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Two-dimensional halide perovskites (2D PVSKs) have drawn tremendous attentions owing to their outstanding ambient stability. However, the random orientation of layered crystals severely impedes the out-of-plane carrier transport and limits the solar cell performance. An in-depth understanding coupled with an effective control of the crystallization in 2D PVSKs is the crux for highly efficient and durable devices. In this contribution, we accidentally discovered that the crystallization of 2D PVSKs can be effectively regulated by so-called ′intermittent spin-coating (ISC)′ process. Combined analyses of in(ex)-situ grazing-incidence wide-angle X-ray scattering with time-of-flight secondary ion mass spectrometry distinguish the interface initialized bottom-up crystallization upon ISC treatment from the bi-directional one in the conventional spin-coating process, which results in significantly enhanced crystal orientation and thus facilitated carrier transport as confirmed by both electrical measurements and ultrafast spectroscopies. As a result, the p-i-n architecture planar solar cells based on ISC fabricated paradigm PEA2MA3Pb4I13 deliver a respectable efficiency of 11.2% without any treatment, which is three-fold improvement over their spin-coated counterparts and can be further boosted up to 14.0% by NH4Cl addition, demonstrating the compatibility of ISC method with other film optimization strategies., Random orientation of layered crystals in 2D perovksites impedes the out-of-plnae carriers transport and hence undermines the photovoltaic perofrmance. Here, the authors discover that the crystallisation can be regulated by intermittent spin-coating strategy.

The physical origin of sub-band gap photoluminescence in Ruddlesden-Poppers two-dimensional (2D) lead halide perovskites (LHPs) is still under debate. In this paper, we studied the photoluminescence features from two different facets of 2D LHP single crystals: the in-plane facet (IF) containing the 2D inorganic layers and the facet perpendicular to the 2D layers (PF). At the IF, the free carriers (FCs) dominate due to the weak electron-phonon coupling in a symmetric lattice. At the PF, the strain accumulation along the 2D layers enhances the electron-phonon coupling and facilitates self-trapped exciton (STE) formation. The time-resolved PL studies indicate that free carriers (FCs) at the IF can move freely and display the trapping by the intrinsic defects. The STEs at the PF are not likely trapped by the defects due to the reduced mobility. However, with increasing STE density, the STE transport is promoted, enabling the trapping of STE by the intrinsic defects.

In polaritons, the properties of matter are modified by mixing the molecular transitions with light modes inside a cavity. Resultant hybrid light-matter states exhibit energy level shifts, are delocalized over many molecular units and have a different excited-state potential energy landscape which leads to modified exciton dynamics. Previously, non-Hermitian Hamiltonians have been derived to describe the excited states of molecules coupled to surface plasmons (i.e. plexcitons), and these operators have been successfully used in the description of linear and third order optical response. In this article, we rigorously derive non-Hermitian Hamiltonians in the response function formalism of nonlinear spectroscopy by means of Feshbach operators, and apply them to explore spectroscopic signatures of plexcitons. In particular we analyze the optical response below and above the exceptional point that arises for matching transition energies for plasmon and molecular components, and study their decomposition using double-sided Feynman diagrams. We find a clear distinction between interference and Rabi splitting in linear spectroscopy, and a qualitative change in the symmetry of the lineshape of the nonlinear signal when crossing the exceptional. This change corresponds to one in the symmetry of the eigenvalues of the Hamiltonian. Our work presents an approach for simulating the optical response of sublevels within an electronic system, and opens new applications of nonlinear spectroscopy to examine the different regimes of the spectrum of non-Hermitian Hamiltonians., 35 pages, 10 figures

Advances in ultrafast spectroscopy can provide access to dynamics involving nontrivial quantum correlations and their evolutions. In coherent 2D spectroscopy, the oscillatory time dependence of a signal is a signature of such quantum dynamics. Here we study such beating signals in electronic coherent 2D spectroscopy of CdSe quantum dots (CdSe QDs) at 77 K. The beating signals are analyzed in terms of their positive and negative Fourier components. We conclude that the beatings originate from coherent LO-phonons of CdSe QDs. No evidence for the quantum dot size dependence of the LO-phonon frequency was identified., 18 pages

The synthesis and crystal structure of rhodamine 590 acid phthalate (RhAP) have been reported. This novel solid-state rhodamine derivative not only has a longer fluorescence lifetime compared to rhodamine solid-state matrixes where emission is quenched but also possesses strong nonlinear optical characteristics. The static and dynamic first- and second-order hyperpolarizabilities were calculated using the time-dependent density functional theory at the B3LYP/6-31+G∗ level. The computed static values of β and γof RhAP by the X-ray diffraction (XRD) structure were 31.9 × 10-30 and 199.0 × 10-36 esu, respectively. These values were about 62 times larger than the corresponding values in urea, an already well-known nonlinear optical material. The second-order hyperpolarizability of the compound was determined experimentally by measuring the two-photon absorption cross section using intensity-modulated light fields. The reported compound, excitable at near-infrared, exhibited frequency upconversion with the two-photon absorption coefficient enhanced by two orders of magnitude compared to that of the dye solution. Hosting the dye in the solid, at high concentrations, exploits the nonlinearity of the dye itself as well as results in significant excitonic effects including formation of broad exciton band and superradiance. (Less)

Quantum dots (QDs) form a promising family of nanomaterials for various applications in optoelectronics. Understanding the details of the excited-state dynamics in QDs is vital for optimizing their function. We apply two-color 2D electronic spectroscopy to investigate CdSe QDs at 77 K within a broad spectral range. Analysis of the electronic dynamics during the population time allows us to identify the details of the excitation pathways. The initially excited high-energy electrons relax with the time constant of 100 fs. Simultaneously, the states at the band edge rise within 700 fs. Remarkably, the excited-state absorption is rising with a very similar time constant of 700 fs. This makes us reconsider the earlier interpretation of the excited-state absorption as the signature of a long-lived trap state. Instead, we propose that this signal originates from the excitation of the electrons that have arrived in the conduction-band edge.

CsPbBr3 quantum dots (QDs) are promising candidates for optoelectronic devices. The substitution of oleic acid (OA) and oleylamine (OLA) capping agents with a quaternary alkylammonium such as di-dodecyl dimethyl ammonium bromide (DDAB) has shown an increase in external quantum efficiency (EQE) from 0.19% (OA/OLA) to 13.4% (DDAB) in LED devices. The device performance significantly depends on both the diffusion length and the mobility of photoexcited charge carriers in QD solids. Therefore, we investigated the charge carrier transport dynamics in DDAB-capped CsPbBr3 QD solids by constructing a bi-sized QD mixture film. Charge carrier diffusion can be monitored by quantitatively varying the ratio between two sizes of QDs, which varies the mean free path of the carriers in each QD cluster. Excited-state dynamics of the QD solids obtained from ultrafast transient absorption spectroscopy reveals that the photogenerated electrons and holes are difficult to diffuse among small-sized QDs (4 nm) due to the strong quantum confinement. On the other hand, both photoinduced electrons and holes in large-sized QDs (10 nm) would diffuse toward the interface with the small-sized QDs, followed by a recombination process. Combining the carrier diffusion study with a Monte Carlo simulation on the QD assembly in the mixture films, we can calculate the diffusion lengths of charge carriers to be ∼239 ± 16 nm in 10 nm CsPbBr3 QDs and the mobility values of electrons and holes to be 2.1 (± 0.1) and 0.69 (± 0.03) cm2/V s, respectively. Both parameters indicate an efficient charge carrier transport in DDAB-capped QD films, which rationalized the perfect performance of their LED device application.

Funder: ERC Starting Grant (No. 717026), Bright and efficient blue emission is key to further development of metal halide perovskite light-emitting diodes. Although modifying bromide/chloride composition is straightforward to achieve blue emission, practical implementation of this strategy has been challenging due to poor colour stability and severe photoluminescence quenching. Both detrimental effects become increasingly prominent in perovskites with the high chloride content needed to produce blue emission. Here, we solve these critical challenges in mixed halide perovskites and demonstrate spectrally stable blue perovskite light-emitting diodes over a wide range of emission wavelengths from 490 to 451 nanometres. The emission colour is directly tuned by modifying the halide composition. Particularly, our blue and deep-blue light-emitting diodes based on three-dimensional perovskites show high EQE values of 11.0% and 5.5% with emission peaks at 477 and 467 nm, respectively. These achievements are enabled by a vapour-assisted crystallization technique, which largely mitigates local compositional heterogeneity and ion migration.

Molecular aggregates on plasmonic nanoparticles have emerged as attractive systems for the studies of cavity quantum electrodynamics. They are highly tunable, scalable, easy to synthesize and offer sub-wavelength confinement, all while giving access to the ultrastrong light-matter coupling regime at room temperature and promising a plethora of applications. However, the complexity of both the molecular aggregate and plasmonic nanoparticle introduces many more processes affecting the excitation and its relaxation, than are present in atom-cavity systems. Here, we follow the complex relaxation pathways of the photoexcitation of such hybrid systems and conclude that while the metal is responsible for destroying the coherence of the excitation, the molecular aggregate significantly participates in dissipating the energy. We rely on two-dimensional electronic spectroscopy in a combined theory-experiment approach, which allows us to ascribe the different timescales of relaxation to processes inside the molecules or the metal nanoparticle. We show that the dynamics beyond a few femtoseconds has to be cast in the language of hot electron distributions and excitons instead of the accepted lower and upper polariton branches, and furthermore set the framework for delving deeper into the photophysics of excitations that could be used in hot electron transfer, for example to drive photocatalytic reactions., revised simulations and interpretation

Recently, metal-halide perovskites have emerged as a candidate for optoelectronic applications such as photodetectors. However, the poor device performance and instability have limited their future commercialization. Herein, we report the spontaneous growth of perovskite/N-rGO hybrid structures using a facile solution method and their applications for photodetectors. In the hybrid structures, perovskites were homogeneously wrapped by N-rGO sheets through strong hydrogen bonding. The strongly coupled N-rGOs facilitate the charge carrier transportation across the perovskite crystals but also distort the surface lattice of the perovskite creating a potential barrier for charge transfer. We optimize the addition of N-rGO in the hybrid structures to balance interfacial structural distortion and the intercrystal conductivity. High-performance photodetection up to 3 × 104 A/W, external quantum efficiency exceeding 105%, and detectivity up to 1012 Jones were achieved in the optimal device with the weight ratio between perovskites and N-rGO to be 8:1.5. The underlying mechanism behind the optimal N-rGO addition ratio in the hybrids has also been rationalized via time-resolved spectroscopic studies as a reference for future applications.

Despite great successes in metal halide perovskites, most of them were achieved in lead-based perovskites (Pb-PVSKs), in which the Pb-toxicity inhibits practical deployments. Various less-toxic substitutes were proposed wherein tin-based PVSKs (Sn-PVSKs) hold the best prospect due to their comparable optoelectronic properties to Pb analogues. Nevertheless, the oxidation proneness of Sn2+ incurs both instability issue and self-doping effect, the latter of which results in high background hole carrier density and hence severe photo-voltage losses. Besides, the unfavorable crystallization process of Sn-PVSKs challenges large-scale manufacturing. Therefore, numerous attempts have been directed at the preparation of highly uniform and oxidation-resistant Sn-PVSK thin films while unveiling the relationships between the optimization strategy and device performance/durability. In this review article, a retrospect is firstly given on the milestones and general properties of paradigm ABX3 structured Sn-PVSKs. Then, various strategies in the categories of synthetic conditions (i.e., additive, solvent and the preparation methods), elemental compositions, device architecture as well as phase composition/distribution will be discussed for diverse optoelectronic applications. Subsequently, the determining mechanisms of electronic structure evolution, photo-physics process and degradation pathways will be thoroughly interpreted. Finally, the conclusions and outlook are given for the guidance of future improvements that depends critically on the strategies.

In this work, we explored the photoinduced charge carriers dynamics rationalizing the photocatalytic oxidation of NO over N-doped Bi2O2CO3/graphene quantum dots composites(N-BOC/GQDs) via time-resolved photoluminescence (TRPL). Under visible light illumination, only GQDs can be photoexcited and inject electrons to N-BOC within 0.5 ns. Under UV light irradiation, the interfacial Z-scheme heterojunction recombination between the electrons in N-BOC and holes in GQDs dominate the depopulation of excited states within 0.36 ns. Such efficient Z-scheme recombination regardless of the large energy difference (1.66 eV) is mediated by the interfacial oxygen vacany defect states characterized by both density functional theory calculations (DFT) and electron paramagnetic resonance (EPR) measurement. This finding provide a novel strategic view to improve the photocatalytic performance of the nanocomposite by interfacial engineering

A common molecular design paradigm for optical power limiting (OPL) applications is to introduce heavy atoms that promote intersystem crossing and triplet excited states. In order to investigate this effect, three multi-branched fluorene molecules were prepared where the central moiety was either an organic benzene unit, para-dibromobenzene, or a platinum(II)&ndash, alkynyl unit. All three molecules showed good nanosecond OPL performance in solution. However, only the dibromobenzene and Pt&ndash, alkynyl compounds showed strong microsecond triplet excited state absorption (ESA). To investigate the photophysical cause of the OPL, especially for the fully organic molecule, photokinetic measurements including ultrafast pump&ndash, probe spectroscopy were performed. At nanosecond timescales, the ESA of the organic molecule was larger than the two with intersystem crossing (ISC) promoters, explaining its good OPL performance. This points to a design strategy where the singlet-state ESA is balanced with the ISC rate to increase OPL performance at the beginning of a nanosecond pulse.

Colloidal all-inorganic perovskite nanocrystals have gained significant attention as a promising material for both fundamental and applied research due to their excellent emission properties. However, reported photoluminescence quantum yields (PL QYs) of blue-emitting perovskite nanocrystals are rather low, mostly due to the fact that the high energy excitons for such wide bandgap materials are easily captured by interband traps, and then decay nonradiatively. In this work, it is demonstrated how to tackle this issue, performing self-assembly of 2D perovskite nanoplatelets into larger size (≈50 nm × 50 nm × 20 nm) cuboid crystals. In these structures, 2D nanoplatelets being isolated from each other within the cuboidal scaffold by organic ligands constitute multiple quantum wells, where exciton localization on potential disorder sites helps them to bypass nonradiative channels present in other platelets. As a result, the cuboid crystals show an extremely high PL QY of 91% of the emission band centered at 480 nm. Moreover, using the same synthetic method, mixed-anion CsPb(Br/Cl) 3 cuboid crystals with blue emission peaks ranging from 452 to 470 nm, and still high PL QYs in the range of 72–83% are produced.

We have prepared electrodes for photo-electro-chemical cells which have enabled to pre-charge colloidal quantum dots in well-controlled fashion. Femtosecond transient absorption measurements were carried out revealing clear speed-up of the photo-induced charge carrier dynamics, particularly the recombination. Such studies allow to understand the behavior of light harvesting materials in operational conditions of optoelectronic devices giving new recipes for improvements.

Halide perovskites have continued to rise as attractive light absorber materials, mainly driven by their potential wide applications in the fields of solar cells, photodetectors, lasers and others. However, Pb-containing perovskites are poisonous and could cause serious potential problems to our environment. Thus, there is a strong desire to develop lead-free perovskites as environmentally friendly alternatives. Here, we have shown a successful synthesis of a non-toxic single crystalline double-halide perovskite, Cs3BiBr6, and analysed its structural characteristics in detail. This perovskite represents a new structure in terms of double-halide perovskites. The crystal structure features isolated BiBr6 polyhedra forming a zero-dimensional halide perovskite. The bandgap of this compound is measured to be 2.55 eV. High thermal stability is also clearly shown in the Cs3BiBr6 single crystal. A photodetector based on this compound is fabricated and tested, which exhibits an impressive detectivity of 0.8 × 109 Jones and low dark current under 400 nm light illumination. Overall, our results show that the Cs3BiBr6 perovskite as a lead-free perovskite has interesting structures and promising properties for optoelectronic devices.

Photo-generated charge carrier dynamics in Ruddlesden-Popper 2D perovskites with linear (n-BA) and branched (iso-BA) butylamine as spacing cations have been studied by using transient absorption and time-resolved photoluminescence spectroscopies. Both n-BA and iso-BA perovskites consist of mixed-phase 2D quantum wells with various layer thicknesses, where the photo-generated charges undergo inter-phase charge transfer from thinner quantum wells to thicker ones. By shortening the spacer from n-BA to iso-BA, the transfer rates are significantly increased, which can also diminish the charge accumulation in thin quantum wells induced by the unbalanced electron and hole charge transfer rates. Under high excitation intensity, the shorter spacing cation is found to further facilitate the energy transfer, which can compete with fast high-order carrier recombination and consequently improve the charge transfer efficiency. Intriguingly, we observe the existence of extra bulk 3D phases embedded within iso-BA perovskites, which can efficiently collect the confined charges within 2D phases and then transport them with faster carrier mobility and slower recombination rates. 2018 The Royal Society of Chemistry. This work was made possible by NPRP grant #7-227-1-034 from the Qatar National Research Fund (a member of Qatar Foundation). Z.L. thanks the support from Inter-Governmental International Cooperation Projects of Science and Technology Commission of Shanghai Municipality (STCSM) under grant No. 17520710100. We also acknowledge financial support by the Swedish Research Council, KAW foundation, Swedish Energy Agency and STINT. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The ELI- ALPS project (GINOP-2.3.6-15-2015-00001) is supported by the European Union and co-financed by the European Regional Development Fund. Scopus

Two-dimensional (2D) spectroscopy has been intensively used to study electronic and vibronic coherences in biological systems and semiconductors. This technique studies coherent as well as incoherent signals that arise from the nonlinear interaction of a sequence of laser pulses. In this paper we present a direct evaluation of the 2D signal based on elementary quantum kinetics in order to compare with the common approximate diagrammatic approaches. Here we consider incoherent action signals such as fluorescence or photocurrent as the observable, which is easily accessible in a measurement. These observables are calculated by solving the time evolution of the density matrix in the Lindblad form, which can take into account all possible decoherence processes. The phase modulation technique is used to separate the relevant nonlinear signals from the other possible interaction pathways. The approach can be used to calculate 2D spectra of any quantum system. For our model system we find a good agreement for the quantum beating between the coupled states.

Cesium lead bromide (CsPbBr3) perovskite nanocrystals (NCs), with large two-photon absorption (TPA) cross-section and bright photoluminescence (PL), have been demonstrated as stable two-photon-pumped lasing medium. With two-photon excitation, red-shifted PL spectrum and increased PL lifetime is observed compared with one-photon excitation. We have investigated the origin of such difference using time-resolved laser spectroscopies. We ascribe the difference to the enhanced size selection of NCs by two-photon excitation. Because of inherent nonlinearity, the size dependence of absorption cross-section under TPA is stronger. Consequently, larger size NCs are preferably excited, leading to longer excited-state lifetime and red-shifted PL emission. In a broad view, the enhanced size selection in two-photon excitation of CsPbBr3 NCs is likely a general feature of the perovskite NCs and can be tuned via NC size distribution to influence their performance within NC-based nonlinear optical materials and devices. 1 2017 American Chemical Society. This work was financed by the Swedish Research Council (VR), Swedish Energy Agency, the Knut and Alice Wallenberg Foundation, STINT grant #CH2015-6232 from the Swedish Foundation for International Cooperation in Research and Higher Education, and NPRP grant # NPRP7-227-1-034 from the Qatar National Research Fund, Interreg Öresund-Kattegat-Skagerrak, and European regional development fund. We acknowledge collaboration within NanoLund. S.C. is grateful for the support from the ELI-ALPS project (GINOP-2.3.6-15-2015-00001) by the European Union and cofinanced by the European Regional Development Fund. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Scopus

All-inorganic colloidal perovskite quantum dots (QDs) based on cesium, lead, and halide have recently emerged as promising light emitting materials. CsPbBr3 QDs have also been demonstrated as stable two-photon-pumped lasing medium. However, the reported two photon absorption (TPA) cross sections for these QDs differ by an order of magnitude. Here we present an in-depth study of the TPA properties of CsPbBr3 QDs with mean size ranging from 4.6 to 11.4 nm. By using femtosecond transient absorption (TA) spectroscopy we found that TPA cross section is proportional to the linear one photon absorption. The TPA cross section follows a power law dependence on QDs size with exponent 3.3 +- 0.2. The empirically obtained power-law dependence suggests that the TPA process through a virtual state populates exciton band states. The revealed power-law dependence and the understanding of TPA process are important for developing high performance nonlinear optical devices based on CsPbBr3 nanocrystals. This work was financed by the Swedish Research Council (VR) the Knut and Alice Wallenberg Foundation, and NPRP Grant # NPRP7-227-1-034 from the Qatar National Research Fund, Interreg Oresund-Kattegat-Skagerrak European regional development fund, and the Program of Study Abroad for Young Teachers by the Agricultural University of Hebei. Keli Han thanks the support from the National Natural Science Foundation of China (Grant No: 21533010). Scopus

The Fano line shape arises from the interference of two excitation pathways to reach a continuum. Its generality has resulted in a tremendous success in explaining the line shapes of many one-dimensional spectroscopies - absorption, emission, scattering, conductance, photofragmentation - applied to very varied systems - atoms, molecules, semiconductors, and metals. Unraveling a spectroscopy into a second dimension reveals the relationship between states in addition to decongesting the spectra. Femtosecond-resolved two-dimensional electronic spectroscopy (2DES) is a four-wave mixing technique that measures the time evolution of the populations and coherences of excited states. It has been applied extensively to the dynamics of photosynthetic units, and more recently to materials with extended band structures. In this paper, we solve the full time-dependent third-order response, measured in 2DES, of a Fano model and give the system parameters that become accessible. (Less)

This work introduces a type of CdS/CuxS quantum dots (QDs) as sensitizers in quantum dot sensitized solar cells by in-situ cationic exchange reaction method where CdS photoanode is directly immersed in CuCl2 methanol solution to replace Cd2+ by Cu2+. The p-type CuxS layer on the surface of the CdS QDs can be considered as hole transport material, which not only enhances the light harvesting of photoanode but also boosts the charge separation after photo-excitation. Therefore, both the electron collection efficiency and power conversion efficiency of the solar cell are improved from 80% to 92% and from 1.21% to 2.78%, respectively.

We investigated the excited-state dynamics of CH3NH3PbBr3 perovskite nanoparticles (NPs) and bulk crystals under various excitation intensity regimes using transient absorption spectroscopy. We confirmed the sub-band gap hole trap states with optical transition to the conduction band in both samples. In bulk crystals, the excited-state dynamics is independent of pump intensity. However, in NPs, pronounced intensity dependence appears. At low intensities, the hole trap states do not affect the excited-state dynamics due to the potential barrier between the photogenerated holes and the surface trap states. When the excitation density is much higher than one per NP, charge accumulation makes hot holes overcome the barrier and get trapped with electrons long living in the conduction band (?10 ns). This explains the high emissive properties of such NPs despite the existence of surface traps. However, in the application of emitting devices requiring high excitation intensity, the surface trapping becomes significant. This Letter was made possible by NPRP Grant # NPRP7-227-1-034 from the Qatar National Research Fund (a member of the Qatar Foundation). The statements made herein are solely the responsibility of the authors. Collaboration within Nano-Lund is acknowledged. Scopus

In a composite film of Cdx Sey Zn1-x S1-y gradient core-shell quantum dots (QDs) and gold nanorods (NRs), the optical properties of the QDs are drastically affected by the plasmonic nanoparticles. We provide a careful study of the two-step formation of the film and its morphology. Subsequently we focus on QD luminescence photoactivation - a process induced by photochemical changes on the QD surface. We observe that even a sparse coverage of AuNRs can completely inhibit the photoactivation of the QDs' emission in the film. We demonstrate that the inhibition can be accounted for by a rapid energy transfer between QDs and AuNRs. Finally, we propose that the behavior of emission photoactivation can be used as a signature to distinguish between energy and electron transfer in the QD-based materials. 2016 IOP Publishing Ltd. This paper was made possible by NPRP grant # NPRP7-227-1-034 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. The work in Lund was funded by the Swedish Research Council and KAW foundation. Collaboration within NanoLund is acknowledged. Scopus

Organic-inorganic halide CH3NH3PbI3 (MAPbI3) perovskite solar cells (PSCs) have attracted intensive attention due to their high power conversion efficiency and low fabrication cost. However, MAPbI3 is known to be very sensitive to humidity, and the intrinsic long-term stability of the MAPbI3 film remains a critical challenge. 2-Aminoethanethiol (2-AET) was used as a ligand to bridge the organic compound (MAI) and inorganic compound (PbI2), which restricted the fast growth of PbI2 to realize the synchronous growth environment of MAI and PbI2 crystals, resulting in the formation of a compact MAPbI3 film with polygonal grains. Due to the compact (PbI2)-2-AET-(MAI) molecule barrier layers in the MAPbI3 structure, the resulting perovskite films showed excellent intrinsic water-resistance, with the MAPbI3 perovskite crystal structure retained for a long time (>10 minutes) after immersion in water. This work makes a step towards obtaining long-term stable MAPbI3 perovskite devices. 2016 Royal Society of Chemistry. Scopus

The energy level structure and dynamics of biomolecules are important for understanding their photoinduced function. In particular, the role of carotenoids in light-harvesting is heavily studied, yet not fully understood. The conventional approach to investigate these processes involves analysis of the third-order optical polarization in one spectral dimension. Here, we record two-dimensional correlation spectra for different time-orderings to characterize all components of the transient molecular polarization and the optical signal. Single- and double-quantum two-dimensional experiments provide insight into the energy level structure as well as the ultrafast dynamics of solvated beta-carotene. By analysis of the lineshapes, we obtain the transition energy and characterize the potential energy, surfaces of the involved states. We obtain direct experimental proof for an excited state absorption transition in the visible (S-2 -> S-n2). The signatures of this transition in pump-probe transients are shown to lead to strongly damped oscillations with characteristic pump and probe frequency dependence.

Quantum dots (QDs) and graphene are both promising materials for the development of new-generation optoelectronic devices. Towards this end, synergic assembly of these two building blocks is a key step but remains a challenge. Here, we show a one-step strategy for organizing QDs in a graphene matrix via interfacial self-assembly, leading to the formation of sandwiched hybrid QD-graphene nanofilms. We have explored structural features, electron transfer kinetics and photocurrent generation capacity of such hybrid nanofilms using a wide variety of advanced techniques. Graphene nanosheets interlink QDs and significantly improve electronic coupling, resulting in fast electron transfer from photoexcited QDs to graphene with a rate constant of 1.3 × 109 s−1. Efficient electron transfer dramatically enhances photocurrent generation in a liquid-junction QD-sensitized solar cell where the hybrid nanofilm acts as a photoanode. We thereby demonstrate a cost-effective method to construct large-area QD-graphene hybrid nanofilms with straightforward scale-up potential for optoelectronic applications.

The ultrafast carotenoid band shift upon excitation of nearby bacteriochlorophyll molecules was studied in three different light harvesting complexes from purple bacteria. The results were analyzed in terms of changes in local electric field of the carotenoids. Time dependent density functional theory calculations based on known and model structures led to good agreement with experimental results, strongly suggesting that the mutual orientation of the pigment molecules rather than the type of the carotenoid molecules determines the extent of the ultrafast band shift. We further estimate that the protein induced local field nearby carotenoid molecule is about 4 or 6 MV/cm, depending on the orientation of the change of the electrical dipole in the carotenoid upon optical transition.

Quantum dot (QD)-metal oxide composite forms a “heart” of the QD-sensitized solar cells. It maintains light absorption and electron-hole separation in the system and has been therefore extensively studied. The interest is largely driven by a vision of harvesting the hot carrier energy before it is lost via relaxation. Despite of importance of the process, very little is known about the carrier relaxation in the QD-metal oxide composites. In order to fill this gap of knowledge we carry out a systematic study of initial electron dynamics in different CdSe QD systems. Our data reveal that QD attachment to ZnO induces a speeding-up of transient absorption onset. Detailed analysis of the onset proves that the changes are caused by an additional relaxation channel dependent on the identity of the QD-ZnO linker molecule. The faster relaxation represents an important factor for hot carrier energy harvesting, whose efficiency can be influenced by almost 50%.

Fluorescence line-narrowing measurements at low temperature were performed on the Fenna−Matthews−Olson complex of Prosthecochloris aestuarii. Superimposed on the phonon wing, several vibronic bands could be observed. By use of these data, the temperature dependence of the lowest-energy absorption band was modeled based on the linear harmonic Franck−Condon approximation. The overall Huang−Rhys factor was estimated to be 0.45. The maximum of the phonon distribution was located at 20 cm-1. Thirty vibrational modes could be observed, and their Franck−Condon factors were estimated. The strongest modes were located at 36, 70, and ∼195 cm-1. For the full width at half-maximum of the inhomogeneous broadening, a value of 80 cm-1 was determined. We did not find any evidence for the presence of different excitonic states in the lowest-energy absorption band.

Polarized, site-selected fluorescence was measured for light-harvesting complex II (LHCII), the major Chl a/b/xanthophyll binding light-harvesting complex of green plants. Upon selective excitation...

The recently suggested possibility that weak vibronic transitions can be excitonically enhanced in light-harvesting complexes is studied in detail. A vibronic exciton dimer model which includes ground state vibrations is investigated using multi-configuration time-dependent Hartree method with a parameter set typical to photosynthetic light-harvesting complexes. Absorption spectra are discussed in dependence on the Coulomb coupling, the detuning of site energies, and the number of vibrational mode. Calculations of the fluorescence spectra show that the spectral densities obtained from the low temperature fluorescence line narrowing measurements of light-harvesting systems need to be corrected for the exciton effects. For the J-aggregate configuration, as in most of the light-harvesting complexes, the true spectral density has larger amplitude than what is obtained from the measurement., revised version (minor)

The concept of a digital cavity is presented. The functionality of a tunable radio-frequency/microwave cavity with unrestricted Q-factor is implemented. The theoretical aspects of the cavity and its potential applications in high resolution spectroscopy and synchronization of clocks together with examples in signal processing and data acquisition are discussed.

The performance of hybrid solar cells composed of polymer and ZnO is mainly hindered by the defects of ZnO. Here, we investigate the effects of ZnO nanoparticle surface modification with poly(ethylene oxide) (PEO) on the performance of bulk heterojunction hybrid solar cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and ZnO nanoparticles. The reference device using ZnO nanoparticles as electron acceptor shows an open-circuit voltage (VOC) of 0.83 V, a short-circuit current (JSC) of 3.00 mA/cm2, a fill factor (FF) of 0.46 and a power conversion efficiency (PCE) of 1.15%. After modification with very small amount of PEO, the PCE will be enhanced, which is attributed to less surface traps of ZnO nanoparticles with PEO modification. With optimized PEO (0.05%) modified ZnO nanoparticles as electron acceptors, the device typically shows a VOC of 0.86 V, a JSC of 3.84 mA/cm2, a FF of 0.51 and a PCE of 1.68% due to less recombination loss of carriers, smaller series resistance, and improved electrical coupling between ZnO nanoparticle and MEH-PPV. However, further increase of PEO content to 0.3% will deteriorate device performance. Funding Agencies|Swedish energy agency (Energimyndigheten)||Swedish Research Council (VR)||Knut, Alice Wallenberg Foundation||Crafoord foundation||VINNOVA

Two photon photoluminescence (2PPL) from single crystals of methyl ammonium lead bromide (CH3NH3PbBr3, MAPbBr3) is studied. We observe two components in the 2PPL spectra, which we assign to the photoluminescence (PL) from the carrier recombination at the band edge and the recombination due to self-trapping of excitons. The PL Stokes shift of self-trapped excitons is about 100 meV from the band-gap energy. Our measurements show that about 15% of the total PL from regions about 40 μm deep inside the crystal is due to the emission from self-trapped exciton. This contribution increases to about 20% in the PL from the regions close to the surface. Time resolved measurements of 2PPL show that the PL due to band-edge recombination has a life time of about 8 ns while the PL lifetime of self-trapped excitons is in the order of 100 ns. Quantification of self-trapped excitons in the materials used in photovoltaics is important as such excitons hinder charge separation. As our results also show that an appreciable fraction of photo-generated carriers get trapped, the results are important in rational design of photovoltaics. On the other hand, our results also show that the self-trapped excitons broaden the emission spectrum, which may be useful in designing broadband light emitting devices.

Recent advances in coherent multidimensional spectroscopy have boosted interest in exciton coherences in light harvesting complexes. We present nonperturbative calculations of two-dimensional (2D) electronic spectroscopy from a fifth-order phase-matching direction. The calculations show clear patterns that correspond to the electronic structure of one- and two-exciton manifolds of a Fenna–Matthews–Olsson light harvesting complex. Such signals can provide new information about the coherent properties of antenna pigment protein complexes.