Overcoming Fill Factor Reduction in Ternary Polymer Solar Cells by Matching the Highest Occupied Molecular Orbital Energy Levels of Donor Polymers

Despite the potential of ternary polymer solar cells (PSCs) to improve photocurrents, ternary architecture is not widely utilized for PSCs because its application has been shown to reduce fill factor (FF). In this paper, a novel technique is reported for achieving highly efficient ternary PSCs without this characteristic sharp decrease in FF by matching the highest occupied molecular orbital (HOMO) energy levels of two donor polymers. Our ternary device—made from a blend of wide-bandgap poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-alt-2,5-dioctyl-4,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (PBDT-DPPD) polymer, narrow-bandgap poly[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2- 6-diyl)] (PTB7-Th) polymer, and [6,6]-phenyl C₇₀-butyric acid methyl ester (PC₇₀BM)—exhibits a maximum power conversion efficiency of 10.42% with an open-circuit voltage of 0.80 V, a short-circuit current of 17.61 mA cm⁻², and an FF of 0.74. In addition, this concept is extended to quaternary PSCs made by using three different donor polymers with similar HOMO levels. Interestingly, the quaternary PSCs also yield a good FF (≈0.70)—similar to those of corresponding binary PSCs. This study confirms that the HOMO levels of the polymers used on the photoactive layer of PSCs are a crucial determinant of a high FF.