Perovskite solar cells (PSCs) have shown remarkable progress in power conversion efficiency (PCE), reaching levels comparable to commercial c-Si solar cells. However, their relatively short lifetime (around one year) hinders widespread adoption. UV degradation is a significant contributor to this instability, causing irreversible damage to the perovskite material, especially crucial in space applications under intense AMO illumination. UV shielding encapsulants can mitigate this, but they often lead to reduced efficiency by blocking valuable UV photons. This work explores a solution combining light trapping techniques with luminescent down-shifting (LDS) layers. LDS layers, particularly those using lanthanide-based materials (like Eu or Tb), efficiently convert high-energy UV photons into lower-energy visible photons usable by the PSC. Reducing the perovskite layer thickness minimizes bulk recombination, but this simultaneously decreases light absorption. Light-trapping (LT) techniques counter this by extending the optical path within the active layer. This paper investigates a checkerboard (CB) grating structure for LT, known for its scalable fabrication methods and effectiveness in enhancing light absorption, in combination with an LDS encapsulant to enhance both performance and stability.

The literature review extensively covers the advancements in perovskite solar cell technology, highlighting the achievements in PCE and the challenges related to long-term stability. It details the impact of UV degradation and its mechanisms, discussing various strategies for UV protection, including the use of encapsulant layers. Existing literature on luminescent down-shifting materials, particularly lanthanide-based complexes, and their applications in enhancing solar cell efficiency is reviewed. Finally, the existing literature on light-trapping techniques in solar cells and their implementation in PSCs is summarized, focusing on the advantages of periodic grating structures and the use of TiO2 as a light-trapping medium.

The study employs a combined optical and electrical modeling approach. First, a checkerboard (CB) grating pattern is designed and optimized using particle swarm optimization for light trapping within the TiO2 electron transport layer (ETL). This optimization considers geometrical parameters like etching depth, side length, grafting period, and width to maximize light absorption and minimize reflection losses. Optical simulations, employing methods like finite-difference time-domain (FDTD) to model light propagation and absorption in the PSC structure with and without the CB pattern, are performed. The optical properties are incorporated into electrical simulations to estimate the device's performance. Secondly, the optical properties of an experimentally developed tri-ureasil modified by lanthanides (t-U(5000)/Eu3+) LDS layer are experimentally characterized, including its absorption and emission spectra, which allows for the accurate incorporation of the LDS effects in the optical model. The coupled optoelectronic simulation combines these optical models with a drift-diffusion solver to analyze electrical characteristics such as short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE). These simulations consider both ultra-thin (250 nm) and conventional (500 nm) perovskite layers to assess the impact of the LT and LDS on devices with varying thicknesses.

The optimized CB grating structure in the TiO2 ETL significantly enhances light absorption in the perovskite layer across the visible spectrum, with the most significant gains observed for thinner (250 nm) perovskite layers. This improvement is attributed to the anti-reflective effect and enhanced light scattering due to the structure's broken symmetry. For the 250 nm perovskite layer, the photocurrent density (Jph) increases by 26%, while for the 500 nm layer, it increases by 19%. The experimentally characterized t-U(5000)/Eu3+ LDS layer effectively blocks approximately 94% of UV radiation (300-400 nm), converting it into visible light. Coupled simulations incorporating both LT and LDS effects demonstrate a combined PCE enhancement of 28.2% for the ultra-thin (250nm) PSC. This includes improvements in Jsc, Voc, and FF. The combined approach improves the performance and stability of the device, particularly its UV resilience. Simulations show a decrease in overall reflection for the 250 nm cell (from ~5 to ~2 mA cm⁻²) compared to the 500 nm cell (from ~3.7 to ~1.6 mA cm⁻²). The angle-resolved photocurrent analysis reveals consistently higher photocurrent across different angles for the cells with the LT structure compared to the planar ones. Fabrication methods such as nanoimprint, laser interference lithography (LIL), and displacement Talbot lithography (DTL) were discussed to emphasize the scalability and feasibility of the CB structure.

The results demonstrate a highly effective strategy for improving the efficiency and stability of PSCs. The combination of light trapping and luminescent down-shifting addresses two major limitations: reduced light absorption in thin perovskite layers and UV-induced degradation. The substantial PCE enhancement (28.2%) in the ultra-thin cell highlights the synergy between the LT and LDS approaches. The findings are particularly relevant for applications requiring high efficiency and stability under intense UV radiation, such as space-based solar energy systems. The superior performance of the ultra-thin PSC with the combined LT and LDS technique underscores the benefits of this novel approach and suggests a path towards more efficient and cost-effective perovskite solar cells.

This work presents a novel approach for improving the performance and stability of perovskite solar cells by combining light trapping and luminescent down-shifting. The checkerboard pattern successfully enhanced light absorption, while the LDS layer effectively converted harmful UV radiation into usable visible light. The combined approach resulted in a significant increase in power conversion efficiency, making this strategy highly promising for various applications, including high-efficiency space solar technology. Future research could focus on exploring other LDS materials and optimizing the CB pattern geometry for even better performance. The successful implementation of these techniques on flexible substrates is also a worthwhile avenue for investigation.

The study relies on optical and electrical modeling, and the experimental validation of the combined LT and LDS approach in real devices is needed to fully confirm the simulated results. The specific choice of materials (TiO2 as ETL, Spiro-OMeTAD as HTL) may affect the generalizability of the findings, and further investigation with different materials is warranted. The long-term stability of the combined LT and LDS structure under various environmental conditions, including thermal stress and humidity, needs additional assessment.