Improving the Photovoltaic Performance of Polymer Solar Cells Based

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Improving the Photovoltaic Performance of Polymer Solar Cells Based on Furan-Flanked Diketopyrrolopyrrole Copolymers via Tuning Alkyl Side Chains Weilong Zhou, Chengzhuo Yu, Huajie Chen, Tao Jia, Weifeng Zhang, Gui Yu, and Fenghong Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00890 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 29, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Improving the Photovoltaic Performance of Polymer Solar

Cells

Based

on

Furan-Flanked

Diketopyrrolopyrrole Copolymers via Tuning Alkyl Side Chain Weilong Zhou,a Chengzhuo Yu,a Huajie Chen,b Tao Jia,a Weifeng Zhang,b Gui Yub and Fenghong Lia* a

State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical

Chemistry, Jilin University, Changchun 130012, P. R. China b

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy

of Sciences, Beijing 100190, P. R. China

*

E-mail: [email protected] (F. H. Li).

ACS Paragon Plus Environment

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ABSTRACT: Two furan-flanked Diketopyrrolopyrrole copolymers, poly{3,6-difuran-2-yl-2,5di(alkyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-altthienylenevinylene} with different alkyl side chains (PDVFs), have been synthesized and applied as a donor in the polymer solar cells (PSCs). The PSC based on a blend of PDVF-8 with 2-octyldodecyl and [6,6]-phenyl-C71-butyric acid methylester (PC71BM) as an active layer has shown a better device performance than the PSC based on a blend of PDVF-10 with 2-decyltetradecyl and PC71BM. Tuning the alkyl side chains attached on the PDVFs leads to an increase of power conversion efficiency from 3.57% (PDVF10) to 4.56% (PDVF-8) due to enhancements of short circuit current and fill factor. Effect of different alkyl side chains on the phase separation of the PDVF:PC71BM thin film has been investigated by using atomic force microscopy, transmission electron microscopy and x-ray photoemission spectroscopy depth profiling in details. Furthermore, impedance spectroscopy was used to analysis the relationship between the phase separation of the PDVF:PC71BM blend films and the PSCs performance.


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1. INTRODUCTION Polymer solar cells (PSCs) have drawn great attention as renewable energy resources owing to their advantages of synthetic variability, light-weight, low-cost, large-area, flexible-devices and roll to roll fabrication.1-6 Typically, mainstream active layer of the PSCs is a bulk heterojunction blend film which is composed of an electron-accepting fullerene derivative and an electron-donating conjugated polymer (CP).7-11 For a better performance of the PSCs, the CPs should have a narrow band gap for efficient solar energy harvesting,12 a low-lying highest occupied molecular orbital (HOMO) to maximize the open circuit voltage (Voc),13 and a lowest unoccupied molecular orbital (LUMO) level that is appropriately offset above the acceptor’s LUMO to drive efficient charge separation while minimizing potential barriers of electron extracton.14, 15 The synthesis of the CPs by alternating electron-rich and electron-poor moieties along the backbone has emerged as an effective way to tune the optical and electronic properties of the CPs.16, 17 Diketopyrrolopyrrole (DPP) is a successful electron deficient unit used in the CPs as an electron donor in the PSCs. The strong electron deficient character of the DPP endows the CPs with an absorption in the near infrared and ambipolar charge transport in organic field effect transistors with good mobilities for holes and electrons, which is favorable for the highly efficient PSCs .17-25 In addition, in the process of CPs’ design, the effect of solubilizing alkyl chain should not be ignored. Type, length and position of the alkyl side chains on a CP backbone can significantly influence the processability, morphology and transport channel formation in CP:fullerene blend film, which determine the PSCs performance.26-32 For donor−acceptor (D−A) CPs including dithieno[2,3-b;7,6-b]carbazole (DTC) and DPP, the alkyl chain on the DTC unit has a strong impact on the film morphology of CP:PC71BM blends. Severe phase separation was found for polymers containing branched alkyl chains while the CPs with straight alkyl chains


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formed uniform films featuring fine phase separation.31 For poly[{2,5-bis(2-hexyldecyl)-2,3,5,6tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl}-alt-{[2,2’-(1,4-phenylene)bisthiophene]5,5’-diyl}] (PDPPTPT):PC71BM system, power conversion efficiency (PCE) is increased from 3.2%, 5.7% to 7.4% by decreasing the side chain length from 2-decyltetradecyl (DT) via 2octyldodecyl (OD) to 2-hexyldecyl (HD).32 Therefore tuning the side chain length of the CPs is critical for achieving high photovoltaic response. The main reason is that the side chains significantly affect the morphology that is formed when spin-coating the blend films. In this contribution, we present two furan-flanked DPP copolymers, poly{3,6-difuran-2-yl2,5-di(alkly)-pyrrolo[3,4-c]pyrrole-1,4-dione-altthienylenevinylene} (PDVFs) with different alkyl branched side chains, which are PDVF-8 with OD and PDVF-10 with DT as shown in Figure 1a. PDVFs are suitable candidates for application in the PSCs owing to the wide light absorption range up to ~900 nm and high hole mobility of 1.65-1.90 cm2 V-1 s-1.33 Therefore, we fabricated the PSCs based on PDVF as an electron donor and [6,6]-phenyl-C71-butyric acid methylester (PC71BM) as an electron accepter. The PCE of 4.56% for the PSC based on PDVF-8 has been achieved while the PCE is 3.57% for the PSC based on PDVF-10. Obviously the different alkyl branched side chains bring about such a change of PCE. In order to explain the influence of the alkyl branched side chains on device performance, phase separation of donor and acceptor in PDVF:PC71BM blend film were investigated using atomic force microscopy (AFM), transmission electron microscopy (TEM) and x-ray photoemission spectroscopy (XPS) depth profiling in details. Furthermore, relation between the phase separation of the blend film and photovoltaic properties is clearly revealed by the impedance spectroscopy (IS).

2. EXPERIMENTAL SECTION


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2.1 Sample Preparation. Synthesis of the PDVFs can be found in Ref.33. The Mn of PDVF10 and PDVF-8 are 26.4 and 63.8 kDa, respectively. PC71BM was purchased from American Dye Source, Inc (USA). In order to optimize the PSCs performance, a series of PDVF:PC71BM (wt/wt) blend solutions were prepared using 1, 2-dichlorobenzene (o-DCB) as solvent and stirred for 12 hours at 120 °C. Finally, the optimal D/A ratio equal to 1:4 was obtained for the PSCs based on both PDVF-10 and PDVF-8. Current density-voltage curves of the PSCs under illumination were showed in Figure S1 and S2. The corresponding photovoltaic results of the PSCs were listed in Table S1and S2. 2.2 Device Fabrication. ITO-patterned glass substrates were consecutively cleaned with acetone and ethanol in an ultrasonic bath. Surface of the glass substrates was treated by UVozone for 20 minutes. A 35 nm thick PEDOT:PSS (Baytron PVP Al 4083) layer was spin-coated onto the cleaned ITO, followed by annealing at 110 °C for 30 minutes in air. Then PDVF:PC71BM blend solution was spin-coated on the surface of PEDOT:PSS layer at 120 °C to form a 90 nm thick film in the nitrogen-filled glove box. Finally, LiF (0.6 nm) and Al (10 nm) were successively deposited by thermal evaporation under high vacuum (1×10-4 pa) onto the surface of the active layer. All devices have an active area of 2.0 × 2.0 mm2. 2.3 Measurements and Characterizations. Current density-voltage (J-V) characteristics of the devices were measured under N2 atmosphere in the glove box by using Keithley 2400 under illumination and dark respectively. Solar cell performance was tested under 1 sun, AM 1.5G full spectrum solar simulator (Photo Emission Tech. Inc., model #SS50AAA–GB) with an irradiation intensity of 100 mW cm-2 calibrated with a standard silicon photovoltaic traced to the National institute of metrology, China. Space charge limited current (SCLC) measurements were explored in device configurations of a) ITO/PEDOT:PSS/PDVF:PC71BM/MoOx/Al for hole-only device


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and b) ITO/Al/LiF/PDVF:PC71BM/LiF/Al for electron-only device. External quantum efficiency (EQE) spectra were measured using Q Test Station 2000 (Crowntech Inc. USA) at room temperature in air. AFM images were measured with an S II Nanonaviprobe station 300 HV (Seiko, Japan) in taping mode. TEM images were measured using JEM-2100F (JEOL, Japan). Ultraviolet photoelectron spectrum (UPS) and XPS experiments were carried out using a VG Scienta R3000 spectrometer in ultrahigh vacuum with a base pressure of 2×10-10 mbar. The measurement chamber is equipped with a monochromatic He (Iα) ultraviolet light source providing photons with 21.22 eV and a monochromatic Al (Kα) X-ray source providing photons with 1486.6 eV. PDVF-8:PC71BM blend film was etched using an ion source (ISE5, Omicron Nano Tech. GmbH) at 5 KeV beam energy and 10 µA current. S2p and C1s XPS spectra were measured after each etching treatment (4 min per etching treatment). IS measurements were implemented using an Impedance/Gain-Phase Analyzer SI1260 (Solartron Metrology, UK) at room temperature in air. The frequency range was from 1 Hz to 1 MHz; DC bias was set at the Voc value of PSCs; Magnitude of the alternative signal was 30 mV. Obtained IS data were fitted by ZView spectrum Analyzer in terms of appropriate equivalent circuits.


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Figure 1. (a) Molecular structures of PDVFs and PC71BM, (b) device architecture of the PSC based on PDVF:PC71BM as an active layer and (c) energy level diagram of the materials used in the PSCs.

3. RESULTS AND DISSCUSSION Figure 1 shows chemical structures of PDVFs, device configuration of the PSCs based on PDVFs and PC71BM, and energy level diagram of the materials used in the PSCs. HOMO level versus vacuum level of the PDVF-10, PDVF-8 and PC71BM thin film are 5.23 eV, 5.19 eV and 6.13 eV respectively obtained from UPS. LUMO level versus vacuum level of the PDVF-10, PDVF-8 and PC71BM thin film can be estimated to be roughly equal to 3.67 eV, 3.63 eV and 3.93 eV, respectively, using the ionization energy (IEorg) and optical band gap (Eg) (details in the Figure S3).


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Figure 2a shows the Ultraviolet−visible (UV-vis) absorption spectra of the PDVF films, which are composed of two absorption bands in the ranges of 300−500 nm and 500−900 nm, respectively. Two absorption peaks at 500−900 nm are attributed to intramolecular charge transfer between D and A units. The other absorption bands at 300-500nm could be ascribed to a π−π* transition.33 Both PDVF-10 and PDVF-8 have the same absorption maxima (λmax), 796 nm, accompanied by a shoulder peak at 719 nm. Compared to PDVF-10, PDVF-8 has relative wider absorption band, indicating a stronger light absorption capability. Generally high absorption coefficient of the CPs corresponds to efficient photon-harvesting and large short-circuit current (Jsc) in the PSCs. As shown in Figure 2b, PDVF-8:PC71BM blend film has a higher absorption coefficient than PDVF-10:PC71BM blend film in 500-900 nm. Therefore we infer that the Jsc of the PSCs based on PDVF-8:PC71BM could be higher than the Jsc of the PSCs based on PDVF10:PC71BM.

Figure 2. (a) UV−vis absorption spectra of PDVF-8 and PDVF-10 films and (b) UV−vis absorption spectra of 90 nm PDVF:PC71BM blend films. Figure 3a shows J−V characteristics of the PSCs (Figure 1b). Corresponding photovoltaic data are summarized in Table 1. The Voc values of both devices are 0.650 V which could be deduced from their close HOMO values. Both Jsc and Fill factor (FF) of the PDVF-8 PSC are higher than those of the PDVF-10 PSC. As shown in Table 1, the Jsc values of the PDVF-8 PSC and PDVF-


8

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10 PSC are 10.56 mA/cm2 and 8.58 mA/cm2, respectively and the FF values of the PDVF-8 PSC and PDVF-10 PSC are 64.1% and 66.6%, respectively. As a result, PDVF-8 PSC has a higher PCE (4.56%) than PDVF-10 PSC (3.57%). Table 1 also presents series resistances (Rs) and shunt resistances (Rsh) of devices obtained from the slops of J-V curves at Voc and Jsc, respectively (Figure 3a). Apparently there is a decrease of Rs and an increase of Rsh for the PDVF-8 PSC compared to the PDVF-10 PSC. In addition, the PDVF-8 PSC exhibits higher rectification ratios at ±1 V than the PDVF-10 PSC (Figure 3b), indicating a better diode quality and suppressed electrical leakage for PDVF-8 PSC. Charge transport property in the PSCs is generally one of important factors for determining the Jsc and FF. Mobilities of electron and hole were measured in the electron-only and hole-only devices using SCLC method (Figure S4a and b) and listed in Table 1. The electron mobility of PDVF-10 device (6.12 ×10-4 cm2 V-1 s-1) is almost equal to PDVF-8 device (6.74 ×10-4 cm2 V-1 s-1). However, the hole mobility of PDVF-10 device (8.67 ×10-5 cm2 V-1 s-1) is lower than PDVF-8 device (1.25 ×10-4 cm2 V-1 s-1). Therefore PDVF-8 PSC has higher Jsc and FF than PDVF-10 PSC. (a)

(b) 102 PDVF-10 PDVF-8

2

2

-2

Current density (mA/cm )

0

Current density (mA/cm )

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


-4 -6 -8 -10 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Voltage (V)

0

10

PDVF-10 PDVF-8

-2

10

-4

10

-6

10

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage(V)

Figure 3. Current density versus voltage characteristics of PSCs based on PDVF:PC71BM under 100 mW cm-2 AM 1.5G illumination (a) and in the dark (b).


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In order to detect a discrepancy of Jsc which easily leads to overvalued PCE, 34 EQE spectra of the PSCs from 300 nm to 900 nm were measured and shown in Figure 4. Jsc values of PDVF10 PSC and PDVF-8 PSC calculated from integration of the EQE spectra are 8.00 mA cm-2 and 9.85 mA cm-2, respectively, which closely match 8.58 mA cm-2 (ca. 7.3% error) and 10.56 mA cm-2 (ca. 7.2% error) obtained from their J–V characteristics under illumination. It means that the Jsc values measured for the PSCs are reliable and PCE values presented in this manuscript are not overvalued. In addition, the Jsc is influenced by efficiency of charge generation, which can be inferred from the spectrally resolved EQE. For our case, the EQE values dramatically increased in the PDVF-8 PSC compared to PDVF-10 PSC in the given wavelength, probably indicating more efficient exciton generation, exciton dissociation or charge extraction in PDVF-8 PSC.35-37 PDVF-10 PDVF-8

40 30 EQE (%)

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20 10 0 300

400

500

600

700

800

900

Wavelength (nm)

Figure 4. EQE spectra of PSCs based on PDVF:PC71BM. Table 1. Photovoltaic Performances of PSCs Based on PDVF:PC71BM Rs [Ω Ω cm2]

Rsh [Ω Ω cm2]

Rectification ratio

µ eb

µ hb

[cm2 V-1 s-1]

[cm2 V-1 s-1]

3.52

11.5

804.7

× 102 2.4×

×10-4 6.12×

×10-5 8.67×

4.49

6.8

1125.7

× 103 1.8×

×10-4 6.74×

×10-4 1.25×

PCE [%]

Devicea

Jsc [mA cm-2]

JscEQE [mA cm-2]

Voc [V]

FF [%]

Max

Aver

PDVF-10

8.58

8.00

0.65

64.1

3.57

PDVF-8

10.56

9.85

0.65

66.6

4.56


10

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a

Device structure: [ITO/PEDOT:PSS/PDVF:PC71BM/LiF/Al]. bCharge carrier mobilities were obtained from electron-only and hole-only devices using SCLC method. In order to confirm that the enhanced Jsc and FF in the PDVF-8 PSC is related with formation of well-connected percolated networks in the blend active layer, AFM and TEM have been used to investigate the morphologies of the PDVF:PC71BM blend films. Figure 5 shows the surface morphology obtained from AFM measurements at ambient. The surface of PDVF-8:PC71BM blend film exhibits smoother and more homogeneous morphology due to a smaller root-meansquare (RMS) roughness (1.91 nm) than PDVF-10:PC71BM blend film (2.02 nm).

Figure 5. AFM height images of PDVF-10:PC71BM film (a) and PDVF-8:PC71BM film (b). From the TEM imaging (Figure 6), we observed a dramatic deference of bulk morphology between PDVF-10:PC71BM blend film and PDVF-8:PC71BM blend film. In contrast to polymer, PC71BM has higher electron density leading to TEM electron beam to be scattered more efficiently by it. Thus, the darker regions in the TEM images are the regions of PC71BM while the lighter regions are the regions of PDVFs. It is clear that both PDVF-10 and PDVF-8 display networked nanoscale fibers inside the blends with the width of 15-20 nm and 5-10 nm, respectively. The significant decrease of fibers width in the PDVF-8:PC71BM blend film indicates that the PDVF-8 is more compatible with PC71BM than PDVF-10. Moreover, the PDVF-8 fibers in the blend have a closer width value to exciton diffusion length (

Improving the Photovoltaic Performance of Polymer Solar Cells Based

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