Title :
Charge-Transfer States and Upper Limit of the Open-Circuit Voltage in Polymer:Fullerene Organic Solar Cells
Author :
Koen Vandewal;Kristofer Tvingstedt;Jean V. Manca;Olle Inganäs
Author_Institution :
Interuniversity Microelectronics Center, Institute for Materials Research in MicroElectronics, vzw, and the Institute for Materials Research, Hasselt University, Belgium
Abstract :
The power conversion efficiency of polymer:fullerene bulk heterojunction solar cells depends on the generated photocurrent and photovoltage. Here we show, using the thermodynamic theory of detailed balance, that the photovoltage in particular is limited by the presence of polymer:fullerene material interaction, resulting in the formation of a weak donor-acceptor charge transfer complex (CTC). Excited CTCs, or charge transfer (CT) states, are visible in highly sensitive measurements of the absorption and photovoltaic action spectrum, or in photoluminescence and electroluminescence measurements. It is shown that photovoltaic and electroluminescent actions of the polymer: fullerene CTC are related by a reciprocity relation. This relation reproduces the measured open-circuit voltage (Voc) of the photovoltaic device under solar conditions. Also, the temperature and illumination intensity dependence of Voc is reproduced by the theory. Assuming perfect conditions for charge generation and recombination, a maximum obtainable Voc value in function of polymer:fullerene CTC properties is derived.
Keywords :
"Voltage","Polymers","Photovoltaic cells","Solar power generation","Photovoltaic systems","Charge transfer","Current measurement","Charge measurement","Electroluminescence","Power conversion"
Journal_Title :
IEEE Journal of Selected Topics in Quantum Electronics
DOI :
10.1109/JSTQE.2010.2043061