Multiple Exciton Generation Process Research Articles

Women in the Singlet Fission World: Pearls in a Semi-Open Shell.

Singlet fission, a multiple exciton generation process, can revolutionize existing solar cell technologies. Offering the possibility to double photocurrent, the process has become a focal point for physicists, chemists, software developers, and engineers. The following review is dedicated to the female investigators, predominantly theorists, who have contributed to the field of singlet fission. We highlight their most significant advances in the subject, from deciphering the mechanism of the process to designing coveted singlet fission materials.

Direct Visualization and Determination of the Multiple Exciton Generation Rate.

Multiple exciton generation (MEG) takes place in competition to other hot carrier cooling processes. While the determination of carrier cooling rates is well established, direct information on MEG dynamics has been lacking. Here, we present a methodology to obtain the MEG rate directly in the initial ultrafast transient absorption dynamics. This method is most effective to systems with slow carrier cooling rates. Perovskite quantum dots exhibit this property and are used to illustrate this approach. They show a delayed carrier concentration buildup following an excitation pulse above the MEG threshold energy, which is accompanied by a faster carrier relaxation, providing a direct evidence of the MEG process. Numerical modeling within a simple framework of two competing cooling mechanisms allows us to extract the MEG rate and carrier energy cooling rates for this material. The presented methodology could provide new insights in carrier generation physics and valuable information for MEG investigations.

Quantum coherence of multiple excitons governs absorption cross-sections of PbS/CdS core/shell nanocrystals

Multiple excitons in semiconductor nanocrystals have been extensively studied with respect to unique carrier dynamics including quantized Auger recombination and implementation in optoelectronic devices such as solar cells and photodetectors. However, the generation mechanism of multiple excitons still remains unclear. Here, we study instantaneous and delayed multiple exciton generation processes in PbS/CdS core/shell nanocrystals. The absorption cross-sections of biexcitons and triexcitons are identical to that of single excitons under instantaneous excitation with a single pulse. In contrast, the delayed excitation using double pulses shows a reduction of the biexciton and triexciton absorption cross-sections. Our theoretical analysis reveals that the excitonic coherence assists the generation of multiple excitons and that the reduction of multiple exciton absorption cross-sections is caused by the reduction of coherent excitation pathways. We clarify that exciton coherences play a key role in multiple exciton generation processes and seamlessly connect the identical and reduced multiple exciton absorption cross-sections.

New insights into the design of conjugated polymers for intramolecular singlet fission

Singlet fission (SF), a multiple exciton generation process that generates two triplet excitons after the absorption of one photon, can potentially enable more efficient solar cell designs by harvesting energy normally lost as heat. While low-bandgap conjugated polymers are highly promising candidates for efficient SF-based solar cells, few polymer materials capable of SF have been reported because the SF process in polymer chains is poorly understood. Using transient spectroscopy, we demonstrate a new, highly efficient (triplet yield of 160–200%) isoindigo-based donor–acceptor polymer and show that the triplet pairs are directly emissive and exhibit a time-dependent energy evolution. Importantly, aggregation in poor solvents and in films significantly lowers the singlet energy, suppressing triplet formation because the energy conservation criterion is no longer met. These results suggest a new design rule for developing intramolecular SF capable low-bandgap conjugated polymers, whereby inter-chain interactions must be carefully engineered.

Recent advance in multiple exciton generation in semiconductor nanocrystals

The multiple exciton generation (MEG), a process in which two or even more electron-hole pairs are created in nanostructured semiconductors by absorbing a single high-energy photon, is fundamentally important in many fields of physics, e.g., nanotechnology and optoelectronic devices. Many high-performance optoelectronic devices can be achieved with MEG where quite an amount of the energy of an absorbed photon in excess of the band gap is used to generate morei additional electron-hole pairs instead of rapidly lost heat. In this review, we present a survey on both the research context and the recent progress in the understanding of MEG. This phenomenon has been experimentally observed in the 0D nanocrystals, such as PbX (X=Se, S, and Te), InX (X=As and P), CdX (X=Se and Te), Si, Ge, and semi-metal quantum dots, which produce the differential quantum efficiency as high as 90%10%. Even more remarkably, experiment advances have made it possible to realize MEG in the one-dimensional (1D) semiconductor nanorods and the two-dimensional (2D) nano-thin films. Theoretically, three different approaches, i.e., the virtual exciton generation approach, the coherent multiexciton mode, and the impact ionization mechanism, have been proposed to explain the MEG effect in semiconductor nanostructures. Experimentally, the MEG has been measured by the ultrafast transient spectroscopy, such as the ultrafast transient absorption, the terahertz ultrafast transient absorption, the transient photoluminescence, and the transient grating technique. It is shown that the properties of nanostructured semiconductors, e.g., the composition, structure and surface of the material, have dramatic effects on the occurrence of MEG. As a matter of fact, it is somewhat hard to experimentally confirm the signature of MEG in nanostructured semiconductors due to two aspects:i) the time scale of the MEG process is very short; ii) the excitation fluence should be extremely low to prevent the multi-excitons from being generated by multiphoton absorption. There are still some controversies with respect to the MEG effect due to the challenge in both the experimental measurement and the explanation of signal data. The successful applications of MEG in practical devices, of which each is composed of the material with lower MEG threshold and higher efficiency, require the extraction of multiple charge carriers before their ultrafast annihilation. Such an extraction can be realized by the ultrafast electron transfer from nanostructured semiconductors to molecular and semiconductor electron acceptors. More recently, an experiment with PbSe quantum dot photoconductor has demonstrated that the multiple charge extraction is even as high as 210%. It is proved that MEG is of applicable significance in optoelectronic devices and in ultra-efficient photovoltaic devices. Although there are still some challenges, the dramatic enhancement of the efficiency of novel optoelectronic devices by the application of MEG can be hopefully realized with the rapid improvement of nanotechnology.

Control of Multiple Exciton Generation and Electron–Phonon Coupling by Interior Nanospace in Hyperstructured Quantum Dot Superlattice

The possibility of precisely manipulating interior nanospace, which can be adjusted by ligand-attaching down to the subnanometer regime, in a hyperstructured quantum dot (QD) superlattice (QDSL) induces a new kind of collective resonant coupling among QDs and opens up new opportunities for developing advanced optoelectric and photovoltaic devices. Here, we report the first real-time dynamics simulations of the multiple exciton generation (MEG) in one-, two-, and three-dimensional (1D, 2D, and 3D) hyperstructured H-passivated Si QDSLs, accounting for thermally fluctuating band energies and phonon dynamics obtained by finite-temperature ab initio molecular dynamics simulations. We computationally demonstrated that the MEG was significantly accelerated, especially in the 3D QDSL compared to the 1D and 2D QDSLs. The MEG acceleration in the 3D QDSL was almost 1.9 times the isolated QD case. The dimension-dependent MEG acceleration was attributed not only to the static density of states but also to the dynamical electron-phonon couplings depending on the dimensionality of the hyperstructured QDSL, which is effectively controlled by the interior nanospace. Such dimension-dependent modifications originated from the short-range quantum resonance among component QDs and were intrinsic to the hyperstructured QDSL. We propose that photoexcited dynamics including the MEG process can be effectively controlled by only manipulating the interior nanospace of the hyperstructured QDSL without changing component QD size, shape, compositions, ligand, etc.

Influence of Li+ doping on the optical properties of PbS

We investigate the influence of Li+ in the optical properties of PbS thin films. PbS and PbS:Li doped films were prepared by chemical bath deposition. Optical studies using the first derivative absorption spectra reveals the multiple exciton generation process associated to the excitonic transitions. The refractive index (n), extinction coefficient (k), and dielectric parameters (ε1 and ε2) were obtained. The analysis of current-temperature measurement in the 12–320 K range shows that the activation energy exhibits lower values with increasing Li+ concentration. The mechanism of current conduction in the films is discussed.

Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%

Multiple exciton generation (MEG) in quantum dots (QDs) has the potential to greatly increase the power conversion efficiency in solar cells and in solar-fuel production. During the MEG process, two electron–hole pairs (excitons) are created from the absorption of one high-energy photon, bypassing hot-carrier cooling via phonon emission. Here we demonstrate that extra carriers produced via MEG can be used to drive a chemical reaction with quantum efficiency above 100%. We developed a lead sulfide (PbS) QD photoelectrochemical cell that is able to drive hydrogen evolution from aqueous Na2S solution with a peak external quantum efficiency exceeding 100%. QD photoelectrodes that were measured all demonstrated MEG when the incident photon energy was larger than 2.7 times the bandgap energy. Our results demonstrate a new direction in exploring high-efficiency approaches to solar fuels. Multiple exciton generation has been shown to improve the performance of quantum-dot-based solar cells. Yan et al. now apply it to photoinduced hydrogen production and present a system using PbS quantum-dot photoelectrodes that yields an external quantum efficiency of over 100%.

Functional Mode Singlet Fission Theory

Singlet fission is a multiple exciton generation process that splits a singlet exciton (S0S1) into a correlated triplet pair (T1T1), affording a route to overcome the long-standing Shockley–Queisser thermodynamic limit for solar energy conversion. A new theory, based on multiconfiguration-constrained density functional theory and functional mode analysis, has been developed to model intermolecular singlet fission in organic photovoltaics. Specifically, constrained density functional theory is first employed to construct molecular orbitals for the six spin configurations comprising T1T1, the diabatic product state. In a subsequent step, linear response time-dependent density functional theory is utilized to formulate the S0S1 diabatic reactant state. Functional mode analysis is then applied to a thermalized ensemble of diabatic energy gaps to ascertain the reaction coordinate for the S0S1 → T1T1 transition. If singlet fission is assumed to follow a direct route, its rate may be evaluated using a modified Jortner formula within strong vibronic coupling regime. In contrast, second-order perturbation theory must be adopted to treat alternate pathways that are mediated by a charge-transfer (CT) intermediate. As shown through numerical simulations of single crystal tetracene, our theory reveals the direct mechanism to be the primary transition path, with an experimentally consistent singlet fission rate of 0.02 ps–1. CT pathways are effectively blocked due to a substantially diminished vibrational resonance among participating states. Our results have broad applicability, as only trivial alterations are needed to enable our new theory to model vibrationally modulated singlet fission using time-delayed pulse sequences.

Singlet Fission: Progress and Prospects in Solar Cells.

The third generation of photovoltaic technology aims to reduce the fabrication cost and improve the power conversion efficiency (PCE) of solar cells. Singlet fission (SF), an efficient multiple exciton generation (MEG) process in organic semiconductors, is one promising way to surpass the Shockley-Queisser limit of conventional single-junction solar cells. Traditionally, this MEG process has been observed as an intermolecular process in organic materials. The implementation of intermolecular SF in photovoltaic devices has achieved an external quantum efficiency of over 100% and demonstrated significant promise for boosting the PCE of third generation solar cells. More recently, efficient intramolecular SF has been reported. Intramolecular SF materials are modular and have the potential to overcome certain design constraints that intermolecular SF materials possess, which may allow for more facile integration into devices.

Efficiency of the coherent biexciton admixture mechanism for multiple exciton generation in InAs nanocrystals

We study the coherent mixing between two-particle (single exciton) and four-particle (biexciton) states of a semiconductor nanocrystal resulting from the Coulomb coupling between states with different numbers of electron-hole pairs. Using a simple model of the nanocrystal wave functions and an envelope function approach, we estimate the efficiency of the multiple exciton generation (MEG) process resulting from such coherent admixture mechanism, including all the relevant states in a very broad energy interval. We show that in a typical ensemble of nanocrystals with an average radius of 3nm, the onset of the MEG process appears about 1 eV above the lower edge of the biexciton density of states. This is due to the angular momentum conservation that imposes selection rules and limits the available MEG pathways, thus taking over the role of momentum conservation that hinders this process in bulk. The efficiency of the MEG process reaches 50% for photon energies around 5 eV. The MEG onset shifts to lower energies and therefore the efficiency increases in a certain energy range as the radius grows. The energy dependence of the MEG efficiency differs considerably between ensembles with small and large inhomogeneity of nanocrystal sizes.

Multiple exciton generation application of PbS quantum dots in ZnO@PbS/graphene oxide for enhanced photocatalytic activity

Multiple exciton generation (MEG), also known as carrier multiplication (CM), is a process in which absorption of a single photon produces more than one electron-hole pairs. We introduce PbS QDs with multiple exciton generation (MEG) property into the construction of a novel ZnO@PbS/GO structure photocatalyst and applied to suspended photocatalytic H2 evolution from water for the first time. The presence of multiple exciton generation process in PbS QDs, which is confirmed by transient absorption spectrum and photocurrent measurements, remarkably improves the photocatalytic efficiency of hydrogen generation combined with the electron-hole separation of GO. This strategy is proposed to open a new way for the photocatalysts design to achieve high efficiency based on QDs materials with MEG property.

Exciton extraction in nanocrystal solar cells

The efficiency of exciton extraction from nanocrystals is defined as the ratio of the number of extracted excitons to the number of absorbed photons. This efficiency is studied here based on a phenomenological model parameterized by recent experimental results in CdSe nanocrystals. Several possibilities of interplay among all possible electronic transition rates (Auger recombination, impact ionization, radiative recombination and electron–hole escape) are explored showing the possibility of obtaining efficiencies above 100% when multiple exciton generation processes are taken into account.

Dynamics of dissipative multiple exciton generation in semiconductor nanostructures

The population dynamics of single exciton and biexciton states in a simple model of a spherical semiconductor nanostructure is modeled numerically in the presence of Coulomb coupling between single and two exciton states and a dissipation channel in order to study the transient biexciton population that occurs in an optically excited semiconductor nanocrystal. The results show that the system evolution strongly changes if the dissipation is included. In a certain range of parameters, the growth of the exciton number (multiple exciton generation process) is fast (on picosecond time scale) and the following decay (Auger process) is much slower (hundreds of picoseconds). In some cases, the maximum occupation of the biexciton state increases when dissipation is included. The dynamics of an ensemble of nanostructures with a certain size dispersion is studied by averaging over the energy of the biexciton state which can be different for each single nanostructure. The validity of Markov and secular approximation is also verified.

Anomalous Independence of Multiple Exciton Generation on Different Group IV−VI Quantum Dot Architectures

Multiple exciton generation (MEG) in PbSe quantum dots (QDs), PbSe(x)S(1-x) alloy QDs, PbSe/PbS core/shell QDs, and PbSe/PbSe(y)S(1-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.