Abstract
Transient optoelectronic measurements were used to evaluate the factors determining the open-circuit voltage of a series of planar photovoltaic devices based on hybrid perovskite layers with varying iodine/bromine ratios. Employing differential charging and transient photovoltage measurements, we used a simple device model based on the charge-carrier-density dependence of nongeminate recombination to re-create correctly not only the measured device open-circuit voltage (VOC) as a function of light intensity but also its dependence on bromine substitution. The 173 (±7) mV increase in device voltage observed with 20% bromine substitution is shown to result from a 227 (±8) mV increase in effective electronic band gap, which was offset in part by a 56 (±5) mV voltage loss due to faster carrier recombination. The faster recombination following 20% bromine substitution can be avoided by indene–C60 bisadduct (ICBA) substitution into the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) electron-collection layer, resulting in a further 73 (±7) mV increase in device VOC. These results are consistent with surface recombination losses at the perovskite/fullerene interface being the primary limitation on the VOC output of bromine-substituted devices. This study thus presents, and experimentally validates, a simple model for the device physics underlying voltage generation in such perovskite-based solar cells and demonstrates that this approach can provide key insights into factors limiting this voltage output as a function of material energetics.
Highlights
Organic−inorganic lead halide perovskites are attracting extensive interest for photovoltaic device applications
It is apparent that substitution of 20% bromide into the material resulted in an increase in VOC from 0.836 (±0.005) V to 1.009 (±0.005) V (Figure 1a), correlated with an increase in the optical band gap from 1.56 (±0.09) eV (795 nm) to 1.68 (±0.09) eV (738 nm), as is apparent from the External quantum efficiency (EQE) spectrum (Figure 1b) and the optical absorbance spectrum (Figure S1)
A further 73 (±7) mV increase in voltage was observed for 20% bromide with the substitution of 20% indene−C60 bisadduct (ICBA) into the phenyl-C61butyric acid methyl ester (PCBM) electron layer, as discussed further below
Summary
Organic−inorganic lead halide perovskites are attracting extensive interest for photovoltaic device applications. One important performance parameter determining photovoltaic device efficiency is the open-circuit voltage, VOC. The remarkably high VOC values achieved for MAPX-based devices, approaching to within 0.4 V of the MAPX optical band gap, are a key factor behind their high device efficiencies.[5−7] at present, there is no consensus on how photovoltage generation in perovskite devices depends on variations in material composition and device architecture. Employing a simple device model, we demonstrate that, by using these transient measurements, it is possible to successfully rationalize the absolute device open-circuit voltage VOC and its variation with light intensity and perovskite-layer halide substitution
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