Nano Energy (2012) 1, 765–768
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journal homepage: www.elsevier.com/locate/nanoenergy
NEWS AND OPINIONS
Quantum dot treatment leads to best solar cells yet Cordelia Sealy Researchers from the University of Toronto and King Abdullah University of Science and Technology (KAUST) have devised a surface treatment for colloidal quantum dot (CQD) ﬁlms that yields the most efﬁcient CQD solar cell yet [A.H. Ip, et al., Nature Nanotechnology (2012), http:// dx.doi.org/10.1038/NNANO.2012.127]. CQD ﬁlms are attractive for solar cell applications because they offer the possibilities of cheap and easy large-area solution processing and bandgap tuning through the quantum size effect. But the high surface-area-tovolume ratio of CQD ﬁlms makes them vulnerable to high densities of trap states if surface passivation is ineffective, leading to charge carrier recombination and poor device performance. Previous efforts to passivate CQD ﬁlms have relied on long insulating ligands, which are attached to the surface through solid-state ligand exchange. Now, however, Edward H. Sargent of the University of Toronto and his colleagues have devised a hybrid passivation scheme based on the introduction of a metal halide salt like CdCl2 during synthesis, while metal cations are introduced to treat surface chalcogens, removing valence-bandassociated trap sites. Finally, the treatment is ﬁnished off with short organic linkers to help the QDs pack closer together. ‘‘By using small chlorine atoms in the solution phase, we are able to passivate the surface sites where bulky organic ligands would be unable to reach,’’ explains ﬁrst author Alex H. Ip of the University of Toronto. ‘‘The simplicity of the technique also lends itself well to scaling-up production for large-scale manufacturing processes.’’ The treatment of the CQD ﬁlm leads to a solar cell that can achieve power conversion efﬁciency (PCE) of 7.0%—a world record for this type of photovoltaic device (Fig. 1). The results, which represent a 37% increase on the previous highest PCE, have been veriﬁed by an accredited photovoltaics lab. ‘‘This is an important benchmark for the ﬁeld,’’ Brian A. Korgel of the University of Texas at Austin told Nano Energy. ‘‘The most important thing about the study,’’ he adds, ‘‘is
that it clearly shows what low efﬁciencies in nanocrystalbased photovoltaics have been due to—poor surface passivation.’’ The results indicate that the approach of developing materials for this type of solar cell with bandgaps essentially free of electronic trap states could yield dividends. ‘‘This work shows that the abundant materials interfaces inside CQDs can be mastered in a robust manner, proving that low cost and steadily-improving efﬁciencies can be combined,’’ says Sargent. But efﬁciencies much higher than 7% will be needed for real world applications like grid-scale power generation, cautions Korgel.
Figure 1 Susanna Thon and Alex Ip with a device and a vial of CQD solution.
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