Graphene has the potential to influence many areas of technology, not least of all the future of solar cell design.
By Dr Craig P. Dawson, 2-DTech.
It is estimated that the total installed PV capacity across the globe will reach the 500GW mark by 2018, according to a report compiled by market analysis firm Solarbuzz. This would be a notable increase on existing renewable resources, but would still not make any major contribution to the world’s ever-increasing energy consumption. As yet a cost-effective and fully efficient way by which to convert solar energy into electrical energy still seems to allude us. Manufacturers of PV infrastructure currently find themselves in an awkward position. They face having to make difficult trade-offs, forced into trying to balance demands to raise energy conversion performance and lower overall cost.
The overwhelming majority of today’s commercial PV deployments rely on crystalline-Silicon (c-Si), which is applied in relatively thick, rather expensive slabs. These c-Si solar cells have the advantage that they can offer up to 25% power conversion efficiency levels, but nevertheless widespread use of this technology for large installations (i.e. ones covering areas above 100m2) is to some extent hampered by the considerable manufacturing costs involved. This is due to the fact that high-purity silicon must be specified in order to attain the expected degree of efficiency.
An alternative to the c-Si approach is to utilise thin film technology. Though thin film solar cells, using semiconductor materials such as copper indium gallium selenide, can be produced more economically, they do not perform in a comparable manner to c-Si solar cells in terms of their light absorption or their conversion efficiency. There have in recent times, however, been encouraging developments made in thin film solar technology, through use of cells with absorbing layers based on Methylammonium Lead Halides (CH3NH3PbX3, where X = I, Br, Cl) which have the perovskite (e.g. CaTiO3) crystal structure.
As they do not require inclusion of any novel or difficult to obtain chemicals, or rely on complex fabrication techniques, the production costs that are associated with perovskite solar cells are far lower than those of c-Si cells. At the same time the perovskite cells manage to deliver comparable power conversion efficiencies to thin film c-Si solar cells and only a few percentage points lower than single cell c-Si solar cells. Perovskite crystals are, as a result of these attributes, already being featured in the latest generation of solar cells. As well as converting incident light into electrical energy very efficiently the perovskite solar cells also offer a very broad spectral response, so the light across an expansive range of wavelengths can be utilised. Some are of the opinion, though, that there are ways in which the conversion efficiencies of this compound can be pushed still further; there is now a growing interest in the role graphene could play within perovskite-based solar cells.
For some time now graphene has been heralded by many as the wonder material that could have huge bearing on many different aspects of modern society. First isolated in 2004, this single atom thick (0.34nm) carbon-based nanostructure is both incredibly strong (with a tensile strength of 1Tpa) and at the same time incredibly light (weighing only 0.77mg per square meter). Of course it is graphene’s electrical properties that are of most importance to the PV industry; it is recognised as the best conductor in existence, with an electrical current density reaching 100MA/cm2 (a million times greater than that of copper) and an intrinsic charge carrier mobility of approximately 105cm2/Vs at room temperature (a hundred times greater than silicon).
The principle challenge in modern PV cell design, with relation specifically to how well sunlight can be translated into electricity, is being able to maximise the cell’s charge collection capacity. Through carefully engineered incorporation of graphene into thin film solar cells, the remarkable electronic and structural properties it possesses could lead to a step change improvement in efficiency levels, as well as the additional benefit of gaining greater surface stability. Unfortunately, so far the research done in this particular area has been limited.
Increased efficiency levels would result in a smaller installed area per unit of electricity generated, thereby reducing the material requirements as well as carbon footprint of the manufacturing process employed. It is also envisaged that through monolayer graphene encapsulation there would be a significant improvement in the durability and lifetime of perovskite solar cells, which can be sensitive to moisture. The encapsulation aspect would prevent moisture ingress and additionally a reduction in the external toxicity of solar cells based on perovskite could be derived, as lead (Pb) compounds present in these cells could be prevented from leaking into the external environment, where they would otherwise present a health hazard.
University of Manchester spin-out, 2-DTech, has recognised the potential that graphene has to aid the PV industry. The company was recently awarded an InnovateUK grant worth £98,000 to carry out research relating to development of a methodology by which graphene nanoparticles could be integrated into the perovskite charge collecting regions of the solar cells. The company has now started to collaborate with solar technology specialist Dyesol on this project
The project being undertaken by 2-DTech and Dyesol will initially look to produce a baseline first generation perovskite solar cell with graphene incorporated into it. Next researchers will over time make a series of small adjustments to the graphene composition to see which combination yields the best power conversion efficiency results for the perovskite solar cells. Finally, the two companies will aim to develop a large scale manufacturing process for monolayer encapsulated graphene-perovskite solar cells, with the end goal of making this a commercially viable product.
Thin film solar cells simply have not, up to this point, been able to offer the sort of power conversion efficiencies that are needed to compete with crystalline alternatives and until recently the progression of thin film solar cell technology has continued at a fairly slow rate. It is possible that graphene could accelerate things significantly - with the nanomaterial being used to enhance the electrical properties of perovskite solar cells. This may hold the key to boosting power conversion efficiency levels beyond their current restrictions, as well as offering some improvements in cell durability and environmental impact. Research now underway in Manchester could, in time, enable advanced PV solutions, employing previously less favoured thin film technology, to be brought to market that have much more attractive cost/performance characteristics.