$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$-based solar cells exceed power conversion efficiencies of 23%. However, the fill factor of these solar cells, with best values around 80%, is relatively low ($\mathrm{Si}$ reaches 84.9%) mostly due to diode factors greater than 1. Recently, we proposed metastable defects, a general feature of the $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ alloy, to be the origin of the increased diode factor even in low injection. Here, we measure the diode factor of the bare absorber layers using excitation-dependent photoluminescence. The increased diode factor above 1 can be well described by the model of metastable defects, as well as a slight excitation dependence within the experimentally accessible range of excitation intensities. We discuss how the excitation dependence of the diode factor depends on the parameters of the metastable defects. Within the same model, we can additionally describe the experimental diode factors of n- and p-type epitaxial $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ films. We find that the diode factors measured optically by photoluminescence impose a lower limit for the diode factor measured electrically on a finished solar cell. Interestingly, the lowest diode factor (optical and electrical) and consequently highest fill factor of 81.0% is obtained by $\mathrm{Ag}$ alloying, i.e., an $(\mathrm{Ag},\mathrm{Cu})(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ absorber. This finding hints to a pathway to increase fill factors and thus efficiencies for $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$-based solar cells.

• Received 27 April 2022
• Revised 5 December 2022
• Accepted 10 January 2023

DOI:https://doi.org/10.1103/PhysRevApplied.19.024052

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Published by the American Physical Society

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Photovoltaic Energy Conversion

Physical Review Applied is pleased to present a Collection on Photovoltaic Energy Conversion, in recognition of the imminent need to harness solar energy, and the key role that Applied Physics plays in that endeavor. Contributions to this collection will be published throughout 2021 and into 2022. The invited articles, plus an editorial by Guest Editors Shanhui Fan and Zetian Mi, are linked below.