Extending Se substitution to the limit: from 5S to 5Se in high-efficiency non-fullerene acceptors.

Based on the newly synthesized seleno[3,2-b]selenophene unit, two near-infrared non-fullerene acceptors (NFAs) of 4Se and 5Se are constructed by replacing four or all sulfurs with selenium in high-efficiency Y-series NFAs. Consequently, binary devices based on 4Se and 5Se afford PCEs of 15.17% and 15.23%, respectively, with a photoelectric response approaching 1000 nm. More excitingly, the energy loss of the 5Se-based device was as low as 0.477 eV along with almost the smallest non-radiative loss of ∼0.15 eV thus far.

Spirocyclic side chain of a non-fullerene acceptor enables efficient organic solar cells with reduced recombination loss and energetic disorder.

Suppressing intramolecular vibration of non-fullerene acceptors (NFAs) by molecular rigidification has been proven to be an effective way to reduce the non-radiative recombination loss and energetic disorder of organic solar cells (OSCs). Thus far, extensive attention has been drawn on rigidifying the fused-ring backbones of NFAs, whereas the highly flexible alkyl side chains are barely concerned. Herein, an effective strategy of side chain rigidification by introducing a spiro-ring is developed for the first time and applied to construct the NFA of Spiro-F. Compared to its counterpart F-2F, the rigid spirocyclic side chain can constrain the vibrational-rotational motion and control the orientation of two highly flexible n-octyl chains effectively. As a result, an optimal molecular packing with enhanced intermolecular actions and lower energetic disorder is achieved by Spiro-F, endowing the OSC based on the as cast blend of PM6:Spiro-F with a significantly improved PCE of 13.56% and much reduced recombination loss compared to that of PM6:F-2F. This work provides a feasible strategy to achieve efficient OSCs through the rigidification of the side chain and could boost the PCEs further if applied to some other efficient systems with smaller bandgaps.