Organic photovoltaic cells with onion-like carbon thin films as hole collection layers

Influence of onion-like carbon (OLC) thin films as insertion layer on the photovoltaic characteristics of soluble zinc phthalocyanine (zinc 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (ZnPc-TB))-based solar cells is investigated. Atmospheric photoelectron yield spectroscopy and ultraviolet-visible absorption spectroscopy reveal that the ionization potential and optical bandgap are 4.84 and 1.70 eV, respectively, for ZnPc-TB and 5.23 and 2.02 eV, respectively, for OLC. The power conversion efficiency of the OLC-inserted solar cell was higher than that of a cell without the OLC interlayer by a factor of 5.3. Improvement in photovoltaic parameters and photocurrent is attributed to an increase in the shunt resistance by a factor of 5.5 due to the OLC thin film which functions as a hole collection layer. The OLC-inserted bulk-heterojunction solar cells comprising ZnPc-TB and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) achieve the best performance with the power conversion efficiency up to 6.9 × 10−2% owing to an enhanced contribution of ZnPc-TB to a photocurrent generation. The OLC thin films acting as the hole collection layer provide a new alternative for solution-processed organic solar cells.