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· H 2 O) 2 and (chla· 2H 2 O) n. There is an equilibrium between these aggregates: q (chla) 2 q (chla· H 2 O) 2 (chla· 2H 2 O) n=2q Among the aggregates, only chlorophyll-a hydrates have photovoltaic effect. The dihydrates, (chla· 2H 2 O) 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· 2H 2 O) n is at 743nm in UV/Vis spectra. The characteristic absorption of chlorophyll aggregates is at 1655 cm -1 in IR spectra. 743nm UV/Vis absorption spectra of chla 1655cm -1 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, SnO 2 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 SnO 2 or metal: chla|SnO 2 or chla|M. Diagram of sandwich cells: SnO 2 |chla|PVA|SnO 2 Diagram of measurement. Measurement of chla photovoltaic cell(cc) X: measuring voltage; Y: measuring current SnO 2 PVA chla Photovoltaic cell
Characters of Chlorophyll-a Photovoltaic Cells Dark rectifying effect : The cells show an obvious rectifying ability in dark, ∆E=E p -E N. ∆E of SnO 2 |chla|PVA|SnO 2 is 850mv ( E N =-600mv, E P =250mv ), which can be decreased to 500mv by adding H 2 Q(hydroqinone) or EDTANa 2 (sodium ethylenediamine tetraacetate), but increased to 1500mv by adding MBBA. The character implies a blocking contact in the cell. V(mv) I(10 -9 A) EpEp ENEN -300 300
Cyclic voltammetry There is neither anodic nor cathodic peak on the cyclic voltammetric curves of the photovoltaic cell, SnO 2 |chla|PVA|SnO 2. Addition of reducing agents such as H 2 Q and EDTANa 2 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. SnO 2 |chla|PVA|SnO 2 SnO 2 |chla|PVA+EDTANa 2 |SnO 2
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. 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· 2H 2 O) n. 3 1.Action spectrum, 2.spectrum sensitivity, 3.absorption spectrum of (chla· 2H 2 O) 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 V oc : open circuit voltage; I sc : short circuit current; V ow : max working voltage; I ow : max working current; P the =I sc V oc : theoretical output power; P max =I ow V ow : the max output power; FF=P max /P the : filling factor; η=P max /P in : the power efficiency. (P in is the light intensity)
Conclusion ☺(☺(chla· 2H 2 O) n is the aggregate to produce the photovoltaic effect. ☺I☺In photovoltaic cell, chla|SnO 2 is p-n type junction. ☺W☺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. ☺P☺Parameters of photovoltaic cells are influenced by the compositions. ☺S☺Study on photovoltaic cells is significant to use solar energy. ☺S☺Study on chlorophyll-a is essential to the imitation of photosynthesis systems. E/v (vs. SCE) 41 2 3