Hole Transport Layer Thickness Research Articles

Effects of the carrier transport layer thickness on the bending durability of flexible OLEDs

In this work, flexible organic light-emitting diode on polyethylene terephthalate (PET) substrate with N, N′-Bis-(1-naphthalenyl)-N, N′-bis-phenyl-(1, 1′-biphenyl)-4, 4′ -diamine (NPB) serve as the hole transport layer (HTL), TPBi as electron transport layer (ETL), and 4-(Dicyanomethylene)- 2-tert-butyl-6-(1, 1, 7, 7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) as dopant were fabricated. And the effects of HTL thickness and ETL thickness on the bending durability were explored and optimized. The bending durability was characterized by the ratio of the luminance after 100 bending cycles (L100) or 1000 cycles (L1000) to that before bending or initial (L0). The results show that for a given thickness of Alq3, the device of structure ‘ITO/NPB/Alq3/Al’ having 80 nm NPB layer exhibited the best bending durability, but low luminance. Overall, the device with 60 nm NPB layer has a good performance both in luminance and bending durability. For a given NPB layer of 60 nm in devices with TPBi as ETL, the optimal TPBi layer thickness for the best bending durability is 25 nm. Experimental measurements of surface of the samples were carried out with an SEM. From the observation results, the main influence of the bending on the device performance comes from the destruction of the organic layer and the aluminum layer.

Numerical analysis of inverted-structure perovskite solar cell based on all-inorganic charge transport layers

We perform a device modeling study on an inverted-structure perovskite solar cell (PSC) with ZnO as an electron transport layer and NiOx as a hole transport layer (HTL). The influence of several parameters on solar cell performance is explored. The simulation indicates that the optimal values for the absorber thickness and the HTL thickness are 600 and 30 nm, respectively. Meanwhile, doping concentrations of NiOx and ZnO can significantly affect the device performance. Hole mobility of HTL larger than 0.1 cm2 / V · s is necessary for the effective hole transporting to the front contact. In addition, a front contact work function higher than 4.8 eV is more suitable for the inverted-structure PSCs. By optimizing these parameters, high-performance inverted-structure PSC with the efficiency of 21.28% is achieved. These research results are conducive to device design and fabrication of inverted PSCs.

Potential Improvement in Fill Factor of Lead-Halide Perovskite Solar Cells

In this study, the origin of the fill factor (FF) in lead-halide perovskite solar cells is discussed based on different thicknesses of 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) as the hole-transporting layer (HTL). As the thickness of HTL is decreased, FF increases and hence, the photovoltaic performance is also improved. This is mainly ascribed to the reduced series resistance with decreasing HTL thickness. Such improvement in FF is examined on the basis of an empirical equation for FF with the diode and photovoltaic parameters. As a result, the thickness-dependent FF can be well explained by this equation. The potential improvements in FF and PCE are further discussed on the basis of the empirical equation.

6‐4: Influence of Hole Transporting Layer Thickness on Quantum Dot Light Emitting Diodes

The quantum dot light emitting diodes (QLEDs) based on solution process are reported for its stability and efficiency. Our work demonstrate that the thickness of hole transporting layer (HTL) play an important role in the QLEDs device. HTL may be corroded by the QD solvent with the process of spin‐coating. And so the quality of HTL, such as roughness, compactness, reliability, becomes worse. The HTL thickness loss more, the quality of QDs layer will be worse. By reducing HTL initial thickness, HTL thickness loss was reduced and the roughness and uniformity of QDs layer which is on the HTL layer was improved. Meanwhile, the efficiency and brightness of QLEDs were improved and the threshold voltage was lowered. However, leakage current increased. In the other hand, if HTL initial thickness is increased, the hole injecting layer (HIL) can be covered completely and leakage current will decrease, meanwhile, the life time can be improved. Therefore, an optimized initial thickness of HTL is very important for QLEDs. In this paper, we also will show the red passive‐matrix QLED device color.

Efficient and Stable Vacuum-Free-Processed Perovskite Solar Cells Enabled by a Robust Solution-Processed Hole Transport Layer.

Here, efficient and stable vacuum-free processed perovskite solar cells (PSCs) are demonstrated by employing solutionprocessed molybdenum tris-[1-(trifluoroethanoyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd-COCF3 )3 )-doped poly(3,4-ethylenedioxythiophene) (PEDOT) film as hole transport layer (HTL). Our results indicate that the incorporation of Mo(tfd-COCF3 )3 dopant can induce p-doping through charge transfer from the highest occupied molecular orbital (HOMO) level of the PEDOT host to the electron affinity of Mo(tfd-COCF3 )3 , leading to an increase in conductivity by more than three orders of magnitude. With this newly developed p-doped film as HTL in planar heterojunction PSCs, a high power conversion efficiency (PCE) up to 18.47 % can be achieved, which exceeds that of the device with commonly used HTL 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9'-spirobifluorene (spiro-OMeTAD). Taking the advantage of the high conductivity of this doped film, a prominent PCE as high as 15.58 % is also demonstrated even when a large HTL thickness of 220 nm is used. Importantly, the high quality film of this HTL is capable of acting as an effective passivation layer to keep the underlying perovskite layer intact during solution-processed Ag-nanoparticles layer deposition. The resulting vacuum-free PSCs deliver an impressive PCE of 14.81 %, which represents the highest performance ever reported for vacuum-free PSCs. Furthermore, the resulting devices show good ambient stability without encapsulation.

Fast Fabrication of a Stable Perovskite Solar Cell with an Ultrathin Effective Novel Inorganic Hole Transport Layer.

With the aim of fabricating simple, reproducible, and scalable perovskite solar cells (PSCs) with least time consumption, a novel CoOx hole transport layer (HTL) was first proposed and introduced in this work. The CoOx HTL thickness was minimized to about 10 nm with complete coverage on the FTO substrate (F-doped SnO2) by direct current magnetron sputtering. The ultrathin HTL could minimize the incident light loss caused by cobalt ion absorption and reduce the carrier transport loss by shortening the transport path. Copper was incorporated into the CoOx lattice to address the low conductivity of the CoOx film and the energy-level mismatch between CoOx and the perovskite material. On the basis of cobalt-copper binary oxide (Co1-yCuyOx), the highest power conversion efficiency (PCE) of about 10% was achieved, which was acceptable for mass production. Moreover, the deposition of such Co1-yCuyOx films takes only 2 min without size limitation of substrates. A well-functioned device based on the Co1-yCuyOx HTL could hence be fabricated within 100 min. Excellent stability was demonstrated as well, with over 90% of the initial PCE remaining after being stored in a dark and humid environment (relative humidity 60%) for 12 days.

Solution-processable MoOx nanocrystals enable highly efficient reflective and semitransparent polymer solar cells

Solution-manufacturing of organic solar cells with best-in-class power conversion efficiency (PCE) will require all layers to be solution-coated without compromising solar cell performance. To date, the hole transporting layer (HTL) deposited on top of the organic bulk heterojunction layer in the inverted architecture is most commonly an ultrathin (<10nm) metal oxide layer prepared by vacuum-deposition. Here, we show that an alcohol-based nanocrystalline MoOx suspension with carefully controlled nanocrystal (NC) size can yield state of the art reflective and semitransparent solar cells. Using NCs smaller than the target HTL thickness (∼10nm) can yield compact, pinhole-free films which result in highly efficient polymer:fullerene bulk heterojunction (BHJ) solar cells with PCE=9.5%. The solution processed HTL is shown to achieve performance parity with vacuum-evaporated HTLs for several polymer:fullerene combinations and is even shown to work as hole injection layer in polymer light emitting diodes (PLED). We also demonstrate that larger MoOx NCs (30–50nm) successfully composite MoOx with Ag nanowires (NW) to form a highly conducting, transparent top anode with exceptional contact properties. This yields state-of-the-art semitransparent polymer: fullerene solar cells with PCE of 6.5% and overall transmission >30%. The remarkable performance of reflective and semitransparent OPVs is due to the uncommonly high fill factors achieved using a carefully designed strategy for implementation of MoOx nanocrystals as HTL materials.

Towards the design of efficient quantum dot light-emitting diodes by controlling the exciton lifetime.

Time-resolved photoluminescence and electroluminescence measurements were used to explore the emission characteristics of excitons in quantum dot light-emitting diodes (QD-LEDs). It is found that the lifetime of excitons in the QDs can be varied by adjusting the distance between the excitons and metal Al mirror, which is due to the effect of local density of optical states (LDOS) on the exciton decay rate. QD-LEDs with different hole transport layer (HTL) thickness, i.e., different distance between QDs and Al reflective anode, have been fabricated and it is found that the HTL thickness affects the device efficiency performance greatly, and the optimal HTL thickness for the red QD-LED (emission peak is at 621 nm) is 80 nm. These results shed light on the factors affecting the efficiency and efficiency roll-off in QD-LEDs, thus may provide a clue for high performance QD-LEDs.

Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells.

Solution-processed organic-inorganic hybrids composing of MoO3 nanoparticles and PEDOT:PSS were developed for use in inverted organic solar cells as hole transporting layer (HTL). The hybrid MoO3:PEDOT:PSS inks were prepared by simply mixing PEDOT:PSS aqueous and MoO3 ethanol suspension together. A core-shell structure was proposed in the MoO3:PEDOT:PSS hybrid ink, where PEDOT chains act as the core and MoO3 nanoparticles connected with PSS chains act as the composite shell. The mixing with PEDOT:PSS suppressed the aggregation of MoO3 nanoparticles, which led to a smoother surface. In addition, since the hydrophilic PSS chains were passivated through preferentially connection with MoO3, the stronger adhesion between MoO3 nanoparticles and the photoactive layer improved the film forming ability of the MoO3:PEDOT:PSS hybrid ink. The MoO3:PEDOT:PSS hybrid HTL can therefore be feasibly deposited onto the hydrophobic photoactive polymer layer without any surface treatment. The use of the MoO3:PEDOT:PSS hybrid HTL resulted in the optimized P3HT:PC61BM- and PTB7:PC61BM-based inverted organic solar cells reaching highest power conversion efficiencies of 3.29% and 5.92%, respectively, which were comparable with that of the control devices using thermally evaporated MoO3 HTL (3.05% and 6.01%, respectively). Furthermore, less HTL thickness dependence of device performance was found for the hybrid HTL-based devices, which makes it more compatible with roll-to-roll printing process. In the end, influence of the blend ratio of MoO3 to PEDOT:PSS on photovoltaic performance and device stability was studied carefully, results indicated that the device performance would decrease with the increase of MoO3 blended ratio, whereas the long-term stability was improved.

Light Harvesting and Charge Recombination in CH3NH3PbI3 Perovskite Solar Cells Studied by Hole Transport Layer Thickness Variation

A tailored optimization of perovskite solar cells requires a detailed understanding of the processes limiting the device efficiency. Here, we study the role of the hole transport layer (HTL) spiro-MeOTAD and its thickness in a mesoscopic TiO2-based solar cell architecture. We find that a sufficiently thick (200 nm) HTL not only increases the charge carrier collection efficiency but also the light harvesting efficiency. This is due to an enhanced reflection of a smooth HTL/Au-electrode interface. The rough CH3NH3PbI3 perovskite surface requires an HTL thickness of >400 nm to avoid surface recombination and guarantee a high open-circuit voltage. Analyses of the electroluminescence efficiency and the diode ideality factor show that the open-circuit voltage becomes completely limited by trap-assisted recombination in the perovskite for a thick HTL. Thus, spiro-MeOTAD is a very good HTL choice from the device physics' point of view. The fill factor analyzed by the Suns-Voc method is not transport limited, but trap-recombination limited as well. Consequently, a further optimization of the device has to focus on defects in the polycrystalline perovskite film.

Characterization of Organic Thin Film Solar Cells of PCDTBT : PC71BM Prepared by Different Mixing Ratio and Effect of Hole Transport Layer

The organic thin film solar cells (OTFSCs) have been successfully fabricated using PCDTBT : PC71BM with different mixing ratios (1 : 1 to 1 : 8) and the influence of hole transport layer thickness (PEDOT : PSS). The active layers with different mixing ratios of PCDTBT : PC71BM have been fabricated using o-dichlorobenzene (o-DCB). The surface morphology of the active layers and PEDOT : PSS layer with different thicknesses were characterized by AFM analysis. Here, we report that the OTFSCs with high performance have been optimized with 1 : 4 ratios of PCDTBT : PC71BM. The power conversion efficiency (PCE = 5.17%) of the solar cells was significantly improved by changing thickness of PEDOT : PSS layer. The thickness of the PEDOT : PSS layer was found to be of significant importance; the thickness of the PEDOT : PSS layer at 45 nm (higher spin speed 5000 rpm) shows higher short circuit current density (Jsc) and lower series resistance (Rs) and higher PCE.

Effect of the Solvent Doping Concentration of PEDOT:PSS Transparent Electrodes and the Hole Transport Layer Thickness on the Characteristics of Organic Solar Cells

Indium tin oxide (ITO) is the most common transparent electrode for organic solar cells due to its high electrical conductivity and transmittance. However, the ITO film significantly increases device costs and its brittleness limits the applications in flexible electronics. Here, a highly conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is studied as an alternative electrode for organic solar cells. The effect of solvent doping concentration on the electrical and surface structural properties of PEDOT:PSS thin films is investigated. It is observed that, with increasing a solvent concentration in PEDOT:PSS films, surface roughness of PEDOT:PSS electrodes increases, causing the change of short circuit current densities in devices. In addition, a light absorption property at short wavelengths is enhanced by decreasing the thickness of hole transport layers of organic solar cells. *E-mail : yhkim113@pknu.ac.kr

Random nanostructure scattering layer for suppression of microcavity effect and light extraction in OLEDs.

In this study, we investigated the effect of a random nanostructure scattering layer (RSL) on the microcavity and light extraction in organic light emitting diodes (OLEDs). In the case of the conventional OLED, the optical properties change with the thickness of the hole transporting layer (HTL) because of the presence of a microcavity. However, OLEDs equipped with the an RSL showed similar values of external quantum efficiency and luminous efficacy regardless of the HTL thickness. These phenomena can be understood by the scattering effect because of the RSL, which suppresses the microcavity effect and extracts the light confined in the device. Moreover, OLEDs with the RSL led to reduced spectrum and color changes with the viewing angle.

All solution-processed stable white quantum dot light-emitting diodes with hybrid ZnO@TiO₂ as blue emitters.

White quantum dot light-emitting diodes (QD-LEDs) have been a promising candidate for high-efficiency and color-saturated displays. However, it is challenging to integrate various QD emitters into one device and also to obtain efficient blue QDs. Here, we report a simply solution-processed white QD-LED using a hybrid ZnO@TiO2 as electron injection layer and ZnCdSeS QD emitters. The white emission is obtained by integrating the yellow emission from QD emitters and the blue emission generated from hybrid ZnO@TiO2 layer. We show that the performance of white QD-LEDs can be adjusted by controlling the driving force for hole transport and electroluminescence recombination region via varying the thickness of hole transport layer. The device is demonstrated with a maximum luminance of 730 cd/m2 and power efficiency of 1.7 lm/W, exhibiting the Commission Internationale de l'Enclairage (CIE) coordinates of (0.33, 0.33). The unencapsulated white QD-LED has a long lifetime of 96 h at its initial luminance of 730 cd/m2, primarily due to the fact that the device with hybrid ZnO@TiO2 has low leakage current and is insensitive to the oxygen and the moisture. These results indicate that hybrid ZnO@TiO2 provides an alternate and effective approach to achieve high-performance white QD-LEDs and also other optoelectronic devices.

In situ growth of double-layer MoO3/MoS2 film from MoS2 for hole-transport layers in organic solar cell

Efficient organic solar cells (OSCs) based on regioregular poly(3-hexylthiophene):fullerene derivative [6,6]-phenyl-C61butyric acid methyl ester composites have been fabricated on fluorine-doped tin oxide (FTO) coated glass substrates by a radio frequency (RF) sputtered and ultraviolet ozone (UVO) treated MoS2 film as the hole-transport layer (HTL). With the help of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Raman spectroscopy, transmission spectra and the Hall-effect system, we find that the deposition temperature can modulate the contents of the various valence states of molybdenum, which can result in changes of the energy level, and the optical and electrical properties of the MoS2 films. MoS2 has been oxidized to a double-layered MoO3–MoS2 film by UVO treatment. Due to the presence of the molybdenum oxidation states Mo5+ and Mo6+, the MoS2 film shows p-type conductive behavior, and its smaller electron affinity can effectively block electron from exciton dissociation. By optimizing the HTL thickness and sputtering deposition temperature, a power conversion efficiency up to 4.15% has been achieved for an OSC that used a double-layered MoO3–MoS2 film as the HTL. Its JSC is bigger than that of the OSC with a pure MoO3 film as the HTL. This shows that this double layer MoO3–MoS2 interface is more favorable for hole-transfer.

Polymer solar cells with electrodeposited CuSCN nanowires as new efficient hole transporting layer

Here, we report for the first time that electrodeposited CuSCN nanowires (NWs) could be successfully used as a new hole transporting layer (HTL) in a conventional polymer solar cell (PSC) with a poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC70BM) blend as an active layer. An efficient PSC with 5.1% power conversion efficiency for 28mm2 active surface area is fabricated. The performances of the PSCs prepared with electrodeposited CuSCN thin films and nanowires are compared and the obtained results clearly show that the nanostructuring of the HTL leads to an efficient hole extraction. The influence of the HTL thickness on the device performances was studied and it has been found that the thickness of the CuSCN nanowire array should not exceed the active layer thickness in order to keep high performances. An extraordinary high open circuit voltage (>900mV) and a fill factor about 60% were measured. It is shown that the electrodeposited CuSCN thin films and NWs could be directly used in the polymer solar cells without any subsequent annealing process, which is a crucial point for the reduction of the device costs.

Examining the effect of additives and thicknesses of hole transport layer for efficient organic solar cell devices

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a conducting polymer, has been receiving a great deal of attention for next generation optoelectronic organic devices. In this report, we discuss the effect of additives along with the thickness of PEDOT:PSS layers on the power conversion efficiency of organic solar cell devices. PEDOT:PSS films treated with high boiling point solvents of dimethyl sulfoxide (DMSO) and ethylene glycol (EG) show a significant enhancement in electrical conductivity without compromising flexibility or optical transparency. The conductivity increased from 0.5 to 517 and 724 S/cm after once and thrice treatment with 4 vol. % EG, respectively. The as-prepared and additives-treated PEDOT:PSS films deposited on glass substrates have been investigated by optical spectroscopy, micro-Raman spectroscopy and atomic force microscopy (AFM). The results indicate that structural and morphological changes were induced by the additive processes. By using DMSO and EG treated PEDOT:PSS as a hole transport layer, organic solar cells with a Poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:[6,6]-phenyl-C71-butyric acid methyl esters (PCDTBT:PC71BM) bulk heterojunction have been fabricated on indium-tin-oxide (ITO) coated glass substrates. The high power conversion efficiency (PCE) of 5.17%, and 5.69%, were observed for PEDOT:PSS hole transport layers treated with DMSO and EG respectively, even though the devices were prepared in air atmosphere.

Three-peak top-emitting white organic emitting diodes with wide color gamut for display application

By changing the thickness of hole transport layer to control the cavity length, a top-emitting white organic light-emitting diode (TWOLED) with three individual narrow peaks matching well with the three primary color filters has been successfully realized. It is very important to carefully design the multimode microcavity for the achievement of the three-peak spectrum. Compared with the bottom-emitting white organic light-emitting diodes (BWOLEDs), the TWOLEDs exhibit improved color purity and a wider color gamut due to the narrow emissive spectrum. The maximum current efficiency and power efficiency of TWOLED reach 28.9cd/A and 27.5lm/W, respectively. It is predicted that this kind of three-peak TWOLEDs is suitable for the high-quality display application.

Multi‐peak and chromaticity‐stable top‐emitting white organic light‐emitting diodes using single blue emitter

AbstractWe have fabricated multi‐peak and chromaticity‐stable top‐emitting white organic light‐emitting diodes (TEWOLEDs) using single blue emitter. Besides the intrinsic emission of blue emitter, the additional emission can be well realized by simply adjusting the thickness of hole transporting layer (HTL), thus modifying the optical cavity length to obtain different resonant wavelengths. The detailed variation process for multi‐peak spectra with the increase of HTL thickness is studied, which provides a guidance for the design of microcavity TEWOLEDs.magnified image(© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Semi-transparent small molecule organic solar cells with laminated free-standing carbon nanotube top electrodes

We have developed semi-transparent n–i–p organic solar cells (OSC) with free-standing multi-wall carbon nanotube (f-CNT) sheets as transparent top electrodes. By a simple and damage-free room temperature orthogonal liquid solution assisted self-laminating process, f-CNT top electrodes are successfully deposited on top of ZnPc:C 60 bulk heterojunction small molecule OSCs. The cells show high fill factors above 58% as well as efficiencies up to 1.5% and greater long-term stability compared to the device having a metal electrode. For the given cell structure with f-CNT semi-transparent electrodes, the influence of an optical spacer on light absorption is studied by a systematic variation of the hole transport layer thickness, supported by optical simulations. The results strongly indicate that OSCs with f-CNT top electrodes and optimized thin film stack are highly promising for semi-transparent OSCs, which can be used in tandem devices, in tinted smart windows, and similar applications by a simple and damage-free process in roll-to-roll configuration that can be scaled to large area manufacturing.