Dilute Oxygen Alloys of ZnS as a Promising Toxic-Free Buffer Layer for Cu(In, Ga)Se2 Thin-Film Solar Cells

Abstract

Using ZnS as a buffer layer in many thin-film solar cells, such as $\text {Cu}{(}\text {In}{,}\,\text {Ga}{)}\text {Se}_{{2}}$ (CIGS), has not been successful as it usually results in a high barrier that suppresses the flow of electrons to the designated contact. To tackle this issue, we analyze dilute oxygen (O) alloys of ZnS as a buffer layer. It exhibits an unusual energy bandgap ( ${E_{G}}$ ) bowing and a sharp increase in electron affinity energy. Such features commonly arise in anion-alloyed compositions due to band anticrossing (BAC) interactions between the introduced defect energy state of O and the extended conduction band edge (CBE) of ZnS that causes a downshift of the CBE. Besides the flexibility of tuning the CBE, this is important to avoid the toxicity of cadmium (Cd) and its compounds. In this article, the band edges of lightly alloyed ${\text {ZnS}_{{1}-{x}}\text {O}_{x}}$ are computed using an atomistic tight-binding (TB) BAC model. Then, a fitting energy band bowing (EBB) model is developed to capture efficiently the nonlinear variations of their ${E_{G}}$ and electron affinity energy. For O composition ranging between 0% and 5%, it is observed that the electron affinity energy sharply increases from 3.3 to 3.98 eV. Also, ${E_{G}}$ drastically reduces from 3.8 to 3.08 eV. Device-wise, by analyzing the effect of dilute O alloys and the doping density of the ZnS buffer layer, it is found that electron transport is remarkably improved with O composition. In ${\text {ZnS}_{{0.95}}\text {O}_{{0.05}}}$ alloy with doping density ${{1} \times {10}^{{18}}\,\,\text {cm}^{-{3}}}$ , the maximum power conversion efficiency (PCE) reaches approximately 23.82%.