Hyuncheol Jeong

Hybrid-type quantum-dot cosensitized ZnO nanowire solar cell with enhanced visible-light harvesting.

A polymer hybrid quantum-dot-sensitized solar cell was developed using CdSe/CdS/ZnO nanowires as a photoanode and regioregular P3HT as a conjugated polymer. The P3HT polymer was used as a hole transport material to replace the liquid electrolyte in quantum dot sensitized solar cells, CdSe/CdS acts as a cosensitizer, which enhances light harvesting in the visible range, and the ZnO nanowires provide a direct pathway for electron transport. Through an adequate cascade bandgap structure of the photoanode, the photoexcited electrons were effectively separated from the electron/hole pairs and transported under illumination. The remaining holes at the anode were transported by the conjugated polymer P3HT without any intermediate potential loss. The fabrication of the hybrid solar cell was optimized with various experimental conditions, including the length of the ZnO nanowires, quantum sensitizers, P3HT filling conditions, and electrolytes. The optimally obtained hybrid solar cells exhibited 1.5% power-conversion efficiency under AM 1.5G of 100 mW/cm(2) intensity. The fabricated hybrid cells exhibited highly durable cell performances, even after 1 month under atmospheric conditions, whereas the liquid junction quantum dot sensitized solar cells exhibited a significant degradation in their performances during the first 2 weeks immediately after fabrication. High open-circuit voltage and fill factor values of our hybrid quantum-dot-sensitized solar cell indicate that the applied hole transport layer efficiently dissociates electron/hole pairs at the interface and retards the interfacial charge recombination.

Irreversible Adsorption Controls Crystallization in Vapor-Deposited Polymer Thin Films

Matrix-assisted pulsed laser evaporation (MAPLE) provides a gentle means for the quasi-vapor deposition of macromolecules. It offers a unique opportunity for the bottom-up control of polymer crystallization as film growth and crystallization occur simultaneously. Surprisingly, with increasing deposition time, it has been shown that crystallization becomes prohibited despite the availability of polymer via continuous deposition. In this Letter, we investigate the molecular origins of suppressed crystallization in poly(ethylene oxide) films deposited by MAPLE atop silicon substrates. We find that suppressed crystallization results from the formation of an irreversibly adsorbed polymer nanolayer at the substrate that forms during deposition. Substrate temperature is shown to influence the stability of the irreversibly adsorbed nanolayer and, hence, polymer thin film crystallization. Our investigation offers new insight into how temperature and interfacial interactions can serve as a new toolbox to tune polymer film morphology in bottom-up deposition.

Anisotropic crystallization in solution processed chalcogenide thin film by linearly polarized laser

The low activation energy associated with amorphous chalcogenide structures offers broad tunability of material properties with laser-based or thermal processing. In this paper, we study near-bandgap laser induced anisotropic crystallization in solution processed arsenic sulfide. The modified electronic bandtail states associated with laser irradiation lead to a distinctive photoluminescence spectrum, compared to thermally annealed amorphous glass. Laser crystalized materials exhibit a periodic subwavelength ripple structure in transmission electron microscopy experiments and show polarization dependent photoluminescence. Analysis of the local atomic structure of these materials using laboratory-based X-ray pair distribution function analysis indicates that laser irradiation causes a slight rearrangement at the atomic length scale, with a small percentage of S-S homopolar bonds converting to As-S heteropolar bonds. These results highlight fundamental differences between laser and thermal processing in thi...

Photoluminescence of Functionalized Germanium Nanocrystals Embedded in Arsenic Sulfide Glass

Embedding metallic and semiconductor nanoparticles in a chalcogenide glass matrix effectively modifies the photonic properties. Such nanostructured materials could play an important role in optoelectronic devices, catalysis, and imaging applications. In this work, we fabricate and characterize germanium nanocrystals (Ge NCs) embedded in arsenic sulfide thin films by pulsed laser ablation in aliphatic amine solutions. Unstable surface termination of aliphatic groups and stable termination by amine on Ge NCs are indicated by Raman and Fourier-transform infrared spectroscopy measurements. A broad-band photoluminescence in the visible range is observed for the amine functionalized Ge NCs. A noticeable enhancement of fluorescence is observed for Ge NCs in arsenic sulfide, after annealing to remove the residual solvent of the glass matrix.