Many commercial solar panels convert light to electricity with an efficiency of about 20%. The efficiency figure has been stuck at that level, more or less, for several years. But that could change thanks to recent developments in materials and cell-building strategies.
Solar researchers now think the way to boost cell efficiencies lies in sandwiching together two different types of solar cells. The combination converts light to electricity more efficiently than either of the two cells working separately. Moreover, the resulting cell structure may cost about the same to make as those used in the solar cells produced commercially today.
A UCLA research group headed by Yang Yang devised a combination perovskite/polymer solar cell with this kind of makeup. In the diagram, PFN is lead iron niobate. PCBM is the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester, a fullerene derivative of the C60 buckyball. It is used as an electron acceptor. CH3NH3PbI3 is the main perovskite solar absorbing material. PEDOT:PSS is a combination of poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate. The top PeDOt:PSS layer is a transparent resistive polymer, the second layer is transparent but conductive. Beneath these layers is a layer formed by spin-coating PFN with a titanium dioxide (TiO2 ) solution for form an interface between the two kinds of cells. PBSeDTEG8:PCBM is the photovoltaic polymer. Indium tin oxide (ITO) forms the backside electrode.
The most promising approach involves using a standard silicon solar cell as a base and building a cell made from perovskite on top of it. Perovskite is a crystalline material (also known as calcium titanium oxide) that is inexpensive. Perovskite cells made in the lab don’t yet have the efficiency of commercial cells but continue to improve.
The intriguing thing about ganging perovskite and silicon cells together is that the resulting structure can be made on conventional solar cell fab lines–most layers in a perovskite cell can be deposited from solution on top of the silicon cell. So theoretically, boosting solar cell efficiency this way entails no big capital expenses for revamped manufacturing facilities.
Combining the two types of cells works because silicon solar cells absorb photons of visible and infrared light, while perovskite cells respond only to the higher end of the visible part of the solar spectrum where the photons have more energy. Because the photons at these wavelengths have more energy, the perovskite cells generate more power per photon of visible light than silicon cells.
One difficulty that has thwarted the construction of perovskite/silicon combo cells has been the lack of a suitable material able to serve as an electrode. Both top and bottom electrodes for the perovskite cell must be transparent so photons reach the silicon cell beneath it. Complicating the search for a suitable electrode material is perovskites’ fragility. Heat can damage them and they readily dissolve in water.
Several research groups have recently made progress on these issues. For example, researchers at Stanford University came up with a way of applying transparent electrodes onto the perovskite cell by pressing a sheet of plastic containing silver nanowires against the top of the cell. The wires transfer from plastic sheet to the perovskite cell the way a temporary tattoo transfers to human skin.
Moreover, the Stanford team used a low-quality silicon cell for their bottom layer. The perovskite/silicon combination was 17% efficient, about 50% better than the silicon cell working alone. The researchers, led by Stanford professor of materials science and engineering Mike McGehee, now think that the technique could make practical the use of low-quality (less pure and less expensive) silicon that previously wasn’t good enough for solar cells.
When perovskite and silicon solar cells are combined, it is important that both kinds of cells produce nearly the same current. Otherwise, the two-cell combination could be less efficient than the cells working alone. Researchers at Stanford devised this example perovskite/silicon module to demonstrate how current-matching at the module level might happen with a mechanically-stacked tandem. In this example, the filtered silicon produces half the photocurrent density of the perovskite, so the silicon cells are twice as large to match the current of the perovskite cells. This example module assumes cells are strung together in series so the total voltage of the module would be the sum of the individual cell voltages.
Silicon cells aren’t the only devices with which perovskite can combine. Another research group at UCLA is working on sandwiching perovskite and plastic solar cells. A problem has been getting a smooth perovskite surface to grow on the plastic cells. (The smoother the surface, the less the leakage currents.) The UCLA group, headed by materials science and engineering professor Yang Yang, came up with a technique for applying a special solvent that helps form layers of perovskite that are gap and void-free. In addition, the process takes place at temperatures low enough to be compatible with the plastic cells beneath the perovskite.
It also looks as though researchers aren’t done improving the efficiency of perovskite cells alone. Recently a group working in South Korea’s Research Institute of Chemical Technology and Sungkyunkwan University were able to devise perovskite cells working at 18% efficiency. They started with a methylammonium lead bromide perovskite and added formamidinium lead iodide to get the higher efficiency. The latter compound also makes the perovskite crystals more stable—otherwise they tend to change phase and lose their photovoltaic properties in the right temperature and humidity conditions over time.
And in fact, the long-term stability of perovskites is still a research topic. Nevertheless, researchers seem to be confident that they can overcome the remaining difficulties and develop useful devices in the next year or two, though there are still questions about the economics of the resulting technology. Some of the materials used in experimental perovskite cells are expensive–fullerene derivatives found in some cells being one example. But researchers are hopeful they can come up with versions of these cells that will be able to compete commercially.
