Suppression of surface recombination in CuInSe_2 (CIS) thin films via Trioctylphosphine Sulfide (TOP:S) surface passivation

Abstract

CuInSe_2 (CIS) solar cells are promising candidates for thin film photovoltaic applications, one key limitation in their performance is surface recombination in these thin films. We demonstrate that passivating CIS films with Trioctylphosphine Sulfide (TOP:S) solution increases photoluminescence (PL) intensity by a factor of ∼30, which suggests that this passivation significantly reduces surface recombination. X-ray photoelectron spectroscopy (XPS) reveals that TOP:S forms both –S and –P bonds on the CIS film surface, which leads to a ∼4-fold increase in the surface Na peak intensity. This value is significantly higher than what would be expected from high temperature annealing alone, which has been linked to improvements in surface morphology and device efficiency in CIGS solar cells. We use Energy-Dispersive X-ray Spectroscopy (EDS) to measure the solid-state transport of Na within CIS films with and without passivation. EDS spectra on CIS film cross-sections reveals a saddle-shaped Na profile in the as-fabricated films and a concentration gradient towards the film surface in the passivated films, with 20% higher surface Na content compared with the unpassivated films. We employ Hybrid (B3PW91) Density Functional Theory (DFT) to gain insight into energetics of Na defects, which demonstrate a driving force for Na diffusion from bulk towards the surface. DFT Calculations with TOP:S-like molecules on the same surfaces reveal a ∼ 1eV lower formation energy for the Na_(Cu) defect. The experiments and computations in this work suggest that TOP:S passivation promotes Na diffusion towards CIS film surfaces and stabilizes surface Na defects, which leads to the observed substantial decrease in surface recombination.