Session P33: Physics of Photovoltaics and other Electricity Production

Light Reflection and Absorption by Free Standing Titania Nanotube Arrays

Newly discovered titania nanotube arrays fabricated by anodization have become the main interest of many research groups around the world, mainly due to their potential use for solar energy harvesting. Light conversion to electron-hole pairs can occur in the surface area of these nanotubes, resulting from their very high aspect ratio and the low recombination probability in the titian surface. In our work we have investigated the transmission of light though the nanotubes for different tube lengths, to explore the penetration depth for various wavelengths in the nanotube array. Specifically, we have investigated the reflection and transmission of light incident from both the open and closed sides of the nanotubes. We find that the reflectivity is generally noticeably smaller for light incident on the open end, but the transmission is about the same, implying greater absorption for light incident on the open side, although for wavelengths close to 400nm, the reflectivity from the open side becomes larger than that from the closed side. We will present a theoretical model to explain our experimental results. The model treats wave propagation along the tubes in the eikonel approximation, and wave propagation transverse to the tubes as Bloch waves.   

A quantum-mechanical study of ZnO and TiO$_{2}$ based DSC

Since the pioneering work of Graetzel, Dye Sensitized Cells (DSCs) have attracted great attention as cheap and effective solar power devices based on wide bandgap metal oxide electrode. Optimization of the DSC is a challenging task as it is a highly complex interacting molecular system. Surface properties of the metal-oxide and proper sensitization with dyes may strongly affect the efficiency. Optimizated DSCs based on TiO2 photoanodes and organic dye have reached conversion efficiency of about 10{\%} whereas the efficiency of ZnO based DSC has been found to be much lower, although this material has photochemical properties similar to TiO2, in general due to the nature of the binding between sensitizer and semiconductor. For this reason understanding how anchoring groups interact with the metal-oxide is fundamental to shed light on the different behaviour of these materials in DSC. Aim of this work is to address the binding of small organic sensitizers, such as catechol and isonicotinic acid molecules, to TiO2 and ZnO surfaces, in terms of geometry, stability, electronic structure and absorption properties. To this end, we employed quantum-mechanical simulations based on hybrid DFT and hybrid TDDFT.   

Band gap engineering and optical properties of tungsten trioxide

Tungsten trioxide (WO3) is a good photoanode material for water oxidation but it is not an efficient absorber of sunlight because of its large band gap (2.6 eV). Recently, stable clathrates of WO3 with interstitial N2 molecules were synthesized [1], which are isostructural to monoclinic WO3 but have a substantially smaller bang gap, 1.8 eV. We have studied the structural, electronic, an vibrational properties of N2-WO3 clathrates using ab-initio calculations and analyzed the physical origin of their gap reduction. We also studied the effect of atomic dopants, in particular rare gases. Substantial band gap reduction has been observed, especially in the case of doping with Xe, due to both electronic and structural effects. Absorption spectra have been computed by solving the Bethe-Salpeter Equation [2] to gain a thourough insight into the optical properties of pure and doped tungsten trioxide. [1] Q. Mi, Y. Ping, Y. Li., B.S. Brunschwig, G. Galli, H B. Gray, N S. Lewis (preprint) [2]D. Rocca, D. Lu and G. Galli, J. Chem. Phys. 133, 164109 (2010)   

Aluminum nanoparticles for plasmonic enhancement of light absorption in organic semiconductors

The energy conversion efficiency in organic photovoltaic (OPV) devices is low, primarily due to the short exciton diffusion length (~10 nm) in organic semiconductors. Much effort is currently devoted to improving the optical absorptivities in OPVs by incorporating plasmonic nanostuctures in or near the active layer, which would allow for significantly thinner absorption layers. We suggest that aluminum nanoparticles are a better choice than gold or silver particles for embedded plasmonic structures in OPV active layers. This is due to the high plasmon frequency of Al, which makes it easy to create good overlap between plasmon resonance and semiconductor absorption band in a random mixture of polymer and nanoparticles. We will present both modeling and experimental data to support this hypothesis.   

Si-SiGe hetero-structure thin-film solar cells using integrated electro-optical modeling

Hetrostructure Si-SiGe thin film solar cells have been designed and optimized using advance electro-optical theoretical modeling and simulation. Some of the key characteristics such as short-circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) have been studied for varying concentration of Ge in SiGe buffer layer. The effect of thickness variation of alloyed layer for varying Ge composition $\sim $0.1---10{\%} has been performed. The improvement in the conversion efficiency of these cells was calculating by tailoring the thickness of p+ doped layer. An approach relying on phenomena of improved absorption of the alloys which leads to a gain in the current was explored. Improved infrared response with higher short circuit current has been obtained for about 25 $\mu $m thick structures. With the optimized Ge concentration, and the incorporated structure design parameters, as much as 4-6{\%} enhancement in the overall efficiency of the solar cells has been calculated compared to that of the conventional single crystal Si solar cells. Moreover, the efficiency of these cells can further be improved because Si-SiGe based solar cells have improved absorption characteristics and offer minimum operating temperature sensitivity. It is believed that with better understanding of the band-gap engineering of SiGe when used as buffer and junction layers, the overall conversion efficiency of such devices can further be improved and could play a critical role to develop low cost and high efficiency solar cells technology. This work is supported by DOE grant DE-FG02-10ER46709 and the state of South Dakota.   

Multiscale Simulation on a Light-Harvesting Molecular Triad

We have investigated the effect of solvation and confinement on an artificial photosynthetic material, carotenoid-porphyrin-C$_{60}$ molecular triad, by a multiscale approach and an enhanced sampling technique (Replica Exchange Method). We have developed a combined approach of quantum chemistry, statistical physics, and all-atomistic molecular dynamics simulation to determine the partial atomic charges of the ground-state triad. The confinement effects on the triad were modeled by imposing three sizes of spherocylindrical nanocapsules. The triad is structurally flexible under ambient conditions and its conformation distribution is manipulated by the choice of water models and confinement. Two types of water models (SPC/E and TIP3P) are used for solvation. We have shown that a slight structural difference in the two water models with the same dipole moment can have great distinction in water density, water orientation and the number of hydrogen bonds in the proximity of a large flexible compound such as the triad. Subsequently, it has direct impact on the position of the triad in a confinement as well as the distribution of conformations at the interface of liquid and solid in a finite-size system.   

The Potential for Hot Carrier Collection from an Amorphous Semiconductor

The quest for clean, inexpensive sources of energy has produced intense interest in low-cost methods for dramatically increasing the efficiencies of solar cells. One such method is to collect carriers before they lose energy to heat. Here we present strong evidence for such hot carrier transfer in an unlikely place, between the amorphous and crystalline regions of nanocrystalline Si. Nanocrystalline Si is a thin film photovoltaic material formed of Si nanocrystallites imbedded in a hydrogenated amorphous Si matrix. Using a combination of photoluminescence quenching and electron spin resonance measurements as a function of nanocrystalline fraction, we observe clear evidence that above a critical fraction carriers excited in the amorphous region transfer to the nanocrystals rather than relaxing to band tail states of the amorphous silicon matrix. The average nanocrystallite spacing is consistent with estimates of the distance hot carriers can transfer in amorphous silicon before thermalization. This result has implications that extend from improving the stability of amorphous silicon under optical illumination to the development of a new paradigm in solar cell design using nanostructured amorphous absorbers.   

Enhanced photocatalytic H$_2$ production by sub-nanometer Au nanoparticles

CdS surfaces were found to exhibit a dramatically enhanced photo-catalytic activity for the water splitting process when loaded with sub-nanometer Au particles, compared to that of bare CdS or CdS modified with other noble metal particles (Pt, Ru, Rh, Pd) with similar co-catalyst loading. As a first step towards understanding the striking photocatalytic activity of Au/CdS, we conducted a detailed characterization of the Au particle samples by combining mass spectroscopy, (scanning) transmission electron microscopy, UV-vis spectroscopy and first-principle calculations. In particular, we carried out systematic studies of the influence of particle size, surface termination and charge state on the structural, electronic and spectroscopic properties of ligand-protected Au nanoparticles using density functional theory and time-dependent density functional theory. The structural and electronic stability of diphosphine-protected Au$_9$ and Au$_{11}$ cations with composition determined from mass spectroscopy, were confirmed from total energy and electronic structure calculations, while the computed optical absorption spectra were found to be in excellent agreement with UV-vis data. Work is underway to study the interaction between Au particles and Cds.   

First-principles study of III-V electrode interfaces for photoelectrochemical hydrogen production

Photoelectrochemical (PEC) cells promise clean, sustainable production of hydrogen fuel using water and sunlight. However, combining solar conversion efficiency with durability in electrolyte solution has proven difficult, in part because the complex chemistry active at the electrode-electrolyte interface remains poorly understood. We use first-principles molecular dynamics simulations and model density-functional calculations to study the structure, stability, and chemical activity of GaP/InP semiconductor electrodes in contact with water. We find that a local bond-topological model is able to capture much of the basic surface chemistry. Interpretation of our results points to the particular importance of surface-adsorbed oxygen in determining the available reaction pathways for photocorrosion and water dissociation. Electronic signatures of the local bond topologies are compared to data from X-ray absorption and emission spectroscopy for insight into actual electrode structure.   

Photoresistance of Li$_{0.2}$Zn$_{0.8}$O thin films and Li$_{0.2}$Zn$_{0.8}$O/Al$_{0.15}$Zn$_{0.85}$O multilayers

Thin films of Li$_{0.2}$Zn$_{0.8}$O and Li$_{0.2}$Zn$_{0.8}$O/Al$_{0.15}$Zn$_{0.85}$O multilayers have been grown on sapphire substrates by pulsed-laser deposition. P-type Li$_{0.2}$Zn$_{0.8}$O is an insulator with the band gap of 3.26eV and n-type Al$_{0.15}$Zn$_{0.85}$O is a transparent conductor. Under the ultraviolet light irradiation, the Li$_{0.2}$Zn$_{0.8}$O films show photoresistance (PR) (PR=(R$_{dark}-$R$_{irradiation})$/R$_{irradiation})$ effect of $\sim $340{\%} at room temperature. In bilayers of Li$_{0.2}$Zn$_{0.8}$O($\sim $110nm)/Al$_{0.15}$Zn$_{0.85}$O($\sim $90nm) where a p-n junction is formed, the photoresistance increases to $\sim $8600{\%}. The photoresistance dependence on wavelength (300-700nm) measurement shows that the photoresistance effect is observed when the light wavelength is below $\sim $380nm. The dependence of the light intensity and the response time of the resistance switching have also been measured and will be discussed. The largely enhanced PR effect in bilayers is probably due to the enhancement of the PR effect in p-n junctions. The increased photosensitivity indicates that the Li$_{0.2}$Zn$_{0.8}$O/Al$_{0.15}$Zn$_{0.85}$O multilayers are promising for UV photodetection applications.   

Materials considerations for the efficiency of photon-enhanced thermionic emission

Photon-Enhanced Thermionic Emission (PETE) is a promising method of solar energy conversion that is based on thermal emission of photoexcited electrons from a high-temperature semiconductor, making it attractive for use in tandem with solar thermal systems. Theoretical efficiencies of PETE devices can exceed those of single junction photovoltaics, but experimental tests of the PETE process have displayed low efficiencies.[1] Here we examine this disconnect between experimental results and theoretical promise. We analyze the effects of real semiconductor parameters on the PETE process and relate them to the ultimate performance of a PETE device, directly translating non-idealities of practical materials into constraints on conversion efficiency. The analysis identifies fundamental challenges to efficient conversion based on PETE and establishes design rules that direct the search for semiconductor systems to form the basis of real devices. We also review experimental work guided by these insights that shows increased emission efficiency due to reduced surface recombination, one of the key challenges for realistic solar energy conversion based on PETE.\\ $\left[1\right]$ J.W. Schwede, {\it et al.}, Nat. Mater. {\bf 9}, 762-767 (2010)   

Designing long wavelength barrier based thermophotovoltaic cells

This work describes the process of designing low source-temperature thermophotovoltaic cells to harvest energy from thermal sources in the 7-10 micron range. Simulations of the bandstructure are performed for strained layer superlattices (SLS) to act as the p, B, and n regions in a TPV barrier diode. Simulations of the band-alignment of these regions are then performed to ensure a high enough barrier in one band, and a smooth transition in the other. The process is performed iteratively to find an ideal match of specific SLS material and doping concentration in each region. Both conduction band barriers and valence band barriers have been investigated. The balance of system and test apparatuses will also be discussed, as well as preliminary results from samples grown via MBE.   

Thought waves remotely affect the performance (output voltage) of photoelectric cells

In our experiments, thought waves have been shown to be capable of changing (affecting) the output voltage of photovoltaic cells located from as far away as 1-3 meters. There are no wires between brain and photoelectric cell and so it is presumed only the thought waves act on the photoelectric cell. In continual rotations, the experiments tested different solar cells, measuring devices and lamps, and the experiments were done in different labs. The first experiment was conducted on Oct 2002. Tests are ongoing. Conclusions and assumptions include: 1) the slow thought wave has the energy of space-time as defined by C1.00007: The mass, energy, space and time systemic theory- MEST. Every process releases a field effect electrical vibration which be transmitted and focussed in particular paths; 2) it has a information of order of tester; 3) the brain (with the physical system of MEST which like a hardware) and consciousness (with the spirit system of the mind, consciousness, emotion and desire-MECD! which like a software) build up a life-informational computer, through some algorithms of DNA and RNA, produce the life-information (include the Genetic code). The Life-Information is a female parent of any information; 4) human can optimize the information. This function is the intelligence; 5) In our experiments, not only thought waves can affect the voltage of the output pho toelectric signals by its energy, but they can also selectively increa! se or decrease those photoelectric currents through remote consciousness interface by a life-information technology.