graphene composite based organic photodetectors: The influence of graphene insertion

graphene composite based organic photodetectors: The influence of graphene insertion

Thin Solid Films 675 (2019) 128–135 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Poly-(...

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Thin Solid Films 675 (2019) 128–135

Contents lists available at ScienceDirect

Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Poly-(3-hexylthiophene)/graphene composite based organic photodetectors: The influence of graphene insertion

T

Anjali Yadava, Aditi Upadhyayaa, Saral Kumar Guptaa, Ajay Singh Vermaa, ⁎ Chandra Mohan Singh Negib, a b

Department of Physics, Banasthali Vidyapith, Rajasthan 304022, India Department of Electronics, Banasthali Vidyapith, Rajasthan 304022, India

A R T I C LE I N FO

A B S T R A C T

Keywords: P3HT/graphene composite Current-voltage characteristics Photodetector Responsivity Detectivity Space charge limited photocurrent

Currently, graphene has emerged as the prominent material for numerous applications in the field of optoelectronic devices. Herein we report an organic photodetector fabricated using solution processed composites of poly-(3-hexylthiophene) (P3HT) and graphene. The UV/Vis/NIR absorption spectra reveal that the composites exhibit absorption in the visible range. Incorporation of graphene introduces slight red shifts in the absorption spectra of P3HT indicative of the increase in the conjugation length of P3HT. The effects of various amount of graphene on electrical performance of devices are investigated. Results show that the device with 5 wt% graphene concentration demonstrates best performance, exhibiting detectivity of 1.8 × 108 Jones and responsivity of 0.25 A/W at –9 V. Intensity and voltage dependent photocurrent studies predict that the photocurrent is not limited by the space charge effect in the device. Impedance spectroscopy analysis is carried out, and the obtained data are fitted to the suitable equivalent circuit to extract the internal device parameters, including junction resistance and capacitance, that are further used to estimate the bandwidth of photodetector. We believe that this report helps in the basic understanding of the impact of graphene insertion in polymer devices that will contribute towards the development of polymer/graphene composite based organic optoelectronic devices.

1. Introduction In recent years, the polymer/carbon derivative composites have been extensively investigated for application in photovoltaic and optoelectronic devices. Graphene the wonder material, one of the most interesting carbon derivative, is an arrangement of sp2 hybridized carbon atoms in the form of two dimensional sheets. Graphene offers remarkable properties, such as high mechanical and chemical stability [1–3]. It afford efficient charge transfer owing to better energy level alignment with other suitable donor materials, such as, poly [2methoxy-5-(2-ethylhexoxy-p-phenylene vinylene] (MEH:PPV), Poly-(3hexylthiophene) (P3HT) etc. It can also provide available delocalized electrons with high mobility, and high current transport capability [4–6]. It can not only offer a direct path for carrier transport but also affords an increased surface junction area for exciton dissociation and transport. Currently, graphene is emerged as innovative material for many scientific and commercial applications [7]. Recently, it has shown potential as a viable alternative for optoelectronic applications including photodetectors. However, weak light absorption and inadequate gain in thin graphene layer limits the performance of ⁎

graphene based devices. Significant efforts have been made to overcome these shortcomings of graphene, such as integrating it with organic semiconductor material [8–11]. On the other hand, the conjugated polymers and their derivatives have been investigated for their potential applications in organic optoelectronics, such as organic photovoltaics, smart windows, and sensors due to their high absorption coefficient, light weight, cost-effectiveness, flexibility, easy solution processability and high throughput fabrication steps. Among the several conjugate polymers, P3HT is a widely recognized conjugated polymer for photovoltaic applications due to its intriguing properties, such as high electrical conductivity, solubility in various solvents and high absorption coefficient (~105 cm−1) in the visible range [11]. In addition, side chain of this polymer assists for dissolving it into various solvents, as well as leading to the Π electron excitation during the photovoltaic process. However, poor electrical conductivity, less mechanical strength and lack of stability in pristine polymers are the major obstacles for commercial applications. The incorporation of graphene in the polymer matrix could result in the substantial properties improvement of the both polymers and

Corresponding author. E-mail address: [email protected] (C.M.S. Negi).

https://doi.org/10.1016/j.tsf.2019.02.013 Received 13 May 2018; Received in revised form 23 January 2019; Accepted 8 February 2019 Available online 23 February 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.

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1. 2. 3. 4. 5.

graphene. These property enhancements pave the path for developments of novel applications. Yu et al. [12], fabricated a solution-cast (grafted graphene P3HT) GP3HT/C60 heterostructure based photovoltaic device, and reported 200% increase in power conversion efficiency compared to their P3HT/C60 counterpart. Strong photoluminescence and fluorescence quenching have been recently reported in P3HT:graphene composites indicating efficient electron transfer at the interface between the two materials [13]. Zhang et al. [14] and Stylianakis et al. [15] fabricated the P3HT-graphene composites based organic bulk heterojunction for photovoltaic and optoelectronic applications. Besides extensive research on the organic based solar cells, conductive polymers have also drawn considerable attention for applications in organic photodiodes [16,17]. In comparison with inorganic photodiodes, organic photodiodes are cheaper to fabricate, less energy intensive, flexible and can offer larger sensing area. Numerous publications so far have been focused on the photodetectors based on the organic bulk heterojunction [18,19]. However, the photodetectors based on polymer/graphene composites got very little attention. In this work, we employed P3HT/graphene composites as the active layer material with the aim to improve the performance of the organic photodetector. The composites comprising of various graphene concentrations were prepared and examined for optical and molecular structural analysis. The photodetectors are optimized by varying the graphene concentration to enhance the photo response. The improvement in the photodetector performance is assessed by measuring the current-voltage (I-V) characteristics of photodetector in dark and under white light illumination. Furthermore, impedance spectroscopy analysis is employed to extract the internal device parameters and bandwidth of photodetector.

D1D2D3D4D5-

ITO/PEDOT:PSS/P3HT/Al ITO/PEDOT:PSS/P3HT:1.25 wt% graphene/Al ITO/PEDOT:PSS/P3HT:2.5 wt% graphene/Al ITO/PEDOT:PSS/P3HT:5 wt% graphene/Al ITO/PEDOT:PSS/P3HT:10 wt% graphene/Al.

2.3. Characterization techniques The prepared films and devices were characterized by various experimental techniques. The optical and molecular structural characterizations of the composites thin films were acquired via Perkin Elmer Lambda 750 Spectrophotometer and Thermo Scientific DXRxi Raman Imaging Microscope, respectively. The I-V measurements were carried out through 2612A Keithley Source meter. To measure the photodiode characteristics, the devices were illuminated by white light lamp source of power 14 mW/cm2 placed at 12 cm from the devices. The impedance spectroscopy was recorded by using an Autolab PGSTAT30 electrochemical work station. The amplitude of ac signal was kept at 10 mV and frequency of signal was scanned from 0.1 Hz to 1 MHz. 3. Results and discussions The normalized absorption spectra of pristine P3HT and P3HT/ graphene composite thin films for wavelength range from 380 nm to 750 nm are shown in Fig. 2(a). The maximum absorption peak (position A) is found at 560 nm along with a shouldered peak (position B) at 604.15 nm for the pristine P3HT thin film, where peak A is attributed to Π-Π* transition, while the peak B related to the n-Π* transitions is attributed to the absorption of interchain stacking in P3HT [20]. In order to study the interaction between pristine P3HT and graphene, variation of peak position, peak absorption intensity, and full width at half maximum (FWHM), with the graphene concentration is also displayed in Fig. 2(b–d). Decrease in FWHM with the increasing graphene concentrations clearly suggests the interaction between graphene and P3HT, as well as indicative of the increase in conjugation length of P3HT [11]. The variation of peak position and peak absorption intensity with graphene content demonstrates that both peaks show slight red shift with increase of graphene concentrations up to 5 wt% in the P3HT/graphene composites. Upon addition of graphene, the P3HT chains uncoil at the graphene surface defects and edge sites thereby increasing their conjunction length leading to the red shift of peaks with respect to P3HT [11]. It can be observed from Fig. 2(b–c) that peak wavelength shift increase with increase in the graphene concentration up to graphene concentration of 5 wt%, however further increase in the graphene concentration reduces the peaks wavelength shift. Moreover, reduction in the absorption intensity is found for the composites thin films, which might be due to the reduction in the optical volume of P3HT in composites thin films. Raman spectroscopy was used to evaluate the interaction between P3HT and graphene. Fig. 3 compares the Raman spectra of P3HT with P3HT/graphene composite thin films at different graphene concentrations (1.25 wt%, 2.5 wt%, 5 wt%, 10 wt%). The samples were excited by 532 nm laser source. In the case of P3HT, the various Raman modes observed and identified are1447 cm−1 (symmetric C]C stretching vibrations) (peak A′), 1379 cm−1 (CeC intraring stretching) (peak B) and 725 cm−1 (CeSeC ring deformation), which is very similar to the previous reported results [21]. Besides this, the similar vibration modes are found for the P3HT/graphene composite thin films, suggesting that the molecular structure of the composites is dominated by the P3HT polymer. Fig. 3(b) shows the variation of intensity for peaks A and B with the graphene concentration. It was observed that the increase in graphene concentration reduces the intensity of both peaks, thus indicating slight change in the chemical structure of P3HT by insertion of graphene [22]. The measured I-V characteristics for devices D1, D2, D3, D4 and D5

2. Experimental details 2.1. Materials and solution processing methods The polymers; Regioregular P3HT and (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) (PEDOT:PSS) and Ortho-dicholorobenzene (ODCB) with molecular weight 147.00 g/mol and 99% purity were purchased from Sigma Aldrich. Multilayer Graphene powder with 99% purity was procured from Redex nanolabs. All the materials were used as obtained without any further purification. Graphene was dispersed in ODCB using ultra probe sonicator (500F Pci analytics) for 2 h, and then subjected to centrifuge for 1 h at 6000 rpm to remove the precipitate from the solution. The P3HT solution was prepared in ODCB with a concentration of 10 mg/ml with the help of magnetic stirring technique for 24 h at 1200 rpm. Next, the dispersed graphene with graphene concentrations (1.25 wt%, 2.5 wt%, 5 wt%, 10 wt%) was mixed with P3HT and stirred for 24 h to form the P3HT/graphene composite solution. 2.2. Device fabrication Indium Tin Oxide (ITO) (surface sheet resistance ~16 Ω/square) coated glass sheets were used as the substrates for the fabrication of devices. A layer of PEDOT:PSS was deposited over the cleaned ITO coated glass substrate at 2000 rpm for 60 s followed by the deposition of active layer (P3HT:graphene composite) at 1500 rpm for 60 s. All the depositions were carried out inside the glove box at room temperature. Finally, Al electrode was deposited via thermal evaporation at a pressure of 10−5 Torr for the completion of the device. All devices were fabricated in the similar manner except varying the graphene concentration in the active layer. The schematic diagram of device and corresponding energy level diagram is shown in Fig. 1. The devices designated as D1, D2, D3, D4 and D5 henceforth of the following architectures were fabricated: 129

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Fig. 1. (a) Schematic diagram of device, (b) corresponding energy level diagram showing transport of charge carriers subsequent to the light excitation, (c) illustration of Schottky barrier formation at P3HT/Al interface before and after intimate contact under thermal equilibrium. Here, ϕs and ϕm represents the work function of P3HT and Al, ᵡs denotes the electron affinity of P3HT.

agglomeration become so heavy such that charge transport hampering dominates over the conductivity enhancement leading to the reduction in the current level in the device. Next, we estimate the various diode parameters, such as ideality factor, barrier height, reverse saturation current and series resistance by analyzing the I-V curves with the Shockley theory, in which the voltage dependence on the current can be expressed as [26];

in the dark and under illumination conditions are displayed in Fig. 4. The exponential dependence of forward current on applied voltage and slow variation of reverse current with applied bias voltage indicate that all devices exhibit rectifying behavior [23,24]. The rectification behavior arises due to the formation of Schottky barrier at P3HT: graphene composite/Al interface (Fig. 1(d)). As can be seen in the Fig. 4(a), current level of the devices with graphene concentration in nanocomposites up to the 5 wt% (for devices D1 to D4) enhances with increase in graphene loading. However, further increase of graphene content reduces the value of current levels. Fig. 4(b) presents the photoresponse characteristics of the devices under the illumination of white light of power 14 mW/cm2. Fig. 4(c) shows the photocurrentvoltage characteristics under reverse bias condition. The variation of photo current with different concentration of graphene as a function of applied voltage is presented in Fig. 4(d). As can be observed, the photocurrent depends on the graphene concentration in a similar way to the dark current. The graphene concentration dependence on current level of devices can be explained as follows; the addition of graphene into P3HT mainly produces two performance determining changes. First, enhancement in the electrical conductivity of the composite owing to giant carrier mobility of graphene as well as the percolative path offered by the network structure of graphene to facilitate the transport of charge carriers. Second is the modification in the nano-morphology of the composite. The addition of graphene leads to the agglomeration of particles resulting in the deterioration of the morphology, which aggravate with the increase in graphene concentration [25]. The agglomeration impedes the charge transport leading to the reduction in current level. Up to 5 wt% concentration, increase in conductivity upon graphene loading dominates over the charge transport hampering due to agglomeration, ultimately results into the enhancement in current level. Beyond 5 wt%

q (V − IRs ) ⎞ − 1⎤, I = Is ⎡exp ⎛ ⎢ ⎥ nkT ⎝ ⎠ ⎣ ⎦

(1)

where, Rs is the series resistance, n is the ideality factor, T is the temperature, k is the Boltzmann constant, q is elementary charge, V is applied voltage and Is is the reverse saturation current, which can be determined by the relationship;

eϕ Is = AA∗ T 2 exp ⎛− B ⎞, ⎝ kT ⎠ ⎜



(2) ∗

here A is the junction area, ϕB is the Schottky barrier height and A is the Richardson's constant. The ideality factor and reverse saturation current is extracted from slope and y-axis intercept, respectively from the semi-logarithmic I-V curves shown in Fig. 6(a) under dark condition for devices D1 and D4, and by knowing the value of reverse saturation current, the barrier height is calculated from Eq. (2). Inverse of the slope of I-V curves in the linear region yields the series resistance. The value of ideality factor (IF) of all devices are found to be larger than 2, which might be attributed to the various phenomena, such as irregularities in the thickness of organic film, inhomogeneity in barrier height, increased recombination due to surface/interface defects and leakage through large series resistance [27]. In Fig. 5(a), the variation of the barrier height and IF of the devices with different graphene 130

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Fig. 2. (a) Normalized absorption spectra of pristine P3HT and P3HT:graphene composites thin films with various graphene concentrations (1.25 wt%, 2.5 wt%,5 wt %, 10 wt%), (b) variation of peak absorption intensity of peak A and B with graphene content, (c) variation of peak positions with graphene content. (d) Variation of FWHM with graphene weight percentage in composites films.

The series resistance decreases with the increase in the graphene concentration up to 5 wt%. Since graphene doping improves the conductivity of the active layer material, thereby reduces the series resistance. Increase in the series resistance above 5 wt% of graphene concentration indicates hindrance of charge transport at large graphene concentration. As obvious, reverse saturation current shows variation exactly opposite that of barrier height. As can be observed from the I-V analysis, the electrical parameters and current values obtained for device D4 are found to be best among all devices under study. Therefore, further electrical characterizations are carried out for device D4. The dark I-V curves analyzed in the light of space charge limited conduction (SCLC) model to understand the

concentration are plotted. The IF of device with graphene doped P3HT devices is larger as compared to the pristine devices. The deterioration in IF might be ascribed to the structural defects originated due to rearrangement of polymer molecules [28]. Structural defects promote recombination, leading to increase in the IF. The barrier height of graphene incorporated devices is less compared to the pristine device. The LUMO and HOMO level of P3HT was found to decrease with the incorporation of graphene [29], this shifts LUMO level of P3HT towards the ITO Fermi level, thereby reduces the work function difference between ITO and composite, consequently decreases barrier height. The variation in series resistance and reverse saturation current of the devices with different graphene concentration is shown in Fig. 5(b).

Fig. 3. Raman spectra of pristine (a) P3HT and P3HT:graphene composites thin films with various graphene concentrations for; 1.25 wt%, 2.5 wt%, 5 wt%, and 10 wt %, and (b) change in intensity versus graphene concentrations for peak A and peak B. 131

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Fig. 4. (a) I-V characteristics of the fabricated devices under dark, (b) I-V characteristics under the illumination condition (c) Photocurrent vs. voltage characteristics of devices with varying graphene content under reverse bias condition (d) variation of photocurrent with graphene content as a function of applied voltage.

electrodes, μ is the carrier mobility and V is the applied voltage. Upon increase in the applied voltage, the injected carrier density keeps growing with the voltage may exceeds over the thermally generated charge carriers density at certain voltage. Thereafter, the current in the device is controlled by the bulk properties of the active layer including mobility, trap density and trap energy distribution. In this regime, rapid rise of current density with voltage as evident by the m > 2 in the Fig. 6(b) is the feature of trap controlled space charge limited condition (TCLC). The current density in the presence of shallow traps at discrete trapping level within the forbidden gap in the organic films is given by [33,34];

charge transport mechanism in the device. The experimental I-V curves are plotted in log-log scale and fitted with the power law profile (J α Vm) as shown in Fig. 6(b) for the pristine device (D1) and optimum device (D4), here exponent m specify the dominant conduction mechanism. Based on the bias dependence on the current, the J-V curve on log-log scale exhibits two distinct charge transport regimes. Similar observation can also be found in the previous reports [22,30,31]. At low voltage, the dominance of thermally generated charge carriers over injected carrier in the active layer results into the linear relationship between current and voltage (m = 1) which is the characteristic of Ohm's law. The current density in this region is described by the equation [32];

V j = qn 0 μ , d

j= (4)

9ɛs θμV 2 , 8 d3

(5)

where, ɛs is the permittivity of the organic semiconductor, μ is the charge carrier mobility, and θ is the trap factor.

where, n0 is the intrinsic carrier density, d is the separation between the

Fig. 5. (a) Variation of (a) ideality factor and barrier height (b) series resistance (Rs) and saturation current (Is) of devices with different graphene concentration. 132

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Fig. 6. (a) Semi logarithmic plot of current density vs. voltage for device consisting pristine P3HT (D1) and device comprising P3HT/5 wt% of graphene content (D4) under dark condition (b) Double logarithmic plot of current density Vs. voltage fitted with the power law profile (J α Vm) for device for device D1 and D4; herein the graph m represents the slope of the lines.

Fig. 7. (a) Variation of photocurrent density with applied voltage at different illumination intensities (b) photocurrent density versus light intensity plot at different voltages, respectively for device D4 fitted with power law (Jph = C Vs Iα). Herein, s andα indicates the exponents of voltage and intensity, respectively.

performance by determining some figure of merit of photodetectors including responsivity and detectivity. Responsivity reflects the efficiency of the photodetectors and is defined as the ratio of the incident light intensity to the measured output photocurrent. It is estimated from experimental data using [37],

0.25 Responsivity Detectivity

0.20

9 Detectivity (10 Jones)

Responsivity(A/W )

0.3

0.15

0.2

R=

0.10 0.1

0.00 0

2

4

6 8 Voltage (V)

10

,

(6)

where, Jph is the photocurrent and Ilight is the light intensity. The Detectivity (D) measures the ability of a photodetector to detect a weak optical signal under the presence of noise. When dark current contribute as the dominant component of the noise, the detectivity then can be expressed as,

0.05

0.0

Jph Ilight

12

Jph

D∗ =

Fig. 8. Responsivity and detectivity versus applied voltage for device D4.

Ilight

(2qJd )2

,

(7)

where Jd is the dark current. The detectivity and responsivity as a function of applied reverse bias voltage calculated using Eq. (6) and Eq. (7) for device D4 is shown in Fig. 8. Responsivity and detectivity initially increases slowly with increasing applied voltage up to 6 V, and then increases significantly with further increase in applied voltage up to 8 V. However, beyond this voltage, variation of responsivity with voltage is relatively slow. On the other hand, detectivity beyond 8 V, start decreasing upon further increase in voltage. At low voltages (< 6 V), the exciton dissociation and photo generated charge carrier transport under applied electric field is rather low, so variation in responsivity and detectivity with voltage is also slow. Increase in voltage enhances exciton dissociation and carrier transport leading to the rapid increase in photocurrent, consequently responsivity and detectivity changes rapidly with the voltage. Beyond

To investigate the SCL photocurrent characteristic in our devices, photocurrent as a function of applied voltage and illumination intensity is shown in Fig. 7. As can be seen, the dependence of photocurrent on applied voltage and light could be fitted with power law (Jph = C Vs Iα), where C is constant, s and α are exponents of voltage and current. Typically SCL photocurrent exhibits square root dependence on applied voltage (s = 1/2) and three-quarter power dependence on illumination intensity (α = 3/4) [35,36]. However, in our case s = 0.55–0.82 and α = 0.82–1.02 indicating space charge effect doesn't occur during the flow of photocurrent. Thus, Current is not limited by space charge effect and the photocurrent transport is governed by both drift and diffusion processes. Optimum device D4 is further characterized in detail to assess its 133

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Fig. 9. (a) Nyquist plot obtained from simulation and experimental data at different voltages for device D4, (b) and (c) Bode plot for simulated and experimental data at different voltages, (d) electrical equivalent circuit model of the photodetector.

and simulated data. The capacitance (Cj) is estimated from the CPE parameter through the Eq. (8),

Table 1 Estimated parameters from impedance spectroscopy study for device D4 at different voltage. Applied voltage (V)

0 1 -1

Rs (Ω)

96.46 2500.6 225.65

RSh (KΩ)

27.337 51.31 156

CPE parameters Q0

η

1.89 × 10−9 1.99 × 10−9 2.32 × 10−9

0.969 0.967 0.951

Cj (μF)

0.0013 0.0014 0.0015

f3dB (kHz)

Cj = RSh

1−η η

(Q ) , 1



(8)

where η (0 < η < 1) and Q0 are the exponent and admittance constant of CPE. The extracted parameters are further used to estimate the photodetector band width by the Eq. (9) [38],

624.94 43.74 349.97

f3dB = 1/2π (RL + Rs ) Cj, 8 V, nearly all the photo generated excitons are dissociated, transported and extracted. Thus, photocurrent and consequently responsivity rise with voltage is comparatively small. As the responsivity vary slowly, whereas the dark current increases noticeably leading to significant increase in the noise current results in the overall reduction in the detectivity for voltage larger than 8 V. Furthermore, EIS was employed to understand the internal device configuration in terms of electrical equivalent circuit and to evaluate the corresponding parameters such as shunt resistance and junction capacitance. Fig. 9(a–c) depicts the recorded Nyquist plots, and Bode plots at different voltage values along with simulated fits obtained from equivalent circuit for the optimized device (D4). Electrical equivalent circuit of the photodetector utilized to fit the experimental data is shown in Fig. 9(d). The circuit consists of the parallel R-C circuit, which is generally corresponds to the shunt resistance (RSh) and the junction capacitance(Cj), in series with the series resistance (RS). The series resistance accounts for the resistance contribution from metallic contacts and wires. The constant phase element (CPE) in the circuit models the electrical behavior of the imperfect capacitor. The junction capacitance includes both space charge and diffusion capacitance. The Nyquist plots and Bode plots clearly demonstrate good quality fit between measured

(9)

where RLrepresents load resistance, respectively. All the extracted parameters and bandwidth is listed in Table 1. Since each method (I-V and impedance) uses different type of lead/wire to connect the device under test and the measuring instrument and the resistance of lead/ wire is inseparable from the resistance of photodetector during the measurement, causing a little discrepancy between the series resistance value obtained from I-V curves and impedance analysis. The calculated bandwidth is comparable to the organic bulk heterojunction photodiode [38]. 4. Conclusions P3HT/graphene composite based photodetectors were fabricated via low-cost solution based spin-coating method and examined their electrical characteristics. The optical and molecular structural studies of the composite thin films reveal interaction between graphene and P3HT. We found that the incorporation of graphene changes dark current, photocurrent and various device parameters. The best electrical performance was achieved for the device with composite having 5 wt% concentration graphene in the active layer. No evidences of space charge limited photocurrent were found from photocurrent134

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intensity analysis. Impedance analysis affords the opportunity to probe the internal capacitor and resistance of the photodetector. The photodetector exhibits good photo response and bandwidth of 0.624 MHz.

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