Supporting Information 1 Enhanced Mobility-Lifetime Products in PbS ...
Supporting Information Enhanced Mobility-Lifetime Products in PbS Colloidal Quantum Dot Photovoltaics Kwang S. Jeong,1 Jiang Tang,2 Huan Liu,2,3 Jihye Kim,1 Andrew W. Schaefer,1 Kyle Kemp,2 Larissa Levina,2 Xihua Wang,2 Sjoerd Hoogland,2 Ratan Debnath,2 Lukasz Brzozowski,2 Edward H. Sargent,2 and John B. Asbury,1
1 Department of Chemistry, The Pennsylvania State University, University Park, PA 16802 2 Department of Electrical and Computer Engineering, University of Toronto, 10 King’s college Road, Toronto, Ontario M5S 3G4, Canada 3 Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan, 430074, PR China
This file includes: Chemicals Materials and Methods Analysis of PbS CQD size distribution Microscopy of ligand exchanged CQD films Mid-IR, near-IR, and TRIR spectra of ligand exchanged CQD films Reference
1
Supporting Information Chemicals
Lead oxide (PbO) (99.9%), oleic acid (90%), bis(trimethylsilyl)sulphide (TMS, synthesis grade), 1-octadecene (90%), 3-mercaptopropionic acid (99%), ethane dithiol (99%), terpineol, Triton-X and all solvents (anhydrous grade) were obtained from Sigma-Aldrich.
Materials and Methods
1. Materials Preparation Synthesis of PbS Colloidal Quantum Dots: PbS colloidal quantum dots (CQDs) were synthesized according to literature procedures.1-3 Briefly, 3 mL of octadecene was degassed and mixed with 0.45 g of PbO and 1.5 g of oleic acid to produce lead oleate stock solution. To 4.5 mL of the lead oleate stock solution was added 15 mL of octadecene and 180 L of TMS diluted in 10 mL octadecene. All synthesis procedures were conducted in a Schlenk line. The PbS CQDs were purified by three successive precipation and redispersion steps using acetone and toluene, respectively. All purification steps were performed in air.
PbS Film Fabrication: Quantum dot films were prepared on TiO2 electrodes by multilayer spincoating of a 50 mg/mL solution of 3.7 nm PbS CQDs in octane. Each layer was deposited at 2500 rpm and treated briefly with 1.15 molar 3-mercaptopropionic (MPA) acid in methanol (10% by volume) or 0.119 molar ethanedithiol (EDT) (1% by volume) in acetonitrile and spin cast. The MPA or EDT samples were rinsed with methanol or acetontile respectively and then a final rinse with octane. To obtain equal absorption of the solar spectrum in each device, CQD layers consisting of a total of 8 layers were used such that the optical densities at 632 nm were matched. The devices were then transferred to a glovebox with N2 atmosphere. Gold contacts, 15 nm thick, were deposited by thermal evaporation at a rate of 0.4 Å/s at a pressureof 1x10-6 mbar. Contact sizes were 0.061 cm2.
2
Supporting Information 2. Device Characterization Field Effect Mobility Measurements: We are most interested in the minority carrier (electron) mobility in the p-type PbS CQD films. Consequently, we chose the ion-gel technique developed by the Frisbie group4,5 because the ion-gel field effect transistor (FET) measurements under inert nitrogen atmosphere permit n-type field effect mobility measurements. The measurements were performed using a homemade setup inside a glovebox. One Keithley 2400 source-meter acquired the drain-source current while another acquired the gate-source current. CQDs were spin-cast onto the precleaned interdigitated gold electrodes following the same procedure used for photovoltaic device fabrication. The film thickness was estimated to be 50 nm. The ion gel was produced using a mixture of 0.195 g PEO, 0.02945 g LiClO4, and 5 mL anhydrous acetonitrile. 15 nm Au / 90 nm Ag electrodes were thermally evaporated onto the ion gel to provide a contact to the gate. The channel width and length were 2 mm and 2.5 μm, respectively; and the area of the gate 1 mm2. The Id-Vg measurements were obtained at Vd =1 V while scanning Vg from 0 V to 2.5 V with 10 s delay per 0.1 V.
PCE Measurement : Current density-voltage characteristics were measured using a Keithley 2400 source-meter in N2 ambient. The solar spectrum at AM1.5 was simulated to within class A specifications (less than 25% spectral mismatch) with a Xe lamp and filters (Solar Light Company Inc.) with measured intensity at 102 mW cm2. The source intensity was measured with a Melles-Griot broadband power meter and a Thorlabs broadband power meter through a circular 0.049 cm2 aperture at the position of the device and confirmed with a calibrated reference solar cell (Newport, Inc.). The accuracy of the power measurement was estimated to be ± 7%. Three strategies were employed in combination to reduce errors in estimating power conversion efficiency: 1)
We calculated the efficiency by dividing the entire power conveyed
through the 0.049 cm2 aperture onto the devices (0.061 cm2 ). 2)
We periodically calibrated the spectral mismatch between our simulator spectrum and the
reference spectrum ASTM G173-03. 3)
The device was then operated at the maximum power point for an
extended period to monitor its static power conversion efficiency.
3
Supporting Information Analysis of CQD Size Distribution We correlated trends in the half-width at half-maximum of the near-IR absorption spectra of the exciton enhanced bandgap transitions of PbS CQDs in octane solution with analysis of nanocrystal size distributions obtained by ultracentrifugation and found that the half-width at half-maxima are indicative of the size distribution. The connection between the nanocrystal size and quantum confined bandgap makes6 analysis of the distribution of bandgaps represented by the half-width at half-maximum of the exciton transition a sensitive measure of the particle size distribution. Taking this approach, typical bandgap half-width at half-maximum energy distributions of our synthesized dots in solution are in the 75 – 100 meV range (See Figure S1). Taking into account approximately 50 meV of homogeneous broadening that is observed in highly monodisperse PbS CQD colloids, the inhomogeneous bandgap energy distribution corresponds to 50 – 75 meV half-width at half-maximum. Using the correlation of exciton energy versus quantum dot diameter developed by the Wise group,6 this energy distribution corresponds to a quantum size range of 3.7 nm
0.18 nm or
5%. Deposition of the quantum
dots into a film is not likely to change the size distribution markedly, but the bandgap energy distribution will increase due to due to inhomogeneous distributions of inter-particle spacing and interactions with ligands.
Figure S1. Near-IR spectrum that is typical of PbS CQDs in octane used as starting materials in the preparation of CQD films for electrical and spectroscopic measurements. The width of the exciton peak is indicative of a 5% distribution of particle sizes.
4
Supporting Information Microscopy of Ligand Exchanged CQD Films
PbS–MPA
PbS–EDT
Figure S2. SEM images of MPA and EDT treated PbS CQD films deposited on TiO2 electrodes. The layer-by-layer deposition approach fills in cracks in previously deposited layers caused by volume contraction in the CQD layer during ligand exchange – providing pin-hole free films. The MPA treated films have lower surface roughness in comparison to EDT treated films.
Figure S3. Atomic force micrograph of MPA treated PbS CQD film. The rms surface roughness is approximately 3 nm and is characteristic of the whole of the film.
5
Supporting Information Characterization of Ligand Exchanged CQD Films Using Mid-IR, Near-IR, and TRIR Spectroscopy
1. Mid-IR Spectroscopy 0.9
absorbance
0.8 0.7
A
0.6 0.5
MPA
Pb O
EDT PrDT Loss of O
0.4 0.3
R C
BDT Pb
PnDT
O
0.2 0.1
HDT
0.0
1,4-BzDT
-0.1 0.9 1000B 0.8 0.7
absorbance
OA
O R C
1,2-BzDT
1500 2000 2500 OA-PbS CQD film
3000
MPA-PbS CQD film after drying
0.6 0.5
MPA-PbS CQD film before drying
0.4 0.3
0.2 0.1 0.0
-0.1 1000 1000
1500 1500
2000 2000
2500 2500
3000 3000
frequency (cm-1)
Figure S4. A. Comparison of IR spectra of PbS CQD films ligand exchanged with the following compounds: MPA, EDT, 1,3-propane dithiol (PrDT), 1,4-butane dithiol (BDT), 1,5pentane dithiol (PnDT), 1,6-hexane dithiol (HDT), ortho-benzene dithiol (1,2-BzDT), and parabenzene dithiol (1,4-BzDT). Ligands that have smaller aliphatic groups exhibit greatly suppressed absorption in the region of the C-H stretch around 2900 cm-1. Ligands with longer aliphatic groups (PnDT, HDT) still exhibit appreciable absorption in the C-H stretch region. Importantly, all ligands (excepting MPA) display complete loss of the carboxylate stretching vibration around 1650 cm-1 that is indicative of highly efficient removal of oleic acid from the surfaces of the quantum dots. The exception to this is MPA because it too contains a carboxylate 6
Supporting Information group that is attached to the quantum dot surfaces. 1,4-BzDT exhibits a small amount of residual oleic acid because it does not have as strong affinity to PbS surfaces in comparison to the other ligands. In total, the data indicate quantitative removal of the original oleic acid ligands. B. Comparison of MPA capped CQD PbS films before and after a 24 hour drying step in which films are stored under inert N2 atmosphere to drive off residual solvent. Before the drying step, some of the carboxylate groups of MPA are not bonded to the CQD surfaces presumably due to the presence of residual methanol (solvent). After the drying step, the carboxylate groups have quantitatively bonded to the PbS surfaces.
7
Supporting Information 2. Near-IR Spectroscopy 1.1 1.0
OA
0.9
MPA
absorbance
0.8
EDT
0.7 0.6 0.5
PrDT
0.07 eV
BDT
0.4
PnDT
0.3
HDT
0.2
1,2-BzDT
0.1
1,4-BzDT
0.0
-0.1 1.1 1.1
1.2 1.2
1.3 1.3
1.4 1.4
1.5 1.5
1.6 1.6
near-IR transition energy (eV)
Figure S5. Comparison of near-IR spectra of the exciton enhanced bandgap transitions of PbS CQD films ligand exchanged with the following compounds: MPA, EDT, 1,3-propane dithiol (PrDT), 1,4-butane dithiol (BDT), 1,5-pentane dithiol (PnDT), 1,6-hexane dithiol (HDT), orthobenzene dithiol (1,2-BzDT), and para-benzene dithiol (1,4-BzDT). The same batch of 3.7 nm PbS CQDs were used as the starting materials for each film. A variation of the bandgap transition of 0.07 eV is observed within the series of ligands. The variation is roughly correlated with the size of the aliphatic group of the dithiol ligands. For example, the bandgap energy increases monotonically in the order of EDT, PrDT, BDT, HDT consistent with decreased electronic coupling in the CQD films treated with bulkier ligands. The larger spacing between quantum dots prevents the excitons from coupling into states of neighboring dots resulting in increased quantum confinement and larger bandgaps. The specific influence of ligand interactions with the quantum dot surfaces also affects the bandgap energy of the dots.
8
Supporting Information 3. TRIR Spectroscopy
absorbance charnge (mO.D.)
0.13 eV 1.0 0.9
500 ns
OA
0.8 0.7
MPA
0.6 0.5
PrDT
0.4 0.3 0.2 0.1 0.0 -0.1 0.1 0.1
EDT BDT PnDT HDT 1,2-BzDT 1,4-BzDT
0.2 0.2
0.3 0.3
0.4 0.4
0.5 0.5
mid-IR transition energy, E (eV)
Figure S6. Comparison of TRIR spectra of PbS CQD films ligand exchanged with the following compounds: MPA, EDT, 1,3-propane dithiol (PrDT), 1,4-butane dithiol (BDT), 1,5-pentane dithiol (PnDT), 1,6-hexane dithiol (HDT), ortho-benzene dithiol (1,2-BzDT), and para-benzene dithiol (1,4-BzDT). The spectra were measured 500 ns following bandgap excitation of the CQD films at 532 nm using a nanosecond Nd:YAG laser. The spectra are plotted with their absolute signal magnitudes measured in the TRIR experiment after small corrections to account for variations in the optical densities of the samples were applied. In all cases, the corrections were no more than 20% of the original signal size. The data reveal marked dependence of the signal size on the ligand treatment which is counter to prior observations of intraband transitions measured in CdSe quantum dots in solution.7 The ligands also strongly influence the maximum of the broad electronic transition observed throughout the mid-IR spectral region. As discussed in the context of Figure 5, the average mid-IR transition energy is somewhat correlated with the bandgap transition of the corresponding CQD films, but the dominant influence on the mid-IR transition energy comes from the ligands. This strong dependence on the surface ligand treatment combined with the observation that the vibrational features of ligands are perturbed by photoexcitation of the CQD films indicates that the broad electronic transitions arise from trapto-band transitions rather than intraband transitions.
9
Supporting Information References (1)
(2)
(3)
(4)
(5) (6)
(7)
Hines, M.; Scholes, G. Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Distribution Adv. Mater. 2003, 15, 1844-1849. Tang, J.; Brzozowski, L.; Barkhouse, D. A. R.; Wang, X.; Debnath, R.; Wolowiec, R.; Palmiano, E.; Levina, L.; Pattantyus-Abraham, A. G.; Jamakosmanovic, D.; Sargent, E. H. Quantum Dot Photovoltaics in the Extreme Quantum Confinement Regime: The Surface-Chemical Origins of Exceptional Air- and Light-Stability ACS Nano 2010, 4, 869-878. Pattantyus-Abraham, A. G.; Kramer, I. J.; Barkhouse, A. R.; Wang, X.; Konstantatos, G.; Debnath, R.; Levina, L.; Nazeeruddin, M. K.; Gratzel, M.; Sargent, E. H. DepletedHeterojunction Colloidal Quantum Dot Solar Cells ACS Nano 2010, 4, 3374-3380. Kang, M. S.; Lee, J.; Norris, D. J.; Frisbie, C. D. High Carrier Densities Achieved at Low Voltages in Ambipolar PbSe Nanocrystal Thin-Film Transistors Nano Lett. 2009, 9, 3848-3852. Kang, M. S.; Sahu, A.; Norris, D. J.; Frisbie, C. D. Size-Dependent Electrical Transport in CdSe Nanocrystal Thin Films Nano Lett. 2010, 10, 3727-3732. Hyun, B.-R.; Zhong, Y.-W.; Bartnik, A. C.; Sun, L.; Abruna, H. D.; Wise, F. W.; Goodreau, J. D.; Matthews, J. R.; Leslie, T. M.; Borrelli, N. F. Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles ACS Nano 2008, 2, 2206-2212. Shim, M.; Shilov, S. V.; Braiman, M. S.; Guyot-Sionnest, P. Long-Lived Delocalized Electron States in Quantum Dots: A Step-Scan Fourier Transform Infrared Study J. Phys. Chem. B 2000, 104, 1494-1496.
10
1 Department of Chemistry, The Pennsylvania State University, University Park, PA 16802 2 Department of Electrical and Computer Engineering, University of Toronto, 10 King’s college Road, Toronto, Ontario M5S 3G4, Canada 3 Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan, 430074, PR China
This file includes: Chemicals Materials and Methods Analysis of PbS CQD size distribution Microscopy of ligand exchanged CQD films Mid-IR, near-IR, and TRIR spectra of ligand exchanged CQD films Reference
1
Supporting Information Chemicals
Lead oxide (PbO) (99.9%), oleic acid (90%), bis(trimethylsilyl)sulphide (TMS, synthesis grade), 1-octadecene (90%), 3-mercaptopropionic acid (99%), ethane dithiol (99%), terpineol, Triton-X and all solvents (anhydrous grade) were obtained from Sigma-Aldrich.
Materials and Methods
1. Materials Preparation Synthesis of PbS Colloidal Quantum Dots: PbS colloidal quantum dots (CQDs) were synthesized according to literature procedures.1-3 Briefly, 3 mL of octadecene was degassed and mixed with 0.45 g of PbO and 1.5 g of oleic acid to produce lead oleate stock solution. To 4.5 mL of the lead oleate stock solution was added 15 mL of octadecene and 180 L of TMS diluted in 10 mL octadecene. All synthesis procedures were conducted in a Schlenk line. The PbS CQDs were purified by three successive precipation and redispersion steps using acetone and toluene, respectively. All purification steps were performed in air.
PbS Film Fabrication: Quantum dot films were prepared on TiO2 electrodes by multilayer spincoating of a 50 mg/mL solution of 3.7 nm PbS CQDs in octane. Each layer was deposited at 2500 rpm and treated briefly with 1.15 molar 3-mercaptopropionic (MPA) acid in methanol (10% by volume) or 0.119 molar ethanedithiol (EDT) (1% by volume) in acetonitrile and spin cast. The MPA or EDT samples were rinsed with methanol or acetontile respectively and then a final rinse with octane. To obtain equal absorption of the solar spectrum in each device, CQD layers consisting of a total of 8 layers were used such that the optical densities at 632 nm were matched. The devices were then transferred to a glovebox with N2 atmosphere. Gold contacts, 15 nm thick, were deposited by thermal evaporation at a rate of 0.4 Å/s at a pressureof 1x10-6 mbar. Contact sizes were 0.061 cm2.
2
Supporting Information 2. Device Characterization Field Effect Mobility Measurements: We are most interested in the minority carrier (electron) mobility in the p-type PbS CQD films. Consequently, we chose the ion-gel technique developed by the Frisbie group4,5 because the ion-gel field effect transistor (FET) measurements under inert nitrogen atmosphere permit n-type field effect mobility measurements. The measurements were performed using a homemade setup inside a glovebox. One Keithley 2400 source-meter acquired the drain-source current while another acquired the gate-source current. CQDs were spin-cast onto the precleaned interdigitated gold electrodes following the same procedure used for photovoltaic device fabrication. The film thickness was estimated to be 50 nm. The ion gel was produced using a mixture of 0.195 g PEO, 0.02945 g LiClO4, and 5 mL anhydrous acetonitrile. 15 nm Au / 90 nm Ag electrodes were thermally evaporated onto the ion gel to provide a contact to the gate. The channel width and length were 2 mm and 2.5 μm, respectively; and the area of the gate 1 mm2. The Id-Vg measurements were obtained at Vd =1 V while scanning Vg from 0 V to 2.5 V with 10 s delay per 0.1 V.
PCE Measurement : Current density-voltage characteristics were measured using a Keithley 2400 source-meter in N2 ambient. The solar spectrum at AM1.5 was simulated to within class A specifications (less than 25% spectral mismatch) with a Xe lamp and filters (Solar Light Company Inc.) with measured intensity at 102 mW cm2. The source intensity was measured with a Melles-Griot broadband power meter and a Thorlabs broadband power meter through a circular 0.049 cm2 aperture at the position of the device and confirmed with a calibrated reference solar cell (Newport, Inc.). The accuracy of the power measurement was estimated to be ± 7%. Three strategies were employed in combination to reduce errors in estimating power conversion efficiency: 1)
We calculated the efficiency by dividing the entire power conveyed
through the 0.049 cm2 aperture onto the devices (0.061 cm2 ). 2)
We periodically calibrated the spectral mismatch between our simulator spectrum and the
reference spectrum ASTM G173-03. 3)
The device was then operated at the maximum power point for an
extended period to monitor its static power conversion efficiency.
3
Supporting Information Analysis of CQD Size Distribution We correlated trends in the half-width at half-maximum of the near-IR absorption spectra of the exciton enhanced bandgap transitions of PbS CQDs in octane solution with analysis of nanocrystal size distributions obtained by ultracentrifugation and found that the half-width at half-maxima are indicative of the size distribution. The connection between the nanocrystal size and quantum confined bandgap makes6 analysis of the distribution of bandgaps represented by the half-width at half-maximum of the exciton transition a sensitive measure of the particle size distribution. Taking this approach, typical bandgap half-width at half-maximum energy distributions of our synthesized dots in solution are in the 75 – 100 meV range (See Figure S1). Taking into account approximately 50 meV of homogeneous broadening that is observed in highly monodisperse PbS CQD colloids, the inhomogeneous bandgap energy distribution corresponds to 50 – 75 meV half-width at half-maximum. Using the correlation of exciton energy versus quantum dot diameter developed by the Wise group,6 this energy distribution corresponds to a quantum size range of 3.7 nm
0.18 nm or
5%. Deposition of the quantum
dots into a film is not likely to change the size distribution markedly, but the bandgap energy distribution will increase due to due to inhomogeneous distributions of inter-particle spacing and interactions with ligands.
Figure S1. Near-IR spectrum that is typical of PbS CQDs in octane used as starting materials in the preparation of CQD films for electrical and spectroscopic measurements. The width of the exciton peak is indicative of a 5% distribution of particle sizes.
4
Supporting Information Microscopy of Ligand Exchanged CQD Films
PbS–MPA
PbS–EDT
Figure S2. SEM images of MPA and EDT treated PbS CQD films deposited on TiO2 electrodes. The layer-by-layer deposition approach fills in cracks in previously deposited layers caused by volume contraction in the CQD layer during ligand exchange – providing pin-hole free films. The MPA treated films have lower surface roughness in comparison to EDT treated films.
Figure S3. Atomic force micrograph of MPA treated PbS CQD film. The rms surface roughness is approximately 3 nm and is characteristic of the whole of the film.
5
Supporting Information Characterization of Ligand Exchanged CQD Films Using Mid-IR, Near-IR, and TRIR Spectroscopy
1. Mid-IR Spectroscopy 0.9
absorbance
0.8 0.7
A
0.6 0.5
MPA
Pb O
EDT PrDT Loss of O
0.4 0.3
R C
BDT Pb
PnDT
O
0.2 0.1
HDT
0.0
1,4-BzDT
-0.1 0.9 1000B 0.8 0.7
absorbance
OA
O R C
1,2-BzDT
1500 2000 2500 OA-PbS CQD film
3000
MPA-PbS CQD film after drying
0.6 0.5
MPA-PbS CQD film before drying
0.4 0.3
0.2 0.1 0.0
-0.1 1000 1000
1500 1500
2000 2000
2500 2500
3000 3000
frequency (cm-1)
Figure S4. A. Comparison of IR spectra of PbS CQD films ligand exchanged with the following compounds: MPA, EDT, 1,3-propane dithiol (PrDT), 1,4-butane dithiol (BDT), 1,5pentane dithiol (PnDT), 1,6-hexane dithiol (HDT), ortho-benzene dithiol (1,2-BzDT), and parabenzene dithiol (1,4-BzDT). Ligands that have smaller aliphatic groups exhibit greatly suppressed absorption in the region of the C-H stretch around 2900 cm-1. Ligands with longer aliphatic groups (PnDT, HDT) still exhibit appreciable absorption in the C-H stretch region. Importantly, all ligands (excepting MPA) display complete loss of the carboxylate stretching vibration around 1650 cm-1 that is indicative of highly efficient removal of oleic acid from the surfaces of the quantum dots. The exception to this is MPA because it too contains a carboxylate 6
Supporting Information group that is attached to the quantum dot surfaces. 1,4-BzDT exhibits a small amount of residual oleic acid because it does not have as strong affinity to PbS surfaces in comparison to the other ligands. In total, the data indicate quantitative removal of the original oleic acid ligands. B. Comparison of MPA capped CQD PbS films before and after a 24 hour drying step in which films are stored under inert N2 atmosphere to drive off residual solvent. Before the drying step, some of the carboxylate groups of MPA are not bonded to the CQD surfaces presumably due to the presence of residual methanol (solvent). After the drying step, the carboxylate groups have quantitatively bonded to the PbS surfaces.
7
Supporting Information 2. Near-IR Spectroscopy 1.1 1.0
OA
0.9
MPA
absorbance
0.8
EDT
0.7 0.6 0.5
PrDT
0.07 eV
BDT
0.4
PnDT
0.3
HDT
0.2
1,2-BzDT
0.1
1,4-BzDT
0.0
-0.1 1.1 1.1
1.2 1.2
1.3 1.3
1.4 1.4
1.5 1.5
1.6 1.6
near-IR transition energy (eV)
Figure S5. Comparison of near-IR spectra of the exciton enhanced bandgap transitions of PbS CQD films ligand exchanged with the following compounds: MPA, EDT, 1,3-propane dithiol (PrDT), 1,4-butane dithiol (BDT), 1,5-pentane dithiol (PnDT), 1,6-hexane dithiol (HDT), orthobenzene dithiol (1,2-BzDT), and para-benzene dithiol (1,4-BzDT). The same batch of 3.7 nm PbS CQDs were used as the starting materials for each film. A variation of the bandgap transition of 0.07 eV is observed within the series of ligands. The variation is roughly correlated with the size of the aliphatic group of the dithiol ligands. For example, the bandgap energy increases monotonically in the order of EDT, PrDT, BDT, HDT consistent with decreased electronic coupling in the CQD films treated with bulkier ligands. The larger spacing between quantum dots prevents the excitons from coupling into states of neighboring dots resulting in increased quantum confinement and larger bandgaps. The specific influence of ligand interactions with the quantum dot surfaces also affects the bandgap energy of the dots.
8
Supporting Information 3. TRIR Spectroscopy
absorbance charnge (mO.D.)
0.13 eV 1.0 0.9
500 ns
OA
0.8 0.7
MPA
0.6 0.5
PrDT
0.4 0.3 0.2 0.1 0.0 -0.1 0.1 0.1
EDT BDT PnDT HDT 1,2-BzDT 1,4-BzDT
0.2 0.2
0.3 0.3
0.4 0.4
0.5 0.5
mid-IR transition energy, E (eV)
Figure S6. Comparison of TRIR spectra of PbS CQD films ligand exchanged with the following compounds: MPA, EDT, 1,3-propane dithiol (PrDT), 1,4-butane dithiol (BDT), 1,5-pentane dithiol (PnDT), 1,6-hexane dithiol (HDT), ortho-benzene dithiol (1,2-BzDT), and para-benzene dithiol (1,4-BzDT). The spectra were measured 500 ns following bandgap excitation of the CQD films at 532 nm using a nanosecond Nd:YAG laser. The spectra are plotted with their absolute signal magnitudes measured in the TRIR experiment after small corrections to account for variations in the optical densities of the samples were applied. In all cases, the corrections were no more than 20% of the original signal size. The data reveal marked dependence of the signal size on the ligand treatment which is counter to prior observations of intraband transitions measured in CdSe quantum dots in solution.7 The ligands also strongly influence the maximum of the broad electronic transition observed throughout the mid-IR spectral region. As discussed in the context of Figure 5, the average mid-IR transition energy is somewhat correlated with the bandgap transition of the corresponding CQD films, but the dominant influence on the mid-IR transition energy comes from the ligands. This strong dependence on the surface ligand treatment combined with the observation that the vibrational features of ligands are perturbed by photoexcitation of the CQD films indicates that the broad electronic transitions arise from trapto-band transitions rather than intraband transitions.
9
Supporting Information References (1)
(2)
(3)
(4)
(5) (6)
(7)
Hines, M.; Scholes, G. Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Distribution Adv. Mater. 2003, 15, 1844-1849. Tang, J.; Brzozowski, L.; Barkhouse, D. A. R.; Wang, X.; Debnath, R.; Wolowiec, R.; Palmiano, E.; Levina, L.; Pattantyus-Abraham, A. G.; Jamakosmanovic, D.; Sargent, E. H. Quantum Dot Photovoltaics in the Extreme Quantum Confinement Regime: The Surface-Chemical Origins of Exceptional Air- and Light-Stability ACS Nano 2010, 4, 869-878. Pattantyus-Abraham, A. G.; Kramer, I. J.; Barkhouse, A. R.; Wang, X.; Konstantatos, G.; Debnath, R.; Levina, L.; Nazeeruddin, M. K.; Gratzel, M.; Sargent, E. H. DepletedHeterojunction Colloidal Quantum Dot Solar Cells ACS Nano 2010, 4, 3374-3380. Kang, M. S.; Lee, J.; Norris, D. J.; Frisbie, C. D. High Carrier Densities Achieved at Low Voltages in Ambipolar PbSe Nanocrystal Thin-Film Transistors Nano Lett. 2009, 9, 3848-3852. Kang, M. S.; Sahu, A.; Norris, D. J.; Frisbie, C. D. Size-Dependent Electrical Transport in CdSe Nanocrystal Thin Films Nano Lett. 2010, 10, 3727-3732. Hyun, B.-R.; Zhong, Y.-W.; Bartnik, A. C.; Sun, L.; Abruna, H. D.; Wise, F. W.; Goodreau, J. D.; Matthews, J. R.; Leslie, T. M.; Borrelli, N. F. Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles ACS Nano 2008, 2, 2206-2212. Shim, M.; Shilov, S. V.; Braiman, M. S.; Guyot-Sionnest, P. Long-Lived Delocalized Electron States in Quantum Dots: A Step-Scan Fourier Transform Infrared Study J. Phys. Chem. B 2000, 104, 1494-1496.
10
