  Non-radiative losses to the open-circuit voltage are a primary factor in
limiting the power conversion efficiency of organic photovoltaic devices. The
dominant non-radiative loss is intrinsic to the active layer and can be
determined from the quasi-Fermi level splitting (QFLS) and the radiative
thermodynamic limit of the photovoltage. Quantification of the QFLS in thin
film devices with low mobility is challenging due to the excitonic nature of
photoexcitation and additional sources of nonradiative loss associated with the
device structure. This work outlines an experimental approach based on
electro-modulated photoluminescence, which can be used to directly measure the
intrinsic non-radiative loss to the open-circuit voltage; thereby, quantifying
the QFLS. Drift-diffusion simulations are carried out to show that this method
accurately predicts the QFLS in the bulk of the device regardless of
device-related non-radiative losses. State-of-the-art PM6:Y6-based organic
solar cells are used as a model to test the experimental approach, and the QFLS
is quantified and shown to be independent of device architecture. This work
provides a method to quantify the QFLS of organic solar cells under operational
conditions, fully characterizing the different contributions to the
non-radiative losses of the open-circuit voltage. The reported method will be
useful in not only characterizing and understanding losses in organic solar
cells, but also other device platforms such as light-emitting diodes and
photodetectors.
