Insight into the Controlled Synthesis of Cu2Zn(Ge,Sn)S4

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Insight Into the Controlled Synthesis of Cu2Zn(Ge,Sn)S4 Nanoparticles with Selective Grain Size Fei Li, Zhiguo Xia, and Quanlin Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05894 • Publication Date (Web): 11 Jul 2016 Downloaded from http://pubs.acs.org on July 14, 2016

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The Journal of Physical Chemistry

Insight Into the Controlled Synthesis of Cu2Zn(Ge,Sn)S4 Nanoparticles with Selective Grain Size Fei Li, Zhiguo Xia*, Quanlin Liu The Beijing Municipal Key Laboratory of New Enrgy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing, 100083, China Supporting Information ABSTRACT: Controlled synthesis of absorber materials Cu2ZnGeS4 (CZGS) has been performed using different Ge precursors, including GeCl4 and the self-synthesized Ge complexes with Ge-Glycolic acid (denoted as Ge-Gly), Ge-Tartaric acid (denoted as GeTar) and Ge-Citric acid (denoted as Ge-Cit). The grain size of as-prepared CZGS nanocrystals (NCs) is dependent on the Ge precursors. All the four Ge precursors enabled the wurtzstannite CZGS phase formation. The Ge-Cit precursor led to the formation of monodispersed NCs owing to the fact that the undissolved metal-Cit complex in OLA absorbed the small CZSG NCs and avoided the irregular crystalline behavior. Other three precursors induced two different sizes and the corresponding reaction mechanism has been proposed. Moreover, the Cu2ZnGe1-xSnxS4 NCs with different Ge/Sn ratios were prepared using the Ge-Cit precursor, verifying the general effect on the phase formation and selective grain sizes. The compositional effect on the band gap variation and morphologies of Cu2ZnGe1-xSnxS4 was also studied.

1. INTRODUCTION The utilization of solar energy is a promising way to meet the urgent need for clean and renewable energy sources1-7. Among numerous types of solar cells8-13, copper-based quaternary chalcogenide semiconductors with the general formula Cu2–II– IV–VI4 (where II = Mn, Fe, Co, Ni, and Zn; IV = Ge, Sn; VI = S, Se) are currently receiving widespread attention due to their excellent light-harvesting properties, earth abundance and low toxicity, which can be expected to replace CdTe and Cu(In,Ga)(SxSe1-x)2 (CIGSSe) for use as absorber materials in thin film solar cell14-19. In previous studies, the theoretical power conversion efficiency (PCE) was predicted to be as high as 32.2% for thin film solar cell20. Until recently, the highest experimentally reached efficiency for Cu2ZnSn(SxSe1-x)4 (CZTSSe) was 12.6%21. In addition, the replacement of tin atoms in the CZTSSe system by germanium is expected to improve the PCE further, and this method provided the performance of 9.4%22. The addition of Ge into the CZTSSe system can not only permit an increase in the band gap of CZTSSe to the ideal value of 1.5 eV, but also restrain the change in oxidation state of Sn during device operation23. Nanocrystal-based route to fabricate semiconductor film in solar cell was put forward and reported in many references24-28. This approach can avoid the requirement for solid-state diffusion and avoid the generation of volatile phases (such as SnS) or kinetically stable impurity phases (such as Cu2SnS3) in the synthesis of Cu2Zn(Ge,Sn)(SxSe1-x)4 (CZGTSSe) 29-30. During the conventional synthesis of Ge-containing CZTSSe nanocrystals (NCs), germanium dichloride-dioxane, GeI4 or GeCl4 were usually employed as Ge precursor14, 22, 31-35. However, the germanium dichloride-dioxane complex and GeI4 are too expensive and the GeCl4 is also air and moisture unstable, which seriously limit their practical usage. Given the shortage of current germanium precursor, it is quite necessary to develop an easy and economical precursor that is robust enough to be used in nanocrystal synthesis. Additionally, the particle size distribution would also have significant impact on solar cell efficiency, especially for the solar cell equipped with bandgap-graded absorbing layer. Hillhouse’s group has reported that the element content was different among the CZTS NCs with different size :

large particles containing primarily Cu and Zn, and small particles of Cu and Sn36. Additionally, the bandgap-grader structure thin film solar cells facilitates both higher short circuit current and open-circuit voltage compared with an ungraded band structured cell37-39, which was consider to be a promising way to improve the PCE of solar cells. However, such a bandgap-grader structure has a strict requirement for precision of composition of absorbing layer40. Therefore, it is necessary to synthesize NCs with uniform size for the precise control of the band gap and compositions of solar cell absorbing layer. Herein, the air stable and inexpensive Ge precursors were prepared by GeO2 and their corresponding organic complex. They were elaborately designed to prepare CZGS NCs with selective grain sizes. Therefore, it can avoid the usage of other ligands. Different Ge precursors enable the same wurtzstannite phase formation but they induce different effects on the size distribution. Thus, the influence of Ge precursors on the phase formation mechanism was comparatively investigated. Moreover, Cu2ZnGe1-xSnxS4 NCs with different Ge/Sn ratios were prepared using the Ge-Cit precursor, and uniform grain size and tunable band gap variations were realized.

2. EXPERIMENTAL DETAILS 2.1. Materials. CuCl (97%), GeO2 (99.99%, 𝛼-quartz type crystal structure), ZnCl2 (98%), Glycolic acid (98%), Citric acid (99.5%), Trisodium citrate dihydrate (99%), Toluene (99%), Chloroform (99%), Ethanol （99.7%） and Tartaric acid (99%) were purchased from Sinopharm, Shanghai, China. Oleylamine (80~90%) and S powder (99.95%) were purchased from Aladdin. Bis(2,4-pentanedionato)tin(IV) dichloride (Sn(acac)Cl2) (98%) was purchased from TCI. GeCl4 was purchased from General Research Institute for Nonferrous Metals, Beijing, China. All reagents and solvents were used as received without any further purification. 2.2. Synthesis of Ge precursors. Preparation of GeGlycolic acid complex (denoted as Ge-Gly) was adapted from a procedure reported by Chiang et al41. The GeO2 powder was added to the deionized water solution of glycolic acid, and then they were heated to reflux for 3h. After that, the as-obtained

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transparent solution was evaporated at 30 ˚C to nearly dry, which yielded colourless square crystals. Finally, the crystals were filtered, washed with water and dried to obtain the Ge-Gly complex for the following use. The synthesis strategy of GeTartaric acid complex (denoted as Ge-Tar) is similar as that for the synthesis of Ge-Gly with the replacement of Glycolic acid with Tartaric acid. For the preparation of Ge-Citric acid complex (denoted as Ge-Cit), GeO2 powder, Citric acid and Trisodium citrate dihydrate were dissolved in deionized water and heated to 70 °C until the transparent solution was obtained. Then, the solution was cooled to room temperature naturally. Ethanol was added into the solution and white precipitate appeared immediately. The white precipitate was filtered, washed with water and dried to obtain the final product. 2.3. Synthesis of CZGS/CZGTS nanocrystals. CZGS NCs were synthesized through a hot-injection method by using different Ge-based precursors. In a typical synthesis, CuCl (0.25 mmol), ZnCl2 (0.125 mmol), GeCl4 or as-prepared Ge-complex (0.125 mmol) and OLA (12 mL) were added in a three-neck flask and evacuated at 120 °C for 30 min. The solution was then heated to 270 °C under the N2 atmosphere. In a separate three-necked flask, a stock solution was made by dissolving S powder (0.5 mmol) into OLA (8 mL) and heating to 150 °C. When the temperature of metal precursor solution reaches 270 °C, the hot stock solution was injected into the flask drop by drop. After injection, the reaction was allowed to proceed for some time with continuous stirring. The reaction was terminated by removal of the heater and the solution was cool to room temperature naturally. The product was washed using chloroform and ethanol by centrifugation two times with the speed at 6000 rpm. The CZGTS nanocrystals with different Ge/Sn compositions were synthesized by varying the Sn(acac)Cl2 amount in the metal precursor solution, and only the Ge-Cit precursor was used. Other experimental details were similar to those as used in the preparation of CZGS NCs. 2.4. Characterizations. X-ray diffraction (XRD) measurements were performed at a PANalytical X’Pert3 Powder diffractometer equipped with a Cu Kα radiation source and operated at 40 kV and 40 mA. Transmission electron microscopy (TEM) was performed on a JEM-2010 operated at 120 keV on 200 mesh carbon coated nickel grids. Dynamic laser scattering (DLS) measurements were performed using a Zetasizer nano ZS to analyze the particle distribution. Energy dispersive spectroscopy (EDS) data were collected using a Hitachi SU8000 Series scanning electron microscope with the accelerating voltage of 20 keV. Infrared data were collected on a spectrum One 7800-350CM FT-IR spectrometer on a laminated diamond mounted in a stainless steel plate in the 3300-1500 cm−1 range with a resolution of 2 cm−1. UV-vis-NIR absorption spectra of colloidal solutions were collected at room temperature using a Varian Cary 5 spectrophotometer.

3. RESULTS AND DISCUSSION As described above, the Glycolic acid, Tartaric acid and Citric acid can react with GeO2 in aqueous media to form air and moisture stable complex for the following CZGS preparation. In such a process, wurtzstannite-type CZGS NCs were prepared in oleylamine solution employed (a) GeCl4; (b) Ge-Gly; (c) Ge-Tar and (d) Ge-Cit as Ge precursors, respectively. Figure 1 gives the XRD patterns of the as-prepared CZGS NCs, which reveal that phase-pure orthorhombic CZGS in wurtzstannite structure were obtained without any secondary phase, and the patterns agree with the standard data of JCPDS No. 26-0572 file (Cu2ZnGeS4). The four different Ge precursors provide the same crystal phase indicating that the wurtzstannite CZGS phase formation is

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insensitive to the Ge precursor selection in the given synthesis procedure.

Figure 1. XRD patterns of as-synthesized CZGS NCs prepared with different Ge precursor: (a) GeCl4; (b) Ge-Gly; (c) Ge-Tar and (d) Ge-Cit, and the JCPDS No. 26-0572 file (Cu2ZnGeS4) was also given as a comparison. To study the morphology of the CZGS NCs as-synthesized via different precursors, TEM images of CZGS NCs employed GeCl4, Ge-Gly, Ge-Tar and Ge-Cit as Ge precursors were obtained, as shown in Figure 2. From the TEM images shown in Figure 2 a, b and c, at least two distinct particle populations can be identified, and the average sizes of big and small particles were measured to be about 15 nm and 5 nm, respectively. We comparatively measured five different regions of the small particles and big particles employed by TEM equipped with a windowless Link EDS analyzer, and the average data were show in Table S1. According to the data, the composition of small particle and big particle was different, which agreed with the result of the Hillhouse’s work. In order to understand whether the particles have the same crystal phase regardless of the different particle size, the high-resolution TEM (HRTEM) images of the particles with selective grain sizes were carried out and shown in the inset. The HRTEM results reveal continuous lattice fringes with an interplanar spacing of 0.32 nm that corresponds to the (100) lattice plane distance for both particle populations, confirming the same crystal phase for both particles sizes. More interestingly, as can be seen in Figure 2 (d), the as-obtained CZGS NCs exhibited uniform particle size when the Ge-Cit precursor was used as Ge source. The size of as synthesized NCs was comparable to that of big NCs formed by employing the other three Ge complex as Ge precursors. However, the TEM images just revealed a small part of products, it is necessary to estimate the size distribution for the total systems. Figure 3 shows the hydrodynamic diameter of the products synthesized with different Ge precursors. The full width at half maximum (FWHM) of nanoparticles synthesized with Ge-Cit was much lower than that of products synthesized with other three Ge precursors, which revealed that Ge-Cit could provide the uniform CZGS NCs. From the XRD patterns shown in Figure 1, it is clear that the Ge precursor have no effect on the CZGS NCs phase composition of but it significantly influences the particle size distribution. In previous report, for application in sintered thin-film, the monodispersity of NCs was considered to be less important than reaction scalability and NC composition tunability23. However, the previous studies in Hillhouse’s group revealed that the element content changed with the particle size variation. Since the bandgap-grader structure thin film solar cell was considered to be a promising way to improve the PCE of solar cells which have a strict requirement for precision of composition of


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absorbing layer, the polydispersity would seriously influence the bandgap-grader thin film solar cell properties. Herein, it is necessary to prepare the uniform size NCs to control precisely the solar cell absorbing layer composition. In this study, the uniform particles were simply obtained by employing Ge-Cit as Ge precursor without addition of other ligand. This would greatly facilitate the procedure and decrease the cost of the CZGS NCs synthesis.

The time evolution XRD patterns were performed to further understand the structural formation process for the reaction products at different reaction time. By visual inspection of the data, special attention should be paid to the three Bragg peaks in the 2 θ region 25˚-32˚. When reaction time was short, the intensity of the peak locked at 28.5˚ was much stronger than its two neighbouring peaks (see Figure 5 a). In a previous study, this result indicates clearly the coexistence of a kesterite type phase and a wurtzstannite type phase42. With the increase of reaction time, the intensity of the peak locked at 27.5˚ become comparable to that of 28.5˚. The change of intensity ratio between 27.5˚ and 28.5˚ indicate that the structure of the NCs can be transformed from the kesterite type to the wurtzstannite type by controlling the reaction time. Also, as can be seen in Figure 5b-d, the ratio variation for the peak intensities at 27.5˚ and 28.5˚for the other three types of products synthesized with different Ge precursors have the same tendency. This indicated that the NCs have the same formation process for different Ge precursors.

Figure 2. TEM images of CZGS NCs as-synthesized with different Ge precursors: (a) GeCl4; (b) Ge-Gly; (c) Ge-Tar; (d) Ge-Cit. Top-down HRTEM image showing the interplanar spacing of nanoparticles with selective grain sizes. Understanding the formation mechanism of different size distribution of the as-prepared CZGS products was important to guide experiments in the future. Therefore, some control experiments were designed and carried out. According to the results of XRD patterns and TEM images, the particle size distribution of NCs was influenced by different Ge precursors. To be precise, the Ge ligands influenced the particle size distribution. Interestingly, the particle size distribution of the as-obtained NCs employed Gly- and Tar2- as ligand was similar to that of employing Cl- rather than Cit3- as ligand. This phenomenon may be caused by the different decomposition behavior of organic ligands. Therefore, the surface chemical structures were preliminarily investigated by using IR spectroscopy. As can be seen in Figure 4b, c and d, when employed Ge-Gly, Ge-Tar, GeCit as Ge precursors, the spectral profiles of the three samples exhibited similar peaks at 2935 cm-1, 2850 cm-1 and 1690 cm-1. Among them, the high wavelength peaks locked at 2935 cm-1 and 2850 cm-1 are attributed to C-H asymmetric stretching and symmetric stretching in the OLA molecule, respectively. The peak locked at 1690 cm-1 should be related with the C=O stretch in the carboxylic acid molecules, and the presence of C=O peak indicated that the organic ligand did not decomposed during reaction. It was found that there was a stronger infrared transmittance at 1690 cm-1 for the samples formed via the Cit3precursor route. This implied that the Cit3- ligand played the key role in the formation of uniform particle size. Moreover, for the as-obtained NCs that employed GeCl4 as precursor, there are just the two high wavelength peaks at 2935 cm-1 and 2850 cm-1 (Figure 4a).

Figure 3. Hydrodynamic diameters of Cu2ZnGeS4 nanocrystals synthesized with different Ge precursors: (a) GeCl4; (b) Ge-Gly; (c) Ge-Tar; (d) Ge-Cit.; the diameters are obtained from dynamic light scattering measurements.



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Figure 4. Infrared transmittance spectra for CZGS NCs synthesized with different Ge precursors: (a) GeCl4; (b) Ge-Gly; (c) Ge-Tar; (d) Ge-Cit.

Figure 5. Time evolution XRD patterns of the CZGS products synthesized with different Ge precursors: (a) GeCl4; (b) Ge-Gly; (c) GeTar; (d) Ge-Cit. As indicated above, the as-obtained four kinds of CZGS NCs can be divided to two groups by the observed size distribution, the sample via the Cit3- precursor route and those employed GeCl4, Ge-Gly and Ge-Tar as precursors. Therefore, the CZGS NCs synthesized via Ge-Tar precursor were selected as representative for the comparison with NCs synthesized with GeCit. Nevertheless, time evolution TEM images of the CZGS products synthesized with different Ge precursor were given in Fig. S1 in the supporting information. Herein, Fig. 6a gives the time evolution TEM images of the CZGS products synthesized with Ge-Tar precursor. It revealed that there was two kinds of grains’ sizes even at a shorter reaction time (5min). The shape of big particles was irregular. With the increase of reaction time, the big particles became regular and exhibited the circular shapes. Since the injection of S takes some time, we could not obtain the product at reaction time below 5 min. Therefore, the observed shape evolution was attributed to the Ostwald ripening stage of NCs. As shown in Fig. 6b, the CZGS NCs synthesized with GeCit possess different evolution rule depending on different reaction time. Indeed, the NCs synthesized using Ge-Cit contain only the big particles from the start to end of the reaction. This result indicated that the Cit3- complex have the important effect on the formation of CZGS NCs before Ostwald ripening. To see the potential reaction mechanism, Figure 6c and d shows the variation of the chemical composition of each product

synthesized with different Ge precursors as a function of reaction time. It should be emphasized that the average composition of the products is measured. By comparing the cation contents, the significant different in the composition change trends is evident for the products synthesized by Ge-Tar and Ge-Cit. From the beginning of reaction, both products were Zn poor, while the products synthesized with Ge-Cit was seriously Ge poor compared with the former Ge rich product. This difference may be derived from the different conditions of metal ions precursors. With the reaction time increase, the ion ration became consistent with the stoichiometric one. During the reaction process, the solution of metal ions is opaque light green due to undissolved copper complex, but upon heating, the precursor dissolves and creates a translucent orange solution, and further slowly changes to dark brown when employed Ge-Tar as Ge precursors. There is obvious precipitation can be seen in the metal ion precursor at 270 ˚C when employed Ge-Cit as precursor. One conceivable explanation is that the precipitation observed prior to injection is due to the formation of metal-Cit complex which is undissolved in OLA. These metal-Cit complex lead to the lower concentration of metal ions in OLA. Additionally, the complex stability constant of Ge-Cit is higher than that of Zn-Cit and this result in the Zn rich Ge poor solution. Therefore, the Ge content was low at the beginning of reaction when employed Ge-Cit, as compared with the NCs synthesized with Ge-Tar. Moreover, the


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productivity of CZGS synthesized with Ge-Cit was about 25%. This productivity was much lower than that of CZGS synthesized by using other three Ge precursors, which have a productivity about 95%. We also studied the precipitate of CZGS synthesized with Ge-Cit, and it was collected during the cleaning of CZGS NCs. The XRD pattern shown in Figure S4 that there was a large amount of nanoscale CZGS existed in the precipitate. This indicated that as-synthesized NCs could be absorbed on the surface of the metal-Cit complex which was undissolved in OLA. Besides, the surface energy increased with the particle size decrease. Therefore, the small particle would be adsorbed

preferentially. Since the large quantity of CZGS was absorbed, the rest of NCs should be of big size, as it was observed. In order to further confirm that the precipitate absorbed small particles, the precipitate was treated by large amount of chloroform to rinse out the small particle. The small particles can be seen (Fig. S4) and proved the hypothesis. As shown in Fig.S1 and Fig. 2, the different size distribution and chemical composition variations are comparatively given for the two precursor types. The CZGS NCs synthesized via the GeCl4, Ge-Gly and Ge-Tar routes have the similar character, while CZGS NCs synthesized from Ge-Cit has the different feature.

Figure 6. Time evolution TEM images of the as-prepared CZGS NCs synthesized with different Ge precursors: (a) Ge-Tar and (b) GeCit. Variations in chemical composition over different reaction time for CZGS NCs synthesized with different Ge precursor: (c) Ge-Tar and (d) Ge-Cit.



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Figure 7. Schematic diagram of the formation mechanisms of the CZGS NCs synthesized with different Ge precursor. Accordingly, we proposed the formation mechanism of different particle size distribution appearance as follows. As discovered in this work, the Ge precursors have no effect on the phase formation process of CZSG NCs but significantly influenced the particle size distribution. As shown in Figure 7, it is proposed that the formation of CZGS NCs experienced the traditional formation route when the GeCl4, Ge-Gly or Ge-Tar acted as Ge precursors. OLA and other fatty amines are known to have an “activating” influence on precursors which facilitate the nucleation43. A large degree of polydispersity was expected without the use of other ligand. In the synthesis of CZGS NCs employed Ge-Cit as precursors, the undissolved metal-Cit complex in OLA emerged prior to injection. Once the NCs were formed, they would be absorbed by the undissolved metal-Cit complex. The small NCs were absorbed preferentially due to the higher surface energy. Therefore, the rest of the small part of the product appeared as big particles. In order to demonstrate the general applicability of Ge-Cit in the successful formation of monodispersed nanoparticles, Cu2ZnGe1-xSnxS4 nanoparticles have been designed and prepared. Moreover, the CZGS-based solid solution compounds depending on different Si/Ge ratios were often designed to adjust the band gap. Therefore, in this work, we synthesized Cu2ZnGe1-xSnxS4 NCs employing Ge-Cit as Ge precursor. As shown in Figure 8, XRD patterns for CZGS NCs can be indexed to JCPDS No. 260572, as mentioned earlier. The similar patterns with peaks shifting to lower diffraction angles obeying Vegard rule upon substitution of Ge by bigger Sn atoms are observed as x varies from 0 (CZGS) to 1 (CZTS). Furthermore, Figure 8b shows the enlarged (100) planes in the range of 26-28 o clearly suggesting the angle shift. This verified the formation of the isostructural Cu2ZnGe1-xSnxS4 solid solutions compounds with controlled unit cell parameters.

The compositional effects on the band gap variation of Cu2ZnGe1-xSnxS4 were examined by using the absorption spectra. The optical band gaps of Cu2ZnGe1-xSnxS4 NCs can be estimated by the related curve (αhν)2 versus photon energy (α = absorbance, h = Planck’s constant, and ν = frequency) by using the intersection of the extrapolated linear portion of the curve15. The band gap increases with the Ge content increase in the synthesis precursors (Figure 8c). As expected, the band gap of the Cu2ZnGe1-xSnxS4 NCs was continuously adjustable from 1.6 eV for single-phased Cu2ZnSnS4 to 2.7 eV for pure Cu2ZnGeS4. The elemental ratio in the final products is not equivalent to the ratio of the precursors. Therefore, the x value was determined and shown in Table S2. Generally, the Ge content in the final products was higher than that of theoretical ratio and result in higher band gap, which was corresponding to the previous study. The elemental ratio in the final products is not equivalent to the ratio of the precursors. Therefore, the x value was determined and shown in Table S2. Generally, the Ge content in the final products was higher than that of theoretical ratio and result in higher band gap, which was corresponding to the previous study. The TEM images of this Cu2ZnGe1-xSnxS4 NCs series were also given to show the sizes and morphology depending on the Ge/Sn ratios (Figure 8d-i). It was clearly found that nearly monodispersed Cu2ZnGe1-xSnxS4 NCs can be obtained via the Ge-Cit precursor route including the end members of Cu2ZnSnS4 by using the Sn(acac)Cl2 precursor, which verified the advantage of the GeCit precursor in the synthesis of monodispersed Cu2ZnGe1-xSnxS4 NCs. Moreover, the products with different Ge/Sn ratio exhibited the similar particle sizes (about 15 nm), as evident in Figure 8d-i and this demonstrated potential application of the solutions as tunable absorber materials in the thin film solar cell.


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Figure 8. (a) XRD patterns of samples with different Ge/Sn ratio. (b) Magnification of the peaks for (100) planes. The diffraction peaks of the obtained samples systemically shift to smaller angles as Sn content increases. (c) The linear extrapolation of (αhν)2 versus photon energy, from which the evaluated band gap also decreases as composition x increase. (d-f) TEM images of Cu2ZnGe1-xSnxS4 nanoparticles with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0.

4. CONCLUSION

*Email:

In summary, we have presented the synthesis, structure and morphology evolution of CZGS NCs prepared via the hotinjection method using different Ge precursors. The grain sizes of as-prepared CZGS NCs is found to be dependent on Ge precursor. There are two different size distribution characters, viz, the sample fabricated via the Cit3- precursor route possesses the mono-dispersed CZGS NCs with size about 15 nm. However, the NCs synthesized by employing GeCl4, Ge-Gly and Ge-Tar precursors, contains two different particles types, big particles about 15 nm and small ones about 5 nm. It is proposed that assynthesized NCs could be absorbed on the surface of the metalCit complex that, in such a case, lead to preferable absorption, separation and cleaning of the small particles. The general applicability of Ge-Cit in the successful formation of monodispersed NCs has been further verified, and the Cu2ZnGe1xSnxS4 NCs with different Ge/Sn ratios were synthesized by using Ge-Cit precursor route. Besides the mono-dispersed particle size and iso-structural solid solution character, the XRD diffraction peaks of Cu2ZnGe1-xSnxS4 NCs shift to smaller angles obeying Vegard rule upon Ge substitution with bigger Sn atoms. The bandgap of the Cu2ZnGe1-xSnxS4 nanocrystals was continuously adjustable from 1.6 eV for pure Cu2ZnSnS4 to 2.7 eV for pure Cu2ZnGeS4, suggesting a kind of potential tunable absorber materials in the thin film solar cell.

■ACKNOWLEDGEMENT The present work was partly supported by the National Natural Science Foundations of China (Grant No. 51572023).

■AUTHOR INFORMATION

[email protected];

■REFERENCES

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The graphic for the TOC:


Insight into the Controlled Synthesis of Cu2Zn(Ge,Sn)S4

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