Universal Encapsulation Adhesive for Lead Sedimentation and Attachable Perovskite Solar Cells with Enhanced Performance

In this work, a modified polyurethane adhesive (PUA) was prepared to realize a convenient encapsulation strategy for lead sedimentation and attachable perovskite solar cells (A‐PSCs). The modified PUA can completely self‐heal within 45 min at room temperature with an efficient lead ion‐blocking rate of 99.3%. The PUA film can be coated on a metal electrode with slight efficiency improvement from 23.96% to 24.15%. The thermal stability at 65 °C and the humidity stability at 55% relative humidity (RH) are superior to the devices encapsulated with polyisobutylene. The PUA film has strong adhesion to the flexible substrate and the initial efficiency of the flexible perovskite module (17.2%) encapsulated by PUA remains 92.6% within 1825 h. These results suggest that PUA encapsulation is universal for rigid and flexible PSCs with enhanced stability and low lead hazards. Moreover, it was found that flexible PSCs can be well attached to various substrates with PUA, providing a facile route for the A‐PSCs in various scenarios without additional encapsulation and installation.


Introduction
][29] Materials with self-healing properties under heatconditions were placed between the fluorine-doped SnO 2 conductive glass (FTO) and the top glass to reduce lead leakage. [13]The film with phosphate groups coated on the front and back surface of the PSC device isolated more than 96% of the lead leakage for damaged device. [15]The low-cost cation exchange resin lead trapping material has been applied into the device stacking layer to improve lead absorption. [16,30]The lead adsorption encapsulants using sulfonated graphene aerogels mixed with polydimethylsiloxane are encapsulated in the flexible PSC, which can capture more than 99% of lead leakage in various simulated device broken conditions. [23]The strategy of integrating impact-resistant lead adsorption ion gel on both sides of the perovskite module demonstrated a significant effect on blocking lead leakage under extreme test conditions. [26]urrently researches have been focusing on providing an additional lead absorbing layer for the device besides the encapsulation layer.The materials used for device encapsulation should have the merits of low permeability of water vapor and oxygen, high visible light transmittance, good aging performance, and chemical inertia with perovskite devices.However, the common encapsulation structure is that the cover glass is bonded to the device substrate with polymers such as epoxy resin, ultraviolet curing resin, and polyisobutylene (PIB). [31,32]These encapsulation methods normally require high pressure, thermal annealing and/or ultraviolet irradiation, which are potential factors that undermine the long-term efficiency and stability of PSCs.It would be In this work, a modified polyurethane adhesive (PUA) was prepared to realize a convenient encapsulation strategy for lead sedimentation and attachable perovskite solar cells (A-PSCs).The modified PUA can completely self-heal within 45 min at room temperature with an efficient lead ionblocking rate of 99.3%.The PUA film can be coated on a metal electrode with slight efficiency improvement from 23.96% to 24.15%.The thermal stability at 65 °C and the humidity stability at 55% relative humidity (RH) are superior to the devices encapsulated with polyisobutylene.The PUA film has strong adhesion to the flexible substrate and the initial efficiency of the flexible perovskite module (17.2%) encapsulated by PUA remains 92.6% within 1825 h.These results suggest that PUA encapsulation is universal for rigid and flexible PSCs with enhanced stability and low lead hazards.Moreover, it was found that flexible PSCs can be well attached to various substrates with PUA, providing a facile route for the A-PSCs in various scenarios without additional encapsulation and installation.
very attractive to fabricate a multifunctional encapsulation layer which can be assembled for PSCs for simultaneous atmosphere insulation and lead sedimentation.It has been proved that the polyurethane has good thermal stability and outdoor stability when used in PSCs encapsulation. [27]n this work, through a chemically modified polyurethane adhesive (PUA), we report a convenient encapsulation route via the direct conglutination of substrate device with the sealing materials for both rigid and flexible PSC devices.The PUA shows self-healing characteristics at room temperature and could be directly placed in contact with the bottom electrode of PSCs.The PCE is enhanced to over 24% with PUA encapsulation.The PUA film has strong adhesion to the flexible substrate, and the initial efficiency of the flexible perovskite module (17.2%) encapsulated by PUA remains 92.6% within 1825 h.The encapsulated PSCs demonstrate efficient lead-absorbing ability in a simulated dripping experiment, and the lead removal rate reached 99.3%.It is worth noting that flexible A-PSCs can be conveniently attached to walls, using this PUA as an electronic source for potential indoor smart facilities.

Result and Discussion
Polyurethane film can be conveniently obtained by the reaction of polyol and polyisocyanate with a catalyst.From the synthesis point of view, this material has an advantage of having an adjustable molecular structure. [33]It can be used for encapsulation of PSCs by selecting appropriate components for reaction to obtain desired properties, such as the functions of capturing heavy metal lead ions, selfhealing, formation of film on various substrates easily, and good processability.We first prepared modified PUA materials using a two-step synthesis method.Polytetramethylene ether glycol (PTMEG) reacted with excess isophorone diisocyanate (IPDI) to form a prepolymer with an isocyanate group at both ends and then the target product was obtained by chain extension.Because the isocyanate groups are highly unsaturated and have strong chemical activity, they are easy to react with compounds containing active hydrogen. [34]herefore, this synthesis requires strict control of impurities such as water and alcohol and the use of urethane-grade solvents as much as possible.The schematic process is shown in Figure 1a.The detailed synthesis process is described in the experimental section in the support information.
Polytetramethylene ether glycol and bis(2-hydroxyethyl) disulfide (HEDS) contain hydroxyl groups at both ends, which can react with IPDI containing binary isocyanate groups in an equivalent amount.[37] The chelation between sulfonic acid groups and lead ions is shown in the infrared absorption spectrum (Figure S1, Supporting Information). [23,38,39]The powder contains monophenol hydroxyl groups, which are supposed to react with IPDI and then be accommodated in the PUA system.IPDI has an asymmetric alicyclic structure, and the activity of its isocyanate groups is different.In this work, an organotin catalyst is used in the process of material preparation.Under the initiation of dibutyltin dilaurate (DBTDL), the activity of phenolic hydroxyl groups is lower than that of primary hydroxyl groups. [40]We adjusted the reaction ratio of HMBSA and IPDI to further control the properties of the synthesized PUA (Figure S2, Supporting Information).
The optimized phenolic hydroxyl of HMBSA can react with an isocyanate of IPDI at a ratio of 1:2.Besides, a stable and well-proportioned PUA solution was obtained by adjusting the proportion of raw materials for PUA synthesis (see the details in the experimental section, Table S1 and Figure S3, Supporting Information).
In the material preparation, IPDI should be in excess and reach the dissolution balance with HMBSA powder and the solvent.PTMEG with a molecular weight of 1000 can easily react with IPDI to obtain high molecular weight products with rapidly increasing viscosity, which significantly reduces the processability.However, the partial replacement of PTMEG by additional HEDS can solve the problem of the system and is beneficial for achieving a stable polymer solution.Because of the low activity of phenolic hydroxyl in HMBSA, it is added into the system after the reaction of PTMEG and IPDI.As shown in Figure 1a, the modified PUA displays an illustrated linear structure with a certain curled shape.The chelation between the negatively charged sulfonic acid groups and lead ions is illustrated in Figure 1b.The lead ions in lead nitrate solution can be instantly adsorbed by the aqueous solution of HMBSA powder.The attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) in Figure 1c shows that there is no characteristic absorption at 2265 cm À1 representing isocyanate groups, [41,42] and the absorption at 1710 cm À1 representing carbonyl groups, indicating the complete reaction of isocyanate and the formation of PUA.
The as-made PUA materials show excellent self-healing properties at room temperature.The newly synthesized PUA solution was heattreated in an oven at 80 °C for 4 h to remove the solvent.PUA then forms a sticky film on the glass substrate.Oblique scratches were made on the film using a blade.The scratched PUA film was gradually heated at different temperatures of 25, 45, 65 and 85 °C, respectively (Figure 2a,b).The higher the temperature, the faster the healing, and finally all the samples reached 100% healing efficiency.The time required for complete healing at four different temperatures is 45, 10, 5, and 1.5 min, respectively.At the cracked positions of the PUA film, the exchange reaction of dynamic reversible disulfide bonds and hydrogen bonds on molecular chains are reconnected. [43]This benefits from the components design that IPDI has an asymmetric alicyclic structure, which can increase the mobility of polymer chains and facilitate the activation of invertible bonds. [44]][50] With the combining of these effects, the self-healing polymer material can resist impact and scratch, and thus prolong its service life.
Different heating time was set to explore the relationship between film-healing performance and heating time.It is found that all six groups of samples heal well in 1 minte (Figure S4, Supporting Information), suggesting that the self-healing performance of the film does not depend on the existence of solvent.No matter how long it takes to remove the solvent by heating, PUA keeps a good self-healing performance, which is also advantageous for the safe encapsulation of perovskite solar cells.
According to the tensile stress-strain curve in Figure 2c, the modified PUA material can be self-supported and deformed with the applied tensile force.From thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis shown in Figure 2d, the modified PUA material has good thermal stability, [51] and its initial decomposition temperature is higher than 200 °C, reaching the first temperature point with the maximum change rate at 249.9 °C, and then continuously decomposing until it disappears completely.In the differential scanning calorimetry (DSC) tests of PUA solid, PUA shows a continuous endothermic phenomenon (Figure S5, Supporting Information), and it is speculated that it has undergone glass transition and melting change under continuous heating.Figure 2e shows the transmittance and reflectance of FTO and of glass/190 lm PUA/FTO (the scheme of encapsulation PSCs from the light entrance surface).The PUAcontaining sample demonstrates the antireflection effect [52] in the visible light region of 400-800 nm, and the maximum antireflection is 2.58% at 559 nm.The water contact angle of the glass is 56.3°with good wettability, while it reaches 97.95°for PUA, suggesting that it is hydrophobic (Figure 2e).The newly synthesized PUA solution has a certain viscosity.The viscosity of the solution can be adjusted and then films with a different thicknesses can be obtained.In the transmission spectra of PUA films with different thicknesses, shown in Figure 2f, the PUA films have strong absorption in the ultraviolet region of 250-400 nm.With the increase of PUA thickness, the zero transmittance in the curve moves to the right.HMBSA powder added in the PUA system used in this work has the function of absorbing ultraviolet rays due to benzophenone ultraviolet absorbers. [53]It can be used in the external encapsulation of PSCs to increase the illumination resistance of the device.
A modified PUA film and soda-lime glass were used in the front and back encapsulation of PSCs to test the lead sedimentation effect.The  PUA film contains active and polar carbamate groups in its molecular chain, and has excellent chemical bonding force with active hydrogencontaining materials such as wood, plastic, glass, and ceramics. [54,55]herefore, viscous PUA can be easily pressed and bonded with PSCs at room temperature.A complementary sample of glass/PIB is that thin strips of PIB are placed around the PSC device and hot pressed at 80 °C.Schematic diagrams of two encapsulation schemes are shown in Figure 3a.
In the water-dripping experiment of lead-leakage testing, six pieces of 3 9 3 cm PSC mini-modules were prepared and the active area of the perovskite layer was 4 AE 0.2 cm 2 .The photographs of the modules before and after the damaging with glass/PIB (top) and glass/PUA (bottom) encapsulations are shown in Figure 3b.The photographs of the remaining four modules are shown in Figure S6, Supporting Information.The damaged modules were placed in a polytetrafluoroethylene mold with an inclination angle of 30°.Nitric acid water (pH = 4.2), as the worse form of acid rain condition, was controlled at 5 mL h À1 for 1 h (Figure S7, Supporting Information).The wetting area of the water drops on the device was about 1 cm 2 to simulate rainfall of 50 mm h À1 . [16]fter 1 h, the perovskite mini-module encapsulated by PIB turned yellow after water spread along the cracks while the PUA encapsulated sample was still black due to the self-healing adhesion effect of PUA (Figure 3c).The lead-leakage concentration of the damaged PIB devices was 1.16-2.85ppm, obtained using an inductively coupled plasma optical emission spectrometer (ICP-OES).In contrast, those of the damaged PUA-devices after encapsulation were reduced by around 100 times to the value of ~0.02 ppm, reaching a lead sedimentary rate up to 99.3%.These results suggest the low-temperature encapsulation route with PUA material can effectively prevent the leakage of lead ions.
We have established an in situ method and equipment to measure the concentration of heavy metal lead by in situ absorption spectrometry.An aqueous solution of xylenol orange (XO) can be used as a complexing indicator for the determination of heavy metal lead, [56] and its maximum absorption wavelength is 580 nm.After acidification with an appropriate amount of low-concentration nitric acid aqueous solution, the pH drops to 4.2 and the purple XO turns yellow and the absorption peak at 580 nm disappears (Figure S8, Supporting Information).Acidic XO can be complexed with lead to obtaining Pb-XO.The characteristic absorption wavelength of the complex is 580 nm, and the concentration of lead can be obtained according to the absorbance at this wavelength.The standard curve for testing lead content is illustrated in Figure S9, Supporting Information.The peak at 580 nm dramatically decreases after adding PUA solution to Pb-XO (Figure 3d), indicating the effective function of capturing lead ions of as-made PUA.The aqueous solution of HMBSA powder can combine with lead ions immediately.The black curve in Figure 3e represents the absorption of Pb-XO at 580 nm.When HMBSA is added to the Pb-XO solution, the absorption peak decreases continuously and the absorbance decreases from 2.16 to 0.02 within 4.1 s (Figure 3e,f).
The device was put into a square cuvette and soaked in the 20.6 mL acidic XO aqueous solution.From the absorption spectrum collected continuously for 2 h, the absorption value of 580.044 nm was taken to obtain the lead leakage of water immersion in two encapsulation methods (Figure 3g).The absorbance at 580 nm indicates that the lead concentration gradually increases to 0.28 within 2 h under the glass/ PIB encapsulation.The absorbance of the device encapsulated with glass/PUA slowly increases to 0.02 at 580 nm, indicating that the glass/PUA encapsulation can prevent the leakage of lead ions in PSCs even in a water immersive condition.
To verify the compatibility of the PUA film as the encapsulation layer of PSCs, the dried PUA film was pasted on the gold electrode of the PSC device to form an architecture of glass/PUA/Au/Spiro-OMeTAD/ Perovskite/SnO 2 /FTO (Figure 4a).There was no additional separation layer between the PUA film and the device.The efficiency distribution of reverse scanning J-V curves of a series of rigid devices with perovskite composition of (FAPbI 3 ) 0.96 (MAPbBr 3 ) 0.04 and effective area of 0.1475 cm 2 before and after encapsulation by PUA film is shown in Figure 4b, and the distributions of open-circuit voltage (V OC ), shortcircuit current (J SC ), and fill factor (FF) are shown in Figure S10, Supporting Information.These devices exhibited an average PCE of 22.49% with a V OC of 1.13 V, a J SC of 24.71 mA, and an FF of 0.806.PCE was enhanced to an average PCE of 22.84% with a V OC of 1.136 V, a J SC of 24.78 mA, and an FF of 0.811 after encapsulation by the PUA film at room temperature (Table S2, Supporting Information).Statistics show that V OC , J SC and FF are all improved after encapsulation and the average efficiency is increased by 0.34%.In addition, the J-V curves of the champion sample before and after encapsulation are shown in Figure 4c.The PCE before encapsulation was 23.96% with V OC of 1.171 V, J SC of 24.79 mA and FF of 0.825, and the PCE after encapsulation was enhanced to 24.15% with V OC of 1.173 V, J SC of 24.85 mA, and FF of 0.828.The slight improvement of device efficiency after encapsulation may be due to the decrease of interface recombination caused by the close contact between PUA and gold electrode and the hole transport layer. [57,58]he thermal stability of devices encapsulated by glass/PIB and glass/PUA were tested at 65 °C and 20% RH environment for over 1000 h (Figure 4d).The PCE of the sample encapsulated with PIB was slightly improved after encapsulation, and began to decrease at 385 h, and finally maintained 54.6% of the original efficiency at 1033 h.The PCE of the PUA-encapsulated device kept 94.3% of original efficiency after being placed on a hot table at 65 °C for more than 1000 h.
The humidity stability of samples was evaluated at 25 °C and 55% RH environment (Figure 4e).Perovskite sealed with glass was eroded by moisture and decomposed in a short time of 288 h.As a practical edge encapsulation material with hot pressed PIB, the water vapor transmission rate is in the range of 10 À2 -10 À3 gm À2 d À1 and could effectively resist dampness invasion. [29]The device in this encapsulation mode shows a 78.4% of the original efficiency in 576 h.The glass/ PUA encapsulation method also has a slight decline in humidity environment, and its decline rate is slower than that of the glass/PIB sample, which finally maintains 80.8% of the original efficiency in 576 h.This could be caused by the gap between the cover glass and the device for PIB samples, which did not exist in the PUA encapsulation.From the above experimental results, the modified PUA synthesized in this paper is a good sealing material with proper efficiency enhancement.Moreover, it can be coated as an adhesive, making the encapsulation process much simpler and time-saving.Compared with the encapsulation process used in some references, PUA encapsulation at room temperature could be more beneficial for the long-term stability of the device (Table 1).
The sealing material is bonded to the device by the PUA encapsulation method.We thus characterized shear strength and obtained the strain curve by using an electronic universal material testing machine (Figure 4f) to evaluate the strength of adhesion with PUA encapsulation. [59]The samples were aged at 25 °C, 55% RH, and 65 °C, 20% RH for 500 h.Three samples were tested in parallel in each group (Figure S11, Supporting Information).It was summarized that the shear strength under the displacement of 1.04 mm was corresponding to the strain of 0.08 (Table S3, Supporting Information).It was found that the shear strength of the sample was kept at 73.1% after being placed in a humid environment for 500 h, while that of the continuously heated sample was enhanced by 4.256 times compared to the original value.Furthermore, the shear modulus in the strain range of 0-0.01 at 55% RH was 1.098 times that of the standard sample, while the sample heated at 65 °C was reduced to 54.9% of the original value.Under continuous heating, there was a little overflow around the bonding of PUA film, the shear resistance was weakened and easy to cause slight displacement, while the adhesion stability of the samples in a humid environment was good.In practice, the adhesion stability could be enhanced with high shear strength due to the intermittent environmental heat during the day and night alternation.
We further verified that the PUA encapsulation scheme mentioned in this paper is very compatible with flexible PSC.A flexible perovskite module with FA 0.8 Cs 0.2 PbI 3 as a perovskite component exhibited a PCE of 17.22% with V OC of 5.307 V, J SC of 4.40 mA, and FF of 0.737.The PUA film was coated on the polyethylene terephthalate (PET) substrate and attached to the electrode of the flexible perovskite module for encapsulation protection.After being placed at 20% RH, and 25 °C for 1825 h, it showed a PCE of 15.96% with V OC of 5.403 V, J SC of 4.03 mA, and FF of 0.734.The efficiency of the module encapsulated by PUA/PET remained 92.6% for about 76 days (Figure 5a).
In the application of PUA in light-weight flexible A-PSCs encapsulation, we mainly considered the adhesion between PUA and different materials.PUA solution can be brushed on wooden cabinets, glass windows and ceramic tiles (Figure S12, Supporting Information).After a short time of solvent removing by hot air blowing, it was found that the flexible PET film substrate could be well attached to the PUA-coated substrate.This provided ideas for attaching the flexible PSC in various scenarios without additional encapsulation.The urethane group contained in PUA film can chemically bond with the adsorbed water-hydroxyl group on the surface of metal, glass, ceramics and devices (Figure 5b), which provides high strength for encapsulation.We present a prototype application of an attachable flexible perovskite module on window glass using PUA, as shown in Figure 5c.The flexible attachable perovskite module can be easily attached on the wall with the green light-emitting diodes (Figure 5d).Thus, with the modified PUA, the flexible devices can be attached to buildings as the distributed energy source for sensors with enhanced stability and inhibited lead leakage.

Conclusion
In summary, we provide a strategy that can be conveniently used for rigid and flexible PSC external encapsulation based on the PUA.Firstly, a stable PUA solution with good processability was obtained by reasonably selecting raw materials, controlling the ratio and  adding sequence of each component.Secondly, detailed characterization proves that the modified PUA has excellent self-healing performance at room temperature, good thermal stability, 2.58% antireflection in the visible light region and strong absorption in the ultraviolet light region.
Compared with the PIB encapsulation, the PUA encapsulation has a lead ion leakage barrier rate of 99.3%, and the lead leakage in the soaking test is far less than that in the control group.The rigid device with an efficiency of 23.96% is improved to 24.15% after being encapsulated by PUA/substrate structure.Moreover, the encapsulation scheme keeps the initial efficiency of 94.3% at 65 °C and aging time of 1000 h.In a 55% RH environment for more than 500 h, 80.8% of the original efficiency was retained, which was proved to be superior to the thermal stability and humidity stability of the glass/PIB encapsulation structure.
In addition, for flexible A-PSCs, this material can also be used for safe and efficient encapsulation.This work provides a new perspective for the general and convenient encapsulation method for flexible and attachable PSCs.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.

Figure 1 .
Figure 1.a) Schematic synthesis route of the universal encapsulation adhesive of PUA.b) Lead adsorption mechanism of HMBSA in the PUA.c) ATR-FTIR spectrogram of PUA film (left), amplified spectrum of N=C=O (top) and C=O (bottom) telescopic vibration.

Figure 2 .
Figure 2. a) Microscope morphologies for gradual recovery of scratched PUA films at different temperatures.b) Healing efficiency at varied temperatures and the time required for complete healing.c) The tensile stress-strain curve of as-made PUA.d) The TG and DTG curve.e) Transmittance and reflectance for FTO glass and FTO glass with 190 lm PUA and soda-lime glass on top (G/PUA/FTO).Insert: the water contact angle of glass and PUA substrate.f) Transmission spectra of PUA with different thickness.

Figure 3 .
Figure 3. a) Illustration of encapsulation architectures of glass/PIB and glass/PUA.b) Photographs of the mini-modules before and after damage under two encapsulation methods.c) The lead-concentration histogram of the collected aqueous solution measured by ICP-OES.Insert: the photograph of the damaged modules with two encapsulation routes after acid water dripping.d) The absorption spectra of acidic xylenol orange (XO) solution, XO and lead nitrate solution and XO, lead and PUA solution in visible light region.e) The in situ curves representing the absorption peaks of lead at different times.f) The absorption intensities at 580 nm after HMBSA aqueous solution is included with XO and lead solution.g) In situ absorption intensities at different soaking times for damaged devices with two encapsulation architectures.

Figure 4 .
Figure 4. a) Schematic architecture of the low-temperature PUA encapsulated PSC.b) Efficiency distribution of a series of rigid devices before and after encapsulation by PUA.c) J-V curves of rigid device with championship efficiency before and after encapsulation by PUA film.d) Normalized efficiency evolution of devices encapsulated with glass/PUA and glass/PIB at 65 °C and 20% RH environment.e) Normalized efficiency evolution of devices encapsulated with glass, glass/PUA and glass/PIB at 25 °C and 55% RH environment.f) The shear strength of glass/190 lm PUA/FTO and the shear strength of the structure after aging at 55% RH, 25 °C and 20% RH, 65 °C for 500 h, respectively.

Figure 5 .
Figure 5. a) J-V curves of the flexible perovskite module before and after PUA encapsulation.Insert: the photograph of flexible perovskite module.b) Adhesion diagram between glass-PUA-flexible perovskite module.c) The process of encapsulation of flexible perovskite module and attached to a window glass with PUA only.d) Photographs of the process of attachable flexible perovskite module encapsulated with PUA film.

Table 1 .
Comparison of encapsulation methods with different encapsulation materials in some recent reports.