This PDF file contains the front matter associated with SPIE Proceedings Volume 9178, including the Title Page, Copyright information, Table of Contents, Authors, and Conference Committee listing.

The hot carrier cell aims to extract the electrical energy from photo-generated carriers before they thermalize to the band edges. Hence it can potentially achieve a high current and a high voltage and hence very high efficiencies up to 65% under 1 sun and 86% under maximum concentration. To slow the rate of carrier thermalisation is very challenging, but modification of the phonon energies and the use of nanostructures are both promising ways to achieve some of the required slowing of carrier cooling. A number of materials and structures are being investigated with these properties and test structures are being fabricated. Initial measurements indicate slowed carrier cooling in III-Vs with large phonon band gaps and in multiple quantum wells. It is expected that soon proof of concept of hot carrier devices will pave the way for their development to fully functioning high efficiency solar cells.

High efficiency photovoltaic devices combine full solar spectrum absorption and effective generation and collection of charge carriers, while commercial success depends on cost effectiveness in manufacturing. Spectrum modification using down shifting has been demonstrated in luminescent solar concentrators (LSCs) since the 1970s, as a cheap alternative for standard c-Si technology. LSCs consist of a highly transparent plastic plate, in which luminescent species are dispersed, which absorb incident light and emit light at a red-shifted wavelength, with high quantum efficiency. Material issues have hampered efficiency improvements, in particular re-absorption of light emitted by luminescent species and stability of these species. In this contribution, approaches are reviewed on minimizing re-absorption, which should allow surpassing the 10% luminescent solar concentrator efficiency barrier.

In this contribution we discuss luminescent down-shifting (LDS) systems consisting of a polymer matrix filled with phosphor particles. It is an elegant approach to make a use of potentially destructive or otherwise wasted high energy photons and diminish charge carrier losses caused by thermalization in photovoltaics. Sub-micron and micron sized particles of strontium aluminate doped with Eu²⁺ and strontium carbonate doped with Eu³⁺ ions are chosen for the application due to their suitable absorption in UV spectral region. These particles exhibit strong luminescence in the visible range between 520 and 650 nm. The systems are carefully designed to meet critical optical requirements such as high transparency in the visible spectrum as well as sufficient absorption of UV light. They are coated on quartz glass substrates (20 x 20 x 1 mm) and can be easily laminated to different kinds of solar cells without any modification to well-established device fabrication processes. Optical characterization further confirms that particles of a few microns in size generate strong light scattering in layers due to the sizes slightly larger than visible light wavelengths. Dried thick layers of 20 to 100 μm are tested with CIGS and organic cells. The concept of light conversion is experimentally proven. However, optical losses cause a reduction in the overall performance of the tested devices. Possible ways to bring down the amount of light scattering and, thus, to increase optical transmission for the studied system are also addressed, and are a subject of future research.

Solar rectifying antennas constitute a distinct solar power conversion paradigm where sunlight’s spatial coherence is a basic constraining factor. In this presentation, we derive the fundamental thermodynamic limit for coherence-limited blackbody (principally solar) power conversion. Our results represent a natural extension of the eponymous Landsberg limit, originally derived for converters that are not constrained by the radiation’s coherence, and are irradiated at maximum concentration (i.e., with a view factor of unity to the solar disk). We proceed by first expanding Landsberg’s results to arbitrary solar view factor (i.e., arbitrary concentration and/or angular confinement), and then demonstrate how the results are modified when the converter can only process coherent radiation. The results are independent of the specific power conversion mechanism, and hence are valid for diffraction-limited as well as quantum converters (and not just classical heat engines or in the geometric optics regime). The derived upper bounds bode favorably for the potential of rectifying antennas as potentially high-efficiency solar converters.

Nanoparticles and nanostructures with plasmonic resonances are currently being employed to enhance the efficiency of solar cells. ^1-3 Ag stripe arrays have been shown theoretically to enhance the short-circuit current of thin silicon layers. ⁴ Monolayers of Ag nanoparticles with diameter d < 300 nm have shown strong plasmonic resonances when coated in thin polymer layers with thicknesses < d.⁵ We study experimentally the diffuse vs. specular scattering from monolayer arrays of Ag nanoparticles (spheres and prisms with diameters in the range 50 – 300 nm) coated onto the front side of thin (100 nm < t < 500 nm) silicon films deposited on glass and flexible polymer substrates, the latter originating in a roll-to-roll manufacturing process. Ag nanoparticles are held in place and aggregation is prevented with a polymer overcoat. We observe interesting wavelength shifts between maxima in specular and diffuse scattering that depend on particle size and shape, indicating that the nanoparticles substantially modify the scattering into the thin silicon film.

The use of plasmonic structures to enhance light trapping in solar cells has recently been the focus of significant research, but these structures can be sensitive to various design parameters or require complicated fabrication processes. Nanosphere lithography can produce regular arrays of nanoscale features which could enhance absorption of light into thin films such as those used in novel solar cell designs. Finite-difference-time-domain simulations are used to model a variety of structures producible by this technique and compare them against the use of mirrors as rear reflectors. Through analysis of these simulations, sensitivity of device performance to parameters has been investigated. Variables considered include the feature size and array period, as well as metal and absorber materials selection and thickness. Improvements in idealized photocurrent density are calculated relative to the use of rear mirrors that are a standard for solar cells. The maximum simulated increase to photocurrent density was 3.58mA/cm² or 21.61% for a 2μm thick Si cell relative to the case where a silver mirror is used as a rear reflector. From this, an initial set of design principles for such structures are developed and some avenues for further investigation are identified.

Metallic nanoparticles exhibiting plasmonic effects as well as dielectric nanoparticles coupling the light into resonant modes have both shown successful application to photovoltaics. On the larger scale, microconcentrator optics promise to enhance solar cell efficiency and reduce material consumption. Here we want to make the link between concentrators on the nano- and on the microscale. From metallic nanospheres we turn to dielectric ones and then look at increasing radii to approach concentrator optics on the mircoscale. The nano- and microlenses are investigated with respect to their interaction with light using 3D simulations with the finite element method. Resulting maps of local electric field distributions reveal the focusing behavior of the dielectric spheres. For larger lens sizes, ray tracing calculations can be applied which give ray distributions in agreement with areas of high electric field intensities. Calculations of back focal lengths using ray tracing coincide with results from geometrical optics simulations. They give us insight into how the focal length can be tuned as a function of particle size, but also depending on the substrate refractive index and the shape of the microlens. Turning from spherical to segment-shaped lenses allows us to approach the realistic case of microconcentrator optics and to draw conclusions about focus tuning and system design. Despite the similarities of focusing behavior we find for the nano- and the microlenses, the integration into solar cells needs to be carefully adjusted, depending on the ambition of material saving, concentration level, focal distance and lens size, all being closely related.

We demonstrate the minority carrier lifetime measurements of polycrystalline silicon nanowires (poly-SiNW) films passivated with aluminum oxide (Al₂O₃) deposited by atomic layer deposition (ALD). The poly-SiNW films were prepared by metal-assisted chemical etching of poly-Si films. The poly-Si films were prepared by solid phase crystallization of a-Si films deposited by radio-frequency sputtering on aluminum induced crystallized poly-Si template. The deposition of an ALD-Al₂O₃ passivation layer and subsequent annealing enabled us to measure effective minority carrier lifetime of the poly-SiNW films. The effective lifetime was found to be 5.76 μs. This result indicates that ALDAl₂O₃ is beneficial to surface passivation of poly-SiNW films.

Polycrystalline silicon nanowires (poly-SiNWs) films were successfully prepared by using metal assisted chemical etching of polycrystalline silicon (poly-Si) films. The poly-Si films were prepared by solid-phase crystallization of amorphous silicon (a-Si) deposited by different deposition techniques on different substrates. In the case of the electron beam evaporated a-Si on a quartz substrate, the formation of poly-SiNWs was not observed and the structure was found to be porous silicon. On the other hand, poly-SiNWs successfully formed from poly-Si on a silicon substrate. We also found that deposition techniques for a-Si films affect the formation of poly-SiNWs.

The effect of tapered shape on electrical properties of heterojunction silicon nanowire (SiNW) solar cells was simulated with a two-dimensional quantum device simulator. When the quantum effect was taken in account, opencircuit voltage (V_oc) and fill factor (FF) of heterojunction SiNW solar cells were drastically improved from 390 to 862 mV and from 0.662 to 0.795, respectively. This is due to the bandgap widening and the enhancement of electric field in the intrinsic SiNW. When a top side diameter (d₁) of the SiNW was set at 2 nm and a bottom side diameter (d₂) was varied from 2 to 6 nm, short-circuit current density (J_sc) was drastically increased from 6.96 to 30.8 mA/cm². The main reason is the absorption enhancement due to a tapered shape with a graded refractive index. Ultimately, conversion efficiency was monotonically increased with increasing d₂ in the range from 2 to 6 nm. The quantum size effect and the tapered shape can enhance conversion efficiency of heterojunction SiNW solar cells.

A crystalline Si based tandem solar cell composed of a nano-wall top cell and a conventional p/n junction bottom cell has been studied. To increase the bandgap of top cell by the quantum confinement effect, the width of nano-walls should decreased to 2 nm or narrower. In this work, we studied how to make nano-walls with a width of several nm by adopting top-down processes. As a result, an array of nano-walls with a width of about 2.0 nm was obtained. The fabrication process of nano-walls is described in detail. In addition, we measured the reflectance of the fabricated samples, and found that the reflectance in the range of 400 nm to 1000 nm was lower than 4%.

We fabricated nanostructured Si/SiO₂ superlattice films for solar cell appliactions. The Si/SiO₂ superlattice films were fabricated by thermal annealing of a-Si/SiO₂ superlattice films. TEM observations revealed the existence of nanocrystalline Si in the Si layer. This sample showed photoluminescence spectrum with peak energy at around 1.5 eV. It was also found that the defect density in the superlattice was reduced by using forming gas annealing. Applying nanosphere lithography and reactive ion etching, we successfully prepared nanostructures on the surface of the superlattice. We also compared the optical properties with the simulation results using rigorous coupled wave analysis.

Tm³⁺ and Yb³⁺ co-doped upconverting (UC) glass phosphors were used to converting near-infrared to visible light and input to a CdS/CdTe solar cell, therefore to enhance solar cell’s response in the near-infrared of the sub-bandgap region. Current-voltage measurements were performed on the solar cell with a UC glass phosphor. A short-circuit photocurrent enhancement of 31 μA was obtained using a Tm³⁺and Yb³⁺ co-doped glass UC phosphor, illuminated by a 980 nm diode laser at 100 mW. This photocurrent response corresponds to external quantum efficiency (EQE) of 0.04 % at 980 nm. For full collection of the UC light in 4π solid angle, the EQE value is expected to reach 1.6 %. The photo-current observed was proportional to the effective UC light intensity from glass UC phosphor. A non-linear relation between the output photo-current and the incident power of the infrared light was observed, similar to the relation between UC intensity and the incident power. UC efficiency of the glass phosphor was calculated using EQE values at both UC wavelengths and at 980 nm.

One of the major concerns in the area of high efficient solar cell production is a substantial shift between the solar radiation spectra and optical absorption spectra of a photoelectric transducer that significantly reduces solar cell efficiency. We propose a concept which based on coating of conventional and cheap photoelectric transducer with a luminophor that transmits longer wavelengths of the sunlight, absorbs shorter wavelengths and converts them into longer ones by the value of the Stocks shift. While photoluminescent light is not collimated and thus losses may reach up to 50% of converted light, it was also proposed to make micropattern formation at photoelectric transducer surface. We propose synthesizing of specific materials based on composite pyrazoline dyes with addition of polymethylmethacrylate, polystyrene and UV-laquers. It was revealed that synthesized luminophor coating are characterized by sufficiently enough Stocks shift (200-400 nm), high quantum yield (near 80%) and stability under circumstances of long term radiation. Further research demonstrated potential of the significant characteristic’s improvement by introducing of organic dye molecules in the white zeolite matrix with additional laser annealing at low intensity. Experimental results have shown that photoluminescent spectrum of pyrazoline dye didn’t change shape, bandwidth and amplitude for last 10 years. It was decided that obtained stability is being caused by porous matrix of white zeolite. Simulation of the solar cell functioning helped to understand physics of the process and simplify problem of microrelief and luminophor optimal parameters search.