Photovoltaics are very far from being profitable at present, only a subsidy policy gives the illusion that it is. The potential for progress, particularly in terms of energy efficiency, is therefore significant. Here is an example of research seeking to improve this criterion… but at what cost?
“At the University of Notre Dame, Indiana, a group of researchers led by Dr Prashant V. Kamat has developed photovoltaic cells by combining semiconductor quantum dots of different sizes and TiO2 nanotubes in place of conventionally used semiconductors. , making them much more efficient. The study, supported by the Office of Basic Energy Sciences of the Department of Energy is published in the Journal of the American Chemical Society.
Scientists use these semiconductor Cadmium Selenide (CdSe) quantum dots rather than other materials because they have the unique advantage of absorbing certain wavelengths of light, depending on their size: smaller quantum dots will absorb shorter wavelengths, larger ones will absorb longer ones. By combining several types of Quantum dots with CdSe, researchers can therefore create photosensitive cells that absorb a larger spectrum of light and are therefore more efficient. The team arranged these quantum dots in an ordered pattern on the surface of a nanometric-thick film, and integrated titanium dioxide (TiO2) nanotubes into them. Quantum dots absorb photons and produce electrons which are then transported by nanotubes and collected by an electrode, thus producing the photocurrent.
Besides the absorption of particular wavelengths, the researchers noticed that the size of the quantum dots has an influence on the performance, by experimenting with four types of these nanoparticles (between 2,3 and 3,7 nm of diameter, they exhibit absorption peaks at wavelengths between 505 and 580 nm). Smaller quantum dots can convert photons to electrons faster, while larger ones absorb a greater percentage of photons. The 3nm diameter quantum dots offer the best compromise. After the development of the first photovoltaic cell composed of different types of quantum dots, the researchers plan for the next steps of their research to create “rainbow” cells, by superimposing layers of quantum dots according to their size: on the outer layer, the smaller ones absorb the blue, and the red light (longer wavelength) passes through this layer to reach the inner layer made up of the larger quantum dots which absorb the red, thus creating a gradient d 'rainbow' absorption, while combining the effects of rapid conversion of small quantum dots and high absorption rate of larger quantum dots.
Current silicon photosensitive cells have an efficiency of 15 to 20%, the rest is lost in heat. Kamat foresees greater efficiency with these new types of “rainbow” photovoltaic cells, which could easily exceed 30%. "
Source: the Adit's BEs