Comparison of small amounts of polycrystalline donor materials in C70-based bulk heterojunction photovoltaics and optimization of dinaphthothienothiophene based photovoltaic

• DNTT shows much higher PCE of 4.28% than pentacene and CuPc in a 5%-donor:C70 BHJ.
• PL spectra indicate the reason for the low performance of pentacene BHJ cell.
• Hole mobility of the 5%-donor blends has a positive relation with the devices’ PCEs.
• This work demonstrates that BHJs with 5%-donor are not suitable to all the donors.

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 (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 30 wt.%. 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 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 ηPCE of a 5 wt.%-DNTT BHJ cell was improved to 4.92% by optimizing the cathode buffer layer.

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