Graphene quantum dots (GQDs) have evolved as a new kind of zero dimensional (0D) carbon-based nanomaterial, consisting of 2-10 monolayer graphene with size ~30 nm. These QDs have unique optical and electronics properties and have been applied in a wide range of applications such as solar cells, photocatalysis, photoelectrochemical (PEC), bioimaging, metal ions detection, light emitting diode (LED), photodetector and electroluminescence devices. More importantly, the optical absorption and emission of GQDs can be tuned in broad spectral range (400-700 nm) by simply doping several elements like S, N, B, F, etc. A group of researchers from the National Institute of Technology, India Department of Physics and Department of Electronics and Communication Engineering have improved the PEC properties of taper-like ZnO nanorod photoanodes by C doping as well as sensitization with S, N co-doped GQDs (SNGQDs). ZnO nanotapers (ZnO NTs) and SNGQDs were synthesized by a low-cost hydrothermal process. SNGQDs sensitized ZnO NTs photoanodes have been studied in quantum dot sensitized solar cells (QDSSC).
For DSSC fabrication, SNGQDs decorated C-ZnO nanotaper (SNGQD/C-ZnO NTs) photoanodes were used as working electrodes and Pt nanoparticle coated FTO were utilized as counter electrode. Both electrodes were sealed with hot melt plastic spacers (25-m thick, Solaronix) and iodine-based electrolyte (Iodolyte Z-50, Solaronix) was poured in between the electrodes.
Atomic force microscopy (AFM) images of SNGQDs were recorded in tapping mode with aBruker Nanoscope-8 Microscope. Scanning electron microscopy study (SEM) of ZnO nanotapers were carried out using a ZEISS SUPRA 40 electron microscope. Transmission electron microscopy (TEM) studies of SNGQDs and ZnO nanotapers were performed using a JEOL JEM-2100F electron microscope. UV-visible absorption spectra were measured in the wavelength range 200-800 nm with a PerkinElmer Lambda 950 spectrometer. Room temperature photoluminescence (PL) measurements were carried out under various excitation wavelengths (340-560 nm) using a PerkinElmer LS 55 fluorescence spectrometer. The chemical composition and bonding of SNGQDs and C-doped ZnO NTs were studied by an X-ray photoelectron spectrometer (XPS) (PHI 5000 Versa Probe II). The current-voltage characteristics (J ?V) of the solar cell devices were measured using a source meter (B2912A, Agilent, USA) under illumination of 100 mW/cm2 light (AM 1.5) from a solar simulator (Enlitech, Taiwan).
The asgrown SNGQDs consisted of 1-4 monolayer of graphene with average diameter 4-25 nm. The chemical bonding and existence of S and N in GQD network was investigated from XPS measurements. UV-visible spectrum of SNGQDs showed four absorption peaks located at 261 nm, 341 nm, 469 nm and 552 nm. These peaks are attributed to the electronic transitions from π?π* in sp2 C=C, n?π* in C=O, n?π* in C=N and n?π* in C=S bonds, respectively. SNGQDs are highly luminescent and demonstrated excitation dependent photoemission in visible spectrum.
To improve the PEC activities of ZnO nanostructured based photoanodes, SNGQDs were attached with C-doped ZnO nanotapers. SNGQD sensitized C-ZnO NTs demonstrated superior photoconversion efficiency (η ~1.46%) and incident-photon-to-current conversion efficiency (IPCE ~ 92.48% at 340 nm) compared to C-ZnO NTs and undoped ZnO NTs. The photovoltaic performances of such SNGQDs sensitized photoanodes was successfully demonstrated in QDSSC applications. The typical QDSSC device showed JSC~3.04 mA/cm2, VOC~0.45 V and power conversion efficiency ηp~0.59% which has been improved significantly compared to other ZnO-based photoanodes. This present study established the potential use of SNGQDs decorated C-ZnO NTs photoanodes for low cost, eco-friendly quantum dot sensitized solar cell application.