Nitrogen-Doped Carbon Dots for “green” Quantum Dot Solar Cells
© Wang et al. 2016
Received: 1 October 2015
Accepted: 5 January 2016
Published: 19 January 2016
Considering the environment protection, “green” materials are increasingly explored for photovoltaics. Here, we developed a kind of quantum dots solar cell based on nitrogen-doped carbon dots. The nitrogen-doped carbon dots were prepared by direct pyrolysis of citric acid and ammonia. The nitrogen-doped carbon dots’ excitonic absorption depends on the N-doping content in the carbon dots. The N-doping can be readily modified by the mass ratio of reactants. The constructed “green” nitrogen-doped carbon dots solar cell achieves the best power conversion efficiency of 0.79 % under AM 1.5 G one full sun illumination, which is the highest efficiency for carbon dot-based solar cells.
KeywordsCarbon dots Nitrogen-doped Quantum dot solar cells
Among the third-generation photovoltaics, quantum dot solar cells (QDSCs) are emerging as a promising candidate due to the unique and versatile characteristics of quantum dots (QDs) including tunable band gap and high absorption coefficient [1–4]. Typically, low-band gap metal chalcogenide (CdS, CdSe, CdTe, PbS, PbSe, CuInS2, etc.) QDs are widely used as sensitizers in QDSCs [5–10]. The QDSCs have been well developed, but they contain highly toxic metals (including Cd, Pb, and In). Hence, the environmentally friendly alternatives (“green” materials) are extremely needed and welcomed to the fabrication of solar cells.
Due to the unique optical and electronic properties, water solubility, excellent biocompatibility, low toxicity, and robust chemical inertness [11–15], carbon dots (CDs) as an emerging carbon-based nanomaterial have been widely researched and applied in many fields [16–22]. One of the CDs applications is tentatively used as sensitizers for QDSCs [23–26]. For instance, Mirtchev et al.  have prepared water soluble CDs as sensitizers for QDSCs achieving power conversion efficiency (PCE) of 0.13 %. Zhang et al.  have also fabricated 0.13 % PCE of QDSCs based on nitrogen-doped CDs (NCDs). Up to now, the low short-circuit current density (J sc) might be the main factor limiting the efficiency of the cells in comparison with the open-circuit voltage (V oc) and fill factor (FF) of metal chalcogenide-based QDSCs. Narrow light-absorption and diverse trap states result in the extremely low J sc of CD-based QDSCs. Meanwhile, CDs’ light-absorption is centered at ultra-violet region and very weak at visible region. Because the existence of trap state defects with different energy levels has been demonstrated by their excitation-dependent fluorescence , partially photoinduced carriers will be recombined by CDs rather than being injected into the transporting media. Accordingly, the PCE of QDSCs is very low. Much effort still needs to improve the practicable sensitizers for QDSCs.
In this study, we have developed NCDs to build up the “green” QDSCs with enhanced efficiency. The NCDs were prepared by direct pyrolysis of citric acid and ammonia. Experimental results show that the NCDs’ excitonic absorption depends on the N-doping content modified by the mass ratio of reactants. The NCDs with optimal excitonic absorption were used as sensitizers in the QDSCs. The constructed NCD solar cell achieves the best PCE of 0.79 % under AM 1.5 G one full sun illumination. It is noting that the obtained efficiency is the highest one in the reported solar cell-based CDs.
Fabrication of Solar Cell
The NCD-sensitized TiO2 photoanodes are fabricated according to the previous report . The cleaned fluorine-doped tin oxide (FTO) glasses are immersed in 40 mM TiCl4 aqueous solution at 70 °C for 30 min and washed with water and ethanol. A 20-nm-sized TiO2 paste is deposited on the FTO glass plate by doctor blade printing technique and then dried at 125 °C for 5 min. The scattering layer of 200-nm-sized TiO2 paste is coated on the top of the first TiO2 layer, followed by sintering in air at 500 °C for 30 min. After cooling to 95 °C, the TiO2 film is immersed in the NCQD acetone solution at room temperature for 2 h. The NCD-sensitized TiO2 is washed with anhydrous ethanol and dried with nitrogen stream. The solar cells are fabricated by sandwiching gel electrolytes between a NCD-sensitized TiO2 electrode and a Pt counter electrode, which are separated by a 25-mm-thick hot-melt ring (Surlyn, Dupont) and sealed by heating. The electrolyte injection hole on the thermally platonized FTO counter electrode is finally sealed with Surlyn sheet and a thin glass by heating.
Characterization and Measurement
The morphology of the materials is investigated by transmission electron microscopy (TEM, FEI Tecnai G-20) and atomic force microscopy (AFM, Asylum Research MFP-3D-BIO). Fourier transform infrared (FT-IR) spectra are obtained on a Bruker VERTEX70 FT-IR spectrometer ranged from 4000 to 400 cm−1. X-ray photoelectron spectra (XPS) are acquired with a Japan Kratos Axis Ultra HAS spectrometer using a monochromatic Al Kα source. The crystallinity of NCDs were characterized by Raman spectroscopy (HORIBA Jobin Yvon HR800) using 514-nm laser as the excitation source. The UV–visible absorption spectrum and diffuse-reflectance spectra are recorded on a UV2501PC (Shimadzu). The photoluminescence (PL) spectra are recorded on a Fluoromax-4 spectrofluorometer (HORIBA JobinYvon Inc.) equipped with a 150 W of xenon lamp as the excitation source. Photocurrent–voltage measurements of solar cells (Keithley2440 sourcemeter) are obtained by using a solar simulator (Newport) with an AM 1.5 G filter under an irradiation intensity of 100 mWcm−2 (550-W Xe source, Abet). The light intensity is calibrated using a standard silicon photovoltaic solar cell. The active cell area is 0.25 cm2. All of the measurements above are performed at room temperature.
Results and Discussion
Relative elemental analysis of the CDs with different N-doping levels by XPS analysis
In summary, we have developed “green” quantum dot solar cells based on nitrogen-doped carbon dots. The optimal NCD-based solar cells achieve a PCE of 0.79 % with J sc of 2.65 mA cm−2, V oc of 0.47 V, and FF of 62.5 %. The obtained PCE is the highest value in the reported QDSCs based on CDs. The study demonstrates that the further optimization of nitrogen dopant can gain a better conversion efficiency of NCD-based solar cells.
We gratefully acknowledge the support from “973 Program – the National Basic Research Program of China” Special Funds for the Chief Young Scientist (2015CB358600), the Excellent Young Scholar Fund from National Natural Science Foundation of China (21422103), Jiangsu Fund for Distinguished Young Scientist (BK20140010), Jiangsu Specially-Appointed Professor Program (SR10800412), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Jiangsu Scientist and Technological Innovation Team (2013).
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