%. Photoinduced absorption spectroscopy and transient photovoltage measurements reveal unambiguously that charges generated in DTDCTB are efficiently transferred to and transported in P3HT and PC61BM. The results also suggest that despite the realization of cascade charge transfer, the bimolecular charge recombination process in the ternary system is still dominated by the P3HT/PC61BM interface. To make the ternary approach more effective, it is important to further engineer the blend morphology so that the low bandgap component forms an interfacial layer between P3HT and PC61BM, thus achieving a morphologically cascade structure to further improve the solar cell performance by reducing the bimolecular recombination of carriers.

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ASSOCIATED CONTENT

S Supporting Information *

Figures for transfer characteristic of DTDCTB bottom-gated FET, AFM images of various blends of active layer, J−V characteristics of various blends, and J−V characteristic of the binary blend device of DTDCTB/PC61BM. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Fax +852-2609-8297; Tel +852-2609-8297; e-mail nzhao@ee. cuhk.edu.hk (N.Z.). *Fax +852-2609-8297; Tel +852-2609-8297; e-mail jbxu@ee. cuhk.edu.hk (J.-B.X.). Author Contributions

L.Y. and H.H.X. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Research Grants Council of Hong Kong through General Research Fund (No. AoE/P-03/ 08, CUHK4179/10E, CUHK4193/11E, N_CUHK405/12, T23-407/13-N). J. B. Xu thanks the National Science Foundation of China for the support, particularly via Grant No. 61229401.

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REFERENCES

(1) Chen, H. Y.; Hou, j.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Polymer Solar Cells with Enhanced Open-Circuit Voltage and Efficiency. Nat. Photonics 2009, 3, 649−653. (2) Hou, J.; Chen, H. Y.; Zhang, S.; Chen, R. I.; Yang, Y.; Wu, Y.; Li, G. Synthesis of a Low Band Gap Polymer and Its Application in Highly Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2009, 131, 15586−15587. (3) Szarko, J. M.; Guo, J.; Liang, Y.; Lee, B.; Rolczynski, B. S.; Strzalka, J.; Xu, T.; Loser, S.; Marks, T. J.; Yu, L.; et al. When Function Follows Form: Effects of Donor Copolymer Side Chains on Film Morphology and BHJ Solar Cell Performance. Adv. Mater. 2010, 22, 5468−5472. (4) Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.; Liu, S.; You, W. Development of Fluorinated Benzothiadiazole as a Structural Unit for a Polymer Solar Cell of 7% Efficiency. Angew. Chem., Int. Ed. 2011, 50, 2995−2998. 20098



Article

Sensitization Using a Small-Bandgap Polymer. Adv. Energy Mater. 2012, 2, 1198−1202. (21) Ye, L.; Xia, H.; Xiao, Y. B.; Xu, J. B.; Miao, Q. Ternary Blend Bulk Heterojunction Photovoltaic Cells with an Ambipolar Small Molecule as the Cascade Material. RSC Adv. 2014, 4, 1087−1092. (22) Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. Efficient Ternary Blend Bulk Heterojunction Solar Cells with Tunable OpenCircuit Voltage. J. Am. Chem. Soc. 2011, 133, 14534−14537. (23) Li, H.; Zhang, Z. G.; Li, Y. F.; Wang, J. Z. Tunable Open-Circuit Voltage in Ternary Organic Solar Cells. Appl. Phys. Lett. 2012, 101, 163302−163304. (24) Street, R. A.; Davies, D.; Khylyabich, P. P.; Burkhart, B.; Thompson, B. C. Origin of the Tunable Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Organic Solar Cells. J. Am. Chem. Soc. 2013, 135, 986−989. (25) Snaith, H. J.; Petrozza, A.; Ito, S.; Miura, H.; Grätzel, M. Charge Generation and Photovoltaic Operation of Solid-State Dye-Sensitized Solar Cells Incorporating a High Extinction Coefficient IndoleneBased Sensitizer. Adv. Funct. Mater. 2009, 19, 1810−1818. (26) Maurano, A.; Hamilton, R.; Shuttle, C. G.; Ballantyne, A. M.; Nelson, J. B.; Regan, B. O.; Zhang, W. M.; McCulloch, I.; Azimi, H.; Morana, M.; et al. Recombination Dynamics as a Key Determinant of Open Circuit Voltage in Organic Bulk Heterojunction Solar Cells: A Comparison of Four Different Donor Polymers. Adv. Mater. 2010, 22, 4987−4992. (27) Wood, J. D.; Jellison, J. L.; Finke, A. D.; Wang, L.; Plunkett, K. N. Electron Acceptors Based on Functionalizable Cyclopenta[hi]aceanthrylenes and Dicyclopenta[de,mn]tetracenes. J. Am. Chem. Soc. 2012, 134, 15783−15789. (28) Lin, L. Y.; Chen, Y. H.; Huang, Z. Y.; Lin, H. W.; Chou, S. H.; Lin, F.; Chen, C. W.; Liu, Y. H.; Wong, K. T. A Low-Energy-Gap Organic Dye for High-Performance Small-Molecule Organic Solar Cells. J. Am. Chem. Soc. 2011, 133, 15822−15825. (29) Huang, J. H.; Velusamy, M.; Ho, K. C.; Lin, J. T.; Chu, C. W. A Ternary Cascade Structure Enhances the Efficiency of Polymer Solar Cells. J. Mater. Chem. 2010, 20, 2820−2825. (30) Ito, Y.; Virkar, A. A.; Mannsfeld, S.; Oh, J. H.; Toney, M.; Locklin, A.; Bao, Z. N. Crystalline Ultrasmooth Self-assembled Monolayers of Alkylsilanes for Organic Field-Effect Transistors. J. Am. Chem. Soc. 2009, 131, 9396−9404. (31) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganäs, O.; Manca, J. V. Relating the Open-Circuit Voltage to Interface Molecular Properties of Donor: Acceptor Bulk Heterojunction Solar Cells. Phys. Rev. B 2010, 81, 125204−125211. (32) Peet, J.; Tamayo, A. B.; Dang, X. D.; Seo, J. H.; Nguyen, T. Q. Small Molecule Sensitizers for Near-Infrared Absorption in Polymer Bulk Heterojunction Solar Cells. Appl. Phys. Lett. 2008, 93, 163306− 163308. (33) Chen, W.; Xu, T.; He, F.; Wang, W.; Wang, C.; Strzalka, J.; Liu, Y.; Wen, J. G.; Miller, D. J.; Chen, J. H.; et al. Hierarchical Nanomorphologies Promote Exciton Dissociation in Polymer/Fullerene Bulk Heterojunction Solar Cells. Nano Lett. 2011, 11, 3707− 3713. (34) Kim, Y. H.; Spiegel, D.; Hotta, S.; Heeger, A. J. Photoexcitation and Doping Studies of Poly(3-hexylthienylene). Phys. Rev. B 1988, 38, 5490−5495. (35) O’Regan, B.; Durrant, J. R.; Sommeling, P. M.; Bakker, N. J. Influence of the TiCl4 Treatment on Nanocrystalline TiO2 Films in Dye-Sensitized Solar Cells. 2. Charge Density, Band Edge Shifts, and Quantification of Recombination Losses at Short Circuit. J. Phys. Chem. C 2007, 111, 14001−14010. (36) Hamilton, R.; Shuttle, C. G.; O’Regan, B.; Hammant, T. C.; Nelson, J.; Durrant, J. R. Recombination in Annealed and Nonannealed Polythiophene/Fullerene Solar Cells: Transient Photovoltage Studies versus Numerical Modeling. J. Phys. Chem. Lett. 2010, 1, 1432−1436. (37) O’Regan, B.; Lenzmann, F. Charge Transport and Recombination in a Nanoscale Interpenetrating Network of n-Type and p-Type Semiconductors: Transient Photocurrent and Photovoltage Studies of

TiO2/Dye/CuSCN Photovoltaic Cells. J. Phys. Chem. B 2004, 108, 4342−4350. (38) Credgington, D.; Hamilton, R.; Atienzar, P.; Nelson, J.; Durrant, J. R. Non-Geminate Recombination as the Primary Determinant of Open-Circuit Voltage in Polythiophene: Fullerene Blend Solar Cells: an Analysis of the Influence of Device Processing Conditions. Adv. Funct. Mater. 2011, 21, 2744−2753.

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