A rigorous proof for a reciprocity theorem that relates the spectral and angular dependences of the electroluminescence of solar cells and light emitting diodes to the spectral and angular quantum efficiency of photocarrier collection is given. An additional relation is derived that connects the open circuit voltage of a solar cell and its electroluminescence quantum efficiency.

State-of-the-art models used for drift-diffusion simulations of organic bulk heterojunction solar cells based on band transport are not capable of reproducing the voltage dependence of dark current density and carrier concentration of such devices, as determined by current-voltage and charge-extraction measurements. Here, we show how to correctly reproduce this experimental data by including an exponential tail of localized states into the density of states for both electrons and holes, and allowing recombination to occur between free charge carriers and charge carriers trapped in these states. When this recombination via tail states is included, the dependence of charge-carrier concentration on voltage is distinctly different from the case of band-to-band recombination and the dependence of recombination current on carrier concentration to a power higher than 2 can be explained.

A consistent mathematical approach is presented that connects the Shockley-Queisser (SQ) theory to the analysis of real-world devices. We demonstrate that the external photovoltaic quantum efficiency $Q_e^{\mathrm{PV}}$ of a solar cell results from a distribution of SQ-type band-gap energies and how this distribution is derived from experimental data. This leads us to the definition of a photovoltaic band-gap energy $E_g^{\mathrm{PV}}$ as a reference value for the analysis of the device performance. For a variety of solar-cell devices, we show that the combination of $Q_e^{\mathrm{PV}}$ and electroluminescence measurements allows for a detailed loss analysis that is fully compatible with the principle of detailed balance.

This article presents a systematic study on the electronic transport mechanisms of ${\mathrm{CuGaSe}}_2$ -based thin film solar cells. A variety of samples with different types of stoichiometry deviations, substrates and buffer layers is investigated. We propose two transport models, namely tunneling enhanced volume recombination and tunneling enhanced interface recombination, which allow to explain the observed features for all devices under consideration. The doping level of the absorber layer turns out to be the most decisive parameter for the electronic loss mechanism. The doping is influenced by the type of stoichiometry deviation as well as by the Na content of the substrate. High doping levels result in tunnel assisted recombination. The best solar cells display the lowest tunneling rates. For these devices treatments of the absorber surface by air-annealing and/or the deposition temperature of the CdS buffer layer are decisive for the final device performance. We use the investigation of the open-circuit voltage relaxation to verify the assumptions on the dominant loss mechanism in the different devices.