Competition Between Charge Collection and Recombination Governing the Fill Factor in Phase‐Separated All‐Polymer Solar Cells

Achieving a high fill factor (FF) in thick photoactive layers remains a critical challenge for polymer solar cells, primarily because the inherently low charge carrier mobility ( µ ) of organic semiconductors imposes a trade‐off between photocurrent generation and FF. Herein, the origin of FF limitation in all‐polymer solar cells employing donor polymers J51, J61, and J71 paired with the acceptor polymer N2200 is systematically investigated. Despite all devices exhibiting similarly suppressed bimolecular recombination with a recombination reduction factor ζ of ∼10 −2 , the photoactive layer thickness dependence of FF was significantly different. Interestingly, the J71/N2200 devices maintain a high FF value of >0.6 even at 200 nm thickness, whereas other devices exhibit substantial FF degradation. The J–V reproduction analysis clarifies that FF degradation originates from isolated domains that impede charge transport, and hence additional recombination. In contrast, the J71/N2200 devices feature a well‐interconnected phase‐separated morphology, which results in efficient charge collection. Quantitative analysis indicates that charge collection should proceed approximately 2500 times faster than recombination to sustain the high FF. These findings establish a practical design rule for polymer solar cells with a thick photoactive layer, which underlines that µ of >10 − 3 cm 2 V −1 s −1 is essential for minimizing recombination‐induced FF losses.