Improving the Understanding of CZTS-Se as a Solar Absorber Material through Single Crystals Formed Using Phase Diagram Analysis
Lead Research Organisation:
University of Liverpool
Department Name: Physics
Abstract
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.
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.
Organisations
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509693/1 | 01/10/2016 | 30/09/2021 | |||
| 1796442 | Studentship | EP/N509693/1 | 01/10/2016 | 30/04/2020 | Theodore Hobson |
| Description | There are several strands to what was discovered and achieved by this award, which may be broadly divided into: 1) basic structural and compositional characterisation of copper-zinc-tin-sulpho-selenide (CZTS-Se); 2) basic structural and compositional characterisation of antimony selenide (Sb2Se3); 3) and the electrical characterisation of single crystals of Sb2Se3. 1) I developed, with others, a novel technique to grow plates of copper-zinc-tin-sulpho-selenide (CZTS-Se) with large crystal grains at a range of compositions using a mixture of molten salts, sodium chloride and potassium chloride, as a solvent. This allowed me to advance the understanding of the structural and compositional properties of alloys formed of CZTS and CZTSe, through characterising these high-quality samples with x-ray diffraction, Raman spectroscopy, scanning electron microscopy and energy-dispersive x-ray spectroscopy. Understanding of the composition and crystal structure of CZTS-Se is crucial to improving photovoltaic devices made from these materials, so my work has contributed to these efforts. The results of the study into CZTS-Se have been published in the journal Crystal Growth and Design. Despite success in using a salt-based solvent to grow crystalline plates, this technique did not prove suitable for forming large CZTS-Se single crystals. Nor did the original method proposed, using Sn as a solvent, succeed as originally hoped. More recent results in the field have indicated that a much greater level of thermal control than we could access was required to succeed. However, my efforts to grow CZTS-Se single crystals were not wasted. I employed the same methods and background work in phase diagrams to another emerging chalcogenide material with applications in photovoltaics, antimony selenide (Sb2Se3), which leads into the next strand of findings. 2) I had consistent success in producing large single crystals of Sb2Se3, with controlled composition, including control of the level of trace impurities. These crystals have proved useful for several studies intending to improve the understanding of Sb2Se3 for photovoltaic applications. A Raman spectroscopy study has allowed us to understand the crystal structure much better, as well as being able to assign the peaks that appear in the Raman spectra to specific atomic vibrations in the material. The fabrication of these crystals also allowed the identification of a key impurity in the source material, which had important impacts on the electronic properties of Sb2Se3. The growth process was published in the conference proceedings of the 45th IEEE PVSC conference, while the Raman study produced a paper manuscript currently in review at the Journal of Materials Chemistry A. 3) The fundamental electrical properties of Sb2Se3 were characterised using the single crystals fabricated in strand 2). Deep-level transient spectrometry has informed us of defects which may form carrier recombination centres that affect photovoltaic performance; capacitance-voltage and hot-point probe measurements have informed us of methods of doping Sb2Se3 to improve conductivity or change the doping type altogether. Photoluminescence has also provided novel data on the defects which may have a more subtle impact on material properties. All of this has relevance for solar device production, and the results have informed the work going on with solar cells in the lab where I am based. The results of the study into the conductivity type in Sb2Se3 have been published in the journal Chemistry of Materials |
| Exploitation Route | By collecting XRD and Raman data from a continuous known composition series from CZTS-CZTSe, our work should allow others to more easily identify the composition and crystal structure of films made from these materials. We see, for instance, a linear shift in the position of the x-ray diffraction (XRD) peaks with S-Se ratio, allowing others to infer the S-Se content from XRD. The techniques applied in this project for growing pure and doped crystals of antimony selenide (Sb2Se3) may be taken forward by others who wish to try different dopants. The Raman study has allowed us to assign peaks in the spectrum to vibrations with specific crystal orientations, providing a reference for researchers who wish to know the crystal orientation of their films, a compliment to XRD. This is especially important as the performance of Sb2Se3 devices is strongly dependent on the orientation of the crystal grains in the film. Our studies of the doping type in single crystals can directly inform the production of thin film devices, providing guidance on how to achieve a particular doping type and density, which other researchers may take forward to better design their devices. |
| Sectors | Chemicals,Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology |
| URL | https://dx.doi.org/10.1109/PVSC.2018.8547622 |