21 research outputs found
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Characterization of Electronic and Ionic Transport in Soft and Hard Functional Materials
Control over concurrent transport of multiple carrier types is desired in both soft and hard materials. For both types of materials, I demonstrate ways to characterize and execute governance over both electronic and ionic transport, and apply these concepts in the fabrication of devices with applications in conducting composites, photovoltaics, electrochemical energy storage, and memristors.
In soft materials, such as polymers, the topology of the binary polymer mesoscale morphology has major implications on the charge/ion transport. Traditional approaches to co-continuous structures involve either using blends of polymers or diblock copolymers. In polymer blends, the structures are kinetically trapped and thus have poor long term stability. In diblock polymers, such morphologies are not universally accessible to non-random coil polymers. I discuss an approach to binary polymer mesoscale morphologies via the assembly of polymer nanoparticles. In this strategy, polymers are assembled into spherical nanoparticles, which are then assembled into hierarchical mesoscale structures. First, I demonstrate, experimentally and computationally, that the electrical transport in semiconducting/insulating polymer nanoparticle assemblies can be predictably tuned according to power law percolation scaling. Then I show that nanoparticle assemblies can be utilized for tunable concurrent transport of electrons and holes for photovoltaics, and for electronic and ionic charges aimed at applications in electrochemical energy storage.
For hard materials, I detail the characterization of mixed electronic and ionic transport in hybrid organic/inorganic lead triiodide perovskites. I used the understanding of mixed electronic and ionic transport in these materials to explain poorly understood phenomena such as photo-instability and current-voltage hysteresis. Then, I show several examples of interfacial materials, and the characterization and implications of their respective work functions, as charge transport materials to control selective charge extraction from perovskites. And finally, I show how interfacial charge transport materials with ionic functionality can be used to change the interfacial chemistry at perovskite/charge transport material interfaces to control both electronic and ionic transport. In this regard, I demonstrate how an adsorbing interface for mobile ions can be used to control current-voltage hysteresis and state-dependent resistance, introducing a novel paradigm of interfacial ion adsorption to fabricate novel perovskite-based memristor devices
High Energy Density in Azobenzene-based Materials for Photo-Thermal Batteries via Controlled Polymer Architecture and Polymer-Solvent Interactions
Energy densities of ~510 J/g (max: 698 J/g) have been achieved in azobenzene-based syndiotactic-rich poly(methacrylate) polymers. The processing solvent and polymer-solvent interactions are important to achieve morphologically optimal structures for high-energy density materials. This work shows that morphological changes of solid-state syndiotactic polymers, driven by different solvent processings play an important role in controlling the activation energy of Z-E isomerization as well as the shape of the DSC exotherm. Thus, this study shows the crucial role of processing solvents and thin film structure in achieving higher energy densities
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Tuning charge transport dynamics via clustering of doping in organic semiconductor thin films
A significant challenge in the rational design of organic thermoelectric materials is to realize simultaneously high electrical conductivity and high induced-voltage in response to a thermal gradient, which is represented by the Seebeck coefficient. Conventional wisdom posits that the polymer alone dictates thermoelectric efficiency. Herein, we show that doping — in particular, clustering of dopants within conjugated polymer films — has a profound and predictable influence on their thermoelectric properties. We correlate Seebeck coefficient and electrical conductivity of iodine-doped poly(3-hexylthiophene) and poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl)-alt-(2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)] films with Kelvin probe force microscopy to highlight the role of the spatial distribution of dopants in determining overall charge transport. We fit the experimental data to a phonon-assisted hopping model and found that the distribution of dopants alters the distribution of the density of states and the Kang–Snyder transport parameter. These results highlight the importance of controlling dopant distribution within conjugated polymer films for thermoelectric and other electronic applications
Fabrication Conditions for Efficient Organic Photovoltaic Cells from Aqueous Dispersions of Nanoparticles
For environmentally friendly and cost-effective manufacturing of organic photovoltaic (OPV) cells, it is highly desirable to replace haloarenes with water as the active layer fabrication solvent. Replacing an organic solvent with water requires retooling the device fabrication steps. The optimization studies were conducted using poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as active layer materials. These materials were dispersed in water as blend and separate nanoparticles using the miniemulsion method. Topologies of the active layers were investigated using atomic force microscopy and electron microscopy techniques. We have identified two essential steps to fabricate efficient OPVs from aqueous dispersions: (1) treatment of the hole-transport layer with UV-O3 to make the surface hydrophilic and (2) the use of an electron-transporting buffer layer for efficient charge extraction. We have also identified relative humidity and substrate temperature as key fabrication parameters for obtaining uniform active layer films. The OPV devices were fabricated using PEDOT:PSS as the hole-transport layer and PCBM as electron-transport layer with Ca/Al as the counter electrode. Efficiencies of 2.15% with a fill factor over 66% were obtained; the efficiency and the fill-factor is the highest among all aqueous processing of P3HT–PCBM nanoparticle solar cells
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Data for Interplay Between Ion Transport, Applied Bias and Degradation under Illumination in Hybrid Perovskite p-i-n Devices
We studied ion transport in hybrid organic inorganic perovskite p-i-n devices as a function of applied bias under device operating conditions. Using electrochemical impedance spectroscopy (EIS) and equivalent circuit modeling, we elucidated various resistive and capacitive elements in the device. We show that ion migration is predictably influenced by a low applied forward bias, characterized by an increased capacitance at the hole transporting (HTM) and electron transporting material (ETM) interfaces, as well as through the bulk. However, unlike observations in n-i-p devices, we found that there is a capacitive discharge leading to ion redistribution in the bulk at high forward biases. Furthermore, we show that a chemical double layer capacitance buildup as a result of ion accumulation impacts the electronic properties of the device, likely by either inducing charge pinning or charge screening, depending on the direction of the ion induced field. Lastly, we extrapolate ion diffusion coefficients (~10-7 cm2 s-1) and ionic conductivities (~10-7 S cm-1) from the Warburg mass (ion) diffusion response, and show that, as the device degrades, there is an overall depletion of capacitive effects coupled with an increased ion mobility.https://scholarworks.umass.edu/data/1004/thumbnail.jp
Solution-Processed Photovoltaics with a 3,6-Bis(diarylamino)fluoren-9-ylidene Malononitrile
3,6-BisÂ(<i>N,N</i>-dianisylamino)-fluoren-9-ylidene
malononitrile (FMBDAA36) was used as an electron donor material in
solution-processed organic photovoltaic devices with configuration
ITO/PEDOT:PSS/(1:3Â[w/w] FMBDAA36:PC<sub>71</sub>BM)/LiF/Al to give
power conversion efficiencies up to 4.1% with open circuit voltage <i>V</i><sub>OC</sub> = 0.89 V, short circuit current <i>J</i><sub>SC</sub> = 10.35 mA cm<sup>–2</sup>, and fill factor
FF = 44.8%. Conductive atomic force microscopy of the active layer
showed granular separation of regions exhibiting easy versus difficult
hole transport, consistent with bulk heterojunction type phase separation
of FMBDAA36 and PC<sub>71</sub>BM, respectively. Single-crystal X-ray
diffraction analysis showed pure FMBDAA36 to form columnar π-stacks
with a 3.3 Ã… intermolecular spacing
Tunable Percolation in Semiconducting Binary Polymer Nanoparticle Glasses
Binary polymer nanoparticle glasses
provide opportunities to realize
the facile assembly of disparate components, with control over nanoscale
and mesoscale domains, for the development of functional materials.
This work demonstrates that tunable electrical percolation can be
achieved through semiconducting/insulating polymer nanoparticle glasses
by varying the relative percentages of equal-sized nanoparticle constituents
of the binary assembly. Using time-of-flight charge carrier mobility
measurements and conducting atomic force microscopy, we show that
these systems exhibit power law scaling percolation behavior with
percolation thresholds of ∼24–30%. We develop a simple
resistor network model, which can reproduce the experimental data,
and can be used to predict percolation trends in binary polymer nanoparticle
glasses. Finally, we analyze the cluster statistics of simulated binary
nanoparticle glasses, and characterize them according to their predominant
local motifs as (<i>p</i><sub><i>i</i></sub>, <i>p</i><sub>1‑<i>i</i></sub>)-connected networks
that can be used as a supramolecular toolbox for rational material
design based on polymer nanoparticles
High Efficiency Tandem Thin-Perovskite/Polymer Solar Cells with a Graded Recombination Layer
Perovskite-containing tandem solar
cells are attracting attention
for their potential to achieve high efficiencies. We demonstrate a
series connection of a ∼90 nm thick perovskite front subcell
and a ∼100 nm thick polymer:fullerene blend back subcell that
benefits from an efficient graded recombination layer containing a
zwitterionic fullerene, silver (Ag), and molybdenum trioxide (MoO<sub>3</sub>). This methodology eliminates the adverse effects of thermal
annealing or chemical treatment that occurs during perovskite fabrication
on polymer-based front subcells. The record tandem perovskite/polymer
solar cell efficiency of 16.0%, with low hysteresis, is 75% greater
than that of the corresponding ∼90 nm thick perovskite single-junction
device and 65% greater than that of the polymer single-junction device.
The high efficiency of this hybrid tandem device, achieved using only
a ∼90 nm thick perovskite layer, provides an opportunity to
substantially reduce the lead content in the device, while maintaining
the high performance derived from perovskites
Interplay between Ion Transport, Applied Bias, and Degradation under Illumination in Hybrid Perovskite p‑i‑n Devices
We
studied ion transport in hybrid organic–inorganic perovskite
p-i-n devices as a function of applied bias under device operating
conditions. Using electrochemical impedance spectroscopy (EIS) and
equivalent circuit modeling, we elucidated various resistive and capacitive
elements in the device. We show that ion migration is predictably
influenced by a low applied forward bias, characterized by an increased
capacitance at the hole-transporting (HTM) and electron-transporting
material (ETM) interfaces, as well as in bulk. However, unlike observations
in n-i-p devices, we found that there is a capacitive discharge leading
to ion redistribution in the bulk at high forward biases. Furthermore,
we show that a chemical double-layer capacitance buildup as a result
of ion accumulation impacts the electronic properties of the device,
likely by inducing either charge pinning or charge screening, depending
on the direction of the ion-induced field. Lastly, we extrapolate
ion diffusion coefficients (∼10<sup>–7</sup> cm<sup>2</sup> s<sup>–1</sup>) and ionic conductivities (∼10<sup>–7</sup> S cm<sup>–1</sup>) from the Warburg mass (ion)
diffusion response and show that, as the device degrades, there is
an overall depletion of capacitive effects coupled with increased
ion mobility
Multiscale Active Layer Morphologies for Organic Photovoltaics Through Self-Assembly of Nanospheres
We address here the need for a general
strategy to control molecular
assembly over multiple length scales. Efficient organic photovoltaics
require an active layer comprised of a mesoscale interconnected networks
of nanoscale aggregates of semiconductors. We demonstrate a method,
using principles of molecular self-assembly and geometric packing,
for controlled assembly of semiconductors at the nanoscale and mesoscale.
Nanoparticles of polyÂ(3-hexylthiophene) (P3HT) or [6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM) were fabricated with targeted
sizes. Nanoparticles containing a blend of both P3HT and PCBM were
also fabricated. The active layer morphology was tuned by the changing
particle composition, particle radii, and the ratios of P3HT:PCBM
particles. Photovoltaic devices were fabricated from these aqueous
nanoparticle dispersions with comparable device performance to typical
bulk-heterojunction devices. Our strategy opens a revolutionary pathway
to study and tune the active layer morphology systematically while
exercising control of the component assembly at multiple length scales