World record again at HZB: Almost 30 % efficiency for next-generation tandem solar cells

The world record cell (here in front of the electron storage ring BESSY II) has an area of about one square centimetre, a typical lab size.

The world record cell (here in front of the electron storage ring BESSY II) has an area of about one square centimetre, a typical lab size. © Amran Al-Ashouri /HZB

The perovskite silicon tandem cell is based on two innovations: A nanotextured front side ( left) and a back side with dielectric reflector (right).

The perovskite silicon tandem cell is based on two innovations: A nanotextured front side ( left) and a back side with dielectric reflector (right). © Alexandros Cruz /HZB

The tandem in the film - perovskit and silicon can be combined to produce efficient solar cells. © HZB

02:14

Three HZB teams led by Prof. Christiane Becker, Prof. Bernd Stannowski and Prof. Steve Albrecht have jointly managed to increase the efficiency of perovskite silicon tandem solar cells fabricated completely at HZB to a new record value of 29.80 %. The value has now been officially certified and is documented in the NREL-charts. This brings the 30 percent mark within reach.

Today's solar modules are mainly made of silicon, and possibilities for further increases in efficiency have already been largely exploited. But since 2008, the material class of "metal halide perovskites" has moved into the focus of research: these semiconductor compounds convert sunlight into electrical energy very well and still offer a lot of room for improvement. In particular, they can be combined with silicon solar cells to tandem solar cells that use sunlight much more efficiently.

A race for records

At HZB, several groups have been working intensively since 2015 on both the perovskite semiconductors and silicon technologies and the combination of both into innovative tandem solar cells. In January 2020, HZB had achieved a record 29.15 % for a perovskite silicon tandem solar cell and published the work in the journal Science. Then, before Christmas 2020, the company Oxford PV was able to announce a certified efficiency of 29.52 %. Since then, the exciting race for new records has been on. "An efficiency of 30 % is like a psychological threshold for this fascinating new technology which could revolutionise the photovoltaic industry in the near future," explains Steve Albrecht, who is working on perovskite thin films at the HySPRINT lab at HZB. Bernd Stannowski, group leader for silicon technology, adds: "I would particularly emphasise the good cooperation between the different groups and institutes at HZB. This is how we managed to develop these new tandem solar cells entirely at HZB and once again get the world record."

Official certification

The recent research and development focus was on the optical improvement of the silicon heterojunction bottom cell. A nanotextured front side and a dielectric back reflector were added. Now came the official confirmation by Fraunhofer ISE CalLab: "Our new perovskite silicon tandem solar cells were independently certified with a world record efficiency of 29.80 %," says a delighted Christiane Becker, an expert on nanostructures in solar cells and their effects on the optical and electrical properties. 

Nanotextured silicon

For the new work, Dr. Philipp Tockhorn (Albrecht group) and doctoral student Johannes Sutter (Becker group) investigated how nanostructures at different interfaces affect the performance of a tandem solar cell, consisting of a perovskite solar cell on top of a silicon solar cell. First, they used a computer simulation to calculate the photocurrent density in the perovskite and silicon subcells for different geometries with and without nanotextures. Then they produced perovskite silicon tandem solar cells with different textures: "Even the nanotexturing on one side improves the light absorption and enables a higher photocurrent compared to a flat reference," says Sutter. And his colleague Tockhorn adds: "Remarkably, the nanotextures also lead to a slight improvement in the electronic quality of the tandem solar cell and to better film formation of the perovskite layers."

Dielectric reflector

Improvements were also made to the back side of the tandem solar cell, which is designed to reflect infrared light back into the silicon absorber. "By using a dielectric reflector, we were able to use this part of the sunlight more efficiently, resulting in a higher photocurrent," says Dr. Alexandros Cruz Bournazou (Stannowski group).

Outlook is bright

The results pave the way for further improvements. The simulations suggest that the performance could be increased even further by nanostructuring the absorber layers on both sides. The researchers are convinced that an efficiency of well over 30 % could be achievable soon. The race is on.

 

arö

You might also be interested in

  • International consortium to advance decarbonisation of the aviation sector
    News
    24.05.2022
    International consortium to advance decarbonisation of the aviation sector
    JOHANNESBURG, SOUTH AFRICA – 24 May 2022: CARE-O-SENE research project will develop advanced catalysts for sustainable aviation fuels

    The company Sasol and Helmholtz-Zentrum Berlin (HZB) will lead a consortium to develop and optimise next-generation catalysts that will play a key role in decarbonising the aviation sector through sustainable aviation fuels (SAF).

  • Thermal insulation for quantum technologies
    Science Highlight
    19.05.2022
    Thermal insulation for quantum technologies
    New energy-efficient IT components often only operate stably at extremely low temperatures. Therefore, very good thermal insulation of such elements is crucial, which requires the development of materials with extremely low thermal conductivity. A team at HZB has now used a novel sintering process to produce nanoporous silicon aluminium samples in which pores and nanocrystallites impede the transport of heat and thus drastically reduce thermal conductivity. The researchers have developed a model for predicting the thermal conductivity, which was confirmed using experimental data on the microstructure of the samples and their thermal conductivity. Thus, for the first time, a method is available for the targeted development of complex porous materials with ultra-low thermal conductivity.
  • Magnetic nanoparticles in biological vehicles individually characterised
    Science Highlight
    17.05.2022
    Magnetic nanoparticles in biological vehicles individually characterised
    Magnetic nanostructures are promising tools for medical applications.  Incorporated into biological structures, they can be steered via external magnetic fields inside the body to release drugs or to destroy cancer cells. However, until now, only average information on the magnetic properties of those nanoparticles could be obtained, thus limiting their successful implementations in therapies. Now a team at HZB conceived and tested a new method to assess the characteristic parameters of every single magnetic nanoparticle.