The incorporation of colloidal semiconductor nanocrystals into the photoabsorbant material of photovoltaic devices may reduce the production costs of solar cells since nanocrystals can be readily synthesized on a large scale and are solution processable. While the lead chalcogenide IV-VI nanocrystals have been widely studied in a variety of photovoltaic devices, concerns over the toxicity of lead have motivated the exploration of less toxic materials. This has led to the exploration of tin and germanium monochalcogenide IV-VI semiconductors, both of which are made up of earth abundant elements and possess properties similar to the lead chalcogenides. This feature article highlights recent efforts made towards achieving synthetic control over nanocrystal size and morphology of the non-lead containing IV-VI monochalcogenides (i.e., SnS, SnSe, SnTe, GeS and GeSe) and their application toward photovoltaic devices.

Despite their extremely low solubility in most solvents, hexagonal grey selenium and tellurium are shown to be remarkably soluble in binary mixtures of thiols and ethylenediamine (en) at room temperature and ambient pressure. A 1 : 4 vol/vol mixture of ethanethiol (EtSH) and en gave saturated solutions of 38 and 9.3 wt% for grey selenium and tellurium, respectively. Crystalline and phase-pure chalcogen is easily recovered from solution by drying and mild heat treatment at 250 C (for selenium) or evaporation at room temperature (for tellurium). To demonstrate utility for these dissolved chalcogens, it was shown that elemental antimony readily reacts with the dissolved selenium to give a stable, solution processable Sb-Se precursor solution. In the same way, elemental tin reacts with the dissolved tellurium to generate a Sn-Te precursor solution. Upon solution deposition and heat treatment to 250 C, these precursor solutions yielded crystalline Sb 2 Se 3 and SnTe.

We have employed a simple modular approach to install small chalcogenol ligands on the surface of CdSe nanocrystals. This versatile modification strategy provides access to thiol, selenol, and tellurol ligand sets via the in situ reduction of R2E2 (R=tBu, Bn, Ph; E=S, Se, Te) by diphenylphosphine (Ph2PH). The ligand exchange chemistry was analyzed by solution NMR spectroscopy, which reveals that reduction of the R2E2 precursors by Ph2PH directly yields active chalcogenol ligands that subsequently bind to the surface of the CdSe nanocrystals. Thermogravimetric analysis, FT-IR spectroscopy, and energy dispersive X-ray spectroscopy provide further evidence for chalcogenol addition to the CdSe surface with a concomitant reduction in overall organic content from the displacement of native ligands. Time-resolved and low temperature photoluminescence measurements showed that all of the phenylchalcogenol ligands rapidly quench the photoluminescence by hole localization onto the ligand. Selenol and tellurol ligands exhibit a larger driving force for hole transfer than thiol ligands and therefore quench the photoluminescence more efficiently. The hole transfer process could lead to engineering long-lived, partially separated excited states.

Molecular stibanates derived from the dissolution of bulk Sb2S3 in a binary ethylenediamine and mercaptoethanol solvent mixture have been studied as capping ligands for colloidal CdSe nanocrystals. A phase transfer ligand exchange strategy was utilized to effectively install the stibanate ligands onto the CdSe nanocrystals to form stable colloidal suspensions in polar solvents, such as formamide. This methodology was very effective in the removal of insulating native ligands on the as-prepared nanocrystals, with the resulting stibanate-capped CdSe nanocrystals giving low organic content thin films upon spin coating with improved interparticle coupling after heating to temperatures <300 °C. 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.

Binary solvent mixtures of alkanethiols and 1,2-ethylenediamine have the ability to readily dissolve metals, metal chalcogenides, and metal oxides under ambient conditions to enable the facile solution processing of semiconductor inks; however, there is little information regarding the chemical identity of the resulting solutes. Herein, we examine the molecular solute formed after dissolution of Sn, SnO, and SnS in a binary solvent mixture comprised of 1,2-ethanedithiol (EDT) and 1,2-ethylenediamine (en). Using a combination of solution (119)Sn NMR and Raman spectroscopies, bis(1,2-ethanedithiolate)tin(II) was identified as the likely molecular solute present after the dissolution of Sn, SnO, and SnS in EDT-en, despite the different bulk material compositions and oxidation states (Sn(0) and Sn(2+)). All three semiconductor inks can be converted to phase-pure, orthorhombic SnS after a mild annealing step (∼350 °C). This highlights the ability of the EDT-en solvent mixture to dissolve and convert a variety of low-cost precursors to SnS semiconductor material.