Solar Nanocomposites with Complementary Charge Extraction Pathways for Electrons and Holes: Si Embedded in ZnS

We propose that embedding silicon nanoparticles (NP) into amorphous, nonstoichiometric ZnS leads to promising nanocomposites for solar energy conversion. Using ab initio molecular dynamics simulations we show that, upon high temperature amorphization of the host chalcogenide, sulfur atoms are drawn to the NP surface. We find that the sulfur content may be engineered to form a type II heterojunction, with complementary charge transport channels for electrons and holes, and that sulfur capping is beneficial to lower the nanoparticle gap, with respect to that of NPs embedded in oxide matrices. Our analysis is conducted using density functional theory with local and hybrid functionals and many body perturbation theory at the $GW$ level.

Figure 1

Ball and stick representation of a Si nanoparticle (${\mathrm{Si}}_{35}$, gray rods) embedded in an $a\text{−}\mathrm{ZnS}$ matrix (${\mathrm{Zn}}_{81}{\mathrm{S}}_{100}$), showing the sulfur atoms (yellow spheres) capping its surface. The highest occupied orbital (whose square modulus is shown in blue) is localized on the surface layer and composed of S lone pairs.

Figure 2

(a) Densities of states (EDOS) renormalized by the number of electrons in the respective model and (b) electronic gaps of Si NPs embedded into $a\text{−}\mathrm{ZnS}$ as a function of the NP diameter, computed using LDA, as defined by Eq. (1). The electronic gaps of Si NPs embedded in $a\text{−}{\mathrm{SiO}}_2$ (from Ref. [12]) are shown for comparison.

Figure 3

(a) Electronic density of states of the Si nanoparticle and host $a\text{−}\mathrm{ZnS}$ matrix for a sample of ${\mathrm{Si}}_{123}{\mathrm{Zn}}_{188}{\mathrm{S}}_{201}$, partitioned into contributions from the nanoparticle, the surface shell ($S$ shell) and the host matrix ($a\text{−}\mathrm{ZnS}$). Isodensity plots of the sum of square moduli of the states at the top of the valence band (blue) and bottom of the conduction band (red) are shown in (b) and (c), respectively. The sums were performed over an energy interval of 0.5 eV, as indicated by vertical bars in (a).

Figure 4

Band offsets between the valence band maximum (VBM) and conduction band minimum (CBM) of the Si nanoparticle and host $a\text{−}\mathrm{ZnS}$ matrix for a sample of ${\mathrm{Si}}_{123}{\mathrm{Zn}}_{188}{\mathrm{S}}_{201}$. The $\mathrm{NP}/\mathrm{ZnS}$ interface is located at 8 Å, while the cell boundary is located at about 10.8 Å.