Photovoltaic effects of metal–chlorophyll-a–metal sandwich cells

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Abstract
The microcrystalline chlorophyll−a film prepared by the method of electrodeposition is shown to have strong photovoltaic effects. The photovoltaic cell (M1‖Chl‖M2) has lamellar arrangement with the Chl−a film sandwiched between two metal electrodes. With dissimilar electrodes of different work functions, φM, the cell usually exhibits a dark rectification behavior. A large forward bias current is seen when the metal with a lower φM is the negative electrode. The rectification is small when the same metal is used as both electrodes. A blocking contact or Schottky barrier is evidently present at the Chl−a‖metal junction particularly fo the metal having a low φM. A p−type semiconduction in Chl−a is implicated. In the photovoltaic mode, cells such as (Al‖Chl‖Hg) and (Al‖Chl‖Au) have an open circuit voltage ranging from 200−500 mV. The power conversion efficiency for these cells is on the order of 10−3% which is among the highest in photovoltaic cells using organic materials. The direction of the photovoltaic current and the shape of its action spectrum are strongly dependent on the electrode materials. The blocking contact at the Chl−a‖metal junction is primarily responsible for the photovoltaic activity of the cell. When this blocking contact is established at the directly illuminated electrode the action spectrum closely matches the absorption spectrum of the Chl−a film. An inverse type response is found when the blocking contact is at the indirectly illuminated electrode with the Chl−a layer essentially acting as an inner filter. The photocurrent is shown to increase when the cell is reverse biased. A minimum quantum yield for charge generation of about 0.03 is found in (Al‖Chl‖Hg) and (Al‖Chl‖Au) cells.