Bulk Minority Carrier Diffusion Length Research Articles

A contactless method for measuring the recombination velocity of an individual grain boundary in thin-film photovoltaics

A cathodoluminescence-based, contactless method for extracting the bulk minority carrier diffusion length and reduced recombination velocity of an individual grain boundary is applied to vapor grown CdTe epitaxial films. The measured diffusion length was within the range of 0.4–0.6 μm and the grain boundary recombination velocity varied from 500 to 750 cm/s. The technique can be used to investigate the effect of grain boundaries on photovoltaic performance.

Design constraints and guidelines for CdS/CdTe nanopillar based photovoltaics

The performance dependence of a CdS/CdTe nanopillar solar cell on various device and materials parameters is explored while examining its performance limits through detailed device modeling. The optimized cell enables efficiencies >∼20% with minimal short circuit current dependence on bulk minority carrier diffusion length, demonstrating the efficient collection of photogenerated carriers, therefore, lowering the materials quality and purity constraints. Given the large p-n junction interface area, the interface recombination velocity is shown to have detrimental effect on the device performance of nanopillar solar cells. In that regard, the CdS/CdTe material system is optimal due to its low interface recombination velocity.

Influence of substitutional metallic impurities on the performances of p-type crystalline silicon solar cells: The case of gold

The influence of a gold bulk contamination on the performances of boron doped p-type crystalline silicon solar cells is investigated for different base doping levels and different kinds of materials, such as float zone Si, Czochralski Si, and multicrystalline Si. Solar cells are made from intentionally contaminated silicon wafers. By monitoring the evolution of the electrically active substitutional gold concentration by means of bulk lifetime and minority carrier diffusion length measurements, this paper highlights the eventual gettering or hydrogenation effects occurring throughout the whole process but also of the danger of such an impurity in materials containing large densities of extended defects generating recombination centers by means of the impurity-defect interaction.

Dependence of the recombination in thin-film Si solar cells grown by ion-assisted deposition on the crystallographic orientation of the substrate

One promising strategy for achieving high-quality polycrystalline silicon thin-film solar cells on glass is based on low-temperature ion-assisted deposition for epitaxial thickening of a thin, large-grained seeding layer on glass. The crystal growth on the seeding layer is influenced by various factors, amongst which the crystal orientation of the grains plays a substantial role. In this paper we investigate how the electronic properties of solar cells grown on ‘‘ideal’’ seeding layers (Si wafers) are influenced by the crystallographic orientation of the substrate. The Si wafers are heavily doped p-type, ensuring that their contribution to the photogenerated current is small. The films grown on (1 0 0)-oriented Si substrates have a very low density of structural defects, while the films grown on (1 1 1)-oriented Si substrates display a high density of twin defects. The electronic properties of the thin-film solar cells were investigated by means of open-circuit voltage measurements as a function of the incident light intensity. The (1 0 0)oriented diodes consistently exhibit a higher Voc than the (1 1 1)-oriented diodes throughout the entire illumination range from 10 � 3 to 10 3 Suns. We determine 7mm as the bulk minority carrier diffusion length of the as-grown (1 0 0)-oriented Si film. A lower bound of 3mm was found for the bulk minority carrier diffusion length in the as-grown (1 1 1)-oriented Si film. The performances of both types of solar cells were improved by hydrogenation in an ammonia plasma. At voltages around the 1-Sun maximum power point the improvement is due to a reduction of non-ideal current mechanisms. The diffusion length of the (1 0 0) diode remains unaffected by hydrogenation while the lower bound of the diffusion length of the (1 1 1) diode improves to 10mm. r 2002 Elsevier Science B.V. All rights reserved.

Porous silicon-based passivation and gettering in polycrystalline silicon solar cells

In this work, we report on the effect of introducing a superficial porous silicon (PS) layer on the electrical characteristics of polycrystalline silicon solar cells. The PS layer was formed using a vapour etching (VE)-based method. In addition to its known anti-reflecting action, the forming hydrogen-rich PS layer acts as a passivating agent for the surface of the cell. As a result we found an improvement of the I– V characteristics in dark conditions and AM1 illumination. We show that when the formation of a superficial PS layer is followed by a heat treatment, gettering of impurities from the polycrystalline silicon material is possible. After the removal of the PS layer and the formation of the photovoltaic (PV) structure, we observed an increase of the light-beam-induced-current (LBIC) for treatment temperatures not exceeding 900 °C. An improvement of the bulk minority carrier diffusion length and the grain boundary (GB) recombination velocity were observed as the temperature rises, although a global decrease of the LBIC current was observed for temperatures greater than 900 °C.

Photocurrent of 1 eV GaInNAs lattice-matched to GaAs

The spectral photocurrent response, or quantum efficiency, of GaInNAs lattice-matched to GaAs with a bandgap of 1 eV has been measured in an electrochemical cell in order to optimize material quality for use in a multijunction solar cell lattice-matched to GaAs or Ge. GaInNAs with about 3% nitrogen grown by MOVPE using dimethylhydrazine as a nitrogen source was found to be p-type with hole concentrations of about 1×10 17 cm −3, possibly due to unintentional carbon doping as observed by SIMS. This unintentionally doped material typically had up to 20% internal quantum efficiencies and an absorption coefficient of about 10 000 cm −1, 200 meV above the bandedge. By doping the material with Si, Se, or Zn, we find that the quantum efficiency is strongly correlated with the carrier concentration because the quantum efficiency is limited by the junction depletion width rather than the bulk minority carrier diffusion length. Low minority carrier lifetimes and mobilities both appear to contribute to these short diffusion lengths. We have observed significant increases in temperature dependent Hall mobilities and photoluminescence intensities due to changes in growth conditions and post-growth anneals.

Multicrystalline silicon material: Effects of classical and rapid thermal processes

For photovoltaic applications silicon is still the predominant material. Besides monocrystalline Czochralski wafers (Cz-Si), multicrystalline sheets (mc-Si) play an important role in terrestrial power applications (almost 50%). Large mc-Si ingots (up to 250 kg) are now produced in large scale by the industry using various directional solidification methods in appropriate crucibles (or molds). However, if the crystallographic properties are now quite satisfactory (columnar structure with large grains of more than 1 cm2, dislocations and intragrains defects), multicrystalline silicon contains larger quantities of impurities than single crystalline silicon which can have detrimental effects on the bulk minority carrier diffusion length (Ln,p). These impurities, including metals as well as high concentrations of carbon and/or oxygen, can degrade the photovoltaic properties of solar cells. Thermal treatments such as gettering, performed in a classical or rapid thermal furnace, studied separately or in conjunction with the doping steps can limit or avoid the degradation of the bulk diffusion length, but its efficiency is strongly dependent on the presence of these impurities in Si.

A study of He + ion-induced damage in silicon by quantitative analysis of charge collection efficiency data

The ion damage is responsible for the progressive reduction of the collection efficiency of a semiconductor device in Ion Beam Induced Charge (IBIC) measurements. This effect was studied by irradiating a silicon n +-p diode with a 1.85 MeV He + microbeam at very low fluences ≤5 × 10 9ions/cm 2 and measuring the decrease of charge pulse height vs ion fluence. The experimental results can be described by a model, which represents the region of maximum damage as an interface with given carrier recombination velocity v s. The model assumes that the ion fluence affects v s, but not the bulk minority carrier diffusion length. The comparison between experiment and theory yields an approximately linear relationship between ion fluence and v s; this yields a value of about 7 for the average number of recombination centers produced by a single He + ion.

P/A1 co-gettering effectiveness in various polycrystalline silicon

The various polycrystalline silicon materials (cast ingots, ribbons) which are commercially available for the solar cells manufacturing differ very much among themselves due to the different growth processes. The resulting microstructure and impurity content will influence differently the material characteristics during thermal treatments inherent to the device manufacturing. As the gettering efficiency depends on the kind of polycrystalline material, the variations observed in the optimal gettering conditions or passivation will be discussed. In this paper, we compare the performances of various types of polycrystalline silicon upon classical and rapid thermal-process-induced co-diffusion of phosphorus and aluminium. We show that a large bulk minority carrier diffusion length enhancement occurs in the case of co-diffusion when compared to the separate diffusion of phosphorus and aluminium.

A generalized approach to surface photovoltage

A detailed majority and minority carrier current balance model for surface photovoltage (SPV) is presented. A rigorous treatment of carrier junction generation and surface recombination is shown to be essential. We show that under weak above band gap photoexcitation, standard constant open-circuit voltage and constant photon flux SPV measurements can both reveal the bulk minority carrier diffusion length, provided the initial band bending Vbi is sufficiently high. However, when Vbi is small, approximate SPV expressions are inapplicable and both standard SPV manifestations will then yield incorrect diffusion length values. We also show that for cases of low surface recombination velocity, the SPV technique can be used to measure the width of the space charge region and Vbi; however, when the surface recombination velocity is large, this approach cannot be used as majority carrier backdiffusion can play a dominant role under high photon energy excitation conditions.

Influence of extended defects and native impurities on external gettering in polycrystalline silicon

The ability of phosphorus diffusion to remove metallic impurities from polycrystalline silicon is analysed in this paper. The intragranular and grain boundary electrical activity into the gettered substrate is investigated using the light-beam-induced current technique. It is pointed out that the bulk minority carrier diffusion length is strongly degraded even during external gettering treatment (15–120 min, 850 and 950 °C): the phorphorus layer does not act as an effective sink for impurities. Our results show also that the grain boundary barriers increase with increasing temperature and annealing duration of these treatments. From these observations, we conclude that the native impurities are activated during subsequent annealing depending on the thermal history of the crystal and that the limiting factor for the gettering efficiency would be the interaction between native point defects and extended defects. In addition, arguments are provided to explain the temperature dependence on external gettering observed in this work.

A direct method for the extraction of diffusion length and surface recombination velocity from an EBIC line scan: planar junction configuration

A direct method of extracting bulk minority carrier diffusion length and surface recombination velocity from an EBIC line scan in the planar configuration is described. The accuracy of the method is verified by 3-D computer simulation and compared with existing methods. It mas found that this method is much simpler to use and gives better accuracy than existing methods. >

A direct and accurate method for the extraction of diffusion length and surface recombination velocity from an EBIC line scan

A new method is proposed for the determination of bulk minority carrier diffusion length and surface recombination velocity. This method uses data from an EBIC line scan in which the current collecting p− n junction or Schottky barrier is parallel to the electron beam. A 3-D computer simulation was used to verify the accuracy of the method. It was found that this method is simpler to use and more accurate than existing methods.

Bulk minority carrier diffusion length and photogenerated carrier profile in silicon photovoltaic cells

The bulk minority carrier diffusion length of single-crystal and polycrystalline ribbon silicon photocells was measured with monochromatic light of various wavelengths and flux densities. We noted that the diffusion length shows a gradual reduction as the illumination wavelength becomes shorter in both types of cells. In addition, the light enhancement phenomenon of the diffusion length in the ribbon cells diminished when shorter wavelength lights were used. In the single-crystal cells, however, no enhancement effect was observed for any wavelength. The interpretation of these results is based on the examination of the photogenerated minority carrier profile as a function of diffusion length and wavelength in the bulk region of the cell. We correlate the measured diffusion length with the depth in which a positive minority carrier concentration gradient exists.

Effect of heat treatment on the bulk diffusion length of EFG ribbon silicon

The effect of thermal anneal on the bulk minority carrier diffusion length, L n , of Edge-defined Film-fed Growth (EFG) ribbon silicon has been investigated. Statistical distributions of L n were gathered after the thermally treated ribbons were fabricated into solar cells by a cold junction formation method (ion implant and pulsed electron beam anneal). The measurements were made by using a bifurcated optical guide arrangement which was specifically designed for ribbon samples with variable surface reflectance. The experimental results indicated that, after a medium temperature (800°C) thermal anneal in a neutral ambient, the form of the L n distribution changed in an overall degradation in the average value. With the same thermal anneal in a gettering ambient, the distribution also altered, but with an overall improvement. The observations will be discussed in terms of the interaction of metallic impurities with the local structural and chemical defects contained in the ribbons.

Effect of short wavelength illumination on the characteristic bulk diffusion length in ribbon silicon solar cells

The influence of short wavelength light on the characteristic bulk minority carrier diffusion length of the ribbon silicon photovoltaic cell has been investigated. We have measured the intensity and wavelength dependence of the diffusion length in an EFG ribbon cell, and compared it with a standard Czochralski grown silicon cell. While the various short wavelength illuminations have shown no influence on the diffusion length in the CZ cell, the diffusion lengths in the ribbon cell exhibit a strong dependence on the volume generate rate as well as on the wavelength of the superimposed lights. We have concluded that the trap-filling phenomenon at various depths in the bulk neutral region of the cell is consistent with the experimental observation.

Energy Dependence of Proton Irradiation Damage in Silicon

The energy dependence of radiation damage in silicon for proton energies in the range 1.35 to 130 MeV has been measured by observing the degradation of the bulk minority carrier diffusion length in silicon solar cells. Variability in proton flux determination at four different accelerators was minimized by employing prebombarded solar cells with known minority carrier diffusion lengths as calibrated solid-state ionization chambers. Where beam intensity measurement comparisons with Faraday cups could be made, agreement to better than 5% was obtained. The quantity characterizing the damage rate is the rate of change of the inverse square diffusion length with flux K≡d(1/L2)/dΦ. The 1-Ω-cm p-type silicon degraded, on the average at a rate six times less rapid than 1-Ω-cm n type, independent of energy. Room temperature annealing gave 30% to 50% decrease in K whenever the diffusion length was measured during and after irradiation. The energy variation of K agrees with the variation predicted by Rutherford scattering below 8 MeV, but decreases less rapidly at higher energies. The measured diffusion lengths increased with excess carrier density n from 2% per decade at n=109cm−3 to 20% per decade at n=1014cm−3. The reported results, obtained at low excess carrier density, can be used to predict solar cell degradation under conditions of outer space illumination if the appropriate excess carrier density is used. Failure to take into account the diffusion length variation will result in an underestimate of the solar cell output of less than 7%.

Solar Cell Degradation under 1-Mev Electron Bombardment

The effect of radiation damage on the important parameters of solar cells has been evaluated for groups of blue-sensitive n-on-p, normal p-on-n, and blue-sensitive p-on-n cells using 1-Mev electrons. The outer space short circuit current, maximum power, junction characteristic, and spectral response are presented quantitatively as a function of radiation flux along with the bulk minority-carrier diffusion length. The rate of change of inverse squared diffusion length with flux is found to be 1.22 × 10−8 for the p-on-n cells, as compared to 1.7 × 10−10 for the n-on-p. The degradation of the spectral response is consistent with the measured diffusion length for both types of cells if one assumes a total effective front layer of 1 μ thickness for the p-on-n cells. As a result of the less rapid degradation of their minority-carrier lifetime, the n-on-p cells exhibit a greater resistance to radiation than the p-on-n cells. Comparing only the two types of blue-sensitive cells, after prolonged bombardment, the flux ratios required to achieve equal values of short-circuit current and maximum power are 17 and 9.5 respectively.