Below the Urbach Edge: Solar Cell Loss Analysis Based on Full External Quantum Efficiency Spectra

We suggest a new solar cell loss analysis using theexternal quantumefficiency (EQE) measured with sufficiently high sensitivity to alsoaccount for defects. Unlike common radiative-limit methods, wherethe impact of deep defects is ignored by exponential extrapolationof the Urbach absorption edge, our loss analysis considers the fullEQE including states below the Urbach edge and uses corrections forband-filling and light-trapping. We validate this new metric on awhole range of photovoltaic materials and verify its accuracy by electricalsimulations. Any deviations between the new metric and experimentalopen-circuit voltage are due to the presence of spatially localizeddefects and are explained as violations of the assumption of flatquasi-Fermi levels through the device.