Solvent Engineering for Ambient-Air-Processed, Phase-Stable CsPbI3

Aug 29, 2016 - However, it is still a challenge to stabilize the desired black α-CsPbI3 perovskites in ambient air for photovoltaic applications. Her...
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Solvent Engineering for Ambient-Air-Processed, Phase-Stable CsPbI3 in Perovskite Solar Cells Paifeng Luo,* Wei Xia, Shengwen Zhou, Lin Sun, Jigui Cheng, Chenxi Xu, and Yingwei Lu Department of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui 230009, People’s Republic of China S Supporting Information *

ABSTRACT: Inorganic CsPbI3 perovskite solar cells (PSCs) owning comparable photovoltaic performance and enhanced thermal stability compared to organic−inorganic hybrid perovskites have attracted enormous interest in the past year. However, it is still a challenge to stabilize the desired black α-CsPbI3 perovskites in ambient air for photovoltaic applications. Herein, sequential solvent engineering including the addition of hydroiodic acid (HI) and subsequent isopropanol (IPA) treatment for fabricating stable and working CsPbI3 PSCs is developed, and a novel low-temperature phase-transition route from new intermediate Cs4PbI6 to stable α-CsPbI3 is also released for the first time. As such, the asprepared PSCs give a relatively high power conversion efficiency (PCE) of 4.13% (reverse scan), and the steady-state power output of 1.88% is confirmed for the selected cell with an initial PCE of 3.13%. To the best of our knowledge, this is the first demonstration of fabricating CsPbI3 inorganic PSCs under fully open-air conditions.

I

component in hybrid PSCs? In other words, can inorganic PSCs also possess comparable optical−electric properties? Recently, Cs/MA/FA-mixed PSCs, such as CsxMA1−xPbI3, CsxFA1−xPbI3, FA0.83Cs0.17Pb(I1−xBrx)3, and Csx(MA0.17FA0.83)100−xPb(I0.83Br0.17)3, have been developed, and further studies found that partially incorporated inorganic Cs cations could effectively improve the device performance and stability.10,19−22 To completely remove the unstable organic component and maximally enhance the thermal stability, all-inorganic CsPbI3 PSCs were successfully achieved under air-free conditions by Snaith last year, and the working PVs showed a preliminary PCE of 2.9%.23 Further research identified that the organic cation is not necessarily essential, and inorganic materials CsPbI3, CsPbBr3, and CsPbI2Br also demonstrated excellent optical−electric properties.23−27 Thus, these innovative works pave the way for development of more thermally stable inorganic PSCs. However, inorganic CsPbI3 materials are still unstable in the desired black perovskite α-phase that usually occurs at high temperatures of above 310 °C and rapidly degrade to the undesired yellow nonperovskite δ-phase in ambient air, unsuitable for the PV applications.23,24,28−30 It has been reported that PVs based on yellow-phase CsPbI3 layers only show poor PV performance with a limited PCE of 0.09%.19 This has led to a major problem in fabricating inorganic PSCs: how to produce and stabilize the black perovskites in ambient air. Recently, partial substitution of the I anion using a relatively smaller Br anion could contract the lattice and stabilize its structure, which should be an effective measure for improving

n recent years, organic−inorganic perovskite solar cells (PSCs) have received great attention in the photovoltaic research community due to their high efficiency, low cost, and easy solution process.1−5 In general, the light- absorbing layers of PSCs typically own a traditional AMX3 perovskite structure, where A can be the organic CH3NH3 (MA) and CHN2H4 (FA) or inorganic Cs; M is Pb or Sn; and X is the halogen I, Br, or Cl. On the basis of the excellent optical−electric properties of perovskite layers, such as a remarkably high absorption coefficient, tunable direct band gap, and long carrier diffusion length,6−8 the power conversion efficiency (PCE) of photovoltaic devices (PVs) has soared from 3.8 to above 21% in just a couple of years.9,10 Because highly efficient PSCs have been successfully achieved, currently, one of the most important tasks facing researchers is to overcome the stability problem. Unfortunately, this key issue of PSCs has yet to be resolved completely even though several effects were made to improve the stability of perovskite materials themselves and their PVs.11−15 From the perspective of materials, organic cation MA- or FAincluded perovskites are unstable, volatile, and hygroscopic in ambient air and are sensitive to the processing conditions on account of their intrinsic structural and thermal instability.16,17 Specifically, Conings et al. proved that MAPbI3 films thermally degrade to PbI2 when the environment temperature is higher than 85 °C.18 However, inorganic semiconductor thin-film solar cells, that is, commercial CuInGaSe2 (CIGS) and CdTe, have demonstrated outstanding long-term stabilities under outdoor testing. Taking advantage of the success of inorganic PVs, therefore, an effective strategy for enhancing the stability of hybrid PSCs is to substitute the organic component with an inorganic cation. Accordingly, a poignant question is put forward for consideration: How important is the organic © 2016 American Chemical Society

Received: July 18, 2016 Accepted: August 29, 2016 Published: August 29, 2016 3603

DOI: 10.1021/acs.jpclett.6b01576 J. Phys. Chem. Lett. 2016, 7, 3603−3608

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a) Crystal structures of α- and δ-CsPbI3 and photographs of S1 samples freshly coated and exposed for 24 h and (b) their XRD patterns. The black dots and star represent the XRD patterns of the TiO2/FTO substrate and the (112) plane of the orthorhombic (Pnma) lattice, respectively.

Figure 2. (a) Photographs of S2 samples freshly coated and exposed for 24 and 72 h and (b) their corresponding XRD patterns; (c) experimental and simulated XRD patterns of the Cs4PbI6 phase.

the stability of PSCs.24−28 However, compared with the ideal solar spectrum (1.1−1.7 eV), the wide band gaps of Br-included perovskites near or above 2.0 eV are only suitable for tandem PV applications. To stabilize the black perovskite phase, Snaith et al. report a solution route to form black CsPbI3 perovskites at room temperature in a totally air-free environment, but the ambient instability problem still remains.23 Herein, a sequential solvent engineering is developed, and the stable and working inorganic CsPbI3 PSCs are first fabricated under fully open-air conditions. Meanwhile, a new intermediate Cs4PbI6 is synthesized, and the novel phase-transition mechanism from Cs4PbI6 to α-CsPbI3 for effectively generating stable black CsPbI3 perovskites is also illustrated here. With the aid of hydroiodic acid (HI) and subsequent isopropanol (IPA) treatment, relatively stable CsPbI3 layers with high film quality are successfully achieved in an air atmosphere. As such, inorganic CsPbI3 PSCs with a relatively high efficiency of 4.13% (reverse scan) are obtained.

Thus far, all of the reported CsPbI3 inorganic PSC studies were carried out under a nitrogen atmosphere in a glovebox or a totally air-free environment.23,29 To expand the practical applications of PSCs, open-air processing for the fabrication of more stable perovskites should be strongly encouraged and explored in-depth at the current stage. However, CsPbI3 materials in a high-temperature black perovskite phase with excellent optical−electric properties quickly degrade in ambient air, which always results in a yellow nonperovskite phase with poor photovoltaic performance at room temperature.23,24,28−30 Because introduction of HI could stabilize the crystal structure and produce uniform films, here a small amount of HI with a concentration of 33 μL/mL is added into the N,Ndimethylformamide (DMF) precursor solution to prepare a CsPbI3 S1 sample, based on the conventional recipe.23 Park et al. have emphasized that the humidity and temperature of the laboratory room play a key role in the fabricating process of perovskites,31 while our experiments are manipulated under fully open-air conditions with humidity below 30% at room 3604

DOI: 10.1021/acs.jpclett.6b01576 J. Phys. Chem. Lett. 2016, 7, 3603−3608

Letter

The Journal of Physical Chemistry Letters

Figure 3. Planar SEM images of films (a) before and (b) after IPA treatment; cross-sectional SEM images of films (c) before and (d) after IPA treatment.

problem. First, different amounts of HI with concentrations of 33, 66, 132, and 165 μL/mL are designed and separately added into the precursor solution. As mentioned above, the CsPbI3 films are very unstable using the 33 μL/mL HI recipe. Surprisingly, when the HI concentration increases to 66 μL/ mL, the stable S2 sample films with a yellow−brown color are obtained, as shown in Figure 2a. From XRD results in Figure 2b, the (012) and (110) planes of the hexagonal Cs4PbI6 perovskite lattice are observed.32−34 When higher concentrations of 132 and 165 μL/mL are applied, yellow−brown precipitation appears in the solution. Using the same simulation method as that of Somma et al., the experimental pattern of the precipitation is in good agreement with the simulated plot based on ICSD#25124 structural data from Figure 2c, exhibiting a hexagonal Cs4PbI6 phase.32−34 Basically, besides CsPbI3, Cs4PbI6 is another ternary compound from solid solutions in the CsI−PbI2 system. Yunakova et al. reported that Cs4PbI6 is much more stable than CsPbI3, and they found that CsPbI3 would pass to Cs4PbI6 when it was heated up to 400 K.30,33 Strikingly, here we first realize the low-temperature solution synthesis of the Cs4PbI6 phase with the addition of 66 μL/mL HI. In addition, weak PbI2 peaks marked with squares are also examined. Second, the optimized precursor films are dipped into the hot IPA solution and annealed at 100 °C for 5 min in the air. After IPA treatment, the coated yellow−brown Cs4PbI6 films rapidly turn into dark brown α-CsPbI3 layers, and there is no obvious appearance change even after for 72 h, as verified by Figure 2a,b. Thus, we speculate that the addition of HI plays a key role during the phase-transition process. Here, the function of water contained in HI (45 wt %) is considered to facilitate the dissolution of CsI. When a small amount of HI (33 μL/mL) is implemented, the reaction should occur as follows

temperature, different from the required air-free environment. The experimental details are given in the Supporting Information (SI). Figure 1 shows the (a) crystal structures of α- and δ-CsPbI3 and photographs of the freshly coated and exposed for 24 h S1 samples and (b) their X-ray diffraction (XRD) patterns. From Figure 1b, we can see that the dark brown S1 sample exposed in the atmospheric environment rapidly degrades to yellow color in 24 h. The just coated CsPbI3 layers after drying (green pattern) show the main peaks at 14.28, 20.28, 24.70, 28.81, 32.24, 35.42, and 41.23°, respectively assigned to the (100), (110), (111), (200), (201), (211), and (220) planes of the cubic (Pm-3m) lattice, except for peaks of the TiO2/FTO substrate marked with black dots.23,28,29 A weak peak marked with a star at 22.64° also can be observed, which belongs to the (112) face of the orthorhombic (Pnma) crystal, indicating that a small amount of black perovskite CsPbI3 begins to degrade. When the fresh layers are exposed to air for 24 h, the cubic black perovskite totally degrades to the orthorhombic yellow nonperovskite. The characteristic peaks at 12.97, 21.67, 22.64, 31.28, and 39.31° are separately ascribed to the (012), (014), (112), (016), and (027) faces of the orthorhombic crystal.23,28,29 Therefore, it can be concluded that the fresh CsPbI3 films own a dominant black perovskite αphase structure that degrades very fast in ambient air. This phenomenon agrees well with the related reports.23,29 Here, the role of HI additive is to improve the solubility of CsI and PbI2 precursors in DMF solution, which enables CsPbI3 to change phases from the yellow nonperovskites to the black perovskites at low temperature. Unfortunately, the ambient instability problem of inorganic CsPbI3 materials still remains, making the fabrication of stable PVs in an air atmosphere challenging. Aiming at the issue of stabilizing black perovskite CsPbI3, sequential solvent engineering including the aid of HI and subsequent IPA solvent treatment is developed to resolve this 3605

DOI: 10.1021/acs.jpclett.6b01576 J. Phys. Chem. Lett. 2016, 7, 3603−3608

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The Journal of Physical Chemistry Letters CsI + PbI 2 → α‐CsPbI3 → δ‐CsPbI3

(1)

On the other hand, continuing to add HI to 66 μL/mL will suppress the solubility of PbI2 and form the complex compound PbI2·DMF,35 in agreement with above XRD result. Therefore, the reaction might change the mechanism and occur as below 4CsI + PbI 2 → Cs4PbI6

(2)

Subsequently, after IPA treatment, the phase transition should be as follows Cs4PbI6 → α‐CsPbI3 + 3CsI

(3)

3CsI + 3PbI 2 → 3α‐CsPbI3

(4)

Cs4PbI6 + 3PbI 2 → 4α‐CsPbI3

(5)

Figure 4. Absorption spectra of δ-CsPbI3 (blue dots) and α-CsPbI3 (red dots) films on TiO2/FTO substrates and the PL spectra of αCsPbI3 layers grown on glass (red line) and TiO2/FTO (green line) substrates; (inset) (αhν)2 vs hν of optical absorption spectra of αCsPbI3 films.

In brief, we present a low-temperature solution synthesis method of the Cs4PbI6 phase and find a new route of preparing stable α-CsPbI3 from Cs4PbI6 intermediate. More importantly, the obtained α-CsPbI3 layers from Cs4PbI6 are very stable, which can even endure in an air atmosphere for 72 h, which suffices to finish the total device preparation. After that, we directly explored the film morphology evolution process of the phase-transition route by scanning electron microscopy (SEM). Generally, smooth, compact, and pinhole-free perovskite layers are encouraged for achieving highly efficient planar PSCs.36,37 Because the film quality of perovskites is a key factor for the fabrication of working PSCs, the morphologies of Cs4PbI6 films before IPA treatment and CsPbI3 layers after IPA treatment are both investigated and shown in Figure 3 (with a HI concentration of 66 μL/mL). From Figure 3a,c, we can see that Cs4PbI6 films are composed of a large number of small grains with sizes below 100 nm. The randomly packed Cs4PbI6 grains are obscure, which should be caused by the incorporation of an amorphous PbI2·DMF complex, as verified above.35 After IPA treatment, the secondary growth phenomenon of grains is observed. As such, they grow up to above 100 nm, and Cs4PbI6 films change to the desired high-quality CsPbI3 layers with well-defined grain contours. As shown in Figure 3b,d, uniform, compact, and fullcovered CsPbI3 absorbers are obtained, with a thickness of ∼200 nm located on the TiO2/FTO substrate. The thickness can be adjusted by changing the solution concentration and coating parameters. However, the film surface is found to be partially dissolved in DMF when coated twice for growing thicker layers. Fortunately, as-coated CsPbI3 films with a thickness of 200 nm already could be used in PVs. The optical properties of δ-CsPbI3 (blue dots, yellow S1) and α-CsPbI3 (red dots, black S2) layers are characterized by the absorption spectra. As shown in Figure 4, the black α-CsPbI3 layers exhibit a strong absorption in the visible light region, extending up to ∼730 nm. Meanwhile, the desired band gap of 1.70 eV as shown in the inset graph is also obtained, consistent with related reports.23,29 The band gap calculation details can be found in the SI. On the contrary, the yellow δ-CsPbI3 films do not have any absorption at these long wavelengths, while they only absorb below ∼430 nm (2.88 eV). Therefore, only black α-CsPbI3 is suitable for the PV applications, and this also explains well the poor efficiency of PCSs based on δ-CsPbI3 in a related report.19 Then, photoluminescence (PL) spectra of αCsPbI3 layers grown on glass (red line) and TiO2/FTO (green line) substrates are also investigated. From Figure 4, we can clearly observe the intense PL emission with peak position at

725 nm, very close to the 730 nm absorption edge. Meanwhile, the PL quenching emission experiment is also designed to investigate the compatibility of α-CsPbI3 absorbers with TiO2 electron acceptors. As shown in Figure 4 (green line), the small emission band of TiO2/α-CsPbI3 bilayers with a red-shifted wavelength of 3.5 nm is detected, indicating good electrontransfer capability. Therefore, the as-obtained α-CsPbI3 layers show good optical properties and should be ideal light absorbers for PSCs. Finally, inorganic PSCs with a regular n−i−p planar structure of FTO/TiO2/α-CsPbI3/spiro-MeOTAD/Ag are fabricated to test their PV performances. Figure 5 shows the (a) crosssectional SEM image and (inset) cell photograph and (b) photocurrent density−voltage (J−V) curves including the reverse and forward scans of CsPbI3 inorganic PSCs, while the device performances detected under open-air conditions (scan speed: 0.1 V/s) are provided in Table 1 and Figure S1 (box plots of cell performances are in the SI). The CsPbI3 PSCs working at reverse scan give an average Voc of 0.65 V, Jsc of 12.19 mA/cm2, fill factor (FF) of 43.95, and PCE of 3.46%. Compared with the results of Snaith fabricated and hence measured under air-free conditions,23 our Voc is a little lower than that of 0.8 V, caused by the thinner absorber layers, and the Jsc is above 12 mA/cm2, while our FF is greatly improved compared with their 0.30, which leads to tremendous enhancement of the device efficiency. Our champion cells demonstrate a high efficiency of 4.13% (with a Voc of 0.66 V, Jsc of 11.92 mA/cm2, and FF of 52.47), higher than the reported PCEs of regular 2.9% and inverted 1.7% (p−i−n structure).23 To the best of our knowledge, this is the first time realizing ambient-air-processed preparation of inorganic CsPbI3 PVs. At the same time, current−voltage hysteresis is also significantly observed when comparing the reverse and forward scans, which once again proves that the polar organic component is not the root cause of generating hysteresis phenomena in hybrid PSCs.23 In addition, the J−V curves at different scan rates and the steady-state power output (SPO) are both examined to confirm our results and avoid the issue of hysteresis. As shown in Figure S2 in the SI, we get efficiencies of 2.48, 3.07, and 3.09%, assigned to the different scan rates (reverse scan) of 1, 0.1, and 0.01 V/s, respectively. The two efficiencies obtained from 0.1 and 0.01 V/s are very close, and only a much higher scan rate of 1 V/s shows a low PCE. As shown in Figure S3a,b 3606

DOI: 10.1021/acs.jpclett.6b01576 J. Phys. Chem. Lett. 2016, 7, 3603−3608

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Figure 5. (a) Cross-sectional SEM image and (inset) cell photographs and (b) J−V curves of the champion CsPbI3 PSCs.

Table 1. Performance Data for CsPbI3 Inorganic PSCs scan direction a

reverse forwarda a

Voc (V)

Jsc (mA/cm2)

FF

PCE (%) [best-PCE]

0.65 ± 0.03 0.62 ± 0.02

12.19 ± 0.73 11.93 ± 1.18

43.95 ± 6.14 31.27 ± 5.77

3.46 ± 0.52 [4.13] 2.31 ± 0.42 [2.83]

Each value represents the average from 14 cells.

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in the SI, we also observe that our cell gives a stabilized PCE of 1.88% based on the initial efficiency of 3.13% and only needs 46 s to obtain the SPO, confirming the stable feature of our CsPbI3 PSCs. Moreover, Hayase et al. recently reported an efficient CsPbI3 cell with a PCE of 4.68% by employing an inverted p− i−n structure and adding MoO3 hole extraction interlayers but still manipulated in the glovebox.29 Thus, we can continue to improve our cell performance by using different device structures and/or optimizing the electron and hole transport layers in the future work. In summary, we developed a new low-temperature solution method for fabricating stable and working CsPbI3 inorganic PSCs and first realized their fabrication under fully open-air conditions. Further study shows that the new synthesized Cs4PbI6 intermediate plays a key role for producing stable black-phase CsPbI3 at room temperature. As a consequence, relatively highly efficient CsPbI3 PVs with a reverse scan PCE of 4.13% are successfully achieved in this work, and the stabilized PCE of 1.88% is also confirmed from the selected cell with an efficiency of 3.13%. On the other hand, our inorganic PSCs are not absolutely stable in ambient air. The CsPbI3 materials usually offer a stability of 72 h when exposed in air with a humidity of less than 30%. Fortunately, such a long time suffices to integrate the whole device. Further work should focus on the structural stability of materials themselves and the replacement of organic carrier transport layers and the encapsulation of PVs. However, we have made an attempt to fabricate CsPbI3 inorganic PSCs through an ambient air process and achieved preliminary success. Therefore, our findings have significant meaning for the research and development of stable and efficient inorganic PSCs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51302058 and U1532140), the Natural Science Foundation of Anhui Province (1508085ME96), and the Applied Technical Achievement Training Program of Hefei University of Technology (JZ2016YYPY0036).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01576. Experimental methods, band gap calculation, and Figures S1−S3, showing box plots of Voc, Jsc, FF, and PCE and J− V curves (PDF) 3607

DOI: 10.1021/acs.jpclett.6b01576 J. Phys. Chem. Lett. 2016, 7, 3603−3608

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DOI: 10.1021/acs.jpclett.6b01576 J. Phys. Chem. Lett. 2016, 7, 3603−3608