TiO2 Photoelectrode Band Gap Modification Using Carbon Quantum Dots (CQDs) for Dye-Sensitised Solar Cells (DSSCs)

In this study, a CQDs at different concentration is used to modify the TiO2 photoelectrode band gap which can improve light absorption of DSSC. The photoelectrode is immersed in different CQDs concentration at 2.5, 5.0, 7.5 and 10 mg/ml to study the effect on TiO2. It was found that photoelectrode with 7.5 mg/ml CQDs was successfully narrowing the TiO2 band gap and generated the highest photocurrent and power conversion efficiency at 17.06 mA/cm2 and 7.23% respectively, compared to pristine TiO2 (PT) at 10.94 mA/cm2 and 4.63% . The band gap narrowing mechanism for CQDs- TiO2 is obtained from the Tauc’s plot method using absorption spectra. The result shows a pristine TiO2 photoelectrode (PT) band gap is 3.38 eV, upon existing of CQDs, the band gap of all photoelectrodes with CQDs at 2.5, 5.0, 7.5 and 10 were reduced to 3.30 eV, 3.28 eV, 3.09 eV, and 3.29 eV respectively. PG 7.5 cell with lowest band gap at 3.09 eV generates effective electron transport from N719 dye to CQDs/ TiO2 layer compared to other photoelectrodes. The band gap narrowing effect is attributed from chemical bonds of Ti-O-C molecules between CQDs/TiO2. Thus, extra energy states are introduced between CQDs and TiO2. The location of these energy will present a quantum confinement effect which narrow the CQDs-TiO2 band gap which extend the light absorption to visible region.