Improving the Understanding of CZTS-Se as a Solar Absorber Material through Single Crystals Formed Using Phase Diagram Analysis

1db29e44-c08e-443d-a9a1-e5d4e39a49de

Closed

EPSRC
EPSRC
EPSRC
EPSRC
EPSRC

No funds listed.

Sept. 30, 2016

April 30, 2020

The aim of this project is examine the electronic and structural characteristics of the low-cost quaternary semiconductor CZTS-Se (chemical formula Cu2ZnSn(S,Se)4). This is intended to inform the use of CZTS-Se for thin-film solar photovoltaic applications. The project aims to gain a deeper understanding of the phase diagram structure and crystallisation processes of CZTS-Se in different solvents, along with the resultant effect on electronic properties. Central to this analysis is the fabrication of large, single crystals of CZTS-Se which would allow characterisation of the material's bulk properties in the absence of surface effects. This is in order to inform solution processing methods of CZTS-Se solar cell fabrication to reach higher efficiencies than the current record.
The motivation for this study is the fact that CZTS-Se devices can be fabricated from cheap, abundant and non-toxic elements. The low price of raw materials coupled with a low disposal cost due to non-toxicity means CZTS-Se devices have the potential to form very cheap solar-power modules. Another advantage to these devices is that as a solid, polycrystalline semiconductor, CZTS-Se does not suffer from the instability issues that limit the viability of organic and perovskite devices at present. This makes it more feasible to create modules that last long enough to provide the necessary returns. Due to these desirable features, if CZTS-Se modules were able to enter the energy market they could significantly reduce the price of solar power around the world. However, despite being a direct-gap semiconductor with a band-gap that is close to optimal, the current record of efficiency for a CZTS-Se solar device is only 12.6%, compared to just over 25% for the champion silicon modules. This low efficiency means that the rate-of-return for CZTS-Se devices is currently too low for them to be economical.

Device efficiency in photovoltaic devices can be understood as a dependence upon 3 parameters: the short circuit current, JSC; the open-circuit voltage, VOC; and the fill factor, FF. CZTS-Se devices have already been able to demonstrate similar values of JSC to devices made of CIGS, an already-established quaternary semiconductor, but the VOC for CZTS-Se is significantly below the equivalent level for CIGS. Addressing the 'VOC- deficit' in CZTS-Se is therefore a major aim of this project. The presence of charge-carrier recombination centres is thought to contribute to the VOC- deficit, where potential sites for recombination include crystal grain boundaries and/or defects in the bulk material. Another factor found to contribute to poor device performance is the formation of secondary compositional and/or structural phases.

As a result, this project is focussed upon removing grain-boundaries altogether by forming a large, single crystal of CZTS-Se. Single crystal growth for CZTS has been successfully demonstrated in the past. Therefore our aim is to develop a process to grow single crystals and measure their photovoltaic properties. These properties can then be compared with polycrystalline CZTS-Se to assess the effect of grain boundaries. A single crystal of CZTS-Se will also allow the effects of bulk defects and secondary phases to be investigated, separate from surface effects. Variations in the elemental composition and fabrication conditions to reduce detrimental bulk defects and secondary phases can then be explored.
Production and characterisation of single crystals requires a deep understanding of the solutions formed from CZTS-Se with a range of solvents. Therefore much of the work of this project will be to characterise phase diagrams of CZTS-Se/solvent systems across a range of temperatures and compositional ratios.
The primary concern of this project is the fundamental analysis of CZTS-Se as a material. However we intend that the insights from this project will be applied to the production of high-quality CZTS-Se absorber layers.