"Stained glass" photovoltaics fuse form and function
By Heidi Hoopes
March 4, 2014
A beautiful stained glass installation, a colorful billboard, or rows of windows on an office building ... all as electricity-generating solar cells? New research at the University of Michigan gives a method for creating such transparent and colorful solar cells using a hybrid silicon/organic composition, and furthermore avoids some of the problems of previous colored and transparent solar cells.
Many existing options for colored solar cells depend upon dyes, which scatter light and blur the background, or plasmonic or photonic filters, which have the drawback where a shifted viewing angle results in unwanted color changes. In order to create a colored solar cell that avoids these problems and is also transparent, Jay Guo and his research team made a couple of key decisions.
First, transparency necessitates a thin semiconductor layer, in the range of 10-20 nm, much thinner than common doped silicon solar cells. This problem is solved using a layer of amorphous silicon (a-Si) rather than crystalline silicon. The flat panel display industry already uses a-Si and thus there is existing technology to deposit large swathes of the substance, such as one might want for creating large solar panel windows.
Second is the problem of creating color. Guo’s research team identified that by depositing different thicknesses of the a-Si they obtained different colors in the resulting panel: 6 nm for blue, 11 nm thick for green, and 31 nm for red, with these being the colors they studied in this experiment.
Using these colored cells, Guo’s team created a small American flag that seemingly is just decorative glass, but under sunlight produced almost 2 mA of current with 2 percent efficiency. In contrast, pure organic solar panels generally obtain about a 10 percent efficiency. By definition light is being reflected back into our eyes when we perceive color, so a colored solar panel will not have the same efficiency as something that is black.
The researchers say this as one of the tradeoffs of swapping some utility for beauty. Moreover, Guo envisions the colored cells, by merits of their appeal, being used on whole sides of buildings rather than just on roofs.
The team also created panels that were reflective, rather than transparent, as shown below.
Additionally, the colored panels did not change colors when viewed up to a 60-70 degree angle. Staring at a panel kitty-corner across the street you’re likely to see the same effect as you would looking at them head on.
One of the novel applications proposed by the research team is a greenhouse roof in which green and ultraviolet light is absorbed for conversion into electricity, while passing the optimal red and blue wavelengths below to the growing plants.
The research was originally published in Scientific Reports.
In the video below, Guo describes his research and some of the merits of the project.
Source: University of Michigan