Charge-transfer excitons (CTEs) immensely enrich property-tuning capabilities of semiconducting materials. However, such concept has been remaining as unexplored topic within halide perovskite structures. Here, we report that CTEs can be effectively formed in heterostructured 2D perovskites prepared by mixing PEA2PbI4:PEA2SnI4, functioning as host and guest components. Remarkably, a broad emission can be demonstrated with quick formation of 3 ps but prolonged lifetime of ~0.5 μs. This broad PL presents the hypothesis of CTEs, verified by the exclusion of lattice distortion and doping effects through demonstrating double-layered PEA2PbI4/PEA2SnI4 heterostructure when shearing-away PEA2SnI4 film onto the surface of PEA2PbI4 film by using hand-finger pressing method. The below-bandgap photocurrent indicates that CTEs are vital states formed at PEA2PbI4:PEA2SnI4 interfaces in 2D perovskite heterostructures. Electroluminescence shows that CTEs can be directly formed with electrically injected carriers in perovskite LEDs. Clearly, the CTEs presents a new mechanism to advance the multifunctionalities in 2D perovskites.

large organic ligands with the formula of A 2 A′ n−1 M n X 3n+1 . [9][10][11][12] Such 2D perovskites can demonstrate remarkable tunabilities on optical properties via controlling morphological structures by selecting different functional A/A′ molecules. [13][14][15] Through down-conversion excitation, 2D perovskites have shown efficient light emission in spontaneous [16,17] and stimulated [18,19] regimes. Recently, amplified spontaneous emission (ASE) have been reported in 2D perovskites [(NMA) 2 (FA)Pb 2 Br 7 and (NMA) 2 (FA)Pb 2 Br 1 I 6 ] with stoichiometrically tunable wavelength from visible to near-infrared spectral range (530-810 nm) with the gain coefficient as high as > 300 cm −1 under pulse laser excitation. [20] On the other hand, it has been found that the edge states can be formed in 2D perovskites with light-emitting properties below the bandgap. [21] The discovery of edge states triggers a fundamental question of whether the gap states can be introduced as a new approach to develop multi-photon up-conversion light emission in solution-processing quasi-2D perovskite films. We should note that an up-conversion ASE was indeed observed at 720 nm in inorganic 3D perovskite (CsPbI 3 ) nanoplates under the infrared pulse laser excitation of 1200 nm. [22] In this work, we explore a new approach to realize multi-photon up-conversion photoluminescence (PL) in quasi-2D perovskite [(PEA) 2 (MA) 4 Pb 5 Br 16 (n = 5)] films by directly exciting broad gap states with continuous-wave (CW) infrared photoexcitation. The broad gap states were adjusted by selecting different n values (n = 1-5) with accelerated crystallization in quasi-2D perovskite films prepared with antisolvent processing method. Here, we found that an up-conversion PL can be observed in 2D perovskite [(PEA) 2 (MA) 4 Pb 5 Br 16 (n = 5)] films under CW 980 nm excitation when broad infrared absorption is appeared between 800 and 1500 nm. To further understand the up-conversion PL under CW infrared excitation, magnetic field effects of PL (namely, magneto-PL) were used to identify whether the gap states function as spatially extended states to generate multiphoton absorption in quasi-2D perovskite films. Furthermore, polarization-dependent PL upon using linearly and circularly polarized photoexcitations was used to explore the effects of orbit-orbit interaction on multi-photon up-conversion PL under A new approach to generate a two-photon up-conversion photoluminescence (PL) by directly exciting the gap states with continuous-wave (CW) infrared photoexcitation in solution-processing quasi-2D perovskite films [(PEA) 2 (MA) 4 Pb 5 Br 16 with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two-photon up-conversion PL occurring in quasi-2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the twophoton up-conversion PL signal. This confirms that the gap states are indeed ...

Orientational manipulation of transition dipole moment (TDM) plays an important role in controlling the polarization of excited states in light emission as well as lasing actions. The present work discovers vertically aligned TDMs in hybrid perovskite films through angle‐resolved photoluminescence (PL) measurements, which show enhanced emission through the film edge. With increasing excitation intensity, the edge emission induced by these vertically aligned TDMs becomes dominant and eventually leads to amplified spontaneous emission (ASE) through the edge view. Meanwhile, polarized emission of both PL and electroluminescence (EL) provides further evidence for vertically aligned TDMs. Surprisingly, the degree of polarization (DOP) through the film edge is increased when grain boundary defects are passivated through either stochiometric engineering or self‐passivation by mobile ions under working conditions. With increasing DOP, ASE threshold of the perovskite film is reduced owing to enhanced collective behaviors of light‐emitting states. This work presents a useful method to manipulate TDMs in organic–inorganic hybrid perovskites.