rGO integrated MEHPPV and P3HT polymer blends for bulk hetero junction solar cells: A comparative insight

Accepted Manuscript Title: rGO integrated MEHPPV and P3HT polymer blends for bulk hetero junction solar cells: A comparative insight Authors: Sumit Kumar, Jitendra Kumar, Shailesh Narayan Sharma, Shubhda Srivastava PII: DOI: Reference:

S0030-4026(18)31461-X https://doi.org/10.1016/j.ijleo.2018.09.148 IJLEO 61587

To appear in: Received date: Accepted date:

25-7-2018 26-9-2018

Please cite this article as: Kumar S, Kumar J, Narayan Sharma S, Srivastava S, rGO integrated MEHPPV and P3HT polymer blends for bulk hetero junction solar cells: A comparative insight, Optik (2018), https://doi.org/10.1016/j.ijleo.2018.09.148 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

rGO integrated MEHPPV and P3HT polymer blends for bulk hetero junction solar cells: A comparative insight Sumit Kumara,b*, Jitendra Kumara*, Shailesh Narayan Sharmab, Shubhda Srivastava b a c

Department of Electronics Engineering, Indian Institute of Technology (IIT-ISM) Dhanbad, Jharkhand 826004, India Advanced Materials and Devices Division, CSIR- National Physical Laboratory, New Delhi 110012, India.

ABSTRACT In this paper, a comparative study of reduced graphene oxide (rGO) integrated with poly[2methoxy,5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEHPPV) and poly(3-hexylthiophene) (P3HT) conducting polymers have been conducted in order to obtain an efficient active layer material for organic photovoltaic (OPV) device fabrication. The structural and morphological properties have been investigated using techniques such as X-ray diffraction, field emission scanning electron microscopy, thermogravimetric analysis, and Raman spectroscopy. The optical and electrical characterizations have been performed to explore the photoluminescence quenching behaviour, electron transfer properties and efficient energy conversion. Moreover, the quantification of charge transport properties was estimated using Stern-Volmer and modified Stern-Volmer plots. The quenching constants obtained for MEHPPV–rGO nanocomposite was 2.41x103 L-1 and 2.72x103 L-1 for P3HT–rGO nanocomposite to better charge transfer capabilities found in nanocomposites. Further, the contact angle and surface energy measurement studies conducted on thin films also proposed that P3HT-rGO nanocomposite pairs are more suitable for solar cell applications as compared to MEHPPV- rGO. However, the I-V results obtained from the fabricated devices indicate that the rGO composites blended with P3HT polymer showed an efficiency of 0.045 % while those composed with MEHPPV shows only 0.024 %.

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Keywords: Graphene Oxide (GO) reduced Graphene Oxide (rGO)

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MEHPPV and P3HT Hybrid nanocomposites Charge transfer properties Organic photovoltaic devices

1. Introduction

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Recently graphene-based organic photovoltaic research has been significantly progressed to solve the energy crisis problem at different levels [1-3]. Especially, in energy-related areas, the incorporation of graphene with the conducting polymers is extensively studied. Graphene and its derivatives have been used as standalone materials in various interfacial layers of the organic photovoltaic (OPV) system such as hole transport layer material (HTL)[4], an electron transport layer material (ETL) [5], and even as an active layer material [6]. Hence, graphene and its derivatives have been investigated curiously in order to improve the charge transport capabilities in organic solar cells [7, 8]. Whereas the utilisation of reduced graphene oxide (rGO) in the active layer in conjunction with the polymers for the solar cell application is still limited. Till date plenty of research articles have been published which demonstrates the strength of graphene in almost every field of science and engineering [9, 10]. Nevertheless, enough data is already available for defining its chemical, physical, mechanical, optical and electrical properties which validate the significant role of graphene-based materials in an abundance of electronic applications like organic transistors, organic light emitting diodes, photovoltaic devices and enormous use in biosensor applications [11-13]. Additionally, the researchers are more prone towards the use of functionalised graphene or other derivatives like graphite oxide, expandable graphite oxides for different purposes [14, 15]. The major restraint in their utilisation was the poor conductivity for which the higher concentrations are required that adversely affects the absorption properties and leads to poor photoconversion process [16].

Fig. 1 Energy conversion process in the BHJ solar cell. Modified and regenerated from Ref. [17]. Therefore rGO-polymer complex based nanocomposites for the bulk-heterojunction (BHJ) solar cell would be employed for attaining the possible improvement in power conversion efficiency (PCE). Such nanocomposites are gaining more attention because of low-cost processing and flexibility as compared to other inorganic based solar cell devices [18]. It has been reported that more than 12% of PCE

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was achieved in polymer-based solar cells [19]. Thus for designing a highly efficient OPV system, a four-step energy conversion process is required which involves the efficient absorption of light on the surface of a material that leads to the generation of excitons in donor: acceptor domains in the initial step. The second step involves the diffusion of excitons at donor: acceptor interfaces. The third step involves the effective dissociation and separation of excitons to enhance the charge carrier generation and minimise the possibilities of recombination states within the species. The fourth step involves the efficient charge transfer capabilities and their collection at both terminals [20]. Based on this mechanism, it is believed that solar cells developed with BHJ structures utilised interpenetrating networks of nanostructured electron donor and electron acceptor materials to enhance the interfacial area between donor and acceptor species for increasing PCE [21]. In this context, the operation of the BHJ based OPV system is described in Fig. 1 for further illustrating the energy conversion process. In present work, rGO-polymer nanocomposites have been used as an active layer material to fabricate OPV cell. These nanocomposites were formed after incorporating the rGO in the matrixes of two different conducting polymers, i.e. MEHPPV and P3HT that has a wide range of applications in organic and hybrid BHJ devices [22, 23]. Here, we investigate the efficient charge transfer properties of these nanocomposite materials when devised for photovoltaic applications. The study is also unique because such simultaneous and corresponding comparative investigation has been carried out for the first time to discuss their charge transport properties in depth. The reduction of GO to rGO was performed using conventional hydrazine hydrate method and confirmed by XRD, FE-SEM, Raman Spectra, TGA and UV-Visible absorption spectroscopic techniques. The charge transfer studies were conducted for calculating the quenching rates as well as an accessible fluorophore in the nanocomposite pairs. Moreover, the qualitative investigation was done using Stern-Volmer theory in which Stern-Volmer plots were evaluated in order to obtain the quenching rates in both cases. While the modified Stern-Volmer plots were described for calculating the accessible and non – accessible fluorophore in the rGOpolymer blends. Based on these interpretations the MEHPPV–rGO and P3HT–rGO based nanocomposites were utilised in fabricating the organic solar cell devices. The electrical properties were identified using current-voltage (I-V) characteristics for calculating the efficiency values. The higher efficiency was obtained in P3HT–rGO based OPV devices as compared to MEHPPV–rGO based devices.

2. Experimental details

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2.1. Materials used

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The materials such as GO powder, conducting polymer MEHPPV powder, regio-regular P3HT powder, poly(3,4-ethylene dioxythiophene)-poly(styrene-sulfonate) (PEDOT: PSS) (1.1% in H2O) procured from Sigma Aldrich and utilised without doing any further purification. All other chemicals, solvents, and reagents utilised during the processes of synthesis, characterisation and fabrication of OPV devices were purchased from Sigma-Aldrich. The GO solution was reduced using hydrazine hydrate and ammonia solution under low temperature and atmospheric conditions as reported elsewhere [24]. The Indium Tin Oxide (ITO) substrates were purchased from TECHNISTRO having a resistivity of 10 ohms/sq approximately and 90% of transmittance.

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2.2. Synthesis of rGO from GO

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For the reduction of GO, 0.1 gm of GO dispersed in 100 ml of water was sonicated in an ultrasonic bath for 30 min, which yielded an inhomogeneous yellow-brown dispersion [25, 26]. Further, 1 ml of hydrazine hydrate was added followed by 15 µl ammonia solution to the above solution. After the few minutes of continuous stirring, the solution was kept at 95 C in a water bath for one hour. The change in colour confirmed the reduction of GO to rGO from brown-yellow (GO) to black (rGO). Digital images are provided for the visual difference between the GO and rGO in Fig. 2. Later, the resultant mixture was dried at 90-100 C to get rGO powder for further use.

Fig. 2 Digital image represents the change in colour after reduction of GO into rGO.

2.3. Synthesis of MEHPPV–rGO and P3HT–rGO nanocomposites The nanocomposites of MEHPPV–rGO and P3HT–rGO were prepared using simple solution processing technique which was subjected to ultrasonication process for 30 min for each case. For the synthesis of nanocomposites, 5 ml of 1 mg ml -1 solution of conducting polymers MEHPPV and regio-regular P3HT in chloroform was prepared and subjected to sonication process for 30 min. Similarly, 1 mg ml-1 of rGO dispersion in chloroform solution was prepared and subjected to sonication for 45 min. Later, with different weight ratios of rGO solution intermixed with the pristine MEHPPV and P3HT solutions to form the respective nanocomposite while keeping the concentration of solutions constant. The film samples from these nanocomposites were spin coated on different substrates for further characterisation purpose.

2.4. Experimental techniques

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The morphological studies of the synthesised materials were done by field emission scanning electron microscopy (FE-SEM). Rigaku’s X-Ray diffractometer (XRD) was used to record XRD patterns and for determining the features in material like crystallinity, phase formation, shifts in diffraction peaks and layer spacing (d spacing) under 2-theta measurements. Raman studies were performed using InVia Raman Microscope, Renishaw using a laser with the excitation wavelength of 514 nm. Perkin Elmer’s UV-visible-NIR spectrophotometer was used for absorption related studies in the wavelength range of 300 nm – 700 nm to observe the red-shift and blueshift in the nanocomposites. Horiba’s Photoluminescence (PL) system was used to carry out the luminescence-based studies to calculate the quenching rate in several batches of different weight ratios of rGO in both conducting polymer matrixes ranging from 0 to 1000 L. The calculations for contact angle measurement, GBX Digidrop instrument was used for obtaining the wettability nature of film whether hydrophobic or hydrophilic and surface energy in each case. The electrical measurements and current-voltage (I-V) characteristics were obtained using Keithley source meter (2420) which was attached to a calibrated light source apparatus that generates energy equivalent to one sun. Both units were interfaced with a computer for corresponding operations. To elucidate the results, thin films were prepared using spin coating unit from HOLMARC (HO-TH-05) on glass substrates having size (10x10x1.1) mm either composed of borosilicate or ITO coated glass substrates at different speeds. A Dektek profilometer was used to measure the thickness of films at various stages.

3. Results & discussion

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In this section, various characterisations have been discussed to confirm the reduction of GO to rGO. The as-prepared rGO was integrated with both the conducting MEHPPV and P3HT polymers and used as an efficient active layer material for fabricating OPV devices. The charge transfer properties of both the nanocomposites were elucidated with the help of miscellaneous optical characterisation techniques. 3.1. XRD analysis

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The crystalline properties of GO, rGO, MEHPPV–rGO and P3HT–rGO nanocomposites were investigated using XRD as shown in Fig. 3a, 3b, 3c, and 3d respectively. In each case, the structural information was calculated using Bragg’s law [27] which is expressed as: λ=2dsin () (1) where  is the Bragg’s diffraction angle, d is the separation between the adjacent layers of GO and λ is the wavelength of the X-ray beam and usually taken as 0.154 nm. Hence, the interlayer distance is measured by using the equation: d=λ/2 sin () = 0.154 nm / 2sin (Ɵ) (2) The GO pattern exhibits two diffraction peaks; the first peak which is very sharp at 2 = 11.4 and another broad peak around 23.32. The spectra in Fig. 3a diffraction peak (001) corresponds to an interlayer spacing of d=0.601 nm due to the presence of the oxygen functional group whereas in the spectra of rGO reflected in Fig. 3b, the diffraction peak correspond to plane (002) at 2 = 23.3 indicates the interlayer spacing d=0.36 nm. The decrement in the d spacing of rGO in contrast to the GO can be utilised as evidence that after the reduction the excess oxygen moieties gets removed to an extent and leads to an irreversible agglomeration of rGO sheets [28]. Moreover, a very weak diffraction peak at 2=42.30 can be observed in Fig. 3b, 3c, and 3d which might have appeared due to the incomplete oxidation. In addition to that, the pattern of MEHPPV–rGO and P3HT–rGO exactly overlap with each other and resembles the spectra of rGO. In fact, the spectra become invisible when drawn altogether with the pattern of rGO because the conducting MEHPPV and P3HT polymers do not exactly show any significant peak.

Fig. 3 XRD spectra: (a) GO (b) rGO (c) MEHPPV–rGO (d) P3HT–rGO. 3.2. FE-SEM analysis The morphology and structure of GO and rGO were recorded using FE-SEM as shown in Fig. 4. The sheet-like structures confirm the formation of GO and rGO. The FE-SEM images of GO nanosheets is depicted in Fig. 4a that shows the flaky texture of the layered microstructure of GO which reveals the crumpled and rippled structure of GO. However, the rGO nanosheets in Fig. 4b shows wrinkled morphology. It represents that the rGO nanosheets are layer structured with irregular folding.

3.3. Raman spectroscopy

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Fig. 4 FE-SEM images: (a) GO (b) rGO.

3.4. TGA analysis

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Further Raman spectroscopy was used to identify the significant structural changes in rGO derived from GO. It is considered as a robust technique to characterise the carbonaceous materials. Moreover, the spectroscopy helps in differentiating the ordered and disordered crystal structures of carbon. Specifically for graphene derivatives, the Raman spectrum shows two bands namely D-band and G-band [29]. Usually, the G-band is assigned to the E2g phonon of C sp2 atoms, whereas the D-band is a breathing mode of k-point phonons of A1g symmetry [30]. The Raman spectra of GO and rGO is depicted in Fig. 5a and 5b which represents the existence of both the bands. The G-band appeared at 1582 cm-1 for GO, while 1585 cm-1 for the rGO. The G-band value of rGO equal to the value of pristine graphite which indicates the reduction of GO. Concurrently, the allocation of D-band for the GO and rGO was observed at 1367 cm-1 and 1356 cm-1 respectively, which corresponds to the defects associated with the in-plane sp2 domain [31]. Additionally, the increased D/G ratio of rGO (1.3) as compared to GO (1) also suggests the contraction in the average size of sp 2 domains during the reduction of GO to rGO [32].

Fig. 5 Raman spectra of GO and rGO.

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Figure 6 depicts the TGA curve for GO and rGO sample. The comparative weight loss in the materials exhibits the effective reduction of oxygen species in the sample because higher oxygen groups lead to the greater weight loss [33]. It is clear from the TGA plot that GO has the highest weight lost while rGO has much lower weight loss which indicates the reduction of GO has taken place.

Fig. 6 TGA curves of GO and rGO.

3.5. UV-visible absorption studies

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Figure 7a, 7b, and 7c showing the absorption spectra of GO and rGO; pristine MEHPPV and MEHPPV–rGO nanocomposite; pristine P3HT and P3HT–rGO nanocomposite. The spectra corresponding to GO and rGO conforms to that available in literature elsewhere [34]. It is also analysed that the peak intensity of absorbance gets reduced in case of rGO and no specific peak was observed. However, a small hump around 308 nm was observed which might have appeared due to the incomplete reduction of rGO and corresponds to n-p*of C=O bond. Whereas the spectrum of pristine MEHPPV and P3HT indicates absorption peak around 535 nm and 450 nm, respectively that can be attributed to π-π* transition that very well agrees with the previously reported literature [27, 35]. While the absorption spectrum of MEHPPV–rGO and P3HT–rGO nanocomposite gets broadens, that shows the contribution of rGO in the polymer matrix. The broadening in absorption spectrum might have occurred due to the effective reduction in the conjugate lengths of polymeric components and delocalisation of exciton wave function to a greater extent [36]. The broadening of the pulse also indicates the distribution of energy along the polymer matrix chain due to the amorphous nature of polymers and suggests that the energy gap will not remain constant along the chain. In addition to it, there is a slight shift in peak positions indicates a favourable condition for solar cell applications. The absorption studies signify that the composites formed with rGO provide enhanced spectrum utilisation which further helps in improving charge transport properties. Hence, the electronic interactions among the donor (polymer): acceptor (rGO) interfaces.

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Fig. 7 UV absorption spectra: (a) GO and rGO (b) Pristine MEHPPV and MEHPPV–rGO nanocomposite (c) Pristine P3HT and P3HT– rGO nanocomposite.

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3.6. Photoluminescence studies

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PL spectra of MEHPPV–rGO and P3HT–rGO nanocomposites are shown in Fig. 8a and 8b which indicates a significant decrease in the intensity on the addition of rGO in the conducting polymer matrixes. It is noteworthy that for very small weight ratios of rGO (typically less than 0.01x) the peak intensities of nanocomposites enhanced rather than getting quenched. The dispersions achieved at lower weight ratios of rGO could have resulted in more scattering towards the emitted light, which in turn enhances the peak intensity [37]. While on the addition of higher values of rGO in the matrixes of polymer enhances the charge transfer properties. Therefore, it is worth mentioning that higher the quenching more will be the charge transfer among the nanocomposite pairs [38]. During PL measurement, the excitation wavelength (Excit) was set up as 500 nm for MEHPPV–rGO nanocomposite and 450 nm for P3HT–rGO nanocomposite while the corresponding emission wavelength profiles (Emiss) achieved for MEHPPV–rGO was at 590 nm and 585 nm for P3HT–rGO nanocomposite.

Fig. 8 PL spectra: (a) MEHPPV–rGO nanocomposite (b) P3HT–rGO nanocomposite. Qualitatively, the PL quenching rates (i.e., PL Quenching Rate= ΔPL/PLInitial; where ΔPL = PLInitial – PLFinal in case of MEHPPV–rGO and P3HT–rGO was computed as 0.699 and 0.748 respectively. Based on PL quenching results, the enhanced charge transfer was achieved in P3HT–rGO nanocomposite as compared to MEHPPV–rGO. Moreover, a red shift was observed in the emission spectra, as there was a difference in the peak positions of absorption and fluorescence spectra in both nanocomposite pairs (i.e., MEHPPV–rGO and P3HT–

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rGO). This could have happened due to the molecular collision and solvent relaxation that results in energy loss and thus contributing to the red-shift. Additionally, the mechanism behind the quenching behaviour was further investigated using the Stern-Volmer (S-V) theory. Generally, the S-V phenomenon states the relation between fluorescence intensity ratio of the fluorophore in the absence and presence of quencher (i.e. IO/IX) to quencher concentration (Q). It can be represented as IO/IX=1+KSV[Q]; where KSV is known as quenching constant. The S-V plot for MEHPPV–rGO and P3HT–rGO nanocomposites is shown in Fig. 9a. It can be easily interpreted from the S-V plot that for the initial values of rGO in both the cases a linear quenching process occurred that is due to the presence of a particular quencher responsible for quenching. However, on the addition of larger weight percentage of rGO in the matrixes of MEHPPV and P3HT polymers, an exponential rise in quenching was observed. It means that some other species of quencher are more effective in the system which works more effectively than the rGO quencher alone, which is still ambiguous. As a matter of the fact that the addition of quencher is for rGO only and might be the new rGO entries leads into the formation of overlay structures of rGO because of attractive and prominent van der Waals forces which help in holding rGO nanosheets together [39]. Hence, the presence of multilayer rGO helps in enhancing the ability of quenching. The values for KSV were calculated as 2.41x103 L-1 for MEHPPV–rGO and 2.72x103 L-1 for P3HT– rGO nanocomposite. The result for KSV reveals that composites prepared with P3HT show better charge transfer capabilities compared to MEHPPV. Subsequently, the modified S-V plot shown in Fig. 9b analysed for calculating the maximum number assessable fluorophore (Fa) in both nanocomposite pairs. The Fa was computed as ~ 0.24 for MEHPPV–rGO and ~ 0.39 for P3HT–rGO nanocomposites. Therefore, P3HT–rGO based nanocomposites are more suitable in contrast to MEHPPV–rGO nanocomposites.

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Fig. 9 (a) Stern-Volmer plot and (b) Modified Stern-Volmer plot of MEHPPV–rGO and P3HT–rGO nanocomposites.

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Moreover, the charge transfer occurrence in the nanocomposite pairs can be verified by eliminating the condition of FRET (fluorescence resonance energy transfer). According to FRET, the energy transfer takes places between the donor species (usually a polymer or dye) and acceptor species (can be organic or inorganic) instead of charge transfer. Additionally, in FRET situation the primary factor that brings the subsequent transfer of energy from absorbing chromophore, i.e. (donor) to acceptor chromophore was due to the resonance interactions among the donor/acceptor pair without having any molecular collision. Secondly, there must be proximity among the donor-acceptor species, as well as the absorption spectrum of acceptor material, must overlap the emission spectrum of donor material [40]. From the above statement, it can be interpreted that FRET is eliminated in both the cases. It means that when rGO was intercalated in the polymer matrixes of MEHPPV and P3HT, then there was no overlapping between the emission spectra of conducting polymer and absorption spectrums of acceptor material. The outmoded FRET condition is depicted in Fig. 10 that represents the fluorescence emission spectra of MEHPPV and P3HT whereas the inset represents the absorption spectrum of acceptor rGO. Hence, the efficient charge transfer takes place instead of energy transfer which is a prerequisite condition for photovoltaic devices.

Fig. 10 Emission spectrum of the donor (Pristine MEHPPV and Pristine P3HT) and the inset shows the absorbance spectrum of the acceptor (rGO) which shows the obsolete FRET condition.

3.7. Contact angle measurements

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It is of great significance to consider the contact angle measurement studies of a thin film to estimate the surface energy and interfacial interactions with the surfaces composed of two or more materials. Moreover, the adhesion between the rGO and conducting polymer is vital for OPV device fabrication which in turn depends on the surface energy [41]. The surface wettability studies of MEHPPV–rGO and P3HT–rGO nanocomposite thin films were conducted to evaluate their surface energy for efficient OPV device fabrication.

Fig. 11 Contact angle measurement: (a) MEHPPV–rGO nanocomposites (b) P3HT–rGO nanocomposites.

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Left Right Angle Angle (L.A) (R.A) Water MEHPPV–rGO 91.2 91.2 Water MEHPPV–rGO 91.9 91.2 Water MEHPPV–rGO 92.5 92.5 Averaged Contact Angle for MEHPPV–rGO

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Figure 11a and 11b show the images captured during the contact angle measurement for MEHPPV–rGO and P3HT–rGO nanocomposites, respectively. De-ionized (DI) water was used for calculating the contact angles with both the surfaces. It is noteworthy that if the wettability of a thin film is higher, the surface tension will be small and vice versa. Therefore, the contact angle for wetting surfaces needs to be smaller than 90, and considered as hydrophilic; whereas for non-wetting surfaces the contact angle would be greater than 90, and considered as hydrophobic [42]. The averaged contact angle for MEHPPV–rGO and P3HT–rGO film was found as 92.08 and 101.07 respectively which state the hydrophobic nature of the surfaces. However, a higher value of contact angle in case of P3HT–rGO represents an improved interaction between the molecules as compared to MEHPPV–rGO surface. This can be attributed to stronger cohesive forces as compared to adhesive forces between the molecules. Additionally, the reversible work of adhesion (WA) was computed as 64.94 and 57.79 for MEHPPV–rGO and P3HT–rGO nanocomposites respectively. The higher values indicate that the films prepared from MEHPPV–rGO nanocomposite are more porous than P3HT–rGO, which suggests the better integration of rGO nanosheets in the matrixes of the P3HT polymer as compared to MEHPPV polymer. Hence, for OPV devices the composites of P3HT– rGO would be more desirable in active layer as compared to MEHPPV–rGO. Table 1 and 2 represents the contact angle measurement whereas Table 3, and 4 represents the measurement of WA and surface energy for MEHPPV–rGO and P3HT–rGO nanocomposite films respectively. Table 1. Contact angle measurement for MEHPPV–rGO nanocomposite film Angle Average (A.A) 92.20 91.55 92.50 92.08

Width (W)

Height (H)

Vol (V)

Area (A)

1.6 1.5 1.5

0.8 0.8 0.8

1.03 1.02 1.02

3.9 3.9 3.9

Volume / Area (V/A) 0.26 0.26 0.26

Table 2. Contact angle measurement for P3HT–rGO nanocomposite film

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Left Right Angle Angle (L.A) (R.A) 1. Water P3HT–rGO 103.7 103.0 2. Water P3HT–rGO 103.2 103.2 3. Water P3HT–rGO 102.6 102.6 Measured Averaged Contact Angle for P3HT–rGO

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Angle Average (A.A) 103.35 103.20 102.60 101.7

Width (W)

Height (H)

Vol (V)

Area (A)

1.4 1.4 1.4

0.9 0.9 0.9

1.09 1.09 1.07

4.1 4.1 4.1

Volume /Area (V/A) 0.25 0.25 0.24

Table 3. Surface energy measurement for MEHPPV–rGO nanocomposite film Sr. No. 1. 2. 3.

Left Right Angle Angle Angle Average (L.A) (R.A) (A.A) Water MEHPPV–rGO 93.49 92.7 93.1 Water MEHPPV–rGO 92.0 92.0 92.0 Water MEHPPV–rGO 92.1 91.3 91.7 Averaged WA & Surface Energy for MEHPPV–rGO

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Cos -0.053 -0.035 -0.030

WA = yL. (1+Cos)

S = yL. (Cos-1)

Temp (C)

H (%)

68.93 70.26 70.64 64.94

-76.67 -75.34 -74.96 -80.66

26.7 26.7 26.7

31.0 31.0 31.0

Table 4. Surface energy measurement for P3HT–rGO nanocomposite film

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Left Right Angle Angle Angle Average (L.A) (R.A) (A.A) Water P3HT–rGO 103.7 103.7 103.7 Water P3HT–rGO 104.4 103.7 104.1 Water P3HT–rGO 102.2 101.5 101.9 Averaged WA & & Surface Energy for P3HT–rGO

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Cos

WA = yL. (1+Cos)

S = yL. (Cos-1)

Temp (C)

H (%)

55.56 55.13 57.85 57.79

-90.04 -90.47 -87.75 -87.81

26.7 26.7 26.7

31.0 31.0 31.0

-0.237 -0.243 -0.205

3.8. Device fabrication and I-V measurements

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The OPV devices were fabricated on the basis of obtained charge transfer properties in the rGO and polymer nanocomposites via photoluminescence quenching phenomenon. Therefore, MEHPPV–rGO and P3HT–rGO nanocomposites have been used as an active layer material for the fabrication of the BHJ device structures. The proposed architecture of the device consists of ITO/PEDOT: PSS/Active layer (MEHPPV–rGO or P3HT–rGO)/Al structure. The device fabrication involves a rigorous cleaning process of ITOcoated glass samples because ITO coated glass works as a substrate as well as a back contact electrode in the device structure. Therefore, the substrates were cleaned first with acetone followed by an IPA solution, and finally, plasma ashing was performed for 10 min. In the next device fabrication step, a thin film of PEDOT: PSS was deposited to generate an HTL layer which acts as a buffer layer between the active layer and ITO substrate. The film deposition process was carried out under N 2 atmosphere using a spin coating unit at a speed of 2,500 rpm at 800 rotations s-1 for 60 s and annealed at 250 C for 25 min. Subsequently, the processed samples were transferred to the glove box without exposing to the external environment for a longer time. Later, the active layer deposition with both the synthesised nanocomposites MEHPPV–rGO and P3HT–rGO was executed by using another spin coating unit of the glove box. The coater speed was adjusted to 1200 rpm for 60 s, and then annealing was performed at 120 C for 20 min to evaporate the excess solvent. It is worth mentioning that the annealing of thin film solar cells helps in improving the charge transfer process that further improves the net efficiency of OPV devices by optimising the donor-acceptor morphology [43]. Further, on the top of the active layer, Al deposition was conducted in to form metal contacts as a terminal electrode to the devices. A thermal evaporation system was utilised for depositing the Al at a vacuum level of approximately 5x10–6 mbar. An approximate thickness of 100 nm for Al was achieved when the deposition rate was 0.01 nm s−1 approximately. The deposition rates can be increased for conducting faster depositions of metal contacts; however, slower rates are desirable because the rates are directly proportional to the grain size of Al on the film surfaces [44]. However, in OPV devices the collection of charge at either terminal depends on the grain size of deposited materials on the substrate. The last step of OPV fabrication includes the protection of device against moisture and other environmental containments to further enhance the life as well as the performance of the device. Hence, the devices were encapsulated using an epoxy-type sealant and cleaned glass plates to cover the developed surfaces on ITO substrates. The original image of fabricated devices with MEHPPV–rGO and P3HT–rGO nanocomposites after the contact formation and encapsulation are depicted in Fig. 12.

Fig. 12 Digital image of fabricated OPV devices: (a) MEHPPV–rGO nanocomposite (b) P3HT–rGO nanocomposite.

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For I-V characteristics, a solar simulator was utilised for measuring the performance of fabricated devices on the basis of essential parameters such as open circuit voltage (V OC), short circuit current (ISC), and fill factor (FF). Based on these parametric values the overall efficiency of devices was calculated and compared [45]. The computed values for the devices fabricated from MEHPPV–rGO and P3HT–rGO nanocomposites are also tabulated in Table 5. Figure 13 elaborates the I-V curves for both the cases. The result reveals that the polymer-based devices fabricated from P3HT–rGO nanocomposite showed better efficiency as compared to those fabricated using MEHPPV–rGO. Table 5. I-V parameters of fabricated OPV devices from MEHPPV–rGO and P3HT–rGO nanocomposites as an active layer material Sr. No. 1 2

Active Layer Composition MEHPPV–rGO P3HT–rGO

ISC (mA) 0.016 0.021

JSC (mA cm-2) 0.457 0.605

VOC (V) 0.50 0.57

Fill Factor 0.31 0.38

ƞ (%) 0.024 0.045

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Fig. 13 I-V curve for MEHPPV–rGO and P3HT–rGO nanocomposite based OPV device.

4. Conclusion

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The present work extends the scope of rGO in the active layer of OPV devices when incorporated with readily available conducting polymers MEHPPV and P3HT. The rGO nanocomposites were prepared using simple ultrasonication technique after its successful reduction from GO. The synthesised rGO was characterised using various techniques to confirm the reduction of GO. However, the charge transport properties were investigated in MEHPPV–rGO and P3HT–rGO nanocomposites using absorption and fluorescence spectroscopy. It has been observed that in both the cases quenching occurs on the addition of rGO and improved charge transfer properties were obtained. From the results of fluorescence spectra, it is found that in both cases the quenching occurs due to the involvement of more than one species in the quenching process. Additionally, the electron transfer was obtained in both the cases and energy transfer was eliminated as a consequence of non-overlap situation between the fluorescence spectra of donor polymer and absorption spectra of acceptor rGO. However, P3HT–rGO nanocomposites are found more suitable for organic solar cell applications instead of MEHPPV–rGO because of the higher quenching rate and improved KSV values. Furthermore, the contact angle measurement study elaborates the better integration of rGO nanosheets in the matrixes of the P3HT polymer as compared to MEHPPV polymer for solar cell device fabrication. The achieved efficiency mark of 0.024% in case of MEHPPV–rGO nanocomposite and 0.045% in case of P3HT–rGO nanocomposites further signify that the blends of rGO integrated with P3HT shows better results in contrast to MEHPPV. Overall, this study provides a thorough investigation of the simultaneous and corresponding integration of rGO in the matrixes of two different conducting MEHPPV and P3HT polymers for achieving efficient energy conversion in OPV devices.

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The authors gratefully acknowledge the support from the Indian Nanoelectronics Users Programme (INUP), IIT Bombay which is duly funded by DeitY, MCIT, and Government of India. The INUP provides all the facilities required for synthesising, fabricating, and characterising the OPV devices at CEN laboratories (Centre of Excellence in Nanoelectronics) of IIT Bombay. The fluorescence-based studies were carried out at IIT-ISM Dhanbad under the DST-FIST project and acknowledge their kind support during the analysis.

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