The Role of Dopant Ions on Charge Injection and Transport in Electrochemically Doped Quantum Dot Films

Abstract

Control over the charge density is very important for implementation of colloidal semiconductor nanocrystals into various optoelectronic applications. A promising approach to dope nanocrystal assemblies is charge injection by electrochemistry, in which the charge compensating electrolyte ions can be regarded as external dopant ions. To gain insight into the doping mechanism and the role of the external dopant ions, we investigate charge injection in ZnO nanocrystal assemblies for a large series of charge compensating electrolyte ions with spectroelectrochemical and electrochemical transistor measurements. We show that charge injection is limited by the diffusion of cations in the nanocrystal films as their diffusion coefficient are found to be ∼7 orders of magnitude lower than those of electrons. We further show that the rate of charge injection depends strongly on the cation size and cation concentration. Strikingly, the onset of electron injection varies up to 0.4 V, depending on the size of the electrolyte cation. For the small ions Li+ and Na+ the onset is at significantly less negative potentials. For larger ions (K+, quaternary ammonium ions) the onset is always at the same, more negative potential, suggesting that intercalation may take place for Li+ and Na+. Finally, we show that the nature of the charge compensating cation does not affect the source-drain electronic conductivity and mobility, indicating that shallow donor levels from intercalating ions fully hybridize with the quantum confined energy levels and that the reorganization energy due to intercalating ions does not strongly affect electron transport in these nanocrystal assemblies.

Spectroelectrochemical Signatures of Surface Trap Passivation on CdTe Nanocrystals

“…This thermally activated detrapping (which is the rate-determining step, RDS) slows down electron transfer enormously by several orders of magnitude. If we assume an electron mobility of 10 –2 cm 2 /(V s) for band edge electrons and thermally activated detrapping from a 0.5 eV deep trap state, then we can estimate that diffusion over the thickness d of a 100 nm thick NC film takes For band edge electrons (where the rate of injection is limited by cation diffusion with a diffusion coefficient of ∼10 –7 cm 2 /s) 28 charge injection over the entire film takes place on a ∼1 ms time scale. So while electron injection/extraction from the conduction band may occur electrochemically reversibly, the filling/emptying of trap states is slow, resulting in the observed strong hysteresis.…”

“…Spectroelectrochemistry is a powerful tool to study important features of nanomaterials, such as the position of the band edges, 26 − 28 the degeneracy of the energy levels 29 and the distribution of trap states within the bandgap. 18 , 30 , 31 Since trapping of photogenerated charge carriers results in a decrease in PLQY, we use in situ photoluminescence spectroelectrochemistry to study the influence of the passivating ligands on the distribution of trap states.…”

Spectroelectrochemistry of Quantum Dots

“…Combination of SEC, differential capacitance and source‐drain electronic conductance measurements was used to determine both the mobility of electrons out‐of‐plane (moving perpendicular through a ZnO QD film) during charge injection and the mobility of electrons in‐plane (moving parallel to the substrate) in a source‐drain configuration. The out‐of‐plane electron mobility was 7 orders of magnitude lower than the in‐plane one …”

“…The out-of-plane electron mobility was 7 orders of magnitude lower than the inplane one. [113] PL-SEC has been used to probe the distribution of Curelated trap states responsible for PL of CuInS 2 NCs. [59] PL line width is correlated to the width of the "bright" Cu + trapstate distribution in the ensemble.…”

Enhancing the stability of the electron density in electrochemically doped ZnO quantum dots

“…film, 30 this shows that electron extraction is predominantly caused by impurities in the electrochemical environment.…”