Defect-Assisted Exciton Transfer across the Tetracene-Si(111):H Interface

Exciton transfers are ubiquitous and extremely important processes, but often poorly understood. A recent example is the triplet exciton transfer in tetracene sensitized silicon solar cells exploited for harvesting high-energy photons. The present ab initio molecular dynamics calculations for tetracene-Si(111):H interfaces show that Si dangling bonds, intuitively expected to hinder the exciton transfer, actually foster it. This suggests that defects and structural imperfections at interfaces may be exploited for excitation transfer.

Figure 1

Scheme showing part of a singlet fission-sensitized silicon solar cell. Absorption of a high-energy photon by the tetracene layer produces a singlet exciton. This singlet exciton undergoes singlet fission to generate two triplet excitons. These excitons are then transferred into the Si solar cell. Enlarged image details (left) show side views of the models used for the interface between Si(111):H and high-density (HD) as well as low-density (LD) Tc phases. Also shown is an interface dangling bond defect (right).

Figure 2

Density of states and band alignment for Tc overlayers on Si(111):H calculated on the HSE and PBE levels of theory. Energies refer to the Si valence band maximum (VBM). Black and orange denote Tc- and Si-related states, respectively. Occupied states are shaded.

Figure 3

Partial density of states (PDOS) for Tc overlayers on defective $\mathrm{Si}(111):\mathrm{H}+{\mathrm{DB}}^{-}$ calculated on the HSE level of theory. Energies refer to the Si VBM. Black and orange denote Tc- and Si-related states. Occupied states are shaded.

Figure 4

Calculated room temperature dynamics of the exciton transfer at the Tc-Si(111):H interface (HD model with DB defect, example trajectory with characteristic snapshots at 0, 250, 310, and 330 fs after the exciton has been trapped in the lowest molecular layer). Blue and red isosurfaces indicate electron and hole localization, respectively.

Figure 5

Evolution of Tc valence band (VB) and conduction band (CB) edge as well as Si DB state energies along the AIMD trajectory shown in Fig. 4 (dashed vertical lines refer to the snapshots). Also shown is the $z$ position of the Si DB surface atom (displacement along the surface normal).

Figure 6

Calculated average times (with standard deviation, assuming a 100% hopping probability) required for exciton transfer across the defective Tc-Si(111):H interface (HD model) in dependence on the simulation temperature. The number of trajectories (out of 50 in total) which did not lead to exciton transfer within 1 ps is also shown.

Figure 7

Energetics of exciton transfer from the Tc layer (HD model) into the defective Si(111):H substrate calculated within PBE and HSE. The PES are calculated starting from the lowest energy configuration of Tc- and Si-localized triplet excitons, i.e., ${\mathrm{Tc}}^{*}\mathrm{Si}$ and ${\mathrm{TcSi}}^{*}$, respectively (see Ref. [35] for atomic coordinates). The interface structures are subsequently fully relaxed for various vertical Si DB atom positions, that serve as reaction coordinate. Solid lines are to guide the eye.