Transient photovoltaic effects in metal–chlorophyll-a–metal sandwich cells

Abstract

Photovoltaic effects in (Cr‖Chl-a‖Hg) and (Cr‖Chl-a‖Hg/In) sandwich cells under cw and microsecond pulsed excitation have been studied. The (Cr‖Chl-a‖Hg) cell exhibits strong rectification in the dark, whereas the effect in the (Cr‖Chl-a‖Hg/In) cell is weak and in the opposite direction. Upon cw excitation, the photovoltaic current in (Cr‖Chl-a‖Hg) quickly reaches a steady state with no obvious transients. The current flow is in the direction from Cr to Hg though the Chl-a film. The action spectrum of the cw as well as the pulsed photovoltaic currents matches closely the absorption spectrum of the Chl-a film. In contrast, the (Cr‖Chl-a‖Hg/In) cell behaves very differently. Transient photovoltaic currents preceding the steady state currents are seen under cw excitation in spectral regions where the Chl-a film absorbs strongly. The transient current flow is in the direction from Cr to Hg/In, but it quickly reverses direction to reach a steady state current of opposite sign. At wavelengths absorbed only slightly by Chl-a, the transients are not apparent, and only the steady state current is seen (Hg/In to Cr). The action spectrum of the steady state current shows an ’’inverse’’ type behavior revealing a minimum in response where Chl-a absorbs most. Under pulsed excitation, both the sign and the shape of the photovoltaic current pulse is seen to depend on the excitation wavelength. The action spectrum of the pulsed photovoltaic current in the (Cr‖Chl-a‖Hg/In) cell actually shows a ’’reversal’’ effect where the sign of the signal changes with wavelength. External bias voltage can be applied and it influences both the direction of the photocurrent as well as the shape of the action spectrum. The rectifying and the photovoltaic properties of both cells are directly related to the existence of potential barriers at the contact between the p-type Chl-a and the electrodes. These barriers appear to resemble the Schottky barriers used to describe the contacts between metals and inorganic semicondutors. The remarkable difference in the two cells can be explained as being due to the presence of a single barrier at the Cr‖Chl-a contact, in the (Cr‖Chl-a‖Hg) cell, and two opposing barriers in the (Cr‖Chl-a‖Hg/In) cell, one at the Cr‖Chl-a contact, as before, and a stronger barrier at the Chl-a‖Hg/In contact. The action spectra clearly reflect the relative strength of the front and the back barriers and the penetration depth of the excitation light. The appearance of the transient photocurrent spike is apparently due to the electrical polarization of the Chl-a microcrystals by light under the influence of the built-in field at the rectifying contacts. The switching effect in the photovoltaic current in the (Cr‖Chl-a‖Hg/In) cell is seen as a result of such polarization and the consequent mutual biasing of the two opposing contact barriers. The power conversion efficiency of the (Cr‖Chl-a/Hg) cell is better than 10−2% based on monochromatic illumination, a value among the best yet reported for organic cells.