Counting sunrays: from optics to the thermodynamics of light

This chapter considers quantum solar energy conversion from a thermodynamic point of view. Starting from geometrical optics, the concept of étendue is used to determine the number of photon states in a beam of light. This naturally leads to the definition of entropy, providing the foundation for the statistical mechanics of light beams. With emphasis on the thermodynamic functions per photon (in particular, the chemical potential), these concepts are illustrated first by comparing the thermodynamic limits of the geometric concentrators with the limits obtained by traditional arguments. The thermodynamic framework is then extended to novel applications. The fluorescent collector is modelled as an open thermodynamic system interacting with a room-temperature heat bath. A detailed thermodynamic description of the operation of a p-n junction solar cell then follows, starting from energy (voltage) rather than from the kinetic argument used by Shockley and Queisser. This provides a novel view of fundamental losses, each identified as a specific form of irreversible entropy generation. The chapter concludes with an analysis of a future photovoltaic device – a hot carrier solar cell where the voltage exceeds the Shockley-Queisser limit. The efficiency of this solar cell, obtained by thermodynamic arguments, is free from specific mechanisms or structures such as selective energy contacts. It is argued that this is the fundamental efficiency limit to the operation of single junction solar cells where thermalization of electron-hole pairs has been reduced or entirely eliminate

Modelling the performance of fluorescent solar collectors

The theoretical power conversion efficiency of a silicon solar cell with a fluorescent solar collector is believed to reach 90% of the maximum efficiency of an ideal silicon solar given by the Shockley-Queisser detailed balance limit, but the practical efficiencies are significantly lower due to several loss mechanisms. This work presents an analytic model which take the non-ideal coupling between the collector and the solar cell mounted at the edge into consideration and it is shown in good agreement with experimental results

Thermodynamic modeling of micro heat engines for power generation

The need for compact, high power-density power sources has led to significant research interest in micro heat engines. However, there is a lack of suitable thermodynamic models which can be used to evaluate the power performance of micro heat engines by taking into consideration the effect of leakage and finite heat input. This work is the first to develop such a thermodynamic model to predict the upper limit of performance of micro heat engines. The model allows investigation of the effects of design parameters such as length and material properties on the theoretical output which can be used for design guidance. Results from this model are further illustrated by comparison to the reported P3 micro engine

All UK electricity supplied by wind and photovoltaics – The 30–30 rule

Based on weather and electricity demand data for the period 1984–2013, we develop a system model based on energy balance to determine the size of photovoltaic and wind generation combined with energy storage to provide a firm power supply for Great Britain. A simple graphical methodology is proposed where the required wind and PV generation capacities can be read off from a “system configuration diagram” as a function of the available storage size. We show, by way of illustration, that a reliable supply would be produced by a system based on PV and wind generators generating some 30% more electrical energy (approximately 100TWh p.a.) than the current electricity supply system if supplemented with 30 days of storage. In terms of generation capacities, the current 82GW of principally thermal generation would then be replaced by about 150GW of wind turbines and 35GW of PV arrays

Photon tunneling into a single-mode planar silicon waveguide

We demonstrate the direct excitation of a single TE mode in 25 nm thick planar crystalline silicon waveguide by photon tunneling from a layer of fluorescent dye molecules deposited by the Langmuir-Blodgett technique. The observed photon tunneling rate as a function of the dye- silicon separation is well fitted by a theoretical tunneling rate, which is obtained via a novel approach within the framework of quantum mechanics. We suggest that future ultrathin crystalline silicon solar cells can be made efficient by simple light trapping structures consisting of molecules on silicon

Light harvesting in silicon(111) surfaces using covalently attached protoporphyrin IX dyes

We report the photosensitization of crystalline silicon via energy transfer using covalently attached protoporphyrin IX (PpIX) derivative molecules at different distances via changing the diol linker to the surface. The diol linker molecule chain length was varied from 2 carbon to 10 carbon lengths in order to change the distance of PpIX to the Si(111) surface between 6 A and 18 A. Fluorescence quenching as a function of the PpIX-Si surface distance showed a decrease in the fluorescence lifetime by almost two orders of magnitude at the closest separation. The experimental fluorescence lifetimes are explained theoretically by a classical Chance-Prock-Silbey model. At a separation below 2 nm, we observe for the first time, a Forster like dipole-dipole energy transfer with a characteristic distance of R-o = 2.7 nm

Silicon photosensitisation using molecular layers

Silicon photosensitisation via energy transfer from molecular dye layers is a promising area of research for excitonic silicon photovoltaics. We present the synthesis and photophysical characterisation of vinyl and allyl terminated Si(111) surfaces decorated with perylene molecules. The functionalised silicon surfaces together with Langmuir-Blodgett (LB) films based on perylene derivatives were studied using a wide range of steady-state and time resolved spectroscopic techniques. Fluorescence lifetime quenching experiments performed on the perylene modified monolayers revealed energy transfer efficiencies to silicon of up to 90 per cent. We present a simple model to account for the near field interaction of a dipole emitter with the silicon surface and distinguish between the 'true' FRET region (<5 nm) and a different process, photon tunnelling, occurring for distances between 10-50 nm. The requirements for a future ultra-thin crystalline solar cell paradigm include efficient surface passivation and keeping a close distance between the emitter dipole and the surface. These are discussed in the context of existing limitations and questions raised about the finer details of the emitter-silicon interaction

Observation of energy transfer at optical frequency to an ultrathin silicon waveguide

Energy transfer from a submonolayer of rhodamine 6G molecules to a 130 nm thick crystalline silicon (Si) waveguide is investigated. The dependence of the fluorescence lifetime of rhodamine on its distance to the Si waveguide is characterized and modeled successfully by a classical dipole model. The energy transfer process could be regarded as photon tunneling into the Si waveguide via the evanescent waves. The experimentally observed tunneling rate is well described by an analytical expression obtained via a complex variable analysis in the complex wavenumber plane

Beyond the Yablonovitch limit: trapping light by frequency shift

It is shown that randomising the photon distribution over the frequency as well as orientation variables dramatically improves the efficiency of optical confinement in a weakly absorbing material such as crystalline silicon. The enhancement in average optical path length over the Yablonovitch limit [E. Yablonovitch, J. Opt. Soc. Am. 72, 899 (1982)] is given by an inverse Boltzmann factor of the frequency shift, making it possible to manufacture, for example, efficient crystalline silicon solar cells of thickness barely 1 micromete