20% increase in energy efficiency after retrofitting solar cells with coffee

In recent years, the perovskite solar cell industry has begun to rise, because monocrystalline silicon and polycrystalline silicon solar cells need to consume a large amount of power during the refining process, and the manufacturing cost is relatively high. Perovskite solar cells have photoelectric conversion close to that of single crystal silicon. Efficiency, but its preparation process is relatively simple, and the cost is relatively low. Therefore, perovskite solar cells have attracted widespread attention from the global academic and industrial circles and have developed rapidly.

In a paper just published in "Joule", a research team from the University of California, Los Angeles, School of Materials Science and Engineering, and Jinzhou Sunshine Energy Company unexpectedly found a way to improve the efficiency of perovskite solar cells from coffee. The corresponding author of this paper is Professor Yang Yang of the University of California, Los Angeles. The research team led by him has observed that the interaction of oxygen atoms in caffeine with lead ions in perovskite materials can significantly improve the thermal stability of perovskite solar cells. Increase the efficiency of solar cells from 17% to 20%, which makes it more likely that perovskite solar cells will replace crystalline silicon cells.

Humans can find a lot of caffeine in coffee and tea. The scientific name of caffeine is 1,3,7-trimethylxanthine. It can be seen from the molecular structure diagram that it contains three methyl groups. In the research led by Professor Yang Yang, it is not the methyl group in the caffeine molecule that plays a key role, but the oxygen atom in the caffeine molecule. These oxygen atoms and carbon atoms form a carbon-oxygen double bond.

We know that there are six electrons in the outermost layer of the oxygen atom. After the carbon-oxygen double bond is formed, four electrons are not paired, and the unpaired electrons in the caffeine oxygen atom can be combined with the lead ions in the perovskite to form a molecular lock.

Perovskite is another protagonist in this study. It is worth noting that there is no calcium or titanium in the perovskite used in this experiment.

Perovskite material is named after Russian mineralogist Lev Perovski. The earliest discovered perovskite materials are composite oxides of calcium and titanium. But later, the concept of perovskite has been greatly extended. It does not specifically refer to perovskite composite oxides, but is used to refer to a series of compounds with the chemical formula ABX3. Here A can be organic such as methylamino. Molecular groups, while B can be a lead atom (or a tin atom), and X generally contains a halogen atom.

In the field of solar cells, organic-inorganic composite perovskites are generally used. Perovskite is generally used as an absorption layer for solar cells. After being irradiated by sunlight, the perovskite will generate electron-hole pairs after absorbing photons. Electrons are negatively charged, while holes can be viewed as positively charged. These electron-hole pairs are separated and become carriers in the solar cell, which flow to the positive and negative electrodes, respectively, so that a photocurrent is formed. Therefore, the physical principle of solar cells is actually the photoelectric effect proposed by Einstein.

A nuclear reaction is taking place on the sun at all times. The sunlight generated by the nuclear reaction illuminates the earth, bringing 1,000 watts of solar radiation power per square meter of ground. The energy of these sunlight can be directly used on the earth to generate electricity, which has created the solar cell industry.

Solar cells are generally stacked with many layers of materials, and the layer that plays the role of light absorption is called an absorption layer.

Solar cells are also named according to the material characteristics of the absorption layer. For example, the absorption layer of a crystalline silicon solar cell is monocrystalline or polycrystalline silicon; the absorption layer of a thin-film solar cell is generally a thin film material with a thickness of several microns; The absorption layer is perovskite.

Among these various types of solar cells, single crystal silicon technology is very mature, because humans have been working on silicon wafers for 60 years, pushing this material to the extreme. However, when refining monocrystalline silicon, a high temperature is required, so the refining of monocrystalline silicon itself is a high energy-consuming industry.

The world record for the photoelectric conversion efficiency of single crystal silicon is 26%, while the world record for the photoelectric conversion efficiency of perovskite is about 24%, with little difference between the two.

But perovskite has unique advantages. Perovskite materials have inherently good optoelectronic properties: Compared to single crystal silicon with an indirect band gap, it is a direct band gap, so the fluorescence efficiency of perovskite is particularly high.

Unfortunately, the area of ​​perovskite batteries that can be realized at present is very small, while the area of ​​single crystal silicon is very large. So from the perspective of photoelectric conversion efficiency, the two are between Pak Chung; but in terms of area, single crystal silicon is still ahead of perovskite. Another disadvantage of perovskite solar cells is that their stability is not good enough. If the stability of perovskite can be improved and its life span mentioned to 20 years, then perovskite is very likely to replace single crystal silicon.

Inspiration from coffee

Yang Yang said in an interview with "Global Science": "I believe that in the near future, perhaps in two or three years, perovskite should catch up with single crystal silicon. The main problem now is that after the area of ​​perovskite batteries is enlarged, , Its photoelectric conversion efficiency will drop. After we take the products of the academic world to the industrial world, sometimes it may not be as good as ideal. So this is a difference between the academic world and the industrial world. "

Professor Yang Yang's research group has been engaged in the research of perovskite solar cells. Yang Yang has always been interested in materials that "generate electricity" or "light-related", partly because he worked in the company of American scientist Professor A. Heeger when he first graduated with his Ph.D. Yang Yang has worked with Allen Haig for more than four years. At first, he began to do conductive polymer materials, and later began to do polymer OLEDs. This is another branch of organic LED. Organic LED was successfully industrialized later, and made into OLED panel, which has many applications on smart phones. Ellen Haig was the 2000 Nobel Prize winner in chemistry for his pioneering contributions in the field of conductive polymers.

One morning, two doctoral students in Yang Yang's research group discussed coffee perovskite while drinking coffee. Wang Rui said, "We all need coffee to refresh ourselves, but what about perovskites? Maybe they also need coffee to perform better?"

Wang Rui's inadvertent sentence reminded Xue Jingjing that caffeine is a common alkaloid, and the unpaired electrons in it can interact with the lead ions in the perovskite material. The carbonyl group on the caffeine molecule can form a molecular lock with the lead ions of perovskite. This can increase the energy barrier required for perovskite decomposition, thereby allowing the perovskite to stabilize. At the same time, such molecular locks can reduce the nucleation rate of perovskite crystals, obtain higher quality perovskite polycrystalline films, and make the grains of perovskite more oriented, thereby improving carrier transport Efficiency, which can improve the photoelectric conversion efficiency of perovskite solar cells.

Increase output power

Yang Yang's research team used a heated method to add caffeine to the perovskite layer of 40 solar cells, and used infrared absorption spectroscopy to determine if the caffeine was successfully combined with the perovskite. They found that the characteristic peak of the carbonyl group in caffeine shifted after it was combined with perovskite, which means that caffeine has successfully combined with perovskite.

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