To Joseph Han’s Presentation about Chlorophyll-A Photovoltaic Cells

1. To Joseph Han’s Presentation about Chlorophyll-A Photovoltaic Cells

2. Photovoltaic Effect 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.

3. 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.

4. 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

5. 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: q (chla)2 <> q (chla· H2O)2<>(chla· 2H2O)n=2q • Among the aggregates, only chlorophyll-a hydrates have photovoltaic effect. The dihydrates, (chla· 2H2O)n , have the strongest photovoltaic effect.

6. Chlorophyll-a Absorption SpectraThe 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

7. 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).

8. SnO2 chla PVA SnO2 Measurement of chla photovoltaic cell(cc) X: measuring voltage; Y: measuring current Photovoltaic cell 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.

9. I(10-9A) EN 300 -300 V(mv) Ep 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.

10. SnO2|chla|PVA|SnO2 SnO2|chla|PVA+EDTANa2|SnO2 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.

11. Open circuit voltage Short circuit current 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.

12. 3 1.Action spectrum, 2.spectrum sensitivity, 3.absorption spectrum of (chla· 2H2O)n 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. • 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.

13. On illumination; In dark 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. 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)

14. E/v (vs. SCE) 2 3 4 1 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.

15. End Thank you