2018钙钛矿的封装-Rongrong Cheacharoen .pdf

144 | Energy Environ. Sci., 2018, 11, 144--150 This journal is ©The Royal Society of Chemistry 2018 Cite this: Energy Environ. Sci., 2018, 11,144 Design and understanding of encapsulated perovskite solar cells to withstand temperature cycling† Rongrong Cheacharoen, a Nicholas Rolston, b Duncan Harwood, c Kevin A. Bush, a Reinhold H. Dauskardt a and Michael D. McGehee * a The performance of perovskite solar cells has rapidly increased above 22%, and their environmental stability is also progressing. However, the mismatch in thermal expansion coefficients and low fracture energy of layers in perovskite solar cells raise a concern as to whether devices can withstand mechanical stresses from temperature fluctuations. We measured the fracture energy of a perovskite film stack, which was shown to produce 23.6% efficiency when incorporated in a monolithic perovskite- silicon tandem. We found that the fracture energy increased by a factor of two after 250 standardized temperature cycles between C040 1C and 85 1C and a factor of four after laminating an encapsulant on top of the stack. In order to observe how the increased mechanical stability translated from film stacks to device performance and reliability, we carried out a comparative study of perovskite solar cells packaged between glass and two commonly used encapsulants with different elastic moduli. We demonstrated that solar cells encapsulated with a stiffer ionomer, Surlyn, severely decreased in performance with temperature cycling and delaminated. However, the solar cells encapsulated in softer ethylene vinyl acetate withstood temperature cycling and retained over 90% of their initial performance after 200 temperature cycles. This work demonstrates a need for an encapsulant with a low elastic modulus to enable mechanical stability and progress toward 25 year operating lifetime. Broader context Hybrid organic–inorganic metal halide perovskite solar cells have rapidly increased in power conversion efficiency up to 22%. However, device stability must be improved to enable commercialization. Also, studies have raised concerns over the potential for delamination in service due to mismatches in thermal expansion coefficients and low fracture energies compared to other solar cell technologies. Despite these concerns, we previously demonstrated that perovskite solar cells exhibit remarkable environmental stability in damp heat and under full sunlight in operation. This work compares the fracture energy of perovskite solar cells before and after temperature cycling and with laminated encapsulants. Furthermore, we subjected encapsulated perovskite solar cells to the IEC 61646 standard test of 200 temperature cycles between C040 1Cand851C and observed no visible delamination and less than a 10% change in performance. Moreover, by performing fracture tests and comparing solar cells with two encapsulants varying in elastic modulus by a factor of 40, we developed a design principle to enable mechanical stability of perovskite solar cells. 1. Introduction Over the past five years, significant research has improved the power conversion efficiency (PCE) of single junction organic– inorganic metal halide perovskite solar cells from 12% to 22.1%. 1 In addition, incorporating perovskite as a wide band- gap absorber on top of a silicon solar cell in a two-terminal tandem has increased the PCE to 23.6%. 2 When perovskites were mechanically stacked in a four-terminal tandem on top of a silicon solar cell, the efficiency surpassed 26%. 3 Perovskite solar cells can also be made on flexible substrates with 17.3% PCE 4 and scaled up to 100 cm 2 active area, showing potential for module-scale production, with 11.2% PCE. 5 Moreover, environmental stability of perovskite solar cells has been demonstrated with proper packaging. With glass–glass encap- sulation, the solar cells fully retained their performance after 1000 hours in 85 1C–85% RH, damp heat, environment. 2 a Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA. E-mail: mmcgehee@stanford.edu b Department of Applied Physics, Stanford University, Stanford, California 94305, USA c D2 Solar, San Jose, California 95131, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c7ee02564e Received 7th September 2017, Accepted 20th October 2017 DOI: 10.1039/c7ee02564e rsc.li/ees Energy 32 however, the perovskite devices remained unchanged after the 140 1C heat treatment with a fracture energy of 0.21 J m C02 in Fig. 2a. Thus, the improvement in fracture energy for the perovskite device resulted from the encapsulant and not the heat treatment. Even with encapsulation, PC 60 BM was still the weakest layer. Fig. S6 (ESI†) shows that the representative fractured surfaces of all four conditions were consistent and uniform, which was verified by XPS in Fig. S7 (ESI†)tobePC 60 BM. Previous reports have shown that PC 60 BM is fragile and susceptible to fracture under applied loads. 33 The encapsulants did not inherently strengthen the PC 60 BM, but their lower elastic moduli allows plastic deformation, where the material is irreversibly length- ened. The energy dissipated upon deformation of the encapsu- lants during mechanical testing could explain the increased fracture energy for the encapsulated perovskite film stack in Fig. 2. Fractured surfaces of film stacks with EVA and Surlyn encapsulants were slightly rougher with R q of 68 nm and 51 nm in Fig. S8 (ESI†) compared to the unencapsulated film stack with R q of 26 nm in Fig. S4a (ESI†). As previously described, the increased roughness after fracture indicates that the crack propagated across a larger thickness. In this case, the fracture path is still within the PC 60 BM layer, and the rougher fracture surface could result from encapsulant deformation during mechanical testing. 2.3 Temperature cycling of encapsulated perovskite solar cells Perovskite solar cells laminated between glass with EVA or Surlyn and edge sealed with butyl rubber were subjected to at least 200 temperature cycles following the temperature profile in Fig. 3a. To investigate how the stiffness of an encapsulant affects device performance with temperature cycling, solar cells encapsulated in EVA and Surlyn were periodically compared. Fig. 2 (a) Fracture energy of control ITO device, after 20 minutes anneal- ing at 140 1C (simulating the condition during lamination) and with the EVA or Surlyn laminated on top. Cartoons illustrate the deformation created by a crack as it propagates through (b) the unencapsulated ITO device and (c) the laminated ITO device. Fig. 3 (a) Temperature profilemeasured by athermocoupleencapsulated in the same glass–glass package as the solar cells in this study as it went through the temperature cycling test. Power conversion efficiency of solar cells during temperature cycling packaged using (b) ionomer Surlyn PV5400 with five solar cells in the dataset (c) ethylene vinyl acetate (EVA) with nine solar cells in the data set. Picture of the encapsulated solar cell before and after 200 temperature cycles in (d) Surlyn and (e) EVA. Energy & Environmental Science Paper Published on 20 October 2017. Downloaded on 12/03/2018 07:10:31. View Article Online This journal is ©The Royal Society of Chemistry 2018 Energy Environ. Sci., 2018, 11, 144--150 | 147 Packaged solar cells were loaded onto a tray that moved between two environmental chambers held at C040 and 85 1C, and they were periodically removed and measured with max- imum power point tracking to obtain stabilized PCE values, as described in the Experimental section. Device performance was plotted as a function of temperature cycles in Fig. 3b and c. Unfortunately, only one out of five solar cells encapsulated in Surlyn retained 90% of their efficiency after 200 thermal cycles (Fig. 3b). Referring to the figures of merit for the Surlyn encapsulated solar cells in Fig. S9 (ESI†), most of the degrada- tion was in the fill factor (FF). Fig. 3d shows delamination after the Surlyn encapsulated solar cell went through 200 tempera- ture cycles, which likely was the cause for the drop in FF. Laser beam induced current mapping on these temperature cycled cells in Fig. S10 (ESI†) further showed no current in the delaminated area, which indicates a loss of Ohmic contact and suggests that delamination occurred within the perovskite solar cell stack. The stiff Surlyn, which adheres well to ITO and glass, 34,35 likely pulls up the sputtered ITO through the PC 60 BM layer. In contrast, EVA packaged devices were mechanically stable during temperature cycling, showing no signs of cracking, blistering, or delamination (Fig. 3e). Pictures of the whole set of EVA encapsulated solar cells after the 200 temperature cycling test can be found in Fig. S11 (ESI†). Moreover, all nine solar cells retained more than 90% of their initial performance after 225 temperature cycles (Fig. 3c) and passed the TC test. All figures of merit of EVA encapsulated solar cells stayed approxi- mately constant throughout the TC test in Fig. S12 (ESI†), which means there is no competing change in performance that makes the solar cells stable or any noticeable degradation modes in the device. We have thus demonstrated a key concept that using a low modulus encapsulant such as EVA enabled encapsulated perovskite solar cells to pass the standardized 200 temperature cycling test. 3. Conclusion Perovskite solar cells have layers with thermal expansion coeffi- cient mismatches and low fracture energies that seemingly limit their potential to be successfully commercialized. This work furthers our understanding on the mechanical properties of encapsulation required for perovskite solar cells to pass the IEC 61646 temperature cycling test. We observed that the high elastic modulus of Surlyn leads to delamination and a drop in perfor- mance when laminated on perovskite solar cells, whereas the lower modulus of EVA dissipates strain and enables encapsulated solar cells to retain 90% of the performance. This work shows promise for the operational stability of perovskite solar cells. 4. Methods 4.1 Sample preparation 4.1.1 Solar cell fabrication. Solar cells were fabricated on patterned indium-doped tin oxide with 10 O per square sheet resistance on 2 cm C2 2cmC2 0.7 mm glass substrate from Xin Yan technology. The substrates were sonicated in Extran, DI water, acetone, and isopropanol. After that, a 1 M solution of nickel nitrate hexahydrate (Sigma-Aldrich) in ethylenediamine (Sigma-Aldrich) and anhydrous ethylene glycol (Sigma-Aldrich) was spun on the 15 minutes UV ozoned substrate at 5000 rpm for 1 minute. The substrates were then annealed at 300 1C for 45 minutes. The substrates with NiO coated were quickly transferred into a dry air box while their temperatures were above 100 1C. The stoichiometric perovskite solution was made by mixing CsI (Sigma-Aldrich, 99.99% trace metals), FAI (Dyesol), PbI 2 (TCI), and PbBr 2 (TCI) in a mixture of N,N- dimethylformamide (Sigma-Aldrich) and dimethyl sulfoxide (Sigma-Aldrich) and letting it stir at room temperature for an hour. The perovskite solution was deposited through a 0.2 mm PTFE filter and spun at 1000 rpm for 14 seconds, and then at 6000 rpm for 30 seconds. 120 mL chlorobenzene was dropped onto the spinning substrates at 5 seconds before the end of the spinning process to enhance crystallization and as an anti- solvent, forming Cs 0.17 FA 0.83 Pb(Br 0.17 I 0.83 ) 3 as the final compo- sition. The films were annealed at 50 1C for 1 minute, then at 100 1C for 50 minutes. After all solution deposition, the substrates were transferred to a dry N 2 box for thermal evapora- tion of 1 nm LiF and then 10 nm of PC 60 BM. Afterwards, 4 nm of stoichiometric SnO 2 and then 2 nm of zinc tin oxide were deposited by pulsed-Chemical Vapor Deposition (CVD) method at 100 1C. Details of the pulsed-CVD method can be found elsewhere. 2 150 nm of indium doped tin oxide was deposited through D.C. sputtering as the top electrical contact. To com- plete the solar cells, silver metal electrode was thermally evaporated around the 1 cm 2 device area to minimize series resistance. 4.1.2 Sample preparation for fracture energy test. Samples for fracture energy testing were fabricated using the exact same steps and thickness as the solar cells, except on a larger glass substrate of 2.5 cm C2 3.5 cm C2 1 mm dimension. NiO, perovskite, LiF, PC 60 BM, SnO 2 , ZnSnO 2 , and ITO were depos- ited onto cleaned glass. 4.2 Glass–glass encapsulation of solar cells Indium solar ribbon (part# WCD102-7747-6022) was soldered on to the evaporated metal electrode of the solar cells prior to assembly. Solar cell substrates were packaged between top and bottom sheets of encapsulant, either ethylene vinyl acetate or ionomer Surlyn 5400 and two sheets of 3 mm thick glass. The butyl rubber edge seal with added desiccant (Quanex, SET LP03) was placed as a frame around outer edge of the glass during assembly. The edge seal was used as a sandwich on both sides of the solar ribbon to minimize moisture ingress. The package was laminated in two steps: pull vacuum for 5 minutes then press with 650 mbar pressure at 140 1C for 20 minutes for the edge seal to soften and the encapsulant to cure. Total bond width of the edge seal is 1 cm, which can prevent moisture from diffusing in at 85 1C–85%RH condition for at least 1000 hours. 36 Optimization steps for the encapsulation are included in the ESI,† section. Paper Energy & Environmental Science Published on 20 October 2017. Downloaded on 12/03/2018 07:10:31. View Article Online 148 | Energy Environ. Sci., 2018, 11, 144--150 This journal is ©The Royal Society of Chemistry 2018 4.3 Temperature cycling test Packaged solar cells were placed on a tray inside a chamber (ESPEC TSE-11A) composed of two different compartments, the top one at 85 1C and the bottom one at C040 1C. The packaged was first equilibrated at 25 1C then was brought up to the top 85 1C chamber within a minute and held there for 20 minutes. Afterwards, the package was moved to the lowerC040 1Cchamber within a minute and also held there for 20 minutes. A cycle was completed when package went through both top and bottom compartments. 4.4 Measuring solar cell performance Current–voltage of a solar cells were performed using Keithley 2400 digital source meter. A 300 W Xenon arc lamp was used to irradiate solar cells from the bottom glass side. A reference KG5 Si photodiode was used to calibrate the solar simulator to match the integrated current measured from external quantum efficiency. The current–voltage was measured from forward to reverse bias between 1.2 V to C00.2 V. Afterwards, the solar cells were stabilized under light at maximum power point bias until reaching stabilized efficiency for 200 seconds. The solar cell current–voltage was then re-measured from forward to reverse, and reverse to forward to check for any hysteresis. The final power convers