Tuning the Excited-State Dynamics of CuI Films with Electrochemical Bias

Abstract

Owing to its high hole conductivity and ease of preparation, CuI was among the first inorganic hole-transporting materials that were introduced early on in metal halide perovskite solar cells, but its full potential as a semiconductor material is still to be realized. We have now performed ultrafast spectroelectrochemical experiments on ITO/CuI electrodes to show the effect of applied bias on the excited-state dynamics in CuI. Under operating conditions, the recombination of excitons is dependent on the applied bias, and it can be accelerated by decreasing the potential from +0.6 to -0.1 V vs Ag/AgCl. Prebiasing experiments show the persistent and reversible "memory" effect of electrochemical bias on charge carrier lifetimes. The excitation of CuI in a CuI/CsPbBr₃ film provides synergy between both CuI and CsPbBr₃ in dictating the charge separation and recombination.

Figure 1

(A) UV–vis absorbance spectrum of the ITO/CuI electrode, annealed at 150 °C for 10 min. (B) Spectroelectrochemical data, recorded for ITO/CuI electrodes in 0.1 M Bu₄NPF₆/DCM electrolyte (5 mV s^–1 sweep rate). The cathodic and anodic half-cycles were recorded on separate electrodes. The absorbance of the excitonic peak at 407 nm is plotted together with the recorded current during the half-cycles.

Figure 2

(A) Time-resolved TA spectra of ITO/CuI electrodes in 0.1 M Bu₄NPF₆/DCM electrolyte after 387 nm laser pulse excitation (4 μJ cm^–2) at an applied potential of +0.2 V vs Ag/AgCl. (B) Bleach recovery profiles monitored at 412 nm at applied potentials from −0.1 to +0.6 V vs Ag/AgCl. (C) Potential dependence of the determined lifetimes. The recovery profiles in (B) were fitted with a monoexponential decay. The error bars represent measurements on three separate electrodes.

Figure 3

Reversibility studies of the effect of applied bias on the kinetics of the bleach recovery. (A) Bleaching recovery profiles monitored at 412 nm at the two end potentials of the stability window. (B) Potential dependence of the determined bleach recovery lifetimes [monoexponential fit of kinetic traces in (A)].

Scheme 1. Effect of Charging with Electrochemical Bias Monitored through the Change in the E_OCP

With time, the original dark potential is achieved.

Figure 4

Bleaching recovery profiles monitored at 412 nm recorded for ITO/CuI electrodes in 0.1 M Bu₄NPF₆/DCM electrolyte that were charged at (A) −0.1 and (C) 0.6 V for 10 min prior to measurements. Relaxation of E_OCP of the electrodes plotted together with the change in the bleaching recovery time after prebiasing at (B) −0.1 and (D) 0.6 V.

Figure 5

(A) Schematic illustration of hole transfer from excited CsPbBr₃ to CuI following the bandgap excitation. (B) Time-resolved TA spectra of ITO/CuI/CsPbBr₃ electrodes in vacuum after 387 nm laser pulse excitation (4 μJ cm^–2). (C) Bleaching recovery profiles monitored at 524 nm for the bleaching recovery of CsPbBr₃. The inset shows the magnification of the 0–200 ps region.