Enhancement of the Open-Circuit Voltage and Hole Conduction of Tetraphenyl Porphyrin/C60 Multilayered Photovoltaic Device by the Insertion of Oxide Hole Collection Layers

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We investigated the photovoltaic properties of organic multilayered photovoltaic devices consisting of Indium--tin-oxide (ITO)/oxide/tetraphenyl porphyrin (H2TPP, ZnTPP)/fullerene (C60)/bathocuproine (BCP)/Al structures. The open-circuit voltage $V_{\text{OC}}$ increases with the thickness of porphyrin layers between 10 and 30 nm. The upper limit of $V_{\text{OC}}$ is attributed to the built-in potential and the energy difference between the highest occupied molecular orbital (HOMO) of H2TPP and the lowest unoccupied molecular orbital (LUMO) of the C60 layer $\Delta E$. The use of oxide hole collection layers, such as NiO and MoO3, is effective for increasing the built-in potential across the organic layers resulting in the improved $V_{\text{OC}}$. The ``kink'' in the $J$--$V$ curve observed at approximately $V_{\text{OC}}$ for the device with a thick H2TPP layer and the device with and without a MoOx layer is analyzed on the basis of the Poole--Frenkel and Schottky models assuming the amorphous porphyrin layers as dielectrics. The resistance of the organic layers is dominated by the field-dependent bulk resistance of H2TPP films for $V<V_{\text{OC}}$, whereas the kink above $V_{\text{OC}}$ was attributed to the relatively high Schottky barrier for holes at the ITO/H2TPP and ITO/MoO3 interfaces.
2011-01-25