Monolithic all-perovskite tandem solar cells, comprising a front subcell with a 1.8 eV wide-bandgap (WBG) perovskite and a back subcell with a 1.2 eV narrow-bandgap (NBG) perovskite, have achieved impressive efficiencies exceeding single-junction perovskite solar cells. However, oxygen-related instability remains a critical hurdle to commercialization. While encapsulation can mitigate oxygen-induced degradation, oxidation can still occur during module processing or via air leakage. Flexible all-perovskite tandem solar cells typically require highly transparent polymer substrates, limiting substrate choices and increasing costs. Therefore, novel device structures that enhance oxygen resistance and reduce material costs are needed. Current all-perovskite tandems utilize a superstrate configuration, where the WBG front subcell is deposited first, followed by the NBG back subcell. This leaves the NBG perovskite, often a mixed lead-tin (Pb-Sn) composition, vulnerable to oxidation. Sn²⁺ in the NBG perovskite oxidizes to Sn⁴⁺ in the presence of oxygen, increasing trap densities and reducing diffusion lengths, leading to device degradation. Strategies like using reducing additives or coating with metal oxides offer limited protection. This study proposes a substrate configuration to address these stability concerns.

Existing literature extensively documents the high efficiency potential of all-perovskite tandem solar cells, exceeding 26.4% in certified record efficiencies. However, these achievements often come at the cost of stability, especially concerning oxygen sensitivity in the narrow bandgap perovskites. Various strategies have been explored to address this challenge. These include the use of encapsulation techniques to create a barrier against oxygen ingress and the addition of reducing additives to the perovskite precursor solution to limit Sn²⁺ oxidation. While these methods have shown some success, they often either limit the flexibility of the devices or do not offer complete protection against oxygen-induced degradation. The review also highlights the limited substrate options in existing designs, which typically rely on transparent conductive substrates. This work directly addresses these shortcomings by employing a novel device architecture that inherently protects the sensitive NBG perovskite from oxidation while expanding the range of possible substrates.

This research employed a substrate configuration, where the NBG back subcell is deposited first, followed by the WBG front subcell and other layers, effectively encapsulating the NBG perovskite. The fabrication process began with depositing a thin layer of copper (Cu) and ITO on a glass substrate. The back subcell (PEDOT:PSS/NBG perovskite/C₆₀) was then deposited using established processes. An ALD-SnO₂ layer was added to serve as part of the tunnel recombination junction (TRJ) and a solvent barrier. Three different recombination layers (RLs) – Au, ITO nanocrystals, and magnetron-sputtered ITO (MS-ITO) – were tested. MS-ITO was selected for its superior performance and stability. The front subcell (NiO/SAM/WBG perovskite/C₆₀/ALD-SnO₂), which incorporates a WBG perovskite composition of FA₀.₈Cs₀.₂Pb(I₀.₆Br₀.₄)₃, was then deposited. Optimizing the annealing process was crucial to prevent damage to the NBG subcell while maintaining the WBG subcell's performance. A lower temperature annealing step was used for the NBG perovskite, followed by a higher temperature, shorter time annealing for the WBG perovskite. Finally, IZO was deposited as the transparent electrode. To improve the performance of the semitransparent WBG subcell, a guanidine tetrafluoroborate (GuaBF₄) additive was incorporated into the perovskite precursor solution. The addition of GuaBF₄ was found to improve the voltage and fill factor, significantly increasing the efficiency of the WBG subcell. The use of GuaBF₄ was studied using XPS, XRD, PL, and TRPL techniques to understand its effect on the perovskite films. This optimized structure was then employed to fabricate substrate-configured all-perovskite tandems on both rigid and flexible substrates (Cu-coated PEN and Cu foil). The devices were characterized using J-V curves, EQE, and stability tests. Density functional theory (DFT) calculations were also performed to study the effect of GuaBF₄ on the perovskite structure.

The study successfully demonstrated high-efficiency all-perovskite tandem solar cells using a novel substrate configuration. A champion device achieved a power conversion efficiency (PCE) of 25.3% under reverse scan, with a V_OC of 2.041 V, a J_SC of 15.6 mA cm⁻², and a fill factor (FF) of 78.9%. The device exhibited minimal hysteresis between reverse and forward scans, with a stabilized PCE of 25.1%. Impressively, unencapsulated devices showed no performance degradation after 1000 hours of storage in dry air, showcasing exceptional oxidation resistance. Flexible devices fabricated on Cu-coated PEN and Cu foil substrates achieved PCEs of 24.1% and 20.3%, respectively, demonstrating the versatility of the substrate configuration. The addition of GuaBF₄ additive to the WBG perovskite significantly improved the V_OC and FF of the semitransparent WBG subcell. XPS analysis showed a chemical interaction between GuaBF₄ and the perovskite surface, resulting in reduced non-radiative recombination and increased carrier lifetime. DFT calculations supported the experimental findings, indicating that GuaBF₄ effectively passivates halogen vacancy defects in the perovskite structure. Comparative stability tests between superstrate and substrate configurations revealed the superior stability of the substrate configuration. Unencapsulated substrate-configured devices remained stable for over 1000 hours in dry air, while superstrate-configured devices degraded significantly within 40 hours. Even under operational conditions, the unencapsulated substrate-configured tandems remained stable for over 200 hours, and encapsulated devices maintained 100% of their initial performance after 600 hours of operation at maximum power point under 1-sun illumination. Bending durability tests on flexible devices showed good performance retention, highlighting their potential for flexible applications.

The findings address the critical challenge of long-term stability in all-perovskite tandem solar cells. By reversing the conventional superstrate configuration to a substrate configuration, the researchers effectively protected the oxygen-sensitive NBG perovskite from degradation. The enhanced stability demonstrated in the unencapsulated devices highlights the efficacy of this architectural approach. The high efficiencies achieved in both rigid and flexible configurations, even on inexpensive substrates like copper foil, significantly advances the potential for commercialization. The use of the GuaBF₄ additive further underscores the optimization possibilities within this design. The improved performance and stability result from its role in passivating defects within the perovskite film, directly impacting the device's efficiency and longevity. These findings significantly advance the development of stable and cost-effective all-perovskite tandem solar cells.

This study successfully demonstrated a highly efficient and stable all-perovskite tandem solar cell architecture. The substrate configuration offers superior oxidation resistance and allows for the use of flexible and cost-effective substrates. The incorporation of GuaBF₄ further enhances device performance and stability. These findings present a significant step towards the commercialization of all-perovskite tandem solar cells. Future research could explore further optimization of the perovskite composition and interfacial engineering, alongside investigating advanced light-trapping strategies to enhance efficiency and flexibility in thinner devices.

The study primarily focused on dry air stability tests. Further investigation of long-term stability under various environmental conditions, including high humidity and UV exposure, is needed for a comprehensive assessment of device durability. While the flexible devices showed good bending durability, the performance under extreme bending conditions could be further improved. The current study focused on devices with specific compositions and layer thicknesses; broader exploration of different material combinations and device parameters may yield even better results. The efficiency of devices on Cu foil was lower than on PEN/Cu; understanding and addressing the reasons for this difference requires further study.