The surface chemistry of CuInS2 nanocrystals used for solar cell applications

In the research field of photovoltaic devices and new solution-based processing methods using sterically stabilized semiconducting nanocrystals, full understanding of the surface chemistry is of the utmost importance. Nuclear Magnetic Resonance (NMR) spectroscopy has developed the last few years into a well-established characterization technique in this field. It can give major insight into the fundamental chemical phenomena that are operating at the nanocrystal surface.[1] Ligands can not only be characterized and quantified, also different types of systems can be defined according to the complex adsorption-desorption phenomena observed in situ. CuInS2-based nanocrystals form an interesting I-III-VI semiconductor compound because they are less toxic and have similar opto-electronic characteristics compared to Cd-based and Pb-based compounds. During the synthesis of CuInS2 NPs, typically oleylamine (OLA) is used as a ligand. An important step in view of further processing and printing is the understanding of the binding of OLA to the CuInS2 surface. Here, we use some advanced NMR techniques to investigate this surface chemistry. As a first observation, we find that OLA ligands bound to CuInS2 nanocrystals show no exchange with a free ligand pool at room temperature. A similar behaviour is typically found with X-type coordinating ligands such as carboxylates or phosphonates. However, it contrasts with the dynamic stabilization of CdSe, CdTe and ZnO nanocrystals by amines, which is typically interpreted as the result of a weak, L-type coordination. To clarify the binding of oleylamine to CuInS2, we performed in-situ heating experiments where the NMR sample is heated up to 130 °C and 1D 1H NMR spectra are recorded in steps of 10 °C. In the case of CdSe stabilized by X-type oleate ligands in a well-purified, apolar dispersion, this heating has no effect on the ligand shell. Opposite from this, we observe a progressive release of OLA upon heating and a, albeit slow yet clearly discernible, readsorption upon cooling. Hence, opposite from oleate ligands, it follows that the OLA/CuInS2 bond can be broken and formed in apolar dispersions without the addition of excess free ligands – a prerequisite in the case of oleate or phosponate ligands.[1] We thus conclude that OLA coordinates CuInS2 nanocrystals as an L-type ligand, yet with a strong activation energy for desorption and adsorption. The proposed surface chemistry is further elaborated in a series of exchange experiments towards thiols and carboxylic acids that are monitored with NMR spectroscopy. In line with the difficult desorption of OLA, the exchanges require harsh conditions (large excess of new ligand, elevated temperatures and multiple iterations) but give satisfying results. As it appears, exchange of oleylamine is possible and is completely reversible, the original ligand density is always restored. Interestingly, there are some variations in the total ligand density when changing to thiols or carboxylic acids, which can be attributed to the complex surface chemistry of the CuInS2 nanocrystals