Investigation of Radical and Cationic Cross-Linking in High-Efficiency, Low Band Gap Solar Cell Polymers

Dithienogermole-co-thieno[3,4-c]pyrroledione (DTG-TPD) polymers incorporating chemically cross-linkable sidechains are reported and their properties compared to a parent polymer with simple octyl sidechains. Two cross-linking groups and mechanisms are investigated, UV-promoted radical cross-linking of an alkyl bromide cross-linker and acid-promoted cationic cross-linking of an oxetane cross-linker. It is found that random copolymers with a 20% incorporation of the cross-linker demonstrate a higher performance in bulk heterojunction solar cells than the parent polymer, while 100% cross-linker incorporation results in deterioration in device efficiency. The use of 1,8-diiodooctane (DIO) as a processing additive improves as-cast solar cell performance, but is found to have a significant deleterious impact on solar cell efficiency after UV exposure. The instability to UV can be overcome by the use of an alternative additive, 1-chloronapthalene, which also promotes high device efficiency. Cross-linking of the polymer is investigated in the presence and absence of fullerene highlighting significant differences in behavior. Intractable films cannot be obtained by radical cross-linking in the presence of fullerene, whereas cationic cross-linking is successful.