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)^[7-9]. Although PEDOT:PSS enables high efficiency, it suffers from several intrinsic drawbacks. Its strong acidity corrodes indium tin oxide (ITO) electrodes and promotes In³⁺ diffusion into the photoactive layer, accelerating device degradation^[10]. Moreover, its hygroscopic nature 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_OC). Additionally, parasitic absorption in the near-infrared 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^[16]. These include tuning alkyl-chain length to optimize packing and film formation^[17], introducing methoxy groups to improve interfacial wettability^[18], halogenation (e.g., F or Cl) to modulate HOMO levels and work-function 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 ITO surface through combined steric and electronic effects. Simultaneously, the phenyl substituent suppresses pre-aggregation in solution via steric hindrance, enabling the formation of denser and more homogeneous SAM films^[35]. As a result, P-4PACz creates smoother, lower-energy ITO surface, leading to improved active layer wetting and interfacial contact. Using P-4PACz as the HTL, bulk-heterojunction OSCs based on PM6:BTP-eC9 photoactive system exhibit enhanced hole extraction, suppressed bimolecular and trap-assisted recombination, and prolonged carrier lifetimes compared to devices employing symmetric 4PACz or PEDOT:PSS. Consequently, the champion P-4PACz-based device shows a PCE of 19.03%, outperforming symmetric 4PACz (champion 18.28%) and PEDOT:PSS (champion 18.22%) controls, and delivers the longest T₈₀ lifetime of up to 782 h under controlled conditions (25 °C, 25% relative humidity, AM 1.5G illumination 100 mW/cm²). These results demonstrate that asymmetric molecular engineering of SAM-based HTLs enables synergistic optimization of interfacial energetics, film morphology, and charge-transport dynamics. This strategy provides a practical pathway toward high-efficiency and stable OSCs compatible with scalable fabrication.

The molecular structures of P4PACz ((4-(3-phenyl-9H-carbazol-9-yl)butyl) phosphonic acid) and 4PACz ((4-(9H-carbazol-9-yl) butyl) phosphonic acid) are presented in Figure 1A. The key feature of this asymmetric modification lies in its subtle yet meaningful influence on the molecular dipole moment characteristics. Although the dipole moment orientation is altered [Figure 1B and Supplementary Figure 1], its magnitude increases only slightly from 2.08 to 2.19 D. Furthermore, the introduction of the phenyl substituent induces a redistribution of the electrostatic potential (ESP). Compared with symmetrical 4PACz, P-4PACz exhibits an expanded negative ESP region extending from the carbazole core toward the phenyl-substituted side, while the positive ESP remains primarily localized on the alkyl chain and phosphonic acid moiety. This spatial redistribution of charge density reflects the asymmetric electronic environment introduced by phenyl substitution [Figure 1C]. Density functional theory (DFT) calculations further reveal that the asymmetric phenyl group induces a downward shift of the Highest Occupied Molecular Orbital (HOMO) level in the self-assembled monolayer [Figure 1D]. Specifically, the calculated HOMO levels of 4PACz and P-4PACz are -5.56 and -5.80 eV, respectively, while the corresponding LUMO levels are -0.799 and -0.725 eV [Supplementary Figure 2]. This modulation of frontier molecular orbitals suggests improved energy-level alignment at the interface.

The SAM films (4PACz and P-4PACz) were fabricated on ITO substrates under identical processing conditions (see Experimental Section) to enable a fair comparison of their intrinsic properties [Figure 2A]. UV-vis absorption spectra [Supplementary Figure 3A] show that P-4PACz exhibits nearly identical absorption features to 4PACz, indicating that the asymmetric phenyl substitution does not significantly alter the optical characteristics of the monolayer. Notably, however, the transmittance of the P-4PACz monolayer in the 400-800 nm range is slightly higher than that of both 4PACz and PEDOT:PSS [Supplementary Figure 3B], which is advantageous for maximizing photon utilization in the active layer.

To understand the origin of the improved film morphology of P-4PACz, we first investigated its intrinsic molecular packing and solution-state behavior. Single-crystal structures of the π-scaffolds and full molecules were analyzed. For the pure π-scaffolds, pristine carbazole exhibits a vertical stacking distance of 3.37 Å and a longitudinal slip of 5.73 Å [Supplementary Figure 4A]. In contrast, the 3-phenyl-substituted carbazole shows a substantially shortened vertical distance of 2.47 Å but an increased slip of 6.46 Å [Supplementary Figure 4B], reflecting a shift in the optimal stacking geometry induced by the phenyl substituent. Notably, when the full molecules (including alkyl chains and phosphonic acid groups) are considered, P-4PACz still exhibits a much shorter vertical distance (2.73 Å) compared to 4PACz (3.29 Å) [Supplementary Figure 4C and D], while the slip distances are approximately 6.02 and 5.96 Å, indicating that the π-π stacking is reasonably moderated. This unique stacking mode also influences the solution-state behavior and the film formation of the SAM. As shown in Supplementary Figure 5, at identical concentration (0.5 mg/mL), the 4PACz solution in ethanol exhibits visible turbidity with undissolved particulates, whereas the P-4PACz solution is completely transparent and clear. This difference is also evident in chloroform, a nonpolar solvent, indicating that the superior solubility of P-4PACz is an intrinsic property of its asymmetric molecular structure rather than being solvent-specific. The reduced aggregation tendency in solutions arising from the weakened intermolecular packing in the solid state.

The surface morphologies of bare ITO, ITO/SAMs, and ITO/PEDOT:PSS were characterized by AFM. As shown in Supplementary Figure 6A-D, the ITO/P4PACz exhibits a reduced arithmetic average roughness (Ra = 4.53 nm) compared to bare ITO (Ra = 4.81 nm) and ITO/4PACz (Ra = 4.67 nm), indicating a smoothing effect that is visually supported by the corresponding height profile in Supplementary Figure 6E. In contrast, the SAM-modified surfaces display Ra values similar to that of pristine ITO, which can be attributed to their ultrathin monolayer structure. In contrast, the PEDOT:PSS exhibits a significantly lower Ra value (Ra = 1.89 nm), reflecting its thicker polymeric film that effectively planarizes the underlying ITO surface^[36]. Notably, the smoother substrate induced by P4PACz translates into improved activelayer morphology. As shown in Supplementary Figure 7, the PM6:BTPeC9 blend deposited on ITO/P4PACz exhibits a lower surface roughness (Ra = 1.82 nm) compared to that on ITO/4PACz (Ra = 1.99 nm), and approaches that of the PEDOT:PSS reference (Ra = 1.78 nm). This suggests that the P4PACz interlayer not only smooths the ITO surface but also promotes the formation of a more uniform and homogeneous bulk heterojunction film. Surface energy plays a critical role in determining both the efficiency and long-term stability of organic solar cells. To quantify the interfacial wettability, contact angle measurements were conducted using water (H₂O) and formamide (FA) as probe liquids. The water contact angles of ITO/P-4PACz and ITO/4PACz were measured to be 86° and 80°, respectively, significantly higher than that of bare ITO (46°) [Supplementary Figure 8 and 9], indicating enhanced hydrophobicity upon SAM modification. Using the Owens-Wendt-Rabel-Kaelble method, the calculated surface energies of ITO/PEDOT:PSS, ITO/4PACz, and ITO/P-4PACz are 76.7, 40.8, and 38.0 mN m^-1, respectively, showing a progressive decrease. Notably, the surface energy of ITO/P-4PACz is closer to that of the active layer PM6:BTP-eC9 (28.9 mN m^-1) [Supplementary Table 1], suggesting improved interfacial compatibility. In addition to increased hydrophobicity, this optimized surface energy alignment facilitates better film formation of the active layer, thereby contributing to enhanced device performance.

The formation of SAMs on ITO substrates was confirmed by XPS. The fullspectrum scans of 4PACz and P4PACz [Supplementary Figure 10] showed marked changes in peak intensities relative to bare ITO, indicating successful SAM coverage. Highresolution O 1s corelevel spectra further revealed systematic variations [Figure 2B and Supplementary Figure 11]: the relative area of the component around 531.5 eV, commonly assigned to surface hydroxyl groups or oxygendeficient sites, decreased progressively from 0.95 (bare ITO) to 0.85 (ITO/4PACz) and to 0.65 (ITO/P4PACz). Moreover, the main O 1s peak for ITO/P4PACz exhibited a noticeable positive bindingenergy shift compared to both bare ITO and ITO/4PACz. The changes suggest that phosphonic acid groups likely consume reactive sites on the ITO surface, thereby contributing to the formation of a more stable interface. Direct evidence for this interfacial bonding is provided by the P 2p spectra [Figure 2C]. Specifically, for P4PACz, the P 2p_3/2 and P 2p_1/2 doublet is observed at 133.85 and 134.75 eV, respectively. Compared with the corresponding peaks of 4PACz (134.05 and 135.25 eV), this represents a systematic negative shift of 0.2-0.5 eV, indicating an increase in electron density around the phosphorus atom in P4PACz upon coordination with the ITO surface. Furthermore, the spectral weight of the lower-binding-energy component (133.85 eV) is more pronounced in P-4PACz, which may reflect an altered distribution of phosphonate binding motifs induced by the asymmetric molecular structure. Furthermore, the In 3d spectra [Figure 2D] reveal that the In 3d_2/3 and In 3d_5/2 peaks of bare ITO are located at 451.725 and 444.225 eV. Upon SAM deposition, these peaks shift to 452.350 and 444.850 eV for 4PACz, and further shift to 452.725 and 445.225 eV for P4PACz. The observed binding energy shifts primarily in the O 1s, P 2p, and In 3d regions collectively indicate an enhanced chemical coupling at the SAM/ITO interface. Notably, the negative shift in the P 2p spectrum suggests increased electron density around the phosphorus atom, consistent with stronger bonding between the phosphonate groups and the ITO substrate. Concurrently, analysis of the C 1s and N 1s spectra [Supplementary Figure 12]^[37] reveals that the area proportion of the C-P component increased from 19% to 23%. This change reflects an altered electronic environment around the phosphorus center, which is consistent with the P 2p shift and further supports the formation of stronger interfacial interactions.

To further investigate the optoelectronic properties of asymmetric SAM, KPFM measurements were conducted. As shown in Figure 3A and B, ITO/P4PACz exhibits the highest surface potential. The work functions (WFs) of the functionalized ITO substrates were characterized by UPS [Figure 3C and Supplementary Figure 13]^[38]. The measured WFs for ITO/P4PACz and ITO/4PACz are -5.13 and -5.01 eV, respectively, which aligns with the trend observed in KPFM. The WFs of bare ITO and PEDOT:PSS were determined to be -4.56 and -4.95 eV, respectively, indicating that P4PACz further deepens the WF compared to both 4PACz and PEDOT:PSS^[39,40]. This leads to improved energy-level alignment across the interface, thereby enhancing hole extraction and promoting more efficient charge carrier transport.

Local conductivity was characterized by C-AFM under a low bias of 8.7 mV. The average current measured on ITO/P4PACz (4.44 nA) was significantly higher than that on ITO/4PACz (3.77 nA) [Figure 3D and E], suggesting enhanced local charge transport. Considering the smoother morphology observed earlier, this improvement likely reflects that the more uniform and continuous monolayer formed by P-4PACz helps to minimize defects that would otherwise impede current flow. Furthermore, electrical characterization based on sandwich-structured devices showed that the conductivities of 4PACz, P4PACz, and PEDOT:PSS are 1.969 × 10^-5, 2.188 × 10^-5, and 1.364 × 10^-5 S·cm^-1, respectively [Figure 3F], consistent with the improved current characteristics observed in device performance.

Binary OSCs with the structure ITO/HTLs/PM6:BTPeC9/PNDIT-F3N/Ag were fabricated, with the device architecture and energy level alignment illustrated in Figure 4A and B, respectively^[41].

The SAM modification process and optimization conditions for the ITO substrates are detailed in Supplementary Table 2, and the J-V curves of the best-performing devices are shown in Figure 4C. The champion device based on 4PACz achieved a PCE of 18.28%, with a J_SC of 27.42 mA cm^-2, an V_OC of 0.865 V, and a fill factor (FF) of 76.54%. In comparison, the champion device employing the optimized P-4PACz as the hole transport layer attained a higher PCE of 19.03%, along with improved J_SC (27.87 mA cm^-2) and FF (78.86%), based on these values, we find that the improvement in FF from 76.54% to 78.86% contributes dominantly to the overall PCE enhancement. According to the relationship PCE = V_OC × J_SC × FF, the increase in FF accounts for approximately 65% of the total efficiency gain, whereas the contributions from V_OC and J_SC are comparatively minor [Supplementary Table 3]. EQE spectra of champion devices based on P-4PACz, 4PACz, and PEDOT:PSS are presented in Figure 4D. The integrated J_SC values derived from the EQE curves are 26.45, 25.90, and 25.00 mA cm^-2, respectively. Notably, the champion P-4PACz-based device exhibited significantly higher EQE responses in the wavelength ranges of 400-450 nm and 500-800 nm. The dark J-V characteristics of the devices are shown in Figure 4E. The P4PACzbased device exhibited a lower current density under both forward and reverse bias, suggesting that the series resistance was reduced and electron injection at the anode interface was more effectively suppressed.

The photovoltaic parameters of devices fabricated with different hole transport layers are statistically summarized in Figure 4F and Supplementary Figure 14. As summarized in Table 1, the P-4PACz-based devices achieved an average PCE of 18.20% (J_SC = 27.73 mA cm^-2, V_OC = 0.866 V, FF = 75.78%), with an average efficiency of 18.20% ± 0.15%, significantly surpassing those of the 4PACz-based (17.52% ± 0.19%) and PEDOT:PSS-based (17.34% ± 0.24%) devices. The performance improvement from 4PACz to P4PACz can be primarily attributed to a notable increase in the FF (from 73.78% ± 0.26% to 75.78% ± 0.25%), accompanied by slight enhancements in both J_SC and V_OC. These collective enhancements suggest more efficient charge extraction and reduced recombination losses. Furthermore, Supplementary Figure 15 shows that P-4PACz exhibits higher V_bi than 4PACz and PEDOT:PSS, which is another key factor contributing to its enhanced performance. To explore the generality of the asymmetric phosphonic acidbased selfassembled interface, we introduced 4PACz, 2PACz, P4PACz, and PEDOT:PSS into other binary systems (PM6:L8BO, PM6:Y6, and PM6:PYDT, chemical structures shown in Supplementary Figure 16), as illustrated in Figure 4G. The corresponding J-V curves are provided in Supplementary Figure 17 and Detailed photovoltaic parameters are summarized in Supplementary Tables 4-6. Importantly, P-4PACz demonstrates universal applicability as an excellent hole-transporting layer across various active layer systems. Unencapsulated devices employing PEDOT:PSS, 4PACz, or P-4PACz as the hole-transport layer were further evaluated under 25 °C, 25% relative humidity, under AM 1.5G illumination (100 mW/cm²); the P-4PACz-based cells delivered the longest T₈₀ lifetime, up to 782 h [Figure 4H].

The exciton dissociation and charge collection efficiencies in organic solar cells incorporating different hole-extraction layers were evaluated by analyzing the photocurrent density as a function of effective bias [Figure 5A]. The charge dissociation probability (P_diss) and charge collection probability (P_coll) are defined as follows: P_diss = J_ph,SC/J_sat and P_coll = J_ph,MPP/J_sat, where J_ph denotes the photocurrent density and J_sat denotes the saturation current density. P_diss is typically evaluated under short-circuit conditions and quantifies the efficiency of exciton dissociation into free charge carriers, whereas P_coll is evaluated at the maximum power point voltage and reflects the efficiency with which photogenerated free charges are collected by the electrodes. Under short-circuit conditions, the P_diss value for devices based on ITO/4PACz, ITO/P-4PACz, and ITO/PEDOT:PSS were 95.03%, 97.27%, and 96.19%, respectively. At the maximum power point, the P_coll values were 84.32%, 88.39%, and 86.84%. These results indicate that the device employing P-4PACz exhibits the highest exciton dissociation efficiency.

The trap-assisted recombination behavior was further investigated by examining the semi-logarithmic dependence of V_OC on light intensity [Figure 5B]. The fitted slope, expressed in units of n·kT/q, approaches the ideal value of 1 kT/q when trap-assisted recombination is minimal. The device employing P-4PACz exhibits the smallest slope (n = 1.17), which is notably lower than those of the 4PACz-based (n = 1.25) and PEDOT:PSS-based (n = 1.21) devices, indicating more effective suppression of trap-assisted recombination in the P-4PACz-based device. The bimolecular recombination behavior was assessed by analyzing the J_SC-P_light relationship, described by the power-law dependence J_SC ∝ P^α [Figure 5C]. A value of α closer to unity signifies weaker trap-assisted recombination. As illustrated in Figure 5C, the P-4PACz-based device achieves the highest α value (α = 0.987), surpassing the 4PACz-based (α = 0.967) and PEDOT:PSS-based (α = 0.975) devices, demonstrating more pronounced suppression of bimolecular recombination in the P-4PACz-based device^[42,43].

The recombination impedance spectra of different devices appear in Figure 5D. The P4PACzbased device exhibits the highest recombination resistance (1.54 kΩ), significantly exceeding those of the 4PACzbased (1.11 kΩ) and PEDOT:PSSbased devices (0.67 kΩ), indicating more effective suppression of carrier recombination^[44]. The P4PACz interlayer can simultaneously optimize energylevel alignment and improve physical contact at the interface. This synergistic effect not only promotes efficient hole extraction but also substantially reduces carrier recombination losses, thereby contributing to higher FF and PCE. To directly visualize interfacial hole transfer, steady-state PL spectroscopy was performed. The PL intensity of the PM6:BTP-eC9 blend was more effectively quenched on the ITO/P-4PACz substrate compared to ITO/4PACz [Supplementary Figure 18].

Transient photovoltage (TPV) decay measurements show that the carrier lifetime of the device based on P-4PACz (5.03 μs) is longer than that of the device based on 4PACz (3.23 μs), which is consistent with the suppression of trap-assisted non-radiative recombination [Figure 5E]. Transient photocurrent (TPC) analysis further indicates that the charge extraction speed of the device based on P-4PACz is accelerated, as reflected in the reduction of the extraction time from 0.28 to 0.19 μs [Figure 5F]. Holes accumulated rapidly at the 4PACz interface, but the charge extraction efficiency was limited. The asymmetric substitution at the P-4PACz interface improved the alignment mode and the change in work function, forming a more efficient charge extraction channel between the electrode and the active layer. This is also reflected in J_SC, ultimately leading to the improvement of the higher PCE of the OSCs.

In summary, we report an easily accessible asymmetric carbazole-based self-assembled monolayer, P-4PACz, featuring a unilateral phenyl substituent that rationally moderates π-π stacking and enhances dispersion and solubility in solution. The resulting SAM exhibits improved molecular packing, higher conductivity, and lower surface energy, which collectively facilitate better active-layer film formation. Furthermore, the asymmetric design deepens the work function and optimizes energy-level alignment with the donor material, thereby promoting efficient hole extraction and charge transport. These benefits synergistically enhance the open-circuit voltage, short-circuit current density, and particularly the fill factor. This simple yet effective strategy of tailoring intermolecular interactions to optimize film formation offers new insights into the design of SAM materials for organic solar cells.