p-Phenylenediaminium iodide capping agent enabled self-healing perovskite solar cell

In this study, p-Phenylenediaminium iodide (PDAI) is used to in-situ growth of 2D (PDA)2PbI4 perovskite layer between (FAPbI3)0.85(MAPbBr3)0.15 3D perovskite and CuSCN as a cheap hole transport layer. The results indicate that the incorporation of 5 mg mL−1 PDAI leads to enlarged grain sizes, compact grain boundaries, reduced trap density, efficient charge extraction, and enhanced stability of perovskite film. Passivation of perovskite film with the appropriate amount of PDAI helps in achieving efficient perovskite solar cell with a PCE as high as 16.10%, a JSC of 21.45 mA cm−2, a VOC of 1.09 V, and FF of 70.21%, with negligible hysteresis and excellent moisture stability which remains 99.01% of its initial PCE value after 5 h in high relative humidity of 90 ± 5% and shows unchanged PCE after 1440 h in low relative humidity of 15 ± 5%. Most strikingly, this ultra-thin 2D passivation layer by the use of PDA cations as a bulky spacer not only passivates the defects on the surface of perovskite film but also induces self-healing properties in PSCs which can be rapidly recovered after keeping away from water vapor exposure. This study introduces the cheap and extra stable perovskite solar cells with outstanding self-healing ability towards commercialization.


Results and discussion
Different concentrations of PDAI solutions (3-15 mg mL −1 ) was used to the in-situ growth of 2D perovskite at (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0. 15 and CuSCN interface. The preparation scheme and the device configuration are illustrated in Fig. 1. The cross-sectional scanning electron microscopy (SEM) image is shown in Fig. 1c. In a 2D perovskite used one ammonium functional group as the long-chain organic cations, two ammonium can interact with each other by van der Waals force. So, space between two [PbX 6 ] 4− octahedra would be expanded due to the repulsive force 18 . When a cation with two ammonium functional groups is used to prepare 2D perovskite, the inorganic frameworks are stack together by the stronger ionic bonds, as illustrated in Supplementary Fig. S1 19 . So, it is anticipated that the 2D perovskite will be formed as an ultra-thin layer, as shown in Fig. 1c 25 . The SEM images of the perovskite films passivated by different concentrations of PDAI are illustrated in Fig. 1d and Supplementary  Fig. S2. For the perovskite film without PDAI post-treatment, there are two separated phases (Fig. 1d). The dark grain region is perovskite crystal, and the bright grain phase is PbI 2 26 . After post-treatment of perovskite film with 5 mg mL −1 of PDAI, the PbI 2 layer has been fully covered (Fig. 1d) 27 . The surface of perovskite film passivated by PDAI is remarkably more compact compared with pristine film. When the solution concentration of PDAI reaches to 5 mg mL −1 , there emerges a new layer that partly covered the (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 film, which is speculated to be a 2D perovskite capping layer growth in the surface or interface of (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 grain boundaries via reaction of PDAI and excessive PbI 2 28 . Surprisingly, the (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 film post-treated with 7 mg mL −1 of PDAI, changed into compact perovskite film with grains larger than 1 μm PDAI solution treatment ( Supplementary Fig. S2b). So, PDAI solution can induce the pristine film second-growth into large grains with fewer grain boundary defects based on Ostwald ripening mechanism 29 . The use of higher PDAI concentrations adversely affects the perovskite morphology and the excessive PDAI is trapped in the GBs, which may be decomposed under illumination and also caused phase separation ( Supplementary Fig. S2c,d) 30 .
The XRD patterns are utilized to investigate the effect of PDAI additive and 2D perovskite formation on the crystallinity of (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 films (Fig. 2a). The XRD patterns show (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 crystal peaks containing three peaks at 14.2°, 28.4°, and 31.9°, related to (110), (220) and (310) crystal face, respectively 30 . Characteristic diffraction peak at 12.7° corresponding to PbI 2 is observed in (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 film due to the excess value of PbI 2 in precursor composition 31 . Compared to pristine film, the intensity of the PbI 2 peak is declined by the 3 mg mL −1 of PDAI, and a new peak at 5.1° is related to the 2D perovskite (PDA)PbI 4 . It means that the excess value of PbI 2 in the pristine film is completely reacted with 3 mg mL −1 of PDAI and created 2D and quasi-2D perovskite. The XRD patterns of 2D perovskite film and PDAI powder are also shown in Supplementary Fig. S3. Furthermore, the characteristic peaks of (110), (220), (310) for perovskite films are notably enhanced by the addition of 5 mg mL −1 PDAI. The FWHM values of the characteristic (110) peak are presented in Supplementary Fig. S4. The smallest FWHM value belongs to the perovskite film passivated by 5 mg mL −1 PDAI, showing its excellent crystallization with a preferred orientation of (110) 30 . However, a new peak appeared at 3.9° in the presence of the higher concentration of PDAI is related to the diffraction pattern of PDAI powder. Figure 2b indicates the normalized UV-Vis absorption spectra of perovskite films. A little improved absorbance is obtained for (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 /(PDA)PbI 4 films prepared in the presence of PDAI near the bandgap absorption region. As the PDAI content is increased up to 5 mg mL −1 , the perovskite film absorption is increased that is related to the PDAI addition impact on the preferred orientation crystal structure and uniform surface morphology 32 . When the PDAI addition amount is increased up to 15 mg mL −1 , the absorbance is sharply decreased, which can relate to the crystallinity decrease, according to the SEM and XRD results. The bandgap of perovskite films can be deduced from the Tauc plot ( Supplementary Fig. S5 Fig. 2c, which shows a peak at 780 nm with the FWHM of about 50 nm 30 . The perovskite films passivated by 3, 5 and 7 mg mL −1 of PDAI indicate an enhanced PL yield in the comparison to the reference film, indicating less involvement in carrier recombination. On the other hand, the (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 film post-treated with 5 mg mL −1 PDAI presents a higher fluorescence peak in comparison to the control film, suggesting the uniform, pinhole-free, reduced density of GBs and compact structure of perovskite resulted in low density of charge-trapping and recombination sites   33,34 . It is reported that the charge recombination is minimized in the presence of ammonium cations which have benzene ring in their structure. It is due to prolonging the electron recombination because of the carrier accumulation in the benzene ring 35 . In the presence of 10 and 15 mg mL −1 PDAI, the large density of unreacted PDAI and relatively lose GBs leads to light scattering and excessive charge-trapping site creation 36,37 . Besides, the PL peak intensity of glass/perovskite/CuSCN samples is significantly decreased in the comparison of glass/perovskite ones, which can be related to the suitable valence band position of the HTMs over perovskite layer. In other words, the extra reduction in the PL spectrum is observed for glass/perovskite/ CuSCN samples, where the 2D perovskite is inserted by in-situ growth as an interface. It is because of the better CuSCN coverage on 2D perovskite and interface engineering. In this study, twelve kinds of devices (A 1 to F 1 and A 2 to F 2 ) were manufacture and studied systematically. Devices are defined as follows. Device A: (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 was used as an absorbent layer without in-situ growth of 2D perovskite on top of it. Device B to F: (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 layer post-treated with different concentrations of PDAI (3, 5, 7, 10 and 15 mg mL −1 ) using spin-coating and in-situ growth of (PDA)PbI 4 on top of the 3D perovskite layer. Devices A 1 to F 1 were fabricated by the spin-coating of CuSCN on top of the perovskite layer as HTL and devices A 2 to F 2 were fabricated without CuSCN HTL. Figure 3e shows the photocurrent density-voltage of the devices. All of the steps related to the preparation of solution precursor and spin-coating of the perovskite layer were performed at ambient conditions. The results indicate that J SC and V OC are increased by spin-coating of 3 mg mL −1 PDAI on the (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 layer (Fig. 3a,b). The higher values of J SC and V OC for device B1 in the comparison of A1 are related to the low density of GBs, large grain size, and the pinhole-free surface of the absorbent layer because of the surface passivation. Furthermore, the enhancement of V OC is due to the in-situ growth of (PDA)PbI 4 as a 2D interface layer between (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0. 15 and CuSCN, which can induce a more effective interaction between absorbent layer and HTL, leading to accelerate the hole transfer to the upper layers by strong contact of perovskite and CuSCN 13 . Additionally, the different trend is obtained for FF change, that it is declined for device B1 and then increased for device C1 (Fig. 3c). It is reasonably assumed that the thickness of (PDA)PbI 4 layer on the perovskite film surface depends on the PDAI amount in the precursor solution. In other words, (PDA)PbI 4 film will be produced with inadequate thickness and incomplete coverage for a small quantity of PDAI (3 mg mL −1 ) in the precursor solution. So, the initial decrease of FF of device C1 might be related to the incomplete conversion of PbI 2 into 2D (PDA)PbI 4 , which is the statement of the thickness and coverage importance for 2D perovskite 21 . By spin-coating of 5 mg mL −1 of PDAI solution on the 3D perovskite film, J SC , V OC and FF values are increased, and the highest performance is achieved for device C1 with an optimal PCE of 16.10% (Fig. 3d), a J SC of 21.45 mA cm −2 , a V OC of 1.09 V, and FF of 70.21%. The device C1 shows a champion efficiency of 17.52%, as shown in Supplementary Table S9. Our results are somewhat different from the similar report on surface modification of (FAPbI 3 ) 0.88 (CsPbBr 3 ) 0.12 with 5-AVAI, which was reported a PCE of 16.75%, a J SC of 21.93 mA cm −2 and a V OC of 1.068 V for champion cell 21 . The higher value of PCE and J SC might be related to different perovskite composition. Contrary, the obtained V OC is greater for our study.
The F1 device shows a decreased PCE of 9.60%. The unreacted diammonium can be trapped in the GBs of perovskite films and make adversely impact on the device performance. Also, the excess value of PDAI can increase series resistance, resulting in a decline of J SC , V OC and PCE.
The IPCE spectra of devices A1 and C1 are summarized in Fig. 3f. The device C1 indicates higher IPCE than A1 between 350 and 750 nm. It is interesting that the absorption range of devices is almost the same, which points out that the 2D perovskite interface layer does not cause a significant change in the (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 bandgap. The integrated currents of devices A1 and C1 are 18.77 and 21.73 mA cm −2 , respectively, which are in good agreement with the corresponding values in Fig. 3a.
J-V hysteresis in PSCs is an important issue since it is related to the stability 38 . The causes of hysteresis have been reviewed as follows: Charge trapping/de-trapping related to the surface defects and corresponding density at the interfaces 39 , Ferroelectric polarization in perovskite materials 40 and ion migration 41 . Accordingly, the J-V curves of devices under different scan directions were recorded after 30 days and shown in Supplementary  Fig. S7. Devices were stored at ambient atmosphere (15 ± 5% RH) in the dark. The corresponding parameters of the devices are inserted in Table 1. The hysteresis index (HI) is calculated as Eq. (1) 42 : where J FS (0.8 V OC ) and J RS (0.8 V OC ) show current density at 80% of V OC for the forward and reverse scan, respectively. Devices A1 (HI = − 0.219) and B1 (HI = − 205) show similar hysteresis. Significantly reduced hysteresis is obtained for device C1 (HI = − 0.027) in comparison to devices A1 and B1, which is attributed to the decrease of the defect and trap sites by PDAI passivation and 2D layer formation as showed by SEM and XRD results. By spincoating of the optimum value of PDAI (5 mg mL −1 ) on pristine film, more compact (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 film with a lower density of GBs is achieved. It is reported that defects are made by vacancies in perovskite. The applied external electric field can create a driving force for ions and vacancies, especially through the GBs. These ions and vacancies can be accumulated at the perovskite-HTL and ETL interface, leading to interrupt the photogenerated electron and hole extraction due to increased capacitance. So, GB engineering is a key issue in hysteresis reduction 38 . It is reported that the passivation agents spun on the pristine film can penetrate the perovskite film via GBs 43 . So, PDAI can passive the iodide-rich trap sites at the GBs and suppress iodide ion migration to the interfaces under applied bias. Also, 2D perovskite ((PDAI)PbI 4 ) will be able to decrease hysteresis through the effective physical barrier providing high activation energy for diffusion to hinder the ion migration and charge accumulation near the perovskite/HTL junctions 44 . It indicates that PDAI not only passivates the GBs in bulk perovskite but also cause to more compact film with a lower density of GBs. So, it can suppress the charges www.nature.com/scientificreports/ and ions accumulations at interfaces and prevents interface electrode polarization. So, interfacial and also bulk perovskite engineering is both supposed by PDAI.
To check the reliability of PCE obtained for the fabricated devices, the stabilized photocurrent was measured at voltages corresponding to maximum output power and plotted as a function of time. Figure 4 displays the J−t and PCE-t plots. The comparison of J−t and PCE-t curves for device A1 and C1 indicates that the J SC and PCE of device A1 are unstable during continuous illumination. Such instability property can be improved by making a 2D layer at perovskite/CuSCN interface because of the halide migration inhibition at GBs 45 .
The time-dependent degradation of devices is investigated to study the correlation between stability and surface passivation. The thermal stability of unencapsulated devices is studied by keeping them at 85 °C with  In contrast, the PCE of device C1 is obtained 90% after 6 cycles. It is interesting that the declined PCE value of device C1 is gradually recovered again after 3 cycles. The raised temperature in the solar cell speeds up FA + and MA + vibration, and thereby throws out the organic cations from the perovskite lattice, leading to irreversible thermal decomposition 46,47 . Comparably, perovskite films post-treated with the appropriate amount of PDAI exhibit some self-healing properties in the thermal aging test (device C1). In this device, the GBs are passivated by PDAI, and the traps in the heterojunction region of GBs are connected together by diammonium cations. In addition, the ammonium both ends of the molecule can crosslink perovskite grains through hydrogen bonds to form a dense perovskite film and suppress the A + migration far from the [PbX 6 ] 4− octahedral frame. So, during the self-healing process, the perovskite film will be reconstructed to the 3D lattice structure. Simultaneously, the main problem of perovskite materials is moisture instability because of the hygroscopic nature of 3D perovskite materials. So, 3D perovskite is decomposed into PbI 2 upon contact with water. The perovskite films (A1 to F1) were kept in home-made equipment with controlled 90 ± 5% RH at room temperature ( Supplementary Fig. S8). Figure 5a shows the optical images obtained during the aging test. The UV-Vis spectra of the films are also illustrated in Fig. 5b-g. It is completely obvious that (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 is decomposed to the PbI 2 yellow phase after 30 d. In agreement with the visual observation, the UV-Vis spectra show a regular decline of absorbance intensity. All of the perovskite films passivated by different concentrations of PDAI are degraded at almost the same rate in the first 10 day. Degradation rate is accelerated in the film B1, D1, E1 and F1 after 20 d (especially for film B1 and F1), whereas no further degradation is obtained for the films C1. In order to more deeply investigate the device stability against the moisture, the devices were aged at low and high relative humidity, and the J-V curves were recorded. As shown in Fig. 4d, and Supplementary Tables S7-S12, the PCE of unencapsulated device A1 is sharply decreased and received to 58.02% of initial PCE during 5 h in high 90 ± 5% RH. The PCEs of devices C1 and D1 are maintained 99.01% and 93.99% of the initial value after 5 h, respectively. At higher concentrations of PDAI (15 mg mL −1 ), the excess amount of diammonium salts is trapped in the GBs and deteriorates device stability. Finally, the comparison of device stability in low 15 ± 5% RH shows that the PDAI-treated PSC is less degraded than the (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 device (device A1) according to Fig. 4e, and Supplementary Tables S13-S18. The pristine device (A1) shows 72.27% of its initial PCEs under a 15 ± 5% RH. Almost no degradation of device C1 under low 15 ± 5% RH corroborates that the PDAI passivation can effectively hinder moisture destruction. Stability tests confirmed that device C1 is the most stable device. It is due to the posttreatment of PDAI on the perovskite film, which leads to the in-situ growth of the hydrophobic 2D interface layer and thus repels water molecules. This will create a layer of hydrophobic and thus repel water molecules. On the other side, water adsorption on the perovskite surface cannot be completely avoided; the passivation of GBs and defects by PDAI can suppress the perovskite solar cell degradation. The findings strongly propose that PDAI passivation can effectively protect the perovskite film from moisture. So, we thought about it to check the self-healing properties of the perovskite films. Figure 6a and b show a comparison of the color changes for perovskite layer without PDAI post-treatment (film A1) and the perovskite layer with 5 mg mL −1 PDAI passivation (film C1) after both were exposed with hot water vapor. In comparison, the film A1 is changed to yellow after 12 s of exposure to water vapor, but the color of the passivated film is remained black during this time and changed to brown after 30 s. So, the pristine film is deteriorated in a humid environment more quickly that perovskite film passivated by PDAI. It is due to the in-situ growth of the 2D (PDAI)PbI 4 layer on top of the 3D perovskite film, which can create a hydrophobic barrier layer on it. After being kept away from water vapor, the color of the pristine film has remained yellow, and it shows no self-healing behavior after 80 s, but the passivated perovskite film is changed to black color after 15 s of healing. The self-healing perovskite film was immediately exposed to the water vapor again. After 20 s of exposure to water vapor, the film color is changed to brownishyellow. After being kept away from water vapor for 70 s, the film color is completely changed to black. This wonderful self-healing behavior is also displayed in 3:46 min video (Supplementary Video S1). Whether the perovskite film in the device structure has such a self-healing ability is a little more complicated. In the configured device, the interaction between different layers is also a critical issue, and how much water molecules are absorbed on the ETL and HTL can affect the stability of the perovskite layer. So, the stability of the perovskite layer in the configured solar cell is also investigated. The self-healing ability of two PSCs spin-coating with 3 and 5 mg mL −1    Figure 6c shows a comparison of the color changes for devices B1 and C1. In the first cycle of water vapor contact, the color of device C1 remains almost constant, and a slight color change is observed for device B1 after 37 s of exposure. After 15 s healing step, the device B1 is refreshed again and became completely black. The frequent self-healing ability of devices is studied by device exposure to hot water vapor for 60 s (Fig. 6d). In the second cycle, the color change of device B1 is more obvious than device C1. The adequate amount of PDAI concentration (5 mg mL −1 ) in device C1 has led to the in-situ growth of an ultra-thin and compact 2D (PDAI) 2 PbI 4 layer on (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 film. This hydrophobic 2D layer on the surface of the 3D perovskite film can partially prevent the penetration of water molecules and provides a sufficient amount of moisture stability. The color of device C1 and B1 is returned black after 20 s and 30 s healing step, respectively. The self-healing ability of the device C1 is also confirmed by the J-V curves before and after hot water vapor exposure (Fig. 6e). The J-V curves  www.nature.com/scientificreports/ indicate that the device C1 can heal itself when it is kept away from hot water vapor and the J-V curve can return to its original shape. This self-healing ability is very suitable for the commercialization of perovskite solar cells since once the device is exposed to the humid environment, the solar cell can self-heal to high PCE again in a short time when it returns to sunlight again. Moreover, The self-healing process is studied by XRD analysis (Fig. 6f). The strong peak at 12.7° indicates that PbI 2 phase is formed during hot water vapor exposure. After self-healing, the PbI 2 peak is disappeared and the perovskite film is recovered to its original crystal phase. To further explain such a self-healing ability in the passivated PSCs, it is necessary to explain the exact mechanism of moisture degradation in the perovskite film. The destruction of perovskite films is more considered in the GBs and interfaces 48 . A possible mechanism for irreversible decomposition of perovskite films has been suggested by Choi and coworkers 49 . It is suggested that the irreversible degradation of perovskite materials only happens while both charges and moisture exist simultaneously. In the first step, perovskite films form hydrates in the presence of water molecules. The [PbX 6 ] 4− octahedra interacts with both organic cations (MA + , FA + ) and H 2 O within the hydrated perovskite 50 . Next, the trapped charge (X − ) at the defects causes to the organic cation deprotonation and producing volatile molecules like CH 3 CH 2 (MA) and HC(= NH)NH 2 (FA) 49 : where TCs denotes trapped charges, and X indicates halide. The MA and FA evaporation can shift the reactions to the right-hand side, leading to irreversible degradation of perovskite film.
The self-healing mechanism of the passivated perovskite film is shown in Fig. 7. Interestingly, the hydrogen atoms available in the diammonium cations (PDA) can interact with iodide in [PbX 6 ] 4− octahedra via hydrogen bonding and create passivation in the GBs of perovskite. The degradation of perovskite is started once the perovskite film is exposed to water vapor. In the pristine film, the perovskite grains start to disintegrate away from each other. So, the MA + and FA + ions are driven through channels and interact with the nearest trapped charges to make volatile molecules. Therefore, the irreversible degradation of pristine perovskite will be happening according to the reaction (2), and the color of the film remains yellow. In the case of passivated perovskite film with PDAI, the GBs are attached strongly because of the presence of two ammonium groups at both ends of the benzene ring. So, the MA + and FA + migrations are partially suppressed through the interconnected GBs. On the other hand, the aromatic ring of phenyl diammonium can suppress the trapped charge migration towards organic www.nature.com/scientificreports/ cations. All of these can inhibit the deprotonation of organic cations and results in organic cation anchors in the nearest place to the [PbX 6 ] 4− octahedra rather than escape away. After being kept away from water vapor, the decomposition reaction can take place in the backward direction again, very similar to the two-step synthesis of perovskite film according to the reaction (3). The consecutive decomposition-recombination mechanism expresses the rapid self-healing process in the scaffold perovskite film with the 5 mg mL −1 PDAI.

Methods
Materials. 5   After that, the CuSCN film was annealed at 60 °C for 15 min. Finally, gold thermal evaporation was performed to generate a 50 nm Au metal layer on top of the HTL as a counter electrode. The whole process of the PSCs fabrication was performed in ambient conditions, except for the gold evaporation process.
Measurement and characterization. The X-ray diffraction (XRD) spectra of the prepared perovskite films were recorded by PANalytical, X'Pert Pro MPD, with an X-ray tube (Cu Kα, λ = 1.5406 Å). The morphology of the films was studied by a field-emission scanning electron microscope (Philips, Model XL30). UV-Vis spectra were observed by the UV-Vis spectrometer (400-1000 nm wavelength range, PerkinElmer Lambda25). Photocurrent density-voltage (J-V) curves were measured by a Keithley 2400 source meter under Am 1.5G (100 mW cm −2 ) simulated light radiation in IRASOL, SIM-1000 system (calibrated by a Thorlabs photodiode) at a scan rate of 50 mV s −1 . The devices were masked with an aperture area of 0.09 cm 2 exposed under illumination. Incident photon to current conversion efficiencies (IPCE) was measured by an IRASOL, IPCE-015 equipment. The moisture stability of unencapsulated films or devices was studied by keeping the films or devices in home-made equipment under a relative humidity (RH) of 90 ± 5% at room temperature (27 ± 2). The UV-visible spectra of perovskite films were recorded every 10 d. The thermal stability of unencapsulated devices was studied by aging the devices at 85 °C under RH of 15 ± 5% for 30 min and then cooling down to room temperature for 1 h. J-V curves were obtained in every cycle. The self-healing ability was studied by exposed the unencapsulated devices or films to hot water vapor and kept theme away in the self-healing step.

Data availability
The datasets generated during the current study are available from the corresponding author on reasonable request.