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Elemental precursor solution processed (Cu Ag)ZnSn(S,Se) Photovoltaic Devices with over 10% Efficiency
Yafang Qi, Qingwen Tian, Yuena Meng, Dongxing Kou, Zheng-Ji Zhou, Wen-Hui Zhou, and Sixin Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017
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ACS Applied Materials & Interfaces
Elemental precursor solution processed (Cu1-xAgx)2ZnSn(S,Se)4 Photovoltaic Devices with over 10% Efficiency Yafang Qiab, Qingwen Tianab, Yuena Meng*ab, Dongxing Kouab, Zhengji Zhouab, Wenhui Zhouab and Sixin Wu*ab a
The Key Laboratory for Special Functional Materials of MOE, Henan University,
Kaifeng, Henan 475004, China b
Collaborative Innovation Center of Nano Functional Materials and Applications,
Henan University, Kaifeng, Henan 475004,China ABSTRACT: The partial substitution of Cu+ with Ag+ into the host lattice of Cu2ZnSn(S,Se)4 thin films can reduce the open-circuit voltage deficit (Voc,deficit) of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. In this paper, elemental Cu, Ag, Zn, Sn, S, and Se powders were dissolved in solvent mixture of 1, 2-ethanedithiol (edtH2) and 1, 2-ethylenediamine (en), and used for the formation of (Cu1-xAgx)2ZnSn(S,Se)4 (CAZTSSe) thin films with different Ag/(Ag+Cu) ratios. The key feature of this approach is that the impurity atoms can be absolutely excluded. Further results indicate that the variations of grain size, band gap and depletion width of CAZTSSe layer are generally determined by Ag substitution content. Benefitting from the Voc enhancement (~50 mV), the power conversion efficiency is successfully increased from 7.39% (x=0) to 10.36% (x=3%), which is the highest efficiency of Ag substituted devices so far. Keywords: kesterite; CZTSSe solar cells; antisite defect; Ag substitution; solution process
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INTRODUCTION Cu2ZnSn(S,Se)4 (CZTSSe) has drawn world-wide attention as an attractive kesterite material due to its low-cost processing techniques and scalable cell efficiency.1-6 The CZTSSe solar cells fabricated by hydrazine solution method has achieved the highest efficiency of 12.6%.7 However, there is still a big gap between CZTSSe solar cells and Cu(In,Ga)Se2 (CIGSe) solar cells (with the efficiency of 22.6%).8,9 The high open circuit voltage deficit (Voc,def) in CZTSSe device is the primary hurdle, which is due to the large amount of CuZn antisite defects and atomic disorder induced by the similar covalent radii between Cu and Zn.10-14 It is critical to alleviate the limitations and suppress the CuZn defects formation in CZTSSe solar cells. Cation substitution with Ge, In, Cd et al. can effectively adjust optoelectronic properties of CZTSSe absorber.15-17 Especially, Silver (Ag) is proved to be an efficient fluxing agent for improving the microstructure and electronic properties of CZTSSe absorbers through occupying Cu sites in the lattice. Theoretical calculation demonstrated that AgZn defect has much higher formation energy than CuZn defect.12 Ag substitution can obviously decrease the band tailing and improve minority carrier lifetime.
Gershon
et
al.
reported
that
the
efficiency
of
Ag-substituted
(Cu1-xAgx)2ZnSnSe4 (CAZTSe) devices can be enhanced dramatically to 10.2% by co-evaporating method.18 Hages et al. prepared CAZTSe absorbers through sulfide nanocrystal inks, and the device efficiency increased to 7.2%.19 Wong et al. reported a molecular solution method for Ag substitution in CZTS by using metal salts as precursor, such as acetate, hydrochloride and nitrate, and the cell efficiency increased from 4.9% to 7.2%.3 However, such metal salt solution can bring undesirable impurity atoms such as O, N, and Cl to the absorber. Efficient Ag substitution for CZTSSe device in simple pure elemental solution system to alleviate the Voc,def and improve photovoltaic performance has not been reported. Herein, we demonstrated a solution-based approach to partially substitute Cu with Ag in CZTSSe thin films. Low-cost elemental Cu, Ag, Zn, Sn, S, and Se powders were used as the starting materials and dissolved in a mixture of 1,2-ethanedithiol
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(edtH2) and 1,2-ethylenediamine (en). CAZTSSe alloy material with different Ag/(Ag+Cu) ratios were prepared subsequently. The key feature of this approach is that impurity atoms can be absolutely excluded, and the elemental sources are substantially cheaper. Systematic studies on the effects of Ag substitution were carried out. The crystal growth of CZTSSe thin films was promoted by Ag substitution. Meanwhile, the depletion width (Wd) at the hetero-junction interface of CZTSSe/CdS was found to be improved by Ag substitution, especially at the optimal Ag/(Ag+Cu) ratio (x=3%). More importantly, the CuZn defect concentration decreases monotonously with the Ag/(Ag+Cu) ratios from 0 to 5% and the Voc,def was gradually decreased. The highest efficiency of Ag substituted CZTSSe device in this work indicates that Ag-doping CAZTSSe alloy material has great potential in alleviating the Voc,def and improving the CZTSSe solar cell efficiency. EXPERIMENTAL SECTION Materials. Copper (Cu, 99.9%), silver (Ag, 99.9%), zinc (Zn, 99.99%), selenium (Se,
99%),
sulfur
(S,
99.95%),
1,2-ethanedithiol
(HSCH2CH2SH,
97%),
1,2-ethylenediamine (H2NCH2CH2NH2, 99%), cadmium sulfate (CdSO4·8/3H2O, 99%), and thiourea (NH2CSNH2, 99%) were purchased from Aladdin Co. Tin (Sn, 99.8%) was obtained from Alfa Aesar Chemical Co. Ammonium hydroxide (NH4OH, 25%) was purchased from Beijing Chemical Works. All chemicals and solvents were commercially available and used as received without further purification. Synthesis of CAZTSSe precursor solution. To prepare a CAZTSSe precursor solution, 1.80 mmol of Cu (or Cu and Ag), 1.24 mmol of Zn, 1.01 mmol of Sn, 0.89 mmol of Se, and 2.59 mmol of S were added into a sample bottle. Afterwards, 0.7 mL of 1,2-ethanedithiol and 5 mL of 1,2-ethylenediamine were mixed into the bottle. Then the solution was magnetically stirred at 60 °C for 12 h until all the solids had dissolved. The ratio of the starting materials follows the target of Cu-poor and Zn-rich stoichiometry [(Ag+Cu)/(Zn+Sn)=0.80 and Zn/Sn=1.23]. To investigate the influence of Ag doping level on the performance of solar cells, we prepared four different Ag doping level precursor solutions [Ag/(Cu+Ag)=0%, 1%, 3%, and 5%]. The digital photograph of the precursor solutions were shown in Fig. S1 (a). All solution
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preparation processes were performed in an argon-filled glovebox (H2O and O2 levels maintained below 1 ppm). Deposition and selenization of CAZTSSe films. First, ∼650 nm thick molybdenum (Mo) was DC-sputtered on a soda lime glass (SLG, 20×20×1.0 mm3). Next, the prepared CZTSSe and CAZTSSe precursor solution with different Ag doping level were spin coated on Mo-coated soda lime glasses at 3000 rpm for 30 s, followed by sintering on a 350 °C hot plate for 2 min. The spin-coating/sintering operations described above were repeated eight times until the CAZTSSe precursor film has a targeted thickness (∼1.5 µm). The above procedure were performed in an argon-filled glovebox ([H2O]
