Osaka, Japan – Just as left and right hands are mirror images, some molecules come in two "handed" forms. This property, called chirality, can influence not only how molecules interact with light but also which electron spins they allow to pass. Researchers at the University of Osaka have developed novel chiral hole-transport materials that shed new light on this unusual effect while also improving the interfaces of perovskite solar cells.
The team built the materials around a chiral "bifacial" indacenodithiophene (IDT) structure, whose two faces carry different chemical groups. Thin films made from the two mirror-image forms showed strong chirality-induced spin selectivity, or CISS, with spin polarization reaching about 60%.
Most strikingly, molecular handedness consistently determined spin preference. The (S,S) form favored negative spin polarization, whereas the mirror-image (R,R) form favored positive polarization. The researchers found the same relationship in two classes of materials they had previously developed – conductive polymers and non-fullerene acceptors—providing a common pattern across three different types of organic electronic materials.
The molecules also produced an unexpected result. The homochiral (R,R) material transported positively charged "holes" nearly three times faster than the racemic and non-chiral counterparts. Whether this improvement is caused directly by CISS remains unclear, but the finding points to an intriguing connection between molecular handedness and charge transport.
When added as an ultrathin layer to perovskite solar cells, the new molecules helped suppress surface defects and promote hole extraction. Cells treated with the homochiral material reached a power conversion efficiency of 20.64%, compared with 19.48% for untreated control devices.
"We are excited to see a consistent relationship between molecular structure and spin preference across three different material classes," says senior author Fumitaka Ishiwari. "The unexpected increase in hole mobility also raises new questions that we hope to answer."
