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

For scavenging energy out of the environment, it is very important for solar cells to maintain the efficiency at low light intensity levels. The high efficiency p-type front junction (18.3% at 1 sun AM1.5) and n-type rear junction (17.3% at 1 sun AM1.5) cells are prepared using an identical cell process based on firing an aluminum paste on the rear. The Al paste acts either as back surface field or rear emitter respectively. A detailed characterization of these cells under low illumination conditions was carried out based on the measured efficiency and suns-V oc . In this paper, it is experimentally and theoretically shown that n-type base cells outperform p-type base cells at such low injection levels. The observed dependence of the performance on the injection level is explained using the Shockley-Read-Hall recombination model where the different capture cross sections for electrons and holes has a decisive influence on the minority carrier bulk lifetime for p-type silicon under low level injection.

Comparison of n- and p-type high efficiency silicon solar cell performance under low illumination conditions

Polysilicon Thin-Film Solar Cells: Influence of the Deposition Rate upon Enhanced Diffusion and on Cell Performance

Properties of p-n Diodes Made in Polysilicon Layers with Intermediate Grain Size

Significant cost reduction of bulk crystalline silicon solar cells requires the removal of the technological barriers that impede the development of a high throughput, low cost, and reliable industrial process on thin substrates. Present industrial surface conditioning and rear surface passivation processes do not meet the requirements for yield and performance on thin substrates. In addition, large-scale production brings about the issue of the environmental impact of PV processing and its related externalities, which may contribute a significant part of the final costs and are so far, underestimated or belittled. In this paper we present an advanced, plasma-based processing technology, suitable for industrial production of bulk silicon solar cells on thin substrates and capable of meeting the PV market growth challenge with a low environmental impact and a competitive cost. The modified processing steps include plasma etching and texturing, dielectric passivation and rear side local contact schemes. Each step is compatible with the standard process sequence, can be integrated separately in the production line and leads to improved performance and/or cost reduction.

Advanced dry processes for crystalline silicon solar cells

The behavior of polycrystalline p-n-junction solar cells with a grain size in the order of 1 /spl mu/m is investigated. The diffusion length appears larger than the average grain size at low doping levels but drastically decreases with increasing doping level, with moderate improvement through hydrogenation. The second diode currents are generally very large, leading to poor open-circuit voltages. We suggest that the zone of enhanced recombination, usually confined to the junction depletion region, extends into the base where very small grains are completely depleted due to carrier trapping at grain boundaries.

Guy Beaucarne

Papers

Comparison of n- and p-type high efficiency silicon solar cell performance under low illumination conditions

Polysilicon Thin-Film Solar Cells: Influence of the Deposition Rate upon Enhanced Diffusion and on Cell Performance

Properties of p-n Diodes Made in Polysilicon Layers with Intermediate Grain Size

Advanced dry processes for crystalline silicon solar cells

On the behaviour of p-n junction solar cells made in fine-grained silicon layers