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Evolution of Na-S(-O) compounds on the Cu2ZnSnS4 absorber surface and their effects on CdS thin film growth Yi Ren, Jonathan James S. Scragg, Marika Edoff, Jes K. Larsen, and Charlotte Platzer-Björkman ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04978 • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on July 1, 2016
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ACS Applied Materials & Interfaces
Evolution of Na-S(-O) Compounds on the Cu2ZnSnS4 Absorber Surface and Their Effects on CdS Thin Film Growth
Yi Ren*, Jonathan J. S. Scragg, Marika Edoff, Jes K. Larsen, and Charlotte PlatzerBjörkman Ångström Solar Center, Solid State Electronics, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Key words: Kesterite, Cu2ZnSnS4, Sodium, surface compounds, CdS, thin film solar cell Abstract Formation of Na-containing surface compounds is an important phenomenon in the Cu2ZnSnS4 (CZTS) quaternary material synthesis for solar cell applications. Still, identification of these compounds and the understanding of their potential influence on buffer layer growth and device performance are scarce. In this work, we discovered the evolution of Na-S(-O) compounds on CZTS surface substantially affect the solution/CZTS interface during the chemical bath deposition of CdS buffer film. We showed that Na2S negatively affects the growth of CdS, and that this compound is likely to form on the CZTS surface after annealing. It was also demonstrated that the Na2S compound can be oxidized to Na2SO4 by air exposure of the annealed CZTS surface or removed by using water dipping instead of commonly used KCN etching process, resulting in significantly better quality of the CdS layer. Lastly, 6.5% CZTS solar cells
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were fabricated with air exposure treatment without incorporation of KCN etching process. This work provides new insight to grow the CdS/CZTS interface for solar cell application, and opens new possibilities for improving likewise Cd-free buffer materials that are grown with similar chemical bath deposition process. 1. Introduction Kesterite Cu2ZnSnS4 (CZTS) is a promising alternative to Cu (In,Ga)(S,Se)2 (CIGS) and CdTe based photovoltaic technology, due to its all earth abundant elements. The typical synthesis method for absorber formation includes CZTS precursor deposition followed by high temperature (> 550 °C) annealing in sulfur excess environment, which yields a current record efficiency for CZTS solar cells of 9.2%.1 The annealing is designed to give a CZTS film with optimal bulk quality, but it is also important to consider the chemistry of the film surface, which is our focus here. In particular, we consider Na-containing surface compounds, since Na released from the soda lime glass (SLG) substrate, is often incorporated into the CZTS film during absorber synthesis.2 Studies concerning the chemistry of the CZTS surface are still very rare, but understanding the CZTS surface is crucial for device refinement, inasmuch as it can substantially affect the quality of the interface with the subsequently applied buffer layer, such as CdS.3 CdS has been successfully developed for CIGS and CdTe solar cells. It is also widely used and has been proven to work for kesterite technology,1 despite the issue of the non-ideal cliff-like band alignment at the CdS/CZTS interface.4,5 Recent work show that alternative buffer, such as chemical bath deposited ZnCdS and atomic layer deposited ZnSnO, can enhance the solar cell efficiency by improving the interface band offset.6,7 To create the CdS/CZTS interface, the chemical bath deposited CdS
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(CBD-CdS) process is still the most common method nowadays. Interestingly a step of “wet chemical cleaning” prior to CBD-CdS is often performed. In principle, the chemical bath solution could already depollute the surface, but still etching with toxic KCN is routinely used in state of the art kesterite solar cells.8,9 The role of KCN treatment for the CZT(S,Se) material is debated. KCN etching was originally introduced to remove Cux(S,Se) secondary phase for Cu-rich CIGS films.10 The need for it is an open question in the CZTS case, since Cu-poor and Zn-rich composition for CZTS synthesis is mainly used. Timmo et al., observed that KCN can dissolve Cu, Sn and chalcogen from Cu-poor CZTSSe monograins, but that removal of Cu seems the same by different etchants, which is in contradiction to reports for CIGS.11 Bär et al., found that KCN etching changed the surface composition of the CZTS film, contributing to a shift of the surface bandgap.12 Buffiere et al., by measuring the etched solution, reported that KCN mainly removed secondary phases rather than changing the composition.13 These discussions reveal that deeper insight and understanding of the surface chemistry of the CZTS absorber is of great importance. In this study, we investigate surface compounds formed on the CZTS film and the changes occurring after air exposure (AE) treatments within a short time scale. We discovered by XPS that Na-S(-O) compounds were evolving on the surface of the CZTS absorber after the annealing process, while carbonate can continuously form on CZTS surface and possibly drive Na out diffusion from CZTS film. We found Na-S(-O) compounds can substantially affect the solution/CZTS interface during the growth of CBD-CdS films. By deliberately adding Na2S, we prove that it can damage the quality of the CdS film, and show that this compound can be oxidized by using AE or removed by
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wet cleaning to greatly improve the quality of the CBD-CdS. The presented work demonstrate the importance of controlling the Na2S surface compounds, and can be expected to also apply to alternative sulfide buffer materials, that are grown in the chemical bath deposition, for solar cell applications. 2. Experimental section 2.1 Sample preparation A baseline Mo back contact of about 350 nm was DC sputtered on soda lime glass (SLG) substrate with the size of 2.5×2.5 cm2. Cu-Zn-Sn-S precursor films were then reactively co-sputtered (Von Ardenne CS600, with a three-target configuration) onto the Mo coated SLG, using an 1:1 H2S/Ar mixed gas, as described in our previous work.14 The power densities on the 4-inch CuS, Zn and Sn targets were 2.06 W/cm2, 5.80 W/cm2 and 1.36 W/cm2 respectively. The thickness of the precursor film was about 1400 nm. The base pressure of the system was below 10-4 Pa. The substrate holder temperature was kept at about 180 °C during the deposition. The as sputtered films were subsequently annealed in a homemade tube furnace. The precursor films were loaded into a closed graphite box of 5×5 cm2 size along with about 80 mg elemental sulfur. In the annealing system, a fast ramping (cooling) thermal process can be realized by transferring the samples from a cold zone into a heated zone, and vice versa. Our baseline annealing process is composed of annealing at 560 °C for 10 min under a static Ar atmosphere (35 kPa).14 XRF confirmed that the composition of the annealed CZTS film was Cu/Sn~1.94±0.02 and Zn/Sn~1.33±0.02. After the annealing process, the CZTS samples were placed under the ambient in the cleanroom at
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Uppsala University with controlled 70% humidity and temperature at 20 °C. The group of the samples is summarized in table 1. Cadmium sulfide (CdS) film was deposited using the chemical bath deposition process (CBD) with a solution containing 0.005M cadmium acetate, 0.07M Thiourea and 1.14M Ammonia. The temperature of the water bath was kept at 60 °C during the CBD. Such conditions start to promote CdS colloids condensing on the substrate after about 2min immersion in the solution based on a previous study.15 Table 1 Summary of the different surface treatment of the annealed CZTS films; “AE” is “air exposure”; the transport time is the estimated time of transporting the sample in ambient from the annealing furnace in the cleanroom to the XPS chamber at Uppsala University. The 2 min KCN etching for sample D extended the transport time for sample A. Sample
AE time (h)
KCN etching time (min)
Trans. time (min)
CBD batch No.
A
0
/
< 10
1
B
1.5
/
