Published on Web 02/16/2008

Charge Carrier Formation in Polythiophene/Fullerene Blend

Films Studied by Transient Absorption Spectroscopy
Hideo Ohkita,*,†,# Steffan Cook,† Yeni Astuti,† Warren Duffy,‡ Steve Tierney,‡
Weimin Zhang,‡ Martin Heeney,‡,|| Iain McCulloch,‡,⊥ Jenny Nelson,§
Donal D. C. Bradley,§ and James R. Durrant*,†
Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United
Kingdom, Merck Chemicals, Chilworth Science Park, Southampton SO16 7QD, United Kingdom,
and Department of Physics, Imperial College London, Prince Consort Road,
London SW7 2BW, United Kingdom
Received August 31, 2007; E-mail: ohkita@photo.polym.kyoto-u.ac.jp; j.durrant@imperial.ac.uk

Abstract: We report herein a comparison of the photophysics of a series of polythiophenes with ionization
potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt % 1-(3-methoxycarbonyl)-
propyl-1-phenyl-[6,6]C61 (PCBM). Three polymers are observed to give amorphous films, attributed to a
nonplanar geometry of their backbone while the other five polymers, including poly(3-hexylthiophene), give
more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by
transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine
polymers, no triplet formation is observed, but rather a short-lived (∼100 ns), broad photoinduced absorption
feature assigned to polymer polarons. For all polymers, the addition of 5 wt % PCBM resulted in 70-90%
quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons.
Remarkably, despite this efficient exciton quenching, the yield of dissociated polymer+ and PCBM- polarons,
assayed by the appearance of a long-lived, power-law decay phase assigned to bimolecular recombination
of these polarons, was observed to vary by over 2 orders of magnitude depending upon the polymer
employed. In addition to this power-law decay phase, the blend films exhibited short-lived decays assigned,
for the amorphous polymers, to neutral triplet states generated by geminate recombination of bound radical
pairs and, for the more crystalline polymers, to the direct observation of the geminate recombination of
these bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic model
for charge generation in polythiophene/PCBM blend films analogous to that reported to explain the
observation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we find
an excellent correlation between the free energy difference for charge separation (∆GCSrel) and yield of the
long-lived charge generation, with efficient charge generation requiring a much larger ∆GCSrel than that
required to achieve efficient PL quenching. We suggest that this observation is consistent with a model
where the excess thermal energy of the initially formed polaron pairs is necessary to overcome their
Coulombic binding energy. This observation has important implications for synthetic strategies to optimize
organic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blend
films, a large ∆GCSrel (or LUMO level offset) is required to achieve efficient charge dissociation.

Introduction physics and photochemistry of photosynthetic reactions centers,

increasingly sophisticated D-A molecular relays have been
Great progress has now been made in our understanding of synthesized and characterized in solution.6,7 Such molecular
photoinduced electron transfer in isolated donor-acceptor (D- relays have been designed for the directional electron transfer
A) molecular systems.1-5 Inspired by the remarkable photo- driven by appropriate energetic cascades, resulting in sequential
†
charge separations and generating a long-lived, spatially sepa-
Department of Chemistry, Imperial College London.
‡ Merck Chemicals. rated charge separated states or “radical pairs”.8-10
§ Department of Physics, Imperial College London.
# Present address: Department of Polymer Chemistry, Graduate School (2) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100,
of Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, 13148-13168.
Japan. (3) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265-322.
| Present address: Department of Materials, School of Engineering and (4) Miller, J. R.; Calcatterra, L. T.; Closs, G. L. J. Am. Chem. Soc. 1984, 106,
3047-3049.
Materials Science, Queen Mary, University of London, Mile End Road, (5) Asahi, T.; Ohkohchi, M.; Matsusaka, R.; Mataga, N.; Zhang, R. P.; Osuka,
London E1 4NS, United Kingdom. A.; Maruyama, K. J. Am. Chem. Soc. 1993, 115, 5665-5674.
⊥ Present address: Department of Chemistry, Imperial College London,
(6) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461.
Exhibition Road, London SW7 2AZ, United Kingdom. (7) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40-48.
(1) Adams, D. M. et al. J. Phys. Chem. B 2003, 107, 6668-6697. (8) Fukuzumi, S. Bull. Chem. Soc. Jpn. 2006, 79, 177-195.
3030 9 J. AM. CHEM. SOC. 2008, 130, 3030-3042 10.1021/ja076568q CCC: $40.75 © 2008 American Chemical Society
Photophysics of Polythiophene/PCBM Blend Films ARTICLES

On the other hand, reports of efficient photoinduced charge only 5 wt % PCBM, indicative of efficient quenching of the
separation in solid-state blend films of conjugated polymers with polymer excitons. Transient absorption spectroscopy is em-
fullerenes11,12 have attracted increasing interest due to their ployed to determine the photogenerated species resulting from
applicability to photovoltaic (PV) solar energy conversion. The this exciton quenching. Remarkably, despite the similar emission
intermixing of the polymer electron donor with the fullerene quenching observed for all polymers studied, the yield of long-
electron acceptor in such blended films (the formation of a “bulk lived photogenerated charges is found to vary by 2 orders of
heterojunction”) can result in a significant enhancement of magnitude, depending upon polymer employed.
photoinduced charge separation in such films relative to more Studies of the photophysics and photochemistry of organic
conventional bilayered heterojunction devices.13 Currently, blend films to date have largely focused on polymer/fullerene
power conversion efficiencies approaching 5% have been and polymer/polymer blend films employing poly(p-phenyle-
reported by several groups for organic PV devices based on nevinylene) (PPV)-based polymers.28-30 Such blend films have
blend films of regioregular poly(3-hexylthiophene) (P3HT) and been shown to give PV devices, although with more modest
1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]C61 (PCBM).14-19 solar energy-conversion efficiencies than polythiophene-based
Most studies of such blend films have focused on the optimiza- devices. In PPV-based polymer/polymer blend films, charge
tion of PV device performance through control of materials photogeneration has been described as a two-step process. Initial
structure, film processing conditions, and blend morphology. charge separation at the donor/acceptor interface has been
Device performance has been shown to be significantly affected suggested to result in the formation of a Coulombically bound
by thermal treatments,19,20 blend composition,21,22 solvents,23 radical pair (BRP) state. Such BRPs are analogous to the
film thickness,24 regioregularity of conjugated polymers,17 and “contact ion pairs” discussed in solution studies of molecular
molecular weight.25 Recent systematic studies with various D-A systems.9,10 For the PPV-based polymer/polymer
conjugated polymers and fullerenes have demonstrated that blends, this BRP state has been shown to exhibit a red-shifted
open-circuit voltage VOC is directly correlated with the HOMO emission band,29-32 analogous to the emission of donor/acceptor
level of the conjugated polymer26 and the LUMO levels of excited-state complexes or “exciplexes”.33,34 This BRP state
fullerene.27 However, systematic studies of the photophysics and can undergo rapid intersystem crossing between its singlet and
photochemistry of polythiophene/PCBM blend films (as a triplet states. Dissociation of this BRP to free charges competes
function of polymer properties) and their correlation with device with its geminate recombination either to the singlet ground
performance have received limited attention to date. Most state S0 or to neutral triplet excitons, depending upon the radical
typically photoluminescence (PL) emission quenching is em- pair spin state. This geminate recombination, which should
ployed to assay the quenching of photogenerated polythiophene be distinguished from the subsequent bimolecular recombina-
excitons by PCBM, with efficient emission quenching being tion of the dissociated free charges, has been suggested to be a
regarded as an indicator of efficient charge photogeneration. In key factor in limiting device performance for PPV-based
this paper we report a detailed study of the photophysics and devices. Similar observations have been made for polyfluorene-
photochemistry of polythiophene/PCBM blend films employing based polymer/polymer blend films35 and for polyfluorene/
eight different thiophene-based polymers. In all cases, efficient fullerene blends.36 In particular, in a study employing three
quenching of the polymer PL is observed by the inclusion of different PPV-based polymers, an inverse correlation was
observed between this exciplex-like emission strength and
(9) Verhoeven, J. W. J. Photochem. Photobiol., C 2006, 7, 40-60. PV device performance.32 However, a clear understanding of
(10) Verhoeven, J. W.; van Ramesdonk, H. J.; Groeneveld, M. M.; Benniston, the parameters determining the efficiency of charge dissocia-
A. C.; Harriman, A. ChemPhysChem 2005, 6, 2251-2260.
(11) Morita, S.; Zakhidov, A. A.; Yoshino, K. Solid State Commun. 1992, 82, tion versus geminate recombination in organic blend films is
249-252. yet to be established. Moreover such studies have not pre-
(12) Smilowitz, L.; Sariciftci, N. S.; Wu, R.; Gettinger, C.; Heeger, A. J.; Wudl,
F. Phys. ReV. B: Condens. Mater. Phys. 1993, 47, 13835-13842. viously focused on polythiophene materials, the polymer class
(13) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Mater. Funct. 2001,
11, 15-26. currently attracting the greatest interest for efficient organic PV
(14) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87, cells.
083506.
(15) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater.
2005, 15, 1617-1622. (28) Müller, J. G.; Lupton, J. M.; Feldmann, J.; Lemmer, U.; Schaber, M. C.;
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Y. Nat. Mater. 2005, 4, 864-868. Phys. 2005, 72, 195208.
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R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nat. R. A. J. Phys. ReV. B: Condens. Mater. Phys. 2005, 72, 045213.
Mater. 2006, 5, 197-203. (30) Morteani, A. C.; Sreearunothai, P.; Herz, L. M.; Friend, R. H.; Silva, C.
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A. J. AdV. Mater. 2006, 18, 572-576. (31) Yin, C.; Kietzke, T.; Kumke, M.; Neher, D.; Hörhold, H.-H. Sol. Energy
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13, 85-88. (33) The term “exciplex” was originally employed in photochemical studies to
(21) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, refer the formation of an excited state molecular complex, observed in
J. R. J. Mater. Sci. 2005, 40, 1371-1376. systems where optical excitation of monomeric molecular species was
(22) Nakamura, J.; Murata, K.; Takahashi, K. Appl. Phys. Lett. 2005, 87, 132105. observed to result in the formation of molecular dimers (see ref 34). As
(23) Al-Ibrahim, M.; Ambacher, O.; Sensfuss, S.; Gobsch, G. Appl. Phys. Lett. such, the use of this term for solid-state molecular systems where optical
2005, 86, 201120. excitation does not result in a significant change in donor-acceptor spatial
(24) Li, G.; Shrotriya, V.; Yao, Y.; Yang, Y. J. Appl. Phys. 2005, 98, 043704. separation is rather questionable.
(25) Hiorns, R. C.; de Bettignies, R.; Leroy, J.; Bailly, S.; Firon, M.; Sentein, (34) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience:
C.; Khoukh, A.; Preud’homme, H.; Dagron-Lartigau, C. AdV. Funct. Mater. London, UK, 1970; Chapter 9.
2006, 16, 2263-2273. (35) Ford, T. A.; Avilov, I.; Beljonne, D.; Greenham, N. C. Phys. ReV. B:
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(27) Kooistra, F. B.; Knol, J.; Kastenberg, F.; Popescu, L. M.; Verhees, W. J. Vanderzande, D.; Bradley, D. D. C.; Nelson, J. AdV. Funct. Mater. 2007,
H.; Kroon, J. M.; Hummelen, J. C. Org. Lett. 2007, 9, 551-554. 17, 451-457.

J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008 3031

ARTICLES Ohkita et al.

We have previously reported photophysical studies of poly- Chart 1. Chemical Structures of Various Polythiophenes with
thiophene/fullerene blend films, employing transient absorption Different IP: from the Top: P(T8T8T0), P(T12NpT12), P(T10PhT10),
spectroscopy with low-intensity excitation pulses. For P3HT/ P(T0T0TT16), P(T12T12TT0), P(T12SeT12), P(T0TT16), P3HT
PCBM blend films, a high yield of photogenerated charge
carriers is observed. These charge carriers exhibit dispersive,
bimolecular recombination characterized by a power-law de-
cay.17 We have recently extended these studies to two higher
ionization potential (IP) thiophene-based copolymers.37 In
contrast to P3HT, quenching of the polymer singlet exciton by
the inclusion of PCBM was shown to generate a high yield of
triplet excitons rather than dissociated polarons. This was
attributed to efficient triplet formation through geminate charge
recombination of bound triplet radical ion pairs,37 analogous to
that also reported for polymer/polymer blend films.35,38 Our
observation of triplet formation rather than polaron formation
for two higher IP polythiophenes indicates that polymer IP may
play a role in influencing the blend photophysics. This result
parallels our recent investigation of a polyfluorene/PCBM blend
film where we concluded that the high IP of the polyfluorene
prevented photoinduced charge separation but rather resulted
in Förster energy transfer to the PCBM and subsequent
intersystem crossing from the PCBM singlet to triplet exciton.39
Analogous energy-transfer dynamics have also been observed
for dye-doped conjugated polymer films.40 These findings have
motivated us to undertake a more in-depth study of the photo-
physical processes in polythiophene/PCBM blend films as a
function of polythiophene IP. To control the polymer IP, we
have chemically modified the backbone structure to achieve a
perturbation of the π electron conjugation. Sterically forcing a
twist between neighboring units has been shown to reduce the
π orbital overlap in the conjugation backbone, thereby raising
the polymer IP.41,42 Alternatively, introducing a nonconjugated
or less conjugated unit as a component of the polymer back-
bone has also been shown to increase the polymer IP.43-45 On
the basis of these strategies, we have prepared a series of
polythiophenes (Chart 1) with IPs ranging from 4.8 to 5.6 eV.
Higher IP polythiophenes are particularly interesting because
of their potential to increase cell VOC relative to that achiev-
able by P3HT.26 These polythiophenes enable us to vary
systematically the energetics of charge separation in blend
films with the electron acceptor PCBM. Charge carriers and
triplet excitons formed in a series of such blend films are
directly observed by transient absorption spectroscopy and
analyzed in terms of a two-step model for charge generation.
In all cases, our studies are limited to comparison of pristine
polymer films with those with 5 wt % added PCBM. The low changes to the polythiophene widely reported to result from
PCBM concentration was selected to avoid the morphological the addition of high PCBM concentrations.21 For all polymers,
this low concentration of PCBM was sufficient to result in
(37) Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Heeney, M.; Tierney, S.; efficient (>70%) emission quenching, indicative of efficient
McCulloch, I.; Bradley, D. D. C.; Durrant, J. R. Chem. Commun. 2006,
3939-3941. polymer exciton quenching. We note that caution should be
(38) Veldman, D.; Offermans, T.; Sweelssen, J.; Koetse, M. M.; Meskers, S. taken in relating measurements taken at this PCBM concentra-
C. J.; Janssen, R. A. J. Thin. Solid Films 2006, 511-512, 333-337.
(39) Cook, S.; Ohkita, H.; Durrant, J. R.; Kim, Y.; Benson-Smith, J. J.; Nelson, tion directly to PV device performance which is typically
J.; Bradley, D. D. C. Appl. Phys. Lett. 2006, 89, 101128. measured at higher (>50 wt %) PCBM concentrations, and
(40) Veldman, D.; Bastiaansen, J. J. A. M.; Langeveld-Voss, B. M. W.;
Sweelssen, J.; Koetse, M. M.; Meskers, S. C. J.; Janssen, R. A. J. Thin emphasize that the primary motivation of these studies is a
Solid Films 2006, 511-512, 581-586.
(41) McCulloch, I.; Bailey, C.; Giles, M.; Heeney, M.; Love, I.; Shkunov, M.; fundamental understanding of the photophysics of polythiophene/
Sparrowe, D.; Tierney, S. Chem. Mater. 2005, 17, 1381-1385. PCBM interfaces rather than a direct correlation with device
(42) Tierney, S.; Heeney, M.; McCulloch, I. Synth. Met. 2005, 148, 195-198.
(43) Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.; performance.
Tierney, S.; McCulloch, I. J. Am. Chem. Soc. 2005, 127, 1078-1079.
(44) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.;
Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, (45) DeLongchamp, D. M.; Kline, R. J.; Lin, E. K.; Fischer, D. A.; Richter, L.
M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, J.; Lucas, L. A.; Heeney, M.; McCulloch, I.; Northrup, J. E. AdV. Mater.
328-333. 2007, 19, 833-837.

3032 J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008

Photophysics of Polythiophene/PCBM Blend Films ARTICLES

Table 1. Characterization of Polythiophene Pristine Films Table 2. Photophysical Properties of Polythiophene Pristine Films
IPb/ absorption PL
Mw Mn Mw/Mn µa/cm2 V-1 s-1 eV cryst.
λmax/ λ0-0 a/ λ0-0 a/ λmax/ ES / ∆EStokes /
P(T8T8T0) 73000 29500 2.48 1 × 10-5 5.6 amorphous
nm nm nm nm eV eV
P(T12NpT12) 22000 9000 2.44 6 × 10-4 5.4 amorphous
P(T10PhT10) 21900 11500 1.90 2 × 10-4 5.4 amorphous P(T8T8T0) 438 - - 572 2.5c 0.66c
P(T0T0TT16) 98100 42000 2.34 2 × 10-1 5.1 crystalline P(T12NpT12) 423 - - 519 2.7c 0.54c
P(T12T12TT0) 51400 27900 1.84 2 × 10-1 5.1 crystalline P(T10PhT10) 416 - - 564 2.6c 0.78c
P(T12SeT12) 46400 23800 1.95 2 × 10-1 5.0 crystalline P(T0T0TT16) 531 ∼580b 621 621 2.1d 0.14d
P(T0TT16) 47600 22900 2.08 3 × 10-1 5.0 crystalline
P(T12T12TT0) 545 ∼595b ∼635b 668 2.0d 0.13d
P3HT 20700 12400 1.67 7 × 10-3 4.8 highly crystalline
P(T12SeT12) 572 618 ∼640b 662 1.9d 0.07d
a Field-effect mobility evaluated in the saturated regime. b Evaluated by
P(T0TT16) 554 ∼595b 633 663 2.1d 0.13d
P3HT 523 ∼600b ∼640b 666 2.0d 0.13d
an ambient ultraviolet photoelectron spectroscopy technique.
aλ
0-0 represents the wavelength of the first shoulder or vibrational band
Experimental Section in the absorption or PL spectra. b Not a peak but a shoulder. c Evaluated
from λmax of the absorption and PL spectra. d Evaluated from λ0-0 of the
Materials. Seven kinds of novel polythiophenes were synthesized absorption and PL spectra.
as reported elsewhere.41-43 1-(3-Methoxycarbonyl)propyl-1-phenyl-[6,6]-
C61 (PCBM) was kindly supplied from the University of Groningen
and used as received.46 To the polythiophene and PCBM was added
chlorobenzene (Aldrich) at a concentration of ∼10 mg mL-1. For
P(T12T12TT0), P(T0T0TT16), P(T0TT16), and P(T12SeT12), the solution
was heated in a water bath at 100 °C for a few minutes to be dissolved
homogeneously. Polymer films were spin-coated onto glass substrates
at a spin rate of 3000 rpm for 90 s from the chlorobenzene solution
under N2 atmosphere. The weight concentration of PCBM in the final
polymer blend films was fixed at 5 wt %. Before the spin-coating, the
substrates were precleaned by sonication in toluene, acetone, and ethanol
for 15 min, respectively. For measurements of J-V characteristics, PV
devices were fabricated as reported previously.17
Measurements. Molecular weight determinations were carried out
in chlorobenzene solution on an Agilent 1100 series HPLC using two Figure 1. Absorption and PL spectra of P3HT (s), P(T0T0TT16) (‚‚‚), and
Polymer Laboratories mixed B columns in series, and the system was P(T10PhT10) (- - -) pristine films at room temperature.
calibrated against narrow-weight polystyrene calibration standards
(Polymer Laboratories). Hole mobility was evaluated from the linear
fit of the root square of the source-drain current in the saturated regime 2. In terms of their materials properties, these polymers can be
versus the gate voltage by the field-effect transistor (FET) measurements effectively divided into two distinct groups. P(T10PhT10), P(T12-
as described elsewhere.41,44 Ionization potential (IP) of the polymer NpT12), and P(T8T8T0) were easily soluble in chlorobenzene at
pristine films was measured by an ambient ultraviolet photoelectron room temperature and formed uniform and smooth films by
spectroscopy (UPS) technique with a UPS spectrometer (Riken-Keiki, spin-coating from the chlorobenzene solution. Differential
AC-2). X-ray diffraction (XRD) measurements were carried out with scanning calorimetry (DSC) data of the bulk powders indicates
an X-ray diffractometer (Philips, PW1710). Absorption and PL spectra
that P(T8T8T0) is amorphous, while P(T10PhT10) and P(T12-
were measured at room temperature with a UV-visible spectropho-
NpT12) do show melting and recrystallization behavior but with
tometer (Shimadzu, UV-1601) and a spectrofluorimeter (Horiba Jobin
Yvon, Spex Fluoromax 1), respectively. The energy-minimized mo- rather low recrystallization enthalpies (8 and 5 J g-1 respec-
lecular structures of oligothiophene units were calculated using MOPAC tively).41 However, XRD data indicated that thin films prepared
program (CambridgeSoft). Transient absorption data were collected with by spin-coating were amorphous (see Supporting Information).
a highly sensitive microsecond transient absorption system under Ar In contrast, P(T12T12TT0), P(T0T0TT16), P(T0TT16), and P(T12-
or O2 atmosphere as described elsewhere.47 The excitation wavelength SeT12) were hardly soluble in chlorobenzene at room temper-
was 420 nm for P(T10PhT10), P(T12NpT12), and P(T8T8T0) and 530 nm ature but were soluble at 100 °C and formed less uniform films.
for P3HT, P(T12T12TT0), P(T0T0TT16), P(T0TT16), and P(T12SeT12) unless XRD indicated that these films are significantly more crystalline
otherwise noted. Low-intensity excitation conditions (5-65 µJ cm-2) (see Supporting Information) and showed peaks similar to those
were employed to ensure that the densities of photogenerated charge of P3HT. As summarized in Table 1, the amorphous polymers
carriers are comparable to those generated under solar irradiation (1017-
exhibited relatively large IPs and lower carrier mobilities, on
1018 cm-3). J-V characteristics of blend films were measured under
50 mW cm-2 simulated sunlight (AM1.5) under N2 atmosphere.
the order of 10-5 to 10-4 cm2 V-1 s-1 as determined by FET
measurements. We note that, due to these relatively low hole
Results mobilities, the amorphous materials are not expected to make
Characterization of Polythiophene Pristine Films. Table efficient PV devices. The more crystalline polymers exhibit
1 summarizes the molecular weights, hole mobilities, and IPs higher carrier mobilities on the order of 10-1 cm2 V-1 s-1 and
of the polymers employed in this study. The polymers exhibited relatively low IPs. It is worth noting that four of the crystalline
weight-average molecular weights (Mw) in the range 20,000- polymers in this study exhibit higher hole mobilities than P3HT.
100,000, with a polydispersity index (Mw/Mn) of approximately The photophysical properties of the pristine polymer films
are summarized in Table 2. Figure 1 shows absorption and PL
(46) Hummelen, J. C.; Knight, B. W.; LePeq, F.; Wudl, F.; Yao, J.; Wilkins, C. spectra of three representative polymer pristine films; P(T10-
L. J. Org. Chem. 1995, 60, 532-538. PhT10) as an amorphous polymer, P(T0T0TT16) as a partially
(47) Ohkita, H.; Cook, S.; Ford, T. A.; Greenham, N. C.; Durrant, J. R. J.
Photochem. Photobiol., A 2006, 182, 225-230. crystalline polymer, and P3HT as a reference. The amorphous

J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008 3033

ARTICLES Ohkita et al.

Table 3. Dihedral Angles in Oligothiophene Units Calculated by

MOPAC-AM1
dihedral angle/deg
T−Ta T−X
P(T10PhT10) 149.2 142.7 b
P(T0T0TT16) 178.9 174.1 c
P3HT 179.1 -

a Dihedral angle of neighboring two thiophene units. b Dihedral angle

between a 3-decythiophene unit (T10) and a phenyl unit (Ph). c Dihedral
angle between a thiophene unit (T0) and a 3,6-dihexadecylthieno[3,2-
b]thiophene unit (TT16).

Figure 2. Minimum-energy molecular structures calculated by the MOPAC

program: (a) 3-mer of P(T10PhT10), (b) 2.5-mer of P(T0T0TT16), (c) 6-mer
of P3HT.
Figure 3. Transient absorption spectra of the P(T10PhT10) pristine film at
P(T10PhT10), P(T12NpT12), and P(T8T8T0) pristine films exhibited 1, 2, 3, 5, and 8 µs after the laser excitation from top to bottom. (Inset)
Transient absorption decays of the P(T10PhT10) pristine film monitored at
structureless absorption bands at 416, 423, and 438 nm, 700 nm under Ar (black line) and O2 (gray line) atmospheres at room
respectively, which are significantly blue-shifted compared with temperature.
the lowest vibrational band of P3HT (∼640 nm). On the other
hand, the more crystalline P(T12T12TT0), P(T0T0TT16), P(T0- inclined at about 30° to the neighboring T10 units, suggesting
TT16), and P(T12SeT12) pristine films exhibited more structured, localization of π-conjugation system. This twisted structure is
red-shifted absorption bands with a shoulder or a vibrational consistent with the amorphous film morphology discussed
band corresponding to the lowest vibronic transition at ∼635, above. In contrast, the dihedral angle of the neighboring two
612, 633, and ∼640 nm, respectively, which is more comparable thiophene (T0) units in the P(T0T0TT16) oligomer was almost
to that of P3HT (∼640 nm). The vibrational structure observed 180°, and that of the T0 unit and a 3,6-dihexadecylthieno[3,2-
is characteristic of ordered polythiophene films and is consistent b]thiophene (TT16) unit was ∼175°, suggesting more complete
with interplanar π-stacking of conjugated planes.48 The corre- delocalization of the π-conjugation system along the more planar
sponding PL spectra show the same trends between the amor- main chain, and consistent with the formation of more crystalline
phous and more crystalline polymers, as summarized in Table films. Furthermore, the dihedral angle of the neighboring two
2. Note that the PL intensity of the amorphous polymers was 3-hexylthiophene units in the P3HT oligomer was almost 180°,
generally much larger than that of the more crystalline polymers. indicating each 3-hexylthiophene unit is aligned in the same
From the absorption and PL spectra, the energy level of the plane, consistent with previous reports.17,50 These calculated
lowest singlet excited state (S1 or “singlet exciton”) ES was dihedral angles are summarized in Table 3. We will discuss in
evaluated to be 2.5-2.7 eV for the amorphous polymer pristine detail later the distinct photophysical processes in pristine and
films, and ∼2.0 eV for the more crystalline polymers. Further- blend films in terms of these molecular structures.
more, the Stokes shifts ∆EStokes, determined from the splitting Transient Absorption of Pristine Films. Transient absorp-
of the absorption and emission bands, were also observed to be tion studies of the organic films were undertaken employing
much larger for the amorphous polymer films compared with low-intensity excitation conditions corresponding to approxi-
those of the more crystalline polymer films. This indicates a mately 1017-1018 absorbed photons cm-3, comparable to the
larger structural relaxation of the amorphous polymers following charge carrier densities generated in polymer/fullerene blend
photoexcitation.49 cells under solar irradiation. We consider first the results
In order to address the origin of the different crystallinities obtained for pristine, amorphous films, again using P(T10PhT10)
observed for the two polymer groups, we calculated molecular as the representative polymer. Pristine films of this polymer
structures of oligomers as model compounds for P(T10PhT10), exhibited a photoinduced transient absorption band at around
P(T0T0TT16), and P3HT. The energy was minimized by MO- 700 nm, as shown in Figure 3. The transient signal decayed
PAC-AM2 calculations from an initial conformation optimized monoexponentially with a lifetime of 7 µs under Ar atmosphere,
with the MM2 force-field. As shown in Figure 2, the P(T10- accelerating to ∼1.5 µs under O2 atmosphere. As already
PhT10) oligomer exhibited a twisted structure in the backbone, reported, the P(T10NpT10) and P(T8T8T0) pristine films also
attributed to the steric hindrance between a hydrogen atom of exhibited similar O2-sensitive transient photoinduced absorption
the phenyl (Ph) unit and the side chain of the 3-decylthiophene bands around 800 nm with lifetimes of several microseconds.37
(T10) unit, while the oligomers for P(T0T0TT16) and P3HT had These transient bands are assigned, as previously,37 to T1 f Tn
a planar backbone. The Ph ring in the P(T10PhT10) oligomer transitions of polymer triplet excitons (3P*) formed in the
amorphous polymer pristine films. No distinct long-lived
(48) Cornil, J.; Beljonne, D.; Calbert, J.-P.; Brédas, J.-L. AdV. Mater. 2001, 13, transients were observed for the pristine films.
1053-1067.
(49) Gierschner, J.; Cornil, J.; Egelhaaf, H.-J. AdV. Mater. 2007, 19, 1-20. On the other hand, the P(T0T0TT16) pristine film, representa-
3034 J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008
Photophysics of Polythiophene/PCBM Blend Films ARTICLES

Table 4. Photophysical Properties of Polythiophene/PCBM Blend

Films
device
fast phase slow phase datad
Qe a / τ/ amplitudeb/ JSC/
% µs assgnmnt ∆µOD Rc mA cm-2
P(T8T8T0) >90 3 3P* e3 0.3 0.002e
P(T12NpT12) ∼80 11 3PCBM* e3 0.3 -
P(T10PhT10) >90 7 3P* 90 0.3 0.02e
P(T0T0TT16) ∼80 ∼0.1 P+/PCBM- 30 0.4 1.8
Figure 4. Transient absorption spectra of the P(T0T0TT16) pristine film at P(T12T12TT0) ∼75 ∼0.1 P+/PCBM- e2 - 0.01
0.3, 0.5, and 1 µs after the laser excitation from top to bottom. (Inset) P(T12SeT12) ∼80 <0.1 - e2 - -
Transient absorption decays of the P(T0T0TT16) pristine film monitored at P(T0TT16) ∼70 ∼0.1 P+/PCBM- 45 0.3 -
960 nm under Ar (black line) and O2 (gray line) atmospheres at room P3HT ∼70 - - 160 0.7 3.4
temperature.
a Steady-state PL quenching of the blend film relative to the correspond-
ing pristine film. b Evaluated from the amplitude of the transient absorbance
power-law decay phase at 1 µs normalized by absorption at the excitation
wavelength. c Exponent of the power law obtained from power-law fits to
this phase. d Measured from bulk heterojunction organic PV devices with
1:1 blend compositions without thermal annealing. Other experimental
conditions as in ref 17. e Note that the low hole mobilities of these
amorphous polymers are expected to limit device photocurrent relative to
the more crystalline materials.

Figure 5. PL spectra of P(T10PhT10)/PCBM blend (solid line) and P(T10-

PhT10) pristine (broken line) films. The PL intensity was corrected for
variation in the absorption at an excitation wavelength of 420 nm. The PL
tail of P(T10PhT10)/PCBM blend film is multiplied by 100 (dotted line).

tive of the more crystalline polymers, exhibited a broader

transient photoinduced absorption band around 900-1000 nm
as shown in Figure 4. This transient optical density was much
smaller than that observed for the amorphous polymers and
decayed rapidly on the hundreds of nanoseconds time scale,
down to less than 10-5 ∆OD within 1 µs. This decay is much
faster than that of triplet excitons observed for the amorphous
polymers. This fast transient was not quenched under O2
atmosphere, as shown in the inset to Figure 4. These observa-
tions suggest that this absorption transient should not be assigned
to triplet excitons. On the other hand, it is too long-lived to be Figure 6. (a) Transient absorption spectra of the P(T10PhT10)/PCBM blend
assigned to singlet excitons; the luminescence lifetime is film at 10, 20, 100, and 800 µs after the laser excitation from top to bottom.
reported to be <1 ns for regiorandom poly(3-octylthiophene) (b) Transient absorption decays of the P(T10PhT10)/PCBM blend film
(P3OT) solution51 and regioregular P3HT pristine film.52 Indeed, monitored at 700 nm under Ar (black line) and O2 (gray line) atmospheres
at room temperature.
no transient absorption ascribable to singlet excitons was
spin-coating solutions resulted in efficient polymer emission
observed for all the polymers studied herein on the time scales
quenching (Qe of 70-90%) for all the polythiophenes studied
studied (>200 ns). A similar absorption band has been reported
herein. In contrast to our recent study of emission quenching
for a regioregular P3HT pristine film by continuous wave
in a polyfluorene/PCBM blend film,39 with the exception of
photoinduced absorption (PIA) measurements at 10 K. The PIA
P(T12NpT12), no PCBM singlet emission (λmax ≈ 700 nm) was
spectrum exhibits an absorption peak around 990 nm (1.25 eV)
observed for any of the blend films studied here. Similarly no
and was assigned to localized polarons in the disordered portions
evidence could be obtained for the appearance of any other lower
of the film.53 Similar rapid decays were observed for the other
energy emission bands analogous to the exciplex-like emission
polythiophene pristine films. Thus, these rapid decays are
reported for PPV-based films. For P(T12NpT12)/PCBM blend
ascribed to the charge recombination of polaron pairs formed
films, a weak 700-nm band was observed for the blend film
in the pristine film. For the P3HT pristine film, no distinct
not present for the pristine film, indicative of singlet energy
transient signal was detected at room temperature within our
transfer from P(T12NpT12) to PCBM. This observation is
time resolution, although a similar, broad, short-lived absorption
consistent with this polymer exhibiting the relatively blue-shifted
transient was observed at 77 K.54
PL maximum of this polymer, resulting in enhanced spectral
PL Quenching of Blend Films. As illustrated in Figure 5,
overlap with PCBM absorption. The efficient polythiophene
and tabulated in Table 4, the inclusion of 5 wt % PCBM in the
emission quenching observed for all blend films indicates good
(50) Chen, T.-A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233- blending of polymer and PCBM species, such that exciton
244. diffusion to the polymer/PCBM interface is not a limiting factor
(51) Kraabel, B.; Moses, D.; Heeger, A. J. J. Chem. Phys. 1995, 103, 5102-
5108. for any of the polymers studied herein.
(52) Kim, Y.; Bradley, D. D. C. Curr. Appl. Phys. 2005, 5, 222-226. Transient Absorption of Blend Films. Figure 6a shows
(53) Korovyanko, O. J.; Österbacka, R.; Jiang, X. M.; Vardeny, Z. V.; Janssen,
R. A. J. Phys. ReV. B: Condens. Mater. Phys. 2001, 64, 235122. transient absorption spectra of the amorphous P(T10PhT10)/

J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008 3035

ARTICLES Ohkita et al.

PCBM blend film. The spectrum at 10 µs exhibited an

absorption peak around 700 nm, similar to that observed for
the P(T10PhT10) pristine film as shown in Figure 2. However,
in contrast to the pristine film, the shape of the transient
spectrum varied with time, with the absorption peak shifting
from 700 nm at 10 µs to ∼900 nm for time delays g100 µs,
demonstrating the formation of two distinct transient species
in the blend film. The time evolution of the transient signal
∆OD at 700 nm was well fitted with a sum of a single-
exponential function and a power equation: ∆OD ) A exp(-
t/τ) + Bt-R. The monoexponential lifetime was τ ) 8 µs under
Ar atmosphere and significantly shortened under O2 atmosphere,
similar with the lifetimes of the 3P* formed in the P(T10PhT10)
pristine film. This fast, monoexponential phase is therefore
assigned to the decay of P(T10PhT10) triplet excitons. Remark-
ably the magnitude of this initial triplet signal (see Figure 6b)
Figure 7. (a) Transient absorption spectra of the P(T0T0TT16)/PCBM blend
is not significantly quenched in the blend compared to the film at 0.3, 0.5, and 1 µs after the laser excitation from top to bottom. (b)
pristine film, despite the strong emission quenching. The power- Transient absorption decays of the P(T0T0TT16)/PCBM blend film monitored
law decay is characteristic of the bimolecular charge recombina- at 960 nm under Ar (black line) and O2 (gray line) atmospheres at room
tion of long-lived, dissociated charged species.55,56 The exponent temperature.
in the power equation was R ) 0.3, which is similar to that
reported for the poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-
p-phenylenevinylene) (MDMO-PPV) and PCBM blend sys-
tem.57 The subunity value for R (R would be expected to equal
1 for ideal bimolecular recombination) has been discussed
previously in terms of polaron trapping in a distribution of
energetic traps in the polymer.55-57 Indistinguishable power-
law decay dynamics were observed under O2 atmosphere,
confirming that this power-law decay cannot be assigned to
triplet excitons. We note, however, that the amplitude of this
power-law transient decreased by ∼30% under the O2 atmo-
sphere. This indicates a reduction in the yield of charge separated
polarons under the O2 atmosphere. The significance of this
observation will be discussed below.
Analogous transient absorption data were obtained for the
other amorphous polymer/PCBM blend films, although the Figure 8. (a) Transient absorption spectra of the P3HT/PCBM blend film
magnitudes of the triplet and polaron transient absorption signals at 1, 2, 3, 5, and 8 µs after the laser excitation from top to bottom. (b)
varied significantly between polymers. For the P(T12NpT12)/ Transient absorption decays of the P3HT/PCBM blend film monitored at
PCBM blend film, PCBM triplet rather than polymer triplet 1000 nm under Ar (black line) and O2 (gray line) atmospheres at room
temperature.
formation was observed.37 As we have discussed in ref 37, the
high polymer/PCBM triplet yields observed for these blend As shown in Figure 7, this initial transient absorption decayed
films, in conjunction with the strong emission quenching, rapidly in less than 1 µs and left a residual long-lived transient
indicate that the triplet states do not originate from direct which extended up to milliseconds. This suggests that there are
intersystem crossing from the polymer singlet exciton. They two independent transient species. The transient decay at 960
are assigned instead to the products of geminate recombination nm could be approximately fitted with a sum of a single-
of triplet BRP states. A detailed model for this behavior is exponential function and a power equation: ∆OD ) A exp(-
discussed below. For both the P(T12NpT12) and P(T8T8T0) blend t/τ) + Bt-R. From the fitting, the lifetime τ and the exponent R
films, the amplitude of the power-law decay assigned to were evaluated to be τ ≈ 0.1 µs and R ) 0.4, respectively,
dissociated polarons was an order of magnitude lower than for suggesting that there are two independent decay pathways. As
the P(T10PhT10) blend films, as detailed in Table 4, and shown in Figure 7, no changes in the decay dynamics were
indicative of large differences in polaron generation yield observed under O2 atmosphere. Thus, neither of these transient
between the three polymers. species can be assigned to triplet excitons.
Turning now to the more crystalline polymers, P(T0T0TT16)/ Blend films for the other more crystalline polymers P(T12T12-
PCBM blend films exhibited a broad transient absorption signal TT0), P(T0TT16), and P(T12SeT12) all exhibit biphasic decays
around 900-1000 nm, similar to, but significantly enhanced in analogous to those shown for the P(T0T0TT16)/PCBM blend
amplitude from, that observed for the P(T0T0TT16) pristine film. film. The amplitude of the power-law decay phase which is
assigned to dissociated polarons varied by an order of magnitude
(54) Cook, S. Ph.D. Thesis, Imperial College London, London, 2006. between polymers, as tabulated in Table 4, and indicates large
(55) Nelson, J. Phys. ReV. B: Condens. Mater. Phys. 2003, 67, 155209.
(56) Nelson, J.; Choulis, S. A.; Durrant, J. R. Thin Solid Films 2004, 451-452, variations in charge dissociation yields between these polymers.
508-514. As shown in Figure 8, the P3HT/PCBM blend film exhibits
(57) Nogueira, A. F.; Montanari, I.; Nelson, J.; Durrant, J. R.; Winder, C.;
Sariciftci, N. S.; Brabec, C. J. Phys. Chem. B 2003, 107, 1567-1573. the largest transient absorption signal of the blend films
3036 J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008
Photophysics of Polythiophene/PCBM Blend Films ARTICLES

obtained where necessary by extrapolation from the power-law

decay observed at the longer time delays to avoid contribution
to the signal from triplet excitons. The 1-µs time delay was
selected to be on the same time scale as charge collection in
P3HT/PCBM PV cells.59 The probe wavelength was selected
to correspond to approximately the polaron absorption maximum
for the films studied, with contributions from both PCBM anion
and polymer cation. It is remarkable that, despite similar
emission quenching for all seven polymers, the yield of this
transient absorption signal assigned to long-lived polarons varies
by over 2 orders of magnitude between polymer blends studied.
Figure 9. Transient absorbance at 1 µs of the blend films plotted against
polymer IPs. The absorbance was corrected for variation in the absorption This range is much greater than any residual uncertainties (for
at the excitation wavelength and an extrapolated value at 1 µs estimated example variations in P+ extinction coefficient) in relating this
from the power-law decay at the longer time domain, measured at a probe ∆OD signal magnitude to charge generation yield and therefore
wavelength of 1000 nm. The open triangles and squares represent data for strongly indicates large variations in charge generation yield,
the amorphous polymers and the more crystalline polymers, respectively.
The arrows below data points indicate upper limit values. ηCS, within this polymer series. P3HT/PCBM blend films
exhibited the highest yield of long-lived polarons, consistent
investigated in this paper. The transient spectrum exhibits a with its well-documented efficient device performance. More
broad transient absorption signal over the wavelength range surprisingly, no correlation is apparent between ∆OD signal
studied with an absorption maximum at ∼1020 nm. Decays magnitude and IP for all the polymers used in this study. It is
measured at 700 and 1000 nm obeyed a power law over the furthermore apparent that the charge generation yield does not
entire measured time range and were well fitted with a single correlate strongly with polymer crystallinity, with the amorphous
power-law component with an exponent R ) 0.7, suggesting polymers exhibiting a range of charge generation yields similar
that there is only one decay process in the blend film. to those of the more crystalline polymers. In contrast, further
Furthermore, the transient signal was not quenched under O2 analyses based upon estimates of the free energy difference for
atmosphere. Therefore, the decay was assigned to the bimo- charge separation do indicate a significant correlation with
lecular recombination of charged species trapped in the blend charge generation yield, as we discuss in detail below.
films, as we have discussed previously. We note that the Discussion
exponent of R ) 0.7 is much larger than that reported for other
polymer/PCBM blend films, including P3HT/PCBM films at We first consider the fundamental photophysics and photo-
higher PCBM concentrations,17,57,58 and approaches the value chemistry of the polymer films addressed in this study, before
of unity corresponding to trap-free (nondispersive) bimolecular moving onto consideration of the relevance of these observations
charge recombination. Thus, the large exponent suggests that to PV device performance.
there are less or shallower trap sites in the P3HT/PCBM Photophysical Characterization of Pristine Films. We start
compared with other blend films. The fitting parameters obtained off our discussion by considering the difference between the
are summarized in Table 4. photophysics of the amorphous pristine polymer films P(T10-
The amplitude of the power-law decays varied sublinearly PhT10), P(T12NpT12), and P(T8T8T0) and that of the more
with excitation density (see Supporting Information) for all the crystalline polymers P(T12T12TT0), P(T0T0TT16), P(T0TT16),
polymer/PCBM films studied, as we have reported previously P(T12SeT12), and P3HT. For the pristine amorphous polymer
for MDMO-PPV/PCBM blends.58 This sublinear behavior has films, triplet excitons were observed as the main transient species
been assigned to the effects of trap filling.58 However, the generated by optical excitation, with a lifetime of several
variation of power-law amplitude between polymers was microseconds. For the more crystalline polymers, on the other
maintained for all excitation densities with, in fact, the largest hand, short-lived (<1 µs) polarons were observed as the main
variation being observed at low excitation densities (for reasons transient species. Herein we discuss the difference in the
of signal-to-noise, the data shown in Table 4 were obtained for transient species on the basis of the molecular structure of the
excitation densities in the range 20-50 µJ cm-2). main chain of the polymers as illustrated in Figure 2.
The amorphous morphology of the first polymer group is
The key parameter of this investigation that is likely to impact
attributed to their twisted backbones which results in a short
PV device performance is the yield of long-lived, dissociated
conjugation length of the main chain, consistent with the blue-
polarons which are required for photocurrent generation. The
shifted absorption band, larger IP, and low carrier mobility.
yield of these polarons (ηCS) can be most readily quantified by
Shorter conjugation lengths are likely to result in more localized
the amplitude of the power-law decay assigned to the bimo-
excitons, consistent with the larger Stokes shift ∆EStokes. This
lecular recombination of these dissociated charges. Figure 9
increased exciton localization is likely to result in a larger energy
shows a graph of the ∆OD amplitude of this power-law transient
gap between the lowest singlet and triplet excitons ∆EST.60,61
absorbance plotted against the polymer IP for the blend films,
As a result, triplet excitons can be expected to be the lowest
monitored at 1 µs and a probe wavelength of 1000 nm. The
excited-state to which photoexcitations can finally relax.
amplitude has been normalized for variations in the optical
Furthermore, the twisted conjugated backbone may enhance the
densities between blend films. The amplitude at 1 µs was
(59) Shuttle, C.; O’Regan, B. C.; Durrant, J. R. Unpublished.
(58) Montanari, I.; Fogueira, A. F.; Nelson, J.; Durrant, J. R.; Winder, C.; Loi, (60) Seixas de Melo, J.; Silva, L. M.; Arnaut, L. G.; Becker, R. S. J. Chem.
M. A.; Sariciftci, N. S.; Brabec, C. Appl. Phys. Lett. 2002, 81, 3001- Phys. 1999, 111, 5427-5433.
3003. (61) Scholes, G. D.; Rumbles, G. Nat. Mater. 2006, 5, 683-696.

J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008 3037

ARTICLES Ohkita et al.

intersystem crossing rate between singlet and triplet excitons; thiophenes such as those studied here suggest that energy
correlated quantum-chemical calculations have demonstrated transfer from 1P* to 1PCBM* is likely to be less efficient for
that the spin-orbit coupling in oligothiophenes increases with this polymer class compared to other more emissive polymers
increase in the twist angle between adjacent thiophene rings.62 such polyfluorenes.39 In either case, the emission quenching is
Thus, we conclude that the efficient triplet formation observed attributed to the generation of a high yield of charged species,
here for the more amorphous polymers is attributable to the P+ and PCBM-.
twisted backbone of the amorphous polymers. We turn now to consideration of the energetics of the exciton
On the other hand, the higher crystallinity of the second and polaron states in these polymer blend films. For the
polymer group is attributed to their relatively planar backbones, amorphous polymers, as mentioned before, the 1P* energy, ES,
resulting in a longer conjugation length of the main chain. This estimated from the absorption and emission spectra, is 2.5-2.7
is consistent with the red-shifted absorption band, smaller IP, eV above the ground state. For these polymers, the triplet energy
and high carrier mobility. In contrast to the amorphous polymers, level ET of 3P* is estimated to be in the range 1-1.7 eV,
excitons will be more delocalized in the partially crystalline assuming a value for ∆EST close to that of oligothiophenes (1-
polymers, consistent with the smaller ∆EStokes and indicative 1.5 eV),60 compatible with their short conjugation length. Similar
of a smaller singlet-triplet exciton energy separation ∆EST. values of ∆EST have been reported for various conjugated
Furthermore, the planar conjugated backbone, favoring inter- polymers except for a ladder-type polyfluorene (MeLPPP) with
planar interaction leading to π stacking in the conjugated rigid and planar backbone structures.63 For the polymers P(T10-
backbone, can be expected to reduce the intersystem crossing PhT10) and P(T8T8T0), our observation of polymer triplet
rate between singlet and triplet excitons. This π-stacking formation in the blend films indicates that their triplet energies
structure can be expected to reduce this intersystem crossing are below that of PCBM (ET ≈ 1.5 eV),64,65 while for P(T12-
rate as twist motions enhancing the spin-orbit coupling are NpT12), the observation of PCBM triplet rather than polymer
effectively suppressed in the π-stacked main chain. Indeed, no triplet formation in the blend indicates that the P(T12NpT12)
triplet formation has been observed in a highly ordered triplet state lies higher in energy than the PCBM triplet state.
regioregular P3HT film.53 Rather, such π stacking can be On the basis of the polymer IPs (Table 1) and PCBM electron
expected to favor interchain charge separation, consistent with affinity (EA ) 3.7 eV),66,67 the energy of the charge separation
the low PL intensity and the transient absorption data reported states P+/PCBM- for these amorphous polymers can be
here and with previous reports of polaron generation in pristine estimated to be in the range 1.7-1.9 eV. Thus, we conclude
regioregular P3HT films.53 We note that the short lifetime of that the 3P*/3PCBM* states are lower in energy than the charge
the photogenerated charges in these pristine films suggests that separated states for these amorphous polymers.
these charges most probably decay via geminate rather than The photophysical data reported herein for the polymer/
bimolecular recombination. The small transient signals observed fullerene blend films can be most readily understood in terms
are indicative of rapid geminate recombination within the of a two-step model for charge dissociation, proceeding via the
instrumental response function. formation of interfacial BRP state, analogous to recent models
Photophysical Processes in Amorphous Blend Films. In proposed for polymer/polymer blend films based on the
this study, the low PCBM concentration employed in the blend observation of exciplex-like emission.29,35 Such “intermediate”
films allows us to assume that the initial photogenerated excited- charge transfer (CT) states have also recently been reported in
state in these films is still the polymer singlet exciton 1P* (the a photophysical study of small band gap copolymers blended
PCBM exhibiting negligible absorption at the excitation wave- with PCBM.68 As we have discussed previously,37 our observa-
length). The efficient emission quenching observed for all blend tion of triplet exciton formation in the presence of quenching
films allows us to conclude that the 1P* states are efficiently of singlet exciton provides strong evidence for the relevance of
quenched by the addition of only 5 wt % PCBM to the polymer this model to polythiophene/PCBM blend films. This model,
film. As such, our discussion focuses not on the diffusion of for the amorphous polymers, is illustrated in Scheme 1. The
excitons to the polymer/PCBM interface but rather upon the initial charge separation results in the formation of a BRP state.
efficiency of charge photogeneration once excitons reach this The Coulombic attraction of these BRPs (∼100-400 meV,
interface. The addition of this low concentration of PCBM is depending upon estimates of the spatial separation of polarons
unlikely to alter the intersystem crossing or internal conversion across the polymer/PCBM interface)69-71 results in a significant
decay rates from the 1P* states, consistent with there being no
(63) Monkman, A.; Burrows, H. D. Synth. Met. 2004, 141, 81-86.
measurable change in polymer absorption spectra. Rather this (64) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc. 1995,
emission quenching is assigned to the generation of polarons, 117, 4093-4099.
(65) Guldi, D. M.; Hungerbühler, H.; Carmichael, I.; Asmus, K.-D.; Maggini,
P+ and PCBM-. From the data we have obtained, we are unable M. J. Phys. Chem. A 2000, 104, 8601-8608.
to distinguish between direct charge separation from the 1P* (66) Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Fromherz, T.;
Rispens, M. T.; Sanchez, L.; Hummelen, J. C. AdV. Funct. Mater. 2001,
and Förster energy transfer from 1P* to 1PCBM* (see ref 39) 11, 374-380.
followed by charge separation from the PCBM singlet state. (67) Mihailetchi, V. D.; Blom, P. W. M.; Hummelen, J. C.; Rispens, M. T. J.
Appl. Phys. 2003, 94, 6849-6854.
However, we note that, except for the P(T12NpT12)/PCBM blend (68) Hwang, I.-W.; Soci, C; Moses, D.; Zhu, Z.; Waller, D.; Gaudiana, R.;
Brabec, C. J.; Heeger, A. J. AdV. Mater. 2007, 19, 2307-2312.
film, the absence of any detectable 1PCBM* emission indicates (69) The Coulombic binding of the initially formed BRPs can be estimated to
that any 1PCBM* states formed by such energy transfer are be 410 meV, assuming a relative dielectric constant of 3.5 (an average
value for P3HT,  ) 3 (ref 70), and PCBM,  ) 4.1 (ref 71)) and spatial
rapidly quenched by charge separation from this state. We separation of 1 nm. Larger spatial separations of 2 or 3 nm would give
furthermore note that the relatively low PL yields for poly- binding energies of ∼210 and 140 meV, respectively.
(70) Chiguvare, Z.; Dyakonov, V. Phys. ReV. B: Condens. Mater. Phys. 2004,
70, 235207.
(62) Beljonne, D.; Shuai, Z.; Pourtois, G.; Bredas, J. L. J. Phys. Chem. A 2001, (71) de Haas, M. P.; Warman, J. M.; Anthopoulos, T. D.; de Leeuw, D. M.
105, 3899-3907. AdV. Funct. Mater. 2006, 16, 2274-2280.

3038 J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008

Photophysics of Polythiophene/PCBM Blend Films ARTICLES

Scheme 1. Energy Diagram for Charge Formation via BRPs On the basis of the data presented herein, the approximate
Proposed for the Amorphous Polymer/PCBM Blend Filmsa
time scales of the processes determining charge photogeneration
at the polymer/PCBM interface are summarized in Scheme 1.
The charge separation time was estimated to be at least <100
ps from the >70% quenching efficiency and a luminescence
lifetime of a few hundred picoseconds reported for poly-
thiophenes.52 This rapid charge formation is consistent with
previous transient studies.77,78 Charge separation is assumed to
result in the formation of Coulombically BRP states. Subsequent
a The thick arrow from 3P* represents our observation of T f T
1 n decay of these BRPs in <100 ns can either result in geminate
absorption decay in transient absorption measurements. The dotted arrow charge recombination or charge dissociation to free charges.
1
represents quenching processes by oxygen molecules. The BRP state is
located slightly higher than the 3BRP state because of the substantial Geminate recombination proceeds either to the neutral triplet
exchange interaction 2J. The gray lines represent charge formation via hot exciton or singlet ground state, depending upon radical pair spin
BRP state. state. The high triplet exciton yields observed for the blend
suggest that geminate recombination proceeds primarily to the
potential energy barrier to dissociation of these BRPs. We note
T1 triplet exciton rather than to the S0 singlet ground state,
that this charge dissociation is also associated with a significant
consistent with the expected energetic stabilization of the 3BRP
increase in entropy (due to the large number of sites the polarons
state relative to 1BRP, and the small energy gap between the
can occupy following dissociation) of similar energetic value 3BRP and 3P* favoring a large triplet recombination rate
to the Coulombic attraction,72 such that the overall thermody-
constant.79 On the basis of this model, our observation of large
namics of this dissociation may be largely neutral in terms of
variations in the yield of long-lived polarons between different
free energy. Following dissociation to free charge separated
polymer blend films can be attributed to variations in the kinetic
polarons (CSfr), these polarons are trapped thermodynamically
competition between geminate recombination and BRP dis-
and kinetically by localization in energetic sub-band gap states
sociation to free charges. The origin of this variation is discussed
(CStr), most probably resulting from local variations in polymer
below in terms of thermodynamics.
conformation.21,57 Subsequent bimolecular charge recombination
Our observation that the yield of long-lived polarons for the
is kinetically limited by the kinetics of detrapping and polaron
P(T10PhT10)/PCBM blend films is quenched by ∼30% in the
diffusion back to the polymer/PCBM interface. Scheme 1 also
presence of oxygen is of particular interest. This quenching may
includes the possible presence of thermally unrelaxed BRP states
be due to a direct quenching of the bound radical states by
(BRP*). The potential importance of such states is discussed
molecular oxygen, although this appears unlikely due to the short
in more detail below.
lifetime of these states. More probably this quenching is
In the model illustrated in Scheme 1, the yield of dissociated
associated with the observed oxygen quenching of the triplet
polarons depends upon the kinetics of charge dissociation
excitons (Figures 4 and 6). As such, our observation of a reduced
relative to the geminate recombination of the BRP to ground
polaron yield in the presence of oxygen implies that recombina-
and to the lower-lying neutral triplet exciton. We note that, with
tion to the triplet exciton is reversible, with a significant
favorable charge dissociation kinetics, this model predicts a
probability of thermally activated charge separation from this
significant yield of dissociated polarons, as observed for the
triplet exciton back to the BRPs. A significant yield for this
P(T0T0TT16)/PCBM blend films herein, even when the neutral
thermally activated pathway would be consistent with the
triplet exciton is thermodynamically lower in energy than the
relatively small free energy difference between the neutral triplet
charge separated polarons. This situation contrasts with molec-
excitons and charge separated species estimated for the amor-
ular D-A systems in solution, where rapid relaxation to the
phous polymer series, as discussed above.
thermodynamically most stable state is typically observed, and
We note that triplet and polaron formation closely related to
the long-lived charge species are only observed when the
spin dynamics in the charge separated state has been reported
system’s energetics have been tuned to ensure that the charge
for several systems, including photosynthetic reaction cen-
separated states lie lower in energy than any molecular triplet
ters,80,81 D-A molecules,82,83 polymer/polymer blend films.29,35
states.73-76 Our observation of long-lived charge separated states
In photosynthetic reaction centers, the back electron transfer
in these blend films, even when these states are thermodynami-
produces a triplet state with an unusual spin polarization,
cally unstable with respect to neutral triplet excitons, can be
showing the charge recombination within a weakly coupled,
understood in terms of the kinetics of diffusion-controlled
spin-correlated radical ion pair.80,84 In D-A molecules, a triplet
bimolecular charge recombination. Polaron trapping on localized
formation is observed following the back electron transfer.82
low-energy trap sites within the polymer phase results in the
slow decay dynamics observed for these dissociated polarons, (77) Ai, X.; Beard, M. C.; Knutsen, K. P.; Shaheen, S. E.; Rumbles, G.;
extending up to millisecond time scales (Figure 6). Ellingson, R. J. J. Phys. Chem. B 2006, 110, 25462-26471.
(78) Kraabel, B.; McBranch, D.; Sariciftci, N. S.; Moses, D.; Heeger, A. J. Phys.
ReV. B: Condens. Mater. Phys. 1994, 50, 18543-18552.
(72) Assuming a polaron size of 1 nm3 and a charge density of 1017 cm-3, the (79) Mataga, N.; Chosrowjan, H.; Taniguchi, S. J. Photochem. Photobiol., C
entropy contribution to the free energy of charge dissociation can be 2005, 6, 37-79.
estimated to be kBT ln 104 ≈ 230 meV. (80) Dutton, P. L.; Leigh, J. S.; Seibert, M. Biochem. Biophys. Res. Commun.
(73) van Hal, P. A.; Knol, J.; Langeveld-Voss, B. M. W.; Meskers, S. C. J.; 1972, 46, 406-413.
Hummelen, J. C.; Janssen, R. A. J. J. Phys. Chem. A 2000, 104, 5974- (81) Rutherford, A. W.; Paterson, D. R.; Mullet, J. E. Biochim. Biophys. Acta
5988. 1981, 635, 205-214.
(74) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am. Chem. (82) Wiederrecht, G. P.; Svec, W. A.; Wasielewski, M. R. Galili, T.; Levanon,
Soc. 2000, 122, 6535-6551. H. J. Am. Soc. Chem. 1999, 121, 7726-7727.
(75) Otsubo, T.; Aso, Y.; Takimiya, K. J. Mater. Chem. 2002, 12, 2565-2575. (83) Wiederrecht, G. P.; Svec, W. A.; Wasielewski, M. R.; Galili, T.; Levanon,
(76) Ramos, A. M.; Meskers, S. C. J.; van Hal, P. A.; Knol, J.; Hummelen, J. H. J. Am. Soc. Chem. 2000, 122, 9715-9722.
C.; Janssen, R. A. J. J. Phys. Chem. A 2003, 107, 9269-9283. (84) Wasielewski, M. R. J. Org. Chem. 2006, 71, 5051-5066.

J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008 3039

ARTICLES Ohkita et al.

Scheme 2. Energy Diagram for Charge Formation via BRPs consistent with the absence of the triplet geminate recombination
Proposed for the More Crystalline Polymer/PCBM Blend Filmsa
pathway. For these polymers, the yield of long-lived polarons
will be determined by the competition between geminate
recombination of BRP to ground versus charge dissociation.
We note that for P3HT, the high yield of polaron generation is
indicative of rapid dissociation of the BRP states, consistent
with the absence of any observable geminate recombination
dynamics on the timescales studied for this polymer.
a The 1BRP state is in thermal equilibrium with the 3BRP state. The Previous studies of PPV and polyfluorene-based donor/
gray lines represent charge formation via hot BRP state. acceptor blends29,35,36,88,89 have reported significant red-shifted
exciplex-like emission and CT absorption, suggesting that the
The triplet formation is attributed to both S-T0 and S-T-1 BRP states observed in these systems are radiatively coupled
mixing between the radical ion pair for D-A molecules with a to the ground state. A sub-band gap absorption has also been
short separation distance because of the high value of the reported for P3HT/PCBM blend films by Fourier-transform
exchange integral J while the triplet states are formed by means photocurrent spectroscopy.90 This may be assigned to ground-
of S-T0 mixing at a long distance >2 nm. For compact D-A state CT absorption indicative of the radiative coupling.
dyads, long-lived intramolecular CT states have been reported However, no distinct red-shifted emission was observed up to
and attributed to triplet CT states that do not interconvert rapidly 850 nm for the polythiophene/PCBM blends studied herein,
with the corresponding singlet CT states because of a relatively indicating that the BRP states proposed here are not strongly
large exchange interaction (e.g.: 200-400 cm-1)85,86 between radiatively coupled to the ground state. This lower radiative
donor and acceptor in the CT states. Exchange interactions of coupling may be associated with the relatively low emission
similar magnitude can be expected for the amorphous polymer/ yields of polythiophenes, with the primary decay pathways for
PCBM blends reported herein, resulting in discrete 1BRP and neutral excitons of polythiophenes being nonradiative (e.g.,
3BRP states as shown in Scheme 1.
intersystem crossing or internal conversion to ground). Alter-
Photophysical Processes in More Crystalline Blend Films. natively, it may result from the degree of spatial overlap of the
The absence of polymer triplet formation in the polymer/PCBM electron and hole orbitals of the radical pair (i.e., the degree of
blend films formed with the more crystalline polymers can be quantum mechanical mixing of the CT and neutral exciton
most readily understood by consideration of the energetics of states). This lack of significant emission from the BRP states
the states involved. For the more crystalline polymers, ES was complicates experimental observation of these states, but is un-
evaluated to be ∼2.0 eV from the absorption and PL spectra. likely to qualitatively change their relevance to device function.
Relatively small ∆EST have been reported for conjugated Relation between Charge Photogeneration and Device
polymers with planar backbone structures including highly Photocurrent. As tabulated in Table 4, and illustrated in Figure
ordered regioregular P3OT films (∆EST ) 0.45 eV)87 and the 9, the yields of dissociated charges, P+/PCBM-, as measured
ladder-type MeLPPP in benzene solution (∆EST ) 0.54 eV).63 by the amplitude of the power-law decay phases observed in
Thus, the polymer triplet energy, ET, for these polymers is the transient absorption data, vary by over 2 orders of magnitude
estimated as ∼1.5 eV. The energy of the charge separated states between the various polymers studied. We note that these studies
can be estimated to be in the range ∼1.2-1.6 eV from IP - were undertaken at low PCBM concentrations (5 wt %), too
EA. It can thus be concluded that, in contrast to the amorphous low for efficient PV device function. We moreover note that
polymers, the charge separated state are energetically likely to the amorphous polymers exhibit charge mobilities too low for
be more stable than the polymer and PCBM triplet states. In efficient PV devices. Nevertheless, the variations in charge
other words, in contrast to the amorphous polymers, geminate photogeneration yield reported here can reasonably be expected
recombination of 3BRP states to yield neutral triplet excitons is to be an impact on device photocurrent generation. While a full
likely to be thermodynamically unfavorable, consistent with the analysis of device behavior is beyond the scope of this paper,
absence of triplet formation apparent from our transient absorp- limited device photocurrent data were obtained, employing 1:1
tion data. blend ratios without annealing. The short-circuit photocurrents
Scheme 2 summarizes the kinetic scheme for charge photo- (JSC) obtained are detailed in Table 4. It is apparent that, for
generation for the more crystalline polymers. Due to the the polymers studied, there is an excellent correlation between
relatively high energy of the triplet excitons, geminate recom- charge generation yields, ηCS and JSC. For the amorphous
bination from the BRP state can only proceed to the singlet polymers, the JSC of the P(T10PhT10)/PCBM blend device was
ground state. The ∼100-ns monoexponential polaron decay 1 order of magnitude larger than that of the P(T12NpT12)/PCBM
phase shown in Figure 7 can be most readily assigned to decay blend device, in good agreement with the variation in ηCS
of these BRP states. This contrasts with the amorphous blends, between these two polymers. For the partially crystalline
where the absence of such a decay phase suggests that the BRPs polymers, the JSC of the P(T0T0TT16)/PCBM blend device was
decay within our instrument response (<100 ns). The longer 1 order of magnitude larger than that of the P(T12T12TT0)/PCBM
lifetime of the BRPs for the more crystalline polymer blends is
(88) Hasharoni, K.; Keshavarz-K., M.; Sastre, A.; González, R.; Bellavia-Lund,
(85) Hviid, L.; Bouwman, W. G.; Paddon-Row, M. N.; van Ramesdonk, H. J.; C.; Greenwald, Y. J. Chem. Phys. 1997, 107, 2308-2312.
Verhoeven, J. W.; Brouwer, A. M. Photochem. Photobiol. Sci. 2003, 2, (89) Goris, L.; Haenen, K.; Nesládek, M.; Wagner, P.; Vanderzande, D.; De
995-1001. Schepper, L.; D’Haen, J.; Lutsen, L.; Manca, J. V. J. Mater. Sci. 2005, 40,
(86) Hviid, L.; Brouwer, A. M.; Paddon-Row, M. N.; Verhoeven, J. W.; 1413-1418.
ChemPhysChem 2001, 2, 232-235. (90) Goris, L.; Poruba, A.; Hod’ákova, L.; Vaněček, M.; Haenen, K.; Nesládek,
(87) Sakurai, K.; Tachibana, H.; Shiga, N.; Terakura, C.; Matsumoto, M.; Tokura, M.; Wagner, P.; Vanderzande, D.; De Schepper, L.; Manca, J. V. Appl.
Y. Phys. ReV. B: Condens. Mater. Phys. 1997, 56, 9552-9556. Phys. Lett. 2006, 88, 052113.

3040 J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008

Photophysics of Polythiophene/PCBM Blend Films ARTICLES

blend device, again in good agreement with ηCS. The P3HT/

PCBM blend films gave the highest JSC and the highest value
for ηCS. This correlation is consistent with a recent study of
PPV-based polymer/polymer blends, which observed as inverse
correlation between exciplex-like emission (indicative of sig-
nificant geminate recombination), and PV device efficiency.32
We note that our studies of device performance for this polymer
series are relatively limited, and that other factors (blend com-
position, annealing, electric fields from the electrodes) are also
likely to impact significantly upon device performance (see for
example ref 91). It has been shown, for example, that the
application of electric fields can assist the dissociation of ana-
logous, luminescence BRPs (referred to therein as exciplex) in Figure 10. Transient absorbance at 1 µs of the blend films plotted against
-∆GCSrel estimated as ES - (IP - EA) where ES is the energy level of
polymer/polymer blend films.30 Nevertheless, it does appear that polymer singlet excitons. The absorbance was corrected for variation in
our transient absorption assay of charge photogeneration in blend the absorption at the excitation wavelength and an extrapolated value at 1
films may be a useful indicator of the photocurrent generation µs estimated from the power-law decay at the longer time domain, measured
performance of such blend films in organic PV devices. at a probe wavelength of 1000 nm. The open triangles and squares represent
data for the amorphous polymers and the more crystalline polymers,
Thermodynamic Origin of the Variation in Charge Pho- respectively. The arrows below data points indicate upper limit values. The
togeneration. At the outset of our investigation, we were low charge generation yield for P(T12NpT12) is assigned to singlet energy
expecting that the charge photogeneration yield ηCS would transfer from P(T12NpT12) to PCBM competing effectively with charge
separation.
correlate either with the energetics of charge generation or with
the degree of delocalization of the polymer polaron, and
the efficiency of exciton quenching at the polymer/PCBM
therefore with polymer crystallinity. As we have discussed
interface. Rather it suggests that the efficiency of dissociation
above, we do not observe any clear correlation between charge
of initially formed BRPs is dependent upon ∆GCSrel. This
generation yield and either polymer IP or crystallinity, as
observation can potentially be rationalized in terms of a model
illustrated in Figure 9. Further calculations were then undertaken
proposed by Friend and co-workers for charge dissociation in
to estimate the free energy difference for charge separation and
polymer/polymer blends.30 In their paper, it was proposed that
evaluate its potential correlation with charge generation yield.
exciton quenching at the donor/acceptor interface initially
This free energy difference ∆GCSrel was estimated as the
formed a thermally “hot” BRP state (BRP* in Schemes 1 and
difference between the singlet and triplet exciton energies (ES
2). This thermally “hot” radical pair has a high probability of
or ET) and the energy of the polaron pairs estimated as IP -
dissociation into free polarons. This dissociation, however,
EA. It should be noted that this free energy difference calculation
competes with thermal relaxation of the BRP. Once thermally
does not take into account the Coulombic attraction between
relaxed, the BRP has insufficient energy to overcome the
the polarons (estimated to be 140-400 meV for the initially
Coulombic attraction of the polarons, but rather primarily
formed polarons, see above), and therefore should only be
undergoes geminate recombination. Within this model, the free
regarded as an indication of the relative rather than absolute
energy of charge separation, ∆GCSrel, will influence the extent
magnitude of the free energy driving force for charge separation.
to which the initially formed BRP is indeed thermally “hot”
Details of these calculations are given in the Supporting
(with a large ∆GCSrel corresponding to more thermal energy in
Information. No correlation was observed between ∆GCSrel and
the initially formed BRPs) and could thereby provide a physical
the ∆OD signal magnitude, employing either the PCBM singlet
mechanism to explain the dependence of the yield of dissociated
exciton energy or the PCBM or polymer triplet exciton energies.
charges upon ∆GCSrel apparent from Figure 10. We note such a
However, employing the polymer S1 singlet exciton energies,
description is analogous to previous reports of “hot” exciton
ES, given in Table 2, a correlation between ∆GCSrel ) ES - (IP
dissociation in conjugated polymers.92,93 At present, the limited
- EA) and the ∆OD signal magnitude is observed, as shown
data set presented herein is insufficient to provide unambiguous
in Figure 10. With the exception of the polymer, P(T12NpT12),
confirmation of the validity of this model. Nevertheless, the
it is striking that the charge separation yield appears to show
correlation shown in Figure 10 is indicative of its potential
correlation with ∆GCSrel, suggesting that the charge separation
relevance to charge generation in polymer/PCBM blend solar
yield increases by 2 orders of magnitude for only a 200 meV
cells. Further experiments to test this model are currently in
increase in free energy driving force. The deviation for P(T12-
progress.
NpT12) is attributed to energy transfer from 1P* to 1PCBM*,
lowering the effective energy of the singlet exciton, and therefore The subtlety of the structure-function relationship which
the effective ∆GCSrel consistent with our observation of PCBM appears to determine the charge photogeneration yield can be
singlet exciton emission for P(T12NpT12)/PCBM blend films. most clearly appreciated by consideration of the two polymers
P(T12T12TT0) and P(T0T0TT16). When blended with 5 wt %
The correlation between ∆GCSrel and the yield of long-lived,
PCBM, the polymer luminescence is quenched in both cases
dissociated polarons contrasts with the lack of variation of
by 75-80%. These two polymers have identical polymer
exciton PL quenching between the different polymers. This
backbones and differ only in the location and length of their
observation strongly indicates the observed dependence of
charge generation upon ∆GCSrel does not result directly from
(92) Arkhipov, V. I.; Emelianova, E. V.; Barth, S.; Bässler, H. Phys. ReV. B:
Condens. Mater. Phys. 2000, 61, 8207-8214.
(91) Koppe, M.; Scharber, M.; Brabec, C.; Duffy, W.; Heeney, M.; McCulloch, (93) Basko, D. M.; Conwell, E. M. Phys. ReV. B: Condens. Mater. Phys. 2002,
I. AdV. Funct. Mater. 2007, 17, 1371-1376. 66, 155210.

J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008 3041

ARTICLES Ohkita et al.

alkyl side chains. MOPAC-AM2 calculations for these polymers Oxygen dependence studies furthermore indicate that this
indicated a slightly more planar backbone configuration for geminate recombination pathway to triplet excitons is reversible,
P(T0T0TT16) compared to that for P(T12T12TT0). They are consistent with our estimates of only a small free energy change
synthesized by similar synthetic strategies. They exhibit similar for this recombination reaction. The large variation in charge
charge carrier mobilities and IPs. They differ only in a small generation yield between the different polythiophenes appears
red-shift of the absorption and PL bands of P(T12T12TT0), to correlate with estimates of the energy difference ∆GCSrel
corresponding to a ∼0.1 eV shift in the energy of their singlet between the polymer singlet exciton ES and the dissociated
excitons. Despite these similarities, their charge photogeneration polarons (as given by IP - EA). This correlation can be most
yields in the blend with 5 wt % PCBM differ by over an order readily understood in terms of the efficiency of dissociation of
of magnitude. This difference in charge photogeneration yield the BRPs being dependent upon the thermal energy of the
was found to be reproducible between different polymer batches. initially formed BRPs, with a large ∆GCSrel resulting in the
Further studies of the P(TnTnTT0) polymer series for alkyl chain initially formed BRPs being thermally hotter, and therefore
lengths up to 16 observed no significant dependence of charge exhibiting a higher charge dissociation yield. These observations
generation yield upon alkyl chain length (data not shown). suggest that, at least for the polythiophene/PCBM blend films
Rather it seems more likely that this difference in charge studied herein, the minimum free energy difference required to
generation performance derives either from the small difference achieve efficient charge dissociation is significantly larger than
in singlet exciton energy, and its resultant influence upon the that required to achieve exciton quenching at the polymer/PCBM
free energy of charge separation, or upon some residual interface. This in turn has important implications for energy
dependence of backbone planarity. level requirements, and specifically LUMO level offset, required
to achieve further advances in PV device performance.
Conclusions
The most striking conclusion from this study is that, despite Acknowledgment. We thank Jessica J. Benson-Smith for
the efficient PL quenching observed for all the polythiophene/ helpful discussions and Dr Tracey Clarke for assistance with
PCBM blends studied, the yield of long-lived, dissociated the PL data for P(T12NpT12). This work was supported by DTI
polarons varies by 2 orders of magnitude depending upon the technology program renewable energy polymer photovoltaics
polythiophene employed. This observation clearly indicates PL project. H.O. is grateful for support from Science Exchange
quenching is not a reliable indicator of dissociated charge Program between the Japan Society for the Promotion of Science
generation in such blend films. This observation can be most (JSPS) and the Royal Society.
readily rationalized in terms of a two-step model for charge
dissociation. Exciton quenching at the polymer/PCBM interface Supporting Information Available: Details about X-ray
initially results in the generation of Coulombically BRP states diffraction measurements of some pristine films, intensity
analogous to the exciplex-like states reported previously for dependence of transient absorption decays for P(T10PhT10)/
PPV-based blend films. Dissociation of these BRPs is in kinetic PCBM, P(T0T0TT16)/PCBM, and P3HT/PCBM blend films,
competition with their geminate recombination either to ground calculations of the free energy difference -∆GCSrel, dependence
or to neutral triplet excitons. This model is in particular of the ∆OD signal magnitude on -∆GCSrel employing either
supported by our observations of high polymer and PCBM triplet the PCBM singlet exciton energy or the PCBM or polymer
yields in the blend films. The strong quenching of the polymer triplet exciton energies, and a full list of authors for ref 1. This
singlet exciton emission and the lack of observation of any material is available free of charge via the Internet at
PCBM emission strongly indicate that these triplets cannot http://pubs.acs.org.
originate from intersystem crossing from singlet excitons, but
rather from geminate recombination from triplet BRP states. JA076568Q

3042 J. AM. CHEM. SOC. 9 VOL. 130, NO. 10, 2008