Chapter 349: Ways to impact the top issue of CNS

Next, Xu Qiu followed the previous strategy and arranged for simulation experimenters to synthesize more than a dozen new non-fullerene receptor materials including IDIC-4F, IDIC-M, ITIC-Th-4F, etc.
He himself studied the IDIC system systematically in the simulation laboratory I, and used 32 times acceleration to test and characterize the FTAZ:IDIC system, including CELIV, SCLC, GIWAXS, DFT, etc. After all, this system showed some differences before Unique features of other ITIC series.
Among them, the results of CELIV and SCLC show that the electron mobility of IDIC is about 3 times that of ITIC.
The results of GIWAXS and DFT respectively proved experimentally and theoretically that IDIC molecules exhibit a tighter molecular arrangement when they are crystallized.
After obtaining these characterization data, Xu Qiu can initially explain the reason for the experimental phenomenon that "IDIC system can produce thick film devices, while the energy loss is relatively low, and the open circuit voltage is relatively high."
Because in the effective layer of the organic photovoltaic device, the positive and negative charges are transported in the separate aggregation phase area formed by the donor and the acceptor. When the crystal area of ​​the acceptor material, the acceptor molecules appear more compact. When the molecules are arranged, the transport of electrons between molecules will be easier, thereby increasing the electron mobility, thereby reducing energy loss, and making thick film devices possible.
Going forward, the reason for the tighter molecular arrangement of IDIC molecules is the change in the side chain. Compared with the phenyl side chain of ITIC, the alkyl side chain of IDIC has a smaller steric hindrance. It is more conducive to stacking between two receptor molecules.
In summary, there is a complete causal chain from microscopic molecular structure to macroscopic device performance:
Side chain changes→intermolecular steric hindrance is reduced→acceptor molecule arrangement in the effective layer becomes tight→electron mobility is improved→thick film devices can be prepared, energy loss is reduced, and open circuit voltage is increased.
Of course, this was only sorted out by Xu Qiu, and a set of statements that he felt was more reasonable might not be completely correct.
After all, things at the micro level are invisible and intangible. It is difficult for anyone to explain the truth.
This is why people who engage in theories will look at each other unpleasantly and will spray at any time.
It is because many things do not have an exact truth.
Unlike those who work with materials, they are relatively simple, crude and objective.
For example, my photoelectric conversion efficiency is 13%. If you don’t believe me, if you think my data is false, then I will do a third-party test. As a result, it is indeed 13%, and the controversy has naturally subsided.
In response to the discovery of the IDIC system, Xu Qiu continued to analyze and deduced, the main direction is how to further increase the electron mobility and further reduce the energy loss.
He found that whether it is ITIC or IDIC, the central D unit is an IDTT unit. In order to ensure the solubility of the molecule, the way to introduce the side chain is through sp3 hybridized carbon atoms.
The sp3 hybridized carbon atom, similar to the structure of methane, will extend two branches in space, and the conjugated structure with the central D unit presents a dihedral angle of about 109 degrees and 28 minutes.
That is, the two introduced side chains and the central D unit conjugate structure are not coplanar.
In this case, even if it is a molecule with alkyl branches like IDIC, when two IDIC molecules are stacked in the vertical direction, they will actually form a larger steric hindrance, but it is more steric than the ITIC. It's smaller.
But in any case, the side chain in the acceptor molecule is still necessary, otherwise the material cannot be dissolved, and it is naturally impossible to prepare battery devices by solution method.
To solve this problem, Xu Qiu temporarily thought of two methods, the main idea is to change the method of introducing side chains.
One method is to use nitrogen atoms to introduce side chains, both of which are sp3 hybridized nitrogen atoms. Because of the existence of lone pairs of electrons, there is only a single side chain, which can reduce steric hindrance to a certain extent.
Another method is to use sp2 hybridized carbon atoms, that is, carbon atoms on the benzene ring or thiophene ring. In this case, the carbon atoms also have only a single side chain, and because it is sp2 hybridized, the side chain It is coplanar with the conjugate structure of the central D unit, which can also greatly reduce steric hindrance.
However, if these two strategies are adopted, it can be regarded as a substantial change to the D unit. The original IDT and IDTT synthesis ideas are definitely useless, the synthesis difficulty will be greatly increased, and the new structure requires a lot of Groping.
Xu Qiu intends to temporarily transfer this preliminary idea to Simulation Lab III, and let the senior experimenters help to explore it.
It's a game of chess, and if it works, it's the best. Even if it doesn't work, it doesn't hurt.
After the work of the ITIC series is completed, he can devote his energy to tackling key problems.
In addition to this method of greatly modifying the molecular structure, Xu Qiu has another feasible strategy that is expected to achieve a breakthrough in device efficiency.
That is to prepare laminated solar cell devices.
The so-called stacked device, as the name implies, is that multiple batteries are connected in series and "stacked" together.
Usually, the devices prepared by Xu Qiu are single junction, which is a battery. If the transmission layer is ignored, the structure is electrode/effective layer/electrode.
If the double-junction laminated battery device is divided into a double-terminal structure and a four-terminal structure, the double-terminal structure is electrode/active layer 1/electrode (charge recombination layer)/active layer 2/electrode, and the four-terminal structure is two "electrodes/ Effective layer/electrode".
If it is a three-junction laminated battery device, there are three effective layers, and a four-junction is four effective layers.
Wujie, I haven't heard of it...
The National Renewable Energy Laboratory (NREL) in the beautiful country, that is, Wei Xingsi's work unit before returning to China, has maintained various world records in traditional inorganic silicon, gallium arsenide, CIGS and other systems throughout the year. According to the data in August 2015 , The highest efficiency of the three-junction device has reached 44.4%, and the four-junction device has reached 46.0%.
Of course, making the efficiency so high is no longer out of commercial application considerations. The main purpose is to explore the boundaries of science.
In other words, I want to know how high the value of this photoelectric conversion efficiency can be stacked with human power.
In practical applications, it doesn't make much sense.
On the one hand, the difference between 30% and 40% is not that big, only one-third away, and unlike the efficiency increase from 1% to 11%, there is a 10 times difference;
On the other hand, for solar cell systems with such a high efficiency, gallium arsenide is basically used. The cost of this thing is very high. It can only be used in military or high-end applications, such as satellites and space stations. Optimized to 100%, there is no civilian value.
This laminated device is a series connection structure, but it is different from the series connection of ordinary dry batteries.
The laminated device is a whole in the space structure, whether it is a "dual terminal" or a "four terminal", a device only absorbs one unit of sunlight.
For example, for a double-junction organic solar cell stack device, when sunlight is incident, it first passes through the top cell and absorbs light of 300-600 nanometers, and then the remaining light is mainly light with a wavelength greater than 600 nanometers, which will be The battery absorbs again.
It sounds very good, it can avoid the problem of narrow light absorption range caused by the exciton absorption characteristics of organic photovoltaic devices, and effectively use solar energy.
But in fact, the performance of stacked devices in the field of organic photovoltaics is not satisfactory.
At present, the efficiency of pure organic photovoltaic stacked devices is only about 12%, which was originally equivalent to 12.21% of single junction devices. When Xu Qiu broke the single junction efficiency to nearly 13%, the performance of stacked devices has actually been Behind the single settlement.
Mainly because of the PCBM fullerene derivative system used all the year round, the acceptor material can hardly absorb visible light and can only rely on the donor material to absorb light.
If taking into account the complementary light absorption, a narrow band gap donor system and a wide band gap donor system are used, since the short-circuit current corresponding to the narrow band gap donor material is usually higher, the current mismatch between the top cell and the bottom cell may occur The problem.
Because the devices are connected in series, according to middle school physics knowledge, the current in the series circuit is equal everywhere.
If the short-circuit current density difference between the upper and lower battery devices is too large, such as one 10 mA per square centimeter and the other 6 mA per square centimeter, the final current shown will be about 6 mA per square centimeter.
For the first battery, about 40% efficiency will be directly lost.
The voltage problem is not big. The batteries are approximately linearly superimposed. For example, one is 0.8 volts and the other is 0.7 volts, so the final result is 1.4, 1.5 volts.
In addition to short-circuit current problems, there are also processing problems.
Most of the laminated devices reported in the current literature in the field of organic photovoltaics are double-junction two-terminal structures. When the laminated devices are prepared, a charge recombination layer is required between the two cells, and conductive electrode materials are usually used.
And this layer of electrodes must be light-transmissive, because if it does not transmit light, the underlying battery is useless and there is no light to absorb.
Light-transmitting electrodes, such as ITO, cannot be prepared by the solution method, and can only be prepared by magnetron sputtering.
In the case of magnetron sputtering, on the one hand, the temperature is high, which may damage the structure of the effective layer. On the other hand, a magnetron sputtering device generally requires 500,000-800,000, which is used to make an ITO electrode, which is a bit of a cannon. The meaning of mosquitoes, unless it is a large research group that can't spend enough money, one will buy one for use.
Based on the difficulty of ITO preparation, the mainstream idea is to use metal electrodes as the charge recombination layer, and the main problem to be solved is light transmittance.
In one method, a thin metal electrode can be used as the charge recombination layer, such as evaporating a few nanometers of silver, which can give consideration to both conductivity and light transmittance.
Metals in daily life do not transmit light. The essence is that metal atoms absorb or reflect incident photons. At the scale of a few nanometers, even metals can transmit light. Of course, the light transmittance is not too high. It may be around 50%.
The advantage of this method is that the preparation process is simple, and only the thickness of the vapor-deposited electrode needs to be modified. The disadvantage is that the light transmittance is not ideal.
Another method is to use silver nanowires, silver nanoparticles, etc. The advantage is that the light transmittance will be better, but the disadvantage is that the preparation process is more complicated.
To sum up, the idea of ​​preparing laminated devices is:
Using a series of newly developed non-fullerene materials of ITIC derivatives, two systems of light absorption and short-circuit current adaptation are found, and then the problems in the processing technology are solved, and the device efficiency is finally achieved.
According to theoretical calculations, if all goes well, the efficiency is expected to reach more than 15%!
This is how Xu Qiu tried to hit the top CNS issue.
Of course, UU reading www.uukanshu.com actually needs to be divided into multiple steps, slowly.
Because the process of laminated devices is very difficult, there are not many research groups that can do laminated devices in China. Xu Zhenghong is regarded as a main force, mainly foreigners.
Xu Qiu intends to start with semi-transparent devices. This is considered a precondition for the preparation of laminated devices. For example, the top cell of the four-terminal method is actually a semi-transparent device.
Moreover, the preparation of translucent devices has other additional benefits:
On the one hand, the near-infrared non-fullerene receptors developed in the research group, such as FN-4F, -4F, and -4Cl, can be used to publish some articles;
On the other hand, this is also one of the highlights of the future commercialization of organic photovoltaics. It can cooperate with Blue River, such as making flexible parts in various colors, and then sticking them on the outside of buildings to decorate and generate electricity.
PS: 4W owed, 3W8 already paid, I can probably pay back the owed tomorrow... This month, the daily average codeword is close to 8,000. Give yourself a thumbs up.
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