High-efficiency Silicon Solar Cells Research Articles

On elimination of inactive phosphorus in industrial POCl3 diffused emitters for high efficiency silicon solar cells

It is compelling to eliminate inactive phosphorus in industrial POCl3 diffused emitters for high efficiency silicon solar cells. Several approaches were reported to significantly reduce inactive phosphorus in the literature. However, the underlying physical mechanisms are not fully understood. In this work, we conduct a systematic investigation and fabricate eight emitters involving two POCl3:O2 ratios at pre-deposition and both inert and oxidising scenarios at drive-in. Characterization techniques such as secondary ion mass spectrometry and X-ray photoelectron spectroscopy are carried out with an emphasis on the phosphosilicate glass. To provide a consistent interpretation of our results and those in the literature, we suggest a probable reaction, “free phosphorus oxidation”, as the dominant mechanism to unveil the role and impact of oxygen during POCl3 diffusion. We further propose that various industrial POCl3 emitters can be fabricated inactive phosphorus free without drive-in or a pre-grown oxide. Among the eight emitters, three emitters are free from inactive phosphorus with a sheet resistance range from 65 to 140Ω/□, featuring a surface doping from 1 × 1020 to 3 × 1020cm−3 and a remarkably low emitter saturation current density down to 51fA/cm2.

An enhanced alneal process to produce SRV < 1 cm/s in 1 Ω cm n-type Si

The alneal is one of the most effective methods of electrically passivating a silicon surface, and has been used by numerous research groups since the 1980s. In this work, we present an enhanced alneal process that substantially improves its effectiveness. Previously, the success afforded by the standard alneal has been attributed to the chemical passivation provided by hydrogenation of the Si-SiO2 interface. However, the work presented here shows that it is possible to enhance the surface passivation by simultaneously introducing a component of Field Effect Passivation (FEP). Where the standard alneal is seen to provide lifetimes of ~2.1ms, equivalent to a surface recombination velocity (SRV) of 3.3cm/s, the enhanced alneal can provide a lifetime of 5.6ms on 1Ωcm, n-type Si, equivalent to a SRV 0.4cm/s. The charge required for this enhanced passivation can be introduced in the order of minutes and has the potential to be introduced at the same time as the aluminium is deposited, thus, resulting in no extra processing time. Secondary ion mass spectroscopy showed that the nature of the charge is likely to be K and Na cations residing at the Si-SiO2 interface. The possibility of increasing the surface passivation beyond that of the standard alneal points to the importance of both chemical and field effect components of passivation, and is therefore of significant interest to high efficiency silicon solar cell research.

Industrially feasible, dopant‐free, carrier‐selective contacts for high‐efficiency silicon solar cells

AbstractDopant‐free, carrier‐selective contacts (CSCs) on high efficiency silicon solar cells combine ease of deposition with potential optical benefits. Electron‐selective titanium dioxide (TiO2) contacts, one of the most promising dopant‐free CSC technologies, have been successfully implemented into silicon solar cells with an efficiency over 21%. Here, we report further progress of TiO2 contacts for silicon solar cells and present an assessment of their industrial feasibility. With improved TiO2 contact quality and cell processing, a remarkable efficiency of 22.1% has been achieved using an n‐type silicon solar cell featuring a full‐area TiO2 contact. Next, we demonstrate the compatibility of TiO2 contacts with an industrial contact‐firing process, its low performance sensitivity to the wafer resistivity, its applicability to ultrathin substrates as well as its long‐term stability. Our findings underscore the great appeal of TiO2 contacts for industrial implementation with their combination of high efficiency with robust fabrication at low cost. Copyright © 2017 John Wiley &amp; Sons, Ltd.

A simple method for pinhole detection in carrier selective POLO-junctions for high efficiency silicon solar cells

Polycrystalline silicon (poly-Si) layers on thin silicon oxide films have received strong research interest as they form excellent carrier selective junctions on crystalline silicon substrates after appropriate thermal processing. Recently, we presented a new method to determine the pinhole density in interfacial oxide films of poly-Si on oxide (POLO)-junctions with excellent electrical properties. The concept of magnification of nanometer-size pinholes in the interfacial oxide by selective etching of the underlying crystalline silicon is used to investigate the influence of annealing temperature on pinhole densities. Eventually, the pinholes are detected by optical microscopy and scanning electron microscopy. We present results on the pinhole density in POLO-junctions with J0 values as low as 1.4 fA/cm2. The stability of this method is demonstrated by proving that no new holes are introduced to the oxide during the etching procedure for a wide range of etching times. Finally, we show the applicability to multiple oxide types and thickness values, differently doped poly-Si layers as well as several types of wafer surface morphologies. For wet chemically grown oxides, we verified the existence of pinholes with an areal density of 2×107cm−2 even already after annealing at a temperature of 750°C (lower than the optimum annealing temperature for these junctions).

Experimental study of the pyramidal texturization on the surface of photovoltaic silicon with cemented carbide micro-milling tool

Some anti-reflection structures can be fabricated on the surface of high-efficiency silicon solar cells. The anti-reflection efficiency can be increased and hence the photoelectric conversion efficiency can be increased. The effects of varied anti-reflection structure are altered on the optical reflectance. The forward stochastic pyramid structure can be fabricated mechanically. Before processing the pyramid structure, a unidirectional V-groove array must be fabricated firstly. The research content of this paper was based on our previous researches. The fabrication of V-groove with different depth on the surface of silicon wafers was explored. Micro-milling tool constituted by cemented carbide was used. The quality of completed V-grooves was measured and analyzed. Then the effect of the structure for the reflectance was evaluated. The calculated reflectance of accomplished V-grooves was compared with that of the theoretical model. A satisfactory value was obtained by contrast with existing technologies. Furthermore, the tools were observed after the processing, to evaluate the tool wear. Results indicated that processing the pyramid structure using the micro-milling tool is feasible.

Bulk Lifetimes up to 20 ms Measured on Unpassivated Silicon Discs Using Photoluminescence Imaging

With high-efficiency silicon solar cells approaching 25% efficiency in mass production, the requirements on the bulk lifetime and its uniformity across the wafer and the ingot increase dramatically. Since some cell architectures require these high lifetimes on starting material, the need arises for characterization methods to measure very high bulk lifetimes that are spatially resolved at an early stage before cell processing. A method based on the spectral ratio of two photoluminescence images is applied here on two unpassivated silicon discs from different positions within a Czochralski-grown phosphorous-doped n-type silicon ingot. The method allows the determination of spatially resolved bulk lifetime images on samples with adequate thickness and can be done within seconds and without the need to passivate surfaces. As-grown bulk lifetimes up to 20 ms are measured on the ingot's central disc, indicating recent improvements in crystallization technology, but are strongly reduced closer to the crown. Evidence suggesting the impact of thermal donors on the lifetime and effective doping concentration near the crown is found from combining spectral photoluminescence and infrared spectroscopy analyses. The technique could find applications in research and development activities, particularly in the optimization of Czochralski silicon crystal growth conditions.

Passivation properties of tunnel oxide layer in passivated contact silicon solar cells

Passivated contact in advanced high-efficiency silicon solar cells based on the full back surface field (BSF) is reported here in based on the application of a tunnel oxide layer that is less than 2nm thick. The open-circuit voltage (Voc) was significantly improved via interface passivation due to insertion of the tunnel oxide layer. During oxide layer growth, a transition region, such as a sub-oxide, was observed at a depth of about 0.75nm in the growth interface between the silicon oxide layer and silicon substrate. The properties of the less than 2nm thick tunnel oxide layer were primarily affected by the characteristics of the transition region. The passivation characteristics of tunnel oxide layer should depend on the physical properties of the oxide. The interface trap density Dit is an important parameter in passivation and is influenced by the stoichiometry of the oxide which in turn strongly affected by the fabrication and the post annealing conditions. During heat treatment of a-Si:H thin films (for the purpose of crystallization to form doped layers), thin film blistering occurs due to hydrogen effusion on flat substrate surfaces. To minimize this behavior, we seek to control the surface morphology and annealing profile. Also, the passivation quality of passivataed contact structure declined for the sample annealed above 900°C. This decline was attributed not only to local disruption of the tunnel oxide layer, but also to phosphorus diffusion. The resistivity of the tunnel oxide layer declined precipitously for the sample annealed above 900°C. On the basis of these, implied Voc over 740mV was achieved in n-type Si wafer through the control of the oxide stoichiometry via optimizing the annealing conditions.

Numerical analysis of monocrystalline silicon solar cells with fine nanoimprinted textured surface

We investigated the surface reflectance of nanoimprinted textures on silicon. Zirconium oxide, which is a wide-bandgap inorganic dielectric material, was used as the texturing material. We performed several calculations to optimize the textures for the production of high-efficiency bulk-type monocrystalline silicon solar cells. Our analysis revealed that nanoimprinted textured solar cells exhibit a lower reverse saturation current density than a solar cell with a conventional etched texture. It was also confirmed that the photocarrier generation rate for a solar cell with a submicron-scale nanoimprinted texture has little dependence on the texture shape. Furthermore, the weighted average reflectance of an optimized nanoimprinted textured solar cell was substantially reduced to 3.72%, suggesting that texture formation by nanoimprint lithography is an extremely effective technology for producing high-efficiency solar cells at a low cost.

Carrier selective contacts:a selection of high efficiency silicon solar cells

Solar cell has two basic units:the photon absorption layer and the contact layer. The contact layer is a region between the highly recombination-active metal interface and the photon absorption layer. It is vital to reduce the recombination loss between the photon absorption layer and the contact layer in pursuit of the higher conversion efficiency of silicon solar cell. In recent years, carrier selective contact is arousing research interest in photovoltaic industry because it is deemed as one of the last remaining obstacles in approaching to the theoretical efficiency limit of silicon solar cell. In this paper, three different types of carrier selective contacts are analyzed, which includes:1) sandwiching a heavily doped thin layer between the photon absorption layer and the metal interface, which is the so-called emitter or back surface field; 2) aligning the conduction bands or the valence bands of two materials; 3) inducing the band bending through a high work function metal oxide contacting crystalline silicon. Based on one-dimensional solar cell simulation software wxAMPS, three different silicon solar cell structures are numerically simulated, which includes:1) diffused homojunction silicon solar cell[(p+)c-Si/(n)c-Si/(n+)c-Si]; 2) silicon heterojunction solar cell with amorphous silicon thin films[(p+)a-Si/(i)a-Si/(n)c-Si/(i)a-Si/(n+)a-Si]; 3) silicon heterojunction solar cell with metal oxide thin films[(n)MoOx/(n)c-Si/(n)TiOx], then the energy band structures and the spatial distributions of carrier concentrations of solar cells in the dark are discussed. The simulation results show that the key factor of carrier selective contacts is the asymmetric spatial distribution of the carrier concentrations, i.e. the asymmetric conductivities of electrons and holes. This leads to the formation of high resistance to electrons and low resistance to holes, or high resistance to holes and low resistance to electrons, so the holes will go through the contact easily and the electrons will be blocked simultaneously, or the electrons will go through the contact easily and the holes will be blocked simultaneously. Therefore a hole selective contact or a electron selective contact is formed, respectively.

Combining light-harvesting with detachability in high-efficiency thin-film silicon solar cells.

Efforts to realize thin-film solar cells on unconventional substrates face several obstacles in achieving good energy-conversion efficiency and integrating light-management into the solar cell design. In this report a technique to circumvent these obstacles is presented: transferability and an efficient light-harvesting scheme are combined for thin-film silicon solar cells by the incorporation of a NaCl layer. Amorphous silicon solar cells in p-i-n configuration are fabricated on reusable glass substrates coated with an interlayer of NaCl. Subsequently, the solar cells are detached from the substrate by dissolution of the sacrificial NaCl layer in water and then transferred onto a plastic sheet, with a resultant post-transfer efficiency of 9%. The light-trapping effect of the surface nanotextures originating from the NaCl layer on the overlying solar cell is studied theoretically and experimentally. The enhanced light absorption in the solar cells on NaCl-coated substrates leads to significant improvement in the photocurrent and energy-conversion efficiency in solar cells with both 350 and 100 nm thick absorber layers, compared to flat-substrate solar cells. Efficient transferable thin-film solar cells hold a vast potential for widespread deployment of off-grid photovoltaics and cost reduction.

Novel Ag-doped glass frits for high-efficiency crystalline silicon solar cells.

Glass frits play an important role in the front contact electrodes of crystalline silicon (c-Si) solar cells. In this work, we developed a novel glass frit by doping Ag into a glass frit in the process of high-temperature synthesis. When the Ag paste including this novel glass frit was used as the front contact electrode of silicon solar cells, the conversion efficiency of poly-crystalline silicon (pc-Si) solar cells was improved by 1.9% compared to the glass frit without Ag. Through SEM characterisation and calculation of series resistance, we further found that the interface between Ag and Si was improved and the contact resistance of Ag and Si was greatly reduced, which were believed to be responsible for the improvement of solar cell performance. This work shows great guidance significance to develop novel and highly efficient commercial glass frits applied in solar cells in the future.

Independent Al2O3/SiNx:H and SiO2/SiN x:H Passivation of p+ and n+ Silicon Surfaces for High-Performance Interdigitated Back Contact Solar Cells

In order to improve interdigitated back contact (IBC) solar cell efficiencies, the entire solar cell surface must be well passivated. Al2O3/SiN x :H and SiO2/SiN x :H passivation stacks have been widely adopted for high-efficiency silicon solar cells. We explored IBC solar cells with 1) only SiO2/SiN x :H; 2) only Al2O3/SiN x :H passivating both diffused surfaces; or 3) independent p+ emitter and n + back surface field (BSF) passivation with Al2O3/SiN x :H and SiO2/SiN x :H respectively. First, stacks were optimized through simulation (using a device model in 3D Quokka) by varying the recombination parameter ${J_o}$ . Second, solar cells were fabricated with a low-cost high-throughput screen-printing technique. Third, simulated ${J_{{\rm{sc}}}}$ , ${V_{{\rm{oc}}}}$ , $FF$ , and $\eta $ values closely matched the experimental results for passivation schemes 1 and 2. Passivation scheme 3 could not be realized experimentally due to fabrication difficulties, while its simulated values were 688 mV, 41.4 mA/cm2, 80.8%, and 23.0%, respectively. It is clear that independent passivation captures material advantages for each diffused region for enhanced solar cell performance compared with conventional passivation of both diffused regions with a single passivation stack.

Silicon-Rich Silicon Carbide Hole-Selective Rear Contacts for Crystalline-Silicon-Based Solar Cells.

The use of passivating contacts compatible with typical homojunction thermal processes is one of the most promising approaches to realizing high-efficiency silicon solar cells. In this work, we investigate an alternative rear-passivating contact targeting facile implementation to industrial p-type solar cells. The contact structure consists of a chemically grown thin silicon oxide layer, which is capped with a boron-doped silicon-rich silicon carbide [SiCx(p)] layer and then annealed at 800-900 °C. Transmission electron microscopy reveals that the thin chemical oxide layer disappears upon thermal annealing up to 900 °C, leading to degraded surface passivation. We interpret this in terms of a chemical reaction between carbon atoms in the SiCx(p) layer and the adjacent chemical oxide layer. To prevent this reaction, an intrinsic silicon interlayer was introduced between the chemical oxide and the SiCx(p) layer. We show that this intrinsic silicon interlayer is beneficial for surface passivation. Optimized passivation is obtained with a 10-nm-thick intrinsic silicon interlayer, yielding an emitter saturation current density of 17 fA cm-2 on p-type wafers, which translates into an implied open-circuit voltage of 708 mV. The potential of the developed contact at the rear side is further investigated by realizing a proof-of-concept hybrid solar cell, featuring a heterojunction front-side contact made of intrinsic amorphous silicon and phosphorus-doped amorphous silicon. Even though the presented cells are limited by front-side reflection and front-side parasitic absorption, the obtained cell with a Voc of 694.7 mV, a FF of 79.1%, and an efficiency of 20.44% demonstrates the potential of the p+/p-wafer full-side-passivated rear-side scheme shown here.

Diffuse reflectors for improving light management in solar cells: a review and outlook

Pigment based diffuse reflectors (DRs) have several advantages over metal reflectors such as good stability, high reflectivity, and low parasitic absorption. As such, DRs have the potential to be applied on high efficiency silicon solar cells and further increase the power conversion efficiency. In this paper, we perform a thorough review on the notable achievements to date of DRs’ application for photovoltaics. We outline unique attributes of these technologies and discuss the theoretical and laboratory development working towards overcoming the challenges of transferring to high efficiency silicon solar cells. In order to understand the potential of DRs for high efficiency silicon solar cells, we provide a qualitative analysis of the impact of front reflection, rear absorption and the angular distribution on the useful light absorption in silicon wafers. By including this discussion, we provide an outlook for the application of DR in reaching maximum photo-current for high efficiency silicon solar cells.

SiO2 passivation layer grown by liquid phase deposition for silicon solar cell application

Surface passivation is one of the primary requirements for high efficient silicon solar cells. Though the current existed passivation techniques are effective, expensive equipments are required. In this paper, a comprehensive understanding of the SiO2 passivation layer grown by liquid phase deposition (LPD) was presented, which was cost-effective and very simple. It was found that the post-annealing process could significantly enhance the passivation effect of the LPD SiO2 film. Besides, it was revealed that both chemical passivation and field-effect passivation mechanisms played important roles in outstanding passivation effect of the LPD SiO2 film through analyzing the minority carrier lifetime and the surface recombination velocity of n-type and p-type silicon wafers. Although the deposition parameters had little influence on the passivation effect, they affected the deposition rate. Therefore, appropriate deposition parameters should be carefully chosen based on the compromise of the deposition rate and fabrication cost. By utilizing the LPD SiO2 film as surface passivation layer, a 19.5%-efficient silicon solar cell on a large-scale wafer (156 mm × 156 mm) was fabricated.

Stabilized 14.0%-efficient triple-junction thin-film silicon solar cell

We report on a high-efficiency triple-junction thin-film silicon solar cell fabricated using the substrate configuration. An undoped hydrogenated amorphous silicon (a-Si:H) solar cell grown using triode plasma-enhanced chemical vapor deposition, which is more stable against light soaking, was applied to the a-Si:H/μc-Si:H/μc-Si:H triple-junction cells with honeycomb-textured substrates. To find the best balance in short circuit density and fill factor, we quantitatively investigated the effect of current mismatch on triple-junction cells. Accordingly, a stabilized efficiency of 14.04% was achieved in an a-Si:H/μc-Si:H/μc-Si:H triple-junction solar cell with a minimum light-induced degradation of 4%, setting a new record in this type of solar cells.

Corona Charge in SiO2: Kinetics and Surface Passivation for High Efficiency Silicon Solar Cells

This manuscript presents a method by which capacitance-voltage measurements can be used in conjunction with Kelvin-probe measurements to calculate the location of charge within a dielectric layer. A first order kinetic model for the transport of charge into SiO2 films after exposure to corona charge deposition is proposed. The rate limiting step for charge migration into oxide films has been observed to be the injection of charge from the surface (air-SiO2 interface). The charge lies preferentially at the air-SiO2 and SiO2-Si interfaces, and its injection into SiO2 has been characterized as having an activation energy of ∼ 50 meV. In this work, corona charge deposition has produced SRV < 2.7cm/s and J01 < 12 fA/cm2 which is within the requirements of high efficiency silicon solar cells. These results contribute to the understanding of corona charge interaction with SiO2 with a view to its potential application as an extrinsic method of passivation for highly efficient silicon solar cells.

반응성 이온 건식식각에서 RF Power 변화에 따른 표면 조직화 개선 연구

A textured front surface is required in high efficiency silicon solar cells to reduce reflectance and to improve light trapping. Wet etching with alkaline solution is usually applied for mono crystalline silicon solar cells. However, alkali texturing method is not appropriate for multi-crystalline silicon wafers due to grain boundary of random crystallographic orientation. Accordingly, acid texturing method is generally used for multi-crystalline silicon wafers to reduce the surface reflectance. To reduce reflectivity of multi-crystalline silicon wafers, double texturing method with combination of acid and reactive ion etching is an attractive technical solution. In this paper, we have studied to optimize RIE condition by different RF power condition (100, 150, 200, 250, 300 W).

High-mobility Hydrogenated Indium Oxide without Introducing Water During Sputtering

The key role of water to obtain high-mobility IO:H (hydrogenated indium oxide) layers has been well documented, but introducing the required tiny amount of water is technologically challenging. We first use simulations to evidence the key role of high mobility for the transparent conductive oxide for high-efficiency crystalline silicon solar cells. Then, we investigate an approach to fabricate high-mobility IO:H that circumvent the introduction of water vapor, relying on water vapor from ambient air. A sputtering tool equipped with a residual gas analyzer allows partial pressure monitoring of hydrogen and water in the system, and to link the gas composition to the properties of the deposited films. To vary the residual water pressure, we varied the pumping time after opening the chamber and before starting the deposition to reach different base pressures (1. 10-5 mbar to 3. 10-7 mbar), which are mostly composed of residual water. An optimum base pressure around 3. 10-6 mbar—and not lower pressures—was found to yield the highest mobility values after annealing. An alternative approach by introducing a small flow of hydrogen together with argon and oxygen is also shown to provide promising results.

Development of n-type Selective Emitter Silicon Solar Cells by Laser Doping Using Boron-doped Silicon Paste

N-type solar cells are being developed as next generation photovoltaic solar cells, and have attracted a significant amount of attention. In addition, the demand for high-efficiency silicon solar cells has increased in order to reduce production costs and save space. In this study, we demonstrate boron laser doping (LD) using a boron-doped NanoGram® Si paste in n-type passivated emitter, rear totally diffused solar cells. The sheet resistance was 54 Ω/sq at the local boron emitter after boron LD. The boron diffusion depth was ∼2.0 μm, and the boron surface concentration was 7 × 1019 atoms/cm3. The n-type solar cells were fabricated using boron LD to have front side boron selective emitters. The characteristics of the newly developed solar cells improved in comparison with those of the reference cells (i.e., those without boron selective emitters), and a maximum efficiency of 19.70% was achieved. The improved efficiency was mainly due to the increase in the short circuit current density and fill factor.