Computer-aided design of zinc phosphide heterojunctions for efficient solar energy conversion

Lead Research Organisation: Cardiff University
Department Name: Chemistry

Abstract

The growing need for energy by our society and the depletion of conventional energy sources demands the development and improvement of safe, renewable and low-cost clean energy technologies. Photovoltaic (PV) technology which makes use of the super-abundant and freely available Sun's energy to generate electricity has obvious economic, environmental and societal benefits. However, in order for PV technology to provide a significant fraction of the world's energy demands, devices must be composed of cheap and earth-abundant materials. Science and engineering are in a unique position to address the challenge to discover, design and develop inexpensive, non-toxic, and earth-abundant new materials that exhibit the ideal electronic properties for PV applications.

This proposal outlines the strategy for the rational design of zinc phosphide (Zn3P2) heterojunctions for the efficient conversion of solar energy into electricity. Zinc phosphide is ideally positioned as a next-generation PV material due to its direct band gap of 1.50 eV, which allows it to absorb a high percentage of the solar spectrum. Zn3P2 also has a high visible-light absorption coefficient, long minority-carrier diffusion length, a large range of potential doping concentrations, and both of its constituent elements are non-toxic, cheap and abundant, which makes Zn3P2 a promising material for cost-effective and scalable thin-film photovoltaic applications. Despite its germane electronic properties, to date, a Zn3P2 device of sufficient efficiency for commercial applications has not been demonstrated. The highest solar energy-conversion efficiencies of 6.0% for multi-crystalline and 4.3% for thin-film cells have been reported. The low efficiencies of the thin film and heterojunction-based Zn3P2 devices have been attributed to poor understanding of the interfaces and band-alignment between the emitter and the absorber layers, to high concentrations of interface trap states (Fermi-level pinning), and/or to inadequate interface passivation. Given their 2-dimensional nature and their typical location buried within bulk materials, interfaces are difficult to resolve or access by purely experimental means.

The goal of this cross-disciplinary project is, therefore, to develop and employ a combination of cutting-edge computational techniques and experiment to design and identify the key interfacial and electronic properties needed for the practical performance of zinc phosphide photovoltaics to achieve improved solar energy-conversion efficiencies. The use of a synergistic computational-experimental approach will help address key questions about the nature of atomic ordering (chemical and structural) and the electronic properties of the surface and interface of epitaxial Zn3P2 films grown on II-VI and III-V substrates, which will unlock a promising pathway towards the development and commercialization of low-cost, high-efficiency and earth-abundant Zn3P2 photovoltaic devices.

The innovation of the proposed project is based on the engineering and transformation of earth-abundant and non-toxic Zn3P2 into a cost-effective, highly efficient and scalable thin-film PV material that provides additional environmental, health and economic benefits to the UK and globally. The main deliverables and benefits of the proposed project include, but are not limited to (i) atomic-level understanding of the surface and interface properties of a Zn3P2 epilayer, which has important implications on device fabrication and performance; and (ii) the growth of high-quality epitaxial Zn3P2 films on II-VI and III-V substrates as proto-types for industrial-scale PV applications.

Planned Impact

The proposed project addresses questions in the UK's energy transition to a sustainable, low-carbon development path and it would have a combination of short- and long-term beneficiaries, and with continued effort, will offer long-term social and economic benefits.

Academic fields - computational and experimental
The most direct impact will be the personal and professional development that is appropriate for the extension of the Fellow's career beyond this fellowship and the training of the one PDRA and a Ph.D. candidate sponsored by Cardiff University. Using a synergistic computational-experimental approach, the knowledge generated on the design and identification of the key interfacial and electronic properties needed for the practical performance of zinc phosphide (Zn3P2) photovoltaics to achieve improved solar energy-conversion efficiencies will be of great interest and benefit to the academic community. The expected substantial Intellectual Property (IP) and know-how to be generated as a result of the research will revitalize the interest of the materials science community in zinc phosphide as well as to provide a template for the development of other new photovoltaic materials.

Industrial/users (medium to long-term)
In the medium term, this work will provide the data and computational tools applicable in many sectors, such as the electronic and solar cell manufacturing industries and it could underpin a new generation of energy materials companies exploiting the identified interface and electronic properties. In the longer term, the new knowledge will lead to the identification of new materials with enhanced properties for use in many current energy generation and storage devices and in future applications. Companies such as IQE plc, a leading global semiconductor company that manufactures advanced epitaxial wafers for a wide range of technology applications, will benefit from the outcomes of the proposed work.

Economy, society, and environment
The proposed research has the potential to contribute significantly in the long term, 10-20 years to solving economic, societal and environmental problems. By increasing the efficiency of zinc phosphide photovoltaics and opening up the opportunity of new advancements from a fundamental understanding of their material properties, the economy and society will benefit from more efficient, cheap, non-toxic and sustainable ways to generate and store energy. This increased efficiency will improve the security of supply and reduce the carbon footprint.

Publications

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Description Zinc phosphide is an attractive earth-abundant solar absorber material for scalable thin-film photovoltaic applications. Despite its ideal optoelectronic properties, problems such as inadequate interface passivation, and low surface stability in the presence of moisture and oxygen remain major problems that severely limits the commercial fabrication
of highly efficient Zn3P2-based photovoltaics. Zinc phosphide nanoparticles and thin films can easily get oxidized when in contact with water and oxygen owing to the higher specific surface area and higher reactivity relative to the bulk. By performing accurate first-principles density functional theory calculations, we are able to unravel the early oxidation mechanism of zinc phosphide surfaces by adsorbed oxygen and water, where we show that the adsorption of oxygen and water species is characterized by a significant charge transfer from the interacting surface species, causing them to be oxidized from Zn2+ to Zn3+ formal oxidation states. These results were published in the Journal of Physical Chemistry Chemical Physics (https://doi.org/10.1039/C9CP03902C). The study also highlighted the need for Zn3P2 nanoparticles to be protected against possible oxidation in the presence of oxygen and moisture via in situ functionalization, wherein the Zn3P2 nanoparticles are exposed to a vapor of organic functional molecules immediately after synthesis. In a follow follow-up study, the interface chemistry between the 4-amino thiophenol (4ATP) molecule and the (001), (101), and (110) surfaces of zinc phosphide (Zn3P2) has been investigated. It was demonstrated that the functionalization of the surfaces of Zn3P2 can be protected against oxidation a the electronic properties are essentially presented for device fabrication. This work is published in the ACS Omega journal (https://doi.org/10.1021/acsomega.9b02736).
Exploitation Route The outcome derived is helping in our experimental synthesis and characterization of zinc phosphide thin films for photovoltaic applications. The derived atomic-level insights should aid the rational design and engineering of heterojunctions (i.e. between the Zn3P2 absorber materials and buffer layer materials (eg, CdS, ZnS, ZnSe, etc) for efficient solar device fabrication.
Sectors Education,Electronics,Energy

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