Efficient Multiple Exciton Generation in Monolayer MoS2.

Utilization of the excess energy of photoexcitation that is otherwise lost as thermal effects can improve the efficiency of next-generation light-harvesting devices. Multiple exciton generation (MEG) in semiconducting materials yields two or more excitons by absorbing a single high-energy photon, which can break the Shockley-Queisser limit for the conversion efficiency of photovoltaic devices. Recently, monolayer transition metal dichalcogenides (TMDs) have emerged as promising light-harvesting materials because of their high absorption coefficient. Here, we report efficient MEGs with low threshold energy and high (86%) efficiency in a van der Waals (vdW) layered material, MoS2. Through different experimental approaches, we demonstrate the signature of exciton multiplication and discuss the possible origin of decisive MEG in monolayer MoS2. Our results reveal that vdW-layered materials could be a potential candidate for developing mechanically flexible and highly efficient next-generation solar cells and photodetectors.

Through structural isomerism: positional effect of alkyne functionality on molecular optical properties.

Literature studies on the effects of alkyne functionality in manipulating the optical properties of donor-π-acceptor-type molecular scaffolds have been scarce compared to those on the alkene functional group. Here, two structurally isomeric donor-acceptor (D-A) dyes were synthesized to study the positional effect of alkyne functionality (triple bond) on their optical, electrochemical and charge generation properties in order to design efficient dyes for possible application in dye sensitized solar cells (DSSCs). These dyes, named CAPC and PACC, contain carbazole and cyanoacrylic acid as the donor and acceptor units, respectively, and the π-conjugation length within the molecules was controlled by the introduction of an alkyne group. The D-π-A design was followed in designing CAPC with the alkyne serving as the π-spacer, while in PACC, alkyne was placed on the donor, which was directly in conjugation with the acceptor. This rendered equal conjugation lengths within the designed dyes. With the help of photophysical characterizations, it was concluded that CAPC featured better characteristics for a DSSC dye than PACC. Our conclusions were further supported by the results of transient absorption spectroscopy, electrochemical analysis, fluorescence lifetime studies and density functional theory.

Nonlinear Optical Properties of CdSe Quantum Dots Having Different Sizes.

We synthesis different sizes of cadmium selenide quantum dots (CdSe QDs) and their linear and nonlinear optical properties. Size-controlled CdSe QDs are characterized by ultraviolet-visible absorption, photoluminescence, transmission electron microscope (TEM), and Z-scan techniques. Redshift was observed in the linear absorption and photoluminescence with an increase in the size of CdSe QDs. TEM images show the sizes of the QDs are 2.2 nm-4.9 nm. Bandgap tuning CdSe QDs of third-order nonlinear optical properties are investigated at 532 nm with an open aperture Z-scan technique using a picosecond laser. These CdSe QDs can be used as optical limiters.