Recent startling interest for lanthanide luminescence is stimulated by the continuously expanding need for luminescent materials meeting the stringent requirements of telecommunication, lighting, electroluminescent devices, (bio-)analytical sensors and bio-imaging set-ups. This critical review describes the latest developments in (i) the sensitization of near-infrared luminescence, (ii) “soft” luminescent materials (liquid crystals, ionic liquids, ionogels), (iii) electroluminescent materials for organic light emitting diodes, with emphasis on white light generation, and (iv) applications in luminescent bio-sensing and bio-imaging based on time-resolved detection and multiphoton excitation (500 references).

/pdf/lanthanide-luminescence-for-functional-materials-and-bio-1w9cppwf8r.pdf

Lanthanide luminescence for functional materials and bio-sciences

Photovoltaic (PV) technologies for solar energy conversion represent promising routes to green and renewable energy generation. Despite relevant PV technologies being available for more than half a century, the production of solar energy remains costly, largely owing to low power conversion efficiencies of solar cells. The main difficulty in improving the efficiency of PV energy conversion lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of a semiconductor material. In recent years, luminescent materials, which are capable of converting a broad spectrum of light into photons of a particular wavelength, have been synthesized and used to minimize the losses in the solar-cell-based energy conversion process. In this review, we will survey recent progress in the development of spectral converters, with a particular emphasis on lanthanide-based upconversion, quantum-cutting and down-shifting materials, for PV applications. In addition, we will also present technical challenges that arise in developing cost-effective high-performance solar cells based on these luminescent materials.

Enhancing solar cell efficiency: the search for luminescent materials as spectral converters

Solar energy represents one of the most abundant and yet least harvested sources of renewable energy. In recent years, tremendous progress has been made in developing photovoltaics that can be potentially mass deployed. Of particular interest to cost-effective solar cells is to use novel device structures and materials processing for enabling acceptable efficiencies. In this regard, here, we report the direct growth of highly regular, single-crystalline nanopillar arrays of optically active semiconductors on aluminium substrates that are then configured as solar-cell modules. As an example, we demonstrate a photovoltaic structure that incorporates three-dimensional, single-crystalline n-CdS nanopillars, embedded in polycrystalline thin films of p-CdTe, to enable high absorption of light and efficient collection of the carriers. Through experiments and modelling, we demonstrate the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates with enhanced carrier collection efficiency arising from the geometric configuration of the nanopillars.

http://nano.eecs.berkeley.edu/publications/NatureMat_2009_SNOPcell.pdf

Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates

Photon upconversion of near-infrared photons is a promising way to overcome the Shockley–Queisser efficiency limit of 32% of a single-junction solar cell. However, the practical applicability of the most efficient known upconversion materials at moderate light intensities is limited by their extremely weak and narrowband near-infrared absorption. Here, we introduce the concept of an upconversion material where an organic near-infrared dye is used as an antenna for the b-NaYF4:Yb,Er nanoparticles in which the upconversion occurs. The overall upconversion by the dye-sensitized nanoparticles is dramatically enhanced (by a factor of ∼3,300) as a result of increased absorptivity and overall broadening of the absorption spectrum of the upconverter. The proposed concept can be extended to cover any part of the solar spectrum by using a set of dye molecules with overlapping absorption spectra acting as an extremely broadband antenna system, connected to suitable upconverters.

/pdf/broadband-dye-sensitized-upconversion-of-near-infrared-light-19zr1jslko.pdf

Broadband dye-sensitized upconversion of near-infrared light

The use of lanthanide ions to convert photons to different, more useful, wavelengths is well-known from a wide range of applications (e.g. fluorescent tubes, lasers, white light LEDs). Recently, a new potential application has emerged: the use of lanthanide ions for spectral conversion in solar cells. The main energy loss in the conversion of solar energy to electricity is related to the so-called spectral mismatch: low energy photons are not absorbed by a solar cell while high energy photons are not used efficiently. To reduce the spectral mismatch losses both upconversion and downconversion are viable options. In the case of upconversion two low energy infrared photons that cannot be absorbed by the solar cell, are added up to give one high energy photon that can be absorbed. In the case of downconversion one high energy photon is split into two lower energy photons that can both be absorbed by the solar cell. The rich and unique energy level structure arising from the 4fn inner shell configuration of the trivalent lanthanide ions gives a variety of options for efficient up- and downconversion. In this perspective an overview will be given of recent work on photon management for solar cells. Three topics can be distinguished: (1) modelling of the potential impact of spectral conversion on the efficiency of solar cells; (2) research on up- and downconversion materials based on lanthanides; and (3) proof-of-principle experiments. Finally, an outlook will be given, including issues that need to be resolved before wide scale application of up- and downconversion materials can be anticipated.

Lanthanide ions as spectral converters for solar cells

Aluminum-induced crystallization of amorphous silicon: influence of materials characteristics on the reaction

Driven by further decreasing the price per watt peak to reach grid parity, the crystalline silicon solar cells roadmap is moving towards thinner wafers while further increasing the efficiency. While 200 μm is the standard thickness used in industrial solar cell production today, solar cells on silicon wafers down to 80 μm are being developed. The influence of rear-side surface recombination on the losses of the solar cell is increasing by going to thinner silicon wafer. In order to decrease this rear-side recombination, new solar cell processes with back-side passivation by a dielectric layer and locally Al-alloyed contacts are being developed. One promising approach involves the local opening of the passivation layer by laser ablation, blanket deposition of Al, and a short thermal anneal. In this paper, we studied the local Al-alloyed contacts formed in this way. We showed that Si dissolution is an important issue and we gave a qualitative explanation for the observed contact structures. Applying dielectric passivation and local Al back surface field (Al-BSF), superior results on very thin multicrystalline (mc) and mono Si solar cells have been reported recently, like 16.6% efficiency on 80 μm thick, 78 cm CZ-Si (4.3 Ω⋅cm) [3].

Local Al-Alloyed Contacts for Next Generation Si Solar Cells

The use of hydrogenated silicon nitride is one of the most significant technological evolutions that has taken place in solar cells industry, due to its ability to act simultaneously as antireflective coating as well as a source of hydrogen for surface and bulk passivation. These very same properties make it an ideal candidate for rear surface passivation in structures with local contacts, yet its application has led so far to results below expectation. This work analyses limits and phenomena that prevent the use of standard nitride as rear surface passivation layer in commercial solar cells and presents a convenient process that can be used to overcome these problems and allows the fabrication of industrial, fully screen printed, PERC-type solar cells on ultrathin substrates. By means of this technology the cell's open circuit voltage shows a significant improvement with respect to the conventional aluminum BSF and is retained all the way down to 100 mum thick devices

Rear Surface Passivation for Industrial Solar Cells on Thin Substrates

Development of interdigitated solar cell and module processes for polycrystalline-silicon thin films

A method for the production of a photovoltaic device, for instance a solar cell, is disclosed. In one aspect, the method comprises providing a substrate having a front main surface and a rear surface. The method further comprises depositing a dielectric layer on the rear surface, wherein the dielectric layer has a thickness larger than about 100 nm. The method further comprises depositing a passivation layer comprising hydrogenated SiN on top of the dielectric layer and forming back contacts through the dielectric layer and the passivation layer. In another aspect, corresponding photovoltaic devices, for instance solar cell devices, are also disclosed.

Guy Beaucarne

Papers

Aluminum-induced crystallization of amorphous silicon: influence of materials characteristics on the reaction

Local Al-Alloyed Contacts for Next Generation Si Solar Cells

Rear Surface Passivation for Industrial Solar Cells on Thin Substrates

Development of interdigitated solar cell and module processes for polycrystalline-silicon thin films

Method for producing photovoltaic cells and photovoltaic cells obtained by such method