Dominant Acceptor Research Articles

NaSbSe2 as a promising light-absorber semiconductor in solar cells: First-principles insights

NaSbSe2 has recently shown great potential to be a light-absorber semiconductor in thin-film solar cells. Our first-principles calculations show that NaSbSe2 has a quasi-direct bandgap (1.11 eV indirect vs 1.18 eV direct gap), which is beneficial for increasing the lifetime of minority carriers. The optical absorption coefficient is high (exceeding 10−4 cm−1 for visible light) because of the direct band-edge transition from the (Sb-5s/5p + Se-4p) valence band to (Sb-5p + Se-4p) conduction band. The formation of the dominant acceptor defects such as NaSb, VNa, and VSb makes it difficult to dope NaSbSe2 to n-type, and thus, only the intrinsic p-type conductivity has been observed. Se-rich conditions are found to produce high concentration of hole carriers and low concentration of recombination-center defects, so we propose that the Se-rich conditions should be adopted for fabricating high efficiency NaSbSe2 solar cells. Furthermore, the mixed-anion NaSb(S,Se)2 alloys are predicted to be highly miscible with a low formation enthalpy and a low miscibility temperature (below room temperature), and their bandgaps can be tuned almost linearly from 1.1 to 1.6 eV, covering the optimal bandgap range for single-junction solar cells. Therefore, we propose that alloying provides a promising method for optimizing the performance of NaSbSe2-based solar cells.

Metal-dependent anaerobic methane oxidation in marine sediment: Insights from marine settings and other systems.

Anaerobic oxidation of methane (AOM) plays a crucial role in controlling global methane emission. This is a microbial process that relies on the reduction of external electron acceptors such as sulfate, nitrate/nitrite, and transient metal ions. In marine settings, the dominant electron acceptor for AOM is sulfate, while other known electron acceptors are transient metal ions such as iron and manganese oxides. Despite the AOM process coupled with sulfate reduction being relatively well characterized, researches on metal-dependent AOM process are few, and no microorganism has to date been identified as being responsible for this reaction in natural marine environments. In this review, geochemical evidences of metal-dependent AOM from sediment cores in various marine environments are summarized. Studies have showed that iron and manganese are reduced in accordance with methane oxidation in seeps or diffusive profiles below the methanogenesis zone. The potential biochemical basis and mechanisms for metal-dependent AOM processes are here presented and discussed. Future research will shed light on the microbes involved in this process and also on the molecular basis of the electron transfer between these microbes and metals in natural marine environments.

Spectra of the D2O dimer in the O-D fundamental stretch region: Vibrational dependence of tunneling splittings and lifetimes.

The fundamental O-D stretch region (2600-2800 cm-1) of the fully deuterated water dimer (D2O)2 is studied using a pulsed supersonic slit jet source and a tunable optical parametric oscillator source. Relatively high spectral resolution (0.002 cm-1) enables all six dimer tunneling components to be observed, in most cases, for the acceptor asymmetric O-D stretch, the donor free O-D stretch, and the donor bound O-D stretch vibrations. The dominant acceptor switching tunneling splittings are observed to decrease moderately in the excited O-D stretch states, to roughly 75% of their ground state values, whereas the smaller donor-acceptor interchange splittings show more dramatic and irregular decreases. Excited state predissociation lifetimes, as determined from the observed line broadening, show large variations (0.2 ≤ τ ≤ 5 ns) depending on the vibrational state, K-value, and tunneling symmetry. Another very weak band is tentatively assigned to a combination mode involving an intramolecular O-D stretch plus an intermolecular twist overtone. Asymmetric O-D stretch bands of the mixed isotopologue dimers D2O-DOH and D2O-HOD are also observed and analyzed.

Correlation of donor-acceptor pair emission on the performance of GaN-based UV photodetector

High responsivity UV photodetector has been realized by fabricating single crystalline epitaxial GaN-based devices grown on silicon substrate by using molecular beam epitaxy system. The influence of trap states existing within the forbidden gap on GaN-based optoelectronic devices has been investigated. The quality and performance of the fabricated GaN based UV photodetector with distinct AlN buffer layer grown at low and high substrate temperature are substantiated. A detailed analysis reveals that high temperature buffer can yield improved crystalline quality of GaN with similar morphological properties as compared to buffer layer grown at low temperature. However, the distinct buffer layer influence the optical properties as the photoluminescence analysis explains that both the films possess superior band-to-band edge emission where GaN grown with high temperature AlN buffer consists of dominant acceptor defect states in comparison to GaN with low temperature AlN buffer layer. The existence of acceptor states has been accredited by the presence of Ga vacancy related states in the band gap. The fabricated devices yield high photoresponsivity of 2.1 and 1.5 A/W at 1 V from GaN films bearing low and high accepter defect states which explain a clear correlation of device performance with trap states within the band-gap region.

A High‐Performance Non‐Fullerene Acceptor Compatible with Polymers with Different Bandgaps for Efficient Organic Solar Cells

Owing to their good polymer compatibility, fullerene derivatives, such as PC61BM and PC71BM, have been the dominant electron acceptors to pair with various polymer donors in polymer solar cells (PSCs). The recent surge of non‐fullerene materials leads to several high‐performance molecular acceptors. Despite their high performance in a given polymer/acceptor system, the generality of these acceptors, i.e., their compatibility with different donor polymers remains uncertain. Here, a high‐performance small molecule acceptor (SMA), BTTIC, is designed and synthesized to combine with three polymers with different bandgaps, namely J71 (1.92 eV), PBDB‐T (1.80 eV), and PTB7‐Th (1.58 eV). Complementary absorption, compatible energy levels, and particularly the favorable morphologies between BTTIC and the three polymers enable high power conversion efficiencies (PCEs), which are 12.8%, 13.2%, and 10.4% for J71:BTTIC‐, PBDB‐T:BTTIC‐, and PTB7‐Th:BTTIC‐based PSCs, respectively, significantly higher than the PCEs of the fullerene‐ or other non‐fullerene‐based counterparts. Moreover, another famous p‐type polymer donor PffBT4T‐2OD, which shows poor solubility in chloroform and has not yet been studied in non‐fullerene PSCs, is also investigated. Processing by dissolving PffBT4T‐2OD and BTTIC in boiling chloroform enables PffBT4T‐2OD:BTTIC‐based PSCs with a PCE of 10.18%, which is significantly higher than that of PSCs (4.78%) before using boiling chloroform processing. The good compatibility of BTTIC with polymers that have either large, moderate, or small bandgaps makes it a promising non‐fullerene acceptor for next‐generation non‐fullerene PSCs.

Recent Progress in Fused-Ring Based Nonfullerene Acceptors for Polymer Solar Cells.

The progress of bulk-heterojunction (BHJ) polymer solar cells (PSCs) is closely related to the innovation of photoactive materials (donor and acceptor materials), interface engineering, and device optimization. Especially, the development of the photoactive materials dominates the research filed in the past decades. Photoactive materials are basically classified as p-type organic semiconductor donor (D) and an n-type organic semiconductor acceptor (A). In the past two decades, fullerene derivatives are the dominant acceptors for high efficiency PSCs. Nevertheless, the limited absorption and challenging structural tunability of fullerenes hinder further improve the efficiency of PSCs. Encouragingly, the recent progresses of fused-ring based A-D-A type nonfullerene acceptors exhibit great potential in enhancing the photovoltaic performance of devices, driving the power conversion efficiency to over 13%. Such kind of nonfullerene acceptors is usually based on indacenodithiophene (IDT) or its extending backbone core and end-caped with strong electron-withdrawing group. Owing to the strong push-pulling effects, the acceptors possess strong absorption in the visible-NIR region and low-lying HOMO (highest occupied molecular orbital) level, which can realize both high open-circuit voltage and short-circuit current density of the devices. Moreover, the photo-electronic and aggregative properties of the acceptors can be flexibly manipulated via structural design. Many strategies have been successfully employed to tune the energy levels, absorption features, and aggregation properties of the fused-ring based acceptors. In this review, we will summarize the recent progress in developing highly efficient fused-ring based nonfullerene acceptors. We will mainly focus our discussion on the correlating factors of molecular structures to their absorption, molecular energy levels, and photovoltaic performance. It is envisioned that an analysis of the relationship between molecular structures and photovoltaic properties would contribute to a better understanding of this kind of acceptors for high-efficiency PSCs.

Iron sulfide formation in young and rapidly-deposited permeable sands at the land-sea transition zone

Organic-poor, permeable quartz sands are often present at land-sea transition zones in coastal regions. Yet, the biogeochemical cycles of carbon, sulfur, and iron are not well studied here. The aim of this work was, therefore, to improve our understanding regarding the chemical processes in these prominent coastal sediments. A 10 m core was collected at a dune base of the barrier island Spiekeroog, Germany, for this purpose. Additionally, groundwater was sampled from a multi-level well for one year to record seasonal hydrochemical variations. Methods included the analyses of geochemical (total carbon, total inorganic carbon, reactive iron, total sulfur, reduced inorganic sulfur) and hydrochemical parameters (field parameters, major ions, DOC, and molecular compositions of DOM), as well as stable sulfur isotopes (δ34S-sulfate, -sulfide, -total reduced inorganic sulfur). Moreover, optically stimulated luminescence (OSL) dating was applied. Results show that the core sediments are very young (<500 a) and were rapidly deposited. They are characterized by remarkably low contents of organic carbon (<0.1% dw.), reactive iron (~10 mmol/kg), and iron sulfides (<3 mmol/kg). Groundwater salinities were low in the top core sediments and increased at depth during most times of the year. However, the sampling site is subject to (seasonally) varying salinities, which could be linked to the biogeochemical cycles. For instance, the infiltration of seawater-derived labile DOM during inundation events drives microbial respiration besides sedimentary organic matter. Oxygen and nitrate were the dominant electron acceptors for the decomposition of organic matter in near-surface groundwater, while sulfate reduction was constrained to the lower brackish sediments. Here, authigenic pyrite formation was inferred based on the detection of dissolved sulfide, intact pyrite framboids, and matching stable sulfur isotope signatures of dissolved and solid sulfides. We concluded that the extremely low organic carbon contents limit pyrite formation in the organic-poor, permeable quartz sands.

Doping properties of cadmium-rich arsenic-doped CdTe single crystals: Evidence of metastable AX behavior

Cd-rich composition and group-V element doping are of interest for simultaneously maximizing the hole concentration and minority carrier lifetime in CdTe, but the critical details concerning point defects are not yet fully established. Herein, we report on the properties of arsenic doped CdTe single crystals grown from Cd solvent by the travelling heater method. The photoluminescence spectra and activation energy of 74 ± 2 meV derived from the temperature-dependent Hall effect are consistent with AsTe as the dominant acceptor. Doping in the 1016 to 1017/cm3 range is achieved for measured As concentrations between 1016 and 1020/cm3 with the highest doping efficiency of 40% occurring near 1017 As/cm3. We observe persistent photoconductivity, a hallmark of light-induced metastable configuration changes consistent with AX behavior. Additionally, quenching experiments reveal at least two mechanisms of increased p-type doping in the dark, one decaying over 2–3 weeks and the other persisting for at least 2 months. These results provide essential insights for the application of As-doped CdTe in thin film solar cells.

Imaging and Manipulating Energy Transfer Among Quantum Dots at Individual Dot Resolution

Many processes of interest in quantum dots involve charge or energy transfer from one dot to another. Energy transfer in films of quantum dots as well as between linked quantum dots has been demonstrated by luminescence shift, and the ultrafast time-dependence of energy transfer processes has been resolved. Bandgap variation among dots (energy disorder) and dot separation are known to play an important role in how energy diffuses. Thus, it would be very useful if energy transfer could be visualized directly on a dot-by-dot basis among small clusters or within films of quantum dots. To that effect, we report single molecule optical absorption detected by scanning tunneling microscopy (SMA-STM) to image energy pooling from donor into acceptor dots on a dot-by-dot basis. We show that we can manipulate groups of quantum dots by pruning away the dominant acceptor dot, and switching the energy transfer path to a different acceptor dot. Our experimental data agrees well with a simple Monte Carlo lattice model of energy transfer, similar to models in the literature, in which excitation energy is transferred preferentially from dots with a larger bandgap to dots with a smaller bandgap.

A first-principles study of carbon-related energy levels in GaN. II. Complexes formed by carbon and hydrogen, silicon or oxygen

This work presents an in-depth investigation of the properties of complexes composed of hydrogen, silicon, or oxygen with carbon, which are the major unintentional impurities in undoped GaN. This manuscript is a complement to our previous work on carbon–carbon and carbon-vacancy complexes. We have employed a first-principles method using Heyd-Scuseria-Ernzerhof hybrid functionals within the framework of generalized Kohn-Sham density functional theory. Two H–C, four Si–C, and five O–C complexes in different charge states have been considered. After full geometry relaxations, formation energies, binding energies, and both thermal and optical transition levels were obtained. The calculated energy levels have been systematically compared with the experimentally observed carbon related trap levels. Furthermore, we computed vibrational frequencies for selected defect complexes and defect concentrations were estimated in the low, mid, and high carbon doping scenarios considering two different cases where electrically active defects: (a) only carbon and vacancies and (b) not only carbon and vacancies but also hydrogen, silicon, and oxygen. We confirmed that CN is a dominant acceptor in GaN. In addition to it, a substantial amount of SiGa–CN complex exists in a neutral form. This complex is a likely candidate for the unknown form of carbon observed in undoped n-type GaN.

Influence of the copper content on the optical properties of CZTSe thin films

We present an optical spectroscopy study of Cu2ZnSnSe4 (CZTSe) thin films deposited on Mo/glass substrates. The [Cu]/[Zn+Sn] ratio in these films varies from nearly stoichiometric to strongly Cu deficient and Zn rich. Increasing Cu deficiency and Zn excess widens the bandgap Eg, determined using photoluminescence excitation (PLE) at 4.2K, from 0.99eV to 1.03eV and blue shifts the dominant band in the photoluminescence (PL) spectra from 0.83eV to 0.95eV. The PL spectra of the near stoichiometric film reveal two bands: a dominant band centred at 0.83eV and a lower intensity one at 0.93eV. The temperature and excitation intensity dependence of the PL spectra help to identify the recombination mechanisms of the observed emission bands as free-to-bound: recombination of free electrons with holes localised at acceptors affected by randomly distributed potential fluctuations. Both the mean depth of such fluctuations, determined by analysing the shape of the dominant bands, and the broadening energy, estimated from the PLE spectra, become smaller with increasing Cu deficiency and Zn excess which also widens Eg due to an improved ordering of the Cu/Zn atoms. These changes in the elemental composition induce a significant blue shift of the PL bands exceeding the Eg widening. This is attributed to a change of the dominant acceptor for a shallow one, and is beneficial for the solar cell performance. Film regions with a higher degree of Cu/Zn ordering are present in the near stoichiometric film generating the second PL band at 0.93eV.

Feedbacks among O2 and CO2 in deep soil gas, oxidation of ferrous minerals, and fractures: A hypothesis for steady-state regolith thickness

O2 and CO2, the two essential reactants in weathering along with water and minerals, are important in deep regolith development because they diffuse to weathering fronts at depth. We monitored the dynamics of these gas concentrations in the hand-augerable zone on three ridgetops—one on granite and two on diabase—in Virginia (VA) and Pennsylvania (PA), U.S.A. and related the gas chemistry to regolith development. The VA granite and the PA diabase protoliths were more deeply weathered than the VA diabase. We attribute this to high protolith fracture density. The pO2 and pCO2 measurements of these more fractured sites displayed the characteristics of aerobic respiration year round. In contrast, the relation of pO2 versus pCO2 on the more massive VA diabase is consistent with seasonal changes in the dominant electron acceptor from O2 to Fe(III), likely regulated by the expansion/contraction of nontronite in the soil BC horizon. These observations suggest that the fracture density is a first order control on deep regolith gas chemistry. However, fractures can be present in protolith but also can be caused by oxidation of ferrous minerals. We propose that subsurface pO2 and weathering-induced fracturing can create positive feedbacks in some lithologies that cause regolith to thicken while nonetheless maintaining aerobic respiration at depth. In contrast, in the absence of weathering-induced fracturing and depletion of pO2, a negative feedback that may be modulated by soil micro-biota ultimately results in thin regolith. These feedbacks may have been important in weathering systems over much of earth's history.

Thermal stability of the prominent compensating (AlZn–VZn) center in ZnO

Electron paramagnetic resonance spectroscopy is used to investigate the thermal stability of the Aluminum–Zinc vacancy (AlZn–VZn) complex created in bulk single crystalline ZnO by room temperature electron irradiation with an energy of 1.2 MeV. Two different stages in the annealing process at 160 and 250 °C with apparent activation energies of EA1 = 1.5 ± 0.2 eV and EA2 = 1.9 ± 0.2 eV, respectively, are observed. The second stage leads to the complete annealing out of the (AlZn–VZn) complex and is accompanied by restoration of the concentration of the AlZn shallow donor centers to its initial value in as-grown (i.e., not irradiated) material. The obtained results prove that the (AlZn–VZn) complex is the dominant acceptor responsible for compensation of n-type-dopants in the studied Al-containing ZnO samples.

Pivotal Role and Regulation of Proton Transfer in Water Oxidation on Hematite Photoanodes.

Hematite is a promising material for solar water splitting; however, high efficiency remains elusive because of the kinetic limitations of interfacial charge transfer. Here, we demonstrate the pivotal role of proton transfer in water oxidation on hematite photoanodes using photoelectrochemical (PEC) characterization, the H/D kinetic isotope effect (KIE), and electrochemical impedance spectroscopy (EIS). We observed a concerted proton-electron transfer (CPET) characteristic for the rate-determining interfacial hole transfer, where electron transfer (ET) from molecular water to a surface-trapped hole was accompanied by proton transfer (PT) to a solvent water molecule, demonstrating a substantial KIE (∼3.5). The temperature dependency of KIE revealed a highly flexible proton transfer channel along the hydrogen bond at the hematite/electrolyte interface. A mechanistic transition in the rate-determining step from CPET to ET occurred after OH(-) became the dominant hole acceptor. We further modified the proton-electron transfer sequence with appropriate proton acceptors (buffer bases) and achieved a greater than 4-fold increase in the PEC water oxidation efficiency on a hematite photoanode.

The impact of Cu on recombination in high voltage CdTe solar cells

Using photoluminescence spectroscopy, we construct a recombination model for state-of-the-art CdTe solar cells doped with Cu. We observe that Cu on Cd sites form a dominant acceptor state about 150 meV from the valence band. Although it is intuitive that this state can increase hole density, we also find that this relatively shallow dopant can also limit lifetime. Consequently, CdTe solar cells doped with Cu could have a lifetime limitation inversely proportional to the hole concentration.

Electronic defect study on low temperature processed Cu(In,Ga)Se2 thin-film solar cells and the influence of an Sb layer

A way to lower the manufacturing cost of Cu(In,Ga)Se2 (CIGS) thin-film solar cells is to use flexible polymer substrates instead of rigid glass. Because such substrates require lower temperature during absorber deposition, the grain growth of the absorber layer can be hindered which leads to a lower cell performance. Partial compensation of this efficiency loss might be accomplished by growing the absorber in the presence of Sb, which is reported to promote grain growth. In this work CIGS solar cells, deposited on glass substrates, at a reduced substrate temperature with a thin Sb layer (7, 12 nm) on top of the Mo contact are investigated. The diffusion profile of Sb is measured with plasma profiling time of flight mass spectrometry. The beneficial effect of Sb on efficiency and grain size is shown in quantum efficiency measurements and with scanning electron microscopy, respectively. Electric spectroscopy is used to explore the possible effects on the defect structure, more in particular on the dominant shallow acceptor. Admittance spectra exhibit a capacitance step to the geometric capacitance plateau at low temperature (5–60 K). Analyzing this capacitance step, we obtained a good estimate of the activation energy of the intrinsic defects that provide the p-type conductivity of the CIGS absorber. The measurements did not show a change in the nature of the dominant acceptor upon Sb treatment.

Effect of limited supply of assimilates on the relationships between their sources and acceptors

Sink activity in young sunflower and bean plants was determined mainly by their growth rate. The organ with higher RGR (stem in sunflower and apical part with expanded trifoliate leaf- in bean plant) was the dominant acceptor even in conditions of limited supply of assimilates.Specific activity of root - donor of phosphorus, depends on the roofs metabolic and physiological activity only in the case, when supply of P to the aerial parts is not a factor limiting photosynthesis. If the activity of the whole root system is relatively low (like in preshaded series of sunflower) the photosynthetic compensation observed in plants replaced into better conditions coincides with compensation of P-absorption (both calculated per g of dry matter). It may explain the existence of a functional balance between shoot and root activity in the case of a changed proportion between their sizes.

Effect of Nitrogen implantation on the optical characteristics of Zn0.85Mg0.15O thin film at low temperature

ABSTRACTThe importance of ZnxMg1-xO is increasing day by day because of its wider bandgap than ZnO. This ternary semiconductor finds its application in the fields of optoelectronics, spintronics, superlattices due to its unique blueshifted UV-luminescent property. n- to p-type conduction which is the motive of the project can be achieved with increasing Mg content in ZnMgO. The optical characteristics of the nitrogen doped ZnxMg1-xO (x=0.85) grown on 2 inch Si &lt;100&gt;wafer by RF sputtering are studied and analyzed thoroughly using low temperature (15K) photoluminescence measurements. Nitrogen implantation was carried out by Plasma immersion Ion Implantation technique on the sample. Rapid Thermal Process was employed to remove defects resulting from implantation. The samples were annealed at 700°C, 800°C, 900°C, and 1000o C for 10 seconds in an oxygen ambient. Photoluminescence (PL) measurements were performed at low temperature (15K) which exhibited acceptor-bound-exciton peak (A°X) and donor-bound-acceptor pair (DAP) at 3.336 eV and 3.236 eV respectively. At 3.364 eV, S peak was found for the sample annealed at 800°C after implantation. This peak was attributed to the existence of ZnO-like composition. Localized and de-localized exciton peaks were found around 3.42 and 3.45 eV respectively. This result is very important because though dominant acceptor peak was not found but proper optimization of the parameters can lead to p-type ZnMgO which is the main motive of this project.

Density and energy level of a deep-level Mg acceptor in 4H-SiC

Reliably determining the densities and energy levels of deep-level dominant acceptors in heavily doped wide-band-gap semiconductors has been a topic of recent discussion. In these discussions, the focus is on both Hall scattering factors for holes and distribution functions for acceptors. Mg acceptor levels in 4H-SiC seem to be deep, and so here the electrical properties of Mg-implanted 4H-SiC layers are studied by measuring Hall effects. The obtained Hall scattering factors are not reliable because they drop to less than 0.5 at high measurement temperatures. Moreover, the Fermi–Dirac distribution function is unsuitable for examining Mg acceptors because the obtained acceptor density is much higher than the concentration of implanted Mg atoms. However, by using a distribution function that includes the influence of the excited states of a deep-level acceptor, the density and energy level of Mg acceptors can be reliably determined.

GGA + U study of native point defects in ZnRh2O4

ZnRh2O4 spinel is the one of the most promising transparent conducting oxides for applications in optoelectronic technology. Energy levels and formation energies of native point defects, i.e. vacancies (V), interstitials (I), and cation antisites in ZnRh2O4 were analysed. Generalized gradient approximation (GGA) was supplemented by the +U on-site corrections imposed on d(Rh) and p(O) states. The inclusion of the pronounced distortions of the anion sublattice was necessary to obtain the correct band gap. U was treated as a free parameter, which allowed for the systematic study of the U-induced changes of the defect states. A diagram of the thermodynamic phase stability of ZnRh2O4 was obtained. ZnRh is the dominant acceptor that can be responsible for the observed p-type conductivity in ZnRh2O4. The low formation energy of ZnRh can make the intentional n-doping difficult. In the O-rich conditions the second important acceptor is VZn. The two dominant donors that can compensate ZnRh in Zn-rich and O-rich conditions are VO and RhZn, respectively. Growth conditions leading to the lowest concentrations of native defects were identified. Due to the pronounced occupancy dependence of the +U term, VO and RhZn are ‘negative-Ueff’ centres.