Enhanced efficiency and high temperature stability of hybrid quantum dot light-emitting diodes using molybdenum oxide doped hole transport layer

High power efficiency (PE) and stability of quantum dot (QD) light-emitting diodes (QLEDs) are important factors for practical use in various displays. However, hybrid QLEDs consisting of an organic hole transport layer (HTL) and an inorganic electron transport layer (ETL) sometimes have poor stability due to the low thermal stability of the organic HTL. To solve the problem, here, we report enhanced efficiency, lifetime, and temperature stability in inverted and hybrid structured QLEDs by adopting a MoO₃-doped HTL. Also, to improve the electron–hole charge carrier balance, a thin insulating interlayer was used between QDs and the ETL. As a result, the QLED with the p-doped HTL exhibited the increased PE by ∼28% and longer lifetime compared to the pristine QLEDs. In addition, the QLED showed stable operation at the high temperature up to 400 K, whereas the control device failed to operate at 375 K. We systematically investigated the effect of the MoO₃-doping on the performance and thermal stability of the QLEDs. We believe that QLEDs with the p-doped HTL can be used for further QLED researches to simultaneously improve the efficiency, lifetime, and high temperature stability, which are highly required for their use in automotive and outdoor displays.

QLEDs introducing a p-doped HTL exhibit stable operation at high temperature up to 400 K.     

The improved performance and mechanism of solution-processed blue PhOLEDs based on double electron transport layers

Performance improved solution-processed blue phosphorescent organic light emitting diodes (PhOLEDs) are demonstrated by adopting a double electron transport layer (ETL) strategy, which consists of TPBi and an additional Alq₃ ETL. With the help of Alq₃ ETL, the performance of the optimal device with a double ETL is significantly enhanced. The maximum luminance of OLEDs is improved from 6787 cd m⁻² to 13 054 cd m⁻², and the maximum current efficiency is increased from 3.9 cd A⁻¹ to 11.4 cd A⁻¹. Furthermore, the difference of carrier injection in the two types of PhOLEDs is explored by using the transient electroluminescence measurement method. The results imply that double ETL can help to balance electron injection and carrier transport, reduce the interface charge accumulation, leading to a high efficiency. The PL decay of the emission layer with different ETL is detected to analyze the effect of the introduced second ETL layer and the interface on the exciton decay of the emission layer. The results show that the introduced interface in devices with a double ETL has an adverse effect on the exciton emission, which contributes to the serious efficiency roll-off of devices with a double ETL.

The performance is improved in solution-processed blue phosphorescent organic light emitting diodes (PhOLEDs) by adopting double electron transport layers (ETL) strategy.     

Improving the performance of quantum-dot light-emitting diodes via an organic–inorganic hybrid hole injection layer

Poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) is a commonly used material for the hole injection layer (HIL) in quantum-dot light-emitting diodes (QLEDs). In this work, we improved the performance of the QLED by using an organic–inorganic hybrid HIL. The hybrid HIL was prepared by mixing PEDOT:PSS with vanadium oxide (V₂O₅), which is a transition-metal oxide (TMO). The hole injection properties of PEDOT:PSS were improved according to the amount of V₂O₅ mixed into the PEDOT:PSS. The maximum luminance and current efficiency were 36 198 cd m⁻² and 13.9 cd A⁻¹, respectively, when the ratio of PEDOT:PSS and V₂O₅ was 10 : 1. Moreover, the operating lifetime exceeded 300 h, which is 10 times longer than the lifetime of the device with only PEDOT:PSS HIL. The improvement was analyzed using ultraviolet and X-ray photoelectron spectroscopy. We found that the density of state (DOS) of PEDOT:PSS near the Fermi energy level was increased by mixing V₂O₅. Therefore, the increase of DOS improved the hole injection and the performance of QLEDs. The result shows that the hybrid HIL can improve the performance and the stability of QLEDs.

The performance of the quantum-dot light-emitting diodes was improved by using an organic–inorganic hybrid hole injection layer.     

High-performance inverted organic light-emitting diodes with extremely low efficiency roll-off using solution-processed ZnS quantum dots as the electron injection layer

The electron-injecting layer (EIL) is one of the key factors in inverted organic light-emitting diodes (OLEDs) to realize high electroluminescence efficiency. Here, we proposed a novel cathode-modified EIL based on ZnS quantum dots (QDs) in inverted OLEDs, and demonstrated that the device performance was dramatically improved compared to traditional ZnO EIL. The EIL of ZnS QDs may greatly promote the electron injection ability and consequently increase the charge carrier recombination efficiency for the device. We also investigated the effects of different pH values (ZnS-A, pH = 10; ZnS-B, pH = 12) on the properties of ZnS QDs. The best inverted phosphorescent OLED device employing mCP:Ir(ppy)₃ as the emission layer showed a low turn-on voltage of 2.9 V and maximum current efficiency of 61.5 cd A⁻¹. Also, the ZnS-A based device exhibits very-low efficiency roll-off of 0.9% and 4.3% at 1000 cd m⁻² and 5000 cd m⁻², respectively. Our results indicate that use of ZnS QDs is a promising strategy to increase the performance in inverted OLEDs.

The electron-injecting layer (EIL) is one of the key factors in inverted organic light-emitting diodes (OLEDs) to realize high electroluminescence efficiency.     

Effects of PEDOT:PSS:GO composite hole transport layer on the luminescence of perovskite light-emitting diodes

Perovskite light-emitting diodes (PeLEDs) employing CH₃NH₃PbBr₃ as the emission layer (EML) and graphene oxide (GO) doped PEDOT:PSS as the hole transport layer (HTL) were prepared and characterized. GO doped in PEDOT:PSS can lead to the increased work function of HTL and lower the hole injection barrier at the HTL/CH₃NH₃PbBr₃ interface, which facilitates the hole injection. Meanwhile, the optimized GO amount in PEDOT:PSS can help to reduce the quenching of luminescence occurring at the interface between HTL and perovskite. The luminance and current efficiency reach the maximum values of 3302 cd m⁻² and 1.92 cd A⁻¹ in PeLED with an optimized GO ratio (0.3), which increase by 43.3% and 73.0% in comparison with the undoped device, respectively. The enhanced luminescence of PeLEDs was caused by the combined effects of enhanced hole injection efficiency and the suppressed exciton quenching occurring at the HTL/EML interface. These results indicate that the introduction of traditional two-dimensional materials is a reasonable method for designing the structure of PeLEDs.

Perovskite light-emitting diodes (PeLEDs) employing CH₃NH₃PbBr₃ as the emission layer (EML) and graphene oxide (GO) doped PEDOT:PSS as the hole transport layer (HTL) were prepared and characterized.     

Improved resistive switching characteristics of a multi-stacked HfO2/Al2O3/HfO2 RRAM structure for neuromorphic and synaptic applications: experimental and computational study

Atomic Layer Deposition (ALD) was used for a tri-layer structure (HfO₂/Al₂O₃/HfO₂) at low temperature over an Indium Tin Oxide (ITO) transparent electrode. First, the microstructure of the fabricated TaN/HfO₂/Al₂O₃/HfO₂/ITO RRAM device was examined by the cross-sectional High-Resolution Transmission Electron Microscopy (HRTEM). Then, Energy Dispersive X-ray Spectroscopy (EDS) was performed to probe compositional mapping. The bipolar resistive switching mode of the device was confirmed through SET/RESET characteristic plots for 100 cycles as a function of applied biasing voltage. An endurance test was performed for 100 DC switching cycles @0.2 V wherein; data retention was found up to 10⁴ s. Moreover, for better insight into the charge conduction mechanism in tri-layer HfO₂/Al₂O₃/HfO₂, based on oxygen vacancies (V_OX), total density of states (TDOS), partial density of states (PDOS) and isosurface three-dimensional charge density analysis was performed using WEIN2k and VASP simulation packages under Perdew–Burke–Ernzerhof _Generalized Gradient approximation (PBE-GGA). The experimental and theoretical outcomes can help in finding proper stacking of the active resistive switching (RS) layer for resistive random-access memory (RRAM) applications.

Atomic Layer Deposition (ALD) was used for a tri-layer structure (HfO₂/Al₂O₃/HfO₂) at low temperature over an Indium Tin Oxide (ITO) transparent electrode.     

Interfacial electronic structure between a W-doped In2O3 transparent electrode and a V2O5 hole injection layer for inorganic quantum-dot light-emitting diodes

The interfacial electronic structure between a W-doped In₂O₃ (IWO) transparent electrode and a V₂O₅ hole injection layer (HIL) has been investigated using ultraviolet photoelectron spectroscopy for high-performance and inorganic quantum-dot light-emitting diodes (QLEDs). Based on the interfacial electronic structure measurements, we found gap states in a V₂O₅ HIL at 1.0 eV below the Fermi level. Holes can be efficiently injected from the IWO electrode into poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(4-sec-butylphenyl)diphenylamine)] (TFB) through the gap states of V₂O₅, which was confirmed by the hole injection characteristics of a hole-only device. Therefore, conventional normal-structured QLEDs were fabricated on a glass substrate with the IWO transparent electrode and V₂O₅ HIL. The maximum luminance of the device was measured as 9443.5 cd m⁻². Our result suggests that the IWO electrode and V₂O₅ HIL are a good combination for developing high-performance and inorganic QLEDs.

Interfacial electronic structure between W-doped In₂O₃ and V₂O₅ has been investigated, and we found gap states that can provide an efficient hole carrier injection pathway.     

Low-temperature atomic layer deposition of Al2O3/alucone nanolaminates for OLED encapsulation

Thin film encapsulation (TFE) is one of the key problems that hinders the lifetime and widespread commercialization of flexible organic light-emitting diodes (OLEDs). In this work, TFE of OLEDs with Al₂O₃/alucone laminates grown by atomic layer deposition (ALD) and molecular layer deposition (MLD) as moisture barriers were demonstrated. The barrier performances of Al₂O₃/alucone laminates with respect to the individual layer thickness and the number of dyads were investigated. It was found that alucone with suitable layer thickness could reduce the permeation to the defect zones of the inorganic layer by prolonging the permeation pathway, sequentially improving the moisture barrier performance. The water vapor transmission rate (WVTR) could be further lowered with increasing the number of dyads of the laminates, the WVTR value reached 1.44 × 10⁻⁴ g per m² per day for laminates with 5.5 dyads. These laminates were incorporated in OLEDs with pixel define layer (PDL), and were found to be able to evidently prolong the lifetime of the OLED.

Al₂O₃/alucone laminates were fabricated by atomic layer deposition (ALD) and molecular layer deposition (MLD), showing good barrier properties. These laminates were found to prolong the lifetime of organic light-emitting diodes (OLEDs) evidently.     

Mixture of quantum dots and ZnS nanoparticles as emissive layer for improved quantum dots light emitting diodes

Recently, quantum dots based light-emitting diodes (QLEDs) have received huge attention due to the properties of quantum dots (QDs), such as high photoluminescence quantum yield (PLQY) and narrow emission. To improve the performance of QLEDs, reducing non-radiative energy transfer is critical. So far, most conventional methods required additional chemical treatment like giant shell and/or ligands exchange. However that triggers unsought shifted emission or reduced PLQY of QDs. In this work, we have firstly suggested a novel approach to improve the efficiency of QLEDs by introducing inorganic nanoparticles (NPs) spacer between QDs, without additional chemical treatment. As ZnS NPs formed a mixture layer with QDs, the energy transfer was reduced and the distance between the QDs increased, leading to improved PLQY of mixture layer. As a result, current efficiency (CE) of the QLED device was improved by twice compared with one using only QDs layer. This is an early report on utilizing ZnS NPs as an efficient spacer, which can be utilized to other compositions of QDs.

Mixture of quantum dots and ZnS nanoparticles as emissive layer for improved QLEDs by decreasing energy transfer between the QDs.     

Interface engineering of graphene–silicon Schottky junction solar cells with an Al2O3 interfacial layer grown by atomic layer deposition

The recent progress in graphene (Gr)/silicon (Si) Schottky barrier solar cells (SBSC) has shown the potential to produce low cost and high efficiency solar cells. Among the different approaches to improve the performance of Gr/Si SBSC is engineering the interface with an interfacial layer to reduce the high recombination at the graphene (Gr)/silicon (Si) interface and facilitate the transport of photo-generated carriers. Herein, we demonstrate improved performance of Gr/Si SBSC by engineering the interface with an aluminum oxide (Al₂O₃) layer grown by atomic layer deposition (ALD). With the introduction of an Al₂O₃ interfacial layer, the Schottky barrier height is increased from 0.843 V to 0.912 V which contributed to an increase in the open circuit voltage from 0.45 V to 0.48 V. The power conversion efficiency improved from 7.2% to 8.7% with the Al₂O₃ interfacial layer. The stability of the Gr/Al₂O₃/Si devices was further investigated and the results have shown a stable performance after four weeks of operation. The findings of this work underpin the potential of using an Al₂O₃ interfacial layer to enhance the performance and stability of Gr/Si SBSC.

One approach to improve Gr/Si SBSC performance is engineering the interface with an interfacial layer. We demonstrate the improved performance of Gr/Si SBSC upon engineering the interface with an aluminium oxide (Al₂O₃) layer grown by atomic layer deposition (ALD)..     

Efficient multi-barrier thin film encapsulation of OLED using alternating Al2O3 and polymer layers

Organic optoelectronic devices, especially for OLEDs, are extremely susceptibility to water vapor and oxygen which limit their widespread commercialization. In order to extend the shelf-lifetime of devices, thin film encapsulation is the most promising and challenging encapsulation process. In this study, dyad-style multilayer encapsulation structures based on alternating Al₂O₃ layer and parylene C have been discussed as gas diffusion barriers, in which dense and pinhole-free Al₂O₃ films were grown by atomic layer deposition (ALD) and flexible parylene C layers were deposited by chemical vapor deposition (CVD). We found the particle in ALD deposited Al₂O₃ films process is the key killer to barrier property. The thickness of Al₂O₃ films is the key factor which limit the amount of strain placed on barrier films. With three dyads of the optimal thickness of 30 nm Al₂O₃ film and 500 nm parylene C, WVTR value is lower than 10⁻⁵ g m⁻² per day. In addition, the lifetime of OLEDs with and without encapsulation was 190 h and 10 h, respectively. All the results show that this TFE structure has the effective encapsulated property and does not cause degradation of the OLED devices.

The work demonstrates the WVTR value of 3 dyads alternative 30 nm Al₂O₃ and 500 nm parylene C encapsulation structure is less than 10⁻⁵ g m⁻² per day. And this TFE technology successfully applies for OLED device encapsulation.     

The recombination zone adjusted by the gradient doping of TPA-DCPP for efficient and stable deep red organic light emitting diodes

Deep-red organic light-emitting diodes (DR-OLEDs) or near-infrared organic light-emitting diodes (NIR-OLEDs) have a wide range of applications in real life, such as special light sources for plant growth in agriculture, optical communications, infrared imaging, infrared medical imaging and other fields. However, the device performance of DR-OLEDs is still far behind that of red, green and blue OLEDs. In addition to the well-known energy gap law, the location of the recombination region also has a significant impact on the device performance. If the recombination area is too close to the cathode, the electrons in the electron transport layer will easily cause exciton quenching. In this study, for the first time, we adopted a quantum well-like structure by changing the host (CBP) and guest (TPA-DCPP) thicknesses as the light-emitting layer to manage the position of the recombination zone, and then improved the carrier injection and transportation as well as increased the exciton recombination rate. Furthermore, we introduced a hole trap layer to reduce the current density and suppress the recombination zone movement; finally, we prepared high-brightness and high-efficiency DR-OLEDs based on the TADF material with a wavelength of 674 nm, a maximum brightness of 1151 cd m⁻² and a maximum EQE of 4.4%.

Efficient deep red organic light-emitting diodes with 4.4% external quantum efficiency (EQE) and a stable electroluminescent spectrum at 674 nm.     

An indenocarbazole-based host material for solution processable green phosphorescent organic light emitting diodes

We designed and synthesized a new host material with a highly soluble and thermally stable indenocarbazole derivative (7,7-dimethyl-5-phenyl-2-(9-phenyl-9H-carbazol-3-yl)-5,7-dihydro-indeno[2,1-b]carbazole) that can make green phosphorescent organic light-emitting diodes (PHOLEDs) in a solution process. In particular, these are used in a blue common layer structure in which green and red-emitting layers are formed by a solution process and blue common layers are thermally evaporated. The new host material possesses excellent hole transport capability and high triplet energy (T₁). Mainly we designed the hole dominant material to keep the exciton forming area away from the hole transport layer (HTL) and emitting layer (EML) interface, an interfacial mixing area to improve device performance. As a result, the greatest lifetime of 1300 hours was achieved and a high current efficiency of up to 66.3 cd A⁻¹ was recorded when we used the optimized device structure of a 5 nm thick bipolar exciton blocking layer (B-EBL). It may be a good agreement of exciton confinement and reduced electron accumulation at the HTL and EML interface.

We designed and synthesized a new host material with a highly soluble and thermally stable indenocarbazole derivative that can make green phosphorescent organic light-emitting diodes (PHOLEDs) in a solution process.     

Retracted Article: Wavelength modulation of ZnO nanowire based organic light-emitting diodes with ultraviolet electroluminescence

Although organic light emitting diodes (OLEDs) can find important applications in display-related fields, it still remains a challenge to fabricate high-efficiency ultraviolet (UV) OLEDs with tunable wavelength. In this work, we demonstrate a facile method to adjust the electroluminescence (EL) peak from an inverted UV-OLED device that has zinc oxide nanowires (ZnO NWs) as an electron injection layer. The organic–inorganic interface between ZnO NWs and the 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ) emission layer employed in this work allows a reduction of the diffusion length of excitons, which further results in a hampered relaxation process of higher energy states as well as a blue shift of the EL spectrum. As a result, the emission peaks of the UV-OLED can be easily adjusted from 383 nm to 374 nm by tuning both the length of the ZnO NWs and the thickness of the TAZ emission layer. Our work reveals an important correlation between emission peaks and exciton diffusion, and presents a novel approach to fabricate high-performance UV-OLEDs with the capability of facilely modifying the emission wavelength.

As organic light emitting diodes (OLEDs) find important applications in display-related fields, we demonstrate the fabrication of an inverted UV-OLED device with tunable wavelength that composes zinc oxide nanowires as an electron injection layer.     

Effect of hydrogen diffusion in an In–Ga–Zn–O thin film transistor with an aluminum oxide gate insulator on its electrical properties

We fabricated amorphous InGaZnO thin film transistors (a-IGZO TFTs) with aluminum oxide (Al₂O₃) as a gate insulator grown through atomic layer deposition (ALD) method at different deposition temperatures (T_dep). The Al₂O₃ gate insulator with a low T_dep exhibited a high amount of hydrogen in the film, and the relationship between the hydrogen content and the electrical properties of the TFTs was investigated. The device with the Al₂O₃ gate insulator having a high H content showed much better transfer parameters and reliabilities than the low H sample. This is attributed to the defect passivation effect of H in the active layer, which is diffused from the Al₂O₃ layer. In addition, according to the post-annealing temperature (T_post-ann), a-IGZO TFTs exhibited two unique changes of properties; the degradation in low T_post-ann and the enhancement in high T_post-ann, as explained in terms of H diffusion from the gate insulator to an active layer.

We fabricated amorphous InGaZnO thin film transistors (a-IGZO TFTs) with aluminum oxide (Al₂O₃) as a gate insulator grown through atomic layer deposition (ALD) method at different deposition temperatures (T_dep).     

Improving charge transport by the ultrathin QDs interlayer in polymer solar cells

Lead sulfide (PbS) quantum dots (QDs) have been incorporated into PTB7:PC₇₁BM BHJ active layers to fabricate polymer solar cells (PSCs) and gather on the top surface of active layers to form an ultrathin interlayer. The PbS QDs ultrathin interlayer with an appropriate thickness increases the carrier transport capacity, exciton dissociation and reduces the carrier recombination, which leads to a higher short circuit current (J_sc) and fill factor (FF). Finally, the power conversion efficiency (PCE) improves from 7.03% (control devices) to 7.87% with an ultrathin interlayer by doping 5% PbS QDs, while the current density (J_sc) and fill factor (FF) enhances from 13.83 mA cm⁻² to 14.81 mA cm⁻² and from 68.70% to 70.85%, respectively.

Lead sulfide (PbS) quantum dots (QDs) have been incorporated into PTB7:PC₇₁BM BHJ active layers to fabricate polymer solar cells (PSCs) and gather on the top surface of active layers to form an ultrathin interlayer.     

Improved performance of a samarium-doped ceria interlayer of intermediate temperature solid oxide electrolysis cells by doping the transition metal oxide Fe2O3

The ionic conductivity of the interlayer in the intermediate temperature solid oxide electrolysis cell (IT-SOEC) affects the polarization resistance of the oxygen electrode. Improving the ionic conductivity of the interlayer can improve the performance of the oxygen electrode. In this work, the ionic conductivity of a samarium-doped ceria (SDC) interlayer is improved by doping the transition metal oxide Fe₂O₃. The experimental results show that the oxygen electrode polarization resistance of the symmetrical cell based on the SDC-Fe₂O₃ interlayer is 0.09 Ω cm⁻² at 800 °C and under the open circuit voltage, which is obviously lower than that of the symmetrical cell based on an SDC interlayer (0.22 Ω cm⁻²). Besides, the electrolysis current of the SOEC based on the SDC-Fe₂O₃ interlayer is 0.5 A cm⁻² at 800 °C and 1.5 V, which is higher than that of the SOEC based on the SDC interlayer (0.3 A cm⁻²). The above results show that improving the ionic conductivity of the SDC interlayer in the SOEC by doping Fe₂O₃ can reduce the polarization resistance of the oxygen electrode and enhance the performance of the SOEC. Thus, this work provides an effective way for improving the performance of the SDC interlayer in the IT-SOEC.

Improving the ionic conductivity of the SDC interlayer in an SOEC by doping Fe₂O₃ can reduce the polarization resistance of the oxygen electrode and enhance the performance of the SOEC.     

Efficient and chromaticity stable green and white organic light-emitting devices with organic–inorganic hybrid materials

Efficient inverted bottom emissive organic light emitting diodes (IBOLEDs) with tin dioxide and/or Cd-doped SnO₂ nanoparticles as an electron injection layer at the indium tin oxide cathode:electron transport layer interface have been fabricated. The SnO₂ NPs promote electron injection efficiently because their conduction band (−3.6 eV) lies between the work function (W_f) of ITO (4.8 eV) and the LUMO of the electron-transporting molecule (−3.32 eV), leading to enhanced efficiency at low voltage. The 2.0% SnO₂ NPs (25 nm) with Ir(ddsi)₂(acac) emissive material (SnO₂ NPs/ITO) have an enhanced current efficiency (η_c, cd A⁻¹) of 52.3/24.3, power efficiency (η_p, lm W⁻¹) of 10.9/3.4, external quantum efficiency (η_ex, %) of 16.4/7.5 and luminance (L, cd m⁻²) of 28 182/1982. A device with a 2.0% Cd-doped SnO₂ layer shows higher η_c (60.6 cd A⁻¹), η_p (15.4 lm W⁻¹), η_ex (18.3%) and L (26 858 cd m⁻²) than SnO₂ devices or control devices. White light emission was harvested from a mixture of Cd–SnO₂ NPs and homoleptic blue phosphor Ir(tsi)₃; the combination of blue emission (λ_EL = 428 nm) from Ir(tsi)₃ and defect emission from Cd–SnO₂ NPs (λ_EL = 568 nm) gives an intense white light with CIE of (0.31, 0.30) and CCT of 6961 K. The white light emission [CIE of (0.34, 0.35) and CCT of 5188 K] from colloid hybrid electrolyte BMIMBF₄–SnO₂ is also discussed.

Efficient inverted bottom emissive organic light emitting diodes (IBOLEDs) with tin dioxide and/or Cd-doped SnO₂ nanoparticles as an electron injection layer at the indium tin oxide cathode:electron transport layer interface have been fabricated.     

The role of cation and anion dopant incorporated into a ZnO electron transporting layer for polymer bulk heterojunction solar cells

Doping is a widely-implemented strategy for enhancing the inherent electronic properties of charge transport layers in photovoltaic devices. A facile solution-processed zinc oxide (ZnO) and various cation and anion-doped ZnO layers were synthesized via the sol–gel method and employed as electron transport layers (ETLs) for inverted polymer solar cells (PSCs). The results indicated that all PSCs with doped ZnO ETLs exhibited better photovoltaic performance compared with the PSCs with a pristine ZnO ETL. By exploring the role of various anion and cation dopants (three compounds with the same Al³⁺ cation: Al(acac)₃, Al(NO₃)₃, AlCl₃ and three compounds with the same Cl⁻ anion: NH₄Cl, MgCl₂, AlCl₃), we found that the work function changed to favor electronic extraction only when the Cl anion was involved. In addition, the conductivity of ZnO was enhanced more with the Al³⁺ cation. Therefore, in inverted solar cells, doping with Al³⁺ and Cl⁻ delivered the best power conversion efficiency (PCE). The maximum PCE of 10.38% was achieved from the device with ZnO doped with Al⁺ and Cl⁻.

The role of cation and anion dopant incorporated into a ZnO layer was systematically investigated. We found that the work function was changed to favor electronic extraction only with Cl anion, while the conductivity change depended on the cation.     

Water assisted atomic layer deposition of yttrium oxide using tris(N,N′-diisopropyl-2-dimethylamido-guanidinato) yttrium(iii): process development,film characterization and functional properties

We report a new atomic layer deposition (ALD) process for yttrium oxide (Y₂O₃) thin films using tris(N,N′-diisopropyl-2-dimethylamido-guanidinato) yttrium(iii) [Y(DPDMG)₃] which possesses an optimal reactivity towards water that enabled the growth of high quality thin films. Saturative behavior of the precursor and a constant growth rate of 1.1 Å per cycle confirm the characteristic self-limiting ALD growth in a temperature range from 175 °C to 250 °C. The polycrystalline films in the cubic phase are uniform and smooth with a root mean squared (RMS) roughness of 0.55 nm, while the O/Y ratio of 2.0 reveal oxygen rich layers with low carbon contaminations of around 2 at%. Optical properties determined via UV/Vis measurements revealed the direct optical band gap of 5.56 eV. The valuable intrinsic properties such as a high dielectric constant make Y₂O₃ a promising candidate in microelectronic applications. Thus the electrical characteristics of the ALD grown layers embedded in a metal insulator semiconductor (MIS) capacitor structure were determined which resulted in a dielectric permittivity of 11, low leakage current density (≈10⁻⁷ A cm⁻² at 2 MV cm⁻¹) and high electrical breakdown fields (4.0–7.5 MV cm⁻¹). These promising results demonstrate the potential of the new and simple Y₂O₃ ALD process for gate oxide applications.

A new water assisted atomic layer deposition (ALD) process was developed using the yttrium tris-guanidinate precursor which resulted in device quality thin films.