Unraveling complex performance-limiting factors of brominated ITIC derivative: PM6 organic solar cells by using time-resolved measurements

Organic solar cells (OSCs) composed of a polymer and nonfullerene acceptor (NFA) have achieved dramatic improvements in power conversion efficiency (PCE), while the roles of many contributing factors are still not fully understood. Charge carrier mobility is conventionally considered a positive factor for NFA-OSC performance, yet this statement is often contradicted by experimental results. In this regard, indacenodithieno[3,2-b]thiophene-based NFAs (ITIC) brominated at the beta, gamma, and delta positions of the terminal IC groups are suitable for a comparative study of the performance-limiting factors. In this work, we evaluated the charge carrier dynamics in these ITIC (beta, gamma, delta):PM6 bulk heterojunction OSCs by using simultaneous time-of-flight (TOF)-time-resolved microwave conductivity (TRMC) and photoinduced charge extraction by linearly increasing voltage (photo-CELIV) techniques. The measured PCEs of these OSCs decreased in the order ITIC-gamma:PM6 (9.3%) > ITIC-beta:PM6 (8.5%) > ITIC:PM6 (7.5%) > ITIC-delta:PM6 (2.3%). The TOF-TRMC/photo-CELIV results revealed that despite having the highest PCE, ITIC-gamma:PM6 showed both low electron and hole mobilities, while its mobility relaxation characteristics were moderate among those of the OSCs. In contrast, ITIC-gamma:PM6 showed the smallest bimolecular recombination coefficient. Finally, fifteen parameters (mobility, etc.) were subjected to a linear regression analysis, which provided a quantitative understanding of the complex PCE-governing factors for NFA OSCs.

Automated summary

This study evaluates the charge carrier dynamics in different structured organic solar cells and reveals the factors limiting the power conversion efficiency (PCE). The results show that the OSC with the highest PCE has low electron and hole mobilities, while the one with higher electron and hole mobilities has a lower PCE. Additionally, the OSC with the highest PCE exhibits the smallest bimolecular recombination coefficient.