Engineering of CIGS nanoparticle inks for colloidal stability, uniform film formation and application as HTL for perovskite solar cells

Abstract

In this work, synthesis of CuIn0.75Ga0.25S2 (CIGS) nanoparticles, the formation of stable dispersion, deposition of high-quality films and, fabrication of thin-film Perovskite solar cells are reported. The stability of nanoparticle ink is crucial in the formation of device-quality films. The chalcogenide-based materials are widely used in thin-film solar cells; in particular, Cu(In,Ga)S2 are used as an absorber and hole transporting layer. In the present study, the nanoparticles of about 20 nm size and bandgap of 1.5 eV are synthesized using a heat-up method. A variety of solvents are used as dispersing media and the stability of the inks is evaluated by precise optical monitoring. We observe a clear dependence of ink stability to the polarity index of the solvent, where the best stability occurs at a polarity index of about 0.26–0.36, corresponding to a range of solvents including chloroform. The thin films that are spin-coated using CIGS chloroform ink show large cracks, presumably due to the high vapor pressure of chloroform and evaporation-induced stress in the film. We resolve this problem through low-temperature deposition, which resulted in highly uniform pin-hole and crack-free films. Finally, the optimum deposition condition is used to fabricate perovskite solar cells having about 16.5% efficiency with CIGS as a hole transport layer.