At present, hydropower, wind power, solar energy and other renewable energy has become an important part of China's energy structure. Solar energy is one of the cleanest and cheapest forms of energy, so how to convert it into electricity that is easier to use has become a hot research topic in photovoltaic field.
In recent decades, solar cells of various materials have proliferated. As a rising star in the field of photovoltaic devices, hybrid perovskite solar cells (hereinafter referred to as perovskite solar cells) have attracted extensive attention since their discovery in 2009 due to their advantages of low cost, good flexibility and large printing area. It was named one of the top 10 Breakthrough technology developments of 2013 by Science journal. Over the past decade, research on perovskite cells has exploded, with the efficiency of their photovoltaics soaring from 2.2 percent to 24.5 percent, close to that of silicon-based solar cells. Therefore, perovskite solar cells are expected to become a major event in the photovoltaic field.
Perovskite solar cells, which contain neither calcium nor titanium
One might reasonably think of calcium and titanium when talking about perovskite solar cells, but the interesting thing is that there is neither calcium nor titanium in these cells. It gets its name from its light-absorbing layer: a perovskite-type substance.
Perovskite, named after the Russian mineralogist Perovski, originally referred to a single mineral called calcium titanate (CaTiO3), but later to ABX3 and similar crystals. In the perovskite solar cells introduced today, cation A is usually organic ion CH3NH3+, C2H5NH3+, B is usually divalent metal ion, such as Pb2+, Sn2+, and X is halogen anion (Cl-, Br-, I-). This material contains both inorganic components and organic molecular groups, so people call this kind of material hybrid perovskite material.
Perovskite solar cell is a device that converts light energy into electricity. Its essence is semiconductor diode, and the power generation principle is based on the photogenerating volt phenomenon of PN junction. P-n junction is a N type doping area (N for Negative state, such as semiconductor containing relatively high concentrations of electrons, negatively charged and the name) and a p-type doped region (P for Positive ", such as semiconductor containing high levels of "holes", equivalent to a Positive charge, positively charged) and the name of close contact, The contact interface is called the heterojunction interface (PN junction). When sunlight hits the PN junction of a semiconductor, it excites to form hole-electron pairs (excitons).
Excitons produced by light are first separated into electrons and holes and then transported to the cathode and anode respectively. The negatively charged free electrons pass through the electron transport layer into the glass substrate and then through an external circuit to the metal electrode. The positively charged holes spread to the hole transport layer and eventually to the metal electrode. At this point, the holes are recombined with the electrons, and the current forms a loop to transport the electricity.
Where is the perovskite battery show?
Perovskite solar cells are regarded as the third generation of solar cells and have outstanding advantages over traditional solar cells. For example, the first generation of monocrystalline silicon solar cells require purity of up to 99.99%, and the production process is complex, high energy consumption and high pollution. Although the production energy cost of the second generation of thin film solar cells has decreased, it still relies on copper, indium and other precious metals, and is accompanied by highly toxic by-products. Perovskite solar cells mostly adopt solvent process, and their raw materials are mostly liquid, which can be prepared at room temperature. Moreover, in the future, large-area flexible solar cells and wearable smart devices can be completely prepared by printing technology.
How close are perovskite solar cells to commercialization?
Although the perovskite the photoelectric conversion efficiency of solar energy has been more than 24%, and in the cost of materials and manufacturing cost has significant market advantage, but the device stability problems have been restricted to its commercial production even in the United States national renewable energy laboratories (NREL) certification is on the table "unstable" label.
In laboratory experiments, it has been found that the efficiency of perovskite solar cells decreases over time when they are made and left at room temperature. The basic reason is that the perovskite material used in the absorption layer is extremely sensitive to the environment of water, heat and oxygen, which makes its structure unstable and easy to produce irreversible degradation. At present, the life span of perovskite solar cells is much lower than that of silicon-based solar cells (25 years). Therefore, in order to achieve large-scale commercial application of perovskite solar cells, it is urgent to solve their own stability problems.
Taking CH3NH3PbI3 (methylamine lead iodine), a typical perovskite battery material, as an example, it will change into solid PbI2 crystals and volatile CH3NH2 and HI gases under the condition of oxygen and water, causing permanent damage to battery devices.
In order to improve the stability of perovskite solar cells, the most commonly used method is to use encapsulation protection and hydrophobic electrode to prevent the erosion of the device by water, in addition to changing the material composition or chemical modification to essentially improve its moisture resistance. In addition, illumination is an inevitable condition for solar cell operation, and the heat generated by continuous illumination under working conditions will accelerate the decomposition of perovskite and induce ion migration. Ion migration is considered to be one of the main reasons for the degradation of perovskite materials and devices. Because organic groups and halogen ions in methylamine lead iodine materials can migrate over long distances through defects and grain boundaries at room temperature, finding an effective method to inhibit ion migration has become the key to solve the stability problems of perovskite materials and devices.
Where is the breakthrough in the new research?
Previous studies have found that the stability of perovskite solar cells declines rapidly under light and increases with increasing light intensity. The stability breakthrough in perovskite solar materials, reported recently in Nature, was the discovery of a way to effectively inhibit ion migration in high light and heat. It was found that after ionic liquid modified perovskite solar cells were continuously operated for more than 1800 hours at the full spectrum sunlight of 70-75℃, the performance of the most stable packaged devices only decreased by about 5%, fully meeting the requirements of stable and efficient photovoltaic devices at room temperature. As a liquid ionic compound, ionic liquid can not only make perovskite grow better in the electron transport layer, but also can have strong electron interaction with TiO2 as the electron transport layer, thus promoting the electron mobility. On the other hand, due to the unfriendly nature of lead-containing materials to the environment, researchers are also trying to achieve lead free. The most direct method is to use Sn element of the same group to replace Pb element, but it will reduce the conversion efficiency of batteries accordingly. At present, some scholars propose to make perovskite solar cells with waste lead to solve the problem of waste lead treatment. However, achieving full lead free remains a challenge in the perovskite solar cell field
Although there are still some "obstacles" in the commercialization of perovskite solar cells, their rapid development speed, as well as the advantages of simple production process, low cost and good flexibility are undeniable. We have reason to believe that with the continuous research of scientists, in the near future, products based on perovskite batteries can enter the lives of ordinary people, let's wait and see.