Thiophene structured additives toward enhanced structural order and reduced non-radiative loss for 19.9 % efficiency organic solar cells

Abstract

Organic semiconductors usually exhibit low structural-order owing to their weak intermolecular interactions, which leads to inferior charge transport and strong carrier recombination in photovoltaics. In this work, three thiophene structured solid additives namely thieno[3,2-b]thiophene (TT), 3,6-dibromothieno[3,2-b]thiophene (TTBB) and 3-bromo-6-iodio-thieno[3,2-b]thiophene (TTBI) are utilized to enhance the molecular interactions in a range of electron donors and acceptors. X-ray technique and molecular dynamics simulations reveal that by adjusting their electrostatic potential and dipole moment, their interacting site and interaction energy with donor (PM6 and D18) or acceptor (L8-BO) can be finely tuned. We find the most electropositive TTBB shows the strongest interaction with PM6, leading to enhanced crystallization; whilst the asymmetric TTBI brings the strongest molecular packing of L8-BO, consequent to more compact π-π stacking and ordered alkyl chain packing. As the results, the bulk heterojunction PM6:L8-BO photovoltaics upon the assistance of TTBI or TTBB obtain improved charge transport and reduced non-radiative energy loss, translating to power conversion efficiency (PCE) values of 18.7 % and 19.1 %. When further taking the synergetic effect of TTBB and TTBI on PM6(D18) and L8-BO respectively, the layer-by-layer deposited PM6(TTBB)/L8-BO(TTBI) and D18(TTBB)/L8-BO(TTBI) allowed superior PCEs of 19.4 % and 19.9 %, which are among the highest values of single-junction organic solar cells.