Understanding the impact of side-chains on photovoltaic performance in efficient all-polymer solar cells

摘要

In order to understand the impact of side-chains on photovoltaic performance and explore efficient all-polymer solar cells, chemical modifications on donor-acceptor based polymers containing benzo[1,2-b:4,5-b ']dithiophene (BDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) backbones were performed. Via side-chain fluorination, the molecular design resulted in lower highest occupied molecular orbital (HOMO) energy levels and enhanced backbone planarity. The intermolecular packing and solid-state ordering were found to significantly improve. These factors are considered as key influences for carrier transport. In contrast, introducing a bulky alkylthio substituent group was found to slightly distort the polymer backbone. As a result of the lower HOMO level, PTF8 exhibits an improved open circuit voltage (V-oc) compared to the template polymer PT8. However, due to the increased crystallinity and aggregation, PTF8 and PTS8 experience an unfavorable phase separation in polymer-polymer bulk heterojunction blends, hindering the PCE to about 4%. Through the introduction of alkylthio side-chains and fluorination, the polymer PTFS8 exhibits an extremely low HOMO level (-5.73 eV). This reduced HOMO level limits charge separation between the donor and acceptor polymers. Without any fluorination and alkylthio side-chains, the wide bandgap polymer PT8 exhibits desired HOMO energy levels and crystallinity, delivering a best PCE of 8% together with a high V-oc of 1.05 V, displaying its great potential for applications in efficient all-polymer optoelectronic devices.