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

A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from thin polycrystalline-silicon (pc-Si) layers. Aluminum-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening is an effective way to obtain large-grained pc-Si layers with excellent properties for solar cells. To obtain efficient solar cells, the electronic quality of the pc-Si material obtained by AIC has to be optimized and the cell design has to be adapted to the material. In this paper, we report on pc-Si solar cells made by AIC in combination with thermal CVD on ceramic alumina substrates. We made pc-Si solar cells on alumina substrates that showed Voc values up to 533 mV and efficiencies up to 5.9%. This is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of silicon was used. We demonstrate that the quality of the pc-Si material can be improved drastically by reducing the substrate roughness using spin-on oxides. We further show that a-Si/c-Si heterojunctions lead to much higher Voc values than diffused homojunctions. A cell concept that incorporates spin-on oxides and heterojunction emitters is therefore best suited to obtain efficient pc-Si solar cells on alumina substrates.

Thin-Film Polycrystalline-Silicon Solar Cells on Ceramic Substrates Made by Aluminum-Induced Crystallization and Thermal CVD

In this work we characterized fine-grained polysilicon layers with a grain size of only 0.2 µm before and after passivation. Plasma hydrogenation led to a higher hydrogen concentration in the first micron of the layer than nitride passivation. The highest efficiency of 5.0 % was reached when nitride passivation was followed by plasma passivation.

Hydrogen Passivation of Thin-film Polysilicon Solar Cells

Efficient thin-film polycrystalline-silicon (pc-Si) solar cells on inexpensive substrates could substantially lower the price of photovoltaic electricity. We recently showed that good solar cells can be made from pc-Si obtained by epitaxial thickening using thermal CVD of a seed layer made by aluminium-induced crystallization (AIC) of amorphous silicon. We already reported cells in substrate configuration with energy conversion efficiencies up to 8.0% for layers on ceramic alumina substrates. However, much higher efficiencies (η > 10%) are needed for this type of pc-Si solar cells to become cost-effective. To achieve these higher efficiencies, cells will probably have to be made in a superstrate configuration on transparent substrates and advanced light trapping will need to be applied. In this paper we report on our recent progress with pc-Si solar cells made on transparent glass-ceramic substrates. So far, our best pc-Si solar cells in substrate configuration on glass-ceramics showed an efficiency of 6.4%. By using plasma texturing to lower the front side reflection, we increased the current density of our cells by roughly 1 mA cm-2. The Jsc is much lower for cells on glass-ceramic than for cells on alumina. This is the result of the better diffuse back reflectance of alumina compared to glass. The Voc and fill factor are comparable for cells on both substrates. To make pc-Si solar cells on glass in superstrate configuration, we will use a-Si/c-Si rear junction emitters. As a first test of the feasibility of this approach, we measured the illuminated IV parameters of pc-Si cells made for the substrate configuration in superstrate configuration. In superstrate configuration, the current density of the cells is much lower than in substrate configuration due to the non-optimized cell design for superstrate illumination. The Voc is slightly smaller in superstrate configuration due to the lower current density. These results indicate that the glass-ceramic substrates are fully compatible to our poly-Si solar cell process. Furthermore, rear-junction poly-Si cells in superstrate configuration should lead to good cell results once the absorber layer thickness is optimized to the diffusion length of the material and light trapping features adapted to the superstrate configuration are applied.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400033297

Processing and Characterization of Efficient Thin-Film Polycrystalline-Silicon Solar Cells

The European project METEOR aims at the development of large-grained poly-Si thin-film solar cells on foreign substrates. A two step process has been used to form the poly-Si films: (1) a thin large-grained poly-Si film (seed layer) is prepared by the aluminium-induced layer exchange (ALILE) process and (2) this seed layer is subsequently used as a template for an epitaxial thickening process. Two different concepts have been investigated: (i) a low-temperature approach using glass substrates (T 1000°C). The surface roughness of the ceramic substrates has a negative impact on the ALILE process. The surface roughness can be reduced by the deposition of an additional oxide layer. At high temperatures thermal CVD has successfully been used for epitaxy. At low temperatures about 73% of the area under investigation have been epitaxially thickened by a 400nm ECRCVD grown film because of the preferential (100) orientation of the seed layer. First poly-Si thin-film solar cells have been prepared at low and high temperatures. The best solar cell so far has reached an open circuit voltage of 428mV and an efficiency of 4.2%.

Guy Beaucarne

Papers

Thin-Film Polycrystalline-Silicon Solar Cells on Ceramic Substrates Made by Aluminum-Induced Crystallization and Thermal CVD

Hydrogen Passivation of Thin-film Polysilicon Solar Cells

Processing and Characterization of Efficient Thin-Film Polycrystalline-Silicon Solar Cells

Crystalline silicon thin-film solar cells on foreign substrates: the european project meteor

Thin-Film Polycrystalline-Silicon Solar Cells Based on a Seed Layer Approach