Non-vacuum deposition of Cu(In,Ga)Se2 absorber layers from binder free, alcohol solutions

Non-vacuum methods for Cu(In,Ga)Se2 (CIGS) absorber deposition have gained wide interest because of their inherent cost and energy saving potential. Here, a solution-based processing route for CIGS absorber layers is presented that employs binder-free solutions of metal salts in non-toxic, alcohol solvents. Despite the low-boiling-point nature of the employed solvents, a residual carbon-rich layer is observed between the CIGS and metal back contact. Based on comprehensive investigations by scanning electron microscopy, energy-/wavelength dispersive X-ray spectroscopy, X-ray fluorescence, X-ray diffraction, thermogravimetric analysis, differential thermal analysis, and extended X-ray absorption fine structure spectroscopy, a formation reaction mechanism through intermediate metalorganic complexes is proposed. In this route, the CIGS layer is formed in selenium atmosphere via a gradual decomposition of the carbon-rich layer comprising carboxylic chelate complexes of metals. A compositional gradient occurs in the CIGS absorber, whereas a significant amount of metals remains in the carbon-rich layer. The incorporation of Ga into CIGS is affected by the initial salt concentration and the selenization temperature. Fabricated solar cells exhibit active area efficiencies of up to 7.7% on 0.3?cm2 area without anti-reflection coating, which is among the highest reported efficiencies for solar cells from a solution process with non-explosive gases or solvents. Copyright (c) 2012 John Wiley & Sons, Ltd.