Analytical Study of Porosity, Dispersion Energy and Dielectric Energy Losses on Performance of Annealed TiO2 - Graphene Dye Sensitized Solar Cell

Abstract

Recent advances in technology have sparked interest in improving the efficiency of dye-sensitive solar cells (DSSC) with great progress, to mitigate challenges such as high recombination rates and low quantum yield. The present study demonstrates a facile synthesis of modified Titanium dioxide (TiO2) compact layers on fluorine tin oxide (FTO) and graphene using the Sol gel Doctor Blade deposition method for solar applications. As deposited films and films subjected to one-step annealing, annealing rates of 1 °C/min and 2 °C/min were studied utilizing a range of techniques, including a Varian 7000e FTIR, UV-VIS spectrophotometer, a Scanning Kelvin Probe Microscope (SKPM), and a Hall Effect setup. FTIR demonstrated considerable absorption at low frequencies (less than 798 cm-1) in TiO2 on graphene heterojunctions, confirming the production of Ti-O and C-O-Ti bonds. The major anatase TiO2 characteristic was detected at 438 cm-1. All films had reflectance between 25 - 30 % in the visible spectrum (550 nm), in accordance to the TiO2 on FTO examination, and average refractive indices ranged from 1.95 to 1.56. The lowest porosity (53%) and maximum dispersion energy (11.30 eV) were found in the film annealed at 1 °C/min. The surface-to-volume energy loss ratio was least for 1oC annealing rate. TiO2 on graphene annealed at a rate of 1 oC/min exhibited a lower VELF/SELF than TiO2 on FTO, implying that an electron dissipates less energy when passing through the TiO2 on graphene film than it does in TiO2 on FTO. The light absorption coefficient (α) and electron diffusion coefficient (D) of TiO2 on graphene rose to 4.637 x 103 and 1.485 x 10-4 (1 oC/min), respectively, whereas TiO2 on FTO values increased to 4.221 x 103 and 1.251 x 10-4 (1 oC/min), in that order. Higher α and D values of TiO2 on graphene indicate enhanced electron transition in the films. Hall Effect measurements on as-deposited and annealed TiO2 on graphene films demonstrated enhanced conductivity at 1 oC/min annealing rate, which was attributed to recrystallization of films due to calcination. Work functions (φ) measurements were -730.8 mV, - 436.3 mV, and -189.2 mV, which reveal an increasing trend towards 1 oC/min annealing rate. This spatial variation and distribution are explained by ensemble modifications to granular tilts and surface slopes using a Smoluchowski smoothing model. The spatially varying levels of electric dipole moments, which are essential to atomic steps, alter φ and induce local fluctuations. The incorporation of TiO2 on graphene photoanode improved h+/e- separation, electron transport, and light absorption due to the high specific surface area of the porous structure and the continuous conduction network on compact TiO2 nanoparticles on graphene acting as an electron leakage barrier. C1 (FTO/TiO2-Pt/FTO), C2 (Gr/TiO2-Pt/FTO), C3 (FTO/TiO2-Pt/Gr), and C4 (Gr/TiO2-Pt/Gr) DSSCs were fabricated (at 1 oC/min) and labeled. Isc and Voc average values were found to be 8.1925 mA and 0.6375 V, respectively. C4 had the highest PCE (%) = 2.973 (3.4 % enhancement on C1). Lower efficiencies noted on C1, C2 and C3 DSSCs could be due to the relatively poor adhesion of TiO2 on FTO resulting to faster back reaction and electron recombination over transport. Tuning annealing rates and coupling TiO2 to graphene improves both dye active sites and crystallinity. Flexible TiO2-graphene nanosheets lower internal resistance and prevent carrier recombination rates. TiO2 on graphene electrode matrix promote C4's power conversion efficiency (PCE) by boosting dye adsorption, electron ejection improving transport.