Role of i-ZnO in Optimizing Open Circuit Voltage of CIGS2 and CIGS Thin Film Solar Cells

It is a customary in the preparation of CuIn
_1-x
Ga
_x
Se
₂
(CIGS) or CuIn
_1-x
Ga
_x
S
₂
(CIGS2) solar cells, to use an un-doped layer of ZnO (i-ZnO) on the CdS layer prior to the deposition of a doped layer (ZnO:AI). This paper presents reasons behind the need for i-ZnO layer and also the effect of its thickness on the open circuit voltage of the CIGS2 based thin film solar cells. It was found that thickness of i-ZnO layer must be optimized depending on the surface roughness of CIGS2 absorber layer. CIGS2/CdS solar cells having optimum i-ZnO thickness were prepared and a photovoltaic conversion efficiency of 11.99% with open circuit voltage, V
_oc
of 830.5 mV under AM 1.5 conditions were obtained. The AM0 efficiency measured at NASA GRC for the same CIGS2 solar cell was 10.25%