Dependence of Crystallite Formation and Preferential Backbone Orientations on the Side Chain Pattern in PBDTTPD Polymers

Alkyl substituents appended to the π-conjugated main chain account for the solution-processability and film-forming properties of most π-conjugated polymers for organic electronic device applications, including field-effect transistors (FETs) and bulk-heterojunction (BHJ) solar cells. Beyond film-forming properties, recent work has emphasized the determining role that side-chain substituents play on polymer self-assembly and thin-film nanostructural order, and, in turn, on device performance. However, the factors that determine polymer crystallite orientation in thin-films, implying preferential backbone orientation relative to the device substrate, are a matter of some debate, and these structural changes remain difficult to anticipate. In this report, we show how systematic changes in the side-chain pattern of poly(benzo[1,2-b:4,5-b′]dithiophene–alt–thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers can (i) influence the propensity of the polymer to order in the π-stacking direction, and (ii) direct the preferential orientation of the polymer crystallites in thin films (e.g., “face-on” vs “edge-on”). Oriented crystallites, specifically crystallites that are well-ordered in the π-stacking direction, are believed to be a key contributor to improved thin-film device performance in both FETs and BHJ solar cells

Ordering Effects in Benzo[1,2-b:4,5- b′]difuran-thieno[3,4- c]pyrrole-4,6-dione Polymers with >7% Solar Cell Efficiency

Benzo[1,2-b:4,5-b\u27]difuran–thieno[3,4-c]pyrrole-4,6-dione (PBDFTPD) polymers prepared by microwave-assisted synthesis can achieve power conversion efficiencies (PCEs) >7% in bulk-heterojunction solar cells with phenyl-C61/71-butyric acid methyl ester (PCBM). In “as-cast” PBDFTPD-based devices solution-processed without a small-molecule additive, high PCEs can be obtained in spite of the weak propensity of the polymers to self-assemble and form π-aggregates in thin films

General Post-annealing Method Enables High-Efficiency Two-Dimensional Perovskite Solar Cells

Two-dimensional (2D) perovskites, with a formula of (RNH3)2MAn-1PbnI3n+1, have shown impressive photovoltaic device efficiency with improved stability. The operating mechanism of such photovoltaic devices is under debate and the scope of incorporated organic cations (RNH3 +) is limited. We report a general post-annealing method to incorporate a variety of organic cations into 2D perovskites, which demonstrate significant device efficiencies (7-12%). A detailed investigation of the archetypical (C4H9NH3)2MA3Pb4I13 (n = 4) reveals that such perovskites thin films contain multiple 2D phases (i.e., 2D quantum wells, n = 2, 3, 4,...). These phases appear to be distributed with decreasing n values from the top to the bottom of the 2D perovskites thin film, enabling efficient energy transfer in the first 500 ps and possible charge transfer at longer time scale, thereby accounting for high device efficiencies. Our post-annealing method is compatible with ambient condition and only requires relatively low annealing temperature for a very short period of time, offering significant prospects for scalable manufacturing of 2D perovskites solar cells

Stable Glasses of Organic Semiconductor Resist Crystallization

The instability of glassy solids poses a key limitation to their use in several technological applications. Well-packed organic glasses, prepared by physical vapor deposition (PVD), have drawn attention recently because they can exhibit significantly higher thermal and chemical stability than glasses prepared from more traditional routes. We show here that PVD glasses can also show enhanced resistance to crystallization. By controlling the deposition temperature, resistance towards crystallization can be enhanced by at least a factor of ten in PVD glasses of the model organic semiconductor Alq3 (Tris(8-hydroxyquinolinato) aluminum). PVD glasses of Alq3 first transform into a supercooled liquid before crystallizing. By controlling the deposition temperature, we increase the glassliquid transformation time thereby also increasing the overall time for crystallization. We thus demonstrate a new strategy to stabilize glasses of organic semiconductors against crystallization, which is a common failure mechanism in OLED (organic light emitting diode) devices.US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, award DE-SC0002161

ELECTROCHEMICALLY ADSORBED Pb ON Ag (111) STUDIED WITH GRAZING-INCIDENCE X-RAY SCATTERING

Nous présentons l'étude par diffraction de rayons X en incidence rasante de l'évolution de couches de plomb sur argent (111) déposées électrochimiquement en fonction du potentiel électrochimique. Les mesures ont été faites avec des couches adsorbées en contact avec la solution dans une cellule échantillon spécialement préparée. Les structures de plomb observées sont fonction du potentiel appliqué et varient d'une couche incommensurable pour un dépôt effectué à sous-potentiel, jusqu'à un ensemble polycristallin orienté au hasard pour des potentiels de dépôt plus faibles. Ces premières expériences montrent la puissance des mesures de diffractions de rayons X in situ pour déterminer les structures de surface résultant d'un dépôt électrochimique.Grazing-incidence x-ray scattering studies of the evolution of electrochemically deposited layers of lead on silver (111) as a function of applied electrochemical potential are presented. Measurements were made with the adsorbed layers in contact with solution in a specially designed sample cell. The observed lead structures are a function of the applied potential and range from an incommensurate monolayer, resulting from underpotential deposition, to randomly oriented polycrystalline bulk lead, resulting from lower deposition potentials. These early experiments demonstrate the ability of in situ x-ray diffraction measurements to determine structures associated with electrochemical deposition

Transformation from crystalline precursor to perovskite in PbCl2 derived MAPbI3

Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide MAPbI3 from the single step deposition of lead chloride and three equivalents of methy lammonium iodide PbCl2 3MAI MA CH3NH3 . The as spun film consists of crystalline MA2PbI3Cl, which is composed of one dimensional chains of lead halide octahedra, coex isting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic device

Use of a Multiple Hydride Donor to Achieve an n Doped Polymer with High Solvent Resistance

The ability to insolubilize doped semiconducting polymer layers can help enable the fabrication of efficient multilayer solution processed electronic and optoelectronic devices. Here, we present a promising approach to simultaneously n dope and largely insolubilize conjugated polymer films using tetrakis[ 4 1,3 dimethyl 2,3 dihydro 1H benzo[d]imidazol 2 yl phenoxy methyl]methane tetrakis O DMBI H , which consists of four 2,3 dihydro 1H benzoimidazole DMBI H n dopant moieties covalently linked to one another. Doping a thiophene fused benzodifurandione based oligo p phenylenevinylene co thiophene polymer TBDOPV T with tetrakis O DMBI H results in a highly n doped film with bulk conductivity of 15 S amp; 8239;cm 1. Optical absorption spectra provide evidence for film retention of amp; 8764;93 after immersion in o dichlorobenzene for 5 min. The optical absorption signature of the charge carriers in the n doped polymer decreases only slightly more than that of the neutral polymer under these conditions, indicating that the exposure to solvent also results in negligible dedoping of the film. Moreover, thermal treatment studies on a tetrakis O DMBI H doped TBDOPV T film in contact with another undoped polymer film indicate immobilization of the molecular dopant in TBDOPV T. This is attributed to the multiple electrostatic interactions between each dopant tetracation and up to four nearby anionic doped polymer segment