In crystalline silicon (c-Si) solar cells, carrier selective contacts are among the remaining issues to be addressed in order to reach the theoretical efficiency limit. Especially in ultra-thin-film c-Si solar cells with small volumes and higher carrier concentrations, contact recombination is more critical to the overall performance. In this paper, the advantages of using TiO<sub>X</sub> as electron-selective layers for contact passivation in c-Si solar cells are analyzed. We characterize the metal/TiO<sub>X</sub>/n-Si electron-selective contact with the contact recombination factor J<sub>0c</sub> and the contact resistivity ρc for the first time. Experimental results show that both J<sub>0c</sub> and ρc decrease after the insertion of TiO<sub>X</sub>. In addition, the effect of post-deposition rapid-thermal-annealing (RTA) at different temperatures is also evaluated. The best J<sub>0c</sub> of 5.5 pA/cm<sup>2</sup> and the lowest ρc of 13.6 mΩ·cm<sup>2</sup> are achieved after the RTA process. This work reveals the potential of TiO<sub>X</sub> as an electron-selective layer for contact passivation to enable high-efficiency ultra-thin c-Si solar cells with a low cost.
State-of-the-art III-V cells have reached the highest energy conversion efficiency among all types of solar cells. However, these cells are not applicable to widespread terrestrial solar energy system yet due to the high cost of epitaxial growth. Ultra-thin film absorbers with advanced light management is one of the most promising solutions to drive down the cost. In this paper, we present an ultra-thin film nano-window gallium arsenide (GaAs) solar cell design. This ultrathin cell consists of a nano-structured Al<sub>0.8</sub>Ga<sub>0.2</sub>As window layer on the front side to reduce the reflection and to trap the light, and a metal reflector on the back side to further increase the light path. The 300 nm thick GaAs cell with Al<sub>0.8</sub>Ga<sub>0.2</sub>As nano-window shows a broad band absorption enhancement from visible to near infrared (NIR), achieving a spectrally averaged absorption of 94% under normal incidence. In addition, this cell shows excellent angular absorption properties, achieving over 85% spectral averaged absorption at up to 60 degree off normal incidence. Meanwhile, this structure with planar junction and nano-window has solved the issue of low fill factor and low open-circuit voltage in nano-structured GaAs solar cell. A nano-window cell with a 3 μm thick GaAs junction demonstrated an open circuit voltage of 0.9V.