Polyazomethine as a component of solar cells-theoretical and optical study

Abstract

A recently synthesized 25Th-cardo polyazomethine (PAZ) and its photocurrent generating junction with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been examined as possible materials for construction of new-generation solar cells. Properties of a 25Th-cardo/PCBM system, where PAZ and PCBM act as donor and acceptor materials, respectively, have been studied theoretically by time-dependent density-functional theory (TD-DFT) and experimentally by means of optical absorption, photocurrent spectroscopy (PCS), and time-resolved luminescence (TRPL). Theoretical calculations show that highest occupied molecular orbital (HOMO) energy levels of PAZ and PCBM are almost equal (values of −6.01 and −5.98eV were obtained with the B3LYP functional and the def2-TZVP orbital basis for PAZ and PCBM, respectively), what suggests a possibility of charge transfer in both directions for this system. The shape of the calculated absorption spectrum is in a qualitative agreement with the experiment. The PCS measurements of the new material show that the external quantum efficiency is the highest (about 0.3%) in the near UV range. The TRPL studies reveal a fast decay of a Langevin type (in picosecond range) of the PAZ-related photoluminescence, which accelerates in presence of PCBM, probably due to the charge transfer to PCBM. In addition, our measurements document a usually neglected process of the hole transfer from a donor to an acceptor. A leakage of holes from PAZ to PCBM is supported by a small energy difference of the HOMO energies, as predicted by theory.