Overcoming the grain size limit to Voc in sustainable photovoltaics

Solar electricity based on wafer silicon is a mature technology that is in widespread use. Although it is getting cheaper due to mass production in China, the international market is still driven by government subsidies. Alternative 'thin film' materials are the best chance of competition, but they have three problems:
a) their voltages are lower than expected due to the small size of the crystal grains in them (several millionths of a meter typically),
b) the materials presently used for thin film solar cells contain rare elements that will pose supply issues as the PV industry expands by several hundred fold over the next 40 years, and
c) production methods must become cheaper and more effective in order to compete in the global market

We will test a new method to make solar cells that promises to overcome all of these limitations. We will work on the archetypal earth abundant semiconductor Cu2ZnSn(S,Se)4 (CZTSS). Solar cells made from it suffer from the typical problems: It underperforms on voltage and has very small crystal grains. It is also difficult to make since it is prone to lose sulphur and selenium (the most successful research labs resort to complex methods involving nanoparticles and dangerous reducing solvents). The efficiency has been limited to 12% for some years now, and this is preventing CZTSS from becoming a production technology.

In this project we will test an alternative method to grow CZTSS. We will explore the possibility of growing large grains of CZTSS on cheap metal sheet with small grains -we expect this could become a workable production route. We will make some hundreds of solar cells to test the hypotheses.

Overall the idea has the potential to increase the efficiency of CZTSS from 12% to 16%, or even higher, making it feasible to open up a pathway for new PV products. A technological lead in this area could give the UK the opportunity to grab back a share of the expanding global PV business.

1. Who will benefit:
- Society in general - from CO2 reductions.
The report of the 40th Session of the International Panel on Climate Change (01 Nov 2014) states: There is a 95% chance that climate change is anthropogenic - continued emissions will increase the likelihood of severe irreversible impacts. It recommends substantial emissions reductions over the next few decades and near zero emissions of CO2 by the end of the century. www.ipcc.ch

- UK and EU industrial capacity - by recovering a share of the growing market for PV from the Far East
- Traditional industries in the UK- by diversifying to high value added products from large volume cheap products such as galvanized sheet.
- New and existing solar electricity module producers - will gain from new manufacturing opportunities.
- Research community for earth abundant PV - gain a new approach to creating viable PV device technologies.
- PV materials and packaging supply chain - gains from increase in production
- PV installation industries - from increased deployment resulting from low cost manufacture.


2. How the benefits will arise:
Increasing the UK market share. Market disruption is the only way to win back PV manufacturing from Far Eastern dominance. We propose a radical shift in technology that could re-invent the industry and allow the UK and Europe to reclaim a market share in a growing industry from China and Taiwan who have led massive expansion of conventional technology.

Transformation of traditional industries to a value added model. e.g. the UK manufactures coated steel sheet at 200,000,000 m2 per annum at <10p/m2, but there is presently no direct manufacturing industry for thin film solar cells - that would sell at £50 - 100 £/m2. Our innovations will a) create a pathway for the transformation to high added value using existing industrial expertise and b) create a new market for low cost sheet and foil materials in high tech manufacture

PV expansion without subsidies. A new route to a revised mass market for PV will eliminate the dependency of the international market on subsidies hence allowing a natural market - led expansion of solar electricity.

Meeting CO2 targets. Expansion of from GWp to TWp production levels, will have a substantial contribution to CO2 targets and the economy. In simple terms, the production of 1 TW of solar electricity will take out the equivalent of ten typical fossil fuel power stations that produce CO2

Training of an essential workforce for advanced industry. We will link the project to the EPSRC Centre for Doctoral Training in New and Sustainable PV - there is an excellent fit. In particular a CDT student at Liverpool, Mr Peter Yates, will work on transmission electron microscope characterisation of the crystal quality of the layers and PV devices. He started in October 2014 and so his post will outlast this research project. We will seek an additional student from the CDT, although this will of course require the approval of the Management Board.