Effect of carbon dot on photovoltaic performance of n-TiO2/p-NiO and n-TiO2/p-CuO heterojunctions in Dye-Sensitized solar cells

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Carbon quantum dot (C-dot) behaves as a photosensitizer and dye-adsorber. Here, effect of C-dot was investigated on dye (N719)-photosensitized solar cells (DSSCs) consisting of n-type TiO2 and p-type metal oxide (NiO or CuO) photoanodes. Power conversion efficiency (PCE) of pristine TiO2 DSSC was enhanced double with adding 10 wt% NiO. Furthermore, the doping of 10 wt% C-dot into TiO2/NiO(10 wt%) nanocomposites increased PCE up to 2.88 times. Then the 14.32% PCE was achieved, since coexistence of C-dot and N719 accelerates the faster and rich charge separation and charge transfer at TiO2/NiO heterojunction. On the other hand, PCE of DSSC consisting of n-TiO2 and p-CuO semiconductors was slightly higher than those of n-TiO2/p-NiO DSSC, but PCE of n-TiO2/p-CuO DSSC decreased after Cdot was added. The calculated energy level alignments exhibit that electrons photogenerated on Cdot and N719 successfully transfer to cathode through NiO and TiO2 in TiO2/NiO/C-dot DSSC, while electrons in a conduction band of TiO2 are recombined with holes in HOMO of C-dot with similar energy level in TiO2/CuO/C-dot nanocomposites. This investigation demonstrates that C-dot provides photo-generated charges and accelerates charge transfer, but it also raises charge recombination, depending on energy level balance with semiconductor components and influencing the performance of DSSC.