Perovskite semiconductors hold large promise for opto-electronic applications such as solar cells and light-emitting diodes (LEDs). In just a few years of research, perovskite LEDs have reached an external quantum efficiency of about 20%, almost at par with incumbent technologies. Yet, these new light sources are still in their infancy, and several fundamental questions remain un-explored and un-resolved.
In the state of the art, the peak external quantum efficiency (EQE) is reached at low current density of 10....100 mA/cm2. At higher current densities, the EQE drops off, a phenomenon known as EQE roll-off. As a result, the achievable brightness is also limited. Furthermore, also the stability of the LEDs under prolonged bias is still insufficient for practical applications, in particular at high current densities.
In this PhD, we will further advance the research on perovskite LEDs towards high efficiency at high current densities (we target current densities of 10 to 100 A/cm2). The first research task is to elucidate the mechanisms that cause the EQE roll off. Possible mechanisms that have been invoked in the literature are Auger recombination, Joule heating or imbalanced charge carrier injection in the active layer, but no consensus has been reached. There may also be a role played by ion migration, as perovskites are ionic semiconductors with some amounts of potentially mobile ions and charged vacancies. These questions will be systematically investigated in this PhD, not only by investigating LED structures, but also by designing and fabricating specific test structures and characterizing them by advanced electrical and optical techniques (such as ultrafast spectroscopic methods). Also, device modeling is foreseen to support the analysis. The second research goal of the PhD is to push out the roll-off to high current densities: we target >10 A/cm2. In our laboratory, we have experience with several types of perovskite active layers (material compositions, film morphologies, phases), and the plan is to compare them in this study. Also, the electron and hole injection and transport layers in perovskite LED stacks are likely of large importance: we plan to investigate the use of oxide semiconductors, amenable to sustaining larger currents than organic ones.
In this PhD, the student will be in charge of the design and realization of the devices in the cleanroom, as well as modeling, characterization by advanced techniques and data interpretation. He/she will be guided by senior researchers with experience in OLEDs and perovskite LEDs. Ample experience with perovskite material systems and device stacks is available.
The candidate PhD student has a background in nano-science, nano-engineering, nano-technology and semiconductor (opto-electronic) devices and physics. Some experience in material engineering and material characterization is a plus. He/she has a strong affinity for semiconductor technology and a keen interest in optical and electro-optical properties of materials. The research will be supported and guided by several experts from different domains in imec. It is part of the plan to make ultra-bright light emitting thin-film diodes and lasers, funded by the ERC Advanced Grant of promotor Paul Heremans.
Required background: Nano-science, Nano-technology, Physics with affinity for opto-electronics and technology
Type of work: 40% device processing, 40% advanced electrical and optical characterization, 10% simulation and modeling, 10% literature study
Supervisor: Paul Heremans
Daily advisor: Weiming Qiu
The reference code for this position is 1812-98. Mention this reference code on your application form.