Completed Research Projects

Study of the Effects of Disorder and State Filling on the Absorption/Emission Spectra of Organic Photovoltaic Devices

(Jan 2019 – Oct 2021) We study two archetypal small molecule bulk heterojunction OPVs that feature different charge transfer CT (state) energetic disorders. We perform comprehensive temperature dependent measurement of the CT state photocurrent and electroluminescence spectra for these devices and reveal that CT spectral width and peak position are affected by the disorder in the blend, which becomes particularly apparent at low temperatures. Assuming a Gaussian CT DOS, we are able to quantify the relative contribution of static and dynamic disorder in the CT spectral broadening. We demonstrate that the complete disorder broadened CT DOS is not represented by the electroluminescence spectra, unlike its photocurrent counterpart, since emission does not always occur from the lowest possible energy state. We connect this scenario to incomplete thermalization of the injected carriers in the CT DOS, a process which is more prevalent at low temperatures and in blends with higher disorder.

Related Publications:

Physical Review Applied (2021): https://doi.org/10.1103/PhysRevApplied.16.044026

Study of Polaritons in Organic Cavities in Ultrastrong Light–Molecule Coupling Regime

(Jul 2019 – Sep 2021) In this study, we use the organic light emitting diode (OLED) material 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) to achieve ultrastrong electronic coupling allowing us to distinctly separate and observe the excited state features of a cavity system. Using pump-probe spectroscopy, we propose a new interpretation of the spectral features in a cavity system and use this framework to investigate polaritonic dynamics.

Also, we compare three cavity systems relevant to organic solar cell active layers, in all cases where the cavity resonance is tuned to an electronic transition of the donor: a donor only, a donor-acceptor bilayer, and a blend. The donor system is P3HT (regioregular poly(3-hexylthiophene-2,5-diyl)), and we incorporated a C60-type fullerene acceptor (C60 or PCBM [6,6]-phenyl-C₆₁-butyric acid methyl ester). Through comparison of the spectroscopic properties of these cavity systems, we demonstrate charge transfer from the lower polariton can compete with the decay of the polariton to the ground state, but only in case of the donor-acceptor blend.

Related Publications:

The Journal of Physical Chemistry Letters (2020): https://doi.org/10.1021/acs.jpclett.0c00247
The Journal of Physical Chemistry Letters (2021): https://doi.org/10.1021/acs.jpclett.1c02644

Study of Organic Donor-Acceptor Blends with Large Frontier Orbital Energy Offsets

(Jan 2017 – Dec 2020) We demonstrated several widegap organic donor-acceptor (DA) systems with large frontier energy offsets, resulting in ultra-broad charge transfer (CT) state spectra. We presented the experimental validation that multiple CT state manifolds can coexist at a single DA interface. Through comprehensive experimentation and molecular dynamics and quantum chemical simulations, we were furthermore able to identify that these broad spectra originate from multiple electronic transitions. In total, we demonstrated six organic small molecules and polymer DA systems with large frontier orbital energy offsets that display unprecedented, more than 2 eV broad, CT state features. We investigated the nature of these sub-bandgap features in detail for the DA system of NPB:HAT-CN (NPB is N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine and HAT-CN is 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile). Overall, we were able to show for the first time a system where (1) multiple CT transitions exist at a single DA interface; (2) that internal quantum efficiency of these transitions is not constant; (3) that internal conversion between CT levels is not efficient.

We also investigated the nature of low-lying CT states in NPB:HAT-CN amorphous DA blends with a multiscale computational approach. We found that wide CT DOS broadening is dominated by static disorder and arises from both conformational and electrostatic landscape variations, which in turn originate from NPB flexibility and HAT-CN quadrupoles. The large broadening and the deep LUMO levels of HAT-CN ensure that a large portion of the CT DOS lies below 1.5 eV, where non-radiative decay to the ground state becomes predominant over charge separation.

Related Publications:

The Journal of Physical Chemistry Letters (2020): https://doi.org/10.1021/acs.jpclett.0c00247
The Journal of Physical Chemistry Letters (2021): https://doi.org/10.1021/acs.jpclett.1c02644

Study of Capacitor-like Charge Buildup in a Bulk Heterojunction Organic Solar Cell

(Jan 2019 – Nov 2020) In this work, we demonstrate that the bulk heterojunction morphology can play an unforeseen role—keeping adjacent free carriers apart via capacitive charging. An active layer morphology consisting of pure and intermixed domains is able to block a key recombination pathway to triplet excitons and suppress bimolecular recombination up to 2000 times greater than predicted by the Langevin model. These dynamics occur concurrently with large electric fields that build-up following charge generation, and are maintained across domains for nanoseconds, resembling the charging of a capacitor. The dielectric constant of the material dictates that the dielectric polarization of intervening amorphous and interfacial domains in the presence of such fields must further protect charges, much like the dielectric layer in a macroscopic capacitor. Rather than being detrimental, we show that this electric field buildup is related to the prevention of non-radiative loss pathways and substantial improvements in device performance, including in thick junction devices.

We also perform a systematic study of blend morphology and dynamics to unravel and clarify the requirements for nanoscale capacitive charging. By altering annealing conditions and donor-acceptor content, the impact of blend morphology on nanoscale electric field buildup is probed by the photoinduced electro-absorption signal it produces in transient absorption spectroscopy. We find that a three-phase morphology defines the energy landscape and is a key requirement for electric fields to build up locally across interfaces. We also identify that a threshold domain purity must be reached—irrelevant of domain size—to turn on the nanoscale charge accumulation phenomenon.

Related Publications:

Journal of the American Chemical Society (2020): https://doi.org/10.1021/jacs.9b12526
The Journal of Physical Chemistry Letters (2020): https://doi.org/10.1021/acs.jpclett.0c03425

Modeling and Simulation of Monolayer WSe₂ channel MOSFET

(Mar 2013 – Jun 2016)

Development of a 1-D self-consistent Schrödinger-Poisson Electrostatics solver for double gate WSe₂ channel MOSFET
Development of a NEGF based Quantum Transport solver using Fast Uncoupled Mode Space (FUMS) method
Development of a compact analytical Transport model of the device
Study of doping concentration, channel length, oxide thickness and material variations on the Threshold voltage, Sub-threshold Swing, on/off current ratio, peak saturation current of the device

Related Publications:

ECS Transactions (2015): https://doi.org/10.1149/06614.0011ecst
Nanotechnology (2018): https://doi.org/10.1088/1361-6528/aab5ac

Ballistic Transport Modeling of Cylindrical CNTFET

(Aug 2011 – Feb 2013)

Development of NEGF based 2-D self-consistent simulator
Study of the effects of gate dielectric, temperature and device dimensions on the in near-ballistic Transport properties of surrounding gate CNTFET

Related Publications:

ECS Transactions (2013): https://doi.org/10.1149/05301.0139ecst
ECS Journal of Solid State Science and Technology (2013): https://doi.org/10.1149/2.010309jss

Modeling and Characterization of III-V High-k Gate-All-Around Nanowire channel MOSFET

(Aug 2011 – Feb 2013)

Development of a 2-D self-consistent Quantum Mechanical device simulator using MATLAB and COMSOL Multiphysics
Investigation of Electrostatics and ballistic Transport
Incorporation of the effects of Dit in simulated C-V characteristics
Calculation of direct gate-leakage current, analytical modeling of threshold voltage and potential profile of the channel
Development of an empirical Transport equation using Gaussian like effective transmission co-efficient
Comprehensive study of Threshold voltage and Transport performances with channel cross-section, oxide thickness, oxide and channel materials, channel doping variation
Development of Uncoupled Mode Space (UMS) approach based 3-D transport simulator
Proposal and modeling of a novel structure “Axially Graded Nanowire FET”

Related Publications:

ECS Transactions (2013): https://doi.org/10.1149/05301.0169ecst
ECS Transactions (2016): https://doi.org/10.1149/07204.0325ecst

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