Two-step sintering of nanocrystalline Cu(In0.7Ga0.3)Se2

A dense selenide ceramic can be obtained by hot-pressing to eliminate the large pores originated from agglomeration of nanparticles. However, the compounds of In and Ga in Cu(In0.7Ga0.3)Se2 (CIGS) easily evaporate after hot-press sintering for a longer time. In this study, a dense CIGS ceramic with uniform microstructure is obtained using a two-step sintering process, first hot-press sintering for a short time to break up the agglomerates and then pressureless sintering at 550 °C under a Se2 atmosphere to densify and enhance grain growth. CIGS with a relative density higher than 75% can be obtained through the hot-press sintering CIGS nano-particles prepared using the heating-up method at 500–550 °C for 10 min. Hot-press sintering effectively breaks up the agglomerates and promotes the nano-particle rearrangement. CIGS relative density is promoted to 91% by sequentially sintering at 550 °C for 1–3 h under Se2 atmosphere. UV–vis–NIR spectroscopy measurements show that the energy gap of the sample after hot pressing and Se2 atmosphere sintering is about 1.11 eV, which is close to the CIGS theoretical optimum band-gap range. The prepared CIGS targets exhibited p-type semiconductor characteristics and the carrier concentration decreased, but the Hall mobility increased, with increasing soaking time at 550 °C.