Low cost and efficient hetero-anthracene based small organic hole transporting materials for solid state photoelectrochemical cells

Abstract

A series of low cost phenothiazine (PTZ) based small organic hole transporting materials (OHTMs) with high conductivity, hole mobility and low crystallinity were synthesized and systematically investigated for solid state photoelectrochemical cells/dye sensitized solar cells (DSSCs). Thermal, optical, photoelectrical and electrochemical properties of all OHTMs were evaluated using thermal techniques, cyclic voltammetry (CV), UV–vis absorption and other techniques. Density functional theory (DFT) was used to calculate optical and electrochemical properties of OHTMs and compared with experimental data. Role of these OHTMs in solid state photoelectrochemical cells with metal-free carbazole dye, SK3 in presence of additives like lithium bis(trifluoromethyl sulfonyl)imide salt (LiTFSI) and tert-butyl pyridine (TBP) are discussed along with the thickness of photoanode. In order to take an insight into photovoltaic performance, electron transfer kinetics and electron recombination in the OHTMs with different thickness of TiO2 thin film based ss-DSSCs were characterized by I–V, electrochemical impedance spectroscopy (EIS) and open circuit voltage decay (OCVD). The hole mobility, conductivity and photovoltaic performance of OHTMs are comparable with commercially available Spiro-OMeTAD. The highest power conversion efficiency (PCE), 4.91% is achieved from thiophene substituted (at 3,6 positions) phenothiazine based SJH-11, is comparable with reference device incorporating Spiro-OMeTAD as HTM with SK3 sensitizer. All the ss-DSSCs with OHTMs exhibit good long-term stability under 1 sun illumination and ambient humidity conditions. The results are promising and have scope for improvement upon optimization of substitution at 3 and 6 positions of PTZ central/core moiety.