Overview on SiN surface passivation of crystalline silicon solar cells

Silicon nitride (SiN) fabricated by plasma-enhanced chemical vapour deposition (PECVD) is increasingly used within the crystalline silicon (c-Si) photovoltaic industry as it offers the possibility to fabricate a surface and bulk passivating antireflection coating at low temperature (450°C). This article presents an overview on the present status of SiN for industrial as well as laboratory-type c-Si solar cells. Topics covered include the fundamentals of the PECVD technology, the present status of high-throughput PECVD machines for the deposition of SiN onto c-Si wafers, and a review of the fundamental properties of Si–SiN interfaces fabricated by PECVD.     

Low temperature surface passivation for silicon solar cells

Surface passivation at low processing temperatures becomes an important topic for cheap solar cell processing. In this study, we first give a broad overview of the state of the art in this field. Subsequently, the results of a series of mutually related experiments are given about surface passivation with direct Plasma Enhanced Chemical Vapour Deposition (PECVD) of silicon oxide (Si-oxide) and silicon nitride (Si-nitride). Results of harmonically modulated microwave reflection experiments are combined with Capacitance-Voltage measurements on Metal-Insulator-Silicon structures (CV-MIS), accelerated degradation tests and with Secondary Ion Mass Spectrometry (SIMS) and Elastic Recoil Detection (ERD) measurements of hydrogen and deuterium concentrations in the passivating layers. A large positive fixed charge density at the interface is very important for the achieved low surface recombination velocities S. The density of interface states D_it is strongly reduced by post deposition anneals. The lowest values of S are obtained with PECVD of Si-nitride. The surface passivation obtained with Si-nitride is stable under typical operating conditions for solar cells. By using deuterium as a tracer it is shown that hydrogen in the ambient of the post deposition anneal does not play a role in the passivation by Si-nitride. Finally, the results of CV-MIS measurements (Capacitance-Voltage measurements on Metal-Insulator-Silicon structures) on deposited Si-nitride layers are used to calculate effective recombination velocities as a function of the injection level at the surface, using a model that is able to predict the surface recombination velocity S at thermally oxidized silicon surfaces. These results are not in agreement with the measured increase of S at low injection levels.     

A new back surface passivation stack for thin crystalline silicon solar cells with screen-printed back contacts

In order to manufacture high-efficiency Si solar cells with a passivated rear surface and local contacts, it is necessary to develop both an excellent rear-passivation scheme compatible with screen-printing technology and a robust patterning technique for local contact formation. In this work, we have fabricated Si solar cells on ∼130 μm thick substrates using manufacturable processing, where rear side was passivated with a plasma-enhanced chemical vapor deposited SiO_x/SiN_x/SiO_xN_y stack and local back contacts using laser. As a result of both the rear surface passivation stack and the laser-fired local contacts, cell efficiencies of up to 17.6% on a 148.6 cm² Float-zone Si wafer and 17.2% for a 156.8 cm² multicrystalline Si wafer were achieved. PC-1D calculations revealed that the cells had a back surface recombination velocity (BSRV) of ∼400 cm/s and a back surface reflectance (BSR) of over 90%, as opposed to standard full Al-BSF cells having a BSRV of ∼800 cm/s and a 70% BSR. This result clearly indicates that the new technique of the passivation scheme and the patterning using laser developed in this study are promising for manufacturing high-efficiency PERC-type thin Si solar cells.     

Highest-quality surface passivation of low-resistivity p-type silicon using stoichiometric PECVD silicon nitride

The surface passivation properties of silicon nitride (SiN) films fabricated by high-frequency direct plasma-enhanced chemical vapour deposition (PECVD) on low-resistivity (1 Ω cm) p-type silicon solar cell substrates have been investigated. The process gases used were ammonia and a mixture of silane and nitrogen. In order to find the optimum set of SiN deposition parameters, a large number of carrier lifetime test structures were prepared under different deposition conditions. The optimised deposition parameters resulted in outstandingly low surface recombination velocities (SRVs) below 10 cm/s. Interestingly, we find the lowest SRVs for stoichiometric SiN films, as indicated by a refractive index of 1.9. In former studies similarly low SRVs had only been obtained for silicon-rich SiN films. The fundamentally different passivation behaviour of our SiN films is attributed to the addition of nitrogen to the process gases.     

Wet-chemical passivation and characterization of silicon interfaces for solar cell applications

The interface properties of silicon solar cell structures were characterized by the two non-destructive and highly surface-sensitive spectroscopic techniques: surface photovoltage and spectroscopic ellipsometry. The resulting charge and density of interface states as well as the microscopic surface roughness and oxide coverage were investigated during silicon wafer preparation and during sample storage in air. The surface state density of hydrogen-terminated silicon surfaces as well as the long-time stability of the hydrogen termination were found to primarily depend on the surface morphology resulting from the wet-chemical oxidation procedures applied before. The smallest interface state densities were obtained by NH₄F treatment subsequent to oxidation in ultra-pure water at 80°C. Surfaces prepared using this procedure are found to be much more stable upon exposition to clean-room air than those prepared by conventionally prepared H-terminated surfaces.The successful application of the new passivation procedures in photovoltaics is shown for selected examples of different solar cell concepts.     

Low temperature silicon oxide and nitride for surface passivation of silicon solar cells

Surface passivation at low processing temperature becomes an important topic for crystalline and multicrystalline silicon solar cells. In this work, silicon oxide (250°C) and silicon nitride (300°C) have been developed by Photo-CVD and PECVD technique respectively. Effects of deposition parameters on the optoelectronic and structural properties of the films have been investigated. Interface-trap density (D_it) and fixed charge density (Qf) have been estimated by high frequency (1 MHz) capacitance-voltage measurement on Metal–Insulator–Silicon structure (CV-MIS). The effect of silicon oxide and silicon nitride on the performance of c-Si solar cells have been studied.     

Low temperature passivation of silicon surfaces by polymer films

A novel surface passivation method for silicon carrier lifetime measurements and solar cells using a polymer film is introduced. It is easy to apply, no special pre-treatment, e.g. no hydrofluoric acid (HF)-treatment, is necessary. The surfaces to be passivated are covered with the polymer solution, dried at 90°C and encapsulated. Surface recombination velocities (S) as low as S=30 cm/s for various doping concentrations have been observed, nearly independent of the bulk injection level. The passivation is stable for at least 6 h. For a polymer-passivated rear contact solar cell the same open circuit voltage is achieved as for a cell with thermally grown oxide.     

Influence of sputter oxygen partial pressure on photoelectrochemical performance of tungsten oxide films

Polycrystalline tungsten oxide films of 1–1.2μm thickness were prepared by reactive sputtering at elevated substrate temperature (270 °C) and under different oxygen partial pressures in the range from 0.8 to 2.1 mTorr. At the lowest partial pressure the films were substoichiometric, showed increased disorder, and exhibited photocurrents of 0.6 mA/cm² at 1.8 V vs SCE in 0.33 M H₃PO₄. At partial pressures of 1.4 mTorr and greater, stoichiometric WO₃ films were produced which exhibited photocurrents of 2.4 mA/cm² at 1.8 V vs SCE. It has been determined that the photoelectrochemical performance of slightly substoichiometric films is adversely affected by changes in optical properties, while the photocurrents of severely substoichiometric films suffer additionally from poor carrier collection.     

Surface modification of aluminum with tin oxide coating

Tin oxide (SnO) was coated on the surface of aluminum spherules with an average particle size of 37 μm by a chemical deposition method to improve the electrochemical properties. The samples were characterized by particle size analysis, X-ray diffraction (XRD), scanning electron microscope (SEM), ac impedance spectroscopy and galvanostatic cycling. Pure aluminum electrode delivers an initial reversible capacity of 779 mAh g⁻¹, whose capacity loss is 58% after 10 cycles. In comparison, 10 wt% SnO–Al composite delivers an initial reversible capacity of 806 mAh g⁻¹ with the capacity loss of 28% after 10 cycles. Results show that SnO coating plays an important role in the improvement of the electrochemical performances. It could not only reinforce the mechanical stability of aluminum particles, but also provide better electronic contacts to the electrode.     

Surface passivation in high efficiency silicon solar cells

Surface passivation for crystalline silicon solar cells is particularly important for devices with open-circuit voltages in excess of 650 mV. Thick passivating thermal oxides, originally developed for use with buried contact solar cells, are shown to produce the most effective and stable surface passivation particularly in conjunction with lightly phosphorus diffused surfaces. However, for improved optical performance, antireflection coatings are only effective with surface oxide thicknesses reduced to 100–200 Å. Thinner passivating oxides cause significant voltage loss, most of which can be recovered through hydrogen passivation. Throughout this study, variation in surface passivation approaches has produced open-circuit voltages ranging from 620 mV to record voltages of 720 mV.     

The effect of rear surface polishing to the performance of thin crystalline silicon solar cells

As the thickness of crystalline silicon solar cells decreases, light loss cannot be avoided due to the absorption limit in long wavelength light. Internal rear side reflection can be enhanced by polishing the rear surface. The rear polishing processes are performed before the texturing and before and after doping the emitter layer to optimize the solar cell fabrication process sequences. All cells made by rear surface polishing showed improved light trapping in long wavelength region (900-1100 nm) compared to that in the conventional cells. However, silicon solar cells fabricated by rear polishing before and after doping have similar (35.5 mA/cm²) or lower (35.26 mA/cm²) short circuit current density compared to the cells produced by the conventional process (35.59 mA/cm²) due to pore damage to the anti-reflection layer and the surface of the emitter layer during rear polishing. This surface damage was effectively prevented adapting the rear surface polishing before the front surface texturing, which led to increasing the current density from 35.59 to 36.29 mA/cm².     

Field-induced surface passivation of p-type silicon by using AlON films

In the present work, we report on the evidence for a high negative charge density in aluminum oxinitride (AlON) coating on silicon. A comparative study was carried out on the composition and electrical properties of AlON and aluminum nitride (AlN). AlON films were deposited on p-type Si (1 0 0) substrate by RF magnetron sputtering using a mixture of argon and oxygen gases at substrate temperature of 300 °C. The electrical properties of the AlON, AlN films were studied through capacitance–voltage (C–V) characteristics of metal–insulator–semiconductor (MIS) using the films as insulating layers. The flatband voltage shift V_FB observed for AlON is around 4.5 V, which is high as compared to the AlN thin film. Heat treatment caused the V_FB reduction to 3 V, but still the negative charge density was observed to be very high. In the AlN film, no fixed negative charge was observed at all. The XRD spectrum of AlON shows the major peaks of AlON (2 2 0) and AlN (0 0 2), located at 2θ value of 32.96° and 37.8°, respectively. The atomic percentage of Al, N in AlN film was found to be 42.5% and 57.5%, respectively. Atomic percentages of Al, N and O in EDS of AlON film are 20.21%, 27.31% and 52.48%, respectively.     

Study on hydrogenated silicon nitride for application of high efficiency crystalline silicon solar cells

The hydrogenated silicon nitride films (SiN_x:H) deposited by plasma enhanced chemical vapor deposition (PECVD) technique is commonly used as an antireflection coating as well as surface passivating layer of crystalline silicon solar cells. The refractive indices of SiN_x:H films could be changed by varying the growth gas ratio R(=NH₃/SiH₄+NH₃) and annealing temperature. For optimum SiN_x:H film, the optical and chemical characterization tools by varying the film deposition and annealing condition were employed in this study. Metal-insulator-semiconductor (MIS) devices were fabricated using SiN_x:H as an insulator layer and they were subjected to capacitance-voltage (C-V) and current-voltage (I-V) measurements for electrical characterization. The effect of rapid thermal annealing (RTA) on the surface passivation as well as antireflection properties of the SiN_x:H films deposited at various process conditions were also investigated for the fabrication of low cost and high efficiency silicon solar cells.     

氮化硅薄膜对硅片背表面的钝化作用

采用等离子体增强化学沉积的方法(PECVD),在低衬体温度下制备不同厚度的双面氮化硅薄膜,通过准稳态电导法(QSSPCD)测试non-diffused和diffused硅片沉积不同厚度双面氮化硅薄膜烧结前后的少子寿命,研究发现,氮化硅薄膜厚度在17 nm左右的时候,背面钝化效果有所下降,超过26 nm的时候,效果基本一致.non-diffused烧结后的少子寿命下降很大,而diffused与之相反.结果表明,采用氮化硅作为背面钝化介质膜,可以改善材料的少子寿命,背面钝化膜可以选择在26～75 nm之间.     

Improving the performance of textured silicon solar cells through the field-effect passivation of aluminum oxide layers and up-conversion via multiple coatings with Er/Yb-doped phosphors

This paper examines the influence of field-effect passivation (from a coating of aluminum oxide) in conjunction with up-conversion (from multiple coatings containing Er/Yb-doped phosphors) on the performance of silicon solar cells. Note that the phosphors were applied to the rear surface of the cells. The surface morphology of the coatings was characterized by scanning electron microscopy and the chemical composition of Er/Yb-doped phosphors coating was examined using energy-dispersive X-ray spectroscopy. The fluorescence emissions of the coatings were examined using photoluminescence and optical image measurements. We examined the influence of field-effect passivation on dark current-voltage as well as photo-current density and external quantum efficiency (EQE). Improvements in photovoltaic performance after applying coatings containing Er/Yb-doped phosphors were estimated in terms of EQE and conversion efficiency. The field-effect passivation of Al₂O₃ and up-conversion provided by Er/Yb-doped phosphors resulted in EQE enhancements over a wavelength range of 600 to 1050 nm. Field-effect passivation was shown to enhance the conversion efficiency by 1.77% (from 16.91% to 17.21%), up-conversion enhanced conversion efficiency by 2.9% (from 17.21% to 17.71%), and a combination of field-effect passivation and up-conversion enhanced conversion efficiency by 4.73% (from 16.91% to 17.71%).     

Thin crystalline silicon solar cell bonded to sintered substrate with aluminum paste

Making thinner wafers is a simple way to reduce the production cost of silicon solar cells. However, thin wafers need to be supported mechanically in order to avoid the problem of breakage. Among the several possible supporting materials, silicon substrate made from the sintering of silicon powder, which is produced during the slicing process is the most favorable one because of its abundance and its similar thermal expansion coefficient with silicon wafers. For the bonding of the substrate and thin silicon wafers, aluminum paste is selected because of its compatibility with silicon and the possible BSF effect. Silicon solar cells of 150 μm with the sintered substrate on the back show 5.42% in solar cell conversion efficiency. Compared to commercial silicon cells, lower J_sc is obtained. This might be due to the poor conduction in the back layer of aluminum, which is absorbed into the supporting substrate during the annealing process.     

Antireflection and surface passivation behaviour of SiO2/Si/SiO2 quantum wells on silicon

Quantum wells (QWs) consisting of Si and Si-related materials (such as SiO₂) are of interest for solar cell work because they can possibly be used as a surface passivating antireflection (AR) coating or as the top cell in an all-silicon tandem solar cell. In this study, we fabricate SiO₂/Si/SiO₂ QW layers by RF magnetron sputtering and thermal oxidation. On high-resistivity (300 Ω cm) n-type silicon wafer substrates, the effective surface recombination velocity provided by our SiO₂/Si/SiO₂ QWs is around 4 cm/s for 13 Å Si thickness and 480 cm/s for 150 Å Si thickness. The parasitic optical absorption in the well-passivating QWs is negligible for terrestrial photovoltaic applications. However, they have very poor AR properties on Si wafers and hence would have to be covered by an additional reflection reducing dielectric film.     

太阳能电池背表面钝化的研究

利用PC1D模拟不同少子寿命的电池效率与背表面复合速率的关系,采用氮化硅和及其与二氧化硅薄膜的叠加层作为背面钝化膜,通过丝网印刷的方法形成条形局域背接触和局域背面点接触,条形接触的面积为背表面的25%,背面点接触孔径为250μm,间距2mm。经过RTP处理之后,两种不同的接触方式存在相同的问题,串联电阻大,并联电阻小,而利用腐蚀浆料的方法形成背面点接触,在电性能参数有少许改善。结果表明,在正常的烧结状态下,常规铝浆很难完全穿透氮化硅薄膜及其叠加层背面钝化层。而利用腐蚀浆料的方法形成背面点接触,在电性能参数有少许改善。     

Hydrogen passivation of defects in polycrystalline silicon solar cells

The investigations of hydrogen passivation of defects in polycrystalline silicon produced by the Czochralski method have been carried on. The results presented give evidence that it is advisable to use this material to create cheap effective solar cells.     

Electrochromism of Li-intercalated Sn oxide films made by sputtering

Sn oxide films were made by reactive rf magnetron sputtering under conditions that led to both electronic and ionic conductivity. Li⁺ intercalation produced electrochromism with coloration efficiency peaked in the infrared.