Abstract
1. Introduction
Originally, the word ‘perovskite’ originally described a mineral calcium titanate (CaTiO3) that was discovered in the Ural Mountains of Russia and named after Russian mineralogist Lev Perovski[
Despite the rapid advancements of 3D-HOIP-based optoelectronic applications, a number of challenges remain including hysteresis, instability and toxicity[
One alternative way to address the stability of 3D-HOIPs is to introduce their two-dimensional (2D) counterparts, which have attracted attention largely due to their better environmental stability[
In this report, we will give a brief summary on the recent progress of optoelectronic properties of 2D perovskites. We will start with an introduction of the layer number n dependent environmental stability and the various methods to synthesize 2D perovskites. The optical and optoelectronic properties and self-trapped states in 2D perovskites will then be discussed. Subsequently, the growth of heterostructures based on 2D perovskites together with their optical and optoelectronic properties will be given. Finally, the outlook of 2D perovskite-based optoelectronic devices will be discussed, which will provide researchers with new insights into the future research direction regarding 2D perovskites.
2. Varying the layer number n and the stability of 2D perovskites
The 2D perovskites can be viewed as the 3D perovskite frameworks being sliced into 2D slabs by the long chain organic spacer chains (Fig. 1(a))[
Figure 1.(Color online) (a) The schematic illustration of the crystal structures of 2D perovskite (BA)2(MA)
Nevertheless, the presence of the long chain spacer cations among inorganic layers would introduce barriers for the carriers to transport across the organic layers, inducing a huge resistivity in the out-of-plane direction[
3. The synthesis of 2D perovskites
Recently, a number of growth methods have been developed to synthesize 2D perovskites with various compositions and morphologies. While 2D perovskite crystals of hundreds of micrometers size have been prepared by a solution method to study their crystal structure, optical properties and band structures (Fig. 2(a))[
Figure 2.(Color online) (a) Scanning electron microscopy (SEM) images of (BA)2(MA)
To obtain pure phase 2D perovskites for the study of their basic optical and electronic properties, a mechanical exfoliation method was adopted to peel 2D perovskite microplates from their respective bulk crystals. Micro-absorption and power-dependent photoluminescence studies reveal that the impurity phases are physically mixed, which allows us to achieve pure phase 2D perovskite microplates via mechanical exfoliation[
The 2D perovskite crystals with the desired shape might find important applications in optoelectronics and thus a series of synthetic strategies have been adopted to achieve 2D perovskites with different morphologies[
4. Optical and optoelectronic properties of 2D perovskites
The naturally formed quantum well structure and large dielectric constant difference between the organic layer and inorganic layer result in the novel and interesting optical and optoelectronic properties in 2D perovskites. By changing the layer number n, the quantum confinement strength can be readily tuned and this leads to a gradual shift of both emission peak and absorption onset in a wider range of wavelength (Figs. 3(a) and 3(b))[
Figure 3.(Color online) (a) Normalized PL spectra of the as-exfoliated (BA)2(MA)
The 2D perovskite-based photodetectors and light emitting devices have been demonstrated with fairly good performance. The responsivity of around 100 A/W has been achieved in pure (C4H9NH3)2PbI4 microplate devices (Fig. 3(c)) while a very high responsivity of 2100 A/W has been reported in graphene-contacted (C4H9NH3)2PbBr4 heterostructures[
5. Self-trapped states in 2D perovskites
Strong electron-phonon interaction is present in both 3D and 2D perovskites, leading to lattice deformation. Self-trapped states are formed when carriers or excitons are localized and trapped by the lattice deformation potential. The self-trapping strongly relies on the dimensionality of the systems. Unlike 3D case where a potential barrier is present for self-trapping, there is no such barrier for one-dimensional systems and a much lower potential barrier or even no potential barrier exists in 2D systems[
Self-trapped excitons can recombine via either radiative or nonradiative pathways depending on the long chain organic molecules and halide anions in 2D perovskites. The self-trapped excitons usually feature as broad emission peaks and a large Stokes shift when compared with exciton emission peak while the self-trapped excitons in some types of 2D perovskites at room temperature only exhibit an asymmetric line shape of PL spectrum[
Figure 4.(Color online) (a) PL spectra of different 2D hybrid perovskites: (i) (BA)2PbCl4, (ii) (BA)2PbBr4, (iii) (BA)2PbI4, (iv) (BA)2PbCl2Br2, (v) BA)2PbBr2I2 and (vi) (BA)2(MA)Pb2Br7 2D microplates and their corresponding PL images as shown in the inset. The scale bars are 2
The emission from self-trapped states also relies on the crystalline quality of 2D perovskites. While only one broad emission peak is present in (PEA)2PbI4 thin films, two distinct peaks are observed with a much weaker strength in (PEA)2PbI4 single crystals, which suggestes that defects can enhance the strength of the self-trapping (Fig. 4(c))[
6. 2D perovskite based heterostructures
The layered nature and rather different electronic band structures of 2D perovskite series (different layer number n) suggest that it is possible to conveniently fabricate heterostructures consisting of 2D perovskites with different layer number n and the functional devices can be achieved with extended functionalities in these 2D perovskite-based heterostructures[
Both lateral and vertical (C4H9NH3)2PbI4/(C4H9NH3)2(CH3-NH3)Pb2I7 heterostructures have been grown by combining the solution method and vapor phase transport method[
Figure 5.(Color online) (a) Schematic illustrations of crystal structure of (BA)2PbI4/(BA)2MAPb2I7 lateral and vertical heterostructures. (b, c) Photographs of the (BA)2PbI4/(BA)2MAPb2I7 lateral and vertical heterostructures, respectively. The boundary of lateral heterostructure are shown in
The 2D perovskite-based heterostructures also have been synthesized on a large scale by pure solution method[
7. Summary and outlook
In summary, we have presented an overview of the recent progress of the optoelectronic applications in 2D perovskites. The 2D perovskites with layered nature exhibit rather unique properties including the naturally formed quantum well structure, extremely large exciton binding energy, strong electron-phonon coupling and a greatly tunable of band gap by either tuning the layer number or the chemical compositions[
Despite the breakthroughs that have been made, several issues in 2D perovskites also need to be addressed to explore more novel and interesting applications based on 2D perovskites. First, although a series of growth strategies have been developed to synthesize 2D perovskites, it is still difficult to obtain 2D perovskites with controlled composition and structures[
Acknowledgements
Dehui Li acknowledges the support from NSFC (No. 61674060) and the Fundamental Research Funds for the Central Universities, HUST (Nos. 2017KFYXJJ030, 2017KFXKJC002, 2017KFXKJC003 and 2018KFYXKJC016).
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