Carrier Recombination Centers Research Articles

Optimized phosphorus diffusion process and performance improvement of c-Si solar cell by eliminating SiP precipitates in the emitter

Phosphorus diffusion is the most common way to form the emitter for p-type crystalline silicon (c-Si) based solar cells. The emitter region is usually known as dead layer, which may result in the band gap narrowing and higher carrier recombination. In this work we have demonstrated that the SiP precipitates are usually formed in the emitter of c-Si during phosphorus diffusion process, which can become the recombination centers for the minority carriers and therefore degrade the performance of c-Si solar cells. By a post-anneal treatment or a higher temperature diffusion with lower concentration of phosphorus source, the SiP precipitates in the emitter region can be effectively eliminated. As a result, the corresponding solar cell based on the modified phosphorus diffusion process exhibits a higher conversion efficiency than the conventional one by a value of 0.2%.

Enhanced Valley Splitting of Transition-Metal Dichalcogenide by Vacancies in Robust Ferromagnetic Insulating Chromium Trihalides.

Recently, single-layer CrI3, a member of the chromium trihalides CrX3 (where X = Cl, Br, or I), has been exfoliated and experimentally demonstrated as an atomically thin material suitable for two-dimensional spintronics. Valley splitting due to the magnetic proximity effect has been demonstrated in a WSe2/CrI3 van der Waals heterojunction. However, the understanding of the mechanisms behind the favorable performance of CrI3 is still limited. Here, we systematically study the carrier mobility and the intrinsic point defects in CrX3 and assess their influence on valley splitting in WSe2/CrI3 by first-principles calculations. The flat-band nature induces extremely large carrier mass and ultralow carrier mobility. In addition, intrinsic point defects-localized states in the middle of the band gap-show deep transition energy levels and act as carrier recombination centers, further lowering the carrier mobility. Moreover, vacancies in CrI3 can enhance ferromagnetism and valley splitting in a WSe2/CrI3 heterojunction, proving that chromium trihalides are excellent ferromagnetic insulators for spintronic and valleytronic applications.

Convenient fabrication of BiOBr ultrathin nanosheets with rich oxygen vacancies for photocatalytic selective oxidation of secondary amines

Photocatalytic oxidation has been widely employed in organic synthesis, by virtue of the green, mild and simple reaction conditions as well as high selectivity. Introducing oxygen vacancies (OVs) with proper concentrations into the photocatalysts has been proven as an effective strategy to boost the catalytic performances. However, the currently used treatment method under high temperature at reducing atmosphere inevitably introduces a large number of OVs at the interior of the catalyst and serving as the recombination centers of carriers. To address this issue, here we develop a facile solvothermal process to prepare ultrathin BiOBr nanosheets with rich surface OVs. This method effectively decreases the bulk of the material and the ratio of interior OVs, rendering most of the OVs exposed on the surfaces which act as exposed catalytic sites and enhance the separation of carriers, therefore significantly elevates the photocatalytic performances. For the photo-oxidation reaction of secondary amines, under the conditions of visible light, ambient temperature and atmosphere, the BiOBr nanosheets featuring rich surface OVs deliver a doubled conversion compared to those with low OV concentrations, and a high selectivity of 99%, a high stability as the performance shows no reduction after 5 times of circular reaction.

Direct formed tri-iodide ions stabilizing colloidal precursor solution and promoting the reproducibility of perovskite solar cells by solution process

Solution process is the most widely used method to prepare perovskite films for efficient solar cells due to its facile fabrication and low-cost. However, the aged perovskite precursor solutions usually suffer from degradation, which impedes the reproducibility of the solution. In this paper, we demonstrate the direct formed tri-iodide ions stabilizing colloidal precursor solution for efficient halide perovskite solar cells. The added I2 molecules dissolved in perovskite precursor sustain the dynamic balance between I− ions and I3− ions by the reversible reaction: I− + I2 ⇌ I3−, which effectively stabilizes the whole I− ion concentration in perovskite solution and prevents the colloidal particles from aggregation. The resulting perovskite films with better crystallization have less bulk defects that act as carrier recombination centers. As a result, the devices using the aged solution with the additional tri-iodide ions show the less efficiency drop from ∼19% to ∼16% with the increased aging time up to 15 days, compared with the devices using the as-prepared precursor solution degrading from ∼18% to ∼6%. Our results represent an important advance in the stabilization of perovskite colloidal precursor and promote the reproducibility of perovskite solar cells by solution process.

Applying BaTiO3-coated TiO2 core–shell nanoparticles films as scaffold layers to optimize interfaces for better-performing perovskite solar cells

In this paper, we replaced mesoporous TiO2 nanoparticles scaffold layers by BaTiO3-coated TiO2 core–shell nanoparticles films which obtained by treating pure mesoporous TiO2 layers with 1.0 wt% barium nitrate solution, successfully realized the aim of optimizing interfaces bonding at TiO2/CH3NH3PbI3. Ultrathin BaTiO3 shell layer can combine better with CH3NH3PbI3 layer so as to reduce the existence of carrier recombination centers. Moreover, better optical absorption and larger fill factor were obtained in this manner by the reason of larger CH3NH3PbI3 grain size and fewer crystal boundaries. Furthermore, photoluminescence spectra and electrochemical impedance spectroscopy verified that our core–shell scaffold material contributes to accelerate carrier separation and retard carrier recombination. As a result, average power conversion efficiency enhanced from 11.20 to 13.76% under ambient conditions, which realized almost a quarter improvement than the devices based on pure mesoporous TiO2 layers. Such results have a certain guiding effect on solving interface defects and carrier recombination.

Effect of oxygen precipitation through annealing process on lifetime degradation by Czochralski-Si crystal growth conditions

In this study, the effects of oxygen precipitation on the minority carrier lifetime were evaluated in conjunction with crystal growth conditions. It was revealed that oxygen precipitates and dislocations formation are suppressed and lifetime deterioration is suppressed in Czochralski (Cz)-Si, which is maintained at high temperature, followed by rapid cooling during crystal growth. In addition, the interface at the oxygen precipitate/Si matrix and dislocations were the dominant carrier recombination centers. From the above, it was suggested that controlling oxygen precipitation by crystal growth conditions can contribute to high-efficiency solar cells.

Ageing of kesterite solar cells 2: Impact on photocurrent generation

As yet results of ageing experiments of Kesterite solar cells have not been published. However, at least in crystalsol detailed investigations of monograin-based solar cells have been performed proving very different stabilities depending on the exact composition of the Kesterites. Completely stable solar cells even without any sealing have been demonstrated internally. However in this paper we report on ageing effects observed for internally called “highly unstable” Cu2ZnSnS4 (CZTS) solar cells aged for 8.5 and 95.5 h at 105 °C in air, already also reported in part 1 of these investigations. During these times the initial efficiency of 5.4% decreased by 76% and another 88%, respectively. The photocurrent generation was investigated in detail by light beam induced current (LBIC) measurements of the front side and electron beam induced current (EBIC) measurements of cross sections of the CZTS/CdS/ZnO active junctions. Besides a pronounced decrease of the LBIC and the EBIC signal also a shift of the EBIC signal much further into the Kesterite material was observed, which can be explained by the in part 1 already found diffusion of copper from the surface of CZTS into the CdS buffer layer acting finally as a strong charge carrier recombination center. Thereby the EBIC signal first observed mainly close to the interface has vanished, only a small EBIC signal in a depth of around 400 nm still remains visible, a depth, from which only minority carriers can still reach this main recombination center in the interface.

Study of gamma-ray radiation effects on the passivation properties of atomic layer deposited Al2O3 on silicon using deep-level transient spectroscopy

Aluminum oxide (Al2O3) has emerged as a potential dielectric material and exhibited an excellent passivation property on silicon surface. However, such oxide layer is extremely sensitive to γ-ray irradiation. In this work, deep-level transient spectroscopy has been applied to study the influence of γ-ray irradiation on the passivation properties of atomic layer deposited Al2O3 on silicon. It is shown that γ-ray irradiation leads to a significant increase of interface state density (Dit). Meanwhile, its energy distribution is broadened and shifts deeper with respect to the top of valence band, and therefore evolves into more efficient recombination centers for carriers. Besides, capacitance–voltage (C–V) curves shows a progressive shift toward the negative voltages with increased radiation doses. This indicates the hole trapping in Al2O3, which can neutralize the negatively charged defects and therefore degrade its field-effect passivation. Hence, the passivation quality of Al2O3 on silicon deteriorates significantly after γ-ray radiation.

Employing tetraethyl orthosilicate additive to enhance trap passivation of planar perovskite solar cells

In this work, trap passivation can be enhanced by introducing the controlled tetraethyl orthosilicate (TEOS) additive into the perovskite precursor. The effect of different concentrations of TEOS additive on interfacial modification and carrier recombination process has been investigated systematically. Silicon dioxide (SiO2), the product of TEOS additive, can precipitate at the grain boundary after the formation of perovskite film, resulting in trap passivation at perovskite interface. The recombination kinetics manifests that a small amount of TEOS additive can significantly prolong carrier lifetime, due to the reduction of trap-states in the perovskite interface, which are supposed to be the carrier recombination centers. Using a typical planar structure of the cells prepared under fully open air condition, the steady-state efficiency of our best-performing devices has improved from 15.96 to 18.38%.

First-Principles Modeling of Defects in Lead Halide Perovskites: Best Practices and Open Issues

Lead halide perovskites are outstanding materials, showing long lifetimes of photogenerated carriers that induce high conversion efficiencies in solar cells and light-emitting devices. Native defects can severely limit the efficiency of optoelectronic devices by acting as carrier recombination centers. The study of defects in lead halide perovskites thus assumes a prominent role in further advancing the exploitation of this class of materials. The perovskites’ defect chemistry has been mainly investigated by computational methods based on density functional theory. The complex electronic structure of perovskites, however, poses challenges to the accuracy of such calculations. In this work, we review the state of the art of defects calculations in lead halide perovskites, discussing the major technical issues commonly encountered and what we believe to be the best practices. By keeping as a test case the prototype MAPbI3 compound, we discuss the impact of the exchange–correlation functional on the electron...

Investigation of Si Dendrites by Electron-Beam-Induced Current

This paper reports on electron-beam-induced current (EBIC) characterization of special multicrystalline Si ingot by dendritic growth under high undercooling. Grain boundaries (GBs), dislocations, and their interaction with carbon related precipitates were investigated. The difference between grains from dendrite and non-dendrite growth was compared. In dendrite grains, parallel twins were frequently found. In non-dendrite grains, irregular GBs of various characters co-existed. Both parallel twins and irregular GBs exhibited dark EBIC contrast at room temperature, indicating the presence of minority carrier recombination centers due to impurity contamination. However, sometimes in non-dendrite grains GBs were visualized with bright EBIC contrast with enhanced collection of charge carriers. The origin of the abnormal bright EBIC contrast was explored and it turned out to be SiC related precipitates, which made GBs conduction channels for electron transport.

Tunable whole visible region color emission, enhancing emission intensity and persistent performance of a self-activated phosphor:Na2CaSn2Ge3O12

The composition and tunable luminescence properties of garnet phosphors are flexible because of their unique A3B2C3O12 structure. Garnet phosphors with tailored luminescence properties can be designed by varying in the A, B, and C cation sublattices. In this research, the self-activated long-persistent phosphor (LPP) Na2CaSn2Ge3O12 (NCSG) was successfully synthesized by a solid-state method. A series of dopants (Mn2+, Pr3+, Tb3+, and Dy3+) were selected as single-component emitters to tailor the emission color of the NCSG phosphor. NCSG emits in yellow at ~ 560 nm and in the near-infrared at ~ 790 nm with long-persistent luminescence (PersL). By doping with these ions, the PersL of the NCSG was significantly improved, and various colors covering the entire visible region from violet to the near-infrared were obtained. The trap types and depths were examined by electron paramagnetic resonance (EPR) and the thermo-luminescence (TL) glow curves. The EPR signals revealed that oxygen vacancies are the trap centers. The TL glow curves showed that the intrinsic and extrinsic luminescence centers both act as charge carrier recombination centers. Furthermore, the PersL mechanism of the NCSG and NCSG:R (R = Mn2+, Pr3+, Tb3+ and Dy3+) phosphors was discussed in detail. The strategies employed in this work can be an effective way to fabricate new multi-wavelength, garnet-based LPPs with potential applications.

Electrically Benign Defect Behavior in Zinc Tin Nitride Revealed from First Principles

Zinc tin nitride (ZnSnN2) is attracting growing interest as a non-toxic and earth-abundant photoabsorber for thin-film photovoltaics. Carrier transport in ZnSnN2 and consequently cell performance are strongly affected by point defects with deep levels acting as carrier recombination centers. In this study, the point defects in ZnSnN2 are revisited by careful first-principles modeling based on recent experimental and theoretical findings. It is shown that ZnSnN2 does not have low-energy defects with deep levels, in contrast to previously reported results. Therefore, ZnSnN2 is more promising as a photoabsorber material than formerly considered.

Ar+ Ion Implantation Induced Surface, Structural and Optical Modifications in Cadmium Selenide Thin Films

Among the groups II–VI semiconductors, cadmium selenide (CdSe) is a direct and wide band gap semiconductor. In comparison to Si films, CdSe semiconducting thin film have superior carrier mobility and, therefore, can be employed in solar cells as an absorber layer. In the present work, firstly thin films of CdSe were electrodeposited on the indium doped tin oxide (ITO) substrates. For the deposition, a bath containing cadmium sulfate and selenium dioxide solution was used. Prior to electrodeposition, ITO substrates were cleaned by an ultrasonicator using de-ionized water and acetone to remove impurities. After successful deposition of the film of thickness 120 nm, these were characterized by UV–Visible spectrophotometer, RF-530 Spectrofluorophotometer and four-probe setup with programmable current–voltage (I–V) source meter. The as-deposited CdSe thin films were implanted with 80 keV Ar+ ions at different fluencies i.e. 1 × 1015 ions/cm2, 5 × 1015 ions/cm2, 1 × 1016 ions/cm2 and 5 × 1016 ions/cm2. The effect of Ar+ ion implantation on the surface properties of CdSe thin films is studied with respect to lattice disorder. Defects created on implantation may give rise to new energy levels within the energy band gap that may serve as the trapping or recombination centers for the charge carriers depending upon the positions of the new energy levels. This alters the properties of the material and hence the characteristics of the device based upon the material.

Accelerated carrier recombination by grain boundary/edge defects in MBE grown transition metal dichalcogenides

Defect-carrier interaction in transition metal dichalcogenides (TMDs) plays important roles in carrier relaxation dynamics and carrier transport, which determines the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in molecular beam epitaxy (MBE)-grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, the carrier recombination rate in MBE-grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE-grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also observed coherent acoustic phonons in both exfoliated and MBE-grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with the previously reported result of theoretical calculation. Our findings provide a useful reference for the fundamental parameters: carrier lifetime and sound velocity and reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics.

Development of a New Fabrication Route for High Quality Visible-Light-Driven Photocatalysts; Atmosphere Controlled Flux Growth for Oxynitride and Nitride Crystals

Photocatalysts and related materials have been attracted a number of research interests due to their various application such as decomposition of organic substances, superhydrophilic surfaces, dye-sensitized solar cells, and hydrogen production by water splitting. In particular, water-splitting by photocatalysts have been investigated because of expectation to supply clean and recyclable hydrogen energy. Generally, photocalysts, as represented by TiO2, are activated by only ultraviolet light illumination due to their wide band gap. Although these UV-light-driven photocatalysts can split water in a proper condition, the solar energy conversion efficiency is rather low because UV light energy is just several percent in total energy of sun light on the earth, and visible light accounts for almost half of the solar energy. From the viewpoint of increase the efficiency and industrial application of solar hydrogen production, visible-light-driven photocatalysts have intensively attracted research interests. In particular, oxynitride and nitride semiconductor photocatalysts are one of promising materials for construction of photocatalytic water splitting systems. Although visible-light-driven photocatalysts with sufficiently small band gap, an appropriate band gap position for water splitting, and stability during the reaction have been developed, it is still required to improve the quantum yield. Generally, lattice defects are not suitable for photocatalysts with high performance because they would become recombination centers of excited carriers. Flux method is a liquid phase crystal growth technique which uses molten metals or molten metal salts as solvents. Flux method can afford binary or multicomponent high-quality crystals at lower temperatures below their melting points. It is interesting that flux-grown crystals usually have idiomorphic shapes with well-developed facets, which represents their high crystallinity. In addition, it provides simple, low-cost and environmentally-benign pathway compared to conventional solid-state-reaction method. Therefore, flux growth is a promising way to prepare high-quality visible-light-driven photocatalyst crystals. Herein, we present growth of oxynitride and nitride crystals for photocatalysts through atmosphere controlled flux method. LaTiO2N, Ta3N5, SrTaO2N, BaTaO2N, and BaNbO2N (and many kinds of oxynitrides and nitrides) crystals were grown under NH3 flow with using flux. Especially LaTiO2N crystals were directly grown from KCl flux and exhibited higher oxygen evolution performance compared to those prepared by conventional methods. It is attributed to elongated lifetime of excited carriers which detected by time-resolve IR spectroscopy. The flux growth is applied to crystal layer (i.e., thin and thick films) fabrication technique and then various photoelectrode consisting of the above-mentioned photocatalysts and current collector were fabricated. The details of flux growth and evaluation (e.g., photocatalytic performance) of the oxynitride and nitride crystals will be presented in 233rd ECS Meeting. This research was supported in part by the Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem), NEDO and JSPS Grant-in-Aid for Scientific Research (A) 25249089.

Deep-level transient spectroscopy characterization of electrical traps in p-type multicrystalline silicon with gettering and hydrogenation process

In p-type multicrystalline silicon solar cells, defects in silicon bulk are well known to act as carrier recombination centers and considerably affect the conversion efficiency of cells. In this study, effective minority carrier lifetime (τeff) mapping and deep-level transient spectroscopy (DLTS) data were utilized to evaluate gettering efficiency and hydrogenation passivation in bulk defects. Microwave photoconductivity decay method was used to illustrate the effect of phosphorus gettering behavior and hydrogenation process on the bulk carrier lifetime of multicrystalline silicon. Results showed that τeff drastically improved after gettering and further increased after hydrogenation. DLTS results further revealed six traps corresponding to interstitial iron (Fei), Fe–O complex, FeB complex, vanadium, Fei complex and divacancy oxygen. The main trap in a nongettered sample was related to Fei, with two deep levels at about EV + 0.373 and EV + 0.470 eV. However, the Fei complex was the main trap in the gettered sample, with two deep traps at about EV+0.390 and EV+0.442 eV. Finally, we concluded that Fei defect and FeB complex can be significantly removed by phosphorus diffusion gettering. Moreover, divacancy-oxygen complexes, vanadium and Fe–O defects were effectively passivated by hydrogenation process. Fei complex, however, was hardly removed by these two process.

Passivation of Surface Recombination at the Si-Face of 4H-SiC by Acidic Solutions

We carried out carrier lifetime measurements for 4H-SiC single crystals in aqueous solutions with various pH by the microwave photoconductivity decay method. For both n- and p-type 4H-SiC, carrier lifetimes measured by Si-face excitation were longer in acidic aqueous solutions compared with carrier lifetimes measured in other solutions. On the other hand, for C-face excitation, carrier lifetimes did not depend on immersion solutions. These results indicate that carrier recombination centers at the surface of the Si-face was passivated by hydrogen ions. We also estimated surface recombination velocities Ss by a numerical analysis. S of the Si-face was reduced from 700 cm/s (in Na2SO4 1 M) to 200 cm/s (in H2SO4 1 M) for the n-type 4H-SiC.

Distribution of microscopic defects in Al-doped CdZnTe crystal

We investigated the distribution of microscopic defects in an Al-doped CdZnTe crystal (CdZnTe:Al), grown under Cd-rich condition by the modified vertical Bridgman method, and their influence on the electrical transport behaviors. These microscopic defects were characterized by photoluminescence (PL), thermally stimulated current spectroscopy and thermoelectric effect spectroscopy. The results show that a low intensity and wide FWHM of (D0, X) peak observed in the low-temperature PL spectrum of CdZnTe:Al sample implies the non-ideal high crystal quality, which is in good agreement with its results of electrical transport properties. The bulk resistivity of CdZnTe:Al sample was evaluated to be in the magnitude of 108 Ω cm with I–V and Hall measurements. Simultaneously, high leakage currents were observed to produce the high noise, which deteriorated the detection performance. Besides, the shallow donor Al Cd + with the activation energy of 13 meV and the relatively shallow acceptor A-center complex (VCd–AlCd) of 112 meV were identified to dominate the low- and high-temperature quenching processes in the temperature-dependent PL spectra, respectively. These microscopic defects as trapping or recombination centers for the charge carriers in CdZnTe:Al detector are responsible for its electrical transport properties, leading to the incomplete charge collection. A further comprehensive investigation on the distribution of microscopic defects was proved to achieve the balance between acceptors and donors in the defects compensation process of CdZnTe:Al sample. In particular, the E DD level at E c − 0.554 eV was attributed to the doubly ionized Cd interstitial (Cd i 2+ ), which pinned the E F level near the mid-gap to achieve the high resistivity performance.

Theory of the carbon vacancy in 4H -SiC: Crystal field and pseudo-Jahn-Teller effects

The carbon vacancy in $4H$-SiC is a powerful minority carrier recombination center and a major cause of degradation of SiC-based devices. Despite the extensiveness of the literature regarding the characterization and modeling of the defect, many fundamental questions persist. Among them we have the shaky connection of the EPR data to the electrical measurements, the physical origin of the pseudo-Jahn-Teller (pJT) effect, the reasoning for the observed sub-lattice dependence of the paramagnetic states, and the severe temperature-dependence of some hyperfine signals which cannot be accounted for by a thermally-activated dynamic averaging between equivalent JT-distorted structures. In this work we address these problems by means of semi-local and hybrid density functional calculations. We start by inventorying a total of four vacancy structures. Diamagnetic states have well defined low-energy structures, whereas paramagnetic states display metastability. This rich structural variety is traced back to the filling of electronic states which are shaped by a crystal-field-dependent pJT effect. From calculated minimum energy paths for defect rotation and transformation mechanisms, combined with the calculated formation energies and electrical levels, we arrived at a configuration-coordinate diagram of the defect. The diagram provides us with a detailed first-principles picture of the defect when subject to optical and thermal excitations. The calculated acceptor and donor transitions agree well with Z$_{1/2}$ and EH$_{6/7}$ trap energies, respectively. From comparison of calculated and measured $U$-values, and correlating the site-dependent formation energies with the relative intensity of the DLTS peaks in as-grown material, we assign Z$_{1}$ (EH$_{6}$) and Z$_{2}$ (EH$_{7}$) signals to acceptor (donor) transitions of carbon vacancies located on the $h$ and $k$ sub-lattice sites, respectively.