An electrochemical impedance study to analyse the phase transformation pattern and stability during electro-crystallization of CIGS photo-voltaic thin films

Abstract

In this study, an effort has been made to gain a thorough knowledge of the phase transformation mechanism of quaternary thin films Cu (In, Ga) Se2 on FTO-glass substrates through electrochemical impedance spectroscopy (EIS). Here, a methodical strategy that starts with the investigation of unitary (Cu, Se, In and Ga), binary (Cu-In, Cu-Se, Cu-Ga, In-Se, In-Ga, and Se-Ga), tertiary (Cu-In-Se, Cu-In-Ga, In-Se-Ga, and Cu-Se-Ga), and finally quaternary (Cu-In-Ga-Se) deposition has been put forward. CuCl2, H2SeO4, InCl3, and GaCl3 were the salts utilized, and their combinations for each stage of deposition. The information obtained from EIS i.e., solution resistance (Rs), double layer capacitance (Cdl), charge transfer resistance (Rct), film resistances (Rfilm), and film capacitances (Cfilm) were used to find the mechanism. For unitary baths, Cu has the highest Rct and high Cdl under the optimized concentration. Binary baths further explained that Se-Ga and In-Ga can be used to perform one stage of deposition considering the low Rct and moderate Cdl. The tertiary bath combinations favored CIS as one stage of deposition in a similar manner (i.e., low Rct and Cdl). While using the quaternary bath, Rct and Cdl were found to have values of 483.6 Ω.cm2 and 285.78 µF/cm2. From the impedance pattern, it was deduced that the deposition takes place in two layers i.e., CIS and CGS tertiary films which eventually give the quaternary composition. Further, the stability of the films was also evaluated in the Na2SO4 solution, the CIGS film is found to be sufficiently stable.