Dual Functional Polymer Interlayer for Facilitating Ion Transport and Reducing Charge Recombination in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) present low-cost alternatives to conventional wafer-based inorganic solar cells and have remarkable power conversion efficiency. To further enhance performance, we propose a new DSSC architecture with a novel dual-functional polymer interlayer that prevents charge recombination and facilitates ionic conduction, as well as maintaining dye loading and regeneration. Poly­(vinylidene fluoride-trifluoroethylene) (p­(VDF-TrFE)) was coated on the outside of a dye-sensitized TiO₂ photoanode by a simple solution process that did not sacrifice the amount of adsorbed dye molecules in the DSSC device. Light-intensity-modulated photocurrent and photovoltage spectroscopy revealed that the proposed p­(VDF-TrFE)-coated anode yielded longer electron lifetime and improved the injection of photogenerated electrons into TiO₂, thereby reducing the electron transport time. Comparative cyclic voltammetry and UV–visible absorption spectroscopy based on a ferrocene–ferrocenium external standard material demonstrated that p­(VDF-TrFE) enhanced the power conversion efficiency from 7.67% to 9.11%. This dual functional p­(VDF-TrFE) interlayer is a promising candidate for improving the performance of DSSCs and can also be employed in other electrochemical devices

Atomic-Scale Interfacial Band Mapping across Vertically Phased-Separated Polymer/Fullerene Hybrid Solar Cells

Using cross-sectional scanning tunneling microscope (XSTM) with samples cleaved in situ in an ultrahigh vacuum chamber, this study demonstrates the direct visualization of high-resolution interfacial band mapping images across the film thickness in an optimized bulk heterojunction polymer solar cell consisting of nanoscale phase segregated blends of poly­(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We were able to achieve the direct observation of the interfacial band alignments at the donor (P3HT)-acceptor (PCBM) interfaces and at the interfaces between the photoactive P3HT:PCBM blends and the poly­(3,4-ethylenedioxythiophene) poly­(styrenesulfonate) (PEDOT:PSS) anode modification layer with an atomic-scale spatial resolution. The unique advantage of using XSTM to characterize polymer/fullerene bulk heterojunction solar cells allows us to explore simultaneously the quantitative link between the vertical morphologies and their corresponding local electronic properties. This provides an atomic insight of interfacial band alignments between the two opposite electrodes, which will be crucial for improving the efficiencies of the charge generation, transport, and collection and the corresponding device performance of polymer solar cells

Tunable Photoinduced Carrier Transport of a Black Phosphorus Transistor with Extended Stability Using a Light-Sensitized Encapsulated Layer

In this article, we propose a novel approach to demonstrate tunable photoinduced carrier transport of a few-layered black phosphorus (BP) field-effect transistor (FET) with extended air stability using a “light-sensitized ultrathin encapsulated layer”. Titanium suboxide (TiO_x) ultrathin film (approximately 3 nm), which is an amorphous phase of crystalline TiO₂ and can be solution processed, simultaneously exhibits the unique dual functions of passivation and photoinduced doping on a BP FET. The photoinduced electron transfer at TiO_x/BP interfaces provides tunable n-type doping on BP through light illumination. Accordingly, the intrinsic hole-dominated transport of BP can be gradually tuned to the electron-dominated transport at a TiO_x/BP FET using light modulation, with enhanced electron mobility and extended air stability of the device. The novel device structure consisting of a light-sensitized encapsulated layer with controllable and reversible doping through light illumination on BP exhibits great potential for the future development of stable BP-based semiconductor logic devices or optoelectronic devices

Quantum Dot Light-Emitting Diode Using Solution-Processable Graphene Oxide as the Anode Interfacial Layer

In this article, the solution processable graphene oxide (GO) thin film was utilized as the anode interfacial layer in quantum dot light emitting diodes (QD-LEDs). The QD-LED devices (ITO/GO/QDs/TPBi/LiF/Al) were fabricated by employing a layer-by-layer assembled deposition technique with the electrostatic interaction between GO and QDs. The thicknesses of GO thin films and the layer number of CdSe/ZnS QD emissive layers were carefully controlled by spin-casting processes. The GO thin films, which act as the electron blocking and hole transporting layer in the QD-LED devices, have demonstrated the advantage of being compatible with fully solution-processed fabrications of large-area printable optoelectronic devices

Spatially Resolved Imaging on Photocarrier Generations and Band Alignments at Perovskite/PbI₂ Heterointerfaces of Perovskite Solar Cells by Light-Modulated Scanning Tunneling Microscopy

The presence of the PbI₂ passivation layers at perovskite crystal grains has been found to considerably affect the charge carrier transport behaviors and device performance of perovskite solar cells. This work demonstrates the application of a novel light-modulated scanning tunneling microscopy (LM-STM) technique to reveal the interfacial electronic structures at the heterointerfaces between CH₃NH₃PbI₃ perovskite crystals and PbI₂ passivation layers of individual perovskite grains under light illumination. Most importantly, this technique enabled the first observation of spatially resolved mapping images of photoinduced interfacial band bending of valence bands and conduction bands and the photogenerated electron and hole carriers at the heterointerfaces of perovskite crystal grains. By systematically exploring the interfacial electronic structures of individual perovskite grains, enhanced charge separation and reduced back recombination were observed when an optimal design of interfacial PbI₂ passivation layers consisting of a thickness less than 20 nm at perovskite crystal grains was applied

Self-Encapsulated Doping of n-Type Graphene Transistors with Extended Air Stability

This paper presents an innovative approach to fabricating controllable n-type doping graphene transistors with extended air stability by using self-encapsulated doping layers of titanium suboxide (TiOx) thin films, which are an amorphous phase of crystalline TiO₂ and can be solution processed. The nonstoichiometry TiOx thin films consisting of a large number of oxygen vacancies exhibit several unique functions simultaneously in the n-type doping of graphene as an efficient electron-donating agent, an effective dielectric screening medium, and also an encapsulated layer. A novel device structure consisting of both top and bottom coverage of TiOx thin layers on a graphene transistor exhibited strong n-type transport characteristics with its Dirac point shifted up to −80 V and an enhanced electron mobility with doping. Most interestingly, an extended stability of the device without rapid degradation after doping was observed when it was exposed to ambient air for several days, which is not usually observed in other n-type doping methods in graphene. Density functional theory calculations were also employed to explain the observed unique n-type doping characteristics of graphene using TiOx thin films. The technique of using an “active” encapsulated layer with controllable and substantial electron doping on graphene provides a new route to modulate electronic transport behavior of graphene and has considerable potential for the future development of air-stable and large-area graphene-based nanoelectronics

Dependence of Nanocrystal Dimensionality on the Polymer Nanomorphology, Anisotropic Optical Absorption, and Carrier Transport in P3HT:TiO₂ Bulk Heterojunctions

It is known that the nanoscale morphological organization of donors or acceptors in bulk heterojunction (BHJ) solar cells is critical to device performance and strongly affects carrier generation, transporting, and collection. This work demonstrates the dependence of nanocrystal dimensionality and organization on the polymer nanomorphology in P3HT:TiO₂ hybrid bulk heterojunctions, which were revealed using grazing-incidence X-ray diffraction (GIXRD) using a synchrotron X-ray beam and electron tomography. We further performed a multiscale molecular dynamic simulation to understand the morphological orientation of a polymer blended with TiO₂ nanoparticles (NPs) or nanorods (NRs). The correlation between polymer nanoscale morphology and the dimensionality and anisotropy of nanocrystals in P3HT:TiO₂ hybrids clearly explains the observation of different optical absorption and carrier transport behaviors in directions perpendicular or parallel to the film substrate. Our results provide crucial information toward understanding the interplay between nanocrystal dimensionality and polymer morphology in developing organic/inorganic hybrid electronic devices such as thin film transistors (TFTs) or photovoltaics (PVs)

Low-Threshold Lasing from 2D Homologous Organic–Inorganic Hybrid Ruddlesden–Popper Perovskite Single Crystals

Organic–inorganic hybrid two-dimensional (2D) perovskites have recently attracted great attention in optical and optoelectronic applications due to their inherent natural quantum-well structure. We report the growth of high-quality millimeter-sized single crystals belonging to homologous two-dimensional (2D) hybrid organic–inorganic Ruddelsden–Popper perovskites (RPPs) of (BA)₂(MA)_{<i><i>n</i></i>−1}Pb_{<i><i>n</i></i>}I_{3<i><i>n</i></i>+1} (<i>n</i> = 1, 2, and 3) by a slow evaporation at a constant-temperature (SECT) solution-growth strategy. The as-grown 2D hybrid perovskite single crystals exhibit excellent crystallinity, phase purity, and spectral uniformity. Low-threshold lasing behaviors with different emission wavelengths at room temperature have been observed from the homologous 2D hybrid RPP single crystals. Our result demonstrates that solution-growth homologous organic–inorganic hybrid 2D perovskite single crystals open up a new window as a promising candidate for optical gain media