Photovoltaics (PV) is the fastest growing electrical energy generation source in the world. The cumulative installed capacity of PV has already surpassed 600 GW by the end of 2019 and the annual PV production is expected to increase in the years to come, ushering in the global energy transition. In such exciting times, the main economic driving force in the PV industry is still the reduction of PV module cost per Watt peak. As the crystalline Si (c-Si) module costs continue to drop, the balance-of-system (BOS) costs become important. Since the BOS costs scale with installation area, higher efficiency modules which produce more power per unit area are preferred. Therefore, developing high-efficiency cell concepts for the future is of great interest in the R&D world.
The PV industry has been transitioning from Al-BSF solar cells 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, with heterojunction contacts based on amorphous Si probably the best-known example. Si heterojunction (SHJ) cells with an interdigitated back-contact (IBC) architecture are the most efficient Si solar cells in the world today, with Kaneka breaking the world record efficiency with its remarkable 26.7% cell , getting quite close to the theoretical and practical limit for single-junction Si solar cells.
To go beyond the fundamental single-junction limit, a tandem device architecture, which employs 2 or more absorber materials of complementary bandgaps in a stack, must be adopted to reduce thermalisation losses and use the solar spectrum more effectively. A wide bandgap perovskite top cell above a c-Si bottom cell is one of the emerging tandem solar cell configurations that has attracted plenty of recent interest, due to its potential of exceeding 30% while combining the attractive properties of perovskites with the well-understood technologies for c-Si. The best monolithic implementation of this material combination was achieved by Oxford PV with an efficiency of 28% .
This master thesis will focus on optical simulations of a complete tandem cell structure at module level. The aim of the simulations is to maximize the photon absorption in the perovskite and c-Si layers by adapting the individual layers. While different configurations are possible, as an example, this simulated module can be made up from bottom-to-top of the following layers: a rear backsheet (or glass), a rear encapsulant, a rear metallic contact, a transparent conduction oxide (TCO), the Si bottom cell with its passivating layers on both sides, an index matching layer in between the 2 cells, the perovskite cell with its hole and electron transport layers, another TCO + local metallic contact, the front encapsulant layer and finally the top glass. Both 2-terminal (2T) and 4-terminal (4T) approaches will be compared. While the monolithic 2T concept requires matching currents in both sub-cells (unlike the 4T tandem cell), it has less stringent conduction requirements for the layers in between the perovskite layer and Si wafer since there's only vertical current transport. A second aspect to be investigated is the surface morphology of the Si bottom cell where different configurations are possible for the front/rear surface: flat/flat or flat/textured or textured/textured. This will be looked at in combination with the angle of incidence of sunlight onto the module. A final feature to be looked at in combination with the previous aspects is the effect of 2-side illumination for bifacial tandem cells which at first sight seems better suited for a 4T application where no current matching is required.
The SUNSOLVE simulation package (available from PVLighthouse), the main tool used for this thesis, allows the distinction between reflection from different interfaces, transmission through the cell and absorption in all the different layers (both useful absorption in the perovskite and Si as well as parasitic absorption in the other layers). Besides setting up and executing these simulations, the work in this master thesis will focus on interpretation of the simulated data to gain understanding of the optics at work in the module and its sub-cells. In addition, specific experiments in combination with optical characterization (spectroscopic ellipsometry, reflection and transmission measurements,) will be used to respectively set up and validate the simulations. The acquired insight from this work can then be used to drive future experimental work on perovskite/Si tandem cells.
 K. Yamamoto, K. Yoshikawa, H. Uzu, and D. Adachi, "High-efficiency heterojunction crystalline Si solar cells," Jpn. J. Appl. Phys., vol. 57, pp. 08RB20-1, 2018.
Type of project: Thesis
Required degree: Master of Engineering Technology, Master of Science, Master of Engineering Science
Required background: Energy, Nanoscience & Nanotechnology
Supervising scientist(s): For further information or for application, please contact: Filip Duerinckx (Filip.Duerinckx@imec.be)