Title image credit: MIC
Solar power is the world’s fastest growing energy generation technology. Solar energy production has grown from around 33.71 TWh in 2010 to 724.09 TWh in 2019. Due to its many benefits, it is a very good candidate for replacing much of the fossil fuels power generation in use today. Currently, most solar panels are made up of silicon photovoltaic cells. These are semiconductor diodes designed to produce direct current when exposed to visible, infrared, or ultraviolet light.
Diodes are made from two types of semiconductor put together. A semiconductor is basically doped semimetal i.e. an element with metallic and nonmetallic properties, with specific impurities purposefully added. The two types are the n-type (which have negative charge carriers in the form of extra electrons, not in covalent bonds), and p-type (which have fewer electrons and use the gaps in the covalent structure, or ‘holes’, as charge-carriers). When these are put together, a neutral so-called ‘depletion zone’ forms where the extra electrons of the n-type fill the holes of the p-type semiconductor. These can be engineered so that when light is shone onto the diode, if the photons are energetic enough they can cause an electron to exit it’s covalent bond in the depletion zone, forming an electron-hole pair, which (due to the charge of either end of the diode) splits and generates a voltage. For a more expansive, visual explanation, click here.
The main problem with current commercial solar cell technology is efficiency. Many factors limit the efficiency of PV cells. The maximum theoretical efficiency (known as the Shockley-Queisser limit) for a single layer solar cell (solely) made from silicon is around 33%, though most commercial cells only have an efficiency of about 22%. The reason for this is that removing an electron from its bond to produce a voltage requires a minimum amount of energy, the ‘band-gap energy’, which for silicon is around 1.12 electron volts. However, the photons from the sun carry a wide range of energies and some do not provide enough energy to release an electron from its bond. Furthermore, if a photon carries more energy than the band-gap energy, then the energy will be converted into thermal energy. Current methods of improving efficiency include using different, more efficient semiconducting materials, using additional layers of the material and using lenses to focus sunlight onto the solar cell. However, these can be very expensive.
Other problems with solar power include the fact that at different places there are variances in solar intensity due to shadows, whether from clouds, trees, mountains or other, as well as night-time hours. Hence reliability is an issue if solar panels were to become the main source of grid electricity. If storage of power could be made more efficient, then this hurdle might be overcome. However, the manufacture of both batteries and solar panels themselves can be harmful to the environment, due to the production of the harmful greenhouse gas nitrogen trifluoride from solar cells and toxic metals such as cadmium, mercury and lead in batteries.
One technology trying to tackle the problem of shadows and varying sunlight is a new device that uses the contrast between bright spots and shadows. The ‘shadow-effect energy generator’ was created by placing a very thin layer of gold on top of silicon. When part of the device is covered by shadow, the excited electrons jump from the silicon to the gold. The voltage of the illuminated metal increases relative to the shaded part, and electrons flow from high voltage to the low voltage, generating a current. Currently, it can only power small electronics and act as a sensor, but someday it may produce energy for the grid where solar panels cannot.
Perovskites, materials with the same type of crystal structure as calcium titanium oxide, are probably the fastest improving solar energy technology. The Perovskite structure is good for solar absorption and is thus very useful for solar cells and panels. The most important part, however, is that thin films (~300 nanometres) can be made cheaply from solutions, and thus can easily coat any surface. Furthermore, they work better than silicon at low light intensities, a factor that is very useful in real-life applications. Due to the low cost of production, and its efficiency and flexibility, it could potentially be used all around our living environments. Improvements in Perovskite efficiency allowed the company OxfordPV to reach 28% efficiency with a commercial Perovskite-based cell. By combining Perovskite with silicon in a ‘tandem’ cell, they can absorb a greater range of light – with silicon absorbing redder light and Perovskite the blue light. Further minor changes can add a small increase to efficiency, for example adding a reflective coating to reflect back unabsorbed light, and modifications to lessen the losses where the metal contacts touch the solar cell, both of which can increase efficiency by around 1-2%. The main problem now is scaling up the technology without losing efficiency and without exponentially increasing costs of production. This is because when scaled up, defects become more pronounced and affect efficiency more, so expensive, high-quality materials and production techniques are needed. However, as new techniques are emerging, scaling up is becoming increasingly cheaper, thus increasing their prospects of mass-commercialisation.
Another potentially revolutionary technology that could improve solar cell efficiency are tiny stem-like SunBOTs. SunBOTs are the first artificial material capable of phototropism, i.e. the ability to move towards light. They consist of a cylinder of polymer of approximately 1mm in diameter embedded with a nanomaterial that responds to light. Their interesting properties arise from the way that the nanomaterial absorbs light and converts it to heat, shrinking the polymer where the temperature increases, i.e. bending it into the light. In lab tests, the tiny cylinders of the material bent to capture around 90% of available light shining onto a surface at a 75-degree angle, compared to 24% captured if they were stationary. Originally the SunBOTs were made from gold and a hydrogel but through testing, other materials have been looked at and combinations have been shown to work even better. Due to the variety of materials that can be used, an added advantage of SunBOTs is that different material allows them to work in different environments, for example floating solar panel arrays on water. SunBOTs are not likely to have an immediate impact, but if they can be successfully implemented into solar panels then they could dramatically improve solar power plants as they could be lined up in rows to cover a surface, potentially allowing solar panels to capture much more sunlight than stationary panels.
Recently, scientists broke the solar cell efficiency record, by using solar cell design with 6 layers of different materials, something thought impossible until recently. They achieved 39.7% efficiency under terrestrial conditions – with normal light from the sun, unlike the 40% efficiency formerly achieved by concentrating light up to 1000x. They used materials from Groups III & V of the Periodic Table which, when combined, produce a wide range of energy band gaps which allow more of the sun’s light to be converted into electricity. But when layers are stacked on top of each other, each layer loses more energy by heat, non-captured photons and reflection than would be the case for single layers. To reduce these problems, extreme precision of manufacturing is needed, but this means that there is a very high cost of production, which is the reason why there is not a highly efficient solar cell option currently commercially available.
As has been shown, new technologies are being developed constantly. In addition, the development of cheaper, more precise manufacturing techniques is going to aid the commercialisation of both these technologies and others – such as cheaper lenses which concentrate the sunlight onto the solar cells thus drastically improving efficiency at a lower cost. As hurdles are being overcome and innovation begins to provide solutions for all sorts of problems, from collection of solar energy to efficient transmission and storage, the future of solar power is looking bright (no pun intended).