Non-uniform distribution in µc-Si1−xGex:H and its influence on thin film and device performance

• Three kinds of the non-uniform distribution in µc-Si1−xGex:H are explored.
• The properties of µc-Si1−xGex:H are mainly governed by Ge composition.
• The infrared response of bottom cell can be enhanced by raising Ge content.
• The 1200 nm a-Si:H/a-Si0.6Ge0.4:H/µc-Si0.5Ge0.5:H solar cell shows an efficiency of 11.35%.

In this work, hydrogenated microcrystalline silicon germanium (µc-Si1−xGex:H) thin films were fabricated by plasma-enhanced chemical vapor deposition (PECVD) and developed as the infrared absorber for thin film silicon solar cells. Three kinds of the non-uniform distribution in µc-Si1−xGex:H thin films including: “the non-uniform distribution of Si and Ge”, the non-uniform distribution of crystallization”, and “the non-uniform distribution of H”, and how these affect the structural, optical and photoelectric properties of µc-Si1−xGex:H thin films have been explored. The results show that the good film quality of µc-Si1−xGex:H is associated with the low crystalline volume fraction and microstructure factor. The band gap of µc-Si1−xGex:H is determined by the proportion of the Ge-related crystalline networks. In addition, it is suggested that the deterioration of the photosensitivity of µc-Si1−xGex:H is mainly due to the increase of the Ge clusters with higher micro-void and defect density. Furthermore, by using µc-Si1−xGex:H bottom sub-cells, the comparable efficiency can be realized under the thin thickness condition. An efficiency of 11.35% in an a-Si:H/a-Si0.6Ge0.4:H/µc-Si0.5Ge0.5:H triple junction structure with total cell thickness as thin as 1200 nm was obtained. It is believed that the µc-Si1−xGex:H thin films can be a better candidate for effective infrared absorber by further improving its microstructure uniformity.