SHORT COMMUNICATION: Surface passivation by rehydrogenation of silicon‐nitride‐coated silicon wafers

A silicon wafer with a silicon nitride layer deposited by low pressure chemical vapour deposition may be subjected to high‐temperature treatments without adversely affecting the electronic properties of the silicon on the condition that a thin oxide is present under the nitride. After high‐temperature treatments there is an apparent degradation in effective lifetime, probably due to a loss of hydrogen from the silicon/oxide interface. Effective lifetimes can be completely recovered by thermal treatment in a hydrogen‐containing ambient. This work has useful applications for solar cells as many of the properties of these nitrides can be used to advantage. Copyright © 2004 John Wiley & Sons, Ltd.     

Bulk and surface passivation of silicon solar cells accomplished by silicon nitride deposited on industrial scale by microwave PECVD

Bulk and surface passivation by silicon nitride has become an indispensable element in industrial production of multicrystalline silicon (mc‐Si) solar cells. Microwave PECVD is a very effective method for high‐throughput deposition of silicon nitride layers with the required properties for bulk and surface passivation. In this paper an analysis is presented of the relation between deposition parameters of microwave PECVD and material properties of silicon nitride. By tuning the process conditions (substrate temperature, gas flows, working pressure) we have been able to fabricate silicon nitride layers which fulfill almost ideally the four major requirements for mc‐Si solar cells: (1) good anti‐reflection coating (refractive index tunable between 2·0 and 2·3); (2) good surface passivation on p‐type FZ wafers (S_eff<30 cm/s); (3) good bulk passivation (improvement of IQE at 1000 nm by 30% after short thermal anneal); (4) long‐term stability (no observable degradation after several years of exposure to sunlight). By implementing this silicon nitride deposition in an inline production process of mc‐Si solar cells we have been able to produce cells with an efficiency of 16·5%. Finally, we established that the continuous deposition process could be maintained for at least 20 h without interruption for maintenance. On this timescale we did not observe any significant changes in layer properties or cell properties. This shows the robustness of microwave PECVD for industrial production. Copyright © 2005 John Wiley & Sons, Ltd.     

Textured silicon surface passivation by high‐rate expanding thermal plasma deposited SiN and thermal SiO2/SiN stacks for crystalline silicon solar cells

Expanding thermal plasma (ETP) deposited silicon nitride (SiN) with optical properties suited for the use as antireflection coating (ARC) on silicon solar cells has been used as passivation layer on textured monocrystalline silicon wafers. The surface passivation behavior of these high‐rate (>5 nm/s) deposited SiN films has been investigated for single layer passivation schemes and for thermal SiO₂/SiN stack systems before and after a thermal treatment that is normally used for contact‐firing. It is shown that as‐deposited ETP SiN used as a single passivation layer almost matches the performance of a thermal oxide. Furthermore, the SiN passivation behavior improves after a contact‐firing step, while the thermal oxide passivation degrades which makes ETP SiN a better alternative for single passivation layer schemes in combination with a contact‐firing step. Moreover, using the ETP SiN as a part of a thermal SiO₂/SiN stack proves to be the best alternative by realizing very low dark saturation current densities of <20 fA/cm² on textured solar‐grade FZ silicon wafers and this is further improved to <10 fA/cm² after the anneal step. Optical and electrical film characterizations have also been carried out on these SiN layers in order to study the behavior of the SiN before and after the thermal treatment. Copyright © 2008 John Wiley & Sons, Ltd.     

Stack system of PECVD amorphous silicon and PECVD silicon oxide for silicon solar cell rear side passivation

A stack of hydrogenated amorphous silicon (a‐Si) and PECVD‐silicon oxide (SiO_x) has been used as surface passivation layer for silicon wafer surfaces. Very good surface passivation could be reached leading to a surface recombination velocity (SRV) below 10 cm/s on 1 Ω cm p‐type Si wafers. By using the passivation layer system at a solar cell's rear side and applying the laser‐fired contacts (LFC) process, pointwise local rear contacts have been formed and an energy conversion efficiency of 21·7% has been obtained on p‐type FZ substrates (0·5 Ω cm). Simulations show that the effective rear SRV is in the range of 180 cm/s for the combination of metallised and passivated areas, 120 ± 30 cm/s were calculated for the passivated areas. Rear reflectivity is comparable to thermally grown silicon dioxide (SiO₂). a‐Si rear passivation appears more stable under different bias light intensities compared to thermally grown SiO₂. Copyright © 2008 John Wiley & Sons, Ltd.     

High‐quality surface passivation of silicon solar cells in an industrial‐type inline plasma silicon nitride deposition system

We have studied the surface passivation of silicon by deposition of silicon nitride (SiN) in an industrial‐type inline plasma‐enhanced chemical vapor deposition (PECVD) reactor designed for the continuous coating of silicon solar cells with high throughput. An optimization study for the passivation of low‐resistivity p‐type silicon has been performed exploring the dependence of the film quality on key deposition parameters of the system. With the optimized films, excellent passivation properties have been obtained, both on undiffused p‐type silicon and on phosphorus‐diffused n⁺ emitters. Using a simple design, solar cells with conversion efficiencies above 20% have been fabricated to prove the efficacy of the inline PECVD SiN. The passivation properties of the films are on a par with those of high‐quality films prepared in small‐area laboratory PECVD reactors. Copyright © 2004 John Wiley & Sons, Ltd.     

20.1%‐efficient crystalline silicon solar cell with amorphous silicon rear‐surface passivation

We have developed a crystalline silicon solar cell with amorphous silicon (a‐Si:H) rear‐surface passivation based on a simple process. The a‐Si:H layer is deposited at 225°C by plasma‐enhanced chemical vapor deposition. An aluminum grid is evaporated onto the a‐Si:H‐passivated rear. The base contacts are formed by COSIMA (contact formation to a‐Si:H passivated wafers by means of annealing) when subsequently depositing the front silicon nitride layer at 325°C. The a‐Si:H underneath the aluminum fingers dissolves completely within the aluminum and an ohmic contact to the base is formed. This contacting scheme results in a very low contact resistance of 3.5 ±0.2 mΩ cm² on low‐resistivity (0.5 Ω cm) p‐type silicon, which is below that obtained for conventional Al/Si contacts. We achieve an independently confirmed energy conversion efficiency of 20.1% under one‐sun standard testing conditions for a 4 cm² large cell. Measurements of the internal quantum efficiency show an improved rear surface passivation compared with reference cells with a silicon nitride rear passivation. Copyright © 2005 John Wiley & Sons, Ltd.     

Al-enhanced PECVD SiNx induced hydrogen passivation in string ribbon silicon

The effectiveness of manufacturable gettering and passivation technologies is investigated for their ability to improve the quality of a promising Si photovoltaic material. The results of this study indicate that a lifetime enhancement of 30 μs is attained when a backside screen-printed aluminum layer and a thin film of SiN_x, applied by plasma-enhanced chemical vapor deposition (PECVD), are simultaneously annealed at 850°C in a lamp-heated belt furnace. Based on the results of this study, a model is proposed to describe the Al-enhanced SiN_x induced hydrogen defect passivation in String Ribbon silicon due to the simultaneous anneal. According to this model, three factors play an important role: i) the release of hydrogen from the SiN_x film into the substrate; ii) the retention of hydrogen at defect sites in silicon; and iii) the generation of vacancies at the Al−Si interface due to the alloying process which increases the incorporation of hydrogen and creates a chemical potential gradient which enhances the migration of hydrogen in the substrate. A PC1D device simulation indicates that screen-printed cell efficiencies approaching 16% can be achieved if the gettering and passivation treatments examined in this study are employed, the substrate thickness is reduced, and a high-quality surface passivation scheme is applied.     

Separation of local bulk and surface recombination in crystalline silicon from luminescence reabsorption

Spontaneous photoemission of crystalline silicon provides information on excess charge carrier density and thereby on electronic properties such as charge carrier recombination lifetime and series resistance. This paper is dedicated to separating bulk recombination from surface recombination in silicon solar cells and wafers by exploiting reabsorption of spontaneously emitted photons. The approach is based on a comparison between luminescence images acquired with different optical short pass filters and a comprehensive mathematical model. An algorithm to separate both front and back surface recombination velocities and minority carrier diffusion length from photoluminescence (PL) images on silicon wafers is introduced. This algorithm can likewise be used to simultaneously determine back surface recombination velocity and minority carrier diffusion length in the base of a standard crystalline silicon solar cell from electroluminescence (EL) images. The proposed method is successfully tested experimentally. Copyright © 2009 John Wiley & Sons, Ltd.     

Role of hydrogen in the surface passivation of crystalline silicon by sputtered aluminum oxide

Aluminum oxide films can provide excellent surface passivation on both p‐type and n‐type surfaces of silicon wafers and solar cells. Even though radio frequency magnetron sputtering is capable of depositing aluminum oxide with concentrations of negative charges comparable to some of the other deposition methods, the surface passivation has not been as good. In this paper, we compare the composition and bonding of aluminum oxide deposited by thermal atomic layer deposition and sputtering, and find that the interfacial silicon oxide layer and hydrogen concentration can explain the differences in the surface passivation. Copyright © 2010 John Wiley & Sons, Ltd.     

Surface recombination and sulfide passivation of GaN

The surface recombination velocity in n-type heteroepitaxial GaN(0001) is shown to decrease dramatically when the surface is chemically treated with aqueous and alcoholic solutions of inorganic sulfides, such as ammonium or sodium sulfide (NH₄)₂S_x and Na₂S). The room-temperature excitonic photoluminescence (PL) intensity increases by a factor of four to six after treatment, and improvements persist for at least seven months in room air. Various other chemicals commonly used in device processing are investigated and shown to change the PL intensity by factors ranging from 0.7 to 2.5, buffered oxide etching being the most beneficial. Schottky barrier diodes using gold as the contact metal are fabricated using a sulfide treatment prior to evaporation. The barrier height from capacitance-voltage measurements is as high as 1.63 ± 0.07 V, the highest value ever achieved on n-GaN. This result is evidence that the effect of surface states on the Fermi level has been substantially reduced by the treatment.     

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Atomic‐layer‐deposited aluminium oxide (Al₂O₃) is applied as rear‐surface‐passivating dielectric layer to passivated emitter and rear cell (PERC)‐type crystalline silicon (c‐Si) solar cells. The excellent passivation of low‐resistivity p‐type silicon by the negative‐charge‐dielectric Al₂O₃ is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al₂O₃ film covered by a 200 nm plasma‐enhanced‐chemical‐vapour‐deposited silicon oxide (SiO_x) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single‐layer of Al₂O₃, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.     

Influence of defect type on hydrogen passivation efficacy in multicrystalline silicon solar cells

We examine the effectiveness of hydrogen passivation as a function of defect type and microstructure at grain boundaries (GBs) in multicrystalline silicon. We analyze a solar cell with alternating mm‐wide bare and SiN_x‐coated stripes using laser‐beam‐induced current, electron backscatter diffraction, X‐ray fluorescence microscopy, and defect etching to correlate pre‐ and post‐hydrogenation recombination activity with GB character, density of iron‐silicide nanoprecipitates, and dislocations. A strong correlation was found between GB recombination activity and the nature/density of etch pits along the boundaries, while iron silicide precipitates above detection limits were found to play a less significant role. Copyright © 2010 John Wiley & Sons, Ltd.     

Extremely low surface recombination velocities on low‐resistivity n‐type and p‐type crystalline silicon using dynamically deposited remote plasma silicon nitride films

Extremely low upper‐limit effective surface recombination velocities (S_eff.max) of 5.6 and 7.4 cm/s, respectively, are obtained on ~1.5 Ω cm n‐type and p‐type silicon wafers, using silicon nitride (SiN_x) films dynamically deposited in an industrial inline plasma‐enhanced chemical vapour deposition (PECVD) reactor. SiN_x films with optimised antireflective properties in air provide an excellent S_eff.max of 9.5 cm/s after high‐temperature (>800 °C) industrial firing. Such low S_eff.max values were previously only attainable for SiN_x films deposited statically in laboratory reactors or after optimised annealing; however, in our case, the SiN_x films were dynamically deposited onto large‐area c‐Si wafers using a fully industrial reactor and provide excellent surface passivation results both in the as‐deposited condition and after industrial‐firing, which is a widely used process in the photovoltaic industry. Contactless corona‐voltage measurements reveal that these SiN_x films contain a relatively high positive charge of (4–8) × 10¹² cm⁻² combined with a relatively low interface defect density of ~5 × 10¹¹ eV⁻¹ cm⁻². Copyright © 2012 John Wiley & Sons, Ltd.     

Passivating thin bifacial silicon solar cells for industrial production

A scheme for passivating thin multi‐crystalline silicon solar cells compatible to mass production is presented. Wafers with a thickness of 180 µm were processed into solar cells. The otherwise severe bowing has been avoided by reduced aluminium coverage on the rear surface. The process scheme includes a silicon nitride firing through step for conventional screen printed contacts, where a silicon nitride layer on the rear surface acts as surface passivation layer and enables a gain in efficiency of 0.6% [abs.]. Copyright © 2007 John Wiley & Sons, Ltd.     

Compositionally graded interface for passivation of HgCdTe photodiodes

A compositionally graded CdTe-Hg_1−xCd_xTe interface was created by deposition of CdTe on p-HgCdTe and subsequent annealing. The compositionally graded layer between CdTe and HgCdTe was formed by an interdiffusion process and was used for passivation. The composition gradient (Δx) in the interfacial region and the width of the graded region were tailored by adopting a suitable annealing procedure. The effect of process conditions on the interfacial profile and photoelectric properties such as lifetime and surface recombination velocity was studied in detail. Surface recombination velocity of the p-HgCdTe could be reduced to the level of 3,000 cm/s at 77 K, which represents very good passivation characteristics. The passivation layer formed by this method can be used for the fabrication of high performance and stable modern infrared detectors. Thus, a passivation process is developed, which is simple, effective, reproducible, and compatible with the HgCdTe device fabrication and packaging processes.     

High‐rate deposition of a‐Si:H thin layers for high‐performance silicon heterojunction solar cells

In this paper, we describe a technique for high‐quality interface passivation of n‐type crystalline silicon wafers through the growth of hydrogenated amorphous Si (a‐Si:H) thin layers using conventional plasma‐enhanced chemical vapor deposition. We investigated the onset of crystallization of the a‐Si:H layers at various deposition rates and its effect on the surface passivation properties. Epitaxial growth occurred, even at a low substrate temperature of 90 °C, when the deposition rate was as low as 0·5 Å/s; amorphous growth occurred at temperatures up to 150 °C at a higher deposition rate of 4·2 Å/s. After optimizing the intrinsic a‐Si:H layer deposition conditions and then subjecting the sample to post‐annealing treatment, we achieved a very low surface recombination velocity (7·6 cm/s) for a double‐sided intrinsic a‐Si:H coating on an n‐type crystalline silicon wafer. Under the optimized conditions, we achieved an untextured heterojunction cell efficiency of 16·7%, with a high open‐circuit voltage (694 mV) on an n‐type float‐zone Si substrate. On a textured wafer, the cell efficiency was further enhanced to 19·6%. Copyright © 2011 John Wiley & Sons, Ltd.     

Passivation of photonic nanostructures for crystalline silicon solar cells

We report on the optical and electrical performances of periodic photonic nanostructures, prepared by nanoimprint lithography (NIL) and two different etching routes, plasma, and wet chemical etching. Optically, these periodic nanostructures offer a lower integrated reflectance compared with the industrial state‐of‐the‐art random pyramid texturing. However, electrically, they are known to be more challenging for solar cell integration. We propose the use of wet chemical etching for fabricating inverted nanopyramids as a way to minimize the surface recombination velocities and maintain a conventional cell integration flow. In contrast to the broadly used plasma etching for nanopatterning, the wet chemically etched nanopatterning results in low surface recombination velocities, comparable with the state‐of‐the‐art random pyramid texturing. Applied to 40‐µm thick epitaxially grown crystalline silicon foils bonded to a glass carrier superstrate, the periodic‐inverted nanopyramids show carrier lifetimes comparable with the non‐textured reference foils (τ_eff = 250 µs). We estimate a maximum effective surface recombination velocity of ~8 cm/s at the patterned surface, which is comparable with the state‐of‐the‐art values for crystalline silicon solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Development of dry processing techniques for CdZnTe surface passivation

A method for passivating the surface of Cd_1−xZn_xTe (CZT) x-ray and gamma ray detectors using relatively simple dry processing techniques has been developed. Leakage currents were significantly reduced for several processing methods. CZT samples were exposed to an oxygen plasma and/or coated with a reactively sputtered silicon nitride layer. Several parameters of the oxygen plasma step were found to be important for achieving enhanced surface resistivity. SiN_X has been previously characterized and was used because of its high dielectric quality and low deposition temperature. Reduction in leakage current after passivation by a factor of as much as twenty is demonstrated. Results are also presented which give a measure of the long-term stability of the passivating layers.     

Evaluation of the influence of an embedded porous silicon layer on the bulk lifetime of epitaxial layers and the interface recombination at the epitaxial layer/porous silicon interface

Porous silicon plays an important role in the concept of wafer‐equivalent epitaxial thin‐film solar cells. Although porous silicon is beneficial in terms of long‐wavelength optical confinement and gettering of metals, it could adversely affect the quality of the epitaxial silicon layer grown on top of it by introducing additional crystal defects such as stacking faults and dislocations. Furthermore, the epitaxial layer/porous silicon interface is highly recombinative because it has a large internal surface area that is not accessible for passivation. In this work, photoluminescence is used to extract the bulk lifetime of boron‐doped (10¹⁶/cm³) epitaxial layers grown on reorganised porous silicon as well as on pristine mono‐crystalline, Czochralski, p⁺ silicon. Surprisingly, the bulk lifetime of epitaxial layers on top of reorganised porous silicon is found to be higher (~100–115 µs) than that of layers on top of bare p⁺ substrate (32–50 µs). It is believed that proper surface closure prior to epitaxial growth and metal gettering effects of porous silicon play a role in ensuring a higher lifetime. Furthermore, the epitaxial layer/porous silicon interface was found to be ~250 times more recombinative than an epitaxial layer/p⁺ substrate interface (S ≅ 10³ cm/s). However, the inclusion of an epitaxially grown back surface field on top of the porous silicon effectively shields minority carriers from this highly recombinative interface. Copyright © 2013 John Wiley & Sons, Ltd.     

物理冶金多晶硅太阳电池叠层钝化减反射结构模拟

采用PC1D模拟软件对p型物理冶金多晶硅太阳电池的SiO2/Si Nx/SiNx叠层钝化减反射结构进行了计算模拟。结果表明:在SiNx/Si Nx双层减反射结构中引入SiO2钝化层后可以明显改善电池的外量子效率与表面减反射效果,并最终提高电池转换效率;随着SiO2膜厚度的增加,电池表面反射率呈先降低后增加的趋势,而电池外量子效率及转换效率则呈现出相反的趋势。二氧化硅膜厚度在2~8 nm时,电池转换效率变化不大,并在6 nm时效率达到最大值18.04%,当二氧化硅膜厚度大于8 nm后电池转换效率会出现明显下降。