Alkyl Substituent Length Dependence of Octaalkylphthalocyanine Bulk Heterojunction Solar Cells

Alkyl substituent length dependences of photovoltaic performance of bulk-heterojunction (BHJ) organic solar cells (OSCs) utilizing 1,4,8,11,15,18,22,25-octaalkylphthalocyanine (C$n$PcH 2 ) mixed in [6,6]-phenyl C 71 butyric acid methyl ester have been studied. By shortening the alkyl substituents, stacking of the discotic C$n$PcH 2 columns is probably changed from 2D rectangular lattices to pseudohexagonal structures, and Davydov splitting at the Q-band of C$n$PcH 2 absorbance spectra decreases, which results in the higher hole mobility and the deeper highest occupied molecular orbital energy levels. As a result, the power conversion efficiencies of C$n$PcH 2 -based BHJ OSCs are improved from 0.3 to 3.7% by changing the alky substituent length.