This PDF file contains the front matter associated with SPIE Proceedings Volume 7411, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Ge nanocrystals (Ge NCs) were grown in a multilayered superlattice using magnetron co-sputtering and subsequent thermal annealing. The purpose is to produce a material in which the band gap can be controlled by controlling the Ge NC size and to investigate the potential of this material for use in tandem solar cells. The presence of size-controlled Ge NCs was revealed by Raman spectroscopy, glancing incidence X-ray diffraction (GIXRD) and Transmission Electron Microscope (TEM), and this was supplemented by the observation of blue shifts in the absorption and photoluminescence (PL) properties. Raman spectra showed Ge-Ge active phonon modes at around 300 cm^-1 implying the formation of high quality Ge NCs. With increasing annealing temperature and duration, more Ge precipitate changed from a non-crystalline phase to a crystalline phase. However, calculation of degree of crystallinity indicated that a considerable amount of non-crystalline Ge remained at our chosen annealing conditions. GIXRD measurements exhibited three Bragg peaks associated with crystalline Ge. TEM images showed direct evidence of the crystal lattice of the Ge NCs. The size of nanocrystals increased with annealing duration indicating nanocrystal growth by diffusion. The growth of nanocrystals was found to be confined by the GeO₂/SiO₂ spacing layers, and the average crystallite size was determined by the thickness of the GeRO layers. However, enhanced interdiffusion at elevated annealing temperature weakened the size confinement effect of the multilayer structure. Hence an optimum annealing condition is needed to produce high quality and reproducible Ge NCs. Our preliminary work indicates that it may be promising to use Ge NCs as absorber materials in tandem solar cells..

Temperature dependent dynamics of phonon-assisted relaxation of hot carriers, both electrons and holes, is studied in a PbSe nanocrystal using ab initio time-domain density functional theory. The electronic structure is first calculated, showing that the hole states are denser than the electron states. Fourier transforms of the time resolved energy levels show that the hot carriers couple to both acoustic and optical phonons. At higher temperature, more phonon modes in the high frequency range participate in the relaxation process due to their increased occupation number. The phonon-assisted hot carrier relaxation time is predicted using non-adiabatic molecular dynamics, and the results clearly show a temperature-activation behavior. The complex temperature dependence is attributed to the combined effects of the phonon occupation number and the thermal expansion. Comparing the simulation results with experiments, we suggest that the multiphonon relaxation channel is efficient at high temperature, while the Auger-like process may dominate the relaxation at low temperature. This combined mechanism can explain the weak temperature dependence at low temperature and stronger temperature dependence at higher temperature.

The performance of light harvesting devices is improved by utilising resonance energy transfer. A hybrid structure of colloidal quantum dots (QDs) and a quantum well (QW) p-i-n heterostructure is investigated. After highly absorbing QDs absorb photons, the excitations are efficiently transferred to a QW p-i-n heterostructure via resonance energy transfer. The generated electron-hole pairs in the heterostructure are subsequently separated by the built-in electric field and collected by the corresponding electrodes. In order to increase the energy transfer rate, the donor-acceptor separation distance is minimised by fabricating channel structures on the heterostructure surface penetrating its active layers. Consequently, a six-fold enhancement of photocurrent conversion efficiency is demonstrated. The proposed hybrid structures offer efficient light harvesting devices where high absorption of the colloidal QDs is utilised and their low charge transfer is overcome.

In this paper we report the growth and characterization of Cu₂ZnSnS₄ (CZTS) nanostructures by co-electrodeposition technique using CuCl₂, SnCl₂ and ZnCl₂ as sources and choline-based ionic liquid (IL) as the electrolyte. X-ray diffraction analysis of CZTS thin films grown by this technique indicated that the films have a kesterite structure with preferred grain orientation along (112). It is found that the energy bandgap of the material is about 1.49eV and the optical absorption coefficient is in the order of 10⁴cm^-1. Anodized aluminum oxide (AAO) was used as the growth mask for the growth of the nanostructures. Anodization of the aluminum foil was carried out in phosphoric acid solution at 1°C and a potential of 40 to 100V was applied. Sulfurization of the rods was performed in elemental sulfur vapor at 450°C for four hours using N₂ as the ambient gas. Experimental results show that nanotubes were formed using the technique and the diameter can be well controlled by varying the applied potential in the anodization process. Electron diffraction experiments show that a mixture of single- and poly-crystalline nanostructures was found.

High-performance dye-sensitized solar cells (DSCs) with nanocomposites as counter electrode are reported, since nanomaterials have high specific surface and could have high catalysis. Nanocomposites of carbon nanotubes (CNTs) and conducting PEDOT:PSS (PEDOT:PSS = poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) were prepared by dispersing CNTs in aqueous solution of PEDOT:PSS. The dispersion is related to the π-π interaction between CNTs and conjugated PEDOT, which is stabilized by the excess PSS in water. Nanocomposite films of CNTs and PEDOT:PSS could be prepared by solution processing. They could have high transparency and high conductivity. Nanocomposite films prepared by drop casting at room temperature were used as the counter electrode of DSCs. The devices exhibited high photovoltaic performance with the energy conversion efficiency of 6.5%, short-circuit current of 15.5 mA cm^-2, open-circuit voltage of 0.66 V, and fill factor of 0.63 under AM1.5 sun light. This performance is close to the devices using conventional platinum as the counter electrode.

Employing DNA molecules provide opportunities for electronics and photonics applications, serving to enhance the device properties as active part of the device or being a linker agent to aid in the self assembly of nanostructures. In this work, the effects of two different sets of biological materials, stand alone DNA sequences and Pt-DNA nanowires on the device properties of bulk heterojunction solar cell devices are being investigated. During the metallization of DNA, a Pt ion activation process over the DNA backbone is followed by a reduction process, where positively charged Pt nanoparticles are assembled on the DNA sequences to form the Pt-DNA complexes via sequential ionic reduction. Pt nanowires 20 nm in diameter are obtained by optimization of the salt reduction parameters of this. Several solar cell devices consisting of Al/P3HT:PCBM/PEDOT:PSS/ITO layers, are fabricated where DNA sequences or the Pt-DNA nanostructures are placed in between the P3HT:PCBM and the PEDOT:PSS layers. Both DNA sequences and Pt-DNA nanostructures are spray coated onto the PEDOT:PSS layer before spin-coating the PEDOT:PSS polymer mixture. The effects of the DNA and Pt-DNA nanostructures are observed from the I-V characteristics under the standard AM1.5G 1 Sun Test Condition. We observe that both DNA sequences and Pt-DNA nanostructures improve the power conversion efficiency (PCE) by %12 and %25 respectively. We believe that this increase in PCE is provided by the enhancement of hole collection and a reduction of the recombination loses. In addition, improvement in the short circuit current (I_sc) is observed for the DNA containing network. Similar improvements in both I_sc and the open circuit voltage (V_oc) are observed for the Pt-DNA containing network. We hypothesize that while the high resistance of the DNA network limits charge collection, comparably low resistance Pt-DNA network improves this feature.

The concept of third generation photovoltaics is to significantly increase device efficiencies whilst still using thin film processes and abundant non-toxic materials. This can be achieved by circumventing the Shockley-Queisser limit for single band gap photovoltaic devices, using multiple energy threshold approaches. Such an approach can be realised either by incorporating multiple energy levels in tandem or intermediate band devices; or by modifying the incident spectrum on a cell by converting either high energy or low energy photons to photons more suited to the cell band gap; or by using an absorber which is heated by the solar photons with power extracted by a secondary structure. These methods have advantages and disadvantages and are at various stages of realisation. The paper discusses and compares these approaches, with some suggested conclusions for the most appropriate approaches.

Transparent erbium-doped fluorozirconate (FZ) glasses are attractive systems for upconversion-based solar cells. Upconverted fluorescence intensity vs. excitation power dependence was investigated for a series of erbium-doped FZ glasses. It was found that the ratio of the 2-photon upconverted emission in the near infrared at 980 nm to the 3-photon upconverted emissions in the visible at 530, 550, and 660 nm decreases with increasing excitation power. The integrated upconverted fluorescence intensity per excitation power shows "saturation" upon increasing the excitation power, while the point of saturation shifts to lower excitation power with increasing erbium doping level. To demonstrate the potential of these upconverters for photovoltaic applications, the external quantum efficiency (EQE) of a commercial monocrystalline silicon solar cell with an Er-doped FZ glass on top of it was measured. For an excitation power of 1 mW at a wavelength of 1540 nm an EQE of 1.6% was found for a 9.1 mol% Er-doped FZ glass. The samples investigated were not optically coupled to the solar cell and no optical coating was applied to the glass surface.

The challenge when applying photonics to photovoltaics is the need to provide broadband, multiple-angle solutions to problems and both plasmonics and biomimetics offer broadband approaches to reducing reflection and enhancing lighttrapping. Over millions of years nature has optimised nanostructures to create black, transparent, white and mirrored surfaces, the antireflective "moth-eye" structures are perhaps the best known of these biophotonic materials. In this paper we use simulated and experimental studies to illustrate how careful optimisation of nanoscale features is required to ensure the optimum match between reflectivity, spectral bandwidth and device quantum efficiencies. In the case of lighttrapping by plasmonic scattering there is more room for design and specific spectral regions can be targeted by precise control of the size, shape and density of particular metal nanoparticles. We describe how the best opportunity for plasmonics within inorganic solar cells appears to be enhanced light-trapping of near-band edge photons.

The Localized Surface Plasmon Resonance (LSPR) phenomenon exhibited in nano-particles, embedded in a dielectric medium has recently been shown to enhance the absorption as well as the photo-generation effect in several lightsensitive structures including solar cells and photo-diodes. The origin of this enhancement has not yet been sufficiently clarified as there appear to be several mechanisms at play, depending on the particular device structure and configuration. We have conducted computer simulation studies on the absorption enhancement in a silicon substrate by nano-shell-related LSPR, based on a Finite Difference, Time-Domain (FDTD) Analysis. Preliminary results of this study show significant enhancement of up to 10X in the near band gap spectral region of Si, using 40-100nm diameter nano-shells. The enhancement was studied as a function of the metallic Shell thickness, the thickness of an externally coating layer of SiO₂, as well as of various nanoshell shapes. The results suggest that the main enhancement mechanism in this case of tubular nanoshells embedded in Si substrate, is that of field-enhanced absorption caused by the strongly LSPR-enhanced electric field extending into the Silicon substrate.

The use of ZnO nanowires has become a widespread topic of interest in optoelectronics. In order to correctly assess the quality, functionality, and possible applications of such nanostructures it is important to accurately understand their electrical and optical properties. Aluminum- and gallium-doped crystalline ZnO nanowires were synthesized using a low-temperature solution-based process, achieving dopant densities of the order of 10²⁰ cm^-3. A non-contact optical technique, photothermal deflection spectroscopy, is used to characterize ensembles of ZnO nanowires. By modeling the free charge carrier absorption as a Drude metal, we are able to calculate the free carrier density and mobility. Determining the location of the dopant atoms in the ZnO lattice is important to determine the doping mechanisms of the ZnO nanowires. Solid-state NMR is used to distinguish between coordination environments of the dopant atoms.

We report on enhancement of thin layer absorption through photonic band-engineering of a photonic crystal structure. We realized amorphous silicon (aSi) photonic crystals, where slow light modes improve absorption efficiency. We show through simulation that an increase of the absorption by a factor of 1.5 is expected for a model film of 100nm of aSi. The proposal is then validated by an experimental demonstration, showing a 50% increase of the absorption of a demonstrator layer of 1μm thick aSi over a spectral range of 0.32 — 0.76μm. This shows new possibilities of increasing the efficiency of thin film photovoltaic cells. Photonic crystal based architecture are proposed and discussed.

Among several factors, like photon capture, photon reflection, carrier generation by photons, carrier transport and collection, the efficiency also depends on the absorption of photons. The absorption coefficient, α, and its dependence on the wavelength, λ, is of major concern to improve the efficiency. Nano-silicon structures (quantum wells and quantum dots) have a unique advantage that multiple direct and indirect band gaps can be realized by appropriate size control of the quantum wells. This enables multiple wavelength photons of the solar spectrum to be absorbed efficiently. We present a theoretical approach to calculate the absorption coefficient using quantum mechanical calculations on the interaction of photons with the electrons of the valence band. In our model, the oscillator strength of the direct optical transitions is enhanced by the quantum confinement effect in Si nanocrystallites. These kinds of quantum wells can be realized in practice in porous silicon. Our theoretical estimations show the absorption coefficient of silicon quantum dots to be comparable to that of bulk silicon, and is also nearly constant over the visible spectrum.