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Analyzing and optimizing carrier behaviors are essential to achieve high electroluminescence performance in perovskite light-emitting diodes (PeLEDs). In this work, a capacitance–voltage (C–V) model for PeLEDs is established to describe carrier behaviors. Four distinct regions in this typical C–V model, including a neutrality region, a barrier region, a carrier diffusion region, and a carrier recombination region, were analyzed. Importantly, the C–V model is implemented to guide the electroluminescence (EL) performance improvement in PeLEDs. By studying the measured C–V characteristics of a typical PeLED, issues of a high hole injection barrier and insufficient recombination are revealed. To address them, one MoO3 interface layer with deep conduction band minimum is designed between a hole transport layer and a hole injection layer to enhance the hole injection. The C–V characteristics for the optimized PeLED confirm the reduced injection barrier and strengthened recombination rate. The optimized PeLED shows an improved external quantum efficiency from 8.34% to 15.82%. The C–V model helps us to quantitatively understand the essential carrier behaviors in PeLEDs and can serve as an efficient method to improve the EL performance of PeLEDs.

Blue InP quantum dot light-emitting diodes (QLEDs) are promising candidates for environmental-friendly displays. To achieve efficient blue InP QLEDs through light extraction, the internal grating structure is a feasible way to extract waveguide modes, but it may bring complicated fabrication process and deteriorated electrical performance. In this work, we proposed an effective strategy to extract light from waveguide modes to air modes by using a thin hole transport layer (HTL), a high-index substrate, and substrate surface-roughening. Through optical tunneling, the thin HTL and the high-index substrate facilitate light transmission from waveguide modes to substrate modes. Thus, substrate surface-roughening can be applied to further extract light from enhanced substrate modes to air modes. As a result, light extraction efficiency has been significantly improved, leading to an external quantum efficiency enhancement from 2.1% to 2.8%, which is a record value among counterparts to date. This light extraction strategy is simple but effective to exploit the potential of high-efficiency (blue InP) QLEDs.

The unbalanced carrier injection is a key factor that deteriorates the performance of blue InP quantum dot light-emitting diodes (QLEDs). Therefore, to achieve efficient blue InP QLEDs, an effective strategy that balances carrier injection through enhancing the hole injection and transport is in demand. In this study, we introduced an ultrathin MoO3 electric dipole layer between the hole injection layer and the hole transport layer (HTL) to form a pair of dipole-induced built-in electric fields with forward resultant direction to enhance hole injection and facilitate the balance of carrier injection. Meanwhile, the p-doping effect by MoO3 leads to increased carrier concentration and decreased trap density of interfacial HTL, therefore improved its effective hole mobility. Consequently, the maximal external quantum efficiency of blue InP QLEDs was enhanced from 1.0% to 2.1%. This work provides an effective method to balance carrier injection by modulating hole injection and transport, indicating the feasibility to realize high-efficiency QLEDs.

The properties of ZnO tetrapod and multipod structures were investigated using scanning electron microscopy, x-ray diffraction, photoluminescence (PL), and electron paramagnetic resonance (EPR) spectroscopy. While there is relationship between g=1.96 EPR and green PL in some of the samples, this is not the case for all the samples. Therefore, the commonly assumed transition between a singly charged oxygen vacancy and photoexcited hole [K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, and J. A. Voigt, Appl. Phys. Lett. 68, 403 (1996)] does not explain the green emission in all ZnO samples. The green emission likely originates from surface defects.

Enhanced generation of carriers when a thermophotovoltaic cell is placed in submicron proximity to a heated surface is demonstrated using custom-designed InAs photodiodes and special silicon-based heater chips produced using microelectromechanical system techniques. The short-circuit current of the photocells is shown to increase sharply (up to fivefold) when the spacing between the heater and photodiode surfaces is reduced, while at the same time, the heater temperature decreases, consistent with increased radiative transfer between the two surfaces. By varying the spacing sinusoidally (at up to 1 kHz), it is demonstrated that the increase in the short-circuit current occurs in phase with the decrease in separation, thereby ruling out thermal effects. It is argued that the increase in short-circuit current is due to increased evanescent coupling of blackbody radiation from the hot surface to the cold photocell, consistent with recent theoretical predictions. The demonstration of this effect is the initia...

A 10 A ultraviolet/ozone grown oxide was used as an atomic diffusion barrier in a Ge/GaAs heterostructure. Good thermal stability of the oxide layer has been demonstrated by the appearance of dendritic crystallization [K. M. Lui, K. P. Chik, and J. B. Xu, J. Appl. Phys. 81, 7757 (1997)], induced by in situ thermal pulse annealing, of the Ge overlayer. In this work, an abrupt heterointerface was revealed by aligned Rutherford backscattering spectroscopy after annealing and compared with a control with no barrier at the interface where considerable diffusion had taken place. Current–voltage measurement indicated good rectifying properties of the oxide barrier heterojunction.

The effect of the growth interruption time during the growth of InGaAs/InAlAs quantum well structures is shown to have a significant effect on both the interband transitions, as determined by photoreflectance, and the electrical properties of the as-grown structure. The results show that, for increasing growth interruption time, the quantum well heterointerfaces become more abrupt and the carrier mobility increases, thereby demonstrating that long interruption times are preferable for the growth of high quality rectangular quantum well structures.

Light blocking induced by top patterned nanostructures is a fundamental limit in solar cells absorption. Here we propose an interstitial lattice patterned organic solar cell which can improve the light blocking of traditional square lattice and achieve broadband absorption enhancement. Compared to square lattice design, the plasmonic mode couplings between individual metallic nanoparticles in the interstitial lattice are more versatile and much stronger. Moreover, plasmonic modes can couple to the guided modes, resulting in large enhancement factor at some wavelengths. The interstitial lattice concept will be a broad interest and great help for high-performance photovoltaics.

Plasmonic back reflectors have recently become a promising strategy for realizing efficient organic solar cell (OSCs). Since plasmonic effects are strongly sensitive to light polarization, it is highly desirable to simultaneously achieve polarization-independent response and enhanced power conversion efficiency (PCE) by designing the nanostructured geometry of plasmonic reflector electrode. Here, through a strategic analysis of 2-dimensional grating (2D) and 3-dimensional patterns (3D), with similar periodicity as a plasmonic back reflector, we find that the OSCs with 3D pattern achieve the best PCE enhancement by 24.6%, while the OSCs with 2D pattern can offer 17.5% PCE enhancement compared to the optimized control OSCs. Importantly, compared with the 2D pattern, the 3D pattern shows a polarization independent plasmonic response, which will greatly extend its uses in photovoltaic applications. This work shows the significances of carefully selecting and designing geometry of plasmonic nanostructures in achieving high-efficient, polarization-independent plasmonic OSCs.

Hybrid transparent monolayer graphene/metal grid is proposed as top electrode of semitransparent organic solar cells. The hybrid electrode using gold grid on flexible polyethylene terephthalate substrate shows very low sheet resistance of 22 ± 3 Ω/□ and high optical transmittance of 81.4%, which is comparable to conventional indium tin oxide/glass electrode. Using lamination process, the layer of poly(3,4-ethylenedioythiophene):poly(styrenesulfonate) doped with D-sorbitol plays an important role in the electrical performance of the laminated devices. In addition, the devices show best power convention efficiency of 3.1% and fill factor of 55.0%, which are much better than those of similar graphene-based semitransparent organic solar cells.

A multiphysics study carries out on organic solar cells (OSCs) by solving Maxwell's and semiconductor equations simultaneously. By introducing a metallic rectangular-grating as the anode, surface plasmons are excited resulting in nonuniform exciton generation. Meanwhile, the internal E-field of plasmonic OSCs is modified with the modulated anode boundary. The plasmonic OSC improves 13% of short-circuit current but reduces 7% of fill factor (FF) compared to the standard one with a planar anode. The uneven photocarrier generation and transport by the grating anode are physical origins of the dropped FF. This work provides fundamental multiphysics modeling and understanding for plasmonic OSCs.

A thin layer of ultraviolet-ozone (UVO) treated gold (Au) is introduced on multi-layer graphene (MLG) to enable the MLG as an effective anode for polymer solar cells (PSCs). By optimizing the Au thickness and the durations of the UVO treatments at different stages, MLG PSCs with enhanced fill factor and power conversion efficiency are obtained, exhibiting better performance compared with MLG devices directly modified with UVO and poly(3,4-ethylenedioythiophene):poly(styrenesulfonate). Further analysis shows that UVO treated Au provides favorable band alignment at the MLG/polymer interface. Moreover, the improved interfacial contact and shortened UVO durations reduce the series resistance of PSCs significantly.

We demonstrate efficiency improvement in polymer solar cells (PSCs) by ∼22% through incorporating Au nanoparticles (NPs) into all polymer layers. Au NPs are found to have distinct mechanisms in improving device performance when incorporated in different polymer layers. Au NPs in poly‐(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) mainly contribute to better hole collection while Au NPs in active layer contributes to the enhanced optical absorption and more balanced charge‐transport. Our theoretical result shows that the absorption enhancement at the active layer is attributed to plasmon resonances with strong near‐field distributions penetrated into absorption polymers. These findings can be applied to design high‐efficiency metallic NPs‐incorporated PSCs.

At 15 K, the high field decay of magneto-electroluminescence (MEL) in tris (8-hydroxyquinoine) aluminum (Alq3)-based organic light-emitting diodes is greatly suppressed by mixing hole transport material N,N′-bis(naphthalen-1-y)-N,N′-bis(phenyl) benzidine (NPB) into Alq3 emission layer. The positive correlation between the MEL decay and the triplet exciton density indicates the reduced triplet exciton density is the major contributing factor. The change of triplet exciton density and then the MEL upon NPB mixing may result from the strengths change of charge-triplets interaction, in which the injected charges flowing through the exciton formation zone without forming static space charges play the most important role.

We investigate near-field multiple scattering effects of plasmonic nanospheres (NSPs) embedded into organic solar cells (OSCs). When NSPs are embedded into a spacer layer, the near-field scattering from the NSPs shows strong direction-dependent features, which significantly affects the optical absorption. When NSPs are embedded into an active layer, the absorption enhancement is attributed to the interplay between longitudinal and transverse modes supported by the NSP chain. The breakdown of electrostatic scaling law is confirmed by our theoretical model and should be accounted for optical designs of OSCs. The work provides the fundamental physical understanding and design guidelines for plasmonic photovoltaics.

We have investigated driving force effects on the ordering of polymer, which is a key factor of self-assembly of soft materials. By turning the substrate up-side-down, the downward driving force can form in solution film-growth process and affect the self-organization of polymer chains and domains. We introduce Brown’s capillarity theory [J. Polym. Sci., Polym. Phys. Ed. 22, 423 (1956)] to describe the film formation. Our results show that the better chain and lamellae packing of polymer make hole transport, carrier balance, and power conversion efficiency of annealed and unannealed devices improve even with thick active-layers as compared to conventional devices.

Doping effects with respect to the electrical properties of morphotropic phase boundary (Bi0.5Na0.5)0.94Ba0.06TiO3 thin films epitaxially grown on CaRuO3 electroded (LaAlO3)0.3(Sr2AlTaO6)0.35 (001) substrates were investigated. Substantial enhancement of ferroelectricity and piezoelectricity has been achieved in La+Ce codoped films with a remanent polarization Pr of 29.5 μC/cm2 and a remanent piezoelectric coefficient d33,f of 31 pm/V, whereas Mn doping seems more favorite to reduce the leakage current by two order of magnitude. Both doped films exhibited diodelike I-V characteristics, which are correlated with resistance switching effect.

The magnetoelectroluminescence (MEL) of organic light emitting devices with a N,N′-bis(l-naphthyl)-N,N′-diphenyl-1,l′-biphentl-4,4′-diamine:tris-(8-hydroxyquinoline) aluminum (NPB:Alq3) mixed emission layer (EML) has been investigated. We find that MEL is maximized when the volume ratio of NPB of the mixed EML reaches 30% and the EML thickness is 40 nm. The features of MEL under various magnetic field strengths are insensitive to the change in EML thickness and mixing ratio. Meanwhile, MEL has a close relationship with the carrier mobility. We have conducted a theoretical study to further verify the relationship. Our experimental and theoretical results confirm that MEL can function as a tool to indicate the mobility.

Organic light emitting devices (OLEDs) with a fluorescence-interlayer-phosphorescence emission layer structure (FIP EML) has been proposed to solve the efficiency roll-off issue effectively. Efficient green OLED based on FIP EML exhibiting only 26% roll-off in the luminance efficiency, which is lower than the typical roll-off of 51% for conventional phosphorescent OLEDs with single EML operated at 5–150 mA/cm2 range, has been demonstrated. Such enhancement should be attributed to the improved carrier balance, the exciton redistribution in recombination zone, the suppression of nonradiative exciton quenching processes, and the elimination of energy transfer loss offered by the FIP EML structure.

Thermal tuning of phononic bandgaps in megahertz range was demonstrated in ferroelectric ceramic-based phononic crystal structure. Temperature variation across ferroelectric phase transition, accompanied by substantial changes in acoustic velocities, leads to a shift in the phononic bandstructure of a two-dimensional (Ba,Sr)TiO3/epoxy composite sample over a range of 10 °C. Experimental results are supported by modelings based on plane-wave expansion calculations. The high tunability of phononic bandstructure is advantageous for active control of ultrasound transmissions.

We report a hybrid nanoparticle/organic device with strong resonant tunneling behavior by introducing ligand-capped Ag nanoparticles between indium tin oxide and hole transport layer in small molecule organic light-emitting devices. The dependences of resonant tunneling current on the thickness of the organic layers are investigated. For the optimized device, a peak-to-valley current ratio as high as 4.5 and narrow peak width about 1.8 V are obtained. Combining with the analysis of current-voltage behavior, the mechanism based on the charge trapping effect of Ag nanoparticles is proposed to interpret the operation of the hybrid device. Finally, the emission characteristics of hybrid devices are also studied.

A comprehensive analysis is given on the modifications of the exciton lifetime and internal quantum efficiency (ηint) for organic light-emitting devices (OLEDs). A linear relation is derived between the exciton lifetime and ηint, which is difficult to measure directly. The internal quantum efficiency can thus be estimated easily through the measurement of the exciton lifetime. The exciton lifetimes for OLEDs with weak or strong microcavity are studied experimentally and theoretically. The modification of the exciton lifetime is well explained through the microcavity effect and surface plasmon resonance. An excellent agreement between the experimental and theoretical results is achieved.

Using the rare-earth special feature of a sharp emission spectrum, voltage-controlled continuous color tuning of organic light-emitting diodes is achieved. Europium(dibenzoylmethanato)3(bathophenanthroline) is used as the strategic starting point close to the red corner of the Commission International de I’Eclairage chromaticity diagram for a wide color tuning. The end point and path of the color tuning can be engineered by doping the hole-transport emitting layer with dyes. The mechanisms of color tuning have been investigated and explained by the efficiency reduction of the europium complex and the extension of carrier recombination zone with driving voltage. The effect of exciplex on the color tuning is also studied.

In this study, free standing crystalline ZnSe nanorings and nanowires have been fabricated on Au coated Si substrates by simple thermal evaporation of ZnSe powders. Ring- or wirelike morphology can be achieved in a controllable manner by using different reactor pressures during growth, while all the other conditions remain the same. Our results show that the ZnSe nanorings are wurtzite phase instead of the zinc-blende phase, observed in typical one-dimensional ZnSe nanostructures. The growth mechanism of the nanorings has been discussed, and the cathodoluminescence of the nanorings has been described.

The efficiency of green organic electroluminescent devices have been improved by cohosting the electron dominant complex, 4,7-diphenyl-1,10-phenanthroline into the traditional electron transporting layer of tris (8-hydroxyquinoline) aluminum. In this cohost strategy, we demonstrate that the luminous efficiency is enhanced by >20% while the driving voltage can be reduced by ∼30% in a uniformly mixed composition as compared to the traditional device configuration. The corresponding device lifetime under atmospheric condition is extended by a factor of ∼1.8, attributed to the reduction of the accumulated positive charges near the electron-hole recombination regime. Results indicate that the knowledge of bulk conductivity engineering of organic n-type transporters is essential in enhancing organic light-emitting devices.