Investigation of optical and electrical properties of new aromatic polyazomethine with thiophene and cardo moieties toward application in organic solar cells

New polyazomethine (25Th-cardo) based on thiophene and cardo moieties was synthesized, characterized and tested as active layer in solar cells. The structure of polymer was characterized by means of FTIR and 1H NMR spectroscopy. The temperatures of 5% weight loss of the polyazomethine range from 435 to 455 °C in nitrogen, depending on the heating rate applied. 25Th-cardo has electrochemical energy band gap of approximately ∼1.85 eV, and HOMO energy level at −5.21 eV. Optical absorption properties of pure 25Th-cardo and of its mixtures with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were investigated on different substrates: glass or glass/PEDOT:PSS. Electrical properties of the two kinds of devices: ITO/25Th-cardo:PCBM/Al and ITO/PEDOT:PSS/25Th-cardo:PCBM/Al were tested by impedance spectroscopy. The investigated polymer solar cells contained different amounts of PCBM. Bulk heterojunction solar cells with the architecture ITO/PEDOT:PSS/25Th-cardo:PCBM/Al gave a power conversion efficiency 3.5 times higher for the mixture of 25Th-cardo to PCBM 1:2 (w/w) than for 1:1 (w/w) under 100 mW/cm2 AM1.5G sunlight illumination in air. The external quantum efficiency value of 25Th-cardo:PCBM solar cell was the highest (about 0.3%) in the near UV range from 3.1 to 3.7 eV.