Zhenbo Deng

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.

Emission Characteristics of PVK Doped TbY(o-MBA)6(phen)2 Systems

Abstract A rare earth complex TbY (o-MBA)6(phen)2 was synthesized, which was first used as an emitting material in electroluminescence. By doping it into the conjugated polymer PVK, single-layer and double-layer devices were fabricated with structures: device A: ITO/PVK: TbY (o-MBA)6(phen)2/BCP/Al; B: ITO/PVK: TbY (o-MBA)6(phen)2/AlQ/LiF/Al; C: ITO/PVK: TbY (o-MBA)6(phen)2/BCP/AlQ/LiF/Al. The characteristics of these devices were investigated. For single-layer and double-layer devices, the emission of PVK was completely restrained, and only the green emission from Tb3+ was observed in electroluminescence. The above mentioned observation is attributed to the different mechanism of electroluminescence and photoluminescence. In photoluminescence process, the energy of Tb complex may come from PVK through Forster energy transfer process, while in electroluminescence process direct sequential charge trapping appeares to be the main operating mechanism. From the optimized device B, brightly green emission can be obtained, and the highest EL brightness of the device reaches 213 cd·m−2 at 14 V.

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.

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.

Quantum well organic light emitting diodes with ultra thin Rubrene layer

Organic light-emitting diodes with ultra thin Rubrene layer have been fabricated with the structure of ITO/hole-transport layer/Rubrene/AlQ/LiF/Al. The hole-transport layer are copper phthalocyanine (CuPc), poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT: PSS), poly(N-vinyl carbazole) (PVK), and N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB), respectively. Compared with the other two devices, when 5,6,11,12-tetraphenylnaphthacene (Rubrene) is inserted between PVK (or NPB) and tris(8-quinolinolato)aluminum (AlQ), Rubrene’s emission get enhanced. The energy transfer mechanism among PVK (NPB), AlQ and Rubrene are revealed by the absorption and emitting spectra. Moreover, the enhancement of Rubrene’s emission can also be ascribed as an organic quantum well structure’s trap effection.

Bright red-to-yellow organic light-emitting devices based on polarization-induced spectral shifts and broadening

We demonstrated red and yellow organic light-emitting devices (OLEDs) with the structure of ITO/NPB/AlQ:DCJTB/AlQ/LiF/Al, where the NPB, AlQ and DCJTB are 4, 4′-bis[N-(1-naphthyl)-N-henylamino] biphenyl, tris(8-quinolinolato)aluminum and 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran, respectively. Electroluminescent (EL) behaviors of these devices have been examined with different concentrations of DCJTB doped into AlQ matrix. The emission color of the devices depends on the doping concentrations of DCJTB. For red and yellow OLEDs, a maximum luminescence of 2750 cd/m2 and 21,700 cd/m2 was obtained, respectively. The peak emission wavelength shift of DCJTB was found to be due to the polarization effects. It is of particular interest that the EL spectrum of DCJTB got broadening with the doping concentrations and current densities of the devices in our experiments.

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.

Organic light-emitting devices based on new rare earth complex Tb(p-CIBA)3phen

A new rare earth complex Tb(p-CIBA)3phen was synthesized and introduced into organic light emitting devices (OLEDs) as emitting material. The Tb(p-CIBA)3phen was doped into PVK to improve the film-forming and hole-transporting property. Two kinds of devices were fabricated. The device structure is as the following. Single-layer device: ITO/PVK: Tb(p-CIBA)3phen/LiF/Al; double-layer device: ITO/PVK: Tb(p-CIBA)3phen/AIQ/LiF/Al. The performances of both devices were investigated carefully. We found that the emission of PVK was completely restrained, and only the green emission was observed from the electroluminescence. The full width at half maximum (FWHM) was less than 10 nm. The highest EL brightness of the single-layer device is 25.4 cd/cm2 at a fixed bias of 18 V, and the highest EL brightness of the double-layer device reaches 234.8 cd/cm2 at a voltage of 20 V.

Electroluminescence Based on Eu0.5La0.5(TTA)3phen Doped Poly N-Vinylcarbazole

Abstract A novel rare earth complex Eu 0.5 La 0.5 (TTA) 3 phen, displaying electroluminescent property, was synthesized, and monolayer and double-layer devices were fabricated by doping it into poly N-vinylcarbazole. The characteristics of these optimized devices were investigated, and the emitting mechanism was explained through the energy band diagram. Optimized double-layer devices with a turn-on voltage of 6.5 V were achieved. At the current density of 68.48 mA·cm −2 , the maximum brightness and the current efficiency of the device reached 238.4 cd·m −2 and 0.35 cd·A −1 , respectively.

Sodium chloride methanol solution spin-coating process for bulk-heterojunction polymer solar cells*

The sodium chloride methanol solution process is conducted on the conventional poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) polymer bulk heterojunction solar cells. The device exhibits a power conversion efficiency of up to 3.36%, 18% higher than that of the device without the solution process. The measurements of the active layer by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and ultraviolet photoelectron spectroscopy (UPS) indicate a slight phase separation in the vertical direction and a sodium chloride distributed island-like interface between the active layer and the cathode. The capacitance–voltage (C–V) and impedance spectroscopy measurements prove that the sodium chloride methanol process can reduce the electron injection barrier and improve the interfacial contact of polymer solar cells. Therefore, this one-step solution process not only optimizes the phase separation in the active layers but also forms a cathode buffer layer, which can enhance the generation, transport, and collection of photogenerated charge carriers in the device simultaneously. This work indicates that the inexpensive and non-toxic sodium chloride methanol solution process is an efficient one-step method for the low cost manufacturing of polymer solar cells.