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– x Te x to the optimal value for photovoltaic absorber. Photovoltaic devices based on crystalline Se 1– x Te 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– x Te 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– x Te 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.