2 Molecular design of non-fullerene acceptors (NFAs) is of vital importance for highefficiency organic solar cells. The branched alkyl chain modification is often regarded as a counter-intuitive approach as which may introduce undesirable steric hindrance that reduces charge transport in NFAs. Here we show the design and synthesis of a highly efficient NFA family by substituting the beta position of thiophene unit on Y6-based A-DAD-A backbone with branched alkyl chains. It was found that such modification of different alkyl chain length could completely change the molecular packing behavior of NFAs, leading to improved structure order and charge transport in thin films. Unprecedented efficiency of 18.32% (certified value of 17.9%) with a fill factor of 81.5% is achieved for single-junction organic solar cells. This work reveals the importance of branched alkyl chain topology in tuning the molecular packing and blend morphology that leads to improved organic photovoltaic performance.

A new acceptor-donor-acceptor-structured nonfullerene acceptor ITCC (3,9-bis(4-(1,1-dicyanomethylene)-3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d':2,3-d']-s-indaceno[1,2-b:5,6-b']-dithiophene) is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V of over 1 V is recorded in photovoltaic devices, suggesting that ITCC has great potential for applications in tandem organic solar cells.