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The depletion region width of metal-oxide/quantum-dot (QD) heterojunction solar cells is increased by a new method in which heavily boron-doped n(+)-ZnO is employed. It is effectively increased in the QD layer by 30% compared to the counterpart with conventional n-ZnO, and provides 41% and 37% improvement of J(sc) (16.7 mA cm(-2) to 23.5 mA cm(-2) ) and power conversion efficiency (5.52% to 7.55%), respectively.

Small-bandgap polymer solar cells (PSCs) with a thick bulk heterojunction film of 340 nm exhibit high power conversion efficiencies of 9.40% resulting from high short-circuit current density (JSC ) of 20.07 mA cm(-2) and fill factor of 0.70. This remarkable efficiency is attributed to maximized light absorption by the thick active layer and minimized recombination by the optimized lateral and vertical morphology through the processing additive.

The ultrafast charge generation process in organic solar cell devices is investigated by transient reflection spectroscopy on five state-of-the-art bulk heterojunction systems. The charge generation process in operating devices is found to be a combination of an ultrafast generation mechanism over several hundred femto-seconds and a slow process from pico-seconds to nanoseconds, limited by exciton diffusion dynamics. In addition, the lack of electric field dependence in the charge dynamics rules out geminate recombination as an important loss mechanism.

optical fi eld. [ 18,19 ] A materials advance is pursued, achieved, and explained in the present work that is crucial to the realization of the inverted, optically-tuned, architecture. We employ a substantially transparent ZnO optical spacer between the photoactive CQD layer and the refl ective metal electrode in an inverted device confi guration. To do so, we develop the means to integrate a low-temperature, solution-processed, n-type ZnO layer as an optical spacer atop the CQD absorber. We show that the spacer offers signifi cant prospects for enhancing light absorption in the CQD layer since careful choice of ZnO thickness enables judicious placement of the optical fi eld maxima within the absorber by optical simulations. Moreover, we demonstrate by experiments that the light harvesting and the photovoltaic effi ciency can be signifi cantly improved in CQD solar cells by tuning the thickness of the active layer and inserting an optical spacer between the active layer and the refl ective electrode. As discussed herein, the new architecture offers an improvement in performance relative to the reference device that goes beyond optical cavity enhancement alone. We investigate using ultraviolet photoemission spectroscopy the detailed electronic structure of each of the materials incorporated into the materials stack in each case. In this way, we offer experimentallysubstantiated spatial band diagrams of the device that detail the origins of the enhanced voltage in the device. Figure 1 shows the device structures studied in this work. The devices were fabricated on indium tin oxide (ITO) coated substrates and all processing steps, except a brief immersion in a 3-mercaptopropionic acid (MPA) solution and the fi nal metal electrode deposition, were performed by spin-coating in air. The ZnO optical spacer was prepared using a diethyl zinc precursor and deposited on the CQD layer. The reactive diethyl zinc precursor decomposes rapidly to form ZnO in the presence of air and only mild thermal annealing (110 °C) is required to prepare the fi lm via this route. The low processing temperature of the ZnO layer is critical to the success of the inverted architecture; other methods of ZnO preparation which

To achieve excellent photoelectrochemical water-splitting activity, photoanode materials with high light absorption and good charge-separation efficiency are essential. One effective strategy for the production of materials satisfying these requirements is to adjust their band structure and corresponding bandgap energy by introducing oxygen vacancies. A simple chemical reduction method that can systematically generate oxygen vacancies in barium stannate (BaSnO3 (BSO)) crystal is introduced, which thus allows for precise control of the bandgap energy. A BSO photoanode with optimum oxygen-vacancy concentration (8.7%) exhibits high light-absorption and good charge-separation capabilities. After deposition of FeOOH/NiOOH oxygen evolution cocatalysts on its surface, this photoanode shows a remarkable photocurrent density of 7.32 mA cm-2 at a potential of 1.23 V versus a reversible hydrogen electrode under AM1.5G simulated sunlight. Moreover, a tandem device constructed with a perovskite solar cell exhibits an operating photocurrent density of 6.84 mA cm-2 and stable gas production with an average solar-to-hydrogen conversion efficiency of 7.92% for 100 h, thus functioning as an outstanding unbiased water-splitting system.

We develop a photovoltaic colloidal quantum dot ink that allows for lossless, single-step coating of large areas in a manufacturing-compatible process. Our materials strategy involves a solution-phase ligand exchange to transport compatible linkers that yield 1-thioglycerol-capped PbS quantum dots in dimethyl sulfoxide with a photoluminescence quantum yield of 24%. A proof-of-principle solar cell made from the ink exhibits 2.1% power conversion efficiency.

A means to control the net doping of a CQD solid is identified via the design of the bidentate ligand crosslinking the material. The strategy does not rely on implementing different atmospheres at different steps in device processing, but instead is a robust strategy implemented in a single processing ambient. We achieve an order of magnitude difference in doping that allows us to build a graded photovoltaic device and maintain high current and voltage at maximum power-point conditions.

Power conversion efficiency up to 8.6% is achieved for a solution-processed tandem solar cell based on a diketopyrrolopyrrole-containing polymer as the low-bandgap material after using a thin polyelectrolyte layer to modify the electron-transport ZnO layers, indicating that interfacial engineering is a useful approach to further enhancing the efficiency of tandem organic solar cells.

Polymer solar cells (PSCs) have recently attracted great attention because of their potential advantages, including flexibility, light weight, and low fabrication cost.[1 4] Photon absorption by conjugated polymers often creates bound electron/hole pairs (i.e., excitons). Charge collection, therefore, requires dissociation of the excitons, a process which is known to be favorable at the interface between conjugated polymers as donors and C60 derivatives as acceptors. Since [6,6]-phenyl-C61-butyric acid ester (PCBM) is still one of the best electron acceptors, most recent efforts have focused on the development of low bandgap donor polymers.[5 22] Examples of low bandgap polymers include conjugated polymers having electron-donating (D) and electron-accepting (A) moieties in the main chain.[21,22] The bandgap of the D- -A polymers can be effectively reduced through intramolecular charge transfer while their physical properties can be readily tuned by tailoring structures of the D and A moieties and/or the linking bridge. An ideal donor polymer should have a broad absorption spectrum for an efficient solar photon harvesting, a low HOMO level for a high opencircuit voltage (VOC), a good compatibility with PCBM for the formation of a bi-continuous network (i.e., bulk heterojunction, BHJ) with a large interface area, and a high hole mobility for an efficient transportation of the photo-generated charge carriers.[ 4] When designing donor polymers, therefore, a delicate balance between the crystallinity and the solubility needs to be taken into consideration.[5] Highly crystalline conjugated polymers with closely packed polymer chains in the solid state often show high hole mobility, but low solubility in organic solvents. Although the high hole mobility is an asset to PSCs, highly crystalline polymers with poor solution-processability rarely form the desired bi-continuous morphology with PCBM, and hence usually exhibit poor photovoltaic performance.

The surface plasmon effect on polymer solar cells and polymer light-emitting diodes is demonstrated by using metal nanoparticles prepared from block copolymer templates. Light absorption of the polymer thin layer is increased with the incorporation of metallic nanostructures, resulting in a significant surface plasmon effect in the optoelectronic devices.

Improved performance in plasmonic organic solar cells (OSCs) and organic light-emitting diodes (OLEDs) via strong plasmon-coupling effects generated by aligned silver nanowire (AgNW) transparent electrodes decorated with core-shell silver-silica nanoparticles (Ag@SiO2 NPs) is demonstrated. NP-enhanced plasmonic AgNW (Ag@SiO2 NP-AgNW) electrodes enable substantially enhanced radiative emission and light absorption efficiency due to strong hybridized plasmon coupling between localized surface plasmons (LSPs) and propagating surface plasmon polaritons (SPPs) modes, which leads to improved device performance in organic optoelectronic devices (OODs). The discrete dipole approximation (DDA) calculation of the electric field verifies a strongly enhanced plasmon-coupling effect caused by decorating core-shell Ag@SiO2 NPs onto the AgNWs. Notably, an electroluminescence efficiency of 25.33 cd A-1 (at 3.2 V) and a power efficiency of 25.14 lm W-1 (3.0 V) in OLEDs, as well as a power conversion efficiency (PCE) value of 9.19% in OSCs are achieved using hybrid Ag@SiO2 NP-AgNW films. These are the highest values reported to date for optoelectronic devices based on AgNW electrodes. This work provides a new design platform to fabricate high-performance OODs, which can be further explored in various plasmonic and optoelectronic devices.

Conjugated polyelectrolytes (CPEs) with π-delocalized main backbones and ionic pendant groups are intensively studied as interfacial layers for efficient polymer-based optoelectronic devices (POEDs) because they facilitate facile control of charge injection/extraction barriers. Here, a simple and effective method of performing precise interfacial energy level adjustment is presented by employing CPEs with different thicknesses and various ion densities under electric poling to realize efficient charge injection/extraction of POEDs. The effects of the CPE ion densities and electric (positive or negative) poling on the energy level tuning process are investigated by measuring the open-circuit voltages and current densities of devices with the structure indium tin oxide/zinc oxide/CPE/organic active layer/molybdenum oxide/gold while changing the CPE film thickness. The performances of inverted polymer light-emitting diodes and inverted polymer solar cells are remarkably improved by precisely controlling the interfacial energy level matching using optimum CPE conditions.

reported under AM1.5 (AM: air mass) illumination, this efficiency is not sufficient to meet realistic specifications for commercialization. The need to improve the light-to-electricity conversion efficiency requires the implementation of new materials and the exploration of new device architectures. Polymer-based photovoltaic cells are thin-film devices fabricated in the metal-insulator-metal configuration sketched in Figure 1a. The absorbing and charge-separating bulk-heterojunction layer with a thickness of approximately 100 nm is sandwiched between two charge-selective electrodes; a transparent bilayer electrode comprising poly(3,4-ethylenedioxylenethiophene):polystyrene sulfonic acid (PEDOT:PSS) on indium tin oxide (ITO) glass for collecting the holes and a lower-work-function metal (here, Al) for collecting the electrons. The work-function difference between the two electrodes provides a built-in potential that breaks the symmetry, thereby providing a driving force for the photogenerated electrons and holes toward their respective electrodes. Because of optical interference between the incident (from the ITO side) and back-reflected light, the intensity of the light is zero at the metallic (Al) electrode; Figure 1a shows a schematic representation of the spatial distribution of the squared optical electric-field strength. [9–11] Thus, a relatively large fraction of the active layer is in a dead-zone in which the photogeneration of carriers is significantly reduced. Moreover, this effect causes more electron–hole pairs to be produced near the ITO/PEDOT:PSS electrode, a distribution which is known to reduce the photovoltaic conversion efficiency. [12,13] This “optical interference effect” is especially important for thin-film structures where layer thicknesses are comparable to the absorption depth and the wavelength of the incident light, as is the case for photovoltaic cells fabricated from semiconducting polymers. In order to overcome these problems, one might simply increase the thickness of the active layer to absorb more light. Because of the low mobility of the charge carriers in the polymer:C60 composites, however, the increased internal resistance of thicker films will inevitably lead to a reduced fill factor. An alternative approach is to change the device architecture with the goal of spatially redistributing the light intensity inside the device by introducing an optical spacer between the active layer and the Al electrode as sketched in Figure 1a. [11] Although this revised architecture would appear to solve the problem, the prerequisites for an ideal optical spacer limit the choice of materials: the layer must be a good acceptor and an electron-transport material with a conduction band edge lower in energy than that of the lowest unoccupied molecular orbital (LUMO) of C60; the LUMO must be above (or close to) the Fermi energy of the collecting metal electrode; and it must be transparent to light with wavelengths within the solar spectrum.

T.L. and X.P. contributed equally to this work. This work was financially supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 51273203, 51261160496, 51473009, 21504066, 21534003, and 21303167), the International Science a Technology Cooperation Program of China (Grant No. 2014DFA52820), the Ministry of Science and Technology (Grant No. 2016YFA0200700), and RGC-NSFC grant N_HKU709/12 from the Research Grants Council of Hong Kong Special Administrative Region, China. This work was also supported by the National Research Foundation of Korea (Grant No. NRF-2015R1C1A1A02036599). X-ray data was acquired at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors thank Chenhui Zhu at beamline 7.3.3, and Cheng Wang at beamline 11.0.1.2 for assistance with data acquisition. Y.S. gratefully acknowledge Dr. Feng Liu for fruitful discussions and valuable suggestions.

Colloidal quantum dot (CQD) optoelectronics offers a compelling combination of low-cost, large-area solution processing, and spectral tunability through the quantum size effect. Since early reports of size-tunable light emission from solution-synthesized CQDs over 25 years ago, tremendous progress has been made in synthesis and assembly, optical and electrical properties, materials processing, and optoelectronic applications of these materials. Here some of the major developments in this field are reviewed, touching on key milestones as well as future opportunities.