Laboratoire de Microélectronique Appliquée, Département d’Electronique. Université Djilali Liabés de Sidi Bel Abbés.22000, Algeria

Steady state photocurrents have been measured in poly-Si TFTs fabricated from crystallized films deposited in an initially amorphous state by LPCVD technique. The transfer characteristics of poly-Si TFT change remarkably by illumination in the subthreshold and off-state regions. It is discovered that the photocurrent Iph is more than three orders of magnitude greater than the dark currents. In the subthreshold and small negative VGS regions, the photocurrent is attributed to thermionic emission of electrons over the GB barriers and ohmic conduction respectively. It depends on the gate bias and channel length. However, Iph in high VGS region is independent on these two parameters. It is therefore set only by the light-induced generation of electrons and holes. In addition, the photocurrent increases linearly with film thickness. The observed decrease for films thinner than 0.1 µm could be due to the increased influence of surface recombination at the poly-Si/SiO2 interfaces

Improved efficiency of Cu(In,Ga)Se2 thinfilm solar cells using a buffer layer alternative to CdS

The chalcopyrite semiconductor CuInSe2 and its constitutes Ga and/or S [Cu (InGa)Se2 or Cu (InGa)(Se,S)2], commonly referred as CIGS have been leading thinfilms for incorporation in high-efficiency photovoltaics. In conventional ZnO-N/i-ZnO/CdS/CIGS solar cells, the traditional CdS buffer is nearly optimum for the commonly used 1.15 eV (CIGS) but less optimal for higher Ga. To overcome this limitation, Cd1-yZnyS is proposed as an alternative buffer layer to replace the standard CdS in CIGS thinfilm solar cells containing an ordered vacancy compound (OVC) layer. Next, the dependence of solar cells performance on the change of Ga and Zn concentrations in absorber and buffer layers, respectively, was investigated using the AMPS-1D software. The results are potential improvement in CIGS efficiency that was obtained with replacement of CdS buffer material by its alternative in one hand, another hand by formation of OVC layer. Lastly, the optimum values of Ga and Zn concentrations were found at 0.7 and 0.6, respectively, leading to a high conversion efficiency of around 23.71%