[show abstract][hide abstract] ABSTRACT: We have determined the Auger recombination kinetics of electrons and holes in colloidal CdSe-only and CdSe/CdS/ZnS core/shell nanoplatelets by time-resolved photoluminescence measurements. Excitation densities as high as an average of 18 electron–hole pairs per nanoplatelet were reached. Auger recombination can be described by second-order kinetics. From this we infer that the majority of electrons and holes are bound in the form of neutral excitons, while the fraction of free charges is much smaller. The biexciton Auger recombination rate in nanoplatelets is more than 1 order of magnitude smaller than for quantum dots and nanorods of equal volume. The latter is of advantage for application in lasers, light-emitting diodes, and photovoltaics.
[show abstract][hide abstract] ABSTRACT: Auger recombination (AR) can be an important loss mechanism for optoelectronic devices, but it is typically not very efficient at low excitation densities. Here we show that in conductive quantum-dot solids, AR is the dominant charge carrier decay path even at excitation densities as low as 10(-3) per quantum dot, and that AR becomes faster as the charge carrier mobility increases. Monte Carlo simulations reveal that this efficient AR results from charge carrier congregation in 'Auger hot spots': lower-energy sites that are present because of energy disorder. Disorder-enhanced AR is a general effect that is expected to be active in all disordered materials. The observed efficient AR is an issue of concern for devices that work at charge carrier densities in excess of ~10(-3) charge carriers per quantum dot. At the same time, efficient carrier congregation could be exploited for fast optical switching or to achieve optical gain in the near infrared.
[show abstract][hide abstract] ABSTRACT: Carrier multiplication, the generation of multiple electron-hole pairs by a single photon, is of great interest for solar cells as it may enhance their photocurrent. This process has been shown to occur efficiently in colloidal quantum dots, however, harvesting of the generated multiple charges has proved difficult. Here we show that by tuning the charge-carrier mobility in quantum-dot films, carrier multiplication can be optimized and may show an efficiency as high as in colloidal dispersion. Our results are explained quantitatively by the competition between dissociation of multiple electron-hole pairs and Auger recombination. Above a mobility of ~1 cm(2) V(-1) s(-1), all charges escape Auger recombination and are quantitatively converted to free charges, offering the prospect of cheap quantum-dot solar cells with efficiencies in excess of the Shockley-Queisser limit. In addition, we show that the threshold energy for carrier multiplication is reduced to twice the band gap of the quantum dots.
[show abstract][hide abstract] ABSTRACT: The cooling and Auger recombination of electron-hole pairs in PbSe quantum dots (QDs) and a series of nanorods (NRs) with similar diameter and varying length was studied by ultrafast pump-probe laser spectroscopy. Hot exciton cooling rates are found to be independent of nanocrystal shape. The energy relaxation rate decreases during cooling of charges, due to reduction of the density of electronic states. Auger recombination occurs via cubic third-order kinetics of uncorrelated charges in the QDs and NRs with length up to 29 nm. On increasing the NR length to 52 nm, a crossover to bimolecular exciton decay is found. This suggests a spatial extent of the one-dimensional exciton of 30-50 nm, which is significantly smaller than the value of 92 nm for the three-dimensional exciton diameter in bulk PbSe. The Auger decay time increases with NR length, which is beneficial for applications in nanocrystal lasers as well as for generation of free charges in photovoltaics.
[show abstract][hide abstract] ABSTRACT: Three conjugated triphenylamine-based poly(azomethine)s were prepared via well-known polycondensation chemistry using cheap and readily available starting materials and the results were contrasted with rrP3HT. Three functionalized diaminetriphenylamines (TPA(X), X = –H, –OMe, –CN) were polymerized in a simple one-step process with 2,3-dihydrothieno[3,4-b][1,4]dioxine-5,7-dicarbaldehyde (ThOx), with water being the only side product. The resulting polymers (TPA(X)ThOx, X = –H, –OMe, –CN) were characterized by GPC, IR and NMR, and show a good thermal stability. The opto-electronic properties could be tuned by changing the functionalization (X = –H, –OMe, –CN) on the triphenylamine moiety. Photovoltaic devices based on TPA(X)ThOx/PCBM (1:2) showed power conversion efficiencies in the range of 0.02–0.04%. TRMC measurements showed that the presence of PCBM as an electron acceptor facilitates the formation of free mobile charges after excitation of the polymer. The low device efficiencies are attributed to a low hole-mobility of the polymer in combination with poor active layer morphology.
[show abstract][hide abstract] ABSTRACT: Bimolecular charge carrier recombination in blends of a conjugated copoly mer
based on a thiophene and quinoxaline (TQ1) with a fullerene derivative
((6,6)-phenyl-C 71 -butyric acidmethyl ester, PC 71 BM) is studied by two complementary
techniques. TRMC (time-resolved microwave conductance) monitors
the conductance of photogenerated mobile charge carriers locally on a timescale
of nanoseconds, while using photo-CELIV (charge extraction by linearly
increasing voltage) charge carrier dynamics are monitored on a macroscopic
scale and over tens of microseconds. Despite these signifi cant differences in
the length and time scales, both techniques show a reduced Langevin recombination
with a prefactor ζ close to 0.05. For TQ1:PC 71 BM blends, the ζ value
is independent of temperature. On comparing TRMC data with electroluminescence
measurements it is concluded that the encounter complex and the
charge transfer state have very similar energetic properties. The ζ value for
annealed poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C 61 -butyric acid methyl
ester (PC 61 BM) is approximately 10 − 4 , while for blend systems containing
an amorphous polymer ζ values are close to 1. These large differences can
be related to the extent of charge delocalization of opposite charges in an
encounter complex. Insight is provided into factors governing the bimolecular
recombination process, which forms a major loss mechanism limiting the
effi ciency of polymer solar cells.
[show abstract][hide abstract] ABSTRACT: Films of colloidal quantum dots (QDs) show great promise for application in optoelectronic devices. Great advances have been made in recent years in designing efficient QD solar cells and LEDs. A very important aspect in the design of devices based on QD films is the knowledge of their absolute energy levels. Unfortunately, reported energy levels vary markedly depending on the employed measurement technique and the environment of the sample. In this report, we determine absolute energy levels of QD films by electrochemical charge injection. The concomitant change in optical absorption of the film allows quantification of the number of charges in quantum-confined levels and thereby their energetic position. We show here that the size of voids in the QD films (i.e. the space between the quantum dots) determines the amount of charges that may be injected into the films. This effect is attributed to size exclusion of countercharges from the electrolyte solution. Further, the energy of the QD levels depends on subtle changes in the QD film and the supporting electrolyte: the size of the cation and the QD ligand length. These nontrivial effects can be explained by the proximity of the cation to the QD surface and a concomitant lowering of the electrochemical potential. Our findings help explain the wide range of reported values for QD energy levels and redefine the limit of applicability of electrochemical measurements on QD films. Finally, the finding that the energy of QD levels depends on ligand length and counterion size may be exploited in optimized designs of QD sensitized solar cells.
[show abstract][hide abstract] ABSTRACT: We report on a photodetector in which colloidal quantum-dots directly bridge nanometer-spaced electrodes. Unlike in conventional quantum-dot thin film photodetectors, charge mobility no longer plays a role in our quantum-dot junctions as charge extraction requires only two individual tunnel events. We find an efficient photoconductive gain mechanism with external quantum-efficiencies of 38 electrons-per-photon in combination with response times faster than 300 ns. This compact device-architecture may open up new routes for improved photodetector performance in which efficiency and bandwidth do not go at the cost of one another.
[show abstract][hide abstract] ABSTRACT: The assembly of quantum dots is an essential step towards many of their potential applications. To form conductive solids from colloidal quantum dots, ligand exchange is required. Here we study the influence of ligand replacement on the photoconductivity of PbSe quantum-dot solids, using the Time Resolved Microwave Conductivity technique. Bifunctional replacing ligands with amine, thiol or carboxylic acid anchor groups of various lengths are used to assemble quantum solid via a layer-by-layer dip coating method. We find that when the ligand lengths are the same, the charge carrier mobility is higher in quantum-dot solids with amine ligands, while in quantum-dot solids with thiol ligands the charge carrier lifetime is longer. If the anchor group is the same, the charge carrier mobility is ligand length dependent. The results show that the diffusion length of charge carriers can reach several hundred nanometers.
[show abstract][hide abstract] ABSTRACT: We have studied the charge transport properties of self-assembled structures of semisynthetic zinc chlorins (ZnChls) in the solid state by pulsed radiolysis time-resolved microwave conductivity measurements. These materials can form either a two-dimensional (2D) brickwork-type slipped stack arrangement or a one-dimensional (1D) tubular assemblies, depending on the exact molecular structure of the ZnChls. We have observed efficient charge transport with mobilities as high as 0.07 cm(2) V(-1) s(-1) for tubular assemblies of 3(1)-hydroxy ZnChls and up to 0.28 cm(2) V(-1) s(-1) for 2D stacked assemblies of 3(1)-methoxy ZnChls at room temperature. The efficient charge transporting capabilities of these organized assemblies opens the way to supramolecular electronics based on biological systems.
Journal of the American Chemical Society 09/2012; 134(39):16147-50. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Organic semiconductors are of great interest for application in cheap and flexible solar cells. They have a typical absorption onset in the visible. Infrared light can be harvested by use of lead-chalcogenide quantum dot sensitizers. However, bulk-heterojunction solar cells with quantum-dot sensitizers are inefficient. Here we use ultrafast transient absorption and time-domain terahertz spectroscopy to show that charge localization on the quantum dot leads to enhanced coulomb attraction of its counter charge in the organic semiconductor. This localization-enhanced coulomb attraction is the fundamental cause of the poor efficiency of these photovoltaic architectures. It is of prime importance for improving solar cell efficiency to directly photogenerate spatially separated charges. This can be achieved when both charges are delocalized. Our findings provide a rationalization in the development of photovoltaic architectures that exploit quantum dots to harvest the near-infrared part of the solar spectrum more efficiently.
[show abstract][hide abstract] ABSTRACT: In this article, a theoretical study of the electronic and spectroscopic properties of well-defined DNA hairpins is presented. The excited states in the hairpins are described in terms of an exciton Hamiltonan model, and the structural dynamics of the DNA model systems is explicitly taken into account by molecular dynamics simulations. The results show that the model reproduces the experimentally observed absorption and circular dichroism spectra accurately in most cases. It is shown that structural disorder leads to excited states that are largely localized on a single base pair, even for regular DNA sequences consisting only of AT base pairs. Variations in the base pair sequence have a significant effect on the appearance of the spectra but also on the degree of delocalization of the excited state.
The Journal of Physical Chemistry B 08/2012; 116(37):11447-58. · 3.61 Impact Factor
[show abstract][hide abstract] ABSTRACT: Using femtosecond transient absorption spectroscopy, we demonstrate that lead chalcogenide nanocrystals show considerable photoinduced absorption (PA) in a broad wavelength range just below the band gap. The time-dependent decay of the PA signal correlates with the recovery of the band gap absorption, indicating that the same carriers are involved. On this basis, we assign this PA signal to intraband absorption, that is, the excitation of photogenerated carriers from the bottom of the conduction band or the top of the valence band to higher energy levels in the conduction and valence band continuum. We confirm our experiments with tight-binding calculations. This broadband response in the commercially interesting near- to mid-infrared range is very relevant for ultra-high-speed all-optical signal processing. We benchmark the performance with bulk Si and Si nanocrystals.
[show abstract][hide abstract] ABSTRACT: Conductive tubes: Self-assembled nanotubes of a bacteriochlorophyll derivative are reminiscent of natural chlorosomal light-harvesting assemblies. After deposition on a substrate that consists of a non-conductive silicon oxide surface (see picture, brown) and contacting the chlorin nanowires to a conductive polymer (yellow), they show exceptional charge-transport properties.
Angewandte Chemie International Edition 05/2012; 51(26):6378-82. · 13.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Supramolecular engineering offers opportunities for creating polymer-based materials with tailored conductive properties. However, this requires an understanding of intermolecular interaction effects on intramolecular charge transport. We present a study of hole transport along molecular wires consisting of fluorene-p-biphenyl or Zn-porphyrin monomer units, in dilute solutions. The intramolecular hole mobility was studied by pulse radiolysis time-resolved microwave conductivity. Experiments were supplemented by charge transport simulations employing a quantum-mechanical description of the hole and a classical description of the polymer and solvent dynamics. The model was parametrized using ab initio and molecular dynamics calculations. It was found that the solvent-induced energy disorder along a polymer chain in common solvents (benzene, cyclohexane, acetonitrile, water) is similar to 1 eV, significantly greater than the values of 0.05-0.2 eV commonly cited in the literature. Environment-induced disorder of this magnitude has profound consequences for intramolecular charge transport. The hole initial state upon injection onto a molecular wire also influences the mobility. Experiments and simulations demonstrate that supramolecular modification of polymers (coordination, rotaxination) can significantly enhance or suppress charge transport. Incorporating a molecular level description of the immediate supramolecular and solvent environment into charge transport models improves their predictive potential, providing a valuable tool for material design.
The Journal of Physical Chemistry C 01/2012; 116:25213-25225. · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report the direct and unambiguous determination of electron transfer rates and efficiencies from PbSe quantum dots (QDs) to mesoporous SnO2 films. We monitor the time-dependent electron density within the oxide with picosecond time resolution using Terahertz spectroscopy, following optical excitation of the QDs using a femtosecond laser pulse. QD-oxide electron transfer occurs with efficiencies of ∼2% in our samples under 800 nm pumping with a marked dependence on QD size, ranging from ∼100 ps injection times for the smallest, ∼2 nm diameter QDs, to ∼1 ns time scale for ∼7 nm QDs. The size-dependent electron transfer rates are modeled within the framework of Marcus theory and the implications of the results for device design are discussed.
[show abstract][hide abstract] ABSTRACT: PbSe quantum-dot solids are of great interest for low cost and efficient photodetectors and solar cells. We have prepared PbSe quantum-dot solids with high charge carrier mobilities using layer-by-layer dip-coating with 1,2-ethanediamine as substitute capping ligands. Here we present a time and energy resolved transient absorption spectroscopy study on the kinetics of photogenerated charge carriers, focusing on 0-5 ps after photoexcitation. We compare the observed carrier kinetics to those for quantum dots in dispersion and show that the intraband carrier cooling is significantly faster in quantum-dot solids. In addition we find that carriers diffuse from higher to lower energy sites in the quantum-dot solid within several picoseconds.
[show abstract][hide abstract] ABSTRACT: We discuss how the mobility of a charge moving along a conjugated polymer chain is affected by different types of disorder. The intrachain mobility has been determined by using pulse radiolysis in combination with microwave conductivity measurements. The intrachain mobility of charges on isolated planar ladder-type poly(p-phenylene) chains in solution is as high as 600 cm2/(V s). In solid samples, the intrachain mobility is only 30 cm2/(V s) due to disorder caused by interactions between different polymer chains. Interestingly, this mobility is 4 orders of magnitude higher than the DC device mobility, which is limited by charge hopping between different chains. Torsional disorder along poly(p-phenylene vinylene) chains limits the intrachain mobility to 60 cm2/(V s), while in polyfluorenes, coiling of the chains strongly reduces the mobility. The results imply that higher charge mobilities can be realized in devices if the polymer chains directly interconnect the electrodes and adopt a straight, planar structure in a homogeneous environment.