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

We investigated the photovoltaic properties of organic multilayered photovoltaic devices consisting of Indium-tin-oxide (ITO)/oxide/tetraphenyl porphyrin (H2TPP, ZnTPP)/fullerene (C-60)/bathocuproine (BCP)/Al structures. The open-circuit voltage VOC increases with the thickness of porphyrin layers between 10 and 30 nm. The upper limit of VOC 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 C-60 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 VOC. The "kink'' in the J-V curve observed at approximately VOC 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 < VOC, whereas the kink above VOC was attributed to the relatively high Schottky barrier for holes at the ITO/H2TPP and ITO/MoO3 interfaces. (C) 2011 The Japan Society of Applied Physics