Indoor Self‐Powered Perovskite Optoelectronics with Ultraflexible Monochromatic Light Source

Self‐powered skin optoelectronics fabricated on ultrathin polymer films is emerging as one of the most promising components for the next‐generation Internet of Things (IoT) technology. However, a longstanding challenge is the device underperformance owing to the low process temperature of polymer substrates. In addition, broadband electroluminescence (EL) based on organic or polymer semiconductors inevitably suffers from periodic spectral distortion due to Fabry–Pérot (FP) interference upon substrate bending, preventing advanced applications. Here, ultraflexible skin optoelectronics integrating high‐performance solar cells and monochromatic light‐emitting diodes using solution‐processed perovskite semiconductors is presented. n–i–p perovskite solar cells and perovskite nanocrystal light‐emitting diodes (PNC‐LEDs), with power‐conversion and current efficiencies of 18.2% and 15.2 cd A−1, respectively, are demonstrated on ultrathin polymer substrates with high thermal stability, which is a record‐high efficiency for ultraflexible perovskite solar cell. The narrowband EL with a full width at half‐maximum of 23 nm successfully eliminates FP interference, yielding bending‐insensitive spectra even under 50% of mechanical compression. Photo‐plethysmography using the skin optoelectronic device demonstrates a signal selectivity of 98.2% at 87 bpm pulse. The results presented here pave the way to inexpensive and high‐performance ultrathin optoelectronics for self‐powered applications such as wearable displays and indoor IoT sensors.


Introduction
Ultraflexible optoelectronics that intimately adhere to skin are highly desirable for new applications such as health monitoring [1,2] and medical treatment [3,4] making them an essential component for the nextgeneration Internet of Things (IoTs).8][9][10][11] These devices demand ultralow bending stiffness, which is achieved by employing soft polymer substrates with a thickness down to ≈1-5 μm.In particular, when the total device thickness is lower than 5 μm, the low bending stiffness makes its skin-adhered state more stable than freestanding, [12] thereby enabling adhesion to complex surfaces like the epidermis, [13][14][15] without causing any uncomfortable sensation.Ultraflexible devices also have an advantage for their excellent biocompatibility due to their low Young's modulus.For example, an ultraflexible amplifier combined with a biocompatible gel electrode was implanted into a hypodermal tissue of a goat, which showed minimal foreign-body reaction even after 4 weeks of implantation. [16]The integration of energy-harvesting components, [17] such as triboelectric nanogenerators, [18] thermoelectric generators, [19] and photovoltaics, [20] could power such devices without downtime.
A great challenge for the fabrication of ultraflexible skin optoelectronics is the required low processing temperature limited by the thermal stability of underlying polymer substrates.As such, organic and polymer semiconductors have been extensively used in devices to fulfill desirable functions. [21,22]These devices have demonstrated remarkable mechanical flexibility enduring a minimum bending radius of 10 μm or less, [14] successfully enabling new applications such as self-powered sensors that integrate organic photovoltaics (OPVs) and organic electrochemical transistors [23] or organic light-emitting diodes (OLEDs). [20]owever, organic devices in skin optoelectronics have intrinsic performance limitations which do not always fulfill specific system requirements.For example, although ultraflexible OPVs can be effectively powered by low-intensity ambient light, the reported highest power conversion efficiency (PCE) is only 15.8%. [21]As a result, a relatively large photovoltaic layout area is required to reach a desirable power output.On the other hand, ultraflexible organic or polymer LEDs typically generate broadband electroluminescence (EL), which would inevitably suffer from spectral distortion due to Fabry-Pérot (FP) interference from the ultrathin substrate. [24,25]It is highly undesirable for many applications such as on-skin displays and photo-plethysmography, as it yields angular-and bending-dependent chromaticity [24,25] and substantially increases the background noise. [26]merging candidates that meet the demanding material requirements for self-powered ultraflexible optoelectronics include II-VI and III-V inorganic quantum dots (QDs) [27][28][29] and lead halide perovskites (LHPs). [30][33][34][35][36][37][38][39] The EL spectral distortion in ultraflexible LEDs resulting from FP interference can be effectively eliminated by fabricating a perovskite nanocrystal LED (PNC-LED) on an ultrathin substrate.Indeed, the ultranarrowband emission of perovskite nanocrystal (<25 nm) enables near interference-free transmittance of the emitted light from a PNC-LED system, as it is considerably smaller than the periodicity of the FP interference (≈50 nm) within the substrate.Nevertheless, the realization of high-efficiency perovskite optoelectronic devices on ultrathin polymer substrates remains challenging due to the limitations regarding material choices and processing conditions. [40,41]ere we report the first self-powered skin optoelectronic system that integrates a high-efficiency solar cell module and a monochromatic nanocrystal LED using solution-processed perovskite semiconductors (Figure 1a,b).Thanks to the excellent power conversion efficiency of the perovskite solar cell (PSC) module, indoor room light is sufficient to power the system.The narrowband emission generated by the PNC-LED enables an FPinterference-free light source, exhibiting consistent EL spectra irrespective of the viewing angle (Figure 1c-e) as well as mechanical compression, highlighting their outstanding reliability under deformation.We demonstrate that the monochromatic emission is particularly useful for selective pulse detection in PPG with 98.4% signal selectivity.

Ultraflexible Perovskite Solar Cells
The realization of high-performance PSCs on ultrathin polymer substrates has been considered to be a challenging task.Indeed, special processing restriction of ultrathin polymer materials hinders direct employment of perovskite materials and device architectures developed on rigid substrates.Conventional ultrathin substrate materials, including polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), have relatively low glass transition temperatures. [41]Thus, most ultraflexible PSCs demonstrated in the literature were fabricated using p-i-n architecture with polymer hole-transporting layer (HTL) materials, thereby undermining current extraction and voltage output by nonradiative recombination. [42,43]igure 2a presents the top-view schematic architecture of our ultraflexible PSCs and the chemical structure of the HTL material.We fabricated these PSCs using an n-i-p architecture, with SnO 2 as the electron-transporting layer, following its success in state-of-the-art high-efficiency PSCs on rigid glass substrates, making use of the high melting temperature (290 °C) of the parylene substrate material used here, which enables favorable crystal formation of the perovskite thin films. [44,45]The perovskite composition and PSC fabrication process were first optimized on glass substrates (Figure S1, Supporting Information).The highest PCE reached in our optimized PSCs was 20.6%, with reasonable fabrication reproducibility (Figure S2, Supporting Information).The developed protocol was subsequently used to fabricate ultraflexible PSCs.
It is noteworthy that although crystalline indium-tin oxide (ITO) has a higher conductivity, it is not compatible with ultraflexible devices due to its brittleness and high deposition temperature.Thus, we developed protocols for the deposition of amorphous ITO (a-ITO) on ultraflexible parylene substrates (Figure S3, Supporting Information) by optimizing process pressure and oxygen flow rate.The deposited a-ITO films reach sheet resistances of down to 50.1 Ω sq −1 and excellent flexibility (Figure S4, Supporting Information).Crystalline perovskite films grown on glass (Figure 2b) and polymer (Figure 2c) substrates have consistent grain sizes of ≈700 nm, as well as comparable X-ray diffraction (XRD) spectra (Figure 2d).We observed that the radio frequency (RF) sputtering power used for a-ITO deposition plays a critical role to obtain high perovskite crystallinity (Figure S5, Supporting Information).Figure 2e presents a cross-sectional transmission electron microscope (TEM) image for a device fabricated on a 1.5 μm thick parylene substrate (also see Figure S6 in the Supporting Information).
Representative current density-voltage (J-V) responses for our fabricated PSCs fabricated on rigid glass and ultraflexible substrates are shown in Figure 2f, corresponding to devices with PCEs of 19.0% and 18.2%, respectively (extracted photovoltaic characteristics see Table 1).The inset shows a photograph of our ultraflexible PSC.Ultraflexible PSCs exhibited comparable performance, together with excellent hysteresis and flexibility (Figures S7-S9, Supporting Information).We note that our ultraflexible PSCs have an improved short-circuit current, J SC , but the fill factor (FF) is lower as compared to the glass PSCs.We attribute the J SC enhancement to a higher transmittance of the thin parylene substrate (Figure S10, Supporting Information), while the higher sheet resistance of the a-ITO layer appears to slightly reduce the device FF (Figure S11, Supporting Information).The intrinsic PCE (IPCE) spectra of our PSCs are shown in Figure 2g and Figure S12 (Supporting Information), in which the integrated J SC value of our ultraflexible PSC is 21.39 mA cm −2 , consistent with that in J-V responses (23.0 mA cm −2 ).We notice that among all ultraflexible (with polymer substrates of a thickness of <3 μm) solar cells in the literature, our PSC showed the best PCE (Figure S13 and Table S1, Supporting Information). [21,30]

Ultraflexible Perovskite Nanocrystal LEDs
Following our earlier discussion, an important motivation for the fabrication of monochromatic light source on ultrathin substrates is to eliminate spectral distortion upon substrate bending, originated from FP interference when the substrate thickness, d, is comparable to the EL wavelength, .Specifically, because the substrate refractive index, n sub , is greater than that in air, n 0 , the propagation of light within the thin substrate is analogous to that in an optical cavity sandwiched between two parallel reflecting interfaces.Waves can transmit through the optical cavity only when they are in resonance with it.For example, consider light at a normal incidence angle passing through a thin plastic film with thickness, d = fd 0 , where d 0 is the reference thickness, and f is the scaling factor.The optical waves would experience constructive interference when 2d = q q , where q is an integer, such that the transmittance spectrum would be disturbed with a variable periodicity, ∆, given by When taking into account all incidence angles, , the offaxis modes yield an angle dependence following 2dn sub cos  = q q , [46] resulting in parabolic ripples that distort emission spectra upon substrate tilting and bending (see Figure 1d,e).Equation (1) clearly suggests that FP interference periodicity ∆ decreases with increasing the scaling factor f. Indeed, Figure S14 (Supporting Information) presents the experimentally characterized transmittance spectra of three parylene thin films with different thicknesses, revealing that, within the visible spectral region, ∆ increases with substrate thickness.Given a robust substrate thickness typically ranging from 1 to 5 μm, our calculations suggest that for visible light LEDs, the EL full width at half maximum (FWHM) needs to be smaller than 25 nm (Figures S15 and S16, Supporting Information).PNC-LEDs are the only known solution-processed semiconductor technology fulfilling this condition, outperforming other candidates, including OLEDs [20] and inorganic QLEDs. [47]igure 3a presents the schematic architecture of our ultraflexible PNC-LEDs and chemical structures of transport materials.The LEDs consist of a thin-film stack of ITO/poly (3,4-  3b; Figure S17, Supporting Information).[39] We optimized the ligand concentration and tail chain length to tune the nanocrystal size without impeding carrier injection in LEDs. [48] double-cation perovskite composition is employed to achieve longer operational lifetimes (Figures S18-S20 and Table S2, Supporting Information). [49]igure 3c shows a representative ultraflexible PNC-LED fabricated on ultrathin polymer substrate, demonstrating bright EL.Luminance and current density as a function of driving voltage are shown in Figure 3d,e, respectively.The LEDs fabricated on both glass and ultraflexible substrates generate decent luminance of greater than 500 cd m −2 at 4 V, with a consistent narrowband emission with an FWHM of ≈23 nm.We demonstrated current efficiencies (EQEs) of 16.8 cd A −1 (4.15%) and 15.2 cd A −1 (3.84%) for rigid and ultraflexible PNC-LEDs, respectively.A degree of enhancement in current efficiency compared to earlier work [50] is attributed to interface engineering and optimization of the nanocrystal ligand and compositions.
We examined the mechanical durability of our fabricated ultraflexible PNC-LEDs.First, the ultraflexible devices were laminated and peeled off from the supportive glass substrates for a bending test (Figure S21, Supporting Information).The freestanding LED device showed stable and consistent luminance-voltage response for up to 1000 cycles of mechanical bending with an 8 mm bending radius.Second, a compression test was carried out by laminating the ultraflexible devices and transferring them to a prestretched elastomer (Figure S22a, Supporting Information).The elastomer strain was then reduced in turn compressing the device (Figure 3f). Figure 3e presents EL spectra of a representative ultraflexible PNC-LED upon compression (for more optoelectronic characteristics, see Figure S22b in the Supporting Information).Remarkably, the EL spectrum remains consistent even under compression, in contrast with other ultraflexible LEDs reported in the literature, [24,25] highlighting its bending-insensitive emission, which has never been demonstrated.
A more rigorous mechanical stability test was thus carried out.Figure 3h presents driving voltage and luminance with respect to the number of compression cycles, under a constant driving current of 2 mA (see also Video S1 in the Supporting Information).The driving voltage remained stable for up to 10 cycles of 33% mechanical compression.Although device luminance dropped in the first cycle of compression, it stabilized afterward.We suppose this comes from the relatively low operational stability of perovskite LEDs, rather than materials' instability induced by mechanical stress upon compression.Our concern for stable luminance measurement is echoed by the stability test of our perovskite LED (Figure S23, Supporting Information).Accordingly, we consider the ultraflexible PNC-LEDs presented here as sufficiently robust for a number of applications, such as PPG measurement.

Selective PPG Measurement with Ultraflexible Monochromatic LEDs
Our angle-independent and bending-insensitive LEDs enabled by narrowband EL open technological opportunities for accurate and selective optical sensing of body signals.Specifically, pulse oximetry requires two reference light sources of red and green color tones, [51] and 2D PPG mapping is generated by local light sources for accurate detection. [52]Thus, in practice, in order to ensure a reliable readout, two optical bandpass (BP) filters are often introduced to selectively screen-reflected light, of which one matches the LED peak maximum, and the other is chosen at a different spectral region as the reference signal. [51,52]As revealed in Figure 4a, a set of BP filters were able to cut off and differentiate EL generated from a PNC-LED, which is not the case for a broadband OLED (yellow spectrum which is distorted by FP interference).EL generated by an ultraflexible PNC-LED is accurately filtered and differentiated by the 520 and 600 nm BP filters (Figure 4b,c), where the former allows complete EL transmission while the latter cuts it off effectively.
The integration of our monochromatic PNC-LED and BP filters represents an ideal platform for PPG measurement.Figure 4d (see also Figure S24a in the Supporting Information) presents a schematic diagram of our PPG measurement setup.A PNC-LED is placed at the finger pulp, emitting light that propagates through the blood vessels.The intensity of the reflected light is then measured by a photodiode (PD).The BP filter placed in front of the PD allows detection of light within a specific spectral region, which differentiates the desirable PPG signal from environmental noise and other vital signals.
Figure 4e shows the fast Fourier transform (FFT) spectra of a representative PPG measurement using a PNC-LED as the light source.The inset (see also Figure S24b in the Supporting Information) compares the original PD signals filtered by the 520 nm (red) and 600 nm (black) BP filters.The resulting FFT spectra yield an excellent signal selectivity of up to 98.7% at a pulse frequency of 87 bpm (beats per minute).

Self-Powered Ultraflexible Light Source
The optoelectronic components and functionalities demonstrated above promise the application of self-powered skin optoelectronics (Figure S25, Supporting Information).We designed and fabricated a perovskite solar cell module connecting six single cells in series to drive the PNC-LED (Figure 5a,b).The current-voltage response of the ultraflexible solar cell module under 1 sun illumination is shown in Figure 5c, generating an output voltage of 4.76 V and a current of 3.1 mA.Considering the device's active area and number of solar cell units, the effective J SC , V OC , FF, and PCE of each cell within the module are 15.4 mA cm −2 , 0.79 V, 0.40, and 4.9%, respectively (Table S3, Supporting Information).The root causes responsible for the lowered device  characteristics include increased surface roughness and high series contact resistance of ITO electrodes in large-area cells.Nevertheless, the ultraflexible solar cell module offers a maximum output power of 5.88 mW (Figure 5a), which is sufficient to drive a PNC-LED.
According to the circuit diagram shown in Figure 5b, we estimate the operating voltage to be 4.1 V by overlaying the currentvoltage responses of our perovskite solar cell module and PNC-LED, under 1 sun illumination.Figure 5d presents LED luminance as a function of light intensity of our solar simulator.The LED luminance increases monotonically with the light intensity, generating a luminance of 50 cd m −2 under 1 sun illumination.Figure 5e features a photograph of a self-powered, all-perovskite device during operation.The ultraflexible LED was successfully powered and generated narrowband EL.The same system remains functional when only powered with indoor ambient light (Figure 5f), which proves its applicability for ultraflexible, selfpowered IoT devices used under indoor lighting conditions.The stable operation of our system under the low intensity of light is also supported by our PCE measurements under different light intensities, showing a stable PCE of 13.7% even under 0.01 sun illumination (see Figure S26 in the Supporting Information).
One of limitations for our current system is its long-term stability.As shown in Figure S23 (Supporting Information), a lifetime maintaining >80% of the initial efficiency (LT80) was measured to be less than 10 min for our perovskite LEDs, while LT80 for our perovskite solar cells was measured to be 6.95 h (Figure S27, Supporting Information).The commercialization of our self-powered skin devices would demand to overcome the low operational stability of our devices.On the other hand, the toxicity of lead is also a great concern for wearable perovskite devices, which are in close contact to skin.To address this challenge, there are a number of approaches proposed to minimize lead leakage from the devices such as adding a lead absorption layer within the encapsulation of the perovskite devices, [53][54][55] using an electron injection layer with lead absorption properties, [56] or mixing lead absorbing molecules into the perovskite precursors. [57,58]We believe that the introduction of these approaches in our ultraflexible perovskite devices could minimize lead leakage to skin, one big step closer to biocompatibility.

Conclusion
In this report, we have developed materials and processes for the fabrication of ultraflexible self-powered skin optoelectronics using solution-processed perovskite semiconductors.Both ultraflexible perovskite solar cells and PNC-LEDs presented here achieved state-of-the-art efficiencies with the solar cell setting new record-high efficiencies.Particularly, spectral distortion of the monochromatic narrowband EL resulting from ultrathinsubstrate-induced FP interference was successfully eliminated, enabling a bending-insensitive light source.We have demonstrated highly selective PPG measurements with our ultraflexible monochromatic LEDs, as well as a self-powered ultraflexible light source under indoor lighting conditions for net-zero-energy IoT devices.We believe the development of skin optoelectronics on ultrathin polymer thin films will be greatly facilitated by the engineering strategies and materials processing approaches presented here.layer.Note that ultraflexible PLED was fabricated according to the previously reported method. [16]ll-Perovskite, Self-Powered, Ultraflexible Device Fabrication: Each of the perovskite devices was fabricated on ultraflexible substrate.The gold wiring was coated on a 1 μm thick parylene formed on 12.5 μm thick polyimide film.Ultraflexible PNC-LED and perovskite solar cell module were connected by gold wiring.PNC-LED and perovskite solar cell module were peeled off from the glass substrate and gold wiring on a 1 μm thick parylene was peeled off from the polyimide film.
Device Characterization: The PSC with an active area of 0.16 cm 2 was characterized under a 1 sun illumination using a solar simulator with black photomask (LED Solar Simulator, Class ABA, MiniSol, The reference cell 91150-KG5, Newport).The J-V characteristics were recorded using a Keithley B1500A analyzer with every 40 mV in an ambient laboratory.Gold butterfly wirings were used for the solid electrical contacts of the PSCs on ultrathin substrates.The 100 nm thick gold was deposited with e-beam evaporation onto 12.5 μm thick polyimide films.One side of the wirings was connected to the electrodes on the freestanding foils using an electrically conductive adhesive transfer tape (3M, ECATT 9703).The other sides were connected to the source meter using alligator clips.For evaluating the PNC-LED characteristics, 2400 source meter (Keithley) and spectrometer (SpectraScan PR 655, JADAK) were used.All measurements were performed in an ambient atmosphere.UV-vis absorption spectra were measured using a UV-vis-NIR spectro-photometer (V670, JASSO).The photoluminescence (PL) quantum yield was measured using the Quantaurus-QY (C11347-11) from Hamamatsu Photonics.Timeresolved photoluminescence (TRPL) spectra were characterized using a Quantaurus-Tau (C11367-31, Hamamatsu Photonics) fluorescence lifetime spectrometer.Scanning electron microscope (SEM) images of the perovskite film were measured at the acceleration of 3 kV (ULTRA plus, Zeiss).To perform perovskite PNC-LED operation experiment with the power of perovskite solar cell module, solar simulator and calibration cells were used to modulate light intensity for simulated sunlight input.The light intensity of the solar simulator was determined by the distance of solar simulator and calibrated by the cells.
Optical Simulation of Ultraflexible LED: Finite element analysis for ultraflexible LED was simulated with SETFOS (FLUXiM) emission module.By expecting the dipole emission model of LED, the module was allowed to deliver emission spectra of ultraflexible LED with different angle.The optical parameters of each layer for the simulation are shown in Table S4 and Figure S28 (Supporting Information).

Figure 1 .
Figure 1.Self-powered skin optoelectronics integrating high-efficiency solar cells and monochromatic LEDs using perovskite semiconductors.a) Schematic illustration of our skin optoelectronic system containing a perovskite solar cell module that powers a monochromatic perovskite nanocrystal LED (PNC-LED).b) A photograph of the self-powered skin optoelectronic system laminated and intimately adhering to skin.Scale bar: 1 cm.c) Schematic comparison between ultraflexible OLED and PNC-LED, in which the latter has a narrowband EL rendered possible by suppressing FP interference from the ultrathin plastic substrate, enabling bending-insensitive light source for accurate and local signal sensing.d,e) Calculated EL spectra as a function of the viewing angle for d) ultraflexible OLED and e) ultraflexible PNC-LED using the dipole emission model.The color bar represents normalized EL intensity.

Figure 2 .
Figure 2. Fabrication and characteristics of ultraflexible perovskite solar cells powering skin optoelectronics.a) Schematic diagram of our PSC fabricated on 1.5 μm thick thermally stable parylene/SU-8 plastic substrate.Scale bar: 1 cm.b,c) SEM images of perovskite films grown on b) glass and c) plastic substrates.Scale bar: 1 μm.d) XRD spectra of perovskite films grown on glass (top) and plastic (bottom) substrates.e) Cross-sectional TEM image of the complete ultraflexible PSC.Scale bar: 1 μm.f) Representative J-V responses for PSCs fabricated on glass (black) and ultraflexible (red) substrates.Inset panel shows photograph of a freestanding PSC device.Scale bar: 1 cm.g) Representative IPCE spectrum and integrated J SC response of our ultraflexible PSC, giving an integrated value of 21.39 mA cm −2 .

Figure 3 .
Figure 3. Ultraflexible perovskite nanocrystal LEDs demonstrating bending-insensitive monochromatic electroluminescence. a) A schematic diagram of PNC-LEDs fabricated on ultrathin parylene/SU-8 plastic substrate.Insets present a cryo-TEM image of perovskite nanocrystals and a photograph of the colloidal solution of perovskite nanocrystals, as well as a schematic illustration of perovskite nanocrystals synthesized by using mixed cations and optimized ligand chain length.b) Cross-sectional TEM image of the fabricated ultraflexible PNC-LED.Scale bar: 1 μm.c) Photograph of ultraflexible PNC-LED under electrical operation.Scale bar: 1 cm.Representative d) Current and e) luminance as a function of voltage responses for our fabricated PNC-LEDs.f) Schematic diagram of the mechanical compression test applied to our ultraflexible PNC-LEDs.g) EL spectra obtained upon mechanical compression.h) Representative luminance and voltage responses for fabricated ultraflexible LEDs as a function of number of compression cycles (33% mechanical compression).Insets show ultraflexible LEDs before and after compression.Scale bar: 1 cm.

Figure 4 .
Figure 4. Pulse-plethysmography (PPG) sensing using a monochromatic PNC-LED as light source.a) Transmittance spectra for our 520 and 600 nm BP filters superimposed on the EL spectra of our PNC-LED (green) and a representative OLED (yellow) fabricated on ultrathin polymer substrates.b,c) Photographs of the EL generated by a PNC-LED (at 4 V) filtered by b) the 520 nm and c) 600 nm BP filters.Scale bar: 1 cm.d) Schematic diagram of our PPG measurement setup.The vessel-reflected light is transmitted through the 520 nm BP filter for PD detection.e) FFT spectra of PPG signals filtered by the 520 nm (red) and 600 nm (black) BP filters, giving an excellent signal selectivity of up to 98.7% at a pulse frequency of 87 bpm.

Figure 5 .
Figure 5. Demonstration of a self-powered ultraflexible skin optoelectronic system.a) Representative photograph of our fabricated self-powered skin optoelectronic system integrating an ultraflexible perovskite solar cell module and PNC-LED light source.Scale bar: 1 cm.b) Circuit diagram of the skin optoelectronic system.c) Measured current density-voltage response of the fabricated ultraflexible perovskite solar cell module made by connecting six cells in series, yielding a maximum power of 5.88 mW under 1 sun irradiation.d) PNC-LED luminance with respect to input light intensity of the solar cell module.e,f) Photographs of the skin optoelectronic system under electrical operation, powered by e) sunlight or f) indoor room light.Scale bar: 1 cm.

Table 1 .
Extracted PV characteristics for PSCs fabricated on glass and ultraflexible substrates.