TSUKUBA, Japan, Aug. 19, 2014 — Spectroscopy may provide a way to identify materials for organic photovoltaics based on their charge formation efficiency. In a recent study of organic solar cells, a team at the University of Tsukuba and the National Institute for Materials Science employed photo-induced spectroscopy and electrochemical spectroscopy, methods which, respectively, determine the change in absorption after femtosecond excitation and 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. Researchers develop method to screen organic materials for organic photovoltaic cells by charge formation efficiency. Courtesy of Yutaka Moritomo/University of Tsukuba. Studying one type of organic polymer film, the team discovered that the charge formation efficiency remained high (0.55) even at low temperatures (80 K). Positive and negative charges are strongly bound in an organic photovoltaic device as an exciton. This charge formation was thought to be 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,” Moritomo said. As the development of organic materials requires both high charge formation efficiency and high charge transport efficiency, the team’s work could enable the high-throughput screening of organic materials for new organic photovoltaic devices. “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,” Moritomo said. The research was published in Applied Physics Letters (doi: 10.1063/1.4892611). For more information, visit www.tsukuba.ac.jp.