Solid-state lighting and displays are becoming ubiquitous in our daily lives, finding their place in televisions, cell phones, high-powered lamps and many common consumer appliances. In the Optoelectronics Group, we investigate a range of polymeric and organic-inorganic hybrid materials for applications in solid-state light-emitting devices. We are devoted to understanding the fundamental physics of light emission in new semiconducting materials, and apply novel design principles to achieve high performance in electroluminescent devices. We employ a range of advanced electrical and optical techniques, including transient optical spectroscopy, time-correlated single photon counting, photoluminescence spectroscopy and transient electrical techniques to investigate the excited-state dynamics and properties in luminescent materials.
We pioneered the world’s first polymer light-emitting diode in 1990, using conjugated poly(p-phenylene vinylene) (PPV). This work has since been successfully commercialised by a spin-off company, Cambridge Display Technology. In the most basic polymer light-emitting diode device structure, a conjugated polymer is sandwiched between an electron-injecting cathode and a hole-injecting anode. The figure below shows the device operation mechanism. First, electrons and holes are injected from the ohmic cathode and anode, respectively, into the organic semiconductor (step 1). The injected carriers, driven by the electric field (and to a smaller extent diffusion), travel toward the opposite electrode (step 2). When opposite carriers meet, they form a coulombically bound neutral exciton (step 3). The excitons then decay radiatively to give photons and hence produce light emission (step 4). In newer structures, charge blocking layers and charge transport layers are implemented to selectively control charge movements and confinement within the active layer to improve exciton formation and light emission.
A hybrid organometal halide perovskite has recently emerged as an excellent semiconductor for photovoltaic applications. In our group, we investigate and exploit the luminescent properties of these direct bandgap semiconductors, and successfully achieved bright perovskite electroluminescence in a light-emitting diode structure.  We designed our perovskite emitter to be sandwiched between two larger bandgap semiconductors, in order to confine charges and achieve better radiative recombination. We have now demonstrated the perovskite to be emissive in a variety of colours, as well as in a range of new device architectures. We look forward to further photo-physical and electrical investigation in this relatively new field of perovskite luminescence, with the view of moving this technology towards commercial viability.
 Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H., Bright light-emitting diodes based on organometal halide perovskite. Nature Nanotechnology 2014, 9 (9), 687-692.
Amplified light emission and Lasing in Photonic Structures
Besides electrically induced light emission in optoelectronic devices, we are also working on photonic structures which allow for optical gain in optically pumped systems. These structures can be used to modify the spectral light output of emissive materials and allow for gain in suitable systems. Requirements for optical gain are high recombination efficiencies and long-lived excited states. We showed that solution processed thin films of organic conjugated polymers can be used as such gain media in optically driven vertical micro-cavities (http://www.nature.com/nature/journal/v382/n6593/abs/382695a0.html), which allowed for the fabrication of flexible conjugated polymer lasers.
Recently, the material class of lead-halide perovskites has received a lot of attention due to their excellent performance in photovoltaic devices. We have demonstrated that these materials can also show efficient radiative recombination of photo-excited states and allow for surprisingly long lifetimes in the order of 100s of nanoseconds. Based on these clean semiconducting properties we fabricated the first optically pumped lasing micro-cavity which uses a hybrid lead-halide perovskite material as gain medium.
In these structures amplification of longitudinal modes occurs at relatively low lasing thresholds around 0.2uJ/pulse, which is comparable to the measured values in the first organic conjugated polymer micro-cavities. (http://www.cam.ac.uk/research/news/revolutionary-solar-cells-double-as-lasers) The demonstration of lasing in these readily prepared spin-coated materials suggests very clean semiconducting properties and makes their application in other photonic applications very promising. We currently aim to develop new types of photonic structures and combine these with optoelectronic devices.