Solution processing of semiconductor thin films offers the potential for reduced cost and high throughput device fabrication as compared to traditional processing techniques. One way that such thin films can be solution deposited is by the use of colloidal semiconductor nanocrystal inks. Nanocrystal-based solar cells derived from such inks have recently reached power conversion efficiencies &gt; 8% as a result of improved surface passivation strategies. The enabling chemistry behind this improved performance has been surface treatment with halides or metal halides. Aside from the obvious improvement in transport properties stemming from decreased interparticle separation associated with these halide or metal halide ligand shells, less is understood about the influence of such ligands on the electronic structure of the nanocrystals. To date, a systematic investigation into the effects of metal halide surface ligands on CdSe nanocrystals is lacking. With this in mind, we developed a simple and robust colloidal route for the installation of halide ligands on the surface of CdSe nanocrystals using CdX2 (X = Cl, Br, I). Photoelectrochemical measurements on solution-cast nanocrystal thin films reveal a striking influence of surface cadmium halide on photocurrent response, with mildly annealed, CdCl2-treated CdSe nanocrystal films showing the greatest enhancement in photocurrent to above band gap illumination. The strong dependence of photoresponse on surface halide is investigated using ultrafast transient absorption, time-resolved photoluminescence, and surface photovoltage spectroscopies. From these measurements, we develop a model that accounts for electron and hole trapping at the nanocrystal surface in which annealed, CdCl2-treated CdSe nanocrystal films undergo a thermal removal of electron trap states without the introduction of deep hole traps, whereas analogous CdBr2 and CdI2 treatments introduce deeper hole traps that prevent surface passivation and large photoresponse.

Investigating the trap state landscape of cadmium halide-treated CdSe nanocrystals

249th National Meeting (2015)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.