Charge Transport and Gradient Doping in Nanostructured Polypyrrole Films for Applications in Photocurrent Generation

Abstract

The investigation of the charge-transport mechanism across disordered conducting and semiconducting materials is of relevance, considering the applications in modern organic and hybrid electronics. The transition from bulk to nm-thick layers may lead to unexpected physical/chemical properties as the device interfaces do influence the overall charge-carrier conduction. Here, we present an investigation of the electrical transport across vertical heterojunctions having disordered nm-thick films (polypyrrole, PPy) as the active material. The PPy structures are prepared by chemical polymerization from the pyrrole vapor phase, resulting in film thicknesses of a few tens of nanometers. The electrical characteristics of the devices are evaluated as a function of voltage and temperature, and the charge transport is found to be strongly influenced by the presence of trap states at the PPy highest occupied molecular orbitalgiving rise to space-chargelimited conduction with exponential distribution of traps. The trapping-state density is calculated, and X-ray photoelectron spectroscopy revealed an increase of disorder and a reduced doping density at the PPy growth interface. As a proof of concept, the PPy films integrated within the as-fabricated vertical heterostructures are applied as photosensitive devices. The observation of photocurrent is correlated to the presence of a gradient in the doping profile (from ca. 27.6 to 17.2% when thickness decreases from 120 to 20 nm). Our findings contribute to the elucidation of the charge-trapping center's origin in the nm-thick PPy films, as well as envision further applications in photoelectrochemistry, solar cells, and water splitting.

Temperature-Independent Polarization of Ultrathin Phthalocyanine-Based Hybrid Organic/Inorganic Heterojunctions

“…It is worth mentioning that the requirement for low roughness is crucial for large-area heterojunctions in which charge transport is prominent, whereas charge accumulation is a minor contribution. There, finite-emission effects and scaling factors based on fractal models are insightful to improve the theoretical approaches of experimental results . Here, we are dealing with large-area devices in which the active regions are vertical metal–insulator–metal (MIM) and metal–insulator–semiconductor (MIS) heterostructures.…”

“…The rNMs can provide an accurate and reliable contact between molecular ensembles and metal electrodes via controlled release of surface-like structures . For electronics, the rNM technology also provides an inherent advantage of reducing devices’ footprint, giving rise to ultracompact circuitry components , such as energy storage units, sensors, inductors, biosensors, capacitors, transistors, memristors, photocurrent generators, and vertical heterojunctions …”

Reorganization Energy upon Controlled Intermolecular Charge‐Transfer Reactions in Monolithically Integrated Nanodevices

“…[34][35][36] Additionally, nCap monolithic integration spares the use of either liquid metal electrodes or scanning probe tips, allowing reliable fabrication of real-life molecular devices that can be operated in different electromagnetic fields, temperatures, pressures, among other external conditions. [37][38][39] In the literature, one may find exciting examples of using the nanomembrane-origami technology to fabricate various monolithically-integrated devices featuring organic/ inorganic hybrid functions. [40][41][42][43] As a validating target for our experimental paradigm, we fabricate nCaps employing copper-phthalocyanine (CuPc) nanometer-thick films.…”

High‐Performance Ultrathin Molecular Rectifying Diodes Based on Organic/Inorganic Interface Engineering

“…[7] As organic materials and devices reach the nanoscale, organic/inorganic interfaces become a fundamental counterpart, being responsible for several functionalities. [8][9][10] In this scenario, understanding the role of the molecular orientation, [11] electrode configuration, [12] and substrates [13] in the charge transfer across organic/inorganic interfaces has enabled the development of new functional device concepts with precise control of their electronic properties. [14,15] The archetypal molecular device displaying interface-driven properties is the molecular rectifying diode, which follows the concept proposed by Aviram and Ratner.…”