Ternary Polymer Solar Cells Enabled Using Two Donor Polymers with Broad Absorption and Cascade LUMO Levels

A polymer donor PTB7-Th with deep lowest unoccupied molecular orbital (LUMO) levels and a long-wavelength absorption spectrum is incorporated into a D18-Cl:N3 binary system as the third component for ternary polymer solar cells (PSCs). We find that PTB7-Th plays a major role in improving the short-circuit current (JSC) of ternary PSCs. The effects of PTB7-Th on absorption, exciton dissociation, charge extraction, charge recombination, morphology, and crystallinity of the photoactive layer have been analyzed in detail. A high JSC generates an advanced power conversion efficiency (PCE) of 16.35% in D18-Cl:PTB7-Th:N3 (0.85:0.15:1.3) ternary PSCs even with a lower open-circuit voltage (VOC). However, the VOC is an obstacle that limits the further improvement of the PCE for these ternary PSCs. We find that the optimized ternary PSCs show comparable charge extraction and longer emission lifetimes compared with the other two PSCs. This study provides an effective and innovative approach to increase JSC and thus the PCE for ternary PSCs by introducing a guest donor with a deeper LUMO level and better miscibility with the acceptor.