Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3699−3703
pubs.acs.org/JPCL
Band Alignment Boosts Charge-Carrier Collection in Sn-based Perovskite over Pb Counterparts Binghan Li,†,‡,§,∥ Ruiying Long,†,‡,§,∥ Qishun Yao,† Zihao Zhu,† and Qixi Mi*,† †
School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China § University of Chinese Academy of Sciences, Beijing 100049, China ‡
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S Supporting Information *
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(4-phenylquinolin-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.
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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. Figure 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
etal 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 longterm device stability,6,7 as well as applications in X-ray detection8 and light-emitting quantum dots.9 Another allinorganic 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., © 2019 American Chemical Society
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. Received: May 17, 2019 Accepted: May 27, 2019 Published: May 27, 2019 3699
DOI: 10.1021/acs.jpclett.9b01405 J. Phys. Chem. Lett. 2019, 10, 3699−3703
Letter
The Journal of Physical Chemistry Letters 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 −3.4 eV, relatively high for perovskite semiconductors. The EVBM vac 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-2-yl)benzene (TQB)15 as a suitable electron transport material for both CsSnBr3 and CsPbBr3. For 16−19 CBM MAPbBr3 with EVBM Au vac = −5.9 eV and Evac = −3.6 eV, and TQB would make contacts with a barrier. We determined EVBM vac of CsSnBr3 by both experimental and computational methods. In Figure 2, ultraviolet photoelectron
Figure 3. (a) Slab models of SnBr2- (top) and CsBr- (bottom) terminated CsSnBr3 (100) surfaces, their electrostatic potential curves starting from the vacuum level (Evac slab), and the macroscopic average inside the slab (Eavg slab). (b) Calculated absolute energy of valence band maximum (EVBM vac ) of CsSnBr3, using the PBE (top) and HSE06 between (bottom) functionals. Dash lines indicate average EVBM vac SnBr2 (yellow) and CsBr (blue) terminations. MA, macroscopic averaging; CLA, core-level alignment; SOC, spin−orbit coupling. ref method, Eref bulk and Eslab are represented by the Sn 1s core levels in the bulk and inside the slab, respectively. For the two slab models, the more electronegative SnBr2 termination consisVBM tently produced more negative Evac than the CsBr termination, and the mean value of the two terminations was adopted as EVBM vac for bulk CsSnBr3. Figure 3b and Table S1 summarize the calculated absolute potentials. The CLA method gave EVBM values 0.1−0.2 eV vac more negative than the MA method.21 Whether or not to include spin−orbit coupling (SOC) had little effect ( 100 cm−1).
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 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
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01405.
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Details for preparation and characterization of materials and devices, and for theoretical calculations; Tables S1 and S2 (PDF) Crystallographic information file for TQB (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Binghan Li: 0000-0001-7826-0679 Qixi Mi: 0000-0001-7644-1417 Author Contributions ∥
These authors contributed equally.
Notes
The authors declare no competing financial interest. 3701
DOI: 10.1021/acs.jpclett.9b01405 J. Phys. Chem. Lett. 2019, 10, 3699−3703
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The Journal of Physical Chemistry Letters
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ACKNOWLEDGMENTS 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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DOI: 10.1021/acs.jpclett.9b01405 J. Phys. Chem. Lett. 2019, 10, 3699−3703
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DOI: 10.1021/acs.jpclett.9b01405 J. Phys. Chem. Lett. 2019, 10, 3699−3703
