Quaternary copper-silver-bismuth-iodide compounds represent promising new wide-bandgap (2 eV) semiconductors for photovoltaic and photodetector applications. Henry J. Snaith, Harry C. Sansom and Laura M. Herz of the University of Oxford, UK, used gas phase co-evaporation to prepare Cu2AgBiI6 thin films and photovoltaic devices.
The results show that the performance of vapor-deposited films is highly dependent on processing temperature, exhibiting increased pinhole density and transformation into a mixture of quaternary, binary and metallic phases depending on the post-deposition annealing temperature. This phase change is accompanied by an increase in photoluminescence (PL) intensity and charge carrier lifetime, as well as the emergence of additional absorption peaks at high energies (~3 eV).
Normally, increased photoluminescence is a desirable property of solar absorber materials, but this change in photoluminescence is attributed to the formation of CuI impurity domains, whose defect-mediated optical transitions dominate the emissive properties of the film. Through optically pumped terahertz probe spectroscopy, it was revealed that CuI impurities hinder the transport of charge carriers in Cu2AgBiI6 films. It was also revealed that the main performance limitation of Cu2AgBiI6 materials is the short electron diffusion length.
Collectively, these findings pave the way to address key issues in copper-silver-bismuth-iodide materials and point to strategies for developing environmentally compatible wide-bandgap semiconductors.
BWJ Putland, M. Righetto, H. Jin, M. Fischer, AJ Ramadan, K.-A. Zaininger, LM Herz, HC Sansom, HJ Snaith, Compositional Transformation and Impurity-Mediated Optical Transitions in Co-Evaporated Cu2AgBiI6 Thin Films for Photovoltaic Applications. Adv. Energy Mater. 2024, 2303313.
DOI: 10.1002/aenm.202303313
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