Silicon Tandem Solar Cells

Project: High-Efficiency Perovskite/Silicon Tandem Solar Cells

PI: Stefaan De Wolf, Derya Baran, Frederic Laquai 

Perovskite solar cells (PSCs) are rapidly emerging as a high-efficiency photovoltaic technology. Owing to their fairly large bandgap, PSCs are particularly attractive for building tandem solar cells, combined with well-established silicon bottom cells. In such a configuration the PSC harvests the blue part of the solar spectrum while letting red and near-infrared (NIR) light pass through to be absorbed in the silicon bottom cell. In this way, such a combination is expected to overcome the single-junction power conversion efficiencies (PCE) limit of silicon solar cells (29.4%). In this context, thanks to its tunable band gap of 1.7–1.8 eV, and high PCE (>20%) perovskite solar cells are indeed very promising as top cells. The most attractive bottom cell technology is that of silicon heterojunction (SHJ) solar cells which offer the highest operating voltages (>750 mV) and efficiencies, thanks to their passivating contacts and the best red response of all silicon solar cells. The most attractive way to combine these two technologies is in a monolithic tandem concept by employing textured SHJ solar cells as the bottom cell and semitransparent perovskite solar cells (PSC) as the top cell.

Currently, both perovskite and silicon solar cells are already being independently developed within the KAUST Solar Center (KSC) and high-efficiency baseline processes are available for these devices, in either type of polarity (electron or hole collection at the front). In this project, we are addressing the challenge of how to integrate both technologies into monolithic perovskite/silicon tandem solar cells. Specific points of attention for this are:

(1) The fabrication of semitransparent PSCs, requiring the deposition of transparent contact stacks onto the perovskite films. To this end, broadband transparent TCOs, ETLs and HTLs will be explored and integrated in our devices, with specific attention to processing compatibility of all layers.

(2) Low-temperature PSC fabrication. This is required for monolithic tandem fabrication, as the SHJ bottom cell, featuring a-Si:H contact layers cannot withstand temperatures above 200 °C.

(3) Vacuum deposited PSCs. In order to reduce any reflection losses, and to be close as current industrial standards, the tandems ultimately will need to use so-called random-pyramid textured bottom cells (feature size of several micron), which makes PSC solution processing a challenge. For this reason, we aim to alter our solution-based PSC processing procedures as much as possible towards vacuum based techniques.

By addressing these challenges, this project aims to realize high PCE tandem devices beyond the best-in-class silicon solar cells (>26.7%) for both n-i-p and p-i-n configurations by optimizing the SHJ and the PSCs towards this end. 

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