Femtosecond Time-Resolved Transient Absorption Spectroscopy with

Subscriber access provided by READING UNIV

Article

Femtosecond Time-Resolved Transient Absorption Spectroscopy with Sub-Diffraction-Limited Spatial Resolution Reveals Accelerated Exciton Loss at Gold-Poly(3-Hexylthiophene) Interface Tahir Zeb Khan, Patrice Donfack, Mahesh Namboodiri, Mehdi Mohammad Kazemi, Sidhant Bom, Veit Wagner, and Arnulf Materny J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11385 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Femtosecond Time-Resolved Transient Absorption Spectroscopy with Sub-Diffraction-Limited Spatial Resolution Reveals Accelerated Exciton Loss at Gold-Poly(3Hexylthiophene) Interface Tahir Zeb Khan, Patrice Donfack, Mahesh Namboodiri, Mehdi Mohammad Kazemi, Sidhant Bom, Veit Wagner, and Arnulf Materny* Physics and Earth Sciences, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany

*Author for correspondence: [email protected] 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

Abstract Molecules are known to change properties when in contact with metal surfaces. Therefore, also dynamics of photo-induced molecular excitons in a semiconductor are expected to be influenced by a metal contact. This effect, which is of considerable interest also for applications, is limited to interface excitons generated within just a few nanometer proximity to a metal layer. Up to now however, a highly localized access to such excitonic events has not been presented and diffraction-limited micro-spectroscopy did not yield any pattern in exciton dynamics other than that of bulk excitons, irrespective of an existing metal interface. In our work, we have combined femtosecond timeresolved spectroscopy with scanning near-field optical microscopy (SNOM) to study the interfacial dynamics of a gold-Poly(3-hexylthiophene) system (Au-P3HT) making use of tip-enhancement of the light fields of the ultrashort laser pulses by a gold rim surrounding the SNOM fiber tip, which collects the signal light. Next to singlet-singlet annihilation of free excitons in P3HT, which is an efficient loss mechanism at the laser powers employed, a direct exciton decay highly confined within the near-field range right at the Au-P3HT interface has been observed. We show that the occurrence of the goldcoated SNOM-tip-induced near-field enhancement of the optical fields permits selective access to the highly confined interfacial exciton decay. The experiments reveal that the early ultrafast exciton loss in P3HT becomes significantly faster at the Au-P3HT interface due to an additional pathway.

2


Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction The interaction of the electronic systems of metals and molecules plays an important role in many processes. Examples are catalysis,1 electronic contributions to surfaceenhanced scattering and emission processes,2 etc. Since also in semiconductor-based electronic devices, the interfaces between the semiconductor active medium and the metal electrode are essential for efficiency-determining processes such as charge transfer, injection or charge collection, a better understanding of the influence of the metal contacts on these elementary processes is of importance. Organic semiconductors are of great interest for the design of electronic devices such as light emitting diodes and solar cells due to the simplicity they offer for application as well as their low manufacturing costs.3 Meanwhile, a lot of work aiming at a better understanding of the elementary processes relevant to the functioning of organic semiconductor devices has focused on investigating the exciton dynamics at the interfaces between the domains of the active polymers.4-19 However, elementary exciton processes at the interface between the metal contact and the polymer layer have not been in the focus of researchers up to now. Many works have shown that the investigation of the energy level alignment at the metal-organic interfaces using different techniques like ultraviolet photoemission spectroscopy (UPS) or X-ray photoemission spectroscopy (XPS), etc.,20-29 yields insights into the magnitudes of energy barriers, which arise from the mismatch between the Fermi levels of the polaron states of the organic semiconductor materials and the metal electrode and which strongly influence charge transfer processes at the metal contact. The presumably best studied model system for electronic devices based on conjugated polymers is the contact between gold and Poly(3-hexylthiophene) (Au-P3HT). P3HT is a very promising material for applications of organic electronics and gold is used due to its very good ohmic contact with P3HT, since the HOMO level of P3HT and the work function of gold are very close to each other.22,24-26 In order to study the charge transport, which occurs in the accumulation layer buried at the interface between organic semiconductor and insulator, Pittner et al.27 have applied electroreflectance (ER) spectroscopy and used it as spatially resolved method for 3



Page 4 of 24

charge modulation analysis. In their work, these authors were able to gain information about the energetic disorder inside the P3HT-SiO2 accumulation layer, which has an effect on the local charge transport. They found that the ER spectral fingerprints of the dielectric function were significantly changed. These changes were shown to depend on the treatment method used for the SiO2 substrate as well as on the distance from the injection contact. Different energetic relaxation processes occurring during the charge transport were assumed to be responsible for the observed effects. Wagner and coworkers28 have also shown that a megahertz operation of the P3HT-based organic thin-film transistors can be applied for very fast switching. The limiting factor for the maximum switching frequency in organic transistors with channels of submicron lengths was shown to be effects at the metal-semiconductor contact described by contact resistance. The considerable number or reported works clearly point to the importance of a better understanding of the processes at metal-organic interfaces. Nevertheless, to our knowledge, an investigation of the dynamics of excitons using a technique, which allows for the selective excitation and probing of the ultrafast processes in the interfacial layer, is still missing. Recently, Marsh et al.6 have studied the exciton dynamics in a prototypical P3HT-only diode, i.e. a P3HT film, which was sandwiched between aluminum and ITO electrodes. To the best of our knowledge, this is the only femtosecond time-resolved investigation of exciton dynamics in organic semiconductors in presence of a metal contact. These authors did not observe a change of the exciton dynamics when the metal electrode was present and also the coating of the ITO glass electrode with a conducting (PEDOT:PSS) polymer layer did not influence the timeresolved results. We would like to point out that the goal of these studies was the investigation of exciton dynamics in situ using a typical electronic device, but not the investigation of exciton dynamics localized at the metal contact. The influence of the metal on the semiconductor exciton dynamics is expected to be limited to a nanometer range only. Despite not having been directly studied or observed optically in previous works, it can be expected that the exciton dynamic picture must be different in organic layers within just a few nanometer to the interface with a metal 4


Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrode, typically within the exciton diffusion length of about 3-10 nm in P3HT. Using a far-field spectroscopic technique for the investigation cannot access the highly spatially confined information due to the spatially averaged detection in the diffraction-limited farfield technique. Hence, the so far widely used approach in the study of the exciton dynamics in organic semiconductors using time resolved spectroscopy with diffractionlimited microscopic techniques4,9 is unable to provide details about local dynamics. Scanning near-field optical microscopy (SNOM) has a considerably higher lateral as well as axial resolution. Up to now, only few researchers have combined SNOM with femtosecond time-resolved spectroscopy, which is due to the technical difficulties inherent to the system.29-36 For instance, Nechay et al.29 used a combination of femtosecond pump-probe spectroscopy and SNOM to investigate the carrier dynamics in GaAs/AlGaAs single quantum wells. They have demonstrated that the observation of how lateral carrier diffusion effects the exciton dynamics is possible due to the increased spatial resolution. In our own work, we have studied very thin layers of an organic semiconductor38 and studied dynamics also using SNOM in combination with different nonlinear spectroscopy techniques32,33 Generally, there are two types of SNOM. The spatial resolution in the one case is realized by a fiber tip with small aperture with a size below the diffraction limit (see e.g. the experiment by Nechay et al.29 mentioned above); light for excitation and/or detection will have to pass the nanometer-sized aperture. In the other case, an apertureless metal tip of only few nanometers in size is used. There, the tip results in a local field enhancement, which increases the spectral response within a spatially restricted volume.30,31 The near-field characteristics due to the tip-enhancement results in an even higher lateral and axial resolution compared to the aperture-SNOM technique. Recently, we have used a metal-coated fiber SNOM tip in order to combine both techniques, aperture SNOM and tip enhancement.32,33 The signal generated in the regime of the field enhanced by the metal rim of the fiber was here efficiently collected by the fiber aperture of the SNOM. This increased axial resolution has been demonstrated for P3HT thin films.33 There, the chemically selective coherent anti-Stokes Raman scattering (CARS) imaging of surface nanostructures resulted in the observation of a particularly increased height resolution of the P3HT topography. 5



Page 6 of 24

The field enhancement can only be used in a sensible way when field-dependent effects changing the observed dynamics are taken into consideration. It is well known that different loss processes such as exciton trapping in defect sites as well as excitonexciton

annihilation

occur

and

compete

with

the

desired

diffusion

and

recombination/dissociation processes in optoelectronic devices that require high excitation densities.33 The annihilation dynamics have previously been widely studied in molecular crystals, biological systems and conjugated polymers.38-40 As has been demonstrated earlier,32 annihilation dynamics are occurring on a short timescale and are strongly depending on the pump laser excitation intensity, which determines the density of the excitons in the excited state. In continuation of our previous works,32,33,37 the work presented in the following combines scanning near-field optical microscopy with femtosecond time-resolved pumpprobe spectroscopy (PP-SNOM) to study concurrent exciton dynamics at the Au-P3HT interfaces, thereby revealing the elementary exciton quenching process localized at the metal surface. Experimental Section Poly-3-hexylthiophene-2,5-diyl (P3HT; Rieke Metals RMI-002EE) was dissolved in chlorobenzene to obtain a solution at a concentration of 35 mg/ml. The solution was stirred for approx. 20 h at a temperature of about 60 °C. As sample carriers, glass substrates were used, which were cut to 25 mm x 25 mm plates and then thoroughly cleaned with acetone, isopropanol, and DI-water successively. For a last cleaning step, the glass substrates put into an ozone cleaner for 10 minutes. Immediately after cleaning, the substrates were spin-coated at room temperature with the P3HT solution for 30 seconds at 1000 rpm. P3HT layers were prepared with a thickness of approx. 170 nm, which was verified with a Dektak surface profilometer. Annealing the coated substrat at 100 °C for 2 minutes was applied in order to remove residual solvent from the P3HT layer. A gold film of 10 nm thickness was deposited on the P3HT layer in a sputter coater (Quorum Q150T S) applying a shadow mask resulting in a metal contact pattern. Thus, the sample provided regions with neat P3HT as well as Au-P3HT as 6


Page 7 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shown in panel c of Figure 1. Both sample preparation and treatment were performed inside a clean room.

Figure 1: Experimental pump-probe SNOM and microscopy setup (a), sketch of nearfield contact of SNOM tip with sample (b), and top view of P3HT film with gold contacts (c). The scheme of the experimental set up is shown in panels a and b of Figure 1. It consisted of a regenerative amplifier Ti:Sapphire laser system (CPA 2010, Clark MXR), which produces laser pulses with 1 mJ energy per pulse and approx. 150 fs duration at a central wavelength of 775 nm with 1 kHz repetition rate. The CPA output pumped two optical parametric amplifiers (OPAs; TOPAS, Light Conversion). The outputs of the OPAs were compressed using a prism compressor setup in order to obtain pulses with approx. 80-90 fs pulse duration. The output of one of the OPAs was used as the pump pulse and that of the other OPA yielded the probe pulse. In our experiments, pump pulses at 520 nm were used for excitation and probe pulses at 650 nm were applied to monitor the dynamics initiated by the pump pulses. The timing of the individual pulses was controlled using retro-reflecting mirrors mounted on computer-controlled delay 7



Page 8 of 24

stages. Pump and probe beams were made collinear and coupled into an inverted microscope (Olympus). From the top, the sample surface could be accessed with a commercial scanning probe microscope (SPM) system (Nanonics Multiview 2000). The sample position was varied using a piezo-controlled XYZ translator. The collinear pump and probe beams were transmitted through the sample and the focus was adjusted to the P3HT film with an objective lens (Olympus PLAN N 10X, NA = 0.25) positioned below the sample. The experimental arrangement allowed for an excitation and probing of excitons close to the upper surface of the P3HT film as well as at the Au-P3HT interface by the light transmitted through the sample. The transmitted light pulses were collected from above the sample in the far field by using a second microscope objective of same type (10X, NA = 0.25) located above the sample or in the near field by positioning a SNOM fiber tip (Nanonics Multi view 2000) in the center of the focus of the transmitted light. Nanonics provided the cantilevered optical fiber SNOM tips, each consisting of a tip itself, a turning fork, and a multimode optical fiber extension. Each tip with a given aperture size was coated with a gold thin film. The tuning fork (cantilever) helped to maintain near-field distance. The multimode optical fiber extension guided the near-field signal, which was collected by the SNOM aperture, towards the detector. The gold rim surrounding the fiber-tip resulted in a localized enhancement of the laser fields resulting in high locally selective signal detection. The pump-pulse frequency was filtered out using a long-pass edge filter (Semrock, 532) while the probe pulse was detected using an avalanche photo diode (STM1DAPD 10, Amplification technologies, Inc). A boxcar amplifier was used to reduce noise as well as background signals. A SNOM tip with an aperture diameter of ~100 nm has been selected for the near-field experiments discussed below. The height of the tip (in the so-called AFM tapping mode) above the sample surface was kept constant using a feedback mechanism. Therefore, also the AFM topography of the investigated samples could be obtained with a spatial resolution, which was limited by the size of the SNOM tip (tip diameter). Results and Discussion In order to shed light onto the effect of the gold contacts on P3HT, we have investigated the early exciton dynamics in P3HT on neat P3HT as well as at the Au-P3HT interface 8


Page 9 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


firstly using a standard microscope observation technique in the far field and secondly applying the SNOM technique as introduced above to access the near field. In our experiments, we have studied the exciton dynamics within a time window from -5 to 15 ps for the delay between the pump (520nm) and the probe (650nm) pulses. Photoexcitation with high pump laser intensity (pulse energy on the order of 1 nJ strongly focused by the microscope objective) is used to primarily generate free excitons in P3HT with a density exceeding 1x1020 cm-3. Given the high exciton density in this case, as excitons diffuse through the polymer matrix, neighboring excitons are close enough to fuse together leading to exciton-exciton annihilation as the predominant exciton relaxation mechanism at early time scales. In the annihilation process, a higher excited singlet state Sn and a ground state So are generated, but the Sn state immediately decays through internal conversion to the initial excited state thus reducing the number of excitons involved in the annihilation process to half of its initial value on the initial excited state. We would like to point out the fact that the dominating role of the annihilation process in the overall dynamics is only due to the high pump laser intensity used in the experiment. When the exciton density decreases other mechanisms will start to be pronounced much more. The relaxation mechanisms from the initial excited state can be described by rate equations, 32,38-40 which have been used to extract respective parameters earlier. Since the goal of our present work was to detect an up to now not observed effect on the exciton dynamics in the interfacial region between gold and semiconductor, we have for simplicity used an exponential fit to the decay observed in the transient absorption, knowing that especially the annihilation process is not correctly described by this. However, we believe that this is sufficient to demonstrate potential changes of the dynamics due to metal-semiconductor interactions and to get a good estimate of the time scale of the metal-induced process. Before presenting the main results obtained, it is worth recalling that the exciton dynamics measured in the far-field are expected to only yield averaged information based on a signal, which is generated and collected from a micrometer-size region; therefore, diffraction-limited microscopy is not suited for a highly localized access to the 9



Page 10 of 24

contribution of the exciton dynamics in the interfacial region. Small differences between the exciton dynamics in neat P3HT and at the Au-P3HT interface cannot be seen in the far-field approach. This is confirmed by the earlier investigations by Marsh et al.6 as well as by our own experiments presented below. Therefore, PP-SNOM had to be used in order to minimize bulk contributions and selectively excite and probe the exciton dynamics, which are influenced by the coupling of the electronic system of P3HT to the metal at the contact interface. Using a SNOM tip with an aperture diameter of approx. 100 nm limits the access to a nanometer-sized region in the center of the illuminated sample spot, which has to be compared to the micrometer-sized region from where the averaged signal field is detected in far-field microscopic studies.4,6,9,32 More importantly, as mentioned earlier, the local field enhancement that arises when the tip is metal-coated increases the depth resolution.33,35 The SNOM tip-induced near-field enhancement is tightly localized within a few nanometers range. The small aperture of the fiber is capable of collecting light only from a small interaction volume resulting in weak signal intensities. Due to the localized enhancement of the pump and probe laser fields, the signal collected is dominated by a signal contribution from only a few molecule layers from the sample surface. We can easily demonstrate this tip-enhancement effect by first choosing high laser pulse energies, which in the far-field microscopy experiment are still safe for the sample. However, when the tip is brought to within a few nanometers above the surface, a highly localized damage only in the near-field range can be observed, which proves the strong confinement of the enhanced field. This increase of the axial resolution had been already demonstrated in recent studies where structure heights of 50 nm and less could be resolved.33 Due to the field enhancement accompanying the PP-SNOM experiment, the upper limit of the applicable laser powers had to be chosen based on the near-field experiments. The pulse intensities were maximized such that no damage occurred (reproducible transients) and at the same time sufficiently high exciton densities ensured that the annihilation process was close to the saturation regime. As a test, we observed the decay times as well as the transient probe pulse absorption while varying the pump laser 10


Page 11 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intensity approx. by a factor of 10. Only a negligible variation of the transients could be observed, which was within the small fluctuation range observed for repeated measurements. A convincing demonstration of the negligible dependence of the saturated annihilation dynamics on the laser power is given in Figure 2 where we compare the transient observed from pure P3HT using a standard micro-pump-probe arrangement with that obtained using the PP-SNOM setup were the pump field is locally enhanced as pointed out above. Here, no change of the dynamics can be observed. After having fulfilled the basic requirements for our highly spatially resolved femtosecond experiments (which proved to be very demanding), the investigation of the P3HT samples could be performed. The obtained results are depicted in Figures 3 and 4.

Figure 2: Pump-probe (PP) transients taken from the P3HT film without gold layer in a micro- and a PP-SNOM arrangement using the same laser powers. We first have investigated the topology as well as the probe absorption by recording the transmitted probe pulse intensity as a function of the surface position using PP-SNOM vs. conventional SNOM (i.e., without the initial pump pulse). Figure 3 compares 11



Page 12 of 24

128×128 pixels images obtained from neat P3HT with those observed from the AuP3HT interface region of the sample. The PP-SNOM image (panel b) was obtained at a fixed probe delay time of 500 fs. For both SNOM (panel a) and PP-SNOM (panel b) measurements, a 100 ms integration time per pixel has been used. Panel c displays an AFM image of the same region, which was recorded using the SNOM tip without laser interaction. Note that the lateral AFM resolution is not very high due to the relatively big size of the fiber tip compared to standard AFM tips. The roughness typically is of the order of 10 nm. Also due to the roughness, time resolved measurements (see Figure 4) have been repeated at several points along the horizontal line shown in panel b.

Figure 3: Investigation of an area of a 170 nm P3HT film partially coated with a 10 nm thick gold contact extending from neat P3HT to an adjacent Au-P3HT interface region (128x128 pixels resolution). Transmission of the 650 nm probe pulse measured using (a) SNOM without and (b) PP-SNOM with the initial 520 nm pump excitation (probe pulse is delayed by 500 fs relative to the initial pump pulse). (c) AFM topographical image using the SNOM tip without laser interaction (color code for transmitted probe intensity: blue = lowest intensity; red = highest intensity). The SNOM (panel a) and PP-SNOM images (panel b) in Figure 3 show the gold-plated P3HT region on the left and the neat P3HT on the right side. The color code shows the variation in the transmitted probe pulse intensity; blue means low and red high intensity, respectively. Metal contacts on the P3HT film have a net shielding effect resulting in a 12


Page 13 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reduced optical transmission in the region of the P3HT coated with gold. This information has to be used for appropriate signal correction for the evaluation of the pump-probe results. Calculating the differential probe-pulse transmission takes into account the shielding effect, which affects the measured pump-probe data by removing background absorption of the probe pulse in the absence of the pump pulse. The SNOM image obtained without the pump pulse shows the probe pulse transmission (panel a), where the probe pulse is not in resonance with the ground state absorption of the P3HT layer. The reduced intensity on the left-hand side of the sample is due to the shielding effect of the gold layer. Panel b displays the PP-SNOM image resulting from the transmitted probe pulse delayed by 500 fs relative to the initial pump pulse. The reduced intensity compared to the image shown in panel a is due to the excited-state absorption and therefore yields a snapshot visualization of the exciton dynamics in the P3HT film. Since now transient absorption of the excitons prepared by the pump pulse contributes, the PP-SNOM images provide chemical contrast. The AFM imaging that was taken with the SNOM tip when both pump and probe beams were blocked (panel c of Figure 3) provides pure topography information. The surface roughness clearly seen in the AFM image can influence the transmitted power density on micro to nanometer scales and thus also vary the initial exciton density due to varying morphology (sample thickness and roughness). Additionally, the roughness of the surface can by itself in general influence the exciton generation as was already mentioned above. An even increased inhomogeneity results from a preparation of very thin P3HT layers, which was avoided in our case. Indeed, point-to-point variations are seen in PP-SNOM transients recorded at different points of both neat P3HT and P3HT coated with gold (see panels b and d of Figure 4, respectively). The obtained 1/e time scales of the overall exciton signal decay vary from approx. 1700 to 2000 fs for the neat P3HT, and 1000 to 1200 fs for the Au-P3HT regions. Besides the fact that we see changed dynamics when gold is in contact with the P3HT, which points to the high axial resolution of the PP-SNOM approach, the high lateral resolution helps to estimate the influence of local surface and interface details on the dynamics of the optically formed excitons. These variations remain however rather 13



Page 14 of 24

small, such that differences, which are due to the metal-semiconductor interaction are not obscured.

Figure 4: PP-SNOM data taken along the horizontal line shown in the image given in panel a (compare panel b of Figure 3). (c) Probe-pulse transmission measured along the black horizontal line shown in the image, taken from the sample at a fixed delay of t = 500 fs after initial pump excitation. Transients recorded at several points along the horizontal line on (b) Au-P3HT and (d) neat P3HT. The transients have been taken along the horizontal line shown in the PP-SNOM image (panel a). Panel c shows the transient absorption of the probe laser for a fixed delay time between pump and probe pulses of 500 fs. The transition from neat P3HT to the gold-coated part can clearly be recognized. The shielding effect is also reflected in the non-corrected transients taken from the two sample areas.

14


Page 15 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In order to obtain a time scale of the overall exciton dynamics for neat P3HT and the AuP3HT interface without the influence of the topographical fine structures observed in the near-field images, many transients in either the neat P3HT or the Au-P3HT interface region of the sample have been averaged. The results are presented in Figure 5 as normalized data for the differential probe pulse transmitted intensity ∆ /

calculated

using the following formula:

Figure 5: Normalized time-dependent differential transmission (∆T/T) of the probe pulse at 650 nm as a function of delay time after pump pulse (520 nm) has prepared the excitons. Results from (a) far-field and (b) near-field experiments on neat P3HT, and from (c) far-field and (d) near-field measurements at the Au-P3HT interface. Insets in (b) and (d) show non-normalized differential transmission ∆T/T comparing far-field to nearfield conditions.

15



Page 16 of 24

pump‐on

pump‐off

(1)

pump‐off

Here,

pump‐on

is the transmitted intensity with initial pump laser interaction and

pump‐off

that without. The results obtained for neat P3HT and gold-coated P3HT from the far field experiments are displayed in panels a and c, and those resulting from the near filed investigation are shown in panels b and d of Figure 5, respectively. At a wavelength of 650 nm, the probe pulse sees the photo-induced absorption in P3HT, which is attributed to charge pairs instantaneously photo generated by the pump pulse interaction.4,6,10 Exponential fits of the time-dependent averaged far-field probe transmissions reveal no difference in the dynamics comparing neat P3HT and Au-P3HT, yielding an overall 1/e exciton loss time scale of approx. 1.9 ps. However, the exponentially fitted averaged probe signal absorption change reveals a clearly faster exciton dynamics in P3HT when it is in contact with gold, with a time scale of approx. 1.1 ps, which means a considerable reduction by approx. 42% as compared to neat P3HT. Drastic changes of the optical density (OD) comparing the far- and near-field experiments are revealed by the insets in panels b and d of Figure 5 that show nonnormalized ∆T/T transients. This important result highlights the more efficient detection of the exciton dynamics in the near-field experiment, which is due to the exclusive access to a small volume where the enhanced pump and probe laser both interact with the sample. In the far-field experiment a considerably spread amount of probe light will be collected, which is weak and not passing through the region where strong pump laser light has generated excitons, which then are responsible for transient absorption. Figure 6 shows the energy level diagrams of gold in panel a, neat P3HT in panel b, and the contact region between gold and P3Ht in panel c. The labels used in this figure are explained in the figure caption. Possible decay pathways for the exciton are indicated for P3HT in panel d and for the interfacial region in panel e. Due to the use of rather high light field densities, the annihilation process will on a short time scale be a dominating process, which we can assume to be comparable in both neat P3HT and Au-P3HT 16


Page 17 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interface (proof for this described above and depicted in Figure 2). The decrease of the decay time constant extracted from the PP-SNOM transients taken from the gold-coated part of the sample by approx. 42% reflects the fast additional decay channel opening up for the excitons due to the coupling to the electronic system of the metal. Our experiments clearly demonstrate the existence of this new pathway in the exciton dynamics and also give an idea about the timescale on which it influences the decay of the excitons in the S1 state.

Figure 6: (a) Energy level diagram of Au, where ɸm, Efm, and E0 are the work function, and the Fermi vacuum levels of pure Au, respectively. (b) Energy level diagram of neat P3HT, where ɸp, Ef, EV and Ec are the work function, chemical potential, and valance and conduction bands of P3HT, respectively. (c) Alignment of energy levels at the AuP3HT interface, where Fermi levels are equal at equilibrium. Exciton decay mechanisms depicting (d) mainly exciton-exciton annihilation in the neat P3HT region, and (e) exciton-exciton annihilation and a concurrent exciton quenching in the Au-coated region of P3HT. In panels d and e, the labels FS, STS, S0, S1, and Sn stand for Frenkel exciton states, self-trapped exciton states, ground, first, and higher excited states, respectively. An intense pump pulse at 520 nm (light grey arrow) is used for high density exciton 17



Page 18 of 24

generation in P3HT on the S1 level where the generated excitons rapidly annihilate one another while being probed by the 650 nm probe pulse (grey arrow) resonant with a higher lying Sn state. For neat P3HT, this surface layer signal is itself still dominated by the initially maximized exciton-exciton annihilation process and hence does not differ from the bulk signal, bearing the same pattern as in the far-field signal. However, for the Au-P3HT interface, excitons generated close to the metal contact will also follow the additional pathway opened by the coupling to the metal, providing an additional exciton loss mechanism that is significant and concurrent with the ongoing annihilation mechanism. Conclusions The influence of metals on the electronic and vibrational properties of molecules is well known. However, an access to the elementary processes entailing this interaction is difficult since the interfacial region has to be selectively probed. We have investigated the change of the ultrafast dynamics of optically excited excitons in P3HT when the organic semiconductor is coated with a thin layer of gold. Earlier experiments using diffraction-limited resolution did not yield differences between the dynamics observed for the semiconductor exciton dynamics of the neat semiconductor and semiconductor in contact to metal. This could also be confirmed in experiments presented in this work where a femtosecond time-resolved transient absorption experiment was accessing the exciton dynamics in a setup using standard microscope optics. Here, no influence of the gold could be observed. The reason for this is the dominating signal contribution from the bulk semiconductor material. In order to specifically excite and probe excitons in the interfacial region between gold and P3HT, we have combined aperture SNOM with tipenhancement of the laser fields making use of the surface enhancement effect arising from a gold coating of the SNOM fiber tip. In order to avoid field-dependent effects on the dynamics of the exciton decay, we have increased the field densities such that they were below the damage threshold of the sample, but high enough to drive the exciton annihilation process close to saturation.

18


Page 19 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Preliminary test experiments have been presented that demonstrate the strongly localized characteristics of the enhanced fields resulting in high axial resolution and confirm that the annihilation processes dominating the exciton dynamics is not influenced by the field enhancement of the pump-probe SNOM (PP-SNOM) experiment. Using PP-SNOM, a clear reduction of the lifetime of the optically prepared excitons could be observed in the interface region of P3HT and gold. While in neat P3HT the exciton dynamics is governed by annihilation dynamics in the observation window up to 15 ps, an additional decay pathway is opened by the metal, which accelerates the exciton relaxation by approx. 42%. The combination of aperture SNOM with a tip-enhancement effect has proven to be a powerful technique in the investigation of ultrafast dynamics in a sub-diffraction-limited range. The experiment has been extremely demanding, but it opens up new possibilities for similar studies. Future projects based on the use of the PP-SNOM technique might for example be: (i) the use of different pump and probe wavelengths would help to gain an even better insight into the processes, which have now been detected at the interface; (ii) different substrates and more complex semiconductor device structures will require a reflection scheme, which has to be realized and tested; (iii) parameters like film thickness, electrode arrangement (coupling) etc. have to be varied systematically. Besides the study of metal-semiconductor interfaces of course also dynamics at other interfaces (e.g. donor-acceptor) should be accessible. Acknowledgements This work was funded by the German Research Foundation (DFG; Grant No. MA 1564/23-1). References [1]

Hashmi, A. S. K.; Hutchings, G. J. Gold Catalysis. Angew. Chem. Int. Ed. 2006, 45, 7896-7936.

19



Page 20 of 24

[2]

McNay, G.; Eustace, D.; Smith, W. E.; Faulds, K.; Graham, D. Surface-Enhanced Raman Scattering (SERS) and Surface-Enhanced Resonance Raman Scattering (SERRS): A Review of Applications. Appl. Spectrosc. 2011, 65, 825-837.

[3]

Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-Based Organic Solar Cells. Chem. Rev. 2007, 107, 1324-1338.

[4]

Grancini, G.; Polli, D.; Fazzi, D.; Cabanillas-Gonzalez, J.; Cerullo, G.; Lanzani, G. Transient Absorption Imaging of P3HT:PCBM Photovoltaic Blend: Evidence for Interfacial Charge Transfer State. J. Phys. Chem. Lett. 2011, 2, 1099-1105.

[5]

Cook, S.; Furube, A.; Katoh, R. Analysis of the Excited States of Regioregular Polythiophene P3HT. Energy Environ. Sci. 2008, 1, 294-299.

[6]

Marsh, R. A.; Hodgkiss, J. M.; Albert-Seifried, S.; Friend, R. H. Effect of Annealing on P3HT:PCBM Charge Transfer and Nanoscale Morphology Probed by Ultrafast Spectroscopy. Nano Lett. 2010, 10, 923-930.

[7]

Guo, J.; Ohkita H.; Benten, H.; Ito, S. Charge Generation and Recombination Dynamics in Poly(3-hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies. J. Am. Chem. Soc. 2010, 132, 6154-6164.

[8]

Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Tierney, S.; Zhang, W.; Heeney, M.; McCulloch, I.; Nelson, J.; Bradley, D. D. Charge Carrier Formation in Polythiophene/Fullerene

Blend

Films

Studied

by

Transient

Absorption

Spectroscopy. J. Am. Chem. Soc. 2008, 130, 3030-3042. [9]

Grancini, G.; Biasiucci, M.; Mastria, R.; Scotognella, F.; Tassone, F.; Polli, D.; Gigli, G.; Lanzani, G.; Dynamic Microscopy Study of Ultrafast Charge Transfer in a Hybrid P3HT/Hyperbranched CdSe Nanoparticle Blend for Photovoltaics. J. Phys. Chem. Lett. 2012, 3, 517-523.

[10]

Kim, I.-S.; Sohn, B.; Hwang, S. I.; Lee, C.-L.; Ko, D.-K. Chirp-independent Timeresolved Spectroscopy Using the Self-Reference Method. Laser Phys. 2014, 24, 045701.

[11]

Kniepert, J.; Schubert, M.; Blakesley, J. C.; Neher, D. Photogeneration and Recombination in P3HT/PCBM Solar Cells Probed by Time-Delayed Collection Field Experiments. J. Phys. Chem. Lett. 2011, 2, 700-705.

[12]

Piris, J.; Dykstra, T. E.; Bakulin A. A.; v. Loosdrecht, P. H.; Knulst, W.; Trinh, M. T.; Schins, J. M.; Siebbeles, L. D. Photogeneration and Ultrafast Dynamics of 20


Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Excitons and Charges in P3HT/PCBM Blends. J. Phys. Chem. C 2009, 113, 14500-14506. [13]

Carrillo, J.-M. Y.; Kumar, R.; Goswami, M.; Sumpter, B. G.; Brown, W. M. New Insights into the Dynamics and Morphology of P3HT:PCBM Active Layers in Bulk Heterojunctions. Phys. Chem. Chem. Phys. 2013, 15, 17873-17882.

[14]

Tapping, P. C.; Kee, T. W. Optical Pumping of Poly(3-hexylthiophene) Singlet Excitons Induces Charge Carrier Generation. J. Phys. Chem. Lett. 2014, 5, 10401047.

[15]

Zhu, X.-Y.; Yang, Q.; Muntwiler, M. Charge-Transfer Excitons at Organic Semiconductor Surfaces and Interfaces. Acc. Chem. Res. 2009, 42, 1779-1787.

[16]

Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H.; Brida, D.; Cerullo, G.; Lanzani, G. Hot Exciton Dissociation in Polymer Solar Cells. Nature Mater. 2013, 12, 29-33.

[17]

Jailaubekov, A. E.; Willard, A. P.; Tritsch, J. R.; Chan, W.-L.; Sai, N.; Gearba, R.; Kaake, L. G.; Williams, K. J.; Leung, K.; Rossky, P. J. Hot Charge-Transfer Excitons Set the Time Limit for Charge Separation at Donor/Acceptor Interfaces in Organic Photovoltaics. Nature Mater. 2013, 12, 66-73.

[18]

Wu, G.; Li, Z.; Zhang, X.; Lu, G. Charge Separation and Exciton Dynamics at Polymer/ZnO Interface from First-Principles Simulations. J. Phys. Chem. Lett. 2014, 5, 2649-2656.

[19]

Wang, H.; Wang, H.-Y.; Gao, B.-R.; Wang, L.; Yang, Z.-Y.; Du, X.-B.; Chen, Q.D.; Song, J.-F.; Sun, H.-B. Exciton Diffusion and Charge Transfer Dynamics in Nano Phase-Separated P3HT/PCBM Blend Films. Nanoscale 2011, 3, 22802285.

[20]

Shaw, P. E.; Lewis, A. J.; Rusekas A.; Samuel I. D. W. Exciton Annihilation and Diffusion in Semiconducting Polymers. Proc. SPIE 2006, 6334, 63340G.

[21]

Ishii H.; Sugiyama K.; Ito E.; Seki K. Energy Level Alignment and Interfacial Electronic Structures at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 1999, 11, 605-625.

[22]

Park Y. D.; Cho J. H.; Kim D. H.; Jang Y.; Lee H. S.; Ihm K.; Kang T-H, Cho K. Energy-Level Alignment at Interfaces Between Gold and Poly(3-Hexylthiophene) 21



Page 22 of 24

Films with Two Different Molecular Structures. Electrochem. Solid-State Lett. 2006, 9, G317-G319. [23]

Kumatani A.; Li Y.; Darmawan P.; Minari T.; Tsukagoshi K. On Practical Charge Injection at the Metal/Organic Semiconductor Interface. Sci. Rep. 2013, 3, 1026.

[24]

Osikowicz W.; de Jong M. P.; Braun S.; Tengstedt C.; Fahlman M.; Salaneck W. R. Energetics at Au Top and Bottom Contacts on Conjugated Polymers. Appl. Phys. Lett. 2006, 88, 193504.

[25]

Park K. D.; Kim Y. H.; Park J. H.; Park J. S.; Lee H. S.; Yim S. J.; Lee Y. H.; Jeong M. S. Ultraviolet Tip-Enhanced Nanoscale Raman Imaging. J. Raman Spectrosc. 2012, 43, 1931-1934.

[26]

Mukherjee A.; Thakur A.; Takashima W.; Kaneto K. Minimization of Contact Resistance between Metal and Polymer by Surface Doping. J. Phys. D: Appl. Phys. 2007, 40, 1789-1793.

[27]

Pittner S.; Lehmann D.; Zahn D. R.; Wagner V. Charge Transport Analysis of Poly(3-Hexylthiophene) by Electroreflectance Spectroscopy. Phys. Rev. B 2013, 87, 115211.

[28]

Wagner V.; Wöbkenberg P.; Hoppe A.; Seekamp J. Megahertz Operation of Organic Field-Effect Transistors Based on Poly(3-hexylthiopene). Appl. Phys. Lett. 2006, 89, 243515.

[29]

Nechay B.; Siegner U.; Achermann M.; Bielefeldt H.; Keller U. Femtosecond Pump-Probe Near-Field Optical Microscopy. Rev. Sci. Instrum. 1999, 70, 27582764.

[30]

Deckert V. Tip-Enhanced Raman Spectroscopy. J. Raman Spectrosc. 2009, 40, 1336-1337.

[31]

Kramer A.; Trabesinger W.; Hecht B.; Wild U. Optical Near-Field Enhancement at a Metal Tip probed by a Single Fluorophore. Appl. Phys. Lett. 2002, 80, 16521654.

[32]

Karki K.; Namboodiri M.; Khan T. Z.; Materny A. Pump-probe Scanning NearField Optical Microscopy: Sub-Wavelength Resolution Chemical Imaging and Ultrafast Local Dynamics. Appl. Phys. Lett. 2012, 100, 153103.

[33]

Namboodiri M.; Khan T. Z.; Bom S.; Flachenecker G.; Materny A. Scanning NearField Optical Coherent Anti-Stokes Raman Microscopy (SNOM-CARS) with 22


Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Femtosecond Laser Pulses in Vibrational and Electronic Resonance. Opt. Express 2013, 21, 918-926. [34]

Ichimura T.; Fujii S.; Verma P.; Yano T.; Inouye Y.; Kawata S. Subnanometric Near-Field Raman Investigation in the Vicinity of a Metallic Nanostructure. Phys. Rev. Lett. 2009, 102, 186101.

[35]

Furusawa K.; Hayazawa N.; Catalan F. C.; Okamoto T.; Kawata S. Tip-Enhanced Broadband CARS Spectroscopy and Imaging Using a Photonic Crystal Fiber Based Broadband Light Source. J. Raman Spectrosc. 2012, 43, 656-661.

[36]

De Serio M.; Zenobi R.; Deckert V. Looking at the Nanoscale: Scanning NearField Optical Microscopy. Trends Anal. Chem. 2003, 22, 70-77.

[37]

Karki K.; Helms G.; Namboodiri M.; Wagner V.; Fritz J.; Materny A. Transient Grating Studies of Femtosecond Processes in Ultra-Thin Layers of PTCDA. Chem. Phys. Chem. 2012, 13, 477-481.

[38]

Braun S.; Salaneck W. R.; Fahlman M. Energy-Level Alignment at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 2009, 21, 1450-1472.

[39]

Cook S.; Furube A.; Liyuan H.; Katoh R. Singlet Annihilation in Films of Regioregular Poly(3-Hexylthiophene): Estimates for Singlet Diffusion Lengths and the Correlation between Singlet Annihilation Rates and Spectral Relaxation. J. Phys. Chem. C 2010, 114, 10962-10968.

[40]

Zaushitsyn Y.; Jespersen K. G.; Valkunas L.; Sundström V.; Yartsev A. Ultrafast Dynamics of Singlet-Singlet and Singlet-Triplet Exciton Annihilation in Poly(3-2‘Methoxy-5’-Octylphenyl)Thiophene Films. Phys. Rev. B 2007, 75, 195201.

23



Page 24 of 24

TOC Graphic

24


Femtosecond Time-Resolved Transient Absorption Spectroscopy with

Recommend Documents