Silicon-based Photovoltaic Windows: Solar Power for All

The extra costs of building equipment linked to energy-efficient buildings tend to discourage both entrepreneurs and individuals. This article focuses on a technology that will help spread solar power generation by using photovoltaic solar windows, which until now, was deemed too expensive. A group of researchers from the University of Minnesota and the University of Milano-Bicocca were able to develop more affordable solar power windows. Co-authored by Francesco Meinardi, Samantha Ehrenberg, Lorena Dhamo, Francesco Carulli, Michele Mauri, Francesco Bruni, Roberto Simonutti, Uwe Kortshagen and Sergio Brovelli, the study entitled “Highly efficient luminescent solar concentrators based on earth-abundant indirect-bandgap silicon quantum dots” was published in Nature Photonics, on February 20, 2017.

Photovoltaic Window Technology

The researchers developed a technology to produce luminescent solar concentrators (LSC) made of silicon nanoparticles. LSCs are devices that concentrate solar radiation and direct it to photovoltaic panels in order to take full advantage of the amount of light that reaches the glass surface. Since its arrival on the market, the solar window technology, launched in 2011 by Nihon Telecommunication Systems, serves as an alternative to photovoltaic panels, and is highly appreciated for its economic and aesthetic benefits. However, collecting sunlight on larger architectural surfaces quickly proved to be unprofitable because of the cost of the window panes compared to the energy savings. Since then, several studies have been carried out in an effort to improve the concept. The use of LSCs greatly optimized the windows’ performance, but remained somewhat unsatisfactory because of reduced transparency of the glass. The problem was subsequently resolved by researchers at the University of Michigan with the production, in 2014, of a new type of transparent solar concentrator. However, in order for this to qualify as a 100% green energy source, the materials used must be environmentally responsible, which is not the case with the products developed to date.

Luminescent solar concentrator (LSC) manufacturing process

First transparent photovoltaic windows

Environmentally Responsible Luminescent Solar Concentrators

Until recently, the best results had been reached using LSCs with relatively complex nanostructures, either composed of potentially toxic elements like cadmium or lead, or rare substances like indium, which is already heavily used in other industries. The researchers at the University of Minnesota chose silicon because it abounds in the environment and is non-toxic. It also works more efficiently by absorbing more light than it emits at different wavelengths. However, silicon, in its normal state, does not emit luminescence, which is the optical property needed to transfer solar energy to photovoltaic cells. To develop this capacity, the researchers reduced the silicon size to a few nanometers, or about one-tenth of a thousandth of the diameter of a human hair. At this scale, silicon acquires new properties. In addition, it becomes a very effective light emitter, as it does not reabsorb its own luminescence. Since luminescence increases the performance of LSC, the main objective of this study was to design a process for the manufacture of LSC based on silicon nanoparticles.

The silicon is reduced to particles with the use of a technological process using a plasma reactor. The powder is converted into an ink-like solution. It is then incorporated into a polymer which can be used to form a sheet of flexible plastic material or to coat a surface with a fine film. The optical characteristics of the silicon nanoparticles resulted in a solar energy capture increase of 5% and also reduced the production cost of photovoltaic glass. The University of Minnesota invented the silicon nanoparticle production process a decade ago and holds a number of patents on this technology.

Silicon synthesis by plasma reactor

Uwe Kortshagen, Professor of Mechanical Engineering at the College of Science and Engineering of the University of Minnesota, and Sergio Brovelli, Physics Professor at the University of Milano-Bicocca and LSC manufacturing expert, met in 2015 to collaborate on the design of this new window. The potential of silicon nanoparticles for this technology was immediately approved and their partnership was born.

The research was funded by the U.S. Department of Energy (DOE), (Office of Basic Science, Centre for Advanced Solar Photophysics, Energy Frontier Research Center), the European Community’s Seventh Framework Program and the National Science Foundation (NSF).