I Have a Research Support System

Xu Qiu's recent target system is J2:IDIC-4F, and these two materials have not been synthesized yet, so he is a little idle.

Of course, paddling is impossible. Xu Qiu looked through the theoretical research literature collected before, and sorted out the testing methods of exciton binding energy and exciton diffusion distance.

He intends to test the water on ITIC first, and then it can be directly and synchronously applied to the IDIC-4F system.

These two experiments are quite important, and they are one of the trump cards he used to sprint for "Natural Energy" or "Joule".

If the final result is the same as he expected, if these two conclusions are separated, it is estimated that there should be no big problem in posting an NC and an AM.

But now it is necessary to combine two important conclusions, plus the J2:IDIC-4F system with an efficiency of 13.5%, to sprint for an article on "Nature·Energy" or "Joule".

No way, it is so difficult to break through the boundaries of the AM level and reach the level of the major sub-journals of "Nature", especially for the not-too-popular field of organic photovoltaics.

Among the two tests, exciton binding energy does not require the purchase of additional materials, so Xu Qiu started with this test first.

Exciton binding energy refers to the energy required for excitons to split into free electrons/holes after organic photoelectric materials generate excitons (bound electron/hole pairs), similar to the concept of chemical reaction activation energy .

For the traditional fullerene system, the donor material is the main light-absorbing material, and the exciton binding energy of the acceptor material is meaningless because it does not absorb light. The polymer donor material, such as the excitons of P3HT, PCE10 and other materials Binding energies are typically around 0.3 eV.

This is why in the traditional organic photovoltaic system, there must be a LUMO energy level difference of at least 0.3 electron volts between the donor material and the fullerene acceptor material, which is used to overcome the exciton binding energy of the donor material itself and ensure the generated The excitons can be split, which also makes the open circuit voltage of the traditional organic photovoltaic system inherently lower by about 0.3 volts.

This LUMO energy level difference of about 0.3 electron volts is also called "driving force".

For non-fullerene systems such as ITIC, the situation is different. Because the acceptor material absorbs light, the exciton binding energy is meaningful.

Moreover, the H43:IT-4F system of the former school girl found that when the HOMO energy level difference between H43 and IT-4F is 0.1 electron volts, it can also show efficient and fast charge splitting and transport.

This indicates that the ITIC non-fullerene system does not seem to require a "driving force" in the process of transporting charges.

Therefore, Xu Qiu guessed that the most likely reason for this phenomenon is that the exciton binding energy of the ITIC non-fullerene system is relatively low, within 0.3 electron volts.

After all, exciton splitting is a thermodynamic process, and the expression formula of exciton binding energy (Eb) is similar to the Arrhenius formula of activation energy, k=Aexp(-Eb/RT).

Under normal sunlight and normal temperature conditions:

Assuming that the exciton binding energy is 0.3 electron volts, about 90% of the generated excitons are in a bound state, and 10% are free electrons/holes. In this case, an additional energy level difference is required as a "driving force";

Assuming that the exciton binding energy is 0.1 electron volts, about 10% of the generated excitons are in a bound state, and 90% are free electrons/holes. In this case, most of the excitons have become free. Electrons/holes naturally do not need energy level difference as a "driving force".

If the latter is the case of the ITIC non-fullerene acceptor system, it can be theoretically explained why efficient and rapid charge splitting and transport can be performed without a large HOMO energy level difference.

Of course, before the test results come out, these are all speculations, and the specific results still need to be proved by experiments.

Practice is the only criterion for testing truth.

In the literature, low-temperature fluorescence luminescence (PL) testing is the most common method for testing exciton binding energy.

The specific operation is to test the PL intensity of the same sample at different temperatures, and then obtain the exciton binding energy through fitting.

In theory, high-temperature PL can also achieve a similar effect.

However, compared with the high temperature test, the low temperature test is more accurate, because the lower the temperature, the higher the PL intensity and the smaller the experimental error.

As for the method of obtaining low temperature, it is naturally cooled with liquid nitrogen.

Most cryogenic experiments use liquid nitrogen.

Because liquid nitrogen is so easy to obtain, it can be directly produced from the air, and the cost is very low, basically just paying for electricity.

At normal pressure, liquid nitrogen has a temperature of minus 196 degrees Celsius, or 77 Kelvin.

In the thermodynamic fitting calculation, the temperature unit used is Kelvin (K), where absolute zero is 0K, and 0 degrees Celsius is about 273K.

In actual operation, it is still difficult to lower the temperature to 77K with liquid nitrogen, but it is relatively easy to reach 100K or 150K, and then only need to slowly increase the temperature to about 200K or 250K.

After confirming the experimental method, Xu Qiu used an eight-pound bottle to make a pot of liquid nitrogen from the Handan campus, and brought it back to Laboratory 216.

Afterwards, he took out the low-temperature test device that Wei Xingsi brought back from the beautiful country.

The structure of this low-temperature test device is not complicated. The lower part is an airtight sample compartment, and the upper part is a liquid nitrogen compartment.

The sample cabin is surrounded by four quartz glass windows, and there is a sample stage with a heater and a thermocouple inside.

The heater is used to increase the temperature of the sample stage, and the thermocouple is used to detect the temperature of the sample stage in real time.

Samples can be placed directly on the sample table, or a sample holder similar to that used in EQE tests can be placed, and then external circuits are used for low-temperature electrical tests. Of course, this is just a PL test, so it doesn’t need to be so complicated, just put the samples directly That's it.

A valve is connected to the outside of the sample chamber, which can be vacuumed, and then during the test, the inside of the sample chamber is kept in a near-vacuum state.

The liquid nitrogen chamber above the sample chamber is mainly used to fill liquid nitrogen to provide a low temperature environment.

The liquid nitrogen chamber and the sample chamber are directly connected by metal for heat conduction.

During the test, because the sample chamber is a near-vacuum environment, the sample stage and the quartz glass are separated in space, and it is difficult for heat conduction to occur between them.

Therefore, there is almost no temperature difference between the inside and outside of the quartz glass window, and fogging does not occur.

In other words, once the quartz window fogs up, the vacuum must not be high enough.

Start the preparations.

First, Xu Qiu asked Mo Wenlin to spin-coat an ITIC sample on a glass substrate;

Then, stick the ITIC sample on the sample stage with copper foil tape;

Next, align the sample with a quartz window, tighten the sample chamber, connect the interface of the sample chamber with a mechanical pump, and start the vacuuming of "duang" and "duang".

After evacuating for about fifteen minutes, close the valve at the interface of the sample cabin to maintain the vacuum environment inside the sample cabin.

Theoretically, it is better to keep vacuuming continuously. After all, even if there is a valve, it cannot ensure that the vacuum degree will not drop all the time.

But in fact it is difficult to do, the main reason is that the test device is a PL instrument, and the optical instrument is very sensitive to the surrounding vibration. If the vacuum pump is placed on the test bench, the test results will definitely be inaccurate. If it is placed on the ground, connect the vacuum pump and the sample chamber. The metal pipe is not long enough.

Moreover, the mechanical pump is placed on the other side of the laboratory, and it is not convenient to move it.

After much deliberation, Xu Qiu still decided to start the test in this way first. If the quartz glass fogs up halfway through the test, it means that the degree of vacuum is probably insufficient, and he will try to solve it at that time.

Finally, turn on the external temperature controller, and the displayed real-time temperature is 294.22 Kelvin (K).

This temperature is quite reasonable. Although it is summer in August and the outside temperature in Shanghai is above 30 degrees Celsius, the air conditioner in the laboratory is always on and the room temperature is 21 degrees Celsius, which should be normal.

The preparations are complete and the experiment begins.

The first step is to fill with liquid nitrogen.

Xu Qiu took a plastic funnel and inserted it above the liquid nitrogen tank, then directly picked up the eight-pound bottle and began to pour the liquid nitrogen.

When liquid nitrogen encounters the "high temperature" of the outside world, it will continue to volatilize, and there will be a burst of splashing.

Xu Qiu's legs were spread wide, and he was in a Zama-step posture, mainly to prevent the liquid nitrogen from splashing on his body. Then, while pouring the liquid nitrogen, he listened carefully to the sound to judge whether the liquid nitrogen was filled.

This process is a bit like pouring boiling water into a thermos at home, judging whether it is full by listening to the sound, the water level is getting higher and higher, the air column is getting shorter and shorter, the vibration frequency is getting higher and higher, and the pitch is gradually getting higher.

Liquid nitrogen, water does not make much difference here, the main source of sound is air.

Not long after, the mouth of the funnel also began to slowly pour out white smoke. Combined with the tone of the voice, Xu Qiu judged that the liquid nitrogen was basically filled and stopped pouring.

After filling it up, Xu Qiu took off the funnel, and slowly inserted a stick from the original cryogenic device into the middle of the liquid nitrogen chamber.

This stick is hollow and capped, and there is a valve near the upper side to control whether the liquid nitrogen tank is connected to the outside world.

It can not only keep the liquid nitrogen container semi-closed to reduce the volatilization rate of liquid nitrogen, but also deliberately let the liquid nitrogen cabin leak to accelerate the volatilization of liquid nitrogen.

At first, the temperature hadn't dropped yet, so Xu Qiu naturally kept the valve half-closed.

It's impossible to fully seal it, because it's liquid nitrogen inside. If it's fully sealed, once the nitrogen volatilized inside can't escape, and the pressure is too high, it will explode directly.

At this point, the temperature indicated by the thermocouple is dropping sharply.

276.93K...

233.17K...

201.61K...

180.88K...

Ten minutes later, the temperature drop began to slow down, and the temperature of 0.1K and 0.2K gradually decreased.

When it reaches about 160K, the temperature drops more slowly, and the range of each decrease is 0.05, 0.08K.

Because energy is conserved, the temperature of liquid nitrogen itself is 77K, and the temperature of the external environment is 294K. The decrease in the temperature of the test chamber is essentially a heat exchange between it and liquid nitrogen. Therefore, if the temperature cannot drop now, it means that The liquid nitrogen has almost evaporated.

Xu Qiu poked it with a stick, and sure enough, the liquid nitrogen tank was almost empty.

So, he picked up the eight-pound bottle and filled up the liquid nitrogen tank again.

The temperature continues to decrease slowly at a rate of about 0.1K.

Ten minutes later, the temperature dropped to about 130K, and it couldn't go any lower, so Xu Qiu added liquid nitrogen again.

This time the temperature dropped to around 120K, and the rate of cooling became slow again. Xu Qiu judged that it should be approaching the limit.

The closer to the 77K temperature of liquid nitrogen itself, the slower the cooling, because it is difficult to completely isolate the heat conduction between the outside world and the sample stage.

Although he didn't reach the ideal 100K, Xu Qiu didn't continue to waste time, but chose to stop.

120K is also enough, as long as starting from 120K, every 5K is used as a sample point, and the temperature is raised to 200K, there are also 17 data points, which is not a small amount of data.