Organic photovoltaic cells are just one type of solar cell that capture sunlight and convert it to useable energy. Though inexpensive and promising in the field of energy conversion, their processes are complex, limiting their appeal for other applications. However, preliminary research from the University of Tsukuba indicates that the power conversion hurdles may soon be a thing of the past.

The issue with organic photovoltaic cells is charge formation and transport, which currently require separate, complex processes. To solve that, the team essentially normalized them, deriving the absolute value of the charge formation efficiency. Doing so requires both photo-induced and electrochemical spectroscopy to determine the change in absorption after femtosecond photo-pulse excitation, followed by the absorption change due to charge injection. The output gives them a measure of how many charges are produced by a single photon.

“This was extremely surprising,” said Professor Yutaka Moritomo, Institute of Materials Science at the University of Tsukuba, since the positive and negative charges are strongly bound in an organic photovoltaic device as an exciton — a bound state of an electron and hole, which are attracted to each other by the electrostatic Coulomb force. “Its charge formation was believed to be too difficult without a thermal activation process,” explained Moritomo. “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.”

In the future, the team’s method will allow for easier screening of organic materials for organic photovoltaic systems. Organic materials tend to have stringent requirements, including high charge formation efficiency. Moritomo’s method found that this is the case, even at low temperatures (an unexpected finding).

“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,” noted Moritomo.