Carbon-Dot/Natural-Dye Sensitizer for TiO2 Solar Cells Prepared by a One-Step Treatment of Celery Leaf Extract

Attempts have been made to create artificial photovoltaic (PV) devices that can harness sunlight in a clean and efficient way by mimicking natural photosynthesis processes. Inspired by the role of chlorophyll in photosynthesis, we introduced pheophytin- or pyropheophytin-capped carbon dots (CDs) into the dye-sensitized solar-cell system. The CD hybrids were synthesized by means of a single-step hydrothermal treatment of celery leaf extract. The effects of hydrothermal temperature on the physical properties of CDs and the survival rate of the natural dyes were studied. CDs heated at 120 degrees C (C-120) display a relatively homogenous smaller particle size distribution and lower photoluminescence emission intensity relative to their counterparts obtained at higher temperatures. Heated at lower temperatures, the achieved CD hybrids contain a higher amount of surviving dye molecules which would be converted into pheophytins or pyropheophytins. The nanocrystalline TiO2 solar cells based on C-120 gave the highest power conversion efficiency (PCE) of 0.48% under AM 1.5G one full sun illumination, which can be compared favorably to the average value of 0.20% normally reported for other biomass-derived carbon-dot-sensitized devices. Besides functioning as a light sensitizer, C-120 CDs with a lower amount of trap states can also work as an effective electron-transfer intermediate to improve the interaction between the dye and TiO2 and suppress the photogenerated carrier recombination, thus improving solar-cell performance.