eV Optical Absorption Research Articles

2D Hemiporphyrazine: A new nanoporous material

Crystalline microporous materials are solids formed by interconnected pores of less than 2 nm in size. Typically, they possess large surface areas desirable for versatile applications such as catalysis, gas adsorption, and energy storage. In the present work, we propose a new porphyrin-based 2D nanoporous crystal, named 2D Porphyrazine (2DP), which is formed by topological assembling H$_{5}$C$_{13}$N$_{4}$ porphyrins. We have considered its monolayer, bi-layer, and molecular crystal (bulk) arrangements. We carried out DFT calculations to investigate 2DP structural and electronic properties. Results show that 2DP is a very stable structure with a direct bandgap of 0.65 eV and significant optical absorption in the visible range. 2DP exhibited satisfactory affinity to lithium atoms. Simulations also showed the existence of proton transfer between nitrogen atoms. It is the first report on the site-specific hydrogen exchange process in 2D crystals.

The Electronic Structures and Energies of the Lowest Excited States of the Ns0, Ns+, Ns− and Ns-H Defects in Diamond

This paper reports the energies and charge and spin distributions of the mono-substituted N defects, N0s, N+s, N-s and Ns-H in diamonds from direct Δ-SCF calculations based on Gaussian orbitals within the B3LYP function. These predict that (i) Ns0, Ns+ and Ns- all absorb in the region of the strong optical absorption at 270 nm (4.59 eV) reported by Khan et al., with the individual contributions dependent on the experimental conditions; (ii) Ns-H, or some other impurity, is responsible for the weak optical peak at 360 nm (3.44 eV); and that Ns+ is the source of the 520 nm (2.38 eV) absorption. All excitations below the absorption edge of the diamond host are predicted to be excitonic, with substantial re-distributions of charge and spin. The present calculations support the suggestion by Jones et al. that Ns+ contributes to, and in the absence of Ns0 is responsible for, the 4.59 eV optical absorption in N-doped diamonds. The semi-conductivity of the N-doped diamond is predicted to rise from a spin-flip thermal excitation of a CN hybrid orbital of the donor band resulting from multiple in-elastic phonon scattering. Calculations of the self-trapped exciton in the vicinity of Ns0 indicate that it is essentially a local defect consisting of an N and four nn C atoms, and that beyond these the host lattice is essential a pristine diamond as predicted by Ferrari et al. from the calculated EPR hyperfine constants.

Two-dimensional monolayer B2P6 with anisotropic elastic properties and carrier mobility

A novel two-dimensional (2D) monolayer B2P6 allotrope is proposed by the first-principles calculations. The monolayer B2P6 is demonstrated as stable by the combination of cohesive energy, phonon dispersion and elastic constants. Moreover, monolayer B2P6 has anisotropic Young's modulus and negative Poisson ratio. The carrier mobility is also anisotropic and can reach thousands of cm2V−1s−1, which is close to that of monolayer black phosphorene. Furthermore, monolayer B2P6 has a direct band gap of 0.71eV (HES06) and effective optical absorption over both infrared and visible light range.The combined direct band gap, ultrahigh mobility and optical properties render monolayer B2P6 a broad application prospect in the field of nano-electronic devices and thin-film solar cells. Finally, calculations for OH adsorption show the possible synthesis of monolayer B2P6 in water environment and also potential applications such as electrocatalytic hydrogen production.

Microdosimetric modeling of the relative efficiency of the optical absorption of LiF:Mg,Ti (TLD-100) detectors exposed to 1H and 4He ions

The Microdosimetric d(z) Model was used in combination with simulated microdosimetric specific energy probability distributions for monoenergetic 1H and 4He ions (energy range = 0.1–1000 MeV/n, target size range = 1–2000 nm) to investigate the possibility of modeling the relative efficiency of two optical absorption (OA) bands (4.77 and 5.08 eV) in the OA energy spectra of LiF:Mg,Ti (TLD-100) thermoluminescent detectors. This work represents the first application of the model to a physical system other than thermally- and optically-stimulated luminescence.The 4.77 eV OA trap is populated by electrons liberated during irradiation and its experimental OA relative efficiency was successfully predicted using a target size of 9 nm in the simulations. On the other hand, the model was not able to reproduce the values of the experimentally-observed proton induced OA efficiency above unity for the 5.08 eV OA band associated with the F centers in LiF:Mg,Ti (electron occupied Fluoride vacancy). The F centers are initially present in the LiF lattice but are also created by the irradiation. Since the absence of defect creation by the irradiation is a necessary condition of the microdosimetric model, the discrepancy between the model results for the F band and the experimental data was not unexpected.The calculations for both OA bands are presented as a function of the particle energy and the simulated microdosimetric target size and will be useful in further applications of the model OA relative efficiencies for charged particles.

Molecular Design of Three-Dimensional Metal-Free A(NH4)X3 Perovskites for Photovoltaic Applications

The intense research activities on the hybrid organic–inorganic perovskites (HOIPs) have led to the greatly improved light absorbers for solar cells with high power conversion efficiency (PCE). However, it is still challenging to find an alternative lead-free perovskite to replace the organohalide lead perovskites to achieve high PCE. This is because both previous experimental and theoretical investigations have shown that the Pb2+ cations play a dominating role in contributing the desirable frontier electronic bands of the HOIPs for light absorbing. Recent advances in the chemical synthesis of three-dimensional (3D) metal-free perovskites, by replacing Pb2+ with NH4+, have markedly enriched the family of multifunctionalized perovskites (Ye et al., Science2018, 361, 151−15530002249). These metal-free perovskites possess the chemical formula of A(NH4)X3, where A is divalent organic cations and X denotes halogen atoms. Without involving transition-metal cations, the metal-free A(NH4)X3 perovskites can entail notably different frontier electronic band features from those of the organohalide lead perovskites. Indeed, the valence and conduction bands of A(NH4)X3 perovskites are mainly attributed by the halogen atoms and the divalent A2+ organic cations, respectively. Importantly, a linear relationship between the bandgaps of A(NH4)X3 perovskites and the lowest unoccupied molecular orbital energies of the A2+ cations is identified, suggesting that bandgaps can be tailored via molecular design, especially through a chemical modification of the A2+ cations. Our comprehensive computational study and molecular design predict a metal-free perovskite, namely, 6-ammonio-1-methyl-5-nitropyrimidin-1-ium-(NH4)I3, with a desirable bandgap of ∼1.74 eV and good optical absorption property, both being important requirements for photovoltaic applications. Moreover, the application of strain can further fine-tune the bandgap of this metal-free perovskite. Our proposed design principle not only offers chemical insights into the structure–property relationship of the multifunctional metal-free perovskites but also can facilitate the discovery of highly efficient alternative, lead-free perovskites for potential photovoltaic or optoelectronic applications.

Nanodiamond surface chemistry controls assembly of polypyrrole and generation of photovoltage

Nanoscale composite of detonation nanodiamond (DND) and polypyrrole (PPy) as a representative of organic light-harvesting polymers is explored for energy generation, using nanodiamond as an inorganic electron acceptor. We present a technology for the composite layer-by-layer synthesis that is suitable for solar cell fabrication. The formation, pronounced material interaction, and photovoltaic properties of DND-PPy composites are characterized down to nanoscale by atomic force microscopy, infrared spectroscopy, Kelvin probe, and electronic transport measurements. The data show that DNDs with different surface terminations (hydrogenated, oxidized, poly-functional) assemble PPy oligomers in different ways. This leads to composites with different optoelectronic properties. Tight material interaction results in significantly enhanced photovoltage and broadband (1–3.5 eV) optical absorption in DND/PPy composites compared to pristine materials. Combination of both oxygen and hydrogen functional groups on the nanodiamond surface appears to be the most favorable for the optoelectronic effects. Theoretical DFT calculations corroborate the experimental data. Test solar cells demonstrate the functionality of the concept.

Studies on CH3NH3PbI3 prepared by low-cost wet chemical technique

Methylammonium lead iodide (CH3NH3PbI3) has attracted tremendous attention in solar cell development due to its direct band gap, ~ 1.50 eV and high optical absorption coefficient, ~ 104–105 cm−1. CH3NH3PbI3 samples were synthesized using a low-cost wet chemical technique and studied their structural, optical, morphological, compositional and thermogravimetric properties. X-ray diffraction pattern of CH3NH3PbI3 revealed a single phase with tetragonal crystal structure; however, the orientation strongly depended on the heating of the sample. Raman spectra exhibited the bending and stretching bonds of Pb–I around 62 and 94 cm−1, respectively, in all samples. The chemical functions of C–H and N–H stretching, C–H and N–H bends, O–H stretch and C–H rock and twist bonds were detected in FTIR spectra. A systematic bond shift revealed higher wavenumber upon increasing the drying temperature demonstrates the growth of stable material. A sharp band edge observed around 1.5 eV confirmed the synthesis of less defective material suitable for high efficiency solar cell development. The content of Pb and I with a desired stoichiometry was determined with EDS measurement. Thermogravimetric analysis demonstrates no weight loss in CH3NH3PbI3 samples up to 220 °C.

Properties of AlN single crystals doped with Beryllium via high temperature diffusion

We report on co-doping of high-quality AlN single crystals by group II Beryllium acceptors by means of diffusion from the vapour phase at a temperature of 1850 °C. We discovered that Be is characterized by the high diffusion length, allowing one to produce Be co-doping of sub-mm-thick AlN wafers. We show that Be diffusion led to the quenching of the visible (VIS) 450 nm (2.75 eV) and deep ultraviolet (UV) 265 nm (4.7 eV) optical absorption bands with simultaneous induction of the absorption band peaked at 248 nm (5 eV). By means of electron paramagnetic resonance (EPR), we also found that the presence of Be impurities compensated the donor type paramagnetic centers. Correlation of the EPR data with the optical absorption allowed us to conclude that Be produced in the AlN via diffusion acted predominantly as an acceptor, inducing the shift of the Fermi level to the lower part of the AlN bandgap. This shift of the Fermi level results in recharging of the deep level defects in the AlN bandgap, which explains the observed quenching of the VIS and UV absorption bands.

Kinetic simulation of the effect of 3.6 eV and 4.2 eV photon excitation on the optical absorption energy spectrum of 137 Cs gamma irradiated LiF:Mg,Ti (TLD-100)

Abstract The effect of photon excitation on the optical absorption (OA) energy spectrum of LiF:Mg,Ti (TLD-100) following gamma ray irradiation to a dose-level of 2500 Gy are presented. Photon energies of 3.6 eV and 4.2 eV were chosen to selectively excite the trapping centers (TCs) associated with the 3.8 eV and 4.3 eV OA bands. The TCs associated with these OA bands are believed to give rise to the electron-only and electron-hole components of composite glow peak 5 in the thermoluminescence (TL) of TLD-100. Kinetic modeling is shown capable of simulating the changes in the optical density of the OA bands induced by the two photon energies with the same values of the recombination coefficients. This result is in contrast to a previous investigation which could not simulate the OA following both gamma irradiation and photon bleaching with the same values of the recombination coefficients. The results reported herein are consistent with : (i) the interpretation of the e-only and e-h charge configurations giving rise to the 3.8 eV and 4.3 eV OA bands and (ii) the possibility that the recombination processes following gamma irradiation occur not only following thermalization of the electron and hole charge carriers but at higher energies as well.

Ultrasensitive Photoresponsive Devices Based on Graphene/BiI3 van der Waals Epitaxial Heterostructures

AbstractIn recent years, bismuth iodide (BiI3), a layered metal halide semiconducting light absorber with a wide bandgap of ≈1.8 eV and strong optical absorption in the visible region, has received greater attention for photovoltaic applications. In this study, ultrasensitive visible‐light photodetectors with graphene/BiI3 vertical heterostructures are achieved by van der Waals epitaxies. The BiI3 films deposited on graphene show flatter morphologies and significantly better crystallinities than that of BiI3 films on SiO2 substrates, mainly due to weak van der Waals interactions at the graphene/BiI3 interface. Hybrid photodetectors with highly crystalline graphene/BiI3 heterostructures demonstrate an ultrahigh responsivity of 6 × 106 A W−1, shot‐noise‐limited detectivity of 7 × 1014 Jones, and a relatively short response time of ≈8 ms. Compared to most previously reported graphene‐based hybrid photodetectors, these devices have comparable photosensitivities but a faster response speed and lower operation voltage, which is quite promising for ultralow intensity visible‐light sensors. Moreover, the electronic structure and interfacial chemistry at the graphene/BiI3 heterojunctions are investigated using photoemission spectroscopy. The results give clear evidence that no chemical interactions occur between graphene and BiI3, resulting in the van der Waals epitaxial growth, and the measured band bending consistently illustrates that a photoinduced charge transfer occurs at the graphene/BiI3 interface.

(Invited) CdTe Based Photocathodes and Photoanodes for Photoelectrochemical Water Splitting Under Sunlight

CdTe based photocathodes and photoanodes for photoelectrochemical water splitting under sunlight Tsutomu Minegishi, Jin Su, Kazunari Domen Photoelectrochemical (PEC) and photocatalytic water splitting reactions are attractive means to obtain energy carriers such as hydrogen and Methylcyclohexane from only water and sunlight. For efficient solar hydrogen production, development of photoelectrodes with long absorption edge wavelength are an essential issue. Cadmium telluride (CdTe) has attractive properties for sunlight driven PEC water splitting such as narrow band gap of 1.5 eV and high optical absorption coefficients of ~104 cm-1 at wavelength shorter than 800 nm.1,2 Owing to these advantageous properties, polycrystalline thin film of CdTe has been employed as an absorber in thin film solar cells nowadays.1,2 Considering the features of CdTe, as mentioned above, the material should be suitable also as photoelectrodes for water splitting working under sunlight. In the present study, PEC properties of surface modified CdTe based photoanodes and photocathodes have been investigated, and overall water splitting by the PEC cell composed of CdTe based photoelectrodes was demonstrated. A CdTe photoelectrode directly prepared on FTO substrate by closed space sublimation (CSS) method showed clear cathodic photo-response in a phosphate buffer aqueous solution because of the p-type semiconducting property of prepared CdTe thin film. The photocurrent density and onset potential of CdTe photocathode were enhanced significantly by a surface modification with 80 nm-thick CdS layer and Pt. A post-deposition calcination of CdTe thin film with existence of CdCl2 as a sintering reagent and introduction of Au and Cu layer between the CdTe layer and FTO substrate resulted in extremely high incident photon-to-current conversion efficiencies (IPCEs) of >95%.3, 4 A Faradic efficiency of the photocathode during hydrogen evolution under simulated sunlight from water was confirmed to be almost unity.3 On the other hand, a CdTe based photoelectrode with CdS layer introduced at between the FTO substrate and CdTe worked as photoanode. It should be noted that a p-n junction formed at CdTe/CdS interface provides photovoltage in the photoanode. The photoanode showed photocurrent at above 0.2 VRHE, indicating that the photoanode possess a sufficient driving force for overall water splitting in bi-photoelectrode PEC cell with the CdTe photocathode. The PEC cell composed of the combination of the CdTe based photocathode and photoanode successfully produced hydrogen and oxygen from water under simulated sunlight without an external bias voltage. 5 1) J. S. Kaper and J. S. Prener, Acta Crystallogr., 7, 246-248 (1954). 2) K. Hashimoto and N.Kishimoto, J. Am. Chem. Soc., 1, 5521-5529 (2016). 3) J. Su, T. Minegishi, M. Katayama and K. Domen, J. Mater. Chem. A, 5, 4486 (2017). 4) J. Su, T. Minegishi and K. Domen, J. Mater. Chem. A, 5, 13154 (2017). 5) J. Su, T. Minegishi, Y. Kageshima, H. Kobayashi, T. Hisatomi, T. Higashi, M. Katayama, and K. Domen, J. Phys. Chem. Lett., 8, 5712–5717 (2017).

Effect of Zn/Sn molar ratio on the microstructural and optical properties of Cu2Zn1-xSnxS4 thin films prepared by spray pyrolysis technique

Quaternary kesterite Cu2ZnSnS4 (CZTS) compound is one of the most promising semiconductor materials consisting of abundant and eco-friendly elements for absorption layer in thin film solar cells. The effect of Zn/Sn ratio on Cu2Zn1-xSnxS4 (0≤x≤1) thin films were studied by deposited by varying molar volumes in the precursor solution of zinc and tin was carried out in proportion of (1-x) and x respectively onto soda lime glass substrates kept at 573 K by using chemical spray pyrolysis technique. The GIXRD pattern revealed that the films having composites of Cu2ZnSnS4, Cu2SnS3, Sn2S3, CuS and ZnS phases. The crystallinity and grain size were found to increase by increasing the x value and the preferential orientation along (103), (112), (108) and (111) direction corresponding to CZTS, Cu2SnS3, CuS, and ZnS phases respectively. Micro-Raman spectra exposed a prominent peak at 332 cm−1 corresponding to the CZTS phase. Atomic force microscopy was employed to study the grain size and roughness of the deposited thin films. The optical band gap was found to lie between 1.45 and 2.25 eV and average optical absorption coefficient was found to be greater than 105 cm−1. Hall measurements exhibited that all the deposited Cu2Zn1-xSnxS4 films were p type and the resistivity lies between 10.9×10−2Ωcm and 149.6× 10−2Ωcm.

Structural, Electronic, and Optical Properties of Cu2NiSnS4: A Combined Experimental and Theoretical Study toward Photovoltaic Applications

Earth-abundant quaternary chalcogenides are promising candidate materials for thin-film solar cells. Here we have synthesized Cu2NiSnS4 nanocrystals and thin films in a novel zincblende type cubic phase using a facile hot-injection method. The structural, electronic, and optical properties are studied using various experimental techniques, and the results are further corroborated within first-principles density functional theory based calculations. The estimated direct band gap ∼ 1.57 eV and high optical absorption coefficient ∼ 106 cm–1 indicate potential application in a low-cost thin-film solar cell. Further, the alignments for both conduction and valence bands are directly measured through cyclic voltametry. The 1.47 eV electrochemical gap and very small conduction band offset of −0.12 eV measured at the CNTS/CdS heterojunction are encouraging factors for the device. These results enable us to model carrier transport across the heterostructure interface. Finally, we have fabricated a CNTS solar cell device for the first time, with high open circuit voltage and fill factor. The results presented here should attract further studies.

Luminescence and Raman Detection of Molecular Cl2 and ClClO Molecules in Amorphous SiO2 Matrix

Chlorine is a common undesirable impurity in synthetic SiO2 glass for ultraviolet optics and optical fibers. It is usually incorporated into glass as bound Si–Cl groups or interstitial Cl2 molecules. We report a high-sensitivity detection of Cl2 in amorphous SiO2 (a-SiO2) by photoluminescence (PL) and also by Raman spectroscopy. The Cl2 PL emission band at 1.22 eV (1016 nm) appears at T < 160 K and shows a characteristic vibronic progression with separations ≈(520–540) cm–1 and an average lifetime of ≈5 ms at 13 K. Its excitation spectrum coincides with the shape of the 3.78 eV (328 nm) optical absorption band of Cl2 in a-SiO2, corresponding to the X → A 1Πu transition to repulsive excited state. Direct X → a singlet-to-triplet excitation was also observed at 2.33 eV (532 nm). Cl2 PL may serve as a sensitive and selective tool for monitoring Cl impurities and their reactions in a-SiO2. A Raman band of Cl2 is found at 546 cm–1. Cl2 photodissociation at energies up to 4.66 eV (266 nm) was not detected, poin...

Optical absorption spectrum and electronic structure of multiferroic hexagonal YMnO3 compound

Optical absorption (OA) spectrum and electronic structure of the hexagonal YMnO3 compound have been investigated by employment of the first-principles calculations based on density functional theory. The calculations were performed upon the ferroelectric structure of the YMnO3, by testing various approximations of the exchange-correlation effects between the Mn d-electrons and considering two types of magnetic ordering of the Mn sub-lattice: (1) collinear anti-ferromagnetic order of the G-type and (2) non-collinear antiferromagnetic order that correspond to magnetic space group P63. The results demonstrate that satisfactory agreement between the theoretical and the experimental OA spectrum can be achieved only if both non-collinear anti-ferromagnetic order of the Mn spins and strong correlations between the Mn d-electrons are taken into account. The latter is found to be best described by effective Hubbard parameter Ueff = 2.55 eV. The principal features of the OA spectrum are interpreted in terms of calculated electronic structure. It is found that the most important, threshold 1.6 eV OA peak is generated by electron transitions from strongly hybridized occupied Mn d- and its neighboring in-plane O p-states to unoccupied Mn d-states. It is also concluded that the electronic gap (calculated as ∼1.1 eV) should be smaller than the optical one (∼1.6 eV).

CuSbSe2 photovoltaic devices with 3% efficiency

Recent technical and commercial successes of existing thin-film solar cell technologies encourage the exploration of next-generation photovoltaic (PV) absorber materials. Of particular scientific interest are compounds that do not exhibit conventional tetrahedral semiconductor bonding, such as CuSbSe2. CuSbSe2 has a 1.1 eV optical absorption onset, a 105 cm−1 absorption coefficient, and a hole concentration of 1017 cm−3. Here, we demonstrate CuSbSe2 PV prototypes with efficiencies >3%, prepared by a self-regulated sputtering process using a conventional substrate device architecture. Bulk recombination, device engineering issues, and a nonideal CuSbSe2/CdS band offset likely limit the promising initial result.

Visible to deep ultraviolet range optical absorption of electron irradiated borosilicate glass**Project supported by the Fundamental Research Funds for the Central Universities of China (Grant No. lzujbky-2014-16).

To study the room-temperature stable defects induced by electron irradiation, commercial borosilicate glasses were irradiated by 1.2 MeV electrons and then ultraviolet (UV) optical absorption (OA) spectra were measured. Two characteristic bands were revealed before irradiation, and they were attributed to silicon dangling bond (E’-center) and Fe3+ species, respectively. The existence of Fe3+ was confirmed by electron paramagnetic resonance (EPR) measurements. After irradiation, the absorption spectra revealed irradiation-induced changes, while the content of E’-center did not change in the deep ultraviolet (DUV) region. The slightly reduced OA spectra at 4.9 eV was supposed to transform Fe3+ species to Fe2+ species and this transformation leads to the appearance of 4.3 eV OA band. By calculating intensity variation, the transformation of Fe was estimated to be about 5% and the optical absorption cross section of Fe2+ species is calculated to be 2.2 times larger than that of Fe3+ species. Peroxy linkage (POL, ≡Si–O–O–Si≡), which results in a 3.7 eV OA band, is speculated not to be from Si–O bond break but from Si–O–B bond, Si–O–Al bond, or Si–O–Na bond break. The co-presence defect with POL is probably responsible for 2.9-eV OA band.

Characterization of Photo-Induced Charge Transfer and Hot Carrier Relaxation Pathways in Spinel Cobalt Oxide (Co3O4)

The identities of photoexcited states in thin-film Co3O4 and the ultrafast carrier relaxation dynamics of Co3O4 are investigated with oxidation-state-specific pump–probe femtosecond core level spectroscopy. A thin-film sample is excited near the 2.8 eV optical absorption peak, and the resulting spectral changes at the 58.9 eV M2,3-edge of cobalt are probed in transient absorption with femtosecond high-order harmonic pulses generated by a Ti/sapphire laser. The initial transient state shows a significant 2 eV redshift in the absorption edge compared to the static ground state, which indicates a reduction of the cobalt valence charge. This is confirmed by a charge transfer multiplet spectral simulation, which finds the experimentally observed extreme ultraviolet (XUV) spectrum matches the specific O2–(2p) → Co3+(eg) charge-transfer transition, out of six possible excitation pathways involving Co3+ and Co2+ in the mixed-valence material. The initial transient state has a power-dependent amplitude decay (190 ± 10 fs at 13.2 mJ/cm2) together with a slight redshift in spectral shape (535 ± 33 fs), which are ascribed to hot carrier relaxation to the band edge. The faster amplitude decay is possibly due to a decrease of charge carrier density via an Auger mechanism, as the decay rate increases when more excitation fluence is used. This study takes advantage of the oxidation-state-specificity of time-resolved XUV spectroscopy, further establishing the method as a new approach to measure ultrafast charge carrier dynamics in condensed-phase systems.

Experimental Synthesis and Properties of Metastable CuNbN2 and Theoretical Extension to Other Ternary Copper Nitrides

Copper nitrides are defect-tolerant semiconductors with properties that are promising for solar energy conversion applications. Currently, there are few known ternary copper nitride materials. Here, we synthesized a previously unreported CuNbN2 using an ion-exchange reaction and subsequently determined its properties. CuNbN2 has a layered delafossite-type structure with NbN6 octahedra arranged in layers separated by linear N–Cu–N bonds. Experimental measurements and theoretical calculations agree that CuNbN2 has a 1.3–1.4 eV optical absorption threshold; theory also indicates that the lowest energy indirect band gap is 0.9 eV. The calculated CuNbN2 electron and hole effective masses are quite isotropic (mout /min = 1.3–2.1) and low (m = 0.3–0.7 me), as for the layered crystal structure. On the basis of these results, we propose a new lattice-matched delafossite tandem solar cell approach with Cu(Nb,Ta)N2 absorbers, p-type CuAlO2 contacts, and n-type ZnO contacts. Interestingly, first-principles calculations indicate that CuNbN2 is thermodynamically unstable with respect to disproportionation, yet the successful synthesis and potentially useful photovoltaic properties of this metastable material are possible. We theoretically examine a wide range of ternary copper nitrides for thermodynamic stability and optoelectronic properties with the goal of accessing their potential for solar energy conversion. It is found that the majority of these materials are thermodynamically unstable but that some of them should have properties that are promising for solar energy conversion applications and thus are worth experimental synthesis attempts.

Formation and annihilation of intrinsic defects induced by electronic excitation in high-purity crystalline SiO2

Formation and thermal annihilation of intrinsic defects in α-quartz were examined using high-purity samples, while minimizing the contributions of reactions involving metallic impurities. Electronic excitation with 60Co γ-rays was employed to avoid radiation-induced amorphization. The results clearly show that formation of oxygen vacancies (SiSi bonds) as a result of decomposition of regular SiOSi bonds (Frenkel process) is the dominant intrinsic defect process. Compared with amorphous SiO2, in α-quartz, the formation yield of SiSi bonds is an order of magnitude smaller, the 7.6 eV optical absorption band is less broadened, and their thermal annihilation is complete at a lower temperature, around the α–β quartz transition. In contrast, radiation-induced interstitial oxygen atoms practically do not form interstitial oxygen molecules.