High-efficiency Si Solar Cells Research Articles

고효율 결정질 실리콘 태양전지 적용을 위한 실리콘 산화막 표면 패시베이션

Minimizing the carrier recombination and electrical loss through surface passivation is required for high efficiency c-Si solar cell. Usually, SiNX, SiOX, SiONX and AlOX layers are used as passivation layer in solar cell application. Silicon oxide layer is one of the good passivation layer in Si based solar cell application. It has good selective carrier, low interface state density, good thermal stability and tunneling effect. Recently tunneling based passivation layer is used for high efficiency Si solar cell such as HIT, TOPCon and TRIEX structure. In this paper, we focused on silicon oxide grown by various the method (thermal, wet-chemical, plasma) and passivation effect in c-Si solar cell.

Polycrystalline silicon passivated tunneling contacts for high efficiency silicon solar cells

Abstract

Investigation of the thickness effect on material and surface texturing properties of sputtered ZnO:Al films for thin-film Si solar cell applications

Transparent conductive Al-doped ZnO (AZO) layers are widely used as the front electrode for thin-film silicon solar cells. For superstrate configurations, the front AZO layer simultaneously provides both electrical conductance and optical scattering. To improve the device performance, a textured surface is needed to enhance the photogeneration inside the solar cell through better light scattering. One approach is to wet-chemically etch the AZO films using hydrochloric (HCl) acid. In this work, AZO films with different initial layer thicknesses (900, 700 and 500 nm) are deposited onto soda-lime glass via magnetron sputtering and their texturing behaviours are compared. It is found that not only the material properties but also the surface texturing process greatly depends on the as-grown thickness of the AZO layer. The increased resistivity is mainly caused by a deteriorating carrier mobility as the thickness reduces. In terms of morphology, thick AZO films (i.e., 900 and 700 nm) show similar texture features after etching. In contrast, the thin AZO films (i.e., 500 nm) show irregularly shaped textures and over-etching, which leads to limited scattering. Hence the AZO layer thickness must be suitably chosen to achieve both good electrical conductance and optical scattering for high-efficiency thin-film Si solar cell applications.

Advanced Passivation Technology and Loss Factor Minimization for High Efficiency Solar Cells.

High-efficiency Si solar cells have attracted great attention from researchers, scientists, photovoltaic (PV) industry engineers for the past few decades. With thin wafers, surface passivation becomes necessary to increase the solar cells efficiency by overcoming several induced effects due to associated crystal defects and impurities of c-Si. This paper discusses suitable passivation schemes and optimization techniques to achieve high efficiency at low cost. SiNx film was optimized with higher transmittance and reduced recombination for using as an effective antireflection and passivation layer to attain higher solar cell efficiencies. The higher band gap increased the transmittance with reduced defect states that persisted at 1.68 and 1.80 eV in SiNx films. The thermal stability of SiN (Si-rich)/SiN (N-rich) stacks was also studied. Si-rich SiN with a refractive index of 2.7 was used as a passivation layer and N-rich SiN with a refractive index of 2.1 was used for thermal stability. An implied Voc of 720 mV with a stable lifetime of 1.5 ms was obtained for the stack layer after firing. Si-N and Si-H bonding concentration was analyzed by FTIR for the correlation of thermally stable passivation mechanism. The passivation property of spin coated Al2O3 films was also investigated. An effective surface recombination velocity of 55 cm/s with a high density of negative fixed charges (Qf) on the order of 9 x 10(11) cm(-2) was detected in Al2O3 films.

High-efficiency heterojunction crystalline Si solar cell and optical splitting structure fabricated by applying thin-film Si technology

Thin-film Si technology for solar cells has been developed for over 40 years. Improvements in the conversion efficiency and industrialization of thin-film Si solar cells have been realized through continuous research and development of the thin-film Si technology. The thin-film Si technology covers a wide range of fields such as fundamental understanding of the nature of thin-film Si, cell/module production, simulation, and reliability technologies. These technologies are also significant for solar cells other than the thin-film Si solar cells. Utilizing the highly developed thin-film Si solar cell technology, we have achieved ∼24% efficiency heterojunction crystalline Si solar cells using 6-in. wafers and >26% efficiency solar cells with an optical splitting structure. These results indicate that further improvement of thin-film Si technology and its synergy with crystalline Si solar cell technology will enable further improvement of solar cells with efficiencies above 26%.

X-ray evaluation of electronic and chemical properties and film structures in SiN passivation layer on crystalline Si solar cells

We investigated the electronic chemical properties and film structures in SiN passivation layers on crystalline Si by X-ray photoelectron spectroscopy (XPS) and X-ray reflectometry (XRR). From XPS analyses, a N-rich component (N–Si–N) and oxygen mixing were observed at the interface. The longest lifetime was obtained after post deposition annealing at 600 °C, where the N-rich layer was the thinnest with small peak height. Therefore, longer lifetime could be obtained as the N concentration at the interface decreased. The film consisted of three SiN layers and one SiO2 layer, as confirmed by XRR profile fitting. From the density profiles, a longer lifetime was obtained when the density gradient in the oxygen mixing layer was small. Results of XPS and XRR suggested that N at the interface significantly affected the lifetime. We believe it is important to understand the electronic chemical properties at the passivation interface to realize high-efficiency Si solar cells.

Laser interference-driven fabrication of regular inverted-pyramid texture on mono-crystalline Si

We show that inverted-pyramid (IP) textures can be fabricated on mono-crystalline Si wafer by laser interference combined with alkali etching. When exposed to three interfering nanosecond-laser beams at 532nm, the surface was periodically melted in accordance with the interference pattern, generating concave holes. Subsequent etching with a KOH solution revealed IP structures as a result of the anisotropic etching. It was found that not only the etching condition but also the in-plane orientation relationship of interfering beams is an important factor to fabricate low-reflectance, uniform IP textures. An average reflectance less than 20% was obtained over the spectral range of 400nm to 1μm. This mask-free process may be effectively utilized for the production of low-cost, high-efficiency crystalline Si solar cells.

Low-refractive-index and high-transmittance silicon oxide with a mixed phase of n-type microcrystalline silicon as intermediate reflector layers for tandem solar cells

Low-refractive-index and high-transmittance silicon oxide (SiOx) with a mixed phase of n-type microcrystalline silicon was developed for intermediate reflector layers (IRLs) of high-efficiency amorphous Si and microcrystalline-Si tandem solar cells. The refractive index, crystalline fraction, and conductivity of the SiOx IRLs were characterized as functions of the deposition conditions. The SiOx films were prepared by plasma-enhanced chemical vapor deposition, at a high pressure of 9Torr and high hydrogen dilution ratio of 400, and using a narrow electrode gap of 12mm. The films showed a refractive index of 1.85, crystalline fraction of ~50%, and conductivity of ~10−7S/cm. Furthermore, the transmittance of the films was more than 90% at wavelengths between 600nm and 1100nm. We present here the procedure to optimize the SiOx IRLs, which is to compare the changes in the top and bottom cells' current densities of the tandem solar cells fabricated with 10–20% thinner absorption layers. A stabilized efficiency of 11.12% and light-induced degradation of −9.3% could be achieved in a tandem module containing an optimized IRL by enhancing the top cell current. The SiOx IRLs developed and the optimization procedure discussed in this paper can be very useful for the fabrication of high-efficiency thin-film Si tandem modules.

Passivated rear contacts for high-efficiency n-type Si solar cells providing high interface passivation quality and excellent transport characteristics

In this work passivated rear contacts are used to replace point contact passivation schemes for high-efficiency n-type crystalline silicon solar cells. Our structure is based on an ultra-thin tunnel oxide (SiO2) and a phosphorus-doped silicon layer, which significantly reduce the surface recombination at the metal–semiconductor interface. The passivation and transport mechanisms of this passivated contact will be addressed within this paper. Particular consideration will be given to the tunnel oxide's impact on interface passivation and on the I–V characteristics of n-type Si solar cells featuring a boron-diffused emitter. It will be shown that the tunnel oxide is a vital part of this passivated contact and that it is required to achieve excellent interface passivation for both open-circuit and maximum power point (MPP) conditions (implied open-circuit voltage iVoc>710mV and implied fill factor iFF>84%). It will also become clear that the transport barrier arising from the tunnel oxide does not constrict the majority carrier flow. Thus, a low series resistance is obtained, which in conjunction with the high iFF enables FFs well above 82%. Investigations on cell levels lead to an independently confirmed conversion efficiency of 23.0% for n-type cells with a boron-diffused emitter and the herein developed passivated rear contact, in which the efficiency is not limited by the electrical properties of our passivated contacts.

Remarkable progress in thin-film silicon solar cells using high-efficiency triple-junction technology

Despite the many advantages of thin-film silicon (Si) solar cells, their low efficiencies remain a challenge that must be overcome. Efficient light utilization across the solar spectrum is required to achieve efficiencies over 15%, allowing them to be competitive with other solar cell technologies. To produce high-efficiency thin-film Si solar cells, we have developed triple-junction solar cell structures to enhance solar spectrum utilization. To maximize the light management, in-house ZnO:Al layers with high haze ratios and high transmittances were developed. In addition, novel doping layers, such as n-type microcrystalline silicon oxide (µc-SiOx:H), which has a very low refractive index, and p-type microcrystalline silicon oxide (µc-SiOx:H), which has a wide bandgap, were successfully applied to the optical reflector and the window layer, respectively. Thin-film quality control techniques for the deposition of hydrogenated amorphous silicon (a-Si:H) in the top cell, hydrogenated amorphous silicon-germanium (a-SiGe:H) or hydrogenated microcrystalline silicon (μc-Si:H) in the middle cell, and hydrogenated microcrystalline silicon (μc-Si:H) in the bottom cell were also important factors leading to the production of high-efficiency triple-junction solar cells. As a result of this work, an initial efficiency of 16.1% (in-house measurement) in the a-Si:H/a-SiGe:H/μc-Si:H stack and a stabilized efficiency of 13.4% (confirmed by NREL) in the a-Si:H/μc-Si:H/μc-Si:H stack were successfully achieved in a small-area triple-junction solar cell with dimensions of 1cm×1cm.

Light Scattering Enhancement by Double Scattering Technique for Multijunction Thin-Film Silicon Solar Cells

Light trapping is an important technique to increase the efficiency of thin-film silicon solar cells. Textured surfaces are known to scatter sunlight while it passes through thin-film solar cells, thereby increasing the optical pathlength and, thus, the photon absorption in the devices. In this paper, microtextured glass superstrates were prepared by the aluminum-induced texturization (AIT) method. These superstrates achieve high transmission haze values of up to 60% while maintaining a high total optical transmission. We demonstrate that both the surface structure and the roughness of the textured glass surface can be controllably adjusted by changing the AIT process parameters. Approximately 900-nm-thick aluminum-doped zinc oxide (AZO) films are deposited onto the microtextured glass surfaces by magnetron sputtering and then further textured using wet-chemical etching in diluted HCl, creating an AZO surface that features both micrometer-scale and submicron-scale structures. Optical spectroscopy and goniophotometer measurements reveal that the light scattering capability of the substrates increases significantly due to the wet-chemical AZO texturization. The combination of microtextured AIT glass, together with the submicron-textured AZO, could be very attractive for high-efficiency double-junction micromorph thin-film Si solar cells, whereby the amorphous Si top cell benefits significantly from the AZO's submicron texture and the microcrystalline Si bottom cell benefits primarily from the microtextured glass surface.

Low cost anisotropic etching of monocrystalline Si (1 0 0): Optimization using response surface methodology

Reduced surface reflectance and enhanced light trapping is required by any high efficiency solar cell. Anisotropic etching was done on silicon (100) by using tetramethyl ammonium hydroxide TMAH, (CH3)4NOH, solution at 85°C. Process variables considered were solution concentration and time proposed by response surface methodology (RSM). An effective surface texture was resulted with reflectance less than 8% without antireflection coating. The antireflection mechanism was also co-related with the etch rate of Si. Optimized values predicted by RSM for time and TMAH concentration were 5min and 3.50% respectively. The technique and optimization of parameters by using response surface methodology (RSM) could be valuable in the texturization process for high-efficiency Si solar cells.

High-efficiency grid-type Si solar cell structure

The demand to reduce Si usage in Si photovoltaic cells in order to improve the cost-effectiveness is ever-increasing in photovoltaic industry. In this study, we propose a new Si photovoltaic cell structure, in which Si grids are formed and reflectors are positioned in between. The optical absorption efficiency of the proposed cell structure was analyzed by making comparisons with simple Si cells with flat and textured surfaces. A quantitative analysis of the optical absorption was made by using a direct absorption calculation algorithm based on the three-dimensional modeling and ray-tracing technique. The proposed grid-type cell structure was shown to be able to yield an improvement in the absorption efficiency per unit Si volume as high as 91% when compared to typical Si cells with pyramidal surface texturing.

Wet-Chemical Preparation of Textured Silicon Solar Cell Substrates: Surface Conditioning and Electronic Interface Properties

The dominance of crystalline silicon (Si) in photovoltaics can be ascribed partly to the extensive knowledge about this material, which has been accumulated in microelectronics technology. Methods to passivate Si interfaces, which were developed for microelectronic device technologies, have been extended to solar cell manufacturing in the past. These methods, however, have been optimised for polished substrates, and do not work so effective with textured surfaces, which commonly used in the fabrication of high efficiency Si solar cells to enhance anti-reflection properties.

High-Efficiency Si Solar Cell Fabricated by Ion Implantation and Inline Backside Rounding Process

We introduce a novel, high-throughput processing method to produce high-efficiency solar cells via a backside rounding process and ion implantation. Ion implantation combined with a backside rounding process is investigated. The ion implantation process substituted for thermal POCl3 diffusion performs better uniformity (<3%). The U-4100 spectrophotometer shows that wafers with backside rounding process perform higher reflectivity at long wavelengths. Industrial screen printed (SP) Al-BSF on different etching depth groups was analyzed. SEMs show that increasing etch depth improves back surface field (BSF). The - measurement revealed that etching depths of 6 μm ± 0.1 μm due to having the highest and , it has the best performance. SEMs also show that higher etching depths also produce uniform Al melting and better BSF. This is in agreement with IQE response data at long wavelengths.

High-efficiency pin-type Amorphous Si Solar Cells Fabricated with a Low-electron-affinity Buffer Layer on the Front Electrode

High-efficiency pin-type Amorphous Si Solar Cells Fabricated with a Low-electron-affinity Buffer Layer on the Front Electrode

Near-infrared emission of Yb^3+ through energy transfer from ZnO to Yb^3+ in glass ceramic containing ZnO nanocrystals

Yb(3+)-doped glass and glass ceramic containing ZnO nanocrystals were prepared by the melting-quenching method and subsequent heat treatment. Intense near-IR emission around 1000 nm that originated from the transition of Yb(3+):(2)F(5/2)→(2)F(7/2) was generated as a result of energy transfer from oxygen interstitials in ZnO nanocrystals to Yb(3+) with energy transfer efficiency of about 10%. The quantum yield for the near-IR emission of Yb(3+) under the excitation of 390 nm was about 16.7%. These materials have potential application in achieving high-efficiency Si solar cells via spectrum modification.

The study of thermal silicon dioxide electrets formed by corona discharge and rapid-thermal annealing

A silicon dioxide (SiO2) electret passivates the surface of crystalline silicon (Si) in two ways: (i) when annealed and hydrogenated, the SiO2–Si interface has a low density of interface states, offering few energy levels through which electrons and holes can recombine; and (ii) the electret’s quasipermanent charge repels carriers of the same polarity, preventing most from reaching the SiO2–Si interface and thereby limiting interface recombination. In this work, we engineer a charged thermal SiO2 electret on Si by depositing corona charge onto the surface of an oxide-coated Si wafer and subjecting the wafer to a rapid thermal anneal (RTA). We show that the surface-located corona charge is redistributed deeper into the oxide by the RTA. With 80 s of charging, and an RTA at 380 °C for 60 s, we measure an electret charge density of 5 × 1012 cm–2, above which no further benefit to surface passivation is attained. The procedure leads to a surface recombination velocity of less than 20 cm/s on 1 Ω-cm n-type Si, which is commensurate with the best passivation schemes employed on high-efficiency Si solar cells. In this paper, we introduce the method of SiO2 electret formation, analyze the relationship between charge density and interface recombination, and assess the redistribution of charge by the RTA.

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 x N 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 cm 2 Float-zone Si wafer and 17.2% for a 156.8 cm 2 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.

Screen printed phosphorus diffusion for low-cost and simplified industrial mono-crystalline silicon solar cells

Screen-printing is considered to be the most efficient technique for large-scale Si solar cell production. Doping technique currently in use, such as POCl 3 diffusion furnace, H 3PO 4 spray doping and spin-on-doping (SOD) tend to be time-consuming and wasteful of source materials. Screen printing technology can be used for the application of doping substance for diffusion. Diffusion paste doping by the support of screen printing has enough potential to overcome the disadvantages of other doping techniques. This paper demonstrates a simple method for creation of P–N junction, with efficiency above 16.1%, using a screen printable phosphorus diffusion paste. To optimize the doping process using screen printing, we varied several experimental conditions, including firing temperatures and time. A fill factor above 77% has been reached, with an efficiency of 16.1% in a 10.0 cm×10.0 cm. The high fill factor with paste doping is mainly due to a contact resistance (3.53 Ω) lower than that for POCl 3 (5.64 Ω). These results could be applied to the manufacturing of low cost, simplified and high efficiency Si solar cells.