scispace - formally typeset
Search or ask a question

Showing papers on "Organic semiconductor published in 2014"


Journal ArticleDOI
TL;DR: It is revealed that solution-processed organic-inorganic halide perovskites (CH3NH3PbX3), which demonstrated huge potential in photovoltaics, also have promising optical gain and may show electrically driven lasing.
Abstract: Low-temperature solution-processed materials that show optical gain and can be embedded into a wide range of cavity resonators are attractive for the realization of on-chip coherent light sources. Organic semiconductors and colloidal quantum dots are considered the main candidates for this application. However, stumbling blocks in organic lasing include intrinsic losses from bimolecular annihilation and the conflicting requirements of high charge carrier mobility and large stimulated emission; whereas challenges pertaining to Auger losses and charge transport in quantum dots still remain. Herein, we reveal that solution-processed organic-inorganic halide perovskites (CH 3 NH 3 PbX 3 where X = Cl, Br, I), which demonstrated huge potential in photovoltaics, also have promising optical gain. Their ultra-stable amplified spontaneous emission at strikingly low thresholds stems from their large absorption coefficients, ultralow bulk defect densities and slow Auger recombination. Straightforward visible spectral tunability (390-790 nm) is demonstrated. Importantly, in view of their balanced ambipolar charge transport characteristics, these materials may show electrically driven lasing. © 2014 Macmillan Publishers Limited.

2,691 citations


Journal ArticleDOI
TL;DR: In this paper, the state-of-the-art in organic field effect transistors (OFETs) are reviewed in light of requirements for demanding future applications, in particular active-matrix addressing for flexible organic light-emitting diode (OLED) displays.
Abstract: Over the past 25 years, organic field-effect transistors (OFETs) have witnessed impressive improvements in materials performance by 3–4 orders of magnitude, and many of the key materials discoveries have been published in Advanced Materials. This includes some of the most recent demonstrations of organic field-effect transistors with performance that clearly exceeds that of benchmark amorphous silicon-based devices. In this article, state-of-the-art in OFETs are reviewed in light of requirements for demanding future applications, in particular active-matrix addressing for flexible organic light-emitting diode (OLED) displays. An overview is provided over both small molecule and conjugated polymer materials for which field-effect mobilities exceeding > 1 cm2 V–1 s–1 have been reported. Current understanding is also reviewed of their charge transport physics that allows reaching such unexpectedly high mobilities in these weakly van der Waals bonded and structurally comparatively disordered materials with a view towards understanding the potential for further improvement in performance in the future.

1,992 citations


Journal ArticleDOI
31 Jan 2014-Science
TL;DR: The time dependence of the separation of photogenerated electron hole pairs across the donor-acceptor heterojunction in OPV model systems is reported, consistent with charge separation through access to delocalized π-electron states in ordered regions of the fullerene acceptor material.
Abstract: Understanding the charge-separation mechanism in organic photovoltaic cells (OPVs) could facilitate optimization of their overall efficiency. Here we report the time dependence of the separation of photogenerated electron hole pairs across the donor-acceptor heterojunction in OPV model systems. By tracking the modulation of the optical absorption due to the electric field generated between the charges, we measure ~200 millielectron volts of electrostatic energy arising from electron-hole separation within 40 femtoseconds of excitation, corresponding to a charge separation distance of at least 4 nanometers. At this separation, the residual Coulomb attraction between charges is at or below thermal energies, so that electron and hole separate freely. This early time behavior is consistent with charge separation through access to delocalized π-electron states in ordered regions of the fullerene acceptor material.

801 citations


Journal ArticleDOI
28 Jan 2014-ACS Nano
TL;DR: The features in the IS responses that are attributed to the ionic and electronic transport properties of the perovskite material and manifest as a slow response at low frequency and an additional RC element at intermediate frequency are explored.
Abstract: Mesoscopic solid-state solar cells based on the inorganic–organic hybrid perovskite CH3NH3PbI3 in conjunction with the amorphous organic semiconductor spiro-MeOTAD as a hole transport material (HTM) are investigated using impedance spectroscopy (IS). A model to interpret the frequency response of these devices is established by expanding and elaborating on the existing models used for the liquid and solid-state dye-sensitized solar cells. Furthermore, the influence of changing the additive concentrations of tert-butylpyridine and LiTFSI in the HTM and varying the HTM overlayer thickness on top of the sub-micrometer thick TiO2 on the extracted IS parameters is investigated. The internal electrical processes of such devices are studied and correlated with the overall device performance. In particular, the features in the IS responses that are attributed to the ionic and electronic transport properties of the perovskite material and manifest as a slow response at low frequency and an additional RC element at...

672 citations


Journal ArticleDOI
TL;DR: In this article, a survey of solution-based processing techniques for plastic electronics relevant on both the commercial and research scale and a set of strategies to control thin film morphology towards enhancing their electronic transport properties.
Abstract: While the chemical structure of organic semiconductors has an obvious effect on their proclivity for charge transport, the ways with which they are processed have a dramatic effect on the performance of plastic electronics devices incorporating them. In some cases, morphological defects and misalignment of crystalline grains can completely obscure the materials' intrinsic charge transport properties. Although some deposition methods, especially vapor-phase ones, can produce single crystals and thus avoid some of these problems, it is desirable to gain a fundamental understanding of how to improve charge transport when using solution-phase deposition techniques. In this review, we present both a survey of solution-based processing techniques for plastic electronics relevant on both the commercial and research scale and a set of strategies to control thin film morphology towards enhancing their electronic transport properties.

500 citations


Journal ArticleDOI
TL;DR: The findings demonstrate g-C3N4 can serve as a multifunctional robust photocatalyst, which could also be used in other systems such as PEC cells or coupled solar cell systems.
Abstract: For the first time, it is demonstrated that the robust organic semiconductor g-C3N4 can be integrated into a nature-inspired water splitting system, analogous to PSII and PSI in natural photosynthesis. Two parallel systems have been developed for overall water splitting under visible light involving graphitic carbon nitride with two different metal oxides, BiVO4 and WO3. Consequently, both hydrogen and oxygen can be evolved in an ideal ratio of 2:1, and evolution rates in both systems have been found to be dependent on pH, redox mediator concentration, and mass ratio between the two photocatalysts, leading to a stable and reproducible H2 and O2 evolution rate at 36 and 18 μmol h–1 g–1 from water over 14 h. Our findings demonstrate g-C3N4 can serve as a multifunctional robust photocatalyst, which could also be used in other systems such as PEC cells or coupled solar cell systems.

455 citations


Journal ArticleDOI
TL;DR: The results demonstrate the utility of this architecture for generating promising materials to be integrated into organic photovoltaic devices, call attention to areas for improvement, and provide guiding principles to sustain the steady increases necessary to move this technology forward.
Abstract: Organic semiconductors incorporated into solar cells using a bulk heterojunction (BHJ) construction show promise as a cleaner answer to increasing energy needs throughout the world. Organic solar c...

447 citations


Journal ArticleDOI
TL;DR: BTBT derivatives and their related materials, including dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), have turned out to be superior organic semiconductors, affording OFETs with very high mobilities.
Abstract: ConspectusThe design, synthesis, and characterization of organic semiconductors applicable to organic electronic devices, such as organic field-effect transistors (OFETs) and organic photovoltaics (OPVs), had been one of the most important topics in materials chemistry in the past decade. Among the vast number of materials developed, much expectation had been placed on thienoacenes, which are rigid and planar structures formed by fusing thiophenes and other aromatic rings, as a promising candidate for organic semiconductors for high-performance OFETs. However, the thienoacenes examined as an active material in OFETs in the 1990s afforded OFETs with only moderate hole mobilities (approximately 0.1 cm2 V–1 s–1). We speculated that this was due to the sulfur atoms in the thienoacenes, which hardly contributed to the intermolecular orbital overlap in the solid state. On the other hand, we have focused on other types of thienoacenes, such as [1]benzothieno[3,2-b][1]benzothiophene (BTBT), which seem to have app...

389 citations


Journal ArticleDOI
TL;DR: In this article, the preferred orientation of the transition dipole moments of heteroleptic iridium complexes (HICs) in organic light-emitting diodes (OLEDs) was investigated.
Abstract: Organic light-emitting diodes (OLEDs) are among the most promising organic semiconductor devices. The recently reported external quantum efficiencies (EQEs) of 29-30% for green and blue phosphorescent OLEDs are considered to be near the limit for isotropically oriented iridium complexes. The preferred orientation of transition dipole moments has not been thoroughly considered for phosphorescent OLEDs because of the lack of an apparent driving force for a molecular arrangement in all but a few cases, even though horizontally oriented transition dipoles can result in efficiencies of over 30%. Here we use quantum chemical calculations to show that the preferred orientation of the transition dipole moments of heteroleptic iridium complexes (HICs) in OLEDs originates from the preferred direction of the HIC triplet transition dipole moments and the strong supramolecular arrangement within the co-host environment. We also demonstrate an unprecedentedly high EQE of 35.6% when using HICs with phosphorescent transition dipole moments oriented in the horizontal direction.

326 citations


Journal ArticleDOI
TL;DR: The shape and the energy distribution of electronic states tailing into the fundamental gap is found to determine both the minimum value of practically achievable injection barriers as well as their spatial profile, ranging from abrupt interface dipoles to extended band-bending regions.
Abstract: Minimizing charge carrier injection barriers and extraction losses at interfaces between organic semiconductors and metallic electrodes is critical for optimizing the performance of organic (opto-) electronic devices. Here, we implement a detailed electrostatic model, capable of reproducing the alignment between the electrode Fermi energy and the transport states in the organic semiconductor both qualitatively and quantitatively. Covering the full phenomenological range of interfacial energy level alignment regimes within a single, consistent framework and continuously connecting the limiting cases described by previously proposed models allows us to resolve conflicting views in the literature. Our results highlight the density of states in the organic semiconductor as a key factor. Its shape and, in particular, the energy distribution of electronic states tailing into the fundamental gap is found to determine both the minimum value of practically achievable injection barriers as well as their spatial profile, ranging from abrupt interface dipoles to extended band-bending regions.

321 citations


Journal ArticleDOI
TL;DR: This article reviews the research efforts of developing N-heteropentacenes into organic semiconductors starting from 2003 with emphasis on the work of the author's group since 2009 and highlights the structure-property relationship and design rationale.
Abstract: Introducing N atoms to the pentacene backbone leads to N-heteropentacenes, whose properties can be tuned by changing the number, position and valence state of N atoms in the pentacene backbone. With a rapid development in recent years, N-heteropentacenes and their derivatives have arisen as a new family of organic semiconductors with high performance in organic thin-film transistors (OTFTs). This article reviews the research efforts of developing N-heteropentacenes into organic semiconductors starting from 2003 with emphasis on the work of the author's group since 2009. The structure-property relationship and design rationale are highlighted based on an overview of reported organic semiconductors of N-heteropentacenes.

Journal ArticleDOI
TL;DR: In this article, the performance of p-and n-type conducting polymer and small molecule organic semiconductors are reviewed primarily in terms of field effect mobility, current on/off ratio, and operating voltage for various OTFT structures.
Abstract: Organic thin film transistor (OTFT) based device modeling and circuit application is a rapidly emerging research area. Taking cognizance of this fact, our paper reviews various basic to advanced OTFT structures, their performance parameters, materials of individual OTFT layers, their molecular structures, OTFT charge transport phenomena, and fabrication techniques. The performance of p- and n-type conducting polymer and small molecule organic semiconductors are reviewed primarily in terms of field effect mobility, current on/off ratio, and operating voltage for various OTFT structures. Moreover, different organic/inorganic materials for realizing the dielectric layer, electrodes, and the substrate in an OTFT are analyzed. Some of the compact models that are essential for predicting and optimizing the device performance are described that takes into account the mobility enhancement factor and channel length modulation. A detailed study of the single gate, dual gate, vertical channel, and cylindrical gate O...

Journal ArticleDOI
TL;DR: Efficient resonant-energy transfer of molecular spin-triplet excitons from organic semiconductor to inorganic semiconductors is reported, enabling luminescent harvesting of triplet exciton energy in light-emitting structures.
Abstract: The efficient transfer of energy between organic and inorganic semiconductors is a widely sought after property, but has so far been limited to the transfer of spin-singlet excitons. Here we report efficient resonant-energy transfer of molecular spin-triplet excitons from organic semiconductors to inorganic semiconductors. We use ultrafast optical absorption spectroscopy to track the dynamics of triplets, generated in pentacene through singlet exciton fission, at the interface with lead selenide (PbSe) nanocrystals. We show that triplets transfer to PbSe rapidly (<1 ps) and efficiently, with 1.9 triplets transferred for every photon absorbed in pentacene, but only when the bandgap of the nanocrystals is close to resonance (±0.2 eV) with the triplet energy. Following triplet transfer, the excitation can undergo either charge separation, allowing photovoltaic operation, or radiative recombination in the nanocrystal, enabling luminescent harvesting of triplet exciton energy in light-emitting structures.

Journal ArticleDOI
TL;DR: In this paper, photochromic molecules can be incorporated at various interfaces of a device, including the electrode/semiconductor interface, or even as a binary mixture in the active layer in order to impart a light responsive nature to the device.
Abstract: Organic semiconductors have garnered significant interest as key components for flexible, low-cost, and large-area electronics. Hitherto, both materials and processing thereof seems to head towards a mature technology which shall ultimately meet expectations and efforts built up over the past years. However, by its own organic electronics cannot compete or complement the silicon-based electronics in integrating multiple functions in a small area unless novel solutions are brought into play. Photochromic molecules are small organic molecules able to undergo reversible photochemical isomerization between (at least) two (meta)stable states which exhibit markedly different properties. They can be embedded as additional component in organic-based materials ready to be exploited in devices such as OLEDs, OFETs, and OLETs. The structurally controlled incorporation of photochromic molecules can be done at various interfaces of a device, including the electrode/semiconductor or dielectric/semiconductor interface, or even as a binary mixture in the active layer, in order to impart a light responsive nature to the device. This can be accomplished by modulating via a light stimulus fundamental physico-chemical properties such as charge injection and transport in the device.

Journal ArticleDOI
TL;DR: This review categorizes the three different exciton types typically encountered in organic semiconductors (Frenkel singlet, Frenkel triplet, and charge transfer) and considers the problem of a localized exciton diffusing in a disordered matrix in detail.
Abstract: The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet ↔ triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion.

Journal Article
TL;DR: In this article, a blue pigment called copper phthalocyanine, commonly used in paints and dyes, appears to satisfy this requirement and can be easily processed into a thin-film form of the type used for device fabrication.
Abstract: Spintronics devices, which exploit the intrinsic spin of electrons as well as their charge, require precise control and read-out of electron spins. For organic semiconductors to find use in spintronic applications, it is desirable to identify molecules that possess long spin relaxation times. This paper establishes that copper phthalocyanine, a blue pigment commonly used in paints and dyes, appears to satisfy this requirement. It is inexpensive and can be easily processed into a thin-film form of the type used for device fabrication, making it a candidate material system for spin-based quantum information processing and other spintronic applications.

Journal ArticleDOI
TL;DR: N-shaped organic semiconductors are synthesized via four steps from a readily available starting material and exhibit preferable ionization potential for p-type operation, thermally stable crystalline phase over 200 °C, and high carrier mobility with small threshold voltages in solution-crystallized field-effect transistors.
Abstract: N-shaped organic semiconductors are synthesized via four steps from a readily available starting material. Such semiconductors exhibit preferable ionization potential for p-type operation, thermally stable crystalline phase over 200 °C, and high carrier mobility up to 16 cm(2) V(-1) s(-1) (12.1 cm(2) V(-1) s(-1) on average) with small threshold voltages in solution-crystallized field-effect transistors.

Journal ArticleDOI
TL;DR: 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.
Abstract: 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.

Journal ArticleDOI
TL;DR: In this article, an overview of recent advances in development of high performance imide/amide-based organic semiconductors for field effect transistors is given, covering naphthalene diimide-, perylene diimides-and amide-based conjugated molecules and polymers for organic semiconductor.
Abstract: Due to their high electron affinities, chemical and thermal stabilities, π-conjugated molecules with imide/amide frameworks have received considerable attentions as promising candidates for high-performance optoelectronic materials, particularly for organic semiconductors with high carrier mobilities. The purpose of this Research News is to give an overview of recent advances in development of high performance imide/amide based organic semiconductors for field-effect transistors. It covers naphthalene diimide-, perylene diimide- and amide-based conjugated molecules and polymers for organic semiconductors.

Journal ArticleDOI
TL;DR: In this paper, the authors reported a Rabi energy splitting (2ℏΩR) of 1.12 eV and record values of the coupling ratio up to 0.6-fold the material band gap in organic semiconductor microcavities and up to 1.5-fold in monolithic heterostructure organic light-emitting diodes working at room temperature.
Abstract: Exciton-polaritons are bosonic quasiparticles that arise from the normal mode splitting of photons in a microcavity and excitons in a semiconductor material. One of the most intriguing extensions of such a light–matter interaction is the so-called ultrastrong coupling regime. It is achieved when the Rabi frequency (ΩR, the energy exchange rate between the emitter and the resonant photonic mode) reaches a considerable fraction of the emitter transition frequency, ω0. Here, we report a Rabi energy splitting (2ℏΩR) of 1.12 eV and record values of the coupling ratio (2ΩR/ω0) up to 0.6-fold the material band gap in organic semiconductor microcavities and up to 0.5-fold in monolithic heterostructure organic light-emitting diodes working at room temperature. Furthermore, we show that with such a large coupling strength it is possible to undress the exciton homogeneous linewidth from its inhomogeneous broadening, which allows for an unprecedented narrow emission line (below the cavity finesse) for such organic LE...

Journal ArticleDOI
TL;DR: It is demonstrated that the nanoconfinement effect combined with the flow-enhanced crystal engineering technique is a powerful and likely material-agnostic method to identify existing polymorphs in OSC materials and to prepare the individual pure forms in thin films at ambient conditions.
Abstract: Understanding crystal polymorphism is a long-standing challenge relevant to many fields, such as pharmaceuticals, organic semiconductors, pigments, food, and explosives. Controlling polymorphism of organic semiconductors (OSCs) in thin films is particularly important given that such films form the active layer in most organic electronics devices and that dramatic changes in the electronic properties can be induced even by small changes in the molecular packing. However, there are very few polymorphic OSCs for which the structure–property relationships have been elucidated so far. The major challenges lie in the transient nature of metastable forms and the preparation of phase-pure, highly crystalline thin films for resolving the crystal structures and evaluating the charge transport properties. Here we demonstrate that the nanoconfinement effect combined with the flow-enhanced crystal engineering technique is a powerful and likely material-agnostic method to identify existing polymorphs in OSC materials a...

Journal ArticleDOI
TL;DR: In this paper, the macroscopic electrical field distribution is determined using capacitance-voltage and Kelvin probe techniques, and charge collection properties are then accessible relying on the relative weight that diffusion and drift have as carrier transport driven forces.
Abstract: Hybrid lead halide perovskites (PVKs) have emerged as novel materials for photovoltaics and have rapidly reached very large solar to electricity power conversion efficiencies. As occurring with other kind of solar technologies establishing the working energy-band diagram constitutes a primary goal for device physics analysis. Here, the macroscopic electrical field distribution is experimentally determined using capacitance-voltage and Kelvin probe techniques. Planar structures comprising CH3NH3PbI3−xClx PVK exhibit p-doping character and form a p-n heterojunction with n-doped TiO2 compact layers. Depletion width at equilibrium within the PVK bulk has an extent about 300 nm (approximately half of the layer thickness), leaving as a consequence a significant neutral zone towards the anode contact. Charge collection properties are then accessible relying on the relative weight that diffusion and drift have as carrier transport driven forces.

Journal ArticleDOI
TL;DR: This mini review will highlight the representative development of high performance n-type and ambipolar organic semiconductors (OSCs) especially for those n- type small OSCs with thin film mobilities >1 cm(2) V(-1) s(-1).
Abstract: Remarkable progress has recently been achieved in n-type and ambipolar OFETs. In this mini review, we will highlight the representative development of high performance n-type and ambipolar organic semiconductors (OSCs) especially for those n-type small OSCs with thin film mobilities >1 cm2 V−1 s−1, and ambipolar small OSCs with both hole and electron mobilities of over 0.1 cm2 V−1 s−1. This overview shall provide a meaningful guideline for further development of high performance n-type and ambipolar materials and devices.

01 Jan 2014
TL;DR: An overview of recent advances in development of high performance imide/amide based organic semiconductors for field-effect transistors for organic semiconductor devices is given.
Abstract: Due to their high electron affi nities, chemical and thermal stabilities, π-conjugated molecules with imide/amide frameworks have received considerable attentions as promising candidates for high-performance optoelectronic materials, particularly for organic semiconductors with high carrier mobilities. The purpose of this Research News is to give an overview of recent advances in development of high performance imide/amide based organic semiconductors for fi eld-effect transistors. It covers naphthalene diimide-, perylene diimide- and amide-based conjugated molecules and polymers for organic semiconductors.

Journal ArticleDOI
TL;DR: The potential of engineering the vacuum electromagnetic environment to modify and to improve properties of materials is illustrated, by injecting carriers into states that are hybridized to the vacuumagnetic field.
Abstract: Organic semiconductors have generated considerable interest for their potential for creating inexpensive and flexible devices easily processed on a large scale [1-11]. However technological applications are currently limited by the low mobility of the charge carriers associated with the disorder in these materials [5-8]. Much effort over the past decades has therefore been focused on optimizing the organisation of the material or the devices to improve carrier mobility. Here we take a radically different path to solving this problem, namely by injecting carriers into states that are hybridized to the vacuum electromagnetic field. These are coherent states that can extend over as many as 10^5 molecules and should thereby favour conductivity in such materials. To test this idea, organic semiconductors were strongly coupled to the vacuum electromagnetic field on plasmonic structures to form polaritonic states with large Rabi splittings ca. 0.7 eV. Conductivity experiments show that indeed the current does increase by an order of magnitude at resonance in the coupled state, reflecting mostly a change in field-effect mobility as revealed when the structure is gated in a transistor configuration. A theoretical quantum model is presented that confirms the delocalization of the wave-functions of the hybridized states and the consequences on the conductivity. While this is a proof-of-principle study, in practice conductivity mediated by light-matter hybridized states is easy to implement and we therefore expect that it will be used to improve organic devices. More broadly our findings illustrate the potential of engineering the vacuum electromagnetic environment to modify and to improve properties of materials.

Journal ArticleDOI
TL;DR: A novel stable bisazide molecule that can freeze the bulk heterojunction morphology at its optimized layout by specifically bonding to fullerenes is reported.
Abstract: level, and operational stability. [ 3 ] Most of BHJ photoactive blends are composed of a mixture of an electron-donor polymer and an electron-accepor fullerene derivative, where the latter material is typically a soluble C 60-fullerene (PC 61 BM) or C 70-fullerene (PC 71 BM) derivative (Figure 1). The BHJ layer is sandwiched between charge carrier selective interlayers and the electrodes. The bottom electrode is typically indium tin oxide (ITO) or other transparent conductors. Interlayers choice governs the polarity of the photovoltaic cells. Metal oxides such as TiO x or ZnO are commonly used as electron selective layer whereas MoO x or conducting polymers (PEDOT:PSS) are used as hole transporting layers. An optimised BHJ layer requires specifi c phase segregation of the BHJ donor-acceptor components to allow optimum charge carrier photogeneration in the blend and charge perco-lation pathways for effi cient electron and hole collection to the respective electrodes. An important morphological parameter of the BHJ blend to achieve large PCEs is that nano-sized fullerene crystallites are necessary within the polymer matrix to prevent electron-hole recombination mechanisms. [ 4–7 ] Thus, the domain size must be in the order of the excitons diffusion length, which typically ranks from 3 to 30 nm. [ 8 ] Such optimal polymer-fullerene blend morphology is achieved with a different efficiency depending on the material combinations. [ 9 ] Optimized phase segregation can be promoted using appropriate solvent(s) and/or specifi c solvent additives during blend deposition as well as post-deposition fi lm processing such as thermal or solvent annealing. [ 10 ] Semicrystalline polymers such as poly(3-hexylth-iophene) (P3HT, Figure 1) tend to expel fullerenes during their crystallization into nano-objects upon drying of the solvent or during post-fi lm deposition thermal annealing. This property enabled to fi nely tune P3HT:PCBM blend morphology and led to a tremendous amount of data concerning OPV cells based on this specifi c polymer. [ 11 ] However, P3HT cells are severely limited in terms of the maximum achievable PCEs. Therefore, low band gap polymers, which can harvest a larger portion of the solar spectrum, were developed to reach greater performances. Unfortunately, several of these high-potential polymers are less crystalline and do not have such a strong tendency for molecular organization. As a consequence, manipulating BHJ morphology of less crystalline/amorphous polymers is not trivial. Solvent additives, such as 1,8-diiodooctane or 1,8-octanedithiol for example, enable to preferentially solvate fullerene derivatives rather than the polymer, were chosen to tune BHJ morphology and achieve effi ciencies >9%. Thus, the major problem that the The use of a bulk heterojunction (BHJ) blend of an electron-donor and an electron-acceptor organic semiconductors to fabricate photovoltaic solar cells and to understand fundamental light-to-charge phenomena in organic solids has attracted the interest of the international scientifi c community for the last 20 years. These efforts recently led to the demonstration of lab-scale organic photovoltaic (OPV) cells with power conversion effi ciencies (PCE) of 9.2% and 10.6% for single cells [ 1 ] and tandem cells [ 2 ] confi gurations, respectively. OPV cells are becoming a credible revolutionary thin-fi lm photovoltaic technology with advantages such as lightweight, mechanical fl exi-bility, roll-to-roll large area and low-cost solar module production. The three major challenges to OPV module realization are the cost of the active/encapsulation layers, effi ciency at module

Journal ArticleDOI
TL;DR: A solution-processable combination of ZnO and PEI is reported, that facilitates electron injection and enables efficient and air-stable inverted devices.
Abstract: Due to their planar nature and their mechanical fl exibility, organic light emitting diodes (OLEDs) open pathway to many new applications in display technology, signage and general lighting. While state-of-the-art OLEDs are mostly fabricated by thermal evaporation in vacuum, solution processing (printing and coating) is widely discussed for future cost reduction in OLED fabrication. So far, vacuum processed devices outperform their solution processed counterparts not least because thermal evaporation allows for stacking an arbitrary number of functional layers while solution deposition is ruled by solvent limitations. In vacuum processed OLEDs, charge carrier injection can be facilitated by utilizing electrically doped organic semiconductor layers. [ 1‐3 ] Alternatively, high-work function metal oxides such as molybdenum oxide (MoO 3 ), [ 4,5 ] tungsten oxide (WO 3 ) [ 6,7 ] or vanadium pentoxide (V 2 O 5 ) [ 8,9 ] can be used for the injection of holes. Recently, we have demonstrated that MoO 3 or WO 3 hole injection layers for OLEDs can also be applied via a facile precursor route from molybdenum(V)ethoxide, tungsten(V)ethoxide or tungsten(VI)ethoxide, respectively. [ 10,11 ] On the other hand, electron injection into the lowest unoccupied molecular orbital (LUMO) level of light emitting or adjacent electron transport materials is much more challenging since it requires low-work function and hence reactive electrodes, interface modifi ers or buffer layers such as calcium,

Journal ArticleDOI
TL;DR: In this article, the state-of-the-art in the field of crystal laser devices is reviewed and an outlook and personal view is provided on the further developments of laser crystals and their applications.
Abstract: Because of long-range order and high chemical purity, organic crystals have exhibit unique properties and attracted a lot of interest for application in solid-state lasers. As optical gain materials, they exhibit high stimulated emission cross section and broad tunable wavelength emission as similar to their amorphous counterpart; moreover, high purity and high order give them superior properties such as low scattering trap densities, high thermal stability, as well as highly polarized emission. As electronic materials, they are potentially able to support high current densities, thus making it possible to realize current driven lasers. This paper mainly describes recent research progress in organic semiconductor laser crystals. The building molecules, crystal growth methods, as well as their stimulated emission characteristics related with crystal structures are introduced; in addition, the current state-of-the-art in the field of crystal laser devices is reviewed. Furthermore, recent advances of crystal lasers at the nanoscale and single crystal light-emitting transistors (LETs) are presented. Finally, an outlook and personal view is provided on the further developments of laser crystals and their applications.

Journal ArticleDOI
TL;DR: In this article, the characteristics of organic single-crystal light-emitting field effect transistors (OSCLEFETs) have been discussed, and various single crystal growth methods that produce different morphologies and geometries of crystals are described.
Abstract: Growth and characterisation of single crystals constitute a major field of materials science. In this feature article we overview the characteristics of organic single-crystal light-emitting field-effect transistors (OSCLEFETs). The contents include the single crystal growth of organic semiconductors and their application to transistor devices. First, we describe various single crystal growth methods that produce different morphologies and geometries of crystals. Using these single crystals we highlight construction and performance of the devices. The single crystal approach not only allows us to study the device performance that reflects the intrinsic nature of the organic semiconductors but also is advantageous to enhancement in the steady device operation. A current-injected laser oscillation in an electronic device configuration remains as a big challenge to be achieved. In this context we briefly mention the spectrally narrowed emissions as well as the possibility of light amplification in the OSCLEFETs.

Journal ArticleDOI
TL;DR: In this paper, a novel technique based upon injection-charge extraction by linearly increasing voltage (i-CELIV) in a metal-insulator-semiconductor (MIS) diode structure is described for studying charge transport in organic semiconductors.
Abstract: A novel technique based upon injection-charge extraction by linearly increasing voltage (i-CELIV) in a metal-insulator-semiconductor (MIS) diode structure is described for studying charge transport in organic semiconductors. The technique (MIS-CELIV) allows selective measurement of both electron and hole mobilities of organic solar cells with active layers thicknesses representative of operational devices. The method is used to study the model high efficiency bulk heterojunction combination poly[N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) at various blend ratios. The absence of bipolar transport in PCDTBT-and-PC70BM-only diodes is shown and strongly imbalanced carrier mobility is found in the most efficient optimized blend ratios. The mobility measurements are correlated with overall device performance and it is found that balanced and high charge carrier mobility are not necessarily required for high efficiencies in thin film organic solar cells.