FIELD OF THE INVENTION
The present invention relates to a copolymer composition. More particularly, the present invention relates to a cost-effective copolymer composition as an n-type semiconducting material for use in All-polymer solar cells which shows high power conversion efficiency in all polymer solar cells.

BACKGROUND AND PRIOR ART OF THE INVENTION
A solar cell is made of two types of semiconductors, called p-type and n-type. p-type material is electron rich and n-type material is electron deficient. For p-type material there are many reports available. For n-type material there are few reports available. Most of the research reports use PCBM ([6,6]-Phenyl-C61-butyric Acid Methyl Ester) as n-type material. But the PCBM has many drawbacks such as phase separation with P-type material, form aggregates and it is costly.

The research based on solution processable organic bulk-hetero junction (BHJ) solar cells is progressing very fast. Continuous efforts have been made to increase the power conversion efficiency (PCE) of BHJ solar cell via development in both the active layer material design (n-type and p-type) as well as the device engineering. In all-polymer BHJ solar cells (PSCs), both components of the active layer i.e. electron donor and acceptor are polymeric semiconductors which have potential advantages over the extensively studied donor polymer /acceptor fullerene composite solar cells. Although significant progress have been made in the development of polymer/fullerene composite solar cell in terms of their high device efficiency over > 9%, the use of fullerene acceptor (PC61BM and PC71BM) has some disadvantages like relatively weak absorption ability in the visible region, high cost of synthesis, and morphological instability of polymer/fullerene blend over time and temperature that limits the performance of the solar cell. On the other hand, non fullerene n-type polymeric acceptors have promising features such as high absorption in visible-infrared region, low cost, high thermal and photochemical stability, mechanical flexibility and synthetic adaptability. Among the various n-type polymeric semiconductors used in all-polymer BHJ solar cells, the polymers based on the naphthalenediimide (NDI) and perylenediimides (PDI) have exhibited the most promising features. They have high electron affinity, good absorption, thermal and photochemical stability, and p-stacking behavior which facilitates favorable solid state packing. Particularly, the low band gap core-substituted NDI donor-acceptor polymers comprising of bithiophene and selenophene donors have attracted much attention due to their high electron mobility in OFET and high PCE efficiency in solar cells. Poly{[N,N'-bis(2-octyl-dodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}, P(NDI2OD-T2) (Polyera ActivInk N2200) is one of the extensively studied high performing n-type polymer reported by Facchetii et al (J. Am. Chem. Soc; 2009, 131, pp 8-9) P(NDI2OD-T2) polymer was extensively utilized in all-PSCs and OFET applications due to its desirable photo-physical and semiconducting properties. It exhibited properties like high electron mobility > 0.85 cm2V-1s-1 in OFET, high PCE efficiency > 5% in all-PSCs, solution processability, high crystalline nature and light absorption capability near visible and infrared region. However, the earlier reports on the all-PSC measurements of P3HT/P(NDI2OD-T2) blend showed very low device efficiency value (PCE, 0.2%) due to aggregation of P(NDI2OD-T2). Later studies showed that the aggregation in P(NDI2OD-T2) could be suppressed significantly in the early stage of film formation by using more polar aromatic solvents. Indeed, the solar cell devices prepared from such non-aggregated solution showed improved PCE due to good intermixing of donor and acceptor components, thereby efficiently harvesting photogenerated excitons at donor-acceptor interface. Beyond the solvent induced morphological control, the structural variations also plays a vital role in improving the blend morphology, bulk crystallinity, molecular orientation, highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) energy level and charge transport properties of polymer. For instance, Jen et al. in Adv Mater. ; 2015; 27(21); pp 3310-3317 reported the significant enhancement in the PCE up to 6.7% of P(NDI2OD-T2) polymer by substituting the bithiophene core with more electron-withdrawing fluorine (F) atom. The (F) substituted P(NDI2OD-T2) polymer exhibited good bulk crystallinity with preferential face-on orientation in the BHJ blend film thereby facilitating improved exciton generation, dissociation and charge transport. Random copolymerization is another promising design strategy utilized for synthesis of NDI containing acceptor polymers which showed significant improvement in the PCE in all-PSCs. NDI based random copolymers were synthesized either by varying two different donor monomers with NDI or by varying the acceptor comonomer (such as PDI) with NDI along with a common donor co component.

Article titled “ N-Type Semiconducting Naphthalene Diimide-Perylene Diimide Copolymers: Controlling Crystallinity, Blend Morphology, and Compatibility Toward High-Performance All-Polymer Solar Cells” by YJ Hwang et al. published in J. Am. Chem. Soc., 2015, 137 (13), pp 4424–4434 reports a series of new semiconducting naphthalene diimide (NDI)-selenophene/perylene diimide (PDI)-selenophene random copolymers, xPDI (10PDI, 30PDI, 50PDI), whose crystallinity varies with composition, and investigated them as electron acceptors in BHJ solar cells. The pairing of the new 30PDI with optimal crystallinity (Lc = 5.11 nm) as acceptor with the same PBDTTT-CT donor yields compatible blends and all-polymer solar cells with enhanced performance (PCE = 6.3%, Jsc = 18.6 mA/cm2, external quantum efficiency = 91%).

A Review article titled “Recent research progress of polymer donor/polymer acceptor blend solar cells” by H Benten et al. published in J. Mater. Chem. A, 2016, 4, 5340-5365 reports an overview of recent progress towards the performance enhancement of polymer/polymer blend solar cells.

Article titled “All-Polymer Solar Cells Based on Absorption-Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%” by L Gao et al. published in Adv Mater.; 2016 ;28(9); pp 1884-1890 reports high PCE of 8.27% with a high FF of 70.24% for all-PSCs by taking a fluorinated medium bandgap copolymer J51as donor and the low bandgap polymer N2200 as acceptor. In addition, the device with an active layer thickness of 300 nm still demonstrated good photovoltaic performance with PCE of ˜4.5%.

Article titled “7.7% Efficient All-Polymer Solar Cells” by YJ Hwang et al. published in Adv Mater.; 2015 ;27(31); pp 4578-84 reports high-performance all-polymer solar cells can be achieved by employing suitable donor and acceptor polymers and by controlling the polymer/polymer blend fi lm self-organization rate through a simple film aging process at room temperature. The resulting all-polymer (PNDIS-HD:PBDTT-FTTE) blend solar cells combined a record 7.7% PCE and 18.8 mA cm-2, short-circuit current density with an EQE of 85%.

Article titled “High-Performance All-Polymer Solar Cells Via Side-Chain Engineering of the Polymer Acceptor: The Importance of the Polymer Packing Structure and the Nanoscale Blend Morphology” by C Lee eta l. published in Adv Mater.; 2015 Apr 17; 27(15); pp 2466-71 reports the effectiveness of side-chain engineering is demonstrated to produce highly efficient all-polymer solar cells (efficiency of 5.96%) using a series of naphthalene diimide-based polymer acceptors with controlled side chains.

Article titled “Manipulating Aggregation and Molecular Orientation in All-Polymer Photovoltaic Cells” by L Ye et al. published in Adv Mater.; 2015;27(39); pp 6046-54 reports replacing the polymeric donor PBDTBDD with its 2D-conjugated polymer PBDTBDD-T, the power conversion efficiency of OPVs featuring the anisotropic polymer acceptor PNDI is drastically boosted from 2.4% up to 5.8%.
Article titled “Combinative Effect of Additive and Thermal Annealing Processes Delivers High Efficiency All-Polymer Solar Cells” by G Shi et al. published in J. Phys. Chem. C, 2015, 119 (45), pp 25298–25306 reports combinative effects of thermal annealing and additive processes on the performance of all-polymer bulk heterojunction (BHJ) solar cells with composites of different donor polymers (PTQ1 poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl], P3HT poly(3-hexylthiophene), PTB7-Th Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]) and poly[1,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)P(NDI2OD-T2) [PolyeraActivInk N2200].
Article titled “High-Performance All-Polymer Solar Cells Based on Face-On Stacked Polymer Blends with Low Interfacial Tension” by H Kang et al. published in ACS Macro Lett., 2014, 3 (10), pp 1009–1014 reports highly efficient all-polymer solar cells with power conversion efficiencies of over 4.5% by highly intermixed blends of PTB7-Th donor and P(NDI2OD-T2) acceptor polymers. The low interfacial tension and the face-on p–p stackings of the all-polymer blends afforded desired nanophase morphology, which facilitates efficient charge transport from the active layer to each electrode.
Article titled “Binary additives synergistically boost the efficiency of all-polymer solar cells up to 3.45%” by P Cheng et al. published in Energy Environ. Sci., 2014,7, 1351-1356 reports that the binary additives synergistically boost the power conversion efficiency of all-polymer solar cells up to 3.45%. They reports that the use of nonvolatile additive PDI-2DTT suppresses aggregation of the acceptor PPDIDTT and enhances donor/acceptor mixing, while the additive DIO facilitates aggregation and crystallization of the donor PBDTTT-C-T as well as improves phase separation. Combination of DIO and PDI-2DTT leads to suitable phase separation and improved and balanced charge transport, which is beneficial to efficiency enhancement.
Article titled “Perylene and naphthalene diimide polymers for all-polymer solar cells: a comparative study of chemical copolymerization and physical blend” by Shuixing Dai et al. published in Polym. Chem., 2015,6, pp 5254-5263 reports Five copolymers, having 4,4,9,9-tetrakis(4-hexylphenyl)-indaceno[1,2-b:5,6-b']-dithiophene as a donor unit, and perylene diimide (PDI) and/or naphthalene diimide (NDI) as acceptor moieties, synthesized by Stille coupling copolymerization, and used as electron acceptors in solution-processed polymer solar cells (PSCs). Among binary blend PSCs using P3HT as a donor and these polymers as acceptors, PPDI25-co-NDI75-based devices (P3HT: PPDI25-co-NDI75 = 3: 1, w/w) yielded the best power conversion efficiency (PCE) of up to 1.54%. Among ternary blend PSCs using P3HT as a donor and PDI polymer PPDI100 and NDI polymer PNDI100 as coacceptors, the P3HT: PPDI100: PNDI100 (3: 0.25: 0.75, w/w) ternary blend afforded the best PCE of 0.83%.
Article titled “A comparison of n-type copolymers based on cyclopentadithiophene and naphthalene diimide/perylene diimides for all-polymer solar cell applications” by B Xiao et al. published in Polym. Chem., 2015,6, pp 7594-7602 reports a comparative study of two solution-processable cyclopenta[2,1-b:3,4-b']dithiophene (CPDT)-based n-type copolymers, PCPDT-NDI and PCPDT-PDI, focusing on their optical, electrochemical and photovoltaic properties. The PCPDT-PDI exhibits a much better photovoltaic performance with a power conversion efficiency of 2.13% when using 1-chloronaphthalene (CN) as an additive to obtain a good film morphology and to improve the electron mobility.
Article titled “High-performance ternary blend all-polymer solar cells with complementary absorption bands from visible to near-infrared wavelengths” by H Benten et al. published in Energy Environ. Sci., 2016,9, 135-140 reports high-performance ternary blend all-polymer solar cells with complementary absorption bands from visible to near-infrared wavelengths. A power conversion efficiency of 6.7% was obtained with an external quantum efficiency over 60% both in the visible and near-infrared regions.
Article titled “One Donor-Two Acceptor (D-A(1))-(D-A(2)) Random Terpolymers Containing Perylene Diimide, Naphthalene Diimide, and Carbazole Units” by Lisa Kozycz et al. published in Journal of Polymer Science Part A Polymer Chemistry; 2014, 52(23); pp 3337-3345 reports a series of one donor–two acceptor (D–A1)-(D–A2) random terpolymers containing a 2,7-carbazole donor and varying compositions of perylene diimide (PDI) and naphthalene diimide (NDI) acceptors synthesized via Suzuki coupling polymerization.
Article titled “Side Chain Optimization of Naphthalenediimide–Bithiophene-Based Polymers to Enhance the Electron Mobility and the Performance in All-Polymer Solar Cells” by W Lee et al. published in Adv. Funct. Mater.; 2016; 26, pp 1543–1553 reports the effects of side chain engineering of P(NDI2OD-T2) polymer (also known as Polyera Activink N2200), which is the most widely used n-type polymer in all-PSCs and organic field-effect transistors (OFETs), on their structural and electronic properties. A series of naphthalenediimide-bithiophene-based copolymers (P(NDIR-T2)) is synthesized, with different side chains (R) of 2-hexyldecyl (2-HD), 2-octyldodecyl (2-OD), and 2-decyltetradecyl (2-DT).
Article titled “Determining the Role of Polymer Molecular Weight for High-Performance All-Polymer Solar Cells: Its Effect on Polymer Aggregation and Phase Separation” by H Kang et al. published in J. Am. Chem. Soc., 2015, 137 (6), pp 2359–2365 reports a series of semicrystalline poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] PPDT2FBT) polymers with different number-average molecular weights (Mn’s) (PPDT2FBTL, Mn = 12 kg/mol; PPDT2FBTM, Mn= 24 kg/mol; PPDT2FBTH, Mn= 40 kg/mol) synthesized, and their photovoltaic properties as electron donors for all-polymer solar cells (all-PSCs) with poly[[N,N'-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (P(NDI2OD-T2)) acceptor.
Most of the aforementioned reports demonstrated the importance of systematic tuning of polymer bulk crystallinity via optimizing the different comonomer compositions in random copolymers. Bulk crystallinity is one of the key factors that is responsible for bulk morphology of donor:acceptor blend in all-PSCs. Optimum crystallinity of acceptor polymer is highly desirable to ensure proper intermixing with donor polymer in the blend to achieve good D/A microphase separation. There are no report for the random copolymer approach whereby perylene diimide is introduced into the Naphthalenediimide-bithiophene P(NDI2OD-T2) polymer. Therefore, there is need for a cost-effective polymeric material which will overcome the prior arts drawback and can lead to good power conversion efficiency in all polymer solar cells.

OBJECTIVE OF THE INVENTION
The main objective of the present invention is to provide a copolymer composition comprising naphthalene diimide, perylene diimide and bithiophene.
Another object of the present invention is to provide copolymer composition for use in solar cell.

SUMMARY OF THE INVENTION
Accordingly, present invention provdes a copolymer composition comprising naphthalene diimide, perylene diimide and bithiophene in the ratio ranging between 1:1:2 to 1.5:0.5:2.
In an embodiment of the present invention, said composition is useful in All-polymer solar cells.
In yet another embodiment of the present invention, said naphthalene diimide is N, N’-Bis (2-octyldodecyl)-2,6-dibromo-1,4,5,8-naphthalenediimide (NDI-2OD-Br2).
In yet another embodiment of the present invention, said perylene diimide is N, N’-Bis (2-ethylhexyl)-1,7-dibromo -3,4,9,10-perylene tetracarboxylicdianhydride (PDI-2EH-Br2).
In yet another embodiment of the present invention, said bithiophene is 5,5'-bis (trimethylstannyl)-2,2'-bithiophene.
In yet another embodiment, present invention provides a solar cell comprising a blend of copolymer composition as an n-type semiconductor material with PTB7-Th as the p type polymer.
In still another embodiment of the present invention, power conversion efficiency (PCE) of said solar cell is in the range of 2.00% to 5.03%.
In yet another embodiment, present invention provides a process for the preparation of copolymer composition comprising the steps of:
i. adding solvent to a mixture of naphthalene diimide, bithiophene and perylene diimide to obtain a reaction mixture;
ii. adding Bis(triphenylphosphine) palladium (II) dichloride (Pd(Ph3)2Cl2) to the reaction mixture of step (a) followed by degasssing and stirring the reaction mixture at temperature in the range of 90 to 95° Cfor the period in the range of 70 to 74 hrs to obtain a reaction mixture;
iii. adding solvent to the reaction mixture of step (b) with stirring, cooling, adding a solution of potassium fluoride in water and stirring followed by work-up to obtain the copolymer composition.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 depicts UV-Vis absorption spectra of dilute solutions of reference P(NDI2OD-T2) and random copolymers in a) chloroform; and b) thin film on glass substrate.
Figure 2 depicts Cyclic voltammograms of reference P(NDI2OD-T2) and random copolymers as thin film on platinum working electrode in 0.1 M n-Bu4NPF6 acetonitrile at a scan rate of 100 mV/s.
Figure 3 depicts Thin-film XRD diffraction patterns of reference P(NDI2OD-T2) and random copolymers.
Figure 4 depicts Current density- Voltage (J – V) characteristics for copolymers using different donors – PTB7 and PTB7-Th.
Figure 5 depicts Dependence of PCE on random copolymer composition for both donors.
Figure 6 represents GPC chromatograms of copolymers recorded using chloroform as eluent.
Figure 7 represents TGA thermograms of copolymers at 10 oC /min under N2 atmosphere.
Figure 8 represents Second heating/cooling curves of quench-cooled samples of the Blend and PTB7-Th in the DSC scans conducted at 10 oC /min under N2 atmosphere
Figure 9 represents Thin-filmXRD diffraction patterns of Blend and PTB7-Th.
Figure 10 represents PL spectra recorded for the blend film of (a) PTB7-Th:P(NDI2OD-T2) (b) PTB7-Th:NDI-Th-PDI15 (c) PTB7-Th:NDI-Th-PDI30 (d) PTB7-Th:NDI-Th-PDI50 blends.
Figure 11 represents The combined PL spectra of donor: acceptor blend film and neat PTB7-Th donor.
Figure 12 represents Current (J) - Voltage (V) characteristics and SCLC fittings of PTB7:reference /copolymerblend films(a) Hole-only device and (b) electron-only device and for PTB7-Th: reference / copolymersblend films (c) Hole-only device and (d) electron-only device.
Figure 13 represents AFM height images (3 µm x 3µm) of PTB7: P(NDI2OD-T2) (1.3:1 w/w) and PTB7:NDI-Th-PDIx (1.3:1 w/w) blend solar cell.
Figure 14 represents AFM height images (3 µm x 3µm) of PTB7-Th: P(NDI2OD-T2) (1.3:1 w/w) and PTB7-Th:NDI-Th-PDIx (1.3:1 w/w) blend solar cell.
Figure 15 represents the process for the synthesis of n-type NDI-bithiophene /PDI-bithiophene random copolymers.

DETAILED DESCRIPTION OF THE INVENTION
Present invention provides a copolymer composition for use in solar cell comprising naphthalene diimide, perylene diimide and bithiophene having high power conversion efficiency in all polymer solar cells.
Naphthalene diimide is N, N’-Bis (2-octyldodecyl)-2,6-dibromo-1,4,5,8-naphthalenediimide (NDI-2OD-Br2).
Perylene diimide is N, N’-Bis (2-ethylhexyl)-1,7-dibromo -3,4,9,10-perylene tetracarboxylicdianhydride (PDI-2EH-Br2).
Bithiophene is 5,5'-bis (trimethylstannyl)-2,2'-bithiophene.
The concentration of said perylene diimide monomer in said copolymer is in the range of 15% to 50%.
Present invention provides a solar cell comprising a blend of first polymer and a copolymer composition of naphthalene diimide, perylene diimide and bithiophene as an n-type semiconductor material.
The power conversion efficiency (PCE) of said solar cell is in the range of 2.00% to 5.30%.
The present invention provides a process for the preparation of copolymer composition comprising the steps of:
a) adding solvent to a mixture of NDI-2OD-Br2 and 5,5'-bis (trimethylstannyl)-2,2'-bithiophene and PDI-2EH-Br2 to obtain a reaction mixture;
b) adding Bis(triphenylphosphine) palladium (II) dichloride (Pd(Ph3)2Cl2) to the reaction mixture of step (a) followed by degasssing and stirring the reaction mixture at temperature in the range of 90 to 95° Cfor the period in the range of 70 to 74 hrs to obtain a reaction mixture;
c) adding solvent to the reaction mixture of step (b) with stirring, cooling, adding a solution of potassium fluoride in water and stirring followed by work-up to obtain the desired copolymer composition.
Solvent is selected from bromobenzene or toluene.
The above process for the synthesis of n-type NDI-bithiophene /PDI-bithiophene random copolymers is shown in figure 15.
From the characterization data of newly synthesized random copolymers it is observed that the bench mark copolymer P(NDI2OD-T2) showed weight average molecular weight (Mw) of 152 kDa with (Mw/Mn) of 5.7; Random copolymers showed lower weight average molecular weight (Mw) and (Mw/Mn) with Mw in the range of 32.4-60.7 kDa and PDI in range of 2.5-3.1 (Table 1); TGA curves showed good thermal stability for all random copolymers with onset decomposition temperature (Td) of over > 400 ?C; DSC curves recorded for reference polymer P(NDI2OD-T2) showed very broad thermal transitions with endothermic peak (Tm) at 291 oC and exothermic peak at (Tc) 271 oC; NDI-Th-PDI 15 and NDI-Th-PDI30 showed endothermic peaks at (Tm, 292 oC and 288 oC respectively) and exothermic peak at (Tc, 278 oC and 267 oC respectively), while only endothermic peak (Tm, 267 oC) is observed for NDI-Th-PDI50; UV-Vis absorption spectra of random copolymers (NDI-Th-PDIx) and reference P(NDI2OD-T2) are recorded in both dilute chloroform solution as well as in thin-film spin coated on glass substrate and are shown in Figure 1. In solution (Figure 1a) all copolymers showed two distict absorption bands; the first high energy absorption band at ~300-425 nm is accounted for by p-p* transition and another low energy band at ~460- 800 nm is assigned to intra-molecular charge transfer (ICT) from bithiophene unit to NDI/PDI. The reference polymer P(NDI2OD-T2) showed typical absorption spectrum with p-p* absorption peak at ~400 nm and ICT peak at 645 nm. It is observed at 630 nm in NDI-Th-PDI15, 618 nm in NDI-Th-PDI30, and 605 nm in NDI-Th-PDI50; Furthermore, an additional broad peak at ~500 nm is obtained for all random copolymers which is absent in P(NDI2OD-T2) and the intensity of this peak at ~500 nm is found to increase with the increase in the content of PDI unit. This additional peak is assigned for the typical absorption of the PDI component in the random copolymer. The spin coated thin-films of all copolymers showed similar absorption spectra as that in solution (Figure 1b); The reference copolymer P(NDI2OD-T2) showed ICT band at 685 nm. On the other hand, random copolymers NDI-Th-PDI15, NDI-Th-PDI30 and NDI-Th-PDI50 exhibited blue shifted ICT band along with decreased absorption coefficient at 670 nm, 655 nm and 630 nm respectively. The optical band gap (Egopt) of random copolymers are calculated from lower energy absorption band edge of thin-film and listed in Table -1. An optical band gap of 1.52 eV is obtained for reference copolymer P(NDI2OD-T2) which is slightly increased to 1.55 eV, 1.57 eV and 1.60 eV in NDI-Th-PDI15, NDI-Th-PDI30 and NDI-Th-PDI50 respectively. The electrochemical redox behavior and electronic energy levels of new n-type random copolymers is analyzed by cyclic voltammetry. Thin-films of polymer are deposited on the platinum working electrode. The measurement is carried out in acetonitrile solvent with ferrocene/ferrocenium as an internal standard and tetrabutylammonium hexafluorophosphate (n-Bu4NPF6 0.1M/ acetonitrile) as supporting electrolyte. Cyclic voltammograms for all copolymers are shown in Figure 2 and the calculated HOMO and LUMO energy levels are given in Table 1.

Table 1: GPC molecular weight, UV-visible absorption, optical band gap, and electronic energy levels of P(NDI2OD-T2) and NDI-Th-PDIx random copolymers.a Weight-average molecular weight (Mw). b Polydispersity index (Mw/ Mn). c The decomposition temperature (5% weight loss) estimated using TGA under N2. ?max UV-visible absorption maxima.

Polymer Mwa (kDa) Mw/Mnb Tdc
(oC) Tmd
(oC) ?Hmd
(J/g)
P(NDI2OD-T2) 152 5.7 422 286.61 6.65
NDI-Th-PDI15 60.7 3.7 437 289.47 12.57
NDI-Th-PDI30 49.8 2.9 435 284.0 6.55
NDI-Th-PDI50 32.4 2.5 436 269.38 4.41

All random copolymers showed quasi-reversible reduction peaks with almost similar values of electrochemical reduction like P(NDI2OD-T2). None of the copolymer showed oxidation peak during anodic scan up to 2V. The lowest unoccupied molecular orbital (LUMO) energy levels were estimated based on the onset value of first reduction peak and reference energy level of ferrocene (4.8 eV below the vacuum level) according to ELUMO (eV) = -e × (Ered onset + 4.8) below the vacuum level. The LUMO and HOMO energy levels of P(NDI2OD-T2) was found to be -3.90 eV and -5.42 eV respectively.
The molecular packing and bulk crystalline nature of reference P(NDI2OD-T2) and new n-type random copolymers were analyzed using wide-angle X-ray diffraction (XRD) measurement. Figure 3 shows the X-ray diffraction patterns of thermally annealed (at 160 oC, 15 min) thin-films of copolymer on glass substrate and relevant data are given in Table 2.
Table 2: UV-visible absorption, optical band gap, electronic energy levels and Thin-film XRD data of thermally annealed random copolymers of P(NDI2OD-T2) and NDI-Th-PDIx random copolymers.
Polymer ?max (nm) Solution ?max (nm)
Thin-film Egopt
(eV) LUMO
(eV) HOMO
(eV) 2??
d-spacing (Å)
(100) (010) d100 d010
P(NDI2OD-T2) 367, 645 381, 685 1.52 3.90 5.42 4.06 23.10 21.72 3.84
NDI-Th-PDI15 367, 630 381, 670 1.55 3.94 5.49 4.21 23.03 20.96 3.85
NDI-Th-PDI30 367, 618 374, 655 1.57 3.94 5.51 4.58 22.59 19.27 3.93
NDI-Th-PDI50 367, 605 368, 630 1.60 4.00 5.60 4.80 21.92 18.38 4.00

The XRD pattern of reference polymer P(NDI2OD-T2) showed lamellar peak (100) at 2? = 4.06o and p-p stacking peak (010) at 2? = 23.10o which corresponded to lamellar packing distance of 21.72 Å and p-p stacking distance of 3.84 Å. The lamellar peak (100) for random copolymers were observed at 2? = 4.21o, 4.58o and 4.80o with d-spacing 20.96 Å, 19.27 Å, and 18.38 Å for NDI-Th-PDI15, NDI-Th-PDI30 and NDI-Th-PDI50 respectively. The lamellar packing distance was found to decrease progressively with increasing incorporation of PDI moiety in the random copolymer chain. It increased from 3.84 Å in reference P(NDI2OD-T2) to 3.85 Å, 3.93 Å, and 4.00 Å in NDI-Th-PDI15, NDI-Th-PDI30, and NDI-Th-PDI50 respectively. Furthermore, relatively broad p-p stacking peaks (010) were observed in NDI-Th-PDI30, and NDI-Th-PDI50 (Figure 3).

The optimum donor:acceptor (D:A) blend ratio and thickness of BHJ active layer films are found to be 1.3:1 (w/w) and 100 to 110 nm, respectively. Figure 4 (top) shows the current density-voltage (J-V) curves and external quantum efficiency (EQE) spectra obtained for PTB7: acceptor polymer blend all-PSCs. The optimized solar cell parameters including short-circuit current density (Jsc), the open-circuit voltage (Voc), fill factor (FF), and PCE, are summarized in Table 3.

Table 3: Photovoltaic properties of PTB7:NDI-Th-PDIx and PTB7: P(NDI2OD-T2) blend all-polymer solar cells.
Active layer Voc
(V) Jsc
(mA/cm2) FF
(%) PCE
(%)
PTB7 Donor
PTB7: P(NDI2OD-T2) 0.807?0.05 6.19?0.07 40.1?0.04 2.00?0.06
PTB7: NDI-Th-PDI15 0.777?0.03 7.66?0.05 52.1?0.02 3.10?0.05
PTB7: NDI-Th-PDI30 0.780?0.02 9.41?0.04 47.8?0.02 3.50?0.06
PTB7: NDI-Th-PDI50 0.774?0.05 7.55?0.05 45.5?0.04 2.65?0.07

The PTB7 is employed as the donor material, which exhibited maximum PCE of 2.06% (Jsc of 6.26 mA/cm2, Voc of 0.81, and FF of 40.5%) with device based on PTB7:P(ND2OD-T2) blend. The device performance of the various PTB7: NDI-Th-PDIx blend devices showed that all the newly synthesized random copolymers (NDI-Th-PDI15, NDI-Th-PDI30, NDI-Th-PDI50) exhibited significant improvement in the PCE as compared to reference polymer P(ND2OD-T2). For instance, PTB7: NDI-Th-PDI15 blend device showed PCE of 3.15% (Jsc of 7.71 mA/cm2,Voc of 0.78, and FF of 52.3%). Further enhancement in the photovoltaic performance was observed in PTB7:NDI-Th-PDI30 blend device which exhibited PCE of 3.56% (Jsc of 9.45 mA/cm2,Voc of 0.78, and FF of 48.0%). However, PTB7: NDI-Th-PDI50 blend with highest incorporation of PDI (50%) showed lower PCE of 2.72% (Jsc of 7.60 mA/cm2, Voc of 0.77, and FF of 45.9%), which was still higher compared to the reference polymer P(ND2OD-T2).

Figure 4 (bottom plot) shows the current density-voltage (J-V) curves and external quantum efficiency (EQE) spectra obtained for PTB7-Th: acceptor polymer blend all-PSCs. A similar trend as was observed with PTB7 as donor was observed with PTB7-Th as the donor component. All the random copolymers exhibited significant improvement in the PCE as compared to reference polymer P(ND2OD-T2). The reference polymer P(ND2OD-T2) exhibited a PCE of 2.97 % when PTB7-Th is used as the donor polymer. PTB7-Th: NDI-Th-PDI15 blend device showed PCE of 4.22%, while PTB7-Th: NDI-Th-PDI30 blend device showed the highest PCE % of 5.03 %. PTB7-Th: NDI-Th-PDI30 blend device exhibited a PCE value of 3.33 %, which was also higher compared to the reference polymer blend device. The photovoltaic properties of PTB-7Th: NDI-Th-PDIx blend all-polymer solar cells are given in table 4.
Table 4: Photovoltaic properties of PTB-7Th: NDI-Th-PDIx and PTB7-Th: P(NDI2OD-T2) blend all-polymer solar cells.
Active layer Voc
(V) Jsc
(mA/cm2) FF
(%) PCE
(%)
PTB7-Th Donor
PTB7-Th: P(NDI2OD-T2) 0.793?0.04 8.96?0.09 41.0?0.03 2.91?0.06
PTB7-Th: NDI-Th-PDI15 0.787?0.05 10.65?0.06 49.3?0.04 4.13?0.09
PTB7-Th: NDI-Th-PDI30 0.792?0.03 11.39?0.04 55.2?0.02 4.98?0.05
PTB7-Th: NDI-Th-PDI50 0.793?0.04 10.10?0.06 40.7?0.05 3.26?0.07

The dependence of the PCE on the varying incorporation of PDI unit in the copolymer is shown in Figure 5. From Figure 5, it can be observed that NDI-Th-PDI30 reflected the best optimized composition and further increase in the content of PDI unit in (NDI-Th-PDIx) random copolymer has not helped to improve the photovoltaic performance.

The bulk electron and hole mobility of polymer/polymer blend films are measured to investigate the effect of charge carrier transport properties on the photovoltaic performance, by space charge limited current (SCLC) technique. All PTB7: acceptor polymer blend film devices are made similarly as the all-PSCs devices. The electron-only device is fabricated using ITO/ZnO/PFN/ active layer /Al structure and hole-only device is fabricated using ITO/PEDOT: PSS/active layer/Au structure. The electron and hole mobility values obtained from PTB7: acceptor polymer blend devices are summarized in Table 5 and their J-V curves with the SCLC fittings are shown in Figure 16.

Table 5: SCLC electron and hole mobilities of PTB7: NDI-Th-PDIx random copolymer blend.
Active layer µh
(cm2/Vs) µe
(cm2/Vs) µh/µe
PTB7 Donor
PTB7: P(NDI2OD-T2) 6.7(?0.4)×10-5 7.5(?0.5)×10-5 0.89
PTB7: NDI-Th-PDI15 0.8(?0.3)×10-4 8.6(?0.4)×10-5 0.93
PTB7: NDI-Th-PDI30 3.8(?0.2)×10-4 0.8(?0.2)×10-4 4.75
PTB7: NDI-Th-PDI50 3.5(?0.3)×10-4 7.6(?0.4)×10-5 4.60
PTB7-Th Donor
PTB7-Th: P(NDI2OD-T2) 0.9(?0.3)×10-4 0.7(?0.4)×10-4 1.28
PTB7-Th: NDI-Th-PDI15 2.6(?0.4)×10-4 0.9(?0.3)×10-4 2.88
PTB7-Th: NDI-Th-PDI30 4.7(?0.5)×10-4 1.1(?0.4)×10-4 4.27
PTB7-Th: NDI-Th-PDI50 4.0(?0.4)×10-4 1.1(?0.3)×10-4 3.63

The reference PTB7: P(NDI2OD-T2) blend device exhibited the hole and electron mobility of 7.0×10-5 cm2/Vs and 8.0×10-5 cm2/Vs respectively. Compared to reference polymer, significant enhancement in the hole mobility values were observed for random copolymers blends (PTB7: NDI-Th-PDIx), which varied from 1.1×10-4 cm2/Vs in NDI-Th-PDI15 to 4.0×10-4 cm2/Vs in NDI-Th-PDI30 and to 3.8×10-4 cm2/Vs in NDI-Th-PDI50. On the other hand, the electron mobility values were found to be almost similar for all copolymer composition blends, except PTB7: NDI-Th-PDI30 which showed 3-fold increase in the electron mobility (1.0×10-4 cm2/Vs). Also PTB7: NDI-Th-PDI30 exhibited highest hole and electron mobility compared to other polymer blend.

The AFM images of donor:acceptor polymer blends are captured, to investigate the surface morphology all-PSCs BHJ devices. The donor:acceptor polymer (1.3:1 w/w) blend films are prepared in identical ways as that for all-PSCs devices. The Figure 15 and 16 shows AFM height images (3 µm x 3µm) of PTB7/PTB7-Th: P(NDI2OD-T2) and PTB7/PTB7-Th:NDI-Th-PDIx blend solar cell. The reference PTB7:P(NDI2OD-T2) or PTB7-Th:P(NDI2OD-T2) blend film showed rather coarsened morphology with average root mean square (RMS) surface roughness of 2.06 nm and 1.87 nm respectively due to the formation of large polymer aggregate domains. The large phase-separated granular morphology observed in PTB7/PTB7-Th:P(NDI2OD-T2) blend film indicated insufficient donor/acceptor intermixing at nanometer scale. The incorporation of large PDI unit in the random copolymer showed dramatic change in the surface morphology of PTB7/PTB7-Th: NDI-Th-PDIx blend films compared to reference blend. The PTB7: NDI-Th-PDI15 and PTB7: NDI-Th-PDI30 blend films showed proper uniform micro-phase separation with smaller phase-separated domain size. Furthermore, the RMS surface roughness decreased to 1.08 nm in PTB7: NDI-Th-PDI15 blend and 1.22 nm in PTB7: NDI-Th-PDI30 blend which suggested the formation of less aggregated domain leading to better donor/acceptor intermixing at nanometer scale. The well-developed nano-scale interpenetrating network observed in PTB7:NDI-Th-PDI15 and PTB7:NDI-Th-PDI30 blend films is beneficial for exciton dissociation and charge carrier transport. Compared to the other two random copolymer blend, the PTB7: NDI-Th-PDI50 blend film showed high RMS surface roughness of 1.42 nm with coarsened morphology, which was still more uniform with smaller phase-separated domain size compared to the reference PTB7:P(NDI2OD-T2) blend.
Examples
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

Example 1: Synthesis of N, N’-Bis (2-octyldodecyl)-2,6-dibromo-1,4,5,8-naphthalenediimide (NDI-2OD-Br2)
2,6-dibromo-1,4,5,8-naphthalenetetracarboxylic acid dianhydride(5 g, 11.73 mmole) was suspended in 25 mL of glacial acetic acid and stirred for a short period of time (1 hr) to get a homogeneous dispersion which was followed by addition of 2-octyldodecyl amine (13.96 g, 46.94 mmol). The reaction mixture was stirred and refluxed (at 120 ?C) to complete dissolution for 3h and cooled to room temperature (25?C). The reaction mixture was concentrated under reduced pressure to about 1/10th of original volume and then precipitated in to methanol to yield reddish brown powder that was filtered and dried under vacuum. The crude product was column purified by using pet ether/ethyl acetate solvent system and again re-crystallized from 1:1 mixture of hexane and acetone to get yellow powder of pure compound. Yield: 1.8 g (15 %, by considering 2, 6 isomer). Melting point (85-86 ?C); 1H NMR (200 MHz, CDCl3) d ppm: 8.98 (s, 2H, aromatic), 4.15 (d, 4H), 1.97 (m, 2H), 1.22, (m, 64 H), 0.86 (m, 12H). 13C NMR (500 MHz, CDCl3) d ppm: 161.16, 161.00, 139.14, 128.36, 127.73, 125.27, 124.06, 45.43, 36.44, 31.90, 31.87, 31.53, 30.01, 29.62, 29.58, 29.52, 29.33, 26.32 29.28, 22.67, 14.11. FTIR (ATR, cm-1): 3050, 2920, 2851,1707, 1652, 1557, 1432, 1363, 1308, 1230, 1199, 990; MALDI-TOF MS (Calcd m/z 985.06); Found m/z = 985.345. Anal. Calcd. for C54 H84 N2O4 Br2: C, 65.84; H, 8.60; N, 2.84. Found C, 66.09; H, 8.67; N, 2.27.

Example 2: Synthesis of N, N’-Bis (2-ethylhexyl)-1,7-dibromo -3,4,9,10-perylene tetracarboxylicdianhydride (PDI-2EH-Br2)
1,7-dibromo-3,4,9,10-peryleneetetracarboxylic acid dianhydride(3 g, 5.61mmole) was suspended in the mixture of 20 mL glacial acetic acid and 60 mL N-methyl pyrrolidineandpurged with argon. The mixture was stirred and heated to 60 ?C for 20 minute to get a homogeneous dispersion which was followed by addition of 2-ethylhexyl amine (2.11 g, 16.36mmol). The reaction temperature was raised to 120 ?C. After 12 hours, the reaction mixture was cooled to room temperature (25 ?C) and poured in to 500 mL water. The water suspension was stirred for 2 hours and filtered on buchner funnel under vacuum. The residue was washed with large amount of water and dried under vacuum. The crude product was column purified by using pet ether/ethyl acetate (1:6) and petether/DCM (50:50) solvent system to get red powder of pure compound.Yield: 1.3 g (29 %), Melting point (225-227?C).
1H NMR (400 MHz, CDCl3) d ppm: 9.35 (d, 2H,), 8.80 (s, 2H), 8.57 (d, 2H), 1.92 (m, 2H), 1.35, (m, 16 H), 0.95 (m, 12H). 13C NMR (400 MHz, CDCl3) d ppm: 163.09, 162.60, 137.96, 132.56, 129.91, 129.01, 128.32, 126.80, 123.04, 122.60, 120.76, 44.42, 37.91, 30.74, 28.66, 24.00, 23.05,14.11, 10.60. FTIR (ATR, cm-1): 2954, 2923, 2857, 1699, 1658, 1586, 1501, 1432, 1386, 1325, 1232, 1181, 1146, 1094, 857, 808, 739, 678; MALDI-TOF MS (Calcd m/z 772.56); Found m/z = 773.98, 795.96 (M+ Na+). Anal. Calcd. for C40H40N2O4 Br2: C, 62.19; H, 5.22; N, 3.63. Found C, 62.28; H, 5.05; N, 3.34.

Example 3: Syntheis of Poly{[N,N'-bis(2-octyl-dodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}, P(NDI2OD-T2)
NDI-2OD-Br2 (0.4g, 0.406 mmole) and 5,5'-bis (trimethylstannyl)-2,2'-bithiophene (0.199g, 0.406 mmole) were taken in air-free Schlenk tube under N2 atmosphere. Dry toluene 18 mL was added in tube followed by purging with nitrogen for half-hour. Bis(triphenylphosphine) palladium (II) dichloride (Pd(Ph3)2Cl2) (15 mg, 0.0211 mmole) was added to the tube quickly by opening rubber septa and the whole mixture was degassed by four freeze-vacuum-thaw cycles. The reaction mixture was stirred at 90?C for 3 days. Bromobenzene (0.2 mL) was then added and reaction mixture was further stirred at 90?C for 12 hours. Upon cooling to room temperature (25 ?C), a solution of potassium fluoride (1g) in 2 mL water was added and stirred for 2 hours. The reaction mixture was extracted with chloroform (250 mL x 3). The organic layer was washed with water, dried over anhydrous sodium sulfate and concentrated on a rotary evaporator. The obtained residue was dried in vacuum oven and subjected to a soxhlet extraction with acetone (48 hours) and chloroform (12 hours). Half of the chloroform was evopoarted on rota and remaining polymer containing chloroform solution was precipitated in 500 mL methanol, stirred for 2 hours, filtered on buchner funnel, washed with plenty of methanol and dried in vacuum. The polymer was obtained as a deep blue solid, Yield: 0.380 g (95%) 1H NMR (400 MHz, CDCl3) d ppm: 8.82 (br , 2H),7.33 (br, 4H), 4.11 (br, 4H), 1.98 (br, 2H), 1.24 (br 64H), 0.84 (br 12 H). (FTIR ATR, cm-1): 2919, 2923, 2857, 1699, 1658, 1586, 1501, 1432, 1386, 1325, 1232, 1181, 1146, 1094, 857, 808, 739, 678; GPC: Mn, 26.4 kDa; Mw, 152 kDa; Mw/Mn, 5.7.

Example 4: Synthesis of Poly {([N N’ -bis(2-octyldodecyl)-naphthalene-1,4,5,8- bis-(dicarboximide)-2,6-diyl]-alt-5,5 '-2,2'-bithiophene)-ran-([N,N'-bis(2-ethylhexyl)-1,7-dibromo-3,4,9,10-perylenediimide]-alt-5,5 '-2,2'-bithiophene)}
All the random copolymers i,e NDI-Th-PDI15, NDI-Th-PDI30 and NDI-Th-PDI50 were synthesized using same procedure as that given for P(NDI2OD-T2), but with different mole ratios of NDI-2OD-Br2 to PDI-2OD-Br2.

Example 5 : Synthesis of NDI-Th-PDI15
NDI-Th-PDI15 was synthesized using (0.199g,0.406 mmole) of 5,5'-bis (trimethylstannyl)-2,2'-bithiophene, (0.340 g,0.345 mmole) of NDI-2OD-Br2, (47 mg, 0.0609 mmole) of PDI-2EH-Br2 and (18 mg) of Pd(Ph3)2Cl2. Yield: 0.360 g (86%) 1H NMR (400 MHz, CDCl3) d ppm: 8.81 (br, 2H naphthalene aromatic), 8.73 (br,2H perylene aromatic), 8.37 (br, 4H perylene aromatic), 7.33 (br, 4H bithiophene), 4.10 (br, 8H), 1.97 (br, 4H), 1.24 (br 80H), 0.84 (br 24 H). (FTIR (ATR, cm-1): 2919, 2851, 1703, 1662, 1571, 1574, 1433, 1377, 1305, 1243, 1188, 1053, 965, 928, 787, 717; GPC: Mn, 16.4 kDa; Mw, 60.7 kDa; Mw/Mn, 3.7

Example 6: Synthesis of NDI-Th-PDI30
NDI-Th-PDI30 was synthesized using (0.250g,0.508 mmole) of 5,5'-bis (trimethylstannyl)-2,2'-bithiophene, (0.350g,0.355 mmole) of NDI-2OD-Br2, (117 mg, 0.152 mmole) of PDI-2EH-Br2 and (18mg) of Pd(Ph3)2Cl2. Yield: 0.450 g (88%) 1H NMR (400 MHz, CDCl3) d ppm: 8.81 (br, 2H naphthalene aromatic), 8.71 (br,2H perylene aromatic), 8.36 (br, 4H perylene aromatic), 7.31 (br, 4H bithiophene), 4.11 (br, 8H), 1.97 (br, 4H), 1.23 (br 80H), 0.84 (br 24 H). (FTIR (ATR, cm-1): 2919, 2852, 1702, 1660, 1572, 1513, 1433, 1401, 1306, 1243, 1186, 1174, 1043, 963, 927, 881, 853, 768, 713, 670; GPC: Mn, 16.8 kDa; Mw, 49.8 kDa; Mw/Mn, 2.9

Example 7: Synthesis of NDI-Th-PDI50
NDI-Th-PDI50 was synthesized using (0.250g,0.508 mmole) of 5,5'-bis (trimethylstannyl)-2,2'-bithiophene, (0.250g,0.245 mmole) of NDI-2OD-Br2, (0.197g, 0.245 mmole) of PDI-2EH-Br2 and (18mg) of Pd(Ph3)2Cl2
1H NMR (400 MHz, CDCl3) d ppm: 8.79 (br, 2H naphthalene aromatic), 8.69 (br,2H perylene aromatic), 8.35 (br, 4H perylene aromatic), 7.33 (br, 4H bithiophene), 4.11 (br, 8H), 1.96(br, 4H), 1.21 (br 80H), 0.81 (br 24 H). (FTIR (ATR, cm-1): 2921, 2852, 1700, 1660, 1587, 1433, 1401, 1374, 1313, 1244, 1188, 1036, 860, 712, 755, 715, 761; GPC: Mn, 12.6 kDa; Mw, 32.4 kDa; Mw/Mn, 2.5

Example 8: Fabrication and characterization of the photovoltaic cells
PTB7 or PTB7-Th as donor and P(NDI2OD-T2) and NDI-Th-PDIx (x = 15, 20, 30 mole% of PDI) acceptors were used. PTB7 or PTB7-Th was blended with each of the four acceptors separately in chloroform, and stirred at 40 °C for more than 24 h in a glovebox. The optimized donor:acceptor (D:A) ratio was 1.3:1 (w/w) and total concentrations of (D+A) in chloroform solution was ?12 mg/ml. The optimized volume fraction of 1,8-diiodooctane (DIO) additives to (D+A) in chloroform solution was 1.25 vol%. PFN (2 mg/ml) was prepared in methanol in the presence of a small amount of acetic acid (2 µl ml-1). Preparation of ZnO sol-gel was done as follows. Zinc acetate dihydrate [Zn(CH3COO).2H2O] (Aldrich, 99.9%) with 0.1 M concentration was first dissolved in anhydrous ethanol [CH3CH2OH] (99.5 + % Aldrich) and rigorously stirred for 2–3 h at 80 °C. Subsequently, ethanolamine was added to the solution as sol stabilizer followed by thorough mixing process with magnetic stirrer for 12-15 h at 60 °C. Inverted type all polymer solar cells were fabricated using an indium tin oxide (ITO)/ZnO/PFN/active layer/MoOx/Al structure. ITO-coated glass substrates were subjected to ultrasonication in soap, deionized water, acetone and in isopropyl alcohol. The substrates were then dried for several hours in an oven at 120 °C. The ITO substrates were treated with UV-ozone before ZnO sol-gel was spin-coated on the ITO-coated glass substrate with 3000 rpm for 60 sec. The ZnO films were annealed at 200 °C for 1 h in the air. The thickness of ZnO film was approximately 30 nm, as determined by a profilometer. PFN was spincoated on ITO at 2,000 rpm for 60 sec and baking for 15 min at 80 °C in N2 glove box. Then, each active blending solution was spin-casted onto an ITO/ZnO/PFN substrate at 2000 rpm for 120s. The final thickness of each films was 100?110 nm. Then, MoO3 (? 10 nm) was thermally deposited in high vacuum (?8 x 10-7 torr). Finally Al (?100 nm) was deposited in same high vacuum (?8 x 10-7 torr), over the MoOx through shadow mask. The active area of the devices was 10 mm2 in all the cases.
The photovoltaic performance of the devices was characterized using a solar simulator (SCIENCTECH SS150 Solar Simulators) with an air-mass (AM) 1.5 G filter. The intensity of the solar simulator was carefully calibrated using an AIST-certified silicon photodiode. The current-voltage behavior was measured using a Keithley 2400 SMU. EQE spectra of fabricated devices were measured using a Keithley 2600 source meter and a CEP-25ML Spectral Response Measurement System which shines light with AM 1.5 G spectral distribution and calibrated using an AIST-certified silicon photodiode to an intensity of 1000 W/m2.

Example 9: SCLC Measurement
The hole and electron mobilities of all-polymer blends and polymer neat films were measured by the space-charge-limited current (SCLC) method using hole only configuration ITO/MoOx (10 nm)/active layer (?100 nm)/Au (200 nm) structure and electron only configuration ITO/ZnO (30 nm)/active layer (?100 nm)//Al (100 nm). In both the cases, active layer was spin-casted exactly the same way as it was done in the case of photovoltaic cells discussed above. Current-voltage measurements in the range of 0-10 V were taken, and the results were fitted to a space-charge-limited function with active area of the devices as 10 mm2. The carrier mobility was extracted by fitting the J-V curves in the near quadratic region according to the modified Mott-Gurney equation: 37

Where J is the current density, e0 (8.85×1014 F/cm) is the permittivity of free space, e is the dielectric constant of the organic semiconductor (assumed to be 3.2), µ is the zero-field mobility, V is the applied voltage, L is the thickness of active layer, and ß is the field-activation factor. The current-voltage curves and SCLC fittings of the data are shown in Supporting Figure 12 and the bulk carrier mobilities are summarized in Table-4.

ADVANTAGES OF THE INVENTION
a. Novel polymeric material.
b. Material has higher molecular weight, improved crystallinity and better film formation capacity.

WE CLAIM
1. A copolymer composition comprising naphthalene diimide, perylene diimide and bithiophene in the ratio ranging between 1:1:2 to 1.5:0.5:2.
2. The composition as claimed in claim 1, wherein said composition is useful in All-polymer solar cells.
3. The copolymer composition as claimed in claim 1, wherein said naphthalene diimide is N, N’-Bis (2-octyldodecyl)-2,6-dibromo-1,4,5,8-naphthalenediimide (NDI-2OD-Br2).
4. The copolymer composition as claimed in claim 1, wherein said perylene diimide is N, N’-Bis (2-ethylhexyl)-1,7-dibromo -3,4,9,10-perylene tetracarboxylicdianhydride (PDI-2EH-Br2).
5. The copolymer composition as claimed in claim 1, wherein said bithiophene is 5,5'-bis (trimethylstannyl)-2,2'-bithiophene.
6. A solar cell comprising a blend of copolymer composition as claimed in claim 1 as an n-type semiconductor material with PTB7-Th as the p type polymer.
7. The solar cell as claimed in claim 6, wherein power conversion efficiency (PCE) of said solar cell is in the range of 2.00% to 5.03%.
8. The present invention provides a process for the preparation of copolymer composition comprising the steps of:
i. adding solvent to a mixture of naphthalene diimide, bithiophene and perylene diimide to obtain a reaction mixture;
ii. adding Bis(triphenylphosphine) palladium (II) dichloride (Pd(Ph3)2Cl2) to the reaction mixture of step (a) followed by degasssing and stirring the reaction mixture at temperature in the range of 90 to 95° Cfor the period in the range of 70 to 74 hrs to obtain a reaction mixture;
iii. adding solvent to the reaction mixture of step (b) with stirring, cooling, adding a solution of potassium fluoride in water and stirring followed by work-up to obtain the copolymer composition.

Dated this ___11th_______ day of _____July________ 2017.

Scientist
Innovation Protection Unit
Council of Scientific and Industrial Research