Efficient Organic Solar Cells Enabled by Chlorinated Nonplanar Small Molecules

Recently, the modification of end-capping groups for non-fullerene small molecules has been shown to be attractive as a promising structural-design strategy to construct effective electron acceptors for organic solar cells (OSCs) due to the tailorable molecular energy levels, tunable molar extinction coefficients, and easy-adjusting phase separation characteristic of the resultant acceptors. The application of chlorine substituents onto the ending groups of non-fullerene acceptors (NFAs) has positive effects such as an enhanced intermolecular charge-transfer effect between the electron-donating and electron-withdrawing structures, thus leading to broadening of the absorption regions. Lately, a series of NFAs containing appropriate nonplanar main chains have achieved promising performances. To further investigate the main-chain twisted NFAs, two chlorinated nonplanar small molecules were designed and prepared. Compared to the mono-chlorinated analogue (i-IEICO-2Cl), the dichlorinated molecule (i-IEICO-4Cl) had a bathochromic-shifted absorption spectrum with enhanced absorption intensity and a deeper molecular energy level. Blending with a medium band gap polymer donor (J52), the device of i-IEICO-4Cl shows more balanced charge mobilities and inhibitory charge recombinations, yielding a high efficiency of 12.16%, which is superior to 11.45% of an i-IEICO-2Cl-based OSC, revealing the effectiveness of chlorinated main-chain twisted acceptors for OSCs.