Chapter 356: Low temperature experiment

Xu Qiu's recent target system is J2: IDIC-4F, and he has nothing to do before these two materials have been synthesized.
Of course, it is impossible to paddle. Xu Qiu looked through the literature on partial theoretical research collected before and compiled the test methods of exciton binding energy and exciton diffusion distance.
He plans to test the water on ITIC first, and then it can be directly applied to the IDIC-4F system simultaneously.
These two experiments are still relatively important. They are one of the cards he used to sprint "Nature·Energy" or "Joule".
If the final result is the same as he expected, and if these two conclusions are disassembled, it is estimated that sending an NC or AM shouldn't be a big problem.
But now it is necessary to combine two important conclusions, plus the 13.5% efficiency J2:IDIC-4F system, to sprint a "Nature·Energy" or "Joule".
There is no way, it is so difficult to break through the AM level and reach the level of "Nature", especially for the organic photovoltaic field that is not too popular.
In the two tests, the exciton binding energy does not require additional purchase of materials, and Xu Qiu started with this test first.
Exciton binding energy refers to the energy required to split the excitons into free electrons/holes after the organic optoelectronic materials produce excitons (trapped electron/hole pairs), similar to the concept of the activation energy of chemical reactions .
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, polymer donor materials, such as P3HT, PCE10 and other materials excitons The binding energy is usually around 0.3 electron volts.
This is why in traditional organic photovoltaic systems, there must be at least 0.3 eV LUMO energy level difference 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 production Excitons can be split, which also makes the open circuit voltage of traditional organic photovoltaic systems inherently less 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 previous 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 shows that the ITIC non-fullerene system does not seem to need a "driving force" in the process of charge transfer.
Therefore, Xu Qiu speculated 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. The expression formula of exciton binding energy (Eb) is similar to the Arrhenius formula of activation energy, k=Aexp(-Eb/RT).
Under normal sun illuminance and normal temperature conditions:
Assuming that the exciton binding energy is 0.3 electron volts, about 90% of the excitons generated 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 excitons generated 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 case of the ITIC non-fullerene acceptor system is the latter, it can also be explained theoretically, why it is possible to carry out efficient and fast charge splitting and transport without a large HOMO energy level difference.
Of course, these are all guesses before the test results come out, and the specific results have to be proved through experiments.
Practice is the only criterion for testing truth.
In the literature, the low-temperature fluorescence luminescence (PL) test is the most common method to test the 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 to 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 low-temperature experiments use liquid nitrogen.
Because liquid nitrogen is so easy to obtain, it can be obtained directly from the air, and the cost is very low. Basically, it pays for electricity.
Under normal pressure, the temperature of liquid nitrogen is minus 196 degrees Celsius, which is 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 difficult to use liquid nitrogen to lower the temperature to 77K. However, it is relatively easy to reach 100K or 150K. After that, the temperature can be slowly raised to about 200K or 250K.
After determining the experimental method, Xu Qiu used an eight-pound bottle to make a pot of liquid nitrogen on the Handan campus and brought it back to the 216 laboratory.
Later, he took out the low temperature test device Wei Xingsi had brought back from Beautiful Country.
The structure of this low-temperature test device is not complicated. The lower part is a closed sample chamber and the upper part is a liquid nitrogen chamber.
The sample compartment is surrounded by four quartz glass windows, and there is a sample stage with heater and 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 sample holders similar to those used in EQE testing can be placed, and then an external circuit can be used for low-temperature electrical testing. Of course, this is just a PL test, so there is no need to be so complicated, just put the sample directly That's it.
A valve is connected to the outside of the sample chamber to vacuum, and then during the test, the sample chamber is kept in a near vacuum state.
The liquid nitrogen chamber above the sample chamber is mainly used for filling liquid nitrogen to provide a low temperature environment.
The liquid nitrogen chamber and the sample chamber are directly connected by metal for heat transfer.
During the test, because the sample chamber is in an approximately vacuum environment, the sample stage and the quartz glass are separated in space, and it is difficult to conduct heat between them.
Therefore, there is almost no temperature difference between the inside and outside of the quartz glass window, and no fogging occurs.
In other words, once the quartz window is fogged, it must be that the vacuum is not high enough.
Start preparations.
First, Xu Qiu asked Mo Wenlin to spin-coat a piece of ITIC sample on the glass substrate;
Then, stick the ITIC sample on the sample stage with copper foil tape;
Next, align the sample with a quartz window, screw the sample chamber tightly, connect the interface of the sample chamber with a mechanical pump, and start vacuuming "duang" and "duang".
After pumping the vacuum for about 15 minutes, close the valve at the interface of the sample chamber to maintain the vacuum environment inside the sample chamber.
In theory, it is better to keep pumping vacuum continuously. After all, even if there are valves, there is no guarantee that the vacuum will not drop.
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 surrounding vibration. If the vacuum pump is placed on the test bench, the test result 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 inconvenient to move around.
After thinking about it, Xu Qiu decided to test it like this first. If the quartz glass is fogged up halfway through the test, it means that the vacuum is mostly insufficient. Then we will find a solution.
Finally, turn on the external temperature controller and display the real-time temperature as 294.22 Kelvin (K).
This temperature is reasonable. Although it is the summer of August and the external temperature of the magic city 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 irrigate liquid nitrogen.
Xu Qiu took a plastic funnel and inserted it above the liquid nitrogen tank, then directly picked up an eight-pound bottle and began to pour the liquid nitrogen.
When the liquid nitrogen encounters the "high temperature" outside, it constantly volatilizes and splashes.
Xu Qiu opened her legs and assumed a posture of zama step, mainly to prevent liquid nitrogen from splashing on her body, and then listened carefully to the sound while pouring the liquid nitrogen to determine whether the liquid nitrogen was full.
This process is a bit like pouring water into a kettle in a normal home, and judging dissatisfaction by listening to the sound. As the water level becomes higher and higher, the air column becomes shorter and shorter, the vibration frequency becomes higher and higher, and the pitch becomes higher gradually.
There is not much difference between liquid nitrogen and water here. The main source of sound is air.
Not long after, the mouth of the funnel began to slowly pour out white smoke. Combined with the tone of the voice, Xu Qiu judged that the liquid nitrogen was basically full and stopped dumping.
After it was full, Xu Qiu removed the funnel and slowly inserted a stick from the original cryogenic device in the middle of the liquid nitrogen tank.
The stick is hollow and capped, and there is a valve on the side near the upper part to control whether the liquid nitrogen tank is connected to the outside.
It can keep the liquid nitrogen container semi-closed to reduce the volatilization rate of liquid nitrogen; it can also deliberately let the liquid nitrogen tank leak to accelerate the volatilization of liquid nitrogen.
In the beginning, the temperature had not yet dropped, Xu Qiu naturally kept the valve half-closed.
It is impossible to be completely enclosed. It is liquid nitrogen inside. If it is completely enclosed, once the nitrogen volatilized inside cannot escape and the pressure is too high, it will explode.
At this time, the temperature displayed by the thermocouple is dropping sharply.
276.93K...
233.17K...
201.61K...
180.88K...
Ten minutes later, the range of temperature drop began to slow down, slowly decreasing at 0.1K and 0.2K.
When it reaches about 160K, the temperature drops more slowly, and the range of each decrease is 0.05 and 0.08K.
Because the 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 the heat exchange between it and the liquid nitrogen, so the temperature cannot fall, which means The liquid nitrogen has almost evaporated.
Xu Qiu stabbed it with a stick. As expected, the liquid nitrogen tank was almost empty.
So he picked up an eight-pound bottle and filled 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 down again. Xu Qiu added liquid nitrogen again.
This time the temperature dropped to about 120K, and the cooling rate became slow again. Xu Qiu judged that it should be reaching the limit.
The closer to the 77K temperature of liquid nitrogen itself, the slower the temperature drop, because it is difficult to completely isolate the heat conduction between the outside world and the sample stage.
Although he did not reach the 100K he had dreamed of, Xu Qiu did not continue to waste time, but chose to stop.
120K is also enough. As long as it starts from 120K, every 5K is used as a sample point, and the temperature rises to 200K. There are also 17 data points. This amount of data is not small.
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