Kenan Gundogdu

Kenan Gundogdu
North Carolina State University | NCSU · Department of Physics

PhD

About

121
Publications
20,758
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4,621
Citations
Introduction
We study ultrafast electronic processes in polymers and their bulk heterojunctions with fullerene blends.
Additional affiliations
January 2009 - present
North Carolina State University
Position
  • Ultrafast Spectroscopy of Polymer Blends
Description
  • We study electron and exciton dynamics in polymers and their blends with fullerenes.
Education
August 1998 - July 2004
University of Iowa
Field of study
  • Physics

Publications

Publications (121)
Preprint
Full-text available
The development and the use of quantum technologies are hindered by a fundamental challenge: Quantum materials exhibit macroscopic quantum properties at extremely low temperatures due to the loss of quantum coherence at elevated temperatures. Here, based on our recent discovery of room temperature superfluorescence in perovskites, we present the Qu...
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The formation of coherent macroscopic states and the manipulation of their entanglement using external stimuli are essential for emerging quantum applications. However, the observation of collective quantum phenomena such as Bose–Einstein condensation, superconductivity, superfluidity and superradiance has been limited to extremely low temperatures...
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Photodetectors that can sense not only light intensity but also light's polarization state add valuable information that is beneficial in a wide array of applications. Polymer semiconductors are an attractive material system to achieve intrinsic polarization sensitivity due to their anisotropic optoelectronic properties. In this report, the thermom...
Article
Recent breakthroughs in material development have increased the demand for characterization methods capable of probing nanoscale features on ultrafast time scales. As the sample reduces to atomically thin levels, an extremely low-level signal limits the feasibility of many experiments. Here, we present an affordable and easy-to-implement solution t...
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Light–matter interactions can create and manipulate collective many-body phases in solids1–3, which are promising for the realization of emerging quantum applications. However, in most cases, these collective quantum states are fragile, with a short decoherence and dephasing time, limiting their existence to precision tailored structures under deli...
Article
The nonequilibrium properties of strongly correlated materials present a target in the search for new phases of matter. It is important to observe the types of excitations that exist in these materials and their associated relaxation dynamics. We have studied the photoexcitations in a spin-orbit assisted Mott insulator α−RuCl3 using time-resolved t...
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Metal halide perovskites have drawn tremendous attention in optoelectronic applications owing to the rapid development in photovoltaic and light‐emitting diode devices. More recently, these materials are demonstrated as excellent gain media for laser applications due to their large absorption coefficient, low defect density, high charge carrier mob...
Article
Two-dimensional (2D) transition metal dichalcogenides (TMDCs) exhibit an electron-hole liquid (EHL) phase transition at unusually high temperatures. Because these materials are atomically thin, optical excitation leads to material expansion. As a result, during the EHL phase transition, the electronic band structure evolves due to both material the...
Article
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Organic solar cells (OSCs) based on D18:Y6 have recently exhibited a record power conversion efficiency of over 18%. The initial work is extended and the device performance of D18-based OSCs is compared with three non-fullerene acceptors, Y6, IT-4F, and IEICO-4Cl, and their molecular packing characteristics and miscibility are studied. The D18 poly...
Preprint
Full-text available
Light-matter interactions can create and manipulate collective many-body phases in solids1-3, which are promising for the realization of emerging quantum applications. However, in most cases these collective quantum states are fragile, with a short decoherence and dephasing time, limiting their existence to precision tailored structures under delic...
Article
Perovskite light-emitting diodes have been gaining attention in recent years due to their high efficiencies. Despite of the recent progress made in device efficiency, the operation mechanisms of these devices are still not well understood, especially the effects of ion migration. In this work, the role of ion migration is investigated by measuring...
Preprint
2D transition metal dichalcogenides (TMDCs) exhibit an electron-hole liquid phase transition at unusually high temperatures. Because these materials are atomically thin, optical excitation leads to material expansion. As a result, during the EHL phase transition the electronic band structure evolves due to both material thermal expansion and renorm...
Article
The development of terahertz (THz) spintronics has created a paradigm shift in the generation of THz radiation through the combination of ultrafast magnetism and spin-based electronics. However, research in this area has primarily focused on all-metallic devices comprising a ferromagnetic thin film adjacent to a non-magnetic heavy metal. Here, we r...
Article
A typical top emitting OLED has a strong microcavity effect because of the two reflective electrodes. The cavity effect causes a serious color shift with the viewing angles and restricts the organic layer thickness. To overcome these drawbacks, we designed a multi-mode OLED structure having dual-dielectric spacer layers, which extend the cavity len...
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Quasi‐2D Ruddlesden–Popper halide perovskites with a large exciton binding energy, self‐assembled quantum wells, and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi‐2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower‐dimensional nano...
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In this work, we studied the optical- and electrical- properties of emerging Cs2AgBiBr6 double perovskite single crystals and demonstrated their potential for detecting ionizing radiation. We prepared Cs2AgBiBr6 double perovskite single crystals from a saturated aqueous solution. Low-temperature photoluminescence (PL) was employed to determine the...
Article
Understanding the correlation between polymer aggregation, miscibility, and device performance is important to establish a set of chemistry design rules for donor polymers with nonfullerene acceptors (NFAs). Employing a donor polymer with strong temperature‐dependent aggregation, namely PffBT4T‐2OD [poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl...
Article
Optical and electrical properties along the b-axis of Fe-doped β-Ga2O3 were studied using low temperature cathodoluminescence (CL) spectroscopy, optical absorption spectroscopy and current-voltage (IV) measurements. The optical absorption spectroscopy showed an absorption edge without near edge shoulder and the corresponding optical bandgap was cal...
Article
Atomically thin (1L) MoS2 emerged as a direct band gap semiconductor with potential optical applications. The photoluminescence (PL) of 1L-MoS2 degrades due to aging related defect formation. The passivation of these defects leads to substantial improvement in optical properties. Here we report the enhancement of PL on aged 1L-MoS2 by laser treatme...
Article
Atomically thin materials exhibit exotic electronic and optical properties. Strong many‐body interactions from the reduced dielectric environment lead to electronic phases that drastically change conductivity and optical response. For example, these many‐body interactions can give rise to the formation of collective states such as Mott metal–insula...
Article
Layered lead halide perovskites have recently been heavily investigated due to their versatile structures, tunable electronic properties and better stability compared with 3D perovskites and have also been effectively incorporated into photovoltaic and light-emitting devices. They are often prepared into thin film form by solution methods and typic...
Article
Hybrid perovskites incorporating conjugated organic cations enable unusual charge carrier interactions among organic and inorganic structural components, but are difficult to prepare as films due to disparate component chemical/physical characteristics (e.g., solubility, thermal stability). Here we demonstrate that resonant infrared matrix-assisted...
Article
Three-dimensional (3D) hybrid organic-inorganic lead halide perovskites (HOIPs) feature remarkable optoelectronic properties for solar energy conversion but suffer from long-standing issues of environmental stability and lead-toxicity. Associated two-dimensional (2D) analogs are garnering increasing interest due to superior chemical stability, stru...
Article
Many-body interactions in photoexcited semiconductors can bring about strongly-interacting electronic states, culminating in the fully-ionized matter of electron-hole plasma (EHP) and electron-hole liquid (EHL). These exotic phases exhibit unique electronic properties, such as metallic conductivity and metastable high photoexcitation density, which...
Article
Adsorption of the acceptor material tetracyanoquinodimethane can control optoelectronic properties of MoS2 by accepting defect generated excess negative charge from the surface that would otherwise interfere with radiative decay processes. Angle Resolved Photoelectron Spectroscopy measurements show that the MoS2 band structure near the Γ point shif...
Article
We present a new series of fullerene derivatives that exhibit solubility in ethanol/water solvent mixtures and implement these materials to fabricate polymer solar cells (PSCs) using environmentally benign solvents. In order to simultaneously optimize the processability of the fullerenes in ethanol/water solvent mixtures and device performance, dif...
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The performance of the highly efficient PBDB-T ITIC system with 11.25% power conversion efficiency is degraded significantly to 4.35% when ITIC is blended with a version of PBDB, dubbed PBDB-O, which has only a minor side-chain modification. We explored the reasons for this difference and investigated the impact of morphology, molecular packing and...
Article
Polymer nonfullerene solar cells are emerging as an alternative of polymer fullerene solar cells. However, maximizing the short-circuit density and open-circuit voltage is a critical issue in these solar cells. Here, using ultrafast spectroscopy, we measured exciton relaxation and charge separation dynamics in polymer nonfullerene blend with low dr...
Article
As many conjugated polymer-based organic photovoltaic (OPV) materials provide substantial solar power conversion efficiencies (as high as 13%), it is important to develop a deeper understanding of how the primary repeat unit structures impact device performance. In this work, we have varied the group 14 atom (C, Si, Ge) at the center of a bithiophe...
Article
Non-fullerene acceptors (NFAs) have been demonstrated to be promising candidates for highly efficient organic photovoltaic (OPV) devices. The tunability of absorption characteristics of NFAs can be used to make OPVs with complementary donor acceptor absorption to cover broad range of solar spectrum. However, both charge transfer from donor to accep...
Article
Resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) is a gentle thin-film deposition technique that combines the facile chemical control of solution processing with the growth control of vapor-phase deposition, yet one that has not been widely applied to crystalline organic-inorganic hybrid materials. In this work, we investigat...
Article
The strong correlation of elementary particles or quasi-particles like electrons, phonons, and excitons may give rise to macroscopic quantum states with exotic functionalities such as superconductivity, Bose-Einstein condensation, Mott insulator, and electron-hole liquid. However, the macroscopic quantum states usually only exist at cryogenic tempe...
Article
The photodegeneration of polymer solar cells (PSCs) based on polymer donor and fullerene acceptor limits their lifetime and reliability. Here, we demonstrate that replacing fullerene acceptors with naphthalenediimide (NDI)-based n-type polymers can significantly enhance the photo-stability of PSCs. As a model system, we compared the photo-stabiliti...
Article
We have developed a simple yet versatile approach for enhancing the performance of all-polymer solar cells (all-PSCs) using highly crystalline small-molecular additive, 6,6’-dithiopheneisoindigo (DTI). The DTI additive in a blend of PTB7-Th donor and P(NDI2HD-T) acceptor enhances the power conversion efficiency of all-PSCs from 5.9 to 6.8%. Based o...
Article
Low-bandgap polymers/molecules are an interesting family of semiconductor materials, and have enabled many recent exciting breakthroughs in the field of organic electronics, especially for organic photovoltaics (OPVs). Here, such a low-bandgap (1.43 eV) non-fullerene electron acceptor (BT-IC) bearing a fused 7-heterocyclic ring with absorption edge...
Article
The performance of all-polymer solar cells (all-PSCs) is often limited by the poor exciton dissociation process. Here, the design of a series of polymer donors (P1–P3) with different numbers of fluorine atoms on their backbone is presented and the influence of fluorination on charge generation in all-PSCs is investigated. Sequential fluorination of...
Article
The performance of polymer‒fullerene bulk heterojunction (BHJ) solar cells is highly affected by the morphology of the blend film. Though the structure of BHJ morphology is well-understood, the relationship between relevant material interactions and BHJ morphology evolutions are poorly understood and seldom explored. In this report, we discuss the...
Article
Transition metal dichalcogenide (TMDC) monolayers present a remarkable multifunctional material with potential to enable the development of a wide range of novel devices. However, the functionalities observed often fall short of the expectation, which hinders the device development. Here we demonstrate that the optical, catalytic, and thermal funct...
Article
Triplet excitons form in quasi-2D hybrid inorganic-organic perovskites and diffuse over 100 nm before radiating with >11% PLQE at low temperatures.
Article
We report the results of the combined experimental and theoretical studies of the low-lying exciton states in crystalline copper phthalocyanine. We derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer exciton state and compare it with temperature dependent optical absorpti...
Article
Full-text available
Fullerene-free organic solar cells show over 11% power conversion efficiency, processed by low toxic solvents. The applied donor and acceptor in the bulk heterojunction exhibit almost the same highest occupied molecular orbital level, yet exhibit very efficient charge creation.
Article
In this study, three boron dipyrromethenes (BODIPY) dyes with extended conjugation and electron donating carbazole groups with different alkyl chain lengths tethered to N-carbazole were synthesized and characterized for dye-sensitized solar cells. The goal was to study the effect of different alkyl chain lengths on dye aggregation at TiO2 surface....
Article
Perovskite solar cells with stabilized power conversion efficiency exceeding 15% have been achieved, using a methylammonium lead iodide (MAPbI3) absorber and CdS as the electron transport layer. X-ray photoelectron spectroscopy reveals a small presence of Cd at the surface of most perovskite films fabricated on CdS. Perovskite films were deliberate...
Article
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Fast and efficient charge separation is essential to achieve high power conversion efficiency in organic solar cells (OSCs). In state-of-the-art OSCs, this is usually achieved by a significant driving force, defined as the offset between the bandgap (Egap) of the donor/acceptor materials and the energy of the charge transfer (CT) state (ECT), which...
Article
Full-text available
It is demonstrated that the luminescence efficiency of monolayers composed of MoS2, WS2, and WSe2 is significantly limited by the substrate and can be improved by orders of magnitude through substrate engineering. The substrate affects the efficiency mainly through doping the monolayers and facilitating defect-assisted nonradiative exciton recombin...
Article
Full-text available
A polymer/PCBM hybrid electron transport layer is reported that enables high‐performance perovskite solar cells with a high power conversion efficiency of 16.2% and with negligible hysteresis. Unlike previous approaches of reducing hysteresis by thermal annealing or fullerene passivation, the success of our approach can be mainly attributed to the...
Article
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We quantitatively evaluate the exciton-exciton annihilation (EEA) and its effect on light emission properties in monolayer TMDC materials, including WS2, MoS2, and WSe2. The EEA rate is found to be 0.3 cm2/s and 0.1 cm2/s for suspended WS2 and MoS2 monolayers, respectively, and subject to the influence from substrates, being 0.1 cm2/s and 0.05 cm2/...
Article
The role of excess excitation energy on long-range charge separation in organic donor/acceptor bulk heterojunctions (BHJs) continues to be unclear. While ultrafast spectroscopy results argue for efficient charge separation through high-energy charge-transfer (CT) states within the first picosecond (ps) of excitation, charge collection measurements...
Article
Full-text available
Charge transfer excitons (CTEs) play an important role in semiconducting polymer-based optoelectronic applications. In organic photovoltaics, they are an intermediate step between tightly bound excitons and free charges. Although CT state energies at the interface of bulk heterojunction organic solar cells have been reported using quantum chemical...
Article
In this study, intriguing difference caused by structural isomerization based on anthracene and phenanthrene stilbazole type ancillary ligands in Ru (II) sensitizers for dye sensitized solar cells (DSCs) has been investigated using femtosecond transient absorption spectroscopy. Both anthracene and phenanthrene based sensitizers HD-7 and HD-8, respe...