Elucidation of hierarchical metallophthalocyanine buffer layers in bulk heterojunction solar cells

Abstract

Vapour deposited metallophthalocyanines (VoPC, NiPC, SnPC, TiPC, FePC, InPC) were inserted as thin p-type buffer layers into organic solar cells consisting of regio-regular poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester in order to modify the band alignment for enhanced photovoltaic responses. It is observed that the VoPC, NiPC and SnPC show higher efficiencies in both ∼5 nm and ∼10 nm thicknesses of film when coated on a PEDOT:PSS layer. The observations were an enhanced efficiency with a maximum J_SC value of 14.80 mA cm⁻², V_OC of 0.60, and FF of 51% with a conversion efficiency of 3.61% in the case of ∼5 nm thick VoPC, while the ∼5 nm thick NiPC shows a J_SC value of 11.15 mA cm⁻² with 3.13% efficiency, and similarly the ∼10 nm thick NiPC has a J_SC value of 8.98 mA cm⁻² with overall efficiency of 2.52%. However, the TiPC, FePC and InPC show poor efficiencies even lower than those of the usual bulk-heterojunction solar cells with deposition of either a ∼5 nm or a 10 nm buffer layer. The enhanced efficiency in VoPC, NiPC and SnPC may be the result of extended absorption, high transparency, planar porphyrin complex orientation and holding p-type properties to transfer the hole efficiently towards the anodes, with additionally a better bicontinuous interpenetrating network with vapour deposited layers, which greatly reduces the exciton loss and improves the charge transport capability.