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Efficient Solution Processable Polymer Solar Cells Using Newly Designed and Synthesized Fullerene Derivatives Surya Prakash Singh, Chittanuru Pavan Kumar, Puvvala Nagarjuna, Jaipal Kandhadi, Lingamallu Giribabu, Malapaka Chandrasekharam, Subhayan Biswas, and Ganesh D. Sharma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03124 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 7, 2016

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The Journal of Physical Chemistry

Efficient Solution Processable Polymer Solar Cells Using Newly Designed and Synthesized Fullerene Derivatives Surya Prakash Singh1*, CH Pavan Kumar#1, P. Nagarjuna#1, Jaipal Kandhadi M.Chandrasekharam1, S. Biswas2, G. D. Sharma*2 1

1

L. Giribabu1,

Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,

Hyderabad -500007, India E-mail: [email protected] 2

Department of Physics, The LNM Institute of Information Technology (Deemed University),

Jamdoli, Jaipur (Rajasthan) 302031, India

# Both contributed equally Abstract Two modified PC60BM and PC70BM fullerene derivates denoted as C60DAM and C70DAM, respectively, were synthesized and characterized. Through the structural modification, the lowest unoccupied molecular orbital (LUMO) energy level of these fullerene derivatives can be raised as much as 0.15 eV with respect to their parent fullerene derivatives which is possibility ascribed to their electron donating nature of substituted moiety. The BHJ polymer solar cells (PSCs) fabricated with poly (3-hexylthiophene) (P3HT) as electron donor and either C60DAM, or C70DAM as electron acceptors showed both higher open circuit voltage (Voc) and short circuit current (Jsc), resulting enhanced power conversion efficiency (PCE) (3.23 % for P3HT:C60DAM and 4.45 % for P3HT: C70DAM) as compared to corresponding PC60BM. The enhanced Jsc and higher Voc values have been ascribed to the stronger absorption in visible region and raise in LUMO level of the modified fullerene derivatives compared to their pristine fullerene derivatives. The BHJ PSC with P3HT:C70DAM blend cast from high boiling point chloronaphthalene (CN), 1,8 diiodoctane (DIO) and 1,8octanedithiol (OT) as additive showed PCE of 5.63 %, 4.75 % and 4.95 %, respectively. Additive treated P3HT:C70DAM interpenetrating network of blend and strong absorption may be the reason in enhancement of PCE.

1. Introduction

Bulk heterojunction (BHJ) polymer solar cells is an emerging photovoltaic technology with bright properties like high efficiency, solution processability, lightness, and low processing

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costs.1-7 BHJ solar cells consist of photoactive blend of a light harvesting conjugated polymer (donor) and an electron accepting fullerene derivative (acceptor). The excitons generated by absorption of light and dissociate at D/A interface into holes and electrons then travel towards the opposite electrodes to generate the photocurrent.8-12 During the last few years the PCEs of BHJ PSCs have been rapidly improved largely owing to development of a wide variety of donors and acceptors with enhanced absorptions in the visible region.13-19 Brabec et al demonstrated to obtained high efficiency, donor materials should have band gap in the range of 1.2 eV to 1.8 eV and between donor and acceptor LUMO offset should be ca, 0.3 eV with an external quantum efficiency of over 65 %.20 Among the fullerene materials investigated so far, two methanofullerene derivatives of [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM) and [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM ) derivates widely used in BHJ solar cells. These fullerene shows high electron affinity and excellent electron accepting properties for efficient electron transport in the active layer of BHJ.21-24 The BHJ solar cells prepared from the blend of poly (3hexylthiphene) (P3HT) and PC60BM showed PCE around 5% by solvent annealing and thermal annealing.25-27 However, further improvement in the PCE of P3HT: PCBM based BHJ is challenging because i) mismatch of its absorption with solar spectrum ii) weak absorption of PCBM derivatives in visible region and iii) low open circuit voltage (Voc). Much efforts has been made towards improving the PCE of PSCs in terms of development of conjugated polymers as donor materials and fullerene derivatives as acceptor materials. In context of best electron acceptor materials, the main characteristic features are expected to be strong absorption in visible region complementary with P3HT donor28-32, good solubility in for solution processable devices33-36, to enhance the Voc suitable redox potentials of donors and acceptors materials are necessary.37-46 A series of high Voc P3HT-based devices recently reported using fullerene derivatives having high LUMO energy level as electron acceptor.47 A PCE of about 5.37 % and 6.48 % have been achieved for the PSCs based on P3HT as electron donor and dihydronaphthyl-based [C60] fullerene bisadduct derivative (NC60BA) and indene-C60 bis adduct (IC60BA) electron acceptors, respectively.48-49 The main difficult part is the synthesis of bis-adducts of fullerenes because it contain mixture of regio-isomers which are not separable. Molecularly engineered fullerenes for BHJ PSCs have been recently reviewed.50 Therefore, it is important to design and synthesize new fullerene acceptors with stronger absorption in visible region and higher lying LUMO energy level for efficient PSCs. A new regioselective 1,2,3-bisazfulleroid: doubly N-bridged bisimino-PCBMs have been reported with less symmetrical and strong absorption at UV-region

compare to mono 2

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functionalized fullerene derivatives.51-54 Herein, we report the synthesis of two new modified fullerene derivatives, PC60BM denoted as C60DAM and PC70BM denoted as C70DAM which strongly absorbs light in the range 350-700 nm. We have done the molecular engineering of PC60BM and PC70BM by maintaining its structural characteristics. The chemical structures are shown in the Figure 1 and detailed synthetic path displayed in Scheme1. 2. Experimental details 2.1 Materials: Catechol, 2-ethylhexylbromide, Aluminium trichloride, para-tolunesulfonylhydrazide, toluene, Dichloromethane (DCM), N,N-dimethylformamide (DMF),1, 2-dichlorobenzene (ODCB),Poly(3-hexyl thiophene-2,5-diyl) (P3HT) fullerene-60 (C60) and fullerene-70 (C70) were purchased from Sigma Aldrich and used without further purification. 1,2-bis(2-ethylhexyloxy)benzene (1) : Catechol (1 g, 9.08 mmol) was dissolved in dry DMF (20 ml), K2CO3 (5.01 g, 36.32 mmol) was added and stirred for 1hr at RT. Then 2-ethyl hexyl bromide (4.03 mL 22.7 mmol) was added drop wise. The reaction mixture was stirred for 16hr at room temperature, the solvent was evaporated. To the resulting residue H2O was added (25 ml) and extracted with EtOAc. The organic layer was dried over Na2SO4. The solvent was removed in vacuo and the residue separated with flash chromatography (silica gel; hexane) to give the expected product. Yield: 2.5gm (82.5%). 1H NMR (CDCl3, 300 MHz): δ 6.84 (br s, 4H), 3.84 (m, 4H), 1.80 (m, 2H), 1.56-1.28 (m, 16H), 0.91 (m, 12H). 13C NMR (CDCl3, 300 MHz)- 149.58,120.84,113.83, 71.55, 39.65, 30.66, 29.20, 23.97, 23.15,14.16, 11.22. MS:(M+H)+ : 334 Methyl 4-(3,4-bis(2-ethylhexyloxy)benzoyl) benzoate (2) : Terephthalic acid monomethyl ester chloride (528 mg, 2.69 mmol) was dissolved in dry CH2Cl2 (12 mL). Aluminium trichloride (398 mg, 2.989 mmol) was added in portions at 0 0C. Subsequently, 1,2-bis(2ethylhexyloxy)benzene (1 g, 2.989 mmol) in CH2Cl2 (10 mL) was added. Then the reaction mixture was stirred for 16hr at room temperature. The reaction mixture turned reddish brown. To the resulting residue H2O was added and extracted with DCM. The organic layer was dried over Na2SO4. Then the solvent was evaporated in vacuo. The product was purified by column chromatography (silica gel; ethyl acetate: hexane = 20: 80) yielded 1.2 g (81%) pure product. 1H NMR (CDCl3, 300 MHz): δ 8.14 (d, 2H), 7.79 (d, 2H), 7.46 (s, 1H), 7.32 (dd, 1H), 6.88 (d, 1H), 3.96 (s, 3H),3.95-3.90 (m, 4H), 1.80 (m, 2H), 1.56-1.28 (m, 16H), 0.91 (m, 12H).

13

C (CDCl3, 300 MHz)- 194.44, 165.97, 153.61, 148.90, 141.97, 132.14, 129.27,

128.97, 128.73, 125.11, 113.15, 110.59, 71.05, 51.93, 39.04, 30.15, 28.66, 22.59, 13.60, 10.74. FT-IR (KBr): 2955, 2928, 2870, 1723, 1599, 1517. MS:(M+H)+ : 334. 3 ACS Paragon Plus Environment


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Methyl4-{(3,4-bis(2-ethylhexyloxyphenyl)(2-tosylhydrazono)-methyl}benzoate

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(3):

Methyl 4-(3,4-bis(2-ethylhexyloxy)benzoyl)benzoate (500 mg, 1.008 mmol) and para-toluene sulfonyl hydrazide (470 mg, 2.52 mmol) were dissolved in methanol (10 mL). The resulting mixture was heated under reflux and reaction was monitored by TLC. After 10hr the conversion almost completed. After cooling to room temperature the solvent was evaporated in vacuo. To the resulting residue H2O was added and extracted with EtOAc. The organic layer was dried over Na2SO4. Purification was enforced by column chromatography (silica gel; EA: Hexane = 40:60) yielded 500 mg (89%) of the title compound. 1H NMR (CDCl3, 300 MHz): δ 7.95 (d, 2H), 7.85 (d, 2H), 7.80 (s, 1H), 7.54 (d, 2H), 7.34 (d, 2H), 6.96 (d, 1H), 6.64 (d, 1H), 6.54 (s, 1H), 3.98-3.90 (m, 5H), 3.76 (m, 2H), 2.43(s, 3H), 1.80 (m, 2H), 1.57-1.29 (m, 16H), 0.90 (m, , 12H). 13C NMR (CDCl3, 300 MHz)- 166.66, 153.31, 150.90, 150.37, 144.26, 140.85, 135.48, 130.88, 129.72, 129.40, 127.91, 127.59, 121.98, 120.84, 113.45, 112.54, 71.68, 71.27, 52.25, 39.50, 30.60, 29.12, 23.88, 23.09, 21.64, 14.08, 11.22. MS:(M+H)+: 666, FT-IR (KBr): 2928, 2854, 1725, 1509 Methyl 4-{(3,4-bis(2-ethylhexyloxyphenyl)-C60} benzoate:(C60DAM) Methyl 4-{(3,4-bis(2-ethylhextloxyphenyl)(2-tosylhydrazono)-methyl}benzoate (200 mg, 0.300 mmol) was dissolved in dry pyridine (3.0 mL) under nitrogen and sodium methoxide (17 mg, 0.300 mmol) was added. The mixture was stirred for 30 min. at room temperature. A solution of (130 mg, 0.180 mmol) of C60 in 10 mL of dry 1, 2-dichlorobenzene (ODCB) was added. The resulting mixture was stirred at 80 0C for 24 h. cooling to room temperature was followed by removal of solvent in vacuo and purified by column chromatography (silica gel; EA: Hexane = 5:95). First fraction was unreacted C60. The second fraction containing brown color was collected and solvent was evaporated. This compound dissolved in toluene and refluxed for 24h for isomerisation. The resulting brown solution was concentrated in vacuo yielded 100 mg (25%) of the title compound. 1H NMR (CDCl3, 300 MHz): δ 8.2 (br s, 4H), 7.61 (m, 2H), 6.98 (d, J=8.0, 1H), 3.98-3.88 (br m, 7H), 1.80 (m, 2H), 1.56-1.28 (m, 16H), 0.91 (m, 12H). FT-IR (KBr): 2951, 2920, 2853, 1721, 1604, 1507, 1458, 1426, 1378, 1271, 1180, 1105, 1018. Anal. Cal. C91H44O4C, 90.98, H, 3.69 Found- C; 87.00, H; 4.09. Methyl 4-{(3,4-bis(2-ethylhexyloxyphenyl)-C70} benzoate: (C70DAM) Methyl 4-{(3,4-bis(2-ethylhextloxyphenyl)(2-tosylhydrazono)-methyl}benzoate (200 mg, 0.300 mmol) was dissolved in dry pyridine (3.0 mL) under nitrogen and to this sodium methoxide (17 mg, 0.300 mmol) was added. The mixture was stirred for 30 min. at RT. A solution of (151 mg, 0.180 mmol) of C70 in 1, 2-dichlorobenzene (ODCB) (10 mL) was added. The resulting mixture was stirred at 80 0C for 24 h, cooled to RT, followed by removal 4 ACS Paragon Plus Environment

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of solvent in vacuo and purified by column chromatography (silica gel; EA: Hexane = 5:95). First fraction was unreacted C70. The second fraction containing brown color was collected and solvent was evaporated. This compound dissolved in toluene and refluxed for 24hr for isomerisation. The resulting brown solution was concentrated in vacuo yielded 100 mg (25%) of the title compound. 1H NMR (CDCl3, 300 MHz): δ 8.2 (br s, 4H), 7.61 (m, 2H), 6.98 (d, 1H), 3.98-3.88 (br m, 7H), 1.80 (m, 2H), 1.56-1.28 (m, 16H), 0.91 (m, 12H). FT-IR (KBr): 2922, 2853, 1723, 1605, 1510, 1458, 1425, 1273, 1180, 1018, Anal. Cal. C101H44O4C, 91.80, H, 3.36 Found- C; 87.22, H; 6.30

2.2 Fabrication of BHJ Solar cells Indium tin oxide (ITO) coated glass substrates were used to fabricate the devices. At first, this glass substrates were initially cleaned with detergent followed by ultrasonication in DI water, acetone and isopropyl alcohol. The cleaned substrate were dried in oven for overnight. An aqueous of PEDOT: PSS (Baytron) was spin casted at 4000 rpm for 50 s to make a film of ~ 60 nm thickness. The PEDOT:PSS coated ITO glass substrate was dried at 100 °C for 10 min in air atmosphere. P3HT and fullerene derivatives i.e. C60DAM and C70DAM were separately dissolved in the THF at a concentration of 15 mg/mL and stirred for 4 hr. Then the solutions P3HT and fullerenes were mixed in different ratio and again stirred for 5 hr before use. The solutions were spin coated on the top of PEDOT: PSS layer at 3000 rpm for 2 min and dried at 40 °C for 4 hr. Finally the aluminum (Al) electrode with thickness of 90 nm was thermally deposited on the active layer through a mask with an area of 5 mm2. The currentvoltage (J-V) measurements were conducted with a computer controlled Keithley source meter. The devices were illuminated by a 100 W light source coupled with AM 1.5 optical filter (supplied by IIT, Mumbai). 3. Results and discussion 3.1 Optical and Electrochemical properties The optical absorption spectra of the both modified fullerene derivatives i.e. C60DAM and C70DAM were shown in Figure 2a and 2b along with PC60BM and PC70BM in THF solution. All the fullerene derivatives displayed strong UV-visible absorption from 200-400 nm as reported earlier in literature. However, the absorption of C60DAM and C70DAM are dramatically enhanced compared to their unmodified counterpart, in the visible region 400700 nm. Relatively lower absorption of PC60BM can be attributed to the high degree of symmetry and leading to the lowest energy transitions being formally dipole forbidden. When



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C60 moiety is replaced with less symmetry C70, these transitions become allowed and dramatic increases in the light absorption is expected, and have been showed as a promising candidate for the enhancement of photovoltaic response in the organic BHJ solar cells.50 The enhancement in the absorption towards longer wavelength is due to the reduced symmetry of C60DAM and C70DAM with respect to their unmodified PCBMs. For OPV devices, the best suited semiconductor materials should have favorable redox potential and excellent thermal stability i.e. a suitable LUMO energy level and temperature dependent morphology changes. The electrochemical properties of C60DAM and C70DAM were examined by cyclic voltammetry. Two reversible reduction waves were observed for both the acceptor which indicates that C60DAM and C70DAM are stable up to the dianion stage. It is clearly observed that both first and second reduction potentials of C60DAM and C70DAM shifted towards negative, the onset reduction potentials of C60DAM and C70DAM were negatively shifted as compared to their unmodified counterpart, enhancing the electron acceptability of the fullerene moiety. Moreover the LUMO energy levels of the fullerene derivatives can be red estimated from onset reduction potential ( E onset ) according to the following expression55

red E LUMO = − q ( E onset + 4.71)eV

red Where the unit of ( Eonset ) is in V vs Ag/ Ag+. With this equation the LUMO level of

C60DAM and C70DAM are -3.78 eV and -3.72 eV, respectively. Therefore the both C60DAM and C70DAM have higher lying LUMO energy levels compare to their counterpart values. The higher value of LUMO level is desirable for their application as electron acceptor in P3HT based polymer solar cells. The difference in LUMO energy levels of donor and acceptor are very important for photoinduced charge transfer in the BHJ solar cells and it should be higher than the exciton binding energy i.e. 0.4-0.5 eV.10 The difference between the LUMO levels of C60DAM, C70DAM and P3HT is more than exciton binding energy and an ultra fast photoinduced charge transfer can be achieved.

3.2 Thermal properties of the C60DAM and C70DAM We have recorded the differential scanning calorimetry (DSC) measurements of

C60DAM, C70DAM, PC60BM and PC70BM. It was observed that in contrast to PC60BM and PC70BM with a crystallization peak around 290°C and 305°C, respectively, no crystallization peak was observed for C60DAM and C70DAM over the whole temperature range of investigation, indicating that both the modified fullerene derivatives are an amorphous material. Thus it is expected that these materials may able to overcome the thermally driven


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crystallization and achieve a high thermally stability in the polymer solar cells.56 Moreover the thermal gravimetric analysis of C60DAM and C70DAM was performed and it is found that the decomposition temperature of C60DAM and C70DAM is as high as about 396 °C, confirming the high thermal stability of C60DAM and C70DAM as compared to their unmodified fullerene counterpart.

3.3 Photoluminescence properties Figure [3] illustrates the emission properties of donor (P3HT) and acceptor (C60DAM and C70DAM) in toluene, chloroform, and 1,2-dichlorobenzene solvents. The pristine P3HT showed emission maxima at 575 nm, 577 nm, and 580 nm in toluene, chloroform, and 1,2dichlorobenzene solvents respectively. In all investigated solvents we identified a slight blue shift (0-5 nm) in absorption spectra by addition of acceptors and fluorescence emission intensity decreased drastically by increasing the concentration of fullerene acceptors. We have adopted Stern-Volmer equation to evaluate the quenching efficiency of fullerene acceptors by eq. 1. I0F/IF = 1+KSV [Q]

(1)

Where I0F and IF are emission intensities in the presence and absence of the quencher, respectively. KSV is the Stern-Volmer quenching constant and [Q] is the concentration of quencher. This equation provides the information regarding intermolecular quenching process, the quenching plots of P3HT with C60DAM and C70DAM in three solvents showed linear plots (Figure 4a and 4b) in the concentration range of acceptor 0 - 2.5 x 10-4 M-1. The quenching efficiency and KSV data were presented in Table [1]. It is evident from Figure 4a and 4b, as the concentration of acceptor increases, the emission intensity is quenched. The calculated KSV value for C60DAM and C70DAM in Toluene, Chloroform, and Odichlorobenzene, is 0.111x103, 0.109x103, 0.108x103 & 0.109x103, 0.106x103, 0.105x103 respectively, where as the KSV value for PC60BM is 0.107x103, and 0.112x103, 0.112x103. The KSV values have correlation with the binding affinity between donor and acceptor.57-58 Based on measured KSV values we noticed that both acceptors (C60DAM and C70DAM) have showed more binding affinity towards the P3HT. The emission intensity is decreased selectively due to electron transfer, which is further evidenced by Gibb’s free energy function (∆G) calculated by eq. 2,

∆G (P3HT→C60DAM) ={ECT (P3HT) + (C60DAM)}-E0-0 (P3HT)

(2)



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∆G values for C60DAM, C70DAM and PC60BM is -0.19 eV, -0.13 eV and -0.06 eV respectively.59 These free energy values more negative compared to reference compound (PC60BM) indicates that, the modified fullerene derivatives are very good electron acceptors than the PC60BM, this is further evidenced by redox potentials which was discussed in electrochemistry section. The emission quenching of P3HT by C60DAM, C70DAM have been carried out in three solvents (Toluene, Chloroform, Orthodichlorobenzene) at room temperature, the quenching is found to be a positive deviation from linearity observed in S-V plots for the P3HT, and the fluorescence emission maxima red shifted with the increasing polarity of solvent (toluene< chloroform < orthodichlorobenzene) which is consistent with literature reports, where as no much differences in quenching efficiencies were observed.60-61

3.4 Electrical properties of C60DAM and C70DAM The current-voltage (J-V) characteristics of the ITO/PEDOT: PSS/ C60DAM and

C70DAM /Al in dark conditions, display rectification effect as shown in Figure 5. As discussed above that the LUMO of both fullerene derivates lie in the range between -3.72 and -3.78 eV, which is very close to the work function of the Al (-4.2 eV) and form the nearly Ohmic contact in the device. However the HOMO energy level of the most of the fullerene derivates is very far from the HOMO level of PEDOT:PSS (-5.0 eV). Consequently, the rectification effect is due to the band bending at the PEDOT: PSS/ modified fullerene interface, forming the Schottky barrier for the hole injection from the PEDOT: PSS into the HOMO level of fullerene. The forward bias corresponds to the negative and positive potential applied to the Al and ITO /PEDOT: PSS electrode, respectively and the device acts as the Schottky barrier device. The electron and hole mobilities for donor and electron components, respectively employed in the BHJ active for organic solar cells is an important factor which influences its photovoltaic performance. We have fabricated the devices having configuration Al/ C60DAM and C70DAM /Al. The electrons are injected from the bottom electrode into the LUMO of fullerene and collected by the top Al electrode, when bottom electrode is negatively biased with respect to top Al electrode. The J-V curves in dark were plotted in log-log scale and are shown in Figure 6. It can be seen from this Figure 6 that in the low voltage region, current density is linearly dependent on the applied voltage with a slope of unity, which corresponds to the Ohmic region. However, in the relatively high voltage region, the current density is linearly dependent to the voltage with a slope of about two. Therefore, the J-V characteristics in this region correspond to the space charge limited current (SCLC) behavior with trap free 8 ACS Paragon Plus Environment

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limit. The electron mobility of the modified fullerene derivatives was estimated fitting the JV characteristics with the following expression, in high voltage region

J SCLC = (9 / 8)ε o ε r µ e (V 2 / d 3 )

(3)

Where JSCLC is the current density in SCLC region, V is applied voltage, εo and εr permittivity of free space and relative dielectric constant of the organic layer, respectively, µe is the electron mobility and d is the thickness of the organic layer. Using εo = 8.85 x10-12 F/m,

εr=3.5 and d=90 nm, the electron mobility, fitting the above equation with J-V characteristics in the SCLC region, are 2.5 x10-4 cm2/Vs and 2.64 x10-4 cm2/Vs for C60DAM and C70DAM respectively. These values are nearly same as for most of the fullerene derivatives reported in literature and indicate that the modification does not affect the electron mobility.

3.5 Optical Properties of P3HT: modified PCBM blends The optical absorption spectra of the P3HT: C60DAM and P3HT: C70DAM cast from THF solution were shown in Figure 7. It was observed that the optical absorption of blends shows the combination of individual components. The absorption peak around 520 nm corresponds to the P3HT. The optical absorption of P3HT: C70DAM shows the more boarder band as compared to that for P3HT: C60DAM which is attributed to the more absorption of

C70DAM in the visible region relative to C60DAM. To investigate the photoinduced charge transfer ability in the BHJ active layer, we have measured the photoluminescence spectra of the P3HT, P3HT: C60DAM and P3HT:

C70DAM blends and shown in Figure 7. It can be seen that the P3HT shows an emission peak around 640 nm which has been significantly quenched for both P3HT: C60DAM and P3HT:

C70DAM blend, indicating the ultra fast photoinduced charge transfer in the blends (as described detailed in photoluminescence section). However the photoluminescence quenching is more for the P3HT: C70DAM blend as compared to P3HT: C60DAM indicates more efficient charge transfer for the P3HT: C70DAM blend as compared to P3HT: C60DAM and enhancement in the short circuit photocurrent is expected and will be discussed in the later part.

3.6 Photovoltaic properties of BHJ solar cells Balance between the absorbance and charge transporting network of the photoactive BHJ layer greatly affect the performance of BHJ device therefore, composition of the electron donor and acceptor materials employed for the photoactive layer in crucial for 9 ACS Paragon Plus Environment


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efficient organic solar cells. Transport of electron ability will be limited at very low concentration of acceptor. Moreover, the absorbance and hole transporting ability decreased when acceptor concentration is too high. Therefore we have chosen different weight ratios of donor: acceptor (1:05, 1:1, 1:1.5, 1: 2, 1: 2.5 and 1:3) to fabricate the BHJ devices and found that the optimized weigh ratio is 1:2 for both C60DAM and C70DAM acceptors. Therefore, we reported the results of BHJ polymer solar cells based on this weight ratio only to balance between the absorbance and charge transporting ability. The current-voltage (J-V) characteristics of the devices under illumination of 100 mW/cm2 based on P3HT as electron donor and PC60BM, PC70BM, C60DAM and C70DAM as electron acceptor are shown in Figure 8a and 8b and corresponding photovoltaic parameters i.e. short circuit photocurrent (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) are presented in Table 2. The PCE of BHJ solar cell devices with P3HT: C60DAM, P3HT: C70DAM, P3HT:PC60BM, and P3HT:PC70BM are 3.23 % (Voc=0.80 V, Jsc=8.08 mA/cm2, FF=0.50), 2.22 % (Voc=0.62 V, Jsc=7.80 mA/cm2, FF=0.46), 4.45 % (Voc=0.82 V, Jsc=9.86 mA/cm2, FF=0.55) and 3.13 % (Voc=0.64 V, Jsc=9.42 mA/cm2, FF=0.52), respectively. Compared to the performance of the non-modified fullerene devices, the devices with modified fullerene derivatives showed both a higher Jsc and Voc, leading to an improved PCE. The higher value of Voc and increment in Jsc for the solar cells based on modified fullerene derivatives is attributed to the high lying LUMO level and broader absorption in the visible region of C60DAM and C70DAM as compared to the PC60BM or PC70BM, which resulted enhancement in the photogeneration of exciton in blend there by resulting slightly higher photocurrent. As mentioned in table 2 that the device based on C70DAM as electron acceptor shows higher PCE than that for the C60DAM, which is mainly due to the higher value of Jsc. The enhancement in Jsc for C70DAM based device may be due to the increased and broad absorption of C70DAM by the virtue of less symmetry C70 moiety, resulting more light harvesting efficiency. Moreover the degree of the photoluminescence quenching is more for the P3HT : C70DAM blend as compared to P3HT: C60DAM blend (as shown in Figure 7) indicates more faster photoinduced charge transfer and also supports the higher value of Jsc and yield enhanced PCE. The electron acceptor C70DAM effectively increases the Voc and Jsc, so the finally the P3HT: C70DAM based device shows PCE of 4.45 % without any additional treatment and solvent additive.

3.7 Effect of solvent additive


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The control of nano-scale morphology of the active layer is one of the important parameters to achieve the high efficiency of BHJ organic solar cells because it directly affects the Jsc and FF. The surface roughness and size of the donor and acceptor domains in the active layer needs to be controlled and strikes a balance between the efficient exciton dissociation and charge transport. The domain size between donor and acceptor should be in the range of exciton diffusion length, for efficient exciton dissociation and also perfect mixing provides the best exciton quenching. Moreover, the efficient charge transfer complex dissociation and charge transport are opposite phases cab be achieved if the morphology of active layer is coarser. Therefore, best device efficiencies can be expected when spatially domain sizes correspond to twice of exciton diffusion length (~20 -40 nm). A number of approaches were widely used in the literature to control the BHJ morphology, i.e. thermal annealing62-66, solvent annealing67-68 and addition of relatively small amounts of high boiling point solvents referred as solvent additives.69-70 Recently, the use of solvent additives has become more widely adopted with significant improvements in PCE of the photovoltaic device. By incorporating a small amount of specific processing additives such as 1-chloro naphthalene (CN) and 1,8, diiodooctane (DIO), into the main solvent, it is promising to control the phase separation in the BHJ active layer and increases the PCE of the PSCs. Therefore by monitoring the solvent quality of additives for polymer and fullerene, both the polymer order and degree of phase separation can be tuned. Since the PSCs based on C70DAM as electron acceptor, we have fabricated the PSCs ITO/PEDOT: PSS/P3HT: C70DAM (1:2 v/v ratio)/Al. The active layer of the devices was prepared by spin coating blend with different additives i.e. 1-chloro naphthalene (CN), 1, 8 diiodooctane (DIO) and 1,8-octanedithiol (OT). Figure 9 shows the J-V characteristics of the PSC under illumination. The photovoltaic performance data of the devices are complied in Table 3. It is observed that incorporating the solvent additive into the host solvent during the thin film processing, the photovoltaic performance of the resultant devices improved significantly. The PSCs without additive treatment is about 4.45 % with Jsc=9.86 mA/cm2. After additive treatment, the Jsc and PCE increased to 10.24 mA/cm2 and 4.75 % for DIO additive, 10.68 mA/cm2 and 4.95 % for OT additive and 11.65 mA/cm2 and 5.63 % for CN additive. The enhancement in the photovoltaic performance for PSCs with the additive could be attributed to the high boiling point of the additive relative to host solvent which is promote the self-organization of both P3HT and C70DAM, to acquire the better nanoscale morphology. The incident photon to current conversion efficiency (IPCE) plots of the solar cells based on the P3HT: C70DAM blends are shown in Figure 11. The PSCs with CN 11 ACS Paragon Plus Environment


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additive shows the highest IPCE values, which is consistent with the tendency of Jsc of corresponding PSCs. In order to get information about the reasons of the enhanced photovoltaic performance of the PSC with additives, absorption spectra and atomic force microscopy (AFM) of the P3HT: C70DAM blend films with and without additives were studied. It can be seen that the blend films cast with additive show stronger absorption band within 350-750 nm and which is more for the film cast with CN additive and could be ascribed to the more ordered structure of P3HT induced by high boiling point solvent additives, beneficial to increase the Jsc and IPCE values for the polymer solar cells with additives. In efficient BHJ organic solar cells, the morphology of the blend thin film plays a crucial role [24]. The BHJ active layer with interpenetrating networks consist of well phase separated domains, provide percolation pathways for charge carrier transport to the respective electrodes, beneficial for high fill factor and PCE of the photovoltaic devices. Additionally, for efficient charge transport towards the electrodes, within the active layer, the hole and electron mobilities must be sufficiently high. We have examined the morphology of the blend layers cast with and without additives through the atomic force microscopy (AFM) measurements and the images for P3HT: C70DAM (with and without CN additives) are shown in Figure 10. Similar effect has been observed for DIO and OT solvent additives. The surface roughness of the film without additive is 1.46 nm from the AFM images, while that of the film with additive of DIO, OT and CN is reduced to 0.87 nm, 0.72 nm and 0.65 nm, respectively. Therefore, using the additives, surface roughness become relatively smoother and the surface of the film with CN additive shows lowest roughness, indicating that the smoother surface of the active layer is beneficial for enhanced FF of the corresponding polymer solar cell. It can also be seen from the AFM images that the interpenetrating network is improved after the solvent additive. The blend without additive displays coarse phase separation between P3HT and C70DAM with bigger size domains. However, solvent additives led to much reasonable phase separation between two materials, improving the D/A interfacial area for exciton dissociation, leading to an increase in Jsc, for these solar cells.

4. Conclusions In conclusions, we have synthesized two modified PC60BM i.e. C60DAM and PC70BM i.e. C70DAM and employed as electron acceptor in combination with P3HT as electron donor for BHJ polymer solar cells. Compared to the parent PC60BM, C60DAM and

C70DAM showed negative shift in the reduction potential, possibility increasing the electron acceptability of fullerene moiety and also leading to the higher value of LUMO energy level. 12 ACS Paragon Plus Environment

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The PCE of the polymer solar cells based on the P3HT: C60DAM and P3HT: C70DAM reached up to 3.23 % and 4.45 % which are higher than that device using their parent PC60BMs. The higher value of PCE is ascribed to the enhanced values of both Jsc and Voc. The higher value of Voc has been resulted from the shift in the LUMO level towards vacuum level whereas the increase in the Jsc has been attributed to the stronger absorption of modified PCBM in visible region. We have also investigated the effect of solvent additive in P3HT:

C70DAM blend solution on the photovoltaic performance of polymer solar cell. The PCE of polymer solar cell P3HT: C70DAM was increased from 4.45 % for the device with no additive to 4.75 % for DIO, 4.95 % for OT and 5.63% for CN additives. Additive treated P3HT:C70DAM interpenetrating network of blend and strong absorption may be the reason in enhancement of PCE. The smoother surface roughness of the blend layer and the interpenetrating networks of active layer are more uniform by using solvent additives confirmed by AFM measurement, which is an essential factor in improving the photovoltaic performance.

Acknowledgement SPS thanks Indo-US Science and Technology Forum (IUSSTF) for providing the BASE fellowship. PN thanks to CSIR for providing senior research fellowship. SPS thanks to XII FY CSIR-INTELCOAT (CSC0114) for financial support. We are also grateful to acknowledge DST-UK ('APEX') for their support.

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60. Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y. “Solvent Annealing” Effect

in

Polymer

Solar

Cells

Based

on

Poly(3-hexylthiophene)

and

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Table 1. Stern-Volumer constant (Ksv) values and quenching efficiency (%) for C60DAM and C70DAM

C60DAM: P3HT (1:5) C70DAM: P3HT (1:5) PC60BM: P3HT (1:5)

Quenching Efficiency (%)

Ksv (103, M)

Acceptor: P3HT

TOLUENE

CHCl3

ODCB

TOLUENE CHCl3

ODCB

0.111

0.109

0.108

83.4

82.1

81.4

0.109

0.106

0.105

82.3

80.7

79.0

0.107

0.112

0.112

80.5

84.5

84.0

Table 2. Photovoltaic parameters of the BHJ polymer solar cells with different blends as from THF without solvent additive Blend

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

P3HT:PC60BM

7.80

0.62

0.46

2.22

P3HT: C60DAM P3HT:PC70BM P3HT: C70DAM

8.08 9.42 9.86

0.80 0.64 0.82

0.50 0.52 0.55

3.23 3.13 4.45

Table 3. Effect of different solvent additives on the photovoltaic parameters of BHJ polymer solar cells based on P3HT: C70DAM blend Solvent additives

Jsc ( mA/cm2)

Voc (V)

FF

PCE (%)

DIO

10.24

0.80

0.58

4.75

OT CN

10.68 11.65

0.80 0.78

0.58 0.62

4.95 5.63


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Figure 1

Figure 1. Structures of new modified PC60BM and PC70BM derivates, i.e. C60DAM and

C70DAM

Scheme 1. Synthetic routes of C60DAM and C70DAM fullerene acceptor



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Figure 2. Optical absorption spectra of (a) PC60BM and C60DAM and (b) PC70BM and C70DAM


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Figure 3. PL spectra of pure P3HT and P3HT:C60DAM, C70DAM at different concentrations (1:05 – 1:5) from top to bottom

Figure 4. Stern−Volmer plots of P3HT quenching by C60DAM and C60DAM in different solvents (Toluene, Chloroform and ODCB) with (7.5x10−3 M) concentration



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Figure 5. Current-voltage (J-V) characteristics of ITO/PEDOT:PSS/ C60DAM or C70DAM/Al devices in dark

Figure 6. Current-voltage characteristics of Al/ C60DAM or C70DAM/Al devices in log-log scale in dark to estimate the electron mobility.


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Figure 7. PL spectra of P3HT, P3HT: C60DAM and P3HT: C70DAM blend thin films cast from THF solvent



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Figure 8. Current-voltage (J-V) characteristics of ITO/PEDOT: PSS/ P3HT: C60DAM or P3HT: C70DAM/Al devices under illumination


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Figure 9. Current-voltage characteristics of ITO/PEDOT: PSS/P3HT: C70DAM/Al devices processed with different solvent additives under illumination

Figure 10. AFM images of the P3HT: C70DAM films with (a) CN and (b) without solvent additives (5µm x 5µm)



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Figure 11. IPCE spectra of ITO/PEDOT:PSS/P3HT: C70DAM/Al devices processed with different solvent additives


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Efficient Solution Processable Polymer Solar Cells ... - ACS Publications

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