Letter pubs.acs.org/JPCL
Unveiling the Crystal Formation of Cesium Lead Mixed-Halide Perovskites for Efficient and Stable Solar Cells Jae Keun Nam,† Myung Sun Jung,† Sung Uk Chai,† Yung Ji Choi,‡ Dongho Kim,‡ and Jong Hyeok Park*,† †
Department of Chemical and Biomolecular Engineering and ‡Department of Chemistry, Yonsei University, Seoul 03722, Korea S Supporting Information *
ABSTRACT: Thermal instability of organic−inorganic hybrid perovskites will be an inevitable hurdle for commercialization. Recently, all-inorganic cesium lead halide perovskites, in particular, CsPbI2Br, have emerged as thermally stable and efficient photovoltaic light absorbers. However, the fundamental properties of this material have not been studied in detail. The crystal formation behavior of CsPbI2Br is investigated by examining the surface morphology, crystal structure, and chemical state of the perovskite films. We discover a previously uncharacterized feature that the formation of black polymorph through optimal annealing temperature proves to be critical to both solar cell efficiency and phase stability. Our optimized planar heterojunction solar cell exhibits a J−V scan efficiency of 10.7% and open-circuit voltage of 1.23 V, which far outperforms the preceding literature.
H
halide perovskites are likely to grow to a level comparable to that of hybrid perovskites. Meanwhile, the formation of perovskite black polymorph is a crucial factor in the fabrication of high-performance solar cells. This requires the annealing procedure at an appropriate temperature and time. To date, the crystallization of hybrid perovskites has been extensively studied and well established, whereas no relevant research on this issue has been reported for all-inorganic perovskites.27,28 As discussed above, CsPbI2Br has attracted research motivation for all-inorganic perovskites. However, the basic understanding of this material has been ignored, and there is still much confusion in the experimental details. We compare the related literature and find that the difference in annealing procedure causes a large variation in the surface morphology and photovoltaic parameters (Table S1). Therefore, the aim of this study is to address the crystal formation behavior of cesium lead mixed-halide perovskites by investigating the surface morphology, crystal structure, and chemical state of CsPbI2Br film and ultimately fabricate highly efficient planar heterojunction solar cells. This study highlights the importance of annealing procedure for the formation of black polymorph of cesium lead mixed-halide perovskites, which greatly enhances the solar cell performance as well as phase stability. We prepare solution-processed CsPbI2Br perovskite films by a one-step spin-coating deposition. To afford a better description of the morphology evolution, as-coated films are annealed at the temperature range from 100 to 350 °C for a brief period (2 min). In Figure 1, scanning electron microscopic
ybrid organic−inorganic perovskites, with the general formula of ABX3 (A is an organic cation, such as methylammonium (MA) or formamidinium (FA), B is typically Pb, and X is a halide) are spotlighted as next-generation photovoltaic materials.1−7 Given their low exciton binding energy and ambipolar charge-transport characteristics, perovskite solar cells have achieved a sharp improvement with a certified power conversion efficiency (PCE) above 20%.8−10 Despite the rapid progress, thermal instability of hybrid perovskites has been a constant issue.11−15 For instance, MAPbI3 film undergoes reversible phase transition at 55 °C and degrades above 85 °C.11,12 FA-based perovskites have much improved thermal stability over MA-based perovskites, which can be increased further by incorporating inorganic cations such as Cs and Rb.16−18 Nonetheless, the intrinsic volatility of organic components in hybrid perovskites remains a technical limitation to satisfy a wide range of operating temperature. In this regard, all-inorganic perovskites are attractive materials for thermally stable photovoltaic light absorbers. Unlike hybrid perovskites, however, studies on these materials have lagged behind due to a lack of efficiency and phase stability. Recently, well-functioning cesium lead halide perovskite solar cells were reported by several groups.19−26 In particular, studies on cesium lead mixed-halide perovskites, CsPbI3−xBrx, clarified the phase stabilization of CsPbI3 by partial incorporation of Br.22,23 These reports highlighted CsPbI2Br as an efficient and stable solar cell material that can be operational even in an ambient atmosphere. Also, CsPbI2Br possesses a bandgap of 1.92 eV, which is still available for a photovoltaic light absorber. Although it is still controversial whether organic cations are vital for the superiority of perovskite materials, given the solar cell performance of the preceding literature, it is surely persuasive that cesium lead © XXXX American Chemical Society
Received: May 3, 2017 Accepted: June 13, 2017 Published: June 13, 2017 2936
DOI: 10.1021/acs.jpclett.7b01067 J. Phys. Chem. Lett. 2017, 8, 2936−2940
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The Journal of Physical Chemistry Letters
Figure 2. XRD patterns of the CsPbI2Br films annealed at the temperature range from 100 to 350 °C.
unfavorable at such a high temperature of 350 °C, a partial amount of CsI or CsBr could be expelled along the (200) planes of CsPbI2Br, resulting in phase segregation into I-rich and Br-rich regions. The PbI2 peak (2θ = 12.2°) is also observed, which is a byproduct of thermal decomposition. In addition, absorbance confirms the effectiveness of crystallinity, as the optimally annealed (280 °C) film exhibits higher light absorption than the less- (260 °C) or over- (330 °C) annealed films, as shown in Figure S1. To elucidate the effect of excess annealing above the optimal temperature on the chemical states of CsPbI2Br, we compare the X-ray photoemission spectroscopy (XPS) of the 280 and 350 °C annealed films. Figure 3 shows the deconvoluted Pb 4f spectra for the surface and bulk region. Detailed information on the measurement and deconvolution is available in the Supporting Information. For the surface measurements, a significant increase in the full width at half-maximum (fwhm) is observed, and we can separate two small peaks from the Pb 4f spectra (Figure 3a). Far more distinguishable peaks are identified in the bulk measurements of both films (Figure 3b). Referring to the literature, these peaks positioned at 136.6
Figure 1. SEM images of CsPbI2Br films annealed at the temperature range from 100 to 350 °C for 2 min. Scale bar: 1 μm.
(SEM) surface images are displaced in order of increasing temperature. Overall, the crystal formation of this material shows a preferential growth of large grains, known as the Ostwald ripening process. Upon annealing above 230 °C, perovskite crystals coalesce to form large grains, and gaps between the grains are simultaneously filled. The film annealed at 280 °C (Figure 1d) exhibits a pinhole-free surface with fully grown grains. Above 330 °C, the film starts to degrade, forming irregular spike-like crystals on the surface (Figure 1e). At a higher temperature of 350 °C, a severely damaged surface is observed (Figure 1f). X-ray diffraction (XRD) of the CsPbI2Br films, as described above, is measured to identify a change in the crystal structure. As shown in Figure 2, a series of XRD patterns presents a clear picture of the phase transition from the nonperovskite yellow polymorph, denoted as δ-phase, to the perovskite black polymorph, denoted as α-phase. The peaks at 2θ = 14.6, 29.5, and 20.8° are assigned to the (100), (200), and (110) planes of CsPbI2Br, respectively.22,23 The 230 °C annealed film is at an intermediate phase, exhibiting both α- and δ-phase characteristic peaks. As the temperature rises, the crystal is preferentially formed along the (100) and (200) planes, and the (110)-oriented growth is suppressed. Also, the peak splitting in the (100) and (200) planes, observed in the 260 °C annealed film, indicates the separate growth of I-rich (2θ = 29.1°) and Br-rich (2θ = 29.8°) crystals. In fact, all of the ionic bonds in the as-coated film must be restructured during the crystallization. This implies the sensitivity of annealing temperature to the phase transition of mixed-halide system. Upon reaching a sufficient temperature above 280 °C, the homogeneous solid solution of α-CsPbI2Br is formed. This is consistent with the high-quality surface morphology shown in Figure 1d. Above 330 °C, the peak splitting in the (100) plane is observed. Because the solid solution of CsPbI2Br is thermodynamically
Figure 3. Pb 4f detailed XPS spectra for the (a) surface and (b) bulk region of the CsPbI2Br films annealed at 280 and 350 °C. 2937
DOI: 10.1021/acs.jpclett.7b01067 J. Phys. Chem. Lett. 2017, 8, 2936−2940
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The Journal of Physical Chemistry Letters and 141.4 eV are assigned to metallic Pb (Pb0), which is formed when Pb cations are highly unsaturated due to a halide deficiency.29−31 In the surface region, Pb0 signal is negligible for the 280 °C annealed film, while small but evident Pb0 peaks are observed for the 350 °C annealed film. Also, in the bulk, the peak area ratio of Pb0/Pb increases from 0.18 (280 °C) to 0.26 (350 °C). As already discussed, an increase in metallic Pb possibly accompanies a halide deficiency. Quantification of the Pb 4f, Cs 3d, I 3d, and Br 3d spectra confirms that the 350 °C annealed film exhibits a significant iodine deficiency in the bulk region (Table 1).24 Taking all of the experimental evidence into account, the annealing procedure by a moderate temperature reduces the possibility of defect formation and preserves the cationic charge of Pb cations in the crystal lattice of CsPbI2Br. Note that small amounts of PbOx (∼137.7 eV) cannot be totally excluded in our data because we prepare the films under an ambient atmosphere.31 Time-resolved photoluminescence (PL) is measured to verify a change in photoexcited carrier dynamics by annealing temperature. As shown in Figure S2 and Table S2, PL profiles show a biexponential decay, which contains fast (τ1) and slow (τ2) components. Notably, the relative amplitude of fast component, which is attributed to nonradiative recombination, is relatively higher in the 260 and 350 °C annealed films.32 A lack of crystallinity and pinholes on the surface, as shown in the XRD and SEM images, could be attributed to the charge recombination in the less-annealed (260 °C) film. In addition, correlated with the XPS, the presence of metallic Pb (Pb0) and halide vacancies in the overannealed (330 and 350 °C) films possibly acts as charge trap sites and lowers the carrier lifetime within the perovskite crystals. Intuitively, we expect that the degree of annealing could have a significant impact on phase stability of CsPbI2Br. To identify this, the as-formed 260, 280, and 330 °C annealed films are stored in a controlled atmosphere chamber and exposed to moderate humidity (20 °C, 30% RH). As shown in Figure 4a, the 280 °C annealed film remains in its initial state, whereas the 260 and 330 °C annealed films undergo a significant degradation within 5 days. This observation supports our speculation that the less- (260 °C) and overannealed (330 °C) films are chemically unstable against moisture. However, once the black polymorph is formed by an optimized annealing procedure, it can remain stable for an extended period in a humid atmosphere. This is also verified by the XRD measurements in the initial state and 10 days after, as shown in Figure 4b, showing no change in the crystal structure. As discussed in the XPS, excess annealing adversely affects the phase stability by reducing the cationic charge of Pb species, thereby weakening the perovskite structure of black polymorph. On the basis of the study, highly efficient planar heterojunction perovskite solar cells are successfully demonstrated. The device configuration is Au/spiro-OMeTAD/ CsPbI2Br/blocking-TiO2/FTO/glass (Figure 5a). Figure 5b shows the current density−voltage (J−V) curve of the
Figure 4. (a) Photographs of the 260, 280, and 330 °C annealed films, showing color changes after storage in a controlled atmosphere chamber (20 °C, RH = 30%) for 10 days. (b) XRD patterns of the 280 °C annealed film, measured at as-formed and after 10 days.
champion cell. By employing optimally annealed CsPbI2Br films, the champion cell exhibits a PCE of 10.7% and an opencircuit voltage (VOC) of 1.23 V, which far outperforms the previously reported records.22−26 External quantum efficiency (EQE) is measured to confirm the validity of the J−V scan efficiency, of which the integrated short-circuit current density (JSC) is calculated to be 11.5 mA cm−2, as shown in Figure 5c. J−V forward scan and stabilized power output (SPO) of the champion cell are shown in Figure S3. Hysteresis behavior is observed, which is typically found in planar heterojunction solar cells. Stabilized PCE and JSC, measured at Vmax of 0.96 V, exhibits 9.5% and 9.8 mA cm−2, respectively. Detailed information on all of the fabricated solar cells is available in Table S3. The optimized annealing procedure could contribute to this superior performance, showing the pinhole-free surface, high crystallinity, and stabilized chemical state, as discussed throughout the study. Obviously, photovoltaic parameters of our cells verify that cesium lead halide perovskites have a high capacity for solar cell materials. The main reason for the discrepancies in the annealing procedure, as shown in Table S1, is assumed to be the difference under atmospheric condition. We prepare the samples under an ambient atmosphere (∼20 °C, RH < 10%). On the contrary, Sutton et al. and Niezgoda et al. performed experiments in an inert gas-filled glovebox, eliminating the presence of oxygen and moisture.22,26 This factor may have a significant impact on the crystal growth and degradation. Therefore, further research is needed to identify the effect of atmospheric condition on the crystal formation of cesium lead mixed-halide perovskites. Because of the insufficient solubility of Br species, our solution-processed CsPbI2Br film is only ∼85 nm thick (Figure
Table 1. Atomic Ratio of I/Pb and Br/Pb, Quantified by the XPS Signal of Pb 4f, I 3d, and Br 3d surface I/Pb Br/Pb
bulk
280 °C
350 °C
280 °C
350 °C
1.52 0.92
1.48 0.70
1.48 0.90
0.95 0.78 2938
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solar cells based on this film shows superb photovoltaic parameters, especially VOC and fill factor (FF). Considering the solar cells we have fabricated, cesium lead halide perovskites present an enormous potential for efficient and stable solar cell materials. Our findings will provide a more precise scheme for understanding the less studied but promising cesium lead halides and will be a significant help in further improvements on all-inorganic perovskites for solar cell application.
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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.7b01067. Experimental section, Figures S1−S4 (absorbance, timeresolved PL, J−V forward scan, SPO, and surface profile), and Tables S1−S3 (comparison of the literature, fitted parameters for the time-resolved PL, and photovoltaic parameters of all the fabricated solar cells). (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Dongho Kim: 0000-0001-8668-2644 Jong Hyeok Park: 0000-0002-6629-3147 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the support from National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF-2016R1A2A1A05005216, NRF2016M3D3A1A01913254, NRF-2015M2A2A6A01045277, NRF-2014M3A7B4051747). This work was also partially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry and Energy (MOTIE), Republic of Korea (20163010012450).
Figure 5. (a) Schematic illustration of the planar heterojunction perovskite solar cell. (b) J−V scan, where the inset shows the four major photovoltaic parameters, and (c) EQE of the champion cell.
S4). Recent studies demonstrated the alternative methods such as vacuum-processed cosublimation and spray-assisted deposition that can overcome the solubility limits of precursor solution.24,33 Advanced techniques will lead to the formation of a sufficiently thick and high-quality cesium lead halide perovskite film. Above all, further understanding of all-inorganic perovskite materials will help to achieve superior efficiency as well as phase stability, as has been achieved for hybrid ones. The Shockley−Queisser limit describes that such a high bandgap material can produce high VOC, which can be applied to various electronic devices. In addition, as a practical use of perovskite materials in industry, cesium lead halide is suitable for a top cell of multiterminal tandem device with c-Si or CIGS solar cell because it possesses a high bandgap of 1.8 to 2.3 eV, depending on the halide composition, whereas hybrid perovskite has relatively low bandgap of ∼1.6 eV.34,35 Given these advantages, all-inorganic cesium lead halide perovskites have a huge potential for broad applications thanks to their distinguishing features from a hybrid one. In closing, we have investigated the crystal formation behavior of cesium lead mixed-halide perovskites by characterizing the differently annealed CsPbI2Br films. This study reveals the complexity of the crystal formation process and its profound influence on both solar cell performance and phase stability. By optimizing the annealing temperature, we fabricate the highly crystalline and defect-free CsPbI2Br film, and the
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