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The performance of polymer light emitting diodes (PLEDs) degrades due to exciton quenching at the interface with charge injection layers and electrodes. We investigate the photo-physics of singlet excitons in Poly (9, 9-dioctylfluorene-alt-benzothiadiazole) (F8BT) conjugated polymer interfaced with various commonly used hole and electron injection layers. Absolute, steady-state and transient photoluminescence (PL) studies are carried out on pristine F8BT film and films with injection layer/F8BT to understand the role of injection layers on exciton quenching. Exciton quenching by the charge injection layers is treated by accounting for both exciton diffusion and the non-radiative transfer of energy to the charge injection layer. The non-radiative transfer of energy is modelled using dipole-dipole interaction theory coupled with diffusion of excitons, from which we obtain the exciton capture radius ( x 0 ) in the range of 1–7 nm. We also correlate x 0 with PL decay time (τ) using the relation τ α 1/ x 0 3 . The steady-state PL yield for each case also shows correlation with the PL decay lifetime. This study provides interesting insight on the selection criterion for injection layer to be used in PLEDs for minimizing optical losses while preserving the electronic injection properties. [ABSTRACT FROM AUTHOR]

Organic molecules store the energy of absorbed light in the form of charge-neutral molecular excitations – Frenkel excitons. Usually, in amorphous organic materials, excitons are viewed as quasiparticles, localized on single molecules, which diffuse randomly through the structure. However, the picture of incoherent hopping is not applicable to some classes of molecular aggregates – assemblies of molecules that have strong near-field interaction between electronic excitations in the individual subunits. Molecular aggregates can be found in nature, in photosynthetic complexes of plants and bacteria, and they can also be produced artificially in various forms including quasi-one dimensional chains, two-dimensional films, tubes, etc. In these structures light is absorbed collectively by many molecules and the following dynamics of molecular excitation possesses coherent properties. This energy transfer mechanism, mediated by the coherent exciton dynamics, resembles the propagation of electromagnetic waves through a structured medium on the nanometer scale. The absorbed energy can be transferred resonantly over distances of hundreds of nanometers before exciton relaxation occurs. Furthermore, the spatial and energetic landscape of molecular aggregates can enable the funneling of the exciton energy to a small number of molecules either within or outside the aggregate. In this review we establish a bridge between the fields of photonics and excitonics by describing the present understanding of exciton dynamics in molecular aggregates.

Vapor-liquid-solid (VLS) growth is the mainstream method in realizing advanced semiconductor nanowires (NWs), as widely applied to many III-V compounds. It is exclusively explored also for antimony (Sb) compounds, such as the relevant GaAsSb-based NW materials, although morphological inhomogeneities, phase segregation, and limitations in the supersaturation due to Sb strongly inhibit their growth dynamics. Fundamental advances are now reported here via entirely catalyst-free GaAsSb NWs, where particularly the Sb-mediated effects on the NW growth dynamics and physical properties are investigated in this novel growth regime. Remarkably, depending on GaAsSb composition and nature of the growth surface, both surfactant and anti-surfactant action is found, as seen by transitions between growth acceleration and deceleration characteristics. For threshold Sb-contents up to 3-4%, adatom diffusion lengths are increased ≈sevenfold compared to Sb-free GaAs NWs, evidencing the significant surfactant effect. Furthermore, microstructural analysis reveals unique Sb-mediated transitions in compositional structure, as well as substantial reduction in twin defect density, ≈tenfold over only small compositional range (1.5-6% Sb), exhibiting much larger dynamics as found in VLS-type GaAsSb NWs. The effect of such extended twin-free domains is corroborated by ≈threefold increases in exciton lifetime (≈4.5 ns) due to enlarged electron-hole pair separation in these phase-pure NWs., (© 2023 The Authors. Small published by Wiley-VCH GmbH.)

Reduced dielectric screening in two-dimensional materials enables bound excitons, which modifies their optical absorption and optoelectronic response. Here, we demonstrate the existence of excitons in the bandgap of the monolayer family of the newly discovered syntheticMoSi2Z4(Z=N, P, and As) series of materials. All three monolayers support several bright and strongly bound excitons with binding energies varying from 1 eV to 1.35 eV for the lowest energy exciton resonances. We show that on increasing the pump fluence or photo-excited carrier density, the lowest energy exciton first undergoes a redshift followed by a blueshift, due to the renormalized exciton binding energies. The exciton binding energy varies as a Lennard-Jones-like potential as a function of the inter-exciton spacing. This establishes an atom-like attractive and repulsive interaction between excitons depending on the inter-exciton separation. Our study shows that theMoSi2Z4series of monolayers offer an exciting test-bed for exploring the physics of strongly bound excitons and their non-equilibrium dynamics., (© 2023 IOP Publishing Ltd.)

InAs(Sb)/GaAs self-assembled quantum dots (QDs) were grown by molecular beam epitaxy though incorporating Sb into InAs with increased beam equivalent pressure ratio, x = Sb 2 /(Sb 2 +As 2), of x = 0, 0.15, 0.25, and 0.5. From photoluminescence measurements, we observed a transition of the band alignment from type-I for the InAs QDs to type-II for InAsSb QDs. Thus, after adding Sb into the QDs these type II InAsSb QDs have different characteristics from the well-studied type-I InAs QDs regarding to their excitation intensity dependence, temperature dependence, and time dependence of the luminescence. Additionally, the Sb distribution in the QDs and the wetting layer was studied. These results indicate that we are able to fabricate InAsSb QDs with a type II band alignment, which may be good optical materials for the development of intermediate-band photovoltaic cells. • Self-assembled InAsSb quantum dots (QDs) are fabricated though incorporating Sb into InAs. • Band alignment transit to type-II for InAsSb QDs after adding Sb into InAs QDs. • Type-II luminescence for InAsSb QDs characterized by blue-shift and nonlinear decay of the emission. • High Sb-content in QDs than wetting layer. • More complex carrier dynamics and thermal quenching for InAsSb QDs. [ABSTRACT FROM AUTHOR]

Monolayer transition-metal dichalcogenides (TMDCs) have recently emerged as promising candidates for advanced photonic and valleytronic applications due to their unique optoelectronic properties. However, the low luminescence efficiency of monolayer TMDCs has significantly hampered their use in these fields. Here it is reported that the photoluminescence efficiency of monolayer WS2 can be remarkably enhanced up to fourfold through the fluorination, surpassing the reported performance of molecular and/or electrical doping methods. Its degree is easily controlled by changing the fluorine plasma duration time and can also be reversibly tuned via additional hydrogen plasma treatment, allowing for its versatile tailoring for interfacial band alignment and customized engineering. The striking photoluminescence improvement occurs via a substantial transition of trions to excitons as a result of the strong electron affinity of fluorine dopants, and the fluorination enables unprecedented detection of n-type NH3 gas in WS2 due to changed excitonic dynamics showing excellent sensitivity (at least down to 1.25 ppm). This work provides valuable strategies and insights into exciton physics in monolayer TMDCs, opening up avenues toward highly-efficient 2D light emitters, photovoltaics, nanosensors, and optical interconnects. [ABSTRACT FROM AUTHOR]

Colloidal semiconductor nanocrystals have attracted continuous worldwide interest over the last three decades owing to their remarkable and unique size- and shape-, dependent properties. The colloidal nature of these nanomaterials allows one to take full advantage of nanoscale effects to tailor their optoelectronic and physical-chemical properties, yielding materials that combine size-, shape-, and composition-dependent properties with easy surface manipulation and solution processing. These features have turned the study of colloidal semiconductor nanocrystals into a dynamic and multidisciplinary research field, with fascinating fundamental challenges and dazzling application prospects. This review focuses on the excited-state dynamics in these intriguing nanomaterials, covering a range of different relaxation mechanisms that span over 15 orders of magnitude, from a few femtoseconds to a few seconds after photoexcitation. In addition to reviewing the state of the art and highlighting the essential concepts in the field, we also discuss the relevance of the different relaxation processes to a number of potential applications, such as photovoltaics and LEDs. The fundamental physical and chemical principles needed to control and understand the properties of colloidal semiconductor nanocrystals are also addressed. [ABSTRACT FROM AUTHOR]

It is demonstrated that the luminescence efficiency of monolayers composed of MoS2, WS2, and WSe2 is significantly limited by the substrate and can be improved by orders of magnitude through substrate engineering. The substrate affects the efficiency mainly through doping the monolayers and facilitating defect-assisted nonradiative exciton recombinations, while the other substrate effects including straining and dielectric screening play minor roles. The doping may come from the substrate and substrate-borne water moisture, the latter of which is much stronger than the former for MoS2 and WS2 but negligible for WSe2. Using proper substrates such as mica or hexagonal boron nitride can substantially mitigate the doping effect. The defect-assisted recombination depends on the interaction between the defect in the monolayer and the substrate. Suspended monolayers, in which the substrate effects are eliminated, may have efficiency up to 40% at room temperatures. The result provides useful guidance for the rational design of atomic-scale light emission devices. [ABSTRACT FROM AUTHOR]

The two classic structural classifications of solid matter, crystalline and amorphous, are quite useful for inorganic solids, from antimony trichloride to zinc dibromide and beyond. However, for many molecular materials, especially those based on components containing ten or more non-hydrogen atoms, ordered single crystals are less common, and intermediate structures that are neither amorphous nor crystalline frequently dominate. Herein, we discuss several situations in which there is local (or partial) ordering in such molecular materials. These materials go beyond the standard concepts concerning polymers and must account for imperfect but substantial local ordering, such as that caused by weak intermolecular interactions (e.g., p stacking, H-bonding, and/or dispersion forces). This local ordering has important implications for, and applications in, materials properties : we specifically consider dielectric response, electron transport, and exciton transfer. Finally, we discuss the fundamental importance of the effective dimensionality of partially ordered domains in molecular materials. [ABSTRACT FROM AUTHOR]

In natural light harvesting systems, the sequential quantum events of photon absorption by specialized biological antenna complexes, charge separation, exciton formation and energy transfer to localized reaction centers culminates in the conversion of solar to chemical energy. A notable feature in these processes is the exceptionally high efficiencies ( $$>$$ 95 %) at which excitation is transferred from the illuminated protein complex site to the reaction centers. The high speed of excitation propagation within a system of interwoven biomolecular network structures, is yet to be replicated in artificial light harvesting complexes. A clue to unraveling the quantum puzzles of nature may lie in the observations of long lived coherences lasting several picoseconds in the electronic spectra of photosynthetic complexes, which occurs even in noisy environmental baths. However the exact nature of the association between the high energy propagation rates and strength of quantum coherences remains largely unsolved. A number of experimental and theoretical studies have been devoted to unlocking the links between quantum processes and information protocols, in the hope of finding the answers to nature's puzzling mode of energy propagation. This review presents recent developments in quantum theories, and links information-theoretic aspects with photosynthetic light-harvesting processes in biomolecular systems. There is examination of various attempts to pinpoint the processes that underpin coherence features arising from the light harvesting activities of biomolecular systems, with particular emphasis on the effects that factors such non-Markovianity, zeno mechanisms, teleportation, quantum predictability and the role of multipartite states have on the quantum dynamics of biomolecular systems. A discussion of how quantum thermodynamical principles and agent-based modeling and simulation approaches can improve our understanding of natural photosynthetic systems is included. [ABSTRACT FROM AUTHOR]

The exciton recombination processes in a series of elastically strained GaAsBi epilayers are investigated by means of time-integrated and time-resolved photoluminescence at T = 10 K. The bismuth content in the samples was adjusted from 1.16% to 3.83%, as confirmed by high-resolution X-ray diffraction (HR-XRD). The results are well interpreted by carrier trapping and recombination mechanisms involving the Bi-related localized levels. Clear distinction between the localized and delocalized regime was observed in the spectral and temporal photoluminescence emission. [ABSTRACT FROM AUTHOR]

Silver phenylselenolate (AgSePh, also known as "mithrene") and silver phenyltellurolate (AgTePh, also known as "tethrene") are two-dimensional (2D) van der Waals semiconductors belonging to an emerging class of hybrid organic-inorganic materials called metal-organic chalcogenolates. Despite having the same crystal structure, AgSePh and AgTePh exhibit a strikingly different excitonic behavior. Whereas AgSePh exhibits narrow, fast luminescence with a minimal Stokes shift, AgTePh exhibits comparatively slow luminescence that is significantly broadened and red-shifted from its absorption minimum. Using time-resolved and temperature-dependent absorption and emission microspectroscopy, combined with subgap photoexcitation studies, we show that exciton dynamics in AgTePh films are dominated by an intrinsic self-trapping behavior, whereas dynamics in AgSePh films are dominated by the interaction of band-edge excitons with a finite number of extrinsic defect/trap states. Density functional theory calculations reveal that AgSePh has simple parabolic band edges with a direct gap at Γ, whereas AgTePh has a saddle point at Γ with a horizontal splitting along the Γ-N 1 direction. The correlation between the unique band structure of AgTePh and exciton self-trapping behavior is unclear, prompting further exploration of excitonic phenomena in this emerging class of hybrid 2D semiconductors.

We extend our recent quantum dynamical study of the exciton dissociation and charge transfer at an oligothiophene–fullerene heterojunction interface (Tamura et al., 2012) [6] by investigating the process using the non-perturbative hierarchical equations of motion (HEOM) approach. Based upon an effective mode reconstruction of the spectral density the effect of temperature on the charge transfer is studied using reduced density matrices. It was found that the temperature had little effect on the charge transfer and a coherent dynamics persists over the first few tens of femtoseconds, indicating that the primary charge transfer step proceeds by an activationless pathway. [ABSTRACT FROM AUTHOR]

The two classic structural classifications of solid matter, crystalline and amorphous, are quite useful for inorganic solids, from antimony trichloride to zinc dibromide and beyond. However, for many molecular materials, especially those based on components containing ten or more non-hydrogen atoms, ordered single crystals are less common, and intermediate structures that are neither amorphous nor crystalline frequently dominate. Herein, we discuss several situations in which there is local (or partial) ordering in such molecular materials. These materials go beyond the standard concepts concerning polymers and must account for imperfect but substantial local ordering, such as that caused by weak intermolecular interactions (e.g., p stacking, H-bonding, and/or dispersion forces). This local ordering has important implications for, and applications in, materials properties : we specifically consider dielectric response, electron transport, and exciton transfer. Finally, we discuss the fundamental importance of the effective dimensionality of partially ordered domains in molecular materials. [ABSTRACT FROM AUTHOR]

Ultrathin layered crystals of coordinated chromium(III) are promising not only as two-dimensional (2D) magnets but also as 2D near-infrared (NIR) emitters due to long-range spin correlation and efficient transition between high- and low-spin excited states of Cr 3+ ions. In this study, we report on the dual-band NIR photoluminescence (PL) of CrPS 4 and show that its excitonic emission bifurcates into fluorescence and phosphorescence depending on thickness, temperature, and defect density. In addition to the spectral branching, the biexponential decay of PL transients, also affected by the three factors, could be well described within a three-level kinetic model for Cr(III). In essence, the PL bifurcations are governed by activated reverse intersystem crossing from the low- to high-spin states, and the transition barrier becomes lower for thinner 2D samples because of surface-localized defects. Our findings can be generalized to 2D solids of coordinated metals and will be valuable in realizing groundbreaking magneto-optic functions and devices.

We measured the exciton dynamics in van der Waals heterojunctions of transition metal dichalcogenides (TMDCs) and organic semiconductors (OSs). TMDCs and OSs are semiconducting materials with rich and highly diverse optical and electronic properties. Their heterostructures, exhibiting van der Waals bonding at their interfaces, can be utilized in the field of optoelectronics and photovoltaics. Two types of heterojunctions, MoS 2 -pentacene and WSe 2 -pentacene, were prepared by layer transfer of 20 nm pentacene thin films as well as MoS 2 and WSe 2 monolayer crystals onto Au surfaces. The samples were studied by means of transient absorption spectroscopy in the reflectance mode. We found that A-exciton decay by hole transfer from MoS 2 to pentacene occurs with a characteristic time of 21 ± 3 ps. This is slow compared to previously reported hole transfer times of 6.7 ps in MoS 2 -pentacene junctions formed by vapor deposition of pentacene molecules onto MoS 2 on SiO 2 . The B-exciton decay in WSe 2 shows faster hole transfer rates for WSe 2 -pentacene heterojunctions, with a characteristic time of 7 ± 1 ps. The A-exciton in WSe 2 also decays faster due to the presence of a pentacene overlayer; however, fitting the decay traces did not allow for the unambiguous assignment of the associated decay time. Our work provides important insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions.

We investigate an artificial molecular dimer made of two dipole coupled cyanine dye monomers in which a strong coherent coupling between electronic and vibrational degrees of freedom arises. Clear signatures of this coupling are reflected in an oscillatory time evolution of the off-diagonal vibronic cross peaks in the two-dimensional optical photon echo spectrum. We find a strong coherence component damped by fast electronic dephasing ( $\approx 50$ fs) accompanied by a much weaker component which decays on the longer time scales (ps) associated to vibrational dephasing. We find that vibronic coupling does not cause longer dephasing times of the dominant photo echo component but additional weak but long-lived components emerge.

Much progress has been made in using hematite (α-Fe2O3) as a potentially practical and sustainable material for applications such as solar-energy conversion and photoelectrochemical (PEC) water splitting; however, recent studies have shown that the performance can be limited by a very short charge-carrier diffusion length or exciton lifetime. In this study, we performed ultrafast studies on hematite nanoparticles of different shapes to determine the possible influence of particle shape on the exciton dynamics. Nanorice, multifaceted spheroidal nanoparticles, faceted nanocubes, and faceted nanorhombohedra were synthesized and characterized by using SEM and XRD techniques. Their exciton dynamics were investigated by using femtosecond transient absorption (TA) spectroscopy. Although the TA spectral features differ for the four samples studied, their decay profiles are similar, which can be fitted with time constants of 1-3 ps, approximately 25 ps, and a slow nanosecond component extending beyond the experimental time window that was measured (2 ns). The results indicate that the overall exciton lifetime is weakly dependent on the shape of the hematite nanoparticles, even though the overall optical absorption and scattering are influenced by the particle shape. This study suggests that other strategies need to be developed to increase the exciton lifetime or to lengthen the exciton diffusion length in hematite nanostructures. [ABSTRACT FROM AUTHOR]

The poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blending is one of the most intensively studied active material systems for bulk-heterojunction (BHJ) organic photovoltaic devices. For the active layer with BHJ structure, the charge tranfer (CT) and exciton dissociation processes between P3HT and PCBM are the important initial steps to form free charge carriers in the active layer. To compare the CT related species from the transient absorption dynamics in P3HT:PCBM blend systems with different orderings of the polymer, we investigated the charge carrier dynamics in P3HT:PCBM blend films with regiorandom and regioregular polymer structures using femtosecond transient absorption spectroscopy. [ABSTRACT FROM PUBLISHER]

Abstract: A method of generation of powerful pulses of ultraviolet and soft X-ray coherent radiation is considered. It consists in employing superradiant indirect exciton recombination in large enough high quality diamond samples. The theory of this transient process is developed and its dynamics is simulated. In result it is shown that this method may probably provide the generation of picosecond pulses of coherent electromagnetic field with peak powers larger than 30W at wavelength 235nm. [Copyright &y& Elsevier]

The carrier recombination dynamics in ZnSe nanowires (NWs) remain poorly understood despite more than a decade of research since their inception in 2001. Herein, through a comprehensive pump fluence‐ and temperature‐dependent two‐photon excitation (TPE) study, a clear picture of the carrier relaxation pathways, intrinsic lifetimes, exciton oscillator strengths, and exciton‐phonon interactions is presented for this NW system. Contrary to a common perception that the higher pump intensities needed to achieve two‐photon‐excited photoluminescence correspond to a higher exciton density threshold (nth) for two‐photon pumped lasing, it is found that a much lower nth is needed to achieve lasing with TPE compared to single‐photon excitation (SPE) of the same ZnSe NWs. This measurement is further supported by the greatly enhanced lasing action photostability characteristics of the ZnSe NWs under TPE. These findings have significant implications on the design and the tailoring of the optoelectronic properties of nanowire lasers. [ABSTRACT FROM AUTHOR]

In single-layer (1L) transition metal dichalcogenides, the reduced Coulomb screening results in strongly bound excitons which dominate the linear and the nonlinear optical response. Despite the large number of studies, a clear understanding on how many-body and Coulomb correlation effects affect the excitonic resonances on a femtosecond time scale is still lacking. Here, we use ultrashort laser pulses to measure the transient optical response of 1L-WS 2 . In order to disentangle many-body effects, we perform exciton line-shape analysis, and we study its temporal dynamics as a function of the excitation photon energy and fluence. We find that resonant photoexcitation produces a blue shift of the A exciton, while for above-resonance photoexcitation the transient response at the optical bandgap is largely determined by a reduction of the exciton oscillator strength. Microscopic calculations based on excitonic Heisenberg equations of motion quantitatively reproduce the nonlinear absorption of the material and its dependence on excitation conditions.

The CsSnBr 3 photodetectors are fabricated by thermal evaporation and 75 °C in situ annealing, and the effect of in situ annealing on the morphology, structure, exciton dynamics and photoresponse of thermally evaporated CsSnBr 3 films are investigated. Especially, temperature dependent steady-state photoluminescence (PL) and transient PL decaying have been analyzed in details for understanding the exciton dynamics. Meanwhile, effect of annealing on the activation energy for trap sites ( E a ), exciton binding energy ( E b ), activation energy for interfacial trapped carriers (Δ E ), trap densities and carriers mobilities are studied and the annealed (A-CsSnBr 3 ) reveals obviously lower E b and trap density together with notably higher carrier mobility than those of the unannealed (UA-CsSnBr 3 ). Temperature dependence of the integrated PL intensity can be ascribed to the combining effect of the exciton dissociation, exciton quenching through trap sites and thermal activation of trapped carriers. The temperature dependent transient PL decaying analysis indicates that the PL decaying mechanism at low and high temperature is totally different from that in intermediate temperature range, in which combing effect of free exciton and localized state exciton decaying prevail. The beneficial effects of the in situ annealing on the photoresponse performance of the CsSnBr 3 films can be demonstrated by the remarkable enhancement of the optimal responsivity ( R ) after in situ annealing which increases from less than 1 A W -1 to 1350 A W -1 as well as dramatically improved noise equivalent power, specific detectivity D * and Gain ( G )., (© 2022 IOP Publishing Ltd.)

Excited states of organic molecules (excitons) are the heart of any organic electroluminescent device. They mediate the conversion of injected charges - electrons and holes - into photons. Phosphorescent emission originating from triplet excitons is especially important, as it is to date the only general route to enable unity charge-to-photon conversion efficiencies. In this paper, we discuss the key aspects of excitons, following the excited state lifecycle. First, we review fundamentals of singlet and triplet exciton formation in organic semiconductors, followed by a discussion of concepts that aim to alter the singlet-to-triplet formation rates to enable higher electroluminescence yields in the fluorescence manifold. Subsequently, we focus on the exciton distribution within the organic semiconductor material during its lifetime. The processes involved ultimately determine organic light-emitting diode (OLED) performance and are especially key in the development of concepts for white emission, where precise balance of the exciton between different emitter species control the emitted color. We close this paper with discussion of non-linear effects at high excitation levels that, to date, limit the high brightness efficiency of phosphorescent OLEDs. [ABSTRACT FROM AUTHOR]

This mini-review on the photophysics of poly-alkyl thiophenes (e.g., P3HT) and its blends with electron-acceptor moeties such as fullerenes (e.g., PCBM) and carbon nanotubes focuses on highlights of recent literature on spectroscopic probes of exciton formation, diffusion, charge-separation, and transport in these materials. The literature in this area is vast: more than 3000 papers have been published in on P3HT (and related materials) and applications to organic solar energy harvesting devices over the last 20 years. Thus, no single review can capture the breadth and depth of this research. Here, we attempt to highlight some of the exciting new research efforts aimed at understanding photophysical processes in organic photovoltaic materials. This mini-review is organized as follows: First, a summary of the theoretical framework commonly used to describe fundamental physical processes of charge generation in organic (polymeric) semiconductor materials is presented. We then discuss recent exciting results on ultrafast spectroscopic probes of exciton dynamics in these materials. Finally, we present highlights of new research on polymer nanostructures (nanoparticles and nanofibers) and their exciting applications to organic photovoltaics. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

Abstract: We review recent progress of using time-resolved two-photon photoelectron spectroscopy (2PPE) to study the energetics and dynamics of excitons at surfaces and interfaces of two prototypical organic semiconductors: C60 and pentacene. For C60 thin films epitaxially grown on Au(111) and Cu(111) surfaces, we observe both charge transfer and exciton states. For excitons in C60, the proximity of a metal surface leads to rapid, exciton band-mediated quenching. At the surface of pentacene thin films we observe a series of charge-transfer excitons where the electron and the photohole are bound across the interface. The ability of 2PPE to measure and directly relate exciton levels to single-electron levels is illustrated. [Copyright &y& Elsevier]