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Page 2 of 14 Yu et al. Energy Mater. 2026, 6, 600044

PEDOT:PSS (champion 18.22%) controls. The superiority of P-4PACz is further validated across multiple
representative systems, including PM6:L8-BO (champion 18.16%), PM6:PY-DT (champion 16.35%), PM6:Y6
(champion 16.73%), demonstrating its broad applicability. In addition to enhanced efficiency, P-4PACz-based
devices exhibit improved operational stability, retaining 80% of their initial PCE after 782 h of continuous
illumination.

INTRODUCTION
Solution-processed organic solar cells (OSCs) are a promising green energy technology for achieving carbon
neutrality due to their lightweight nature, mechanical flexibility, and compatibility with low-cost and
large-area manufacturing processes [1-3] . Recent breakthroughs in non-fullerene acceptors and device
engineering have enabled their PCEs [4-6] . However, high efficiency alone is insufficient for practical
deployment; long-term operational stability remains a critical requirement. One of the major bottlenecks
limiting OSC stability is the widely used hole transport layer (HTL), poly(3,4-ethylenedioxythiophene):
polystyrene sulfonate (PEDOT:PSS) . Although PEDOT:PSS enables high efficiency, it suffers from several
[7-9]
intrinsic drawbacks. Its strong acidity corrodes indium tin oxide (ITO) electrodes and promotes In 3+
diffusion into the photoactive layer, accelerating device degradation . Moreover, its hygroscopic nature
[10]
facilitates moisture and oxygen ingress, which exacerbates photo-oxidative instability. From an electronic
perspective, the relatively high HOMO level of PEDOT:PSS leads to suboptimal energy-level alignment,
limiting the achievable open-circuit voltage (V ). Additionally, parasitic absorption in the near-infrared
OC
region reduces photon flux reaching the active layer, constraining the short-circuit current density (J ).
SC
Collectively, these limitations compromise both the efficiency and long-term stability of OSCs, underscoring
the urgent need for alternative HTLs that simultaneously provide high performance and enhanced durability.

Since 2018, SAMs have emerged as a highly promising class of HTLs for OSCs due to their molecular
tunability, ultralow material consumption, and compatibility with solution processing. Their well-defined
molecular structures enable precise control over interfacial energetics and charge-transport properties.
Among them, carbazole-based phosphonic acid derivatives, particularly 2PACz, have achieved
state-of-the-art device performance and established a benchmark for SAM-based HTLs [11-15] . To further
optimize device performance, extensive molecular engineering strategies have been explored . These
[16]
include tuning alkyl-chain length to optimize packing and film formation , introducing methoxy groups to
[17]
improve interfacial wettability , halogenation (e.g., F or Cl) to modulate HOMO levels and work-function
[18]
alignment [19,20] , and asymmetric substitution to tailor molecular dipoles and interfacial energetics [21,22] . More
recently, extended asymmetric π-conjugated frameworks have been designed to enhance molecular ordering
and facilitate interfacial charge transport [23-26] . However, practical translation to large-area OSCs requires not
only improved interfacial properties but also structural simplicity, good solubility, and uniform film
formation [27-33] . For example, 4PACz, an extended derivative of 2PACz, improves charge extraction through
enhanced steric interactions but suffers from limited solubility and strong aggregation tendency, leading to
non-uniform films and interfacial defects .
[34]

To address these challenges, we report an asymmetric SAM molecule, P-4PACz, featuring a phenyl
substituent at the 3-position of the carbazole core. Different from other asymmetric strategies that rely on
strengthening intermolecular π-π stacking or modifying charge distribution via halogen substitution, this
asymmetric molecule is characterized by the following features: (1) it strategically modulates π-π stacking to
mitigate common issues such as poor solubility and excessive pre-aggregation in solution; (2) it maintains
appropriately balanced intermolecular interactions during assembly, enabling the formation of ordered
self-assembled monolayers that further influence active layer morphology. This structural modification
increases the molecular dipole moment from 2.08 to 2.19 D, strengthening phosphonic acid anchoring to the