Enhanced Power Conversion Efficiency of Low Band- Gap Polymer Solar Cells by Insertion of Optimized Binary Processing Additives

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1 Enhanced Power Conversion Efficiency of Low Band- Gap Polymer Solar Cells by Insertion of Optimized Binary Processing Additives Dong Hwan Wang, Aung Ko Ko Kyaw, Jean-Rémi Pouliot, Mario Leclerc, and Alan J. Heeger* Organic solar cells from a bulk-heterojunction (BHJ) material based on blends of donor polymer and a fullerene derivative as acceptor have been developed through the past decades. [ 1 11 ] Recently, the power conversion efficiency (PCE) of single BHJ solar cells >8% have been reported in solution-processible polymer or small-molecule solar cells. [ 1 15 ] However, due to the narrow absorption spectrum of the donor polymers and the energy losses from recombination the single BHJ solar cells have thus far demonstrated limited PCE or photocurrent density ( J SC ). Therefore, the tandem cell architecture (two cells are stacked and connected in series) to increase the open circuit voltage and to increase the bandwidth of the light harvesting. [ 3,6,16,17 ] Tandem cells with PCE > 10% have been recently reported. [ 18,19 ] The tandem cell structure is a promising alternative to bring about high efficiency organic solar cells, especially, since the open circuit voltage ( V OC ) is the sum of the two individual sub-cells in optimized devices. To realize the tandem cell structure, an efficient low band-gap polymer (band-gap of E g = 1. ev to 1.5 ev) is needed to absorb the longer wavelength photons. One of the promising candidates for the low band-gap polymer is a PDPPFTF, which contains the diketopyrrolopyrrole (DPP). [ 0 3 ] The semiconducting properties and the solubility can be enriched by two furan units. The PCE of the PDPPFTF:[6,6]-phenyl C 71 -butyric acid methyl ester (PC 70 BM) BHJ solar cells is sharply increased when the BHJ dissolved in chlorobenzene (CB) solvent with 1-chloronaphthalene (CN) additive due to the improved surface morphology which has been correlated to increased J SC in previous research. [ 4 ] Matching the current density of the two sub-cells (large bandgap and low band-gap) is another critical issue for achieving efficient tandem devices. Therefore, further device optimization with reproducible PCE and large cell area of the low band-gap PDPPFTF are needed to directly apply as the sub-cell in the tandem structure. Dr. D. H. Wang, Dr. A. K. K. Kyaw, Prof. A. J. Heeger Center for Polymers and Organic Solids University of California at Santa Barbara Santa Barbara, CA, , USA ajhe1@physics.ucsb.edu J.-R. Pouliot, Prof. M. Leclerc Department of Chemistry Université Laval Quebec City, QC, G1V 0A6, Canada DOI: /aenm Here, we have been demonstrated enhanced PCE of the low band-gap polymer solar cell (PDPPFTF:PC 70 BM BHJ) fabricated by insertion of binary processing additives of 1,8-diiodooctane (DIO)/CN in CB solvent with an optimized ratio. The binary and ternary solvent mixture using chloroform (CF) solvent can improve the morphology of the BHJ blends films as previously reported. [ 5 ] However, any remaining residual CF may cause problems in the PDPPFTF system, including low reproducibility of the performance due to the fast evaporation of the solution during spin-casting. It is, therefore, difficult to accurately control the film thickness and film homogeneity. In this research, the low band-gap polymer solar cells are optimized by binary additive (DIO/CN) with optimized ratio, donor-acceptor ratio, and BHJ film thickness. Additionally, the electron transport layer of TiO x serves to enhance J SC as reported earlier for large band-gap polymers. [ 6,7 ] These results support the fabrication of efficient low band-gap polymer based sub-cells for high-performance tandem devices. The molecular structures of low band-gap polymer, PDPPFTF, the acceptor, PC 70 BM and the proposed device architecture are shown in Figure 1a. The devices are fabricated on glass/indium tin oxide (ITO; 150 nm) substrates with subsequent deposition of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) ( 35 nm), followed by deposition of the BHJ layer ( 100 nm) and the TiO x ( 10 nm) ETL capped with Al (100 nm). As shown in the UV-vis spectra of Figure 1 b, PDPPFTF shows a red-shift during film solidification, whereas in solution, the wavelengths for maximum absorption are almost similar with maximum at 80 nm. Figure 1 c shows the highest occupied molecular orbital (HOMO) ( 5.9 ev), lowest unoccupied molecular orbital (LUMO) ( 3.91 ev) energy levels, and band-gap (1.38 ev) of PDPPFTF as obtained by cyclic voltammetry (CV) (the CV data are shown in Figure S1a, Supporting Information). Furthermore, our synthesized PDPPFTF polymer exhibits M n = 7 kda, and yield = 83% as shown in Table S1 (Supporting Information). Figure shows current density voltage ( J V ) curves of the PDPPFTF:PC 70 BM BHJ solar cells based on different fabrication conditions of processing additive, donor and acceptor ratio, and BHJ film thicknesses, respectively. As shown in Figure a, the BHJ device without additive exhibits low PCE = 0.9% ( V OC = V, J SC = 4.5 ma cm, and FF = 0.33). On the other hand, the BHJ devices with single additive of 3% DIO or 3% CN (v/v) show improved PCE = 4.% ( V OC = V, J SC = ma cm, and FF = 0.56) or 4.0% ( V OC = 0.77 V, J SC = 10.5 ma cm, and FF = 0.54), respectively. The BHJ Adv. Energy Mater. 014, 4, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (1 of 6)

2 Figure 1. a) Molecular structure of low band-gap donor polymer of PDPPFTF and the PC 70 BM acceptor, and a schematic of the structure of the PDPPFTF:PC 70 BM BHJ solar cells. b) Normalized UV-vis absorption spectra of PDPPFTF. c) Band diagram levels of the proposed device structure. solar calls using binary processing additives of 3% DIO/3% CN in CB (v/v) exhibits enhanced PCE = 4.4% with the following parameters: V OC = V, J SC = ma cm, and FF = The PDPPFTF:PC 70 BM BHJ film with 3% DIO/3% CN reveals maximum absorption peak near 80 nm as shown in UV-vis spectra of Figure S1b (Supporting Information). From these PCE results, the binary processing additives are more efficient than the single additive at increasing J SC. As shown in the J V curves of Figure b, the BHJ solar cells with the weight ratio of 1: between donor and acceptor exhibits higher PCE than the 1:1 ratio due to the improved FF: from 0.40 to Moreover, the BHJ devices show enhanced PCE when the thickness is optimized at 100 nm because of the higher J SC from 10.6 ma cm ( 10 nm) and ma cm ( 85 nm) to ma cm as shown in J V curves of Figure c. Therefore, we found the optimized fabrication conditions of the low band-gap polymer solar cells of PDPPFTF:PC 70 BM through the 1: weight ratio and the thickness of 100 nm with the binary processing additives of 3% DIO/3% CN in CB solvent. At this current stage, the FF near 0.60 has to be improved through further investigation. The remarkable differences in the PCE in the device and efficiency parameters without and with binary processing additives originate from the improved nanomorphology of the BHJ films. Figure 3 shows the atomic force microscopy (AFM) images of PDPPFTF:PC 70 BM BHJ dissolved in CB without processing additive and dissolved in CB with binary processing additives (3% DIO/3% CN in CB). The AFM scan size is 5 μ m 5 μm. The BHJ film without processing additive exhibits non-optimized morphology between the PDPPFTF and PC 70 BM, which appeared as large microsized aggregated domains in the film with a root mean square (rms) thickness variation of 9. nm as shown in Figure 3 a,b. Although the BHJ with single additive of 3% DIO or 3% CN (v/v) exhibited relatively improved nanomorphology, the aggregated regions are still located on the surface and there are little fibrillar structures as shown in Figure S (Supporting Information). In contrast, the BHJ film with the insertion of binary processing additives (3% DIO/3% CN in CB) exhibited improved nanoscale phase separation with elongated fibrillar structures which result in efficient charge separation due to the interpenetrating nanomorphology with a length scale of 0 nm. The rms thickness variation value 1.1 nm was obtained from Figure 3 c,d. Therefore, the binary processing additives in BHJ film resulted in a well-defined nanomorphology with smaller domains, which lead to higher J SC and FF ( of 6) wileyonlinelibrary.com 013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Energy Mater. 014, 4,

3 Figure. J V curves of the PDPPFTF:PC 70 BM BHJ solar cells based on different optmized fabrication conditions of: a) without, with single (3% DIO or 3% CN), and binary processing additives (3% DIO/CN) in CB; b) PDPPFTF and PC 70 BM ratios of 1:1, 1:1.5, 1:, and 1:.5; and c) BHJ film thicknesses of 85 nm, 100 nm, and 10 nm, respectively. To confirm the interlayer effects at this low band-gap poly mer BHJ system, an electron transport layer (ETL) of TiO x was spin-cast with a thickness of 10 nm between BHJ and Al cathode to induce electron transport and hole blocking effects. [ 3,7 ] As shown in Figure 4 a, the BHJ device with TiO x exhibits enhanced PCE = 4.7% from the following para meters: V OC = V, J SC = ma cm, and FF = The incident photon-to-current-conversion efficiency (IPCE) spectra of BHJ solar cells fabricated without and with TiO x are displayed in Figure 4 b. The devices with TiO x exhibit average IPCE over 40% from the wavelength between 400 nm and 600 nm region and the maximum peak approaches 50% at 470 nm. Also, the measured J SC (11.33 ma cm ) from the IPCE is well matched with the observed J SC (11.35 ma cm ) from the J V curves with only 0.% error as shown in Table S (Supporting Information). From the insertion of TiO x interlayer, the J SC is improved 7.5% for the PDPPFTF:PC 70 BM solar cells as shown in Figure 4 b. The average PCE with single additive (DIO and CN) and binary additives (DIO/ CN) from the 10 devices are summarized in Figure S3 (Supporting Information). Figure 5a,b shows the J V curves and PCE range of the PDPPFTF:PC 70 BM BHJ solar cells as a function of volume ratio (v/v) of the binary processing additives (DIO/CN) from 0% to 5% in CB. By using an optimized ratio of 3% DIO/3% CN in CB, the device performance is sharply improved over cells made without a processing additive or over cells made using a single additive of DIO and CN, respectively. Whereas increasing the content of DIO/CN up to 5% in CB, the PCE is reduced from 4.7% to 3.9% due to the gradually decreased J SC from to 9.48 ma cm with little change in V OC or the FF. These data are summarized in Table 1. Moreover, the BHJ device with 3% DIO/3% CN (binary) reveals a reduced series resistance ( R s =.3 Ω cm ) compared to the device with 3% DIO (single) ( R s = 5.0 Ω cm ) or 5% DIO/5% CN (binary) ( R s =.6 Ω cm ) from Table 1 and Figure S4 (Supporting Information) of the J V curves in the dark. Therefore, the enhanced PCE of the PDPPFTF:PC 70 BM BHJ solar cells is achieved by optimizing the ratio of the binary processing additives and the use of an ETL of TiO x, which correlated to the increased J SC with reduced series resistance and finer phase-separated BHJ nanomorphology. In conclusion, we fabricated PDPPFTF:PC 70 BM BHJ solar cells with enhanced PCE through the use of binary processing additives of DIO/CN in CB solvent with an optimized 3% ratio (v/v). The BHJ device with binary processing additives exhibits increased PCE = 4.7% with optimized J SC of ma cm compared to the BHJ without additive (4.5 ma cm ) and with single additive (10.98 ma cm ). Furthermore, binary processing additives affects smoother surface (rms thickness variation: 1.1 nm) and improved phase separation with percolated nanomorphology in the BHJ material, whereas the BHJ material dissolved in only CB exhibits microscale aggregated surface regions (rms: 9.1 nm). The ETL of TiO x also works well between BHJ and Al cathode and increases J SC by 7.5% compared to the device without TiO x. The optimized fabrication Adv. Energy Mater. 014, 4, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (3 of 6)

4 Figure 3. AFM images of a,b) PDPPFTF:PC 70 BM BHJ dissolved in CB without processing additive and c,d) PDPPFTF:PC 70 BM BHJ dissolved in CB with binary processing additive (3% DIO/ 3% CN). Panels (a,c): height images; panels (b,d): phase images. The scan size is 5 μ m 5 μm. conditions of PDPPFTF:PC 70 BM BHJ solar cells indicate that this donor system is a promising candidate as a sub-cell for high-performance tandem devices. Experimental Section Synthesis of Low Band-Gap Polymer PDPPFTF : All reagents or starting materials were purchased from Sigma Aldrich and used without further purifi cation. The synthesis procedure for the PDPPFTF decribed in detail in ref. [ 4 ] was used. 3,6-bis(5-bromofuran--yl)-,5- bis(-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(h,5h)-dione (130.1 mg, 0. mmol),,5-bis(trimethylstannyl)-thiophene (8.0 mg, 0. mmol), Pd (dba) 3 (3.7 mg) and P(o-tol) 3 (4.9 mg) were added in a dry/nitrogen purged flask. Three nitrogen purge/vacuum cycles under vigorous stirring were done before adding dry/nitrogen saturated chlorobenzene (4 ml). The mixture was stirred for 48 h at 110 C. After cooling to room temperature, the mixture was precipitated in a methanol:water solution (10:1) then fi ltered. The residue was loaded in a Soxhlet thimble. Successive extractions with acetone, hexanes and chloroform and then precipitation in methanol of the latter afforded a dark purple material. (100 mg, 83%) Fabrication of Low Band-Gap Polymer Solar Cells : The glass/ ITO substrate ( 0 Ω / ) were prepared by Thin Film Devices, Inc. and cleaned with deionized (DI) water, acetone, and isopropyl alcohol with ultrasonication for 30 min. The hole transport layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (AI 4083) was spin-coated with a thickness of 35 nm at 5000 rpm for 40 s. The blended ratio of PDPPFTF:PC 70 BM BHJ solution was prepared from 1:1 to 1: with an overall concentration of 30 mg ml 1 in CB solvent without additive, with single (DIO or CN), and binary processing additives (DIO/CN) with ratios from 0% to 5% in CB (v/v). The BHJ solution was stirred at 00 rpm on the 60 C hotplate overnight. The PDPPFTF:PC 70 BM BHJ was spin-coated before using a PTFE fi lter of 0.45 μ m with a thickness from 85 nm to 10 nm at 500 to 1500 rpm. The BHJ fi lm was then heated to 70 C for 10 min to dry residual solvents. The ETL of TiO x was spin-coated with a thickness of 10 nm. After that, Al cathode ( 100 nm) was deposited by thermal evaporator with a rate of 3 Å s 1. Measurement and Characterization : Before the measuring the devices, the light source was calibrated using by silicon reference cells with an (4 of 6) wileyonlinelibrary.com 013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Energy Mater. 014, 4,

5 Figure 4. a) J V curves of PDPPFTF:PC 70 BM BHJ solar cells (dissoved in CB with 3% DIO/3% CN) without and with electron transport layer (ETL) of TiO x. b) IPCE spectra of the devices without and with ETL of TiO x and measured J SC (by IPCE). intensity of 100 mw cm at the solar simulator (AM 1.5 Global). Also, the J V curves of the solar cells were measured with a Keithley 400 Sourcemeter. All the device areas were determined by the mm aperture during the measurement for accurate PCE. To confi rm the J SC values at the J V curves, the IPCE was analyzed using by a QE measurement system (PV measurements, Inc.) after the calibration of a monochromatic power-density. The surface of the PDPPFTF:PC 70 BM BHJ fi lms were analyzed by AFM (AFM Asylum MFP3D) to characterize Figure 5. a) J V curves and b) PCE ranges of the PDPPFTF:PC 70 BM BHJ solar cells as a function of binary processing additive (DIO/CN) ratios from 0% to 5% in CB. the nanomorphology depending on without and with binary processing additives. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Table 1. The effi ciency parameters of the low band-gap polymer solar cells from a PDPPFTF/PC 70 BM BHJ with TiO x dissolved in CB without, with single (DIO), and binary processing additives (3% and 5% DIO/CN). Processing Additive condition PDPPFTF: PC70 BM BHJ solar cell V OC [V] J SC [ma cm ] FF PCE [%] R s [ Ω cm ] R sh [k Ω cm ] CB (Only) CB 3% DIO (single) CB 3% DIO/3% CN (binary) CB 5% DIO/5% CN (binary) Adv. Energy Mater. 014, 4, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (5 of 6)

6 Acknowledgements D.H.W. and A.K.K.K. contributed equally to this work. Research at UCSB, including the fabrication and testing of solar cells and the measurements and analysis, was supported by the Air Force Offi ce of Scientifi c Research (AFOSR FA ), Dr. Charles Lee, Program Offi cer. A.K.K.K. thanks Agency for Science Technology and Research (A*STAR) of Singapore for a postdoctoral fellowship. Authors are grateful to support from the National Science Foundation under Grant No. NSF DMR , to the Materials Research Laboratory (MRL) and to the MRL staff. Received: July 17, 013 Revised: September 13, 013 Published online: October 5, 013 [1] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 70, [] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 005, 4, 864. [3] J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, A. J. Heeger, Science 007, 317,. [4] S. H. Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, A. J. Heeger, Nat. Photonics 009, 3, 97. [5] D. H. Wang, D. Y. Kim, K. W. Choi, J. H. Seo, S. H. Im, J. H. Park, O O. Park, A. J. Heeger, Angew. Chem. Int. Ed. 011, 50, [6] L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G. Li, Y. Yang, Nat. Photonics 01, 6, 180. [7] Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan, A. J. Heeger, Nat. Mater. 01, 11, 44. [8] D. H. Wang, J. K. Kim, J. H. Seo, I. Park, B. H. Hong, J. H. Park, A. J. Heeger, Angew. Chem. Int. Ed. 013, 5, 874. [9] R. C. Chiechi, J. C. Hummelen, ACS Macro Lett. 01, 1, [10] A. Marrocchi, D. Lanari, A. Facchetti, L. Vaccaro, Energy Env. Sci. 01, 5, [11] P. Kumar, S. Chand, Prog. Photovoltaics 01, 0, 377. [1] B. Walker, C. Kim, T.-Q. Nguyen, Chem. Mater. 011, 3, 470. [13] Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Nat. Photonics 01, 6, 591. [14] A. K. K. Kyaw, D. H. Wang, D. Wynands, J. Zhang, T.-Q. Nguyen, G. C. Bazan, A. J. Heeger, Nano Lett. 013,13, [15] D. H. Wang, A. K. K. Kyaw, V. Gupta, G. C. Bazan, A. J. Heeger, Adv. Energy Mater. 013, 3, [16] V. S. Gevaerts, A. Furlan, M. M. Wienk, M. Turbiez, R. A. J. Janssen, Adv. Mater. 01, 4, 130. [17] S. Sista, Z. R. Hong, L. M. Chen, Y. Yang, Energ. Environ. Sci. 011, 4, [18] J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C. C. Chen, J. Gao, G. Li, Y. Yang, Nat. Commun. 013, 4, [19] J. You, C.-C. Chen, Z. Hong, K. Yoshimura, K. Ohya, R. Xu, S. Ye, J. Gao, G. Li, Y. Yang, Adv. Mater. 013, 5, [0] Y. Zou, D. Gendron, R. Neagu-Plesu, M. Leclerc, Macromolecules 009, 4, [1] J. C. Bijleveld, A. P. Zoombelt, S. G. J. Mathijssen, M. M. Wienk, M. Turbiez, D. M. d. Leeuw, R. A. J. Janssen, J. Am. Chem. Soc. 009, 131, [] W. Li, W. S. C. Roelofs, M. M. Wienk, R. A. J. Janssen, J. Am. Chem. Soc. 01, 134, [3] J. C. Bijleveld, V. S. Gevaerts, D. Di Nuzzo, M. Turbiez, S. G. J. Mathijssen, D. M. d. Leeuw, M. M. Wienk, R. A. J. Janssen, Adv. Energy Mater. 010,, E4. [4] C. H. Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee, J. M. J. Fréchet, J. Am. Chem. Soc. 010, 13, [5] L. Ye, S. Zhang, W. Ma, B. Fan, X. Guo, Y. Huang, H. Ade, J. Hou, Adv. Mater. 01, 4, [6] J. Jo, A. Pron, P. Berrouard, W. L. Leong, J. D. Yuen, J. S. Moon, M. Leclerc, A. J. Heeger, Adv. Energy Mater. 01,, [7] J. Y. Kim, S. H. Kim, H.-H. Lee, K. Lee, W. Ma, X. Gong, A. J. Heeger, Adv. Mater. 006, 18, (6 of 6) wileyonlinelibrary.com 013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Energy Mater. 014, 4,