CI(G)S Thin Film Optoelectronic Devices

Abstract

The escalating global energy demand has accelerated the development of renewable and sustainable energy sources. Solar energy, specifically thin-film solar cells, has emerged as a promising alternative to traditional fossil fuels due to its abundance, environmental friendliness, and flexibility. Copper-indium-gallium selenide (Cu(In,Ga)Se2, CIGS) thin-film solar cells have attracted significant attention for their high efficiency, tunable bandgap, and potential for flexible applications. Moreover, CuInSe2 (CISe) near-infrared (NIR) photodetectors have demonstrated potential for various applications such as remote sensing, security, and biomedical imaging. This thesis focuses on the fabrication, characterization of flexible CIGS solar cells and CISe NIR photodetectors. First, to overcome the absence of sodium (Na) and to prevent undesired impurity diffusion from stainless steel (STS) substrates, a bi-layer molybdenum (Mo) structure consisting of a 1-μm-thick Mo and Na-doped Mo (Mo:Na) with variable thickness was utilized as the back contact layer in flexible CIGS solar cells. The short circuit current density and power conversion efficiency (PCE) of the CIGS on the Mo/Mo:Na bi-layer structure were improved by 8.1 and 69.4%, respectively, compared with those of CIGS solar cells fabricated without a Mo:Na layer. The impact of Na diffusion-assisted MoSe2 layer formation and Ga grading on junction properties have been evaluated. Furthermore, Thermal admittance spectroscopy has been analyzed to unveil the consequence of Na no shallow/deep-level defects and correlate with the CIGS/Mo junction properties. Second, low bandgap CISe thin-film photodetectors for operating in NIR were demonstrated. Low bandgap CISe thin films were grown to improve the photo-absorption properties in the NIR using a three-stage co-evaporation process. The longest cut-off wavelength of 1300 nm was obtained from the CISe thin film having a Cu/In ratio of 0.87. The photocurrent of the photodetector showed a linear dependence on an optical power intensity up to 2.33 mW optical illumination and responsivity of 0.60 A/W under -0.4 V. In addition, due to the advantages of n-Si, as abundance in nature, low cost, environmental friendliness, and compatibility with the Si-based read-out-circuit (ROIC) chips, we investigated the growth of p-CISe/n-Si heterojunction for Near-infrared photodetection. This work shows that a CISe-based photodetector can be a prominent candidate for a photodetector operating in NIR.