I Have a Research Support System

In the evening, when Xu Qiu returned to the dormitory, he saw his roommate Sun Yifan sitting at the table with a lamp on, rubbing the furnace stones in front of the computer.

Sun Yifan greeted: "Xu Qiu, you're back. Did you see the list sent by the counselor? Our direct Ph.D. students have no human rights. They only have the second prize, and the first prize was taken away by the master-doctorate students."

Xu Qiu shrugged and echoed, "Yeah, I thought we were the first prize...but the first and second prizes are about the same, and the difference is estimated to be one or two thousand yuan at most. Sprinkle the water."

"That's true," Sun Yifan nodded, and asked casually, "By the way, Xu Qiu, have you applied for a national award? I remember that you seem to have published a lot of undergraduate articles."

"Yes, I have applied." Xu Qiu replied. He looked behind Sun Yifan for a while. The game had just started, and both sides had 30 HP, so he couldn't see anything. He had played WAR3 before, but he hadn't touched WOW and Hearthstone after Blizzard.

Sun Yifan said suddenly: "I heard that Mu Xue, the monitor of our doctoral class, has also applied for the national award."

Xu Qiu was stunned for a moment, and asked, "Mu Xue, didn't she come from Qingbei University, how did she apply?"

Sun Yifan shook his head: "Then I don't know."

The two chatted for a few more words, and when Xu Qiu heard someone coming out of the bathroom outside, he took a shower with his toiletries and his smart card.

Normally, Xu Qiu and Sun Yifan didn't communicate much, not as frequently as their roommates when they were undergraduates. Of course, it may also be because Tao Yan is more familiar with each other.

After taking a shower, Xu Qiu went to bed and entered the simulation laboratory.

These days, for semi-transparent battery devices, he has made a lot of attempts in the simulation laboratory and obtained some preliminary work results.

The preparation of semi-transparent devices is not difficult. The only difference from traditional devices is that the electrodes are replaced with semi-transparent electrode materials, including thin-layer metals, silver nanowires or ITO.

Among them, the preparation of ITO is too difficult and requires the use of magnetron sputtering and other methods, so Xu Qiu only tried spin-coating silver nanowires and vapor-depositing thin-layer metal electrodes.

Previously, Xu Qiu originally thought that spin-coated silver nanowires could have both the advantages of electrical conductivity and light transmission, making them a perfect translucent electrode material.

However, it was found in the actual operation process that the result was not ideal.

He used the silver nanowire solution purchased by Blue River to spin-coat the film. The light transmittance is satisfactory, and it looks very transparent, but the conductivity is poor, and the prepared device is directly in the open circuit state.

In order to find out the reason, Xu Qiu tested the resistance of the silver nanowire thin film with a multimeter, and found that its resistance value was at the level of thousands of ohms, while the resistance value of ordinary ITO electrodes was generally at the level of several ohms, which was three orders of magnitude worse. above.

Later, he further explored and observed with various electron microscopes such as AFM, TEM, and SEM. Anyway, the electron microscope in the simulation laboratory does not cost money, so he simply used it to test it all.

The final results show that many of the silver nanowires obtained by spin coating are approximately parallel or stacked together, and there are still some broken nanowires.

For things like nanowires to have good electrical conductivity, each nanowire must be in good contact with each other to form a grid-like structure.

Therefore, it is easy to understand the low conductivity of the current microstructure. The possible reason is the orientation and damage of the silver nanowires during the spin coating process.

In order to try to solve this problem, Xu Qiu browsed the Xiaochongzi website.

I found that other people had similar problems when spin-coating silver nanowires, including poor conductivity and poor stability, and there was no good solution.

Therefore, Xu Qiu checked some literatures and found that the common methods for preparing silver nanowire conductive films are scrape coating, drip coating, and "slot-die" methods.

The so-called "slot-die" method is a bit like an advanced version of scraping coating, that is, a narrow slot is placed at a certain height above the substrate, and the effective layer or transport layer solution is continuously injected into the narrow slot, and these solutions will be drawn from the narrow slot. It drips in the groove and falls on the substrate to form a film. Because it is a narrow groove, the formed film is also a thin and narrow film. At this time, let the solution move the narrow groove synchronously at a certain speed during the dripping process, so that the solution after dripping can fill the area swept by the narrow groove, that is, a thin film can be obtained.

The difference between "slot-die" and squeegee coating is that squeegee coating is to drop the solution on the substrate first, while "slot-die" is that the solution is in a narrow groove, and then flows out a little bit, so the process difficulty is slightly higher. .

Xu Qiu's operations are actually his routine steps when encountering experimental failures:

First, deduce the possible reason according to the experimental phenomenon;

Then use a cheap method (multimeter) to initially confirm the cause;

Then use expensive methods (various electron microscopes) to confirm the reason. Of course, if the team has no money, this step can be omitted;

Next, go to Xiao Chongzi to see if other people have similar situations and how they solved them;

Finally, look through the literature to find a solution or alternative.

These orders are not completely fixed and can be adjusted according to actual needs.

In addition, the reason why I went to Xiaochongzi first is because this is a Chinese forum about scientific research, and there are endless bigwigs. If you can accurately search for problems that others have solved, it will save a lot of time, similar to the program Use the wheels made by others in the circle.

After all, it takes a lot of time to search for SCI papers. In many cases, it may take an hour to find a few related documents, but in the end they are found to be useless and cannot solve the problem.

Of course, not every time after this operation, the problem of experimental failure can be solved.

For example, this time, Xu Qiu operated fiercely like a tiger, and finally came to the conclusion that PLAN A (Plan A) was on the street.

That is, based on the existing equipment in the laboratory, it is not suitable to use this method of silver nanowire thin film to prepare the top electrode of semi-transparent devices.

However, Xu Qiu didn't panic at all.

Because he still has PLAN B, he didn't bother to optimize the silver nanowire preparation process. He just gave up PLAN A temporarily and started with the method of thin-layer metal electrodes. If necessary in the future, it is not too late to try again.

Before the formal experiment, Xu Qiu made a brief summary of the existing semi-transparent device literature.

He found that the concept of translucent devices existed several years ago, and there are many literatures. There are more than ten articles in the first and second sections, many of which were published by Yang Yang, a large foreign group.

However, in the previous fullerene-only system, although the average transmittance (AVT) in the visible light range can be made very high, even up to 50%, the efficiency (PCE) has not been improved.

With AVT alone, without PCE, this is almost the same as "only face, no lining". For example, 50% AVT with 1% PCE does not make much sense. Photovoltaic devices must return to the competition of efficiency in the end.

At present, the best work is a translucent device based on PCE10:PCBM, the efficiency is only 7%, and the AVT is only 25%. The electrode they use is a thin layer of 10 nanometer silver electrode.

During the official experiment, Xu Qiu tried three kinds of thin-layer electrodes, which are commonly used gold, silver, and aluminum. Using PCE10:IEICO-4F and PCE10:FNIC-4F as standard samples, metal electrodes with different thicknesses were prepared. devices, ranging from 5 nanometers to the normal 100 nanometers.

The final result, taking the PCE10:IEICO-4F system as an example.

Under the condition of electrode thickness of 100 nanometers, the highest device efficiencies of gold, silver and aluminum electrodes are 12.3%, 12.4% and 12.5%, respectively, and the device efficiencies of the three electrodes are equivalent. At this time, the AVT of the device is about 0, that is, the device is almost completely impenetrable to visible light.

Under the condition of 50nm, the highest efficiencies are 12.0%, 12.1% and 12.0%, respectively, and the device efficiencies of the three electrodes are still comparable. The AVT of the device is also about 0 at this time.

Under the condition of 30 nanometers, the highest efficiency is 10.8%, 11.0% and 6.2%, respectively, and the device efficiency of the three electrodes is differentiated. Among them, the device with gold and silver as the electrode has no obvious attenuation of efficiency, while the device with aluminum as the electrode, the efficiency attenuation More serious. At this time, the AVT of the device has reached 5%-10%, and part of the visible light can pass through, and the things behind the device can be seen vaguely.

Under the condition of 20 nanometers, the highest efficiency is 9.4%, 10.2% and 0%, respectively. The devices with gold and silver as electrodes have no obvious efficiency attenuation, while the devices with aluminum as electrodes have broken the circuit. At this time, the AVT of the device has reached 10%-20%, and part of the visible light can pass through, and the things behind the device can be seen vaguely.

Under the condition of 10 nanometers, the highest efficiency is 8.0%, 9.0% and 0%, respectively. Devices with gold and silver electrodes also begin to differentiate, and devices with silver electrodes have higher efficiency. At this time, the AVT of the device reaches 20%-40%, and a large part of visible light can pass through, so that you can see things behind the device more clearly.

Under the condition of 5 nanometers, the highest efficiency is 5.3%, 1.2% and 0%, respectively. The gold electrode device can still maintain a certain device efficiency, while the efficiency of the silver electrode device drops directly, close to an open circuit. At this time, the AVT of the device has reached 30%-45%, and a large part of visible light can pass through, so that the things behind the device can be clearly seen.

In addition, in the process of reducing the thickness of the electrode, the efficiency of the device decays mainly due to the decrease of the short-circuit current density, and the two parameters of the open-circuit voltage and the fill factor remain almost unchanged.

For these experimental phenomena:

The level of primary school students can draw relatively simple conclusions.

Wow, I broke the world record again.

This is also very normal. With the resources Xu Qiu currently possesses, it's fine not to choose a certain field. Once a certain field is chosen, it means that the world record in this field will change hands.

After all, he has temporarily become a leader in the large field of organic photovoltaics.

At the level of middle school students, you can think a little deeper.

For the preparation of translucent devices, the aluminum electrodes are rubbish. Before the electrodes are translucent, the devices are already on the street.

At the level of college students, think about it further.

The current system uses silver electrodes better, and the device performance is the best one in the appropriate range of 10-20 nanometers, which may be related to the fact that it is the metal material with the best conductivity.

At the level of a master's student, think about it further.

At a thickness of 5 nanometers, the performance of gold electrodes surpasses that of silver, which shows that whether metal electrodes can conduct electricity in a thin layer state, density may be a very critical factor.

Because the density of gold is 19.32 grams per cubic centimeter, the density of silver is 10.49 grams per cubic centimeter, and aluminum is only 2.70 grams per cubic centimeter.

The essence of metal conduction is to conduct electrons through the formation of "electron gas" between metal atoms. There is a prerequisite for conduction here, that is, the metal atoms must be continuous without too many gaps.

When the metal film is relatively thick, it doesn’t matter if the density is lower. Anyway, close packing can be approximated. Even if there is an occasional defect, there are other metal atom brothers next to it to pass it on its behalf;

When the metal layer is relatively thin, the metal material with low density will be more likely to appear uneven and not dense during evaporation. At this time, defects occur. Smooth conduction of electrons, resulting in an open circuit.

At the level of a doctoral student, you can think about it further.

Compared with opaque metal electrodes, one of the main sources of device performance loss caused by semi-transparent metal electrodes is the decrease in electrode conductivity, which directly affects the ability of electrodes to collect charges, resulting in a decrease in short-circuit current density, which eventually leads to the attenuation of device efficiency. .

But there is another source of performance loss, and that is less secondary reflected light from opaque metal electrodes.

How should this be understood?

When sunlight is incident on the surface of a photovoltaic device, part of the light will be directly reflected, scattered and lost, and part of it will be converted into heat and lost, after which the sunlight will reach the effective layer.

Part of the light reaching the active layer will be absorbed by the active layer to form excitons, which will then be split and transported to form a current, while another part will pass through the active layer and reach the position of the top electrode.

If the top electrode is made of gold, silver, or aluminum with a thickness of 100 nanometers, that is, it is opaque, because the metal will reflect light, and this part of the light that passes through the effective layer will return to the effective layer, which is the so-called "secondary electrode". Sub-reflection" will also contribute to the device efficiency to a certain extent.

Now that the metal electrode becomes translucent, this part of the light will be partly "secondarily reflected", and part of it will be lost through the metal electrode.

At Xu Qiu's level, he integrated the above and thought of a multi-layer thin-layer electrode structure.

The structure of this thin layer of metal is to first evaporate a thin layer of relatively dense gold, such as one nanometer thick, and then evaporate silver with better conductivity, such as 10-20 nanometers.

The expected effect of such a structure is:

The ultra-thin gold layer provides a dense nucleation center, thereby reducing the penetration thickness of the silver film into the transmission layer and the effective layer, improving the uniformity of the silver film at a low thickness, and ensuring the formation of a continuous silver film, thus obtaining both Thin metal electrodes with high transmittance and low resistance.

Of course, the analysis belongs to the analysis, and the specific results still have to be proved by practice.

PS: Today there are three changes in 4D, the second and third changes are at 7 o'clock and 16 o'clock respectively.