Uncovering Unique Screening Effects in 2D Perovskites: Implications for Exciton and Band Gap Engineering

Abstract

Coulomb interactions govern most optoelectronic properties in semiconductors including bandgap, excitons, polaron formation, light absorption, interaction of carriers with defects, charge and energy transport. A clear signature of the strength of many-body Coulomb interactions is the exciton binding energy (Eb). As such, the importance of Eb is perhaps second only to the fundamental band gap (Eg) in understanding the physics and efficiency of low-dimensional semiconductors. However, despite their importance, Eg and Eb are difficult to measure. Here, by choosing 2D halide perovskites as the material and electroabsorption as the experimental probe, we measure Eg and Eb with a precision that is an order of magnitude better than typical methods. The Eb values are both lower than previous literature reports and lower than expected from standard theory. Using dielectric spectroscopy, density functional theory, and quantum mechanical modeling, we demonstrate these low Eb values are a consequence of unique screening effects, such as superlattice screening and phonon screening. We demonstrate a clear correlation between Eb and Eg and provide design principles in order to a priori tune Eg and Eb to their optimal values. As such, this work lays the blueprint for Eg-Eb engineering of low-dimensional semiconductors, as an even more useful replacement to simply band gap engineering.

Ruddlesden–Popper Perovskite Alloys: Continuous and Discontinuous Tuning of the Electronic Structure

“…As shown in Figure a, the exciton absorption monotonically blueshifts with increasing Cl concentration, and so does the position of the FK feature in the EA (Figure b) as well as their separation, which is the exciton binding energy. In two-dimensional semiconductors, E g and E b are correlated since both strongly depend on the polarizability (or dielectric constant) of the semiconducting layer, and a strong linear correlation is observed here in accordance with a larger trend that we observed in a complementary study . The values of E b and E g for this alloy series are listed in Table and plotted as a function of the Br:Cl ratio in Figure c.…”

“…In Figure 4, the absorption and EA of the PEA 2 BX 4 structures are shown for BX 4 = SnI 4 , Pb 0.5 Sn 0.5 I 4 , PbI 4 , PbBr 2 I 2 , PbBr 4 , PbBr 2 Cl 2 , and PbCl 4 . The non-alloyed 2D HP compositions have been reported previously by our group, 34,44,46 but we repeat those results here to provide context for the results on alloyed structures. PEA 2 SnI 4 , PEA 2 PbI 4 , PEA 2 PbBr 4 , and PEA 2 PbCl 4 all have similar absorption and EA lineshapes that resemble the schematic in Figure 3, and this allows for unambiguous assignments of E 1s (open circle) and E g (asterisk) according to the established EA theory.…”

“…In two-dimensional semiconductors, E g and E b are correlated since both strongly depend on the polarizability (or dielectric constant) of the semiconducting layer, 49 and a strong linear correlation is observed here in accordance with a larger trend that we observed in a complementary study. 46 The values of E b and E g for this alloy series are listed in Table 1 and plotted as a function of the Br:Cl ratio in Figure 5c. As discussed in the introduction, lowtemperature EA allows for far more precise measurements of the one-electron band gap E g than traditional spectroscopies; thus, the results in Figure 5 are among the clearest evidence of E g and E b tunability in HP alloys to-date.…”