Chapter 62: Perovskite transport layer

　　 "Sister Feifei Wu, can you introduce the field of perovskite solar cells you are doing?"
　　 "Yes, perovskite materials are a class of substances with the general molecular formula ABX3," Wu Feifei said:
　　"For the field of perovskite solar cells, the commonly used molecular structures of A are CH3NH3 and H2NCHNH2, abbreviated as MA and FA respectively.
　　 B is usually lead element, X is halogen element, iodine element is usually used, and the solvent is usually DMF.
　　 The two most classic molecular structures of perovskite materials are MAPbI3 and FAPbI3, which are made by mixing MAI or FAI with lead iodide, respectively.
　　The main problem in this field is the toxicity of lead and the instability of devices in air or high humidity environments. "
　　 "Can they be used as transmission layer materials for organic photovoltaic devices?" Xu Qiu asked.
"From the perspective of solvents, it is feasible, because the effective layer solution of spin-coated organic devices is chloroform, chlorobenzene, etc., which cannot dissolve perovskite materials.
　　 But from the perspective of light absorption, the two classic perovskite materials, MAPbI3 and FAPbI3, are probably not suitable.
　　Because these two molecules are mainly used to make the effective layer, their band gap is about 1.5-1.7 electron volts, and they have good light absorption properties in the visible light range.
　　If it is used as a transmission layer, the intensity of sunlight entering the effective layer will decrease and the photoelectric conversion efficiency will be reduced.
　　 Our commonly used transmission layer materials, such as PEDOT:PSS, zinc oxide, etc., are usually materials with a band gap greater than 2 electron volts. "Wu Feifei said.
　　 Xu Qiu thought for a while and asked:
　　 "So is there a way to increase the band gap of perovskite materials to more than 2 electron volts?"
　　 "There are ways. Modifying A, B, and X will affect the physical and chemical properties of the formed perovskite material, including changing the band gap."
　　Wu Feifei found a document in her file bag, handed it to Xu Qiu, and said:
　　 "Well, let me give you a review. There are physicochemical properties of perovskite materials reported in recent years. You can compare and screen them."
"Ok."
　　……
　　 The first problem that Xu Qiu faces now is to find a perovskite material with a high band gap.
　　 For semiconductors, if the energy of a photon is lower than its band gap, it cannot be absorbed by the semiconductor.
"For example, the band gap of silicon is about 1.1-1.3 electron volts, so only photons with energy exceeding 1.1-1.3 electron volts can be absorbed by silicon, which corresponds to light with a wavelength of less than 1000 nanometers.
　　 Therefore, silicon materials can absorb part of ultraviolet, visible light (390-780 nanometers) and part of infrared, covering the entire solar spectrum almost perfectly, which is the reason for the high photoelectric conversion efficiency of silicon cells.
　　 Xu Qiu looked through the literature and listed a series of perovskite materials, some with a band gap of more than 3.0, some close to 1.0, and other materials with multiple band gap values.
　　 He took a closer look, and it turned out that different researchers used different test methods, using UV photoelectron spectroscopy, cyclic voltammetry, and light absorption edge conversion methods.
　　 It seems that the band gap data can only be used as a reference.
　　 looked for a while, he suddenly realized a problem, he had to check what reagents were in the laboratory first.
　　 Otherwise, I finally found the right material, but found that the laboratory did not have it, so it would be embarrassing.
　　 "Wu Feifei, what do we have for perovskite medicines?" Xu Qiu asked.
　　 "Now I often do two standard systems, namely MAI, FAI and lead iodide. As for the others..." Wu Feifei paused and said:
　　 "Anyway, they are all in my box, you can look for it. My box is a translucent buckle box with WFF marked with a marker."
"Ok."
　　 Xu Qiu took protective measures and entered the glove box.
　　 He found Feifei Wu’s special kit, which contained many white plastic bottles of medicine.
　　 There are many bottles of MAI and lead iodide. It seems that these two are the most commonly used.
　　 After turning for a long time, Xu Qiu found only two useful medicines, lead chloride and lead bromide, which can be mixed with MAI and FAI.
　　 There are four perovskite materials, namely MAPbBr3, MAPbCl3, FAPbBr3 and FAPbCl3.
　　 Xu Qiu unscrewed the caps of the lead chloride and lead bromide reagent bottles, confirmed that there were medicines inside, and then put them back.
　　 Then he left the glove box and asked Feifei Wu to get the formula:
　　 10% of the total mass fraction of MAI and lead iodide are mixed in equivalents, dissolved in DMF solvent, and the solution is stirred evenly, spin-coated at 3000r.p.m. for 30 seconds.
　　Of course, this is the recipe of MAPbI3, so I can only refer to it.
　　 Xu Qiu imitated her formula and also used 10% mass fraction as the condition to calculate the material ratios in the four solutions.
　　 Then, he returned to the glove box again and prepared 1 ml of each of the four perovskite solutions.
　　 In addition, because the previously prepared effective layer solution took too long to be used, it was uncertain whether it could still be used, so he also prepared the PTB7-TH:PC[70]BM solution under the optimal conditions.
　　……
　　 After the solution was dissolved, Xu Qiu took a clean glass piece and decided to try the spin coating of the perovskite solution first.
　　 For the first piece, he placed the substrate, dropped the solution, and started spin coating.
　　The glue machine stopped, and he used tweezers to remove the substrate, and found that the film on it was not in good shape, the surface was foggy, and it felt like a layer of hoarfrost had formed.
　　 In the second film, he changed one solution, still the same situation~EbookFREE.me~ continued to change the other two solutions, white mist still appeared.
　　 What is going on?
　　The film that I painted is not right.
　　 At least, a transparent film is needed.
　　 Xu Qiu had no perovskite spin coating experience, so he had to call Feifei Wu.
　　 After seeing the substrate, she said:
　　"You can try the anti-solvent method. After spin-coating, add dropwise anti-solvent toluene to quickly crystallize the perovskite material, which may improve the morphology of the film.
　　 However, this method is more time-tested and needs to explore the conditions, and the best conditions for different systems may be different.
　　My MAPbI3 system is usually done after starting spin coating, in my heart, read from 1 to 5, and then quickly drop toluene on the substrate. "
　　 "Okay, I'll try it." Xu Qiu said.
　　At this time, he suddenly thought, can he copy Feifei Wu's skills?
　　 "System, can Wu Feifei's skills be copied?"
　　【Yes. 】
　　 [The solution preparation skills are detected, whether to record? 】
　　 [Spin ​​coating skills detected, whether to record? 】
　　……
　　 Xu Qiu was confused by a series of system prompts and asked:
　　 "After recording, will the original Chen Wanqing's skills be overwritten?"
　　 [There are two types of situations. For indifferent skills, such as cleaning substrates, they will be covered; for differentiated skills, such as spin coating, they will not be covered and will be recorded separately. 】
　　 "Understood, there are different skills, all recorded."
　　【Recording completed. 】
　　 "With that indifferent skill, can the one with the highest proficiency of the two be retained?"
　　【Yes, the record is complete. 】