Effect of Acceptor Lamination on Photocarrier Dynamics in Hole

Article pubs.acs.org/JPCC

Effect of Acceptor Lamination on Photocarrier Dynamics in Hole Transporting Hexabenzocoronene Nanotubular Self-Assembly Yusuke Wakikawa,† Tadaaki Ikoma,*,†,‡,⊥ Yohei Yamamoto,§ Takanori Fukushima,∥ Takuzo Aida,¶ and Kimio Akiyama# †

Graduate School of Science and Technology and ⊥Center for Instrumental Analysis, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan ‡ CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan § Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan ∥ Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan ¶ Department of Chemistry and Biotechnology, School of Engineering, and Center for NanoBio Integration, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan # Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: Measurements of transient photoconductance under an external magnetic field were used to investigate photocarrier dynamics in low-dimensional hexabenzocoronene (HBC) self-assemblies, which are a promising material group for highly efficient solar cells achieved by bottom-up technology, and to clarify the effect of lamination with electron acceptor layer on the surfaces of HBC nanotubes. In an HBC column without an acceptor, the carrier generation yield from a geminate electron−hole (e-h) pair is dependent on the external electric and magnetic fields. The time dependence of the magnetic field effect on geminate e-h pair dynamics in the HBC column structure was analyzed to estimate the recombination rate constants of the singlet and triplet e-h pairs (krS and krT), which were 1.5 × 108 and 1.2 × 108 s−1 respectively. The same kinetic analysis with consideration of the electric field effect on the photocarrier generation yield provided an electric field dependent dissociation rate constant in the range of 107−108 s−1 in the HBC column structure. However, neither electric nor magnetic field effects on the carrier generation process were observed in acceptor-appended HBC nanotubes. The disappearance of the external field effects in acceptor-appended HBC indicates that the geminate recombination is reduced substantially by a well-organized donor/acceptor heterojunction with an interval of a few nanometers due to some σ-bonds. However, efficient nongeminate recombination with a ratio of krS:krT = 1.0:0.8 in the acceptorappended HBC nanotubes was also elucidated by the incident photon density and magnetic field effects, which is an inherent nature in materials with high carrier density.

1. INTRODUCTION Bottom-up approach for the development of nanomaterials with controlled structure at a molecular level has attracted much attention due to the technical benefits of scaling down device dimensions. The self-assembly process of a low molecular weight compound is well-known as one of the bottom-up approaches to create new functional nanostructures. Müllen and co-workers have reported that hexabenzocoronene (HBC) attached with long alkyl chains self-assembles into discotic columnar structures, due to π-stacking interactions between the aromatic cores, as shown in Figure 1.1−4 The discotic columns, in which the stacking arrangement of HBC molecules is tilted at close to 45° with respect to the columnar axis, exhibit one-dimensional transport of hole carriers with high mobility,5 and the photovoltaic response of a thin film of HBC doped with perylene dye has been reported.6 Recently, novel molecular-based photoconductors have also been © 2013 American Chemical Society

constructed through the self-assembly process of gemini-shaped amphiphilic HBC (Figure 1).7−10 HBC derivatives that are covalently bonded with electron acceptors such as trinitrofluorenone (TNF) and fullerene (C60) can assemble to form several micrometer long nanotubes with uniform diameter and wall thickness under select conditions, where an electrondonating (D) graphitic bilayer composed of π-stacked HBC is laminated with the electron-accepting (A) layers. A well-defined nanoscale D/A heterojunction is organized in the selfassembled HBC nanotubes, although the formation of charge transfer complexes between the D and A components is favored. The acceptor-appended HBC nanotubes have excellent photoconductive properties with on/off ratios in the range of Received: March 16, 2013 Revised: June 15, 2013 Published: June 17, 2013 15295

dx.doi.org/10.1021/jp402640k | J. Phys. Chem. C 2013, 117, 15295−15305

The Journal of Physical Chemistry C

Article

nation, as demonstrated using the TNF-appended HBC (HBCTNF) nanotube.13 The time-resolved MPC method is based on the angular momentum conservation of electron spin during electron transfer among molecules without large spin−orbit coupling.14 Therefore, the MPC experiment is expected to enable us to investigate mainly recombination of e-h pair with a spin restriction rule, which is not easy to clarify with external electric field dependence of PC used very often. The external electric field perturbs the probability of dissociation rather than recombination because of deforming the Coulomb potential of the e-h pair to be dissociative. The external magnetic field also manipulates the potentials of the e-h pair and modulates the spin dynamics of the pair by the Zeeman interaction, which results in the change of recombination efficiency.48 Furthermore, the real time observation of MPC by a time-resolved technique in this paper has an advantage of being able to distinguish between the geminate and nongeminate recombinations than steady state measurement. To elucidate the effect of the acceptor layer lamination on the surface of a molecular nanowire of a low dimensional HBC assembly, we examine the spin and charge dynamics of photocarriers in acceptorappended HBC nanotubes using nanosecond time-resolved MPC measurements. A one-dimensional columnar object of πstacked HBC with six long alkyl chains of −(CH2)13CH3 (HBC-C14)2 and a HBC nanotube without an acceptor layer (HBC-TEG) are also investigated for comparison.

2. EXPERIMENTAL SECTION 2.1. Film Device Preparation. Film samples were prepared by depositing drops of a solution including HBC assemblies on quartz glass plates coated with comb-shaped indium tin oxide (ITO) electrodes, of which the thickness and gap interval were 100 nm and 5 μm, respectively. The HBC assemblies maintained their structure and were randomly oriented in the deposited film, even after evaporation of the solvent, which was confirmed by absorption spectra that indicated the π−π stacking of HBC15,16 and by microscopic observation of the HBC bundles on the micrometer scale. 2.2. Time-Resolved MPC Measurements. To observe the transient phenomena of the photogenerated carriers, timeresolved PC measurement with current (i-PC) and charge (QPC) were conducted using an RC circuit comprising the sample device with a capacitance (C) of ca. 150 pF, a suitable resistance (R), and a variable DC power supply (PLE-160−0.45, Matsusada Precision), as illustrated in Figure 2. A resistance of R = 50 Ω was used for i-PC measurements, while for the QPC measurements R = 1 MΩ was employed to form an integrating circuit with a long RC time constant. A static magnetic field (B) was applied to a film sample fixed in the center of an electromagnet (TM-YSV5410−061.5, Tamagawa Factory). The B strength, of which the direction was parallel to the external electric field (E) in order to prevent the Hall effect, was measured with a gauss meter (HGM-8201-8R-10 V, Tamagawa Factory). The second and third harmonics of a nanosecond YAG laser (FTSS 355-50-I and FDSS 532-150-I, CryLas) with a pulse width of 2 ns were utilized as the excitation light. The incident light intensity was controlled in the range of 1014 photons cm−2 pulse−1 using a neutral density (ND) filter. The voltage generated between both ends of the R (ΔV), which is induced by the drift of photocarriers and corresponds to iR in i-mode and Q/C in Q-mode, was monitored as a function of the delay time after laser flash (t) with a 1 GHz digital oscilloscope (DPO7104, Tektronix).

Figure 1. (a) Molecular structures of HBC derivatives. (b) Schematic representation of the columnar assembly of HBC-C14 and coaxial nanotubes consisting of HBC-TEG, HBC-TNF, and HBC-C60.

103 to 104. The introduction of the acceptor layer quenches the fluorescence of the HBC nanotube, due to an electron transfer from HBC to the electron-accepting molecule,7 which indicates the excited singlet state of HBC dissociates into a singlet geminate electron−hole (e-h) pair. The HBC nanotubes generally exhibit hole conductivity that originates from the one-dimensional helical arrays of π-stacked HBC,11,12 while the ambipolar carrier transport properties in C60-appended HBC (HBC-C60) nanotubes, due to the electron-transporting component of C60, have been investigated using a field-effect transistor (FET).9 One of the most essential points in the development of highefficiency solar cells that employ new functional molecules is to clarify the potential of the materials on the basis of the photocarrier dynamics, which can be divided into carrier generation and transport processes. Transient photoconductance (PC) measurement is a powerful tool to directly observe not only the carrier transport, but also the carrier generation, superior to conventional methods to get overall photoconductance.46,47 In addition, time-resolved PC measurements under an external magnetic field (hereafter referred to as timeresolved magnetophotoconductance (MPC) measurement) provide unique information regarding spin-dependent recombi15296



Article

Figure 2. (a) Device structure with a film sample and (b) block diagram of the apparatus used for time-resolved MPC measurements. Figure 3. Time profiles of (a) i- and (b) Q-PC signals for the HBC assemblies measured at E = 1.0 × 104 V cm−1 and B = 0 mT. The black line in (b) is a time profile of the pulse laser used for excitation. Incident photon density was 2.2 × 10 14 and ∼1.8 × 10 14 photon·cm−2·pulse−1 for the HBC-C14 column and acceptorappended HBC nanotubes, respectively.

Unwanted influences, due to drifting in the device characteristics induced by long term illumination under an electric field, were removed using zero voltage correction by subtracting the dark level at every measurement, in addition to measurement of the PC signal at zero magnetic field just before each measurement at nonzero magnetic field. The time-resolved PC measurements were performed at room temperature. 2.3. ESR and Optical Measurements. Light-induced electron spin resonance (ESR) measurements were performed using an X-band spectrometer (E-109E, Varian) in combination with a dye laser pumped by a XeCl excimer laser (HE-300 and HE-400, Lumonics). The film samples for ESR measurements were sealed in quartz glass tubes without an electrode under a pressure of 10−3 Pa using a vacuum line equipped with a diffusion pump. Absorption spectra of the HBC assemblies on quartz glass plates without electrodes were measured with the reflection mode under room temperature using a UV−vis spectrometer (V-660, Jasco). Fluorescence spectra of the film samples were detected using a multichannel emission spectrometer (PMAC10027, Hamamatsu) and the decays of fluorescence were observed using a photomultiplier (R7400U-01, Hamamatsu).

nongeminate recombination of free carriers under the E-field. The prompt signals of the acceptor-appended HBC nanotubes were much greater in intensity than that of HBC-C14 column structures. In addition, the nonexponential decay properties of the i-PC signal became pronounced by introduction of the acceptor layer. The electron acceptor has been known to enable a significant increase in the efficiency for electron transfer (ϕ0) from excitons,18,19 with the electron in the acceptor molecule and the hole remaining in the donor molecule. Nonexponential decay can be understood by nongeminate recombination of the free carriers. The high carrier density in the acceptor-appended HBC nanotubes enhances the probability of nongeminate e-h pair formation, which is a bimolecular process that exhibits nonexponential behavior.13 Reliable measurements of photocurrent and photocharge for the HBC-TEG nanotubes under the same experimental condition as the other HBC assemblies could not be achieved, due to a substantial dark current under the external electric field.20 3.2. Incident Photon Density Dependence. Nongeminate recombination for the acceptor-appended HBC nanotubes was also confirmed from the incident photon density (p) dependence of the Q-PC signal, as illustrated in Figure 4. The initial Q-PC signals in both the HBC-C14 column structure and acceptor-appended HBC nanotubes were linearly increased with the laser intensity, which indicates carrier generation through a one photon process. The slope for the HBC-C14 column structure does not change significantly with the delay time, while that for the acceptor-appended HBC nanotubes decreased with time. The slope function fslope, in Figure 4 can be expressed as

3. RESULTS 3.1. Photocurrent and Photocharge. Figure 3 shows time profiles of the i- and Q-PC signals observed under E = 1.0 × 104 V cm−1 without the B-field. The irradiation at wavelengths (λ) of 355 and 532 nm correspond to selective excitation of the HBC and C60 chromophores, respectively (see Figure S-1 in Supporting Information).15−17 Prompt photocurrents in the i-PC measurements were observed at the time of laser flash, at t = 0, after which the currents decayed with the delay time. The Q-PC measurements also showed prompt signals at t = 0 and the signals further increased with the delay time. The prompt signals of i- and Q-PC (Qp) are due to initial photocarrier generation. The decay of the i-PC signal and the delayed increasing component of the Q-PC signal (Qd) reflect carrier transport that is carrier drift motion involving the 15297



Article

where p0 is the base photon density, and k1 and k2 are the rate constants for the first and second order decay of free carriers, respectively. The derivation of eq 1 and symbol definitions are given in section S2 of Supporting Information. fslope is a function that monotonically decreases with t and p. The features of the slope function coincide with the observed pdependence for the acceptor-appended HBC nanotubes. The speed of nongeminate recombination is accelerated by a high free carrier density (nc), so that the t-dependent reduction of the slope due to nongeminate recombination becomes pronounced in the range of high photon density. However, fslope can be approximated by the following function, which is independent of t and p when k2 is negligible. lim fslope (t ) =

k 2→ 0

1 p0

(2)

The constant slope is consistent with the case for the HBCC14 column structure. Thus, nongeminate recombination in the HBC-C14 column structure is less effective than that in the acceptor-appended HBC nanotubes. The influence of nongeminate recombination was also observed in the time dependence of MPC, which is discussed later. 3.3. Electric Field Dependence. The applied E-field dependence of the dissociation efficiency (ϕd) from a geminate e-h pair observed in the selective excitation of HBC is presented in Figure 5. The relative ϕd value could be estimated from the prompt Q-PC signal using eq 3:

ϕd ∝

Figure 4. Incident photon density dependence of the relative Q-PC signal intensity for (a) the HBC-C14 column structure, (b) HBC-TNF nanotubes, and (c) HBC-C60 nanotubes observed during selective excitation of HBC at λ = 355 nm. The Q-PC signal intensities were normalized using that detected by excitation with the smallest photon density.

k12e k1t (e k1t − ϕ2p0) p0 (1 − ϕ2p0){k1e k1t + k 2′(e k1t − 1)p}2 ∵ ϕ2p =

k 2ϕcp k1 + k 2ϕcp

=

k 2′p k1 + k 2′p

E

(3)

The ϕd efficiency observed in the HBC-C14 column is significantly increases with an increase of the E-field. However, ϕd for the acceptor-appended HBC nanotubes was mostly independent of the E-field. The dependence of ϕd on the applied E-field can be simulated using the Onsager model.21−23 The model describes the diffusive motion of a geminate electron−hole (e-h) pair, bounded by their Coulomb interaction under application of an E-field (see section S3 in Supporting Information). The solid curves in Figure 5 show simulations using a typical dielectric constant of ε = 3.0. From the Onsager simulations, the initial separation distances were estimated to be 3.7 nm for the HBC-C14 column structure and more than 30 nm for the acceptor-appended HBC nanotubes. The simulations also provided the carrier generation efficiencies for the geminate e-h pairs in the HBC-C14 column structure and the acceptor-appended HBC nanotubes at zero electric field, which were 10−3 and ca. 1, respectively. The r0 distance of the acceptor-appended HBC nanotubes is much larger than the length between HBC and the acceptor molecules (ca. 2−3 nm). Such an unrealistic r0 estimated by the simulation may stem from an oversimplification of recombination in the Onsager model. This problem is basically solved by adopting the finite recombination time24 or the distance-dependent recombination rate.49 In these modified Onsager models, the E-dependence of dissociation efficiency tends to vanish with increasing intrinsic ϕd at zero electric field. Also, various theories have been proposed to explain for the E-independence of ϕd by considering excess energy,50 energetic disorder,51 and special polymer/fullerene interface.52−54 However, these sophisticated models are not applicable to the acceptor-appended HBC nanotubes. The fact that no excitation wavelength dependence of ϕd was observed in the acceptor-appended HBC nanotubes

Figure 5. External electric field dependence of the dissociation efficiency from a geminate e-h pair for the HBC assemblies (solid marks) and the maximum |MPC| due to the geminate e-h pair for the HBC-C14 column structure (open circles) observed in the selective excitation of HBC at λ = 355 nm. The solid curves represent simulations according to the Onsager model using parameters of ε = 3.0 and T = 300 K.

fslope (t , p) =

Q p(E , t0)

(1) 15298



Article

structure could be observed by application of an external Bfield, although this was unclear from only photocurrent and photocharge measurements, due to the very low carrier density. In contrast, the MPC in the acceptor-appended HBC nanotubes at B = 50 mT increased slowly with the delay time from the beginning (Figure 6a). The increasing speed is almost the same as that of the positive component for the HBC-C14 column structure. The similar time-dependence of the positive components in the HBC assemblies and the effective nongeminate recombination of the acceptor-appended HBC nanotubes leads to identification of the observed MPC for the acceptor-appended HBC nanotubes as that of the spin selective nongeminate recombination. In contrast to HBC-C14, no observation of a fast component is consistent with the negligible geminate recombination of the acceptor-appended HBC nanotubes derived from the E-independency of the dissociation yield for the geminate e-h pair. Figure 7 shows the B-dependence of the MPC observed by excitation of the HBC component. A steep change due to

denies the possibility of excess energy. The well-organized structure of the nanotube assemblies may exclude both the large energetic disorder and secondary layer next the interface. Hence, the observed E-independence of ϕd, in acceptor-append HBC nanotubes, should be interpreted in terms of a slow recombination rate constant of the e-h pair. 3.4. Magnetic Field Dependence. Figure 6 shows time evolutions for the MPC effect on the Q-PC signal measured at B = 50 mT. The percentage MPC is defined by MPC(B , t ) =

Q (B , t ) − Q (0, t ) × 100 Q (0, t )

(4)

where Q is the intensity of the Q-PC signal. The MPC of the HBC-C14 column structure decreased until 500 ns, where it reached a maximum value of −1.59%. After the maximized time, the MPC increased with the delay time and the phase became positive after ca. 10 μs (Figure 6b). Figure 5 (open circles) shows that the absolute maximum value of the negative MPC for the HBC-C14 column structure increased with decrease in the E-field, which indicates that the negative fast component is related with the spin selective recombination of the geminate e-h pair. The Hall effect can be ignored in a parallel configuration between the B- and E-fields (B||E); therefore, the observed MPCs in the low fields can be understood by the spin selective recombination and coherent hyperfine interaction of the e-h pair,27−30 which induces a steep change of the Q-PC signal at low B-fields, as described in the next section. The origin of the positive slow component can be assigned to the recombination of the nongeminate e-h pair, because the nongeminate pair can be formed even at later times and the population of nongeminate e-h pairs with triplet spin character is statistically higher than that with singlet character. The nongeminate recombination in the HBC-C14 column

Figure 7. External magnetic field dependence of the MPC for (a) the HBC-C14 column structure, (b) HBC-TNF nanotubes, and (c) HBCC60 nanotubes observed during selective excitation of HBC at λ = 355 nm. E-fields of 1.0 × 105 and 1.0 × 104 V cm−1 were applied to films of the HBC-C14 column structure and acceptor-appended HBC nanotubes, respectively. Incident photon density was 2.2 × 1014 and ∼1.8 × 1014 photon·cm−2·pulse−1 for the HBC-C14 column and acceptor-appended HBC nanotubes, respectively.

hyperfine coupling appeared for all the HBC assemblies in the low field region of less than 50 mT. The low MPC for hyperfine coupling was detected from 50 ns in the case of HBC-C14, while it was not apparent in the acceptor-appended HBC nanotubes during early excitation. Therefore, it can be concluded that the MPC signals at 50 mT shown in Figure 6 are time evolutions of the MPC effect caused by the hyperfine mechanism. However, there was a difference in the MPC curves detected at high fields above 50 mT. The MPC signals of the

Figure 6. (a) Time profiles of the MPC for the HBC assemblies observed at B = 50 mT with selective excitation at λ = 355 nm. E-fields of 1.0 × 105 and 1.0 × 104 V cm−1 were applied to films of the HBCC14 column structure (red), and the acceptor-appended HBC nanotubes (blue and green), respectively. (b) Time profile of the MPC for HBC-C14 observed over long time under the same measurement condition as that for (a). 15299



Article

generation efficiency. The one-dimensional D/A heterojunction with a particular interval of ca. 2.5 nm in the well-organized supramolecules not only increase the dissociation yield of ex into e-h, but also depresses the recombination of e-h, which is supported by the observation of fluorescence quenching (see section S1 in Supporting Information) and the E-independency of the carrier generation yield from the geminate e-h pair for the acceptor-appended HBC nanotubes. Such negligible recombination of the geminate e-h pair in acceptor-appended HBC nanotubes corresponds with the non-negative MPC due to the geminate recombination for the acceptor-appended HBC films, and is also consistent with the lack of decay of the transient optical absorption signal for the photogenerated holes of HBC-C60 nanotubes in the region of 10−10 s.38 The nonexponential decay of i-PC and the growth of Q-PC signals with the delay time indicates that nongeminate recombination occurs in the carrier transport of the self-assemblies. The time dependent slope for plots of incident light power dependence on the carrier density also indicate nongeminate recombination in the time scale of 10−8−10−6 s. The sign of the observed MPC due to the e-h pair mechanism shows that the carriers are generated from the 1ex state and disappear through a preferential recombination from the 1e-h pair. To quantitatively discuss the carrier dynamics, kinetic analysis of the time dependence of low field MPC was conducted by dividing the observation time into two regions that correspond to geminate and nongeminate recombinations. Scheme 2 illustrates the dynamics of e-h pairs with singlet (S) and triplet (T) character.

HBC-TNF and HBC-C60 nanotubes respectively increased and decreased beyond 50 mT, where the rate of coherent spin conversion between the singlet and triple states of e-h pair caused by the hyperfine interaction did not change. The same B-dependence of MPC with a steep positive effect at low fields and a gentle decrease at high fields was observed, even for the selective excitation of C60 in HBC-C60 (see Figure S-3 in Supporting Information). No selective excitation dependence indicates that the excited triplet state of C60 in the nanotube is not involved in photocarrier generation, although free C60 has an almost unit quantum yield for the generation of the excited triplet state.32−35 The observed high field MPCs of HBC-TNF and HBC-C60 are likely to be interpreted by the spin relaxation31 and Δg mechanisms, respectively, with the assistance of light-induced ESR measurements described in section S5 of Supporting Information. The lack of an ESR signal due to the photocarriers for HBC-TNF is consistent with fast spin relaxation in the HBC-TNF nanotubes. The difference between the g factors of electrons and holes in the pair of HBCC60 (Δg = 0.002(5)) is also supported by light-induced ESR measurements and the values reported for C60 anion radicals.36,37

4. DISCUSSION 4.1. Formulation for Photocarrier Dynamics. The photocarrier dynamics in HBC assemblies derived from the photon density, and the E- and B-dependency of the photocharge signals are illustrated in Scheme 1. Scheme 1. Photocarrier Dynamics in HBC assemblies

Scheme 2. Spin and Charge Dynamics around the ElectronHole Pair

The four spin states are degenerate in the absence of the Bfield, so that intersystem crossing (kisc) can occur among those spin states. However, intersystem crossing between the S and T± states is prohibited in the presence of the B-field, more than the hyperfine field (ahf), because of the large energy gap due to the Zeeman interaction of gμBB. Because of no spin restriction of free carriers, the dissociation (kd) and formation (kf) rate constants of the S state can be set to the same as those of the T states,39,40 although it has been recently reported that these constants are spin-dependent in organic semiconductor of πconjugated polymer.41 However, the recombination rate constant of the S state (krS) is different from those of the T states (krT). The time evolution of the e-h pair density (ne‑h) in these two limited cases can be expressed with the following rate equations: At B = 0

In HBC-C14, the singlet exciton (1ex) of HBC created by UV irradiation has two pathways of returning to the singlet ground state (1g), accompanied with fluorescence (see Figure S-1 in Supporting Information) and the generation of a geminate 1e-h pair. Geminate e-h pairs with singlet and triplet character not only recombine, but also dissociate to free carriers of electrons (e) and holes (h). The presence of the geminate eh pair as a precursor for free carriers was confirmed by the Efield dependence measurements. The B-field dependence of MPC for HBC-C14 confirms intersystem crossing by hyperfine interactions with nuclear spins in the e-h pair and the formation of a nongeminate e-h pair from uncorrelated e and h. The i- and Q-PC measurements showed that the addition of an electronaccepting molecule at the end of the long aliphatic chain connected with HBC significantly increased the carrier 15300



Article

⎧ dn S ⎪ e ‐ h = − (k rS + 3k isc + kd)neS‐ h + k isc ∑ neT‐jh ⎪ dt j =+,0, −‐ ⎪ ⎨ + k f nc2 , ⎪ ⎪ dneT‐jh T = k iscneS‐ h − (k rT + k isc + kd)ne ‐jh + k f nc2 ⎪ ⎩ dt

(5a)

When |B| is much greater than |ahf| ⎧ dn S ⎪ e ‐ h = −(k rS + k isc + kd)neS‐ h + k iscneT‐0h ⎪ dt ⎪ + k f nc2 ⎪ ⎪ ⎧ dn T0 ⎨ ⎪ e ‐ h = k iscneS‐ h − (k rT + k isc + kd)neT‐0h ⎪ ⎪ dt 2 ⎪⎪ ⎨ + k f nc ⎪⎪ ⎪ ⎪ dn T± ⎪ ⎪ e ‐ h = −(k rT + kd)neT‐±h + k f nc2 ⎩ ⎩ dt

Figure 8. (a) Electric field dependence of the time evolution of MPC for the HBC-C14 column structure observed at B = 50 mT during excitation of HBC at λ = 355 nm and (b) the rate constants of the geminate e-h pair. Solid curves in (a) represent the curves calculated using the rate constants shown in (b).

that should be a function of E, because the Onsager analysis of HBC-C14 indicates that kd increases with the E-field. The rate constant for intersystem crossing (kisc) determined by the hyperfine interaction was estimated to be 6.4 × 107 s−1 from WPP of the ESR spectrum for the HBC-TEG nanotubes at 298 K. Figure 8a shows the simulation of MPC curves calculated using the sets of rate constants in Figure 8b and Table 1. The

(5b)

In addition, the rate equation for the density of free carrier must be taken into account: dnc = kd ∑ nei ‐ h − 4k f nc2 dt i = S,T ,T ,T +

(6)

‐

0

The magnetic field dependence of the photocharge density is caused by the difference between krS and krT under cooperation with the B-field dependent intersystem crossing. Hence, the spin selectivity in recombination defined by α=

Table 1. Kinetic Parameters for Photocarrier Dynamics Obtained from Simulations of the Time Profiles for Q-PC and MPC parameters

k rS k rS

+

7

3k rT

kisc/10 s kd/107 s−1 krS/107 s−1 krT/107 s−1 α βn0/108 s−1 Qp/pC Qd/pC

(7)

is an important parameter. To calculate the Q-PC signal using the carrier density, i.e., the solution of eqs 5 and 6, nc must be integrated:31 Q (t ) ≃ qμd ES

∫0

t

nc(τ )dτ

in

t ≪ τc

(8)

t

MPCg (B , t ) =

t

HBC-TNF

HBC-C60c

0.64 0.4−8.5b 15 12 0.294 a a a

 a a a 0.293 2.00 24 13

a a a a 0.298 (0.296) 5.00 (0.70) 73 (14) 11 (15)

a

Could not be estimated. bThe value varied with the strength of applied electric field. cThe values in the brackets were obtained by selective excitation of C60 at λ = 532 nm.

simulations reproduced the observed MPCs very well. However, it should be noted that a unique set of rate constants could not be obtained without simultaneous fitting of the Efield dependence of ϕd (see Figure 5), because the magnitude of MPC was sensitive only to the balance between kr and kd. It has been clarified that the reduction of |MPC| by half under the E-field range of (0.6−2.2) × 105 V cm−1 is caused by an increment in kd of more than one order, due to the sizable krT, even though it is less than krS. The recombination occurs from the 3e-h pair to 3ex of HBC, as well as from the 1e-h pair to 1g. Under low E-fields kd governs the apparent time-growth of the negative MPC, but under high E-fields, the increasing behavior is likely to be determined by kisc, which is the smallest among the four rate constants. In contrast to HBC-C14, neither E- nor B-field effects due to the geminate e-h pair dynamics could be detected for the acceptor-appended HBC nanotubes. This is mainly because recombination becomes slow compared with dissociation. The separation distance between HBC and acceptor of a few nanometers is not a valid cause for the reduction in

t

∫0 nc(0, τ ) dτ

HBC-C14

a

where q, μd, S, and τc represent respectively the elementary charge, the drift mobility, the cross section of the current, and the time constant of integral circuit which is equal to RC (see Figure 2). 4.2. Geminate Pair Dynamics. For HBC-C14, as shown in Figure 8a, the time evolution of the negative MPC for photocharge due to the geminate e-h pair mechanism is observed until 500 ns after the laser flash. The maximum of |MPC| decreases in intensity with increase in the E-field. As for the geminate e-h pair, the formation dynamics from the free carriers in Scheme 2 and eqs 5 and 6 can be ignored. In addition, the initial condition of ne‑hS = n0 and ne‑hT = 0 for the geminate e-h pair generated from the 1ex of HBC enables an analytical solution nc(t) of eqs 5 and 6 (see section S6 in Supporting Information) to be obtained. The formula for simulation of the time evolution for the observed MPC is

∫0 nc(B , τ ) dτ − ∫0 nc(0, τ ) dτ

−1

× 100 (9)

A negative MPC is achieved with α > 1/4, which indicates that krS is faster than krT. kd is another unknown rate constant 15301



Article

recombination, because of the effective electron transfer from 1 ex of HBC to the acceptors. The lack of fluorescence from HBC indicates that electronic coupling between 1ex of HBC and the acceptor is sufficient for electron transfer. Superexchange interaction through several covalent σ-bonds in the linker is a probable reason for the efficient electron transfer. Within the framework of the Marcus theory for nonadiabatic electron transfer, it is considered that some activation energy is induced by significant reorganization of the e-h pair surroundings, which hinders recombination. In addition, high electron mobility in the acceptor layer of C60 or TNF could also assist in the prevention of recombination.55 4.3. Nongeminate Pair Dynamics. Nongeminate e-h pair dynamics appear in the photocharge signal and MPC measured in the period of carrier transport. The MPC of HBC-C14 involves both geminate and nongeminate e-h pair dynamics, but those of acceptor-appended HBCs reflect only the nongeminate e-h pair dynamics. For ease of simulation, we have focused on analysis of the time profiles of Q-PC and MPC for the acceptor-appended HBC nanotubes. Calculation of the time evolution for the carrier density during transport is required for the simulation. Formation of the nongeminate e-h pair continues as long as the carriers exist; therefore, a steady state approximation was employed for the time variation of the nongeminate e-h pair density:13

d ne ‐ h = 0 dt

Q (t ) =Q p + Q d ln(1 + βn0t ) ∵ Qd =

(14)

profiles calculated using eq 14 with three parameters of Qp, Qd, and βn0 listed in Table 1. The calculated curves simulate the time profiles of the Q-PC signal well for acceptor-appended HBC nanotubes observed at 0 mT, whereas an exponential function corresponding to a first order decay of the free carrier was unsatisfactory (see Figure S-5 in Supporting Information). The nongeminate recombination, which is a second order reaction, as represented in eq 11, plays a dominant role in the decay dynamics of free carriers in the acceptor-appended HBC nanotubes, which is in agreement with the results of flashphotolysis time-resolved microwave conductivity of HBC-TNF nanotubes.43 Qp is a value proportional to n0; therefore, the relative magnitude of β should be reflected by the ratio of βn0/ Qp. The average value of βn0/Qp for HBC-C60 estimated by the selective excitations is (0.9 ± 0.1) × 109 s−1 V−1, while that for HBC-TNF is 1.3 × 109 s−1 V−1. The difference in β would originate in the electronic character of acceptor and/or the structure of nanotubes. On the other hand, the βn0 value for HBC-C60 is 2.5 times larger than that for HBC-TNF. From comparison with the difference in β, therefore, it can be said that the initial density of photocarriers is a main factor that determines the recombination speed in the acceptor-appended HBC nanotubes. To obtain information on the spin dynamics in the e-h pair, i.e., the intermediate state for the nongeminate recombination, further simulations of the MPC time profiles due to the nongeminate e-h pair mechanism were conducted by adding a correction term for the prompt Q-PC signal, which is regarded to be independent of the B-field, into eq 9

kk k f k r(B) d nc = − f r nc2 = −βnc2 , ∵ β(B) = dt kd + k r kd + k r(B) (11)

In the case of kd ≪ kr, the reduced decay rate constant β is approximated to kf, which may be equal to the concept in Langevin recombination theory.41 For kd ≫ kr, β is proportional to kr dependent upon the spin state of the e-h pair and the external B-field. However, under a simple statistical limit regarding the population ratio among the spin states of the e-h pair (S:T+:T0:T− = 1:1:1:1), all the recombination rate constants become a single value in the absence of a B-field.

(12)

Hence, eq 11 can be solved analytically under this condition. In the presence of a B-field larger than ahf, the four rate constants are categorized into two constants: ⎧ k S + k rT ⎪ k rS = k rT0 = r 2 kr = ⎨ ⎪ T± T ⎩kr = kr

, t≥0

Figure 9. Time profiles of the Q-PC signals for the HBC assemblies observed at B = 0 mT and E = 1.0 × 104 V cm−1. The solid curves represent simulations calculated using the parameters listed in Table 1.

(10)

k rS + 3k rT 4

β

Qp and n0 are the prompt Q-PC component and the initial density of the free carrier generated from dissociation of the geminate e-h pair, respectively. Figure 9 shows the Q-PC

By this approximation, the rate equation for the carrier densities is significantly reduced:

k r = k rS = k rT0 = k rT± =

qμd ES

⎡ MPCng (B , t ) = ⎢ ⎢⎣

(13)

Such spin-dependent recombination makes it impossible to solve eq 11 analytically. Therefore, nc(B,t) was calculated numerically using a Runge−Kutta method. Substitution of the analytical form of nc(0,t) into eq 8 gives a representation for the Q-PC signal:

∫0

⎡ /⎢ ⎢⎣

t

∫0

nc(B , τ ) dτ −

∫0

t

nc(0, τ ) dτ +

t

⎤ nc(0, τ ) dτ ⎥ ⎥⎦

Qp ⎤ ⎥ × 100 Q dβn0 ⎥⎦ (15)

15302



Article

No analytical form of nc (B,t) exists; therefore, the integrations in eq 15 were performed using a trapezoidal method. The correction term is a constant that can be calculated from the parameters optimized by the simulation of the Q-PC signal at B = 0 mT; therefore, the α value is only a variable parameter in the simulation of MPC. The size of MPC due to the nongeminate e-h pair mechanism increased as the |α − 1/4| value increased (Figure S-6 in Supporting Information). This V-shaped trend of MPC against the recombination anisotropy differs from that for geminate e-h, in which the population ratio among the spin states deviates from the statistical limit. The magnitude of MPC was also sensitive to the βn0 value, but kisc has no significant influence on the time profiles of MPC (Figure S-6). This is because β is a branching ratio of the recombination against the dissociation and the statistical limit of population quenches the kisc-dependence. Figure 10 shows simulations and the observed MPC time

approach to examine the recombination property of organic semiconductor.

5. CONCLUSION The effect of acceptor-layer lamination on the photocarrier dynamics in HBC nanotubular self-assemblies, which are good hole transporters, were investigated using time-resolved MPC techniques that provide information regarding the charge and spin dynamics of photocarriers. The electric and magnetic field effects on the carrier generation yield from photoinjected geminate e-h pairs in HBC-C14 were demonstrated. In contrast, no significant external field effects on the generation yield were observed for HBC-TNF and HBC-C60. These results lead to the conclusion that lamination of the acceptor-layer not only enhances the quantum yield of geminate e-h pairs from the 1 ex state of HBC, but also significantly reduces recombination from the geminate pair. Several flexible chemical bonds between the donor and acceptor layers are a key structure to prevent recombination of the geminate e-h pairs. Some kinetic parameters of the geminate e-h pairs in HBC-C14 and nongeminate e-h pairs in acceptor-appended HBC were obtained by simulation of the MPC profiles. The spin selective recombination ratio of krS:krT was 1.0:0.8 for all the HBC assemblies, and the generation of 3ex from substantial recombination of the 3e-h pair was noted.

■

ASSOCIATED CONTENT

S Supporting Information *

Appendix S1, optical absorption and emission spectra; Appendix S2, slope function in photon density plot; Appendix S3, Onsager analysis; Appendix S4, magnetic field dependence of MPC of HBC-C60 in C60 selective excitation; Appendix S5, light-induced electron spin resonance spectra of HBC-TEG; Appendix S6, analytical solution of geminate pair dynamics; Appendix S7, simulation of Q-PC using a first order decay function of carrier density. Appendix S8, simulations for the time profiles of MPC due to the nongeminate e-h pair mechanism. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

Figure 10. Time profiles of the MPC signals measured for the (a) HBC-TNF and (b,c) HBC-C60 nanotubes observed at B = 50 mT and E = 1.0 × 104 V cm−1. The light wavelengths employed in (a) and (b,c) were 355 and 532 nm, respectively. The colored curves represent the simulations.

ACKNOWLEDGMENTS We thank Prof. Dr. Kazunari Shinbo (Niigata Univ.) for helping measurements of absorption spectra and Dr. Tomoaki Miura (Niigata Univ.) for his helpful discussion about spin dynamics of radical pair. Y.W. (Niigata) thanks the JSPS Young Scientist Fellowship. This work was financially supported by a Grant-inAid (23-7285) from JSPS, a CREST from JST, and a grant from Nihon Kagaku Kenkyukai.

profiles. The optimized α values listed in Table 1 were obtained from the best fittings. The α values of acceptor-appended HBC nanotubes are mostly the same that for HBC-C14 with 0.29, which corresponds to krS:krT = 1.0:0.8. The good accordance between the simulated and observed time profiles indicates that the positive MPC in the range of several microseconds is due to the spin selective recombination of the nongeminate e-h pair. Since the ratio between the singlet and triplet recombinations is an important factor even in organic light emitting diode,44,45 the time-resolved MPC can be used as a complementary

■

REFERENCES

(1) Watson, M. D.; Fechtenkotter, A.; Müllen, K. Big Is beautiful ″Aromaticity″ Revisited from the Viewpoint of Macromolecular and Supramolecular Benzene Chemistry. Chem. Rev. 2001, 101, 1267− 1300. (2) Wu, J. S.; Pisula, W.; Müllen, K. Graphenes as Potential Material for Electronics. Chem. Rev. 2007, 107, 718−747. 15303



Article

(3) van de Craats, A. M.; Warman, J. M.; Müllen, K.; Geerts, Y.; Brand, J. D. Rapid Charge Transport along Self-Assembling Graphitic Nanowires. Adv. Mater. 1998, 10, 36−38. (4) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson, M.; Müllen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. Meso-Epitaxial Solution-Growth of Self-Organizing Discotic LiquidCrystalline Semiconductors. Adv. Mater. 2003, 15, 495−499. (5) Piris, J.; Debije, M. G.; Watson, M. D.; Müllen, K.; Warman, J. M. The Mobility and Decay Kinetics of Charge Carriers in Discotic Hexabenzocoronenes. Adv. Funct. Mater. 2004, 14, 1047−1052. (6) Schmidt-Mende, L.; Fechtenkotter, A.; Müllen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Self-Organized Discotic Liquid Crystals for High-Efficiency Organic Photovoltaics. Science 2001, 293, 1119−1122. (7) Yamamoto, Y.; Fukushima, T.; Suna, Y.; Ishii, N.; Saeki, A.; Seki, S.; Tagawa, S.; Taniguchi, M.; Kawai, T.; Aida, T. Photoconductive Coaxial Nanotubes of Molecularly Connected Electron Donor and Acceptor Layers. Science 2006, 314, 1761−1764. (8) Yamamoto, Y.; Fukushima, T.; Saeki, A.; Seki, S.; Tagawa, S.; Ishii, N.; Aida, T. Molecular Engineering of Coaxial Donor-Acceptor Heterojunction by Coassembly of Two Different Hexabenzocoronenes: Graphitic Nanotubes with Enhanced Photoconducting Properties. J. Am. Chem. Soc. 2007, 129, 9276−9277. (9) Yamamoto, Y.; Zhang, G. X.; Jin, W. S.; Fukushima, T.; Ishii, N.; Saeki, A.; Seki, S.; Tagawa, S.; Minari, T.; Tsukagoshi, K.; Aida, T. Ambipolar-Transporting Coaxial Nanotubes with a Tailored Molecular Graphene-Fullerene Heterojunction. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 21051−21056. (10) Hill, J. P.; Jin, W. S.; Kosaka, A.; Fukushima, T.; Ichihara, H.; Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. SelfAssembled Hexa-Peri-Hexabenzocoronene Graphitic Nanotube. Science 2004, 304, 1481−1483. (11) Yamamoto, Y.; Fukushima, T.; Jin, W. S.; Kosaka, A.; Hara, T.; Nakamura, T.; Saeki, A.; Seki, S.; Tagawa, S.; Aida, T. A Glass Hook Allows Fishing of Hexa-Peri-Hexahenzocoronene Graphitic Nanotubes: Fabrication of a Macroscopic Fiber with Anisotropic Electrical Conduction. Adv. Mater. 2006, 18, 1297−1300. (12) Hara, T.; Furukawa, K.; Nakamura, T.; Yamamoto, Y.; Kosaka, A.; Jin, W. S.; Fukushima, T.; Aida, T. Possible One-Dimensional Helical Conductor: Hexa-Peri-Hexabenzocoronene Nanotube. J. Phys. Soc. Jpn. 2008, 77, 034710−1−6. (13) Wakikawa, Y.; Ikoma, T.; Yamamoto, Y.; Fukushima, T.; Aida, T. Magnetic Field Effect on the Photocarriers in Self-Assembled Hexabenzocoronene Nanotubes. Synth. Met. 2010, 160, 275−279. (14) Shiraishi, M.; Ikoma, T. Molecular Spintronics. Physica E 2011, 43, 1295−1317. (15) Fleming, A. J.; Coleman, J. N.; Dalton, A. B.; Fechtenkotter, A.; Watson, M. D.; Müllen, K.; Byrne, H. J.; Blau, W. J. Optical Spectroscopy of Isolated and Aggregate Hexabenzocoronene Derivatives: A study of Self-Assembling Molecular Nanowires. J. Phys. Chem. B 2003, 107, 37−43. (16) Piris, J.; Debije, M. G.; Stutzmann, N.; van de Craats, A. M.; Watson, M. D.; Müllen, K.; Warman, J. M. Anisotropy in the Mobility and Photogeneration of Charge Carriers in Thin Films of Discotic Hexabenzocoronenes, Columnarly Self-Assembled on Friction-Deposited Poly(tetrafluoroethylene). Adv. Mater. 2003, 15, 1736−1740. (17) Hare, J. P.; Kroto, H. W.; Taylor, R. Preparation and Uv Visible Spectra of Fullerenes C60 and C70. Chem. Phys. Lett. 1991, 177, 394− 398. (18) Im, C.; Emelianova, E. V.; Bässler, H.; Spreitzer, H.; Becker, H. Intrinsic and Extrinsic Charge Carrier Photogeneration in PhenylSubstituted Polyphenylenevinylene-trinitrofluorenone Blend Systems. J. Chem. Phys. 2002, 117, 2961−2967. (19) Im, C.; Lupton, J. M.; Schouwink, P.; Heun, S.; Becker, H.; Bässler, H. Fluorescence Dynamics of Phenyl-Substituted Polyphenylenevinylene-Trinitrofluorenone Blend Systems. J. Chem. Phys. 2002, 117, 1395−1402. (20) We could detect weak i- and Q-PC signals using a top contact device of HBC-TEG. However it was impossible to compare the data

of HBC-TEG with those of other HBC assemblies, because of lot of differences in experimental condition such as applied external fields, excitation efficiency, electrode materials, and so on. (21) Onsager, L. Initial Recombination of Ions. Phys. Rev. 1938, 54, 554−557. (22) Melz, P. J. Photogeneration in Trinitrofluorenone-Poly(NVinylcarbazole). J. Chem. Phys. 1972, 57, 1694−1699. (23) Mozumder, A. Effect of an External Electric Field on the Yield of Free Ions. I General Results from the Onsager Theory. J. Chem. Phys. 1974, 57, 4300−4304. (24) Noolandi, J.; Hong, K. M. Theory of Photogeneration and Fluorescence Quenching. J. Chem. Phys. 1979, 70, 3230−3236. (25) Braun, C. L. Electric-Field Assisted Dissociation of ChargeTransfer States as a Mechanism of Photocarrier Production. J. Chem. Phys. 1984, 80, 4157−4161. (26) Wang, Y.; Suna, A. Fullerenes in Photoconductive Polymers. Charge Generation and Charge transport. J. Phys. Chem. B 1997, 101, 5627−5638. (27) Ito, F.; Ikoma, T.; Akiyama, K.; Tero-Kubota, S. Spin Dynamic Study on the Electric Field Dependence of Carrier Generation. J. Phys. Chem. B 2005, 109, 7208−7213. (28) Hayashi, H.; Nagakura, S. Theoretical-Study of Relaxation Mechanism in Magnetic-Field Effects on Chemical-Reactions. Bull. Chem. Soc. Jpn. 1984, 57, 322−328. (29) Ogiwara, T.; Ikoma, T.; Akiyama, K.; Tero-Kubota, S. Spin Dynamics of Carrier Generation in a Photoconductive C60-doped Poly(N-Vinylcarbazole) Film. Chem. Phys. Lett. 2005, 411, 378−383. (30) Ito, F.; Ikoma, T.; Akiyama, K.; Watanabe, A.; Tero-Kubota, S. Carrier Generation Process on Photoconductive Polymer Films as Studied by Magnetic Field Effects on the Charge-Transfer Fluorescence and Photocurrent. J. Phys. Chem. B 2005, 109, 8707− 8717. (31) Ikoma, T.; Ogiwara, T.; Takahashi, Y.; Akiyama, K.; TeroKubota, S.; Suzuki, T.; Wakikawa, Y. Giant Magnetoresistance due to Electron-Hole Pair Mechanism in Poly(N-Vinylcarbazole). Synth. Met. 2010, 160, 285−290. (32) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. Photophysical Properties of C60. J. Phys. Chem. 1991, 95, 11−12. (33) Dimitrijevic, N. M.; Kamat, P. V. Triplet Excited-State Behavior of Fullerenes -Pulse-Radiolysis and Laser Flash-Photolysis of C60 and C70 in Benzene. J. Phys. Chem. 1992, 96, 4811−4814. (34) Wasielewski, M. R.; Oneil, M. P.; Lykke, K. R.; Pellin, M. J.; Gruen, D. M. Triplet States of Fullerenes C60 and C70 -Electron Paramagnetic Resonance Spectra, Photophysics, and Electronic Structures. J. Am. Chem. Soc. 1991, 113, 2774−2776. (35) Terazima, M.; Hirota, N.; Shinohara, H.; Saito, Y. Photothermal Investigation of the Triplet State of C60. J. Phys. Chem. 1991, 95, 9080−9085. (36) Allemand, P. M.; Srdanov, G.; Koch, A.; Khemani, K.; Wudl, F.; Rubin, Y.; Diederich, F.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. The Unusual Electron Spin Resonance of Fullerene C60 Anion Radical. J. Am. Chem. Soc. 1991, 113, 2780−2781. (37) Dubois, D.; Jones, M. T.; Kadish, K. M. Electroreduction of Buckminsterfullerene, C60, in Aprotic Solvents: Electron-Spin Resonance Characterization of Singly, Doubly, and Triply Reduced C60 in Frozen Solutions. J. Am. Chem. Soc. 1992, 114, 6446−6451. (38) Moritomo, Y.; Yonezawa, K.; Ito, M.; Kamioka, H.; Yamamoto, Y.; Fukushima, T.; Aida, T. Interface Dependence of Charge Formation Dynamics in Hexabenzocoronene−C60. Appl. Phys. Express 2012, 5, 062401−1−062401−3. (39) Steiner, U. E.; Ulrich, T. Magnetic Field Effects in Chemical Kinetics and Related Phenomena. Chem. Rev. 1989, 89, 51−147. (40) Hayashi, H. Introduction to Dynamics Spin Chemistry: Magnetic Field Effects on Chemical and Biochemical Reactions; World Scientific Pub. Co. Inc.: Singapore, 2004. (41) McCamey, D. R.; Lee, S. Y.; Paik, S. Y.; Lupton, J. M.; Boehme, C. Spin-Dependent Dynamics of Polaron Pairs in Organic Semiconductors. Phys. Rev. B 2010, 82, 125206. 15304



Article

(42) van der Holst, J. J. M.; van Oost, F. W. A.; Coehoorn, R.; Bobbert, P. A. Electron-Hole Recombination in Disordered Organic Semiconductors: Validity of the Langevin Formula. Phys. Rev. B 2009, 80, 235202. (43) Saeki, A.; Yamamoto, Y.; Y., K.; Fukushima, T.; Aida, T.; Seki, S. Photoconductivity of Self-Assembled Hexabenzocoronene Nanotube: Insight into the Charge Carrier Mobilities on Local and Long-Range Scales. J. Phys. Chem. Lett. 2011, 2, 2549−2554. (44) Wohlgenannt, M.; Tandon, K.; Mazumdar, S.; Ramasesha, S.; Vardeny, Z. V. Formation Cross-Sections of Singlet and Triplet Excitons in π-Conjugated Polymers. Nature 2001, 409, 494−497. (45) McCamey, D. R.; Seipel, H. A.; Paik, S. Y.; Walter, M. J.; Borys, N. J.; Lupton, J. M.; Boehme, C. Spin Rabi Flopping in the Photocurrent of a Polymer Light-Emitting Diode. Nat. Mater. 2008, 7, 723−728. (46) Borseberger, P. M.; Richert, R.; Bässlar, H. Dispersive and Nondispersive Charge Transport in a Molecularly Doped Polymer with Superimposed Energetic and Positional Disorder. Phys. Rev. B 1993, 47, 4289−4295. (47) Mozer, A. J.; NSarcifci, N. S.; Pivrikas, A.; Ö sterbacka, R.; Juška, G.; Brassat, L.; Bässlar, H. Charge Carrier Mobility in Regioregular Poly(3-Hexylthiophene) Probed by Transient Conductivity Techniques: A Comparative Study. Phys. Rev. B 2005, 71, 035214. (48) Ikoma, T. In Selectivity, Control, and Fine Tuning in High-Energy Chemistry; Stass, D. V., Feldman, V. I., Eds.; Research Signpost: Kerala, 2011; p 251. (49) Wojcik, M.; Tachiya, M. Geminate Charge Recombination with Distance-Dependent Intrinsic Reaction Rate: Escape Probability and its Electric Field Effect. Radiat. Phys. Chem. 2005, 74, 132−138. (50) Arkhipov, V. I.; Emelianova, E. V.; Bässlar, H. Hot Exciton Dissociation in a Conjugated Polymer. Phys. Rev. Lett. 1999, 82, 1321− 1324. (51) Rubel, O.; Baranovskii, S. D.; Stolz, W.; Gebhard, F. Exact Solution for Hopping Dissociation of Geminate Electron-Hole Pairs in a Disordered Chain. Phys. Rev. Lett. 2008, 100, 196602−1−196602−4. (52) Arkhipov, V. I.; Heremans, P.; Bässlar, H. Why Is Exciton Dissociation So Efficient at the Interface between a Conjugated Polymer and an Electron Acceptor? Appl. Phys. Lett. 2003, 82, 4605− 4607. (53) Baranovskii, S. D.; Wiemer, M.; Nenashev, A. V..; Jansson, F.; Gebhard, F. Calculating the Efficiency of Exciton Dissociation at the Interface between a Conjugated Polymer and an Electron Acceptor. J. Phys. Chem. Lett. 2012, 3, 1214−1221. (54) MacMahon, D. P.; Cheung, D. L.; Troisi, A. Why Holes and Electrons Separate So Well in Polymer/Fullerene Photovoltaic Cells. J. Phys. Chem. Lett. 2011, 2, 2737−2741. (55) Tachiya, M. Breakdown of the Onsager Theory of Geminate Ion Recombination. J. Chem. Phys. 1988, 89, 6929−6935.

15305


Effect of Acceptor Lamination on Photocarrier Dynamics in Hole

Recommend Documents