New crystal materials could double the solar cell's efficiency

Although it has been promoted for a long time to replace fossil fuels, the contribution of solar power is still very modest. One of the reasons for the efficiency of solar panels is still very low.

A new material has just been published by researchers at Purdue University and the National Renewable Energy Laboratory (NREL) to show that it is capable of producing twice as much electricity than silicon, the material. The main is being used to make solar panels today.

Limitations of silicon Solar batteries

The silicon solar cell has a maximum efficiency of only 33%.

The performance of traditional solar cells currently reaches a maximum of 1/3, which scientists call the Shockley-Queisser Limit. The cause of this phenomenon lies in the physical nature of silicon - the energy gap (band gap) of electrons in atoms. Basically, the principle of solar power is that the valence electrons must be excited into free electrons to produce a directional charge current (ie, current). This stimulation is caused by photons in the sun's rays.

Since the silicon band gap is so large, there are very few chemotherapy electrons that get enough energy from photons to turn into free electrons. Besides, for silicon, these free electrons have very short lifetime (only about 1 picosecond - or 10-12 seconds) and the distance they travel is only 10 nanometers long. After going all the way up, free electrons lose all their energy due to the photons (from the Sun). The result is very low efficiency of silicon solar cells because most of the received solar energy is converted into thermal energy.

Perovskite crystal material solution

In order to overcome this situation, Libai Huang, assistant professor of chemistry at Purdue, and his colleagues developed a new technique based on microscopes and fast lasers, to track the distance traveled. and the speed of free electrons carrying energy in the lattice. Thanks to the above technique, they recorded the characteristics of many different materials. Finally, what the scientists found was a temporary material named "perovskite hybrid" (hybrid perovskite). This material is made from lead (Pb), Iodine (I) and methyl-ammonium (CH ₃ NH ₃ ).

Hybrid crystal structure of perovskite hybrid.

A special feature of the material is that it has a crystal structure similar to perovskite (CaTiO ₃ ), a compound with a special structure consisting of "cages" surrounding free atoms in the center. With hybrid perovskite, the Pb and I atoms form a "cage" that encapsulates the CH ₃ NH ₃ cluster in the middle. It is this perovskite structure that allows free electrons to travel longer distances and survive longer before losing all energy.

Ms. Huang described in more detail: "The distance that free electrons need to travel must be at least equal to the thickness of the solar panel (to generate electricity), about 200 nanometers. This is the number of materials that "Perovskite can be achieved. In addition, these electrons can survive up to 100 picoseconds, 100 times longer than silicon."

Fast laser imaging shows the lifetime of free electrons in the lattice.

Kai Zhu, the NREL report co-author, is excited with the result: "This study shows that free electrons carrying energy in a standard perovskite crystalline thin film can move a longer distance or by the thickness of the battery, which is essential for making an efficient solar cell, this information also indicates that the potential for developing solar cells applies structure. perovskite is very good ".

However, one disadvantage of this material is that it uses Pb, an environmentally hazardous element. Now Purdue researchers are trying to develop a material with a similar perovskite structure but without Pb. And the final detail is the finishing of the product. Mr. Zhu summed up: "The next step is to find or develop a material or structure suitable for adequate energy levels to extract these free electrons in order to generate electricity in secondary circuits. This will not be simple. "