70 research outputs found

    Physics of organic-organic interfaces

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    Low-Temperature Behaviour of Charge Transfer Excitons in Narrow-Bandgap Polymer-Based Bulk Heterojunctions

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    Photoluminescence studies of the charge transfer exciton emission from a narrow-bandgap polymer-based bulk heterojunction are reported. The quantum yield of this emission is as high as 0.03%. Low temperature measurements reveal that while the dynamics of the singlet exciton is slower at low temperature, the dynamics of the charge transfer exciton emission is temperature independent. This behavior rules out any diffusion process of the charge transfer excitons and energy transfer from these interfacial states toward lower lying states. Photoluminescence measurements performed on the device under bias show a reduction (but not the total suppression) of the charge transfer exciton recombination. Finally, based on the low temperature results the role of the charge transfer excitons and the possible pathways to populate them are identified

    Physics of organic-organic interfaces

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    This thesis focused on the experimental study of a variety of organic-organic heterojunctions that have applications in organic devices. Organic-organic interfaces are inhered for plastic electronics, which is promising alternatives to epitaxially grown, inorganic electronics. However, before organic-based electronics become widespread in daily live much effort is required. Not all organic semiconductors-based devices perform sufficiently to be of commercial interest. Thus, the understanding and the control of the processes determining the properties of organic semiconductors is crucial to the further development of organic electronics. In this work, by combining spectroscopy and microscopy techniques, the nature of the interactions between the materials and the influence on the final properties of the hetrojunction was investigated.

    Inorganically Functionalized PbS-CdS Colloidal Nanocrystals:Integration into Amorphous Chalcogenide Glass and Luminescent Properties

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    Inorganic semiconductor nanocrystals (NCs) with bright, stable, and wavelength-tunable luminescence are very promising emitters for various photonic and optoelectronic applications. Recently developed strategies for inorganic surface capping of colloidal NCs using metal chalcogenide complexes have opened new perspectives for their applications. Here we report an all-inorganic surface functionalization of highly luminescent IR-emitting PbS-CdS NCs and studies of their luminescence properties. We show that inorganic capping allows simple low-temperature encapsulation of inorganic NCs into a solution-cast IR-transparent amorphous As2S3 matrix. The resulting all-inorganic thin films feature stable IR luminescence in the telecommunication wavelength region. The high optical dielectric constant of As2S3 also helps reduce the dielectric screening of the radiating field inside the quantum dot, enabling fast radiative recombination in PbS-CdS NCs.</p

    Ambipolar all-polymer bulk heterojunction field-effect transistors

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    We demonstrate solution processable all-polymer based field-effect transistors (FETs) exhibiting comparable electron and hole mobilities. The semiconducting layer is a bulk heterojunction of poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (n-type polymer) and regioregular poly(3-hexylthiophene) (p-type polymer). These polymers form a type-II heterojunction as revealed by the faster photoluminescence dynamics of the blend compared to the pristine materials. An electron mobiliy of 4 x 10(-3) cm(2)/V s and a hole mobility of 2 x 10(-3) cm(2)/V s were extracted from the transfer characteristics of bottom contact FETs. The balanced mobilities suggest that the active layer is a fine network of the two components, as confirmed by atomic force microscopy phase images

    Charge-Separation Dynamics in Inorganic–Organic Ternary Blends for Efficient Infrared Photodiodes

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    Knowledge about the working mechanism of the PbS:P3HT:PCBM [P3HT = poly(3-hexylthiophene), PCBM = [6,6]-phenyl-C61-butyric acid methyl ester] hybrid blend used for efficient near-infrared photodiodes is obtained from time-resolved photoluminescence (PL) studies. To understand the role of each component in the heterojunction, the PL dynamics of the ternary (PbS:P3HT:PCBM) blend and the binary (PbS:P3HT, PbS:PCBM and P3HT:PCBM) blends are compared with the PL of the pristine PbS nanocrystals (NCs) and P3HT. In the ternary blend the efficiency of the charge transfer is significantly enhanced compared to the one of PbS:P3HT and PbS:PCBM blends, indicating that both hole and electron transfer from excited NCs to the polymer and fullerene occur. The hole transfer towards the P3HT determines the equilibration of their population in the NCs after the electron transfer towards PCBM, allowing their re-excitation and new charge transfer process.

    Photoluminescence of conjugated polymer blends at the nanoscale

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    Here we report on a combined photoluminescence and morphological study of a polymer–polymer blend composed of a copolymer of derivatives of polyspirobifluorene and polyfluorene (PBFF) and a derivative of polyphenylene vinylene (MDMO-PPV). Evidence of partial Förster energy transfer from PBFF to MDMO-PPV is revealed by steady-state and time-resolved photoluminescence. Atomic force microscopy (AFM) on blends with different weight ratios of host (PBFF) and guest (MDMO-PPV) polymers shows that in blends with a MDMO-PPV concentration higher than 10% two phases are present. The nature of the phases is identified by means of scanning near-field optical microscopy (SNOM). Using this technique the topography and the local photoluminescence of the thin film are simultaneously mapped with a spatial resolution of ~100 nm. The local measurements reveal that the PL signal of MDMO-PPV is present in the whole surface, but with variations of the PL intensity and spectral shape. The correlation of the SNOM, time-resolved and steady-state photoluminescence measurements allows us to identify the two phases, one rich in PBFF and the other rich in MDMO-PPV.
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