Organic film evolution and recombination losses in highly efficient perovskite/organic tandem solar cells

Perovskite/organic tandem solar cells are a promising strategy to surpass the efficiency limits of single-junction devices, yet their performance is restricted by recombination losses in the organic subcells. Here, we investigate these losses by tracking film evolution from the very initial stage of organic film formation. We strategically manipulated donor and acceptor ratios to modulate film growth characteristics, while employing in situ techniques to monitor the real-time crystallization dynamics. Our research findings underscore that the variance in donor content within the organic blend exerts a fine-tuning effect on the solution-to-solid transformation process. When the donor content is inadequate, the acceptor molecules tend to aggregate, disrupting molecular packing and lowering crystallinity. These morphological changes hinder exciton dissociation, thereby leading to charge recombination and deteriorating overall device performance. Optimizing film morphology and crystallization reduces recombination losses, enabling perovskite/organic tandem solar cells with a record 26.42% power conversion efficiency.