Yufeng Hu

High-efficiency THz modulator based on phthalocyanine-compound organic films

We report a high efficiency, broadband terahertz (THz) modulator following a study of phthalocyanine-compound organic films irradiated with an external excitation laser. Both transmission and reflection modulations of each organic/silicon bilayers were measured using THz time-domain and continuous-wave systems. For very low intensities, the experimental results show that AlClPc/Si can achieve a high modulation factor for transmission and reflection, indicating that AlClPc/Si has a superior modulation efficiency compared with the other films (CuPc and SnCl2Pc). In contrast, the strong attenuation of the transmitted and reflected THz waves revealed that a nonlinear absorption process takes place at the organic/silicon interface.

Negative differential resistance and carrier transport of electrically bistable devices based on poly(N-vinylcarbazole)-silver sulfide composites

An electrically bistable device has been fabricated based on poly(N-vinylcarbazole) (PVK)-silver sulfide (Ag2S) composite films using a simple spin-coating method. Current–voltage (I-V) characteristics of the as-fabricated devices exhibit a typical electrical bistability and negative differential resistance (NDR) effect. The NDR effect can be tuned by varying the positive charging voltage and the charging time. The maximum current ratio between the high-conducting state (ON state) and low-conducting state (OFF state) can reach up to 104. The carrier transport mechanisms in the OFF and ON states are described by using different models on the basis of the experimental result.

High sensitivity and fast response solution processed polymer photodetectors with polyethylenimine ethoxylated (PEIE) modified ITO electrode

Most organic photodetectors utilize a bulk heterojunction (BHJ) photo-active film due to its high exciton dissociation efficiency. However, the low dark current density, a key role in determining the overall performance of photodetectors, is hardly achieved in the BHJ structure since both the donor and acceptor domains are in contact with the same electrode. The most popular strategy to overcome this problem is by fabricating bilayer or multilayer devices. However, the complicated fabrication process is a challenge for printing electronics. In this work, we demonstrate a solution processed polymer photodetector based on a poly (3-hexylthiophene) (P3HT): (phenyl-C61-butyric-acid-methyl-ester) (PC61BM) blend film with polyethylenimine ethoxylated (PEIE) modified ITO electrode. The transparent PEIE efficiently blocks the unnecessary electronic charge injection between the active film and the electrode, which dramatically decrease the dark current. Under illumination, the photoexcited charges accumulated in the PEIE modified ITO region finally can tunnel through the barrier with the help of the applied reverse bias, leading to a large photocurrent. Therefore, the resulting polymer photodetector shows a 2.48 × 104 signal-to-noise ratio (SNR) under -0.3 V bias and an 11.4 MHz bandwidth across the visible spectra under a small reverse bias of 0.5 V. The maximum EQE of 3250% in the visible wavelength is obtained for the polymer photodetector at -1 V under 370 nm (3.07 μW/cm2) illumination. This solution processed polymer photodetector manufacturing is highly compatible with the flexible, low-cost, and large area organic electronic technologies.

Ligand-free rutile and anatase TiO2 nanocrystals as electron extraction layers for high performance inverted polymer solar cells

Ligand-free rutile and anatase TiO2 nanocrystals have been synthesized through a hydrolytic sol–gel reaction. The morphology, crystal structure, elemental composition and band structure of the obtained nanocrystals are characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and UV-visible absorption spectroscopy. These two kinds of nanocrystals could serve as electron extraction layers for improving the performance in inverted polymer solar cells. Compared with the device fabricated by using amorphous TiO2 (6.11%) and rutile TiO2 (6.93%), the device based on anatase TiO2 shows a significant enhancement in power conversion efficiency (7.85%). Meanwhile, the ideal current–voltage model for a single heterojunction solar cell is applied to clarify the junction property of the cell. The model demonstrates that the device based on anatase TiO2 has effective electron extraction and hole-blocking properties.

Investigation on Thermal Degradation Process of Polymer Solar Cells Based on Blend of PBDTTT-C and BM

The effects of thermal treatment on the photovoltaic performance of conventional and inverted polymer solar cells (PSCs) based on the combination of poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thie-no[3,4-b]thiophene))-2,6-diyl] (PBDTTT-C) and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) are investigated. The transient photoconductivity, the absorption spectra, and the transmission electron microscopy (TEM) images have been employed to study the thermal degradation of the inverted PSCs. The degradation is attributed to the inefficient charge generation and imbalance in charge-carrier transport, which is closely associated with the morphological evolution of the active layer with prolonged heating time.

High sensitivity, fast response and low operating voltage organic photodetectors by incorporating a water/alcohol soluble conjugated polymer anode buffer layer

Low dark current density plays a key role in determining the overall performance of organic photodetectors (OPDs). However, both the donor domains and acceptor domains in the bulk heterojunction, which has high exciton dissociation efficiency, are in contact with the two electrodes. Therefore, the undesirable charge injection from the electrodes to the active layer is hard to avoid, leading to a high dark current density in most OPDs. In this work, we fabricate the OPDs based on a conventional poly(3-hexylthiophene) (P3HT)/(phenyl-C61-butyric-acid-methyl-ester) (PC61BM) bulk heterojunction. By incorporating a water/alcohol soluble conjugated polymer (WSCP), poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), interlayer between the anode and the active layer, the dark current density is effectively reduced from 0.07 mA cm−2 to 1.92 × 10−5 mA cm−2 under a −0.5 V bias. The resulting OPDs show a 1.93 × 105 signal-to-noise ratio (SNR), a 10 MHz bandwidth, and a 9.10 × 1012 Jones detectivity at a low reverse bias of −0.5 V (at 550 nm). Our research provides a promising way for high performance OPDs.

Enhanced amplified spontaneous emission from morphology-controlled organic–inorganic halide perovskite films

Organic–inorganic tri-halide perovskites (MAPbX3, where MA = CH3NH3, and X = Cl, Br, I) have shown promise as laser gain media. The laser characteristics of perovskite films are susceptible to their crystallinity and morphology. Herein, we demonstrate the morphology and crystallinity of perovskite films could be well-controlled in a modified sequential deposition process by using binary solvent mixtures involving N,N-dimethylmethanamide (DMF) and dimethylsulfoxide (DMSO). The highly crystalline and notably smooth MAPbI3 films on glass substrates yield outstanding amplified spontaneous emission (ASE) performance. The binary solvent engineering approach opens up new opportunities for the development of low-cost and high-efficient ASE devices based on perovskite materials.

Plasma treatment of ITO cathode to fabricate free electron selective layer in inverted polymer solar cells

With Ar plasma treatment of the indium tin oxide (ITO) cathode, we achieve efficient inverted bulk heterojunction solar cells based on poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester, which do not require electron selective layer. The plasma treatment improves the power conversion efficiency of the device from 1.07% to 3.57% with a fill factor of 66%, open-circuit voltage of 0.60 V, and short-circuit current of 9.03 mA cm−2. This result is comparable to the regular inverted devices with an additional electron selective layer. The Kelvin probe detects a reduction in the ITO work function of ∼0.45 eV after plasma treatment, which finally leads to an increase in the built-in potential and faster carrier extraction. As a result, good device performance is achieved. Because the electron selective layer becomes unnecessary, our strategy suggests a simple way to achieve efficient inverted organic solar cells.

Synthesis of ultrathin two-dimensional organic–inorganic hybrid perovskite nanosheets for polymer field-effect transistors

In this study, free-standing phenylethylammonium lead halide perovskite (i.e. (PEA)2PbX4 (PEA = C8H9NH3, X = Cl, Br, and I)) nanosheets (NSs) with few-layer thickness were synthesized using a facile antisolvent method. The as-prepared (PEA)2PbX4 NSs could be well-dispersed in organic solvents such as toluene and hexane. By incorporating the (PEA)2PbX4 NSs into poly(3-hexylthiophene) (P3HT) in toluene, (PEA)2PbX4 NSs: P3HT composite films were fabricated as channel layers for field-effect transistors (FETs) through spin coating. All of the resultant FETs show promising hole transport and current saturation behaviour at room temperature. Notably, the FET based on (PEA)2PbI4 NSs: P3HT exhibited the best hole mobility, μh, of 1.43 × 10−1 cm2 V−1 s−1 and threshold voltage, Vth, of 5.19 V. These results pave a new way for the application of 2D organic–inorganic hybrid perovskite NSs as channel materials in high-performance FETs.

Temperature dependent amplified spontaneous emission of vacuum annealed perovskite films

Hybrid organic–inorganic halide perovskite crystals exhibit potential applications in amplified spontaneous emission (ASE) devices. The enhancement of the photoluminescence (PL) and full width at half maximum (FWHM) with the temperature dependent phase transition are beneficial to investigate the low threshold of ASE devices. However, the ASE performance and temperature dependent transition characteristics of perovskite films critically depend on morphology and crystallinity. In this paper, we propose vacuum-assisted thermal treatment to effectively prepare smooth, condense and well-crystalline MAPbI3 perovskite films. An obvious temperature dependent phase transition occurs in the vacuum annealed MAPbI3 perovskite films, leading to an increase in PL intensity, which could non-monotonically lower the threshold for ASE. Investigating the transient performance at the temperature dependent phase transition underlines an important step in optimizing these novel materials.