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For understanding the operation of perovskite solar cells and light-emitting diodes, knowledge of the dielectric properties is indispensable. The dielectric properties of perovskites are frequency dependent due to the presence of moving ions, which complicates the interpretation of impedance spectra. Using Au/CsPbI2Br/Au capacitors with varied layer thickness as a model system, we demonstrate that in the dark, an extended Maxwell circuit consistently describes the impedance data. From the thickness dependence of the resistivities, both the electronic and ionic conductivities are obtained, whereas the combination of electronic and ionic capacitances with the characteristic frequencies for space-charge formation determines the ion diffusion coefficient and ion density. At low frequencies, a slow transient process with a fixed time constant of ∼0.1 s occurs, governed by the electronic conductivity, being independent of illumination strength and sample thickness. As a possible mechanism, we propose the spatial reorganization of ions within the ion accumulation layer at the electrode/perovskite interface.

Hybrid organic-inorganic perovskites are promising materials for the application in solar cells and light-emitting diodes. However, the basic current-voltage behavior for electrons and holes is still poorly understood in these semiconductors due to their mixed electronic-ionic character. Here, we present the analysis of space-charge-limited electron and hole currents in the archetypical perovskite methyl ammonium lead iodide (MAPbI3). We demonstrate that the frequency dependence of the permittivity plays a crucial role in the analysis of space-charge-limited currents and their dependence on voltage scan rate and temperature. Using a mixed electronic-ionic device model based on experimentally determined parameters, the current-voltage characteristics of single-carrier devices are accurately reproduced. Our results reveal that in our solution processed MAPbI3 thin films transport of electrons dominates over holes. Furthermore, we show that the direction of the hysteresis in the current-voltage characteristics provides a fingerprint for the sign of the dominant moving ionic species., Space-charge-limited currents are widely used to characterize charge transport in semiconductors. Here, the authors characterize space-charge-limited electron and hole currents in metal-halide perovskites, applicable in emerging solar cells. The currents are strongly influenced by the high permittivity and ion motion.

In amorphous organic semiconductor devices, electrons and holes are transported through layers of small organic molecules or polymers. The overall performance of the device depends both on the material and the device configuration. Measuring a single device configuration requires a large effort of synthesizing the molecules and fabricating the device, rendering the search for promising materials in the vast molecular space both nontrivial and time-consuming. This effort could be greatly reduced by computing the device characteristics from the first principles. Here, we compute transport characteristics of unipolar single-layer devices of prototypical hole- and electron-transporting materials, N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) and 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) using a first-principles multiscale approach that requires only the molecular constituents and the device geometry. This approach of generating a digital twin of the entire device can be extended to multilayer stacks and enables the computer design of materials and devices to facilitate systematic improvement of organic light-emitting diode (OLED) devices.

Electron trapping is a well-recognized issue in organic semiconductors, in particular in conjugated polymers, leading to a significant electron mobility reduction in materials with electron affinit...

The steady-state charge-carrier mobility in semiconductors can be extracted from the space-charge-limited current in single-carrier devices. However, in many cases, a built-in voltage is present, which should be known accurately to obtain the correct mobility. Here, it is demonstrated that band bending at the injecting electrode has important consequences for the built-in voltage and the analytical description of the current-voltage characteristics. It is shown that the built-in voltage can be accurately determined from the slope of the current-voltage characteristics on a semilogarithmic scale. Knowing the effect of band bending on the injected current, a simple analytical equation for the drift-diffusion space-charge-limited current above the built-in voltage is derived, which allows for improved determination of the charge-carrier mobility from experimental data.

This perspective article discusses the various contributions to the external quantum efficiency (EQE) of polymer light-emitting diodes (PLEDs). The EQE of an organic light-emitting diode is governed by a number of parameters, such as the electrical efficiency, the photoluminescence quantum yield (PLQY), the optical outcoupling efficiency and the spin statistics for singlet exciton generation. In the last decade the electrical efficiency has been determined from a numerical PLED device model. More recently, an optical model to simulate the fraction of photons outcoupled to air for PLEDs with a broad recombination zone has been developed. Together with the directly measured PLQY the EQE of a PLED can then be estimated. However, it has been observed that the measured EQEs of fluorescent PLEDs, including the model system super-yellow poly(p-phenylene vinylene) (SY-PPV) often exceed the expected values. To solve this discrepancy, we demonstrate that the electrical PLED model has to be expanded by the inclusion of triplet-triplet annihilation (TTA), which is shown to be responsible for a substantial EQE enhancement. Experimentally, it is obtained that TTA contributes to a singlet-exciton generation efficiency of ∼40% in SY-PPV PLEDs, giving rise to an EQE of ∼4% instead of the expected value of 2.5%. This article is protected by copyright. All rights reserved.

The incorporation of electron-accepting units into π-conjugated systems is well-established as a powerful approach to tune the physical properties and frontier orbital energy levels of molecules. To realize semiconductors with novel functions, naphtho[1,2-c:5,6-c’]bis[1,2,5]thiadiazole (NTz) has been utilized as an effective electron-accepting unit. To enhance the electron-accepting nature of NTz in this work, the synthesis of fluorinated naphthobisthiadiazole (FNTz) was accomplished by the sequential introduction of amino functional groups into 1,5-difluoronapthalene derivatives to form fluorinated tetraaminonapthalene derivatives, followed by the formation of thiadiazole rings. Organic solar cells based on our synthesized FNTz acceptor in combination with poly(3-hexylthiophene) (P3HT) as a donor exhibit a significant improvement of power conversion efficiency (PCE) compared to the corresponding nonfluorinated NTz-based cells, reaching a high PCE of up to 3.12%. Investigation of the blend-film properties and device physics unambiguously reveals that the blend films based on P3HT and the FNTz-based compound shows good film morphologies and thus efficient charge generation and transport characteristics. These results demonstrate the potential of FNTz for an electron-accepting unit in organic semiconductors. A procedure that adds fluorine atoms to carbon-based ring systems can make it easier to optimize low-cost solar cells. Organic photovoltaics normally contain junctions between electron-rich ‘donor’ molecules and ‘acceptor’ compounds able to accommodate negative charges. Yutaka Ie from Osaka University in Japan and colleagues have found that modifying acceptor compounds so that they capture more electrons enhances solar energy conversion efficiency in organic devices. The team accomplished this with a multi-step synthesis that converts ordinary naphthalene rings into rings containing fluorine, the strongest electron-attracting element in the periodic table, nitrogen and sulfur. Combined microscopy and photovoltaic measurements revealed that the fluorine atoms, which impart Teflon-like properties to many molecules, helped the new acceptor blend smoothly with donor molecules into the active layer of a thin-film device. A new electron-accepting π-conjugated compound containing fluorinated naphthobisthiadiazole (FNTz) is designed and synthesized for application as an acceptor in organic solar cells (OSCs). Physical measurements show that the introduction of fluorine atoms in the naphthobisthiadiazole (NTz) unit has considerable influence on the absorption behavior and frontier orbital energy levels of molecules. In OSCs, the FNTz-based acceptor in combination with poly(3-hexylthiophene) (P3HT) as a donor exhibits a significant improvement in power conversion efficiency compared to the corresponding nonfluorinated NTz-based acceptor. This study demonstrates the potential of FNTz as an electron-accepting unit in organic semiconductors.

Polymer light-emitting diodes (PLEDs) are attractive for use in large-area displays and lighting panels, but their limited stability under current stress impedes commercialization. In spite of large efforts over the last two decades a fundamental understanding of the degradation mechanisms has not been accomplished. Here we demonstrate that the voltage drift of a PLED driven at constant current is caused by the formation of hole traps, which leads to additional non-radiative recombination between free electrons and trapped holes. The observed trap formation rate is consistent with exciton-free hole interactions as the main mechanism behind PLED degradation, enabling us to unify the degradation behaviour of various poly(p-phenylene) derivatives. The knowledge that hole trap formation is the cause of PLED degradation means that we can suppress the negative effect of hole traps on voltage and efficiency by blending the light-emitting polymer with a large-bandgap semiconductor. Owing to trap-dilution these blended PLEDs show unprecedented stability.

Amorphous small-molecule organic materials are utilized in organic light emitting diodes (OLEDs), with device performance relying on appropriate chemical design. Due to the vast number of contending materials, a symbiotic experimental and simulation approach would be greatly beneficial in linking chemical structure to macroscopic material properties. We review simulation approaches proposed for predicting macroscopic properties. We then present a library of OLED hosts, containing input files, results of simulations, and experimentally measured references of quantities relevant to OLED materials. We find that there is a linear proportionality between simulated and measured glass transition temperatures, despite a quantitative disagreement. Computed ionization energies are in excellent agreement with the ultraviolet photoelectron and photoemission spectroscopy in air measurements. We also observe a linear correlation between calculated electron affinities and ionization energies and cyclic voltammetry measurements. Computed energetic disorder correlates well with thermally stimulated luminescence measurements and charge mobilities agree remarkably well with space charge–limited current measurements. For the studied host materials, we find that the energetic disorder has the greatest impact on the charge carrier mobility. Our library helps to swiftly evaluate properties of new OLED materials by providing well-defined structural building blocks. The library is public and open for improvements. We envision the library expanding and the workflow providing guidance for future OLED material design.

From a design, optimization and fabrication perspective, an organic light-emitting diode consisting of only one single layer of a neat semiconductor would be highly attractive. Here, we demonstrate an efficient and stable organic light-emitting diode based on a single layer of a neat thermally activated delayed fluorescence emitter. By employing ohmic electron and hole contacts, charge injection is efficient and the absence of heterojunctions results in an exceptionally low operating voltage of 2.9 V at a luminance of 10,000 cd m−2. Balanced electron and hole transport results in a maximum external quantum efficiency of 19% at 500 cd m−2 and a broadened emission zone, which greatly improves the operational stability, allowing a lifetime to 50% of the initial luminance of 1,880 h for an initial luminance of 1,000 cd m−2. As a result, this single-layer concept combines high power efficiency with long lifetime in a simplified architecture, rivalling and even exceeding the performance of complex multilayer devices. Long-lived, efficient organic light-emitting diodes based on a simple design of a single layer of an active light-emitting medium sandwiched between two contacts and no additional charge injection and transport layers are reported.

Organic semiconductors, which serve as the active component in devices, such as solar cells, light-emitting diodes and field-effect transistors

Fullerenes and their derivatives are well-known electron-transport materials for organic (opto)electronic devices, but the extent to which they can transport holes is strongly disputed. To resolve the matter, this study quantifies bulk hole transport in a fullerene derivative. Remarkably, hole transport here is trap-free, with a high mobility equal to that for electrons. This demonstration that fullerenes can exhibit balanced, bipolar charge transport, which is unusual for organic semiconductors, is interesting for application in organic photovoltaics and ambipolar transistors.

Organic semiconductors, which serve as the active component in devices, such as solar cells, light-emitting diodes and field-effect transistors1, often exhibit highly unipolar charge transport, meaning that they predominantly conduct either electrons or holes. Here, we identify an energy window inside which organic semiconductors do not experience charge trapping for device-relevant thicknesses in the range of 100 to 300 nm, leading to trap-free charge transport of both carriers. When the ionization energy of a material surpasses 6 eV, hole trapping will limit the hole transport, whereas an electron affinity lower than 3.6 eV will give rise to trap-limited electron transport. When both energy levels are within this window, trap-free bipolar charge transport occurs. Based on simulations, water clusters are proposed to be the source of hole trapping. Organic semiconductors with energy levels situated within this energy window may lead to optoelectronic devices with enhanced performance. However, for blue-emitting light-emitting diodes, which require an energy gap of 3 eV, removing or disabling charge traps will remain a challenge. Water clusters induce hole traps in organic semiconductor thin films. Detrimental effects of hole and electron traps on charge transport can be avoided by using materials with ionization energy and electron affinity within an energy window of 2.4 eV.

Charge transport in the amorphous organic small molecules α-NPD (N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) and Spiro-TAD (2,2′,7,7′-tetrakis(N,N-diphenylamino)-9,9-spirobifluorene) is investigated in solution-processed films and compared to charge transport in vacuum-deposited films of the same molecule. By optimizing the solution-deposition conditions, such as solvent and concentration, equal charge-transport parameters for solution-processed and vacuum-deposited films are demonstrated. Modeling of the charge carrier transport characteristics was performed by drift-diffusion simulations. The dependence of the charge carrier mobility on temperature, carrier density, and electric field was found to be the same for vacuum deposition and solution processing. In both material processing cases, hole mobilities of 4 × 10−8 m2 V−1 s−1 for spiro-TAD and 0.9 × 10−8 m2 V−1 s−1 for α-NPD are obtained, demonstrating that solution processing can be a viable alternative to vacuum deposition in terms of charge transport.

Top-emitting organic light-emitting diodes can achieve high efficiencies due to the strong cavity effect resulting from the relatively thick semi-transparent metallic top electrode. The strong cavity resonance, however, simultaneously brings along negative side effects such as pronounced angular-dependent emission and spectral narrowing. In this work, through numerical simulations, we demonstrate that top-emitting organic light-emitting diodes using a thin Au(2 nm)/Ag(7 nm) top electrode can achieve light-outcoupling efficiency comparable to a thick silver electrode, while reducing spectral narrowing. This can be realized by tuning the organic capping layer thickness without affecting the electrical properties, which can be applied to diodes based on either intrinsic or efficiently doped charge transport layers.

To eliminate quenching of excitons at the metallic cathode of a polymer light-emitting diode (PLED) the emitting layer is separated from the cathode by a hole-blocking layer (HBL). We investigate a wide range of single-layer and bilayer PLEDs with different thicknesses consisting of a poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) emitting layer and a 20 nm poly(9,9'-dioctylfluorene) (PFO) HBL. The highest efficiency for both single-layer and bilayer devices is achieved when the total polymer layer thickness is similar to 90 nm. As a result, addition of an HBL to reduce cathode quenching is only effective when the luminescence enhancement due to microcavity effects in PLEDs is restored. The relative efficiency enhancement in bilayer devices as compared to single-layer devices varies from 283% for a 30 nm active layer to 20% for a 250 nm device. (C) 2016 Elsevier B.V. All rights reserved.

In 1962, Mark and Helfrich demonstrated that the current in a semiconductor containing traps is reduced by N/N-t(r), with N the amount of transport sites, N-t the amount of traps and r a number that depends on the trap energy distribution. For r > 1, the possibility opens that trapping effects can be nearly eliminated when N and Nt are simultaneously reduced. Solution-processed conjugated polymers are an excellent model system to test this hypothesis, because they can be easily diluted by blending them with a high-bandgap semiconductor. We demonstrate that in conjugated polymer blends with 10% active semiconductor and 90% high-bandgap host, the typical strong electron trapping can be effectively eliminated. As a result we were able to fabricate polymer light-emitting diodes with balanced electron and hole transport and reduced non-radiative trap-assisted recombination, leading to a doubling of their efficiency at nearly ten times lower material costs.

This project received funding from the European Union Horizon 2020 research and innovation programme under Grant Agreement No. 646176 (EXTMOS). D.A. thanks the BMBF grant InterPhase (FKZ 13N13661) and the European Union Horizon 2020 research and innovation programme “Widening materials models” under Grant Agreement No. 646259 (MOSTOPHOS). This research was supported by the King Abdullah University of Science and Technology (KAUST), via the Competitive Research Grants (CRG) Program. D.A. acknowledges KAUST for hosting his sabbatical.

Perovskite-based light-emitting diodes (PeLEDs) are drawing great attention due to their remarkable performance and ease of processing. Nevertheless, a critical aspect is the perovskite film formation on top of solution-processed anodes such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Use of conventional PEDOT:PSS anodes gives rise to high leakage currents that mask the hole transport properties of the perovskite semiconductor. Here, we show a feasible approach to overcome this constraint by implementing a solution-processed, self-compensated, hole-doped triarylamine-fluorene copolymer (p-pTFF-C2F5SIS) with a work function of 5.85 eV as the anode for polymer–perovskite blend LED devices. Highly efficient hole injection was obtained, near that of evaporated MoOx. Hole-only devices reveal that the hole transport in the polymer–perovskite blend is trap-limited. PeLEDs with the ultrahigh-workfunction p-pTFF-C2F5SIS anode show much lower leakage and much better stability in current-voltage and light output characteristics than those with the PEDOT:PSSH anode.

Barrier-free (Ohmic) contacts are a key requirement for efficient organic optoelectronic devices, such as organic light-emitting diodes, solar cells, and field-effect transistors. Here, we propose a simple and robust way of forming an Ohmic hole contact on organic semiconductors with a high ionization energy (IE). The injected hole current from high-work-function metal-oxide electrodes is improved by more than an order of magnitude by using an interlayer for which the sole requirement is that it has a higher IE than the organic semiconductor. Insertion of the interlayer results in electrostatic decoupling of the electrode from the semiconductor and realignment of the Fermi level with the IE of the organic semiconductor. The Ohmic-contact formation is illustrated for a number of material combinations and solves the problem of hole injection into organic semiconductors with a high IE of up to 6 eV. It is shown that Ohmic contacts for the injection of hole carriers into organic semiconductors with high ionization energy can be formed by adding ultrathin interlayers with higher ionization energy.

Negative differential capacitance (NC) occurring at low frequencies in organic light-emitting diodes (OLEDs) is a poorly understood phenomenon. We study the origin of the NC effect by systematically varying the number of electron traps in OLEDs based on the polymeric semiconductor poly(p-phenylene vinylene). Increasing the electron trap density enhances the NC effect. The magnitude and observed decrease of the relaxation time is consistent with the (inverse) rate of trap-assisted recombination. The absence of NC in a nearly trap-free light-emitting diode unambiguously shows that trap-assisted recombination is the responsible mechanism for the negative contribution to the capacitance in bipolar organic diodes. Our results reveal that the NC effect can be exploited to quantitatively determine the number of traps in organic semiconductors in a nondestructive fashion.

Advanced science 4(5), 1600393 (2017). doi:10.1002/advs.201600393, Published by Wiley-VCH, Weinheim

Photosystem I (PSI) is a key element in Nature's photosynthesis and an attractive building block for bio‐inspired solar energy conversion systems. In article number 1600393, Ryan C. Chiechi, Andreas Herrmann, and co‐workers describe how phage display enables the selection of small peptides that allow almost perfect orientation of PSI on semiconductor electrodes in solid state bio‐photovoltaic devices.

Electron and hole transport properties of the organic small molecule N,N0 -Di(1-naphthyl)-N,N0 - diphenyl-(1,10 -biphenyl)-4,40 -diamine are investigated by space-charge-limited current measurements. The hole transport shows trap-free behavior with a mobility of 2.3 108m2 /Vs at vanishing carrier density and electric field. The electron transport, on the other hand, shows heavily trap-limited behavior, which leads to highly unbalanced transport. A trap concentration of 1.3 1024m3 was found by modeling the electron currents, similar to the universal trap concentration found in conjugated polymers. This indicates that electron trapping is a generic property of organic semiconductors, ranging from vacuum-deposited small-molecules to solution-processed conjugated polymers.

The incorporation of an acceptor unit into π-conjugated systems is an effective approach to tune both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of organic semiconducting materials. We report on the design of a series of fluorine-substituted benzodioxocyclohexene-annelated thiophene acceptor units and the synthesis of donor–acceptor (D–A) type copolymers based on these acceptor units and dithienosilole as a donor unit. Physical measurements of the copolymers indicate that the D–A characteristics are increased by increasing the number of introduced fluorine atoms and the HOMO and LUMO energy levels of the copolymers are fine-tuned depending on the acceptor units. Organic photovoltaics based on blend films of these D–A copolymers and [6,6]-phenyl-C71-butyric acid methyl ester show photovoltaic responses with a power conversion efficiency of up to 7.30%. Investigation of the device physics shows that the performance is mainly limited by the hole transport, which provides insight in the direction of material design toward the improvement of OPV performance. These results demonstrate the potential of fluorine-substituted benzodioxocyclohexene-annelated thiophene as an acceptor unit in organic semiconducting materials.

The large multiprotein complex, photosystem I (PSI), which is at the heart of light-dependent reactions in photosynthesis, is integrated as the active component in a solid-state organic photovoltaic cell. These experiments demonstrate that photoactive megadalton protein complexes are compatible with solution processing of organic-semiconductor materials and operate in a dry non-natural environment that is very different from the biological membrane.

This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p-phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap-free space-charge-limited conduction, with the mobility depending on the electric field and charge-carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm⁻³. The trapped electrons recombine with free holes via a non-radiative trap-assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap-assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge-carrier mobilities and amount of trapping centers known from charge-transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non-radiative trap-assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single-layer polymer LEDs.