Enhanced Stability of MAPbI3 Perovskite Solar Cells using Poly(p-chloro-xylylene) Encapsulation

We demonstrated an effective poly(p-chloro-xylylene) (Parylene-C) encapsulation method for MAPbI3 solar cells. By structural and optical analysis, we confirmed that Parylene-C efficiently slowed the decomposition reaction in MAPbI3. From a water permeability test with different encapsulating materials, we found that Parylene-C-coated MAPbI3 perovskite was successfully passivated from reaction with water, owing to the hydrophobic behavior of Parylene-C. As a result, the Parylene-C-coated MAPbI3 solar cells showed better device stability than uncoated cells, virtually maintaining the initial power conversion efficiency value (15.5 ± 0.3%) for 196 h.

and 1492 cm −1 , corresponding to the vibration energy of chlorine and phenyl groups, respectively 26 . The two C-H stretching modes related to methyl groups were also observed at 2857 cm −1 and 2927 cm −1 27 . Figure 1b shows a cross-sectional scanning electron microscopy (SEM) image of the Parylene-C-coated MAPbI 3 solar cell, and we confirmed that the thickness of the Parylene-C coating was about 700 nm. The rough surface of the Parylene-C layer was generated by the focused ion beam milling process (Fig. S2). Generally, Parylene-C shows a low surface roughness value of 4.1 ± 0.4 nm because of using a vapor-deposition technique 28 .
First, we measured the time-dependent absorption of bare and Parylene-C-coated MAPbI 3 in order to trace the decomposition reaction in ambient conditions; we also took photographs simultaneously (Fig. S3). The perovskite films were exposed to a room temperature (26.1 °C ± 2 °C) environment with 40-50% relative humidity for 196 h. As shown in Fig. 2a, we observed that the bare MAPbI 3 changed from dark brown to yellow, and the absorption peak at 762 nm gradually decreased. We also found that a new absorption peak at 510 nm became dominant, as the entire film of bare MAPbI 3 became yellow (dashed circle in Fig. 2a). In contrast, the Parylene-C-coated MAPbI 3 film exhibited no color changes, and the absorption also remained unchanged after 196 h in air (Fig. 2b).
To understand these absorption changes, X-ray diffraction (XRD) analysis of the samples exposed to air for 26 d was performed (Fig. 2c). The predominant XRD peaks observed in the as-prepared bare MAPbI 3 (black, top) significantly decreased in the aged bare MAPbI 3 (blue, middle) and a new peak appeared at 12.64°. According to a previous report, this peak corresponds to the (001) diffraction of PbI 2 , which is a well-known by-product of the degradation of MAPbI 3 perovskite 29 . We confirmed that the absorption change below 550 nm (2.25 eV) in Fig. 2a was due to the formation of PbI 2 because the band gap energy of PbI 2 is about 2.30 eV. In contrast, the diffraction patterns of the Parylene-C-coated MAPbI 3 film were almost identical to those of the as-prepared MAPbI 3 , even after 26 d (red, bottom). This result indicates that the Parylene-C passivation layer effectively slowed the decomposition reaction in the MAPbI 3 perovskite.
We further confirmed the passivation effect of Parylene-C using time-dependent PL as a function of air-exposure time. In Fig. 3a, the PL intensity of the bare MAPbI 3 decreased dramatically over time, and the PL peak position was slightly blue-shifted (black dashed line). According to a previous report, the blue-shifted PL peak of aged MAPbI 3 was related to PbI 2 formation 17 , and we observed an increased PL intensity at 510 nm from the aged MAPbI 3 (Fig. S4), which is consistent with the band gap of PbI 2 . In contrast, Parylene-C-coated MAPbI 3 maintained its initial PL intensity and peak position for 196 h (Fig. 3b), showing the effective passivation effect of Parylene-C on MAPbI 3 .
Time-resolved photoluminescence (TRPL) measurements were conducted simultaneously on both bare and Parylene-C-coated MAPbI 3 in Fig. 3c,d. The integrated PL intensity of the bare MAPbI 3 was zero after 120 h; thus, we compared the TRPL spectra taken for 120 h (excluding data taken after 196 h). As shown in Fig. 3c, the PL decay curves of the bare MAPbI 3 decreased gradually with increasing exposure time, whereas those of the Parylene-C-coated MAPbI 3 remained unchanged for 120 h (Fig. 3d). We calculated the average lifetime from two decay components fitted to a bi-exponential function and plotted the changes as a function of air-exposure time (Fig. S5). The carrier lifetime in the bare MAPbI 3 decreased remarkably from the initial value of 8.67 ns to 0.99 ns because of the decomposition reaction. In contrast, Parylene-C-coated MAPbI 3 initially exhibited a longer carrier lifetime than the bare film, i.e., 9.19 ns, maintained for 120 h.
Finally, we investigated the photovoltaic performance of Parylene-C-coated MAPbI 3 solar cells in terms of the effect of Parylene-C encapsulation. The solar cell performance was measured under the AM 1.5 G illumination with a power density of 100 mW cm −2 (see Supplementary Information for details). In Fig. S6, the comparison of current density-voltage (J-V) curves before and after Parylene-C deposition revealed that the MAPbI 3 PSC was not damaged during the vapor-deposition process. Figure 4 shows time-dependent photovoltaic characteristics for the bare, polymethyl methacrylate (PMMA)-coated, and Parylene-C-coated MAPbI 3 PSCs. We selected PMMA for comparison, as it is the most commonly used polymer for PSC encapsulation. Interestingly, www.nature.com/scientificreports www.nature.com/scientificreports/ the PMMA-coated PSCs showed a 30% drop from their initial efficiency (Fig. 4b) because chlorobenzene, a pre-dissolution of PMMA and Spiro-OMeTAD, can damage Spiro-OMeTAD during PMMA coating 30 . This result indicates that the use of PMMA limits the selection of hole-transport materials in solar cells, although it is among the most widely used of various passivation polymers. On the contrary, Parylene-C can be used in combination with various hole-transport materials without affecting the initial device performance. In terms of device stability, the solar cell performance of the bare and PMMA-coated PSCs decreased dramatically after exposure to ambient conditions, and these cells were completely degraded within 196 h (Fig. 4a,b). In contrast, the PCE of the Parylene-C-coated PSC exhibited no notable change over a period of 196 h (Fig. 4c).
To explain how Parylene-C can efficiently retain the initial performance of MAPbI 3 PSC, we further performed FTIR spectroscopy. As shown in Fig. S7, two N-H stretching modes around 3150 cm −1 were remarkably  www.nature.com/scientificreports www.nature.com/scientificreports/ suppressed for aged MAPbI 3 , and a new absorption peak appeared at 3466 cm −1 , corresponding to the vibration energy of hydroxyl groups (−OH). This finding agrees well with a previous report 31 on hydrated compounds in aged MAPbI 3 resulting from adsorption of water vapor. Note that humid conditions are the main cause of degradation in MAPbI 3 perovskite, and therefore, to improve stability of the PSC, it is necessary to prevent water penetration. Fortunately, Parylene-C has been known to have low water permeability of 0.14 cm 3 ·mil/(100 in 2 24 hr ·atm) at 23 °C 32 , which correlated well with our contact angle data (the inset of Fig. 4c). The surface of PMMA showed a contact angle of 72°, which were in the hydrophilic range (<90°) 33 . In contrast, Parylene-C showed hydrophobic behavior, with a considerably higher contact angle of 121°.
We tested the water permeability of these encapsulating materials (Video S1). We prepared Spiro-OMeTADand PMMA-coated MAPbI 3 as well as bare and Parylene-C-coated MAPbI 3 . The bare and Spiro-OMeTAD-coated MAPbI 3 perovskite films turned yellow on exposure to water drops, revealing that the hole transporting material could not protect against reaction with water. Although the PMMA-coated MAPbI 3 initially remained unchanged, the decomposition reaction occurred within 6 min. Yoo et al. reported that a part of PMMA molecular chain is hydrophilic; thus, water molecules can hydrolyze the ester groups in PMMA and break down the PMMA structure 30 . This is consistent with our results, indicating that PMMA is unable to protect perovskite from water damage. Unlike the other tested materials, Parylene-C effectively passivated the perovskite film against water droplet exposure for 30 min. These results indicate that Parylene-C was able to slow down the degradation of PSC because it could effectively passivate MAPbI 3 PSC from reaction with water.
For further analysis, we compared the photovoltaic parameters of the as-prepared (0 h) and aged (after 196 h) PSCs with different encapsulating layers. In Fig. 4e,f, the bare and PMMA-coated PSCs showed remarkable decreases in Jsc and PCE values after 196 h. Interestingly, the Voc values hardly changed after 196 h for both the bare and PMMA-coated solar cells (Fig. 4d). Conings et al. reported similar small decreases of Voc, from aged MAPbI 3 PSCs, and revealed that Jsc was influenced more by degradation because perovskite degradation increased resistance and recombination near the interface between the perovskite and carrier transporting layers 17 . On the basis, we considered that the huge decrease of Jsc in our data was also attributable to perovskite layer degradation. Lastly, the Parylene-C-coated PSC showed no noticeable change to any of the parameters; therefore, we confirmed that Parylene-C could effectively encapsulate the MAPbI 3 solar cells for 196 h.

conclusions
We demonstrated successful encapsulation of MAPbI 3 solar cells by Parylene-C deposition. By structural and optical analyses, we systemically investigated the origin of the decomposition reaction in MAPbI 3 and confirmed that Parylene-C can efficiently slow down the decomposition reaction in the MAPbI 3 films. In particular, Parylene-C can efficiently isolate MAPbI 3 perovskite from reaction with water, owing to its hydrophobic character, and as a result, the Parylene-C-coated MAPbI 3 solar cells maintained almost the initial PCE values (15.5 ± 0.3%) for 196 h. On the basis of this work, we believe that the (p-xylylene) type polymers have shown the potential to improve the lifetime of organo-lead halide perovskite in future photovoltaic applications.