Fabricating Device Architectures
Organic and Hybrid Photovoltaics
Unlike traditional solar cells, which are based on an active layer of inorganic material such as silicon, organic solar cells (also known as OPVs) use semiconducting organic molecules and polymers to absorb light and convert it into electricity. The active layer of an OPV is typically a mixture of two materials, which act as electron-donor and –acceptor, in order to separate the photo-generated exciton into free charge carriers for extraction at the anode and cathode respectively.
Device optimisation has shown that the most efficient structure for the active layer is a bulk heterojunction, where the two materials are blended together at the nanoscale. This structure maximises exciton dissociation, through the large donor/acceptor interface, and aids charge transport through interpenetrating networks of donor- and acceptor-rich domains. The importance of morphology in determining efficiency means that structural optimisation is an essential step to obtaining the best possible efficiency from every novel material developed for OPV applications.
OPVs offer several potential advantages over traditional solar cells, due to the low cost of the materials, their controllable opto-electronic properties, and the ease of large-scale high-throughput OPV fabrication through industrial roll-to-roll printing processes. In order to compete economically with traditional solar cells and non-renewable energy sources, OPVs must reach efficiencies greater than the current record of 10%, and be able to operate for long periods of time without degradation. To this end, we must endeavour to understand the structure-property relationships that determine device efficiency and ensure long-term photo-chemical stability.
Organic Chemical Sensors
Unlike large stand-alone analytical chemistry systems (mass spectrometers, chromatographs, IR spectrometers, etc.), organic chemical sensors based on π-conjugated system can provide us with a special offer to enable a low-cost device architecture/fabrication and a low-power consumption. Because the physical and electrical properties of π-conjugated molecules are easily tuned by a different chemical structure via low-cost synthetic process, we typically try to change the chemical structures of molecules depending on target molecules to optimize a four S’s— sensitivity, selectivity, speed and stability a sensitivity and responsibility. In addition, we can make an use of a variety of advantages of their excellent physical and optical characteristics, such as, lightweight, flexibility and transparency.
However, the corresponding organic sensor materials/devices still suffer from poor sensitivity and selectivity because of low conductivity and physical absorbance. Thus, in order to achieve the desirable organic chemical sensors, we may require a cooperative system which is consisting of a new class of π-conjugated systems as a signal deliver and a novel stimulus-to-signal transducing system as a stimulus receptor. Recently, our group and international collaborator (Prof. Kwanghee Lee’s group, GIST, South Korea) undertake the project of ‘innovative plastic sensors’ which aims to develop a cooperative blend system incorporated with the efficient stimulus receptor and signal delivers for fine discrimination of gas phase molecules. By exploiting an electrostatic interaction in the stimulus receptor, which can allow a redistribution of electron density in the signal deliver when the gas phase molecules are attached to the film surface, the changed electrical signal can be collected through the signal delivers. We can call it as a cooperative stimulus-to-signal transducer (CSST) as shown in Figure 1.
Organic Field Effect Transistors
Transistors are the fundamental building block of contemporary circuitry. The so-called field effect is the changing conductivity of the semiconducting layer on application of an electric field orthogonal to its surface. Organic Field Effect Transistors comprise of an organic layer, separated from a gate electrode by an insulating material (usually SiO2). Two additional electrodes (source and drain) are used to move charges across the channel. On application of a gate voltage, charges are injected into the organic material and move from source to drain on application of a second voltage. Measures of merit for OFETs are the on/off ratio (ratio of current when off to current when on) and mobility (ease of charge movement through the device).
In our group we look to improve device performance through manipulation of molecular orientation. For insoluble small molecules such as copper phthalocyanine and pentacene, we have demonstrated the importance of self-assembled monolayers and interlayers on molecular orientation, which inturn determine charge transport through the active layer.
We have used our zone-casting apparatus to control the molecular orientation of small molecule TIPS-pentacene. When uniaxially aligned needles of TIPS pentacene align parallel to the source and drain electrodes charge carrier mobility is improved by an order of magnitude and on/off ratio by 104.
Organic Light Emitting Diodes
Organic Light Emitting Diodes (OLEDs) are an emergent technology, which are promising for cheap, lightweight, flexible and efficient displays and lighting sources. In particular, current lighting requirements account for a significant fraction of energy consumption, which could be considerably reduced by the use of highly efficient organic solid-state lighting systems such as white OLEDs. OLEDs comprise of thin films of organic material sandwiched between two electrodes (one of which is transparent).
Electrons are injected into the LUMO of the organic materials from the cathode and holes into the HOMO of the organic materials from the anode. Under an applied voltage, these charges move through the device where they form coulombically bound hole-electron pairs known as excitons. These can decay radiatively to form photons of light which can be of different colours depending on the bandgap of the material.
Because the organic material itself generates coloured light, it can work without a backlight unlike LCD monitors. This gives the added benefit of decreased power consumption, and increased contrast with deeper blacks, richer colours and improved viewing angles. Our OLED research in the nanoanalysis group consists of 1) controlling the organic thin film nanostructure to improve OLED properties such as stability, colour richness and device efficiency and 2) using new electrode and interlayer materials and studying their impact on device performance and charge injection, transport and recombination.