Researchers have developed two novel hole-transporting materials (HTMs) based on a spiro[fluorene-9,9′-xanthene] (SFX) core that could significantly improve the stability and cost-effectiveness of perovskite solar cells (PSCs). The new HTMs, dubbed XC2-M and XC2-H, incorporate N-methylcarbazole and N-hexylcarbazole units, respectively. These materials exhibited impressive performance, with the XC2-H-based device retaining an incredible 88% of its initial power conversion efficiency (PCE) after 720 hours of storage in ambient air without encapsulation. This surpasses the stability of both the spiro-OMeTAD (55% retention) and XC2-M (68% retention) devices. The exceptional stability of XC2-H is attributed to its unique film morphology and high hydrophobicity, which act as a protective barrier against moisture. Moreover, the new HTMs can be synthesized through a simple, cost-effective process, making them promising candidates for large-scale production of high-performance and stable PSCs.
Towards Stable and Affordable Perovskite Solar Cells
Perovskite solar cells (PSCs) have emerged as a promising technology in the renewable energy landscape, with power conversion efficiencies (PCEs) now rivaling those of conventional silicon-based solar cells. However, a key challenge in the widespread adoption of PSCs is the development of stable and cost-effective materials, particularly for the hole-transporting layer (HTL) that plays a crucial role in charge extraction and device performance.
Introducing Novel Spiro-Based HTMs
In a recent study, researchers from Mahidol University in Thailand have developed two new hole-transporting materials (HTMs) based on the spiro[fluorene-9,9′-xanthene] (SFX) core, a versatile building block known for its simple and economical synthesis. These HTMs, dubbed XC2-M and XC2-H, feature N-methylcarbazole and N-hexylcarbazole units, respectively, as peripheral substituents.
Exceptional Stability and Cost-Effectiveness
The key advantage of these new HTMs lies in their exceptional long-term stability. The XC2-H-based device exhibited an impressive 88% retention of its initial PCE after being stored in ambient air (30-40% humidity) for 720 hours without encapsulation. In contrast, devices based on the widely used spiro-OMeTAD and the XC2-M HTM retained only 55% and 68% of their initial PCEs, respectively, under the same conditions.
The superior stability of XC2-H is attributed to its highly hydrophobic nature and the formation of a compact, smooth film on the perovskite layer. This unique morphology is believed to act as a protective barrier against moisture, preventing degradation of the perovskite material. Additionally, the new HTMs can be synthesized through a simple, three-step process with good overall yields (around 40%) and low production costs, making them more economically viable alternatives to spiro-OMeTAD.
Enhancing Device Performance
Despite the XC2-H device exhibiting slightly lower initial PCE (10.2%) compared to the XC2-M (13.5%) and spiro-OMeTAD (12.2%) devices, the exceptional stability of XC2-H is a significant advantage. The researchers attribute the performance difference to the balance between efficient hole extraction and the compact, uniform film morphology. The XC2-M and spiro-OMeTAD materials showed more intimate contact with the perovskite layer, leading to better charge extraction and higher initial PCEs.
Paving the Way for Practical PSCs
The development of these new SFX-based HTMs with their unique combination of properties – high stability, cost-effectiveness, and appropriate energy level alignment – represents an important step towards realizing practical and sustainable perovskite solar cell technology. By addressing the long-standing challenges of HTL stability and affordability, the researchers have demonstrated a viable approach to unlocking the full potential of PSCs for large-scale deployment.
Going forward, further optimization of the HTM’s molecular structure and device architecture could lead to even higher efficiencies and lifetimes, bringing us closer to the widespread adoption of this promising solar technology.
Author credit: This article is based on research by Jeeranun Manit, Pongsakorn Kanjanaboos, Phiphob Naweephattana, Atittaya Naikaew, Ladda Srathongsian, Chaowaphat Seriwattanachai, Ratchadaporn Supruangnet, Hideki Nakajima, Utt Eiamprasert, Supavadee Kiatisevi.
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