There’s been a lot of excitement about perovskite-based solar technology lately, with some observers already declaring it the future of solar PV. So, are silicon PV’s days really numbered?
As much as people love the idea of solar power, the fact is that clean energy also has to be cheap if it’s going to compete against fossil fuel generation. It is fortunate then that, despite a lack of consistent political support, the price of crystalline silicon (c-Si) PV modules – which accounts for the vast majority of PV systems around the world – has plummeted in the past four decades. Technological innovation, market competition and economies of scale have driven the module cost down from more than US$50 per peak watt production in 1976 to less than US$0.70 on average in 2015.
But these PV modules first have to be fabricated, and this requires an initial energy input, which comes with an ecological footprint. Manufacturing c-Si PV involves high-temperature processes as well as environmentally unfriendly chemicals, which affect the energy payback time of these modules. A recent study from the Fraunhofer Institute for Solar Energy Systems estimated the energy payback time of current c-Si PV is two years on average.
Why is perovskite PV attracting so much interest?
What if we could produce PV modules at a lower cost and with less energy? This is the prospect that low-cost thin films offer. Imagine your solar panels were printed in the way bank notes are in Australia. Once freed from their high-temperature processing and rigid nature, solar panels could gain flexibility and offer new potential applications ranging from consumer electronics to solar powered textiles, all at a very low cost. However, historically there has always been a trade-off between panel efficiency and module energy cost. Despite their promise for low processing cost, the efficiency of organic (OPV) and dye-sensitised solar cells (DSC) remains low (10-12 per cent) compared to that of c-Si PV (25 per cent). This is where the perovskite PV technology could become a game changer.
What is perovskite PV?
When people talk about “˜perovskite solar cells’, they are referring to the device’s photoactive layer, and particularly to the crystal structure of the material constituting this layer. These “˜perovskites’ have the same crystallographic arrangement as a mineral called “˜calcium titanate’, but, contrary to what has sometimes been reported, this mineral is never present in a solar cell. The perovskites we’re talking about are actually hybrid organic-inorganic halide analogues of calcium titanate – but rather than dragging around an expression like “˜solar cells using hybrid materials having the same crystal structure as calcium titanate’, researchers understandably abbreviate it to “˜perovskite solar cells’.
Perovskites have been around for a while, but it was only in 2006 that Professor Miyasaka’s team at Toin University of Yokohama first applied these materials to solar cells, with a device efficiency of only 2.2 per cent at the time. Since that first attempt, perovskite PV has come a long way: in December 2015, a research team led by Prof. Gretzel and Prof. Hagfeldt from the Ecole Polytechnique Fédérale de Lausanne (EPFL) reported a record of 21 per cent.
Why has perovskite PV progressed so rapidly?
The swift surge of perovskite PV can partly be explained by the ease of fabrication of these devices, in which the crystalline perovskite layer can be formed from solution at a temperature as low as 100°C. The materials show outstanding opto-electronic properties, with a wide bandgap (1.56 eV for the most commonly used perovskite, CH3NH3PbI3), a high absorption coefficient and an excellent charge carrier diffusion. What this means is that a 300 nm thin perovskite layer can absorb photons from a large section of the light spectrum and efficiently carry the generated charges across to the collecting layers.
Last but not least, a number of DSC and OPV researchers who saw the potential of perovskite PV started working with perovskites, bringing in their years of solar device experience, which contributed to the rapid emergence of the new PV technology.
When might perovskite modules be hitting the market?
It is hard not to get enthused by the potential this technology offers. But “potential’ is the key word here, as this young technology has yet to reach maturity and a number of shortcomings have yet to be overcome.
First of all, there’s the lead conundrum. The most commonly used organic-inorganic halide perovskites are constituted of lead (CH3NH3PbI3), which is toxic. This raises some thorny questions. Will researchers hit a roadblock when reaching market deployment and having to face legislation such as the European Restriction on Hazardous Substances? Or will perovskite PV modules be exempted from stringent legislations? In CdTe (cadmium telluride) thin films, for instance, the toxicity of cadmium seems to have had little consequence on its market adoption.
In perovskite PV, the water solubility of the lead-based degradation products represents an additional environmental risk. One interesting strategy could consist in recycling lead from lead-acid battery waste to produce perovskite PV. Ideally, lead-free perovskites could replace their lead-based analogues, but these more sustainable devices currently exhibit lower efficiencies.
Importantly, the complete life cycle analysis of PV modules should be considered, since other PV technologies also use or generate toxic substances in their manufacturing processes, and this more complete picture used to compare the impact with that of fossil fuel generated electricity.
This leads to another issue that perovskite PV currently faces, which is its short device lifetime. The most commonly used perovskite material (CH3NH3PbI3) reacts with water, and ambient humidity is sufficient to degrade the device. Many research groups have been working on designing new device architecture to prolong the lifetime, as well as on encapsulation techniques to create an effective barrier between the module and its environment. Using current encapsulation techniques, perovskite PV has only been able to demonstrate stability over a few months so far. This means that there is still significant progress to be made to meet the 20-year lifetimes of silicon PV.
This technology is still at a very early stage. This is particularly true because most techniques used to fabricate these devices are lab-oriented and cannot be employed to produce large-scale devices. Extraordinary efficiencies have been reported, but these usually concern devices of less than 0.25 cm2 in size. Is it really fair to compare the efficiency of these tiny devices to that of >143 cm2 c-Si PV modules? This doesn’t mean that suitable manufacturing techniques – such as printing – are not possible. But they have not been able to give “˜record’ efficiencies that make the headlines yet.
To transform innovative technologies into successful business stories, all these practical industrial concerns need to be thoroughly addressed, and this requires funding. Companies like Oxford PV in the UK or Dyesol in Australia are investing in perovskite PV and pushing for its commercialisation. The Australian Renewable Energy Agency recently showed its support with the announcement of $892,000 for the testing of perovskite solar cell performance. And hopefully there will be more to come!
What can be done in the meantime?
Perovskites have the potential to reconcile the low-cost PV and c-Si PV technologies by, for example, targeting different segments of the market. Crystalline silicon PV is well established and is likely to remain the favourite technology for rooftop applications. Because of their lightweight and flexible nature, perovskite PV cells are more suitable for consumer driven electronics. Thanks to their semi-transparency, they could also become an ideal candidate for integrated building applications. What’s more, rather than competing, these technologies could take advantage of each other. In tandem perovskite/Si PV, the perovskite material can be cleverly tuned to absorb parts of the light spectrum that are normally not converted into electricity by c-Si PV. This would lead to an overall increase of the module efficiency with a low additional manufacturing cost.
There is great potential for collaboration between perovskite and c-Si PV researchers. Whether through collaboration or competition, the PV module price will carry on going down, becoming more accessible for people who can’t afford and wish to have access to a cleaner form of energy.