Recently, record efficiencies of 5.4% and 4.4% have been achieved for both wafer-based and thin-film CuO respectively. Efficiency enhancement in both classes of device has largely been driven by interface engineering which involves controlling the chemistry and band-alignment at the hetero-interface using buffer layers to improve open-circuit voltage (V). However, short-circuit currents (J) for both device architectures are still below the 14 mA/cm theoretical entitlement for a 2.1 eV bandgap absorber. Quantum efficiency analysis indicates that increasing charge collection length by reducing bulk recombination is an important next step towards higher J.
Furthermore, reducing bulk recombination can also allow the electron quasi-Fermi level in the absorber be pushed closer to the conduction band, further increasing V. To this end, the ability to engineer CuO in ways that can mitigate the effects of deleterious bulk defects is crucial. In addition, using appropriate tools to characterize relevant bulk electronic properties such as defect levels and bulk minority carrier lifetime (τ) is important so that the absorber can be systematically optimized for PV devices.
In this work, we measure a high τ of up to 10 μs in thermally oxidized 100 μm CuO bare wafers using the microwave reflection photoconductance decay technique. This is achieved by tailoring the oxygen partial pressure and temperature profile in the furnace during the post-annealing step. We use complementary characterization techniques to elucidate the relationship between the growth process, defect-structure and electronic properties of CuO. Spectrally-resolved room-temperature photoluminescence (PL) reveals a broad mid-gap defect band centered at 1.2 eV and samples with lower lifetime show comparatively stronger defect-related PL emission. This suggests that radiative recombination from mid-gap states is detrimental and dominates recombination activity in lower lifetime samples. Spatial PL mapping is used to gain insights into the structural dependence of defect-assisted recombination and PL images show that grain boundaries are relatively more recombination active than the bulk, pointing towards the need of larger grain sizes for higher device efficiencies. We will also present PV device characteristics to compare the effects of post-annealing conditions. We conclude by discussing how the framework used in this study can be applied more broadly to advance other Earth-abundant materials.