Molecular design and synthesis of diphenylamine based metal-free organic chromophores for dye sensitized solar cells (DSSCs)

Abstract

Dye-sensitized solar cell (DSSC), which belongs to third generation solar cells has emerged as an attractive and promising low-cost solar device for harvesting solar energy. Grätzel and O’Brian invented DSSC in the year 1991, from that onwards it has been a promising technology attracted by both academic and industrial clad. Sensitizer is one of the vital components of DSSCs. It absorbs solar energy effectively to inject electrons into the TiO2 layer thereby producing electric energy. Here, sensitizer (dye) plays a crucial role in achieving high injection rate, thereby increasing the efficiency of the cell. Among the metalfree sensitizers n- type organic chromophores are of great importance. Even though, there are lots of studies on-going in this field, there is an ample scope for investigation of efficient sensitizers as they affect directly on PCE. Also, development of novel co-sensitizers for DSSCs sensitized with metal-based dyes, is an equally important area of research study. In this context, it was planned to design, synthesize and investigate the photovoltaic performance of new organic dyes based on diphenylamine as n-type sensitizers/co-sensitizers for DSSCs. Based on the detailed literature survey, twenty seven new n-type organic diphenylamine based sensitizers/co-sensitizers were designed using various molecular engineering strategies. They were successfully synthesized following appropriate synthetic protocols. Further, they were well-characterized by (FTIR, NMR and MS) spectral, optical and electrochemical analyses. The results revealed that, all the new dyes displayed the λabs and λemi in the range of 350-560 nm and 430-690 nm, respectively. Their band gaps were calculated to be in order of 1.90-3.09 eV. DFT study has been employed to optimize the molecular geometries and to apprehend the effect of structures of organic sensitizers/cosensitizers on their photovoltaic performances in devices. Also, TD-DFT simulations were carried out for the selected dyes. Finally, the synthesized dyes were subjected to the photovoltaic studies as sensitizers/co-sensitizers in DSSCs. Among newly synthesized organic sensitizers, the cell fabricated with D22 carrying cyanoacetic acid as an anchoring unit displayed the optimum PCE of 5.909 %. Whereas, the co-sensitization studies indicated that, under the same conditions, the device co-sensitized using D26 with HD-2 sensitizer showcased upheld PCE of 10.55 % showing an increment of 3.18 %. To sum up, by appropriately optimizing the molecular structures of organic chromophores, it is possible to further ameliorate the photovoltaic performance of the cells.