Abstract
The application of atomic layer deposition(ALD) in copper indium gallium selenide(CIGS) solar cells is rapidly growing both in the industry as well as in the academic research. Buffer layers less than 10 nm by ALD have been demonstrated to be working. Materials of the ALD thin buffer layers reported include CdS, ZnO, ZnS, ZnSnO, and TiO2. However, the established ideas of traditional buffer layers apparently are being challenged as the thickness gets thinner and thinner. To answer the question of the working mechanism of very thin buffer layers, we investigate the ideal 'virtual buffers' in this study. By creating a very thin 'virtual buffer layer' on CIGS with a positive fixed charge on the interface, an internal electric field necessary for current collection is established. Assuming a fixed charge density of 10∼12cm-2, a cell efficiency of over 18% can be achieved, outperforming the reference cell with CdS buffer layer. The strong interface charge capped on the surface of CIGS is critical for the operation of this type of cell, whereas the thickness of the virtual buffer layer can be as thin as the interface charge is maintained. The idea of a virtual buffer layer provides another picture for further research work dealing with thin buffer layers less than 10 nm.