Gitti L. Frey

Inorganic solution-processed hole-injecting and electron-blocking layers in polymer light-emitting diodes

The use of the solution-processed layered transition metal dichalcogenide (LTMDC) MoS2 as a hole-injecting electrode in polymer light-emitting diodes (LEDs) is reported. MoS2 functions as a very high work function metal and, in combination with an electron-blocking layer in the form of MoO3, provides good LED performance. In this study we investigated model LED devices with a single semiconductor layer, namely, the electron transporting polymer poly-[2,7-(9,9′-di-n-octylfluorene)-3,6-benzothiadiazole]. LED operation was successfully modeled using experimentally determined work functions, carrier mobilities, and barrier properties. Good agreement between experiment and model allows us to demonstrate that the MoS2 and the MoO3 layers act as a high work function hole-injection layer (MoS2) and an electron extraction barrier layer (MoO3), respectively. They improve device performance by allowing the buildup of electron density at the oxide/emissive layer interface which generates a local field, enhancing hole...

Facile infiltration of semiconducting polymer into mesoporous electrodes for hybrid solar cells

Hybrid composites of semiconducting polymers and metal oxides are promising combinations for solar cells. However, forming a well-controlled nanostructure with bicontinuous interpenetrating networks throughout the photoactive film is difficult to achieve. Pre-structured “mesoporous” metal oxide electrodes can act as a well-defined template for latter polymer infiltration. However, the long range infiltration of polymer chains into contorted porous channels has appeared to elude the scientific community, limiting the advancement of this technology. Here we present a structural and electronic characterisation of poly(3-hexylthiophene) (P3HT) infiltrated into mesoporous dye-sensitized TiO2. Through a combination of techniques we achieve uniform pore filling of P3HT up to depths of over 4 μm, but the volumetric fraction of the pores filled with polymer is less than 24%. Despite this low pore-filling, exceptionally efficient charge collection is demonstrated, illustrating that pore filling is not the critical issue for mesoporous hybrid solar cells.

Patterned electrode vertical field effect transistor fabricated using block copolymer nanotemplates

We report the design and implementation of a vertical organic field effect transistor which is compatible with standard device fabrication technology and is well described by a self consistent device model. The active semiconductor is a film of C60 molecules, and the device operation is based on the architecture of the nanopatterned source electrode. The relatively high resolution fabrication process and maintaining the low-cost and simplicity associated with organic electronics, necessitates unconventional fabrication techniques such as soft lithography. Block copolymer self-assembled nanotemplates enable the production of conductive, gridlike metal electrode. The devices reported here exhibit On/Off ratio of 104.

Understanding and controlling organic-inorganic interfaces in mesostructured hybrid photovoltaic materials.

The chemical compositions and structures of organic-inorganic interfaces in mesostructurally ordered conjugated polymer-titania nanocomposites are shown to have a predominant influence on their photovoltaic properties. Such interfaces can be controlled by using surfactant structure-directing agents (SDAs) with different architectures and molecular weights to promote contact between the highly hydrophobic electron-donating conjugated polymer species and hydrophilic electron-accepting titania frameworks. A combination of small-angle X-ray scattering (SAXS), scanning and transmission electron microscopy (SEM, TEM), and solid-state NMR spectroscopy yields insights on the compositions, structures, and distributions of inorganic and organic species within the materials over multiple length scales. Two-dimensional NMR analyses establish the molecular-level interactions between the different SDA blocks, the conjugated polymer, and the titania framework, which are correlated with steady-state and time-resolved photoluminescence measurements of the photoexcitation dynamics of the conjugated polymer and macroscopic photocurrent generation in photovoltaic devices. Molecular understanding of the compositions and chemical interactions at organic-inorganic interfaces are shown to enable the design, synthesis, and control of the photovoltaic properties of hybrid functional materials.

Self-assembled conjugated polymer–surfactant–silica mesostructures and their integration into light-emitting diodes

A self-assembly process for the preparation of functional mesoscopically ordered semiconducting polymer–silica nanocomposite thin films is reported. The nanocomposites are prepared by introducing pre-synthesized semiconducting polymers into a tetrahydrofuran (THF)–water homogeneous sol solution containing silica precursor species and a surface-active agent. Depending on the concentration of the surface-active agent, it was possible to prepare materials with three different types of mesostructural order: i) a 2D hexagonal mesophase silica with conjugated polymer guest species incorporated within the hydrophobic cylinders organized in domains aligned parallel to the substrate surface plane; ii) a lamellar mesophase silica with the layers oriented parallel to the substrate surface and the conjugated polymer guest species incorporated in the hydrophobic layers; or iii) an apparent intermediate phase consisting of a mixture of the hexagonal and lamellar phases in addition to worm-like aggregates with no appreciable orientational order. The continuous through-film conductive pathway provided by the intermediate phase has allowed the integration of ordered semiconducting polymer–silica nanocomposites into opto-electronic devices. By comparison, the lamellar mesostructure prevents through-film conduction, with the result that no light emission occurs. Blue-, green- and red-emitting diodes comprising blue-emitting poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), green-emitting poly(9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-(2,1′,3)-thiadiazole) (F8BT), and red-emitting poly[2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEHPPV) confined within the 2D hexagonal silica nanostructure were fabricated with luminances of ca. 3 cd m–2 at 15 V. Device performances provide criteria for optimizing the selection of synthesis chemistries, processing conditions, compositions, and structures, for light-emission properties sought.

Spontaneous interlayer formation in OPVs by additive migration due to additive–metal interactions

The presence of interlayers between the active layer and the electrode are known to modify the metal work-function and enhance carrier extraction, consequently improving OPV device performance. Spontaneous formation of interlayers by surface-enrichment of suitable additives eliminates separate processing steps and hence is technically advantageous and cost effective. However, surface enrichment is limited to additives with low surface energy. Here we show that additive migration to the organic/electrode interface could be induced by additive–metal interactions, modulated by the interactions between the additive and the underlying substrate. In this study, additive migration induced by metal evaporation is studied by blending P3HT with PEG, an established interlayer material with a surface energy higher than that of P3HT. XPS analysis reveals that, as expected, PEG is not present on the surface of the organic spun film. However, Ca or Al evaporation induces a significant migration of PEG to the organic/metal interface. In contrast, Au evaporation does not induce such migration. The comparison between Al, Ca and Au, metals with significantly different reduction potentials, revealed that the driving force for PEG migration is its chemical interaction with the deposited metal atoms. The extent of PEG migration was also found to depend on the type of underlying substrate, ITO/PEDOT:PSS or ITO. Finally, the PEG interlayer is shown to reduce the Al work function confirming that spontaneous additive migration induced by metal–additive interactions could be harnessed for charge extraction in organic electronic devices.

Atomic layer deposition of zinc oxide onto and into P3HT for hybrid photovoltaics

Hybrid organic–inorganic bulk heterojunction (BHJ) photovoltaic devices continue to be a promising alternative for present semiconductor solar cell technology. However, to become competitive hybrid devices must improve their currently low efficiencies. A major challenge to overcome is the control over the hybrid morphology, and more specifically, directing interpenetrated nano-scale phase separated continuous networks through the active layer. Here we demonstrate that atomic layer deposition (ALD) can be used to deposit hybrid BHJ photovoltaic films with exceptional control over film composition and morphology. The BHJ is prepared by exposing a pre-formed conjugated polymer film to an ALD alternating sequence of a metal oxide precursor and water. In this study ZnO was grown from cycles of diethyl zinc (DEZ) and water inside pre-formed P3HT films. We find that DEZ diffuses into the amorphous regions of the P3HT film, followed by oxidation by water to form ZnO crystalline particles (5–10 nm). Importantly, the inorganic crystalline phase is formed within the polymer amorphous regions while the ordered polymer domains are maintained. Investigation of the growth mechanism and control over the number of ALD cycles allowed us to direct a BHJ morphology with: (I) a continuous ZnO network through the P3HT film; (II) a descending concentration gradient of ZnO from the top surface down to the substrate; and (III) a dense ZnO electron transporting layer on the polymer film surface. The successful morphology-control is manifested in efficient photocurrent generation, evident from time resolved microwave-photoconductivity measurements and device performances that are similar to those reported for intensely optimized conjugated polymer/metal oxide solar cells.

Hybrid mesostructured electrodes for fast-switching proton -based solid state electrochromic devices

Tungsten oxide, the most commonly used electrochromic material, reversibly changes its color from transparent to blue under applied bias. The rate of color modulation, a parameter critical for device applications, is determined by the rate of cation injection into WO3 and its diffusion inside the solid. Here we show that fast switching could be obtained by processing mesostructured hybrid electrochromic electrodes with high organic–inorganic interfacial area and intimate contact between an organic electrolyte and WO3. The hybrid electrode is prepared by infiltrating a polymer electrolyte, Nafion, into a highly porous sol–gel processed WO3 matrix. Energy filtered-transmission electron microscopy is used to map the distribution of Nafion in the micro- and nano-sized pores. The images corroborate the formation of a continuous Nafion network through the inorganic scaffold and high WO3–Nafion interfacial area, for proton conductivity and proton transfer, respectively. The use of a polymer electrolyte, in contrast to commonly used liquid electrolytes, allows integration of the hybrid electrode into fully solid state devices. The device shows dramatically reduced response times compared to the corresponding bi-layer WO3–Nafion electrodes, associated with the mesostructured morphology of the solid hybrid electrode.

Enhanced reversible electrochromism via in situ phase transformation in tungstate monohydrate

This study demonstrates that realizing the correlation between in situ crystallographic structure modifications of an electrochromic material and its functionality leads to improved performances, which can then contribute to a variety of energy-efficient applications.

Self-assembled lamellar MoS2, SnS2 and SiO2 semiconducting polymer nanocomposites.

Lamellar nanocomposites based on semiconducting polymers incorporated into layered inorganic matrices are prepared by the co-assembly of organic and inorganic precursors. Semiconducting polymer-incorporated silica is prepared by introducing the semiconducting polymers into a tetrahydrofuran (THF)/water homogeneous sol solution containing silica precursor species and a surface-active agent. Semiconducting polymer-incorporated MoS2 and SnS2 are prepared by Li intercalation into the inorganic compound, exfoliation and restack in the presence of the semiconducting polymer. All lamellar nanocomposite films are organized in domains aligned parallel to the substrate surface plane. The incorporated polymers maintain their semiconducting properties, as evident from their optical absorption and photoluminescence spectra. The optoelectronic properties of the nanocomposites depend on the properties of both the inorganic host and the incorporated guest polymer as demonstrated by integrating the nanocomposite films into light-emitting diodes. Devices based on polymer-incorporated silica and polymer-incorporated MoS2 show no diode behaviour and no light emission due to the insulating and metallic properties of the silica and MoS2 hosts. In contrast, diode performance and electroluminescence are obtained from devices based on semiconducting polymer-incorporated semiconducting SnS2, demonstrating that judicious selection of the composite components in combination with the optimization of material synthesis conditions allows new hierarchical structures to be tailored for electronic and optoelectronic applications.