Stabilization of Charge-Transfer Excited States in Biological Systems: A Computational Focus on the Special Pair in Photosystem II Reaction Centers.

Charge-transfer (CT) excited states play an important role in many biological processes. However, many computational approaches often inadequately address the equilibration effects of nuclear and environmental degrees of freedom on these states. One prominent example of systems in which CT states are of utmost importance is reaction centers (RC) in photosystems. Here we use a multiscale approach combined with time-dependent density functional theory to explore the lowest CT excited state of the special pair PD1-PD2 in the Photosystem II-RC of a cyanobacterium. We find that the nonequilibrium CT excited state resides near the Soret band, making an exciton the lowest-energy excited state. However, accounting for nuclear and state-specific dielectric equilibration along the CT potential energy surface (PES), the CT state PD1--PD2+ stabilizes energetically below the excitonic state. This underscores the crucial role of state-specific solvation in mapping the PES of CT states, as demonstrated in a simplified dimer model.

Dielectric Screening and Charge-Transfer in 2D Lead-Halide Perovskites for Reduced Exciton Binding Energies.

Layered lead-halide perovskites have shown tremendous success as an active material for optoelectronics. This is attributed to the electronic structure of the inorganic sublattice and large exciton binding energies due to quantum and dielectric confinement. Expanding functionalities for applications that depend on free-carrier generation requires new material design routes to decrease the binding energy. Here we use electronic structure methods with model Bethe-Salpeter equation (BSE) to examine the contributions of the dielectric screening and charge-transfer excited-states to the exciton binding energy of phenylethylammonium (PEA2PbBr4) and naphthlethylammonium (NEA2PbBr4) lead-bromide perovskites. Our model BSE calculations show that NEA introduces hole acceptor states which impose charge-transfer character on the exciton along with larger dielectric screening. This substantially decreases the exciton binding compared to PEA. This result suggests the use of organic cations with high dielectric screening and hole acceptor states as a viable strategy for reducing exciton binding energies in two-dimensional halide perovskites.

Charge-Transfer Luminescence in a Molecular Donor-Acceptor Complex: Computational Insights.

Donor-acceptor molecular complexes are a popular class of materials utilizing charge-transfer states for practical applications. A recent class of donor-acceptor dyads based on the fluorescent BODIPY functionalized with triphenylamine (TPA) shows the peculiar property of dual fluorescence. It is hypothesized that instead of the sensitized charge-transfer state being optically dark, it provides an additional bright radiative pathway. Here we use time-dependent density functional theory to characterize the energetic alignment of excitonic and charge-transfer states in a BODIPY-TPA molecular complex. We observe that using a long-range exchange corrected functional in combination with state-specific solvation scheme gives a qualitatively correct alignment of the exciton and charge-transfer states and an enhancement in oscillator strength for the equilibrium solvated charge-transfer state, in agreement with experiment. This work provides rationalization of charge-transfer state emission and provides a foundation to explore charge-transfer using ab initio excited-state nonadiabatic dynamics.

Induced Chirality in Halide Perovskite Clusters through Surface Chemistry.

Chiroptical properties are of interest for various applications, including structure determination, polarized photodetectors, and spintronics. Inducing chiroptical activity into semiconductors is challenging because of difficulties in creating asymmetric crystal structures. One promising method is to use chirality transfer by deploying chiral organic molecules as capping ligands for nanocrystals. Experimentally, chiral-capped nanocrystals show emergent chiroptical signatures, but the mechanisms for chirality transfer remain unclear. Here we utilize atomistic modeling using time-dependent density functional theory calculations to explore chirality transfer in CsPbX3 (X = Cl, I) clusters capped with chiral diaminocyclohexane (DACH) enantiomers. When DACH enantiomers are bound to the cluster surface, the perovskite optical transitions gain chiral signatures. This observed chirality transfer is best rationalized by chiral molecular dipole-cluster transition dipole coupling. With multiple DACH molecules bound to the cluster surface, anisotropy factors are found to increase proportionally to the surface ligand density, providing mechanistic insight toward improving chiroptical functionality in semiconductor nanomaterials.

Cesium-Coated Halide Perovskites as a Photocathode Material: Modeling Insights.

Photocathodes emit electrons when illuminated, a process utilized across many technologies. Cutting-edge applications require a set of operating conditions that are not met with current photocathode materials. Meanwhile, halide perovskites have been studied extensively and have shown a lot of promise for a wide variety of optoelectronic applications. Well-documented halide perovskite properties such as inexpensive growth techniques, improved carrier mobility, low trap density, and tunable direct band gaps make them promising candidates for next-generation photocathode materials. Here, we use density functional theory to explore the possible application of pure inorganic perovskites (CsPbBr3 and CsPbI3) as photocathodes. It is determined that the addition of a Cs coating improved the performance by lowering the work function anywhere between 1.5 and 3 eV depending on the material, crystal surface, and surface coverage. A phenomenological model, modified from that developed by Gyftopoulos and Levine, is used to predict the reduction in work function with Cs coverage. The results of this work aim to guide the further experimental development of Cs-coated halide perovskites for photocathode materials.

Highly efficient photoelectric effect in halide perovskites for regenerative electron sources.

Electron sources are a critical component in a wide range of applications such as electron-beam accelerator facilities, photomultipliers, and image intensifiers for night vision. We report efficient, regenerative and low-cost electron sources based on solution-processed halide perovskites thin films when they are excited with light with energy equal to or above their bandgap. We measure a quantum efficiency up to 2.2% and a lifetime of more than 25 h. Importantly, even after degradation, the electron emission can be completely regenerated to its maximum efficiency by deposition of a monolayer of Cs. The electron emission from halide perovskites can be tuned over the visible and ultraviolet spectrum, and operates at vacuum levels with pressures at least two-orders higher than in state-of-the-art semiconductor electron sources.

Excited-State Properties of Defected Halide Perovskite Quantum Dots: Insights from Computation.

CsPbBr3 quantum dots (QDs) have been recently suggested for their application as bright green light-emitting diodes (LEDs); however, their optical properties are yet to be fully understood and characterized. In this work, we utilize time-dependent density functional theory to analyze the ground and excited states of the CsPbBr3 clusters in the presence of various low formation energy vacancy defects. Our study finds that the QD perovskites retain their defect tolerance with limited perturbance to the simulated UV-vis spectra. The exception to this general trend is that Br vacancies must be avoided, as they cause molecular orbital localization, resulting in trap states and lower LED performance. Blinking will likely still plague CsPbBr3 QDs, given that the charged defects critically perturb the spectra via red-shifting and lower absorbance. Our study provides insight into the tunability of CsPbBr3 QDs optical properties by understanding the nature of the electronic excitations and guiding improved development for high-performance LEDs.

Chiroptical characterization tools for asymmetric small molecules - experimental and computational approaches for electronic circular dichroism (ECD) and anisotropy spectroscopy.

Synchrotron radiation electronic circular dichroism (SRECD) and anisotropy spectroscopy for both enantiomers of a group of small non-planar chiral molecules are reported here. The experimental SRECD spectra are compared to computational ECD spectra generated using time-dependent density functional theory and a thermal averaging over relevant molecular configurations. The combination of these experimental and computational characterization methodologies for such molecules enables the prediction and understanding of the spectral behavior of other small molecules, in addition to chiroptically characterizing members of the mandelic acid family substructure. Enantiomeric purity of samples can be evaluated in comparison with these spectra and the extent of photolytic enantioinduction can also be predicted using these experimental/calculated SRECD and anisotropy spectra.

Hot Carrier Cooling and Recombination Dynamics of Chlorine-Doped Hybrid Perovskite Single Crystals.

Controlling the photoexcited properties and behavior of hybrid perovskites by halide doping has the potential to impact a wide range of emerging technologies, including solar cells and radiation detectors. Crystalline samples of methylammonium lead bromide substituted with chlorine (MAPbBr3-xClx) were examined by transient reflectivity spectroscopy and nonadiabatic molecular dynamics simulations. At picosecond time scales, the addition of chlorine to the perovskite crystal increased the observed rate of hot carrier cooling and the calculated electron-phonon coupling constants. Chlorine-doped samples also exhibit a slower surface recombination velocity and a smaller ambipolar mobility.

Role of the Metal-Semiconductor Interface in Halide Perovskite Devices for Radiation Photon Counting.

Halide perovskites are promising optoelectronic semiconductors. For applications in solid-state detectors that operate in low photon flux counting mode, blocking interfaces are essential to minimize the dark current noise. Here, we investigate the interface between methylammonium lead tri-iodide (MAPbI3) single crystals and commonly used high and low work function metals to achieve photon counting capabilities in a solid-state detector. Using scanning photocurrent microscopy, we observe a large Schottky barrier at the MAPbI3/Pb interface, which efficiently blocks dark current. Moreover, the shape of the photocurrent profile indicates that the MAPbI3 single-crystal surface has a deep fermi level close to that of Au. Rationalized by first-principle calculations, we attribute this observation to the defects due to excess iodine on the surface underpinning emergence of deep band-edge states. The photocurrent decay profile yields a charge carrier diffusion length of 10-25 µm. Using this knowledge, we demonstrate a single-crystal MAPbI3 detector that can count single γ-ray photons by producing sharp electrical pulses with a fast rise time of <2 µs. Our study indicates that the interface plays a crucial role in solid-state detectors operating in photon counting mode.

Polarons in Halide Perovskites: A Perspective.

Metal halide perovskites (MHPs) have rapidly emerged as leading contenders in photovoltaic technology and other optoelectronic applications owing to their outstanding optoelectronic properties. After a decade of intense research, an in-depth understanding of the charge carrier transport in MHPs is still an active topic of debate. In this Perspective, we discuss the current state of the field by summarizing the most extensively studied carrier transport mechanisms, such as electron-phonon scattering limited dynamics, ferroelectric effects, Rashba-type band splitting, and polaronic transport. We further extensively discuss the emerging experimental and computational evidence for dominant polaronic carrier dynamics in MHPs. Focusing on both small and large polarons, we explore the fundamental aspects of their motion through the lattice, protecting the photogenerated charge carriers from the recombination process. Finally, we outline different physical and chemical approaches considered recently to study and exploit the polaron transport in MHPs.

Correlation of Spatiotemporal Dynamics of Polarization and Charge Transport in Blended Hybrid Organic-Inorganic Perovskites on Macro- and Nanoscales.

Progress in flexible organic electronics necessitates a full understanding of how local inhomogeneities impact electronic and ionic conduction pathways and underlie macroscopic device characteristics. We used frequency- and time-resolved macro- and nanoprobe measurements to study spatiotemporal characteristics of multiscale charge transport dynamics in a series of ternary-blended hybrid organic inorganic perovskites (HOIPs) (MA0.95-xFAxCs0.05PbI3). We show that A-site cation composition defines charge transport mechanisms across broad temporal (102-10-6 s) and spatial (millimeters-picometers) scales. Ab initio molecular dynamic simulations suggest that insertion of FA results in a dynamic lattice, improved ion transport, and dipole screening. We demonstrate that correlations between macro- and nanoscale measurements provide a pathway for accessing distribution of relaxation in nanoscale polarization and charge transport dynamics of ionically conductive functional perovskites.

Optoelectronic Properties of Two-Dimensional Bromide Perovskites: Influences of Spacer Cations.

Two-dimensional (2D) halide perovskites have displayed unique emission properties, making them potential candidates for next-generation light-emitting devices. Here, we combine nonadiabatic molecular dynamics and time-domain density functional theory to investigate the fundamental mechanisms of carrier recombination processes. Considering monolayer bromide perovskites with dissimilar organic spacer molecules, n-butylammonium (BA) and phenylethylammonium (PEA) cations, we find a strong correlation between temperature-induced structural fluctuations and nonradiative carrier recombination rates in these materials. The more flexible geometry of (BA)2PbBr4 compared to that of (PEA)2PbBr4, results in faster electron-hole recombination and shorter carrier lifetime, diminishing the photoluminescence quantum yield for softer 2D perovskites. Reduced structural fluctuations in relatively rigid (PEA)2PbBr4 not only indicate of a longer carrier lifetime but also suggest a narrower emission line width, implying a higher purity of the emitted light. Our ab initio modeling of excited state properties in 2D perovskites conveys material designing strategies to fine-tune perovskite emissions for solid-state lighting applications.

Tuning Electronic Structure in Layered Hybrid Perovskites with Organic Spacer Substitution.

Two-dimensional layered halide organic perovskites (LHOPs) are promising candidates for many optoelectronic applications due to their interesting tunable properties. They provide a unique opportunity to control energy and charge dynamics via the independent tunability of the energy levels within the perovskite and the organic spacer for various optoelectronic applications. In the perovskite layer alone, one can replace the Pb (Sn), the halide (X = I, Br, Cl), the organic component, and the number of layers between the organic spacer layers. In addition, there are many possibilities for organic spacer layers between the perovskite layers, making it difficult for experimental methods to comprehensively explore such an extensive combinatorial space. Of particular technological interest is alignment of electronic levels between the perovskite layer and the organic spacer layer, leading to desired transfer of energy or charge carriers between perovskite and organic components. For example, as band edge absorption is almost entirely attributed to the perovskite layer, one way to demonstrate energy transfer is to observe triplet emission from organic spacers. State-of-the-art computational chemistry tools can be used to predict the properties of many stoichiometries in search for LHOPs that have the most promising electronic-structure features. In this first-principles study, we survey a group of π-conjugated organic spacer candidates for use in triplet light-emitting LHOPs. Utilizing density functional theory (DFT) and time-dependent DFT, we calculate the first singlet (S1) and triplet (T1) excitation energy in the ground-state geometry and the first triplet excitation energy in the excited-triplet-state relaxed geometry (T1*). By comparing these energies to the known lowest exciton energy level of PbnX3n+1 perovskite layers (X = I, Br, Cl), we can identify organic spacer and perovskite layer pairings for possible transfer of Wannier excitons from the inorganic perovskite lattice to spin-triplet Frenkel excitons located on the organic cation. We successfully identify ten organic spacer candidates for possible pairing with perovskite layers of specific halide composition to achieve triplet light emission across the visible energy range. Molecular dynamics simulations predict that finite temperatures and perovskite environment have little influence on the average excitation energies of the two common organic spacers naphthylethylammonium (NEA) and phenelethylammonium (PEA). We find significant thermal broadening up to 0.5 eV of the optical excitation energies appearing due to finite temperature effects. The findings herein provide insights into alignment of electronic levels of the conjugated organic spacer with the layer.

Lattice Expansion in Hybrid Perovskites: Effect on Optoelectronic Properties and Charge Carrier Dynamics.

Hybrid halide perovskites frequently undergo structural expansion due to various stimuli, significantly affecting their electronic properties and in particular their charge carrier dynamics. It is essential to atomistically model how geometric changes modify electronic characteristics that are important for applications such as light harvesting and lighting. Using ab initio simulations, here we investigate the structural dynamics and optoelectronic properties of FAPbI3 under tensile strain. The applied strain leads to elongation of the Pb-I bonds and a decrease in the level of PbI6 octahedral tilting, which manifests as blue-shifts in band gaps. Nonadiabatic molecular dynamics simulations further reveal that charge carrier recombination rates moderately decrease in these expanded lattices. The complex influence of lattice dynamics on electron-phonon scattering results in a longer carrier lifetime, which is advantageous for efficient solar cells. By providing detailed information about the structure-property relationships, this work emphasizes the role of controlled lattice expansion in enhancing the electronic functionalities of hybrid perovskites.

Cation Alloying Delocalizes Polarons in Lead Halide Perovskites.

Recently, mixed-cation perovskites have promised enhanced performances concerning stability and efficiency in optoelectronic devices. Here, we report a systematic study on the effects of cation alloying on polaronic properties in cation-alloyed perovskites using first principle calculations. We find that cation alloying significantly reduces the polaron binding energies for both electrons and holes compared to pure methylammonium lead iodide (MAPbI3). This is rationalized in terms of crystal symmetry reduction that causes polarons to be more delocalized. Electron polarons undergo large Jahn-Teller distortions (∼15-30%), whereas hole polarons tend to shrink the lattice by ∼5%. Such different lattice distortion footprints could be utilized to distinguish the type of polarons. Finally, our simulations show that Cs, formamidinium (FA), and MA mixtures can effectively minimize polaron binding energy while weakly affecting band gap, in a good agreement with experimental findings. These modeling results can guide future development of halide perovskite materials compositions for optoelectronic applications.

Geometry Distortion and Small Polaron Binding Energy Changes with Ionic Substitution in Halide Perovskites.

Halide perovskites have demonstrated remarkable performance in optoelectronic applications. Despite extraordinary progress, questions remain about device stability. We report an in-depth computational study of small polaron formation, electronic structure, charge density, and reorganization energies of several experimentally relevant halide perovskites using isolated clusters. Local lattice symmetry, electronic structure, and electron-phonon coupling are interrelated in polaron formation in these materials. To illustrate this, first-principles calculations are performed on (MA/Cs/FA)Pb(I/Br)3 and MASnI3. Across the materials studied, electron small polaron formation is manifested by Jahn-Teller-like distortions in the central octahedron, with apical PbI bonds expanding significantly more than the equatorial bonds. In contrast, hole polarons cause the central octahedron to uniformly contract. This difference in manifestation of electron and hole polaron formation can be a tool to determine what is taking place in individual systems to systematically control performance. Other trends as the anion and cations are changed are established for optimization in specific optoelectronic applications.

Interlayer-Decoupled Sc-Based Mxene with High Carrier Mobility and Strong Light-Harvesting Ability.

Two-dimensional (2D) van der Waals (vdW) layered materials offer a unique combination of electronic and structural properties attractive for technological applications. Most of them show strong vdW interactions, which lead to interlayer-coupled optoelectronic properties due to quantum confinement. Here we present a systematic computational study of one Mxene, 2D double-metal-layered scandium chloride carbides (Sc2CCl2). Unlike conventional quantum-confined nanosystems, 2D Sc2CCl2 exhibits weak vdW interactions with robust interlayer-decoupled optoelectronic properties and extremely high and anisotropic carrier mobilities of about 1-4.5 × 104 cm2 V-1 s-1 that consequently produce comparatively large drain currents. Specifically, the 2D Sc2CCl2 family has strong light-harvesting ability and could be utilized as efficient donor materials in excitonic solar cells. Overall, in combination with high structural stability against ambient conditions, interlayer-decoupled robust optoelectronic properties potentially relax the requirements for the fabrication of high-quality monolayers and for selection of suitable substrates and suggest promising next-generation optoelectronic applications.

Excited-state vibrational dynamics toward the polaron in methylammonium lead iodide perovskite.

Hybrid organic-inorganic perovskites have attractive optoelectronic properties including exceptional solar cell performance. The improved properties of perovskites have been attributed to polaronic effects involving stabilization of localized charge character by structural deformations and polarizations. Here we examine the Pb-I structural dynamics leading to polaron formation in methylammonium lead iodide perovskite by transient absorption, time-domain Raman spectroscopy, and density functional theory. Methylammonium lead iodide perovskite exhibits excited-state coherent nuclear wave packets oscillating at ~20, ~43, and ~75 cm-1 which involve skeletal bending, in-plane bending, and c-axis stretching of the I-Pb-I bonds, respectively. The amplitudes of these wave packet motions report on the magnitude of the excited-state structural changes, in particular, the formation of a bent and elongated octahedral PbI64- geometry. We have predicted the excited-state geometry and structural changes between the neutral and polaron states using a normal-mode projection method, which supports and rationalizes the experimental results. This study reveals the polaron formation via nuclear dynamics that may be important for efficient charge separation.

High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells.

Three-dimensional organic-inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley-Queisser limit for a single­junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden-Popper phases--layered two-dimensional perovskite films--have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.