Researchers at the University of Osaka have developed chiral hole-transport materials (HTMs) that combine strong spin-selective charge transport with effective perovskite surface passivation, lifting device efficiency to 20.64% from 19.48% for an untreated control.
The work builds on the group's chiral "bifacial" indacenodithiophene (IDT) molecular platform, in which introducing different substituents on the two faces of the IDT core creates chirality and enables chirality-induced spin selectivity (CISS) - a phenomenon in which spin-polarized current passes preferentially through a chiral material. The team had previously shown strong CISS responses in bifacial-IDT conjugated polymers and non-fullerene acceptors. Here, they extended the design to hole-transport materials by adding triarylamine units, producing two mirror-image forms, (S,S)-1 and (R,R)-1, with spin polarization around 60%. Across all three material classes, the researchers found the same correspondence between molecular handedness and spin polarity: the (S,S) form always yields negative spin polarization, the (R,R) form always yields positive. It's the first time this configuration-to-spin-polarity link has been shown to hold across three distinct molecular platforms built on the same chiral core.
Applied as an ultrathin passivation layer on top of a FA0.85MA0.1Cs0.05PbI3 perovskite layer, the homochiral (R,R)-1 outperformed both the racemic and non-chiral (meso) versions of the same molecule, as well as an untreated control. Devices passivated with (R,R)-1 showed the highest short-circuit current density, an open-circuit voltage 0.023 V above the control, and improved fill factor, which the researchers attribute to more effective suppression of defect-related recombination at the perovskite surface and more efficient hole extraction. Racemic and meso versions gave intermediate efficiencies of 20.21% and 19.71%, respectively, indicating that homochirality itself, and not just the passivating molecule's chemistry, contributes to the improvement.
Space-charge-limited-current measurements showed the homochiral (R,R)-1 film had roughly three times the hole mobility of the racemic or meso films, consistent with faster photoluminescence quenching and shorter carrier lifetimes measured for the same sample. The authors are careful to note that a direct causal link between this mobility boost and the CISS effect has not been established, as film packing and interfacial states can also affect mobility, but the pattern is consistent with a CISS-assisted, spin-selective transport mechanism. Ambient stability testing showed no meaningful difference between the treatments, with all devices retaining about 70% of their initial efficiency after 45 days, which the team attributes to moisture uptake by the passivator's hydrophilic side chains rather than to the chirality of the treatment.
"We are excited to see a consistent relationship between molecular structure and spin preference across three different material classes," said author Fumitaka Ishiwari. "The unexpected increase in hole mobility also raises new questions that we hope to answer."
