Band Alignment Boosts Charge-Carrier Collection in Sn-Based

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Band Alignment Boosts Charge-Carrier Collection in Sn-Based Perovskite over Pb Counterparts Binghan Li, Ruiying Long, Qishun Yao, Zihao Zhu, and Qixi Mi J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01405 • Publication Date (Web): 27 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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The Journal of Physical Chemistry Letters

Band Alignment Boosts Charge-Carrier Collection in Sn-Based Perovskite over Pb Counterparts Binghan Li,a,b,c,‡ Ruiying Long,a,b,c,‡ Qishun Yao,a Zihao Zhu ,a and Qixi Mi,a,* a

School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210,

China b

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

c

University of Chinese Academy of Sciences, Beijing 100049, China

AUTHOR INFORMATION Corresponding Author * [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 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

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ABSTRACT. The diverse elemental compositions endow metal halide perovskites with tailorable electronic structures and broad optoelectronic applications. For Sn-based perovskites, their bandedge positions, which govern interfacial charge-carrier transport, are less well studied than their Pb counterparts. In this work, the valence band maximum (VBM) of CsSnBr3 was experimentally and theoretically determined to be −5.2 eV, to which Au forms a good contact. The conduction band minimum (CBM) of CsSnBr3 at −3.4 eV is matched by 1,3,5-tris(4phenylquinolin-2-yl)benzene (TQB), an organic electron transport material and a ligand to Sn(II). Thanks to proper band alignment, the device structure Al/TQB/CsSnBr3/Au constitutes a photodetector responsive to the entire visible spectrum without a bias voltage, and outperforms Pb-based devices under similar conditions. Our results highlight the advantage of combined experimental and theoretical tools in understanding intrinsic material properties and guiding device fabrication.

TOC GRAPHICS


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Metal halide perovskites are noted for exceptional optoelectronic properties1,2 and rich applications in high-performance solar cells,3 photodetectors,4 and luminescent devices.5 In particular, lead halide perovskites feature a unique electronic band structure where the valence band maximum (VBM) consists of the Pb2+ s and halide p orbitals, and the conduction band minimum (CBM) mainly involves the empty Pb2+ p orbital. As a result, the bandgap energy of lead halide perovskites can be tuned by using various combinations of halides, and also by replacing the toxic Pb2+ with its lighter congener Sn2+. The univalent A-site cation, typically methylammonium (MA+) or Cs+, also indirectly affects the VBM and CBM of the perovskite by adjusting its crystal structure. The structural flexibility in metal halide perovskites results in diverse material compositions and properties. In addition to the well-studied MAPbI3 benchmark, all-inorganic perovskites such as CsPbBr3 show distinct advantages in terms of long-term device stability,6,7 as well as applications in X-ray detection8 and light-emitting quantum dots.9 Another all-inorganic perovskite CsSnBr3 has lately been studied10 to reveal excellent semiconducting properties and thermal stability that are comparable or superior to those of MAPbI3. The bandgap energy, which can be directly measured, determines the spectral range of light absorption and emission of the perovskite material, and hence its application areas. By contrast, the bandedge positions are less evident properties that underlie charge-carrier collection and injection in optoelectronic devices. Inappropriate band alignment could explain why solar cells made of bromide perovskites (e.g., MAPbBr3, CsPbBr3, and CsSnBr3) usually suffer from large losses in open-circuit voltages.11 In this study, we aimed at finding out whether optimized contact materials, based on accurate VBM and CBM values, can help Sn-based perovskite devices to achieve the performance levels of their Pb counterparts. Crystal photodetectors were chosen as



devices for proof of concept, because the device structure and interfaces can be well controlled for all Sn- and Pb-based perovskites.

E vs. vacuum (eV)

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a

b

c

−4.2 −3.4 −3.4

−4.2 −3.4

−5.2 −5.2 Al TQB

Au

CsSnBr3

−3.3 −5.6

Al TQB

−4.2 −3.4 −3.6 −5.2

−5.2 −5.9

Au

CsPbBr3

Al TQB

Au

MAPbBr3

Figure 1. Band alignment diagrams for several device structures consisting of (a) CsSnBr3, (b) CsPbBr3, or (c) MAPbBr3 as the active layer and 1,3,5-tris(4-phenylquinolin-2-yl)benzene (TQB) as the electron transport layer, showing absolute energy levels with an uncertainty of ~0.1 eV.

Fig. 1 illustrates the interfacial energetics in three device structures studied herein. The absolute energy of valence band maximum (EVBM vac ) of CsSnBr3 was experimentally found to be −5.2 eV and corroborated by theoretical calculations. It was then straightforward to employ Au to form an ohmic contact and a semitransparent top layer for incident light. Based on a bandgap energy Eg = 1.8 eV for CsSnBr3,2 its absolute energy of conduction band minimum (ECBM vac ) was derived to be VBM −3.4 eV, relatively high for perovskite semiconductors. The Evac and ECBM vac values of CsPbBr3

have been reported to be −5.6 and −3.3 eV,7,12-14 respectively. After a survey of literature in polymer solar cells and light emitting diodes, we identified 1,3,5-tris(4-phenylquinolin-2yl)benzene (TQB)15 as a suitable electron transport material for both CsSnBr3 and CsPbBr3. For VBM 16-19 Au and TQB would make contacts with MAPbBr3 with Evac = −5.9 eV and ECBM vac = −3.6 eV,

a barrier.


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DOS (eV-1)

10

5

0

E vs. EF (eV) -10 -5

-15

5 0 VBM Evac

Intensity

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Ecutoff

-5.2 eV

x100 0

-5

-10 -15 -20 E vs. vacuum (eV)

-25

Figure 2. The valence-band electronic structure of CsSnBr3, calculated as the distribution of density of states (DOS, top panel), or measured by ultraviolet photoemission spectroscopy (UPS) relative to the vacuum level (bottom panel). Inset: The experimental spectrum near the valence band maximum (VBM) is magnified by 100 times for clarity.

We determined EVBM vac of CsSnBr3 by both experimental and computational methods. In Fig. 2, ultraviolet photoelectron spectroscopy (UPS) reveals the electronic structure of the valence band of CsSnBr3. The deepest energy level that can be excited by UPS is the negative of He I photon energy (21.22 eV), appearing in the spectrum as a cutoff energy for photoelectrons with low kinetic energy. The UPS signals in the high-energy end are less clear cut, because the VBM of CsSnBr3 consists of a single energy-dispersed band,20 unlike metals with a high density of states (DOS) near the Fermi level (EF). To pinpoint the VBM, we compared the UPS data with the calculated distribution of DOS of CsSnBr3 in Fig. 2. The excellent agreement between the experimental and theoretical results helped us determine EVBM vac = −5.2 eV.



b

E (eV)

5 Evac slab

0

Eavg slab

-5

0

E vs. vacuum (eV)

a

-10

Eavg slab

-5

E vs. vacuum (eV)

Evac slab

0

-10

-2

EVBM vac

PBE = -4.9 ± 0.1 eV

-4 -6 -8 0

5

E (eV)

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

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-2 -4 -6 -8

0

10 20 30 40 z along [001] (Å)

50

HSE06 EVBM = -5.2 ± 0.1 eV vac

MA

MA SOC

CLA

CLA SOC

Figure 3. (a) Slab models of SnBr2- (top) and CsBr- (bottom) terminated CsSnBr3 (100) surfaces, vac their electrostatic potential curves starting from the vacuum level (Eslab ), and the macroscopic avg

average inside the slab (Eslab ). (b) Calculated absolute energy of valence band maximum (EVBM vac ) of CsSnBr3, using the PBE (top) and HSE06 (bottom) functionals. Dash lines indicate average EVBM vac between SnBr2 (yellow) and CsBr (blue) terminations. MA: macroscopic averaging; CLA: core-level alignment; SOC: spin-orbit coupling.

We also carried out ab initio computational study for EVBM vac of CsSnBr3 by building slab models having SnBr2 or CsBr termination exposed to a vacuum layer. (Fig. 3a) Density functional theory VBM (DFT) calculations generated the VBM energy Ebulk of bulk CsSnBr3, as well as vacuum energy

Evac slab from the slab models. These two energies must be subtracted by their respective references, VBM : before deriving Evac VBM ref vac ref EVBM vac = (Ebulk − Ebulk ) − (Eslab − Eslab ) .

(1)


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ref We worked out the energy references Eref bulk and Eslab using two methods. In the macroscopic

averaging21 (MA) method described in Fig. 3a, the three-dimensional electrostatic potential field is first averaged within the xy plane to yield a potential curve along the z direction.22 The avg

macroscopic average Eslab of the electrostatic potentials inside the slab is taken as the energy ref . Similar macroscopic averaging of electrostatic potentials in bulk CsSnBr3 led to reference Eslab ref ref ref Ebulk = 0 by convention. In the core-level alignment23,24 (CLA) method, Ebulk and Eslab are

represented by the Sn 1s core levels in the bulk and inside the slab, respectively. For the two slab VBM models, the more electronegative SnBr2 termination consistently produced more negative Evac

than the CsBr termination, and the mean value of the two terminations was adopted as EVBM vac for bulk CsSnBr3. Fig. 3b and Table S1 summarize the calculated absolute potentials. The CLA method gave VBM Evac values 0.1–0.2 eV more negative than the MA method.21 Whether or not to include spin–

orbit coupling (SOC) had little effect ( 100 cm−1).

Fig. 5 plots the internal quantum efficiencies (IQE) of photodetectors made of CsSnBr3 and MAPbBr3 crystals, as a result of dividing the external quantum efficiencies (EQE) by the transmittance (20–50%) of the top Au electrode. In the absence of a bias voltage, the Al/TQB/CsSnBr3/Au device is responsive to the entire visible spectrum, coincident with the absorption spectrum of the black CsSnBr3 crystal. The IQE curve reaches its peak value of 7% at λ = 700 nm, and follows the steep drop of the absorption edge of CsSnBr3 at longer λ, because of ineffective light absorption. For above-bandgap photons, CsSnBr3 and CsPbBr3 possess absorption coefficients as high as 104–105 cm−1 and thus low (≤1 μm) penetration depths. Therefore, the response of the Al/TQB/CsSnBr3/Au device to excitation at λ < 660 nm is the overall effect of four individual steps: 1) photogeneration of free charge carriers; 2) efficient collection of hole at the



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CsSnBr3/Au interface; 3) diffusion of electrons across the 1-mm-thick crystal plate; and 4) efficient collection of electrons arriving at the TQB/CsSnBr3 interface. In addition to our previous discussion on interfacial collection of electrons and holes (Steps 2 and 4), the facile generation (Step 1) and diffusion (Step 3) of charge carriers in CsSnBr3 are also noteworthy. Halide perovskites are excitonic materials and a binding energy (Eb) is required to separate the photoinitiated exciton into free charge carriers. We have recently found10 Eb = 20 meV for CsSnBr3, lower than the thermal energy at room temperature (kBTrt = 26 meV), which means that charge separation of excitons in CsSnBr3 is spontaneous. For Sn-based perovskites, it is a general concern32,33 that p-type defects in the form of Sn4+ or Sn vacancies create trap states that induce charge-carrier recombination. From our observation of electron diffusion across 1-mm thickness of CsSnBr3 as minority carriers, it can be inferred that trap states in the CsSnBr3 crystal plate have been substantially diminished and that higher IQE values can be achieved by thinner plates or films of CsSnBr3. By contrast, the absorption spectrum of CsPbBr3 features a prominent exciton peak, reflecting a relatively high exciton binding energy Eb. This value may be comparable to Eb = 60 meV for MAPbBr3,34 which is much greater than that of CsSnBr3 and can favor charge-carrier recombination. The spectral response of the Al/TQB/CsPbBr3/Au photodetector is weaker and narrower than its CsSnBr3 counterpart, and shows a divergence between the two regimes of abovebandgap and near-bandedge excitation, suggesting less efficient charge-carrier diffusion in bulk CsPbBr3 than in CsSnBr3. For the Al/TQB/MAPbBr3/Au device, no photocurrent could be detected in the entire visible range, which indicates hindered electron collection at the TQB/MAPbBr3 interface. This result is consistent with the band alignment diagram in Fig. 1c showing an uphill CBM offset from MAPbBr3 to TQB.


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In conclusion, we performed experiments and computations on CsSnBr3 and located its VBM and CBM at −5.2 eV and −3.4 eV vs. vacuum, respectively. We further fabricated CsSnBr3 crystal plates into photodetectors of the device structure Al/TQB/CsSnBr3/Au with aligned band positions. Without an external bias, the CsSnBr3-based photodetector is responsive to the entire visible spectral range and exhibits a maximum IQE of 7% at 700 nm, significantly enhanced from photodetectors made of CsPbBr3 or MAPbBr3 in terms of both efficiency and spectral range. These findings suggest that the VBM of Sn-based perovskites generally lie higher than their Pb counterparts, and that the polypyridyl compound TQB is suitable for transporting high-lying electrons in CsSnBr3 and CsPbBr3 by energy-level matching and coordinating interactions. Lastly, the functional CsSnBr3 photodetector underscores the importance of interfacial engineering and clarifies undue concerns about bulk defects in this material. Further improvements in Sn-based perovskite photodetector and solar cells can be achieved by using a proper hole transport material and reducing the active layer thickness.

ASSOCIATED CONTENT Supporting Information. Details for preparation and characterization of materials and devices, and for theoretical calculations, Tables S1 & S2 (PDF) Crystallographic information file for TQB (CIF) AUTHOR INFORMATION Corresponding Author



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[email protected] Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is financially supported by a State Key Research Project (No. 2016YFA0204000) from the Ministry of Science and Technology of China and a Shell–CAS Frontier Sciences Program (No. PT78963), and technically supported by the Analytical Instrumentation Center and the High-Performance Computing (HPC) Platform of ShanghaiTech University. The authors thank Dr. Peihong Cheng, Dr. Na Yu, Mr. Zhifang Shi, and Ms. Qi Wei for assistance with experiments and calculations.

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Band Alignment Boosts Charge-Carrier Collection in Sn-Based

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