Investigation of Band Alignments in Antimony Selenide Solar Cells

Lead Research Organisation: University of Liverpool
Department Name: Physics

Abstract

The field of photovoltaics has been growing rapidly over the last few decades with production of silicon and cadmium telluride based devices increasing steadily whilst also increasing in efficiency. However, despite the success of these types of devices there is still a big push to find a more sustainable alternative. The production of both cells involves the use of expensive materials and very toxic chemicals which can be hugely problematic when aiming to meet the demands of terawatt scale-up. This has never been more important given the government's commitment to reducing emissions by 80% by the year 2050, or that the UK is set to miss its target of 15% of energy produced by renewable sources by 2020.
Antimony Selenide (Sb2Se3) thin films are one of the most promising candidates- antimony selenide has favourable optoelectronic properties including a direct band gap in the region of 1.2eV and an absorption coefficient >105 cm-1. It also boasts a rare crystal structure which is made up of 1-D nanoribbons which is suggested to significantly reduce losses at grain boundaries[1]. The current record efficiency for Sb2Se3 sits at 6.5% [2] which is still some way off Cadmium Telluride (22.1% [3]) but considering the relative immaturity of its development as an absorber material it is very promising. One of the most crucial mechanisms in the working of a solar cell is the positions of the valence and conduction bands and how these bands line up at the interface between different materials. Unfavourable band line-ups can lead to significant losses in efficiency and these band positions can be affected by a number of factors such as crystal structure and chemical composition. One of the best ways to measure band positions is through photoemission spectroscopy (PES) and inverse photoelectron spectroscopy (IPES), particularly X-ray photoemission spectroscopy (XPS). XPS is a surface sensitive technique that involves bombarding the surface of the material with X-rays which causes electrons to be emitted. By analysing the kinetic energy of these emitted electrons, information on the energy levels in the material can be inferred and compared to DFT calculations.
It is commonly the case that there is a divide between the bottom-up approach in photovoltaics - the investigation and modelling of fundamental properties of the constituent materials, and the top-down approach of device optimisation. It is striking that in all the literature on Sb2Se3 as an absorber layer only a handful of papers include XPS measurements and none of these has carried out a thorough investigation beyond identifying the existence of antimony and selenium in the sample. This project aims to harness powerful fundamental techniques such as XPS to investigate the band structure at interfaces between layers in the cell, and carry this information all the way through to the production of cells and characterisation of their performance. In particular, we will aim to measure any differences in band line-up when pairing Sb2Se3 with different window layers, including CdS, TiO2 and ZnO, making use of Kraut's method [5].
Measuring band line-ups is, unfortunately, a rather complex problem. By carrying out ionisation potential measurements on the individual layers we can determine the 'natural' band alignments, however, this does not take into account the effect of charge transfer between materials at the interface. Instead, by growing extremely thin layers (<10nm) of one material on another we can carry out XPS across the interface. This would also have the added benefit of providing information on the effect of charge transfer on the band alignments at the interface. However, there are pitfalls to this method too - the morphology of the extremely thin layers required to carry out such a surface sensitive technique is likely to be different to the bulk structure incorporated in an actual device.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R51231X/1 01/10/2017 30/09/2021
1946949 Studentship EP/R51231X/1 01/10/2017 30/09/2021 Huw Shiel
 
Description Paper published on the effect of chemical etches on the surface chemical composition of antimony selenide (Sb2Se3) films and on the performance of Sb2Se3 solar cells.
Manuscript completed on the true nature of the band alignments between Sb2Se3 and its two most common partner layers and its relation to device performance.
Exploitation Route Improved understanding of the effect of the chemical etches will allow groups to tailor their back contact treatment to their needs and potentially boost device efficiency.
Understanding the true band alignments will help in identifying the optimum device structure for Sb2Se3 solar cells.
Sectors Energy