Improved TCO characteristics of ZnO:Si films via utilization of Si4+ ionized donor states and its application in n-SZO/p-Si heterojunction solar cells

Abstract

Utilizing Si4+ ionized states providing two free electrons per atom, ZnO:Si (SZO) thin films have been grown as a superior transparent conducting oxide (TCO) material ideal for Si-based electronic devices. Energetically favoured substitutional doping of Zn2+ by Si3+ and Si4+ states facilitates increase in concentration (ne) and mobility (μ) of the charge carriers. At increased dopant incorporation at the interstitial sites Si2+ and Si1+ states are proportionally contributed which are not electronically active and act as scattering centres for the existing free electrons. Further, the formation of non-conducting SiOx bonds and their segregation at grain boundaries restrict increase in charge carrier concentration and reduce their mobility. The ideal dopant incorporation in ZnO:Si network accommodates ~1.6 ± 0.07 at.% Si. A minimal resistivity, (ρ)min ~7.16 × 10−5 Ωcm corresponding to the highest mobility, (μ)max ~85 V−1cm2s−1 and superior carrier density, (ne)max ~1.0 × 1021 cm−3 signify a highly degenerate semiconductor characteristic of the SZO films with reduced effective mass of electron, m* ~0.15 m0. Significantly high magnitude of Figure-of-Merit (FoM: Φ) ~9.7 × 10−2 Ω−1, even at moderately low growth temperature ~150 °C demonstrates excellent TCO characteristics of SZO films. The n-SZO/p-Si solar cells reveal increased VOC ~312 mV and JSC ~27.9 mA/cm2 via moderate widening in optical band gap and a significant increase in carrier concentration in the SZO component, respectively. The n-SZO/p-Si heterojunction solar cells with a FF ~0.499 and photovoltaic conversion efficiency η ~4.34% has been obtained with improved SZO layer grown by optimized doping via consumption of Si4+ donor states.