Development Imaging Techniques Illuminate Performance of Multi-Junction Solar Cells

A pioneering development at the Fraunhofer Institute for Solar Energy Systems is set to transform the analysis of multi-junction solar cells, particularly those utilizing perovskite-silicon and perovskite-perovskite-silicon architectures. These advancements in imaging technology provide unprecedented insights into the performance of individual sub-cells, effectively bridging a critical gap in the characterization of these high-efficiency solar devices.

Understanding Multi-Junction Solar Cells

Multi-junction solar cells are engineered to surpass the efficiency limits of traditional single-junction photovoltaic cells. By stacking layers of semiconductors with varying bandgaps, these devices can capture a wider spectrum of solar radiation. For instance, tandem solar cells typically feature two junctions—an upper layer designed to absorb high-energy photons while transmitting lower-energy photons to a second layer. Triple-junction cells take this concept further, potentially achieving efficiencies of up to 51%, compared to about 29% for conventional silicon cells. This enhanced performance is primarily due to reduced thermal and transmission losses, making multi-junction designs particularly attractive for both terrestrial and space applications [Energy.gov].

The Challenges of Performance Analysis

Despite their advantages, characterizing the performance of individual sub-cells within these multi-junction architectures has been a significant challenge. Traditional measurement approaches tend to provide averaged data across the entire device, obscuring localized defects and performance disparities. This limitation is especially pronounced in complex configurations like perovskite-silicon tandems, where junctions are fabricated in direct contact with each other. The inability to pinpoint where efficiency losses occur hinders optimization efforts [PV Magazine].

Innovative Imaging Techniques for Enhanced Analysis

The new imaging methodologies developed by the Fraunhofer team are game-changers for the solar energy sector. By employing advanced techniques such as Suns-PL Imaging (Photoluminescence Imaging) and electroluminescence/photoluminescence (EL/PL) imaging, researchers can now detect and analyze performance losses at the sub-cell level without causing damage to the devices.

• Suns-PL Imaging: This technique allows for precise localization of electrical losses within tandem solar cells, enabling detailed mapping of performance variations.
• EL/PL Imaging: When combined with lock-in thermography, this method facilitates comprehensive characterization, providing insights into built-in voltage levels and identifying critical defects like shunts that can impair efficiency.

These imaging techniques not only enhance understanding of how various materials and surface treatments impact performance but also assist in developing strategies to mitigate defects and optimize interfaces. For example, researchers have found that substituting traditional materials with alternatives such as piperazine-1,4-diium chloride can significantly enhance the stability of perovskite layers [Nature].

Implications for Future Development

The ability to accurately analyze sub-cell performance marks a material advancement in the commercialization of multi-junction solar technologies. With current record efficiencies nearing 35.9% for triple-junction cells, these imaging techniques promise to streamline the optimization process, reducing development timelines and costs. As manufacturing costs decline and efficiency becomes increasingly critical for lowering the levelized cost of energy, innovations like these position perovskite-based multi-junction cells as formidable competitors against traditional III-V technologies [Ossila].

Conclusion: Current Changes in Solar Technology