Colloidal CdSe quantum dots (QDs) are suitable as electron acceptors in polymer/nanoparticle bulk heterojunction hybrid solar cells. For this application, a thick organic ligand shell which is typically surrounding the QDs after synthesis needs to be removed. Ligand exchange with pyridine is the most widely used method for this purpose. Although this approach is already 15 years old, detailed studies on the effectiveness of ligand exchange with pyridine for solar cell applications are still missing. In the present work hybrid solar cells were prepared from CdSe QDs initially capped with oleic acid (OA), and the impact of single and multiple pyridine treatment was thoroughly investigated. NMR was applied to determine the composition of the ligand shell as well as to distinguish the bound and free ligands before and after ligand exchange. It is shown that after a single pyridine treatment some amount of OA is still present in the samples. By using thermal gravimetric analysis (TGA) we could obtain also quantitative information about the effectiveness of subsequent pyridine treatments. In a series of one-, two-, and threefold ligand exchange, the estimated surface coverage by OA decreased from 26% to 12%, whereas that of pyridine increased from 54% to 80%. Laboratory solar cells with pyridine-capped CdSe QDs and poly(3-hexylthiophene) (P3HT) were characterized by current-voltage (I-V) measurements, and in order to get deeper insight into charge carrier generation and recombination processes, CdSe/P3HT blends were studied by light-induced electron spin resonance (l-ESR). Although repeated pyridine treatment was found to have a beneficial effect in the sense that more complete ligand exchange was achieved, which in turn enabled more efficient charge transfer, the performance of the solar cells was found to be reduced. This fact correlates with increased aggregation tendency of repeatedly pyridine-treated particles, negatively influencing the morphology of the blends, as well as with a larger amount of surface defects in particles stabilized by the weak pyridine ligand shell.

We report unusual spectral features in the resonant Raman scattering spectra of colloidal CdSe nanoparticles as small as 2–3 nm. High-frequency shoulders of the longitudinal optical phonon peak and its overtones were observed and their dependence on the excitation wavelength, temperature, nanoparticle size, and surface passivation with ZnS shell studied. As the probable origin of the uncommon spectral feature the participation of acoustic phonons and manifestation of the density of surface-related vibrational states is discussed.

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High mobility, solution-processed field-effect transistors are important building blocks for flexible electronics. Here we demonstrate the alignment of semiconducting, colloidal ZnO nanorods by a simple solvent evaporation technique and achieve high electron mobilities in field-effect transistors at low operating voltages by electrolyte-gating with ionic liquids. The degree of alignment varies with nanorod length, concentration and solvent evaporation rate. We find a strong dependence of electron mobility on the degree of alignment but less on the length of the nanorods. Maximum field-effect mobilities reach up to 9 cm(2) V(-1) s(-1) for optimal alignment. Because of the low process temperature (150 °C), ZnO nanorod thin films are suitable for application on flexible polymer substrates.

involve a variety of interactions and driving forces between inorganic hard core, organic soft surface-coating ligands, and surrounding solvent molecules. [2,[7][8][9] As a consequence, a thorough understanding of a solvent-mediated assembly process is required to produce NC solids with programmable properties.Here, we have chosen lead sulfide (PbS) nanocrystals as a model system for the investigation of the in situ self-assembly due to the possibility to produce monodisperse particles in a colloidal solution with a precisely controlled size. Additionally, the unique electronic properties of semiconductor PbS NCs with tunable bandgap make them attractive for many potential technological applications, including solar cells, light-emitting diodes, transistors, photodetectors, etc. [10][11][12][13] The structure and degree of order in NC superlattices depend on several factors, such as NC size and shape, ligand length, grafting density, ligand-solvent interactions, NC concentration, solvent evaporation rate (in case of evaporative assembly), and cell geometry or substrate where the assembly takes place. Thus, recent studies on lead chalcogenide NC assemblies revealed the formation of face-centered cubic (fcc) or body-centered cubic (bcc) superstructures as well as lattice distorted facecentered tetragonal (fct) and body-centered tetragonal (bct) superlattices. [14][15][16][17][18] However, their assembling mechanism remains largely unresolved mainly due to the challenge in measuring intermediate transitions between the initial colloidal state and the final superlattice state, especially in real time.Recently, in situ scattering methods have emerged as a powerful characterization tool to study the nucleation and growth of nanoparticle superlattices. [19][20][21][22] Small-angle X-ray scattering (SAXS) allows for monitoring the self-assembly dynamics with subsecond temporal resolution over large sample volumes under controlled conditions (e.g., controlled solvent vapor environment and temperature) and provides realtime information on long range order in the assembled structures. Often, the grazing-incidence small-angle X-ray scattering (GISAXS) technique is applied for in situ measurements, where a drop of nanocrystal suspension is placed on a substrate and evaporated in a controlled manner. [16,20,23,24] However, at small incident angles of the X-ray beam GISAXS is able to probe only up to several monolayers and is thus limited to the surface regime. For bulk measurements transmission SAXS is Self-assembled nanocrystal superlattices have attracted large scientific attention due to their potential technological applications. However, the nucleation and growth mechanisms of superlattice assemblies remain largely unresolved due to experimental difficulties to monitor intermediate states. Here, the self-assembly of colloidal PbS nanocrystals is studied in real time by a combination of controlled solvent evaporation from the bulk solution and in situ small-angle X-ray scattering (SAXS) in transmission geometry. For the fi...