Study of a low power plasma reactor for the synthesis of zinc oxide as window layers in Cu(In,Ga)Se2 solar cells

The low power plasma reactor is an original process which allows the deposition of ZnO controlled thickness films from an aqueous precursor solution in a cold plasma only in order to reach high growth rates. The quality of the deposited material (purity, crystallinity, size of the grains…) depends primarily on the interactions in the reactor between the droplets and the plasma. In this study, the parameters of the deposition (composition of the gases, pressure, power, temperature…) were studied and controlled. The doping characteristics are mainly influenced by the concentration of the precursors in the solution and by the method of injection. The final optimizations allowed high growth rates ranging from 0.6 to 1 nm/s. X-ray Diffraction results show a good crystallinity of the deposited layer (würtzite structure). According to the transmittance measurements, the films present a good transparency and a calculated optical gap value ranging between 3.2 and 3.3 eV. This deposition technique using plasma is fast, flexible, low power consuming and easily adaptable. Cu(In,Ga)Se2 solar cells with a ZnO window layer have successfully been achieved by using the low power plasma reactor and their performances show an efficiency of 11%.