Experiments showed that a coating just one molecule thick can significantly enhance the performance of an organic photovoltaic cell (OPV). In performance, the coating outperformed PEDOT:PSS, which is the leading material used for the same task. In addition to improving the performance of OPVs, the single-molecule coating developed by scientists at King Abdullah University of Science and Technology (KAUST) could be used to improve other devices that rely on organic molecules, such as LEDs and photodetectors. When light enters a photovoltaic cell, its energy frees an electron and leaves behind a positive gap, known as a hole. In OPVs, PEDOT:PSS is widely used to ease the transfer of holes into an electrode. However, the material is expensive and acidic, and it can degrade the cell’s performance over time. KAUST scientists have developed an organic solar cell with an electrode coating just one molecule thick that outperforms the most common photovoltaic cells. Courtesy of KAUST Discovery via Lin et. al. In its search for an alternative to PEDOT:PSS, the KAUST team used a thin coating of a hole-transporting molecule, Br-2PACz, which binds to an indium tin oxide (ITO) electrode to form a single-molecule layer, or coating. Tests showed that an organic solar cell using Br-2PACz achieved a power conversion efficiency (PCE) of 18.4%, compared to 17.5% PCE achieved by an equivalent cell using PEDOT:PSS. The Br-2PACz coating material also caused less electrical resistance than PEDOT:PSS and improved hole transport/extraction, and it allowed more light to shine through to the absorbing layer. Br-2PACz also improved the structure of the light-absorbing layer, an effect that the researchers said could be related to the coating process. “We were very surprised indeed by the performance enhancement,” researcher Yuanbao Lin said. “We believe Br-2PACz has the potential to replace PEDOT:PSS due to its low cost and high performance.” The single-molecule coating could also improve solar cell recyclability; the researchers found that the ITO electrode could be removed from a cell, stripped of its coating, and reused to exhibit equally strong performance. The use of PEDOT:PSS can roughen the surface of the ITO. This can cause it to perform poorly if reused in another cell. Although OPVs do not yet rival the performance of silicon cells, they could be easier and cheaper to manufacture on a large scale using printing techniques. The research was published in ChemSusChem (www.doi.org/10.1002/cssc.202100707).
Photosensitizers