Photoinduced Charge Carrier Generation in a Poly(3-hexylthiophene) and Methanofullerene Bulk Heterojunction Investigated by Time-Resolved Terahertz Spectroscopy

Observation of an Exciton-Plasma Transition in a Molecular Semiconductor

The nonfullerene electron acceptors (NFAs) for organic solar cells are attracting intense research efforts due to their impressive performance. Understanding the temporal evolution of the excited states in NFAs is essential to gain insights into the working mechanism of these state-of-the-art devices. Here we characterized the photoconductivities of a neat Y6 film and a Y6:PM6 blend film using time-resolved terahertz spectroscopy. Three different types of excited states were identified based on their distinct terahertz responses, i.e., plasma-like carriers, weakly bound excitons, and spatially separated carriers. Under high-intensity excitation, the many-body interaction of excitons in the Y6 film leads to the plasma-like state, giving rise to a terahertz response characteristic for a dispersive charge transport. This transient state decays quickly into exciton gas due to fast Auger annihilation. Under low-intensity excitation, only isolated excitons are created and the plasma state is absent.

Charge-transfer electronic states in organic solar cells: a TDDFT study

The prediction of new organic photovoltaic materials in organic solar cells (OSCs) must include a precise description of charge-transfer states because they are involved in electron-transfer processes such as charge separation and charge recombination which govern the device efficiency. Also, as the experimental performance of an optoelectronic device is measured for nonequilibrium nanostructures, computational approaches need models that can incorporate morphology effects. Usually, this aspect is treated by molecular dynamics simulation (MDS) methodologies; however, methodologies and formalisms to calculate the electron-transfer processes are still controversial and sometimes do not connect their information with the phase morphologies. In this work we propose a simple and fast characterization of electron-transfer processes to find the rate constants by analysing the distribution of vertical excitation energies of both local excitation (LE) and charge-transfer (CT) states using TD-DFT calculations in the donor-acceptor pair structures which were extracted from MDS. This proposal assumes that conformational changes are prevented and equilibria are not achieved while the electron-transfer events take effect, and thus the only pathway that connects the LE and CT states is their surface crossing point where an ideal distribution might exist. Different density functionals and dialectric models were tested. The results indicate a close relationship between the proposal and experimental data for electron-transfer events, suggesting the application of this method in the rational design of new photovoltaic materials.

Ultrafast Charge Generation Enhancement in Nanoscale Polymer Solar Cells with DIO Additive

We study the ultrafast photoexcitation dynamics in PBDTTT-C-T (P51, poly(4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)-benzo[1,2-b:4,5-b']dithiophene-alt-alkylcarbonyl-thieno[3,4-b]thiophene)) film (~100 nm thickness) and PBDTTT-C-T:PC71BM (P51:PC71BM, phenyl-C71-butyric-acid-methyl ester) nanostructured blend (∼100 nm thickness) with/without DIO(1,8-diiodooctane) additives with sub-10 fs transient absorption (TA). It is revealed that hot-exciton dissociation and vibrational relaxation could occur in P51 with a lifetime of ~160 fs and was hardly affected by DIO. However, the introduction of DIO in P51 brings a longer lifetime of polaron pairs, which could make a contribution to photocarrier generation. In P51:PC71BM nanostructured blends, DIO could promote the Charge Transfer (CT) excitons and free charges generation with a ~5% increasement in ~100 fs. Moreover, the dissociation of CT excitons is faster with DIO, showing a ~5% growth within 1 ps. The promotion of CT excitons and free charge generation by DIO additive is closely related with active layer nanomorphology, accounting for Jsc enhancement. These results reveal the effect of DIO on carrier generation and separation, providing an effective route to improve the efficiency of nanoscale polymer solar cells.

Evaluation-oriented exploration of photo energy conversion systems: from fundamental optoelectronics and material screening to the combination with data science

Light is a form of energy that can be converted to electric and chemical energies. Thus, organic photovoltaics (OPVs), perovskite solar cells (PSCs), photocatalysts, and photodetectors have evolved as scientific and commercial enterprises. However, the complex photochemical reactions and multicomponent materials involved in these systems have hampered rapid progress in their fundamental understanding and material design. This review showcases the evaluation-oriented exploration of photo energy conversion materials by using electrodeless time-resolved microwave conductivity (TRMC) and materials informatics (MI). TRMC with its unique options (excitation sources, environmental control, frequency modulation, etc.) provides not only accelerated experimental screening of OPV and PSC materials but also a versatile route toward shedding light on their charge carrier dynamics. Furthermore, MI powered by machine learning is shown to allow extremely high-throughput exploration in the large molecular space, which is compatible with experimental screening and combinatorial synthesis.

Open-Circuit Voltage of Organic Photovoltaics: A Time-Dependent and Unrestricted DFT Study in a P3HT/PCBM Complex

Solar cells based on blends of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, P3HT/PCMB, constitute one of the most efficient polymer photovoltaic cell types. One of the main factors that determine the efficiency of the solar cells is the open-circuit voltage, V_OC. In this work, we provide an analysis of the parameters affecting the V_OC in a P3HT/PCBM complex. Electronic transitions, excited states, and electron transfer parameters are evaluated under the classical Marcus formalism via the time-dependent and unrestricted CAM-B3LYP/6-31G* methods. The charge-recombination driving force is found to mainly affect the charge-recombination rate constant and, in turn, V_OC. Even though other parameters also determine the value of V_OC like density of states, dimensions of the cell, and microstructure of the donor/acceptor interface, the current work highlights the understanding attained by modeling charge-transfer parameters. The analysis reported here encourage further quantum-chemical investigations in organic photovoltaics with the aim of estimating and improving V_OC, such that more efficient organic solar cells may be predicted.

Complex Photoconductivity Reveals How the Nonstoichiometric Sr/Ti Affects the Charge Dynamics of a SrTiO3 Photocatalyst

Strontium titanate (SrTiO₃) is a perovskite that is important in water-splitting photocatalytic chemistry. Although excess Sr is known to improve the photocatalytic activity, its effect on charge dynamics remain largely unaddressed. Herein, we present a detailed analyses of gigahertz complex transient photoconductivity (Δσ) measured using time-resolved microwave conductivity (TRMC). We show that charge carrier trapping associated with the emergence of an anomalous positive imaginary part and the first-order rate constant of the normal positive real part of Δσ dramatically decreased with increasing Sr/Ti ratio. The second-order rate constant attributed to charge recombination simultaneously decreased, and these rate constants were well correlated with the improved hydrogen evolution rate of aqueous SrTiO₃ suspensions with a Pt co-catalyst. These findings provide a fresh perspective on the stoichiometry-carrier dynamics relationship paramount for the optimization of composition-engineered photocatalysts and reveal the broad implications for mechanistic studies based on TRMC evaluation.

Direct Observation of Structure and Dynamics of Photogenerated Charge Carriers in Poly(3-hexylthiophene) Films by Femtosecond Time-Resolved Near-IR Inverse Raman Spectroscopy

The initial charge separation process of conjugated polymers is one of the key factors for understanding their conductivity. The structure of photogenerated transients in conjugated polymers can be observed by resonance Raman spectroscopy in the near-IR region because they exhibit characteristic low-energy transitions. Here, we investigate the structure and dynamics of photogenerated transients in a regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) blend film, as well as in a pristine P3HT film, using femtosecond time-resolved resonance inverse Raman spectroscopy in the near-IR region. The transient inverse Raman spectrum of the pristine P3HT film at 50 ps suggests coexistence of neutral and charged excitations, whereas that of the P3HT:PCBM blend film at 50 ps suggests formation of positive polarons with a different structure from those in an FeCl₃-doped P3HT film. Time-resolved near-IR inverse Raman spectra of the blend film clearly show the absence of charge separation between P3HT and PCBM within the instrument response time of our spectrometer, while they indicate two independent pathways of the polaron formation with time constants of 0.3 and 10 ps.

A time-resolved millimeter wave conductivity (TR-mmWC) apparatus for charge dynamical properties of semiconductors

This article demonstrates a contactless, time-resolved, millimeter wave conductivity apparatus capable of measuring photoconductivity of a diverse range of materials. This cavity-less system determines the time-dependent magnitude of a sample's charge carrier density-mobility product by monitoring the response of a continuous, millimeter-wave probe beam following excitation of the sample by an ultrafast laser pulse. The probe beam is tunable from 110 GHz to 170 GHz and the sample response data can be obtained over the sub-nanosecond to millisecond time interval. This system has been tested on silicon wafers, S-I GaAs, perovskite thin films, SiO₂-Ge(nc), and CdS_xSe_1-x nanowire samples. We demonstrate a minimum detectable photoconductance change of ∼1 µS, an estimated time resolution for conductance decay of ∼100 ps, and a dynamic range greater than 57 dB. The calibration constant of the system, needed for quantitative calculation of photoconductivity from experimental data, has been determined using silicon wafers. This system has several advantages over currently used microwave and terahertz techniques, such as facile tunability of probe frequency and substantially wider time range for study of decay kinetics, while maintaining an open sample environment that enables characterization of a wide range of sample sizes under controlled environmental conditions.

Mobility dependent recombination models for organic solar cells

Modern solar cell technologies are driven by the effort to enhance power conversion efficiencies. A main mechanism limiting power conversion efficiencies is charge carrier recombination which is a direct function of the encounter probability of both recombination partners. In inorganic solar cells with rather high charge carrier mobilities, charge carrier recombination is often dominated by energetic states which subsequently trap both recombination partners for recombination. Free charge carriers move fast enough for Coulomb attraction to be irrelevant for the encounter probability. Thus, charge carrier recombination is independent of charge carrier mobilities. In organic semiconductors charge carrier mobilities are much lower. Therefore, electrons and holes have more time react to mutual Coulomb-forces. This results in the strong charge carrier mobility dependencies of the observed charge carrier recombination rates. In 1903 Paul Langevin published a fundamental model to describe the recombination of ions in gas-phase or aqueous solutions, known today as Langevin recombination. During the last decades this model was used to interpret and model recombination in organic semiconductors. However, certain experiments especially with bulk-heterojunction solar cells reveal much lower recombination rates than predicted by Langevin. In search of an explanation, many material and device properties such as morphology and energetic properties have been examined in order to extend the validity of the Langevin model. A key argument for most of these extended models is, that electron and hole must find each other at a mutual spatial location. This encounter may be limited for instance by trapping of charges in trap states, by selective electrodes separating electrons and holes, or simply by the morphology of the involved semiconductors, making it impossible for electrons and holes to recombine at high rates. In this review, we discuss the development of mobility limited recombination models from the early Langevin theory to state-of-the art models for charge carrier recombination in organic solar cells.

Light Harvesting for Organic Photovoltaics

The field of organic photovoltaics has developed rapidly over the last 2 decades, and small solar cells with power conversion efficiencies of 13% have been demonstrated. Light absorbed in the organic layers forms tightly bound excitons that are split into free electrons and holes using heterojunctions of electron donor and acceptor materials, which are then extracted at electrodes to give useful electrical power. This review gives a concise description of the fundamental processes in photovoltaic devices, with the main emphasis on the characterization of energy transfer and its role in dictating device architecture, including multilayer planar heterojunctions, and on the factors that impact free carrier generation from dissociated excitons. We briefly discuss harvesting of triplet excitons, which now attracts substantial interest when used in conjunction with singlet fission. Finally, we introduce the techniques used by researchers for characterization and engineering of bulk heterojunctions to realize large photocurrents, and examine the formed morphology in three prototypical blends.

Planarity and multiple components promote organic photovoltaic efficiency by improving electronic transport

Establishing how the conformation of organic photovoltaic (OPV) polymers affects their electronic and transport properties is critical in order to determine design rules for new OPV materials and in particular to understand the performance enhancements recently reported for ternary blends. We report coupled classical and ab initio molecular dynamics simulations showing that polymer linkage twisting significantly reduces optical absorption efficiency, as well as hole transport rates in donor polymers. We predict that blends with components favoring planar geometries contribute to the enhancement of the overall efficiency of ternary OPVs. Furthermore, our electronic structure calculations for the PTB7-PID2-PC71BM system show that hole transfer rates are enhanced in ternary blends with respect to their binary counterpart. Finally, our results point at thermal disorder in the blend as a key reason responsible for device voltage losses and at the need to carry out electronic structure calculations at finite temperature to reliably compare with experiments.

The Driving Force of Photoinduced Charge Separation in Metal-Cluster-Encapsulated Triphenylamine-[80]fullerenes

Understanding photoinduced charge separation in fullerene-based dye-sensitized solar cells is crucial for the development of photovoltaic devices. We investigate here how the driving force of the charge separation process in conjugates of M@C₈₀ (M=Sc₃ N, Sc₃ CH, Sc₃ NC, Sc₄ O₂ , and Sc₄ O₃ ) with triphenylamine (TPA) depends on the nature of the metal cluster. Both singlet and triplet excited-state electron-transfer reactions are considered. These results based on TD-DFT calculations demonstrate that the driving force of charge separation in TPA-M@C₈₀ can be tuned well by varying the structure of the metal cluster encapsulated inside the fullerene cage.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.

Theoretical estimation of the rate of photoinduced charge transfer reactions in triphenylamine C60 donor-acceptor conjugate

Fullerene-based molecular heterojunctions such as the [6,6]-pyrrolidine-C60 donor-acceptor conjugate containing triphenylamine (TPA) are potential materials for high-efficient dye-sensitized solar cells. In this work, we estimate the rate constants for the photoinduced charge separation and charge recombination processes in TPA-C60 using the unrestricted and time-dependent DFT methods. Different schemes are applied to evaluate excited state properties and electron transfer parameters (reorganization energies, electronic couplings, and Gibbs energies). The use of open-shell singlet or triplet states, several density functionals, and continuum solvation models is discussed. Strengths and limitations of the computational approaches are highlighted. The present benchmark study provides an overview of the expected performance of DFT-based methodologies in the description of photoinduced charge transfer reactions in fullerene heterojunctions. © 2016 Wiley Periodicals, Inc.

Synthesis and ultrafast spectroscopic study of new [6,6]methanofullerenes

Ultrafast transient absorption and terahertz spectroscopic studies have been performed on new [6,6]methanofullerenes synthesized by the reaction of diazomethane with fullerene[60] via eco-friendly methodology, amine-assisted 1,3 dipolar cycloaddition (AACA).

Use of ultrafast time-resolved spectroscopy to demonstrate the effect of annealing on the performance of P3HT:PCBM solar cells

The organic solar cells of heterojunction system, ITO/PEDOT:PSS/P3HT:PCBM/Al, with a thermal annealing after deposition of Al exhibit better performance than those with an annealing process before deposition of Al. In this study, ultrafast time-resolved spectroscopy is employed to reveal the underlying mechanism of annealing effects on the performance of P3HT:PCBM solar cell devices. The analyses of all decomposed relaxation processes show that the postannealed devices exhibit an increase in charge transfer, in the number of separated polarons and a reduction in the amount of recombination between excited carriers. Moreover, the longer lifetime for the excited carriers in postannealed devices indicates it is more likely to be dissociated into photocarriers and result in a larger value for photocurrent, which demonstrates the physical mechanism for increased device performance.

Ultrafast Terahertz Photoconductivity of Photovoltaic Polymer-Fullerene Blends: A Comparative Study Correlated with Photovoltaic Device Performance

Ultrafast photoinduced carrier dynamics in prototypical low band gap polymer:fullerene photovoltaic blend films PTB7:PC70BM and P3HT:PC70BM is investigated using ultrafast terahertz (THz) spectroscopy. The subpicosecond and few-picosecond decays of THz-probed photoconductivities for both compounds are observed, attributed to the rapid formation of polaron pairs by exciton-exciton annihilation and subsequent polaron pair annihilation, respectively. The transient THz photoconductivity spectra of PTB7:PC70BM are well described by the Drude-Smith (DS) model, directly yielding the important charge transport parameters such as charge carrier density, momentum scattering time, and effective localization. By comparison with P3HT:PC70BM, we find that in PTB7:PC70BM the mobile charge carrier photoconductivity is significantly enhanced by a factor of 1.8 and prevails for longer times after charge formation, due to both improved mobile charge carrier yield and lower charge localization. In PTB7:PC70BM, a strong dependency of electron momentum scattering time on electron density was found, well parametrized by the empirical Caughey-Thomas model. The difference in ultrafast photoconductivities of both P3HT:PC70BM and PTB7:PC70BM is found to correlate very well with the performance of photovoltaic devices based on those materials.

How high local charge carrier mobility and an energy cascade in a three-phase bulk heterojunction enable >90% quantum efficiency

Charge generation in champion organic solar cells is highly efficient in spite of low bulk charge-carrier mobilities and short geminate-pair lifetimes. In this work, kinetic Monte Carlo simulations are used to understand efficient charge generation in terms of experimentally measured high local charge-carrier mobilities and energy cascades due to molecular mixing.

Synthesis of structurally well-defined and liquid-phase-processable graphene nanoribbons

The properties of graphene nanoribbons (GNRs) make them good candidates for next-generation electronic materials. Whereas 'top-down' methods, such as the lithographical patterning of graphene and the unzipping of carbon nanotubes, give mixtures of different GNRs, structurally well-defined GNRs can be made using a 'bottom-up' organic synthesis approach through solution-mediated or surface-assisted cyclodehydrogenation reactions. Specifically, non-planar polyphenylene precursors were first 'built up' from small molecules, and then 'graphitized' and 'planarized' to yield GNRs. However, fabrication of processable and longitudinally well-extended GNRs has remained a major challenge. Here we report a bottom-up solution synthesis of long (>200 nm) liquid-phase-processable GNRs with a well-defined structure and a large optical bandgap of 1.88 eV. Self-assembled monolayers of GNRs can be observed by scanning probe microscopy, and non-contact time-resolved terahertz conductivity measurements reveal excellent charge-carrier mobility within individual GNRs. Such structurally well-defined GNRs may prove useful for fundamental studies of graphene nanostructures, as well as the development of GNR-based nanoelectronics.

Ultrafast energy transfer in ultrathin organic donor/acceptor blend

It is common knowledge that poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend, a prototype system for bulk heterojunction (BHJ) solar cells, consists of a network of tens of nanometers-large donor-rich and acceptor-rich phases separated by extended finely intermixed border regions where PCBM diffuse into P3HT. Here we specifically address the photo-induced dynamics in a 10 nm thin P3HT/PCBM blend that consists of the intermixed region only. Using the multi-pass transient absorption technique (TrAMP) that enables us to perform ultra high sensitive measurements, we find that the primary process upon photoexcitation is ultrafast energy transfer from P3HT to PCBM. The expected charge separation due to hole transfer from PCBM to P3HT occurs in the 100 ps timescale. The derived picture is much different from the accepted view of ultra-fast electron transfer at the polymer/PCBM interface and provides new directions for the development of efficient devices.

Ultrafast photoconductivity of graphene nanoribbons and carbon nanotubes

We present a comparative study of the ultrafast photoconductivity in two different forms of one-dimensional (1D) quantum-confined graphene nanostructures: structurally well-defined semiconducting graphene nanoribbons (GNRs) fabricated by a "bottom-up" chemical synthesis approach and semiconducting carbon nanotubes (CNTs) with a similar bandgap energy. Transient photoconductivities of both materials were measured using time-resolved terahertz spectroscopy, allowing for contact-free measurements of complex-valued photoconductivity spectra with subpicosecond time-resolution. We show that, while the THz photoresponse seems very different for the two systems, a single model of free carriers experiencing backscattering when moving along the long axis of the CNTs or GNRs provides a quantitative description of both sets of results, revealing significantly longer carrier scattering times for CNTs (ca. 150 fs) than for GNRs (ca. 30 fs) and in turn higher carrier mobilities. This difference can be explained by differences in band structures and phonon scattering and the greater structural rigidity of CNTs as compared to GNRs, minimizing the influence of bending and/or torsional defects on the electron transport.

Electron and Hole Contributions to the Terahertz Photoconductivity of a Conjugated Polymer:Fullerene Blend Identified

Time-resolved terahertz spectroscopy was employed for the investigation of charge-transport dynamics in benzothiadiazolo-dithiophene polyfluorene ([2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]) (APFO-3) polymers with various chain lengths and in its monomer form, all blended with an electron acceptor ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM). Upon photoexcitation, charged polaron pairs are created, negative charges are transferred to fullerenes, while positive polarons remain on polymers/monomers. Vastly different hole mobility in polymer and monomer blends allows us to distinguish the hole and electron contributions to the carrier mobility.

Photoexcitation dynamics in films of C60 and Zn phthalocyanine with a layered nanostructure

We elucidate photoexcitation dynamics in C(60) and zinc phthalocyanine (ZnPc) from picoseconds to milliseconds by transient absorption and time-resolved terahertz spectroscopy. Autoionization of C(60) is a precursor to photocarrier generation. Decay of the terahertz signal is due to decreasing photocarrier mobility over the first 20 ps and thereafter reflects recombination dynamics. Singlet diffusion rates in C(60) are determined by modeling the rise of ground state bleaching of ZnPc absorption following C(60) excitation. Recombination dynamics transform from bimolecular to monomolecular as the layer thickness is reduced, revealing a metastable exciplex at the C(60)/ZnPc interface with a lifetime of 150 μs.

Direct observation of sub-100 fs mobile charge generation in a polymer-fullerene film

The formation of mobile charges in a roll-to-roll processed poly-3-hexylthiophene-fullerene bulk heterojunction film is observed directly by using transient terahertz spectroscopy with sub-100 fs temporal resolution. The transient terahertz ac conductivity reveals that 20% of the incident pump photons are converted into highly delocalized charges within the 40 fs, 3.1 eV pump pulse duration, which then rapidly becomes localized within 120 fs. Approximately 2/3 of these carriers subsequently decay, possibly into an exciton, on a 1 ps time scale.

Structural variability and dynamics of the P3HT/PCBM interface and its effects on the electronic structure and the charge-transfer rates in solar cells

Using a range of realistic interface geometries obtained from a molecular dynamics simulation we study the effects of different microscopic atomic arrangements on the electronic structure and charge transfer rates of the prototypical photovoltaic interface between P3HT (poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C(61)-butyric acid methyl ester). The electronic structures of charge-transfer (CT) states belong to two groups that can be denoted as "charge-separated" and "charge-bridging" states. For the former group of structures, which may lead to fully separated charges, the ranges and the average values of internal reorganization energy, the electronic coupling and the charge separated states energy are evaluated. A range and distribution of absolute charge separation (CS) and recombination (CR) rates are computed using the Marcus-Levich-Jortner rate equation. Due to the variety of P3HT/PCBM interface structures, a very broad range of CS (7.7 × 10(9)-1.8 × 10(12) s(-1)) and CR (2.5 × 10(5)-1.1 × 10(10) s(-1)) "instantaneous" rates are computed. However, the energetic parameters affecting the rate evolve in time due to the dynamic nature of the interface with a characteristic timescale of about 10 ns. For this reason the slowest CR instantaneous rates are not observed and the minimum CR rate observed is determined by the rate of conformational rearrangement at the interface. The combination of these observations provides a more general framework for the interpretation of experimental spectroscopic data, suggesting that the analysis based on simple first order rates may be insufficient to describe charge transfer in organic solar cell interfaces.

Charge separation in semicrystalline polymeric semiconductors by photoexcitation: is the mechanism intrinsic or extrinsic?

We probe charge photogeneration and subsequent recombination dynamics in neat regioregular poly(3-hexylthiophene) films over six decades in time by means of time-resolved photoluminescence spectroscopy. Exciton dissociation at 10 K occurs extrinsically at interfaces between molecularly ordered and disordered domains. Polaron pairs thus produced recombine by tunneling with distributed rates governed by the distribution of electron-hole radii. Quantum-chemical calculations suggest that hot-exciton dissociation at such interfaces results from a high charge-transfer character.

Ultrafast charge separation at a polymer-single-walled carbon nanotube molecular junction

We have investigated the charge photogeneration dynamics at the interface formed between single-walled carbon nanotubes (SWNTs) and poly(3-hexylthiophene) (P3HT) using a combination of femtosecond spectroscopic techniques. We demonstrate that photoexcitation of P3HT forming a single molecular layer around a SWNT leads to an ultrafast (∼430 fs) charge transfer between the materials. The addition of excess P3HT leads to long-term charge separation in which free polarons remain separated at room temperature. Our results suggest that SWNT-P3HT blends incorporating only small fractions (1%) of SWNTs allow photon-to-charge conversion with efficiencies comparable to those for conventional (60:40) P3HT-fullerene blends, provided that small-diameter tubes are individually embedded in the P3HT matrix.

Probing the momentum relaxation time of charge carriers in ultrathin layers with terahertz radiation

We report on the development of a terahertz time-domain technique for measuring the momentum relaxation time of charge carriers in ultrathin semiconductor layers. Making use of the Drude model, our phase sensitive modulation technique directly provides the relaxation time. Time-resolved THz experiments were performed on n-doped GaAs and show precise agreement with data obtained by electrical characterization. The technique is well suited for studying novel materials where parameters such as the charge carriers' effective mass or the carrier density are not known a priori.

Supramolecular nanoarchitectures for light energy conversion

Recent developments in synthetic and supramolecular techniques have made it possible to control precisely, organize and arrange molecules at the nanometre level. Such synthetic and supramolecular strategies enable us to construct photofunctional molecular architectures for light energy conversion, such as photovoltaics. In photovoltaic cells, processes such as light-harvesting, charge separation for carrier generation, and carrier transport are generally required. Therefore, the construction of supramolecular assemblies based on these three processes is interesting and promising for the future development of photovoltaics. In this perspective, the focus is on the recent developments of supramolecular systems for light energy conversion, which are mainly composed of porphyrin dyes and nanocarbon materials, such as fullerenes and carbon nanotubes. The specific topics are as follows: (i) preparation, photodynamics, and photoelectrochemistry of self-assembled porphyrin nanoparticles prepared by simple blend, (ii) highly organized supramolecular nanoassemblies of porphyrins and fullerenes using gold nanoparticles, dendritic and polypeptide structures, (iii) the supramolecular formation and photoelectrochemical property of carbon nanotubes, and (iv) supramolecular photofunctional nanorods of porphyrins.

Insights on interfacial charge transfer across P3HT/fullerene photovoltaic heterojunction from Ab initio calculations

The interfacial charge-transfer mechanism of the P3HT/fullerene photovoltaic heterojunction is elucidated using density functional theory calculations. Our findings indicate that an efficient adiabatic electron transfer is highly probable due to the presence of an extended electronic state that has a significant probability distribution across the interface in the lowest excited state. Furthermore, efficient exciton dissociation is possible because this bridging state has significant overlap with near-degenerate unoccupied states that are localized on the fullerene.