Article pubs.acs.org/JPCC
Precursor Stack Ordering Effects in Cu2ZnSnSe4 Thin Films Prepared by Rapid Thermal Processing Andrew Fairbrother,† Lionel Fourdrinier,‡ Xavier Fontané,† Victor Izquierdo-Roca,† Mirjana Dimitrievska,† Alejandro Pérez-Rodríguez,†,§ and Edgardo Saucedo*,† †
Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, Barcelona, Spain AC&CS − CRM Group, Boulevard de Colonster B57, 4000 Liege, Belgium § IN2UB, Departament d’Electrònica, Universitat de Barcelona, C. Martí Franquès 1, 08028 Barcelona, Spain ‡
S Supporting Information *
ABSTRACT: Solar cells based on Cu2ZnSnSe4 thin film absorber layers have shown promise as an alternative to more mature thin film technologies because they are composed of more earth abundant elements. To increase device efficiencies there is still much to be investigated about its properties, and film and device processing. Rapid thermal processing is a more industrially viable method of forming thin film absorber layers than time intensive conventional thermal processing. However, optimized conditions for conventional processing are not readily transferable to rapid thermal processing. Thermal processing of this material is complicated through loss of volatile components such as Zn and Sn−Se, in addition to decomposition at elevated temperatures. In this study the effect of stack order has been investigated for Cu−Zn(O)−Sn precursor stacks selenized by rapid thermal processing to form Cu2ZnSnSe4 thin films. Precursor stack ordering is shown to have significant effects on the film properties, including precursor alloy formation, composition and elemental loss, morphology, and secondary phase formation and distribution. Optoelectronic properties of devices prepared from these films also show a dependence on precursor stack order. Most notable is the poor performance of devices with Zn as a bottom layer, due to excessive ZnSe formation at the back contact region. The viability of ZnO as a precursor layerin place of volatile Znis also investigated, and shown to not completely react in the Se atmosphere, leaving residual oxygen in the Cu2ZnSnSe4 films. The best performing device has a conversion efficiency of 4.3%, and uses a stack order of glass/Mo/Sn/Zn/Cu.
1. INTRODUCTION Photovoltaic energy conversion is gaining an increasingly important role in local and national energy planning, with significant amounts of funding dedicated to research and commercialization. Due to a variety of limiting factors for silicon-based technologies the development of thin film solar cell technologiesprincipally Cu(In,Ga)Se2, CdTe, and aSi:Hhas increased significantly in the past decade. While at a relatively early stage of industrialization, there is some uncertainty about the long-term viability of the technologies from a resource point of view because of the use of scarce and near-critical elements such as In, Ga, Cd, and Te.1 Thus, the search for alternative thin film absorber materials has begun with significant effort in recent years, of which Cu2ZnSnSe4 (CZTSe) has received considerable attention.2−5 While a promising material, the record device efficiency is just 9.2%,2 far below the record efficiencies for Cu(In,Ga)Se2- and CdTebased technologies. CZTSe thin films and devices have been prepared by a wide variety of techniques including physical vapor deposition,2,6 chemical vapor deposition, electroplating,3 and solvent and nanoparticle printing.7 Additionally, these methods have been employed with use of both metallic or multinary compound © 2014 American Chemical Society
precursors. In nearly all cases a high-temperature process is used to react the precursors to form CZTSe and improve crystalline quality of the films. Herein lays a significant challenge for this technology: while at elevated temperatures there exists the possibility for decomposition and loss of volatile components such as metallic Zn and Sn−Se.8−10 Decomposition may also occur in the conventionally employed Mo back contact region.11,12 In addition to affecting CZTSe growth, elevated temperatures also lead to the selenization of the Mo back contact, which is increased at higher temperatures.6 It is important to sufficiently anneal the thin films for high crystalline quality grains, but also to limit the detrimental effects of elemental loss and decomposition. Rapid thermal processing (RTP) is advantageous in this regard because processing times are much shorter than conventional thermal processes (CTP), thus allowing kinetically driven grain growth processes to proceed more quickly than thermodynamically driven decomposition reactions. Received: April 15, 2014 Revised: June 27, 2014 Published: July 7, 2014 17291
dx.doi.org/10.1021/jp503699r | J. Phys. Chem. C 2014, 118, 17291−17298
The Journal of Physical Chemistry C
Article
2. EXPERIMENTAL SECTION 2.1. Precursor Film Deposition. To form CZTSe thin films as absorber layers for solar cells, precursor metallic stacks were annealed in a selenium-containing atmosphere. The precursor stacks were prepared by DC-magnetron sputtering deposition of Cu, Zn, and Sn (Alliance Concepts Ac450) and pulsed DC-magnetron sputtering deposition of ZnO (Alliance Concepts CT100) onto Mo-coated (t = 800 nm, R□ = 0.35 Ω/ □) soda-lime glass substrates. This deposition process is described in greater detail in ref 13. Here, and throughout this work, annealed films are referred to by their precursor stack order, in the order of bottom/middle/top layers. A total of six different stack orders were tested: Zn/Cu/Sn, Zn/Sn/Cu, Sn/ Cu/Zn, Sn/Zn/Cu, Sn/Cu/Zn/ZnO, and Sn/Cu/ZnO. Cu was not used as a bottom layer in any of the stacks because these sequences were selected to protect at least one of the volatile elements (Zn or Sn−Se), and also investigate the loss dynamics of Zn or Sn as a top layer. The total thickness of the precursor stack was approximately 600 nm (slightly higher when using ZnO instead of Zn), and the composition of the films was near the range of those reported as ideal for highefficiency solar cell devices, namely Cu-poor and Zn-rich:2,6 Cu/(Zn+Sn) = 0.70−0.85 and Zn/Sn = 1.10−1.30. For the stack order with both Zn and ZnO, Zn was deposited to obtain a stoichiometric value with Sn (Zn/Sn ≈ 1.00), and the Znexcess (≈ 0.16) was deposited in the form of ZnO. While oxygen is generally expected to be detrimental to device performance because of the possible formation of high bandgap oxide phases, ZnO was introduced as a Zn precursor in two of the stack orders to minimize Zn-loss during annealing. Zn has the lowest vapor pressure of the metals in the Cu−Zn−Sn system, and is therefore most susceptible to loss during annealing, at least before the less volatile ZnSe phase forms. In principle ZnO may react to form ZnSe and then CZTSe during the RTP, and thus displace all oxygen in the final film. 2.2. Thermal Processing. Precursors were then selenized by rapid thermal processing (RTP) in an AnnealSys AS-ONE100 furnace in a graphite receptor with 8 cm3 volume. Approximately 25 mg of Se (Neyco, 99.999% purity) was placed near the precursor films, and after purging and attaining a sufficient base vacuum (
