Comparison of small amounts of polycrystalline donor materials in C ₇₀-based bulk heterojunction photovoltaics and optimization of dinaphthothienothiophene based photovoltaic

Comparative studies of the effects of a series of polycrystalline donors on the performance of 95 wt.%-C₇₀-based bulk-heterojunction (BHJ) photovoltaics were conducted. A BHJ based on the wide band-gap molecule dinaphthothienothiophene (DNTT) shows power conversion efficiency (η_PCE) 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 30 wt.%. The low-lying DNTT ionization potential and the high μ_h in the DNTT blend support the excellent DNTT device performance. The low performance of BHJs with 5 wt.% PEN and 5 wt.% 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 5 wt.% donor has a positive correlation with the corresponding device performance. The η_PCE of a 5 wt.%-DNTT BHJ cell was improved to 4.92% by optimizing the cathode buffer layer.