Electron Spin Knows Carrier Dynamics in Organic Solar Cell
T. Ikoma1,2
1
2
Department of Chemistry, Niigata University, Niigata 950-2181, Japan
CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan
Renewable energy sources are again attracting considerable attention these
days. Harvesting energy directly from the sunlight using photovoltaic cell has been
widely recognized as an essential component of the future global energy production.
The power conversion efficiency of silicon solar cells is already close to the theoretical
upper limit of 30 %. On the other hand, the conversion efficiency of organic solar cell is
much lower than that of inorganic ones, although organic materials offer a number of
benefits. In order to break the plateau of the conversion efficiency of the organic solar
cells, it is necessary to know the reaction of charged carriers in solar cell device.
Chemist is a kind of freak thinking of chemical reaction from various
viewpoints. Conservative quantity during the reaction is an important aspect to
understand the reaction mechanism and to develop new reactions. Among a lot of
conservative properties such as element, mass, charge, energy and so on, momentum is
a unique one because of its vector property. The momentum analysis in reactions
therefore has been succeeding to understand chemistry in gas phase. However, the
conservation law of momentum is likely to be underrated in condensed phase chemistry,
because a lot of collisions of atoms and molecules randomize the vector properties,
which apparently makes the linear momentum negligible. Nevertheless, the electron and
nuclear spins remain their angular momenta even though molecules suffer from the
many collisions. Therefore, we can elucidate history of chemical reaction by observing
the electron spin angular momentum. Measurement of magnetization is a good way to
observe the spin angular momentum because electron spin is accompanied with the
magnetic moment. Also as the magnetic moment in molecule is oriented and quantized
to an external magnetic field, which is so called Zeeman effect, the magnetic field can
influence on the chemical reaction that occurs via transition or intermediate states with
unpaired electrons [1]. In this paper, we have investigated external magnetic field
effects on the device performance of organic solar cells, in order to clarify the carrier
dynamics involving unpaired electrons.
[1] M. Shiraishi and T. Ikoma, Physica E, 43, 1295 (2011).