Crystal structure of Cu$_2$Te predicted within adaptive genetic algorithm*

Cu$_2$Te is one of the most commonly used conductive back-contacting materials for high-efficiency CdTe-based solar cells. However, the detailed crystal structure of Cu$_2$Te is still undetermined blocking property investigations of the Cu$_2$Te-based solar cell systems. Some models have been proposed but all of them have positive formation energy [1]. We have performed adaptive genetic algorithm crystal structure search to find low energy crystal structures of Cu$_2$Te. We found a new layered-structure edged by Te atoms with negative formation energy from first-principles calculations within local density approximation. This layered structure consists of tilted Cu$_2$Te ribbon arrays. Structural and electronic properties of the newly found Cu$_2$Te structure will be discussed in detail.\\[4pt] [1] J. L. F. Da Silva, S.-H. Wei, J. Zhou, and X. Wu, Appl. Phys. Lett. 91, 091902 (2007)