Virtually Bare Nanocrystal Surfaces - Significantly Enhanced Electrical Transport in CuInSe2 and CuIn(1-x)Ga(x)Se2 Thin Films upon Ligand Exchange with Thermally Degradable 1-Ethyl-5-thiotetrazole

摘要

We present a facile and safe ligand exchange method for readily synthesizedCuInSe2 (CIS) and CuIn(1-x)Ga(x)Se2 (CIGS) nanocrystals (NCs) from oleylamineto 1-ethyl-5-thiotetrazole which preserves the colloidal stability of thechalcopyrite structure. 1-ethyl-5-thiotetrazole as thermally degradable ligandis adapted for the first time for trigonal pyramidal CIS NCs (18 nm), elongatedCIS NCs (9 nm) and CIGS NCs (6 nm). The exchanged NC solutions are spin-coatedonto Si/SiO2 substrates with predefined gold electrodes to yield ordered NCthin films. These films are thermally annealed at 260 C to completely remove1-ethyl-5-thiotetrazol leaving virtually bare NC surfaces. We measure thecurrent-voltage characteristics of the NC solids prior to ligand thermolysis inthe dark and under illumination and after thermolysis of the ligand in the samemanner. The conductivity of trigonal pyramidal CIS NCs increases by four ordersof magnitude from 1.4*10E-9 S/cm in the dark to 1.4*10E-5 S/cm for ligand-freeilluminated NC films. Elongated CIS NC films show an increase by three ordersof magnitude and CIGS NC films exhibit improved conductivity by two orders ofmagnitude. The degree of conductivity enhancement thereby depends on the NCsize accentuating the role of trap-states and internal grain boundaries inligand-free NC solids for electrical transport. Our approach offers for thefirst time the possibility to address chalcopyrite materials' electricalproperties in a virtually ligand-free state.