Let the sun shine! Innovations in solar cell design
As the need to move away from fossil fuels increases, innovations in generating renewable energy are necessary. Solar energy is a very promising method to provide clean energy. Today, we take a look at some innovations in this field.
Researchers at the University of Stanford designed a new solar cell inspired by the compound eyes of insects in an attempt to pave the way for the development of solar panels based on the promising material perovskite.
Perovskites are promising, low-cost materials that convert sunlight into electricity with the same efficiency as the more conventional material silicon. However, perovskites, which have a brittle, salt-like crystal structure, are very unstable and fragile, barely surviving the manufacturing process. The flat design of most solar devices, like rooftop panels, doesn’t work for perovskite solar cells.
By packing tiny solar cells together, like micro-lenses of insect eyes, the researchers hope to overcome the durability challenge. The honeycomb shape has built-in redundancy; if one segment breaks, hundreds of others will operate. The perovskite cells are protected by the scaffold wall made of inexpensive epoxy resin around them.
The researchers exposed the perovskite cells to temperature of 85 degrees Celsius (185 degrees Fahrenheit) and humidity for six weeks. Despite the extreme conditions, the cells continued to generate relatively high rates of efficiency.
The device combines low-cost organic (carbon containing) materials with electrodes of graphene, a transparent and flexible material made from inexpensive and abundant carbon sources.
Solar cells made of organic compounds offer various advantages over inorganic silicon solar cells. They are cheaper and easier to manufacture, lightweight and flexible, as well as less fragile. In addition, many organic materials absorb ultraviolet and infrared light.
When developing transparent organic solar cells, researchers tend to use indium tin oxide as electrode, which is conductive and transparent, but also stiff and brittle. In addition, the material is expensive and relatively rare.
Graphene forms an alternative material, but the main problems with intergrading this material were that the organic materials are too sensitive to deposit the graphene on, and as an electrode, graphene needs to play different roles. In solar cells, only one electrode has to let electrons flow out easily, so to have both electrodes made out graphene, the function of the material has to be changed so that the electrons know which way to go.
The MIT researchers tackled both these problems. For the bottom electrode, graphene is grown on copper foil, on which a layer of polymer is deposited to support it. They then use an acidic solution to etch the copper foil off the back, ending up with a graphene-polymer stack that they transfer to water for rinsing. They then simply scoop up the floating graphene-polymer stack with the substrate and remove the polymer layer using heat or an acetone rinse. The result: a graphene electrode resting on the substrate.
For the top electrode, the same method is used, but this one can’t be scooped out of the water. Instead, the researchers press a thin frame of silicon rubber onto it, which is later peeled off using a little heat. The graphene is then adhered to the so-called hole transport layer using a spray-on polymer.
The current power conversion efficiencies (PCE), which is the part of the incoming solar power that is converted to electricity, ranges between 2.8 and 4.1 per cent, but the theoretical PCE achievable at the current level of transparency is 10 per cent.
For more innovative solar cells, click here.
Photos: Dauskardt Lab (Stanford University) / Stuart Darsch (MIT) / Opo Terser