High-performance barrier using a dual-layer inorganic/organic hybrid thin-film encapsulation for organic light-emitting diodes

We report an inorganic/organic hybrid barrier that combines the alternating deposition of a layer of ZrO₂ using low temperature atomic layer deposition and a 16-μm-thick layer of UV-curable NOA63 epoxy using spin-coating. The effective water vapor transmission rates of single ZrO₂ film was improved by adding solution epoxy from 3.03 × 10⁻³ g/m² day to 1.27 × 10⁻⁴ g/m² day in the hybrid NOA63/ZrO₂/NOA63/ZrO₂ films at 20 °C and a relative humidity of 60%. In consequence, the organic light-emitting diodes encapsulated with inorganic/organic hybrid barriers were undamaged by environmental oxygen and moisture and their luminance decay time improved by a considerable extent.     

Highly reliable hybrid nano-stratified moisture barrier for encapsulating flexible OLEDs

Herein, a novel thin-film encapsulation for flexible organic light-emitting diodes (FOLEDs) is proposed, and its long-term reliability in tensile stress conditions was tested. The hybrid nano-stratified moisture barrier consists of 2.5 dyads of an Al₂O₃/ZnO nano-stratified structure and a S-H nanocomposite organic layer. The nano-stratified structure is prepared by low-temperature atomic layer deposition and the S-H nanocomposite by spin-coating at a thickness of 30 and 120 nm, respectively. An optical transmittance of 89.05% was measured with the 2.5-dyad hybrid nano-stratified moisture barrier with a total thickness of 330 nm. A low water vapor transmission rate (WVTR) of 1.91 × 10⁻⁵ g/m²day was recorded based on an electrical Ca test at 30 °C and 90% R.H. without losing its properties after a bending test. With this highly reliable hybrid nano-stratified moisture barrier, FOLEDs were successfully encapsulated. After 30 days under conditions of 30 °C and 90% R.H. with tensile stress, the J-V-L performances of the FOLEDs were comparable to those of the initial state without dark spots. These results suggest that this hybrid nano-stratified moisture barrier is an excellent method for encapsulating FOLEDs.     

Thickness dependent barrier performance of permeation barriers made from atomic layer deposited alumina for organic devices

Organic devices like organic light emitting diodes (OLEDs) or organic solar cells degrade fast when exposed to ambient air. Hence, thin-films acting as permeation barriers are needed for their protection. Atomic layer deposition (ALD) is known to be one of the best technologies to reach barriers with a low defect density at gentle process conditions. As well, ALD is reported to be one of the thinnest barrier layers, with a critical thickness – defining a continuous barrier film – as low as 5–10 nm for ALD processed Al₂O₃. In this work, we investigate the barrier performance of Al₂O₃ films processed by ALD at 80 °C with trimethylaluminum and ozone as precursors. The coverage of defects in such films is investigated on a 5 nm thick Al₂O₃ film, i.e. below the critical thickness, on calcium using atomic force microscopy (AFM). We find for this sub-critical thickness regime that all spots giving raise to water ingress on the 20 × 20 μm² scan range are positioned on nearly flat surface sites without the presence of particles or large substrate features. Hence below the critical thickness, ALD leaves open or at least weakly covered spots even on feature-free surface sites. The thickness dependent performance of these barrier films is investigated for thicknesses ranging from 15 to 100 nm, i.e. above the assumed critical film thickness of this system. To measure the barrier performance, electrical calcium corrosion tests are used in order to measure the water vapor transmission rate (WVTR), electrodeposition is used in order to decorate and count defects, and dark spot growth on OLEDs is used in order to confirm the results for real devices. For 15–25 nm barrier thickness, we observe an exponential decrease in defect density with barrier thickness which explains the likewise observed exponential decrease in WVTR and OLED degradation rate. Above 25 nm, a further increase in barrier thickness leads to a further exponential decrease in defect density, but an only sub-exponential decrease in WVTR and OLED degradation rate. In conclusion, the performance of the thin Al₂O₃ permeation barrier is dominated by its defect density. This defect density is reduced exponentially with increasing barrier thickness for alumina thicknesses of up to at least 25 nm.     

A flexible moisture barrier comprised of a SiO2-embedded organic–inorganic hybrid nanocomposite and Al2O3 for thin-film encapsulation of OLEDs

We demonstrated a high performance flexible multi-barrier containing a silica nanoparticle-embedded organic–inorganic hybrid (S–H) nanocomposite and Al₂O₃. The multi-barrier was prepared by low-temperature Al₂O₃ atomic layer deposition and with a spin-coated S–H nanocomposite. The moisture barrier properties were investigated with a water vapor transmission rate (WVTR), estimated by a Ca test at 30 °C, 90% R.H.. Moisture diffusion was effectively suppressed by the sub-700 nm thick multi-barrier incorporating well-dispersed silica nanoparticles in the organic layer. A low WVTR of 1.14 × 10^?5 g/m² day and average transmittance of 85.8% in the visible region were obtained for the multi-barrier. After bending under tensile stress mode, the moisture barrier property of the multi-barriers was retained. The multi-barrier was successfully applied to thin-film encapsulation of OLEDs. The thin-film encapsulated OLEDs showed practicable current–voltage–luminance (I–V–L) characteristics and stable real operation over 700 h under ambient conditions.     

Atomic layer deposited TiOx/AlOx nanolaminates as moisture barriers for organic devices

Atomic layer deposited nanolaminates of alternating AlO_x and TiO_x thin-films are investigated as moisture barriers for organic electronic devices. Direct encapsulation on organic light emitting diodes (OLEDs) is tested in aging experiments and compared to calcium corrosion tests of equivalent barrier films. This allows for a direct assessment of moisture barrier performance in simple as well as more complex systems. Thickness variations are performed for the nanolaminate single and total layer thickness, with an optimum single layer thickness of 1–2 nm observed. This correlates to the maximum number of dyads once completely closed single layers are produced. For large single layer thickness and low dyad count, strong lateral diffusion from the edges occurs in the OLEDs, which likely correlates to poor mechanical stability. At optimum single layer thickness, barriers remain mechanically and chemically stable up to 100 nm total thickness. OLEDs encapsulated with such nanolaminate barriers show no significant degradation after 2500 h of continuous aging.     

Improvement of the morphological and electrical characteristics of Al3+, Fe3+ and Bi3+-doped TiO2 compact thin films and their incorporation into hybrid solar cells

Compact titanium dioxide (TiO₂) hole-blocking layers are commonly employed in organic-inorganic solar cells, however, their importance in terms of morphology and electrical conductivity is frequently overlooked in this novel type of solar energy converters. In this work, single TiO₂ thin films were prepared by a sol-gel method, observing large pinhole densities and low electrical conductivities. As a means to solve the morphological issue, the deposition of a second TiO₂ film was explored, which effectively reduced the surface irregularities obtained in single oxide films. The limited electrical conductivity of single and double layers was successfully increased by doping with the trivalent cations of aluminum, iron (III) and bismuth (III), observing an increase from 2.48 × 10⁻⁸ S/cm for an undoped TiO₂ double layer to 51.41 × 10⁻⁸ S/cm for a Fe³⁺-doped TiO₂ double layer. The incorporation of these hole blocking layers in hybrid solar cells led to further insights in the important role of trivalent doping cations in the transference and transport of electrons on the surface and in the bulk of the prepared TiO₂ compact films.     

Life prediction for CIGS solar modules part 2: degradation kinetics,accelerated testing,and encapsulant effects

Copper indium gallium di‐selenide (CIGS) solar cells from Global Solar Energy, Inc. and development cells from another manufacturer are laminated in non‐barrier packaging and exposed to a variety of temperature and humidity conditions, and measured degradation rates fit to a kinetic expression. The kinetic constants are then used in a life model that is based on time‐dependent mass and energy balances governing module temperature and the diffusion of water through a module package subjected to weather conditions. Cells with an aluminum‐doped zinc oxide window layer degrade approximately 25× faster than cells having an indium tin oxide window layer, and so require much lower permeability barrier films in a flexible package. The electrically conducting adhesive used to connect bottom and top contact to solder‐coated ribbons can be a major factor in degradation. This difference may only be apparent at lower temperatures, which drastically affects real‐world lifetime. Preliminary Florida and Arizona data confirm this. The key assumption in the model is that the instantaneous rate of degradation is proportional to the relative saturation of the encapsulant with water (0–100%), and not the absolute concentration of water in the encapsulant (10⁻³–10⁻² g/cm³). This leads to the model prediction that it is only the diffusion time constant t_c = L_ES_E/WVTR that determines the degradation and life of a packaged cell and that thicker encapsulants with higher water solubility will significantly extend cell life. Experimental data for three different encapsulant materials with and without a moisture barrier film confirm these predictions. Copyright © 2011 John Wiley & Sons, Ltd.     

In situ vapor-deposited parylene substrates for ultra-thin,lightweight organic solar cells

We fabricate the thinnest (1.3 μm) and lightest (3.6 g/m²) solar cells yet demonstrated, with weight-specific power exceeding 6 W/g, in order to illustrate the lower limits of substrate thickness and materials use achievable with a new processing paradigm. Our fabrication process uniquely starts with growth of an ultra-thin flexible polymer substrate in vacuum, followed by deposition of electrodes and photoactive layers in situ. With this process sequence, the entire cell—from transparent substrate to active layers to encapsulation—can be fabricated at room temperature without solvents and without breaking vacuum, avoiding exposure to dust and other contaminants, and minimizing damage risk associated with handling of thin substrates. We use in situ vapor-phase growth of smooth, transparent, and flexible parylene-C films to produce ultra-thin, lightweight molecular organic solar cells as thin as 2.3 μm including encapsulation with a second parylene-C film. These parylene-based devices exhibit power conversion efficiencies and fabrication yields comparable to glass-based cells. Flexible solar cells on parylene membranes can be seamlessly adhered to a variety of solid surfaces to provide additive solar power.     

Indoline-based donor molecule for efficient co-evaporated organic photovoltaics

We demonstrated the use of an asymmetrical donor–acceptor-type indoline dye—D131, developed for dye-sensitized solar cells, as an electron donor and fullerene C₇₀ as an electron acceptor for thermal co-evaporated bulk-heterojunction organic solar cells (OSCs). In spite of the presence of intermolecular hydrogen bonds among D131 molecules, they can be thermally evaporated in high vacuum at a relatively low temperature of 220 °C. The blend ratio and thickness of the active layer of D131/C₇₀ blend films in OSCs were optimized to achieve a maximum power-conversion efficiency of 4.5% with a short-circuit current of 9.1 mA cm^?2, an open-circuit voltage of 0.89 V, and a fill factor of 0.56 under AM 1.5G solar illumination (100 mW cm^?2), which is the best value reported so far for OSCs based on indoline-based donor materials.     

Annealing effects on physical properties of doped CdTe thin films for photovoltaic applications

Polycrystalline Cadmium Telluride (CdTe) thin films were prepared on glass substrates by thermal evaporation at the chamber ambient temperature and then annealed for an hour in vacuum ~1×10⁻⁵ mbar at 400 °C. These annealed thin films were doped with copper (Cu) via ion exchange by immersing these films in Cu (NO₃)₂ solution (1 g/1000 ml) for 20 min. Further these films were again annealed at different temperatures for better diffusion of dopant species. The physical properties of an as doped sample and samples annealed at different temperatures after doping were determined by using energy dispersive x-ray analysis (EDX), x-ray diffraction (XRD), Raman spectroscopy, transmission spectra analysis, photoconductivity response and hot probe for conductivity type. The optical band gap of these thermally evaporated Cu doped CdTe thin films was determined from the transmission spectra and was found to be in the range 1.42–1.75 eV. The direct energy band gap was found annealing temperatures dependent. The absorption coefficient was >10⁴ cm⁻¹ for incident photons having energy greater than the band gap energy. Optical density was observed also dependent on postdoping annealing temperature. All samples were found having p-type conductivity. These films are strong potential candidates for photovoltaic applications like solar cells.     

Reduced water vapor transmission rates of low-temperature-processed and sol-gel-derived titanium oxide thin films on flexible substrates

Sol-gel-derived, crack-free, and condensed TiO_x thin films with improved barrier properties were successfully fabricated on polymeric substrates with a simple two-step heat treatment at low temperatures. To assess the barrier properties of the TiO_x thin films, Ca corrosion tests were conducted and their water vapor transmission rates (WVTRs) were measured. We found that the two-step heat treatment (at 45 °C for 90 min and 110 °C for 60 min) produces a close-packed TiO_x structure that substantially reduces the WVTRs of the coated polymeric substrates. The WVTRs of 86 nm thick TiOx thin films on polyethylene naphthalate (PEN) substrates at a relative humidity (RH) of 90% were found to be 0.133 g m⁻² day⁻¹ at 38 °C and 0.0387 g m⁻² day⁻¹ at 25 °C. In addition, the WVTR value of the TiO_x thin films on PEN substrates are stable with respect to bending: it was found to increase by only ∼13% after 100 repetitions of bending with a 20 mm radius.     

Highly Conductive and Environmentally Stable Organic Transparent Electrodes Laminated with Graphene

Improving the lifetime and the operational and thermal stability of organic thin‐film materials while maintaining high conductivity and mechanical flexibility is critical for flexible electronics applications. Here, it is reported that highly conductive and environmentally stable organic transparent electrodes (TEs) can be fabricated by mechanically laminating poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) films containing dimethylsulfoxide and Zonyl fluorosurfactant (PDZ films) with a monolayer graphene barrier. The proposed lamination process allows graphene to be coated onto the PDZ films uniformly and conformally with tight interfacial binding, free of wrinkles and air gaps. The laminated films exhibit an outstanding room‐temperature hole mobility of ≈85.1 cm² V^?1 s^?1 since the graphene can serve as an effective bypass for charge carriers. The significantly improved stability of the graphene‐laminated TEs against high mechanical/thermal stress, humidity, and ultraviolet irradiation is particularly promising. Furthermore, the incorporation of the graphene barrier increases the expected lifetime of the TEs by more than two orders of magnitude.     

Efficient planar organic solar cells with the high near-infrared response

Efficient planar organic solar cells extending the response into the near-infrared (NIR) were fabricated using the highly ordered Titanyl phthalocyanines (TiOPc) films as the donor layer. This type of films obtained through the weak epitaxy growth (WEG) method presents good continuity and integrity with the low density of grain boundaries. More importantly the films own a strong absorption in the NIR (750–950 nm) and a broad absorption spectrum from 550 to 950 nm. Meanwhile the high external quantum efficiency (EQE) is obtained in the NIR with the peak value over 38% and the EQE is over 18% in the entire response range, which could benefit from the long exciton diffusion length and the high carrier mobility of the highly ordered films. Thereby the fabricated planar solar cells achieve a high short-circuit current density (J_sc) of 9.26 mA cm^?2 and a power conversion efficiency (PCE) of 2.67%.     

Tandem organic solar cells formed in co-deposited films by doping

Tandem organic solar cells, in which two single p⁺in⁺-homojunctions are connected by a heavily-doped n⁺p⁺-ohmic interlayer, were formed in co-deposited films consisting of fullerene and α-sexithiophene simply by doping with molybdenum oxide and cesium carbonate. The single and tandem cells showed open-circuit voltages of 0.85 V and 1.69 V and conversion efficiencies of 1.6% and 2.4%, respectively.     

High performance inverted perovskite solar cells using PEDOT:PSS/KCl hybrid hole transporting layer

PEDOT:PSS is one of the most widely used hole transporting layer for inverted perovskite solar cells. Yet the performances of the corresponding perovskite solar cells are not satisfactory. Here, we demonstrate that KCl modified PEDOT:PSS film can promote the crystallization of perovskite film and enlarge the perovskite crystals. At the same time, KCl can diffuse into the perovskite film and effectively passivate the defects. As a result, inverted perovskite solar cells fabricated on 10 mg mL⁻¹ PEDOT:PSS/KCl films exhibit an average power conversion efficiency of 16.24 %, which is enhanced by 17.77 % compared with the reference perovskite solar cells. Open circuit voltage of 1.009 V and power conversion efficiency of 17.09 % have also been demonstrated using the optimized 10 mg mL⁻¹ PEDOT:PSS/KCl films.     

High rate (~7 nm/s), atmospheric pressure deposition of ZnO front electrode for Cu(In,Ga)Se2 thin‐film solar cells with efficiency beyond 15%

Undoped zinc oxide (ZnO) films have been grown on a moving glass substrate by plasma‐enhanced chemical vapor deposition at atmospheric pressure. High deposition rates of ~7 nm/s are achieved at low temperature (200 °C) for a substrate speed from 20 to 60 mm/min. ZnO films are highly transparent in the visible range (90%). By a short (~minute) post‐deposition exposure to near‐ultraviolet light, a very low resistivity value of 1.6·10⁻³ Ω cm for undoped ZnO is achieved, which is independent on the film thickness in the range from 180 to 1200 nm. The photo‐enhanced conductivity is stable in time at room temperature when ZnO is coated by an Al₂O₃ barrier film, deposited by the industrially scalable spatial atomic layer deposition technique. ZnO and Al₂O₃ films have been used as front electrode and barrier, respectively, in Cu(In,Ga)Se2 (CIGS) solar cells. An average efficiency of 15.4 ± 0.2% (15 cells) is obtained that is similar to the efficiency of CIGS reference cells in which sputtered ZnO:Al is used as electrode. Copyright © 2013 John Wiley & Sons, Ltd.     

Spontaneous growth by sol-gel process of low temperature ZnO as cathode buffer layer in flexible inverted organic solar cells

In this study, the sol–gel method was employed to prepare zinc oxide (ZnO) thin films as cathode buffer layers for inverted organic solar cells (IOSCs). We used a low temperature sol-gel process for the synthesis of ZnO thin films, in which the molar ratio of zinc acetate dihydrate (ZAD) to ethanolamine (MEA) was varied; subsequently, using the thin films, we successfully fabricated inverted solar cells on flexible plastic substrates. A ZnO sol–gel was first prepared by dissolving ZAD and MEA in ethylene glycol monomethyl ether (EGME). The molar ratios of ZAD to MEA were set as 1:1.2, 1:1, and 1:0.8, and we investigated the characteristics of the resulting ZnO thin films. We investigated the optical transmittance, surface roughness, and surface morphology of the films. Then, we discussed the reasons about the improvement of the device efficiency. The devices were fabricated using the ZnO thin films as cathode buffer layers. The results indicated that the morphology of the thin films prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 changed to a rippled nanostructure after two-step annealing. The PCE was enhanced because of the higher light absorption in the active layer caused by the nanostructure. The structure of the inverted device was ITO/ZnO/P3HT:PC₆₁BM/MoO₃/Ag. The short-circuit current densities (8.59 mA/cm² and 8.34 mA/cm²) of the devices with films prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 ratios, respectively, and annealed at 125 °C were higher than that of the device containing the ZnO thin film that was annealed at 150 °C. Inverted solar cells with ZnO films that were prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 and annealed at 125 °C exhibited PCEs of 3.38% and 3.30%, respectively. More than that, PCEs of the flexible device can reach up to 1.53%.     

Anti-Dissociation Passivation via Bidentate Anchoring for Efficient Carbon-Based CsPbI2.6Br0.4 Solar Cells

Molecular passivation on perovskite surface is an effective strategy to inhibit surface defect-assisted recombination and reduce nonradiative recombination loss in perovskite solar cells (PSCs). However, the majority of passivating molecules bind to perovskite surface through weak interactions, resulting in weak passivation effects and susceptible to interference from various factors. Especially in carbon-based perovskite solar cells (C-PSCs), the molecular passivation effect is more susceptible to disturbance in subsequent harsh preparation of carbon electrodes via blade-coating route. Herein, bidentate ligand 2,2′-Bipyridine (2Bipy) is explored to passivate surface defects of CsPbI_2.6Br_0.4 perovskite films. The results indicate that compared with monodentate pyridine (Py), bidentate 2Bipy shows a stronger chelation with uncoordinated Pb(II) defects and exhibits a greater passivation effect on perovskite surface. As a result, 2Bipy-modified perovskite films display a significantly boosted photoluminescence lifetime, accompanied by excellent anchoring stability and anti-dissociation of passivating molecules. Meanwhile, the moisture resistance of the 2Bipy-modified perovskite films is also significantly enhanced. Consequently, the efficiency of C-PSCs is improved to 16.57% (J_sc = 17.16 mA cm⁻², V_oc = 1.198 V, FF = 80.63%). As far as it is known, this value represents a new record efficiency for hole transport material-free inorganic C-PSCs.     

Improved barrier and mechanical properties of Al2O3/acrylic laminates using rugged fluorocarbon layers for flexible encapsulation

Thin film encapsulation (TFE) with high barrier performance and mechanical reliability is essential and challenging for flexible organic light-emitting diodes (OLEDs). In this work, SF₆ plasma treatments were introduced for surface modifications of acrylic in Al₂O₃/acrylic laminates fabricated by atomic layer deposition (ALD) and ink-jet printing (IJP). It was found that micro-/nano-structures and surface fluoridation appeared on the surface of acrylic, and could be modulated by the discharge power and irradiation of plasma treatment. The water vapor transmission rates (WVTR) of Al₂O₃/acrylic multi-layers decreased evidently because of reducing surface polarity and strong cross-link of acrylic after plasma treatments. Furthermore, the rugged surface and relieved residual stress resulted from etching and heating of acrylic could enhance the mechanical property remarkably. The plasma treated Al₂O₃/acrylic multi-layers with only 3 dyads exhibited a low WVTR value of 1.02 × 10⁻⁶ g/m²/day and more stable mechanical property under 200 iterations blending test by comparative measurements, proving that the introduction of SF₆ plasma surface modifications could improve simultaneously the barrier performance and mechanical reliability prominently of the inorganic/organic multi-layers with no need of extra neutral axis design.     

Acenaphtho[1,2-b]quinoxaline diimides derivative as a potential small molecule non-fullerene acceptor for organic solar cells

We designed and synthesized a small molecule acenaphtho[1,2-b]quinoxaline diimide derivative AQI-T2 as an electron-accepting material for non-fullerene organic solar cells. This molecule exhibits a relatively broad absorption band from 300 to 650 nm, with a moderately low-lying lowest unoccupied molecular orbital energy level of −3.64 eV. Non-fullerene organic solar cells with conventional structure using PTB7-Th as the electron donor and AQI-T2 as the electron acceptor exhibited moderate photovoltaic performances. The best performance was attained from the pristine device, which showed a power conversion efficiency of 0.77% with a relatively high open-circuit voltage of 0.86 V, a short circuit current of 2.04 mA cm⁻² and a fill factor of 43.98%. These results indicated that this n-type molecule can be a promising electron-accepting material for non-fullerene organic solar cells.