Understanding how particles move through a device is critical to improving the efficiency of solar cells. KAUST researchers have worked with an international team of scientists to develop a series of design guidelines to improve the performance of molecular materials.
When a packet of light, or photon, is absorbed by a semiconductor, it creates a pair of particles known as an exciton. An electron is part of this pair; The other is the positively charged equivalent called a hole. Excitons are electrically neutral, so it is impossible to set them in motion by applying an electric field. Instead, the excitons “hop” through random movement or diffusion. Dissociation of excitons in charges is necessary to generate a current, but is highly unlikely in an organic semiconductor.
“Typically, we have to mix two semiconductors, a so-called electron donor and an electron acceptor, in order to efficiently generate free charges,” explains Yuliar Firdaus. “The donor and acceptor materials intrude. Maximizing the exciton diffusion length – the distance the exciton can travel before recombining and being lost – is critical to optimizing the performance of the organic solar cell.
Many earlier organic solar cells were made by mixing a polymer with molecules known as fullerenes. More recently, replacing the fullerene with other organic materials such as small molecules without fullerene has resulted in impressive improvements in device efficiency.
Firdaus and colleagues combined measurements of the photocurrent with ultrafast spectroscopy to calculate the diffusion length of a variety of non-fullerene molecules. They observed very long exciton diffusion lengths in the range of 20 to 47 nanometers – an improvement in the range of 5 to 10 nanometers characteristic of fullerenes.
To better understand this improvement, the team compared data describing the crystallographic structure of the molecules with quantum chemical calculations. In this way, they were able to identify key relationships between the chemical structure of the molecule and the diffusion length. Using these compounds, scientists developed a set of rules to aid in the synthesis of improved materials and ultimately to aid in the design of organic photovoltaic devices with improved conversion efficiency.
“Next, we want to investigate how film processing processes can affect the exciton transfer rate of the existing low-molecular acceptors,” says Firdaus. “We are also interested in translating the rules of molecular design to synthesize new acceptor materials with better performance.”
King Abdullah University of Science and Technology
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Trina Solar and Tongwei combine the 210 integrated industrial chain
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