Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 121

(a)

1

S+P
S
P

0.8

0.6

0.4

0.2

0

(b)

PB

-0.5

-1

0.5

0 1

Time / ps

Fig. 5.13 a Normalized transient absorption signals of singlet exciton (closed circles) and
polaron (closed triangles) generated in RRa-P3HT:PCBM (50:50 w/w) blend films excited at
400 nm. The closed circles are obtained by subtracting the transient signal of polaron at 1600 nm
(closed triangles) from that at 1000 nm (solid line, singlet exciton and polaron). The subtracted
signals (closed circles) are fitted with a monoexponential function: DOD(t) = A exp(-t/s D ). The
transient rise signals at 1600 nm are fitted with an exponential function and a constant:
DOD(t) = A[1 - exp(-t/s R )] ? B s D = s R = 0.2 ps). b Normalized transient absorption
signals of photobleaching at 470 nm (closed squares) for RRa-P3HT:PCBM (50:50 w/w) blend
films excited at 400 nm. The photobleaching signals are fitted with a constant: DOD(t) = con-
stant. The white broken lines represent the best-fitting curves. The dotted line indicates the
instrument response function of the transient absorption spectroscope. Reprinted with permission
from [ 19 ]. Copyright 2010 American Chemical Society

can promptly encounter a PCBM molecule without exciton migration. Further-
more, as shown in Fig. 5.13 b, no decay is observed for the photobleaching at
470 nm, suggesting that all the singlet excitons are converted into polarons
without deactivating to the ground state. We therefore conclude that both the
exciton diffusion efficiency (g ED ) and the charge transfer efficiency (g CT ) are as
high as 100 % in RRa-P3HT:PCBM blend films.
In RR-P3HT:PCBM blend films, as shown in Fig. 5.7 a, P3HT singlet excitons
are also already quenched to *50 % even at 0 ps, and decays on a time scale of