RF Magnetron Sputtering of Transparent Conducting Oxides and CdTe/CdS Solar Cells

Abstract

The applicability of radio frequency magnetron sputtering (RFMS) for the development of: a) transparent conducting oxides (TCOs) and b) fully sputtered CdTe/CdS solar cells is demonstrated.

TCO materials - In2O3:Sn (ITO), SnO2:F (FTO), ZnO:Al (AZO) and ZnO:F (FZO) - were investigated with respect to key deposition parameters in an attempt to generate films with low resistivities and high transmittances. Minimum resistivity values of 1.2 x 10^-4
Ohm.cm and 4.7 x 10^-4 Ohm.cm were achieved for films of ITO and AZO respectively while maintaining transmittances of > 80%. Such films are viable for incorporation into CdTe based solar cells as front contact layers. A model for the dielectric permittivity for TCO materials is presented based on classical Lorentz and Drude models of bound and free electron behaviour, and a model of inter-band transitions that describes behaviour in the vicinity of a direct band-gap. The model is successfully applied to the tting of transmittance data for TCO films and used to extract opto-electronic properties.

The results of a fully-sputtered CdTe prototype device structure are presented; a maximum conversion eciency of 12.5% is achieved. Further investigations, via XRD, into the effect of sputter pressure on CdTe films indicates that a 10 mTorr Ar pressure is best for optimising device effciency. J-V-T and C-V measurements show that at room temperature, current transport in the sputtered devices is dominated by Shockley-Read-Hall recombination and that the CdTe layer, under zero applied bias, is fully depleted with a carrier concentration of 4 x 10^14 cm^-3. Cross-sectional SEM and TEM show that both CdS and CdTe layers undergo significant recrystallisation during post-deposition CdCl2 treatment.

A multi-layer optical model of transmittance is developed based on a transfer matrix method and using optical data acquired from spectrophotometry and ellipsometry. The model is used to predict the fraction of transmitted light received by the CdTe absorber in a fully sputtered device and it is indicated that through further thinning of the CdS layer, to below 50 nm, significant gains in transmittance, upwards of 20%, may be achieved. It is established that the further development of sputtered CdTe/CdS solar cells requires a significant improvement in the uniformity of the current CdCl2 based post-deposition treatment.