Position Of Valence Band Maximum Research Articles

Influence of Pressure on Structural, Electronic, Optical, and Elastic Properties of Lead-Free Chalcogen Perovskite LaLuS3 via First-Principles Calculations: Implications for Optoelectronic Applications

Abstract Chalcogen perovskites have garnered increasing attention as promising materials for optoelectronic applications. In this study, we employed the first-principles method to investigate the structural, electronic, optical, and elastic properties of LaLuS3 under hydrostatic pressure at various levels. Through a thorough analysis of the calculated electronic structures, we observed that LaLuS3 exhibits direct band gaps, with the magnitudes of these gaps changing as the pressure varied. Specifically, the band gaps decrease by 2.19 eV, 2.025 eV, 1.365 eV, and 0.6043 eV at hydrostatic pressures of 0%, 10%, 20%, and 30% GPa, respectively. Furthermore, we observed shifts in the conduction band minimum and valence band maximum positions, indicating the potential of LaLuS3 for perovskite-based devices. This suggests that external pressure can serve as a powerful tool for designing new functional materials with intriguing properties. Our investigation also revealed promising optical properties of LaLuS3 under high pressures, further affirming its potential for optoelectronic and solar cell applications. The optical functions of the material are enhanced with increasing pressure, particularly in the ultraviolet range, highlighting its suitability for a wide range of optoelectronic devices. Moreover, while maintaining mechanical stability, hydrostatic pressure exerts a significant influence on the mechanical properties of LaLuS3. Lastly, our calculations on anisotropy demonstrate that applied pressure can enhance the anisotropic nature of LaLuS3. This comprehensive study underscores the efficacy of hydrostatic pressure as a systematic approach to modifying the photovoltaic performance of halide perovskites.

Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting.

Transition metal oxynitrides are a promising class of functional materials for photoelectrochemical (PEC) applications. Although these compounds are most commonly synthesized via ammonolysis of oxide precursors, such synthetic routes often lead to poorly controlled oxygen-to-nitrogen anion ratios, and the harsh nitridation conditions are incompatible with many substrates, including transparent conductive oxides. Here, we report direct reactive sputter deposition of a family of zirconium oxynitride thin films and the comprehensive characterization of their tunable structural, optical, and functional PEC properties. Systematic increases of the oxygen content in the reactive sputter gas mixture enable access to different crystalline structures within the zirconium oxynitride family. Increasing oxygen contents lead to a transition from metallic to semiconducting to insulating phases. In particular, crystalline Zr2ON2-like films have band gaps in the UV-visible range and are n-type semiconductors. These properties, together with a valence band maximum position located favorably relative to the water oxidation potential, make them viable photoanode candidates. Using chopped linear sweep voltammetry, we indeed confirm that our Zr2ON2 films are PEC-active for the oxygen evolution reaction in alkaline electrolytes. We further show that high-vacuum annealing boosts their PEC performance characteristics. Although the observed photocurrents are low compared to state-of-the-art photoanodes, these dense and planar thin films can offer a valuable platform for studying oxynitride photoelectrodes, as well as for future nanostructuring, band gap engineering, and defect engineering efforts.

Time evolution of the defect states at the surface of MoS2

MoS2 has generated significant attention due to its unique electronic properties and versatile applications. Being a van der Waals material, MoS2 is expected to exhibit an inert surface due to lack of dangling bond. However, our photoemission study finds MoS2 to be highly sensitive toward residual gases. The position of the valence band maximum (VBM) shifts even in a vacuum of 10−10 Torr. We find this to be due to CO adsorption causing unintentional electron doping. The time evolution of the position of VBM is exponential, and it reaches two different saturation points, depending on whether the sample is exposed to ultraviolet (UV) radiation or not. Our XPS (x-ray photoemission spectroscopy) study shows no time-dependent escape of sulfur, which was in a previous study attributed to a VBM shift. The VBM shift can be reversed by annealing, sputtering, and UV light, which desorb CO gases. The study shows that the MoS2 surface is easily doped, which offers the possibility of using it as a sensor but in many other applications could diminish device performance and needs to be considered.

Effect of uniaxial strain on Hole mobility of Sb<sub>2</sub>Se<sub>3</sub>

Antimony selenide (Sb<sub>2</sub>Se<sub>3</sub>) is a simple-phase, element-rich, and economically friendly material for solar cell absorption layers, with broad application prospects. However, the weak conductivity of Sb<sub>2</sub>Se<sub>3</sub> has become a significant factor limiting the performance of solar cell devices. Carrier mobility is an important electrical parameter for both materials and devices, and strain can change carrier mobility. Therefore, studying the effect of strain on the carrier mobility of Sb<sub>2</sub>Se<sub>3</sub> is of practical significance. In this work, using density functional theory and deformation potential theory, we systematically investigate the influence of uniaxial strain on the band structure, bandgap width, iso-surface, and effective mass of Sb<sub>2</sub>Se<sub>3</sub>. We analyze the effects of three types of uniaxial strains along the <i>x-</i>, <i>y-</i>, and <i>z-</i>direction on the carrier mobilities along the <i>x-</i>, <i>y-</i>, and <i>z-</i>direction, which are denoted by μ<sub>x</sub>, <i>μ</i><sub><i>y</i></sub>, and <i>μ</i><sub><i>z</i></sub>, respectively. It is found that under these strains, the valence band maximum (VBM) position of Sb<sub>2</sub>Se<sub>3</sub> remains unchanged, and the bandgap decreases with the increase of strain along the y- and z-direction, while it increases along the <i>x-</i>direction. The variation in bandgap may be related to the coupling strength between the Sb-5p orbital and Se-4p orbital of the conduction band minimum (CBM). For fully relaxed Sb<sub>2</sub>Se<sub>3</sub>, its iso-surface exhibits a distorted cylindrical shape, with low dispersion along the <i>z</i>-axis and high dispersion along the x- and y-axis, where μ<sub>x</sub> is greater than <i>μ</i><sub><i>y</i></sub> and <i>μ</i><sub><i>z</i></sub>, suggesting that the x-direction should be considered as the specific growth direction for Sb<sub>2</sub>Se<sub>3</sub> experimentally. When the strain is applied along the x- and z-direction, <i>μ</i><sub><i>x</i></sub> gradually increases with strain increasing, while it decreases when the strain is applied along the <i>y-</i>direction. Taking into account the combined effects of strain on bandgap, iso-surface, density of states, and mobility, this study suggests that the optimal performance of Sb<sub>2</sub>Se<sub>3</sub> solar cell absorber layer material can be realized when the strain is applied along the <i>y</i>-axis, with a compressive strain of 3%.

Enhancement of photoelectrochemical performance for nitrogen-doped titanium dioxide-based thin films produced by a facile air-based sputtering deposition

This work explores the use of air as a reactive gas in the sputtering deposition of N-doped TiO2-based bilayer, gradient, and multilayer thin films for photoelectrochemical evaluations. By merely adjusting the air/Ar flow ratio, crystalline phases, nitrogen doping contents, electrical properties, optical bandgaps, and valence band maximum (VBM) positions of N–TiO2 films were tailored. The N–TiO2/TiN(O) bilayer on the glass substrates exhibited a peak photocurrent density of 776 μA cm−2 at an air/Ar ratio of 1.60, which may be due to suitable bandgap and carrier mobility of the TiO2 overlayer as the bandgap was affected by the nitrogen content and rutile fraction in the TiO2 overlayer. To further improve the performance of N–TiO2, stepwise and sequential gradient structures with gradient bandgaps and nitrogen contents were produced, which could reach photocurrent densities of 805 ± 4 and 837 ± 3 μA cm−2, respectively. The gradient structures lead to enhanced light absorption efficiency and reduce carrier recombination. The N–TiO2/TiN(O) multilayer structures were also designed and revealed the highest photocurrent density of 865 ± 7 μA cm−2. Such evident enhancement is mainly due to the thin conductive while translucent TiN(O) layers between the photoelectrochemical-active N–TiO2 layers provide multiple low-impedance channels in enhancing carrier transport.

Physical properties of LaBO3 (B = Mn, Fe, Co) thin films grown on SrTiO3 by pulsed laser deposition technique

Realizing an oxide thin film with proper stoichiometry is one of the most challenging objects in materials science. Owing to the growth dynamics as well as kinetics, the physical properties of thin films often differ from their bulk counterparts. Here, we report pulsed laser-deposited thin films of LaBO3 (B = Mn, Fe, Co) grown on a SrTiO3 (001) substrate under various thermodynamic conditions. Structural, magnetic, and optical studies have been carried out. The x-ray diffraction study confirms that an appropriate choice of growth thermodynamics may help one to realize epitaxially grown films on the SrTiO3 substrate with out-of-plane lattice parameters 3.976, 3.984, and 3.825 Å for LaMnO3 (LMO), LaFeO3 (LFO), and LaCoO3 (LCO), respectively. A mixed valence state of Mn2+, 3+, 4+ for LMO, a Fe3+ state for LFO, and a mixed state of Co2+, 3+ for LCO have been confirmed by x-ray photoelectron spectroscopy, which is in good agreement with the Ellingham diagram. The optical study showed a bandgap of 1.2, 2.5, and 1.5 eV for LMO, LFO, and LCO, respectively. Ultraviolet photoelectron spectroscopy (UPS) shows a glimpse of the valence band maximum and Fermi level position. UV, UPS, and photoconductive study simultaneously results in a type II band bending, i.e., staggered type bending is observed at these interfaces. Room temperature weak ferromagnetism along with the insulating nature and a sign of photovoltaic application of these thin films fascinate to carry forward rigorous study from fundamental as well as technological points of view.

Natural band alignment of MgO1−xSx alloys

We have calculated formation enthalpies, bandgaps, and natural band alignment for MgO1−xSx alloys by first-principles calculation based on density functional theory. The calculated formation enthalpies show that the MgO1−xSx alloys exhibit a large miscibility gap, and a metastable region was found to occur when the S content was below 18% or over 87%. The effect of S incorporation for bandgaps of MgO1−xSx alloys shows a large bowing parameter (b ≃ 13 eV) induced. The dependence of the band lineup of MgO1−xSx alloys on the S content by using two different methods and the change in the energy position of the valence band maximum (VBM) were larger than those of the conduction band minimum. Based on the calculated VBM positions, we predicted that MgO1−xSx with S content of 10%–18% can be surface charge transfer doped by high electron affinity materials. This work provides an example to design for p-type oxysulfide materials.

Single-walled AlX (X=S, Se) nanotubes for photocatalytic water splitting: Analysis by density functional theory

Here the geometry, electronic structure and photocatalytic properties of AlX (X = S, Se) nanotubes were calculated using the HSE06 functional. To analyze the stability of the nanotubes, we calculated the strain energies, which decrease with increasing radius. The calculated band gap values for both AlS and AlSe nanotubes in the most stable structure are 2.27 eV and 2.04 eV, respectively, both of which are narrower than the band gap of the monolayer. Interestingly, Strain modulation also allows AlX nanotubes to meet the redox potential of spontaneous hydrolysis in different aqueous solutions of acid and neutral, where AlS nanotubes have −6.26 eV and −3.82 eV at the valence band maximum and conduction band minimum positions, respectively, at 5% tensile strain; while AlSe nanotubes have −6.18 eV and −3.99 eV at the band edge positions, respectively, satisfying the hydrolysis potentials at pH = 0 and 7. These unique and excellent properties also make AlX (X = S, Se) nanotubes promising as the next generation of one-dimensional photocatalysts.

Room-temperature synthesized amorphous P-type transparent Ga2O3-Cu2S alloy thin films with tunable optoelectronic properties

Ultra-wide bandgap Ga2O3 has many intriguing properties which make them potentially suitable for optoelectronic applications. However, its functionalities are largely hindered by the lack of p-type Ga2O3 based materials. In this work, we synthesized amorphous p-type transparent (Ga2O3)1−x(Cu2S)x alloy thin films with x<∼0.5 by magnetron sputtering at room temperature. The optoelectronic properties of these alloy thin films were investigated by a combination of analytical techniques. The optical bandgap shows a reduction from ∼4.8 eV to ∼2.5 eV with increasing x, while the hole concentration N increases from 1019 cm−3 (x=∼0.2) to ∼2 × 1021 cm−3 (x = ∼0.5), with their hole mobility μ ∼ 0.3 cm2 V−1 s−1. The hole transport in these amorphous alloys follows the variable-range-hopping mechanism in the temperature range of 120–300 K. We observe that the valence band maximum (VBM) position moves up rapidly by ∼3 eV with x > 0.2 to ∼5.5 eV below the vacuum level, making the formation of native shallow acceptors more energetically favorable and hence enhancing their p-type conductivity. Our results show that electrical and optical properties of these amorphous p-type transparent (Ga2O3)1−x(Cu2S)x alloy thin films are potentially important in bipolar devices applications, e.g., Ga2O3 based p-n heterojunction power devices and high efficiency solar cells.

Hybridization and localized flat band in the WSe2/MoSe2 heterobilayer

Nearly localized moiré flat bands in momentum space, arising at particular twist angles, are the key to achieve correlated effects in transition-metal dichalcogenides. Here, we use angle-resolved photoemission spectroscopy (ARPES) to visualize the presence of a flat band near the Fermi level of van der Waals WSe2/MoSe2 heterobilayer grown by molecular beam epitaxy. This flat band is localized near the Fermi level and has a width of several hundred meVs. By combining ARPES measurements with density functional theory calculations, we confirm the coexistence of different domains, namely the reference 2H stacking without layer misorientation and regions with arbitrary twist angles. For the 2H-stacked heterobilayer, our ARPES results show strong interlayer hybridization effects, further confirmed by complementary micro- Raman spectroscopy measurements. The spin-splitting of the valence band at K is determined to be 470 meV. The valence band maximum (VBM) position of the heterobilayer is located at the Γ point. The energy difference between the VBM at Γ and the K point is of −60 meV, which is a stark difference compared to individual single monolayer WSe2 and monolayer WSe2, showing both a VBM at K.

Wide gap p-type NiO-Ga2O3 alloy via electronic band engineering

Gallium oxide (Ga2O3) has gained significant interest in recent years due to its wide bandgap and related unique properties, making it suitable for many high power and deep ultra-violet optoelectronic and photodetection devices. Nevertheless, Ga2O3 can only be doped effectively n-type, and its full potential in device applications is severely limited by the lack of a reliable p-type material. Here, we report on modifying electronic bands of Ga2O3 through alloying with NiO to achieve p-type conducting Ga2O3-NiO alloy (NixGa1−xO) thin films. We find that room temperature sputter-deposited stoichiometric and O-rich alloys with low Ni content (x < 0.22) have an amorphous structure. In contrast, films with higher Ni content (x ≳ 0.22) are polycrystalline with the rocksalt (RS) NiO structure. O-rich RS-alloys are p-type with resistivity ~20 Ω-cm (for x~0.6) and decreases to< 10 Ω-cm with increasing x. Optically, p-type O-rich films with x ≥ 0.46 have strong sub-gap absorption in the low energy region (˂ 3.5 eV) due to a high concentration of Ni vacancies VNi and this results in a low transmittance which also decreases with x from ~70 % (x = 0.3) to ~ 40 % (x = 1). The band gap Eg of the alloy films exhibits a wide tunability with a monotonic decrease with increasing x from 4.98 (x = 0) to 3.53 (x = 1). The Eg for the RS alloys follows the virtual crystal approximation with a small bandgap bowing of 0.36 eV and an extrapolated Eg of 4.6 eV for RS Ga2O3. Furthermore, we find that the amorphous and RS crystalline alloys have a type II (staggered) band offset (ΔEV~1.8 eV and ΔEC~1.4 eV) with a stepwise upshift of the valence band maximum (VBM) position from ~8–6.2 eV below the vacuum level at the amorphous to RS transition (x~0.2). This significant uplift of the VBM is believed to be responsible for the measured p-type conductivity of the O-rich RS alloys. It also suggests that deep acceptors in Ga2O3 would become shallow in these alloys and hence effective acceptor doping to further improve their p-type conductivity is possible.

Chemical Vapor Deposition of Wide-Bandgap p-Type Semiconductor Cuprous Iodide for Optoelectronic Device Applications

Continuous, pinhole-free thin films of transparent conductive materials (TCMs) with p- and n-type conductivity are critical components of optoelectronic devices including photovoltaics (PV), transparent electronics, and LEDs [1]. TCMs with n-type conductivity are widely available in the form of semiconducting oxides [2]; however, p-type TCMs (hole transport materials, HTMs) with suitable conductivity are comparatively rare [1]. Particularly in the rapidly growing field of thin film perovskite PV, device-quality thin films of HTMs with high stability and conductivity are urgently needed to replace the organic HTMs typically employed in perovskite PV research [3]. As such, techniques to deposit inorganic HTM thin films are desired.With a focus on applying HTMs in optoelectronic devices at the commercial scale, these materials must be deposited by a scalable technique yielding thin, continuous, pinhole-free films. Chemical vapor deposition (CVD) is a highly scalable thin film deposition technique widely used in industry, with a proven ability to yield high-quality thin films on large-area substrates [4]. A variety of n-type TCMs are accessible by CVD techniques, but few CVD methods exist for p-type TCMs; examples include atomic layer deposition of NiOx and VOx [5].The cuprous halides (CuX, X = Cl, Br, I), which are optically transparent p-type semiconductors with bandgaps ~3 eV, are promising inorganic HTM candidates. Cuprous iodide is of particular interest due to its low resistivity (~10-2 Ohm·cm), high hole concentration and mobility (~1019 cm-3 and ~1-10 cm2V-1s-1, respectively), and its valence band maximum position (ionization potential: 5.0-5.4 eV), which is well-aligned with common perovskite absorber materials [6]. While CuI crystallite arrays have been obtained by metal-organic CVD [7], CVD of continuous CuI thin films has only been achieved by a two-step vapor conversion process, requiring initial deposition of a copper chalcogen followed by vapor-phase conversion to the halide [6].Our group has recently published a CVD technique enabling direct deposition of continuous CuBr thin films via CVD reaction between hydrogen bromide and vinyltrimethylsilane(hexafluoroacetylacetonato)copper(I) [Cu(hfac)(vtms)] [8], and this technique has been extended to deposition of CuI. X-ray photoelectron spectroscopy indicates pure CuI films with an atomic ratio of approximately 1:1 Cu:I, and x-ray diffraction confirms deposition of γ-CuI. Similar to our observations for CuBr films, substrate identity has significant effects on CuI film continuity. Progress towards CVD of continuous CuI films on substrates of interest for practical application in optoelectronic devices will be discussed, with a particular focus on perovskite PV.[1] A. N. Fioretti and M. Morales-Masis, "Bridging the p-type transparent conductive materials gap: synthesis approaches for disperse valence band materials," J. Photon. Energy, 10, 042002 (2020).[2] M. Morales-Masis, S. De Wolf, R. Woods-Robinson, J. W. Ager, and C. Ballif, "Transparent Electrodes for Efficient Optoelectronics," Adv. Electron. Mater., 3, 1600529 (2017).[3] B. Gil, A. J. Yun, Y. Lee, J. Kim, B. Lee, and B. Park, "Recent Progress in Inorganic Hole Transport Materials for Efficient and Stable Perovskite Solar Cells," Electron. Mater. Lett., 15, 505 (2019).[4] R. Gordon, "Chemical Vapor Deposition of Coatings on Glass," J. Non-Cryst. Solids, 218, 81 (1997).[5] J. A. Raiford, S. T. Oyakhire, and S. F. Bent, "Applications of atomic layer deposition and chemical vapor deposition for perovskite solar cells," Energy Environ. Sci., 13, 1997 (2020).[6] R. Heasley, L. M. Davis, D. Chua, C. M. Chang, and R. G. Gordon, "Vapor Deposition of Transparent, p-Type Cuprous Iodide Via a Two-Step Conversion Process," ACS Appl. Energy Mater., 1, 6953 (2018).[7] V. Gottschalch, S. Blaurock, G. Benndorf, J. Lenzner, M. Grundmann, and H. Krautscheid, "Copper iodide synthesized by iodization of Cu-films and deposited using MOCVD," J. Cryst. Growth, 471, 21 (2017).[8] C. M. Chang, L. M. Davis, E. K. Spear, and R. G. Gordon, "Chemical Vapor Deposition of Transparent, p-Type Cuprous Bromide Thin Films," Chem. Mater., 33, 1426 (2021).

Ge incorporation in kesterite thin films by solution processing route: An in-depth study of structural and optoelectronic properties

Herein, we report a promising solution-processing method for the controlled incorporation of germanium (Ge) into Cu2ZnSnS4 (CZTS) films using a nontoxic molecular ink. The effect of Ge concentration on structural, optical, electrical and electronic properties of the films were systematically investigated. The successful Ge alloying in CZTS was confirmed by XRD analysis and exploring the sensitivity of Raman scattering. The as-synthesized Cu2ZnSn1-xGexS4 (CZTGS; x = 0, 0.1, 0.15, 0.2 and 0.25) films exhibit strong absorption in the visible region with the gradual increment in the band gap energy from 1.46 to 1.61 eV by increasing x from 0 to 0.25. A decrease in Urbach energy at higher [Ge]/[Sn + Ge] ratio supports the reduction of overall disorder into the system. The information about the correct oxidation states of the constituent elements and optimal chemical composition of the CZTGS films was obtained from X-ray photoelectron spectroscopy (XPS). The decrease in SnII/SnIV signal ratio with the increase in Ge further supports the suppression of deep-trap SnZn antisite defects. The XPS valence band spectra reveal that the position of valence band maximum ascended from 0.09 to 0.15 eV with increasing Ge content in the films. Several relevant optical constants were determined of the CZTGS films which are important for applications in solar cell devices as well as for simulation purpose.

Effect of semiconducting nature of ZnO interfacial layer on inverted organic solar cell performance

The light-soaking effect is one of the major drawbacks for inverted organic solar cells (OSCs) if metal oxides are used as the electron transport layer (ETL). The oxide ETL primarily originates the above effect from the energy barrier, deep level defects, and excess carriers tunneling. Here, electron-beam evaporated high-quality pristine and post-treated e-ZnO thin films are utilized to fabricate inverted OSC as the ETL between the transparent cathode and active bulk-heterojunction PBDB-T-2Cl:PC61BM layer to study the influence on device performance. Various experimental techniques, including AFM, XRD, XPS, and UPS, are utilized to identify the surface and semiconducting properties of differently treated interfacial e-ZnO films precisely. XPS results reveal the variation of oxygen vacancies and adsorbed oxygen species on the surface of e-ZnO layers. The semiconducting nature of various e-ZnO thin films for the use of ETL are also probed with the help of UPS results, which accurately locate the valence band maximum and Fermi level position. After correlating the property of e-ZnO systematically with the respective OSC device performance, it is found that the deeper valence band top and higher n-type nature of e-ZnO is desirable to depict the light soaking free highest solar cell efficiency and large open-circuit voltage of about 0.97 V in a single junction. The presence of lesser chemisorbed oxygen species over the e-ZnO surface might be an added advantage to demonstrate the light soaking free operation in inverted OSC devices.

Effect of the valence band maximum control of Cu(In,Ga)Se2 photoelectrode surface on water splitting

The effects of valence band maximum (VBM) position control on the Cu(In,Ga)Se2 (CIGS) photoelectrode surface during the photoelectrochemical (PEC) water splitting using CIGS thin films were studied. First, a hole-blocking layer was obtained by replacing Se with S atoms to achieve VBM position control. The corresponding increase in photocurrent density suppressed the recombination at the photoelectrode/electrolyte interface. Subsequently, the CdS layer with lower VBM position than the CIGS layer was deposited on the CIGS photoelectrode. The results showed that the onset potential and photocurrent density increased due to the VBM position of the CdS layer. This study shows that water splitting could be effectively improved by controlling the VBM position of the photoelectrode surface.

Cs2SnI6 nanocrystals enhancing hole extraction for efficient carbon-based CsPbI2Br perovskite solar cells

Carbon electrode-based perovskite solar cells (C-PSCs) promise long-term stability due to the chemically inertness and moisture resistance of carbon electrodes. However, the larger energy level mismatch between the perovskite and carbon electrode in C-PSCs results in lower hole extraction efficiency and poorer photovoltaic performance compared with the conventional metal electrode-based ones. In this work, a novel hole transport material (HTM) of Cs2SnI6 nanocrystals (CSI NCs) is synthesized and employed in CsPbI2Br C-PSCs. The suitable valence band maximum position of CSI (−5.55 eV) constitutes a gradient energy level alignment in CsPbI2Br/CSI/carbon, which effectively accelerates the hole extraction from perovskite to carbon electrode and reduces nonradiative recombination loss at the interface. With the introduction of CSI NC HTM between CsPbI2Br perovskite film and carbon electrode, the power conversion efficiency (PCE) of resulting cell devices are increased from 13.16% (Jsc of 14.17 mA cm−2, Voc of 1.221 V, FF of 76.05%) to 14.67% (Jsc of 14.51 mA cm−2, Voc of 1.267 V, FF of 79.81%). To the our best knowledge, this is the first time that CSI was used as a HTM in PSCs and it has shown attractive prospects in improving hole extraction efficiency and photovoltaic performance in resulting PSCs.

Exfoliation methods for compositional and electronic characterization of interfacial Mo(Sex,Sy) in Cu(In,Ga)(Se,S)2 solar cells by X‐ray and UV photoelectron spectroscopy

Mo(SexSy) is a transition metal dichalcogenide typically applied as a back contact interlayer in Cu(In,Ga)(Se,S)2 (CIGSSe) solar cells. Band alignment at the buried Mo/CIGSSe junction mediated by Mo(SexSy) is important for current transport and enables quasi‐ohmic behavior between the CIGSSe absorber and the Mo back electrode. Furthermore, the S/(Se + S) ratio is a crucial parameter that determines the height of the valence band offset at the CIGSSe/Mo(SexSy) interface. Because the interlayer is formed during rapid thermal processing, an MoSe2 or MoS2 thin film grown on free substrate surfaces will not be representative for a realistic solar cell device. Thus, for fundamental thin‐film material analysis, as well as functional characterization and modeling, appropriate preparation and analytical techniques are required in order to prevent artifacts. In principal, the weak van der Waals forces between two‐dimensional stacked Mo(SexSy) sheets allow the implementation of exfoliation procedures to generate free Mo(SexSy) surfaces out of CIGSSe solar cell layer stacks. In this article, two different exfoliation‐based Mo(Sex,Sy) preparation methods are investigated and evaluated with respect to subsequent surface analytical characterization by X‐ray and ultraviolet photoelectron spectroscopy. A special focus is laid on an artifact‐free characterization of chemical and electronical properties of the exposed layers for a number of samples. In a first instance, the compositional Se/S and (Se + S)/Mo ratios at the surface are quantitatively analyzed on the basis of dedicated peak‐fitting routines. Artifacts from carbonaceous contamination due to different exfoliation glues can be prevented through a detailed comparative analysis of carbon 1s and KLL Auger peaks. Furthermore, a significant surface band bending is observed that can be reduced by low‐energy Ar ion in situ sputtering. A simple model for the sputter removal of a charged surface layer is presented, which allows to approximately calculate the absolute valence band maximum (VBM) positions required for band alignment and numerical device simulations. The presented exfoliation surface analysis methodology is important for the whole CIGS(Se) solar cell community and may be of general interest for emerging applications of further 2D transition metal dichalcogenides as well.

C-doped TiO2 nanotubes with pulsed laser deposited Bi2O3 films for photovoltaic application

Anodization was used to obtain a nanotubular TiO 2 photoanode on F–SnO 2 glass. Subsequent annealing in the CH 4 atmosphere promoted the C-doping and improved the crystallinity of the TiO 2 nanotubes. The pulsed laser deposition was applied to cover the nanotubes with Bi 2 O 3 , serving as a hole transport material. X-ray photoelectron spectroscopy analyses of the doped samples reveal a shift in the valence band's maximum position towards lower binding energy as compared to those observed for the undoped samples (annealed in the air). The doping positively affects the absorption by shifting the absorption edge to 567 nm. I – V measurements under illumination show that the C-doping of TiO 2 increases the current density following the absorbance results. The highest open circuit voltage was reached for the samples with the 300 °C-deposited Bi 2 O 3 layer, pointing to better quality of the p-n junction, hence of the contact between Bi 2 O 3 and TiO 2 . This in situ annealing provided the formation of close contact between Bi 2 O 3 and TiO 2 , which enabled a faster charge transport as compared to the contact obtained with no annealing or even with post annealing. • C-doping of anodized TiO 2 NTs supported on FTO by annealing in CH 4 . • PLD to cover TiO 2 NTs with Bi 2 O 3 film, serving as a hole transport material. • XPS: Shift of VBM to lower BE for the doped vs. the undoped NTs (annealed in air). • DRS: Absorption red shift from 430 nm (undoped NTs) to 567 nm (doped NTs). • Best PV performances: C-doped TiO 2 with Bi 2 O 3 deposited at 300 °C in situ .

Oxygen incorporation in AlN films grown by plasma-enhanced atomic layer deposition

Oxygen is often detected as an impurity in III-nitride films deposited by atomic layer deposition (ALD). The presence of oxygen has deep and unfavorable influences on the structural and optical properties of AlN. We have studied the oxygen incorporation in AlN films prepared by hollow cathode plasma-assisted ALD by alternating H2 plasma. We report a decrease in the O concentration in the film upon the addition of H2 plasma flow. However, increasing the H2 plasma flow does not further decrease the O incorporation. Film composition became almost constant at the surface and beneath the surface of AlN films deposited using N2/H2 plasma. Only samples grown with N2/H2 plasma showed decreases in oxygen concentration from the surface to the bulk film. Oxygen produces important modifications in the structural, chemical, and electrical properties. The Al 2p, N 1s, and O 1s high-resolution x-ray photoelectron spectra represent that AlN films are composed of Al–N, Al–O, and N–Al–O bonds. X-ray photoelectron spectroscopy data suggest that the composition of the AlN film changes to AlON + AlN when N2 plasma is used. The valence band maximum position of the AlN film is found to be at ∼1.7 eV for oxygen atomic concentration ∼20 at. %. The TO and LO phonon modes are present in AlN films with oxygen contamination ∼20 at. % and disappear at high oxygen contamination (∼55 at. %).

Machine learning-enabled band gap prediction of monolayer transition metal chalcogenide alloys.

Monolayer transition metal dichalcogenide (TMD) alloys with tunable direct band gaps have promising applications in nanoelectronics and optoelectronics. The composition-dependent band gaps of ternary, quaternary and quinary monolayer TMD alloys have been systematically studied combining density functional theory and machine learning models in the present study. The excellent agreement between the DFT-calculated band gaps and the ML-predicted values for the training, validation and test datasets demonstrates the accuracy of our machine learning based on a neural network model. It is found that the band gap bowing parameter is closely related to the difference between the band gaps of the endpoint material compositions of the monolayer TMD alloy and increases with increasing band gap difference. The band gap bowing effects of monolayer TMD alloys obtained by mixing different transition metals are attributed to the conduction band minimum positions, while those of monolayer TMD alloys obtained by mixing different chalcogen atoms are dominated by the valence band maximum positions. This study shows that monolayer TMD alloys with tunable direct band gaps can provide new opportunities for band gap engineering, as well as electronic and optoelectronic applications.