Letter pubs.acs.org/NanoLett
Engineering Charge Injection and Charge Transport for High Performance PbSe Nanocrystal Thin Film Devices and Circuits Soong Ju Oh,† Zhuqing Wang,† Nathaniel E. Berry,† Ji-Hyuk Choi,†,∥ Tianshuo Zhao,† E. Ashley Gaulding,† Taejong Paik,§ Yuming Lai,‡ Christopher B. Murray,†,§ and Cherie R. Kagan*,†,‡,§ †
Department of Materials Science and Engineering, ‡Department of Electrical and Systems Engineering, and §Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ∥ Complex Assemblies of Soft Matter, CNRS-SOLVAY-UPENN UMI 3254, Bristol, Pennsylvania 19007, United States S Supporting Information *
ABSTRACT: We study charge injection and transport in PbSe nanocrystal thin films. By engineering the contact metallurgy and nanocrystal ligand exchange chemistry and surface passivation, we demonstrate partial Fermi-level pinning at the metal−nanocrystal interface and an insulator-to-metal transition with increased coupling and doping, allowing us to design high conductivity and mobility PbSe nanocrystal films. We construct complementary nanocrystal circuits from n-type and p-type transistors realized from a single nanocrystal material by selecting the contact metallurgy. KEYWORDS: PbSe, field-effect transistor, charge injection, charge transport, bandlike transport, circuit films as the ligand length defines the interparticle distance and, therefore, film conductivity.6 Surface passivation, most notably metal halide treatment, is known to fill the surface trap states that remain after synthesis or form during ligand exchange to further improve charge transport in NC thin films.2,3,14,17−20 Metal electrodes introduced in constructing devices control charge injection. The work function of the metal defines the barrier height for electron and hole injection. In ideal Mott− Schottky theory, the barrier height for electron injection is assumed to be the difference between the metal work function and the semiconductor electron affinity.21 If Fermi level pinning occurs, the barrier for electron injection becomes completely independent of or more weakly dependent on the metal work function. In the cases of Mott−Schottky and partial Fermi-level pinning, a low (high) work function metal is favorable for electron (hole) injection as it presents a low barrier for electron (hole) injection.22 We use temperature-dependent electrical measurements to show that in PbSe NC devices partial Fermi-level pinning occurs at the metal−NC interface, consistent with previous reports,12 and we show that the nature of charge transport evolves from hopping to bandlike for different surface ligand treatments and with surface passivation, characteristic of an insulator-to-metal transition. By engineering the metal contacts, ligand exchange chemistries, and postdeposition surface passivation used, we tune the polarity of charge carrier injection
L
ead chalcogenide nanocrystals (NCs) have been shown to be promising candidate materials for electronic,1 optoelectronic,2,3 and thermoelectric4 applications due to their unique size-dependent properties and solution processability. Although these applications have different physical mechanisms and may require different approaches to enhance device performance, the one common and important mechanism to be investigated is charge transport. Indeed, the actual charge or current flowing in devices arises from charge injection across metal−NC interfaces and charge transport within NC thin films. Charge transport in lead chalcogenide NC thin-film electronic devices has been studied for both scientific interest and technological applications.1,5−11 However, there have been few studies probing the mechanism of charge injection in NC devices, except for a couple of examples in optoelectronic devices.12,13 This is largely because conductive NC thin films with high mobility charge transport had not been achieved until recently,8,9,14 and because the barriers to charge injection are not as evident in narrow bandgap, lead chalcogenide materials. Here, we separate the contributions of charge injection and transport to the total polarity and magnitude of the current in lead chalcogenide NC thin-film devices by probing charge carrier flow in the field-effect transistor (FET) geometry [Figure 1] and by varying the NC ligand exchange chemistry, surface passivation, and contact metallurgy. Ligands on the surface of NCs have two main roles in influencing the electronic properties of NCs and their thin films: (1) ligands may shift the Fermi energy and conduction band and valence band edges by donating or accepting electrons or holes or by creating or passivating midgap and shallow trap states1,15,16 and (2) ligands may also alter the electronic properties of NC thin © 2014 American Chemical Society
Received: July 3, 2014 Revised: September 29, 2014 Published: October 9, 2014 6210
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Figure 1. Schematic of PbSe NC field-effect transistors with different contact metallurgy (left) and different organic (O) and inorganic (I) ligand chemistries (right).
the first excitonic resonance in the absorption spectra relative to that for the native OA-capped PbSe NC thin films, mostly resulting from increased electronic coupling at shorter interparticle distances.1,8,9,25,33,34 Different ligand exchange chemistries give rise to shifts in the position of the conduction band and valence band, decreasing the energy of the conduction band and valence band as we move from hydroxide to halide to thiocyanate (a pseudohalide) to organic ligands. The trend is in good agreement with previous report.35 Representative transfer characteristics for PbSe NC thin-film FETs with various ligand treatments and contact metallurgy are exemplified in Figure 2 and in Supporting Information Figure S3. One noticeable phenomenon is that for organic ligands such as EDT [Figure 2A, Supporting Information Figure S3A,B], BDT [Supporting Information Figure S3C,D,)], and MPA [Supporting Information Figure S3E,F)], the FET characteristics show low current levels and are not significantly affected by the selection of contact metal.36 These NC FETs also show ambipolar characteristics with large hysteresis. The insensitivity of the electron and hole currents to the selection of contact metal for these organic ligand exchanges is summarized and highlighted in the yellow regions in Figure 2E,F. Representative transfer characteristics for NC FETs with the short inorganic thiocyanate, halide, and hydroxide ligands formed by treatment with NH4SCN, NH4Cl, TBAB, TBAI and TMAOH and with different metal contacts are shown in Figure 2B−D and Supporting Information Figure S3G,H.16,31 For these ligand treatments, the FET characteristics show a strong dependence on metal work function. As we previously reported, SCN-treated PbSe NC FETs fabricated with Au source and drain electrodes [Figure 2B] show ambipolar characteristics with some variation in the predominance of hole and electron currents due to oxygen exposure and to the inherent Pbenrichment of the NC surface in synthesis, respectively.8,37,38 However, here we show SCN-treated PbSe NC FETs fabricated with lower work function Al, Ag, and Cr metal contacts form ntype devices, whereas FETs with high work function Pd contacts are p-type. As reported, the halides comprised of chloride [Figure 2C], bromide, and iodide [Supporting Information Figure S3G,H] bind to surface lead atoms and donate an electron to the NC.15,16 For these halide-treated NC thin films, low work function metals form FETs with high electron currents and as the contact metal is changed from Al to Cr, Ag, Au, and Pd, the electron current decreases
and transport from electrons to holes to design n-type and ptype semiconducting NC devices with conductivities and mobilities among the highest reported. Using these n-type and p-type PbSe NC FETs, we build integrated complementary-metal-oxide-semiconductor (CMOS) circuits. Six-nanometer PbSe NCs capped with oleic acid (OA) are synthesized as previously reported.8 To construct FETs, 20 nm of Al2O3 is grown by atomic layer deposition on the surface of 250 nm thermal SiO2 on heavily n-doped silicon wafers that are used as the substrates, the (Al2O3/SiO2) gate dielectric stack, and (n+ Si) gate electrode of the devices. PbSe NCs are deposited by spin-coating to form 20−30 nm thin-film FET channels. Fused quartz substrates are similarly coated with NC thin films for conductivity measurements. All the substrates are treated with mercaptopropyl trimethoxysilane (MPTS) to improve NC adhesion and reduce device hysteresis.8 These OA-capped NCs form thin films with commonly observed hexagonal NC packing. After NC film deposition, the substrates are briefly immersed in a solution containing the following investigated ligands: benzenedithiol (BDT),23,24 ethanedithiol (EDT),12,25 mercaptopropionic acid (MPA),2,26 ammonium thiocyanate (NH4SCN),8,27−30 ammonium chloride (NH4Cl), tetrabutyl ammonium bromide (TBAB),3,16 tetrabutyl ammonium iodine (TBAI),16,31 and tetramethylammonium hydroxide (TMAOH).16,31 Due to the weak binding of OA ligands on {100} NC facets, the OA ligands are easily removed during ligand exchange and the structure of most compact, ligandexchanged NC thin films is transformed to square ordered packing [Supporting Information Figure S1]. Most notably, SCN and TBAI treated NC assemblies show the most ordered structures. To further modify the doping and surface passivation of the NC thin films, after ligand exchange we explore treatment of the NC thin films with PbCl2.14 To investigate source and drain electrodes varying in their work function, Pd (5.5 eV), Au (5.1 eV), Ag (4.7 eV), Cr (4.5 eV), and Al (4.2 eV)21,32 are deposited by thermal evaporation after NC film deposition and ligand exchange. Cyclic voltammetry (CV) [Supporting Information Figure S2A] in combination with absorption spectroscopy [Supporting Information Figure S2B] are used to characterize the electronic structure of PbSe NC thin films and the energy level alignment with the investigated metal electrodes [see Supporting Information discussion and Figure S2C]. All the ligand exchange processes give rise to a red-shift in the position of 6211
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[highlighted in red in Figure 2E]. Here, the current and effective mobility decrease results from limited electron injection. In the case of TMAOH, a known p-dopant,31 relatively higher hole currents are observed with high work function Au or Pd contacts, but the current level decreases as the metal work function changes for Ag, Cr, and Al contacts [Figure 2D] and hole injection is limited [highlighted in blue in Figure 2F]. From our observations, we conclude that for PbSe NC thin films with organic ligands, charge transport limits device currents due to the relatively large interparticle distances [Figure 3A,B]. In this regime, charge injection does not affect the total current flow, as charge transport is the limiting process. However, for PbSe NC thin films with short inorganic ligands carrier hopping rates are high due to reduced or eliminated interparticle spacing, and therefore charge injection starts to play an important role and can limit the total current flow. For example, the electron (hole) current is limited by injection at the semiconductor-metal contact for n-type (ptype) PbSe NC thin films with Au (Al) metal contacts, whereas the hole (electron) current is limited by charge transport in the channel [Figure 3C,D]. We measure the temperature dependence of the electrical characteristics of NC FETs to explore charge transport within the NC thin films (described below) and carrier injection at the metal−NC interface [Supporting Information Figures S4 and S5]. We show using the temperature dependence of the current in the off-state and in the electron38 and hole accumulation regimes of the FETs that while the electron and hole currents in PbSe NC FETs exchanged with compact, inorganic ligands depend on the metal work function, the Fermi level is partially pinned [see Supporting Information discussion and Figure S5]. For example, the effective work function difference between Al and Au contacts in PbSe NC FETs is extracted as 90 meV, instead of an ideal 900 meV, consistent with the previous report.12 We note that devices fabricated with Al contacts show
Figure 2. PbSe NCs FETs treated with (A) EDT (B) NH4SCN, (C) NH4Cl, and (D) TMAOH and fabricated with (red) Pd, (orange) Au, (blue) Ag, (green) Cr, and (black) Al top contacts. (E) Electron currents at VG = 50 V and VDS = 50 V for PbSe NCs FETs with different metal contacts and treated with (red) BDT, (green) MPA, (orange) EDT, (blue) TBAI, (purple) TBAB, (black) NH4Cl, and (gray) SCN. (F) Hole currents at VG = −50 V and VDS = −50 V for PbSe NCs FET with different metal contacts and treated with (red) BDT, (green) MPA, (orange) EDT, and (brown) TMAOH.
Figure 3. Band diagram depicting charge injection and transport in PbSe NC thin films with long ligands organic for FETs fabricated with (A) high work function Au contacts and (B) low work function Al contacts, and with short inorganic ligands for FETs fabricated with (C) Au and (D) Al contacts. 6212
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4F]. This is consistent with a loss of surface lead atoms as lead oleate39 upon SCN ligand treatment, leaving exposed some unpassivated selenium atoms, and with lead binding to surface selenium atoms that exist after NC synthesis or are generated during ligand exchange, repairing the surface after PbCl2 treatment [Figure 4A]. Figure 5A and B show the transfer
significant degradation over time, even in the
