Material and Electrical Properties of Liquid Phase Crystallised Silicon Thin-film Solar Cells on Glass

Abstract

The recently developed liquid-phase diode laser crystallisation process for polycrystalline Si thin-film solar cells on glass has emerged as a promising technology, outperforming the solid-phase crystallisation process. This thesis aims to study the materials and electrical properties of diode laser crystallised polycrystalline Si thin-film on glass. Firstly, the grain growth mechanisms in liquid-phase crystallisation are investigated. The quickest undercooling is realised through the interface between the film and glass. Thus, grain nucleation and growth are initiated in surface normal direction of the film; subsequent lateral epitaxial growth proceeds along the laser scan direction with appropriate scan speed and power. When a scan speed of 15–100 cm/min is used, high quality lateral grains, with sizes up to a few tenths of millimetre are formed. Parallel grain boundaries, mostly Σ3 twin boundaries, are observed to form at a high density. The highest photoluminescence signal and mobility were observed for a twin-free grain with (100) orientation in the normal direction. However, the percentage of (100) oriented grains is very limited; rather, X-ray diffraction results show predominant (110) orientation in the film. The second part of this thesis aims to optimise silicon thin-film quality. Microstructure and crystallographic orientations are strongly dependent on the deposition temperatures. A higher density of grain boundaries is obtained when the laser crystallised film is deposited below 450 °C, which limits the solar cell performance by grain boundary recombination. Performance degradation is expected, due to severe shunting. Increasing the percentage of twin-free (100) oriented grains is attempted by applying a SiOx capping layer prior to crystallisation, and producing up to 97 per cent occupancy of the (110) orientation. Thus, a lower contribution of the grain boundary recombination in the presence of the 100 nm thick capping layer and improved 1-Sun voltage are achieved. Textured glass is compatible with diode laser crystallisation, which shows improved absorption, as well as improved 1-Sun voltage. Finally, atomic probe microscopy characterisation of defects is performed. A potential barrier height up to 15 mV is measured across some grain boundaries, which is expected to increase the carrier recombination.