Molecular engineering and investigation of new efficient photosensitizers/co-sensitizers based on bulky donor enriched with EDOT for DSSCs

Abstract

Herein, we report design, synthesis and photovoltaic performance of four novel metal-free heteroaromatic photosensitizers coded IA 1-4 with (D-D)2-D-A architecture carrying electron donating triphenylamine coupled with EDOT core which is directly connected to four different electron withdrawing/anchoring groups, viz. rhodamine-3-acetic acid, cyano acetic acid, 2-methyl quinoline-6-carboxylic acid and 1-phenyl-pyrazol-5-one-3-carboxylic acid without any π-spacer. The newly designed IA 1-4 were applied as sensitizers and co-sensitizers in DSSCs. Their structures were confirmed by FT-IR, 1H NMR, MS and elemental analyses. The photosensitizers were subjected to optical and electrochemical studies in order to investigate their absorption/emission behavior as well as HOMO/LUMO energies. The UV–Vis revealed that IA 1-4 exhibited at 464, 497, 531 and 412 nm,λmax respectively. Their optical band gap is in the range of 1.95–2.28 eV. From the energy level diagram of the named photosensitizer, it is clear that all the dyes have good thermodynamically favorable ground and excited state oxidation potentials for electron injection into CB edge of TiO2 as well as dye regeneration. The photovoltaic performance studies indicate that, dye IA-1 anchored with cyanoacetic acid displayed the highest IPCE (61.5%), resulting in PCE of 5.92% (JSC = 12.3 mA cm−2, VOC = 0.68 V, FF = 70.29%). Furthermore, when IA-1 was used as a co-sensitizer with MH-13, a total PCE of 8.43% (JSC = 22.8 mA cm−2, VOC = 0.66 V, FF = 54.1%) was achieved to gain more insights into the equilibrium molecular geometry (EMG), thermodynamic parameters and vertical electronic excitation, DFT studies were performed using GAUSSIAN 09 software to calculate the EMG of each photosensitizer, its and electron cloud delocalization of HOMO/LUMO levels and vertical electronic excitation. Results from FMO revealed that dyes IA 1-4 showed effective charge separation, which translated into greater ICT behavior with IA-1 being the highest in photovoltaic performance.