Comprehensive review of research on heterojunction solar cell technologies

Expectations of future developments in this area

Scientist the Nankai University (China) have in an in Nanophotonics published work created a comprehensive overview of the current research on silicon tandem solar cells with heterojunction (SHJ-TSCs), showed the opportunities and challenges of this technology and formulated their expectations for future developments in this area.

Perovskite solar cell at the HZB Institute for Si-PVPhoto © Gerhard Hofmann for Solarify

According to their analysis, the major challenge with perovskite / SHJ-TSCs is the loss of open circuit voltage (Voc), which is mainly caused by non-radiative recombination at the grain boundary and the interface. A number of strategies have been used to address this problem, including grain size increase, surface passivation, 2D / 3D heterojunction technology, and ion compensation.

However, passivation of defects has proven to be the most effective means of improving Voc. Much research effort is currently focused on Lewis bases / acids, alkali metal ions (Na, K, Ru, Cs), ligand passivation, halogen ions (Cl, Br), PbI2, 2D perovskites, insulating polymers, and guanidinium-based additives, the report says.

Another problem is the production of perovskite top cells on textured c-Si cells, where the vacuum deposition method can give good results, according to the study. “In order to further improve the performance of the TSCs, the optical and electronic losses must be minimized. With these issues resolved, power conversion efficiency (PCE) in excess of 35% can be expected, ”the report said. In order to advance manufacturing and commercialization on a larger scale, the long-term stability and toxicity of the material must also be optimized, according to the Chinese scientists.

In contrast to perovskites, III-V compound semiconductor materials with their proven reliability and adjustable band gap show a high degree of efficiency and a promising potential for industrial applications. However, they are associated with high costs.

To date, many strategies have been used to fabricate III-V / SHJ TSCs, including heteroepitaxial growth, wafer bonding, and mechanical stacking. The heteroepitaxial integration approach, which relies on controlling the dislocation density at the interface, has been used to fabricate devices, but the quality of these devices has been poor. Approaches based on surface activated bonding (SAB) appear to be the most versatile for making highly efficient III-V / SHJ TSCs. However, this approach is not suitable for textured SHJ solar cells, and the need for a clean room environment and tunnel junction can also be costly.

Therefore, mechanical stacking using a transparent, conductive adhesive and an arrangement of metal nanoparticles could be a new direction of research for high-performance III-V / SHJ-TSCs, the report said. However, he concludes that all of the approaches described need to be improved in terms of performance and cost before they can be used on a large scale in commercial applications.

From the article in Nanophotonics

“It is expected that silicon heterojunction solar cells (SHJ) will dominate the photovoltaic market due to their stable and high efficiency. So far, the highest efficiency of SHJ solar cells with interdigital back contact (IBC) has reached 26.7%, which corresponds approximately to the theoretical Shockley-Queisser limit (SQ) of 29.4%. To break this limit, multiple cells, made up of two or three stacked sub-cells, have been developed that can make full use of sunlight by absorbing different parts of the solar spectrum.

This article provides a comprehensive overview of the current research on SHJ-based tandem solar cells (SHJ-TSCs), including perovskite / SHJ-TSCs and III-V / SHJ-TSCs. First we give a brief introduction to the structures of SHJ-TSCs, followed by a discussion of the manufacturing processes. After that, we will focus on the various materials and processes that have been researched to optimize electrical and optical performance. Finally, we highlight the opportunities and challenges of SHJ-TSCs and give an outlook on the future development directions in this area.

Crystalline silicon solar cells have dominated the photovoltaic market for decades. The system components, including installation, cabling and inverters, make up the main costs of photovoltaics as these costs are area-dependent. Improving power conversion efficiency (PCE) is the most effective way of reducing LCOE. The silicon heterojunction solar cells (SHJ) include c-Si / a-Si and c-Si / MoOx heterojunction solar cells. Due to their high performance, SHJ solar cells represent the new research direction. The previous world record of 26.7% cell efficiency was set by Kaneka (Japan) in 2017. The company used an SHJ structure combined with an interdigital back contact (IBC). However, this efficiency is only 2.7% below the theoretical efficiency limit of 29.4% for silicon single-junction solar cells. That means that he is really close to his performance limit. So there is an urgent need for new methods.

A well-known strategy for overcoming the theoretical efficiency limit of Shockley-Queisser (SQ) is the use of multi-junction solar cells. In these components, an upper cell with a wide band gap absorbs high-energy photons and a lower cell with a narrow band gap absorbs low-energy photons. This structure can minimize thermalization losses and improve the use of the solar spectrum. According to simulation results, the maximum PCE of tandem solar cells (TSC) based on silicon with double junction is 45% and that of TSC based on silicon with triple junction is 50%.

Various top-cell materials have been investigated for silicon-based TSCs, but current research is focused on III-V and perovskite semiconductors. III-V solar cells are known for their high efficiency and excellent reliability, which reduces investment risk and improves the stability of tandem modules. A PCE value of 35.9% was determined for a mechanically stacked GaInP / GaAs / Si tandem module with four connections, the highest PCE value for silicon-based TSCs. In the meantime, the perovskite solar cell is another ideal candidate for SHJ-based tandem solar cells (SHG-TSCs) due to their tunable band gap, their ease of manufacture and their high PCE of 25.5%. Using a low temperature solution method, perovskite solar cells can be well compatible with c-Si / a-Si SHJ solar cells. Oxford PV has achieved a PCE record of 29.5% in one sun for bipolar perovskite / SHJ TSC. However, the high Voc loss due to non-radiative recombination and the high sensitivity of perovskite to low humidity still pose major challenges that require further research and development.

This article provides an overview of current research activities focused on perovskite / SHJ-TSCs and III-V / SHJ-TSCs. The text of this article is divided into six parts: Section 2 discusses various structures related to SHJ-TSCs and manufacturing strategies that have been developed to achieve ideal SHJ-TSCs. In the following three sections, these strategies are explained in detail using specific examples. The last section closes with personal comments on the directions of future research on such new solar cells … “

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