Researchers at the State University of New York at Buffalo, University of California, Los Alamos National Laboratory, National Taiwan University and Brookhaven National Laboratory have demonstrated a strategy to extend the optical activity of chiral perovskites into the visible spectral range by introducing charge-transfer states via molecular doping.
Chiral perovskites are semiconductors with broken mirror symmetry that can selectively interact with circularly polarized light. However, their practical use has been limited by their wide bandgaps, which typically restrict photoresponse to the ultraviolet region. To address this limitation, the team incorporated the electron-accepting molecule 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) into a chiral perovskite matrix.
This host-guest system gives rise to a new optically active charge-transfer (CT) state, which introduces a distinct absorption band spanning the 550-750 nm wavelength range. This newly formed CT state exhibits circular dichroism, indicating that the chiral information of the perovskite host is effectively transferred to the otherwise non-chiral F4TCNQ dopant through electronic coupling.
Spectroscopic and theoretical analyses provide insight into the underlying mechanism. Transient absorption spectroscopy confirms the formation of interfacial charge-transfer states, while time-dependent density functional theory (TDDFT) simulations reveal strong electron-hole wavefunction overlap between host and guest in a closely packed configuration. This overlap is critical for enabling optically active transitions. Structural characterization via grazing-incidence X-ray scattering further supports the formation of a densely packed crystal structure, consistent with F4TCNQ incorporation between perovskite chains.
The electronic interaction between the host and guest effectively lowers the energy required for optical excitation by enabling electron transfer from the chiral perovskite to the dopant molecule. As a result, visible photons - previously insufficient to excite carriers in the chiral matrix - can now drive transitions through the CT state.
Device integration highlights the functional implications of this approach. Photodetectors fabricated using the doped films demonstrate selective detection of circularly polarized light across both the UV and visible regions. The preservation of circular dichroism in both the intrinsic perovskite absorption and the CT band enables broadband chiral light sensing, while the presence of the dopant also contributes to enhanced electrical conductivity.
“We were able to transfer the properties of chirality to a non-chiral molecule,” says study author Wanyi Nie. “The resulting material retains the handedness that makes chiral semiconductors promising building blocks for next-generation electronics, while adding the ability to respond to visible light.”
“The core physics is associated to the electron transfer that carries chirality from the chiral perovskite host to the non-chiral dopant molecule,” Nie says.
Overall, this work establishes a route for extending the spectral range of chiral semiconductors via electronically coupled host - guest systems. By engineering charge-transfer states that inherit chirality, the approach enables simultaneous broadband absorption and polarization sensitivity, with potential applications in advanced optoelectronics, including polarized light detection and optical communication systems.
