Non-Stoichiometric Amorphous Indium Selenide Thin Films as a Buffer Layer for CIGS Solar Cells with Various Temperatures in Rapid Thermal Annealing

The conventional structure of most of copper indium gallium diselenide (CuIn_{1− x} Ga _x Se₂, CIGS) solar cells includes a CdS thin film as a buffer layer. Cd-free buffer layers have attracted great interest for use in photovoltaic applications to avoid the use of hazardous and toxic materials. The RF magnetron sputtering method was used with an InSe₂ compound target to prepare the indium selenide precursor. Rapid thermal annealing (RTA) was conducted in ambient N₂ gas to control the concentration of volatile Se from the precursor with a change in temperature. The nature of the RTA-treated indium selenide thin films remained amorphous under annealing temperatures of ≤700 °C. The Se concentration of the RTA-treated specimens demonstrated an opposite trend to the annealing temperature. The optical transmittance and band gap energies were 75.33% and 2.451– 3.085 eV, respectively, and thus were suitable for the buffer layer. As the annealing temperature increased, the resistivity decreased by an order-of-magnitude from 10⁴ to 10¹ Ω-cm. At lower Se concentrations, the conductivity abruptly changed from p-type to n-type without crystallite formation in the amorphous phase, with the carrier concentration in the order of 10¹⁷ cm⁻³.