Research on Materials and Interface Synergistic Engineering for Efficient and Stable Perovskite-Organic Tandem Solar Cells

Abstract

As the core of next-generation thin-film photovoltaics, perovskite solar cells (PSCs) have opened a new pathway towards low-cost, high-efficiency photovoltaic applications, owing to their exceptional optoelectronic properties, solution-processability, and the abundance of material systems. Their high defect tolerance and tunable direct bandgap have enabled a remarkable leap in certified power conversion efficiency (PCE) from 3.8% to over 27.3% within just over a decade, surpassing the decades-long development trajectory of traditional technologies like polycrystalline silicon, demonstrating immense potential to disrupt the current photovoltaic industry landscape. However, single-junction perovskite cells are still constrained by the Shockley-Queisser (S-Q) theoretical efficiency limit, and long-term operational stability remains the most critical bottleneck for their commercialization. Therefore, developing tandem solar cells based on perovskites has become a strategic direction to break the efficiency limit and enhance device reliability simultaneously.

Among various perovskite-based tandem architectures, perovskite-organic tandem solar cells (POTSCs) have attracted significant attention due to their unique advantages. By combining a high-efficiency wide-bandgap (WBG) perovskite top cell with a spectrally complementary narrow-bandgap (NBG) organic bottom cell, POTSCs theoretically can achieve over 40% efficiency. More importantly, both perovskite and organic photovoltaic materials share common merits, such as low-temperature solution-processability, easily tunable bandgaps, and suitability for fabricating lightweight, flexible devices. This enables POTSCs to achieve ultra-high efficiency while remaining inherently compatible with low-cost, large-scale production technologies such as roll-to-roll processing. Compared to the complex process of perovskite-silicon tandems or the greater stability challenges of all-perovskite tandems, POTSCs offer a technological pathway with a better balance among efficiency, cost, and process complexity.

Despite their promising prospects, the commercialization of POTSCs faces multiple challenges rooted in materials and interfacial physics. Regarding efficiency and charge dynamics, the limited exciton diffusion length in the organic subcell and the disordered donor-acceptor network within its bulk-heterojunction (BHJ) lead to inefficient charge extraction and significant recombination losses, which are the primary reasons for low fill factors (FF). Simultaneously, energy-level misalignment and interfacial defects in the interconnecting layer (ICL) cause significant charge accumulation and tunneling barriers, resulting in additional voltage loss and parasitic absorption. Regarding stability, degradation mechanisms driven by water/oxygen ingress are coupled with the intrinsic ion migration issue in perovskite materials. Particularly in wide-bandgap perovskite subcells, the migration of uncoordinated Pb²⁺ ions and halide vacancies at the interface, driven by light and electrical stress, leads to increased non-radiative recombination centers, aggravated phase segregation, and severe interfacial energetic disorder. This constitutes the fundamental physical cause of continuous degradation in device performance.

Previous research often employed isolated, "point-to-point" strategies, failing to systematically address these interconnected mechanistic issues. To enhance efficiency, standard methods such as optimizing perovskite film morphology or adjusting the organic phase-separation scale can partially improve charge generation but often neglect optoelectronic synergistic loss mechanisms in the ICL, resulting in limited optimization outcomes. To improve stability, traditional physical encapsulation only delays external water/oxygen ingress. Commonly used single-molecule passivation, such as alkylammonium salts, can temporarily passivate surface defects, but their dynamic physical adsorption tends to fail under prolonged stress and may disrupt the continuity of interfacial energy levels, potentially increasing charge transport resistance. For enhancing the stability of the organic subcell, traditional binary blend systems struggle to simultaneously optimize exciton dissociation, charge transport, and hydrophobicity at the molecular scale. These strategies fail to achieve synergistic regulation from the root causes of charge transport kinetics and ion migration thermodynamics, thus generally suffer from the limitation of "addressing one issue while neglecting another," making it difficult to achieve a simultaneous breakthrough in both efficiency and stability.

This work proposes a multi-scale, mechanism-driven, synergistic strategy spanning from the bulk to the interface to systematically address the challenges. Firstly, in the design of the organic active layer, we constructed a quaternary all-polymer bulk heterojunction. Its multiple donor-acceptor combinations optimize the energy level gradient, providing more efficient channels for exciton dissociation and charge transport. Simultaneously, the interpenetrating network formed by the all-polymer system exhibits stronger hydrophobicity and morphological stability, suppressing water/oxygen penetration pathways at the source. Secondly, in the engineering of the interconnecting layer, we precisely tuned the interface dipole and optimized energy level alignment at the atomic scale by introducing a self-assembled monolayer (Me-4PACz) and controlling the MoOx thickness, thereby reducing charge recombination and optical parasitic absorption losses and achieving a record-high FF (83.62%). Most critically, we proposed a multidentate anchoring-bridging strategy to stabilize the perovskite interface. This strategy utilizes molecules with multiple coordination sites, (3,3,3-trifluoropropyl) trimethoxy silane, to form a periodic passivation array with strong chemical bonds on the perovskite surface, permanently saturating dangling bonds. Furthermore, its fluorinated side chains reconfigure the interfacial dielectric environment, significantly increasing the activation energy for halide-vacancy migration and fundamentally suppressing the degradation pathways initiated by ion migration.

In conclusion, this study achieves a synchronous leap in the efficiency of POTSCs (>26%), fill factor (>83%), and long-term stability (T₉₂>1000 hours) through a tripartite mechanistic innovation: optimizing bulk charge dynamics, minimizing opto-electrical losses in the interconnecting layer, and thoroughly suppressing interfacial ion migration. This work not only provides a set of generalizable design principles for tandem devices but also lays a solid foundation at the physical mechanism level for the commercialization of next-generation high-performance, high-reliability tandem photovoltaic technology.

Date of Award	6 May 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zonglong ZHU (Supervisor)