Controlling Energy Levels and Blend Morphology for All-Polymer

Aug 22, 2016 - We investigate the photovoltaic properties and charge dynamics of all polymer solar cells (all-PSCs) based on poly[(N ...
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Controlling Energy Levels and Blend Morphology for All-Polymer Solar Cells via Fluorination of a Naphthalene Diimide-Based Copolymer Acceptor Mohammad Afsar Uddin,†,‡ Youngkwon Kim,§ Robert Younts,∥ Wonho Lee,§ Bhoj Gautam,∥ Joonhyeong Choi,§ Cheng Wang,⊥ Kenan Gundogdu,*,∥ Bumjoon J. Kim,*,§ and Han Young Woo*,† †

Department of Chemistry, Korea University, Seoul 02841, Republic of Korea Department of Cogno-Mechatronics Engineering, Pusan National University, Miryang 627-706, Republic of Korea § Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea ∥ Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, United States ⊥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡

S Supporting Information *

ABSTRACT: We investigate the photovoltaic properties and charge dynamics of all polymer solar cells (all-PSCs) based on poly[(N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′-(2,2′-bithiophene)] (P(NDI2OD-T2)) and its fluorinated analogue, poly[(N,N′bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)2,6-diyl)-alt-5,5′-(3,3′-difluoro-2,2′-bithiophene)] (P(NDI2OD-T2F)), as the acceptor polymer and poly[4,8bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)dione] (PBDTTTPD) as the donor polymer. The PBDTTTPD:P(NDI2OD-T2)-based device has a high open-circuit voltage (VOC) of 1.03 V but suffers from low power conversion efficiency (PCE) of 2.02% with a short-circuit current density (JSC) and fill factor (FF) of 4.45 mA cm−2 and 0.44, respectively. In a stark contrast, the PCE of PBDTTTPD:P(NDI2OD-T2F)-based PSC dramatically increases to 6.09% (VOC = 1.00 V, JSC = 11.68 mA cm−2, and FF = 0.52). These results are attributed to the fluorination, which removes the energetic barrier for hole transfer and promotes the formation of the donor/acceptor blend morphology with suppressed phase separation and enhanced intermixed phases. The detailed charge dynamics examined by femtosecond transient absorption spectroscopy suggests the significantly increased hole transfer efficiency and larger populations of long-lived polarons for PBDTTTPD:P(NDI2ODT2F).

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INTRODUCTION Bulk-heterojunction (BHJ) polymer solar cells (PSCs), in which electron-donating conjugated polymers and electronaccepting fullerene derivatives are often used as an active layer, have been extensively studied during the past ∼10 years,1 demonstrating power conversion efficiencies (PCEs) over ∼10%.2 However, the use of fullerene-based acceptors (e.g., phenyl-C61-butyric acid methyl ester (PCBM)) has several disadvantages including (i) limited tunability of the chemical structures and the energy levels, (ii) poor mechanical and thermal stabilities caused by brittleness and the rapid diffusion property of fullerene, (iii) low light absorbance in the visible range, and (iv) low solution viscosity, which are considered as a big hurdle for their use in the large-scale printing processed PSCs.3−7 To overcome these problems, all-polymer solar cells (all-PSCs), composed of binary blend of conjugated polymers used as both donor and acceptor, have been investigated to © XXXX American Chemical Society

replace the fullerene-based PSCs. In recent years, the PCEs of all-PSCs have been significantly improved up to 6−8% by developing new n-type conjugated polymers with high electron mobility and favorable molecular interaction with the donor polymer.8−27 Moreover, all-PSCs have several advantages in terms of enhancing short-circuit current (JSC) and open-circuit voltage (VOC) because the energy levels of both donor and acceptor polymers can be easily tuned through the push−pull strategy with different combinations of monomers, optimizing the energy difference between the highest occupied molecular orbital (HOMO) of the polymer donor and the lowest unoccupied molecular orbital (LUMO) of the polymer acceptor.28−34 At the same time, achieving optimal BHJ blend Received: July 1, 2016 Revised: August 6, 2016

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Figure 1. (a) Molecular structures, (b) normalized UV−vis absorption spectra of PBDTTTPD, P(NDI2OD-T2), and P(NDI2OD-T2F) in film, and (c) energy level diagram.

have deep HOMO levels, while maintaining sufficient HOMO−HOMO level offset. Recently, Jen and co-workers reported that the crystalline properties of fluorinated NDI polymers substantially improved PCEs of all-PSCs together with side chain engineering.8 While the importance of the fluorination of polymer acceptors has been well demonstrated in-depth studies of the fluorination effect on the n-type polymer properties and their photovoltaic characteristics in terms of energy levels, charge generation, and transfer dynamics need to be investigated. In this report, we described the effect of fluorination in controlling both optical and electronic properties of a NDIbased polymer acceptor. In particular, we systematically investigated the fluorination effect on the donor/acceptor interfacial interactions and the resulting morphological characteristics as well as the charge dynamics affecting the performances of all-PSCs. Two different NDI-based n-type polymers, poly[(N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl)-alt-5,5′-(2,2′-bithiophene)] (P(NDI2OD-T2))25,55−59 and the fluorine-incorporated poly[(N,N ′-bis(2-octyldodecyl)na phthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′-(3,3′-difluoro-2,2′-bithiophene)] (P(NDI2OD-T2F)), were synthesized for this comparison study. Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione] (PBDTTTPD)30,60,61 was chosen as the donor material because it shows a complementary absorption with those of the acceptor polymers and the lowlying HOMO level of PBDTTTPD is expected to produce a high VOC value in all-PSCs. Surprisingly, the PCEs of PBDTTTPD:P(NDI2OD-T2) and PBDTTTPD:P(NDI2OD-

morphology is a critical factor for efficient exciton dissociation and charge transport in the PSCs.2,35−37 Controlling the BHJ morphology of all-PSCs is a great challenge because of the significantly reduced entropic contribution to the Gibbs free energy, favoring the demixing of the two different polymers.31 In addition, the orientation of polymer acceptors in thin film should be carefully controlled to enhance the JSC and fill factor (FF) values of the all-PSCs.10,13,15,16,24,38 In recent years, many researchers have attempted to improve the PCEs of all-PSCs by tuning the chemical structures of ntype polymers.8,10,14,15,18,19,21,39 In particular, such efforts have been focused on developing new naphthalene diimide (NDI)based copolymers by varying electron-donating units and side chain engineering.8,10,15,39 Among various strategies to modify the chemical structures, the fluorination of the polymer backbone has been proved to be very effective in enhancing the performance of polymer:fullerene BHJ systems.40−53 For example, Yu et al. experimentally demonstrated that the fluorination of benzodithiophene−thienothiophene copolymers lowered both their LUMO and HOMO levels because of the strong electron-withdrawing ability of fluorine atoms.52 Subsequently, a higher VOC was obtained in the fluorinated copolymer-based PSCs. And, the JSC and FF could increase owing to the improved blend morphology that was attained by decreasing the surface tension of polymer by fluorination. Similar enhancements in photovoltaic properties by fluorination were reported for the poly(3-hexylthiophene)-based polymers and the benzothiadizole copolymer studies.51,53,54 However, the photovoltaic effects of fluorination on the n-type polymers in all-PSCs have been studied little. For producing high VOC in all-PSCs, both donor and acceptor polymers should B

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configuration of ITO/PEDOT:PSS/active layer/LiF/Al. Two different active layers of PBDTTTPD:P(NDI2OD-T2) and PBDTTTPD:P(NDI2OD-T2F) were prepared, and their photovoltaic properties were compared. Detailed procedures for device fabrication are described in the Experimental Section. The optimized blend ratio of polymer donor:polymer acceptor was determined to be 1.4:1.0 (w/w) (see Table S1), and the optimized thickness of the polymer blend film (spin-cast from chloroform solution) was in the range 90−100 nm, regardless of the types of polymer acceptors. Finally, 1,8-diiodooctane (DIO, 1 vol %) was used as a processing additive to modulate the blend morphology. Figure 2a shows the current density versus voltage (J−V) curves and the external quantum efficiency (EQE) of PBDTTTPD:P(NDI2OD-T2) and PBDTTTPD:P(NDI2ODT2F) based all-PSCs. The detailed photovoltaic parameters of the all-PSC devices are listed in Table 2. The PBDTTTPD:P(NDI2OD-T2) device had high VOC of 1.03 V, but the resulting PCE (2.02%) was low with poor JSC and FF values of 4.45 mA cm−2 and 0.44, respectively. The use of the solvent additive affected the device performance slightly. In contrast, the PCE value of PBDTTTPD:P(NDI2OD-T2F)-based PSC dramatically increased to 6.09% (VOC = 1.00 V, JSC = 11.68 mA cm−2, and FF = 0.52). In this case, the use of the DIO additive had a significant improvement in the device performance from 4.26 to 6.09%. Notably, this PCE value was similar to or even higher than the PCE values of 4−6% reported previously for the PBDTTTPD:PCBM PSCs in the literature.30,61,62 This remarkable enhancement of PCE in the PBDTTTPD:P(NDI2OD-T2F)-based PSC is mainly attributed to the improvement of JSC and FF values. The JSC enhancement is well matched with the changes in their spectral responses in the EQE curves (Figure 2b). Clearly, the EQE values of the PBDTTTPD:P(NDI2OD-T2F) device are much higher than those of PBDTTTPD:P(NDI2OD-T2) for the entire range of wavelengths. In particular, the maximum EQE (EQEmax) of PBDTTTPD:P(NDI2OD-T2F) was measured to be as high as 62.4% at λ = 570 nm while that of PBDTTTPD:P(NDI2ODT2) was only 22.6%. The immediate impact of fluorination on the electronic structure of the polymers is the shift in the electronic energy levels due to strong electronegativity of fluorine atoms. Table 1 summarizes the HOMO and LUMO energy levels of the three polymers as measured by cyclic voltammetry (CV) (Figure S2). PBDTTTPD, P(NDI2OD-T2), and P(NDI2OD-T2F) have the LUMO/HOMO levels of −3.51/−5.61, − 3.78/−5.75, and −3.90/−5.99 eV, respectively. By introducing fluorine atoms to the bithiophene moiety in the acceptor polymer, both the HOMO and LUMO energy levels of P(NDI2OD-T2F) decreased compared to those of P(NDI2OD-T2). In particular, the HOMO level significantly decreased from −5.75 eV (P(NDI2OD-T2)) to −5.99 eV (P(NDI2OD-T2F)). Considering a deep HOMO level of −5.61 eV of PBDTTTPD, the HOMO−HOMO offset (0.38 eV) between PBDTTTPD and P(NDI2OD-T2F) would be sufficient for producing efficient exciton dissociation by hole transfer and generating free charge carriers at the donor/acceptor interfaces. In contrast, HOMO− HOMO offset between PBDTTTPD and P(NDI2OD-T2) is very small, 0.14 eV, and probably is not enough to provide a driving force for hole transfer across the interface. These results indicate that the fluorination of NDI-based polymer acceptors is very effective in tuning the energy levels and producing an optimized pair with the donor polymer with a deep HOMO

T2F) devices showed largely different values of 2.02% and 6.09%, respectively. This dramatic contrast was caused by the different behaviors (with and without fluorination) associated with the alignment of the energy levels and the blend morphologies. Fluorination leads to efficient charge separation (especially hole transfer) by decreasing the HOMO level of P(NDI2OD-T2F). In addition, the fluorination produced more favorable interfacial interaction in the PBDTTTPD:P(NDI2OD-T2F) blend and generated the desired BHJ morphology with suppressed macrophage separation and well-developed intermixed phase. Charge generation, transfer, and recombination dynamics in both blends were also significantly dependent on the energy level alignment and blend morphology, as measured by transient absorption spectroscopy (TAS). The TAS data show a remarkable enhancement in hole transfer efficiency and longer polaron lifetime, leading to high JSC and FF in all-PSCs with fluorinated acceptors. Thus, the fluorination of a polymer backbone may suggest an effective design strategy of N-type polymers for obtaining both high VOC and JSC values.

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RESULTS AND DISCUSSION Figure 1 shows the molecular structures of donor and acceptor polymers, PBDTTTPD, P(NDI2OD-T2), and P(NDI2ODT2F), used in this study. PBDTTTPD and P(NDI2OD-T2) were synthesized using procedures described in our previous literature.16,60 P(NDI2OD-T2F) was synthesized by the Stille cross-coupling between N,N-bis(2-octyldodecyl)-2,6-dibromonaphthalene-1,4,5,8-bis(dicarboximide) and 5,5′-bis(trimethylstannyl)-3,3′-difluoro-2,2′-bithiophene with Pd2(dba)3 as a catalyst in chlorobenzene using a microwave reactor (∼60% yield). The number-average molecular weight (Mn) and molecular-weight distribution (polydispersity index, Đ) were determined to be 20 kDa (Đ = 2.9), 42 kDa (Đ = 2.6), and 30 kDa (Đ = 2.8) by gel permeation chromatography (GPC) for PBDTTTPD, P(NDI2OD-T2), and P(NDI2OD-T2F), respectively (Table 1). P(NDI2OD-T2) and P(NDI2OD-T2F) have Table 1. Summary of Physical, Optical, and Electrochemical Properties of Polymers polymers PBDTTTPD P(NDI2ODT2) P(NDI2ODT2F)

λabs (in film) [nm]

Egopt b [eV]

LUMOc [eV]

HOMOc [eV]

2.9 2.6

614 627

1.85 1.50

−3.51 −3.78

−5.61 −5.75

2.8

695

1.60

−3.90

−5.99

Mna [kDa]

Đ

20 42 30

a

Determined by GPC with o-dichlorobenzene as the eluent at 80 °C. Determined by UV−vis absorption onsets in the polymer films. c LUMO and HOMO levels were measured by cyclic voltammetry. a b

reasonably high Mn values and similar Đ values, thus minimizing the effects of molecular weight on the polymer properties. Figures 1b and 1c show the UV−vis absorption spectra of the three polymers and their HOMO and LUMO energy level alignments. The absorption maxima were measured at 614, 627, and 695 nm in film for PBDTTTPD, P(NDI2OD-T2), and P(NDI2OD-T2F), respectively. The overall absorption range of P(NDI2OD-T2F) was blue-shifted by the fluorination compared to that of P(NDI2OD-T2). To examine the fluorination effect on the device properties, the conventional type all-PSCs were fabricated with the C

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Figure 2. (a) J−V curves and (b) EQE spectra of PBDTTTPD:P(NDI2OD-T2) and PBDTTTPD:P(NDI2OD-T2F) all-PSCs with and without DIO.

Table 2. Summary of Photovoltaic Characteristics of PBDTTTPD:P(NDI2OD-T2) and PBDTTTPD:P(NDI2OD-T2F) AllPSC Devices all-PSCs (PBDTTTPD: acceptor) P(NDI2OD-T2) P(NDI2OD-T2) P(NDI2OD-T2F) P(NDI2OD-T2F) a

DIO (vol %) 0 1 0 1

VOC (V) 1.03 1.03 0.99 1.00

JSC (mA cm−2)

FF

4.37 4.45 9.38 11.68

0.42 0.44 0.46 0.52

PCEmax (%) 1.88 2.02 4.26 6.09

(1.87 (1.98 (4.22 (5.88

± ± ± ±

0.02) 0.05) 0.05) 0.12)

a

EQEmax (%)

calcd JSC (mA cm−2)

22.6 23.5 51.0 62.4

4.11 4.33 9.24 11.23

Average value of PCE ± standard deviation for ∼20 different measurements.

Figure 3. AFM height images of (a) PBDTTTPD:P(NDI2OD-T2) (RMS roughness = 2.93 nm) and (b) PBDTTTPD:P(NDI2OD-T2F) (RMS roughness = 1.75 nm). The scale bars are 500 nm. (c) R-SoXS profiles of PBDTTTPD:P(NDI2OD-T2) and PBDTTTPD:P(NDI2OD-T2F) blends.

level. Thus, the dramatically enhanced JSC and FF values of the PBDTTTPD:P(NDI2OD-T2F) all-PSC device might be related to favorable offset between the HOMO−HOMO energy levels of the donor−acceptor pairs. The energetic cost to the VOC by the decreased HOMO (donor) − LUMO (acceptor) energy difference is very small with only 30 meV (1.03 V → 1 V) loss due to fluorination. At first, this seems surprising because the LUMO level of P(NDI2OD-T2F) was substantially lowered by 0.12 eV. However, as recently discussed by Burke et al., VOC depends primarily on the interfacial charge transfer (CT) state energy and not to the HOMO−LUMO offset of the donor/acceptor molecules. Thus, the large decrease in the LUMO of the acceptor polymer did not lead to a similarly large sacrifice from VOC.63 Structural characterization further elucidates the origin of stark contrast in the performances of the PBDTTTPD:P(NDI2OD-T2) and PBDTTTPD:P(NDI2OD-T2F) devices. We performed grazing incidence X-ray scattering (GIXS), atomic force microscopy (AFM), and resonant soft X-ray scattering (R-SoXS) measurements to investigate the microstructures and blend morphologies of the thin film blends. The blend films were prepared under identical device fabrication conditions by spin-coating from chloroform solution. The two-

dimensional GIXS images of PBDTTTPD:P(NDI2OD-T2) and PBDTTTPD:P(NDI2OD-T2F) blend films are displayed in Figure S3. Both blend films show a preferential face-on π−π stacking at the same peak position (qz = 1.71 Å−1 and d010 = 3.67 Å) in the out-of-plane direction (qz), but the peak intensity is much stronger for the PBDTTTPD:P(NDI2OD-T2) than PBDTTTPD:P(NDI2OD-T2F) blend film. Also, PBDTTTPD:P(NDI2OD-T2) blend film shows the more pronounced lamellar scattering peaks (h00) up to the third order (300) in the qz compared to PBDTTTPD:P(NDI2ODT2F), suggesting the stronger self-aggregation behavior of P(NDI2OD-T2) polymer in the blend film.25,60 In addition, PBDTTTPD:P(NDI2OD-T2) blend film had stronger (001) peak from polymer acceptor in the in-plane direction (qxy) than that in PBDTTTPD:P(NDI2OD-T2F) blend film.64 The reduced self-aggregation behavior of P(NDI2OD-T2F) could enhance the interaction with PBDTTTPD and affect the phase separation in the PBDTTTPD:P(NDI2OD-T2F) blend by promoting the formation of intermixed phase of donor and acceptor polymers. Figure 3 shows the AFM surface morphology of PBDTTTPD:P(NDI2OD-T2) and PBDTTTPD:P(NDI2ODT2F) films. The PBDTTTPD:P(NDI2OD-T2) film has a more D

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Figure 4. TA spectra for (a) pristine PBDTTTPD film, (b) PBDTTTPD:P(NDI2OD-T2) blend film, and (c) PBDTTTPD:P(NDI2OD-T2F) blend film with a pump excitation 2.0 eV.

(NDI2OD-T2F) blends to compare the intermixing and exciton separation (Figure S6). Although there is some spectral overlap, excitation wavelengths were chosen to predominately excite either the donor (electron transfer) at 540 nm or the acceptor (hole transfer) at 385 and 375 nm for the nonfluorinated and fluorinated acceptors, respectively. Although this comparison is only approximate, the measured PL quenching efficiencies from electron transfer in the PBDTTTPD:P(NDI2OD-T2F) and PBDTTTPD:P(NDI2OD-T2) blend films were 95% and 86%, respectively. The slightly diminished PL quenching for PBDTTTPD:P(NDI2OD-T2) blend may be related (partially) to the larger phase separation, suppressing exciton dissociation at the PBDTTTPD/P(NDI2OD-T2) interface. The measured PL quenching from hole transfer results in 50% and 6% for PBDTTTPD:P(NDI2OD-T2F) and PBDTTTPD:P(NDI2OD-T2), respectively. While PL quenching from electron transfer is slightly increased, the quenching from hole transfer drastically improves upon fluorination. Last but not least in order to thoroughly investigate the effect of the different energetic alignment and morphologies for charge generation and recombination dynamics, TAS measurements were performed. In these experiments by predominantly photoexciting either the donor or the acceptor moieties, we measured the electron and hole transfer processes. It is not possible to solely excite the donor or acceptor and completely isolate the electron and hole transfer dynamics because of the spectral overlap between the donor and acceptor polymers. However, by tuning the excitation pulse to the spectral regions where there is a large contrast in the absorption of the donor and acceptor, we were able to deduce information about electron and hole transfer at the interfaces. In the first set of experiments, the donor polymer was predominantly excited with 2.0 eV (620 nm) pump pulses, creating a photoexcitation density of ∼1016 cm−3. In the second set of experiments, the acceptor polymer was preferentially excited with 3.31 eV (375 nm) and 3.22 eV (385 nm) for P(NDI2OD-T2F) and P(NDI2OD-T2), respectively, with a photoexcitation density ∼1017 cm−3. The TAS results for the pristine polymer donor and blend films with donor excitation are shown in Figure 4. Reduced absorption in the visible region (>1.9 eV) is due to bleaching of the ground state (GSB) for all thin films. In the near-IR region, several features contribute to the transient spectra. Photoinduced absorption (PIA) due to the donor polymer singlet excitons (EXD) is centered at ∼1.05 eV. A

distinctly phase-separated structure with larger aggregates, which is partly attributed to the stronger aggregation of P(NDI2OD-T2) than its fluorinated analogue as shown in the GIXS results. PBDTTTPD:P(NDI2OD-T2) also showed a rougher surface (root-mean-square (RMS) roughness = 2.93 nm) compared to the PBDTTTPD:P(NDI2OD-T2F) film (RMS roughness = 1.75 nm). This difference in the morphological characteristics upon fluorination was also elucidated by the R-SoXS measurement.38,65,66 Figure 3c and Figure S4 show the R-SoXS scattering profiles of the two blends of PBDTTTPD:NDI-based copolymers acquired at 287.5 eV, in which the maximum scattering contrast between the two polymers was observed.61 The PBDTTTPD:P(NDI2OD-T2) blend film had a distinct peak at q = 0.0068 Å−1, thus representing a domain scale (d = 2π/q) of 92 nm. In contrast, the PBDTTTPD:P(NDI2OD-T2F) sample showed very weak and broad scattering intensity profile without the distinct maximum peak, indicating that PBDTTTPD:P(NDI2OD-T2F) has a significantly reduced pure donor and acceptor domains with suppressed macrophase separation and thus better intermixed phase relative to PBDTTTPD:P(NDI2OD-T2).10,67 To understand this morphological difference between the two polymer blends, the interfacial tensions (γ) between PBDTTTPD and P(NDI2OD-T2) and between PBDTTTPD and P(NDI2OD-T2F) were compared by measuring the contact angles (Figure S5 and Table S2).61 The γ values of the two different blends were estimated based on the surface tension of each polymer by the Wu model.68,69 The surface tension of P(NDI2OD-T2F) (23.60 mN/m) was lower than that of P(NDI2OD-T2) (24.30 mN/m) because the presence of fluorine groups lowers the surface tension. Interestingly, the surface tension of P(NDI2OD-T2F) was very similar to that of PBDTTTPD (23.59 mN/m), producing a very low γ value of 0.01 mN/m. This extremely low γ value decreased significantly the energy penalty associated with producing large interfacial area, resulting in the well-intermixed BHJ morphologies with smaller phase-separated domains in the PBDTTTPD:P(NDI2OD-T2F) blend.70−72 This difference in the interfacial interactions between two different blends was also reflected in the results of the GIXS, AFM, and RSoXS measurements. The well-intermixed morphology is important for efficient charge dissociation in all-PSCs. Photoluminescence (PL) quenching was also measured to investigate both electron and hole transfer process in PBDTTTPD:P(NDI2OD-T2) and PBDTTTPD:PE

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Figure 5. Time evolution of (a) exciton absorption and (b) polaron absorption for PBDTTTPD:P(NDI2OD-T2) and PBDTTPD:P(NDI2ODT2F) blended thin films with a pump excitation 2.0 eV.

Figure 6. (a) Time traces of TA at ∼1.45 eV for PBDTTTPD:P(NDI2OD-T2F), PBDTTTPD:P(NDI2OD-T2), and P(NDI2OD-T2) films. (b) Deconvoluted EXA and CS contribution to the TA dynamics in the blends of PBDTTTPD:P(NDI2OD-T2F) and PBDTTTPD:P(NDI2OD-T2). Pump excitation was 3.31 eV for PBDTTTPD:P(NDI2OD-T2F) and 3.22 eV for PBDTTTPD:P(NDI2OD-T2).

weaker PIA feature centered at 1.4 eV was observed from the excited-state absorption of the donor polarons in chargeseparated (CS) states. These assignments of the exciton and polaron PIA spectral features are consistent with the previous results of TAS for polymer blends.13,73,74 In blends, the singlet exciton PIA peak decays faster and the polaron peak lasts for a longer period (Figure 4), which further confirms the assignment. By measuring the time evolution of the EXD and CS spectral features after donor excitation, we determined the effect of fluorination on the electron transfer dynamics. To isolate the contributions of the EXD and CS to the PIA, the near-IR spectra were modeled as a linear combination of the EXD spectral line shape at 1 ps from the pristine PBDTTTPD film and a Gaussian distribution centered at ∼1.4 eV, representing additional PIA contributions from the CS states.75,76 Figure 5 shows the resulting evolution of the EXD and CS populations upon donor excitation after spectral deconvolution. For both of the blended films, the depopulation dynamics of the donor singlet exciton was very similar. This indicates that photoinduced electron transfer is not altered upon fluorination. Figure 5b shows the polaron generation and decay dynamics. Two important observations are as follows: (i) As shown in the inset in Figure 5b, polarons form promptly with a rise time of 0.98 ps in PBDTTTPD:P(NDI2OD-T2), whereas for the PBDTTTPD:P(NDI2OD-T2F) blend, polaron formation continues over a longer time period characterized by a rise time of 7.5 ps and reaches to higher amplitude in ultrafast time

scales. (ii) The second observation is the large difference in the amount of residual charges in the two blends. Within the first 5 ns after excitation, a significant portion of the polaron population decays with a decay time constant of 415 ps in PBDTTTPD:P(NDI2OD-T2), whereas the population is 3.3 times larger and its decay is much slower (945 ps decay time constant) in PBDTTTPD:P(NDI2OD-T2F). Therefore, upon fluorination, the charge generation process becomes more efficient, yielding more longer-lived polarons, which may originate from favorable film morphology. In the second set of TAS experiments, we predominantly excited the acceptor polymers and monitor the hole transfer process, illuminating the difference in charge dynamics from deepening the acceptor HOMO with fluorination. Figure S7 shows the near-IR TA spectra in the range of 0.85−1.6 eV for the pristine acceptor polymer films and the D:A blended polymer films. Both acceptor-only films have two characteristic singlet exciton absorption peaks featured at 1.15 and 1.45 eV, denoted by EXA, showing very similar dynamics regardless of fluorination. For the blended thin films, two main spectral features were observed. Although the excitation energy for predominant acceptor absorption was selected, the donor singlet exciton absorption band (EXD) at 1.05 eV was still observed. However, this feature was significantly suppressed and short-lived as compared to the measurement with direct donor excitation (2.0 eV) shown in Figure 4. The other broad PIA feature at 1.45 eV is attributed to both the donor polaron (Figure S7) and the acceptor singlet exciton. The same F

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[3,4-c]pyrrole-4,6-dione (TPD), and 4,9-dibromo-2,7-bis(2-octyldodecyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone were purchased from SunaTech, Inc., and used as received without purification. 5,5′-Bis(trimethylstannyl)-2,2′-bithiophene and 5,5′-bis(trimethylstannyl)-3,3′-difluoro-2,2′-bithiophene were synthesized by the literature procedure.46 All reagents were purchased from SigmaAldrich unless specified and used as received. PBDTTTPD and P(NDI2OD-T2) were synthesized by our previous literature procedure.16,60 Synthesis of P(NDI2OD-T2F). The polymer P(NDI2OD-T2F) was synthesized by modification of the previous literature method.8 In a glovebox, 5,5′-bis(trimethylstannyl)- 3,3′-difluoro-2,2′-bithiophene (0.230 g, 0.436 mmol), 4,9-dibromo-2,7-bis(2-octyldodecyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (0.429 g, 0.436 mmol), 2 mol % of tris(dibenzylideneacetone)dipalladium(0), and 8 mol % of tri(o-tolyl)phosphine were added in a 5 mL microwave vial. The vial was sealed, and chlorobenzene (2.5 mL) was added. The polymerization reaction was carried out in the microwave reactor: 10 min at 80 °C, 10 min at 100 °C, and 40 min at 140 °C. The polymer was end-capped by addition of 0.1 equiv of 2-(tributylstannyl)thiophene and further reacted at 140 °C for 20 min. The resulting solution was cooled down, 0.2 equiv of 2-bromothiophene was added by syringe, and again reacted at 140 °C for 20 min. The crude polymer was precipitated into the mixture of methanol:HCl (350 mL:10 mL) and then purified by Soxhlet extraction with acetone, hexane, and chloroform. The dissolved portion in CHCl3 was concentrated under reduced pressure and precipitated into cold methanol. The resulting polymer was dried under vacuum for 24 h. Yield: 60%. 1H NMR (300 MHz, CDCl3): δ (ppm) 8.90−8.60 (br, 2H), 7.25−7.20 (br, 2H), 4.25−3.90 (br, 4H), 2.12−1.75 (br, 2H), 1.74−1.00 (br, 64H), 0.95− 0.70 (br, 12H). Characterizations. 1H and 13C NMR spectra were recorded on a JEOL (JNM-AL300) FT NMR system operating at 300 and 75 MHz, respectively. UV−vis spectra were obtained with a JASCO V-630 spectrophotometer. The number- and weight-average molecular weights of the polymers were determined by GPC with odichlorobenzene as the eluent on an Agilent GPC 1200 series, relative to a polystyrene standard. CV experiments were performed with Versa STAT 3 analyzer. All CV measurements were carried out in 0.1 M tetrabutylammonium tetrafluoroborate (Bu4NBF4) in acetonitrile with a conventional three-electrode configuration employing a platinum wire as a counter electrode, platinum electrode coated with a thin polymer film as a working electrode, and Ag/Ag+ electrode as a reference electrode (scan rate: 50 mV/s). AFM measurements were performed using a Veeco Dimension 3100 instrument in tapping mode. The samples were prepared by spin-cast onto the PEDOT:PSS/ ITO-coated glass. RSoXS measurements were performed at BL 11.0.1.2 in the Advanced Light Source (USA) using a series of photon energies to determine the maximum scattering contrast between the donor and the acceptors. RSoXS samples were prepared on a PEDOT:PSS/glass substrate under same fabrication condition for optimized devices. Then, the active layers were floated on water and transferred to a 1.0 × 1.0 mm, 100 nm thick Si3N4 membrane supported by a 5 × 5 mm, 200 μm thick Si frame (Norcada Inc.). GIXS measurements were performed at beamline 3C in the Pohang Accelerator Laboratory (South Korea). GIXS samples were prepared by spin-coating onto a PEDOT:PSS/Si substrate. X-rays with a wavelength of 1.1179 Å were used. The incidence angle (∼0.12°) was chosen to allow for complete penetration of X-rays into the film. Solar Cell Device Fabrication and Measurements. The conventional type all-PSC devices were fabricated with a decvice architecture of ITO/PEDOT:PSS/polymer blend films/LiF/Al. The ITO-coated glass substrates were subjected to ultrasonication in acetone, deionized water, and isopropyl alcohol. After drying the substrates for 1 h in an oven at 80 °C, the ITO substrates were treated with UV-ozone. A filtered dispersion of PEDOT:PSS in water (PH 500) was spin-cast at 3000 rpm on the top of ITO substrate with ∼30 nm thickness and heated at 150 °C for 20 min in air. For BHJ architectures, donor polymer (PBDTTTPD) and acceptor polymers (P(NDI2OD-T2), P(NDI2OD-T2F)) were blended in chloroform.

deconvolution method as previously described was used to remove the contributions from the neighboring EXD band from the spectral band at ∼1.45 eV, resulting in the dynamics shown in Figure 6a. Using the EXA lifetime obtained from the acceptor-only films and the CS polaron lifetime from the donor excitation of blended films, the charge generation process and the resulting evolution of the TA signal at 1.45 eV were modeled (see Supporting Information), as shown in Figure 6. Our analysis shows that 89% of the initial acceptor excitons undergo charge separation in PBDTTTPD:P(NDI2OD-T2F), whereas only 33% charges separate in PBDTTTPD:P(NDI2OD-T2). These results indicate that insufficient HOMO−HOMO energy offset substantially hinders the hole transfer in the PBDTTTPD:P(NDI2OD-T2) blend and agree well with the EQE spectra (Figure 2b) which show little photoresponse from P(NDI2OD-T2) in the wavelength range of 650−800 nm. From the modeled CS dynamics as shown in Figure 6b, the hole transfer time was calculated to be 13 and 63 ps, and the percentage of long-lived charges was 62 and 43% for PBDTTTPD:P(NDI2OD-T2F) and PBDTTTPD:P(NDI2OD-T2), respectively. In the PBDTTTPD:P(NDI2OD-T2) blend, excitons formed in the acceptor domain do not efficiently undergo charge transfer, and the small percentage of excitons that separate quickly recombine, hampering overall charge generation. In contrast, the hole transfer in the PBDTTTPD:P(NDI2OD-T2F) blend was more efficient due to the increase in the HOMO−HOMO energy offset, producing much faster hole transfer and larger populations of long-lived charges. These enhanced charge transfer efficiencies as well as reduced charge recombination resulted in a dramatic increase in the performance of the PBDTTTPD:P(NDI2OD-T2F)-based all-PSCs.

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CONCLUSION In this study, a highly efficient all-PSC with a PCE value of over 6% was demonstrated based on the fluorinated NDI-based acceptor, P(NDI2OD-T2F) and PBDTTTPD as a donor, showing a stark contrast to the nonfluorinated P(NDI2ODT2)-based all-PSC (PCE = 2.02%). This dramatic enhancement in the PCE value of PBDTTTPD:P(NDI2OD-T2F) device was mainly attributed to the following effects: (1) sufficient HOMO−HOMO offset between the donor and acceptor polymers for efficient hole transfer and (2) optimized BHJ blend morphology with the well-developed intermixed phases, both of which were driven by the fluorination of the acceptor polymer. The fluorination reduced the surface tension of P(NDI2OD-T2F), producing a pair of donor/acceptor polymers with very low interfacial tension and enhanced miscibility, supported by the AFM and RSoXS results. The effects of fluorination on the charge generation (electron/hole transfer) and recombination dynamics in all-PSCs were quantitatively investigated by femtosecond TAS. A long-lived and higher polaron population was clearly observed in PBDTTTPD:P(NDI2OD-T2F), benefited from the sufficient HOMO−HOMO energy offset between the donor and acceptor by fluorination and the well-developed intermixed blend morphology. Our results suggest a guideline of a simple but effective strategy of chemical modulation and proper choice of material pairs for high-performance all-PSCs.

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EXPERIMENTAL SECTION

Materials. 2,6-Bis(trimethyltin)-4,8-bis(5-ethylhexyl-2-thienyl)benzo[1,2-b:4,5-b′]dithiophene (BDTT), 1,3-dibromo-5-octylthienoG

DOI: 10.1021/acs.macromol.6b01414 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules The solutions were stirred at 45 °C for more than 1 h in a glovebox under nitrogen. The optimum donor:acceptor (D:A) blending ratio was 1.4:1 (w/w), and total concentration (D + A) was 13 mg/mL with 1.0 vol % DIO. The solutions were then filtered by a 0.45 μm polytetrafluoroethylene syringe filter. Each prepared blend solution was spin-cast onto the PEDOT:PSS layer in a nitrogen-filled glovebox. The thickness of polymer blend films was found to be in the range 90−100 nm. After drying at room temperature in the glovebox, the substrate was placed in an evaporation chamber under high vacuum (∼10−6 Torr) to deposit LiF (0.8 nm) and Al (100 nm). The active area of the fabricated devices was 0.09 cm2. The J−V characteristics of the devices were measured by a Keithley 2400 SMU under an air mass (AM) 1.5G solar simulator (100 mW cm−2, Peccell: PCE-L01, Class AAB ASTM standards) in air. The intensity of solar simulator was calibrated carefully by using an AIST-certified silicon photodiode equipped with a KG-5 color filter. The EQE data were obtained using a spectral measurement system (K3100 IQX, McScience Inc.) with a monochromatic light from a xenon arc lamp at 300 W filtered by a monochromator (Newport) and an optical chopper (MC 2000 Thorlabs) under ambient conditions. Calculated JSC values acquired by integrating the product of EQE and the AM 1.5G solar spectrum are in good agreement with the measured JSC within 2% error. Transient Absorption Spectroscopy. A transient absorption setup consists of a pump−probe spectrometer (Ultrafast Helios system), an amplified Ti:saphhire laser (Coherent LIBRA HE, 4 mJ, 1 kHz, 100 fs), and an optical parametric amplifier (OPA, Coherent Opera Solo). The output of the Ti:saphhire laser was split into two different paths. One beam was used to pump the OPA to generate tunable pump pulses. The other beam was used for white light continuum generation to probe the excited state dynamics in a broad spectral range from 0.8 to 2.6 eV. Sapphire plate and flint glasses were used for white light generation in the visible and IR region of the spectral range, respectively. Pump and probe beams were focused on the sample, and transmitted probe light was collected by a chargecoupled device (CCD). The pump pulses were tuned to 2.0 eV (620 nm), 3.3 eV (375 nm), and 3.2 eV (385 nm) in order to selectively excite the donor (PBDTTTPD), fluorinated acceptor P(NDI2ODT2F), and nonfluorinated acceptor P(NDI2OD-T2), respectively.

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(KETEP), Korea (No. 20133030011330). This work was also supported by Office of Naval Research (ONR) grant N000141310526 (B.R.G., R.Y., and K.G).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01414. Synthetic scheme, 1H NMR, CV data, summary of photovoltaic parameters with different D:A blend ratios, 2D-GIXS images, R-SoXS profiles, contact angle measurements, PL spectra, TAS spectra (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.G.). *E-mail: [email protected] (B.J.K.). *E-mail: [email protected] (H.Y.W.). Author Contributions

M.A.U. and Y.K. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) of Korea (2015R1A2A1A15055605, 2012M3A6A7055540). This work was also supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning H

DOI: 10.1021/acs.macromol.6b01414 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01414 Macromolecules XXXX, XXX, XXX−XXX