Synthesis Of CuInS₂ And Cu₂ZnSnS₄ Nanostructures In Polyalcohol And Fabrication Of Corresponding Thin Films Based On Inks

Multi-chalcogenide materials, such as CuInS2 (CIS) and Cu2ZnSnS4 (CZTS), are promising solar cell absorber layer materials with great research and application prospects. The syntheses of their nanostructures and thin films have attracted much attention during these years, especially thin films syntheses based on nanoparticle inks and photovoltaic devices fabrication in nanoscales. This paper focuses on the syntheses of compound nanostructures and corresponding thin films based on inks. On one hand, by using polyalcohol (mostly diethylene glycol (DEG)) as solvent, CIS and CZTS nanostructures are synthesized via simple precursors; on the other hand, corresponding inks are made by obtained nanostructures and used to fabricate the final stable thin films via sulfurization. In the meanwhile, metallic inks made by nano metal alloys are also used to fabricate CZTS thin films and CIS thin films with special nanostructures. Primary significant results are summarized as follows:(1) Using DEG as solvent, CIS nanostructures with different phase structures and mophologies are synthesized through different agents. Large diameter hexagonal plates with chalcopyrite structure are obtained under "free growth" condition without any agent, while wurzite structures are formed when triethanolamine (TEA) is used as agent. To add polyvinylpyrrolidone (PVP) or hexadecyl trimethyl ammonium bromide (CTAB), both chalcopyrite structures are obtained, but with PVP, the CIS appear polyhedrons with sizes about 100 nm, while with CTAB, flower-like structures with diameters about 500 nm are obtained. It is indicated that the synthesized CIS nanostructures can be adjusted both on phase and morphologies through different agents.(2) CIS thin films are synthesized based on inks which made by as-synthesized CIS nanostructures. Chalcopyrite phase CIS thin films with good crystalline and optical properties are obtained. The density and uniformity of the thin films are strongly related to the size and shape of the nanostructures. Small size and regular shape are benefit to the sulfurization. This result also indicates the importance of the adjustment of the size and shape of CIS nanostructures. Further, the as-synthesized CIS nanostructures are used to fabricate FTO/TiO2/CIS/C and FTO/TiO2/In2S3/CIS/C heterojunctions. Obvious photovoltaic properties with a highest converse efficiency of 0.29% are obtained under illumination.(3) CIS thin films with nanorod array structures are obtained via Cu-In alloy precursors sulfurization under H2S gas in RTP. This is the first report of CIS nanorod arrays synthesized by non-vacuum and free template route. The obtained CIS thin films are composed by three layers. The nanorod arrays are at the top, and compact films with large grains compose the middle layer, while porous films composed by small grains are at the bottom. Compared with normal thin films, the nanorod array structures have lower reflectance in the visible spectral region, resulting in a good light trapping effect. The obtained CIS thin films with nanorod arrays show a conversion efficiency of 0.34% in a two-electrode photoelectrochemical cell. Free standing CIS thin films with the same structures are achieved on Si wafer substrates, which is also the first report in corresponding field.(4) DEG is used as solvent to synthesize CZTS nanostructures. It is found that Zn has a very slow reaction rate under this condition. The products are mainly Cu2SnS3 after 30 min reaction. The ratio of Zn raise up with the reaction time expended, but it is difficult to obtain pure CZTS phase. The as-synthesized Cu2SnS3 can be used as inks sulfurized with ZnO thin films to fabricate CZTS thin films. However, since the low reactivity between the two compounds during the sulfurization, the thin films are composed with small grains, and can hardly form the ideal compact and uniform films.(5) Cu-Sn metallic inks combination with ZnO are involved to synthesize CZTS thin films. This is a kind of new method based on inks to form CZTS thin films. The optimum reaction parameters for syntheses Cu-Sn alloy particles using reduce method are obtained after systematic studies. The as-synthesized Cu-Sn alloy particles are mainly Cu6Sn5 phase with regular spherical shape about 50-80 nm. The Cu-Sn metallic inks are applied to combine with ZnO thin films through different deposition routes. It is found that the react activity and size of the ZnO particles in ZnO layer have large influences on the final CZTS films. Compact and uniform CZTS thin films with pure phase and good crystalline are synthesized based on spin-coating ZnO films. The obtained thin films covered on n-type Si to form Si/CZTS heterojunction shows obvious photovoltaic property under illumination.