Ternary CuZnS Nanocrystals: Synthesis, Characterization, and


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Ternary CuZnS Nanocrystals: Synthesis, Characterization, and Interfacial Application in Perovskite Solar Cells Jiangsheng Li,† Chaoyang Kuang,† Min Zhao,† Chengjie Zhao,† Le Liu,† Fushen Lu,*,‡ Ning Wang,† Changshui Huang,*,† Chenghao Duan,‡ Hongmei Jian,† Lili Yao,† and Tonggang Jiu*,† †

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Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, Shandong 266101, P. R. China ‡ Department of Chemistry, College of Science, Shantou University, Shantou 515063, P. R. China S Supporting Information *

ABSTRACT: Ternary CuZnS nanocrystals (NCs) are synthesized via a facile, scalable, noninjection method at low temperatures for the first time, wherein sodium ascorbate plays the dual roles of reducing agent and capping ligand in the preparation process. These NCs can be dispersed well in a polar solvent like dimethyl sulfoxide, and the average size is ∼4 nm as measured by transmission electron microscopy. The results of X-ray diffraction and X-ray photoelectron spectroscopy indicate that the crystal structure of CuZnS NCs displays covellite CuS-like structure and the Zn element partly occupies the Cu position. Also, the crystal structure of CuZnS NCs is completely converted from a covellite CuS structure into a digenite Cu9S5 structure when the NCs are treated above 350 °C. Moreover, CuZnS NCs demonstrate favorable hole transport properties. When it is employed in MAPbI3-based perovskite solar cells as a hole transport layer, a peak power conversion efficiency of 18.3% is achieved. Simultaneously, the devices based on CuZnS exhibit a remarkably reduced J− V hysteresis. The results indicate that CuZnS is a promising hole transport layer for enhancing perovskite solar cell performance and presents great potential for optoelectronic applications, as well.



INTRODUCTION Nanometer-scale semiconductor materials have attracted intense attention because of their extremely small size and large surface-to-volume ratio, which have led to differences between their chemical and physical properties and those of the bulk with the same chemical composition in the past several decades.1−5 To date, photovoltaic devices based on ternary inorganic nanocrystals (NCs) such as CuInSe2, CuInS2, and AgInS2 have raised considerable concerns attributed to their high absorption coefficients and adjustable band gaps.6−9 CuZnS, a good alternative for large-scale commercial applications, is derived from CuInS by replacing In(III) with Zn(II) to overcome the problems of using a relatively rare element and the high cost.10,11 In addition, CuZnS NCs are regarded as an alloy material with a “combined structure” of CuxS and ZnS, which falls into the category of excellent p-type inorganic materials.11,13 A large transmission in the visible range, rectification properties, and certain photovoltaic effects, as well as nontoxic features, are detected in this kind of material.14 The optical and electrical properties of CuZnS NCs can be adjusted by altering the Cu/Zn ratios in element compositions.11 The composition ratio change and function control rely on the synthesis of an inorganic nanocrystal with high quality and process parameters for easy fabrication, which are essential for fabricating optical and electrical devices.12,15,16 © XXXX American Chemical Society

Advances in the synthesis of ternary semiconductor nanocrystals have been demonstrated.17−19 Most preparation methods are confined to the hot injection method and relative derivatives that are forced to confront the severe reaction condition that includes a high-temperature, oxygen-free, and anhydrous environment,20 which makes it unfavorable for application in industrial-scale production. Other methods such as pulsed laser deposition, electrochemical deposition, chemical spray pyrolysis, and photochemical deposition are reported as common techniques for the preparation of thin films, which are relatively expensive and require complicated operation and accurate and reliable control.21−25 Currently, organic−inorganic perovskite solar cells have emerged at the forefront of new energy research. The hole transport materials for perovskite solar cells are particularly limited to several organic hole conductors such as spiroOMeTAD, PEDOT:PSS, and PTAA.26−31 Despite their promising properties, they are still not regarded as ideal hole-conducting materials to further improve the performance of perovskite solar cells. For example, spiro-OMeTAD needs LiTFSI to improve its conductivity. It is very difficult to control the oxidation of spiro-OMeTAD, which is important Received: April 15, 2018

A

DOI: 10.1021/acs.inorgchem.8b01030 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Schematic illustrations of the formation of CuZnS NCs and photograph of the NC solution.

while its contents were being stirred. The solution was kept at 160 °C for 3 h. During this process, the CuZnS NCs grew as the sublimed sulfur solution was dropped into the metal source solution. The nanocrystals were simultaneously attacked by threonic acid ion, which can prevent nanocrystals from aggregating. The acetate group from the starting materials was also found to be the surface capping ligand as proven by 1 H nuclear magnetic resonance measurement. According to the results described above, we proposed the possible growth mechanism illustrated in Figure 1. The formation of CuZnS NCs could be understood as a kinetically controlled reduction−nucleation−crystallization mechanism. First, metal ions reacted with sodium ascorbate, forming metal−ascorbate complexes. During the reaction, ascorbate reacted with Cu(II), turning into a metal−dehydroasorbic complex and Cu(II) partly turning into Cu(I). At the same time, dehydroasorbic ion was hydrolyzed and the lactone ring subsequently opened, thus yielding the metal−threonic complex. The process is reflected in the variation of the reaction color, which changed from wathet blue to yellow and then dark brown and finally stabilized as black. When the sublimed sulfur solution was added, the reaction materials in the solution organized themselves to form the nanocrystal spontaneously. When the crystal growth followed the process of the reaction, the tiny crystal nucleus grew to nanocrystals of different sizes. Compared with small nanocrystals, the large ones have a lower surface free energy. On the basis of Ostwald ripening,38 the large nanocrystals may grow at the cost of the small ones. At the same time, the surface ligands, including threonic acid and the excessive ascorbate group adsorbing on the surface of nanocrystals, can efficiently prevent nanocrystals from aggregating. The obtained CuZnS nanocrystals can be readily dispersed in polar solvent H2O, DMSO, etc., because of the many hydroxy groups from threonic acid and sodium ascorbate. Our method of using the small ligand sodium ascorbate in a low-temperature solution is different from the hot injection method, in which the NCs are capped with longchain ligands such as oleylamine, trioctylphosphine oxide, or oleic acid. They act as insulators and have to be removed during an additional ligand-exchange processing step before being employed in photovoltaic devices. Moreover, the conditions of low temperatures and the solution process occurring in an atmosphere can be scaled up compared to the conventional hot injection method that requires a hightemperature, severe oxygen-free, and anhydrous environment. Transmission electron microscopy (TEM) images of the CuZnS NCs are provided in Figure 2a. The average diameter of CuZnS NCs is 4.1 nm, with a wide size distribution of 20% shown in Figure S1. Figure 2b presents a high-resolution TEM (HRTEM) image of CuZnS nanocrystals and the lattice fringes with a d spacing of 2.05 Å corresponding to (008) planes of the hexagonal covellite structure. The inset is a selected-area electron diffraction (SAED) image of a few nanocrystals that

for obtaining a high hole transport efficiency, not to mention the hysteresis and high cost of those organic hole transport materials.32,33 In comparison with those organic materials, inorganic hole transport materials usually have the advantages of high chemical stability, hole mobility, and low cost. However, only a few inorganic materials such as MoO3, CuI, CuSCN, and NiOx have been reported to function as hole transport materials in perovskite solar cells.25,33−37 The selection of inorganic hole transport materials requires suitable band alignment and processing compatibility with perovskite films. So far, the resulting power conversion efficiencies (PCEs) based on the inorganic hole transport layer in perovskite solar cells are lower than those based on the organic hole transport layer.37 Therefore, the development of new inorganic hole transport materials is urgently needed for further enhancement of the performance of perovskite solar cells. Herein, a new method for preparing the ternary CuZnS nanocrystals was introduced in which sodium ascorbate played dual roles of both the capping agent and the reducing agent to control the size of the ternary nanocrystal and perform the reaction at low temperatures. Compared with the conventional hot injection method, neither inert gas protection nor anhydrous solvents are needed. The results of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicated that the crystal structure of as-prepared CuZnS NCs displayed a covellite CuS-like structure and the Zn element partly occupied the Cu position. Also, the crystal structure of CuZnS NCs converted to a Cu9S5-like structure completely when thermally treated above 350 °C. Moreover, the film of CuZnS NCs can be used as a hole transport layer in MAPbI3-based perovskite solar cells with the peak PCE reaching 18.3% and showed almost no hysteresis. It implied that CuZnS has the potential to be used as a highly effective and hysteresis-free hole transport layer in photovoltaic devices.



RESULTS AND DISCUSSION First, Cu(OAc)2·H2O, Zn(OAc)2·2H2O, and sodium ascorbate were dissolved in dimethyl sulfoxide (DMSO). The solution was heated at 160 °C for 10 min. During this process, sodium ascorbate, serving as a weak reducing agent and the capping agent of the precursor, can be oxidized easily and converted to a lactone ring at high temperatures. The lactone ring will continue to hydrolyze partly and open, turning to threonic acid ion. Before the sublimed sulfur solution was dropped into the metal source solution, the color of the reaction solution changed from wathet blue to yellow, and then dark brown, and finally stabilized as black over the rest of the reaction time. The black color of the solution indicated the beginning of formation of CuZnS NCs with DMSO working as the solvent and sulfur source in this process. Cu(II) was partly reduced to Cu(I) by sodium ascorbate during the heating process. Then sublimed sulfur was dropped into a three-neck flask over 5 min B

DOI: 10.1021/acs.inorgchem.8b01030 Inorg. Chem. XXXX, XXX, XXX−XXX

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determined by high-resolution spectra in Figure 3. The narrow doublet peaks in the Cu 2p spectrum appear at 932.3 eV (2p3/

Figure 3. XPS spectra of CuZnS nanocrystals, calibrated by referencing the C 1s peak at 284.8 eV: (a) full scan and (b) Cu 2p, (c) Zn 2p, and (d) S 2p core levels.

Figure 2. (a) Transmission electron microscopy image of CuZnS nanocrystals. (b) High-resolution transmission electron microscopy image of CuZnS nanocrystals and an inset showing the SAED pattern. (c) XRD pattern of the as-synthesized CuZnS nanocrystals. (d) Raman spectra of as-synthesized CuZnS nanocrystals.

2) and 952.1 eV (2p1/2). The peak separation of 19.8 eV suggests that the Cu(I) exists in the CuZnS NCs. Weak bands at 944.7 and 962.6 eV can be assigned to the satellite peaks of Cu(II), an indication of Cu(II) in the structure of CuZnS NCs. Therefore, we propose that the Cu atoms in CuZnS contain Cu(I) and Cu(II). The peaks of Zn 2p appear at binding energies of 1022.2 and 1045.3 eV, which correspond to Zn(II) with a peak splitting of 23.1 eV. The binding energies of 161.7 and 163.0 eV with a doublet separation of 1.3 eV can be assigned to the sulfur spectrum.42,43 The peak at 166.5 eV may come from the oxidation of the remaining sublimed sulfur. To further understand the effect of thermal treatments on the ternary crystal structure, we studied X-ray diffraction of the CuZnS film after it had been annealed at different temperatures for 30 min in an inert environment. The XRD spectra of the CuZnS NC film are shown in Figure 4. As analyzed above, the unannealed sample is indexed to the hexagonal covellite CuS-like crystal structure (JCPDS Card 06-0464). It is observed that no change in the crystal structure happens when the sample is annealed at 150 °C compared with that of the unannealed one. While annealing the sample at 200 °C, we

also present the crystallinity of the obtained products. Furthermore, the Cu:Zn:S stoichiometric ratio was close to 7:3:7 as determined by HRTEM−EDX analysis. The Cu:Zn ratio was consistent with the ICP-OES analysis in which the ratio is 7:3. XRD was employed to analyze the composition of the nanocrystals and ensure the structural homogeneity of the samples. As shown in Figure 2c, a resultant XRD diffractogram shows major reflections at 2θ values corresponding to the (102), (103), (006), (110), (108), and (116) planes, which can be indexed to the hexagonal covellite CuS-type crystal structure with the following lattice constants: a = b = 3.792 Å, and c = 16.34 Å (JCPDS Card 06-0464). The sharp diffraction peaks suggest that the obtained nanocrystals were quite crystalline. Also, no other crystalline compounds, such as Cu2S and ZnS, were observed. Moreover, Raman spectroscopy was utilized to obtain further insight into phase identification, and the results are shown in Figure 2d. There are two Raman peaks at 267 and 469 cm−1. The peak at 267 cm−1 shifts slightly compared with that at 260 cm−1, which corresponds to A1gTO of ZnS.39 It can be indexed to the A1gTO of CuZnS NCs, and the shift is affected by the existence of the copper atom of CuZnS NCs. Also, the peak at 469 cm−1 can be assigned to S−S stretching in CuZnS NCs with a small shift compared with a previous report of the Raman spectrum of Cu2S (472 cm−1) and CuS (474 cm−1).40,41 Hence, the studies mentioned above confirm the structure of CuZnS NCs, and the zinc atoms partly replace the copper atoms in the hexagonal covellite CuS crystal structure. XPS was performed to investigate the chemical composition of the CuZnS NCs. A survey spectrum of the synthesized nanocrystals identified the presence of Cu, Zn, S, O, and C, which were calibrated by referencing the C 1s peak at 284.8 eV, and the oxidation states of the constituent elements were

Figure 4. XRD patterns of as-synthesized CuZnS nanocrystals annealed at different temperatures for 30 min in an inert environment. For reference, the XRD patterns of hexagonal covellite CuS (JCPDS Card 06-0464) and digenite Cu9S5 (JCPDS Card 47-1748) are shown, as well. C

DOI: 10.1021/acs.inorgchem.8b01030 Inorg. Chem. XXXX, XXX, XXX−XXX

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of Figure 5b. The spectrum shows that the CuZnS NCs absorb light in the region of 300−500 nm within an absorption shoulder at a short wavelength of 340 nm. The spectrum reaches a minimum around 674 nm then rises over 1000 nm for longer wavelengths because of intraband absorption. Such data indicate that the characteristics of covellite-like CuZnS NCs, which originate from the band transitions from valence states to the unoccupied states,47 are similar to those in previous reports of CuS. The direct band gap value of CuZnS NCs is evaluated according to the equation (αhν)2 = D2(hν − Eg) (where α is the adsorption coefficient, D is a constant, hν is the discrete photoenergy, and Eg is the band gap energy). The right part of Figure 6 shows the plot of (αhν)2 versus hν in

observe a weak peak reaching 2θ = 46.2°, which is assigned to the Cu9S5-type crystal structure indicating the change in the crystal structure. With an increasing annealing temperature, the intensity and width the planes of digenite Cu9S5 (JCPDS Card 47-1748) can be noted and become stronger, suggesting the formation of a digenite Cu9S5-like crystalline phase in CuZnS NCs at temperatures ranging from 200 to 250 °C. At the same time, the main peaks at 29.3°, 32.9°, 47.9°, 52.7°, and 59.4° from (102), (103), (110), (108), and (116) planes of the hexagonal covellite CuS structure, respectively, became weak. Finally, the main diffraction peaks of the CuS-like crystalline phase disappeared when the temperature further increased to 350 °C, indicating that the crystal structure of CuZnS NCs converted to the Cu9S5-like crystalline phase completely. The annealing process can remove long-chain ligands for the fabrication of devices. The removal of long-chain ligands can be beneficial for the transportation of charge, which can improve the performance of PSCs. Surface investigation of CuZnS NCs was performed by FTIR, and the results are shown in Figure 5. It reveals the

Figure 5. (a) FTIR spectra of CuZnS nanocrystals. (b) Ultraviolet− visible absorption spectra of the obtained CuZnS nanocrystals (left). The right panel shows (αhν)2 vs hν for the CuZnS nanocrystals. The band gap is estimated to be 2.24 eV.

Figure 6. (a) Device architecture. (b) Corresponding EQE spectrum of the devices with or without CZS. (c and d) J−V curves obtained from forward bias to short circuit and from short circuit to forward bias of devices without or with CZS.

presence of various bands at 614, 668, 1111, 1367, 1462, 1539, 1618, 1694, 2356, 2904, and 3444 cm−1. The band around 3444 cm−1 can be attributed to O−H stretching vibrations of the ascorbate and threonate group. The signal at 2904 cm−1 showed C−H stretching of the methoxyl group and methylene group, typical for threonic acid ion. Peaks observed at 2356 and 1618 cm−1 are due to CC stretching vibrations in the ascorbate ligands.44 The shoulders at 1694 cm−1 originated from conjugated carbonyl stretches in the ascorbate group. In addition, the band associated with the intraring C−C stretching has been reported to be between 1450 and 1365 cm−1, indicating the ring opening reaction partly at high temperatures. In the spectrum, the bands originating from this stretching can be found at either 1462 or 1367 cm−1. The band at 1111 cm−1 predominantly arises from C−O stretching in the secondary alcohol group of threonic acid and ascorbate ligands that attached to the CuZnS NCs surface.45,46 Weak additional bands at 668 and 614 cm−1 are assigned to the metal−S bond of CuZnS NCs. The presence of ascorbic acid and threonic acid at the CuZnS NC surface indicates that they act as stabilizers to protect NCs from aggregation. This also explains the especially good solubility and stability of the NC dispersion in polar solvents. Ultraviolet−visible (UV−vis) absorption measurement is one of the most important approaches for discovering the optical properties of semiconductor nanocrystals and has been used to study as-synthesized CuZnS NCs. The UV−vis spectrum of the CuZnS NC film is shown in the left panel

which Eg can be determined to be 2.24 eV.48 The value is between that of CuS (Eg = 2.0 eV) and ZnS (Eg = 3.9 eV).49 It indicates the atomic interaction in the crystal structure of the ternary CuZnS NCs. The resulting CuZnS layers were fabricated into solar cell devices to determine their potential application. The structure of photovoltaic devices is shown in Figure 6a. The CuZnS layer was spin-coated on the P3CT-K layer that is on top of ITO/glass, followed by the spinning ∼350 nm thick MAPbI3 perovskite, PC61BM, and ZnO layer. Thermal evaporation of 100 nm thick aluminum through a shadow mask was then performed, which also served as a charge collection grid. The details of these procedures can be found in the experimental section. Corresponding external quantum efficiency (EQE) spectra of the device with or without CuZnS are displayed in Figure 6b. From the EQE curves, the integrated photocurrents of 19.9 and 21.7 mA/cm2 were obtained for the P3CT-K-based and P3CT-K/CuZnS-based devices, respectively. The J−V curves of the devices based on P3CT-K or P3CT-K/CZS with different scanning directions were recorded, as shown in panels c and d of Figure 6. The PCE values obtained from the forward bias scan [forward bias to short circuit (FB to SC)] and the reverse bias scan [short circuit to forward bias (SC to FB)] of the device with CuZnS were 18.3 and 17.6%, respectively. Those of the devices without CuZnS were 16.5 and 14.9%, respectively. The difference in PCE values between forward D

DOI: 10.1021/acs.inorgchem.8b01030 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry bias scans and the reverse bias scans in devices with CuZnS is smaller than that of devices without CuZnS. It is proposed that the insertion of CuZnS can strengthen the contact between P3CT-K and the active layer, resulting in an easier hole transfer pathway and reduced charge accumulations.50 Therefore, the J−V hysteresis was improved significantly. Notably, we exhibited the details of devices with or without CuZnS in Figure S2. As one can see, the average Jsc and PCE of P3CT-K/CuZnS-based devices were higher than those of control devices. Moreover, a smaller standard deviation of device parameters for the P3CT-K/CuZnS-based devices compared with that of the control devices indicated a higher degree of reproducibility of the devices with CuZnS. As a result, CuZnS presents effective properties of hole collection and transport, which can be considered as a promising material for application in environmentally friendly solar cells at a low cost because of its nontoxicity, facile preparation method, and ambient condition stability. The lifetime measurement was performed and is exhibited in Figure S3. All devices were stored and measured in the nitrogen-filled glovebox without any encapsulation. As displayed in the figure, the PCE of devices based on CuZnS remained 89% after storage for 72 h, which was much better than those based on pristine P3CT-K (69% remaining), which indicated that inserting CuZnS is an effective way to improve the stability of devices.

Changshui Huang: 0000-0001-5169-0855 Tonggang Jiu: 0000-0001-9608-4429 Author Contributions

J.L. and C.K. contributed equally to this work. The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the Major Basic Research Program of the Shandong Natural Science Foundation (ZR2017ZB0313) and the Natural Science Foundation of China (51672288). This study was also supported by the DICP QIBEBT (UN201705), Dalian National Laboratory For Clean Energy and Scientific Research Staring Foundation of Qingdao Institute of Bioenergy, Youth Innovation Promotion Association of Chinese Academy of Sciences and Bioprocess Technology, Chinese Academy of Sciences.





CONCLUSIONS In conclusion, we report a facile solution method for obtaining ternary CuZnS NCs at low temperatures in which sodium ascorbate acts as the reducing agent and the capping ligand. The structural characterizations confirmed the morphology, structure, and composition of the as-obtained nanocrystals. The CuZnS nanocrystals can be dispersed well in a polar solvent and had an average particle size of 4.1 nm, which presents possible applications in several fields. The XRD and XPS results indicate that CuZnS NCs display covellite CuStype crystal structure. Inside, Cu was composed of Cu(I) and Cu(II). Also, when treated above 350 °C, the covellite CuS structure of CuZnS NCs converted to the digenite Cu9S5 structure completely. Interestingly, as-prepared CuZnS NCs can be used as a hole transport layer in MAPbI3-based perovskite solar cells. A PCE of 18.3% was obtained, and the device exhibited almost no hysteresis. All the results implied that ternary material CuZnS nanocrystals may facilitate the hole transport and have great potential to be applied in highefficiency and low-cost photoelectronic devices.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01030. Experimental details and Figures S1−S3 (PDF)



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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ORCID

Fushen Lu: 0000-0002-3323-7181 E

DOI: 10.1021/acs.inorgchem.8b01030 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b01030 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Perovskite Solar Cells Based on Dual Doping Graphdiyne. Nano Energy 2018, 46, 331−337.

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DOI: 10.1021/acs.inorgchem.8b01030 Inorg. Chem. XXXX, XXX, XXX−XXX