A TU Delft team of researchers has examined how perovskite materials could be optimized in order to achieve improved performance in bifacial two-terminal perovskite-silicon tandem solar cell technologies. They targeted bifacial two-terminal tandem PV optimization by considering aspects not included in previous studies, such as the effect rear irradiance has on the optimal bandgap energy and thickness of the perovskite cell in this kind of a tandem module.
The bifacial perovskite/silicon cell structure used in the simulations. Image from: Solar Energy Materials and Solar Cells
The team's findings show that bifacial tandems have over a 25% gain in energy yield compared to bifacial single junction modules and up to 5% gain compared to monofacial tandem modules.
The scientists' recent work was mostly focused on the modeling aspects of photovoltaic multijunction devices to gain a better understanding of their working principle under real-world conditions and get insights into their optimal design. Their experimentally validated modeling work further refines the existing design guidelines for perovskite-silicon tandems.
The research group used a reference 32.5%-efficient perovskite-silicon tandem cell developed by German research center Helmholtz-Zentrum Berlin (HZB) to optimize the design of a perovskite cell in a bifacial monolithic 2T tandem module under various conditions. The team said that the reference cell was adjusted by reducing the wafer thickness and adding glass and encapsulation and that the bifacial modules were created by removing the silver layer and adding a second encapsulant layer.
The simulations were conducted via the PVMD Toolbox, which is a comprehensive modeling software to simulate building-integrated and tandem PV systems. The Advanced Semiconductor Analysis (ASA) was used to calculate the electrical properties of the cells and the calibrated lumped element method (CLEM) was utilized for energy yield simulations.
The simulated modules comprised 72 perovskite-silicon tandem cells with a size of 15.7 cm x 15.7 cm. The panels were assumed to be deployed at 0.5 m above the ground, with the distance between the modules being 1 m in the east-west and 8 m in the north-south direction.
The bifacial PV system simulated with the proposed module configuration was optimized to operate in four different geographical locations, namely Delft (the Netherlands), Shanghai (China), Lagos (Nigeria), and Lisbon (Portugal). The aim was to identify the optimal perovskite for the different operating conditions.
Through their analysis, the scientists found that the optimal bandgap energy of the perovskite material used in the top device of the tandem cell lies in the range of 1.61–1.65 eV for all locations, with the optimal thickness being 650–750 nm. They also explained that the bandgap energy is slightly lower for locations with a higher air mass, due to the red shift in the spectrum. They gathered that the energy yield varies greatly for different locations but has a lower dependency on the ground material. This is because the rear side irradiance is converted less efficiently than the front side irradiance, due to thermalization and reflection losses.
They also found that the best perovskite cell configuration may vary depending on the location, but they also ascertained the loss in energy yield when using a standardized module is smaller than 3% for all analyzed scenarios.
Looking ahead, the researchers are planning to expand their analysis to production costs and most cost-competitive tandem designs.
