Bulk Diffusion Length Research Articles

Measurement of local recombination activity in high diffusion length semiconductors

We present a conceptual approach for the localisation and characterisation of local sites of recombination in high diffusion length semiconductors under photovoltaic field conditions. While established imaging techniques operate in this very regime of uniform “1 sun” illumination, inevitable lateral diffusion of charge carriers veils the origin and severity of localised recombination sites. To reduce this limitation due to lateral diffusion the natural choice is using focussed charge carrier excitation and detection in combination with scanning the specimen. The resulting photoluminescence intensity maps are of high spatial resolution and may be composed of a superposition of a multitude of recombination active defects influencing each other due to the high bulk diffusion length. We demonstrate the feasibility of a self-consistent calibration of the setup quantum efficiency in such experimental condition which delivers a charge carrier density map in absolute units. A solution is presented to disentangle the superposition of local sites of recombination to isolate the actual recombination activity of every site. We demonstrate the feasibility of the approach experimentally on the high diffusion length semiconductor silicon.

MTF-Based Approach to Determining the Bulk Diffusion Length of Photogenerated Charge Carriers in the Absorber Material of MCT-Based Infrared Focal Plane Arrays

MTF-Based Approach to Determining the Bulk Diffusion Length of Photogenerated Charge Carriers in the Absorber Material of MCT-Based Infrared Focal Plane Arrays

Role of Alkali Metal in BiVO4 Crystal Structure for Enhancing Charge Separation and Diffusion Length for Photoelectrochemical Water Splitting.

Alkali metal (Na or K) doping in BiVO4 was examined systematically for enhancing bulk charge separation and transport in addition to improving charge transfer from the surface. The alkali metal-doped BiVO4 thin film photoanodes having nanostructured porous grain surface morphology exhibited better photocurrent density than pristine BiVO4. In particular, Na:BiVO4/Fe:Ni/Co-Pi photoanode showed a significantly improved photocurrent of 3.2 ± 0.15 mA·cm-2 in 0.1 M K2HPO4 electrolyte at 1.23 VRHE under 1 sun illumination. The depth-dependent Doppler broadening spectroscopy measurements confirmed the significant reduction in Bi- and V-based defect density with Na metal doping, and this led to a higher bulk diffusion length of charge pairs (four times that of the pristine one). Na doping led to reduced surface defects resulting in improved surface charge transfer based on cyclic voltammetry experiments. The density functional theory calculations confirmed the improved performance in Na-doped BiVO4 photoanodes achieved through interband formation and reduction in the band gap.

Numerical simulations of two-photon absorption time-resolved photoluminescence to extract the bulk lifetime of semiconductors under varying surface recombination velocities

We investigate the limitations of two-photon absorption time-resolved photoluminescence to measure the low-injection bulk lifetime of different semiconductor materials under varying surface recombination. The excitation source is assumed to be a sub-bandgap pulsed laser and the localized absorption and carrier generation was modeled using a focused TEM00 Gaussian beam under the assumption of diffraction-limited performance. The subsequent carrier kinetics were simulated by applying the finite-difference time-domain method to the continuity equation. Three typical semiconductor materials were modeled: direct bandgap low-mobility material (such as CZTS), direct bandgap high mobility (such as GaAs), and indirect bandgap high mobility (such as float-zone silicon). The extracted effective lifetime as a function of surface recombination velocity was compared to the bulk lifetime and the effective lifetime calculated using an analytical 1D approximation. For the direct bandgap materials, focusing inside the material yields an effective lifetime within a few percent of the bulk lifetime, regardless of the surface recombination velocity, while for excitation close to the surface it is up to 30% lower than the bulk lifetime at high surface recombination velocities (>104 cm/s). For the indirect bandgap material, the effective lifetime is dominated by the surface, making the bulk lifetime inaccessible, even at surface recombination velocities of 100 cm/s. Finally, we use the 1D approximation to find under what conditions the bulk lifetime can be extracted by this method and determine that both the bulk diffusion length and the product of the bulk lifetime and surface recombination velocity must be much less than twice the device thickness.

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.

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.

Effective Diffusion Length and Bulk Saturation Current Density Imaging in Solar Cells by Spectrally Filtered Luminescence Imaging

Most methods for interpreting electroluminescence (EL) or photoluminescence (PL) images of solar cells evaluate the local diode voltages but not the local luminescence intensity itself. One exception is the Fuyuki approximation, which assumes that the local value of the luminescence signal is proportional to the local effective diffusion length. This dependence has been derived for infinitely thick solar cells and neglects self-absorption of the luminescence photons. However, for real solar cells and imaging conditions, with increasing diffusion length, the luminescence signal approaches a limiting value; hence, the Fuyuki approximation no longer holds. In this paper, we compare EL and PL images of multicrystalline solar cells using different kinds of light filtering and find that gentle shortpass filtering is useful for avoiding optical artifacts. Based on earlier calculations, a physically founded formula for the dependence of the gently shortpass-filtered luminescence signal on the bulk diffusion length, for a given rear surface recombination velocity, is presented. Since this formula only barely allows us to calculate the diffusion length from the luminescence signal, a simplified approximate formula is proposed, and its accuracy is checked. This method is tested on EL and ${V_{{{\rm oc}}}}$ PL images of solar cells. We find that for a typical industrial multicrystalline Al-backside solar cell, the obtained effective diffusion length images correlate well with such images obtained by spectral LBIC image evaluation. In addition, the saturation current density images correlate well with such images obtained by dark lock-in thermography, which show a much lower spatial resolution. The main limitation of the proposed method is that it is basically approximate and needs some fitting parameters.

Impact of Iron Precipitation on Phosphorus-Implanted Silicon Solar Cells

Ion implantation is a promising method to implement a high-performance emitter for crystalline silicon solar cells. However, an implanted emitter redistributes and mitigates harmful metal impurities to a different degree than a diffused one. This paper quantitatively assesses the effect of iron contamination level on the bulk diffusion length and open-circuit voltage of phosphorus-implanted solar cells manufactured with varying gettering parameters. By synchrotron-based micro-X-ray fluorescence measurements, we directly observe a process-dependent iron precipitate size distribution in the implanted emitters. We show that controlling the iron precipitate size distribution is important when optimizing final cell performance and discover a tradeoff between large shunting precipitates in the emitter and a high density of recombination active small precipitates in the wafer bulk. We present a heterogeneous iron precipitation model capable of reproducing the experimentally measured size distributions. We use the model to show that the dominant gettering mechanism in our samples is precipitation and that implanted emitters with surface phosphorus concentrations around $2{\times}10^{19}$ cm $^{-3}$ induce little-to-no segregation-based gettering. Based on this finding, we discuss optimal gettering strategies for industrial silicon solar cells with implanted emitters.

Economic feasibility of bifacial silicon solar cells

AbstractBifacial solar cells and modules are a promising approach to increase the energy output of photovoltaic systems, and therefore decrease levelized cost of electricity (LCOE). This work discusses the bifacial silicon solar cell concepts PERT (passivated emitter, rear totally diffused) and BOSCO (both sides collecting and contacted) in terms of expected module cost and LCOE based on in‐depth numerical device simulation and advanced cost modelling. As references, Al‐BSF (aluminium back‐surface field) and PERC (passivated emitter and rear) cells with local rear‐side contacts are considered. In order to exploit their bifacial potential, PERT structures (representing cells with single‐sided emitter) are shown to require bulk diffusion lengths of more than three times the cell thickness. For the BOSCO concept (representing cells with double‐sided emitter), diffusion lengths of half the cell thickness are sufficient to leverage its bifacial potential. In terms of nominal LCOE, BOSCO cells are shown to be cost‐competitive under monofacial operation compared with an 18% efficient (≙ pMPP = 18 mW/cm2) multicrystalline silicon (mc‐Si) Al‐BSF cell and a 19% mc‐Si PERC cell for maximum output power densities of pMPP ≥ 17.3 mW/cm2 and pMPP ≥ 18.1 mW/cm2, respectively. These values assume the use of $10/kg silicon feedstock for the BOSCO and $20/kg for the Al‐BSF and PERC cells. For the PERT cell, corresponding values are pMPP ≥ 21.7 mW/cm2 and pMPP ≥ 22.7 mW/cm2, respectively, assuming the current price offset (≈50%, at the time of October 2014) of n‐type Czochralski‐grown silicon (Cz‐Si) compared with mc‐Si wafers. The material price offset of n‐type to p‐type Cz‐Si wafers (≈15%, October 2014) currently accounts for approximately 1 mW/cm2, which correlates to a conversion efficiency difference of 1%abs for monofacial illumination with 1 sun. From p‐type mc‐Si to p‐type Cz‐Si (≈30% wafer price offset, October 2014), this offset is approximately 2.5 mW/cm2 for a PERT cell. When utilizing bifacial operation, these required maximum output power densities can be transformed into required minimum rear‐side illumination intensities for arbitrary front‐side efficiencies ηfront by means of the performed numerical simulations. For a BOSCO cell with ηfront = 18%, minimum rear‐side illumination intensities of ≤ 0.02 suns are required to match a 19% PERC cell in terms of nominal LCOE. For an n‐type Cz‐Si PERT cell with ηfront = 21%, corresponding values are ≤ 0.11 suns with 0.05 suns being the n‐type to p‐type material price offset. This work strongly motivates the use of bifacial concepts to generate lowest LCOE. Copyright © 2016 John Wiley & Sons, Ltd.

Analysis of charge-carrier diffusion in the photosensing films of HgCdTe infrared focal plane array photodetectors

The lateral diffusion of minority charge carriers in the photosensitive films of photovoltaic HgCdTe infrared focal plane array (IR FPA) detectors with a continuous (without mesa insulation of the diodes) absorber layer was experimentally studied and analyzed. The experimental study was performed using the spot-scan technique, implemented by scanning a narrow strip-shaped illumination spot with a selected FPA diode at various levels of diode photocurrents in the detector. The local diffusion lengths of the charge carriers in the film regions beneath and outside the FPA diodes were estimated. The values obtained for the bulk diffusion length of minority carriers in the HgCdTe material of the examined middle-wave and long-wave IR FPA detectors were found to be in good agreement with previously reported data. Moreover, the estimated local diffusion length of minority carriers in the film region beneath the back-biased photodiodes proved to be consistent with a theoretical estimate of this length.

Determination of charge-carrier diffusion length in the photosensing layer of HgCdTe n-on-p photovoltaic infrared focal plane array detectors

In the present paper, we propose a method for evaluating the bulk diffusion length of minority charge carriers in the photosensing layer of photovoltaic focal plane array (FPA) photodetectors. The method is based on scanning a strip-shaped illumination spot with one of the detector diodes at a low level of photocurrents jph being registered; such scanning provides data for subsequent analysis of measured spot-scan profiles within a simple diffusion model. The asymptotic behavior of the effective (at jph ≠ 0) charge-carrier diffusion length ld eff as a function of jph for jph → 0 inferred from our experimental data proved to be consistent with the behavior of ld eff vs jph as predicted by the model, while the obtained values of the bulk diffusion length of minority carriers (electrons) in the p-HgCdTe film of investigated HgCdTe n-on-p FPA photodetectors were found to be in a good agreement with the previously reported carrier diffusion-length values for HgCdTe.

Determination of Diffusion Length for the Finite Thickness Normal-Collector Configuration Using EBIC Technique

This paper presents a study of the extraction of the diffusion length in the normal-collector configuration with finite thickness with the use of the electron beam-induced current (EBIC) line scan. The EBIC data used were obtained from a commercially available device simulator. It was found that the surface recombination at the free surfaces affects the EBIC data, and therefore also affects the value of the extracted diffusion length and the linearization coefficient α. The effects of the surface recombination at the free surfaces and the sample thickness on the diffusion length extracted and the linearization coefficient α were researched and are discussed in this paper. Furthermore, the conditions at the free surfaces for a direct and accurate extraction of the bulk diffusion length at different sample thicknesses are suggested.

Lifetime Measurements on Attached Epilayers and Detached Epifoils Grown on Reorganised Porous Silicon Showing a Bulk Lifetime Exceeding 100 μs

This paper discusses on-going efforts towards reliable lifetime measurements on epilayers and the subsequent decoupling of the bulk and surface recombination components, both while it is still attached to the p+ substrate on which it is grown (“attached epilayer”) and after its detachment from the substrate (“detached epifoil”). For the “attached epilayers”, microwave photoconductance decay (^-PCD) and simulation-assisted photoluminescence (sim-PL) were applied together with a variation in the epilayer thickness to evaluate the bulk lifetime (Tbulk) and total effective surface recombination velocity (Stot) of p-type epilayers. By applying linear fits to reciprocal effective lifetime versus reciprocal epilayer thickness data, Stot of all samples were reliably extracted, resulting in Stot of ∼265cm/s in the absence of porous silicon at the epilayer/substrate interface compared to ∼9220cm/s when present and ∼775cm/s when shielded by a thin back surface field. Based on these Stot values, sim-PL was used to estimate Tbulk to be ∼160 |us in the porous silicon area. For the n-type “detached epifoils”, quasi-steady state photoconductance (QSSPC) was used. However, reliable Tbulk could not be extracted, despite the lower Stot because Tbulk was too high. However, a lower limit to Tbulk was estimated to be >138 μs. Such high bulk lifetimes imply bulk diffusion lengths that are an order of magnitude longer than the epilayer thicknesses, potentially leading to high cell efficiencies.

Excellent Average Diffusion Lengths of 600 μm of N-Type Multicrystalline Silicon Wafers After the Full Solar Cell Process Including Boron Diffusion

In this paper we investigate the material quality of n- and p-type multicrystalline silicon wafers after different high- temperature steps, as applied during cell processing. Both materials start with a high initial bulk diffusion length of around 440μm (harmonic mean of the whole wafer) which is further improved by the solar cell process. A diffusion length of 510μm was measured after phosphorus and boron diffusion and firing in the n-type material. The p-type wafers showed diffusion lengths of 540μm after phosphorus diffusion and firing. These diffusion lengths were measured at a generation rate of 1/20 sun close to maximum power point injection conditions of a solar cell. At higher injection levels both materials reach 600μm diffusion length. The high material quality of n-type material maintained after the high temperature boron diffusion is remarkable. An efficiency analysis shows that these excellent diffusion lengths allow for high efficiency devices exceeding 20% efficiency.

Hollow Anatase TiO2 Single Crystals and Mesocrystals with Dominant {101} Facets for Improved Photocatalysis Activity and Tuned Reaction Preference

Faceting photocatalysts has attracted increasing interest to improve photocatalytic activity by optimizing surface charge carrier separation/transfer. In principle, a high photocatalytic activity is co-contributed by both high surface separation/transfer and low bulk recombination of charge carriers. However, little effort focuses on lowering bulk recombination of charge carriers in faceted photocatalysts. In this work, we report the synthesis of hollow anatase TiO2 single crystals and mesocrystals with dominant {101} facets by a new route with PO43–/F– as morphology controlling agent. It is found that with respect to solid crystals, being hollow crystals and mesocrystals can substantially improve photocatalytic activity (O2/H2 evolution from water splitting, CH4 generation from photoreduction of CO2) as a result of the synergistic effects of shortened bulk diffusion length of carriers for the decreased bulk recombination and increased surface area. Furthermore, the photocatalysis reaction preference towa...

Three-dimensional simulation of electron beam induced current collected by a nano-contact: Diffusion and collection analysis

Three-dimensional simulation is used to study the electron beam induced current collected by a nano-contact forming a nano-junction. For the calculation the surface recombination velocity is supposed to be either infinite or zero. The influence of the bulk diffusion length, energy of the incident electron beam, shape and size of the depletion zone beneath the nano-contact on the induced current collected by the nano-junction is studied. From the fit of the simulated data, the minority carrier effective diffusion length is extracted, and its variation with the electron beam energy is compared with experimental measurements obtained with the nano-EBIC (electron beam induced current) technique that we have developed by combining scanning probe microscopy and scanning electron microscopy. The effective diffusion length is an increasing function with electron beam energy up to a certain value beyond which it decreases. The simulation shows that the same behavior also occurs in the case of the conventional detection by a semi-infinite Schottky contact perpendicular to the electron beam. This is the first time, to our knowledge, that this behavior is pointed out.

Analysis of Electron-Beam Crystallized Large Grained Si Films on Glass Substrate by EBIC, EBSD and PL

The properties of electron-beam crystallized, large-grained silicon layers of about 10 µm thickness on glass have been studied by combining EBIC, EBSD and photoluminescence. It is found that most grains are free of dislocations. From a detailed analysis based on the dependence of EBIC collection efficiency on beam energy we conclude that the recombination properties of the layers are mainly determined by the bulk diffusion length. The estimated bulk diffusion length in the dislocation-free layer regions is in the range of roughly 5 – 7 µm, depending on the recombination velocity assumed for the rear surface. In dislocated regions the diffusion length drops to 1 µm or less. Close to some twin boundaries, an unsusual improvement of the electrical layer properties has been observed. In addition, wave-like inhomogeneities of the layer properties have been established, resulting probably from instabilities during the crystallization process.

Determination of diffusion length in photovoltaic crystalline silicon by modelisation of light beam induced current

The diffusion length of minority carriers is one of the most important electrical parameters to qualify silicon for photovoltaic applications. One way to evaluate this parameter is to analyse the decay of the current induced when a focused beam is scanned away from the collector using Light Beam Induced Current (LBIC) technique. The LBIC signal was numerically calculated with 2D-DESSIS software under different boundary conditions, as a function of material thickness and surface recombination velocity in order to verify the limitations of analytical models and to fit the LBIC signal measured in thin silicon samples. Samples with thickness ranging from 55 μm to 2500 μm were evaluated with diffusion length values ranging from 70 μm to 2.5 mm. Analytical expressions of the Internal Quantum Efficiency (IQE) were also used to extract the minority carrier bulk and effective diffusion lengths from surface averaged spectral response and reflectivity data in thick solar cells.

Note on the interpretation of electroluminescence images using their spectral information

We show how to use the reciprocity between electroluminescence and solar cell quantum efficiency for the simulation and interpretation of electroluminescence images. The analysis of the spectral properties of the electroluminescence images obtained by using different short- and long-pass filters yields information about surface recombination velocity, bulk diffusion length and reflection of the back side in flat and textured solar cells.

Minority carrier bulk lifetimes through a large multicrystalline silicon ingot and related solar cell properties

The bulk lifetime tau(n) and diffusion length L-n of minority carriers vary through the height of a cast multicrystalline silicon ( mc-Si) block. This variation is due to the segregation of metallic impurities during the directional solidification and the native impurity concentrations increase from the bottom to the top of the ingot, which is solidified last, while the ingot bottom, which is solidified first, is contaminated by the contact with the crucible floor. It is of interest to verify if a correlation exists between the bulk lifetime tau of as cut wafers and the conversion eficiency. of solar cells. In a very large ingot (> 310 kg), it was found that tau(0), in raw wafers, tau(dif) in phosphorus diffused ones and Ln in diffused wafers are smaller in the top and in the bottom of the ingot. The same evolution is observed in solar cells, however the diffusion length values L-cel in the central part of the ingot are markedly higher than those found in diffused wafers, due to the in-diffusion of hydrogen from the SiN-H antireflection coating layer. The variations of. and those of tau(0), along the ingot height, are well correlated, suggesting that the evaluation of tau(0) can predict the properties of the devices. In addition, segregation phenomena around the grain boundaries are observed at the bottom of the ingots, due to a marked contamination by the crucible floor, and at its top where impurities are accumulated. These phenomena are linked to the long duration of the solidification process and the large amount of imperfect silicon used to cast the ingot.