Hybrid perovskites have emerged as a transformative material for tandem solar cells due to their tunable bandgap, high absorption coefficients, and solution-processability. In tandem architectures, perovskites serve as either top or bottom absorbers, complementing other photovoltaic materials such as silicon, CIGS, or organic semiconductors. Their role is critical in maximizing photon utilization across the solar spectrum while minimizing thermalization and transmission losses. This article examines the material-specific requirements for hybrid perovskites in tandem configurations, focusing on bandgap tuning, interfacial engineering, and optical management.

Bandgap tuning is essential for optimizing the performance of perovskite-based tandem solar cells. The ideal bandgap for a top-cell perovskite absorber in a tandem with silicon (1.1 eV) ranges between 1.6 eV and 1.8 eV to ensure efficient spectral splitting. Methylammonium lead iodide (MAPbI3) has a bandgap of approximately 1.55 eV, which is slightly low for optimal current matching in silicon-perovskite tandems. By incorporating bromide, the bandgap can be widened; for example, MAPb(I1-xBrx)3 achieves tunability from 1.55 eV to 2.3 eV. Formamidinium lead iodide (FAPbI3) offers a narrower starting bandgap (1.45 eV), but alloying with cesium and bromide enhances phase stability while enabling precise bandgap adjustments. Triple-cation perovskites (CsFA MA) further improve stability and allow fine-tuning within the 1.6–1.8 eV range. For all-perovskite tandems, the bottom cell typically requires a low-bandgap perovskite (1.2–1.4 eV), achieved through tin-lead alloying (e.g., FASn0.5Pb0.5I3). However, tin-based perovskites face challenges such as oxidation and defect-mediated recombination, necessitating careful compositional engineering.

Interfacial engineering plays a pivotal role in mitigating losses at the perovskite/subcell and perovskite/transport layer boundaries. In monolithic tandems, the interconnecting layer must facilitate efficient charge recombination while minimizing optical and resistive losses. Commonly used recombination layers combine transparent conductive oxides (e.g., ITO) with thin metal oxides (e.g., MoOx, TiOx) or organic materials (e.g., PEDOT:PSS). For perovskite-silicon tandems, the top perovskite cell must be processed without damaging the underlying silicon heterojunction or textured surface. Atomic layer deposition (ALD) of SnO2 or sputtered ITO are preferred for their conformal coverage and low parasitic absorption. In perovskite-perovskite tandems, the interlayer must prevent halide or metal interdiffusion while maintaining high conductivity. Thin metal films (e.g., Au, Ag) or doped organic layers can serve as effective recombination mediators. At the electron and hole transport layers, energy level alignment is critical to minimize voltage losses. For wide-bandgap perovskites, nickel oxide (NiOx) and PTAA are common hole transport materials, while C60 or SnO2 are used for electron extraction. Surface passivation techniques, such as post-deposition treatments with alkylammonium halides, reduce non-radiative recombination at these interfaces.

Optical management is crucial to maximize light absorption and minimize reflection in tandem devices. In perovskite-silicon tandems, the perovskite top cell must be optically thick enough to absorb high-energy photons while remaining thin enough to avoid excessive resistive losses. Anti-reflection coatings and textured surfaces enhance light trapping, particularly in silicon bottom cells with pyramidal structures. For perovskite-perovskite tandems, the optical design must balance absorption between the two subcells, often requiring sophisticated optical modeling to optimize layer thicknesses. The use of photonic structures, such as distributed Bragg reflectors or plasmonic nanoparticles, can selectively enhance absorption in specific spectral regions. Additionally, transparent electrodes with high conductivity and low absorption, such as ultrathin metal films or metal nanowire networks, are essential to maintain high current densities in the top cell.

Stability remains a key challenge for perovskite-based tandem solar cells. Wide-bandgap perovskites, particularly those with high bromide content, are prone to phase segregation under illumination, leading to voltage losses. Encapsulation strategies must address moisture and oxygen sensitivity while maintaining optical transparency. For tin-containing low-bandgap perovskites, antioxidant additives (e.g., SnF2) and reduced-dimensional perovskite capping layers improve operational stability. Thermal management is also critical, as the additional layers in tandem devices can exacerbate heat buildup, accelerating degradation.

Recent advancements in perovskite tandem solar cells have demonstrated remarkable efficiencies, with perovskite-silicon tandems exceeding 33% and all-perovskite tandems reaching over 26%. These achievements highlight the potential of hybrid perovskites to surpass the efficiency limits of single-junction devices. However, scalability and long-term stability under real-world conditions remain areas of active research. Innovations in compositional engineering, interfacial design, and optical optimization will further solidify the role of hybrid perovskites in next-generation tandem photovoltaics.

The integration of hybrid perovskites into tandem solar cells represents a convergence of material science, device physics, and optical engineering. By addressing the unique challenges of bandgap tuning, interfacial losses, and light management, researchers continue to unlock new pathways toward high-efficiency, cost-effective multijunction photovoltaics. The ongoing development of stable, high-performance perovskite compositions and advanced device architectures ensures that tandem solar cells will remain at the forefront of renewable energy technology.