Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post

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Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange Han Wang, Derrick J. Butler, Daniel B. Straus, Nuri Oh, Fengkai Wu, Jiacen Guo, Kun Xue, Jennifer D. Lee, Christopher B. Murray, and Cherie R. Kagan ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09055 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange Han Wang†, Derrick J. Butler‡, Daniel B. Straus‖, Nuri Oh†⊥, Fengkai Wu‡, Jiacen Guo‡, Kun Xue†, Jennifer D. Lee‖, Christopher B. Murray‡‖, Cherie R. Kagan†‡‖

†Department of Electrical and Systems Engineering, ‡Department of Materials Science and Engineering, ‖Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ⊥Division of Materials Science and Engineering, Hanyang University, Seoul, 133-791, Republic of Korea * To whom correspondence should be addressed. E-mail: [email protected]

Phone : (215) 573-4384. Fax : (215) 573-2068

ABSTRACT Colloidal semiconductor nanocrystals (NCs) are a promising materials class for solutionprocessable, next-generation electronic devices. However, most high-performance devices and

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circuits have been achieved using NCs containing toxic elements, which may limit their further device development. We fabricate high mobility CuInSe2 NC field-effect transistors (FETs) using a solution-based, post-deposition, sequential cation exchange process that starts with electronically-coupled, thiocyanate (SCN)-capped CdSe NC thin films. First Cu+ is substituted for Cd2+ transforming CdSe NCs to Cu-rich Cu2Se NC films. Next, Cu2Se NC films are dipped into a Na2Se solution to Se-enrich the NCs thus compensating the Cu-rich surface, promoting fusion of the Cu2Se NCs, and providing sites for subsequent In-dopants. The liquid-coordinationcomplex trioctylphosphine-indium chloride (TOP-InCl3) is used as a source of In3+ to partiallyexchange and n-dope CuInSe2 NC films. We demonstrate Al2O3-encapsulated, air-stable CuInSe2 NC FETs with linear (saturation) electron mobilities of 8.2 ± 1.8 cm2/Vs (10.5 ± 2.4 cm2/Vs) and with current modulation of 105, comparable to that for high-performance Cd-, Pb-, and -As based NC FETs. The CuInSe2 NC FETs are used as building blocks of integrated inverters to demonstrate their promise for low-cost, low-toxicity NC circuits. TABLE OF CONTENTS

KEYWORDS: nanocrystals ·doping · solution-process · copper indium diselenide · stoichiometry · field effect transistors · integrated circuits


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The booming market for Internet of Things (IoT) devices is spurring a strong demand1 for lower-cost, lower-process temperature material substitutes to traditional Si-based technologies that are compatible with fabrication on flexible polymer substrates. Colloidal semiconductor nanocrystals (NCs) have gained attention in recent years as a promising alternative electronic materials class2 for their (i) size- and composition-dependent electronic properties, which provides flexibility in bandgap engineering;3 (ii) high surface-to-volume ratio, which enables a variety of surface modification methods to be used to engineer material properties4 including the energy, type, concentration, and mobility of carriers; and (iii) dispersibility in solvents,5 which enables low-cost, large-area methods for device fabrication.6–8 Exploitation of these traits has led to the progress in NC thin-film field-effect transistors (FETs)9 and integrated circuits,10,11 with reports of carrier mobilities exceeding 1 cm2/Vs in Cd-12–17 and Pb-18–20 chalcogenides and in InAs21,22 NC devices. However, the oxidative, photolytic instability of Cd- and Pb- based NCs23 creates concern over the release of Cd2+ and Pb2+ ions, two of ten substances on the Restriction of Hazardous Substances (RoHS) guide impacting the electronics industry and, while the selection of InAs NCs removes heavy metal ions, it introduces arsenic, another toxic element.24 To broaden the potential applications for NC-based devices, particularly in wearable or implantable electronics, developing non-toxic NC materials that yield devices with comparable performance is attracting increased focus in the NC community. Ternary I-III-VI2 CuInE2 (E = S or Se) NCs have drawn much interest as possible nontoxic NC alternatives and have been explored for their potential use in biomedical imaging,25 light emitting diodes,26 and solar energy conversion.27–30 The size- and stoichiometry-dependent bandgap31 and carrier type32 and the respectable, single crystal mobility33 of CuInE2 suggest it is a promising material for NC electronic devices. Synthetic routes have been developed to prepare


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CuInE2 NCs directly from precursors, although with relatively poor control over sample size distribution and shape34 and indirectly during synthesis, through cation exchange35,36 of highquality NCs conserving the mother NC anion lattice.37 However, to the best of our knowledge, there has not been a demonstration of high mobility (> 0.1 cm2/Vs) CuInE2 NC devices, using either direct synthesis or indirect cation exchange methods. Here, we report high electron mobility CuInSe2 NC thin film FETs and integrated circuits via a solution-based, post-deposition, sequential cation exchange process that begins with thiocyanate- (SCN-) capped CdSe NC thin films which are known to yield high mobility CdSe NC devices.12 We first fully exchange Cd2+ with Cu+, using a Cu+ precursor solution38 to form a Cu-rich Cu2Se NC thin film. Na2Se treatment is carried out to compensate the Cu-rich nonstoichiometry. More importantly, Se-enrichment creates an unstable NC surface, promoting NC fusion, and leaves sites for In-dopants in the next cation exchange step. Finally, we transform the fused Cu2Se NC to In-rich CuInSe2 NC thin films by treating the film with a solution of the liquid coordination complex (LCC) trioctylphosphine-indium chloride (TOP-InCl3),39 taking advantage of the weak In-P bond within the LCC to drive cation exchange and add excess Indopants at the NC surface. We fabricate FETs using CuInSe2 NC thin films and encapsulate the FETs with Al2O3 through atomic layer deposition (ALD). The elevated processing temperatures in the ALD provide in-situ thermal activation of the In-dopants, while the encapsulation process further passivates trap states,18 improves doping efficiency,40 and makes the FETs air-stable. The performance of these n-type CuInSe2 NC FETs is comparable to current Cd-, Pb-, and -As containing NC FETs. We show the feasibility of CuInSe2 NC FETs for low-voltage operation and fabricate CuInSe2 NC inverters, demonstrating their promise for fabricating low-cost, lowtoxicity integrated circuits.


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RESULTS AND DISCUSSION Structural, Analytical, and Optical Characterization of the Post-Deposition Cation Exchange Process: The scheme for the post-deposition exchange process to transform CdSe into CuInSe2 NC thin films is shown in Figure 1A-E. We map the process step-by-step structurally, using high-resolution transmission electron microscopy (HRTEM, Figure 2A-2C) and wideangle X-Ray scattering (WAXS, Figure 2D); and spectroscopically, via optical absorption (Figure 2E, solid) and photoluminescence (PL) measurements (Figure 2E, dashed) and through energy-dispersive X-Ray spectroscopy (EDS, Supporting Information Figure S1).

Figure 1. Schematic of sequential post-deposition cation exchange to transform CdSe to CuInSe2 NC thin films. (A) A SCN-capped CdSe NC thin film is immersed into a methanolic Na2Se solution to remove SCN ligands. (B) A Na2Se treated CdSe NC thin film is immersed into a methanolic [Cu(CH3CN)4]PF6 solution to substitute Cd2+ for Cu+ and form a Cu-rich Cu2Se NC thin film. (C) A Cu-rich Cu2Se NC thin film is immersed into a methanolic Na2Se solution to


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compensate non-stoichiometry, drive interparticle fusion, and provide binding sites for the subsequent introduction of In3+. (D) A fused Cu2Se NC thin film is immersed in a dilute solution of the TOP-InCl3 LCC complex in octadecene to partially cation exchange Cu+ for In3+ and form an In-rich CuInSe2 NC thin film. (E) Final CuInSe2 NC thin film.

SCN-capped CdSe NCs are selected to provide a solution-processable, compact ligandexchanged NC dispersion known to form large-area, uniform, and crack-free films that are used to fabricate high-mobility NC FETs.11 To start, a dispersion of SCN-capped wurtzite CdSe NCs (~4 nm) is deposited by spin-coating to form thin films using a previously published method.41 The HRTEM image, the WAXS pattern, and the optical absorption and PL spectra of the starting CdSe NC thin films are shown in Figure 2A, 2D, and 2E, while the EDS spectrum is shown in Supporting Information Figure S1A, respectively. As the coverage of SCN- ligands may passivate and block NC surface sites from further surface treatment, we first immerse the SCNcapped CdSe NC film in a methanolic Na2Se solution (shown in Figure 1A), similar to the work of Oh et al. using Se2- to replace SCN- ligands and form a Se-rich shell at the surface of lead chalcogenide NCs.42 FTIR spectra (Supporting Information Figure S2A) confirms the total loss of the SCN- ligand and EDS data show a reduction in the Cd:Se ratio from 1:0.87 ± 0.02 to 1:1.05 ± 0.03 after Na2Se treatment, consistent with the addition of Se2- to surface Cd atoms. The absorption spectrum of the CdSe NC films after Na2Se treatment (Supporting Information Figure S2B) reveals a ~16 nm redshift and a ~14 nm broadening of the first excitonic peak. However, the WAXS pattern of Na2Se treated CdSe NC thin films (Supporting Information Figure S2C) shows no significant sharpening of the diffraction peak, yielding a Scherrer grain size of 3.2 nm. The smaller Scherrer grain size compared to the TEM seen size is consistent with broadening of


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the diffraction peak from defects including stacking faults.5 The absorption and WAXS data suggest the redshift and the broadening of the first excitonic peak for Na2Se-treated CdSe NC films mainly originates from Se-rich shell growth and enhanced interparticle coupling instead of NC fusion, in contrast to the significant grain growth seen upon Na2Se treatment of lead chalcogenide NC films.42 The wurtzite CdSe structure lacks inversion symmetry along the caxis.43 We hypothesize that the addition of excess Se2- to surface Cd sites alters the local surface charge on the polar all-Cd facets,44 reducing or even removing the dipole interaction, while increasing the repulsive electrostatic interaction between NCs, thus weakening the driving force for oriented attachment.45,46 It is also possible that a Se-rich CdSe NC surface is relatively more energetically stable47 than that for PbSe NCs and thus limits CdSe NC fusion.


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Figure 2. High-resolution TEM images of assemblies of (A) SCN-capped CdSe NCs, (B) cationexchanged, fused Cu2Se NCs after the Na2Se treatment, and (C) cation-exchanged, fused CuInSe2 NCs. (D) WAXS patterns and (E) absorption and PL spectra of SCN-capped CdSe NC films (red), fused Cu2Se NC films (green), and fused CuInSe2 NC films (blue). Next, we soak Na2Se-treated CdSe NC thin films in a methanolic tetrakis(acetonitrile)copper(i) hexafluorophosphate solution, as shown in Figure 1B, to exchange Cd2+ with Cu+ ions and form Cu2Se NC thin films. EDS spectra in Supporting Information Figure S1B show a Cu:Se ratio of 2.47 ± 0.12:1, consistent with the Se-rich shell in the mother CdSe NCs providing binding sites and thus Cu-enriching the NC surface. No signature of Cd is detected by EDS indicating complete replacement of Cd2+ by Cu+. The WAXS pattern, shown in Supporting Information Figure S3, matches well with Berzelianite Cu2Se (JCPDS PDF-Card 01088-2043). The calculated 3.9 nm Scherrer grain size suggests no significant fusion happens during this stage. Though cation exchange usually maintains the anion structure, it is interesting to point out that in our case the hexagonally close packed (hcp) structure of the mother CdSe NCs is transformed to a face-centered cubic (fcc) structure in the product Cu2Se NCs. From a thermodynamic point of view, it is not uncommon to see the anion lattice change motif to maintain energetic favorability as Cu2Se with a hcp anion lattice is a metastable phase.48 Another Se-enrichment step by Na2Se treatment (Figure 1C) is carried out following the cation exchange for Cu+ to tune the stoichiometry and importantly, to introduce an unstable Serich NC surface42 that drives NC fusion to improve charge transport and to introduce sites for subsequent In-doping. Quantitative analysis of EDS spectra in Supporting Information Figure S1C indicates that the Na2Se treatment reduces the Cu:Se ratio to 1.96 ± 0.02:1. The HRTEM image (Figure 2B) together with an average line narrowing of the Cu2Se diffraction peaks in the


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WAXS pattern of 57.3% (Figure 2D), yields a Scherrer grain size of 9.9 nm, consistent with the fusion of the Cu2Se NCs in thin films. Unlike the wurtzite CdSe NC case which is stable upon Na2Se treatment, the fusion of cubic Cu2Se NCs is consistent with dipolar interactions and/or a high-energy Se-rich surface driving NC fusion. Strain has been reported in cation-exchanged Cu2Se NCs with low ligand coverage49 and is known to enhance dipolar interactions.44 The absorption spectrum of the fused Cu2Se NC thin film shows a strong plasmon resonance (Figure 2E). We hypothesis the plasmon resonance is caused by Se-enrichment of Cu2Se NC thin films introducing additional Cu vacancies (V′Cu) and therefore holes in the system.50 As the measurement is conducted in air, we also cannot rule out the contribution of oxidization to this plasmon resonance. Cu+ is easily oxidized to Cu2+, which would increase the concentration of V′Cu and enhance the plasmon resonance.50 The WAXS pattern of Cu2Se NCs measured in open air (Supporting Information Figure S4) confirms possible oxidization of NCs. No PL is detected at this stage. The partial cation exchange of Cu+ for In3+ is conducted following the Na2Se surface treatment of Cu2Se NCs by soaking thin films in a dilute TOP-InCl3 LCC solution in octadecene (Figure 1D-E). Quantitative EDS analysis of CuInSe2 NC thin films (Supporting Information Figure S1D) yields a Cu:In:Se ratio of 1.55 ± 0.2:1.65 ± 0.1:2. The Se-rich surface of the mother NCs provides binding sites and is consistent with the formation of an In-rich NC surface during the cation exchange process. The relative metal-rich stoichiometry could be the result of Se loss upon introduction of TOP to the Cu2Se NCs.51 The lack of P and Cl signals indicates the TOP and Cl- from the LCC are not introduced to the surface of the NCs as an additional ligand during the cation exchange process. We perform inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements to further verify the In-rich stoichiometry, yielding a


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Cu:In:Se ratio of 1.24:1.48:2. HRTEM images of CuInSe2 NC assemblies (Figure 2C) and the cross-sectional image of the NC thin films (Supporting information Figure S5) confirm that the fused structure of the Cu2Se NCs is maintained in the CuInSe2 NC films. The WAXS pattern of the product NC film (Figure 2D) shows the formation of the tetragonal, chalcopyrite CuInSe2 (JCPDS PDF-Card 00-040-1487) structure. Scherrer analysis of the WAXS pattern indicates a grain size of the cation-exchanged CuInSe2 NCs in the films to be approximately 10.1 nm, again consistent with the film maintaining the fused NC structure. The WAXS pattern and similar grain size found before and after In3+ cation exchange give no evidence of significant crystalline or amorphous In2Se3 phase formation. Optical absorption spectra of the CuInSe2 NC thin films (Figure 2E) shows a band edge characteristic of a semiconductor and no signature of the plasmon resonance of the parent Cu2Se NC thin film. The absorption edge calculated from a Tauc plot is 1.10 eV (1127 nm), close to the value of 1.04 eV (1192 nm) for bulk CuInSe2.27 No significant blue shift is observed in the optical absorption spectra from quantum-confinement as the Scherrer grain size is similar to the bulk Bohr radius of 10.6 nm. Still, no PL is detected. Cation-exchanged CuInSe2 NC Thin Film Electronic Performance: To characterize the electronic properties of the CuInSe2 NC thin films, we fabricate FETs. A heavily n-doped Si wafer with 300 nm thermally grown SiO2 is used as the gate and gate oxide layers of the FETs. 40 nm thick, solution-deposited and cation-exchanged CuInSe2 NC thin films are fabricated, and 50 nm Au is thermally evaporated through shadow masks to define the source and drain electrodes. As-cation-exchanged CuInSe2 NC FETs show n-type behavior (see representative transfer curve in Figure 3A (light blue)), consistent with an In-rich and Se-poor stoichiometry.52 The forward scan is used in extracting electron mobility (µe) and found in the saturation regime (VDS=50 V) to be 3.6 × 10-3 ± 1.1 × 10-3 cm2/Vs. Linear regime characteristics are not shown and


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linear µe is not reported as the small currents levels are dominated by leakage currents. Annealing the CuInSe2 NC FETs at 250 ℃ for 90 min in a nitrogen-filled glove box increases the µe by a factor of 103, yielding a linear regime µe=5.8 ± 1.8 cm2/Vs (VDS=1 V, Supporting Information Figure S6A) and saturation regime µe=6.7 ± 1.8 cm2/Vs (VDS=50 V, Figure 3A (blue)). The annealed FETs are not stable in air. Gradually doping by oxygen switches the device polarity to p-type (Supporting Information Figure S6B), as is commonly seen in I-III-VI2 NC devices.32

Figure 3. (A) Transfer curves (VDS=50 V) for (light blue) as-cation-exchanged, (blue) annealed and (dark blue) ALD encapsulated CuInSe2 NC FETs with W/L ratio of 15. Forward scans are represented by solid lines while reverse scans are represented by dotted lines. B) Cross-sectional SEM image of an Al2O3-encapsulated, CuInSe2 NC FET. C) Air stability of representative Al2O3-encapsulated, CuInSe2 NC FETs.


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To further passivate NC surface states,18,53 improve the efficiency of doping created by non-stoichiometry,40 and prevent NC thin films from oxidizing,18,53 we encapsulate the CuInSe2 NC devices with 50 nm amorphous Al2O3 deposited by ALD. ALD is also used to replace the annealing process described above, as the FETs are exposed to the same 250 ℃ temperature excursion for 90 min during encapsulation. A cross-sectional SEM image of an ALD Al2O3encapsulated CuInSe2 NC FET is cut and imaged using the Focused Ion Beam (Figure 3B). Encapsulation of the CuInSe2 FET further boosts µe, consistent with trap state passivation and increased doping efficiency. For 20 devices with channel lengths ranging from 48 µm to 150 µm, the average µe is 8.2 ± 1.8 cm2/Vs in the linear regime (VDS=1 V, Supporting Information Figure S6C) and 10.5 ± 2.4 cm2/Vs in the saturation regime (VDS=50 V, Figure 3A (dark blue)). The current modulation for the encapsulated CuInSe2 NC FET is 105, which is an order of magnitude lower than that of the parent CdSe NC devices, but still higher than most reported NC devices.13,14,18,21 Output characteristics of a representative, encapsulated CuInSe2 NC FET are shown as an inset in Supporting Information Figure S6C. Transmission line measurements are conducted (Supporting Information Figure S6D) and confirm Ohmic contact formation in the FETs. The performance of the encapsulated devices is maintained after 7 days of air exposure, with no significant observable change in µe and threshold voltage (VT) (Figure 3D). Annealing is effective at increasing device mobility and therefore we evaluate the effect of heat on the structural and optical properties of the NC thin films. After ALD-encapsulation of CuInSe2 NC thin films, Scherrer analysis of WAXS patterns (Supporting Information Figure S7A) yields a grain size of 10.4 nm, similar to that found in unannealed films. HRTEM images (Supporting Information Figure S7B) for CuInSe2 NC thin films annealed at the same temperature and for the same time period as the ALD process, also show no significant grain


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growth after heat treatment. The lack of grain growth is consistent with the CuInSe2 NCs having an In-rich and not a Se-rich surface, as surface In has been shown to stabilize the surface and limit fusion of NCs.54 The absorption edge of ALD Al2O3-encapsulated CuInSe2 NC thin films are also largely unchanged, having a bandgap calculated from a Tauc plot (Supporting Information Figure S7C) of 1.07 eV (1159 nm). Interestingly, weak PL is detected at 1222 nm (Supporting Information Figure S7C), suggesting that surface trap states are passivated by the ALD process.55 The 63 nm Stokes shift in our NC films is smaller than that of directlysynthesized quantum confined (2.5-3.5 nm) CuInSe2 NCs56 and is consistent with the trend of smaller Stokes shift seen for larger CuInSe2 NCs.56 In-Doping via TOP-InCl3 Complexes: In our system, it is interesting to note that both EDS and ICP-OES quantitative analysis methods show an In-rich and Se-poor CuInSe2 NC stoichiometry. It is well known that Se-poor CuInSe2 tends to show n-type behavior as a V··Se serves as a donor, and In can effectively n-dope bulk CuInSe2 by forming In··Cu, a lower ionization energy donor.52 Here, we attribute the high mobility of the fused CuInSe2 NC thin films in FETs to the effective In-doping introduced by the TOP-InCl3 complex. Trioctylphosphine (TOP) forms an LCC with InCl3. By analyzing the 31P NMR and 115In NMR spectra, it has been reported that the simple molecular adduct InCl3+L[InCl3L] (L = trioctylphosphine or trioctylphosphine oxide) forms.57 TOP-InCl3 is selected as a cation exchange precursor because the chemical hardness of TOP (6 eV) is much closer to that for Cu+ (6.28 eV) than that for In3+ (13 eV).39 These differences in chemical hardness are consistent with TOP forming a stronger bond with Cu+ than with In3+, driving cation exchange by stripping Cu+ from the lattice and forming stable TOP-CuCl complexes while simultaneously introducing In3+ into the lattice.


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The surface of the Cu2Se NC is made Se-rich by Na2Se treatment before In3+ cation exchange. The standard bond dissociation energy for In-Se is 247 kJ/mol,58 while the standard bond dissociation energy for In-P is 197.9 kJ/mol.58 Moreover, as TOP is a relatively bulky ligand, it may effectively elongate the In-P bond in the TOP-InCl3 LCCs.59 We therefore suggest that the actual In-P bond dissociation energy in the TOP-InCl3 complex is even smaller than 197.9 kJ/mol. As a result, when we introduce the TOP-InCl3 LCC during the cation exchange process, it is energetically favorable to break the In-P bond within the TOP-InCl3 LCC and form In-Se bond at the Se-rich NC surfaces, contributing to the observed In-rich NC stoichiometry. Prior to heat treatment, added In-dopants may not all occupy effective doping sites on the surface. In addition, shallow V′Cu acceptor states may exist, as unlike II-VI materials in which typical energies of cation vacancy formation are large, theoretical calculation suggests the energy to form V′Cu in CuInSe2 is small.60 These contributions are consistent with the weak n-type behavior of as cation-exchanged CuInSe2 NC thin films. Akin to the structural reorganization and dopant activation realized by annealing bulk semiconductors following ion implantation, upon heat treatment, reconfiguration and diffusion of excess surface In-dopants will be activated. The excess In may also occupy and eliminate shallow V′Cu acceptor states to form shallow In··Cu donor states instead.52 The effective doping of the NCs films upon annealing raises the Fermi Level and passivates trap states, allowing electrons to occupy higher densities of states, increasing the mobility of carrier transport.9 We design a series of control experiments to verify the In-doping effect by the TOP-InCl3 LCC. Since surface Se2- are proposed to present binding sites for excess In-dopants, we control the In stoichiometry in CuInSe2 NC thin films either by eliminating the Na2Se treatment before the In3+ cation exchange, or by replacing the Na2Se treatment before the In3+ cation exchange


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with a Na2S treatment (note: the In-S bond dissociation energy is 289 kJ/mol, also larger than InP bond dissociation energy). These films are integrated in FETs and all samples are encapsulated with 50 nm of Al2O3 deposited by ALD at 250 °C. We correlate the NC stoichiometry with µe and VT found from FET device characteristics (Figure 4A), and use the threshold voltage shift ΔVT to calculate the excess electron concentration (Δn) as: 𝛥𝑛 =

𝛥𝑉𝑇𝐶𝑂𝑋 𝑞𝑑

, 40

where COX is the unit capacitance of the oxide (11.6 nF/cm2) and d is the thickness of the thin film. Table 1 reports the surface treatments and their corresponding FET µe, VT, and Δn. We also calculate the 𝛥𝑛 from capacitance-voltage (C-V) measurements of these devices (Supporting Information Figure S8). Table 1. Device Characteristics for Al2O3-encapsulated CuInSe2 NC FETs with different surface treatments (Δn calculated from ΔVT). Surface treatment Before In3+ cation exchange N. A.

µe cm2/Vs 0.7 ± 0.2

VT V 26.7 ± 2.1

Δn cm-3 0

Na2S

2.3 ± 0.5

22.8 ± 1.4

7.0 × 1017

Na2Se

10.5 ± 2.4

19.1 ± 1.8

1.36 × 1018

Omitting the chalcogen surface treatment presents the fewest chalcogen binding sites for In3+ amongst the control samples, and is consistent with the smallest measured In:Cu ratio of 0.85 ± 0.08:1 by EDS (Figure 4B). FETs have the smallest µe=0.7 ± 0.2 cm2/Vs and the largest VT=26.7 ± 2.1V. Na2S surface treatment leads to 34.3% line narrowing of the Cu2Se WAXS pattern (Supporting Information Figure S9, Scherrer grain size of 6.2 nm), consistent with less fusion of NCs treated with Na2S in comparison to that for Na2Se treatment (57.3% line


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narrowing, Scherrer grain size of 9.9 nm).42 It also provides excess surface S2- for In-dopants binding, consistent with an increase in the In:Cu ratio to 0.92 ± 0.08:1 (Figure 4B) and yielding a higher µe=2.3 ± 0.5 cm2/Vs and a smaller VT=22.8 ± 1.4V. The additional electron concentration is 7.0 × 1017 electrons/cm3, as calculated from VT (Figure 4C), and 3.6 × 1018 electrons/cm3, as calculated from C-V measurement (Supporting Information Figure S8B). The larger Δn calculated from C-V measurements is not surprising as parasitic capacitance in the thin film transistor configuration will increase the measured capacitance.61 Finally, Na2Se surface treatment of CuInSe2 NC films introduces the largest degree of fusion and excess surface Se2- for In-dopant binding, yielding the largest In:Cu ratio of 1.07 ± 0.10:1 (Figure 4B). This treatment produces the best performing CuInSe2 NC FETs with the highest µe=10.5 ± 2.4 cm2/Vs and the lowest VT=19.1 ± 1.8V. The additional electron concentration, compared to that for samples without surface chalcogen treatment, increases to 1.36 × 1018 electrons/cm3, as calculated from VT (Figure 4C), and 6.0 × 1018 electrons/cm3, as calculated by C-V measurement (Supporting Information Figure S8B). The increase in In concentration and therefore doping for Se-treated in contrast to Streated NCs is consistent with a previous observation for In-doping of similarly treated CdSe nanostructures, and attributed to the smaller electronegativity and difference in atomic radii for Se.62 The dependence of the In:Cu ratio and therefore the electron concentration on chalcogen surface treatments, supports the hypothesis that surface chalcogens provide binding sites for Indopant addition from the TOP-InCl3 LCCs in the cation exchange process to create effective ndoping for CuInSe2 NC thin film upon heat treatment. We note that the increase in surface chalcogens also leads to greater NC fusion, which contributes to the increase in µe and In-doping efficiency.63 Since µe and doping concentration and efficiency and fusion are inter-dependent, it


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is not possible to separate their contributions to FET performance. We also note that all of our CuInSe2 NC samples are metal-rich. Se vacancies (V··Se) also serve as donors, however their higher, 80 meV ionization energy in comparison to the 8 meV for In··Cu donors52 in bulk CuInSe2 and the trend of increasing carrier concentration in the control samples with chalcogen treatments that reduce V··Se, suggest In··Cudonors are more likely to serve as the dominant, effective dopant in CuInSe2 NC films and devices.

Figure 4. (A) Transfer curves (VDS=50 V) of CuInSe2 NC FETs with (cyan) no surface chalcogen treatment; (light blue) Na2S surface treatment; (dark blue) Na2Se surface treatment. (B) In:Cu ratio and (C) excess electron concentration (Δn) calculated from ΔVT for CuInSe2 NC films and FETs with different surface treatments.


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Cation-Exchanged CuInSe2 NC Integrated Inverters: The gate oxide layer of the CuInSe2 NC FETs is scaled down to reduce device operating voltage, important to FET integration in circuits and other IoT devices. The low operating voltage FETs are built on rigid substrates. A slightly modified device structure10 with a gate oxide stack consisting of 15 nm Al2O3 (bottom) and 10 nm SiO2 (top) is adopted. The measured unit capacitance of the gate oxide stack is 186 nF/cm2. The device schematic and the transfer and output curves of the CuInSe2 NC FETs built on the thin gate oxide stack are shown in Figure 5A. For 20 of devices with channel lengths ranging from 40 µm to 120 µm, the linear regime µe is 8.0 ± 1.8 cm2/(Vs) (VDS=0.1 V) and saturation regime µe is 8.8 ± 0.8cm2/(Vs) (VDS=3 V).

Figure 5. (A) Transfer (dark blue: VDS=3 V; dark cyan: VDS=0.1 V) and output (inset) curves of a CuInSe2 NC FET fabricated on an ALD grown oxide stack of 10 nm SiO2 on top of 15 nm Al2O3. Device schematic is shown inset. Devices are built on rigid wafer substrate. (B) Photo of


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two CuInSe2 NC integrated inverters. (C) Transfer curve of a CuInSe2 NC integrated inverter. Inset is the inverter schematic. We fabricate proof-of-concept inverters to demonstrate that CuInSe2 NC-based FETs can be further used in integrated circuits. CuInSe2 NC inverters are fabricated using a saturated-load design with a channel width to length ratio for the load transistor of 10 and for the driver transistor of 40, yielding an ideal gain of -2.10 The photograph of fabricated inverters is shown in Figure 5B. The CuInSe2 NC inverter voltage transfer characteristic (VTC) is shown in Figure 5C. For the 10 inverters we fabricate, we find a gain of -1.72 ± 0.26 V/V with an average output swing of 2.07 ± 0.12 V. The output swing makes use of 68.9 ± 4.0 % of the available supplied voltage. The gain and VTC for our CuInSe2 NC inverters are comparable to that for CdSe NC inverters with the same circuit topology,10 indicating a sufficiently small single transistor variation in µe and VT, as well as a low VT and high current modulation in scaled CuInSe2 NC FETs. CONCLUSIONS In summary, we demonstrate and characterize a post-deposition cation exchange method to form fused CuInSe2 NC thin films. Our mechanistic investigation demonstrates that TOPInCl3 complexes both mediate the In3+ cation exchange and also dope the CuInSe2 NC. With the help of a post-cation exchange ALD encapsulation process, CuInSe2 NC FETs and integrated circuits are fabricated and show performance comparable to previously reported II-VI, IV-VI, and III-V NC devices containing toxic Pb-, Cd-, and -As elements as summarized in Supporting Information Table S1. Because our NC devices are free of highly toxic elements, our findings show the potential application of CuInSe2 NC thin films for large-area, flexible, wearable, and implantable electronic technologies.


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EXPERIMENTAL SECTION Materials. Trioctylphosphine oxide (TOPO, 90%), octadecylamine (ODA, 90%), 1-octadecene (ODE, 90%), tributylphosphine (TBP, 97%), trioctylphosphine (TOP, 90%), (3-mercaptopropyl)trimethoxysilane (MPTS, 95%), sodium sulfide (Na2S, 99.8%), tetrakis(acetonitrile)copper(i) hexafluorophosphate (99.8%), selenium powder, anhydrous hexane, anhydrous 2-propanol, anhydrous methanol, anhydrous toluene, and anhydrous acetone are purchased from SigmaAldrich. Anhydrous dimethylformamide (DMF, 99.8%), Octadecylamine (ODA, 90%) and anhydrous tetrahydrofuran (THF, 99.9%) are purchased from Acros. Sodium selenide (Na2Se, 99.8%) and indium chloride (InCl3, 99.999%, ultra dry) are purchased from Alfa Aesar. CdSe NC Synthesis and Ligand Exchange. To synthesize CdSe NCs, 40 g TOPO, 40 g ODA and 4.20 g cadmium stearate (synthesized following a previously reported method11) are loaded in a 3-neck flask and held under vacuum for 1 hour at 135 °C. Then, the solution is heated up to 320 °C under nitrogen and 20.0 mL of 1.25 M Se in TBP (prepared following a previously reported method11) is quickly injected into the flask, nucleating NCs. The reaction is kept at 290 °C for 15 mins to further grow NCs to and stopped by slowly injecting 50 mL of anhydrous toluene, resulting in NC samples with batch-to-batch size ranging from 3.6-4.1 nm in diameter. The solution is transferred into a nitrogen-filled glovebox. The CdSe NCs are precipitated by ethanol and redispersed in hexanes. The CdSe NCs are further washed by precipitation/redispersion with ethanol/hexanes, acetone/hexanes and 2-propanol/hexanes. The final dispersion of CdSe NCs are kept in hexanes with an optical density (OD) ~60 in glovebox for long term storage and diluted to OD 20 before ligand exchange.


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The ligand exchange of CdSe NCs is conducted in a nitrogen-filled glovebox. 0.6 mL of CdSe NCs (OD 20 in hexanes) is added to a centrifuge tube followed by the addition of 1.8 mL of hexanes and 1.2 mL of 10 mg/mL NH4SCN in acetone solution. The mixture is vortexed for 2 min and then centrifuged for 1 min to precipitate the NCs. The ligand-exchanged NCs are further washed by vortexing in THF and toluene. The final ligand-exchanged CdSe NCs are dispersed in 0.1 mL DMF and spin-coated on various MPTS treated substrates (prepared by immersing various substrates into MPTS in toluene solutions for 12 h). Na2Se/Na2S Treatment. Na2Se/Na2S treatments are conducted in a nitrogen-filled glovebox by soaking the NC thin films (CdSe or Cu2Se) into 10 mM methanolic Na2Se/Na2S solutions for 15 min at room temperature. Complete surface treatment is found for lower 1 mM concentrations and shorter treatment times, but the higher concentration and longer treatment time is selected to guarantee the re-use of the methanolic Na2Se/Na2S solution stored in glovebox for up to a few weeks. Thin films are then thoroughly washed by methanol after the treatment. Cation Exchange precursor preparation for Cu+/In3+. The Cu+ cation exchange is conducted by soaking Se-enriched CdSe NC thin films in jars with an 0.8 mg/mL methanolic tetrakis(acetonitrile)copper(i) hexafluorophosphate solution for 30 min on a hot plate in the glovebox held at 60 °C. The cation exchange can also be achieved at higher tetrakis(acetonitrile)copper(i) hexafluorophosphate concentrations of up to 4.8 mg/mL or at room temperature for longer reaction time. Then thin films are thoroughly washed by methanol. The In3+ cation exchange is conducted by soaking the Cu2Se NC thin films in a TOP-InCl3 LCC solution in ODE for 6 h on a hot plate in the glovebox held at 125 °C. The final CuInSe2 NC thin film is washed by hexane followed by methanol. The LCC precursor is prepared by adding 5 mmol of InCl3 to 5 mL of 90% TOP. The precursor solution is stirred overnight at 100 °C to


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dissolve the InCl3. Then, 15 mL of degassed ODE is added to dilute the LCC precursor and increase the total volume of the solution. The diluted LCC precursor is kept in glovebox and reused for up to a month. Fabrication of low operating voltage FETs and integrated inverters. 20 nm of Al is thermally deposited through a shadow mask to form the FET gate on substrates composed of heavily n-doped Si wafer with 300 nm thermally grown SiO2. Oxygen plasma is used to selectively increase the native oxide thickness on Al. 15 nm of Al2O3 and subsequently 10 nm of SiO2 is grown by ALD to form the gate oxide stack. The substrate is soaked in an MPTS in toluene solution for 12 h. Then, spin-coating and the cation exchange of NC thin films are conducted on the substrate. 50 nm of Au is thermally evaporated using a shadow mask to define the source and drain electrodes. Finally, 50 nm of Al2O3 is grown by ALD at 250 °C to encapsulate the devices and inverters. Characterization. Absorption measurements are carried out in air using an Agilent Cary 5000 UV-Vis-NIR absorption spectrometer. PL measurements are performed in air using a HoribaJobin-Yvon Fluorolog FL3 spectrofluorometer. Samples are excited using a CW 378 nm diode laser (Picoquant) and PL is detected using a Hamamatsu R928 and R5509-73 photomultiplier tubes. HRTEM measurements are carried out using a JEOL 2010F electron microscope operating at 200 kV. NC thin films are directly prepared on a Cu grid coated with an amorphous carbon thin film, then the sequential treatments are identically carried out by immersing the TEM grids into each solution. WAXS measurements are carried out in a Rigaku Smartlab diffractometer operating at 40 kV and 30 mA in reflection geometry. Samples are sealed in nitrogen-filled plastic bags during WAXS measurements.


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EDS measurements are carried out with an Oxford Instruments energy-dispersive spectrometer in a FEI Quanta 600 ESEM. The accelerating voltage is 10 kV and the integration time is 30 s. The stoichiometric ratios are obtained by averaging the values obtained from three different areas (2000 m x 1500 m) of the sample. ICP-OES measurement is conducted on a Spectro Genesis spectrometer with a concentric nebulizer. The cross-sectional SEM image is prepared in a focused-ion beam system (FIB, FEI Strata DB235). 300 nm Pt/C mixtures are in-situ deposited onto the sample surface to create a protective layer. Ga ions under the accelerating voltage of 30 kV are used to mill a square hole with dimensions of 1 µm in width, 4 µm in length and 3 µm in depth. Then the cross-section image is taken at a 52° tilt of the stage. The FET and integrated inverter measurements are carried out on a Karl Suss PM5 probe station mounted in a nitrogen-filled glovebox, using a model 4156C Agilent semiconductor parameter analyzer. Capacitance-voltage measurements are carried out using a HP Model 4192A LF impedance analyzer with source and drain electrodes electrically shorted and connected to the low terminal, while the gate is connected to the high terminal.

ACKNOWLEGEMENT We thank Q. Zhao, F. Stinner, N. Cui, and C. Zeng for helpful discussions. The authors are grateful for primary support of this work from the National Science Foundation (NSF) MRSEC Program under Award No. DMR-1720530 for NC synthesis, post-deposition cation exchange, WAXS, EDS, HRTEM, device fabrication, and electronic property measurements. The optical absorption and PL measurements are supported by Center for Advanced Solar


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Photophysics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. FIB cutting and imaging and ICP-OES are supported by the Office of Naval Research Multidisciplinary University Research Initiative Award No. N00014-18-1-2497. Electron microscopy/FIB is performed in facilities supported by the NSF MRSEC program DMR-1720530. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ASSOCIATED CONTENT Supporting Information The Supporting Information Figure S1-S9 is available free of charge via the Internet at http://pubs.acs.org. The authors declare no competing financial interest.

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