Cd-free CIGS solar cells with buffer layer based on the In2S3 derivatives

Abstract

This study guided by device evaluations was conducted to reveal the reasons for the loss of the photo-generated carriers in CIGS cells with the buffer based on In₂S₃ derivatives. Chemical bath deposited In_x(OOH,S)_y films have been employed as a Cd-free buffer layers. When compared to solar cells with CdS buffer layer, the Cu_0.9(In_0.7,Ga_0.3)Se_2.1 (E_g = 1.18 eV) cells with the In_x(OOH,S)_y buffer exhibited strong voltage-dependent carrier collection and poor spectral response above 500 nm, presumably, due to energy barrier at the junction. In order to improve the charge collection by upward shift of the conduction band minimum of CIGS absorber, In_x(OOH,S)_y/Cu_0.9(In_0.55,Ga_0.45)Se_2.1 (E_g = 1.30 eV) solar cells were also fabricated and their spectral responses were examined. When compared to the Cu_0.9(In_0.7,Ga_0.3)Se_2.1 cells, the improved spectral response and voltage dependent carrier collection were obtained. Nevertheless, considerable loss in charge collection above 500 nm was still observed. The efficiency reached 9.3% while the Cu_0.9(In_0.7,Ga_0.3)Se_2.1 cell exhibited only the efficiency of 3.4%. Finally, CIGS (E_g = 1.18 eV) solar cells with n-ZnO/i-ZnO/In_x(OOH,S)_y/CdS/CIGS and n-ZnO/i-ZnO/CdS/In_x(OOH,S)_y/CIGS configurations were fabricated. The influence of the TCO/buffer interface on the device characteristics was also addressed by means of comparison between the characteristics of two cells employing different interfaces. A 13.0% efficient cell has been achieved from n-ZnO/i-ZnO/CdS/In_x(OOH,S)_y/CIGS configuration. The obtained data suggested that the limitation of the device efficiency was mainly related to the i-ZnO/In_x(OOH,S)_y interface. The experimental results provide the knowledge base for further optimization of the interface properties to form high-quality p–n junction in the CIGS solar cells employing the CBD In₂S₃ buffer layer.