Isomerism: Minor Changes in the Bromine Substituent Positioning Lead to Notable Differences in Photovoltaic Performance

An isomerism strategy was employed to develop single, end-group bromine-substituted non-fullerene two isomeric acceptors, 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo [3,4-e]thieno[2, '' 3 '':4',5'] thieno[2',3':4,5]pyrrolo [3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl) bis(methanylylidene))bis(4-bromo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)dimalononitrile (BTIC-2Br-beta) and 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno [2,'' 3 '':4',5'] thieno[2',3':4,5]pyrrolo[3,2-g]thieno [2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5-bromo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)dimalononitrile (BTIC-2Br-gamma), for organic solar cells, aimed to examine the improvement of power conversion efficiency (PCE). The effects of isomerism on their optical, electronic, charge dynamics, morphological, and photovoltaic properties were systematically investigated. When blended with a donor poly{[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c']dithiophene-1,3-diyl]]} (PBDB-TF), BTIC-2Br-gamma-based devices exhibited an outstanding PCE of 16.52%, they exhibited an outstanding PCE of 16.52%, which was the highest recorded value among brominated acceptors, compared with 8.11% obtained for BTIC-2Br-pbased devices. Crystallographic analysis of BTIC-2Br-gamma single-crystal demonstrated that the entire molecular backbone presented a plane structure between the core and end groups. Moreover, multiple intermolecular interactions such as Br center dot center dot center dot pi and CN center dot center dot center dot H existing in the solid-state allowed BTIC-2Br-gamma to form a three-dimensional (3D) network-packing structure, providing more electron transport channels. Our morphology investigations revealed that the BTIC-2Br-gamma-blend films displayed tailored crystallite with distinct fibrillary nanostructures, and the low miscibility of PBDB-TF and BTIC-2Br-gamma obtained by the contact angle could assist the formation of the fibrillary interpenetrating networks to achieve effective charge transport pathways. Compared with BTIC-2Br-beta, BTIC-2Br-gamma possessed a higher extinction coefficient, more balanced charge transport, and weaker bimolecular recombination. This work shows that subtle changes in bromine position can significantly improve the photovoltaic efficiencies of organic solar cells (OSCs); therefore, it provides a new guideline for the rational design of efficient fused-ring electron acceptors (FREAs).

Automated summary

An isomerism strategy was employed to develop two isomeric acceptors with single end-group bromine substitution, BTIC-2Br-beta and BTIC-2Br-gamma, for organic solar cells. Comparative studies revealed that BTIC-2Br-gamma exhibited higher power conversion efficiency and superior charge transport characteristics compared to BTIC-2Br-beta when blended with PBDB-TF as the donor material.