Deposition and Properties of TCOs

a-Si:H/c-Si heterojunction solar cells require different contacting schemes as compared to conventional solar cells with diffused emitters due to the low emitter conductivity. Apart from back-contacted solar cells it is common to use a transparent conducting oxide (TCO) instead of silicon nitride as an anti-reflection (AR) layer. The choice of materials is vast, with materials based on indium oxide and zinc oxide being the most prominent choice. The optical and electrical properties of these films both play a significant role for the solar cell but they are strongly related, meaning that one cannot optimize them independently. Too high carrier concentrations for instance lead to a lower refractive index of the TCO even for light with a wavelength well below 1100 nm, which results in a worsened AR effect. It is therefore advantageous to use materials with moderate carrier concentrations. The challenges for the deposition of these materials are mainly the low thickness required for an optimum AR effect, for which properties are still influenced by inferior film growth during the nucleation phase, and the allowed substrate temperature of around 200 °C which is limited by the thermal stability of the a-Si:H/c-Si interface.