Extensive Properties Investigation of AgNWs‐PEDOT:PSS Based Transparent Conductive Electrode for Ultraflexible Organic Photodetector Applications

In this work, the optoelectrical, mechanical, and material properties of silver nanowires/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (AgNWs‐PEDOT:PSS) hybrid electrode, solution processed on 5 µm parylene‐C substrate for ultraflexible organic photodetector (OPD) applications, are investigated. The fabricated transparent conductive hybrid electrode exhibits low sheet resistance (Rs) of 27 Ω Sq.−1 and shows an average optical transmission of 90% in the visible spectrum. Besides the comparable work function (WF) of 5 eV when compared with the standard indium tin oxide (ITO) electrode, the surface root mean square (RMS) roughness of the hybrid electrode is effectively reduced from 8 nm to 4 nm by spin coating a planarization layer of PEDOT:PSS. With a stable value in the relative change in resistance over continuous 50 k bending cycles at a curvature radius of 0.1 mm, the hybrid electrode exhibits excellent mechanical characteristics. Finally, using this hybrid electrode it is shown that an ultraflexible OPD fabricated in ambient atmospheric conditions demonstrates current density to voltage (J–V) characteristics comparable with standard ITO‐based OPD. Further, the ultraflexible OPD can withstand more than 5 k continuous bending cycles with similar J–V characteristics before and after bending.


Introduction
[3][4][5][6] Unlike conventional inorganic photodetectors DOI: 10.1002/aelm.202300559(PDs), which are rigid and brittle, OPDs with organic semiconductor (OSC) active materials can be fabricated onto ultraflexible substrates thus enabling them to conform to the intricate forms and contours of curvilinear shapes, as instance the human body, without compromising their performances. [7]Therefore, flexible OPDs have opened up new opportunities toward the design and their use in several wearable electronic devices including continuous health monitoring systems. [8,9]For example, ultraflexible OPDs integrated into wearable pulse oximeter device systems including skin patches are used to monitor blood oxygen saturation levels by sensing either the reflected or transmitted light from the biological tissues. [10,11][14] Transparent conductive electrodes (TCEs) are considered to be one of the most crucial part of the OPDs as they play a significant role in improving the OPD efficiency by effectively taking part in light transmission and charge extraction. [15,16]Typically, TCEs are designed to exhibit high transparency thereby allowing the incoming light to pass through them with minimal loss and hence play a critical role in maintaining the overall transparency of the OPD.Furthermore, in order to enable the efficient extraction of photogenerated charge carriers by the absorbed light, TCEs should possess excellent electrical conductivity.For the TCEs with low electrical conductivity, the charge carriers may undergo considerable recombination or become trapped resulting in the loss of photocurrents and reduced device efficiencies. [17,18]On the other hand, the high electrical conductivity of the TCEs minimizes signal loss and capacitive effect and allows for fast response times, thus enhancing the overall performance of the OPD. [19,20]However, the transparency and electrical conductivity of the TCEs are inversely related.Higher conductivity of TCEs often results in low transparency as it causes more light to be scattered or absorbed. [21,22]Hence there is a need to choose suitable TCE materials that can provide high conductivity and flexibility while maintaining the optical transparency.
In addition to excellent optical and electrical properties, OPDs with high mechanical flexibility require TCEs to withstand mechanical deformation while maintaining the conductivity under bending conditions.Standard TCEs rely on transparent metal oxides such as ITO due to their excellent optoelectronic properties, such as low sheet resistance (10-20 Ω Sq. −1 ) and high optical transmission (>80%) in the visible range. [23]Therefore, ITO coated on plastic substrates such as polyethylene terephthlate (PET) or polyethylene naphthalate (PEN) is still being used for flexible optoelectronic applications. [24]However, the intrinsic brittle nature of ITO thin films often leads to cracking when subjected to continuous bending and hence this property has limited its use in long-time flexible device operation. [25]Additionally, ITO suffers from chemical instability, [26] including toxicity of indium. [27]Moreover, the limited natural resource of indium may become soon an issue for industrial applications and commercialization of optoelectronic devices.Further, ITO requires complex vacuum processing conditions such as sputtering that drives up the manufacturing cost. [28]Another crucial factor for ITO is its lack of adhesion to organic and polymer layers that further limits the manufacturing of ITO-based flexible devices on plastic substrates. [29,30]For all these reasons, increasing efforts have been made by researchers, and several materials are being explored and investigated to replace conventional ITO.Materials such as graphene, [31] carbon nanotubes, [32] metal nanowires (NWs), [33] metal networks, [34] and conducting polymers [35] have proven their ability to replace conventional ITO by offering excellent optoelectrical and flexible properties.Additionally, these materials are compatible with organic polymer materials for device applications and can be solution-processed thus resulting in the reduction of the processing complexity and scaling down the cost compared to traditional ITO.
Among the various solution-processable materials, 1D metal structures such as AgNWs with high aspect ratio are considered to be superior and are perceived as promising candidates for TCEs due to some of their remarkable properties such as low percolation threshold, high electrical conductivity, high optical transparency, and excellent flexibility. [36,37]The typical R s values of AgNWs thin film lies in the range of 15-25 ΩSq.−1 and transparency of 80-90% in the visible spectrum range. [38]enerally, the free volume present in between the AgNWs thin film allows light to pass through thereby increasing the transparency, whereas the continuous 3D network of AgNWs allows high lateral electrical conductivity thus enabling efficient charge transport. [39]Moreover, spin coating of AgNWs offers versatile thin film processability with the advantage of thickness tunability and uniform surface properties. [40]On the other hand, one of the major drawbacks of using AgNWs as TCEs is that the electrical conduction of the AgNWs is affected by the junction resistance or contact resistance [41] between the nanowires and the packing density. [42]Though the packing density could be optimized by tuning the film thickness of AgNWs thin films, the reduction of junction resistance involves several complex processing conditions that include high-temperature annealing, [43] photonic curing, [44] electron-beam induced welding, [45] plasmonic welding, [46] soldering [47] and focused ion-beam welding, [48] to weld the junctions of these nanowires.Most of these techniques are critical and include high-temperature processing conditions (>180 °C) and the local heating caused during the process may critically damage the vulnerable ultrathin plastic substrates such as parylene-C underneath the electrode and hence limit the practical application of AgNWs as efficient TCEs. [49]52] Thus, considering the above two limitations, we have explored the combination of AgNWs with conducting polymers such as PEDOT:PSS.[55] Additionally, benefiting from past printed electronics development, PEDOT:PSS can increase the adherence of Ag-NWs onto the substrate without undergoing any surface treatment for AgNWs. [56]Better substrate adhesion helps to prevent the delamination of the film when subjected to repetitive bending tests.Alongside, PEDOT:PSS incorporated in between Ag-NWs not only helps to reduce the interfacial resistance between the nanowires by acting as electrical conduit but also helps in maintaining the overall stability and electrical conductivity of the film. [38]Further, embedding PEDOT:PSS into the AgNWs network helps to maintain a smooth morphology of the film by filling the free volume between the nanowires. [57]In addition to this, PEDOT:PSS helps to create well-dispersed AgNWs in the film volume thereby preventing AgNWs from any agglomeration and hence promotes electrical conductivity and mechanical intergrity of the TCE film. [58,59]Furthermore, AgNWs-PEDOT:PSS hybrid films can be created utilizing a straightforward, inexpensive solution-based techniques enabling simple scaling and mass production. [60]Hence, AgNWs when combined with PEDOT:PSS offer exceptional optical, electrical, and mechanical properties thus providing a viable solution towards the fabrication of flexible TCEs for ultraflexible OPD applications.
Further, for an ultraflexible OPD choosing the right substrate material is essential to ensure mechanical flexibility, optoelectrical performance, and overall efficient device operation.Some of the commonly used flexible substrates that satisfy these requirements include PET, PEN, polyimide (PI), polydimethylsiloxane (PDMS), flexible glass, parylene, and elastomers.63][64] In this work, we have fabricated AgNWs-PEDOT:PSS based ultraflexible TCE spincoated on 5 μm parylene-C substrate and have investigated its optoelectrical, mechanical, and material characteristics.Finally, using this electrode we have fabricated an ultraflexible OPD, and its performance is evaluated and compared with standard ITO-based OPD.

Results and Discussion
Thin films of AgNWs-PEDOT:PSS with different thicknesses of 50, 70, 100, 160, and 240 nm were spin-coated on parylene-C with glass as carrier substrate.Three different samples for each thickness of such hybrid electrode were prepared and the standard deviation values for optical transmission and R s data were calculated.The results showed that the average optical transmission data values for the films with thicknesses of 50, 70, and 100 nm are greater than 85%, while thicker films of 160 and 240 nm showed less than 80% transmission value throughout the visible range.The decrease in optical transmission of AgNWs-PEDOT:PSS based TCEs with the increase in thickness is majorly associated with the light scattering effect due to the increase in packing density of the randomly oriented AgNWs network [65,66] and light absorption from the PEDOT:PSS component of the electrode. [67]The optical transmission spectrum for different thicknesses of the hybrid electrode is shown in Figure S1 (Supporting Information).In order to evaluate the electrical conductivity of the AgNWs-PEDOT:PSS hybrid electrode, the R s was measured as a function of film thickness.As the thickness of the hybrid electrode was increased from 50 to 240 nm, the R s decreased from 64 to 8 Ω Sq. −1 The decrease in R s value with the increase in thickness is associated with the well connected percolation network of AgNWs embedded within PEDOT:PSS that permits efficient charge conductivity.Figure 1a represents the variation of R s and transmittance with light wavelength for various thickness of the AgNWs-PEDOT:PSS based TCEs.
Furthermore, in order to evaluate the optimal trade-off between the optical and electrical properties of the hybrid electrode, we calculated the figure of merit (FOM) value, [44,68] which is the electrical-to-optical conductivity ratio defined by: Where  DC is the direct current electrical conductivity and  OP is the optical conductivity of the transparent film.Furthermore, the transmittance and the  OP are related according to the following equation: Where T is the transmittance value corresponding to the wavelength at 550 nm, t is the thickness of the transparent film and Z 0 is the impedence of free space whose value is 377 Ω.In addition to this the direct conductivity and the R s are related by the following equation: Now solving for t and substituting the parameters in Equation (2), the transmittance can be re-written as: Finally, substituting the value for Z 0 = 377 Ω in Equation ( 4) and rearranging, the FOM for transparent electrode can be written as: It is clear from the equation that the higher the FOM, the better is the optoelectrical properties for the TCEs. Figure 1b summarizes the FOM values for different thickness of the AgNWs-PEDOT:PSS based TCEs.The hybrid electrode with a thickness of 100 nm showed relatively higher FOM value of 106 when compared with various other thickness values.The lowest value of FOM is 84, corresponding to the hybrid electrode with the thickness of 70 nm.Though the FOM values of the hybrid electrode with different thicknesses are associated closely to each other, we have chosen an optimum thickness of 100 nm for our ultraflexible OPD application owing to its better trade-off between the optoelectrical properties.However, the FOM for the hybrid electrode with a thickness of 100 nm is still three times less when compared with standard ITO with a FOM value of 353, that has an average R s value of 17 Ω Sq. −1 and transmission value of 94% at 550 nm.Hence there is still a scope to further increase the FOM value by lowering the R s value of the AgNWs-PEDOT:PSS TCE film.Despite high-temperature annealing conditions that can damage the plastic substrates beneath the electrode, lowtemperature processing conditions have to be explored that can efficiently reduce the R s without compromising the optical properties.
Apart from transparency and conductivity, flexibility is an inevitable requirement to realize flexible optoelectronic devices.Moreover, the flexible nature of the TCE is associated with its ability to undergo mechanical deformation while maintaining its electrical properties.Therefore, the electrical properties of the hybrid electrode were evaluated under steady and continuous bending conditions and the performance was compared with standard ITO electrode on PET substrate.For this, we first spin coated the hybrid electrode on 5 μm parylene-C coated on carrier glass substrate.The parylene-C along with the hybrid electrode was then peeled off from the carrier glass substrate to realize standalone flexible AgNWs-PEDOT:PSS based TCE.The R s of the hybrid electrode after peel off showed negligible change, thus indicating minimum occurrence of stress or cracking within the hybrid electrode during peeling.The flexibility measurements for the hybrid electrode were performed using U-folding tests.The initial resistance (R 0 ) values of the hybrid electrode and standard ITO were measured and the resistance data (R) for both hybrid and ITO electrode were recorded at different radii of curvature and the relative change in resistance ( The relative change in resistance for the ITO electrode showed a gradual increase in value when the bending radius decreased below 6.6 mm.However, on reducing the bending radius below 2.6 mm the relative change in resistance increased dramatically.On the other hand, the relative change in resistance of the AgNWs-PEDOT:PSS hybrid electrode remained relatively stable (less than 0.2) throughout the tests even at an extremely small bending radius of 0.1 mm.The plot for the relative change in resistance for the ITO electrode and the hybrid electode with different bending radii is shown in Figure 2a.In addition to this, the absolute change in resistance of the hybrid electrode with various radius of curvature and the different images taken during the bending tests are shown in Figure S2a,b (Supporting Information), respectively.Furthermore, we understand that the long-term reliability of the TCEs under continuous bending stress is particularly significant for the practical utilization of the flexible electrodes.Considering this, continuous bending measurements for the hybrid electrode were made for 50 k cycles at a constant bending radius of 0.1 mm, and the relative change in resistance with bending was recorded.The relative change in resistance for the hybrid electrode with the number of cycles during continuous bending tests is shown in Figure 2b.From Figure 2a,b, it is obvious and clear that the hybrid electrode can withstand more than 50 k bending cycles continuously, without any significant difference in the relative change in resistance values even with small bending radius of 0.1 mm.This indicates excellent mechanical stability and robustness of the hybrid TCE film thereby demonstrating its suitability for ultraflexible applications.
The performance of the OPD is greatly influenced by the WF value of the TCE as it influences the charge carrier extraction processes including alignment of energy levels and dipole interface effects [69] at the electrode/OSC interface.Moreover, in an OPD the efficient charge carrier extraction occurs when there is a sufficient match between the WF of the hybrid electrode and the highest occupied molecular orbital (HOMO) level of the donor polymer or lowest unoccupied molecular orbital (LUMO) level of the acceptor polymer. [70]Further, alignment of energy levels of the electrode and the neighbouring polymer layer reduces recombination losses and ensures good ohmic contact. [20,70]In our case, the electrical conductivity of the AgNWs-PEDOT:PSS film is associated with the local contact between the AgNWs and hole charge extraction from the PEDOT:PSS layer. [71,72]The WF of the hybrid electrode was measured to be 5 eV, which was comparable to the measured WF value of the standard ITO (4.8 eV).However, after coating PEDOT:PSS as planarization layer over the hybrid electrode, the WF increased to 5.1 eV, which is only 0.1 eV more than the hybrid electrode alone.This small energy difference between the WF values of the hybrid electrode before and after planarization ensures an ohmic contact between the two layers, thereby facilitating efficient hole charge transport.75] Apart from the WF, another important parameter that determines the efficiency of the OPD is the surface roughness and peak-to-valley height of the hybrid electrode.High surface roughness and peak-to-valley height can generate conductive shunt pathways leading to high leakage currents resulting in poor efficiencies. [76]Hence reduction in the surface roughness and peak-to valley-height in AgNWs:PEDOT:PSS based TCE is a critical parameter to realize efficient OPD.Therefore, in order to unravel the topography properties of the hybrid electrode, atomic force microscopy (AFM) measurements were performed.][79][80] However, inorder to further reduce the surface roughness, an extra layer of 60 nm PEDOT:PSS was coated over the hybrid electrode.As a result, the RMS surface roughness was reduced from 8 to 4 nm, and peak-to-valley height was decreased down to ≈37 nm.The AFM images of the hybrid electrode before and after 60 nm PEDOT:PSS planarization are shown in Figure 3a,b, respectively.Further, as PEDOT:PSS shows optical absorption in the visible range, [81] the effect of planarization layer on the optical transmission of the hybrid electrode was further evaluated.It was observed that after planarization, the average optical transmission of the hybrid electrode in the visible range decreased from 89% to 85%.However, this range is still suitable for optoelectronic applications.
Thus, coating of an additional layer of PEDOT:PSS over the hybrid electrode facilitates planarization on the hybrid electrode surface with reduced surface roughness without compromising its optical transparency and its electron blocking property effectively suppresses electron charge carrier density reaching the anode during the operational mode of the OPD.The increase in the thickness of the PEDOT:PSS planarization layer to 100 and 200 nm further decreased the RMS surface roughness value of the hybrid electrode to 3 and 2 nm, respectively, and peak-tovalley height to ≈29 and ≈24 nm, respectively (Figure S3a,b, Supporting Information).Despite the significant reduction in the surface roughness, we also observed a drastic decrease in the optical transmission values, thus making them less suitable for practical optoelectronic applications.The transmission spectrum of the hybrid electrode corresponding to the various thicknesses of the planarization layers are shown in Figure S4 (Supporting Information).
In addition to the surface roughness and peak-to-valley height, the surface morphology of the TCE plays a very important role in determining the performance metrics of the OPD.The scanning electron microscopy (SEM) image of hybrid electrode for the optimum thickness of 100 nm is shown in Figure 3c.The average diameter of AgNWs in the AgNWs-PEDOT:PSS TCE is ≈40 nm and the average length extends to several microns.This high aspect ratio of AgNWs in the polymer matrix not only ensures high percolation threshold but also renders ultraflexibilty without breaking and thus prevents from any short-circuit through the semiconducting active layer during mechanical tests.Further, we observe that the AgNWs-PEDOT:PSS thin film consists of a porous structure where the AgNWs are well distributed within the PEDOT:PSS polymer matrix.The SEM image showing the average diameter of AgNWs and the surface topography of the hybrid electrode film is shown in Figure S5, (Supporting Information).
Further, inorder to understand the possibility of any electronic interaction that could exist between AgNWs and PEDOT:PSS under illumination, we investigated the Raman spectra of AgNWs-PEDOT:PSS thin film.During the experiment, we first investigated the Raman fingerprints of the pristine PEDOT:PSS film as a reference and then have compared the spectra with the Raman peaks obtained for the hybrid electrode cointaining AgNWs and PEDOT:PSS.Greenlight (514.4 nm) was used as the excitation source during the experiments.The Raman spectra recorded for pristine PEDOT:PSS were comparable with the spectra from the literature. [82]In the Raman spectra of pristine PEDOT:PSS, the vibrational modes of PEDOT centered ≈1538, 1448, 1362, 1259, and 1498 cm −1 correspond to the C  ═C  asymmetrical, C  ═C  symmetrical, C  ─C  stretching, C  ─C  inter-ring stretching vibrations and antisymmetrical C=C vibration modes respectively.The vibrational modes of PSS were centered ≈986, 1130, and 1565 cm −1 , respectively. [82,83]Further, while comparing the Raman spectra of pristine PEDOT:PSS with AgNWs-PEDOT:PSS hybrid electrode, we see consistent small Raman shifts throughout the spectrum.These shifts in the Raman peak positions are associated with the electronic interactions that occur between the silver atoms present in the AgNWs and the thiophene rings of PEDOT. [84]Moreover, the Raman shifts corresponding to the peaks at 1361 and 1254 cm −1 for the AgNWs-PEDOT:PSS film are associated with the decrease in the intensity of C─C single bonds and increase in the intensity of C=C bonds due to the changes in the conformational linear structure of benzoid ring in the presence of AgNWs. [82]urther, the important performance metrics that determine the overall performance of the OPD include EQE, responsivity (R), and detectivity. [85]However, these performance metrics rely on the dark current density (J d ) and photogenerated current density (J ph ) values of the OPD.Low J d and high J ph values lead to high ON/OFF ratio, resulting in high EQE, responsivity, and detectivity metrics. [74]As the TCE takes an important role in charge carrier extraction process, its influence in determining both the J d and J ph values is significant.Hence, in our case we carried out experiments to better understand the role of AgNWs-PEDOT:PSS based TCE in determining the current efficiencies of the OPD.At first, two OPDs in the standard configuration i) AgNWs-PEDOT:PSS/PEDOT:PSS (as planarization and extraction layer)/PM6:Y12 (bulk heterojunction (BHJ) active layer)/Al (Figure 4a) and ii) ITO/PEDOT:PSS/PM6:Y12 (BHJ active layer)/Al were fabricated and characterized in ambient air.
The energy level diagram of the OPD is shown in Figure 4b.The active area of both OPDs was designed to be 0.25 cm 2 .The (J − V) characteristics of the OPDs with ITO and AgNWs-PEDOT:PSSbased electrodes are shown in Figure 4c.At first the J-V characteristics of both OPDs under white light illumination with an input light power density (P in ) of 1 mW cm −2 and under a voltage sweep from −2 to +2 V are compared.The J d values of the AgNWs-PEDOT:PSS and the ITO based OPD at −1.7 V bias were 2.8 and 0.9 μA cm −2 , respectively.The marginal lower value in the J d for the ITO based OPD is due to the smooth surface morphology of the ITO when compared with the hybrid electrode based OPD.Furthermore, under illumination, the J ph value of AgNWs-PEDOT:PSS-based OPD at −1.7 V bias was 100 μA cm −2 which was two times higher when compared with the J ph value recorded for the ITO based OPD that was 58 μA cm −2 .The lower J ph value for the ITO based OPD could be attributed to the charge recombinations at the ITO-PEDOT:PSS interface due to the higher WF difference (0.3 eV) between the ITO and PEDOT:PSS and due to the ITO degradation from to the acidic attack from PEDOT:PSS.On the other hand, the lower WF difference (0.1 eV) and smaller interfacial dipole effects between the hybrid electrode and PE-DOT:PSS facilitate better charge extraction towards the anode electrode resulting in relatively higher J ph values.
Further, the ON/OFF ratio of the OPD defined by the ratio of J ph in the light state and in the dark state is evaluated.We observed a comparable ON/OFF ratio from the AgNWs:PEDOT:PSS-based OPD and ITO-based OPD.However, the magnitudes were less than two orders in both the case.Such a low value in ON/OFF ratio, in both the OPDs, could be attributed to the high J d values due to the presence of leakage currents and low J ph values due to the presence of charge carrier traps, associated with the water/oxygen species [85][86][87] at the PEDOT:PSS/BHJ interface and in the bulk of the OSC polymer material since the OPD was fabricated and characterized in the ambient air.
In general, the J d values in an OPD could be effectively supressed by incorporating efficient charge blocking layers, on either sides of the BHJ, that selectively annihilate the charge carrier extraction toward the electrodes resulting in low leakage currents. [88]Further, the oxygen or moisture-induced trap states could be suppressed by employing inert fabrication conditions for the OPD.However, during the operational mode of our OPD in the dark and applied reverse bias conditions, although the electron charge carrier density is effectively suppressed by the PE-DOT:PSS layer, the hole charge carrier density in the device is still present due to the absence of hole blocking layer (HBL) at the BHJ/cathode interface.This results in hole charge carrier current dominating the J d in our OPD.Hence incorporating suitable HBL at the BHJ/cathode interface, the hole current density could be supressed. [88]The energy level diagrams demonstrating the performance of our OPD in the dark state in the absence and presence of HBL is shown in Figure S7a,b (Supporting Information) respectively.Further, the energy level diagram showing the carrier dynamics of the OPD in the light state and the effect of charge trapping due to oxygen species at the PEDOT:PSS/BHJ interface is shown in Figure S8 (Supporting Information).
In the real time scenario, an OPD should have the ability to detect weak optical signals despite the presence of noise.Thus, the specific detectivity (D*) of the OPD was evaluated from the responsivity values under different wavelengths of light in the visible and near-infrared (NIR) region as the BHJ (PM6:Y12) used in this study exhibits a wide absorption spectrum.The absorption spectrum of the PM6:Y12 BHJ polymer blend is shown in Figure S9 (Supporting Information).In order to evaluate D*, at first the current-voltage characteristics (I-V) of the OPD were recorded under the illumination from LED light sources of different wavelengths, and the J ph and R values were calculated.The P in of all the LED light sources was fixed at 10 μW cm −2 .The D* was then evaluated according to the following: The R value of an OPD at a particular wavelength () is given by the equation: Assuming, that J d is the main contribution of shot noise in the OPDs while operating in the reverse bias, the shot noise limited specific detectivity (D * sh ) for an OPD is given by: [90] From Equation ( 6) the R values of the OPD lengths were obtained from the J-V characteristics of the OPD and by substituting the R values in Equation ( 7), the D * sh values of the OPD corresponding to different wavelengths of light were evaluated.Finally, the graph of D * sh versus light wavelength was plotted as shown in FigureS10a (Supporting Information).
Initially, the D * sh values saw a marginal increase from 7.5 × 10 11 jones for deep blue light (455 nm) till 1.1 × 10 12 Jones for the green light (528 nm).This initial increase in D * sh values from deep blue to green light is due to the increase in R values of the OPD.Further, we observe that the D * sh values decrease steadily from yellow (590 nm) to NIR light (940 nm).This is due to the steady decrease in J ph resulting in low R values.We believe that the decrease in J ph values are attributed to the increase in the degradation factor of the OPD with the number of measurements as the measurements were performed in air.
Further, in order to evaluate their ultraflexibility, the OPD composed of AgNWs-PEDOT:PSS based TCE was subjected to continuous 5 k bending cycles at a radius of curvature of 0.2 mm.The J-V characteristics of the OPD before and after the bending test were evaluated under white light illumination with P in value of 10 mW cm −2 .Figure 4d shows the J-V characteristics of the OPD before and after bending.At a bias of −1.7 V, the J d and J ph values of the OPD before bending were 2.8 and 196 μA cm −2 , respectively.The J d and J ph values of the OPD after bending were 4.3 and 76 μA cm −2 respectively.It was observed that the J ph values after bending decreased 2.5 times when compared with the values recorded before bending.Initially, we assumed that the decrease in the J ph values after bending must be associated either with the mechanical stress or air degradation of the BHJ resulting in poor J ph values during the bias-voltage operation mode of the OPD.[93][94] Therefore, in order to better understand if the mechanical tests are explaining OPD's degradation, we fabricated and tested a new OPD under similar processing and operating conditions (i.e., biased at −1.7 V and 10 mW cm −2 white light illumination) and have recorded the J d and the J ph values every 15 min under non-bending conditions in ambient air.The J d and J ph values recorded for the first measurement were 1.18 and 186 μA cm −2 , respectively.These values are similar when compared with the data obtained from OPD before bending.Furthermore, the J d and J ph values recorded after 15 min during the second measurement were 1.21 and 74 μA cm −2 , respectively.Those values are similar to the data obtained for our OPD after bending.Hence these results show that the decrease in the J ph values after bending is associated with the air degradation of the BHJ semiconducting polymers under applied bias-voltage.The graph of variation of current density with time for the OPD under white light illumination with several P in values is shown in Figure S10b (Supporting Information).It is evident from the data that the J ph values show a steady decrease with time and is associated with the severe air degradation of the BHJ polymers with time leading to lower performance metric values for the OPD.However, we believe that this issue could be addressed by employing inert fabrication conditions for the OPD and by suitable encapsulation techniques.Thus, we show here and corroborate that the J ph degradation is not due to any physical/mechanical stress that might occur in the BHJ semiconducting polymers film during bending, but due to the degradation of the OSC polymers during the bias-voltage operation mode of the OPD.
However, we believe that owing to the excellent flexible nature of the TCE, the fabrication of the OPD in inert conditions with suitable encapsulation techniques will result in excellent device characteristics.Thus, despite the effect of atmospheric degradation, the obtained results illustrate the ultraflexible nature of the OPD incorporated with AgNWs-PEDOT:PSS as ultraflexible and transparent conductive electrode, thereby demonstrating its potential in future flexible OPD applications.

Conclusion
In conclusion, we have investigated the optoelectrical, mechanical, and material properties of AgNWs-PEDOT:PSS-based hybrid electrode fabricated on 5 μm parylene-C substrate.A successful implementation of such hybrid electrode in an OPD device has been demonstrated, verifying its suitability for ultraflexible OPD applications.The hybrid electrode exhibits excellent average R sh value of 27 Ω Sq. −1 and an average optical transmission of 90% in the visible region, thus indicating its suitability for optoelectronic applications.Moreover, the relative change in resistance for the AgNWs-PEDOT:PSS hybrid electrode shows a steady value of less than 0.2 even after 50 k cycles of continuous U-folding tests for a radius of curvature as low as 0.1 mm.These experimental results demonstrate the ultraflexible nature of the hybrid electrode.Furthermore, the high aspect ratio of AgNWs ensures ultraflexibility to the hybrid electrode thus avoiding any breaking of AgNWs on bending.In addition, the surface roughness of the hybrid electrode is effectively reduced from 8 nm to 4 nm after planariza-tion using a PEDOT:PSS layer.Further, the Raman spectra results showed evidence of electronic interactions between AgNWs and the PEDOT polymer chains under illumination.These extensive investigations encouraged us to integrate such hybrid electrode in OPD devices.
Further, the low energy level difference between the hybrid electrode and the PEDOT:PSS planarization layer ensures good ohmic contact and efficient charge carrier extraction, resulting in higher photogenerated currents under illumination when compared with the ITO based OPD.Moreover, the AgNWs-PEDOT:PSS hybrid electrode based OPD shows similar ON/OFF ratio when compared with standard ITO, thus indicating the potential replacement of ITO for ultraflexible OPD applications.Furthermore, within the limits of the atmospheric degradation of the OSC polymers under bias voltage, the ultraflexible OPD shows a minimal drift in the J d and J ph values before and after bending under white light illumination, thus indicating excellent stability conditions of the electrode under mechanical deformation.Therefore, by virtue of the high performance and simple fabrication technique, AgNWs:PEDOT:PSS hybrid electrodes are best suited for the practical use in ultraflexible OPD applications.Finally, the demonstration of such extensive properties investigation gives helpful guidelines for the design and development of ultraflexible devices.We then believe that it will accelerate the rise of ultraflexible OPD devices in next future applications.

Experimental Section
AgNWs-PEDOT:PSS Electrode Fabrication: AgNWs-PEDOT:PSS soultion (CLEVIOS HYE) was directly purchased from Heraeus Deutschland GmbH & Co. KG, Germany.Thin films of AgNWs-PEDOT:PSS were spin coated onto 5 μm parylene-C coated on carrier glass substrates.Finally, the parylene-C layer consisting of hybrid electrode was peeled-off from carrier glass to realize flexible TCE.To achieve this, at first glass slides with dimensions 25 × 75 × 1 mm 3 were cleaned subsequently in acetone, isopropanol, and DI water in an ultrasonic bath for 10 min each, followed by N 2 blow dry and finally subjected to a dehydration bake at 130 °C for 10 min.A thin layer of 2% soap solution was spin coated over glass at 3000 rpm for 1 min.5 m Parylene-C (dichloro-p-cyclophane) as dimer from Specialty Coating Systems) was deposited via chemical vapor deposition (CVD) equipment, from Specialty Coating Systems (SCS).50, 70, and 100 nm TCE electrodes were fabricated by spin coating at a spinning speed of 800, 600, and 400 rpm for 1 min, respectively.160 and 240 nm electrodes were fabricated at 400 rpm (bilayer) and 400 rpm (trilayer) for 1 min, respectively.The electrodes were subsequently annealed at 130 °C for 10 min.Finally, the electrode on parylene-C is peeled off from the carrier glass substrate to realize the ultraflexible TCE.The video demonstration showing the peeling process of parylene-C alone from the carrier glass substrate is shown in movie S1.mp4 (Supporting Information) and the standalone ultraflexible TCE after peel off from carrier glass substrate is shown in movie S2.mp4 (Supporting Information), respectively.
Opto-Electrical and Mechanical Characterization: Optical transmission tests of the hybrid electrode and standard ITO were performed using UV-Vis spectrophotometer (Shimadzu UV-2600).R s was measured from a custom made four-point probe set up in connection with a 2612 Keithley source meter unit.The tool was calibrated against standard ITO with a sheet resistance of 12.45 Ω Sq. −1 Mechanical characterization of the ultraflexible TCE and OPDs was realized by a custom designed equipment.During the flexibility measurements, the electrical resistance of the film in the flat and bending condition was recorded using a 2635 Keithley source meter unit.

Surface Analysis, WF, Raman Spectroscopy, and Thickness Measurements:
AFM imaging to estimate the surface roughness and peak-to-valley height was performed using an equipment from Veeco in surface tapping mode.On the other hand, the surface morphology of the AgNWs-PEDOT:PSS was examined by SEM using a CARL ZEISS/Ultra 55 electron microscope in the InLens detector setup.The WF measurements were carried out in air using a Kelvin Probe apparatus (APSO2 KP Technology).Raman Spectroscopy was done using Horiba Jobin Yvon LabRAM HR 800 UV system and the thickness of the electrode films was determined by a Dektak XT surface profilometer (Bruker).
Organic Active Layer Preparation: PM6 (20 mg) as polymer donor and BTP-4F-C12 (also known as Y12) (24 mg) as non-fullerene polymer acceptor (both purchased from Brilliant Materials), with broad range absorption in the visible and NIR range were solubilized separately in 1 mL each o-Xylene at 40 °C and kept for overnight under stirring.Finally, the solutions were mixed in a 1:1 volume ratio to obtain a total concentration of 22 mg mL −1 .Then, 0.5% of p-anisaldehyde solution (purchased from Merck) was added and the resulting solution was stirred at 40 °C for 4 h.
Organic Photodetector Fabrication: OPDs with the active area 0.25 cm 2 were fabricated on 5 μm parylene-C and were finally peeled-off from carrier glass substrate to realize stand alone ultraflexible OPDs.To achieve this, first Cr/Au (5 nm chromium and 120 nm gold) as bottom contact was vacuum evaporated using a thermal evaporator (Boc Edwards Auto 500) over the parylene-C using a metal shadow mask.The flexibility tests for the Cr/Au electrode are shown in Figure S6 (Supporting Information).AgNWs-PEDOT:PSS solution was then spin coated (400 rpm for 60 s) and the film was thermally annealed at 130 °C for 10 min to obtain a transparent uniform film of 100 nm.Furthermore, inorder to reduce the surface roughness and peak-to-valley height of the hybrid electrode film, 60 nm PEDOT:PSS (Clevios P VP Al4083 grade from Heraeus) was spin coated (2000 rpm for 60 s) over the hybrid electrode as a planarization layer.Further, the film was annealed at 130 °C for 10 min.For the formation of the active layer, the donor-acceptor polymer blend consisting of PM6:Y12 was spin coated (1000 rpm for 60 s) and later was thermally annealed at 110 °C for 10 min.The thickness of the active layer was ≈100 nm.After forming PM6:Y12 organic active layer, a 150 nm of aluminium (Al) was thermally vacuum evaporated as the cathode for the OPDs.The hybrid electrode and the active layer were solution processed in air under normal clean room conditions (45% RH, 20 °C).The video demonstration showing the ultraflexible nature of the OPD is shown in movie S3 (Supporting Information).
OPD Sensing Characterization: All measurements were conducted under ambient air conditions in a lab-made 3D-printed black box.During the measurements, the box was covered from all sides so as to ensure the absence of any stray light.The OPD was kept at a fixed distance of 3.6 cm from the white light commercial LED source (350-800 nm spectral range, LUW CR7P OSRAM Opto Semiconductors).The intensity calibration was done against a silicon photodiode (S1223 from Hamamatsu) biased at −1.7 V. Furthermore, the I − V characteristics of the OPD were recorded using a 2612 Keithley source meter unit.The input light power densities of various LED light sources were calibrated against a standard Si photodiode (S1223 from Hamamatsu).The different LEDs used for the experiment were, deep blue (455 nm), signal blue (470 nm), green (528 nm), yellow (590 nm), orange (617 nm), red (625 nm), hyper red (656 nm), far-red (730 nm), NIR (850 nm) and NIR (940 nm).

Figure 1 .
Figure 1.a) R s and optical transmittance curves for different thicknesses of AgNWs-PEDOT:PSS-based TCE.The error bars show the standard deviation for three seperate measurements.b) FOM as a function of thickness for the AgNWs-PEDOT:PSS-based TCE.

Figure 2 .
Figure 2. a) Plot of relative change in resistance for the AgNWs-PEDOT:PSS and ITO electrodes with different radius of curvature.Inset (A) shows the gradual increase in the relative change in resistance for the ITO electrode below 6.6 mm, whereas the AgNWs-PEDOT:PSS electrode maintained sufficiently stable values in the relative change in resistance.Inset (B) indicates that the relative change in resistance of the hybrid electrode is less than 0.2 even at a radius of curvature of 0.1 mm.b) Plot shows the variation in the relative change in resistance of the hybrid electrode with the number of continuous bending cycles.Inset (A) shows the bent condition (in blue points) and flat condition (in red points) of the hybrid electrode during continuous bending tests.The relative change in resistance of the hybrid electrode during continuous bending cycles remained below 0.2.

Figure 3 .
Figure 3. a) and b) are the AFM images of the AgNWs-PEDOT:PSS hybrid electrode before and after coating 60 nm of PEDOT:PSS as a planarization layer respectively.c) SEM image of the AgNWs-PEDOT:PSS-based hybrid electrode (bar scale is 1 μm).d) Raman spectra of pristine PEDOT:PSS and AgNWs:PEDOT:PSS thin films.

Figure 4 .
Figure 4. a) Flexible OPD stack with AgNWs-PEDOT:PSS as bottom transparent anode.b) Energy level diagram of the OPD.c) J-V characteristics of the AgNWs-PEDOT:PSS based OPD and ITO-based OPD in dark and white light illuminated conditions (1 mW cm −2 ).d) J-V characteristics of the AgNWs-PEDOT: PSS-based OPD before and after 5 k continuous bending cycling tests (radius of curvature of 0.2 mm).
For the D * sh measurements the LED light sources operating in the visible and NIR wavelength regions were purchased from OSRAM Opto Semiconductors, with OSLON80 4± PowerStar Colors/ILH-ON04-xxxx-SC211-xx Series for LEDs operating under visible light and OSLON 4 PowerStar IR/ILH-IO04-xxxx-SC201-xx Series for LEDs operating under NIR light respectively.