Increase In Diffusion Length Research Articles

A Chemically Orthogonal Hole Transport Layer for Efficient Colloidal Quantum Dot Solar Cells.

Colloidal quantum dots (CQDs) are of interest in light of their solution-processing and bandgap tuning. Advances in the performance of CQD optoelectronic devices require fine control over the properties of each layer in the device materials stack. This is particularly challenging in the present best CQD solar cells, since these employ a p-type hole-transport layer (HTL) implemented using 1,2-ethanedithiol (EDT) ligand exchange on top of the CQD active layer. It is established that the high reactivity of EDT causes a severe chemical modification to the active layer that deteriorates charge extraction. By combining elemental mapping with the spatial charge collection efficiency in CQD solar cells, the key materials interface dominating the subpar performance of prior CQD PV devices is demonstrated. This motivates to develop a chemically orthogonal HTL that consists of malonic-acid-crosslinked CQDs. The new crosslinking strategy preserves the surface chemistry of the active layer beneath, and at the same time provides the needed efficient charge extraction. The new HTL enables a 1.4× increase in charge carrier diffusion length in the active layer; and as a result leads to an improvement in power conversion efficiency to 13.0% compared to EDT standard cells (12.2%).

Preparation of Elastic and Antibacterial Chitosan-Citric Membranes with High Oxygen Barrier Ability by in Situ Cross-Linking.

Chitosan–citric biomembranes Ch-CA-Gx (x = 0–3) were prepared by a simple cross-linking. The dependence of mechanical property, water-resisting capacity, microstructural characteristic, oxygen barrier ability, and thermal properties of membranes on the content of glycerin was investigated. The results revealed that vacuum drying at 80 °C can lead to low-yield amidation and the Maillard reaction, thus affecting the thermal stability and water resistance of biomembranes. Owing to the ionic cross-linking and amidation, the chitosan–citrate complex showed weaker compatibility when the glycerin content increased, thereby leading to discontinuity of microstructure in the Ch-CA-Gx (x = 1–3) membranes, which was in line with the weaker mechanical properties and water-resisting abilities of membranes, compared to Ch-CA-G0. Chitosan membranes showed interestingly high oxygen barrier capabilities under 40 and 80% relative humidity (RH) conditions, probably attributed to the increased diffusion length arising from the hydrogen-bonding, ionic, and covalent cross-linking. The oxygen transmission rates of Ch-CA-Gx were below 0.1 cm3 m–2 day–1 at 40% RH. The Ch-CA-Gx membranes showed a good elasticity assigned to the reversibly cross-linked structure. The membranes presented strong antibacterial activities against Staphylococcus aureus and Escherichia coli bacteria, probably owing to the citric acids. The results demonstrated that these materials have potential applications as membranes or protecting coatings for food packaging and successful cross-linking by means of amidation, and the Maillard reaction under the condition of vacuum drying can be probably applied as a green and alternative method for the fabrication of mechanically tough and antibacterial membranes, fibers, and gels.

Cascade surface modification of colloidal quantum dot inks enables efficient bulk homojunction photovoltaics

Control over carrier type and doping levels in semiconductor materials is key for optoelectronic applications. In colloidal quantum dots (CQDs), these properties can be tuned by surface chemistry modification, but this has so far been accomplished at the expense of reduced surface passivation and compromised colloidal solubility; this has precluded the realization of advanced architectures such as CQD bulk homojunction solids. Here we introduce a cascade surface modification scheme that overcomes these limitations. This strategy provides control over doping and solubility and enables n-type and p-type CQD inks that are fully miscible in the same solvent with complete surface passivation. This enables the realization of homogeneous CQD bulk homojunction films that exhibit a 1.5 times increase in carrier diffusion length compared with the previous best CQD films. As a result, we demonstrate the highest power conversion efficiency (13.3%) reported among CQD solar cells.

High-performance numerical modeling of toxic-free CZTS solar cell structure

A systematic numerical study of a toxic-free CZTS solar cell model based on the ZnO buffer layer is numerically investigated and analysed using the SCAPS-1D simulation platform. The proposed toxic-free CZTS model exhibits an improvement in the current density (Jsc) of about 1.41 mA/cm2 and 6.33% in quantum efficiency (QE) compared to the conventional toxic structure. The simulation results of the eco-friendly model reveal an optimum efficiency of about 19.96% at a bandgap energy of 1.42 eV and with absorber thickness as low as 1600 nm. It has been observed that increasing the doping level from 1015 cm−3 to 1018 cm−3 results in an efficiency enhancement of about 2.5%. To maintain the high-performance of the device, the ZnO buffer layer must be as thin as 100 nm with a bandgap energy of 3.3 eV. The increase in buffer layer thickness above 100 nm leads to deterioration in the device performance. This might be ascribed to the increase of the recombination rate due to the increase of carrier diffusion length away from the junction at thicker ZnO thicknesses.

Spin Hall effect investigated by spin Hall magnetoresistance in Pt100−xAux/CoFeB systems

We studied spin Hall effect (SHE) in Pt100-xAux with various Au compositions x for Pt-Au/CoFeB systems by means of spin Hall magnetoresistance (SMR). We synthesized low resistive, flat, and highly (111)-textured Pt-Au alloy films by sputtering using low pressure Kr gas. We found that spin Hall angle (θSH), spin Hall conductivity (σSH), and spin diffusion length (λS) increase by alloying Pt with Au at certain x. The enhancement of σSH is consistent with the theoretically predicted increase in the intrinsic spin Hall conductivity, however quantitatively, there are some discrepancies in θSH and λS between our result and the experimental and theoretical results in the literatures 19 and 20. The discrepancy between our results and previous results would be related to the difference of the nanostructure in Pt-Au alloys related to the scattering from film surfaces and grain boundaries and the degree of phase separation in Pt-Au alloy systems.

Optical and structural properties of dislocations in InGaN

Threading dislocations in thick layers of InxGa1−xN (5% < x < 15%) have been investigated by means of cathodoluminescence, time-resolved cathodoluminescence, and molecular dynamics. We show that indium atoms segregate near dislocations in all the samples. This promotes the formation of In-N-In chains and atomic condensates, which localize carriers and hinder nonradiative recombination at dislocations. We note, however, that the dark halo surrounding the dislocations in the cathodoluminescence image becomes increasingly pronounced as the indium fraction of the sample increases. Using transmission electron microscopy, we attribute the dark halo to a region of lower indium content formed below the facet of the V-shaped pit that terminates the dislocation in low composition samples (x < 12%). For x > 12%, the facets of the V-defect featured dislocation bundles instead of the low indium fraction region. In this sample, the origin of the dark halo may relate to a compound effect of the dislocation bundles, of a variation of surface potential, and perhaps, of an increase in carrier diffusion length.

Quantifying brain microstructure with diffusion MRI: Theory and parameter estimation.

We review, systematize and discuss models of diffusion in neuronal tissue, by putting them into an overarching physical context of coarse-graining over an increasing diffusion length scale. From this perspective, we view research on quantifying brain microstructure as occurring along three major avenues. The first avenue focusses on transient, or time-dependent, effects in diffusion. These effects signify the gradual coarse-graining of tissue structure, which occurs qualitatively differently in different brain tissue compartments. We show that transient effects contain information about the relevant length scales for neuronal tissue, such as the packing correlation length for neuronal fibers, as well as the degree of structural disorder along the neurites. The second avenue corresponds to the long-time limit, when the observed signal can be approximated as a sum of multiple nonexchanging anisotropic Gaussian components. Here, the challenge lies in parameter estimation and in resolving its hidden degeneracies. The third avenue employs multiple diffusion encoding techniques, able to access information not contained in the conventional diffusion propagator. We conclude with our outlook on future directions that could open exciting possibilities for designing quantitative markers of tissue physiology and pathology, based on methods of studying mesoscopic transport in disordered systems.

Useful studies on the optical properties of annealed and non annealed InGaN/GaN single QW LEDs, with different dopings in the barrier

In this paper we have studied comprehensively annealed and non annealed single quantum well (QW) light emitting diode (LED) structures with different doping concentrations in the barriers. Computations have been carried out through the self-consistent solutions of Schrödinger and Poison equations. Wide changes in the transition energies (TE) and the transition probabilities (TP) are observed due to the doping variation. On annealing, with the increase of diffusion length, the results show that depending on the doping concentration, the TE increases monotonically as well as it goes through red-blue shifts. It is also observed that, for lower doping, TP increases and/or decreases with the diffusion length. The variation of TP also depends on the operating current. After a critical doping in the barriers, on annealing, the optical performances of QW LEDs deteriorate. Through change in doping and annealing, the optical performance of the InGaN/GaN QW LEDs may be improved. This information should be of importance to the optoelectronic designer.

Enhanced external quantum efficiency from Cu2ZnSn(S,Se)4 solar cells prepared from nanoparticle inks

Cu2ZnSn(S,Se)4 (CZTSSe) thin film photovoltaic absorber layers are fabricated by selenizing Cu2ZnSnS4 (CZTS) nanoparticle thin films in a selenium rich atmosphere. The selenium vapor pressure is controlled to optimize the morphology and quality of the CZTSSe thin film. The largest grains are formed at the highest selenium vapor pressure of 226 mbar. Integrating this photovoltaic absorber layer in a conventional thin film solar cell structure yields a champion short circuit current of 37.9 mA/cm2 without an antireflection coating. This stems from an improved external quantum efficiency characteristic in the visible and near-infrared part of the solar spectrum. The physical basis of this improvement is qualitatively attributed to a substantial increase in the minority carrier diffusion length.

Nonlinear optical susceptibilities in the diffusion modified AlxGa1–xN/GaN single quantum well

Under thermal treatment of the post growth AlGaN/GaN single quantum well, the diffusion of Al and Ga atoms across the interface is expected to form the diffusion modified quantum well with diffusion length as a quantitative parameter for diffusion. The modification of confining potential and position-dependent effective mass in the quantum well due to diffusion is calculated taking the Fick's law. The built-in electric field which arises from spontaneous and piezoelectric polarizations in the wurtzite structure is included in the effective mass equation. The electronic states are calculated from the effective mass equation using the finite difference method for several diffusion lengths. Since the effective well width decreases with increasing diffusion length, the energy levels increase with it. The intersubband energy spacing in the conduction band decreases with diffusion length due to built-in electric field and reduction of effective well width. The linear susceptibility for first-order and the nonlinear second-order and third-order susceptibilities are calculated using the compact density matrix approach taking only two levels. The calculated susceptibilities are red shifted with increase in diffusion lengths due to decrease in intersubband energy spacing.

Increasing resolution of selectivity in alkene hydrogenation via diffusion length in core-shell MFI zeolite

We designed a core-shell zeolite structure comprises of palladium-deposited ZSM-5 core and silicalite-1 (S-1) shell which favors selectivity towards light olefin in hydrogenation via increased diffusion length. A well designed S-1/Pd/ZSM-5 core-shell structure was prepared via secondary crystallization of S-1 layer on the Pd/ZSM-5 core. The catalytic and selectivity performance of the S-1/Pd/ZSM-5 composite was evaluated in catalytic hydrogenation of alkenes in liquid phase. The synthesized S-1/Pd/ZSM-5 gives a much higher selectivity towards 1-hexene (87%) over cyclohexene (13%) even though both reactants are able to enter the 10-membered ring channels of the core-shell structure. The zeolitic core-shell composite also showed an increasing selectivity towards 1-hexene over 1-heptene as the S-1 layers built up, even though both are linear alkenes with similar kinetic diameter that are accessible to the MFI framework. In this work, we demonstrate a strong correlation between the thickness of the S-1 shell layer and the selectivity towards light olefins due to faster mass transfer rate. The design of the core-shell MFI structure is a new example of how selectivity in a zeolite-catalyzed reaction can be changed and enhanced without relying on typical molecular size exclusion process.

Effect of Extended Conjugation of N-Heterocyclic Carbene-Based Sensitizers on the Performance of Dye-Sensitized Solar Cells.

We report the synthesis, characterization, and photovoltaic properties of four ruthenium complexes (CI101, CBTR, CB111, and CB108) having various N-heterocyclic carbene ancillary ligands, pyridine-imidazole, -benzimidazole, -dithienobenzimidazole, and -phenanthroimidazole, respectively. These complexes were designed to investigate the effect of extended conjugation ordained from ring fusion on the power conversion efficiencies of the solar cells. The device sensitized by CB108, the pyridine-phenanthroimidazole conjugated complex, showed an improved efficiency (9.89%) compared to those of pyridine-benzimidazole conjugated system (CBTR, 9.72%) and the parent unfused ring system (CI101, 6.24%). Surprisingly, the sulfur-incorporated pyridine-dithienobenzimidazole system (CB111, 9.24%) exhibited a little lower efficiency than that of N719 (9.41%). The enhanced photovoltaic performance of CB108 was mainly attributed to the increase in electron lifetime and diffusion length confirmed by the electrochemical impedance spectroscopy.

Minority carrier diffusion length, lifetime and mobility in p-type GaAs and GaInAs

Minority carrier diffusion lengths and lifetimes were determined for p-type Ga(1-x)InxAs with an In-content of 0 ≤ x ≤ 0.53 by cathodoluminescence and time-resolved photoluminescence measurements respectively under low injection conditions; the resulting minority carrier mobilities are also reported. Highly p-doped samples (3 × 1018 cm−3) demonstrate a constant minority carrier diffusion length of (5.0 ± 0.7) μm and a constant lifetime of (3.7 ± 0.7) ns for an In-content up to 21%. Lower doped samples (3 × 1017 cm−3), on the other hand, show an increase in minority carrier diffusion length and lifetime with In-content from (6.3 ± 0.2) μm and (6.2 ± 0.5) ns respectively for GaAs to (14 ± 2) μm and (24.4 ± 0.5) ns respectively for Ga0.79In0.21As. Increasing the In-content to 53% results in a drop in the minority carrier diffusion length independently of the p-doping concentration .This is interpreted as a change in the energy of the Shockley-Read-Hall trap levels within the bandgap as a function of indium concentration.

Correlation between Photoluminescence and Carrier Transport and a Simple In Situ Passivation Method for High-Bandgap Hybrid Perovskites.

High-bandgap mixed-halide hybrid perovskites have higher open-circuit voltage deficits and lower carrier diffusion lengths than their lower-bandgap counterparts. We have developed a ligand-assisted crystallization (LAC) technique that introduces additives in situ during the solvent wash and developed a new method to dynamically measure the absolute intensity steady-state photoluminescence and the mean carrier diffusion length simultaneously. The measurements reveal four distinct regimes of material changes and show that photoluminescence brightening often coincides with losses in carrier transport, such as in degradation or phase segregation. Further, the measurements enabled optimization of LAC on the 1.75 eV bandgap FA0.83Cs0.17Pb(I0.66Br0.34)3, resulting in an enhancement of the photoluminescence quantum yield (PLQY) of over an order of magnitude, an increase of 80 meV in the quasi-Fermi level splitting (to 1.29 eV), an increase in diffusion length by a factor of 3.5 (to over 1 μm), and enhanced open-circuit voltage and short-circuit current from photovoltaics fabricated from the LAC-treated films.

Alternative luminescence image evaluation – Comparison with lock-in thermography

It was shown recently that previous approaches for quantitative luminescence image evaluation based on the model of independent diodes inevitably lead to erroneous results for imaging saturation current densities. However, the pioneering work of Fuyuki on electroluminescence imaging of solar cells was not based on this model. There it was claimed that the calibration constant of the local luminescence signal should be proportional to the local effective diffusion length. With increasing diffusion length, however, the luminescence reaches a saturation value. Recently a simple formula for the calibration constant was proposed, which describes both the proportionality for small diffusion lengths and the saturation property for large ones. While this non-linear Fuyuki PL evaluation method was previously only applied to standard BSF cells, in this work it is applied to modern multicrystalline PERC cells processed from different high performance multicrystalline silicon materials. The results are compared to that of dark lock-in thermography (DLIT) and Laplacian PL investigations of these cells. It is found that saturation current density images obtained from the alternative luminescence evaluation methods are close to that based on DLIT in low lifetime regions, but show a much better spatial resolution. In high lifetime regions, however, this alternative PL evaluation is generally less reliable than DLIT. These alternative kinds of luminescence image evaluation may replace previous approaches of quantitative PL evaluation based on the model of independent diodes.

Thermal annealing performance of sulfur-hyperdoped black silicon fabricated using a Nd:YAG nanosecond-pulsed laser

Compared to femtosecond-pulsed laser and ion implantation with chemical texturing methods, the nanosecond (ns)-pulsed laser method is inexpensive and efficient for fabricating a microstructured and hyperdoped crystalline silicon surface (usually known as black Si), facilitating the industrial manufacturing of optoelectronic devices. However, the ns-pulsed laser method has been rarely studied in material post-processing and device manufacturing. In this study, a microstructured and sulfur-hyperdoped p-type Si〈100〉 wafer surface was fabricated using a neodymium-doped yttrium aluminum garnet (Nd:YAG) ns-pulsed laser irradiation in sulfur hexafluoride gas. The effects of thermal annealing on the morphological, optical (visible and infrared (IR) spectra), and electrical properties of black Si samples were evaluated. The results show that the small particles and protrusions, overlying conical structures of the samples, decreased after the annealing. The light reflectance of the samples in the wavelength range 400–1000nm decreased by ∼2%, and a high IR absorptance (λ=1100–2400nm) was observed. The IR absorptance decreased slowly from 86% to 60% with an annealing time of 30 (or 60) min and temperature increase from 575K to 800K (corresponding to the increase in diffusion length of S-dopant from 0.09nm to 68.63nm). Furthermore, sheet carrier concentration and resistivity of the black Si samples increased and decreased with increasing S-dopant diffusion length, respectively. The improvements in electrical property indicate that thermal annealing can enhance the activation of S-dopant and electrical rectification of the samples. These are important for developing Si-based broad-spectrum solar cells and IR detectors.

Nano-EBIC analysis: An attempt to describe the surface recombination effects by the modified Donolato probability

Donolato minority carrier collection probability [Sol. Stat. Electron. 28, (1985) 1143–115] is used to describe the surface recombination effects between zero and infinite values of the surface recombination velocity. The probability function is modified for a nano-electrode. An opening angle and a “specific” length are introduced. The opening angle leads to the emerged charge to “see” the electrode, and the “specific” length describes an average distance travelled by the charge at the surface sample before being collected by the nano-contact. Results show that both minority carrier effective diffusion length and collection efficiency maximum increase when the “specific” length is increased. The effective diffusion length varies from 0.3 μm for infinite velocity to 0.9 μm for velocity zero. The latter is 10% smaller than the bulk diffusion Length because of both the nano-scale contact and the bi-dimensional diffusion space which affect the collection probability, and hence the collection efficiency profile.

Damping and spin mixing conductance in Ni80Fe20/CuIr structures: effect of Ir doping

Ni80Fe20(Py) thin films of different thicknesses have been sputtered on thermally oxidized Si substrates and capped with Pt or Cu doped with , and of Ir. Vibrating sample magnetometery has been used to measure the magnetization at saturation. Microstrip line ferromagnetic resonance (MS-FMR) has been used to investigate the Py and capping layer thickness dependence of the damping parameter enhancement due to the spin pumping. The experimental data have been analyzed using a model for spin pumping that includes the effective spin mixing conductance at the Py/HM (HM = Pt and CuIr) interface and the spin-diffusion length of HM. The spin mixing conductance (spin diffusion length) increases (decreases) as Ir doping increases. For samples capped with Pt (CuIr(6%)), the obtained values of spin mixing conductance and the spin diffusion length have been estimated to be 25 nm−2 (9.87 nm−2) and 1.05 nm (2.8 nm), respectively. MS-FMR measurements revealed that the effective magnetization varies linearly with the effective Py thickness due to the perpendicular surface anisotropy.

Quantum Efficiency Measurements and Modeling as Tools to Monitor Air Annealing of Cu2SnS 3 Solar Cells

Air annealing of chalcopyrite solar cells at either 200 °C or higher is often known to increase their power conversion efficiency. In this paper, we investigate the nature of this effect for Cu2SnS3 (CTS) solar cells by modeling the experimental external quantum efficiency. We find that the cell efficiency increase stems from increased diffusion length and depletion width and decreased interface recombination at the p-n junction. The increased diffusion length is also reproduced when only the absorber layer is air annealed. When solar cells are annealed in N2, no increase in diffusion length is measured. Hence, we attribute the increase in diffusion length to passivation of the grain boundaries in the bulk by oxygen. The larger depletion width on air and N2 annealing in the devices is independent of the CdS buffer layer thickness and occurs in its absence. We ascribe it to copper diffusion from the absorber layer to the n-type buffer and window layers. Interface recombination positively correlates with increasing buffer layer thickness. Based on our modeling, we conclude that the CTS absorber layer is still too highly doped to obtain large depletion widths and is highly recombination active at the p–n interface.

The mechanism of growth of ZnO nanorods by reactive sputtering

DC reactive magnetron sputtering of zinc target in argon-oxygen sputtering atmosphere has been used to grow ZnO thin films/nanorods on Si in a wide substrate temperature range of 300–750°C and under different sputtering conditions, namely, DC power, sputtering pressure and oxygen percentage in the sputtering atmosphere. Powder X-ray diffraction, Raman spectroscopy and a combination of top-down and cross-sectional scanning electron microscopy studies of ZnO films and nanorods grown under different conditions, have shown that substrate temperature critically controls their growth behavior and morphology, eventually resulting in the growth of vertically c-axis oriented, highly aligned and separated ZnO nanorods at substrate temperatures of 700–750°C. The strongly substrate temperature dependent growth of nanorods is explained by considering that the growth above 600°C, takes place in the ‘desorption regime’, in which, the surface diffusion length decreases exponentially with temperature. The diameter of nanorods increases with increase of DC power or decrease of sputtering pressure, which is attributed to the increase of surface diffusion length at higher deposition flux. The morphology of ZnO nanorods is not significantly affected by oxygen percentage in the sputtering atmosphere, since it does not influence the deposition flux.