Cuprous oxide (Cu₂O) is a p-type metal-oxide semiconductor that has a fundamental, direct energy bandgap of 2.1 eV and cubic crystal structure. The native p-type conductivity of Cu₂O is related to the occurrence of acceptor-like copper vacancies (V_Cu). Cu₂O has been suggested to be suitable as a solar cell absorber for a long time but but device efficiencies have been limited so far to less than 10%¹. Nevertheless, it is an active topic of investigation for the fabrication of solar cells and photocatalysis but also for CO₂ reduction. Cu₂O has also been used as an archetype for the study of Rydberg excitons with very large principal quantum numbers of n = 25 and giant wavefunction extensions in excess of 2 µm in gem crystals of Cu₂O ². More recently Rydberg exciton-polaritons were detected in a SiO₂/Ta₂O₅/Cu₂O/Ta₂O₅/SiO₂ Fabry-Pérot cavity using natural gem Cu₂O³.

Here I will describe how it is possible to obtain Cu₂O layers that do not contain Cu₄O₃, CuO or Cu that is critical for the observation of excitons in Cu₂O layers but also for the fabrication of solar cells.  Furthermore, I will show that Cu₂O layers obtained under optimum conditions exhibit a detailed spectral structure and distinct peaks at 2.75, 2.55 and 2.21 eV corresponding to the blue, indigo and yellow direct transitions of Cu₂O as observed by ultrafast pump-probe spectroscopy at room temperature. A lower energy transition at 1.8 eV is attributed to carrier recombination via states located ~ 0.4 eV below the conduction band whose origin remains controversial and is discussed in conjunction with electronic structure calculations as it is very important from an applied and fundamental point of view.   

¹ S. Shibasaki et al. Appl Phys Lett 119(24), 242102 (2021).

^\2T. Kazimierczuk et al. Nature 514(7522), 343–347 (2014).

³K. Orfanakis et al. Nat Mater 21(7), 767–772 (2022).