Improved Environmental Stability and Solar Cell Efficiency of (MA,FA

Jun 26, 2019 - There is strong interest in improving the environmental stability of hybrid perovskite solar cells while maintaining high efficiency. H...
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Improved Environmental Stability and Solar Cell Efficiency of (MA,FA)PbI3 Perovskite Using a Wide-Band-Gap 1D Thiazolium Lead Iodide Capping Layer Strategy Lili Gao,†,‡ Ioannis Spanopoulos,‡ Weijun Ke,‡ Sheng Huang,§ Ido Hadar,‡ Lin Chen,† Xiaolei Li,† Guanjun Yang,*,† and Mercouri G. Kanatzidis*,‡ †

State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China ‡ Department of Chemistry and Argonne−Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois 60208, United States § Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, 5 Zhongguancun South Street, Haidian District, Beijing 100081, China S Supporting Information *

ABSTRACT: There is strong interest in improving the environmental stability of hybrid perovskite solar cells while maintaining high efficiency. Here, we solve this problem by using epilayers of a wide-band-gap 1D lead iodide perovskitoid structure, based on a short organic cation, namely, thiazole ammonium (TA) in the form of lead iodide (TAPbI3). The 1D capping layer serves to passivate threedimensional (3D) perovskite films, which promotes charge transport, improves carrier lifetime, and prevents iodide ion migration of the 3D (MA,FA)PbI3 film (MA = methylammonium, FA = formamidinium). Furthermore, the corresponding device achieved considerable efficiency and better environmental stability than the -based analogue, delivering a champion PCE value of 18.97% while retaining 92% of this efficiency under ambient conditions in air for 2 months. These findings suggest that utilization of a 1D perovskitoid is an effective strategy to improve the environmental stability of 3D-based perovskite solar cell devices maintaining at the same time their high efficiency.

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dimensional (2D) perovskites have been widely utilized in 2D/ 3D staking structure PSCs, which combine the advantages of enhanced air stability of the 2D perovskites as well as the strong light absorption and good charge transport carrier properties of the 3D perovskite materials.28−32 Owing to the hydrophobic nature of the large organic cations in 2D perovskites, mixed 2D/3D PSCs exhibit significantly enhanced long-term stability by effectively blocking the moisture invasion pathway to the vulnerable 3D perovskite segments.29,33 Some of the instability in perovskites is contributed to the high energies of the 5s2 (Sn2+) or 6s2 (Pb2+) orbitals, which are near the top of the valence bands and are thus more reactive.34,35 However, in addition to the impressive stability of the 2D perovskites, they can also bring some unfavorable character-

rganic−inorganic halide perovskites have become one of the most promising materials for solar cell applications because of their unique optical and electronic properties, such as high absorption coefficient, optical tuneability, long-range charge transport, and low-cost solution processability.1−6 The most prominent of them are the three-dimensional (3D) organic−inorganic halide perovskites, which have a general formula of ABX3 (A = methylammonium (MA), formamidinium (FA), or Cs; B = Sn2+ or Pb2+; X = Cl, Br, or I).7−13 The certified power conversion efficiency (PCE) of 3D-based halide perovskite solar cells (PSCs) has reached a value of 24.2%.14 Although having rapidly achieved high PCEs, because of their inherent structural characteristics, PSCs still face the challenge of longterm environmental stability, which degrades device efficiency.15−18 Compared to 3D materials, low-dimensional perovskites containing long-chain organic cations possess superior environmental stability19 and tunable optical properties.20−27 Two© 2019 American Chemical Society

Received: April 29, 2019 Accepted: June 26, 2019 Published: June 26, 2019 1763

DOI: 10.1021/acsenergylett.9b00930 ACS Energy Lett. 2019, 4, 1763−1769

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Cite This: ACS Energy Lett. 2019, 4, 1763−1769

Letter

ACS Energy Letters istics into the device assembly because of the presence of bulky organic cations where they can, e.g., decrease carrier mobility, cause charge accumulation, and hinder charge separation and charge extraction. Here we present an alternative approach in improving solar cell stability without sacrificing efficiency by using an epilayer of 1D perovskitoid phases.36 It has been observed by Bi et al.37 that a 1D layer of FEAPbI3 (FEA: 2,2,2trifluoroethan-1-amine) on the top of the 3D MAPbI3 phase can increase substantially the air stability of the assembled device, while Fan et al.27 used PZPY (2-(1H-pyrazol-1yl)pyridine) for the assembly of a 1D layer of PZPYPbI3 on top of a 3D perovskite layer and the corresponding solar cell device exhibited improved air and heat stability as compared to the pristine 3D-based device. This strategy could be a superior choice for the acquisition of both highly efficient and stable PSCs. In previous work, we defined a class of organic−inorganic metal halides that are not proper perovskites because they do not exhibit exclusively corner-sharing MX6 octahedral connectivity. The MX6 octahedra in these compounds feature edge- and face-sharing motifs or combinations of corner-, edge-, and face-sharing connectivity36 that cannot be properly called perovskites. They are collectively referred to as “perovskitoids”36 and can exhibit much higher environmental stability because the edge- and face-sharing motifs tend to stabilize (lower) the 5s2 (Sn2+) or 6s2 (Pb2+) orbital energies that typically dominate the top of the valence bands in proper perovskites. For example, the 1D perovskitoid structures of CsPbI3 and CsSnI3 are orders of magnitude more stable than the corresponding 3D perovskites.38 In order to take advantage of the extra stability, we sought a short organic conjugated molecule as a cation suitable in allowing good electron transfer properties39,40 but at the same time bulky enough to create a perovskitoid structure with the aim to study it as a capping layer on a 3D (MA,FA)PbI3 film. An organic spacer molecule that satisfies both of these parameters is thiazole ammonium iodide (TAI). Its reaction with lead iodide in a hydrohalide solution gave rise to a one-dimensional (1D) structure with a formula of TAPbI3 featuring face-sharing PbI6 octahedra and a wide-band-gap of 2.72 eV. After determining the single-crystal structure, we used it as an ultrathin film for the fabrication of a 1D/3D solar cell architecture and evaluated its performance in terms of PCE and environmental stability. The construction of 1D/3D stacked structures was achieved by the reaction of TAI with PbI2 on the surface of the 3D perovskite film, and the corresponding assembled devices achieved a high efficiency of 18.97%. Moreover, the 1D/3D devices exhibit remarkably improved long-term air stability retaining 92% of their initial efficiency after 2 months of exposure in air with a relative humidity of 20 ± 10%, at a temperature of 20 ± 5 °C, which is 32% higher than that of a 3D MAPbI3-based device only. These results verify that stable wide-band-gap 1D perovskitoid structures can be exploited to create highly efficient and more stable PSCs. The crystal structure of the TAI-based yellow 1D perovskite, with a formula of TAPbI3, is shown in Figure 1. The structure consists of 1D chains of face-sharing [PbI3]− octahedra, where thiazole ammonium cations located in the interlayer space charge balance the structure. The phase crystallizes in the centrosymmetric space group P-6 (174) in high yield and purity, as shown by the comparison of the experimental and calculated powder X-ray diffraction (PXRD) patterns of Figure 1d. The optical absorption properties of TAPbI3 are shown in

Figure 1. (a−c) Part of the TAI-based 1D perovskitoid structure viewed along different directions and (d) comparison of the calculated and as-made PXRD patterns of the TAPbI3 single crystals; inset: optical absorption spectrum showing the absorption edge at 2.72 eV.

the inset of Figure 1d, with a band gap of 2.72 eV. The crystals appear indefinitely stable in the air, and their PXRD pattern is unchanged after exposure to ambient air for over 70 days (Figure S2). As we will show below, this material forms in situ on the surface of (MA,FA)PbI3 as an epilayer when TAI is added on it. The 1D/3D perovskite films were prepared through a twostep process. First, the 3D perovskite film, from a solution containing MAI, FAI, and PbI2 with a molar ratio of 0.9:0.1:1.1, was fabricated by the gas-pump drying method.41 Second, an isopropanol solution of TAI was spin-coated on the surface of the 3D perovskite film to form the 1D TAPbI3 epilayer. Figure 2 shows the morphologies of the 1D/3D perovskite films. Without the 1D capped layer, the 3D perovskite film exhibits a pinhole-free morphology with grain size of around 350 nm, Figure 2a. The atomic force microscopy (AFM) image shows a smooth surface, where the root-mean-square (RMS) is only 7.44 nm, Figure 2b. The compact and smooth morphology is favorable for a planar perovskite device.42 After coating the (MA,FA)PbI3 film surface with an isopropanol TAI solution, an equally smooth pinhole-free surface of the capping layer can be obtained. The concentration of the TAI solution is optimized at 2.0 mg/mL, and the morphology/smoothness of the surfaces improves in going from 0.5 to 2.0 mg/mL, as clearly shown in Figure 2c−h. After TAI treatment, a compact capping layer grows on the top of the 3D perovskite surface, which serves as a selfencapsulating layer to protect 3D bulk perovskite from moisture. Scanning electron microscope (SEM) cross-sectional images of the films are shown in Figure S3a−d. The 3D perovskite cross section is uniform, and the thickness is about 400 nm. With TAI treatment, the cross section is irregular. The capping layer with 0.5 mg/mL TAI is discontinuous and cannot fully cover the (MA,FA)PbI3 surface. The thicknesses of capping layers with 1.0 and 2.0 mg/mL TAI are about 12 and 20 nm, respectively. In order to shed light on the composition and coverage of the capping layer on the (MA,FA)PbI3 surface, we carried out 1764

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Figure 3. XRD spectra of 1D/3D perovskite films with different TAI concentrations: (a) concentrations of 0, 0.5, 1.0, and 2.0 mg/ mL, (b) 1D TAPbI3 perovskitoid, 0, 5, 10, 20, and 30 mg/mL, and (c) magnification of the peak marked by the box in (b).

3D structure, forming a 1D/3D stacking perovskite architecture. The UV−vis absorption spectrum of the MA0.9FA0.1PbI3 film with the 1D TAPbI3 passivation layer is almost the same as that of the neat MA0.9FA0.1PbI3 film; see Figure 4a,b. As

Figure 2. SEM and AFM images of 1D/3D perovskite films passivated with different TAI concentrations: (a,b) morphologies of the 3D perovskite film; (c,d) 3D perovskite film passivated with 0.5 mg/mL TAI; (e,f) 3D perovskite film passivated with 1.0 mg/ mL TAI; (g,h) 3D perovskite film passivated with 2.0 mg/mL TAI.

X-ray photoelectron spectroscopy (XPS) measurements on the 1D TAPbI3, (MA,FA)PbI3, and TAI treated 3D perovskite films. Comparing the C 1s binding energy peak profiles in each case (see Figure S4), it seems that indeed the TAI passivated 3D film has the same composition with the 1D TAPbI3-only based film as the C 1s binding energies are the same for both the CC double bond and the C−N bond.32 This verifies the coverage of the 3D film with the 1D structure. The formation of the 1D TAPbI3 on the surface of the (MA,FA)PbI3 films can be observed using X-ray diffraction (XRD) as films are deposited from solutions of increasing TAI concentration. Figure 3a shows that X-ray reflections from the 1D TAPbI3 structure are growing in intensity as the thickness of the layer increases. Specifically, by increasing the TAI concentration to 10 mg/mL, the main (strongest) Bragg diffraction peak of the 1D structure is observed at 11.28° and is increasing in intensity as the TAPbI3 layer thickness (higher concentration of TAI) increases; see Figure 3b,c. These results verify that an ultrathin 1D perovskitoid layer deposits on the

Figure 4. Optical properties of 3D and 1D/3D perovskite films with 1.0 mg/mL TAI: (a) absorption spectra and (b) band gaps. (c) PL spectra of 1D/3D perovskite films, (d) PL lifetime of a 1D/ 3D perovskite film, (e) PL spectra, and (f) TRPL of a 1D/3D perovskite film coated with a layer of spiro-MeOTAD.

expected, the ultrathin wide-band-gap 1D perovskitoid layer does not affect the optical properties of the absorbing layer. This is further supported by the PL measurements, where the PL peak positions of both the 1D/3D perovskite film and the 3D perovskite film are centered at the same energy, Figure 4c. Nonetheless, the existence of the 1D TAPbI3 capping layer affects the electronic properties of the films as the 1D/3D perovskite film displays a stronger photoluminescence (PL) intensity than the 3D perovskite film (see Figure 4c), 1765

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dramatically suppressed. This demonstrates that the more stable 1D perovskitoid layer can prevent iodide migration from the absorbing (MA,FA)PbI3 layer to the electrodes, strongly inhibiting silver electrode degradation. In order to evaluate the photovoltaic performance of the 1D/3D perovskite structure, we fabricated PSCs using fluorine-doped tin oxide (FTO)/compact TiO2 (electron transport layer, ETL)/3D perovskite/1D TAPbI3/spiroOMeTAD (HTL)/Au. Figure 5a shows the cross configuration

indicating that the 1D TAPbI3 epilayer is helpful in carrier production. Furthermore, time-resolved PL (TRPL) measurements reveal that the 1D/3D perovskite film has a significantly longer average lifetime than the standard 3D perovskite film, Figure 4d. The calculated average lifetimes, τaverage, for all perovskite samples fitted by an biexponential formula32 are summarized in Table S1. On the basis of τaverage, the 1D/3D perovskite film exhibits a longer lifetime of ∼114 ns than the 3D perovskite film alone, whose average lifetime is 67 ns. This clearly suggests that the 1D TAPbI3 layer with its wide band gap of 2.73 eV not only does not interfere with the generation carriers but is also helpful in improving the photoexcited carrier lifetimes. Therefore, the 1D layer appears to be effective in passivating the surface of the 3D perovskite film by reducing the defects and suppressing the nonradiative recombination.32,43 By comparison, we investigated another set of film samples by coating a well-known hole transport layer (HTL) over them and measured the influence of the 1D TAPbI3 layer on the charge transport dynamics at the interface of perovskite and HTL layers, as the inset schematic shows in Figure 4e. 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-MeOTAD) was chosen as the HTL. The PL intensity of the 1D/3D perovskite film was lower than that of the pure 3D perovskite film, Figure 4e. In addition, a slightly shorter lifetime of 5.7 ns was obtained for the 1D/3D/HTL sample than for the 3D/HTL sample (7.0 ns) (see Figure 4f), which indicates that the 1D perovskitoid layer is generally beneficial to hole injection into the HTL. The faster hole extraction is attributed to the reduced interfacial nonradiative recombination, which suppresses the trap states on the 3D perovskite surface by deposition of the 1D capping layer material.32,43 In order to further confirm the charge transport between 1D/3D and the HTL, valence band maximum (VBM) values were measured by photoemission yield spectroscopy in air (PYSA). The results show that VBMs of separate 3D and 1D films are −5.45 and −5.72 eV, respectively. However, the VBM of the combined 1D/3D perovskite film is −5.50 eV, which is only 0.05 eV higher than that of 3D, as shown in Figure S5a. The VBM of the 1D/3D film therefore can match well with the HOMO level of the hole transport material (HTM, spiroMeOTAD). The schematic diagram of energy levels of the device layers is shown in Figure S5b and further supports that the appropriateness of the resulting band structure is for efficient charge transport and improved device performance. Some reports have shown that ion migration and ionic defects in 3D perovskite films could lead to device hysteresis,44 perovskite degradation, and efficiency loss in PSCs.45,46 Ions in the perovskite can react with metal electrodes through migration. One way to overcome this deficiency is the introduction of one additional layer between the perovskite and the HTL.22,31,47 In order to test if the 1D perovskitoid epilayer can also serve as a blocking layer for ion migration, we prepared the structures of FTO/perovskite (3D and 1D/3D)/ Ag and examined them by energy dispersive spectroscopy (EDS) elemental analysis after 1 week inside of a nitrogenfilled glovebox. The results are shown in Figure S6a,b. It can be observed after 7 days that interdiffusion has occurred, Ag is present in the 3D perovskite layer, and I is also present in the Ag layer. Interdiffusion between Ag and I can be easily identified in Figure S6a. However, Figure S6b shows that in the 1D/3D perovskite layer the interdiffusion phenomenon is

Figure 5. (a) Cross section of a 1D/3D stacking device structure and the crystal structure of a 1D/3D stacking perovskite, (b) J−V curves of the best fabricated 1D/3D PSCs under reverse and forward scans, (c) stabilized photocurrent measurement at the maximum power point (0.89 V) of 1D/3D PSCs, (d) corresponding IPCE measurement and integrated Jsc of 1D/3D PSCs, and (e) statistics of the PCE distribution of the fabricated 1D/3D PSCs (60 devices).

of the planar stacking in these devices. The best cell employing the 1D/3D perovskite material exhibited 18.97% efficiency with a short-circuit current density (Jsc) of 22.81 mA cm−2, an open-circuit voltage (Voc) of 1.08 V, and a fill factor (FF) of 0.77; see Figure 5b. Different devices prepared with different concentrations of the TAI solutions to deposit the 1D epilayers and a device lacking an 1D epilayer (control device) are shown in Figure S7. The solar cells showed little hysteresis effect with forward scan, yielding an efficiency of 18.02% with a Jsc of 22.55 mA cm−2, a Voc of 1.08 V, and a FF of 0.74. The 1D/3D device showed a suppressed photocurrent hysteresis with a low calculated hysteresis factor48 of 5.01%. The negligible hysteresis can be attributed to the 1D TAPbI3 layer, which prevents the migration of iodide ions, which generally induces device hysteresis. The inhibition of such migration avoids ion 1766

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The 1D TAPbI3 perovskitoid structure has a wide band gap of 2.73 eV and can be deposited as a capping layer on a 3D (MA, FA)PbI3 perovskite film to create high-performance solar cells with significantly improved stability. The 1D capping layer promotes better charge transport and suppresed ion migration in the 3D perovskite while preserving its beneficial optical properties. The assembled 1D/3D stacked structure PSCs achieved not only a high efficiency of ∼19% but also an impressive long-term operating stability in air at 1500 h, maintaining 92% of the initial efficiency under ambient conditions, RH = 20 ± 10% and temperature = 20 ± 5 °C. Furthermore, the devices maintained 84 and 70% of their initial efficiency after treatment for 500 h under 70 ± 10 °C and RH = 70 ± 10%, respectively. The successful and beneficial introduction of a TAI-based 1D TAPbI3 structure in a 3D-based device sets the stage for the utilization of other 1D wide-band-gap structures for further enhancement of both stability and performance in the corresponding solar cells.

accumulation at the interfaces between the perovskite and HTL. Figure 5c shows the steady-state photocurrent efficiency of the 1D/3D PSC. It exhibits a stabilized PCE of 18.52% at the maximum power point of 0.89 V, which is comparable to the device performance parameters extracted from the J−V curve. Figure 5d shows the corresponding IPCE spectrum with a comparable integrated current density of 21.51 mA cm−2. Figure 5e shows the statistics of the PCE distribution for the fabricated 1D/3D PSCs, which verifies the good reproducibility of our 1D/3D PSCs. Furthermore, the best J−V curves and reproducibility of different TAPbI3 passivated devices are also evaluated; see Figure S7a,b. From the above results, we can see that a compact thin 1D perovskitoid layer promotes multiple aspects of device performance. Low-dimensional perovskite and perovskitoid materials are generally more heat and moisture stable than the 3D (MA,FA)PbI3 materials, a fact that initially motivate this study.22,31,49 We evaluated the stability of 1D/3D stacked layer PSCs under different conditions. The unencapsulated 1D/3D devices were exposed in ambient conditions in air for 1500 h. Our best-performing device maintained 92% of its initial efficiency after 1500 h of storage in the dark with humidity of 20 ± 10% at 20 ± 5 °C; however, the corresponding standard 3D-based PSCs (lacking a 1D TAPbI3 epilayer) dropped to 60% of its original efficiency, Figure 6a.

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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/acsenergylett.9b00930. Experimental details, fitting parameters for the timeresolved PL decay from 1D/3D perovskite films with and without HTL, chemical structure and properties of TAI, stability PXRD of TAPbI3 perovskite, cross section images of 1D/3D perovskite film, XPS spectra of 1D, 3D, and TAI treated perovskite films, XRD spectra of 1D/3D perovskite films with different concentrations, band alignment of every device layer, EDS elemental analysis of FTO/perovskite/Ag structure, J−V curves of 1D/3D staking devices with different TAI concentrations passivated, and parameter statistics of 1D/3D staking devices with different TAI concentration passivated (PDF) Crystal data of TAI-based 1D perovskite (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ioannis Spanopoulos: 0000-0003-0861-1407 Weijun Ke: 0000-0003-2600-5419 Ido Hadar: 0000-0003-0576-9321 Xiaolei Li: 0000-0003-4913-0028 Guanjun Yang: 0000-0002-7753-3636 Mercouri G. Kanatzidis: 0000-0003-2037-4168

Figure 6. Stability performance: (a) 3D and 1D/3D PSCs with 1.0 mg/mL (b) 3D and 1D/3D PSCs stored in a 70 ± 10 °C environment, and (c) 3D and 1D/3D PSCs stored in a 70 ± 10% humidity environment.

Notes

In another set of testing conditions, the unencapsulated 3D and 1D/3D devices were heated in air at 70 ± 10 °C. The 1D/ 3D PSCs held 84% of the initial efficiency after 500 h, while 3D PSCs suffered 50% efficiency loss, Figure 6b. When the unencapsulated 3D and 1D/3D devices were also stored in a high-humidity environment, RH = 70 ± 10%, they retained 70% of the initial efficiency after 500 h of exposure, compared to 35% of the control 3D PSCs, Figure 6c. These results suggest that the epilayer of the wide-band-gap 1D perovskitoid material serves to improve the environmental and thermal stability of 3D perovskite-based devices.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Program for Support of Top-notch Young Professionals. The work at Northwestern University was supported by the ONR (N00014-17-1-2231), and PYSA measurements were carried out with equipment acquired by ONR Grant N00014-18-1-2102. The authors thank the Instrument Analysis Center of Xi’an Jiaotong University for the XPS, UV−vis, PL, and TRPL testing. 1767

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ACS Energy Letters L.L.G. gratefully acknowledges financial support from the Joint Educational Ph.D. Program of Chinese Scholarship Council (CSC).

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