Silicon quantum dots (SiQDs) with diameters less than 5 nm are promising for next-generation photovoltaics with attractive features that include gap tunability, an optimum stability against oxidation [1], multi-exciton generation [2], and environmental neutral footprint. However, their optical gaps are simply too large and current SiQD-based solar cells perform poorly due to low carrier mobilities [3,4]. These drawbacks have motivated a multi-functionalization scheme in which the synergism between the dot, ligand, bridge and matrix lead to a substantial improvement of absorption as well as a desirable charge dynamic for free carrier extraction.

As a proof of concept, we have carried out ab initio calculations on 2.6 nm SiQDs embedded in P3HT with the triphenylamine (TPA) and the C8H8N2S molecules serving as the terminating ligand and dot-to-dot bridge, respectively. By using a conjugating vinyl bond connection and establishing a type-II energy level alignment at the dot-ligand interface, low-energy photons can be absorbed via direct generation of spatially separated excitons. Both local and spatially separated excitons will subsequently evolve to charge transfer states with the electron and hole localized on the dot and within the matrix, respectively. While the electrons can efficiently hop between neighboring dots connected by the bridge molecules via a superexchange mechanism, hole mobility is also high due to the delocalized wave function in the matrix. Although efficient polaron dissociation in such systems is a challenge, we show that this can be solved through the introduction of an external electric field.

References:

[1] Li, H. et al., ACS Nano 6, 9690, 2012.

[2] Nozik, A. J., Chem. Rev. 110, 6873, 2010

[3] Niesar, S. et al., Green 1, 339, 2011.

[4] Liu, C.-Y. et al., Nano Lett. 9, 449, 2009.

Huashan Li

Colorado School of Mines

Appearances