Ultrasensitive photodetectors enabled by pressure-induced atomic-level optimization of graphene-based heterojunctions

Domain: biology
Published: 2026-03-23
Editorial outcome: minor_revision

Abstract
Photodetectors with high photoresponsivity are crucial in a wide range of emerging fields.
Here, we report the realization of photodetectors based on co-assembled composite
membranes consisting of copper phthalocyanine (CuPc) molecules and graphene oxide (GO)
sheets, showing a photoresponsivity as high as ~10⁷ A/W under 3 nW illumination and a
moderate external pressure (~1 atm) in the visible range. We attribute the high
photoresponsivity to the pressure-induced optimization of the atomic-level heterojunctions
between photoactive CuPc molecules and adjacent GO sheets in the photodetector, which
enhances the electronic coupling at the heterojunction for charge transport. The
photodetectors show good flexibility for electronic skin applications, enabling simultaneous
contact sensing and non-contact perception under ambient illumination down to dim
moonlight. The developed strategy provides a promising perspective for fabricating next-
generation photoelectric conversion devices with potential applications spanning intelligent
sensing, aerospace technologies, and defence-related photonic platforms.

Introduction
Photodetectors that convert light into electrical signals are essential for diverse advancing
technologies, such as living cell and biomedical imaging, optical communication, night vision, and
security systems1-5. High photoresponsivity (the responsivity of the photodetector), i.e., the ratio
of the net photocurrent to the incident-light power, is of great importance in practical applications.
Intensive efforts and significant achievements have been made6-16; however, there is only little
progress on the raising of photoresponsivity under the common range of illumination powers (3
nW ‒ 10 mW) in the past decade (Fig. 1d), thus calling for new innovative strategy.

In optoelectronic devices, particularly those based on organics, photoactive materials absorb light
to produce excitons (coupled electrons and holes), and the electrons/holes resulting from the
dissociated excitons transport to the respective acceptor material through a heterojunction17-20. The
performance of the heterojunction is crucial for the photoresponsivity21-24. However, unfavourable
factors, such as defects and unevenness in the photoactive and/or acceptor materials, can degrade
the electronic coupling at the heterojunction for charge separation and transport16,25-27. Clearly,
these unfavourable factors are at the atomic/molecular level, making it very challenging to reduce
the detrimental effects. Fortunately, for materials with a planar architecture, a macroscopic
manipulation (i.e., applying an external pressure) can drive atomic/molecular-level modifications.
Based on our previous works on graphene oxide (GO)28-30, we recently fabricated a composite
membrane consisting of planar structures of copper phthalocyanine (CuPc) molecules and GO
sheets, where CuPc and GO serve as typical photoactive and electron-acceptor materials,
respectively31. We expected that a simple macroscopic manipulation could reduce some
detrimental effects to make a more compact CuPc‒GO heterojunction, thus leading to an even
higher photoresponsivity.

Electronic skins (e-skins) hold great transformative potentials32,33. Current e-skin technologies
predominantly rely on contact-based sensing modalities to acquire environmental information34,35.
Although this mode is effective for static scenarios, it fails to address dynamic environments
requiring non-contact interactions, such as robot-assisted navigation or preemptive hazard
detection in vehicular safety systems. Conventional optical e-skins, employing radar, laser, or
infrared light, require additional active illumination, and face critical limitations under passive
natural light conditions36. Clearly, if there is an ultrasensitive photodetector to detect direction and
distance, as well as flexibility and piezoelectricity to monitor the subjected pressure, we can obtain
a dual-mode e-skin simultaneously realizing contact and non-contact sensing under ambient
illumination.

Here, we present a photodetector based on a co-assembled composite membrane consisting of
CuPc molecules and GO sheets (CuPc@GO), exhibiting high responsivity under 3 nW ‒ 10 mW
incident powers and moderate external pressure within the near-ultraviolet, visible and infrared
bands. We associate the high photoresponsivity to the optimization of the atomic-level CuPc‒GO
heterojunctions via a macroscopic manipulation ‒ by simply applying a modest external pressure
(~1 atm), a strong electronic coupling at the heterojunction is established and a ~105 to 106-fold
improvement in photoresponsivity is obtained. The photodetector is successfully used in dual-
mode e-skins, enabling flexibility together with simultaneous contact and non-contact sensing,
within the irradiation range from daylight to moonlight.

Results
High photoresponsivity obtained under external pressure

The central component of the photodetector is a CuPc@GO membrane fabricated via a drop
casting method. Briefly, a CuPc suspension was mixed with a GO dispersion. The mixture was
then dripped onto release paper and dried, yielding a composite membrane. The membrane was
subsequently employed to fabricate a CuPc@GO photodetector.

To ensure that the embedded CuPc molecules contact the GO sheets well, we applied an external
force (i.e., pressure) on the CuPc@GO membrane. Henceforth, we refer to the CuPc@GO
membranes with and without the pressure as optimized and unoptimized membranes, respectively.
Simultaneously, we illuminated the optimized CuPc@GO membrane with a beam of light on the
opposite side from the pressure application site, with the illuminated area of 1 cm2 identical to the
area under external pressure (Fig. 1a). In this paper, the illumination wavelength is 635 nm (visible
light) unless otherwise explicitly stated. The current‒voltage (I‒V) performance with an incident
power E = 3 nW was examined firstly (Fig. 1b). As the bias ranges from -1.2 to 1.2 V, the
in
photocurrent increases with the increasing pressure (Fig. 1b and Supplementary Fig. 1). When the
external pressure (denoted as P) reaches 240 kPa, the photocurrent increases to 4.41 × 10-2 A under
a bias of 1.2 V, in contrast to the dark current of 4.11 × 10-3 A. Notably, the photocurrent and dark
current in the unoptimized case are only 5.35 × 10-8 A and 1.87 × 10-8 A, respectively. Thus, the
light response is boosted by more than 105 times under P = 240 kPa, while it is only modest in the
absence of the external pressure.

The responsivity (R) of the photodetector under a certain pressure, denoted by R , was then
P
computed by the following definition:
𝑅 = (𝐼p−𝐼0,p) / 𝐸in
where I denotes the measured photocurrent, I is the dark current obtained without illumination
p 0,p
(both I and I were obtained at a bias of 1.2 V) at the same external pressure, and E is the incident
p 0 in
power. R exhibits an observable increase as the pressure increases until P = 60 kPa, followed by
P
a plateau-like behaviour when P = 60 ‒ 240 kPa (Fig. 1c). Since 60 kPa and 240 kPa correspond
to ~0.6 atm and 2.4 atm respectively, the results indicate that a modest external pressure (~1 atm)
can significantly raise the photodetector responsivity. Hereafter we refer the photoresponsivity at
P = 240 kPa to as R for ease of description. Remarkably, R reaches 1.33 × 107 A/W, exceeding the
responsivity of other photodetectors by more than one order of magnitude at E = 3 nW.
in
The photodetector achieves a remarkable photoresponsivity across a wide range of incident powers
from 3 nW to 10 mW (Fig. 1d), showing the highest values of R among state-of-the-art
photodetectors reported in recent years within the given incident-power range37 (see
Supplementary Table 1 for details). The relationship between R and incident power can be linearly
fitted well in double logarithmic coordinates, consistent with the power law of photocurrent versus
incident light intensity38. Further, we also characterized the comprehensive photodetection
performance of the photodetector, and the specific detectivity (D*) of the CuPc@GO photodetector
is a function of incident optical power, among which it can be 9.71 × 1013 Jones under 3 nW
illumination (Fig. 1f). This D* value is closely correlated with the ultra-low noise-equivalent power
(NEP) of 7.71 × 10-13 W under 3 nW illumination, as the two key figures of merit are inherently
linked through the noise current and R (Supplementary Fig. 2). We also conducted experiments
and calculated the linear dynamic range (LDR) and bandwidth (f ), which are 138.4 dB and 5.6
c
kHz, respectively (Supplementary Figs. 3&4).

The device works with high photoresponsivity across a broad wavelength range. The values of R
exceed 106 A/W at E = 3 nW for all three typical wavelengths tested, i.e., 635 nm, 395 nm, and
in
808 nm (Fig. 1e). As E increases, the R values for these three wavelengths converge and become
in
almost indistinguishable at E = 10 mW. The pressure-induced enhancement rate, defined as R/R ,
in 0
where R is the responsivity without pressure application, was then calculated. For low incident
0
powers, the enhancement rate reaches approximately 106 for the incident wavelength of 635 nm,
whereas those for the other two wavelengths remain on the order of ~105. As E increases, the
in
enhancement rates for different wavelengths converge, reaching ~6 × 105 at E = 10 mW.
in
The CuPc@GO detector achieves a lower detection limit of only 100 pW (Fig. 1g) for illumination
power at room temperature without requiring an additional photomultiplier, which reaches the
upper limit of the working range for single-photon detectors39. Moreover, the optimization greatly
raises the response speed of the photodetector – both the rise time (τ) and decay time (τ ) were
r d
decreased by 3 ‒ 4 orders of magnitude, from several hundreds of milliseconds to several tens of
microseconds (Fig. 1h and Supplementary Fig. 5).

The high photoresponsivity shows strong stability (Supplementary Figs. 6&7) and reproducibility
among different production batches (Supplementary Figs. 8&9), insensitive to ambient humidity
while sensitive to the oxidation degree of GO (Supplementary Figs. 10&11).

Characterization of CuPc@GO membrane under pressure

The X-ray diffraction (XRD) pattern (Fig. 2a) indicates an intersheet spacing of 0.826 nm for the
unoptimized CuPc@GO membrane, which is slightly larger than that for the pure GO membrane
(0.731 nm). This suggests that the CuPc molecules, which have a planar structure and single-atom
thickness, intercalate into the GO membrane and form a single-molecule layer sandwiched
between two adjacent GO sheets. The intercalation of CuPc is also supported by scanning electron
microscopy (SEM), where a well-defined layered structure (Fig. 2b) of the CuPc@GO membrane
(unoptimized) was clearly revealed. Compared with the pure GO membrane, a new sharp
diffraction peak in the XRD pattern (Fig. 2a) appears at ~26° for the CuPc@GO membrane,
corresponding to an interplanar spacing of ~0.334 nm, indicating the stacking of CuPc aromatic
rings with the unoxidized regions40,41 of the GO sheets.

The XRD patterns after the pressure application are shown in Fig. 2a. Notably, as the pressure
increases, the intersheet spacing of the composite membrane decreases continuously (Fig. 2c), and
the peak at ~26° corresponding to an interplanar spacing of 0.334 nm becomes more apparent.
These phenomena indicate that the confined space between the GO sheets is gradually regulated
such that more π‒π stacks are enabled between the aromatic rings of the CuPc molecules and the
GO sheets.

We further performed femtosecond transient absorption (TA)42 spectroscopy measurements (with
an external pressure ~25 kPa; see Methods). The TA spectra of the optimized (Fig. 2d) and
unoptimized (Supplementary Fig. 12) membranes pumped by a 400 nm laser pulse show clear
excited-state absorbance (ESA) signals peaking at ~650 nm, whereas ground-state bleaching (GSB)
and stimulated emission (SE) signals are not observed. The ESA signals span a broad range of
wavelengths, from ~550 nm to 800 nm (and beyond), and exhibit a large overlap with the typical
absorption spectrum of the CuPc molecule (Supplementary Fig. 13), indicating that the
photocurrent is mainly caused by the optical absorption of CuPc molecules. The absence of GSB
and SE signals can be attributed to aggregation-induced quenching43 of excitons in CuPc molecules
because of their strong π‒π stacking with neighbouring GO sheets, which leads to ultrafast charge
dynamics and is beneficial to the high photoresponsivity.

Multiexponential fitting with time constants of τ , τ , and τ was applied to the TA kinetics data
1 2 3
(Fig. 2e). The same abrupt increase in the ESA signal (with a time scale of ~ 0.9 ps) can be observed
for both the optimized and unoptimized CuPc@GO membranes, implying the immediate
generation of charge carriers right after the application of the laser pump. Then, two different
charge dynamics processes can be identified: i) the ultrafast transfer of the charge carrier from the
CuPc molecules to the neighbouring GO sheets, of which the characteristic time τ is reduced from
1
0.95 ps for the unoptimized membrane to 0.32 ps for the optimized membranes. ii) the intra-sheet
and inter-sheet charge transport (bridged by the CuPc molecule), of which the characteristic time
τ is reduced from 4.33 ps to 2.56 ps. Another possible process involved in the TA kinetics is the
2
charge trapping and exciton recombination due to the presence of inevitable trap states in the GO
sheets, of which a much larger characteristic time τ (~15‒16 ps) is almost independent of the
3
pressure.

Mechanism of the pressure-enhanced photoresponsivity

To elucidate the underlying mechanism, we performed density functional theory (DFT)
calculations on the unit cell of the composite membrane ‒ a sandwich complex where one CuPc
molecule is placed between adjacent GO sheets. Here we used a simplified GO model44: one
epoxide group and one hydroxy group were located closely to each other and separately on both
sides of the GO sheet, leaving a relatively large region unoxidized. This model is consistent with
previous reports that the oxidized groups tend to gather and there are large unoxidized regions on
the GO sheets40,41. Considering the observation of remarkable π‒π stacking interactions between
the CuPc molecules and the GO sheets (Fig. 2b,c), the CuPc molecule was placed in the middle of
the unoxidized regions of the GO sheets. Two typical states were considered: the coupling state
and the uncoupling state, corresponding to the cases of small and large intersheet distance,
respectively (see Fig. 3a,b). In the coupling state, a CuPc molecule has an equal distance of 0.327
nm with each GO sheet, yielding the intersheet distance of 0.654 nm according to structural
optimization (Fig. 3a). The coupling state corresponds to the pressure-optimized configuration.

The well-dispersed valence and conduction bands (Fig. 3c) and the delocalized electronic
wavefunction distribution (Fig. 3a) clearly indicate strong electronic coupling at the CuPc‒GO
interface. Consequently, a charge-transport channel is well formed perpendicular to the GO sheets,
which is bridged by the CuPc molecule. The valence and conduction bandwidths are 13.8 meV
and 8.2 meV, respectively, which corresponds to a coupling at the CuPc‒GO interface of 3.5 meV
for the hole and 2.1 meV for the electron according to the one-dimensional tight-binding model.
This result is consistent with our ultraviolet photoelectron spectroscopy (UPS) data, which show
a shift in the valence band position due to pressure optimization (Supplementary Fig. 14). In
addition, we calculated the intra-sheet and inter-sheet electric conductivities to quantitively
characterize the relative contributions of intra- and inter-sheets to charge transport as well as the
influence of pressure (see Supplementary Fig. 15). The inter-sheet conductivity is still lower than
the intra-sheet conductivity even after the optimization, indicating that the inter-sheet transport
still represents the main bottleneck limiting charge transport in the overall GO‒CuPc‒GO pathway.

In the typical uncoupling state, the intersheet distance was set to be 0.826 nm, corresponding to
the intersheet spacing of the unoptimized CuPc@GO membrane (see Fig. 2b). The valence and
conduction bandwidths are sharply reduced to 0.9 meV and 1.6 meV, respectively. We also show
the case of pure GO system (parallel GO sheets without CuPc) as the control group in Fig. 3c,
where the bandwidths are also negligible due to the absence of intersheet interactions. These results
indicate negligible electronic coupling in the uncoupling case, as also demonstrated by the much
more localized electronic wavefunction distribution along the perpendicular direction (Fig. 3b).
Now we can illustrate how efficient charge-transport pathways are established in the CuPc@GO
membrane and why the applied pressure serves as a key. As shown in Fig. 3d, the CuPc molecules
are distributed between the GO sheets. There are two processes associated with the transport of
charge carriers: (i) the electron transfer from the CuPc molecule onto the neighbouring GO sheet
due to exciton dissociation; and (ii) the intra-sheet and inter-sheet (via a GO‒CuPc‒GO pathway)
charge transport. In the absence of the external pressure, due to the relatively large distance
between CuPc and GO sheets, there are rare coupling states to form efficient pathways for charge
transfer and intersheet charge transport (Fig. 3e). However, through the application of pressure,
most hindrance mentioned above are removed and much more coupling states with efficient
charge-transport pathways are well established (Fig. 3d), thus resulting in a high photocurrent.

Dual-mode-sensing electronic skin of the photodetector

We fabricated a CuPc@GO-based electronic skin (CuPc@GO e-skin) which can work in both non-
contact and contact sensing modes (Fig. 4a). We note that the CuPc@GO photodetector has good
flexibility with high photoresponsivity remaining at various bending/twisting angles (Fig. 4b and
Supplementary Figs. A16&17). Thus, the photodetector can have more applications requiring
flexibility45 such as the e-skin. Standard models with sizes of ~10 cm were used to testify the dual-
mode sensing under ambient illumination with E = 10 μW, corresponding to the ambient
in
illumination just at dawn (~9.5 lux for the CuPc@GO membrane with an area of 1 × 1 cm2). As e-
skin approaches the models, the photocurrents decrease commenced at the distances of 15 mm, 12
mm, and 5 mm, for cuboid, hemisphere and stick, respectively, exhibiting dependence on both
object geometry and separation distance (Fig. 4c). Once the e-skin contacts the models, an abrupt
current increase is observed (Fig. 4c), which also exhibits a shape-dependent manner mainly due
to the piezoelectricity of the CuPc@GO membrane (see Ref. 31 and Supplementary Table 2). The
e-skin can also distinguish the hardness of objects ‒ an object with higher hardness can induce a
more abrupt current increase as the e-skin contacts the object (Fig. 4d). As the target size increases,
the e-skin shows an extended range of object discrimination, enabling non-contact detection at
distances up to 2 m for the target with a size of ~1 m (Fig. 4e). Remarkably, the e-skin can “see”
the object even under a power of 900 pW (Fig. 4f), corresponding to the ambient illumination
under the moonlight (~0.01 Lux for the CuPc@GO membrane).

We designed a control board for signal acquisition and actuation of the robotic manipulator (Fig.
4g and Supplementary Fig. 18), which was subsequently installed on a robotic manipulator
incorporating the CuPc@GO e-skin into the fingertips (Fig. 4h). A series of actions including
approaching, grasping, moving and releasing were conducted by using the robotic manipulator
according to the e-skin feedback, and the featured responses in the photocurrent are explicated in
Fig. 4i, with the middle fingertip as a typical example to illustrate the idea (Fig. 4j). Firstly, the
middle finger was controlled to approach the target object, inducing a decreasing photocurrent.
According to the results of standard models, a 1.9% reduction in the photocurrent serves as a
threshold for impending mechanical contact. Once the current decreased lower than this threshold,
the approaching velocity was initially halved and then progressively attenuated to 0.9 mm/s,
thereby minimizing impulse transfer to the object. The manipulating examples with a banana (Fig.
4i,j) and an egg (Supplementary Fig. 19) the successfully touching on the objects while avoiding
damage. During the releasing process, all fingers retract simultaneously, exhibiting fast decreases
in the current (Fig. 4i,j and Supplementary Fig. 19).

Methods
Materials

The Copper phthalocyanine and Graphite were provided by Aladdin Co., Ltd. (Shanghai, China)
with a purity beyond 99.95%. Potassium persulfate (K S O ), Phosphorus pentoxide (P O ),
2 2 8 2 5
Sulfuric acid (H SO ), Potassium permanganate (KMnO ) and Hydrogen peroxide (H O ) were
2 4 4 2 2
purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received. Milli-
Q water (18.25 MΩ, Millipore, US) was used to prepare GO aqueous dispersions. The
programmable robotic manipulator (uHand 2.0) was provided by Hiwonder Tech Co., Ltd.
(Shenzhen, China).

Preparation of CuPc@GO composite membranes and photodetectors

A GO suspension (5 mg/mL) was prepared with GO synthesized from natural graphite powder
using an improved Hummers method. A CuPc suspension was prepared by adding 28.68 mg of
powder to a centrifuge tube containing 50 mL of water, followed by an ultrasonic treatment for 1
hour. The CuPc dispersion was thoroughly mixed with the 5 mg/mL GO dispersion in equal
volumes. Ultrapure water of the same volume was subsequently added, and the resulting mixture
was processed with ultrasonic treatment for 1 hour to yield a stable CuPc@GO suspension. The
composite suspension was then divided into equal-volume droplets, which were dripped onto
release paper for self-assembly and dried in an oven at 70℃ for 12 hours. This process resulted in
the formation of the CuPc@GO composite membrane which has a thickness of ~1 μm with good
flexibility. Next, the membrane was peeled off from the release paper and then trimmed into square
pieces measuring 1 × 1 cm2. The CuPc@GO membrane within this photodetector served as the
photon-sensing layer.

Measurement of the photodetector properties

The I‒V characteristics and photocurrent versus time curves of the photodetectors were measured
using a Keithley 2461 source meter (Tektronix, USA) following rigorous experimental procedures.
To ensure accurate signal transmission and to minimize external interference, a gold probe was
employed during the photocurrent measurements. The external pressure was applied by the weight
unless otherwise explicated stated and was calibrated with commercial pressure transducers. The
incident light sources were monochromatic lasers (Thorlabs, USA) with an adjustable power
density and were precisely calibrated with photodiode power detectors (Thorlabs, S170C and
PM100D, USA) with 10 nW detection limit. The 3-nW incident power was achieved by attenuating
the 10 nW source through a neutral density filter with 30% transmittance. The illumination position
and area (1 cm2) were kept consistent throughout all the measurements, except for the case of 100
pW incident power which was achieved by reducing the effective optical area of the device to 0.1
× 0.1 cm² via a mask. Notably, no light source other than the laser was present in the measured
environment. Moreover, the photodetectors were tested under conditions consisting of room
temperature (25°C), 20% humidity, and atmospheric pressure. The rise and decay times were
obtained using an oscilloscope (Tektronix, MSO44 4-BW-200).

Characterization of the CuPc@GO membranes

Lateral images of the CuPc@GO membranes were acquired via secondary electron imaging in the
scanning electron microscope (ZEISS Gemini 300, Germany) at an accelerating voltage of 3 kV.
Patterns of X-ray diffraction were obtained via XRD (Rigaku Ultimate IV, Japan) with a Cu Kα
radiation source (λ = 0.15418 nm) in the 2θ range of 5°~40°. Before the XRD experiments were
conducted, pressures with different values (ranging from 20 to 240 kPa) were applied to the
CuPc@GO membrane for 5 minutes. There is no pressure applied on the sample during the XRD
experiments.

Transient absorption (TA) spectroscopy

Femtosecond TA spectroscopy was carried out using a Helios ultrafast pump‒probe system, which
included an oscillator and a Coherent Opera Solo optical parametric amplifier. The titanium
sapphire amplifier in the system generates 800 nm pulses at a repetition rate of 1 kHz, with each
pulse having a duration of 70 fs. These pulses are split by a beam splitter: one portion is directed
to a TOPAS parametric amplifier (Coherent) to produce the pump pulses while the other is focused
onto sapphire and YAG plates, generating a white supercontinuum that covers the 420–780 nm and
800–1400 nm ranges, respectively. The time delay between the pump and probe pulses has a
maximum value of 8 ns when a motorized optical delay line is used.

In order to meet the requirements of the TA spectroscopy measurement, both the membrane
thickness and the pressure applied are different from those of other experiments: i) the TA
measurement requires the sample surface to be very flat, so the CuPc@GO suspension was
carefully deposited via the drop-casting technique onto a 2 cm × 2 cm high-clarity quartz glass
substrate, followed by drying in an oven at 70°C. This results in a membrane thickness of ~ 100
nm, much thinner than that in other experiments (~1 μm). ii) Considering the fragility of the glass,
we applied a small pressure of ~25 kPa on the sample for 5 minutes. During the TA spectroscopy
measurements, the pressure was removed.

Dual-mode sensing of the electronic skin

The flexible CuPc@GO photodetector was mounted at the fingertip of a robotic manipulator to
maintain its optimal operational state during photodetection. For non-contact sensing, the robotic
system drove the fingertip in a programmed, controlled trajectory, advancing it toward the target
object until the initial surface contact was established. After contact, the system applied a
controlled compressive force by advancing the fingertip an additional 1 mm. During the entire
process, the current signals were monitored in real-time. Dynamic analysis on the current variation
enables the real-time capture of the mechanical interaction state between the fingertip and the
object, facilitating closed-loop feedback for precise adjustment of the motion trajectory of the
manipulator. This adaptive strategy ensures both accuracy and efficiency in the process of object
grasping. Environmental illumination remained stable throughout all experimental procedures.

Theoretical calculations

DFT calculations were performed with the Vienna Ab initio Simulation Package (VASP)46.
Projector augmented wave (PAW) pseudopotentials were applied. The generalized gradient
approximation (GGA) of the Perdew–Burke–Ernzerhof (PBE) formalism was used to treat the
exchange‒correlation interaction between electrons47, and the DFT-D3 method of Grimme with a
zero-damping function48 was applied for dispersion correction. The electron wavefunction was
expanded on a plane-wave basis set with an energy cutoff of 450 eV. The Brillouin zones were
sampled by k-point grids of 2 × 2 × 3 for structural optimization49.

For structural optimization of the state without an external pressure, the periodicity along the
direction perpendicular to the GO sheets was set to 0.826 nm, consistent with the XRD result,
whereas those along the other two directions were calculated to fully relax the stress. To describe
the state with an external pressure, the restriction of the periodicity along the z-axis was released,
and the structure was reoptimized via DFT calculations, resulting in a new periodicity of 0.654 nm,
which represents the deformation influenced by the external pressure. After the entire structural
optimization process, the number of k-points in the Brillouin zone was increased, and non-self-
consistent calculations were performed to obtain the energy band. By using the one-dimensional
tight-binding model, the intersheet electronic coupling 𝑡 for electrons/holes was then
e⁄h
calculated with the width of the conduction/valence band 𝑊 , that is, 𝑡 = 𝑊 ⁄4.
CB⁄VB e⁄h CB⁄VB

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors reported the fabrication of the photodetectors based on copper phthalocyanine/graphene oxide heterojunctions and investigated their photoelectrical properties. They found that high photoresponsivity can be achieved by enhancing electronic coupling between photoactive copper phthalocyanine molecules and adjacent graphene oxide sheets through pressure. The further demonstrated dual-mode electronic skin application, operable under illumination ranging from daylight to moonlight. Their study is interesting and useful, supported by rigorous experimental validation, theoretical modeling, and practical demonstrations. I recommend acceptance pending revisions to s to address the following technical and conceptual aspects:\n1. While the drop-casting method is simple, its reproducibility over larger areas and feasibility for industrial-scale production should be discussed, along with potential challenges in uniformity and yield.\n2.Address the long-term stability of the pressure-optimized heterojunctions under cyclic mechanical stress (beyond Supplementary Fig. 6).\n3.Elaborate on how this strategy (pressure-optimized heterojunctions) could be generalized to other optoelectronic systems (e.g., perovskites or 2D materials).\n4.Discuss potential limitations (e.g., sensitivity to humidity or oxidation of GO) and mitigation strategies.\n5.Clarify whether the 100 pW detection limit (Fig. 1f) requires cryogenic conditions or is achievable at room temperature.\n6. Given graphene’s inherent high conductivity, additional experimental or theoretical evidence quantifying the relative contributions of intra-sheet versus inter-sheet (GO‒CuPC‒GO pathway) electron transport would further solidify the proposed mechanism.", "reviewer_2": "The authors proposed a photodetector based on the CuPC/GO heterojunction using the vertical device structure. The device demonstrated significant responsivity enhancement after applying external forces. This work lacks sufficient innovation and the conclusions need additional data for support. I do not think it is suitable for publication in Nature Communications.\n1. The main novelty of this paper is the use of external pressure to enhance device performance. However, applying pressure to improve the contact and reduce the interlayer distance in layered materials is a known and expected phenomenon. Furthermore, this work did not address the key concerns regarding the reproducibility of the method and the device-to-device variation under pressure. The authors show that the performance of one device is enhanced by more than 10^5 times when a pressure of 240 kPa is applied, but they don't provide data to show whether other devices would require the same pressure to achieve a similar performance improvement and whether this improvement could be controlled or repeated under multiple pressure-release cycles. The absence of these data makes the approach for performance enhancement is not a fundamentally new and reliable principle. I think the novelty of using external pressure is not solid.\n2. Some basic and crucial performance characterizations are missing. For example, there are no results about the detectivity, LDR, or frequency response of the device, which are essential for evaluating a photodetector's performance. As a flexible device, the mechanical stability, including performance under repeated bending and complex strain, is not fully explored. Furthermore, the authors claim that the piezoelectricity of the CuPC@GO membrane is the reason for the abrupt current increase during contact sensing in Fig. 4c, however, there is no characterization about the piezoelectricity in this work. And I did not find any works reporting about the piezoelectricity of the CuPc and GO.\n3. In the DFT calculation, the authors used pristine graphene instead of GO due to that GO sheets have \"large unoxidized regions\" and \"remarkable π-π stacking interactions\". However, this is a significant simplification that ignores the effects of oxygen-containing functional groups in GO, which could act as charge trapping sites. The authors should provide a clear explanation for the use of pristine graphene.\n4. The authors claimed that the photodetector outperforms all current state-of-the-art devices under illumination powers from 3 nW to 10 mW. However, some published works have reported a higher photoresponsivity (Nat Commun 2021, 12, 1798). A more comprehensive and up-to-date literature review should be provided.\n5. The paper's organization could be improved to enhance the readability. The current figure sequence is confusing. For example, Fig. 1d was cited as the first figure in the main text. The Fig. 2d and 2e appeared after the Figure 3. The illumination intensity with unit of nm/cm2 of μW/cm2 is more common for the comparison of responsivity while also providing the applied voltages.", "reviewer_3": "In this article, the authors claim to propose a dual-mode electronic skin (e-skin) based on the CuPc@GO photodetector. However, there are numerous loopholes:\n1) In the experimental part, important information about the protocols used is missing. Common parameters for photodetectors, such as specific detectivity (D*), response bandwidth fc, the optical effective area Ad and noise equivalent power (NEP) must be thoroughly characterized. The authors have only presented data on responsivity and response time, which is insufficient to support the claimed potential for future applications. I believe this literature would be highly beneficial for the authors to deepen their understanding of photodetectors. (Wang, F., Zhang, T., Xie, R. et al. How to characterize the figures of merit of two-dimensional photodetectors. Nat. Commun. 14, 2224 (2023). https://doi.org/10.1038/s41467-023-37635-1.)\na) The photoelectric properties shown in Fig.1 are difficult to regard as those of the CuPc@GO because they strongly reflect the properties of CuPc. It is recommended to make and measure the lateral device, using CuPc or GO.\nb) In Fig. 1b, what is the mechanism behind the phenomenon where the device exhibits current at 0V without applied pressure, but this current disappears after pressure is applied?\nc) In Fig. 1c, how did the authors manage to detect light as weak as 3 nW, which is notoriously difficult to measure with conventional power meters?\nd) In Fig. 1d, why does the photoresponsivity reported in the latest literature appear lower than in earlier studies? Has the performance in this field been deteriorating?\ne) In Fig. 1f, it is highly unusual that the device exhibits current at the milliampere level alongside significant noise under illumination as low as 100 pW. The authors must provide a plausible explanation for this anomalous data.\nf) In Fig. 1g, when calculating the photoresponse time, it is advisable to normalize the data to clearly illustrate the range from 10% to 90% of the photocurrent variation.\n2) Basic characterization experiments were conducted, but some results lack clarity, making analyzing the material's electrical performance less robust.\na) As a photoactive material, providing the ultraviolet absorption spectrum, UPS data, and energy band diagram of CuPc@GO is essential. Unusually, the experimental work utilizes GO, while the simulations are based on graphene. Such limited computational modeling is unconvincing. Synchrotron radiation experiments are necessary to characterize the bonding and carrier distribution in the heterojunction material.\nb) Schematic of the network-like pathways of electron transport in fig. 3d-e is confusing; the electrical conductivity of graphene oxide is relatively poor. What role does graphene oxide play in the process of carrier separation? Furthermore, when asymmetric electrodes with significant potential barriers are employed, it is crucial to determine whether the photocurrent generation originates from the material itself or from the Schottky barrier at the electrode interface. Comparative experiments are necessary to clarify this mechanism.\n3) For applications in flexible electronic skin, comprehensive characterization of mechanical properties is essential. The authors merely conducted simple tests at different bending angles, which is far from sufficient. It is necessary to evaluate the maximum number of bending cycles at various angles and quantify the variance in photoresponsivity during bending. Additionally, for Fig. 4g, which demonstrates the integration of electronic skin onto a robotic hand, please provide the circuit design diagram and physical images of the data acquisition system used in this setup.\nOther suggestions:\n1) Several data points in the table in the supplementary information are incorrect. Moreover, the authors compared device parameters under several different light intensity ranges, yet data for their device are missing in multiple segments.\n2) It is surprising that the authors even misspelled the abbreviation of their commonly used material, copper phthalocyanine (CuPc)."}, "author_response": "Responses to Reviewer’ Comments\nReviewer #1 (Remarks to the Author):\nThe authors reported the fabrication of the photodetectors based on copper\nphthalocyanine/graphene oxide heterojunctions and investigated their photoelectrical\nproperties. They found that high photoresponsivity can be achieved by enhancing\nelectronic coupling between photoactive copper phthalocyanine molecules and adjacent\ngraphene oxide sheets through pressure. The further demonstrated dual-mode electronic\nskin application, operable under illumination ranging from daylight to moonlight. Their\nstudy is interesting and useful, supported by rigorous experimental validation,\ntheoretical modeling, and practical demonstrations. I recommend acceptance pending\nrevisions to s to address the following technical and conceptual aspects:\nWe sincerely appreciate the Reviewer’s recognition of the novelty, originality, and\nsignificance of our work, as well as the constructive suggestions. These suggestions\nhave deepened our mechanistic understanding and helped us to substantially improve\nthe work. We have conducted additional experiments/analyses and carefully revised the\nmanuscript accordingly.\n1. While the drop-casting method is simple, its reproducibility over larger areas and\nfeasibility for industrial-scale production should be discussed, along with potential\nchallenges in uniformity and yield.\nReply: We thank the Reviewer for this very helpful suggestion. In accordance with the\nReviewer’s suggestions, we designed cross-batch validation experiments spanning\nmultiple production batches. By fabricating a CuPc@GO film with area (3 × 3 cm²), 9\ntimes larger than the original film (1 × 1 cm²) and cutting it into 9 freestanding films,\nwe encapsulated them in a device with pressure of 240 kPa to fabricate CuPc@GO\nphotodetectors and test their performance. A total of three batches were prepared, with\neach batch taking ten days to complete, and all three batches were finished within one\nmonth. All tested CuPc@GO photodetectors demonstrate stabilized photoresponsivity\n(R) values around 1.32 × 10⁷ A/W with variations less than 4.1% (Figs. R1&R2). The\nhigh degree of consistency across different production batches not only demonstrated\nthe reproducibility but also served as a key indicator for assessing the processing\nscalability. The reproducibility experiments, together with ease and high-yield of the\nprocessing of CuPc@GO membranes, confirm the basis for scalable manufacturing and\nlarge-scale productions.\nfrom different batches (1-9 belong to Batch 1, 10-18 belong to Batch 2, and 19-27\nbelong to Batch 3). R denotes the photoresponsivity under pressure (240 kPa)\ntreatments. Coefficient of Variation (CV), expressed as a percentage, represents the\nratio of standard deviation to mean and indicates the degree of variation among different\nR.\n2\n3\nThere are nine samples in each batch.\nWe also fully recognize that progress towards practical applications necessitates\nscalability considerations for large-scale fabrication. At the material preparation level,\nboth CuPc and GO exhibit excellent solution processability. This implies that uniform\nCuPc@GO composite films can be continuously and efficiently fabricated on flexible\nor rigid substrates via low-cost roll-to-roll coating or large-area inkjet printing\ntechniques. This non-vacuum, low-temperature process offers significant advantages\nover traditional vacuum deposition techniques in terms of equipment investment and\nenergy consumption. Additionally, at the device integration level, the simple planar\ndevice structure we employ can be combined with established photolithography or\nscreen-printing electrode patterning processes. This enables the parallel fabrication of\nlarge-scale photodetector arrays, meeting the demands of imaging or sensing\napplications.\nChanges made:\nPlease kindly see Page 6, Lines 14-17 in revised manuscript: “The ultrahigh\nphotoresponsivity shows strong stability (Supplementary Figs. 6&7) and excellent\n5\nreproducibility among different production batches (Supplementary Fig. 8),\ninsensitive to ambient humidity while sensitive to the oxidation degree of GO\n(Supplementary Figs. 10&11).”\nPage 13, Lines 17-23 in revised manuscript: “Moreover, the strategy to obtain an\natomic-scale sandwich with the drop-casting method together with a macroscopic\nmanipulation to greatly optimize the heterojunction (i.e., an external pressure)\nensures its reproducibility over larger areas and feasibility for industrial-scale\nproduction, while also offering potential for the improvement of uniformity and yield.\nThis method is also expected to be a substantial step toward fabricating other\nultrahigh-performance optoelectronic devices.”\nFig. S8 and its caption in supplementary materials: “Supplementary Fig. 8 |\nPhotoresponsivity of the CuPc@GO photodetector for the samples from different\nbatches (1-9 belong to Batch 1, 10-18 belong to Batch 2, 19-27 belong to Batch 3). R\ndenote the photoresponsivity under pressure (240 kPa). Coefficient of Variation (CV),\nexpressed as a percentage, represents the ratio of standard deviation to mean and\nindicates the degree of variation among different R.”\n2. Address the long-term stability of the pressure-optimized heterojunctions under\ncyclic mechanical stress (beyond Supplementary Fig. 6).\nReply: We thank the Reviewer for the suggestions. In practical applications, we\nencapsulated the CuPc@GO composite film within a device maintained at pressure of\n~240 kPa to fabricate the CuPc@GO photodetector. No extra pressure was applied to\nthe photodetector in use. According to the Reviewer’s suggestion, we conducted\nvarious cyclic mechanical stress experiments. To test the long-term stability of the\nCuPc@GO photodetector, we performed systematic experiments by applying a\ncyclically pressure of 240 kPa. The operational stability of CuPc@GO photodetectors\nwas investigated through cyclically alternating between two conditions: i) pressurized\n(240 kPa) and illuminated (3 nW) simultaneously; ii) non-pressurized and in the dark\nenvironment. Following 1500 complete compression-release cycles with synchronized\nillumination modulation, the device exhibited remarkable photocurrent retention (＞\n95.0%), demonstrating exceptional stability in both photoresponsive behaviour and\nstructural integrity under external pressure.\nbrown squares indicate the magnified areas around 500 cycles, 1000 cycles and 1500\ncycles, respectively. Insets are enlarged plots corresponding to the three regions\nenclosed by the brown square.\nChanges made:\nPlease kindly see Page 6, Lines 14-17 in revised manuscript: “The ultrahigh\nphotoresponsivity shows strong stability (Supplementary Figs. 6&7) and excellent\nreproducibility among different production batches (Supplementary Fig. 8), insensitive\nto ambient humidity while sensitive to the oxidation degree of GO (Supplementary Figs.\n10&11).”\nFig. S7 and its caption in supplementary materials: “Supplementary Fig. 7 | Long-\nterm cycling stability diagram of the CuPc@GO photodetector under conditions of i)\nwith pressure (240 kPa) and illumination (3 nW), and ii) without pressure and\nillumination. The brown squares indicate the magnified areas around 500 cycles, 1000\ncycles and 1500 cycles, respectively. Insets are enlarged plots corresponding to the\nthree regions enclosed by the brown square.”\n3.Elaborate on how this strategy (pressure-optimized heterojunctions) could be\ngeneralized to other optoelectronic systems (e.g., perovskites or 2D materials).\nReply: We thank the Reviewer for the helpful suggestion. The proposed strategy can be\ngeneralized to other optoelectronic systems, but the efficacy will be largely reduced\nwithout an extra design as in the present work. We note that the key of high pressure-\ninduced performance is the unique properties of the CuPc@GO membrane: i) The GO\nsheet’s excellent flexibility facilitates local configuration under mechanical stimulus,\nenabling a better fit with the small CuPc molecule and numerous inter-layer interactions\nvia π‒π stacking. ii) Both the GO sheet and the sandwiched CuPc molecule have an\n7\natomically planar structure, thus facilitating the stress transfer across the heterojunction\ninterface when external pressure is applied.\nIn order to demonstrate the uniqueness of the CuPc@GO system, we performed new\nexperiments of the perovskite photodetector (with the help of Prof. Wenjun Wu in\nSchool of Chemistry and Molecular Engineering, East China University of Science and\nTechnology) based on all-inorganic perovskite (CsPbI Br) following the Reviewer’s\n2\nsuggestion. The perovskite photodetector was fabricated via the following procedure.\nFirst, a TiO₂ paste was printed onto the FTO conductive glass using screen-printing\ntechnology, leveled for 30 minutes, and heated at 80°C for 10 minutes to remove most\nof the terpineol. The film was subsequently sintered at 500°C for 30 minutes to obtain\na mesoporous TiO₂ (m-TiO₂) layer. A 100 μL aliquot of the CsPbI₂Br precursor solution\nwas dropped on the FTO/m-TiO₂ substrate and spin-coated at 1000 rpm for 10 seconds\nand 3000 rpm for 30 seconds. Starting at the 5th second of spin-coating, 100°C hot air\nwas blown continuously toward the substrate until the film turned brownish. The\nsubstrate was then pre-annealed on a hotplate at 50°C for 1.5 minute, followed by\nannealing at 270°C for 20 minutes to form the CsPbI₂Br film. Finally, the device was\nencapsulated with a packaging methodology identical to the CuPc@GO counterparts.\nThe photoresponsivity (R) showed a progressive enhancement with increasing pressure\n(P): Relative to the non-pressurized case, R increased 2.2-fold for P = 240 kPa and\n2.6-fold for P = 500 kPa. The perovskite crystal structure was found to collapse under\nhigher external pressure (Fig. R4).\nIt can be concluded that photoelectronic devices with rigid or non-planar active\nmaterials can exhibit reduced efficacy under pressure-optimizing strategy, as\ndemonstrated in our perovskite-based photodetector.\nFig. R4. Photoresponse of the perovskite (CsPbI Br) photodetector. (A)\ndifferent pressures (0, 240, 300, 400 and 500 kPa). (B) Responsivity (R) of the\nperovskite photodetector as a function of the applied pressure. The inset represents a\nschematic of the device structure.\n8\n4.Discuss potential limitations (e.g., sensitivity to (1) humidity or (2) oxidation of GO)\nand mitigation strategies.\nReply: We thank the Reviewer for the helpful suggestion. Following the suggestion, we\nsystematically evaluated the photoresponse characteristics of CuPc@GO\nphotodetectors under varying humidity conditions, together with the influence of\ngraphene oxide (GO) oxygen content. The CuPc@GO photodetector is insensitive to\nthe ambient humidity. Additionally, the reduction of oxidation of GO will slightly\nreduce the performance of the photodetector.\nThe characterization of CuPc@GO photodetectors under varying humidity conditions\nis demonstrate firstly. Multiple CuPc@GO photodetectors were exposed to\nenvironments with controlled relative humidities of 11.3%, 43.1%, 75.7%, and\n96.0 % for up to 12 hours (the saturated salt solution method standard is based on\nrelevant recommendations of the International Organization of Legal Metrology,\nensuring the accuracy and consistency of humidity measurement.), following which the\nchanges in their responsivity (R) values were quantitatively measured. As illustrated in\nFig. R5, the R of the photodetector remained remarkably stable at approximately 1.34\n× 10⁷ A/W with variations below 3.4% across all tested humidity levels. This finding\nconfirms that the CuPc@GO photodetector possesses excellent sealing performance,\nwhich effectively inhibits the penetration of water molecules and their potential\ninterference with the internal microstructure of the active film. Such superior sealing\ncapability endows the photodetectors with promising applicability for wearable\napplications, as they can maintain reliable performance.\nFig. R5. Photoresponsivity (R) of the CuPc@GO photodetector under different\nhumidity conditions. The relative humidity was controlled using different saturated\nsalt solutions. Saturated solutions of LiCl, K₂CO₃, NaCl, and KNO₃ generate stable\nrelative humidity levels of 11.3 %, 43.1 %, 75.7 %, and 96.0 %, respectively, at 10°C.\nThe CuPc@GO photodetector was placed under each humidity condition for 12 hours\n9\nbefore photoresponse evaluation.\nThe influence of oxygen content in the graphene oxide (GO) on device performance is\nthen investigated. Here, we prepared a series of CuPc@GO films with varying\nreduction degrees. The existing method for preparing GO employed an improved\nHummer's method [J. Am. Chem. Soc. 80, 1339 (1958)], upon which a temperature\ngradient process is subsequently applied to produce GO with lower oxygen content.\nThis was achieved by subjecting the films to thermal reduction at different temperatures\n(90°C, 110°C, 130°C, 150°C, 170°C, and 190°C) for 6 hours, where a higher\ntemperature corresponds to a greater extent of reduction and thus a lower oxygen\ncontent. A film only dried at 70°C served as the non-reduced reference. After the\nreduction process, all samples were encapsulated following the original method to\nensure consistency in device structure. Subsequently, the photoresponse characteristics\nof the fabricated devices were measured under identical test conditions, and the oxygen\ncontent of GO in each CuPc@GO film was quantitatively calibrated via XPS analysis.\nFig. R6. Photoresponsivity (R) of the CuPc@GO photodetector under different\noxygen contents. As the oxygen content decreases, (A) the dark current and the\nphotocurrent increase, thus leading to (B) an overall decline in photoresponsivity.\nThe experimental results (Fig. R6) show that, as the thermal reduction temperature\nincreases from 70°C to 190°C (the original structure is compromised when the\ntemperature exceeds 190°C), the oxygen content of GO gradually decreases from 31.0%\nto 6.7%, and both dark current and photocurrent increase. As the oxygen content\nreduces, the interconnection of carbon network is restored and the charge carriers\nbecome delocalized, thus increasing electric conductivity and current of the device. The\ndark current increases more quickly, resulting a slightly decrease in responsivity from\n1.34 × 10⁷ A/W to 6.25 × 10⁶ A/W. We are grateful for the Reviewer's suggestion on\nmitigation strategies, which guided us in identifying an appropriate operating\ntemperature range for the CuPc@GO photodetector.\nChanges made:\nPlease kindly see Page 6, Lines 14-17 in revised manuscript: “The ultrahigh\n10\nphotoresponsivity shows strong stability (Supplementary Figs. 6&7) and excellent\nreproducibility among different production batches (Supplementary Fig. 8), insensitive\nto ambient humidity while sensitive to the oxidation degree of GO (Supplementary\nFigs. 10&11).”\nFig. S10 and its caption in supplementary materials: “Supplementary Fig. 10\n|Photoresponsivity of the CuPc@GO photodetector under different humidity conditions.\nThe relative humidity was controlled using different saturated salt solutions. Saturated\nsolutions of LiCl, K₂CO₃, NaCl, and KNO₃ generate stable relative humidity levels of\n11.3 %, 43.1%, 75.7 %, and 96.0 %, respectively, at 10°C. The CuPc@GO\nphotodetector was placed under each humidity condition for 12 hours before\nphotoresponse evaluation.”\nFig. S11 and its caption in supplementary materials: “Supplementary Fig. 11 |\nPhotoresponsivity of the CuPc@GO photodetector under different oxygen contents. As\nthe oxygen content decreases, (a) the dark current and the photocurrent increase, thus\nleading to (b) an overall decline in photoresponsivity.”\n5.Clarify whether the 100 pW detection limit (Fig. 1f) requires cryogenic conditions or\nis achievable at room temperature.\nReply: All experiments, including the “100 pW detection” experiment, were performed\nat room temperature. The “room-temperature operation” of our detector has been\nexplicitly stated in the revised manuscript.\nChanges made:\nPlease kindly see Page 6, Lines 6-9 in revised manuscript where we have added\n“at room temperature”: “The CuPc@GO detector achieves a lower detection limit of\nonly 100 pW (Fig. 1g) for illumination power at room temperature without requiring\nan additional photomultiplier, which reaches the upper limit of the working range for\nsingle-photon detectors.”\n6. Given graphene’s inherent high conductivity, additional experimental or theoretical\nevidence quantifying the relative contributions of intra-sheet versus inter-sheet (GO‒\nCuPC‒GO pathway) electron transport would further solidify the proposed mechanism.\nReply: In the theoretical calculation of our original manuscript, we use pristine\ngraphene, instead of the GO, to illustrate the key physical mechanism underlying the\nexperimental observation. Considering the considerable difference in the electric\nconductivity between graphene and GO, in the revised manuscript, we have conducted\nnew DFT calculations based on GO to achieve closer agreement with the experiment.\nThe conclusion that pressure-induced enhancement of inter-sheet charge transport\nremains valid when replacing pristine graphene with GO.\nFigure R7 shows that when optimization (pressure) is applied, the inter-sheet electric\nconductivity is enhanced by several orders of magnitude, while the intra-sheet\n11\nconductivity changes negligibly. The inter-sheet conductivity is still lower than the\nintra-sheet conductivity even after the optimization, indicating that the inter-sheet\ntransport still represents the main bottleneck limiting charge transport in the overall\nGO‒CuPc‒GO pathway.\nWe computed the electrical conductivity σ by using the Boltzmann transport equation\nwithin the constant relaxation time approximation. Consequently, the direct output of\nour calculation is the ratio σ/τ , which represents the ideal conductivity governed solely\n0\nby the material band structure. It is difficult for DFT calculations to evaluate the real\nvalues of the relaxation time τ , which encompasses all scattering processes (e.g.,\n0\nelectron-phonon, impurity, and defect scattering). Additionally, it is also challenging to\nperform thorough investigations of excited electron transport by means of time-\ndependent density function theory combined with non-adiabatic molecular dynamics.\nGiven these limitations, the ground-state DFT calculations employed in our study\nrepresent a balanced alternative for describing charge transport properties.\nFig. R7. Intra-sheet (A) and inter-sheet (B) electric conductivities obtained by DFT\ncalculations.\nWe also carefully re-examine our experimental data of femtosecond transient\nabsorption (TA) spectra measurements. It is noted that, for the time constants from\nmulti-exponential fitting of the TA data, 𝜏 and 𝜏 are sensitive to the optimization of\n1 2\nthe membrane. As such, according to the theoretical analysis above, 𝜏 and 𝜏 both\n1 2\nhave a contribution from inter-sheet electron transport. While 𝜏 remains almost\n3\nindependent of the optimization, it is difficult to attribute 𝜏 to purely intra-sheet\n3\nelectron transport because of its much larger value compared to 𝜏 and 𝜏 . As a matter\n1 2\nof fact, all the electron processes observed from the TA measurements have both inter-\nsheet and intra-sheet contributions, which form the “net-like” electron transport\nchannels in the membrane.\nChanges made:\nPlease kindly see Page 8, line 18- Page 9, line 2 in the revised manuscript where\nwe have rewritten: “To elucidate the underlying mechanism, we performed density\n12\nfunctional theory (DFT) calculations on the unit cell of the composite membrane … The\ninter-sheet conductivity is still lower than the intra-sheet conductivity even after the\noptimization, indicating that the inter-sheet transport still represents the main\nbottleneck limiting charge transport in the overall GO‒CuPc‒GO pathway.”\nFig. S15 and its caption in supplementary materials: “Supplementary Fig. 15 |\nIntra-sheet (a) and inter-sheet (b) electric conductivities obtained by DFT calculations.”\nWe sincerely thank the Reviewer for the very helpful suggestions and very constructive\ncomments, which help us to make a more efficient and pleasant manuscript. According\nthe Reviewer’s comments, we have revised the manuscript. We think that all\nambiguities in the manuscript have been clarified. We hope that the Reviewer will be\nsatisfied with our response and will now recommend its publication in Nature\nCommunication.\nThe authors proposed a photodetector based on the CuPC/GO heterojunction using the\nvertical device structure. The device demonstrated significant responsivity\nenhancement after applying external forces. This work lacks sufficient innovation and\nthe conclusions need additional data for support. I do not think it is suitable for\npublication in Nature Communications.\nWe sincerely appreciate the Reviewer’s valuable, constructive feedbacks, which have\nsignificantly strengthened this work. We have carefully addressed all points the\nReviewer raised in the response below and the revised manuscript, and hope that our\nresponses and manuscript refinements have resolved all the concerns.\n1. The main novelty of this paper is the use of external pressure to enhance device\nperformance. However, (1) applying pressure to improve the contact and reduce the\ninterlayer distance in layered materials is a known and expected phenomenon.\nFurthermore, (2) this work did not address the key concerns regarding the\nreproducibility of the method and the device-to-device variation under pressure. (3) The\nauthors show that the performance of one device is enhanced by more than 10^5 times\nwhen a pressure of 240 kPa is applied, but they don't provide data to show whether\nother devices would require the same pressure to achieve a similar performance\nimprovement and (4) whether this improvement could be controlled or repeated under\nmultiple pressure-release cycles. The absence of these data makes the approach for\nperformance enhancement is not a fundamentally new and reliable principle. I think the\nnovelty of using external pressure is not solid.\nReply: We sincerely thank the Reviewer for this comment, which allows us to elaborate\non the novelty and key point of our work more clearly in the revised manuscript. We\nagree that “applying pressure to improve the contact and reduce the interlayer distance\nis a known and expected phenomenon”. However, we would like to clarify that the key\nin this work lies in the synergy between the unique properties of the CuPc@GO\nmembrane in the photodetector we fabricated and the external pressure to greatly\noptimize the heterojunction, rather than merely applying external pressure.\nConsequently, a huge improvement in the photoresponsivity (105 to 106-fold) has been\nachieved with only a modest external pressure (~1 atm). Below we will show that the\noptimization of interlayer distance through external pressure also works effectively for\nthe perovskite photodetector, but the enhancement rate in the photoresponsivity is only\n2.6.\nThe unique properties of the CuPc@GO membrane lie in two folds. i) The GO sheet\nhas excellent flexibility, making the local configuration easy in response to the\nmechanical stimulus. ii) Both the GO sheet and the sandwiched CuPc molecule have\nan atomically planar structure, thus facilitating stress transfer across the heterojunction\ninterface when external pressure is applied. This response enables a better fit with the\n14\nsmall CuPc molecule under mechanical stimulus, including the establishment of\nnumerous inter-layer interactions via π‒π stacking, as indicated in Figs. 3d,e in the\nmanuscript. Consequently, once optimized by pressure, the “compact” CuPc‒GO\nheterojunction is formed and sufficient vertical GO‒CuPc‒GO transport channels are\nwell established, leading to hyper-efficient transport of free charge carriers and thus the\nultrahigh photoresponsivity.\nFig. 3 d, e, in the manuscript. Schematic of the network-like pathways of charge\ntransport in the optimized (d) and unoptimized (e) CuPc@GO membranes. τ , τ are the\n1 2\ncharacteristic times associated with the charge-carrier transfer from the CuPc molecule\nto the GO sheet, and the intra-sheet and CuPc-bridging inter-sheet charge transport,\nrespectively.\nFor other optoelectronic devices with rigid and/or non-planar and/or homogenous bulk\narchitectures, the pressure-optimizing strategy may also work to some extent, but the\nefficacy will be largely reduced. As an example, we performed additional experiment\nby using the all-inorganic perovskite (CsPbI Br) as the optoelectronic material. A\n2\nhigher pressure (~500 kPa) is required, yielding only a 2.6-fold improvement in\nphotoresponsivity. For an even higher external pressure, the crystal structure of\nperovskite would be destroyed (Fig. R1).\nFig. R1. Photoresponse of the perovskite (CsPbI Br) photodetector. (A)\n2\npressures (0, 240, 300, 400 and 500 kPa). (B) Responsivity (R) of the perovskite\nphotodetector as a function of applied pressure and its schematic device structure.\nThe perovskite photodetector was fabricated (with the help of Prof. Wenjun Wu in\nSchool of Chemistry and Molecular Engineering, East China University of Science and\nTechnology) based on all-inorganic perovskite (CsPbI Br) following the Reviewer’s\n2\nsuggestion. The perovskite photodetector was fabricated via the following procedure.\nFirst, a TiO₂ paste was printed onto the FTO conductive glass using screen-printing\ntechnology, leveled for 30 minutes, and heated at 80°C for 10 minutes to remove most\nof the terpineol. The film was subsequently sintered at 500°C for 30 minutes to obtain\na mesoporous TiO₂ (m-TiO₂) layer. A 100 μL aliquot of the CsPbI₂Br precursor solution\nwas dropped on the FTO/m-TiO₂ substrate and spin-coated at 1000 rpm for 10 seconds\nand 3000 rpm for 30 seconds. Starting at the 5th second of spin-coating, 100°C hot air\nwas blown continuously toward the substrate until the film turned brownish. The\nsubstrate was then pre-annealed on a hotplate at 50°C for 1.5 minute, followed by\nannealing at 270°C for 20 minutes to form the CsPbI₂Br film. Finally, the device was\nencapsulated with a packaging methodology identical to the CuPc@GO counterparts.\n(2) Following the Reviewer’s suggestions referring to the reproducibility and the\ndevice-to-device variation under pressure, we have performed systematic\nreproducibility assessments on CuPc@GO photodetectors fabricated across multiple\nproduction batches. Through randomized sampling of twenty-seven independently-\nprepared devices (in three different batches), we quantitatively evaluated the pressure-\ndependent evolution of photoresponsivity (R) (Fig. R2). All specimens demonstrated\nremarkable consistency in baseline photoresponsivity (R ) prior to pressure application.\nR values stabilized near 1.32 × 10⁷ A/W for all tested devices.\nThe observed response uniformity under applied pressure substantiates exceptional\ndevice-to-device reproducibility and high-quality film assembly characteristics.\nMoreover, batch-independent performance alignment reveals remarkable process\nrobustness, as the photoresponsivity maintains below 4.1% variance between different\nproduction lots.\nfrom different batches (1-9 belong to Batch 1, 10-18 belong to Batch 2, 19-27\nbelong to Batch 3). R denotes the photoresponsivity under pressure (240 kPa)\ntreatments. Coefficient of Variation (CV), expressed as a percentage, represents the\nratio of standard deviation to mean and indicates the degree of variation among different\nR.\n17\n18\nunder external pressure. There are nine samples in each batch.\nChanges made:\nPlease kindly see Page 6, Lines 14-17 in revised manuscript: “The ultrahigh\nphotoresponsivity shows strong stability (Supplementary Figs. 6&7) and excellent\nreproducibility among different production batches (Supplementary Fig. 8),\ninsensitive to ambient humidity while sensitive to the oxidation degree of GO\n(Supplementary Figs. 10&11)”\nFig. S8 and its caption in supplementary materials: “Supplementary Fig. 8 |\nPhotoresponsivity of the CuPc@GO photodetector for the samples from different\nbatches (1-9 belong to Batch 1, 10-18 belong to Batch 2, 19-27 belong to Batch 3). R\ndenote the photoresponsivity under pressure (240 kPa). Coefficient of Variation (CV),\nexpressed as a percentage, represents the ratio of standard deviation to mean and\nindicates the degree of variation among different R.”\n(3) On the comment “when a pressure of 240 kPa is applied, but they don't provide data\nto show whether other devices would require the same pressure to achieve a similar\nperformance improvement”. We take perovskite-based photodetectors as a\n20\nrepresentative alternative optoelectronic system. These photodetectors were fabricated\nemploying a packaging methodology identical to the CuPc@GO counterparts and\nsubjected to an external pressure. As the pressure increased up to 500 kPa, a value far\nexceeding 240 kPa, photoresponsivity (R) increased monotonically, achieving a 2.6-\nfold improvement relative to the non-pressurized devices. For an even higher external\npressure, the crystal structure of perovskite would be destroyed (Fig. R1). Consequently,\nphotodetectors with different components require different pressures to achieve their\nbest performances.\n(4) On the comment \"whether this improvement could be controlled or repeated under\nmultiple pressure-release cycles”. In practical applications, we encapsulated the\nCuPc@GO composite film within a device maintained at pressure of 240 kPa to\nfabricate the CuPc@GO photodetector. No extra pressure was applied to the\nphotodetector in use. According to the Reviewer’s suggestion, we take the typical\npressure of 240 kPa as an example to discuss whether this improvement could be\ncontrolled or replicated under multiple pressure-release cycles. The operational stability\nof CuPc@GO photodetector was systematically investigated through continuous\noperational cycling under controlled stimuli: alternating between pressurized (240 kPa)\nilluminated condition (3 nW) and non-pressurized dark condition (Fig. R4). Following\n1500 complete compression-release cycles with synchronized illumination modulation,\nthe devices exhibited remarkable photocurrent retention (＞ 95.0%), demonstrating\nexceptional stability in both photoresponsive behavior and structural integrity. This\ncyclic durability establishes CuPc@GO photodetector as robust candidate for flexible\noptoelectronic systems requiring repeated mechanical deformation.\nFig. R4. Long-term cycling stability diagram of the CuPc@GO photodetector. The\nbrown squares indicate the magnified areas around 500 cycles, 1000 cycles, and 1500\ncycles, respectively. Insets denote the enlarged plots corresponding to the three regions\nenclosed by the brown square.\nPlease kindly see Page 6, Lines 14-17 in revised manuscript: “The ultrahigh\nphotoresponsivity shows strong stability (Supplementary Figs. 6&7) and excellent\nreproducibility among different production batches (Supplementary Fig. 8), insensitive\nto ambient humidity while sensitive to the oxidation degree of GO (Supplementary Figs.\n10&11)”\nFig. S7 and its caption in supplementary materials: “Supplementary Fig. 7 | Long-\nterm cycling stability diagram of the CuPc@GO photodetector under conditions of i)\nwith pressure (240 kPa) and illumination (3 nW), and ii) without pressure and\nillumination. The brown squares indicate the magnified areas around 500 cycles, 1000\ncycles and 1500 cycles, respectively. Insets are enlarged plots corresponding to the\nthree regions enclosed by the brown square.”\n2. Some basic and crucial performance characterizations are missing. For example,\nthere are no results about the (3) detectivity, (1) LDR, or (2) frequency response of the\ndevice, which are essential for evaluating a photodetector's performance. As a flexible\ndevice, (4) the mechanical stability, including performance under repeated bending and\ncomplex strain, is not fully explored. (5) Furthermore, the authors claim that the\npiezoelectricity of the CuPC@GO membrane is the reason for the abrupt current\nincrease during contact sensing in Fig. 4c, however, there is no characterization about\nthe piezoelectricity in this work. And I did not find any works reporting about the\npiezoelectricity of the CuPc and GO.\nReply：(1) We have conducted experiments to obtain the LDR, frequency response,\nand detectivity (D*) of the CuPc@GO photodetector.\nFig. R5 shows the photocurrent I as a function of the incident power. The I can be\nph ph\nwell fitted by an exponential function of the incident power E , i.e., I = 0.45 E 0.019.\nin ph in\nThis relationship does not show linear behaviour like that of a silicon-based\nphotodetector. It is widely known that I ∝ E α (an equivalent expression is R∝ E -(1-\nph in in\nα)) where E is the incident power and 0 < α ≤ 1. The larger the deviation of α from\nin\nunity, the stronger the nonlinear effect of the photodetector is. The power exponent α is\nequal to 1 for ideal photodetector and can be very close to 1 for silicon photodiodes and\nsome 3D organic photodiodes [Science 370, 698 (2020)]. However, a nonunity\nexponent α is often found in low dimensional photodetectors as a result of the complex\nprocess of carrier generation, trapping, and recombination within semiconductors [Adv.\nSci. 4, 1700323 (2017)]. In literature, it has been reported that a 2D photodetector based\non based\non black-phosphorus exhibits an α close to zero (α ≈ 0.1) and high responsibility [Adv.\nMater. 28, 3481 (2016)].\nph\nCuPc@GO photodetector (under 635 nm illumination and pressure of 240 kPa)\nwith power ranging from 3 nW to 100 mW. I = I - I ,where I denotes the measured\nph p 0,p p\nphotocurrent, I is the dark current obtained without illumination.\n0,p\nThe photocurrent as a function of power shows linear behaviour in a wide range when\nplotted on the double logarithmic coordinates (note that the y axis is also logarithmic,\nrather than linear). As those in the references [Nat. Commun. 14, 5352 (2023) and Adv.\nFunct. Mater. 35, 2418614 (2025)], we may obtain a linear dynamic range (LDR) based\non the equation:\n𝐿𝐷𝑅 = 20log\n𝐸max,\n𝐸min\nwhere 𝐸 and 𝐸 represent the maximum and minimum illumination power within\nmax min\nthe linear range, respectively. E = 50 mW, and E = 3 nW, and the value of LDR is\nmax min\n138.4 dB.\nChanges made:\nPlease kindly see Page 5, Lines 32-34 in revised manuscript: where we have added\n“We also conducted experiments and calculated the linear dynamic range (LDR) and\n(fc), which are 138.4 dB and 5.6 kHz respectively (Supplementary Figs. 3&4).”\nFig. S3 and its caption in supplementary materials: “Supplementary Fig. 3 |\nPhotogenerated current as a function of incident power of the CuPc@GO\nphotodetector (under 635 nm illumination and pressure of 240 kPa) with power ranging\nfrom 3 nW to 100 mW.”\n(2) Different frequencies ranging from 1 Hz to 7.5 kHz under an illumination power of\n3 nW were applied to the CuPc@GO photodetector to obtain the detection bandwidth\n(fc). The light was generated by a tuneable laser and modulated in frequency using a\nsignal generator. The photocurrent of the photodetector as a function of time was\nmeasured using a Keithley 2461 source meter (Tektronix, USA) following rigorous\nexperimental procedures. Fig. R6 shows the normalized responsivity with respect to the\nlaser frequency. The −3 dB cutoff frequency was determined to be 5.6 kHz, which\n23\nsignificantly exceeds typical light source frequencies used in practical applications.\nFig. R6. Frequency response of the CuPc@GO photodetector with respect to the\nlaser frequency. The cutoff frequency is determined at the −3 dB point, where the\nphotogenerated current obtained under 1 Hz laser irradiation is set to 1. The\nphotodetector was tested under 240 kPa and 3 nW.\nChanges made:\nPlease kindly see Page 5, Lines 32-34 in revised manuscript: where we have added\n“We also conducted experiments and calculated the linear dynamic range (LDR) and\n(fc), which are 138.4 dB and 5.6 kHz respectively (Supplementary Figs. 3&4).”\nFig. S4 and its caption in supplementary materials: “Supplementary Fig. 4 |\nFrequency response of the CuPc@GO photodetector with respect to the laser frequency.\nThe cutoff frequency is determined at the −3 dB point, where the photogenerated\ncurrent obtained under 1 Hz laser irradiation is set to 1. The photodetector was tested\nunder 240 kPa and 3 nW.”\n(3) Specific detectivity (D*) measures the quality of a photodetector's ability to detect\nweak light signals. The D* of CuPc@GO photodetector, defined by the formula 𝐷∗ =\n√𝐴𝑑𝛥𝑓𝑅\n, was calculated as 9.71 × 1013 Jones under 3 nW illumination (Fig. R7). Here,\n𝑖𝑛\n𝐴 denotes the active area of photodetector, 𝛥𝑓 the measurement bandwidth defined as\nd\nthe difference between the lowest frequency obtained (10 Hz) and the detection\nbandwidth (𝑓𝑐 = 5.6 kHz ) (Fig. R8), 𝑅 the current responsivity, and 𝑖 the noise\nn\ncurrent corresponding to 𝛥𝑓.\nincident optical power.\nChanges made:\nPlease kindly see Page 5, Lines 26-29 in revised manuscript: “Further, we also\ncharacterized the comprehensive photodetection performance of the photodetector, and\nthe specific detectivity (D*) of the CuPc@GO photodetector is a function of incident\noptical power, among which it can be 9.71 × 1013 Jones under 3 nW (Fig. 1f).”\nFig. 1f and its caption in revised manuscript: “Fig. 1f |Specific detectivity (D*) of\nthe CuPc@GO photodetector as a function of incident optical power.”\nThe noise-equivalent power (NEP) is a fundamental figure of merit for a photodetector,\ndefined as the incident optical power required to generate a signal equal to the inherent\nnoise of photodetector. The NEP of the CuPc@GO photodetector was calculated to be\n7.71 × 10-13 W under 3 nW illumination. This value was determined using the standard\nNEP formula for a photodetector characterized by its current responsivity: 𝑁𝐸𝑃 =\n𝑖𝑛,\n𝑅\nwhere 𝑖 denotes the noise current and 𝑅 the current responsivity (Fig. R2). We note\nn\nthat i is obtained corresponding to the measurement bandwith Δf, which is defined as\nn\nthe difference between the lowest frequency obtained (10 Hz) and the detection\nbandwidth (𝑓𝑐 = 5.6 kHz).\nphotodetector under dark conditions with 240 kPa.\nChanges made:\nPlease kindly see Page 5, Lines 29-32 in revised manuscript: where we have added:\n“This D* value is closely correlated with the ultra-low noise-equivalent power (NEP)\nof 7.71 × 10⁻¹³ W under 3 nW illumination, as the two key figures of merit are inherently\nlinked through the noise current and R (Supplementary Fig. 2).”\nFig. S2 and its caption in supplementary materials: “Supplementary Fig. 2\n|Frequency dependent noise power density of the CuPc@GO photodetector under dark\nconditions with 240 kPa.”\n(4) According to the Reviewer’s suggestion, we explored the effect of bending/twisting\non the performance of the CuPc@GO photodetector. During experiment, the\nencapsulated flexible CuPc@GO photodetectors were subjected to over 1500\nbending/twisting cycles. The photoresponse of the device was measured under a\npressure of 240 kPa after they were subjected to different numbers of bending/twisting\ncycles. The effect of bending angle was assessed at 30°, 60°, and 90°. As a result, the\ndevice maintained 95.0% of its photoresponsivity after over 6000, 2000 and 1000\nbending cycles in the 30°, 60° and 90° angle test, respectively (Fig. R9). Another\nverification was conducted where the CuPc@GO photodetector was subjected to the\ncyclic twist with a fixed twist angle of 30° (Fig. R10). The responsivity of the\nphotodetector retains 95.2% of its initial value after 1300 consecutive torsion cycles,\ndemonstrating excellent stability against repeated torsional deformations. In the\nmeasurements, both bending and twisting were applied by using an FT2000 flexible\nelectronic tester (from Shanghai Mifang Electronic Technology Co., Ltd).\nbending loadings. (A‒C), (D‒F), (G‒I) corresponds to the photodetector at bending\nangles of 30°, 60°, and 90°, respectively. After bending CuPc@GO photodetector by 30°\nfor 6500 times, the performance was retained at 95.2% of the original (non-bending)\nvalue. After 2200 cycles of bending at 60°, the performance was retained at 95.3%.\nAfter bending CuPc@GO photodetector by 90° for 1000 times, the performance was\nretained at 95.7%. The applied external pressure for all cases is 240 kPa.\nFig. R10. Durability evaluation of the CuPc@GO photodetector after twisting\ncyclic torsional rotations. After twisting CuPc@GO photodetector by 30° for 1300\ntimes, the performance was retained at 95.2% under 240 kPa.\nThese systematic mechanical stability characterizations confirm that the CuPc@GO\nflexible photodetector exhibits outstanding resistance to both torsion and bending\ndeformations. The minimal degradation in photoresponsivity after repeated mechanical\n27\ncycling verifies the structural integrity of the CuPc@GO film, the reliability of the\nphotodetector's electrode/film interface, and the robustness of the overall photodetector\narchitecture.\nChanges made:\nPlease kindly see Page 10, Lines 18-20 in revised manuscript: “The CuPc@GO\nphotodetector has excellent flexibility, with ultrahigh photoresponsivity remaining at\nvarious bending/twisting angles (Fig. 4a and Supplementary Fig. 16&17)”\nFig. S16 and its caption in supplementary materials: “Supplementary Fig. 16 |\nDurability evaluation of the CuPc@GO photodetector under bending loading. (a‒c),\n(d‒f), (g‒i) corresponds to the photodetector at bending angles of 30°, 60°, and 90°,\nrespectively. After bending CuPc@GO photodetector by 30° for 6500 times, the\nperformance was retained at 95.2% of the original (non-bending) value. After 2200\ncycles of bending at 60°the performance was retained at 95.3%. After bending\nCuPc@GO photodetector by 90° for 1000 times, the performance was retained at\n95.7%. The applied external pressure for all cases is 240 kPa.”\nFig. S17 and its caption in supplementary materials: “Supplementary Fig. 17 |\nDurability evaluation of the CuPc@GO photodetector under twisting strain. After\ntwisting CuPc@GO photodetector by 30° for 1300 times, the performance was retained\nat 95.2% under 240 kPa.”\n(5) On the characterization about the piezoelectricity in this work, a periodic and step-\nlike response of the output voltage can be clearly observed, which is well correlated\nwith the applied pressure (see Fig. R11). The output voltage exhibits a positive\ncorrelation with external pressure. Under constant pressure, it remains stable.\nFor details, please refer to the article [Int. J. Mol. Sci. 25, 12 (2024)]. This indicates that the\nCuPC@GO membrane has piezoelectric properties, which is the reason for the sudden\nincrease in current during the contact sensing process shown in Figure 4c.\nFig. R11. Output voltages of the CuPc@GO device under different pressures.\n3. In the DFT calculation, the authors used pristine graphene instead of GO due to that\nGO sheets have \"large unoxidized regions\" and \"remarkable π-π stacking interactions\".\nHowever, this is a significant simplification that ignores the effects of oxygen-\ncontaining functional groups in GO, which could act as charge trapping sites. The\nauthors should provide a clear explanation for the use of pristine graphene.\ngraphene instead of the GO to illustrate the key physical mechanism underlying the\nexperimental observation. Given the significant difference in electrical conductivity\nbetween graphene and GO, we have performed new DFT calculations on GO to\nestablish a more accurate correlation with the observed data. The conclusion that\npressure-induced optimization enhances inter-sheet charge transport still holds true\nwhen GO is used instead of pristine graphene.\nChanges made:\nPlease kindly see Page 8, line 18-Page 9, line 2 in the revised manuscript: “To\nelucidate the underlying mechanism, we performed density functional theory (DFT)\ncalculations on the unit cell of the composite membrane … The inter-sheet conductivity\nis still lower than the intra-sheet conductivity even after the optimization, indicating\nthat the inter-sheet transport still represents the main bottleneck limiting charge\ntransport in the overall GO‒CuPc‒GO pathway.”\nFig. S15 and its caption in supplementary materials: “Supplementary Fig. 15 |\nIntra-sheet (a) and inter-sheet (b) electric conductivities obtained by DFT calculations.”\n4. The authors claimed that the photodetector outperforms all current state-of-the-art\ndevices under illumination powers from 3 nW to 10 mW. However, some published\nworks have reported a higher photoresponsivity (Nat Commun 2021, 12, 1798). A more\ncomprehensive and up-to-date literature review should be provided.\nReply: We thank the Reviewer for recommending that interesting literature. For the\nliterature mentioned by the Reviewer, although a higher photoresponsivity (5.1 × 107\nA/W) was reported, its incident power is less than 0.2 pW (the incident-power density\nis 10-8 W/cm2). Here we note that the area of that device is 100 × 20 μm2, while optical\neffective area A was not given in that literature. According to the schematic diagram\nd\nand the device area larger A in the literature, we can estimate the upper limit of the\nd\nincident power as 0.2 pW.\nAs suggested by the Reviewer, we have conducted a more thorough and up-to-date\nliterature review and found a new literature at E = 28.8 nW [Light Sci. Appl. 14, 201\nin\n(2025)] which has been added in Fig. 1d. Still, we have not found any literature\nreporting a higher responsibility than our work within the working region of our\nphotodetector (3 nW ‒ 10 mW).\nChanges Made:\nPlease kindly see Fig. 1d where we have a new data at E = 28.8 nW.\nin\nSupplementary Table 1 in supplementary material where we have added a new\nliterature at E = 28.8 nW [Light Sci. Appl. 14, 201 (2025)].\nin\nReference [Nat. Commun. 12, 1798 (2021)] cited as Ref. 36 in the revised\nmanuscript.\n29\n5. The paper's organization could be improved to enhance the readability. The current\nfigure sequence is confusing. For example, Fig. 1d was cited as the first figure in the\nmain text. The Fig. 2d and 2e appeared after the Figure 3. The illumination intensity\nwith unit of nm/cm2 of μW/cm2 is more common for the comparison of responsivity\nwhile also providing the applied voltages.\nReply: We thank the Reviewer for the suggestions. Following the suggestions, we have\nreorganized the manuscript so that the figure sequence follows the sequence of the text\nsequence.\nWe note that both incident illumination power and illumination intensity were used in\nthe literatures [incident illumination power: Nat. Nanotechnol. 7, 363 (2012); Nat.\nCommun. 7, 13278 (2016); Nat. Photonics 14, 578 (2020); Nat. Nanotechnol. 12, 668\n(2017) and Nat. Nanotechnol. 8, 497 (2013); illumination intensity: Science 324, 1542\n(2009); Nat. Commun. 6, 6972 (2015); Sci. Adv. 6, eaaw8065 (2020); Nat. Commun. 15,\n9011 (2024) and Nat. Commun. 12, 3667 (2021)] To our understanding, incident\nillumination power is preferred when evaluating the overall performance of a\nphotodetector, whereas illumination intensity is more relevant when analyzing the\nphotophysical behavior of the materials used in a photodetector. Our study here\nevaluates the photodetector’s overall performance by measuring its responsivity to\nincident illumination power under varying light signal conditions. Given the limited\nunderstanding of the photophysical properties of the CuPc@GO, our future work will\nsystematically vary the illumination intensity to investigate the atomic-level\noptimization of the material.\nChanges Made:\nThe original Fig. 1f and Fig. 1g have been changed to Fig. 1h and Fig. 1g\nrespectively, and the D* diagram has been added as Fig. 1f. Their captions in revised\nmanuscript: “Fig. 1g, Specific detectivity (D*) of the CuPc@GO photodetector as a\nfunction of incident optical power. Fig. 1f, Photocurrent vs. time under a 635 nm laser\nwith P = 240 kPa, showing the photodetector is able to detect the illumination as low\nas 100 pW. Fig. 1h, Rise (τ ) and decay (τ ) times of the photodetector (E = 3 nW, P =\nr d in\n240 kPa).”\nFig. 2 have been reorganized so that the figure sequence follows the sequence of\nthe text sequence. The original Fig. 4g and 4h have been changed to Fig. 4j and 4i\nrespectively. Meanwhile, the original Fig. 4i has been transformed into Fig. S19 in\nsupplementary material, and mechanical hand-related detail diagrams have been added\nas Fig. 4g and 4h in the revised manuscript. Their captions in revised manuscript have\nalso been added: “Fig. 4g, Diagram of the custom-designed control board for signal\nacquisition and robotic hand control. Fig. 4h, Photography of the robotic hand with\nintegration of the control board and the CuPc@GO photodetectors. Fig. 4i, Current\nsignals of the manipulator system executing approaching, grasping, moving, and\nreleasing actions for banana in the low-illuminance environment (10 μW). During the\n30\nbanana manipulation, all fingers need to be in contact with the target and apply force\nonto it. Fig. 4j, Current signals of the middle figure in the CuPc@GO e-skin\nmanipulator system executing approaching, grasping, moving, and releasing actions\nfor a banana under a low illuminance environment (10 μW). Insets: Schematic of the\nsteps in the food-grasping experiment (right); the speed changing over time during the\nprocess of the middle finger approaching the object, and the relative current change\nover time (left). The relative current change is defined by the ratio of the reduction in\ncurrent to the initial current.”\nFig. S19 and its caption in supplementary materials: “Supplementary Fig. 19 |\nCurrent signals of the manipulator system executing approaching, grasping, moving,\nand releasing actions for egg in the low-illuminance environment (10 μW). During the\negg manipulation, the thumb, index, and middle fingers sequentially contact and apply\nforce to grasp the egg, while the ring finger contacts without exerting force, and the\nlittle finger remains uninvolved.”\nWe sincerely thank the Reviewer for the very helpful suggestions and very constructive\ncomments, which help us to make a more efficient and pleasant manuscript. According\nthe Reviewer’s comments, we have revised the manuscript. We think that all\nambiguities in the manuscript have been clarified. We hope that the Reviewer will be\nsatisfied with our response and will now recommend its publication in Nature\nCommunication.\nIn this article, the authors claim to propose a dual-mode electronic skin (e-skin) based\non the CuPc@GO photodetector. However, there are numerous loopholes:\nWe sincerely appreciate the Reviewer’s valuable, constructive feedbacks, which have\nsignificantly strengthened this work. We have carefully addressed all points the\nReviewer raised in the response below and the revised manuscript, and hope that our\nresponses and manuscript refinements have resolved all the concerns.\n1) In the experimental part, important information about the protocols used is missing.\nCommon parameters for photodetectors, such as (1) specific detectivity (D*), (2)\nresponse bandwidth fc, (3) the optical effective area Ad and (4) noise equivalent power\n(NEP) must be thoroughly characterized. The authors have only presented data on\nresponsivity and response time, which is insufficient to support the claimed potential\nfor future applications. I believe this literature would be highly beneficial for the\nauthors to deepen their understanding of photodetectors. (Wang, F., Zhang, T., Xie, R.\net al. How to characterize the figures of merit of two-dimensional photodetectors. Nat.\nCommun. 14, 2224 (2023). https://doi.org/10.1038/s41467-023-37635-1.)\nReply：We sincerely thank the Reviewer for this very helpful suggestion and the useful\nreference. We have performed additional experimental and theoretical investigations to\nobtain the specific detectivity (D*), response bandwidth fc, the optical effective area\nAd, and noise equivalent power (NEP) of the CuPc@GO photodetector.\n(2) Different frequencies ranging from 1 Hz to 7.5 kHz under 3 nW were applied to the\nCuPc@GO photodetector to obtain the detection bandwidth (fc). The light was\ngenerated by a tunable laser and modulated in frequency using a signal generator. The\nphotocurrent of the photodetectors as a function of time was measured using a Keithley\n2461 source meter (Tektronix, USA) following rigorous experimental procedures. The\nphoto-induced current consistently decreases as the laser frequency increases. The −3\ndB cutoff frequency was determined as 5.6 kHz, which significantly exceeds typical\nlight source frequencies used in practical applications (Fig. R1).\nlaser frequency. The cutoff frequency is determined at the −3 dB point, where the\nphotogenerated current obtained under 1 Hz laser irradiation is set to 1. The\nphotodetector was tested under 240 kPa and 3 nW.\nChanges made:\nPlease kindly see Page 5, Lines 32-34 in revised manuscript: where we have added\n“We also conducted experiments and calculated the linear dynamic range (LDR) and\nbandwidth (fc), which are 138.4 dB and 5.6 kHz respectively (Supplementary Figs.\n3&4).”.\nFig. S4 and its caption in supplementary materials: “Supplementary Fig. 4 |\nFrequency response of the CuPc@GO photodetector with respect to the laser frequency.\nThe cutoff frequency is determined at the −3 dB point, where the photogenerated\ncurrent obtained under 1 Hz laser irradiation is set to 1. The photodetector was tested\nunder 240 kPa and 3 nW.”\nReference [Nat. Commun. 14, 2224 (2023)] has been cited as Ref. 5 in the revised\nmanuscript.\n(3) The optical effective area A , also the active area of photodetector, is 1 cm2, which\nd\nis defined by a square mask with side length of 1 cm. The incident optical power was\ncalibrated through commercial photodetector (Thorlabs, S170C and PM100D, USA)\nwith a same effective area by using the mask.\n(4) The noise-equivalent power (NEP) is a fundamental figure of merit for a\nphotodetector, defined as the incident optical power required to generate a signal equal\nto the inherent noise of photodetector. The NEP of the CuPc@GO photodetector was\ncalculated to be 7.71 × 10-13 W under 3 nW illumination. This value was determined\nusing the standard NEP formula for a photodetector characterized by its current\nresponsivity: 𝑁𝐸𝑃 =\n𝑖𝑛\n, where 𝑖 denotes the noise current and 𝑅 the current\nn\n𝑅\nresponsivity (Fig. R2). We note that i is obtained corresponding to the measurement\nn\nbandwith Δf, which is defined as the difference between the lowest frequency obtained\n33\n(10 Hz) and the detection bandwidth (𝑓𝑐 = 5.6 kHz).\nFig. R2. Frequency dependent noise power density of the CuPc@GO\nphotodetector under dark conditions with 240 kPa.\nChanges made:\nPlease kindly see Page 5, Lines 29-32 in revised manuscript: where we have added:\n“This D* value is closely correlated with the ultra-low noise-equivalent power (NEP)\nof 7.71 × 10⁻¹³ W under 3 nW illumination, as the two key figures of merit are inherently\nlinked through the noise current and R (Supplementary Fig. 2).”\nFig. S2 and its caption in supplementary materials: “Supplementary Fig. 2\n|Frequency dependent noise power density of the CuPc@GO photodetector under dark\nconditions with 240 kPa.”\n(1) Specific detectivity (D*) measures the quality of a photodetector's ability to detect\nweak light signals. The D* of CuPc@GO photodetector, defined by the formula 𝐷∗ =\n√𝐴𝑑𝜟𝒇𝑅\n, was calculated as 9.71 × 1013 Jones. Here, 𝐴 denotes the active area of\nd\n𝑖𝑛\nphotodetector, 𝛥𝑓 the measurement bandwidth (10 Hz-5.6 kHz), 𝑅 the current\nresponsivity, and 𝑖 the noise current corresponding to 𝛥𝑓. Moreover, as shown in Fig.\nn\nR3, R and D* exhibit a positive correlation with respect to power.\nincident optical power.\nChanges made:\nPlease kindly see Page 5, Lines 26-29 in revised manuscript: where we have added:\n“Further, we also characterized the comprehensive photodetection performance of the\nphotodetector, and the specific detectivity (D*) is a function of incident optical power,\namong which it can be 9.71 × 1013 Jones under 3nW illumination (Fig. 1f).”\nFig. 1f and its caption in revised manuscript: “Fig. 1f |Specific detectivity (D*) of\nthe CuPc@GO photodetector as a function of incident optical power.”\na) The photoelectric properties shown in Fig.1 are difficult to regard as those of the\nCuPc@GO because they strongly reflect the properties of CuPc. It is recommended to\nmake and measure the lateral device, using CuPc or GO.\nReply: The origin of exceptional optoelectronic performance was investigated through\ncontrolled fabrication of different materials. For comparison, we attempted to fabricate\nstandalone CuPc devices via multiple methods, i.e., solution-processed methodologies\nusing the same dip-coating method as that for the CuPc@GO composite film, and spin-\ncoating. All products exhibited discontinuous surface morphology even with optimized\ndeposition parameters (Fig. R4), fundamentally precluding reliable device integration\nand making it impossible to characterize its optoelectronic properties.\nFig. R4. Film formation characteristics of CuPc without GO. (A) Photograph of\ndrop casting film formation (B) Photograph of spin coating film formation.\nPure GO does not possess photoelectric activity. We also fabricated GO-only devices\nusing the same fabrication protocol and measured their photocurrent responses before\nand after pressure application (Fig. R5). The GO-only devices exhibited no detectable\nphotoresponse—pressure application merely enhanced their electrical conductivity.\nConsistent with the design rationale of our system, the sandwich structure formed by\nthe self-assembly of CuPc and GO enables atomic-level interfacial optimization, which\n35\nis responsible for the significant enhancement of the photoresponse. In the CuPc@GO\nphotodetector, for the CuPc who is a typical organic photoactive molecule, light\nadsorption leads in the formation of an electrically neutral, spatially localized electron-\nhole pair. Exciton dissociation requires a donor‒acceptor heterojunction which provides\nthe interfacial energy offsets to facilitate the generation of free electron and hole.\nThe CuPc molecule acts as the electron-donor at the heterojunction interface, and GO is\nchosen as the electron-acceptor materials owing to their strong electron-accepting\nabilities and exceptional flexibility, enabling superior compatibility with small CuPc\nmolecules and significantly enhancing the number of π-π stacking interactions.\nConsequently, once pressure-optimised, a “compact” CuPc-GO heterojunction forms,\nestablishing sufficient vertical GO-CuPc-GO transport pathways to achieve highly\nefficient free carrier transport. It is precisely this sandwich structure that underpins our\noutstanding optoelectronic performance.\nOur photodetector adopts a vertical structure, employing a design where the CuPc@GO\nphotoactive layer is sandwiched between top and bottom electrodes, with charge\ntransport occurring perpendicular to the active layer. In contrast, lateral devices utilise\na three-electrode system comprising two electrodes positioned on either side of the\ndevice and a third electrode for controlling electric field distribution, where carriers rely\non channel transport. Consequently, the photodetector we have fabricated differs from\nlateral devices. We are grateful for the Reviewer’s suggestions. Given the complex\nequipment requirements and operational mechanisms of lateral devices, we are\nconsidering applying our system to the study of lateral devices. This may form part of\nour future research agenda. We appreciate the valuable input of the Reviewer.\nFig. R5. Photoresponse of GO device. The photocurrent and corresponding dark\ncurrent of GO at different pressures (P) of 0 and 240 kPa, under illumination of 635 nm,\nwith an incident power E = 3 nW.\nin\nb) In Fig. 1b, what is the mechanism behind the phenomenon where the device exhibits\ncurrent at 0V without applied pressure, but this current disappears after pressure is\napplied?\noptimization of the Schottky barriers.\nSchottky barriers, which exist at both Au/CuPc@GO and ITO/CuPc@GO interfaces,\nare different in height and width because of the different work function of Au (𝜑 ≈\nAu\n5.1 eV) and ITO (𝜑 ≈ 4.8 eV), together with the quality of contact with CuPc@GO\nITO\nat the interfaces, respectively.\nIn the unoptimized case, when bias is not applied (𝑉 = 0), it is difficult to collect charge\ncarriers by thermal activation at both Au/CuPc@GO and ITO/CuPc@GO interfaces\nbecause of the Schottky barriers, leading to a small photocurrent or dark current of the\ndevice. When a small positive bias (e.g., 𝑉 = Δ𝜑⁄𝑒 = (𝜑 −𝜑 )⁄𝑒 ≈ 0.3 V ) is\nAu ITO\napplied, the Schottky barrier at the ITO/CuPc@GO interface is lowered such that more\ncharge carriers can be collected, leading to a larger current of the device. In contrast,\nwhen a small negative bias (e.g., 𝑉 = −Δ𝜑⁄𝑒 ≈ −0.3 V) is applied, the barrier at the\nITO/CuPc@GO interface is raised while that at the Au/CuPc@GO is not reduced\nenough, resulting in a current even smaller than the case of 𝑉 = 0 is obtained (see Fig.\nR6). This leads to the unsymmetric characteristics of the I-V curves observed in the\ndevice under positive and negative biases (see Fig. R7).\nIn the optimized case, the unsymmetric characteristics mentioned above can be\nremoved by the improved quality of contact with CuPc@GO, leading to smaller\nSchottky barriers width at both Au/CuPc@GO and ITO/CuPc@GO interfaces (see Fig.\nR6). In this context, carriers are prone to be collected by quantum tunneling through\nthe thinner Schottky barriers, which are less impacted by the bias applied to the devices,\nthus leading to symmetric I-V curves (see Fig. R7).\nFig. R6. Schematics of the energy diagrams of the devices in the unoptimized and\noptimized cases.\ncharacteristics and the work function deference 𝚫𝝋 of the two electrodes.\nc) In Fig. 1c, how did the authors manage to detect light as weak as 3 nW, which is\nnotoriously difficult to measure with conventional power meters?\nReply: The detection limit of the commercial power meter (Thorlabs, S170C and\nPM100D, USA) was 10 nW. Therefore, the 3 nW incident power could not be directly\ncalibrated. Instead, it was obtained by attenuating a 10 nW source using a neutral\ndensity filter with 30% transmittance.\nChanges made:\nPlease kindly see Page 14, Lines 22-24 in methods: where we have added “The 3\nnW incident power was achieved by attenuating the 10 nW source through a neutral\ndensity filter with 30% transmittance.”\nd) In Fig. 1d, why does the photoresponsivity reported in the latest literature appear\nlower than in earlier studies? Has the performance in this field been deteriorating?\nReply: Regarding Fig. 1d, the apparent decrease in photoresponsivity reported in recent\nliterature compared to earlier studies is indeed a noteworthy observation. We have also\nnoted this trend, though the underlying reasons do not appear to be fully understood.\nExploring this phenomenon served as one of the motivations for the present work, as\nwe suspected that certain factors might have been overlooked.\nBased on extensive literature research, the reasons may include the following: the\nprocessing technology of photodetectors has progressed tremendously owing to\nadvances in material science and device engineering in the past two decades. Many\nconventional performance parameters have approached their practical limits.\nConsequently, many of the researches have shifted their focus toward developing\nmaterials and devices with distinctive and/or integrated functionalities, such as\nflexibility, color sensitivity, biosafety, ultrafast response, and ultrahigh specific\n38\ndetectivity (D*).\nOur group have worked on interfacial physics over two decades. We understand that\nthe manipulation of interfacial behaviour is important to improve the device\nperformance. For the photodetector, the heterojunction at the donor‒acceptor interface\nplays a key role in both charge separation and transport. However, improving the\nheterojunction performance usually requires the effective manipulation of\nconnections/interactions at the heterojunction interface on the atomic scale, which is\nhighly difficult.\nWe realized that: i) both the GO sheet and the CuPc molecule have an atomically planar\nstructure, which facilitates the stress transfer across the heterojunction interface when\nexternal pressure is applied; ii) the GO sheet exhibits mechanical flexibility, enabling\nlocal deformation in response to applied mechanical stimulus; iii) Due to the small size\nand non-bonded interactions with GO sheets, CuPc molecules can be repositioned to\nbetter conform to confined spaces, thereby enhancing π‒π stacking interactions under\nmechanical stimulation. These characteristics enables easy manipulation of the atomic\ninterface of the CuPc‒GO heterojunction by a macroscopic stimulation (i.e., an external\nforce). With the synergy between the unique properties of the CuPc@GO membrane in\nthe photodetector we fabricated and a modest external pressure (~1 atm), we obtain a\nhuge improvement in the photoresponsivity (105 to 106-fold).\nWe think that the strategy we presented here provides a new perspective for fabricating\nnext-generation photodetectors.\ne) In Fig. 1f, it is highly unusual that the device exhibits current at the milliampere level\nalongside significant noise under illumination as low as 100 pW. The authors must\nprovide a plausible explanation for this anomalous data.\nReply: We thank the Reviewer for the helpful suggestion. Here, the current response is\ncontributed to the illumination under a voltage bias on the device. To make it clear, we\nadded the curve of both illumination condition and dark condition in the figure. It is\nobvious that the current response (net photocurrent) as well as its noise is at the level\nof microampere with illumination of 100 pW at 1.2 V bias. With an ultra-high\nresponsivity, the mean current response is 0.032 mA, which is higher than the 3σ of\nnoise (~0.019 mA). Therefore, the 100 pW illumination is detectable for the CuPc@GO\ndetector.\nkPa, showing the photodetector is able to detect the illumination as low as 100 pW. The\nred line indicates the condition of illumination, where the high/low level represents the\non/off condition.\nChanges made:\nPlease kindly see Fig. 1g and its caption in revised manuscript: “Fig. 1g |\nPhotocurrent (blue line) and dark current (grey line) vs. time under a 635 nm laser with\nP = 240 kPa, showing the photodetector is able to detect the illumination as low as 100\npW. The mean current response is 0.032 mA, which is higher than the 3σ of noise (0.019\nmA). The red line indicates the input of illumination.”\nf) In Fig. 1g, when calculating the photoresponse time, it is advisable to normalize the\ndata to clearly illustrate the range from 10% to 90% of the photocurrent variation.\nReply: We fully agree that normalizing the data is crucial to clearly illustrate the 10%–\n90% range of photocurrent variation, which is a standard method for accurately\ndetermining photoresponse kinetics. We have reprocessed and redrawn Fig. 1g by\nnormalizing the photocurrent data (Fig. R9). And it was repositioned to the position of\nFig. 1h. This revision explicitly highlights the transition process from 10% to 90% of\nthe photocurrent variation, making the extraction of rise time and decay time more\nintuitive and credible. The normalized curves clearly show the rapid photoresponse\nbehavior of the CuPc@GO devices, which is consistent with the performance trends\nreported in the original manuscript. The revised Fig. 1g has been updated in the\nmanuscript.\nr d in\nChanges made:\nWe have normalized the response time data to clearly illustrate the range from 10%\nto 90% of the photocurrent variation.\n2) Basic characterization experiments were conducted, but some results lack clarity,\nmaking analyzing the material's electrical performance less robust.\na) As a photoactive material, providing (1) the ultraviolet absorption spectrum, (2) UPS\ndata, and (3) energy band diagram of CuPc@GO is essential. (4) Unusually, the\nexperimental work utilizes GO, while the simulations are based on graphene. Such\nlimited computational modeling is unconvincing. (5) Synchrotron radiation\nexperiments are necessary to characterize the bonding and carrier distribution in the\nheterojunction material.\nReply: We sincerely thank you for this very helpful suggestion.\n(1-3) We have tried to perform new experiments on the ultraviolet absorption spectra,\nUPS data, and tried to analyze the energy band diagram of CuPc@GO before and after\npressure, obtaining the effects induced by pressure.\nIn order to meet the requirements for UV-Vis spectrometer, the film thickness was\nadjusted compared to that used in previous experiments. Specifically, the CuPc@GO\nsuspension was carefully deposited onto a 2 cm × 2 cm high-purity quartz glass\nsubstrate using the drop-casting method and dried at 70°C, resulting in a film thickness\nof ~100 nm. This is significantly thinner than the ~1 μm films typically used in\nCuPc@GO photodetectors. As is shown in Figure R10, the CuPc@GO spectra is a\nsimple combination of the pristine CuPc and GO case. Through the Tauc plot method,\nwe obtained a bandgap of 3.53 eV for CuPc@GO without applied pressure, which is\nconsistent with the results of pristine GO (4.35 eV).\nfilms under pressurized/unpressurized conditions, with bandgap widths derived from\nTauc plot analysis. (B) UV-Vis spectra of the GO film under unpressurized condition,\nwith bandgap widths derived from Tauc plot analysis. (C) UV-Vis spectra of the CuPc\nfilm under unpressurized condition.\nWe then tried to perform experiment to investigate the effect of external pressure.\nUnfortunately, the encapsulated high-pressure apparatus (e.g., polyimide-based\nsubstrates) cannot be accommodated within the sample chamber in the UV-Vis\nspectrometer, making in situ characterization of the CuPc@GO membrane under\npressure unfeasible. As an alternative, we turned to ex situ measurements, where the\nmembrane was first pressurized, then transferred to the instrument. To estimate the\neffect of pressure, we measured the thickness of the membrane by using Atomic Force\nMicroscope (AFM) after it is pressurized. As shown in Fig. R11, the thickness reaches\nsaturation after 5 minutes. Of note, in the UV-Vis experiment, the sample was required\nto be placed in the test chamber for 5 minutes before the measurement. As a result, the\npressure effect disappears before the measurement begins. Our experiment shows that\nthe UV-Vis absorption spectra of the pre-pressurized sample does not exhibit significant\nchanges.\nFig. R11. Change of film thickness (∆ℎ) over time after applying an external pressure\non a silicon wafer, for the CuPc@GO membrane with original thickness of ~100 nm\n(A) and ~1 μm (B). Triangles indicate the time point and the corresponding thickness\nfor different experiments, with the green and red ones representing the the pressure-\ninduced effect can be, and cannot be observed, respectively. TA is the abbreviation of\n42\nfemtosecond transient absorption (TA) spectra, which we have described in the\nmanuscript.\nTable1. Spectra Test Instructions\nMethod Sample Thickness Pre-Test Pressure-induced effect\ncan be observed\nTA 100 nm 2 min Yes\nUV-Vis 100 nm 5 min NO\nUPS 1 μm Vacuum for 12 h Yes\nFor the UPS experiment, the sample of ∼1 μm thickness CuPc@GO film was first\ncharacterized without external pressure. Similar to the UV-Vis experiments, the\ninstrument requires 12 hours of evacuation before sample testing. Through fitting and\nenergy calibration analysis of the valence band region in the UPS spectra, the energy\noffset between the valence band maximum (VBM) and the Fermi level (E ) of the\nF\nCuPc@GO film was determined to be 4.43 eV.\nFor pressure-dependent measurements, the sample was first pressurized, then\ndepressurized, and transferred to the instrument for equivalent vacuum treatment prior\nto ex situ measurements. Upon this process, a 0.3 eV shift in the valence band position\nof CuPc@GO was observed. Due to its amorphous nature, this system lacks a strictly\ndefined valence band top, and substantial tail states are expected to be present. The\nobserved UPS shift is likely due to changes in the band-tail states near the valence band\nedge. This 0.3 eV shift under pressure can be attributed to the enhanced electronic\ncoupling between CuPc and GO. These UPS results (Fig. R12) show that pressurization\nstrengthens the electronic coupling within the system, thereby facilitating electronic\ntransports. It is noteworthy that the encapsulated high-pressure apparatus (e.g.,\npolyimide-based substrates) cannot be accommodated within the UPS analysis chamber,\nthereby rendering in situ characterization unfeasible. Additionally, when these thin\nsamples undergo off-site measurements after pressurisation, they do not fully revert to\ntheir 1-micrometre thickness state during vacuum extraction, thereby retaining partial\npressure-induced effects.\nWe note that the conventional method to retrieve the energy band diagram, i.e., using\nthe UPS results to derive VBM and UV-Vis spectrometer to derive the band gap, is not\nsuitable for our doped materials. This is mainly because the Tauc plot method [Nat.\nCommun. 11, 2304 (2020); Science 358, 1192 (2017) and Nat. Commun. 13, 5216\n(2022)] employed for bandgap analysis, utilizing multiple idealized assumptions, is\nonly applicable to homogeneous bulk materials. Here the CuPc@GO membrane is an\natomic sandwiched structure with complex internal interactions. Therefore, the\nproposed results, i.e., the bandgap shift for the CuPc-doped GO and the VBM shift for\nthe pressurized CuPc@GO, only show some qualitative changes other than the major\nfeature of GO. Further investigation into the band structure of the CuPc@GO is\nprecluded by its complicated doped structure.\npressurized/unpressurized conditions. (A, B) CuPc@GO films under unpressurized\nUPS spectra and valence band positions. (C, D) CuPc@GO films under pressurized\nUPS spectra and valence band positions.\nChanges made:\nPlease kindly see Fig. S13 and its caption in supplementary materials:\n“Supplementary Fig. 13 | Ultraviolet Photoelectron Spectra (UPS) of the CuPc@GO\nfilm under pressurized/unpressurized conditions. (a, b) CuPc@GO films under\nunpressurized UPS spectra and valence band positions. (c, d) CuPc@GO films under\npressurized UPS spectra and valence band positions.”\nFig. S14 and its caption in supplementary materials: “Supplementary Fig. 14 |\nUltraviolet-Visible Spectra (UV-Vis). (a) UV-Vis spectra of the CuPC@GO films under\npressurized/unpressurized conditions, with bandgap widths derived from Tauc plot\nanalysis. (b) UV-Vis spectra of the GO film under unpressurized condition, with\nbandgap widths derived from Tauc plot analysis. (c) UV-Vis spectra of the CuPc\nfilm under unpressurized condition.”\n(4) On the comment “Unusually, the experimental work utilizes GO, while the\nsimulations are based on graphene. Such limited computational modeling is\nunconvincing.”\nReply: In the theoretical calculation of our original manuscript, we used pristine\ngraphene instead of the GO, to illustrate the key physical mechanism underlying the\nexperimental observation. Considering the considerable difference in the electric\nconductivity between graphene and GO, in the revised manuscript, we have conducted\nnew DFT calculations based on GO to make a closer correlation with the experiment.\ncharge transport is unchanged when using GO instead of pristine graphene.\nChanges made:\nPlease kindly see Page 8, line 18-Page 9, line 2 in the revised manuscript where we\nhave rewritten: “To elucidate the underlying mechanism, we performed density\nfunctional theory (DFT) calculations on the unit cell of the composite membrane … The\ninter-sheet conductivity is still lower than the intra-sheet conductivity even after the\noptimization, indicating that the inter-sheet transport still represents the main\nbottleneck limiting charge transport in the overall GO‒CuPc‒GO pathway.”\nFig. S15 and its caption in supplementary materials: “Supplementary Fig. 15 |\nIntra-sheet (a) and inter-sheet (b) electric conductivities obtained by DFT calculations.”\n(5) Synchrotron radiation can be used to characterize interface bonding and carrier\ndistribution. We have tried to perform measurements in the BL09U Photoemission\nElectron Microscopy (PEEM) station of the Shanghai Synchrotron Radiation Facility\n(SSRF) (see Fig. R13). The PEEM station employs a surface-sensitive, in situ, and\ndynamic imaging technique that probes charge carrier distribution and related\nproperties on sample surfaces by detecting electrons excited through high voltage.\nFig. R13. The equipment diagram of the BL09U photoemission electron\nmicroscopy (PEEM) station at the Shanghai Synchrotron Radiation Facility\n(SSRF).\nIn the experiment, the CuPc@GO film (1 × 1 cm2) was prepared using the same\nfabrication process as described in the manuscript. It was subsequently trimmed to 0.8\n× 0.8 cm2 to fit the sample holder (Figs. R14A–C). This film was mounted on a silicon\nwafer, loaded into the sample holder, and vacuum-packed for 24 hours before being\ntransferred to the measurement station. A 100 W high-voltage mercury lamp, placed\napproximately 10 cm from the sample, was used to irradiate the surface for parameter\ncalibration (Fig.R14D). The initial surface temperature reached about 52.1°C, with\nfurther temperature accumulation under prolonged irradiation. Additionally, a high\n45\nvoltage of –20 kV was required to extract photoelectrons. The combined effect of above\noperation caused carbonization of the CuPc@GO film (Figs. R14E and F), preventing\nsuccessful measurements. Although the experiment did not yield viable data, we have\nmaintained close collaboration with the SSRF team and continue to discuss alternative\ncharacterization strategies with synchrotron specialists. In light of the timeline for\nmanuscript revision, we are submitting the current results at this stage. We would also\nsincerely appreciate any further guidance from the reviewers regarding potential\noptimizations to the testing protocol.\nFig. R14. (A) Photograph of the CuPc@GO film produced by trimming the original 1\n× 1 cm² film into 0.8 × 0.8 cm². (B) Photograph of the PEEM sample holder, together\nwith the 0.8 × 0.9 cm² silicon wafer placed prior to sample loading. (C) Mounted film\nfrom (A) secured in the sample holder, with an exposed circular measurement window\nof 0.7 cm in diameter. (D)High-voltage mercury lamp used for parameter calibration\nbefore sample testing. (E) Condition of the sample inside the chamber after\nsimultaneous exposure to mercury lamp irradiation and a –20 kV bias. (F) Carbonized\nsample after testing.\nIn summary, methods such as UPS and ultrafast spectra characterization can provide\nevidence to support the conclusions, and we are still planning to develop ultra-thin\nencapsulation technologies (such as molecular layer deposition MLD) in subsequent\nworks to reduce the total thickness of the device and create conditions for in-situ\nsynchrotron characterization.\nb) (A) Schematic of the network-like pathways of electron transport in fig. 3d-e is\n46\nconfusing; the electrical conductivity of graphene oxide is relatively poor. What role\ndoes graphene oxide play in the process of carrier separation? Furthermore, (B) when\nasymmetric electrodes with significant potential barriers are employed, it is crucial to\ndetermine whether the photocurrent generation originates from the material itself or\nfrom the Schottky barrier at the electrode interface. Comparative experiments are\nnecessary to clarify this mechanism.\nReply: Graphene oxide plays an important role in the process of charge separation\n(exciton dissociation), by forming strong π‒π stacking interactions with the CuPc\nmolecule by the unoxidized regions on the GO sheet. We have emphasized this point in\nthe revised manuscript.\nOn the comment “Schematic of the network-like pathways of electron transport in fig.\n3d-e is confusing; the electrical conductivity of graphene oxide is relatively poor.”\nWe have calculated the intra-sheet and inter-sheet electric conductivity of CuPc@GO,\nas is shown in Fig. R15. Clearly, even in the presence of the oxidized sites, the GO sheet\nexhibits a not-too-low electric intra-sheet conductivity. Under pressure optimization,\nthe inter-sheet electric conductivity is enhanced by several orders of magnitude, while\nthe intra-sheet conductivity changes negligibly. The inter-sheet conductivity is still\nlower than the intra-sheet conductivity even after the optimization, indicating that the\ninter-sheet transport still represents the main bottleneck limiting electron transport in\nthe overall GO‒CuPc‒GO pathway.\nWe computed the electrical conductivity σ by using the Boltzmann transport equation\nwithin the constant relaxation time approximation. Consequently, the direct output of\nour calculation is the ratio σ/τ , which represents the ideal conductivity governed solely\n0\nby the material band structure. It is difficult for DFT calculations to evaluate the real\nvalues of the relaxation time τ , which encompasses all scattering processes (e.g.,\n0\nelectron-phonon, impurity, and defect scattering). Additionally, it is also challenging to\nperform thorough investigations of excited electron transport by means of time-\ndependent density function theory combined with non-adiabatic molecular dynamics.\nGiven these limitations, the ground-state DFT calculations employed in our study\nrepresent a balanced alternative for describing charge transport properties.\nDFT calculations.\nChanges made:\nPlease kindly see Page 8, line 18-Page 9, line 2 in the revised manuscript: “In\naddition, we calculated the intra-sheet and inter-sheet electric conductivities to\nquantitively characterize the relative contributions of intra- and inter-sheets to charge\ntransport as well as the influence of pressure (see Supplementary Fig. 15). The inter-\nsheet conductivity is still lower than the intra-sheet conductivity even after the\noptimization, indicating that the inter-sheet transport still represents the main bottleneck\nlimiting charge transport in the overall GO-CuPc-GO pathway.”\nFig. S15 and its caption in supplementary materials: “Supplementary Fig. 15 |\nIntra-sheet (a) and inter-sheet (b) electric conductivities obtained by DFT calculations.”\n(B) To distinguish whether the photocurrent originates from the bulk material or the\nelectrode Schottky barrier, we conducted a comparative experiment by replacing the\nmetallic electrodes with non-metallic carbon electrodes, while maintaining identical\nfabrication protocols for the CuPc@GO active layer and encapsulation. Pressure-\ndependent responsivity (R) analysis demonstrated statistically invariant performance of\n1.33 × 10⁷ A/W (Fig. R16) between the two different configurations. This electrode-\nindependent response behavior conclusively establishes the bulk material\nphotoconduction as the dominant mechanism, effectively eliminating Schottky junction\ncontributions.\nphotocurrent/responsivity of the CuPc@GO photodetector after electrode replacement.\nThe carbon electrode excludes the metal-semiconductor Schottky barrier, leading to an\naltered response characteristic. (B) Illustration of the CuPc@GO photodetector\narchitecture after replacing the conventional gold (Au) electrodes with carbon\nelectrodes.\n3) For applications in flexible electronic skin, comprehensive characterization of\nmechanical properties is essential. The authors merely conducted simple tests at\ndifferent bending angles, which is far from sufficient. It is necessary to evaluate the\nmaximum number of bending cycles at various angles and quantify the variance in\nphotoresponsivity during bending.\nAdditionally, for Fig. 4g, which demonstrates the integration of electronic skin onto a\nrobotic hand, please provide the circuit design diagram and physical images of the data\nacquisition system used in this setup.\nReply: We thank the Reviewer for the constructive suggestions. We have conducted\nmore experiments to quantify the variance of the photodetector in photoresponsivity.\nThen, we explored the effect of cyclic bending on the CuPc@GO photodetector. For\nthe photoresponse measurement under cyclic bending conditions, three distinct bending\nangles (30°, 60°, and 90°) were used to represent different bending levels (Fig. R17). As\na result, the device maintained 95.0% of its photoresponsivity after over 6000,\n2000 and 1000 bending cycles in the 30°, 60° and 90° angle test, respectively (Fig. R18).\nAnother verification was conducted where the CuPc@GO photodetector was subjected\nto the cyclic twist with a fixed twist angle of 30° (Fig. R18). The responsivity of the\nphotodetector retains 95.2% of its initial value after 1300 consecutive torsion cycles,\ndemonstrating excellent stability against repeated torsional deformations. In the\nmeasurement, the loadings of bending and the twisting were applied by using an\nFT2000 flexible electronic tester (Shanghai Mifang Electronic Technology Co., Ltd).\nloading. (A‒C), (D‒F), (G‒I) corresponds to the photodetector at bends bending angles\nof 30°, 60°, and 90°, respectively. After bending CuPc@GO photodetector by 30° for\n6500 times, the performance was retained at 95.2% of the original (non-bending) value.\nAfter 2200 cycles of bending at 60°, the performance was retained at 95.3%. After\nbending CuPc@GO photodetector by 90° for 1000 times, the performance was retained\nat 95.7%. The applied external pressure for all cases is 240 kPa.\nFig. R18. Durability evaluation of the CuPc@GO photodetector under twisting\nstrain. After twisting CuPc@GO photodetector by 30° for 1300 times, the performance\nwas retained at 95.2% under 240 kPa\nChanges made:\nPlease kindly see Page 10, Lines 18-20 in revised manuscript: “The CuPc@GO\nphotodetector has excellent flexibility, with ultrahigh photoresponsivity remaining at\n50\nvarious bending/twisting angles (Fig. 4a and Supplementary Figs. 16&17)”\nFig. S16 and their caption in supplementary materials: “Supplementary Fig. 16 |\nDurability evaluation of CuPc@GO photodetector under bending loading. (a‒c), (d‒\nf), (g‒i) corresponds to the photodetector at bending angles of 30°, 60°, and 90°,\nrespectively. After bending CuPc@GO photodetector by 30° for 6500 times, the\nperformance was retained at 95.2% of the original (non-bending) value. After 2200\ncycles of bending at 60°the performance was retained at 95.3%. After bending\nCuPc@GO photodetector by 90° for 1000 times, the performance was retained at\n95.7%. The applied external pressure for all cases is 240 kPa.”\nFig. S17 and their caption in supplementary materials: “Supplementary Fig. 17 |\nDurability evaluation of the CuPc@GO photodetector under twisting strain. After\ntwisting CuPc@GO photodetector by 30° for 1300 times, the performance was retained\nat 95.2% under 240 kPa.”\nThese systematic mechanical stability characterizations confirm that the CuPc@GO\nflexible photodetector exhibits outstanding resistance to both torsion and bending\ndeformation. The minimal degradation in photoresponsivity after repeated mechanical\ncycling verifies the structural integrity of the CuPc@GO film, the reliability of the\nphotodetector’s electrode/film interface, and the robustness of the overall photodetector\narchitecture. Such excellent mechanical stability is crucial for the photodetector to meet\nthe practical application requirements in wearable electronics, flexible optoelectronic\nsystems, and other scenarios involving dynamic mechanical stress, further supporting\nthe feasibility of the CuPc@GO for flexible optoelectronic applications.\nOn the comment “the circuit design diagram and physical images of the data acquisition\nsystem used in this setup”, as shown in Fig. R19, a custom-designed control board was\napplied for control of the robotic hand with 5 degrees of freedom acquiring the signal\nfrom the CuPc@GO-based e-skin mounted at the fingertips. In detail, the control board\ncontains five headstages, a microcontroller (MCU, STM32F103, STMicroelectronics,\nItaly), a multi-channel 16-bit analog-digital converter (AD7175-8, Analog Devices inc,\nUnited states), a 16-bit digital-analog converter (AD5663, Analog Devices, United\nstates), Bluetooth module, signal condition circuit, power management circuit and Li-\nbattery. Each finger in the robotic hand is driven by a servo motor, which is controlled\nthrough pulse width modulation (PWM) generated by the MCU. To eliminate the noise\ninduced from the transmission cable during measurements, the headstages are\nintroduced to pre-amplify the current signal of the CuPc@GO-based e-skin and convert\ncurrent signal to voltage signal through a transimpedance amplifier. Then the amplified\nsignals are acquired by the analog-digital converter. The voltage bias is adjusted by the\ndigital-analog converter, whose output is modulated and calibrated by the signal\ncondition circuit. An operational amplifier (OPA) in the headstage works as a follower\nto apply the voltage bias. The acquired data is transmitted by the Bluetooth module\nthrough serial communication. A host computer receives and records the data by using\na custom-designed MATLAB program. To ensure the accuracy of the output voltage\nand the acquired data, both the ADC and DAC refer to a high precise voltage reference\n51\n(ADR4520, Analog Devices, United states). The robotic hand and the control board are\nsupplied by a Li-ion battery with a power management circuit, which generates DC\nsources with voltages includes 3.3 V, ±2.5 V, ±5 V. During experiment, an embedded\nprogram is developed in the MCU to ensure the fast response for controlling the\nmovement of the fingers according to the feedback signal from the CuPc@GO-based\ne-skin.\nFig. R19. (A) Diagram of the custom-designed control board for signal acquisition and\nrobotic hand control; (B) Photography of the robotic hand with integration of the control\nboard and the CuPc@GO photodetectors.\nChange made:\nPlease kindly see Page 10, Lines 39-41, Line 1 in revised manuscript: “We\ndesigned a control board for signal acquisition and actuation of the robotic\nmanipulator (Fig. 4g), which was subsequently installed on a robotic manipulator\nincorporating the CuPc@GO e-skin into the fingertips (Fig. 4h).”\nFig. 4g and its caption in revised manuscript: “Diagram of the custom-designed\ncontrol board for signal acquisition and robotic hand control.”\nFig. 4h and its caption in revised manuscript: “Photography of the robotic hand\nwith integration of the control board and the CuPc@GO photodetectors.”\nOther suggestions:\n1) Several data points in the table in the supplementary information are incorrect.\nMoreover, the authors compared device parameters under several different light\nintensity ranges, yet data for their device are missing in multiple segments.\nReply: We have re-ranged the data in Table S1 with the segments of input illumination\nof 1 nW-10 nW, 10 nW-100 nW, 100 nW-1 μW, 1 μW-100 μW and 100 μW-10 mW,\nrespectively, and now all data points in the table in the supplementary information\nshows reasonably.\ndensity to illumination power using “illumination power = power density × effective\narea”, where the power density was provided, and the effective area was directly\nprovided by or obtained through the data shown in the original reports. For the cases\nwhere the effective area was not directly provided, we have listed both the data source\nand our computing method by which the effect area was derived in Table S1.\nChange Made:\nWe have re-ranged the data in Table S1 with the segments of input illumination of\n1 nW-10 nW, 10 nW-100 nW, 100 nW-1 μW, 1 μW-100 μW and 100 μW-10 mW,\nrespectively, and now all data points in the table in the supplementary information\nshows reasonably.\nSupplementary Table 1 and its caption in supplementary material: “Supplementary\nTable 1 | Metrics and physical parameters of current state-of-the-art photodetectors.”\n2) It is surprising that the authors even misspelled the abbreviation of their commonly\nused material, copper phthalocyanine (CuPc).\nReply: Thanks. We have changed CuPC to CuPc in all the files. We used CuPC in the\nprevious manuscript since the following literatures used it [Inorg. Chem. 47, 1255\n(2008); Adv. Energy Mater. 5, 1501320 (2015); Adv. Sci. 11, 2304735 (2024)]. We also\nfound many literatures used CuPc [Nature 400, 245 (1999); Nat. Mater. 8, 115 (2009)\nand Nat. Commun. 11, 3592 (2020)].\nWe sincerely thank the Reviewer for the very helpful suggestions and very constructive\ncomments, which help us to make a more efficient and pleasant manuscript. According\nthe Reviewer’s comments, we have revised the manuscript. We think that all\nambiguities in the manuscript have been clarified. We hope that the Reviewer will be\nsatisfied with our response and will now recommend its publication in Nature\nCommunication.\n53"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors provided detailed answers to the referees’ comments and revised the manuscript accordingly. Especially, my main questions are fully addressed. Now I recommend the manuscript for publication in Nature Communications.", "reviewer_2": "The authors have addressed most of my concerns. However, further clarification is still needed regarding the piezoelectricity of the CuPC@GO membrane. The authors cited the fig2e from the reference paper to demonstrate the piezoelectric response. These voltage responses with pressure in fig2e could sometimes originate from other mechanisms, such as the triboelectric effect or the electret behavior. Because that the cited paper did not provide direct characterization of the intrinsic piezoelectric properties, such as PFM phase/amplitude loops or d33 meter measurements, I suggest the authors verify the piezoelectricity of the CuPC@GO membrane using these direct methods to support their claim, or further clarify the working mechanism of the device as a strain sensor.", "reviewer_3": "In this revision, the authors have addressed the questions properly. In this case, it is ready to accept this work in the current format."}, "author_response": "Reviewer #2 (Remarks to the Author):\nThe authors have addressed most of my concerns. However, further clarification is\nstill needed regarding the piezoelectricity of the CuPC@GO membrane. The authors\ncited the fig2e from the reference paper to demonstrate the piezoelectric response.\nThese voltage responses with pressure in fig2e could sometimes originate from other\nmechanisms, such as the triboelectric effect or the electret behavior. Because that the\ncited paper did not provide direct characterization of the intrinsic piezoelectric\nproperties, such as PFM phase/amplitude loops or d33 meter measurements, I suggest\nthe authors verify the piezoelectricity of the CuPC@GO membrane using these direct\nmethods to support their claim, or further clarify the working mechanism of the\ndevice as a strain sensor.\nResponse: We sincerely thank the reviewer for the positive feedback. According to the\nreviewer’s suggestion, we have performed characterization of the intrinsic\npiezoelectric properties of the CuPc@GO membrane by employing the d meter.\nexhibit piezoelectricity with the piezoelectric coefficient d of 0.7 to 1.6 (pC/N),\n33\nwhich are comparable to the d values (1.95–2.09 pC/N) of the MoS /WS\n33 2 2\nheterobilayer [Science 376, 973–978 (2022)]. It is noteworthy that even the multilayer\ngraphene oxide (GO) exhibits piezoelectricity [d = 0.59 pC/N; see: Physica Scripta\n33\n101, 075928 (2026)]. It appears that the stacking between CuPc and GO enhances the\nout-of-plane piezoelectricity as compared with the multilayer GO, reminiscent of the\ncase of MoS /WS heterobilayer (individual layers of WS and MoS have d = 0, but\n2 2 2 2 33\nthe heterobilayer has a non-zero value of d ; see the Science paper mentioned above).\nprevious paper (Int. J. Mol. Sci. 25, 6713-6724 (2024)) and the abrupt current\nincreases during contact sensing in Figs. 4c-f of this manuscript.\nTable 1. Piezoelectric coefficient d (in pC/N) of CuPc@GO membranes obtained by\n33\nthe quasi-static d meter. We totally prepared three samples of the CuPc@GO\n33\nmembrane; for each sample, we chose three areas for measuring the d . The values of\n33\nd for the three samples are listed in descending order of the average value of d .\n33 33\nSample Area 1 Area 2 Area 3 Average\nNumber\n1 1.6 1.4 1.2 1.4\n2 1.1 1.1 0.9 1.0\n3 0.7 0.9 0.8 0.8"}]

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