Conjugated Polymer-Based Flexible Photovoltaic Cells with Controlled Nanostructures
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Conjugated Polymer-Based Flexible Photovoltaic Cells with Controlled Nanostructures Myung-Su Kim1, Jin-Sung Kim2, Jae Cheol Cho1, Max Shtein1,3, L. Jay Guo2,3, and Jinsang Kim1,3 1 Materials Science and Engineering, University of Michigan, Ann Arbor, MI, 48109 2 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, 48109 3 Macromolecular Science and Engineering, University of Michigan, Ann Arbor, MI, 48109 ABSTRACT We demonstrate that conjugated polymers (CPs)-based flexible solar cells with well-defined interdigitated donor-acceptor interfaces enhance charge separation and transport. The welldefined straight donor-acceptor interfaces are achieved successful application of nanoimprinting technology to rationally designed energy harvesting and hole transporting conjugated polymers. Nanoimprinting enables the precise and direct nano-scale control of the shape of the donoracceptor interface on both rigid and flexible substrates.1 Comparison between the performances of the solar cells having imprinted different feature sizes revealed that the short circuit current can be systematically increased by the interfacial area of the heterojunction without affecting the open circuit voltage. The results also showed that the vertically oriented heterojunction facilitate charge transport and allow synergistically improved fill factor, open circuit current, and ensuing energy conversion efficiency beyond the gain of the interfacial area of the heterojunction. INTRODUCTION Conjugated polymer solar cells offer an attractive alternative to silicon-based photovoltaic (PV) technology for low-cost solar energy conversion due to easy processability, compatibility with flexible substrates, and the tunable optoelectronic properties of the CPs. However, the power conversion efficiency remains low. The relatively low power conversion efficiency exhibited by polymer-based solar cells stems from the fact that photo-excited electrons in a conjugate (CP) are not free from holes. Instead they are strongly bound with holes to form excitons.2 To break apart the electron-hole pair within its short lifetime and a short diffusion length, effective electron acceptors have been investigated as a dispersed second component in the bulk heterojuncion CP-based solar cells.1,2,3 Once electrons and holes are separated, efficient transport of the charges to electrodes is another critical step in energy conversion. However, in most bulk heterojunction CP-based solar cells, large amounts of electron acceptors above their percolation threshold have been mixed to provide a pathway for electrons to reach the cathode. Therefore, the major transport mechanism of the charges in bulk heterojunction CP solar cells is ineffective electron hopping among dispersed electron acceptors in CP matrix.4 In this context, producing ordered continuous nano-scale multi-domains with large interfacial area is necessary to realize effective separation and transport of the charges. EXPERIMENT Polymer synthesis and characterization Thermally de
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