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Quasi-Two-Dimensional (Quasi-2D) Halide Perovskite Single Crystal Photodetector Kai Wang, Congcong Wu, Dong Yang, Yuanyuan Jiang, and Shashank Priya ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01999 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 23, 2018
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ACS Nano
Quasi-Two-Dimensional (Quasi-2D) Halide Perovskite Single Crystal Photodetector Kai Wang,#* Congcong Wu,#* Dong Yang, Yuanyuan Jiang, Shashank Priya*
Center for Energy Harvesting Materials and System (CEHMS), Virginia Tech, Blacksburg, VA 24061, USA #
Equal contribution.
*Corresponding authors. E-mail addresses:
[email protected] (K. Wang),
[email protected] (C. Wu),
[email protected] (S. Priya).
ABSTRACT The robust material stability of the quasi-two-dimensional (quasi-2D) metal halide perovskite has opened the possibility for their usage instead of three-dimensional (3D) perovskite. Further, devices based on large area single crystal membrane have shown increasing promise for photoelectronic applications. However, growing inch-scale quasi-2D perovskite single crystal membrane (quasi-2D PSCM) has been fundamentally challenging. Here we report a fastsynthetic method for synthesizing inch-scale quasi-2D PSCM, namely (C4H9NH3)n(CH3NH3)n1PbnI3n+1
(index n=1, 2, 3, 4, and ∞), and demonstrate their applications in single-crystal
photodetector. Quasi-2D PSCM has been grown at the water-air interface where spontaneous alignment of alkylammonium cations and high chemical potentials enables the uniform orientation and fast in-plane growth. Structural, optical and electrical characterizations have been conducted as a function of quantum well thickness that is determined by the index n. It is shown that the photodetector based on the quasi-2D PSCM with the smallest quantum well thickness (n=1) exhibits a striking low dark current of ~10-13 A, higher on/off ratio of ~104 and faster response time in comparison to those of photodetectors based on the quasi-2D PSCM with larger quantum well thickness (n>1). Our study paves the way towards the merging the gap between single crystal devices and the emerging quasi-2D perovskite materials.
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KEYWORDS: quasi-two-dimensional halide perovskite, water-air interface, fast grow, single crystal membrane, photodetector The progress in understanding the organic-inorganic hybrid metal halide perovskites has triggered the advances in the photovoltaics and generated opportunities in designing applications such as gamma-ray detection,1 X-ray imaging,2,3 monochromatic photodetector4/laser5, lightemitting diodes6,7 and subwavelength photonic devices with advanced functionalities in infrared to terahertz region.8 The intrinsic material stability issues still limit the real applications of the prototype three-dimensional (3D) halide perovskite materials.9 Alternatively, quasi-twodimensional (quasi-2D) perovskite material with robust chemical stability, optoelectronic tunability and multiple quantum well (MQW) structures have been considered as one of the top choices for next-generation electronics and optoelectronics.10 Quasi-2D perovskite has a general formula of (RNH3)n(CH3NH3)n-1MnX3n+1 (M is metal, X is halide), where the more hydrophobic long alkyl chain or aromatic group “R-” is introduced on the ammonium cation at the crystallographic A-site to form a large organic layer that separates the neighboring inorganic corner-sharing octahedral MX4- network.11 Featured by the natural MQW structure, the organic layer acts as a barrier layer whereas the inorganic layer act as the quantum well layer. The quantum well thickness is thus determined by the thickness of the inorganic layer, namely the index n, which in turn dominates the material stability, electronic band structure, optical bandgap and the degree of dielectric and quantum confinement.11 The dielectric and quantum confinement are well-known phenomena in quasi-2D perovskite MQW, which drastically enlarge the exciton binding energy and confine the charge transport within the quantum well plane. In this layered structure, an intuitively large transport anisotropy is expected in the quasi-2D perovskite MQW.12 As a result, optoelectronic devices with randomly orientated polycrystalline quasi-2D perovskite typically deliver poor performance due to the insufficient charge transport properties. To date, various efforts have been made to improve the charge transport in quasi-2D polycrystalline perovskites via either alloying with 3D perovskite13 or aligning the crystal orientation by high-temperature processing techniques.14,15 However, the polycrystalline thin film still suffers from orders-of-magnitude lower carrier diffusion length,16 boundary-induced low chemical durability17 and heavy defect density,18 with respect to their single crystal counterparts. Therefore, optoelectronic devices based on single crystal is of great potential for boosting the device performance. 2 ACS Paragon Plus Environment
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In parallel, the urgency in fabrication of single crystal device is however restricted by the challenges in growing large-area perovskite single-crystal membrane (PSCM). Conventional strategies for PSCM includes the ultrasonic pulse initiated nucleation,19 space-confined inverse temperature crystallization,20 surface-tension driven nucleation,21 capillary top-seeded-solutiongrowth molding22 cast-capping method23,24 and geometrically-confined in-plane growth,25 where the crystal geometry is mostly manipulated through the space confinement by various “containers”. However, it is practically difficult to construct an optimal container of large aspect ratio on inch-scale in basal plane and nanometer-scale in height. Microscopically, the surface morphology of the PSCM grown inside the containers is largely dependent on the nanostructures of the container surface. The exfoliation of the PSCM through physical release from the container wall also causes the inevitable structural damage to the PSCM during the bond cleavage between the perovskite and the wall, especially in the limit of thin film.26 Such a physical exfoliation induced structural damage is even expected to be more serious in quasi-2D PSCM because the weaker van der Waals interaction between the organic layers is more vulnerable to be broken. Therefore, free-standing quasi-2D PSCM without exfoliation induced structural damage is of great potential for being incorporated into electronic devices.26,27 Unfortunately, the crystallization thermodynamics for quasi-2D PSCM still remains ambiguous and becomes complicated by their characteristic layered structure related facet-control.28 Here we report a rapid synthetic road towards achieving free-standing quasi-2D PSCM, namely, (C4H9NH3)n(CH3NH3)n-1PbnI3n+1 (n=1, 2, 3, 4 and ∞) from the water-air interface and their application in single crystal photodetectors. During the crystallization, the self-assembly of the C4H9NH4+ precursor cation at the water-air interface acts as a soft template guiding the growth of nanostructures underneath. The higher solvation energy of precursor molecules at the asymmetric water-air interface delivers higher chemical potentials, leading to a low energy barrier and faster in-plane growth. Consequently, inch-size free-standing quasi-2D PSCM with large aspect ratio of ~104 has been achieved at the water-air interface. Optical, electronic and photodetector performance dependence on the quantum well thickness have been discussed. We found the photodetector device based on quasi-2D PSCM with the smallest quantum well thickness (n=1) exhibits a striking low dark current of ~10-13 A, higher on/off ratio of ~104 and faster response time (rise time of ~1.7 µs and drop time of 3.9 µs) in comparison to those of photodetectors based on the quasi-2D PSCM with larger quantum well thickness (n>1). 3 ACS Paragon Plus Environment
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RESULTS AND DISCUSSION The crystallization phenomenon has been understood by invoking the density functional theory and the classical nucleation theory.29 The driving force for precipitation of solid phase in a solution is the supersaturation quantified in terms of the difference in chemical potential of a molecule in solution and in the bulk of the crystal phase, and thermodynamically proportional to
temperature (T) and Napierian logarithm of supersaturation ratio ( () ).30 Higher is the
supersaturation higher is the tendency towards crystallization. In the case of halide perovskite,
recent progresses have suggested that the supersaturation can be achieved by elevating the temperature i.e., the so called “inverse temperature crystallization” method31 or by adding certain chemicals32 to induce complex dissolution. Both methods take advantages of the dissociation of precursor-solvent complex triggered by either higher temperature or chemical reaction for releasing higher content of free precursor molecules that enlarges the supersaturation ratio (). We noticed that at the water-air interface, the spontaneously oriented alkyl ammonium cations could template the facet orientation of the nucleus cluster. Meanwhile the breakdown of translational and rotational symmetry at the water-air interface as well as the surface tension induced higher chemical potentials of precursor molecules could also increase the nucleation probability and accelerate the epitaxial crystal growth.33 To maximize the growth-rate and maintain the floatation of the crystal membrane, we chose water as the solvent because of its higher surface tension coefficient of 72.86 mN·m-1 than commonly used organic solvents such as dimethyl sulfoxide (σ = 42.8 mN·m-1) or dimethylformamide (σ = 35.2 mN·m-1). The crystallization process occurs in two steps, nucleation and crystal-growth. Particularly at the initial state, at the water-air interface, since the hydrophilic ammonium cationic heads are more attractive to the water molecules through the static coulomb interaction,34 the alkyl ammonium cation surfactants are consequently aligned in a “head-down” pattern at the water-air interface, as shown in Figure 1a.35 A local monolayer of the oriented alkyl ammonium cations can thus act as a template to direct the nucleation followed by the subsequent growth. The nucleation rate , is exponentially related to the energy barrier for nucleation Δ,36 and can be expressed by an Arrhenius-type equation:37 = Λ ⋅ exp
(1)
where Λ depends on supersaturation, is the Boltzmann constant and T is temperature. In a
simplified model, we consider a given solution system containing only one type of precursor 4 ACS Paragon Plus Environment
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molecules (A), which can exist in the form of (i) free precursor molecules ‘A’, (ii) ‘A-cluster’ nucleus and (iii) solvated complex ‘A-jS’ where one ‘A’ is intermolecular bonded with j solvent molecules (‘S’) and has a complex binding energy of . Then the nucleation barrier can be expressed as (derived in Supporting Information, Note 1): Δ =
!" − − "$ + & ln )
+
3
* - . ⋅/* +01 ),
2
(2)
where " (>0) is the cohesive energy of precursor molecules in the cluster, "$ (>0) is the energy is the surface tension coefficient, 56 and 5$ are the total molar
of precursor molecules,
concentrations of solvent and precursor molecules. In comparison to the bulk solution, the molecules at the water-air interface experience the extra tensile elastic stress (Figure 1c) that
increases the energy of precursor molecules by 7 (>0) and leads to the reduced barrier (Figure
1e) as
Δ89:;
