Effective D-A-D type chromophore of fumaronitrile-core and terminal alkylated bithiophene for solution-processed small molecule organic solar cells

A new and novel organic π-conjugated chromophore (named as RCNR) based on fumaronitrile-core acceptor and terminal alkylated bithiophene was designed, synthesized and utilized as an electron-donor material for the solution-processed fabrication of bulk-heterojunction (BHJ) small molecule organic solar cells (SMOSCs). The synthesized organic chromophore exhibited a broad absorption peak near green region and strong emission peak due to the presence of strong electron-withdrawing nature of two nitrile (–CN) groups of fumaronitrile acceptor. The highest occupied molecular orbital (HOMO) energy level of –5.82 eV and the lowest unoccupied molecular orbital (LUMO) energy level of –3.54 eV were estimated for RCNR due to the strong electron-accepting tendency of –CN groups. The fabricated SMOSC devices with RCNR:PC60BM (1:3, w/w) active layer exhibited the reasonable power conversion efficiency (PCE) of ~2.69% with high short-circuit current density (JSC) of ~9.68 mA/cm2 and open circuit voltage (VOC) of ~0.79 V.


Results and Discussion
The thermal stability of the synthesized organic chromophore, RCNR has been analysed by the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under N 2 atmosphere. The TGA plot (Fig. 2) reveals that RCNR starts to decompose over ~300 °C. The decomposition temperature (T d ) of the RCNR is found as ~368 °C, indicating a relatively high thermal-stability of the organic chromophore which is expedient for the solution-processed device fabrication and the operation of organic solar cells 29 . From differential scanning calorimetry (DSC) measurement (Fig. 2 inset), the RCNR show numbers of melting phase transitions (T m ) at ~69 °C, ~161 °C, and ~172 °C, with no signs of a glass-transition temperature (T g ), while an isotropic transition phase is observed after ~270 °C 30 . The increase in the thermal transition temperatures is an indication of enhanced intermolecular connectivity and thin film crystallinity in RCNR, which is attributed to the presence of induced π − π stacking 31 . The difference in film crystallinity is an important factor for solution-processed organic solar cells, as it shows a direct effect on the surface roughness of the thin film morphology and consequently, the solar cell device performance 32 . The presence of terminal alkyl chains of organic chromophore induces the solubility in common organic solvents. Additionally, different melting transitions suggest the occurrence of various liquid-crystalline (LC) phases of RCNR via self-assembly behavior 33 . Generally, self-assembly behavior is the result of electrostatic interactions which might be due to the result of π − π staking and hydrogen-bonding ability of the organic conjugated molecules 34,35 . This clearly indicates the interconversion of different LC phases from smectic C to smectic A to nematic phase as a function of temperature 36,37 .
UV-Vis absorption spectra (Fig. 3a) of RCNR have displayed a good absorption in dilute chloroform solution (1 × 10 −5 M) and thin film state. In chloroform solution, the two distinct peaks are observed. The spectra shows a relatively small absorption peak at λ max ≈ 368 nm and another broad absorption peak at λ max ≈ 465 nm. The molar absorption coefficient (ε ) in solution is calculated as ~1.58 × 10 4 M −1 cm −1 which indicates a strong intramolecular charge transfer (ICT) interaction behavior between thiophene donor and fumaronitrile-acceptor 38,39 . However, a slight red shift with broad absorption spectrum (Table 1) is observed for the chromophore in the solid thin film as compared to chloroform solution which might be due to an aggregation in the solid thin film state 40 . RCNR indicates an ordered and planar structure due to the presence of alkyl side chains, resulting in a good intermolecular electron-delocalization and  hence, evolves the self-assembly behavior 41,42 . Moreover, an optical band gap (E g opt ) of ~2.03 eV is calculated by the absorption edge (λ edge ) from solid thin film absorption by the formula: The photoluminescensce spectra (Fig. 3b) of the synthesized organic chromophore has shown a good potential of light emitting properties in solution as well as solid thin film state. A single storng green emission peak at ~649 nm is recorded in chloroform solution at the room temperature which shows a slight red-shift in thin film spectra. This strong emission of RCNR is due to the intramolecular planarization or aggregation of organic chromophore 43 . It clearly indicates the fluorescence quenching after mixing with PC 60 BM acceptor, suggesting the electron transfer from donor to acceptor and the fast charge-transfer which is enough to compete with the radiative recombination of the excitons 44,45 .
The redox properties of the organic chromophore are measured by cyclic voltammetry (CV) studies of RCNR thin film (Fig. 4) 6 ] − at a potential scan rate of 100 mV/s. The oxidation and reduction peaks are situated at the onset value of E ox = + 1.42 ± 0.02 eV and E red = -0.86 ± 0.02 eV. Hence, the RCNR solid thin film exhibits HOMO and LUMO of -5.82 eV and -3.54 eV, respectively. The observed electrochemical band gap is found to be E g el = 2.28 eV. The difference of HOMO and LUMO energy level is a crucial factor for determining the energy band gap which indicates the electrons delocalization in the solid thin films 46,47 .
Solution-processed BHJ small molecule organic solar cells are fabricated using RCNR as an electron-donor and [6,6]-phenyl C 61 -butyric acid methyl ester (PC 60 BM) as an electron-acceptor with a standard device structure of ITO/PEDOT:PSS (~80 nm)/RCNR:PC 60 BM blend (~60 nm)/Ag (~100 nm). The blended active layers of the solar cell devices are developed by spin-casting the various (1:1, 1:2, 1:3, 1:4, w/w) mixtures of the RCNR with PC 60 BM. The photovoltaic properties (Table 2) of the fabricated solar cell devices of RCNR have been examined by the current density (J)-voltage (V) measurements (Fig. 5) under the 1 sun light (100 mW/cm 2 , 1.5 AM). The PCE of ~2.69% is achieved by the SMOSC devices fabricated with RCNR:PC 60 BM (1:3, w/w) active layer ratio, whereas the other fabricated SMOSC devices exhibit inferior PCEs of ~1.50% for RCNR:PC 60 BM (1:1, w/w), ~2.0% for RCNR:PC 60 BM (1:2, w/w) and ~2.23% for RCNR:PC 60 BM (1:4, w/w) active layer ratios. The SMOSC fabricated with RCNR:PC 60 BM (1:3, w/w) active layer presents the J SC of ~9.68 mA/cm 2 , and high V OC of ~0.792 V. Herein, the presence of -CN groups connecting with vinyl double bond enhances the conjugation length of chromophore and hence, better electron-delocalization which might affect the open-circuit voltage and short-circuit  density of the solar cell devices 8,48 . Moreover, the presence of the terminal side chains has a strong impact on the aggregation and self-organizing behavior of the electron-donor molecules in BHJ thin films and hence, increases the photocurrent-density of the devices due to better charge transport 49 . The thin film morphology of the devices might be related to the lowering of the V OC value at low concentration ratios (1:1, 1:2, w/w) of RCNR in the blended active layers. The atomic force microscopy (AFM) analysis is used to investigate the morphological behavior of the blended active layer RCNR:PC 60 BM (1:1, 1:2, 1:3, 1:4, w/w) thin films, as shown in Fig. 6. The RCNR:PC 60 BM (1:3, w/w) blended active layer ( Fig. 6(e,f)) clearly exhibits a homogeneous and smooth morphology of low root-mean-square surface roughness (R rms = 2.06 nm) in nanoscale phase separation which contributes to good miscibility of donor-acceptor, high exciton-dissociation rate and better charge transport. On the other hand, other blended active layers of RCNR:PC 60 BM (1:1, 1:2 and 1:4, w/w) record high R rms values of 9.20 nm, 2.63 nm, 3.29 nm, respectively. These results show that the RCNR:PC 60 BM (1:3, w/w) active layer is the optimized one for homogeneous miscibility between donor and acceptor yielding a smooth thin film morphology and a large donor-acceptor (D− A) interface area with nanoscale phase separation 33 and high exciton-dissociation rate, which eventually assists to achieve the best performance of organic solar cell devices. In addition, the morphological analysis reveals that RCNR:PC 60 BM (1:3, w/w) film depicts the lowest R rms of ~2.06 nm as compared to other blended RCNR:PC 60 BM (1:1, 1:2 and 1:4, w/w) active layers, suggesting the homogenous nature of RCNR and PC 60 BM molecules in the blended layer which provides enough surface area for exciton-dissociation 40 . For all the fabricated SMOSCs devices, the fill factor (FF) value is rather low due to a number of factors like unfavourable domain size, film morphology, misalignment of energy levels, large series-resistance, etc 44 . The active layer RCNR:PC 60 BM (1:3, w/w) device shows a minimum series-resistance and hence, a maximum FF of ~0.35. Furthermore, the lower values of FF are related to increase in the series-resistance of RCNR:PC 60 BM/ ITO in SMOSCs, resulting in a higher recombination rate over the surface of RCNR:PC 60 BM blended active layers 6 . Due to spontaneous phase-segregation process in the blended active layers of RCNR and PC 60 BM, a bicontinuous network structure might form which creates the percolation channels for the efficient charge carrier collection within the active layer of BHJ solar cells 50 . On the other hand, the improvement in the V OC value might be due to the presence of two -CN groups which induce the better film morphology and strong intermolecular charge-transfer (ICT) between RCNR and PC 60 BM 9,51 . Thus,   Table 2. Summary of J-V curves of the fabricated SMOSCs. the presence of two strong electron-withdrawing -CN groups might have electrostatic-attractions with PC 60 BM which improves the film-morphology of the blended active layers and ultimately increases the photocurrent-density for the better performance of solar cell devices 52 .

Experimental Methods
Instruments. Unless otherwise noted, the chemicals and reagents were purchased from commercial sources as Sigma-Aldrich, Alfa-aesar and TCI chemical companies and used as received. Thin layer chromatography (TLC) was performed on Merck TLC-plates of aluminum coated with silica gel 60 F254. Flash column chromatography was performed on a column packed with silica gel (300-400 mesh).

1-(5-(thiophen-2-yl) thiophen-2-yl) hexan-1-one (2).
Hexanoyl chloride (4.07 mL, 20.0 mmol) was added to a solution of 2,2'-bithiophene 1 (3.17 g, 19.1 mmol) in anhydrous benzene (20 mL) at the room temperature. Then TiCl 4 (2.25 mL, 20.5 mmol) was added slowly to the reaction mixture at 0°C and was stirred for 15 min at 0 °C. After completion of the reaction, cold water was added into the reaction mixture to quench the reaction. The resulting mixture was diluted with CH 2 Cl 2 (50 mL), washed successively with water (200 mL) and saturated aqueous solution of NaHCO 3 (100 mL), then dried over MgSO 4 followed by an evaporation under vacuum to afford a yellow solid (5.00 g, 85%), anticipated as the desired ketone intermediate 2 which was used directly for next step of the reaction.

2,3-bis(4-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)phenyl)fumaronitrile (RCNR).
In a 50 mL round bottom flask, monomer 5 (0.46 g, 1.22 mmol) and monomer 7 (0.198 g, 0.51 mmol) with triphenylphosphine (0.034 g, 0.03 mmol) were mixed and then subjected to three cycles of evacuation and nitrogen purging in anhydrous toluene (~10 mL) solvent. Aqueous solution of potassium carbonate (2 M, ~5 mL) was added by syringe to the reaction mixture and was stirred at 110 °C for 12 h. The reaction mixture was cooled down to the room temperature followed by the addition of water. Subsequently, an organic phase was extracted with dichloromethane (~20 mL) and the reaction mixture was washed with brine and distilled water and dried over magnesium sulfate. The solution was filtered and evaporated in vacuum to achieve a red colored residue, which was then recrystallized several times in dichloromethane and methanol Device fabrication. For the fabrication of SMOSCs, the indium tin oxide (ITO) glass substrate was first cleaned with detergent, ultrasonicated in water, acetone and isopropyl alcohol and subsequently dried overnight in an oven. PEDOT:PSS thin film (thickness ~80 nm) was coated on ITO substrates by spin-coating the solution with a speed of ~4000 rpm for 40 s and thereafter, annealed at 130 °C for 10 min in a vacuum oven. The active RCNR:PC 60 BM layer (thickness ~60 nm) with w/w blending ratio of 1:1, 1:2, 1:3 or 1:4 in o-dichlorobenzene solution (10 mg/ml) was again spin-coated on PEDOT:PSS film-coated ITO at a scan rate of ~700 rpm for 40 s. The fabricated active layer was heated at 80 °C for 10 min to evaporate the residual solvent. Finally, the silver cathode (thickness ~100 nm) was deposited through a shadow mask by thermal evaporation under a vacuum of about 3 × 10 -6 Torr. The active area of device was measured as ~1.5 cm 2 . The photovoltaic properties of the cells were measured under simulated AM 1.5 radiation at 100 mW/cm 2 using 1000 W metal halide lamp (Phillips) which was served as a simulated sun light source and its light intensity (or radiant power) was adjusted with a Si photo detector fitted with a KG-5 filter (Schott) as a reference, calibrated at NREL (USA). The power conversion efficiency (η) is calculated by the following equation: where J SC is the short-circuit photocurrent density, V OC is the open-circuit voltage, FF is the fill factor, and P in is the incident radiation power.

Conclusions
A novel, symmetric D-A-D type fumaronitrile-acceptor based organic π -conjugated chromophore (RCNR) is synthesized and applied as an electron-donor material for the solution-processed fabrication of SMOSCs. The synthesized organic chromophore presents a broad absorption peak near green region and strong emission peak due to the presence of two strong electron-withdrawing − CN groups. The cyclic voltammetry study of RCNR shows relatively deep HOMO of − 5.82 eV and LUMO of − 3.54 eV, which suggests a strong electron-accepting tendency of -CN groups. The fabricated SMOSC device of active layer RCNR:PC 60 BM (1:3, w/w) achieves a reasonable PCE of ~2.69% with J SC of ~9.68 mA/cm 2 and V OC of ~0.79 V. The variation in the concentration of PC 60 BM acceptor in blended active layers has considerably affected the thin film morphology and hence, the performance of the fabricated solar devices.