A Detailed Investigation on the Phase Transformation Mechanism During Electrodeposition of Cu(In,Ga)Se2 Films

Abstract

CIGS-based solar cells, which belong to the second generation of solar cells, have achieved a power conversion efficiency of over 23% through the use of vacuum-based fabrication techniques. Despite their impressive performance, these vacuum-based techniques face a significant challenge in terms of their high cost. One potential solution to this challenge is the adoption of scalable non-vacuum-based techniques, such as electrodeposition. Electrodeposition has emerged as a promising alternative, as it offers a more cost-effective approach compared to vacuum-based methods. In fact, CIGS devices fabricated using electrodeposition techniques have already surpassed the efficiency achieved by vacuum-based techniques by a margin of 15%. In these devices, CIGS thin film is utilized as the absorber, playing a crucial role in capturing sunlight and converting it into electricity. There have been no attempts made to comprehend the process by which the CIGS film forms using electrochemical impedance spectroscopy (EIS). While examining the stability of the device or the alloy film alone is important, it is also possible to perform EIS at different stages of deposition during the formation of the CIGS film. This approach can help determine a more feasible pathway for electrochemical deposition. Additionally, studying the stability of various combinations of baths (including unitary, binary, ternary, and quaternary) in a Na2SO4 solution can provide valuable information about the stability behavior of individual species as well as combinations of species. By employing equivalent EIS circuits, it is possible to estimate the dominant mechanism involved when a particular species undergoes transformation. This allows for an understanding of how different species interact when considered individually or combined with one or two other species. Such an analysis can shed light on how the phase transformation mechanism is influenced, for example, by the addition of a species, which may enhance or hinder the overall deposition process. Consequently, utilizing these combinations of baths can offer a comprehensive understanding of all the mechanisms involved in electrochemical deposition. Unfortunately, no in-depth investigation has been conducted thus far to provide a concrete understanding of the electrochemical phenomena underlying the formation of the CIGS film. In this study, an attempt has been made to acquire an in-depth understanding of the electroplating mechanism of Cu (In, Ga) Se2 quaternary thin films on FTO-glass substrates. A systematic approach starting with the study of unitary, binary, ternary, and finally quaternary deposition of the elements have been performed through cyclic voltammetry (CV). The salts used were CuCl2, H2SeO3, InCl3 and GaCl3 for each stage of deposition with varying combinations and proportions. CVs were first done for unitary Cu, Se, In and Ga elements, then binary combinations of Cu-In, Cu-Se, Cu-Ga, In-Se, In-Ga, and Se-Ga system, ternary baths of Cu-In-Se and Cu-In-Ga, In-Se-Ga and Cu-Se-Ga systems. A thorough structural and elemental analysis, carrier densities, type of conductivity was carried out for the as-deposited films. In ternary baths, only Cu-In-Se films did show good photo-electrochemical (PEC) behavior. For this bath, an attempt was made to check whether by increasing In content would affect the PEC behavior. The deposition was done in presence of surfactants and was found that the films were compact and finer with Cu-poor surface (In-rich). Single step deposition of CIGS was found to be impossible, hence Ga was attempted to incorporate in the films through a two-stack approach. From the position and intensity of the cathodic peaks and the phase analysis, plausible mechanism has been proposed for the quaternary alloy. After that an effort has been made to gain a thorough knowledge on the phase transformation mechanism of quaternary thin films through electrochemical impedance spectroscopy (EIS). Here also, a methodical strategy that starts with the investigation of unitary, binary, ternary and finally quaternary deposition. The information obtained from EIS i.e. solution resistance (Rs), double layer capacitance (Cdl), charge transfer resistance (Rct), film resistances (Rfilm), film capacitances (Cfilm) were used to find the mechanism. For unitary baths, Cu has 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 ternary 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 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 ternary films which eventually give the quaternary composition. Further the stability of the films was also evaluated in Na2SO4 solution, the CIGS film is found to be sufficiently stable.