Coadditive Engineering with 5-Ammonium Valeric Acid Iodide for

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Co-additive Engineering with 5-Ammonium Valeric Acid Iodide for Efficient and Stable Sn Perovskite Solar Cells Md. Emrul Kayesh, Kiyoto Matsuishi, Ryuji Kaneko, Said Kazaoui, Jae-Joon Lee, Takeshi Noda, and Ashraful Islam ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02216 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Energy Letters

Co-additive Engineering with 5-Ammonium Valeric Acid Iodide for Efficient and Stable Sn Perovskite Solar Cells Md. Emrul Kayesh1, 2, Kiyoto Matsuishi3*, Ryuji Kaneko1, Said Kazaoui4, Jae-Joon Lee5, Takeshi Noda1, Ashraful Islam1* 1Photovoltaic

Materials Group, Center for Green Research on Energy and Environmental

Materials, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, 305-0047, Japan 2Graduate

School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 3058573, Japan

3Faculty

of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

4Research

Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

5Department

of Energy & Materials Engineering & Research Center for Photoenergy Harvesting

and Conversion Technology, Dongguk University, Seoul 04620, Republic of Korea

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ABSTRACT: Sn-based perovskite solar cells (PSCs) featuring high performance and long-term stability are very challenging, because Sn2+ is relatively prone to oxidation. Here, we have performed a coadditive engineering with 5-ammonium valeric acid iodide (5-AVAI) for FASnI3 based perovskite film. From the morphological, structural and elemental analyses, we observed that 5-AVAI affects the crystal growth of perovskite through its hydrogen bond with I- of SnI64octahedral. As a result, pin hole free homogeneous and stable Sn-based perovskite film form over large area with lower Sn4+ content. This made us able to enhance the power conversion efficiency (PCE) for Sn-based PSCs up to 7% in 0.25 cm2 aperture area. Most importantly, the 5-AVAI added PSCs showed a record stability with maintaining their initial PCE under 1-sun continuous illumination at maximum power point tracking for 100 hours. Table of Content (TOC) graphic

Organic-inorganic halide perovskites have become promising candidates for the green energy technology due to their low cost of fabrication process and favorable optoelectronic properties for solar cells. Due to the continuous research efforts from all over the world, the power conversion efficiency (PCE) of Pb-based perovskite solar cells (PSCs) has been improved from 3.8% to over 23% within few years.1-2 However, the toxicity of Pb makes a serious concern for inauguration of


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ACS Energy Letters

PSCs in the market. Therefore, researchers have turned their concentration towards Pb-free PSCs to replace Pb with other suitable divalent metal cations. In this regard, Sn, Ge, Cu and Bi have been effectively considered.3-6 Among them, Sn has been demonstrated as the most promising candidate because Sn-based perovskite has similar or even superior optoelectronic properties than Pb such as optimum optical band gap, low exciton binding energy, and higher charge carrier mobilities.7-9 Despite favorable optoelectronic properties, the performance of Sn-based PSCs is still far below the Pb-based devices.10 This is because the Sn-based perovskites suffer from some fundamental limitations such as inability to form pin-holes free uniform films, facial tendency to oxidation from Sn2+ to Sn4+ and poor stability.11-12 The oxidation of Sn2+, which is also known as the Sn2+ vacancies, causes unwanted p-type doping in perovskite films, resulting in loses of suitable semiconducting properties for Sn-based perovskite materials.13 To lower the Sn4+ content in final film, Mathews et al. used excess amount of SnF2 into the precursor solution and demonstrated that the addition of SnF2 is essential for continuous film morphology with lower Sn4+ content.3 After that, the addition of excess amount of SnF2 is commonly used for Sn-based and other mixed metal-based PSCs.14-15 Later, several other additives such as SnCl2, SnBr2, H3PO2, pyrazine, hydrazine vapor have also been used with SnF2 to further reduce the Sn4+ concentration and to improve the film morphology.16-20 In this regard, in our previous work, we observed that the addition of N2H5Cl into FASnI3 precursor solution not only reduce the Sn4+ content but also assist the formation of uniform and pin-holes free FASnI3 films21. Mixing of organic cation has also been reported to enhance the performance and stability of Snbased PSCs. For instant, Huang et al. replaced part of methylamonium (MA+) by formamidinium (FA+) for Sn-based perovskite films and found enhanced performance and reproducibility.22 To enhance the stability, recently the two-dimensional (2D)/ three-dimensional (3D) composite


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perovskites concept from the Pb-based PSCs has been implemented for Sn-based PSCs.23-24 In this case, a low dimensional perovskite is formed on the top of 3D perovskite by replacing small organic cation with bulkier organic cation which effectively protects the inner layer from ambient environment. For example, Shao et al. formed 2D/3D perovskite interface layer by introducing phenylethylammonium iodide into FASnI3 precursor solution and improved the PCE up to 9% with 2 h light soaking stability in nitrogen filled glove box.24 But they did not show the maximum power point tracking (MPPT) stability of PSCs which is considered as the standard degradation test for any photovoltaic solar cells. During writing this manuscript, Diau et al. reported a PCE of 9.6% with one-hour operational stability at MPPT in 0.022 cm2 cell area by using two coadditive for FASnI3 based PSCs.25 Hydrophobic long carbon chain organic additives with bifunctional groups, like 5- ammonium valeric acid iodide (5-AVAI) and butylphosphonic acid 4-ammonium chloride (4ABPCl), are well known for improving crystallinity and stability of Pb-based perovskite by cross linking adjacent grains and forming protective layer.26-27 Nazeeruddin et al. have used the hydrophobic nature of 5AVAI to form an ultra-stable 2D perovskite with PbI2 and showed one-year stable PSCs.26 In contrast, Graetzel et al. demonstrated the cross-linking properties of 4-ABPCl to fabricate compact and passive film morphology for stable Pb-based PSCs.27 Therefore, these dual beneficial aspects of additives for cross-linking grains through hydrogen bond formation and forming inert passive layer on the surface might be simultaneously enhances the performance and stability of Sn-based PSCs, which has not been explored yet. In this work, we studied the effects of 5-AVAI on the passivation of grain boundaries, film formation and crystallinity, as well as on the device performance and stability of Sn-based PSCs. We found that the 5-AVAI coordinated with SnI6-4 through hydrogen bond formation and


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ACS Energy Letters

effectively passivated grain boundaries of FASnI3 films. As a result, the PCE of PSCs improved from 3.4 % to 7.0 % in 0.25 cm2 cells. In addition, we observed highly stable PSCs with a record 100 h operational stability without losing the initial efficiency at MPPT condition. For preparation of FASnI3 films, we followed our previously reported one-step anti-solvent method21. We added 5-AVAI into FASnI3 precursor solution by maintaining a 1:1 molar ratio of organic cations (FAI+5-AVAI) and SnI2. From our experiment, we optimized the molar concentration of 5-AVAI as 3 mol%. For simplicity, hereafter we refer to the perovskite film prepared by addition of only SnF2 and SnF2 + 5-AVAI as pristine and with 5-AVAI, respectively. Figure 1a compares the proton nuclear magnetic resonance (1H NMR) spectra of FAI, 5-AVAI, FAI+5-AVAI, 5-AVAI+SnI2, FAI+SnI2, and FAI+SnI2+5-AVAI in deuterated dimethyl sulfoxide-d6 (DMSO-d6) solution. For FAI-SnI2 1H NMR spectra, the proton resonance peaks at 7.8 parts per million (ppm), 8.4 ppm and 8.7 ppm can be assigned for the -CH-, -NH2 and =NH2 for FA, respectively.28 But with the addition of 5-AVAI into FAI-SnI2 solution, including the above three peaks, a new peak appeared at 7.5 ppm. By comparing the 1H NMR peaks of FAI+5AVAI, 5-AVAI +SnI2 and FAI +SnI2+5-AVAI solution in DMSO-d6, we assigned the peak at 7.5 ppm for coordination of functional groups of 5-AVAI with iodide ions. From this result, we proposed that the 5-AVAI undergoes a hydrogen bond interaction (O-H…I and N-H…I) by the carboxylic acid (-COOH) and the ammonium (-NH3+) end groups of 5-AVAI with iodide from SnI64- octahedra. Graetzel et al. also observed similar phenomena for Pb-based perovskite system in using 4-ABPCl as additive.27 The pristine FASnI3 film exhibited relatively larger grains but very poor film coverage with numerous pinholes compare with 5-AVAI added FASnI3 films (Figure 1b, Figure S1). The average grain size was about 603 nm for pristine FASnI3 film (Figure S2a). However, after incorporating


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optimum amount (3 mol%) of 5-AVAI into FASnI3 precursor solution, a homogeneous and pinholes free uniform film was obtained (Figure 1b). This is because the bifunctional 5-AVAI additive makes a coordination with SnI2 which delays the perovskite crystallization rate and directs the growth of perovskite grains with full coverage. However, we observed a reduced grain size of perovskite with increasing amount of 5-AVAI (average grain size 418 nm, 343 nm and 128 nm for 3 mol%, 5 mol% and 10 mol% of 5-VAI, respectively (Figure S2b-d)), possibly due to increasing number of heterogeneous nucleation sites with increasing 5-AVAI amount.29-30 X-ray diffraction (XRD) patterns of pristine and with 5-AVAI (3 mol%) FASnI3 films showed similar characteristic XRD peaks located at14.10, 24.5°, 28.30, 31.80, 40.500 and 42.90 which can be assigned to 100, 102, 200, 122, 222 and 213 crystal planes, respectively, for the orthorhombic perovskite phase (Figure 1c).18,31 More in details, the FASnI3 films with 3 mol% 5-AVAI show a change in intensity of (100), (102) and (200) peaks as compared with pristine FASnI3 XRD pattern. With the addition of 5-AVAI the diffraction intensity of (102) plane decreased whereas the intensities of (100) and (200) plans increased relatively to those of other peaks. This result indicates a preferred orientation along the direction with the addition of 5-AVAI into the precursor solution.32 We also measured the XRD for the film fabricated from precursor solution containing 1:1 molar ratio of 5-AVAI: SnI2 (Figure 1c). Here we observed some sharp peaks at lower diffraction angle (2θ

Coadditive Engineering with 5-Ammonium Valeric Acid Iodide for

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