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Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGS) and CdTe photovoltaic semiconductors have reached remarkable laboratory efficiencies. It is surprising that these thin-film polycrystalline solar cells can reach such high efficiencies despite containing a high density of grain boundaries (GBs), which would seem likely to be nonradiative recombination centers for photo-generated carriers. In this paper, we review our atomistic theoretical understanding of the physics of grain boundaries in CIGS and CdTe absorbers. We show that intrinsic GBs with dislocation cores exhibit deep gap states in both CIGS and CdTe. However, in each solar cell device, the GBs can be chemically modified to improve their photovoltaic properties. In CIGS cells, GBs are found to be Cu-rich and contain O impurities. Density-functional theory calculations reveal that such chemical changes within GBs can remove most of the unwanted gap states. In CdTe cells, GBs are found to contain a high concentration of Cl atoms. Cl atoms donate electrons, creating n-type GBs between p-type CdTe grains, forming local p-n-p junctions along GBs. This leads to enhanced current collections. Therefore, chemical modification of GBs allows for high efficiency polycrystalline CIGS and CdTe thin-film solar cells. [ABSTRACT FROM AUTHOR]

Recently, Cu delafossites, CuMO2, have received much attention as photo-catalysts for hydrogen production by water splitting due to their unique properties such as stability in most aqueous solutions and p-type conductivity with excellent hole mobility. However, due to their large optical bandgaps, they can absorb sun light only in the ultraviolet region. Hence, it is necessary to tailor their optical properties to enhance their optical absorption in the visible light regions. In this paper, we report on the effects of Bi alloying on the electronic and optical properties of Cu delafossites by density functional theory. We find that Bi alloying can lead to improved optical absorption as compared to the pristine Cu delafossites. We further find that the lone pair Bi-6s electrons create occupied delocalized anti-bonding states on the top of the valence band, leading to further improved hole mobility. [ABSTRACT FROM AUTHOR]

We present density-functional total-energy calculations to investigate the passivation effects of impurity elements such as Br, Cl, S, O, H, and Na on double-positioning twin boundaries in CdTe. We find that Br, Cl, S, and O atoms present passivation effects on the boundaries with different degrees, whereas H and Na do not passivate the boundaries. Of all these impurities, Cl atoms present the best passivation effects on the double-positioning twin boundaries. The superior passivation effects are realized by either terminating the Cd atoms with dangling bonds, or substituting the Te atoms with dangling bonds in the double-positioning twin boundaries in CdTe by Cl atoms. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using the combination of high-resolution transmission electron microscopy, first-principles density-functional total-energy calculations, and image simulations, we studied the atomic structure and effects of double-positioning twin boundaries in CdTe. The structure for the double-positioning twin boundaries is found to contain more Te dangling bonds than Cd dangling bonds. The two sides of the boundaries have the same polarities. The structure produces energy states in the band gap that are detrimental to the electronic properties of CdTe. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have fabricated CdTe thin-film solar cells using cobalt-phthalocyanine (CoPC) and Au as Cu-free back contacts on F-doped SnO2-coated soda lime glass substrates. The CdTe solar cells using CoPC/Au back contacts showed 80-120mV improvements on open circuit voltage (VOC) as compared to the cells using Au-only back contacts. The best small area cell using CoPC/Au back contacts has demonstrated an efficiency of 14.3% with an open circuit voltage of 815mV, a short circuit current of 24.3mA/cm², and a fill factor of 72.3% under AM1.5 illumination. Accelerated life testing performed at 200°C and open circuit biasing revealed degradations, partially caused by the presence of oxygen and/or moisture. [ABSTRACT FROM AUTHOR]

We report the first-principles electronic structure of BiVO4, a promising photocatalyst for hydrogen generation. BiVO4is found to be a direct band gap semiconductor, despite having band extrema away from the Brillouin zone center. Coupling between Bi 6s and O 2p forces an upward dispersion of the valence band at the zone boundary; however, a direct gap is maintained via coupling between V 3d, O 2p, and Bi 6p, which lowers the conduction band minimum. These interactions result in symmetric hole and electron masses. Implications for the design of ambipolar metal oxides are discussed. [ABSTRACT FROM AUTHOR]

Quasicrystals are solids that exhibit symmetries long thought forbidden in nature. Since their discovery in a rapidly solidified Al–Mn alloy in 1984, the central issue in the field has been to understand why they form. Are they energetically stable compounds or stabilized by entropy? In recent years, major strides have been made in determining atomic structure, largely by direct imaging using advanced electron microscopy. One system is now known to be energetically stabilized, and quasicrystals are therefore firmly established as a new physical state of matter. They represent a unique packing of atomic clusters some tens of atoms in size, with substantial localized fluctuations, referred to as phasons. Understanding phasons may in future allow their unique macroscopic properties to be tailored for useful materials applications. [ABSTRACT FROM AUTHOR]

Polytypoid structures in the AlN crystals containing oxygen were investigated by electron diffraction and high-resolution electron microscopy. The polytypoids were revealed to consist of flat and corrugated inversion domains. The flat inversion domain boundaries were found to have three different interfacial structures. The distance between two neighbouring flat inversion domain boundaries was found to be either 5c or (5 +- 0.5)c , where c is the lattice parameter along the c axis of the wurtzite 2H-AlN crystal. Structural models which explain well the electron diffraction patterns and high-resolution electron microscopy images of the polytypoids are proposed. [ABSTRACT FROM AUTHOR]

CoAl2O4–Fe2O3 p-n nanocomposite electrodes were deposited on Ag-coated stainless-steel substrates and annealed at 800 °C. Their photoelectrochemical (PEC) properties were investigated and compared with that of p-type CoAl2O4 films. We found that the nanocomposite electrodes exhibit much improved PEC photoresponse as compared to the reference p-type CoAl2O4 electrodes. We speculate that the enhancement is due to the formation of a three-dimensional junction between p-type CoAl2O4 and n-type Fe2O3 nanoparticles, which improves electron-hole separation, thus reducing charge recombination upon light illumination. [ABSTRACT FROM AUTHOR]

Through density functional theory calculations, we demonstrate that enhancement of optical absorption and optimization of the fundamental band gap for Cu delafossites can be achieved through alloying group IIIA and IIIB delafossites. These alloys significantly improved the flexibility in designing delafossite-based photoelectrodes for application in photoelectrochemical decomposition of water by visible spectra of solar light. [ABSTRACT FROM AUTHOR]

We report the band structures and excitonic properties of delafossites CuMO2 (M=Al, Ga, In, Sc, Y, Cr) calculated using the state-of-the-art GW-BSE approach. We evaluate different levels of selfconsistency of the GW approximations, namely G0W0, GW0, GW, and QSGW, on the band structures and find that GW0, in general, predicts the band gaps in better agreement with experiments considering the electron-hole effect. For CuCrO2, the HSE wave function is used as the starting point for the perturbative GW0 calculations, since it corrects the band orders wrongly predicted by PBE. The discrepancy about the valence band characters of CuCrO2 is classified based on both HSE and QSGW calculations. The PBE wave functions, already good, are used for other delafossites. All the delafossites are shown to be indirect band gap semiconductors with large exciton binding energies, varying from 0.24 to 0.44 eV, in consistent with experimental findings. The excitation mechanisms are explained by examining the exciton amplitude projections on the band structures. Discrepancies compared with experiments are also addressed. The lowest and strongest exciton, mainly contributed from either Cu 3d→Cu 3p (Al, Ga, In) or Cu 3d→M 3d (M=Sc, Y, Cr) transitions, is always located at the L point of the rhombohedral Brillouin zone. [ABSTRACT FROM AUTHOR]

The authors present their first-principles calculations of doping effects in ZnO with group-IB elements such as Cu, Ag, and Au. The calculated transition energies [variant_greek_epsilon](0/-) for substitutional Cu, Ag, and Au are 0.7, 0.4, and 0.5 eV, respectively. The calculated formation energies are very low for these group-IB elements on the substitutional sites, but rather high at the interstitial sites under oxygen-rich growth conditions. Under the conditions, the formation of major hole-killer defects, such as oxygen vacancies and Zn interstitial, are suppressed. Thus, Ag may be a good candidate for producing p-type ZnO. [ABSTRACT FROM AUTHOR]

We propose an approach to stabilize the cubic zinc-blende (ZB) phase of semiconductor compounds that are usually more stable in the hexagonal wurtzite (WZ) phase. This approach is based on impurity doping and we take advantage of the band offset between the ZB and WZ phases. We show that introduction of donors should stabilize the one with lower conduction band (ZB), whereas holes should stabilize the one with higher valence band (WZ). A mechanism to invert the valence band offset is proposed in order to stabilize the ZB phase through holes. We used GaN, ZnO, and AlN as examples. [ABSTRACT FROM AUTHOR]

The atomic structure and electronic properties of crystalline-amorphous interfaces in silicon heterojunction solar cells are investigated by high-resolution transmission electron microscopy, atomic-resolution Z-contrast imaging, and electron energy-loss spectroscopy. With these combined techniques, we directly observe abrupt and flat transition from crystalline Si to hydrogenated amorphous Si at the interface of Si heterojunction solar cells. We find that high-quality hydrogenated amorphous Si layers can be grown abruptly by hot-wire chemical vapor deposition on 200 °C (100) Si substrates after a two-step pretreatment of the substrate, comprised of exposure to hot-wire decomposed H2-diluted NH3 followed by atomic H etching. [ABSTRACT FROM AUTHOR]

The microstructure and chemistry of CuInSe2 single-crystals and Cu(In,Ga)Se2 thin films from high-efficiency devices are investigated by transmission electron microscopy and x-ray energy-dispersive spectroscopy. We find strong chemical fluctuations at the nanoscale, which result in a lattice comprising a mixture of relatively Cu-poor and Cu-rich nanodomains in both cases. These nanodomains are crystallographically coherent, and no structural lattice defects are found at the interfaces between them. These nanodomains may interconnect, forming three-dimensional, interpenetrating Cu-poor and Cu-rich percolation networks. Such interconnected structures may play a role in the high device performance of Cu(In,Ga)Se2 thin-film photovoltaics. [ABSTRACT FROM AUTHOR]

The light-emitting diodes (LEDs) used in indoor testing of perovskite solar cells do not expose them to the levels of ultraviolet (UV) radiation that they would receive in actual outdoor use. We report degradation mechanisms of p-i-n–structured perovskite solar cells under unfiltered sunlight and with LEDs. Weak chemical bonding between perovskites and polymer hole-transporting materials (HTMs) and transparent conducting oxides (TCOs) dominate the accelerated A-site cation migration, rather than direct degradation of HTMs. An aromatic phosphonic acid, [2-(9-ethyl-9H-carbazol-3-yl)ethyl]phosphonic acid (EtCz3EPA), enhanced bonding at the perovskite/HTM/TCO region with a phosphonic acid group bonded to TCOs and a nitrogen group interacting with lead in perovskites. A hybrid HTM of EtCz3EPA with strong hole-extraction polymers retained high efficiency and improved the UV stability of perovskite devices, and a champion perovskite minimodule—independently measured by the Perovskite PV Accelerator for Commercializing Technologies (PACT) center—retained operational efficiency of >16% after 29 weeks of outdoor testing. [ABSTRACT FROM AUTHOR]

We present a density-functional theory study on the electronic structure of pure and 3d transition metal (TM) (Sc, Ti, Cr, Mn, and Ni) incorporated α-Fe2O3. We find that the incorporation of 3d TMs in α-Fe2O3 has two main effects such as: (1) the valence and conduction band edges are modified. In particular, the incorporation of Ti provides electron carriers and reduces the electron effective mass, which will improve the electrical conductivity of α-Fe2O3. (2) The unit cell volume changes systematically such as: the incorporation of Sc increases the volume, whereas the incorporation of Ti, Cr, Mn, and Ni reduces the volume monotonically, which can affect the hopping probability of localized charge carriers (polarons). We discuss the importance of these results in terms of the utilization of hematite as a visible-light photocatalyst. [ABSTRACT FROM AUTHOR]

Describes the atomic structure of the high-perfection decagonal quasicrystal Al72Ni20Co8. Extensive broken decagonal symmetry for the entire cluster; Z-contrast imaging of clusters with no strong broken symmetry.

Even though the quantum size effect on the electronic gap of nano-structures is well-understood, its implication on the optical absorption near band gap energies is still a challenging question, especially for metal-oxides. A unique class of highly stable, self-saturated and self-charge-compensated delafossite nanocrystals has been identified in this paper. The structural and electronic properties of these nanocrystalline Cu-based delafossites have been studied using density functional theory (DFT). To better estimate the electronic excitation energies, and consequently the optical gap, a time-dependent DFT has also been employed. The goal here is to study whether a nano-phase can enhance the optical absorption of near band gap energies of delafossite nano-structures compared to their bulk state to enable their application as a photocatalyst. [ABSTRACT FROM AUTHOR]

Photoelectrochemical (PEC) water reduction is a long-term strategical technology for hydrogen production. In this work, we synthesize a series of compact and nano/mesoporous Nb-substituted BaBiO3 [i.e., Ba2Bi(Bi1-xNbx)O6, 0 ≤ x ≤ 0.93, BBNO] thin films using cost-effective chemical solution methods. The synthesized BBNO alloy based thin films demonstrate tunable bandgaps from 1.41 eV (x = 0) to 1.89 eV (x = 0.93) to efficiently absorb the solar spectrum and p-type conductivities suitable for hydrogen production. The photoelectrodes with a configuration fluorine-doped SnO2/BBNO (0 ≤ x ≤ 0.93)/Pt produce cathodic photocurrents of 0.05-1 mA·cm-2 at 0 VRHE (volt versus reversible hydrogen electrode) measured in a neutral (pH = 7.2) phosphate buffer and under simulated AM 1.5G illumination (100 mW·cm-2). The BaBiO3 without Nb alloying based electrode delivers the best photocurrent of 1 mA·cm-2 at 0 VRHE but is subjected to severe corrosions during the PEC related tests. Alloying Nb has an obvious influence on enhancing the material stability against corrosion. With Nb alloying, the screen-printed nanoporous BBNO (x = 0.6, bandgap = 1.62 eV) based photoelectrode generates a better photocurrent of 0.2 mA·cm-2 at 0 VRHE with a highly positive onset at 1.5 VRHE enabling unbiased water reduction. [ABSTRACT FROM AUTHOR]

Mixed tin (Sn) and lead (Pb) based perovskite thin films have been prepared by solution processing combining methylammonium lead iodide (MAPbI3) and formamidinium tin iodide (FASnI3) precursors. Optical response in the form of complex dielectric function (ε = ε1 + iε2) spectra and absorption coefficient (α) spectra of (FASnI3)1-x(MAPbI3)x based perovskite films have been extracted over a spectral range 0.74 to 5.89 eV using spectroscopic ellipsometry. Absorption band edge energy changes as a function of composition for films including FASnI3, MAPbI3, and mixed x = 0.20, 0.35, 0.40, and 0.6 (FASnI3)1-x(MAPbI3)x perovskites. (FASnI3)0.60(MAPbI3)0.4 is found to have the minimum absorption band edge energy near ~1.2 eV. [ABSTRACT FROM AUTHOR]

The development of highly stable and efficient wide-bandgap (WBG) perovskite solar cells (PSCs) based on bromine-iodine (Br–I) mixed-halide perovskite (with Br greater than 20%) is critical to create tandem solar cells. However, issues with Br–I phase segregation under solar cell operational conditions (such as light and heat) limit the device voltage and operational stability. This challenge is often exacerbated by the ready defect formation associated with the rapid crystallization of Br-rich perovskite chemistry with antisolvent processes. We combined the rapid Br crystallization with a gentle gas-quench method to prepare highly textured columnar 1.75–electron volt Br–I mixed WBG perovskite films with reduced defect density. With this approach, we obtained 1.75–electron volt WBG PSCs with greater than 20% power conversion efficiency, approximately 1.33-volt open-circuit voltage (Voc), and excellent operational stability (less than 5% degradation over 1100 hours of operation under 1.2 sun at 65°C). When further integrated with 1.25–electron volt narrow-bandgap PSC, we obtained a 27.1% efficient, all-perovskite, two-terminal tandem device with a high Voc of 2.2 volts. [ABSTRACT FROM AUTHOR]

Thin-film solar cells based on Methylammonium triiodideplumbate (CH3NH3PbI3) halide perovskites have recently shown remarkable performance. First-principle calculations show that CH3NH3PbI3 has unusual defect physics: (i) Different from common p-type thin-film solar cell absorbers, it exhibits flexible conductivity from good p-type, intrinsic to good n-type depending on the growth conditions; (ii) Dominant intrinsic defects create only shallow levels, which partially explain the long electron-hole diffusion length and high open-circuit voltage in solar cell. The unusual defect properties can be attributed to the strong Pb lone-pair s orbital and I p orbital antibonding coupling and the high ionicity of CH3NH3PbI3. [ABSTRACT FROM AUTHOR]

High efficiency CdTe solar cells typically use oxygenated CdS (CdS:O) window layers. We synthesize CdS:O window layers at room temperature (RT) and 270 °C using reactive sputtering. The band gaps of CdS:O layers deposited at RT increase when O2/(O2+Ar) ratios in the deposition chamber increase. On the other hand, the band gaps of CdS:O layers deposited at 270°C decrease as the O2/(O2+Ar) ratios increase. Interestingly, however, our high temperature closed-space sublimation (CSS) processed CdTe solar cells using CdS:O window layers deposited at RT and 270 °C exhibit very similar cell performance, including similar short-circuit current densities. To understand the underlying reasons, CdS:O thin films deposited at RT and 270 °C are annealed at temperatures that simulate the CSS process of CdTe deposition. X-ray diffraction, atomic force microscopy, and UV-visible light absorption spectroscopy characterization of the annealed films reveals that the CdS:O films deposited at RT undergo grain regrowth and/or crystallization and exhibit reduced band gaps after the annealing. Our results suggest that CdS:O thin films deposited at RT and 270 °C should exhibit similar optical properties after the deposition of CdTe layers, explaining the similar cell performance. [ABSTRACT FROM AUTHOR]

Aluminum doped zinc oxide (AZO) has attained prominence as being a very good transparent conducting oxide for optoelectronics and photovoltaic applications. In this work, we report on the synthesis and characterization of AZO films with c-axis preferred orientation using magnetron radio frequency (RF) sputtering. It was found that the degree of the c-axis preferred orientation can be controlled by varying the growth conditions, such as working pressure, RF power, and substrate temperatures. The preferred orientation increased as the working pressure decreased, while it increased as the RF power and substrate temperature increased. Electrical and optical measurements have revealed that the growth conditions and c-axis preferred orientation have strong influence on the physical properties of the synthesized AZO thin films. The thin films with increased c-axis preferred orientation exhibited enhanced carrier mobility. [ABSTRACT FROM AUTHOR]

CdCl2 activation near 400 °C is known to be critically important for obtaining high efficiency CdS/CdTe solar cells. However, this treatment step behaves differently on high-temperature-grown CdTe than on lower-temperature-grown CdTe layers such as those grown by sputtering. On sputtered films, the post-deposition activation produces grain-boundary passivation, sulfur diffusion into CdTe, and substantial grain growth. Nevertheless, we find the CdCl2 process for sputtered films to be characterized by a single activation energy that we interpret as applying to S diffusion into CdTe. We find this activation energy to hold for CdCl2 treatments from 370 to 440 °C. The completed CdS/CdTe solar-cell structures showed somewhat poorer initial performance with activation above 420 °C, but, in this case, the cell efficiency increased after accelerated life testing at 85 °C, open-circuit biasing and one-sun illumination. With an optimized CdCl2 activation process, the use of oxygenated sputtered CdS, and low-iron soda-lime glass, cell efficiencies of 14.5% were achieved. [ABSTRACT FROM AUTHOR]

The stability, transparency, and conductivity of ZnO are suggested to be tailored by alloying with MgO or CdO to meet wide applications. Our Monte Carlo simulation based on first-principle cluster expansion methods partially explain the solubility and stability data scattered in extensive experiments and further reveal that CdxZn1-xO has much higher solubility than prevalent MgxZn1-xO in a large range of experimentally achievable temperature (400K-1200K). Furthermore, first-principles calculations based on hybrid functional methods show that CdxZn1-xO has better n-type doping properties than MgxZn1-xO. The optical gap of CdxZn1-xO could be ~1.5 eV higher than its fundamental gap due to large Moss-Burstein shift. We thus predict that CdxZn1-xO has great potential to be a better transparent conducting oxide than MgxZn1-xO. [ABSTRACT FROM AUTHOR]

We report on the synthesis of amorphous copper tungsten oxide thin films with tunable band gaps. The thin films are synthesized by the magnetron cosputtering method. We find that due to the amorphous nature, the Cu-to-W ratio in the films can be varied without the limit of the solubility (or phase separation) under appropriate conditions. As a result, the band gap and conductivity type of the films can be tuned by controlling the film composition. Unfortunately, the amorphous copper tungsten oxides are not stable in aqueous solution and are not suitable for the application of photoelectrochemical splitting of water. Nonetheless, it provides an alternative approach to search for transition metal oxides with tunable band gaps. [ABSTRACT FROM AUTHOR]

Using ab initio density function theory total energy calculations, we have investigated the mechanism of Fe segregation into Si Σ5(310) grain boundary (GB). We find that the segregation is site selective at the GB-Fe will only segregate to specific sites. We further find that the choice of the segregation site is determined by the segregation-induced stress and effective crystal-field-induce splitting of Fe d orbital at that site. Our results suggest that the revealed mechanism of Fe segregation into the GB should be general for other 3d transition metals with partially filled 3d orbits and for other grain boundaries. [ABSTRACT FROM AUTHOR]

The dynamics of partial dislocations in CdTe have been observed at the atomic scale using aberration-corrected scanning transmission electron microscopy (STEM), allowing the mobility of different dislocations to be directly compared: Cd-core Shockley partial dislocations are more mobile than Te-core partials, and dislocation cores with unpaired columns have higher mobility than those without unpaired columns. The dynamic imaging also provides insight into the process by which the dislocations glide. Dislocations with dangling bonds on unpaired columns are found to be more mobile because the dangling bonds mediate the bond exchanges required for the dislocations to move. Furthermore, a screw dislocation has been resolved to dissociate into a Shockley partialdislocation pair along two different directions, revealing a way for the screw dislocation to glide in the material. The results show that dynamic STEM imaging has the potential to uncover the details of dislocation motion not easily accessible by other means. [ABSTRACT FROM AUTHOR]

Mixed tin (Sn)-lead (Pb) perovskites with high Sn content exhibit low bandgaps suitable for fabricating the bottom cell of perovskite-based tandem solar cells. In this work, we report on the fabrication of efficient mixed Sn--Pb perovskite solar cells using precursors combining formamidinium tin iodide (FASnI3) and methylammonium lead iodide (MAPbI3). The best-performing cell fabricated using a (FASnI3) 06(MAPbI3)0.4 absorber with an absorption edge of ~1.2 eV achieved a power conversion efficiency (PCE) of 15.08 (15.00)% with an open-circuit voltage of 0.795 (0.799)V, a short-circuit current density of 26.86(26.82) mA/cm², and a fill factor of 70.6(70.0)% when measured under forward (reverse) voltage scan. The average PCE of 50 cells we have fabricated is 14.39 ± 0.33%, indicating good reproducibility. [ABSTRACT FROM AUTHOR]

Through density functional theory calculations, we show that the alloy perovskite system BaZr1-xTixS3 (x < 0.25) is a promising candidate for producing high power conversion efficiency (PCE) solar cells with ultrathin absorber layers. To maximize the minority carrier lifetime, which is important for achieving high PCE, the defect calculations show that BaZr1-xTixS3 films should be synthesized under moderate (i.e., near stoichiometric) growth conditions to minimize the formation of deep-level defects. The perovskite BaZrS3 is also found to exhibit ambipolar self-doping properties, indicating the ability to form homo p-n junctions. However, our theoretical calculations and experimental solid-state reaction efforts indicate that the doped perovskite BaZr1-xTixS3 (x > 0) may not be stable under thermal equilibrium growth conditions. Calculations of decomposition energies suggest that introducing compressive strain may be a plausible approach to stabilize BaZr1-xTixS3 thin films. [ABSTRACT FROM AUTHOR]

The surface structures of ionic zinc-blende CdTe (001), (110), (111), and (211) surfaces are systematically studied by first-principles density functional calculations. Based on the surface structures and surface energies, we identify the detrimental twinning appearing in molecular beam epitaxy (MBE) growth of II-VI compounds as the (111) lamellar twin boundaries. To avoid the appearance of twinning in MBE growth, we propose the following selection rules for choosing optimal substrate orientations: (1) the surface should be nonpolar so that there is no large surface reconstructions that could act as a nucleation center and promote the formation of twins; (2) the surface structure should have low symmetry so that there are no multiple equivalent directions for growth. These straightforward rules, in consistent with experimental observations, provide guidelines for selecting proper substrates for high-quality MBE growth of II-VI compounds. [ABSTRACT FROM AUTHOR]

Computational, thin-film deposition, and characterization approaches have been used to examine the ternary halide semiconductor Cs3Sb2I9. Cs3Sb2I9 has two known structural modifications, the 0-D dimer form (space group P63/mmc, no. 194) and the 2-D layered form (P3m1, no. 164), which can be prepared via solution and solid-state or gas-phase reactions, respectively. Our computational investigations suggest that the layered form, which is a one-third Sb-deficient derivative of the ubiquitous perovskite structure, is a potential candidate for high-band gap photovoltaic (PV) applications. In this work, we describe details of a two-step deposition approach that enables the preparation of large grain (>1 µm) and continuous thin films of the lead-free layered perovskite derivative Cs3Sb2I9. Depending on the deposition conditions, films that are c-axis oriented or randomly oriented can be obtained. The fabricated thin films show enhanced stability under ambient air, compared to methylammonium lead(II) iodide perovskite films stored under similar conditions, and an optical band gap value of 2.05 eV. Photoelectron spectroscopy study yields an ionization energy of 5.6 eV, with the valence band maximum approximately 0.85 eV below the Fermi level, indicating near-intrinsic, weakly p-type character. Density functional theory (DFT) analysis points to a nearly direct band gap for this material (less than 0.02 eV difference between the direct and indirect band gaps) and a similar high-level of absorption compared to CH3NH3PbI3. The photoluminescence peak intensity of Cs3Sb2I9 is substantially suppressed compared to that of CH3NH3PbI3, likely reflecting the presence of deep level defects that result in nonradiative recombination in the film, with computational results pointing to Ii, ISb, and VI as being likely candidates. A key further finding from this study is that, despite a distinctly layered structure, the electronic transport anisotropy is less pronounced due to the high ionicity of the I atoms and the strong antibonding interactions between the Sb s lone pair states and I p states, which leads to a moderately dispersive valence band. [ABSTRACT FROM AUTHOR]

Computational, thin-film deposition, and characterization approaches have been used to examine the ternary halide semiconductor Cs3Sb2I9. Cs3Sb2I9 has two known structural modifications, the 0-D dimer form (space group P63/mmc, no. 194) and the 2-D layered form (P3̅m1, no. 164), which can be prepared via solution and solid-state or gas-phase reactions, respectively. Our computational investigations suggest that the layered form, which is a one-third Sb-deficient derivative of the ubiquitous perovskite structure, is a potential candidate for high-band gap photovoltaic (PV) applications. In this work, we describe details of a two-step deposition approach that enables the preparation of large grain (>1 μm) and continuous thin films of the lead-free layered perovskite derivative Cs3Sb2I9. Depending on the deposition conditions, films that are c-axis oriented or randomly oriented can be obtained. The fabricated thin films show enhanced stability under ambient air, compared to methylammonium lead(II) iodide perovskite films stored under similar conditions, and an optical band gap value of 2.05 eV. Photoelectron spectroscopy study yields an ionization energy of 5.6 eV, with the valence band maximum approximately 0.85 eV below the Fermi level, indicating near-intrinsic, weakly p-type character. Density functional theory (DFT) analysis points to a nearly direct band gap for this material (less than 0.02 eV difference between the direct and indirect band gaps) and a similar high-level of absorption compared to CH3NH3PbI3. The photoluminescence peak intensity of Cs3Sb2I9 is substantially suppressed compared to that of CH3NH3PbI3, likely reflecting the presence of deep level defects that result in nonradiative recombination in the film, with computational results pointing to Ii, ISb, and VI as being likely candidates. A key further finding from this study is that, despite a distinctly layered structure, the electronic transport anisotropy is less pronounced due to the high ionicity of the I atoms and the strong antibonding interactions between the Sb s lone pair states and I p states, which leads to a moderately dispersive valence band. [ABSTRACT FROM AUTHOR]

Lead halide perovskite solar cells with the high efficiencies typically use high-temperature processed TiO2 as the electron transporting layers (ETLs). Here, we demonstrate that low-temperature solution-processed nanocrystalline SnO2 can be an excellent alternative ETL material for efficient perovskite solar cells. Our bestperforming planar cell using such a SnO2 ETL has achieved an average efficiency of 16.02%, obtained from efficiencies measured from both reverse and forward voltage scans. The outstanding performance of SnO2 ETLs is attributed to the excellent properties of nanocrystalline SnO2 films, such as good antireflection, suitable band edge positions, and high electron mobility. The simple lowtemperature process is compatible with the roll-to-roll manufacturing of low-cost perovskite solar cells on flexible substrates. [ABSTRACT FROM AUTHOR]

Recent theoretical and experimental reports have shown that the perovskite CH3NH3PbI3 exhibits unique ambipolar self-doping properties. Here, we show by density-functional theory calculation that its sister perovskite, CH3NH3PbBr3, exhibits a unipolar self-doping behavior--CH3NH3PbBr3 presents only good p-type conductivity under thermal equilibrium growth conditions. We further show that despite a large bandgap of 2.2 eV, all dominant defects in CH3NH3PbBr3 create shallow levels, which partially explains the ultra-high open-circuit voltages achieved by CH3NH3PbBr3-based thin-film solar cells. Our results suggest that the perovskite CH3NH3PbBr3 can be both an excellent solar cell absorber and a promising low-cost hole-transport material for lead halide perovskite solar cells. [ABSTRACT FROM AUTHOR]

When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to earner collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells. [ABSTRACT FROM AUTHOR]

Through the use of aberration corrected scanning transmission electron microscopy, the atomic configuration of CdTe intragrain Shockley partial dislocation pairs has been determined: Single Cd and Te columns are present at opposite ends of both intrinsic and extrinsic stacking faults. These columns have threefold and fivefold coordination, indicating the presence of dangling bonds. Counterintuitively, density-functional theory calculations show that these dislocation cores do not act as recombination centers; instead, they lead to local band bending that separates electrons and holes and reduces undesirable carrier recombination. [ABSTRACT FROM AUTHOR]

Using first-principles band structure calculations, we investigate the crystal-field and spin-orbit splittings at the valence-band edge of ZnO and their dependence on the strain. Different from other conventional semiconductors, the variation of the valence-band splitting of ZnO shows a strong nonlinear dependence on the strain and the slope of the crystal-field splitting as a function of strain can even change sign. Our analysis shows that this unusual behavior in ZnO is due to the strong coupling between Zn 3d states and oxygen 2p states. A mapping of the valence-band ordering in ZnO under different strain levels is provided that will be useful in designing ZnO-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

In the present Letter, we report on a combined ab initio density functional theory calculation, multislice simulation, and electron holography study, performed on a &Sgr;9 grain boundary (GB) in a CuGaSe2 bicrystal, which exhibits a lower symmetry compared with highly symmetric &Sgr;3 GBs. We find an electrostatic potential well at the &Sgr;9 GB of 0.8 V in depth and 1.3 nm in width, which in comparison with results from &Sgr;3 and random GBs exhibits the trend of increasing potential-well depths with lower symmetry. The presence of this potential well at the &Sgr;9 GB can be explained conclusively by a reduced density of atoms at the GB. Considering experimental limitations in resolution, we demonstrate quantitative agreement of experiment and theory. [ABSTRACT FROM AUTHOR]

The kesterite Cu2ZnSnS4 (CZTS) is attracting considerable interest because first-principles calculations predict that its electronic properties must be similar to their associated chalcopyrite Cu(In,Ga)Se2 (CIGS) compounds [Chen et al., Phys. Rev. B 81, 245204 (2010)]. Here, the authors report on first experimental evidence of the close resemblance in the luminescence of Cu-poor kesterites and Cu-poor chalcopyrites used in photovoltaic applications. Microluminescence measurements suggest that even the very distinct electronic structure of grain boundaries in CIGS is present, to some extent, in CZTS. The similarities between CIGS and CZTS become more pronounced as the efficiency of the CZTS solar cells gradually increases. The implications of these results for the future development of CZTS solar cells are discussed. [ABSTRACT FROM AUTHOR]

The intrinsic and extrinsic doping properties of BiVO4, i.e., the formation energies and transition energy levels of defects and impurities, have been studied systematically by first-principles density-functional theory. We find that for doping caused by intrinsic defects, O vacancies are shallow donors and Bi vacancies are shallow acceptors. However, these defects compensate each other and can only lead to moderate n-type and p-type conductivities at Bi-rich and O-rich growth conditions, respectively. To obtain BiVO4 with high n-type and p-type conductivities, which are required for forming Ohmic contacts, extrinsic doping using foreign impurities is necessary. Our results reveal that Sr, Ca, Na, and K atoms on Bi sites are very shallow acceptors and have rather low formation energies. The calculated Fermi-level pinning positions predict that doping of these impurities under oxygen-rich growth conditions should result in outstanding p-type conductivity. Substitutional Mo and W atoms on V sites are very shallow donors and have very low formation energies. Fermi-level pinning position calculations expect the doping of Mo and W under oxygen-poor growth conditions to produce excellent n-type conductivity. Also discussed is the dependence of formation energies and transition energies of defects on the atomic size and atomic chemical potential trends. [ABSTRACT FROM AUTHOR]

To deploy Li-ion batteries in next-generation vehicles, it is essential to develop electrodes with durability, high energy density, and high power. Here we report a breakthrough in controlled full-electrode nanoscale coatings that enables nanosized materials to cycle with durable high energy and remarkable rate performance. The nanoparticle electrodes are coated with Al2O3using atomic layer deposition (ALD). The coated nano-LiCoO2electrodes with 2 ALD cycles deliver a discharge capacity of 133 mAh/g with currents of 1400 mA/g (7.8C), corresponding to a 250% improvement in reversible capacity compared to bare nanoparticles (br-nLCO), when cycled at this high rate. The simple ALD process is broadly applicable and provides new opportunities for the battery industry to design other novel nanostructured electrodes that are highly durable even while cycling at high rate. [ABSTRACT FROM AUTHOR]

Ga–N co-doped ZnO thin films with reduced bandgaps were deposited on F-doped tin-oxide-coated glass by radio-frequency magnetron sputtering at different substrate temperatures in mixed N2 and O2 gas ambient. We found that Ga–N co-doped ZnO films exhibited enhanced crystallinity when compared to undoped ZnO films grown under the same conditions. Furthermore, Ga–N co-doping ensured enhanced N-incorporation ZnO thin films as the substrate temperature is increased. As a result, Ga–N co-doped ZnO thin films exhibited much improved photoelectrochemical (PEC) response, compared to ZnO thin films. Our results therefore suggest that the passive co-doping approach could be a means to improve PEC response for bandgap-reduced wide-bandgap oxides through impurity incorporation. [ABSTRACT FROM AUTHOR]

Substitutional doping can improve the electronic properties of α-Fe2O3for the solar photoelectrochemical (PEC) applications. Generally speaking, nonisovalent substitutional doping helps to enhance the electronic conductivity of α-Fe2O3. However, we found that the introduction of strain in the lattice, which is achieved by isovalent substitutional doping of an Al, can also improve the electronic properties. α-Fe2O3films with the Al dopant atomic concentration varying from 0 to 10% were prepared by electrodeposition, and their performance for photoelectrochemical hydrogen production was characterized. Results indicate that the incident photon conversion efficiency (IPCE) for ∼0.45 at-% Al substitution increases by 2- to 3-fold over undoped samples. Density-functional theory (DFT) was utilized to interpret the experimental findings. It was shown that although no substantial change to the electronic structure, a contraction of the crystal lattice due to the isovalent replacement of Fe3+by an Al3+benefits the small polaron migration, resulting in an improvement in conductivity compared to the undoped samples. [ABSTRACT FROM AUTHOR]

Due to its large band gap (3.2 eV), TiO2 cannot absorb sun light effectively. To reduce its band gap, various approaches have been attempted; most of them are using doping to modify its band structure. Using first-principles band structure calculations, we show that unlike the rutile phases, the band gap of TiO2 in the anatase phase can be effectively reduced by applying stress along a soft direction. We propose that this approach of tuning the band gap by applying stress along soft direction of a layered semiconductor is general and should be applicable to other anisotropic materials. [ABSTRACT FROM AUTHOR]