I Have a Research Support System

Next, Xu Qiu followed the previous strategy and arranged for simulation experimenters to synthesize more than a dozen new non-fullerene acceptor materials, including IDIC-4F, IDIC-M, and ITIC-Th-4F.

He himself systematically studied the IDIC system in the simulation laboratory I. Using 32 times acceleration, he performed tests on the FTAZ:IDIC system including CELIV, SCLC, GIWAXS, and DFT. After all, this system showed some differences before Exclusive 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 prove experimentally and theoretically that IDIC molecules will show a tighter molecular arrangement during crystallization.

After obtaining these characterization data, Xu Qiu can preliminarily explain the reason for the experimental phenomenon that "IDIC system can prepare thick film devices, while the energy loss is relatively low and the open circuit voltage is relatively high".

Because inside the active layer of the organic photovoltaic device, the positive and negative charges are transported in the separate aggregated phase region formed by the donor and the acceptor. When the crystal region of the acceptor material, the acceptor molecules present a more compact When the molecules are arranged, the transport of electrons between the molecules will be easier, thereby improving the electron mobility, thereby reducing energy loss, and thick film devices can be prepared.

Taking a step forward, the reason for the tighter molecular arrangement of IDIC molecules is the change of side chains. Compared with the phenyl side chains of ITIC, the alkyl side chains of IDIC have smaller steric hindrance. It is more conducive to stacking between two receptor molecules.

To sum up, there is a complete causal chain from the microscopic molecular structure to the macroscopic device performance:

Change of side chain → reduction of intermolecular hindrance → tight arrangement of acceptor molecules in the effective layer → increase of electron mobility → thick film devices can be prepared, energy loss is reduced, and open circuit voltage is increased.

Of course, this is just what Xu Qiu sorted out, and what he thinks is more reasonable is not necessarily completely correct.

After all, things at the micro level are invisible and intangible, and it is difficult for anyone to explain the truth.

This is also why people who engage in theories look at each other disliked, and will vent their anger at any time.

It is because there is no definite truth about many things, and the public says the public is right and the woman is right.

Unlike those who work on materials, they are simpler and more objective.

For example, my photoelectric conversion efficiency is 13%. If you don’t believe me, if you think my data is fake, then I will do a third-party test. The result is indeed 13%, and the debate will naturally subside.

Regarding the discovery of the IDIC system, Xu Qiu continued to analyze and deduce, 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 side chains 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 of the central D unit presents a dihedral angle of about 109 degrees 28 minutes.

That is, the two introduced side chains and the central D unit conjugated structure are not coplanar.

In this case, even for molecules with alkyl branches like IDIC, when two IDIC molecules are stacked in the vertical direction, it will actually form a larger steric hindrance, but it is more steric hindrance than ITIC. Just smaller.

But in any case, the side chain in the acceptor molecule is still necessary, otherwise the material will not be dissolved, and naturally it is impossible to prepare battery devices by the 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, which are also 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 atom also has only a single side chain, and because it is sp2 hybridized, the side chain It is coplanar with the conjugated structure of the central D unit, which can also greatly reduce the steric hindrance.

However, if these two strategies are adopted, it can be regarded as a major change to the D unit. The original IDT and IDTT synthesis ideas will definitely not work, and the difficulty of synthesis will increase greatly. It is a brand new structure that requires a lot of groping.

Xu Qiu intends to temporarily hand over this preliminary idea to Simulation Lab III, and let the senior experimenters help to explore it.

It can be regarded as a casual move, if it has an effect, it is naturally the best, even if it has no effect, it doesn't matter.

After the work of the current ITIC series is completed, he can devote his energy to tackling key problems.

In addition to this means of substantially modifying the molecular structure, Xu Qiu has another feasible strategy, which is expected to achieve a breakthrough in device efficiency.

That is to prepare tandem solar cell devices.

The so-called laminated device, as the name suggests, is a plurality of batteries connected in series and "stacked" together.

Usually, the devices prepared by Xu Qiu are all single-junction, that is, a battery. If the transport layer is ignored, then the structure is electrode/active layer/electrode.

If it is a double-junction laminated battery device, it 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)/effective layer 2/electrode, and the four-terminal structure is two "electrodes/ active layer/electrode".

If it is a three-junction laminated battery device, then there are three effective layers, and a four-junction is four effective layers.

Wujie, I haven't heard of it yet...

The National Renewable Energy Laboratory (NREL) in the beautiful country, which is the work unit of Wei Xingsi before returning to China, has maintained various world records in traditional inorganic silicon, gallium arsenide, CIGS and other systems all year round. 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 not out of commercial application considerations, but the main purpose is to explore the boundaries of science.

In other words, I just want to know how high the value of this photoelectric conversion efficiency can be piled up with the power of human beings.

In practical application, it doesn't make much sense.

On the one hand, the difference between 30% and 40% is not that big, only one-third of the difference, and it’s not like there is a 10-fold difference in efficiency from 1% to 11%;

On the other hand, for such a high-efficiency solar cell system, gallium arsenide is basically used. The cost of this thing is very high, and it can only be used in military or high-end applications, such as satellites and space stations. Even if the efficiency Even if it is optimized to 100%, it has no civilian value.

This stacked device is a series structure, but it is different from the series connection of ordinary dry batteries.

The laminated device is a whole in terms of spatial structure, whether it is "double-terminal" or "four-terminal", one 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 rest is mainly light with a wavelength greater than 600 nanometers, which will be absorbed by the bottom cell. The battery sucks again.

It sounds good, and it can avoid the problem of narrow light absorption range brought about by the exciton absorption characteristics of organic photovoltaic devices, and effectively use sunlight 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 is equivalent to 12.21% of single-junction devices. When Xu Qiu broke through the single-junction efficiency to nearly 13%, the performance of stacked devices has actually improved. Behind the single knot.

The main reason is that the PCBM fullerene derivative system that has been used for many years, the acceptor material can hardly absorb visible light, and can only rely on the donor material to absorb light.

If a narrow-bandgap donor system and a wide-bandgap donor system are used in consideration of light absorption complementarity, since the short-circuit current corresponding to the narrow-bandgap donor material is usually higher, there may be a current mismatch between the top cell and the bottom cell. 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, for example, one is 10 mA per square centimeter and the other is 6 mA per square centimeter, then the final displayed current will be around 6 mA per square centimeter.

For the first cell, there is an immediate loss of about 40% efficiency.

The problem of voltage is not so big. The batteries are approximately linearly superimposed. For example, one is 0.8 volts and the other is 0.7 volts, so in the end it will look like 1.4 and 1.5 volts.

In addition to the problem of short-circuit current, there are also problems in processing technology.

Most of the laminated devices reported in the existing literature in the field of organic photovoltaics have a double-junction and two-terminal structure. When preparing a laminated device, a charge recombination layer is required between the two cells, and a conductive electrode material is usually used.

And this layer of electrode must be transparent, because if it is not transparent, the lower battery will be useless, and there will be 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 and other methods.

For magnetron sputtering, on the one hand, the high temperature may damage the structure of the effective layer. On the other hand, a magnetron sputtering equipment generally costs 500,000 to 800,000 yuan, which is used to make an ITO electrode, which is a bit of a cannonball. Mosquito means that unless it is a large research group that cannot run out of funds, it will buy one for use.

Based on the current situation of ITO preparation difficulties, the mainstream idea is to use metal electrodes as the charge recombination layer, and the main problem to be solved is light transmission.

One of the methods is to use a thin metal electrode as a charge recombination layer, such as evaporating a few nanometers of silver, which can take into account both electrical conductivity and light transmittance.

Metals in daily life are opaque. The essence is that metal atoms absorb or reflect incident photons. On the scale of a few nanometers, even metals can transmit light. Of course, the light transmittance will not be too high. Probably around 50%.

The advantage of this method is that the preparation process is simple, and only the thickness of the evaporation electrode needs to be modified. The disadvantage is that the light transmission is not ideal.

Another method is to use silver nanowires, silver nanoparticles and other methods. The advantage is that the light transmission will be better, and the disadvantage is that the preparation process is more complicated.

To sum up, the idea of ​​​​preparing stacked devices is:

Using a series of newly developed ITIC derivative non-fullerene materials, two systems with light absorption and short-circuit current adaptation are found, and then the processing technology problems are solved, and finally a leap in device efficiency is achieved.

According to theoretical calculations, if all goes well, the efficiency is expected to reach more than 15%!

This is the way Xu Qiu tried to attack the top publications of CNS.

Of course, in practice, it must be divided into multiple steps, and it must be done gradually.

Because the process of stacked devices is very difficult, there are not many research groups that can do stacked devices in China. Xu Zhenghong is a main force, mainly foreigners are doing it.

Xu Qiu plans to start with translucent devices, which is a prerequisite for the preparation of stacked devices, such as the top cell of the four-terminal method, which is actually a translucent device.

Moreover, the preparation of semi-transparent devices has other additional benefits:

On the one hand, the near-infrared non-fullerene acceptors developed in the research group, such as FN-4F, IEICO-4F, and IEICO-4Cl, can be used to publish some articles;

On the other hand, this is also one of the bright spots in the future commercialization of organic photovoltaics. We can cooperate with Blue River, for example, to make flexible parts into various colors, and then stick them on the outside of buildings for decoration and power generation. .

PS: I owe more than 4W, and I have paid 3W8. I will probably be able to repay the owe more tomorrow... The average daily codeword this month is close to 8,000, give yourself a thumbs up.