Sustainable Development Analysis of Solar Cells

On May 25, 2023, Nature published three articles on the latest research progress of solar cells, which discussed the current and future development directions from the perspectives of different types of battery design, efficiency improvement and sustainable development of solar cells, which also laid the foundation for the further commercial application of solar cells! The first units to publish the articles are the Shanghai Institute of Microsystem and Information Technology of the Chinese Academy of Sciences, Southern University of Science and Technology, and Nanjing University of Technology.

Specifically: Professor He Zhubing's team at Southern University of Science and Technology has made important breakthroughs in the field of trans-perovskite photovoltaic cells, and research teams such as the Shanghai Institute of Microsystem and Information Technology have successfully developed flexible single-crystal silicon solar cell technology. The team of Huang Xiaomeng of Tsinghua University has cooperated to reveal the global soil carbon storage mechanism based on deep learning methods. The above three research results are all published in Nature magazine, and the results of the Shanghai Institute of Microsystem and Information Technology were selected as the cover of the current issue of Nature magazine.

Nature: He Zhubing's team at Southern University of Science and Technology has made important breakthroughs in the field of trans-perovskite photovoltaic cells

Since 2022, a large amount of capital has poured into the wave of industrialization of perovskite photovoltaic technology, among which trans-perovskite photovoltaic cells have attracted the most attention due to their simple device structure, significant cost reduction potential and diversity of key material selection. Professor He Zhubing's team has focused on the research of inverse perovskite cells from the beginning, and has accumulated a solid theoretical and process technology foundation in the synthesis and screening of key materials, device structure design and device physical analysis (Adv. Energy Mater. 2019, 9, 1803872; Adv. Mater. 2019, 31, 1902781; Adv. Mater. 2019, 31, 1805944; Adv. Funct. Mater. 2019, 29, 1808855; Adv. Energy Mater. 2018, 8, 1703519; Adv. Mater. 2018, 30, 1800515; Adv. Energy Mater. 2017, 7, 1700722; Adv. Mater. 2017, 29, 1603923, etc.), and has made sustained and solid progress. However, the stability of key materials, especially hole transport materials, the cost of synthesis and the interface reaction with perovskite make the current inverse perovskite device structure still not the best choice for industrialization. Therefore, a simpler device structure without a hole transport layer has attracted attention. In order to construct the key ITO/Perovskite Schottky junction in the device, perovskite needs to be regulated to be a strong p-type semiconductor. As we all know, due to the low tolerance of lattice impurity ions, controllable doping of perovskite conductivity type is still a key problem. At the same time, as a non-luminescent deep energy level defect, perovskite bulk phase grain boundary defects are still the main reason hindering the further improvement of device performance.

Figure 1. Trans-perovskite photovoltaic technology based on a new "molecular extrusion" process In response to the above two problems, Professor He Zhubing's team proposed a new "molecular extrusion" process strategy based on the idea of ​​chemical coordination. The p-type acridine small molecules with phosphate anchoring groups are perfectly squeezed to the grain boundaries and bottom during the perovskite film formation process, thereby fully covering and passivating the perovskite grain boundaries and surfaces, and reducing the density of deep energy level defect states to the order of ~1013. At the same time, it was found that there was an obvious electron transfer based on the "charge transfer complex (CTC)" mechanism between the perovskite grain surface and the acridine molecules, thereby achieving strong p-type doping of the perovskite, constructing a Schottky junction with an energy level mismatch of only 0.21 eV, and significantly improving the interface hole transfer efficiency. This process strategy "kills two birds with one stone" and solves the above two problems simultaneously! In the field of perovskite cells without a preset hole transport layer, this work has raised the device efficiency record from 22.20% to 25.86%, and the third-party certified efficiency has reached 25.39%, which is also the world record for the entire trans-perovskite cell. Based on perfect grain boundary and surface passivation, after 1000 hours of standard sunlight exposure, the device efficiency still maintains 96.6% of the initial efficiency. After 500 hours of exposure to the reference cell without grain boundary passivation, the device efficiency decays by more than 20%. This work uses infrared atomic force microscopy supplemented by secondary ion mass spectrometry to directly present the distribution of acridine molecules at the grain boundaries and surfaces of perovskite films, clarifying previous speculations about the distribution of functional molecules in cells without hole transport layers, and pointing out that continuous molecular extrusion thin layers are the key factors for high-performance devices. Because the acridine small molecules used are stable, simple in structure and easy to synthesize, and the device structure is more simplified, the new "molecular extrusion" process reported in this work will have a profound impact on the industrialization investment of perovskite cells. Two national invention patents have been applied for this work.

Figure 2. Distribution of acridine molecules (DMAcPA) in perovskite films based on infrared atomic force microscopy (A-L) and secondary ion mass spectrometry (M-N) tests. Postdoctoral fellows Dr. Tan Qiong (device preparation and characterization) and Dr. Li Zhaoning (molecular design and synthesis) from the Department of Materials Science and Engineering of SUSTech are the co-first authors, He Zhubing is the corresponding author, and SUSTech is the first and only corresponding unit of the paper. Among the co-authors, Assistant Professor Luo Guangfu was responsible for the density functional calculations in the paper, doctoral students Zhang Xusheng and Chen Guocong completed the characterization of infrared atomic force microscopy and ultraviolet photoelectron spectroscopy respectively, and other graduate students participated in the structural and physical property tests of this work. The characterization and analysis of deep energy level defect states was strongly supported by Professor Chen Tao and graduate student Che Bo from the Department of Materials Science and Engineering of USTC. This work also received continuous guidance and encouragement from Academician Yu Shuhong of the Institute of Innovative Materials. The above research was supported by the key and general projects of the Joint Fund of the National Natural Science Foundation of China and the Shenzhen Key Laboratory.

As early as the 1950s, researchers at Bell Labs in the United States invented single-crystal silicon solar cells, which achieved a breakthrough in converting sunlight into electrical energy using single-crystal silicon wafers and were successfully used in artificial satellites. At that time, the photoelectric conversion efficiency was only about 5%. In recent years, through the collaborative innovation of material structure engineering and high-end equipment development, researchers have increased the photoelectric conversion efficiency of single-crystal silicon solar cells to 26.8%, close to the theoretical limit of 29.4%, and the manufacturing cost and comprehensive power generation cost have dropped significantly, achieving grid parity in most parts of my country. At the same time, the market share of single-crystal silicon solar cells in the photovoltaic market has also risen to more than 95%. In addition to the large-scale application of conventional solar cells in ground photovoltaic power stations and distributed photovoltaics, flexible solar cells also have huge development space in the fields of wearable electronics, mobile communications, vehicle-mounted mobile energy, photovoltaic building integration, aerospace, etc. However, commercial high-efficiency, lightweight, large-area, low-cost flexible solar cells have not yet been developed at home and abroad to meet the application needs in this field.

The research team of Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences (hereinafter referred to as Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences) found through high-speed camera observation that the fracture of single-crystal silicon solar cells under bending stress always starts from the "V"-shaped groove at the edge of the single-crystal silicon wafer. This area is defined as the "mechanical short board" of the silicon wafer. Based on this phenomenon, the research team innovatively developed edge smoothing technology to process the sharp "V"-shaped grooves on the surface and sides of the silicon wafer edge into smooth "U"-shaped grooves, changing the structural symmetry on the mesoscopic scale. Combined with finite element analysis, molecular dynamics simulation under dynamic stress loads and residual stress analysis of spherical aberration transmission electron microscopy, it was found that the "brittle" fracture behavior of single-crystal silicon was transformed into "elastic-plastic" secondary shear band fracture behavior. At the same time, since the rounding treatment is limited to the edge area of ​​the silicon wafer, it does not affect the light absorption capacity of the surface and back of the silicon wafer, thereby maintaining the photoelectric conversion efficiency of the solar cell unchanged. This structural design can significantly improve the "flexibility" of silicon wafers. A 60-micron-thick single-crystal silicon solar cell can be folded like an A4 paper, with a minimum bending radius of less than 5 mm (Figure 1a); it can also be bent repeatedly, with a bending angle of more than 360 degrees (Figure 1b). The relevant results were published in Nature on May 24, 2023 and were selected as the cover of the issue.

Figure 1. a, The bending radius of the flexible solar cell silicon wafer is less than 5 mm; b, The bending angle of the flexible solar TV exceeds 360 degrees.

This work realizes the manufacturing of flexible single-crystal silicon solar cells through simple process processing, and verifies the feasibility of mass production in the mass production line, providing a feasible technical route for the development of lightweight and flexible single-crystal silicon solar cells. The large-area flexible photovoltaic modules developed by the research team have been successfully applied to near-space aircraft, building photovoltaic integration and vehicle photovoltaic fields.

The first completion unit of this work is the Shanghai Institute of Microsystem and Information Technology of the Chinese Academy of Sciences, and the first author is Associate Researcher Liu Wenzhu of the Shanghai Institute of Microsystem and Information Technology of the Chinese Academy of Sciences, Associate Professor Liu Yujing of Changsha University of Science and Technology, Dr. Yang Ziqiang of Saudi Aramco, and Professor Xu Changqing of Nanjing Normal University. Theoretical calculations were completed in cooperation with Associate Professor Ding Bin of Beijing University of Aeronautics and Astronautics and Professor Xu Changqing of Nanjing Normal University. Residual stress analysis was completed in cooperation with Professor Liu Xiaochun and Associate Professor Liu Yujing of Changsha University of Science and Technology. The high-speed camera was used to shoot the instantaneous fracture process of the silicon wafer by Dr. Yang Ziqiang of Saudi Aramco.

Researcher Di Zengfeng, the corresponding author of this article, said: "The understanding of the fracture behavior of solar cell silicon wafers with sharp 'V'-shaped grooves on the surface inspired the research team to change the morphology of the edge area of ​​the silicon wafer, and process the sharp 'V'-shaped groove into a smooth 'U'-shaped groove, so that the bending strain can be effectively dispersed, effectively suppressing the strain fracture behavior, and improving the flexibility of the silicon wafer, and finally realizing a high-efficiency, lightweight, and flexible single-crystal silicon solar cell."

The corresponding author of this article, researcher Liu Zhengxin, introduced: "Since the rounding strategy is only implemented on the edge of the silicon wafer, it basically does not affect the photoelectric conversion efficiency of the solar cell. At the same time, it can significantly improve the flexibility of the solar cell. It has broad application prospects in space applications, green buildings, portable power supplies, etc. in the future."

Since its establishment in 2010, the New Energy Technology Center of the Shanghai Institute of Microsystem and Information Technology of the Chinese Academy of Sciences has focused on the research and development of amorphous silicon/monocrystalline silicon heterojunction (Silicon Heterojunction) solar cells, and has achieved many original scientific research results. In the past three years, it has published papers in top international academic journals such as Nature, Nature Energy, and Joule as the first corresponding unit. At the same time, many important research results have been applied on a large scale in the fields of large-scale industrialization, near-space development, and renewable energy power supply for polar research stations.

The amount of soil organic carbon stored on the earth is four times that of terrestrial vegetation organic carbon. A small proportion of the loss may also accelerate climate warming. Promoting soil carbon sequestration helps reduce the concentration of carbon dioxide in the atmosphere and is one of the natural solutions to climate change. An international research team led by Professor Huang Xiaomeng and doctoral student Tao Feng from the Department of Earth System Science at Tsinghua University and Professor Luo Yiqi from Cornell University conducted in-depth interdisciplinary research in the fields of ecology and computer science, and used artificial intelligence and data assimilation technology to reveal the decisive role of microbial carbon use efficiency in global soil organic carbon storage.

The study is based on the soil carbon cycle theory of the past two hundred years, integrates the world's largest soil organic carbon database, and combines advanced artificial intelligence and data assimilation technology to systematically evaluate the relative contribution of various soil carbon cycle processes to global soil organic carbon storage for the first time. The study reveals the relationship between microbial carbon use efficiency and soil organic carbon storage, providing a scientific theoretical basis for influencing microbial processes, promoting soil carbon fixation and achieving carbon neutrality through land management. In addition, the mechanism model constructed by the study and the new paradigm of integrating ecological big data with artificial intelligence also provide new ideas for research in other related fields.

The results were published in the journal Nature on May 24 under the title "Microbial carbon use efficiency promotes global soil carbon storage".

Microorganisms are not only the main decomposers of organic matter in the soil, but also directly produce soil organic matter through their growth and death. Analyzing the dual control mechanism of microbial processes on soil organic carbon storage and quantitatively evaluating their relative contributions are the key to understanding the soil carbon cycle and its response to climate change.

To this end, the research team used microbial carbon utilization efficiency as a variable, integrated the dual control mechanism of microbial processes on soil organic carbon storage, and explored its relationship with global soil organic carbon reserves. Microbial carbon utilization efficiency represents how microorganisms allocate carbon between biosynthesis and mineralization decomposition in metabolism. High microbial carbon utilization efficiency means that the accumulation of soil organic carbon is promoted by increasing biomass, thereby producing more apoptotic products and organic byproducts. On the other hand, this may also mean that more enzymes that promote the decomposition of organic matter are synthesized, and ultimately accelerate the loss of soil organic carbon.

The study integrated the mechanistic model describing the complex soil carbon cycle with more than 50,000 soil carbon observation data, and determined the most likely control path of microbial processes on soil organic carbon storage under the Bayesian framework. The results show that on a global scale, microbial carbon utilization efficiency is positively correlated with soil organic carbon storage, and a higher carbon allocation ratio to organic synthesis in microbial metabolism ultimately leads to the accumulation of soil organic carbon rather than loss.

The research team further extended the data-model fusion results at the site scale to the global scale based on the "process-driven and data-driven fusion deep learning modeling (PRODA) method" independently developed by the team, and obtained the spatial distribution pattern of seven types of soil carbon cycle processes including microbial carbon utilization efficiency, and quantitatively evaluated their relative contributions to global soil organic carbon storage and spatial distribution. Microbial carbon utilization efficiency presents a pattern of low values ​​at low latitudes and high values ​​at high latitudes around the world (Figure 3), reflecting the adaptability of microbial physiology to temperature - in tropical regions, microorganisms reduce the carbon allocation ratio to organic synthesis to adapt to the higher energy required to maintain metabolism in high temperature environments. "The PRODA method creatively uses artificial intelligence technology combined with process models to reveal the spatial pattern of soil carbon cycle processes, which is crucial for the reasonable simulation of soil carbon storage using process models." Professor Huang Xiaomeng of Tsinghua University said.

The study also found that microbial processes play the most critical role in soil carbon storage. Accurately describing the spatial pattern of microbial carbon utilization efficiency is the key to accurately simulate the global soil organic carbon storage and spatial distribution, and its importance is more than 4 times that of other processes such as soil organic matter decomposition and plant carbon input. Our team has made a breakthrough in solving the difficult problem of assessing the relative importance of microbial processes and other processes to soil carbon storage at a global scale. Professor Luo Yiqi of Cornell University said.

Tao Feng, a 2018 direct doctoral student at Tsinghua University, is the first author of the paper, and Professor Luo Yiqi of Cornell University and Professor Huang Xiaomeng of Tsinghua University are co-corresponding authors. More than 30 collaborators from China, the United States, Germany, France, Sweden, Switzerland, Australia, Italy and the United Kingdom participated in this study. The research was supported by the National Natural Science Foundation of China, the National Key R&D Program and the China Scholarship Council.