Doping Gradient Research Articles

Balance between thermal stability and operation speed realized by Ti gradient doping in Sb2Te3 phase-change memory

Phase-change memory (PCM) is one of the leading candidates for the next generation nonvolatile memory. As a growth-dominated crystalline material, Sb2Te3 is of rapid crystallization speed while its poor thermal stability limits its application. Doping Ti can significantly enhance its amorphous stability but inevitably slows down its crystallization speed. How to balance the contradiction between thermal stability and operation speed remains challenging. In this work, we proposed a gradient Ti-doped Sb2Te3 phase-change material and device. This gradient doping strategy compensates for the negative effect of Ti doping on the crystallization rate of Sb2Te3 via the template effect of the lower doping concentration layer. Very small lattice mismatch between the Sb2Te3 layers with different Ti doping concentrations is verified by x-ray diffraction characterization. The crystallization temperature of a gradient Ti-doped Sb2Te3 thin film is raised up to 172.6 °C and the same 50 ns operation speed as a pure Sb2Te3 device is achieved in the corresponding PCM device. Furthermore, the gradient distribution of Ti elements and the corresponding progressive crystallization phenomenon are verified by transmission electron microscopy revealing the microscopic origin of rapid crystallization speed. Therefore, with our gradient doping strategy, the amorphous stability is improved without sacrificing the crystallization speed in PCM.

Heterogeneous doping via charge carrier transport improves Photoelectrochemical H2O oxidative H2O2 synthesis

Photoelectrochemical (PEC) water splitting into H2O2 and H2 has attracted significant attention due to its low cost and sustainability. However, slow charge carrier transport and water oxidation kinetics limit the selectivity for H2O2 production and solar conversion efficiency. Herein, we propose a heterogeneous doping approach that combines surface gradient doping with bulk doping to improve the charge carrier transport in the photoelectrode. Inducing gradient Nb and homogeneous Mo doping into BiVO4 photoanode (G-Nb/Mo:BVO) can promote charge separation and carrier transfer, leading to the high selectivity for H2O2 generation and suppression of O2 production. Consequently, the heterogeneously doped G-Nb/Mo:BVO photoanode presents an average Faraday efficiency (FE) of over 80% for H2O2 production in a wide potential of 0.6–1.8 VRHE with the maximum FE of 83.7% at 1.2 VRHE under AM 1.5G illumination. More importantly, H2O2 production rate can reach 1.23 μmol min−1 cm−2 at 1.23 VRHE, representing the best H2O2 production rate reported for the photoelectrodes. Density functional theory calculations prove that Mo and Nb co-doping increases the reactivity of BiVO4 and improves 2e− water oxidation reaction activity and selectivity. This work demonstrates that heterogeneous doping provides a cost-effective strategy to break performance trade-offs by improving charge carrier separation and transport in light absorbers, and modulating the selectivity and activity of water oxidation reaction to generate H2O2, which can be extended to other photocatalytic reactions.

Low-strain Co-free Li-rich layered cathode with excellent voltage and capacity stability

Owing to the inherent advantages of low cost and high capacity, cobalt (Co)-free lithium (Li)-rich layered oxides have become one of the most promising cathodes for next-generation high-energy lithium-ion batteries. However, these familial cathodes suffer from serious voltage decay due to many reasons, such as oxygen release and transition metal (TM) migration, which are closely related to nanoscale strain evolution. Here, by combining the synergistic effects of surface integration, bulk doping, and concentration gradient, we successfully construct a Co-free Li-rich layered cathode with a very small volumetric strain (1.05%) between 2.0 and 4.8 V, approaching the critical value of zero strain. Various characterizations indicate that the constructed zero-strain cathode can significantly suppress the TM migration, interfacial reactions, and structural degradation including cracks, lattice defects, phase evolution, and nanovoids, leading to improved voltage stability of Co-free Li-rich layered oxides during the prolonged cycles. This work provides a strategy to eliminate the lattice strain of Li-rich layered cathodes and facilitates the up-scaled application of the as-prepared cathode materials.

Double function-layers construction strategy promotes the cycling stability of LiCoO2 under high temperature and high voltage

Lithium cobalt oxide (LCO), as a popular commercial material, can release higher capacity by increasing the charging voltage. However, deep delithiation leads to the dissolution of elements and structural disorder. Meanwhile, high-temperature performance is still in urgent need to be improved in practical application. Here, we use LiNO3, NaF and H3BO3 as raw materials to construct the outer LiF-rich layer and the inner B, F gradient doped hierarchical nanostructures through stirring and calcination. The electrochemically stable LiF-rich surface helps to form a favorable cathode electrolyte interphase (CEI) layer to prevent electrolyte penetration. The gradient doping of B and F in the subsurface region increases the stability of the lattice at high temperature and high potential. The unique structure enables treated LCO to demonstrate stable cycling under high voltage (4.6 V) with a capacity retention of 93.2% after 800 cycles and displayed 95.2% retention for 100 cycles under high temperature (45 °C). The postmortem analyses after long cycles prove the strategy of surface modification helps to protect the structure and reduce adverse reactions. This study provides a simple and extendable method for improving the cycling stability of cathodes at high temperature and high voltage.

Photovoltaics literature survey (no. 182)

Photovoltaics literature survey (no. 182)

Constructing gradient structure to increase efficiency for carbon-based hole transport layer free all-inorganic perovskite solar cells using SCAPS-1D

Carbon-based all-inorganic perovskite solar cells (C-IPSCs) without hole transport layer (HTL) are of great interest because of their excellent stability and low production costs. In contrast to hybrid PSCs with HTLs, C-IPSCs have not yet attained great conversion efficiency. In this work, a new C-IPSC with gradient doping structure and gradient energy band structure using CsPbI3 as an absorber is proposed and numerically simulated by SCAPS-1D. The C-IPSC was optimized from various aspects including absorber layer thickness, doping gradient, doping concentration, defect, and energy band gradient. The optimal thickness of CsPbI3 is determined based on the optical absorption properties. Through a thorough analysis of the energy band structure, the intensity of the built-in electric field, and the recombination rate, it is found that the presence of the doping gradient introduces the built-in electric field, which promotes the movement of photogenerated carriers between the layers and significantly improves the efficiency of C-IPSCs. In order to address the issue of energy level mismatch, we propose to construct an energy band gradient. The presence of the energy band gradient improves the energy level matching of the device and provides paths for the movement of holes and electrons. In addition to the doping concentration, and defect density, we have chosen according to the optical performance and manufacturing level. Through a series of optimization, the C-IPSC with gradient structure has an efficiency of 21.15%. This study demonstrates that gradient doping structure and gradient energy band structure have great potential in future C-IPSCs with CsPbI3.

Investigation of Antimony ions doping on crystal structure and enhanced microwave dielectric performance of MgTa2O6 ceramics

Dielectric ceramics are promising in large-scale commercial millimeter-wave communication technology, such as 5G and the upcoming 6G, thanks to their excellent frequency selection characteristics and environmental stability. In this work, various contents of SbO bonds were introduced into the MgTa2O6 lattice using the solid-phase reaction method to investigate the effects on the lattice and microwave dielectric properties. XRD confirms that Sb ions successfully occupy Ta sites in the lattice and cause lattice shrinkage and crystallinity deterioration, which leads to a slight decrease in the quality factor. Furthermore, DFT calculations reveal that the doping leads to electron-biased aggregation toward O atoms, causing higher SbO ionicity, but also attenuates the degree of ionization of Ta and Mg ions, which makes the dielectric constant of the doped samples vary non-monotonically with gradient doping. Satisfactorily, Sb doping substantially enhanced the thermal stability of the ceramics, with TCF values reduced from 36 × 10−6 °C-1 to 15 × 10−6 °C-1.

A Gradient Doping Strategy toward Superior Electrochemical Performance for Li-Rich Mn-Based Cathode Materials.

Lithium-rich layered oxides (LLOs) are concerned as promising cathode materials for next-generation lithium-ion batteries due to their high reversible capacities (larger than 250mA h g-1 ). However, LLOs suffer from critical drawbacks, such as irreversible oxygen release, structural degradation, and poor reaction kinetics, which hinder their commercialization. Herein, the local electronic structure is tuned to improve the capacity energy density retention and rate performance of LLOs via gradient Ta5+ doping. As a result, the capacity retention elevates from 73% to above 93%, and the energy density rises from 65% to above 87% for LLO with modification at 1 C after 200 cycles. Besides, the discharge capacity for the Ta5+ doped LLO at 5 C is 155mA h g-1 , while it is only 122mA h g-1 for bare LLO. Theoretical calculations reveal that Ta5+ doping can effectively increase oxygen vacancy formation energy, thus guaranteeing the structure stability during the electrochemical process, and the density of states results indicate that the electronic conductivity of the LLOs can be boosted significantly at the same time. This strategy of gradient doping provides a new avenue to improve the electrochemical performance of the LLOs by modulating the local structure at the surface.

Investigation of High-Efficiency and Stable Carbon-Perovskite/Silicon and Carbon-Perovskite/CIGS-GeTe Tandem Solar Cells

The primary purpose of recent research on solar cells is to achieve a higher power conversion efficiency with stable characteristics. To push the developments of photovoltaic (PV) technology, tandem solar cells are being intensively researched, as they have higher power conversion efficiency (PCE) than single-junction cells. Perovskite solar cells (PSCs) are recently used as a top cell of tandem solar cells thanks to their tunable energy gap, high short circuit current, and low cost of fabrication. One of the main challenges in PSCs cells is the stability issue. Carbon perovskite solar cells (CPSCs) without a hole transport material (HTM) presented a promising solution for PSCs’ stability. The two-terminal monolithic tandem solar cells demonstrate the commercial tandem cells market. Consequently, all the proposed tandem solar cells in this paper are equivalent to two-terminal monolithic tandem devices. In this work, two two-terminal tandem solar cells are proposed and investigated using the SCAPS-1D device simulator. Carbon perovskite solar cell (CPSC) without hole transport material (HTM) is used as the top cell with a new proposed gradient doping in the perovskite layer. This proposal has led to a substantial enhancement of the stability issue known to be present in carbon perovskite cells. Moreover, a higher PCE, exceeding 22%, has been attained for the proposed CPSC. Two bottom cells are examined, Si and CIGS-GeTe solar cells. The suggested CPSC/Si and CPSC/CIGS-GeTe tandem solar cells have the advantage of having just two junctions, which reduces the complexity and cost of solar cells. The performance parameters are found to be improved. In specific, the PCEs of the two proposed cells are 19.89% and 24.69%, respectively.

Performance enhancement in MA0.7FA0.3PbI3 based perovskite solar cell by gradient doping

The mixed organic cation-based perovskite solar cells have attracted researcher attention due to their stability as compared to other perovskite material-based cells. The present manuscript has investigated the cell structures, i.e., FTO/SnO2/MA0.7FA0.3PbI3/HTL free/Au and FTO/SnO2/ MA0.7FA0.3PbI3 /CuInSe2 (HTL)/Au using SCAPS software. The performance of both the structures is studied using uniform doping and gradient doping method. The effect of absorber thickness and uniform doping in the absorber layer resulted in an efficiency of 13.21% for HTL free structure and 19.78% with HTL structure. Furthermore, the effect of doping gradient, average doping gradient, defect density in the absorber and interface defect density are studied in both the structures. After optimization of these parameters, an improved performance is obtained, i.e., efficiency of 18.08% for HTL free and 23.97% with HTL. The present study simplifies the manufacturing techniques by diving the absorber layer into four sublayers of equal thickness with the optimised value of the doping gradient. It is suggested that by considering two sublayers, one can achieve the highest cell performance using gradient doping method. Our results provide an effective technique to the experimentalist for fabricating the highly efficient device.

Suppressing interface recombination in CZTSSe solar cells by simple selenization with synchronous interface gradient doping.

The main bottleneck in the development of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is their very low VOC due to severe carrier recombination. Specifically, due to the poor defect environment and unfavorable band structure, carrier recombination at the front interface is considered to be one of the most serious issues. Thus, to reduce the interface recombination and VOC deficit, we propose a convenient and effective strategy for Cd gradient doping near the front interface during selenization. The formed Cd gradient significantly reduced the CuZn defects and related [2CuZn + SnZn] defect clusters near the CZTSSe-CdS heterojunction, thus significantly suppressing the interface recombination near the heterojunction. Benefitting from the formed Cd gradient, a champion device with 12.14% PCE was achieved with the VOC significantly improved from 432 mV to 486 mV. The proposed element gradient doping strategy can offer a new idea for selenization and element gradient doping in other photoelectric devices.

Boosting the Photoelectrochemical Performance of Au/ZnO Nanorods by Co-Occurring Gradient Doping and Surface Plasmon Modification.

Band bending modification of metal/semiconductor hybrid nanostructures requires low-cost and effective designs in photoelectrochemical (PEC) water splitting. To this end, it is evinced that gradient doping of Au nanoparticles (NPs) inwards the ZnO nanorods (NRs) through thermal treatment facilitated faster transport of the photo-induced charge carriers. Systematic PEC measurements show that the resulting gradient Au-doped ZnO NRs yielded a photocurrent density of 0.009 mA/cm2 at 1.1 V (vs. NHE), which is 2.5-fold and 8-fold improved compared to those of Au-sensitized ZnO and the as-prepared ZnO NRs, respectively. The IPCE and ABPE efficiency tests confirmed the boosted photoresponse of gradient Au-incorporated ZnO NRs, particularly in the visible spectrum due to the synergistic surface plasmonic effect of Au NPs. A gradient Au dopant profile promoted the separation and transfer of the photo-induced charge carriers at the electrolyte interface via more upward band bending according to the elaborated electrochemical impedance spectroscopy and Kelvin probe force microscopy analyses. Therefore, this research presents an economical and facile strategy for preparing gradient plasmonic noble NP-incorporated semiconductor NRs, which have excellent potential in energy conversion and storage technologies.

The effects of source doping concentration and doping gradient on the ON-state current of Si nanowire TFETs

The tunnel field-effect transistor (TFET) is considered a promising next-generation transistor due to its potentially limit-breaking low subthreshold swing and better immunity against short-channel effects. However, the low ON-state current (ION) of TFETs has been a critical problem. In this work, we investigated the effects of the source doping concentration and the source doping gradient (SDG) on the ION of n-type Si gate-all-around (GAA) nanowire (NW) TFETs using an ATLAS device simulator. Unexpectedly, we found that increasing the source doping concentration does not necessarily improve ION, especially for TFETs with a large SDG. Moreover, although reducing the SDG indeed increases ION, for TFETs with low source doping concentration (e.g., 1 × 1019 cm−3), the improvement in ION by reducing the SDG becomes insignificant.

Further Boosting Solar Cell Performance via Bandgap-Graded Ag Doping in Cu2ZnSn(S,Se)4 Solar Cells Compared to Uniform Ag Doping.

Cu2ZnSn(S,Se)4 (CZTSSe) is a hopeful substitution to commercialized Cu(In,Ga)Se2 (CIGSe) devices with similar structure and optoelectronic properties and has advantages of nontoxicity, low cost, and abundant reserves. However, the traditional flat bandgap structure of the CZTSSe absorber layer does not efficiently enhance the collection of photogenerated electrons and decrease recombination. Graded bandgap engineering toward the interfaces of CIGSe solar cells is the key to realize high-efficiency devices. In this study, we obtained (Cu1-xAgx)2ZnSn(S,Se)4 (CAZTSSe) absorber layers with high-concentration Ag doping at both ends of the absorption layer and undoped or low-concentration Ag doping in the middle part through a new miscible layered precursor method. This bandgap structure suppressed CuZn defects, delayed Fermi level pinning near the CZTSSe/CdS interface, sustained good electrical conductivity and light absorption in the middle of the absorption layer, improved the conversion efficiency of incident light, and inhibited recombination of carriers toward the Mo back electrode. In addition, we also compared the performance of undoped, uniformly Ag-doped, and V-type Ag-doped CZTSSe devices to acquire a deeper understanding of the reasons for the enhanced performance. It can be found that compared with undoping, the open-circuit voltage (Voc) of the best devices with uniform doping (x = 15%) increased from 379 to 386 mV, the fill factor (FF) increased from 44.70 to 54.14%, and the photoelectric conversion efficiency (PCE) increased from 4.63 to 6.21%. More surprisingly, the Voc of the optimal CAZTSSe devices (sample D) with Ag-graded doping was increased to 413 mV and the FF was increased to 59.63%. It also achieved an impressive PCE of 7.35%. The above results prove the importance of tuning Ag gradient doping of CZTSSe films for improving solar cell performance.

Low Operation Voltage White Organic Light‐Emitting Diodes with Excellent Color Stability based on Gradient‐Doping Sensitization System

AbstractWith tremendous progresses in material synthesis and device engineering, white organic light‐emitting diodes (OLEDs) have achieved continuous breakthroughs in brightness and efficiencies. Regrettably, as another important index, color stability is still a thorny problem restricting the industrial development of white OLEDs. Herein, a gradient‐doping sensitization system is constructed to realize low operation voltage white OLEDs with excellent color stability by gradient doping europium complex into light‐emitting layer (EML) as sensitizer. The gradient‐doping sensitization system realizes the precise regulation of carriers’ distribution and the reasonable assignment of recombination zone in different EMLs, thus leading to low turn‐on voltage of 2.3 V and negligible chromaticity coordinates variation of (0.005, 0.002) over wide brightness range of 1000–20 000 cd m−2. Meanwhile, the obtained white OLED realizes the maximum brightness of 67 384 cd m−2, current efficiency (ηc) of 65.27 cd A−1, power efficiency (ηp) of 73.24 lm W−1, and external quantum efficiency of 29.93%. The gradient‐doping sensitization system provides an efficient and reliable strategy for designing white OLEDs with improved color stability and reduced power consumption.

Decoupling light absorption and carrier transport via heterogeneous doping in Ta3N5 thin film photoanode

The trade-off between light absorption and carrier transport in semiconductor thin film photoelectrodes is a major limiting factor of their solar-to-hydrogen efficiency for photoelectrochemical water splitting. Herein, we develop a heterogeneous doping strategy that combines surface doping with bulk gradient doping to decouple light absorption and carrier transport in a thin film photoelectrode. Taking La and Mg doped Ta3N5 thin film photoanode as an example, enhanced light absorption is achieved by surface La doping through alleviating anisotropic optical absorption, while efficient carrier transport in the bulk is maintained by the gradient band structure induced by gradient Mg doping. Moreover, the homojunction formed between the La-doped layer and the gradient Mg-doped layer further promotes charge separation. As a result, the heterogeneously doped photoanode yields a half-cell solar-to-hydrogen conversion efficiency of 4.07%, which establishes Ta3N5 as a leading performer among visible‐light‐responsive photoanodes. The heterogeneous doping strategy could be extended to other semiconductor thin film light absorbers to break performance trade-offs by decoupling light absorption and carrier transport.

Meso-pore generating P doping for efficient photoelectrochemical water splitting

Hematite (Fe2O3) has been widely used as a photoanode in photoelectrochemical water splitting (PEC) for green hydrogen production. Here, for the first time, we investigate how a simple in-situ phosphorus (P) doping strategy improves the overall PEC performance of hematite with a systematic analysis of the various effects on the PEC performance. By introducing enriched FePO4 regions on the Ti-doped FeOOH surface and subsequent high-temperature annealing via P-doping, meso-porous P,Ti co-doped Fe2O3 (P,Ti-Fe2O3) nanorods were fabricated. P,Ti-Fe2O3 exhibited four-fold and two-fold increased BET surface area and electrical active area, respectively, compared to that of Ti-Fe2O3. Benefiting from the nano-structuring and efficient P doping effects [e.g., increased carrier density (Nd=3.48168 ×1020 cm−3), enhanced charge separation (ηbulk= 38.7% and ηsurface= 79.1%), and steeper band bending (Wd=3.910 nm)], the resulting P,Ti-Fe2O3 photoanode exhibited 94% improved photocurrent density of 2.50 mA cm−2 compared to that of Ti-Fe2O3 (@ 1.23 VRHE) under 1 sun illumination. With the deposition of the NiFeOx cocatalyst, the NiFeOx/P,Ti-Fe2O3 photoanode exhibited excellent photocurrent density of 3.54 mA cm−2 (@ 1.23 VRHE) with a remarkable cathodic shift (180 mV) of the onset potential, marking the highest value among P doped hematite studies. This study suggests a new paradigm of P doped hematite with mesopores and gradient doping properties affordable in a cost-efficient way, achieving an excellent PEC water splitting performance.

Improve the efficiency of organic light-emitting diodes using a novel hole injection layer with triple-layer gradually doped structure

In this study, we reported the fabrication of an organic light-emitting diodes (OLED) using the metal oxide Nb-doped ZnO (NZO) and the common hole transport layer of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) to form a novel hole injection layer with gradually-doped triple-layer structure (HITL). The comparison between the stepwise doped double-layer and uniformly doped device showed that the HITL structure could effectively improve the current efficiency of the OLED device. The results of both current density-voltage measurement by hole-only device (HOD) and the UV photoelectron spectroscopy (UPS) confirmed that the highest occupied molecular orbital (HOMO) energy level could help to improve the hole injection performance. In addition, the non-destructive analysis method of capacitance-voltage was used to observe the movement of charge carriers during the operation of the device. The comparison study indicated that by the stepwise doped double-layer injection layer, the influence of the doping gradient on the carrier injection was confirmed, and a smaller doping gradient was found to show the better performance of carrier injection. Based on the results obtained from this study, the maximum luminance and current efficiency of OLED device with HITL was found to be enhanced by approximately 2.48 times and 32.6% respectively when compared with the device without hole injection layer. Various measurements carried out in this study showed that the HITL structure could effectively improve the characteristics of hole carriers, and thereby the better charge balance effect and a more stable current efficiency performance could be achieved by reducing the current efficiency roll-off effect. • In this study, we demonstrated the performances of Alq 3 -based OLED using a novel hole injection layer with triple-layer gradually-doped structure (HITL). • The HITL structure can effectively improve the characteristics of hole carriers, thereby obtaining a better charge balance effect, reducing the efficiency roll-off effect, and obtaining a more stable efficiency performance. • Compared with the double-layer stepwise doped injection layer, the influence of the doping gradient on the carrier transport is confirmed, and a smaller doping gradient has a better performance of carrier injection and transport.

In situ surface engineering enables high interface stability and rapid reaction kinetics for Ni-rich cathodes

Layered oxide cathodes with high Ni content promise high energy density and competitive cost for Li-ion batteries (LIBs). However, Ni-rich cathodes suffer from irreversible interface reconstruction and undesirable cracking with severe performance degradation upon long-term operation, especially at elevated temperatures. Herein, we demonstrate in situ surface engineering of Ni-rich cathodes to construct a dual ion/electron-conductive NiTiO3 coating layer and Ti gradient doping (NC90–Ti@NTO) in parallel. The dual-modification synergy helps to build a thin, robust cathode–electrolyte interface with rapid Li-ion transport and enhanced reaction kinetics, and effectively prevents unfavorable crystalline phase transformation during long-term cycling under harsh environments. The optimized NC90–Ti@NTO delivers a high reversible capacity of 221.0 mAh g−1 at 0.1C and 158.9 mAh g−1 at 10C. Impressively, it exhibits a capacity retention of 88.4% at 25 ​°C after 500 cycles and 90.7% at 55 ​°C after 300 cycles in a pouch-type full battery. This finding provides viable clues for stabilizing the lattice and interfacial chemistry of Ni-rich cathodes to achieve durable LIBs with high energy density.

Growth of Large Ti:Sapphire Crystals by the Method of HDC with Gradient Doping

Growth of Large Ti:Sapphire Crystals by the Method of HDC with Gradient Doping