Trapped-electron doping of photovoltaic sandwich cells containing microcrystalline chlorophyll a

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Abstract
Cr‖Chla‖Hg sandwich cells with microcrystalline chlorophyll a exhibit rectifying I‐V characteristics, whereas Au‖Chla‖Hg cells show Ohmic behavior. We present a comparative study of their ac and dc electrical properties. The impedances of both structures increase with age. The aging process can be reversed by light, suggesting that a trapped space charge controls the impedance levels. The results are consistent with an interpretation that the space charge is trapped electrons which function like a dopant in the Chl layer. The results with the Cr cells show the presence of an interfacial barrier region at the Cr‐Chl contact which is responsible for the photovoltaic activity of these cells. The extent of this region is governed by the negative space charge. In a cell with approximately 7×1016 electrons/cm3, the width of the barrier is about 450 Å.