Carrier Energy Levels Research Articles

Optimizing Energy Levels and Improving Film Compactness in PbS Quantum Dot Solar Cells by Silver Doping.

PbS quantum dot (QD) solar cells harvest near-infrared solar radiation. Their conventional hole transport layer has limited hole collection efficiency due to energy level mismatch and poor film quality. Here, how to resolve these two issues by using Ag-doped PbS QDs are demonstrated. On the one hand, Ag doping relieves the compressive stress during layer deposition and thus improves film compactness and homogeneity to suppress leakage currents. On the other hand, Ag doping increases hole concentration, which aligns energy levels and increases hole mobility to boost hole collection. Increased hole concentration also broadens the depletion region of the active layer, decreasing interface charge accumulation and promoting carrier extraction efficiency. A champion power conversion efficiency of 12.42% is achieved by optimizing the hole transport layer in PbS QD solar cells, compared to 9.38% for control devices. Doping can be combined with compressive strain relief to optimize carrier concentration and energy levels in QDs, and even introduce other novel phenomena such as improved film quality.

Are Coarse-Grained Structures as Good as Atomistic Ones for Calculating the Electronic Properties of Organic Semiconductors?

The quality of amorphous molecular morphologies obtained with a recently introduced coarse-grained model, representing molecules in terms of connected anisotropic beads (Phys. Chem. Chem. Phys. 2019, 21, 26195), is benchmarked against reference atomistic data. Typical small-molecule organic semiconductors in their pristine and doped forms are chosen as a challenging and technologically relevant case study for our comparison, which includes both structural features and the resulting electronic properties, such as charge carrier energy levels, energetic disorder, and intermolecular charge transfer couplings. Our analysis shows that our accurate coarse-grained model leads to molecular glasses that are very similar to native atomistic samples, with the discrepancy being further reduced upon back-mapping. The electronic properties computed for back-mapped morphologies are almost indistinguishable from the atomistic reference, especially for multibranched poly(hetero)cyclic hydrocarbons usually employed as organic semiconductors. This study provides a proof of principle for highly accurate large-scale simulations of complex molecular systems at a reduced computational cost.

Heterocyclic amino acid molecule as a multifunctional interfacial bridge for improving the efficiency and stability of quadruple cation perovskite solar cells

Regulating the charge extraction ability of electron transport materials and constructing a chemically linked interface are imperative to enhance the power conversion efficiencies (PCEs) and improve the device stability of perovskite solar cells (PSCs). Herein, a heterocyclic amino acid molecule of 3-amino-4-pyrazolecarboxylic acid (APA) is incorporated into the SnO2/perovskite interface to improve the device performance via a multifunctional interfacial bridge. The carboxylic group and pyrazole N in APA significantly improve the optoelectronic properties of SnO2, such as carrier mobility, conductivity, energy levels and trap-state. Meanwhile, the pre-buried APA effectively passivates the buried perovskite interface, enhances the crystallinity of perovsite film and boosts the carrier extraction efficiency through chemically linking SnO2 and perovskite. As a result, the champion device delivers an impressive PCE of 24.71 % along with a fill factor of 83.56 %, which is one of the highest efficiencies for RbCsFAMA quadruple cation PSCs. Moreover, over 83 % and 80 % of their initial efficiencies are retained after 2400 h of storage and 500 h of continuous maximum output power point tracking under 1 sun illumination (white light LED array) for the unencapsulated devices, respectively. This work offers a facile approach to construct a robust interfacial bridge for efficient and stable PSCs.

Toward Understanding Chalcopyrite Solar Cells via Advanced Characterization Techniques

AbstractIn response to global environmental concerns, the importance of renewable energy, especially photovoltaic technology, is widely emphasized. As such, various efforts are made to improve the efficiency of solar cells, leading to significant technological advances supported by effective analysis and characterization techniques. Understanding these characterization techniques is important when looking to overcome technological limits and set the direction of technological development. Because the electrical properties of device, such as the carrier lifetime, doping density, energy levels, and surface potential, are generally determined based on the crystallographic phase, morphology, and compositional distribution, characterization techniques are developed to provide quantitative information on the absorber, surface, and interfaces of solar cells in order to improve their overall performance. In this review, basic to advanced characterization techniques for chalcopyrite solar cells are introduced. Because chalcopyrite solar cells exhibit high efficiency and are already commercialized, this can be a good lesson for studying the technological development history regarding efficiency improvement with characterization of thin‐film solar cells. Based on the various characterization techniques reviewed in this paper, structure−property relationships are established, thus providing a foundation for the development of strategies to improve processing conditions and thus increase the efficiency of solar cells.

Changing Spin and Orbital Ground State Symmetries in Colloidal Nanoplatelets with Magnetic Fields

The symmetry of the electronic ground state is of paramount importance in determining the magnetic, optical and electrical properties of semiconductor nanostructures. Here it is shown theoretically that non-trivial spin and orbital symmetries can be induced in colloidal nanoplatelets by applying out-of-plane magnetic fields. Two scenarios are presented. The first one deals with two electrons confined inside a platelet. Here, the strong electron-electron exchange interaction reduces the interlevel energy spacing set by lateral quantum confinement. As a result, relatively weak magnetic fields suffice to induce a singlet-to-triplet spin transition. The second one deals with type-II core/crown nanoplatelets. Here, the crown has doubly-connected topology, akin to that of quantum rings. As a result, the energy levels of carriers within it undergo Aharonov-Bohm oscillations. This implies changes in the ground state orbital symmetry, which switch the exciton and trion optical activity from bright to dark.This article is protected by copyright. All rights reserved.

Hot carrier exploitation strategies and model for efficient solar cell applications

Hot carriers are electrons or holes that are created in semiconductors upon the absorption of photons with energies greater than the fundamental bandgap. The excess energy of the hot carrier cools to the lattice temperature via carrier–phonon scattering and wasted as heat in [the] picoseconds timescale. The hot-carrier cooling represents a severe loss in the solar cells that have significantly limits their power conversion efficiencies. Hot carrier solar cells aim to mitigate this optical limitation by effective utilization of carriers at elevated energies. However, exploitation of hot carrier energy is extremely challenging as hot carriers rapidly lose their excess energy in phonon emission and therefore requires a substantial delay of carrier cooling in absorber material. In this paper a simple model was formulated to study the kinetic energies and hence the energy levels of the photo excited carriers in the quantum dots (QDs) whereas Schaller model was used to investigate the threshold energies of considered QDs. Results strongly indicate low threshold photon energies within the energy conservation limit for PbSe, PbTe, PbS, InAs, and InAs QDs. These materials seem to be good candidates for efficient carrier multiplication. It is found also that PbSe, PbTe, PbS, InAs, ZnS and InAs QDs exhibit promising potential for possible hot carrier absorber due to their widely spaced energy levels predicted to offer a large phononic gap between the optical and acoustic branches in the phonon dispersion. This in principle enhances phonon bottleneck effect that dramatically slows down hot carrier cooling leading to retention of hot carriers long enough to enable their exploitation. Two novel strategies were employed for the conversion of hot carriers into usable energies. The first approach involves the extraction of the energetic hot carriers while they are ‘hot’ to create higher photo voltage while the second approach uses the hot carrier to produce more carriers through impact ionization to create higher photo current. These mechanisms theoretically give rise to high overall conversion efficiencies of hot carrier energy well above Shockley and Queisser limit of conventional solar cells.

Resolving Donor–Acceptor Interfaces and Charge Carrier Energy Levels of Organic Semiconductors with Polar Side Chains

AbstractOrganic semiconductors consisting of molecules bearing polar side chains have been proposed as potential candidates to overcome the limitations of organic photovoltaics owing to their enhanced dielectric constant. However, introducing such polar molecules in photovoltaic devices has not yet resulted in higher efficiencies. A microscopic understanding of the impact of polar side chains on electronic and structural properties of organic semiconductors is paramount to rationalize their effect. Here, the impact of such side chains on bulk heterojunction overall morphology, molecular configurations at donor–acceptor (DA) interfaces, and charge carrier energy levels is investigated. The multiscale modeling approach used allows to resolve DA interfaces with atomistic resolution while taking into account the large‐scale self‐organization process which takes place during the processing of an organic thin film. The polar fullerene‐based blends are compared to the well‐studied reference system, poly(3‐hexyl‐thiophene) (P3HT):phenyl‐C61‐butyric acid methyl ester (PCBM). Introduction of polar side chains on a similar molecular scaffold does not affect molecular orientations at the DA interfaces; such orientations are, however, found to be affected by processing conditions and polymer molecular weight. Polar side chains, instead, are found to impact considerably the charge carrier energy levels of the organic blend, causing electrostatic‐induced broadening of these levels.

(Invited) Single Nanoelectrode Photodissolution Mechanisms

The nanoscale stability of metal and metal oxide surfaces is crucial to the performance and degradation of catalysis supports and battery electrodes. Our lab is developing single particle spectroelectrochemical methods to untangle the complex dynamics of dissolution of plasmonic metal nanoparticles. We are particularly interested in plasmon enhanced electrochemical processes. I will discuss our recent results in which we vary the electrolyte anion chemistry, ground state carrier energy levels, and excited state carrier levels in order to study the photodissolution of nanorods into more complex shapes. We also identify the role of hot carriers in the photo-electrochemical dissolution process.

Dopant‐Dependent Increase in Seebeck Coefficient and Electrical Conductivity in Blended Polymers with Offset Carrier Energies

AbstractThe strong and counterproductive interrelationship of thermoelectric parameters remains a bottleneck to improving thermoelectric performance, especially in polymer‐based materials. In this paper, a compositional range is investigated over which there is decoupling of the electrical conductivity and Seebeck coefficient, achieving increases in at least one of these two parameters while the other is maintained or slightly increased as well. This is done using an alkylthio‐substituted polythiophene (PQTS12) as additive in poly(bisdodecylquaterthiophene) (PQT12) with tetrafluorotetracyanoquinodimethane (F4TCNQ) and nitrosyl tetrafluoroborate (NOBF4) as dopants. The power factor increases two orders of magnitude with the PQTS12 additive at constant doping level. Using a second pair of polymers, poly(2,5‐bis(3‐dodecylthiophen‐2‐yl)thieno[3,2‐b]thiophene (PBTTTC12) and poly(2,5‐bis(3‐dodecylthiothiophen‐2‐yl)thieno[3,2‐b]thiophene, (PBTTTSC12), with higher mobilities, decoupling of the Seebeck coefficient and electrical conductivity is also observed and higher power factor is achieved. Distinguished from recently reported works, these two sets of polymers possess very closely offset carrier energy levels (0.05–0.07 eV), and the microstructure, assessed using grazing incidence X‐ray scattering, and mobility evaluated in field‐effect transistors, are not adversely affected by the blending. Experiments, calculations, and simulations are consistent with the idea that blending and doping polymers with closely spaced energy levels and compatible morphologies to promote carrier mobility favors increased power factors.

(Invited) Sub-Nm Quantification of Point Defects in Ceramics and Semiconductors Using Atom Probe Tomography

Many material properties are dependent upon local defect chemistry and associated charge compensations. Properties such as electronic and ionic transport, intergranular fracture, dielectric constant, and electrical breakdown are dependent upon the accumulation of point defects at internal interfaces such as grain boundaries and heterojunctions. Quantification of these point defect accumulations has been difficult, either due to light element sensitivity, detectability limits, or the 3-D nature of the internal interfaces. Recent work has shown that Atom Probe Tomography (APT) has the requisite counting statistics, 3-dimensionality, detectability limits, and spatial resolution to quantify point defect accumulations at grain boundaries and heterojunctions. Utilizing the 3-D atom by atom nature of APT data has enabled the conversion of analytical characterization data directly into space charge voltages and band alignments, ultimately enabling the direct relationship between materials characterization and materials or device properties. The ability of APT to quantify point defect distributions in 3-D has limitations that are dependent upon the type of defect. Individual vacancies on either oxygen or metal cation sites (Vo ** or Vm ’) have been quantified with APT at grain boundaries and heterojunctions. Quantification of vacancies are heavily dependent upon the detection efficiency of the APT experiment and upon the volume being sampled. Interstitial oxygen or metal cations (Oi ” or Mi *) can be observed but again is limited to the counting statistics of the volume being analyzed. However, their exact location can not be determined at this point as APT is limited in the necessary spatial resolution. Substitutional cations and defect pairs (MN ’) can be quantified as long as the cations are distinguishable in the mass spectrum. Substitutional defects coupled with vacancies (common ordered defect pairs) have been quantified at individual grain boundaries and, in some cases, allow for the calculation of electronic energy levels at interfaces. Schottky defects (Vo ** + Vm ’’) may be quantified using APT, as these defect pairs result in a change in atomic density, however, their quantification is limited by the detection efficiency of the APT instrument. Frenkel pairs on either the oxygen or metal cation sites (Vo ** + Oi ”) do not result in a change in atomic density and are only dependent upon site occupancies. Similar to interstitials, APT does not have the spatial resolution or detection efficiency necessary to identify individual Frenkel pairs. Such analyses will have to wait for Atomic Scale Tomography, of which APT may play a part. Ultimately, the combination of APT’s 3-D spatial and chemical resolution combined with analytical techniques such as EELS that allows for cation valence and free carrier energy levels to be determined will be the most complete solution to defect chemistry quantification.

(Plenary) Conjugated Polymers for Selective Chemical Sensing and Energy Conversion

The ability to control the carrier energy levels, functional group polarity, and film morphology make organic and polymeric semiconductors (OSCs) especially attractive for chemical sensing and for various energy interconversions. Their mechanical flexibility, low temperature processing, potential printability, capability of blending to form composites, predictable activity in various electronic circuit configurations, and use of common elements are additional attractive features. In this presentation, we focus on thiophene polymers and small conjugated molecular solids designed for maximum and selective responses for vapor sensing, with additional extensions to biosensing and thermoelectrics. Over the past several years, we have reported the electronic signaling by OSCs of exposures to vapors of interest for environmental health and security, including ammonia and phosphonates. These responses were selective over common interferents such as solvents and water. In our most recent work on vapor sensors, we demonstrated the selective and quantitative detection of nitrogen dioxide (NO2), an important component of polluted air. Two thiophene polymers, poly(bisdodecylquaterthiophene) and poly(bisdodecyl thioquaterthiophene) (PQT12 and PQTS12, respectively), were used as active layers in organic field-effect transistors (OFETs) to detect NO2 at parts-per-million concentrations. Responses of several hundreds of percent current increases were obtained, among the highest sensitivities reported for an NO2-responsive device based on an organic semiconducting film. From measurements of cyclic voltammetry and the electronic characteristics, we found that the introduction of sulfurs into the side chains induces traps in films of the PQTS12 and decreases domain sizes, both of which could contribute to the higher sensitivity of PQTS12 to NO2 gas. The ratio of responses of PQTS12 and PQT12 is higher for exposures to lower concentrations, allowing us to distinguish responses to low concentrations for extended times from exposures to high concentrations from shorter times, important for monitoring of indoor air quality, and contributing to the selectivity of the detection over likely interferent vapors. We used the same class of polymers for thermoelectric studies as well. Doping the polymer with sulfur in side chains (PQTS12) with the strong oxidant nitrosonium tetrafluroborate (NOBF4), we obtained a very high conductivity up to 350 S cm-1, which is the highest reported nonionic conductivity among films made from dopant-polymer solutions. We found the combination of efficient charge transfer, tighter π-π stacking and strong intermolecular coupling is responsible for the high conductivity. Furthermore, the high conductivity is stable in air without extrinsic ion contributions that are associated with the polymer most often used for high conductivity, poly(ethylenedithiothiophene) (PEDOT:PSS). The thermoelectric power factor compared favorably with prior reports for p-type polymers that were made by the alternative process of immersion of polymer films into dopant solutions, and fit the established models for thermoelectric polymers. Our work on biosensors has emphasized the role of dielectric receptor layers and the interfaces of those layers with analyte media and the OSCs. We have investigated the perturbations of OSC currents in OFETs by protein biomarkers of brain injury, especially glial fibrillary acidic protein (GFAP). We show that this protein, when bound to its antibody in a supporting polymer matrix on the OFET, causes an on-current change consistent with the charge that it delivers to the device surface, using p- and n-type OSCs. The use of an optimized hydrophobic bilayer on the OSC to protect it from the hydrophilic receptor layer, or the offset of the receptor layer from the OSC altogether, added to the device reliability. The incorporation of ion-complexing poly(ethylene glycol) (PEG) additives increased sensitivity.

Mathematical modelling of optical properties of CdSe/ZnS core shell quantum dot

In this paper, the electron and hole energies are computed for core/shell quantum dots (CSQDs) using two ways. First, we have resolved the three dimensional Schrödinger equation in the spherical coordinate system. The second approach is based on a combination of coordinate transformation and the finite difference method. The carrier energy levels of CdSe/ZnS CSQD are investigated, then the transition energies and the oscillator strength are deduced.

Optical refrigeration with coupled quantum wells.

Refrigeration of a solid-state system with light has potential applications for cooling small-scale electronics and photonics. We show theoretically that two coupled semiconductor quantum wells are efficient cooling media for optical refrigeration because they support long-lived indirect electron-hole pairs. Thermal excitation of these pairs to distinct higher-energy states with faster radiative recombination allows an efficient escape channel to remove thermal energy from the system. This allows reaching much higher cooling efficiencies than with single quantum wells. From band-diagram calculations along with an experimentally realistic level scheme we calculate the cooling efficiency and cooling yield of different devices with coupled quantum wells embedded in a suspended nanomembrane. The dimension and composition of the quantum wells allow optimizing either of these quantities, which cannot, however, be maximized simultaneously. Quantum-well structures with electrical control allow tunability of carrier lifetimes and energy levels so that the cooling efficiency can be optimized over time as the thermal population decreases due to the cooling.

Ruderman-Kittel-Kasuya-Yosida interaction between diluted magnetic semiconductor quantum dots embedded in semiconductor

Using the Keldysh Green's function method, we calculated the Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetic interaction between the localized spins of two diluted magnetic semiconductor (DMS) quantum dots embedded in a two-dimensional semiconductor. It was shown that the RKKY interaction is strongly dependent on the hybridization between the quantum-confined states in DMS quantum dots and the band in the semiconductor, the discrete energy levels in DMS quantum dots and the carrier density of the semiconductor. Since the carrier energy levels in DMS quantum dots may be adjusted by the applied voltage gates, the RKKY interaction of the present system is gate-controllable. It provides an alternate way to get the controllable RKKY magnetic interaction in semiconductor nanostructures for potential application in quantum information processing.

Observation of large Zeeman splitting in GaGdN/AlGaN ferromagnetic semiconductor double quantum well superlattices

Symmetric GaGdN/AlGaN (Gd concentration: 2%) and GaN/AlGaN double quantum well superlattices (DQW-SLs) were grown by radio-frequency plasma-assisted molecular-beam epitaxy on GaN (0001) templates. Atomic steps were observed on all the sample surfaces by atomic force microscope. X-ray diffraction θ/2θ scan curves exhibited well-defined satellite structures. Room temperature ferromagnetism was confirmed for the GaGdN/AlGaN DQW-SL samples by using alternating gradient magnetometer. Strong photoluminescence was observed from both GaGdN and GaN QWs at higher energy side of GaN excitonic peak. Magneto-photoluminescence spectra for GaGdN/AlGaN DQW-SL samples showed a large magnetic field dependence of the excitonic energy by applying a magnetic field up to 7T. The observed strong redshift of excitonic PL indicated an enhancement of Zeeman splitting of the free carrier energy levels in magnetic GaGdN/AlGaN DQW-SL. Enhanced g-factor was estimated to be about 60 for GaGdN/AlGaN DQW-SL sample with QW thickness of 1nm.

Quantification of graphene based core/shell quantum dots from first principles

Density functional calculations are performed to study the electronic structure of recently proposed graphene/graphane based core/shell quantum dots, which have a type I band alignment and exhibit quantized carrier energy levels. Strong confinement is robust with shell thickness. The bandgap, band offset, and the number of confined carrier orbitals with different size and geometry are determined. Our findings indicate that these core/shell dots are potentially well suited for the design of advanced diode lasers and room-temperature single electron devices. The proposed method to determine the number of confined orbitals is applicable for other quantum dot systems.

Host engineering for improving the performance of blue phosphorescent organic light-emitting devices

We report an effective means to greatly improve the performance of blue phosphorescent organic light-emitting devices by doping well-known electron- or hole-transporting triplet host material, specifically, 3-(4-biphenyl)-4-phenyl-5-(4- tert-butylphenyl)-1,2,4-triazole (TAZ) or 4,4′,4′′-tris( N-carbazolyl)triphenylamine (TCTA), into emissive layer to form mixed host structures. A superior device performance with high current and power efficiencies of 47.4 cd/A and 37.3 lm/W, respectively, and more importantly, a theoretical maximum of external quantum efficiency of 21.9%, can be achieved for an optimized TCTA-doped device. On the other hand, doping of TAZ into hole-transporting host material would degrade device performance. It is found that, in addition to the well-known effects of charge carrier balance, both energy levels and triplet energies of mixed host materials play crucial roles in determining the device performance. As confirmed by the time-resolved phosphorescence decay studies, the lifetime of FIrpic is strongly dependent on the triplet energy of host materials and the shorter lifetime of triplet excitons with high Stern–Volmer quenching constant of 0.1564% −1 in TAZ-doped films is the origin of the low electroluminescence efficiency of the TAZ-doped devices.

Electron and hole energy levels in InAs/GaAs quantum dots: Size and magnetic field effects

We present a systematic study on the influence of strain, size, and magnetic field on the electronic properties of InAs/GaAs quantum dots. Using a 40-band k.p model, we have calculated the band diagram of strained InAs, and extract the band parameters which are useful for the electronic properties of InAs/GaAs quantum dots. Then, using an exact numerical diagonalization method on Fourier–Bessel function basis over a large cylinder domain, we calculated numerically the electron and hole eigenenergies and associated wave functions. We considered thereafter the effect of an external applied magnetic field, strain and quantum dot size variation on the charge carrier energy levels. It is clearly found that the strain strongly modifies the quantum dot potential profile, leading to a different electron and hole energy distribution. Our results revealed also that the electron and hole energy spectra change significantly when varying the quantum dot size as well as the magnetic field. Given this striking nanostructure size and magnetic field energy dependent property, these systems provide the opportunity to control and tune their optical and electronic properties through these parameters.

Numerical Analysis of Multilayer Organic Light-Emitting Diodes

We make a comparative analysis of two different multilayer organic light-emitting diodes (OLEDs), one of which has an emitting layer (EML), and the other has a carrier transport controlling layer (CTCL) embedded between a hole transport layer (HTL) and an electron transport layer (ETL). The key differences between them lie in the carrier mobilities (relatively low for EML but high for CTCL) and energy levels of the middle layer. An in-depth numerical analysis has been done to provide guidelines for the design of trilayer OLED structures, especially in the context of mobility and energy level offsets. Furthermore, we focus on the transient response and carrier charge and discharge dynamics of those devices. Other than the HTL/CTCL/ETL structure, the transient current balance of the HTL/EML/ETL structure is shown to be much affected by the energy level offsets at the organic/organic interfaces due to the slow carrier dynamics in the EML. It is also demonstrated that the electroluminescence (EL) delay upon turn-on is mainly determined by the electron transport passing through the ETL and further EML, while the fast EL decay upon turn-off is by the rapid discharge of the abrupt accumulation of carriers at the organic/organic interfaces.

Theoretical modelling of electron transport in InAs/GaAs quantum dot superlattices

AbstractA theoretical model describing the electron transport in InAs/GaAs quantum dot infrared photodetectors, modelled as ideal quantum dot superlattices, is presented. The carrier wave functions and energy levels were evaluated using the strain dependent 8‐band k · p Hamiltonian and used to calculate all intra‐ and inter‐period transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi's golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. The populations of energy levels were then found from a system of rate equations, and the electron current in the superlattice was subsequently extracted. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)