ALD deposited Bi-functional ZnTiO as a buffer and a transparent conducting oxide layer for $Cu(In,Ga)Se_2$ thin film solar cells

Abstract

PV devices, especially $Cu(In,Ga)Se_2$ solar cells have received lots of attention due to long-term stability in outdoor condition, and material properties such as the tunable band gap energy and the high absorption coefficient. Attributed to those material properties, the CIGS solar cells have achieved the honor of being one of the five (CIGS, Si, Perovskite, CdTe, GaAs) solar cells that exhibit over 20% of power conversion efficiency. Despite of the great honor, there are two disadvantages: it utilizes CdS buffer layer which contains environmentally hazardous element (Cd) and Al doped ZnO window layer which shows free carrier absorption in long-wavelength region. In this Master’s thesis, thus, to solve the two problems we adopt Bi-functional ZnTiO layers as a buffer layer and a transparent conducting oxide layer. Mostly, we focus on the developing Cd-free ZnTiO buffer and its related wet pre-treatment effects, and partially on ZnTiO transparent conducting oxide in this thesis. Here, we report on the development of the novel Cd-free ZnTiO buffer layer deposited by atomic layer deposition. PV-device targeted properties such as band gap energy and electrical properties of buffer layer were optimized by tuning the relative concentration ratio of Zn and Ti elements. In order to improve the quality of the CIGS/ZnTiO interface the effects of the pre-treatment of CIGS absorber with the aqueous solution of $NH_4OH$, aqueous solution of $Cd^{2+}$, and $H_2O$ on the PV performance of Cd-free solar cells were studied. For this purpose, the quantification of the optoelectronic structure of the CIGS/ZnTiO interface was provided and pre-treatment effects on the chemical and electronic surface structure of absorber were investigated. The XPS depth profile and UPS analyses provide an explanation to the origin of the increase of device efficiencies induced with treatment of absorbers. The experimental results suggest that the reason of the increase the PCE of solar cells is the removing the Na compounds, that provides the strong p-n junction by reducing the shunt leakage, and the water is main factor that impact to this process. We also proof this conclusion by comparative characterization of devices fabricated on SLG (containing Na) and Corning Glass (Na-free). Application of the $H_2O$ treatment enables increase efficiency values of up to 9.03% for SLG-based device, while a cell based on Na-free glass shown a negligible PCE improvement. The obtained results provide a simple and attractive approach toward designing next generation CIGS solar cells with bi-functional ZnTiO layer, which able to replace both a buffer and a trans-conducting oxide layers. We fabricated CIGS solar cells with Bi-functional ZnTiO layer as a buffer and a transparent conducting oxide layer. We obtained improved current density (Jsc) compared to conventional CIGS solar cells structure, which is mainly attributed to high transmittance of ZnTiO TCO layer without free carrier absorption in long-wavelength region. However, the lowered open circuit voltage ($V_{OC}$) was main reason why CIGS solar cells with Bi-functional ZnTiO layer couldn’t surpass conventional structured CIGS solar cells, which resulted from Zn diffusion in to CIGS absorber layer during deposition of TCO under relative high temperature ($240^\circ C$). As a result, the diffusion barrier at the CIGS/buffer or TCO/buffer should be included to reduce VOC loss.