Electroabsorption Spectroscopy Studies of (C4H9NH3)2PbI4 Organic

Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES

Letter

Electroabsorption Spectroscopy studies of (C4H9NH3)2PbI4 Organic-Inorganic Hybrid Perovskite Multiple Quantum-Wells Eric Amerling, Sangita Baniya, Evan Lafalce, Chuang Zhang, Zeev Valy Vardeny, and Luisa Whittaker-Brooks J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01741 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Electroabsorption

Spectroscopy

Studies

(C4H9NH3)2PbI4

Organic-Inorganic

of

Hybrid

Perovskite Multiple Quantum-Wells

Eric Amerling,† Sangita Baniya, ‡§ Evan Lafalce, ‡§ Chuang Zhang, ‡ Zeev Valy Vardeny,* ‡ and Luisa Whittaker-Brooks*†

†

Department of Chemistry, University of Utah, Salt Lake City, UT, 84112

‡

Department of Physics, University of Utah, Salt Lake City, UT, 84112

§

The authors contributed equally to this work

Corresponding Author * Email: [email protected] (Z.V.V)

*Email: [email protected] (L.W.B)

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

ABSTRACT

Two-dimension (2D) organic-inorganic hybrid perovskite multiple quantum wells which consist of multilayers of alternate organic and inorganic layers, exhibit large exciton binding energies of order of 0.3 eV due to the dielectric confinement between the inorganic and organic layers. We have investigated the exciton characteristics of 2D butylammonium lead iodide, (C4H9NH3)2PbI4 using photoluminescence and UV-Vis absorption in the temperature range of 10K to 300K, and electroabsorption spectroscopy. The evolution of an additional absorption/emission at low temperature indicates that this compound undergoes a phase transition at ≈250K. We found that the electroabsorption spectrum of each structural phase contains contributions from both quantum confined exciton Stark effect and Franz-Keldysh oscillation of the continuum band, from which we could determine more accurately the 1s exciton, continuum band edge, and the exciton

binding

energy.

TOC GRAPHICS

KEYWORDS electroabsorption, 2D organic-inorganic quantum-wells, exciton binding energy, Stark effect, Franz-Keldysh oscillation, polarization changes


2

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Research on organic-inorganic hybrid perovskites (OIHP) has become one of the trendiest foci in condensed matter physics in the past few years. Although OIHP have been studied extensively, much of their recent research spotlight has been sparked by the significantly high power conversion efficiencies attained with three dimension (3D) methylammonium lead iodide (CH3NH3PbI3) as the absorber material in solar cells (circa 22.1%).1 The observed high power conversion efficiencies in 3D OIHP is attributed to their superb optoelectronic properties such as tunable band gaps between 1.2 and 2.3 eV,2-3 large absorption coefficient,4 ambipolar charge transport,5-7 limited charge recombination8-10 that leads to long carrier lifetimes,11 and a relatively small exciton binding energy of 5-30 meV.8 However, these properties, particularly their small exciton binding energy may be a disadvantage for applications beyond solar cells such as light emitting diodes, for example. Given the rich structural diversity of OIHP, much research efforts have shifted towards investigating the optoelectronic properties of the low-dimensional analogues.

These low-dimensional OIHPs are characterized by confined excitons in their

quantum structures, which are self-organized building blocks where the [PbI6]4- octahedra may form zero-dimensional (0D, quantum dots), one-dimensional (1D, quantum wires) or twodimensional (2D, quantum wells) compounds.12 The excitons in these compounds are confined to the [PbI6]4- inorganic layer, whereas the organic moiety acts as a potential barrier. This quantum confinement enables the formation of stable excitons with relatively large binding energies (Eb = 200 ~ 400 meV) and oscillator strengths (f ≈ 0.7 per formula unit).13-15 Another interesting feature observed in these quantum confined structures is that they tend to experience the ‘image charge effect’ (known as the ‘dielectric confinement effect’), which is triggered by the high dielectric mismatch between the [PbI6]4- inorganic and the organic moiety layers thus strengthening the 2D exciton confinement.16-19


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

Among OIHP that crystallize in a low-dimensional structure, 2D OIHP have captivated the attention of the condensed matter physics community due to the intriguing optoelectronic properties that develop upon exciton confinement. These remarkable optoelectronic properties range from bright photoluminescence (PL) and electroluminescence,14, scintillation,23-25 to unique optical nonlinearities.26

20-22

increased

These properties make 2D OIHP ideal

candidates for integration into functional devices such as light emitting diodes, field-effect transistors, and polariton lasers.14, 20, 22, 27-31

Although the excitonic and electronic structures of various 2D OIHP have been previously investigated,12-14, 16, 18, 21-22, 27, limited efforts have been devoted to understanding the excitons and band structure of 2D butylammonium lead iodide–the smallest possible 2D OIHP quantum well to crystallize.32 In the present study we investigate the exciton and continuum band of 2D butylammonium lead iodide (C4H9NH3)2PbI4 quantum wells using cw methods such as optical absorption (OA) and PL, as well as nonlinear optical technique such as electroabsorption (EA). Systematic temperature-dependent optical spectroscopy studies of (C4H9NH3)2PbI4 thin films reveal distinctive exciton and binding energy characteristics due to the presence of two different crystal phases. Likewise, understanding the exciton nature (Wannier-type vs Frenkel-type) as well as the extent to which the dielectric confinement effect influences the 2D exciton character and binding energy in (C4H9NH3)2PbI4 quantum wells is of paramount significance for photovoltaic applications.


4

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(i)

Structural characterization

A continuous and uniform (C4H9NH3)2PbI4 thin film (as evidenced in the SEM image presented in Figure S1) is readily fabricated by spincoating a precursor consisting of C4H9NH3I and PbI2. (C4H9NH3)2PbI4 crystallizes in an orthorhombic structure that consists of single [PbI4]2- large sheets of corner-shared [PbI6]4- octahedra stacked along the c-axis, separated by C4H9NH3+ layers on either side of the inorganic octahedra. The 2D nature is imparted by slicing the tetragonal structure of 3D CH3NH3PbI3 along the surfaces, and replacing the CH3NH3+ with C4H9NH3+ organic moieties.33-35 The structure is held together via N-H···I weak hydrogen bonds through the NH3 ligands of the organic C4H9NH3+ cations, ionic interactions, and weak van der Waals forces between adjacent C4H9NH3+ cations (Figure 1A).

Figure 1. (A) Schematic crystal structure of (C4H9NH3)2PbI4. (B) X-ray diffraction pattern of (C4H9NH3)2PbI4 thin film.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

Structural characteristics of the as-fabricated 2D (C4H9NH3)2PbI4 thin films can be resolved via X-ray diffraction (XRD) studies, as shown in Figure 1B. All experimental reflection lines can be assigned to the Pcba space group symmetry of the room temperature orthorhombic (C4H9NH3)2PbI4 crystal phase.33-35 Furthermore, all diffraction peaks are exclusively associated with the {00l} crystallographic family of planes. The fact that only reflections associated with the {00l} are observed indicates that (C4H9NH3)2PbI4 thin films preferentially grow along the [110] direction. This growth direction corresponds to [PbI6]4- layers -within the (C4H9NH3)2PbI4 thin film - oriented parallel to the substrate. As a caveat, (C4H9NH3)2PbI4 undergoes a reversible structural phase transition at ≈ 250 K, associated with changes in the ordering and hydrogen bonding of the organic moiety.34

Figure S2 shows the experimental and simulated XRD

diffraction pattern for a (C4H9NH3)2PbI4 thin film at 100 K and 300 K highlighting the (002) scattering peak. Here, we observe a shift associated with the (002) diffraction plane due to the evolution of two distinct structural phases upon undergoing the phase transition at ≈ 250 K. Structurally, the low and high temperature phases are both orthorhombic, however, the low temperature phase possesses distinctive exciton and electronic properties when compared with the room temperature phase (vide infra). Moreover, (C4H9NH3)2PbI4 is a “natural” quantum well structure formed through self-assembled layers of dielectrically dissimilar inorganic [PbI6]4- wells (εw = 6.1) and organic C4H9NH3+ barrier layers (εb = 2.1).14 This large difference in dielectric constant results in reduced screening of the Columbic interaction between electrons and holes that results in larger exciton binding energies.14,19, 36


6

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(ii)

Absorption and photoluminescence spectroscopies

To investigate the exciton and continuum band in (C4H9NH3)2PbI4 thin films, careful absorption studies were performed. The OA spectra for a (C4H9NH3)2PbI4 thin film as a function of temperature are shown in Figure 2A.

The absorption spectrum at room temperature is

dominated by a prominent band at 2.43 eV which corresponds to a strong exciton resonance. The optical band edge of (C4H9NH3)2PbI4 may be at ~2.4 eV, which is consistent with other layered perovskite sharing that same [PbI6]4- sheet system14 ; but more precise measurements are needed to confirm it (see below). The OA spectra also display a broad band at ≈ 2.85 eV which is indicative of 2D-type interband absorption response. Moreover, as shown in Figure 2A, the intensity of the exciton band at 2.43 eV decreases as the temperature, T is reduced from 300K to 50K. At T

Electroabsorption Spectroscopy Studies of (C4H9NH3)2PbI4 Organic

Recommend Documents