Perovskite Solar Cells
The rapid improvement of perovskite solar cells has made them the rising star of the photovoltaics world and of huge interest to the academic community. Since their operational methods are still relatively new, there is great opportunity for further research into the basic physics and chemistry around perovskites. Furthermore, there is huge potential for engineering better, more efficient solar cells which are expected to reach in excess of 20% power conversion efficiency.
Why are perovskite solar cells so significant?
There are two key graphs which demonstrate why perovskite solar cells have attracted such prominent attention in the short time since the breakthrough paper of 2012.
The first of these graphs, which uses data taken from NREL solar cell efficiency tables, demonstrates the power conversion efficiencies of the perovskite based devices over recent years in comparison to other technologies.
The graph shows a meteoric rise compared to most other technologies over a relatively short period of time. Although it could be argued that more resources and better infrastructure for solar cell research has been available in the last few years, the dramatic rise in efficiency is still incredibly significant and impressive. This suggests that with continued research, efficiency of perovskite based solar cells can continue to rise at this rate over the coming years.
The second key graph below is the open circuit voltage compared to the band gap for a range of technologies that the perovskites compete with.
This graph demonstrates how much of a photon’s energy is lost in the conversion process from light to electricity. For standard excitonic-based, organic-based solar cells this loss can be as high as 50% of the absorbed energy. However, for the perovskite based solar cells the loss is far less. Perovskite based solar cells are fast approaching the same level of photon energy utilisation as the current leading monolithic crystalline technologies such as silicon and GaAs. Furthermore, they also have the potential for much lower processing costs.
Currently, there is not known to be a significant negative aspect of perovskite based solar cells. Although lifetimes of the cells aren’t yet proven, there is no evidence to suggest their lifetime is any less than that of pure organic devices. The use of lead in perovskite compounds is not ideal, but it is used in much smaller quantities than that which is currently present in either lead or cadmium based batteries and there is potential for a lead alternative to be used in perovskite compounds instead. Finally, there has also been little discussion of the optical density of these materials which, although higher than silicon, is still lower than other active materials. As a result, the perovskite devices require thicker light-harvesting layers which may cause some fabrication limitations; particularly for solution processed devices where creating such thick layers with high uniformity can be difficult.
A key development will therefore be the improvement of precursor materials for solution based perovskite deposition and associated coating and processing techniques. Although at present the best perovskite solar cells are vacuum deposited, solution processed devices will ultimately yield lower production costs. While vacuum based processes are relatively easy to scale up, the capital equipment cost of doing so can rapidly become astronomical.
To enable a truly low cost-per-watt will require perovskite solar cells to have the much heralded trio of high efficiency, long lifetimes and low manufacturing costs. This has not yet been achieved for other thin film technologies but perovskite based devices so far demonstrate enormous potential for achieving this.
What are perovskites?
The term perovskite and perovskite structure are often used interchangeably. Technically, 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 formed 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). However, since most people in the solar cell world aren’t involved with minerals and geology, perovskite and perovskite structure are used interchangeably.
The perovskite lattice arrangement is demonstrated below, but it must be considered that, 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).
Dependant on which atoms/molecules are used in the structure, perovskites can have an impressive array of interesting properties including superconductivity, giant magnetoresistane, spin dependent transport (spintronics) and catalytic properties. Perovskites therefore represent an exciting playground for physicists, chemists and material scientists.
In the case of perovskite solar cells, the most efficient devices so far have been produced with the following combination of materials in the usual perovskite form ABX3:
- A = An organic cation - methylammonium (CH3NH3)+
- B = A big inorganic cation - usually lead(II) (Pb2+)
- X3= A slightly smaller halogen anion – usually chloride (Cl-) or iodide (I-)
Since this is a relatively general structure, these perovskite based devices can also be given a number of different names which can either refer to a more general class of materials or a specific combination. As an example of this we’ve created the below table to highlight how many names can be formed from one basic structure.
|Organo||Metal||Trihalide (or trihalide)|
|Methylammonium||Lead||Iodide (or triiodide)|
|Plumbate||Chloride (or trichloride)|