Organic solar cells - devices based on carbon compounds that convert light into electricity - are considered a promising alternative to silicon panels. They are lighter, cheaper and more flexible, but usually fall short of silicon in the efficiency of converting light into current and last less long. One way to improve their performance is to apply a special thin coating between the light-absorbing layer and the charge-collecting electrode. How easily the charge reaches the electrode depends on how the molecules of this coating are arranged.
A group of scientists from the Beijing University of Chemical Technology, the Chinese Academy of Sciences and Linkoping University (Sweden) developed a two-stage approach to controlling the orientation of the molecules in such a coating. First, the light-absorbing layer is treated with the solvent methanol, and then the coating material, dissolved in another solvent - trifluoroethanol - is applied and distributed by spin coating. As a result, the molecules of the intermediate layer line up more vertically compared with conventional application methods.
The method was tested on organic solar cells of different compositions and with different coating materials - it worked in all the tested combinations. According to an article in the journal Nature Energy, the proposed method made it possible to achieve a certified efficiency of 20.65% and a fill factor of 82.7% in binary organic solar cells. The fill factor is a measure of how fully a cell uses its potential; the higher it is, the smaller the losses.
The authors explain that sum-frequency spectroscopy (a way of observing the orientation of molecules) showed a predominantly perpendicular arrangement of the molecules of the intermediate layer. This increased the surface potential and reduced resistance to charge transfer. The researchers stress the importance of controlling molecular orientation to improve device performance, but note that their approach has yet to be tested on larger panels and under real production conditions. There is as yet no talk of industrial production or of when such panels might appear on the market.