9,9-bifluorenylidene derivatives as novel hole-transporting materials for potential photovoltaic applications

Novel 9,9'-bifluorenylidene derivatives were designed to study the effect of alkyl chain length on selected physical properties. The structure of the synthesized compounds was confirmed by using NMR spectroscopy (H-1, C-13, H-H COSY, H-C HMQC, H-C HMBC) and elemental analysis. They showed high thermal stability and undergo decomposition in the range of 388-400 degrees C. As was revealed by DSC investigations, they can be converted from crystalline to amorphous materials with relatively high glass transition temperature. The replacement of the alkyl chains from ethyl to butyl resulted in a significant negative impact on melting and glass transition temperatures. The synthesized derivatives undergo reversible electrochemical oxidation and reduction and showed a very low energy band gap (1.47 and 1.79 eV). They intensively absorb the light up 550 nm and also exhibited a week absorption band in the range of 550-750 nm. Their hole transporting ability was tested in perovskite solar cells. Additionally, the effect of the doping concentration of Li+ on photovoltaic device performance for these compounds was investigated. It should be stressed found that 9,9'-bifluorenylidene derivative substituted with ethyl units applied as hole transporting materials in perovskite solar cells demonstrated the highest device efficiency of 7.33% higher than of the spiro-OMeTAD utilized for preparation of the reference cell (4.40%).