Our research is focused on the spectroscopy of molecular materials to explore fundamental scientific issues related to using the functional molecules for electronic applications. Our work lies at the interface of physics, materials, and physical chemistry. Research in molecular electronics has a very broad scope with many promising applications, including: solar cells, displays, transistors, biosensors and photonic device applications. Despite the diversity of uses, all these applications are based on thin films of functional materials such as organic semiconductors and organic/inorganic hybrid materials. In each case their performance is critically dependent upon the structural and optoelectronic properties of molecules, the precise arrangement, packing and interactions between the molecules. Our principal research focuses on this fundamental issue, seeking to develop a systematic, microscopic understanding of the relationship between nanostructures and optoelectronic properties of molecular semiconductors and to correlate it with the device functionality and performance.
In this regard, our team has been developing advanced optical and structural probes for molecular semiconductors. For example, we have developed and established Raman spectroscopy as an advanced structural nanoprobe for conjugated molecular semiconductors. Utilising selective resonant and polarisation dependent excitations, together with in situ control of temperature, pressure, electrical, and electrochemical potential, we have demonstrated its unique capability to elucidate the properties of molecular semiconductors. These include chemical structure, molecular conformation, order, orientation, fundamental photo- and electro-chemical processes and stability - all of which are critically important to the performance of a wide range of optical and electronic organic semiconductor devices.
Our ambition now is to extend our expertise towards the field of Nanoscale Functional Materials including organic and organic/inorganic, perovskites, bio-nanomaterials for hybrid electronics targeting for photo-electron conversion and bio applications, paralleled with developing novel spectroscopic Nanometrology for these functional materials.
Interfacial energetics and tail (trap) states
We measure the energetics of organic and perovskite semiconductor materials and at their interfaces, by Ambient Photoelectron Spectroscopy (APS) and Kelvin probe. Some examples are shown below; Figure (a) dark work-function and LUMO/HOMO values of 1µm thick organic bulk heterojunction films, (b) thickness dependent Fermi-level with different contact layers, in which a large change in the work function is found below 200 nm, but almost consistent over 200nm, and (c) Tail states in different organic bulk heterojunction martials.
We have also successfully identified the origin of traps in organic BHJ PV devices, causing a significant initial drop in device efficiency. These traps were most likely caused by light-induced bond disruption or cleavage at the site of the solubilizing side chain of PC71BM electron acceptor. [Cha, H. et al, Adv. Mater., 29(33), (2017)] The light-induced chemical bond alteration and conformational twisting of the electron donor molecules were also found to be responsible for device trap formation [Luke, J. et al, Adv. Ener. Mater. 1803755 (2019)]. For the perovskite/charge injection interlayer interfaces, we found that the perovskite deposited on NiOx have lower trap density than that deposited on PEDOT:PSS, with relatively higher density of trap states preferentially formed near the interface of PEDOT:PSS and the perovskite. [Lee, S. et al., ADVANCED SCIENCE, 5(11), 10 pages. doi:10.1002/advs.201801350]
Charge accumulation leading to recombination loss
Increasing the open circuit voltage (Voc) is one of the key strategies for further improvement of the efficiency of perovskite solar cells. It requires fundamental understanding of the complex optoelectronic processes related to charge carrier generation, transport, extraction and their loss mechanisms inside a device upon illumination. We have shown the important origin of Voc losses in Perovskite based solar cells, which results from undesirable positive charge (hole) accumulation at the interface between the perovskite photoactive layer and the charge extraction layer. Using Surface Photovoltage (SPV) measurement, we show strong correlation between the thickness-dependent SPV and device performance, unravelling that the interfacial charge accumulation leads to charge carrier recombination and results in a large decrease in Voc for the PEDOT:PSS/MAPI inverted devices. In contrast, accumulated positive charges at the TiO2/MAPI interface modify interfacial energy band bending, which leads to an increase in Voc for the TiO2/MAPI conventional devices. Our results provide an important guideline for better control of interfaces in perovskite solar cells to improve device performance further.
In-situ biosensing of metabolites
We demonstrate the use of in-situ resonance Raman spectroscopy to probe subtle molecular structural changes of PEDOT:PSS associated with its doping level. We demonstrate how such doping level changes of PEDOT:PSS can be used, for the first time, on operational organic electrochemical transistors (OECTs) for sensitive and selective metabolite sensing whilst simultaneously performing amperometric detection of the analyte. By changing the electrolyte to cell culture media, the selectivity of in-situ resonance Raman spectroscopy is emphasized as it remains unaffected by other electroactive components in the electrolyte. The application of this molecular structural probe highlights the importance of developing biosensing probes that benefit from high sensitivity of the material’s structural and electrical properties whilst being complimentary with the electronic methods of detection.
Degree of Molecular Order
Ordering of molecules in semiconducting materials can have significant effects on their optoelectronic properties. For example, thin films of regioregular poly(3-hexylthiophene) (RR-P3HT) can exhibit a high degree of molecular order (π–π stacking of molecules). This high degree of molecular order can lead to an increase in absorption at longer wavelength and a dramatic increase in charge carrier mobility as compared to its disordered form. Understanding of this molecular order is important to clarify the structure–property relationship in thin films and to make use of these thin films as active layers in various devices.
Natures of Electronic Transitions
A strong resonant enhancement in the Raman scattering intensity occurs when the energy of the excitation photon matches the energy of a dipole-allowed electronic transition of the molecule. This enhancement is observed for those Raman-active vibrational normal modes which map onto the geometric distortion of the molecule accompanying the electronic transition. In order to elucidate the natures of the different electronic transitions we can use 457 and 785 nm excitations, which allow us to selectively probe the high and low energy absorption bands.
We can probe the effect of photodegradation on the molecular structure of polymer chains in order to understand how different units react with with oxygen and light at molecular level. RBy comparing experimentally observed Raman spectra to theoretical spectra obtained from Density Functional Theory (DFT) simulations of likely degradation products, we identify the nature of photo-oxidised species. This information can assist the development of analogue polymers modified to hinder or avoid that degradation process, potentially allowing the fabrication of high-efficiency long-lifetime OPV devices.
One of the most promising research topics in the field of energy materials is the production of solar cells based on Organic Molecules. Organic Photovoltaic (OPV) offers a wide variety of advantages, especially the possibility to create low-cost, light-weight, transparent and flexible devices which can be embedded into everyday objects for clean energy harvesting.
In our research group we study new organic molecules for OPVs, with a special focus on Non-Fullerene Acceptors, to gain nanoscale insight into their photo-conversion mechanisms. In particular, the fact that our devices are based on bulk heterojunction active layers, i.e. an interpenetrating network of different semiconductor molecules, complicates the process of electron-hole separation and extraction at the interfaces and makes detailed studies necessary to optimise device architectures and production routes.
In our laboratories we fabricate solar cells by solution-based processes like spin-coating and characterise them by a variety of techniques. Some of the highlights of our equipment are Kelvin Probe and Ambient Pressure Photoemission Spectroscopy to investigate materials energy levels, Raman Spectroscopy for a thorough analysis of molecular vibrational modes and Atomic Force Microscopy to get surface images with nanometric resolution, as well as standard testing methods like electrical measurements in the Solar Simulator.
Since one of the most important requirements to achieve an effective large-scale OPV production is a stable long-term device performance, our research also aims at an in-depth understanding of the degradation processes undergone by the molecules. Methods like temperature-dependent Raman help us to probe the ageing mechanisms in the materials in order to improve their stability under operating conditions.
Perovskite Solar Cells
With the sharp growing demands on clean energy, Perovskite, a type of crystal structure with ABX3 composition, is one of the most promising materials for future solar cell markets. Perovskite exhibits many unusual properties which enable it to surpass other materials. The high charge mobility, low fabrication-cost and facile tenability makes perovskite solar cells (PSCs) reaching the efficiency of 25.2% (NREL Oct 2019), one of the highest performing single junction solar cell. Though the state-of-the-art with PSCs is highly competitive to alternative materials in the market, PSCs are still falling behind their theoretical limit. In our group, we focus on two different aspects to improve PSCs further by understanding the operation mechanism inside.
PSCs composed of multiple thin films to assist the photocharge separation and extraction to electrodes. Different types of thin film materials can affect the performance and behaviour of the solar cells. By studying the energetics of these different materials alone or in the cells, our group is able to detail the effect of energetics to cell performance. Then, systematically tuning the thin films energetics to meet the best performing condition. Besides energetics, the dynamic of charges is our second focus. Monitoring the charge flows of cells under lighting condition reveals the charge separation mechanism. Our lab facilitates the advance technique to measure not a full cell but half, so we are able to distinguish each layers contribution to the full device performance stack by stack.
Our highlight equipment, Ambient Pressure Photoemission Spectroscopy 04 (APS04), is capable of measuring the material energetics, such as work function and highest occupied molecular orbital, in high resolution. Other than energetics, it can also measure the dynamic process of the devices under white light or UV light condition. These provide the essentials to comprehend devices’ performance. Our lab also has Raman system for sensing molecular vibrational modes and structural deformation under energetic tuning, Atomic Force Microscopy for morphological checking and work function scanning and Time-Correlated Single Photon Counting system for high time resolution photonic properties.
Find out more about our Research
Efficient Charge Carrier Injection and Balance Achieved by Low Electrochemical Doping in Solution‐Processed Polymer Light‐Emitting Diodes
Twist and Degrade—Impact of Molecular Structure on the Photostability of Nonfullerene Acceptors and Their Photovoltaic Blends
Impact of Initial Bulk‐Heterojunction Morphology on Operational Stability of Polymer:Fullerene Photovoltaic Cells