Fast photoresponse in organic semiconductors: understanding the
mechanisms and structure-property relationships
O. Ostroverkhovaa, S. Shcherbynaa, D. G. Cookea, R. Egertona, R. R. Tykwinskib, J. E. Anthonyc,
F. A. Hegmann*a
a
Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2J1
b
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
c
Department of Chemistry, University of Kentucky, Lexington, Kentucky USA 40506-0055
ABSTRACT
We present a comprehensive study of ultrafast time-resolved photoconductivity in pentacene and functionalized
pentacene single crystals and thin films measured using optical pump-terahertz probe technique. By investigating the
wavelength and temperature dependence of the transient photoconductivity, we reveal a sub-picosecond wavelengthindependent charge carrier photogeneration and band-like charge transport in both single crystal and thin film samples.
The amplitude and decay dynamics of the photoconductivity transients are correlated with the morphology of the films,
assessed by atomic force and electron microscopy.
Keywords: Organic semiconductors, pentacene, molecular electronics, transient photoconductivity, terahertz
spectroscopy
1. INTRODUCTION
Organic semiconductors have gained considerable interest due to their potential applications in molecular electronics
and photonics.1,2 Organic thin films are technologically attractive due to their easy processing, low cost and potentially
tunable properties. In order to develop organic electronic devices, it is important to clarify the mechanisms of
conductivity in organic materials. However, using device structures to characterize a material’s intrinsic electronic
properties is complicated due to the effects of metal-organic interfaces at contacts, device geometry and defects in the
overall electronic response of the device. As a result, despite substantial experimental efforts, intrinsic semiconductor
band transport characterized by carrier mobility increasing with decreasing temperature over a wide temperature range
has not yet been observed in organic thin films,2 except those containing radical ion salts.3 In particular, most studies
carried out on polyacene thin films reported thermally-activated charge transport (mobility increasing with
temperature)4,5 or, in exceptional cases, temperature-independent mobility.6
Recently, ultrafast techniques that employ terahertz (THz) pulses for assessing electronic properties of materials have
been developed.7 In particular, optical pump – THz probe time-resolved techniques allow probing of the transient
photoconductivity with sub-picosecond (sub-ps) time resolution and, therefore, represent a sensitive non-contact tool for
studying the dynamics of mobile charge carriers in materials before they are trapped at defect sites. Optical pump – THz
probe techniques have been widely utilized in studies of ultrafast carrier dynamics in inorganic semiconductors,
quantum dots, superconductors, liquids, and insulators,8-14 and more recently in organic single crystals15,16 and
semiconducting polymers.17
In this paper, we report on photoexcited carrier dynamics on sub-ps time scales in polyacene thin films studied by
optical pump–THz probe time-resolved spectroscopy. We measure the wavelength and temperature dependence of the
transient photoconductivity in pentacene and functionalized pentacene thin films which reveal for the first time bandlike transport in polyacene thin films. We also compare the transient photoconductive response in thin films to that
observed in single crystal samples of functionalized pentacene. In addition, we correlate the structure and morphology of
*
hegmann@phys.ualberta.ca; phone: 1 780 492-7852; fax 1 780 492-0714
thin films revealed by atomic force and scanning electron microscopy with linear optical absorption spectra and transient
photoconductivity.
2. EXPERIMENTAL
2.1 Materials
In our studies, we used thin films of regular pentacene (Pc) and pentacene derivative functionalized with the
triisopropylsilylethynyl (TIPS) side groups (FPc).18,19 Pc was obtained from Aldrich and used without further
purification. All Pc and most FPc films were prepared on mica, glass or KCl substrates by thermal evaporation of the
corresponding powder heated to 200-250 ºC in high vacuum of 10-6-10-7 Torr at a deposition rate of 0.3 Å/s. Some
relevant information about the films discussed in this paper is given in Table 1, where different samples of the same
batch (prepared on the same substrate under the same deposition conditions) are presented as, for example, Pc 1, 1’.
During the vacuum deposition of all Pc films (Pc 1-4) and FPc films 1 and 3, the substrate was maintained at room
temperature (25 ºC), while the FPc 2 film was obtained at the substrate temperature of 85 ºC. FPc 4 film was cast from
THF solution on a glass substrate at room temperature. The thickness of the films was measured with a profilometer
(Tencor Instruments) and ranged between 150 nm and 1.3 m (Table 1). FPc single crystal samples were grown from
solution and typically had dimensions of 2 mm x 4 mm and a thickness of 0.3-0.5 mm;14,18,20 8 samples were studied and
yielded similar results.
It is well-known that polycyclic aromatic hydrocarbons including pentacene and its derivatives can be unstable in the
presence of light and oxygen. 21,22 Although we did not observe any photobleaching and degradation in the Pc films, the
FPc films showed signs of partial photobleaching under ambient light within several months. Therefore, all the
measurements reported here were carried out using freshly prepared samples that were stored in the dark during the
experiments, so that no day-to-day variation in optical and photoconductive properties of studied samples was observed.
Furthermore, many FPc thin films slowly degraded under pulsed laser illumination, with the transient photoconducitivity
signals reduced to about 80% of their initial values, obtained in a fresh sample, after ~15-30 min of pulsed illumination
with the laser fluences typically used in our experiments (Sec.2.4), depending on the film. In order to avoid this problem,
the pulsed illumination of the same spot on a FPc film was limited to ~2-10 min, depending on the film, after which the
sample was translated to a new region. Interestingly, more crystalline FPc films exhibited much slower photodegradation
than the amorphous ones, and in the cases of the film FPc 4 and FPc single crystal samples, we did not observe any
photodegradation of the photoresponse even after many hours of pulsed illumination. In addition, all the FPc crystals, the
FPc 4 film and all of the Pc films were very stable with respect to the photo-oxidation in air under ambient light,
exhibited the same values of photoconductivity when exposed to air or kept in vacuum, and did not show any signs of
degradation over a period of at least 9 months thus far.
Table 1. Thin film and single crystals samples used in our study.
Sample
Pc 1,1’
Pc 2
Pc 3,3’
Pc 4
FPc 1
FPc 2
FPc 3,3’
FPc 4
FPc crystals
Preparation
method
evaporation
evaporation
evaporation
evaporation
evaporation
evaporation
evaporation
solution
solution
Substrate
mica
KCl
mica
glass
mica
mica
mica
glass
--
Substrate
temperature (ºC)
25
25
25
25
25
85
25
25
--
Thickness (m)
 (cm2/(Vs))*
0.25
0.3
0.15
0.25
0.52
0.8
0.33
1.3
300-500
0.02-0.03
0.03
0.03-0.04
0.03
0.01
0.06
0.02-0.03
0.06
0.15-0.2
*The product of mobility () and photogeneration efficiency () was obtained from the measurements of transient photoconductivity
by optical pump-THz probe time-resolved technique (Sec. 2.4 and 3.4).
2.2 Optical absorption
For measurements of the optical absorption spectra of the films, the light from a tungsten source was coupled to a fiber
and was at normal incidence to the samples. The light transmitted through the samples was collected by another fiber and
analyzed using a spectrometer (Ocean Optics). All spectra obtained from the thin film samples were referenced to those
of the corresponding substrates.
2.3 Morphology characterization
The atomic force microscopy (AFM) images were obtained using Digital Instruments Multimode AFM in a tapping
mode, with Si cantilevers oscillating at a frequency of 100-300 kHz. The lateral resolution was about 15 nm.
The secondary electron micrographs were obtained using Hitachi S-4800 (cold tip field emission) scanning electron
microscope (SEM). No conductive coating was applied to a sample surface. The electron beam was normally incident on
the sample, and the accelerating voltage was 1.5-3 kV.
2.4 Optical pump – THz probe time-resolved experiment
A detailed description of the optical pump-THz probe experimental setup was reported elsewhere. 13,14 Briefly, an
amplified Ti:sapphire laser source (800 nm, 100 fs, 1 kHz) was used to generate optical pump pulses in the wavelength
region of =400-800 nm through various wave-mixing schemes as well as THz probe pulses generated via optical
rectification in a 0.5 mm-thick ZnTe crystal. The samples were mounted on 1-1.5 mm apertures in an optical cryostat
(sample in vapor) in the temperature dependence studies and in air in all other studies, and both the THz probe and
optical pump pulses were at normal incidence to the surface of the films or the a-b plane of the FPc crystal samples. The
electric field of the THz pulse transmitted through the samples was detected by free-space electro-optic sampling in a 2mm-thick ZnTe crystal and monitored as a function of delay time (t) with respect to the optical pump pulse. The range
of optical pump fluences employed in our experiments was 0.4-1.5 mJ/cm2. In the approximation of a thin film on an
insulating substrate, the differential transmission (-T/T0, where T0 is the amplitude of the THz pulse transmitted through
unexcited sample) due to optical excitation of mobile carriers at small |T/T0| is related to the transient photoconductivity
as follows:13,15,16 ph=(T/T0)(1+N)/(Z0d), where Z0=377  is the impedance of free space, N is the refractive index of
the substrate at THz frequencies, and d is the film thickness. Using this expression, the product of the charge carrier
mobility and photogeneration efficiency is calculated as follows:15,23
h 1  N 
T
,
  
T0 eF 1  exp[ d ] Z 0
where e is the electric charge, h is the Planck constant,  is the light frequency,  is the absorption coefficient, and F is
the incident fluence.
3. RESULTS
3.1 Sub-picosecond photoconductivity
Figure 1(a) shows the electric field (T(t)) of the THz pulse transmitted through a Pc thin film in the absence of optical
excitation at room temperature (solid line; peak value of T0). The spectrum of this THz pulse peaks at ~1 THz and
extends to ~2.5 THz (inset of Fig. 1(a)). Optical excitation of the sample results in a change in the transmitted electric
field (T(t)), as shown in Fig. 1(a) (dashed line), due to the transient photoconductivity (i.e. mobile photocarriers). We
note that the polarization of photogenerated neutral excitons by the THz electric field would result in a phase shift
between the T(t) and T(t).24 However, in the absence of this phase shift, as is the case for all our samples (e.g. Fig.1(a)
and Ref.15), the optically induced relative change in the THz peak amplitude (-T/T0) provides a direct measure of the
transient photoconductivity. Moreover, for |T/T0|<<1 the photoconductivity (ph) is directly proportional to a
differential THz transmission -T/T0.13,14,16
Figure 1(b) illustrates the differential THz transmission (-T/T0) under optical excitation with ~580 nm at an
incident fluence (F) of ~1 mJ/cm2 at room temperature as a function of delay time (t) between the optical pump and
THz probe pulses, obtained in a FPc crystal sample and FPc 3’ and Pc 1’ thin films. The inset of Fig. 1(b) shows the
onset of the photoresponse, normalized to its maximum value at t=0, and reveals a fast photogeneration process for
mobile carriers with characteristic times below ~400 fs limited by the time resolution of our setup in all our samples.
The decay dynamics of the transient photoconductivity yield information about the nature of charge transport, trapping
and recombination.25-27 In FPc crystals, the transient photoconductivity exhibited a fast initial decay during the first few
picoseconds, followed by slow decay best described by a power law function (t-), attributed to dispersive transport
(discussed below).15,27 Photoconductivity decay dynamics in both FPc and Pc films depended on the film structure and
morphology and ranged between fast single-exponential (~exp[-t/]) with time constants ~0.5-1.5 ps (e.g. Fig. 1(b)),
most likely due to deep-level traps at the interfaces between crystallites, and longer power law (t-) with ~0.5 (Sec.
3.4).
From the maximum of the differential transmission (-T/T0), the product of charge carrier mobility and
photogeneration efficiency can be calculated (Sec. 2.4). At room temperature and for optical excitation at ~580 nm, we
obtain ~0.19, 0.02 and 0.025 cm2/(Vs) for a FPc crystal, Pc 1’ and FPc 3’ films, respectively.
T(t)x200
1.0
0.5
0.5 1.0 1.5 2.0
Frequency, THz
2.5
3
0.0
T(t)
-0.5
-1.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Time t, ps
16
12
0.0
-0.6
-3
0.0
0.0
(b)
4
20
FPc crystal
Pc 1'
FPc 3'
-0.4
-0.2
0.0
0.2
Delay time t, ps
2
-3
(a)
298 K 1.0
FPc crystal
Pc 1'
FPc 3' 0.5
-T (a.u.)
T0
1.5
-T/T0 (10 ) (films)
0.5
FFT amplitude, a.u.
THz electric field, a.u.
1.0
2.0
8
1
4
0
-T/T0 (10 ) (FPc crystal)
5
1.5
0
0
2
4
6
8
Delay time t, ps
Figure 1. (a) THz electric field (T(t)) transmitted through an unexcited Pc film (solid line) and the change in THz electric field
(T(t)) under optical excitation (dashed line). The inset shows the spectrum of the transmitted THz pulse. (b) Relative change in the
peak of THz transmission under optical excitation with ~580 nm due to transient photoconductivity, obtained in FPc crystals and
FPc 3’ and Pc 1’ thin films. Single-exponential fits to the thin film data with ~0.64 ps and 0.97 ps, respectively, are also shown.
The offset along the y-axis is for clarity. The inset illustrates the 0.4 ps onset of photoconductivity in these systems.
3.2 Wavelength dependence
In order to gain insight into the photogeneration process, we repeated the experiment at various pump wavelengths.
Due to the short temporal width of the optical pulses (~100 fs), the illumination is not monochromatic, as illustrated in
Fig. 2(a), which shows the spectra of the optical pump pulses utilized in our experiment. Typical optical absorption
spectra of the thin film and single crystal samples are shown in Fig. 2(b). No transient photoresponse was obtained from
FPc and Pc thin films upon excitation at 800 nm – the spectral region in which there is too little absorption (Fig. 2(b)).
At all wavelengths of optical excitation within the absorption spectra, we observed a photoinduced change in THz
transmission with a fast onset and decay dynamics similar to that shown in Fig. 1(b). Furthermore,  calculated from
our data is wavelength-independent within our experimental error, as shown in Fig. 2(c).
3.3 Temperature dependence
The temperature dependence of the photoresponse provides valuable information about the mechanism of
photoconductivity.28 In both single crystals and thin films, the transient photoconductivity increased as the temperature
Norm. Intensity
1.0
(a)
0.5
0.0
Absorbance
1.5
FPc crystal
FPc 3
Pc 4
1.0
(b)
0.5
(c)
0.1
FP c crystal
Pc 1
FP c 3
2
 (cm /(Vs))
0.0
0.01
400
500
600
700
800
900
W avelength  (nm )
Figure 2. (a) Normalized intensity spectra of selected optical pump pulses. Spectra of pulses centered at 550 nm and 590 nm are
not included for clarity. (b) Absorption spectra of representative FPc crystal, FPc and Pc thin film samples. (c) Product of mobility
and photogeneration efficiency ) measured in a FPc crystal, and FPc 3 and Pc 1 thin film samples at room temperature.
decreased, as demonstrated in Fig. 3. In particular, Fig. 3(a) shows the photoinduced change in THz transmission (T/T0) as a function of pump-probe delay time (t) obtained at temperatures of 298, 40 and 10 K upon optical excitation
at 400 nm in a FPc crystal sample. Figure 3(b) illustrates -T(t)/T0 obtained in Pc 1’ thin film at temperatures of 298,
120, 80 and 20 K using 580-nm excitation. The photoconductivity decay dynamics did not change appreciably with the
temperature in either films or single crystal samples. At all temperatures studied, the transients obtained from the FPc
crystals exhibited a fast (<1 ps) initial decay followed by slow decay, which persisted over time scales from ~1 ps to at
least ~600 ps and could be fitted with the power law function t-, where  ranged between 0.5 and 0.7 (e.g.=0.55 at
80 K as illustrated in the inset of Fig. 3(a)).15 In the Pc 1’ film, the decay dynamics could be described by a single
exponential (exp[-t/]) throughout the entire temperature range, with a characteristic time constant ~1-1.7 ps.
The temperature dependence of  calculated from the data for four FPc crystal samples, Pc 1 and 1’ thin films, and
FPc 3’ thin film is shown in Fig. 4. As the temperature decreased from 298 K to 5 or 10 K, 
from ~0.2
cm2/(Vs) to ~2-3 cm2/(Vs) in FPc crystals and from ~0.02 cm2/(Vs) to ~0.07 cm2/(Vs) in Pc thin films (with a similar
trend seen in FPc films). It should be emphasized that this band-like mobility which increases as the temperature
decreases (Fig.4) has not been previously observed in polyacene thin films. In addition, even though the band-like
mobility has been obtained in several organic crystals,15,16, 28-31 only a few studies reported the band-like behavior over a
wide temperature range,15,16,28 as observed in our experiments (Fig.4). The thin film data in Fig. 4 can be fitted with the
power law function ~(Tº)-n, with the exponent n~0.32, similar to n~0.27 reported from transient photoconductivity
measurements of Pc single crystals.16 However, in FPc crystals, the temperature dependence over the entire temperature
range can be better described by a single exponential ~exp[-aTº] rather than a power law, as illustrated in the inset of
Fig. 4. The physical reason for this difference in temperature dependence of  between single crystals and thin films is
currently unknown and requires further investigation. However, assuming  is independent of temperature25,32,33 or at
least does not increase as the temperature decreases, the band-like temperature dependence of the mobility obtained in
thin films (Fig. 4) is especially remarkable since no special purification and/or substrate surface-pretreatment that could
lead to improved transport properties34,35 were performed.
2
-3
-T, a.u.
-2
3
=0.55±0.01
0.01
1
1
(b)
5
FPc crystal
80 K
-T/T0 (10 )
4
-T/T0 (10 )
6
FPc crystal (a)
10 K 1
40 K
298 K
-
t
0.1
5
10
100
Delay timet, ps
4
Pc 1'
20 K
80 K
120 K
298 K
3
2
1
0
0
0
5
10
15
20
0
Delay time t, ps
2
4
6
8
Delay time t, ps
Figure 3. Differential transmission of the peak of the THz probe pulse as a function of delay time and temperature for (a) FPc
crystal under optical excitation with F~0.4 mJ/cm2 at 400 nm at temperatures of 298, 40 and 10 K. The inset shows the power
law fit (t-, =0.55) to the data obtained at 80 K, and (b) Pc 1’ thin film under optical excitation with F~1 mJ/cm2 at 580 nm at
temperatures of 298, 120, 80 and 20 K. Single-exponential fits to the data are also shown.
1
FPc crystals
2
, cm /(Vs)
FPc crystals
1
0.1
0.1
0
50 100 150 200 250 300
n=0.32±0.03
FPc and Pc films
0.01
10
100
o
Temperature T , K
Figure 4. Temperature dependence of the product of charge carrier mobility and photogeneration efficiency for several FPc crystal
samples (closed symbols: squares and diamonds – samples 1 and 2 at 800-nm excitation; triangles and circles – samples 3 and 4 at
400-nm excitation), Pc thin films (open symbols: circles – sample 1’ at 580-nm excitation, squares – sample 1 at 400-nm
excitation) and a FPc 3’ film (open triangles at 580-nm excitation). A power law fit (~(Tº)-n with n=0.32) to the data obtained in
the Pc thin film (sample 1) at 580-nm excitation is also shown. The inset shows the same data obtained in FPc crystals re-plotted
on a semi-log scale, where a solid line is a guide for the eye.
3.4 Structure-property relationships
3.4.1 Pentacene films
Atomic force microscopy images of the films Pc 1, 3 and 4 are shown in Fig. 5. All our Pc films exhibited dendritic
grain structure, characteristic for polycrystalline pentacene films,4,36 with an average grain size of ~1-1.2 m (Pc 4),
~0.8-0.9 m (Pc 2) and ~0.7-0.8 m (Pc 1).
Figure 6(a) shows the differential transmission (-T/T0) obtained in films Pc 1, 3 and 4 upon optical excitation with
~1.5 mJ/cm2 at ~580 nm. Despite differences in the grain size of the Pc films grown on different substrates (Fig.5), the
observed sample-to-sample variation in the amplitude and decay dynamics of the transient photoconductivity could not
be attributed to any influence of the substrate, since only slightly different transients could be obtained from the films
grown on different substrates under the same conditions. Moreover, slight differences in the decay dynamics were
observed even in different films grown on the same substrate (e.g. Pc 3 and 3’ in the inset of Fig, 6(a)), which confirms
(c)
(b)
(a)
210
260
0
0
250
0
2 m
Figure 5. Atomic force microscopy images of the Pc films: (a) Pc 4 (on glass); (b) Pc 2 (on KCl); (c) Pc 1 (on mica). Each image
has a dimension of 2x2 m and has a vertical scale given in nm.
that the substrate does not contribute significantly into sample-to-sample variation in our signals. Indeed, the product of
charge carrier mobility () and photogeneration efficiency () calculated from the amplitude of the -T/T0 transients
exhibited similar values of ~0.02-0.04 cm2/(Vs) in all Pc films studied (Table 1) and did not reflect small differences in
the grain size of the films. The transient photoconductivity decay could be described either by the single exponential
(~exp[-t/]) with the time constant ~0.8-1.5 ps (Fig.1(b)) or a bi-exponential (Aexp[-t/1]+Bexp[-t/2]) with the
contribution of the fast decay component with 1~0.6-1.2 ps dominating over that of the slow decay component with
2~5-16 ps with A/(A+B)>0.75, depending on the sample (Fig. 6(a)). Therefore, even in our best Pc films, the deep
trapping, most likely at grain boundaries, prevented observation of the carrier transport on time scales above ~20-30 ps
(inset of Fig. 6(a)). From the bi-exponential fits shown in Fig. 6(a) for the films Pc 1 (on mica), 3 (on mica) and 4 (on
glass), we obtain 1=1.14 ps, 2=16 ps (A/(A+B)=0.83), 1=0.82 ps, 2=12 ps (A/(A+B)=0.86), and 1=0.64 ps, 2=5 ps
(A/(A+B)=0.79), respectively. Although it is not straightforward to relate the charge carrier mobility and a grain size of
the polycrystalline films,4 longer decays of the transient photoconductivity due to fewer grain boundaries that serve as
1.0
Pc 4
Pc 1
Pc 3
4
0.8
-T, a.u.
-3
-T/T0 (10 )
(b)
(a)
5
Pc 3
0.6
Pc 3'
0.4
0.2
3
0.0
0.1
2
1
10
Delay time t, ps
100
1
0
0
5
10
15
20
Delay time t, ps
25
2 m
Figure 6. (a) Differential transmission (-T/T0) as a function of delay time (t) obtained in Pc 1, 3 and 4 thin films under optical
excitation at ~580 nm in air at room temperature. Fits with a bi-exponential function are also shown (parameters are listed in
the text). The transients are offset along y-axis for clarity. Inset shows the -T transients normalized at their values at t=0
obtained under the same conditions in the Pc 3 and 3’ films.
(b) Scanning electron micrograph of the Pc 4 film. The accelerating voltage is 1.5 kV, and the amplification is 25k.
trapping sites would be expected in the films with a larger grain size. However, although the grain size in our Pc films
grown on a glass substrate was slightly larger than in those grown on mica, we did not observe longer decays in the
former case (Fig. 6). We note that since the measured photoresponse is averaged over the circular area of ~1-1.5 mm
diameter (imposed by the size of the aperture – see Sec. 2.4 for experimental details), it could be influenced by the
presence of non-uniform areas that contain small random domains separating the crystallites, as revealed by the scanning
electron micrographs (Fig. 6(b)), which could mask the grain size dependence of the conductivity in our Pc films.
3.4.2 Functionalized pentacene films
The structure and morphology of the FPc thin films were independent of the substrate used, but sensitive to the
preparation conditions and the film thickness. Figure 7 shows the absorption spectra of the dilute solution of the FPc
molecules in THF (solid line), thin film FPc 1 (dash-dotted line) (Fig. 7(a)), and thin films FPc 2, 3 and 4 (Fig. 7(b),
dash-dotted, solid and dashed lines, respectively). As seen from Fig.7(a), the absorption spectrum of the FPc 1 film is
very similar to that of the FPc molecules in solution. The red-shift of the spectrum, commonly observed in polyacenes in
the solid-state phase compared to solution,3,37 was only ~5 nm, which could indicate the dominance of the amorphous
FPc 1
FPc in THF
FPc 2
FPc 3
FPc 4
1.5
(a)
Absorbance
Absorbance
2.0
1.5
1.0
(b)
1.0
0.5
0.5
0.0
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength, nm
Figure 7. Absorption spectra of: (a) FPc inTHF solution (solid line) and FPc 1 thin film (dash-dotted line); (b) FPc 2, 3 and 4 films
(dash-dotted, solid and dashed lines, respectively).
(b)
(a)
168 nm
1 m
300 nm
Figure 8. Atomic force microscopy images of: (a) FPc 1 film; (b) FPc 2 film.
1 m
phase. Indeed, at random molecular distribution, the bulky TIPS side groups 18 act as a “solvent” by preventing the stacking and molecular interaction, which leads to a loss of the “solid-state” features. In contrast, the films FPc 2, 3 and 4
exhibit ~50-70 nm red-shift (Fig. 7(b)) due to efficient molecular orbital overlap leading to enhanced intermolecular
interaction that determine the displacement of the solid-state absorption spectra with respect to that of solution.37
The difference in the FPc film morphology that manifests itself in the optical absorption spectra (Fig.7) can be directly
observed in the AFM images (Fig. 8). For example, the surface topology of the film FPc 1 (Fig. 8(a)) exhibits smooth
featureless domains with sparse crystallites separated by 1-2 m, while that of the film FPc 2 (Fig. 8(b)) shows a much
better crystalline coverage.
(a)
(b)
FPc 4
FPc 2
FPc 3
0.1
0.01
0
0
2
4
Delay time t, ps
FPc 4, =0.53±0.02
-
~t
4
-2
FPc crystal, =0.62±0.02
1
-T, a.u
-3
-T/T0 (10 )
8
6
FPc 2
=0.53±0.02
1E-3
0.1
1
10
Delay time t, ps
100
Figure 9. (a) Differential transmission (-T/T0) as a function of time delay (t) observed in the films FPc 2, 3 and 4. Single
exponential fit (exp[-t/], =1.35 ps) to the data obtained in FPc 3 film as well as power law fits (t-) obtained in FPc 2 and 4
films are also shown. Transients are offset along y-axis for clarity. (b) Log-log plot of the differential transmission data (offset
along y-axis for clarity) obtained in FPc 2 and 4 films and a FPc crystal sample. Power law fits and corresponding values of 
are also included.
(a)
(b)
2 m
2 m
Figure 10. Scanning electron microscopy images of the films: (a) FPc 2, voltage 3 kV, amplification 25k and (b) FPc 3,
voltage 1.5 kV, amplification 25k.
The transient photoconductivity signals obtained in various FPc films differed by the amplitude and decay dynamics,
depending on the structure and morphology of the film. Figure 9(a) illustrates the differential transmission -T/T0
transients for the films FPc 2,3, and 4 obtained at 580-nm excitation. The product of mobility and photogeneration
efficiency () calculated from the amplitude of the -T/T0 transients (Sec.2.4) were 0.06, 0.03 and 0.06 cm2/(Vs) for the
films FPc 2, 3 and 4, respectively (Table 1). The  product obtained from the amplitude of the transient
photoconductivity in the “amorphous” film FPc 1 was 0.01 cm2/(Vs), the smallest value of all FPc films studied, and
the decay dynamics observed in this film could not be analyzed due to insufficient signal-to-noise ratio. Among the
samples FPc 2, 3, and 4, the film FPc 3 showed the fastest decay dynamics described by a single-exponential (exp[-t/])
with ~1.35 ps (Fig. 9(a)). In contrast, in films FPc 2 and FPc 4, slow component of the decay dynamics appeared and
persisted over at least ~100 ps. This component could be best described by the power law function t- with ~0.5 (inset
of Fig. 9(b)). Interestingly, this behavior is very similar to that obtained in FPc single crystals (whose representative
transient photoconductivity decay is also included in Fig. 9(b)), in which the decay dynamics were temperatureindependent and characterized with the power law decay with ~0.5-0.7 (Fig. 3(a)).15,20,23
The difference in the decay dynamics observed, for example, in the films FPc 2 and 3 can be related to their structure.
The SEM scans obtained in these films are shown in Fig. 10(a) and (b), respectively. While the film FPc 3 appears to
contain small randomly interconnected domains, the film FPc 2 consists of larger crystallites (see also Fig. 8(b)), which
could be a reason for longer charge carrier lifetimes manifested in longer power law decays of transient
photoconductivity observed in the film FPc 2 (Fig. 9(b)).
4. DISCUSSION AND CONCLUSIONS
It is conventional to describe charge photogeneration in organic solids by electric-field-assisted38 and temperatureassisted exciton dissociation as described by the Onsager model.3,39 However, the sub-ps, wavelength-independent
photogeneration in the absence of an external electric field and the increase in photoconductivity as the temperature
decreases, as observed in our experiments, indicate that one of the primary photoexcitations in FPc and Pc thin films and
FPc single crystals is mobile charge carriers,15,16,25,33 and not only excitons.3,17,40 On our time scales, the observed
temperature dependence of the photoconductivity in both crystals and thin films is consistent with both a temperatureindependent charge photogeneration efficiency25,32,33 and the nearly small molecular polaron (MP) transport model,
which treats the motion of a charge carrier as a heavy quasiparticle (formed as a result of interaction of a charge carrier
with vibrational modes) with a temperature-dependent effective mass (m*) described by an exponential (m*~exp[aTº]) or
power law (m*~(Tº)b).39 Furthermore, the propagation of the MP through a crystal is described as a multi-step tunneling
process via isoenergetic sites,39 which agrees with the temperature-independent power-law decay27 of the transient
photoconductivity over at least three orders of magnitude in time, as observed in FPc crystals (inset of Fig. 3(a)) and
more crystalline FPc 2 and 4 thin films. In other films, characterized by smaller grain sizes and/or less uniform structure,
fast carrier capture due to deep traps at the grain boundaries results in a single- or bi-exponential decay and prevents
observation of MP transport on time scales >5-30 ps after excitation.
The room-temperature mobility values ( ~0.2 cm2/(Vs) and ~0.02-0.06 cm2/(Vs) for the crystals and thin films,
respectively), estimated from the peak values of transient photoconductivity under assumption of photogeneration
efficiency =1, are within the range of those obtained in similar systems using other experimental techniques.4,34,41
However, most studies reported thermally activated charge transport and, therefore, the “effective” trap-limited mobility
values, whereas our measurements assess “intrinsic” transport, characterized by higher mobility over time scales shorter
than typical carrier trapping times. If <1, which would be the case if the initial photoexcitation resulted in a significant
exciton population density (which the THz probe is not sensitive to in our case) and which would also be further reduced
as a result of partial carrier loss due to recombination and trapping occurring at times <400 fs after the photoexcitation
(shorter than the time resolution of our experiment), then our reported mobility values would be much higher.
In summary, sub-ps wavelength-independent charge photogeneration in FPc single crystals and FPc and Pc thin films
suggests that mobile charge carriers are a primary photoexcitation in these materials. An increase in mobility as the
temperature decreased was observed in all samples, indicative of band-like charge transport. In FPc crystals, charge
carrier decay dynamics were temperature-independent and were characterized by a fast initial decay (<1 ps) followed by
slow power-law decay (>1 ps) due to dispersive charge transport via multi-step tunneling consistent with the nearly
small molecular polaron model. In thin films, the charge carrier dynamics depended on the structure and morphology of
the film and ranged between fast single-exponential (~1 ps) observed in the films with smaller domains due to deep
trapping at the grain boundaries and power-law observed in more uniform FPc films with larger grain sizes.
ACKNOWELEDGMENTS
The authors are grateful to Steven Launspach of National Institute of Nanotechnology (Edmonton, Canada) for
technical assistance with the AFM and SEM characterization. We also thank Hue Nguyen of Nanofabrication Center for
the film thickness measurements. This work was supported by NSERC, CFI, CIPI, iCORE, and ONR. O.O.
acknowledges the Killam Trust for a Killam Memorial Postdoctoral Fellowship.
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