Size-dependent photocurrent switching in chemical bath deposited CdSe quantum dot films

Abstract

Size-dependent photocurrent switching has been investigated in chemical bath deposited CdSe quantum dot (QD) films with band gaps 2.26, 2.09, and 1.81 eV (corresponds to nanoparticles' average diameter of 4, 5, and 10 nm). CdSe films generate only anodic photocurrent (exhibit n-type semiconductor behavior) in the solution which contains only acceptor of photoholes (SO 3 2− anions), whereas cathodic photocurrent (corresponding to p-type behavior) arises after immersion of the films in polyselenide electrolyte (containing Se n 2− /Se 2− redox system). Appearance of the cathodic photocurrent is related to chemisorptions of Se 2− and Se n 2− anions, as revealed by the cadmium underpotential deposition (UPD). Photocurrent switching from anodic to cathodic becomes more pronounced with decreasing of CdSe nanoparticle size because small quantum dots with their broadened band gaps have more favorable conduction band energy for electron injection to polyselenide anions. On the contrary, particle size does not play a significant role for the injection of photoholes into the electrolyte because the position of the valence band is weakly size-dependent, and anodic photocurrent is determined primarily by the real surface area of the electrode, which was found to be greater than the geometrical one by 1-2 orders of magnitude from cadmium UPD. Effective charge separation at the highly developed CdSe-electrolyte interface contributes to high incident photon-to-current conversion efficiency of photocurrent (IPCE~40 %).

Effective p-type photocurrent sensitization of n-Bi2O3 with p-CuBi2O4 and p-CuO: Z-scheme photoelectrochemical system

“…Photocurrent spectra were obtained using a setup equipped with a high-intensity grating monochromator (spectral resolution 1 nm), a 250-W halogen lamp, and a light chopper. The spectral dependences of the incident photon-to-current conversion efficiency (IPCE) were calculated from the photocurrent spectra and light intensity distribution at the monochromator output [11].…”

“…Signal acquisition time was equal to 60 s. The excitation spot diameter was about 1 μm. Spectral calibration was done using a built-in gas-discharge lamp providing accuracy better than 2.5 cm −1 (0.1 nm) [11].…”

Underpotential Deposition of Cadmium on Colloidal CdSe Quantum Dots: Effect of Particle Size and Surface Ligands

“…Spittel et al proposed potential-modulated absorption spectroscopy for evaluation of the absolute energy level positions in QD from absorption changes upon polarization. Malashchonak et al employed photocurrent measurements to evaluate LUMO positions of CdSe QDs …”

“…The applicability of QDs in such applications is determined by the energy of occupied and unoccupied orbitals which control the optical transitions, charge transfer, and nanoparticle redox chemistry. The valence and conduction band edge energies [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy] on the absolute energy scale were measured by different techniques, including electrochemical or photoelectrochemical methods, − ultraviolet photoelectron spectroscopy, − and tunneling spectroscopy. − Electrochemical methods are relatively simple; they provide information on QD redox behavior , and allow determining so-called electrochemical band gap of QDs. ,, The basic electrochemical technique involves cyclic polarization of QDs on conducting substrate or in dissolved state in electrolyte solution. This method was applied to characterize redox behavior of CdS, CdSe, ,,,− Cu–Zn–S/Se QDs, and so forth.…”

“…Chalcogenide nanoparticles characterization was also performed using other electrochemical and spectroelectrochemical methods. − Volk et al applied energy-resolved electrochemical impedance spectroscopy for studying electronic structure of PbS nanocrystals. Spittel et al proposed potential-modulated absorption spectroscopy for evaluation of the absolute energy level positions in QD from absorption changes upon polarization.…”