Two-step fabrication of thin film encapsulation using laser assisted chemical vapor deposition and laser assisted plasma enhanced chemical vapor deposition for long-lifetime organic light emitting diodes

A thin film encapsulation layer was fabricated through two-sequential chemical vapor deposition processes for organic light emitting diodes (OLEDs). The fabrication process consists of laser assisted chemical vapor deposition (LACVD) for the first silicon nitride layer and laser assisted plasma enhanced chemical vapor deposition (LAPECVD) for the second silicon nitride layer. While SiN_x thin films fabricated by LAPECVD exhibits remarkable encapsulation characteristics, OLEDs underneath the encapsulation layer risk being damaged during the plasma generation process. In order to prevent damage from the plasma, LACVD was completed prior to the LAPECVD as a buffer layer so that the laser during LACVD did not damage the devices because there was no direct irradiation to the surface. This two-step thin film encapsulation was performed sequentially in one chamber, which reduced the process steps and increased fabrication time. The encapsulation was demonstrated on green phosphorescent OLEDs with I–V-L measurements and a lifetime test. The two-step encapsulation process alleviated the damage on the devices by 19.5% in external quantum efficiency compared to the single layer fabricated by plasma enhanced chemical vapor deposition. The lifetime was increased 3.59 times compared to the device without encapsulation. The composition of the SiNx thin films was analyzed through Fourier-transform infrared spectroscopy (FTIR). While the atomic bond in the layer fabricated by LACVD was too weak to be used in encapsulation, the layer fabricated by the two-step encapsulation did not reveal a Si–O bonding peak but did show a Si–N peak with strong atomic bonding.     

Electronic and chemical properties of ZnO in inverted organic photovoltaic devices

Photo-conversion efficiency of inverted polymer solar cells incorporating pulsed laser deposited ZnO electron transport layer have been found to significantly increase from 0.8% to up to 3.3% as the film thickness increased from 4 nm to 100 nm. While the ZnO film thickness was found to have little influence on the morphology of the resultant ZnO films, the band structure of ZnO was found to evolve only for films of thickness 25 nm or more and this was accompanied by a significant reduction of 0.4 eV in the workfunction. The films became more oxygen deficient with increased thickness, as found from X-ray photoelectron spectroscopy (XPS) and valence band XPS (VBXPS). We attribute the strong dependence of device performance to the zinc to oxygen stoichiometry within the ZnO layers, leading to improvement in the band structure of ZnO with increased thickness.     

Dispersion control of Ag nanoparticles in bulk-heterojunction for efficient organic photovoltaic devices

This research focuses on the effect of different capping agents on Ag nanoparticles (NPs), for the improved efficiency of organic photovoltaic cells. Ag NPs were produced by solution chemistry of the polyol process, and then successfully capped with oleylamine (OA), polyvinylpyrrolidone (PVP), or thiol terminated polystyrene (PS-SH), as proven by FT-IR spectra. These Ag NPs with different capping agents were finally embedded in the photoactive layer of poly(3-hexylthiophene):6,6-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells. Because of the presence of a suitable capping agent that prevents aggregation, the dispersity of the Ag NPs in organic solvent was significantly improved, in the sequence of OA, PVP, and PS-SH. The photovoltaic cells exhibit increased performance from 3.11% to 3.49%, at an optimized blend ratio of Ag NPs (2.5 wt%) capped with PS-SH. This enhancement is mainly attributed to the improved short circuit current (increased from 8.49 mA/cm² to 9.29 mA/cm²) and extinction with effective light scattering, caused by improved dispersion of the Ag NPs in BHJ films, through reducing unwanted particle aggregation.     

All thermal-evaporated surface plasmon enhanced organic solar cells by Au nanoparticles

Au nanoparticles (NPs) are fabricated on indium-tin-oxide substrates by a thermal evaporation method and incorporated to an efficient small molecule organic solar cell (OSC). This renders an all thermal evaporated surface plasmon enhanced OSC. The optimized device shows a power conversion efficiency of 3.40%, which is 14% higher than that of the reference device without Au NPs. The improvement is mainly contributed to the increased short-circuit current which resulted from the enhanced light harvesting due to localized surface plasmon resonance of Au NPs and the increased conductivity of the device.     

Performance enhancement in organic photovoltaic devices using plasma-polymerized fluorocarbon-modified Ag nanoparticles

In this work, Ag nanoparticles were modified by an ultra-thin plasma-polymerized fluorocarbon film (CF_X) to form a composite CF_X-modified Ag nanoparticles/indium tin oxide (ITO) anode for application in organic photovoltaic (OPV) devices. A CF_X-modified Ag nanoparticles/ITO anode exhibited a superior surface work function of 5.4 eV suited for application in OPV devices. The performance of zinc phthalocyanine:fullerene-based OPV devices showed a significant improvement when the structural identical cells are made with the CF_X-modified Ag nanoparticles/ITO. This work yielded a promising power conversion efficiency of 3.5 ± 0.1%, notably higher than that with a bare ITO anode (2.7 ± 0.1%).     

Comparative study of ZnO thin films prepared by plasma deposition and electron beam evaporation for use in photovoltaic devices

Undoped zinc oxide thin films were grown at room temperature using two techniques: plasma deposition (PD) and electron beam evaporation in an argon atmosphere. PD offers some advantages, such as low ion damage and low deposition temperature. The optical transmittance of the films deposited by both methods was higher than 80% in the near UV–VIS range; the energy band gap and index of refraction agree with values reported in the literature. The resistivity of films grown by PD was 3.1 × 10⁻² Ω cm, lower than the value of 1.2 × 10⁻¹ Ω cm found for plasma assisted e‐beam evaporated films. Copyright © 2010 John Wiley & Sons, Ltd.     

Thin-film silicon formation on foreign substrates by rapid thermal chemical vapour deposition for photovoltaic application

Polycrystalline silicon thin-film layers were deposited on foreign substrates such as SiO₂, alumina, mullite and graphite. The deposition studies were carried out in a single-wafer, horizontal, rapid thermal chemical vapour phase reactor at temperatures ranging from 900°C to 1250°C at atmospheric pressure. We employed the gas precursor trichlorosilane and the layers were doped with boron from the dopant source trichloroborine rarified in a hydrogen carrier gas. The surface structures and grain sizes of the thin films obtained were evaluated by Nomarski microscopy and scanning electron microscopy characterization methods. X-ray diffraction analyses were used to determine the preferential crystalline orientations at various operational parameters. Furthermore, electrical properties in terms of Hall mobility and lifetimes of the minority carriers were investigated by means of Van-der-Pauw and photoconductivity decay methods, respectively. Generally, it has been shown that at elevated deposition temperatures maximum grain sizes of 3–20 μm for 10-μm thick layers can be found, depending critically on the type of the substrate. For polycrystalline silicon deposited at 1100°C on silicon dioxide, alumina, and graphite substrates, a preferred crystallographic orientation of (220) was observed, implying columnar grain structures. © 1998 John Wiley & Sons, Ltd.     

Tin naphthalocyanine complexes for infrared absorption in organic photovoltaic cells

In this work, we examine the optical properties of tin naphthalocyanine dichloride (SnNcCl₂), and its performance as an electron donor material in organic photovoltaic cells (OPVs). As an active material, SnNcCl₂ is attractive for its narrow energy gap which facilitates optical absorption past a wavelength of λ = 1100 nm. We demonstrate a power conversion efficiency of η_P = (1.2 ± 0.1)% under simulated AM1.5G solar illumination at 100 mW/cm² using the electron donor–acceptor pairing of SnNcCl₂ and C₆₀ in a bilayer device architecture. While some phthalocyanines have been previously used to improve infrared absorption, this is often realized through the formation of molecular dimers. In SnNcCl₂, the infrared absorption is intrinsic to the molecule, arising as a result of the extended conjugation. Consequently, it is expected that SnNcCl₂ could be utilized in bulk heterojunction OPVs without sacrificing infrared absorption.     

Integrating nanostructured electrodes in organic photovoltaic devices for enhancing near-infrared photoresponse

We introduce a simple methodology to integrate prefabricated nanostructured-electrodes in solution-processed organic photovoltaic (OPV) devices. The tailored “photonic electrode” nanostructure is used for light management in the device and for hole collection. This approach opens up new possibilities for designing photonically active structures that can enhance the absorption of sub-bandgap photons in the active layer. We discuss the design, fabrication and characterization of photonic electrodes, and the methodology for integrating them to OPV devices using a simple lamination technique. We demonstrate theoretically and experimentally that OPV devices using photonic electrodes show a factor of ca. 5 enhancement in external quantum efficiency (EQE) in the near infrared region. We use simulations to trace this observed efficiency enhancement to surface plasmon polariton modes in the nanostructure.     

Critical light instability in CB/DIO processed PBDTTT-EFT:PC71BM organic photovoltaic devices

Organic photovoltaic (OPV) devices often undergo ‘burn-in’ during the early stages of operation, this period describing the relatively rapid drop in power output before stabilising. For normal and inverted PBDTTT-EFT:PC₇₁BM OPVs prepared according to current protocols, we identify a critical and severe light-induced burn-in phase that reduces power conversion efficiency by at least 60% after 24 hours simulated AM1.5 illumination. Such losses result primarily from a reduction in photocurrent, and for inverted devices we correlate this process in-situ with the simultaneous emergence of space-chare effects on the μs timescale. The effects of burn in are also found to reduce the lifetime of photogenerated charge carriers, as determine by in-situ transient photovoltage measurements. To identify the underlying mechanisms of this instability, a range of techniques are employed ex-situ to separate bulk- and electrode-specific degradation processes. We find that whilst the active layer nanostructure and kinetics of free charge generation remain unchanged, partial photobleaching (6% of film O.D.) of PBDTTT-EFT:PC₇₁BM occurs alongside an increase in the ground state bleach decay time of PBDTTT-EFT. We hypothesise that this latter observation may reflect relaxation from excited states on PBDTTT-EFT that do not undergo dissociation into free charges. Owing to the poor lifetime of the reference PBDTTT-EFT:PC₇₁BM OPVs, the fabrication protocol is modified to identify routes for stability enhancement in this initially promising solar cell blend.     

Double-heterojunction bipolar transistors in InP/GaInAs grown by metal organic chemical vapour deposition

Double-heterojunction bipolar transistor structures in InP/GaInAs have been grown by low-pressure metal organic chemical vapour deposition. Good control of the Zn dopant in the GaInAs base layer was achieved, and devices with current gains up to 300 at current densities of 1.4kA/cm2 have been demonstrated.     

Microstructured ZnO coatings combined with antireflective layers for light management in photovoltaic devices

We describe microstructured ZnO coatings that improve photovoltaic (PV) device performance through their antireflective properties and their tendency to scatter incoming light at large angles. In many PV devices, reflection from the transparent conductive top contact significantly degrades performance. Traditional quarter‐wave antireflective (AR) coatings reduce surface reflection but perform optimally for only a narrow spectral range and incident illumination angle. Furthermore, in some types of devices, absorption far from the junction increases the rate of recombination, and light management strategies are required to remedy this. The randomly patterned, microstructured ZnO coatings described in this paper, formed via a simple wet etch process, serve as both an AR layer with superior performance to that of a thin film AR coating alone as well as a large angle forward scatterer. We model formation of the coatings and evaluate their AR properties. When combined with a traditional quarter‐wave MgF₂ coating, these microstructured ZnO coatings increase short circuit currents of example Cu(In,Ga)Se₂ (CIGS) devices by over 20% in comparison to those of uncoated devices at normal incidence. A similar improvement is observed for illumination angles of up to 60°. While demonstrated here for CIGS, these structures may prove useful for other PV technologies as well. Published 2016. This article is a U.S. Government work and is in the public domain in the USA. Progress in Photovoltaics: Research and Applications Published by John Wiley & Sons Ltd.     

Determining emissive dipole orientation in organic light emitting devices by decay time measurement

We present a method to detect anisotropy in the distribution of the transition dipole moment in organic light emitting diodes (OLEDs). The method is based on the dependency of the exciton decay rate on the optical environment and the orientation of the dipole transition moment, also called the Purcell effect. We use this method to demonstrate a preferential orientation of the small molecule emitter Ir(MDQ)₂(acac) in a TPBi matrix. The outcoupling improvement for OLEDs that could be obtained with perfectly oriented transition dipoles is estimated by simulation. For perfectly planar structures this shows an EQE in air of up to 34%.     

Realizing both improved luminance and stability in organic light-emitting devices based on a solution-processed inter-layer composed of MoOX and Au nanoparticles mixture

We report on the solution-processed mixture of Au nanoparticles (NPs) and MoO_X as an inter-layer in organic light-emitting devices (OLEDs), leading to the enhanced light emission and good stability. An impressive enhancement in current efficiency and power efficiency is achieved up to 70% and 100%, respectively. A systematic study to the effect of the Au NPs:MoO_X inter-layer on OLEDs demonstrates that the improved electrical properties is mainly ascribed to the enhanced hole injection due to the high work function of MoO_X and the good conductivity of Au NPs, and the enhanced optical properties is mainly attributed to the localized surface plasmon induced by Au NPs, which makes a great contribution to the improved efficiency. Besides, Au NPs:MoO_X inter-layer also behaves superior to the frequently-used polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) in device stability. The decay ratio for Au NPs:MoO_X based device is 60% after 80 h, while it is nearly dying out for the device with PEDOT:PSS inter-layer.     

Polypyrrole top-contact electrodes patterned by inkjet printing assisted vapor deposition polymerization in flexible organic thin-film transistors

Highly conductive polymer, polypyrrole (PPy) was successfully patterned as source and drain (S/D) electrodes for flexible pentacene thin film transistors in top-contact structure by combining inkjet printing and vapor deposition polymerization. Facile inkjet printing of initiator and subsequent exposure of pyrrole monomers resulted in selective absorption and polymerization of pyrrole monomers on the patterned initiator region. Pentacene transistors based on printed PPy electrodes exhibited higher electrical characteristics than that of the devices with thermally evaporated Au electrodes. Improved performance of the devices based on PPy electrodes could be attributed to the reduction of contact resistance at the interface between polymer and organic semiconductor. For the replacement of metal electrodes, vapor deposition polymerization assisted inkjet printing technique can provide a versatile method to utilize highly conductive polymer as a functional electrode of flexible organic electronic devices.     

Controlled positioning of metal nanoparticles in an organic light-emitting device for enhanced quantum efficiency

It is shown that there exists an optimum distance between the plane where nanoparticles (NPs) are positioned and the active layer of Au-NP-embedded organic light-emitting devices (OLEDs) for the maximum external quantum efficiency. Au NPs are precisely positioned in a specific plane in the hole-transport layer using a dry, room-temperature aerosol technique at atmospheric pressure. By controlling the position of the Au NPs and their density, we optimize the external quantum efficiency of the Au-NP-embedded OLEDs, with the maximum efficiency being 38% larger than that of the control device without Au NPs. In contrast to commonly employed methods to incorporate metal NPs in an organic layer, such as vacuum thermal evaporation or spin coating, the aerosol-deposited Au NPs do not penetrate into the underlying organic layer, not only allowing for precise control of the vertical (perpendicular to the substrate surface) position of the Au NPs, but also minimizing damage to the hole-transport organic material. Our electrical and optical characterizations show that the existence of the optimal distance occurs by the competition between the increased electron–hole recombination probability caused by the electrostatic effects of holes trapped in the Au NPs and the metal induced quenching.     

Enhancement of the electroluminescence of organic light emitting devices based on Ir(ppy)3 by doping with metallic and magnetic nanoparticles

Magnetic nanoparticles embedded in the active layer of the Organic Light Emitting Diodes (OLEDs) significantly increases the electroluminescence and the charge transport without influencing the transparency of these devices. A brief comparison was done in order to identify which parameter influences these properties, by comparing the CoFe₂O₄ magnetic nanoparticles with CoFe₂ metallic magnetic nanoparticles, the latter one being obtained by thermal reduction in hydrogen of cobalt ferrite nanoparticles. CoFe₂ have shown a better efficiency of the metallic nanoparticles where probably the main advantage is the higher magnetization property instead of the coercive field. Concerning the charge transport across the OLEDs, these nanoparticles reduce the electron injection, acting as filling traps, which directly increases the electroluminescence and the current at the same voltage.     

Enhanced hole extraction by interaction between CuI and MoO3 in the hole transport layer of organic photovoltaic devices

The selection of materials for use of a hole transport layer is crucial to improve the photovoltaic performances by means of efficient hole extraction. Herein, we investigate how the formation of a hybrid dual hole transport interlayer consisting of copper (I) iodide (CuI) and molybdenum oxide (MoO₃) affects the efficiency of the device based on poly(3-hexylthiophene)(P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends. The rough surface of a CuI layer was observed when prepared on indium tin oxide (ITO) substrates, but became smooth by the thermal evaporation of MoO₃ on the rough CuI surface, forming a dual layer. The devices incorporated with the layer show an enhancement in efficiency compared to the devices with the CuI or MoO₃ alone layer, which is attributed to enhanced hole extraction. Our X-ray photoelectron spectroscopy (XPS) results show that Mo⁵⁺ defect states are increased by the interaction between MoO₃ and CuI at the interface, giving rise to an increase in gap states, which we attribute to the improvement of hole extraction.     

Characteristics of thin film transistors fabricated in polysilicon films deposited by plasma enhanced chemical vapor deposition

Thin-film transistors (TFTs) fabricated in polysilicon films deposited by plasma enhanced chemical vapor deposition (PECVD) were characterized. The transistors were fabricated using a low temperature process (i.e., <- 700° C). The characteristics of the devices were found to improve as the deposition temperature of the polysilicon film increased. The best characteristics (μ _FE of 15 cm²/V _s andV _TH of 2.2V) were measured in the devices fabricated in the film deposited at 700° C. The devices fabricated in the PECVD polysilicon films were compared to those fabricated in polysilicon films deposited by thermal CVD in the same reactor in order to decouple the effect of the plasma. A coplanar electrode structure TFT with adequate characteristics (μ _FE of 8 cm²/V _s) was also demonstrated in the PECVD polysilicon films.