Virtually Bare Nanocrystal Surfaces: Significantly Enhanced Electrical Transport in CuInSe₂ and CuIn_1−xGa_xSe₂ Thin Films upon Ligand Exchange with Thermally Degradable 1‐Ethyl‐5‐Thiotetrazole

Abstract

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.

Quantifying Angular Correlations between the Atomic Lattice and the Superlattice of Nanocrystals Assembled with Directional Linking

“…34 Since the optoelectronic properties of PbS NC ensembles bear many opportunities for applications in solar cells or photodetectors, a number of ligand exchange procedures with small organic or inorganic molecules as well as single atom passivation strategies have been developed, all of which greatly increase the carrier mobilities within the SL of NCs. 28,33,[35][36][37][38][39][40][41][42][43][44] Due to the short interparticle spacing imposed by these ligands, structural coherence is mostly lost in such superlattices, but in rare cases it has been demonstrated that significant long-range order and even mesocrystallinity can be preserved. 25,35,45 However, a persisting problem of these protocols is that they are prone to introduce defects in the superlattice structure with some degree of granularity and significantly smaller grain sizes, which poses difficulties in determining the angular correlation with a meaningful statistical distribution.…”

Tetrazoles: Unique Capping Ligands and Precursors for Nanostructured Materials

“…Therefore, pre-synthesized NPs should be subjected to ligand exchange with tetrazoles, such as the substitution of weakly bonding citrate on the surface of gold nanocrystals, [ 30 ] or oleylamine on CuInSe 2 and CuIn 1− x Ga x Se 2 NPs. [ 31 ] …”

“…This strategy has recently been realized by the Weller group with the synthesis of 1-ethyl-5-thiotetrazolecapped CuInSe 2 (CIS) and CuIn 1− x Ga x Se 2 (CIGS) NPs. [ 31 ] These particles, originally synthesized at 240 °C www.small-journal.com four orders of magnitude higher than that of fi lms with OLAcapped NPs.…”

Postdeposition Ligand Exchange Allows Tuning the Transport Properties of Large‐Scale CuInSe2 Quantum Dot Solids

“…In general, hopping occurs to a site with the combination of shortest distance and lowest activation energy, described by the hopping probability P ∝ exp (−2 d / a − Δ E / k B T ), where d is the hopping distance, a the localization length, and Δ E is the activation energy, which depends on the electronic environment . An obvious approach to enhance the hopping transport is the reduction of the interparticle distance by utilizing short connecting ligands . Another, presently less elaborated approach is the tuning of the activation energy, mainly the electronic states between the QD's conduction and valence band states (CB and VB) .…”