Impact of the Nature of the Side-Chains on the Polymer-Fullerene Packing in the Mixed Regions of Bulk Heterojunction Solar Cells

T Wang and MK Ravva and JL Bredas, ADVANCED FUNCTIONAL MATERIALS, 26, 5913-5921 (2016).

DOI: 10.1002/adfm.201601134

Polymer-fullerene packing in mixed regions of a bulk heterojunction solar cell is expected to play a major role in exciton-dissociation, charge-separation, and charge-recombination processes. Here, molecular dynamics simulations are combined with density functional theory calculations to examine the impact of nature and location of polymer side-chains on the polymer fullerene packing in mixed regions. The focus is on poly-benzo1,2-b: 4,5-b' dithiophene-thieno3,4-c pyrrole-4,6-dione (PBDTTPD) as electron-donating material and 6,6-phenyl-C-61-butyric acid methyl ester (PC61BM) as electron- accepting material. Three polymer side-chain patterns are considered: i) linear side-chains on both benzodithiophene (BDT) and thienopyrroledione (TPD) moieties; ii) two linear side-chains on BDT and a branched side- chain on TPD; and iii) two branched side-chains on BDT and a linear side-chain on TPD. Increasing the number of branched side-chains is found to decrease the polymer packing density and thereby to enhance PBDTTPD-PC61BM mixing. The nature and location of side-chains are found to play a determining role in the probability of finding PC61BM molecules close to either BDT or TPD. The electronic couplings relevant for the exciton-dissociation and charge-recombination processes are also evaluated. Overall, the findings are consistent with the experimental evolution of the PBDTTPD-PC61BM solar-cell performance as a function of side-chain patterns.

Return to Publications page