The tandem devices can only be understood and be fully exploited when understanding all the optical and electrical losses
Silicon solar cells have dominated the photovoltaics (PV) market, owing to their proven reliability, continued cost reduction and steady efficiency increase of around 0.5% per year. Tandem solar cells, consisting of absorbers of two different but complementary bandgaps, are widely considered to be the next-generation cell technology that would enable efficiencies well beyond what is possible with a single bandgap absorber, by reducing thermalization losses.
Tandem devices in 2-terminal, 4-terminal and more recently 3-terminal configurations have been explored, with bandgap-tunable perovskites or III-V materials as the top absorber, and silicon or CIGS as the bottom absorber. Besides the absorber materials, the tandem device stack consists also of several charge transport layers, optical buffers and/or recombination layers. The device architecture also depends on the chosen electrical configuration. Optical design of the tandem stack, charge carrier recombination in the bulk and at the heterointerfaces and the charge transport properties are crucial in maximizing the tandem solar cell performance. In addition, for perovskite-based tandems, the impact of field-induced ion migration, light-soaking effects and thermal stress on the electrical properties and stability of the device must be understood.
The heart of this PhD thesis is two-fold: (1) detailed optical, electrical and material characterization of the absorbers, charge transport and recombination layers, their heterointerfaces, and fully fabricated devices, and (2) modeling of the device physics of tandem solar cells in different electrical configurations (i.e., 2, 3 and 4-terminal). Characterization techniques such as ellipsometry, X-ray diffraction, UV photoelectron spectroscopy, time-resolved photoluminescence (PL), PL quantum yield, and electrical measurements (e.g., illuminated current-voltage measurements, external quantum efficiency and impedance spectroscopy) will be used to gain insights into specific material or device properties. Simulation software such as Sunsolve and Sentaurus TCAD will be available to build a physics-based opto-electrical model of the tandem solar cells, based on the properties of the different layers and their interfaces, including for e.g., ion migration and phase segregation effects in perovskite absorbers. The developed models will be validated using functional devices, and subsequently used to analyze the main loss factors and suggest improvements to the device design. Comparison of the different electrical configurations would be done in both monofacial and bifacial modes, and configuration-dependent device optimization will be done. Gaining such insights into the device characteristics is key to boosting the performance of tandem solar cells.
Required background: M.Sc. in Physics or Electrical Engineering, with a strong background or interest in device modelling. Curious, autonomous, and a team player.
Type of work: 40% modelling, 40% fabrication and characterization, 20% literature
Supervisor: Bart Vermang
Co-supervisor: Jef Poortmans
Daily advisor: Hariharsudan Sivaramakrishnan Radhakrishnan, Filip Duerinckx