The transient photovoltage (TPV) and charge extraction (CE) techniques have been frequently used in the past to determine charge carrier lifetime and density, respectively, in thin-film solar cells (such as organic and perovskite solar cells), allowing for the underlying charge carrier recombination properties in these systems to be clarified [1]. In the ideal case, the lifetime obtained from TPV, using a small perturbation of the carrier density at open-circuit, reflects the bulk recombination of photo-generated charge carriers in the active layer. In organic solar cells, this lifetime is generally strongly dependent on the steady-state light intensity (i.e. the open-circuit voltage). A simultaneous determination of the corresponding charge carrier density with CE at different steady-state light intensities (open-circuit voltages) then allows for the recombination rate and recombination order of the charge carriers to be obtained. Recently, however, the relevance of both the TPV lifetime and the associated recombination order, obtained in combination with CE, as material or device figure of merits for the understanding of the recombination in solar cells and other photoactive material systems has been questioned [2].

In this work, we review and expand the underlying theory of TPV and CE [3]. Based on fundamental electrical transient theory, we derive expressions for the associated TPV lifetime and CE carrier density, allowing for their physical meaning in thin-film solar cells to be assessed. The derived theoretical framework is verified by numerical transient drift-diffusion simulations and demonstrative experiments on organic solar cells. Based on the theoretical findings, different regimes for the extracted lifetime and charge carrier density in TPV and CE, respectively, can be distinguished. At low steady-state light intensities, the determination of the carrier lifetime and the carrier density is strongly distorted by capacitive effects associated with non-uniform carrier profiles. On the other hand, at high light intensities, the CE carrier density is susceptible to incomplete charge extraction. In order to identify the “working dynamic range” for the accurate determination of both the carrier lifetimes and the extracted carrier densities, TPV and CE measurements over a wide range of steady-state light intensities are required.