Influence of HOMO Energy Alignment of Multiple Polymer Donors on Charge Transport and Fill Factor in Ternary All-Polymer Blend Solar Cells

Optimizing the fill factor (FF) in ternary all-polymer blend solar cells is challenging because incorporating a third polymer changes the blend nanomorphology and charge-transport energy levels relative to those of the donor–acceptor (D:A) host binary device. In this study, two ternary blend systems, D2x:D1–x:A, were carefully designed based on two host D polymers─poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T) and its fluorinated derivative (PBDB-T-2F)─which exhibited comparable morphological characteristics but differed in their highest occupied molecular orbital (HOMO) energy-level alignment with a second polymer donor (D2), poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene))-alt-(2,2-ethyl-3(or 4)-carboxylate-thiophene)] (PTO2). The contribution of the HOMO energy difference (ΔHOMO) between D and D2 on charge transport and the FF was evaluated. For the ternary system with ΔHOMO = 0.26 eV, the FF decreased for D2 fractions (x) > 0.5, reaching a minimum at x = 0.9, owing to hole trapping within the insufficiently percolated D networks. In contrast, no reduction in the FF was observed for the ternary system where ΔHOMO = 0.08 eV because hole trapping was largely mitigated. These distinctly different FF dependences on the loading amounts of D2 underscore the importance of precise energy-level matching in maintaining a high FF in ternary all-polymer blend systems, providing a guideline for polymer selection with excellent FF tolerance across various blend compositions.