Hole Trapping in Lead Halide Perovskite Nanocrystal-Viologen Hybrids and Its Impact on Back Electron Transfer

Control of forward and back electron transfer processes in semiconductor nanocrystals is important to maximize charge separation for photocatalytic reduction/oxidation processes. By employing methyl viologen as the electron acceptor, we have succeeded in mapping the electron transfer from excited CsPbI₃ nanocrystals to viologen as well as the hole trapping process. The electron transfer to viologen is an ultrafast process (k_et = 2 × 10¹⁰ s^-1) and results in the formation of extended charge separation as electrons are trapped at surface-bound viologen sites and holes at iodide sites. The I₂^─• formation, which is confirmed through the transient absorption at 750 nm, provides a convenient way to probe trapped holes and its participation in the back electron transfer process. By employing a series of mixed halide compositions, we were able to tune the bandgap and valence band energy of the perovskite donor. The back electron transfer rate constant (k_bet = 1.3-2.6 × 10⁷ s^-1) is nearly three orders of magnitude smaller than that of forward electron transfer, thus extending the lifetime of the charge-separated state. The weak dependence of the back electron transfer rate constant on the valence band energy suggests that trapping of holes at halide (I or Br) sites is involved in the back electron transfer process. The ability to extend the lifetime of the charge-separated pair can offer new strategies to improve the redox properties of semiconductor-based photocatalytic systems.