Nguyen Group Research

The Nguyen Group conducts research on organic electronic devices, such as photovoltaics, light-emitting diodes, and field-effect transistors. We characterize devices and the organic, semiconducting materials utilized in these devices by a variety of electrical, optical, and structural measurements to elucidate the influence of chemical structure on the properties and performance of devices. With an ever-evolving understanding of the underlying mechanisms that govern device performance, we can design new materials, processes, and device architectures to progress this field toward commercially viable, light-weight, and flexible organic electronics.

Organic Electronics

Organic Photovoltaics

A bulk heterojunction organic photovoltaic (BHJ OPV) device consists of a ~100 nm thick film comprising a blend of an electron donating and electron accepting material that phase segregate to form nanoscale, bicontinuous domains. Upon exposure to light, a photon absorbed by one of the materials promotes an electron from its highest occupied molecular orbital (HOMO) to its lowest unoccupied molecular orbital (LUMO), forming a bound electron–hole pair known as an exciton. This exciton can diffuse within the film and may reach an interface between the electron donor and electron acceptor. At this interface, there is an energetic driving force for the exciton to split into free charges, with the electron in the acceptor and the hole in donor. Once charges are formed, an electric field created by two electrodes of differing work functions provides a driving force to sweep out the generated charges, with the electrons travelling through the acceptor domains to the cathode and the holes travelling through the donor domains to the anode.

In order to maximize quantum efficiency and therefore the current generated by a device, each of these processes (photon absorption, exciton diffusion, and charge generation, transport, and collection) must occur efficiently. OPV materials’ absorption spectra must have good overlap with the terrestrial solar spectrum in order to maximize the number of photons absorbed by the device. We monitor the absorption characteristics of OPV materials by UV-vis spectroscopy. The exciton diffusion length limits the domain size of an organic material that can efficiently harvest photons. Measurement of the exciton diffusion length is challenging; in the Nguyen group, we conduct steady-state and time resolved photoluminescence measurements, Monte Carlo simulation, and modeling of the external quantum efficiency in order to study exciton diffusion length in organic films. Charge generation and transport are characterized by several methods in our lab, which are described in more detail in the Charge Transport and Recombination section. The nanoscale morphology of phase separated donor and acceptor domains can also greatly influence the basic processes governing OPV efficiency, and so we characterize the nanoscale morphology of organic semiconducting films in order to develop structure-property-performance relationships for OPVs.

In order to test our materials, we fabricate small solar cells on glass substrates that contain a transparent conductive material, indium tin oxide. We first add a transparent conductive polymer, which serves as one electrode, then cast our organic semiconductor blend from a single solution via spin-coating. The two materials phase separate into an interconnected network of donor and acceptor phases upon drying by evaporation of the solvent, which, as mentioned above, strongly influences the performance of OPV devices. We work to control the nanoscale morphology of the blend film by adjusting the parameters of our organic film processes, including the solvents, solvent additives, concentration, blend ratio, spin rate, and temperature. A low work function, reflective metal contact is then added to the top surface via thermal evaporation through a shadow mask. The end result is an organic film sandwiched between two electrodes. To test the cells, we use a lamp that mimics the spectrum and intensity of sunlight, and measure the current generated as a function of voltage. Comparing the maximum power generated to the total power available from the incident light gives an efficiency value. We use other tools such as charge transport and recombination studies and nanoscale morphological characterization to understand the origin of the OPV performance and to find news ways of obtaining higher efficiency solar cells.

Charge Transport and Charge Recombination

Charge carrier transport and carrier recombination govern the operation of all electronic devices, including those utilizing organic semiconductors. Thus, understanding charge transport and charge recombination in organic semiconductors is a prerequisite for successfully designing future high performance organic electronic devices such as solar cells, field-effect transistors or light emitting diodes.

In our group, we study charge carrier transport by fabricating field-effect transistors and (single-carrier) diodes. Understanding the energetics of the organic materials allows us to isolate either hole or electron transport by choosing electrode materials with the correct work functions relative to the frontier energy levels of a given organic compound. Analysis of the current–voltage characteristics of these devices provides information about how fast these charge carriers are transported through the organic material in the precence of an applied electric field, which is known as their mobility. Furthermore, with an understanding of the mobility’s dependence on temperature, we can get information about the width of the density of states (DOS) and the activation energy (Ea) in the organic semiconductor systems. By characterizing mobility values in pristine donor/acceptor materials and blends, we can not only understand the underlying mechanisms that dictate device performances, but also provide insights on the design and synthesis of new and improved materials.

We employ a variety of techniques to understand the recombination mechanisms in organic semiconductors. Studying double carrier devices enables us to investigate the process of holes recombining with electrons. This process is a fundamental loss mechanism in organic solar cells, but is essential to the operation of light emitting diodes. Additionally, we probe recombination mechanisms by looking at the photoluminescence, electroluminescence, quantum efficiency, and impedance response of organic electronic devices as a function of temperature and excitation energy.

Morphological Characterization

The performance of organic electronic devices is profoundly influenced by the nanoscale morphology of the organic film. For example, the active layer of a bulk heterojunction organic photovoltaic (BHJ OPV) device is a ~100 nm thick film consisting of a blend of an electron donating and electron accepting material that phase segregates to form nanoscale, bicontinuous domains. The size, purity, order, and distribution of these nanoscale domains all affect each of the processes that govern solar cell efficiency. Therefore, having control over nanoscale morphology is of great importance.

Given the complicated nature of organic thin films, no one technique can fully describe the nanoscale morphology. In order to probe the nanoscale morphology of organic films, we employ a variety of different characterization techniques. By utilizing a variety of different techniques, we can more completely describe a given system in order to develop structure-property-processing relationships that will guide the development of future materials and devices. By comparing the nanoscale morphology of related OPV systems that perform differently, we can identify morphological characteristics that either help or hurt device performance, and then tune our fabrication procedures accordingly. The figure below is an example of how the nanoscale morphology of an organic film (in this case a small molecule donor:fullernene acceptor BHJ OPV) can be studied with several different techniques, which each provide unique structural information about the system.

Interfaces in Organic optoelectronics

The operation of organic electronic devices strongly depends on the charge injection or charge extraction properties of the metal contact. The interfaces between metal electrodes and organic semiconducting materials play a crucial role in these processes. The graphs (from left to right) below illustrate device architectures of organic light-emitting diodes (OLEDs), solar cells (OPVs) and field-effect transistors (OFETs), respectively. Generally speaking, good energetic alignment between the electrode work function (WF) and the conduction (valence) band of the electron (hole) conducting material is desired to maximize device performance. At the same time, the unique requirements of each application (photovoltaics, light-emitting diodes, and field-effect transistors) limit the range of suitable electrodes. For example, to produce electroluminescence in the PLED (left graph), electrons have to be injected from the top cathode to recombine with holes injected from the bottom anode. An opposite process occurs in the OPV (middle graph) where charges created in the active layer must be extracted at the electrodes to result in the photoconductivity. In the OFET (right graph), the current is formed via charges injected from the source electrode under the modulation of gate bias. In OLEDs and n-type or ambipolar FETs, for instance, efficient injection of electrons from a metal electrode to the LUMO of the organic material is necessary. A reactive, low WF metal electrode provides a suitable contact to the LUMO of organic materials but simultaneously raises a problem of air stability. Thus it is favorable to have stable materials with tunable WFs so that charge injection can be preferentially improved without complicating the device structure and fabrication procedure. An emerging alternative to a low WF metal is a solution processable interlayer between the organic film and the metal electrode that effectively modifies the electrode work function while maintaining its otherwise desirable characteristics such as stability and cost.

In reality, the charge injection efficiency is often limited by the choice of metal electrodes, undesirably affecting the ultimate device performance. To circumvent this shortage, interlayers are introduced to deal with charge injection issues. For instance, interlayers consisting of a thin organic film or DNA film have been incorporated in OLEDs, OFETs, or OPVs from solution processing. As a result the work function of the top electrodes is modified such that charge injection barriers are effectively reduced. This modification leads to improving the luminance efficiency (in the case of OLEDs) and charge carrier mobility in OFETs. To understand the impact of interlayers on the device performance, we employ a combination of techniques including ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), AFM, (scanning) Kelvin probe microscopy, electrostatic force microscopy, conducting AFM and contact angle measurements, etc. From these measurements, we aim to attain insights into the interfacial properties at the metal/organic interface.

Solution-processed Organic Ratchets

Organic-ionic ratchets are asymmetric devices that allow for the generation of electrical power from electromagnetic noise signals by means of a charge pump mechanism. The ionic-ratchets are based on organic field-effect transistors (OFETs) structure and realized by doping the organic semiconductor with ions. The symmetry breaking of ratchets comes from the asymmetric non-linear conductance of an organic semiconductor: ions blend subjected to a voltage stress. With asymmetric source and drain contacts, the ratchets work like a charge pump and thus convert random electromagnetic noise signals between the gate and source to dc current between source and drain without bias applied between the source and the drain. The ionic-ratchets can deliver large electronic currents that are relevant for energy-harvesting applications including radio-frequency identification tags.

To tackle the loss of ratchet functionality due to the relaxation of ions in the ionic ratchet system, an alternative approach utilizing built-in asymmetry of metal/semiconductor interface can also render ratchet effect, which has been shown to have significantly improved device stability compared to the ionic counterpart.

Upper-left: The schematic diagram of the organic-ionic ratchets. Upper-middle: The flexible ratchets fabricated on 3M tape with pencil-drawn graphene as electrodes. Upper-right: The electrical characteristics of the organic-ionic ratchets. The ratchet can output maximum power of about 50 μW. Lower: Schematic illustration of asymmetric contact ratchet and stability of its electrical characteristics.

 

Organic Field-Effect Transistors (OFETs)

Organic FETs (OFETs) are promising to be integrated in plastic electronics and bioelectronics due to their unique properties including flexibility, transparency and solution-processibility. In our lab, we focus on elucidating the molecular assembly, charge transport and charge trapping in high mobility OFETs fabricated from donor-acceptor (D-A) conjugated polymers. Those fundamental understandings are crucial for molecular designs and device engineering of OFETs toward practical applications. Using high-resolution atomic force microscopy (AFM), we revealed the alignment of polymer fibers at the resolution of a few nanometers. This alignment facilitates the fast intrachain charge transport along the polymer backbone, which occasionally hopping between polymer chains. The energetic barrier for carrier hopping is relatively low for high mobility systems. Device stability is also important for practical applications. With a thorough study, we found that electron trapping and formation of -SiO- charges are the origins of the electrical instability and non-ideal current-voltage behaviors in p-type OFETs fabricated from low band-gap D-A polymers.

 (Left) High-resolution AFM image of a thin-film OFET fabricated from a D-A polymer with mobility of ~10 cm2/Vs; (right) A cartoon drawing of charge transport mechanism in high-mobility OFETs with aligned polymer chains: predominant intrachain charge transport with occasionally hopping at low activation energy.

Electron Transport in Biofilms

Microbial fuel cell (MFC) technology is considered as a viable candidate to provide an economical pathway for wastewater treatment because it utilizes anaerobic bacteria to digest organic matters in wastewater while it is capable of producing electricity to self-sustain treatment plants. Though scientifically interesting and economically attractive, commercially full-scale and highly effective MFC reactors are not yet viable due to multiple fundamental and technological challenges. Among them, the low power density of MFCs is the critical one. The lack of a fundamental understanding in charge transport within biofilms and charge transfer at the bacteria/electrode interface is considered as of the reasons preventing a breakthrough in power output of MFC technology. In our lab, we design and perform measurements with controlled experimental conditions to gain insight into both the short and long-range electronic transport processes in biofilms. We investigate the electronic properties of the biofilms in the dry state, at various hydration levels, and under MFC operational conditions where foods and electrolytes are present. The detailed characterization of morphological and electrical properties of biofilms at nanoscale and in the bulk is carried out to determine the short and long-range charge transport mechanisms.

 (a) Operational principle of a MFC; (b) three proposed mechanisms of electron transfer from cells to the anode surface: (i) direct transfer, (ii) indirect transfer through shuttle redox compounds, and (iii) conductive pili nanowires; (c) a schematic drawing showing long-range electron transport through the biofilm to the anode surface; and (d) schematic drawings of conducting atomic force microscopy (c-AFM) measurement of single cell bacteria with the membrane proteins to support direct electron transfer from inner cell to electrode surface

Organic Electronic Devices for Tactile Sensing Applications

Tactile sensors that translates distributed mechanical signals into electronic signals, have widespread applications in industry, healthcare, consumer electronics, and robotics. With new advances in high-mobility organic semiconductor, solution-processing and device architectures of organic field-effect transistor (OFET), opportunities emerge for integration of flexible and compliant OFET-based capacitive tactile sensor arrays. By utilizing highly compliant polymer and introducing micro/nano-engineering for one of the key elements of OFET, the dielectric layer, we aim to realize highly sensitive OFET pressure sensor, readily to be further fashioned into tactile sensor pixels, which are of great utility for electronic skin and sensing tools for a variety of applications.

OFET-based sensor design integration into applications for electronic palpation in biomedicine, wearable computing, and robotics.

Novel doping of Organic Semiconductors

Lewis acids have primarily been used in various syntheses for both organic and organometallic complexes. A few years ago, a new application for lewis acids was discovered: as a dopant for polymers with lewis basic binding sites. Traditionally, doping has involved a one-electron transfer process from the polymer to the dopant, creating a hole in the polymer which contributes to improved electrical properties in the polymer. Here however, UV-vis and EPR data both show no one-electron transfer reaction occurring during doping by a lewis acid such as tris(pentafluorophenyl)borane (BCF). Yet in addition to a shift in the main band absorption peak, improved electrical properties such as an increase in free charge carrier concentration and a decrease in mobility and activation energy are observed. Our group aims to understand the fundamental doping mechanism and its effects on the polymer’s electrical properties.

Current density–voltage (open symbols; J-V) and luminance– voltage (closed symbols; L-V) plots for devices containing 0.00 (black circles), 0.01 (red squares), and 0.02 mol equivalents (green triangles) B(C6F5)3.

Exciton Diffusion in Organic Semiconductors

Organic semiconductors have relatively low dielectric constants and weak intermolecular forces, which confines Coulombically-bound electron-hole pairs (excitons) to a single molecule, or perhaps two. Although spatially confined, the energy contained in these excitons can be transferred to other nearby molecules via exciton diffusion, which follows a ‘random walk’ motion. In OPVs an exciton is formed by the absorption of solar irradiation. In order to separate into free charges, however, that exciton must find an interface where there is an energetic driving force for charge separation. If the exciton does not find a suitable interface during its excited-state lifetime, it will decay radiatively or nonradiatively, thus unable to contribute to the solar cell’s photocurrent. On the other hand, in organic light-emitting diodes (OLEDs) excitons are formed when charges injected from opposite electrodes come into close contact. For optimal device operation an exciton should immediately emit a photon (radiative decay). If it does not, then the exciton has a chance to diffuse throughout the active layer and potentially undergo unwanted bimolecular processes, such as exciton-charge annihilation. Thus, exciton diffusion is a critical factor in the performance of optoelectronic devices.

                  In the Nguyen group we use a time-correlated single-photon counting technique, coupled with Monte Carlo simulations, to measure the exciton diffusion length of organic semiconductors. Besides just measuring exciton diffusion lengths, our group has investigated what factors influence exciton diffusion, such as temperature, crystallinity, thermal annealing, and solution-processing additives. Some of our current work concerns the fundamental understanding of the mechanisms that contribute to exciton diffusion, i.e. Förster and Dexter energy transfer, and how they are influenced by molecular and electronic structure. In our lab we are also studying the fundamental photophysical properties of a new class of molecules designed for OLEDs so that we can facilitate the rational design of newer, better materials. Finally, our group has recently gained an interest in the phenomenon of triplet-triplet annihilation photon upconversion (TTA-UC), a process whereby diffusion results in two optically generated triplet excitons encountering each other (TTA), leading to the formation of one higher energy singlet exciton. Converting low energy photons into higher energy photons has the potential to enhance solar cell performance and perhaps even drive UV-catalyzed reactions from just solar irradiation.