(Cu1‐xAgx)2ZnSnS4 (CAZTS) and (Cu1‐xAgx)2ZnSnSe4 (CAZTSe) solid solutions with 0 ≤ x ≤ 1.0 were synthesized. Their crystal structures were analyzed by Rietveld refinement of X‐ray diffraction data. The refined lattice constants a of the kesterite‐type CAZTS and CAZTSe increase with Ag content (x) increasing, while their lattice constants c slightly decrease. Therefore, by increasing x, the c /a ratio decreases from 2 for Cu‐based compounds to a value lower than 1.9 for Ag‐based compounds. The band‐gap energies (Eg) of CAZTS and CAZTSe solid solutions were determined by their diffuse reflectance spectra. The reflectance edges of the CAZTS and CAZTSe shifted to the longer wavelength side with Ag content increasing at 0.0 ≤ x ≤ 0.2, while they shifted to the shorter wavelength side at 0.2 ≤ x ≤ 1.0. By increasing x, the Eg of CAZTS decreases from 1.49 eV (x=0.0) to 1.47 (x=0.2) and then increases to 2.01 eV (x=1.0); and the Eg of CAZTSe decreases from 0.98 eV (x=0.0) to 0.95 (x=0.2) and then increases to 1.34 eV (x=1.0). There is small bowing in the band gaps of the CAZTS and CAZTSe systems. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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