Poly(3-hexylthiophene) Regioregularity
A R T I C L E S
suggests that high molecular weight (M > 20 000 g/mol), broad
n
polydispersity (PDI), and high RR (>95%) are optimal for solar
cell performance, as can be supported via both observed
increases in efficiency when these parameters are satisfied and
extrapolation from structure-function relationships observed in
1
5-18
pristine samples of P3HT.
However, all of these conclu-
sions come with caveats. First, while high molecular weight
has been shown to improve charge-carrier mobility and optical
properties in pristine, high-RR P3HT films, solar cells using
P3HT of significantly lower molecular weight (∼11 000 g/mol)
have been reported to give efficiencies over 4% under optimized
13
processing conditions. Second, a systematic study of the effect
of the polydispersity of P3HT has not been reported, although
it does appear that a broad mix of high- and low-molecular-
weight P3HT in a given sample improves the performance of
Figure 1. I-V characteristics of optimized devices made from 86, 90, and
1
2
P3HT-PCBM-composite solar cells. Finally, the only sys-
tematic study of the effect of P3HT RR, which points to
increasing efficiency with increasing RR, is based on a
comparison of devices for which the processing conditions had
96% RR P3HT blended with PCBM at a 55:45 weight ratio. For 86% RR (9):
2
V
(
oc ) 0.62 V, Jsc ) -11.7 mA/cm , FF ) 0.54, PCE ) 3.9%. For 90% RR
2
b): Voc ) 0.63 V, Jsc ) -9.8 mA/cm , FF ) 0.61, PCE ) 3.8%. For 96%
RR (2): Voc ) 0.58 V, Jsc ) -11.8 mA/cm , FF ) 0.55, PCE ) 3.8%.
2
1
3
not been optimized. In view of the enormous effects that are
solar cells is not yet known. In this work, we examined three
samples of P3HT having similar molecular weight and PDI that
were polymerized by the same polymerization method but in
which the RR varied from 86 to 96%. This RR range was
selected in order to look at a high-RR P3HT (96%), a sample
with a RR similar to that of Rieke P3HT (90%), and a sample
with a significantly lower RR than has been reported in most
P3HT-PCBM solar cells (86%). The lowest RR value was
attained via a copolymerization of 2-bromo-3-hexylthiophene
and 5-bromo-3,3′-dihexyl-2,2′-bithiophene (see the Supporting
Information). Solar cell performance and long-term thermal
stability were examined within the context of variations in the
fundamental properties of the polymers induced via changes to
the primary structure in the form of non-head-to-tail “defect”
linkages.
1
9,20
21,22
known for variations in solvent,
blend ratio,
and electrode structure,
such unoptimized results are not necessarily definitive. The study
spin
20,23
4,24,25
26,27
speed,
annealing conditions,
1
3
by Kim et al. examined the range of RR from 90.7-95.4%
and showed that samples in the 90.7-93.0% range have
efficiencies of under 2%, which is lower than the 4-5%
efficiencies reported in the literature using P3HT from Rieke
It has also been shown that a copolymer
analogue of P3HT with an effective RR of 91% was capable of
3,28
Metals (RR ∼92%).
28
producing solar cells with a PCE of 4.5%. It is therefore clear
that the definitive effect of P3HT RR on fullerene-composite
(
12) Ma, M.; Kim, J. Y.; Lee, K.; Heeger, A. J. Macromol. Rapid Commun.
2
007, 28, 1776–1780.
(
13) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant,
J. R.; Bradley, D. D. C.; Giles, M.; McCullough, I.; Ha, C. S.; Ree,
M. Nat. Mater. 2006, 5, 197–203.
Results
(14) Urien, M.; Bailly, L.; Vignau, L.; Cloutet, E.; de Cuendias, A.; Wantz,
Solar Cell Performance. The performance of each polymer
was independently optimized according to annealing conditions
while the polymer-PCBM blend ratio was kept constant at 55:
G.; Cramail, H.; Hirsch, L.; Parneix, J. Polym. Int. 2008, 57, 764–
7
69.
(
15) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fr e´ chet,
J. M. J.; Toney, M. F. Macromolecules 2005, 38, 3312–3319.
4
5 by weight and the devices all had similar thicknesses (∼100
(
16) Zen, A.; Pflaum, J.; Hirschmann, S.; Zhuang, W.; Jaiser, F.; Asawa-
pirom, U.; Rabe, J. P.; Scherf, U.; Neher, D. AdV. Funct. Mater. 2004,
nm). Figure 1 shows the I-V characteristics of the most efficient
P3HT-PCBM bulk heterojunction (BHJ) devices made from
86, 90, and 96% RR P3HT, which achieved peak PCEs of 3.9,
1
4, 757–764.
(
17) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.;
Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen,
R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999,
3
.8, and 3.8%, respectively, under AM 1.5G illumination with
-2
an intensity of 100 mW cm . First, it is noteworthy that P3HT
with RR as low as 86% can still achieve close to 4% PCE. A
second observation is that the optimized annealing time was
different for the three polymers. At an annealing temperature
of 150 °C, the device with 96% RR P3HT required the shortest
annealing time (30 min) to reach the highest efficiency, whereas
the devices with 90 and 86% RR P3HT required longer
annealing times (60 and 120 min, respectively). The slightly
lower Jsc in the device with 90% RR may be due to the lower
4
01, 685–688.
(
(
(
(
(
(
(
(
18) Barta, P.; Cacialli, F.; Friend, R. H.; Salaneck, W. R.; Zagorska, M.;
Prori, A. Synth. Met. 1999, 101, 296–297.
19) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.;
Durrant, J. R. Appl. Phys. Lett. 2005, 86, 063502.
20) Chu, C.-W.; Yang, H.; Hou, W. J.; Huang, J.; Li, G.; Yang, Y. Appl.
Phys. Lett. 2008, 92, 103306.
21) Nakamura, J.; Murata, K.; Takahashi, K. Appl. Phys. Lett. 2005, 87,
1
32105.
22) Chirvase, D.; Parisi, J.; Hummelen, J. C.; Dyakonov, V. Nanotech-
nology 2004, 15, 1317–1323.
23) Shrotriya, V.; Yao, Y.; Li, G.; Yang, Y. Appl. Phys. Lett. 2006, 89,
molecular weight of the polymer, which may influence the
0
63505.
11,12
parameters of device performance,
but the high fill factor
24) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87,
0
83506.
(FF) of this device compensates for this reduction in current,
allowing it to attain an efficiency comparable to those of the
devices made from 86 and 96% RR P3HT. In addition, there
appears to be a trend in which higher RR gives a lower Voc. On
average, 96% RR P3HT devices had a Voc of 0.58 V whereas
25) Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.;
Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J. Nano Lett. 2005, 5,
5
79–583.
(
(
(
26) Ko, C.-J.; Lim, Y.-K.; Chen, F.-C.; Chu, C.-W. Appl. Phys. Lett. 2007,
9
0, 063509.
27) Kim, J. Y.; Kim, S. H.; Lee, H.-H.; Lee, K.; Ma, W.; Gong, X.; Heeger,
A. J. AdV. Mater. 2006, 18, 572–576.
8
6% RR devices had a Voc of 0.62V.
The thermal stabilities of the photovoltaic devices were also
examined, and Figure 2 shows for each RR level the evolution
28) Sivula, K.; Luscombe, C. K.; Thompson, B. C.; Fr e´ chet, J. M. J. J. Am.
Chem. Soc. 2006, 128, 13988–13989.
J. AM. CHEM. SOC. 9 VOL. 130, NO. 48, 2008 16325