Monoammonium Porphyrin for Blade-coating Stable Large-area

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Monoammonium Porphyrin for Blade-coating Stable Large-area Perovskite Solar Cells with >18% Efficiency Congping Li, Jun Yin, Ruihao Chen, Xudong Lv, Xiaoxia Feng, Yiying Wu, and Jing Cao J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 15, 2019

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Journal of the American Chemical Society

Monoammonium Porphyrin for Blade-coating Stable Large-area Perovskite Solar Cells with >18% Efficiency Congping Li1,†, Jun Yin2,†, Ruihao Chen2,†, Xudong Lv1, Xiaoxia Feng1, Yiying Wu3, Jing Cao1,* 1 State

Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P.R. China. 2 Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, P.R. China. 3 Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States Dedicated to Professor Jin-Shun Huang on the occasion of his 80th birthday ABSTRACT: Efficient control of crystallization and defects of perovskite films are the key factors towards the performance and stability of perovskite solar cells (PSCs), especially for the preparation of large-area PSCs devices. Herein, we directly embedded surfactant-like monoammonium zinc porphyrin (ZnP) compound into the methylammonium (MA+) lead iodide perovskite film to blade-coat large-area uniform perovskite films as large as 16 cm2. Efficiency as high as 18.3% for blade-coating large-area (1.96 cm2) PSCs with ZnP was unprecedentedly achieved, while the best efficiency of fabricated small-area (0.1 cm2) device was up to 20.5%. The detailed analyses demonstrated the functions of ZnP in crystallization control and defects passivation of perovskite surfaces and grain boundaries. As the consequence, the ZnPencapsulated devices retained over 90% of its initial efficiency after 1000 hours with a humidity of about 45% at 85 °C. This research presents a facile way to achieve the synergistic effect of large-area coating, morphology tailoring and defect suppression based on the molecular encapsulation strategy for perovskite films, further improving the photovoltaic performance and stability of PSCs.

deteriorating the resulting photovoltaic performance and stability.26-28 Several approaches had tried to resolve these issues. For instance, introducing polymers or larger organic molecules on the surface of perovskite film efficiently tuned the film growth and/or surface composition.25,29-33 However, these treatments only regulated the surficial behavior of perovskite film. The large number of defects within perovskite film could be one of the main factors to cause the decay of cell performance and stability. To treat this issue, a series of small organic molecules were used to modify the perovskite film.26,27,34-36 However, the small molecules are difficult to inhibit the escape of organic cation within perovskite film during thermal treatment.

INTRODUCTION Organic-inorganic hybrid perovskite solar cells (PSCs) have drawn enormous interests as the promising photovoltaics with the certified photovoltaic performance over 22%.1 Such a featuring development originates from the intrinsic advantages of the perovskite materials, such as high absorption coefficient, long hole/electron diffusion lengths, adjustable band gap, and idea charge mobility, etc.2-7 The easy solution processibility of perovskite films affords the great promise for future commercialization of PSCs.8-12 However, the polycrystalline property of low temperature solution processed perovskite film inevitably leads to the presence of crystallographic defects at perovskite surfaces and grain boundaries (GBs).13-17 Thus, controlling the film growth and defects within perovskite films are very essential to prepare the efficient and stable PSCs.

More importantly, most of the reported efficient PSCs in the laboratory are still limited the active area to about 1 cm2 produced by spin-coating. Blade coating as a scalable and simple thin film deposition technique can achieve the preparation of large-area PSCs. However, the poorer film quality with defects and pinholes produced by this method generally also resulted in the presence of lower cell performance. Thus, how to choose a suitable additive in perovskite film is becoming of great concern to achieve the synergistic function of morphology tailoring and defect suppression of perovskite film, finally fabricating efficient and stable large-area PSCs.

The theoretical studies indicated that the formation of defects at perovskite surfaces and GBs are mainly induced by the terminal iodide or the ammonium ions.18-20 The ammonium ion, e.g., methylammonium (MA+), are unstable under high heat and/or moisture because it can be easily released from the perovskite lattice.21-23 This process further causes the generation of undercoordinated Pb and Pb-I antisite defects,24,25 adversely 1

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Porphyrins and their derivatives with excellent photoelectric properties and thermal stability,37-39 render them as promising additive candidates in perovskite films to resolve these issues. Thus, a monoammonium porphyrin (ZnP, the structure shown in Figure 1a) was selected as dopant into MAPbI3 film in this work. As expected, such a modification successfully achieved efficient crystallization control of perovskite film. Especially, the ZnP compound can efficiently attach on the surfaces of perovskite nucleus, preventing the cationic escape from perovskites, thus realizing a perfectly molecular encapsulation to avoid the defects formation (Figure 1b and 1c). Finally, the pinhole-free and uniform perovskite film by doping ZnP with an area of 16 cm2 was successfully fabricated. And a high efficiency of 18.3% with a large-area of 1.96 cm2 was unprecedentedly obtained. The small-area (0.1 cm2) PSCs was up to 20.5%. Moreover, the remarkably improved stability toward moisture and heating were achieved unprecedentedly. This is the first time reporting of the fabrication of efficient and stable large-area PSCs by corporation of large molecule. This work provides a new choice for the mass production of perovskite solar cell industrialization.

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coating method.40 Detailed prepared processed were afforded in the Experiment Section. We firstly employed X-ray diffraction (XRD) measurements to assess the function of ZnP on the generation of perovskite phase and crystallization. As prepared in Figures 2a and S1, the samples without and with ZnP doping all revealed the crystallographic phases of 3D perovskite. The similar optical absorption spectra (Figure 2b and S2) further suggested that the ZnP might only attach on the surface of perovskite crystalline, because of the strong ionic bond between surficial iodide anions from perovskite crystalline and ZnP as the cation components (Figure 1c). To verify the above hypothesis, the distribution of Zn and Pb elements by EDX mapping (Figure 2c and 2d) was examined to show the presence of ZnP within perovskite film. Furthermore, the variation of work function of the perovskite film without and with ZnP doping was also measured by scanning Kelvin probe microscopy (SKPM). As displayed in Figures 3, S3 and S4, the contact potential difference (CPD) of the control film without ZnP is ~ -240 mV, the CPD for the ZnPencapsulated film changes to ~ 550 mV (Figures 3c and 3f). This result clearly verified that the ZnP units were uniformly anchored on the surfaces of perovksite nucleus, thus achieving an efficient molecular encapsulation of perovskite surface and GBs (Figure 1c).

RESULTS AND DISCUSSION

Figure 1. (a) Structure of ZnP. (b) Scheme illustration of MAPbI3 film with ZnP doping. (c) Structure of perovskite encapsulated by ZnP.

Figure 2. (a) XRD patterns and (b) UV-vis spectra of perovskite without and with ZnP doping. (c, d) Elemental maps of perovskite with 0.05% ZnP doping.

The effect of introducing ZnP on perovskite films. To evaluate whether the introduction of ZnP into MAPbI3 film could achieve the synergistic effect of morphology tailoring and defect control of perovskite film, we synthesized the ZnP by a facile synthetic route (The detailed synthesis process prepared in Experimental Section) and directly dissolved it in perovskite precursor solution. The added content of ZnP in the precursor solutions was expressed in terms of wt% with respect to the PbI2 content. The ZnP content was range of 0.01-0.1% in this work. To detailed investigate the function of ZnP, the prepared precursor solution was firstly coated on the FTO/TiO2 substrate using the reported one-step spin-

To further explore the effect of ZnP on the crystallization behavior of perovskite film, we measured the Scanning Electron Microscope (SEM) images of the MAPbI3 films without and with adding of ZnP. As shown in Figure 3g, 3h and S5, with the increase of content of ZnP from 0 to 0.05%, the remarkably reduced pinholes and more uniform grain size of the films with ZnP were observed than that of pure perovskite film. This result reflected that the incorporation of ZnP can well control the perovskite grain growth and reduce the pinholes formation to yield the uniform size and compact perovskite film. Unfortunately, when the doping amount of ZnP was further increased to 0.1%, the grain size was 2

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Journal of the American Chemical Society high quality perovskite film, further improving cell performance.

further reduced and the large number of pinhole and defects formed (Figure S5), consistent with the reported results.41

Figure 4. (a) IPCE, (b) best J-V data tested in reverse (RS) and forward (FS) scans and (c) histograms of cell efficiencies among 30 smaller-area cells without and with ZnP doping. (d) Transient photoluminescence of perovskite films with and without ZnP doping.

Figure 3. Contact potential differences measured on the surface of perovskite films without (a-c) and with (d-f) ZnP doping. SEM images of perovskite films without (g) and with (h) ZnP doping.

The effect of introducing ZnP on cell performances of smaller-area PSCs. While the positive effect of introducing ZnP on the formation of perovskite film was well demonstrated, to examine the effect of ZnP on the photovoltaic performances of PSCs, we fabricated the commonly used mesoporous PSCs with FTO/compact TiO2/mesoporous TiO2/perovskite/Co(II)- and Co(III)based porphyrins8,28 (Figure S6)/Au configuration (Figure S7). As depicted in Figures 4a, the incident photon-tocurrent conversion efficiency (IPCE) spectra of the corresponding devices without and with ZnP doping were in excellent accordance with their corresponding optical absorption spectra (Figures 2b). The photovoltaic performances of these devices were offered in Fig. 4, S8 and Table S1. With the increment of the content of ZnP from 0 to 0.05%, the enhanced cell performances were observed. The best efficiency of the PSC device with 0.05% ZnP doping was up to 20.5%, much higher than that of the control device with MAPbI3, 18.8%. The champion efficiency was further evaluated by the stabilized efficiency output with 19.9% (Figure S9). The remarkably reduced hysteresis of modified PSC was observed (Figure 4b and Table S2). Measurements over 30 devices revealed an average efficiency of 19.4% ± 1.1% for ZnP-based PSCs. In contrast, the control PSCs offered an average efficiency of 17.8% ± 1.1% (Figure 4c and S10). When the doping amount of ZnP was increased to 0.1%, a significant drop in the device's performance was observed. This could be assigned to the formation of much more pinholes within perovskite film (Figure S5), and thereby yielding new recombination sites. These results clearly implied that the introduction of a suitable amount of ZnP can produce

The transient photoluminescence (PL) spectra were further utilized to assess the reason behind the improved efficiency. As shown in Figure 4d, the doped perovskite film revealed a significantly increased decay time than that of pure perovskite film (38.2 vs 74.6 ns for the films without and with ZnP doping), indicating dramatically reduced the defects-assisted recombination in the modified perovskite film. Thus, we can conclude that the introduction of monoammonium porphyrin can achieve an efficiently molecular encapsulation to control the crystallization and defects formation of perovskite film and thus improving the cell performances. To further demonstrate the advantage of monoammonium porphyrin, we also prepared the tetra-ammonium zinc porphyrin (Figure S11) as additive. Although the presence of similar morphologies of perovskite films with ZnP and ZnP4 doping (Figure S12a and S12b), the significantly reduced decay time (46.2 vs 74.6 ns for the films with ZnP4 and ZnP doping, Figure S12c) suggested the presence of much more defects within the perovksite film with ZnP4 doping than that of the film with ZnP. As shown in Figure S3 and S13, the similar CPD values of perovskite without and with ZnP4 doping further indicated that there was no ZnP4 on the surface of ZnP4modified perovskite film. These results clearly exhibited that the introduction of ZnP4 only passivated the GBs of perovskite film, not modifying the surface of perovskite film because of the existence of unbalanced surficial charge (Figure S11). In contrast, the corporation of ZnP can realize the synergistic passivation of perovskite surfaces and GBs, thus improving the cell performance (Figure S12d). The introduction of ZnP to fabricate large-area PSCs by blade-coating. Blade coating as a scalable and

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decrease the loss of surface organic cation, which was well demonstrated by the disappear of the XRD peaks of PbI2 by doping ZnP into perovskite film (Figure 5e).

simple thin film deposition technique can prepare the large-area PSCs. However, the poorer film quality with larger number of pinholes produced by this method generally resulted in the presence of lower cell performance. To resolve this issue, Huang et al. introduced a surfactant additive into perovskite precursor solution to promote the production of highly efficient large-area PSCs.41,42 The porphyrins with high molecular weight may enhance the adhesion of precursor solution of perovskite, and thus controlling film growth and defects formation of perovskite films.

The corresponding cell performances were further evaluated. As shown in Figures 5f, S16 and Table S3, it was found that the introduction of ZnP readily further improved the overall performance of PSCs. The best efficiency of the doped device with an area of 1.96 cm2 reached 18.3%, almost consistent with the highest efficiency of large-area PSCs produced by blade coating technique.41,42 In contrast, the efficiency of the control device with MAPbI3 was 16.0%. This result verifies that the introduction of monoammonium porphyrin into perovskite film offers a novel approach to achieve the larger areas device application. Stable ZnP-based PSCs by blade-coating. The ZnP groups were anchored on the surface and GBs of perovskite film, could prevent the attracting of moisture and escaping of organic cation from perovskite film while under thermal treatment,33 thereby improving the moisture and thermal stability. To verify this hypothesis, the stabilities of the PSCs without and with ZnP doping were investigated (all PSCs without encapsulation in this work).

Figure 5. (a) Scheme illustration of blade-coating. (b) Image of 16 cm2 perovskite film prepared by blade-coating. SEM images of perovskite films without (c) and with (d) ZnP doping. (e) XRD patterns of perovskite without and with ZnP doping. (f) Best J-V data of mesoporous PSCs (the area of 1.96 cm2) with ZnP doped perovskite. To evaluate whether the introduction of ZnP could generate positive function on the perovskite film formation, the large-area devices fabricated by blade coating technique were performed. As illustrated in Figure 5a, 5b and S14, it was exciting that we successfully achieved the fabrication of mirror-like perovskite films with an area of 16 cm2 by blade-coating technique. The remarkably reduced contact angle of ZnP-modified perovskite precursor solution on FTO/TiO2 substrate indicated the surfactant-like behavior of ZnP can support the perovskite precursor solution to be easily spread and achieved full coverage on the substrate during bladecoating (Figure S15). The SEM images further clearly confirmed that the introduction of ZnP can reduce the surficial pinholes, generating a more uniform grain size of perovskite film (Figure 5c and 5d). Certainly, the efficient coverage of ZnP on the surface of perovskite crystals can

Figure 6. (a) The stability measurements performed at (a) the humidity about 45% at room temperature, (b) 85 °C in N2 environment, (c, d) 85 °C with the humidity of 45%. Firstly, the moisture stability of these PSCs was carried out with the humidity of 45%. As illustrated in Figure 6a, the ZnP-based PSC device exhibited the obviously enhanced moisture stability by remaining 90% of its origin efficiency after 2,000 h. Under the same conditions, the reference PSC device maintained only