Influence of Fullerene Acceptor on the Performance, Microstructure, and Photophysics of Low Bandgap Polymer Solar Cells

The morphology, photophysics, and device performance of solar cells based on the low bandgap polymer poly[[2,6'-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl (PBDTTT-EFT) (also known as PTB7-Th) blended with different fullerene acceptors: Phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), phenyl-C₇₁ -butyric acid methyl ester (PC₇₁BM), or indene-C₆₀ bisadduct (ICBA) are correlated. Compared to PC₇₁ BM-based cells - which achieve a power conversion efficiency (PCE) of 9.4% - cells using ICBA achieve a higher open-circuit voltage (V_OC) of 1.0 V albeit with a lower PCE of 7.1%. To understand the origin of this lower PCE, the morphology and photophysics have been thoroughly characterized. Hard and soft X-ray scattering measurements reveal that the PBDTTT-EFT:ICBA blend has a lower crystallinity, lower domain purity, and smaller domain size compared to the PBDTTT-EFT:PC₇₁BM blend. Incomplete photoluminescence quenching is also found in the ICBA blend with transient absorption measurements showing faster recombination dynamics at short timescales. Transient photovoltage measurements highlight further differences in recombination at longer timeframes due to the more intermixed morphology of the ICBA blend. Interestingly, a mild thermal treatment improves the performance of PBDTTT-EFT:ICBA cells which is exploited in the fabrication of a homo PBDTTT-EFT:ICBA tandem solar cell with PCE of 9.0% and V_OC of 1.93 V.