Understanding cooling of hot charge carriers in semiconductor quantum dots (QDs) is of fundamental interest and useful to enhance the performance of QDs in photovoltaics. We study electron and hole cooling dynamics in PbSe QDs up to high energies where carrier multiplication occurs. We characterize distinct cooling steps of hot electrons and holes and build up a broadband cooling spectrum for both charge carriers. Cooling of electrons is slower than of holes. At energies near the band gap we find cooling times between successive electronic energy levels in the order of 0.5 ps. We argue that here the large spacing between successive electronic energy levels requires cooling to occur by energy transfer to vibrational modes of ligand molecules or phonon modes associated with the QD surface. At high excess energy the energy loss rate of electrons is 1–5 eV/ps and exceeds 8 eV/ps for holes. Here charge carrier cooling can be understood in terms of emission of LO phonons with a higher density-of-states in the valence band than the conduction band. The complete mapping of the broadband cooling spectrum for both charge carriers in PbSe QDs is a big step toward understanding and controlling the cooling of hot charge carriers in colloidal QDs.Keywords: carrier cooling; carrier dynamics; electronic structure; nanocrystal; quantum dot; transient absorption spectroscopy;

The implementation of next generation ultrathin electronics by applying highly promising dimensionality-dependent physical properties of two-dimensional (2D) semiconductors is ever increasing. In this context, the van der Waals layered semiconductor InSe has proven its potential as photodetecting material with high charge carrier mobility. We have determined the photogeneration charge carrier quantum yield and mobility in atomically thin colloidal InSe nanosheets (inorganic layer thickness 0.8–1.7 nm, mono/double-layers, ≤ 5 nm including ligands) by ultrafast transient terahertz (THz) spectroscopy. A near unity quantum yield of free charge carriers is determined for low photoexcitation density. The charge carrier quantum yield decreases at higher excitation density due to recombination of electrons and holes, leading to the formation of neutral excitons. In the THz frequency domain, we probe a charge mobility as high as 20 ± 2 cm2/(V s). The THz mobility is similar to field-effect transistor mobilities extracted from unmodified exfoliated thin InSe devices. The current work provides the first results on charge carrier dynamics in ultrathin colloidal InSe nanosheets.

We show that black phosphorus is a highly efficient infrared emitter. To study the carrier dynamics, excess electron–hole pairs were generated in bulk black phosphorus by irradiation with 3 MeV electron pulses. The transient microwave conductivity due to excess charges was measured as a function of time for different initial charge densities at temperatures in the range 203–373 K. A new global analysis scheme, including the treatment of intrinsic carriers, is provided, which shows that the recombination dynamics in black phosphorus, a low bandgap semiconductor, is strongly influenced by the presence of intrinsic carriers. The temperature dependence of the charge mobility and charge carrier decay via second-order radiative recombination is obtained from modeling of the experimental data. The combined electron and hole mobility was found to increase with temperature up to 250 K and decrease above that. Auger recombination is negligible for the studied densities of excess electron–hole pairs up to 2.5 × 1017 cm–3. For this density the major fraction of the excess electrons and holes undergoes radiative recombination. It is further inferred that for excess charge densities of the order of 1018 cm–3 electrons and holes recombine with near unity radiative yield. The latter offers promising prospects for use of black phosphorus as efficient mid infrared emitter in devices.

In semiconductor quantum dots (QDs), charge carrier cooling is in direct competition with processes such as carrier multiplication or hot charge extraction that may improve the light conversion efficiency of photovoltaic devices. Understanding charge carrier cooling is therefore of great interest. We investigate high-energy optical transitions in PbSe QDs using hyperspectral transient absorption spectroscopy. We observe bleaching of optical transitions involving higher valence and conduction bands upon band edge excitation. The kinetics of rise of the bleach of these transitions after a pump laser pulse allow us to monitor, for the first time, cooling of hot electrons and hot holes separately. Our results show that holes cool significantly faster than electrons in PbSe QDs. This is in contrast to the common assumption that electrons and holes behave similarly in Pb chalcogenide QDs and has important implications for the utilization of hot charge carriers in photovoltaic devices.Keywords: band structure; charge carrier cooling; electron acceptor; nanocrystal; quantum dot; transient absorption spectroscopy;

In a conventional photovoltaic device (solar cell or photodiode) photons are absorbed in a bulk semiconductor layer, leading to excitation of an electron from a valence band to a conduction band. Directly after photoexcitation, the hole in the valence band and the electron in the conduction band have excess energy given by the difference between the photon energy and the semiconductor band gap. In a bulk semiconductor, the initially hot charges rapidly lose their excess energy as heat. This heat loss is the main reason that the theoretical efficiency of a conventional solar cell is limited to the Shockley–Queisser limit of ∼33%. The efficiency of a photovoltaic device can be increased if the excess energy is utilized to excite additional electrons across the band gap. A sufficiently hot charge can produce an electron–hole pair by Coulomb scattering on a valence electron. This process of carrier multiplication (CM) leads to formation of two or more electron–hole pairs for the absorption of one photon.In bulk semiconductors such as silicon, the energetic threshold for CM is too high to be of practical use. However, CM in nanometer sized semiconductor quantum dots (QDs) offers prospects for exploitation in photovoltaics. CM leads to formation of two or more electron–hole pairs that are initially in close proximity. For photovoltaic applications, these charges must escape from recombination. This Account outlines our recent progress in the generation of free mobile charges that result from CM in QDs. Studies of charge carrier photogeneration and mobility were carried out using (ultrafast) time-resolved laser techniques with optical or ac conductivity detection.We found that charges can be extracted from photoexcited PbS QDs by bringing them into contact with organic electron and hole accepting materials. However, charge localization on the QD produces a strong Coulomb attraction to its counter charge in the organic material. This limits the production of free charges that can contribute to the photocurrent in a device.We show that free mobile charges can be efficiently produced via CM in solids of strongly coupled PbSe QDs. Strong electronic coupling between the QDs resulted in a charge carrier mobility of the order of 1 cm2 V–1 s–1. This mobility is sufficiently high so that virtually all electron–hole pairs escape from recombination.The impact of temperature on the CM efficiency in PbSe QD solids was also studied. We inferred that temperature has no observable effect on the rate of cooling of hot charges nor on the CM rate.We conclude that exploitation of CM requires that charges have sufficiently high mobility to escape from recombination. The contribution of CM to the efficiency of photovoltaic devices can be further enhanced by an increase of the CM efficiency above the energetic threshold of twice the band gap. For large-scale applications in photovoltaic devices, it is important to develop abundant and nontoxic materials that exhibit efficient CM.

The nature and decay dynamics of photoexcited states in CdSe core-only and CdSe/CdS core/shell nanoplatelets was studied. The photophysical species produced after ultrafast photoexcitation are studied using a combination of time-resolved photoluminescence (PL), transient absorption (TA), and terahertz (THz) conductivity measurements. The PL, TA, and THz exhibit very different decay kinetics, which leads to the immediate conclusion that photoexcitation produces different photophysical species. It is inferred from the data that photoexcitation initially leads to formation of bound electron–hole pairs in the form of neutral excitons. The decay dynamics of these excitons can be understood by distinguishing nanoplatelets with and without exciton quenching site, which are present in the sample with close to equal amounts. In absence of a quenching site, the excitons undergo PL decay to the ground state. In nanoplatelets with a quenching site, part of the initially produced excitons decays by hole trapping at a defect site. The electron that remains in the nanoplatelet moves in the Coulomb potential provided by the trapped hole.

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.

Carrier multiplication—the generation of multiple electron–hole pairs by a single photon—is currently of great interest for the development of highly efficient photovoltaics. We study the effects of infilling PbSe quantum-dot solids with metal oxides by atomic layer deposition on carrier multiplication. Using time-resolved microwave conductivity measurements, we find, for the first time, that carrier multiplication occurs in 1,2-ethanedithiol-linked PbSe quantum-dot solids infilled with Al2O3 or Al2O3/ZnO, while it is negligible or absent in noninfilled films. The carrier-multiplication efficiency of the infilled quantum-dot solids is close to that of solution-dispersed PbSe quantum dots, and not significantly limited by Auger recombination.Keywords: atomic layer deposition; carrier multiplication; multiple exciton generation; quantum dot solid; third-generation solar cell; time-resolved microwave conductivity;

Carrier multiplication (CM)—the Coulomb scattering whereby a sufficiently energetic charge excites a valence electron—is of interest for highly efficient quantum dot (QD) photovoltaics. Using time-resolved microwave conductivity experiments on 1,2-ethanedithiol-linked PbSe QD solids infilled with Al2O3 or Al2O3/ZnO by atomic layer deposition, we find that CM and hot-carrier cooling are temperature independent from 90–295 K and that spontaneous phonon emission limits the yield of charges resulting from the CM–cooling competition.Keywords: carrier multiplication; hot-carrier cooling; quantum-dot solid; temperature dependence; time-resolved microwave conductivity;

The mobility and spatial distribution of photoexcited electrons in CdSe/CdS core/shell nanorods was studied using optical-pump THz-probe spectroscopy. Measurements were conducted on two samples, differing in rod length. After photoexcitation the hole localizes in the CdSe core within a picosecond, while the electron delocalizes around the core. Analysis of the THz mobility with a model of one-dimensional electron diffusion on a finite rod yields an electron delocalization of ∼25% into the CdS shell and a mobility of 700 cm2/(V s). This is one and a half times the mobility value for bulk CdS, which can be due to quantum confinement effects on electron–phonon scattering and electronic structure.

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.Keywords: Auger recombination; CdSe nanoplatelet; charge carrier; exciton; photoluminescence;

The assembly of quantum dots is an essential step toward 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 solids 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.Keywords: carrier lifetime; carrier mobility; photoconductivity. layer-by-layer assembly; quantum dots; solar cells

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.Keywords: bulk heterojunction; infrared photovoltaic; organic semiconductor; quantum dot; terahertz spectroscopy; ultrafast spectroscopy

Multiple exciton generation (MEG) in PbSe quantum dots (QDs), PbSexS1−x alloy QDs, PbSe/PbS core/shell QDs, and PbSe/PbSeyS1−y core/alloy-shell QDs was studied with time-resolved optical pump and probe spectroscopy. The optical absorption exhibits a red-shift upon the introduction of a shell around a PbSe core, which increases with the thickness of the shell. According to electronic structure calculations this can be attributed to charge delocalization into the shell. Remarkably, the measured quantum yield of MEG, the hot exciton cooling rate, and the Auger recombination rate of biexcitons are similar for pure PbSe QDs and core/shell QDs with the same core size and varying shell thickness. The higher density of states in the alloy and core/shell QDs provide a faster exciton cooling channel that likely competes with the fast MEG process due to a higher biexciton density of states. Calculations reveal only a minor asymmetric delocalization of holes and electrons over the entire core/shell volume, which may partially explain why the Auger recombination rate does not depend on the presence of a shell.

We show that in films of strongly coupled PbSe quantum dots multiple electron–hole pairs can be efficiently produced by absorption of a single photon (carrier multiplication). Moreover, in these films carrier multiplication leads to the generation of free, highly mobile charge carriers rather than excitons. Using the time-resolved microwave conductivity technique, we observed the production of more than three electron–hole pairs upon absorption of a single highly energetic photon (5.7Eg). Free charge carriers produced via carrier multiplication are readily available for use in optoelectronic devices even without employing any complex donor/acceptor architecture or electric fields.

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.

A new generally applicable method to calculate the relative energetic stability of localized and delocalized charges in a system of two molecules is presented. The relative stability of localized and delocalized charges was calculated for π-stacked triphenylenes at varying twist angles and intermolecular distances. The reliability of the new method was validated by comparison with results from Hartree–Fock calculations on particular configurations. According to the calculations, charges are localized for larger twist angles that are typical for triphenylene derivatives in the liquid crystalline phase. In contrast, significant charge delocalization is expected for eclipsed stacking of triphenylene units in a covalent organic framework. This can give rise to band-like motion of delocalized charges with a mobility of the order of 10 cm2 V–1 s–1 or more.

Highly photoconductive films of CdSe nanocrystals have been prepared by exchanging the original bulky ligands with 1,2-ethanedithiol (EDT) and 1,2-ethanediamine (EDA). Different methods to achieve this exchange, layer-by-layer (LbL) deposition and soaking of drop-casted films, have been compared in detail. Introduction of EDT and EDA by the soaking method results in a broadening of the optical absorption due to disorder in the film. In contrast, the width of the absorption features is unaffected in the LbL films, while the position of the first optical absorption peak is red-shifted by tens of millielectronvolts. The photoluminescence is completely quenched for the LbL films. These findings are characteristic for strong and homogeneous electronic coupling between the quantum dots (QDs) in the LbL films. The photoconductivity of these films was studied with the time-resolved microwave conductivity (TRMC) technique. With this electrodeless technique effects of electrode injection on charge transport are avoided, so that information about the intrinsic mobility of charge carriers is obtained. We find that in simple drop-casted films the conductivity is mainly imaginary and dominated by the polarizability of photogenerated excitons. When the orginal ligands are exchanged by soaking or by the LbL procedure, the conductivity becomes real and dominated by interparticle transport of free charge carriers. It is found that the product of the exciton dissociation yield and the charge carrier mobility is 4 × 10−3 cm2/(V s) in the LbL grown films with EDA capping molecules. This implies that a surprisingly high fraction of free carriers is generated or, alternatively, that the carrier mobility is higher than all previously reported mobility values for layers of CdSe QDs.

Thermal annealing of thin films of CdSe/CdS core/shell quantum dots induces superordering of the nanocrystals and a significant reduction of the interparticle spacing. This results in a drastic enhancement of the quantum yield for charge carrier photogeneration and the charge carrier mobility. The mobile electrons have a mobility as high as 0.1 cm2/(V·s), which represents an increase of 4 orders of magnitude over non-annealed QD films and exceeds existing literature data on the electron mobility in CdSe quantum dot films. The lifetime of mobile electrons is longer than that of the exciton. A fraction of the mobile electrons gets trapped at levels below the conduction band of the CdSe nanocrystals. These electrons slowly diffuse over 50−300 nm on longer times up to 20 μs and undergo transfer to a TiO2 substrate. The yield for electron injection in TiO2 from both mobile and trapped electrons is found to be >16%.Keywords: annealing; photoconductivity; quantum dot; supercrystal; TiO2

Incoherent hopping models are commonly used to describe charge transport in many classes of electrically conducting materials. An important parameter in these models is the charge carrier hopping rate or its inverse, the charge carrier lifetime at a localization site. Among the most common approaches to calculating the charge hopping rate are those based on the hydrogen molecular ion (H2+) theory and on the Marcus theory of electron transfer reactions. The former is typically used when describing doped inorganic crystalline semiconductors, while the latter is commonly employed for organic systems. In the present paper the limitations of these approaches are examined and a generalized expression for the charge carrier lifetime at a localization site is proposed, which includes the expressions found from H2+ theory and Marcus theory as limiting cases. Charge transport simulations based on all three expressions are compared.

Using a tight-binding model of charge transport in systems with static and dynamic disorder, we present a theoretical study of the positive charge transfer in molecular assemblies that involve a hole donor and an acceptor connected by fluorene and phenyl bridges. Two parameters that determine the rate of charge transfer within the proposed model are the charge transfer integral between neighboring units and the site energies. Fluctuations in the values of the charge transfer integral and the energy landscape for hole transport were calculated by taking into account variations of the dihedral angle between neighboring units and electrostatic interaction of positive charge moving along the bridge and the negative charge that remains on the hole donor. Analysis of the dynamics of hole transfer and the distribution of the positive charge during this process allows the conclusion that the rapid fall of the hole transfer rate coefficient observed in experiments with short bridges (three and four structural units for systems with fluorene and phenyl bridges, respectively) can be attributed to the electrostatic interaction. This interaction is responsible for the formation of the effective barrier between donor and acceptor with the height that increases as the number of structural bridge units remains less than 3 (fluorene bridge) or 4 (phenyl bridge). For longer bridges, however, the effective barrier changes only weakly and now the charge transport is mostly dominated by the fluctuation-assisted incoherent hole migration along the bridge. The latter mechanism exhibits much weaker dependence of the rate coefficient on the bridge length in agreement with the available experimental results.

Efficient exciton diffusion in light-harvesting organic materials is of great importance for the development of cheap and flexible photovoltaic devices. Bioinspired molecular materials such as derivatives of phthalocyanine and porphyrin offer promising prospects for the realization of efficient exciton diffusion. The chemical composition of these molecules is found to strongly affect the molecular organization in thin films and in turn the efficiency of exciton diffusion. High singlet exciton diffusion coefficients exceeding 10−6 m2 s−1 have been found. This value is similar to that for the natural chlorosomal bacterio-chlorophyll, which exhibits the highest exciton diffusion coefficient for self-assembled systems.

The photogeneration quantum yield and dynamics of charge carriers and excitons in thin films of neat regioregular poly(3-hexylthiophene) (P3HT) and blends with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were studied with ultrafast optical pump−probe spectroscopy. In neat P3HT the quantum yield for direct photogeneration of charge carriers amounts to 0.15 per absorbed photon. The remaining fraction of absorbed photons leads to formation of excitons. Recombination of charges reduces the quantum yield to about 25% of its initial value on a time scale of 100 ps followed by decay to a no longer observable yield after 1 ns. Addition of 50% PCBM by weight leads to ultrafast (<200 fs) formation of charge pairs with a total quantum yield of 0.5. The presence of 50% PCBM causes exciton decay to be about an order of magnitude faster than in neat P3HT, which is expected to be at least in part due to interfacial exciton dissociation into charge carriers. The yield of charges in the blend has decayed to about half its initial value after 100 ps, while no further decay is observed within 1 ns. The small fraction (∼1%) of excitons in neat P3HT that is probed by photoluminescence measurements has a lifetime of 660 ps, which significantly exceeds the 200 ps lifetime of nonfluorescent excitons that are probed by transient absorption measurements. The nonfluorescent excitons have a diffusion coefficient of about 2 × 10−4 cm2/s, which is an order of magnitude smaller than reported values for fluorescent excitons. The interaction radius for second-order decay of photoexcitations is as large as 8−17 nm, in agreement with an earlier result in the literature.

Efficient exciton diffusion in light-harvesting organic materials is of great importance for the development of cheap and flexible photovoltaic devices. Bioinspired molecular materials such as derivatives of phthalocyanine and porphyrin offer promising prospects for the realization of efficient exciton diffusion. The chemical composition of these molecules is found to strongly affect the molecular organization in thin films and in turn the efficiency of exciton diffusion. High singlet exciton diffusion coefficients exceeding 10−6 m2 s−1 have been found. This value is similar to that for the natural chlorosomal bacterio-chlorophyll, which exhibits the highest exciton diffusion coefficient for self-assembled systems.