Energy Band Offsets Research Articles

Microwave-Assisted Synthesis of CdS-MOF MIL-101 (Fe) Composite: Characterization and Photocatalytic Performance.

The present study outlines the synthesis of cadmium sulfide-metal-organic framework (CdS-MOF) MIL-101 (Fe) heterojunctions achieved via fast microwave-assisted reactions. Thus, different CdS-MOF MIL-101 (Fe) ratios were prepared to study their effectiveness as photocatalysts. These compounds were employed in the photocatalytic degradation of methylene blue (MB) under UV and visible irradiation. Structural, morphological, textural, compositional, and optical properties of the synthesized compounds determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV-vis spectroscopy, Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy were utilized to characterize the structural, morphological, textural, compositional, and optical properties. Electrochemical impedance spectroscopy (EIS) was also employed to determine photovoltage and photocurrent densities. The resulting valence band offset (Vfb) and band gap energy values were utilized to construct an energy band scheme. Our research revealed that the CdS-MOF MIL-101 (Fe) heterojunction enhances the efficiency of electron-hole pair separation, thereby mitigating charge carrier recombination effects. Moreover, the type I electronic band structure established an efficient reaction mechanism, effectively suppressing the recombination of photogenerated electron-hole pairs. The photocatalytic system demonstrated exceptional behavior, achieving complete MB removal within 30 min of reaction time and exhibiting outstanding stability and reusability after four reaction cycles. These findings highlight the potential of the synthesized compounds in the field of wastewater treatment for organic pollutants, offering a promising alternative to current environmental issues.

Insight into the Dependence of Photovoltaic Performance on Interfacial Energy Alignment in Solar Cells with Mobile Ions

A key aspect in the optimization of perovskite (PVK) solar cells is the mitigation of interface losses, which are determined by energy band offsets and recombination. Counterintuitively, recent experimental evidence on inorganic PVK solar cells shows that increasing band offsets may improve open‐circuit voltage at efficiency levels over 20%. In order to improve the knowledge of the impact of interfaces, the authors model solar cells consisting of an intrinsic absorber material containing mobile ions, sandwiched between two wide‐gap charge transport layers. The results show that minimizing band offsets decreases interface losses in various scenarios, but cannot be adopted as a universal optimization rule. For instance, even in the absence of interface recombination, unequal majority carrier band offsets avoid high injection conditions. Moreover, assuming a fixed band offset at one interface, the band offset at the opposite interface should be reduced if it shows the highest interface recombination, or increased if it shows the lowest recombination. Remarkably, it is found that solar cells with mobile ions hold the potential to outperform of ion‐free counterparts, depending on the extent to which the interplay between band offsets and Debye layers result in the establishment of a majority carrier concentration in the intrinsic absorber.

Construction of type-II SnO2/InGaN nanorods heterostructure toward high photoelectrochemical performance

Exploring highly efficient and stable photoelectrode material is essential for high-performance photoelectrochemical (PEC) water-splitting applications. III-nitride semiconductors, particularly InGaN, have been considered as prospective materials for PEC hydrogen evolution. However, their surface states and other recombination centers, which enhance the charge recombination kinetics, are bottlenecks for the high PEC performance. In this work, we report the construction of type-II heterojunction by sputter depositing SnO2 on InGaN nanorods (NRs) to promote interfacial carrier transport and thereby enhance PEC performance. The energy band offsets at the SnO2/InGaN NRs interface were analyzed by x-ray photoelectron spectroscopy. Type-II heterojunction was defined at the SnO2/InGaN NRs interface with a valence band offset of 0.77 eV and conduction band offset of 0.25 eV. The photocurrent density of the SnO2/InGaN NRs photoanode is 7.09 mA/cm2 at 0.77 V vs Ag/AgCl electrode with 80 nm SnO2 thickness, which is ∼14-fold higher than that of the pristine InGaN NRs photoanode. Furthermore, the applied bias photo-to-current efficiency of SnO2/InGaN NRs photoanode records 3.36% at 0.77 V vs Ag/AgCl electrode. The enhanced PEC performance is mainly ascribed to the formation of high-quality SnO2/InGaN NRs heterojunction that enforces the directional charge transfer and substantially boosts the separation of photogenerated electron–hole pairs at the interface of InGaN NRs and SnO2. Overall, this work sheds light on the promising strategy to design and fabricate III-nitride nanostructures-based photoelectrodes for feasible PEC water-splitting applications.

The Impact of Multifunctional Ambipolar Polymer Integration on the Performance and Stability of Perovskite Solar Cells

AbstractEffective passivation of grain boundaries in perovskite solar cells is essential for achieving high device performance and stability. However, traditional polymer‐based passivation strategies can introduce challenges, including increased series resistance, disruption of charge transport, and insufficient passivation coverage. In this study, a novel approach is proposed that integrates a multifunctional ambipolar polymer into perovskite solar cells to address these issues. The ambipolar polymer is successfully incorporated into both the perovskite film and the hole transport layer (HTL), enabling comprehensive restoration of defect sites within the perovskite active layer. Moreover, this approach yields additional advantages for perovskite devices, such as enabling bidirectional charge transport, limiting pinhole formation at the HTL, reducing lithium‐ion migration from the HTL to the perovskite, and minimizing both the band offset and surface energy difference between the perovskite film and HTL interface. With these benefits, the ambipolar polymer integrated device achieves a power conversion efficiency (PCE) of 24.0%. Remarkably, it also exhibits enhanced long‐term stability, preserving 92% of its initial PCE after 2000 h under ambient conditions, and 80% of its initial PCE after 432 h under harsh conditions (at 85 °C and 85 ± 5% RH).

Thickness control of organic semiconductor-incorporated perovskites.

Two-dimensional organic semiconductor-incorporated perovskites are a promising family of hybrid materials for optoelectronic applications, owing in part to their inherent quantum well architecture. Tuning their structures and properties for specific properties, however, has remained challenging. Here we report a general method to tune the dimensionality of phase-pure organic semiconductor-incorporated perovskite single crystals during their synthesis, by judicious choice of solvent. The length of the conjugated semiconducting organic cations and the dimensionality (n value) of the inorganic layers can be manipulated at the same time. The energy band offsets and exciton dynamics at the organic-inorganic interfaces can therefore be precisely controlled. Furthermore, we show that longer and more planar π-conjugated organic cations induce a more rigid inorganic crystal lattice, which leads to suppressed exciton-phonon interactions and better optoelectronic properties as compared to conventional two-dimensional perovskites. As a demonstration, optically driven lasing behaviour with substantially lower lasing thresholds was realized.

Boosting efficiency of eco-friendly perovskite solar cell through optimization of novel charge transport layers.

Formamidinium tin triiodide (FASnI3) is a suitable candidate for the absorber layer in perovskite solar cells (PSC) because of its non-toxicity, narrow band gap, thermal stability and high carrier mobility. This study focuses on the analysis and improvement in the performance of FASnI3-based PSCs using various inorganic charge transport materials. The copper-based materials such as Cu2O, CuAlO2, CuSCN and CuSbS2 are introduced as hole transport layers due to their earth abundance, ease of manufacturing, high charge mobilities and chemical stability. Similarly, fullerene derivates (PCBM and C60) are deployed as electron transport layers due to their mechanical strength, thermal conductivity and stability. The effect of these materials on optical absorption, quantum efficiency, energy band alignment, band offsets, electric field and recombination are studied in detail. The reasons for the low performance of the cell are identified and improved through design optimization. The PSC performance is analysed in both inverted and conventional architecture. Among all the structures, the best result is achieved through ITO/CuSCN/FASnI3/C60/Al with an efficiency of 27.26%, Voc of 1.08 V, Jsc of 29.5 mAcm-2 and FF of 85.6%.

Ultrathin SnO2 electron transport layers for perovskite solar cells by combustion method at low temperature

Electron transport layers can collect photo-generated electrons and suppress interfacial recombination, which are crucial for efficient perovskite solar cells. Tin oxide is a suitable electron transport material for its wide bandgap, high electron mobility, low photocatalytic activity and appropriate energy band offsets. For the deposition of tin oxide layers, SnCl2 precursor is usually used for the conventional solution spin-coating method. In this work, we developed an in-situ synthesized precursor, and ultrathin tin oxide layers were obtained by the combustion process at 150 °C. Furthermore, the devices with these tin oxide layers can achieve the PCE of 19.6%, while conventional tin oxide layers with SnCl2 precursor lead to the PCEs of 16.9% and 14.9% with and without plasma treatment. This improvement can be related to the enhanced photoelectron collection and superior interfacial contact. This work provides a facile method for fabricating SnO2 electron transport layers of perovskite solar cells at low temperature.

Energy band offsets and charge transport process in an InAsSb based nBn structure: A numerical simulation

We report on the valence energy band offset (ΔEvC-A) between the InAsSb contact (C) and the InAsSb absorber (A) layers, in the C–InAsSb/B–AlAsSb/A-InAsSb nBn structure. ΔEv(C-A) is analyzed, numerically, for different thicknesses of the AlAsSb barrier (B) layer and different doping levels of the barrier and the contact layers, under equilibrium and at room temperature. A value of ΔEv(C-A) around 0,331 meV, corresponding to a near flat band situation, was obtained with donor's concentration of about 1016 cm−3 in the contact. First, simulations were carried at room temperature, under reverse bias, and for a 50 nm and a 100 nm barrier's thicknesses. The temperature dependence of ΔEv(C-A), under reverse bias, was analyzed between 150 K and 300 K. Finally, the reverse current-voltage characteristics of the structure were simulated in the dark and under illumination. Our results are compared to experimental results encountered in the literature and for the same system.

Demonstration on dynamic evolution of energy band offsets based in NiOx/Ga2O3 P-N heterojunction interface under heat effect

NiOx/Ga2O3 heterostructure is widely utilized to Ga2O3 electrics devices, which can work in high power density and at severe ambient temperature. However, the heat effect on band offsets in NiOx/Ga2O3 junction has rarely been reported before, and little study on the interfacial properties is definitive. Here, a novel in-situ temperature-dependent XPS characterization technique was performed to investigate the heat effect on band offsets with the temperature from 300 K to 473 K. It indicates that the conduction band offset (ΔEc) and valence band offset (ΔEv) decreases from 2.06 to 1.56 eV, and from 2.74 to 2.24 eV, respectively. It means that the heat effect advances suppression in band bending. In addition, the NiOx/Ga2O3 pn diode was fabricated and the temperature dependence of I-V measurement was performed to verify the discovery based on XPS analysis. Furthermore, the temperature coefficient is extracted to be −4 meV·K−1, which are projected to determine the band offsets at a specific temperature. This work opens scenario to study interfacial properties and makes suggestion for designing in Ga2O3 power electronics.

Association study between the optical-photocatalysis behaviors and the carrier transfer characteristics of g-C3N4/Sr2MgSi2O7:(Eu,Dy)

Continuous photocatalysis is achieved by using light-storing assisted photocatalytic composites, but little is known about the interactions between photocatalysts and long afterglow materials. g-C3N4/Sr2MgSi2O7:(Eu,Dy) with a sustained reaction activity was prepared by pyrolysis polymerization, and its optical-photocatalysis behaviors were studied. It was found that the light absorption ability and NO removal effect were improved, but the luminescence-afterglow performance became worse. The interface model of g-C3N4/Sr2MgSi2O7:(Eu,Dy) was constructed for the first time. On the basis of experimental analyses, its electronic structure and carrier transfer paths were studied by First-principles calculations, and reasons behind the changes in macro performance were revealed. The results showed that Eu 4 f electrons are difficult to return to the Eu 5d orbits after being captured by C 2p orbits, leading to the reduction of light energy radiation; the built-in electric field and energy band offsets between g-C3N4 and Sr2MgSi2O7:(Eu,Dy) promote the transfer of photoinduced carriers, which follows a traditional type-II mechanism, resulting in enhanced photocatalytic activity and decreased luminescence-afterglow performance. This paper reveals the relationship between optical-photocatalysis behaviors and the electronic structure of g-C3N4/Sr2MgSi2O7:(Eu,Dy). It provides a reference for designing and optimizing light-storing assisted photocatalytic composites.

Dark current simulation and analysis for InAs/GaSb long wavelength infrared barrier detectors

The authors reported a heterojunction dark current simulation method for InAs/GaSb long wavelength infrared barrier detectors. Calculations using the dark current model with energy band offset have shown that the theoretical and experimental data of the model are highly consistent. The energy band offsets of absorption and barrier region in the detectors were calculated by the 8-band k·p method. Then, the dark current model of heterojunction was established and compared with the homojunction model previously adopted. The agreement between the theoretical predictions and the experimental measurement led us to propose a barrier device structure with low dark current in long wavelength infrared domain. The results show that under a small reverse bias (V < -100 mV), the G-R and surface leakage current dominate, while the tunneling dark current occupies a leading position with a large bias (V > -320 mV) at 77 K. The dark current density under −10 mV bias voltage is 1.16 × 10-4A/cm2.

Interpretation of slow electroluminescence and open circuit voltage transient response in Cs-based perovskite solar cells

The desirable small hysteresis in the current–voltage characteristics of perovskite solar cells is often understood as a result of small ionic concentration or mobility and low interface charging by depleted/accumulated ions. However, devices having very small apparent hysteresis at practical scan rates can exhibit strong ionic effects seen in the transient response to excitation events. We explore Cs-based double-cation perovskite solar cells showing vanishing hysteresis and nearly hour-long responses to light- and voltage-step excitation, which are tracked by the evolution of open-circuit voltage and injected current, together with electroluminescence emission, respectively. The observed responses, including the increase of electroluminescence with time, are explained by the modulation of the electric field within the perovskite by mobile ions under the condition of interface recombination of mobile charge carriers dominating overall recombination. This is further explored by a numerical model containing mobile ions, which requires that charge carriers recombine predominantly at the interfaces of the device in order to obtain transient responses comparable to the experiments. Further support for the predominance of interface recombination is experimentally obtained by steady-state photocarrier grating characterization, which yields diffusion lengths of photogenerated carriers above 1 μm, i.e., well above the 300 nm perovskite layer thickness. The numerical model further shows that within the case of dominating interface recombination, the shape of both the light- and voltage-step transient responses are strongly determined by the energy band offsets between the perovskite and the contact layers of the solar cell.

Tuning Organic Electrochemical Transistor (OECT) Transconductance toward Zero Gate Voltage in the Faradaic Mode.

In this work, we investigate material design criteria for low-powered/self-powered and efficient organic electrochemical transistors (OECTs) to be operated in the faradaic mode (detection at the gate electrode occurs via electron transfer events). To rationalize device design principles, we adopt a Marcus-Gerischer perspective for electrochemical processes at both the gate and channel interfaces. This perspective considers density of states (DOS) for the semiconductor channel, the gate electrode, and the electrolyte. We complement our approach with energy band offsets of relevant electrochemical potentials that can be independently measured from transistor geometry using conventional electrochemical methods as well as an approach to measure electrolyte potential in an operating OECT. By systematically changing the relative redox property offsets between the redox-active electrolyte and semiconducting polymer channel, we demonstrate a first-order design principle that necessary gate voltage is minimized by good DOS overlap of the two redox processes at the gate and channel. Specifically, for p-type turn-on OECTs, the voltage-dependent, electrochemically active semiconductor DOS should overlap with the oxidant form of the electrolyte to minimize the onset voltage for transconductance. A special case where the electrolyte can be used to spontaneously dope the polymer via charge transfer is also considered. Collectively, our results provide material design pathways toward the development of simple, robust, power-saving, and high-throughput OECT biosensors.

Band alignment analysis of CuGaO2/β-Ga2O3 heterojunction and application to deep-UV photodetector

Single-oriented CuGaO2 films have been successfully grown on β-Ga2O3 (2¯01) substrate by reactive deposition epitaxy. The energy band offsets and alignment at CuGaO2/β-Ga2O3 heterojunction are investigated by x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). A type-II band alignment is identified at CuGaO2/β-Ga2O3 heterojunction with valence band offset (VBO) of 1.63 eV and conduction band offset (CBO) of 0.45 eV. The CuGaO2/β-Ga2O3 heterojunction based ultraviolet photodetector is prepared which exhibits an obvious ultraviolet (UV) photoresponse at zero bias voltage. The combination between β-Ga2O3 and wide bandgap delafossite oxide semiconductor may open up possibilities for next generation self-power deep UV optoelectronic devices in future.

Band offsets, electron affinities and optical dynamics at the CdBr2/SiO2 interfaces

Herein, the structural, optical, dielectric and optical conductivity parameters of the CdBr2/SiO2 interfaces are reported. Thin films of CdBr2 are coated with 50 nm thick SiO2 under a vacuum pressure of 10−5 mbar. The structural morphological and optical investigations have shown that CdBr2 exhibit hexagonal structure of lattice parameters of a=b=7.20Å and =13.86Å, energy band gap of 3.32 eV and Urbach energy of 0.84 eV. The interfacing of CdBr2 films with SiO2, shortened the lattice parameters, blue-shifted the energy band gap and decreased the Urbach energy value. The electron affinity of CdBr2 which is determined here is found to be 3.27 eV. The CdBr2/SiO2 interfaces displayed a conduction and a valence band offsets of 2.37 eV and 3.21 eV, respectively. In addition, the dielectric dispersion and optical conduction analyses using the Drude-Lorentz approach have shown that the drift mobility and plasmon frequency (Wpi) of the free charge carriers at the CdBr2/SiO2 interfaces varies in the range of 28.59–22.87 cm2/Vs and 0.41–6.47 GHz, respectively. The effectiveness of the plasmon frequency to limit signal propagation in the heterojunction devices is confirmed by the impedance spectroscopy technique which showed radiowave (RF) band filter characteristics at the targeted Wpi. The energy band offsets, the optical conductivity parameters and the RF filtering properties nominates the CdBr2/SiO2 heterojunctions for use in thin film transistor technology.

Terahertz Pulse Emission from Semiconductor Heterostructures Caused by Ballistic Photocurrents.

Terahertz radiation pulses emitted after exciting semiconductor heterostructures by femtosecond optical pulses were used to determine the electron energy band offsets between different constituent materials. It has been shown that when the photon energy is sufficient enough to excite electrons in the narrower bandgap layer with an energy greater than the conduction band offset, the terahertz pulse changes its polarity. Theoretical analysis performed both analytically and by numerical Monte Carlo simulation has shown that the polarity inversion is caused by the electrons that are excited in the narrow bandgap layer with energies sufficient to surmount the band offset with the wide bandgap substrate. This effect is used to evaluate the energy band offsets in GaInAs/InP and GaInAsBi/InP heterostructures.

Experimental and theoretical band alignment of Ta2O5/ZnO stack for heterostructured devices applications

Oxide semiconductor interfaces exhibit a promising expectation in electronic devices. The band alignment at the heterointerface plays a key role in determining the carrier transport and device performances. In this study, a Ta2O5/ZnO heterojunction is fabricated by using radio-frequency magnetron sputtering and ion beam sputtering technologies on a α-sapphire substrate. The energy-band offsets are determined by using the X-ray photoelectron spectroscopy and electron affinity rule. The heterojunction between Ta2O5 and ZnO shows a Type Ⅰ band alignment accompanied with a conduction band offset of 0.33±0.03 eV and a valence band offset of −0.38±0.03 eV. In all, this work could provide an indication for their heterostructured devices fabrications, owing to its large conduction band offset and excellent electron confinement across the heterointerface of Ta2O5/ZnO.

Understanding VOC and performance deficit in wide bandgap perovskite photovoltaic devices

Wide bandgap metal halide perovskites materials are of interest for application as top subcells in multijunction devices. In contrast to smaller gap perovskite devices that perform fairly close to their internal Shockley-Queisser limit, wide gap versions show substantial deficits. To understand the origin of the limitations in more detail we simulated the performance of wide bandgap devices as a function of the energy band offsets at the contacts. Our results show that traditionally-used charge transport layers such as spiro-OMeTAD and TiO2 limit the performance of wide bandgap absorbers through a subtle interplay between effects on VOC and FF. We investigate the optimal characteristics for the charge transport layers as function of absorber bandgap and identify the needed band offsets at the p- and n-interfaces. Our results call for more research on charge transport layers to minimize the VOC and performance deficit in wide bandgap devices.

Single-Crystalline All-Oxide α-γ-β Heterostructures for Deep-Ultraviolet Photodetection.

Recent advancements in gallium oxide (Ga2O3)-based heterostructures have allowed optoelectronic devices to be used extensively in the fields of power electronics and deep-ultraviolet photodetection. While most previous research has involved realizing single-crystalline Ga2O3 layers on native substrates for high conductivity and visible-light transparency, presented and investigated herein is a single-crystalline β-Ga2O3 layer grown on an α-Al2O3 substrate through an interfacial γ-In2O3 layer. The single-crystalline transparent conductive oxide layer made of wafer-scalable γ-In2O3 provides high carrier transport, visible-light transparency, and antioxidation properties that are critical for realizing vertically oriented heterostructures for transparent oxide photonic platforms. Physical characterization based on X-ray diffraction and high-resolution transmission electron microscopy imaging confirms the single-crystalline nature of the grown films and the crystallographic orientation relationships among the monoclinic β-Ga2O3, cubic γ-In2O3, and trigonal α-Al2O3, while the elemental composition and sharp interfaces across the heterostructure are confirmed by Rutherford backscattering spectrometry. Furthermore, the energy-band offsets are determined by X-ray photoelectron spectroscopy at the β-Ga2O3/γ-In2O3 interface, elucidating a type-II heterojunction with conduction- and valence-band offsets of 0.16 and 1.38 eV, respectively. Based on the single-crystalline β-Ga2O3/γ-In2O3/α-Al2O3 all-oxide heterostructure, a vertically oriented DUV photodetector is fabricated that exhibits a high photoresponsivity of 94.3 A/W, an external quantum efficiency of 4.6 × 104%, and a specific detectivity of 3.09 × 1012 Jones at 250 nm. The present demonstration lays a strong foundation for and paves the way to future all-oxide-based transparent photonic platforms.

Numerical analysis of dopant‐free asymmetric silicon heterostructure solar cell with SiO 2 as passivation layer

Conventional silicon heterojunction solar cells employ defects-prone a-Si:H layers for junction formation and passivation purposes. Substituting these layers with hole-selective MoOx and electron-selective TiOx can reduce parasitic absorption and energy band offsets issues associated with doped silicon films. In this work, dopant-free asymmetric heterostructure Si solar cells are studied with and without SiO2 passivation layer, and their performance has been compared. The inclusion of ultrathin SiO2 insulator as a passivation layer promotes significant band bending that induces interface inversion of crystalline silicon as well as maintains the electric field required to tunnel charge carriers. The energy band diagram studies and variation of oxide thickness show that the IV characteristics of the solar cell critically depend on the insulator thickness; as the carriers tunnelling through the insulator becomes negligible at larger thicknesses. The simulated structure with MoOx as front hole-selective contact and without any passivation exhibited conversion efficiency of 15.73%, which improved to 18.69% by incorporating passivated a-Si:H. However, by employing rear SiO2/TiOx stack with the front SiO2/MoOx, the device performance enhanced to open-circuit voltage of 785 mV, short-circuit current density of 41 mA/cm2, fill factor of 77%, and simulated conversion efficiency of 24.83%, which is ∼10% enhancement in the performance as compared to reference device employing traditional a-Si:H with dopant-free films. Novelty Statement For the first time, a dopant-free asymmetric silicon heterostructure solar cell (DASH) employing silicon oxide (SiO2) as a passivation layer has been physically modelled using Silvaco TCAD. The dopant-free MoOx and TiOx as hole- and electron-selective contacts have been incorporated. An ultrathin SiO2 promotes band bending that induces interface inversion of absorber as well as facilitating carrier tunnelling. An efficiency of 24.83% has been numerically attained which is the best performance among DASH structures designed with oxide passivation. The 2-D cross-section view of the proposed device employing dopant-free layers and oxide passivated film is illustrated in Figure 1 for which we have applied memory intensive numerical method based on the computation of Poisson, charge transport and continuity equations in the Silvaco ATLAS module. In order to undertake in-depth analysis of the proposed structure, a simple device based on MoOx/cSi wafer was first designed and simulated in order to understand the charge transport behaviour. The electrical and optical parameters for various layers have been shown in the Table and the current density due to tunnelling of charge carriers across Silicon oxide (SiO2) passivated layer has been calculated via Fowler-Nordheim approximation. The reference structure with traditional intrinsic a-Si:H has also been simulated.