Fullerene Functionalized Thiophene Derivative as an Acceptor Material for Organic Photovoltaics

Organic photovoltaic devices (OPV) have become a most active research area due to their promising advantages such as ease of processing, low cost synthesis and clean-renewable energy sources [1]. Introduction of Bulk hetero junction solar cells (BHJ) in OPV leads to an improved power conversion efficiency (PCE), due to its interpenetrating network structure of active layer [2,3]. The active layer of BHJ solar cell is comprised of a donor and acceptor material. From the material point of view, the development of OPVs is mainly due to the development in the synthesis of new donor materials for the active layer. The progress in acceptor moiety is relatively low. Fullerene derivatives have gained much interest as an acceptor material due to its structural uniqueness and electronegative character [4].


Introduction
Organic photovoltaic devices (OPV) have become a most active research area due to their promising advantages such as ease of processing, low cost synthesis and clean-renewable energy sources [1]. Introduction of Bulk hetero junction solar cells (BHJ) in OPV leads to an improved power conversion efficiency (PCE), due to its interpenetrating network structure of active layer [2,3]. The active layer of BHJ solar cell is comprised of a donor and acceptor material. From the material point of view, the development of OPVs is mainly due to the development in the synthesis of new donor materials for the active layer. The progress in acceptor moiety is relatively low. Fullerene derivatives have gained much interest as an acceptor material due to its structural uniqueness and electronegative character [4].
Fullerenes and their derivatives exhibit non linear optical properties [5,6]. The photo physical properties of the fullerenes can be tailored by the chemical modification of the buckyball [7][8][9][10]. Fullerene as such cannot be used as an acceptor material for OPVD due to its less solubility in common organic solvents. [6,6]-phenyl-C 60 -butyric acid methyl ester (PC 60 BM)is one of the chemically modified fullerenes gained much interest as the acceptor material in OPV due to its low lying lowest unoccupied molecular orbital (LUMO), high electron mobility and compatibility with most of the donor materials, which are soluble in most of the common organic solvents [11]. Even though these fullerene derivatives are well explored in the field of OPV, researchers have been trying to replace PC 60 CBM with its higher fullerene analogue C70PCBM [12,13] in order to improve the efficiency of the devices. More over these fullerene derivatives are also used in organic field effect transistors [14].
The main drawback of C 60 PCBM is its insufficient absorption in the visible region. This arises due to the structural symmetry of the C60PCBM which forbids low-energy transitions. One of the strategy to overcome these drawbacks is functionalization of fullerene to extend the absorption spectra to UV-V is region [15,16]. Several approaches have been introduced to modify C60PCBM chemically such as substitution of phenyl group to other aromatic groups or varying alkyl chain length etc [10,17]. Other than these C60PCBM like molecules, indene-fullerenes, endohedrall fullerenes etc. have been synthesized for various organic electronics applications [16,18]. Kim et al. [19] reported carbazole substituted fullerenes such as fulleropyrrolidine derivative.
Fulleropyrrolidine derivatives can be used for OPV application due to their high electron affinity and good charge transportability [20,21]. Here in this work fulleropyrrolidine derivative of 4, 7-di (thiophen-2-yl)-2, 1, 3-benzothiadiazole (DTBT) was synthesized by 1, 3-dipolar cycloaddition reaction. DTBT units were widely used as electron accepting units in organic electronics. Functionalization of DTBT with fullerene could result a strong acceptor unit that can be used for OPV application. Structural, optical, and electrical characterization of the functionalized DTBT was carried out and photovoltaic property was evaluated by fabricating a PV device with P 3 HT as the donor.

Experimental section
Materials O-phenylenediamine, Thiophene, tributylchlorostannane, Tetrakis (triphenylphosphine) palladium (Pd(PPh3)4), N-methylglycien, triphenylphosphine and tetrabutylammoniumhexafluorophosphate (TBAPF 6 ) were purchased from Sigma Aldrich and used without further purification. n-Bun-BuLi solution (1.6M in hexane) was purchased from across chemicals and used as such. Solvents and other reagents were purchase locally and distilled before use.

Synthesis
Synthesis of 2,1,3-benzothiadiazole: o-phenylenediamine (1g, 9.2472mmol) and triethylamine (3.7g, 36.9890mmol) in 30mL DCM were taken in a round bottom flask. 2.2 gram (18.4950mmol) of thionyl chloride was added drop wise to the reaction mixture at 0 °C. Temperature of the reaction mixture was maintained at 0 °C for 30min, followed by refluxing for 5h at 50 °C. The solvent was removed using a rotary evaporator and quenched the reaction mixture with DI water. A pH of 1 was achieved using concentrated hydrochloric acid (HCl). Product was extracted with dichloromethane (3×50mL) and dried over sodiumsulphate (NaSO 4 ). Further the product was purified by column chromatography using hexane/ethyl acetate as eluting solvent. Yield= 87% ( Figure 1).

Synthesis of 4, 7-dibromo-2, 1, 3-benzothiadiazole:
A solution of Br 2 in HBr (5mL) was added slowly to a mixture of 2, 1, 3-benzothiadiazole (0.5g, 3.6717mmol) and 10mL of HBr (48%) at 0 °C. After stirring the reaction mixture at room temperature for 3h, refluxed for 6h at 110 °C, reaction mixture was cooled to room temperature and excess Br 2 was removed by saturated solution of sodium bisulphite. The mixture was filtered and the precipitate was washed with copious amount of DI water and cold diethyl ether. The purification of the product was carried out by recrystallization from ethanol. Yield=81% (Figure 2).

Synthsis of tributyl (thiophen-2-yl) stannane:
Thiophene (2.0g, 23.7699mmol) and 15mL freshly distilled dry THF were taken under inert atmosphere. 16.2mL (26.1469mmol) of n-BuLi 1.6M in hexane was added drop wise at 0 °C and for 1h at 0 °C. The reaction mixture was then cooled down to -78 °C and tributylchlorostannane (6.4mL, 23.7699mmol) was added. The mixture was stirred for another 12h at room temperature. The reaction mixture was quenched with 100mL of ice cooled water, and the organic layer was extracted with diethyl ether (3×30mL). The solvent was removed from the combined organic layer by rotary evaporation and obtained the product as brown oil (88%). The crude product was used in next step without further purification ( Figure 3).

Res Dev Material Sci
5.3593mmol) were taken in dry toluene (15mL). After purging the reaction mixture for 10 minutes with nitrogen, catalyst Pd (PPh3)4 (4mol%) was added. The reaction mixture was heated to 80 °C and stirred for 24h. After cooling to room temperature, the reaction mixture was poured in to cold water (100mL). The organic layer was extracted with dichloromethane (3×50mL) and the product was purified by column chromatography. Yield (72%) (Figure 4).

Functionalization of fullerene on DTBT (F-DTBT):
5-[7-(thiophen-2-yl)-2,1,3-benzothiadiazol-4-yl]thiophene-2-carbaldehyde (0.3g, 0.0913mmol) was added to a dried two neck round bottom flask containing 40mL of chlorobenzene. The solution was purged with nitrogen gas for 10 min. C 60 (1.96g. 0.2739mmol) and N-methylglycien (0.82g, 0.9134mmol) was then added to the solution. The system was further purged with nitrogen gas for another 15 min. The reaction mixture was heated to reflux for 24h. After cooling the system to room temperature, chlorobenzene was removed using rotary evaporator under reduced pressure. The crude product was purified by column chromatography on silica gel using toluene and carbon disulfide as the eluting solvents ( Figure  6).  Optical studies of the F-DTBT were carried out using UVvisible spectroscopy. Absorption spectrum of F-DTBT in dilute chlorobenzene solution is as shown in (Figure 7). An absorption peak is observed at about 320nm, which is the common absorption peak for C 60 molecules. The absorption peak at about 425nm has been observed after the incorporation of DTBT on fullerene. This absorption hump suggests that functionalization of PC 60 BM with DTBT resulted in the broadening of absorption spectra. Optical band gap of the molecule is obtained from the absorption onset and is determined as 2.38eV.

Results and Discussion
Electronic structure of F-DTBT was determined using cyclic voltammetry (CV). CV experiment was carried out in acetonitrile solvent using TBAPF 6 as the supporting electrolyte. Cyclic

Research & Development in Material Science
voltammogram of F-DTBT is as shown in (Figure 8). F-DTBT shows three quasi reversible one electron reduction waves during negative sweep. The reduction peaks are observed at about -1.10V, -1.44V and -1.77V. By considering the onset of first reduction, LUMO energy level was calculated. The onset of first reduction potential of F-DTBT is observed at -0.80V with a LUMO level of -3.6eV. HOMO level of F-DTBT was calculated by adding the optical band gap with LUMO level and is determined as -6.16eV. It is reported that, the HOMO-LUMO levels of C 60 PCBM is -6.1eV and-3.78eV respectively. Hence, F-DTBT has a deeper HOMO level and higher LUMO level than that of C60PCBM. This higher LUMO of DTBT functionalized fullerene will results in well energy level matching with the donor molecules of high HOMO. Thermal properties of F-DTBT were studied by thermo gravimetric analysis. TGA thermogram of F-DTBT is as shown in (Figure 9). 10% weight loss of F-DTBT occurs at about 315 °C. BHJ solar cells were fabricated using F-DTBT as the acceptor material and P 3 HT as the donor material for active layer. The configuration of the fabricated solar cell is ITO/PEDOT: PSS/P 3 HT: F-DTBT (1:1 wt%)/Al ( Figure 10). The transparent ITO coated glass plates were cleaned by ultra sonication with acetone, DI water and isopropanol respectively. About 30nm of PEDOT: PSS was spin coated on the cleaned ITO glass plates at 5000rpm for 60s. These spin coated samples were then annealed at 150 °C for 30 min on a hot plate. Active material for the device was obtained by dissolving P 3 HT and F-DTBT in chlorobenzene (15mgmL -1 , 1:1 weight ratio) with overnight stirring. The active material was spin coated at 1500rpm for 10s and dried at 130 °C for 15min. Aluminum electrodes were deposited on the active layer using a multi source thermal evaporator at a base pressure of 5×10 -6 Torr.     AFM image of the top surface of a BHJ film on ITO substrate is as shown in (Figure 11). It shows a mean roughness of 4.3nm. The bright features in this phase image, which correspond to bumps in topography, indicate that FDTBT-rich domains are present at the top surface. It also shows that it is not a homogeneous dispersion having a mean diameter of 92±33 nm which could impact the performance of the device. For an efficient device, acceptor material should form a homogeneous dispersion on the surface such that can be in contact with the cathode for efficient extraction of electrons from the device. This may be one of the reasons for the moderate performance of the device. PV characteristics of the fabricated device are as shown in (Figure 12 & 13). Short circuit current density and open circuit voltage of the device are 6.1mAcm-2 and 0.61V respectively. Fill factor of the fabricated PV device is determined as 29%. Power conversion efficiency of the fabricated Res Dev Material Sci photovoltaic device is obtained as 1.1%. Further optimization of device architecture using hole transport layers and exciton blocking layers will enhance the performance of the solar cells.

Conclusion
A functionalized fullerene derivative, F-DTBT was designed and synthetic routes were developed. This thermally stable fullerene derivative shows absorption peaks around 320 and 425nm. In addition to the C 60 absorption peak, another absorption peak at 425 is observed after the functionalization. Electrochemical studies show that, these functionalized materials have higher LUMO levels compared to the C 60 . BHJ solar cells were fabricated using F-DTBT as the acceptor and P 3 HT as the donor materials and PV parameters are evaluated. With F-DTBT as an acceptor material, PCE of 1.10% with a V oc of 0.61V, a J sc of 6.1mAcm -2 and a fill factor of 29% were obtained.