Role of Additives on the Performance of CsPbI3 Solar Cells - The

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C: Energy Conversion and Storage; Energy and Charge Transport 3

The Role of Additives on the Performance of CsPbI Solar Cells Do Yeon Heo, Sang Mok Han, Nam Sub Woo, Young Ju Kim, Tae-Yoon Kim, Zhengtang Tom Luo, and Soo Young Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04613 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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The Journal of Physical Chemistry

The Role of Additives on the Performance of CsPbI3 Solar Cells Do Yeon Heo,1 Sang Mok Han,2 Nam Sub Woo,2 Young Ju Kim,2 Tae-Yoon Kim,3 Zhengtang Tom Luo4,* and Soo Young Kim1,* 1

School of Chemical Engineering and Materials Science, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea

2

Korea Institute of Geoscience and Mineral Resources, 905 Yeongilman-daero, Heunghaeeup, Buk-gu, Pohang-si, Gyeongsangbuk-do, 37559, Republic of Korea

3

Korea Marine Equipment Research Institute, 1st Floor, Business Support Building, Korea

Rural Community Corporation 466, Saemangeumbuk-ro, Gunsan-si, Jeollabuk-do, 54004, Republic of Korea 4

Department of Chemical and Biomolecular Engineering, the University of Hong Kong Science and Technology, Clear Water Bay, Kowloon, Hong Kong

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ABSTRACT: The role of additives on the performance of CsPbI3 perovskite solar cells (PSCs) was investigated. Different kinds of cations and anions were used as additives in a N,N-dimethylformamide (DMF) solution containing CsI and PbI2 (1:1 molar ratio). These include HI, HBr, HCl, NH4I, NH4Br, and NH4Cl. Additive cations (H+ and NH4+) as well as halide ions (I-, Br-, and Cl-) are important for the properties of PSCs. Especially, the addition of iodine ion showed good characteristics compared to Br- and Cl-. Among the CsPbI3 layers prepared with different kinds of additives and annealed at different temperatures, the X-ray diffraction peaks of CsPbI3 were clearly found at 14° and 28° for the sample annealed at 150 °C with 50 µL HI, suggesting the formation of a cubic structure at the low temperature of 150 °C. The field emission scanning electron microscopy images indicate that the surface of the perovskite layer with hydrogen halide additive (H+ based additive) is more uniform than that with ammonium additive. The roughness profiles determined by atomic force microscopy indicate that the CsPbI3 film with HI additive shows the least roughness among the samples with “H+” based additives. Therefore, the best power conversion efficiency (PCE) of 4.72% is obtained for CsPbI3 PSCs annealed at 150 °C with HI (50 µL). The “H+”-based additives seem to react with PbI2 in DMF solution, increasing the solubility of PbI2 and thus lowering the processing temperature. Furthermore, the PCE of CH3NH3PbI3-xClx PSCs decreased from 7.45 to 0.23%, whereas that of CsPbI3 PSCs with 50 µL HI only decreased from 3.55 to 2.78% after exposing the samples to air for 3 h. These results indicate that “H+” based additives, especially HI, have more impact on the CsPbI3 PSCs in terms of lowering the processing temperature and improving the performance.


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INTRODUCTION In 2017, the efficiency of the perovskite solar cells (PSCs) was renewed. The power conversion efficiency (PCE) of 3.8%,1 which was reported for the first time in 2009, was rapidly increased to 22.1%.2 PSCs are a type of dye-sensitized solar cell (DSSCs). DSSCs use liquid dyes as light absorbers, whereas PSCs use a solid perovskite crystal structure for absorbing light. Perovskite has an ABX3 structure, where A is an organic cation, B is a metal cation, and X is a halide ion. Whereas organic dyes of DSSCs have low light absorption, perovskites have high absorption coefficients in the visible light range. Thus, they can absorb light adequately at a thin thickness to generate a large amount of electric charge.3-6 Further, one of the advantages of perovskites is that it is easy to control the optical properties by changing the composition. PSCs with these characteristics are fabricated at a low temperature with high efficiency and low manufacturing cost. Therefore, PSCs are expected to replace the commercialized copper indium gallium selenide, cadmium telluride, and silicon-based solar cells. One of the disadvantages of PSCs is their short lifetimes. In order to extend the stability, the “inverted” planar heterojunction structure was studied to eliminate the hysteresis.7 Conventional PSCs contain titanium dioxide (TiO2) and additives, which are corrosive and lower their lifetime.8-10 In contrast, the “inverted” planar heterojunction solar cells do not require corrosive additives, and therefore, they have longer lifetimes than conventional PSCs. There have been various studies on “inverted” planar heterojunction solar cells.11-21 The “inverted” planar heterojunction devices using CH3NH3PbI3 as a light absorber were reported to show a high efficiency of 18.1%.7 The PSCs based on CH3NH3PbI3 show excellent light absorption characteristics owing to the suitable band gap (1.4–1.6 eV) for absorbing light over the entire visible range. Furthermore, their manufacturing process is simple. However,



the methylammonium cation in the structure is highly hygroscopic and this perovskite is not stable when exposed to air or moisture.22 In order to overcome the disadvantage in CH3NH3PbI3 PSCs,

Cs-based PSCs are being developed.

CsPbI3, one of the materials used as a light absorber in Cs-based PSCs, has two phases including a black-phase (cubic α-CsPbI3) and a yellow-phase (orthorhombic δ-CsPbI3).23 The δ-CsPbI3 phase has a large band gap of 2.82 eV at room temperature, which is unsuitable for light absorption. In contrast, α-CsPbI3 has a band gap of 1.73 eV, which is more suitable for PSCs. However, it has been reported that α-CsPbI3 could appear at a high temperature (> 300 °C).24,25 In order to decrease the process temperature for the formation of α-CsPbI3, hydroiodic acid (HI) was added to the dimethylformamide (DMF) solution of the Cs precursors, thereby reducing its solubility.23,26 It has been reported that the rapid crystallization induced by the addition of HI could decrease the formation temperature of αCsPbI3. As a result, a uniform and smooth black-phase of α-CsPbI3 was found to appear at low temperatures (150 °C). However, the role of the cation and anion of the added salt, HI is not yet clear. Therefore, it is necessary to investigate the role of additives in the formation of the desired α-CsPbI3. In this study, the role of additives in CsPbI3 PSCs was investigated. Different kinds of cations and anions including HI, HBr, HCl, NH4I, NH4Br, and NH4Cl were added into the DMF solution of the Cs precursor. The perovskite layer was synthesized by a one-step coating. As an “H+”-based cation additive, 33, 50, 66 µL of HI, HBr, and HCl, respectively, were added into 1:1 CsI: PbI2 solutions. As an “NH4+”-based cation additive, solutions containing 0.1 M and 0.3 M of NH4I, NH4Br, and NH4Cl were used. Then, the efficiency of the CsPbI3 PSCs was evaluated as a function of the annealing temperature and the amount of additives. It is shown that the power conversion efficiency of perovskite solar cell improved


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by the incorporation of additives. Based on these results, the role of the additives in Cs-based perovskite solar cells is discussed.

EXPERIMENTAL DETAILS Materials. Unless stated otherwise, all materials were purchased from Sigma-Aldrich. Patterned indium tin oxide (ITO) glass (15 Ω sq-1) was purchased from Woo Yang GMS. Poly(3,4-ethylenedioxythiophene): polystyrene sulfonic acid (PEDOT: PSS, Clevious P VP. Al 4083) was purchased from HERAEUS. It was water solution and its viscosity is 5 Min- 12 Max mPas. The [6,6]-phenyl-C60 butyric acid methyl ester (PCBM) was purchased from Nanoholdings. Bathocuproine (BCP) and LiF were purchased from TAEWON SCIENTIFIC CO. (TASCO). Fabrication of the CsPbI3 PSCs. ITO-coated glass was used as the substrate. It was cleaned by ultrasonication in acetone, isopropyl alcohol, and deionized (DI) water for 15 min each. The substrates were then treated with UV-Ozone for 15 min.27 Hole transport layer of PEDOT:PSS was spin-coated at 4000 rpm for 30 s and heated at 150 °C for 15 min in air. The substrates were loaded into a N2-filled glove box. For the synthesis of the CsPbI3 precursor, CsI and PbI2 were dissolved in anhydrous DMF in a 1:1 molar ratio. Then, 33, 50, and 60 µL, respectively, of HI, HBr, and HCl were added to the solution. We also prepared a series of samples by adding each of the 0.1 M and 0.3 M NH4I, NH4Br, and NH4Cl solution into the perovskite precursor solution. Then, the solutions of the perovskite precursor were spin-coated onto PEDOT: PSS layer on ITO-coated glass at 6000 rpm for 30 s. The substrates were subsequently annealed at 100, 125, and 150 °C. Then, on top of the perovskite film, a PCBM solution (40 mg/mL) in chlorobenzene was spin-coated at 750 rpm for 15 s and the substrates were heated at 60 °C for 5 min. Subsequently, films of BCP (3 nm), LiF (1 nm), 5 ACS Paragon Plus Environment


and Al (100 nm) were deposited under vacuum (10-6 Torr) on top of the electron transport layer (PCBM layer) using a thermal evaporator, as shown in Fig. 1. BCP and LiF were used as hole blocking layers and Al was used as electrode. Fabrication of the CH3NH3PbI3-xClx PSCs. To compare the stability, we also prepared PSCs of CH3NH3PbI3-xClx. The process of device fabrication was the same as that used for fabricating the CsPbI3 PSCs. For the synthesis of the CH3NH3PbI3-xClx precursor, CH3NH3I, lead(II) iodide (PbI2), and lead(II) chloride (PbCl2) in 4:1:1 molar ratio were dissolved in anhydrous DMF. The precursor solution was then spin-coated onto the PEDOT: PSS layer at 4000 rpm for 30 s. Then, the electron transport layer, hole blocking layer, and electrode were deposited using the same method as in the case of CsPbI3 PSCs.

RESULTS AND DISCUSSION We measured the X-ray diffraction (XRD) patterns of the perovskite layer annealed at 100, 125, and 150 °C as shown in Figure 2. The CsPbI3 films were prepared on the ITO glass using 33 or 50 µL quantities of HI, HBr, or HCl.

Figure 2(a) shows the XRD peaks of

CsPbI3 layer with 33 µL of HI. The XRD peak at 2θ = 12.2° indicative of cubic PbI2 appears at 100 °C, which however disappears at 125 and 150 °C.22,23 However, peaks corresponding to CsPbI3 black phase are not observed at 14° and 28° for samples annealed at 125 and 150 °C.23,28 Furthermore, the peaks are noisy and unclear. Therefore, it is apparent that the perovskite film is not formed well when 33 µL of HI is used, regardless of the annealing temperature. Further, after annealing the film of CsPbI3 precursors with 50 µL of HI at 150 °C, peaks corresponding to black phase CsPbI3 appeared clearly and no PbI2 peak is found, as shown in Figure 2(b). In samples annealed at 100 and 125 °C, the peak of PbI2 appeared, suggesting that CsPbI3 perovskite film was not fully formed. In the case of the


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perovskite layer with 33 µL HBr additive (Figure 2(c)), the peak of the black phase is hardly detected in samples annealed at 100 and 125 °C. However, weak peaks of the black phase appear after annealing at 150 °C. In the case of 50 µL HBr additive (Figure 2(d)), the peaks of the CsPbI3 black phase appear after annealing at 125 °C and the peak intensity increases with the annealing temperature. Note that the PbI2 peak appears for all the samples with HBr additive, regardless of the annealing temperature, suggesting that PbI2 did not dissolve completely and remained in the CsPbI3 perovskite film. In the case of HCl additive, similar XRD peaks as with the HBr additive are observed (Figures 2(e) and (f)). These results indicate that the addition of 50 µL of the additive induced the formation of more black phase CsPbI3 than that induced by 33 µL of the additives. In addition, even when the same quantity of the additives, 50 µL was added to the CsPbI3 precursor, the CsPbI3 black phase was largely observed to form at 150 °C. The UV-visible absorption spectrums of the perovskite film with and without additive are shown in Figure S1. The films were prepared using (a) HI, (b) HBr, (c) HCl, and (d) no additive on the glass. All of the films show strong absorption at 300-450 nm, especially for HI. In the case of HCl, the absorption is similar to (d) no additive due to annealing. After annealing, the color of HCl film changed to brown, but it is not darker than the other films of HI and HBr. The XRD patterns of the CsPbI3 layer with NH4I, NH4Br, and NH4Cl additives were measured and shown in Figure S2. Similar peaks are observed regardless of the additive concentration and annealing temperature. The peaks corresponding to the CsPbI3 yellow phase are located at 9.82, 13.07, 22.72, 25.70, and 26.39°, as reported previously.24,28 Despite varying the concentration of the additives and annealing temperature, the peaks indicating PbI2 and CsPbI3 yellow phase are found in all the samples with the NH4-based additives. These results indicated that no cubic phase of CsPbI3 is formed at the temperature of 100, 125,



and 150 °C when the NH4X (X = I, Br, and Cl) additives are added. This suggests that the PCE value of a PSC based on CsPbI3 with “NH4+”-based additives would be low. The field emission scanning electron microscopy (FE-SEM) images of the CsPbI3 perovskite layer prepared using each additive were measured. In the case of “H+”-based additives (Figure 3), a large quantity of yellow phase is generally observed in the samples annealed at 100 °C. The yellow phases are clearly observed in Figures 3(b-1), 3(c-1) and 3(f1). Furthermore, many pinholes are observed in the samples annealed at 100 °C. These results indicate that annealing at 100 °C is not adequate to form a CsPbI3 film. Thus, PCE of PSCs formed at 100 °C is expected to be low. As the annealing temperature was increased to 125 and 150 °C, the yellow phase changed to black phase and the crystal size increased along with a reduction in the pinholes. This is clearly observed in Figures 3(c-1), 3(c-2) and 3(c-3). These results suggest that PCEs of PSCs would be improved with an increase in the annealing temperature. Among the samples annealed at 150 °C, the least amount of the yellow phase and the least number of pinholes are found in Figure 3(b-3). Furthermore, the crystal size of the sample shown in Figure 3(b-3) is larger than that in Figure 3(a-3). Thus, the charge carrier extraction at the interface between the perovskite layer and the charge collection layer could be improved in the CsPbI3 with 50 µL HI additive annealed at 150 °C. A similar phenomenon is observed for “NH4+”-based additives (Figure S3). As shown, the surfaces of films were burned by the electron beam, suggesting that the films including “NH4+”-based additives are not perfect. By comparing the SEM images in Figure 3 and Figure S3, it is clear that the surface of the perovskite layer with H+ additive is more uniform than that with ammonium additive. It is considered that the addition of ammonium halide to the perovskite solution results in a rough phase because ammonium ions break the perovskite structure. Therefore, “H+”-based additives are thought to be more effective in CsPbI3 film than the “NH4+”-based additives. 8 ACS Paragon Plus Environment

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To confirm the roughness of the CsPbI3 with 50 µL “H+”-based additives annealed at 150 °C, the atomic force microscopy (AFM) measurement was performed. AFM images and roughness profiles are shown in Figure 4. The roughness of the perovskite layers were 17.68, 3.13, 6.82, and 10.19 nm for samples without and with HI, HBr and HCl, respectively. It was confirmed that the roughness decreased when the additive was added. The grain size is observed to reduce after the incorporation of additives in the CsPbI3 film. However, the roughness decreased upon the addition of the “H+”-based additives. It is believed that the “H+”-based additives could dissolve the PbI2 remaining in the CsPbI3 precursor, thereby reducing the roughness. Among the samples with “H+”-based additives, the CsPbI3 film with HI additive shows the lowest roughness. The roughness of CsPbI3 is an important factor in PSCs. Therefore, these results suggest that the “H+”-based additives could reduce the roughness of the perovskite layer, leading to increased efficiency. Figure 5 and Figure S4 show current density-voltage (J-V) curves of the CsPbI3 PSCs with “H+”- and “NH4+”-based additives. As a reference, CsPbI3 PSCs without any additive was also fabricated. The open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), and PCE values of PSCs with additives are summarized in Table 1 and Table S1, respectively. The perovskite layers were annealed at 100, 125, and 150 °C, respectively. It was shown that PCE values were decreased above 150 °C (Table S2). It is observed that the black phase of CsPbI3 PSCs without the additive appeared after annealing at 335 °C, which is relatively higher than the black phase formation temperature of CsPbI3 with an additive. Furthermore, the PCE of CsPbI3 PSCs without additives is as low as 0.4%. These results imply that the incorporate of additives in CsPbI3 PSCs is necessary to increase the PCE and decrease the perovskite formation temperature. Among the series of PSCs with different amounts of the “H+”- and “NH4+”-based additives, the best performance is observed for CsPbI3 PSCs annealed at 150 °C with HI 50 µL. Specifically, the JSC, VOC, FF, and PCE values are 10.00 9 ACS Paragon Plus Environment


mA/cm2, 0.75 V, 0.63, and 4.72%, respectively. The next best PCE is achieved for CsPbI3 PSCs with 50 µL of HBr annealed at 150 °C, with the JSC, VOC, FF, and PCE values being 9.93 mA/cm2, 0.67 V, 0.39, and 2.56%, respectively. It is interesting to note that the sequence of the highest PCE value of PSCs with “H+”-based additives corresponds to the ascending order of roughness (Fig. 4). For each additive, the highest PCE value is generally observed for the sample annealed at 150 °C, for which the XRD peaks are sharp and clear, as shown in Figure 2. These results suggest that HI could render the CsPbI3 surface uniform, thereby increasing the PCE value. The J-V curves for the PSCs including NH4I, NH4Br, and NH4Cl as additives are shown in Figure S4. It was detected that the perovskite layer with “NH4+”based additives changed rapidly to yellow phase regardless of the annealing temperature. Therefore, the PCEs of the PSCs (0.28%) with “NH4+”-based additives are as low as the PCE of CsPbI3 PSCs annealed at 110 °C without additives.29 This implies that NH4I, NH4Br, and NH4Cl additives do not affect the PCE of CsPbI3 PSCs. It seems that the crystal structure of the CsPbI3 perovskite layer with “NH4+”-based additives is orthorhombic instead of cubic so that the device does not function as a PSC. The “H+”-based additives seem to react with PbI2 in DMF solution, thereby increasing the solubility of PbI2 when the solutions were fabricated for one-step coating.9 Therefore, the role of the additive in the Cs-based PSCs is considered to be lowering the processing temperature of the CsPbI3 layer, the processing of which usually requires a high temperature to produce a uniform and cubic crystal structure. Unlike the solutions containing HI and HBr, the solution containing HCl did not dissolve the solute perfectly. Figure S5 shows the FE-SEM images of the perovskite layers with 50 µL of HI and HBr annealed at 150 °C. Large grains of the perovskite are formed owing to the additives.30,31 We speculate that the crystallinity of the perovskite increased and a uniform surface was obtained as a result of spin-coating of a well-


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dissolved solution. However, the CsPbI3 layer with HBr additive has more pinholes than that with HI additive. Therefore, the PCE of PSCs with HI is higher than that with HBr (Figure 5). To compare the change of the PCE as a function of the additive concentration and annealing temperature, the PCEs were summarized. Figure 6(a) shows the PCEs of the CsPbI3 PSCs with HI additive. For 33 µL of HI addition the PCE of the PSCs decreased with increasing temperature. In contrast, when 50 or 66 µL of HI was used as an additive, the PCE of the PSCs increased with temperature. Further, the highest PCE is achieved at 150 °C. Figure 6(b) and (c) show the comparison of the PCEs of the CsPbI3 PSCs with HBr and HCl additives, respectively. For both cases, the PCE values generally increase with annealing temperature and the highest PCE is achieved at 150 °C, as with the PSCs with HI additive. These results imply that the perovskite layer with additives is affected by the annealing temperature. We confirmed the stability characteristics of CH3NH3PbI3-xClx PSCs and CsPbI3 PSCs in air for 3 h. The active layer of the CsPbI3 PSCs was annealed at 150 °C with 50 µL HI. The device characteristics were evaluated every 30 min in air and shown in Figure 7. The initial PCE of CH3NH3PbI3-xClx PSC is 7.45%, which is higher than that of the CsPbI3 PSC (3.55%) (Figure 7(a)). The FF of CH3NH3PbI3-xClx PSCs decreased rapidly from 0.69 to 0.23, but that of CsPbI3 PSCs decreased slightly from 0.51 to 0.46 after 3 h (Figure 7(b)). The JSC and VOC of CH3NH3PbI3-xIx PSCs also decreased to a greater extent than those of CsPbI3 PSCs (Figure 7(c) and (d)). Therefore, the PCE of CH3NH3PbI3-xClx PSCs decreased from 7.45 to 0.23%, whereas that of CsPbI3 PSCs only decreased from 3.55 to 2.78% after 3 h, as shown in Figure 7(e). However, it was confirmed that the color of perovskite phase (CsPbI3 and CH3NH3PbI3-xClx) was not changed after 3 h in air (Figure S6). Comparison of the characteristics of CH3NH3PbI3-xIx PSCs with those of CsPbI3 PSCs confirms that the stability of latter is better than that of the former. 11 ACS Paragon Plus Environment


CONCLUSION The role of additives in Cs-based PSCs was investigated. Among the CsPbI3 layers obtained with different kinds of additives followed by annealing at different temperatures, the XRD peaks of CsPbI3 annealed at 150 °C with 50 µL HI were clearly observed at 14° and 28°, suggesting the formation of the cubic structure at the low temperature of 150 °C. In contrast, XRD peaks of the CsPbI3 layer annealed at 150 °C with NH4I, NH4Br, and NH4Cl additives appeared at 9°, 13°, 22°, 25°, and 26°, indicating the yellow phase of CsPbI3 with orthorhombic structure. This implies that a high annealing temperature over 150 °C is required to form α-CsPbI3 with a cubic structure. According to FE-SEM images, the surfaces of the perovskite layers with “H+” additives are more uniform than those with ammonium additives. AFM roughness profiles indicated that the CsPbI3 film with HI additive shows the least roughness among the samples with “H+”-based additives. The black phase of CsPbI3 PSCs without any additive appeared at 335 °C and the PCE value of this PSC is as low as 0.4%. Among samples with “H+”- and “NH4+”-based additives at different amounts, the best performance was observed for CsPbI3 PSCs annealed at 150 °C with HI (50 µL). The JSC, VOC, FF, and PCE of this PSC are 10.00 mA/cm2, 0.75 V, 0.63, and 4.72%, respectively. The “H+”-based additives seem to react with PbI2 in DMF solution, increasing the solubility of PbI2 when the solutions were fabricated for one-step coating. Thus, the role of “H+”-based additives in the Cs-based PSCs is speculated to be lowering the process temperature of the CsPbI3 layer; this process usually requires a high temperature to generate a uniform and cubic crystal structure. Halide is also important to the characteristics of PSCs. The best effect among halide ions is shown in iodine case. The HI additive could render the CsPbI3 surface uniform, thereby increasing the PCE value. Furthermore, the PCE of CH3NH3PbI3-xClx PSCs


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decreased from 7.45 to 0.23%, whereas that of CsPbI3 PSCs with 50 µL HI only decreased from 3.55 to 2.78% after exposing the samples to ambient air for 3 h. These results indicate that the “H+”-based additives, especially HI, are more effective for fabricating CsPbI3 PSCs in terms of lowering the process temperature and increasing the performance.

ASSOCIATED CONTENT Supporting Information Available XRD, FE-SEM images, and current density-voltage curves/table of CsPbI3 perovskite solar cells with NH4X (X=I, Br, and Cl), FE-SEM images of the perovskite layer with HI and HBr are available in Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author S. Y. K. ([email protected]), Tel: 82-2-820-5875, Fax: 82-2-824-3495. Z. T. L. ([email protected]), Tel: 852-2358-8823, Fax: 852-2358-0054.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This research was supported by the Basic Research Laboratory of the NRF funded by the Korean government, MSIT(2018R1A4A1022647), the Korea Agency for Infrastructure Technology Advancement grant funded by the Ministry of Land, Infrastructure and Transport (17IFIP-B133622-01), and the Chung-Ang University Research Grants in 2018. 13 ACS Paragon Plus Environment


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Lam, Y. M. The Origin of High Efficiency in Low-Temperature Solution-Processable Bilayer Organometal Halide Hybrid Solar Cells. Energy Enviro. Sci. 2014, 7, 399-407. 10. Jeng, J.-Y.; Chiang, Y.-F.; Lee, M.-H.; Peng, S.-R.; Guo, T.-F.; Chen, P.; Wen, T.-C. CH3NH3PbI3 Perovskite/Fullerene Planar-Heterojunction Hybrid Solar Cells. Adv. Mater. 2013, 25, 3727-3732. 11. Le, Q. V.; Nguyen, T. P.; Choi, K. S.; Cho, Y.-H.; Hong, Y. J.; Kim, S. Y. Dual Use of Tantalum Fisulfides as Hole and Electron Extraction Layers in Organic Photovoltaic Cells. Phys. Chem. Chem. Phys. 2014, 16, 25468–25472. 12. Lee, C. Y.; Le, Q. V.; Kim, C.; Kim, S. Y. Use of Silane-Functionalized Graphene Oxide on Organic Photovoltaic Cells and Organic Light-Emitting Diodes. Phys. Chem. Chem. Phys. 2015, 17, 9369–9374. 13. Zhang, W.; Pathak, S.; Sakai, N.; Stergiopoulos, T.; Nayak, P. K.; Noel, N. K.; Haghighirad, A. A.; Burlakov, V. M.; deQuilettes, D. W.; Sadhanala, A.; et al. Enhanced Optoelectronic Quality of Perovskite Thin Films with Hypophosphorous Acid for Planar Heterojunction Solar Cells. Nature communications 2015, 6, 10030. 14. Le, Q. V.; Shin, J. W.; Jung, J.-H.; Park, J.; Ozturk, A.; Kim, S. Y. Control of the Crystal Growth Shape in CH3NH3PbBr3 Perovskite Materials. J. Nanosci. Nanotechnol. 2017, 17, 8169-8174. 15. Zhao, L.; Luo, D.; Wu, J.; Hu, Q.; Zhang, W.; Chem, K.; Liu, T.; Liu, Y.; Zhang, Y.; Liu, F.; et al. High-Performance Inverted Planar Heterojunction Solar Cells Based on Lead Acetate Precursor with Efficiency Exceeding 18%. Adv. Func. Mater. 2016, 26, 35083514. 16. Huang, J.; Wang, M.; Ding, L.; Yang, Z.; Zhang, K. Hydrobromic Acid Assisted Crystallization of MAPbI3-xClx for Enhanced Power Conversion Efficiency in Perovskite Solar Cells. RSC Adv. 2016, 6, 55720-55725. 15 ACS Paragon Plus Environment


17. Hasani, A.; Gavgani, J. N.; Pashaki, R. M.; Baseghi, S.; Salehi, A.; Heo, D.; Kim, S. Y.; Mahyari, M. Poly(3,4 ethylenedioxythiophene):Poly(Styrenesulfonate)Iron(Ⅲ) Porphyrin Supported on S and N Co-Diped Graphene Quantum Dots as a Hole Transport Layer in Polymer Solar Cells. Sci. Adv. Mater. 2017, 9, 1616-1625. 18. Abdi-Jalebi, M.; Dar, M. I.; Sadhanala, A.; Senanayak, S. P.; Frankevičius, M.; Arora, N.; Hu, Y.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Grätzel, M.; et al. Impact of Monovalent Cation Halide Additives on the Structural and Optoelectronic Properties of CH3NH3PbI3 Perovskite. Adv. Energy Mater. 2016, 6, 1502472. 19. Le, Q. V.; Choi, J.-Y.; Kim, S. Y. Recent Advances in the Application of TwoDimensional Materials as Charge Transport Layer in Organic and Perovskite Solar Cells. FlatChem 2017, 2, 54-66. 20. Kim, Y. G.; Kwon, K. C.; Le, Q. V.; Hong, K.; Jang, H. W.; Kim, S. Y. Atomically Thin Two-Dimensional Materials as Hole Extraction Layers in Organolead Halide Perovskite Photovoltaic Cells. J. Power Sources 2016, 319, 1-8. 21. Kwon, K. C.; Hong, K.; Le, Q. V.; Lee, S. Y.; Choi, J.;. Kim, K.-B; Kim, S. Y.; Jang, H. W. Inhibition of Ion Migration for Reliable Operation of Organolead Halide PerovskiteBased Metal/Smiconductor/Metal Broadband Photodetectors. Adv. Fun. Mater. 2016, 26, 4213-4222. 22. Seetharaman, M.; Nagarjuna, S. P.; Kumar, P. N.; Singh, S. P.; Deepa, M.; Namboothiry, M. A., G. Efficient Organic-Inorganic Hybrid Perovskite Solar Cells Processed in Air. Phys. Chem. Chem. Phys. 2014, 16, 24691-24696. 23. Eperon, G. E.; Paternò, G. N.; Sutton, R. J.; Zampetti, A.; Haghighirad, A. A.; Cacialli, F.; Snaith, H. J. Inorganic Caesium Lead Iodide Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 19688-19695.


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24. Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and NearInfrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019-9038. 25. Moller, C. K. Crystal Structure and Photoconductivity of Caesium Plumbohalides. Nature 1958, 182, 1436. 26. Kim, Y. G.; Kim, T.-Y.; Oh, J. H.; Choi, K. S.; Kim, Y.-J.; Kim, S. Y. Cesium Lead Iodide Solar Cells Controlled by Annealing Temperature. Phys. Chem. Chem. Phys. 2017, 19, 6257-6263. 27. Le, Q. V.; Nguyen, T. P.; Jang, H. W.; Kim, S. Y. The Use of UV/Ozone-Treated MoS2 Nanosheets for Extended Air Stability in Organic Photovoltaic Cells. Phys. Chem. Chem. Phys. 2014, 16, 13123-13128 28. Nikl, M.; Nitsch, K.; Chval, J.; Somma, F.; Phani, A. R.; Santucci, S.; Giampaolo, C.; Fabeni, P.; Pazzi, G. P.; Feng, X. Q. Optical and Structural Properties of Ternary Nanoaggregates in CsI-PbI2 Co-Evaporated Thin Films. J. Phus.: Condens. Matter 2000, 12, 1939-1946 29. Choi, H.; Jeong, J.; Kim, H.-B.; Kim, S.; Walker, B.; Kim, G.-H.; Kim, J. Y. CesiumDoped Methylammonium Lead Iodide Perovskite Light Absorber for Hybrid Solar Cells. Nano Energy 2014, 7, 80-85 30. Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J.-C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; et al. High-Efficiency Solution-Processed Perovskite Solar Cells with Millimeter-Scale Grains. Science 2015, 347, 522-525. 31. Ki, T.; Pan, Y.; Wang, Z.; Xia, Y.; Chen, Y.; Huang, W. Additive Engineering for Highly Efficient Organic-Inorganic Halide Perovskite Solar Cells: Recent Advances and Perspectives. J. Mater. Chem. A 2017, 5, 12602-12652.



(a)

(b)

Figure 1. Schematic (a) structure (b) constructed device of the inverted planar PSC. The PSC is composed of ITO/PEDOT: PSS/CsPbI3/PC60BM/BCP/LiF/Al. The crystal structure of CsPbI3 perovskite is also shown. There were 4 pixels on the one ITO substrate and the illuminated areas of each pixels were 0.04 cm2.


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Figure 2. XRD spectra of the CsPbI3 layers spun-coat on PEDOT: PSS layers. The CsPbI3 solutions used for spin-coating contained (a) HI, 33 µL, (b) HI, 50 µL, (c) HBr, 33 µL, (d) HBr, 50 µL, (e) HCl, 33 µL, and (f) HCl, 50 µL, respectively. The samples were annealed at different temperatures (100, 125, and 150 °C). ▼ : (100) and (200) peaks of the perovskite structure. * : Cubic PbI2 peak.



Figure 3. FE-SEM images of the CsPbI3 layers coated on PEDOT: PSS layers. The additives are (a) HI, 33 µL, (b) HI, 50 µL, (c) HBr, 33 µL, (d) HBr, 50 µL, (e) HCl, 33 µL, and (f) HCl, 50 µL. The annealing temperatures are 100, 125, and 150 °C. The spots in the FE-SEM images are the yellow phase of CsPbI3 layers. The scale bar is 2 µm.


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Figure 4. AFM images and roughness profiles of the CsPbI3 layers coated on PEDOT: PSS layers. The perovskite solutions were prepared by adding 50 µL of (a) HI, (b) HBr, or (c) HCl as an additive. The layers were annealed at 150 °C after spin-coating. The images and roughness profiles of CsPbI3 layer without the additive is also shown in (d) for comparison. The CsPbI3 layer without additive is annealed at 335 °C to obtain the black phase.



Figure 5. Current density-voltage curves of the CsPbI3 PSCs. The additives in the CsPbI3 solutions are (a) HI, 33 µL, (b) HI, 50 µL, (c) HBr, 33 µL, (d) HBr, 50 µL, (e) HCl, 33 µL, and (f) HCl, 50 µL. The annealing temperatures are 100, 125, and 150 °C. The maximum PCE values of CsPbI3 PSCs with HI, HBr, and HCl additives are 4.72, 2.56, and 2.11%, respectively.


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Figure 6. Change in the PCE of PSCs with different additive concentrations (33 µL, 50 µL, and 66 µL) as a function of the annealing temperature. The additives are (a) HI, (b) HBr, and (c) HCl. The PCE values generally increase with annealing temperature and the highest PCE of PSCs is observed at 150 °C.



Figure 7. (a) Current density-voltage curves of CH3NH3PbI3-xClx and CsPbI3 PSCs. The CsPbI3 layer with 50 µL HI was annealed at 150 °C. The change in (b) FF, (c) JSC, (d) VOC, and (e) PCE of CH3NH3PbI3-xClx and the CsPbI3 PSCs after storing in air for 3 h are shown.


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Table 1. Photovoltaic Performance of the CsPbI3 PSCs with HI, HBr, and HCl Additives The average values are given from 20 cells for each type of device with AM 1.5G solar spectrum. Additives HI, 33 µL

HI, 50 µL

HI, 66 µL

HBr, 33 µL

HBr, 50 µL

HBr, 66 µL

HCl, 33 µL

HCl, 50 µL

HCl, 66 µL

Temperature (°C)

Voc (V)

Jsc (mA/cm2)

100 125 150 100 125 150 100 125 150 100 125 150 100 125 150 100 125 150 100 125 150 100 125 150 100 125 150

0.81 ± 0.06 0.81 ± 0.04 0.80 ± 0.04 0.74 ± 0.07 0.76 ± 0.10 0.75 ± 0.09 0.83 ± 0.04 0.68 ± 0.06 0.67 ± 0.07 0.72 ± 0.07 0.71 ± 0.07 0.73 ± 0.05 0.81 ± 0.04 0.77 ± 0.01 0.67 ± 0.08 0.73 ± 0.10 0.68 ± 0.09 0.66 ± 0.05 0.83 ± 0.06 0.76 ± 0.08 0.70 ± 0.04 0.64 ± 0.03 0.73 ± 0.02 0.75 ± 0.03 0.62 ± 0.03 0.69 ± 0.05 0.61 ± 0.05

6.11 ± 0.34 5.15 ± 0.11 2.68 ± 0.48 2.61 ± 0.62 3.34 ± 0.31 10.00 ± 0.13 3.48 ± 0.20 4.72 ± 0.61 4.84 ± 0.59 2.73 ± 0.72 5.63 ± 0.73 4.15 ± 0.38 0.18 ± 0.03 2.90 ± 0.40 9.93 ± 0.18 1.10 ± 0.66 2.70 ± 0.12 2.03 ± 0.84 2.55 ± 0.65 4.49 ± 0.33 5.60 ± 0.61 0.29 ± 0.20 2.39 ± 0.23 3.00 ± 0.14 0.39 ± 0.52 2.87 ± 0.54 1.31 ± 0.60


FF

PCE (%)

0.44 ± 0.04 2.17 ± 0.32 0.47 ± 0.02 1.94 ± 0.45 0.45 ± 0.02 0.98 ± 0.22 0.46 ± 0.03 0.89 ± 0.76 0.45 ± 0.04 1.14 ± 0.30 0.63 ± 0.04 4.72 ± 0.52 0.49 ± 0.01 1.40 ± 0.18 0.61 ± 0.08 1.97 ± 0.19 0.50 ± 0.07 1.64 ± 0.54 0.41 ± 0.01 0.81 ± 0.08 0.39 ± 0.05 1.55 ± 0.49 0.50 ± 0.06 1.53 ± 0.23 0.37 ± 0.09 0.055 ± 0.08 0.17 ± 0.01 0.38 ± 0.22 0.39 ± 0.01 2.56 ± 0.50 0.40 ± 0.02 0.30 ± 0.16 0.45 ± 0.08 0.83 ± 0.80 0.59 ± 0.03 0.78 ±0.12 0.59 ± 0.03 1.24 ± 0.37 0.34 ± 0.03 1.16 ± 0.25 0.54 ± 0.04 2.11 ± 0.16 0.38 ± 0.01 0.069 ± 0.25 0.38 ± 0.07 0.67 ± 0.60 0.58 ± 0.04 1.30 ± 0.15 0.31 ± 0.05 0.074 ± 0.01 0.44 ± 0.01 0.88 ± 0.03 0.41 ± 0.01 0.34 ± 0.14


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Figure 1. Schematic (a) structure (b) constructed device of the inverted planar PSC. The PSC is composed of ITO/PEDOT: PSS/CsPbI3/PC60BM/BCP/LiF/Al. The crystal structure of CsPbI3 perovskite is also shown. There were 4 pixels on the one ITO substrate and the illuminated areas of each pixels were 0.04 cm2. 164x180mm (150 x 150 DPI)

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Figure 2. XRD spectra of the CsPbI3 layers spun-coat on PEDOT: PSS layers. The CsPbI3 solutions used for spin-coating contained (a) HI, 33 µL, (b) HI, 50 µL, (c) HBr, 33 µL, (d) HBr, 50 µL, (e) HCl, 33 µL, and (f) HCl, 50 µL, respectively. The samples were annealed at different temperatures (100, 125, and 150 °C). ▼ : (100) and (200) peaks of the perovskite structure. * : Cubic PbI2 peak. 131x165mm (120 x 120 DPI)


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Figure 3. FE-SEM images of the CsPbI3 layers coated on PEDOT: PSS layers. The additives are (a) HI, 33 µL, (b) HI, 50 µL, (c) HBr, 33 µL, (d) HBr, 50 µL, (e) HCl, 33 µL, and (f) HCl, 50 µL. The annealing temperatures are 100, 125, and 150 °C. The spots in the FE-SEM images are the yellow phase of CsPbI3 layers. The scale bar is 2 µm. 122x183mm (120 x 120 DPI)



Figure 4. AFM images and roughness profiles of the CsPbI3 layers coated on PEDOT: PSS layers. The perovskite solutions were prepared by adding 50 µL of (a) HI, (b) HBr, or (c) HCl as an additive. The layers were annealed at 150 °C after spin-coating. The images and roughness profiles of CsPbI3 layer without the additive is also shown in (d) for comparison. The CsPbI3 layer without additive is annealed at 335 °C to obtain the black phase. 317x211mm (120 x 120 DPI)


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Figure 5. Current density-voltage curves of the CsPbI3 PSCs. The additives in the CsPbI3 solutions are (a) HI, 33 µL, (b) HI, 50 µL, (c) HBr, 33 µL, (d) HBr, 50 µL, (e) HCl, 33 µL, and (f) HCl, 50 µL. The annealing temperatures are 100, 125, and 150 °C. The maximum PCE values of CsPbI3 PSCs with HI, HBr, and HCl additives are 4.72, 2.56, and 2.11%, respectively. 187x216mm (120 x 120 DPI)



Figure 6. Change in the PCE of PSCs with different additive concentrations (33 µL, 50 µL, and 66 µL) as a function of the annealing temperature. The additives are (a) HI, (b) HBr, and (c) HCl. The PCE values generally increase with annealing temperature and the highest PCE of PSCs is observed at 150 °C. 190x65mm (150 x 150 DPI)


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Figure 7. (a) Current density-voltage curves of CH3NH3PbI3-xClx and CsPbI3 PSCs. The CsPbI3 layer with 50 µL HI was annealed at 150 °C. The change in (b) FF, (c) JSC, (d) VOC, and (e) PCE of CH3NH3PbI3-xClx and the CsPbI3 PSCs after storing in air for 3 h are shown. 300x154mm (120 x 120 DPI)



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Role of Additives on the Performance of CsPbI3 Solar Cells - The

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