Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 102

Chapter 5
Exciton and Charge Dynamics in Polymer
Solar Cells Studied by Transient
Absorption Spectroscopy
Hideo Ohkita and Shinzaburo Ito
Abstract Transient absorption spectroscopy is a powerful tool for studying the
photovoltaic conversion events in polymer solar cells, which occur on a wide
temporal scale from *10 -15 s for exciton generation by photon absorption to
*10 -5 s for charge collection to the electrode. This chapter describes first how to
assign transient species such as singlet and triplet excitons, polarons, and other
charge carriers by transient absorption spectroscopy. On the basis of the assign-
ments, the exciton and charge dynamics are analyzed to discuss photophysical
fundamental events related to the photovoltaic conversion such as exciton delo-
calization, energy transfer, triplet formation, and charge generation and recom-
bination. Finally, the relevance to the device performance is summarized.
5.1 Introduction
In recent years, polymer solar cells are attracting increasing interest because of
their potential advantages: lightweight, flexibility, and high throughput and large-
area production based on the printing and coating techniques [ 1 - 3 ]. Although the
power conversion efficiency (PCE) of polymer solar cells was less than 1 % in the
1990s, it has steadily increased every year in the past decade owing to extensive
research. In particular, regioregular poly(3-hexylthiophene) (RR-P3HT) has been