1. Colloidal Quantum Dots for Solar Technologies
Haiguang Zhao, Federico Rosei 
Chem 
Volume 3, Issue 2, Pages (August 2017)
DOI: /j.chempr
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2. Chem 2017 3, DOI: ( /j.chempr )
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3. Figure 1
Schematic Representation of the Synthesis of Colloidal QDs via Various Approaches and Their Emission Range
(A) A synthetic apparatus used in the preparation of various structured QDs. The bare QDs can be synthesized via a cation exchange approach. The core-shell structure can be obtained via both a cation exchange approach and SILAR approach.
(B) The emission range for representative QDs.
Chem 2017 3, DOI: ( /j.chempr )
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4. Figure 2 Morphologies and Optical Spectra of Colloidal PbS NCs
(A) High-resolution transmission electron microscopy images of colloidal PbS NCs with an exciton absorption at 1,440 nm.
(B) The selected area electron diffraction spacing (inset on the single-particle close up on the right) correspond to bulk lattice parameters. Room temperature optical characterization of toluene solutions of PbS NCs.
(C) Absorption spectra spanning the range of tunable sizes.
(D) Band-edge absorption and photoluminescence peaks for a sample ∼6.5 nm in diameter. Adapted with permission from Hines et al.44 Copyright 2003 John Wiley and Sons.
Chem 2017 3, DOI: ( /j.chempr )
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5. Figure 3
Colloidal Perovskite CsPbX3 NCs (X = Cl, Br, I) Exhibit Size- and Composition-Tunable Bandgap Energies Covering the Entire Visible Spectral Region with Narrow and Bright Emission
(A) Colloidal solutions in toluene under a UV lamp (λ = 365 nm).
(B) Representative photoluminescence spectra (λexc = 400 nm for all but 350 nm for CsPbCl3 samples).
(C) Typical optical absorption and photoluminescence spectra.
(D) Time-resolved photoluminescence decays for all samples shown in (C) except CsPbCl3. Adapted with permission from Protesescu et al.49 Copyright 2015 American Chemical Society.
Chem 2017 3, DOI: ( /j.chempr )
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6. Figure 4 Reaction Scheme and Reaction Evolution during Cation Exchange
(A) A schematic illustration of the cation-exchange reaction by the Pb-halide precursor and ZnSe QDs.
(B) Evolution of absorption spectra of PbSe QDs exchanged from ZnSe in a PbI2/oleylamine heterogeneous solution at different injection temperatures.
(C) The diameter of the resulting PbSe QD domain as calculated from standard sizing curves. The colored markers correspond with the temperatures used in (B). The shaded box is the ZnSe QD precursor diameter.
(D) Absorption spectrum of precursor ZnSe QDs. Adapted with permission from Kim et al.51 Copyright 2015 American Chemical Society.
Chem 2017 3, DOI: ( /j.chempr )
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7. Figure 5
Schematic Representation of the Structure and Energy-Level Alignment in Pure QDs and Different Core-Shell QDs Systems
Different colors indicate different materials.
Chem 2017 3, DOI: ( /j.chempr )
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8. Figure 6 Morphologies and Emission Spectra of Lead Chalcogenide QDs
(A and B) High resolution transmission electron microscopy images of (A) PbSe/CdSe and (B) PbTe/CdTe core-shell particles in the <110> directions. Adapted with permission from Lambert et al.76 Copyright 2009 American Chemical Society.
(C) Photoluminescence spectra of PbS/CdS QDs synthesized via the cation exchange approach. Adapted with permission from Zhao et al.55 Copyright 2011 Royal Society of Chemistry.
Chem 2017 3, DOI: ( /j.chempr )
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9. Figure 7
Scheme of the Charge-Transfer Process in Typical QD (Bare, Core-Thin Shell, and Core-Thicker Shell QD)-Based Solar Cells Sensitized with Various Types of QDs and with the Photoanode Composed of TiO2 Nanoparticles
Chem 2017 3, DOI: ( /j.chempr )
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10. Figure 8
Electron Dynamic in QDs Capped with Ligands with Different Lengths
(A) Qualitative prediction of electronic coupling in short (represented as 3-MPA) versus long ligands (represented as 8-MOA).
(B) The steady-state absorption spectrum of (a) CdSe QDs plotted alongside the difference absorption spectrum of CdSe-(16-MHA)-TiO2 measured at (b) 1 ps, (c) 10 ps, (d) 100 ps, and (e) 1,000 ps via transient absorption spectroscopy.
(C) Kinetic traces for (a) CdSe linked to TiO2 with 16-MHA and (b) CdSe linked to SiO2 with 3-mercaptotrimethoxysilane (3-MPS). The red traces represent the exponential fitting equation.
(D) Experimental values of ket plotted against the barrier width, determined by measuring the length of the extended molecule. Black dots are experimental values, the blue line is the fit, and red represents the error bars determined from four separate measurements. The exponential fit equation is presented on the plot.
Adapted with permission from Hines et al.77 Copyright 2015 American Chemical Society.
Chem 2017 3, DOI: ( /j.chempr )
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11. Figure 9
Electron Dynamic in Core-Shell PbS/CdS QDs in the Presence of Metal Oxide
(A) Scheme of the PbS/CdS core-shell QDs bound to the TiO2 or SnO2 surface through mercaptoacetic acid ligand. The position of the electronic conduction bands is sketched (not to scale) as a function of core size, shell thickness, and QD-oxide distance. Electron injection rate is assumed to increase from 1 to 4, as confirmed by experimental findings and theoretical calculations.
(B and C) Fluorescence decays of PbS/CdS QDs grafted on silica, TiO2, and SnO2 for different PbS core diameters: (B) 3.0 nm and (C) 4.2 nm. The shell thickness is approximately 0.2–0.3 nm. The excitation wavelength is λex = 444 nm. All measurements were carried out at ambient temperature.
(D–F) kt as a function of core diameter for three different shell thicknesses: (D) 0 nm; (E) 0.3 nm; (F) 0.6 nm.
(G) Tunneling rate kt as a function of shell thickness for a fixed core size of 3.0 nm. Black circles, TiO2, experiment; red squares, TiO2, theory; blue triangles, SnO2, experiment.
Adapted with permission from Zhao et al.78 Copyright 2014 Royal Society of Chemistry.
Chem 2017 3, DOI: ( /j.chempr )
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12. Figure 10 Band Alignment and Charge Transfer in “Giant” CdSe/CdS QDs
(A) A band alignment diagram of bulk CdSe and CdS; the band offset at the CdSe-CdS interface is not well known and in our calculations varied from 0 to 0.32 eV.
(B) Spatial probability distribution, ρ(r), of the hole (gray area) and electron (colored areas) for R = 1.5 nm and H = 1.6 nm (blue area), H = 2.8 nm (green area), and H = 7.6 nm (red area); ρ(r) ∝ r2|ψe,h|2, where ψe,h are the electron (e) and the hole (h) wave functions. Donor-acceptor system.
(C) The hole donor is at the CdSe core, and the acceptor is localized at the end of the ligand chain.
(D) Hole acceptors FcC3SH, FcC6SH, and alkylSH.
(E) Energy positions of the conduction and valence bands of CdSe and CdS and the oxidation potentials of FcSH and alkylSH.
(F) PLQY as a function of bound acceptor ligands per QD (N) for the nine donor−acceptor systems made with three donor particles and three acceptor molecules. Inset: the same data and fit is plotted on a logarithmic scale in the x axis to give a better representation of the effect of low N.
(G and H) Transmission electron microscopy images of CdSe/CdS NCs. CdSe/xCdS NCs with core radius, R = 1.5 nm, and increasing shell thickness (G, x = 4; H, x = 7; CdS monolayers, corresponding to shell thickness of 1.6 and 2.8 nm, respectively). Scale bars correspond to 20 nm (G and H) and 2 nm (inset of G). Schematic representations of the NCs structure are reported for each set of NCs for a fixed core radius.
(A, B, G, and H) Adapted with permission from Brovelli et al.72 Copyright 2011 Nature Publishing Group.
(C–F) Adapted with permission from Ding et al.80 Copyright 2015 American Chemical Society.
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13. Figure 11 Scheme and Solar Cell Performance in Liquid-Junction QDSCs
(A) Schematic of the device architecture of liquid-junction QDSCs.
(B) J-V curves for the champion cells, recorded under standard conditions (AM 1.5 G, 100 mW/cm2). The blue curve is the certified J-V curve.
(C) Corresponding IPCE spectra.
(B and C) Adapted with permission from Du et al.30 Copyright 2016 American Chemical Society.
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14. Figure 12 Device Architecture and Performance of Solid-State QDSCs
(A) Schematic of the device architecture of solid-state QDSCs.
(B) Hysteresis-free J-V characteristics of solar cell devices.
(C) Model showing the device architecture of the solar cell used for performance testing. Inset shows the actual sample device.
(D) Current density versus voltage performance testing of best-performing devices under dark and AM 1.5 G illumination conditions. Both devices have the same structure, with similar thickness, and underwent the same solid-state ligand exchange process using 1,2-ethanedithiol. Arrows indicate the voltage points where light-generated current equals the dark current and gives an estimate of the built-in potential of the device.
(A and B) Adapted with permission from Lan et al.31 Copyright 2016 American Chemical Society.
(C and D) Adapted with permission from Neo et al.59 Copyright 2014 American Chemical Society.
Chem 2017 3, DOI: ( /j.chempr )
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15. Figure 13
PEC Device Architecture and Performance Based on Core-Shell PbS/CdS QDs
(A) Scheme of the PEC device.
(B) J-V curves under chopped (black) and constant (red) illumination (100 mW cm−2).
(C) Photocurrent density versus the applied voltage (versus reversible hydrogen electrode [RHE]) for the TiO2/core/shell/CdS/ZnS in the dark (black line) under AM 1.5 G illumination at 100 mW cm−2 (blue line) and 800 mW cm−2 (red line).
(D) Measured current density as a function of time for the samples of TiO2/core/shell/CdS/ZnS, TiO2/core/shell/CdS, and TiO2/core/shell at 0.2 V versus RHE under 100 mW cm−2 illumination with AM 1.5 G filter.
Adapted with permission from Lei et al.86 Copyright 2016 John Wiley and Sons.
Chem 2017 3, DOI: ( /j.chempr )
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16. Figure 14
Energy Levels and Performance of the PEC Device for Hydrogen Production Based on “Giant” Core-Shell QDs
(A) Approximate energy levels (correspond to pH 13) of TiO2, CdSe, CdS, together with related characteristic redox potentials. The bandgap values of CdSe and CdS correspond to QD and bulk semiconductors, respectively. The arrows indicate the electron and hole transfer process.
(B and C) Photocurrent density-potential dependence of TiO2 sensitized by CdSe#2 (R, 1.65), Giant#1 (R/H 1.85/3.2), Giant#2 (R/H 1.65/4.3) (B) without and (C) with ZnS (two cycles) in the dark and under AM 1.5 G illumination at 100 mW/cm2, with Pt as the counter electrode.
(D) Measured photocurrent density of Giant#2, with different ZnS SILAR cycles and without ZnS, as a function of time at 0.2 V versus RHE under 100 mW/cm2 illumination with AM 1.5 G filter.
Adapted with permission from Adhikari et al.32 Copyright 2016 Springer.
Chem 2017 3, DOI: ( /j.chempr )
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17. Figure 15 Schematic Representation of a QD-Based LSC
The numbers indicate the typical processes of energy loss in an LSC. (1) Unabsorbed light; (2) light reflects from the top surface; (3) the light was absorbed by the QDs, but there is partial loss because of the non-unity of fluorescence quantum yield; (4) re-emitted incident light escapes from the surface because of the escape cone (the angle larger than the critical angle); and (5) light is re-absorbed by another QD.
Adapted with permission from Zhao et al.92 Copyright 2015 John Wiley and Sons.
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18. Figure 16 Large-Area LSC Based on Stokes-Shift-Engineered QDs
(A) Left: photograph of a QD-PMMA-based LSC (dimensions: 21.5 × 1.35 × 0.5 cm) comprising CdSe/CdS QDs (R/H, 1.5/4.2 nm) illuminated by a UV lamp emitting at 365 nm (top) and under ambient illumination (bottom). Scale bar, 5 cm. Right: the same LSC during measurement of the concentration factor with illumination from a solar simulator (1.5 AM).
(B) Optical absorption spectra of the QD solution (dotted line) and the QD-PMMA composite (same as in A) (solid line) showing a minimal contribution from scattering and normalized photoluminescence spectra (excitation at 473 nm) collected at the edge of the LSC when the excitation spot is located at distances d = 0 cm (black line) or d = 20 cm (purple line) from the edge. The lack of a d-dependent change in the shape of the photoluminescence spectrum suggests that losses to re-absorption by the QD material are negligibly small. Non-normalized photoluminescence spectra as a function of d (0–20 cm) are shown in the inset (logarithmic scale is used on the intensity axis); they indicate that the overall photoluminescence intensity drops with increasing d as a result of scattering at optical imperfections within the PMMA matrix and photon escape from the waveguide.
Adapted with permission from Meinardi et al.93 Copyright 2014 Nature Publishing Group.
Chem 2017 3, DOI: ( /j.chempr )
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