Topic title: Thin and flexible heterojunction interdigitated back-contact (HJ-IBC) cells on epitaxial Si substrates with world-record efficiency
PV is the fastest growing electrical energy generation source with 39% of newly-added global electricity generation capacity in 2017 coming from PV alone. The cumulative installed capacity of PV has already surpassed 400 GW and in the time span 2025-2030, annual PV production is expected to reach 1 TW/year, ushering in the global energy transition. In such exciting times, the main economic driving force in the PV industry is the reduction of PV module cost per Watt peak, which translates into higher efficiency cells and better utilisation of silicon, which still constitutes about a third of the final module cost. Moreover, in the age of TW-scale PV deployment, it is essential that the CO2-footprint of PV manufacturing itself is kept under check.
Towards these targets, imec has been involved in the development of disruptive wafering technologies, in particular, the epitaxial Si (epi-Si) lift-off technology , which enables the production of thin (30-100 µm) epi-Si substrates without any kerf loss, while also short-cutting the majority of the energy- and capital-intensive conventional PV manufacturing value chain. This could potentially enable silicon utilisation rate down to just 1 g/Wp and a reduction in energy usage and thus the CO2-footprint by about 2 times.
Meanwhile, the PV industry is also transitioning from Al-BSF solar cells (the current work horse) to PERC solar cells in order to increase the module efficiencies. The focus of next-generation solar cell technology development has now turned towards reducing contact recombination. For this, solar cells with passivated contacts are being widely investigated, such as (1) tunnel oxide + poly-Si contacts, (2) low-temperature all-Si heterojunction contacts, and (3) transition metal oxide-based carrier-selective contacts. At imec, all three routes are being investigated. Si heterojunction (HJ) cells with an interdigitated back-contact (IBC) architecture are the most efficient silicon solar cells in the world today, with Kaneka breaking the world record efficiency with its remarkable 26.7% cell , . However, such high efficiencies must be married to an industrially-viable processing sequence.
This PhD thesis proposal focuses on the implementation and advancement of the HJ-IBC cell technology on advanced thin and flexible kerfless epitaxial Si substrates. The first part of this work will focus on the investigation of the loss mechanisms in HJ-IBC cell technology of imec, in particular the charge carrier transport and passivation properties of the thin film contact layers. New materials and advanced architectures such as bifacial IBC will also be investigated. Based on the understanding gained, improvements to the device performance will be made step-by-step using standard Cz wafers, targeting efficiencies of ~26%. In the second part of this work, the developed high-efficiency HJ-IBC cell technology will be applied to advanced epi-Si substrates. The behaviour of this new type of Si material in different process steps will be studied. Both electronic and mechanical proprties of thinner substrates up to 50 µm thickness will be evaluated, in both freestanding and bonded configurations. The limitations of the epi-Si material and of handling thin substrates will be identified and addressed. This work is expected to culminate in the attainment of a world-record efficiency on thin epi-Si foils using a rather simplified process sequence. Integration of such thin and flexible cells on curved modules, ready for vehicle-integrated PV (VIPV) applications will be started as the outlook of this PhD work.
 H. Sivaramakrishnan Radhakrishnan, V. Depauw, K. Van Nieuwenhuysen, I. Gordon, and J. Poortmans, "Epitaxial Si lift-off technology: Current status and challenges," Photovoltaics Int., vol. 38, no. September, pp. 44–56, 2017.
 M. A. Green et al., "Solar cell efficiency tables (version 50)," Prog. Photovoltaics Res. Appl., vol. 25, no. 7, pp. 668–676, 2017.
 K. Yoshikawa et al., "Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26%," Nat. energy, vol. 2, pp. 1–8, 2017.
Required background: Master in Science or Master in Engineering
Type of work: 15% literature study + 15% modeling + 70% experimental
Supervisor: Jef Poortmans
Daily advisor: Hariharsudan Sivaramakrishnan Radhakrishnan
The reference code for this position is 1812-70. Mention this reference code on your application form.