Analysis of ITO thin film layers and interfaces in heterojunction solar cells by AFM, SCM and SSRM methods

Atomic force microscopy (AFM), scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM) were used for the analysis of the morphology and electrical properties of heterojunction (HJ) solar cell structures made of Indium Tin Oxides (ITO) layers deposited on p-type polished Si substrates with a buffer layer of n-type hydrogenated amorphous Si (a-Si:H). By means of AFM measurements it is shown that the a-Si:H layer can be deposited on a Si substrate quite homogeneously with a roughness of only a few nanometers. In contrast, the morphology of the ITO layers depends on the deposition temperature and can be varied during the HJ solar cell processing. Cross section analysis of the HJ structures by SCM shows penetration of n-type carriers into the (p)Si bulk substrate and the formation of the resulting n-p electrical junction depends strongly on the ITO deposition temperature and the presence of the (n)a-Si:H layer. Complementary cross section SSRM measurements show only a highly n-type doped region confined to the ITO layer. Finally it is concluded that the properties of the HJ solar cell structures depend dramatically on the properties of the ITO layers and can be modified by variation of the ITO deposition temperature. (c) 2005 Elsevier B.V. All rights reserved