A Facile Secondary Deposition for Improving Quantum Dot Loading in

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A Facile Secondary Deposition for Improving Quantum Dot Loading in Fabricating Quantum Dot Solar Cells Wei Wang, Lianjing Zhao, Yuan Wang, Weinan Xue, Fangfang He, Yiling Xie, and Yan Li J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10901 • Publication Date (Web): 24 Feb 2019 Downloaded from http://pubs.acs.org on February 24, 2019

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Journal of the American Chemical Society

A Facile Secondary Deposition for Improving Quantum Dot Loading in Fabricating Quantum Dot Solar Cells

Wei Wang,# Lianjing Zhao,# Yuan Wang, Weinan Xue, Fangfang He, Yiling Xie, and Yan Li*

Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China

ABSTRACT: A sufficient loading of pre-synthesized quantum dots (QDs) on mesoporous TiO2 electrodes is the prerequisite for the fabrication of high performance QD sensitized solar cells (QDSCs). Here, we provide a general approach for increasing QD loading on mesoporous TiO2 films by surface engineering. It was found that the zeta potential of pre-sensitized TiO2 can be effectively adjusted by surfactant treatment, on the basis of which additional QDs are successfully introduced onto photoanodes during secondary deposition. The strategy developed, that is, the secondary deposition incorporating surfactant treatment, makes it possible to load various QDs onto photoanodes regardless of the nature of QDs. In standard AM 1.5G sunlight, a certified efficiency of 10.26% for the QDSC with Cu2S/brass counter electrodes was achieved by the secondary deposition of Zn-Cu-In-Se QDs.

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INTRODUCTION The urgent demands for energy consumption and environmental protection are spurring the design of high performance and cost effective photovoltaics.13 Since semiconductor quantum dots (QDs) possess uniquely tunable optoelectronic properties and tailorable surface characters, QDs based solar cells (QDSCs) now are considered as one of the most potentially competitive third-generation solar cells.47 The fabrication of QDSCs has been extensively investigated. Most of the relevant research focused on the adjustment of bandgap alignment of QD materials and heterointerface engineering during device construction, by which the highest power conversion efficiencies (PCE) of 13.4% for solid-state QDSCs and 12.7% for liquidjunction QDSCs were obtained.815 Still, the PCE of QDSCs available is far below the theoretical value, which can be ascribed mainly to insufficient light harvesting as well as the charge recombination of cell devices.16 Being the first step of photovoltaic conversion process, light harvesting is the foundation of the development of high performance solar cells.1718 To make a full use of solar light, ideally, QDs should bear an appropriate band edge for a wide light harvesting range and an efficient electron injection.1920 Meanwhile, a sufficient loading of QDs is highly desired to approach a complete absorption of the photons with the energy above the band gap of QDs.2122 The direct growth of QDs on mesoporous TiO2 films by chemical bath deposition or successive ion layer and adsorption reaction (SILAR) provides a well-established method for achieving a high QD coverage on TiO2.2328 However, the direct growth commonly leads to uncontrollable optoelectronic features of QDs and a large number of defects in QDs, leaving PCE values mostly below 7%.2328 By the fact that the main drawbacks of the direct growth could be effectively overcome by the adoption of pre-synthesized QDs, a post-synthesis strategy involving surface ligand-assisted assembly was developed to deposit pre-synthesized QD sensitizers on TiO2 electrodes.9, 1112, 21, 29 Accordingly, a PCE beyond 11% was achieved using Zn-Cu-In-Se (ZCISe) QD sensitizers.30 It is noteworthy that the loading of QDs by the postsynthesis assembly currently is lower than that by direct growth. An even better PCE is supposed to be obtained by increasing pre-synthesized QD loadings. Recently, secondary deposition was attempted to improve the loading of pre-synthesized


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QDs in QDSCs.15 CdSe QDs as the secondary sensitizers were introduced into ZCISe QD presensitized electrodes, achieving a PCE up to 12.7% under standard AM 1.5G illumination.15 Unfortunately, this approach at present can only be applied to the loading of two types of QDs, during which one of them possessing a relatively stronger interaction with TiO2 is introduced onto the TiO2 pre-sensitized by another QDs. For the construction of high performance QDSCs, there is an urgent need to develop a general post-synthesis approach so as to improve the loading of pre-synthesized QDs, either the same or different. Herein, we present a simple but general method for the improvement of QD loading in QDSCs by surface engineering. With the aid of the treatment of QD presensitized TiO2 films by ammonium cationic surfactants, additional ZCISe, Zn-Cu-In-S (ZCIS) or CdSe QDs were successfully introduced onto ZCISe presensitized electrodes by secondary deposition (Scheme 1). Owing to improved sunlight harvesting, all the resultant QDSCs possessing a higher QD loading show the significantly increased short-circuit current density (Jsc) and PCE. Among various QDSCs prepared with Cu2S/brass counter electrodes (CEs), a certified PCE of 10.26% under simulated AM 1.5G test conditions was achieved based on the secondary deposition of ZCISe QDs.

Scheme 1. Schematic illustration of secondary deposition strategy for fabricating quantum dot sensitized photoanodes by surface ligand-assisted assembly.



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RESULTS AND DISCUSSIONS A variety of oil-soluble QDs, including ZCISe, ZCIS and CdSe QDs, were synthesized in organic media following the procedure described elsewhere.3032 The TEM images and absorption spectra of the QDs obtained were illustrated in Figure S1 in the Supporting Information (SI). The ZCISe QDs with an average diameter of about 4 nm have an absorption spectrum with an onset at about 980 nm and a shoulder at about 800 nm. The absorption onset of ZCIS QDs with the size distribution centered at 4.4 nm is 820 nm, and a shoulder around 680 nm also appears on its absorption spectrum. By contrast, a sharp excitonic absorption peak at 600 nm is observed on the spectrum of CdSe QDs with the particle size of about 4.2 nm. Surface ligand-assisted assembly has been recognized as a feasible method for tethering pre-synthesized QDs on TiO2 films.21, 29 Surface ligands, like typical 3-mercaptopropionic acid (MPA), possess two functional groups, thiol and carboxyl. The former coordinates preferentially with QDs and the latter has a strong coordination capability with TiO2, providing the driving force to anchor QDs onto the TiO2 surface.15, 33 It is generally acknowledged that colloidal QDs dispersed in polar solvents are stabilized electrostatically by the repulsive interaction between charged species adsorbed on QDs.34 Similarly, QDs coated by MPA should be sparsely adsorbed on TiO2 films because of electrostatic repulsion.3536 By the neutralization of the original electrostatic field of QD presensitized TiO2 films, there is a good chance to introduce additional QDs onto the bare surface of TiO2 during secondary deposition. To tether QDs onto the TiO2 surface by ligand-assisted assembly, the oleylamine (OAm) capped QDs prepared were first converted to MPA-modified QDs by ligand exchange.15, 2931 As illustrated in Scheme 1, an aqueous dispersion of MPA-modified ZCISe QDs was then dropped onto the mesoporous TiO2 films loaded on FTO glass substrates, anchoring ZCISe QDs on TiO2 films. The ZCISe pre-sensitized TiO2 electrodes were subsequently immersed in the

solution

of

various

ammonium

salt

cationic

surfactants,

such

as

hexadecyltrimethylammonium chloride (HTAC), cetyltrimethylammonium bromide (CTAB) or dodecyltrimethylammonium bromide (DTAB), followed by a secondary deposition of MPAmodified ZCISe, ZCIS or CdSe QDs. After the final coating of ZnS by the SILAR method, the ZCISe/AX/ZCISe, ZCISe/AX/ZCIS or ZCISe/AX/CdSe photoanodes were obtained and AX


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means the surfactant adopted. For comparison, the direct secondary deposition without surfactant treatment was also applied. The resultant photoanodes are denoted as ZCISe/ZCISe, ZCISe/ZCIS or ZCISe/CdSe photoanodes. To evaluate the effectiveness of the secondary deposition, ZCISe QD sensitized TiO2 photoanodes based on single deposition, denoted as ZCISe photoanodes, were prepared as a reference. Inductively coupled plasma-optical emission spectrometry (ICP-OES) was used to identify the influence of surfactant treatment on secondary deposition, with the data listed in Tables S1 to S3. There is no obvious change in In element content between ZCISe/ZCISe obtained in direct secondary deposition and ZCISe photoanodes obtained in single deposition. After the surfactant treatment before secondary deposition, a significant increase in In element content could be found for ZCISe/AX/ZCISe photoanodes. Similar phenomenon occurs in the secondary deposition of ZCIS QDs on ZCISe pre-sensitized electrodes (Table S2). Because of the differential coordination mode of MPA between ZCISe and CdSe QDs, an obvious increase in Cd content appears in the direct secondary deposition of CdSe QDs on the ZCISe presensitized electrodes.15 Nevertheless, an even higher Cd content could be detected in ZCISe/AX/CdSe photoanodes (Table S3), confirming the effectiveness of surfactant treatment in the secondary deposition. It can thus be concluded that the treatment of QD presensitized electrodes by ammonium surfactants promotes the QD loading on TiO2 films during the secondary deposition regardless of the nature of QDs involved. By the consideration that HTAC, CTAB and DTAB play a similar role in QD secondary loading, HTAC was selected as a representative cationic surfactant to optimize photoanode performance in the following work.

Figure 1. SEM cross-sectional image (a) and elemental mapping images (b-g) of ZCISe/HTAC/ZCISe photoanode. The increased QD loading mentioned above can also be confirmed by the TEM images of



the representative ZCISe/HTAC/ZCISe photoanode as shown in Figure S2. Compared with the reference ZCISe photoanode, a higher coverage of QDs on TiO2 could be observed on the ZCISe/HTAC/ZCISe photoanode. The diameter of QDs is about 4 nm, while that of TiO2 varies from 20 to 40 nm. Also, the elemental mapping images illustrated in Figure 1 show an even distribution of Ti, Zn, Cu, In and Se elements. The uniform dispersion of ZCISe within the TiO2 films in the ZCISe/HTAC/ZCISe photoanodes indicates that no aggregation of QDs occurs after the surfactant treatment and the subsequent secondary deposition. To study the mechanism of surfactant treatment in secondary deposition, zeta potential measurement was performed and the corresponding data are shown in Figure 2. By the negative zeta potential of 51.0 mV observed, the ZCISe QDs modified by MPA should carry negative charge on their surface. It has been proven that QDs can support approximately a few tens of charges per particle.37 After the first deposition of a certain amount of ZCISe QDs on TiO2 surface with an original positive zeta potential of 26.4 mV, the surface of TiO2/ZCISe obtained becomes negatively charged, suppressing the further adsorption of QDs because of electrostatic repulsion. The high adsorption energy of cation surfactant HTAC on TiO2/ZCISe facilitates the introduction of HTAC carrying positive charge onto the surface of TiO2/ZCISe, which was confirmed by the sign variation of the zeta potential of TiO2/ZCISe/HTAC. In the secondary deposition, the negatively charged ZCISe QDs are therefore deposited continuously on the positively charged TiO2/ZCISe/HTAC surface, by which a higher loading of ZCISe QDs could be obtained.

Figure 2. Zeta potential of substances involved in secondary deposition with surfactant treatment.


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By increasing QD loading, the performance of QDSCs is supposed to be improved. Therefore, the photoanodes prepared were adopted to assemble sandwich-type QDSC devices with Cu2S/brass CEs and polysulfide electrolyte. Fifty parallel cells based on each photoanode were constructed, and their J-V curves and relevant parameters are supplied in Figures S3 to S11. The corresponding data of the champion cell of each photoanode are shown in Figure 3 and Table 1. The reference cells based on ZCISe photoanodes have an average PCE of 9.35% with Jsc of 25.44 mA·cm–2, Voc of 0.638 V and FF of 0.576. Basically, there is no essential difference in photovoltaic performance between ZCISe/ZCISe photoanode and ZCISe photoanode based QDSCs, which could be ascribed to their nearly equivalent loading of QDs. By contrast, a significantly improved PCE up to 10.42%, derived mainly from the increase in Jsc to 27.54 mA·cm–2, could be obtained on ZCISe/HTAC/ZCISe photoanodes. By Figure 3b and 3c and Table 1, a similar improvement in Jsc and PCE can also be observed on ZCISe/HTAC/ZCIS or ZCISe/HTAC/CdSe electrodes. According to the ICP-OES data mentioned before, it is not surprising that the direct secondary deposition of CdSe QDs on the ZCISe presensitized electrode boosts the value of Jsc and PCE. Even so, a much better PCE up to 10.36% along with the Jsc of 27.16 mA·cm–2 could be obtained on ZCISe/HTAC/CdSe electrodes. Therefore, it can be deduced that the increased loading of QDs resulting from surfactant treatment could enhance the PCE of QDSCs. The PCE of the QDSCs based on titanium-mesh-supported mesoporous carbon CEs (MC/Ti CEs) was reported to be higher than that of the devices based on Cu2S/brass CEs. By this consideration, MC/Ti CEs were also adopted to fabricate ZCISe/HTAC/ZCISe based QDSC devices, with the corresponding photovoltaic data shown in Figure 3, Figure S12, Table 1 and Table S4. Notably, ZCISe/HTAC/ZCISe based devices with MC/Ti CEs show a champion PCE as high as13.50% (Jsc = 27.71 mA·cm−2, Voc = 0.745 V and FF = 0.654). The significantly improved PCE over the devices with Cu2S/brass CEs results mainly from the increase in Voc and FF, consistent with the results reported in the literatures.38 The threedimensional interconnected mesoporous framework of MC/Ti CEs was suggested to be responsible for the electrocatalytic activity for polysulfide reduction and excellent conductivity, resulting in an increase in FF. Besides, it was reported that MC/Ti CEs could downshift the



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redox potential of polysulfide, thus efficiently improving Voc.

Figure 3. Champion J-V curves of QDSC devices under standard AM 1.5G illumination: a) ZCISe, ZCISe/ZCISe and ZCISe/HTAC/ZCISe photoanodes based devices with Cu2S/brass CEs, and ZCISe/HTAC/ZCISe based devices with MC/Ti CEs; b) ZCISe, ZCISe/ZCIS, ZCISe/HTAC/ZCIS and ZCIS based devices with Cu2S/brass CEs; c) ZCISe, ZCISe/CdSe, ZCISe/HTAC/CdSe and CdSe based devices with Cu2S/brass CEs. Table 1. Photovoltaic parameters of various QDSCs. Cells

Voc (V)

Jsc (mA·cm2)

FF

PCE (%)

ZCISea)

0.638(0.646)

25.44(25.60)

0.576(0.575)

9.35±0.10(9.50)

ZCISe/ZCISea)

0.640(0.647)

25.82(25.61)

0.577(0.584)

9.53±0.10(9.67)

ZCISe/HTAC/ZCISea)

0.634(0.636)

27.54(27.65)

0.597(0.602)

10.42±0.10(10.59)

ZCISe/HTAC/ZCISeb)

0.747(0.745)

27.54(27.71)

0.648(0.654)

13.34±0.11(13.50)

ZCISa)

0.633(0.631)

22.72(22.87)

0.613(0.625)

8.82±0.11(9.02)

ZCISe/ZCISa)

0.641(0.638)

26.31(26.66)

0.582(0.591)

9.82±0.13(10.06)

ZCISe/HTAC/ZCISa)

0.634(0.637)

26.62(26.60)

0.597(0.606)

10.08±0.12(10.27)

CdSea)

0.627(0.630)

14.86(15.09)

0.675(0.681)

6.29±0.10 (6.48)

ZCISe/CdSea)

0.640(0.640)

26.56(26.59)

0.587(0.597)

9.98±0.11(10.16)

ZCISe/HTAC/CdSea)

0.632(0.634)

27.11(27.16)

0.592(0.602)

10.14±0.14(10.36)

a)Based

on fifty parallel QDSCs with Cu2S/brass CEs; b) based on five parallel QDSCs with MC/Ti CEs.

The improvement in Jsc can be reflected by the incident photon-to-electron conversion efficiency (IPCE) curves illustrated in Figure 4 and the integral currents of IPCE curves listed in Table S5. The ZCISe, ZCISe/ZCISe and ZCISe/HTAC/ZCISe based cells all have a similar photocurrent onset at about 1100 nm. The ZCISe/ZCISe based cells show the nearly equivalent absorbance over the whole spectrum range. Obviously, the IPCE values of the ZCISe/HTAC/ZCISe based cell are higher than those of the ZCISe based one in the wavelength


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ranging from 350 to 1000 nm. Similarly, the ZCISe/HTAC/ZCIS based cells show higher IPCE values than both ZCISe and ZCISe/ZCIS based cells from 350 to 820 nm. Because of the introduction of additional CdSe, higher IPCE values from 350 to 630 nm can already be obtained on the ZCISe/CdSe based cell when compared with the ZCISe based cell. Still, the IPCE values of ZCISe/HTAC/CdSe are slightly higher than those of the ZCISe/CdSe based cell in the same spectrum range. The variation of the integral current in each case is consistent with that of the measured currents in photovoltaic performance, respectively.

Figure 4. IPCE curves of various QDSC devices: a) ZCISe, ZCISe/ZCISe and ZCISe/HTAC/ZCISe ZCISe/HTAC/ZCIS

photoanode and

ZCIS

based

QDSC

devices;

b)

ZCISe,

ZCISe/ZCIS,

based

QDSC

devices;

c)

ZCISe,

ZCISe/CdSe,

ZCISe/HTAC/CdSe and CdSe based QDSC devices. The light-harvesting capability of QD sensitizers depending on their absorption wavelength range and loading in the TiO2 film has a vital role in IPCE.3941 As can be seen from in Figure 5a, the absorption profiles of ZCISe, ZCISe/ZCISe, and ZCISe/HTAC/ZCISe based films vary similarly, indicating that no particle aggregation between QDs occurs during secondary deposition. Over the full range of absorption wavelength, the absorbance of both ZCISe/ZCISe and ZCISe/HTAC based films is nearly the same as that of the ZCISe based film. However, the absorbance of the ZCISe/HTAC/ZCISe based film is significantly higher than that of ZCISe or ZCISe/ZCISe based one. Similarly, the higher absorbance can also be observed for the ZCISe/HTAC/ZCIS based film compared with the ZCISe based film (Figure 5b). For the CdSe system (Figure 5c), the distinct exitonic peak of CdSe QDs could be observed in the absorption profiles of both ZCISe/CdSe and ZCISe/HTAC/CdSe based films. Among ZCISe, ZCISe/CdSe and ZCISe/HTAC/CdSe based films, the ZCISe/HTAC/CdSe based film exhibits the highest absorbance especially below 700 nm. The improved light harvesting efficiency of



photoanodes at increased loading of QD sensitizers is favorable for the Jsc boost.

Figure 5. UV-vis absorption spectra: a) ZCISe, ZCISe/ZCISe, ZCISe/HTAC/ZCISe and ZCISe/HTAC based films; b) ZCISe, ZCISe/ZCIS, ZCISe/HTAC/ZCIS and ZCIS based films; c) ZCISe, ZCISe/CdSe, ZCISe/HTAC/CdSe and CdSe based films with an active area of 1.6 cm2. Apart from light-harvesting efficiency, the IPCE is also related to electron-injection and charge-collection efficiencies.39,

4243

Electrochemical impedance spectroscopy (EIS) was

adopted to examine the dynamic information on charge collection and recombination of the secondary deposition of various QDs, with the results illustrated in Figure 6 and Figures S13 to S16.4447 For simplicity, the following discussion focuses on the secondary deposition of ZCISe QDs. The values of chemical capacitance (Cμ) of the ZCISe/HTAC/ZCISe photoanode are close to those of the reference ZCISe photoanode over the full range of forward biases applied, demonstrating that HTAC treatment basically has no influence on the conduction band of TiO2 in the resultant photoanodes. In Figure 6b and Table S6, the recombination resistance (Rrec) of the ZCISe/HTAC/ZCISe based device was slightly lower than that of the reference ZCISe based cell over the full range of the forward biases applied, leading to a relatively shorter electron lifetime (τn = Rrec × Cμ). Consequently, the charge recombination at the TiO2/QD/electrolyte interface of the ZCISe/HTAC/ZCISe based device increases in small quantities compared with that of the reference device. Such a result is also reflected by the shorter τn of the ZCISe/HTAC/ZCISe based cell at the same voltages obtained from open-circuit voltage decay (OCVD) characterization (Figure 6d and Figure S17). The slightly higher charge recombination of the ZCISe/HTAC/ZCISe based cell might be attributed to the presence of HTAC as a charge recombination center during charge transfer since a reduction of photoluminescence intensity within a certain range was observed upon mixing of HTAC and


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ZCISe QD dispersion (Figure S18).

Figure 6. EIS data of reference ZCISe and ZCISe/HTAC/ZCISe based QDSC devices: a) capacitance; b) recombination resistance; c) Nyquist plots under −0.60 V forward bias; d) electron lifetime plots derived from OCVD characterization. To comprehensively examine the photovoltaic performance of the QDSC devices fabricated, the stability of ZCISe/HTAC/ZCISe based devices with Cu2S/brass CEs or MC/Ti CEs was evaluated under continuous irradiation of one simulated sunlight of 100 mWcm2 at room temperature. The results are shown in Figure S19. The Cu2S/brass CE based QDSC device maintains its initial efficiency in about 30 min with the nearly unchanged Jsc, Voc and FF, retaining 85% of the initial efficiency to 45 min. Without a proper seal to prevent the leakage of polysulfide electrolyte through Ti meshes, currently the efficiency of the MC/Ti CE based device could only be measured immediately after the construction of the device. In summary, the pre-sensitized TiO2 electrodes were prepared by depositing MPAmodified ZCISe QDs on TiO2 films, followed by the treatment of ammonium salt cationic surfactants. With the subsequent secondary deposition, additional MPA-modified ZCISe, ZCIS or CdSe QDs were introduced into the ZCISe pre-sensitized electrodes. On the basis of zeta



potential characterization, the successful secondary deposition of QDs could be ascribed to the neutralization of the original electrostatic field of QD presensitized TiO2 films by surfactant treatment. By the general strategy mentioned above, that is, surfactant treatment and secondary deposition, a high loading of the same or different pre-synthesized QDs on mesoporous TiO2 electrodes was readily achieved. Despite the slightly increasing charge recombination, the significantly improved Jsc and PCE indicate that the enhancement of sunlight utilization resulting from a higher QD loading has a positive decisive role in QDSC performance. Accordingly, a certified PCE of 10.26% under simulated AM 1.5G illumination was achieved on the Cu2S/brass CE based QDSC device by the secondary deposition of ZCISe QDs. Besides, a high initial PCE of 13.50% was obtained on the MC/Ti CE based QDSC, although the stability of the QDSCs at present is far from satisfactory. EXPERIMENTAL SECTION Chemicals. 1-octadecene (ODE) of 90 wt% and diphenylphosphine (DPP) of 98 wt% were obtained from J&K. Oleic acid (OA) of 90 wt%, sulfur (S) of 99.99 wt%, copper iodide (CuI) of 99.998 wt% and 3-mercaptopropionic acid (MPA) of 99 wt% were received from Alfa Aesar. Hexadecyltrimethylammonium chloride (HTAC), cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB) and poly(vinylpyrrolidone) (PVP) of 8000 molecular weights were received from Aladdin China. Oleylamine (OAm) of 80-90 wt% were purchased from Acros Organics. Indium acetate (In(OAc)3) of 99.99 wt% and selenium powder (Se) of 100 mesh and 99.99 wt% were received from Aldrich. Zinc acetate dehydrate (Zn(OAc)2·2H2O) and sodium sulfide nonahydrate (Na2S·9H2O) were obtained from Sinopharm Chemical Reagent Co., Ltd. Trioctylphosphine (TOP) of 97 wt% and cadmium oxide (CdO) of 99.99 wt% were purchased from Strem Chemicals. Water-soluble Zn-Cu-In-Se QD synthesis. Oil-soluble OAm-capped ZCISe QDs were first prepared according to conventional hot-injection method.30,48 Typically, 0.1 M Zn precursor was obtained by the dissolution of Zn(OAc)2·2H2O into the mixture of ODE and OAm with a volume ratio of 9:1. The bright yellow Se-DPP solution was obtained by the dissolution of 42.5 mg Se powder into the mixture of OAm and DPP (VOAm:VDPP=1:1) under ultrasonication. 59.5 mg In(OAc)3, 28.0 mg CuI and 0.8 mL as-prepared Zn precursor were added into a flask


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containing 4 mL OAm and 3 mL ODE. When the mixture was heated to 180 oC under inert atmosphere, the Se precursor prepared was rapidly injected into the mixture. After a 5 min reaction at 180 oC, oil-soluble OAm-capped ZCISe QDs were received. Following the ligand exchange method reported, MPA-modified ZCISe QDs which are water-soluble were finally obtained and stored for further use. Water-soluble Zn-Cu-In-S QD synthesis. The method for synthesizing MPA-modified ZnCu-In-S (ZCIS) QDs is similar to that for water-soluble ZCISe QDs.31 First, 0.1 M S precursor was prepared by dissolving S powder into an OAm solution. A mixture of 59.5 mg In(OAc)3, 28.0 mg CuI, 0.8 mL Zn precursor, 4.0 mL OAm and 1.0 mL ODE were introduced into a flask. When the mixture was heated to 160 °C under inert atmosphere, a S stock solution was rapidly injected into the mixture. After a 5 min reaction, the ZCIS QDs capped by OAm were received. To obtain MPA-modified ZCIS QDs, the phase transfer procedure similar to that for watersoluble ZCIS QDs was then conducted. Finally, MPA-modified ZCIS QDs were obtained and stored for further use. Water-soluble CdSe QD synthesis. The OAm-capped CdSe QDs with the first excitonic absorption peak at about 600 nm were prepared following a hot-injection approach.32 Typically, to obtain 0.4 M Cd precursor, 256 mg CdO powder was dissolved into the mixture of 2.5 mL OA and 2.5 mL ODE at 280 oC under inert atmosphere. 17.5 mg Se powder, 0.5 mL TOP and 4.5 mL OAm were added into a flask and heated to 250 oC under inert atmosphere. Subsequently, 0.5 mL Cd precursor was injected into the solution above. The reaction system was maintained for about 4 min to yield CdSe QDs. The following purification and ligand exchange procedures for MPA-modified CdSe were the same as the process for ZCISe QDs. Photoanode Fabrication and solar cell construction. Mesoporous TiO2 film electrodes with an active area of 0.235 cm2 (Figure S20) were fabricated with successive multilayer screen printing method.30, 4950 The TiO2 film electrode obtained contains a transparent layer of 20 μm in thickness and a light scattering layer of 10 μm in thickness. 45 μL MPA-modified ZCISe QDs were first dropped onto the TiO2 film prepared, which was kept at 50 oC for 2 h. The excess of ZCISe QDs was removed from the surface of TiO2 film by washing successively with water and ethanol, followed by drying with air flow to give ZCISe QD alone sensitized TiO2



film electrodes. Then, ZCISe QD sensitized electrodes were treated by ammonium salt by immersing the ZCISe alone sensitized electrodes into the 2.75 mM ammonium salt methanol solution (such as HTAC, CTAB or DTAB) for 8 min. After rinsing with ethanol, the electrodes treated by ammonium salt were dried with air flow. Finally, 45 μL MPA-modified ZCISe, ZCIS or CdSe QD dispersion was subsequently dropped onto the surface of ammonium salt treated electrodes above again and kept for 10 min. The electrodes obtained were washed successively with water and ethanol, followed by drying with air flow. After coating with the ZnS blocking layer by SILAR method, the ZCISe/ZCISe, ZCISe/ZCIS or ZCISe/CdSe-sensitized electrodes were obtained. The sandwich-structure QDSCs were fabricated by combining the complete photoanode and Cu2S/brass or MC/Ti CEs using a binder clip. The Cu2S/brass CEs were obtained following previous literatures.15, 30 MC/Ti CEs with a carbon film area of 0.322 cm2 (Figure S20) were fabricated through successive multilayer screen printing method.30, 38 The modified polysulfide electrolyte containing 2.0 M S, 2.0 M Na2S, 0.2 M KCl and 20 wt% PVP was injected into the gap between photoanode and CEs, by which the integrated solar cells were obtained.52 Characterization. The UV-vis absorption spectrum was obtained on a spectrophotometer (UV2600, Shimadzu). The photoluminescence spectrum was collected on a fluorescence spectrophotometer (EI FLS 980). The TEM image was photographed on a microscope (JEM2100, JEOL). The ICP-OES was obtained from Agilent 725ES with the samples for ICP-OES prepared by dissolving the sensitized photoanode in aqua regia. During the first and secondary deposition of QDs on TiO2 films, TiO2 particles with or without QD loading were scraped off FTO glass and dispersed in deionized water. The obtained dispersions of 1.0 mg·mL1 then were subjected to zeta-potential measurement on a Zetasizer NanoZS Instrument (Malvern Instruments, UK). For ZCISe QDs or HTAC, their dispersions or aqueous solutions at the same concentration were employed directly. To test the PCE of solar cells, the J-V curves of devices were obtained on a Keithley 2400 sourcemeter under simulated AM 1.5G illumination by a solar simulator (model 94022A, Oriel). The light intensity of 100 mW·cm2 applied was calibrated by a NREL standard Si solar cell. For the ZCISe/HTAC/ZCISe based devices, the J-V curves were also measured by sweeping the applied voltage from short circuit to open


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circuit or vice versa with different delay times (Figure S21). As shown in Figure S20, the photoactive area determined by a metal mask is 0.2008 cm2 during measurement. The IPCE curve was tested on a Keithley 2000 multimeter under the irradiation of a 300 W tungsten lamp with a DK240 monochromator. The characterization of EIS was conducted on an electrochemical workstation (Zennium, Zahner). The EIS curve recorded under dark conditions was achieved at the forward bias from 0.35 to 0.60 V with the potential amplitude of 20 mV over the frequency ranging from 1 MHz to 0.1 Hz. To test the OCVD of devices, the sample was exposed constantly to a 100 mW·cm2 AM 1.5 G irradiation and the attenuation was examined at extending time after the light was turned off.



ASSOCIATED CONTENT Supporting Information: Additional TEM images, absorption spectra, ICP-OES data, J-V curves and photovoltaic parameters of various QDSCs, EIS data, OCVD analyses, and so forth. The material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Y. Li) ORCID Yan Li: 0000-0003-3031-6252 Author Contributions #

W. Wang and L. Zhao contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research is supported by National Natural Science Foundation of China (21421004 and 21771063), the State Key Research Development Program of China (2016YFA0204200), Shanghai Municipal Science and Technology Major Project (Grant No.2018SHZDZX03) and the Programme of Introducing Talents of Discipline to Universities (B16017). Meanwhile, we thank Dr. Donghui Long and Dr. Mingqi Chen for providing mesoporous carbon materials.

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For Table of Contents Only



Scheme 1.


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Figure 1.



Figure 2.


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Figure 3.



Figure 4.


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Figure 5.



Figure 6.


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A Facile Secondary Deposition for Improving Quantum Dot Loading in

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