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

Abstract

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.