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.
Nanoanalysis Research Videos
Nonfullerene Acceptor‐Based Organic Solar Cell Review
Chiara introduces review paper on the Recent Progress and Challenges in OPV
A Commercial Benchmark:Organic Solar Cells for Indoor Light
Joel gives an overview of his recently published work.
Introducing one of our recent photodetector publications
Nanoanalysis Lab Tour
A tour of the Nanoanalysis lab and instruments we use to characterise of novel electronic devices
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.
Find out more about our Research
Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic solar cells
Molecular understanding of a π-conjugated polymer/solid-state ionic liquid complex as a highly sensitive and selective gas sensor
Molecular-level electrochemical doping for fine discrimination of volatile organic compounds in organic chemiresistors
Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells
Towards Efficient Integrated Perovskite/Organic Bulk Heterojunction Solar Cells: Interfacial Energetic Requirement to Reduce Charge Carrier Recombinat
Origin of Open-Circuit Voltage Losses in Perovskite Solar Cells Investigated by Surface Photovoltage Measurement
A highly sensitive molecular structural probe applied to in situ biosensing of metabolites using PEDOT:PSS
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