G. Smestad, "Luminescence as a predictor
of quantum solar energy conversion", Thesis No. 1263
(1994), The
Swiss Federal Institute of Technology (EPFL), Lausanne,
Swizerland, thesis advisor: Prof. Dr. Michael Grätzel.
This thesis presents experimental data which supports the
idea that room temperature photoluminescence measurements
can be used to estimate the maximum thermodynamically allowed
chemical potential and solar conversion efficiency for quantum
solar converters. Experimental photoluminescence spectra obtained
for silicon, chlorophyll based photosynthesis and for Ruthenium
dye sensitized TiO2 photoelectrochemical solar cells are compared
to predictions made using optical absorption data. The absorption
data includes measurements of transmission and reflection,
as well as those using the action (or induced photo - product)
spectrum. Predictions are made using the measured quantum
absorptivity, the Generalized Planck equation and Detailed
Balance techniques that relate the chemical potential of the
excited state to the photoluminescent emission and photocurrent
available for performing work. Although the absorptivity obtained
from the optical measurements can be used to predict much
of the photoluminescence curve, the predicted values may deviate
from measurements at long wavelengths. The photoluminescence
spectrum for silicon, the Ruthenium dye cell, and for photosynthesis
may also be predicted from action spectra. Both techniques
yield a reasonable fit to the experimental spectral distribution
and the predicted chemical potentials and voltages are consistent
with electrical measurements. Experimental data is also presented
on porous silicon solar cells, vacuum photodiodes and silver
oxide (and chloride) based photochemistry in relation to the
use of photoluminescence signal strength as an indicator of
voltage or free energy generation. The novelty of this thesis
is the description of all these varied types of quantum photoconverters
within the same text and photoluminescence framework.
In chapter one, an introduction is made to the various types
of solar conversion systems and their relationship and relevance
to luminescence studies. Energy band diagrams are presented
for each system and a discussion is made of the recombination
pathways that produce work, and the pathways which dissipate
energy. At the end of this chapter, a general quantum solar
converter is presented and related to thermodynamic principles
such as the generation of chemical potential and to the creation
of an excited state concentration gradient. This general quantum
converter consists of a light activated "pump" which maintains
the concentration gradients and chemical potentials.
In chapter two, a more detailed theoretical framework is presented
which introduces the previously derived Generalized Planck
equation and its relationship to absorptivity, emissivity
and the generation of a chemical potential. This equation
is connected to the photoluminescence efficiency of the light
absorber and to the current -voltage characteristics of a
quantum converter. This connection is possible using assumptions
regarding the non-radiative dissipation (or recombination)
pathways relative to the generation of luminescence. These
assumptions are examined using conventional p-n junction solid
state theory and the transport equation, and found to be similar
to the assumptions used in other device analysis techniques.
The relationship between the photoluminescence and the kinetics
of the conversion process is then outlined. Specifically,
the radiative lifetime and injection rate constants are described
for a general photochemical converter and applied to the dye
sensitized photoelectrochemical cell. Lastly, a brief theory
of phototube operation is presented which allows for the understanding
of synergistic effects between heat and light in a vacuum
photodiode photoconverter. The thermal and photochemical reactions
for the silver based materials used in the phototube photodiode
are also presented.
In sections three and four, experimental methods and results
are presented for the systems described in the previous chapters.
The apparatus is described for the measurement of transmission,
absorptivity, photoluminescence, photo-product spectra and
current-voltage characteristics. The photoluminescence of
silicon, Ruthenium dye cells and photosynthesis is found to
be consistent with predictions based on the optical absorptivity
and induced photocurrent, or photo-product spectra. A novel
concept introduced by this thesis is the use of the photo-product
spectra to estimate the emissivity and absorptivity used in
the Generalized Planck equation. Solar conversion efficiency
predictions based on this equation are applied to these three
conversion systems and are found to be consistent with the
experimental performance. Specifically, the maximum allowed
chemical potentials for the silicon solar cell are found to
be 0.6-0.65 eV , while those for photosynthesis and the dye
sensitized cell are found to be near 1.3 eV. The porous silicon
solar cell and phototube photodiode results highlight the
finding that the chemical potentials predicted from the analysis
of photoluminescence are generally much higher than for the
actual device. This is due to the converter configuration
and kinetic properties such as resistivity. In the case of
the vacuum photodiode, the configuration of the converter
allows for heat and light to simultaneously be converted to
work. Although this has been predicted using photoluminescence
based models, this thesis presents the first clear demonstration
of a synergistic effect in a "hot" electron device. In hopes
of utilizing this effect in a photochemical system, experiments
were conducted using the same metal oxide (Ag2O) as was used
as the phototube photodiode absorber. It is found, however,
that although the oxide powder reacts at low temperatures,
the action of light does not conclusively change the reaction
thermodynamics or kinetics.
In chapter five, a discussion of the results in the previous
chapters is presented, as are ramifications for the application
of the theoretical formalism to practical quantum solar converters.
Separate conclusions at the end of the thesis summarize the
knowledge gained by examining all the various types of converters
together. A table of symbols is given before the curriculum
vitae to facilitate the description of the concepts presented
and discussed.
For a table of contents (table des matières)
and additional information about the thesis, see the
EPFL website.
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