Nonlinear Band Gap Tunability in Selenium-Tellurium Alloys and Its Utilization in Solar Cells

In this Letter, we report the alloying of the high-band-gap photovoltaic elemental absorber selenium with the isomorphic low-band-gap semiconductor tellurium to tune the band gap energy of Se_1-xTe_x to the optimal value for photovoltaic absorber. Photovoltaic devices based on crystalline Se_1-xTe_x alloys are promising candidates for extremely cheap and highly scalable solar cells, offering simple low-temperature fabrication and intrinsic stability. We explore the electro-optical properties of Se_1-xTe_x alloys and show that the tellurium red shifts the band gap in a nonlinear manner, faster than expected, due to significantly nonlinear change of the conduction band energy, allowing them to easily reach the desired band gap of 1.2-1.4 eV. On the basis of these results, we rationally design and demonstrate the fabrication of simple Se_1-xTe_x photovoltaic devices, showing significantly improved current density in comparison to pure selenium. Furthermore, we identify and analyze the main factors limiting the device efficiency and suggest a few approaches for future improvements of such photovoltaic devices.