Key Takeaways & Executive Findings
- •• • The AOCzPA SAM with hydrogen-bond networks achieves a PCE of 21.56% in 1.77 eV wide-bandgap PSCs, with V_OC of 1.35 V and FF of 85.76%, demonstrating minimal voltage losses and suppressed non-radiative recombination, critical for high-efficiency tandem top cells. • • The hydrogen-bond network narrows the energy offset to 0.42 eV, aligning HOMO levels for barrier-free hole extraction, which reduces interfacial charge transfer losses and enhances fill factor, directly impacting module-level power output. • • The design suppresses detrimental self-aggregation via twisted carbazole dimer conformation, preventing π–π stacking and creating an amorphous, homogeneous molecular distribution, which improves interfacial adhesion and long-term operational stability under thermal stress. • • The amide groups form cooperative hydrogen bonds with TCO surfaces (C=O···HO–In/Sn and N–H···O–In/Sn), strengthening anchoring and mechanical robustness, essential for scalable fabrication and durability in tandem devices.
Abstract
All-perovskite tandem solar cells (TSCs) are poised to surpass the Shockley–Queisser limit of single-junction perovskite solar cells (PSCs) by integrating wide- and narrow-bandgap subcells to broaden spectral utilization. However, their performance remains constrained by interface charge transfer losses and non-radiative recombination in wide-bandgap subcells. Self-assembled monolayers (SAMs) serve as effective hole-selective contacts, yet conventional designs suffer from uncontrolled intermolecular interactions due to amphiphilic characteristics, leading to detrimental self-aggregation, suboptimal molecular packing, and weakened interfacial adhesion. In a recent breakthrough published in Nature Energy, Wang et al. introduced a rational molecular design that integrates amide units as dual hydrogen-bond donors and acceptors into a bicarbazole-based biphosphonic acid dimer (AOCzPA). This design suppresses self-aggregation via a twisted conformation of the C–C-linked carbazole dimer, enhancing steric hindrance and preventing π–π stacking. The amide groups establish an expansive, cooperative hydrogen-bonding network, forming intramolecular bonds, intermolecular connections, and strengthened bonds with hydroxylated transparent conductive oxides (TCO) via C=O···HO–In/Sn and N–H···O–In/Sn. This network impedes long-range crystalline order, creating an amorphous, homogeneous molecular distribution without nanovoids. Consequently, the energy band at the perovskite interface bends upward, narrowing the energy offset to 0.42 eV and aligning HOMO levels for barrier-free hole extraction. The strategy yields exceptional performance: 1.77 eV single-junction wide-bandgap PSCs achieve a PCE of 21.56%, V_OC of 1.35 V, and FF of 85.76%, indicating low voltage losses and suppressed non-radiative recombination. This work advances SAM design from monolayer assembly to networked interface engineering, enhancing mechanical and chemical robustness and minimizing hole-transport losses.
1. Introduction
All-perovskite tandem solar cells (TSCs) offer a pathway to exceed the Shockley–Queisser limit of single-junction devices by stacking wide- and narrow-bandgap subcells. However, their commercial viability is hindered by interface charge transfer losses and non-radiative recombination, particularly in wide-bandgap subcells. Conventional self-assembled monolayers (SAMs) used as hole-selective contacts suffer from uncontrolled intermolecular interactions due to their amphiphilic nature, leading to self-aggregation and suboptimal molecular packing. These issues weaken interfacial adhesion and degrade both power conversion efficiency (PCE) and operational stability, posing a critical bottleneck for tandem device performance.
To address this, Wang et al. introduced a rational molecular design that integrates amide units as dual hydrogen-bond donors and acceptors into a bicarbazole-based biphosphonic acid dimer (AOCzPA). This design deliberately moves beyond conventional single-mode anchoring by establishing an expansive, cooperative hydrogen-bonding network. The twisted conformation of the carbazole dimer suppresses π–π stacking, while the amide groups form intramolecular and intermolecular bonds, as well as strengthened bonds with the TCO substrate. This network creates an amorphous, homogeneous molecular distribution, reducing energy offsets to 0.42 eV and enabling barrier-free hole extraction. The strategy significantly enhances interfacial robustness and minimizes transport losses, representing a paradigm shift from monolayer assembly to networked interface engineering.
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Luo Huaiqing, Zhou Qisen, Chen Wei (2026). Engineering Hydrogen-Bond Networks in Self-Assembled Molecules Boosts All-Perovskite Tandem Solar Cell Efficiency. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4136-y
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Frequently Asked Questions
What is the specific role of the amide units in the AOCzPA molecule in enhancing device performance?
The amide units (–CO–NH–) act as dual hydrogen-bond donors (N–H) and acceptors (C=O), establishing a cooperative hydrogen-bonding network. This network disperses localized intermolecular interactions of phosphonic acid groups, forming intramolecular bonds, intermolecular connections among adjacent SAMs, and strengthened bonds with the TCO surface (C=O···HO–In/Sn and N–H···O–In/Sn). This enhances molecular packing homogeneity, suppresses self-aggregation, and reduces the energy offset to 0.42 eV, leading to improved hole extraction and a high fill factor of 85.76%.
How does the hydrogen-bond network affect the long-term operational stability of the devices under thermal stress?
The hydrogen-bond network creates an amorphous, homogeneous molecular distribution without nanovoids, which enhances the mechanical and chemical robustness of the SAM layer on TCO. This structural reinforcement improves adhesion and reduces degradation pathways, but the text notes that operational stability under thermal stress (e.g., MPP tracking at 85°C) appears to be moderate. Further validation under combined heat and humidity (85°C/85% RH) is required to assess commercial viability.
What are the main challenges for scaling this molecular design to large-area modules?
Scaling from spin-coated laboratory cells to large-area modules via slot-die coating or inkjet printing is challenging because fabricating a highly uniform hydrogen-bond network at the square-meter scale is technically difficult. The uniformity of the SAM layer is critical for consistent performance, and any inhomogeneities could lead to localized defects and reduced efficiency.
How does the AOCzPA design compare to conventional SAMs in terms of suppressing non-radiative recombination?
Conventional SAMs suffer from uncontrolled intermolecular interactions, leading to self-aggregation and suboptimal packing, which increases non-radiative recombination. The AOCzPA design suppresses π–π stacking via a twisted carbazole dimer and establishes a hydrogen-bond network that creates a homogeneous molecular distribution. This reduces energy offsets to 0.42 eV, aligning HOMO levels and minimizing non-radiative recombination, as evidenced by the high V_OC of 1.35 V and FF of 85.76%.
What are the implications of the 0.42 eV energy offset for hole extraction efficiency?
The 0.42 eV energy offset between the perovskite HOMO and the SAM HOMO is sufficiently small to allow barrier-free hole extraction, minimizing interfacial charge transfer losses. This alignment is crucial for achieving high fill factors and open-circuit voltages, as demonstrated by the exceptional FF of 85.76% and V_OC of 1.35 V in wide-bandgap PSCs, which are among the best reported for this bandgap.
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