Abstract
AbstractArguably the most controllable way to control the charge density in various semiconductors, is by electrochemical doping. However, electrochemically injected charges usually disappear within minutes to hours, which is why this technique is not yet used to make semiconductor devices. In this manuscript, electrochemical doping of different semiconductor films (ZnO Quantum Dots (QDs), PbS QDs, and P3DT) is investigated in various high melting‐point nitrile‐based solvents. It is shown that by charging the semiconductors at elevated temperatures, then cooling down to room temperature where these solvents are frozen, the doping stability increases immensely. Measurements performed in cyanoacetamide show that ion transport is entirely halted at room temperature, and that the n‐type conductivity is stable for days, and only drops marginally (≈10%) in several weeks. For p‐doped P3DT films, the conductivity is even completely stable during the entire 76 days of the measurement. In an ambient atmosphere, the p‐type doping is stable, while the n‐type doping disappears in several hours, as electrons react with molecular oxygen. Finally, a pn‐junction diode made of a PbS QD film is demonstrated. These results highlight the possibility of using solidified electrolytes for electrochemical doping and for obtaining semiconductor devices wherein the doping density is controlled electrochemically.
Highlights
Our previous results showed that succinonitrile, despite a melting point of the pure solvent of 57 °C, was not fully crystallized at RT when used as an electrolyte solvent to dope ZnO quantum dots (QDs) films
We showed that the stability of n-doping in ZnO QD films is related to both the electrochemical stability windows and the melting point of the solvents
The best results for ZnO QD films were obtained with cyanoacetamide, as the electrolyte ions are completely immobilized at room temperature
Summary
Our previous results showed that succinonitrile, despite a melting point of the pure solvent of 57 °C, was not fully crystallized at RT when used as an electrolyte solvent to dope ZnO QD films. The film is charged at 140 °C when the solvent is liquid, the potentiostat is disconnected and the film is quickly removed from the cell, causing the temperature to drop and the cyanoacetamide to solidify Since in this case we cannot measure the potential versus a reference electrode we use source–drain electronic conductivity measurements to test the doping stability, employing a home-built interdigitated electrode (IDE) (Figure 5c, see an image of the film in Figure S11 in the Supporting Information). As the conductivity does not decrease when the film is taken out of the glovebox, it shows that cyanoacetamide is a great diffusion barrier against impurities that oxidize at these potentials (such as water) These measurements show that it is possible to obtain stable electrochemically doped semiconductor films made of both QDs (PbS and ZnO) and conductive polymers (P3DT) by the use of cyanoacetamide at room temperature. This shows that a diode is formed, albeit there is room for improvement
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