One-Step Synthesis of New Electron Acceptor for High Efficiency

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One-Step Synthesis of New Electron Acceptor for High Efficiency Solution Processable Organic Solar Cells Puvvala Nagarjuna, Anirban Bagui, Ashish Garg, Vinay Gupta, and Surya Prakash Singh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08167 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 12, 2017

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The Journal of Physical Chemistry

One-Step Synthesis of New Electron Acceptor for High Efficiency Solution Processable Organic Solar Cells P. Nagarjuna,†,‡ Anirban Bagui,*,† Ashish Garg,§ Vinay Gupta,∥ and Surya Prakash Singh*,†,‡ †

Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India

‡

Academy of Scientific and Innovative Research (AcSIR), New Delhi, India

§

Department of Materials Science and Engineering and Samtel Center for Display Technologies, Indian Institute of Technology Kanpur, Kanpur, 208016, India

∥CSIR-National

Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi, 110012, India

Corresponding author: [email protected], [email protected] Abstract Here, we demonstrate a new fulleropyrrolidine electron acceptor coded as DIF-ful-C60, following one-step synthetic protocol. The absorption spectrum of DIF-ful-C60 suggests broader absorption over PC61BM while the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the molecule are -5.32 and 3.60 eV, respectively. The solution processed bulk-heterojunction polymer solar cells fabricated using a blend of DIF-ful-C60 with PTB7 and PTB7-Th as electron donors show power conversion efficiencies (PCEs) of 6.8 and 8.6%, respectively, which is highest reported so far for fulleropyrrolidine type electron acceptors. In comparison, the control devices made from PTB7:PC61BM and PTB7-Th:PC61BM blends showed PCEs of 6.2 and 7.9%, respectively. The increase in efficiency of the devices based on DIF-ful-C60 is due to increase in the short circuit current as well as open circuit voltage of ca. 0.8V. 1.

INTRODUCTION Solar photovoltaics is perhaps the most attractive renewable energy generation method to

meet mankind’s increasing energy demand by harnessing the abundantly available solar energy which can also help us to mitigate the environmental concerns due to fossil fuel based energy. Among various technologies explored and developed over last few decades, last decade has seen extensive research on organic photovoltaics due to their low production cost, light-weight, possibility of making flexible devices, and potential for large area applications.13

The most explored configuration of organic photovoltaic devices is the bulk heterojunction 1 Environment ACS Paragon Plus


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(BHJ) solar cells,4 in which a photo-active layer is a mixture of two phases, first being the electron donor and other being the acceptor. The soluble π-conjugated polymers and fullerene derivatives, particularly phenyl-C61-butyric acid methyl ester (PC61BM) have been broadly utilized so far as electron donor and acceptor candidates, respectively with PCE approaching over 10%.5-7 Recently, several research groups have developed fulleropyrrolidine derivatives having substituent like planar phenyl rings and heterocyclic compounds and long alkyl chains, which are used as electron acceptors in BHJ solar cells and showed good photovoltaic performance in the devices (Figure 1, Table S1).2,

8-16

The PCE of these new

fulleropyrrolidine derivatives gradually increases over 7%. In 2014, Aso et al. have reported a new N-phenyl fulleropyrrolidine derivative and achieved highest PCE of 7.3% using poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) as a donor polymer.12 Since, most of the light absorption and charge carrier generation takes places in the donor material i.e. the polymers, it is a key component in the BHJ design and hence, tremendous research has been carried out on development of donor materials. However, acceptors are equally important due to their impact in carrier transport and energy level alignment with the polymers influencing the device parameters significantly. Till now, fullerene derivatives have attracted most of the attention as electron acceptors in organic solar cells (OSCs) due to their unique nature, such as π-electron system, better light harvesting properties, higher electron mobility, and appropriate LUMO energy levels, which are compatible with most of conjugated polymer donors.17 Recently various research groups widely synthesized different types of fullerene acceptors such as methanofullerenes,18-20 diarylmethanofullerene,21 indene bis adducts,22 and fulleropyrrolidine derivatives.23 In this regard, fulleropyrrolidine derivatives are very interesting due to the advantage of its easier one-pot synthetic protocol, also known as Prato synthesis.24, 25 In general, Prato synthetic route involves only aldehydes and amino acids, which reduces production cost. Besides, several analogues can be prepared in a short period of time along with additional advantages of highly stable derivatives with high purity and good yield.

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N

1

PCE 0.47% Sol. Energ. Mater. Sol. Cells, 2007, 91, 892

O O

N

2

O

S

5

S

MeO

O

S

S

PCE 2.5% ACS. Appl. Mater. Interfaces, 2012, 4, 6133

O

4

N

3

PCE 1.6% Chem. Lett, 2015, 14, 527

O N

C6H13C6H13

O

O

6

N

N

S

.

PCE 2.8% J. Phys. Chem. C, 2012, 116, 8881

PCE 2.9% Chem. Lett, 2013, 42, 1209

PCE 3% Tetrahedron, 2012, 68, 3605

MeO C6H13

O 7

O N

PCE 3.4% J. Mat. Chem. 2010, 20, 9226

S

8

N

S

PCE 3.4% J. Mat. Chem. A, 2015, 3, 14401

S

9

C6H13

S

N

N O

O

O

PCE 3.5% ACS. Appl. Mater. Interfaces, 2016, 8, 8481

10

N

PCE 7.3% J. Mat. Chem. A, 2014, 2, 20889

Figure 1. Different types of fulleropyrrolidine derivatives as reported in the literature. In this work, we have followed the following strategies to improve the electron accepting nature of our new trifluoromethyl derived fulleropyrrolidine (DIF-ful-C60)- 1) direct attachment of more electronegative atoms to the fullerene cage, 2) functionalization of fullerene with more electron accepting groups and 3) functionalization with heterocyclic compounds.26 The attachment of –F, –CF3 groups to pristine buckminsterfullerene (C60) enhances the electron accepting character of the resultant molecule due to the high electron deficient property of the ligands.27 The presence of two trifluoromethyl groups in DIF-fulC60 derivative shows significant change in the electrochemical and photo-physical properties and the main advantage of this derivative is the formation of smooth thin-film at metal-active layer interface with low roughness. DIF-ful-C60 offers good performance as acceptor in BHJ solar cells with PTB7 and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-



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b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate2-6-diyl)] (PBDTTT-EFT, also known as PTB7-Th) as electron donors (Figure 2). High PCEs of 6.8 and 8.6% are achieved from the OSCs fabricated using PTB7:DIF-ful-C60 and PTB7-Th:DIF-ful-C60 bulk composite films, respectively.

Figure 2. Molecular structures of (a) newly developed electron acceptor coded as DIF-fulC60 and the polymer donors (b) PTB7 and (c) PTB7-Th used for solar cell fabrication in this study. 2.

EXPERIMENTAL SECTION

2.1 Synthesis of DIF-ful-C60 The derivative DIF-ful-C60 was synthesized following Prato one step synthetic protocol. A mixture of C60 (100 mg, 0.138 mmol), 2,5-bis(triflouromethyl)benzaldehyde (33.58 mg, 0.138 mmol), and N-methylglycine (113 mg, 1.278 mmol) in dry toluene (10 mL) was refluxed for 20 h. Once the reaction mixture was cooled down to room temperature, it was filtered and solvent was removed by rotary evaporator. The crude product was purified by column chromatography (silica gel; hexane/ethyl acetate, 9:1) afforded DIF-ful-C60 (35% yield) as a brown solid. 1H NMR (300 MHz, CDCl3): δ ppm 8.82 (s, 1H), 7.94-7.87 (d, J= 8.4 Hz 1H), 7.76-7.71 (d, J= 8.4 Hz 1H), 5.47 (s, 1H), 5.04-4.97 (d, J= 9.6 Hz 1H), 4.39-4.34 (d, J= 9.6 Hz 1H), 2.73 (s, 3H) (Figure S1); FT-IR (cm-1) KBr–3446.72, 2918.78, 2845.72, 2780.86, 1633.28, 1460.22, 1423.10, 1331.40, 1310.09, 1261.83, 1171.08, 1134.24, 1087.81, 1037.81; MALDI TOF: C71H9N6F6calc:–989.83 (m/z), found 989.49 (m/z) (Figure S2). 2.2 Fabrication and characterization of solar cells


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The BHJ solar cells were made with device configuration ITO/ZnO/photo-active layer/MoO3/Ag based on DIF-ful-C60 blended with PTB7 and PTB7-Th while control samples were based on a blend of PC61BM with PTB7 and PTB7-Th. Commercially available ITO coated glass substrates with sheet resistance of ~12Ω/□ were first patterned by conventional photo-lithography method to obtain the desired bottom electrode pattern and necessary inter-connections. The substrates were then RCA-cleaned followed by UV-ozone treatment for 15 min under an oxygen flow rate of 350 mL/min. Subsequently, 30 nm thick o

ZnO seed layer was spin coated at 3000 rpm for 60 s on ITO followed by annealing at 240 C for 15 min. The ZnO solution was prepared by dissolving 0.384 g of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) in 4.895 ml of 2-methoxy ethanol (CH3OCH2CH2OH) as solvent with addition of 0.105 mL of monoethanolamine (MEA) (H2NCH2CH2OH) as stabilizer followed by overnight stirring at room temperature before film deposition. On spin coated ZnO electron transport layer, the photo-active layers of blend of donor:DIF-ful-C60 were spin coated at 700 rpm for 20 s followed by drying. The blend was dissolved in different weight ratios in ortho-dichlorobenzene (o-DCB) solution along with addition of 3% (vol) diiodooctane (DIO) to improve the solubility and stability. After this, a 3 nm thin MoO3 film was thermally evaporated at 0.1- 0.2 Ås-1 followed by thermal evaporation of 90 nm thin Ag film through a shadow mask at 0.5-1.0 Ås-1 to form the anode. During the film deposition, evaporation chamber was kept at a base pressure below 3×10-6 mbar. The film thicknesses of different constituent layers were measured by Bruker’s Dektak XT stylus profilometer. The transmission characteristics of thin films were investigated with Perkin Elmer’s Lamda-750 spectrometer, whereas morphological studies were conducted in tapping mode using Asylum Research’s MFP-3D stand-alone atomic force microscopy (AFM) system. The incident-photon-to-collected-electron efficiency (IPCE) was measured with a lock-in detector after illumination with a monochromatic light from a tungsten lamp. The current-voltage characteristics of the devices were measured with a Keithley 2400 source meter using a xenon-arc-lamp-based solar simulator (Newport's Oriel) as a light source for solar cell testing under illumination at 1.5G AM. All the measurements were carried out in ambient conditions. 3.

RESULTS AND DISCUSSION

3.1 Optical properties



The UV-visible absorption spectrum of newly synthesized DIF-ful-C60 was measured by dissolving it in dichloromethane (DCM) solution at 10-5 mol·L-1 concentration and the result is shown in Figure 3. The molecule shows a broad absorption in the wavelength range of 290-400 nm with two closely lying sharp peaks at 228 and 254 nm and two overlapping weak peaks at 311 and 326 nm. The absorption intensity of DIF-ful-C60 is comparatively higher than that of PC61BM measured at same concentration, which is attributed to the contribution from the trifluoromethyl functional groups. The value of molar extinction coefficient (ε) is calculated to be 1.2 × 105 Mol-1cm-1 at 254 nm. The new acceptor also showed better absorption coefficient than PC61BM in thin film of similar thicknesses as shown in Figure 3b. It is noteworthy to mention that both acceptors showed red-shifted peaks in thin film compared to liquid state because of more delocalization of π-electron in solid state. A stronger absorption of DIF-ful-C60 is expected to benefit the photovoltaic performance of solar cell devices made from this acceptor by improving light coupling with solar spectrum. (a)

12

DIF-ful-C60

-1

DIF-ful-C60 -1

PC61BM

1.5

5

-1

Mol cm )

10

4

(b) absorption coefficient (x10 cm )

PC61BM

ε (x10

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8 6 4 2 0

1.0

0.5

0.0

250

300

350 400 450 Wavelength (nm)

500

550

400

500

600

700

800

Wavelength (nm)

Figure 3. The UV-visible absorption spectrum of DIF-ful-C60 in comparison to PC61BM in (a) liquid state and (b) thin film form. The newly developed molecule shows broader absorption. 3.2 Electrochemical properties The cyclic voltammetric measurements of fulleropyrrolidine derivative DIF-ful-C60 was carried out at room temperature in o-DCB solvent containing tetrabutylammonium hexafluorophosphate (TBAPF6) of concentration 0.1 M. The saturated calomel, platinum wire and glassy carbon were used as reference electrode, counter electrode, and working electrode, respectively. The cyclic-voltammogram of DIF-ful-C60 is shown in Figure 4 along with PC61BM. The plot for DIF-ful-C60 shows three quasi-reversible reduction waves at -0.66, 1.04 and -1.52 V due to the presence of electronegative CF3 groups.28 The LUMO energy


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level is calculated by taking the onset reduction potential (Ered/onset), ferrocene as internal standard (4.8 eV) using the empirical formula for estimation of LUMO, i.e., LUMO = −e (E1red + 4.8)

(1)

As Table 1 suggests, DIF-ful-C60 shows up-shifted LUMO energy level as compared to PC61BM and hence an enhancement in the open circuit voltage (VOC) of the device is expected. DIF- ful-C60 PC61BM

Current (µA)

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-2.0

-1.5

-1.0 Potential (V)

-0.5

0.0

Figure 4. Cyclic voltammograms of DIF-ful-C60 and PC61BM. Table 1. The HOMO and LUMO values of DIF-ful-C60 and PC61BM Fullerene

E1red (V)

E1ox (V)

LUMO (eV)

HOMO (eV)

DIF-ful-C60

-0.66

1.67

-3.57

-5.90

PC61BM

-0.56

1.68

-3.67

-5.91

PTB7-Th

–

–

-3.33

-5.30 29

3.3 Light J-V characteristics Polymer BHJ solar cells in inverted architecture were fabricated with device configuration of ITO/ZnO/active layer/MoO3/Ag with device structure and the corresponding energy band diagram shown in Figure 5a and b, respectively. The photovoltaic measurements were carried out under 1 sun illumination using standard AM1.5G filter. The current densityvoltage (J-V) characteristics of PTB7:DIF-ful-C60 and PTB7-Th:DIF-ful-C60 based OSCs have been shown in Figure 6 and the photovoltaic parameters are reported in Table 2. The



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data was compared with that of control samples based on blend of PTB7 or PTB7-Th with PC61BM.

Figure 5. (a) Schematic representation of OSC, (b) corresponding energy band diagram. Experiments were carried out in two steps to achieve maximum device performance i.e. firstly, the Donor/Acceptor (D/A) weight ratio was varied to get the best possible device followed by optimization of annealing temperature. The devices were made with three different D/A weight ratios – 1:1, 1:1.5 and 1:2. For PTB7:DIF-ful-C60 based OSCs, the D:A weight ratio of 1:1.5 showed the best photovoltaic performance with PCE of 6.4%, shortcircuit current density (JSC) of 15.6 mA·cm-2, VOC of 0.82 V and fill factor (ff) of 50% (Figure S3, Table S2). When the D:A weight ratios were kept at 1:1 and 1:2, the devices showed PCEs of 5.8 and 3.7 %, respectively. The value of VOC also dropped down when the acceptor content in the active layer blend is higher. For the aforementioned set of devices, o

active layers were dried at the room temperature (~25 C). In the next step, PTB7:DIF-fulC60 (1:1.5) active layers were dried at 40 and 80 oC for 1 h along with reference sample dried at RT (Figure S4). The devices annealed at 40 oC showed the best performance with a highest PCE of 6.8 %, VOC of 0.82 V, JSC of 16.0 mA·cm-2 and ff of 52%. The PTB7:DIF-fulC60 (1:1.5) based solar cells annealed at 40 oC also showed lowest series resistance (RS) of 8.2 Ω˖cm-2 and highest shunt resistance (RSH) of 0.32 KΩ˖cm-2 possibly because of improved metal-semiconductor interface and reduced charge recombination, respectively. However, annealing at higher temperature (80 oC) deteriorates the device performance as can be seen from Figure S4. In contrast, the control samples of PTB7:PC61BM blend showed an efficiency of 6.2% with lower VOC and JSC. However, the control devices still show higher ff and better parasitic resistances suggesting towards scope of further optimization of the devices based on the new acceptor. Further, the solar cell devices were also fabricated using PTB7-Th as electron donor material in optimized experimental conditions and the results are shown in Figure 6. The optimized PTB7-Th:DIF-ful-C60 based BHJ OSC devices showed a


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highest PCE of 8.6 % with a JSC of 16.01 mA·cm-2, VOC of 0.82 V and a high ff of 65.5%; whereas the reference solar cell made from PTB7-Th:PC61BM blends showed a highest efficiency of 7.9 % with reduced JSC of 15.14 mA·cm-2 and ff of 62.1 %. All the photovoltaic parameters are summarized in Table 2. In order to check the reproducibility of device performance, it is imperative to study a large number of solar cells. Herein OSCs were fabricated on at least twelve different substrates from four independent runs for each category. As each substrate was designed to contain four pixels/cells, so the photovoltaic performance of new acceptor was studied on total 48 cells for each set. Figure 6b shows the statistical distribution of PCEs obtained from PTB7:DIF-fulC60 and PTB7-Th:DIF-ful-C60 based OSCs constructed in optimized experimental conditions. The average PCE based on 48 devices for each category is provided in Table 2 with standard deviation. (b)

(a)

PTB7:DIF-ful-C60

16

0

PTB7-Th:DIF-ful-C60

14

2

Current density (mA/cm )

-5

PTB7:DIF-ful-C60

12

PTB7-Th:DIF-ful-C60

10

Counts

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


PTB7:PC61BM PTB7-Th:PC61BM

-10

8 6 4

-15

2 0 0.0

0.2

0.4 0.6 Voltage (V)

0.8

6.2 6.4 6.6 6.8 7.0 8.0 8.2 PCE (%)

8.4

8.6

8.8

Figure 6. (a) The light J-V characteristics of champion devices made from PTB7:DIFful-C60 and PTB7-Th:DIF-ful-C60 blends along with reference solar cells, (b) PCE histograms for PTB7:DIF-ful-C60 and PTB7-Th:DIF-ful-C60 based OSCs made in optimized experimental conditions. Table 2. Photovoltaic parameters of the solar cells with different D/A weight ratio. Active layer

D/A

Annealing VOC o

JSC

ff 2

PCEa

PCEb

RS

RSH -2

(Ω˖cm ) (KΩ˖cm-2)

ratio temp. ( C) (V)

(mA/cm ) (%)

(%)

(%)

PTB7:DIF-full-C60

1:1.5

40

0.82

-15.97

51

6.8

6.5 ± 0.2

8.2

0.32

PTB7:PC61BM

1:1.5

40

0.70

-15.40

58

6.2

5.9 ± 0.3

7.4

0.34

PTB7-Th:DIF-full-C60

1:1.5

40

0.82

-16.01

65

8.6

8.4 ± 0.1

6.9

0.73

PTB7-Th: PC61BM

1:1.5

40

0.84

-15.14

62

7.9

7.7 ± 0.2

5.9

0.76

a = highest PCE from each, b = average PCEs calculated based on 48 devices for each category.



3.4 IPCE measurements To further examine the cause behind improvement in the efficiencies of the devices based on new acceptor, we conducted the quantum efficiency measurements. The IPCE spectra of the devices with highest PCE made from two different donors and acceptors are shown in Figure 7. The best performing PTB7:DIF-ful-C60 based solar cell with efficiency of 6.8 % exhibited an IPCE over 60 % in the wavelength range of 539-690 nm. On the other hand, PTB7-Th:DIF-ful-C60 based device with a PCE of 8.6 % showed the IPCE over 72 % in the wavelength range of 539 - 690 nm. One should note that these quantum efficiencies are higher than those depicted by the blend devices based on PC61BM. Although the PTB7:DIFful-C60 shows much higher quantum efficiency over the PTB7:PC61BM devices, the efficiency is not enhanced commensurately due to lower ff. The maximum values of JSC estimated by integrating the IPCE spectra over the wavelength range of interest (300-900 nm) coupling with AM 1.5G solar spectrum approximately matches to the values measured during light J-V characterization. 90 PTB7:DIF-ful-C60

EQE (%)

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80

PTB7-Th:DIF-ful-C60

70

PTB7:PC61BM

60

PTB7-Th:PC61BM

50 40 30 20 10 0 300

400

500 600 700 Wavelength (nm)

800

Figure 7. The IPCE curves of the polymer solar cells fabricated from PTB7:DIF-fulC60 and PTB7-Th: DIF-ful-C60 blends along with respective reference devices. 3.5 AFM measurement Further, to see whether the change in the performance of the two devices with different acceptors is also related to change in the active layer morphology, we conducted AFM measurements of the film surfaces in tapping mode to visualize the nanoscale film morphology in the lateral direction of the blend active layers. Figure 8 shows the topological 10 Environment ACS Paragon Plus

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images and the corresponding phase images of PTB7-Th based blend films as obtained from AFM measurement on 5×5 µm2 scanning area. The root-mean-square (RMS) roughness for PTB7-Th:DIF-ful-C60 film is 1.8 ± 0.2 nm, whereas RMS roughness of reference PTB7Th:PC61BM film is found to be comparatively higher 2.9 ± 0.3 nm. Similar trend in film roughness is also observed for PTB7 based blend films as shown in Figure S5. This is possibly because of better solubility of DIF-ful-C60 molecule in o-DCB and improved miscibility with the donor molecule compared to that of PC61BM. A smooth active layer surface is known to improve the interface with the top contact and the HTL, which could eventually help in improved charge collection and reduced recombination.30,31

Figure 8. AFM topographical images of (a) PTB7-Th:PC61BM and (b) PTB7-Th:DIF-fullC60; corresponding phase images blend films. 4.

CONCLUSION To conclude, we have designed and synthesized a new trifluoromethyl substituted

fulleropyrrolidine DIF-ful-C60 to be used as electron acceptor in OSCs. The molecule has better electron accepting nature due to the presence of two trifluoromethyl groups. It showed higher molar extinction coefficient compared to conventional PC61BM. High PCE of 6.8% was achieved from PTB7:DIF-ful-C60 based BHJ solar cell with D/A ratio of 1:1.5 (w/w) at an annealing temperature of 40 oC. The champion device made from PTB7-Th:DIF-ful-C60 blend showed a PCE of 8.6%, which is highest reported so far from this class of electron acceptors. Better solubility of this molecule in organic solvents also gave rise to smooth film morphology, which is beneficial for efficient charge collection. Because of broader absorption, promising PCE and high VOC in case of solar cells, using DIF-ful-C60 over other



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conventional fullerene based acceptors could be an improved choice for fabrication of solution processed BHJ solar cells. ACKNOWLEDGEMENT PN thanks the CSIR, Government of India for providing SRF. AB would like to thank Department of Science & Technology (DST), Government of India for his INSPIRE Faculty fellowship (DST/INSPIRE/04/2017/000087). We also gratefully acknowledge DST-UK (APEX-Phase-II) for their support. SUPPORTING INFORMATION The Supporting Information is available free of charge on the journal website and from the corresponding author. It contains optimization of device fabrication process and structural characterization by 1H NMR, and MALDI-TOF (PDF). REFERENCES (1)

Guldi, D. M.; Illescas, B. M.; Atienza, C. M.; Wielopolski, M.; Martin, N., Fullerene

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One-Step Synthesis of New Electron Acceptor for High Efficiency

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