In the future, windows may not only serve the primary functions of allowing light indoors and conserving heat or cooling, but they may also capture enough solar power to meet all of a building’s energy needs. In other words, windows of the future may pull double duty as solar collectors.


Toward that objective, a recent study conducted by researchers at the Institute of Photonic Sciences (ICFO) in Barcelona, Spain, has fabricated an optimal organic solar cell with a high level of transparency and high power-conversion efficiency. This new discovery appears to be a promising step toward more affordable, clean, integrated renewable energy.


Today’s commercial solar panels are, for the most part, composed of wafer-based crystalline silicon solar cells, which are quite efficient in converting solar radiation into electrical power with roughly 15 percent conversion efficiency. However, several obstacles stand in the way of maximum conversion. They must be precisely oriented to receive direct sunlight. Even then, they are limited in their ability to absorb diffused light. In addition, they are heavy, opaque and take up a lot of space.


Organic solar cell technology has been around for about 30 years. It is starting to attract substantial interest due to lower production cost. While organic cells have not yet reached the efficiency values of silicon-based cells, organic photovoltaic (OPV) cells have proven to be lighter in weight, more flexible, capable of adapting to curved surfaces and even more sensitive to low light and indirect sunlight, making them one of the most appealing photovoltaic technologies for many everyday applications, such as semitransparent windows for homes and commercial buildings.


OPV, like any other photovoltaic technology, achieves its maximum light-to-electricity conversion efficiency with opaque devices. To turn such cells into transparent ones, the back metal electrode must be thinned down to just a few nanometers, drastically reducing the device’s capacity to collect sunlight. 


ICFO researchers, however, have been able to implement a semi-transparent cell incorporating a photonic crystal and reach a cell performance almost as good as its opaque counterpart. By adding the extra photonic crystal to the cell, ICFO’s scientists were able to increase the amount of infrared and ultraviolet light absorbed by the cell, reaching 5.6 percent efficiency while preserving a transparency almost indistinguishable from normal glass. Because of the results in efficiency and transparency, these cells are an extremely competitive product for building-integrated photovoltaic (BIPV) technologies. To reach the adequate architectural look, the color of the cells may be tuned by simply changing the layer configuration of the photonic crystal.


“Applications for this type of technology in BIPV are just a few steps away, but the technology has not reached its saturation point yet,” said Jordi Martorell, a leader of the ICFO study. “ICFO’s discovery opens the path for innovation to other industrial applications of transparent photovoltaics. In the midterm, we expect to reach the extremely high transparencies and efficiencies needed to power up devices such as displays, tablets and smartphones.”


The future looks promising for these devices. A recently approved European project will allow European researchers and industry leaders to study the capabilities of these cells, improving stability and lifetimes and obtaining the material needed to substantially raise the efficiency.