Inverted device architecture for high efficiency single-layer organic light-emitting diodes with imbalanced charge transport.

Many wide-gap organic semiconductors exhibit imbalanced electron and hole transport, therefore efficient organic light-emitting diodes require a multilayer architecture of electron- and hole-transport materials to confine charge recombination to the emissive layer. Here, we show that even for emitters with imbalanced charge transport, it is possible to obtain highly efficient single-layer organic light emitting diodes (OLEDs), without the need for additional charge-transport and blocking layers. For hole-dominated emitters, an inverted single-layer device architecture with ohmic bottom-electron and top-hole contacts moves the emission zone away from the metal top electrode, thereby more than doubling the optical outcoupling efficiency. Finally, a blue-emitting inverted single-layer OLED based on thermally activated delayed fluorescence is achieved, exhibiting a high external quantum efficiency of 19% with little roll-off at high brightness, demonstrating that balanced charge transport is not a prerequisite for highly efficient single-layer OLEDs.

Water binding and hygroscopicity in π-conjugated polyelectrolytes.

The presence of water strongly influences structure, dynamics and properties of ion-containing soft matter. Yet, the hydration of such matter is not well understood. Here, we show through a large study of monovalent π-conjugated polyelectrolytes that their reversible hydration, up to several water molecules per ion pair, occurs chiefly at the interface between the ion clusters and the hydrophobic matrix without disrupting ion packing. This establishes the appropriate model to be surface hydration, not the often-assumed internal hydration of the ion clusters. Through detailed analysis of desorption energies and O-H vibrational frequencies, together with OPLS4 and DFT calculations, we have elucidated key binding motifs of the sorbed water. Type-I water, which desorbs below 50 °C, corresponds to hydrogen-bonded water clusters constituting secondary hydration. Type-II water, which typically desorbs over 50-150 °C, corresponds to water bound to the anion under the influence of a proximal cation, or to a cation‒anion pair, at the cluster surface. This constitutes primary hydration. Type-III water, which irreversibly desorbs beyond 150 °C, corresponds to water kinetically trapped between ions. Its amount varies strongly with processing and heat treatment. As a consequence, hygroscopicity-which is the water sorption capacity per ion pair-depends not only on the ions, but also their cluster morphology.

Double-type-I charge-injection heterostructure for quantum-dot light-emitting diodes.

Enforcing balanced electron-hole injection into the emitter layer of quantum-dot light-emitting diodes (QLEDs) remains key to maximizing the quantum efficiency over a wide current density range. This was previously thought not possible for quantum dot (QD) emitters because of their very deep energy bands. Here, we show using Mesolight® blue-emitting CdZnSeS/ZnS QDs as a model that its valence levels are in fact considerably shallower than the corresponding band maximum of the bulk semiconductor, which makes the ideal double-type-I injection/confinement heterostructure accessible using a variety of polymer organic semiconductors as transport and injection layers. We demonstrate flat external quantum efficiency characteristics that indicate near perfect recombination within the QD layer over several decades of current density from the onset of device turn-on of about 10 µA cm-2, for both normal and inverted QLED architectures. We also demonstrate that these organic semiconductors do not chemically degrade the QDs, unlike the usual ZnMgO nanoparticles. However, these more efficient injection heterostructures expose a new vulnerability of the QDs to in device electrochemical degradation. The work here opens a clear path towards next-generation ultra-high-performance, all-solution-processed QLEDs.

Role of Linker Functionality in Polymers Exhibiting Main-Chain Thermally Activated Delayed Fluorescence.

Excellent performance has been reported for organic light-emitting diodes (OLEDs) based on small molecule emitters that exhibit thermally activated delayed fluorescence. However, the necessary vacuum processing makes the fabrication of large-area devices based on these emitters cumbersome and expensive. Here, the authors present high performance OLEDs, based on novel, TADF polymers that can be readily processed from a solution. These polymers are based on the acridine-benzophenone donor-acceptor motif as main-chain TADF chromophores, linked by various conjugated and non-conjugated spacer moieties. The authors' extensive spectroscopic and electronic analysis shows that in particular in case of alkyl spacers, the properties and performance of the monomeric TADF chromophores are virtually left unaffected by the polymerization. They present efficient solution-processed OLEDs based on these TADF polymers, diluted in oligostyrene as a host. The devices based on the alkyl spacer-based TADF polymers exhibit external quantum efficiencies (EQEs) ≈12%, without any outcoupling-enhancing measures. What's more, the EQE of these devices does not drop substantially upon diluting the polymer down to only ten weight percent of active material. In contrast, the EQE of devices based on the monomeric chromophore show significant losses upon dilution due to loss of charge percolation.

Overcoming the water oxidative limit for ultra-high-workfunction hole-doped polymers.

It is widely thought that the water-oxidation reaction limits the maximum work function to about 5.25 eV for hole-doped semiconductors exposed to the ambient, constrained by the oxidation potential of air-saturated water. Here, we show that polymer organic semiconductors, when hole-doped, can show work functions up to 5.9 eV, and yet remain stable in the ambient. We further show that de-doping of the polymer is not determined by the oxidation of bulk water, as previously thought, due to its general absence, but by the counter-balancing anion and its ubiquitously hydrated complexes. The effective donor levels of these species, representing the edge of the 'chemical' density of states, can be depressed to about 6.0 eV below vacuum level. This can be achieved by raising the oxidation potential for hydronium generation, using large super-acid anions that are themselves also stable against oxidation. In this way, we demonstrate that poly(fluorene-alt-triarylamine) derivatives with tethered perfluoroalkyl-sulfonylimidosulfonyl anions can provide ambient solution-processability directly in the ultrahigh-workfunction hole-doped state to give films with good thermal stability. These results lay the path for design of soft materials for battery, bio-electronic and thermoelectric applications.

Improving organic photovoltaic cells by forcing electrode work function well beyond onset of Ohmic transition.

As electrode work function rises or falls sufficiently, the organic semiconductor/electrode contact reaches Fermi-level pinning, and then, few tenths of an electron-volt later, Ohmic transition. For organic solar cells, the resultant flattening of open-circuit voltage (Voc) and fill factor (FF) leads to a 'plateau' that maximizes power conversion efficiency (PCE). Here, we demonstrate this plateau in fact tilts slightly upwards. Thus, further driving of the electrode work function can continue to improve Voc and FF, albeit slowly. The first effect arises from the coercion of Fermi level up the semiconductor density-of-states in the case of 'soft' Fermi pinning, raising cell built-in potential. The second effect arises from the contact-induced enhancement of majority-carrier mobility. We exemplify these using PBDTTPD:PCBM solar cells, where PBDTTPD is a prototypal face-stacked semiconductor, and where work function of the hole collection layer is systematically 'tuned' from onset of Fermi-level pinning, through Ohmic transition, and well into the Ohmic regime.

Surface Doping of Organic Single-Crystal Semiconductors to Produce Strain-Sensitive Conductive Nanosheets.

A highly periodic electrostatic potential, even though established in van der Waals bonded organic crystals, is essential for the realization of a coherent band electron system. While impurity doping is an effective chemical operation that can precisely tune the energy of an electronic system, it always faces an unavoidable difficulty in molecular crystals because the introduction of a relatively high density of dopants inevitably destroys the highly ordered molecular framework. In striking contrast, a versatile strategy is presented to create coherent 2D electronic carriers at the surface of organic semiconductor crystals with their precise molecular structures preserved perfectly. The formation of an assembly of redox-active molecular dopants via a simple one-shot solution process on a molecularly flat crystalline surface allows efficient chemical doping and results in a relatively high carrier density of 1013 cm-2 at room temperature. Structural and magnetotransport analyses comprehensively reveal that excellent carrier transport and piezoresistive effects can be obtained that are similar to those in bulk crystals.

Nearly 100% Photocrosslinking Efficiency in Ultrahigh Work Function Hole-Doped Conjugated Polymers Using Bis(fluorophenyl azide) Additives.

Self-compensated (SC) hole-doped conjugated polyelectrolytes with high work functions can provide efficient hole-injection and -collection layers for organic and other semiconductor devices. If these films can be photocrosslinked, the semiconductor overlayer can be deposited from a wider range of solvents, enabling flexibility in device design and fabrication. However, a generic photocrosslinking methodology for these materials is not yet available. Here, we demonstrate that sFPA82-TfO, the recently developed bis(fluorophenyl azide) photocrosslinker that is also i-line compatible, can surprisingly give 100% efficient photocrosslinking for SC hole-doped conjugated polyelectrolytes, i.e., one crosslink per reactive moiety, using mTFF-C2F5SIS-Na, a triarylamine-fluorene copolymer, as the model polyelectrolyte, without degrading its ultrahigh work function of 5.75 eV. The photocrosslinking efficiency is much higher than in the corresponding undoped polyelectrolyte and nonconjugated polyelectrolyte films, where the efficiency is only 20%. We attribute this improvement to the formation of smaller ion multiplet clusters in the hole-doped polyelectrolyte, as suggested by molecular dynamics simulations and infrared spectroscopy, which prevents occlusion of the ionic crosslinker. Photocrosslinking of the SC hole-doped mTFF-C2F5SIS-Na film used as a hole-injection layer in 100 nm-thick PFOP diodes suppresses the leakage current by over 3 orders of magnitude compared to those without crosslinking, to below 30 nA cm-2 at ±2 V. Photocrosslinking of the same film used as the hole-collection layer in PBDTTPD:PC61BM solar cells produces a higher photocurrent density, fill factor, and power conversion efficiency.

Multivalent anions as universal latent electron donors.

Electrodes with low work functions are required to efficiently inject electrons into semiconductor devices. However, when the work function drops below about 4 electronvolts, the electrode suffers oxidation in air, which prevents its fabrication in ambient conditions. Here we show that multivalent anions such as oxalate, carbonate and sulfite can act as powerful latent electron donors when dispersed as small ion clusters in a matrix, while retaining their ability to be processed in solution in ambient conditions. The anions in these clusters can even n-dope the semiconductor core of π-conjugated polyelectrolytes that have low electron affinities, through a ground-state doping mechanism that is further amplified by a hole-sensitized or photosensitized mechanism in the device. A theoretical analysis of donor levels of these anions reveals that they are favourably upshifted from ionic lattices by a decrease in the Coulomb stabilization of small ion clusters, and by irreversibility effects. We attain an ultralow effective work function of 2.4 electronvolts with the polyfluorene core. We realize high-performance, solution-processed, white-light-emitting diodes and organic solar cells using polymer electron injection layers with these universal anion donors, demonstrating a general approach to chemically designed and ambient-processed Ohmic electron contacts for semiconductor devices.

Doped polymer semiconductors with ultrahigh and ultralow work functions for ohmic contacts.

To make high-performance semiconductor devices, a good ohmic contact between the electrode and the semiconductor layer is required to inject the maximum current density across the contact. Achieving ohmic contacts requires electrodes with high and low work functions to inject holes and electrons respectively, where the work function is the minimum energy required to remove an electron from the Fermi level of the electrode to the vacuum level. However, it is challenging to produce electrically conducting films with sufficiently high or low work functions, especially for solution-processed semiconductor devices. Hole-doped polymer organic semiconductors are available in a limited work-function range, but hole-doped materials with ultrahigh work functions and, especially, electron-doped materials with low to ultralow work functions are not yet available. The key challenges are stabilizing the thin films against de-doping and suppressing dopant migration. Here we report a general strategy to overcome these limitations and achieve solution-processed doped films over a wide range of work functions (3.0-5.8 electronvolts), by charge-doping of conjugated polyelectrolytes and then internal ion-exchange to give self-compensated heavily doped polymers. Mobile carriers on the polymer backbone in these materials are compensated by covalently bonded counter-ions. Although our self-compensated doped polymers superficially resemble self-doped polymers, they are generated by separate charge-carrier doping and compensation steps, which enables the use of strong dopants to access extreme work functions. We demonstrate solution-processed ohmic contacts for high-performance organic light-emitting diodes, solar cells, photodiodes and transistors, including ohmic injection of both carrier types into polyfluorene-the benchmark wide-bandgap blue-light-emitting polymer organic semiconductor. We also show that metal electrodes can be transformed into highly efficient hole- and electron-injection contacts via the self-assembly of these doped polyelectrolytes. This consequently allows ambipolar field-effect transistors to be transformed into high-performance p- and n-channel transistors. Our strategy provides a method for producing ohmic contacts not only for organic semiconductors, but potentially for other advanced semiconductors as well, including perovskites, quantum dots, nanotubes and two-dimensional materials.

Madelung and Hubbard interactions in polaron band model of doped organic semiconductors.

The standard polaron band model of doped organic semiconductors predicts that density-of-states shift into the π-π* gap to give a partially filled polaron band that pins the Fermi level. This picture neglects both Madelung and Hubbard interactions. Here we show using ultrahigh workfunction hole-doped model triarylamine-fluorene copolymers that Hubbard interaction strongly splits the singly-occupied molecular orbital from its empty counterpart, while Madelung (Coulomb) interactions with counter-anions and other carriers markedly shift energies of the frontier orbitals. These interactions lower the singly-occupied molecular orbital band below the valence band edge and give rise to an empty low-lying counterpart band. The Fermi level, and hence workfunction, is determined by conjunction of the bottom edge of this empty band and the top edge of the valence band. Calculations are consistent with the observed Fermi-level downshift with counter-anion size and the observed dependence of workfunction on doping level in the strongly doped regime.

Suppressing recombination in polymer photovoltaic devices via energy-level cascades.

An energy cascading structure is designed in a polymer photovoltaic device to suppress recombination and improve quantum yields. By the insertion of a thin polymer interlayer with intermediate energy levels, electrons and holes can effectively shuttle away from each other while being spatially separated from recombination. An increase in open-circuit voltage and short-circuit current are observed in modified devices.

A general method for transferring graphene onto soft surfaces.

Recent advances in chemical vapour deposition have led to the fabrication of large graphene sheets on metal foils for use in research and development. However, further breakthroughs are required in the way these graphenes are transferred from their growth substrates onto the final substrate. Although various methods have been developed, as yet there is no general way to reliably transfer graphene onto arbitrary surfaces, such as 'soft' ones. Here, we report a method that allows the graphene to be transferred with high fidelity at the desired location on almost all surfaces, including fragile polymer thin films and hydrophobic surfaces. The method relies on a sacrificial 'self-releasing' polymer layer placed between a conventional polydimethylsiloxane elastomer stamp and the graphene that is to be transferred. This self-releasing layer provides a low work of adhesion on the stamp, which facilitates delamination of the graphene and its placement on the new substrate. To demonstrate the generality and reliability of our method, we fabricate high field-strength polymer capacitors using graphene as the top contact over a polymer dielectric thin film. These capacitors show superior dielectric breakdown characteristics compared with those made with evaporated metal top contacts. Furthermore, we fabricate low-operation-voltage organic field-effect transistors using graphene as the gate electrode placed over a thin polymer gate dielectric layer. We finally demonstrate an artificial graphite intercalation compound by stacking alternate monolayers of graphene and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). This compound, which comprises graphene sheets p-doped by partial hole transfer from the F4TCNQ, shows a high and remarkably stable hole conductivity, even when heated in the presence of moisture.

High internal quantum efficiency in fullerene solar cells based on crosslinked polymer donor networks.

The power conversion efficiency of organic photovoltaic cells depends crucially on the morphology of their donor-acceptor heterostructure. Although tremendous progress has been made to develop new materials that better cover the solar spectrum, this heterostructure is still formed by a primitive spontaneous demixing that is rather sensitive to processing and hence difficult to realize consistently over large areas. Here we report that the desired interpenetrating heterostructure with built-in phase contiguity can be fabricated by acceptor doping into a lightly crosslinked polymer donor network. The resultant nanotemplated network is highly reproducible and resilient to phase coarsening. For the regioregular poly(3-hexylthiophene):phenyl-C(61)-butyrate methyl ester donor-acceptor model system, we obtained 20% improvement in power conversion efficiency over conventional demixed biblend devices. We reached very high internal quantum efficiencies of up to 0.9 electron per photon at zero bias, over an unprecedentedly wide composition space. Detailed analysis of the power conversion, power absorbed and internal quantum efficiency landscapes reveals the separate contributions of optical interference and donor-acceptor morphology effects.

Surface-directed spinodal decomposition in poly[3-hexylthiophene] and C61-butyric acid methyl ester blends.

Demixed blends of poly[3-hexylthiophene] (P3HT) and C61-butyric acid methyl ester (PCBM) are widely used in photovoltaic diodes (PV) and show excellent quantum efficiency and charge collection properties. We find the empirically optimized literature process conditions give rise to demixing during solvent (chlorobenzene) evaporation by spinodal decomposition. Ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) results are consistent with the formation of 1-2 nm thick surface layers on both interfaces, which trigger the formation of surface-directed waves emanating from both film surfaces. This observation is evidence that spinodal demixing (leading to a bicontinuous phase morphology) precedes the crystallization of the two components. We propose a model for the interplay of demixing and crystallization which explains the broadly similar PV performance for devices made with the bottom electrodes either as hole or electron collector. The process regime of temporal separation of demixing and crystallization is attractive because it provides a way to control the morphology and thereby the efficiency of PV devices.

Interplay of processing, morphological order, and charge-carrier mobility in polythiophene thin films deposited by different methods: comparison of spin-cast, drop-cast, and inkjet-printed films.

The dependence of morphology and polymer-chain orientation of regioregular poly(3-hexylthiophene) (rrP3HT) thin films on processing conditions have been widely studied. However, their possible variation across the film thickness direction remains largely unknown. We report here a marked difference in the optical dielectric (n,k) spectra between the top and bottom interfaces of spin-cast (sc) rrP3HT films deposited from chlorobenzene solutions. These spectra were obtained from reflection variable-angle spectroscopic ellipsometry using a self-consistent graded optical model with self-imposed Kramers-Krönig consistency. The top interface shows a red-shifted absorption that is characteristic of better order than at the bottom, across a wide range of film thicknesses. This disparity diminishes in drop-cast (dc) and multipass inkjet-printed (ijp) films, and disappears in amorphous films such as those of polystyrene and of a green-emitting phenyl-substituted poly(p-phenylenevinylene). The (n,k) spectra also reveal that crystallinity increases across sc < dc < ijp films. This is supported by cross section scanning electron microscopy of the cleaved edges and measurement of the microroughness of both the film interfaces. Furthermore, optical anisotropy decreases across sc > dc > ijp films. Finally, near-edge X-ray absorption fine structure spectroscopy also shows the frontier chains in ijp and dc films are more isotropically oriented than those in sc films. These results suggest that semicrystalline conjugated polymer films can be produced far from equilibrium. This explains the marked variation in their (opto)electronic properties between the top and bottom surfaces that has sometimes been found depending on the film deposition method. In particular, an unusually pronounced crystallization is induced by ijp. We label this marked ijp-induced crystallization the "ijp morphology", which appears to be general, as it is found also in single-inkjet-droplet films. It appears also to be responsible for the lower field-effect mobility measured for ijp films deposited on a variety of linear and circular electrode arrays. This however can fortuitously be reversed by annealing in solvent vapor. As all films were deposited in the low Peclet-number regime, we can rule out surface skin formation. We attribute the extensive crystallization to the non-uniform drying of picoliter droplets, further promoted by repeated film swelling-deswelling cycles in multipass-ijp films.

High-performance polymer semiconducting heterostructure devices by nitrene-mediated photocrosslinking of alkyl side chains.

Heterostructures are central to the efficient manipulation of charge carriers, excitons and photons for high-performance semiconductor devices. Although these can be formed by stepwise evaporation of molecular semiconductors, they are a considerable challenge for polymers owing to re-dissolution of the underlying layers. Here we demonstrate a simple and versatile photocrosslinking methodology based on sterically hindered bis(fluorophenyl azide)s. The photocrosslinking efficiency is high and dominated by alkyl side-chain insertion reactions, which do not degrade semiconductor properties. We demonstrate two new back-infiltrated and contiguous interpenetrating donor-acceptor heterostructures for photovoltaic applications that inherently overcome internal recombination losses by ensuring path continuity to give high carrier-collection efficiency. This provides the appropriate morphology for high-efficiency polymer-based photovoltaics. We also demonstrate photopatternable polymer-based field-effect transistors and light-emitting diodes, and highly efficient separate-confinement-heterostructure light-emitting diodes. These results open the way to the general development of high-performance polymer semiconductor heterostructures that have not previously been thought possible.

Role of delta-hole-doped interfaces at Ohmic contacts to organic semiconductors.

An electromodulated absorption spectroscopy study of the contact between an organic semiconductor (OSC) poly(2,5-dialkoxy-p-phenylenevinylene) and p-doped poly(3,4-ethylenedioxythiophene) electrodes of different work functions (phivac) reveals direct evidence for the formation of a hole-doped layer at the OSC interface in equilibrium with high-phivac electrodes. When the hole density at this interface exceeds a few 10(11) cm(-2), degenerate "bandlike" polaron states emerge. This appears to be crucial to furnish efficient carrier injection into the bulk of the OSC to achieve Ohmic injection. The gap measured by ultraviolet photoemission between the electrode Fermi level and the OSC transport level (typically pinned at 0.6 eV) does not reflect the true injection barrier.

Direct evidence for the role of the madelung potential in determining the work function of doped organic semiconductors.

The work function of a model degenerately doped organic semiconductor p-doped poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) can be systematically tuned over an eV-wide range by exchanging excess matrix protons with spectator cations, without altering the organic semiconductor doping level or polaron density. Ultraviolet photoelectron spectroscopy reveals this to arise not from an interface dipole, but from a bulk effect due to a shift in the Madelung potential set up by the local counter- and spectator-ion structure at the polaron sites. Electrostatic modeling of this potential is in agreement with the observed shift in carrier energetics. The spectator cations also cause a systematic shift in electron-phonon coupling and carrier delocalization, as revealed by infrared and Raman phonon modes, and charge-modulated absorption, which can be related to disorder in this potential.

Deoxidation of graphene oxide nanosheets to extended graphenites by "unzipping" elimination.

Low-temperature scanning tunneling microscopy on alkyl-surface-functionalized graphene oxide nanosheets reveals the formation of low-dimensional graphenite nanostructures with extended pi-conjugation at deoxidation temperatures above 150 degrees C. The elimination of these alkyl chains from the surface of the nanosheets does not occur uniformly, but in distinctive patterns that correspond to the formation of an underlying network of graphenite one-dimensional "tracks" and "dots." Atomic-resolution imaging of these graphenite regions reveals a defective honeycomb lattice characteristic of single-layer graphenes. These extended graphenite structures percolate the nanosheet even for moderate levels of deoxidation and regraphenization of the basal plane. The formation of extended conjugation indicates a regioselective rather than random elimination of the oxygen atoms and alkyl chains. The resultant network morphology allows bandlike transport of charge carriers across the sheets despite defects and disorder. The sub-meV apparent activation energies for the field-effect mobilities at low temperatures (70-30 K) for both electrons and holes rule out significant electron-phonon coupling. This suggests a remarkable potential for electronic applications of these solution-processable functionalized graphene oxide nanosheets.