Search

The efficient harvesting of hot carrier energy in semiconductors is typically inhibited by their ultrafast thermalization process. Recently, highly promising experiments reported on the slowdown of the intraband relaxation in hybrid metal halide perovskites. In this work, we report on the presence of long-lived hot carriers in weakly confined colloidal nanocrystals (NCs) of formamidinium lead iodide perovskite (FAPbI3). The effect is apparent from the excitation-dependent lengthening of the rise time and broadening of the high-energy tail of the transient absorption bleaching signal, yielding a retardation of the carrier relaxation by 2 orders of magnitude compared to typical time scales in colloidal semiconductor NCs. Three distinct cooling stages are observed, occurring at sub-picosecond, ∼5 ps, and ∼40 ps time scales, which we attribute to scattering from LO-phonons, contribution from a hot phonon bottleneck effect and Auger heating, respectively. Thermalization appears also influenced by the FAPbI3 NCs purity, with trapping at unreacted precursor impurities further reducing the carrier energy loss rate. © 2017 American Chemical Society. 121 22 12434 12440

The photovoltaic performance of blends of a series of 4,4′-Difluoro-4-bora-3a,4a-diaza-s-indacenes)-based (BODIPY) conjugated polymers donors with fullerene electron acceptors is investigated. Despite the high Voc values observed, solar cell device yields relatively low power conversion efficiencies. Our study takes into account the materials’ structure-property relationship, light harvesting capabilities, charge transport, collection properties and morphological characteristics to elucidate factors affecting the photovoltaic performance in this class of polymers. We show that elimination of low molecular weight species and suitable electrodes for hole collection can be used to overcome some of the observed limitations on photovoltaic performance. © 2017 Elsevier B.V. 226 25 30

We report on an extensive spectroscopic investigation of the impact of substitutional doping on the optoelectronic properties of PbS colloidal quantum dot (CQD) solids. N-doping is provided by Bi incorporation during CQD synthesis as well as post-synthetically via cation exchange reactions. The spectroscopic data indicate a systematic quenching of the excitonic absorption and luminescence and the appearance of two dopant-induced contributions at lower energies to the CQD free exciton. Temperature-dependent photoluminescence indicates the presence of temperature-activated detrapping and trapping processes of photoexcitations for the films doped during and after synthesis, respectively. The data are consistent with a preferential incorporation of the dopants at the QDs surface in the case of the cation-exchange treated films versus a more uniform doping profile in the case of in-situ Bi incorporation during synthesis. Time-resolved experiments indicate the presence of fast dopant- and excitation-dependent recombination channels attributed to Auger recombination of negatively charged excitons, formed due to excess of dopant electrons. The data indicate that apart from dopant compensation and filling of dopant induced trap states, a fraction of the Bi ionized electrons feeds the QD core states resulting in n-doping of the semiconductor, confirming reported work on devices based on such doped CQD material.

A systematic investigation of Förster resonant energy transfer (FRET) is reported within a hybrid prototype structure based on nitride single quantum well (SQW) donors and light emitting polymer acceptors. Self-consistent Schrödinger-Poisson modeling and steady-state and time-resolved photoluminescence experiments were initially employed to investigate the influence of a wide structural parameter space on the emission quantum yield of the nitride component. The optimized SQW heterostructures were processed into hybrid structures with spin-casted overlayers of polyfluorenes. The influence of important unexplored aspects of the inorganic heterostructure such as SQW confinement, content, and doping on the dipole-dipole coupling was probed. Competing mechanisms to the FRET process associated with interfacial recombination and charge transfer have been studied and their implications to device applications exploiting FRET across heterointerfaces have been discussed. © 2015 AIP Publishing LLC. 143 21 Cited By :2

We report an optical investigation of conjugated polymer (P3HT)/fullerene (PCBM) semiconductor blends sensitized by near-infrared absorbing quantum dots (PbS QDs). A systematic series of samples that include pristine, binary and ternary blends of the materials are studied using steadystate absorption, photoluminescence (PL) and ultrafast transient absorption. Measurements show an enhancement of the absorption strength in the near-infrared upon QD incorporation. PL quenching of the polymer and the QD exciton emission is observed and predominantly attributed to intermaterial photoinduced charge transfer processes. Pump-probe experiments show photo-excitations to relax via an initial ultrafast decay while longer-lived photoinduced absorption is attributed to charge transfer exciton formation and found to depend on the relative ratio of QDs to P3HT:PCBM content. PL experiments and transient absorption measurements indicate that interfacial charge transfer processes occur more efficiently at the fullerene/polymer and fullerene/nanocrystal interfaces compared to polymer/nanocrystal interfaces. Thus the inclusion of the fullerene seems to facilitate exciton dissociation in such blends. The study discusses important and rather unexplored aspects of exciton recombination and charge transfer processes in ternary blend composites of organic semiconductors and near-infrared quantum dots for applications in solution-processed photodetectors and solar cells. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 1 5 802 812 Cited By :56

We report the processing conditions for inkjet-printed active layers of organic photovoltaic (OPV) devices comprising bulk-heterojunction blends of the low bandgap conjugated polymer poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b: 2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1, 3-benzothiadiazole)-5,5′-diyl] (Si-PCPDTBT) as well as poly[N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) used as electron donors with [6,6]-phenyl-C71-butyric acid methyl ester (PC 70BM) as electron acceptor. By controlling the inkjet printing processing conditions, we gain a significant improvement in device power conversion efficiency (PCE) to previously reported work using these materials. We obtain Si-PCPDTBT:fullerene and PCDTBT:fullerene-based inkjet-printed OPVs with power conversion efficiency of ~3% and ~4%, respectively. © 2014 Elsevier B.V. 130 474 480 Cited By :10

Electronic and interface properties of spin-coated poly(9,9- dioctylfluorenyl-2,7-diyl) (PFO) films on GaN have been investigated in terms of their potential for optoelectronic applications. The PFO/GaN interface was studied by photoemission spectroscopy showing a type-II energy alignment with band offsets suitable for efficient photocurrent generation. The light harvesting potential is further supported by fluorescence experiments that show evidence of photo-induced electron transfer from PFO to GaN. The impact of polymer film thickness was probed using emission anisotropy and ellipsometry, indicating the presence of an ordered planar phase of PFO. The study has implications to hybrid optoelectronic devices employing the two important materials. © 2013 American Institute of Physics. 102 6 Cited By :4

The photophysics of bulk heterojunctions of a high-performance, low-gap silicon-bridged dithiophene polymer with oleic acid capped PbS quantum dots (QDs) are studied to assess the material potential for light harvesting in the visible- and IR-light ranges. By employing a wide range of nanocrystal sizes, systematic dependences of electron and hole transfer on quantum-dot size are established for the first time on a low-gap polymer-dot system. The studied system exhibits type II band offsets for dot sizes up to ca. 4 nm, whch allow fast hole transfer from the quantum dots to the polymer that competes favorably with the intrinsic QD recombination. Electron transfer from the polymer is also observed although it is less competitive with the fast polymer exciton recombination for most QD sizes studied. The incorporation of a fullerene derivative provides efficient electron-quenching sites that improve interfacial polymer-exciton dissociation in ternary polymer-fullerene-QD blends. The study indicates that programmable band offsets that allow both electron and hole extraction can be produced for efficient light harvesting based on this low-gap polymer-PbS QD composite. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 3 11 1490 1499 Cited By :18

Electron spin polarizations of 32% are obtained in a GaAs quantum well via electrical injection through a reverse-biased Fe/AlGaAs Schottky contact. An analysis of the transport data using the Rowell criteria demonstrates that single step tunneling is the dominant transport mechanism. The current-voltage data show a clear zero-bias anomaly and phonon signatures corresponding to the GaAs-like and AlAs-like longitudinal-optical phonon modes of the AlGaAs barrier, providing further evidence for tunneling. These results provide experimental confirmation of several theoretical analyses indicating that tunneling enables significant spin injection from a metal into a semiconductor., Comment: 4 pages, 4 figures, submitted to APL

We report on excitation dynamics in pristine and bulk heterojunction films of the low bandgap silicon-bridged dithiophene copolymer poly[(4,4′-bis(2- ethylhexyl)dithieno[3,2-b:2′, 3′-d]silole)-2,6-diyl-alt-(4,7-bis(2- thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] with methanofullerene derivatives. The combination of ultrafast transient transmission and photoluminescence allows us to probe the relaxation of both exciton and polaron states in a relatively wide spectral and temporal range. Measurements reveal that the majority of excitations undergo ultrashort non-radiative relaxation while a small fraction of the photoexcited species decays slowly within hundreds of ps. In the blend films, significantly longer decays are observed suggesting the presence of long lived holes and/or charged-transfer type of excitons. © 2012 American Institute of Physics. 100 15 Cited By :3

We report electrical spin injection from a ferromagnetic metal contact into a semiconductor light emitting diode structure with an injection efficiency of 30% which persists to room temperature. The Schottky barrier formed at the Fe/AlGaAs interface provides a natural tunnel barrier for injection of spin polarized electrons under reverse bias. These carriers radiatively recombine, emitting circularly polarized light, and the quantum selection rules relating the optical and carrier spin polarizations provide a quantitative, model-independent measure of injection efficiency. This demonstrates that spin injecting contacts can be formed using a widely employed contact methodology, providing a ready pathway for the integration of spin transport into semiconductor processing technology., Comment: 14 pages including 3 figures, version accepted by Applied Physics Letters - A. Hanbicki, et al. Appl. Phys. Lett. 80 (7), p.TBD (2002)

The spin-polarized light-emitting diode (spin-LED) is a very effective tool for accurately quantifying electrical spin injection in a model independent manner. We resolve and identify various components which occur in the electroluminescence (EL) spectra of GaAs quantum-well-based spin-LEDs, and examine the circular polarization of each. While a number of components exhibit significant circular polarization, the values do not necessarily reflect the electrical spin injection efficiency. We show that a reliable measure of spin injection efficiency can be obtained only if one takes care to spectroscopically resolve and accurately identify the free exciton or free carrier components of the EL spectrum, and exclude other components. © 2001 American Institute of Physics. 79 19 3098 3100 Cited By :62

We have studied the room-temperature non-radiative energy transfer processes in hybrid structures composed of (Ga, In)N/GaN single quantum wells and semiconducting polymer blend films placed in nanometre-scale proximity. The blends consist of three polyfluorene materials with concentrations adjusted so that they emit white light. Power-dependent photoluminescence (PL) measurements are used to investigate the process of energy transfer from the quantum wells to the different components of the polymer blend. We show that energy distribution among the hybrid structures involves competition between nanoscale range non-radiative energy transfer processes from the inorganic well to the polymer components and within the blend itself. © 2009 IOP Publishing Ltd. 20 27 Cited By :16

We report on novel InGaN/GaN quantum well/polyfluorene heterostructures where efficient Förster energy transfer from the well to the organic layer occurs. We show that Mott-Wannier excitons dominate the quantum well luminescence in the quantum wells in the 77 to at least 225 K range and are responsible for the efficient energy channeling to the polyfluorene films. © 2007 American Institute of Physics. 893 355 356 Sponsors: Austrian Research Centers (ARCS) Infineon Austrian Fed. Minist. Educ., Sci. Cult. (BMBMWK) FFG Austrian Nano Initiative Cited By :1

Hybrid inorganic/organic semiconductor structures offer the prospect of new devices that can combine the excellent electrical conductivity of inorganic materials with the attractive luminescence properties of organic materials. Consequently, devices based on dipole-dipole energy transfer from inorganic semiconductors to conjugated polymers can produce highly efficient emission across the entire visible spectrum. We have investigated hybrid structures containing InGaN/GaN single quantum wells (QWs) that are spaced from fluorene-based polymer films by thin cap layers of GaN. Provided their electronic states are matched [1, 2], the close proximity of the polymer film to the QW promotes efficient non-radiative (Forster) energy transfer, resulting in a strong visible emission from the hybrid devices.

We report on studies of electrical spin injection from ferromagnetic Fe contacts into semiconductor light emitting diodes containing single layers of InAsGaAs self-assembled quantum dots (QDs). An oblique magnetic field is used to manipulate the spin of the injected electrons in the semiconductor. This approach allows us to measure the injected steady-state spin polarization in the QDs, Pspin as well as estimate the spin losses in the QD spin detector. After subtraction of magneto-optical effects not related to spin injection, we measured a Pspin of 7.5% at 15 K and estimated an injected spin polarization before QD recombination of around 20%. © 2006 American Institute of Physics. 88 2 1 3 Cited By :28

Electrical spin injection and band offsets in an n-type ferromagnetic semiconductor heterostructure of CdCr2Se4-AlGaAs-GaAs are studied. Epitaxially grown CdCr2Se4 films exhibit hysteresis behavior with significant remanence, an in- plane easy axis with a coercive field of 125 Oe. The cross section of the LED and the band alignment were shown. © 2003 IEEE. Sponsors: Magnetics Society of the Institute of Electrical and Electronics Engineers Conference code: 114081

PbS quantum dots (QDs) of different sizes capped with short (NH4)(3)AsS3 inorganic ligands are produced via ligand exchange processes from oleate-capped PbS QDs. The solid-state photophysical properties of the control organic-capped and the inorganic-ligand-capped QDs are investigated to determine their potential for optoelectronic applications. Ultrafast transient transmission shows that in the oleate-capped QDs, carrier recombination at sub-nanosecond scales occurs via Auger recombination, traps, and surface states. At longer times, intense signals associated with radiative recombination are obtained. After ligand exchange, the QDs become decorated with (NH4)(3)AsS3 complexes and relaxation is dominated by efficient carrier transfer to the ligand states on timescales as fast as approximate to 2 ps, which competes with carrier thermalization to the QD band edge states. Recombination channels present in the oleate-capped QDs, such as radiative and Auger recombination, appear quenched in the inorganic-capped QDs. Evidence of efficient carrier trapping at shallow ligand states, which appears more intense under excitation above the (NH4)(3)AsS3 gap, is provided. A detailed band diagram of the various relaxation and recombination processes is proposed that comprehensively describes the photophysics of the QD systems studied. 4 7 1301547 1301547 PT: J TC: 8 J9: ADV ENERGY MATER

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

We present the highlights of a research programme on hybrid inorganic-organic light emitters. These devices combine recent developments in III-V nitride technology (including UV emitting micro-arrays and specifically tailored quantum wells) with conjugated polymers to access the entire visible spectrum. Two types of devices are studied, those based on down conversion of the quantum well emission by radiative transfer and those based on non-radiative resonant energy transfer. The spectral and operating characteristics of the devices are described in detail. Selectable colour micro-arrays and bar emitters are demonstrated. The nature of the non-radiative energy transfer process has also been studied and we find transfer efficiencies of up to 43% at 15 K, with a 1/R2 dependence on the distance between quantum well and polymer layer, suggesting a plane-plane interaction. The relative importance of the non-radiative resonant energy transfer process increases with temperature to be up to 20 times more efficient, at 300 K, than the radiative transfer process. © 2008 IOP Publishing Ltd. 41 9 Cited By :39

Progress toward electrically driven conjugated polymer lasers should benefit from the identification of materials that exhibit good charge transport properties coupled with large optical gain. Here, we report a detailed study of a blue-emitting fluorene-based polymer that combines a large hole mobility with desirable stimulated emission characteristics. Laser oscillation is achieved within distributed feedback structures and leads to low threshold, tunable lasers operating in the blue spectral region. Our results establish this material as an attractive gain medium with which to try and construct a polymer laser diode. © 2006 American Institute of Physics. 88 8 Cited By :22