Jacek J. Jasieniak

Size-Dependent Valence and Conduction Band-Edge Energies of Semiconductor Nanocrystals

Through the use of photoelectron spectroscopy in air (PESA), we investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS, and PbSe semiconductor quantum dots (QDs). The results are compared to those of previous studies, based on differing experimental methods, and to theoretical calculations based on k·p theory and state-of-the-art atomistic semiempirical pseudopotential modeling. To accurately map out the energy level landscapes of QDs as a function of size, the QDs must be passivated by comparable surface chemistries. This is highlighted by studying the effect of surface chemistry on the valence band-edge energy in an ensemble of 4.7 nm CdSe QDs. An energy level shift as large as 0.35 eV is observed for this system through modification of surface chemistry alone. This shift is significantly larger than the size-dependent valence band-edge shift that is observed when comparable surface chemistries are used.

A new method to position and functionalize metal-organic framework crystals

With controlled nanometre-sized pores and surface areas of thousands of square metres per gram, metal-organic frameworks (MOFs) may have an integral role in future catalysis, filtration and sensing applications. In general, for MOF-based device fabrication, well-organized or patterned MOF growth is required, and thus conventional synthetic routes are not suitable. Moreover, to expand their applicability, the introduction of additional functionality into MOFs is desirable. Here, we explore the use of nanostructured poly-hydrate zinc phosphate (α-hopeite) microparticles as nucleation seeds for MOFs that simultaneously address all these issues. Affording spatial control of nucleation and significantly accelerating MOF growth, these α-hopeite microparticles are found to act as nucleation agents both in solution and on solid surfaces. In addition, the introduction of functional nanoparticles (metallic, semiconducting, polymeric) into these nucleating seeds translates directly to the fabrication of functional MOFs suitable for molecular size-selective applications.

Solution-Processed Sintered Nanocrystal Solar Cells via Layer-by-Layer Assembly

Solar cells made by high temperature and vacuum processes from inorganic semiconductors are at a perceived cost disadvantage when compared with solution-processed systems such as organic and dye-sensitized solar cells. We demonstrate that totally solution processable solar cells can be fabricated from inorganic nanocrystal inks in air at temperature as low as 300 °C. Focusing on a CdTe/ZnO thin-film system, we report solar cells that achieve power conversion efficiencies of 6.9% with greater than 90% internal quantum efficiency. In our approach, nanocrystals are deposited from solution in a layer-by-layer process. Chemical and thermal treatments between layers induce large scale grain formation, turning the 4 nm CdTe particles into pinhole-free films with an optimized average crystallite size of ∼70 nm. Through capacitance-voltage measurements we demonstrate that the CdTe layer is fully depleted which enables the charge carrier collection to be maximized.

Mapping the optical properties of CdSe/CdS heterostructure nanocrystals: the effects of core size and shell thickness.

Here we present the first comprehensive report on CdSe/CdS heterostructure nanocrystals. The effects of core size and shell thickness on the optical properties of CdSe/CdS heterostructure nanocrystals are investigated. We report a reliable synthetic method to grow thick CdS shells on CdSe cores with sizes ranging from 2.5-4.7 nm. We provide a calibration curve, which enables determination of CdS shell thickness (+/-0.1 nm) over a wide range of core sizes, circumventing the need for time-consuming HRTEM analyses. Epitaxial growth of the shells was verified by HRTEM, XRD, and SAED. In-situ reaction measurements revealed the average per particle (p) deposition rates for cadmium and sulfur to be k(Cd) = 5.38 x 10(-25) mol s(-1) p(-1) and k(S) = 4.83 x 10(-24) mol s(-1) p(-1). Faster sulfur deposition rates are attributed to the absence of strong sulfur binding ligands in the growth medium. Through the rigorous use of high resolution transmission electron microscopy, a direct link between the dimensions of the heterostructures and their band-edge transition energies, quantum yields, and excited state lifetimes is established. The experiments show that the band-edge transition energies of the core samples, which initially span approximately 431 meV, condense to span only 163 meV after the growth of a 6 monolayer-thick CdS shell. Furthermore, shifts in the band-edge transition energies were found to be extremely sensitive to core size. The QY of the as-prepared core/shells ranged from 25 to 60%. The QYs and band-edge lifetimes of the core/shells were found to depend upon the ligands adsorbed to the particle surface. These data prove that one or both of the charge carriers still has access to the particle surface despite the presence of a 2.2 nm thick CdS shell.

Photoinduced Charge Generation in a Molecular Bulk Heterojunction Material

Understanding the charge generation dynamics in organic photovoltaic bulk heterojunction (BHJ) blends is important for providing the necessary guidelines to improve overall device efficiency. Despite more than 15 years of experimental and theoretical studies, a universal picture describing the generation and recombination processes operating in organic photovoltaic devices is still being forged. We report here the results of ultrafast transient absorption spectroscopy measurements of charge photogeneration and recombination processes in a high-performing solution-processed molecular BHJ. For comparison, we also studied a high-performing polymer-based BHJ material. We find that the majority of charge carriers in both systems are generated on <100 fs time scales and posit that excited state delocalization is responsible for the ultrafast charge transfer. This initial delocalization is consistent with the fundamental uncertainty associated with the photon absorption process (in the visible, λ/4π > 30 nm) and is comparable with the phase-separated domain size. In addition, exciton diffusion to charge-separating heterojunctions is observed at longer times (1-500 ps). Finally, charge generation in pure films of the solution processed molecule was studied. Polarization anisotropy measurements clearly demonstrate that the optical properties are dominated by molecular (Frenkel) exictons and delocalized charges are promptly produced (t < 100 fs).

Solution-processed nanocrystal quantum dot tandem solar cells.

J.J.C., W.N.W., and R.S.H. contributed equally to this work. This work was supported in part by Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). J.J.C. acknowledges support from the NSF IGERT fellowship. W.N.W. and R.S.H. acknowledge support from undergraduate research fellowships from the Cornell Engineering Learning Initiative and the KAUST-CU center. Y.F.L. acknowledges fellowship from Agency of Science, Technology and Research (A*STAR), Singapore. J.L.L. and J.A.M. acknowledge support from the U.S. National Science Foundation through grants DMR-1006633 and DMR-0706508. J.J. acknowledges funding through the Flexible Electronics Theme of the CSIRO Future Manufacturing Flagship, aCSIRO Office of Chief Executive Postdoctoral Fellowship and the Australian Department of Innovation, Industry, Science and Research, International Science Linkage Grant, CG100059. The authors thank Prof. Rene A. J. Janssen and David Moore for a critical reading of the manuscript and useful discussions.

Layer-by-Layer Assembly of Sintered CdSexTe1–x Nanocrystal Solar Cells

Alloying is a versatile tool for engineering the optical and electronic properties of materials. Here, we explore the use of CdTe and CdSe nanocrystals in developing sintered CdSe(x)Te(1-x) alloys as bandgap tunable, light-absorbing layers for solution-processed solar cells. Using a layer-by-layer approach, we incorporate such alloyed materials into single- and graded-composition device architectures. Nanostructured solar cells employing CdSe(x)Te(1-x) layers are found to exhibit a spectral response deeper into the IR region than bulk CdTe devices as a result of optical bowing and achieve power conversion efficiencies as high as 7.1%. The versatility of the layer-by-layer approach is highlighted through the fabrication of compositionally graded solar cells in which the [Se]:[Te] ratio is varied across the device. Each of the individual layers can be clearly resolved through cross-sectional imaging and show limited interdiffusion. Such devices demonstrate the importance of band-alignment in the development of highly efficient, nanostructured solar cells.

Highly Luminescent Metal–Organic Frameworks Through Quantum Dot Doping

The incorporation of highly luminescent core-shell quantum dots (QDs) within a metal-organic framework (MOF) is achieved through a one-pot method. Through appropriate surface functionalization, the QDs are solubilized within MOF-5 growth media. This permits the incorporation of the QDs within the evolving framework during the reaction. The resulting QD@MOF-5 composites are characterized using X-ray fluorescence, cross-sectional confocal microscopy, energy-dispersive X-ray spectroscopy, scanning electron microscopy, and small-angle X-ray scattering. The synergistic combination of luminescent QDs and the controlled porosity of MOF-5 in the QD@MOF-5 composites is harnessed within a prototype molecular sensor that can discriminate on the basis of molecular size.

Cu2ZnSnS4xSe4(1–x) Solar Cells from Polar Nanocrystal Inks

A facile ligand exchange method for dispersing Cu2ZnSnS4 (CZTS) nanocrystals (NCs) in environmentally benign polar solvents, such as ethanol or n-propanol, at high concentrations (up to 200 mg/mL) is demonstrated. This approach has been applied to CZTS nanocrystals synthesized via scalable, noninjection methods to formulate colloidally stable inks that are suitable for the solution processing of solar cell devices. Unlike other inks currently used to fabricate NC solar cells, the CZTS nanocrystal ink developed here circumvents the need for hydrazine, pyridine, or thiol coordinating solvents. By combining our polar CZTS inks with optimized selenization procedures, substrate CZTSSe solar cells have been successfully fabricated with device efficiencies of 7.7%.

A Hyperbranched Conjugated Polymer as the Cathode Interlayer for High‐Performance Polymer Solar Cells

An alcohol-soluble hyperbranched conjugated polymer HBPFN with a dimethylamino moiety is synthesized and used as a cathode interlayer. A PCE of 7.7% is obtained for PBDTTT-C-T/PC71 BM based solar cells. No obvious interfacial dipole is found at the interface between the active layer and HBPFN however, an interfacial dipole with the cathode could be one of the reasons for the enhanced performance.