Synthesis of hexagonal structured wurtzite and chalcopyrite CuInS2 via a simple solution route
© Sheng et al; licensee Springer. 2011
Received: 11 July 2011
Accepted: 25 October 2011
Published: 25 October 2011
Wurtzite semiconductor CuInS2 [CIS] has been reported in recent years. As a kind of metastable structure, it is a great challenge to synthesize pure wurtzite CIS at low temperature. In this paper, via a simple and quick solution route, we synthesize both wurtzite- and chalcopyrite-structure CIS. Well-controlled wurtzite CIS hexagonal plates are obtained when an appropriate agent is added. The influence of the used agent triethanolamine [TEA] has also been studied, and it turns out that without TEA, chalcopyrite CIS with a kind of rare morphology is formed through this method.
Ternary • I-III-VI2 semiconductors have garnered great interest due to their promising photovoltaic applications [1, 2]. Meanwhile, the growing need for highly efficient and low-cost photovoltaic devices continues to drive new research in developing non-vacuum techniques. Thus, solution routes to fabricate I-III-VI2 semiconductor nanocrystals have been greatly developed because nanocrystal synthesis can utilize lower-cost processing and device fabrication can benefit from roll-to-roll or solution-phase processing such as spin-coating [3–10]. Among the kinds of I-III-VI2 semiconductors, one good example is CuInS2 [CIS] which has high optical absorption coefficient (>105 cm-1) and desirable bandgap (approximately 1.45 eV) that matches well with solar spectra [11, 12]. Therefore, researches on nanocrystal CIS synthesis have attached great attention [13–18].
It was reported that CIS has three crystal structures [19, 20]: these are (1) the chalcopyrite structure [CH-CIS], stable from room temperature to 1,253 K; (2) the zinc-blende structure, stable between 1,253 and 1,318 K; and (3) an unknown structure, existing from 1,318 K to melting temperature. It was found that the unknown structure could be turned into wurtzite phase, while Cu and In atoms occupy the cation sublattice positions disorderly . Moreover, previous studies have proved that both the zinc-blende and wurtzite structures are metastable at room temperature since they may transform into chalcopyrite phase as the temperature recurred to room temperature [19, 20]. As a result, CH-CIS is believed to be the most common phase and extensively used in CIS solar cells.
Recently, Pan et al. reported the synthesis of zinc-blende- and wurtzite-structure CIS [WZ-CIS] nanocrystals by a hot-injection method , which brings great interest in these two structures, especially in WZ-CIS. Since wurtzite phase allows flexibility of stoichiometry, it provides the ability to tune the Fermi energy over a wide range, which is beneficial for device fabrication . Some groups have reported to synthesize WZ-CIS nanocrystals in recent years [23–29]. Most reported works are also synthesized in an oil system since former researches find out that solvents like ethanolamine, ethylenediamine, and isopropanolamine are beneficial for the formation of WZ-CIS . However, it is still a challenge for the synthesis of metastable WZ-CIS at room temperature, especially via a simple method and low-cost precursors.
In our study, we synthesize pure and well-controlled WZ-CIS via a simple and quick solution route in a polyalcohol system under low temperature. We find that the agent triethanolamine [TEA] plays an important role in the synthesis of WZ-CIS phase. Without TEA, large-diameter hexagonal-structure CH-CIS is obtained. It is a kind of rare morphology in CH-CIS since most other groups pay more attention to controlling the sizes and shapes of the particles and focus on the synthesis of nanosheets, nanorods, quantum dots, and others [13–18]. The growth process and mechanism of hexagonal-structure CH-CIS are discussed.
All the reagents are used as received without any further purification. The reagents are as follows: copper(I) dichloride [CuCl 2H2O], indium(III) trichloride [InCl3 4H2O], thiourea [TA], diethylene glycol [DEG], and TEA.
Nanostructured WZ-CIS samples are synthesized via a simple and quick solution route. One mmol of CuCl 2H2O and 1 mmol of InCl3 4H2O are dissolved into 40 mL DEG in a three-neck flask. This solution is stirred under N2, while the temperature rises to 180°C. After adding 3 mL TEA while stirring, the solution turns into a deeper color but still clear without any precipitation. Then, stoichiometric amounts of TA dissolved in 10 mL DEG is slowly added into the former solution. After a 2-h reaction at 180°C, the flask is removed from the heater and cooled at room temperature. The precipitates are separated by centrifugation, washed with ethanol for three to five times, and dried at 80°C for 5 h. Meanwhile, CH-CIS samples are synthesized via a similar route, but without adding TEA.
The as-prepared products are characterized by X-ray diffraction [XRD], scanning electron microscopy [SEM], and transmission electron microscopy [TEM]. XRD is carried out to study the crystal structures of all the samples by using an X'Pert PRO (PANalytical, Almelo, The Netherlands) diffractometer equipped with a Cu Kα radiation source. Data are collected by step-scanning of 2θ from 10° to 70° with a step of 0.02° and a counting time of 1 s per step. Morphology of the products is investigated by SEM and TEM. The SEM images are taken by SEM S4800 (Hitachi, Tokyo, Japan). The TEM images and high resolution TEM [HRTEM] are acquired by Tecnai F20 (FEI, Hillsboro, OR, USA).
Results and discussion
Structure characterization of WZ-CIS
The influence of TEA
A former study has reported that solvents like ethanolamine, ethylenediamine, and isopropanolamine are beneficial for the formation of WZ-CIS [24, 25]. The pivot is that these solvents play important roles as ligand and reducing agent which reduce Cu2+ to Cu+. Qi et al. have synthesized WZ-CIS in 2009 using ethanolamine as solvent . They claim that the successful synthesis of WZ-CIS strongly depends on the formation of coordination between Cu2+ or Cu+ and -NH2. In our work, TEA is the pivotal agent. TEA is a kind of versatile ligand that readily forms coordination compounds with almost all metal ions . It can play a similar role as ethanolamine, which is coordinating with Cu+ and facilitating the formation of WZ-CIS. Also, the experiment phenomena show clearly that only when the Cu source and TEA have an appropriate ratio and form a clear solution before the S source is added can pure WZ-CIS be obtained. Otherwise, when the precursor solution is turbid, CH-CIS will co-exit with WZ-CIS. Moreover, TEA also provides ligand for In3+ to limit the size and control the morphology of the products. It can be presumed that TEA coordinates with Cu+ and In3+, changing and controlling the relationship of release rates between these two cations, and makes Cu+ and In3+ occupy the cation sublattice positions disorderly when they react with S2-.
The growth process of CH-CIS hexagonal plates
In conclusion, WZ-CIS with a well-controlled hexagonal structure is synthesized via a simple and quick solution route. It is found that the agent TEA plays a key role in forming the wurtzite phase and controlling the size of the products since it coordinates with Cu+ and also provides ligand for In3+ to limit and control the morphology of the product. Without TEA, the products appeared as CH-CIS phase, and the hexagonal structures had much larger diameters. The growth process shows that the fabrication of CH-CIS hexagonal structures is due to hexagonal phase CuS self-aggregation and reaction with In3+. After a short time, about 20 min, nearly pure CH-CIS phase with hexagonal structures is formed.
This work is supported by the National Basic Research Program of China or the 973 Program (No. 2007CB613403), the Innovation Team Project of Zhejiang Province (2009R50005), and the Fundamental Research Funds for the Central Universities.
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