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).

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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

N-type thin-film polycrystalline-silicon solar cells were fabricated on alumina substrates. The polycrystalline silicon material was made by overdoping of AIC seed layers with phosphorus atoms followed by epitaxial thickening with low-pressure chemical vapor deposition (LPCVD) at 1000°C. The cells have i/p+ amorphous Si heterojunction emitters and show an average V OC of 455 mV, an average J SC of 16.5 mA cm−2 and a record efficiency of 5.0 %. The excess out-diffusion of phosphorus from the seed layers during the epitaxial growth resulted in a non-ideal doping profile, which is believed to be the main limitation of the current density besides the material quality. In addition, the non-optimized heterojunction emitter deposition for n-type polycrystalline silicon and the lack of superior light trapping also limited the present cell performance.

N-type thin-film polycrystalline-silicon solar cells using a seed layer approach

The purpose of this paper is twofold. Firstly, it presents a concise overview of the benefits to be expected from the development of a thin-film crystalline Si solar cell technology on a low-cost substrate as well as the technical and physical challenges encountered. It becomes clear that a large number of options are open in terms of substrate, deposition technology and solar cell technology. Secondly, focus is placed on the approach chosen by the thin-film crystalline Si solar cell team of IMEC and the results and progress of understanding achieved by the group over the last five years. Basically, their technical strategy is based on an evolutionary scheme for the cell design. More specifically, a transition is made from a classical two-side contacted cell design to a one-side contacted cell type which paves the way for a monolithic module design. For the technical realisation of the thin crystalline Si layers, chemical vapour deposition is expected to be the preferred technique, at least in a short and mid-term perspective.

Advanced concepts and options for thin-film crystalline Si solar cells

Cellule solaire avec une couche épaisse de passivation d'oxyde de silicium et de nitrure de silicium et son procédé de fabrication

Long diffusion length is essential to reach high efficiencies (≫15%) on rear-junction solar cells. As a rule of thumb the minority carrier diffusion length should be at least three times as long as the wafer thickness. Thanks to the smaller capture cross-section of impurities measured in n-type material, n-type silicon can exhibit a diffusion length exceeding 600 μm even in its multi-crystalline form. High-efficiency rear-junction cells become feasible at reasonable cost with wafers as thick as 200 μm. The development of the p+ emitter on the rear is essential for this type of cell, and Al alloying is one of the techniques of choice to realize it. After optimizing the emitter, we obtained very good values for Voc (∼615 mV) and FF (≫80%). Afterwards, focus was laid on improving the short-circuit current. This was achieved by an optimization of the phosphorus-diffused front-surface field, and by thinning down the wafers. These two actions had the consequence of increasing independently of each other the Jsc by 2.3 mA.cm−2 for the optimization of the front-surface field, and by 3 mA.cm−2 when wafer thickness decreased from 300 μm to 150 μm. The characteristics obtained are observed to be extremely dependent on the wafer quality, which varies a lot along the ingot. An efficiency of 15.0% (30.7 mA.cm−2) was reached on rear-junction n-type 150-μm-thick 5×5-cm2 2.5-ohms.cm multi-crystalline Si.

15% multicrystalline n-type silicon screen-printed solar cells with Al-alloy emitter

Thin - film solar cell technologies based on Si with a thickness of less than a few micrometers combine the low - cost potential of thin - film technologies with the advantages of Si as an abundantly available element in the earth’s crust and a readily manufac turable ma te rial for photovoltaics (PVs). In recent years, several technologies have been developed that promise to take the per form ance of thin - film silicon PVs well beyond that of the currently established amorphous Si PV technology. Thin - film silicon, like no other thin - film ma te rial, is very effective in tandem and triple - junction solar cells. The research and development on thin crystalline silicon on foreign substrates can be divided into two different routes: a low - temperature route compatible with standard float glass or even plastic substrates, and a high - temperature route (600C). This ar ticle reviews the ma te rial properties and technological challenges of the different thin - film silicon PV ma te rials.

Guy Beaucarne

Papers

N-type thin-film polycrystalline-silicon solar cells using a seed layer approach

Advanced concepts and options for thin-film crystalline Si solar cells

Cellule solaire avec une couche épaisse de passivation d'oxyde de silicium et de nitrure de silicium et son procédé de fabrication

15% multicrystalline n-type silicon screen-printed solar cells with Al-alloy emitter

Amorphous Silicon, Microcrystallineand Thin - Film PolycrystallineSolar Cells