A team from Yale University has discovered a modification of perovskite that can increase the stability and efficiency of perovskite solar cells.
The team explained that light usually generates an exciton in most semiconductor materials, a state where an electron is bound to an electron hole via an electrostatic force. 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 by 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.
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 the team.
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.