Near room-temperature synthesis of transparent conducting fluorine-doped ZnO thin films as window layer for solar cells

Abstract

Highly transparent and conducting fluorine-doped ZnO (FZO) thin films were developed by 13.56 MHz RF magnetron sputtering near room-temperature (∼45 °C) on glass substrates. The effects of sputtering gas pressures in the range (10–40) mTorr were studied on the morphological, electronic, and optical characteristics of the FZO thin films. At a plasma pressure of 20 mTorr, the optimum FZO films were obtained with a wide optical band gap Eg ∼3.28 eV and high electrical conductivity of ∼102 S cm−1 via minimization of strain in the network containing 1.2 at.% F. A structurally favored c-axis orientated F-doped ZnO network has been achieved with an average grain size of ∼29 nm, high electrical mobility, μ ∼13 cm2 V−1 s−1 and carrier concentration, ne ∼1020 cm−3 that provided an excellent conduction channel for the stacked-layer devices. Using the optimized FZO films as a transparent conducting (TCO) window layer of the nc-Si solar cell in ‘Glass/FZO/p-nc-SiOX:H (∼20 nm)/i-nc-Si (∼2000 nm)/n-a-Si:H (∼30 nm)/Ag/Al’ configuration, a photovoltaic conversion efficiency (η) of 5.68% with short-circuit current density, JSC ∼13.36 mA cm−2, fill factor, FF ∼58.4%, and open-circuit voltage, VOC ∼727.5 mV was obtained.