Influence of carbonyl group on photocurrent density of novel fluorene based D-π-A photosensitizers: Synthesis, photophysical and photovoltaic studies

Herein, we developed novel and promising low-cost metal free organic dyes 7a-c and 10a-c via a simple, highly efficient, and high yield through two-step methodology based on Suzuki coupling reaction. These new dyes characterized with D-π-A architecture, utilized a binary π-conjugated spacer comprised of fluorenone and thiophene or spiro fluorene and thiophene. This construction would help in studying the impact of π-spacers on the photovoltaic (PV) parameters of fabricated DSSCs. The highest photocurrent efficiency (PCE) of 3.5%, JSC (7.80 mA cm−2), VOC (0.62 V) and FF (69.11%) was achieved by photosensitizer 10b incorporating spiro fluorene π-spacers moiety compared to PCE of 2.70%, JSC (6.45 mA cm−2), VOC (0.57 V) and FF (71.50%) for dye 7b with fluorenone π-spacers at the AM1.5 G conditions and using a volatile redox electrolyte (iodide/triiodide). DFT/TD-DFT calculations were used to clarify the relation between the molecular structure, photophysical and photoelectrochemical properties utilizing GAUSSIAN 09 programming. Interestingly, DFT results showed a distribution of a frontier LUMO isosurface between the carbonyl group of fluorenone and the acceptor part in case of photosensitizers 7a-c compared to 10a-c, which shows a complete frontier LUMO shift to the acceptor part only. This confirms the effect of carbonyl group on trapping the electrons. This causes a weak electron injection into TiO2 surface, which lowered the photocurrent conversion efficiency of 7a-c dyes compared to 10a-c. The dyes structures were confirmed by UV–vis, emission spectrophotometry, FT-IR, ESI-MS and 1H-NMR.