Charge carrier dynamics and self-trapping on ${\mathrm{Sb}}_2{\mathrm{S}}_3\left(100\right)$

Antimony sulfide (${\mathrm{Sb}}_2{\mathrm{S}}_3$) is a promising material for solar energy conversion. It consists of earth-abundant elements with a low toxicity and high stability. Furthermore, it shows suitable optical and electronic properties like a large absorption coefficient and a convenient band gap of 1.7 eV. Nevertheless, so far, the obtained power conversion efficiencies and the open circuit voltages are far below the theoretical limits and need to be increased for a viable application of ${\mathrm{Sb}}_2{\mathrm{S}}_3$-based cells at the large scale. To achieve this it is important to identify the dominating loss mechanism. Here time-resolved two-photon photoemission experiments are presented from the (100) surface of a cleaved ${\mathrm{Sb}}_2{\mathrm{S}}_3$ single-crystal to investigate the charge carrier dynamics after photoexcitation. Based on these measurements an ultrafast relaxation within below 150 fs towards the conduction band minimum is observed. This is followed by a decay within 1 ps into states, which are located in the band gap 0.06 eV and 0.44 eV below the conduction band minimum where the charge carriers have long lifetimes of 27 ps and 63 ps, respectively. Based on the energetic position, the energy width and the formation time of the lower-lying gap state a self-trapping mechanism of free charge carriers by optical phonons with a frequency of 1.44 THz is proposed, and the results are discussed in this context. These findings provide evidence that the poor performance of ${\mathrm{Sb}}_2{\mathrm{S}}_3$ in solar devices can be traced back to intrinsically formed traps that can hardly be avoided.

Figure 1

Crystal structure of antimony sulfide. (a) Crystal structure of antimony sulfide (${\mathrm{Sb}}_2{\mathrm{S}}_3$) with the ribbons pointing along $\vec{b}$. The Sb atoms are colored in brown and the S atoms in lemon. The cleavage plane (blue dashed line) and the orthorhombic unit cell (black line) are marked. (b) Brillouin zone with the high-symmetry directions along ($\mathrm{\Gamma }\mathrm{Y}$) and across ribbons ($\mathrm{\Gamma }\mathrm{Z}$) indicated.

Figure 2

Valence band spectrum of ${\mathrm{Sb}}_2{\mathrm{S}}_3$. The valence band spectrum is shown close to the VBM. Data were taken at $\overline{\mathrm{\Gamma }}$ with the HHG light source using a photon energy of 15.3 eV and $p$-polarization. Linear extrapolation of the falling edge allows to determine the onset of the VBM (blue dashed line).

Figure 3

Time-dependent energy distribution of photoexcited electrons in ${\mathrm{Sb}}_2{\mathrm{S}}_3$. (a) The principle of tr-2PPE. Absorption of a pump photon leads to the excitation of an electron from the VB to the CB, where the population is probed by absorption of a probe photon as function of time delay. (b) The energy distribution of electrons after photoexcitation with a 3-eV pump pulse is shown color-coded as function of energy and time delay. The energy positions where the charge carrier dynamics were analyzed are indicated. The data were integrated between $0{\AA }^{-1}$ and $0.12{\AA }^{-1}$. (c) Spectra taken at selected times after photoexcitation. For the data presented in (b,c) the intensity at negative time delays was subtracted from the raw data.

Figure 4

Relaxation dynamics of photoexcited charge carriers. (a)–(c) Time-dependent electron population at selected energies. Fits based on rate equations are superimposed to the data (black dashed lines). In (b) and (c) the transients at 0.64 eV and 0.58 eV together with the respective fits are scaled by a factor of 0.01 and shifted vertically for better visualization.

Figure 5

Relaxation pathways and self-trapping: At $t_0$ an electron is excited from the VB to the CB due to absorption of a pump photon. In the 1-step process the photogenerated electron-hole pair is immediately trapped by local lattice distortions and forms a STE. In the 2-step process a free electron and hole are trapped separately within the lattice before they merge to form a STE. The trapping of free charge carriers is marked by red and orange arrows, and charge carrier relaxation is indicated by pink arrows. The measured time constants are indicated.