Toward Efficient Triple-Junction Polymer Solar Cells through Rational Selection of Middle Cells

The photon energy losses of polymer solar cells (PSCs) routinely drag their experimental power conversion efficiencies (PCEs) far below the theoretical limits. We report herein efficient triple-junction PSCs (TJ-PSCs) with mitigated energy losses through rational selection of subcells. We reveal that avoiding strong photon competition between the front and middle cells is critical in balancing the absorption rate among subcells with realistic layer thicknesses. Efficient TJ-PSCs are achieved by stacking a front cell of PBDB-T-2F:PC71BM, a middle cell of PBDB-T:HF-TCIC, and a rear cell of PTB7-Th: IEICO-4F in series and connecting them with two functional interconnection layers. A PCE of 13.09% is obtained from champion devices, representing one of the best TJ-PSCs among the reported studies. It accounts for a 35% improvement in efficiency over those of single-junction PSCs with the same absorption range, which is mainly attributed to the reduced nonabsorbing and thermalization losses of TJ-PSCs.