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

Solar modules made from thin-film crystalline-silicon layers of high quality on glass substrates could lower the price of photovoltaic electricity substantially. One possibility is to use polycrystalline-silicon thin films, but the difficulty of this approach is to obtain a sufficiently high material quality and to achieve an effective passivation of the numerous grain boundaries. However, the utilization of a monocrystalline-silicon thin film instead of a polycrystalline-silicon layer should enable simpler device processes, higher process yield and a single-crystal-like efficiency due to the intrinsic higher quality of the material. In this paper, 10-micron thick monocrystalline-silicon layers on transparent glass-ceramic substrates were made by epitaxial thickening of thin monocrystalline-silicon seed layers. By using a very simple cell structure that does not incorporate any light trapping features, silicon single-crystal solar cells on glass-ceramics presented efficiencies of up to 7.5%. However, efficiencies above 9% should be realizable by simply improving the contacting scheme. In addition, by integrating an efficient light trapping scheme and by optimizing the minority carrier diffusion length, the efficiency potential could be as high as 15% for films with a thickness between five to ten microns on glass-ceramics.

Thin-film monocrystalline-silicon solar cells on transparent high-temperature glass-ceramic substrates

Publisher Summary
This chapter discusses industrial technologies for manufacturing crystalline silicon solar cells at low costs. Typical efficiency of commercially produced crystalline silicon solar cells lies in the range of 14%–17%. Because the efficiency of the cell influences the production cost at any production stage, considerable efforts are directed toward efficiency improvement of solar cells. The required near future efficiency goals for industrial cells are 18–20% on monocrystalline, and 16–18% on multicrystalline silicon. Based on laboratory scale achievements, one can consider that production type cells able to fulfill the efficiency goal should have features including front surface texturing, optimized emitter surface concentration and doping profile, effective front surface passivation, fine line front electrode, and front electrode passivation. This chapter discusses the concepts of cell processing, substrates, etching, texturing, optical confinement, and junction formation in detail. It then explains front surface passivation and functions of antireflection coating. Techniques of gettering by phosphorous diffusion and gettering by aluminum treatment are also discussed. Concepts related to screen-printed solar cells, buried contact solar cells, solar cells on silicon ribbons, and back-contacted solar cells are explained in detail in the chapter.

IIb-2 – Low Cost Industrial Technologies of Crystalline Silicon Solar Cells

Thin film silicon solar cells, consisting of an epitaxially grown active layer on a low quality highly doped silicon substrate, incorporate many attractive features usually associated with their sister cells based on bulk silicon. However, the efficiency of the current epitaxial semi-industrial screen printed cells is limited to 11-12% mainly due to optical shortcomings. This paper will give an overview of our work aimed at tackling the 2 most important problems: (i) Finding and implementing an adequate front surface texture and (ii) the simulation, fabrication and incorporation of an intermediate reflector. The former issue has been addressed by the development of plasma texturing based on halogen species. This method allows us to fulfil the sometimes contradictory requirements for the textured surface, i.e. a uniform and reduced reflection, a strong lambertian character to scatter the light and a limited removal of silicon. It will be shown that the scattering efficiency is dependent on both the wavelength of the impinging light and on the silicon removal during the texturing process. The second and main issue of this work is the limited absorption volume of the epitaxial layer. To resolve this drawback, an intermediate reflector is placed at the epi/substrate interface to enhance the path length of the low energy photons through the epi-layer. In practice, a multi-layer porous silicon stack is created by electrochemical anodization of the substrate. The reflection at the epi/reflector/substrate interface is a combination of several different effects including a Bragg mirror and Total Internal Reflection (TIR). Measurements of the external reflectance as well as extraction of the internal reflection parameters are used to clarify the issue. Advanced structures, including chirped porous silicon stacks, are introduced. Finally, the benefits of the reflector on the level of the epitaxial silicon solar cell are analysed. Efficiencies close to 14% are obtained for epitaxial cells incorporating an advanced porous Si reflector.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400033261

Plasma Texturing and Porous Silicon Mirrors for Epitaxial Thin Film Crystalline Silicon Solar Cells

We have reported before on the use of (Al2O3)3(TiO2)1-x pseudo-binary alloys, deposited by spin coating and thermally treated, for field-induced surface passivation of p-type silicon. These layers show fixed negative charge densities in the order of 1012 cm -2 and yield low surface recombination velocities, but the passivating properties are degrading with time. It was found now that a treatment at high temperature (720degC-1000degC) in oxygen substantially reduces this degradation. The influence of temperature, oxygen flow and duration of the treatment on the effective lifetime is studied, as are effects occuring at the PBA/Si interface. A first optimization of this process leads to surface recombination velocities below 150 cm/s on low-resistivity FZ material and around 300 cm/s on CZ months after deposition. Experiments on the exact nature of the degradation mechanism are ongoing

Progress on the Pseudo-Binary (Al2O3)3(TiO2)2-System for Surface Passivation of P-Type Silicon

The price of photovoltaic electricity could be lowered substantially if efficient solar cells could be made from polycrystalline-silicon (pc-Si) thin films on inexpensive substrates. A promising way to make pc-Si layers for solar cells is to use aluminum-induced crystallization (AIC) of amorphous silicon in combination with thermal CVD. To obtain efficient pc-Si solar cells, the material quality has to be optimized and cell processes different from those applied for standard bulk-Si solar cells have to be developed. In this paper, we present our best pc-Si solar cells so far and we discuss some key features of our solar cell process. We also discuss the structural and electronic quality of our pc-Si layers. We obtained efficiencies of up to 8% on polycrystalline-silicon solar cells made by AIC in combination with thermal CVD on alumina substrates. By replacing conventional diffused emitters by a-Si/c-Si heterojunction emitters, much higher V oc values were obtained. The use of plasma texturing led to higher current densities due to an enhanced coupling of light into the pc-Si layers. The efficiency of our pc-Si solar cells is limited by the presence of a high intragrain defect density in our layers. A detailed TEM study revealed that most of these defects are already present in the AIC seed layers prior to epitaxial thickening. To improve the material quality of our pc-Si layers, we will therefore need to improve the seed layer quality.

Guy Beaucarne

Papers

Thin-film monocrystalline-silicon solar cells on transparent high-temperature glass-ceramic substrates

IIb-2 – Low Cost Industrial Technologies of Crystalline Silicon Solar Cells

Plasma Texturing and Porous Silicon Mirrors for Epitaxial Thin Film Crystalline Silicon Solar Cells

Progress on the Pseudo-Binary (Al2O3)3(TiO2)2-System for Surface Passivation of P-Type Silicon

Processing and characterization of efficient thin-film polycrystalline silicon solar cells