Ultraviolet Light-Activated Charge Modulation Heterojunction for Versatile Organic Thin Film Transistors.

Organic thin film transistors (OTFTs) are a promising technology for the application of photosensors in smart wearable devices. Light-induced electrical behavior of OTFTs is explored to achieve diverse functional requirements. In most studies, OTFTs show an increased drain current (ID) under light irradiation. Here, we use an ultraviolet (UV) light absorption top layer, tris(8-hydroxyquinoline) aluminum (Alq3), to improve the UV light response of poly(3-hexylthiophene-2,5-diyl) (P3HT)-based OTFTs. Unexpectedly, the Alq3-covered device operated at the accumulation mode demonstrates a decreased ID during the UV light irradiation. N,N'-Ditridecyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI, electron acceptor), pentacene (electron donor), and lithium fluoride (LiF, insulator) as an interlayer were inserted between the P3HT and the Alq3 layers. The PTCDI/Alq3-covered device also shows an unusual decrease in ID under the UV light but an increase in ID under the green light. The pentacene/Alq3-covered device shows an increased ID during the UV light irradiation and, unexpectedly, a memory effect in ID after removing the UV light. The LiF/Alq3-covered device exhibits an electrical behavior similar to the bare P3HT-based device under the UV light. Results of spectroscopic analyses and theoretical calculations have shown that the occurrence of charge transfer at heterojunctions during the UV light irradiation causes charge modulation in the multilayered P3HT-based OTFTs and then results in an unusual decrease or memory effect in ID. In addition, the unexpected ID reduction can be observed in the Alq3-covered poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]-based OTFTs under UV light. The features, including opposite electrical responses to different wavelengths of light and optical memory effect, provide the multilayered P3HT-based OTFTs with potential for various optical applications, such as image recognition devices, optical logic gates, light dosimeters, and optical synapses.

Enhanced Functionality of Dual-Gate Organic Transistors Based on Semiconducting/Insulating Polyblend-Induced Asymmetric Charge Modulation Layers.

Dual-gate organic thin-film transistors (DG-OTFTs) with enhanced functionality, including large current enhancement behavior, highly efficient threshold voltage controllability, and self-contained dual-mode logic gate features, are reported. These DG-OTFTs are based on a semiconducting/insulating polyblend-based active layer with asymmetric top and bottom charge modulation layers (atb-CMLs). The atb-CMLs are automatically generated through the preparation of multilayer stacks of phase-separated semiconducting poly(3-hexylthiophene) (P3HT):insulating poly(methylmethacrylate) (PMMA) polyblend layer, poly(vinylidene fluoride) (PVDF) layer, and cross-linked-poly(4-vinylphenol) (cPVP) layer. They consist of a thin PMMA bottom layer and an uneven-shaped PMMA:PVDF miscible mixture-based top layer. The presence of the polarizable insulating PMMA, PVDF, and PMMA:PVDF mixture regions causes the bottom and top CMLs to experience electrical polarization, which induces the dipole field to achieve efficient charge modulation functions in DG-OTFTs. Owing to the presence of atb-CMLs, the DG-OTFTs exhibit unprecedented electrical characteristics, such as the easy depletion of the bottom channel by the top gate potential. However, the top channel can work properly only when given a bottom gate potential (either positive or negative). Given these unusual electrical features, the design of the fundamental dual-mode logic gates (e.g., AND and OR gates) can be achieved with just one DG transistor. This finding opens an interesting direction for the preparation of DG-OTFTs with diverse operating modes and increasing functionality, thereby widening the application potential of such transistors.

Synergistic Effects of Binary-Solvent Annealing for Efficient Polymer-Fullerene Bulk Heterojunction Solar Cells.

Conjugated polymer-fullerene-based bulk-heterojunction (BHJ) organic solar cells (OSCs) have attracted tremendous attention over the past two decades because of their potential to develop low-cost and easy methods to produce energy from light. The complicated microstructure and morphology with randomly organized architecture of these polymer-fullerene-based active layers (ALs) is a key factor that limits photovoltaic performance. In this study, a binary-solvent annealing (BSA) approach was established to improve the poly(3-hexylthiophene):indene-C60 bisadduct-based AL for efficient BHJ-type OSCs by varying the second solvents with different boiling points (BP). Thus, we were able to change the evaporation behavior of cosolvents and consequently obtain the various microstructural properties of the AL. An in-depth study was conducted on the solvent-evaporation driven morphology of the active layer under various cosolvent conditions and its effect on the photovoltaic parameters of OSCs. Under the BSA processes, we found that the specimens with low-BP second solvents allows us to observe a more ideal AL for increasing photon absorption and efficient charge transport and collection at the respective electrodes, resulting in enhanced PCE of the corresponding OSCs. By contrast, the specimens with high-BP second solvents exhibit random microstructures, which are detrimental to charge transport and collection and lead to diminished PCE of the corresponding OSCs. By appropriately selecting the composition of a binary solvent, BSA can be employed as an easy method for the effective manipulation of the microstructures of ALs. BSA is a promising technique for the performance enhancement of not only OSCs but also other organic/polymeric-based electronic devices.

Spontaneous Formation of an Ideal-Like Field-Effect Channel for Decay-Free Polymeric Thin-Film Transistors by Multiple-Scale Phase Separation.

We demonstrate semiconducting polymer-based thin-film transistors (PTFTs) with fast switching performance and an uncommon nondecaying feature. These PTFTs based on widely studied poly(3-hexylthiophene) are developed by incorporating the insulating polymer into the active channel and subjecting the compound to specific, spontaneous multiple-scale phase separation (MSPS). An in-depth study is conducted on the interfacial and phase-separated microstructure of the semiconducting/insulating blending active layer and its effect on the electrical characteristics of PTFTs. The polyblends exhibit a confined crystallization behavior with continuously semiconducting crystalline domains between scattered insulator-rich domains. The insulator-rich domains can block leakage current and strengthen the gate control of the channel. A small amount of the insulating polymer penetrates the bottom of the active channel, resulting in effective interface modification. We show specific MSPS morphology of the present blending films to reduce charge trapping effects, enhance charge accumulation, and create a high-seed switching channel. The findings enable us to develop the required morphological conceptual model of the ideal-like field-effect-modulated polymer-based active channel. The polyblend-based PTFTs with MSPS morphology also have promising sensing functions. This study offers an effective approach for overcoming the major drawbacks (instability and poor switching) of PTFTs, thus allowing such transistors to have potential applications.

A nanoscale study of charge extraction in organic solar cells: the impact of interfacial molecular configurations.

In the optimization of organic solar cells (OSCs), a key problem lies in the maximization of charge carriers from the active layer to the electrodes. Hence, this study focused on the interfacial molecular configurations in efficient OSC charge extraction by theoretical investigations and experiments, including small molecule-based bilayer-heterojunction (sm-BLHJ) and polymer-based bulk-heterojunction (p-BHJ) OSCs. We first examined a well-defined sm-BLHJ model system of OSC composed of p-type pentacene, an n-type perylene derivative, and a nanogroove-structured poly(3,4-ethylenedioxythiophene) (NS-PEDOT) hole extraction layer. The OSC with NS-PEDOT shows a 230% increment in the short circuit current density compared with that of the conventional planar PEDOT layer. Our theoretical calculations indicated that small variations in the microscopic intermolecular interaction among these interfacial configurations could induce significant differences in charge extraction efficiency. Experimentally, different interfacial configurations were generated between the photo-active layer and the nanostructured charge extraction layer with periodic nanogroove structures. In addition to pentacene, poly(3-hexylthiophene), the most commonly used electron-donor material system in p-BHJ OSCs was also explored in terms of its possible use as a photo-active layer. Local conductive atomic force microscopy was used to measure the nanoscale charge extraction efficiency at different locations within the nanogroove, thus highlighting the importance of interfacial molecular configurations in efficient charge extraction. This study enriches understanding regarding the optimization of the photovoltaic properties of several types of OSCs by conducting appropriate interfacial engineering based on organic/polymer molecular orientations. The ultimate power conversion efficiency beyond at least 15% is highly expected when the best state-of-the-art p-BHJ OSCs are combined with present arguments.

Electron transport properties in fluorinated copper-phthalocyanine films: importance of vibrational reorganization energy and molecular microstructure.

Electron transport (ET) properties of a series of fluorinated copper-phthalocyanine (F(16)CuPc) thin films, which were deposited at different substrate temperatures (T(sub)) ranging from 30 to 150 degrees C, have been investigated by quantum mechanical calculations of the reorganization energy (lambda(reorg)), X-ray diffraction (XRD), atomic force microscopy (AFM), and microRaman spectroscopy. Density functional theory calculations were used to predict the vibrational frequencies, normal mode displacement vectors, and electron-vibrational lambda(reorg) for the F(16)CuPc molecule. The electron mobilities (mu(e)) of F(16)CuPc thin films are strongly dependent on the T(sub), and the value of mu(e) increases with increasing T(sub) from 30 to 120 degrees C, at which point it reaches its maximum value. The importance of electron-vibrational coupling and molecular microstructures for ET properties in F(16)CuPc thin films are discussed on the basis of theoretical vibrational lambda(reorg) calculations and experimental observations of resonance Raman spectra. We observed a good correlation between mu(e) and the full-width-at-half-maximum of the vibrational bands, which greatly contributed to lambda(reorg) and/or which reflects the molecular microstructural quality of the active channel. In contrast, the crystal size analysis by XRD and surface grain morphology by AFM did not reveal a clear correlation with the ET behaviours for these different F(16)CuPc thin films. Therefore, we suggest that for organic films with weak intermolecular interactions, such as F(16)CuPc, optimized microscopic molecular-scale parameters are highly important for efficient long-range charge transport in the macroscopic devices.