Low-cost CIGS solar cells by paste coating and selenization

A simple process for the deposition of Cu(In,Ga)Se2 (CIGS) absorber layers is described. A low-cost CIGS precursor paste deposited by simple and fast doctor blade technique is subsequently selenized under selenium vapour in a quartz tube at 10 mbar (10 min at 550 °C). The precursor paste is prepared with metal chlorides and nitrates dissolved in alcohol. The solution is then mixed with a cellulose solution to adjust the viscosity for optimal deposition. The conversion of the precursor to the CIGS phase was confirmed by X-ray diffraction (XRD). Grain size and morphology were characterised with electron microscopy. A double-layer structure formed during selenization, with a CIGS layer on top of an amorphous carbon layer. Auger electron spectroscopy (AES) shows a decreasing Ga/In ratio from the carbon–CIGS interface towards the CIGS surface. The layer structure grown on Mo-coated glass substrates (conventional dc-sputtering) was processed to solar cells by depositing a CdS buffer layer (chemical bath deposition) and ZnO/ZnO:Al front contacts (conventional rf-sputtering). A maximum efficiency of 6.7% was achieved with approximately 0.5-μm-thick absorber layers. Quantum efficiency measurements reveal photon absorption losses for the longer wavelengths, which are attributed to the thin layers.