Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 122

Fig. 5.14 Normalized
transient absorption signals of
RR-P3HT:PCBM blend films
excited at 400 nm measured
at 1200 nm (singlet excitons,
closed circles) and 720 nm
(polarons, open circles): a 5
wt % PCBM, b 20 wt %
PCBM, c 50wt % PCBM
before thermal annealing, and
d 50 wt % PCBM after
thermal annealing. The
transient absorption decay at
1200 nm is fitted with a
double exponential function:
DOD(t) = A D1 exp(-t/
s D1 ) ? A D2 exp(-t/s D2 ). The
transient absorption decay at
720 nm is fitted with a double
exponential function:
DOD(t) = A R [1 - exp(-t/
s R )] ? B. The gray lines
represent the best-fitting
curves [ 19 ]

(a)

(b)

(c)

(d)

0 50 100

Time / ps

picoseconds, followed by efficient P3HT polaron generation. Figure 5.14 shows
the time evolution of the singlet exciton band at 1200 nm and the polaron band at
720 nm. The decay of P3HT singlet excitons can be fitted by a double exponential
function with a short lifetime of *10 ps and an intrinsic exciton lifetime of
330 ps. On the other hand, a part of P3HT polarons are promptly generated even at
0 ps and the others are gradually generated with the same time constant as the
short lifetime of P3HT singlet excitons. The delayed polaron formation (*10 ps)
is much slower than that observed for RRa-P3HT:PCBM blend films (\1 ps).
Furthermore, the rise and decay constants depend on the P3HT domain size: both
increase with increasing P3HT concentration and slightly increase after the ther-
mal annealing. We therefore assign the delayed formation to the polaron gener-
ation via the exciton migration to the interface of RR-P3HT/PCBM. In other