Electrochemical CO2 conversion can be coupled with a photovoltaic cell and provide a pathway to utilize solar energy for the chemical synthesis. Ideally, such artificial photosynthesis system want to use CO2 and H2O as feed-stock molecules to produce value-added chemicals such as fuels or raw chemicals. My research team reported a monolithic and stand-alone device composed of a photovoltaic cell module, an Au CO2 reduction, a cobalt oxide anode accomplishing over 4 % conversion efficiency for CO2 conversion to CO production. To improve the solar to chemical conversion efficiency and to increase the feasibility further, we have developed efficient electrocatalysts and replaced the photovoltaic cell with Si modules, achieving ~ 8% of solar-to-CO conversion efficiency.

In addition, in this talk, metal-based electrocatalysts interacting with p-block elements or surface mediated molecules will be discussed for selective CO or C2+ (i.e. ethylene) production from CO2 reduction. The experimental results and theoretical simulation with various different types of metal catalysts (Ag, Zn, and Cu) give insights how to suppress the hydrogen evolution reaction (HER) is crucial to achieve efficient CO2 reduction catalysts. Monodispersed Ag nanoparticles are suggested to have the special interaction between the surface Ag and the surface mediated molecules which can modify the local electronic structure favoring for the selective CO production (up to 95 % of Faradaic efficiency). In addition, in the case of selective ethylene production, special Cu nanostructure formed by in-situ electrochemical fragmentation is demonstrated to be effective for increasing C-C bond coupling (up to 73 % of Faradaic efficiency) and selective ethylene production (up to ~ 60 % of Faradaic efficiency). In-situ X-ray absorption spectroscopy (XAS) studies are performed to understand the catalyst activity. Our series of studies suggests the modification of the metal nanoparticle surface by oxygen atom or surface mediated molecules can be effective strategies to increase CO2 reduction reaction activity and stability.

Chemically synthesized quantum dots (QDs) can potentially enable new classes of highly flexible, spectrally tunable lasers processible from solutions [1,2]. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage and an important challenge - realization of lasing with electrical injection - is still unresolved. A major complication, which hinders the progress in this field, is fast nonradiative Auger recombination of gain-active multicarrier species such as trions (charged excitons) and biexcitons [3,4]. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination by “softening” the electron and hole confinement potentials [5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [6], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a “zero-threshold-optical-gain” concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [7]. Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting recent developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices operating across a wide range of wavelengths and fabricated on virtually any substrate using a variety of optical-cavity designs.

[1] Klimov, V. I.et al., Optical gain and stimulated emission in nanocrystal quantum dots. Science290, 314 (2000).

[2] Klimov, V. I.et al., Single-exciton optical gain in semiconductor nanocrystals. Nature447, 441 (2007).

[3] Klimov, V. I. et al., Quantization of multiparticle Auger rates in semiconductor quantum dots. Science287, 1011 (2000).

[4] Robel, I., et al., Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett.102, 177404 (2009).

[5] Y.-S. Park, et al., Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).

[6] Lim, J., et al., Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater.17, 42 (2018).

[7] Wu, K., et al., Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol.12, 1140 (2017).

Colloidal semiconductor nanocrystals have gained interest since their optical and electronic properties can be tuned by varying their shape, size and composition. Recently, 2D square and honeycomb superlattice of lead- and cadmium-chalcogenide quantum dots (QDs) have been prepared. These superstructures are formed by assembling PbSe nanocrystals in a monolayer at the toluene suspension air/interface after which the nanocrystals attach via their four vertical {100} facets [1],[2]. Afterward, cation exchange transforms PbSe into zinc blend CdSe. Theoretical studies show that these 2-D systems have distinct band structures compared to continuous nanosheets, with the appearance of Dirac cones in the case of the honeycomb [3]. Strong electronic coupling via the atomic connections of the QDs in the superstructure may result in a higher mobility compared to the self-assembled lead chalcogenide QDs that are less strongly coupled due to the (in) organic ligands [4].  

In our research, we use electrolyte-gated transistors to study the optoelectronic properties and transport characteristics of 2-D PbSe and CdSe superstructures [5]. The potential of the gate electrode determines the Fermi level with respect to the conduction band (CB) or valence band (VB) of the superstructure. First, to monitor the stability of the superlattice under electron injection we measure the differential capacitance as a function of gate voltage. Second, the conductivity of the network is measured as a function of the Fermi level position. To quantify band occupation into the superlattice, the optical absorption quenching employed. Finally, the mobility of the system is calculated from conductivity and charge density.

We reported the first study of electron transport in a 2-D PbSe system with a square geometry in which band occupation is assured by the electron density of 8 electrons per nanocrystal . The electron mobility between 5 and 18 cm²/Vs is observed for these supersructures [6].­­

In our recent work, we study the electron transport of CdSe superlattices with square and honeycomb geometry. The band occupation is assured by the number of 2 electrons per nanocrystal. The electron mobility of 1 and 10 cm²/Vs is achieved for square and honeycomb geometry respectively.   

1) W.H. Evers et al., Nano Lett., 13, (2013).

2) M.P. Boneschanscher et al., Science, (2014).

3) E. Kalesaki et al., Phys. Rev. B 88, (2013).

4) W.H. Evers et al., Nature Communications 6, (2015).

5) D. Vanmaekelbergh et al., Electrochemica Acta, 53, (2007).

6) M. Alimoradi Jazi et al., Nano Lett., 17, (2017)

Nanocrystal building blocks can be assembled to make an artificial, nanocrystal solid. The choice of building block and the way they are assembled set up pathways to make new and unique materials with tailored properties. A case in point are superlattices of semiconductor nanocrystals or quantum dots (QDs), which find applications in, e.g., photodetectors, solar cells and field-effect transistors. Quantum dots offer the appealing combination of a tunable band gap, a high absorption coefficients, and a suitability for solution-based processing. QD films are typically produced through, e.g., spincoating, dropcasting or spraycoating. This results in disordered nanocrystal stacks, where poor electronic transport can be caused by excessive surface defects or restricted dot-to-dot hopping. To disentangle such effects, we analyzed the delocalization and transport of charge carriers in 2D superlattices of epitaxially connected QDs. In the case of PbS and PbSe QDs, such superlattices can be formed over several square micrometer. Using elemental analysis and structural analysis by in-situ XRF and GISAXS, respectively, we show that such lattices keep their structural integrity in a wide temperature window, ranging up to 310 ºC and more; an ideal starting point to assess the effect of gentle thermal annealing on the superlattice properties. We find that annealing such superlattices at temperatures ranging from 75-150 ºC induces a marked redshift of the QD band-edge transition. In fact, the band-edge found after annealing agrees, opposite from state-of-the-art literature, with theoretical predictions on charge carrier delocalization in such epitaxially connected superlattices. In addition, we observe a 1000-fold increases of the charge carrier mobility after mild annealing. While the superstructure remains intact at these temperatures, an XRD rocking curve analysis indicates that annealing markedly decreases the density of grain boundaries. This indicates that the presumably epitaxial connections between QDs in as-synthesized superlattices still form a major source of grain boundaries and defects, to an extend that carrier delocalization over multiple QDs is prevented and dot-to-dot transport remains strongly restricted.

The dynamics of photoluminescence (PL) from nanocrystal quantum dots (QDs) is significantly affected by reversible trapping of photo-excited charge carriers. This process occurs after up to 50% of the absorption events, depending on the type of QD considered, and can extend the time between photo-excitation and relaxation of the QD by orders of magnitude. Although many opto-electronic applications require QDs assembled into a QD solid, until now reversible trapping has been studied only in (ensembles of) spatially separated QDs. Here, we study the influence of reversible trapping on the excited-state dynamics of CdSe/CdS core/shell QDs when they are assembled into close-packed “supraparticles”. Time- and spectrally resolved PL measurements reveal competition between spontaneous emission, reversible charge carrier trapping, and Förster resonance energy transfer between the QDs. While Förster transfer causes the PL to redshift over the first 20–50 ns after excitation, reversible trapping stops and even reverses this trend at later times. We can model this behavior with a simple kinetic Monte Carlo simulation by considering that charge carrier trapping leaves the QDs in a state with zero oscillator strength from which no energy transfer can occur. Our results highlight that reversible trapping significantly affects the energy and charge carrier dynamics for applications where QDs are assembled into a QD solid.

Hot carriers refer to electrons (holes) that are formed from the thermalization of non-equilibrium photoexcited carrier populations following the absorption of above-bandgap photons. These hot carriers (HCs) later equilibrate within few picoseconds with the semiconductor lattice through carrier cooling processes such as carrier phonon scattering, Auger process, etc. The mechanisms and dynamics of HC cooling in semiconductors are of fundamental importance for enhancing device functionalities. Colloidal lead halide perovskite nanocrystals recently emerged as promising candidate materials for many optoelectronic applications. Building efficient and long-lasting devices from perovskite nanocrystals however remains a challenge. In this work, we study the size and temperature dependent cooling process of HC in CsPbBr₃ nanocrystals using time-domain density functional theory. We provide detailed insights into the mechanism of cooling by analyzing the effect of the passivating ligands (alkyl ammonium) in this process. We demonstrate that the inorganic lattice plays a much larger role in mediating the thermalization to the band edge.

Colloidal semiconductor nanocrystals, specifically quantum dots (QDs), are of interest to numerous scientific disciplines due to their highly tunable optical and electronic properties. For many years, various chemical treatments have been developed to fabricate conductive QD arrays for use in solar cells.  While such investigations have lead to increased solar cell performance there is much about how the chemical treatments modify the optical and electrical properties that are not well understood.  We have developed a simple, robust, and scalable solution-phase X-type ligand exchange method for PbS QDs that replaces native surface ligands with functionalized cinnamate ligands, yielding highly tunable, well-defined organic/inorganic hybrid chemical systems. We explore a library of functionalized cinnamic acid molecules to systematically tune PbS QD surface chemistry, and find that thin films of fully ligand exchanged QDs exhibit remarkable band edge shifts: the band edge position of QDs can be tuned over 2.0 eV.  We have also developed simple methods to impurity dope PbS and PbSe QDs with both n and p-type dopants.  We show how the dopants are incorporated and result in doped QDs with well behaved properties.   

Nanocrystals can be used to achieve transition in the mid and far infrared. This is in particular promising for the design of low cost infrared photodetectors. However more need to be understood on the dynamic of carriers in narrow band gap nanocrystals. This question is nevertheless quite challenging since many conventional experimental methods such as time resolved photoluminescence cannot be used because of the low PL efficiency in the infrared nanocrystals and because of difficulties to build infrared optical setup. We have investigated two complementary ways to probe carrier dynamics in narrow band gap colloidal materials which are time resolved photoemission and transient photocurrent. To illustrate these experiments, two systems have been investigated. First I will show that pump probe experiment can be conducted while exciting 2D HgTe nanoplatelets in the near IR and while looking at relaxation with a photoemission probe. This method is very efficient to determine without contact majority and minority carrier lifetime.In the second part of the talk, I will discuss how it is possible to determine band structure parameter such as the Urbach energy from transient photocurrent measurements conducted on HgTe nanocrystals. To do so, we have built a broad bandwidth setup (from ns to ms) and bring evidence for the multi-trapping transport regime.These two methods are extremely complementary to optical time resolved spectroscopy often limited to short dynamics (ns and less)

Nonresonant excitation of colloidal quantum dots (QDs) creates hot carriers that subsequently cool down to the band edges or are trapped in localized states. Carrier cooling and trapping typically happens on timescales from femtoseconds to picoseconds, orders of magnitude faster than the nanosecond to microsecond timescales of radiative recombination. Understanding cooling and trapping is relevant for (hot) carrier extraction in photovoltaics and to increase the luminesence output of QDs used as phosphor.

We investigate carrier cooling and trapping in InP QDs with a ZnSe_1-xS_x shell. Undoped QDs are compared to Cu⁺-doped QDs, where the Cu⁺ ion serves as a designed hole trap. Using pump probe transient absorption spectroscopy with femtosecond time resolution, we are able to monitor the population of electrons in the conduction band.

Our comparative study shows that hot electron cooling is almost an order of magnitude slower in the Cu⁺-doped QDs than in the undoped QDs. We ascribe this to rapid hole trapping on the Cu⁺ ion. This confirms the model in which hot electron cooling goes via an Auger-like process, where the hot electron transfers its excess energy to the hole which subsequently relaxes by phonon coupling. In our Cu⁺-doped QDs the hole is trapped on a Cu⁺ ion on sub-picosecond timescales, so the Auger cooling pathway is unavailable to the hot electron. Instead it must cool down via another, slower, pathway, most likely by coupling to high-energy vibrations at the surface of the QDs. This must also mean that hole trapping on the Cu⁺ ion is faster than the Auger cooling timescale of the undoped QDs, which is of the order of 300 fs. Our results provide insight in the behaviour of hot electrons and holes in the short time period after excitation of both Cu⁺-doped and undoped InP QDs.

The operational lifetime and scale-up of quantum dot sensitized solar cells (QDSCs) are largely limited by the hole transport layer, which is usually an aqueous polysulfide solution. The liquid, and alkaline nature of this electrolyte makes it difficult to develop laminated sealed devices. Further, the polysulfide electrolyte composition alters with use (there are observable color and viscosity variations!) thus impacting the cell performance. Besides the photoanode, the counter electrode (CE) also plays a key role in controlling cell response. While a variety of CEs have been attempted in the past: metallic coatings, carbon nanomaterials and conducting polymers, poly(3,4-ethylenedioxypyrrole) or PEDOP has rarely been used in QDSCs.

In view of the above described issues, in this report, QDSCs with a photoanode comprising of N-doped graphene quantum dots (N-GQDs) and cadmium sulfide (CdS)/titania (TiO₂) based solar cells with electropolymerized PEDOP@carbon cloth as CE and a polysulfide/SiO₂ gel with an electrolyte additive, namely, sodium poly(styrene sulfonate) or NaPSS were developed. The proportion of NaPSS is optimized on the basis of cell performance. A significant improvement in QDSC performance is obtained by incorporating NaPSS in the gel. By using impedance spectroscopy, the role of NaPSS in improving the cell performance is determined. NaPSS increases the recombination resistance for back electron transfer at the photoanode/electrolyte interface, thus increasing the power conversion efficiency to nearly 7% from 6.1% (when no NaPSS is present). NaPSS also imparts an enhanced operational life to the QDSC. Apart from NaPSS, the effectivity of PEDOP as a CE for QDSCs is studied by comparing impedance parameters, electrocatalytic activities and electrical conductivities of PEDOP films with different dopants. Ionically conducting and electrically conducting dopants were attempted. These studies provide a deeper understanding of factors that limit QDSC performances, and help in overcoming them.

Control over the energy alignment of Quantum Dots (QD) heterostructures can be used to unlock new functionalities for QD based optoelectronic devices: from improved carrier separation in type-II heterostructure, to control over hot-carrier transfer in type I heterostructures and lowered Carrier Multiplication threshold in quasi-type-II heterostructures.

We investigated the carrier dynamics in QD heterojunction films composed of PbSe and CdSe QDs. We demonstrate that such films tend to form a type I band alignment in which fast and efficient hot electron transfer from PbSe QDs to CdSe QDs is observed by transient absorption (TA) measurements. ­ The efficiency of the hot electron transfer process increases with excitation energy as a result of the more favorable competition between hot-electron transfer and electron cooling. The experimental picture is supported by time-domain density functional theory calculations, showing that electron density is transferred from lead selenide to cadmium selenide quantum dots on the sub-picosecond timescale. Hot-electron solar cells have been proposed as a route towards higher efficiency solar cells, and our observation reveals the possibility to achieve and control hot-electron transfer via energy-structure engineering in QD heterojunctions.

We next attempted to switch the energy alignment between the PbSe QDs and CdSe QDs, using the size-dependence of their energy structure as well as using tailored ligands to shift the energy levels through their surface dipoles. Spectroelectrochemical measurements reveal that we can shift the type I alignment to a type II alignment and TA measurements demonstrate a much-improved efficiency of “cold” electron transfer.

We thus proved that a combination of size-variation and control over surface-passivation allows to span the range between type-I and type-II alignment. One particularly interesting configuration is that of quasi-type-II alignment, where the conduction electron levels are resonant, as this could potentially be used for optimal Carrier Multiplication.

Metal-organic frameworks (MOFs) are coordination polymers consisting of metal ions connected by organic ligands. Besides the traditional applications in gas storage and separation as well as catalysis, the long-range crystalline order in MOFs and the tunable coupling between their organic and inorganic constituents have recently led to the design and synthesis of (semi-)conducting MOFs, opening the path for their application in opto-electronics. Yet, despite being a critical aspect for the development of MOF based electronics, the true nature of charge transport in MOFs, i.e. whether hopping or band-like transport occurs, has remained unresolved to date.

Here we report a time-resolved high-frequency (terahertz) conductivity study of a newly developed Fe₃(THT)₂ (THT=2,3,6,7,10,11-hexathioltriphenylene) two-dimensional MOF.  The novel π-d conjugated samples, synthesized through interfacial method at room temperature, are obtained as a large-area, free-standing film with tunable geometry (size and thickness). The Fe₃(THT)₂ films are conductive (~1 S/cm), porous (specific surface area of 526±5 m²/g) and semiconducting (with a ~250 meV direct bandgap). We demonstrate for the first time band-like charge transport in MOFs. This finding is directly apparent from the Drude-type high-frequency (terahertz) photo-conductivity response obtained in the samples; revealing free-moving, delocalized charge carriers displaying ~200 cm²/Vs mobilities at room temperature; a record mobility for MOFs. The temperature dependence of the mobility reveals that the main scattering mechanism limiting the mobility and hence band-like charge transport in this material is related to impurity scattering, so that material improvements may further increase the mobility.

The demonstration of band-like charge transport in MOFs reveal the potential of (porous) conductive MOFs to be employed as active materials in opto-electronics devices.

Understanding the charge transport rates across molecules and molecule-electrode interfaces  is important in many areas of research including chemistry, biology, and nanoscience.^1-2 A crucial parameter is the tunnelling decay coefficient (β, in Å^-1 or n_C^-1) which determines how quickly the current across the junction decreases as a function of the length of the molecule. Usually, the value of β can be changed by changing the chemical structure of the molecular backbone,^3-4 but β also depends on the type of the binding with the electrodes for conjugated systems.³ We have reported that SAMs of S(CH₂)_nX where X = H, F, Cl, Br, or I, have increasingly high currents with increasing polarizability of X.⁵ Here we report a new approach to tune β by simply changing X. In this system, eutectic gallium-indium alloy (EGaIn) was used as the top electrode, a monolayer of S(CH₂)_nX was self-assembled on a Ag surface which also served as the bottom electrode. We found that as the polarizability of X increases from X = F to I, β decreased from 0.97 ± 0.04 n_C^-1 to 0.34 ± 0.01 n_C^-1 and the dielectric constant ε_r increased from 2.5 ± 0.6 to 8.9 ± 1.6, respectively. DFT calculations show that the electrostatic potential profile of the SAM depends on X. More specifically, we found that the HOMO-1 is the dominant conduction orbital that is highly effected by X resulting in the tunnelling barrier height and thus the decay coefficient. In other words, the value of β can be controlled by using one polarizable atom without the need to change the molecular backbone.

References:

(1)           Stubbe, J. et al. Chem. Rev. 2003, 103, 2167.

(2)           Heitzer, H. M. et al. Acs Nano 2014, 8, 12587.

(3)           Kim, B. et al. Am. Chem. Soc. 2011, 133, 19864.

(4)           Xie, Z. et al. Acs Nano 2015, 9, 8022.

(5)           Wang, D. et al. Adv. Mater. 2015, 27, 6689.

Among many other applications, room temperature ionic liquids (ILs) are used as electrolytes for storage and energy conversion devices. In this context, rechargeable batteries are extended and useful devices to store energy. Since their discovery in 1970s, Li-ion batteries have become popular energy storage solutions and nowadays represent a promising alternative to conventional devices. The increasing requirements and power of these batteries, and disadvantages such as degradation or high flammability, make essential the search of alternative materials. Thanks to the high abundancy of Na as a raw material, Na-ion batteries have attracted intense attention as potential candidates for the replacement of Li-ion batteries. Other alternatives to Li-ion batteries based on multivalent metal cations are emerging in recent years. The ability of these metal species to transfer more than one electron can be useful to obtain faster charge rates. In this work, we investigate at microscopic level the structural and dynamical properties of 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide [C₄PYR]⁺[Tf₂N]^- IL-based electrolytes for metal-ion batteries. We carried out molecular dynamics simulations of electrolytes mainly composed of [C₄PYR]⁺[Tf₂N]^- IL with the addition of Mⁿ⁺-[Tf₂N]^- metal salt (M = Li⁺, Na⁺, Ni²⁺, Zn²⁺, Co²⁺, Cd²⁺, and Al³⁺, n = 1, 2, and 3) dissolved in the IL. The addition of low salt concentration lowers the charge transport and conductivity of the electrolytes. This effect is due to the strong interaction of the metal cations with the [Tf₂N]^- anions, which allows for molecular aggregation between them. We analyze how the conformation of the [Tf₂N]^- anions surrounding the metal cations determine the charge transport properties of the electrolyte. We found two main conformations based on the size and charge of the metal cation: monodentate and bidentate (number of oxygen atoms of the anion pointing to the metal atoms). The microscopic local structure of the Mⁿ⁺-[Tf₂N]^- aggregates influences the microscopic charge transport as well as the macroscopic conductivity of the total electrolyte.

Acknowledgements. The research leading to these results has received funding from the Andalucía Region (FQM-1851).

While charge transport in highly ordered system is generally well understood, correct modelling of conductivity in highly disordered system quite often presents an important theoretical but also experimental challenge. Slow conductance relaxation has been studied in many disordered insulators using field effect measurements. After a quench at low temperatures, a change in the gate voltage is accompanied by a sudden increase in the conductivity, slowly decreases with a roughly logarithmic dependence on time. Memory effects and aging are often seen in the same type of experiments. These glassy behaviour has been interpreted in different ways, but there is a growing tendency to explain them in terms of electron glasses, i.e., systems with states localized by the disorder and long-range Coulomb interactions between carriers.

The use of local probe techniques, such as scanning Kelvin probe microscopy (SKPM), presents two advantages as compared with the conductance measurements performed so far: i) it allows a study of the phenomena at the nanometer scale, and ii) samples with larger resistances can be measured. In the present work Dynamic AFM is used to characterize the electronic properties of two quite different nanoscale highly disordered and low conductivity systems. On the one hand highly resistive granular metal grains[1], and on the other Graphene Oxide islands[2].

We apply Electrostatic and Kelvin Force Microscopy to these samples. In addition, their time evolution is studied using “movies” where topography and surface potential are acquired simultaneously. Our AFM studies suggest evidence of the formation of an electron glass on the materials studied. This evidence includes the presence of domains on the surface potential, uncorrelated with topograph. The fluctuations of the surface potential are compatible with variations of the Coulomb energy of a single charge over the distance between domains. At the same time, the fact that the fluctuations are larger than kT and that time correlations are dominated by a broad distribution of characteristic times can be naturally explained within the electron glass model. When the conducting polymers are excited with light the surface potential relaxes logarithmically with time, as usually observed in electron glasses.

[1] M. Ortuño, E. Escasaín, E. López-Elvira, A. Somoza, J. Colchero and E. Palacios-Lidón. Scientific Reports, Scientific Reports 6, article #21647 (2016).

[2]M.F. Orihuela, A.M. Somoza, J. Colchero, M. Ortuño, E. Palacions-Lidón, Physical Review B 95(20) 205427 (2017).

Constructing a materials-specific theory of charge dynamics in organic single materials is a complex prob- lem, where the computation of accurate structural and vibrational properties needs to be coupled to ways of determining the charge mobility characteristics. In particular one needs an accurate method for describing excitations, which is also scalable to reasonably large systems. Here I will discussed how different flavours of constrained density functional theory (CDFT) can achieve such goal.

Firstly I will consider the most conventional form of CDFT, which allows one to calculate the energy of systems with displaced electron densities (e.g. in a charge transfer process). Such scheme can be used to extract a number of quantities important for charge dynamics. Here I will make examples of the calculation of 1) the charge transfer energies of molecules on surfaces, so to derive accurate level alignments [1,2]; 2) the quasi-particle gap renormalisation in molecular crystals [3]; 3) the reorganisation energy of molecules in the gas phase and on surfaces [4].

Then I will move to show a recently implemented scheme, which uses CDFT to compute elementary excitations in molecules [5]. This method, which we have named excitonic DFT (XDFT), calculates the M-particle excited state of an N-electron system, by optimizing a constraining potential to confine N−M electrons within the ground-state Kohn-Sham valence subspace. The efficacy of XDFT will be demonstrated by calculating the lowest single-particle singlet and triplet excitation energies of the well-known Thiel molecular test set, with results which are in excellent agreement with time-dependent density functional theory (TDDFT).

[1]A.M.Souza, I.Rungger, C.D.Pemmaraju, U.Schwingenschloegl and S.Sanvito, Constrained-DFTmethod for accurate energy-level alignment of metal/molecule interfaces, Phys. Rev. B 88, 165112 (2013).

[2]  Subhayan Roychoudhury, Carlo Motta and Stefano Sanvito, Charge transfer energies of benzene physisorbed on a graphene sheet from constrained density functional theory, Phys. Rev. B 93, 045130 (2016).

[3] A. Droghetti, I. Rungger, C.D. Pemmaraju and S. Sanvito, Fundamental gap of molecular crystals via constrained Density Functional Theory, Phys. Rev. B 93, 195208 (2016).

[4] Subhayan Roychoudhury, David D. O’Regan and Stefano Sanvito, Wannier-function-based constrained DFT with nonorthogonality-correcting Pulay forces in application to the reorganization effects in graphene- adsorbed pentacene, Phys. Rev. B 97, 205120 (2018).

[5] Subhayan Roychoudhury, Stefano Sanvito and David D. ORegan, XDFT: an efficient first-principles method for neutral excitations in molecules, arXiv:1803.01421 (2018).

The development of robust, chemically-sensitive techniques is crucial for the advancement of single-molecule electronics. Studies in single-molecule junctions largely rely on indirect electrical characterization to statistically evaluate the chemistry and quality of the established circuits. One fundamental challenge is the direct, quantitative determination of charge-vibrational coupling for well-defined single-molecule junctions. The ability to record molecular charge-vibrational coupling for individual species grants access to the determination of maximal charge transport efficiencies for specific molecular configurations and currents. Here we explore the charge-vibrational coupling for current-carrying tethered molecules by combined vibrational and metal-molecule-metal junction current-voltage spectroscopy. By inspecting the steady-state vibrational distribution during charge transport in a bis-phenyl-ethynyl-anthracene derivative by Raman scattering, we deduce a coupling constant of ≈0.35 vibrational excitations per charge carrier. Furthermore we follow the conformational response of a two-state molecular switch. Specifically, we remove the ground state polarizability and symmetry of a known p-terphenyl-4,4´´-dithiol (TPD) molecule by employing the 2,2´,5´,2´´-tetramethylated (TM-TPD) derivate. Whereas the highly sterically hindered, non-planar TM-TPD, lacking π-conjugation, in its pristine conformation does not exhibit a Raman signature, a marked on/off modulation of the single-molecule Raman signal exceeding a factor of 100 is achieved via redox state control by means of the applied voltage.

Support by the Deutsche Forschungsgemeinschaft (DFG) via SPP 1234 (Grant RE2592) & Munich Centre for Advanced Photonics (MAP), the European Research Council via Advanced Grant MolArt (n° 247299) and Chinese Scholarhip Council (H.B., Y.G.) is gratefully acknowledged.

Key refs.: JACS 140 (2018) 4835 | Nature Comm. 7 (2016) 10700 | Nature Nanotechn. 7 (2012), 673

Electron transport (ETp), i.e., electronic conduction, across protein monolayers in a solid state–like configuration is surprisingly efficient, comparable, length-normalized, to completely conjugated molecules. This is amazing as apparently nature does not use this capability, except for electron transfer, ET, within and some times between redox proteins, a process that is coupled to ionic transport.

Nature regulates ET via redox chemistry, while for ETp a redox process is not a necessary condition. This allows studying ETp via non-redox proteins, such as rhodopsins and albumins; remarkably, ETp is quite efficient also across these proteins.

If contact to the external world does not limit ETp, i.e., intra-protein transport dominates, there seems to be no barrier for transport. As ETp is temperature-independent, it may well be coherent…. This may be important also for electron transfer, ET, which involves injection and extraction of electrons, the analogue of which for ETp is the coupling to the electrodes. Then, efficient ETp via non-redox proteins suggests why there are redox centres in ET proteins, ito protect it from high reducing power.

I will discuss experimental data that illustrate our main results as well as some more recent ones on multi-heme proteins and on protein multilayers, which raise even more questions.^1,2

* work with Mordechai Sheves & Israel Pecht, Weizmann Inst.

References

C. Bostick et al.  Rep. Prog.Phys., 81 (2018) 026601, “Protein bioelectronics:a review of what we do and do not know” doi.org/10.1088/1361-6633/aa85f2 ,

N. Amdursky et al., Adv. Mater. 42, (2014) 7142 “Electronic Transport via Proteins” 10.1002/adma.201402304

We report recent experimental and theoretical results for molecular junctions based on \pi conjugated wires. These wires represent a class of linear molecules whose transport properties can be understood in the framework of the topological Su-Schrieffer-Heeger model for polyacetylene. We present an in-depth theoretical analysis based on tight-binding and ab-initio simulations of their coherent transport properties and show that, under certain conditions and depending on the chain parity (even /odd) and length, the conductance at the Fermi level can depend very weakly or even increase with the wire length. For short odd chains, we also provide experimental evidence of the role of the external environment in their charge transport properties: we study conductance trends in single-molecular junctions of polymethine dyes and prove that the trends can also be altered by the choice of the embedding solvent. Overall, our results suggest a way of enabling efficient electron transport at the nanoscale with one-dimensional wires.Iryna Davydenko

The extraordinary absorption cross-section and high photoluminescence (PL) quantum yield of perovskite nanocrystals make this type of material attractive to a variety of applications in optoelectronics. For the same reasons, nanocrystals are also ideally suited to function as nanoantennae to excite nearby single dye molecules by fluorescence resonant energy transfer (FRET). Here, we demonstrate that FAPbBr3 perovskite nanocrystals, of cuboidal shape and approximately 10 nm in size, are capable of selectively exciting single Cyanin 3 molecules at a concentration a hundredfold higher than standard single-molecule concentrations. This FRET antenna mechanism increases the effective brightness of the single dye molecules one hundredfold. Photon statistics and emission polarization measurements provide evidence for the FRET process by revealing photon-antibunching with unprecedented fidelity and highly polarized emission stemming from single dye molecules. Remarkably, the quality of single-photon emission improves two-fold compared to emission collected directly from the nanocrystals since the higher excited states of the dye molecule act as effective filters to multiexcitons. The same process gives rise to efficient deshelving of the molecular triplet state by reversed intersystem crossing, translating into a reduction of the PL saturation of the dye, thereby increasing the maximum achievable PL intensity of the dye by a factor of five.

Perovskite based solar cells have shown impressive progress in recent years. Now that high efficiencies are in reach, more and more effort is put into understanding the underlying device physics and to improve the devices intrinsic and extrinsic stability.

Touching on both of these subjects, I want to first discuss recent findings regarding the implementation of metal oxide layers into perovskite solar cell device stacks. These can be used to form impenetrable barrier layers which prevent the ingress of humidity as well the egress of perovskite decomposition products. Using this strategy, the overall decomposition of perovskite can be significantly suppressed, leading to outstanding solar cell device stability.

On the other hand, we find for a variety of systems that directly interfacing the perovskite to metal oxide layers can trigger a complex variety of reactions, which significantly alter the composition of the perovskite at the interface and lead to the presence of degradation products.  These thin interlayers play an important role for film formation and charge extraction and will therefore influence the overall device performance.

Photovoltaic devices based on hybrid metal halide perovskites are rapidly improving in power conversion efficiency. While these materials are generally viewed as three-dimensional, the effects of nano-scale interfaces within the bulk films are still poorly understood. Such interfaces may appear between the perovskite layers and the charge extraction layers, or within the perovskite layers, at grain boundaries, of when passivating or hydrophobic interlayers are included.

We show here that photon reabsorption in lead iodide perovskite layers is strongly influenced by layer thickness and interfaces formed with charge-extraction layers^[1]. Such photon re-absorption is found to reduce the apparent bi-molecular charge-carrier recombination rate constant with increasing film thickness, while the intrinsic value can be fully explained as the inverse process of absorption^[2].

In addition, we discuss the role of nanoscale interfaces^[3,4], energetic disorder^[5], and passivating interlayers^[6] on the dynamics of charge-carriers in various metal halide perovskites. Lowering of the perovskite dimensionality is shown to have effects similar to those known for classic inorganic semiconductors, such as enhancing bimolecular and Auger recombination and reducing trap-mediated recombination through surface passivation. Such extrinsic effects will also reduce the charge-carrier mobility below the maximum attainable value near 100cm²/(Vs) for MAPbI₃^[7].

[1] T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P. Müller-Buschbaum, M. B. Johnston, and L. M. Herz,Nano Lett. 17, 5782 (2017).

[2] C. L. Davies, M. R. Filip, J. B. Patel, T. W. Crothers, C. Verdi, A. D. Wright, R. L. Milot, F. Giustino, M. B. Johnston, and L. M. Herz, Nature Communications 9, 293 (2018).

[3] D. P. McMeekin, Z. Wang, W. Rehman, F. Pulvirenti, J. B. Patel, N. K. Noel, S. R. Marder, M. B. Johnston, L. M. Herz, and H. J. Snaith, Adv. Mater., 29 (2017), p. 1607039.

[4] R. L. Milot, R. J. Sutton, G. E. Eperon, A. A. Haghighirad, J. M. Hardigree, L. Miranda, H. J. Snaith, M. B. Johnston, and L. M. Herz, Nano Letters, 16 (2016), pp. 7001-7007.

[5] A. D. Wright, R. L. Milot, G. E. Eperon, H. J. Snaith, M. B. Johnston, and L. M. Herz, Adv. Func. Mater., 27 (2017), p. 1700860.

[6] Z. Wang, Q. Lin, F. P. Chmiel, N. Sakai, L. M. Herz, and H. J. Snaith, Nature Energy, 2 (2017), p. 17135.

[7] L. M. Herz, ACS Energy Lett., 2 (2017), pp. 1539-1548.

The optical and transport properties of lead-halide perovskites (LHPs) have been used as a basis for new solar cell technologies showing record improvements in efficiencies. In the search for the microscopic origins of this success, many recent studies suggest that structurally dynamic effects are active already at room temperature and standard operating conditions and may affect device performance and/or stability. Here, we explore this issue using first-principles calculations based on density functional theory. In particular, we focus on ion migration and dynamic distortions.

The optical and transport properties of lead-halide perovskites (LHPs) have been used as a basis for new solar cell technologies showing record improvements in efficiencies. In the search for the microscopic origins of this success, many recent studies suggest that structurally dynamic effects are active already at room temperature and standard operating conditions and may affect device performance and/or stability. Here, we explore this issue using first-principles calculations based on density functional theory. In particular, we focus on ion migration and dynamic distortions.

In this talk, I will discuss our group’s recent experimental and computational work on understanding electronic and phononic structure nanocrystal thin films and charge transport in these thin films. Using electrochemical-based approaches, we show that we can quantify the electronic density of states and also examine charge-transfer processes across interfaces. Using inelastic x-ray scattering, we quantify the phononic denisty of states. We combine density functional theory calculations and kinetic Monte Carlo simulations to develop a first-principles model for charge transport in nanocrystals solids. We show that these simulations explain temperature-dependent time-of-flight measurements of electron and hole mobility performed on lead sulfide nanocrystal thin films. The combination of experimental and computational work highlights the importance of electron-phonon interactions in nanoscale transport and enables us to determine the relative impact of energetic and positional disorder on transport, providing us with design guidelines on parameters to consider when optimizing nanocrystal synthesis, nanocrystal surface treatments, and nanocrystal thin film preparation for different device applications.

For the fabrication of an integrated solar-to-chemical system, different components should be interfaced together in an orchestrated manner. Photoelectrodes need to absorb in the visible range, with a valence and a conduction band suited for the target reaction. Moreover, the presence of catalysts is required to manage the intrinsic energetic hurdle. Herein, we address the study of the major challenges, namely performance, stability, and interfaces to enable fabrication of integrated solar-to chemical systems. Novel scientific directions for the synthesis of functional interfaces and the development of new tools for their characterization will be addressed. Specifically, we will present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. These findings are confirmed by in situ electrochemical atomic force microscopy (EC-AFM), which reveals that degradation under operating conditions occurs via dissolution of the film, starting at exposed facets of grains in polycrystalline thin films. In addition, we will present the correlation between morphological and functional heterogeneity in this material by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate.

Insights into corrosion mechanisms and nanoscale heterogeneity aid development of protection strategies and provide information on how local functionality affects the macroscopic performance. 

Metal-halide perovskites are promising materials for opto-electronic applications. Their mechanical and electronic properties are directly connected to the nature of their lattice vibrations. Whereas the mid infrared (IR) range contains mainly information on the internal vibrations of the methylammonium cation, the lead-halide lattice vibrations are located in the far IR.

We will report far-IR spectroscopy measurements of lead halide perovskite thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra.¹ We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites. We determine the frequencies of both the transversal and longitudinal optical phonons, and use them to calculate the static dielectric constants, polaron masses, and upper limits for the phonon-scattering limited charge carrier mobilities. Furthermore, we compare our experimental results to molecular dymanics simulations of lead halide perovskites. Our findings are important for the basic understanding of charge transport processes and mechanical properties in metal halide perovskites.

(1) Sendner, M.; Nayak, P. K.; Egger, D. A.; Beck, S.; Müller, C.; Epding, B.; Kowalsky, W.; Kronik, L.; Snaith, H. J.; Pucci, A.; Lovrincic, R. Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport. Mater Horiz 2016, 3 (6), 613–620.

Solar cells based on metal-halide perovskite absorber layers have resulted in outstanding photovoltaic devices with long non-radiative lifetimes as a crucial feature enabling high efficiencies. Long non-radiative lifetimes occur if the transfer of the energy of the electron-hole pair into vibrational energy is slow, due to, e.g., a low density of defects, weak electron phonon coupling or the release of a large number of phonons needed for a single transition. Here, we discuss the implications of the known material properties of metal-halide perovskites (such as permittivities, phonon energies and effective masses) and combine those with basic models for electron-phonon coupling and multiphonon-transition rates in polar semiconductors. We find that the low phonon energies of MAPbI₃ lead to a strong dependence of recombination rates on trap position, which can be readily deduced from the underlying physical effects determining non-radiative transitions. Here, we show that this is important for the non-radiative recombination dynamics of metal-halide perovskites, as it implies that these systems are rather insensitive to defects that are not at midgap energy. This can lead to long lifetimes, which indicates that the low phonon energies are likely an important factor for the high performance of optoelectronic devices with metal halide perovskites. 

In conventional solar cells (SCs), above-bandgap “hot” carriers (HCs) rapidly lose their excess energy to vibrations in the semiconductor lattice via electron-phonon coupling. This thermalisation to the band edge forms the main part of the Shockley-Queisser limiting efficiency. Semiconductors with reduced carrier cooling rates are desirable to exceed this limit via a hot-carrier architecture, where the hot carriers are extracted before they are fully cooled. Lead-halide perovskites are solution processible solar cell materials, which exhibit exceptional power conversion efficiencies and a tunable band gap. Recent measurements indicate slow cooling at high carrier densities in these material systems. Here we use ultrafast infrared intraband spectroscopy to directly compare the dynamics of carrier cooling in a range of five commonly studied lead-halide perovskites: FAPbI₃, FAPbBr₃, MAPbI₃, MAPbBr₃ and CsPbBr₃. We measure this cooling as occuring within ~100-900 fs, depending on both the carrier density, n^hot (slower cooling at higher n^hot) and choice of cation (with the slowest cooling in the all-inorganic Cs-based system). These observations support the existence of a “hot-phonon bottleneck” and assert the role of lattice vibrations towards HC cooling.

Metal-halide perovskites show exceptional optoelectronic properties for next generation photovoltaics and light-emitting diodes. Recently, monovalent cation substitution has been reported to generate luminescence very efficiently, yet the underlying photo-physics remain to be understood.

Here, we study the origin of this increased brightness by combining transient absorption and photoluminescence (PL) to track charge carrier dynamics in thin films. Unexpectedly, we find that the recombination regime changes from the previously-reported second to first order regime dynamically within tens of nanoseconds after excitation, in line with fluence-dependent PLQE measurements. In temperature-dependent PL we find a redshift of the luminescence with decreasing temperature, directly mapping localized shallow traps. Supported by DFT calculations and transistor measurements we propose that energetic disorder in the distribution of electronic states leads to spatial accumulation of charges, creating n- and p-type regions. Our results indicate that strong luminescence can be achieved in mixed-cation perovskites even at low carrier densities and thereby provides a roadmap for highly efficient LEDs.