Diffusion Length Of Minority Charge Carriers Research Articles

Extraordinarily High Minority Charge Carrier Lifetime Observed in Crystalline Silicon

Recent progress in surface passivation technology and wafer pretreatment already resulted in significant improvements in the achievable minority charge carrier lifetime of crystalline silicon. Herein, this is further exemplified by studying the lifetime on lowly doped crystalline silicon wafers passivated by poly‐Si. To ensure credible lifetime measurements multiple measurement techniques are compared and good agreement between the investigated approaches is found. The resulting lifetime curves are analyzed in detail and the main limitation is very likely caused by silicon bulk recombination—most likely due to impurities. This analysis indicates that even very low impurity concentrations can be a limiting factor at the extraordinary high level of charge carrier lifetime observed in this study. Despite these limitations, lifetimes of 0.18 s on p‐type and 0.5 s on n‐type crystalline silicon wafers are measured, which to our knowledge exceed previously reported lifetimes. In both cases, these measured lifetimes correspond to an effective minority charge carrier diffusion length of ≈2.5 cm.

Photonic crystal based photoelectrochemical cell for solar fuels

AbstractThe solar fuel production efficiency of photoelectrochemical cells based on TiO2 is primarily limited by the low optical absorption coefficient and concomitant long penetration depth of visible light compared with the minority charge carrier diffusion length. A TiO2 photoelectrode consisting of a slanted conical‐pore photonic crystal immersed in water and integrated on a reflective back‐contact is proposed to reduce the light penetration depth by more than 10 times and branch the electron and hole transports into dimensions matching their diffusion lengths. This brings the photoelectrolytic process into the high‐efficiency operating regime. The maximum achievable photocurrent density for a lightly doped TiO2 with the imaginary part of the dielectric constant 0.01 in the visible range reaches 7.65 mA cm‐2.

Electrochemical Fabrication of Cu2o-Nanowires-Based Photo-Cathodes for Enhanced Solar Water Splitting

Solar-power driven photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes is one of the most promising approaches for energy conversion, in terms of production of renewable hydrogen with a minimum carbon footprint. There are, however, enormous challenges to be overcome to obtain sufficient solar water-splitting efficiency, such as over-potential losses due to fast recombination rates of charge carriers, electrode photo-degradation, limited light-harvesting capacities, to name a few. In this work, we report an extensive study on Cu2O based photocathode for hydrogen evolution reaction (HER) in terms of enhancing its solar-to-hydrogen (STH) conversion efficiency, while using low-cost fabrication methods. We have addressed the inefficiency of Cu2O which mainly arises due to incompatible light absorption and charge carrier diffusion lengths, by fabricating highly reproducible Cu2O-nanowire arrays on Cu substrate. The reported optimal thickness of Cu2O for efficient absorption of sunlight is around 2-4 µm, while the minority charge carrier diffusion length is limited to 200-250 nm only. The nanostructured Cu2O, especially in the form of nanowire arrays, improves its performance through morphology control, by improving the light-harvesting capacity along the full length of wire and providing a shorter diffusion path length corresponding to the radial distance in the wire, towards the electrolyte. We prepared the photocathode by anodizing Cu foil to Cu(OH)2 nanowires at 10 mA/cm2, followed by oxidation to Cu2O nanowires in an inert atmosphere. Experimental measurements showed that the process was highly reproducible and resulted in an increased current density from 0.5 mA/cm2 for planar Cu2O to 82.3 mA/cm2 for Cu2O nanowires, both at 1 V cathodic bias. The photocurrent density was enhanced from 0.15 mA/cm2 for planar Cu2O to 5.54 mA/cm2 for the nanowire morphology, while using visible light source in all cases. All the photoelectrodes were reproducible in the regime of 4 µm thicknesses of Cu2O nanowires, oxidized over Cu-foil substrates. The STH efficiency calculated for all the fabricated photoelectrodes was as high as 3.54±0.21%, in comparison to 0.24% for the planar Cu2O photoelectrodes. Keywords: Cu2O, nanowires, photocathode, solar water splitting, photoelectrochemistry, STH efficiency, PEC.

Light-induced changes in the minority carrier diffusion length of Cu(In,Ga)Se2 absorber material

In this study strong evidence for an illumination-induced change in minority charge carrier diffusion length is given for Cu(In,Ga)Se2 solar cells. After annealing under illumination (light soaking) the cells show the well-known metastable increase in open circuit voltage, but also a metastable reduction in current collection efficiency (which can be reversed by annealing in the dark). Partly, this can be attributed to an increase in doping density causing a reduced space charge region width as verified by capacitance-voltage profiling. Nevertheless, by using time-resolved photoluminescence and electron-beam-induced current measurements we found that the changes in doping density and space charge region width are not sufficient to describe the modification in current collection efficiency. Additionally there seems to be a reduction in minority carrier diffusion length and lifetime after white light soaking. This can be explained by a metastable change of electronic defects as found in temperature-dependent admittance spectroscopy. Device simulations confirm the impact of the found defects on the photocurrent.

Photocurrent Mapping in Single-Crystal Methylammonium Lead Iodide Perovskite Nanostructures.

We investigate solution-grown single-crystal methylammonium lead iodide (MAPbI3) nanowires and nanoplates with spatially resolved photocurrent mapping. Sensitive perovskite photodetectors with Schottky contacts are fabricated by directly transferring the nanostructures on top of prepatterned gold electrodes. Scanning photocurrent microscopy (SPCM) measurements on these single-crystal nanostructures reveal a minority charge carrier diffusion length up to 21 μm, which is significantly longer than the values observed in polycrystalline MAPbI3 thin films. When the excitation energy is close to the bandgap, the photocurrent becomes substantially stronger at the edges of nanostructures, which can be understood by the enhancement of light coupling to the nanostructures. These perovskite nanostructures with long carrier diffusion lengths and strong photonic enhancement not only provide an excellent platform for studying their intrinsic properties but may also boost the performance of perovskite-based optoelectronic devices.

Grain boundary light beam induced current: A characterization of bonded silicon wafers and polycrystalline silicon thin films for diffusion length extraction

The charge carrier lifetime and accordingly the diffusion length in polycrystalline semiconductor materials is known to be detrimentally influenced by disordered interfaces like grain boundaries (GBs). The GB light beam induced current (GB‐LBIC) technique is suitable for the extraction of the minority charge carrier diffusion length in unprocessed polycrystalline materials. This measurement method is based on the GB itself acting as a charge collector. A spatially‐resolved light beam induced current can thus be measured even without a collecting p–n junction or Schottky contact, and without biasing the sample. In this contribution we present a simulation based analysis of measured GB‐LBIC line scans on bonded silicon wafers and polycrystalline silicon thin films with different laser wavelengths and intensities.

High-Efficiency Multicrystalline Silicon Solar Cells: Potential of n-Type Doping

In this study, we demonstrate the potential of multicrystalline (mc) n-type silicon for the fabrication of highly efficient mc-Si solar cells. High-quality mc n-type silicon wafers are obtained from a research ingot crystallized in a high-purity crucible, using high-purity granular silicon as seed layer in the crucible bottom and high-purity polysilicon feedstock for the block. An mc p-type silicon block crystallized under identical conditions (same seed and feedstock, crucible system, and temperature profiles) serves as reference and enables measurements of the interstitial iron and chromium concentrations by metastable defect imaging. In combination with 2-D simulations for in-diffusion and precipitation of chromium, the limitation of n-type high-performance mc silicon by these metals is assessed after different solar cell processing steps. Material-related efficiency losses are assessed by an “efficiency limiting bulk recombination analysis,” which combines injection-dependent photoluminescence imaging of minority charge carrier diffusion length with PC1D cell simulations. Finally, based on this material, boron-diffused front-junction mc n-type silicon solar cells with a full-area passivated rear contact (TOPCon) are fabricated. The record cell features an efficiency of 19.6%, which is the highest efficiency reported for an mc n-type silicon solar cell.

Residual metallic contamination of transferred chemical vapor deposited graphene.

Integration of graphene with Si microelectronics is very appealing by offering a potentially broad range of new functionalities. New materials to be integrated with the Si platform must conform to stringent purity standards. Here, we investigate graphene layers grown on copper foils by chemical vapor deposition and transferred to silicon wafers by wet etching and electrochemical delamination methods with respect to residual submonolayer metallic contaminations. Regardless of the transfer method and associated cleaning scheme, time-of-flight secondary ion mass spectrometry and total reflection X-ray fluorescence measurements indicate that the graphene sheets are contaminated with residual metals (copper, iron) with a concentration exceeding 10(13) atoms/cm(2). These metal impurities appear to be partially mobile upon thermal treatment, as shown by depth profiling and reduction of the minority charge carrier diffusion length in the silicon substrate. As residual metallic impurities can significantly alter electronic and electrochemical properties of graphene and can severely impede the process of integration with silicon microelectronics, these results reveal that further progress in synthesis, handling, and cleaning of graphene is required to advance electronic and optoelectronic applications.

High dose proton implantations into silicon: a combined EBIC, SRP and TEM study

AbstractProton (H+) implantations are used in power semiconductor devices to introduce recombination centers (Hazdra et al., Microelectron. J. 32(5), 449–456 (2001)) or to form hydrogen related donor complexes (Zohta et al., Jpn. J. Appl. Phys. 10, 532–533 (1991)). Proton implantations are also used in the 'smart cut' process to generate defects that can be used to cleave thin wafers (Romani and Evans, Nucl. Instrum. Methods Phys. Res. B 44, 313–317 (1990)). However, the implantation damage resulting from H+implantations is not completely understood. In this study, protons with energies from 400 keV up to 4 MeV and doses up to 1016 H+/cm² were implanted into highly ohmic boron doped m:Cz silicon (100). Electron Beam Induced Current (EBIC) measurements were performed to locally determine the minority charge carrier diffusion length. The diffusion length decreases with increasing implantation dose and incorporated damage. Spreading Resistance Profiling (SRP) measurements were performed to analyze the charge carrier concentration profiles for different annealing procedures. The electrical activation and growth of the defect complexes varies strongly with the annealing parameters. Transmission Electron Microscopy measurements were made to investigate the microscopic structures formed by the high dose implantation processes. Due to the high local damage density resulting from low energy and high dose H+ implants, platelet structures are formed. During high‐energy high‐dose H+implantations, the implanted hydrogen generates strain in the crystal lattice resulting in changes in the distances between atomic planes. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical simulation of cross section electron-beam induced current in thin-film solar-cells for low and high injection conditions

Electron-beam induced current measurements (EBIC) in the cross-section configuration can be used to characterize electronic properties of thin-film solar-cells with a spatial resolution in the submicrometer range. Assuming low injection conditions and complete charge carrier collection in the depletion region, the minority charge-carrier diffusion length and width of the space charge region can be extracted from EBIC data using an analytical expression. In the present work, we evaluate the validity of the assumptions underlying the analytical description by using numerical device simulation to describe EBIC profiles perpendicular to the pn-junction of thin-film solar cells. We find that under low injection conditions, the analytical description provides good results if the minority charge-carrier diffusion length in the absorber layer is significantly larger than the width of the space charge region. On the other hand, the analytical description of the EBIC profiles deviates significantly from the numerical simulation for short diffusion lengths and also for high injection conditions. Experimental EBIC profiles of Cu(In,Ga)Se2 solar cells are evaluated to obtain local minority carrier diffusion-lengths and to illustrate high-injection and low-injection effects in the measurements.

Selective photodetectors based on GaAlAs/GaAs and GaN/GaInN heterostructures

We fabricated selective photodetectors on the basis of graded-gap GaAlAs/GaAs heterostructures and quantum-confined GaInN/GaN structures. At λ= λ max the monochromatic photosensitivity is in the range of 0.1–0.7 A/W (GaAlAs/GaAs), 0.15 A/W (GaInN/GaN). The methods are developed for control over the photosensitivity band width and its energy position in the 0.35–1.1 μm by varying technological factors (such as gap gradient, spacing between the metallurgical boundary and p–n junction, minority charge carrier diffusion length).

Investigation of the spatial resolution of the light-addressable potentiometric sensor

The spatial resolution of the light-addressable potentiometric sensor (LAPS) is investigated both theoretically and experimentally. For a theoretical analysis, the diffusion equation for minority charge carriers in the semiconductor was solved. The results suggest that by thinning the semiconductor wafer, the spatial resolution of the LAPS is no longer limited by the bulk minority charge carrier diffusion length. Spatial resolution in the micrometer range should thus be possible. For an experimental analysis, the effective diffusion length of light-generated charge carriers parallel to the sensor surface was measured. The results show that by increasing the doping density and by thinning the semiconductor substrate, spatial resolution of about 15 μm is obtained.

Ebic contrast in a polycrystalline semiconductor: Grain size dependence

Using the perturbation method (Born approximation), a theoretical model has been developed to account for the EBIC intensities and contrasts likely to occur in an n-type polycrystalline semiconductor, as a function of the grain size. The influence of the minority charge carrier diffusion length and the primary electron beam energy have also been examined, respectively.

Studies on ferric oxide electrodes for the photo-assisted electrolysis of water

Results of the photoelectrochemical studies carried out with sintered discs of pure Fe 2O 3 and of Fe 2O 3 doped with TiO 2, SnO 2, ZrO 2 and Ta 2O 5 are compared and discussed. Flat band potential, saturation current and minority charge carrier diffusion length are higher for TiO 2-doped Fe 2O 3 samples than for other doped specimens. Ta 2O 5-doped Fe 2O 3 samples and SrFe 2O 4 have been found to be photo-inactive. The results are analysed in terms of depletion layer theory.