Characterization of Cr Doped CuGaS2 Thin Films Synthesized By Chemical Spray Pyrolysis

Abstract

Addition of an impurity or intermediate band in a semiconductor can extend its optical functionalities to novel application such as intermediate band solar cells (BSCs). For this purpose, the optical and electronic characterization were performed on Chromium (Cr) doped (1-4%) chalcopyrite CuGaS2 (CGS) thin films synthesized by chemical spray pyrolysis technique on glass substrates. The structural and chemical characterization studied in the past confirmed that the prepared films are in tetragonal chalcopyrite structure with polycrystalline nature [1]. In the present study, electronic transitions studied by photo-modulated reflectance (PR) measurements showed widened bandgaps when Cr was added, and agrees well with our calculation based on density functional theory (DFT). Native defect-related transitions typically observed within the bandgap of the host CGS semiconductor were reduced in the Cr added films. This observation is consistent with photo-luminescence (PL) spectra measured at room temperature. An additional signature of an impurity band emerged in the PR transitions for Cr-added samples. Analysis of spin-resolved density of state calculation suggests that the IB originates from spin-polarized bands. Introduction. Addition of an intermediate band (IB) such as an impurity band can extend the functional limit posed by the conventional two-band character of semiconductor materials. For example, the concept of intermediate band solar cells (IBSC) makes use of an intermediate step to excite electrons from the valence band (VB) to the conduction band (CB) by two-step photon absorption (TSPA) of long wavelength photons that otherwise transpire through the material [2]. Detailed balance calculation predicts conversion efficiency of 65.1% for a three band solar cell (e.g. VB, IB, CB) under maximal solar concentration which is much higher than the conventional solar cell consisting two bands (e.g. VB and CB) [3]. The key operational demonstration of TSPA both at low [4] and room temperature [5-7] has been presented in a few IB systems of thin films and nanostructures despite much enhancement is needed in the magnitude of the TSPA driven current for practical application.