With a global market share of about 90%, crystalline silicon is by far the most important photovoltaic technology today. This article reviews the dynamic field of crystalline silicon photovoltaics from a device-engineering perspective. First, it discusses key factors responsible for the success of the classic dopant-diffused silicon homojunction solar cell. Next it analyzes two archetypal high-efficiency device architectures – the interdigitated back-contact silicon cell and the silicon heterojunction cell – both of which have demonstrated power conversion efficiencies greater than 25%. Last, it gives an up-to-date summary of promising recent pathways for further efficiency improvements and cost reduction employing novel carrier-selective passivating contact schemes, as well as tandem multi-junction architectures, in particular those that combine silicon absorbers with organic–inorganic perovskite materials.

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High-efficiency crystalline silicon solar cells: status and perspectives

Atomic-layer-deposited aluminium oxide (Al2O3) is applied as rear-surface-passivating dielectric layer to passivated emitter and rear cell (PERC)-type crystalline silicon (c-Si) solar cells. The excellent passivation of low-resistivity p-type silicon by the negative-charge-dielectric Al2O3 is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al2O3 film covered by a 200 nm plasma-enhanced-chemical-vapour-deposited silicon oxide (SiOx) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single-layer of Al2O3, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Ever since the first publications by R.J. Schwartz in 1975, research into back-contact cells as an alternative to cells with a front and rear contact has remained a research topic. In the last decade, interest in back-contact cells has been growing and a gradual introduction to industrial applications is emerging. The goal of this review is to present a comprehensive summary of results obtained throughout the years. Back-contact cells are divided into three main classes: back-junction (BJ), emitter wrap-through (EWT) and metallisation wrap-through (MWT), each introduced as logical descendents from conventional solar cells. This deviation from the chronology of the developments is maintained during the discussion of technological results. In addition to progress on manufacturing these cells, aspects of cell modelling and module manufacturing are discussed and an outlook towards industrial implementation is presented. Copyright © 2005 John Wiley & Sons, Ltd.

Back-contact solar cells : A review

A wafer-based monocrystalline silicon photovoltaics road map: Utilizing known technology improvement opportunities for further reductions in manufacturing costs

Silicon heterojunction solar cells have record-high open-circuit voltages but suffer from reduced short-circuit currents due in large part to parasitic absorption in the amorphous silicon, transparent conductive oxide (TCO), and metal layers. We previously identified and quantified visible and ultraviolet parasitic absorption in heterojunctions; here, we extend the analysis to infrared light in heterojunction solar cells with efficiencies exceeding 20%. An extensive experimental investigation of the TCO layers indicates that the rear layer serves as a crucial electrical contact between amorphous silicon and the metal reflector. If very transparent and at least 150 nm thick, the rear TCO layer also maximizes infrared response. An optical model that combines a ray-tracing algorithm and a thin-film simulator reveals why: parallel-polarized light arriving at the rear surface at oblique incidence excites surface plasmons in the metal reflector, and this parasitic absorption in the metal can exceed the absorption in the TCO layer itself. Thick TCO layers—or dielectric layers, in rear-passivated diffused-junction silicon solar cells—reduce the penetration of the evanescent waves to the metal, thereby increasing internal reflectance at the rear surface. With an optimized rear TCO layer, the front TCO dominates the infrared losses in heterojunction solar cells. As its thickness and carrier density are constrained by anti-reflection and lateral conduction requirements, the front TCO can be improved only by increasing its electron mobility. Cell results attest to the power of TCO optimization: With a high-mobility front TCO and a 150-nm-thick, highly transparent rear ITO layer, we recently reported a 4-cm2 silicon heterojunction solar cell with an active-area short-circuit current density of nearly 39 mA/cm2 and a certified efficiency of over 22%.

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Infrared light management in high-efficiency silicon heterojunction and rear-passivated solar cells

New passivation layers for the back side of silicon solar cells have to show high performance in terms of electrical passivation as well as high internal reflectivity. This optical performance is often shown as values for the back side reflectance Rb which describes the rear internal reflection. In this paper, we investigate in detail the meaning of this single-value parameter, its correct determination and the use in one-dimensional simulations with PC1D. The free-carrier-absorption (FCA) as non-carrier-generating absorption channel is analyzed for solar cells with varying thickness. We apply the optical analysis to samples with different thickness, silicon oxide layer thickness, rear side topography as well as passivation layers (SiO2, SiNx, SiC and stack systems). Additionally, the optical influence of the laser-fired contacts (LFC) process is experimentally investigated. Finally, we show that with correct parameters, the one-dimensional simulation of very thin silicon solar cells can successfully be performed. Copyright © 2007 John Wiley & Sons, Ltd.

Theory and experiments on the back side reflectance of silicon wafer solar cells

Laser Chemical Processing (LCP) is presented as a novel microstructuring method for multiple applications. Via the combination of a chemical liquid jet and a laser beam, thermochemical and photochemical reactions can be initiated. Due to the free choice of the chemistry for the carrier liquid and the laser source, efficient processes can be devised for a large variety of applications. We present some examples for the microstructuring of silicon with the focus on etching, selective doping of phosphorous and the combination of etching and doping.

Laser Chemical Processing (LCP)—A versatile tool for microstructuring applications

The introduction of selective emitters underneath the front contacts of solar cells can considerably increase the cell efficiency. Thus, cost-effective fabrication methods for this process step would help to reduce the cost per W p of silicon solar cells. Laser Chemical Processing (LCP) is based on the waterjet-guided laser (LaserMicroJet®) developed and commercialized by Synova S.A., but uses a chemical jet. This technology is able to perform local diffusions at high speed and accuracy without the need of masking or any high-temperature step of the entire wafer. We present experimental investigations on simple device structures to choose optimal laser parameters for selective emitter formation. These parameters are used to fabricate high-efficiency oxide-passivated LFC solar cells that exceed 20% efficiency.

Laser-doped silicon solar cells by Laser Chemical Processing (LCP) exceeding 20% efficiency

Study on the edge isolation of industrial silicon solar cells with waterjet-guided laser

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.

Daniel Kray

Papers

Theory and experiments on the back side reflectance of silicon wafer solar cells

Laser Chemical Processing (LCP)—A versatile tool for microstructuring applications

Laser-doped silicon solar cells by Laser Chemical Processing (LCP) exceeding 20% efficiency

Study on the edge isolation of industrial silicon solar cells with waterjet-guided laser

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