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Page 4 of 14 Yu et al. Energy Mater. 2026, 6, 600044

Kelvin probe force microscopy (KPFM) characterization was carried out on an Oxford Instruments Cypher
ES system, employing a Ti/Ir-coated conductive cantilever to simultaneously acquire topographic and
surface potential maps. X-ray photoelectron spectroscopy (XPS) data were acquired using a Kratos Axis
Ultra Imaging XPS spectrometer (Kratos Analytical Ltd., UK) equipped with a 300 W Al Kα X-ray source.
The base pressure during data acquisition was maintained at ~3 × 10 mbar. All binding energies were
-9
referenced to the C 1s peak of adventitious carbon at 284.8 eV. Ultraviolet photoelectron spectroscopy (UPS)
measurements were performed using a He I radiation source (21.22 eV) on a Thermo Scientific GENESIS
500 system (Thermo Fisher Scientific, USA), with the sample biased at -10.0 V. Current density-voltage (J-V)
characteristics were measured under simulated AM 1.5 G illumination (100 mW cm ) using a
-2
computer-controlled Keithley 2400 (Keithley Instruments, USA) source measurement unit coupled to a
Newport Oriel solar simulator (Newport Corporation, USA). Light intensity was calibrated against a certified
NREL silicon reference cell. External quantum efficiency (EQE) spectra were recorded using an Enlitech
QE-R system (Enlitech, Taiwan, China) and calibrated with a certified single-crystal silicon reference cell.
Steady-state photoluminescence (PL) spectra were recorded using an Edinburgh Instruments fluorescence
spectrometer (Edinburgh Instruments, UK), with excitation provided by a Xe 900 xenon lamp and detection
performed by an RP928 photomultiplier tube. All optical and electrical characterizations were carried out at
room temperature (25 °C). Electrochemical impedance spectroscopy (EIS) measurements were conducted on
the OSC devices using a CHI760E (Shanghai Chenhua, China) precision electrochemical impedance
analyzer. All EIS measurements were performed under dark conditions. Impedance spectra probing
recombination kinetics were acquired at V over a frequency range of 10 MHz to 1 Hz, whereas spectra
OC
reflecting charge carrier transport properties were measured under zero applied bias across a frequency range
of 10 MHz to 0.01 Hz. The built-in potential (V ) was determined via Mott-Schottky analysis from
bi
capacitance-voltage (C-V) measurements, which were performed under dark conditions across a voltage
range of -0.2 to 1.0 V at a frequency of 10 kHz, using the standard relationship 1/C vs. V. The V was
2
bi
extracted from the linear region of the 1/C -V plot as the x-intercept. The light-intensity dependence of the
2
V was evaluated inside a nitrogen-filled glovebox under standard test conditions (AM 1.5G, 100 mW/cm ).
2
OC
Illumination intensity was systematically varied from 0.3 to 100 mW/cm using a calibrated silicon reference
2
solar cell equipped with a KG5 filter and controlled via a Keithley 2400 source measure unit.
Photocurrent density (J ) and effective voltage (V ) were determined from current density-voltage curves in
ph
eff
light and dark states with a high applied voltage from -1 to 3 V. J is defined as J = J - J , where J and
ph
light
ph
light
dark
J are the values of current density under the illumination of AM 1.5G, 100 mW/cm and in dark,
2
dark
respectively. V = V 0 - V , where V 0 refers to the voltage while J = J , and V are the applied voltage. At
dark
light
app
app
eff
a high effective voltage, all free charges in the film are completely expelled and gathered by the electrodes, a
state that is temperature-independent and is referred to as the saturation current (J ).
sat
RESULTS AND DISCUSSION
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