Efficiency potential of organic bulk heterojunction solar cells

In order for organic bulk heterojunction solar cells to compete with the traditional inorganic cells, power conversion efficiencies of more than 10% are desirable. Nowadays, efficiencies up to 5% are reached and the question about the limits for the attainable efficiency of organic cells arises. In this paper, we study the efficiency potential of organic bulk heterojunction solar cells. We make realistic assumptions to predict efficiencies obtainable in the near future, and calculate the upper-limit. We study the influence of the difference between the lowest unoccupied molecular orbital (LUMO)-energy levels of donor and acceptor, and the absorption window on the efficiency. Ideal material characteristics are obtained from these calculations, giving an idea how the ideal organic solar cell should look like. The calculations show that nowadays an efficiency of 5.8% for the single junction bulk heterojunction solar cell should be possible. Considering parameters which are credible to be achieved in the future, an organic solar cell of 15.2% is in reach, with an optimal bandgap of 1.5 eV for the absorber. We also consider the situation where both the nand p-type materials are absorbers. All calculations are not only done for a single junction cell, but also for tandem solar cells. For a tandem structure of organic cells, we find in a realistic scenario a maximum attainable efficiency of 10.1% and an efficiency of 23.2% in an optimistic scenario with optimal bandgaps E-g1 = 1.7 eV and E-g2 = 1.1 eV Copyright (c) 2007 John Wiley & Sons, Ltd.