Effect of the order-disorder transition on the optical properties of Cu2ZnSnS4

The effect of the order-disorder transition on the band gap of kesterite Cu2ZnSnS4, an interesting material for solar cells, has been investigated by optical spectroscopy. The band gap energy (E-g) decreases continuously with increasing annealing temperature, T-a, and reaches its minimum at T-a similar to 273 degrees C. E-g is about 200 meV higher in the most ordered state, than in the fully disordered state. Its value and the transition kinetic depend on the sample stoichiometry. A simplified model able to explain the order degree and stoichiometry effects on the E-g value is developed. Ordering results in narrower Raman peaks without affecting the shape of the photoluminescence spectrum-except for the change in E-g-or the characteristic energy of the exponential tail below the fundamental absorption edge. Although a prolonged annealing increases the order degree, the material properties are still influenced by residual disorder as well as by defects related to the off-stoichiometry composition. Published by AIP Publishing.