Efficient Perovskite Solar Cells Based on Multilayer Transparent Electrodes through Morphology Control

A multilayer transparent electrode WO₃/Ag/WO₃ (WAW) has been introduced into perovskite solar cells (PSCs). It is found that the substrate has an obvious effect on the perovskite morphology and crystallization and thus power conversion efficiency (PCE) of the PSCs. The precursor composition and its effect on the morphology, crystal, and device properties of the perovskite films based on WAW and ITO electrodes have been investigated in detail. When the CH₃NH₃I (MAI):PbI₂ molar ratio is 1.04:1, the perovskite film shows flat and dense morphology formed by the complete reaction of MAI and PbI₂, and PSC device shows the maximum PCE value of 9.73%, comparable with the controlled device with the MAI:PbI₂ molar ratio of 1:1 based on ITO electrode (10.51%). Meanwhile, a flexible PSC based on WAW transparent electrode has also been fabricated, which exhibits a PCE of 8.04%, indicating that WAW multilayer transparent electrodes have the potential application in PSCs, especially in flexible PSCs

Pixeled Electroluminescence from Multilayer Heterostructure Organic Light-Emitting Transistors

Improved performance of multilayer heterostructure organic light-emitting transistors (OLETs) was observed in brightness and external quantum efficiency (EQE) by inserting an ultrathin MoO_<i>x</i> layer and TPBI buffer layer. With in-plane emission mainly beneath the drain electrode with a maximum width of 120 μm, an EQE of 0.16% at a brightness of 238 cd/m² was obtained. Different sizes of pixeled OLETs were fabricated by restricting the pixel length by narrowing the width of the gate electrode perpendicular to the source/drain electrodes. Light emission pixels with sizes from 25 to 400 μm have been successfully demonstrated. The maximum width of the emission zone was not affected, and the maximum EQE and the corresponding brightness presented an increasing tendency for pixeled OLETs. The results in our work are helpful for developing a new generation of OLET-based display technology

Dual-Functional WO₃ Nanocolumns with Broadband Antireflective and High-Performance Flexible Electrochromic Properties

The three-dimensional, high-porous, and oriented WO₃ nanocolumn film with broadband antireflective and high-performance flexible electrochromic dual-functionalities is achieved by utilizing a simple, one-step, room-temperature glancing angle deposition without any catalysts and templates. It is found that the WO₃ nanocolumn film is effective in increasing the optical transparency in the visible range, enhancing the color-switching response time as well as improving the mechanical flexibility and electrochemical cycling stability in comparison to dense WO₃ film. The further optical, morphological, and electrode reaction kinetics analyses reveal that these improvements can be attributed to its unique porous nanocolumn arrays, which reduce the refractive index, facilitate the interfacial charge-transfer and ion-penetration, and alleviate the internal stress of the film under the bending treatment. These results would provide a simple and effective guidance to design and construct low-cost, robust, flexible, stable, and transparent electrochromic smart windows

Low-Work-Function, ITO-Free Transparent Cathodes for Inverted Polymer Solar Cells

A low-work-function, indium tin oxide (ITO)-free transparent cathode having a tin oxide (SnO_X)/Ag/SnO_X/bismuth oxide (Bi₂O₃) (SASB) structure is developed without using annealing treatment. This represents the first time that Bi₂O₃ has been introduced to lower the work function of transparent electrodes. The SASB transparent cathode exhibits excellent photoelectric properties with a maximum transmittance of ∼88%, a low sheet resistance of ∼9.0 Ω·sq^–1, and a suitable work function of 4.22 eV that matches the lowest unoccupied molecular orbital level of the acceptor for exacting electrons efficiently. The power conversion efficiency of the polymer solar cell with the SASB electrode is 6.21%, which is comparable to that of ITO-based devices. The results indicate that SASB is a good alternative to ITO as transparent cathodes in optoelectronic devices

Harvesting Triplet Excitons with Exciplex Thermally Activated Delayed Fluorescence Emitters toward High Performance Heterostructured Organic Light-Emitting Field Effect Transistors

The utilization of triplet excitons plays a key role in obtaining high emission efficiency for organic electroluminescent devices. However, to date, only phosphorescent materials have been implemented to harvest the triplet excitons in the organic light-emitting field effect transistors (OLEFETs). In this work, we report the first incorporation of exciplex thermally activated delayed fluorescence (TADF) emitters in heterostructured OLEFETs to harvest the triplet excitons. By developing a new kind of exciplex TADF emitter constituted by m-MTDATA (4,4′,4″-tris­(<i>N</i>-3-methylphenyl-<i>N</i>-phenylamino)­triphenylamine) as the donor and OXD-7 (1,3-bis­[2-(4-<i>tert</i>-butylphenyl)-1,3,4-oxadiazo-5-yl]­benzene) as the acceptor, an exciton utilization efficiency of 74.3% for the devices was achieved. It is found that the injection barrier between hole transport layer and emission layer as well as the ratio between donor and acceptor would influence the external quantum efficiency (EQE) significantly. Devices with a maximum EQE of 3.76% which is far exceeding the reported results for devices with conventional fluorescent emitters were successfully demonstrated. Moreover, the EQE at high brightness even outperformed the result for organic light-emitting diode based on the same emitter. Our results demonstrate that the exciplex TADF emitters can be promising candidates to develop OLEFETs with high performance

Efficient and Stable Red Emissive Carbon Nanoparticles with a Hollow Sphere Structure for White Light-Emitting Diodes

Red-emissive solid-state carbon nanoparticles (CNPs) with a hollow sphere structure for white light-emitting diodes (WLEDs) were designed and synthesized by molecular self-assembly and microwave pyrolysis. Highly ordered graphite-like structures for CNPs were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible (UV–vis) spectroscopy. The emission mechanism of the red-emissive solid-state CNPs was investigated in detail by steady-state and time-resolved photoluminescence (PL) spectroscopy. The as-prepared CNPs showed a red emission band centered at 620 nm with excitation wavelength independence, indicating uniform size of sp² carbon domains in the CNPs. The CNPs also had a PL quantum yield (QY) of 17% under 380 nm excitation. Significantly, the PL QY of the organosilane-functionalized CNPs was 47%, which is the highest value recorded for red-emissive solid-state carbon-based materials under UV-light excitation. More importantly, the red-emissive CNPs exhibited a PL QY of 25% after storage in air for 12 months, indicating their excellent stability. The red-emissive CNP powders were used as environmentally friendly and low-cost phosphors on a commercial 460 nm blue GaN-based chip, and a pure white light with CIE coordinates of (0.35, 0.36) was achieved. The experimental results indicated that the red-emissive CNP phosphors have potential applications in WLEDs

Bifunctional MoO₃–WO₃/Ag/MoO₃–WO₃ Films for Efficient ITO–Free Electrochromic Devices

Dielectric–metal–dielectric (DMD) trilayer films, served as both electrochromic (EC) film and transparent conductor (TC), have exhibited great potential application in low–cost, ITO–free electrochromic devices (ECDs). However, recent reports on the DMD–based ECDs revealed that the response time and the optical modulation properties were not very satisfactory. Here, the mixed MoO₃–WO₃ materials were first introduced as the dielectric layer to construct an EC–TC bifunctional MoO₃–WO₃/Ag/MoO₃–WO₃ (MWAMW) film, which demonstrates strong and broad–band optical modulation in the visible light region, fast color-switching time (2.7 s for coloration and 4.1 s for bleaching), along with high coloration efficiency (70 cm² C^–1). The electrical structure and electrochemical reaction kinetics analysis revealed that the improved EC performances are associated with the increased electron intervalence transition together with the fast charge–transfer and ion–diffusion dynamics

Improved Performance of Organic Light-Emitting Field-Effect Transistors by Interfacial Modification of Hole-Transport Layer/Emission Layer: Incorporating Organic Heterojunctions

Organic heterojunctions (OHJs) consisting of a strong electron acceptor 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) and an electron donor N,N′-di­(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) were demonstrated for the first time that they can be implemented as effective modification layers between hole transport layer (HTL) and emission layer in the heterostructured organic light-emitting field effect transistors (OLEFETs). The influence of both HAT-CN/NPB junction (npJ) and NPB/HAT-CN junction (pnJ) on the optoelectronic performance of OLEFETs were conscientiously investigated. It is found that both the transport ability of holes and the injection ability of holes into emissive layer can be dramatically improved via the charge transfer of the OHJs and that between HAT-CN and the HTL. Consequently, OLEFETs with pnJ present optimal performance of an external quantum efficiency (EQE) of 3.3% at brightness of 2630 cdm^–2 and the ones with npJs show an EQE of 4.7% at brightness of 4620 cdm^–2. By further utilizing npn OHJs of HAT-CN/NPB/HAT-CN, superior optoelectronic performance with an EQE of 4.7% at brightness of 8350 cdm^–2 and on/off ratio of 1 × 10⁵ is obtained. The results demonstrate the great practicality of implementing OHJs as effective modification layers in heterostructured OLEFETs

High-Performance NiO/Ag/NiO Transparent Electrodes for Flexible Organic Photovoltaic Cells

Transparent electrodes with a dielectric–metal–dielectric (DMD) structure can be implemented in a simple manufacturing process and have good optical and electrical properties. In this study, nickel oxide (NiO) is introduced into the DMD structure as a more appropriate dielectric material that has a high conduction band for electron blocking and a low valence band for efficient hole transport. The indium-free NiO/Ag/NiO (NAN) transparent electrode exhibits an adjustable high transmittance of ∼82% combined with a low sheet resistance of ∼7.6 Ω·s·<i>q</i>^–1 and a work function of 5.3 eV after UVO treatment. The NAN electrode shows excellent surface morphology and good thermal, humidity, and environmental stabilities. Only a small change in sheet resistance can be found after NAN electrode is preserved in air for 1 year. The power conversion efficiencies of organic photovoltaic cells with NAN electrodes deposited on glass and polyethylene terephthalate (PET) substrates are 6.07 and 5.55%, respectively, which are competitive with those of indium tin oxide (ITO)-based devices. Good photoelectric properties, the low-cost material, and the room-temperature deposition process imply that NAN electrode is a striking candidate for low-cost and flexible transparent electrode for efficient flexible optoelectronic devices

Efficient Inorganic Perovskite Light-Emitting Diodes with Polyethylene Glycol Passivated Ultrathin CsPbBr₃ Films

Efficient inorganic perovskite light-emitting diodes (PeLEDs) with an ultrathin perovskite emission layer (∼30 nm) were realized by doping Lewis base polyethylene glycol (PEG) into CsPbBr₃ films. PEG in the perovskite films not only physically fills the crystal boundaries but also interacts with the perovskite crystals to passivate the crystal grains, reduce nonradiative recombination, and ensure efficient luminance and high efficiency. As a result, promoted brightness, current efficiency (CE), and external quantum efficiency (EQE) were achieved. The nonradiative decay rate of the PEG:CsPbBr₃ composite film is 1 order of magnitude less than that of the neat CsPbBr₃ film. After further optimization of the molar ratio between CsBr and PbBr₂, a peak CE of 19 cd/A, a maximum EQE of 5.34%, and a maximum brightness of 36600 cd/m² were achieved, demonstrating the interaction between PEG and the precursors. The results are expected to offer some helpful implications in optimizing the polymer-assisted PeLEDs with ultrathin emission layers, which might have potential application in see-through displays