Stable and Highly Flexible Perovskite Solar Cells with Power Conversion Efficiency Approaching 20% by Elastic Grain Boundary Encapsulation

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jul 2021Stable and Highly Flexible Perovskite Solar Cells with Power Conversion Efficiency Approaching 20% by Elastic Grain Boundary Encapsulation Chengda Ge†, Ziqi Yang†, Xiaoting Liu, Yilong Song, Anran Wang and Qingfeng Dong Chengda Ge† State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 †C. Ge and Z. Yang contributed equally to this work.Google Scholar More articles by this author , Ziqi Yang† State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 †C. Ge and Z. Yang contributed equally to this work.Google Scholar More articles by this author , Xiaoting Liu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Yilong Song State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Anran Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Qingfeng Dong *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000335 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Here, we show that flexible perovskite solar cells (PSCs) with high operational stability and power conversion efficiency (PCE) approaching 20% were achieved by elastic grain boundary (GB) encapsulation. An introduction of trimethyltrivinyl-cyclotrisiloxane (V3D3) and solvent annealing (SA) resulted in an in situ cross-linking reaction between GBs and enlarged grain size that enabled oriented charge-transport properties to be achieved synchronously, leading to reduced sheet resistance with a high fill factor (FF) up to 82.93% in flexible PSCs. Meanwhile, the insulating elastic V3D3 for GB encapsulation suppressed the carrier recombination substantially, as well as ion migration, which enhanced both the mechanical and the operational stability dramatically in flexible PSCs. Additionally, more than 73% of its initial PCE was retained even after 10,000 bending cycles, with 83% of its stabilized PCE remaining after 300 h of continuous operation at maximum power point (MPP) condition under one sun illumination without cooling. Download figure Download PowerPoint Introduction Flexible solar cells (FSCs), which could be fabricated by the scalable roll-to-roll technology, are receiving sustained attention for their merits, including lightweight, flexibility, portability, and compatibility with curved surfaces.1–3 Inorganic-semiconductor-based FSCs, such as c-Si,4,5 CIGS,6–8 and AsGa,9–11 have achieved good efficiency,12 but the low flexibility and high-cost limit their practical application. On the other hand, organic solar cells, as types of attractive FSC candidates, show excellent flexibility.13 However, their efficiencies are still much lower than those of commercialized FSCs.14,15 Recently, FSCs based on metal halide hybrid perovskite materials attracted much attention due to their solution processability,16–18 low cost,17–19 and rapidly improved device efficiency.20 Several investigators have reported efficient single-junction flexible perovskite solar cells (PSCs) by using commercial polymer/indium tin oxide (ITO) substrate with a power conversion efficiency (PCE) of >19%.21,22 However, the inorganic nature of the high modulus microscopic structures for both perovskite grains and the ITO electrode itself constrains the flexibility of PSCs. Furthermore, most of efficient, flexible PSCs based on commercialized ITO substrate undoubtedly degrade after only hundreds of bending cycles due to the ceramic nature of ITO and degradation of fragile perovskite film during bending.23–28 In fact, it is more challenging for PSCs to possess high flexibility besides high device efficiency, since high-quality pin-hole-free perovskite films are relatively difficult to fabricate on hydrophobic flexible substrates under low film-forming temperature, compared with their rigid-substrate counterparts.29 More importantly, the operational stability of flexible PSCs still lags behind, compared with the rigid ones, which restrains the development of flexible PSCs substantially. Therefore, investigations concerning the operation stability of flexible PSCs are urgently required. The suppression of ion migration is believed to be an essential parameter to consider regarding stability improvement of flexible PSCs30 due to their fragile grain boundaries (GBs) with much lower crystallinity, compared with the rigid PSCs; it is presumed that the hysteresis behavior at the GBs facilitates ion migration pathways in flexible devices.31 In this work, highly flexible PSCs with good operational stability were realized by introducing in situ cross-linked elastic GB encapsulation, formed by thermal excitation with a unique cross-linker that favored enrichment of the cross-linking network, followed by a solvent annealing (SA)-induced phase separation between the inorganic grains and the organic cross-linkers. Also, the SA process-induced high-quality perovskite films with both enhanced crystallinity and charge-transport properties, which led to high efficiency and good mechanical properties of the PSCs. Meanwhile, the insulating GB encapsulation resulted in a suppressed ion migration rate and reduced carrier recombination across GBs, which enhanced the device performance considerably. Therefore, flexible PSCs displayed PCE approaching 20% with dramatically improved mechanical property >73% of the initial PCE value, which was retained even after 10,000 bending cycles. Moreover, significant enhancement of operational stability was achieved under continuous output at a maximum power point (MPP) condition under one sun illumination without cooling. Experimental Methods Device fabrication All solvents used in the experiments were purchased from Sigma-Aldrich (Shanghai, China). Trimethyltrivinyl-cyclotrisiloxane (V3D3) was bought from TCI (Shanghai, China). The patterned ITO/glass substrates were cleaned by sonication in isopropyl alcohol and acetone. A 2 mg/mL of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)]amine (PTAA) solution was spin-coated on flexible substrates at 6000 rpm for 60 s, followed by a thermal annealing treatment at 100 °C for 10 min. The perovskite layer used in our flexible PSC is Cs0.05(FA0.87MA0.13)0.95Pb(I0.87Br0.13)3. CsI was dissolved in dimethyl sulfoxide (DMSO) in advance; methylamine hydrobromide (MABr), lead bromide (PbBr2), formamidinium iodide (FAI), and lead iodide (PbI2) were dissolved in a mixed solvent of dimethylformamide (DMF) and DMSO (v/v 4∶1), to form the perovskite precursor solution (1.4 mmol/mL).32 The conjugated electron-interface layer material, PFN-Br,33,34 was spin-coated on the PTAA layer at 4000 rpm for 30 s, and perovskite films were obtained by a sequential spin coating of 120 μL perovskite solution and added antisolvent, chlorobenzene, to induce rapid crystallization of the perovskite precursor solution.35 Then SA was performed by adding 2 μL DMSO as solvent vapor at 100 °C for 20 min, followed by thermal annealing treatment for 40 min to create the reference or the prototype device (control). For the fabrication of the new cross-linked device, the V3D3 monomer was added directly into the perovskite precursor solution at a concentration of 2‰ or was added into the antisolvent at a concentration of 0.3 mg/mL to cross-link the polymer films. After antisolvent treatment, the film was placed in an N2-filled glovebox at room temperature for 25 min and then preheated at 80 °C for 1 min, followed by annealing in the same way as the reference device. After that, 20 nm C60, 8 nm BCP (2,9-dimethyl-4,7-diphenyl-1,10-PhenanthrolineSynonym), and 80 nm Cu were deposited on the perovskite film with an ITO/Cu overlap area of 10 mm2 via step by step thermal vacuum evaporation to generate the highly flexible PSCs. Characterizations Current–voltage (J–V) characteristics of flexible PSCs were recorded in N2-filled glovebox (Mikrouna Co., Ltd., Shanghai, China) by using Keithley 2400 under an AAA solar simulator (Crowntech. Inc., Pennsylvania, USA; 100 mW/cm2, AM 1.5G irradiation). The scan range was from −0.1 to 1.2 V, and the delay time was 33 ms. A 4 mm2 photomask was used during measurement. The standard silicon cell was corrected for National Renewable Energy Laboratory as reference cell calibration. The external quantum efficiency (EQE) measurement was performed on the QTest Hifinity5 system (Crowntech. Inc.), and monochromatic light was calibrated by the reference silicon cell before EQE measurement. The scanning Kelvin probe force microscopy (KPFM) was used together with atomic force microscopy (AFM; Bruker Dimension FastScann, Massachusetts, USA) to carry out electrical potential distribution measurement of the amplitude modulation (AM-KPFM) mode. X-ray diffraction (XRD) indexing and data collection were performed on an Ultima VI diffractometer with Cu Kα (λ = 0.15418 nm) at 293 K. Results and Discussion Preparation of elastic GBs and morphology characterization Branched olefin-functionalized monomers could be introduced into a growing polymer intentionally for an in situ cross-link polymerization, initiated by heating without the production of harmful byproducts such as H2O. Meanwhile, the multifunctional monomers would generate highly branched dendrimer networks over a long-range (Figure 1a). Based on this principle, a multifunctional alkene semiorganic monomer, V3D3, was used as the cross-linker, as illustrated in Figure 1b, to form three-dimensional (3D) cross-linked networks directly by low-temperature thermal-initiated polymerization. It is worth noting that the organosilicon, V3D3, was different from SiO2-based doping materials such as tetraethoxysilane (TEOS) used in rigid PSCs to improve the electrochemical performance reported previously36 ( Supporting Information Figure S3), which preferred to form a rigid core–shell structure with perovskite grain instead of flexible elastic-GBs. Figure 1 | (a) Illustration of the SA treatment. (b) The polymerization reaction of V3D3. (c–g) Top view of SEM images of different perovskite films with various methods. SA, solvent annealing; V3D3, trimethyltrivinyl-cyclotrisiloxane; SEM, scanning electron microscopy. Download figure Download PowerPoint It is difficult to establish an in situ cross-linking reaction when the ratio of the doping monomers is relatively low in the perovskite precursor. Here, we employed SA treatment to increase the monomers in GBs effectively, which enabled the cross-linking reaction to form elastic GB encapsulation, as illustrated in Figure 1a. The enrichment of the insulating polymers was evidenced by scanning electron microscopy (SEM) analysis, as shown in the images in Figures 1c–1g. The cross-linking organic composition is distributed homogeneously over the whole perovskite film without SA (Figure 1e). However, the cross-linking organic composition was enriched in GBs efficiently, forming domain structures with the adoption of the SA method. The SEM images demonstrated that the SA treatment facilitated a potent phase separation of the inorganic grains and the organic cross-linkers to enable in situ formation of elastic GBs. Multiple characterization methods, including KPFM, energy-dispersive X-ray (EDX) spectra, and transmission electron microscopy (TEM), were carried out to characterize the structures using different scales. From KPFM images (Figures 2a–2f), the surface potential was similar on the surface of both grain and GBs for the control (prototype) perovskite films. In contrast, we noted a sharply varied surface potential indicative of the differences in composition between the grains and GBs. This observation was ascribed to the enriched cross-linker composition in GBs, just as expected, which was also consistent with the findings of the SEM images. KPFM measurement was used to study the differences of surface potential between grain and GBs in this work, with the measured values being relative, rather than absolute. EDX was used to investigate the distribution of insulating polymers in the perovskite films. The doping ratio was increased to 2 mol %, in order to make the element content reach the EDX detection limit. The characteristic elements of silicon (Si) and oxygen (O) were identified in the white region at the grain or domain boundary, while the dark region showed almost no signal response from the characteristic Si and O elements (Figures 2g–2i), consistent with the observations in Figure 1. Meanwhile, the bright region outside the perovskite grains in the TEM images (Figures 2j) indicated further that the polymer was covered around the perovskite grains, suggesting the formation of elastic GBs. Figure 2 | Characterization of elastic GB encapsulation. (a and b) Height and (d and e) KPFM mode AFM images for perovskite films with or without elastic GBs and the distribution of (c) height and (f) the surface potential. (g) SEM, (h and i) EDX images, and (j) TEM images of perovskite films with elastic GBs. GB, grain boundary; KPFM, Kelvin probe force microscopy; AFM, atomic force microscopy; SEM, scanning electron microscopy; EDX, energy-dispersive X-ray. Download figure Download PowerPoint Evaluation of defect and electrical properties The SA treatment of the cross-linked perovskite films induced continuous cross-linked elastic insulating networks across the GBs, and also realized secondary grain growth successfully with a significant increase in grain size. The insulating GB encapsulation promoted vertical charge transport and suppressed carrier recombination across GBs. Meanwhile, it contributed significantly to the reduction of trap-state density and improvement of electron mobility with structure retention, evidenced by XRD patterns (Figure 3a), which was a key factor considered for improving the charge-transport properties in the sandwiched architecture of the PSC device. The space-charge-limited current (SCLC) method was used to evaluate the effect of the cross-linking process. According to described previous methods, the trap-state density was estimated by the formula37–39: V TFL = e n t L 2 2 ɛ ɛ 0 (1)where nt is the trap-state density, and VTFL is the trap-filled limited voltage. The VTFL could be acquired from the dark J–V curve of the electron-only device with a device structure, ITO/C60/Perovskite/C60/BCP/Cu (Figure 3c). After V3D3 treatment, nt was reduced significantly to 6.74 × 1015 cm−3, compared with 1.24 × 1016 cm−3 in regular perovskite film. This result was consistent with the time-resolved photoluminescence (TRPL) measurement (Figure 3b), in which a longer fluorescence lifetime was apparent of the cross-linked perovskite film, compared with the control sample, which also indicated a marked reduction in defect concentration.40 Furthermore, there were two orders of magnitude increment in electron mobility of the V3D3 cross-linked devices, estimated as 1.03 × 10−3 cm2 V−1 s−1, compared with the reference device, 1.02 × 10−5 cm2 V−1 s−1, which were calculated using the Mott–Gurney law, as follows:38,41 J D = 9 ɛ ɛ 0 μ V b 2 8 L 3 (2) Figure 3 | Characterization of perovskite films with V3D3 cross-linking. (a) XRD measurement of control and cross-linked perovskite films. (b) TRPL measurement of control and cross-linked films. (c) Dark-state J–V measurement of an electron-only device. (d) Vertical direction ion migration measurement. (e and f) Comparison of transverse and longitudinal conductivity. V3D3, trimethyltrivinyl-cyclotrisiloxane; XRD, X-ray diffraction; TRPL, time-resolved photoluminescence; J–V, current–voltage characteristic. Download figure Download PowerPoint It is noteworthy that the insulating GB encapsulation suppressed the ion migration substantially in a vertical direction across the polycrystalline perovskite film, which benefited efficient defect passivation, as well as blockage of the ion migration pathways. Figure 3d shows the current tracking under 0.1 sun illumination with and without cross-linking under the vertical electric field, used to evaluate the current contributed by the ion migration effect. The current variation in the initial stage was reduced significantly in the cross-linking films (Figure 3d), which paralleled a diminution of the ion migration effect in a vertical direction across the perovskite film. Meanwhile, the insulation of the elastic GB encapsulation reduced the charge concentration at GBs profoundly, which suppressed the charge recombination across GBs, resulting in an improved vertical charge-transport property (Figures 3e and 3f), as well as blocking the lateral charge-transport pathways. These insulation properties are always preferred in photovoltaic device systems, as they contribute to an optimized fill factor (FF) of these devices, which, favorably, was the main achievement of our fabricated GB encapsulation device. Characterization of photovoltaic performance The insulating elastic GB encapsulation, which suppressed carrier recombination, as well as the ion migration effect, led to a dramatic enhancement of the photovoltaic performance of our flexible PSCs, compared with the prototypes without elastic GBs (Figure 4b). The inverted planar device structure of ITO/PTAA/PFN-Br/PVKcross-linker/C60/BCP/Cu (4a) was used to evaluate the performance of the flexible PSCs. After introducing V3D3 cross-linking into the fabrication process, the flexible PSC showed a JSC of 22.33 mA/cm2, a VOC of 1.08 V, an FF of 82.93%, and a champion PCE of 20%, which was a demonstration of a significant increase in PCE, compared with the reference device with a JSC of 21.49 mA/cm2, a VOC of 1.06 V, an FF of 81.83%, and a lower PCE of 18.64%. Here, the reference device was already well optimized as the control device and had approached the best value of flexible PSCs reported earlier. Notably, the highest FF of 82.93% was realized in our flexible PSCs, which was the highest value ever reported in flexible PSCs, and similar to the best value obtained in rigid devices. The excellent FF was believed to be a benefit from our insulating GB encapsulation-induced low-trap density and an outstanding charge carrier transport properties. Figure 4 | Photovoltaic performance of flexible perovskite solar cells. (a) Structure of flexible perovskite solar cells. (b) J–V measurement of control and cross-linked device. (c) Image of flexible PSCs sample. (d) EQE measurement of our flexible PSCs. J–V, current–voltage characteristic; EQE, external quantum efficiency. Download figure Download PowerPoint Mechanical property and stability test The elastic cross-linking strategy adapted in the PSCs fabrication enabled the flexible “grain-elater-grain” encapsulation to release mechanical stress generated during bending, as well as thermal expansion of flexible PSCs, which contributed significantly to excellent device stabilities under both continuous bending and operational conditions. As shown in Figure 5d, regular flexible PSCs showed an apparent decay within 700-times of bending cycles, similar to most results reported previously. On the contrary, devices with V3D3 showed dramatically enhanced bending stability under the same test condition with a small bending radius, and restoration of >70% of the initial PCE, even after 10,000 bending cycles with a bending radius of 2.5 mm, which, to our knowledge, was one of the best values in flexible PSCs on ITO substrate (Table 1), indicating that the elastic GB structure was recoverable and eliminated the damage of the fragile GBs during bending effectively. In order to investigate further the degradation mechanism, the transformation of grain structures was tracked in stretched perovskite films by SEM measurements. The reference perovskite film stretched ruinously by the universal material testing machine during the evaluation of the damage pathways. The film broke randomly and even destroyed the grain itself (Figure 5a). In contrast, the elastic-V3D3 cross-linked perovskite films showed distinct separation of grains or domain boundaries, but retained grain structure under stretching (Figure 5b), leading us to infer that the degradation mechanism was attributed to the high rigidity of the GBs in regular perovskite films, while the elastic ones preferred to release the applied stress and deformation with a recoverable characteristic. Figure 5 | Mechanical and operational stability of FSCs. (a) Brittle rupture of flexible PSCs after a tensile experiment. (b) Regular fracture after a tensile experiment. (c) Continuous output at MPP of flexible PSCs. (d) Bending test of the control device and cross-linked device in which the bending radius is 2.5 mm. FSCs, Flexible solar cells; PSCs, perovskite solar cells; MPP, maximum power point. Download figure Download PowerPoint Table 1 | Summary of Some Classic Research About Flexible PSCs Year Substrate PCE (%) Times of Bending/Retention of PCE (%) Bending Radius (mm) Stability Test Condition Retention of PCE (%) References 2013 PET/ITO 6.4 Not given Not given NA for FSC — 42 2016 PET/ITO 16.09 300/>91 5 NA for FSC — 39 2017 PEN/ITO 15.4 300/>60 4, 8, 12 NA for FSC — 43 2017 PET/ITO 18.4 1000/88.6 4.4 Storage/100 days 88.8 44 2018 PET/ITO 18.4 5000/83 4 Storage/60 days ∼80 45 2019 PET/ITO 18.1 Not given Not given Storage/960 h 72 46 2019 PET/ITO 15.12 5000/85 2.5 NA for FSC — 47 2019 Mica/ITO 18 5000/91.7 5 Storage/85 °C, 85% RH >60 48 2019 PEN/ITO 19.11 2000/93 10 Storage/85 °C, 1000 h 68 22 2019 PEN/ITO 19.51 6000/95 8 Storage/1000 h 90 21 2019 PEN/ITO 20 10,000/73.5 2.5 Operation/300 h/MPP 83 This work Abbreviations: NA, not applicable; PET, polyethylene terephthalate; PEN, polyethylene naphthalate; RH, relative humidity. The flexible PSCs also showed high continuous operational stability at MPP conditions under one sun illumination without a cooling stage when the device achieved a measured, balanced temperature of ∼64 °C. On the contrary, the PSCs without the cross-linking degraded rapidly in the first 10 h (∼10 h). The cross-linked flexible PSCs with the elastic GBs retained ∼83% of its initial efficiency, following 300 h of continuous operation after a balanced thermal point, as shown in Figure 5c. It was believed that the enhanced stability was benefited from the high-quality perovskite films with elastic GB encapsulation, as well as the suppressed ion migration rate. Besides, ITO with a flexible polymer substrate always has poor heat stability, unlike rigid substrates, due to the significant differences in the coefficients of thermal expansion between layers. Moreover, the increased absorption of UV light in polymer substrates other than glass would cause a further increase in substrate temperature49; thus, stress damage is inevitable. Here, the grain–elater–grain hybrid structures in the lateral direction overcame this issue effectively by retaining the grain structure during expansion and enhanced the operational stability, compared with the prototype grain–grain structures. Conclusions The SA-induced in situ cross-linked elastic GB encapsulation, formed by V3D3, enabled high-quality perovskite films in terms of optimized grain structure and improved charge-transport properties. Meanwhile, this approach suppressed the ion migration rate and reduced carrier recombination, which enhanced the device performance dramatically, including device efficiency, bending stability, as well as operational stability. All these improvements indicated that the cross-linked insulating elastic GB encapsulation strategy provides a prospective pathway to develop high-performance flexible PSCs. Supporting Information Supporting Information is available. Conflicts of Interest The authors declare no conflict of interest. Data availability The data that support the plots within this work are available from the corresponding author upon request. Acknowledgments Q.D. conceived the idea, supervised the project, and conducted the initial experiment. C.G. and Z.Y. fabricated the devices, performed the characterization, and did most of the experiments. X.L., Y.S., and A.W. contributed to the device characterization. Q.D. and C.G. wrote the paper. This work was supported by the National Natural Science Foundation of China (no. 21875089).