As the solar energy industry booms in the United States, Yale researchers have found a way to modify a mineral structure that could produce more efficient solar cells.
A group of researchers, working in the Transformative Materials and Devices lab at the Yale School of Engineering and Applied Science, have discovered a perovskite-structured compound — based on the perovskite mineral — that can increase the stability and efficiency of perovskite power cells. Their study was published in the journal Nanoscale last March.
“There were two goals for the study,” said Yifan Zheng GRD ’17, one of the study’s authors. “The first is to make the electron hole transport layer more conductive. The second is to better the morphology of the perovskite film.”
Perovskite solar cells contain a unique perovskite-structured compound composed of metal and halide that allows it to function as a semiconductor of light, according to Jaemin Kong, a postdoctoral associate and one of the study’s authors. The perovskite absorber has an optical bandgap of 1.5 to 2.3 electron-volts, which allows it to effectively absorb the sun’s visible light. Perovskite-structured compounds presents an attractive option for incorporation into solar cells, as the material’s properties can increase a cell’s power conversion efficiencies.
Power conversion efficiency refers to the amount of light that can be converted to electricity, Zheng explained. He added that the conversion efficiency of the perovskite solar cell has risen from 5 percent in 2009 to 20 percent, which makes it more attractive than alternative options like organic polymer solar cells, which can take over 50 years to reach a 20 percent conversion efficiency.
Perovskite-structured compounds also possess other key advantages over traditional solar cell materials like crystalline silicon. The constituent elements of their internal crystal structure can be regulated to produce a wide range of optical properties. This means that solar cells can be designed to appear with certain colors, which can in turn increase their market value, according to Di Huang GRD ’17 and the lead author of the study.
Kong said that light usually generates an exciton in most semiconductor materials, a state wherein an electron is bound to an electron hole via an electrostatic force. However, in order to produce usable electricity, the bound electron-hole pair has to be separated into a free electron and free electron hole.
This is usually done with electron acceptors, which can overcome the binding energy holding the electron-hole pair together. However, since perovskite semiconductors possess exciton binding energies as low as 16 meV, they generally do not require the use of electron acceptors, which eases the process of generating electricity.
“The exciton generated by light is readily separated into free charge carriers even under thermal agitation at room temperature, so we don’t need to put more effort [into] setting [the] bound electron and hole free,” Kong said. “It is a great merit to [be able to] easily take free charge carriers out of the light absorbing medium.”
In order to properly operate, perovskite solar cells require the extraction of electrically charged particles, called charge carriers, to an external circuit. The electron and electron hole act as these charge carriers. Scientists typically use the hole transport layer to extract charge carriers from the solar cell’s perovskite layer to its electrodes. While PEDOT:PSS, a polymer mixture, has typically been used as a hole transport material, its low conductivity hampers the transfer of electric charge, according to Huang.
The study found that coating PEDOT:PSS with dimethyl sulfoxide (DMSO), an organic solvent, can increase its conductivity by an order of two to three magnitudes, which in turn can improve its function as a hole transfer material. According to the study, DMSO was selected on the basis of its “distinctive advantages,” such as a high boiling point, that enable the formation of strong dipole-charge interactions between the PEDOT:PSS and DMSO, which in turn enhances the mobility of the charge carriers.
Despite the study’s results, however, perovskite as a solar cell material still comes with a few built-in disadvantages, according to Huang. It is highly sensitive to moisture, which weakens its durability. As such, perovskite solar cells are sealed so as to prevent degradation, according to Zheng. Due to trace amounts of lead, the cells can also be toxic, though their toxicity levels have declined over the past few years. Moreover, the perovskite layer can be improved via crystallization techniques to further increase the power conversion efficiency of the solar cell.
“To achieve the commercialization of the perovskite solar cell, there [is] still more work to do,” Huang said.
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Correction, April 29: A previous version of this article suggested that perovskite solar cells are mineral-based. In fact, perovskite solar cells do not contain perovskite mineral, but rather incorporate perovskite-structured compounds.