Functionalized 2D-MoS2-Incorporated Polymer Ternary Solar Cells

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Functionalized 2D-MoS2 Incorporated Polymer Ternary Solar Cells: Role of Nanosheets Induced Long range Ordering of Polymer Chains on Charge Transport Razi Ahmad, Ritu Srivastava, Sushma Yadav, Suresh Chand, and Sameer Sapra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08725 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 5, 2017

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ACS Applied Materials & Interfaces

Functionalized 2D-MoS2 Incorporated Polymer Ternary Solar Cells: Role of Nanosheets Induced Long range Ordering of Polymer Chains on Charge Transport Razi Ahmada,b , Ritu Srivastavaa*, Sushma Yadavb, Suresh Chanda, and Sameer Saprab a

Center for Organic Electronics, Physics of Energy Harvesting Division, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi-110012, India b

Department of Chemistry, Indian Institute of Technology Delhi, New Delhi, India-110016

Abstract In this paper, we demonstrated the enhancement in power conversion efficency (PCE) of solar cells based on Poly(3-hexylthiophene-2,5-diyl) (P3HT): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) by incorporation of functionalized 2D-MoS2 nanosheet (NS) as an additional charge transporting material. The enhancement in PCE of ternary solar cells arises due to the synergic enhancement in exciton dissociation and improvement in mobility of both electrons and holes, through the active layer of the solar cells. The improved hole mobility is attributed to the formation of long range ordered nano fibrillar structure of polymer phases and improved crystallinity in the presence of 2D-MoS2 NS. The improved electron mobility arises due to the highly conducting 2D network of MoS2 NS which provides additional electron transport channels within the active layer. The nanosheets incorporated ternary blend solar cells exhibit 32 % enhancement in PCE relative to the binary blend P3HT:PC71BM. Keywords: polymer ternary solar cells, MoS2 nanosheets, AFM, nanofibrillar, GIXRD, charge transport

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction Organic solar cells (OSCs) based on blends of light harvesting component of conjugated polymer and fullerene derivative as an electron acceptor have received enormous attention due to their ease of fabrication by low-cost solution processability on large area of flexible substrates.1–4 OSCs are considered as a promising candidate to fulfill the clean and low-cost energy demands. The bulk heterojunction (BHJ) configuration of OSCs provides an enormous donor-acceptor interface for efficient exciton dissociation to overcome the short exciton diffusion length of organic materials.3,5 Over the past decade, tremendous efforts have been made to achieve highly efficient BHJ solar cells by combination of several ways for example, introduction of new low band gap polymers with extended light absorption to the longer wavelengths,6–8 optimized nanomorphology,9–11 interface engineering,12–14 and advances in the new device structures15 such as tandem geometry. Despite the significant advancement in the fabrication processes and materials, the photovoltaic performance is still limited by insufficient light absorption and poor charge transport through the active layer.16–18 To address these serious issues a straight forward approach of the ternary blend solar cells has emerged as an attractive strategy where the third component either act as a complementary light absorber or additional transporting channel. Recently, an enhancement in the power conversion efficiency of the ternary solar cells has been achieved in comparison to binary blend solar cells. It is due to enhanced spectral response by incorporation of the third component such as low band gap polymers,19–22 small molecules,23,24 quantum dots,25–27 and plasmonic metal nanoparticles17,28 via surface plasmon resonance effect. The enhancement in carrier transport through the active layer have been mainly achieved by introduction of additional charge transporting materials such as carbon nanotubes (CNTs),29,30 and graphene.31–33 The 1D nanostructure of CNTs provide large surface area for exciton dissociation and exhibit superior charge carrier mobility to transfer the 2 ACS Paragon Plus Environment

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photogenerated charge carriers towards the electrodes. However, the use of CNTs is problematic due to lack of bandgap along with poor solubility in the active layer leading to agglomeration and bundling which act as recombination center for charge carriers and decrease the device performance significantly.34,35 Therefore, due metallic nature of CNTs, the highly conductive carbon nanotube films are being identified as potential candidates for transparrent electrodes in large area flexible organic electronic devices.36 Two-dimensional (2D) networks of graphene have received significant attention due to excellent charge carrier mobility,37 and solution phase synthesis which is compatible with organic solvents.38 The planar geometry of 2D graphene sheets provides a large surface area in comparison to CNTs for efficient exciton dissociation. The graphene nanosheets owing highly conductive 2D networks, transport charge carriers more efficiently than CNTs.31 In this context, graphene can be considered as a promising material to facilitate the efficient exciton dissociation, and charge transport inside the active layer of the solar cells. The use of graphene nanosheets into the active layer as a ternary component in the OSCs was first reported by Jun et al.31 The author demonstrated enhanced performance of P3HT:PCBM solar cells by incorporating nitrogen doped reduced graphene oxide (rGO) into the active layer. The modification in energy level position of rGO by nitrogen doping facilitates selective charge transport through the active layer. Later Robaeys et al32 demonstrated enhanced performance of P3HT:PCBM based solar cells by incorporating solution-processed graphene flakes into the active layer as a ternary component. The addition of graphene flakes improves the crystallinity, and morphology of polymer leading to enhancement in charge carrier mobility. Moreover, graphene acts as a bridge to connect the phase separated donor and acceptor domains which provide interconnected percolation networks for efficient charge transport in the active layer via graphene networks. However, due to zero bandgap of graphene, the light harvesting contribution is negligible and it acts as recombination center 3 ACS Paragon Plus Environment


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for charge carriers since the energy offset is favourable only for electron transport.33 In this context, the finite band gap with favourable energy band offset along with high charge carrier mobility is highly desirable for additional charge transporting material in ternary solar cells. Unlike the zero band gap of graphene, the 2D transition metal chalcogenides (TMDs) such as monolayer MoS2 have a direct band gap of 1.9 eV compared to the indirect bandgap of 1.2 eV in bulk state.39 The light harvesting capability, and high in-plane charge carrier mobility40 of around 200-500 cm V s make it a promising candidate to act as a complementary light absorber and additional charge transporting material for the efficient ternary solar cells. However, the utilization of MoS2 nanosheets (NS) as a charge transport material inside the active layer of OSCs is limited due to lack of solubility in organic solvents which is compatible with polymer-fullerene blends. Recently, our group reported41 functionalized MoS2 NS of one or few monolayers which is able to form a stable dispersion, even of high concentration, in non-polar organic solvent such as dichlorobenzene. In this paper, we demonstrated an improvement in PCE of P3HT:PC71BM based BHJ-OSCs by incorporating dodecanethiol (DDT) functionalized MoS2 NS into the active layer. The addition of DDT-MoS2 NS into active layer allows polymer chain alignment into the long range ordered morphology leading to improved hole mobility due to crystallization of the polymer phase.The 2D networks of NS provide additional continuous percolation pathways between donor-acceptor interfaces for efficient exciton dissociation, and electron transport. A synergic enhancement in short-circuit current density, and fill factor of MoS2 incorporated solar cell led to an increase of about 32 % in PCE with 3.7 % compared to 2.8 % of the reference cell. These results open up a new direction towards optimization of high-efficiency ternary based OSCs.

Results and Discussion


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Synthesis and Characterization of DDT Functionalized MoS2 Nanosheets The DDT functionalized MoS2 NSs were synthesized by liquid phase exfoliation of MoS2 powder in DCB:DDT mixture (see experimental section for details). During sonication process, the thiol group of DDT ligands immediately get adsorbed at the sulfur vacancies on the surface and edges of MoS2 NSs. The hydrophobic alkyl chains exposed in DCB, facilitate stable dispersion by preventing from restacking of exfoliated nanosheets. The exfoliated NSs were purified by repeated cycle of precipitation and dispersion. The purified solid product was dispersed in DCB (shown in the inset of Figure 1a) for further uses. Figure 1a shows a transmission electron microscopy image of DDT-MoS2 NS deposited on the holey carbon grid. The TEM image revealed that the exfoliated material exists as thin sheets with a lateral size of 200 nm. The appearance of the transparent surface is clearly visible suggesting mono or few layer NSs are present in the dispersion. The high magnification TEM image is shown in Figure 1b displays lattice fringes of highly crystalline MoS2 NS. Figure 1c shows highresolution TEM image of MoS2 NSs in which regular atomic arrangements with hexagonal closed pack symmetry throughout the sheet are clearly illustrated being consistent with the 2H phase of MoS2.42 In the atomic resolution image, the lattice spacing of 2.7 Å is assigned to (100) planes of 2H phase MoS2 which is consistent with previous reports.43,44 The selected area electron diffraction pattern (Figure 1d) reveals the regular hexagonal arrangement of the spots which confirms the single crystalline nature of exfoliated NSs. The absorption spectrum of MoS2 NSs is shown in Figure 2. The four characteristic peaks in the spectrum located at 665 nm, 606 nm, 450 nm, and 390 nm can be assigned to A, B, C, and D transitions, respectively.42 The sharp excitonic doublet peak at 665 nm, and 606 nm are attributed from the direct interband transition at K point in the Brillouin zone. The peaks at 450 nm, and 390 nm are assigned to the direct transition from the deep valence band to the conduction band.45



Figure 1: Typical (a) low magnification (b) high magnification TEM images of DDT-MoS2 NS. (c) high-resolution TEM image of single layer nanosheet and (d) corresponding selected area electron diffraction pattern.

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The influence of incorporation of 2D-MoS2 NSs on the optical properties of P3HT was investigated by UV-Vis absorption spectroscopy. The absorption spectra of pristine P3HT and hybrid P3HT:MoS2 films with varying concentrations of NSs are presented in Figure 3a. The absorption spectra display several characteristic features of the pristine P3HT film, the two distinct features at 510 nm, and 550 nm with a shoulder at 600 nm assigned as the (0-2), (0-1), and (0-0) absorption band, respectively.46 The weak feature centered at 550 nm arise due to the absorption from extended conjugation of P3HT in solid state47 and the shoulder at 600 nm is attributed to the interchain vibrational absorption48 of ordered P3HT chains. In comparison to the pristine P3HT, the P3HT:MoS2 hybrid display enhancement in absorption intensity with increasing concentration of NSs. Moreover, upon NSs addition the vibronic shoulder becomes sharper and more pronounced at the higher contents of NSs. The subsequent appearance of the sharp vibronic band (0-0) at 600 nm, which is a good sign of ordered packing of the polymer chains, show that the crystallinity of P3HT in hybrid film has been substantially increased. The crystallinity of the P3HT film is directly correlated with the ordered packing of the polymer chains and consequently, related to the intensity of the vibronic shoulder at 600 nm. Thus the absorption spectra suggest that the incorporation of NSs has led to a significant increase in the absorption intensity with more pronounced vibronic peak, resulting from the increased ordering of polymer chains49,50 which further enhances the absorption coefficient of hybrid films. Furthermore, we use normalized absorption spectra of P3HT and hybrid films to compare the aggregation state of P3HT quantitatively, in the presence of NSs. According to the Spanos model, the intensity ratio of 0-0 and 0-1 vibronic peaks are related to the free exciton bandwidth (W) of the aggregates by following expression.51,52

=

./

./



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where A0-0 and A0-1 are the intensities of (0-0), and (0-1) transitions, respectively, is the vibrational energy of symmetric vinyl stretch (taken as 0.178 eV).46 The exciton band width is related to the conjugation length and interchain ordering. A decrease in W indicates increase in the conjugation length and ordering. The variation of free exciton bandwidth for P3HT as a function of increasing concentration of NSs are shown in Figure 4 (black curve). From the Figure 4 it can be seen that the exciton band width gradually decreases with increasing concentration of NSs. In particular, W value is decreased from 129 meV for pristine P3HT film to 75 meV for P3HT:MoS2 hybrid film containing 5 wt % of NSs. The significant reduction in exciton bandwidth indicates the increased conjugation length and ordering of polymer chains in the presence of NSs. Furthermore, we have compared the NSs induced ordering of P3HT component in P3HT:PC71BM film. The UV-Vis absorption spectra of NSs incorporated P3HT:PC71BM film are shown in Figure 3c. As compared to the binary (P3HT:MoS2) film, the ternary (P3HT:PC71BM:MoS2) films show similar trend of enhancement in absorption intensity with increasing concentration of NSs. The variation of free exciton bandwidth for P3HT component in ternary film with increasing concentration of NSs are shown in Figure 4 (triangles). In the absence of NSs, the exciton bandwidth for P3HT component in P3HT:PC71BM film is higher than exciton bandwidth in pristine P3HT film, consistent with the previous report53 suggesting that the exciton bandwidth in the blend film never achieves same value that is reached without PC71BM. However, after the addition of increasing concentration of NSs, ternary blend show similar trend of decrease in exciton bandwidth as in binary blend, suggesting that the NSs induced ordering of P3HT chains is not effected by PC71BM.


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Figure 4: The variations of free exciton band width (W) for P3HT (spheres) and P3HT:PC71BM (triangles) films with increasing concentration of NSs. Ternary Solar Cells



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The impact of incorporation of 2D-MoS2 NSs on the photovoltaic performance of P3HT: PC71BM based devices with device structure ITO/PEDOT: PSS/P3HT: PC71BM: MoS2/Ca/Al was measured by current density-voltage (J-V) characteristics. The schematic of device structure and energy level diagram of each component used for the fabrication of devices are shown in Figure 5a, and 5b, respectively. The J-V characteristics of pristine P3HT and MoS2 incorporated P3HT:PCBM based ternary solar cells with varying concentrations of MoS2 NSs under AM 1.5 G conditions with an illumination intensity of 100 mW/cm2 are displayed in Figure 5c. The detailed electrical parameters of the fabricated photovoltaic devices are presented in Table 1. The values of all the parameters listed in Table 1 show an average of all parameters over five identical devices at each MoS2 concentration. The highest performing photovoltaic device based on pristine P3HT: PC71BM exhibits a short-circuit current density (

) of 7.97 mA/cm2, an open circuit voltage ( ) of 0.63 V, and a fill factor (!! ) of 55.1 % which results in a power conversion efficiency (PCE) of 2.8 %. After incorporation of varying amount of 2D-MoS2 NSs, the highest performing ternary solar cell based on P3HT:PC71BM:MoS2 show a gradual increase in both , and !! eventually improving

PCE. The best performing device with 3 wt % MoS2 NSs exhibit the highest value of , and FF at 9.5 mA/cm2, and 61.1 %, respectively with

of 0.63 V results in a very promising

PCE of 3.7 %. The PCE of the highest performing device based on ternary blends comprising P3HT:PC71BM and 3 wt % MoS2 NSs is improved by 32 % as compared to the highest performing pristine device based on binary P3HT: PC71BM blends which is mainly attributed to the synergic enhancement in and FF. It is well established that the in the polymer based solar cell is correlated to the light absorption capacity, and optimized morphology of active layer; the efficiency of exciton dissociation, and charge transport through the active layer of the device. After incorporation of 2D-MoS2 NSs into the active layer, the light absorption capacity of the composite films significantly improved compared to the pristine


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device which could promote exciton generation rate. The presence of 2D-MoS2 NS in the ternary blend provides additional P3HT:MoS2 interface, thus a large interfacial area for charge separation which accelerates the rate of exciton dissociation. Moreover, the highly conducting 2D network of MoS2 NS offers new interconnected percolation networks for charge carrier transport, and collection which could lead to improved electron mobility resulting in larger . (a)

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Figure 5: (a) Schematic of the device structure (b) energy level diagram of each component of the ternary solar cell (c) current density-voltage (J-V) characteristics of the ternary solar cell with different concentrations of nanosheets under AM 1.5 G condition with an illumination intensity of 100 mW/cm2. Table 1: Photovoltaic parameters of the fabricated devices based on P3HT:PC71BM binary blend and P3HT:PC71BM:MoS2 ternary blends with different concentrations of MoS2 NS. The values given in the table correspond to the mean and standard deviation of five devices of each kind. The PCE values in parentheses represents the PCE of highest performing device. The active area of the devices was 0.06 cm2.

In order to understand the effects of MoS2 incorporation on the exciton generation and dissociation processes in the P3HT: PC71BM based devices, we have calculated maximum exciton generation rate #$%&' (, and exciton dissociation probability )#, *(. The photocurrent density #+ ( versus effective voltage #,-- ( for pristine P3HT: PC71BM and ternary P3HT:PC71BM:MoS2 based devices are shown in Figure 6. + is calculated as, + = . / 0 where . , and 0 are current densities under illumination, and under dark, respectively. ,-- = / , where is the compensation voltage at which + = 0 and is the applied voltage.18 Figure 6 shows the linear increase in + with increasing ,-- upto 0.1 V and the + attain saturation values at the higher ,-- . Compared to the pristine device, the saturation current density significantly increases from 86.7 A/cm2 to 103.6 A/cm2 for the


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MoS2 incorporated device. The maximum exciton generation rate can be calculated as + = 2$%&' 3, where 2 is the electronic charge, and 3 is the thickness (120 nm ) of the active layer. The calculated value of $%&' increases from 4.15 × 10 27 m s for pristine device to 4.94 × 10 27 m s for MoS2 incorporated device. The enhancement in $%&' is attributed to the increased light absorption by MoS2 incorporation which is well matched with UV-Vis absorption spectra. Further, the exciton dissociation probability )#, *( which depends upon the electric field, and temperature are given by power law equation + = 456 where 7 is the recombination parameter. The )#, *( could be obtained by the ratio of + ⁄8&9 , under 8; condition.18 The )#, *( is increased from 82.9 % for the pristine device to 91.6 % for the MoS2 incorporated ternary device. The enhancement in )#, *( is attributed to the decrease in recombination rate which supports the improved fill factor of devices with MoS2 NS.

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Figure 6: The photocurrent density #J=> ( versus effective voltage #V?@@ ( curve of pristine and MoS2 incorporated ternary solar cell. Electrons and Holes Mobility Measurements To gain deeper insights of the influence of MoS2 incorporation on the charge carrier transport inside the active layer, we have fabricated electron and hole only devices in order to 13 ACS Paragon Plus Environment


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characterize the electron and hole mobility. The energy levels of PC71BM based electron-only devices with device structure ITO/Cs2CO3/PC71BM:MoS2/Al are displayed in Figure 7d. The ITO/Cs2CO3 bilayer electrode acts as a hole blocking layer and can inhibit30 injection of holes into the PC71BM layer. The energy level diagram of P3HT:MoS2 based hole-only devices with device structure (ITO/PEDOT:PSS/P3HT:MoS2/Au) is shown in Figure 7c. According to the energy level diagram, electron injection from Au electrode is restricted and only holes can be injected from ITO/PEDOT:PSS bilayer electrode into the active layer. The J-V characteristics of hole-only, and electron-only devices are presented in Figure 7a, and 7b, respectively, which display improved electron and hole transport with increasing concentrations of MoS2 NS into the respective active layers. The charge carrier mobility can be calculated from space charge limited current model (SCLC) by using Mott-Gurney equation given below:54 9 . = CD C E 8 3 where CD is a relative dielectric constant of the organic material, C is permittivity of free space, E is charge carrier mobility, is applied voltage, and 3 is the thickness (120 nm) of the active layer. (a)

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Figure 7: J-V characteristics of (a) P3HT based hole-only devices, and (b) PC71BM based electron-only devices with different content of MoS2 NS. Energy level diagrams of (c) hole only, and (d) electron-only devices. The calculated electron, and hole mobilities of PC71BM, and P3HT based devices, respectively, with varying concentrations of MoS2 are presented in Table 2. The values of hole and electron mobilities listed in Table 2 show an average over five identical devices at each MoS2 concentration. From the table, we can see that both electron and hole mobilities of the respective devices gradually increase with the increase in the concentration of MoS2 NS 15 ACS Paragon Plus Environment


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into the respective active layer and attain a maximum value at the concentration of 3 wt % of MoS2 NS. In particular, for best performing device of each kind, the electron mobility is increased from 1.44 × 10 −3 cm V s for pristine PC71BM based device to 2.9 × 10 −3 cm V s for MoS2 (3 wt %) incorporated device. The enhancement in electron mobility is attributed to the presence of additional electron transport pathway due to highly conducting 2D networks of MoS2 NS. However, the electron mobility of hybrid device containing 5 wt % of MoS2 NS is lower than that with 3 wt % MoS2 NS. The decrease in electron mobility at the higher content of MoS2 (5 wt %) can be anticipated owing formation of additional NSrich domain which does not contribute efficiently to the transportation pathway of the charge carriers. These results are in agreement with the previous reports, suggesting that nanomaterial rich domain destruct the transportation pathway for charge carriers, and thus significantly deteriorate the device performance.26,55 Table 2: The calculated hole mobility of P3HT based hole-only devices and electron mobility of PC71BM based electron-only devices with varying content of MoS2 NSs. The mobilities values given in the table correspond to the mean and standard deviation of five devices of each kind. The values in parentheses represents the mobility of highest performing device of each kind.


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Figure 8: Tapping mode AFM height and phase images of (a, e) pristine P3HT (b, f) 1 wt %, (c, g) 3 wt % and (d, h) 5 wt % MoS2 NS. The corresponding orientational map analysis of (i) pristine P3HT (j) 1 wt % (k) 3 wt % and (l) 5 wt % MoS2 NS.



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Figure 9: Polar curve of orientation distribution extracted from the orientational map of (a) pristine P3HT (b) 1 wt % (c) 3 wt % and (d) 5 wt % MoS2 NS. (e) orientational order parameter (S2D) of pristine and hybrid films as a function of NS concentration. Similar to the improved electron mobility, the hole mobility of the highest performing pristine P3HT based device increased from 4.6 × 10 − 4 to 8.1 × 10 −4 cm V s for the device with 3 wt % MoS2 NS. The improved hole mobility in P3HT is directly associated with the crystallization of the polymer. The crystallization of conjugated polymers involves various processing conditions which result in the ordering of polymer chains leading to the formation of various ordered morphologies such as fibers,56 ribbons,57 and sheets.58 The assembly of polymer chains into the ordered structure is expected to have a dominating influence on the hole transport through the polymer domains. The crystallization of P3HT, and improved hole mobility upon graphene32 addition was also reported by Robaeys et al. Thus the improved hole mobility in the hybrid film most likely originates from the ordering of polymer chain, and improved crystallization of the hybrid films by 2D-MoS2 NS. To understand the effect of 2D-MoS2 NS incorporation on the morphology of P3HT film, we characterize the morphology of the pristine P3HT, and hybrid films by using atomic force microscopy (AFM). Figure 8 displays tapping mode AFM height, and phase images of pristine P3HT and P3HT:MoS2 hybrid films with varying concentrations of NS inside the P3HT matrix. In particular, the morphology of pure P3HT film is smooth and featureless and the absence of


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any ordered structure in the phase image clearly demonstrates the random orientations of P3HT chains. Interestingly, in the presence of 2D NSs (1 wt %) the morphology of blend film changes from the random orientation of polymer chains to highly ordered and small nanofibrillar structure. The effect is more pronounced at a concentration of 3 wt % MoS2 where long and aligned nanofibers of P3HT chains are clearly visible. The nano-fibrillar networks are characteristic of ordered packing of P3HT chains which could promote facile charge transport through the film. The self-assembly of P3HT chains, either in solution or thin films, into nanorods or nanofiber structure can occur under various circumstances, including ultrasonication induced ordering,56 the addition of small amounts of poor solvents,59 thermal annealing,5 and hydrophobic interactions between side chains of polymer and hydrophobic surface due to surface treatment effects.60 In general the P3HT chains self-assemble into nanorods or nanofibers preferably with edge-on orientation via

π −π

stacking.57 The self-

assembly of polymer chains into nanorods and fibers by hydrophobic interactions between polymer side groups, and hydrophobic surfaces are well established.61–63The difference in nano-morphology of the pristine, and hybrid film is attributed to the presence of functionalized 2D-MoS2 NS. On the basis of the previous reports61,63 we believe that the selfassembly of P3HT chains into isolated 1D fibres are induced by functionalized MoS2 NSs. More precisely, the edge-on orientation assembly of P3HT chains are triggered by hydrophobic interactions between hexyl side chains of P3HT, and long alkyl chains of DDT present at the surface of MoS2 NSs. The large surface area of 2D NSs acts as nucleation centre and strong hydrophobic interaction provide driving force for self-assembly of P3HT chains into nanofiber structure. The presence of optimum amount of NSs is enough to accelerate the nucleation and growth of P3HT chains into long and highly aligned interconnected network of P3HT nanofibers.



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AFM phase images shown in Figure 8e-h were further analysed by using an opensource image processing algorithm (GT Fiber software).52,64,65 The processed AFM phase images generated by GT Fiber software are shown in Figure 8i-l. According to the figure, the pristine film display random orientation of polymer chains which on increasing concentration of MoS2 NS get aligned as a long range nanofibrillar structure. The degree of alignment is further quantified by the orientational order parameter (F0 ), given by following equation64 F0 = 2HIJ KL / 1 where KL is the angle between fiber segment, and population director ( n⃗( which is the average orientation of the population. The F0 value lies between 0 to 1. For fully random orientation of fibers F0 = 0, whereas for totally aligned fibers F0 = 1. The fiber orientation distribution is expressed in polar coordinates shown in Figure 9a-d. The degree of alignment is correlated with F0 value which is extracted from orientation distribution curve. The variation of F0 with the concentration of MoS2 NS is shown in Figure 9e. It can be seen that the F0 value increases with increasing concentration of MoS2 NS and attains a maximum value of 0.54 at 3 wt % MoS2 NS suggesting a highly aligned, and long range nanofibrillar structure are obtained at this concentration. The high degree of ordering can be seen in the AFM images where long and interconnected networks of P3HT fibers appear in bundles of parallel orientation. The high degree of alignment in 3 wt % MoS2 NSs is attributed to the presence of long and interconnected network of P3HT fibers. It is reported46 that the long rod like object more easily align themselves in a dispersion and in thin film. Furthermore, the aggregates of interconnected fibers can act as physical bridge between neighbouring fibers which could increase the degree of alignment. Grazing Incident X-ray Diffraction Pattern of P3HT Films


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The effects of incorporation of 2D-MoS2 NS on the crystallization of P3HT film were examined by grazing incident X-ray diffraction (GIXRD) measurement. The diffraction patterns of pristine P3HT and P3HT:MoS2 hybrid films at different concentrations of NS are shown in Figure 10. The GIXRD pattern of pure P3HT film displays weak diffraction peak at diffraction angle 2K =5.3° which is assigned to (100) reflection, and associated with the lamellar arrangement of polymer chains oriented along the crystallographic direction perpendicular to the polymer backbone.56 In comparison to the pristine P3HT film, the intensity of (100) reflection of the hybrid film gradually increases with increasing concentration of NS. Along with reflection at (100) weak higher order reflections5 are also observed corresponding to (200), and (300) planes centered at diffraction angles of 10.7°, and 15.9° respectively in the hybrid films which are not detected in the pristine P3HT film. The enhancement in intensity of (100) reflection and presence of higher order reflections in the GIXRD patterns of hybrid films indicate that the ordering and crystallization of P3HT is more pronounced in the presence of MoS2 NS which is also observed in AFM images and consistent with the UV-Vis absorption measurements. In the XRD pattern of hybrid film, in addition to the characteristic peak of P3HT the weak peak at diffraction angle of 14.37° is also present and assigned to (002) reflection of MoS266 which further confirms the NSs induced crystallization of hybrid film. Thus, the ordering and crystallization of P3HT in the hybrid film result in the enhancement of hole mobility. However, hole mobility in hybrid film with 5 wt % MoS2 NS is lower than that in 3 wt % MoS2 NS, even though hybrid film containing 5 wt % MoS2 NS exhibits higher crystallinity with distinct nano-fibrillar networks. It suggests that a high degree of ordered structure may not necessarily provide high charge carrier mobility.56 Taking account of these results we believe that the excess amount of NSrich domains may hinder hole transport, even though highly crystalline polymer phases are present. Our results suggest that optimum concentration of 3 wt % MoS2 NS in the hybrid 21 ACS Paragon Plus Environment


film would lead to the formation of long-range ordered nano fibrillar structure which can remarkably provide efficient transportation pathways for charge carriers and thus results improved device performance.

(100) 12

P3HT 1 wt % MoS2

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2 θ (degree) Figure 10: The GIXRD pattern of pristine P3HT and P3HT:MoS2 hybrid film with different concentrations of MoS2 NS.

Experimental Section Synthesis of Functionalized MoS2 Nanosheets DDT functionalized MoS2 NSs were synthesized by liquid phase exfoliation of bulk MoS2 powder in a solvent mixture of DDT and DCB as reported41 by our group. In brief, 100 mg MoS2 powder was dispersed in 1 mL DDT and the solution was heated at 60 °C for 4 h over magnetic stirrer inside a nitrogen filled glove box. After that 9 mL DCB was added to it and the mixture was placed under sonication in a sonic bath for 6 h, followed by centrifugation to remove unexfoliated residue. The top 90 % optically transparent dark green supratenant of suspended MoS2 was collected and stored for further uses. The DDT functionalized NSs were purified by adding an excess of acetone, the flocculated NSs were separated by centrifugation 22 ACS Paragon Plus Environment

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and redispersed in DCB under mild sonication. The purified sample was stable for several months without any sign of aggregation. Device Fabrication The photovoltaic devices (device structure of ITO/PEDOT:PSS/P3HT:PC71BM/MoS2 Ca/Al) with an active layer of binary P3HT:PC71BM and ternary P3HT:PC71BM/MoS2 with different concentration of MoS2 NS were fabricated according to the following procedure. The P3HT and PC71BM (1:1 weight ratio) with concentration of 25 mg/mL were dissolved in a solution of DCB containing various amounts of MoS2 NS. The four different ternary blend solutions of P3HT:PC71BM:MoS2 with 0, 1, 3 and 5 wt % of NS was prepared by stirring for 24 h at 60 ⁰C inside nitrogen filled glove box. The patterned indium tin oxide (ITO) coated glass substrates were thoroughly cleaned by ultrasonication with detergent, deionized water, and acetone for 30 min each. The final cleaning was completed with vapors of boiling isopropanol followed by drying in vacuum oven. The cleaned substrate was treated with UV ozone for 15 min. A 30 nm hole transport layer of poly(3,4-ethylenedioxythiophene)poly(styrene sulfonate) (PEDOT:PSS) was spin coated on cleaned ITO substrates at 4000 rpm for 90 s and then baked at 120 °C for 30 min in a vacuum oven. The 120 nm thick active layer of binary P3HT:PC71BM and ternary blends of P3HT:PC71BM:MoS2 with varying concentration of MoS2 NS dispersed in DCB, was spin coated at 900 rpm for 2 min on PEDOT:PSS layer. The active layer was annealed at 120 °C for 10 min on a hot plate. Finally, calcium interfacial layer with thickness 10 nm and Al cathode of thickness 100 nm were deposited on top of the active layer by thermal evaporation technique in a glove box at a chamber pressure of ~10−7 Torr. The active area of device was found to be 0.06 cm2. Characterization Techniques



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The transmission electron microscopic (TEM) images of MoS2 NSs ware recorded on FEI Tecnai G2 F 30 STWIN operating at an accelerating voltage of 300 kV. Samples were prepared on holey carbon supported Cu grids by drop casting 10 µl of the MoS2 solution dispersed in dichlorobenzene. UV-Vis absorption spectroscopy was carried out using UV-Vis spectrophotometer (Shimadzu 2401 PC). Atomic force microscopy (AFM) images were taken using an AFM Nano First-3100 operated in tapping mode. The GIXRD pattern of polymer films were obtained on high resolution X-ray diffraction system (HRXRD, Panalytical X'Pert PRO MRD 40). The thickness of the active layers was measured by ellipsometer. The J – V characteristics of the solar cells were measured by using a Keithley 2420 source meter. The light source was calibrated using silicon reference cell with an AM 1.5G solar simulator under illumination intensity of 100 mW/cm2.

Conclusions In summary, we have demonstrated successful incorporation of DDT functionalized 2DMoS2 NSs as an additional charge transporting materials into the active layer of P3HT:PC71BM based OSCs in order to boost the device performance. The incorporation of NSs enables synergic enhancement in exciton generation, and dissociation, along with improved hole and electron transport through the active layer which led to the enhancement in PCE. The improved hole mobility is attributed to the crystallization of P3HT by selforganization of their chains into ordered nano fibrillar structure. The ordering of P3HT chains could be associated with hydrophobic interactions between hexyl side chains of P3HT and alkyl chains of DDT molecules present at MoS2 surface.

In addition, due to highly

conductive 2D network of NS, it provides additional percolation pathway for efficient transport of electrons within the active layer. The highest performing ternary OSCs containing 3 wt % MoS2 NS exhibited a higher value of , and FF than highest performing binary P3HT:PC71BM blend leading to an enhancement by 32 % in PCE from 2.8 % to 3.7 24 ACS Paragon Plus Environment

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%. Our results demonstrated that the exploitation of functionalized MoS2 NS as an additional charge transporting material is a promising approach towards optimization of the efficient ternary solar cell.

Author Information Corresponding Author Dr. Ritu Srivastava, Principal Scientist Flexible Organic Energy Devices National Physical Laboratory, New Delhi, India Phone- +91-11-45608644 Fax: +911145609310. Email: [email protected]

Acknowledgements The authors are grateful to the Director NPL, New Delhi, India for the facility. R.A and S.Y gratefully acknowledge the financial support from the Council of Scientific and Industrial Research (CSIR), New Delhi, for the award of RA and SRF, respectively. The research was partially funded from DRDO grant ERIPR/ER/1000389/M/01/1407.

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Functionalized 2D-MoS2-Incorporated Polymer Ternary Solar Cells

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