Efficiency of all-inorganic perovskite solar cells improved by passivation

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All-inorganic perovskite solar cells have drawn increasing attention because of their outstanding thermal stability. A research team led by scholars from City University of Hong Kong (CityU) has recently developed a new type of all-inorganic perovskite solar cell through passivation. The novel solar cell achieved a remarkable power conversion efficiency of 16.1% with improved photostability, representing the most efficient all-inorganic perovskite solar cells of its kind to date. Their research results will provide insight into the development of perovskite solar cells with high efficiency and stable performance in the future.

Professor Alex Jen Kwan-yue, CityU’s Provost and Chair Professor of Chemistry and  Materials Science, together with Dr Zhu Zonglong, Assistant Professor of Department of Chemistry, led the research team. They were joined by fellow researchers from South China University of Technology, University of Washington, and National Taiwan University. Their research findings were published in the scientific journal Nature Communications, titled “Highly efficient all-inorganic perovskite solar cells with suppressed non-radiative recombination by a Lewis base”.

Thermal stability vs power conversion efficiency

In terms of the power conversion efficiency, organic-inorganic hybrid perovskite solar cells perform better than all-inorganic perovskite solar cells currently. However, the all-inorganic perovskite solar cells outperform the hybrid ones in thermal stability. Professor Jen pointed out that one of the indicators to determine whether solar cells are ready for practical applications is their good performance at high temperature up to 85 degrees Celsius. For organic-inorganic hybrid perovskite solar cells, they can generally only be used at 60 to 65 degrees Celsius or below.

“The organic part of the organic-inorganic hybrid perovskite solar cells will decompose when the temperature reaches 85 degrees Celsius. In contrast, all-inorganic perovskite cells show better thermal stability in resisting high temperatures. This enables them to be deployed in harsh environments such as deserts,” explained Dr Zhu.

The secret to enhance efficiency: passivating the perovskite surface with small-molecule additives

Therefore, scientists have been actively looking for solutions to improve the power conversion efficiency of all-inorganic perovskite in recent years. The research team spent a year and developed a novel type of all-inorganic perovskite solar cell with improved power conversion efficiency and stability. Their secret lies in the simple molecular passivation strategy. Passivation is a method to reduce the activity of the material surface, often used on organic solar cells.

Passivation method
The research team uses a simple molecular passivation strategy to reduce surface defects of perovskite. (Photo source: Nat Commun 11, 177 (2020).http://doi.org/10.1038/s41467-019-13909-5)

 

Professor Jen explained that defects might form on the surface or the grain boundaries during the rapid crystallisation of perovskite in its fabrication process. These defects will “trap” the electrons and prevent them from moving freely to form a current, resulting in severe open-circuit voltages loss. This is the main reason for low power conversion efficiency in all-inorganic perovskite solar cells.

crystalization
The grain size of the perovskite film before (fig a) and after the 6TIC-4F treatment (fig b). Researchers discover that grain size increased after passivation.  Both the grain boundaries defects and open-circuit voltages loss reduced. (Photo source: Nat Commun 11, 177 (2020). https://doi.org/10.1038/s41467-019-13909-5)

 

To solve the defects problem, the team added a kind of small molecule additives called “6TIC-4F” to passivate the perovskite surface during its production. They discovered that the grain size of the perovskite film increased after the 6TIC-4F treatment. Hence both the defects in the grain boundaries and open-circuit voltages loss reduced.

Experiment results showed an improved open-circuit voltage from 1.10 V to 1.16 V after the passivation of 6T1C-4F. Apart from open-circuit voltage, the power conversion efficiency is also affected by fill factor (whether the electrons are effectively collected) and short-circuit current. All these indicators have shown improvements after the passivation of 6T1C-4F.

Experiment results after passivation
Improvements show in all the above indicators after passivation (orange lines), comparing to data without passivation (green lines). This shows that the power conversion efficiency of all-inorganic perovskite solar cell is improved. (Photo source: Nat Commun 11, 177 (2020). https://doi.org/10.1038/s41467-019-13909-5)

 

The newly developed all-inorganic perovskite solar cell has achieved a remarkable power conversion efficiency of 16.1%, the best among inverted all-inorganic perovskite solar cells reported so far. The device was sent to the National Institute of Metrology in Beijing for certification, and a certified efficiency of 15.6% was obtained.

The results also showed that the photostability of the cell has also improved. After 350 hours of continuous illumination, the power conversion efficiency has only decreased by about 15%. Moreover, 6TIC-4F passivation has enhanced the cell’s resistance to moisture, oxygen, and light.

Great application potentials

Professor Jen noted that the breakthrough of this research lies in successfully fabricating all-inorganic perovskite solar cells with high power conversion efficiency and stability by a very simple method. “We believe that the power conversion efficiency has the potential to be further improved,” he said.

He added that they adopted the inverted configuration in the design of this all-inorganic perovskite solar cell, so that it is compatible with the fabrication of tandem solar cells. Tandem solar cells can absorb the different spectrum of sunlight simultaneously, hence the improvement in the efficiency of power conversion, which is estimated to be over 30%.

As a leading expert and highly cited scholar in the field of perovskite research, Professor Jen pointed out that research on perovskite solar cells has just started for about a decade, but their power conversion efficiency has already increased rapidly from 3.8% to more than 25%. This efficiency rate is catching up with the silicon-based solar cells that have been commonly used and developed for 50 years.

Moreover, since perovskite is produced from solution, “just like the ink used in newspaper printing, it can be ‘printed’ on plastic films as flexible solar cells, or can be painted on window glass as a coating to absorb sunlight. The application potentials will be huge,” Professor Jen said.

He emphasized that the production of perovskite solar cells is not complicated at all, and mass production is possible to reduce costs. The energy required to produce perovskite solar cells is only one-tenth of that for silicon-based solar cells. The commercialization and wide usage of perovskite solar cells can help solve the energy crisis.

“The most difficult part of this research is to strike a balance between the stability and power conversion efficiency of the system. One of our long-term research goals is to enhance the potential for commercialization of inorganic perovskites,” added Dr Zhu.

Professor Jen, Dr Zhu, and Professor Yip Hin-Lap from South China University of Technology are the corresponding authors of the paper. The first authors are research fellow Dr Zhang Jie and PhD student Wang Jing, both from CityU’s Department of Chemistry. Other co-authors included Professor Xue Qifan and Zhou Yingzhi from South China University of Technology, Professor Li Xiaosong and Dr Liu Hongbin from University of Washington, as well as Professor Chueh Chu-Chen from National Taiwan University.

CityU Alex Jen
Major members of CityU’s research team (from left): Dr Zhu Zonglong, Dr Zhang Jie and Professor Alex Jen Kwan-yue.

 

The study was supported by CityU, Innovation and Technology Fund, National Science Foundation, and Natural Science Foundation of Guangdong Province.

DOI number: 10.1038/s41467-019-13909-5

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