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Potassium Incorporation for Enhanced Performance and Stability of Fully Inorganic Cesium Lead Halide Perovskite Solar Cells Jae Keun Nam, Sung Uk Chai, Wonhee Cha, Yung Ji Choi, Wanjung Kim, Myung Sun Jung, Jeong Kwon, Dongho Kim, and Jong Hyeok Park Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00050 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017

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Potassium Incorporation for Enhanced Performance and Stability of Fully Inorganic Cesium Lead Halide Perovskite Solar Cells

Jae Keun Nam†, Sung Uk Chai†, Wonhee Cha‡, Yung Ji Choi‡, Wanjung Kim†, Myung Sun Jung†, Jeong Kwon§, Dongho Kim‡, and Jong Hyeok Park*,† †

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea ‡

Department of Chemistry, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea §

Department of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 16419, Republic of Korea *Correspondence to Prof. J. H. Park (email: [email protected])

ABSTRACT

Thermally unstable nature of hybrid organic-inorganic perovskites has been a major obstacle to fabricating the long-term operational device. A cesium lead halide perovskite has been suggested as an alternative light absorber, due to its superb thermal stability. However, the phase instability and poor performance are hindering the further progress. Here, cesium lead halide perovskite solar cells with enhanced performance and stability are demonstrated via incorporating potassium cations. Based on Cs0.925K0.075PbI2Br, the planar-architecture device achieves a power conversion efficiency of 10.0%, which is a remarkable record in the field of inorganic perovskite solar cells. In addition, the device shows an extended operational lifetime

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against air. Our research will stimulate the development of cesium lead halide perovskite materials for next-generation photovoltaics.

KEYWORD: perovskite solar cells, cesium lead halide, compositional engineering, phase stability


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Hybrid organic-inorganic perovskite solar cells are now taking center stage as nextgeneration photovoltaics. Since the first perovskite solar cells were reported, enormous research efforts have achieved remarkable photovoltaic performance with a certified power conversion efficiency above 20%.1-8 A perovskite has small exciton binding energy and ambipolar characteristics, which leads to the facile charge carrier formation and transport.9-12 Thanks to its desirable properties, hybrid perovskite light absorbers have exhibited superior performance. The progress on deposition techniques enables to produce a high-quality perovskite layer with large grain sizes. Representatively, two-step sequential method, vapor deposition and selective solvent washing method have been highlighted.13-16 Compositional engineering has also been studied such as mixed-halide, mixed-cation, and non-stoichiometric perovskites, where a well-tailored perovskite shows the enhancement of both photovoltaic performance and stability.6, 17-22 Apart from the performance, the long-term stability is vital for commercial development. Thermal stability issues of hybrid perovskite solar cells have been continuously proposed.23-26 Evidently, a hybrid perovskite has a vulnerability to heat. Due to a thermally unstable nature of organic materials, a methylammonium lead iodide film, which is most widely used as a light absorber, undergoes thermal decomposition above 85 °C.23 As an alternative, a cesium lead halide perovskite has been studied as thermally stable light absorber.27-31 However, application into the photovoltaic device is quite challenging. Specifically, CsPbBr3 has a too large bandgap of 2.3 eV, unable to absorb the lights with long-range wavelength. Although CsPbI3 has more suitable bandgap 1.73 eV, it suffers from the notable phase instability. A cesium cation has not enough size for holding PbI6 octahedra, implied by the tolerance factor barely over 0.8.27, 31, 32 To overcome the notable investigated.29,

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phase instability,

mixed-halide CsPbI3-xBrx

perovskites

were

Incorporation of bromine into CsPbI3 leads to stabilizing the perovskite


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structure. Both works emphasized that CsPbI2Br has a reasonable bandgap of 1.92 eV and considerably high ambient stability. Still, the current experimental level is not enough for fabricating highly efficient and stable inorganic perovskite solar cells. Formulation of the perovskite structure may provide a solution for further stabilized cesium lead halide. In the field of hybrid perovskites, many researchers proposed that partial substitution of A-site cations in ABX3 perovskite is a fine method for stabilizing the crystal. For example, formamidinium based perovskite obtain the ambient stability by mixing with methylammonium.6 Inorganic cations such as cesium and rubidium were also employed, showing improved efficiency and thermal stability.17, 20, 33 Here, we report a compositional engineering approach for cesium lead halide perovskites, by means of incorporating potassium cations. Motivated by the works on hybrid perovskites, we inferred that partial substitution of A-site cation in ABX3 perovskite by smaller cation also bring the advantageous features on the crystals in the cesium lead halide system. Following the previous work, CsPbI2Br was chosen as a reference, which can be handled in dehumidified atmosphere.29 By varying the stoichiometric ratio of potassium, Cs1-xKxPbI2Br films were investigated. At optimal condition (x = 0.075), Cs0.925K0.075PbI2Br planar-architecture perovskite solar cells achieve a maximum power conversion efficiency (PCE) of 10.0% and an average of 9.1%, exceeding the reference CsPbI2Br devices with an average of 8.2%. Stability against air is also improved, compared to pristine CsPbI2Br perovskites. For the film preparation, we first attempted to follow the procedure reported elsewhere.29 All the preparation steps were conducted in dehumidified atmosphere (~20 °C, RH < 20%). As already reported, we also observed a low solubility of perovskite precursors, by which only a ~100 nm layer is formed through a one-step spin coating method. Unlike previous work, the


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perovskite film was degraded during an annealing step at 350 °C. We assume that our experimental condition (ambient air, versus a nitrogen-filled glove box) changes a chemical response to thermal stress. Consequently, we modified the annealing condition, which was optimized to be 280 °C for 10 min. As-formed films show brownish color, along with little optical haze on the surface (see Figure S1 in the Supporting Information). X-ray diffraction (XRD) patterns of the black phase Cs1-xKxPbI2Br films are shown in Figure 1a (x = 0, 0.025, 0.05, 0.075, 0.1). The shift of the peak position was investigated by a closer look for the (100) peak. Figure 1b shows the Gaussian fitting curves for (100) peaks, identifying a maximum peak position and a full width at half maximum (FWHM) of each curve (see Table S1 in the Supporting Information). For x = 0.075, the largest shift of the peak position to higher angle was observed, which could be correlated with a significant decrease in the lattice constants. According to Bragg’s law, the lattice constants for a cubic structure are calculated to be 6.0341 Å, which is very similar to the reported value, and 6.0137 Å for x = 0 and x = 0.075, respectively.30 Presumably, the contraction of cubic volume may contribute to stabilizing the perovskite phase.17 Additionally, XRD peak intensity is significantly increased for x = 0.075 compared to x = 0, indicative of an increase in the crystallinity that the preferred orientation is arranged. Supplementary XRD patterns for films with x > 0.1 are placed in Figure S2 in the Supporting Information. For the purpose of identifying the chemical states of CsPbI2Br and Cs0.925K0.075PbI2Br, X-ray photoelectron spectra (XPS) were investigated. Figure 1c-1f show the XPS spectra for each element, calibrated based on C 1s (284.8 eV). Photoemission of K 2p could not be resolved due to a low atomic percentage, thus being out of the detection limits and overlapping with that of C 1s (see Figure S3 in the Supporting Information). Energy dispersive spectroscopy (EDS) was performed to identify the presence of potassium (see Figure S4 in the


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Supporting Information). Notably, a shift of the peak position to higher binding energy is observed in Cs0.925K0.075PbI2Br for Pb 4f and Br 3d. For the photoemission of Pb 4f, the peak position shifts to higher binding energy from 137.8 eV to 138.2 eV. For the photoemission of Br 3d, the peak position shifts to higher binding energy from 67.95 eV to 68.35 eV, respectively. The shifts certainly originate from the insertion of potassium cations, as the previous research confirmed that the binding energies of lead cations and halides did not alter by a little perturbation on stoichiometric ratio between I and Br in mixed-halide perovskites.33 The shifts in binding energy are clear evidences for the changes in chemical bonding properties between lead cations and halides. We presume that the greater part of potassium cations could be located in Asites of the perovskite, since excess PbI2 is scarcely detected in XRD measurements. This leads to the contraction of perovskite cubic volume, associating with the difference between ionic radius of cesium (1.67 Å) and potassium (1.38 Å). Phase stability of perovskites is determined by the volumetric ratio between PbX6 octahedra and A-site cations.32 An increase in cationic charge of lead ions, confirmed by XPS shifts, could contribute to tightening halides and shrinking the PbX6 (X = I or Br) octahedral volume. This could be advantageous that A-site cations strongly hold the corner-shared PbX6 octahedra, leading to the enhancement of the phase stability. Photophysical characteristics for potassium-containing cesium lead halides were also examined. Figure 2a shows absorbance spectra of Cs1-xKxPbI2Br films, of which the thicknesses are carefully controlled to be same. A significant increase in absorbance intensity is observed for x = 0.075 over the entire wavelength, which could be merit for the formation of photoexcited charge carriers in the perovskite light absorber. As shown in Figure 2b and 2c, time-resolved PL decay profiles were measured for CsPbI2Br and Cs0.925K0.075PbI2Br films, with and without a blocking-TiO2 electron extraction layer, to reveal the changes in charge carrier injection.10, 22, 34-


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Photoluminescence (PL) spectra were measured to be ~654 nm and ~663 nm for

Cs0.925K0.075PbI2Br and CsPbI2Br, respectively (see Figure S5 in the Supporting Information). The PL profiles, monitored at the peak emission, exhibit bi-exponential decays with an average lifetime of ~11 ns and ~14 ns for pristine Cs0.925K0.075PbI2Br and CsPbI2Br, respectively (see Table S2 in the Supporting Information). A slight decrease in the charge carrier lifetime might be derived from residual potassium cations, which fail to be located in A-site of the perovskite. However, it is remarkable that Cs0.925K0.075PbI2Br film with the electron extraction layer possesses a large amount of charge transport components (τCT = 0.74 ns, 16%), outweighing CsPbI2Br film (τCT = 1.3 ns, 3%). This indicates that far more efficient carrier extraction occurs in Cs0.925K0.075PbI2Br film, which could be the evidence for enhanced photovoltaic performance. Scanning electron microscopy (SEM) surface images of CsPbI2Br and Cs0.925K0.075PbI2Br films are shown in Figure 3a and 3b. Apparently, CsPbI2Br film shows a rough surface with small crystals, whereas Cs0.925K0.075PbI2Br film shows a smoother morphology with larger crystals. On account of the uniform distribution of potassium cations by EDS maps (Figure S3), we can conclude that the morphology refinement is additional benefits of potassium incorporation. Comparable researches on hybrid perovskites also suggested that small amounts of metal ion additives could modify the crystallization process, forming highly crystalline, uniform perovskite layers.40,

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This surely dedicates to improving the photo-induced charge

carrier generation and transportation. To confirm the effectiveness of potassium incorporation on photovoltaic performance, planar-architecture perovskite solar cells were fabricated. Cross-sectional SEM images of the device are shown in Figure 3c and 3d. Full coverage of the perovskite layer is observed in the device, of which the film thickness is measured to be ~100 nm. At a scan rate of 0.12 V s-1 with


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an aperture area of 0.15 cm2, the Cs0.925K0.075PbI2Br devices achieve a maximum PCE of 10.0% and an average of 9.1%, whereas the reference CsPbI2Br devices show an average of 8.2%. Surely, these are outstanding records among fully inorganic perovskite solar cells. Current density-voltage (J-V) curves for the champion device are shown in Figure 4a. Hysteresis behavior is observed in a forward scan, which is typically found in the planar-architecture device. Stabilized power output (SPO) exhibits over 9% in PCE (see Figure S6 in the Supporting Information). In Figure 4b, External quantum efficiency (EQE) was measured to guarantee the validity of the J-V scan efficiency. Integrated short-circuit current density (Jsc) is calculated to be 11.6 mA cm-2, which exactly corresponds to Jsc from the J-V scans. Histograms of the J-V scan efficiencies for each batch of 20 devices are shown in Figure 4c and 4d. Detailed information on the individual device for each batch is summarized in Table S3 and S4 in the Supporting Information. Clearly, Jsc is a primary parameter for the enhanced performance. It attributes to the fact that more photoexcited carriers could be generated in Cs0.925K0.075PbI2Br film and then more electrons could be injected into the blocking-TiO2 layer, which are confirmed by absorbance, PL decay profiles, and EQE spectra (see Figure S7 in the Supporting Information). Finally, Air stability of the device based on CsPbI2Br and Cs0.925K0.075PbI2Br was evaluated. Without any encapsulation, the devices were stored in a constant temperaturehumidity chamber, where an interior atmosphere was controlled to be 20 °C and RH = 20%. Figure 5 shows normalized Jsc, open-circuit voltage (Voc), fill factor (FF), and PCE as a function of time. The normalized PCE for the CsPbI2Br device gradually decreased and reached half of its initial value in 72 hours. However, Cs0.925K0.075PbI2Br device maintained 80% of its initial value for 120 hours. We infer that the phase transition from perovskite black phase to non-perovskite yellow phase is suppressed by potassium incorporation, as mentioned above in XRD and XPS


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analysis. Still, in the moisture-rich condition (RH > 50%), films were rapidly transformed to yellow phase within a few hours. Further tailoring inorganic perovskites is necessary for possessing high structural stability. Advances in deposition techniques will bring far more enhanced photovoltaic performance, attributed by controllable thickness, high crystallinity, and surface morphology, as it has achieved for hybrid organic-inorganic perovskite solar cells. Thanks to its inherent thermal stability, a fully inorganic perovskite has a potential for applying to commercial products prior to organic-containing materials. Furthermore, it is well-studied that a perovskite light absorber with a bandgap near 1.8 eV is desirable for the silicon-perovskite tandem device, which is considered as a practical use for the industrial application.42 For sure, cesium lead halide perovskites possess suitable features for this application.27-30 In summary, we have achieved high performance fully inorganic cesium lead halide perovskite solar cells. By partial substitution of cesium by potassium (Cs0.925K0.075PbI2Br), an average PCE is increased from 8.2% to 9.1%, including the champion device of 10%. Potassium incorporation in cesium lead halide perovskite solar cells facilitates photoexcited charge carrier formation and transportation, as shown by enhanced Jsc. Moreover, the device shows extended operational lifetime against air. Incorporation of potassium cations leads to the contraction of PbX6 octahedral volume, thereby improving the phase stability. Our research opens the possibility of further improvement on fully inorganic perovskite solar cells.


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EXPERIMENTAL SECTION Device fabrication The FTO glass (TEC15, Pilkington) was cleaned in sequence with detergent, acetone and ethanol then treated with ultraviolet ozone for 30 min. A compact hole-blocking TiO2 layer (bl-TiO2) was formed by a spin coating of 0.2 M titanium diisopropoxide bis(acetylacetonate) (75 wt% in isopropanol, Sigma-Aldrich) solution in 1-butanol (Sigma-Aldrich) on the FTO glass at 2000 rpm for 30 sec, then dried at 125 °C for 10 min. The film was annealed at 500 °C for 30 min. After cooling to room temperature, the film was treated with a 20 mM aqueous TiCl4 (99.8%, Sigma-Aldrich) solution at 90 °C for 30 min then cleaned with ethanol, followed by annealing again at 500 °C for 30 min. To prepare a Cs1-xKxPbI2Br precursor, CsI (SigmaAldrich), PbI2 (TCI), PbBr2 (Sigma-Aldrich), and KI (Sigma-Aldrich) were dissolved in dimethylformamide (DMF, Sigma-Aldrich) with an appropriate molar ratio to form a 0.4

M

solution, followed by stirring at 90 °C for 1 hour before use. The solution was then spin coated on bl-TiO2/FTO/glass at 1500 rpm for 40 sec. Right after the spin coating, the film was annealed at 280 °C for 10 min. A 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD, Merck) solution was spin coated on perovskite/bl-TiO2/FTO/glass at 4000 rpm for 30 sec to form a hole transporting layer. To prepare a spiro-OMeTAD solution, spiroOMeTAD/chlorobenzene (72.3 mg/ml, Sigma-Aldrich) was mixed with 17.5 µl of lithium bis(trifluoromethanesulfonyl)imide/acetonitrile (520 mg/ml, both from Sigma-Aldrich) and 28.8 µl of 4-tert-butylpyridine (Sigma-Aldrich). Finally, a gold counter electrode was deposited by thermal evaporation. All the fabrication steps were conducted in dehumidified atmosphere (~20 °C, RH < 20%).


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Characterization Current density-voltage (J-V) characteristics were measured by a solar simulator (Sol 3A, Class AAA, Newport) under 1.5G air mass one sun (100 mW/cm2) illumination from a filtered 450 W xenon lamp (6279NS, Newport), calibrated by the NREL-calibrated silicon reference solar cell. To fix the active area, a light blocking mask was placed beneath the device with an aperture area of 0.15 cm2. A QE/IPCE measurement system (Scan 100, Zolix) was used to measure the external quantum efficiency (EQE) accompanied by data acquisition system, xenon light source, and reference silicon solar cell. Surface morphologies and the device structure were observed by a field-emission scanning electron microscopy (FE-SEM, JSM-7001F, JEOL). The structural characteristics were investigated by X-ray diffractometer (XRD, SmartLab, Rigaku) with a Cu-Kα radiation (1.5418 Å). Chemical states were identified by an X-ray photoemission spectroscopy (XPS, K-alpha, Thermo Fisher). Absorbance was measured by a UV-vis spectroscopy (V-650, Jasco). Energy dispersive spectroscopy (EDS, X-Max, Oxford) was performed to confirm the uniform distribution of potassium cations.

Time-resolved photoluminescence measurement Photoluminescence (PL) spectra were measured by a fluorescence spectrophotometer (F7000, Hitachi). The spontaneous PL decay was recorded by a time-correlated single-photon counting (TCSPC) technique. A mode-locked Ti:sapphire laser (MaiTai BB, Spectra-Physics) was used as the excitation light source, which generates ultrashort pulses of 80 fs FWHM with a high repetition rate of 80 MHz. This repetition rate can be adjusted to the range of 1 MHz–800 kHz by an in-house-built pulse picker. The frequency of the output pulse was then doubled to


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420 nm by a 1 mm thick BBO crystal (EKSMA). To obtain polarization-independent PL decays, a pump pulse was vertically polarized by a Glan-laser polarizer and a sheet polarizer was placed in the PL pathway at a complementary angle of 54.7 °. Finally, PL was collected by a microchannel plate photomultiplier (MCP-PMT, R3809U-51, Hamamatsu) with a thermoelectric cooler (C4878, Hamamatsu), integrated to a TCSPC module (SPC-130, Becker Hickl GmbH). The overall instrumental response function was ~30 ps FWHM.

Air stability evaluation Without any encapsulation, the devices based on CsPbI2Br and Cs0.925K0.075PbI2Br were stored in a constant temperature-humidity chamber. An interior atmosphere was controlled to be 20 °C and RH = 20%. J-V characteristic was periodically measured to follow the changes in photovoltaic parameters.


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ASSOCIATED CONTENT Supporting Information Available: [Figure S1-S7 include photographs of perovskite films, supplementary XRD and XPS spectra, EDS maps, PL spectra, SPO for the champion device, EQE spectra, and statistics for photovoltaic parameters. Table S1-S4 include XRD analysis, time-resolved PL decay profiles, detailed information on the individual device for each batch.] This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Correspondence to Prof. J. H. Park (email: [email protected]) Notes The authors declare no competing financial interests Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT Prof. J. H. Park acknowledges the support from National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF-2013R1A2A1A09014038, 2015M1A2A2074663). Prof. D. Kim acknowledges the support from the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT Future Planning (MSIP) of Korea under contracts


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NRF-2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System).


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Figure 1. (a) XRD patterns and (b) the Gaussian fitting curves for (100) peaks for Cs1-xKxPbI2Br films (x = 0, 0.025, 0.05, 0.075, 0.1). XPS spectra of CsPbI2Br (black) and Cs0.925K0.075PbI2Br (red) for (c) Cs 3d, (d) Pb 4f, (e) I 3d, and (f) Br 3d.


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Figure 2. (a) Absorbance spectra of Cs1-xKxPbI2Br films (x = 0, 0.025, 0.05, 0.075, 0.1). Timeresolved PL decay profiles of (b) CsPbI2Br and (c) Cs0.925K0.075PbI2Br films with and without a bl-TiO2 layer.


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Figure 3. SEM surface images of (a) CsPbI2Br and (b) Cs0.925K0.075PbI2Br films. Cross-sectional SEM images of planar-architecture perovskite solar cells based on (c) CsPbI2Br and (d) Cs0.925K0.075PbI2Br.


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Figure 4. (a) J-V curves for the Cs0.925K0.075PbI2Br champion device in a reverse (solid lines) and a forward direction (dashed lines) at a rate of 0.12 V s-1 and with an aperture area of 0.15 cm2. (b) EQE spectra and integrated Jsc for the Cs0.925K0.075PbI2Br champion device. Histograms of the JV scan efficiencies for each batch of 20 devices based on (c) Cs0.925K0.075PbI2Br and (d) CsPbI2Br.


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Figure 5. Normalized Jsc, Voc, FF, and PCE for planar-architecture CsPbI2Br (black) and Cs0.925K0.075PbI2Br (red) perovskite solar cells without encapsulation, under T = 20 °C and RH = 20%.


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