Spray-On Solar Cells Getting Closer to a Reality
The creation process of perovskite solar cells. Source: Segawa Laboratory at Tokyo University
Imagine a future when solar cells can be sprayed or printed onto the windows of skyscrapers or atop sports utility vehicles -- and at prices potentially far cheaper than today’s silicon-based panels.
It’s not as far-fetched it seems. Solar researchers and company executives think there’s a good chance the economics of the $42 billion industry will soon be disrupted by something called perovskites, a range of materials that can be used to harvest light when turned into a crystalline structure.
The hope is that perovskites, which can be mixed into liquid solutions and deposited on a range of surfaces, could play a crucial role in the expansion of solar energy applications with cells as efficient as those currently made with silicon. One British company aims to have a thin-film perovskite solar cell commercially available by the end of 2018.“This is the front-runner of low-cost solar cell technologies,” said Hiroshi Segawa, a professor at the University of Tokyo who’s leading a five-year project funded by the Japanese government that groups together universities and companies such as Panasonic Corp. and Fujifilm Corp. to develop perovskite technology.
Comparing efficiencies of solar cells based on silicon and perovskite. Source: Martin Green at UNSW Sydney
Not everyone is sold on perovskite as a game-changer from the industry’s heavy reliance on silicon photovoltaic cells. That said, recent research pointing to the material’s potential continues to grip the solar energy research community.
The World Economic Forum picked the material as one of its top 10 emerging technologies of 2016. Meanwhile, solar panel makers and top universities in Europe, the U.S. and Asia are racing to commercialize the technology, with researchers churning out as many as 1,500 papers a year on the material.
Generic structure of a standard (non-inverted) perovskite solar cell.
The terms "perovskite" and "perovskite structure" are often used interchangeably. Technically, a perovskite is a type of mineral that was first found in the Ural Mountains and named after Lev Perovski who was the founder of the Russian Geographical Society. A perovskite structure is any compound that has the same structure as the perovskite mineral. True perovskite (the mineral) is composed of calcium, titanium and oxygen in the form CaTiO3. Meanwhile, a perovskite structure is anything that has the generic form ABX3 and the same crystallographic structure as perovskite (the mineral).
One possible form of perovskite that is being tested for solar applications is Methylammonium Lead triiodide (CH3 NH3 PbI3).
The perovskite lattice arrangement is demonstrated below. As with many structures in crystallography, it can be represented in multiple ways. The simplest way to think about a perovskite is as a large atomic or molecular cation (positively-charged) of type A in the centre of a cube. The corners of the cube are then occupied by atoms B (also positively-charged cations) and the faces of the cube are occupied by a smaller atom X with negative charge (anion).
A generic perovskite crystal structure of the form ABX3. Note however that the two structures are equivalent – the left hand structure is drawn so that atom B is at the <0,0,0> position while the right hand structure is drawn so that atom (or molecule) A is at the <0,0,0> position. Also note that the lines are a guide to represent crystal orientation rather than bonding patterns.
The main issues for practical device fabrication of perovskite solar cells are film quality and thickness. The light-harvesting (active) perovskite layer needs to be several hundred nanometres thick – several times more than for standard organic photovoltaics. Unless the deposition conditions and annealing temperature are optimised, rough surfaces with incomplete coverage will form. Even with good optimisation, there will still be a significant surface roughness remaining. Therefore, thicker interface layers than might normally be used are also required. However, the fact that efficiencies of over 11% have already been achieved for spin coated devices is highly encouraging.
Recent improvements to device processing have led to significant increases in the surface coverage while reducing the surface roughness. One method for improving the surface coverage and the roughness is to add small amounts of acids such as hydoriodic, hydrobromic, or hydrochloric acid. These materials are byproducts of the synthesis of methylammonium halides however the presence of these acids have an impact upon the solubility of the lead components. We previously discussed this in a post about the purity of MAI vs lead chloride solubility. Another method is by precise control of the timing of precipitation of the salts, this is done by solvent quenching methods with precise timings of the quench and volumes of the quenching solvents needed to give the optimal performance. To help with our understanding of this we decided to build the new Ossila syringe pump which has allowed us to use this quenching process to push in house power conversion efficiency values over 16%.
Even now there are further improvements being made in all areas of perovskite processing which are of great interest; some of these include mixed phase perovskites, two dimensional perovskite structures and inorganic perovskites.
The advances have raised the possibility that perovskite cells could one day be placed on top of cars, windows, and walls. Oxford Photovoltaics Ltd., a spin-off from the University of Oxford, says it’s developing thin-film perovskite solar cells able to be printed directly onto silicon solar cells. In December, Oxford PV said it got 8.1 million pounds ($10 million) of additional funding from investors including Statoil ASA.
“We expect to have a product that meets industry requirements by the end of 2017,” Frank Averdung, chief executive officer at Oxford PV said by email. “Adding some time for qualification, certification and production, our first product could be commercially available towards the end of 2018.”
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