Abstract
Comparative studies of the effects of a series of polycrystalline donors on the performance of 95wt.%-C70-based bulk-heterojunction (BHJ) photovoltaics were conducted. A BHJ based on the wide band-gap molecule dinaphthothienothiophene (DNTT) shows power conversion efficiency (ηPCE) of up to 4.28%. The photovoltaic parameters are superior to those of devices using the similar molecule pentacene (PEN) or polycrystalline copper phthalocyanine (CuPc) for donor concentrations from 5 to 30wt.%. The low-lying DNTT ionization potential and the high μh in the DNTT blend support the excellent DNTT device performance. The low performance of BHJs with 5wt.% PEN and 5wt.% CuPc may stem from strong exciplex recombination in the PEN:C70 blend and limited hole mobility combined with geminate polaron-pair recombination in the CuPc:C70 blend. The zero-field hole mobility of the blends with 5wt.% donor has a positive correlation with the corresponding device performance. The ηPCE of a 5wt.%-DNTT BHJ cell was improved to 4.92% by optimizing the cathode buffer layer.
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