Effective and stable hole-transporting materials (HTMs) are necessary for obtaining excellent planar perovskite solar cells (PSCs). Herein, we reported a solution-processed composite HTM consisting of a polymer poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) and a small-molecule copper phthalocyanine–3,4′,4″,4‴-tetrasulfonated acid tetrasodium salt (TS–CuPc) with optimized doping ratios. The composite HTM is crucial for not only enhancing the hole transport and extraction but also improving the perovskite crystallization. In addition, the composite HTM can weaken the indium tin oxide erosion by reducing the acidity and increasing the dispersibility of the PEDOT:PSS aqueous dispersion via incorporating suitable TS–CuPc. Consequently, a highly efficient device was fabricated with a power conversion efficiency (PCE) of 17.29%. Its short-circuit current (JSC) is 22.23 mA/cm2, and its open-circuit voltage (VOC) is 1.01 V. Meanwhile, it exhibited a higher fill factor (FF) of 77% and improved cell stability. The developed composite HTM provides a good potential anode interfacial layer for fabricating outstanding PSCs.Keywords: high efficiency; high stability; hole-transporting layer; perovskite solar cells (PSCs); small molecule-polymer;

Photovoltaic performance of planar perovskite hybrid solar cells (pero-HSCs) has been improved by mixing CH3NH3PbIxCl3−x and an electron donor polymer [N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiaz-ole)] (PCDTBT). PCDTBT contains lone pairs of electrons due to the presence of S and N atoms, which could passivate the trap states of the perovskite layer and thus reduce the number of film defects. A Stonehenge-like structure could be formed by the interaction of CH3NH3PbIxCl3−x and PCDTBT, developing more ordered orientation crystallization and a high quality film morphology. The doped solar cells are characterized by their excellent photovoltaic properties and enhanced stability. When the doping concentration is 0.3 mg mL−1, the fabricated solar cell device exhibits an outstanding power conversion efficiency (PCE) of 15.76%, which represents a significant improvement with respect to the magnitude of 16% obtained for the reference device.

Poly(3,4-ethylene dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a widely utilized hole-transporting material (HTM) in planar photovoltaic devices, such as organic solar cells (OSCs) and perovskite solar cells (PSCs). However, the hygroscopic nature of PEDOT:PSS aqueous dispersions may restrict their future application. Therefore, it is necessary to develop other effective and stable HTMs to achieve high-performance photovoltaic devices. Herein, we demonstrate a facile route to deposit solution-processed MoO3, GeO2, V2O5 and CrO3 thin films as hole-transporting layers by directly dissolving their commercial powders in deionized water. Among these, the solution-processed V2O5 (sV2O5) film exhibited the highest work function of 5.2 eV, and the best hydrophobicity, with a contact angle of 77.2°. The sV2O5-based OSCs and PSCs presented power conversion efficiencies (PCEs) of 8.36% and 14.13%, respectively. Notably, the PEDOT:PSS V2O5 composite HTM based device obtained a maximum PCE of 18.03% with a Voc exceeding 1.0 V. These aqueous-solution-processed HTMs have potential applications in green and low-cost photovoltaic devices by virtue of their simple and ecofriendly preparations.

All-inorganic perovskite materials, i.e. cesium lead halide (CsPbX3 (X = I, Br, Cl)), have attracted much attention in the application of photoelectronic devices, especially in solar cells and light-emitting diodes (LEDs). However, the solubility issue of CsPbX3 restricts their utilization in solution-processed devices. Herein, we demonstrated efficient all-inorganic perovskite LEDs (PeLEDs) via co-evaporation of cesium bromide (CsBr) and lead bromide (PbBr2) based on a vacuum thermal evaporation process. The molar ratio of CsBr to PbBr2, which can be adjusted via the co-evaporation ratio, is proved to be very critical to the device performance. Excess CsBr in the perovskite layer causes poor surface morphology and affects the charge transport. With an optimization of the molar ratio, the PeLEDs based on the equimolar CsBr and PbBr2 exhibit the best green electroluminescence (EL) performance with a maximum external quantum efficiency (EQE) of 1.55%. Meanwhile, the full width at half-maximum (FWHM) of the green EL spectrum is as narrow as 18.5 nm, which can offer high color purity and large color space in display applications.

The high cost of vacuum thermal evaporation and the challenging fabrication of multilayer devices by the solution processing method restrict the commercialization of organic light-emitting diodes (OLEDs). Herein, we introduce a flash evaporation method where an organic film pre-coated on a silicon wafer is re-deposited by sudden exposure to high temperature (∼1000 °C) in a rough vacuum to fabricate small molecule-based multilayer OLEDs. The flash-evaporated organic films maintain the original molecular structure after flash evaporation. Compared with the random molecular orientation of spin-coated small molecule films, flash-evaporated films have a high degree of molecular orientation perpendicular to the substrate surface. As a result, flash-evaporated OLEDs exhibit improved efficiency with low roll-off compared with spin-coated devices. The successful fabrication of a flexible, large-area (20 × 20 mm2) OLED suggests the great potential of the flash evaporation method for fabricating flexible and large area OLEDs with low cost in the future.

The fabrication and device parameters of inverted planar heterojunction (PHJ) organic–inorganic lead mixed-halide (CH3NH3PbI3−xClx) perovskite based solar cells (PSCs) using a:CuAlO2 as the hole selective buffer layer between the ITO electrode and PEDOT:PSS were demonstrated. Thin films of a:CuAlO2 were derived from a pre-fabricated polycrystalline CuAlO2 ceramic target by using the direct current (d.c.) magnetron sputtering technique. The one-step spin coating method was used to prepare the perovskite layer. A short circuit current density (Jsc) of 21.98 mA cm−2, an open circuit voltage (Voc) of 0.88 V, a fill factor (FF) of 0.75 and a power conversion efficiency (PCE) of 14.52% were achieved for the optimized device. These improved device parameters were also accompanied by improved stability as a result of sandwiching the ambient stable a:CuAlO2 layer with decent conductivity between the ITO and the PEDOT:PSS layers. The versatility of this material application was also demonstrated as a similar improvement in device performance and stability, which was observed by using the prepared a:CuAlO2 in another perovskite solar cell system based on CH3NH3PbI3 prepared by the two-step spin-coating method.

Carrier injection plays an important role in determining the device performance of organic light-emitting diodes (OLEDs). 1,4,5,8,9,11-Hexaazatriphenylene hexacarbonitrile (HAT-CN) has been widely used as an effective material to promote the hole injection when fabricating vacuum deposited OLEDs. However, serious crystallization occurs in solution-processed HAT-CN films, which weakens its hole injection ability in OLEDs. Herein, we demonstrate a solution-processed composite film as the hole injection layer (HIL) in OLEDs developed by mixing HAT-CN with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). The crystallization is suppressed effectively by optimizing the mixing ratio. In addition, a doping of HAT-CN:F4-TCNQ composite film contributes to a decreased hole injection barrier, which can be reflected from the current density–voltage curve. Furthermore, HAT-CN:F4-TCNQ is successfully combined with MoO3 doped copper phthalocyanine-3,4′,4′′,4′′′-tetra-sulfonated acid tetra sodium salt (TS-CuPc) as bi-HIL in solution-processable blue phosphorescent OLEDs, which exhibit a maximum current efficiency of 16.7 cd A−1.

A new red phosphorescent material Ir(dmppm)2(dmd), which is a pyrimidine-based iridium(III) complex, has been synthesized and successfully used to fabricate solution-processed red and white organic light-emitting diodes (OLEDs). Due to its excellent solubility in common organic solvents and its good compatibility with the host material, a record current efficiency of 27.2 cd A−1 so far with satisfactory Commission International de l'Eclairage (CIE) coordinates of (0.60, 0.40) has been achieved for partially solution-processed red OLEDs by using Ir(dmppm)2(dmd) as a dopant. Furthermore, the fabricated two-component “warm-white” OLEDs based on the Ir(dmppm)2(dmd) red emitter demonstrate a maximum current efficiency of 28.9 cd A−1, which can meet the call for physiologically-friendly indoor illumination.

Blue phosphorescent organic light-emitting diode (PhOLED) with a high maximum external quantum efficiency (EQE) of 26.6% was achieved using a new material, 2,8-bis(9,9-dimethylacridin-10(9H)-yl)dibenzo[b,d]furan (DBF-DMS) with a small bandgap, as the host. The device with DBF-DMS showed improved performance compared with that with 1,3-di-9-carbazolylbenzene, which is ascribed to the enhancement in carrier injection and transporting abilities and material stability of DBF-DMS. A lifetime of more than 100 h (time to 50% of the initial luminance, 1000 cd/m2 with an EQE of 19.6%) in the other DBF-DMS-based device is obtained by further utilizing better device structure. This is a report indicating that host material with a small bandgap like DBF-DMS can be successfully utilized toward blue PhOLEDs with high performance.

An approach to achieve improved performance in pentacene-based organic field effect transistors (OFETs) using high-k AlOx prepared by a low temperature sol–gel technique as a thin buffer layer on a SiO2 gate dielectric was demonstrated. The maximum processing temperature for the AlOx thin layer was 150 °C. The resulting all-inorganic SiO2/AlOx bilayer gate dielectric system exhibited a low leakage current density <1 × 10−8 A cm−2 under an applied electric field strength of 1.8 MV cm−1, a smooth surface with an rms of 0.11 nm and an equivalent dielectric constant (k) of 4.13. The OFET fabricated as a result of this surface modification exhibited a significantly improved field effect mobility of 0.81 cm2 V−1 s−1 when compared with a reference device with a SiO2 single layer gate dielectric, which had a lower mobility of 0.28 cm2 V−1 s−1.

Anode modification by doping silver nano-particles (Ag NPs) into poly(3,4-ethylene dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) and cathode interfacial modification by inserting solution-processed bathophenanthroline (sBphen) in CH3NH3PbI3−xClx based planar perovskite solar cells are investigated. Prior to the optical effect such as localized surface plasmon resonance, the Ag-NPs distributed in PEDOT:PSS mainly cause an improvement in the electrical property of PEDOT:PSS–Ag NPs composite films. The sBphen interfacial layer modified the surface morphology of perovskite/phenyl-C61-butyric acid methyl ester (PC61BM) films by filling the voids on the surface of perovskite/PC61BM effectively, which led to an obvious improvement in the fill factor. Accordingly, an efficient device with a power conversion efficiency of 15.75% was achieved due to the simultaneous cathode and anode interfacial modification.

Green phosphorescent inverted organic light-emitting diodes (IOLEDs) with 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN)/aluminium/n-doped 4,7-diphenyl-1,10-phenanthroline (Bphen) used as an electron injection layer (EIL) were demonstrated. The IOLED shows the lowest driving voltage of 4.5 V at 10000 cd m−2 to date. The electron injection effects of different interlayers were further investigated by ultraviolet photoelectron spectroscopy (UPS) and evaluating the electron injection efficiency. For application in large-sized OLEDs, a 120 × 120 mm2 flexible IOLED was successfully fabricated based on this inverted structure.

The authors develop an aqueous solution-processed hole injection layer, MoO3 doped copper phthalocyanine-3,4′,4′′,4′′′-tetra-sulfonated acid tetra sodium salt (TS-CuPc), in organic light-emitting diodes (OLEDs) via an environmentally-friendly and easy fabrication process. The generation of a charge transfer complex in TS-CuPc:MoO3 composite films is confirmed by absorption spectra and X-ray photoemission spectroscopy (XPS) measurements. Enhanced hole injection in OLEDs is attributed to the decreased hole barrier at the ITO side, which is in agreement with the Schottky thermal emission evaluation. The efficient modification of ITO by TS-CuPc:MoO3 is further confirmed by ultraviolet photoemission spectroscopy (UPS) measurements.

The authors demonstrate a honeycomb structured organic light-emitting diode (OLED) with high enhancements greater than 2.0 fold and 2.3 fold in current efficiency and power efficiency, respectively. The dispersion relationships in both planar and nano-honeycomb structured OLEDs are calculated through numerical simulations utilizing the finite-difference time-domain method and measured through the electroluminescence spectra. There is good agreement between the numerically calculated and the experimentally measured dispersion relationships for the nano-honeycomb structured OLEDs. Improved light out-coupling efficiency is mainly attributed to the efficient extraction of the waveguide and the surface plasmon polariton (SPP) loss modes in the devices. Particularly, most of the extracted energy is verified to be originated from the SPP loss mode in honeycomb structured OLEDs.

The electrical doping nature of a strong electron acceptor, 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN), is investigated by doping it in a typical hole-transport material, N,N′-bis(naphthalen-1-yl)-N,N′-diphenylbenzidine (NPB). A better device performance of organic light-emitting diodes (OLEDs) was achieved by doping NPB with HATCN. The improved performance could, in principle, arise from a p-type doping effect in the codeposited thin films. However, physical characteristics evaluations including UV–vis absorption, Fourier transform infrared absorption, and X-ray photoelectron spectroscopy demonstrated that there was no obvious evidence of charge transfer in the NPB:HATCN composite. The performance improvement in NPB:HATCN-based OLEDs is mainly attributed to an interfacial modification effect owing to the diffusion of HATCN small molecules. The interfacial diffusion effect of the HATCN molecules was verified by the in situ ultraviolet photoelectron spectroscopy evaluations.Keywords: carrier injection; charge transfer; doping; electron acceptor; interfacial diffusion; organic light-emitting diodes;

Perovskite film generally has rough surface morphology due to the voids between the grain domains. Smoothed interface contact between the perovskite layer and the top electrode is critical for planar perovskite solar cells. We reported high efficiency bromine–iodine based perovskite solar cells with a flattening cathode interface by incorporating a solution-processed bathocuproine (sBCP) interfacial layer at the cathode side. Compared with vacuum evaporated bathocuproine (eBCP), sBCP demonstrated an excellent surface modification effect at the cathode side with very smaller charge transfer resistance. Accordingly, a high fill factor exceeding 85% and a power conversion efficiency exceeding 13% in CH3NH3PbI3−xBrx based perovskite solar cells were achieved. The largely improved fill factor was attributed to the smooth film morphology and full surface coverage of perovskite films modified by the solution-processed BCP layer.

In this work, NH2CHNH2PbI3 (FAPbI3) was employed for light harvesting in inverted planer perovskite solar cells for the first time. Except for the silver cathode, all layers were solution-processed under or below 140 °C. The effect of the annealing process on device performance was investigated. The FAPbI3 solar cells based on a slowed-down annealing shows superior performance compared to the CH3NH3PbI3 (MAPbI3)-based devices, especially for the short circuit current density. A power conversion efficiency of 13.56% was obtained with high short circuit current density of 21.48 mA cm−2. This work paves the way for low-temperature fabrication of efficient inverted planer structure FAPbI3 perovskite solar cells.

Aluminum and silver (Al/Ag) stacked films are utilized as the anode in ITO free top-emitting organic light-emitting devices (TEOLEDs). Serious short circuit issues can be resolved since the stacked metal films can increase the crystallinity and smoothen the surface morphology to suppress the poor infiltration between pure Ag and glass substrates. Optical simulations are carried out based on a transfer matrix method and microcavity effect to guide the real fabrications of the fluorescent TEOLEDs. The stacked Al (56 nm)/Ag (44 nm) anode based TEOLEDs demonstrate a better device performance than that of the Al-only anode based devices. The proposed stacked metal electrode provides a simple and convenient way to fabricate TEOLEDs with suppressed electrical short circuits.

•An optical energy loss mechanism in OLEDs is introduced based on CPS theory.•Theoretical calculations of both OCE and EQE in OLEDs are carried out.•The simulation results are further validated experimentally.An optical energy loss mechanism including the surface plasmon polariton (SPP) loss, wave guide (WG) mode and substrate mode in organic light-emitting diodes (OLEDs) is introduced based on CPS theory. The theoretical calculations of both the out-coupling efficiency (OCE) and the external quantum efficiency (EQE) of OLEDs are proposed. MATLAB tools are applied to simulate the optical model and provide the results of the two efficiencies. It is demonstrated that, the OCE and the EQE in a green phosphorescence OLED with optimized device structure can reach up to 20% and 27%, respectively (intrinsic quantum efficiency q = 90% assumed). The simulation results based on the theoretical model are further validated experimentally.

The authors demonstrate an aqueous solution-processed Cs2CO3 thin film with an adjustable work function via MoO3 and/or Na2WO4 doping. The doped Cs2CO3 as a cathode interfacial layer is successfully used in poly(3-hexyl-thiophene) (P3HT)/indene-C60 bisadduct (IC60BA) heterojunction based solar cells with improved open-circuit voltage and unaffected short-circuit current density. X-ray photoelectron spectroscopy (XPS) evaluation was conducted to verify the formation of the new composites of W–O–Cs and Mo–O–Cs after doping of MoO3 and/or Na2WO4 into Cs2CO3. The change of the work function of MoO3- and/or Na2WO4-doped Cs2CO3 was further confirmed by ultraviolet photoelectron spectroscopy (UPS) measurements.

An intermediate connector (IC) consisting of lithium (Li) doped 4,7-diphenyl-1,10-phenanthroline (BPhen)/Al/tetrafluoro-tetracyanoquinodimethane (F4-TCNQ)/1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) is developed for fabricating tandem white organic light-emitting diodes (WOLEDs). An investigation of the charge generation and separation process in Bphen:Li/Al/F4-TCNQ/HAT-CN, which is carried out by analysis of the current–voltage and capacitance–voltage characteristics, shows that the proposed IC structure is suitable as a connecting unit in tandem OLEDs. The tandem WOLED based on a silicon compound host material of 10-phenyl-2′-(triphenylsilyl)-10H-spiro [acridine-9,9′-fluorene] (SSTF) with the proposed IC structure exhibits a maximum current efficiency of 159.2 cd A−1 and a maximum power efficiency of 69.4 lm W−1. For application in large-area OLEDs, a 150 × 150 mm2 tandem lighting panel with maximum efficiencies of 231.8 cd A−1 and 52.9 lm W−1, correlated color temperature of 3000 K and Commission International de I'Eclairage (CIE) coordinates of (0.34, 0.45) is also demonstrated.

Organic light-emitting diodes (OLEDs) have attracted tremendous interest and have already become a prevalent technology in MP3 players, smartphones and cameras. In response to the calls for the large-scale application of OLEDs, the complicated and costly processes for preparing a device is a major challenge which should be addressed. Herein, a novel bipolar host material, 26PyzCz, which contains a pyrazine/carbazole hybrid, has been designed and synthesized. 26PyzCz-based single-layer (SL) fluorescent (F)–phosphorescent (P) OLEDs with various colors have been successfully fabricated. Green and orange SL phosphorescent OLEDs (PHOLEDs) have exhibited efficiencies as high as 63.3 and 62.1 cd A−1 at 1000 cd m−2, and 55.7 and 53.8 cd A−1 at 10000 cd m−2, respectively. Meanwhile, a SL warm white OLED based on fluorescent blue and phosphorescent orange has demonstrated excellent performance, with a maximum current efficiency of 27.5 cd A−1 and a maximum power efficiency of 21.6 lm W−1. In addition, the charge carrier behavior have been evaluated by impedance spectroscopy, which revealed that the dopant trapping effect plays a critical role in charge balance and exciton generation in the SL PHOLEDs.

•Charge transport and electronic traps in P3HT:PCBM blends was evaluated in dark and illumination.•Charge transport properties were analyzed based on current–voltage characteristics measurements.•Electronic trap density was evaluated by a differential method.•The charge transport was strongly associated with the trap states distribution.A direct comparison of charge transport and electronic traps in representative polymer–fullerene blend, poly (3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM), is carried out in dark and illuminated conditions based on the measurements of temperature-dependent current–voltage characteristics. In dark condition, the charge transport presents a transition from Ohmic to trap-limited current. While the trap-filled space charge limited current is observed under illumination at the same applied bias. From evaluations of trap density and energy distribution by a differential method, it is reveal that the diverse charge transport in dark and illuminated conditions is mainly caused by the different trap states distribution, which strongly affects the space charges and the electrical field in P3HT: PCBM blends.Graphical abstract

This paper investigates the effects of localized surface plasmon resonance (LSPR) in an inverted polymer/fullerene solar cell by incorporating Au and/or Ag nanoparticles (NPs) into the TiO2 buffer layer. Enhanced light harvesting via plasmonic resonance of metal NPs has been observed. It results in improved short-circuit current density (Jsc) while the corresponding open-circuit voltage (Voc) is maintained. A maximum power conversion efficiency of 7.52% is obtained in the case of introducing 30% Ag NPs into the TiO2, corresponding to a 20.7% enhancement compared with the reference device without the metal NPs. The device photovoltaic characteristics, photocurrent properties, steady-state and dynamic photoluminescences of active layer on metal NP-doped TiO2, and electric field profile in metal NP-doped TiO2 layers are systematically investigated to explore how the plasmonic effects of Au and/or Ag NPs influence the OSC performance.Keywords: localized surface plasmon resonance; metal nanoparticles; polymer solar cells;

A simple and cheap method for depositing solution-processed GeO2 (sGeO2) film is proposed utilizing the weak solubility of GeO2 in water. X-ray photoelectron spectroscopy analysis reveals that a pure GeO2 thin film can be formed by casting its aqueous solution. This method can avoid the difficulty of vacuum evaporation by its high melting point. The sGeO2 film has been used successfully as an anode interfacial layer in poly(3-hexylthiophene) (P3HT) and indene-C60 bisadduct (IC60BA)-based bulk heterojunction organic solar cells with improved power conversion efficiency and device stability compared with that using conventional poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS); the improvement of the power conversion efficiency and the device stability are estimated to be 9% and 50%, respectively. The calculations of optical intensity in a whole cell demonstrate that a thin layer of sGeO2 could function as an optical spacer in the based bulk heterojunction (BHJ) organic solar cells (OSCs) for enhancing the light harvesting in the active layer. Interfacial evaluation by impedance spectroscopy shows that the sGeO2-based cell exists less charge carrier recombination and lower contact resistance. More importantly, the sGeO2 film processing is very simple and environmentally friendly, which has potential applications in green and low-cost organic electronics in the future.Keywords: device stability; GeO2 aqueous solution; interfacial layer; optical space effect; organic solar cells; solution processing;

A high efficiency phosphorescent organic light-emitting diode (OLED) has been fabricated by introducing a double exciton-blocking layer (d-EBL) between the hole-transporting layer and the light-emitting layer in the device. The device exhibits a yellow emission with a maximum current efficiency of 58.5 cd/A at 117 cd/m2, corresponding to the power efficiency of 50.9 lm/W, which is two times improved compared with that of devices having only one traditional single exciton-blocking layer (s-EBL). The efficiency improvement has been investigated through the electroluminescence (EL) spectral analyses in the phosphorescent guest-doped and the non-doped OLEDs. The results demonstrate that the electrons are blocked and the excitons are confined more effectively in the d-EBL-based devices than that in the s-EBL-based devices. In addition, over two times improvement in the lifetime is also achieved in the devices with the d-EBL compared with the devices having a traditional s-EBL.Graphical abstractHighlights► A double exciton-blocking layer is used in a PHOLED. ► Remarkable enhancements in EL efficiency and stability are achieved. ► The mechanism of better electron and exciton confining capability is investigated. ► The exciton–exciton annihilation can be suppressed as well in the device.

•A physical investigation was carried out in MoO3-doped pentacene films.•The charge transport properties were analyzed based on current-voltage characteristics measurements.•Trap density in MoO3-doped pentacene was evaluated by a differential method.•Conducting mechanism in MoO3-doped pentacene was investigated via Mo 3d valence state analysis.Molybdenum trioxide (MoO3) doped organic semiconductors have shown attractive applications in organic electric devices. The authors carried out an investigation on the origin of enhanced photoelectric characteristics in MoO3-doped pentacene films. Electrical properties including charge transport, trap density and conductivity in bulk MoO3-doped pentacene films were investigated through fundamental measurements of current-voltage characteristics. Electrical structure and conducting mechanism in MoO3-doped pentacene films were further evaluated by X-ray diffraction and X-ray photoelectron spectroscopy measurements. The experimental results suggest that the improved conductivity in MoO3-doped pentacene film was partly associated with the increased ratio of low Mo oxidation state (Mo4+) with a fact of better conducting property of MoO2 than that MoO3.Graphical abstract

The authors demonstrate an effective anode interfacial layer based on aqueous solution-processed MoO3 (sMoO3) in poly (3-hexylthiophene) (P3HT) and indene-C60 bisadduct (ICBA) based bulk-heterojunction organic solar cells (PSCs). Various sMoO3 concentration (0.03–0.25 wt%) was obtained by dissolving MoO3 powder into deionized water directly with weak solubility. The characteristics of sMoO3 films evaluated by atomic force microscope (AFM) and scanning electron microscope (SEM) suggest that the sMoO3 films continuously cover the entire indium tin oxide (ITO) surface. The sMoO3 based PSCs exhibit comparable power conversion efficiency with poly (3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT:PSS) based devices. However, even more importantly, the stability of sMoO3 based devices have been greatly improved in air under continual light-illumination at 52 mW/cm2. Further evaluations on Mo valence states and work function of sMoO3 films by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) demonstrate that the aqueous solution-processed MoO3 could act as an better anode interfacial layer than the conventional PEDOT:PSS.Graphical abstractHighlights► Water-soluble MoO3 (sMoO3) was used to form a thin layer in organic solar cells. ► The preparation of sMoO3 layer is simple and environment-friendly. ► sMoO3 based devices are comparable to that of PEDOT:PSS in conversion efficiency. ► A better stability was obtained from sMoO3 based devices.

•A single layer graphene was modified with TiOx and PEDOT:PSS.•The sheet resistance of the modified graphene was reduced by 86% from 628 Ω/sq to 86 Ω/sq.•The work function of the modified graphene was increased by 0.82 eV from 4.30 eV to 5.12 eV.•Enhanced charge injection and transport were achieved from the modified graphene.There are many challenges for a direct application of graphene as the electrodes in organic electronics due to its hydrophobic surfaces, low work function (WF) and poor conductance. The authors demonstrate a modified single-layer graphene (SLG) as the anode in organic light-emitting diodes (OLEDs). The SLG, doped with the solution-processed titanium suboxide (TiOx) and poly(3,4-ethylenedio-xythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS), exhibits excellent optoelectronic characteristics with reduced sheet resistance (Rsq), increased work function, as well as over 92% transmittance in the visible region. It is notable that the Rsq of graphene decreased by ∼86% from 628 Ω/sq to 86 Ω/sq and the WF of graphene increased about 0.82 eV from 4.30 eV to 5.12 eV after a modification by using the TiOx–PEDOT:PSS double interlayers. In addition, the existence of additional TiOx and PEDOT:PSS layers offers a good coverage to the PMMA residuals on SLG, which are often introduced during graphene transfer processes. As a result, the electrical shorting due to the PMMA residues in the device can be effectively suppressed. By using the modified SLG as a bottom anode in OLEDs, the device exhibited comparable current efficiency and power efficiency to those of the ITO based reference OLEDs. The approach demonstrated in this work could potentially provide a viable way to fabricate highly efficient and flexible OLEDs based on graphene anode.Graphical abstract

An ultrathin layer of indium trichloride (InCl3) is thermally evaporated on the indium tin oxide (ITO) anode to enhance the hole injection in simplified phosphorescent organic light-emitting diodes (PHOLEDs). Comparing with the device with ultraviolet (UV)-ozone treatment, the device modified by InCl3 exhibits a maximum current efficiency of 82.2 cd/A measured at about 2000 cd/cm2 and 36% improvement in power efficiency measured at 20 mA/cm2. More importantly, more than three times improvement in half lifetime estimated at an initial luminance of 1000 cd/cm2 is achieved. The investigations using ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and the bias- and temperature-dependent current density–voltage characteristics in the related hole-dominated devices have revealed that the improved device performance is mainly attributed to the enhanced hole injection resulting from the lowered hole injection barrier height in the InCl3-modified devices.Keywords: hole injection; indium trichloride; phosphorescent OLEDs; surface modification; thermal evaporation; work function;

An InCl3 dipole layer is inserted into a copper phthalocyanine (CuPc) and fullerene (C60) based organic photovoltaic cell (OPV) to modify the indium-tin-oxide (ITO) anode surface. The work function of the ITO is improved from 4.63 eV to 5.47 eV. In addition, a 30% enhancement in absorption coefficient is achieved due to the strong interaction between CuPc and InCl3 molecules, which induces a configuration change of the CuPc stacks from perpendicular to parallel along the ITO substrate. Therefore, the power conversion efficiency of the OPV devices has a 30% improvement because of the improved work function of the ITO anode and the enhanced absorption coefficient of the devices.Graphical abstractHighlights► The work function of InCl3 modified ITO can be increased from 4.63 eV to 5.47 eV. ► Optical absorption enhancement can be obtained from a CuPc film grown on the ITO. ► 30% improvement in performance of the OPV devices can be then achieved.

Photovoltaic performance of planar perovskite hybrid solar cells (pero-HSCs) has been improved by mixing CH3NH3PbIxCl3−x and an electron donor polymer [N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiaz-ole)] (PCDTBT). PCDTBT contains lone pairs of electrons due to the presence of S and N atoms, which could passivate the trap states of the perovskite layer and thus reduce the number of film defects. A Stonehenge-like structure could be formed by the interaction of CH3NH3PbIxCl3−x and PCDTBT, developing more ordered orientation crystallization and a high quality film morphology. The doped solar cells are characterized by their excellent photovoltaic properties and enhanced stability. When the doping concentration is 0.3 mg mL−1, the fabricated solar cell device exhibits an outstanding power conversion efficiency (PCE) of 15.76%, which represents a significant improvement with respect to the magnitude of 16% obtained for the reference device.

Anode modification by doping silver nano-particles (Ag NPs) into poly(3,4-ethylene dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) and cathode interfacial modification by inserting solution-processed bathophenanthroline (sBphen) in CH3NH3PbI3−xClx based planar perovskite solar cells are investigated. Prior to the optical effect such as localized surface plasmon resonance, the Ag-NPs distributed in PEDOT:PSS mainly cause an improvement in the electrical property of PEDOT:PSS–Ag NPs composite films. The sBphen interfacial layer modified the surface morphology of perovskite/phenyl-C61-butyric acid methyl ester (PC61BM) films by filling the voids on the surface of perovskite/PC61BM effectively, which led to an obvious improvement in the fill factor. Accordingly, an efficient device with a power conversion efficiency of 15.75% was achieved due to the simultaneous cathode and anode interfacial modification.

The authors demonstrate an aqueous solution-processed Cs2CO3 thin film with an adjustable work function via MoO3 and/or Na2WO4 doping. The doped Cs2CO3 as a cathode interfacial layer is successfully used in poly(3-hexyl-thiophene) (P3HT)/indene-C60 bisadduct (IC60BA) heterojunction based solar cells with improved open-circuit voltage and unaffected short-circuit current density. X-ray photoelectron spectroscopy (XPS) evaluation was conducted to verify the formation of the new composites of W–O–Cs and Mo–O–Cs after doping of MoO3 and/or Na2WO4 into Cs2CO3. The change of the work function of MoO3- and/or Na2WO4-doped Cs2CO3 was further confirmed by ultraviolet photoelectron spectroscopy (UPS) measurements.

Carrier injection plays an important role in determining the device performance of organic light-emitting diodes (OLEDs). 1,4,5,8,9,11-Hexaazatriphenylene hexacarbonitrile (HAT-CN) has been widely used as an effective material to promote the hole injection when fabricating vacuum deposited OLEDs. However, serious crystallization occurs in solution-processed HAT-CN films, which weakens its hole injection ability in OLEDs. Herein, we demonstrate a solution-processed composite film as the hole injection layer (HIL) in OLEDs developed by mixing HAT-CN with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). The crystallization is suppressed effectively by optimizing the mixing ratio. In addition, a doping of HAT-CN:F4-TCNQ composite film contributes to a decreased hole injection barrier, which can be reflected from the current density–voltage curve. Furthermore, HAT-CN:F4-TCNQ is successfully combined with MoO3 doped copper phthalocyanine-3,4′,4′′,4′′′-tetra-sulfonated acid tetra sodium salt (TS-CuPc) as bi-HIL in solution-processable blue phosphorescent OLEDs, which exhibit a maximum current efficiency of 16.7 cd A−1.

Green phosphorescent inverted organic light-emitting diodes (IOLEDs) with 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN)/aluminium/n-doped 4,7-diphenyl-1,10-phenanthroline (Bphen) used as an electron injection layer (EIL) were demonstrated. The IOLED shows the lowest driving voltage of 4.5 V at 10000 cd m−2 to date. The electron injection effects of different interlayers were further investigated by ultraviolet photoelectron spectroscopy (UPS) and evaluating the electron injection efficiency. For application in large-sized OLEDs, a 120 × 120 mm2 flexible IOLED was successfully fabricated based on this inverted structure.

An intermediate connector (IC) consisting of lithium (Li) doped 4,7-diphenyl-1,10-phenanthroline (BPhen)/Al/tetrafluoro-tetracyanoquinodimethane (F4-TCNQ)/1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) is developed for fabricating tandem white organic light-emitting diodes (WOLEDs). An investigation of the charge generation and separation process in Bphen:Li/Al/F4-TCNQ/HAT-CN, which is carried out by analysis of the current–voltage and capacitance–voltage characteristics, shows that the proposed IC structure is suitable as a connecting unit in tandem OLEDs. The tandem WOLED based on a silicon compound host material of 10-phenyl-2′-(triphenylsilyl)-10H-spiro [acridine-9,9′-fluorene] (SSTF) with the proposed IC structure exhibits a maximum current efficiency of 159.2 cd A−1 and a maximum power efficiency of 69.4 lm W−1. For application in large-area OLEDs, a 150 × 150 mm2 tandem lighting panel with maximum efficiencies of 231.8 cd A−1 and 52.9 lm W−1, correlated color temperature of 3000 K and Commission International de I'Eclairage (CIE) coordinates of (0.34, 0.45) is also demonstrated.

Perovskite film generally has rough surface morphology due to the voids between the grain domains. Smoothed interface contact between the perovskite layer and the top electrode is critical for planar perovskite solar cells. We reported high efficiency bromine–iodine based perovskite solar cells with a flattening cathode interface by incorporating a solution-processed bathocuproine (sBCP) interfacial layer at the cathode side. Compared with vacuum evaporated bathocuproine (eBCP), sBCP demonstrated an excellent surface modification effect at the cathode side with very smaller charge transfer resistance. Accordingly, a high fill factor exceeding 85% and a power conversion efficiency exceeding 13% in CH3NH3PbI3−xBrx based perovskite solar cells were achieved. The largely improved fill factor was attributed to the smooth film morphology and full surface coverage of perovskite films modified by the solution-processed BCP layer.

In this work, NH2CHNH2PbI3 (FAPbI3) was employed for light harvesting in inverted planer perovskite solar cells for the first time. Except for the silver cathode, all layers were solution-processed under or below 140 °C. The effect of the annealing process on device performance was investigated. The FAPbI3 solar cells based on a slowed-down annealing shows superior performance compared to the CH3NH3PbI3 (MAPbI3)-based devices, especially for the short circuit current density. A power conversion efficiency of 13.56% was obtained with high short circuit current density of 21.48 mA cm−2. This work paves the way for low-temperature fabrication of efficient inverted planer structure FAPbI3 perovskite solar cells.

Organic light-emitting diodes (OLEDs) have attracted tremendous interest and have already become a prevalent technology in MP3 players, smartphones and cameras. In response to the calls for the large-scale application of OLEDs, the complicated and costly processes for preparing a device is a major challenge which should be addressed. Herein, a novel bipolar host material, 26PyzCz, which contains a pyrazine/carbazole hybrid, has been designed and synthesized. 26PyzCz-based single-layer (SL) fluorescent (F)–phosphorescent (P) OLEDs with various colors have been successfully fabricated. Green and orange SL phosphorescent OLEDs (PHOLEDs) have exhibited efficiencies as high as 63.3 and 62.1 cd A−1 at 1000 cd m−2, and 55.7 and 53.8 cd A−1 at 10000 cd m−2, respectively. Meanwhile, a SL warm white OLED based on fluorescent blue and phosphorescent orange has demonstrated excellent performance, with a maximum current efficiency of 27.5 cd A−1 and a maximum power efficiency of 21.6 lm W−1. In addition, the charge carrier behavior have been evaluated by impedance spectroscopy, which revealed that the dopant trapping effect plays a critical role in charge balance and exciton generation in the SL PHOLEDs.

A new red phosphorescent material Ir(dmppm)2(dmd), which is a pyrimidine-based iridium(III) complex, has been synthesized and successfully used to fabricate solution-processed red and white organic light-emitting diodes (OLEDs). Due to its excellent solubility in common organic solvents and its good compatibility with the host material, a record current efficiency of 27.2 cd A−1 so far with satisfactory Commission International de l'Eclairage (CIE) coordinates of (0.60, 0.40) has been achieved for partially solution-processed red OLEDs by using Ir(dmppm)2(dmd) as a dopant. Furthermore, the fabricated two-component “warm-white” OLEDs based on the Ir(dmppm)2(dmd) red emitter demonstrate a maximum current efficiency of 28.9 cd A−1, which can meet the call for physiologically-friendly indoor illumination.

The authors develop an aqueous solution-processed hole injection layer, MoO3 doped copper phthalocyanine-3,4′,4′′,4′′′-tetra-sulfonated acid tetra sodium salt (TS-CuPc), in organic light-emitting diodes (OLEDs) via an environmentally-friendly and easy fabrication process. The generation of a charge transfer complex in TS-CuPc:MoO3 composite films is confirmed by absorption spectra and X-ray photoemission spectroscopy (XPS) measurements. Enhanced hole injection in OLEDs is attributed to the decreased hole barrier at the ITO side, which is in agreement with the Schottky thermal emission evaluation. The efficient modification of ITO by TS-CuPc:MoO3 is further confirmed by ultraviolet photoemission spectroscopy (UPS) measurements.

The fabrication and device parameters of inverted planar heterojunction (PHJ) organic–inorganic lead mixed-halide (CH3NH3PbI3−xClx) perovskite based solar cells (PSCs) using a:CuAlO2 as the hole selective buffer layer between the ITO electrode and PEDOT:PSS were demonstrated. Thin films of a:CuAlO2 were derived from a pre-fabricated polycrystalline CuAlO2 ceramic target by using the direct current (d.c.) magnetron sputtering technique. The one-step spin coating method was used to prepare the perovskite layer. A short circuit current density (Jsc) of 21.98 mA cm−2, an open circuit voltage (Voc) of 0.88 V, a fill factor (FF) of 0.75 and a power conversion efficiency (PCE) of 14.52% were achieved for the optimized device. These improved device parameters were also accompanied by improved stability as a result of sandwiching the ambient stable a:CuAlO2 layer with decent conductivity between the ITO and the PEDOT:PSS layers. The versatility of this material application was also demonstrated as a similar improvement in device performance and stability, which was observed by using the prepared a:CuAlO2 in another perovskite solar cell system based on CH3NH3PbI3 prepared by the two-step spin-coating method.

The authors demonstrate a honeycomb structured organic light-emitting diode (OLED) with high enhancements greater than 2.0 fold and 2.3 fold in current efficiency and power efficiency, respectively. The dispersion relationships in both planar and nano-honeycomb structured OLEDs are calculated through numerical simulations utilizing the finite-difference time-domain method and measured through the electroluminescence spectra. There is good agreement between the numerically calculated and the experimentally measured dispersion relationships for the nano-honeycomb structured OLEDs. Improved light out-coupling efficiency is mainly attributed to the efficient extraction of the waveguide and the surface plasmon polariton (SPP) loss modes in the devices. Particularly, most of the extracted energy is verified to be originated from the SPP loss mode in honeycomb structured OLEDs.