There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Probability Distributions.- Linear Models for Regression.- Linear Models for Classification.- Neural Networks.- Kernel Methods.- Sparse Kernel Machines.- Graphical Models.- Mixture Models and EM.- Approximate Inference.- Sampling Methods.- Continuous Latent Variables.- Sequential Data.- Combining Models.

Pattern Recognition and Machine Learning

This paper reviews the history, the present status and possible future developments of photovoltaic (PV) materials for terrestrial applications. After a brief history and introduction of the photovoltaic effect theoretical requirements for the optimal performance of materials for pn-junction solar cells are discussed. Most important are efficiency, long-term stability and, not to be neglected, lowest possible cost. Today the market is dominated by crystalline silicon in its multicrystalline and monocrystalline form. The physical and technical limitations of this material are discussed. Although crystalline silicon is not the optimal material from a solid state physics point of view it dominates the market and will continue to do this for the next 5–10 years. Because of its importance a considerable part of this review deals with materials aspects of crystalline silicon. For reasons of cost only multicrystalline silicon and monocrystalline Czochralski (Cz) crystals are used in practical cells. Light induced instability in this Cz-material has recently been investigated and ways to eliminate this effect have been devised. For future large scale production of crystalline silicon solar cells development of a special solar grade silicon appears necessary. Ribbon growth is a possibility to avoid the costly sawing process. A very vivid R&D area is thin-film crystalline silicon (about 5–30 μm active layer thickness) which would avoid the crystal growing and sawing processes. The problems arising for this material are: assuring adequate light absorption, assuring good crystal quality and purity of the films, and finding a substrate that fulfills all requirements. Three approaches have emerged: high-temperature, low-temperature and transfer technique. Genuine thin-film materials are characterized by a direct band structure which gives them very high light absorption. Therefore, these materials have a thickness of only one micron or less. The oldest such material is amorphous silicon which is the second most important material today. It is mainly used in consumer products but is on the verge to also penetrate the power market. Other strong contenders are chalcogenides like copper indium diselenide (CIS) and cadmium telluride. The interest has expanded from CuInSe 2 , to CuGaSe 2 , CuInS 2 and their multinary alloys Cu(In,Ga)(S,Se) 2 . The two deposition techniques are either separate deposition of the components followed by annealing on one hand or coevaporation. Laboratory efficiencies for small area devices are approaching 19% and large area modules have reached 12%. Pilot production of CIS-modules has started in the US and Germany. Cadmium telluride solar cells also offer great promise. They have only slightly lower efficiency and are also at the start of production. In the future other materials and concepts can be expected to come into play. Some of these are: dye sensitized cells, organic solar cells and various concentrating systems including III/V-tandem cells. Theoretical materials that have not yet been realized are Auger generation material and intermediate metallic band material.

Photovoltaic materials, history, status and outlook

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.

/pdf/status-and-prospects-of-al2o3-based-surface-passivation-kquhly1a8k.pdf

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

The minority carrier lifetime in boron-doped oxygen-contaminated Czochralski (Cz) silicon is strongly reduced under illumination or carrier injection. This process can be fully reversed by a 200 °C anneal step. In several recent studies it was shown that boron and oxygen are the major components of the underlying metastable Cz-specific defect. The energy level of the defect in its active state A was determined to be around midgap [Schmidt et al., J. Appl. Phys. 86, 3175 (1999)] while the energy level of the defect in its passive state P is very shallow. The Cz-specific defect in its passive state can be identified with the shallow thermal donor. The kinetics of the excess carrier-induced transformation from state P to state A can be described using recombination-enhanced defect reaction theory. On the basis of these experimental facts different solutions for the reduction or elimination of the metastable defect are suggested. Two promising solutions are discussed in more detail: the use of gallium-doped C...

Minority carrier lifetime degradation in boron-doped Czochralski silicon

The field-effect passivation of the interface of thermal oxides on silicon is experimentally investigated by depositing corona charges on the oxide of solar cells and of lifetime test structures. The open circuit voltage of solar cells with interdigitated rear contacts can be increased by +12 mV or decreased by −34 mV, respectively, by depositing positive or negative corona charges on top of the front oxide. The resulting effective surface recombination velocity, Seff, is determined on carrier lifetime test structures for different injection levels and charge densities using microwave-detected photoconductance decay and a new expression for the Auger-limited bulk lifetime. Seff can be varied between 24 cm/s and 538 cm/s on a 1 Ω cm p-type wafer with a thermal oxide of 105 nm thickness. The measurements are compared with theoretical predictions of an analytical model for the calculation of the surface recombination. Measured values for the capture cross sections and interface trap densities are used for th...

Field-effect passivation of the SiO2Si interface

Degradation of carrier lifetime in Cz silicon solar cells

By combining data from temperature- and injection-dependent lifetime spectroscopy (TDLS and IDLS) measured by means of the microwave-detected photoconductance decay technique and the quasi-steady state photoconductance technique, respectively, the exact electronic structure of the metastable defect in standard boron-doped Czochralski (Cz) silicon has been determined. A detailed Shockley-Read-Hall analysis of the entire TDLS curve reveals that the Cz-specific defect acts as an attractive Coulomb center (/spl sigma//sub n/(T)=/spl sigma//sub n0/T/sup -2/) which is localized in the upper band gap half at E/sub C/-E/sub t/=0.41 eV and has an electron/hole capture cross section ratio k:=/spl sigma//sub n///spl sigma//sub p/ of 9.3. A new routine for data evaluation allows a transparent SRH analysis of IDLS and TDLS data and enables the accuracy and consistency of the determined defect parameters to be assessed. For the metastable defect in boron-doped Cz-Si perfect agreement between IDLS and TDLS has been found, which demonstrates the excellent performance of lifetime spectroscopy.

Advanced lifetime spectroscopy: unambiguous determination of the electronic properties of the metastable defect in boron-doped CZ-Si

Light-induced degradation (LID) has been identified to be a critical issue for solar cells processed on boron-doped silicon substrates. Typically, Czochralski-grown silicon (Cz-Si) has been reported to suffer from stronger LID than block-cast multicrystalline silicon (mc-Si) due to higher oxygen concentrations. This work investigates LID under conditions practically relevant under module operation on different cell types. It is shown that aluminium oxide (AlOx) passivated mc-Si solar cells degrade more than a reference aluminium back surface field mc-Si cell and, remarkably, an AlOx passivated Cz-Si solar cell. The defect which is activated by illumination is shown to be doubtful a sole bulk effect while the AlOx passivation might play a certain role. This work may contribute to a re-evaluation of the suitability of boron-doped Cz- and mc-Si for solar cells with very high efficiencies. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Stefan Rein

Papers

Minority carrier lifetime degradation in boron-doped Czochralski silicon

Field-effect passivation of the SiO2Si interface

Degradation of carrier lifetime in Cz silicon solar cells

Advanced lifetime spectroscopy: unambiguous determination of the electronic properties of the metastable defect in boron-doped CZ-Si

Light‐induced degradation of PECVD aluminium oxide passivated silicon solar cells