Article pubs.acs.org/JPCC
Distance Dependence of Exciton Dissociation at a Phthalocyanine− C60 Interface G. J. Dutton and S. W. Robey* National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg, Maryland 20899, United States S Supporting Information *
ABSTRACT: Exciton dissociation at donor−acceptor (DA) interfaces is critical for the operation of organic photovoltaic (OPV) devices, yet a detailed physical understanding of this process is lacking. This work examines an important aspect of this process, namely the dependence of the exciton dissociation rate on distance from the DA interface. Time-resolved two-photon photoemission (TR-2PPE) measurements were performed on bilayer H2Pc\C60 heterojunctions fabricated using organic molecular beam epitaxy (MBE) with varying H2Pc thickness. In the measurements, the dynamics of the H2Pc S1 exciton population created with a 1.55 eV pump pulse were monitored via photoemission with a delayed UV probe pulse. The depth sensitivity of TR-2PPE, due to the short electron attenuation length, provides the means to follow excited state dynamics as a function of H2Pc thickness. Analysis of the S1 population decay as a function of H2Pc thickness revealed that the electron transfer rate for the first H2Pc layer, adjacent to C60, is kCT = (2.3 ± 0.4) × 1012 s−1. Exciton dissociation is reduced by a factor of at least 10 for the second H2Pc layer and beyond.
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INTRODUCTION Exciton dissociation and charge transfer at donor−acceptor interfaces in organic photovoltaic (OPV) structures is required to produce free carriers. Typically, it is assumed that excitons created in the bulk diffuse to interfacial regions where dissociation is efficient.1,2 Factors that may influence the range of exciton dissociation near OPV interfaces were recently investigated theoretically by Caruso et al.3 They noted that the binding energy of the charge transfer exciton decreases with distance from the interface, leading to more effective long-range exciton dissociation. In addition, recent experimental studies of polymer bulk heterojunctions (BHJ)4−10 revealed ultrafast charge separation that is difficult to rationalize in terms of point-like exciton diffusion, also suggesting the possibility of more complex exciton behavior prior to dissociation. Delocalization, particularly in the excited state, was suggested to account for this ultrafast charge transfer in poly(3hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and poly[[9-(1-octylnonyl)-9H-carbazole-2,7diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:[6,6]-phenyl-C 61 -butyric acid methyl ester (PCDTBT:PCBM) blends that occurred faster than expected for exciton diffusion.6−10 For solid state OPV heterojunction structures, exciton dissociation processes may thus be enhanced through delocalization of the exciton and charge transfer states4,5 that may, in addition to reducing exciton binding energies,11,12 reduce the limitations of exciton diffusion to interfaces. A detailed understanding of exciton dissociation near donor−acceptor interfaces may thus suggest new avenues to increase OPV efficiency. The work described here is motivated by the desire to produce a deeper understanding of the dependence of exciton dissociation on the separation from donor−acceptor interfaces. This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society
Distance-dependent electron transfer in donor−bridge− acceptor (D−B−A) systems has been an active and fruitful area of investigation in the photophysics of donor−acceptor systems. Studies as a function of bridge coupling and length revealed that well-conjugated bridges can produce efficient charge transfer over extended distances.13−23 More recently, the emphasis on photovoltaic systems has led to investigations of the impact of the sensitizer coupling groups on the performance of dye-sensitized solar cells, producing similar conclusions.24−26 Much of this work on charge separation in D−B−A systems was performed for molecules in solution using ultrafast optical techniques combined with variation of the molecular bridge via chemical synthesis to investigate distance-dependent dynamics. For solid state OPV systems, all-optical techniques may also been used in conjunction with layered samples27 to increase the sensitivity to interfacial processes. In the work reported here, however, we achieved the required interfacial sensitivity through the surface/interface sensitivity of ultrafast timeresolved two-photon photoemission (TR-2PPE). TR-2PPE has proven to be a very useful technique for understanding interfacial processes at metal−organic interfaces28−37 and organic donor−acceptor interfaces of interest for photovoltaics.38−41 (This is a representative sample of recent studies and by no means a complete list.) Distance-dependent dynamics at metal−fullerene interfaces were examined in some previous TR-2PPE studies.29,30 Organic−organic donor−acceptor H2Pc\C60 heterojunctions were fabricated with controlled H2Pc thicknesses using organic molecular beam epitaxy (OMBE). In the TR-2PPE measureReceived: November 8, 2013 Published: November 13, 2013 25414
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(STM) measurements.42,43 Those investigations revealed the growth of bulk-like Pc phases for CuPc and ZnPc on C60, with “upright” molecular orientations where the axis corresponding to the a-axis of the Pc bulk phase is perpendicular to the surface.44 Deposition at a slightly elevated substrate temperature produced a nearly perpendicular molecular orientation42,43 with the π-stacking direction along the surface and a molecular surface density very close to that expected for bulk phases (∼1 Pc per 0.5 nm2).44 The CuPc and ZnPc surface structures formed on C60 are very similar,42,43 and combined with very similar bulk structures for most Pc’s,44 we expect that H2Pc growth on C60 will produce analogous surface phases. This expectation is supported by nearly identical polarized ultraviolet photoemission spectra measured for CuPc\C60 and H2Pc\C60 as a function of thickness. More details on the H2Pc \C60 interface growth are provided in the Supporting Information. The C60 coverage on Ag(111) was calibrated based on the measured work-function change and shifts in image state energies in 2PPE spectra as a function of C60 deposition. For H2Pc on C60, UPS measurements of the C60 HOMO intensity, the H2Pc HOMO intensity, and the change in work function with H2Pc coverage were used to calibrate the deposition required for 1 ML. These data are included in the Supporting Information, Figures SI1−SI3. The initial linear regions of the coverage-dependent data were extrapolated to provide ML coverage estimates. The 1 ML deposition point determined in this manner agreed well with H2Pc source calibration via a quartz crystal monitor (QCM) at the sample position combined with the H2Pc structure determined from STM. The coverage-dependent UPS and 2PPE measurements also reveal another important point. For both photoemission measurements the signal from the underlying C60 was completely attenuated, and the H2Pc intensity became saturated with the completion of the first H2Pc layer. This indicates that the photoemission intensity in the UPS measurements and, more importantly, TR-2PPE is dominated by the outermost H2Pc layer due to electron attenuation in the ≈1 nm thick Pc molecular layer, consistent with previous studies.29,39,40 Thus, as the H2Pc\C60 interface is formed by OMBE, the TR-2PPE measurement will predominantly sample the dynamics of the S1 population in the outermost H2Pc layer, providing a picture of the layer-dependent exciton dynamics with increasing H2Pc deposition. UPS, 2PPE, and inverse photoelectron spectroscopy (IPES)45,46 data were combined to determine the electronic structure at the H2Pc\C60 interface. The band alignment was found to be identical to that discussed previously for the CuPc/ C60 interface.38,39 A qualitative illustration of the molecular arrangement at the interface is provided in Figure 1a, and a schematic representation of the interfacial electronic structure is given in Figure 1b. Time-Resolved Two-Photon Photoemission. TR-2PPE measurements were performed using the Ti:sapphire fundamental at 800 nm (hνpump = 1.55 eV) to excite π → π* Q-band transitions from the H2Pc a1u HOMO to the e1g LUMO, populating H2Pc S1 exciton levels. The third harmonic of the Ti:sapphire fundamental at 267 nm, or hνprobe = 4.65 eV, was then used to probe the dynamics of this S1 population. The work function (WF) for the H2Pc\C60 structures varied from 4.4 ± 0.1 eV for 1 ML to 4.2 ± 0.1 eV for 8 ML. The wavelength combination described above was chosen as a compromise to match the pump to the H2Pc Q-band
ments, H2Pc S1 excitons were excited using subpicosecond pulses in the near-IR, and the decay of this S1 population was probed with photoemission via a time-delayed UV probe pulse, revealing the dynamics of the excited S1 exciton states. This combination of techniques allowed us to directly monitor changes in the exciton dynamics as the donor−acceptor interface region is created. We previously investigated the energy dependence of exciton dissociation at CuPc\C60 interfaces by following changes in the decay rate as the initial S1 population relaxed to lower energy.38,39 Ultrafast exciton dissociation with rates of up to (5−7) × 1012 s−1 were observed for the interfacial CuPc layer, consistent with a subsequent investigation of the Pc\C60 interface that also employed TR-2PPE, combined with ultrafast second harmonic generation.41 Our previous study of the CuPc \C 60 interface focused on heterojunctions with CuPc thicknesses of 1 ML (interface) and 5 ML (bulk). Electron transfer in D−B−A systems is typically assumed to decay nearly exponentially with distance, that is, electron transfer rate ∝ exp(−βd) where d is the bridge length. The distance variation in our previous work was too coarse to allow us to draw conclusions about distance dependence in exciton dissociation because typical values of the decay constant, β, lie in the range from ≈1 Å−1 to about 0.1 Å−1.13−23 Thus, in the work discussed here, we extended our previous measurements to probe exciton dynamics near the H2Pc/C60 interface on a finer Pc thickness scale. We find that the charge transfer rate at the H2Pc/C60 interface is kCT = (2.3 ± 0.3) × 1012 s−1, consistent with the values measured for the CuPc\C 60 interface. 39 More importantly, we find that the charge transfer rate decreases by at least an order of magnitude for the second, and successive, H2Pc layers. We begin with a discussion of experimental details, including background on the interfacial electronic and molecular structure at the H2Pc\C60 interface. We briefly discuss the growth of H2Pc on C60 as revealed by STM, UPS, and 2PPE measurements. Pump−probe TR-2PPE data as a function of H2Pc thickness are presented that reveal the dynamics of S1 excitons created near the interface. These data are interpreted following the picture developed in our previous work39 based on decay via a combination of bulk-like intraband (IBR) or vibronic relaxation plus donor−acceptor exciton dissociation \electron transfer near the interface. The initial decay of the H2Pc S1 population is then analyzed as a function of distance from the interface to determine the distance dependence of exciton dissociation. Possible correlations with the Pc\C60 interfacial molecular structure and a brief comparison with the distance-dependent dynamics observed at C60−metal interfaces are discussed.
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EXPERIMENTAL METHODS H2Pc\C60 Bilayer Formation. Bilayer structures for TR2PPE measurements were formed by depositing between 1 monolayer (ML) and 70 ML of H2Pc on 20 ML thick C60 films on Ag(111). A ML is defined as a complete H2Pc layer on C60 with the structure determined from STM (see below). The bilayers were characterized using ultraviolet photoelectron spectroscopy (UPS), employing a He resonance lamp equipped with a linear polarizer, and by 2PPE with excitation using a Ti:sapphire oscillator (∼35 fs pulse width, 700−900 nm wavelength range). The molecular structure at the Pc\C60 interfaces was investigated in previous scanning tunneling microscopy 25415
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dependence. We also verified that this pump−probe scheme produced no measurable, overlapping photoemission response from C60, as noted in our previous work.38,39 This is consistent with negligible absorption in C60 at the 800 nm pump wavelength.47 More importantly, the highest occupied levels in C60 are 1 eV below the highest occupied levels of H2Pc (Figure 1b) so pump−probe excitations in C60 will lead to photoemission final states at least 1 eV below those due to excitation of H2Pc S1 excitons, ruling out the possibility of overlapping intensity. Weak intensity from C60 was observed due to a twophoton, probe-only process, but this was easily eliminated by reducing the probe power. Reducing the probe power was also desirable in order to limit the background due to the probeinduced one photon photoemission. TR-2PPE data were acquired with two formats. In the first, a series of spectra were collected at fixed pump−probe delays with excitation using combinations of the pump-plus-probe, the pump-only, and the probe-only wavelengths. The one-photon photoemission background at the lowest kinetic energies in the spectra was minimized by lowering the probe power, as discussed above, and the remaining small contribution was removed by subtracting the probe-only spectra from the pumpplus-probe intensity, giving difference spectra containing only the pump-induced response. For some samples, small shifts (several 10s of meV) between the pump-plus-probe and probeonly spectra were present due to photovoltage produced by the pump. This resulted in a weak remnant background that was not removed in the difference spectra, at the lowest energies in the spectra provided below, but this residual background was negligible in the region of the H2Pc S1 exciton population of primary interest. The pump-only and probe-only background spectra were also used to monitor and normalize the spectra for laser intensity variations. In a second set of TR-2PPE measurements, cross-correlation (CC) curves were acquired at specific energies across the spectra with typical steps of 100 meV. The intensity was measured at a fixed energy in the 2PPE spectrum as the pump− probe delay was varied from 0 to a maximum of 250 ps. The intensity at negative time (probe before pump) was measured before and after CC scans to correct for drift in the laser output. We verified that the CC curves scaled linearly with laser power confirming that higher order processes, such as exciton− exciton annihilation, were not significant. Our previous investigation of the energy dependence of exciton dissociation at the CuPc\C60 interface38,39 employed a pump wavelength of 750 nm, well into the Q-band absorption. For this, more detailed, investigation of the impact of H2Pc thickness we found it desirable to employ a pump wavelength closer to 800 nm. This was motivated by the significantly increased long-term stability of the Ti:sapphire oscillator at this wavelength. Measurements of optical absorption for H2Pc in the literature indicate 800 nm is close to the threshold for the Q-band, with weak absorption at this energy that depends on the exact polymorph of H2Pc.48−53 To ensure that measurements at 800 nm were indicative of the dynamics measured with excitation deeper into the Q-band absorption, we compared the pump-induced spectra and dynamics for 800 nm excitation with those at 750 nm. TR-2PPE spectra at these two energies contained pump-induced intensity that was, in both cases, consistent with excitations from the H2Pc HOMO (see below). The loss in intensity due to reduced Q-band absorption at 800 nm compared to 750 nm was compensated by the increase in laser power at 800 nm.
Figure 1. (a) Schematic illustration of the molecular orientation at the H2Pc\C60 interface determined with STM.41,42 (b) Schematic illustration of the interfacial electronic structure determined with UV photoemission and 2PPE from this work, inverse photoemission,45,46 and optical absorption. The excitation scheme used in the TR-2PPE measurements is also indicated, along with processes involved in H2Pc S1 dynamics at the interface. The arrows represent the (red) pump and (blue) probe excitations (hνprobe = 3hνpump). ICT = interfacial charge transfer states, PL = photoluminescent levels, kCT = charge transfer rate constant, and kRel = rate constant for intraband and\or vibronic relaxation. The energies are based on CuPc and C60 HOMO and LUMO levels (electron, hole polarons) measured with UPS, IPES, and 2PPE. The position of the ICT states was estimated as described in the text, consistent with an exciton binding energy of about 200−300 meV below the edge of the C60 LUMO (polaron) levels.
absorption while suppressing the one-photon background signal from the UV probe, which had a photon energy slightly above the sample work function, to a manageable level. The TR-2PPE excitation scheme is included in Figure 1b. The pump beam was focused to a diameter of about 100 μm, producing a peak power density of
