Electron–hole analysis for substituent effects in donor–acceptor conjugated polymers

The development of an all-electron donor (D) and deficient-electron acceptor (A) for preparing conjugated polymer fragments, known as D-A type polymers, has received considerable attention in organic photovoltaics. As experimental studies for such development can be expensive, theoretical calculations are employed to predict the product performance at a low cost. This study uses time-dependent density functional theory for electron–hole analysis to explore excitation characteristics. Particularly, we elucidate the effects of substituents (–OCH3 and –F) on the photoelectric efficiency of 1,3-bis(2-ethylhexyl)− 4 H,8 H-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione–3′,4′-dioctyl-2,2′:5′,2′:5′,2′-quaterthiophene polymer. Energy levels, optical absorption, electron–hole distribution, and charge transfer are investigated based on the optimized structure of the polymers under periodic boundary conditions with different substituents. Results indicate that the –OCH3 substituent increases the highest occupied molecular orbital energy level, reduces the band gap, and enhances the charge transfer and electron concentration of the D fragment, whereas the –F substituent improves its open circuit voltage. The theoretical analysis suggests that this compound with substituents has practical development value.