Virtually Bare Nanocrystal Surfaces: Significantly Enhanced Electrical Transport in CuInSe2 and CuIn1−xGaxSe2 Thin Films upon Ligand Exchange with Thermally Degradable 1-Ethyl-5-Thiotetrazole



A facile and safe ligand exchange method for readily synthesized CuInSe2 (CIS) and CuIn1-xGaxSe2 (CIGS) nanocrystals (NCs) from oleylamine to 1-ethyl-5-thiotetrazole, preserving the colloidal stability of the chalcopyrite structure, is presented. 1-Ethyl-5-thiotetrazole as thermally degradable ligand is adapted for the first time for trigonal pyramidal CIS (18 nm), elongated CIS (9 nm) and CIGS NCs (6 nm). Exchanged NC solutions are processed onto gold electrodes yielding ordered thin films. These films are thermally annealed at 260 °C to completely remove 1-ethyl-5-thiotetrazol leaving individual closely assembled NCs with virtually bare surfaces. The current–voltage characteristics of the NC solids are measured prior to ligand thermolysis in the dark and under illumination and after ligand thermolysis in the same manner. The conductivity of trigonal pyramidal CIS increases by four orders of magnitude (1.4 × 10−9 S cm−1 (dark) to 1.4 × 10−5 S cm−1 (illuminated)) for ligand-free NC films. Elongated CIS NC films show a three orders of magnitude conductivity increase and CIGS NC films exhibit improved conductivity by two orders of magnitude. 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. This approach for the first time offers the possibility to address chalcopyrite materials’ electrical properties in a virtually ligand-free state.