High‐Performance Air‐Stable 2D‐WSe2/P3HT Based Inorganic–Organic Hybrid Photodetector with Broadband Visible to Near‐IR Light Detection

Photodetectors that can achieve high‐speed photoresponse and high responsivity with broadband detection are essential for bio‐health monitoring, imaging, chemical sensing, and many other applications. Herein a high‐performance inorganic–organic hybrid photodetector based on a heterojunction of exfoliated 2D tungsten diselenide (WSe2) layers and solution‐processable poly(3‐hexylthiophene‐2,5‐diyl), P3HT is reported. The heterojunction shows enhanced exciton harvesting through long‐range energy transfer from P3HT to WSe2, which is confirmed using photoluminescence quenching and fluorescence decay measurements. The detector shows broadband light detection from visible to near‐infrared (NIR) (400−1100 nm), and air‐stable device performance. The device exhibits the highest responsivity of 17.6 AW−1 for low incident light intensity (<10−5 W cm−2) at 640 nm wavelength of light. Furthermore, a fast photoresponse speed with a rise/fall time of 9.5/5.1 µs, which retained its performance for more than four months under ambient conditions is demonstrated. The superior device performance presented here using an inorganic–organic hybrid heterojunction is the key to producing novel high‐performance and air‐stable photodetectors with broad spectral bandwidth.


Introduction
Over the past few years, light detection in visible to IR spectral regions using two-dimensional (2D) layered materials has been of massive interest to researchers working in the field of photonics and optoelectronics.Speaking of the much-discussed light-matter interaction cross-section or optical absorption in 2D-transition metal dichalcogenides (2D-TMDs), even in atomically thin samples, the light absorption can be up to 10% at excitonic resonances.Such absorbance is at least one order higher in magnitude than that observed in Si or conventional narrow-band gap semiconductors like GaAs. [1][7][8] As a prominent member amongst the TMDs, bulk WSe 2 has an indirect bandgap of 1.3 eV, whereas, in its monolayered form, it has a direct bandgap of 1.6 eV.Furthermore, high room temperature carrier mobility (100 cm 2 V −1 s −1 ) [9] of atomically thin WSe 2 film makes it a favorable choice for PD applications. [10,11]To improve its photodetection performance, WSe 2 has been used in conjunction with various inorganic semiconducting platforms (e.g., monolayer WSe 2 with Nitrogendoped graphene QDs, [12] WSe 2 /SnS 2 heterostructure, [13] ZnO NW-WSe 2 vdW heterostructure [14] ).However, to date, these approaches have had limited success in delivering simultaneous improvements across responsivity, dark current, speed, linear dynamic range, and device stability -the key figures of merit of a PD.In particular, either the photoresponsivity or the response speed was found to be compromised. [15,16]Therefore, a strategy that could offer simultaneous enhancement of key figures-ofmerit of PD would represent significant progress in the field and a major step towards deployment and commercialization.
Organic semiconductors offer unique physiochemical properties such as high light absorption coefficient, solution processability, and mechanical flexibility, along with cost-effectiveness.Integrating organic semiconductors with 2D semiconductors offers promising functionalities pertaining to both systems and quite often enables complementary benefits.In particular, inorganic-organic HJ interfaces have the potential for interfacial charge transfer by various means, e.g.excited-state charge transfer, Förster-type resonant energy transfer (FRET), and hybrid charge transfer exciton dissociation.These processes have been experimentally demonstrated in some TMD-organic hybrid structures.19] Here, we report a high-performance hybrid inorganic-organic HJ PD based on exfoliated 2D-WSe 2 layers and solutionprocessable poly(3-hexylthiophene-2,5-diyl), P3HT.The PD exhibits a broad spectral response covering the visible to nearinfrared (NIR) region (400 to 1100 nm), with peak responsivity at 750 nm.The device exhibits the highest responsivity of 17.6 A W -1 at low light intensity and corresponding specific detectivity D* of 6.7 × 10 9 Jones.The enhanced photoresponse of the HJ device is due to the long-range energy transfer from P3HT to WSe 2, which enables exciton harvesting even from a thick P3HT layer.A fast photoresponse with a 9.5 μs rise time and 5.1 μs fall time is recorded for incident light at 20 kHz modulation frequency.The shelf life of the HJ device is found to be significantly prolonged, with stable and repeatable electrical and optical characteristics for more than four months.Our results show that these simultaneous enhancements of key figures-of-merit of PD are higher by an order of magnitude than the best-reported 2D-organic hybrid photodiodes.

Photodetector Architecture and Characterization
The WSe 2 /P3HT hybrid HJ PD device was fabricated using mechanically exfoliated n-type WSe 2 flakes and solution-processable P3HT polymer solution.The schematic illustration of the WSe 2 /P3HT HJ device is presented in Figure 1 The active region of the device is defined by the area between the two electrodes for the WSe 2 /P3HT HJ.The exfoliated WSe 2 flakes were marked prior to the device fabrication and characterized using an atomic force microscope (AFM), as shown in Figure 1(e).Figure 1(f) shows the height scan across various positions on the flake.The WSe 2 flake has different thicknesses at different regions, which shows the multi-layered nature of the exfoliated flake.
To ascertain the formation of the heterostructure, the crosssectional field emission scanning electron microscopy (FE-SEM) image of the WSe 2 /P3HT heterojunction and electron probe microanalysis (EPMA) mapping image of one of the heterostructure devices were obtained and is shown in Figure 2(a-i).The FE-SEM cross-sectional image in Figure 2(a) shows the P3HT layer on top of the WSe 2 flake forming the heterostructure.Figure 2(b-i) shows the SEM image and the element-wise EPMA mapping image of a WSe 2 /P3HT hybrid heterostructure device.Figure 2(c,d) confirms the presence of W and Se corresponding to WSe 2 in the overlap area, whereas Figure 2(e) shows the elements of metal contacts.The blue rectangular region in Figure 2(f) shows the absence of Al and hence the etched-out Al 2 O 3 region through which P3HT and WSe 2 can be seen forming a heterojunction.Figure 2(h,i) corresponds to the elements C and S present in P3HT.Thus, the EPMA mapping of the individual elements present in the heterostructure device clearly shows the formation of the WSe 2 /P3HT heterostructure.
The Raman spectra of WSe 2 /P3HT HJ and FTIR spectra of P3HT were also measured.The peak at 250 cm −1 corresponds to in-plane vibrational E 2g mode, and the peak at 256 cm −1 corresponds to out-of-plane vibrational A 1g mode of WSe 2 , [20,21] confirming the existence of atomically thin flakes (see Figure S2 in the Supporting Information).First, we investigate the photoinduced charge transfer and exciton dissociation characteristics of the hybrid WSe 2 /P3HT HJ.A steady-state photoluminescence (PL) and fluorescence lifetime imaging microscopy (FLIM) of pure P3HT and hybrid WSe 2 /P3HT regions were measured.For this purpose, a P3HT layer was spin-coated on a SiO 2 /Si wafer with pre-transferred WSe 2 flakes.The steady-state PL measurements were carried out with two different thicknesses of spincoated P3HT layer, i.e., 20 and 30 nm, respectively while ensuring similar thickness for WSe 2 flakes.Figure 3(a,b) shows the PL spectrum (at 514 nm excitation wavelength) of pure P3HT and WSe 2 /P3HT HJ based on 20 and 30 nm thick P3HT layers, respectively.Pure P3HT has two emission peaks centered at 652 and 710 nm, which correspond to 0-0 and 0-1 transition, [22] respectively.Both of these peaks are significantly quenched in the WSe 2 /P3HT HJ.It is important to note that the magnitude of PL quenching is more pronounced for a thinner P3HT film.This PL quenching can be attributed to two processes: i) the exciton diffusion from P3HT to the WSe 2 /P3HT interface, where they get dissociated into free charge carriers; ii) the exciton formed in P3HT layer undergo Förster energy transfer to WSe 2 .To further investigate the quenching process, we performed excitation-dependent PL measurements.In the PL spectra of Figure 3(a,b), no emission peaks corresponding to WSe 2 were observed at a 514 nm excitation wavelength, thus, the measurements were repeated with a higher excitation wavelength of 785 nm. Figure 3(c) shows the PL spectra of WSe 2 , WSe 2 /P3HT, and P3HT region obtained using a 785 nm laser source.P3HT did not show any emission peak from 800 to 1000 nm, whereas the PL spectra of WSe 2 and WSe 2 /P3HT were found to be completely overlapping, which implies that there is no charge/energy transfer from WSe 2 to P3HT at their hetero-interface.It may be argued that the most probable reason for PL quenching in WSe 2 /P3HT heterojunction is the long-range resonant energy transfer from P3HT to WSe 2 .For this to happen, one necessary condition is that the emission spectra of the donor (here P3HT) must overlap with the absorption spectra of the acceptor (here WSe 2 ).P3HT has emission spectra ranging from 600-850 nm, and multilayer WSe 2 shows sig-nificant absorption in the same spectral region (see Figure S3 in the Supporting Information).Since the prepared P3HT/WSe 2 HJ satisfies this necessary condition for FRET, the hetero-energy transfer process can provide an additional path for exciton migration.In FRET, the energy transfer between donor and acceptor takes place through the non-radiative coupling of their electric dipole moments.The energy transfer efficiency between two point dipoles separated by a distance "r" is proportional to 1/r 6 as predicted by the Förster theory.However, point-to-point dipole approximation does not apply to a planar heterojunction as the molecules in a thin film are arranged with a high degree of order and alignment.The dipoles in the donor can interact with any of the dipoles from the acceptor film, resulting in an extended dipole coupling.This results in a reduction of the power dependency on "r" from1/r 6 to 1/r 4 for a monolayer of acceptors and further reduces to 1/r 3 in case of slab of acceptors. [23,24]So, longrange energy transfer can take place in planar heterojunction beyond the Förster radius (R o ) through extended dipole coupling.In the presence of the long-range energy transfer, the effective exciton diffusion length can be an order of magnitude higher than the intrinsic exciton diffusion length (exciton diffusion length in P3HT can be up to 20 nm [25] ).The efficacy of energy transfer depends on the Förster radius.A larger R o means a stronger coupling and results in a higher percentage of excitons harvested from a longer range.Using the absorption coefficient of WSe 2 from the literature, a Förster radius of ≈3.5 nm was estimated for the current WSe 2 /P3HT heterostructure (see Section S3 in the Supporting Information for more details).Such a value of R o implies an effective diffusion length of tens of nm for efficient energy transfer from P3HT to WSe 2 .This type of long-range energy transfer has been shown to exist in organic planar heterojunctions where the ability to harvest exciton from a distance of 100 nm away from the interface has been reported. [23]o get further insight into the exciton dissociation process, fluorescence lifetime image microscopy (FLIM) and decay profiles of the WSe 2 /P3HT HJ and pure P3HT regions of 20 nm P3HT coated sample were investigated, as presented in Figure 3(d,e).The FLIM image in Figure 3(d) shows pure P3HT (region 1) and WSe 2 /P3HT HJ (region 2).The decay kinetics in Region 1 was fitted using a triexponential fit, while the decay profile in Region 2 was fitted using a bi-exponential fit (see Section S4 in the Supporting Information).The measurements were repeated in three identical samples, and the results were averaged to extract an average decay lifetime of 389 and 214 ps for pure P3HT and WSe 2 /P3HT HJ, respectively (see Figure S5 and Table S1 in the Supporting Information).This shortening of the lifetime in the HJ provides evidence of long-range energy transfer and dissociation and interfacial charge transfer between P3HT and WSe 2 .The PL decay arises from three channels; the natural PL decay from the top surface of the P3HT/air interface, the photoinduced electron transfer from P3HT to WSe 2 at the hetero-interface which separates holes and electrons at the P3HT/WSe 2 interface, and the long-range energy transfer from P3HT to WSe 2 .

Dark and Light Characteristics and Responsivity
The electrical and photodetection characteristics of the WSe 2 /P3HT hybrid HJ device were investigated under dark and different illumination conditions.For the performance comparison of the hybrid device, we have performed control measurements on the bare WSe 2 device (see Section S5 in the Supporting Information).Figure 4(a) shows the I-V curves of the WSe 2 /P3HT hybrid HJ device for incident light of different wavelengths ranging from 400 to 1100 nm with a fixed optical power of ≈0.2 mW cm −2 .A significant photocurrent change was observed in the hybrid device for the entire wavelength range covering visible to infrared light, and the photocurrent was more pronounced at the reverse bias voltages.The maximum photocurrent in the hybrid device was observed at 750 nm wavelength.
Two of the most significant figures of merit for a PD are its responsivity (R) and external quantum efficiency (EQE).Responsivity is described as the ability of the PD to produce photocurrent under a specific wavelength of light, and it is expressed as R = I ph /P where I ph is the photocurrent and P is the optical power incident on the device. [2,26]The EQE is defined as the ratio of the number of electron-hole pairs that contribute to the net photocurrent to the number of incident photons.The EQE can be extracted from responsivity using the equation: EQE = R (hc /q) where h, , q and c are the Planck constant, excitation wavelength, elementary charge and speed of light respectively. [27,28] under 0.2 mW cm −2 optical intensity compared to a responsivity of 0.82 A W −1 for the bare WSe 2 device at −1 V bias under identical experimental conditions.The photo-responsivity of the hybrid device with a lower P3HT thickness (90 nm) was also measured and it showed a responsivity of 1.07 A W −1 at 750 nm wavelength.The detailed photoconduction mechanism of the hybrid device is explained in the discussion section.The corresponding EQE values are estimated to be 237%, 178%, and 137% for WSe 2 /P3HT hybrid HJ with 150 nm, 90 nm P3HT coating, and bare WSe 2 devices, respectively, at −1 V bias (see Figure S8 in the Supporting Information).These high EQE values suggest P3HT thickness-dependent exciton harvesting and the presence of a photoconductive gain mechanism (EQE > 100%) in the devices.In general, 2D semiconductors are prone to trap states where one type of photogenerated carrier can get trapped, and the other type of carrier can recirculate many times in the channel giving rise to a photoconductive gain.This type of photoconductive gain has been observed in many other 2D hybrid heterojunction devices which resulted in an EQE higher than 100%. [29,30]o investigate the power dependence of photocurrent (I ph ) on the incident light intensity (P in ), the photocurrent of the hybrid device was plotted as a function of excitation light intensity.The photocurrent can be worked out by subtracting the dark current from the light current ( I ph = I light − I dark ). Figure 5 shows the photocurrent versus light intensity graph at an excitation wavelength of 640 nm with illumination intensity varying from 140 nW cm −2 to 0.5 W cm −2 .The photocurrent of the hybrid device initially increases with linear dependence and then transits into a sublinear regime under high irradiation power densities.On increasing the light intensity from 560 nW cm −2 to 55 μW cm −2 , the photocurrent quickly rises from 1.1 × 10 −11 to 6.3 × 10 −10 A, but on further increasing the light intensity, the photocurrent increases gradually.The two regimes were fitted separately according to a power law I ph ∼ P  in where  is an exponent.It may be noted from Figure 5 that for low optical power, photocurrent changes almost linearly with incident power with  = 0.96, but with further increase in optical power, the dependence on optical power becomes sublinear with  reaching a value of 0.3.This sublinear dependence of photocurrent versus intensity is due to the increased bimolecular recombination processes as a result of more charge carrier generation at high optical power and the presence of some trap states between the conduction band edge and Fermi level.[33] So, the incident optical power is a crucial parameter that defines the optimum operation condition for better PD performance of such HJ devices.We extracted the responsivity of the WSe 2 /P3HT hybrid HJ device as a function of light intensity under 640 nm light, and the variation is shown in the inset of Figure 5.The device shows excellent, almost flat responsivity at very low incident light intensities (< 10 −5 W cm −2 ), resulting in a high responsivity of ≈17.6A W −1 corresponding to an incident light intensity of 4.3 μW cm −2 .As incident intensity increases, a dramatic decrease in the responsivity is recorded.This could be attributed to the higher rate of scattering and bimolecular recombination of photogenerated charge carriers under intense optical illumination. [32,34]

Stability, Noise, and Detectivity
The variation in dark current and photocurrent of the HJ device was monitored over a prolonged period of time (≈125 days) before calculating the other device parameters and is statistically depicted in Figure 6(a).During this period, no apparent degradation of the device's electrical characteristics was observed.The change in the dark and photocurrent of the HJ device with time is very minimal, with a standard deviation of 0.03 and 0.11 nA, respectively.The hybrid device also exhibits good photocurrent switching behavior for repeated on/off cycles of laser illumination (Figure S9, Supporting Information).This shows that the fabricated WSe 2 /P3HT hybrid HJ device maintains good stability and sensitivity over time, even without any encapsulation layer.
Another key parameter for a PD is its specific detectivity, and it measures the ability of a photodetector to detect low optical signals.One can calculate the specific detectivity using the noise equivalent power (NEP) as per the formula: [35] where A device is the active area of the device.The NEP value can be estimated by using the formula, NEP = √ S I R , [35] where S I is the current noise spectral density and R is the responsivity at a specific wavelength and incident light intensity.Most of the time shot noise from the dark current is assumed to have a dominant contribution to the total noise spectral density, which gives an overestimation of the specific detectivity as this assumption is not necessarily true.Therefore, the value of S I was experimentally determined by performing low-frequency noise (LFN) measurements at room temperature on the hybrid HJ device.The LFN measurements were performed in the dark condition from 1 to 100 Hz frequency range.A constant current of 2 nA was applied to the device from a constant current source, and the variation in spectral power density of voltage fluctuations (S V ) was measured as a function of frequency.The S V can then be converted to S I using the formula [36] : where  is the ideality factor for the device, T is the temperature and I s is the saturation current.For the accuracy of the measured data, 20 noise spectra were averaged to get the final noise spectral density.The observed trend in NEP and specific detectivity, as seen in Figure 6(c), is a direct result of the inverse dependence of NEP on spectral responsivity (R ) of the PD device measured at an optical intensity of 0.2 mW cm −2 .However, for a low incident optical intensity of 4.3 μW cm −2 , a maximum detectivity of 6.7 × 10 9 Jones was obtained.

Temporal Response
The fast response speed is a crucial performance parameter for modern-day PDs in various applications.To examine the response speed of the WSe 2 /P3HT hybrid HJ PD, the transient photoresponse of the device was obtained using a 660 nm modulated laser source.The laser source was externally modulated using a function generator, and the data was recorded in a digital oscilloscope.Figure 7(a) summarizes the time response of the WSe 2 /P3HT hybrid HJ device at different modulation frequencies up to 30 kHz.As can be seen from Figure 7(a), the prepared HJ PD device exhibits excellent time response with good repeatability and stability up to a very high modulation frequency of 30 kHz.Although the output signal from the detector decreases gradually with increasing modulation frequency, it still shows good switching behavior till 30 kHz modulation frequency.To determine the speed of the hybrid device, one cycle of the time response curve was chosen, and the corresponding rising and falling edges were fitted.The rise and fall time is the time it takes for the detector output signal to increase from 10 to 90% of peak signal and to decrease from 90 to 10% of peak signal, respectively. [37]Figure 7(b) shows the rise and fall time under 20 kHz modulation frequency as the on/off saturation levels for this output signal are clearly visible.The HJ device exhibits a fast response with a rise/fall time of 9.5/5.1 μs, which is several orders of magnitude faster than the response speed values reported for other 2D-inorganic/organic hybrid PD devices (Table 1).For WSe 2 /PANI [16] based photodiode, the response speed was of the order of seconds, whereas for MoS 2 /Rhodamine 6G [38] based photodiode, the rise time was quite fast, but the fall time showed a trailing behavior.The fast photoresponse speed observed in the current device is a result of the faster detrapping of holes in the presence of the organic layer. [39]Since the fermi level is close to the conduction band in n-type WSe 2 , most of the electron traps will be hence filled and can be neglected.It is, therefore, that the response time of the device will depend on the hole trapping and detrapping lifetimes.As shown in Figure S13 (Supporting Information), the trap state can be filled by hole trapping from the WSe 2 valence band or from the photogenerated hole on the HOMO of P3HT during photoexcitation.But the presence of P3HT HOMO with intermediate energy will accelerate the a) Specific detectivity was calculated assuming the shot noise assumption as the major component in NEP.This can give an overestimation of the specific detectivity.detrapping process of holes, leading to a fast response time.One thing to note here is the slower rise time than the fall time for this HJ device.This kind of unusual behavior has also been reported in other heterostructure photodetector devices. [7,33]The rise time of a photodetector is determined by the carrier collection time.In this lateral heterostructure device design, carriers are generated in a wide area, which may require a longer time to transport to the electrodes, leading to a slower rise in the photocurrent.On the other hand, in such type of heterostructure, the space charge area is narrow and the carrier recombination process is mainly restricted to this narrow region.Additionally, as explained earlier, the presence of the organic layer accelerates detrapping of holes and leads to faster recombination time, thus resulting in a faster fall time of the device.The photoresponse of the device was also tested by continuously varying the frequency of a modulated light source which is shown in Figure 7(c).A −3 dB bandwidth of 35 kHz was obtained for the HJ photodetector, which corresponds to a response speed ( r ≈ 0.35/f −3dB ) of ≈10 μs.This value is in good agreement with the extracted response speed of the device obtained by fitting the rising and falling edge of the transient curve.

Discussion
The increase in the responsivity and EQE in the hybrid device is primarily due to three reasons: i) the enhanced light absorption by the organic P3HT layer, which has a good absorption in the visible range, [44,45] and ii) effective separation of photogenerated electron-hole pairs at the HJ interface and iii) long-range energy transfer from P3HT to WSe 2 .WSe 2 has good absorption from the visible to NIR range, with a few-layer WSe 2 having a strong excitonic absorption peak at ≈750 nm which gets red-shifted with increasing thickness. [46,47]Figures S11 and S12 in the Supporting Information shows the absorption spectra of the P3HT layer and differential reflectance spectra of exfoliated WSe 2 flake and WSe 2 /P3HT heterostructure, respectively.Since our WSe 2 flake shows graded thickness with some portions having a few-layer thickness, it is expected to absorb strongly in the visible region.P3HT has a broad absorption peak in the visible region.It exhibits a main peak at 520 nm and two vibronic shoulder peaks at 552 and 605 nm.As the P3HT thickness increases, the carrier generation in the bulk also contributes to the photocurrent and a large percentage of it comes from the light beyond the absorption maximum of P3HT.For devices with a thick P3HT layer, the photocurrent maximum not only depends on the absorbance but also on the recombination processes occurring at the surface and near the electrodes.It has been shown that with increasing thickness of P3HT, the photocurrent maximum is red-shifted. [48]he device operating mechanism of the hybrid junction can be understood on the basis of the photocarrier generation processes occurring in the heterostructure device.Figure 8 shows the schematic representation of the energy band diagram and charge/energy transfer across the WSe 2 /P3HT hybrid interface.Here, the typical values of LUMO (−3.0 eV) and HOMO (−5.0 eV) levels of P3HT and the valence band (−5.3 eV) and conduction band (−4.0 eV) levels of layered WSe 2 have been considered, in accordance with which, the WSe 2 /P3HT HJ is expected to have a type-II band alignment. [49,50]The layered WSe 2 flake with an average thickness of 25 nm can be deemed as a semiconductor with a bandgap of 1.3 eV.Due to the unregulated thickness gradient across the exfoliated WSe 2 flake, some portions of it also show characteristics pertaining to 3L or 4L, which support direct electronic transitions.However, for depicting the charge transfer across the point of contact in WSe 2 /P3HT HJ, the bulk form of WSe 2 has been considered.Photo-excitation of the prepared hybrid device with a light of favorable energy (400−700 nm) leads to the formation of excitons predominantly in the P3HT region.Here, for WSe 2 /P3HT HJ, the electron affinities of WSe 2 and P3HT have an energy difference (∆E) greater than the exciton binding energy (E b ) of P3HT (usually lying in the range of 0.28−0.33eV).So, once the photogenerated exciton in P3HT diffuses towards the HJ interface, it can dissociate into free electron and hole (shown in Figure 8(a)).Since the conduction band of WSe 2 lies between the LUMO of P3HT and the fermi level of Cr metal, transport of photo-excited electrons takes place from WSe 2 to the Cr/Au electrode, thus increasing the current in the circuit.The excitons generated within the exciton diffusion length from the heterointerface can contribute to the photocurrent by this process.Upon photoexcitation, photocarriers are also generated in WSe 2 (Figure 8(b)).Due to the low exciton binding energy in multilayered WSe 2 , excitons are readily dissociated to free electron-hole pair which are then collected through the respective electrodes.Another possibility is the long-range energy transfer from P3HT to WSe 2, as shown in Figure 8(c).The process of long-range energy transfer has already been discussed in the previous section and can enhance the photocurrent in the current HJ device.In the presence of long-range energy transfer, excitons can be harvested from a thick P3HT layer.As the thickness of the P3HT layer increases, more and more excitons can be transferred from P3HT to WSe 2 through the long-range energy transfer which eventually leads to the enhanced photoresponsivity of the HJ device.The electrical characteristics and the operation mechanism of the HJ PD are consistent with the PL quenching and decay measurements shown in Figure 3.
The prepared WSe 2 /P3HT HJ PD device has a much better response speed compared to other reported inorganic-organic hybrid PD devices.This fast response of the prepared HJ device is attributed to the built-in electric field at the WSe 2 /P3HT interface and the combined charge carrier mobilities of the two materials.The built-in electric field efficiently separates the photogenerated electron-hole pairs which are swiftly collected by the respective electrodes.Although multilayer WSe 2 has a high electron mobility of ≈100 cm 2 V −1 s −1 , [9,51] P3HT is known to have a moderate hole mobility of ≈10 −2 cm 2 V −1 s −1 due to in-plane charge transport along the direction of the substrate.This imbalance of electron and hole mobilities in HJ results in transit time dominated by slower charge carrier mobility. [52]Nevertheless, WSe 2 /P3HT HJ device demonstrates fast operation speed compared to other reported 2D inorganic-organic PD devices operational in this wavelength range.Therefore, the current device provides a strong foundation for the realization of stable and repeatable broadband photodetection with high responsivity without compromising the operation speed.Further efforts are devoted toward improving the detectivity of such inorganic-organic HJ devices whilst maintaining other device performance parameters.

Conclusion
In summary, we have successfully demonstrated a fast WSe 2 /P3HT-based inorganic-organic hybrid PD with high responsivity.The photodetection analysis of the hybrid device reveals that it responds to a broad wavelength range from visible to NIR region (400−1100 nm).The incorporation of an organic P3HT layer significantly boosted the responsivity of the hybrid device as compared to the bare WSe 2 device.The hybrid device achieved the highest maximum responsivity of 17.6 A W −1 at low light intensity (4.3 μW cm −2 ).The formation of the type ΙΙ HJ and long-range energy transfer from P3HT to WSe 2 helped achieve a high EQE and responsivity.The device demonstrates a rise/fall time of 9.5/5.1 μs with a good response up to a light modulation frequency of 30 kHz.We believe that the photodetection performance of the prepared hybrid PD can be further improved by optimizing the thickness of the P3HT layer.The results presented here showcase the potential of such inorganic/organic hybrid devices to achieve high responsivity without compromising the response speed.Further, the strategy adopted here and the availability of an abundant number of organic semiconductors and 2D inorganic materials can promote novel design strategies for miniaturized, high-performance PDs based on inorganic/organic HJs.

Experimental Section
Fabrication of the Photodetector: The WSe 2 /P3HT inorganic-organic HJ PD device was fabricated using a series of fabrication steps on a Si substrate with a thermal oxide of 200 nm thickness.n-type WSe 2 flakes were mechanically exfoliated from the bulk crystal and transferred to the substrate.Two extended contact electrodes were patterned using a direct laser writing process, with one electrode in contact with the WSe 2 flake and the other with a few micron separations with the flake.A 10/80 nm Cr/Au metal stack was deposited by a radio frequency sputtering technique as the conducting electrode material.Then a 50 nm-thick insulating layer of Al 2 O 3 was deposited using the atomic layer deposition (ALD) technique.Afterward, a rectangular window was patterned on top of the device, exposing a portion of the WSe 2 flake and one electrode (electrode with a few microns separation from the flake), followed by etching of the Al 2 O 3 layer within this window using buffer HF solution.The P3HT (average M w 68 000-80 000) solution was prepared by dissolving P3HT powder in dichlorobenzene (20 mg/mL) and stirred overnight using a magnetic stirrer.For fabricating the WSe 2 /P3HT HJ, the electrode-patterned substrate was placed on a hot plate, and the solution of P3HT was spin-coated (at 1000 rpm).Subsequently, the sample was annealed at 120 °C for 20 min in a N 2 gas environment.For comparison, a bare WSe 2 PD device without a P3HT layer was also fabricated using similar fabrication steps.
Material and Device Characterization: The n-type WSe 2 crystal was commercially purchased from 2D Semiconductors.Raman spectra of the 2D-WSe 2 /P3HT HJ were obtained using a 514 nm laser source in a Horiba Raman system (LabRam HR Evolution).The thickness of the WSe 2 flakes was measured using atomic force microscopy (AFM) (Bruker Corporation, USA), and the images were processed using the Nanoscope Analysis software.The cross-sectional image of the heterostructure was obtained using a field emission scanning electron microscope (FESEM TESCAN), and the elemental map was obtained using an electron probe micro-analyzer (EPMA-1720 HT).The fluorescence lifetime image microscopy (FLIM) and decay profiles were obtained using PicoQuant MicroTime-200 instrument.A 485 nm pulsed laser source with a pulse width of 84 ps was used as the excitation source, and a 690/70 bandpass filter was used as an emission filter.The electrical measurements were carried out in a Keithley parameter analyzer (4200A-SCS).For optical characterization and photodetection, the Sciencetech quantum efficiency system (PTS-2-QE), Thorlab's four-channel fiber-coupled laser source (MCLS1), and PicoQuant pulse diode laser source (PDL 800-D) were used.Averages were taken for at least three devices for responsivity measurement.For the transient photocurrent measurements, light signals were modulated using a waveform generator (Keithley 3390), and the output result was recorded in a digital oscilloscope (Tektronix MDO3104; 1 GHz).For the low-frequency noise measurement, the power spectral density of voltage fluctuations across the device was measured using a low noise amplifier (Stanford Research Systems, model 560) and a dynamic signal analyzer (Stanford Research Systems, model 785).
(a), whereas Figure 1(b) shows the cross-sectional view of the device.The detailed fabrication process flow is explained in Figure S1 in the Supporting Information.Devices were fabricated on a Si/SiO 2 substrate, where the WSe 2 flakes were transferred by mechanical exfoliation.Two metal electrodes consisting of Cr/Au were patterned by a direct laser writing process and deposited using the Radio-frequency (RF) sputtering technique.A top ALD-coated Al 2 O 3 insulation mask layer with a rectangular etched-out window in the Al 2 O 3 was created.Thus ensuring that the spin-coated P3HT can make electrical contact only with a portion of WSe 2 and with one Cr/Au electrode in the exposed area.The left image in Figure 1(c) shows the optical microscopic image of the Al 2 O 3covered device where a rectangular window was etched out exposing a portion of the flake and one electrode.The optical image of the complete HJ device after P3HT coating is shown in the right image of Figure 1(c).

Figure 1 .
Figure 1.a) 3D Schematic illustration of the WSe 2 /P3HT hybrid HJ device.b) cross-sectional view of the HJ device.c) Optical microscopic image(left) of the Al 2 O 3 coated WSe 2 flake with two Cr/Au electrodes, where one electrode can be seen in direct contact with the flake while the other electrode was patterned at a distance for making contact only with the P3HT layer.A rectangular etched-out window in the Al 2 O 3 layer can also be seen in the same image, showing the exposed area of the sample.Optical microscope image of the complete hybrid device after P3HT coating (right).d) Chemical structure of P3HT.e) AFM image of the WSe 2 flake used for HJ device fabrication.f) Thickness profile along the lines drawn in (e).It may be easily seen that the WSe 2 flake is multi-layered with different thicknesses, approximately ranging from 4.6 to 34 nm at different positions.

Figure 2 .
Figure 2. a) Cross-sectional FE-SEM image showing the interface of WSe 2 and P3HT on SiO 2 /Si substrate.b) Top-view SEM image of one WSe 2 /P3HT heterojunction device; and the element-wise EPMA mapping image of WSe 2 /P3HT showing the presence of c) W and d) Se in WSe 2 , e) Au in metal contacts, f) Al and g) O in Al 2 O 3 , and h) C and i) S in P3HT.

Figure 3 .
Figure 3. PL spectra of pure P3HT and WSe 2 /P3HT HJ under excitation at 514 nm for P3HT coating of a) 20 and b) 30 nm, respectively.A significant PL intensity quenching was observed in WSe 2 /P3HT HJ owing to long-range Förster resonance energy transfer from P3HT to WSe 2 , c) PL spectra of WSe 2 , WSe 2 /P3HT, and P3HT obtained using a 785 nm laser excitation, d) FLIM map image showing regions with pure P3HT (region 1) and WSe 2 /P3HT (region 2), e) Decay profiles, and fitted curves for P3HT and WSe 2 /P3HT.The samples were excited with a 485 nm pulsed excitation source, and a 690/70 bandpass filter was used at the emission.

Figure 4 (
Figure 4(b) shows the spectral response of the WSe 2 /P3HT hybrid PD in comparison to the bare WSe 2 device.Although both the PD devices have a broadband response from the visible to NIR range, it is clearly evident that the WSe 2 /P3HT hybrid device is superior in terms of responsivity in the entire wavelength range.A maximum responsivity of 1.44 A W −1 is obtained for the WSe 2 /P3HT hybrid device at 750 nm wavelength

Figure 4 .
Figure 4. a) I-V characteristics of the WSe 2 /P3HT hybrid HJ device under the illumination of light with different wavelengths.The HJ device is sensitive to wavelengths ranging from visible to NIR. b) The spectral responsivity of the WSe 2 /P3HT hybrid device with different P3HT thicknesses in comparison to the bare WSe 2 device at −1 V.The spectral responsivity was measured at 0.2 mW cm −2 light intensity.

Figure 5 .
Figure5.Intensity-dependent photocurrent in WSe 2 /P3HT hybrid HJ device for an illumination wavelength of 640 nm.For low optical intensities, the dependence of photocurrent on intensity is linear, which becomes sub-linear at higher intensities.The inset image shows the responsivity variation with increasing incident light intensity.The responsivity is high (17.6A W −1 ) and almost flat at very low incident light intensity (<10 −5 W cm −2 ).The responsivity decreases sublinearly at higher intensities.

Figure 6 .
Figure 6.a) Variation in dark current and photocurrent of the WSe 2 /P3HT hybrid HJ device over a prolonged period of time which shows a minimal deviation from values at day zero.b) Low-frequency noise spectra of the hybrid device.c) NEP and specific detectivity of the hybrid device as a function of wavelength.The lowest NEP of 1.79 × 10 −12 W Hz −1/2 and maximum D* of 5.47 × 10 8 Jones were obtained at 750 nm wavelength of light for an optical intensity of 0.2 mW cm −2 .

Figure 6 (
b) shows the current noise spectra of the WSe 2 /P3HT hybrid device.The noise spectra show a 1/f  type behaviour with  = 1.4.The value of S I per unit bandwidth (1 Hz) is 6.6 × 10 −24 A 2 Hz −1 , and this value was used to calculate the NEP and consequently the specific detectivity (D*) of the WSe 2 /P3HT hybrid device (FigureS10(a,b) in the Supporting Information shows NEP versus frequency and specific detectivity versus frequency, respectively).The NEP and specific detectivity values thus obtained are displayed in Figure6(c) as a function of wavelength in the entire operating region of the hybrid PD device.

Figure 7 .
Figure 7. a) Transient photoresponse of the hybrid device under the light of different modulation frequencies.The device shows good response up to a light modulation frequency of 30 kHz. b) The rise time (9.5 μs) and fall time (5.1 μs) for one cycle of the output signal.c) Normalized response as a function of the frequency of the modulated source; a −3 dB cut-off frequency of 35 kHz was obtained.

Figure 8 .
Figure 8. Band alignment of WSe 2 /P3HT heterostructure showing the formation of type ΙΙ heterojunction.a) Photoexcitation leads to the formation of excitons mainly in P3HT regions, which diffuse to the interface and get dissociated because of the sufficient energy offset between LUMO of P3HT and CB WSe 2 (∆E > E b ).b) Photocarrier generation in WSe 2 , c) Long-range energy transfer from P3HT to WSe 2 which enables exciton harvesting from thick P3HT layer.S 1 and S 0 represent the first excited state and ground state, respectively.

Table 1 .
Comparison of the performance parameters of photodiodes based on WSe 2 and other organic-inorganic hybrid heterojunctions.