Comparative Study of Al2O3 and HfO2 for Surface Passivation of Cu(In,Ga)Se2 Thin Films: An Innovative Al2O3/HfO2 Multistack Design

In Cu(In,Ga)Se₂ (CIGS) thin-film solar cells, interface recombination is one of the most important limiting factors with respect to device performance. Herein, metal–insulator–semiconductor samples are used to investigate and compare the passivation effects of Al₂O₃ and HfO₂ at the interface with CIGS. Capacitance–voltage–frequency measurements allow to qualitatively and quantitatively assess the existence of high negative charge density (Q _f ≈ −10¹² cm⁻²) and low interface-trap density (D _it ≈ 10¹¹ cm⁻² eV⁻¹). At the rear interface of CIGS solar cells, these, respectively, induce field-effect and chemical passivation. A trade-off is highlighted between stronger field-effect for HfO₂ and lower interface-trap density for Al₂O₃. This motivates the usage of Al₂O₃ to induce chemical passivation at the front interface of CIGS solar cells but raises the issue of its processing compatibility with the buffer layer. Therefore, an innovative Al₂O₃/HfO₂ multistack design is proposed and investigated for the first time. Effective chemical passivation is similarly demonstrated for this novel design, suggesting potential decrease in recombination rate at the front interface in CIGS solar cells and increased efficiency. 300 °C annealing in N₂ environment enable to enhance passivation effectiveness by reducing D _it while surface cleaning may reveal useful for alternative CIGS processing methods.