21.7%! New method reshapes perovskite solar cell fabrication process

The researchers used coating printing, vacuum deposition and other technologies to realize the preparation of large-area all-perovskite tandem photovoltaic modules for the first time in the world, opening up a new way of mass production and commercialization of large-area perovskite tandem cells. path. Certified by an international authoritative third-party testing organization, the stable photoelectric conversion efficiency of this module is as high as 21.7%, which is the world's highest efficiency of known perovskite photovoltaic modules.

Solar cells can directly convert solar energy into electricity, which is an important way to obtain clean energy.

The cost of photovoltaic power generation depends on the photoelectric conversion efficiency of solar cells. Studies have shown that for every 1% increase in conversion efficiency, the cost of power generation can be reduced by 7%, but the photoelectric conversion efficiency of crystalline silicon solar cells currently encounters development bottlenecks. The key to Internet access will also provide important scientific and technological support for the realization of the "dual carbon" goal.

Recently, the research group of Professor Tan Hairen from the School of Modern Engineering and Applied Sciences of Nanjing University and scholars from the University of Oxford in the United Kingdom used coating printing, vacuum deposition and other technologies to realize the preparation of large-area all-perovskite stacked photovoltaic modules for the first time in the world. A new path for mass production and commercialization of large-area perovskite tandem cells.

Certified by an international authoritative third-party testing organization, the stable photoelectric conversion efficiency of this module is as high as 21.7%, which is the world's highest efficiency of known perovskite photovoltaic modules. This achievement was included in the latest issue of "Solar Battery World Records", and the relevant results were recently published in the international authoritative academic journal "Science".

Lower production cost and more energy saving

The development of clean, low-cost solar photovoltaic power generation is an important way and technical guarantee to achieve carbon peaking and carbon neutrality. In the first quarter of 2022, my country's photovoltaic power generation was 84.1 billion kWh, a year-on-year increase of 22.2%.

"However, with the development of technology, traditional crystalline silicon single junction solar cells have also encountered two development bottlenecks. First, the existing industrial production capacity has approached the limit of the photoelectric conversion efficiency of crystalline silicon single junction solar cells; second, the cost High energy and high energy consumption, the process of refining quartz sand into industrial silicon and making monocrystalline silicon requires a high temperature of more than 1000 °C, while the preparation of perovskite solar cells requires about 100 °C." As the corresponding author of this study , Tan Hairen said frankly that perovskite solar cells with lower production costs and more energy saving are regarded as new opportunities for the development of the photovoltaic industry in recent years, and the structural optimization and technological innovation of perovskite tandem cells will accelerate the photovoltaic industry to achieve cost reduction Synergistic.

Previously, Tan Hairen's research group proposed a new tunneling structure, which broke through the preparation problem of all-perovskite stacks, developed a new method to enhance the passivation of defects on the surface of perovskite grains, and created the photoelectric conversion of all-perovskite stacks. The world record efficiency of 26.4% has surpassed the highest certified efficiency of single-junction perovskite cells for the first time in the world. The relevant results have been published in international authoritative academic journals such as Nature.

"Although small-area perovskite cells in the laboratory have achieved high conversion efficiency, the commercialization of large-area perovskite photovoltaic cell blocks still faces many challenges." Tan Hairen did not deny that although previous research has produced 1 The high-efficiency perovskite tandem battery is about square centimeters, but the mass-produced preparation method and the long-term stability of the interconnect structure in the battery block are the key bottlenecks to realize industrialization.

Multiple technologies allow the material to form a uniform film

To achieve mass production, it is first necessary to solve the problem of large-area and uniform preparation of wide-bandgap perovskite films.

"Wide-bandgap perovskite contains high bromide components, its solubility is low, the solvent selection space is small, the crystallization control is not easy, and it is difficult to obtain high-quality, uniform and dense films. International research on its mass production preparation technology Almost blank." Tan Hairen pointed out.

In response to the above challenges, the research team proposed a mass-produced all-perovskite tandem battery preparation scheme for the first time. They used coating printing, vacuum deposition and other preparation technologies to replace the spin-coating film formation process commonly used in the laboratory, and prepared a 20 square centimeter all-perovskite tandem battery.

"Previously, we used a spin coating process, that is, first apply the perovskite solution on the glass substrate, and then use the machine to quickly drive the entire glass substrate to rotate, and use centrifugal force to distribute the solution on the substrate to form a thin film, but this method will In addition, the spin coating machine rotates very fast, so it is difficult to drive a large-area glass substrate to rotate, which determines that it is not suitable for mass production of perovskite solar cells." Tan Hairen said.

In order to allow the perovskite solution to form a uniform film over a large area, the research team first used a doctor blade coating process. Tan Hairen explained that they dropped the solution on the transparent conductive glass, and then scraped it forward with a blade, which formed a uniform wet film on the surface of the glass. In this way, they completed the hole transport layer, calcium Brushing the titanium layer, and then preparing the electron transport layer and tunneling structure by vacuum deposition to protect the first layer of perovskite, then coating the hole transport layer and the second layer of perovskite, and vacuum-evaporating the electron transport layer After and metal electrodes, a perovskite solar cell block frame is "out of the oven" like building blocks.

It is not enough to build a "house", it also has to be well-proportioned and strong. Tan Hairen said that when the perovskite tandem battery block was initially prepared, the film was still uneven due to the long crystallization time of the solution. "Later, I thought that if it could be like printing paper, the ink would dry immediately after printing, which might improve the film. quality and productivity”.

Aiming at the difficulty of controlling the crystallization of wide-bandgap perovskite in the coating process, after several attempts, the team increased the cesium content of the A-site cation in the perovskite component to 35%, and combined with the blade coating method of air-assisted crystallization to accelerate the solution After volatilization, a flat and dense wide-bandgap perovskite film with the best crystallinity was finally obtained, which laid the foundation for the mass production of all-perovskite stacked components.

Why does cesium become the "Son of Heaven" to make batteries rapidly and stably formed? Tan Hairen introduced: "Cesium is an inorganic ion and is not easily volatile. It will improve the thermal stability of the device, reduce lattice strain, improve the photostability of the device, reduce the crystallization barrier, and speed up the nucleation rate of the device."

Avoid different materials "harming" each other

"Theoretically, the photoelectric conversion efficiency of the current single-layer perovskite solar cell is only 33% at the highest, while the double-layer structure can reach up to 45%. The higher the power generation efficiency, the lower the cost." Long-term in-depth research, Let Tan Hairen discover that in order to achieve the leap from "one to two" in the internal structure of perovskite cells, we must also consider how to "harmoniously coexist" between device materials.

"In a tandem perovskite photovoltaic module, there is a complex interconnection structure in the connection region of each two sub-cells. Due to the direct contact between the perovskite light-absorbing layer and the back metal electrode in the interconnection region, the halogen ions in the perovskite will Interdiffusion with the metal in the electrode will cause the metal material to be corroded and the electrical properties of the perovskite material to decline, which will affect the photoelectric conversion efficiency of the battery block." Tan Hairen said, in order to overcome this problem, the team used the perovskite light-absorbing layer and the back metal. Between the electrodes, a layer of tin dioxide electron transport layer was prepared by atomic layer deposition.

"Tin dioxide is a semiconductor material that can be grown in a low temperature environment and has good electrical conductivity. It does not affect the ohmic contact between the metal electrode in the interconnect area and the transparent conductive oxide electrode on the front surface. At the same time, the tin dioxide electron transport layer It can be conformally deposited in the interconnected regions between sub-cells, blocking the direct contact between perovskite and metal. As an electron transport layer in the active region of the cell, it also prevents the oxidation of narrow-bandgap perovskite by air, realizing atmospheric The interconnection preparation, testing and packaging of components under the conditions of operation." Tan Hairen explained.

This innovative module structure design significantly improves the fabrication repeatability, photovoltaic performance and stability of the module. As determined by the Japan Electrical Safety and Environmental Technology Laboratory, the photoelectric conversion efficiency of this all-perovskite tandem solar cell block is 21.7%, which is the world's highest efficiency reported for perovskite photovoltaic modules. "Solar battery world record table" included.

The potential demonstrated by large-area perovskite tandem photovoltaic modules inspired the team to have greater fighting spirit. Tan Hairen said that if we want to promote the industrialization of this technology, we must do more research and development in the process of printing and preparing perovskite. It is relatively simple to prepare 20 square centimeters of ink, but if it is expanded to 1 square meter, which technical conditions need to be innovated, it still needs continuous verification.