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Article pubs.acs.org/JPCC
Improving the Photovoltaic Performance of Polymer Solar Cells Based on Furan-Flanked Diketopyrrolopyrrole Copolymers via Tuning the Alkyl Side Chain Weilong Zhou,† Chengzhuo Yu,† Huajie Chen,‡ Tao Jia,† Weifeng Zhang,‡ Gui Yu,‡ and Fenghong Li*,† †
State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun 130012, People’s Republic of China ‡ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *
ABSTRACT: Two furan-flanked diketopyrrolopyrrole copolymers, poly(3,6-difuran-2-yl-2,5-di(alkyl)-pyrrolo[3,4-c]pyrrole1,4-dione-altthienylenevinylene) with different alkyl side chains (PDVFs), have been synthesized and applied as a donor in polymer solar cells (PSCs). The PSC based on a blend of PDVF-8 with 2-octyldodecyl and [6,6]-phenyl-C71-butyric acid methyl ester (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% (PDVF-10) to 4.56% (PDVF-8) due to enhancements of short circuit current and fill factor. The 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 analyze the relationship between the phase separation of the PDVF/ PC71BM blend films and the PSCs performance. 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 PSC’s performance.26−32 For donor−acceptor (D−A) CPs including dithieno[2,3-b;7,6b]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, whereas the CPs with straight alkyl chains formed uniform films featuring fine phase separation.31 For the 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 2-octyldodecyl (OD) to 2-hexyldecyl (HD).32 Therefore, tuning the side chain length of the CPs is critical for achieving high photovoltaic
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, flexibility, and roll-to-roll fabrication.1−6 Typically, the 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 extraction.14,15 The synthesis of the CPs by alternating electron-rich and electronpoor 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 electrondeficient 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 © 2016 American Chemical Society
Received: January 27, 2016 Revised: February 16, 2016 Published: February 19, 2016 4824
DOI: 10.1021/acs.jpcc.6b00890 J. Phys. Chem. C 2016, 120, 4824−4832
Article
The Journal of Physical Chemistry C
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.
PDVF-10 and PDVF-8. Current density−voltage curves of the PSCs under illumination are shown in Figures S1 and S2. The corresponding photovoltaic results of the PSCs are listed in Tables S1 and S2. 2.2. Device Fabrication. Indium−tin oxide (ITO)patterned glass substrates were consecutively cleaned with acetone and ethanol in an ultrasonic bath. The surface of the glass substrates was treated by UV−ozone for 20 min. A 35 nm thick poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS) (Baytron PVP Al 4083) layer was spin-coated onto the cleaned ITO, followed by annealing at 110 °C for 30 min in air. Then PDVF/PC71BM blend solution was spincoated on the surface of PEDOT/PSS layer at 120 °C to form a 90 nm thick film in the nitrogen-filled glovebox. 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 glovebox 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 no. 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 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., U.S.A.) at room temperature in air. AFM images were measured with an S II Nanonaviprobe station 300 HV (Seiko, Japan) in taping mode. TEM images
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-yl-2,5-di(alkyl)-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-C71butyric acid methyl ester (PC71BM) as an electron acceptor. 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 PDVF10. 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 was investigated using atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray photoemission spectroscopy (XPS) depth profiling in details. Furthermore, the relation between the phase separation of the blend film and photovoltaic properties is clearly revealed by the impedance spectroscopy (IS).
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Synthesis of the PDVFs can be found in ref 33. The Mn’s of PDVF-10 and PDVF-8 are 26.4 and 63.8 kDa, respectively. PC71BM was purchased from American Dye Source, Inc. (U.S.A.). In order to optimize the PSCs performance, a series of PDVF/PC71BM (w/w) blend solutions were prepared using 1,2-dichlorobenzene (o-DCB) as solvent and stirred for 12 h at 120 °C. Finally, the optimal D/A ratio equal to 1:4 was obtained for the PSCs based on both 4825
DOI: 10.1021/acs.jpcc.6b00890 J. Phys. Chem. C 2016, 120, 4824−4832
Article
The Journal of Physical Chemistry C
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 3. Current density vs voltage characteristics of PSCs based on PDVF/PC71BM under 100 mW cm−2 AM 1.5G illumination (a) and in the dark (b).
Table 1. Photovoltaic Performances of PSCs Based on PDVF/PC71BM PCE [%] devicea
Jsc [mA cm−2]
JscEQE [mA cm−2]
Voc [V]
FF [%]
max
av
Rs [Ω cm2]
Rsh [Ω cm2]
rectification ratio
μe [cm2 V−1 s−1]b
μh [cm2 V−1 s−1]b
PDVF-10 PDVF-8
8.58 10.56
8.00 9.85
0.65 0.65
64.1 66.6
3.57 4.56
3.52 4.49
11.5 6.8
804.7 1125.7
2.4 × 102 1.8 × 103
6.12 × 10−4 6.74 × 10−4
8.67 × 10−5 1.25 × 10−4
a
Device structure: [ITO/PEDOT/PSS/PDVF/PC71BM/LiF/Al]. bCharge carrier mobilities were obtained from electron-only and hole-only devices using the SCLC method.
and PC71BM thin film are 5.23, 5.19, 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, 3.63, and 3.93 eV, respectively, using the ionization energy (IEorg) and optical band gap (Eg) (details in the Figure S3). 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 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 300−500 nm 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, a 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 the 500−900 nm region. 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 PDVF-10/PC71BM.
were measured using a 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. S 2p and C 1s 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, U.K.) 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.
3. RESULTS AND DISCUSSION 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, 4826
DOI: 10.1021/acs.jpcc.6b00890 J. Phys. Chem. C 2016, 120, 4824−4832
Article
The Journal of Physical Chemistry C
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 PDVF10 PSC in the given wavelength, probably indicating more efficient exciton generation, exciton dissociation, or charge extraction in PDVF-8 PSC.35−37 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 conditions. 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). 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 the 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 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 PDVF8 fibers in the blend have a closer width value to exciton diffusion length (
