Spray-ion layer gas reaction (ILGAR) -: A novel low-cost process for the deposition of chalcopyrite layers up to the micrometer range for photovoltaic applications

A novel variant of the ion layer gas reaction (ILGAR) process is described. Up to now, only layers with a maximum thickness of about 100 nm could be deposited by ILGAR in a reasonable time. Replacing of dipping by spraying on a warm substrate accelerated the application of the precursor solution (e.g., metal chloride in water) by a factor of at least 100. For multinary products, all precursors are applied either simultaneously as a mixed solution or sequentially. In intervals, the spraying is stopped and the reaction gas H2S is allowed to flow over the substrate. A special, yet simple setup is shown, which allows also a recycling of unconverted precursor. In about an hour time, a micrometer thick layer of mixed or stacked CuS/In2S3/(Ga2S3) (from CuCl2, InCl3 and in some cases GaCl3, respectively) is deposited. The latter is converted to CuInS2 (CIS) or Cu(In,Ga)S-2 (CIGS) by annealing in H2S/Ar at 550 degreesC. The thermodynamically stable product distribution for this process is calculated. The influence of the process parameters, substrate temperature during precursor deposition, annealing temperature, CuCl2/InCl3 ratio and gallium addition, is studied. The layers are used as absorbers in chalcopyrite thin film solar devices. The grain size necessary for this purpose is already sufficient (ca. 0.5-0.7 mum diameter). First solar cells based on spray-ILGAR-CIS and -CIGS have been prepared and compared. The efficiency has already reached 3.4%. An unusual layer morphology is obtained when the substrate temperature is too high ( >90 degreesC). In this case, the layer consists of ideal hollow spheres with a hole in them. An explanation for this phenomenon is given.