- Nano Express
- Open Access
Solution-Processed Cu2ZnSn(S,Se)4 Thin-Film Solar Cells Using Elemental Cu, Zn, Sn, S, and Se Powders as Source
- Jing Guo†1,
- Yingli Pei†1,
- Zhengji Zhou1Email author,
- Wenhui Zhou1,
- Dongxing Kou1 and
- Sixin Wu1Email author
© Guo et al. 2015
- Received: 2 June 2015
- Accepted: 10 August 2015
- Published: 21 August 2015
Solution-processed approach for the deposition of Cu2ZnSn (S,Se)4 (CZTSSe) absorbing layer offers a route for fabricating thin film solar cell that is appealing because of simplified and low-cost manufacturing, large-area coverage, and better compatibility with flexible substrates. In this work, we present a simple solution-based approach for simultaneously dissolving the low-cost elemental Cu, Zn, Sn, S, and Se powder, forming a homogeneous CZTSSe precursor solution in a short time. Dense and compact kesterite CZTSSe thin film with high crystallinity and uniform composition was obtained by selenizing the low-temperature annealed spin-coated precursor film. Standard CZTSSe thin film solar cell based on the selenized CZTSSe thin film was fabricated and an efficiency of 6.4 % was achieved.
- Thin film
- Solar cells
- Solution process
Quaternary semiconductor Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) compounds have received considerable interest as new generations of photovoltaic absorbing materials, due primarily to their suitable band gaps, high absorption coefficient, and low material cost . Recently, various approaches have been developed to fabricate the absorber layers, briefly including vacuum-based deposition and non-vacuum-based solution process; both strategies have yielded a remarkable improvement in photovoltaic performance [2–8]. Compared to vacuum-based approaches, non-vacuum technologies such as electrodeposition approach [9–11], milling dispersion approach , nanoparticle-based approach [13–16], hydrazine-based approach [17–19], and sol–gel approach [20–24] are more feasible for industrial production. Among those solution-based process, the hydrazine-based deposition has made the great progress, achieving the power conversion efficiency (PCE) of 12.6 % . However, as a result of the high toxicity and dangerous instability of explosible hydrazine, the non-hydrazine solvent is more desirable for practical application. Therefore, some non-hydrazine solvents, such as the mixtures of ethanol and water and amine-thiol mixture, have been tried presently to dissolve the metallic oxide or metal salt for preparing the CZTS precursor solution [26–28].
For CZTS (e) thin film solar cell, it is very crucial to precisely control the elemental composition of quaternary compounds, which regulate the band gap of the semiconductor and further dominate the device performance. From this point of view, the ideal precursor is prepared using elemental Cu, Zn, Sn, and S (e) rather than metallic oxide or metal salt to avoid involving impurity. Furthermore, the elementary metal powder are substantially inexpensive and easy industrially available. Lately, the Pan’s research group has firstly reported an approach to fabricate CZTSe films which used the mixture of thioglycolic acid and ethanolamine to dissolve the Cu, Zn, Sn, and Se powder . In order to adjust the viscosity of the solution for subsequent spin coating, another organic solvent of 2-methoxyethanol was added into the mixed solution.
In this paper, we presented a more convenient and quicker method to fabricate the high-quality CZTSSe thin film. 1,2-ethanedithiol and 1,2-ethylenediamine were adopted as a facile and low-toxic solution to dissolve the low-cost Cu, Zn, Sn, S, and Se powders as starting materials. A solar cell efficiency of 6.4 % was obtained using this novel solution deposition and process procedure of CZTSSe active layer.
Cu (99.9 %, Aladdin), Zn (99.9 %, Aladdin), Sn (99.8 %, Alfa Aesar), Se (99 %, Alfa Aesar), and S (99.9 %, Aladdin) powders are analytical reagents. 1,2-ethanedithiol (HSCH2CH2SH, AR), 1,2-ethylenediamine (H2NCH2CH2NH2,AR), ammonium hydroxide (NH4OH, 25 %), cadmium sulfate (AR), and thiourea (AR) were purchased from Alfa Aesar. All chemicals and solvents were commercially available and used as received without further purification.
Preparation of CZTSSe Precursor Solution
Cu (1.10 mmol), Zn (0.76 mmol), Sn (0.62 mmol), S (1.50 mmol), and Se (1.50 mmol) were added into a 25-ml round flask. Then, 0.5 ml of 1,2-ethanedithiol and 5 ml of 1,2-ethylenediamine were injected into the flask. The mixture was magnetically stirred on a 70 °C hot plate for 1.5 h and a clear orange-colored solution was obtained.
Fabrication of CZTSSe Thin Film and Solar Cell Device
The CZTSSe precursor solution was spin-coated on a 2 × 2 cm molybdenum-sputtered soda lime glass (SLG) substrate, followed by heating at 310 °C on a hot plate in argon atmosphere. This coating and sintering procedure was repeated several times till the desired film with thickness of 1.6 μm was obtained. Finally, the thin film was annealed at 550 °C in a graphite box containing 200 mg of Se powder for 15 min. CZTSSe thin film solar cells were fabricated using the selenized CZTSSe thin films by successively depositing the following additional layers: chemical bath deposition (CBD) of ∼60 nm cadmium sulfide (CdS), sputtering of ∼70 nm intrinsic zinc oxide (ZnO), and ∼200 nm indium-doped tin oxide (ITO). On the top of the device, Al collection grid electrodes were deposited by thermal evaporation. No anti-reflection coating was utilized.
Thermogravimetric analysis (TGA) was performed by a TGA/SDTA851e of Mettler-Toledo. The powder X-ray diffraction (XRD) patterns were taken with a Bruker D8 Advance X-ray diffractometer. The Raman spectra were measured by a Renishaw in via Raman microscope using an excitation laser with a wavelength of 532 nm. The scanning electron microscope (SEM) images were collected using a Nova NanoSEM 450. Photocurrent density-voltage curves were recorded under the standard AM1.5 illumination (100 mW · cm−2) with a Keithley 2400 source meter. The external quantum efficiency (EQE) spectrum was measured using a Zolix SCS100 QE system equipped with a 150-W xenon light source and a lock-in amplifier.
Compared with hydrazine-based solution method for CZTSSe thin film, the efficiency of photovoltaic device present in our study is still limited, which may be mainly attributed to the existence of small-grained bottom layer in the selenized CZTSSe thin films. It has been reported that a thick small-grained bottom layer would increase the series resistance of devices and further degrade the photoelectric conversion efficiency of CZTSSe solar cells . Therefore, in order to further improve the performance of solar cells, it is necessary to optimize the selenization conditions for reducing or even completely eliminating the small-grained bottom layer of CZTSe films, which is the ongoing research in our laboratory.
In summary, a reproducible and lower-toxicity solution-based process for the fabrication of the CZTSSe absorber layer, involving simultaneous dissolution of elemental Cu, Zn, Sn, S, and Se powders in the mixed 1,2-ethanedithiol and 1,2-ethylenediamine solution followed by deposition of a precursor solution, has been presented. After the preheating and post-selenization processes, an extremely dense and compact CZTSSe thin film with high crystallinity has formed. The CZTSSe thin film solar cells fabricated using this process exhibits an efficiency of 6.4 %, which is expected to further enhance by optimizing the composition and selenization of the CZTSSe film.
This project is supported by the National Natural Science Foundation of China (21203053, 21271064, and 61306016), the Joint Talent Cultivation Funds of NSFC-HN (U1204214), and the Program for Changjiang Scholars and Innovative Research Team in the University (PCS IRT1126) of Henan.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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