Comparative Study of Al_2O_3 and HfO_2 for Surface Passivation of Cu(In,Ga)Se_2 Thin Films: An Innovative Al_2O_3/HfO_2 Multistack Design

摘要

In Cu(In,Ga)Se2 (CIGS) thin-film solar cells, interface recombination is one of themost important limiting factors with respect to device performance. Herein,metal–insulator–semiconductor samples are used to investigate and comparethe passivation effects of Al_2O_3 and HfO_2 at the interface with CIGS.Capacitance–voltage–frequency measurements allow to qualitatively and quantitativelyassess the existence of high negative charge density (Q_f≈-10~(12) cm~(-2))and low interface-trap density (D_(it) ≈ 10~(11) cm~(-2) eV~(-1)). At the rear interface ofCIGS solar cells, these, respectively, induce field-effect and chemical passivation.A trade-off is highlighted between stronger field-effect for HfO_2 and lowerinterface-trap density for Al_2O_3. This motivates the usage of Al_2O_3 to inducechemical passivation at the front interface of CIGS solar cells but raises the issueof its processing compatibility with the buffer layer. Therefore, an innovativeAl_2O_3/HfO_2 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 CIGSsolar cells and increased efficiency. 300 °C annealing in N_2 environment enable toenhance passivation effectiveness by reducing Dit while surface cleaning mayreveal useful for alternative CIGS processing methods.