The 70th Japan Society of Applied Physics Spring Meeting 2023
Highlighted Presentations Press Release
April 18, 2023

1000-h Light-Soaking Stability in Perovskite/Silicon Tandem Solar Cells

The next generation of solar cells, “tandem solar cells,” areaccustomed to 1000 h of light durability
Toshiba Energy Systems & Solutions Corporation / Toshiba Corporation
Miyuki Shiokawa, Takeshi Gotanda

Summary• Developed a perovskite/silicon tandem solar cell with an initial power conversion
efficiency (PCE) of 21.2% and high light durability.
• Conducted a continuous light-soaking stability test with a perovskite/silicon tandem solar
cell exposed to 1SUN-equivalent illumination including ultraviolet (UV) light without UV
cut-off filter.
• Achieved the world's first high durability of perovskite/silicon tandem solar cells, withover 1000 h (T90 :>1000 h) required before the power output reached 90% of the initialvalue.
The research group of Toshiba Energy Systems & Solutions et al. has developed perovskite/silicon tandem solar cellswith a two-terminal structure. This research group has succeeded in improving the durability by applying a uniquetechnology to increase the stability of ions that constitute the perovskite. The perovskite/silicon tandem solar cell with an
initial power conversion efficiency (PCE) of 21.2% was continuously exposed to 1SUN-equivalent illumination including
ultraviolet (UV) light without UV cut-off filter, resulting in a degradation rate of 9.57% after 1000 h. This is the first time inthe literature that perovskite/silicon tandem solar cells have achieved such high photovoltaic durability and the timerequired for the power output to reach 90% of the initial value is over1000 h (T90 :>1000 h).

Details

Next generation tandem solar cells with high efficiency and reliability
For governments worldwide, carbon neutrality is a high priority goal. In its “Green Growth Strategy
Through Achieving Carbon Neutrality in 2050,” Japan's Ministry of Economy, Trade, and Industry(METI) has set a goal of increasing the share of renewable energy in Japan’s power generation to50–60% by 2050 (*1). To achieve this goal, it is necessary to promote renewable energy as themain source of electricity.
Sunlight is a renewable energy source, and the industry continues to innovate its photovoltaic (PV)technology. Currently, the development of “tandem solar cells,” in which two solar cells are stacked
on top of each other to achieve a higher power conversion efficiency, is accelerating. The higher
power conversion efficiency and reliability of solar cells are essential in reducing the cost of power
generation and promoting their widespread use in society. A research group by Toshiba Energy
Systems & Solutions Corporation and others has developed perovskite/silicon tandem solar cellswith a two-terminal structure to improve their power conversion efficiency (PCE) and durability.
The perovskite/silicon tandem solar cell which the research group has been developing, has astructure with a perovskite layer (band gap of approximately 1.7 eV) in the top cell and a siliconlayer (band gap of approximately 1.1 eV) in the bottom cell. The perovskite and silicon layers
absorb short-wavelength and long-wavelength lights, respectively, enabling more efficient power
generation. The same perovskite/silicon tandem solar cell developed by the research grouprealized to add 8.2% abs by the perovskite layer from the 17.3% PCE of the silicon layer solely.
The developers stated that the perovskite/silicon tandem solar cell they developed with a twoterminal structure achieves a PCE of 25.5%, which is one of the highest values worldwide for theadd-on point to the PCE of the bottom cell.
World's first 1000-hour continuous light-soaking test
The research group conducted a 1000-hour continuous light-soaking test on a perovskite/silicontandem solar cell under development. The perovskite layer of the tandem solar cell utilized in thecontinuous light-soaking test was prepared using a common anti-solvent method. The device wereencapsulated in cylindrical glass tubes and continuously irradiated with 1SUN-equivalent
illumination (AM 1.5 G, 1000 W/m2, without ultraviolet (UV) cut-off filter).
The results of the continuous light-soaking test demonstrated that the degradation rate was lessthan 10% after 1000 h of testing using devices with an initial PCE of 21.2 % (Figure). The very low
degradation rate was the result of the research and development for improving durability that theresearch group had conducted so far. Previous studies have reported that the migration (diffusion)of ions that constitute the perovskite layer has a negative impact on durability. It is important tosuppress the migration of ions to improve durability. A research group has previously reported thation migration can be blocked using the feature of sputtering films (*2); however, sputtering candamage the perovskite layer. Therefore, a research group has independently developed lowdamage sputtering and is using it to improve durability while reducing damage to the perovskitelayer. Furthermore, inside the perovskite layer, lattice defects in the grain boundary (*3) are
passivated (*4) with PEAI (Phenylethylamonium iodide) to improve ionic stability. “The passivationof lattice defects prevents the migration of ions, and this leads to improved durability,” researcherssaid.

It is also important to note that the continuous light-soaking test is an evaluation without the use ofUV cut-off filters by developers. “In tandem solar cells, the perovskite layer absorbs shortwavelength light, including ultraviolet light, to increase the power generation efficiency. However, inthe case of perovskite, a few materials can be degraded by ultraviolet light. To avoid degradation
due to testing, UV cut-off filters that cut ultraviolet light may be attached to the outside of the solar
cell for continuous light-soaking test; however, we did not use these filters. In other words, thisexperiment proves that our perovskite/silicon tandem solar cells can combine high-efficiency power
generation with high durability in natural sunlight,” researchers said.
There are precedents for continuous light-soaking tests, and research institutes worldwide havepublished their results (*5). Looking solely at the evaluation time, the German research institute
HZB (Helmholtz-Zentrum Berlin) has 300 h, EPFL (Ecole Polytechnique Fédérale de Lausanne) at
the Swiss Federal Institute of Technology Lausanne has 250 h, the University of North Carolina in
the US has 100 h, the University of Toronto has 400 h, and the same study has 1000 hour.
Comparing the results of 1SUN-equivalent illumination (AM 1.5G, 1000 W/m2, without UV cut-offfilter) expose to devices with initial PCE of 20% or higher among these precedents as well as thisstudy, the University of Toronto showed no degradation at 400 h, while the same study also
showed no degradation at 400 h. Furthermore, the same study did not utilize lithium fluoride (LiF),
which causes low durability instead of high efficiency, or MA (methylammonium) ions, whichconstitute perovskite that is another low-durability material, was maintained a low level content.
Therefore, the degradation rate after long-term continuous light-soaking test was kept low by
combining it with the aforementioned ion migration suppression technology. This research is thefirst demonstration test worldwide to report a degradation rate of 10% or less over 1000 h, farahead of other research institutes globally.
The achievement of world-class demonstration time and durability for tandem solar cells, whose
development is currently accelerating, is an important result, stated the developers. We willcontinue to aim for even higher efficiency and durability in the future, and the developers, areenthusiastic. Tandem solar cells with a two-terminal structure that offers high efficiency and highdurability are expected to replace conventional crystalline silicon PV facilities for private power
generation and medium- and large-scale applications. It is an innovative technology that can beapplied to infrastructure to support society.
Annotation*1 https://www.meti.go.jp/english/press/2020/1225_001.html*2 Sputtering: A physical vapor deposition (PVD) method is used to form thin films on the surface of materials.
*3 Grain boundary: A boundary created by grains with different crystal orientation.
*4 Passivation: Surface inerting. Stabilization of the surface energy state of a material.
*5 A. A.-Ashouri et al., Science 370, 1300(2020)F. Sahli et al. Nature Materials, 17, 820-826 (2018).
B. Chen et al, Joule, 4(2020).
Y. Hou et al, Science 367, 1135(2020)

FigureContinuous light-soaking test degradation rate was within 10% after 1000 H of light exposure with 1SUN-equivalentillumination (AM 1.5G, 1000 W/m2, without UV cut-off filter). Initial PCE was 21.2%. No degradation after 300 H.
Degradation rate after 750 H was 5.59%. Degradation rate after 1000 H was 9.57%. (T95 : >718 h, T90 : >1000+ h)