Effective Diffusion Length Research Articles

Defect characterization of UMG mc-Si solar cells using LBIC and luminescence imaging techniques

ABSTRACTUpgraded metallurgical-grade silicon (UMG Si) solar cells with different ranges of efficiencies were characterized through electroluminescence imaging (ELi) and light-beam induced current (LBIC) measurements. The results showed a good correlation between the EL intensity and the efficiency of the solar cells. ELi images gave a bright contrast at the defects, grain boundaries and intragrain defects, and dark contrast inside the grain bodies. Metallic impurities are much more present in some cells due to the directional solidification of the Si ingot. Local short-circuit current mapping with LBIC measurements revealed a bright zone in the neighborhoods of the defects due to the depletion of impurities. Internal quantum efficiencies (IQE) and effective diffusion lengths (Leff) were calculated using different excitation wavelengths. High resolution LBIC measurements revealed micrometric clusters of impurities around intragrain defects.

Effective diffusion length and elementary surface processes in the concurrent growth of nanowires and 2D layers

In this work, we revisit models of the diffusion-induced growth of nanowires (NWs) accompanied by the growth of crystalline layers on the nonactivated areas of the substrate surface. In the conventional approach, the presence of the surface steps is taken into account implicitly by introducing effective diffusion length of adatoms before desorption or incorporation. In contrast, here we treat adatom incorporation as an outcome of a sequence of elementary surface processes on the terraces and step edges. The surface diffusion problems are formulated and solved for two modes of the lateral growth at the substrate surface: (i) submonolayer growth of 2D islands and (ii) permanent presence of the steps forming a closed loop around the NW base. In the limiting cases where the substrate adatoms with high probability either reach the sidewall of a NW or leave the substrate surface by desorption or attachment to 2D islands, our analysis demonstrates the correspondence between this approach and the conventional one under the relevant and physically transparent choice of the effective diffusion length. It has been shown that at relatively large NW heights the steps act as the sinks not only for the atoms impinging the substrate surface but also for the atoms impinging the NW sidewalls. This explains the failure of the simple material balance arguments to predict the NW height evolution observed in the experiments on the Au-assisted growth of InAs and GaAs NWs.

An equivalent 1D nanochannel model to describe ion transport in multilayered graphene membranes

Multilayered graphene-based membranes are promising for a variety of applications related to ion or molecule transport, such as energy storage and water treatment. However, the complex three-dimensional cascading nanoslit-like structure embedded in the membrane makes it difficult to interpret and rationalize experimental results, quantitatively compare with the traditional membrane systems, and quantitatively design new membrane structures. In this paper, systematic numerical simulations were performed to establish an equivalent one-dimensional (1D) nanochannel model to represent the structure of multilayered graphene membranes. We have established a quantitative relationship between effective diffusion length Leff and cross-section area Aeff of the 1D model and our recently developed two dimensional (2D) representative microstructure for graphene membranes. We find that only in the cases of a relatively large lateral size L (> ~100 nm) and a small slit size h (< 2 nm), the effective diffusion length Leff and Aeff can be calculated by an over-simplified but often used model. Otherwise, they show complex dependence on all three structural parameters of the 2D structural model. Our equivalent 1D nano-channel model can reproduce experimental results very well except for h < 0.5 nm. The discrepancy could be attributed to the anomalous behaviour of molecules under nano-confinement that is not considered in our simulations. This model can also be extended to multilayered membranes assembled by other 2D materials.

Assessing clogging of laminated hydrophobic membrane during fecal sludge drying

A new sanitation technology has been proposed in which a laminated hydrophobic membrane contains and enhances drying of fecal sludge in a toilet, with particular focus on application to urban regions of low-income countries. The proposed technology uses a laminated hydrophobic membrane liner as an integral component of container-based sanitation systems. The focus of this study is to quantitatively evaluate the laminate's clogging after repeated use, which will affect replacement interval and might limit the laminate's application in container-based toilets. The membrane of the laminated hydrophobic membrane used in this process is hydrophobic and only allows vapor transport. Drying of water vapor using the laminated hydrophobic membrane occurs due to moderate temperature or humidity gradients, while other constituents such as aqueous dissolved solutes of fecal sludge are retained. Controlled laboratory experiments evaluated repeated use of a laminated hydrophobic membrane for fecal sludge drying, with mild brushing/rinsing of the laminate between each application. Drying occurred at a constant rate as long as the fecal sludge moisture content exceeded 11.6 (g/g), below which water activity <1. Over five drying cycles, at a significance level of α = 0.05 the dimensionless drying rate in the constant-rate period was not reduced. While scanning electron microscopy and energy dispersive X-ray analyses of used laminated hydrophobic membrane showed deposition of fecal sludge on the inner fabric of the laminate, particulate accumulation was never sufficient to alter the fecal sludge drying rate. Experiments with only water indicated that the fecal sludge increased the effective diffusion length through the laminate by 10–30%. These data demonstrate that clogging of the laminated hydrophobic membrane is minor over five cycles of fecal sludge drying with mild rinsing between cycles, indicating that use of the laminate may be feasible in many field applications.

Impact of Dielectric Layers on Liquid-Phase Crystallized Silicon Solar Cells

Liquid-phase crystallization (LPC) of thin silicon layers on glass substrates is a technique to fabricate solar cells with low energy and material consumption and open-circuit voltages comparable to multicrystalline silicon wafer cells. We studied the impact of different passivation layers deposited with plasma-enhanced chemical vapor deposition (PECVD) on the cell quality. Silicon nitride (SiNx) and ultrathin silicon oxide (SiO x ) layers with varying thicknesses were used. In addition, we plasma-treated the SiN x surface to form a thin silicon oxynitride (SiO x N y ) passivation layer. Plasma oxidation is an attractive alternative to PECVD, since thicknesses of ultrathin layers can be controlled easier as compared with PECVD. We found that the cell performance is influenced by the passivation layer. Particularly, cells on the 9 nm PECVD SiO x passivation layer are worse than cells on the 11 nm SiO x N y layer. We modeled cell parameters employing ASPIN3 to estimate effective diffusion lengths. Using transmission electron microscopy as well as electron energy loss spectroscopy, we studied the effect of the plasma treatment on the morphology and elemental distribution at the interface between passivation layer and LPC-Si absorber. Our best interdigitated back contacted solar cell with an efficiency >14% is based on the SiO x N y layer.

Fractal analysis of collision cascades in pulsed-ion-beam-irradiated solids

The buildup of radiation damage in ion-irradiated crystals often depends on the spatial distribution of atomic displacements within collision cascades. Although collision cascades have previously been described as fractals, the correlation of their fractal parameters with experimental observations of radiation damage buildup remains elusive. Here, we use a pulsed-ion-beam method to study defect interaction dynamics in 3C-SiC irradiated at 100 °C with ions of different masses. These data, together with results of previous studies of SiC and Si, are analyzed with a model of radiation damage formation which accounts for the fractal nature of collision cascades. Our emphasis is on the extraction of the effective defect diffusion length from pulsed beam measurements. Results show that, for both Si and SiC, collision cascades are mass fractals with fractal dimensions in the range of ~1–2, depending on ion mass, energy, and the depth from the sample surface. Within our fractal model, the effective defect diffusion length is ~10 nm for SiC and ~20 nm for Si, and it decreases with increasing cascade density. These results demonstrate a general method by which the fractal nature of collision cascades can be used to explain experimental observations and predict material’s response to radiation.

Determination of transport properties in optoelectronic devices by time-resolved fluorescence imaging

In this article, we introduce time-resolved fluorescence imaging as an optical characterization method for optoelectronic devices. Under wide-field illumination, it allows obtaining time-resolved photoluminescence maps with a temporal resolution of 500 ps and a micrometric spatial resolution. An experiment on a GaAs-based solar cell is presented here as a proof of concept. Thanks to a model including diffusion and recombination of minority charge carriers, we fit the experimental photoluminescence transients and extract key optoelectronic properties for the considered device. For various fluence levels, we determine an intrinsic bulk recombination lifetime τn = 75 ns, a constant effective diffusion length Leff = 190 μm, which is characteristic for the lateral transport inside the solar cell, and an injection-dependent contact recombination velocity Sn, taking its values between 7 × 104 and 3 × 105 cm/s, which is explained by the saturation of defects. The wide-field illumination notably avoids lateral diffusion artefacts leading to a significant underestimation of τn.

Surface effects on exciton diffusion in non polar ZnO/ZnMgO heterostructures

The diffusion of excitons injected in ZnO/Zn0.92Mg0.08O quantum well heterostructures grown by metal-organic-vapor-phase-epitaxy on non-polar ZnO substrates is investigated at room temperature. Cathodoluminescence linescans in a field-emission-gun scanning-electron-microscope are performed across cleaved cross-sections. A 55 nm diffusion length is assessed for excitons in bulk ZnMgO. When prepared as small angle bevels using focused ion beam (FIB), the effective diffusion length of excitons is shown to decrease down to 8 nm in the thinner part of the slab. This effect is attributed to non-radiative surface recombinations, with a 7 × 104 cm s−1 recombination velocity estimated at the FIB-machined ZnMgO surface. The strong reduction of the diffusion extent in such thin lamellae usually used for transmission electron microscopy could be use improve the spatial resolution of cathodoluminescence images, often limited by diffusion processes.

New insight into the ultra-long lifetime of excitons in organic–inorganic perovskite: Reverse intersystem crossing

Recently, an effective exciton diffusion length L exceeding 100 μm has been reported for organic–inorganic halide perovskites owing to both the high mobility and ultra-long lifetime of the excitons; however, the origin of ultra-long L is still unclear in nature. In some photoelectric materials, reverse intersystem crossing (RISC) from the triplet to the singlet state can enhance the quantum yield of photoluminescence greatly. In this study, our theoretical investigation indicated that the energy difference ΔEst between the singlet state and the triplet state of CH3NH3PbI3 was less than 0.1 eV, which represents one crucial prerequisite for the occurrence of RISC. Meanwhile, the experimental results showed that the photoluminescence lifetime increased with the increasing temperature, a typical feature of RISC. Based on this study, we put forward the hypothesis that the ultra-long lifetime of excitons in organic–inorganic halide perovskite might be caused by the RISC process. This may provide a new insight into the important photophysical properties of such novel photovoltaic materials.

Dynamic annealing in Ge studied by pulsed ion beams

The formation of radiation damage in Ge above room temperature is dominated by complex dynamic annealing processes, involving migration and interaction of ballistically-generated point defects. Here, we study the dynamics of radiation defects in Ge in the temperature range of 100–160 °C under pulsed beam irradiation with 500 keV Ar ions when the total ion fluence is split into a train of equal square pulses. By varying the passive portion of the beam duty cycle, we measure a characteristic time constant of dynamic annealing, which rapidly decreases from ~8 to 0.3 ms with increasing temperature. By varying the active portion of the beam duty cycle, we measure an effective diffusion length of ~38 nm at 110 °C. Results reveal a major change in the dominant dynamic annealing process at a critical transition temperature of ~130 °C. The two dominant dynamic annealing processes have an order of magnitude different activation energies of 0.13 and 1.3 eV.

Advanced Local Characterization of Silicon Solar Cells

Solar cells made from multicrystalline silicon material, which still represent the majority of solar cells produced today, are by nature inhomogeneous devices. Their bulk excess carrier lifetime, which decisively influences the short circuit current density, the saturation current density, and the effective bulk diffusion length, may vary from position to position by an order of magnitude or more. Moreover, such cells may contain ohmic shunts and, in particular in the edge regions, positions of significant depletion region recombination current. The classical local characterization method for solar cells is light beam‐induced current (LBIC) mapping. This method does not give direct information on factors influencing the dark current density. A reliable quantitative local characterization of inhomogeneous cells can be performed by combining dark lock‐in thermography (DLIT) with electro‐ and photoluminescence (EL and PL) imaging. In this contribution, the well‐established and the most recent findings on the application of these methods are reviewed and typical application examples are presented. It is shown that luminescence imaging is most appropriate for local series resistance imaging and for qualitatively imaging of all kinds of recombination‐active defects. DLIT, on the other hand, shows a lower spatial resolution, but is most appropriate for imaging local two‐diode parameters like depletion region‐ and bulk saturation current densities, the latter allowing to draw conclusions also to the local short circuit current density. A comprehensive local characterization of silicon solar cells is possible by applying both DLIT and luminescence imaging, which complement each other.

Potential of interdigitated back-contact silicon heterojunction solar cells for liquid phase crystallized silicon on glass with efficiency above 14%

Liquid phase crystallization of silicon (LPC-Si) on glass is a promising method to produce high quality multi-crystalline Si films with macroscopic grains. In this study, we report on recent improvements of our interdigitated back-contact silicon heterojunction contact system (IBC-SHJ), which enabled open circuit voltages as high as 661mV and efficiencies up to 14.2% using a 13µm thin n-type LPC-Si absorbers on glass. The influence of the BSF width on the cell performance is investigated both experimentally and numerically. We combine 1D optical simulations using GenPro4 and 2D electrical simulations using Sentaurus™ TCAD to determine the optical and electrical loss mechanisms in order to estimate the potential of our current LPC-Si absorbers. The simulations reveal an effective minority carrier diffusion length of 26µm and further demonstrate that a doping concentration of 4 × 1016cm−3 and a back surface field width of 60µm are optimum values to further increase cell efficiencies.

Current Matching in Multifold DBP/C70 Organic Solar Cells With Open-Circuit Voltages of up to 6.44 V

In this paper, we demonstrate a novel method for achieving high open-circuit voltages ( V oc ) in organic solar cells based on tetraphenyldibenzoperiflanthen (DBP) as donor and fullerene (C70) as acceptor molecules, by fabrication of multifold bilayer single cells stacked on top of each other. As devices based on the material combination of DBP and C70 show relatively high open-circuit voltages of 0.87 V for single junction cells, and as both materials show broad absorption in the visible region with pronounced peaks, they become ideal candidates as active layer materials in tandem stacked solar cells. By using a ten-fold bilayer structure of DBP and C70, sandwiched between two electrodes, we reach, in this paper, an open-circuit voltage of up to 6.44 V for a single device, and thus crossing the 5 V limit that is required to power up several low-power consuming devices. Furthermore, by conducting drift-diffusion-based device modeling of the multifold devices, considering also the optical absorption profile in the stacked tandem cell and the effective exciton diffusion lengths, we demonstrate that effective current matching can be obtained in the devices through thickness optimization for each single cell. As a result, we demonstrate that the efficiency of these novel devices can be improved from 3.1% to 4.4% (best performing devices) in the case of a fivefold device structure, mainly due to the strong increase in the short-circuit current density, and thus lead to efficient small molecule-based solar cells with high open-circuit voltages.

Light induced degradation: kinetic model and grain boundary impact on sponge-LID

High-performance multi-crystalline silicon material (HPmc-Si) dominates the market for casted p-type silicon. Solar cells made from HPmc-Si material might suffer from light induced degradation due to the so called sponge-LID mechanism. In this work, we present a kinetic sponge-LID model showing that the degradation follows a pairing reaction involving two reactants. This implies that sponge-LID is based on a different reaction scheme compared to the well-known models for boron-oxygen- or iron-boron-degradation. Based on our model, degradation rates are investigated reading the influence of temperature and illumination on the degradation. Finally, we present statistical results implying that Sigma-3 grain boundaries are less affected of the degradation than other grain boundary types. Using a detailed spatially resolved analysis of the effective carrier diffusion length, the different behaviour of grain boundaries and intra-grain regions is quantified.

Tailoring Perovskite Thin Films to Rival Single Crystals

High temperature epitaxial growth has been a long-standing requirement for forming semiconductor thin films with exceptional optoelectronic properties. In this issue of Joule , Brenes and coworkers have demonstrated that atmospheric and photoinduced post-treatments of solution-processed polycrystalline metal halide perovskites can improve the diffusion length, carrier lifetime, and surface recombination velocity to values previously only achievable by single crystal materials.

Pt-decorated zinc oxide nanorod arrays with graphitic carbon nitride nanosheets for highly efficient dual-functional gas sensing

In this work, well-aligned ZnO nanorods were grown on the substrate of exfoliated g-C3N4 nanosheets via a microwave-assisted hydrothermal synthesis, and then Pt/ZnO/g-C3N4 nanostructures were obtained after the deposition of Pt nanoparticles. The growth of vertically ordered ZnO nanorods was occurred on g-C3N4 nanosheets through the bonding interaction between Zn and N atoms, which was confirmed by XPS, FT-IR data and molecular orbital theory. The Pt/ZnO/g-C3N4 nanostructures sensor exhibited the remarkable sensitivity, selectivity, and fast response/recovery time for air pollutants of ethanol and NO2. The application of Pt/ZnO/g-C3N4 nanostructures could be used as a dual-functional gas sensor through the controlled working temperature. Besides, the Pt/ZnO/g-C3N4 nanostructures sensor could be applied to the repeating detection of ethanol and NO2 in the natural environment. The synergistic effect and improved the separation of electron-hole pairs in Pt/ZnO/g-C3N4 nanostructures had been verified for the gas sensing mechanism. Additionally, Pt/ZnO/g-C3N4 nanostructures revealed the excellent charge carriers transport properties in electrochemical impedance spectroscopy (EIS), such as the longer electron lifetime (τn), higher electron diffusion coefficient (Dn) and bigger effective diffusion length (Ln), which also played an important role for Pt/ZnO/g-C3N4 nanostructures with striking gas sensing activities.

Impact of defects on exciton diffusion in organic light-emitting diodes

Abstract The impact of defect concentration and current density on the effective singlet exciton diffusion length in 4’-bis(carbazol-9-yl)biphenyl (CBP) is quantified by analyzing the electroluminescent characteristics of several sets of OLEDs. The defect concentration and effective diffusion length are determined through fitting of the defect and CBP emission bands in the electroluminescence spectra under constant current operation using an analytical model derived based on the competition between exciton diffusion and energy transfer to defects. Defect concentrations of 3 ± 1 × 10 18 cm −3 , 2 ± 1 × 10 18 cm −3 and 0.3 ± 0.7 × 10 18 cm −3 are calculated in three sets of OLEDs, in which the effective diffusion length decreases as the defect concentration increases. Modelling the dependence of the effective diffusion length on defect concentration a “defect free” diffusion length of 4.5 ± 0.3 nm is obtained for CBP singlet excitons in these devices operated under low current density. We also show that the driving voltage scales linearly with the defect concentration.

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.

Near-field transport imaging applied to photovoltaic materials

We developed and applied a new analytical technique—near-field transport imaging (NF-TI or simply TI)—to photovoltaic materials. Charge-carrier transport is an important factor in solar cell performance, and TI is an innovative approach that integrates a scanning electron microscope with a near-field scanning optical microscope, providing the possibility to study luminescence associated with recombination and transport with high spatial resolution. In this paper, we describe in detail the technical barriers we had to overcome to develop the technique for routine application and the data-fitting procedure used to calculate minority-carrier diffusion length values. The diffusion length measured by TI agrees well with the results calculated by time-resolved photoluminescence on well-controlled gallium arsenide (GaAs) thin-film samples. We report for the first time on measurements on thin-film cadmium telluride using this technique, including the determination of effective carrier diffusion length, as well as the first near-field imaging of the effect of a single localized defect on carrier transport and recombination in a GaAs heterostructure. Furthermore, by changing the scanning setup, we were able to demonstrate near-field cathodoluminescence (CL), and correlated the results with standard CL measurements. The TI technique shows great potential for mapping transport properties in solar cell materials with high spatial resolution.

Analysis by Monte-Carlo simulation of uncapped nanocrystals density effects on the collection efficiency

The effects of nanocrystals density on the electron beam induced current are studied by the use of Monte-Carlo simulation. The nanocrystals are considered as recombination centers for captured charges and are distributed randomly at the sample surface. The induced current is collected by a nanocontact. The surface recombination velocity at the free surface is supposed to be zero. For a given electron beam energy, simulated results show a decrease of the carrier collection efficiency and shrinkage of its profile when the nanocrystals density increases. As a consequence, the minority carrier effective diffusion length which is extracted from simulated results is a decreasing function. This is not surprising because it is well known that an increase of recombination centers leads to reduction of the minority carrier lifetime and then a reduction of their diffusion length. These effects are highlighted because of both collecting electrode small size and perpendicular configuration.