Comparison between Low Tin Content and Standard (10 %) ITO as a Transparent Electrode in Hetero-Junction Crystalline Silicon Solar Cells

27th European Photovoltaic Solar Energy Conference and Exhibition; 2079-2082
ITO is the front transparent conductive oxide (TCO) of choice for hetero-junction a-Si:H/c-Si solar cells because of its excellent combination of high electrical conductivity, good transparency in the visible part of the solar spectrum, and high work function. Unfortunately, the industrially established, DC magnetron sputtered, standard ITO grade, produced using a target with 10wt% SnO2, has certain drawbacks. Its low resistivity is mainly due to a relatively high free charge carrier concentration (FCC), up to 1.2E+21 cm-3. Consequently, it displays an increased absorption of photons at a wavelength above 900 nm due to free electrons. Additionally, because of the associated decrease in refractive index, a higher fraction of the incident light is reflected away from the silicon wafer. This results in significant optical losses in the NIR portion of the spectrum. Several publications exist which explore hydrogenated indium oxide (In2O3:H), a thin film with a much lower FCC and a very high carrier mobility (up to a factor of 2.5 higher), as an alternative for standard ITO. Pure In2O3 ceramic targets are rather brittle, however, hence more difficult to manufacture. Currently this composition is only available in a planar format so the cost effective mass production of In2O3:H thin films becomes questionable. This work explores the potential of ITO with lower Sn doping content (3 wt% of SnO2 in the ceramic target) as an alternative to the standard ITO (90/10). This material can still be produced as rotary targets which enables deposition of equivalent thickness coatings at a much lower cost. The main focus of this paper is a comparison of the electro-optical properties of thin films prepared by DC magnetron sputtering with sintered ceramic ITO rotary targets having various SnO2 concentrations.