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Effect of a Band-Gap Gradient on the Radiative Losses in the Open-Circuit Voltage of Solar Cells

Sevan Gharabeiki1,*, Francesco Lodola1, Tilly Schaaf1,2, Taowen Wang1, Michele Melchiorre1, Nathalie Valle3, Jérémy Niclout3, Manha Ali4, Yucheng Hu4 et al.

Gunnar Kusch4, Rachel A. Oliver4, and Susanne Siebentritt1,†

  • *Contact author: sevan.gharabeiki@uni.lu
  • †Contact author: susanne.siebentritt@uni.lu
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PRX Energy 4, 033006 – Published 25 July, 2025

DOI: https://doi.org/10.1103/r25l-ftmp

Abstract

The radiative open-circuit voltage loss in a solar cell occurs because the absorptance spectrum near the band gap shows a gradual increase rather than a sharp step-function-like transition. This broadening effect has been attributed to band-gap fluctuations and/or to Urbach tails. In this work, we use modeling based on Planck’s generalized law to distinguish between these two effects. Our results demonstrate that Urbach tails have only a minimal effect on the absorptance-edge broadening and clarify that even an ideal direct semiconductor, with no band-gap fluctuations, shows broadening at the absorptance onset. Furthermore, state-of-the-art inorganic thin-film solar cells often incorporate a band-gap gradient across their thickness, which can further contribute to the absorptance broadening. Using Cu(In,Ga)Se2 (CIGSe) absorbers as a case study, we perform a comprehensive analysis of voltage losses through absolute photoluminescence and electroluminescence spectroscopy, combined with photospectrometry and high-spatial-resolution cathodoluminescence measurements. This study has two main purposes: (i) to present a detailed loss analysis based on Planck’s generalized law and (ii) to quantify the radiative losses arising from the band-gap gradient. We find that the loss analysis based on the combination of radiative, generation, and nonradiative losses covers the entire losses and does not leave out any other significant contribution. Samples with a graded band-gap profile show more pronounced broadening of the absorptance onset and up to 16 mV higher radiative losses compared to the samples with uniform band gap. There is also an indication that band-gap-graded samples have larger lateral band-gap inhomogeneity. Needs text.

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Popular Summary

Radiative open-circuit voltage loss, a source of performance loss in solar cells, reduces the maximum voltage they can produce. They are caused by a gradual, rather than abrupt, onset of light absorption. Here, the authors investigate what causes that gradual onset of absorption using advanced modeling and spectroscopy techniques. The results show that even an ideal direct semiconductor displays absorption onset broadening. Many solar cells employ a gradual change in the band gap of the material across its thickness, which helps reduce the loss of photogenerated electrons. This bandgap variation, however, increases the absorption onset broadening and thus the radiative voltage loss. This study provides a framework for understanding and quantifying radiative open-circuit voltage losses, offering valuable insights for designing more efficient solar cells through the careful management of band-gap gradients.

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PRX Energy 4, 033006– Published 25 July, 2025
  • Received 18 March 2025
  • Revised 29 May 2025
  • Accepted 23 June 2025
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