Scanning probe analysis of polydiacetylene nanowires and poly(3-hexylthiophene) thin films

In molecular and organic electronic systems, the electrode material has considerable influence on the performance of the resulting device. The advent of scanning tunneling microscopy (STM) and its various spectroscopic extensions has allowed for exploration of the polymer-electrode interface with atomic-scale resolution. Here I present the use of STM to analyze such systems. Specifically, STM and microwave-frequency alternating current STM (ACSTM) of conducting polymers on different substrates can shed new light on how the electrode material electronically affects the adhered polymer structure. The polymers used in this work are polydiacetylene nanowires and poly(3-hexylthiophene) (P3HT) monolayer, bilayer, and thin films. For both polymers, we can use the convolution of electronic and topographic information inherent in STM topography images to extrapolate information about the electronic structure. It is also possible to acquire information about the work function, the density of states (DOS), relative energy level positions, and the differential capacitance via spectroscopic measurements. In particular, capacitance imaging requires a novel technique known as ACSTM that can be used to probe relative carrier concentration. This thesis presents analyses of PDA and P3HT on graphite and molybdenum disulfide. For P3HT thin films, gold and platinum substrates are also studied. The results indicate a strong substrate-dependent charge transfer that is further illuminated through ACSTM and other spectroscopic investigations. In this work, preliminary investigations of photovoltaic P3HT:fullerene films are also discussed

Optical tuning of the diamond Fermi level measured by correlated scanning probe microscopy and quantum defect spectroscopy

Quantum technologies based on quantum point defects in crystals require control over the defect charge state. Here we tune the charge state of shallow nitrogen-vacancy and silicon-vacancy centers by locally oxidizing a hydrogenated surface with moderate optical excitation and simultaneous spectral monitoring. The loss of conductivity and change in work function due to oxidation are measured in atmosphere using conductive atomic force microscopy (C-AFM) and Kelvin probe force microscopy (KPFM). We correlate these scanning probe measurements with optical spectroscopy of the nitrogen-vacancy and silicon-vacancy centers created via implantation and annealing 15-25 nm beneath the diamond surface. The observed charge state of the defects as a function of optical exposure demonstrates that laser oxidation provides a way to precisely tune the Fermi level over a range of at least 2.00 eV. We also observe a significantly larger oxidation rate for implanted surfaces compared to unimplanted surfaces under ambient conditions. Combined with knowledge of the electron affinity of a surface, these results suggest KPFM is a powerful, high-spatial resolution technique to advance surface Fermi level engineering for charge stabilization of quantum defects

(3-Aminopropyl)trimethoxysilane Surface Passivation Improves Perovskite Solar Cell Performance by Reducing Surface Recombination Velocity

We demonstrate reduced surface recombination velocity (SRV) and enhanced power-conversion efficiency (PCE) in mixed-cation mixed-halide perovskite solar cells by using (3-aminopropyl)trimethoxysilane (APTMS) as a surface passivator. We show the APTMS serves to passivate defects at the perovskite surface, while also decoupling the perovskite from detrimental interactions at the C60 interface. We measure a SRV of ~125 + 14 cm/s, and a concomitant increase of ~100 meV in quasi-Fermi level splitting in passivated devices compared to the controls. We use time-resolved photoluminescence and excitation-correlation photoluminescence spectroscopy to show that APTMS passivation effectively suppresses non-radiative recombination. We show that APTMS improves both the fill factor and open-circuit voltage (VOC), increasing VOC from 1.03 V for control devices to 1.09 V for APTMS-passivated devices, which leads to PCE increasing from 15.90% to 18.03%. We attribute enhanced performance to reduced defect density or suppressed nonradiative recombination and low SRV at the perovskite/transporting layers interface.Comment: 22 pages, 6 figure

Hydration of a side-chain-free n-type semiconducting ladder polymer driven by electrochemical doping

We study the organic electrochemical transistors (OECTs) performance of the ladder polymer, poly(benzimidazobenzophenanthroline) (BBL) in an attempt to better understand how an apparently hydrophobic side-chain-free polymer is able to operate as an OECT with favorable redox kinetics in an aqueous environment. We examine two BBLs of different molecular masses from different sources. Both BBLs show significant film swelling during the initial reduction step. By combining electrochemical quartz crystal microbalance (eQCM) gravimetry, in-operando atomic force microscopy (AFM), and both ex-situ and in-operando grazing incidence wide-angle x-ray scattering (GIWAXS), we provide a detailed structural picture of the electrochemical charge injection process in BBL in the absence of any hydrophilic side-chains. Compared with ex-situ measurements, in-operando GIWAXS shows both more swelling upon electrochemical doping than has previously been recognized, and less contraction upon dedoping. The data show that BBL films undergo an irreversible hydration driven by the initial electrochemical doping cycle with significant water retention and lamellar expansion that persists across subsequent oxidation/reduction cycles. This swelling creates a hydrophilic environment that facilitates the subsequent fast hydrated ion transport in the absence of the hydrophilic side-chains used in many other polymer systems. Due to its rigid ladder backbone and absence of hydrophilic side-chains, the primary BBL water uptake does not significantly degrade the crystalline order, and the original dehydrated, unswelled state can be recovered after drying. The combination of doping induced hydrophilicity and robust crystalline order leads to efficient ionic transport and good stability.Comment: 24 pages, 5 figure

Interfacial charge transfer in nanoscale polymer transistors

Interfacial charge transfer plays an essential role in establishing the relative alignment of the metal Fermi level and the energy bands of organic semiconductors. While the details remain elusive in many systems, this charge transfer has been inferred in a number of photoemission experiments. We present electronic transport measurements in very short channel ($L < 100$ nm) transistors made from poly(3-hexylthiophene) (P3HT). As channel length is reduced, the evolution of the contact resistance and the zero-gate-voltage conductance are consistent with such charge transfer. Short channel conduction in devices with Pt contacts is greatly enhanced compared to analogous devices with Au contacts, consistent with charge transfer expectations. Alternating current scanning tunneling microscopy (ACSTM) provides further evidence that holes are transferred from Pt into P3HT, while much less charge transfer takes place at the Au/P3HT interface.Comment: 19 preprint pages, 6 figure

Space Charge at Nanoscale: Probing Injection and Dynamic Phenomena Under Dark/Light Configurations by Using KPFM and C-AFM

International audienc

Atomic-scale investigation of polydiacetylene nanowires by scanning tunneling microscopy and spectroscopy

Nanowires comprised of polydiacetylene, a conjugated polymer, have been analyzed at the nanoscale using scanning tunneling microscopy (STM) and spectroscopy. STM analysis shows that these nanowires exhibit unique electronic behavior due to the different substrate electrode materials used, particularly graphite and molybdenum disulfide. The change in charge transfer behavior is evidence of the importance of polymer-electrode interactions. Nanowires are also shown to randomly desorb due to an interaction with the STM tip. A single disruption often results in the entire nanowire desorbing, and the underlying monolayer is reordered within milliseconds. Additionally, spectroscopic data has been acquired using a novel technique called alternating current STM (ACSTM). ACSTM allows for the acquisition of differential capacitance information. Analysis of the nanowires yields a peak in differential capacitance, as is typical of metal/insulator/semiconductor structures. The ACSTM is sensitive to both carrier concentration and dopant type, making it ideal for future metrological applications