Optoelectronic evaluation of the nanostructuring approach to chalcopyrite-based intermediate band materials

Abstract

Nanostructured chalcopyrite compounds have recently been proposed as absorber materials for advanced photovoltaic devices. We have used photoreflectance (PR) to evaluate the impact of interdiffusion phenomena and the presence of native defects on the optoelectronic properties of such materials. Two model material systems have been analyzed: (i) thin layers of CuGaSe 2 ( E g =1.7 eV) and CuInSe 2 (1.0 eV) in a wide/low/wide bandgap stack that have been grown onto GaAs(0 0 1) substrates by metalorganic chemical vapor deposition (MOCVD); and (ii) thin In 2S 3 samples ( E g =2.0 eV) containing small amounts of Cu that have been grown by co-evaporation (PVD) intending to form Cu x In y S z ( E g ∼1.5 eV) nanoclusters into the In 2S 3 matrix. The results have been analyzed according to the third-derivative functional form (TDFF). The valence band structure of selenide reference samples could be resolved and uneven interdiffusion of Ga and In in the layer stack could be inferred from the shift of PR-signatures. Hints of electronic confinement associated to the transitions at the low-gap region have been found in the selenide layer stack. Regarding the sulphide system, In 2S 3 is characterized by the presence of native deep states, as revealed by PR. The defect structure of the compound undergoes changes when incorporating Cu and no conclusive result about the presence of ternary clusters of a distinct phase could be drawn. Interdiffusion phenomena and the presence of native defects in chalcopyrites and related compounds will determine their potential use in advanced photovoltaic devices based on nanostructures.