Article Cite This: Chem. Mater. 2018, 30, 6566−6574
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Band Gap Tailoring and Structure-Composition Relationship within the Alloyed Semiconductor Cu2BaGe1−xSnxSe4 Garrett C. Wessler,† Tong Zhu,† Jon-Paul Sun,† Alexis Harrell,† William P. Huhn,† Volker Blum,*,†,‡ and David B. Mitzi*,†,‡ †
Department of Mechanical Engineering and Materials Science and ‡Department of Chemistry, Duke University, Durham, North Carolina 27708, United States
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S Supporting Information *
ABSTRACT: Recently, the I2−II−IV−VI4 (I = Cu, Ag; II = Ba, Sr; IV = Ge, Sn; VI = S, Se) materials family was identified as a promising source of potential new photovoltaic (PV) and photoelectrochemical (PEC) absorbers. These materials avoid the pitfalls of the successful photovoltaic semiconductors Cu(In,Ga)(S,Se)2 and CdTe, as they do not contain scarce (In, Te) or toxic (Cd) elements. Furthermore, ionic sizes and coordination preferences are very different for the I, II, and IV cations in the I2−II−IV−VI4 family, providing an intriguing avenue to avoid intrinsic antisite disordering that limits efficiency improvement in Cu2ZnSn(S,Se)4 (where Cu and Zn can easily substitute for one another). Here, we experimentally and computationally explore alloys Cu2BaGe1−xSnxSe4 (CBGTSe, 0 ≤ x ≤ 1) to fine-tune the structural, optical, and electronic properties for the relatively large band gap (Eg = 1.91(5) eV) unalloyed compound Cu2BaGeSe4 (CBGSe). We show that CBGTSe maintains the P31 crystal structure type of the parent CBGSe up to x ≤ 0.70. A minimum band gap value of 1.57(5) eV can be reached at x = 0.70 before the structure transforms to the Ama2 structure type. The experimental and theoretical investigations demonstrate the potential of CBGTSe for thin-film PV and PEC absorbers.
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INTRODUCTION Thin-film photovoltaic (PV) technologies based on chalcogenide semiconductors, most importantly Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe, have had success in the research and industrial spaces, with lab-scale power conversion efficiencies exceeding 22%.1 However, CdTe and CIGSSe may ultimately be limited in their global deployment by the reliance of each material on scarce (i.e., In, Te) or toxic (i.e., Cd) elements. Thin-film solar cells based on the kesterite Cu2ZnSn(S,Se)4 (CZTSSe) absorber emerged from the desire to replace the toxic and scare elements with benign and earth-abundant elements Zn and Sn in these PV technologies. While there is a great interest in CZTSSe as an absorber material, the efficiency of research-scale devices has stagnated at 12.6% for the past several years.2,3 This efficiency stagnation stems at least in part from significant CuZn and ZnCu antisite disordering in CZTSSe, leading to potential fluctuations in the electronic band structure and band tailing, which in turn effectively limit the PV device open circuit voltage (Voc).4 Additionally, CuSn, SnCu, ZnSn, and SnZn antisite defects can contribute defect energy levels deep within the band gap of CZTSSe and increase recombination in CZTSSe PV devices, further hampering the power conversion efficiency.5 In the CZTSSe lattice, Cu, Zn, and Sn cations have a similar tetrahedral coordination and comparable ionic radii. These similarities contribute to small antisite defect formation energies and their prevalence in CZTSSe.5 One method to © 2018 American Chemical Society
suppress the antisite disorder is to replace the Zn ion with a much larger alkaline earth cation such as Ba or Sr, introducing structural and ionic size diversity into the lattice. Cu2BaSnS4 (CBTS) and Cu2SrSnS4 (CSTS) structures adopt the trigonal space group P31,6,7 whereas the fully Se-substituted relatives Cu2BaSnSe4 (CBTSe) and Cu2SrSnSe4 (CSTSe) crystallize in the related orthorhombic Ama2 structure type (structural differences between these structures and CZTSSe are shown in Figure S1 of the Supporting Information, SI).8,9 These and other semiconductors within the larger family of I2−II−IV− VI4 (I = Cu, Ag; II = Ba, Sr; IV = Ge, Sn; VI = S, Se) materials form structures in which the II atom is 8-fold coordinated while the I and IV atoms maintain the tetrahedral coordination as in CZTSSe. The difference in coordination and ionic size between the II atom and the I/IV atom is predicted to hinder the formation of antisite defects and can decrease the amount of band tailing within films and devices based on these semiconductors.10,11 For example, Cu2BaSn(S,Se)4 (CBTSSe) has garnered interest as a potential alternative solar0 absorber to CZTSSe.11−18 Initial P31 space group-based CBTSSe solar cells and photoelectrochemical (PEC) devices have exceeded 5% power conversion efficiency and a photocurrent of 12 mA/ cm2 at 0 V vs the reversible hydrogen electrode (RHE), Received: August 8, 2018 Revised: August 28, 2018 Published: August 28, 2018 6566
DOI: 10.1021/acs.chemmater.8b03380 Chem. Mater. 2018, 30, 6566−6574
Article
Chemistry of Materials
Aesar, 99.999%), and Se (Alfa Aesar, 99.999%) in an agate mortar and pestle and cold-pressing into a pellet inside a nitrogen-filled glovebox. Prior to weighing, the BaSe and CuSe precursor powders were first baked on a hot plate, in an inert atmosphere, at 175 °C to remove slight SeO2 impurities. The pellets were loaded into prebaked quartz tubes, evacuated to ∼5 × 10−6 Torr and flame-sealed under dynamic vacuum using a hydrogen−oxygen torch. The samples were then heated to 600 or 650 °C (x = 1.00) with a ramping rate of 100 °C/h and held at these temperatures for 24−60 h. The grinding, pressing, and heating processes were repeated 1−2 times for each sample without exposure to air in order to improve phase purity. During the first heating, the pellets expanded and became fragile as the initial reaction proceeded, with an average mass loss of 6.9 mg noted. After the first heating, the pellets and powders were dark gray. During subsequent heating steps, the pellets did not deform and
