Redox-active ions unlock substitutional doping in halide perovskites.

Electrical doping of metal halide perovskites (MPHs) is a key step towards the use of this efficient and cost-effective semiconductor class in modern electronics. In this work, we demonstrate n-type doping of methylammonium lead iodide (CH3NH3PbI3) by the post-fabrication introduction of Sm2+. The ionic radius of the latter is similar to that of Pb2+ and can replace it without altering the perovskite crystal lattice. It is demonstrated that once incorporated, Sm2+ can act as a dopant by undergoing oxidation to Sm3+. This results in the release of a negative charge that n-dopes the material, resulting in an increase of conductivity of almost 3 orders of magnitude. Unlike substitution doping with heterovalent ions, furtive dopants do not require counterions to maintain charge neutrality with respect to the ions they replace and are thus more likely to be incorporated into the crystalline structure. The incorporation of the dopant throughout the material is evidenced by XPS and ToF-SIMS, while the XRD pattern shows no phase separation at low and medium doping concentrations. A shift of the Fermi level towards a conduction energy of 0.52 eV confirms the doping to be n-type with a charge carrier density, calculated using the Mott-Schottky method, estimated to be nearly 1017 cm-3 for the most conductive samples. Variable-temperature conductivity experiments show that the dopant is only partially ionized at room temperature due to dopant freeze-out.

Probing proton diffusion as a guide to environmental stability in powder-engineered FAPbI3 and CsFAPbI3 perovskites.

Formamidinium lead iodide-based solar cells show promising device reliability. The grain imperfection can be further suppressed by developing powder methodology. The water uptake capability is critical for the stability of α-formamidinium lead triiodide (FAPbI3) thin films, and elucidating the migration of hydrogen species is challenging using routine techniques such as imaging or mass spectroscopy. Here, we decipher the proton diffusion to quantify indirect monitoring of H migration by following the N-D vibration using transmission infrared spectroscopy. The technique allows a direct assessment of the perovskite degradation associated with moisture. The inclusion of Cs in FAPbI3, reveals significant differences in proton diffusion rates, attesting to its impact. CsFAPbI3's ability to block the active layer access by water molecules is five times higher than α-FAPbI3, which is significantly higher than methylammonium lead triiodide (MAPbI3). Our protocol directly probes the local environment of the material to identify its intrinsic degradation mechanisms and stability, a key requirement for optoelectronic applications.

Narcissistic self-sorting of n-acene nano-ribbons yielding energy-transfer and electroluminescence at p-n junctions.

The 2,3-didecyloxy derivative of an n-type anthracene (n-BG) and a p-type tetracene (p-R) have been synthesized and their self-assembly into nano-ribbons studied. Hyperspectral fluorescence imaging revealed their narcissistic self-sorting, leading to separated nanoribbons emitting with very different colors (blue or green for n-BG, depending on the growth solvent, and red for p-R). It is unique that the usual origins of self-sorting, such as specific H-bonding, different growth kinetics, or incompatible steric hindrance can be ruled out. Hence, the narcissistic behaviour is herein proposed to originate from a so-far unconsidered cause: the discrepancy between the quadrupolar character of n-BG and dipolar character of p-R. At the p-n junctions of these nanoribbons, inter-ribbon FRET and electro-luminescence switch-on were observed by fluorescence/luminescence microscopy.

Supramolecular gating of TADF process in self-assembled nano-spheres for high-resolution OLED applications.

Acridine-based donor-acceptor chromophores exhibiting E-type delayed fluorescence were substituted with bis-biuret H-bonding motifs to induce the formation of hollow spheres which can be deposited from solution to form the active component of OLED devices. In solution, the contribution of the delayed component is sensitive to disruption of the aggregates.

Comment on "Eppur si Muove: Proton Diffusion in Halide Perovskite Single Crystals": Eppur Non si Muove: A Critical Evaluation of Proton Diffusion in Halide Perovskite Single Crystals.

A recent report by Cahen and co-workers is examined that finds the diffusion constant for proton migration in methylammonium lead triiodide single crystals to be 2 × 105 -fold greater than that previously reported by Sadhu et al. By comparing the conversion of single crystals versus microcrystalline samples, it is concluded that proton diffusion in macroscopic single crystals is accelerated by the presence of defects acting as high-diffusivity paths.

Observing the Migration of Hydrogen Species in Hybrid Perovskite Materials through D/H Isotope Exchange.

Unlike heavier elements, the migration of hydrogen species in perovskite materials cannot be directly tracked using imaging or mass spectrocopy techniques. Our results show that quantitative analysis of D/H exchange in PbCH3ND3I3 allows indirect monitoring of H migration by following the N-D vibration using polarization-modulated infrared reflection-absorption spectroscopy. Kinetic analysis shows that the isotope exchange process is pseudo-first order and particularly sensitive to the intensity of light and relative humidity, and, to a lesser degree, sample thickness. In the presence of light (450 nm), the D/H exchange is accelerated up to 10-fold with respect to samples in the dark but slows again for higher light intensities.. The technique also allows the direct assessment of the efficiency of protective layers toward deterioration of hybrid organic-inorganic perovskite devices by moisture. Comparison of different monolayer-forming fluorinated molecules reveals important differences in rates attesting to variations between their efficiency in blocking access to the active layer by water molecules.

Role of Oxide/Metal Bilayer Electrodes in Solution Processed Organic Field Effect Transistors.

High performance, air stable and solution-processed small molecule 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) based organic field-effect transistors (OFETs) with various electrode configurations were studied in detail. The contact resistance of OFET devices with Ag, Au, WO3/Ag, MoO3/Ag, WO3/Au, and MoO3/Au were compared. Reduced contact resistance and consequently improved performance were observed in OFET devices with oxide interlayers compared to the devices with bare metal electrodes. The best oxide/metal combination was determined. The possible mechanisms for enhanced electrical properties were explained by favorable morphological and electronic structure of organic/metal oxide/metal interfaces.

Application of Rubrene Air-Gap Transistors as Sensitive MEMS Physical Sensors.

Micro-electromechanical systems (MEMS) made of organic materials have attracted efforts for the development a new generation of physical, chemical, and biological sensors, for which the electromechanical sensitivity is the current major concern. Here, we present an organic MEMS made of a rubrene single-crystal air-gap transistor. Applying mechanical pressure on the semiconductor results in high variations in drain current: an unparalleled gauge factor above 4000 has been measured experimentally. Such a high sensitivity is induced by the modulation of charge injection at the interface between the gold electrode and the rubrene semiconductor as an unusual transducing effect. Applying these devices to the detection of acoustic pressure shows that force down to 230 nN can be measured with a resolution of 40 nN. This study demonstrates that MEMS based on rubrene air-gap transistors constitute a step forward in the development of high-performance flexible sensors.

A Multifunctional Interlayer for Solution Processed High Performance Indium Oxide Transistors.

Multiple functionality of tungsten polyoxometalate (POM) has been achieved applying it as interfacial layer for solution processed high performance In2O3 thin film transistors, which results in overall improvement of device performance. This approach not only reduces off-current of the device by more than two orders of magnitude, but also leads to a threshold voltage reduction, as well as significantly enhances the mobility through facilitated charge injection from the electrode to the active layer. Such a mechanism has been elucidated through morphological and spectroscopic studies.

Kinetic Isotope Effects Provide Experimental Evidence for Proton Tunneling in Methylammonium Lead Triiodide Perovskites.

The occurrence of proton tunneling in MAPbI3 hybrid organic inorganic perovskites is demonstrated through the effect of isotopic labeling of the methylammonium (MA) component on the dielectric permittivity response. Deuteration of the ammonium group results in the acceleration of proton migration (inverse primary isotope effect), whereas deuteration of the methyl group induces a normal secondary isotope effect. The activation energies for proton migration are calculated to be 50 and 27 meV for the tetragonal and orthorhombic phases, respectively, which decrease upon deuteration of the ammonium group. The low activation barrier and the deviation from unity of the ratio of the pre-exponential factors (AH/AD = 0.3-0.4) are consistent with a tunneling mechanism for proton migration. Deuteration of the PEDOT:PSS hole transport layer results in a behavior that is intermediate between that of the deuterated and undeuterated perovskite, due to extrinsic ion migration between the two materials.

Frequency-Selective Photobleaching as a Route to Chromatic Control in Supramolecular OLED Devices.

We report a series of molecules that spontaneously self-organize into small electroluminescent domains of sub-micrometer dimensions when dissolved in tetrahydrofuran. The self-assembled spherical aggregates have an average diameter of 300 nm and exhibit efficient energy transfer from the blue to the green or red component. The aggregates can be chromatically addressed or patterned by selective bleaching of the energy-acceptor component using a laser source. This allows the fabrication of electroluminescence devices by directly photopatterning the active layer without the need of additional steps. Submicron features (700 nm) can be achieved using a collimated light source.

Crucial Role of the Electron Transport Layer and UV Light on the Open-Circuit Voltage Loss in Inverted Organic Solar Cells.

Understanding the degradation mechanisms in organic photovoltaics is crucial in order to develop stable organic semiconductors and robust device architectures. The rapid loss of efficiency, referred to as burn-in, is a major issue to be addressed. This study reports on the influence of the electron transport layer (ETLs) and UV light on the drop of open-circuit voltage (Voc) for P3HT:PC60BM-based devices. The results show that Voc loss is induced by the UV and, more importantly, that the ETL can amplify it, with TiOx yielding a stronger drop than ZnO. Using impedance spectroscopy (IS) and X-ray photoelectron spectroscopy (XPS), different degradation mechanisms were identified according to whether the ETL is TiOx or ZnO. For TiOx-based devices, the formation of an interface dipole was identified, resulting in a loss of the flat-band potential (Vfb) and, thus, of the Voc. For ZnO-based devices, chemical modifications of the metal oxide and active layer at the interface were detected, resulting in a doping of the active layer which impacts the Voc. This study highlights the role of the architecture and, more specifically, of the ETL in the severity of burn-in and degradation pathways.

Long-Term Stable Organic Photodetectors with Ultra Low Dark Currents for High Detectivity Applications.

Printed organic photodetectors can transform plastic, paper or glass into smart surfaces. This innovative technology is now growing exponentially due to the strong demand in human-machine interfaces. To date, only niche markets are targeted since organic sensors still present reduced performances in comparison with their inorganic counterparts. Here we demonstrate that it is possible to engineer a state-of-the-art organic photodetector approaching the performances of Si-based photodiodes in terms of dark current, responsivity and detectivity. Only three solution-processed layers and two low-temperature annealing steps are needed to achieve the performance that is significantly better than most of the organic photodetectors reported so far. We also perform a long-term ageing study. Lifetimes of over 14,000 hours under continuous operation are more than promising and demonstrate that organic photodetectors can reach a competitive level of stability for successful commercialization of this new and promising technology.

Piezoelectric polymer gated OFET: Cutting-edge electro-mechanical transducer for organic MEMS-based sensors.

The growth of micro electro-mechanical system (MEMS) based sensors on the electronic market is forecast to be invigorated soon by the development of a new branch of MEMS-based sensors made of organic materials. Organic MEMS have the potential to revolutionize sensor products due to their light weight, low-cost and mechanical flexibility. However, their sensitivity and stability in comparison to inorganic MEMS-based sensors have been the major concerns. In the present work, an organic MEMS sensor with a cutting-edge electro-mechanical transducer based on an active organic field effect transistor (OFET) has been demonstrated. Using poly(vinylidenefluoride/trifluoroethylene) (P(VDF-TrFE)) piezoelectric polymer as active gate dielectric in the transistor mounted on a polymeric micro-cantilever, unique electro-mechanical properties were observed. Such an advanced scheme enables highly efficient integrated electro-mechanical transduction for physical and chemical sensing applications. Record relative sensitivity over 600 in the low strain regime (<0.3%) was demonstrated, which represents a key-step for the development of highly sensitive all organic MEMS-based sensors.

Electroluminescence from Spontaneously Generated Single-Vesicle Aggregates Using Solution-Processed Small Organic Molecules.

Self-assembled aggregates offer great potential for tuning the morphology of organic semiconductors, thereby controlling their size and shape. This is particularly interesting for applications in electroluminescent (EL) devices, but there has been, to date, no reports of a functional EL device in which the size and color of the emissive domains could be controlled using self-assembly. We now report a series of molecules that spontaneously self-organize into small EL domains of sub-micrometer dimensions. By tailoring the emissive chromophores in solution, spherical aggregates that have an average size of 300 nm in diameter and emit any one color, including CIE D65 white, are spontaneously formed in solution. We show that the individual aggregates can be used in EL devices built either using small patterned electrodes or using a sandwich architecture to produce devices emitting in the blue, green, red, and white. Furthermore, sequential deposition of the three primary colors yields an RGB device in which single aggregates of each color are present in close proximity.

Lanthanum Hexaboride As Novel Interlayer for Improving the Thermal Stability of P3HT:PCBM Organic Solar Cells.

For efficient organic photovoltaic (OPV) solar cells, a low work function electrode is necessary to enhance the built-in voltage of the active layer, thereby improving the overall efficiency. Calcium is often used for this purpose in the laboratory; however, its development on a larger scale is impaired by its high reactivity with oxygen and water and the resulting low stability of solar cells under operation. The influence of a novel interlayer, lanthanum hexaboride (LaB6), on the electronic properties of OPV is studied in this work. Similarly to calcium, when LaB6 is used as an interlayer, it enhances the built-in voltage in the device, leading to a higher fill factor (FF) and optimal open circuit voltage (V(oc)). As a result, optimized LaB6-based devices present significantly improved power conversion efficiencies. More importantly, while calcium/aluminum (Ca/Al) and aluminum (Al) cathodes lose their capacity to enhance the internal electrical field during thermal aging, the LaB6/aluminum (LaB6/Al) electrode remains stable. This remarkable effect results in a highly stable V(oc) and flat-band potential during aging.

Solution-Processed Small-Molecule Bulk Heterojunctions: Leakage Currents and the Dewetting Issue for Inverted Solar Cells.

In organic photovoltaic (PV) devices based on solution-processed small molecules, we report here that the physicochemical properties of the substrate are critical for achieving high-performances organic solar cells. Three different substrates were tested: ITO coated with PEDOT: PSS, ZnO sol-gel, and ZnO nanoparticles. PV performances are found to be low when the ZnO nanoparticles layer is used. This performance loss is attributed to the formation of many dewetting points in the active layer, because of a relatively high roughness of the ZnO nanoparticles layer, compared to the other layers. We successfully circumvented this phenomenon by adding a small quantity of polystyrene (PS) in the active layer. The introduction of PS improves the quality of film forming and reduces the dark currents of solar cells. Using this method, high-efficiency devices were achieved, even in the case of substrates with higher roughness.

Thermal stabilisation of polymer-fullerene bulk heterojunction morphology for efficient photovoltaic solar cells.

A novel stable bisazide molecule that can freeze the bulk heterojunction morphology at its optimized layout by specifically bonding to fullerenes is reported. The concept is demonstrated with various polymers: fullerene derivatives systems enable highly thermally stable polymer solar cells.

New synthetic routes towards soluble and dissymmetric triphenodioxazine dyes designed for dye-sensitized solar cells.

New π-conjugated structures are constantly the subject of research in dyes and pigments industry and electronic organic field. In this context, the triphenodioxazine (TPDO) core has often been used as efficient photostable pigments and once integrated in air stable n-type organic field-effect transistor (OFET). However, little attention has been paid to the TPDO core as soluble materials for optoelectronic devices, possibly due to the harsh synthetic conditions and the insolubility of many compounds. To benefit from the photostability of TPDO in dye-sensitized solar cells (DSCs), an original synthetic pathway has been established to provide soluble and dissymmetric molecules applied to a suitable design for the sensitizers of DSC. The study has been pursued by the theoretical modeling of opto-electronic properties, the optical and electronic characterizations of dyes and elaboration of efficient devices. The discovery of new synthetic pathways opens the way to innovative designs of TPDO for materials used in organic electronics.