Sample composition determined by EDX, as well as FWHM of (112) peaks, XRD peak ratios I₍₁₁₂₎/I₍₂₂₀₎, optical band gap and photoluminescence peak position.

Sample	Cu/(Zn + Sn)	Zn/Sn	Cu/Sn	FWHM(112) (deg)	I (112)/I(220)	E g (eV)	PL max (eV)
No alkali	0.73	1.63	1.92	0.15	2.69	1.52	—
Li	0.73	1.74	2.00	0.20	2.43	1.51	1.35
Na	0.75	1.70	2.01	0.15	2.53	1.52	1.33
K	0.80	1.73	1.89	0.11	3.64	1.43	1.33