An (ITO or AZO)/ZnO/Cu(In1−xGax)Se2 superstrate thin film solar cell structure prepared by spray pyrolysis

Feasibility of preparing a thin film (ITO or AZO)/ZnO/Cu(In_1-xGa_x)Se₂ (CIGS) heterojunction solar cell using low cost non-vacuum technique is demonstrated. Low resistive (10^-4 Ohm-cm) Indium tin oxide (ITO) (>;80%T) and ZnO:Al (AZO) (10^-3 Ohm-cm) (>;75%T) films of ~700 nm thickness were deposited on to glass substrates using Ultrasonic Spray Pyrolysis (USP) at 420°C and 475°C respectively. Over this a 300 nm i-ZnO buffer layer was deposited at 400°C by USP. XRD and optical data results indicates that ITO and AZO, i-ZnO crystallites exhibits cubic and hexagonal wurtizite structures with direct band gap of 3.75 and 3.32, 3.31 eV respectively. FE-SEM analysis showed pyramidal and columnar structure for ITO and AZO, i-ZnO. Composition obtained from EDAX showed the amount of tin and aluminum doped into the ITO and AZO films. Little or no interdiffusion was found at the ITO/ZnO interface, allowing the use of high temperatures for deposition. Preliminary depositions of Cu(In_1-xGa_x)Se₂ (x=1 to 0) layer over glass showed polycrystalline chalcopyrite in nature with preferred (112) orientation. Raman studies showed a peak corresponding to A1 mode CIGS phase at ~170 cm^-1. A 700 nm graded CIGS layer showed high optical absorption coefficient value of 10⁶ cm^-1 with a band gap of 1.25 eV. Graded CIGS layer was deposited over ITO/ZnO and AZO/ZnO layers at a substrate temperature of 350°C after an appropriate surface treatment. XRD/FESEM analyses revealed the crystallinity and graded distribution of gallium and indium.