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FEATURE ARTICLE
Confocal ultrafast pump–probe spectroscopy: a new technique to explore nanoscale composites
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Tersilla Virgili,*a Giulia Grancini,a Egle Molotokaite,a Inma Suarez-Lopez,a Sai Kiran Rajendran,a Andrea Liscio,b Vincenzo Palermo,b Guglielmo Lanzani,ac Dario Polliac and Giulio Cerulloa Received 2nd December 2011, Accepted 27th December 2011 DOI: 10.1039/c2nr11896c This article is devoted to the exploration of the benefits of a new ultrafast confocal pump–probe technique, able to study the photophysics of different structured materials with nanoscale resolution. This tool offers many advantages over standard stationary microscopy techniques because it directly interrogates excited state dynamics in molecules, providing access to both radiative and non-radiative deactivation processes at a local scale. In this paper we present a few different examples of its application to organic semiconductor systems. The first two are focussed on the study of the photophysics of phase-separated polymer blends: (i) a blue-emitting polyfluorene (PFO) in an inert matrix of PMMA and (ii) an electron donor polythiophene (P3HT) mixed with an electron acceptor fullerene derivative (PCBM). The experimental results on these samples demonstrate the capability of the technique to unveil peculiar interfacial dynamics at the border region between phase-segregated domains, which would be otherwise averaged out using conventional pump–probe spectroscopy. The third example is the study of the photophysics of isolated mesoscopic crystals of the PCBM molecule. Our ultrafast microscope could evidence the presence of two distinctive regions within the crystals. In particular, we could pinpoint for the first time areas within the crystals showing photobleaching/stimulated emission signals from a charge-transfer state. a Istituto di Fotonica e Nanotecnologie (IFN) CNR, Dipartimento di Fisica, Politecnico di Milano, P.zza L. Da Vinci 32, 20133 Milano, Italy. E-mail:
[email protected]; Fax: +390223996126; Tel: +390223996076
Tersilla Virgili graduated in Physics at the University of Bologna, Italy, in 1996. In 2000 she obtained a PhD in Physics from the University of Sheffield, UK, under the supervision of Prof. Donal Bradley. Since 2001 she has been a permanent researcher at the Institute of Photonics and Nanotechnologies (IFN) of the National Research Council of Italy. Her research activity involves different Tersilla Virgili research fields such as condensed matter physics, ultrafast spectroscopy, studies on organic microcavities working in weak or strong coupling and realization and characterization of organic devices.
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b Istituto Sintesi Organica e Fotoreattivit a, Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy c Center for Nanoscience and Technology CNST-IIT@POLIMI, Via Pascoli 70/3, 20133 Milano, Italy
Giulia Grancini was born in Pavia in 1984. In 2008 she received a master’s degree in Physical Engineering from Politecnico of Milan. From January 2009 to December 2011 she pursued a PhD in Physics at Politecnico of Milan. From September 2010 to March 2011 she joined the Physics Department at Oxford University, where she has developed expertise on the fabrication and electronic characterization Giulia Grancini of dye-sensitized solar cells. She is now a post doc in the Center for Nanoscience and Technology at the Italian Institute of Technology in Milan. Her main activities are devoted to the spectroscopic investigation of the photophysics of the interface in low dimensional multi-component systems, addressing in particular to the charge generation mechanism in polymer-based systems for organic photovoltaics. Nanoscale, 2012, 4, 2219–2226 | 2219
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Introduction Nowadays there are several microscopic techniques allowing the visualization of the nanoscopic structure of matter. In particular, fundamental and applied research in organic semiconductors not only calls for morphological characterization but also for spectroscopic investigations at the nanoscale. These materials, in the solid state, are generally amorphous or polycrystalline, with local order only achieved in mesoscopic domains with size ranging from a few tens to a few hundreds of nanometres. In addition, many devices use blends of different molecules, which undergo phase separation into domains of varying size and shape.1,2 The nature of these mesoscopic structures has a decisive influence on excited state dynamics, which in turn determines fluorescence quantum yield, charge carrier mobility and generation efficiency.3
Femtosecond transient absorption spectroscopy provides a wealth of information on the photophysics of organic semiconductors.4 The photogenerated excited singlet states can undergo a variety of processes, such as mono- and bimolecular decay, internal conversion, energy and charge transfer or intersystem crossing to a triplet state. Understanding the dynamics and efficiency of such processes is crucial, both from a fundamental point of view and for optimizing the efficiency of organic optoelectronic devices, which are based on these effects.5 Transient absorption spectroscopy is usually performed over relatively large sample areas, with diameter of the order of several tens of microns, thus obtaining a macroscopic information that is averaged over many mesoscopic domains. An instrument combining the temporal resolution of femtosecond spectroscopy with a microscopic spatial resolution would allow the study of excited state dynamics in individual domains. The
Vincenzo Palermo is group leader of the Nanochemistry lab at ISOF institute of the National Research Council of Italy. He works on the production and nanoscale characterization of new materials for optoelectronics. He obtained his PhD in 2003 at the University of Bologna. He previously worked at the University of Utrecht (the Netherlands) and at National Research Council (Canada). Vincenzo Palermo He is actually head of the CNR research unit ‘‘Functional Organic Materials’’, and coordinator of the project of the European Science Foundation GOSPEL and of the International Training Network GENIUS, and member of the scientific committee of EUROGRAPHENE.
Dario Polli was born in Milano (Italy) on September 5, 1976. In 2001 he graduated in Electronic Engineering at the Politecnico di Milano and in Physical Engi neering at the Ecole Centrale de Paris (France). He received a PhD degree in Physics from the Politecnico di Milano in 2005, where he is now Assistant Professor of Physics. His research activity is focused on the generation of tunable Dario Polli femtosecond pulses and on their application to ultrafast pump– probe spectroscopy of carotenoids, light-harvesting complexes and organic molecules. Other research topics include confocal and near-field microscopy, coherent Raman spectroscopy, non-linear optics and micromachining.
Guglielmo Lanzani is Director of the nascent Center for Nanoscience and Technology of the Italian Institute of Technology, and Associate Professor in Physics at the Politecnico di Milano. His activity in experimental physics concerns the science and technology of carbon based and nanostructured semiconductors, in particular photo-physics, optoelectronics, and photonics, including device manufacturing and characterization.
Giulio Cerullo is Full Professor of Physics in the Physics Department, Politecnico di Milano (Italy). His research activity has mainly focused on the physics and applications of ultrashort pulse lasers, covering a wide range of aspects. His current scientific interests concern generation of fewoptical-cycle pulses, ultrafast spectroscopy with time resolution down to a few femtoseconds, nonlinear microscopy and optical waveguide writing by ultrashort pulses. He has published more than 230 papers in international journals.
Guglielmo Lanzani
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Giulio Cerullo
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derived information would enable one to establish a link between the observed dynamical optical properties and the environment in which they occur,6 providing important feedback to material scientists who strive to control the morphology and the supramolecular organization of organic thin films in order to optimize the device performance.7 A few examples of time-resolved microscopes have been reported in the literature. Femtosecond confocal microscopy based on a laser oscillator was used to study single metal8,9 or semiconductor10 nanostructures or to image graphene flakes.11 Scanning near-field optical microscopy (SNOM) allows breaking the diffraction limit,12 exciting the surface of interest through a tapered and metalized optical fiber having an aperture at its apex, which is scanned in the optical near field of the sample. Time-resolved SNOM has achieved a combined 100 nm spatial and 100 fs temporal resolution and has been used to image the dynamics of bulk13 or nanostructured14,15 semiconductors. These instruments, however, due to their limited spectral tunability and/or complicated experimental setup, have not been systematically used for the study of organic semiconductors. In this paper we review the design and applications of an ultrafast confocal microscope specifically optimized for the study of nanostructured organic composites.16 The instrument delivers simultaneously high temporal (150 fs) and spatial (300 nm) resolution over the visible spectral range (450–750 nm). The basic characteristics of the experimental setup are discussed and its dynamical imaging abilities are shown. The measurements provide information not available with other stationary microscopy techniques, allowing a completely new insight into the local structure of organic materials. We present applications of this innovative tool to the study of excited state dynamics in conjugated polymer blends and in fullerene microcrystals of interest for optoelectronics and photovoltaics.
Experimental section The set-up The experimental setup of the ultrafast confocal microscope is shown in Fig. 1. It is driven by 10 mJ, 150 fs pulses at 1 kHz repetition rate and 800 nm wavelength derived from an amplified Ti:sapphire laser (Quantronix model Integra-C). The fraction of the pulse energy reflected by a first beam splitter is frequency doubled by a b-barium borate crystal to generate the pump pulses at 400 nm. The transmitted part is focused in a sapphire plate to produce a single-filament white light continuum spanning the 450–800 nm wavelength range, used as the probe pulse. An interference filter with 10 nm bandwidth selects single probe wavelengths. Pump and probe pulses, synchronized by a computer-controlled delay line, are collinearly recombined by a dichroic beam splitter and focused on the sample by a high numerical aperture air microscope objective (0.75 numerical aperture, magnification 100). The sample films are prepared on a reflective metal substrate, so that reflection becomes equivalent to a double-pass transmission. The film thickness ensures that all possible metal induced effects are negligible. The probe light, collected by the same microscope objective in a standard epi configuration, is then transmitted by a beam splitter, spectrally filtered to reject the pump light, focused on the 50 mm core of an This journal is ª The Royal Society of Chemistry 2012
Fig. 1 Experimental set-up of the ultrafast confocal microscope.
optical fiber, serving as the pinhole of the confocal microscope, and sent on a photodetector. Note that, due to the chromatic aberrations of the focusing objective, pump and probe beams have different focal planes. In our experiment we optimize the focusing of the probe on the sample; the pump will be out of focus and will excite a larger portion of the sample than that actually probed (global pump), thus providing a spatially uniform excitation of the locally probed area. The sample is raster scanned by a piezotranslator with nm accuracy on a 100 mm 100 mm scan area, allowing the acquisition of threedimensional linear transmission T(x,y,l) images as a function of sample position (x,y) and probe wavelength l. By modulating the pump beam at 500 Hz with a mechanical chopper and demodulating the collected probe light at the same frequency, we also simultaneously register four-dimensional differential transmission DT/T(x,y,l,s) images, which also depend on the pump–probe delay s. Practically one collects, for a given probe wavelength l, either DT/T(x,y) maps at a given delay s or DT/T(s) time traces at a given position (x,y) on the sample. The typical sensitivity is DT/T z 104. As demonstrated below, the instrumental temporal and spatial resolutions are 150 fs and 300–400 nm, depending on the probe wavelength. Materials Thin films of polyfluorene (PFO)/polymethyl methacrylate (PMMA) blends with few mm thickness were obtained by drop casting a solution of 50 mg PMMA and 5 mg PFO in 1 ml of toluene. The samples were prepared and measured under ambient conditions. Standard pump–probe spectroscopy of pure PFO films and PFO isolated in PMMA was performed as outlined in ref. 22. Thin films of regioregular poly(3-hexylthiophene) (P3HT) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were obtained from a solution (weight ratio 1 : 1) in dichlorobenzene (15 mg ml1) heated and stirred for one hour at 70 C. The solution was slowly spin coated (800 rpm for 10 seconds) on a metal substrate, leading to a sample with homogeneous thickness of a few hundreds of nanometres. The sample was prepared in an inert gas atmosphere inside the glove box. By Nanoscale, 2012, 4, 2219–2226 | 2221
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thermal annealing (at 140 C for 12 minutes under nitrogen atmosphere) we thermodynamically altered the phase separation of the blend, ensuring high crystalline order and purity within each phase. Therefore our sample is made of PCBM-rich clusters immersed in mostly fibrillar-like P3HT crystals and in an amorphous P3HT region.17–19 The samples were measured at room temperature and in air. Highly ordered mesoscopic crystals of PCBM were grown on SiOx using an original technique of dip coating in saturated solvent vapours, as described in detail in previous works.20 This technique allows the growth of PCBM crystals having a size between 1 and 80 mm in diameter and from 20 to 500 nm in thickness. Spatial and temporal resolution In order to assess the performance of our ultrafast confocal microscope, we used a film of a PFO/PMMA blend, which will be described more in detail in the following section. Fig. 2(a) shows a linear transmission map of a 20 20 mm2 area of the sample at 600 nm probe wavelength. Due to phase separation, PFO disks of a few mm diameter are formed in the PMMA matrix. The contrast is rather low because neither PFO nor PMMA absorbs at this wavelength, and can be attributed to local variations in scattering. Fig. 2(b) shows the DT/T map at s ¼ 200 fs delay: we observe a strong difference between PMMA, which displays zero photoinduced signal being transparent at the pump wavelength, and PFO, which exhibits a negative signal due to photoinduced absorption at this wavelength (PA see Fig. 3(a)). The contrast and the spatial resolution of the DT/T image (in Fig. 2(b)) are much higher than those of the linear transmission map (in Fig. 2(a)), thus allowing a clearer visualization of the PFO distribution in the blend. Note that the ultrafast microscope also allows us to differentiate between the PFO and the impurities in the blend: the latter appear dark as the PFO in the linear transmission map (see black arrows in Fig. 2(a)) but provide zero signal in the DT/T map (Fig. 2(b)). Fig. 2(c) shows a horizontal
Fig. 3 (a) Differential transmission spectra at 200 fs probe delay for the isolated polymeric chain (form A, blue line) and for the aggregated polyfluorene (form B, red line). Linear (b) and differential (c and d) transmission maps of the PFO–PMMA blend at 640 nm (b and d) and 510 nm (c) probe wavelengths. The black contours in (b) indicating the PFO regions were also reported in white in panel (d) to highlight the vanishing differential signal at the disks borders at this wavelength.
cut of Fig. 2(b), across the transition from PFO to PMMA, which occurs within 500 nm (90% to 10% of the signal). Assuming a much smaller interface area and thus a step spatial signal, one can deconvolve from the data a Gaussian spatial point spread function with 300 nm FWHM. Fig. 2(d) shows a DT/T(s) time trace at a single position inside one of the PFO islands (indicated with a gray spot in Fig. 2(b)). The PA signal rises within 150 fs, demonstrating the high temporal resolution of the microscope. The combination of spatial and temporal resolution together with the probe wavelength tunability enables a wealth of novel investigations on the photophysics of nanostructured materials.
Pump–probe confocal microscopy of organic blends Photophysics of the PFO/PMMA blend
Fig. 2 Linear (a) and differential (b) transmission maps of the PFO (disks)–PMMA (surrounding background) blend. (c) Horizontal cut of (b) along the blue dashed line, highlighting the high spatial resolution. (d) Pump–probe dynamics on a PFO disk showing a rise time within 150 fs.
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Understanding the structure and optical properties of phaseseparated organic semiconductor blends is of both fundamental and practical interest. In most photonic and optoelectronic applications (LEDs, solar cells, etc.) the use of mixtures of polymers as active material is nowadays indispensable. In such samples, phenomena occurring at interfaces between two materials are extremely complex and poorly understood. Here we study a PFO/PMMA blend which has attracted a great attention as an active component for photonic applications such as amplifiers, optical switches or gain media.21–23 We discover, thanks to our ultrafast confocal microscope, the presence of interface regions at the border of phase-segregated islands, where the photophysical properties of polyfluorene are dramatically changed as compared to those of the bulk.24 The PFO/PMMA blend has been previously studied by conventional, spatially averaged pump–probe.25 It was found that, according to the position in the sample, the PFO polymer This journal is ª The Royal Society of Chemistry 2012
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chains present two different forms of aggregation. In the first form (called A in the following) the chains are substantially isolated due to their dispersion in the PMMA inert matrix, so that no interchain interaction is present. In the second form (called B) the PFO chains are strongly aggregated and interchain interaction occurs. Fig. 3(a) shows the spatially averaged DT/T spectra at s ¼ 200 fs probe delay for the two forms. In the case of isolated chains (form A, blue line), we observe a stimulated emission (SE) signal over a broad spectral region (420–610 nm) and a structured photoluminescence spectrum (not shown here), indicating that the PFO is probably in b-phase conformation.26 For the aggregated chains (form B, red line) SE is overwhelmed by the PA band due to the charges generated by interchain dissociation. The presence of this PA band is detrimental for photonic applications such as lasers and amplifiers and therefore the PFO/PMMA blends, containing isolated PFO chains, provide superior photonic properties with respect to the pure PFO film. The sample used for this experiment presents large millimetric areas (visible by the naked eye) dominated by either the inert matrix or the polymer. However the spatial distribution and the control of isolated and aggregated chains in these systems is an open question. Using ultrafast confocal microscopy, we focus our attention on the PFO-rich regions to investigate whether the phase separation occurs even at the nanoscale. Fig. 3(b) shows the T(x,y) map on a 14 14 mm2 area at l ¼ 510 nm probe wavelength; the PFO islands appear dark due to increased scattering. Fig. 3(c) and (d) show the DT/T(x,y) maps for s ¼ 200 fs pump–probe delay at 510 nm and at 640 nm, respectively. At 510 nm probe wavelength (Fig. 3(c)) we can clearly discriminate between isolated and aggregated PFO phases (see DT/T spectra in Fig. 3(a)): for the isolated chains the signal is positive (SE), while for the aggregated ones the signal is negative (PA band due to interchain charges). Using the conventional 100 mm spatial resolution of pump–probe spectroscopy, the spatially averaged DT/T signal is negative, suggesting the presence of largely aggregated PFO chains. Instead the picture that comes out with higher spatial resolution is completely different, with nanoscale phase separation appearing within the PFO microislands. These islands display a spatially varying DT/T signal, with a negative area in the centre (red in Fig. 3(c)) surrounded by a positive ring (blue).24 We are thus able to highlight an interface region at the border of phase-segregated islands, where there is an efficient mixing of the two polymers, resulting in isolation of PFO chains. The same difference is evident in Fig. 3(d) where the strong negative signal in the centre of the micro-islands (yellow) is in contrast with the almost zero signal at the borders (black). Our time-resolved microscope shows that even a blend of an active polymer into an inert matrix displays peculiar dynamics at the interface, which could not be observed with standard optical microscopy. Such behaviour is due to the strong dependence of PFO charge separation physics on interchain interactions.25 This has important implications for photonics and optoelectronics, where conjugated polymer chain isolation and intimate polymer intermixing are crucial for device performance.
these systems the role of interface physics is of fundamental interest, since it strongly affects the overall device performance. The introduction of the bulk heterojunction (BHJ) concept, where donor/acceptor components are intimately mixed together in a fine dispersed phase separated morphology, has boosted the organic-based polymer solar cell efficiency compared to bilayer technology. The most studied system comprises a light-absorbing electron-donor polymer mixed with a soluble PCBM, acting as the electron acceptor. Since it has not yet been possible to replace the fullerene derivatives with equally efficient electron acceptors, the preferred way to improve the solar cell efficiency27 is through the choice of the polymer. Ideally the sun’s maximum photon flux, lying around 650–700 nm, should be covered. In the past decades, since the introduction of the BHJ technology, semicrystalline regioregular P3HT has attracted a lot of attention as a donor material, as it has an absorption edge around 650 nm, combined with a high hole mobility.28 Promising power conversion efficiencies, well over 3%, have been reported for P3HTbased solar cells.29,30 A prerequisite for optimising the efficiency is understanding the relation between the morphology of the active film and device efficiency. Recent experimental investigations suggest that increasing phase segregation can reduce geminate recombination and improve the overall charge photogeneration yield.31 Our ultrafast confocal microscope can spatially resolve the photophysical properties of the different P3HT:PCBM crystalline phases. We demonstrate that the morphology not only plays a role in controlling the transport properties, but also directly influences the kinetics of charge separation/recombination after thermalization.32 We identify a peculiar long-lived charge-transfer state (CTS), not affected by geminate recombination, located at the border region between P3HT:PCBM microscopic phases; it displays a highly polarized component, and it is described by a quantum superposition of the donor (P3HT) and acceptor (PCBM) states. The linear transmission image at l ¼ 640 nm, displayed in Fig. 4(a), shows the morphology of the blend; the whitish areas represent the PCBM-rich crystals, while the dark areas indicate the P3HT-rich domains. The DT/T map (s ¼ 200 fs, l ¼ 640 nm) in Fig. 4(b) reveals regions of positive signal (red) embedded in a mainly negative background (blue/black). The positive signal is due to SE from amorphous regions of P3HT, as reported previously.33,34 In this phase the singlet excitons do not dissociate, thus preventing the photovoltaic operation. In order to understand the physics of the interface, three different dynamics have been recorded from a PCBM-rich aggregate region (named A in Fig. 4), a P3HT-rich crystal (named C in Fig. 4) and from the interface region (named B in Fig. 4). It is clear that the dynamics recorded at the interface is not just a linear superposition of those in the P3HT and PCBM-rich regions. This means that the signal is not a simple spatial average of the two neighbouring crystalline phases. This long-lived PA signal has been assigned to a peculiar coherent interfacial CTS,32 consisting of partially separated, coulombically bound charge pairs, where the hole is localized on the polythiophene and the electron is on the fullerene.
Photophysics of the P3HT:PCBM photovoltaic blend
Photophysics of PCBM single crystals
The blend of regioregular P3HT with PCBM is one of the most commonly used in efficient plastic photovoltaic solar cells. In
PCBM35 is nowadays the best electron acceptor in BHJ solar cells in terms of device performance. The energy gap between the
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Fig. 4 Linear transmission image (a) and DT/T(x,y,s ¼ 200 fs) map (b) at l ¼ 640 nm for the P3HT/PCBM blend. (c) DT/T dynamics at l ¼ 640 nm for the PCBM-rich aggregate region (point A), the interfacial region (point B) and the P3HT-rich crystal (point C).
highest-occupied molecular orbital (HOMO) and the lowestunoccupied molecular orbital (LUMO) of PCBM is reduced compared with that of standard C60, allowing for increased photoelectric conversion efficiencies. Despite the large number of papers studying the optoelectronic properties of PCBM in blends and solar cells, the exact molecular packing of PCBM is still not clear. In blends with other molecules, PCBM can either disperse on molecular scale in the donor matrix or phase separate forming irregular crystals or rounded agglomerates, depending on deposition and post-treatment (e.g. annealing) conditions.36–38 The different results were mainly due to the fact that PCBM tends to form disordered, amorphous aggregates composed of nanocrystals with random orientations, whose structure depends sensitively on the solvent used for the deposition, and often includes solvent molecules in the lattice. It is thus important to find new ways to form large crystals of PCBM, having welldefined size and orientation. The preparation of molecular solids with high degrees of symmetry, thanks to their simple physical and chemical properties, makes it possible to gain a better understanding of intermolecular interactions also in the low-symmetry analogues. Here we study the photophysics of single micrometre crystals of PCBM. This study perfectly fits the general purpose of understanding how to manipulate the crystalline content in a PCBM-based blend in order to improve the BHJ efficiency.39 Fig. 5(a) shows the DT/T spectrum of PCBM obtained in a spatially averaged pump–probe experiment. The spectrum 2224 | Nanoscale, 2012, 4, 2219–2226
shows a large PA band all over the spectral visible region as previously reported40 due to the transition from the photoexcited S1 state to higher lying singlet states. Fig. 5(b) shows the linear transmission map (20 mm 20 mm) at l ¼ 520 nm, revealing a hexagonal crystal with 8 mm long side. The crystal is not homogeneous, displaying high (area B in Fig. 5(b)) and low (area A) transmission. This is related to different local scattering and/or absorption coefficients, likely due to a different fraction of solvent molecules, which are known to be included in the crystals.20 This presence could change the thickness and the molecular packing of the PCBM inside the crystal. The DT/T signal changes in sign in the two regions (see Fig. 5(c)): it is positive in area A and negative in area B. Cook et al.41 have studied the different optical behaviours of a pristine PCBM film where the fullerene molecules are expected to be closely packed and a film of PCBM molecules dispersed in polystyrene. They found that when the molecules of PCBM are well packed the film can be considered crystalline and strong intermolecular interactions result in the formation of a CTS. The absorption/emission transition of this state is around 500 nm,42–44 and it is not visible when the molecules of PCBM are isolated. The same photophysics is also present in our crystals: in area A the PCBM molecules are more closely packed, thus giving rise to a photobleaching (PB)/SE signal from the CTS, which overwhelms the singlet–singlet PA which instead dominates in the less aggregated areas of the PCBM crystal (area B). The presence of this state is consistent also with previous measurements done in crystalline PCBM.45,46 However, the PB/SE signal from the CTS was never observed before, probably due to its reduced intensity and extension in the crystal volume, which would vanish in a spatially averaged pump–probe. The ultrafast confocal microscope thus allows mapping with diffraction-limited resolution the photophysics of different regions of the PCBM crystal, showing how the molecular packing affects the photophysics via formation of a CTS.
Fig. 5 (a) Differential transmission spectrum at 200 fs probe delay for the microcrystal of PCBM taken with the conventional pump–probe (spatial resolution of 120 mm). Linear (b) and differential (c) transmission maps of the microcrystal at 520 nm probe wavelength. (d) DT/T dynamics in the ‘‘A’’ and ‘‘B’’ regions indicated in (b) and (c).
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Conclusions In this article we have described a new spectroscopic tool based on a time-resolved microscope, combining high temporal (150 fs) and spatial (300 nm) resolution. We have shown the potential of this technique by presenting a few examples: (i) we have studied the photophysics at the interface between the PMMA and PFO domains, revealing the efficient mixing of the two polymers at the borders that results in the isolation of the PFO chains; (ii) we have highlighted the creation of a peculiar long-lived CTS at the interface of a P3HT/PCBM blend for photovoltaic applications; and (iii) we have investigated the spatially varying photophysics inside a microcrystal of PCBM. We found areas where the packing of PCBM molecules leads to the formation of a CTS with a strong PB/SE signal. The CTS transitions have been previously observed; however the presence of a positive DT/T signal has never been reported due to the lack of spatial resolution. The innovative instrument described in this work represents a powerful new tool for the investigation of energy relaxation and charge transfer dynamics in the phase separated organic film, extending it to a previously unexplored spatial range. We expect that our ultrafast confocal microscopy technique will allow imaging the spatial variation of excited state generation and deactivation not only due to composition change but also due to local interactions, opening up a wealth of new investigations in nanostructured organic materials.
Acknowledgements T.V. and I.S.L. thank the FONDAZIONE CARIPLO for funding the ‘‘Local micro-tailoring of conjugated polymer emission by spatially resolved nanoparticles implantation for next-generation light-emitting devices’’ 2009-2562 project. S.K.R. thanks the European Union for funding via FP7 ITN project Icarus (237900). V.P. and A. L. thank R. Dabirian, M. Gazzano and P. Samorı. D.P. acknowledges financial support by the HFSP program grant number RGP0005 and the ‘‘5 per mille junior’’ research grant by Politecnico di Milano.
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