Solvent Effects
Conditions for spin coating and evaporation affect device efficiency. Solvent and additives influence the donor-acceptor morphology. Additives slow down evaporation, leading to more crystalline polymers and thus improved hole conductivities and efficiencies. Typical additives include 1,8-octanedithiol, ortho-dichlorobenzene, 1,8-diiodooctane (DIO), and nitrobenzene. The effect of the DIO was attributed to the selective solubilization of the PCBM component. Additives can also lead to big increases in efficiency for polymers. For HXS-1/PCBM solar cells, the effect was also correlated with charge generation, transport and shelf-stability. Other polymers such as PTTBO also benefit significantly from DIO, achieving PCE values of more than 5% from around 3.7% without the additive.
Small differences in polymer structure can also lead to significant changes in crystal packing which inevitably affect device morphology. In the case of PCPDTBT Vs PSBTBT, there is a significant difference caused by the difference in bridging atom between the two polymers (C vs. Si) which implies that better morphologies are achievable with the PCPDTBT:PCBM solar cells containing additives as opposed to the Si system which achieves good morphologies without any help from additional substances.
Read more about this topic: Polymer Solar Cell
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