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

The reduction in electronic recombination losses by the passivation of silicon surfaces is a critical enabler for high-efficiency solar cells. In 2006, aluminum oxide (Al2O3) nanolayers synthesized by atomic layer deposition (ALD) emerged as a novel solution for the passivation of p- and n-type crystalline Si (c-Si) surfaces. Today, high efficiencies have been realized by the implementation of ultrathin Al2O3 films in laboratory-type and industrial solar cells. This article reviews and summarizes recent work concerning Al2O3 thin films in the context of Si photovoltaics. Topics range from fundamental aspects related to material, interface, and passivation properties to synthesis methods and the implementation of the films in solar cells. Al2O3 uniquely features a combination of field-effect passivation by negative fixed charges, a low interface defect density, an adequate stability during processing, and the ability to use ultrathin films down to a few nanometers in thickness. Although various methods can be used to synthesize Al2O3, this review focuses on ALD—a new technology in the field of c-Si photovoltaics. The authors discuss how the unique features of ALD can be exploited for interface engineering and tailoring the properties of nanolayer surface passivation schemes while also addressing its compatibility with high-throughput manufacturing. The recent progress achieved in the field of surface passivation allows for higher efficiencies of industrial solar cells, which is critical for realizing lower-cost solar electricity in the near future.

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Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells

Black silicon (BSi) represents a very active research area in renewable energy materials. The rise of BSi as a focus of study for its fundamental properties and potentially lucrative practical applications is shown by several recent results ranging from solar cells and light-emitting devices to antibacterial coatings and gas-sensors. In this paper, the common BSi fabrication techniques are first reviewed, including electrochemical HF etching, stain etching, metal-assisted chemical etching, reactive ion etching, laser irradiation and the molten salt Fray-Farthing-Chen-Cambridge (FFC-Cambridge) process. The utilization of BSi as an anti-reflection coating in solar cells is then critically examined and appraised, based upon strategies towards higher efficiency renewable solar energy modules. Methods of incorporating BSi in advanced solar cell architectures and the production of ultra-thin and flexible BSi wafers are also surveyed. Particular attention is given to routes leading to passivated BSi surfaces, which are essential for improving the electrical properties of any devices incorporating BSi, with a special focus on atomic layer deposition of Al2O3. Finally, three potential research directions worth exploring for practical solar cell applications are highlighted, namely, encapsulation effects, the development of micro-nano dual-scale BSi, and the incorporation of BSi into thin solar cells. It is intended that this paper will serve as a useful introduction to this novel material and its properties, and provide a general overview of recent progress in research currently being undertaken for renewable energy applications.

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Black silicon: fabrication methods, properties and solar energy applications

From today's viewpoint future solar cells will be thinner, of higher efficiency and produced in greater numbers. A solar cell concept able to fit these developments could be the passivated emitter and rear cell. With a new laser-based process (laser-fired contacts), the rear point contact pattern of this concept can be implemented industrially. After the deposition of a dielectric passivation layer and a metal layer on top, a Nd-YAG laser is used to alloy the contact points through the dielectric layer. Excellent efficiencies have been achieved which approach closely those of reference cells processed by photolithography. This demonstrates the high potential of the new laser-based technique. Copyright © 2002 John Wiley & Sons, Ltd.

Laser‐fired rear contacts for crystalline silicon solar cells

The Passivated Emitter and Rear Cell (PERC): From conception to mass production

New concepts in silicon solar cell design require dry processing technologies. For this reason two reactive ion etching (RIE) processes have been developed: one for surface cleaning and one for the removal of phosphorous glass (PSG). However, damage is induced in silicon during reactive ion etching which deteriorates solar cell performance. Damage caused by SF6 RIE cleaning has been investigated by means of secondary ion mass spectroscopy, positron annihilation, and minority charge carrier lifetime measurements. Particles contained in the etch gas can be detected up to a depth of 50–80 nm in the silicon sample. A two layer model of vacancy distribution has been established: A layer of high vacancy concentration (1019 cm−3) up to a depth of 20 nm is followed by a second layer that extends over a depth of 1 μm with a vacancy concentration of 1016 cm−3. Effective minority charge carrier lifetimes decrease to about 10% of the lifetime of the wet etched control during RIE. If a heavily damaged layer of 20 nm i...

Low damage reactive ion etching for photovoltaic 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

The use of hydrogen for passivation of multicrystalline silicon in solar cell technology is described. Three kinds of hydrogen incorporation into mc-Si solar cells have been evaluated: hydrogen diffusion out of a SiN-layer (SiN:H), low-energy hydrogen ion implantation (HII), and remote plasma hydrogen passivation (RPHP). Best results were obtained by RPHP, whereas using HII, damage exceeded the passivation effect to some extent. While SiN:H passivates more the grains, RPHP acts particular on grain boundaries. Combining hydrogen passivation by SiN:H and RPHP leads to optimal bulk passivation. We have investigated the influence of RPHP to various mc-Si materials by solar cell and diffusion length measurements. CVD layers and ribbon material show the strongest increase in performance. A boost of up to 77.0 mV in the open-circuit voltage and 2.0% in the efficiency of solar cells has been achieved. Electromagnetically casted Si has shown an improvement of 2.1% in efficiency after RPHP treatment. But even mc-Si of higher quality, like mc-Baysix and Eurosolare mc-Si could be rectified by RPHP. Applying a standard cell process for material assessment efficiencies of 16.9% have been obtained including a RPHP step. Thus the potential for high-efficiency mc-Si solar cells is shown.

Hydrogen passivation of multicrystalline silicon solar cells

New processing schemes for fabricating the rear contact pattern of the PERC-structure (passivated emitter and rear cell) are demonstrated. Both, thermally-grown silicon oxide (SiO/sub 2/) and plasma-deposited silicon nitride (SiN/sub x/) are used as the passivating rear layer. The first processing scheme utilizes plasma etching of the dielectric layer through a mask. The plasma process was optimized in order to reduce the damage in the silicon base of the cell. Efficiencies of 21.5% and 21.7% have been achieved for SiN/sub x/ and SiO/sub 2/ rear layers, respectively. The second approach uses a laser beam to remove the dielectric layer for the rear contact pattern. Efficiencies of 19.7% and 21.3% have been achieved for SiN/sub x/ and SiO/sub 2/ rear layers, respectively. Reference cells with the same front structure but conventionally processed rear (photo resist, wet-chemical etching) show only a slightly higher efficiency of 22.0% on cells with a SiO/sub 2/ passivation layer. This proves that both approaches have a very high potential.

Ralf Lüdemann

Papers

Laser‐fired rear contacts for crystalline silicon solar cells

Low damage reactive ion etching for photovoltaic applications

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

Hydrogen passivation of multicrystalline silicon solar cells

New simplified methods for patterning the rear contact of RP-PERC high-efficiency solar cells