Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 103

studied thoroughly as a donor material in polymer/fullerene solar cells [ 4 - 9 ]. It is
noteworthy that this polymer has a good balance between solubility and opto-
electronic properties [ 10 - 13 ], while most conjugated polymers generally have
faced a trade-off between them. Regioregular P3HT is likely to be crystalline in
solid films even with high solubility in various organic solvents, because each
monomer unit attached with a hexyl group is regularly bound in a head-to-tail
linkage. Consequently, RR-P3HT:PCBM solar cells have been reported to be
strongly dependent on the fabrication conditions, particularly annealing treat-
ments. After thermal or solvent annealing, RR-P3HT:PCBM solar cells exhibit a
reproducible PCE approaching 5 % and excellent external quantum efficiency
(EQE) up to [80 % [ 5 - 9 ]. Therefore, this polymer solar cell is still being inten-
sively studied as a benchmark. Thereafter, owing to syntheses of various new
materials, optimization of blend morphology, and development of new device
structures, a certified PCE in excess of 8 % has been listed in the solar cell
efficiency table (version 37) [ 14 ]. In 2012, a certified PCE of 10.0 % has been
listed in the solar cell efficiency table (version 39) [ 15 ].
For the improvement of the device efficiency, new materials and device
structures are developed mainly on the basis of macroscopic properties of
J-V characteristics. However, the J-V characteristics just provide us the final result
of a series of fundamental photovoltaic conversion events including photon
absorption, exciton generation, exciton migration, charge separation, charge
recombination, charge dissociation, charge transport, and charge collection. In
other words, the essential problem causing a poor device performance cannot be
specified only from the J-V characteristics. Thus, it is of particular importance to
elucidate each fundamental event to design new materials and develop new device
structures rationally. Photovoltaic conversion events occur in a range of over nine
orders of magnitude on a temporal scale from *10 -14 s for ultrafast charge
separation to *10 -5 s for charge collection to the electrode. Transient absorption
spectroscopy is therefore a powerful tool for studying such conversion events in
photovoltaic devices directly. By analyzing the transient spectra in detail, we can
assign all the transient species such as exciton, polaron pair, free polaron, and
trapped polaron separately and discuss their dynamics quantitatively [ 16 - 21 ]. This
chapter describes the assignment of excitons and charge species and the photo-
physics of photovoltaic conversion events in polymer solar cells studied by tran-
sient absorption spectroscopy.
5.2 Temporal Scale of Photovoltaic Conversion
Here, we describe the temporal scale of photovoltaic conversion events in polymer
solar cells. Figure 5.1 shows a schematic illustration of the most simple polymer
solar cells with a bilayered structure of the hole-transporting (donor) and electron-
transporting (acceptor) materials, which is similar to silicon-based solar cells
consisting of p-type and n-type semiconductors. In polymer solar cells, as shown in