Toward Highly Thermal Stable Perovskite Solar Cells by Rational Design of Interfacial Layer

Strain Effects on Flexible Perovskite Solar Cells

Flexible perovskite solar cells (f-PSCs) as a promising power source have grabbed surging attention from academia and industry specialists by integrating with different wearable and portable electronics. With the development of low-temperature solution preparation technology and the application of different engineering strategies, the power conversion efficiency of f-PSCs has approached 24%. Due to the inherent properties and application scenarios of f-PSCs, the study of strain in these devices is recognized as one of the key factors in obtaining ideal devices and promoting commercialization. The strains mainly from the change of bond and lattice volume can promote phase transformation, induce decomposition of perovskite film, decrease mechanical stability, etc. However, the effect of strain on the performance of f-PSCs has not been systematically summarized yet. Herein, the sources of strain, evaluation methods, impacts on f-PSCs, and the engineering strategies to modulate strain are summarized. Furthermore, the problems and future challenges in this regard are raised, and solutions and outlooks are offered. This review is dedicated to summarizing and enhancing the research into the strain of f-PSCs to provide some new insights that can further improve the optoelectronic performance and stability of flexible devices.

Advances in Hole Transport Materials for Layered Casting Solar Cells

Huge energy consumption and running out of fossil fuels has led to the advancement of renewable sources of power, including solar, wind, and tide. Among them, solar cells have been well developed with the significant achievement of silicon solar panels, which are popularly used as windows, rooftops, public lights, etc. In order to advance the application of solar cells, a flexible type is highly required, such as layered casting solar cells (LCSCs). Organic solar cells (OSCs), perovskite solar cells (PSCs), or dye-sensitive solar cells (DSSCs) are promising LCSCs for broadening the application of solar energy to many types of surfaces. LCSCs would be cost-effective, enable large-scale production, are highly efficient, and stable. Each layer of an LCSC is important for building the complete structure of a solar cell. Within the cell structure (active material, charge carrier transport layer, electrodes), hole transport layers (HTLs) play an important role in transporting holes to the anode. Recently, diverse HTLs from inorganic, organic, and organometallic materials have emerged to have a great impact on the stability, lifetime, and performance of OSC, PSC, or DSSC devices. This review summarizes the recent advances in the development of inorganic, organic, and organometallic HTLs for solar cells. Perspectives and challenges for HTL development and improvement are also highlighted.

Digital Twin Modeling of Flexible Perovskite Nano-Films with In-Situ Mechanical Microscopy Validation

Flexible perovskite solar cells introduce opportunities for high throughput, high specific weight, and short energy payback time photovoltaics. However, they require additional investigation into their mechanical resiliency. This work investigates the mechanical properties and behaviors of perovskite thin films and builds a robust model for future research. A two-pronged approach was utilized. Perovskite thin films were flexed in a three-point bend mode with in-situ SEM. Novel insights into the perovskite mechanical behaviors with varying substrate layers were gained. Modeling and validation, the second prong, was completed with finite element analysis. Model coupons of the imaged perovskite architectures were built, with sensitivity analysis completed to provide mechanical property estimates. The results demonstrate that mechanical degradation of perovskite thin films on polyethylene terephthalate (PET) primarily presents as a crack in the grain boundaries between crystals. Perovskite thin films on Indium Tin Oxide (ITO) and PET primarily crack in a periodic pattern regardless of the placement of perovskite crystals.

Key Factors Affecting the Stability of CsPbI3 Perovskite Quantum Dot Solar Cells: A Comprehensive Review

The power conversion efficiency of CsPbI₃ perovskite quantum dot (PQD) solar cells shows increase from 10.77% to 16.2% in a short period owing to advances in material and device design for solar cells. However, the device stability of CsPbI₃ PQD solar cells remains poor in ambient conditions, which requires an in-depth understanding of the degradation mechanisms of CsPbI₃ PQDs solar cells in terms of both inherent material properties and device characteristics. Along with this analysis, advanced strategies to overcome poor device stability must be conceived. In this review, fundamental mechanisms that cause the degradation of CsPbI₃ PQD solar cells are discussed from the material property and device viewpoints. In addition, based on detailed insights into degradation mechanisms in CsPbI₃ PQD solar cells, various strategies are introduced to improve the stability of CsPbI₃ PQD solar cells. Finally, future perspectives and challenges are presented to achieve highly durable CsPbI₃ PQD solar cells. The investigation of the degradation mechanisms and the stability enhancement strategies can pave the way for the commercialization of CsPbI₃ PQD solar cells.

Advances in SnO2 for Efficient and Stable n-i-p Perovskite Solar Cells

Perovskite solar cells (PSCs) based on the regular n-i-p device architecture have reached above 25% certified efficiency with continuously reported improvements in recent years. A key common factor for these recent breakthroughs is the development of SnO₂ as an effective electron transport layer in these devices. In this article, the key advances in SnO₂ development are reviewed, including various deposition approaches and surface treatment strategies, to enhance the bulk and interface properties of SnO₂ for highly efficient and stable n-i-p PSCs. In addition, the general materials chemistry associated with SnO₂ along with the corresponding materials challenges and improvement strategies are discussed, focusing on defects, intrinsic properties, and impact on device characteristics. Finally, some SnO₂ implementations related to scalable processes and flexible devices are highlighted, and perspectives on the future development of efficient and stable large-scale perovskite solar modules are also provided.

Are Metal Halide Perovskite Solar Cells Ready for Space Applications?

The appeal of metal halide perovskite solar cells (PSCs) has been widely demonstrated in the field of photovoltaic technology. On account of the excellent optical and electrical properties, as well as compatibility with flexible substrates, the PSCs also hold the highest record of specific power for lightweight solar cell devices, suggesting excellent promise in space applications. Hence, there is increasing interest in the performance of PSCs in space environments where radiation beams and thermal cycling can cause extreme stress on the devices. In this Perspective, we provide a brief summary of the research on PSCs for space applications. The radiation tolerance and thermal stability of PSCs and the fundamental mechanisms are discussed and analyzed. Key challenges facing PSC technology toward future space applications are demonstrated. This Perspective features the prospect of PSCs as the next frontier in space PV technology.

Auger Electron Spectroscopy Analysis of the Thermally Induced Degradation of MAPbI3 Perovskite Films

Organometal halide perovskites are highly promising materials for photovoltaic applications due to the rapid growth of power conversion efficiency in recent years. However, thermal stability is still a major hurdle for perovskite solar cells toward commercialization. Herein, we first explore the slow thermal response of the CH₃NH₃PbI₃ perovskite crystal investigated via Auger electron spectroscopy (AES). AES image mapping directly observes the evolution of morphology and elemental distribution over time. The AES small spot analysis demonstrates the precise initial degradation position of perovskite with both information regarding physical changes in crystals and chemical changes in elemental bonding at the nanometer scale. X-ray photoelectron spectroscopy (XPS) was used to confirm the surface chemical bonding and composition of the perovskite crystals. This work provides the first insights into the physical and chemical changes of perovskites investigated by AES upon long-term exposure to heat under ambient conditions.

Strain analysis and engineering in halide perovskite photovoltaics

Halide perovskites are a compelling candidate for the next generation of clean-energy-harvesting technologies owing to their low cost, facile fabrication and outstanding semiconductor properties. However, photovoltaic device efficiencies are still below practical limits and long-term stability challenges hinder their practical application. Current evidence suggests that strain in halide perovskites is a key factor in dictating device efficiency and stability. Here we outline the fundamentals of strain within halide perovskites relevant to photovoltaic applications and rationalize approaches to characterize the phenomenon. We examine recent breakthroughs in eliminating the adverse impacts of strain, enhancing both device efficiencies and operational stabilities. Finally, we discuss further challenges and outline future research directions for placing stress and strain studies at the forefront of halide perovskite research. An extensive understanding of strain in halide perovskites is needed, which would allow effective strain management and drive further enhancements in efficiencies and stabilities of perovskite photovoltaics.

Multi-Walled Carbon Nanotube-Assisted Encapsulation Approach for Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) are regarded as the next-generation thin-film energy harvester, owing to their high performance. However, there is a lack of studies on their encapsulation technology, which is critical for resolving their shortcomings, such as their degradation by oxygen and moisture. It is determined that the moisture intrusion and the heat trapped within the encapsulating cover glass of PSCs influenced the operating stability of the devices. Therefore, we improved the moisture and oxygen barrier ability and heat releasing capability in the passivation of PSCs by adding multi-walled carbon nanotubes to the epoxy resin used for encapsulation. The 0.5 wt% of carbon nanotube-added resin-based encapsulated PSCs exhibited a more stable operation with a ca. 30% efficiency decrease compared to the ca. 63% decrease in the reference devices over one week under continuous operation. Specifically, the short-circuit current density and the fill factor, which are affected by moisture and oxygen-driven degradation, as well as the open-circuit voltage, which is affected by thermal damage, were higher for the multi-walled carbon nanotube-added encapsulated devices than the control devices, after the stability test.

Engineering bandgap of CsPbI3 over 1.7 eV with enhanced stability and transport properties

Potential multijunction application of CsPbI₃ perovskite with silicon solar cells to reach efficiencies beyond the Shockley-Queisser limit motivates tremendous efforts to improve its phase stability and further enlarge its band gap between 1.7 and 1.8 eV. Current strategies to increase band gap via conventional mixed halide engineering are accompanied by detrimental phase segregation under illumination. Here, ethylammonium (EA) in a relatively small fraction (x < 0.15) is first investigated to fit into three-dimensional CsPbI₃ framework to form pure-phase hybrid perovskites with enlarged band gap over 1.7 eV. The increase of band gap is closely associated with the distortion of Pb-I octahedra and the variation of the average Pb-I-Pb angle. Meanwhile, the introduction of EA can retard the crystallization of perovskite and tune the perovskite structure with enhanced phase stability and transport properties.

Polymer Modification on the NiOx Hole Transport Layer Boosts Open-Circuit Voltage to 1.19 V for Perovskite Solar Cells

Inverted-structure perovskite solar cells (PVSCs) applying NiO_x as the hole transport layer (HTL) have attracted increasing attention. It is still a challenge to optimize the contact between NiO_x and the perovskite layer and to suppress energy loss at the interface. In this study, interface engineering was carried out by modifying the NiO_x layer with different polymers such as polystyrene, poly(methyl methacrylate) (PMMA), or poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) to improve the surface contact between NiO_x and the perovskite, to decrease the defect states, and to make the energy level alignment better. The NiO_x/PMMA-based device presents a V_oc as high as 1.19 V because of the improved interfacial contact and the interaction of the carbonyl and methoxy group with Pb²⁺. The NiO_x/PTAA-based device with the structure ITO/NiO_x/PTAA/(MAPbI₃)_0.95(MAPbBr₂Cl)_0.05/PCBM/BCP/Ag exhibits the highest power conversion efficiency of 21.56% with a high V_oc of 1.19 V. The enhanced performance can be attributed to the deepened highest occupied molecular orbital level of NiO_x/PTAA, which matched well with that of the perovskite and suppressed interface energy loss as well. This work provides a facile approach for efficiently improving the V_oc of NiO_x-based PVSCs.

Tungsten-Doped Zinc Oxide and Indium-Zinc Oxide Films as High-Performance Electron-Transport Layers in N-I-P Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted tremendous research attention due to their potential as a next-generation photovoltaic cell. Transition metal oxides in N–I–P structures have been widely used as electron-transporting materials but the need for a high-temperature sintering step is incompatible with flexible substrate materials and perovskite materials which cannot withstand elevated temperatures. In this work, novel metal oxides prepared by sputtering deposition were investigated as electron-transport layers in planar PSCs with the N–I–P structure. The incorporation of tungsten in the oxide layer led to a power conversion efficiency (PCE) increase from 8.23% to 16.05% due to the enhanced electron transfer and reduced back-recombination. Scanning electron microscope (SEM) images reveal that relatively large grain sizes in the perovskite phase with small grain boundaries were formed when the perovskite was deposited on tungsten-doped films. This study demonstrates that novel metal oxides can be used as in perovskite devices as electron transfer layers to improve the efficiency.