Colloidal-quantum-dot (CQD) optoelectronics offer a compelling combination of solution processing and spectral tunability through quantum size effects. So far, CQD solar cells have relied on the use of organic ligands to passivate the surface of the semiconductor nanoparticles. Although inorganic metal chalcogenide ligands have led to record electronic transport parameters in CQD films, no photovoltaic device has been reported based on such compounds. Here we establish an atomic ligand strategy that makes use of monovalent halide anions to enhance electronic transport and successfully passivate surface defects in PbS CQD films. Both time-resolved infrared spectroscopy and transient device characterization indicate that the scheme leads to a shallower trap state distribution than the best organic ligands. Solar cells fabricated following this strategy show up to 6% solar AM1.5G power-conversion efficiency. The CQD films are deposited at room temperature and under ambient atmosphere, rendering the process amenable to low-cost, roll-by-roll fabrication.

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.

The vortex state, characterized by a curling magnetization, is one of the equilibrium configurations of soft magnetic materials and occurs in thin ferromagnetic square and disk-shaped elements of micrometre size and below. The interplay between the magnetostatic and the exchange energy favours an in-plane, closed flux domain structure. This curling magnetization turns out of the plane at the centre of the vortex structure, in an area with a radius of about 10 nanometres--the vortex core. The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position. The sense of gyration is determined by the vortex core polarization. Here we report on the controlled manipulation of the vortex core polarization by excitation with small bursts of an alternating magnetic field. The vortex motion was imaged by time-resolved scanning transmission X-ray microscopy. We demonstrate that the sense of gyration of the vortex structure can be reversed by applying short bursts of the sinusoidal excitation field with amplitude of about 1.5 mT. This reversal unambiguously indicates a switching of the out-of-plane core polarization. The observed switching mechanism, which can be understood in the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application in data storage.

In this communication, we report the synthesis of a novel diketopyrrolopyrrole-diketopyrrolopyrrole (DPP-DPP)-based conjugated copolymer and its application in high-mobility organic field-effect transistors. Copolymerization of DPP with DPP yields a copolymer with exceptional properties such as extended absorption characteristics (up to ~1100 nm) and field-effect electron mobility values of >1 cm(2) V(-1) s(-1). The synthesis of this novel DPP-DPP copolymer in combination with the demonstration of transistors with extremely high electron mobility makes this work an important step toward a new family of DPP-DPP copolymers for application in the general area of organic optoelectronics.

Solvent additive processing can lead to drastic improvements in the power conversion efficiency (PCE) in solution processable small molecule (SPSM) bulk heterojunction solar cells. In situ grazing incidence wide-angle X-ray scattering is used to investigate the kinetics of crystallite formation during and shortly after spin casting. The additive is shown to have a complex effect on structural evolution invoking polymorphism and enhanced crystalline quality of the donor SPSM.

Spin-coating is extensively used in the lab-based manufacture of organic solar cells, including most of the record-setting solution-processed cells. We report the first direct observation of photoactive layer formation as it occurs during spin-coating. The study provides new insight into mechanisms and kinetics of bulk heterojunction formation, which may be crucial for its successful transfer to scalable printing processes.

and renewable or earth abundant materials. [ 1,2 ] For photovoltaic panels, the energy recovery time is the length of operation of a panel required to generate the energy used to manufacture the panel. In thin fi lm inorganic panels, this time has dropped to about one year. [ 3 ] One of the revolutionary appeals of roll-to-roll manufacturing of organic photovoltaics (OPV) is the potential to achieve energy recovery times as low as 10 d. [ 4 ] Tremendous advances have recently been made in OPV effi ciencies, [ 5 ] all based on optimization of the bulk-heterojunction (BHJ) motif. [ 6,7 ] The performance of a BHJ requires the optimization of both its molecular-scale order and its nanometer-scale domain structure. [ 8 ] In general, the optimized structure is not at equilibrium. Many techniques have been employed for BHJ fi lm optimization, [ 9 ] including solvent choice and post deposition thermal [ 10 ] or vapor annealing. [ 11 ] Recently, formulations using low volatility liquid additives to achieve higher effi ciencies without thermal treatment have become popular. [12][13][14] Although additives such as 1,8-octanedithiol (ODT) and 1-chloronaphthalene (CN) have almost ubiquitous benefi cial effects, there is no consensus on either the origin or mechanism of their effi cacy. In some systems, additives cause smaller domains, [ 15,16 ] while in others they cause larger domains. [ 12,17 ] The effi cacy of ODT and diiodooctane has been attributed to selective solvation of the fullerene, enabling improved polymer order. CN, however, is typically a good solvent for both polymer and fullerene and thus must act via a different mechanism.We recently employed real-time optical techniques to study the mechanism by which the additives ODT and CN infl uence the solidifi cation of the poly(3-hexylthiophene):phenyl-C61butyric-acid-methyl-ester (P3HT:PCBM) BHJ. [ 18 ] Both additives promoted a greater than 5× improvement in device effi ciency over the additive-free fi lm in the absence of thermal annealing. Post-deposition characterization with ultraviolet-visible absorption spectroscopy (UV-Vis) and grazing incidence X-ray diffraction (GIXD) established that the additives increased the polymer's local order and crystallinity. Additionally, energy-fi ltered transmission electron microscopy (EF-TEM) revealed that the The most successful active fi lm morphology in organic photovoltaics is the bulk heterojunction (BHJ). The performance of a BHJ arises from a complex interplay of the spatial organization of the segregated donor and acceptor phases and the local order/quality of the respective phases. These critical morphological features develop dynamically during fi lm formation, and it has become common practice to control them by the introduction of processing additives. Here, in situ grazing incidence X-ray diffraction (GIXD) and grazing incidence small angle X-ray scattering (GISAXS) studies of the development of order in BHJ fi lms formed from the donor polymer poly(3-hexylthiophene) and acceptor phenyl-C61-butyric acid methyl e...

The optical, structural and charge transport properties of solution-processed films of copper(I) thiocyanate (CuSCN) are investigated in this work. As-processed CuSCN films of ~20 nm in thickness are found to be nano-crystalline, highly transparent and exhibit intrinsic hole transporting characteristics with a maximum field-effect mobility in the range of 0.01-0.1 cm(2) V(-1) s(-1).