Improve the efficiency of silicon solar cells

Solar energy is one of the most environmentally friendly sources, which in principle could provide all the energy needed for the planet. Solar cells use the photovoltaic effect to convert solar energy into usable electrical energy, and therefore are a key technology to provide the world with cheap and reliable energy. Silicon solar cells are a very promising technology where significant technological improvements are still possible to ensure further price reductions and an increase in solar plant construction.

Reducing losses due to material surfaces and interfaces

Ruy Sebastián Bonilla aims to improve the efficiency of silicon solar cells, reducing losses due to materials surfaces and interfaces. The research proposed here will expand the current understanding of the mechanisms of charge loss for semiconductor surfaces. This work will provide the necessary knowledge to improve the manufacture of a variety of optoelectronic devices. The benefits of this research include not only the scientific advancement of semiconductors and physical dielectrics, but also the potential for improved performance and a reduction in the manufacturing cost of these devices. Greater absorption of solar energy allows a direct reduction of CO2 emissions and long-term energy security.

Most common solar cells are based on the photovoltaic effect, in which the light that impinges on a two layer semiconductor device produces a potential difference between the layers that conducts a current through an external circuit. The silicon solar cells available today have a conversion efficiency of 15-20%.

Evolution of solar cells is as follows:

  • First-generation solar cells are constructed from semiconductor silicon wafers.
  • Second generation introduces thin film technology, and has several drawbacks:
    The semiconductor layers are deposited by high vacuum, which is complex and expensive.
    They are placed on a glass substrate that requires processes to establish the electrical contacts.
  • The third generation of solar cells is also based on thin sheets, but does not have the same difficulties since the layers of semiconductor material are deposited directly on a metal. In addition, the semiconductor layers are applied by an ink jet containing nanometric semiconductor particles employing a rotary printing process similar to that used to print newspapers and magazines, which is much cheaper. The layers deposited on the metal are of CuInGaSe and CdS that substitute the different types of silicon P and N of the traditional cells. In addition, a layer of ZnO is applied which acts as an electrode, the metal being the other electrode.

Conventional solar panels filter ultraviolet light or absorb the silicon and convert it into a heat that does not serve for electricity. But if nanoparticles are used, they can take advantage of that ultraviolet light and turn it into electricity, so that much more sunlight is used. By integrating a thin layer of silicon nanoparticles one nanometer in size into silicon solar cells, the energy efficiency is improved by 60% in the ultraviolet spectrum range. Another type of solar cells used are organic solar cells, which have advantages as they are thinner, more flexible and easier to produce. These cells can improve, for example, some household appliances. A technique has been developed to improve the efficiency of these organic solar cells by protecting them with a layer containing a mixture of inorganic nanoparticles of cadmium selenide and an organic polymer. It has also been experimented with semiconductor polymers that include small fragments of silver, capable of absorbing solar energy and generating electricity more efficiently and economically than conventional methods.

Silver nanoparticles allow polymers to capture a wide range of wavelengths of sunlight that would otherwise not be harnessed. The addition to the polymer of these nanoparticles would increase by 12% the electric generation. In addition to using nanoparticles to improve the efficiency of solar cells, a low-cost technique is also being developed to manufacture nanowire solar cells, which could reduce the production costs of the cells while maintaining their efficiency levels. For the manufacture of solar nanowire cells, cadmium sulfide semiconductors are used for the core and copper sulfide for the structure. These cells are cheap and easy to manufacture, and have an energy conversion efficiency of 5.4%, comparable to that of flat solar cells.

This poor performance may be due to surface recombination and poor control over the quality of p-n junctions in high temperature processes. To overcome this, the p-n junctions of conventional solar cells are replaced by a radial p-n junction in which a n-type silicon layer forms a layer around a p-type silicon nanowire core. This makes each of the nanowires act as a photovoltaic cell and improves the light capture efficiency of silicon cells. For the manufacture of nanowires, cadmium sulfide and copper sulfide are used in combination with a chemical solution which consists of immersing the cadmium sulfide nanowires in a copper chloride solution resulting in cation exchange converting the surface layer of cadmium sulfide into a copper sulfide shell.

It is thought that the energy conversion efficiency of the nanowires of solar cells could be improved by increasing the amount of material in the copper sulfide layer. To be viable it is necessary to achieve an energy conversion efficiency of at least 10%. Therefore, research in this field is necessary and will help us find more viable solutions for the use of photovoltaic energy.

Dr. Ruy Sebastian Bonilla