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Highly Efficient Zn-Cu-In-Se Quantum Dot Sensitized Solar Cells through Surface Capping with Ascorbic Acid Hua Zhang, Wenjuan Fang, Wenran Wang, Nisheng Qian, and Xiaohe Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18033 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Highly Efficient Zn-Cu-In-Se Quantum Dot Sensitized Solar Cells through Surface Capping with Ascorbic Acid Hua Zhang,* Wenjuan Fang, Wenran Wang, Nisheng Qian, and Xiaohe Ji Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT The balance between band structure, composition and defect is essential for improving the optoelectronic properties of ternary and quaternary quantum dots and the corresponding photovoltaic performance. In this work, ascorbic acid (AA) as capping ligand is introduced into the reaction system to prepare green Zn-Cu-In-Se (ZCISe) quantum dots. Results show that the addition of AA can increase the Zn content while decrease In content, resulting in the enlarged band gap, high conduction band energy level and suppressed charge recombination. When AA/Cu ratio is 1, the quantum dots possess the largest band gap of 1.49 eV and the assembled quantum dot sensitized solar cells exhibit superior photovoltaic performance with ~ 17% increment mainly contributed by the dramatically increased current density. The new record efficiency of 10.44% and 13.85% are obtained from the ZCISe cells assembled with brass and titanium mesh based counter electrodes respectively. KEYWORDS: ascorbic acid, capping ligand, quaternary semiconductor, high efficiency, quantum dot sensitized solar cells

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INTRODUCTION Due to the increasing exhaustion of fossil fuels and global warming, developing affordable and renewable clean energy devices, especially for solar cells, has undoubtedly been considered as one of the most promising method.1-3 Quantum dot sensitized solar cells (QDSCs), the next generation photovoltaic devices, have gained considerable attention due to their merits of tunable band gap (Eg), high absorption coefficient, easy fabrication, and multiple exciton generation.4-7 In the past decade, the development of liquid junction QDSCs has mainly been contributed by designing and adopting of new types of quantum dot (QD) sensitizers, tailoring and band-engineering of QDs, suppressing charge recombination, and improving QD loading amount. The accumulated efforts have received an extremely appealing high short-circuit current density (Jsc) and open-circuit voltage (Voc), accordingly leading to the highest power conversion efficiency (PCE) of 12.75%.4,8-13 Nevertheless, the gained PCEs of the optimized QDSCs are still far lower than those of other emerging solar cells such as monolithic perovskite/silicon tandem solar cells with 25.2% PCE and organic solar cells with 14.03% PCE.14,15 As a Cd- and Pb-free “green” sensitizer for potentially commercial application in QDSCs, CuInS2 and CuInSe2 (CISe) are important ternary I-III-VI QDs possessing high absorption coefficient and narrow Eg.6,16-18 Especially, CISe QDs have attracted more and more interests for photovoltaic utilization, biological labeling and photodetectors, owing to the relatively narrower Eg (1.05 vs. 1.52 eV) and bio-compatibility.13,19-21 However, the plain CISe QDs have high density of surface


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and intrinsic defects, leading to much serious charge recombination, and reducing the photovoltaic performance accordingly.22-24 In addition, because there is small gap between the conduction band edge of CISe QDs and that of TiO2, the injection rate of photogenerated electrons from CISe to TiO2 is poor and thus bringing forward low photocurrent and low photovoltage in the resultant QDSCs. Therefore, how to improve the quality and make balance between wide absorption and high injection rate of CISe QDs has still being challenging. Structure and composition control, change of capping ligand, cosensitizing and overcoating wide Eg passivation layer (such as ZnS, ZnTe) are effective for excellent optical properties and satisfactory photovoltaic performance.24-27 For instance, Lenysak et al. have presented a sequential partial cation exchange method to prepare CISe QDs. The subsequent overcoating ZnS shell and careful tuning experimental parameters enabled the absorption extending to a wide NIR range from 990 to 1210 nm and QDs more stable.28 Zhong’s group has adopted DPP-Se as highly reactive anion precursor to fabricate oleylamine capped Zn-Cu-In-Se (ZCISe) QDs with absorption onset extending to the near infrared region. The constructed QDSCs showed extremely high PCE over 11%.10 Afterwards, Wang et al. developed a ZCISe and CdSe co-sensitized photoanode and therefore boosted the QDSC efficiency to over 12%.4 The results have demonstrated a great potential for further improving the application and performance of green QDs based QDSCs. It has been reported that the increase of Zn or decrease of Cu content to a certain extent in ZCISe QDs would be favorable for large Eg and high quality which are associated with high photovoltaic performance.10,24 There is no doubt that



exploring the alloying strategy and balancing between band structure, composition and defect are still required for further promoting the properties of ZCISe QDs as light harvesters and the resulting device performance. Meanwhile, the influence of In content should be discussed in depth. Colloidal QDs were commonly synthesized via hot injection during which surface ligands played a critical role in the nature and property of QDs including particle size, morphology, solubility, surface chemical state, optical property, band structure, and so on.29-33 As an example, ligands of carboxylic acid and organic amines have been used by Pan’s group to fabricate colloidal CsPbBr3 nanoparticles.30 The findings confirmed that organic amines could effectively tune the anisotropic nanoplates growth, while carboxylic acid could module the size of isotropic nanocubes. The results have clarified the influence of ligand chemistry on crystal growth and stabilization of nanocrystals. Ascorbic acid (AA) is a typical enediol-structured organic acid with aromatic structure and multiple hydroxyl. Previous work have investigated the role of AA in the synthesis of QDs.34-37 Wang and co-workers have synthesized well-defined copper nanocubes by using AA as a reducing agent via a simple one-pot solution-phase method.38 Zhao’s group has found that AA served as an efficient and nontoxic electron donor for scavenging photogenerated holes of CdS QDs under mild solution medium.39 In all these works, however, AA did not act as ligands. Actually, similar to the coordination of QD with nitrogen in amine, the oxygen atom in AA would coordinate with QDs, which possibly affecting the nature of QDs, including structure, 4 ACS Paragon Plus Environment

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composition, morphology, defects and property. In the view of its significant structural features, additional and novel employment of AA and its elegant integration into QDs is sure to be interesting. Thus, if AA was introduced into the reaction system and capped with QDs, AA modified ZCISe QDs would be well developed and three beneficial effects could be anticipated: (1) tuning the composition, surface chemical state and band structure of ZCISe QDs; (2) balancing the wide absorption (related with narrow Eg) and the higher conduction band edge (related with large Eg) of ZCISe sensitizers; (3) reducing the defects of QDs, and thus inhibiting charge recombination process. In this work, AA aqueous solution was added during preparation of ZCISe via hot injection, resulting in the oleylamine (OAm) and AA co-modified ZCISe QDs (in brief AA-ZCISe). The experimental results manifest that the amount of AA could subtly control and balance the composition and band structure of ZCISe QDs without changing the molar ratio of cation and anion precursors. The QDs obtained at the AA/Cu molar ratio of 1 showed the largest Eg of 1.49 eV and the highest conduction band edge with high quality. The modifying with AA can observably improve the loading amount of sensitizers and the electron lifetime of the assembled QDSCs, accordingly improving Jsc and suppressing the charge recombination at the photoanode/electrolyte and QD/TiO2 interfaces. The assembled AA-ZCISe QDSCs showed the extremely high and record efficiency of 13.85% with Jsc of 26.49 mA cm-2, Voc of 0.770 V, and FF of 0.679. The photovoltaic performance of AA modified AA-ZCISe QDSC is dramatically higher than that of AA-absent ZCISe QDSC with ~ 5 ACS Paragon Plus Environment


17% improvement. EXPERIMENTAL SECTION Materials. Cuprous iodide (CuI, 99.998%) and 3-mercaptopropionic acid (MPA, 99%) were obtained from Alfa Aesar. Indium acetate (In(OAc)3, 99.99%), selenium powder (200 mesh, 99.99%), and 1-octadecene (ODE, 90%) were gained from Aldrich. Zinc acetate dihydrate (Zn(OAc)2·2H2O, AR) were received from Sinopharm. Diphenylphosphine (DPP, 99%) and oleylamine (OAm, 97%) were attained from J&K. Ascorbic acid (AA, 99%) was purchased from Aladdin. No further processing was carried out to all chemicals. Synthesis of ZCISe QDs. Before fabricating Zn-Cu-In-Se (ZCISe) QDs, some precursor solutions were first prepared as follows. 0.1 M Zn stock solution: 0.22 g of Zn(OAc)2·2H2O was dissolved into a mixture containing 1 mL of OAm and 9 mL of ODE; DPP-Se precursor solution: 0.3 mmol of selenium powder was dissolved into a mixture containing 0.3 mL of DPP and 0.5 mL of OAm via ultrasonic irradiation; AA precursor solution: different amounts of AA depending on the AA/Cu molar ratio from 0.5 to 2 were put into 0.5 ml of deionized water to form a clear and transparent solution. AA modified ZCISe QDs were synthesized according to a modified literature method reported by Du and Zhang.10,24 First, Cu (0.07 mmol), In (0.1 mmol) and Zn (0.04 mmol) precursors with the molar ratio fixed at 0.7:1:0.4 were separately transferred to a 50 mL three-necked round bottom flask containing 2.0 mL of OAm followed by introducing the freshly prepared AA precursor solution. The air was


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driven away from the flask and high purity nitrogen was introduced into the system at 110 oC. After the reaction mixture got clear at 180 oC, fresh DPP-Se stock solution were injected and stayed for 5 min before the removal of heat source. When the temperature decreased to about 90 oC, 15 mL of petroleum ether was poured into the mixture followed by decantating and centrifugating with the addition of ethanol and acetone. The as-prepared oil soluble QDs are termed as AA-ZCISe, precisely 0.5-ZCISe or 1-ZCISe et al., depending on the AA/Cu molar ratio from 0.5 to 2. For preparing common ZCISe QDs with no AA modification, the experimental conditions and procedures were similar to that for AA-ZCISe QDs with the absence of AA precursor solution. The samples are termed as regular 0-ZCISe. For improving loading amount on TiO2 mesoporous film, ligand exchange procedure using MPA as capping ligand was carried out for all ZCISe QDs according to our previous work.40 Construction of Solar Cells. TiO2 mesoporous film with thickness of 25 ± 1.5 μm (20 μm transparent layer and 5 μm light scattering layer) were prepared through screen printing method. First, 20 μL of 0.02 M MgCl2 aqueous solution was pipetted onto TiO2 film with an area of 0.235 cm2 at room temperature followed by putting them into 50 oC oven for 3 h. Second, QDs aqueous solution were pipetted onto Mg2+ decorated TiO2 mesoporous film and stayed for 1.5 h till the loading was saturated. And finally, ZnS passivation layers were overcoated on the QD-sensitized photoanodes via alternately dipping into 0.1 M Zn(OAc)2 methanol solution and 0.1 M Na2S aqueous solution with five cycles. The solar cells were constructed through assembling a counter electrode and a QD-sensitized photoanode with a long tail clip 7 ACS Paragon Plus Environment


and syringing into polysulfide/sulfide aqueous electrolyte (including 2.0 M Na2S, 2.0 M S, and 0.2 M KCl). The detailed synthesis of counter electrodes and characterization could be found in Supporting Information. RESULTS AND DISCUSSION Effects of AA on Optical Property and Band Structure of ZCISe QDs. How the amount of AA concisely controls the absorption and emission of ZCISe QD sensitizers was first studied and the absorption curves are shown in Figure 1a. For simplicity, the QDs prepared at different ratio of AA/Cu from 0 to 2 are termed as 0-ZCISe, 0.5-ZCISe, 1-ZCISe and so on. It can be seen that the absorption from all samples have similar shape, in which a wide peak and long tail can be resolved. This indicates the formation of alloyed structure instead of core/shell structure in the ZCISe QDs in accordance with that in previous work.10,24 When AA was absent, the absorption onset of the prepared 0-ZCISe is located ~ 975 nm. When AA was introduced with AA/Cu ratio from 0.5 to 2, all the samples show blue-shifted absorption onsets compared with that of 0-ZCISe. Most dramatically, the sample of 1-ZCISe shows the largest blue shift to ~ 930 nm with 45 nm decrements. The corresponding Eg of various QDs were estimated from extrapolation the linear portion of the plot of (ahv)2 vs hv with results shown in Figure 1b, in which the x-axis intercept of an extrapolated line from the linear regime of the curve represented the Eg value. It is clear that when AA/Cu was increased from 0 to 1, 1-ZCISe shows the largest Eg of ~ 1.49 eV, 70 meV larger than that of 0-ZCISe (~ 1.42 eV). When AA/Cu ratio was further increased from 1 to 2, the Eg gradually and slightly decreases 8 ACS Paragon Plus Environment

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with ~ 20 meV each time, but still larger than that of 0-ZCISe. It should be noted that when AA/Cu increased from 0 to 0.5, 0.5-ZCISe shows the largest increase of ~ 50 meV, indicating the critical role of AA in improving Eg. It is well known that composition and/or size generally influence the absorption and Eg of ternary and quaternary QDs. To investigate the mechanism, chemical composition of various ZCISe QDs were measured by inductively coupled plasma - atomic emission spectrometry (ICP-AES) with the results shown in Table 1 and the change tendency shown in Figure S1 (Supporting Information). It can be seen that when AA/Cu ratio was increased from 0 to 1, Zn content dramatically increased from 0.074 (7.4% of Se content) to 0.084 (8.4%), inversely, the In content dramatically decreased from 48.5% to 47.8%. For Cu content, the small increase of 0.1% could be ignored. When the ratio was 1.5, the small change in Zn content of ~ 0.1% could be ignored, suggesting the saturation of Zn. But the In content dramatically decreased and Cu increased. The total metal/Se ratio slightly changed, in which the maximum of 84.4% could be observed at AA/Cu ratio of 1. Thus, it can be concluded that the blue shift of absorption onset and increase in Eg at AA/Cu of 1 are dominantly attributed to the increased Zn and the relative red shift at AA/Cu larger than 1 to the increased Cu. It is also consistent with the well-known non-linear effect of composition on band energy of alloyed QDs and previous reports.41 It should be noted that although there is more Cu content in ZCISe at AA/Cu of 1.5, the Eg is still larger than that of AA-free QDs. It might be from the synergistic effects of increased Zn and decreased In contents in a certain range with the addition of AA, also suggesting the role of In in determining



the band structure of ZCISe QDs.

Figure 1. (a) UV-vis absorption of ZCISe QDs prepared at different AA/Cu molar ratios and (b) estimation of the optical Eg from extrapolation the linear portion of the plot of (ahv)2 vs hv. UPS spectra of (c) 0-ZCISe and (d) 1-ZCISe QDs, and (e) the schematic diagrams of the corresponding energy level distribution. (f) PL spectra of 0-ZCISe and 1-ZCISe QDs with excitation at 468 nm. The PL lifetime decay spectra and the corresponding fitting curves of 0-ZCISe (g) and 1-ZCISe (h) QDs.


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Table 1. The normalized chemical compositions in molar ratio of various ZCISe QDs determined by ICP-AES. AA/Cu

Zn

Cu

In

Se

Total M/Se

0

0.074

0.282

0.485

1

0.841

1

0.084

0.283

0.478

1

0.844

1.5

0.085

0.311

0.447

1

0.842

Ultraviolet photoelectron spectroscopy (UPS) characterization was used to reveal the conduction band (CB) and valence band (VB) edge positions of typical QDs of 0-ZCISe and 1-ZCISe with results shown in Figure 1c and 1d, respectively. The ionization potentials (equivalent to VB energy level) of 1-ZCISe and 0-ZCISe QDs were calculated to be - 4.91 eV and - 4.89 eV respectively by subtracting the excitation energy (21.22 eV) from the width of UPS spectra. The CB energy level is thus estimated to be at - 3.42 eV and - 3.47 eV. Figure 1e shows the schematic diagrams of the corresponding energy level distribution. It is well known that bulk ZnSe has CB energy level centered at ~ - 3.40 eV which is higher than that of CISe. Due to quantum size effect, the CB energy level of nano-scaled ZnSe will further upshift. When CISe is doped with Zn, the band gap will get larger and the CB energy level will upshift. Thus it is easy to understand that the higher CB energy level at 3.42 eV and larger band gap for 1-ZCISe than that of 0-ZCISe is strongly dependent on the larger amount of Zn content due to the addition of AA. It has also been confirmed by the ICP, UV and UPS results. Accordingly the higher CB edge position 11 ACS Paragon Plus Environment


in 1-ZCISe is favorable for a quicker and more effective injection of photogenerated electrons from the CB of QDs to that of TiO2 matrix. Figure 1f shows the PL spectra of the prepared 0- and 1-ZCISe QDs. Two peaks can be clearly resolved from two samples in the curves, in which the short-wavelength peaks represent band gap emission and the long-wavelength one for defect-related emission. The deconvolution of PL spectra are shown in Figure S2 to distinguish the two components. Compared with 0-ZCISe QDs, the band gap emission of 1-ZCISe blue shifted from ~ 896 nm to ~ 857 nm and for defect-related emission from ~ 953 nm to ~ 929 nm which is well consistent with UV-vis absorption results and calculated Eg as discussed above. All the results tell us that the addition of AA during preparation has played an important role in controlling the chemical composition of ZCISe with nearly no change in size as discussed below, especially for increasing Zn and decreasing In, consequently leading to the subtle tuning of absorption, emission, Eg, energy level position, and etc. 41 When AA/Cu ratio was 1, 1-ZCISe QDs show the largest Eg, and the resultant upward CB position potentially promotes the driving force for electron injection from QDs into TiO2, consequently improving the electron injection rate and photovoltaic performance. PL lifetime decay is commonly taken to evaluate the quality of QDs, because long decay lifetime can be used as an indicator of better performing materials, further resulting in the high performance QDSCs. PL lifetime of QD is the order of ns to ps where the longer one represent the defect-related emission including surface defects and intrinsic defects. Figure 1g and 1h show the PL lifetime decay of 0-ZCISe and 12 ACS Paragon Plus Environment

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1-ZCISe respectively measured at the long-wavelength peak centered at ~ 923 nm. The curves can be fitted well to a biexponential function and then the average lifetimes (τav) are calculated with corresponding parameters shown in the insets of Figure 1g and 1h, where τ1 and τ2 are the lifetimes and A1 and A2 are the contributions from the two components. The defect-related emission of the long-wavelength peak could be further confirmed by the lifetimes which are so large to be ns degree. In complex quaternary ZCISe QDs, surface defects are dominantly non-radiative recombination, and several intrinsic defect levels with different emission wavelength and lifetime are located near the CB and VB.41 The difference in defect emission wavelength is very small that may not be resolved from PL at room temperature. But the difference in lifetime of τ1 and τ2 corresponding to different defect states could be resolved from the fitting results. The detailed defect states are not be concerned in this work. The values of τ1, τ2 and τav of 1-ZCISe are much longer than those of 0-ZCISe QDs, revealing that 1-ZCISe QDs have relatively higher quality and reduced defects due to the addition of AA. In summary, the addition of AA should effectively tune the composition, absorption, emission and quality of QDs, thus being beneficial to the efficient carrier separation and the suppression of charge recombination.42 The Surface State and Phase of ZCISe QDs. The surface chemical state of the prepared oil soluble ZCISe QDs was studied by IR with results shown in Figure 2a. The peaks at 2922 cm-1, 2853 cm-1, 1462 cm-1 match well with the IR standard spectrum of OAm. They could be observed from both 1-ZCISe and 0-ZCISe QDs, representing that two samples are capped with OAm surface ligands. It should be 13 ACS Paragon Plus Environment


noted that there are some differences in the curve of 1-ZCISe. First, the strong peak at 3440 cm-1 assigned to O-H stretching vibration can be obviously seen. It may come from AA. Second, in IR standard spectrum of AA, the peaks appearing at ~ 1755 and ~ 1670 cm-1 can be assigned to ester group and C=O group in aromatic structure respectively.43,44 But in 1-ZCISe QDs the two peaks could not be seen while a new peak at ~ 1634 cm-1 appears. It might be derived from the complexation of ester group (possibly oxygen atom in carbonyl) with QDs. The results show that AA group exists on the surface of 1-ZCISe QDs, suggesting that AA plays a ligand role to modify the surface of 1-ZCISe QDs together with OAm and the resulting QDs are co-capped by both OAm and AA. It has been confirmed by Sitko and coauthors that the oxygen-containing groups in aqueous graphene oxide can coordinate with metal ions such as Zn, Cu ions.45 Similar to that in graphene oxide, the oxygen-containing groups in AA could also combine with metal ions during ZCISe preparation before the decomposition of residual AA molecular at high temperature. The coordination behavior of AA could be confirmed by the color change of reactants after injection of AA to the mixture containing cation precursors (shown in Figure S3a-3d, Supporting Information). The removal of blue color (possibly coming from the oxidation of Cu+ ions) represents the complextion of Cu ions with AA. And the complextion of Zn and In ions could also be possible due to the characteristic of AA and the resultant composition tuning. It should be noted that the stability of 1-ZCISe QDs is improved as shown in Figure S3e. It shows that after 5 days, oil-soluble 0-ZCISe solution has become opaque, suggesting the slight aggregation, while 1-ZCISe solution was still


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transparent. Furthermore, the ligand-exchanged 0-ZCISe QDs were usually assembled within two days to get high efficiencies. But for ligand-exchanged 1-ZCISe QDs which had been preserved for one week, the assembled QDSCs still showed the similar and high efficiencies. Figure 2b shows the thermal gravimetric analysis (TGA) curves of 0-ZCISe and 1-ZCISe QDs. From the initial stage of area I, we can see that the two samples have the similar weight loss as far as 10% within 220 oC. It is due to the removal of moisture, crystal water and the surface adsorbed organic matter. However, in 220 - 400 oC, the weight loss of 0-ZCISe is faster than that of 1-ZCISe, may be ascribed to the easier removal of amino group than that of hydroxyl group. Finally they all drop to ~ 80% at 400 oC. With the temperature exceeding 400 - 700 oC

(area II), additional weight loss of ~ 5% for 1-ZCISe and ~ 7% for 0-ZCISe are

observed respectively, mainly due to the carbonization of the organics.46 Furthermore, the X-ray diffraction (XRD) patterns of 0-ZCISe and 1-ZCISe QDs are shown in Figure S4a (Supporting Information). It can be found that the two samples are almost the same with chalcopyrite structure (JCPDS 35-1349), indicating that the addition of AA will not change the crystal structure of ZCISe QDs. For further analyzing the ZCISe QDs, valance states of Cu in the system were characterized by X-ray photoelectron spectroscopy (XPS) as shown in Figure S4b and S4c (Supporting Information). It is worthy to note that two binding energy peaks at 931.6 and 951.2 eV appear in the two samples. There are no satellite peaks between them, suggesting the absence of Cu2+ in the samples. Figure S4d and S4e show the C1s peaks. The shift of



C=O related peak from 289.0 to 288.3 eV is possibly the result of complextion of metal with AA.

Figure 2. (a) IR spectra of oil-soluble 0-ZCISe, 1-ZCISe QDs, and the reagent AA respectively; (b) TGA curves of 0-ZCISe and 1-ZCISe QDs. Deposition of QDs on TiO2 Mesoporous Film. Particle size and surface chemical state are the two basic factors for QDs to deposit onto TiO2 mesoporous film, therefore affecting the amount of QD loading and current density. Figure 3a and 3b show the transmission electron microscopy (TEM) images of 0-ZCISe and 1-ZCISe QDs, in which they have close diameters of ~ 3.7 and 3.4 nm respectively. The well resolved lattice fringes with 0.33 nm spacing in the insets can be assigned to (112) crystal facets and also imply the high crystallization. After loading of the as-prepared QDs onto mesoporous TiO2 film (with TEM of TiO2 shown in Figure 3c with particles size of 20 - 40 nm), ZCISe sensitized photoanodes were fabricated with the TEM images shown in Figure 3d and 3e. It can be seen that TiO2 film are all densely and uniformly covered with 1-ZCISe and 0-ZCISe QDs. Figure 3f shows the UV-vis absorption spectra of various ZCISe sensitized films with AA/Cu from 0 to 2. From


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the optical spectra, it can be found that the light-harvesting range of all ZCISe QDs have been extended to the NIR region with wavelength through to ~ 960 nm, and this results are nearly close to that of UV-vis with a slight blue-shift after addition of AA. Moreover, it can be confirmed that when AA was added, the absorption intensity becomes stronger, unveiling the high QDs loading, possibly leading to the higher photocurrent for the assembled solar cells as discussed below.

Figure 3. TEM images of 0-ZCISe (a), 1-ZCISe (b) QDs, blank TiO2 film (c), and the corresponding high resolution TEM in the insets. TEM images of 0-ZCISe (d) and 1-ZCISe (e) sensitized TiO2 film, and (f) UV-vis absorption spectra of the QD-sensitized photoanodes. Photovoltaic Performance of ZCISe QDSCs. To investigate the influence of the subtly controlled structure and composition of ZCISe QDs on the photovoltaic performance, the sensitized TiO2 photoanodes were assembled with common



Cu2S/brass counter electrodes using polysulfide electrolyte as redox couple. Figure 4a-4c shows the J−V curves and corresponding parameter (Table 2) changes of ZCISe prepared with different AA/Cu molar ratios. It can be found that when AA was absent, the 0-ZCISe QDSC has average PCE of 9.30% with Jsc of 25.82 mA cm-2, Voc of 0.603 V and fill factor (FF) of 0.597. When AA was added, all the assembled QDSCs showed higher PCE. When AA/Cu ratio was set at 0.5, the PCE was increased to 9.78% with dramatically increased Jsc to 27.10 mA cm-2. When the ratio was further increased to 1, the highest PCE to date of 10.44% from pure Zn-Cu-In-Se QDSC was obtained and the average PCE also reached to the maximum of 10.36% (Jsc = 27.45 mA cm-2, Voc = 0.614 V, and FF = 0.615) which is ~ 11% higher than that of 0-ZCISe QDSC. When the ratios were larger than 1, the PCE gradually decreased to 9.77% at ratio of 2, but still larger than that of AA-absent 0-ZCISe QDSC. The parameters of Jsc and Voc have the same change tendency as that of PCE, and there was only slight difference in FF of which the change at ratio of 0.5 could be ignored. It should be noted that the increased PCE was mainly attributed to the dramatically increased Jsc which showed ~ 6.3% growth for 1-ZCISe compared with 0-ZCISe. The results can be further verified by incident photon-to-current conversion efficiency (IPCE) as shown in Figure 4d. It can be seen that all samples showed broad photocurrent responses extending to ~ 1000 nm and slightly wider one for AA-ZCISe. In the range of 400 ~ 900 nm, relatively high IPCE values for AA-ZCISe could be obviously seen, and 1-ZCISe reached to the maximum of ~ 80%. Meanwhile, the integrated Jsc according to IPCE spectra were 25.24 mA cm-2 for 0-ZCISe and 27.15 mA cm-2 for


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1-ZCISe respectively, very close to that obtained from J-V measurements. Table 2. Average photovoltaic parameters (brackets for maximum) extracted from J-V results of ZCISe QDs based QDSCs with different AA/Cu molar ratios under the illumination of 1 full sun intensity. AA/Cu

Voc / V

Jsc / mA cm-2

0.603±0.008

25.82±0.09

0.597±0.005

9.30 ± 0.10

(0.613)

(25.85)

(0.597)

(9.38)

0.609±0.005

27.10±0.69

0.590±0.008

9.78 ± 0.10

(0.607)

(27.53)

(0.591)

(9.87)

0.614±0.004

27.45±0.37

0.615±0.009

10.36 ± 0.08

(0.608)

(27.71)

(0.620)

(10.44)

0.614±0.002

27.24±0.41

0.607±0.003

10.16±0.11

(0.613)

(27.53)

(0.608)

(10.27)

0.604±0.006

26.64±0.27

0.608±0.012

9.77±0.05

(0.599)

(26.62)

(0.620)

(9.81)

FF

PCE / %

0

0.5

1

1.5

2

From the above results of various characterization, the dramatically higher Jsc and the superior photovoltaic performance of 1-ZCISe QDSCs in comparison with 0-ZCISe QDSCs are contributed by several factors closely related with the composition and band structure. First, the enlarged Eg and upshifted CB edge favor the photogenerated electrons injection into TiO2. Second, AA-ZCISe QDs show higher quality which could enhance the band gap emission accompanied by the depressed defects-related emission and long PL lifetime decay. Third, AA-ZCISe 19 ACS Paragon Plus Environment


QDs with narrower size distribution and AA surface modification can give rise to more QDs loaded onto TiO2 film, leading to greater light harvesting capacity.

Figure 4. (a) J−V curves of ZCISe with different AA/Cu molar ratios. Average PCE and FF (b), average Voc and Jsc (c) dependent on the quantity of AA in the modified ZCISe QDs. (d) Corresponding IPCE curves. During the last two years, titanium (Ti) mesh based counter electrodes have been developed to dramatically improve the efficiency of QDSCs in which Ti served as substrate and carbon or/and CuxS as catalyst.10 In order to get better photovoltaic performance, we assembled the 0-ZCISe and 1-ZCISe sensitized photoanodes with highly efficient counter electrodes of Ti mesh supported graphene hydrogel (GH-Cu2S/Ti) respectively with the results shown in Figure S5 (Supporting Information). It is found that after AA was added with AA/Cu molar ratio of 1, the


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maximum PCE of 1-ZCISe QDSC significantly increased from 11.83% to 13.85% (of ~ 17% increment) with Voc of 0.770 V, Jsc of 26.49 mA cm-2 and FF of 0.679. To our best knowledge, this is the highest efficiency among all kinds of QDs based solar cells. Open-Circuit Voltage Decay and Electrochemical Impedance Spectroscopy. Besides for various characterization of QD sensitizers for investigating the mechanism of the extremely superior photovoltaic performance from 1-ZCISe QDSCs, open circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS) have also been studied to deeply understand the intrinsic nature of the whole device. Figure 5a shows the OCVD results. Apparently, it could be informed that the Voc decay rate of 1-ZCISe was much slower than that of 0-ZCISe QDSC. Accordingly, the electron lifetime (τn) inside devices has been calculated in the light of equation:  n  (k BT / e )(dV oc / dt )1 , where kB is the Boltzmann constant, T is the absolute temperature, and e is the elementary charge quantity. From the calculated results shown in Figure 5b, the longer electron lifetime of 1-ZCISe QDSC than that of 0-ZCISe can be obviously confirmed. All the results show that AA modified ZCISe QDSC exhibited high electron-injection efficiency, long electron lifetime and suppressed charge recombination. For EIS analysis, Nyquist plots of 0-ZCISe and 1-ZCISe QDSCs were recorded under different forward bias ranging from - 0.3 V to - 0.6 V in dark condition with each curve shown in Figure S6 (Supporting Information), then chemical capacitance (Cµ) and charge transfer resistance (Rrec) were calculated with results shown in Figure 5c and 5d respectively. The Nyquist plots received at the forward bias of - 0.6 V are provided in Figure 5e and parameters in Table 3. From Figure 5c, it can be seen that there was nearly no



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difference between Cµ for the two samples. However, the difference in Rrec could be obviously seen in the range of 0.4 - 0.6 V from Figure 5d. For example, the calculated Rrec at 0.6 V for 1-ZCISe was 259.7 Ω cm2 which is greatly higher than that of 0-ZCISe (190.7 Ω cm2). The larger Rrec values from 1-ZCISe QDSC demonstrate that the charge recombination at the interface between TiO2/electrolyte and QD/electrolyte is more difficult. Furthermore, the electron lifetimes (τn) calculated by the equation

 n  R rec  C  are 2006.18 and 1396.88 ms for 1-ZCISe and 0-ZCISe QDSCs, respectively. The longer lifetime of 1-ZCISe is in good agreement with the results from OCVD.

Table 3. Simulated parameters of series resistance (Rs), charge transfer resistance (Rrec) and capacitance (Cµ) of 0-ZCISe and 1-ZCISe QDSCs under the forward bias of - 0.6 V. AA/Cu ratio

Rs / Ω cm2

Cµ / mF cm-2

Rrec / Ω cm2

τn / ms

0-ZCISe

7.90

7.33

190.70

1396.88

1-ZCISe

6.79

7.73

259.70

2006.18


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Figure 5. (a) OCVD curves of 0-ZCISe and 1-ZCISe QDSCs and (b) the corresponding electron lifetime extracted from OCVD measurements. EIS characterization of 0-ZCISe and 1-ZCISe QDSCs: (c) chemical capacitance Cμ; (d) recombination resistance Rrec; (e) Nyquist plots of cells at the forward bias of - 0.6 V and equivalent circuit.

CONCLUSIONS In conclusion, ascorbic acid (AA) has been successfully introduced into the reaction system to subtly control the composition and band structure of Zn-Cu-In-Se (ZCISe) QDs with properties dependent on the amount of AA. Experimental results show that AA serves as ligand and can improve the quality and effectively increase the Zn content while decrease In content. In comparison with common ZCISe, all AA 23 ACS Paragon Plus Environment


modified QDs possess enlarged Eg and higher conduction band energy level, and the largest Eg of 1.49 eV is obtained at AA/Cu ratio of 1. PL, J-V, OCVD, EIS results demonstrate that due to the enhanced driving force for electron injection, long electron lifetime, and suppressed charge recombination, AA modified ZCISe QDSCs exhibit superior photovoltaic performance with ~ 17% increment mainly contributed by the dramatically increased current density. Upon optimization, the highest conversion efficiency (PCE) of 10.44% with brass based counter electrode and 13.85% with Ti mesh based counter electrode have been recorded respectively for pure Zn-Cu-In-Se based QDSCs. This work is important to find the interesting role of AA and to develop a new route for precisely controlling the intrinsic nature of QDs therefore improving the photovoltaic performance of solar cells. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ Preparation of counter electrodes; characterization of the samples and devices; Figure S1 to Figure S6. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Phone, Fax: 86-64253681


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ORCID Hua Zhang: 0000-0002-4065-8179 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by the Science and Technology Commission of Shanghai Municipality (No. 18ZR1409200).

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