I Have a Research Support System

Senior Wu Feifei, can you introduce to me the field of perovskite solar cells you are working on?

Yes, perovskite materials are a type of substance 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, referred to as MA and FA, respectively.

B is usually lead element, X is a halogen element, usually iodine element, and the solvent is usually DMF.

The molecular structures of the two most classic perovskite materials are MAPbI3 and FAPbI3, which are made by mixing MAI or FAI with lead iodide, respectively.

The main problems in this field are the toxicity of lead and the instability of devices in air or high humidity environment.

Can they be used as transport 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 and the like, which cannot dissolve perovskite materials.

However, from the perspective of light absorption, the two classic perovskite materials, MAPbI3 and FAPbI3, may not be suitable.

Because these two molecules are mainly used as effective layers, 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, it will reduce the intensity of sunlight incident on the effective layer and reduce the photoelectric conversion efficiency.

Our commonly used transport 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:

Then is there a way to increase the bandgap width of perovskite materials to more than 2 electron volts?

There is a way. Modifying A, B, and X will all affect the physical and chemical properties of the formed perovskite material, including changing the forbidden band width.

Wu Feifei dug out a document in her file bag, handed it to Xu Qiu, and said:

Well, let me give you a review, which includes the physical and chemical properties of perovskite materials reported in recent years, and you can compare them.

OK.

...

The first problem Xu Qiu faces now is to find a perovskite material with a high band gap.

For a semiconductor, if the energy of the photon is lower than its forbidden band width, 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, corresponding to light with a wavelength less than 1000 nanometers.

Therefore, silicon materials can absorb part of ultraviolet rays, visible light (390-780 nanometers) and part of infrared rays, covering almost the entire solar spectrum perfectly, which is also 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 bandgap exceeding 3.0, some close to 1.0, and some materials with multiple bandgap values.

He took a closer look and found that different researchers used different testing methods, such as ultraviolet photoelectron spectroscopy, cyclic voltammetry, and light absorption edge conversion.

It seems that the data of the forbidden band width can only be used as a reference.

After flipping through it for a while, he suddenly realized a problem, he had to check what reagents were in the laboratory first.

Otherwise, it would be embarrassing if you finally found the right material and found that the laboratory didn't have it.

Wu Feifei, perovskite medicine, what do we have? 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:

It's all in my box anyway, you can look for it, my box is a translucent lock box with WFF marked on it with a marker.

OK.

Xu Qiu took protective measures and entered the glove box.

He found Wu Feifei's special kit, which contained many medicines in white plastic bottles.

There are many bottles of MAI and lead iodide, and it seems that these two are the most commonly used by her.

After searching for a long time, Xu Qiu only found two medicines that are useful to him, lead chloride and lead bromide, which can be mixed with MAI and FAI.

Thus four perovskite materials were obtained, namely MAPbBr3, MAPbCl3, FAPbBr3 and FAPbCl3.

Xu Qiu unscrewed the caps of the lead chloride and lead bromide reagent bottles, checked that there were medicines inside, and then put them back.

Then he left the glove box and asked Wu Feifei to get the formula:

10% of the total mass fraction of MAI and lead iodide were mixed in equal amounts, dissolved in DMF solvent, and the solution was stirred evenly, and then spin-coated at a speed of 3000r.p.m. for 30 seconds.

Of course, this is the formula of MAPbI3, it can only be used as a reference.

Xu Qiu imitated her formula, and also used 10% mass fraction as a condition to calculate the ratio of materials 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 effective layer solution prepared before was too long, it is not sure whether it can still be used, so he also prepared the PTB7-TH:PC[70]BM solution under optimal conditions.

...

After the solution was dissolved, Xu Qiu took a clean glass piece and decided to try spin-coating the perovskite solution first.

For the first piece, he placed the substrate, added the solution dropwise, and started spin coating.

When the dispenser stopped, he took off the substrate with tweezers and found that the film on it was not in good shape, the surface was foggy, and it felt like a layer of hoarfrost.

The second tablet, he changed to another solution, still the same situation.

Continue to change the other two solutions, white mist will still appear.

What's going on here?

The coated film doesn't look right.

At least, to get a transparent film.

Xu Qiu had no perovskite spin coating experience, so he had to call Wu Feifei.

After seeing the substrate, she said:

You can try the anti-solvent method. After spin-coating, add the anti-solvent toluene dropwise to make the perovskite material crystallize quickly, which may improve the morphology of the film.

However, this method is more of a test of opportunity and needs to explore the conditions, and the optimal conditions of different systems may also be different.

For my MAPbI3 system, the general way to do it is to silently read from 1 to 5 in my heart after the start of spin coating, and then quickly drop toluene on the substrate.

Okay, I'll give it a try. Xu Qiu said.

At this time, he suddenly thought, can Wu Feifei's skills be copied?

System, can Wu Feifei's skills be copied?

【Can. 】

[Solution preparation skill detected, is it recorded? 】

[Spin-coating skill detected, is it recorded? 】

...

Xu Qiu was confused by a series of system prompts, and asked:

After recording, will the original skills of Chen Wanqing be overwritten?

[It is divided into two types. For non-differential skills, such as cleaning substrates, it will be covered; for differentiated skills, such as spin coating, it will not be covered and will be recorded separately. 】

Understood, there are differences in skills, all recorded.

[The recording is complete. 】

The indiscriminate skill, can you keep the one with the highest proficiency of the two?

[Yes, the recording is complete. 】