Ambipolar Charge Carrier Transport Properties at the S‑Benzyl‑l‑cysteine-Induced 2D/3D Halide Perovskite Interface

We fabricated a 2D/3D perovskite film by adding a molecular spacer, S-benzyl-l-cysteine (SBLC), during MAPbI3 perovskite film formation and investigated the effect of SBLC on the electronic conduction with a field-effect transistor. The addition of SBLC resulted in a larger grain size on the top surface and activated the electronic conduction modulated by the gate electric field. The field-effect modulation is attributed to the presence of the 2D perovskite surface layer induced by the SBLC molecular spacer on the 3D MAPbI3 perovskite bulk, resulting from the passivation of interfacial localized states and suppression of ion migration within the MAPbI3 perovskite film. Illumination with green light activated hole transport, resulting in ambipolar transport in the SBLC-induced MAPbI3 perovskite 2D/3D film, explaining that the SBLC treatment reduced the hole trap density of states. Illumination with green light maximized the effect of the surface 2D layer of the 2D/3D MAPbI3 perovskite film on the field effect-induced charge transport, enabling ambipolar electronic transport with the 2D/3D configuration due to suppression of the ionic behavior on the surface. This study highlights the electronic and structural effects of the organic molecular spacer on the electrical properties of the 3D bulk layer, hinting that defect-induced electronic and structural instability in the 3D bulk can be overcome by the passivation of the 3D bulk surface by the formation of the 2D/3D perovskite interface

Memory and Photovoltaic Elements in Organic Field Effect Transistors with Donor/Acceptor Planar-Hetero Junction Interfaces

Interfacial charge transfer at organic/organic planar-hetero junctions allows access to device structures that create new opportunities for flexible electronic devices. Fundamental characteristics of a pentacene/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) interface are explored via a comprehensive study of charge transfer between these two materials using the field-effect transistor (FET) geometry both in the dark and under illumination. Organic memory elements in a field effect transistor are demonstrated for a device fabricated with a pentacene/PCBM interface. Electric field induced charge transfer at the interface, in the dark, induced a nonvolatile memory effect with a large hysteresis characterized by a memory window of 43 V in the transfer characteristics. A photoinduced threshold voltage shift induced by exciton dissociation at the interface, in the absence of a gate electric field, is consistent with the formation of the photoinduced conducting channel in pentacene

Creating and Optimizing Interfaces for Electric-Field and Photon-Induced Charge Transfer

We create and optimize a structurally well-defined electron donor–acceptor planar heterojunction interface in which electric-field and/or photon-induced charge transfer occurs. Electric-field-induced charge transfer in the dark and exciton dissociation at a pentacene/PCBM interface were probed by <i>in situ</i> thickness-dependent threshold voltage shift measurements in field-effect transistor devices during the formation of the interface. Electric-field-induced charge transfer at the interface in the dark is correlated with development of the pentacene accumulation layer close to PCBM, that is, including interface area, and dielectric relaxation time in PCBM. Further, we demonstrate an <i>in situ</i> test structure that allows probing of both exciton diffusion length and charge transport properties, crucial for optimizing optoelectronic devices. Competition between the optical absorption length and the exciton diffusion length in pentacene governs exciton dissociation at the interface. Charge transfer mechanisms in the dark and under illumination are detailed

Anisotropic Assembly of Conjugated Polymer Nanocrystallites for Enhanced Charge Transport

The anisotropic assembly of P3HT nanocrystallites into longer nanofibrillar structures was demonstrated via sequential UV irradiation after ultrasonication to the pristine polymer solutions. The morphology of resultant films was studied by atomic force microscopy (AFM), and quantitative analysis of intra- and intermolecular ordering of polymer chains was performed by means of static absorption spectroscopy and quantitative modeling. Consequently, the approach to treat the precursor solution enhanced intra- and intermolecular ordering and reduced the incidence of grain boundaries within P3HT films, which contributed to the excellent charge carrier transport characteristics of the corresponding films (μ ≈ 12.0 × 10^–2 cm² V^–1 s^–1 for 96% RR P3HT)

Photoinduced Anisotropic Assembly of Conjugated Polymers in Insulating Polymer Blends

Low-dose UV irradiation of poly­(3-hexylthiophene) (P3HT)-insulating polymer (polystyrene (PS) or polyisobutylene (PIB)) blend solutions led to the formation of highly ordered P3HT nanofibrillar structures in solidified thin films. The P3HT nanofibers were effectively interconnected through P3HT islands phase-separated from insulating polymer regions in blend films comprising a relatively low fraction of P3HT. Films prepared with a P3HT content as low as 5 wt % exhibited excellent macroscopic charge transport characteristics. The impact of PS on P3HT intramolecular and intermolecular interactions was systematically investigated. The presence of PS chains appeared to assist in the UV irradiation process of the blend solutions to facilitate molecular interactions of the semiconductor component, and to enhance P3HT chain interactions during spin coating because of relatively unfavorable P3HT–PS chain interactions. However, P3HT lamellar packing was hindered in the presence of PS chains, because of favorable hydrophobic interactions between the P3HT hexyl substituents and the PS chains. As a result, the lamellar packing <i>d</i>-spacing increased, and the coherence length corresponding to the lamellar packing decreased, as the amount of PS in the blend films increased

Grafting of Polyimide onto Chemically-Functionalized Graphene Nanosheets for Mechanically-Strong Barrier Membranes

A series of polyimide (PI) nanocomposite films with different loadings of aminophenyl functionalized graphene nanosheets (AP-rGO) was fabricated by in situ polymerization. AP-rGO, a multifunctional carbon nanofiller that can induce covalent bonding between graphene nanosheets and the PI matrix, was obtained through the combination of chemical reduction and surface modification. In addition, phenyl functionalized graphene nanosheets (P-rGO) were prepared by phenylhydrazine for reference nanocomposite films. Because of homogeneous dispersion of AP-rGO and the strong interfacial interaction between AP-rGO and the PI matrix, the resulting nanocomposite films that contained AP-rGO exhibited reinforcement effects of mechanical properties and oxygen barrier properties that were even better than those of pure PI and the reference nanocomposite films. In comparison to the tensile strength and tensile modules of pure PI, the composite films that contained AP-rGO with 3 wt % loading were increased by about 106% (262 MPa) and 52% (9.4 GPa), respectively. Furthermore, the oxygen permeabilities of the composites with 5 wt % filler content were significantly decreased, i.e., they were more than 99% less than the oxygen permeability of pure PI

Tunable Exciton Dissociation and Luminescence Quantum Yield at a Wide Band Gap Nanocrystal/Quasi-Ordered Regioregular Polythiophene interface

A comprehensive understanding of the effect of polymer chain aggregation-induced molecular ordering and the resulting formation of lower excited energy structures in a conjugated polymer on exciton dissociation and recombination at the interface with a wide-bandgap semiconductor is provided through correlation between structural arrangement of the polymer chains and the consequent electrical and optoelectronic properties. A vertical diode-type photovoltaic test probe is combined with a field effect current modulating device and various spectroscopic techniques to isolate the interfacial properties from the bulk properties. Enhanced energy migration in the quasi-ordered (poly­(3-hexylthiophene)) (P3HT) film, processed through vibration-induced aggregation of polymer chains in solution state, is attributed to the presence of the aggregation-induced interchain species in which excitons are allowed to migrate through low barrier energy sites, enabling efficient iso-energetic charge transfer followed by the downhill energy transfer. We discovered that formation of nonemissive excitons that reduces the photoluminescence quantum yield in the P3HT film deactivates exciton dissociation at the donor (P3HT) close to the acceptor (ZnO) as well as in the P3HT far away from the ZnO. In other words, exciton deactivation in its film state arising from the quasi-ordered structural arrangement of polymer chains in solution is retained at the donor/acceptor interface as well as in the bulk P3HT. Effect of change in the highest occupied molecular orbital level and the resulting energy band bending at the P3HT/ZnO interface on exciton dissociation is also discussed in relation to the presence of vibration-induced aggregates in the P3HT film

Competition between Charge Transport and Energy Barrier in Injection-Limited Metal/Quantum Dot Nanocrystal Contacts

Injection-limited contacts in many of electronic devices such as light-emitting diodes (LEDs) and field effect transistors (FETs) are not easily avoided. We demonstrate that charge injection in the injection-limited contact is determined by charge transport properties as well as the charge injection energy barrier due to vacuum energy level alignment. Interestingly, injection-limited contact properties were observed at 5 nm diameter lead sulfide (PbS) quantum dot (QD)/Au contacts for which carrier injection is predicted to be energetically favorable. To probe the effect of charge transport properties on carrier injection, the electrical channel resistance of PbS nanocrystal (NC) FETs was varied through thermal annealing, photoillumination, ligand exchange, surface treatment of the gate dielectric, and use of different sized PbS NCs. Injection current through the PbS/Au contact varied with the FET mobility of PbS NC films consistent with a theoretical prediction where the net injection current is dominated by carrier mobility. This result suggests that the charge transport properties, that is, mobility, of QD NC films should be considered as a means to enhance carrier injection along with the vacuum level energy alignment at the interface between QD NCs and metal electrodes. Photocurrent microscopic images of the PbS/Au contact demonstrate the presence of a built-in potential in a two-dimensionally continuous PbS film near the metal electrodes

Additive-Free Hollow-Structured Co₃O₄ Nanoparticle Li-Ion Battery: The Origins of Irreversible Capacity Loss

Origins of the irreversible capacity loss were addressed through probing changes in the electronic and structural properties of hollow-structured Co₃O₄ nanoparticles (NPs) during lithiation and delithiation using electrochemical Co₃O₄ transistor devices that function as a Co₃O₄ Li-ion battery. Additive-free Co₃O₄ NPs were assembled into a Li-ion battery, allowing us to isolate and explore the effects of the Co and Li₂O formation/decomposition conversion reactions on the electrical and structural degradation within Co₃O₄ NP films. NP films ranging between a single monolayer and multilayered film hundreds of nanometers thick prepared with blade-coating and electrophoretic deposition methods, respectively, were embedded in the transistor devices for <i>in situ</i> conduction measurements as a function of battery cycles. During battery operation, the electronic and structural properties of Co₃O₄ NP films in the bulk, Co₃O₄/electrolyte, and Co₃O₄/current collector interfaces were spatially mapped to address the origin of the initial irreversible capacity loss from the first lithiation process. Further, change in carrier injection/extraction between the current collector and the Co₃O₄ NPs was explored using a modified electrochemical transistor device with multiple voltage probes along the electrical channel