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

Abstract

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.

Crosslinked Semiconductor Polymers for Photovoltaic Applications

“…This is in agreement with the results of Heeney and co-workers [32] discussed above, who have shown that just 20% of crosslinkable groups result in sufficient device stabilization.…”

“…Heeney and co-workers [32] compared derivatives of the low bandgap polymer PDTG-TPD with both bromine and oxetane functional groups (Figure 2). The pure polymers can both readily be crosslinked.…”

“…[14,18,22,25] In an attempt to systematically assess the relation between different functional groups and the stability of the crosslinked OSCs, comparative studies were performed by several groups. [22,25,32] Krebs and co-workers [22] compared bromine-, azide-, vinyl-, and oxetane-functionalized derivatives of the low bandgap polymer TQ1 (Figure 2). Crosslinking via UV-illumination resulted in solvent resistant films and did not change the optical absorption spectra, therefore excluding damage to the π-conjugated polymer backbone.…”

Stability issues of the next generation solar cells

“…Bulk heterojunction system exhibits less thermal stability since it likely undergoes macrophase segregation of the blend components with fullerene domain growing with time and thermal annealing . Hence, various strategies have been developed to stabilize the photoactive layer morphology by restricting the molecular diffusion within the blend such as introducing intermolecular cross‐linking , compatibilizer , and side‐chain engineering .…”

Enhanced miscibility and strain resistance of blended elastomer/π‐conjugated polymer composites through side chain functionalization towards stretchable electronics

“…Reactive bonding has been explored in polymeric solar cells with side chain functionalization by bromide, oxetane, azide and other groups . Devices with crosslinked polymers demonstrated enhanced thermal stability, resistance to solvents and overall lifetime improvement . Immiscible blends of elastomers and semicrystalline insulators commonly fracture at polymer interfaces, and improved polymer compatibility has been proven to increase cracking resistance .…”