CZ Silicon Research Articles

Removing the effect of striations in n-type silicon solar cells

Several industrial n-type Czochralski silicon ingots were analysed on wafer and cell levels with ECN׳s bifacial n-type solar cell process. In some of the ingots, the solar cell performance in the very top drop of about 1% absolute with respect to cell from the middle part of the ingot. These cells show typical ring shaped pattern. After receiving a post-process anneal treatment at 200°C, the efficiency nearly completly recover. We demonstrated that the improvement is due to bulk lifetime enhancement. The recovery is stable in storage conditions, under illumination and high temperature treatments up to 600°C. The same effect cannot be reproduced in p-type Cz silicon solar cells with similar ring shaped patterns. This indicates that the defects responsible for lifetime and efficiency degradation in wafers affected by ring patterns differ in n-type and p-type.

Tunnel oxide passivated contacts as an alternative to partial rear contacts

Recently, n-type Si solar cells featuring a passivated rear contact, called TOPCon (Tunnel Oxide Passivated Contact) were reported. The high conversion efficiency of 24.4% and very high FF>82% demonstrates that the efficiency potential of this full-area passivated rear contact is as good as or even better than that of partial rear contact (PRC) schemes like PERL (passivated emitter and rear locally diffused) and in addition avoids complex structuring steps and features a 1D carrier transport. Likewise, a boron-doped passivated rear contact for p-type solar cells (p-TOPCon) is proposed as an alternative to p-PRC cells. The optimum device design of PRC cells has to account for two opposing effects: a low-loss 3D carrier transport requires a high base doping but Shockley–Read–Hall (SRH) recombination within the base due to the formation of boron–oxygen complexes in standard Cz silicon calls for a low base doping level. This conflict might be overcome by p-TOPCon because its performance is less sensitive to base doping. This will be discussed on the base of experimental results. It is shown that its high implied fill factor (iFF) of 84% combined with the 1D carrier transport in the base translates into a higher FF potential. First investigations on planar solar cells prove the good performance of the p-TOPCon with respect to passivation and carrier transport. A Voc of 694mV and a FF of 81% underline the efficiency potential of this rear contact.

Improved Hydrogen Capping Effect in n-Type Crystalline Silicon Solar Cells by SiN(Si-Rich)/SiN(N-Rich) Stacked Passivation

The effect of hydrogen capping of SiN(Si-rich)/SiN(N-rich) stacks for n-type c-Si solar cells was investigated. Use of a passivation layer consisting of Si-rich SiN with a refractive index (n) of 2.7 and N-rich SiN with a refractive index of 2.1 improved the thermal stability. A single SiN passivation layer with a refractive index of 2.05 resulted in an initial lifetime of 200 μs whereas the layer with a refractive index of 2.7 resulted in a high initial lifetime of 2 ms, but the layer degraded rapidly after firing. A stacked passivation layer with refractive indices of 2.1 and 2.7 had a stable lifetime of 1.5 ms with an implied open-circuit voltage (iVoc) of 720 mV after firing. The thermally stable passivation mechanism with changing amounts of Si–N and Si–H bonding was analyzed by Fourier-transform infrared (FTIR) spectroscopy. Incorporation of the SiNx stack layer (2.7 + 2.1) into the passivated rear of n-type Cz silicon screen-printed solar cells resulted in energy conversion efficiency of 19.69%. Improved internal quantum efficiency in the long-wavelength range above 900 nm, with Voc of 630 mV, is mainly because of superior passivation of the rear surface compared with conventional solar cells.

Impact of heavy doping with donors on CZ silicon properties

This work reports a detailed characterization of 200 mm silicon wafers heavily doped with antimony, arsenic or phosphorus, in the range 1–50 mΩ•cm, corresponding to 7 × 1019 – 2 × 1017 at·cm−3. Grown‐in voids (revealed as light‐scattering objects on the wafer surface) are shown to be controlled by the dopant concentration. For As‐doped wafers the voids show a pronounced increase in their density up to [As] = 1.7 × 1019 cm−3, but at higher concentration the void density drops sharply. The same behaviour was found for P‐doped wafers where a sharp drop occurs at [P] > 2.9 × 1019 cm−3. In the case of antimony, the explored concentration range was sufficient only to reveal the increasing side of the curve. AFM characterization confirmed the morphology of voids as also seen in lightly‐doped silicon. Both single and double voids were observed. The oxygen precipitate density (BMD) was measured after submitting the wafers to a stabilization and growth thermal cycle. Overall it ranges from 1 × 106 ( = detection limit) to 1.5 × 1010 counts/cm3. The relationship between BMD density and oxygen concentration follows the classical S‐shaped curve, indicating that also for heavily doped N+ type wafers the oxygen content plays a major role on BMD formation.

Characterization of Silicon Oxides Formed by Light-Induced Anodisation for Silicon Solar Cell Surface Passivation

ABSTRACTIn this paper we report the properties of the anodic silicon dioxide film formed using light-induced anodisation (LIA) method and its potential to be used as surface passivation layer of p-type silicon surfaces of silicon solar cells. The high uniformity of the formed oxide is possibly due to the uniform drift of the positive charge carriers in the silicon to the surface being anodised. The oxide grows at higher rate than that in nitric acid, an oxide layer with thickness of 18 nm can be formed by anodising for 10 min with 15 V bias in 0.5 M sulphuric acid. After annealing in oxygen and then forming gas at 400 °C for 30 min, an average effective carrier lifetime of 120 μs was measured by quasi-steady state photoluminance on 180 μm p-type 3-5 Ohm cm Cz silicon wafers, with a value of 110 μs being measured for the same wafers passivated by a thermally-grown oxide of the same thickness. The properties of the anodic silicon dioxide layers formed by LIA have been characterized by ellipsometry, x-ray photoelectron spectroscopy, quasi-steady state photoluminance and Fourier transform infrared spectroscopy.

Electrical and Chemical Studies on Al2O3 Passivation Activation Process

Al2O3 deposited using Atomic layer Deposition (ALD) technique is known as an excellent material for p-type silicon surface passivation. However a post-deposition annealing step is needed to make the passivation effective. Thanks to coupled electrical measurements (capacitance and photoconductance) and chemical analyses (X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS)) carried out on the same p-type Cz silicon sample, a closer explanation of this activation process is given. The presence of hydrogen and oxygen is correlated to the evolution of the electrical parameters and the minority carrier lifetime for 0 to 60min 450°C annealing.

Cz シリコン単結晶の凝固プロセス中に水素を添加することで達成できるプロセスゾーンの拡大

Cz シリコン単結晶の凝固プロセス中に水素を添加することで達成できるプロセスゾーンの拡大

Concentrator bifacial Ag-free LGCells

Results obtained for the bifacial low concentrator Ag free Cz silicon solar cells based on Indium–Tin–Oxide(ITO)/(p+nn+)Cz–Si/Indium–Fluorine–Oxide (IFO) structure (“n-type” cell) as well as on IFO/(n+pp+)Cz–Si/ITO structure (“p-type” cell) are presented in this work. The (p+nn+)Cz–Si and (n+pp+)Cz–Si structures were produced by diffusion of boron and phosphorus from deposited B- and P-containing glasses followed by an etch-back step. The n+ surface of the structures was textured, whereas the p+ surface remained planar. Transparent Conducting Oxide (TCO) films, which act as antireflection electrodes, were deposited by ultrasonic spray pyrolysis method on both sides. The contact pattern of copper wire was attached by the low-temperature (160°C) lamination method simultaneously to the front and rear TCO layers as well as to the interconnecting ribbons arranged outside the structure. The shadowing from the contacts is in the range of ∼4%. The resulting solar cells showed front/rear efficiencies of 18.6–19.0%/14.9–15.3% (“p-type” cell) and 17.3–17.7%/18.1–18.5% (“n-type” cell) respectively at 1–5 suns. At 20–50% albedo of 1-sun illumination, similar energy production corresponds to 21.6–26.8% (n-type cell) and 21.6–26.1% (“p-type” cell) efficiency of a monofacial cell.

Crystal growth of MCZ silicon with ultralow carbon concentration

In this study, we investigated the evaporation of carbon monoxide (CO) from silicon melt during crystal growth by evaluating the carbon concentrations in the crystals using photoluminescence (PL) spectroscopy. In order to achieve greater carrier lifetimes in magnetic-field-induced Czochralski (MCZ) silicon for high-power insulated-gate bipolar transistor (IGBT) devices, we focused on the reduction of carbon impurities in MCZ silicon, that act as heterogeneous nucleation sites for oxygen precipitates. To obtain MCZ silicon with a carbon concentration lower than that of floating-zone (FZ) silicon, it is necessary to prevent the back-diffusion of CO from the hot graphite components into the melt and promote CO evaporation from the melt. By promoting CO evaporation, we managed to grow 6-in. CZ silicon crystals with a carbon concentration lower than 1.0×1014atoms/cm3 at a solidified fraction of 80%.

Numerical and experimental studies on the Black Periphery Wafer in CZ Si growth

In Cz silicon crystal growth, in order to study the causes and solutions for the lower minority carrier lifetime on the periphery of the wafer (Black Periphery Wafer, BPW), experiments and numerical simulations are conducted for the measuring and modeling of the solid/liquid interface, the heater power, the impurity concentration, the minority carrier lifetime and the point defects. The solid/liquid interface and heater power in modeling are consistent with the experimental results. The real CZ Si production processes can be effectively reproduced by our modeling. One main cause of Black Periphery Wafer is determined as the OSF-ring. In order to eliminate the OSF-ring, a higher pulling rate is proposed so that to increase the V/G ratio. Experimental measurements and modeling results proved that, with proper higher pulling rate, the BPW has been eliminated effectively and the minority carrier lifetime on the periphery of the wafer has been increased.

Light-induced anodisation of silicon for solar cell passivation

This paper reports a new method for forming anodic oxides on silicon surfaces using the light-induced current of pn-junction solar cells to make p-type silicon surfaces anodic. The light-induced anodisation process enables anodic oxide layers as thick as 79 nm to be formed at room temperature in a faster, more uniform, and controllable manner compared to previously reported clip-based anodisation methods. Although the effective minority carrier lifetime decreased immediately after light-induced anodisation from initial values measured with an 17 nm thermally grown oxide on both wafer surfaces, the 1-sun implied open circuit voltage of wafers on which the thermally grown oxide on the p-type surface was replaced by an anodic oxide of the same thickness could be returned to its initial value of ∼635 mV (for 3–5 Ω-cm Cz silicon wafers) after a 400 °C anneal in oxygen and then forming gas. The passivation of the formed anodic oxide layers was stable for a period of 50 days providing the oxide was protected by a 75 nm thick silicon nitride capping layer.

Passivation of boron-doped р+-Si emitters in the (p+nn+)Si solar cell structure with AlOx grown by ultrasonic spray pyrolysis

AlOx/(p+nn+)Cz–Si/IFO and ITO/AlOx/(p+nn+)Cz–Si/IFO solar cell structures have been fabricated from n-type Czochralski (Cz) silicon wafers through boron and phosphorus diffusion for producing the p+-Si emitter and n+-Si layer, respectively. The In2O3:F (IFO), AlOx, and In2O3:Sn (ITO) films have been grown by ultrasonic spray pyrolysis at 475°С, 330°С, and 375°С, respectively. The AlOx film thickness was varied in the range 27–108nm, the annealing time at 330°С in an Ar+5% O2 atmosphere containing vapor of a 2M H2O solution in methanol – in the range 0–18min, and the sheet resistance of the p+-Si emitter – in the range 28–133Ω/□ (varied through layer-by-layer chemical etching). We have studied the internal quantum efficiency (IQE) spectrum, photocurrent JIQE of the structures (evaluated from the IQE spectrum), as well as their photovoltage and pseudo-fill factor evaluated from Suns–Voc measurements. The results demonstrate that the level of p+-Si surface passivation increases with increasing AlOx film thickness and as a result of annealing. The optimal sheet resistance of the emitter is ∼65Ω/□. The pseudo-efficiency of the optimized ITO/AlOx/(p+nn+)Cz–Si/IFO structures was 20.2% under front illumination.

Improved surface quality of anisotropically etched silicon {111} planes for mm-scale optics

We have studied the surface quality of millimetre-scale optical mirrors produced by etching CZ and FZ silicon wafers in potassium hydroxide to expose the {111} planes. We find that the FZ surfaces have four times lower noise power at spatial frequencies up to 500 mm−1. We conclude that mirrors made using FZ wafers have higher optical quality.

Infrared Defect Dynamics of Irradiation Induced Complexes in CZ Silicon - C-Rich Case

Irradiation induced complexes of C-rich case in silicon crystal were examined by the highly sensitive and accurate infrared absorption spectroscopy. Low impurity concentration and high quality crystal was used and low dose was employed to make the reaction simple. Almost all possible absorption lines were revealed and their absorbance determined. The conversion coefficient from absorbance to the complex concentration was estimated by the plausible assumptions. The reaction was discussed in terms of concentration rather than absorbance. Intra-group reaction, chain reaction of successive addition of oxygen or self-interstitial, reaction yield and competition between the parallel reactions were described.

Formation of Single and Double Donor States of Trivacancy-Oxygen Complexes in P-Type Silicon

Deep level transient spectroscopy (DLTS) has been applied to study the formation of trivacancy-oxygen complexes in Si. Samples of p-type Czochralski grown (Cz) silicon, with a boron doping concentration of ~1x1015cm-3, have been irradiated at room temperature (RT) with 1.8 MeV protons to a dose of 5×1012cm-2. Two new energy levels at Ev+0.24 eV and Ev+0.11 eV (Evdenotes the valence band edge) emerge when the divacancy (V2) to divacancy-oxygen (V2O) transition takes place during post-irradiation annealing. The concentration is ~30% relative to V2(or V2O) and further, the two new levels exhibit an almost one-to-one correlation in strength. The present results strongly support that both levels are related to the same defect with a possible identification as single and double donor states of V3O.

Deposition temperature independent excellent passivation of highly boron doped silicon emitters by thermal atomic layer deposited Al2O3

In this work, we demonstrate that by using H2O based thermal atomic layer deposited (ALD) Al2O3 films, excellent passivation (emitter saturation current density of ∼28 fA/cm2) on industrial highly boron p+-doped silicon emitters (sheet resistance of ∼62 Ω/sq) can be achieved. The surface passivation of the Al2O3 film is activated by a fast industrial high-temperature firing step identical to the one used for screen printed contact formation. Deposition temperatures in the range of 100-300 °C and peak firing temperatures of ∼800 °C (set temperature) are investigated, using commercial-grade 5″ Cz silicon wafers (∼5 Ω cm n-type). It is found that the level of surface passivation after activation is excellent for the whole investigated deposition temperature range. These results are explained by advanced computer simulations indicating that the obtained emitter saturation current densities are quite close to their intrinsic limit value where the emitter saturation current is solely ruled by Auger recombination. The process developed is industrially relevant and robust.

Infrared signals correlated with self-interstitial clusters in neutron-irradiated silicon

Using infrared spectroscopy we have investigated the defect spectrum of neutron-irradiated Czochralski–silicon (Cz–Si). The study was focused on three weak signals, mainly on a band at 533 cm−1, as well as on two other bands at 582 and 592 cm−1. The band at 533 cm−1 disappears from the spectra at ~170 °C exhibiting similar thermal stability with the Si-P6 electron paramagnetic resonance spectrum, previously correlated with a di-interstitial defect. The suggested model for the latter defect, comprising two self-interstitials placed symmetrically a lattice site Si atom, is very similar with that of the allene molecule. This allowed the calculation of the vibrational frequency of the suggested di-interstitial structure giving a value close to the 533 cm−1, in further support of the above assignment. The band at 582 cm−1 is stable up to 550 °C. The possible correlation of its origin to large self-interstitial clusters is examined. Also, the origin of the 592 cm−1 band, which is stable up to 200 °C is discussed, with indications tentatively pointing to a CV pair.

Plasma hydrogenated, reactively sputtered aluminium oxide for silicon surface passivation

AbstractThe intentional addition of hydrogen during reactive sputtering of AlOx films has led to a dramatic improvement of the surface passivation of crystalline silicon wafers achieved with this technique. The 5 ms effective minority carrier lifetime measured on 1.5 Ω cm n‐type CZ silicon wafers is close to the 6 ms of a control wafer coated by atomic layer deposition (ALD) of AlOx. Hydrogen‐sputtered films also provide excellent passivation of 1 Ω cm p‐type silicon, as demonstrated by an effective lifetime of 1.1 ms. It is likely that the improved passivation is related to the formation of an interfacial silicon oxide layer, as indicated by FTIR measurements. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon solar cells based on all-laser-transferred contacts

Crystalline silicon solar cells based on all-laser-transferred contacts (ALTC) have been fabricated with both front and rear metallization achieved through laser induced forward transferring. Both the front and rear contacts were laser-transferred from a glass slide coated with a metal layer to the silicon substrate already processed with emitter formation, surface passivation, and antireflection coating. Ohmic contacts were achieved after this laser transferring. The ALTC solar cells were fabricated on chemically textured p-type Cz silicon wafers. An initial conversion efficiency of over 15% was achieved on a simple cell structure with full-area emitter. Further improvements are expected with optimized laser transferring conditions, front grid pattern design, and surface passivation. The ALTC process demonstrates the advantage of laser processing in simplifying the solar cell fabrication by a one-step metal transferring and firing process. Copyright © 2013 John Wiley & Sons, Ltd.

High efficiency PERL cells on CZ P-type crystalline silicon using a thermally stable a-Si:H/SiNx rear surface passivation stack

For the purpose of manufacturing high efficiency p-type CZ crystalline silicon (c-Si) solar cells with conventional plated metallization and a passivated rear surface and local contacts, a thermally stable passivating system of plasma-enhanced chemical vapor deposition (PECVD) deposited a-Si:H/SiNx:H stack was developed in this work. Good thermal stability of this passivating stack capable of withstanding over 90min thermal annealing at 400°C enabled the implementation of hydrogenation processes to significantly enhance minority carrier lifetimes (τeff) in the bulk while simultaneously achieving compatibility with the metal contacting schemes that require relatively low sintering temperatures. Point local back surface field (LBSF) regions with effective surface recombination velocities (SRV) of less than 4800±400cm/s were formed by laser-doping (LD) technique with optimized laser parameters on test wafers. The combination of the optimized a-Si:H/SiNx:H rear surface passivating stack, hydrogenation of the bulk and high quality LBSF formed by LD technique resulted in passivated emitter and rear locally diffused (PERL) cells with efficiency of up to 20.28% on a 13cm2 industrial B-doped CZ silicon wafer. This is close to the record 20.3% efficient Pluto–PERL cells reported earlier this year and using the identically front surface designs. τeff of 1.7ms with corresponding upperbound for the SRV of 5.8cm/s was demonstrated with the same passivating stack on 1Ωcm p-type FZ wafers.