One merit of organic–inorganic hybrid perovskites is their tunable bandgap by adjusting the halide stoichiometry, an aspect critical to their application in tandem solar cells, wavelength-tunable light emitting diodes (LEDs), and lasers. However, the phase separation of mixed-halide perovskites caused by light or applied bias results in undesirable recombination at iodide-rich domains, meaning open-circuit voltage (VOC) pinning in solar cells and infrared emission in LEDs. Here, we report an approach to suppress halide redistribution by self-assembled long-chain organic ammonium capping layers at nanometer-sized grain surfaces. Using the stable mixed-halide perovskite films, we are able to fabricate efficient and wavelength-tunable perovskite LEDs from infrared to green with high external quantum efficiencies of up to 5%, as well as linearly tuned VOC from 1.05 to 1.45 V in solar cells.Keywords: halide migration; LEDs; Mixed-halide perovskite; phase separation; solar cells;

We demonstrate that reversible chemical reactions occur at TiO2/gas and CH3NH3PbI3/gas interfaces on a time scale of seconds to minutes. The chemisorption strongly affects their electronic properties, mainly acting to deplete TiO2 of free electrons and passivate surface traps on the perovskite. Although the chemistry is not directly probed, we infer that reversible chemistry occurs at the solid-state TiO2/CH3NH3PbI3 interface. Equilibrium or steady-state concentrations established for adsorbed species associated with each material would be voltage- and illumination-dependent due to free or photocarriers being a main reactant. Interfacial chemistry provides an additional physical mechanism to explain the origins of normal and anomalous hysteretic current–voltage characteristics of perovskite devices. Furthermore, chemical reactions help us to understand why measured perovskite ion-transport properties and the nature of hysteresis are highly dependent on interfaces.

The smooth surface of crystalline rubrene films formed through an abrupt heating process provides a valuable platform to study organic homoepitaxy. By varying growth rate and substrate temperature, we are able to manipulate the onset of a transition from layer-by-layer to island growth modes, while the crystalline thin films maintain a remarkably smooth surface (less than 2.3 nm root-mean-square roughness) even with thick (80 nm) adlayers. We also uncover evidence of point and line defect formation in these films, indicating that homoepitaxy under our conditions is not at equilibrium or strain-free. Point defects that are resolved as screw dislocations can be eliminated under closer-to-equilibrium conditions, whereas we are not able to eliminate the formation of line defects within our experimental constraints at adlayer thicknesses above ∼25 nm. We are, however, able to eliminate these line defects by growing on a bulk single crystal of rubrene, indicating that the line defects are a result of strain built into the thin film template. We utilize electron backscatter diffraction, which is a first for organics, to investigate the origin of these line defects and find that they preferentially occur parallel to the (002) plane, which is in agreement with expectations based on calculated surface energies of various rubrene crystal facets. By combining the benefits of crystallinity, low surface roughness, and thickness-tunability, this system provides an important study of attributes valuable to high-performance organic electronic devices.Keywords: crystals; electron backscatter diffraction; Homoepitaxy; line defect; roughness; rubrene; screw dislocation; thin films;

In this work, we discovered a very efficient method of crystallization of thermally evaporated rubrene, resulting in ultrathin, large-area, fully connected, and highly crystalline thin films of this organic semiconductor with a grain size of up to 500 μm and charge carrier mobility of up to 3.5 cm2 V–1 s–1. We found that it is critical to use a 5 nm-thick organic underlayer on which a thin film of amorphous rubrene is evaporated and then annealed to dramatically influence the ability of rubrene to crystallize. The underlayer property that controls this influence is the glass transition temperature. By experimenting with different underlayers with glass transition temperatures varying over 120 °C, we identified the molecules leading to the best crystallinity of rubrene films and explained why values both above and below the optimum result in poor crystallinity. We discuss the formation of different crystalline morphologies of rubrene produced by this method and show that field-effect transistors made with films of a single-domain platelet morphology, achieved through the aid of the optimal underlayer, outperform their spherulite counterparts with a nearly four times higher charge carrier mobility. This large-area crystallization technique overcomes the fabrication bottleneck of high-mobility rubrene thin film transistors and other related devices and, given its scalability and patternability, may lead to practical technologies compatible with large-area flexible electronics.

Hybrid organic–inorganic halide perovskite semiconductors are attractive candidates for optoelectronic applications, such as photovoltaics, light-emitting diodes, and lasers. Perovskite nanocrystals are of particular interest, where electrons and holes can be confined spatially, promoting radiative recombination. However, nanocrystalline films based on traditional colloidal nanocrystal synthesis strategies suffer from the use of long insulating ligands, low colloidal nanocrystal concentration, and significant aggregation during film formation. Here, we demonstrate a facile method for preparing perovskite nanocrystal films in situ and that the electroluminescence of light-emitting devices can be enhanced up to 40-fold through this nanocrystal film formation strategy. Briefly, the method involves the use of bulky organoammonium halides as additives to confine crystal growth of perovskites during film formation, achieving CH3NH3PbI3 and CH3NH3PbBr3 perovskite nanocrystals with an average crystal size of 5.4 ± 0.8 nm and 6.4 ± 1.3 nm, respectively, as confirmed through transmission electron microscopy measurements. Additive-confined perovskite nanocrystals show significantly improved photoluminescence quantum yield and decay lifetime. Finally, we demonstrate highly efficient CH3NH3PbI3 red/near-infrared LEDs and CH3NH3PbBr3 green LEDs based on this strategy, achieving an external quantum efficiency of 7.9% and 7.0%, respectively, which represent a 40-fold and 23-fold improvement over control devices fabricated without the additives.Keywords: in situ preparation; light-emitting diodes; nanocrystal synthesis; organic−inorganic hybrid perovskites; perovskite stability;

Organic–inorganic hybrid perovskite materials are emerging as semiconductors with potential application in optoelectronic devices. In particular, perovskites are very promising for light-emitting devices (LEDs) due to their high color purity, low nonradiative recombination rates, and tunable bandgap. Here, using pure CH3NH3PbI3 perovskite LEDs with an external quantum efficiency (EQE) of 5.9% as a platform, it is shown that electrical stress can influence device performance significantly, increasing the EQE from an initial 5.9% to as high as 7.4%. Consistent with the enhanced device performance, both the steady-state photoluminescence (PL) intensity and the time-resolved PL decay lifetime increase after electrical stress, indicating a reduction in nonradiative recombination in the perovskite film. By investigating the temperature-dependent characteristics of the perovskite LEDs and the cross-sectional elemental depth profile, it is proposed that trap reduction and resulting device-performance enhancement is due to local ionic motion of excess ions, likely excess mobile iodide, in the perovskite film that fills vacancies and reduces interstitial defects. On the other hand, it is found that overstressed LEDs show irreversibly degraded device performance, possibly because ions initially on the perovskite lattice are displaced during extended electrical stress and create defects such as vacancies.

•Porous scattering films are obtained by immersion precipitation of colorless polyimide for flexible substrate loss recovery.•The porous scattering films are applied to the flexible substrate backside of silver nanowires in neat colorless polyimide.•The scattering devices allow for 74% and 68% increase in EQE for green and white OLEDs, respectively, compared to glass/ITO.•The outcoupling efficiency of the porous scattering films remains unharmed after 5000 cycles at 2 mm bending radius.We demonstrate an upscalable approach to increase outcoupling in organic light-emitting diodes (OLEDs) fabricated on flexible substrates. The outcoupling enhancement is enabled by introducing a thin film of microporous polyimide on the backside of silver nanowire (AgNW) electrodes embedded in neat colorless polyimide. This porous polyimide film, prepared by immersion precipitation, utilizes a large index contrast between the polyimide host and randomly distributed air voids, resulting in broadband haze (>75%). In addition, the composite polyimide/AgNW scattering substrate inherits the high thermal (>360 °C), chemical, and mechanical stability of polyimides. The outcoupling efficiency of the composite scattering substrate is studied via optical characterization of the composite substrate and electron microscopy of the scattering film. The flexible scattering substrates compared to glass/indium tin oxide (ITO) allows for a 74% enhancement in external quantum efficiency (EQE) for a phosphorescent green OLED, and 68% EQE enhancement for a phosphorescent white OLED. The outcoupling enhancement remains unharmed after 5000 bending cycles at a 2 mm bending radius. Moreover, the color uniformity over viewing angles is improved, an important feature for lighting applications.Download high-res image (410KB)Download full-size image

Conventional solar cells absorb photons with energy above the bandgap of the active layer while sub-bandgap photons are unharvested. One way to overcome this loss is to capture the low energy light in the triplet state of a molecule capable of undergoing triplet–triplet annihilation (TTA), which pools the energy of two triplet states into one high energy singlet state that can then be utilized. This mechanism underlies the function of an organic intermediate band solar cell (IBSC). Here, we report a solid-state organic IBSC that shows enhanced photocurrent derived from TTA that converts sub-bandgap light into charge carriers. Femtosecond resolution transient absorption spectroscopy and delayed fluorescence spectroscopy provide evidence for the triplet sensitization and upconversion mechanisms, while external quantum efficiency measurements in the presence of a broadband background light demonstrate that sub-bandgap performance enhancements are achievable in this device. The solid-state architecture introduced in this work serves as an alternative to previously demonstrated solution-based IBSCs, and is a compelling model for future research efforts in this area.

We report a comprehensive study of the chemistry of perovskite optoelectronic device degradation and show that redox reactions are fundamental to the degradation process for CH3NH3PbI3, CsPbI3, and CsPbBr3 perovskites with Ag, Al, Yb, or Cr contacts. Using in situ X-ray diffraction measurements, we study the chemistry of CH3NH3PbI3 perovskite devices equipped with Al electrodes; we find that Al0 rapidly reduces Pb2+ to Pb0, converting CH3NH3PbI3 first to (CH3NH3)4PbI6·2H2O and then to CH3NH3I. In situ scanning electron microscopy measurements show that moisture enables continued reaction of the Al and perovskite layers by facilitating ion diffusion, before serving as a decomposition reagent for the perovskite film. Redox reactions follow what is expected based on standard electrochemical potentials for Al, Cr, and Yb; for Ag, the redox chemistry is enabled by the presence of iodide. We emphasize that critical chemical reactions can stem from intrinsic interfacial interactions between the layers in a device and not necessarily from external agents; degradation studies must consider the device as an entity, rather than focusing only on the stability of perovskite films.

Here we investigate the photophysical properties of Au(0)@Au(I)-thiolate nanoclusters by controlling the degree of aggregation, and measure electrochemical energy levels to design a metal nanocluster-based thin film LED (MNC-LED) structure. These efforts allow us to implement MNC-LEDs with luminance exceeding 40 cd m−2 and external quantum efficiency exceeding 0.1% with clearly visible orange emission. It is also demonstrated that by varying the sizes of nanoclusters, the electroluminescence spectrum of the device can be tuned to the infrared emission, indicating the possibility of exploiting metal nanocluster emitters for use over a wide spectral range.

A current bottleneck in the thin film photovoltaic field is the fabrication of low cost electrodes. We demonstrate ultrasonically spray coated multiwalled carbon nanotube (CNT) layers as opaque and absorptive metal-free electrodes deposited at low temperatures and free of post-deposition treatment. The electrodes show sheet resistance as low as 3.4 Ω □−1, comparable to evaporated metallic contacts deposited in vacuum. Organic photovoltaic devices were optically simulated, showing comparable photocurrent generation between reflective metal and absorptive CNT electrodes for photoactive layer thickness larger than 600 nm when using archetypal poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C61-butyric acid methyl ester (PCBM) cells. Fabricated devices clearly show that the absorptive CNT electrodes display comparable performance to solution processed and spray coated Ag nanoparticle devices. Additionally, other candidate absorber materials for thin film photovoltaics were simulated with absorptive contacts, elucidating device design in the absence of optical interference and reflection.

Despite high internal quantum efficiencies, planar organic light-emitting diodes (OLEDs) typically suffer from limited outcoupling efficiencies. To improve this outcoupling efficiency, we have developed a new thin (∼2 μm) light scattering layer that employs air voids (low-index scattering centers) embedded in a high-index polyimide matrix to effectively frustrate the substrate-trapped light, increasing the outcoupling efficiency. The porous polyimide scattering layers are created through the simple and scalable fabrication technique of phase inversion. The optical properties of the scattering layers have been characterized via microscopy, transmittance/haze measurements, and ellipsometry, which demonstrate the excellent scattering properties of these layers. We have integrated these films into a green OLED stack, where they show a 65% enhancement of the external quantum efficiency and a 77% enhancement of the power efficiency. Furthermore, we have integrated these layers into a white OLED and observed similar enhancements. Both the green and white OLEDs additionally demonstrate excellent color stability over wide viewing angles with the integration of this thin scattering layer.