While the energy conversion efficiency of perovskite solar cells (PVSCs) – a future for solar cells – has improved significantly over the past decade, the problems of instability and potential environmental impacts have yet to be overcome. Recently, scientists from the City University of Hong Kong (CityU) developed a novel method that can simultaneously fix lead leakage from PVSCs and the stability problem without compromising efficiency. This paves the way for the real application of perovskite photovoltaic technology.
The research team is led by Professor Alex Jen Kwan-yue, CityU Provost and Chair of Chemistry and Materials Science, along with Professor Xu Zhengtao and Dr. Zhu Zonglong from the Department of Chemistry. Their research results were recently published in the journal Nature Nanotechnology with the title “2D metal-organic framework for stable perovskite solar cells with minimized lead leakage”.
Currently, the highest energy conversion efficiency of PVSCs is comparable to the latest silicon-based solar cells. However, the perovskites used contain lead components which raise concerns about possible environmental pollution. “As the solar cell ages, the lead species can enter the soil through the devices, such as rainwater, and pose a toxicity hazard to the environment,” said Professor Jen, an expert on PVSCs. “Large scale commercial use of PVSCs requires not only high energy conversion efficiency, but also long-term device stability and minimal environmental impact.”
In collaboration with Professor Xu, whose specialist knowledge is materials synthesis, Professor Jen and Dr. Zhu joined the team to address the above challenges by applying two-dimensional (2D) organometallic frameworks (MOFs) to PVSCs. “We are the first team to produce PVSC devices with minimized lead leakage, good long-term stability and, at the same time, high power conversion efficiency,” said Professor Jen, summarizing her research breakthrough.
Multifunctional MOF layer
Organometallic frameworks (MOF) have previously been used as scaffolds to template the growth of perovskites. Scientists have also used them as additives or surface modifiers to passivate the defects of perovskites (to reduce the reactivity of the material surface) to improve device performance and stability.
However, most 3D MOFs are quite electrically insulating with low charge carrier mobility and therefore unsuitable for use as charge transport materials.
However, the MOFs created by Professor Xu are different. They are honeycomb-like 2D structures that are equipped with numerous thiol groups as a key functionality. They have suitable energy levels so that they can be an electron extraction layer (also called an “electron collection layer”) in which electrons are eventually collected by the electrode of the PVSCs. “Our molecularly engineered MOFs have the property of a multifunctional semiconductor and can be used to improve charge extraction efficiency,” said Professor Xu.
Capture the lead ions to prevent contamination
More importantly, the dense arrays of thiol and disulfide groups in the MOFs can “trap” heavy metal ions at the perovskite-electrode interface to reduce lead leakage.
“Our experiments showed that the MOF used as the outer layer of the PVSC device captured over 80% of the leaked lead ions from the mined perovskite and formed water-insoluble complexes that would not contaminate the soil,” said Professor Jen. In contrast to the physical encapsulation methods used in other studies to reduce lead leakage, this chemical in-situ sorption of lead by the MOF component integrated in the device proved to be more effective and sustainable for long-term practical applications.
Long-term operational stability achieved
In addition, this MOF material could protect perovskites from moisture and oxygen while maintaining high efficiency.
The power conversion efficiency of your MOF modified PVSC device could reach 22.02% with a fill factor of 81.28% and an open circuit voltage of 1.20V. Both the conversion efficiency and the recorded open circuit voltage are among the highest values available for the planar inverted PVSCs. At the same time, the device showed superior stability in an ambient environment with a relative humidity of 75%, with 90% of its original efficiency being maintained after 1,100 hours. In contrast, the power conversion efficiency of the PVSC without MOF dropped significantly to less than 50% of its original value.
In addition, their device maintained 92% of its initial efficiency when continuously exposed to light for 1,000 hours at 85 ° C. “This level of stability has already met the standard for commercialization set by the International Electrotechnical Commission (IEC),” said Dr. Zhu.
“This is a very significant result that has proven that our MOF method is technically feasible and has the potential to commercialize PVSC technology,” added Professor Jen.
Highly efficient PVSCs for clean energy applications
It took the team nearly two years to complete this promising research. Your next step will be to further improve energy conversion efficiency and explore ways to reduce production costs.
“We hope that making these types of PVSCs will be similar to ‘printing’ newspapers in the future and that it will be easily scalable in production, facilitating the large-scale deployment of high-efficiency PVSCs for clean energy applications,” concluded Professor Jen.
Professor Jen, Professor Xu and Dr. Zhu are the corresponding authors of the paper. The first author of the paper is CityU graduate student Wu Shengfan. Other members of the CityU research team include Li Zhen, Li Mu-Qing, Diao Yingxue, Francis Lin, Liu Tiantian, Zhang Jie and Qi Feng. Colleagues from Xi’an Jiaotong University, the University of Washington and the University of California-Irvine are also involved in the collaboration.
The study received various financial support, including CityU, the Hong Kong Research Grants Council, the Guangdong Major Project for Basic and Applied Basic Research, and the National Science Foundation.