Enhancing Loading Amount and Performance of Quantum Dot

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Enhancing Loading Amount and Performance of Quantum Dot Sensitized Solar Cells Based on Direct Adsorption of Quantum Dots from Bi-Component Solvents Wenran Wang, Huashang Rao, Wenjuan Fang, Hua Zhang, Mengsi Zhou, Zhenxiao Pan, and Xinhua Zhong J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03713 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 2, 2019

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The Journal of Physical Chemistry Letters

Enhancing Loading Amount and Performance of Quantum Dot Sensitized Solar Cells based on Direct Adsorption of Quantum Dots from Bi-Component Solvents Wenran Wang,†,‡ Huashang Rao,† Wenjuan Fang,‡ Hua Zhang,‡ Mengsi Zhou,‡ Zhenxiao Pan*,†, Xinhua Zhong*,†,‡ †College

of Materials and Energy, South China Agricultural University, 483 Wushan Road,

Guangzhou 510642, China ‡School

of Chemistry and Molecular Engineering, East China University of Science and

Technology, Shanghai 200237, China Email: [email protected] (for Z. P.); [email protected] (for X. Z.)

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Intrinsically weak interaction between oil-soluble QDs and TiO2 in direct adsorption process limits QD loading and the performance of quantum dot sensitized solar cells (QDSCs). Herein, underlying chemistry and mechanisms governing QD adsorption on TiO2 were studied in order to improve QD loading and cell performance. Experimental results indicate that solvent polarity plays the crucial role in determining QD loading. Compared with single-component solvents, substantially greater QD loading can be realized at critical point (CP) of bi-component solvents, where QDs become metastable and start to precipitate. Through this strategy, average efficiency of 12.24 % was obtained for ZCISe QDSCs, which is comparable to those based on capping ligand induced self-assembly route. This report demonstrates the great potential of bi-component solvents at CP for high QD loading and excellent cell performance, as well as a platform for assembling functional composites with use of different nanocrystals and substrates.

Table of Contents (TOC)

2


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Due to the superior optoelectronic properties of quantum dot light-harvesting materials as well as the low cost for solution-processable production, quantum dot sensitized solar cells (QDSCs) have been regarded as a promising candidate among the new generation photovoltaic techniques in solving the crises of global warming and energy crisis.1–4 A rapid evolution of QDSC have been witnessed with power conversion efficiencies (PCEs) increasing from less than 5% to the state-of-art 12.65% in just five years.5–13 However, the challenge of further improving photovoltaic performance to make it competitive to other types of solar cells,14–17 as well as the simplification of device fabrication process remain urgent.18,19 High QD loading amount and the formation of a full monolayer of QD on the surface of TiO2 film electrode is an inevitable route for high efficiency QDSCs.18 Accordingly, the mode of QD sensitizers tethering on TiO2 film electrodes is directly related to the performance of QDSCs since this process determines the loading amount as well as the distribution of QD sensitizers on the surface of TiO2 substrate.18 Unambiguously, direct adsorption (DA) by immobilizing initial oil-soluble QD on TiO2 electrode is a facile deposition approach.20–32 On one hand, DA route avoids the tedious ligand exchange process adopted in the mostly common deposition method, capping ligand induced self-assembling (CLIS), for high efficiency QDSCs; on the other hand, DA route can fully take the potential of pre-prepared QDs in comparison with the in situ growth QD methods.33–36 Furthermore, the intimate contact between QD and TiO2 matrix in DA approach favors electron injection from QD to TiO2.37–39 Unfortunately, low loading amount and poor reproducibility are still bottlenecks for the DA route, and therefore limits the photovoltaic performance of the resultant cell devices. With the initiative of simplifying QD deposition process and further improving cell performance, it is meaningful to restudy the underlying chemistry and mechanisms governing the poor QD loading for the DA mode. Recently, the influence of various experimental variables in DA process on the QD loading was investigated systematically by a series of research groups as well as our own group.20–32 These results indicated that DA mode shares the featured merits of facile operation and favored electron injection from QD to TiO2 substrates. In addition, the distinctive drawbacks of QD agglomeration during deposition process can be avoided by controlling sensitization time and the concentration of QD dispersions.32 The major obstacle is that DA failed to achieve high QD loading amount compared with the mainstream deposition method CLIS.32 QD deposition is in essence a solid-liquid adsorption process, so the excavation of loading 3



driving force is the key to improve the loading amount of QD solute. From the equilibrium point of view, a weakened interaction of QD with solvent molecule and the depressed ligand solvation energy will favor the loading process. It is therefore feasible and promising to de-stabilize the dispersed QDs by tuning the properties of solvent. In the past years, the rare investigations concerning the influence of solvents on QD loading behavior in DA mode had hardly figured out why various solvents behaved so distinctly, and the elementary findings cannot provide theoretical guidance.20-32 For example, toluene was usually applied as single-component solvent to disperse QD in early works, only leading to very limited QD loading amount and poor PCE.20–22,24,26 The utilization of dichloromethane as single-component solvent seemed to contribute to greatest QD loading amount among all single-component solvents by DA mode.23,28,30 Regrettably, the loading amount was still inferior to the one realized by CLIS mode.32 On a brighter note, the utilization of bi-component solvents provides broadened choices and resulted in precise tuning of solvent properties. Sykora et al. reported that toluene/butylamine bi-component solvent in DA mode resulted in obvious improvement of light harvesting efficiency (LHE).25 Similar increase of optical gain was obtained with hexane/butylamine.31 These findings emphasized on the ligand exchange process in the presence of nitrogen-containing molecules, and a large number of bi-component solvent combinations remained unexplored.40 Herein, a facile QD direct adsorption route based on bi-component solvents as well as the related adsorption mechanism were explored. The results demonstrated that the increase of solvent polarity is the key to an optimized DA process. Specifically, by precisely tuning the component ratio of bi-component solvents exactly at the critical point (CP), where the QDs become metastable and start to precipitate, an extremely high QD loading amount can be realized. At this critical state, the bi-component solvent exhibits the largest acquirable polarity, and the surface ligands around QDs adopt a contracted configuration, therefore losing their function as colloidal stabilizer. As a result, the intermolecular force between QDs and TiO2 substrate is thoroughly excavated. Meanwhile the solvation energy of ligands and the surface energy of QDs are converted into the Gibbs free energy change for QD loading. A series of bi-component solvents can result in high QD loading amount at this critical point, verifying the universality and regularity of this solvent engineering strategy. The optimized CdSe QDSCs via DA mode based on hexane/acetone system resulted in a record CdSe QD fractional coverage of 38% and exhibited an average PCE of 6.53% (Voc = 0.621 V, Jsc = 15.87 mA/cm2, FF = 0.663), and the Zn−Cu−In−Se (ZCISe) QDSCs based on hexane/methyl 4


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acetate exhibited an average PCE of 12.24±0.19 % (Voc= 0.744 V, Jsc = 25.33 mA/cm2, FF = 0.649). These results are comparable to those based on state-of-the-art CLIS route (CdSe QDSCs: PCE = 6.51%, Voc = 0.615 V, Jsc = 15.85 mA/cm2, FF = 0.669, and ZCISe QDSCs: PCE = 12.15%, Voc = 0.743 V, Jsc = 25.20 mA/cm2, FF = 0.648). The QD direct adsorption (DA) route based on single-component solvents was illustrated to provide primary understanding about the whole process. In DA route, the loading of CdSe QDs was realized by immersing TiO2 film (4 nm in thickness) electrodes in as-prepared QD dispersions for 3h, and QD loading amount is directly reflected in UV-Vis spectra by the absorbance value of sensitized TiO2 films at the first excitonic peak (around 616 nm). Typically, the nonpolar solvents for dispersing QDs are classified as “A type solvents” hereafter, including dichloromethane, hexane, toluene, cyclohexane, tetrahydrofuran, and chloroform in this investigation. All of the A type solvents exhibit good solubility for the original OAm-capped CdSe QDs. The UV-Vis spectra of six QD dispersions in the investigated solvents coincided (Figure 1a), indicating that the solvents did not influence the electronic structure of CdSe QDs notably. The results in Figure 1b indicated that sole dichloromethane leaded to the highest QD loading amount with an absorbance value (A) of 0.47 among all solvents tested. For other A type solvents, QD loading amounts were dramatically lower with absorbance values below 0.25. All the absorbance values were inferior to the one based on CLIS route (A = 0.59), indicating that DA route based on single-component solvents was still less efficient compared with CLIS.

b)

a)

Figure 1. Absorption spectra of (a) OAm-capped CdSe QDs dispersed in a series of single-component solvents including dichloromethane (dic), hexane (hex), toluene (tol), cyclohexane (cyc), tetrahydrofuran (tet), and chloroform (chl), and (b) CdSe QD sensitized TiO2 films via DA route in these A type solvents or CLIS route. These results deducted the 5



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absorbance of blank TiO2 film. Inset: photographs of corresponding films. The influence of solvent molecule properties on QD loading amount in DA process was hardly explored. The low QD loading amount should not be attributed to the poor contact of TiO2 substrate and solvents because it is observed that all the A type solvents used here can spread on TiO2 substrate. Although dichloromethane and chloroform have similar molecular structure, it was observed that the resultant QD loading amounts were quite distinctive. Gómez et al. put forward an opinion that when toluene was used, the fast agglomeration of QDs would block the entrance of TiO2 nanochannel, thus forming a bottleneck for further QD penetration.24 However, we found that when TiO2 underwent long enough sensitization time in QD-toluene dispersion and then further underwent the second sensitization process in QD-dichloromethane dispersion, significantly greater QD loading amount was observed (Figure S1). This observation verified that the blocked nanochannel could not be the source of inefficient QD loading. The fact that sole dichloromethane served as the best single-component solvent in DA process is in accord with previous reports.30,32 Note that the polarity of dichloromethane is distinctively larger than other solvents (the related dielectric constant (ε)41 and polarity index (ETN) values42 were available in Table 1). The preliminarily observed high QD loading amount based on sole dichloromethane highlighted polarity as the key solvent feature for a DA process. Table 1. Dielectric Constant (at 20 oC) and Polarity Index (at 25 oC) for studied Solvents. solvent

dielectric constant (ε)

polarity index (ETN)

dichloromethane

9.1

–0.330

hexane

1.9

–0.009

toluene

2.2

–0.097

cyclohexane

2.0

–0.010

tetrahydrofuran

7.6

–0.210

chloroform

4.9

–0.265

acetone

20.7

–0.366

methanol

31.2

–0.796

ethanol

25.7

–0.680

isopropanol

18.3

–0.573

n-butanol

17.1

–0.625

n-butylamine

4.9

–0.173

6


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methyl acetate

6.7

–0.294

The problem is that the loading driving force deriving from sole dichloromethane was still not large enough. If greater polar single-component solvents were directly applied, the QDs cannot disperse homogeneously. It is noted that the utilization of bi-component solvents allows the tuning of solvent polarity continuously and precisely, so that QD loading driving force can be further excavated. In this section, QD loading amount was investigated when bi-component solvents were applied. The polarity of dispersions was tuned by adding various polar solvents in the above-mentioned six nonpolar solvents. A series of high-polar solvents, including acetone, methanol, ethanol, isopropanol, n-butanol, and n-butylamine, are defined as “B type solvents” hereafter. The feature of B type solvents lies in their inter-solubility with A type solvents as well as their precipitation ability towards the dispersed CdSe QDs (with a few exceptions). The QD dispersions based on bi-component solvents were prepared by adding corresponding B type solvent into QD-A dispersion, and the volume fraction of solvent B is denoted as VB. Specifically, the VB value at which CdSe QDs become metastable and the dispersion started to become cloudy is defined as the “critical point” (CP) of that bi-component solvent. The photographs of QD dispersion at CP was illustrated in Figure 2a. The influence of VB at bi-component solvent system on the CdSe QD loading amount on TiO2 film was scrutinized with use of hexane/acetone bi-component solvent as an example, and the analysis of maximum absorbance value of sensitized TiO2 films at 616 nm (A) versus VB is illustrated in Figure 2a with the detailed absorption spectra and photographs available in Figure 2b. The polarity of bi-component solvent enhanced with the increase of VB value, as indicated by the increasingly negative polarity index values (ETN, Figure 2a). It is observed that as VB increased from 0 to 0.448, the absorbance (A) of corresponding sensitized TiO2 films exhibited a very limited increase (from 0.15 to 0.25). This phenomenon indicated that simply using large-polar B type solvents as addictive can expedite QD loading to some extent. This is consistent with a recent investigation.40 Unexpectedly, further increase of VB approaching CP (0.534) resulted in a boost of the QD loading amount, and the maximum value occurred at CP (A = 0.63). It is worthy to mention that in the case of hexane/acetone system, the absorbance is higher than that based on CLIS (0.63 vs 0.59). Correspondingly, the QD fractional coverage is 0.38 for DA and 0.36 for CLIS (detailed calculation is available in Supporting Information). To the best of our knowledge, this QD loading amount and QD coverage level is among the highest in the fields of QDSC.18,43 It is noted that the presence of QD agglomeration is inevitable at the CP in a bi-component solvent system. Experimental 7



results indicated that the QD loading amount remained constant whether or not removing the QD agglomeration in the dispersion system via a centrifugation purification process (Figure S2). This indicates that the presence of already agglomerated QDs in the dispersion system at CP does not influence the QD loading process and the loading amount. Therefore, the occurrence of QD agglomeration can serve as a signal of CP and offer a convenient way to estimate it, just like an end point in a chemical titration. The slump of QD loading mount at VB > CP should be ascribed to the sharp loss of QD dispersion concentration (Figure S3). On account of the above observations, it is summarized that the largest loading amount occurs at CP, and an extremely narrow VB region, which can lead to high QD loading amount, locates near CP. N

0.00 0.7

-0.05

ET

-0.10

-0.15

-0.20

-0.25

b)

a) CP

0.6

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.5 0.4 0.3

hex

hex/acet

0.2 0.1 0.0 0.0

0.1

0.2

0.3

VB

0.4

0.5

0.6

0.7

B

Figure 2. (a) The dependence of absorbance of CdSe sensitized films at 616 nm on acetone volume fraction (VB) in the acetone/hexane dispersion system. The polarity index values (ETN) of bi-component solvents were calculated using the empirical Snyder formula.44 Inset: photographs of CdSe QDs dispersed in hexane (hex) and hexane/acetone (hex/acet) at critical point (CP). (b) UV-Vis spectra of corresponding sensitized films at a series of VB. The results deducted the absorbance of blank TiO2 film. Inset: photographs of corresponding films. The above results indicated that when bi-component solvents are applied in DA route, it is critical to cautiously tune VB to around CP in order to get high QD loading amount. Inspired by above observations, QD loading amounts were investigated based on a series of bi-component solvent combinations at corresponding CPs. The analysis of maximum absorbance at 616 nm (A) in DA based on various bi-component solvents at CP was illustrated in Figure 3 with detailed composition of CP available in Table S1 and the detailed 8


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spectral information available in Table S2 and Figure S4. The results demonstrated that a series of profoundly high absorbance values can be obtained when VB located at corresponding CPs, and the absorbance values based on bi-component solvents are essentially higher than the ones based on sole A type solvents. In addition, QD loading amounts at CP were influenced by the choice of solvent combinations. Among all, acetone serves as the best B type solvent with the highest absorbance values under each chosen solvent A. The next are alcohols, while n-butylamine is the least efficient. In previous results, dichloromethane has been confirmed to be the best single-component solvent for QD loading. Encouragingly, the absorbance values based on a number of bi-component solvents (dichloromethane/acetone, dichloromethane/isopropanol, toluene/acetone,

hexane/acetone,

toluene/isopropanol,

hexane/ethanol,

cyclohexane/acetone,

hexane/isopropanol, cyclohexane/ethanol,

cyclohexane/isopropanol, tetrahydrogenfuran/acetone, and chloroform/acetone) exceed that of sole dichloromethane (A = 0.47) and approach the one based on CLIS route (A = 0.59). Notably, the best hexane/acetone results in an absorbance of 0.63 and surpasses CLIS. Therefore, DA mode based on bi-component solvents is highlighted by the intrinsic merits of universality, effectiveness, tunability, and feasibility.

single-component solvents acet meth etha isop buta buty

0.8

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

CLIS sole dic

0.4 0.2 0.0 dic

hex

tol

cyc

tet

chl

Figure 3. Statistical analysis of the maximum absorbance of CdSe QD sensitized TiO2 films at 616 nm via DA mode based on a series of single and bi-component solvents at CP. The abbreviations are: dichloromethane (dic), hexane (hex), toluene (tol), cyclohexane (cyc), tetrahydrofuran (tet), chloroform (chl), acetone (acet), methanol (meth), ethanol (etha), isopropanol (isop), n-butanol (buta), and n-butylamine (buty). The two dashed lines represent the absorbance values based on sole dichloromethane in DA mode and water in CLIS mode. 9



Direct adsorption (DA) of QDs is in essence a solute transportation process where QDs leave liquid phase and deposit on solid surface. In our case, the CdSe QDs are capped with oil-soluble ligands (oleylamine, oleic acid, and trioctylphosphine) that contain long hydrocarbon tails.45 Therefore, the obtained QDs are uncharged,46,47 and DA process is driven by intermolecular forces, which is also termed as van der Waals forces. Unfortunately, the affinity of the capping ligands with solvent molecules is strong in a nonpolar solvent. As a result, the hydrocarbon chains adopt a stretched configuration to increase the contact with liquid environment (Figure 4a), elastically repelling one another and stabilizing colloidal NC dispersions.46 This also indicated that the dispersed QDs exhibited negative solvation energy. The solvent molecules can compete with the substrate for the adsorption sites or avoid the adsorption of the substrate, maintaining it soluble in the liquid phase.32 Keep these in mind, the key to realize efficient DA process relies on the following two factors: (i) the destruction of interaction between ligands and solvent molecules, and (ii) the excavation of van der Waals forces between QDs and TiO2 substrates. Evidently the utilization of polar solvents is a powerful technique to break the interaction between solute and solvent molecules. Previous investigations have depicted the behavior of QDs in polar solvents.46,48 Specifically, as the solvent polarity increases, due to the incompatibility of surface ligands to polar solvent molecules, a drive to minimize contact with surrounding liquid arises, so the depth that solvent molecules can penetrate into the void space between the adjacent ligands becomes shallower, eventually the ligands adopt a contracted configuration (Figure 4a). The comprehensive result is that the QDs become less stable with reduced solubility and overcome solvation energy. At this stage, our results indicated that the UV-Vis spectra and morphology of QDs remained unchanged (Figure S3 and Figure S5). It is also observed that the PL intensity encounters a drastic decrease at CP (Figure S6), which can be ascribed to the loss of surface ligands that passivate QD surface trap states.32,38,39,49,50 Correspondingly, the state change of surface ligands is advantageous for the excavation of loading driving force. Computational simulations proved that the presence of a protective ligand layer around QD in nonpolar solvents provides dominant elastic repulsion force and preserves the colloidal system by screening van der Waals attractions.51 In polar solvents, however, the contraction and partial detachment of these ligands dramatically weaken the 10


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repulsion, in turn, enable efficient attractions among the adjacent particles (Figure 4b). The most universal version of van der Waals force is dispersion force, which originates from the continuous electronic movement of atoms or molecules even they process no permanent dipole moment. The potential energy of dispersion interactions between two species, Udis, can be expressed as:

Udis = −

1 (4)2



3𝑎 1  𝑎 2 2𝑟 6

𝐼 𝐼

 (𝐼 1+ 2𝐼 ) 1

(1)

2

where ε is the dielectric constant of medium, α1 and α2 are the polarizabilities and I1 and I2 are the ionization potentials of QD and TiO2 substrate, r is the distance between two dispersive center.52 The negative sign indicates that the dispersion interactions contributes to the attraction of adjacent species. In addition, compared with nonpolar solvents, the utilization of polar solvent generates another type of intermolecular forces: inductive forces. The electric dipole of a molecule possessing a permanent dipole moment μ can induce a dipole moment in an adjacent molecule, and the potential energy of induction, Uind, between two species can be expressed as:

Uind = −

1 (4)2



𝑎1  𝜇 22 + 𝑎2 𝜇 12

(2)

𝑟6

where α1 and α2 are the polarizabilities and μ1 and μ2 are the permanent dipole moment of two species, and r is the distance between two dipole centers.52 From a microscopic view, strong dipole-dipole

interaction

serving

as

the

driving

force

for

CdTe

nanoparticle

self-organization53 and PbS nanoparticle aggregation54 have been observed. It should be noted that the two kinds of intermolecular forces are extremely short-range in action depending on 1/r6, so the dramatically reduced average r value between QD and TiO2 due to the contraction of ligands effectively promotes the attractions that serve as loading driving force. As for the inductive forces, both Cd and Se are soft categories according to soft hard acid base (HSAB) theory,55 so CdSe material shares a large α value and thus can be effectively induced by polar solvent molecules. Then the induced QDs possessing larger dipole moment therefore show extra driving force of loading. Additionally, the reduced steric hindrance enables a close-compacted QD sensitized film, which is the premise for a high QD loading amount.18,32 11



As the increase of solvent polarity and the excavation of attractive intermolecular forces, the destabilized QDs show the tendency to reduce their exposed surface area in contact with liquid and eventually leave the liquid phase. At this stage, the attractions of QD-TiO2 and QD-QD become prominent, therefore two pathways determine the fate of metastable QDs: the loading of QDs on TiO2 substrates, and the agglomeration of QDs due to hydrophobic interactions. At VB < CP, QD loading amount increased over solvent polarity (Figure 2a). At VB > CP, the severe loss of QD concentration due to the undesired QD agglomeration resulted in inefficient DA mode (Figure S3). The maximum QD loading amount at CP therefore can be understood as the optimized state that maximizes QD loading while minimizes QD loss. Having analyzed the driving force in detail, a thorough understanding of Gibbs free energy change (∆G) in DA process is also meaningful because ∆G is directly linked with equilibrium position and QD loading amount. The Gibbs free energy diagram of CdSe QDs at different stages of DA process is depicted in Figure 4c. The organic ligands around CdSe QDs are stretched when dispersed in an A type solvent with the negative solvation energy Esol.46 In this case, the total free energy G1 of the stable QDs in a sole nonpolar solvent is low, therefore the process for QDs to leave liquid phase and to load on TiO2 substrate is not favored due to the rather low ∆G1 (∆G1 = G1 − G2, Figure 4c). With the introduction of polar solvent at CP, the surface ligands contract and the solvation energy Esol of ligands is overcome. At the same time, the partial detachment of surface ligands due to the presence of polar solvent molecules (Figure S6) results in uncoordinated QD surface states and therefore an increase of QD surface energy Esur.46 As a result, the total free energy G3 of the QDs in a bi-component solvent exhibits a remarkable elevation contributed by Esol and Esur. Meanwhile, G4 value of loaded QDs further declined due to the contribution of more negative potential energy of Udis and Uind.56 The above two factors result in increased ∆G2 (∆G2 = G3 – G4) and therefore accounted for the greater QD loading amount based on bi-component solvent system (Figure 4c). The analysis can also illustrate the reason why dichloromethane serves as the best single-component solvent for QD loading. Dichloromethane shares a moderate polarity (Table 1), and the QDs dispersed in dichloromethane should adopt an incompletely contracted surface ligand configuration.48 In the case of n-butylamine, the presence of nitrogen atoms and the alkaline nature enable it to competitively replace the original long aliphatic chain, and its short chain can not provide sufficient solvation energy, which is the 12


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reason for the precipitation of QDs.57 The universal lower QD loading amount based on n-butylamine compared with other B type solvents should be ascribed to the low Esol from its short chain as well as the low polarity of n-butylamine and the related bi-component solvents (Table

1),

and

in

the

extreme

case,

the

loading

amount

based

on

dichloromethane/n-butylamine is lower than sole dichloromethane (Figure 3, Figure S4). The above results indicated that due to the different solvent polarity and solvation, different bi-component solvents can contribute to divergent alteration of ∆G2, and the detailed mechanism demands further investigation. It is important to note that the critical change of QD surface ligands at CP and the elimination of ligand solvation energy and surface energy push the QDs up to a metastable state possessing a large Gibbs free energy, and the “activated” QDs are vital for high QD loading amount. The key to high QD loading amount in bi-component solvent based DA mode therefore lies in the reasonably tailoring of solvent polarity with composition exactly at CP. a)

b)

c)

Figure 4. (a) Stretched and contracted configurations of QD surface ligands dispersed in single- (left) and bi-component solvents at CP (right), respectively. (b) The interaction potential U with interparticle distance r for oil-soluble ligand-capped nanocrystals in ideal good and non- solvents.46 (c) Gibbs free energy (G) diagram of QDs in a DA process.

13



Efficient DA process was applied to assemble CdSe QD sensitized TiO2 photoanode in the construction of QDSCs by using the hexane/acetone bi-component solvent system with Cu/brass as counter electrode. The influence of VB on the photovoltaic performances of QDSCs is illustrated in Figure 5 with the detailed information available in Table S3 and Figure S7. The results indicated that the PCE values are prevailingly dominated by Jsc values (Figure 5a), which is in accordance with our previous report.32 Both PCE and Jsc values are positively related to the QD loading amount, and the observed systematic larger Voc at greater QD loading amount and Jsc values (Figure 5b) should be ascribed to efficient charge accumulation and retarded charge recombination at higher QD coverage.32,58 In addition, QD loading amount does not notably influence FF values (Figure 5b) because series and shunt resistances will not observably change over QD loading amount.32 The champion cell performance occurs at CP (PCE = 6.53%, Voc= 0.621 V, Jsc = 15.87 mA/cm2, FF = 0.663), which is comparable to the one based on CLIS (PCE = 6.51%, Voc = 0.615 V, Jsc = 15.85 mA/cm2, FF = 0.669, see Table S4 and Figure S8). CP

a)

6.0

0.64

18

PCE

0.80

b)

0.60

0.76

15

4.5

12 3.0

9

Voc

0.56

0.72

FF

0.52

2

Jsc 1.5 0.0

CP

FF

PCE (%)

21

Voc (V)

7.5

J sc (mA/cm )

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Figure 5. Average photovoltaic parameter analysis for CdSe QDSCs via DA mode based on the hexane/acetone bi-component solvent at various VB. (a) PCE and Jsc, (b) Voc and FF. We endeavored in applying the experiences obtained from CdSe to Zn−Cu−In−Se (ZCISe) QDSCs to get higher PCEs. The absorption spectrum of ZCISe QD dispersion is available in Figure S9. The introduction of above-mentioned B type solvents, however, contributes to an irreversible ZCISe QD agglomeration, presumably due to severe loss of surface diphenylphosphine ligands.59 Herein, a weak polar solvent, methyl acetate, was applied as B 14


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type solvent to maintain the stability of ZCISe QD dispersions.60 The CP for ZCISe QD dispersion in hexane/methyl acetate bi-component solvent is 0.630. The average cell performance via DA mode based on hexane/methyl acetate and Ti mesh-supported mesoporous carbon (MC/Ti) counter electrodes is PCE = 12.24±0.19 %, Voc = 0.744 V, Jsc = 25.33 mA/cm2 and FF = 0.649 with detailed performance for individual cells available in Table S5 and Figure S10. It is highlighted that this performance via DA mode is comparable to those based on state-of-the-art CLIS route (PCE = 12.15±0.20 %, Voc = 0.743 V, Jsc = 25.20 mA/cm2, FF = 0.648). Meanwhile, this performance is also among the best results for QDSCs. The realization of high efficiency QDSC devices demonstrates one of the successful applications of DA mode based on bi-component solvents. In summary, a facile QD direct adsorption route at the CP of a bi-component solvent system was developed. The results indicated that the polarity of solvents is beneficial for the efficient QD loading on TiO2 film electrode in the DA process. Upon excavation of loading driving force, the utilization of bi-component solvents in DA route can result in considerably higher QD loading amount compared with those based on single-component solvents. The maximum loading amount appears at the critical point when the QDs become unstable and begin to precipitate. The marked feature of QDs at this critical state is that the surface ligands around QDs adopt a contracted configuration and lose the function as colloidal stabilizer. As a result, the intermolecular attractions between QDs and TiO2 substrate is thoroughly excavated, meanwhile the solvation energy of ligands and surface energy of QDs are converted to the Gibbs free energy change for QD loading. A number of bi-component solvent systems are promising to realize high QD loading amount at the critical point, verifying the universality and regularity of this strategy. The optimized CdSe QDSCs based on hexane/acetone and Cu/brass counter electrodes exhibited a PCE of 6.53% (Voc= 0.621 V, Jsc = 15.87 mA/cm2, FF = 0.663). Meanwhile, the optimized ZCISe QDSCs based on hexane/methyl acetate and Ti mesh-supported mesoporous carbon counter electrodes exhibited an average PCE of 12.24%, which can be comparable to those obtained via the state-of-art CLIS deposition method, and also among the highest values for QDSCs. Furthermore, it is expected that the developed QD deposition approach from bi-component solvent at the critical point is promising to extend to other systems of divergent nanocrystals and substrates, opening up a new nanomaterial deposition method at liquid-solid interfaces.

15



Experimental Section Synthesis, Purification and Water-Solubilization of CdSe QDs for Direct Adsorption (DA) and Capping Ligand-Induced QD Self-Assembly (CLIS). Oil-soluble OAm-capped CdSe QDs for DA and CLIS modes were synthesized through a high temperature pyrolysis route according to our previous report and all chemicals used were identical to literature methods.45 In DA mode, for CdSe QD purification, the crude reaction mixtures were precipitated with addition of methanol/acetone (v/v = 4/1), followed by re-dispersion of pellet in dichloromethane after centrifugation. Sole acetone was used for the second precipitation step, and the pellet was dispersed in a kind of nonpolar A type solvent. The A type solvents are dichloromethane, hexane, toluene, cyclohexane, tetrahydrofuran, and chloroform in this work. The CdSe QDs dispersed in bi-component solvents were prepared by adding a kind of polar B type solvent into QD-A dispersion. The B type solvents include acetone, methanol, ethanol, isopropanol, n-butanol, and n-butylamine in this work. In CLIS mode, the purification and water-solubilization procedures are followed from our previous report.40 Synthesis, Purification and Water-Solubilization of Zn−Cu−In−Se (ZCISe) QDs for Direct Adsorption (DA) and Capping Ligand-Induced QD Self-Assembly (CLIS). Oil-soluble OAm-capped ZCISe QDs for DA and CLIS modes were synthesized through a high temperature pyrolysis route according to our previous report and all chemicals used were identical to literature methods.7 In DA mode, for ZCISe QD purification, the crude dispersions were precipitated with addition of ethanol, followed by re-dispersion of pellet in dichloromethane or hexane after centrifugation. The ZCISe QDs dispersed in hexane/methyl acetate were prepared by adding methyl acetate into ZCISe-hexane dispersion. In CLIS mode, the purification and water-solubilization procedures are available from previous report.7 Sensitization of TiO2 Photoanode and Construction of QDSCs. The preparation of TiO2 mesoporous electrodes was the same as our previous works.18,32 The electrodes with an area of 0.235 cm2 for photovoltaic measurements were composed of 10 µm-thick transparent layer and 5 µm-thick light scattering layer. In UV-Vis absorption characterization, the adopted TiO2 film electrodes possess only a 4.0 μm transparent layer with active area of 1.6 cm2 (2.0 cm  0.8 cm). The sensitization process in DA or CLIS 16


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approaches was realized by immersing TiO2 mesoporous film electrodes in corresponding QD dispersions in organic solvent or water for 3h. A moderate concentration window of all CdSe QD dispersions (26.0 ± 4.0 μM) was adopted for each chosen solvent in order to achieve high QD loading amount and prevent the undesired QD agglomeration simultaneously.32 In the case of ZCISe QDs, one batch of QDs synthesized according to the approach in ref. 7 were eventually dissolved in 10 mL of dichloromethane or 8 mL of hexane. For solar cell fabrication, a ZnS barrier layer was overcoated on the sensitized photoanodes after QD deposition by alternately dipping the QD sensitized electrode into 0.1 M Zn(OAc)2 methanol solution and 0.1 M Na2S aqueous solution (four SILAR cycles for CdSe and six cycles for ZCISe). The ZnS coated CdSe sensitized film electrodes were further coated with SiO2 layer by immersing them in 0.01 M tetraethyl orthosilicate ethanol solution at 35 oC for 2h according to our modified method.61 Sandwich-structured solar cells were fabricated by assembling Cu2S/brass counter electrode and QD-sensitized TiO2 film electrode using a 60 μm-thick Scotch spacer with a binder clip and filled with aqueous polysulfide electrolyte solution (2.0 M Na2S, 2.0 M S, and 0.2 M KCl). Cu2S counter electrodes were prepared by immersing brass foils in 1.0 M HCl solution at 90 °C for 30 min, followed by reacting them with fresh polysulfide aqueous solution. The method to prepare the Ti mesh-supported mesoporous carbon (MC/Ti) counter electrodes is available in a previous report.62 Characterization. The UV-Vis absorption spectra was recorded from a UV-Vis spectrophotometer (Shimadzu UV-3101 PC). The photovoltaic properties and J-V curves of the QDSCs were measured by Keithley 2400 source meter equipped with a 150 W AM 1.5 G solar simulator (Oriel, model no. 94022A). Calibration was taken by an NREL standard Si solar cell to set the power of the simulated solar light to 100 mW/cm2. During the measurement, the photoactive area was defined by a shading mask of 0.235 cm2. Transition electron microscopy (TEM) images were obtained using a JEOL JEM-2100 instrument with accelerating voltage of 200 kV. For sample preparation, a few drops of CdSe QD dispersions were taken and dipped on a TEM grid.

17



Acknowledgements This research is supported by the National Natural Science Foundation of China (Nos. 51732004, 21703071)

Supporting Information Absorption spectra of CdSe QD sensitized TiO2 films based on one- and two-step loading, calculation of QD coverage on TiO2 surface, absorption spectra of QD dispersions and QD sensitized TiO2 films under various conditions, the information of constitution of critical point (CP) of CdSe QD dispersions based on various bi-component solvents represented by VB, wide-field TEM images of CdSe QDs dispersed in hexane/acetone, PL emission spectra of CdSe QDs dispersed in hexane/acetone close to CP, details of photovoltaic performance of QDSCs under various experimental conditions, including J-V curves. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Kamat, P. V.; Tvrdy, K.; Baker, D. R.; Radich, J. G. Beyond Photovoltaics: Semiconductor Nanoarchitectures for Liquid-Junction Solar Cells. Chem. Rev. 2010, 110, 6664–6688. (2) Albero, J.; Clifford, J. N.; Palomares, E. Quantum Dot Based Molecular Solar Cells. Coord. Chem. Rev. 2014, 263, 53–64. (3) Green, M. A.; Bremner, S. P. Energy Conversion Approaches and Materials for High-Efficiency Photovoltaics. Nat. Mater. 2017, 16, 23–34. (4) Nozik, A. J.; Beard, M. C.; Luther, J. M.; Law, M.; Ellingson, R. J.; Johnson, J. C. Semiconductor Quantum Dots and Quantum Dot Arrays and Applications of Multiple Exciton Generation to Third-Generation Photovoltaic Solar Cells. Chem. Rev. 2010, 110, 6873–6890. (5) Jiao, S.; Du, J.; Du, Z.; Long, D.; Jiang, W.; Pan, Z.; Li, Y.; Zhong, X. Nitrogen-Doped Mesoporous Carbons as Counter Electrodes in Quantum Dot Sensitized Solar Cells with a Conversion Efficiency Exceeding 12%. J. Phys. Chem. Lett. 2017, 8, 559−564. (6) Du, Z.; Pan, Z.; Fabregat-Santiago, F. ; Zhao, K.; Long, D.; Zhang, H.; Zhao, Y.; Zhong, X.; Yu, J. -S.; Bisquert, J. Carbon Counter-Electrode-Based Quantum-Dot-Sensitized Solar Cells with Certified Efficiency Exceeding 11%. J. Phys. Chem. Lett. 2016, 7, 3103–3111. (7) Du, J.; Du, Z.; Hu, J. -S.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.; Dong, H.; Sun, L.; Zhong X. et al.; Zn−Cu−In−Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%. J. Am. Chem. Soc. 2016, 138, 4201–4209. (8) Ren, Z.; Wang, Z.; Wang, R.; Pan, Z.; Gong, X.; Zhong, X. Effects of Metal Oxyhydroxide Coatings on Photoanode in Quantum Dot Sensitized Solar Cells. Chem. Mater. 2016, 28, 2323–2330. (9) Li, W.; Pan, Z.; Zhong, X. CuInSe2 and CuInSe2-ZnS based High Efficiency “Green” Quantum Dot Sensitized Solar Cells. J. Mater. Chem. A 2015, 3, 1649–1655. 18


Page 18 of 22

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Zhao, K.; Yu, H.; Zhang, H.; Zhong, X. Electroplating Cuprous Sulfide Counter Electrode for High-Efficiency Long-Term Stability Quantum Dot Sensitized Solar Cells. J. Phys. Chem. C 2014, 118, 5683−5690. (11) Zhang, H.; Cheng, K. Hou, Y.; Fang, Z.; Pan, Z.; Wu, W.; Hua, J.; Zhong, X. Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by Postsynthesis Assembly Approach. Chem. Commun. 2012, 48, 11235–11237. (12) Wei, H.; Wang, G.; Shi, J.; Wu, H.; Luo, Y.; Li, D.; Meng, Q. Fumed SiO2 Modified Electrolytes for Quantum Dot Sensitized Solar Cells with Efficiency Exceeding 11% and Better Stability. J. Mater. Chem. A 2016, 4, 14194–14203. (13) Wang, W.; Feng, W.; Du, J.; Xue, W.; Zhang, L.; Zhao, L.; Li, Y. Cosensitized Quantum Dot Solar Cells with Conversion Efficiency over 12%. Adv. Mater. 2018, 30, 1705746–1705752. (14) Xu, J.; Voznyy, O.; Liu, M.; Kirmani, A. R.; Walters, G.; Munir, R.; Abdelsamie, M.; Proppe, A. H.; Sarkar, A.; García de Arquer, F. P.; et al. 2D Matrix Engineering for Homogeneous Quantum Dot Coupling in Photovoltaic Solids. Nat. Nanotech. 2018, 13, 456–462.

(15) Meng, L.; Zhang, Y.; Wan, X.; Li, C.; Zhang, X.; Wang, Y.; Ke, X.; Xiao, Z.; Ding, L.; Xia, R.; et al. Organic and Solution-Processed Tandem Solar Cells with 17.3% Efficiency. Science 2018, 361, 1094–1098. (16) Jeon, N. J.; Na, H.; Jung, E. H.; Yang, T.-Y.; Lee, Y. G.; Kim, G.; Shin, H.-W.; Seok, S. II; Lee, J.; Seo, J. A Fluorene-Terminated Hole-Transporting Material for Highly Efficient and Stable Perovskite Solar Cells. Nature Energy 2018, 3, 682−689. (17) Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Fujisawa, J.; Hanaya, M. Highly-Efficient Dye-Sensitized Solar Cells with Collaborative Sensitization by Silyl-Anchor and Carboxy-Anchor Dyes. Chem. Commun. 2015, 51, 15894−15897. (18) Li, W.; Zhong, X. Capping Ligand-Induced Self-Assembly for Quantum Dot Sensitized Solar Cells. J. Phys. Chem. Lett. 2015, 6, 796–806. (19) Du, Z.; Tong, J.; Guo, W.; Zhang, H.; Zhong, X. Cuprous Sulfide on Ni Foam as a Counter Electrode for Flexible Quantum Dot Sensitized Solar Cells. J. Mater. Chem. A 2016, 4, 11754–11761. (20) Zaban, A.; Mićić, O. I.; Gregg, B. A.; Nozik, A. J. Photosensitization of Nanoporous TiO2 Electrodes with InP Quantum Dots. Langmuir 1998, 14, 3153–3156. (21) Yu, P.; Zhu, K.; Norman, A. G.; Ferrere, S.; Frank, A. J.; Nozik, A. J. Nanocrystalline TiO2 Solar Cells Sensitized with InAs Quantum Dots. J. Phys. Chem. B 2006, 110, 25451–25454. (22) Lee, H. J.; Yum, J. -H.; Leventis, H. C; Zakeeruddin, S. M.; Haque, S. A.; Chen, P.; Seok, S. Il; Grätzel, M.; Nazeeruddin, M. K. CdSe Quantum Dot-Sensitized Solar Cells Exceeding Efficiency 1% at Full-Sun Intensity. J. Phys. Chem. C 2008, 112, 11600–11608. (23) Giménez, S.; Mora-Seró, I.; Macor, L.; Guijarro, N.; Lana-Villarreal, T.; Gómez, R.; Diguna, L. J; Shen, Q.; Toyoda, T.; Bisquert, J. Improving the Performance of Colloidal Quantum-Dot-Sensitized Solar Cells. Nanotechnology 2009, 20, 295204–295209. (24) Guijarro, N.; Lana-Villarreal, T.; Mora-Seró, I.; Bisquert, J.; Gómez, R. CdSe Quantum Dot-Sensitized TiO2 Electrodes: Effect of Quantum Dot Coverage and Mode of Attachment. J. Phys. Chem. C 2009, 113, 4208–4214. (25) Fuke, N.; Hoch, L. B.; Koposov, A. Y.; Manner, V. W.; Werder, D. J.; Fukui, A.; Koide, N.; Katayama, H.; Sykora, M. CdSe Quantum-Dot-Sensitized Solar Cell with ~100% Internal Quantum 19



Efficiency. ACS Nano 2010, 4, 6377–6386. (26) Pernik, D. R.; Tvrdy, K.; Radich, J. G.; Kamat, P. V. Tracking the Adsorption and Electron Injection Rates of CdSe Quantum Dots on TiO2: Linked versus Direct Attachment. J. Phys. Chem. C 2011, 115, 13511–13519. (27) McDaniel, N.; Fuke, N.; Makarov, N. S.; Pietryga, J. M.; Klimov, V. I. An Integrated Approach to Realizing High-performance Liquid-junction Quantum Dot Sensitized Solar Cells. Nat. Commun. 2013, 4, 2887–2896. (28) Guijarro, N.; Guillén, E.; Lana-Villarreal, T.; Gómez, R. Quantum Dot-sensitized Solar Cells Based on Directly Adsorbed Zinc Copper Indium Sulfide Colloids. Phys. Chem. Chem. Phys. 2014, 16, 9115–9122. (29) Liu, F.; Zhu, J.; Wei, J.; Li, Y.; Hu, L.; Huang, Y.; Takuya, O.; Shen, Q.; Toyoda, T.; Zhang, B.; et al. Ex Situ CdSe Quantum Dot-Sensitized Solar Cells Employing Inorganic Ligand Exchange To Boost Efficiency. J. Phys. Chem. C 2014, 118, 214–222. (30) Kim, J.-Y.; Yang, J.; Yu, J. H.; Baek, W.; Lee, C.-H.; Son, H. J.; Hyeon T.; Ko, M. J. Highly Efficient Copper-Indium-Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers. ACS Nano 2015, 9, 11286–11295. (31) Malak, S. T.; Lafalce, E.; Jung, J.; Lin, C. H.; Smith, M. J.; Yoon, Y. J.; Lin, Z.; Vardeny, Z. V.; Tsukruk, V. V. Enhancement of Optical Gain Characteristics of Quantum Dot Films by Optimization of Organic Ligands. J. Mater. Chem. C 2016, 4, 10069–10081. (32) Wang, W.; Jiang, G.; Yu, J.; Wang, W.; Pan, Z.; Nakazawa, N.; Shen, Q.; Zhong, X. High Efficiency Quantum Dot Sensitized Solar Cells Based on Direct Adsorption of Quantum Dots on Photoanodes. ACS Appl. Mater. Interfaces 2017, 9, 22549−22559. (33) Niitsoo, O.; Sarkar, S. K.; Pejoux, C.; Rühle, S.; Cahen, D.; Hodes, G. Chemical Bath Deposited CdS/CdSe-Sensitized Porous TiO2 Solar Cells. J. Photochem. Photobiol. A 2006, 181, 306–313. (34) Becker, M. A.; Radich, J. G.; Bunker, B. A.; Kamat, P. V. How Does a SILAR CdSe Film Grow? Tuning the Deposition Steps to Suppress Interfacial Charge Recombination in Solar Cells. J. Phys. Chem. Lett. 2014, 5, 1575–1582. (35) Fan, S. Q.; Kim, D.; Kim, J. J.; Jung, D. W.; Kang, S. O.; Ko, J. Highly Efficient CdSe Quantum-Dot-Sensitized TiO2 Photoelectrodes for Solar Cell Applications. Electrochem. Commun. 2009, 11, 1337−1339. (36) Yan, K. Y.; Chen, W.; Yang, S. H. Significantly Enhanced Open Circuit Voltage and Fill Factor of Quantum Dot Sensitized Solar Cells by Linker Seeding Chemical Bath Deposition. J. Phys. Chem. C 2013, 117, 92−99. (37) Guijarro, N.; Shen, Q.; Giménez, S.; Mora-Seró, I.; Bisquert, J.; Lana-Villarreal, T.; Toyoda, T.; Gómez, R. Direct Correlation between Ultrafast Injection and Photoanode Performance in Quantum Dot Sensitized Solar Cells. J. Phys. Chem. C 2010, 114, 22352–22360. (38) Kalyuzhny, G.; Murray, R. W. Ligand Effects on Optical Properties of CdSe Nanocrystals. J. Phys. Chem. B 2005, 109, 7012–7021. (39) Hines, D. A.; Kamat, P. V. Quantum Dot Surface Chemistry: Ligand Effects and Electron Transfer Reactions. J. Phys. Chem. C 2013, 117, 14418−14426. (40) Torresan, M. F.; Baruzzi, A. M.; Iglesias, R. A. Enhancing the adsorption of CdSe quantum 20


Page 20 of 22

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dots on TiO2 nanotubes by tuning the solvent polarity. Solar Energy Materials & Solar Cells. 2017, 164, 107–113. (41) Smallwood, I. Handbook of organic solvent properties; Arnold: London, 1996. (42) Reichardt, C. A Chiral Hypervalent Iodine(III) Reagent for Enantioselective. Angew. Chem. Int. Ed. 1979, 18, 98−110. (43) Zhang, H.; Cheng, K.; Hou, Y. M.; Fang, Z.; Pan, Z. X.; Wu, W. J.; Hua, J. L.; Zhong, X. H. Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by a Postsynthesis Assembly Approach. Chem. Commun. 2012, 48, 11235–11237. (44) Issaq, H. J. A century of separation science; Marcel Dekker: New York, 2002. (45) Meng, X.; Du, J.; Zhang, H.; Zhong. X. Optimizing the Deposition of CdSe Colloidal Quantum Dots on TiO2 Film Electrode via Capping Ligand Induced Self-assembly Approach. RSC Adv. 2015, 5, 86023–86030. (46) Boles, M. A.; Ling, D.; Hyeon, T.; Talapin, D. V. The Surface Science of Nanocrystals. Nature Mater. 2016, 15, 141–153. (47) Nag, A.; Kovalenko, M. V.; Lee, J. -S.; Liu, W.; Spokoyny, B.; Talapin, D. V. Metal-free Inorganic Ligands for Colloidal Nanocrystals: S2–, HS–, Se2–, HSe–, Te2–, HTe–, TeS32–, OH–, and NH2– as Surface Ligands. J. Am. Chem. Soc. 2011, 133, 10612–10620. (48) Leekumjorn, S.; Gullapalli, S.; Wong, M. S. Understanding the Solvent Polarity Effects on Surfactant-Capped Nanoparticles. J. Phys. Chem. B 2012, 116, 13063–13070. (49) Bullen, C.; Mulvaney, P. The Effects of Chemisorption on the Luminescence of CdSe Quantum Dots. Langmuir 2006, 22, 3007–3013. (50) Munro, A. M.; Plante, I. J. -L.; Ng. M. S.; Ginger, D. S. Quantitative Study of the Effects of Surface Ligand Concentration on CdSe Nanocrystal Photoluminescence. J. Phys. Chem. C 2007, 111, 6220–6227. (51) Lee, C. K.; Hua, C. C. Nanoparticle Interaction Potentials Constructed by Multiscale Computation. J. Chem. Phys. 2010, 132, 224904−224912. (52) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 3rd ed.; Wiley-VCH Verlag: Weinheim, 2003. (53) Tang, Z.; Kotov, N. A.; Giersig, M. Spontaneous Organization of Single CdTe Nanoparticles into Luminescent Nanowires. Science 2002, 297, 237–240. (54) Shanker, G. S.; Swarnkar, A.; Chatterjee, A.; Chakraborty, S.; Phukan, M.; Parveen, N.; Biswas, K.; Nag, A. Electronic Grade and Flexible Semiconductor Film Employing Oriented Attachment of Colloidal Ligand-Free PbS and PbSe Nanocrystals at Room Temperature. Nanoscale 2015, 7, 9204–9214. (55) Pearson, R. G. Hard and Soft Acids and Bases. J. Am. Chem. Soc. 1963, 85, 3533–3539. (56) Boles, M. A.; Engel, M.; Talapin, D. V. Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials. Chem. Rev. 2016, 116, 11220–11289. (57) Ning, Z.; Dong, H.; Zhang, Q.; Voznyy, O.; Sargent, E. H. Solar Cells Based on Inks of n-Type Colloidal Quantum Dots. ACS Nano 2014, 8, 10321–10327. (58) Wang, W.; Du, J.; Ren, Z.; Peng, W.; Pan, Z.; Zhong, X. Improving Loading Amount and Performance of Quantum Dot Sensitized Solar Cells through Metal Salt Solutions Treatment on 21



Photoanode. ACS Appl. Mater. Interfaces 2016, 8, 31006–31015. (59) Anderson, N. C.; Hendricks, M. P.; Choi, J. J.; Owen, J. S. Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding. J. Am. Chem. Soc. 2013, 135, 18536–18548. (60) Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M. Quantum dot-induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics. Science 2016, 354, 92–95. (61) Zhao, K.; Pan, Z.; Mora-Seró, I.; Canovas, E.; Wang, H.; Song, Y.; Gong, X.; Wang, J.; Bonn, M.; Bisquert. J.; et al. Boosting Power Conversion Efficiencies of Quantum-Dot-Sensitized Solar Cells Beyond 8% by Recombination Control. J. Am. Chem. Soc. 2015, 137, 5602–5609. (62) Du, Z.; Pan, Z.; Fabregat-Santiago, F.; Zhao, K.; Long, D.; Zhang, H.; Zhao, Y.; Zhong, X.; Bisquert, J. Carbon Counter-Electrode-Based Quantum-Dot-Sensitized Solar Cells with Certified Efficiency Exceeding 11%. J. Phys. Chem. Lett. 2016, 7, 3103−3111.

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Enhancing Loading Amount and Performance of Quantum Dot

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