Jun Sang Cho1,Hyun Kyung So1,Eugenia Vasileiadou2,Mercouri Kanatzidis2,Myung Hwa Jung1,Joon Ik Jang1
Sogang University1,Northwestern University2
Jun Sang Cho1,Hyun Kyung So1,Eugenia Vasileiadou2,Mercouri Kanatzidis2,Myung Hwa Jung1,Joon Ik Jang1
Sogang University1,Northwestern University2
The Ruddlesden-Popper (R-P) series of metal organic halide perovskites can host tightly bound excitons even at room temperature due to strong quantum confinement. [1] This implies that two-dimensional (2D) confinement can also enhance the stability of their Coulomb bound state, the so-called biexciton. This confinement effect is supposed to increase when decreasing the perovskite layer number (n) in the Br-based R-P perovskite series, (BA)<sub>2</sub>(MA)<sub>n-1</sub>Pb<sub>n</sub>Br<sub>3n+1</sub> (BA = CH<sub>3</sub>(CH<sub>2</sub>)<sub>3</sub>NH<sub>3</sub>, MA = CH<sub>3</sub>NH<sub>3</sub>, n = 1–3). Recently, it was shown that biexcitons are formed by binding of two dark excitons, which are stable up to around 100 K in (BA)<sub>2</sub>PbBr<sub>4</sub> (n = 1). [2] However, there is no comprehensive study on the stability of biexcitons upon tuning the confinement effect by varying the layer number in the R-P series. Herein, we investigate the impact of quantum confinement on the biexciton binding energy as a function of n = 1–3 by employing precision photoluminescence (PL) spectroscopy and second harmonic generation (SHG) measurement. Based on the series of PL spectra obtained as a function of excitation intensity, polarization, and temperature, the biexciton binding energies are precisely determined to be 18.7, 25.5, and 12.0 meV at 10 K for n = 1, 2, and 3, respectively. Therefore, our results unexpectedly indicate that the quantum confinement effect seems to be strongest at n = 2, not n = 1. However. we show that this anomaly arises basically from the unusually small exchange interaction at n = 2, which is 6.9 meV. This value is much smaller than 21.8 meV (n = 1) and 12.6 meV (n = 3). Temperature-dependent SHG and PL spectra clearly demonstrate that this small dark-bright splitting energy of 6.9 meV is associated with a newly discovered structural phase transition near 100 K, occurring only in the n = 2 perovskite. We will discuss the nature of the structural phase transition and its impact on the dark-bright splitting that directly affects the biexciton binding energies of the series. Our study will deepen the understanding of the excitonic fine structure and structural phase transition as a function of layer number in these important materials and potentially pave a way toward 2D excitonics.<br/><br/>References<br/>[1] C. C. Stoumpos et al., Chem. Matter. <b>28</b>, 2852 (2016)<br/>[2] W. Choi et al., J. Am. Chem. Soc. <b>143</b>, 19785 (2021)