Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.257
Publication date: 18th July 2019
The emergence of non-fullerene acceptors pushed organic photovoltaics (OPVs) to new heights, with efficiencies over 16% now being reported for single-junction devices [1]. However, there are still very few systems that maintain their performance at technically relevant thicknesses of 300 nm and more. One reason why efficient thick-film OPVs are so challenging is because of undesirable space-charge effects. There are basically two reasons for the accumulation of space charge in OPVs: unintentional doping [2,3] and imbalanced transport of electrons and holes [4]. In both cases, the electric field in the active layer is redistributed such that the photocurrent becomes space-charge limited, i.e., it shows a square-root dependence on the voltage. But then how can we distinguish whether the device limitations are caused by doping or imbalanced charge transport?
Here, we show that this can be done by determining the width of the space-charge region (w) as a function of the photogeneration rate (G). First, we show analytically that only imbalanced charge transport leads to a characteristic G-1/4 dependence of w. We then present a simple and robust method how the build-up of space charge with increasing generation can be monitored in a real operating device. The method is based on a light-intensity dependent measurement of the external quantum efficiency (EQE). Using a numerical reconstruction approach [5], we are able to attribute changes in the height and the spectral shape of the EQE with light intensity to variations of the space-charge region width. We demonstrate our approach for 300 nm thick OPV devices with a 10 times higher electron than hole mobility. We show that the photocurrent is completely governed by the mobility mismatch, while doping, but also other effects reducing the photocurrent as a function of voltage, such as a field-dependent generation, can be ruled out.
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