Rational Design, Synthesis, and Structure-Property Relationship Studies of a Library of Thermoplastic Polyurethane Films as an Effective and Scalable Encapsulation Material for Perovskite Solar Cells

Hybrid organic-inorganic metal halide perovskite solar cell (PSC) technology is experiencing rapid growth due to its simple solution chemistry, high power conversion efficiency (PCE), and potential for low-cost mass production. Nevertheless, the primary obstacle preventing the upscaling and widespread outdoor deployment of PSC technology is the poor long-term device stability, which stems from the inherent instability of perovskite materials in the presence of oxygen and moisture. To address this issue, in this work, we have synthesized a series of thermoplastic polyurethanes (TPUs) through a rational design by utilizing polyols having different molecular weights and diverse isocyanates (aromatic and aliphatic). Thorough characterization of these TPUs (ASTM and ISO standards) along with structure-property relationship studies were carried out for the first time and were then used as the encapsulation material for PSCs. The prepared TPUs were robust and adhered well with the glass substrate, and the use of low temperature during the encapsulation process avoided the degradation of the perovskite absorber and other organic layers in the device stack. The encapsulated devices retained more than 93% of their initial power conversion efficiency (PCE) for over 1000 h after exposure to harsh environmental conditions such as high relative humidity (80 ± 5% RH). Furthermore, the encapsulated perovskite absorbers showed remarkable stability when they were soaked in water. This article demonstrates the potential of TPU as a suitable and easily scalable encapsulant for PSCs and pave the way for extending the lifetime and commercialization of PSCs.

Recent Advances in UV-Cured Encapsulation for Stable and Durable Perovskite Solar Cell Devices

The stability and durability of perovskite solar cells (PSCs) are two main challenges retarding their industrial commercialization. The encapsulation of PSCs is a critical process that improves the stability of PSC devices for practical applications, and intrinsic stability improvement relies on materials optimization. Among all encapsulation materials, UV-curable resins are promising materials for PSC encapsulation due to their short curing time, low shrinkage, and good adhesion to various substrates. In this review, the requirements for PSC encapsulation materials and the advantages of UV-curable resins are firstly critically assessed based on a discussion of the PSC degradation mechanism. Recent advances in improving the encapsulation performance are reviewed from the perspectives of molecular modification, encapsulation materials, and corresponding architecture design while highlighting excellent representative works. Finally, the concluding remarks summarize promising research directions and remaining challenges for the use of UV-curable resins in encapsulation. Potential solutions to current challenges are proposed to inspire future work devoted to transitioning PSCs from the lab to practical application.

The current state of the art in internal additive materials and quantum dots for improving efficiency and stability against humidity in perovskite solar cells

The remarkable optoelectronic capabilities of perovskite structures enable the achievement of astonishingly high-power conversion efficiencies on the laboratory scale. However, a critical bottleneck of perovskite solar cells is their sensitivity to the surrounding humid environment affecting drastically their long-term stability. Internal additive materials together with surface passivation, polymer-mixed perovskite, and quantum dots, have been investigated as possible strategies to enhance device stability even in unfavorable conditions. Quantum dots (QDs) in perovskite solar cells enable power conversion efficiencies to approach 20%, making such solar cells competitive to silicon-based ones. This mini-review summarized the role of such QDs in the perovskite layer, hole-transporting layer (HTL), and electron-transporting layer (ETL), demonstrating the continuous improvement of device efficiencies.

Ambient Spray Coating of Organic-Inorganic Composite Thin Films for Perovskite Solar Cell Encapsulation

Perovskite solar cells (PSCs) are developing rapidly in recent years, showing remarkable power conversion efficiency (PCE) of 25 %, which is comparable to crystalline silicon solar cells. However, since perovskite and other functional layers are very sensitive to the environment with high humidity, illumination, and heat, PSCs meet great challenges in device stability, which significantly limit their industrialization and commercialization. Encapsulation has become an effective strategy to enhance the stability of PSCs, and various encapsulation techniques have been developed, such as atomic layer deposition and glass-glass technology. Most of the current encapsulating methods are either time-consuming and sophisticated processes, or exhibit rigid configuration, which is unsuitable for flexible and curved devices. Here, an ambient spray coating method was developed to fabricate organic-inorganic composite film for direct encapsulation of PSCs. By systematical optimization of the film composition, thickness, and microstructures, a superhydrophobic encapsulating thin film with high compactness and homogeneity was achieved. As a result, the hybrid encapsulating film with polystyrene (PS)-4033/PS-4033-SiO₂ significantly improved the stability of PSCs in humid environment (60-70 % relative humidity, 35 °C) by showing about 10 times longer lifetime than that of the unencapsulated devices, which was mainly attributed to complementary effects from the high compactness of PS and high hydrophobicity of SiO₂ . This work suggests that direct deposition of organic-inorganic composite on devices as encapsulating films is an efficient strategy to enhance the device stability, and this method shows great promises of application in flexible and large-area devices.

Encapsulation and Stability Testing of Perovskite Solar Cells for Real Life Applications

With the progress in the development of perovskite solar cells, increased efforts have been devoted to enhancing their stability. With more devices being able to survive harsher stability testing conditions, such as damp heat or outdoor testing, there is increased interest in encapsulation techniques suitable for this type of tests, since both device architecture compatible with increased stability and effective encapsulation are necessary for those testing conditions. A variety of encapsulation techniques and materials have been reported to date for devices with different architectures and tested under different conditions. In this Perspective, we will discuss important factors affecting the encapsulation effectiveness and focus on the devices, which have been subjected to outdoor testing or damp heat testing. In addition to encapsulation requirements for these testing conditions, we will also discuss device requirements. Finally, we discuss possible methods for accelerating the testing of encapsulation and device stability and discuss the future outlook and important issues, which need to be addressed for further advancement of the stability of perovskite solar cells.

Improving the Efficiency, Stability, and Adhesion of Perovskite Solar Cells Using Nanogel Additive Engineering

Additive engineering has been applied widely to improve the efficiency and/or stability of perovskite solar cells (PSCs). Most additives used to date are difficult to locate within PSCs as they are small molecules or linear polymers. In this work, we introduce, for the first time, carboxylic acid-functionalized nanogels (NGs) as additives for PSCs. NGs are swellable sub-100 nm gel particles. The NGs consist of poly(2-(2-methoxyethoxy) ethyl methacrylate)-co-methacrylic acid-co-ethylenegylcol dimethacrylate (PMEO₂MA-MAA-EGD) particles prepared by a scalable synthesis, which have a diameter of 40 nm. They are visualized in the perovskite films using SEM and are located at the grain boundaries. X-ray photoelectron and FTIR spectroscopy reveal that the NGs coordinate with Pb²⁺ via the -COOH groups. Including the NGs within the PSCs increased the grain size, decreased nonradiative recombination, and increased the power conversion efficiency (PCE) to 20.20%. The NGs also greatly increase perovskite stability to ambient storage, elevated temperature, and humidity. The best system maintained more than 80% of its original PCE after 180 days of storage under ambient conditions. Tensile cross-cut tape adhesion tests are used to assess perovskite film mechanical integrity. The NGs increased both the adhesion of the perovskite to the substrate and the mechanical stability. This study demonstrates that NGs are an attractive alternative to molecularly dispersed additives for providing performance benefits to PSCs. Our study indicates that the NGs act as a passivator, stabilizer, cross-linker, and adhesion promoter.

Encapsulation Strategies for Highly Stable Perovskite Solar Cells under Severe Stress Testing: Damp Heat, Freezing, and Outdoor Illumination Conditions

A key direction toward managing extrinsic instabilities in perovskite solar cells (PSCs) is encapsulation. Thus, a suitable sealing layer is required for an efficient device encapsulation, preventing moisture and oxygen ingression into the perovskite layer. In this work, a solution-based, low-cost, and commercially available bilayer structure of poly(methyl methacrylate)/styrene-butadiene (PMMA/SB) is investigated for PSCs encapsulation. Encapsulated devices retained 80% of the initial power conversion efficiency (PCE) at 85 °C temperature and 85% relative humidity after 100 h, while reference devices without SB (only PMMA) suffer from rapid and intense degradation after only 2 h, under the same condition. In addition, encapsulated devices retained 95% of the initial PCE under -15 °C freezing temperature after 6 h and retained ∼80% of the initial PCE after immersion in HCl (37%) for 90 min. Moreover, applying an additional aluminum metal sheet on the PMMA/SB protective bilayer leads to the improvement of device stability up to 500 h under outdoor illumination, retaining almost 90% of the initial PCE. Considering the urge to develop reliable, scalable, and simple encapsulation for future large-area PSCs, this work establishes solution-based bilayer encapsulation, which is applicable for flexible solar modules as well as other optoelectronic devices such as light-emitting devices and photodetectors.improvement of device stability up to 500 h under outdoor illumination, retaining almost 90% of the initial PCE. Considering the urge to develop reliable, scalable, and simple encapsulation for future large-area PSCs, this work establishes solution-based bilayer encapsulation, which is applicable for flexible solar modules as well as other optoelectronic devices such as light-emitting devices and photodetectors.

Efficient and Stable Perovskite Solar Cells with a Superhydrophobic Two-Dimensional Capping Layer

A two-dimensional (2D) perovskite layer is considered to be a desirable defect passivation structure for the lifelong stability of perovskite solar cells (PSCs). However, the efficiency could be compromised behind the traditional PSCs. Herein, we solve this issue by employing a highly hydrophobic organic cation, 2-[4-(trifluoromethyl)phenyl]ethanamine (CF₃-PEA), to form a 2D (CF₃-PEA)₂PbI₄ to effectively passivate the 3D MAPbI₃ with fewer defects. The new 2D/3D-structured PSCs show reduced charge recombination, an elongated carrier lifetime, efficient charge generation and transport. Those excellent characters lead to a significant enhancement of the efficiency from 17.9% for pristine PSCs to 21.43% for 2D/3D PSCs. Benefiting from the high hydrophobicity of CF₃-PEA, the cells show remarkably improved stability by maintaining 83% of the original efficiency exposed to 80% R.H. and 50 °C for 600 h and 87% under 1 sun illumination for 600 h, which makes our PCSs among the most efficient and stable MAPbI3 solar cells.

Ethyl cellulose based self-healing adhesives synthesized via RAFT and aromatic schiff-base chemistry

In this work, reversible addition-fragmentation chain transfer (RAFT) polymerization and Schiff base chemistry was combined to fabricate self-healing adhesives. An esterification reaction was first performed to prepare ethyl cellulose based macroinitiators. Then, a "grafting from" RAFT of vanillin methacrylate and lauryl methacrylate was used to obtain graft copolymers. DSC result showed that the glass transition temperature was manipulated via changing the ratio of vanillin and fatty acids moieties. NMR spectrum analysis demonstrated the presence of aldehyde groups, which were available for the dynamic crosslinking to generate a network as self-healing adhesives. The adhesive test showed that the shear strength could reach 0.81 MPa with a self-healing efficiency of 98.7 %. The bottlebrush structures of copolymers and reversibility of Schiff base chemistry might collaboratively contribute to the high self-healing efficiency. This study provides a facile way to fabricate high-performance self-healing adhesives from ethyl cellulose and renewable resources.