High-quality Surface Passivation Research Articles

Interplay of annealing temperature and doping in hole selective rear contacts based on silicon-rich silicon-carbide thin films

We present a detailed optimization of a hole selective rear contact for p–type crystalline silicon solar cells which relies on full-area processes and provides full-area passivation. The passivating hole-contact is based on a layer stack comprising a chemically grown thin silicon oxide, an intrinsic silicon interlayer, and an in-situ boron-doped non-stoichiometric silicon-rich silicon-carbide layer on top. After deposition, the structure is annealed at 775–900°C to diffuse dopant impurities to the c-Si wafer and a hydrogenation step is carried out. It is shown that hydrogenation is essential to obtain high quality surface passivation. In particular, we compare the effect of annealing in forming gas and annealing with a silicon-nitride overlayer as hydrogen source. We present a systematic optimization of the hole-selective contact, for which we varied the doping concentration, annealing parameters and report the implied open circuit voltage (iVoc) and combined specific contact resistivity (ρc). It is observed that for highly doped layers the optimum annealing temperature for high quality surface passivation is 800°C while for lowly doped layers the optimum annealing condition shifts to 850°C. Excellent surface passivation and efficient current transport is evidenced by an iVoc value of 718mV which corresponds to a saturation current density (J0) of 11.5fA/cm2 and a ρc of 17mΩcm2 on p−type wafers. Moreover, the evolution of the boron diffusion profiles with different annealing conditions is investigated. Finally, we demonstrate proof-of-concept p−type hybrid solar cells employing the full-area hole-selective rear contact presented here and standard heterojunction front electron contact. The excellent efficiency potential of our passivating rear contact is highlighted by conversion efficiencies up to of 21.9%, enabling Voc of 708mV, FF of 79.9% and Jsc of 38.7mA/cm2.

Effects of epitaxial growth on the optimum condition of intrinsic amorphous silicon oxide buffer layers for silicon heterojunction solar cells

Intrinsic amorphous silicon oxide (a-Si1−xOx:H) buffer layers were deposited on both sides of crystalline silicon (c-Si) wafers using plasma-enhanced chemical vapor deposition (PECVD) technique. The input gas flow ratio of carbon dioxide (CO2) to silane (SiH4) was varied in a wide range to study the passivation and structural properties of the a-Si1−xOx:H buffer layers. In this work, when the a-Si1−xOx:H layer was quite thick (>15nm), an extremely high effective lifetime of ~10ms was achieved on the n-type float-zone c-Si (~3Ω-cm, ~280μm) at moderate CO2/SiH4 flow ratios, resulting in an exceptionally low surface recombination velocity (<1.4cm/s). However, when CO2/SiH4 flow ratio was either rather low (<0.13) or extremely high (>0.47), the surface passivation quality would deteriorate significantly. In addition, a certain amount of epitaxial phase (epi-Si) was observed in some excellent buffer layers made at the moderate CO2/SiH4 ratios. Moreover, it was found that the epi-Si content could be gradually suppressed by slightly increasing the CO2/SiH4 ratio without affecting passivation quality. When the a-Si1−xOx:H buffer layer thickness was kept at only a few nanometers as required by silicon heterojunction (SHJ) solar cells, the PECVD optimum condition (CO2/SiH4 ratio) for buffer layers was revealed by applying the a-Si1−xOx:H buffer layers directly in a practical SHJ solar cell. We found that when the a-Si1−xOx:H buffer layer containing a certain amount (~22%) of epi-Si was employed at the back-surface-field side of the solar cell, a high open-circuit voltage (VOC) and a high fill factor (FF) were obtained at the same time. By contrast, at the emitter side of the solar cell, only the buffer layer without any epi-Si can be used to provide high-quality surface passivation for an excellent SHJ solar cell.

Ozone‐Based Atomic Layer Deposition of Al2O3 from Dimethylaluminum Chloride and Its Impact on Silicon Surface Passivation

Dimethylaluminum chloride (DMACl) as an aluminum source has shown promising potential to replace more expensive and commonly used trimethylaluminum in the semiconductor industry for atomic layer deposited (ALD) thin films. Here, the Al2O3 DMACl‐process is modified by replacing the common ALD oxidant, water, by ozone that offers several benefits including shorter purge time, layer‐by‐layer growth, and improved film adhesion. It is shown that the introduction of the ozone instead of water increases carbon and chlorine content in the Al2O3, while long ozone pulses increase the amount of interfacial hydrogen at silicon surface. These are found to be beneficial effects regarding the surface passivation and thus final device operation. Heat treatments (at 400 and 800 °C) are found to be essential for high quality surface passivation similar to ALD Al2O3 deposited from conventional precursors, which is correlated with the changes at the interface and related impurity distributions. The optimal deposition temperature is found to be 250 °C, which provides the best chemical passivation after thermal treatments.

Silicon/Organic Heterojunction for Photoelectrochemical Energy Conversion Photoanode with a Record Photovoltage.

Silicon (Si) is a good photon absorption material for photoelectrochemical (PEC) conversion. Recently, the relatively low photovoltage of Si-based PEC anode is one of the most significant factors limiting its performance. To achieve a high photovoltage in PEC electrode, both a large barrier height and high-quality surface passivation of Si are indispensable. However, it is still challenging to induce a large band bending and passivate Si surface simultaneously in Si-based PEC photoanodes so far, which hinders their performance. Here, we develop a simple Si/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) heterojunction with large band banding and excellent surface passiviation for efficient PEC conversion. A chemically modified PEDOT:PSS film acts as both a surface passiviation layer and an effective catalyst simultaneously without sacrificing band bending level. A record photovoltage for Si-based PEC photoanodes as high as 657 mV is achieved via optimizing the PEDOT:PSS film fabrication process. The density of electron state (DOS) measurement is utilized to probe the passivation quality of the organic/inorganic heterojunction, and a low DOS is found in the Si/PEDOT:PSS heterojunction, which is in accordance with the photovoltage results. The low-temperature solution-processed Si/organic heterojunction photoanode provides a high photovoltage, exhibiting the potential to be the next-generation economical photoanode in PEC applications.

Over 21% Efficiency of n-Type Monocrystalline Silicon PERT Photovoltaic Cell With Boron Emitter

We have developed an n-type monocrystalline silicon photovoltaic cell achieving the conversion efficiency of 21.3% by passivated emitter and rear totally diffused structure with a total area of 239 cm2. The center area of the cell showed 21.7% (226.3 cm2). The main developments for the cell related to an emitter are a boron diffusion process and a passivation process of the boron layer. The developed cells contain a deeply diffused and lightly doped boron emitter with a high-quality surface passivation layer to obtain high efficiency. An atmospheric pressure chemical vapor deposition (APCVD) process is used for the fabrication of a boron emitter. This process is suitable for mass production lines. An atomic layer deposition Al2O3 layer is applied for a passivation of the boron layer. In addition, the ohmic contacts between the cell and electrodes are improved by changing a temperature profile for an electrode firing process.

Effects of Low Temperature Anneal on the Interface Properties of Thermal Silicon Oxide for Silicon Surface Passivation.

High quality surface passivation has gained a significant importance in photovoltaic industry for reducing the surface recombination and hence fabricating low cost and high efficiency solar cells using thinner wafers. The formation of good-quality SiO2 films and SiO2/Si interfaces at low processing temperatures is a prerequisite for improving the conversion efficiency of industrial solar cells with better passivation. High-temperature annealing in inert ambient is promising to improve the SiO2/Si interface. However, annealing treatments could cause negative effects on SiO2/Si interfaces due to its chemical at high temperatures. Low temperature post oxidation annealing has been carried out to investigate the structural and interface properties of Si-SiO2 system. Quasi Steady State Photo Conductance measurements shows a promising effective carrier lifetime of 420 μs, surface recombination velocity of 22 cm/s and a low interface trap density (D(it)) of 4 x 10(11) states/cm2/eV after annealing. The fixed oxide charge density was reduced to 1 x 10(11)/cm2 due to the annealing at 500 degrees C. The FWHM and the Si-O peak wavenumber corresponding to the samples annealed at 500 degrees C reveals that the Si dangling bonds in the SiO2 films due to the oxygen defects was reduced by the low temperature post oxidation annealing.

Prototyping of nanophotonic grating back contacts for light trapping in planar silicon solar cells

Nanophotonic gratings can be used for efficient light trapping in crystalline silicon solar cells. A common challenge is the high quality surface passivation, when applying such gratings. In this article, we report on the prototyping of the application of nanophotonic grating back contacts to planar silicon heterojunction solar cells for improved light trapping. These back contacts are processed after the Si wafer passivation, omitting any direct texturing of the absorber layer. Thereby, maintaining the integrity of the planar passivation layer of high quality and simultaneous light trapping is realized. The induced light‐trapping effect is further studied by photoluminescence imaging, external quantum efficiency measurements as well as absorptance measurements of solar cells applying nanophotonic grating back contacts of different grating periods. A clear correlation is found for the three characterization methods which allows the optimal period for efficient light trapping in the investigated solar cells to be identified. For the optimized grating period of the nanophotonic grating back contact an increased short‐circuit current density is obtained and the power conversion efficiency is enhanced by 0.4%, absolute.

GaAs Solar Cells Grown by Hydride Vapor-Phase Epitaxy and the Development of GaInP Cladding Layers

The high cost of high-efficiency III–V photovoltaic devices currently limits them to niche markets. Hydride vapor-phase epitaxy (HVPE) growth of III–V materials recently reemerged as a low-cost, high-throughput alternative to conventional metal–organic vapor-phase epitaxy (MOVPE) growth of high-efficiency solar cells. Previously, we demonstrated unpassivated HVPE-grown GaAs p-n junctions with good quantum efficiency and high open-circuit voltage $(V_{{\rm oc}})$ . In this work, we demonstrate the growth of GaInP by HVPE for use as a high-quality surface passivation layer to GaAs solar cells. Solar cells grown with GaInP window layers show significantly improved quantum efficiency compared with unpassivated cells, increasing the short-circuit current $(J_{{\rm SC}})$ of these low-cost devices. These results show the potential of low-cost HVPE for the growth of high-quality III–V devices.

Boron-Doped Silicon Surfaces From B$_{\bf 2}$H $_{\bf 6}$ Passivated by ALD Al$_{\bf 2}$O$_{\bf 3}$ for Solar Cells

A p+ - doping method for silicon solar cells is presented whereby boron atoms from a pure boron (PureB) layer deposited by chemical vapor deposition using B2H6 as precursor were thermally diffused into silicon. The applicability of this doping process for the doped surfaces of silicon solar cells was evaluated in terms of surface morphology after thermal diffusion, the boron dopant profiles, and sheet resistances, as well as the recombination parameter $J_{0{\rm p+}}$ , when the doped layers were passivated by Al2O3 films prepared by atomic layer deposition. Adequate surface passivation could be achieved with a surface recombination contribution to $J_{0{\rm p+}}$ of <20 fA/cm2 for most samples. However, when a boron-rich layer (BRL) was present at the Si surface, a much higher recombination current density was observed, proving that a BRL was detrimental to the high-quality surface passivation of boron-diffused surfaces. It was found that sufficient O2 in the furnace during the thermal diffusion process could eliminate any potential residual BRL, while excessive O2 concentration results in boron depletion and a higher sheet resistance. Therefore, in addition to optimizing the initial PureB layer thickness and thermal budget to control the dopant profiles, the O2 concentration during the diffusion must also be well controlled.

Electronic Properties of Al p+ Surfaces Formed by Laser Doping from Aluminium Oxide Precursors: Implications for PERC Cell Design and Performance

Aluminium oxide (Al2O3)functions doubly as a high-quality surface passivation material for crystalline silicon and as an aluminium (Al) p-type precursor for laser doping. Thus, p+ doping based on laser ablation of Al2O3 thin-films deposited on a silicon substrate is an attractively simplified process for concurrent local contact definition and aligned surface doping. A number of studies have demonstrated this process, but a careful examination of the influence of laser parameters on the electronic properties of the Al laser doped p+ surface itself, including the influence of post-doping annealing, has yet to be presented. Such information is valuable for establishing process windows and for providing parameters by which the contact geometry can be optimized and the performance of locally Al2O3laser doped solar cells predicted. In this work, we present accurate characterization of the electronic properties of primary importance to solar cell performance: effective surface recombination and contact resistivity. Recombination at the Al2O3laserdoped p+ surface is found to exceed that of equivalently-doped p+ silicon from furnace diffused boron, while contact resistivity to vacuum evaporated Al is up to two orders of magnitude less than screen-printed, fired Al. Based on this characterization, computersimulations demonstrate that with optimized rear contact geometries, an industrially relevant PERC cell can approach 21 % efficiency, and the high-performance UNSW PERL structure can exceed 24 %.

Low Surface Recombination Velocity on P-Type Cz-Si Surface by Sol-Gel Deposition of Al2O3 Films for Solar Cell Applications.

High quality surface passivation has gained a significant importance in photovoltaic industry for fabricating low cost and high efficiency solar cells using thinner and lower cost wafers. The passivation property of spin coated Al2O3 films with a thickness of about 50 nm on p-type Cz-Si wafers has been investigated as a function of annealing temperatures. An effective surface recombination velocity of 55 cm/s was obtained for the films annealed at 500 °C. The chemical and field effect passivation was analyzed by C-V measurements. A high density of negative fixed charges (Qf) in the order of 9 x 10(11) cm(-2) was detected in Al2O3 films and its impact on the level of surface passivation was demonstrated experimentally. The C-V curves show density of the interface state (Dit) of 1 x 10(12) eV(-1)cm(-2) at annealing temperature of 500 °C. During annealing, a thin interfacial SiOx is formed, and this interfacial layer is supposed to play a vital role in the origin of negative QF and Dit. The homogeneous SiOx interlayer result in higher passivation performance due to both the increase of negative Qf and the decrease of Dit.

Ultralow Interface State Density Achieved by Light-Induced Anodization of Aluminum on Silicon Solar Cell Surfaces

A light-induced anodization (LIA) method, which uses the light-induced current of a silicon solar cell to anodize aluminum and form an anodic aluminum oxide (AAO) layer that can reduce recombination at a p-type surface of the cell, is reported. This method can result in anodic oxides with uniform properties over the surfaces of industrially sized silicon wafers since the current flows through the wafer rather than in the surface aluminum layer, as it does when the aluminum is directly electrically contacted. Uniform AAO layers can be formed within 4 min on p-type silicon surfaces by anodizing 300-nm-thick aluminum layers, and when formed over a thin interfacial silicon dioxide layer, the interface state density at the p-type silicon wafer interface can be reduced to values as low as 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ·eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and a fixed charge density of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . Photoconductance carrier lifetime measurements indicate that these AAO dielectric stack layers are capable of passivating silicon surfaces to a level similar to that provided by silicon nitride layers, which are deposited using plasma-enhanced chemical vapor deposition, although at potentially a much-reduced cost. The high-quality surface passivation, rapid room temperature processing, and large-area uniformity of LIA make it a promising method for producing passivation materials that can further improve efficiency and potentially reduce the cost of photovoltaic solar cells and other p-n junction devices.

Determining the Electronic Performance Limitations in Top-Down-Fabricated Si Nanowires with Mean Widths Down to 4 nm

Silicon nanowires have been patterned with mean widths down to 4 nm using top-down lithography and dry etching. Performance-limiting scattering processes have been measured directly which provide new insight into the electronic conduction mechanisms within the nanowires. Results demonstrate a transition from 3-dimensional (3D) to 2D and then 1D as the nanowire mean widths are reduced from 12 to 4 nm. The importance of high quality surface passivation is demonstrated by a lack of significant donor deactivation, resulting in neutral impurity scattering ultimately limiting the electronic performance. The results indicate the important parameters requiring optimization when fabricating nanowires with atomic dimensions.

In situ growth of SiNx as gate dielectric and surface passivation for AlN/GaN heterostructures by metalorganic chemical vapor deposition

SiNx grown in situ by metalorganic chemical vapor deposition (MOCVD) has shown great potential as a high-quality gate dielectric and surface passivation for AlN/GaN heterostructure transistors. In this paper, we present a thorough study on how the growth conditions affect the film quality of SiNx and correlate the observed material properties with the electrical characteristics of the heterostructures. Lowering the growth pressure and SiH4/NH3 ratio can improve the SiNx/AlN interface roughness, leading to a reduced interfacial trap state density. The gate leakage current can be suppressed by increasing the resistivity of SiNx, which can be tailored with growth temperature and SiH4/NH3 ratio.

Back amorphous‐crystalline silicon heterojunction (BACH) photovoltaic device with facile‐grown oxide ‐ PECVD SiNx passivation

AbstractThis article reports on the integration of facile native oxide‐based passivation of crystalline silicon surfaces within the back amorphous‐crystalline silicon heterojunction solar cell concept. The new passivation scheme consists of 1‐nm thick native oxide and nominally 70‐nm thick PECVD silicon nitride. The low temperature passivation scheme provides uniform high quality surface passivation and low parasitic optical absorption. The interdigitated doped hydrogenated amorphous silicon layers were deposited on the rear side of the silicon wafer using the direct current saddle field PECVD technique. A systematic analysis of a series of back amorphous‐crystalline silicon heterojunction cells is carried out in order to examine the influence of the various cell parameters (interdigital gap, n‐doped region width, ratio of widths of p, and n‐doped regions) on cell performance. A photovoltaic conversion efficiency of 16.7 % is obtained for an untextured cell illuminated under AM 1.5 global spectrum (cell parameters: VOC of 641 mV, JSC of 33.7 mA‐cm − 2 and fill factor of 77.3 %). Copyright © 2014 John Wiley &amp; Sons, Ltd.

Surface passivation for single-molecule protein studies.

Single-molecule fluorescence spectroscopy has proven to be instrumental in understanding a wide range of biological phenomena at the nanoscale. Important examples of what this technique can yield to biological sciences are the mechanistic insights on protein-protein and protein-nucleic acid interactions. When interactions of proteins are probed at the single-molecule level, the proteins or their substrates are often immobilized on a glass surface, which allows for a long-term observation. This immobilization scheme may introduce unwanted surface artifacts. Therefore, it is essential to passivate the glass surface to make it inert. Surface coating using polyethylene glycol (PEG) stands out for its high performance in preventing proteins from non-specifically interacting with a glass surface. However, the polymer coating procedure is difficult, due to the complication arising from a series of surface treatments and the stringent requirement that a surface needs to be free of any fluorescent molecules at the end of the procedure. Here, we provide a robust protocol with step-by-step instructions. It covers surface cleaning including piranha etching, surface functionalization with amine groups, and finally PEG coating. To obtain a high density of a PEG layer, we introduce a new strategy of treating the surface with PEG molecules over two rounds, which remarkably improves the quality of passivation. We provide representative results as well as practical advice for each critical step so that anyone can achieve the high quality surface passivation.

Surface Passivation for Single-molecule Protein Studies

Single-molecule fluorescence spectroscopy has proven to be instrumental in understanding a wide range of biological phenomena at the nanoscale. Important examples of what this technique can yield to biological sciences are the mechanistic insights on protein-protein and protein-nucleic acid interactions. When interactions of proteins are probed at the single-molecule level, the proteins or their substrates are often immobilized on a glass surface, which allows for a long-term observation. This immobilization scheme may introduce unwanted surface artifacts. Therefore, it is essential to passivate the glass surface to make it inert. Surface coating using polyethylene glycol (PEG) stands out for its high performance in preventing proteins from non-specifically interacting with a glass surface. However, the polymer coating procedure is difficult, due to the complication arising from a series of surface treatments and the stringent requirement that a surface needs to be free of any fluorescent molecules at the end of the procedure. Here, we provide a robust protocol with step-by-step instructions. It covers surface cleaning including piranha etching, surface functionalization with amine groups, and finally PEG coating. To obtain a high density of a PEG layer, we introduce a new strategy of treating the surface with PEG molecules over two rounds, which remarkably improves the quality of passivation. We provide representative results as well as practical advice for each critical step so that anyone can achieve the high quality surface passivation.

Detailed study of the effects of interface properties of ozone-based atomic layer deposited AlOx on the surface passivation of crystalline silicon

The effects of interface properties such as a negative fixed charge density and an interface trap density on the surface passivation of crystalline Si by O3-based batch ALD AlOx were studied. High-quality surface passivation with Smax of ∼10 cm/s was obtained from the AlOx samples deposited at 200 °C after annealing. This feature is attributed to the excellent field effect passivation by the high negative fixed charge density of ∼−5 × 1012 cm−2 and chemical passivation, which reduces the interface trap density to ∼1 × 1011 eV−1 cm−2. The annealed AlOx samples deposited at 200 °C also show high thermal stability during firing at 850 °C. Additionally, we found that the formation of a thin SiOx interlayer is essential for the formation of a high negative fixed charge density that induces strong field effect passivation, and that defect passivation at the Si/SiOx interface by diffused hydrogen from AlOx layers is the origin of chemical passivation.

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

High quality surface passivation (Seff &amp;lt; 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 (&amp;gt;±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.

Spectroscopic investigations of dark Si nanocrystals in SiO2 and their role in external quantum efficiency quenching

The percentage of dark silicon nanocrystals, i.e., the nanocrystals that are not able to radiatively recombine after absorption of a photon, is investigated by combining measurements of external and internal quantum efficiencies. The study is conducted on samples prepared by co-sputtering and subsequent heat treatments. We show that the external quantum efficiency is mainly limited by the presence of dark nanocrystals, which induce losses after direct excitation and also, as we propose, by indirect excitation enabled by energy migration. The percentage of dark nanocrystals can be decreased by high quality surface passivation as a result of low-temperature annealing in ambients of O2 and H2. By using a non-passivated sample as a reference, the relation between the size of a nanocrystal and its probability of being dark is studied. Larger nanocrystals are demonstrated to function more likely as dark centers. The study shows that high external quantum efficiencies of Si nanocrystal ensembles can be realized for small, well passivated Si nanocrystals under suppression of excitation diffusion.