N-type Electron Transport Layer Research Articles

Low-Temperature Processed TiOx Electron Transport Layer for Efficient Planar Perovskite Solar Cells.

The most frequently used n-type electron transport layer (ETL) in high-efficiency perovskite solar cells (PSCs) is based on titanium oxide (TiO2) films, involving a high-temperature sintering (>450 °C) process. In this work, a dense, uniform, and pinhole-free compact titanium dioxide (TiOx) film was prepared via a facile chemical bath deposition process at a low temperature (80 °C), and was applied as a high-quality ETL for efficient planar PSCs. We tested and compared as-deposited substrates sintered at low temperatures (< 150 °C) and high temperatures (> 450 °C), as well as their corresponding photovoltaic properties. PSCs with a high-temperature treated TiO2 compact layer (CL) exhibited power conversion efficiencies (PCEs) as high as 15.50%, which was close to those of PSCs with low-temperature treated TiOx (14.51%). This indicates that low-temperature treated TiOx can be a potential ETL candidate for planar PSCs. In summary, this work reports on the fabrication of low-temperature processed PSCs, and can be of interest for the design and fabrication of future low-cost and flexible solar modules.

Enhanced Efficiency of InP-Based Red Quantum Dot Light-Emitting Diodes.

Due to the inherent toxicity of cadmium selenide (CdSe)-based quantum dots (QDs), Cd-free alternatives are being widely investigated. Indium phosphide (InP) QDs have shown great potential as a replacement for CdSe QDs in display applications. However, the performance of InP-based quantum dot light-emitting diodes (QLEDs) is still far behind that of the CdSe-based devices. In this study, we wanted to show the effects of different approaches to improving the performance of InP-based QLED devices. We investigated the effect of magnesium (Mg) doping in ZnO nanoparticles, which is used as an n-type electron transport layer, in balancing the charge transfer in InP-based QLED devices. We found that an increasing Mg doping level can broaden ZnO band gap, shift its energy levels, but most importantly, increase its resistivity; as a result, the electron current density is significantly reduced and the device efficiency is improved. We also investigated the effect of high-photoluminescence quantum yield emitters and different QLED architectures on the device performance. Through optimizing QD structures and devices, red InP QLEDs with the current efficiencies as high as 11.6 cd/A were fabricated.

High Thermally Stable n-Type Semiconductor up to 850 K Based on Dianionic Naphthalenediimide Derivative

Electrostatic cation–anion interaction is effective to form a tightly bounded π-molecular assembly, which enhances the thermal stability and carrier transport property. Dianionic bis(benzenesulfonate)–naphthalenediimide (BSNDI2–) formed simple 2:1 cation–anion pairs of (Na+)2(BSNDI2–) (1), (K+)2(BSNDI2–) (2), and (NH4+)2(BSNDI2–) (3), and their redox behaviors, thermal stabilities, crystal structures, electron transport properties, and dielectric constants were compared to those of neutral bis(phenyl)–naphthalenediimide (4). Crystals 1, 2, and 3 had quite high thermal stabilities up to 850, 810, and 600 K, respectively, even though organic molecules. A two-dimensional (2D) n-type electron transport layer consisting of NDI π cores was sandwiched between networks of highly polarized electrostatic cation–anion pairs showing 2D herringbone (1), one-dimensional π-stacking (2), and brickstone-like 2D π-stacking (3) interactions. The values of electron mobility in polycrystalline 1, 2, 3, and 4 reached 0.22, >0....

Water/Alcohol Soluble Thickness-Insensitive Hyperbranched Perylene Diimide Electron Transport Layer Improving the Efficiency of Organic Solar Cells.

The electron transport layer (ETL) is very crucial for enhancing the device performance of polymer solar cells (PSCs). Meanwhile, thickness-insensitive and environment-friendly water/alcohol soluble processing are two essential requirements for large-scale roll-to-roll commercial application. Based on this, we designed and synthesized two new n-type ETLs with tetraethylene pentamine or butyl sulfonate sodium substituted tetraethylene pentamine as the branched side chains and high electron affinities perylene diimide (PDI) as the central core, named as PDIPN and PDIPNSO3Na. Encouragingly, both PDIPN and PDIPNSO3Na can effectively reduce the interfacial barrier and improve the interfacial contact. In addition, both of them can exhibit strong n-type self-doping effects, especially the PDIPN with higher density of negative charge. Consequently, compared to bare ITO, the PCE of the devices with ITO/PDIPN and ITO/PDIPNSO3Na ETLs has increased to 3–4 times. Our research results indicate that n-type self-doping PDI-based ETL PDIPN and PDIPNSO3Na could be promising candidates for ETL in environment-friendly water/alcohol soluble processing large-scale PSCs.

A new generation of organic light-emitting materials and devices

Organic light-emitting diodes (OLEDs) have been a research focus for almost 30 years because of their great potential applications in flat-panel displays, solid-state lighting, and wearable electronics. The recombination of holes and electrons within the devices under electrical excitation typically generates 25% singlet excitons and 75% triplet excitons. However, only 25% singlet excitons can be utilized to emit light, and the other 75% triplet excitons are generally wasted through non-radiative transition to give a maximum internal quantum efficiency of about 25% for the first generation fluorescent OLEDs. To improve the internal quantum efficiency, phosphorescent materials have been utilized to exploit the non-radiative triplet state by introducing noble heavy metal atoms like iridium (Ir) and platinum (Pt) to increase spin-orbit interactions. Overall electroluminescence (EL) internal quantum efficiency has been successfully improved to 100% since both singlet and triplet excitons could be harvested due to their radiative triplet excitons and efficient intersystem crossing (ISC) from the singlet excited state to the triplet excited state. Nevertheless, noble metals are indispensable for those phosphorescent materials, which are expensive and nonrenewable. In the past few years, metal-free thermally activated delayed fluorescence (TADF) emitters that can also realize 100% exciton utilization through efficient reverse intersystem crossing (RISC) of triplet excitons are regarded as promising materials for next-generation OLEDs. Besides, the intersystem crossing of excitons between higher energy Tm and Sn with close energy levels may also happen in hybridized local and charge-transfer (HLCT) excited state, where the local excited (LE) state contributes to a high efficiency fluorescence radiative decay, while the charge transfer (CT) state ensures the generation of singlet excitons in high yield through the reverse intersystem crossing from high-lying CT-based triplet excited state back to the CT-based singlet excited state. 75% Non-luminescent triplet excitons can theoretically be transferred to singlet ones via reverse intersystem crossing to give a comparable internal quantum efficiency to that of the phosphorescent devices. In addition, an organic open-shell molecule with one unpaired electron in the highest singly occupied molecular orbital (SOMO) could also be used as an emitter to circumvent the transition problem of triplet. Intermixing of electron-donating and electron-accepting molecules as an emitting layer could also break the fluorescence internal quantum efficiency limit of 25% by using the high reverse intersystem crossing efficiency of the intermolecular excited state (that is, exciplex state). Most recently, a simple strategy towards high-performance fluorescent OLEDs was reported by stacking p-type hole-transport layer and n-type electron-transport layer, resulting in a planar pn heterojunction configuration similar to their inorganic counterparts, namely light emitting diodes (LEDs). Like this, there is a great and urgent demand for low-cost high-performance light-emitting materials, simple device structure and simple manufacturing process in the field of OLED display. This project will focus on the development of noble metal free organic light-emitting materials and their high performance devices, including development of a new generation of organic light-emitting materials and corresponding host materials with high efficiency and practical lifetimes, investigation of their light emitting mechanisms, structure-property relationships, excited states and their regulation processes, etc. Based on the developed materials and devices, full-color OLED display panels will also be designed and developed.

P-i-n/n-i-p type planar hybrid structure of highly efficient perovskite solar cells towards improved air stability: synthetic strategies and the role of p-type hole transport layer (HTL) and n-type electron transport layer (ETL) metal oxides

There has been fast recent progress in perovskite solar cells (PSCs) towards low cost photovoltaic technology. Organometal mixed halide (MAPbX or FAPbX) perovskites are the most promising light absorbing material sandwiched between the electron transport layer (ETL) and hole transport layer (HTL). These two layers play a critical role in boosting the power conversion efficiency (PCE) and maintaining air stability. However, the device stability is a serious issue in regular as well as p-i-n inverted type perovskite solar cells. This mini-review briefly outlines the state-of-art of p-i-n/n-i-p type planar hybrid perovskite solar cells using MAPbX/FAPbX perovskite absorbing layers. Later, we will focus on recent trends, progress and further opportunities in exploring the air stable hybrid planar structure PSCs.

Controlling the conduction band offset for highly efficient ZnO nanorods based perovskite solar cell

The mechanism of charge recombination at the interface of n-type electron transport layer (n-ETL) and perovskite absorber on the carrier properties in the perovskite solar cell is theoretically studied. By solving the one dimensional diffusion equation with different boundary conditions, it reveals that the interface charge recombination in the perovskite solar cell can be suppressed by adjusting the conduction band offset (ΔEC) at ZnO ETL/perovskite absorber interface, thus leading to improvements in cell performance. Furthermore, Mg doped ZnO nanorods ETL has been designed to control the energy band levels. By optimizing the doping amount of Mg, the conduction band minimum of the Mg doped ZnO ETL has been raised up by 0.29 eV and a positive ΔEC of about 0.1 eV is obtained. The photovoltage of the cell is thus significantly increased due to the relatively low charge recombination.

High current conduction with high mobility by non-radiative charge recombination interfaces in organic semiconductor devices

We report a unique non-radiative p-n-p junction structure to provide high current conduction with high mobility in organic semiconductor devices. The current conduction was improved by increasing p-n junctions made with intrinsic p-type hole transport layer and n-type electron transport layer. The excellent hole mobility of 5.3×10−1cm2/Vs in this p-n-p device configuration is measured by the space charge limited current method with an electric field of 0.3MV/cm. Enhanced current conduction of 248% at 4.0V was observed in fluorescent blue organic light-emitting diodes with introduction of non-radiative p-n-p-n-p junction interfaces. Thereupon, the power efficiency at 1000cd/m2 was improved by 22% and the driving voltage also was reduced by 17%, compared to that of no interface device. Such high current conduction with high mobility is attributed to the carrier recombination at p-n-p interfaces through coulombic interaction. This non-radiative p-n-p junction structure suggested in this report can be very useful for many practical organic semiconductor device applications.

Operational stability of electrophosphorescent devices containing p and n doped transport layers

The operational stability of low-operating voltage p-i-n electrophosphorescent devices containing fac-tris(2-phenylpyridine) iridium as the emissive dopant is investigated. In these devices, Li-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) served as an n-type electron transport layer, or as an undoped hole blocking layer (HBL), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane doped 4,4′,4″-tris(3-methylphenylphenylamino) triphenylamine served as a p-type hole transport layer. The glass transition temperature of BPhen can be increased by the addition of aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolate (BAlq), resulting in improved morphological stability, thereby reducing device degradation. When thermally stable BAlq was used as a HBL in both p-i-n and undoped devices, the extrapolated operational lifetime (normalized to an initial luminance of 100 cd/m2) of the p-i-n and undoped devices are 18 000 and 60 000 h, respectively, indicating that the presence of p and n dopants can accelerate device degradation.