Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)
Publication date: 5th February 2015
Charge transport in bulk heterojunction solar cells is often affected by dispersion. Dispersive transport causes the transient extraction photocurrent to reduce over time, inhibiting charge extraction and likely harming device performance. In organic semiconductors, it has been suggested that dispersive transport is caused by a time-dependent mobility, as predicted by the well-known Gaussian Disorder Model. According to that model, the mobility of charge carriers reduces as they lose energy within the density of states. Therefore, decaying photocurrent transients are apparently explained through energetic relaxation. However, we report measurements in three bulk heterojunction photovoltaic blends that are inconsistent with this hypothesis, and therefore argue that an alternative explanation is needed in the studied systems.
We measured charge extraction transients in solar cells made from poly[N-9’’-hepta-decanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT):[6,6] phenyl-C70-butyric-acid-methyl-ester (PC70BM), poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):PC70BM, and poly[3,6-dithi- ophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene] (PDPP-TNT):PC70BM. If energetic relaxation is the cause of dispersive transport, then the mobility should change as a function of the transit time, because longer transit times allow for greater relaxation to occur. We varied the transit time by changing the electric field and device thickness, and we observed no dependence upon either. We tested different photon energies to determine whether “hot carriers” take longer to relax, and we observed no dependence. Finally, we tested the time-dependence of mobility with the photo-CELIV technique, and again observed no dependence.
We conclude that there is negligible energetic relaxation on the timescale needed to explain dispersive transport. We propose an alternative mechanism for the decaying photocurrent: that charge carriers are increasingly captured by trap states. Consequently, trap states appear to be more important than thermalisation in the studied devices, and efforts should be directed towards reducing the density of trap states rather than managing high-energy “hot” carriers.
Schematic illustration of two pathways to dispersive transport. (a) Thermalisation causes the mobility to decrease with time, whereas (b) trapping causes the loss of carrier density.
[1] Philippa, B.; Stolterfoht, M.; Burn, P. L.; Juška, G.; Meredith, P.; White, R. D.; Pivrikas, A. The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells. Scientific Reports 2014, 4, 5695. [2] Philippa, B.; Vijila, C.; White, R. D.; Sonar, P.; Burn, P. L.; Meredith, P.; Pivrikas, A. Time-independent charge carrier mobility in a model polymer:fullerene organic solar cell. Organic Electronics 2015, 16, 205–211.
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