Thesis presented September 02, 2014

Abstract:
The main objective of this PhD thesis was directed toward establishing and explaining the relationships between synthesis conditions of CZTSSe, its physical properties and performance of photovoltaic devices. To tackle on this task the first approach was to understand the formation mechanism of the material in relation to the growth conditions. CZTSSe is synthesized by two-step selenization process, where a first step of precursor deposition by PVD is required, followed by a second step of annealing. Different precursor stacking orders have been studied in order to understand the sequence of reactions that, starting from their deposition, lead to the final CZTSSe layer. This study made step-by-step has required a strong effort on the material characterization at each step of the synthesis. The result demonstrated that in the case of two-step process, the final material is independent of the precursor deposition. The possible beneficial involvements due to incorporation of sodium in CZTSSe are also disclosed. This study is carried out by synthesizing CZTSSe on different sodium-containing substrates: in this way sodium migrates from the substrates to the absorber. After quantification of Na in CZTSSe right after growth, the latter is characterized to evaluate its quality and employed in a full solar cell to check on its photovoltaic properties. Results demonstrated that, as for CIGS technology, sodium is beneficial for CZTSSe allowing increasing the open circuit voltage and efficiency. Molybdenum is the most used back contact in CZTSSe based solar cells. However, it has been suggested recently that Mo is not stable at the interface with CZTSSe. In addition, to the best of our knowledge, no experimental study has been carried out so far to test whether solar cells built on another back contact could exhibit better photovoltaic properties. For this purpose, various metals (Au, W, Pd, Pt, and Ni) are deposited on top of Mo, and it is demonstrated that it is possible to synthesize device-quality CZTSSe thin films on W, Au, and Pt back contacts. It is shown that that W and Au back contacts allow enhancing the photogenerated current, but that Mo remains the best back contact in terms of power conversion efficiency. The effects of [S]/([S]+[Se]) ratio tuning on CZTSSe based solar cell performances have been studied by solar cell capacitance simulator (SCAPS) to find out the optimum absorber composition. Two different kind of approach have been studied: linear variation of the chalcogens ratio, and a parabolic variation. The simulations lead to an efficiency of 16.5% (with open-circuit voltage of 0.56 V, short-circuit current of 37.0 mA/cm2 and fill factor of 79.0%) when the sulfur content is linearly decreased from the back contact towards the buffer layer. Based on these results, we propose that bandgap engineering based on the control of [S]/([S]+[Se]) ratio in the absorber is a powerful tool which allows increasing the performances of CZTSSe based solar cells without changing the absorber material quality.

Keywords:
Thin film, CZTS, Solar cell

On-line thesis.