Effects of the Isomerized Thiophene-Fused Ending Groups on the Performances of Twisted Non-Fullerene Acceptor-Based Polymer Solar Cells.

摘要： Recently, benefiting from the merits of small molecule acceptors (NFAs), polymer solar cells (PSCs) have achieved tremendous advances. From the perspective of the structural characteristics of the π-conjugated acceptor-donor-acceptor (A-D-A)-type of organic molecules, the backbone’s planarity, as well as the terminal groups and their substituents, have strong influences on the performances of the constructed NFAs. Through enlarging the dihedral angle of the conjugated main-chain of NFAs, a certain degree of enhanced photovoltaic parameters have been achieved. To further probe the influences of ending groups on the performances of nonplanar NFAs, we synthesized two new NFAs of i-cc23 and i-cc34 with isomerized thiophene-fused ending groups and twisted π-conjugated main-chain. Compared to the i-cc23 containing 2-(6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene)malononitrile ending group, the 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile contained acceptor i-cc34 has a relatively higher molar extinction coefficient, bathochromic-shifted absorption spectrum, and deepened energy levels. When mixed with PBDB-T in solar cells, the i-cc23-based device achieved an excellent open-circuit voltage (VOC) of 1.10 V and a moderated power conversion efficiency of 7.34%. Although the VOC of i-cc34 related device was decreased to 0.96 V, the short-circuit current density and fill factor were improved, giving rise to enhanced efficiency of 9.51%. Apart from the distinct photovoltaic performances, the two isomers-based devices exhibit high radiative efficiency of 8×10-4, leading to a very small non-radiative loss of 0.19 V. Our results emphasize the importance of the isomerized thiophene-fused ending groups on the performances of nonplanar NFAs-based PSCs.