Mapping Losses through Empirical Extraction of the Spatial External Luminescence Efficiency

The performance of photovoltaic cells is determined by the loss mechanisms and charge transport properties within the device. Thus, quantitative assessment of these mechanisms is fundamental for device optimization and the evaluation of new materials and interfaces. This paper introduces a method for the in-depth quantitative mapping of loss and charge transport mechanisms under operando conditions. The spatial external luminescence efficiency (SELE) is defined as the probability for an electron-hole pair that is photogenerated at a specific depth within a device to contribute to its photoluminescence. By coupling incident wavelength-dependent external luminescence efficiency measurements with optical modeling, we demonstrate the extraction of the SELE of a GaAs wafer. The SELE directly maps the competition between radiative and nonradiative recombination within the device and can thus be used to study the properties of surfaces and interfaces. Moreover, since the external luminescence efficiency is related to the obtainable photovoltage from the device, the SELE can be used to map the spatial contribution to the device photovoltage. Furthermore, the SELE is calculated by using device simulations that account for photon recycling. These simulations validate the extraction method and attribute the features in the profile to important physical quantities, such as the surface recombination velocity and carrier lifetime.