Charge Injection Barrier Research Articles

Fully Solution‐Processed Semi‐Transparent Perovskite Solar Cells With Ink‐Jet Printed Silver Nanowires Top Electrode

Owing to the sensitivity of the perovskite thin film to solvent, preparation of metal top electrode by solution process is of great challenging. This is the key technology for the realization of fully solution processed perovskite solar cells. In this paper, we report the preparation of transparent silver nanowires (AgNW) top electrode for perovskite solar cells using inkjet printing process. Experiment results demonstrate that low device performance with low fill factor was obtained when the AgNW is directly printed onto the PC61BM layer. This is ascribed to the mismatched work functions of the AgNW electrode and PC61BM layer, and the solvent assisted chemical corrosion of the AgNW electrode by halogen anions. By inserting a thin layer of polyethylenimine (PEI), the charge injection barrier between PC61BM and AgNW electrode was minimized. More importantly, such a thin PEI layer suppresses the chemical corrosion of AgNW electrode during printing, yielding a condensed and uniform AgNW networks. The introduction of a thin PEI layer greatly improves the device performance and stability. A high power conversion efficiency of 14.17% with an averaged light transmittance of 21.2% was achieved for the PEI/AgNW cells. In addition, improved performance stability was measured for the PEI/AgNW cells.

Revealing the Working Mechanisms of Planar Perovskite Solar Cells With Cross-Sectional Surface Potential Profiling

With the rapid improvement in the efficiencies of perovskite solar cells (PSCs), understanding their working mechanisms becomes more and more critical for their further optimization. In this work, Kelvin probe force microscopy (KPFM) is utilized to characterize the surface potential (SP) distribution on the cross-sectional surface of planar PSCs. Devices with regular structure Indium Tin Oxide (ITO)/ZnO/perovskite/2,2′,7,-7′-tetrakis(N,N-di-p-metho-xyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD)/Au and inverted structure Fluorine doped Tin Oxide (FTO)/nickel oxide ( $\text{NiO}_{x}$ )/perovskite/ZnO/Ag are studied. The analysis of KPFM measurement indicates that, while p-n junction forms at the perovskite/ZnO interface in regular planar PSCs, p-i-n junction exists in inverted planar PSCs with electric field peaks at perovskite/ $\text{NiO}_{x}$ and perovskite/ZnO interfaces. In addition, charge injection barriers in planar PSCs are revealed with relative SP profiles obtained under different biases. In regular PSCs, charge injection barriers are only observed at the perovskite/ZnO interface and the contact between perovskite and spiro-OMeTAD is an ideal ohmic contact, while in inverted PSCs, charge injection barriers are observed at both perovskite/ZnO and perovskite/ $\text{NiO}_{x}$ contacts. The appearance of additional charge injection barriers in inverted PSCs exacerbates the charge recombination in these devices and leads to their relatively low open-circuit voltages. Our result indicates that the inefficient charge extraction is a major problem affecting the performance of planar PSCs.

Light-Induced Anion Phase Segregation in Mixed Halide Perovskites

Hybrid lead halide perovskites such as MAPbI3 (MA = CH3NH3+) and their mixed halide analogues represent an emerging class of materials for solar energy conversion. Intriguing aspects include sizable carrier diffusion lengths, large optical absorption coefficients, and certified power conversion efficiencies that now exceed 22%. Halide-composition-tunable band gaps also make MAPb(I1–xBrx)3 systems ideal candidates for tandem solar cells. Unfortunately, preventing the effective integration of MAPb(I1–xBrx)3 into working devices are intrinsic instabilities due to light-induced halide phase segregation. Namely, under illumination, mixed halide perovskites reversibly segregate into low-band-gap I-rich and high-band-gap Br-rich domains. Under electrical bias, halide migration has also been proposed as the source of undesirable charge injection barriers that degrade photovoltaic performance. In this Perspective, we review the origin of light-induced halide phase segregation, its effects on photovoltaic response, and ongoing research to suppress its influence on the optical and electronic response of mixed halide perovskites.

Narrow Band Gap Conjugated Polyelectrolytes.

Two essential structural elements define a class of materials called conjugated polyelectrolytes (CPEs). The first is a polymer framework with an electronically delocalized, π-conjugated structure. This component allows one to adjust desirable optical and electronic properties, for example the range of wavelengths absorbed, emission quantum yields, electron affinity, and ionization potential. The second defining feature is the presence of ionic functionalities, which are usually linked via tethers that can modulate the distance of the charged groups relative to the backbone. These ionic groups render CPEs distinct relative to their neutral conjugated polymer counterparts. Solubility in polar solvents, including aqueous media, is an immediately obvious difference. This feature has enabled the development of optically amplified biosensor protocols and the fabrication of multilayer organic semiconductor devices through deposition techniques using solvents with orthogonal properties. Important but less obvious potential advantages must also be considered. For example, CPE layers have been used to introduce interfacial dipoles and thus modify the effective work function of adjacent electrodes. One can thereby modulate the barriers for charge injection into semiconductor layers and improve the device efficiencies of organic light-emitting diodes and solar cells. With a hydrophobic backbone and hydrophilic ionic sites, CPEs can also be used as dispersants for insoluble materials. Narrow band gap CPEs (NBGCPEs) have been studied only recently. They contain backbones that comprise electron-rich and electron-poor fragments, a combination that leads to intramolecular charge transfer excited states and enables facile oxidation and reduction. One particularly interesting combination is NBGCPEs with anionic sulfonate side groups, for which spontaneous self-doping in aqueous media is observed. That no such doping is observed with cationic NBGCPEs indicates that the interplay between electrostatic forces and the redox chemistry of the organic semiconducting chain is essential for stabilizing the polaronic states and increasing the conductivity of the bulk. Capitalizing upon the properties of NBGCPEs has resulted in a range of new applications. When doped, they can be introduced as interlayers in organic and perovskite solar cells. Single-walled carbon nanotubes can be n- or p-doped with NBGCPEs, depending on whether the same backbone contains attached cationic or anionic side groups, respectively. The resulting dispersions can be used to fabricate flexible thermoelectric devices in which the n- and p-semiconductor legs are nearly identical in terms of chemical composition. Electrostatic interactions with negatively charged cell walls, in combination with the long-wavelength absorption and high photothermal efficiencies, have been used to create effective agents for photothermal killing of bacteria. Additionally, recent results have shown that cationic NBGCPEs can effectively n-dope graphene and that this doping is temperature-dependent. The preferential charge carriers can therefore be chosen to be electrons or holes depending on the applied temperature.

Interface-Engineered Charge-Transport Properties in Benzenedithiol Molecular Electronic Junctions via Chemically p-Doped Graphene Electrodes.

In this study, we fabricated and characterized vertical molecular junctions consisting of self-assembled monolayers of benzenedithiol (BDT) with a p-doped multilayer graphene electrode. The p-type doping of a graphene film was performed by treating pristine graphene (work function of ∼4.40 eV) with trifluoromethanesulfonic (TFMS) acid, producing a significantly increased work function (∼5.23 eV). The p-doped graphene-electrode molecular junctions statistically showed an order of magnitude higher current density and a lower charge injection barrier height than those of the pristine graphene-electrode molecular junctions, as a result of interface engineering. This enhancement is due to the increased work function of the TFMS-treated p-doped graphene electrode in the highest occupied molecular orbital-mediated tunneling molecular junctions. The validity of these results was proven by a theoretical analysis based on a coherent transport model that considers asymmetric couplings at the electrode-molecule interfaces.

In-situ analysis of microwave conductivity and impedance spectroscopy for evaluation of charge carrier dynamics at interfaces

A unique concerted analysis comprising non-contact microwave conductivity measurements and impedance spectroscopy was developed to simultaneously assess the charge carrier mobility and injection barriers. The frequency dependence of the microwave conductivity as well as the electrical current was analyzed by applying sinusoidal voltage to determine the equivalent circuit parameters. Based on the temperature dependence of the circuit parameters, the energy of the injection barrier was estimated to be 0.4 eV with the Richardson–Schottky model, and the band-like transport was confirmed with the negative temperature coefficient with the β value of 1.4 in the intra-layer conduction of C8-BTBT. In contrast, the increase in the resistance of the C8-BTBT layer with decreasing temperature implied the occurrence of hopping-like transport in the inter-layer conduction of C8-BTBT.

Patterned water dispersible conducting polymer electrode in organic thin film transistor through a parylene lift-off process

Availability of non-destructive micro-patterning techniques is essential for the realization of organic electronic technologies that are compatible with chemically sensitive materials. Organic thin film transistors (OTFTs) with conductive polymer composite poyaniline:polystyrene sulphonic acid (PANi-PSS) electrodes that are fabricated by a simple modified Parylene lift-off process is reported. The functionality of the conducting and semiconducting material is not affected by this technique and it is fully amenable for patterning on a flexible substrate. This patterning technique for conducting polymer electrode, without exposure to harmful solvents, results in a resolution of 5μm. Water dispersible nanostructured conducting PANi-PSS composite for patterning was synthesized by bulk polymerization of aniline templated with polystyrene sulphonic acid. For the formation of stable dispersion of PANi-PSS with desired conductivity, concentration of the precursor acid used in the synthesis was optimized. The conductivity of films thus obtained from a stable dispersion was increased by more than two orders of magnitude by secondary doping with dichloroacetic acid. Elucidation of the mechanism by which this conductivity improvement occurs was done using Ultraviolet Visible spectroscopy and X-ray diffraction analysis. Morphology characterization of the synthesized nanoparticles and thin films in OTFT was carried out using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Electrical characteristics of bottom contact pentacene TFTs with PANi-PSS electrodes and pentacene active material is superior to those with palladium electrodes due to a lower charge injection barrier. Contact resistance was extracted in both the cases by Transfer Line Method (TLM).

Orientation-Dependent Work-Function Modification Using Substituted Pyrene-Based Acceptors.

The adsorption of molecular acceptors is a viable method for tuning the work function of metal electrodes. This, in turn, enables adjusting charge injection barriers between the electrode and organic semiconductors. Here, we demonstrate the potential of pyrene-tetraone (PyT) and its derivatives dibromopyrene-tetraone (Br-PyT) and dinitropyrene-tetraone (NO2-PyT) for modifying the electronic properties of Au(111) and Ag(111) surfaces. The systems are investigated by complementary theoretical and experimental approaches, including photoelectron spectroscopy, the X-ray standing wave technique, and density functional theory simulations. For some of the investigated interfaces the trends expected for Fermi-level pinning are observed, i.e., an increase of the metal work function along with increasing molecular electron affinity and the same work function for Au and Ag with monolayer acceptor coverage. Substantial deviations are, however, found for Br-PyT/Ag(111) and NO2-PyT/Ag(111), where in the latter case an adsorption-induced work function increase of as much as 1.6 eV is observed. This behavior is explained as arising from a face-on to edge-on reorientation of molecules in the monolayer. Our calculations show that for an edge-on orientation much larger work-function changes can be expected despite the prevalence of Fermi-level pinning. This is primarily ascribed to a change of the electron affinity of the adsorbate layer that results from a change of the molecular orientation. This work provides a comprehensive understanding of how changing the molecular electron affinity as well as the adsorbate structure impacts the electronic properties of electrodes.

Chemisorption of Pentacene on Pt(111) with a Little Molecular Distortion

We investigated the adsorption of pentacene on the (111) surface of platinum, which is an archetypal system for a junction with a low charge-injection barrier. We probed the structural and electronic configurations of pentacene by scanning tunnelling microscopy (STM), X-ray photoemission spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. We simulated the interface by means of ab initio methods based on the density functional theory (DFT) framework, while including the dispersion forces. We found that the molecules adsorb at the bridge site of the close-compact atom rows with the long axis parallel to the substrate’s <110> directions, in a slightly distorted geometry, driven by the good match between the position of the carbon atoms of the molecule and the underlying lattice of the surface. Most importantly, a chemical bond is formed at the interface, which we attribute to the high chemical reactivity of the Pt substrate.

Determination of charge transport activation energy and injection barrier in organic semiconductor devices

Charge carrier transport in organic semiconductor devices is thermally activated with characteristic activation energies in the range of 0.2–0.6 eV, leading to strongly temperature-dependent behaviour. For designing efficient organic semiconductor materials and devices, it is therefore indispensable to understand the origin of these activation energies. We propose that in bilayer organic light-emitting diodes (OLEDs) employing a polar electron transport layer, as well as in metal-insulator-semiconductor (MIS) devices, the hole injection barrier Einj and the hole mobility activation energy Eμ can be decoupled from each other if temperature-dependent capacitance-frequency (C-f-T) and MIS-CELIV (charge extraction by linearly increasing voltage) experiments are combined. While the C-f-T signal contains information of both injection and transport, the CELIV current is expected to be insensitive to the electrode injection properties. We employ numerical drift-diffusion simulations to investigate the accuracy of this analytical parameter extraction approach and to develop criteria for its validity. We show that the implicit assumption of constant charge density and field profiles leads to systematic errors in determining the activation energies. Thus, one should be aware of the intrinsic limitations of the analytical Arrhenius fit, and for more accurate parameter determination a full drift-diffusion modelling is advised. Applying the analytical method to a standard bilayer OLED, we find that the total activation energy of 0.5 eV for the hole current can be split into contributions of ≈0.25 eV each for injection barrier and mobility. Finally, we also discuss the broader applicability of this method for other device stacks and material combinations.

Charge transfer properties of diphenyl substituted cyclopentadithiophene organic semiconductors: The role of fluorine and malononitrile substitutions and crystal ordering

Abstract The geometry and electronic structures of four cyclopentadithiophene (CPD) derivatives in their crystals are systematically studied through density functional theory (DFT) calculations. The factors influencing the CPDs' semiconductor nature and charge carrier mobility are analyzed in detail, revealing the different roles of fluorine and dicyanomethylene in the CPDs examined in this study. Malononitrile substitutions show a large effect in lowering the molecular LUMO energy level but induce a smaller charge transfer integral due to the resulting reduced crystal order caused by the resulting steric hindrance. In comparison, although F substitution lowers the LUMO level more weakly than C(CN)2 substitution, it allows the molecule to retain a more planar configuration with more orbital delocalization, and the crystals of the F substituted compounds are more ordered than those of the C(CN)2 substituted compounds. Overall, this comparison can explain why F substitution is a more effective method to cause a transition of a p-type semiconductor to n-type. Crystal ordering plays a significant role in determining the charge transfer mobility in devices. In spite of the advantage offered by dicyanomethylene substitution in terms of charge injection barrier compared to fluorine substitution, the reduced crystal order caused by C(CN)2 limits the charge transfer in CPDs with C(CN)2 substitution. The validity of these findings can be generalized, which is expected to contribute to the rational design of n-type semiconductors based on CPDs.

Estimation of the Charge Injection Barrier at a Metal/Organic Semiconductor Interface Based on Accumulated Charge Measurement: The Effect of Offset Bias Voltages

The effect of the offset bias voltage on the threshold voltage of the hole injection into the organic-semiconductor (OS) layer was examined in detail in the accumulated charge measurement (ACM) for the n-type Si/SiO2/OS/Ag (OS = zinc phthalocyanine [ZnPc] or metal-free phthalocyanine [H2Pc]) capacitor. The threshold highly depends on the offset bias voltages, when the OS layer is in the hole-depletion regime. On the contrary, the threshold was nearly constant when the OS layer operated in the hole-accumulation regime. The hole injection barrier of the Ag/OS interface was obtained by the threshold in the accumulation regime. The obtained values were 0.41 and 0.05 eV for H2Pc/Ag and ZnPc/Ag interfaces, respectively. The study revealed that accurate estimation of the injection barrier is possible by examining the offset voltage dependence in the ACM.

Significant reduction in the hole-injection barrier by the charge-transfer state formation: Diindenoperylene contacted with silver and copper electrodes

Charge-carrier injection in organic electronics is critically influenced by the interface electronic states formed at the organic semiconducting thin films and metal electrode contacts. A practical solution for the issue is to form a charge-transfer (CT) state at the interfaces, which will produce a substantial density of gap states in the vicinity of the Fermi level of metal electrodes and thus reduce significantly the charge-injection barrier at the interfaces. In this study, we use ultraviolet photoelectron spectroscopy and demonstrate the formation of a CT state at diindenoperylene (DIP) and Ag (or Cu) electrode contacts. Remarkably, the CT state occurs irrespective of the electrode crystallinity, crystal grain orientations, or the molecular orientations in the DIP film, hence both the top- or bottom-metal contact conditions result in the formation of the CT state. Theoretical calculation simulations show the sign of strong interaction between those metal atoms and DIP leading to the CT state formation, analogous to the surface-induced aromatic stabilization proposed by Heimel et al. in a literature [Nat. Chem., 5(2013)187.]. Our results aid in understanding the mechanism of the CT state formation and propose the DIP-CT interlayers being novel candidates for the efficient hole-injection layers used in organic electronics.

Density functional study of metal–phosphorene interfaces

Properties of interfaces of a phosphorene monolayer with six different low-index metal surfaces are calculated using density functional methods. Pd(111), Pd(110), Pd(100), Ti(0001), Au(110) and Ni(110) surfaces have been considered as these metals have been used as electrodes in experimental studies of phosphorene-based field effect transistor (FET) devices. In order to understand the chemistry of metal–phosphorene bonding, adsorption of individual atoms of these four metals on a phosphorene monolayer has also been studied. In addition to structural and electronic properties, barriers for charge injection at these metal–phosphorene interfaces have been studied by calculating the Schottky and tunneling barrier heights. Ti appears to be the best choice for metal electrode in phosphorene devices.

A DFT Study on the Electronic Structures and Conducting Properties of Rubrene and its Derivatives in Organic Field-Effect Transistors

We systematically studied the electronic structures and conducting properties of rubrene and its derivatives reported recently, and disscussed the influences of electron-withdrawing groups and chemical oxidation on the reorganization energies, crystal packing, electronic couplings, and charge injection barrier of rubrene. Hirshfeld surface analysis and quantum-chemical calculations revealed that the introduction of CF3 groups into rubrene decreases the H···H repulsive interaction and increases intermolecular F···H/H···F attractive interactions, which resulted in the tight packing arrangement and the increase of the electronic couplings, and finally cause the higer intrinsic hole-mobility in bis(trifluoromethyl)-dimethyl-rubrene crystal (μh = 19.2 cm2 V−1 s−1) than in rubrene crystal (μh = 15.8 cm2 V−1 s−1). In comparison, chemical oxidation reduces charge-carrier mobility of rubrene crystal by 2~4 orders of magnitude and increased the hole and electron injection barrier, which partly explains the rubrene-based field-effect transistor performance degrades upon exposure to air. Furthermore, we also discussed the influence of structural parameters of carbon nanotube (CNT) electrode on charge injection process, which suggests that the regulation of CNT diameters and increasing in thickness is an effective strategy to optimize CNT work functions and improve n-type OFET performances based on these organic materials.

Novel liquid crystalline organic semiconducting oligomers incorporating N-heterocyclic carbazole moieties for fluorescent OLEDs

ABSTRACTA novel class of nematic liquid crystalline organic semiconducting oligomers incorporating N-heterocyclic carbazole moieties has been synthesised using simple and highly efficient reaction pathways. The electroluminescent colour of these novel oligomers can be varied in a controlled manner by molecular design. The values of the ionisation potential and the electron affinity of these electroluminescent oligomers can also be matched by structural design to the Highest Occupied Molecular Orbital (HOMO) energy level of the electron-blocking layer and the Lowest Unoccupied Molecular Orbital (LUMO) energy level of electron-transporting layer in the Organic light emitting diodes to create low charge-injection barriers for electrons and holes, respectively leading to electroluminescence with an efficacy up to 4.1 cd A−1.

Effects of the interfacial charge injection properties of silver nanowire transparent conductive electrodes on the performance of organic light-emitting diodes

We investigated the charge injection properties of silver nanowire networks (AgNWs) in a composite-like structure with poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS). The composite films acted as the anodes and hole transport layers (HTLs) in organic light-emitting diodes (OLEDs). The current density (J)–voltage (V)–luminance (L) characteristics and power efficiency (ε) of the OLEDs were measured to determine their electrical and optical properties. The charge injection properties of the AgNWs in the OLEDs during operation were characterized via impedance spectroscopy (IS) by determining the variations in the capacitances (C) of the devices with respect to the applied V and the corresponding frequency (f). All measured results were compared with results for OLEDs fabricated on indium tin oxide (ITO) anodes. The OLEDs on AgNWs showed lower L and ε values than the OLEDs on ITO. It was also observed that AgNWs exhibit excellent charge injection properties and that the interfaces between the AgNWs and the HTL have very small charge injection barriers, resulting in an absence of charge carrier traps when charges move across these interfaces. However, in the AgNW-based OLED, there was a large mismatch in the number of injected holes and electrons. Furthermore, the highly conductive electrical paths of the AgNWs in the composite-like AgNW and PEDOT:PSS structure allowed a large leakage current of holes that did not participate in radiative recombination with the electrons; consequently, a lower ε was observed for the AgNW-based OLEDs than for the ITO-based OLEDs. To match the injection of electrons by the electron transport layer (ETL) in the AgNW-based OLED with that of holes by the AgNW/PEDOT:PSS composite anode, the electron injection barrier of the ETL was decreased by using the low work function polyethylenimine ethoxylated (PEIE) doped with n-type cesium carbonate (Cs2CO3). With the doped-PEIE, the performance of the AgNW-based OLED was significantly enhanced through the balanced injection of holes and electrons, which clearly verified our analysis results by IS.

An analytical solution for contact resistance of staggered organic field-effect transistors

We have developed analytical models for bias dependent contact resistance (RC) and output characteristics of staggered organic field-effect transistors (OFETS) based on a bulk resistance-approximated and mobility-modified current-crowding method. Numerical evaluations of RC and its resistive components show that the bias dependency of the bulk resistance is negligible. Consequently, the properties of the active layer interfaces determine RC and its characteristics. Effective parameters include a normally constant charge injection barrier at the organic-metal interface (Eb) and a gate induced surface carrier-concentration (PS0) at the organic-insulator boundary. The energy barrier pertains to the fabrication process, and its related resistance (rc) can be determined as the fitting parameter of the theoretical model. However, PS0 is strongly gate bias dependent and the results of the numerical model indicate that the resulting component (rch) is dominant and has a considerable effect on RC and its characteristics. More importantly, PS0 as the key parameter of the contact resistance is analytically expressible and by using a proposed mobility-modified current-crowding model, the contact resistance can be analytically formulated. Accordingly, the output characteristics of the OFETs in the triode region can be also analytically modeled using the developed relation of RC.

Contact resistance at planar metal contacts on bilayer graphene and effects of molecular insertion layers

The possible origins of metal–bilayer graphene (BLG) contact resistance are investigated by taking into consideration the bandgap formed by interfacial charge transfer at the metal contacts. Our results show that a charge injection barrier (Schottky barrier) does not contribute to the contact resistance because the BLG under the contacts is always degenerately doped. We also showed that the contact-doping-induced increase in the density of states (DOS) of BLG under the metal contacts decreases the contact resistance owing to enhanced charge carrier tunnelling at the contacts. The contact doping can be enhanced by inserting molecular dopant layers into the metal contacts. However, carrier tunnelling through the insertion layer increases the contact resistance, and thus, alternative device structures should be employed. Finally, we showed that the inter-band transport by variable range hopping via in-gap states is the largest contributor to contact resistance when the carrier type of the gated channel is opposite to the contact doping carrier type. This indicates that the strategy of contact resistance reduction by the contact-doping-induced increase in the DOS is effective only for a single channel transport branch (n- or p-type) depending on the contact doping carrier type.

Estimation of Charge-Injection Barriers at the Metal/Pentacene Interface through Accumulated Charge Measurement

The charge-injection barrier from metal electrodes to thin-film pentacene is investigated using accumulated charge measurements. When a gold electrode is deposited on a pentacene film, the interface forms a Schottky contact with a hole-injection barrier of 0.2 eV. However, interfacial carrier motion is reversible between charge injection and discharge. The result suggests that the reported electrical hysteresis in typical pentacene transistors is caused by carrier traps that are localized primarily in the SiO2/pentacene interface. The Ag/pentacene junction has a large barrier height of 0.5 eV. The barrier height is significantly reduced, and an Ohmic contact is realized by using molybdenum oxide (MoO3) as a buffer layer.