Passivation Of Silicon Solar Cells Research Articles

Ultrathin Self-Assembled Monolayer for Effective Silicon Solar Cell Passivation.

Passivation technology is crucial for reducing interface defects and impacting the performance of crystalline silicon (c-Si) solar cells. Concurrently, maintaining a thin passivation layer is essential for ensuring efficient carrier transport. With an ultrathin passivated contact structure, both Silicon Heterojunction (SHJ) cells and Tunnel Oxide Passivated Contact (TOPCon) solar cells achieve an efficiency surpassing 26%. To reduce production costs and simplify solar cell manufacturing processes, the rapid development of organic material passivation technology has emerged. However, its widespread industrial production is hindered by environmental safety concerns, such as strong acid corrosion and biological and ecological safety issues. Here, we discovered a low-cost self-assembled monolayer (SAM) hole-selective transport material known as 2PACz ([2-(9H-carbazol-9-yl) ethyl] phosphonic acid) with phosphate groups to form c-Si solar cells for the first time. The ultrathin film of 2PACz with phosphate groups can establish strong and stable P-O-Si bonds on the silicon surface. Meanwhile, like 2PACz, a uniform ultrathin film with a carbazole function group can offer electron-localizing and thus hole-selective properties, which provides ideas for studying dopant-free silicon solar cells. As a result of such interfacial passivation engineering, it plays an important role in repairing porous structures, such as pyramid-textured silicon surfaces, and cutting losses during the commercialization of c-Si solar cells. Crucially, this advancement offers insights for the development of new high-efficiency ultrathin film passivation methods in the postsilicon era.

Silicon‐Inspired Analysis of Interfacial Recombination in Perovskite Photovoltaics

AbstractPerovskite solar cells have reached an impressive certified efficiency of 26.1%, with a considerable fraction of the remaining losses attributed to carrier recombination at perovskite interfaces. This work demonstrates how time‐resolved photoluminescence spectroscopy (TRPL) can be utilized to locate and quantify remaining recombination losses in perovskite solar cells, analogous to methods established to improve silicon solar cell passivation and contact layers. It is shown how TRPL analysis can be extended to determine the bulk and surface lifetimes, surface recombination velocity, the recombination parameter, J0, and the implied open‐circuit voltage (iVoc) of any perovskite device configuration. This framework is used to compare 18 carrier‐selective and passivating contacts commonly used or emerging for perovskite photovoltaics. Furthermore, the iVoc values calculated from the TRPL‐based framework are directly compared to those calculated from photoluminescence quantum yields and the measured solar cell Voc. This simple technique serves as a practical guide for screening and selecting multifunctional, passivating perovskite contact layers. As with silicon solar cells, most of the material and interface analysis can be done without fabricating full devices or measuring efficiency. These purely optical measurements are even preferable when studying bulk and interfacial passivation approaches, since they remove complicating effects from poor carrier extraction.

TiO2 Antireflection Coating Deposited by Electro-Beam Evaporation: Thin Film Thickness Effect on Weighted Reflectance and Surface Passivation of Silicon Solar Cells

Titanium dioxide was extensively used in solar cell industry and currently has been studied to produce passivated contacts in PERC/PERT and TOPCon solar cells. The aim of this paper was to analyze the impact of the thickness of TiO2 thin films deposited by electro-beam evaporation on the weighted reflectance and the surface passivation on silicon solar cells. Thin films with different thicknesses were deposited to produce PERT solar cells, varying from 50 to 90 nm. The surface passivation was enhanced as the thickness was increased. For instance, at 400 nm, the internal quantum efficiency increased from 71% to 76% when the thickness of the TiO2 was augmented from 50 nm to 90 nm. The lowest weighted reflectance was obtained in samples with 80 nm thick TiO2 films. Considering the compromise between antireflection properties and surface passivation, the highest efficiency solar cells were produced with 80 nm thick TiO2.

In situ phosphorus-doped polycrystalline silicon films by low pressure chemical vapor deposition for contact passivation of silicon solar cells

In situ phosphorus (P)-doped polycrystalline silicon (poly-Si) films by low pressure chemical vapor deposition (LPCVD) were studied in this work for the fabrication of poly-Si passivating contacts. In situ doping was targeted for enabling the full potential of the high-throughput LPCVD technique, as it could allow leaner fabrication of industrial solar cells featuring poly-Si passivating contacts than the more common ex situ doping routes. By careful optimization of the deposition temperature and the flows of the carrier gas (H2) and the dopant precursor (PH3), high doping in the poly-Si layers was achieved with active P concentrations up to 1.3⋅1020 cm−3. While reduction in the deposition rate (rdep) and thus in the throughput is a known problem when growing in situ P-doped films by LPCVD, this reduction could be limited, and the resulting rdep was equal to 0.078 nm/s. The developed poly-Si films were characterized both structurally and in terms of their passivation potential in poly-Si contacts. The latter yielded recombination current densities down to 1.5 fA/cm2 in passivated (J0,p) and 25.6 fA/cm2 in screen-printing metallized (J0,m) regions on saw-damage removed (SDR) Cz-Si surfaces, accompanied by a contact resistivity (ρc,m) of 4.9 mΩ⋅cm2. On textured Cz-Si surfaces, the corresponding values were J0,p = 3.5 fA/cm2, J0,m = 56.7 fA/cm2, and ρc,m = 1.8 mΩ⋅cm2. Optical impact of the developed poly-Si films was also assessed and a short circuit density loss of 0.41 mA/cm2 is predicted per each 100 nm of poly-Si applied at the rear side of solar cells.

Nitrogen Ion Implantation Effect on Silicon Solar Cells for Surface Passivation

We investigated the surface passivation of silicon solar cells by nitrogen ion implantation. The nitrogen ion was implanted with the energy of 20 keV at a twist angle of 22.5° from the Si 〈100〉 direction and a tilt angle of 7°. The ion concentration was varied at 1013, 1014, and 1015 N+/cm2. The implanted cell showed the implanted region located below the anti-reflection coating layer. For the low dose cell of 1013 N+/cm2, the nitrogen ion implantation showed increased quantum efficiency, minority carrier lifetime, and conversion efficiency compared to the reference cell. However, as the ion dose was increased, the quantum efficiency, minority carrier lifetime, photoreflectance, and cell conversion efficiency of the fabricated cell decreased. This indicates that nitrogen ion implantation can passivate the surface to increase the cell conversion efficiency with a critical dose of ions.

Silicon nitride cover layer prepared by silane-free plasma chemical vapor deposition for high quality surface passivation of silicon solar cells

Passivated emitter and rear contact (PERC) silicon solar cell is one of the most promising high-efficiency solar cells, which uses aluminum oxide (Al2O3) and hydrogenated silicon nitride (SiNx:H) stack as the rear passivation layer. The SiNx:H not only serves as a protective layer but can provide hydrogen atoms that further passivate the dangling bonds on silicon wafer surface. In this study, the SiNx:H cover layers are deposited using high density plasma chemical vapor deposition with a silane-free gas mixture of tetramethylsilane (TMS) and ammonia (NH3). Effects of the TMS gas flow rate on properties of the SiNx:H cover layers and effective minority carrier lifetime of silicon wafers are investigated. The experimental results show that the film density is positively related to the SiN bond proportion. The secondary ion mass spectrometry measurement shows that the dense SiNx:H can make hydrogen atoms move toward wafer surface instead of out-diffusion to the environment. The best SiNx:H film is deposited at a TMS flow rate of 35 sccm. The minority carrier lifetime can be improved from 70 μs for as-cleaned wafer to 373 μs for wafer/Al2O3 and to 1006 μs for wafer/Al2O3/SiNx:H. The surface recombination rate of 23 cm/s is obtained. Therefore, this study demonstrates the dense, silane-free SiNx:H films with high passivation quality for PERC solar cells.

Dielectric surface passivation for silicon solar cells: A review

Silicon wafer solar cells continue to be the leading photovoltaic technology, and in many places are now providing a substantial portion of electricity generation. Further adoption of this technology will require processing that minimises losses in device performance. A fundamental mechanism for efficiency loss is the recombination of photo‐generated charge carriers at the unavoidable cell surfaces. Dielectric coatings have been shown to largely prevent these losses through a combination of different passivation mechanisms. This review aims to provide an overview of the dielectric passivation coatings developed in the past two decades using a standardised methodology to characterise the metrics of surface recombination across all techniques and materials. The efficacy of a large set of materials and methods has been evaluated using such metrics and a discussion on the current state and prospects for further surface passivation improvements is provided.

Emitter passivation of silicon solar cell via organic coating at room temperature

We present a simple method for passivation of silicon solar cells at room temperature. Oleylamine has been used as passivation agent on the n-type emitter of silicon solar cell surface. The desired effect is seen in the form of apparent enhancement in efficiency of the solar cell after coating. The efficiency increases by an amount of 14 % as compared to the one without any passivation applied and antireflection coating. The efficiency was found to decrease marginally and stabilized later. Device performance was monitored for 100 h and the efficiency was found higher as compare to bare solar cell without passivation coating.

Impact of interstitial oxygen trapped in silicon during plasma growth of silicon oxy-nitride films for silicon solar cell passivation

Low temperature oxidation of silicon in plasma ambient is a potential candidate for replacing thermally grown SiO2 films for surface passivation of crystalline silicon solar cells. In this work, we report the growth of silicon oxy-nitride (SiOxNy) film in N2O plasma ambient at 380 °C. However, this process results in trapping of interstitial oxygen within silicon. The impact of this trapped interstitial oxygen on the surface passivation quality is investigated. The interstitial oxygen trapped in silicon was seen to decrease for larger SiOxNy film thickness. Effective minority carrier lifetime (τeff) measurements on n-type float zone silicon wafers passivated by SiOxNy/silicon nitride (SiNv:H) stack showed a decrease in τeff from 347 μs to 68 μs, for larger SiOxNy film thickness due to degradation in interface properties. From high frequency capacitance-voltage measurements, it was concluded that the surface passivation quality was governed by the interface parameters (fixed charge density and interface state density). High temperature firing of the SiOxNy/SiNv:H stack resulted in a severe degradation in τeff due to migration of oxygen across the interface into silicon. However, on using the SiOxNy/SiNv:H stack for emitter surface passivation in screen printed p-type Si solar cells, an improvement in short wavelength response was observed in comparison to the passivation by SiNv:H alone, indicating an improvement in emitter surface passivation quality.

Graded silicon nitride films: Optics and passivation

The authors describe a method for graded thin-film deposition requiring a single step that exploits the noninstantaneous replacement of reactant gases. They deposit the graded silicon-nitride films by plasma enhanced chemical vapor deposition. Channeling Rutherford backscattering measurements of the graded films find the N:Si ratio increases sixfold from the c-Si surface to air interfaces. The refractive index at the crystalline silicon/film interface was 2.96 reducing monotonically to 1.95 at the surface. The graded films achieve improved optics and surface passivation for silicon solar cells with measured surface recombination velocity of 3.9 cm/s.

Characterization of Al2O3 Thin Films Prepared by Thermal ALD

Thermal Atomic Layer Deposition was used to deposit Al2O3 layers with thickness ranging from 2 to 100nm for surface passivation of silicon solar cells. Various characterization techniques were used to evaluate the chemical, physical and optical properties of the layers and interfaces. Minority carrier lifetime around 2ms was measured for an optimal thickness of 15nm for as-deposited layers on high resistivity n-type silicon substrate. An annealing step at 400°Cincreases lifetime up to 5.7ms for the same structure.

Manipulation of stored charge density and polarity in anodic aluminium oxide for silicon solar cell passivation

This paper introduces a method to manipulate stored charge density and polarity in anodic aluminium oxide (AAO) for silicon solar cell passivation. The method involves the use of a pulse current source to anodise aluminium layers instead of the typically used constant current/voltage source, with the test structures experiencing positive and negative cycles periodically. By tuning the positive cycle percentage, it is demonstrated that the effective stored charge can be manipulated in a range from -5.2×1011 to 2.5×1012 q/cm2 when the AAO is formed over a thin silicon dioxide layer. Photoconductance decay measurements on symmetrical n-type test structures with ∼ 150Ω/□ boron-diffused surfaces were used to estimate the recombination current density associated with the diffused surface (Jop+), which was then correlated to the stored charge. As the effective stored charge changed from positive towards negative values, the Jop+ was reduced from 210 fA/cm2 to 100 fA/cm2. The stored charge in various test structures was monitored constantly to study the stability. It was found that the positive stored charge deteriorated with time while the negative charge was relatively stable.

A technique for field effect surface passivation for silicon solar cells

The recombination of electric charge carriers at the surface of semiconductors is a major limiting factor in the efficiency of optoelectronic devices, in particular, solar cells. The reduction of such recombination, commonly referred to as surface passivation, is achieved by the combined effect of a reduction in the trap states present at the surface via a chemical component, and the reduction in the charge carriers available for a recombination process, via a field effect component. Here, we propose a technique to field effect passivate silicon surfaces using the electric field effect provided by alkali ions present in a capping oxide. This technique is shown to reduce surface recombination in a controlled manner, and to be highly stable. Surface recombination velocities in the range of 6–15 cm/s are demonstrated for 1 Ω cm n-type float zone silicon using this technique, and they are observed to be constant for over 300 days, without the use of any additional surface chemical treatment. A model of trapping-mediated ionic injection is used to describe the system, and activation energies of 1.8–2 eV are deduced for de-trapping of sodium and potassium alkali ionic species.

Characterization of Al2O3 surface passivation of silicon solar cells

A study of the passivation of silicon surface by aluminum oxide (Al2O3) is reported. A correlation of fixed oxide charge density (Qf) and interface trap density (Dit) on passivation efficiency is presented. Low surface recombination velocity (SRV) was obtained even by as-deposited Al2O3 films and this was found to be associated to the passivation of interface states. Fourier transfer infrared spectroscopy spectra show the existence of an interfacial silicon oxide thin layer in both as-deposited and annealed Al2O3 films. Qf is found positive in as-deposited films and changing to negative upon subsequent annealing, providing thus an enhancement of the passivation in p-type silicon wafers, associated to field effects. Secondary ion mass spectrometry analysis confirms the correlation between Dit and hydrogen concentration at the Al2O3/Si interface. A lowest SRV of 15cm/s was obtained after an anneal at 400°C in nitrogen atmosphere.

Manipulation of K center charge states in silicon nitride films to achieve excellent surface passivation for silicon solar cells

High quality surface passivation (Seff < 5 cm/s) was achieved on polished float zone and textured p- and n-type solar grade Czochralski silicon substrates by externally injecting and storing positive or negative charges (>±8 × 1012 cm−2) into a dual layer stack of Plasma Enhanced Chemical Vapor Deposition (PECVD) Silicon Nitride (SiNx)/PECVD Silicon Oxide (SiO2) films using a corona charging tool. We demonstrate long term stability and uniform charge distribution in the SiNx film by manipulating the charge on K center defects while negating the requirement of a high temperature thermal oxide step.

Advanced Bulk Defect Passivation for Silicon Solar Cells

Through an advanced hydrogenation process that involves controlling and manipulating the hydrogen charge state, substantial increases in the bulk minority carrier lifetime are observed for standard commercial grade boron-doped Czochralski grown silicon wafers from 250-500 μs to 1.3-1.4 ms and from 8 to 550 μs on p-type Czochralski wafers grown from upgraded metallurgical grade silicon. However, the passivation is reversible, whereby the passivated defects can be reactivated during subsequent processes. With appropriate processing that involves controlling the charge state of hydrogen, the passivation can be retained on finished devices yielding independently confirmed voltages on cells fabricated using standard commercial grade boron-doped Czochralski grown silicon over 680 mV. Hence, it appears that the charge state of hydrogen plays an important role in determining the reactivity of the atomic hydrogen and, therefore, ability to passivate defects.

Electric Field Effect Surface Passivation for Silicon Solar Cells

Effective reduction of front surface carrier recombination is essential for high efficiency silicon solar cells. Dielectric films are normally used to achieve such reduction. They provide a) an efficient passivation of surface recombination and b) an effective anti-reflection layer. The conditions that produce an effective anti-reflection coating are not necessarily the same for efficient passivation, hence both functions are difficult to achieve simultaneously and expensive processing steps are normally required. This can be overcome by enhancing the passivation properties of an anti-reflective film using the electric field effect. Here, we demonstrate that thermally grown silicon dioxide is an efficient passivation layer when chemically treated and electrically charged, and it is stable over a period of ten months. Double layers of SiO2 and SiN also provided stable and efficient passivation for a period of a year when the sample is submitted to a post-charge anneal. Surface recombination velocity upper limits of 9 cm/s and 19 cm/s were inferred for single and double layers respectively on n-type, 5 Ωcm, Cz-Si.

Enhanced rear‐side reflection and firing‐stable surface passivation of silicon solar cells with capping polymer films

AbstractLow refractive index polymer materials have been investigated with a view to form the back surface mirror of advanced silicon solar cells. SiOx:H or AlOy SiOx:H polymer films were spun on top of an ultra‐thin (<10 nm) atomic‐layer‐deposited (ALD) Al2O3 layer, itself deposited on low‐resistivity (1 Ω cm) p‐type crystalline silicon wafers. These double‐layer stacks were compared to both ALD Al2O3 single layers and ALD Al2O3/plasma‐enhanced chemical vapour deposited (PECVD) SiNx stacks, in terms of surface passivation, firing stability and rear‐side reflection. Very low surface recombination velocity (SRV) values approaching 3 cm/s were achieved with ALD Al2O3 layers in the 4–8 nm range. Whilst the surface passivation of the single ALD Al2O3 layer is maintained after a standard firing step typical of screen printing metallisation, a harsher firing regime revealed an enhanced thermal stability of the ALD Al2O3/SiOx:H and ALD Al2O3/AlOy SiOx:H stacks. Using simple two‐dimensional optical modelling of rear‐side reflection it is shown that the low refractive index exhibited by SiOx:H and AlOy SiOx:H results in superior optical performance as compared to PECVD SiNx, with gains in photogenerated current of ∼0.125 mA/cm2 at a capping thickness of 100 nm. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anodic Aluminum Oxide Passivation For Silicon Solar Cells

The requirement to form localized rear metal contact regions for higher silicon solar cell efficiencies places demand on patterning techniques in terms of the small size of the openings and the ability to perform the patterning at commercial wafer processing rates. We suggest here the possibility of using a self-patterning approach which offers the potential of enhanced surface passivation and nanoscale patterning achieved using a single electrochemical anodization process. It is shown that when nanoporous anodic aluminum oxide (AAO) layers are formed by anodizing an aluminum layer over an intervening SiO2 or SiN x dielectric layer, the implied open-circuit voltages of p-type silicon test structures can be increased by an average of 40 and 47 mV, respectively. Capacitance-voltage measurements show that these passivating AAO dielectric stack layers store positive charges, which differs from what is observed for Al2O3 layers deposited by plasma-enhanced chemical vapor deposition or atomic layer deposition. Furthermore, we show that the magnitude of the stored charge in the dielectric stacks depends on the anodization conditions, highlighting the possibility of controlling the charge storage properties of these layers for specific cell requirements. Although the passivating properties of the anodized aluminum layer appear to be strongly influenced by charge effects, it is also possible that hydrogenation effects may play a role as has been previously observed for other electrochemical processes, such as metal plating.

Advances in the Surface Passivation of Silicon Solar Cells

The surface passivation properties of aluminium oxide (Al2O3) on crystalline Si are compared with the traditional passivation system of silicon nitride (SiNx). It is shown that Al2O3 has fundamental advantages over SiNx when applied to the rear of p-type silicon solar cells as well as to the p+ emitter of n-type silicon solar cells. Special emphasis is paid to the transfer of Al2O3 into industrial solar cell production. We compare different Al2O3 deposition techniques suitable for mass production such as ultrafast spatial atomic layer deposition, inline plasma-enhanced chemical vapour deposition and reactive sputtering. Finally, we review the most recent cell results with Al2O3 passivation and give a brief outlook on the future prospects of Al2O3 in silicon solar cell production.