Tailoring Mo(S,Se)2 structure for high efficient Cu2ZnSn(S,Se)4 solar cells
Introduction
Cu(In,Ga)Se2 (CIGS) solar cells are one of the most promising thin film photovoltaic devices and the best efficiency has reached 22.6% [1]. However, In and Ga are rare elements, thus the maximum power production capacity of CIGS solar cells will be limited to ~ 100 GW per year [2]. Kesterite structure Cu2ZnSn(S,Se)4 (CZTSSe) is a suitable alternative material with the more earth-abundant and low-cost elements Zn and Sn to replace the rare elements In and Ga [3], [4]. Thus, CZTSSe solar cells will be much cheaper. In the past few years, CZTSSe solar cells have attracted much attention and the highest efficiency of CZTSSe solar cells is 12.6% [5], [6].
Like CIGS solar cells, Mo film is served as back contact in CZTSSe solar cells. Generally, a Mo(S,Se)2 interfacial layer can be formed between CIGS or CZTSSe absorber and Mo back contact during the growth of CIGS or CZTSSe film [7], [8]. It is well known that a thin Mo(S,Se)2 layer can act as a buffer layer to convert the Schottky contact to a quasi-ohmic contact, which is beneficial to the transportation of charge carriers [9], [10], [11]. However, an over thick Mo(S,Se)2 layer will block the transportation of charge carriers, which is detrimental to the performance of devices [7], [9], [12]. Generally, to promote the growth of grains, suppress the surface decomposition, and avoid the formation of VSe or VS defects, CZTSSe films are often prepared at high temperature under high Se or S partial pressure [8], [12], [13], [14], [15]. As a consequence, an over thick Mo(S,Se)2 layer will be formed, which will increase the contact resistance between CZTSSe absorber layer and Mo back contact electrode [8], [12]. Thus, it is crucial to control the thickness of Mo(S,Se)2 interfacial layer for high efficient CZTSSe solar cells. Shin et al. indicated that the formation of MoSe2 during selenization process under Se ambient can be divided into three step processes: diffusion of Se through Cu2ZnSnSe4 (CZTSe), diffusion of Se through MoSe2 which is growing with continued annealing, and reaction between Se and Mo [8]. Thus, the formation of Mo(S,Se)2 can be suppressed by such processes. For example, an intermediate layer, such as TiN, TiB2, ZnO, and MoO2, deposited between CZTSSe absorber and Mo back contact acts as a Se diffusion barrier to inhibit the formation of Mo(S,Se)2 [12], [16], [17], [18], [19]. However, these intermediate layers may induce a rather high series resistance in the device and degrade the device performance [16], [17]. Lopez-Marino et al. and Schnabel et al. indicated that a direct contact between CZTSSe and Mo(S,Se)2 is necessary for achieving high device performance, though a thin MoO2 or TiN intermediate layer can effectively suppress the formation of Mo(S,Se)2 [18], [19]. Thus, a top sacrificial Mo cap layer was deposited on MoO2 or TiN layer to control the thickness of Mo(S,Se)2 layer, and thereby the performance parameters of fill factor (FF) and open voltage circuit (VOC) are increased. On the other hand, Li et al. proposed a new method to suppress the formation of MoSe2 by a dense temporary alloy layer formed during a low temperature annealing process under Ar atmosphere [20]. As further selenizing the samples under high Se pressure, the thickness of MoSe2 interfacial layer was tailored to less than 10 nm and the conversion efficiency was improved from 5.6% to 8.7%. However, this process cannot work well in some other processes, such as selenizing the precursors prepared by sol-gel method. Though preparation of CZTSSe films under low Se pressure can prevent the formation of Mo(S,Se)2 layer and a CZTSe solar cells with the efficiency of 8.2% was fabricated under low Se pressure [21], the CZTSe surface may decompose and more defects exist in the absorber layer. Thus, it is necessary to find a way to easily control the thickness of Mo(S,Se)2 interfacial layer while selenizing the precursors at high temperature under high Se pressure.
In this study, we present an effectively and easily controlled way to tailor the preferred orientation and thus tailoring the thickness of Mo(S,Se)2 by modifying the surface morphology of Mo back contact. Though Shin et al. indicated that the preferred orientation of CIGS film can be changed by the modification of Mo morphology [22], they didn’t disclose the influence on the formation of MoSe2 with different orientation by other factors, such as thickness of MoSe2. This paper makes detail analysis to the influence of preferred orientation of Mo(S,Se)2 on the thickness of Mo(S,Se)2 and thereby the influence on the device performances. In other work, such as work done by Lin et al., they also proposed a method to decrease the thickness of MoSe2 in CIGS solar cells [23]. They attribute this decrease to the increase of Na content in the CIGS absorber with the increase of thickness of Mo barrier layer prepared at higher working pressure, which will promote the formation of Na2SeX at grain boundaries during selenization. Thus, the dense of CIGS absorber is increased and the grain boundary diffusion of Se is reduced, which will reduce the growth rate of MoSe2. In our study, however, we found the decrease of thickness of Mo(S,Se)2 is because of the change of the preferred orientation of Mo(S,Se)2, which is introduced by the different morphologies of Mo films. As a consequence, the thickness of Mo(S,Se)2 was tailored from 1500 nm to 200 nm, the series resistance of CZTSSe solar cells was reduced to a low level (~ 0.49 Ω cm2), and thereby the conversion efficiency of CZTSSe solar cells was increased from 6.98% to 9.04%.
Section snippets
Experimental
Soda-lime glasses (SLG) were used as substrates. The SLG were cleaned by electronic cleaning agent followed by ultrasonic cleaning, and then the SLG were dried with a nitrogen flux. Mo films were deposited on the cleaned SLG by DC-magnetron sputtering Mo target. One-layer-structured and two-layer-structured Mo films with different surface morphologies were prepared at different deposition conditions, respectively, as shown as Fig. 1(a)-(c). First, a one-layer-structured Mo film named as Mo-1
Results and discussion
Fig. 1(d) shows the surface morphology of sample Mo-1, which is composed of large worm-like grains with ca. 200 nm long by ca. 100 nm wide (Fig. S1). And Fig. 1(d) shows clearly the presence of voids between grains. To modify the surface morphologies of Mo films, a thin top layer (ca. 100 nm) was deposited at working power of 2.5 and 0.5 W/cm2 respectively under the same working pressure of 1.5 Pa, which were named as Mo-2 and Mo-3, respectively. Fig. 1(e) and (f) show the selected SEM surface
Conclusions
This work provides a simple and effective way to adjust the preferred orientation of Mo(S,Se)2, and thus the thickness of Mo(S,Se)2 interfacial layer by modifying the morphology of the Mo back contacts. As a consequence, the series resistance RS of CZTSSe solar cell is reduced and the conversion efficiency is improved. Two-layer-structured Mo films were introduced to modify the surface morphologies of Mo films. The bottom 1200 nm thick Mo layers were deposited with working power of 2.5 W/cm2
Acknowledgments
This work was supported by the National Science Foundation of China (51572132, 61674082), Tianjin Natural Science Foundation of Key Project (16JCZDJC30700), and YangFan Innovative and Entrepreneurial Research Team Project (2014YT02N037).
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