  We present a facile and safe ligand exchange method for readily synthesized
CuInSe2 (CIS) and CuIn(1-x)Ga(x)Se2 (CIGS) nanocrystals (NCs) from oleylamine
to 1-ethyl-5-thiotetrazole which preserves the colloidal stability of the
chalcopyrite structure. 1-ethyl-5-thiotetrazole as thermally degradable ligand
is adapted for the first time for trigonal pyramidal CIS NCs (18 nm), elongated
CIS NCs (9 nm) and CIGS NCs (6 nm). The exchanged NC solutions are spin-coated
onto Si/SiO2 substrates with predefined gold electrodes to yield ordered NC
thin films. These films are thermally annealed at 260 C to completely remove
1-ethyl-5-thiotetrazol leaving virtually bare NC surfaces. We measure the
current-voltage characteristics of the NC solids prior to ligand thermolysis in
the dark and under illumination and after thermolysis of the ligand in the same
manner. The conductivity of trigonal pyramidal CIS NCs increases by four orders
of magnitude from 1.4*10E-9 S/cm in the dark to 1.4*10E-5 S/cm for ligand-free
illuminated NC films. Elongated CIS NC films show an increase by three orders
of magnitude and CIGS NC films exhibit improved conductivity by two orders of
magnitude. The degree of conductivity enhancement thereby depends on the NC
size accentuating the role of trap-states and internal grain boundaries in
ligand-free NC solids for electrical transport. Our approach offers for the
first time the possibility to address chalcopyrite materials' electrical
properties in a virtually ligand-free state.
