Electronic doping of semiconductor nanomaterials can be efficiently achieved using electrochemistry. However, the injected charge carriers are usually not very stable. After disconnecting the cell that is used for electrochemical doping the carrier density drops, typically in several minutes. While there are multiple possible causes for this, we demonstrate here, using n-doped ZnO quantum-dot films of variable thickness that the dominant mechanism is reduction of solvent impurities by the injected electrons. We subsequently investigate two different ways to enhance the doping stability of ZnO QD films. The first method uses preemptive reduction of the solvent impurities; the second method involves a solid covering the QD film, which hinders impurity diffusion to the film. Both methods enhance the doping stability of the QD films greatly@en

Semiconductor films that allow facile ion transport can be electronically doped via electrochemistry, where the amount of injected charge can be controlled by the potential applied. To apply electrochemical doping to the design of semiconductor devices, the injected charge has to be stabilized to avoid unintentional relaxation back to the intrinsic state. Here, we investigate methods to increase the stability of electrochemically injected charges in thin films of a wide variety of semiconductor materials, namely inorganic semiconductors (ZnO NCs, CdSe NCs, and CdSe/CdS core/shell NCs) and organic semiconductors (P3DT, PCBM, and C₆₀). We show that by charging the semiconductors at elevated temperatures in solvents with melting points above room temperature, the charge stability at room temperature increases greatly, from seconds to days. At reduced temperature (-75 °C when using succinonitrile as electrolyte solvent) the injected charge becomes entirely stable on the time scale of our experiments (up to several days). Other high melting point solvents such as dimethyl sulfone, ethylene carbonate, and poly(ethylene glycol) (PEG) also offer increased charge stability at room temperature. Especially the use of PEG increases the room temperature charge stability by several orders of magnitude compared to using acetonitrile. We discuss how this improvement of the charge stability is related to the immobilization of electrolyte ions and impurities. While the electrolyte ions are immobilized, conductivity measurements show that electrons in the semiconductor films remain mobile. These results highlight the potential of using solidified electrolytes to stabilize injected charges, which is a promising step toward making semiconductor devices based on electrochemically doped semiconductor thin films.

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