Application of carbon quantum dots for dye-sensitized solar cells

Carbon nanodots were prepared according to our previous report (Loukanov et al., 2016).Citric acid (1 g) was diluted with 10 ml distilled water and ethylenediamine (0.2 ml, 0.18 g) was injected to the solution under vigorous stirring. The clear transperant solution mixture became a yellowish brown gum after microwave irradiation for 3 minutes at microwave oven (750 W). The obtained yellowish brown solid was dissolved in Milli-Q and dialyzed against pure water through a dialysis membrane. The diazotization reaction was performed in ice bath at temperature between 0 and 5 °C. While 20 mL of concentrated hydrochloric acid was diluted with about 60 g of crushed ice to which 2.5 g sodium nitrite dissolved in 10 mL of water was added. 5 – 7 mL of such prepared nitrous acid (blue color) was added to 20 mL solution of 5-amino–fluorescein with concentration 5 mg/mL (the color of precursor solution was changed to orange – yellow). The prepared diazonium ion was slowly added dropwise to 20 mL solution of naked CDs (with concentration 20 mg/mL) at alkaline pH (pH ~ 9). The color mixture was changing from light yellow to dark red. The obtained Fluorescein–N=N–CDs were purified from the reaction mixture by centrifugation and washed with acetone (as described above). 200 mg carbon nanodots modified with diazofluorescein (or Fluorescein–N=N–CDs) were dissolved in 2 mL MilliQ water. The prepared aqueous solution was injected in a triple flask, which contains 1 mmol metal chloride (iron or copper) in 50 mL solution of ethylene glycol under Ar atmosphere. The reaction mixture was refluxed for 4 hours at ~ 180 – 190 °C. The color changed from a dark black to a bright yellow in approximately two hours. The resulting solution was then cooled down to room temperature and analyzed. The obtained [Fluorescein–N=N–CDs]Me²⁺ nanoparticles were purified from the reaction mixture by centrifugation and washing with acetone.

The as-prepared TiO₂ electrode and 22 mg of C-dots were put into 50 ml of distilled water. Then they were hydrothermally treated at 100 °C for 4 h. After that, the C-dots sensitized TiO2 electrodes were rinsed with absolute ethanol for several times and dried under vacuum. The liquid electrolyte contained: (i) 0.6 M of tetrapropylammonium iodide, 0.1 M of I₂, 0.1 M of KI, 0.5 M of 4-tert-butylpyridine in acetonitrile, or (ii) aqueous buffer solution saturated with oxygen at alkaline pH. QDSSCs were assembled by dropping a drop of liquid electrolyte above the C-dots sensitized TiO₂ porous film electrode. A graphite counter electrode was placed above it. The two electrodes were clipped together and an adhesive was used as sealant to prevent the electrolyte solution from leaking. The device structure of QDSSCs is shown on Fig. 2. I-V measurements were obtained by using a solar simulator (Newport) with an AM 1.5 G filter under an irradiation intensity of 100 mW cm^–2. The light intensity was calibrated using a standard silicon photovoltaic solar cell. The active cell area was 0.15 cm².

The luminescence studies on dyes adsorbed onto semiconductor electrodes have shown that the excited states could be efficiently quenched on these surfaces. With semiconductors, oxidation of the dye takes place through transfer of an electron from a molecule’s excited energy level to the conduction band of the semiconductor. In an electrochemical cell using semiconductor as bulk electrodes, the excited-state charge injection manifests itself as photocurrents, measurable quantitatively under anodic polarization. Exposure of the classic dye-sensitized solar cell assembly to visible light lead to a sequence of reactions. Figure 2 shows schematically these processes. We first consider the reactions that take place at the anode, where the absorption of the light by the dye S leads to formation of its electronically excited state S*:

The injected electrons travel through the mesoporous network of particles to reach the back-collector electrode to pass through the external circuit. The oxidized dye is reduced rapidly to the ground state by the donor (iodide) present in the electrolyte:

The spectral response of the dye-sensitized solar cell depends on the absorption properties of the dye. Characterization of the cell depends on a number of experimentally accessible parameters, including the photocurrent and photopotentials measured under different conditions (open and closed circuit, under monochromatic light or sunlight illumination): I_oc, V_oc, I_sc and V_sc. The term incident photon-to-electrical conversion efficiency is a quantum-yield term for the overall charge-injection collection process measured using monochromatic light (single wavelength source).

The solar-cell current voltage and associated parameters show that all hybrid nanocomposite improve the efficiency compared to the uncoated (TiO₂) devices. QDSSCs device has the highest efficiency owing to a combination of relatively high open-circuit voltage (V_oc) and short-circuit current (J_sc) associated with relatively low series resistance (R_s) and high shunt resistance (R_sh), as it is shown in Table 1:

Sample J_sc V_oc FF PCE Rs Rsh

[mAcm^–2] [mV] [%] [k] [k]

Uncoated 0.083 15 0.34 0.0004 0.27 0.30

C-dots 0.690 440 0.42 0.1120 0.85 9.18
The power conversion efficiency of 0.112 % for the QDSSCs device is the best one obtained in our experiments.

Conclusion
We have developed C-dots/TiO₂ quantum dot-sensitized solar cells, which showed a synergetic power conversion activity, due the doping of nitrogen in the nanoparticles. This is attributed to the lower work function, induced by N doping. So, the performance of the device is depended on the absorbance and functional groups on the carbon nanodot surface.
References
Li, H., Kang, Z., Liu, Y., Lee, S-T., (2012) Carbon nanodots: synthesis, properties and applications. J. Mater. Chem. 46, 24230-24253.

Loukanov, A., Sekiya, R., Yoshikawa, M., Moriyasu, Y., Nakabayashi, S., (2016) Photosensitizer-conjugated ultrasmall nanodots as multifunctional fluorescent probes for bioimaging. Journal of Phys. Chem.

Mirtchev, P., Henderson, E., Soheilnia, N., Yip, C., Ozin, G., (2012) Solution phase synthesis of carbon quantum dots as sensitizers for nanocrystalline TiO₂ solar cells. J. Mater. Chem. 22, 1265-1269.

Zhang, Y. et al., (2013) N-doped carbon quantum dots for TiO₂-based DSSC, Nano Energy, 5, 545-552.