Non‐vacuum methods for formation of Cu(In, Ga)(Se, S)₂ thin film photovoltaic absorbers

Abstract

Polycrystalline thin films of copper indium diselenide and its alloys with gallium and sulphur (CIGS) have proven to be suitable for use as absorbers in high-efficiency solar cells. Record efficiency devices of 20% power conversion efficiency have been produced by co-evaporation of the elements under high vacuum. However, non-vacuum methods for absorber deposition promise significantly lower capital expenditure and reduced materials costs, and have been used to produce devices with efficiencies of up to 14%. Such efficiencies are already high enough for commercial up-scaling to be considered and several companies are now trying to develop products based on non-vacuum deposited CIGS absorbers. This article will review the wide range of non-vacuum techniques that have been used to deposit CIGS thin films, highlighting the state of the art and efforts towards commercialization.

From Metals to Semiconductors: Challenges in Electrodeposition for Photovoltaic Applications

“…Since the pioneering work by Bhattacharya (16), the deposition kinetics has been largely investigated (5,24,31,34,36) and references therein. However, the nucleation and growth mechanism is still a matter of discussion, especially the incorporation of indium.…”

“…It has a quasi amorphous structure and exhibits a reduced band gap close to 1.3 eV (21,22,25,30). , 19 (24) 1-10 (2009) CI(G)S. During the last decades, researches concerning I-III-VI 2 , such as CuInSe 2 or Cu(In,Ga)Se 2 , compounds have been widely developed (4,5,31). These compounds have appropriate optical absorption coefficients, the bandgap can be tuned depending on the composition between 1eV for CuInSe 2 and 1.7 eV for CuGaSe 2 .…”

“…Electrodeposition, largely applied for metal and metallic alloy deposition, is developing quickly for semiconductor thin film deposition since the end of the 70's, as reviewed in (3)(4)(5)11,12). The two first important elemental semiconductors prepared by electrodeposition, were silicon and germanium from molten salt solutions (3,11).…”

“…Cu(II) + Se(IV) + Se(0) ads + 6 e -> CuSe 2 / surf [5] This compound acts as a catalytic site for the incorporation of indium, which occurs, beyond a given potential threshold close to -0.6V/MSE, i.e. at a much higher potential than its equilibrium potential, thanks to the free energy of formation of In-Se bonds (38):…”

“…These layers are often prepared by ultra-high vacuum co-evaporation methods, which allow high efficiency, high control, but at high costs. Soft chemical processes (screen printing, electrodeposition, chemical bath deposition, sol-gel) enable low cost and scalable processes (3)(4)(5)(6). Electrochemical deposition is also emerging for semiconductor oxide preparation (7)(8)(9)(10).…”