• Holder, Emma
• Duck, Benjamin
• Khan, Hareem
• Anderson, Kenrick

Abstract

While the global energy demand continues to rise steadily, a heightened consciousness regarding the negative environmental effects of current energy technologies has resulted in a shifting emphasis towards exploring cleaner, low-emission alternatives like photovoltaic (PV) technologies.Considering the current situation, where traditional commercial PV technologies are nearing their maximum efficiency using crystalline silicon (c-Si), researchers are turning their attention to newer PV technologies. Notably, technologies based on organometal halide perovskite have undergone rapid advancements, achieving comparable power conversion efficiencies (PCE) of 3.8% to 25.5% over the past decade and are showing immense potential for future commercialization with their relatively cost-effective materials and facile fabrication processes (Murugan, 2021). This progress has motivated the development of tandem configurations. Capitalizing on the current state-of-the-art Si technologies, and adding a perovskite top cell with tuneable, high band gap characteristics, carefully designed tandem solar technologies further pave the way for highly efficient energy solutions with low manufacturing costs. However, the long-term durability of the perovskite component remains an issue.Significant efforts have been devoted to enhancing the inherent stability of perovskite-based devices. Equally important is the establishment of effective barrier systems like those shown in Figure 1, which shield the devices from external deteriorating factors like oxygen, moisture, UV light, and heat. Exposure to these elements leads to processes such as photo-oxidation, hydrolysis, thermal stress, and defect formation.Multiple well-developed commercial solutions exist for encapsulation which necessitate the consideration of factors such as cost, processing temperature, elastic modulus, adhesion, and barrier permeation rates. Given the susceptibility of perovskites to heat due to their thermally sensitive organic cations and/or hole transport layer (HTL), adopting a sealing process with a temperature requirement not exceeding 200 ⁰C is crucial (Corsini, 2020).Figure 1. Current encapsulation schemes utilised to improve extrinsic stability.One of the most widely used and effective encapsulation methods is the glass-glass packaging method where the device structure is placed between two glass sheet layers and edge sealed with a suitable thermo-curable or UV-curable adhesive. Glass has incredibly good barrier properties and therefore the choice of the edge sealant is vital to reduce the lateral moisture and oxygen-diffusive pathways. Lamination is necessary to prevent the escape of volatile by-products from the perovskite layer, which can lead to degradation.Ethylene-vinyl acetate (EVA) remains the dominant encapsulation in the market due to its well-known properties and low cost-to-performance ratio. The fact that it yellows as it ages and releases acetic acid as a by-product has rendered EVA incompatible with perovskite-based technologies. UV- curable epoxies are a particularly viable and prevalent choice for edge sealing options as heat is not applied during processing, although they tend to be costlier, and their high elastic modulus may allow delamination under thermal cycling conditions. Similarly, polyisobutylene (PIB) has gained immense popularity due to its remarkable moisture barrier properties, however, the fact that its non-transparent limits its use to inverted structures.Emerging contenders for encapsulation materials to replace EVA include thermoplastic polyurethane (TPU) and polyolefin-based options, such as polyolefin elastomer (POE) and thermoplastic polyolefin (TPO) (Oreski et al., 2020). EPE is a new, multi-layered version of EVA and POE, namely EVA-POE-EVA, which combines the benefits of good adherence to glass from EVA and exceptional water barrier properties from POE.The ability to independently assess the durability and lifespan of encapsulant materials is crucial. Calcium tests are a preferred method due to the reactive nature of calcium metal. A thin film of silver calcium, typically around 200 nm, quickly becomes transparent when exposed to moisture. By sandwiching a calcium layer between two glass pieces using encapsulation materials, the oxidation of this layer can be significantly minimised or prevented. To effectively monitor track the gradual oxidation and consequent degradation of the thin film, optical images are commonly captured and juxtaposed.Images of the encapsulated devices were taken under uniform backlit conditions before and after environmental testing (damp heat and thermal cycling). Calculating the percentage change of area is important as typical ingress patterns commence from the outer periphery and advance inwards, reducing the size of calcium film. Electrical-based methods add an additional level of accuracy and sensitivity to the durability testing. In this approach, central reg...

Topics

• perovskite
• impedance spectroscopy
• silver
• thin film
• Oxygen
• glass
• reactive
• glass
• layered
• Silicon
• defect
• Calcium
• durability
• thermoplastic
• susceptibility
• elastomer
• temperature-programmed oxidation