A company called SolarWindow Technologies is planning to harvest solar energy from windows by coating them with organic PV materials dissolved in a liquid. These coatings will be semi-transparent. That is the PV material will absorb light in a certain frequency range and convert it to electric current and the rest of the light will pass through to the interior of the building. The occupants of the building will still be able to look outside even though the window glass will appear colored rather than completely transparent.
The SolarWindow Technologies web site is high in promotional hype and low in technical detail. They give no details on the composition, the efficiency, or expected lifetime of their solar coatings. They emphasize the fact that on tall buildings with lots of windows they will be able to harvest a lot more solar energy than a roof top installation (up to fifty times more according their promotional material). This claim may well be true, but cost per kWh and expected lifetime will be the keys to whether this technology is economically practical.
According to this press release manufacturing readiness is still more than two years away. However, based on projected costs the company is estimating a one year payback time for their solar windows. I assume this estimate applies to the extra cost associated with adding solar coatings, wiring, inverters and so forth to the base cost of the windows. In this case the payback time would apply to new buildings or to already planned window replacements in existing buildings. Presumably the economics of replacing an entire set of existing windows with SolarWindows would be significantly less favorable.
The question of performance profile over time of this technology is also interesting. Ideally the coating would last as long as the windows themselves. If the expected useful lifetime of the coatings is significantly less than that of the windows, then the economics of this technology will be less favorable.
Organic solar cells have been a subject of research for a long time, but no products based on this technology have reached the market. It remains to be seen whether SolarWindow Technologies will be the first company to pass the mark.