###### Abstract  
This review summarizes all studies which used dielectric-based materials as a passivation layer at the rear surface of copper indium gallium (di)selenide, Cu(In,Ga)Se\({}_{2}\), (CIGS)-based thin film solar cells, up to 2019. The results regarding the kind of dielectric materials, the deposition techniques, contacting approaches, the existence of additional treatments, and current-voltage characteristics (J-V) of passivated devices are emphasized by a detailed table. The techniques used to implement the passivation layer, the contacting approach for the realization of the current flow between rear contact and absorber layer, additional light management techniques if applicable, the solar simulator results, and further characterization techniques, i.e., external quantum efficiency (EQE) and photoluminescence (PL), are shared and discussed. Three graphs show the difference between the reference and passivated devices in terms of open-circuit voltage (\(V_{\infty}\)), short-circuit current (\(J_{\infty}\)), and efficiency (\(\eta\)), with respect to the thicknesses of the absorber layer. The effects of the passivation layer at the rear surface are discussed based on these three graphs. Furthermore, an additional section is dedicated to the theoretical aspects of the passivation mechanism.  
CIGS; solar cells; review; passivation +
Footnote †: journal: Applied Physics Letters  
_Applied Physics Letters_  
n-type buffer layer, generally cadmium sulfide (CdS). This process is completed with deposition of a transparent conducting oxide (TCO) window layer. In order to increase the efficiency, in some studies, an anti-reflection coating layer is also applied as a final touch [4].  
As mentioned earlier, the biggest competitor of the CIGS solar cells is Si PV technology. In order to compete with high-efficiency and low-cost Si PV, the power conversion efficiency of CIGS solar cells should be increased, and the thickness of the absorber layer should be decreased [1]. Considering the fact that In and Ga are critical raw materials, reducing the thickness of the absorber layer has another positive outcome: decreasing the amount of In and Ga used. However, decreasing the thickness of the absorber layer causes insufficient absorption and the detrimental impact of back-surface recombination [5]. Consequently, the power conversion efficiency of the thin CIGS-based solar cells decreases. If the thickness of the absorber layer is decreased, the electrical quality of these solar cells, hence, the power conversion efficiency, should be kept the same or, if possible, improved.  
The traditional way to enhance the electrical quality of the CIGS solar cell is gallium (Ga) grading. For thick absorber layers, the Ga gradient is in the neutral region of the absorber layer and creates a back surface field (BSF). If the absorber layers are thinner, they become completely depleted, and the BSF is created in the space charge region (SCR), improving \(V_{oc}\) for thin absorber layers [6]. Ga grading is not a complete solution for passivation of the rear surface because an additional counteract electric field (E-field) occurs, which causes an overall weaker E-field in SCR [6]. Consequently, a new and improved solution to passivate the rear surface of CIGS solar cells was sought. Introducing a passivation layer with point openings that ensures electrical contact between the rear contact and the absorber layer is one of the solutions to this problem (Figure 2). In this paper, an overview of the papers that use this approach for rear surface passivation of CIGS solar cells to date is given.