Reaction pathways and optoelectronic characterization of single-phase Ag 2 ZnSnS 4 nanoparticles
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School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA Davidson School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, USA a) Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] 2
Received: 19 July 2019; accepted: 8 October 2019
Sputtered thin films of Ag2ZnSnS4 (AZTS) have shown promising semiconducting properties in spite of the films containing SnS2, SnSx, or ZnS as impurity phases. In this study, reaction pathways were identified to produce single-phase AZTS nanoparticles as precursors for forming dense, single-phase films. The morphology, composition, and phase evolution during nanoparticle formation in an oleylamine-based solvothermal reaction process were determined using surface-enhanced Raman spectroscopy (SERS) and transmission and scanning transmission electron microscope (TEM/STEM). The reaction pathways for AZTS nanoparticles were found to be different from Cu2ZnSnS4 nanoparticles in oleylamine, which may explain the difficulty in creating (Ag, Cu) 2ZnSnS4 solid solutions in the nanoparticle synthesis. The single-phase AZTS nanoparticle films have a band gap (2.16 eV) slightly higher than sputtered films, and photoelectrochemical (PEC) measurements demonstrated a current of 0.1 mA/cm2 in K2SO4 solution even as porous nanoparticle films, suggesting the potential of this material in solar energy conversion when converted into a dense film.
Introduction Although CZTSSe has been the subject of significant research in the last 15 years for its light absorption properties in thinfilm solar cells [1, 2], the efficiency of Cu2ZnSn(S, Se)4 (CZTSSe) solar cells has remained at 12.6% since 2013, limited by intrinsic defects [3]. With the realization that Cu1 and Zn21 antisite defects make the open circuit voltage (Voc) the bottle neck of CZTSSe solar cells [4, 5, 6, 7, 8, 9, 10], researchers have focused their effort in reducing the defect density by substituting atoms with different sizes onto Cu, Zn, or Sn sites to increase the antisite defect formation energy. The modified materials based on CZTSSe include Cu2Zn(Ge, Sn)(S, Se)4 [11, 12, 13], Cu2CdSn(S, Se)4 [14], and (Cu, Ag)2ZnSnSe4 (CAZTSe) [10, 15, 16, 17, 18]. One of the successful examples is substituting Ag for Cu, whose covalent radius is about 30% larger than that of Cu. This resulting Ag2ZnSnSe4 (AZTSe) has been proved to have lower densities of donor- or acceptor-type defect states than pure CZTSSe [18, 19]. Silver-containing CZTSSe solar cells were first reported in 2016 by two groups. Hages et al. [10] reported p-type CZTSSe with partial substitution of Cu with up to 50% Ag, and Gershon
ª Materials Research Society 2019
et al. [16, 20] reported n-type AZTSe in the absence of Cu. The n-type AZTSe layer was fabricated through coevaporation of Ag, Zn, Sn, and cracked Se sources, and the resulting device achieved about 5% efficiency. As an alternative to vacuum-based deposition techniques, solution processing offers a cost-effective option for deposit
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