Effects of replacing benzodithiophene with a benzothiadiazole derivative on an efficient wide band-gap benzodithiophene-alt-pyrrolo [3,4-c]pyrrole-1, 3(2H,5H)-dione copolymer

In this study, we investigated the property modulation of a high energy converting, wide-band-gap, alternating polymer, P(BDTT-PPD), comprising benzodithiophene (BDTT) and pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (PPD) derivatives by replacing BDTT with 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT). The new alternating polymer, named P(DTBT-PPD), was prepared by polymerizing distannyl DTBT and dibromo PPD derivatives, with the aim of making a PPD-based, low-band-gap polymer for solar cell applications. Polymer P (DTBT-PPD) displayed an intense absorption band between 300 and 750 nm with two distinct absorption maxima at 439 and 605 nm as a film. The calculated optical band-gap (E-g) was 1.64 eV. The determined highest occupied and lowest unoccupied molecular (HOMO and LUMO, respectively) orbital energy levels of P(DTBT-PPD) were -5.30 and -3.66 eV, respectively. Solution-processed organic solar cells (OSCs), made with P (DTBT-PPD):PC70BM ([6,6]-Phenyl C-71 butyric acid methyl ester), provided a maximum power-conversion efficiency (PCE) of 2.23%. P(DTBT-PPD) displayed much lower E-g (approximate to 0.4 eV), slightly higher HOMO level (approximate to 0.14 eV), and considerably lower PCE (approximate to 4%), than P(BDTT-PPD). The increased curvature of P(DTBT-PPD) chains could be the main reason for their lower PCE than P(BDTT-PPD).