Boron‐doped selective silicon epitaxy: high efficiency and process simplification in interdigitated back contact cells

The present research and development activities in crystalline silicon photovoltaics include the exploration of doping technologies alternative to the mainstream diffusion process. The goal is to identify those technologies with potential to increase the solar cell efficiency and reduce the cost per watt peak. In that respect, this work presents the selective epitaxial growth of silicon as a candidate for boron doping; showing the results of the evaluation of boron‐doped silicon epitaxial emitters on slurry and diamond‐coated wire‐sliced Czochralski material, their integration in interdigitated back contact solar cells, and the development of a novel process sequence to create the interdigitated rear junction of these devices using selective epitaxial growth. Boron‐doped silicon epitaxy is demonstrated to perform in the high efficiency range (>22%), and the use of selective epitaxial growth is proposed as a route for the simplification of the interdigitated back contact solar cell flow. Copyright © 2013 John Wiley & Sons, Ltd.     

Add‐on laser tailored selective emitter solar cells

An elegant laser tailoring add‐on process for silicon solar cells, leading to selectively doped emitters increases their efficiency η by Δη = 0.5% absolute. Our patented, scanned laser doping add‐on process locally increases the doping under the front side metallization, thus allowing for shallow doping and less Auger recombination between the contacts. The selective laser add‐on process modifies the emitter profile from a shallow error‐function type to Gaussian type and enables excellent contact formation by screen printing, normally difficult to achieve for shallow diffused emitters. The significantly deeper doping profile of the laser irradiated samples widens the process window for the firing of screen printed contacts and avoids metal spiking through the pn‐junction. Copyright © 2010 John Wiley & Sons, Ltd.     

Passivating thin bifacial silicon solar cells for industrial production

A scheme for passivating thin multi‐crystalline silicon solar cells compatible to mass production is presented. Wafers with a thickness of 180 µm were processed into solar cells. The otherwise severe bowing has been avoided by reduced aluminium coverage on the rear surface. The process scheme includes a silicon nitride firing through step for conventional screen printed contacts, where a silicon nitride layer on the rear surface acts as surface passivation layer and enables a gain in efficiency of 0.6% [abs.]. Copyright © 2007 John Wiley & Sons, Ltd.     

Efficient silicon heterojunction solar cells based on p‐ and n‐type substrates processed at temperatures < 220°C

We present the optimization and characterization of heterojunction solar cells consisting of an amorphous silicon emitter, a single crystalline absorber and an amorphous silicon rear side which causes the formation of a back surface field (a‐Si:H/c‐Si/a‐Si:H). The solar cells were processed at temperatures <220°C. An optimum of the gas phase doping concentration of the a‐Si:H layers was found. For high gas phase doping concentrations, recombination via defects located at or nearby the interface leads to a decrease in solar cell efficiency. We achieved efficiencies >17% on p‐type c‐Si absorbers and >17·5% on n‐type absorbers. In contrast to the approach of Sanyo, no additional intrinsic a‐Si:H layers between the substrate and the doped a‐Si:H layers were inserted. Copyright © 2006 John Wiley & Sons, Ltd.     

Silicon solar cells based on all‐laser‐transferred contacts

Crystalline silicon solar cells based on all‐laser‐transferred contacts (ALTC) have been fabricated with both front and rear metallization achieved through laser induced forward transferring. Both the front and rear contacts were laser‐transferred from a glass slide coated with a metal layer to the silicon substrate already processed with emitter formation, surface passivation, and antireflection coating. Ohmic contacts were achieved after this laser transferring. The ALTC solar cells were fabricated on chemically textured p‐type Cz silicon wafers. An initial conversion efficiency of over 15% was achieved on a simple cell structure with full‐area emitter. Further improvements are expected with optimized laser transferring conditions, front grid pattern design, and surface passivation. The ALTC process demonstrates the advantage of laser processing in simplifying the solar cell fabrication by a one‐step metal transferring and firing process. Copyright © 2013 John Wiley & Sons, Ltd.     

Progress on large area n‐type silicon solar cells with front laser doping and a rear emitter

We report on the progress of imec's n‐type passivated emitter, rear totally diffused rear junction silicon solar cells. Selective laser doping has been introduced in the flow, allowing the implementation of a shallow diffused front surface field and a reduction of the recombination current in the contact area. Simplifications have been implemented towards a more industrial annealing sequence, by replacing expensive forming gas annealing steps with a belt furnace annealing. By applying these improvements, together with an advanced texturing process and emitter passivation by atomic layer deposition of Al₂O₃, 22.5% efficient cells (three busbars) have been realized on commercial 156 · 156 mm² Czochralski‐Si. This result has been independently confirmed by ISE CalLab. Copyright © 2016 John Wiley & Sons, Ltd.     

19% efficient n‐type Czochralski silicon solar cells with screen‐printed aluminium‐alloyed rear emitter

High and stable lifetimes recently reported for n‐type silicon materials are an important and promising prerequisite for innovative solar cells. To exploit the advantages of the excellent electrical properties of n‐type Si wafers for manufacturing simple and industrially feasible high‐efficiency solar cells, we focus on back junction n⁺np⁺ solar cells featuring an easy‐to‐fabricate full‐area screen‐printed aluminium‐alloyed rear p⁺ emitter. Independently confirmed record‐high efficiencies have been achieved on n‐type phosphorus‐doped Czochralski‐grown silicon material: 18·9% for laboratory‐type n⁺np⁺ solar cells (4 cm²) with shadow‐mask evaporated front contact grid and 17·0% for front and rear screen‐printed industrial‐type cells (100 cm²). The electrical cell parameters were found to be perfectly stable under illumination. Copyright © 2006 John Wiley & Sons, Ltd.     

Investigation of laser‐fired rear‐side recombination properties using an analytical model

This paper presents the application of the analytical model for locally contacted rear sides recently published by Fischer to the determination of recombination losses of solar cells with fixed metallization fraction, but varying contact pitch. After the successful experimental validation of the model on oxide‐passivated solar cells with ohmic contacts, the model was used for a detailed investigation of rear sides prepared by the laser‐fired contacts (LFC) method. In this way the surface recombination velocity (SRV) at the very contact areas was extracted for a broad base doping range. The determined parameterization allows the calculation of the SRV of any LFC rear side concerning base doping and contact pitch. The excellent passivation quality of the alnealed oxide with LFC contacts is shown: on 1 (100) Ω cm FZ an effective SRV of only 35 (4·3) cm/s could be measured with 1000 µm contact pitch. Copyright © 2006 John Wiley & Sons, Ltd.     

Investigation of the impact of the rear‐dielectric/silver back reflector design on the optical performance of thin‐film silicon solar cells by means of detached reflectors

Thin‐film silicon solar cells often rely on a metal back reflector separated from the silicon layers by a thin rear dielectric as a back reflector (BR) design. In this work, we aim to obtain a better insight into the influence of the rear‐dielectric/Ag BR design on the optical performance of hydrogenated microcrystalline silicon (µc‐Si:H) solar cells. To allow the application of a large variety of rear dielectrics combined with Ag BRs of diverse topographies, the solar cell is equipped with a local electrical contact scheme that enables the use of non‐conductive rear dielectrics such as air or transparent liquids of various refractive indices n. With this approach, detached Ag BRs having the desire surface texture can be placed behind the same solar cell, yielding a direct and precise evaluation of their impact on the optical cell performance. The experiments show that both the external quantum efficiency and the device absorptance are improved with decreasing n and increasing roughness of the BR. Calculations of the angular intensity distribution of the scattered light in the µc‐Si:H are presented. They allow for establishing a consistent picture of the light trapping in the solar cell. Copyright © 2013 John Wiley & Sons, Ltd.     

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Multicrystalline silicon solar cells exceeding 20% efficiency

This paper presents the first conversion efficiency above 20% for a multicrystalline silicon solar cell. The application of wet oxidation for rear surface passivation significantly reduces the process temperature and therefore prevents the degradation of minority‐carrier lifetime. The excellent optical properties of the dielectrically passivated rear surface in combination with a plasma textured front surface result in a superior light trapping and allow the use of substrates below 100 μm thickness. A simplified process scheme with laser‐fired rear contacts leads to conversion efficiencies of 20·3% for multicrystalline and 21·2% for monocrystalline silicon solar cells on small device areas (1 cm²). Copyright © 2004 John Wiley & Sons, Ltd.     

Co‐optimization of emitter profile and metal grid of selective emitter silicon solar cells

Co‐optimization of the metallization and emitter dopant profile is fully investigated for selective emitter crystalline silicon solar cells. The simulation parameters for the laser doping selective emitter, metallization by plating, silicon nitride passivation, and aluminum back surface field are identified and reached. Internal light flux reflection is also considered in the model. In particular, the influence of the increased light trapping ability of a textured surface on the optimization results is clarified by comparing a cell with a non‐textured surface. In this paper, the optimization results, including the electrical performances of a solar cell are discussed in detail. On the basis of these simulation results, an optimized metallization and emitter dopant profile is proposed. Copyright © 2011 John Wiley & Sons, Ltd.     

Bifacial silicon solar cells with 21·3% front efficiency and 19·8% rear efficiency

We introduced a triode structure with p–n junctions on both sides into single‐crystalline bifacial silicon solar cells in order to improve solar cell performance. These fabricated bifacial silicon solar cells have an energy conversion efficiency of 21·3% under front 1 sun illumination (the standard 1 kW/m² AM 1·5 global spectrum at 25°C) and 19·8% under rear 1 sun illumination tested at the Japan Quality Assurance Organization. The total of the front and rear conversion efficiencies is the highest ever reported for bifacial silicon solar cells. Copyright © 2000 John Wiley & Sons, Ltd.     

High‐efficiency microcrystalline silicon single‐junction solar cells

This short communication highlights our latest results towards high‐efficiency microcrystalline silicon single‐junction solar cells. By combining adequate cell design with high‐quality material, a new world record efficiency was achieved for single‐junction microcrystalline silicon solar cell, with a conversion efficiency of 10.69%, independently confirmed at ISE CalLab PV Cells. Such significant conversion efficiency could be achieved with only 1.8 µm of Si. Copyright © 2013 John Wiley & Sons, Ltd.     

Front and rear silicon‐nitride‐passivated multicrystalline silicon solar cells with an efficiency of 18.1%

A solar cell process designed to utilise low‐temperature plasma‐enhanced chemical vapour deposited (PECVD) silicon nitride (SiN_x) films as front and rear surface passivation was applied to fabricate multicrystalline silicon (mc‐Si) solar cells. Despite the simple photolithography‐free processing sequence, an independently confirmed efficiency of 18.1% (cell area 2 × 2 cm²) was achieved. This excellent efficiency can be predominantly attributed to the superior quality of the rear surface passivation scheme consisting of an SiN_x film in combination with a local aluminium back‐surface field (LBSF). Thus, it is demonstrated that low‐temperature PECVD SiN_x films are well suited to achieve excellent rear surface passivation on mc‐Si. Copyright © 2002 John Wiley & Sons, Ltd.     

Development of back‐junction back‐contact silicon solar cells based on industrial processes

We have presented simplified industrial processes to fabricate high performance back‐junction back‐contact (BJBC) silicon solar cells. Good optical surface structures (solar averaged reflectance 2.5%) and high implied open‐circuit voltage (0.695 V) have been realized in the BJBC cell precursors through wet chemical processing, co‐diffusion, P ion implantation and annealing oxidation, as well as laser patterning and plasma enhanced chemical vapour deposition passivation processes. We have achieved a certified high efficiency of close to 22% on BJBC silicon solar cells with the size of 4.04 cm² by using screen printing and co‐firing technologies. The manufacturing process flow further successfully yields efficiency of around 21% BJBC silicon solar cells with enlarged sizes of 6 × 6 cm². The present work has demonstrated that the commercialization of low‐cost and high‐efficiency BJBC solar cells is possible because we have used processes compatible with existing production lines. Copyright © 2017 John Wiley & Sons, Ltd.     

Investigation of laser ablation on boron emitters for n‐type rear‐junction PERT type silicon wafer solar cells

n‐type silicon wafer solar cells are receiving increasing attention for industrial application in recent years, such as the n‐type rear‐junction Passivated Emitter Rear Totally‐diffused (PERT) solar cells. One of the main challenges in fabricating the n‐PERT solar cells is the opening of the rear dielectric for localized contacts. In this work laser ablation is applied to locally ablate the rear dielectric. We investigate the laser damage to the emitter at the laser‐ablated regions using the emitter saturation current density, J_0e,laser, extracted by two approaches. J_0e,laser is observed to be injection dependent due to high J₀₂ recombination caused by laser damage to the space charge region. By using the optimized laser ablation parameters, n‐PERT solar cells with an efficiency of up to 21.0% are realized. Copyright © 2015 John Wiley & Sons, Ltd.     

Screen‐printed Emitter‐Wrap‐Through solar cell with single step side selective emitter with 18.8% efficiency

A fabrication process for Emitter‐Wrap‐Through solar cells on monocrystalline material with high quality gap passivation by wet thermal silicon dioxide is investigated. Masking and structuring steps are performed by screen‐printing technology. Via‐holes are created by an industrially applicable high‐speed laser drilling process. The cell structure features a selective emitter structure fabricated in a single high temperature step: a highly doped emitter at the via‐holes and the rear side, allowing for a low via‐hole resistivity as well as a low resistivity contact to screen‐printed pastes, and a moderately doped front side emitter exhibiting high quantum efficiency in the low wavelength range. Therefore a novel approach is applied depositing either doped or undoped PECVD silicon dioxide layers on the front side. It is shown that doping profiles advantageous for the EWT‐cell structure can be achieved. The screen‐printed aluminum paste is found to penetrate the underlying thermal dioxide layer at appropriate contact firing conditions leading to a zone of high recombination in the overlap region of aluminum and silicon dioxide. It is shown that conventional PECVD‐anti‐reflection silicon nitride acts as effective protection layer reducing the recombination in this region. Designated area conversion efficiencies up to 18.8% on FZ material are obtained applying the single step side selective emitter fabrication technique. Copyright © 2010 John Wiley & Sons, Ltd.     

Advantages of photoplating for laser doped solar cells

The use of laser doping in solar cell fabrication has received increased attention in recent years, especially due to its ability to form a selective emitter without subjecting the wafer to prolonged high temperature processes and likely corresponding degradation. At the University of New South Wales, a novel laser doping method was developed that combines the formation of the selective emitter with a self‐aligned metallisation pattern. This paper investigates the influence of different plating methods on shunt and enhanced recombination regions. It was found that the pseudo fill factors of solar cells plated using photoplating are higher compared to those plated with the electroless method, indicating reduced shunting and/or junction recombination. The superiority of the photoplating method is even more pronounced at high laser powers. According to our knowledge this is the first time photoplating and electroless plating techniques have been compared for laser‐doped solar cells. Copyright © 2011 John Wiley & Sons, Ltd.     

Stack system of PECVD amorphous silicon and PECVD silicon oxide for silicon solar cell rear side passivation

A stack of hydrogenated amorphous silicon (a‐Si) and PECVD‐silicon oxide (SiO_x) has been used as surface passivation layer for silicon wafer surfaces. Very good surface passivation could be reached leading to a surface recombination velocity (SRV) below 10 cm/s on 1 Ω cm p‐type Si wafers. By using the passivation layer system at a solar cell's rear side and applying the laser‐fired contacts (LFC) process, pointwise local rear contacts have been formed and an energy conversion efficiency of 21·7% has been obtained on p‐type FZ substrates (0·5 Ω cm). Simulations show that the effective rear SRV is in the range of 180 cm/s for the combination of metallised and passivated areas, 120 ± 30 cm/s were calculated for the passivated areas. Rear reflectivity is comparable to thermally grown silicon dioxide (SiO₂). a‐Si rear passivation appears more stable under different bias light intensities compared to thermally grown SiO₂. Copyright © 2008 John Wiley & Sons, Ltd.     

Efficiency (>15%) for thin‐film epitaxial silicon solar cells on 70 cm2 area offspec silicon substrate using porous silicon segmented mirrors

We report on the beneficial use of embedded segmented porous silicon broad‐band optical reflectors for thin‐film epitaxial silicon solar cells. These reflectors are formed by gradual increase of the spatial period between the layer segments, allowing for an enhanced absorption of low energy photons in the epitaxial layer. By combining these reflectors with well‐established solar cell processing by photolithography, a conversion efficiency of 15·2% was reached on 73 cm² area, highly doped offspec multicrystalline silicon substrates. The corresponding photogenerated current densities (J_sc) were well above 31 mA/cm² for an active layer of only 20 µm. Copyright © 2010 John Wiley & Sons, Ltd.