Pulsed radiofrequency glow discharge optical emission spectrometry for the direct characterisation of photovoltaic thin film silicon solar cells

Abstract

The increasing demand for photovoltaic devices has created a silicon supply shortage, providing a great opportunity for hydrogenated amorphous silicon (a-Si:H) used in thin film technology. The potential of continuous and pulsed radiofrequency glow discharge optical emission spectrometry (rf-GD-OES) for the characterisation of thin film solar cells (TFSC), based on a-Si:H, has been investigated in this work. Qualitative in-depth profiles of TFSC obtained by both GD modes were carefully compared, in terms of signal intensity, penetration rate, emission yield, and depth resolution. The influence of rf-GD parameters operating in continuous mode was studied using three types of samples: B doped, P doped and the complete photovoltaic TFSC device based on a-Si:H. Moreover, the effect of different roughness of the Zn substrate on the depth resolution of the a-Si:H layer was evaluated, along with plasma cleaning conditions optimisation. Additionally, intensity signals and relative depth resolution were investigated for different pulsed GD experimental conditions, such as the pulse frequency (from 500 Hz to 10 kHz), duty cycle (in the range of 0.1875-0.5) and rf forward power (from 25 W to 75 W). 450 Pa and 25 W were selected as the optimum conditions for continuous rf-GD-OES analysis, whereas 450 Pa, 75 W, 1000 Hz and a duty cycle of 0.5 were selected for pulsed rf-GD-OES work. Results show that rf-GD-OES is a powerful tool for direct depth-profiling analysis of a-Si:H TFSC, allowing to discriminate the different parts of the photovoltaic devices: the first contact layer with ZnO₂ and Al₂O₃, the a-Si:H layer (where it can be distinguished between the B doped, the intrinsic a-Si:H and the P doped films), the back contact layer and, finally, the Zn substrate. Moreover, diffusion processes between the coating layers, which could have an important influence on the final efficiency of photovoltaic devices, can be identified and could be studied by rf-GD-OES.