174 research outputs found
Modeling Space-Charge Limited Currents in Organic Semiconductors: Extracting Trap Density and Mobility
We have developed and applied a mobility edge model that takes into account
drift and diffusion currents to characterize the space charge limited current
in organic semiconductors. The numerical solution of the drift-diffusion
equation allows the utilization of asymmetric contacts to describe the built-in
potential within the device. The model has been applied to extract information
of the distribution of traps from experimental current-voltage measurements of
a rubrene single crystal from Krellner et al. [Phys. Rev. B, 75(24), 245115]
showing excellent agreement across several orders of magnitude of current.
Although the two contacts are made of the same metal, an energy offset of 580
meV between them, ascribed to differences in the deposition techniques
(lamination vs. evaporation) was essential to correctly interpret the shape of
the current-voltage characteristics at low voltage. A band mobility 0.13 cm2/Vs
for holes was estimated, which is consistent with transport along the long axis
of the orthorhombic unit cell. The total density of traps deeper than 0.1 eV
was 2.2\times1016 cm-3. The sensitivity analysis and error estimation in the
obtained parameters shows that it is not possible to accurately resolve the
shape of the trap distribution for energies deeper than 0.3 eV or shallower
than 0.1 eV above the valence band edge. The total number of traps deeper than
0.3 eV however can be estimated. Contact asymmetry and the diffusion component
of the current play an important role in the description of the device at low
bias, and are required to obtain reliable information about the distribution of
deep traps
Multi-phase semicrystalline microstructures drive exciton dissociation in neat plastic semiconductors
The optoelectronic properties of macromolecular semiconductors depend
fundamentally on their solid-state microstructure. For example, the
molecular-weight distribution influences polymeric- semiconductor properties
via diverse microstructures; polymers of low weight-average molecular weight
(Mw) form unconnected, extended-chain crystals, usually of a paraffinic
structure. Because of the non-entangled nature of the relatively short-chain
macromolecules, this leads to a polycrystalline, one-phase morphology. In
contrast, with high-Mw materials, where average chain lengths are longer than
the length between entanglements, two-phase morphologies, comprised of
crystalline moieties embedded in largely unordered (amorphous) regions, are
obtained. We investigate charge photogeneration processes in neat regioregular
poly(3-hexylthiophene) (P3HT) of varying Mw by means of time-resolved
photoluminescence (PL) spectroscopy. At 10 K, PL originating from recombination
of long-lived charge pairs decays over microsecond timescales. Both the
amplitude and decay rate distribution depend strongly on Mw. In films with
dominant one-phase chain-extended microstructures, the delayed PL is suppressed
as a result of a diminished yield of photoinduced charges, and its decay is
significantly faster than in two-phase microstructures. However, independent of
Mw, charge recombination regenerates singlet excitons in torsionally disordered
chains forming more strongly coupled photophysical aggregates than those in the
steady-state ensemble, with delayed PL lineshape reminiscent of that in
paraffinic morphologies at steady state. We conclude that highly delocalized
excitons in disordered regions between crystalline and amorphous phases
dissociate extrinsically with yield and spatial distribution that depend
intimately upon microstructure.Comment: 19 pages, 4 figure
Solid solutions of rare earth cations in mesoporous anatase beads and their performances in dye-sensitized solar cells
Solid solutions of the rare earth (RE) cations Pr3+, Nd3+, Sm3+, Gd3+, Er3+ and Yb3+ in anatase TiO2 have been synthesized as mesoporous beads in the concentration range 0.1-0.3% of metal atoms. The solid solutions were have been characterized by XRD, SEM, diffuse reflectance UV-Vis spectroscopy, BET and BJH surface analysis. All the solid solutions possess high specific surface areas, up to more than 100 m2/g. The amount of adsorbed dye in each photoanode has been determined spectrophotometrically. All the samples were tested as photoanodes in dye-sensitized solar cells (DSSCs) using N719 as dye and a nonvolatile, benzonitrile based electrolyte. All the cells were have been tested by conversion efficiency (J-V), quantum efficiency (IPCE), electrochemical impedance spectroscopy (EIS) and dark current measurements. While lighter RE cations (Pr3+, Nd3+) limit the performance of DSSCs compared to pure anatase mesoporous beads, cations from Sm3+ onwards enhance the performance of the devices. A maximum conversion efficiency of 8.7% for Er3+ at a concentration of 0.2% has been achieved. This is a remarkable efficiency value for a DSSC employing N719 dye without co-adsorbents and a nonvolatile electrolyte. For each RE cation the maximum performances are obtained for a concentration of 0.2% metal atoms. © 2015, Nature Publishing Group. All rights reserved
Recommended from our members
Tuning the bandgap of Cs2AgBiBr6 through dilute tin alloying.
The promise of lead halide hybrid perovskites for optoelectronic applications makes finding less-toxic alternatives a priority. The double perovskite Cs2AgBiBr6 (1) represents one such alternative, offering long carrier lifetimes and greater stability under ambient conditions. However, the large and indirect 1.95 eV bandgap hinders its potential as a solar absorber. Here we report that alloying crystals of 1 with up to 1 atom% Sn results in a bandgap reduction of up to ca. 0.5 eV while maintaining low toxicity. Crystals can be alloyed with up to 1 atom% Sn and the predominant substitution pathway appears to be a ∼2 : 1 substitution of Sn2+ and Sn4+ for Ag+ and Bi3+, respectively, with Ag+ vacancies providing charge compensation. Spincoated films of 1 accommodate a higher Sn loading, up to 4 atom% Sn, where we see mostly Sn2+ substitution for both Ag+ and Bi3+. Density functional theory (DFT) calculations ascribe the bandgap redshift to the introduction of Sn impurity bands below the conduction band minimum of the host lattice. Using optical absorption spectroscopy, photothermal deflection spectroscopy, X-ray absorption spectroscopy, 119Sn NMR, redox titration, single-crystal and powder X-ray diffraction, multiple elemental analysis and imaging techniques, and DFT calculations, we provide a detailed analysis of the Sn content and oxidation state, dominant substitution sites, and charge-compensating defects in Sn-alloyed Cs2AgBiBr6 (1:Sn) crystals and films. An understanding of heterovalent alloying in halide double perovskites opens the door to a wider breadth of potential alloying agents for manipulating their band structures in a predictable manner
Electronic Doping and Enhancement of n‐Channel Polycrystalline OFET Performance through Gate Oxide Modifications with Aminosilanes
Self-assembled monolayers (SAMs) are widely employed in organic field-effect transistors to modify the surface energy, surface roughness, film growth kinetics, and electrical surface potential of the gate oxide to control the device's operating voltage. In this study, amino-functionalized SAM molecules are compared to pure alkylsilane SAMS in terms of their impact on the electrical properties of organic field-effect transistors, using the n-type polycrystalline small molecule semiconductor material N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8). In order to understand the electronic impact of the amino groups, the effect of both the number of amino-containing functional groups and the SAM molecular length are systematically studied. Though amino-functionalized SAM materials have been studied previously, this study is, for the first time, able to shed light on the nature of the doping effect that occurs when the gate oxide is treated with polar aminosilane materials. By a comprehensive theoretical study of the interface on the molecular level, it is shown that the observed shift in the threshold voltage is caused by free charges, which are attracted to the PTCDI-C8 and are stabilized there by protonated aminosilanes. This attraction and the voltage shift can be systematically tuned by varying the length of the neutral terminal chain of the aminosilane. © 2021 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb
Structural origin of gap states in semicrystalline polymers and the implications for charge transport
We quantify the degree of disorder in the {\pi}-{\pi} stacking direction of
crystallites of a high performing semicrystalline semiconducting polymer with
advanced X-ray lineshape analysis. Using first principles calculations, we
obtain the density of states of a system of {\pi}-{\pi} stacked polymer chains
with increasing amounts of paracrystalline disorder. We find that for an
aligned film of PBTTT the paracrystalline disorder is 7.3%. This type of
disorder induces a tail of trap states with a breadth of ~100 meV as determined
through calculation. This finding agrees with previous device modeling and
provides physical justification for the mobility edge model.Comment: Text and figures are unchanged in the new version of the file. The
only modification is the addition of a funding source to the acknowledgment
On the Potential of Optical Nanoantennas for Visibly Transparent Solar Cells
This study aims to determine the maximum possible energy conversion efficiency of visibly transparent solar cells using the detailed balance limit (also known as the Shockley–Queisser limit) and compare it to the efficiency of traditional single-junction solar cells. To achieve this, a new optical nanoantenna has been designed to absorb incoming light selectively, enhancing the average visible transmission while maintaining high absorption in the infrared and UV regions. The color appearance of the antennas has also been evaluated through colorimetrical characterization. Our findings indicate that it is possible to achieve high average visible transparency and energy conversion efficiency of over 80 and 18%, respectively, by carefully selecting semiconductor materials. Such solar cells are versatile enough to be integrated seamlessly into smart windows, agrivoltaic concepts in open and protected cultivation, mobile devices, and appliances without compromising their appearance or functionality. The dimensions and optics of the proposed antennas and visibly transparent solar cells have been thoroughly discussed
Organic Electronics for Point-of-Care Metabolite Monitoring.
In this review we focus on demonstrating how organic electronic materials can solve key problems in biosensing thanks to their unique material properties and implementation in innovative device configurations. We highlight specific examples where these materials solve multiple issues related to complex sensing environments, and we benchmark these examples by comparing them to state-of-the-art commercially available sensing using alternative technologies. We have categorized our examples by sample type, focusing on sensing from body fluids in vitro and on wearable sensors, which have attracted significant interest owing to their integration with everyday life activities. We finish by describing a future trend for in vivo, implantable sensors, which aims to build on current progress from sensing in biological fluids ex vivo.A.M.P and R.O. gratefully acknowledge the support by the Marie Curie Innovative Training Network (ITN) project OrgBio 607896. O.P. gratefully acknowledges support from the Knut and Alice Wallenberg Foundation (KAW 2014.0387) for postdoctoral research at Stanford University
- …