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Photo Conversion Efficiency of Solvent Engineered and Nitrogen Functionalized Graphene Quantum Dots Sensitized Solar Cells. We discussed compositional, optical and electronic properties of N-GQDs synthesized in different solvents, and their role in the photoconversion efficiency of quantum dots sensitized solar cells (QDSSCs). Solar cells fabricated from R-GQDs synthesized in DMF solvent show much superior performance (∼5.3%) compared to the solar cells fabricated from G-GQDs (∼0.7%) synthesized in water solvent, which is attributed to higher Jsc, Voc and fill factor. [Display omitted] • N-GQDs are used as non-toxic and highly-stable photosensitizers in QDSSC. • N-GQDs were synthesized in different solvents i.e·H 2 O (G-GQDs) and DMF (R-GQDs) • Solvents used strongly influence photo-physical and electronic properties of N-GQDs. • R-GQDs facilitate more light absorption and better charge transport than in G-GQDs. • R-GQDs used as photosensitizer in QDSSC exhibit photoconversion efficiency of > 5% W e report for the first time that the choice of solvent used in the synthesis of nitrogen-doped graphene quantum dots (N-GQDs) has a significant effect on the photovoltaic performance of quantum dot sensitized solar cells (QDSSC) where N-GQDs work as photosensitizers. Switching from protic water (H 2 O) solvent to aprotic N-N dimethylformamide (DMF) causes significant change in structural, photophysical and electronic properties of N-GQDs working as active materials in QDSSC. Unlike traditional dye-sensitized solar cells, N-GQDs replace toxic and readily degradable organic dyes as an active material, due to enhanced stability, cost-efficient fabrication, and environment-friendly processing. We discussed compositional, optical, and electronic properties of N-GQDs synthesized in different solvents, and their role in the photoconversion efficiency of quantum dots sensitized solar cells. Spectroscopic results reveal that nitrogen-doped graphene quantum dots with green emission (G-GQDs) have relatively shorter effective conjugation length and lower content of pyrrolic nitrogen groups because water is a polar protic solvent, which suppresses the water elimination reaction by donating protons to the reaction medium. On the contrary, DMF, a polar aprotic solvent, cannot donate hydrogen, leading to enhanced nitrogen units in the graphene quantum dots framework and red emission (R-GQDs), thus improving carrier density and transport properties. QDSSCs fabricated from R-GQDs exhibit photovoltaic performance (PCE > 5.0%) superior to other previously reported GQD sensitized solar cells. [ABSTRACT FROM AUTHOR]

A unique delay-dependent dynamic switching of optical nonlinearity in terms of saturation absorption (SA) and excited state absorption (ESA) in graphene oxide (GO) and reduced graphene oxide (RGO2 and RGO100) is achieved with an optical pump and white light super-continuum probe only above a threshold pump intensity. Infrared and chemical reduction are used to obtain RGO2 and RGO100 respectively. The switching regime of probe wavelength can be modulated by varying either pump wavelength or degree of reduction. When pumped at 415 nm, the threshold pump intensity to obtain switching property decreases to 9 GW/cm 2 for RGO2 from 18 GW/cm 2 in GO and the tunability range shifts from 471 to 526 nm for as grown GO to 519–623 nm in maximally reduced RGO2. Though the saturation intensity of intrinsic non-degenerate two photon absorption (nd-TPA) is found to be lower in GO (4.3 GW/cm 2 ) than RGO2 (18.2 GW/cm 2 ), nd-TPA coefficient increases from 0.0015 cm/GW (GO) to 0.0026 cm/GW (RGO2) with increasing reduction. The detailed kinetics of the scattering processes show variation as a function of pump power, probe wavelength and degree of reduction. A model band diagram based on amorphous-carbon model and a Tauc analysis are used to explain the unusual nonlinear optical properties. [ABSTRACT FROM AUTHOR]

Utilizing the superior luminescence properties and high color purity of inorganic perovskite nanocrystals (NCs), we report the fabrication of color-saturated CsPbBr 3−x I x (x = 0–3)/ZnO heterojunctions based white light emitting diodes (LEDs) on a flexible platform. Here, bandgap engineered mixed halide perovskite NCs have been prepared by solution phase synthesis technique that was spin coated on sputtered ZnO film, acting as an electron injector layer as well as an efficient luminescent material. Broad (~ 350–700 nm) electroluminescence (EL) emission features covering the near UV and entire visible region of the electromagnetic spectrum have been successfully achieved at an ambient (humidity ~ 50–55%) condition with comparatively lower forward bias (V th ~ 2 V). Finally, we demonstrate the significant enhancement (up to ~ 75%) of EL intensity by applying an external stress owing to the piezoelectric properties of ZnO films. The modulation of energy band alignment in CsPbBr 3 /ZnO heterojunction due to the local strain induced positive piezo-potential in ZnO leads to an enhanced radiative recombination efficiency in CsPbBr 3 domain, resulting in superior EL emission characteristics. The combinatorial fascinating optical properties of inorganic perovskites and piezo-phototronic effect of ZnO provide a promising platform to fabricate highly luminescent LEDs that may find perspective applications in high-resolution flexible display devices and wearable electronics. [Display omitted] • Band gap engineered mixed halide inorganic perovskite NCs (CsPbBr 3−x I x (x = 0–3)) by solution phase synthesis technique. • Perovskite/ZnO p-n junction based moisture resistant broad white LEDs under fairly lower forward bias of ~ 2V. • Successful tuning of EL emission by increasing the I-/Br- ratio of mixed halide perovskites through anion exchange reaction. • Enhancement of EL intensity up to ~ 75% by applying an external stress owing to the piezoelectric properties of ZnO films. [ABSTRACT FROM AUTHOR]

Abstract Highly luminescent, crystalline graphene quantum dots (GQDs) of homogenous size and shape with high yield have been successfully synthesized by a one-pot, facile and rapid synthesis technique. The characteristic blue emissions of GQDs from the crystalline sp2 graphene core could be tuned from green to yellow wavelength, by modulating sp3-oxygeneous surface states. The progressive evolution of surface functional groups on annealing at different temperatures has been corroborated by X-ray photoelectron spectroscopy (XPS). Temperature dependent as well as excitation dependent photoluminescence (PL) behavior of the samples exhibits distinctly different thermal quenching characteristics. A transition from positive to negative thermal quenching phenomena with progressive annealing up to 300 οC and crossovers at intermediate temperatures have been observed. With the help of schematic energy band diagram, we have explained the novel fluorescence mechanism in GQDs. Au/GQD/Au UV-photodetector device has been fabricated and its performance has been studied. Tunable emission of solution processed GQDs from UV to red wavelength can be very attractive for their use in bio-imaging and optoelectronics devices. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]