Exploring the Origin of Instability in Thin-Film Solar Cells Based on Wide-Gap (Ag,Cu)(In,Ga)Se2 via Repeated Electrical Characterisation

Sammanfattning

Wide-gap (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells are a promissing top-cell option for two-junction tandem photovoltaic devices. They can be fabricated with a bandgap of 1.63 eV, which gives theoretically the highest efficiency in tandem cell applications. However, the occurence of ordered vacancy compound (OVC) phases within the absorber layer brings important challenges to device output and stability. This study considers the effect of the absorber stoichiometry and sodium incorporation on the performance of ACIGS solar cells across three absorber categories: close-to-stoichiometric (CH), OVC, and bilayer absorbers. Nearly 200 solar cell devices were studied using current-density vs. voltage (JV) measurements and external quantum efficiency (EQE) measurements. Additional treatments, such as long-term storage, light soaking, and dark annealing, are also applied to assess the metastable behaviour and device stability. For CH-type absorbers, a clear optimal compositional window was identified between [I]/[III] ratios of 0.70 and 0.92, with the highest fabricated device efficiency exceeding 13%. Sodium incorporation was found to strongly affect the device, with Na-free cells produced on alumina substrates showing poor performance. OVC-type absorbers revealed two types of electrical conductivity behaviour at either side of the identified [I]/[III] threshold ratio of 0.19. N-type absorber conductivity observed for [I]/[III]<0.19 always led to non-functional devices, whereas p-type absorber conductivity for [I]/[III]>0.19 often resulted in operational solar cells. OVC-type absorbers exhibited metastable behaviour, including EQE improvement after repeated measurements and a decrease in carrier collection with illumination intensity. This changes are in contrast with the relative stability of CH-type devices. Moreover, for OVC/CH bilayer-type devices, the solar cell performance was shown to depend on the Na amount, with the optimum NaF layer thickness being equal to 8 nm for Na-free alumina substrate. Overall, [I]/[III] compositional ratio and Na supply are demonstrated to be paramount factors for good ACIGS solar cell performance, proving that proper control over them is essential for high device efficiency and stability.