So far, there has been no major commercial breakthrough in perovskite solar cells. Lithuanian scientists want to change this. They are based on a new, innovative polymer.
Organic-inorganic hybrid perovskite solar cells are considered to be a promising solution in photovoltaics and a possible alternative to expensive silicon-based cells. They are cheaper to manufacture, more flexible to use and have higher energy conversion efficiency. Their main disadvantage is their poor stability. While silicon solar cells hardly lose their efficiency for 20 years or more, the perovskite cell curve breaks down after weeks or months. Numerous working groups have dealt with the issue of perovskite stabilization – with varying degrees of success.
A group of chemists from the Kaunas University of Technology in Lithuania who have been studying solar energy for a long time are now reporting innovative approaches. Scientists synthesized a new class of carbazole-based cross-linking materials that are resistant to various environmental influences, including the powerful solvents used in solar cell manufacturing. Carbazoles are ring structures made of carbon and nitrogen, hence the name. They can be made quite cheaply.
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The physics behind it: Next-generation stacked solar cells can have two architectural structures, namely regular (clamp) and inverted (pins) systems. With the latter, the hole transporting materials are deposited beneath the perovskite absorber layer. A semiconductor doped with the rates of electron donors is called an n-type semiconductor; here the electrons conduct the current. In the case of p-semiconductors, atoms were added as electron acceptors; here the voids (“holes”) are ladders. In layer i the level of doping is low.
New materials developed in the laboratories helped the perovskite cells with inverted architecture to achieve an initial efficiency of 16.9% when used as hole transporting layers. Scientists expect that optimization can achieve higher levels of performance.
But the road was rocky. “While pin cells have many advantages over regular architecture perovskite solar cells, they also have serious disadvantages,” explains Vytautas Getautis, professor at KTU’s Department of Chemical Technology. “Therefore, the hole transporting compounds must be able to withstand the strong polar solvents used to form the light-absorbing layer of the perovskite on top.”
The new polymer solves a number of difficulties
To solve this problem, polymers are often used as hole transport materials in pin architectures. However, it is not easy to form a polymeric layer therefrom due to solubility problems. The challenge is also to reproduce the reaction in order to synthesize the same structure. Therefore, KTU scientists produced a hole transporting layer from carbazole-based molecules, which was then thermally polymerized in situ to achieve a cross-linking effect.
The cross-linked polymer has a three-dimensional structure. It is highly resistant to various influences, including strong solvents used to form the light-absorbing layer of the perovskite. “We used several groups of molecules and developed materials that, when used as a hole transporting layer, can improve the efficiency of the inverse perovskite solar cell to almost 17%,” says Šarūnė Daškevičiūtė-Gegužienė from Kaunas University of Technology.
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What’s next? The research group synthesized many new compounds. In any event, Getautis expects that the new solar cells – as soon as they are ready for market launch – can provide an inexpensive alternative to silicon cells.
“Our research area aims to improve existing perovskite solar device technologies and in this area we achieved the best results with the self-assembled monolayer technology. But science often moves in many directions because we need to find ways to make the best use of solar energy, ”says Getautis.
While perovskite cells are relatively new to silicon-based solar technologies, there are several companies that have already marketed various products based on perovskite technology. They produce flexible, translucent interior elements, portable modules or various architectural solutions. And this is just the beginning.
“Solar energy is completely environmentally friendly – it is non-polluting and the installed solar systems require little maintenance,” says Getautis. “In the light of current events and the energy crisis, more and more people are interested in installing solar power plants in their homes or in obtaining shares in a solar farm. This is the future of energy. ” He believes that by 2050 about half of the electricity used on Earth will be generated from solar energy – and by then perovskite solar cells will play a major role.
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