Surface Passivation and Detrimental Heat-Induced Diffusion Effects in RbF-Treated Cu(In,Ga)Se₂ Solar Cell Absorbers

The remarkable efficiency improvements of Cu(In,Ga)Se₂-based chalcopyrite thin film solar cells to more than 23 % in the past few years have been largely attributed to post deposition treatments (PDTs) of the absorbers with heavy alkali metal compounds such as KF, RbF and CsF ^[1]. Therefore, understanding the effect of alkali PDTs on the electronic properties and defect physics of the chalcopyrite absorber surfaces are of utmost importance in view of the p/n junction formation and device optimization of such solar cells. In the present work, we adopt a combined analytical approach using scanning tunneling spectroscopy (STS) with a lateral resolution in the nanometer-regime and X-ray photoelectron spectroscopy (XPS) to compare the defect electronic properties and chemical composition of RbF-treated and non-treated Cu(In,Ga)Se₂ absorber surfaces ^[2]. Our STS results show that RbF PDT is effective in passivating the electronic defect levels at the absorber surface, as indicated by the low differential conductance within the bandgap. This is achieved by preventing surface oxidation and the formation of In, Ga and Se oxides on RbF-treated samples, as opposed to bare Cu(In,Ga)Se₂ surfaces. The passivating effect of RbF PDT is confirmed by ab-initio density functional theory (DFT) calculations which show that the chalcopyrite surfaces after the PDT are significantly less prone to oxidation and the incorporation of oxygen into such surfaces is energetically less favored. Furthermore, our XPS data indicate the presence of chemisorbed Rb species on the surface with a bonding configuration similar to a RbInSe₂ bulk compound, which is the likely cause of the observed surface passivation. A quantitative analysis of the XPS data shows that the Rb coverage is only in the sub-monolayer regime.<br/>During device fabrication, the chalcopyrite absorber usually undergoes several heating steps. Hence, to understand the effect of heat on RbF-treated samples, they were investigated before and after annealing under ultra-high vacuum conditions up to 320 °C. Our measurements indicate that there is a homogeneous distribution of Rb on the surface both before and after annealing, albeit with an increased concentration at the surface after the heat treatment. This suggests that the heat treatment leads to elemental diffusion of Rb from the bulk to the absorber surface. Additional depth-resolved magnetic sector secondary ion mass spectroscopy (SIMS) measurements highlighted that this diffusion in the bulk occurs predominantly along the grain boundaries. Annealing the samples to temperatures higher than 100 °C is found to lead to the formation of metallic Rb species on the surface, which results in a significant increase in the electronic defect levels and/or surface dipole formation as shown by STS and XPS measurements. Thus, heat-induced diffusion of Rb towards the surface effectively degrades the surface and will likely cause the deterioration of the absorber-window interface due to increased recombination losses at the p/n-junction. Our results strongly suggest that RbF PDT is a double-edged sword, where in addition to its advantageous surface passivation effect, a potential detrimental effect needs to be considered, especially during further device fabrication steps (e.g. the sputter deposition of ZnO on RbF-treated Cu(In,Ga)Se₂ thin films) at elevated temperatures.<br/><br/>[1] Nakamura, M. et al., <i>IEEE Journal of Photovoltaics</i>, <i><b>9</b></i><i>(6)</i>, 1863–1867 (2019).<br/>[2] Elizabeth, A. et al., <i>ACS Applied Materials and Interfaces</i>, <i><b>14</b></i><i>(29)</i>, 34101–34112 (2022).