Organic photovoltaic cells, which use polymeric materials to capture sunlight, could be effective energy conversion devices.
A roadblock for the successful creation of these devices has been the complexity of the power conversion process, which involves separate charge formation and transport processes, the American Institute of Physics reported. To solve this problem the researchers developed a method to determine the absolute value of the charge formation efficiency using two types of spectroscopy.
The two types used by the Japanese researchers are photo-induced spectroscopy to determine the change in absorption after femtosecond photo-pulse excitation as well as electrochemical spectroscopy to look at the absorption change due to charge injection
"By qualitative analysis of the spectral change, we can deduce how many charges are produced by one photon -- its charge formation efficiency," said Professor Yutaka Moritomo, Institute of Materials Science at the University of Tsukuba.
The researchers say this is a huge step forwards, and it led to the discovery that charge formation efficiency remains high even when exposed to low temperatures.
"This was extremely surprising," Moritomo said. "Its charge formation was believed to be too difficult without a thermal activation process. But our work shows that the charge formation process of an organic photovoltaic device is purely quantum mechanical, and any theoretical model should explain the high charge formation efficiency at low temperatures."
The team's work will enable the screening of organic material for new photovoltaic devices. The criteria they will be judged on will be high charge formation efficiency and high charge transport efficiency.
"Now that we have a method to determine the key physical parameter, charge formation efficiency, we're exploring the interrelation between it and the nanoscale structure of the organic photovoltaic device to clarify the mechanism of the charge formation," concluded Moritomo.
The findings were published August 19 2014 in Applied Physics Letters.
