In this exclusive column Dr. Nigel Salter, Managing Director of Manchester-based graphene manufacturer 2-DTech, explains how the incorporation of graphene into thin film photovoltaics may boost their conversion efficiencies and help us meet rising global demand for energy.
By Dr Nigel Salter, Managing Director of 2-DTech
Global energy demands are set to rise at a startling rate over the next few decades, with increased population levels plus continued global economic development both contributing to this increase. Based on figures by the US Energy Information Administration, estimates have been made that by the year 2050 our total energy consumption will be approximately 33TWy.
This massive projected escalation in our energy requirements will put considerable strain on the Earth’s available energy resources. It will call for a significant ramp up in the contribution made by renewables, such as wind and solar power. According to projections made by the International Energy Agency, in the 2050 timeframe photovoltaic technology could constitute in the region of 16 per cent of the world’s electricity generation, but at the moment this seems quite a long way from being realised.
Crystalline silicon photovoltaics uneconomic
Though growing, solar has yet to make a serious contribution to the world’s total power generation capacity. Finding a cost-effective method by which to efficiently convert solar energy into electricity is still proving difficult. Photovoltaic manufacturers are having to make compromises – either pushing the power conversion envelope and having to deal with high unit costs, or conversely sacrificing conversion performance in order to keep these costs in check. Neither situation is anywhere near ideal of course.
The vast majority of photovoltaic infrastructure currently deployed is dependent on the long-established use of crystalline silicon (c-Si). Though c-Si photovoltaics can deliver power conversion efficiencies of up to a highly respectable 25 per cent, widespread proliferation of this technology is being somewhat held back. This is due to the high production costs associated with this approach.
As it is relatively poor at absorbing light energy, c-Si has to be applied in thick (around 200mm), rigid slabs. Also the silicon needs to have an extremely high degree of purity. It is these two criteria that make it expensive to implement – meaning that it is very difficult to apply this technology to large installations (where areas of 100m2 and above need to be covered). Other, more economical methods of harnessing solar energy are therefore being sought.
Thin film technology an attractive alternative
Thin film technology appears to present the industry with an attractive alternative to c-Si. Thin film photovoltaics, utilising copper indium gallium selenide or cadmium telluride compounds and such like, are substantially cheaper to fabricate than c-Si. These materials are also much less fragile – making them easier to handle and less prone to damage. There is the unfortunate drawback though that so far they have not been able to match c-Si’s elevated conversion efficiencies.
Recent progress has been made that could boost conversion efficiencies in thin film technology. This is being achieved through research done into the employment of perovskites and related materials within photovoltaic cells’ absorbing layers.
Photovoltaic cells which feature perovskite can deliver power conversion efficiencies that are closer to c-Si photovoltaic cells than has previously been possible via thin film technology. This material’s expansive spectral response is another advantage – thereby increasing the scope of wavelengths from which energy can be derived.
Incorporating graphene into thin film photovoltaics
Some are of the opinion that the incorporation of graphene into thin film photovoltaics will be the next step towards achieving parity with c-Si photovoltaics in terms of power conversion efficiency. Graphene has a super-thin carbon-based nanostructure with many remarkable characteristics. With a single atom thickness (just 0.34nm) it is understandably referred to as a two dimensional (2D) material.
Though it weighs very little (a mere 0.77mg per square metre) it is extremely strong (with a 1Tpa tensile strength). As well as its mechanical robustness, graphene has exceptional electrical properties (supporting current densities of 100MA/cm2 and offering an intrinsic charge carrier mobility that reaches the 105cm2/Vs mark) – and it is these which are certain to be of value to the photovoltaics business.
Prospects in the future
Manchester-based graphene manufacturer 2-DTech recently embarked on a year-long collaborative project with solar technology specialist Dyesol that will see graphene incorporated into perovskite charge collecting regions of solid-state dye-sensitised photovoltaic cells.
Through this it is envisaged that the cells’ charge collection capabilities will be enhanced. The core objective of the 2-DTech/Dyesol project is to eventually reach a stage where it is possible to develop a fabrication process capable of producing monolayer encapsulated graphene-perovskite photovoltaic cells on a large scale. This may provide the impetus needed to help accelerate the uptake of solar technology.
The emergence of a new breed of thin film photovoltaics, where graphene has been integrated into perovskite, will have the potential to deliver a compelling combination of cost-effectiveness and high-conversion performance. Through research projects like the one discussed here, marked improvements to the effectiveness of solar technology can over time be achieved.