Abstract
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.
Highlights
As power-conversion efficiencies of organic solar cells surpass 18% [1,2], it has become essential to comprehend and eradicate every mechanism contributing to efficiency reduction
As the VOC is defined by the quasi-Fermi-level splitting (QFLS) of electrons and holes in the device, an accurate quantification of the QFLS is key for understanding nonradiative recombination processes in photovoltaic devices
Under electromodulated-photoluminescence conditions, the QFLS varies little across the device for each voltage and much less over the applied voltage range compared with the 10-meV variance in QFLS under Jidnajrk = JSACM1.5 conditions, seen in panel (b)
Summary
As power-conversion efficiencies of organic solar cells surpass 18% [1,2], it has become essential to comprehend and eradicate every mechanism contributing to efficiency reduction. Traditional photoluminescence measurements (as applied to inorganic semiconductors) are not valid as a method of determining the QFLS in organic semiconductor blends, since the contribution to charge generation is simultaneously overestimated for excitons and underestimated for CT states These discrepancies can be circumvented by employing Rau’s reciprocity principle between the charge collection of photogenerated carriers (under illumination) and the electroluminescent emission in the dark. Rau’s theory assesses the nonradiative losses by providing an expression for both the radiative limit and the nonradiative losses of the open-circuit voltage of a device [32] This has been successfully employed in conjunction with electroluminescent external quantum efficiency measurements to quantify the QFLS and related losses in a wide variety of semiconductor-based solar cells [33,34,35,36]. The electromodulated-photoluminescence measurements have been found to successfully predict the QFLS and related losses over the range of devices used
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