Semiconductor inks for solution processed electronic thin‐films

Solution processing of inorganic semiconductors for the fabrication of affordable, scalable electronic thin‐films is emerging as a commercially viable alternative to traditional high vacuum deposition methods. Using this approach, semiconductor thin‐films can be printed onto a various substrates (e.g. plastics, glasses, or textiles) at low temperatures using existing high‐throughput technologies. In this work, we have developed a number of strategies to synthesize, understand and enhance nanocrystalline and molecular inks for the deposition of high‐quality inorganic semiconducting films. ❧ Compositionally controlled SnₓGe₁₋ₓSe nanocrystal inks were synthesized for the first time as an environmentally sound alternative to lead‐ and cadmium‐based chalcogenides. The band gaps, lattice parameters, and morphologies of the alloys were easily tuned via nanocrystal composition. Interestingly by alloying we showed facile band gap tuning, which is otherwise difficult to achieve in layer structure IV–VI semiconductors. ❧ As synthesized, semiconductor nanocrystal inks are not ready for use in nanocrystal‐based devices due the presence of electrically insulating long‐chain organic surface ligands. While these native ligands ensure chemical and colloidal stability, they must be removed in order to enhance the conductivity of the nanocrystalline inks for electrically conductive films. With this in mind, we have developed a number of ligand exchange techniques whereby the electronically insulating ligands on cadmium selenide (CdSe) nanocrystals are replaced with smaller molecules to enhance the electron mobility of the resulting thin‐films. In one approach we installed small chalcogenol ligands via the in situ reduction of R₂E₂ (R = ᵗBu, Bn, Ph; E = S, Se, Te) by diphenylphosphine (Ph₂PH). The nanocrystal photophysics were investigated using time‐resolved and low temperature photoluminescence measurements. It was found that the phenylchalcogenol ligands rapidly quench the photoluminescence by hole localization onto the ligand. In another approach, molecular stibanates, derived from the dissolution of bulk Sb₂S₃ in a binary ethylenediamine and mercaptoethanol solvent mixture were employed as capping ligands. Photoelectrochemical measurements on stibinate‐capped CdSe nanocrystal films showed that this novel ligand leads to a > 25‐fold increase in photocurrent response relative to as‐prepared CdSe nanocrystal films. ❧ Molecular inks are an attractive alternative to nanocrystalline based inks because they do not require laborious synthesis, purification or surface tuning steps. In this work, tin sulfide based inks were made by dissolving bulk tin precursors in an ethylenediamine (en) / ethanedithiol (EDT) solvent mixture. Despite numerous publications utilizing this solvent system, there is no direct information regarding the chemical identity of the dissolved species. Bisethanedithiolate tin(II) was identified as the single tin species present in all three (Sn, SnS and SnO) / en / EDT solutions, despite the various bulk xiv precursor compositions (Sn, SnO and SnS) and oxidation states (Sn⁰ and Sn²⁺). All three inks can be converted to SnS using a mild annealing step (~350 ℃). Therefore, using a simple “dissolve and recover” process, cheap bulk materials can be used as elemental sources for high quality semiconducting inks and thin‐films.