Understanding structure-property correlation of metal free organic dyes using interfacial electron transfer measurements

Abstract

Triphenylamine-bithiazole based two metal-free organic dyes were designed and synthesized employing D-π-A strategy for dye sensitized solar cells (DSSCs) which differed in the anchoring group attached to it. The photophysical, electrochemical and photovoltaic properties of the dyes were investigated in detail. The HOMO-LUMO analysis proved that the energetics of both the dyes were properly placed with respect to TiO2 and iodide/triiodide electrolyte in order to have efficient electron injection and regeneration, respectively. Photovoltaic characteristics of the dyes were studied in the presence of a coadsorbent exhibiting a maximum efficiency of 4.7%. The electron lifetimes obtained from open circuit voltage decay (OCVD) measurements shows that addition of co-adsorbent CDCA helped in reducing aggregation of both TPA-bithiazole dyes thereby improving the photovoltage for devices with co-adsorbent and a better efficiency in comparison to the dye alone devices. Fundamental insights into the interfacial charge transfer processes occurring in devices were obtained using electrochemical impedance spectroscopy (EIS) measurements. EIS measurements indicated that the device sensitized by the compound having cyanoacrylic acid as anchoring group in presence of coadsorbent exhibited the largest resistance for recombination between the electrons injected to TiO2 and electrolyte leading to a better lifetime and photovoltage.