NANO LETTERS
High Carrier Densities Achieved at Low Voltages in Ambipolar PbSe Nanocrystal Thin-Film Transistors
2009 Vol. 9, No. 11 3848-3852
Moon Sung Kang, Jiyoul Lee, David J. Norris,* and C. Daniel Frisbie* Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, 421 Washington AVenue SE, Minneapolis, Minnesota 55455 Received June 26, 2009; Revised Manuscript Received August 17, 2009
ABSTRACT Efficient transport of both electrons and holes with high carrier densities is a requirement for obtaining light-emitting transistors from films of colloidal semiconductor nanocrystals. Such devices offer an approach to efficient electrically pumped nanocrystal lasers with tunable emission. Here, we report a low-voltage ambipolar thin-film transistor that features high carrier mobility and high induced carrier density by combining a PbSe nanocrystal film with a high-capacitance ion-gel gate dielectric layer (∼22 and ∼9 µF/cm2 for electron and hole accumulation, respectively). At operation voltages below 2.5 V, electron and hole densities higher than ∼1014 carriers/cm2 could be achieved in the PbSe nanocrystal film, which corresponds to ∼3 electrons or holes per particle. Carrier mobilities were also dependent on charge density and were as high as 0.4 and 0.02 cm2/(V s) for electrons and holes, respectively.
Semiconductor nanocrystals (NCs) can exhibit tunable electronic and optical properties due to their nanometer size.1,2 To exploit these properties in electronic and optoelectronic devices, methods have been developed to prepare nearly monodisperse colloidal NCs and to deposit these NCs in close-packed assemblies.3 The resulting thin films can exhibit efficient charge transport4-8 with carrier mobilities approaching those observed in organic semiconductors.6,9 Consequently, NCs have been applied to many standard electronic devices, including thin-film transistors,6 solar cells,10 light-emitting diodes,11,12 and photodetectors,13 and a basic understanding of charge transport in NC films has begun to emerge.14-16 These successes have encouraged researchers to consider additional NC devices that can further enhance NC behavior. In particular, light-emitting transistors (LETs) offer exciting possibilities by combining the functionality of a transistor with that of a light-emitting device. The source, drain, and gate electrodes of the transistor can provide exquisite control of charge transport of both electrons and holes as well as their recombination.17,18 Thus, if LETs based on NC films could be obtained, the NC emission intensity could be modulated by controlling carrier densities.19 More importantly, such LETs would offer a route to electrically pumped NC lasers if a suitable optical cavity could be integrated and charge-exciton quenching could be minimized.20,21 However, several requirements must be satisfied before NC-based LETs can be realized. First, ambipolar films must * Corresponding authors,
[email protected] and
[email protected]. 10.1021/nl902062x CCC: $40.75 Published on Web 09/23/2009
2009 American Chemical Society
be prepared that contain a balanced number of highly mobile electrons and holes that can transport across the long channel of the transistor (>1 µm). Second, these electrons and holes must then recombine within individual NCs to generate light. Third, the channel should accumulate high carrier densities so that this emission is intense. Finally, in practice, all of this should be achieved with low operation voltages. As a first step toward developing NC-based LETs, here we report low-voltage ambipolar NC thin-film transistors (TFTs) with high carrier mobility and high carrier density for electrons and holes. We selected PbSe NCs because they have demonstrated the best electrical properties in terms of mobility and conductivity.6,9 In addition, PbSe NC films allow the injection of both electrons and holes,22 and charge transport characteristics have been varied from n-type to ambipolar to p-type conduction by controlling film fabrication conditions (e.g., annealing time, air exposure, postdeposition chemical treatment, etc.).6,9,23 Consequently, different research groups have already shown ambipolar conduction with PbSe NC films.6,23-25 However, the mobilities of the carriers in these ambipolar devices have unfortunately been low, indicating inefficient charge transport. To alleviate this problem, we have employed a highcapacitance ion gel as the gate dielectric. Such materials can exhibit ultrahigh capacitance because mobile ions in the gel can create a nanometer-thick double layer in response to an electric field.26 As a result, when an ion gel is placed next to a film of PbSe NCs, high carrier densities (>1014 carriers/ cm2) can be induced in the film. This facilitates the charge
Figure 1. (a) Chemical structure of the ionic liquid and triblock copolymer ion gel components: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) and poly(styrene-block-(methyl methacrylate)-block-styrene) (PS-PMMA-PS). (b) Optical absorbance spectrum of PbSe NCs in tetrachloroethylene (TCE). The lowest energy absorption peak occurs at 1655 nm with a fwhm
