Researchers from Paderborn University have developed a new design for solar cells that can harness more energy from sunlight than ever before. The key to their innovation is a thin layer of organic material called tetracene that enhances the efficiency of silicon solar cells. Their findings have been published in the journal Physical Review Letters.
A new design for solar cells
According to Prof Dr. Wolf Gero Schmidt, physicist and Dean of the Faculty of Natural Sciences at Paderborn University, solar energy is a huge and largely untapped clean and renewable energy source. He says, “The annual energy of solar radiation on Earth amounts to over one trillion kilowatt-hours and thus exceeds the global energy demand by more than 5,000 times. Photovoltaics, i.e., the generation of electricity from sunlight, therefore offers a large and still largely untapped potential for the supply of clean and renewable energy.”
However, he adds, the silicon solar cells currently used to convert sunlight into electricity have some limitations. One of them is that they waste some of the energy from short-wave radiation as heat instead of using it for electricity.
The Role of Tetracene
To overcome the problem of energy wastage, Schmidt and his team have devised a way to add an organic layer made from the semiconductor tetracene to the silicon solar cell. This layer absorbs the short-wave light and transforms it into high-energy electronic excitations, called excitons. These excitons then split into two low-energy excitations in the tetracene layer. If these excitations can be transferred to the silicon solar cell, they can be converted into electricity more efficiently and increase the overall output of usable energy.
The secret of rapid energy transfer
However, transferring excitations from tetracene to silicon is a complex process. It requires complex computer simulations to understand and optimize. Schmidt and his team have used the Paderborn Center for Parallel Computing (PC2), the university’s high-performance computing center, to conduct their simulations.
They have discovered that the key to speeding up the energy transfer is the presence of special defects at the interface between the tetracene film and the solar cell. These defects are unsaturated chemical bonds that occur when hydrogen atoms are removed from the surface. These defects create electronic interface states that have varying energy levels. These energy fluctuations act like a lift that transports the excitations from the tetracene to the silicon.
Interestingly, these defects are usually considered a source of energy loss in solar cells. But in this case, they are essential for the rapid energy transfer. Schmidt says, “The results of our computer simulations are truly surprising. They also provide precise indications for the design of a new type of solar cell with significantly increased efficiency.”
Study abstract:
Exciton transfers are ubiquitous and extremely important processes, but often poorly understood. A recent example is the triplet exciton transfer in tetracene sensitized silicon solar cells exploited for harvesting high-energy photons. The present ab initio molecular dynamics calculations for tetracene-Si(111):H interfaces show that Si dangling bonds, intuitively expected to hinder the exciton transfer, actually foster it. This suggests that defects and structural imperfections at interfaces may be exploited for excitation transfer.
