Medium Bandgap Small Molecule Acceptors With Isomer‐Free Chlorinated End Groups Enabling High‐Performance Tandem Organic Solar Cells

Abstract Tandem organic solar cell (TOSC), composed of the front and rear cells with complementary absorption, is effective device structure for surpassing the Shockley–Queisser limit of single‐junction organic solar cells (OSCs). However, most of the medium bandgap (≈1.6 eV) organic photovoltaic materials for front cells in the TOSCs show considerable voltage losses. In this work, two medium bandgap (1.63 eV) isomeric small molecule acceptors m ‐DTC‐Cl‐1 and m ‐DTC‐Cl‐2 are synthesized with different chlorine substitution positions in 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile (IC). The different chlorine substituted positions in IC groups show significant influences on the physicochemical properties, charge dynamics, morphology, and photovoltaic performance of the acceptors. Consequently, the OSC with PTO2 as polymer donor and m ‐DTC‐Cl‐2 as acceptor delivers a champion power conversion efficiency (PCE) of 14.1% with a high open circuit voltage of 1.05 V and a low nonradiative energy loss of 0.25 eV, which indicates that the OSC is an ideal candidate for the application as front cell in the TOSCs. Then, a monolithic TOSC is fabricated with the OSC based on PTO2: m ‐DTC‐Cl‐2 as front cell and the OSC based on PTB7‐Th:BTPV‐4Cl as rear cell, which demonstrates a high PCE of 18.8%.