Crystallization properties of Cu2ZnGeSe4

Highlights: • Synthesis of thin film Cu₂ZnGeSe₄ layers by H₂Se selenization of stacked metals. • Stack with Ge on top - > slower reaction (15 min). • Stack with Ge on bottom - > faster reaction (3 min). • Copper germanide phase formed very rapidly in selenization process. -- Abstract: We have studied the crystallization reaction of polycrystalline Cu₂ZnGeSe₄ solar cell absorbers fabricated by H₂Se selenization of sequentially deposited metal layer stacks. We have executed a stop experiment, stopping the crystallization reaction at different times during the process, then analyzing the subsequent X-ray diffraction patterns. We have found that mainly Cu₃Ge and ZnSe phases form very rapidly at temperatures below 350 °C. Depending on the order of the sequentially deposited metal layer stack, the formation reaction proceeds at different speeds. Putting the Ge layer in the bottom and the Cu layer on top leads to a very fast nucleation reaction of Cu₂ZnGeSe₄, leading to small grains that have obtained their final size already after 3 min of selenization at 460 °C. The inverse stack with Ge on top and Cu in the bottom delays the nucleation of Cu₂ZnGeSe₄, leading to a somewhat slower formation reaction and larger Cu₂ZnGeSe₄ grains, which obtain their final grain size only after 15 min of selenization at 460 °C.