Acceptor Interfaces in All-Polymer

Jul 25, 2017 - We report a pentafluorobenzene-based additive (FPE) to control the donor/acceptor (D/A) interfacial morphology via quadrupolar electros...
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Morphological Control of Donor/Acceptor Interfaces in All-Polymer Solar Cells Using a Pentafluorobenzene-Based Additive Hong Il Kim,†,§ Minjun Kim,†,§ Cheol Woong Park,† Hae Un Kim,† Han-Koo Lee,‡ and Taiho Park*,† †

Department of Chemical Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 37673, Korea ‡ Pohang Accelerator Laboratory, 80 Jigokro-127-beongil, Nam-gu, Pohang, Gyeongbuk 37673, Korea S Supporting Information *

ABSTRACT: We report a pentafluorobenzene-based additive (FPE) to control the donor/acceptor (D/A) interfacial morphology via quadrupolar electrostatic interactions between donor and acceptor polymers in all-polymer solar cells (allPSCs). The morphology changes are investigated using a combination of atomic force microscopy, grazing incidence wide-angle X-ray scattering, and near-edge X-ray absorption fine-structure spectroscopy. Unlike a conventional solvent additive, such as 1,8-diiodooctane, a bicontinuous interpenetrating morphology without large-scale phase separation and an enhanced π−π stacking with face-on orientation are found in the FPE processed blended films. These morphology changes improve the charge carrier extraction and charge transport between D/A interfaces to achieve an increase in the photovoltaic performance of all-PSCs.

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INTRODUCTION All-polymer solar cells (all-PSCs), which comprise binary blends of donor and acceptor polymers, have attracted considerable attention because of their advantages over conventional fullerene-based polymer solar cells (PSCs), such as readily tunable molecular energy levels and complementary absorption.1−4 All-PSCs have considerably better mechanical durability and higher stretchability than fullerene-based PSCs. Therefore, all-PSCs have great potential for applications in flexible and portable photovoltaic devices.5−7 Despite these versatile advantages, most all-PSCs still exhibit low power conversion efficiencies (PCEs) that are far behind fullerene-based PSCs, and only a few all-PSCs show high PCEs over 6%.8−12 In general, the PCEs of all-PSCs are limited by low short-circuit current densities (JSC ≤ 10 mA/cm2) and fill factors (FF ≤ 60%). This characteristic is mainly attributed to the unfavorable blend morphology of the donor and acceptor polymers.13,14 The energetically favored demixing of the two different polymers causes large-scale phase separations that lead to inefficient charge dissociation at the donor/acceptor (D/A) interface because of dominant nongeminate recombination processes.15−19 Thus, controlling the blend morphology of allPSCs is a great challenge when obtaining highly efficient allPSCs. Various approaches have been attempted to improve the blend morphology of the interconnected network of polymer donors and acceptors, such as adjusting the polymer molecular weight,20−24 D/A blending ratios,11,25,26 thermal annealing,27−29 and solvent additive processing.29−31 Among these processing methods, solvent additive processing is considered © 2017 American Chemical Society

as an effective and simple way to control the nanoscale structure of phase separation and polymer ordering in D/A blend systems.29−31 Neher et al.32 demonstrated that 1chloronaphthalene (CN) (1:1 p-xylene:CN) can suppress the preaggregation of the acceptor polymer, poly[[N,N′-bis(2octyldodecyl)napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]alt-5,5′-(2,2′-bithiophene)] (P(NDI2OD-T2)) in a poly(3hexylthiophene-2,5-diyl) (P3HT)/P(NDI2OD-T2) blend film and showed that this phenomenon exhibited a large increase in JSC. Hou et al.33 also reported an increased FF in a blend system consisting of benzo[1,2-b:4,5-b′]dithiophene (BDT)−thiophene donor polymer (PBDTBDD-T) and BDT−naphthalene diimide (NDI) acceptor polymer (PBDTNDI-T) after processing chlorobenzene (CB) with 3 vol % CN and showed that this process induced high domain purity and improved the molecular ordering of the blend film. Kim et al.34 showed that the addition of 1,8-diiodooctane (DIO; 1.25 vol %) and diphenyl ether (DPE; 1.0 vol %) increased JSC because of the enhanced crystallinity of the P(NDI2OD-T2) acceptor in a P(NDI2OD-T2)-based all-PSC system. However, these solvent additives have not been able to considerably improve π−π intermolecular interactions between donor and acceptor polymers. Furthermore, few investigations exist on the use of processing solvent additives to tune interactions between the D/A interfaces in all-PSCs. Received: April 27, 2017 Revised: July 23, 2017 Published: July 25, 2017 6793

DOI: 10.1021/acs.chemmater.7b01718 Chem. Mater. 2017, 29, 6793−6798

Article

Chemistry of Materials

CB with a concentration of 18 wt %. These solutions were stirred at 70 °C for 12 h inside a glovebox filled with nitrogen gas. Fabrication of Devices. The patterned ITO-coated glass substrates were cleaned by detergent, ultrasonicated in deionized water, rinsed with deionized water followed by acetone and isopropanol, and subsequently dried overnight in an oven at 150 °C. After oxygen plasma cleaning for 15 min, a ZnO electron transporting layer was spin-cast at 6000 rpm for 30 s on top of the ITO-coated glass substrate and was baked at 200 °C for 1 h in the air. The weight ratio of PTB7-Th to P(NDI2OD-T2) was 1:1 [PTB7-Th (9 mg) and P(NDI2OD-T2) (9 mg) were dissolved in 1 mL of chlorobenzene]. The weight ratio of PTB7-Th to P(NDI2OD-T2) was 1:1 [PTB7-Th (9 mg) and P(NDI2OD-T2) (9 mg) were dissolved in 0.97 mL of chlorobenzene and 0.03 mL of DIO]. The weight ratio of PTB7-Th to P(NDI2OD-T2) was 1:1 [PTB7-Th (9 mg) and P(NDI2OD-T2) (9 mg) were dissolved in 0.97 mL of chlorobenzene and 0.03 mL of FPE]. The PTB7-Th:P(NDI2OD-T2) solutions were deposited onto the ZnO layer by spin-coating at 1200 rpm for 40 s (∼100 nm). Then, the samples were dried under N2 atmosphere at room temperature. Finally, to complete the solar cell device, the 20 nm of MoO3 and 100 nm of Ag layer were thermally evaporated with a shadow mask at a base pressure of 6.0 × 10−7 Torr. The overlapping area between the cathode and anode defined a photoactive area of 0.04 cm2. Measurements. The samples for the ultraviolet−visible (UV−vis) spectra absorption measurement were prepared on the precleaned quartz glass substrates using the same method as the device fabrication. The UV−vis spectra were measured using a Shimadzu UV-2550 ultraviolet−visible spectrophotometer. Photoluminescence (PL) spectra were measured on a Horiba LabRAM HR-800 spectrometer. The atomic force microscopy (AFM) examinations were conducted using a Nanoscope IV (Digital Instruments). Grazing incidence wide-angle Xray scattering (GIWAXS) and near-edge X-ray absorption finestructure spectroscopy (NEXAFS) experiments were performed using synchrotron radiation on bean lines 3C and 4D, respectively, at the Pohang Accelerator Laboratory (PAL) in Korea. The current density−voltage (J−-V) characteristics were measured using Keithley 236 source measure unit (Keithley Instruments) under illumination of simulated solar light at 100 mW/cm2 (AM 1.5G) using a 1000 W xenon-lamp-based solar simulator (Newport Corp.). The illumination intensity used was calibrated by using a NREL certified standard Si photodiode detector with an integrated KG5 optical filter (PV Measurements).

Herein, we systematically investigated the morphological control of D/A interfaces in all-PSCs using 1,2,3,4,5pentafluoro-6-phenoxybenzene (FPE) as an additive. The pentafluorophenyl group has a positive quadrupolar sign that interacts attractively with electron-rich aromatic rings, and the phenyl group has a negative quadrupolar sign that interacts attractively with electron-deficient aromatic rings.35 In particular, these quadrupolar electrostatic interactions between phenyl and perfluorophenyl groups enable the formation of intermolecular face-to-face stacking.36,37 We suggest that the introduction of FPE as an additive could form a selective π−π interaction38 between donor and acceptor polymers that would facilitate the optimal D/A morphology and a bicontinuous interpenetrating D/A network without large-scale phase separation (Figure 1). Therefore, FPE additive is expected to

Figure 1. Schematic illustration of selective quadrupolar electrostatic interaction between PTB7-Th and P(NDI2OD-T2).

increase the miscibility and D/A interfacial area, indicating more efficient exciton dissociation and bicontinuous pathways for carrier migration to the respective electrodes, favoring high FF and JSC values by suppressing charge accumulation and nongeminate recombination. In addition to controlling the morphology, the FPE additive is expected to improve vertical charge transport, which benefits organic solar cells by inducing face-to-face stacking between donor and acceptor polymers.39 We chose poly[[4,8-bis[5-(2-ethylhexyl)thiophene-2-yl]benzo[1,2-b:4, 5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th) as a donor polymer and P(NDI2OD-T2) as an acceptor polymer. The blend system of these two polymers have been extensively studied in all-PSCs because they exhibit high charge carrier generation and high collection efficiency and are comparable to those for polymer/fullerene blended solar cells.40 We prepared blend films processed with DIO, which is a conventional additive in PSCs, to conduct a comparative study on additive effects on all-PSCs.

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RESULTS AND DISCUSSION The chemical structures of materials employed in this work are shown in Figure 2a. The donor polymer (PTB7-Th (Mn = 42k; PDI = 2.5)) and the acceptor polymer (P(NDI2OD-T2) (Mn = 75k; PDI = 2.5)) were purchased from 1-Material Inc. The FPE additive was synthesized according to a previously reported method.42 The experimental details of FPE are described in Supporting Information Figure S1. The electrostatic potential (ESP) map of FPE was calculated to visualize and understand the electrostatic state by using density functional theory (DFT) calculations at the B3LYP/6-31G level (Figure 2b). When going from the phenyl group to the pentafluorophenyl group, a gradual color change from red to blue above the ring surface is observed. This phenomenon indicates that the sign of the quadrupole changes from negative to positive and that the FPE additive can form an electrostatic attraction with both donor and acceptor polymers. The optical properties of the allpolymer blend films were determined by UV−vis absorption and PL spectra. All blend films show almost identical absorption spectra, thus indicating that both DIO and FPE additives do not affect the absorption behavior of the PTB7Th/P(NDI2OD-T2) polymer blend films (Figure S2). The PL spectra of the blend films show a significant emission difference of around 980 nm with the degree of PL quenching decreasing

EXPERIMENTAL SECTION

Materials. Patterned indium tin oxide (ITO) glass with a sheet resistance of 11 Ω/□ was purchased from JM International Co. P(NDI2OD-T2) was purchased from 1-Material Inc. PNDIS-HD was synthesized according to a previously reported method (Mn = 37 kDa; PDI = 2.91).41 CB and DIO were purchased from Sigma-Aldrich. The precursor solution of ZnO was prepared as previously reported.10 A weight ratio of PTB7-Th to P(NDI2OD-T2) (1:1) was dissolved in 6794

DOI: 10.1021/acs.chemmater.7b01718 Chem. Mater. 2017, 29, 6793−6798

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with the FPE additive could increase the photovoltaic performance by improving charge transport at the interface between the active layer and counter electrode. GIWAXS analysis was performed to investigate microstructural changes, such as packing structure and crystal orientation, in the blend films (Figures 4 and S3). The blend

Figure 2. (a) Chemical structures of materials used in this study. (b) ESP map of FPE. (c) PL spectra of PTB7-Th:P(NDI2OD-T2) blend films spin-cast from a blend solution.

in the following order: CB + FPE > CB + DIO > CB processed films (Figure 2c). This result indicates that a blend film processed with FPE generates a more efficient charge transfer at the PTB7-Th/P(NDI2OD-T2) interface than the other two blend films; therefore, the FPE processed film has a fine blend film morphology. To investigate the effects of additives on the phase separation of the donor and acceptor polymers, we obtained AFM topographical images (5 μm × 5 μm) of the all-polymer blend films processed with CB, CB + DIO, and CB + FPE (Figure 3).

Figure 4. GIWAXS images of PTB7-Th:P(NDI2OD-T2) blend films processed with (a) CB, (b) CB + DIO, and (c) CB + FPE. (d) GIWAXS out-of-plane patterns of PTB7-Th:P(NDI2OD-T2) blend films processed with CB, CB + DIO, and CB + FPE, respectively.

films have π−π stacking (010) peaks at ∼17 nm−1 in the out-ofplane direction and the alkyl stacking (100) peak at ∼2.6 nm−1 in the in-plane direction, thus indicating that the polymers preferentially adopt a face-on orientation to the substrate.44 The CB + DIO processed blend film exhibits a slightly increased (010) peak intensity in the out-of-plane direction with the evidence of enhanced crystallinity for P(NDI2ODT2); this finding is in accordance with a previous report.45 Interestingly, the CB + FPE processed blend film exhibits a stronger (010) peak intensity for π−π stacking compared with the other two blend films. Furthermore, the peaks (100) of alkyl stacking in the in-plane direction of blend films are shifted to high qxy values in the following order: CB < CB + DIO < CB + FPE (Figure S4). These results indicate that treatment with FPE is effective in promoting an ordered π−π stacking with face-on orientation among the polymer chains. Therefore, the electrostatic attraction induced by the FPE could improve the face-to-face interactions between the donor and acceptor polymers. NEXAFS was performed to investigate the molecular orientation of the top surface and bulk state of the blend films. The NEXAFS spectra were obtained over the incident Xray angle (θ) range from 30° to 70°, which provides information on the average tilt angle (α) of the polymer chains from the dichroism of the C 1s → π* resonant transitions perpendicular to the plane of the conjugated planes (see the Supporting Information for experimental details). To determine the orientation of both the top surface and bulk state of the blend film, NEXAFS spectra were measured from the partial electron yield (PEY) and total electron yield (TEY) modes, with PEY acquisition having a probing depth of 6 nm and TEY acquisition probing depth of ∼50 nm (Figures 5 and S5). The average α of CB, CB + DIO, and CB + FPE processed

Figure 3. AFM height and 3D images of PTB7-Th:P(NDI2OD-T2) blend films processed with (a, d) CB, (b, e) CB + DIO, and (c, f) CB + FPE.

Although the AFM images of both CB and CB + DIO processed films show distinct phase-separated domain structures, the CB + FPE processed film exhibits a fine surface morphology. The surface root-mean-square (RMS) roughness of the blend fims processed with CB, CB + DIO, and CB + FPE are 6.07, 5.89, and 1.85 nm, respectively. This fine surface morphology is consistent with the PL quenching results that suggest that FPE could have a role in mixing donor and acceptor polymers at the nanometer scale, which is beneficial for efficient exciton dissociation and charge transport for high JSC and FF.43 Furthermore, the smooth surface of the blend film 6795

DOI: 10.1021/acs.chemmater.7b01718 Chem. Mater. 2017, 29, 6793−6798

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Figure 6. (a) J−V characteristic curves, (b) IPCE spectra, (c) statistical analysis of JSC, and (d) VOC on the light intensity for all-PSCs based on PTB7-Th and P(NDI2OD-T2) processed with CB, CB + DIO, and CB + FPE, respectively.

Figure 5. NEXAFS TEY mode of PTB7-Th:P(NDI2OD-T2) blend films processed with (a) CB, (b) CB + DIO, and (c) CB + FPE; and (d) intensities of π* transitions vs incidence angle.

blend films calculated from the linear fitting to eq S1 are 63.6°, 61.6°, and 50.5°, respectively (α = 90° edge-on and α = 0° faceon orientation). The dichroic ratios (R) were also determined using eq S2 to compare the orientation tendency of the blend films. The calculated R values of the blend films processed with CB, CB + DIO, and CB + FPE are 0.28, 0.22, and −0.13, respectively (R = 0.7 edge-on, R = 0 random, and R = −1 faceon orientation). The trend of R is similar to α, thus indicating that the polymer chains in the blend film processed with FPE adopt a face-on orientation on average in the bulk state; this finding is consistent with the GIWAXS results. The PEY mode results are similar to those of the TEY mode, but slight increases in the distribution of edge-on orientation are observed in all blended films. The correlation between the morphological evolution and their photovoltaic performances was explored by evaluating allPSCs based on the PTB7-Th:P(NDI2OD-T2) processed with CB, CB + DIO, and CB + FPE. The device fabrication conditions are described in the Supporting Information. The detailed photovoltaic performances of the all-PSC devices are summarized in Tables 1 and S1. Figures 6 and S6 show the J−V

JSC values (Figures 6c and S7). The JSC values match well with the integrated JSC values obtained from the EQE spectra within 1% error. The dramatic increases of the JSC and FF values for CB + FPE are consistent with the morphological analysis data, which indicate that the processing additive provides efficient charge transport and collection in the vertical direction. In addition, we investigate thermal stability of all-PSCs based on PTB7-Th:P(NDI2OD-T2) processed with CB and CB + FPE, respectively (Figure S8). As the annealing temperature increases, performance of the CB processing device sharply decreases as compared to the CB + FPE processing device, which is due to the increase of domain size and roughness (Figure S9). We applied FPE additive to another D/A polymers blend system to gain the reliability of the effect of FPE additive. We chose a blend system consisting of PTB7-Th and naphthalene diimide (NDI)−selenophene copolymer (PNDIS-HD), which is one of the recently reported high efficiency blend systems.10 The photovoltaic results of PTB7Th:PNDIS-HD devices are provided in Figure S10 and Table S2. The FPE additive also increases the JSC and FF values of the PTB7-Th:PNDIS-HD device and, consequently, improves the efficiency by 30.0% (from 4.10% to 5.33%). Studies on the FPE additive in various blend systems are under current research work to gain generality in all-PSCs. Bimolecular recombination is determined by the light intensity dependence of V OC (Figure 6d). 46,47 When bimolecular recombination is the only loss mechanism, VOC on the light intensity is given by the following equation:

Table 1. Summary of the Photovoltaic Parameters for AllPSCs Based on PTB7-Th and P(NDI2OD-T2) Processed with CB, CB + DIO, and CB + FPE, Respectively

a

solvents

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

CB CB + DIO CB + FPE

7.3 8.8 9.5

0.80 0.80 0.80

49.3 56.8 58.8

2.9 (2.7)a 4.0 (3.9)a 4.5 (4.4)a

Data in parentheses are the averaged results from 20 devices.

VOC =

Egap q

−

2 kT ⎡ (1 − PD)γNc ⎤ ⎥ ln⎢ q ⎣ PDG ⎦

(1)

where q is the elementary electric charge, Egap is the energy difference between the lowest unoccupied molecular orbital (LUMO) of the electron-acceptor material and the highest occupied molecular orbital (HOMO) of the electron-donor material, kT/q is 0.02586 V, γ is the recombination constant, PD is the dissociation probability, NC is the density of states, G is proportional to light intensity, and the slope of the natural logarithm of VOC on the light intensity gives nkT/q. Figure 6d shows the relationship between VOC and Plight in the all-PSCs

characteristic curves and the incident photon to converted electron (IPCE) spectra of all-PCSs based on PTB7-Th:P(NDI2OD-T2) processed with CB, CB + DIO, and CB + FPE. The resulting PCE values increase remarkably from 2.9% with a VOC of 0.80 V, JSC of 7.3 mA/cm2, and FF of 49.3% (CB) to the following: 4.0% with a VOC of 0.80 V, JSC of 8.8 mA/cm2, and FF of 56.8% (CB + DIO); 4.5% with a VOC of 0.80 V, JSC of 9.5 mA/cm2, and FF of 58.8% (CB + FPE). This remarkable increasing tendency of the PCE is mainly due to the increased 6796

DOI: 10.1021/acs.chemmater.7b01718 Chem. Mater. 2017, 29, 6793−6798

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processed with CB, CB + DIO, and CB + FPE. As the slope approaches 1 kT/q, the bimolecular recombination is dominant. The device processed with the CB + FPE exhibits a slope of 1.13 kT/q, while slopes of devices processed with CB and CB + DIO are 1.43 and 1.75 kT/q, respectively. These results speculate that CB + FPE processed device reduces the interfacial trap densities at the D/A interfaces, thereby suppressing the trap-assisted recombination and enhancing JSC and FF.

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CONCLUSIONS

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ASSOCIATED CONTENT

REFERENCES

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We designed and synthesized a novel FPE additive to control the blend morphology of PTB7-Th and P(NDI2OD-T2). In combination with microstructure analyses and PL quenching results, we found that FPE processed films form an optimal morphology and have bicontinuous interpenetrating networks without large-scale phase separation for efficient charge carrier extraction and transport between D/A interfaces. In addition to the desired phase separation, processing with FPE also promotes enhanced π−π stacking with a face-on orientation in the bulk state of blend films; this result was obtained by GIWAXS and NEXAFS measurements. Therefore, all-PSC devices that employ FPE as a processing additive exhibit an improved JSC with FF yielding a PCE of 4.5%, which represents a 55.2% improvement over the control device via efficient charge transport and collection in the vertical direction. We expect that our results will provide valuable information to morphological control strategies for achieving highly efficient all-PSCs.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01718. Detailed methods of NEXAFS, synthetic route for the FPE, UV−vis absorption spectra, diffraction profiles, GIWAXS in-plane patterns, NEXAFS PEY mode, thermal stability analysis, AFM height images, statistical analysis, J−V characteristic curves, and summaries of the photovoltaic parameters (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Taiho Park: 0000-0002-5867-4679 Author Contributions §

H.I.K. and M.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the Center for Advanced Soft Electronics under the Global Frontier Research Program (2012M3A6A5055225) and the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) of Korea (2015M1A2A2056216). 6797

DOI: 10.1021/acs.chemmater.7b01718 Chem. Mater. 2017, 29, 6793−6798

Article

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DOI: 10.1021/acs.chemmater.7b01718 Chem. Mater. 2017, 29, 6793−6798