Optimization of the electrical resistivity of magnetron sputtered Aluminium and Boron co-doped zinc oxide thin films for solar cells

Abstract

The aim of this research was to optimize the electrical resistivity of transparent Al-B co- doped ZnO thin films for application as a transparent front electrode in thin film solar cells, as a cheaper alternative to the existing standard indium tin oxide (ITO) which is expensive. Particular aims of the research were to investigate the effect of oxygen flow rate (OFR), substrate position and doping level on the electrical resistivity, optical properties and structure of Al-B co-doped ZnO thin films. Also, to prepare reference thin films of undoped ZnO and single (Al-, B-) doped ZnO under similar preparation conditions. Reactive direct current magnetron sputtering method was used to prepare films on a soda-lime glass substrate using metallic targets of Zn and alloys of Zn/Al and Zn/B in Ar-O2 mixture. Films were characterized by X-ray diffraction, UV–visible spectroscopy, stylus profilometry and four point probe method. Results showed that both Al-B co- doped ZnO thin films and reference films had hexagonal wurtzite crystal structure with a preferential (002) orientation. Besides, their resistivities were strongly dependent on substrate position, OFR and doping levels. Transparent conductive undoped ZnO thin films (237 – 240 nm thick) had relatively high optimal resistivity of 2 x10-2 Ω cm; while transparent conductive 2 at. % Al-doped ZnO films (178 -188 nm thick) and 3 at. % B-doped ZnO films (247 - 256 nm thick) had a magnitude lower optimal resistivity of 4 x10-3 Ω cm and 2 x10-3 Ω cm, respectively. A two magnitude lower optimal resistivity of 8 x10-4 Ω cm was obtained for transparent conductive ZnO films co-doped with 2 at. % Al and 1 at. % B (film thickness ~ 405 - 408 nm). The order of magnitude (~10 -4 Ω cm) is comparable to that of ITO (1-2 x10-4 Ω cm), currently dominating the solar cell market. Thus, Al-B co-doped ZnO films can potentially replace ITO for solar cell applications. Further work using radio frequency (RF) magnetron sputtering with ceramic targets, along with optimization of sputtering pressure and target-substrate distance is recommended.