Solar cells using N,N’-substituted thiourea-based lead sulfide quantum dots

Typically lead sulfide quantum dots (PbS QDs) for solar cell incorporation are synthesized with the aid of the highly toxic and reactive sulfur precursor bis(trimethylsilyl) sulfide (TMS) (Hines et al. Adv. Mater. 2003). Recently, Hendricks et al. (Science 2015) proposed a new synthesis scheme that makes use of a wide variety of N,N'-substituted thioureas. By varying the thiourea substituents, the reaction rate can be tuned and this allows control over the QD size while maintaining extremely narrow size dispersions and full conversion of reactants. This, together with a high reproducibility makes it more appealing for larger volume synthesis than the TMS-based synthesis. However, various synthesis methods for the same material can give raise to different QD surface terminations which, in turn, will influence the final device performance. Perhaps, because of that, there have been no reports on PbS solar cells, which make use of this thiourea-based synthesis. In this contribution, we synthetized PbS QDs with a ~1.35 eV band gap by both TMS and thioureas and incorporated them into solar cells. Using nuclear magnetic resonance and Rutherford backscattering spectroscopy, we show that in both cases, synthesis and workup methods can be tuned to yield PbS with identical surface chemistry. Under such conditions, the solar cells show similar performance. That finding tells us that the resulting QD surface chemistry is key for high performance and not the synthesis route it-self