Article pubs.acs.org/cm
Microreactor Chemical Bath Deposition of Laterally Graded Cd1−xZnxS Thin Films: A Route to High-Throughput Optimization for Photovoltaic Buffer Layers Kevin M. McPeak,†,⊗ Borirak Opasanont,† Tomohiro Shibata,‡,# Dong-Kyun Ko,§,△ Matthew A. Becker,∥ Soma Chattopadhyay,‡,# Holt P. Bui,⊥ Thomas P. Beebe, Jr.,⊥ Bruce A. Bunker,∥ Christopher B. Murray,§ and Jason B. Baxter*,† †
Department of Chemical & Biological Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States ‡ Advanced Photon Source, Argonne National Laboratory, Bldg 433B, CSRRI-IIT, MRCAT Sector 10, Argonne Illinois 60439, United States # Physics Department, Illinois Institute of Technology, Chicago, Illinois 60616, United States § Department of Chemistry and Department of Materials Science & Engineering, University of Pennsylvania, 231 S. 34th Street, Philadelphia, Pennsylvania 19104, United States ∥ Physics Department, University of Notre Dame, Notre Dame, Indiana 46556, United States ⊥ Department of Chemistry and Biochemistry, University of Delaware, 175 Brown Laboratory, Newark, Delaware 19716, United States S Supporting Information *
ABSTRACT: Cd1−xZnxS (CdZnS) is a promising replacement for the CdS buffer layers in copper indium gallium (di)selenide (CIGS) solar cells because the wider band gap of CdZnS offers improved optical transmittance of blue light. Chemical bath deposition (CBD) is the state-of-the-art deposition method for CdS and CdZnS. However, CBD of CdZnS is poorly understood, and relationships between bath composition and stoichiometry, microstructure, and optoelectronic properties of the deposited film are lacking. We introduce CBD using a continuous flow microreactor as a new technique to rapidly explore a wide variety of deposition conditions on a single substrate using spatially dependent characterization. X-ray diffraction and X-ray absorption spectroscopy indicate that the film is a mixture of nanocrystalline CdZnS and amorphous Zn(O,OH,S). Over the length of a single substrate, films showed increasing Zn:Cd ratio in the nanocrystalline phase, increasing amorphous content, and increasing quantum confinement, and resultant monotonic increase in band gap from 2.42 to 2.75 eV. Microreactor CBD (μR-CBD) enables rapid identification of CdZnS compositions that are ideal candidates for thin film photovoltaics, as well as determination of the CBD conditions required to deposit them. KEYWORDS: CdZnS, chemical bath deposition, microreactor, photovoltaic, cadmium zinc sulfide, zinc oxysulfide
1. INTRODUCTION
properties across the substrate. We also use X-ray diffraction and X-ray absorption spectroscopy to show strong evidence for a nanocrystalline CdZnS solid solution mixed with amorphous Zn(O,OH) across a wide range of stoichiometries. CdZnS is a II−VI semiconductor that has shown promise as an alternative buffer layer to CdS in thin film photovoltaics.2−5 CdS provides a high quality p-n junction when interfaced with absorbers such as copper indium gallium (di)selenide (CIGS). State-of-the-art CIGS thin film solar cells have shown record
This paper introduces microreactor chemical bath deposition (μR-CBD) as a new technique to deposit ternary semiconductor thin films with graded composition along the length of a single substrate. The graded composition results in graded material properties; and spatially dependent characterization of the substrate enables rapid and unprecedented understanding of the effect of growth conditions on optical, electronic, and structural properties of the material. μR-CBD uses a continuous flow microreactor (CFμR), which we have previously described for deposition of ZnO nanowire arrays.1 Here we report the deposition of Cd1−xZnxS (CdZnS) thin films with wide variations in stoichiometry, microstructure, and optoelectronic © 2013 American Chemical Society
Received: July 25, 2012 Revised: December 12, 2012 Published: January 8, 2013 297
dx.doi.org/10.1021/cm3023506 | Chem. Mater. 2013, 25, 297−306
Chemistry of Materials
Article
allows the relationship between bath composition and material properties to be probed by spatial characterization. A few reports exist on the use of continuous flow systems to deposit materials on substrates confined within narrow channels,21,22 although no reports have been published in which a continuous flow reactor was used to intentionally grow films with wide variations in material properties across the substrate. μR-CBD provides a straightforward method to probe the influence of bath composition on material properties.
efficiencies of 19.9−20.3% using a very thin (
