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Tuning Carrier Mobilities and Polarity of Charge Transport in Films of CuInSexS2–x Quantum Dots Sergiu Draguta, Hunter McDaniel,* and Victor I. Klimov* Solution-processed semiconductor quantum dots (QDs) are an emerging class of materials with a wide range of applications in electronics, optics, and optoelectronics. The ability to adjust semiconductor band gap and thereby tailor the absorption spectrum with nanocrystal size makes QDs particularly useful for photovoltaic engineering. On the other hand, the high photoluminescence (PL) efficiency also makes QDs attractive as active materials for light emitting diodes, solid-state lighting, and displays. Among various QD systems, CuInSexS2–x (CISeS) QDs are especially suitable for light harvesting due to their direct and near-optimal, yet tunable, band gap and high absorption cross-section. In addition to controlling band-gap with size,[1] the use of alloyed CISeS QDs offer an additional means of tunability with anion composition (x). For example, increasing the ratio of Se to S (that is, increasing x) decreases band-gap from the visible to the near infrared (NIR).[2] Yet another means for tunability is provided by control of the cation composition. For example, it has been shown that modifying the copper to indium ratio in bulk CuInSe2 affects charge transport.[3] In addition to these attractive qualities, CISeS QDs have the further advantage of much lower cytotoxicity compared with CdSebased nanostructures due to the absence of heavy metals.[4] Relatively high power conversion efficiencies have been demonstrated for CISeS-based solar cells ranging from 20% for bulk thin films[5] to 9% for sintered nanocrystal films,[6] and 5%–7% for liquid-junction QD-sensitized devices.[7,8] The best thin film CISeS QD solar cells without sintering perform modestly well with efficiencies around 1.6%.[9] In general, avoiding sintering is preferred to reduce the time and energy required to fabricate devices. This can also enable exploiting novel concepts derived from effects of quantum confinement such as spectrally tunable absorption onset (especially useful, e.g., in multijunction devices) and carrier multiplication,[10] which is generation of multiple excitons by single absorbed photons. The primary factor limiting the performance of unsintered CISeS nanocrystal solar cells is poor charge transport.[9] In order to better understand charge transport in films of coupled CISeS QDs and devise effective approaches for its control, we incorporate these films into field-effect transistors (FETs) and evaluate their performance as a function of QD surface properties, heat treatment, and the type of source and drain electrodes. We observe that as-synthesized CISeS QDs show fairly good p-type charge transport (hole mobilities of ≈10−4 cm2 V–1 s–1) despite the presence of fairly bulky native ligands comprising S. Draguta, Dr. H. McDaniel, Dr. V. I. Klimov Center for Advanced Solar Photophysics Los Alamos National Laboratory Los Alamos, NM 87545, USA E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201404878
Adv. Mater. 2015, DOI: 10.1002/adma.201404878
1-dodecanethiol (DDT) and oleylamine (OLA). After QD recapping with shorter 1,2-ethanedithiol (EDT) molecules for enhancing interdot coupling, the mobility increases by more than two orders of magnitude to ≈0.03 cm2 V–1 s–1. Cation exchange with Cd2+ (in solution prior to film deposition) results in ambipolar behavior with high hole and electron mobilities of ≈0.02 cm2 V–1 s–1. Annealing with indium contacts switches charge transport to n-type with the highest mobility of ≈0.02 cm2 V–1 s–1. For this study, we synthesize CISeS QDs with 60% Se cations (x = 1.2). Their peak emission wavelength is λ = 960 nm, which corresponds to spectral energy of ≈1.3 eV (Figure 1a; red trace). The absorption onset is characterized by a similar wavelength (Figure 1a; black trace). The fabricated QDs have a shape of a tetrahedron with a mean height of ≈4.4 nm (Figure 1b). To fabricate FETs (Figure 1c,d), we use a heavily doped silicon substrate as a bottom gate electrode and a 500 nm layer of SiO2 (dry thermal oxide) provides a low-leakage gate dielectric. Films of CISeS QDs were deposited onto the substrate by spin coating at 2500 rpm. In order to make a consistent QD layer thickness of ≈50 nm, 4–5 cycles of spin coating separated by 5 min of annealing of the film at 100 °C were repeated. For ligand exchange, a 0.5 mmol solution of the new ligand in methanol was spin coated during each cycle after spin coating QDs. We observe the film roughness by atomic force microscopy to typically be on the order of the QD size indicating that the layer-by-layer approach fills voids created by volume contraction during ligand exchange. The introduction of the new ligand was always followed by rinsing with neat methanol in order to replace and remove excess ligands. The annealing step was helpful in preventing dissolution of QDs upon deposition of subsequent layers even after ligand exchange. We tested a number of different ligands including EDT, ethylenediamine (EDA), ammonium thiocyanate (NH4SCN), and sodium sulfide (Na2S). Worth noting, the Murray group reported photoconductivity of large CuInSe2 nanocrystals treated with NH4SCN; however, they did not quantify mobility or charge carrier polarity.[11] We observed the strongest gating effect and highest mobilities with EDT and so this is our focus here. Results for the other ligands can be found in the Supporting Information, Figure S1. We note that as-synthesized CISeS QDs are not as predisposed to ligand exchange as PbSe or PbS[12] QDs and required the 100 °C annealing step mentioned. This is possibly caused by incomplete exchange due to the presence of “crystal-bound” DDT molecules on QD surfaces.[13] Unless stated otherwise, carrier mobilities were measured in the linear regime from the slope of the drain current (ID) versus gate voltage (VG) dependence assuming a gradual channel approximation.[14,15] We started our investigation by using films of as prepared CISeS QDs with original DDT and OLA passivation. Films of PbSe QDs with native ligands such as oleic acid and
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QDs or, alternatively, might be indicative that the effect of apparent doping relates not to 4.4 nm QDs interior charges but rather to charges located 960 nm CuInSe1.2S0.8 within intra-gap states that are likely of surface origin. The latter scenario would suggest that the observed charge transport is not due to intrinsic quantized QD states but rather intra-gap surface states. There is an ample amount of evidence in the literature that the involvement of surface states in “dark” charge transport (that is, transport measured 1.0 1.5 2.0 2.5 3.0 without illumination) is a general property Photon Energy (eV) of QD films as indicated, for example, by c d 150 nm thick Au or In extreme sensitivity of conductivity to various surface treatments[23] and a large difference (drain/source) in carrier mobilities measured with and ~50 nm without illumination.[24,25] thick source Similar to PbS and PbSe QDs,[14,26] we 66 µm CISeS observe that CISeS QDs are moderately QDs sensitive to air exposure, showing a large drain 300 nm SiO2 increase in conductivity over time under air ++ (Supporting Information, Figure S2). Such p Si substrate behavior is likely related to degenerate p-type 300 µm doping by oxidation.[27] In the related material system of Cu2S nanocrystals, the formaFigure 1. CISeS QD characteristics and FET architecture. a) Absorption (black) and photolution of copper vacancies due to oxidation also minescence (red) spectra for typical CISeS QDs used in this study. b) Transmission electron leads to an increase in the concentration of microscopy image of the same QDs. c) Top-down optical image of a QD-FET. d) Cross-sectional holes.[28,29] structure of a QD-FET (not to scale). In order to decrease the distance between CISeS QDs in the film and thus increase interdot coupling, trioctylphosphine are normally insulating and do not exhibit we replace the bulky native ligands with shorter molecules. a gating effect.[16,17] On the other hand, the films of as-synthesized CISeS QDs are fairly conductive and show a pronounced gating effect (Figure 2a). a 2.0 b 0 The long insulating “native” ligands (in in air 2 minutes 1.5 as-synthesized this case DDT and OLA) should present -200 1.0 large barriers to charge mobility, yet we 0.5 observe fairly efficient p-type transport -400 VG= 0.0 with mobilities of 1.3 × 10−4 cm2 V–1 s–1, V = 0V 0 G -0.5 which is the highest reported for OLA capped -600 -10 V +/-10V [ 1,9,18,19 ] -1.0 -20 V +/-20 V QDs. We speculate that the pyramidal -30 V +/-30 V -1.5 -800 shape of CISeS QDs could lead to closer prox-40 V +/-40 V -2.0 imity of some QD surfaces (see Figure 1b) -40 -20 0 20 40 -40 -30 -20 -10 0 and hence more intimate contact comVDS (V) VDS (V) pared to spherical QDs even with the native c d 600 ligands. We do not believe that these QDs are 0.0 VDS= -10V unusually poorly passivated based on their -0.2 500 colloidal stability in solution and consistently -0.4 400 bright PL. -0.6 300 These measurements suggest that asV = -0.8 EDT treated EDT treated G -0 V prepared CISeS QDs are p-doped. However, -1.0 200 -10 V while exhibiting the signatures of apparent -20 V -1.2 100 -30 V doping in transport measurements, these -40 V -1.4 dots do not show typical signs of charges -40 -30 -20 -10 0 -20 -10 0 10 20 being present in the QD interior such as VDS (V) VG (V) PL quenching by Auger recombination or bleaching of band-edge absorption, observed, Figure 2. P-type transport, typical of CISeS QDs. a) Drain current (I ) versus source–drain D for example, in the cases of chemical[20] or voltage (VSD) for the untreated QD film, b) for the film exposed to air for 2 minutes, and [ 21,22 ] electrochemical doping. This might be c) for the air-free EDT-treated QD film. d) Drain current versus gate voltage (VG) for the device due to low doping levels in as-synthesized characterized in c.
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observe that the CISeS-Cd QDs show ambipolar conductivity (Figure 3a), which indiVDS=20V 8 VG= cates switching from the apparent p-type to 4 0V 0.2 VG= +/-10 V the apparent intrinsic transport, which might 6 0 V +/-20 V 3 +/-10 V again point toward its surface character. This 0.0 +/-30 V -20 -10 0 10 20 +/-20 V VG (V) 4 behavior is unique to the QD form of CISeS, +/-30 V 2 +/-40 V as there have been no previous reports of ambi2 1 polar transport in the bulk form of this material. For example, in bulk CuInSe2, Cd has been 0 0 -40 -20 0 20 40 -40 -20 0 20 40 shown to be a donor impurity,[33] thus changing VDS (V) VDS (V) the majority carrier from holes to electrons. c d Here, we speculate that the Cd2+ surface treat40 3 V =20V ment compensates acceptor-like surface states V = DS G 10 0V 2 that results in an approximately intrinsic film. 10 V VG= 30 8 1 20 V After EDT-treatment (Figures 3b and 3c), the 0V 30 V 10 V 40 V 6 0 -20 -10 0 10 20 measured FET saturation mobilities for elec20 20 V VG (V) trons and holes are 0.018 cm2 V–1 s–1 and 30 V 4 40 V 0.024 cm2 V–1 s–1, respectively. These values are 10 2 comparable to what has been reported for lead 0 chalcogenide QDs using a similar EDT treat0 0 10 20 30 40 ment procedure.[12] The highest electron and 0 10 20 30 40 VDS (V) VDS (V) hole mobilities reported for EDT-only treated PbS QDs are around 10−4 cm2 V–1 s–1,[34] while Figure 3. FET performance for ambipolar and n-type CISeS QD films. a) Drain current 2 –1 –1 2+ 2+ versus source–drain voltage for Cd -exposed QDs, and b) for Cd -exposed QDs treated for PbSe QDs they are 0.07 cm V s for 2 –1 –1 [12] electrons and 0.03 cm V s for holes. In with EDT with the drain current versus gate voltage shown inset. c) Drain current versus source–drain voltage for indium-annealed FETs, and d) for indium-annealed FETs treated another example, EDT-treated InAs QD FETs with EDT with the drain current versus gate voltage shown inset. showed an even lower mobility of around 10−5 cm2 V–1 s–1.[35] Doping I-III-VI2 QDs n-type would be of great use in It was shown previously that inter-QD spacing decreases by 1.6 nm when oleic acid is exchanged with EDT.[30] We find engineering p–n junctions in solar cells. Prior studies have shown that bulk CuInSe2 can be doped p-type or n-type by that the most common QD film treatment using EDT has the greatest effect on transport, with the mobility (measured in exploiting “intrinsic” defects (that is, via deviation from stoithe linear regime) increasing by more than two orders of magchiometry)[3,36–38] or by extrinsic dopants such as Zn,[39] Cd,[33] nitude to 0.029 cm2 V–1 s–1 (Figure 2c). NH4SCN and EDA also or Cl.[40] In the case of CISeS QDs, the apparent p-type doping affected the mobility but to a much lesser degree (Supporting of as prepared materials would need to be overcompensated Information, Figure S1). in order to switch the conductivity to n-type. If we hypotheAmbipolar Behavior: Decreasing the distance between QDs by size that the p-type conductivity of as-synthesized QDs arises ligand exchange increases charge mobility but does not appear from indium vacancies on the QD surfaces, we might expect to affect the apparent nature of majority charge carriers (that to achieve the effect of overcompensation by introducing addiis, holes in the present case). A p-type behavior in bulk CISeS tional indium into the film as was previously demonstrated is related to indium vacancies (e.g., when the copper to indium by Kagan and co-workers for CdSe QDs.[31] In this published ratio is greater than one)[3] but we should be careful in ascribing approach, indium is incorporated into the QD film by annealing the entire QD-FET fabricated using indium as the source- and bulk self-doping processes to QD films that are highly granular drain-contact material.[31,41,42] The low melting point of indium and likely have a different carrier transport mechanism. Considering the involvement of surface states in charge conductallows for such diffusion without significant sintering. ance the manipulation of surface properties of the QDs can In our studies, we decided to pursue a similar strategy. Because directly affect both the type and the level of apparent doping of indium is a component element of CISeS QDs, we suspected the film.[16,17] There are a few reported methods of filling surthat the crystal lattice would accommodate indium diffusion and allow for filling of vacancies. The effect of indium on transport face trap states in a film including halide passivation,[26] atomic was tested by thermally evaporating indium as the source and layer deposition (ALD),[14] and diffusion of metal ions from drain contacts (by shadow mask). In order to ensure complete device contacts to reduce trap densities.[31] indium diffusion across the channel, the distance between the In order to better passivate surfaces and thereby reduce the contacts for these FETs is smaller (30 µm) and we deposited density of trap states and/or their occupancy, we exposed the enough indium in each contact (1 mm wide by 150 nm thick) to QDs to Cd2+ ions based on a previous report.[32] This surface diffuse throughout the interior of the entire QD film. After contreatment results in 22% of QD cations being exchanged with tact deposition, the films were annealed at 250 °C for 1 h. cadmium based on inductively coupled plasma atomic emisIn a control study with indium source and drain contacts sion spectroscopy. We know that this treatment is effective at but without annealing, the measured conductivity is p-type passivating CISeS QD surfaces because it leads to increases (Supporting Information, Figure S3). However, after annealing at in both the PL quantum yield and the exciton lifetime.[32] We
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250 °C, the conductivity of the film switches to n-type (Figure 3c). These observations support our hypothesis that cation stoichiometry and specifically the stoichiometry of the QD surface is at least one of the factors controlling the type and the level of doping of CISeS QD films. This assessment is somewhat similar to one derived from previous studies of bulk CISeS where though the defects are not of primarily of surface origin as in QDs but distributed over the entire semiconductor volume. Specifically, for bulk materials it was observed that indium-rich samples were n-type while in the copper-rich samples the conductivity was p-type.[3] A transition from p-type to n-type was observed for both as-synthesized QD films as well as for EDT-treated films (Figure 3d). From the measured ID–VG characteristics, we derive linear electron mobilities of 2.4 × 10−3 cm2 V–1 s–1 and 1.8 × 10−2 cm2 V–1 s–1 for as-synthesized QD films and EDTtreated films, respectively. We note that the n-type films are sensitive to air-exposure with increasing conductivity and diminished gating effect over time, similar to the p-type devices. It is known that annealing can increase mobility of the QD films by decreasing inter-particle distance.[23,43,44] However, it was also shown that overheating QD films might lead to decomposition of ligands and QD sintering which eventually leads to the loss of quantum confinement and restoration of bulk-material properties.[45–47] To verify that QD integrity is preserved in our samples after annealing, we conduct an additional control experiment in which CISeS QD FETs with gold contacts are heated to 250 °C for 1 h. Following this procedure, we detect only a slight increase in the mobility (Supporting Information, Figure S3). We also observe that the absorption spectra of films treated in the same way experience only a small (
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Tuning Carrier Mobilities and Polarity of Charge Transport in Films of CuInSexS2–x Quantum Dots Sergiu Draguta, Hunter McDaniel,* and Victor I. Klimov* Solution-processed semiconductor quantum dots (QDs) are an emerging class of materials with a wide range of applications in electronics, optics, and optoelectronics. The ability to adjust semiconductor band gap and thereby tailor the absorption spectrum with nanocrystal size makes QDs particularly useful for photovoltaic engineering. On the other hand, the high photoluminescence (PL) efficiency also makes QDs attractive as active materials for light emitting diodes, solid-state lighting, and displays. Among various QD systems, CuInSexS2–x (CISeS) QDs are especially suitable for light harvesting due to their direct and near-optimal, yet tunable, band gap and high absorption cross-section. In addition to controlling band-gap with size,[1] the use of alloyed CISeS QDs offer an additional means of tunability with anion composition (x). For example, increasing the ratio of Se to S (that is, increasing x) decreases band-gap from the visible to the near infrared (NIR).[2] Yet another means for tunability is provided by control of the cation composition. For example, it has been shown that modifying the copper to indium ratio in bulk CuInSe2 affects charge transport.[3] In addition to these attractive qualities, CISeS QDs have the further advantage of much lower cytotoxicity compared with CdSebased nanostructures due to the absence of heavy metals.[4] Relatively high power conversion efficiencies have been demonstrated for CISeS-based solar cells ranging from 20% for bulk thin films[5] to 9% for sintered nanocrystal films,[6] and 5%–7% for liquid-junction QD-sensitized devices.[7,8] The best thin film CISeS QD solar cells without sintering perform modestly well with efficiencies around 1.6%.[9] In general, avoiding sintering is preferred to reduce the time and energy required to fabricate devices. This can also enable exploiting novel concepts derived from effects of quantum confinement such as spectrally tunable absorption onset (especially useful, e.g., in multijunction devices) and carrier multiplication,[10] which is generation of multiple excitons by single absorbed photons. The primary factor limiting the performance of unsintered CISeS nanocrystal solar cells is poor charge transport.[9] In order to better understand charge transport in films of coupled CISeS QDs and devise effective approaches for its control, we incorporate these films into field-effect transistors (FETs) and evaluate their performance as a function of QD surface properties, heat treatment, and the type of source and drain electrodes. We observe that as-synthesized CISeS QDs show fairly good p-type charge transport (hole mobilities of ≈10−4 cm2 V–1 s–1) despite the presence of fairly bulky native ligands comprising S. Draguta, Dr. H. McDaniel, Dr. V. I. Klimov Center for Advanced Solar Photophysics Los Alamos National Laboratory Los Alamos, NM 87545, USA E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201404878
Adv. Mater. 2015, DOI: 10.1002/adma.201404878
1-dodecanethiol (DDT) and oleylamine (OLA). After QD recapping with shorter 1,2-ethanedithiol (EDT) molecules for enhancing interdot coupling, the mobility increases by more than two orders of magnitude to ≈0.03 cm2 V–1 s–1. Cation exchange with Cd2+ (in solution prior to film deposition) results in ambipolar behavior with high hole and electron mobilities of ≈0.02 cm2 V–1 s–1. Annealing with indium contacts switches charge transport to n-type with the highest mobility of ≈0.02 cm2 V–1 s–1. For this study, we synthesize CISeS QDs with 60% Se cations (x = 1.2). Their peak emission wavelength is λ = 960 nm, which corresponds to spectral energy of ≈1.3 eV (Figure 1a; red trace). The absorption onset is characterized by a similar wavelength (Figure 1a; black trace). The fabricated QDs have a shape of a tetrahedron with a mean height of ≈4.4 nm (Figure 1b). To fabricate FETs (Figure 1c,d), we use a heavily doped silicon substrate as a bottom gate electrode and a 500 nm layer of SiO2 (dry thermal oxide) provides a low-leakage gate dielectric. Films of CISeS QDs were deposited onto the substrate by spin coating at 2500 rpm. In order to make a consistent QD layer thickness of ≈50 nm, 4–5 cycles of spin coating separated by 5 min of annealing of the film at 100 °C were repeated. For ligand exchange, a 0.5 mmol solution of the new ligand in methanol was spin coated during each cycle after spin coating QDs. We observe the film roughness by atomic force microscopy to typically be on the order of the QD size indicating that the layer-by-layer approach fills voids created by volume contraction during ligand exchange. The introduction of the new ligand was always followed by rinsing with neat methanol in order to replace and remove excess ligands. The annealing step was helpful in preventing dissolution of QDs upon deposition of subsequent layers even after ligand exchange. We tested a number of different ligands including EDT, ethylenediamine (EDA), ammonium thiocyanate (NH4SCN), and sodium sulfide (Na2S). Worth noting, the Murray group reported photoconductivity of large CuInSe2 nanocrystals treated with NH4SCN; however, they did not quantify mobility or charge carrier polarity.[11] We observed the strongest gating effect and highest mobilities with EDT and so this is our focus here. Results for the other ligands can be found in the Supporting Information, Figure S1. We note that as-synthesized CISeS QDs are not as predisposed to ligand exchange as PbSe or PbS[12] QDs and required the 100 °C annealing step mentioned. This is possibly caused by incomplete exchange due to the presence of “crystal-bound” DDT molecules on QD surfaces.[13] Unless stated otherwise, carrier mobilities were measured in the linear regime from the slope of the drain current (ID) versus gate voltage (VG) dependence assuming a gradual channel approximation.[14,15] We started our investigation by using films of as prepared CISeS QDs with original DDT and OLA passivation. Films of PbSe QDs with native ligands such as oleic acid and
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QDs or, alternatively, might be indicative that the effect of apparent doping relates not to 4.4 nm QDs interior charges but rather to charges located 960 nm CuInSe1.2S0.8 within intra-gap states that are likely of surface origin. The latter scenario would suggest that the observed charge transport is not due to intrinsic quantized QD states but rather intra-gap surface states. There is an ample amount of evidence in the literature that the involvement of surface states in “dark” charge transport (that is, transport measured 1.0 1.5 2.0 2.5 3.0 without illumination) is a general property Photon Energy (eV) of QD films as indicated, for example, by c d 150 nm thick Au or In extreme sensitivity of conductivity to various surface treatments[23] and a large difference (drain/source) in carrier mobilities measured with and ~50 nm without illumination.[24,25] thick source Similar to PbS and PbSe QDs,[14,26] we 66 µm CISeS observe that CISeS QDs are moderately QDs sensitive to air exposure, showing a large drain 300 nm SiO2 increase in conductivity over time under air ++ (Supporting Information, Figure S2). Such p Si substrate behavior is likely related to degenerate p-type 300 µm doping by oxidation.[27] In the related material system of Cu2S nanocrystals, the formaFigure 1. CISeS QD characteristics and FET architecture. a) Absorption (black) and photolution of copper vacancies due to oxidation also minescence (red) spectra for typical CISeS QDs used in this study. b) Transmission electron leads to an increase in the concentration of microscopy image of the same QDs. c) Top-down optical image of a QD-FET. d) Cross-sectional holes.[28,29] structure of a QD-FET (not to scale). In order to decrease the distance between CISeS QDs in the film and thus increase interdot coupling, trioctylphosphine are normally insulating and do not exhibit we replace the bulky native ligands with shorter molecules. a gating effect.[16,17] On the other hand, the films of as-synthesized CISeS QDs are fairly conductive and show a pronounced gating effect (Figure 2a). a 2.0 b 0 The long insulating “native” ligands (in in air 2 minutes 1.5 as-synthesized this case DDT and OLA) should present -200 1.0 large barriers to charge mobility, yet we 0.5 observe fairly efficient p-type transport -400 VG= 0.0 with mobilities of 1.3 × 10−4 cm2 V–1 s–1, V = 0V 0 G -0.5 which is the highest reported for OLA capped -600 -10 V +/-10V [ 1,9,18,19 ] -1.0 -20 V +/-20 V QDs. We speculate that the pyramidal -30 V +/-30 V -1.5 -800 shape of CISeS QDs could lead to closer prox-40 V +/-40 V -2.0 imity of some QD surfaces (see Figure 1b) -40 -20 0 20 40 -40 -30 -20 -10 0 and hence more intimate contact comVDS (V) VDS (V) pared to spherical QDs even with the native c d 600 ligands. We do not believe that these QDs are 0.0 VDS= -10V unusually poorly passivated based on their -0.2 500 colloidal stability in solution and consistently -0.4 400 bright PL. -0.6 300 These measurements suggest that asV = -0.8 EDT treated EDT treated G -0 V prepared CISeS QDs are p-doped. However, -1.0 200 -10 V while exhibiting the signatures of apparent -20 V -1.2 100 -30 V doping in transport measurements, these -40 V -1.4 dots do not show typical signs of charges -40 -30 -20 -10 0 -20 -10 0 10 20 being present in the QD interior such as VDS (V) VG (V) PL quenching by Auger recombination or bleaching of band-edge absorption, observed, Figure 2. P-type transport, typical of CISeS QDs. a) Drain current (I ) versus source–drain D for example, in the cases of chemical[20] or voltage (VSD) for the untreated QD film, b) for the film exposed to air for 2 minutes, and [ 21,22 ] electrochemical doping. This might be c) for the air-free EDT-treated QD film. d) Drain current versus gate voltage (VG) for the device due to low doping levels in as-synthesized characterized in c.
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observe that the CISeS-Cd QDs show ambipolar conductivity (Figure 3a), which indiVDS=20V 8 VG= cates switching from the apparent p-type to 4 0V 0.2 VG= +/-10 V the apparent intrinsic transport, which might 6 0 V +/-20 V 3 +/-10 V again point toward its surface character. This 0.0 +/-30 V -20 -10 0 10 20 +/-20 V VG (V) 4 behavior is unique to the QD form of CISeS, +/-30 V 2 +/-40 V as there have been no previous reports of ambi2 1 polar transport in the bulk form of this material. For example, in bulk CuInSe2, Cd has been 0 0 -40 -20 0 20 40 -40 -20 0 20 40 shown to be a donor impurity,[33] thus changing VDS (V) VDS (V) the majority carrier from holes to electrons. c d Here, we speculate that the Cd2+ surface treat40 3 V =20V ment compensates acceptor-like surface states V = DS G 10 0V 2 that results in an approximately intrinsic film. 10 V VG= 30 8 1 20 V After EDT-treatment (Figures 3b and 3c), the 0V 30 V 10 V 40 V 6 0 -20 -10 0 10 20 measured FET saturation mobilities for elec20 20 V VG (V) trons and holes are 0.018 cm2 V–1 s–1 and 30 V 4 40 V 0.024 cm2 V–1 s–1, respectively. These values are 10 2 comparable to what has been reported for lead 0 chalcogenide QDs using a similar EDT treat0 0 10 20 30 40 ment procedure.[12] The highest electron and 0 10 20 30 40 VDS (V) VDS (V) hole mobilities reported for EDT-only treated PbS QDs are around 10−4 cm2 V–1 s–1,[34] while Figure 3. FET performance for ambipolar and n-type CISeS QD films. a) Drain current 2 –1 –1 2+ 2+ versus source–drain voltage for Cd -exposed QDs, and b) for Cd -exposed QDs treated for PbSe QDs they are 0.07 cm V s for 2 –1 –1 [12] electrons and 0.03 cm V s for holes. In with EDT with the drain current versus gate voltage shown inset. c) Drain current versus source–drain voltage for indium-annealed FETs, and d) for indium-annealed FETs treated another example, EDT-treated InAs QD FETs with EDT with the drain current versus gate voltage shown inset. showed an even lower mobility of around 10−5 cm2 V–1 s–1.[35] Doping I-III-VI2 QDs n-type would be of great use in It was shown previously that inter-QD spacing decreases by 1.6 nm when oleic acid is exchanged with EDT.[30] We find engineering p–n junctions in solar cells. Prior studies have shown that bulk CuInSe2 can be doped p-type or n-type by that the most common QD film treatment using EDT has the greatest effect on transport, with the mobility (measured in exploiting “intrinsic” defects (that is, via deviation from stoithe linear regime) increasing by more than two orders of magchiometry)[3,36–38] or by extrinsic dopants such as Zn,[39] Cd,[33] nitude to 0.029 cm2 V–1 s–1 (Figure 2c). NH4SCN and EDA also or Cl.[40] In the case of CISeS QDs, the apparent p-type doping affected the mobility but to a much lesser degree (Supporting of as prepared materials would need to be overcompensated Information, Figure S1). in order to switch the conductivity to n-type. If we hypotheAmbipolar Behavior: Decreasing the distance between QDs by size that the p-type conductivity of as-synthesized QDs arises ligand exchange increases charge mobility but does not appear from indium vacancies on the QD surfaces, we might expect to affect the apparent nature of majority charge carriers (that to achieve the effect of overcompensation by introducing addiis, holes in the present case). A p-type behavior in bulk CISeS tional indium into the film as was previously demonstrated is related to indium vacancies (e.g., when the copper to indium by Kagan and co-workers for CdSe QDs.[31] In this published ratio is greater than one)[3] but we should be careful in ascribing approach, indium is incorporated into the QD film by annealing the entire QD-FET fabricated using indium as the source- and bulk self-doping processes to QD films that are highly granular drain-contact material.[31,41,42] The low melting point of indium and likely have a different carrier transport mechanism. Considering the involvement of surface states in charge conductallows for such diffusion without significant sintering. ance the manipulation of surface properties of the QDs can In our studies, we decided to pursue a similar strategy. Because directly affect both the type and the level of apparent doping of indium is a component element of CISeS QDs, we suspected the film.[16,17] There are a few reported methods of filling surthat the crystal lattice would accommodate indium diffusion and allow for filling of vacancies. The effect of indium on transport face trap states in a film including halide passivation,[26] atomic was tested by thermally evaporating indium as the source and layer deposition (ALD),[14] and diffusion of metal ions from drain contacts (by shadow mask). In order to ensure complete device contacts to reduce trap densities.[31] indium diffusion across the channel, the distance between the In order to better passivate surfaces and thereby reduce the contacts for these FETs is smaller (30 µm) and we deposited density of trap states and/or their occupancy, we exposed the enough indium in each contact (1 mm wide by 150 nm thick) to QDs to Cd2+ ions based on a previous report.[32] This surface diffuse throughout the interior of the entire QD film. After contreatment results in 22% of QD cations being exchanged with tact deposition, the films were annealed at 250 °C for 1 h. cadmium based on inductively coupled plasma atomic emisIn a control study with indium source and drain contacts sion spectroscopy. We know that this treatment is effective at but without annealing, the measured conductivity is p-type passivating CISeS QD surfaces because it leads to increases (Supporting Information, Figure S3). However, after annealing at in both the PL quantum yield and the exciton lifetime.[32] We
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250 °C, the conductivity of the film switches to n-type (Figure 3c). These observations support our hypothesis that cation stoichiometry and specifically the stoichiometry of the QD surface is at least one of the factors controlling the type and the level of doping of CISeS QD films. This assessment is somewhat similar to one derived from previous studies of bulk CISeS where though the defects are not of primarily of surface origin as in QDs but distributed over the entire semiconductor volume. Specifically, for bulk materials it was observed that indium-rich samples were n-type while in the copper-rich samples the conductivity was p-type.[3] A transition from p-type to n-type was observed for both as-synthesized QD films as well as for EDT-treated films (Figure 3d). From the measured ID–VG characteristics, we derive linear electron mobilities of 2.4 × 10−3 cm2 V–1 s–1 and 1.8 × 10−2 cm2 V–1 s–1 for as-synthesized QD films and EDTtreated films, respectively. We note that the n-type films are sensitive to air-exposure with increasing conductivity and diminished gating effect over time, similar to the p-type devices. It is known that annealing can increase mobility of the QD films by decreasing inter-particle distance.[23,43,44] However, it was also shown that overheating QD films might lead to decomposition of ligands and QD sintering which eventually leads to the loss of quantum confinement and restoration of bulk-material properties.[45–47] To verify that QD integrity is preserved in our samples after annealing, we conduct an additional control experiment in which CISeS QD FETs with gold contacts are heated to 250 °C for 1 h. Following this procedure, we detect only a slight increase in the mobility (Supporting Information, Figure S3). We also observe that the absorption spectra of films treated in the same way experience only a small (
