Electron-Selective TiO(2) Contact for Cu(In,Ga)Se(2) Solar Cells

The non-toxic and wide bandgap material TiO(2) is explored as an n-type buffer layer on p-type Cu(In,Ga)Se(2) (CIGS) absorber layer for thin film solar cells. The amorphous TiO(2) thin film deposited by atomic layer deposition process at low temperatures shows conformal coverage on the CIGS absorber layer. Solar cells from non-vacuum deposited CIGS absorbers with TiO(2) buffer layer result in a high short-circuit current density of 38.9 mA/cm(2) as compared to 36.9 mA/cm(2) measured in the reference cell with CdS buffer layer, without compromising open-circuit voltage. The significant photocurrent gain, mainly in the UV part of the spectrum, can be attributed to the low parasitic absorption loss in the ultrathin TiO(2) layer (~10 nm) with a larger bandgap of 3.4 eV compared to 2.4 eV of the traditionally used CdS. Overall the solar cell conversion efficiency was improved from 9.5% to 9.9% by substituting the CdS by TiO(2) on an active cell area of 10.5 mm(2). Optimized TiO(2)/CIGS solar cells show excellent long-term stability. The results imply that TiO(2) is a promising buffer layer material for CIGS solar cells, avoiding the toxic CdS buffer layer with added performance advantage.