Title: Thin Film Electronics – Limits to Performance
Applications for thin film semiconductor devices are huge, covering displays, solar cells and transistor/logic devices. The electronic performance of thin film semiconductors is however is generally not as high as for the ‘single crystal’ electronics of silicon and III-V materials, because disorder, in the bulk and at interfaces, tends to compromise electronic functionality. Nevertheless, two families of materials now perform very well, and in this talk I will explore current successes and future opportunities.
Organic LEDs and solar cells have shown remarkable improvements in performance, with close to unity quantum yields. However, the voltage efficiencies are still relatively low, and there are real opportunities to make step changes in performance. For LEDs, phosphorescent emission is widely used to avoid triplet exciton losses. Charge injection is often across the singlet energy gap, but the triplet emission is lowered by the exchange energy and this raises the drive voltage. For solar cells, performance is limited by relatively low luminescence yields, which reduces open circuit voltages, currently by 0.3 V and more. I will discuss new materials and device designs.
Lead halide perovskites now provide remarkably high performance thin film solar cells; single junction perovskite cells demonstrate efficiencies above 20% and, with bandgap tuning, provide the high gap semiconductor for tandems with silicon. This is enabled by a remarkable tolerance to the presence of chemical and structural defects, as evident most clearly from the very high luminescence efficiencies now achieved, close to 100%. This enable thin film LEDs and I will discuss recent results which demonstrate very high internal quantum efficiencies.
Professor Sir Richard Friend FRS FREng holds the Cavendish Professorship of Physics at the University of Cambridge and is Director of the Winton Programme for the Physics of Sustainability. He has pioneered the physics, materials science and engineering of semiconductor devices made with carbon-based semiconducting polymers. His research group was first to demonstrate using polymers efficient operation of field-effect transistors and light-emitting diodes. These advances revealed that the semiconductor properties of this broad class of materials are unexpectedly clean, so that semiconductor devices can both reveal their novel semiconductor physics, including their operation in efficient photovoltaic diodes, optically-pumped lasing, directly-printed polymer transistor circuits and light-emitting transistors.Professor Friend has also been directly involved in the process of commercialisation of this technology, forming several spin-out companies from the University of Cambridge including Cambridge Display Technology, Plastic Logic and Eight19