Tandem - Page 21

Oxford PV recently stated that it hopes to deliver perovskite-silicon tandem solar cells to high end solar module manufacturers in the first half of 2021, now less than a year away.

The group expects these solar cells to have an efficiency between 26-27%, to increase in efficiency by 1% per year as the company improves its manufacturing techniques. It was said that initially, a 400 watt 60-cell solar module will probably be available.

An international research team has developed a new type of solar cell that can both withstand environmental hazards and is 26.7% efficient in power conversion.

Structure and photovoltaic performance for the perovskite-Si tandem device. Image by KAIST

The researchers, led by Byungha Shin, a professor from the Department of Materials Science and Engineering at KAIST, focused on developing a new class of light-absorbing material, called a wide bandgap perovskite. The material has a highly effective crystal structure that can process the power needs, but it can become problematic when exposed to environmental hazards, such as moisture. Researchers have made some progress increasing the efficiency of solar cells based on perovskite, but the material reportedly has greater potential than what was previously achieved.

Researchers at The Australian National University (ANU) have announced an impressive achievement - a silicon/perovskite tandem solar cell with a conversion efficiency of 27.7%.

Professor Kylie Catchpole says this would only need to improve slightly - to around 30% - before the technology could be rolled out around the world. "In comparison, typical solar panels being installed on rooftops at the moment have an efficiency around 20%" Professor Catchpole said.

The groups of Steve Albrecht and Bernd Stannowski at HZB have reached a record efficiency of 29.15% of its tandem solar cell made of perovskite and silicon.

The illustration shows the structure of the tandem solar cell: between the thin perovskite layer (black) and the silicon layer (blue) are functional intermediate layers. © Eike Köhnen/HZB

This value has been officially certified by the CalLab of the Fraunhofer Institute for Solar Energy Systems (ISE) and means that surpassing the 30% efficiency mark is now within reach.

A research group led by Prof. Liu Shengzhong from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) and Prof. Yang Dong at Shaanxi Normal University have developed high efficiency semi-transparent perovskite solar cells by using MoO₃ sandwiched gold nanomesh (MoO₃/Au/MoO₃) multilayer as the transparent electrode. Combined with a superior heterojunction silicon solar cell, a high efficiency four-terminal perovskite/silicon tandem solar cell was obtained.

Tandem/multijunction structure has been proven to be an effective way to break the Shockley-Queisser limit. To obtain a high efficiency tandem solar cell, the key is to fabricate transparent electrode with high conductivity as well as high transparency through a mild method.

Researchers from King Abdullah University of Science and Technology (KAUST) in Saudi Arabia and University of Twente in the Netherlands have developed transparent conductive films that let through a broader range of the solar spectrum, which are set to increase the power conversion efficiency of perovskite-based multijunction solar cells beyond 30%.

A comparison of the used device test structures with different silicon bottom cells. Image taken from Advanced Functional Materials

Performance of perovskite-based tandem solar cells rests on the ability of the top cell to harvest the blue portion of the solar spectrum while letting through the near-infrared light. Conversely, the bottom cell only needs to absorb near-infrared light. "The semitransparency of the top cell depends on the optical bandgap and thickness of the perovskite thin film as well as the characteristics of the transparent electrodes, especially their sun-exposed side," explains study lead Stefaan De Wolf from theKAUST Solar Center.

A team led by Prof. Steve Albrecht from the HZB has announced a new world-record: a tandem solar cell with certified efficiency of 23.26% that combines the semiconducting materials perovskite and CIGS. One reason for this success lies in the cell's intermediate layer of organic molecules: they self-organize to cover even rough semiconductor surfaces. Two patents have been filed for these layers.

Perovskite-based solar cells have experienced an incredibly rapid increase in efficiency over the last ten years. The combination of perovskites with classical semiconductor materials such as silicon and copper-indium-gallium-selenide (CIGS) compounds in tandem solar cells promises low-cost, high-performance solar modules for the future. However, losses at the electrodes between the two semiconductors considerably reduce the efficiency.

Solliance and U.S-based MiaSolé announced a new record - power conversion efficiency of 23% on a flexible tandem solar cell: a top flexible semi-transparent perovskite solar cell with a bottom flexible copper indium gallium selenide (CIGS) cell.

This achievement comes only 9 months after the January 2019 announcement by Solliance and MiaSolé regarding a flexible solar cell with an impressive power conversion efficiency of 21.5%. The solar cell, similarly to this newly announced one, combined two thin-film solar cell technologies into a 4 terminal tandem solar cell stack: a top flexible semi-transparent perovskite solar cell with a bottom flexible copper indium gallium selenide (CIGS) cell.

Several research institutions in South Korea are actively conducting research and development on next-generation solar cells, heightening expectations for commercialization. The research team led by Prof. Yoon Soon-gil of Chungnam National University has developed a new graphene electrode to produce perovskite solar cells at a low temperature. In addition, the team led by Prof. Choi Kyoung-jin of the School of Materials Science and Engineering at UNIST has developed a new concept tandem solar cell using transparent conductive adhesives (TCA).

The graphene electrode developed by Professor Yoon's team can help create a perovskite solar cell at a low temperature and can raise both safety and economic efficiency.

Parasitic absorption in transparent electrodes is one of the main roadblocks to enabling power conversion efficiencies (PCEs) for perovskite'based tandem solar cells beyond 30%. To reduce such losses and maximize light coupling, the broadband transparency of such electrodes should be improved, especially at the front of the device.

Erkan Aydin and coworkers from KAUST Photovoltaics Laboratory have recently shown the excellent properties of Zr'doped indium oxide (IZRO) transparent electrodes for such applications, with improved near'infrared (NIR) response compared to conventional tin'doped indium oxide (ITO) electrodes. Optimized IZRO films feature very high electron mobility (up to '77 cm² V'1 s'1), enabling highly infrared transparent films with a very low sheet resistance ('18 Ω ^'1 for annealed 100 nm films). For devices, this translates to a parasitic absorption of only '5% for IZRO within the solar spectrum (250'2500 nm range), to be compared with '10% for commercial ITO.