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To Joseph Han’s Presentation about
Chlorophyll-A
Photovoltaic Cells
Photovoltaic effect: When photons are absorbed by
photo-sensitizer, a voltage difference across a junction is
produced.
The voltage difference is caused by the internal drift of
electrons which accepted the light energy and leaved the
normal position.
Photoelectric effect: If the electrons are let though a
circuit, a current can be formed.
Photovoltaic effect is the basic physical process through
which a solar cell converts sunlight into electricity.
Photo-sensitizer
• Inorganic photo-sensitizer: These include
metals, semiconductors and inorganic compounds.
The early solar cells are based on this kind of
materials.
• Organic photo-sensitizer: there are many
organic pigments, for example, porphyrin and
pyridine, that can be used for photovoltaic effect
research.
• Chlorophyll
We used Chlorophyll-a as the photo-sensitizer.
Properties Of Chlorophyll-a
Structure of chlorophyll-a: It is composed
of porphyrin ring and the hydrophobic phytyl chain which
is mobile under normal conditions.
Porphyrin ring
Phytyl chain
Chlorophyll-a Hydrate Aggregates
• Chlorophyll-a can form different aggregates, for example,
(chla)2, (chla· H2O)2 and (chla· 2H2O)n.
• There is an equilibrium between these aggregates:
 2qH 2O
 2qH 2O
q (chla)2 <
> q (chla· H2O)2 <
> (chla· 2H2O)n=2q
 2qH 2O
 2qH 2O
• Among the aggregates, only chlorophyll-a hydrates have
photovoltaic effect. The dihydrates, (chla· 2H2O)n , have the
strongest photovoltaic effect.
Chlorophyll-a Absorption Spectra
The absorption spectra of chlorophyll-a changes in different solvents.
The characteristic absorption of (chla· 2H2O)n is at 743nm in UV/Vis
spectra.
The characteristic absorption of chlorophyll aggregates is at 1655 cm-1 in IR
spectra.
743nm
1655cm-1
UV/Vis absorption spectra of chla
IR spectra of chla
Experiments
• Extract and purify chlorophyll-a and phosphate using column
chromatography.
• Analyze chlorophyll-a with CS-930 Dual-wavelength Thin Layer
Chromatographer (Japan), DMS-200 UV/Vis Spectrometer (Varian, USA),
and FTS-40 IR Spectrometer (BIO-RAD, USA).
• Synthesize liquid crystal materials: MBBA (n-(p’methoxylbenzylidence)-butylanine).
• Prepare chlorophyll-a electrodes with substrates, SnO2 glass or metals,
by means of electrodepositing at 1000 V/cm.
• Make PVA (polyvinyl alcohol) film.
• Construct chlorophyll-a photovoltaic cells.
• Measure photovoltaic effect and the properties of the chlorophyll-a
cells, using the BAS 100A electrochemical analyzer (Bioanalytical System
Inc, USA) or HA-501 Potentiostat/Galvanostat, B-104 Function
Generator (Hokuto Denko, Japan), x-y Auto-balance Recorder (Tokyo,
Japan), and x-t Recorder (Dahua, China).
Photovoltaic Cells
• Composition of chlorophyll-a electrode: Chlorophyll-a was
electrodeposited onto substrate SnO2 or metal: chla|SnO2
or chla|M.
• Diagram of sandwich cells: SnO2|chla|PVA|SnO2
• Diagram of measurement.
SnO2
chla
PVA
SnO2
Measurement of chla photovoltaic cell(cc)
Photovoltaic cell
X: measuring voltage; Y: measuring current
Characters of Chlorophyll-a
Photovoltaic Cells
Dark rectifying effect:
The cells show an obvious
rectifying ability in dark, ∆E=Ep-EN.
• ∆E of SnO2|chla|PVA|SnO2 is
850mv (EN=-600mv, EP=250mv), which
can be decreased to 500mv by
adding H2Q(hydroqinone) or
EDTANa2 (sodium
ethylenediamine tetraacetate), but
increased to 1500mv by adding
MBBA.
• The character implies a blocking
contact in the cell.
I(10-9A)
EN
300
-300
V(mv)
Ep
Cyclic voltammetry
• There is neither anodic nor cathodic peak on the cyclic
voltammetric curves of the photovoltaic cell,
SnO2|chla|PVA|SnO2.
• Addition of reducing agents such as H2Q and EDTANa2
make both peaks to appear.
• Potentials should be controlled within anodic and cathodic
peaks to avoid the contribution of chemical reactions to the
photovoltaic cells.
SnO2|chla|PVA|SnO2
SnO2|chla|PVA+EDTANa2|SnO2
Output characters
• Open circuit voltage: increase sharply on
illumination and slowly reaches maximum; Drop to original
after the light is off.
• Short circuit current: a pulse appear on illumination,
then drop to a steady value; return to original when light
off.
Open circuit voltage
Short circuit current
Action spectrum and spectrum sensitivity
• Action spectrum:
measure the short circuit
current at different
wavelength of light. The
curve of current vs.
wavelength is the action
spectrum of the cell.
3
• Spectrum sensitivity:
the photocurrent induced by
unit light intensity.
• Both action spectrum and
spectrum sensitivity of chla
photovoltaic cell well match
the absorption spectrum of
(chla· 2H2O)n.
1.Action spectrum, 2.spectrum sensitivity,
3.absorption spectrum of (chla· 2H2O)n
Parameters of Photovoltaic Cells
•
•
Working curves: dark curves show very
small current; illuminating curves show
photocurrent at different voltages.
Cell parameters: eight parameters can
be gained from the working curves. They
are changed with the constructer of the
cells.
On illumination;
In dark
Voc: open circuit voltage; Isc: short circuit current; Vow: max working voltage; Iow: max working current; Pthe=IscVoc:
theoretical output power; Pmax=IowVow: the max output power; FF=Pmax/Pthe: filling factor; η=Pmax/Pin: the power efficiency.
(Pin is the light intensity)
Conclusion
☺(chla· 2H2O)n is the aggregate to produce the photovoltaic effect.
☺In photovoltaic cell, chla|SnO2 is p-n type junction.
☺When illuminated, electrons in chlorophyll-a are excited from
ground state to singlet or triplet states. Electrons can be transferred
to the PVA film which is conductive in wet conditions.
☺Parameters of photovoltaic cells are influenced by the
compositions.
☺Study on photovoltaic cells is significant to use solar energy.
☺Study on chlorophyll-a is essential to the imitation of
photosynthesis systems.
E/v (vs. SCE)
2
4
1
3
End