Abstract
Microstructural changes and the understanding of their effect on photocurrent generation are key aspects for improving the efficiency of organic photovoltaic devices. We analyze the impact of a systematically increased amount of the solvent additive diiodooctane (DIO) on the morphology of PBDTTT-C:PC71BM blends and related changes in free carrier formation and recombination by combining surface imaging, photophysical and charge extraction techniques. We identify agglomerates visible in AFM images of the 0% DIO blend as PC71BM domains embedded in an intermixed matrix phase. With the addition of DIO, a decrease in the size of fullerene domains along with a demixing of the matrix phase appears for 0.6% and 1% DIO. Surprisingly, transient absorption spectroscopy reveals an efficient photogeneration already for the smallest amount of DIO, although the largest efficiency is found for 3% DIO. It is ascribed to a fine-tuning of the blend morphology in terms of the formation of interpenetrating donor and acceptor phases minimizing geminate and nongeminate recombination as indicated by charge extraction experiments. An increase in the DIO content to 10% adversely affects the photovoltaic performance, most probably due to an inefficient free carrier formation and trapping in a less interconnected donor-acceptor network.
Highlights
Microstructural changes and the understanding of their effect on photocurrent generation are key aspects for improving the efficiency of organic photovoltaic devices
In the present work we report on the use of a systematically increasing fraction of the solvent additive DIO in PBDTTT-C:PC71BM solar cells
Photophysical and charge extraction techniques we found a multi-tiered effect of DIO on the blend microstructure and correlated changes in photogeneration, free carrier formation and recombination
Summary
Andreas Zusan1*, Bjorn Gieseking1*, Mario Zerson[2], Vladimir Dyakonov[1,3], Robert Magerle2 & Carsten Deibel1{. Collins et al and Hedley et al suggest that DIO reduces fullerene domain size, with only minor effects on domain composition and crystallinity[19], and supports the formation of elongated fiber-like polymer-rich and fullerene-rich domains in the optimized blend[20], respectively While all these studies address the comparison of the active layer without additive to the one with the optimum amount of DIO, little is known about the direct correlation between the additive content and the effects of morphological changes on free charge carrier formation and recombination losses, leading to the distinct maximum of the PCE for a certain additive concentration. The further increase in the DIO content up to 10% reduces the PCE, which appears to result from once again less interconnected pure domains
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