Naphthodithiophene-Diketopyrrolopyrrole Small Molecule Donors for Efficient Solution-Processed Solar Cells
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Naphthodithiophene-Diketopyrrolopyrrole Small Molecule Donors for Efficient SolutionProcessed Solar Cells Stephen Loser1,2, Carson J. Bruns1, Hiroyuki Miyauchi1, Rocío Ponce Ortiz1, Antonio Facchetti1,2,3*, Samuel I. Stupp1,2,4,5*, and Tobin J. Marks1,2 1
Department of Chemistry, Northwestern University, 2145 North Sheridan Road, Evanston, Illinois 60208 USA 2 The Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208 USA 3 Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077 USA, 4 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 USA 5 Department of Medicine and the Institute for BioNanotechnology in Medicine, Northwestern University, Chicago, Illinois 60611 USA ABSTRACT We report the synthesis, characterization, and implementation of various naphtho[2,3b:6,7-b’]dithiophene (NDT)-based donor molecules for organic photovoltaics (OPVs). When NDT(TDPP)2 (TDPP = thiophene-capped diketopyrrolopyrrole) contains all branched 2ethylhexyl chains and is combined with the electron acceptor PC61BM, a power conversion efficiency (PCE) of 4.06±0.6% is achieved. This respectable PCE is attributed to the broad, high oscillator strength visible absorption, the ordered molecular packing, and a high hole mobility of 0.04 cm2⋅V-1⋅s-1. We find utilizing linear C-12 side chains on either the TDPP or NDT framework dramatically increases the d-spacing, which directly correlates with inferior OPV device performance. This leads to the conclusion that the selection of an appropriate side chain plays a key role in determining OPV device performance of a small molecule donor. INTRODUCTION Solution-processed organic photovoltaics (OPVs) have the potential to be cost-effective, renewable energy sources due to their amenability to high-throughput roll-to-roll processing, earth-abundant constituents, and architectural tunability over multiple length scales.1 However, a major challenge for OPVs to attain widespread implementation is to advance the power conversion efficiencies (PCEs) from recently achieved and impressive values near 7%2 to ≥ 10%.3 Bulk-heterojunction (BHJ) solution-processed OPVs typically consist of an active layer having a phase-separated blend of an electron donor polymer and an electron acceptor fullerene placed between a tin-doped indium oxide (ITO) anode and an Al cathode. Strategies to enhance BHJ OPV performance include anode and cathode modifications with interfacial layers that block minority charge carriers while extracting majority charge carriers,4 and/or developing new low bandgap polymeric donors having broad optical cross-sections and high carrier mobility.5 Emerging and promising alternatives to polymeric OPV electron donors are smallmolecule donors, offering the attraction of facile synthesis and purification, less batch-to-batch variation in properties, and intrinsic monodispersity.6 Nevertheless, compared to polymeric donors, solution-processable molecular donors have not been extensively investigat
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