Suchi Guha

Understanding charge transport in lead iodide perovskite thin-film field-effect transistors

Band-like charge transport is observed in lead halide perovskite field-effect transistors. Fundamental understanding of the charge transport physics of hybrid lead halide perovskite semiconductors is important for advancing their use in high-performance optoelectronics. We use field-effect transistors (FETs) to probe the charge transport mechanism in thin films of methylammonium lead iodide (MAPbI3). We show that through optimization of thin-film microstructure and source-drain contact modifications, it is possible to significantly minimize instability and hysteresis in FET characteristics and demonstrate an electron field-effect mobility (μFET) of 0.5 cm2/Vs at room temperature. Temperature-dependent transport studies revealed a negative coefficient of mobility with three different temperature regimes. On the basis of electrical and spectroscopic studies, we attribute the three different regimes to transport limited by ion migration due to point defects associated with grain boundaries, polarization disorder of the MA+ cations, and thermal vibrations of the lead halide inorganic cages.

Squeezing Organic Conjugated Molecules— What Does One Learn?**

The effects of hydrostatic pressure on conjugated polymers, in particular polyphenyls (see Figure for monomer unit) have been studied using photoluminescence, absorption, photo-induced absorption, and Raman spectroscopy. The effect of pressure on the singlet and triplet excitons and polarons allows an understanding of localized and delocalized electronic states. Changes in the intensity ratios of Raman bands that correspond to vibrations of a perpendicular and a coplanar array of phenyl rings in the chain, and comparison with calculated intensities, demonstrate the influence of pressure on the polymers conformation.

Conformations in dioctyl substituted polyfluorene: A combined theoretical and experimental Raman scattering study

The structural properties of polyfluorenes (PF) are extremely sensitive to the choice of functionalizing side chains. Dioctyl substituted PF (PF8) adopts metastable structures that depend upon the thermal history and choice of solvents used in film forming conditions. We present a detailed study of the changes in the backbone and side chain morphology in PF8, induced by the various crystallographic phases, using Raman scattering techniques. The vibrational frequencies and intensities of fluorene oligomers are calculated using hybrid density-functional theory with a 3-21G(*) basis set. The alkyl side chains are modeled as limiting conformations: all anti, anti-gauche-gauche, and end gauche representations. The calculated vibrational spectra of single chain oligomers in conjunction with our experimental results demonstrate the beta phase, which is known to originate in regions of enhanced chain planarity as a direct consequence of the alkyl side chain conformation.

Quantum dots by ultraviolet and x-ray lithography

Highly luminescent semiconductor quantum dots have been synthesized in porous materials with ultraviolet and x-ray lithography. For this, the pore-filling solvent of silica hydrogels is exchanged with an aqueous solution of a group II metal ion together with a chalcogenide precursor such as 2-mercaptoethanol, thioacetamide or selenourea. The chalcogenide precursor is photodissociated in the exposed regions, yielding metal chalcogenide nanoparticles. Patterns are obtained by using masks appropriate to the type of radiation employed. The mean size of the quantum dots is controlled by adding capping agents such as citrate or thioglycerol to the precursor solution, and the quantum yield of the composites can be increased to up to about 30% by photoactivation. Our technique is water-based, uses readily available reagents, and highly luminescent patterned composites are obtained in a few simple processing steps. Polydispersity, however, is high (around 50%), preventing large-scale usage of the technique for the time being. Future developments that aim at a reduction of the polydispersity are presented.

Electrical and Optical Properties of Diketopyrrolopyrrole-Based Copolymer Interfaces in Thin Film Devices

Two donor-acceptor diketopyrrolopyrrole (DPP)-based copolymers (PDPP-BBT and TDPP-BBT) have been synthesized for their application in organic devices such as metal-insulator semiconductor (MIS) diodes and field-effect transistors (FETs). The semiconductor-dielectric interface was characterized by capacitance-voltage and conductance-voltage methods. These measurements yield an interface trap density of 4.2 × 10(12) eV⁻¹ cm⁻² in TDPP-BBT and 3.5 × 10¹² eV⁻¹ cm⁻² in PDPP-BBT at the flat-band voltage. The FETs based on these spincoated DPP copolymers display p-channel behavior with hole mobilities of the order 10⁻³ cm²/(Vs). Light scattering studies from PDPP-BBT FETs show almost no change in the Raman spectrum after the devices are allowed to operate at a gate voltage, indicating that the FETs suffer minimal damage due to the metal-polymer contact or the application of an electric field. As a comparison Raman intensity profile from the channel-Au contact layer in pentacene FETs are presented, which show a distinct change before and after biasing.

Polarization-induced transport in ferroelectric organic field-effect transistors

Ferroelectric dielectrics, permitting access to nearly an order of magnitude range of dielectric constants with temperature as the tuning parameter, offer a great platform to monitor the changes in interfacial transport in organic field-effect transistors (OFETs) as the polarization strength is tuned. Temperature-dependent transport studies have been carried out from pentacene-based OFETs using the ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as a gate insulating layer. The thickness of the gate dielectric was varied from 20 nm to 500 nm. By fits to an Arrhenius-type dependence of the charge carrier mobility as a function of temperature, the activation energy in the ferroelectric phase is found to increase as the thickness of the PVDF-TrFE layer decreases. The weak temperature-dependence of the charge carrier mobility in the ferroelectric phase of PVDF-TrFE may be attributed to a polarization fluctuation driven transport, which results from a coupling of the charge ca...

Crystallization of amorphous silicon by self-propagation of nanoengineered thermites

Crystallization of amorphous silicon (a-Si) thin film occurred by the self-propagation of copper oxide/aluminum thermite nanocomposites. Amorphous Si films were prepared on glass at a temperature of 250°C by plasma enhanced chemical vapor deposition. The platinum heater was patterned on the edge of the substrate and the CuO∕Al nanoengineered thermite was spin coated on the substrate that connects the heater and the a-Si film. A voltage source was used to ignite the thermites followed by a piranha solution (4:1 of H2SO4:H2O2) etch for the removal of residual products of thermite reaction. Raman spectroscopy was used to confirm the crystallization of a-Si.

Role of the triplet state in the green emission peak of polyfluorene films: A time evolution study

The blue emission of ethyl-hexyl substituted polyfluorene (PF2/6) films is accompanied by a low energy green emission peak around 500 nm in inert atmosphere. The intensity of this 500 nm peak is large in electroluminescence (EL) compared to photoluminescence (PL) measurements. Furthermore, the green emission intensity reduces dramatically in the presence of molecular oxygen. To understand this, we have modeled various nonradiative processes by time dependent quantum many body methods. These are (i) intersystem crossing to study conversion of excited singlets to triplets leading to a phosphorescence emission, (ii) electron-hole recombination (e-hR) process in the presence of a paramagnetic impurity to follow the yield of triplets in a polyene system doped with paramagnetic metal atom, and (iii) quenching of excited triplet states in the presence of oxygen molecules to understand the low intensity of EL emission in ambient atmosphere, when compared with that in nitrogen atmosphere. We have employed the Pariser-Parr-Pople Hamiltonian to model the molecules and have invoked electron-electron repulsions beyond zero differential approximation while treating interactions between the organic molecule and the rest of the system. Our time evolution methods show that there is a large cross section for triplet formation in the e-hR process in the presence of paramagnetic impurity with degenerate orbitals. The triplet yield through e-hR process far exceeds that in the intersystem crossing pathway, clearly pointing to the large intensity of the 500 nm peak in EL compared to PL measurements. We have also modeled the triplet quenching process by a paramagnetic oxygen molecule which shows a sizable quenching cross section especially for systems with large sizes. These studies show that the most probable origin of the experimentally observed low energy EL emission is the triplets.

Raman Spectroscopic Studies of Polyfluorenes

Polyfluorenes reveal a complex interplay between emissive properties and intra- and intermolecular structure. Vibrational frequencies and intensities determined by Raman spectroscopy are strongly influenced by variations in the backbone as well as side chain conformations. Changes in the structural and electronic properties of two side group substi- tuted polyfluorenes (PF): ethyl-hexyl substituted PF (PF2/6) and dioctyl substituted PF (PF8) as a function of solvent, thermal cycling, and hydrostatic pressure via Raman scattering are presented. The vibrational frequencies and intensities of fluorene oligomers with various alkyl side chain conformations are calculated using hybrid density-functional theory. A comparison of the computed vibrational spectra of single chain fluorene oligomers with our experimental data shows that the conformational isomers in PF8 are a direct consequence of the side chain conformation.

On the structure of oligophenylenes

We performed Raman spectroscopy on polycrystalline powders of bi-, para-ter-, para-quarter- and para-sexiphenyl under hydrostatic pressure up to 70 kbar and from 10 K to 300 K. We show that the crystallographic phase transition temperatures can be determined with our experimental method. Furthermore, we report a structural transition with pressure that we attribute to a change of the interring torsion potential from W- to U-shaped. The experimental findings are confirmed by calculated Raman spectra and interring torsion potentials for the isolated oligomers as well as for the corresponding polymer in crystalline phase.