Robert P. H. Chang

Bithiophene Imide and Benzodithiophene Copolymers for Efficient Inverted Polymer Solar Cells

Bithiophene imide (BTI) and benzodithiophene (BDT) copolymers are synthesized for application in organic photovoltaic (OPV) cells. The electron deficiency of the BTI units leads to polymers with a low-lying HOMOs (∼-5.6 eV). Inverted solar cells are fabricated to investigate the OPV performance of the BTI-based polymers and achieve power conversion efficiencies up to 5.5%, with substantial V(oc)s above 0.9 V which are among the highest V(oc)s reported to date for polymer/PCBM solar cells. The results indicate that the BTI is a promising building block for constructing polymer donors for OPV applications.

Ultraflexible Polymer Solar Cells Using Amorphous Zinc−Indium−Tin Oxide Transparent Electrodes

Polymer solar cells are fabricated on highly conductive, transparent amorphous zinc indium tin oxide (a-ZITO) electrodes. For two representative active layer donor polymers, P3HT and PTB7, the power conversion efficiencies (PCEs) are comparable to reference devices using polycrystalline indium tin oxide (ITO) electrodes. Benefitting from the amorphous character of a-ZITO, the new devices are highly flexible and can be repeatedly bent to a radius of 5 mm without significant PCE reduction.

Review of the synthesis and properties of colloidal quantum dots: the evolving role of coordinating surface ligands

This review highlights the developments in synthetic methods for colloidal quantum dots that have expanded the range of achievable sizes, shapes, materials, and surface chemistries over the past 30 years, and how these methods have enabled optimization of properties like photoluminescence quantum yield, monodisperse size distributions, and conductivity in the solid state.

Quasi-reversible point defect relaxation in amorphous In-Ga-Zn-O thin films by in situ electrical measurements

Quasi-reversible oxygen exchange/point defect relaxation in an amorphous In-Ga-Zn-O thin film was monitored by in situ electrical property measurements (conductivity, Seebeck coefficient) at 200u2009°C subjected to abrupt changes in oxygen partial pressure (pO2). By subtracting the long-term background decay from the conductivity curves, time-independent conductivity values were obtained at each pO2. From these values, a log-log “Brouwer” plot of conductivity vs. pO2 of approximately −1/2 was obtained, which may indicate co-elimination (filling) of neutral and charged oxygen vacancies. This work demonstrates that Brouwer analysis can be applied to the study of defect structure in amorphous oxide thin films.

Ultra-sharp plasmonic resonances from monopole optical nanoantenna phased arrays

Diffractively coupled plasmonic resonances possess both ultra-sharp linewidths and giant electric field enhancement around plasmonic nanostructures. They can be applied to create a new generation of sensors, detectors, and nano-optical devices. However, all current designs require stringent index-matching at the resonance condition that limits their applicability. Here, we propose and demonstrate that it is possible to relieve the index-matching requirement and to induce ultra-sharp plasmon resonances in an ordered vertically aligned optical nano-antenna phased array by transforming a dipole resonance to a monopole resonance with a mirror plane. Due to the mirror image effect, the monopole resonance not only retained the dipole features but also enhanced them. The engineered resonances strongly suppressed the radiative decay channel, resulting in a four-order of magnitude enhancement in local electric field and a Q-factor greater than 200.

Multi-scale order in amorphous transparent oxide thin films

Nominally “diffraction amorphous” materials represent a pervasive challenge in establishing classical structure-property relationships. This stems from the difficulty in defining the structure of nominally amorphous materials and experimentally differentiating the short-range (<10u2009A) and medium-range (10 to 30u2009A) order as a function of process parameters which are important due to their influence on physical, chemical, or transport properties. Herein, we report on the determination of short- and medium-range order in nominally amorphous zinc and tin co-substituted indium oxide In1.4Zn0.3Sn0.3O3 (a-ZITO) thin films grown by pulsed laser deposition. These thin films are being explored as transparent conductors that exhibit changes in transport properties depending on deposition temperature while remaining “diffraction amorphous.” Reduced density function (RDF) analysis from electron diffraction accompanied by density functional theory molecular dynamics simulation of a liquid quench suggests ordering at sho...

Efficiency Enhancement in Dye-Sensitized Solar Cells by Three-Dimensional Photonic Crystals

We have proposed a new dye-sensitized solar cell (DSSC) structure that employs three-dimensional (3D) photonic crystals (PCs) to enhance the light absorption and improve the power conversion efficiency (PCE) by using coherent scattering phenomena. All the DSSC structures with the 3D PC layer exhibited higher short-circuit current densities and higher PCE (10.8%) than those of traditional DSSCs (9.5%) because light that passed through the photoanode was diffracted, thereby making it possible to reuse it. The PCE is improved without affecting the delicate kinetic balance between the charge separation and recombination that is required to improve light-harvesting efficiency (LHE).

Perfect coupling of light to a periodic dielectric/metal/dielectric structure

Using the finite difference time domain method, it is demonstrated that perfect coupling can be achieved between normally incident light and a periodic dielectric/metal/dielectric structure. The structure serves as a diffraction grating that excites modes related to the long range surface plasmon and short range surface plasmon modes that propagate on continuous metallic films. By optimizing the structural dimensions, perfect coupling is achieved between the incident light and these modes. A high Q of 697 and an accompanying ultrasharp linewidth of 0.8u2009nm are predicted for a 10u2009nm silver film for optimal conditions.