HKUST engineering researchers crack the code to boost solar cell efficiency and durability
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Assistant Professor Lin Yen-Hung of the Department of Electronic and Computer Engineering and the State Key Laboratory of Advanced Displays and Optoelectronics Technologies (right), Electronic and Computer Engineering PhD student CAO Xueli (center), and Senior Manager of the State Key Laboratory of Advanced Displays and Optoelectronics Technologies Dr. Fion YEUNG (left)
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Photovoltaic (PV) technologies, which convert light into electricity, are increasingly applied worldwide to generate renewable energy. Researchers at the School of Engineering of the Hong Kong University of Science and Technology (HKUST) have developed a molecular treatment that significantly enhances the efficiency and durability of perovskite solar cells. Their breakthrough will potentially accelerate the large-scale production of this clean energy.
A key to the solution was their successful identification of critical parameters that determine the performance and lifespan of halide perovskites, a next-generation photovoltaic material which has emerged as one of the most promising materials in PV devices for its unique crystal structure. The findings have been published in Science.
Led by Assistant Professor LIN Yen-Hung of the Department of Electronic and Computer Engineering and the State Key Laboratory of Advanced Displays and Optoelectronics Technologies, the research team investigated various ways of passivation, a chemical process that reduces the number of defects or mitigates their impact in materials, thereby enhancing the performance and longevity of devices comprising these materials. They focused on the “amino-silane” molecular family for passivating perovskite solar cells.
“Passivation in many forms has been very important in improving the efficiency of perovskite solar cells over the last decade. However, passivation routes that lead to the highest efficiencies often do not substantially improve long-term operational stability,” Prof. Lin explained the problem.
For the first time, the research team showed how different types of amines (primary, secondary, and tertiary) and their combinations can improve perovskite films’ surfaces where many defects form. They achieved this using both “ex-situ” (outside the operating environment) and “in-situ” (within the operating environment) methods to observe molecules’ interactions with perovskites. From there, they identified molecules that substantially increase photoluminescence quantum yield (PLQY), i.e. the quantity of photons emitted during materials excitation, indicating fewer defects and better quality.
“This approach is crucial for the development of tandem solar cells, which combine multiple layers of photoactive materials with different bandgaps. The design maximizes the use of the solar spectrum by absorbing different parts of sunlight in each layer, leading to higher overall efficiency,” Prof. Lin elaborated on the application.
In their solar cell demonstration, the team fabricated devices of medium (0.25 cm²) and large (1 cm²) sizes. The experiment achieved low photovoltage loss across a broad range of bandgaps, maintaining a high voltage output. These devices reached high open-circuit voltages beyond 90% of the thermodynamic limit. Benchmarking against about 1,700 sets of data from existing literature showed that their result was among the best reported to date in terms of efficiency in energy conversion.
Even more critically, the study demonstrated remarkable operational stability for amino-silane passivated cells under the International Summit on Organic Solar Cells (ISOS)-L-3 protocol, a standardized testing procedure for solar cells. Approximately 1,500 hours into the cell aging process, the maximum power point (MPP) efficiency and power conversion efficiency (PCE) remained at high levels. For the best-passivated cells to decrease to 95% of their initial values, the champion MPP efficiency and the champion PCE were recorded at 19.4% and 20.1% respectively – among the highest (when factored for the bandgap) and the longest metrics reported to date.
Prof. Lin emphasized that their treatment process not only boosts the efficiency and durability of perovskite solar cells, but is also compatible with industrial-scale production.
“This treatment is similar to the HMDS (hexamethyldisilazane) priming process widely used in the semiconductor industry,” he said. “Such similarity suggests that our new method can be easily integrated into existing manufacturing processes, making it commercially viable and ready for large-scale application.”
The team included Electronic and Computer Engineering PhD student CAO Xue-Li, Senior Manager of the State Key Laboratory of Advanced Displays and Optoelectronics Technologies Dr. Fion YEUNG, along with collaborators from Oxford University and the University of Sheffield.
Amino-silane molecules and their fabrication and optoelectronic properties
Amino-silane molecules and their fabrication and optoelectronic properties
Credit
HKUST
Journal
Science
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Bandgap-universal passivation enables stable perovskite solar cells with low photovoltage loss
More electricity from the sun
Surface modification for more effective textured perovskite/silicon tandem solar cells
Wiley
A coating of solar cells with special organic molecules could pave the way for a new generation of solar panels. As a research team reports in the journal Angewandte Chemie, this coating can increase the efficiency of monolithic tandem cells made of silicon and perovskite while lowering their cost—because they are produced from industrial, microstructured, standard silicon wafers.
In solar cells, light “knocks” electrons out of a semiconductor, leaving behind positively charged “holes”. These two charge carriers are separated from each other and can be collected as current. Tandem cells were developed to better exploit the entire spectrum of sunlight and increase solar cell efficiency. Tandem cells are made of two different semiconductors that absorb different wavelengths of light. Primary contenders for use in this technology are a combination of silicon, which absorbs mostly red and near-infrared light, and perovskite, which very efficiently uses visible light. Monolithic tandem cells are made by coating a support with the two types of semiconductor, one on top of the other. For a perovskite/silicon system, this is usually achieved by using silicon wafers that are produced by the zone melting process and have a polished or nanostructured surface. However, these are very expensive. Silicon wafers produced by the Czochralski process with micrometer-scale pyramidal structural elements on their surfaces are significantly cheaper. These microtextures result in better light capture because they are less reflective than a smooth surface. However, the process of coating these wafers with perovskite results in many defects in the crystal lattice, which affect the electronic properties. Transfer of the released electrons is impeded, and electron-hole recombination increasingly occurs through processes that do not emit light. Both the efficiency and the stability of the perovskite layer are decreased.
Headed by Prof. Kai Yao, a Chinese team at Nanchang University, Suzhou Maxwell Technologies, the CNPC Tubular Goods Research Institute (Shaanxi), the Hong Kong Polytechnic University, the Wuhan University of Technology, and Fudan University (Shanghai) has now developed a strategy for surface passivation that allows the surface defects of the perovskite layer to be smoothed out. A thiophenethylammonium compound with a trifluoromethyl group (CF3-TEA) is applied by a dynamic spray coating process. This forms a very uniform coat—even on microtextured surfaces.
Due to its high polarity and binding energy, the CF3-TEA coating very effectively weakens the effects of the surface defects. Nonradiative recombination is suppressed, and the electronic levels are adjusted so that the electrons at the interface can be more easily transferred to the electron-capturing layer of the solar cell. Surface modification with CF3-TEA allows perovskite/silicon tandem solar cells based on common textured wafers made of Czochralski silicon to attain a very high efficiency of nearly 31% and maintain long-term stability.
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About the Author
Dr Kai Yao is a Professor at Nanchang University (China) with appointments in Institute of Photovoltaics. His research interest is in the field of organic–inorganic hybrid materials for optoelectronic applications. He is also Director of emerging photovoltaic technologies research group at the Nanchang University.
Journal
Angewandte Chemie International Edition
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Surface Molecular Engineering for Fully Textured Perovskite/Silicon Tandem Solar Cells
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