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Solution Processable n-Type Perylene Diimide Copolymers for Organic Photovoltaics Ziqi Liang,* Russell A. Cormier, Alexandre M. Nardes, and Brian A. Gregg* National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401 [email protected]; [email protected] ABSTRACT Perylene diimides are known as promising n-type semiconductor building blocks. Here we report the synthesis and characterization of a set of three soluble poly(perylene diimide)s and their preliminary characterization in organic photovoltaic cells. These polymers are made through the polycondensation of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) with a variety of poly(ethylene glycol) (PEG)- or poly(propylene glycol) (PPG)-based diamine comonomers. The flexible spacer offers increased solubility in organic solvents and allows the perylene core to assume a conformation that promotes favorable cofacial π−π interactions. Mixtures of these polymers with the hole-transporting polymer, poly(3-hexylthiophene) (P3HT) result in significant fluorescence quenching. However, the phase separation occurs on a scale too large for a bulk heterojunction solar cell. The PPGylated poly(perylene diimide) shows an unusually low free electron concentration (~1.0 × 1012 cm-3) and therefore makes an excellent model system for future doping studies. These new polymers may have promise as stable electron-conductive layers with large light-absorptivities in solution-processable applications of organic electronics. INTRODUCTION Fused aromatic rings such as perylene diimides (PDIs) that provide a deep-lying highest occupied molecular orbital (HOMO) energy level and a rigid, planar core have emerged as viable building blocks to n-type organic semiconductors for photovoltaic (PV) and other applications. PDIs combine unique liquid crystalline behavior, high electron affinity/mobility, and excellent photochemical and thermal stability together in a single molecular architecture [1,2]. A critical challenge, however, is the poor solubility of these fused arylene systems in organic solvents, rendering it difficult to employ cost-effective solution-printing techniques [3]. The solubility issue of these arylene molecules has been addressed by employing a diimide structure whose imide nitrogens are modified with alkyl tails, or by attaching substituents to the main core at the bay positions. Unfortunately, many of these tail structures and side groups create steric hindrance that inhibits the extended π-stacking required for superior optoelectronic properties. PDI derivatives have been exploited as light-absorbing and electron-conductive components in both vacuum-deposited bilayer [4−7] and solution-processed bulk-heterojunction (BHJ) [8−10] PV devices, in conjunction with p-type molecular semiconductors or holetransporting polymers. One obvious advantage of PDIs over commonly used electron acceptor, methanofullerene derivatives (e.g., PC60BM), is their excellent photo- and thermo-stability. In the blend case, PDIs form large, mesoscopic aggregates upon annealing, l