Influence of CIGS film thickness on the microstructure, bulk optoelectronic, and surface electrical properties

The present work addresses the changing microstructure, bulk optoelectronic, and surface electrical properties of sputter deposited Cu(In,Ga)Se<inf>2</inf> (CIGS) films on the soda lime glass (SLG) substrates as a function of altered film thickness. To this end, these films were deposited by varying the deposition time from 180 to 540 min in radio frequency (RF) magnetron sputtering and by using a low substrate temperature of 523 K. A range of experimental techniques were then used to investigate the microstructure, bulk optoelectronic, and surface electrical properties of these films. All these films are found to grow in single phase, chalcopyrite crystal structure, and with dominant (112) orientation, though the overall composition of these films is found to slightly alter. Moreover, a semimetallic to semiconductor transition in the thickening CIGS film is seen, which is then correlated with their hole concentration. Based on these observations and the positions of the Fermi energy, a possible theory regarding the presence of the types, quantities, and positions of the electronic defect states and their variations by thickening the CIGS film is discussed. Both the transport and optical band gaps (E<inf>g</inf>) are found to follow the same trend, which could also be related with the compositional variation in these films. Moreover, CIGS film deposited for 540 min is found to yield a film thickness of about 2.85 μm, with hole concentration of 2.17 × 10¹⁷ cm⁻³ and optical band gap of 1.16 eV; optoelectronic properties appropriate for an absorber layer in photovoltaic application.