Controlling Open-Circuit Voltage in Organic Solar Cells by Terminal

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Controlling Open-Circuit Voltage in Organic Solar Cells by Terminal Fluoro-Functionalization of Narrow-Bandgap #-Conjugated Molecules Seiichi Furukawa, Hideaki Komiyama, and Takuma Yasuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06758 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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The Journal of Physical Chemistry

Controlling Open-Circuit Voltage in Organic Solar Cells by Terminal Fluoro-Functionalization of Narrow-Bandgap π-Conjugated Molecules Seiichi Furukawa,†,§ Hideaki Komiyama,§,‡ and Takuma Yasuda*,†,§

†

Department of Applied Chemistry, Graduate School of Engineering, §INAMORI Frontier

Research Center (IFRC), and ‡International Institute for Carbon-Neutral Energy Research (WPII2CNER), Kyushu University, 744 Motooka, Nishi-ku Fukuoka 819-0395, Japan.

ABSTRACT: A series of narrow-bandgap π-conjugated small molecules composed of benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) chromophoric units with different electron-withdrawing fluorinated end groups was designed and synthesized as donor materials for systematically studying their structure–property relationship in organic solar cells (OSCs). The terminal fluoro-functionalization of the π-conjugated BDT-DPP backbone resulted in systematic changes in the HOMO and LUMO energy levels of the resulting materials, as well as in their subsequent OSC device performance. These materials possessed relatively low HOMO energy levels ranging from −5.23 to −5.47 eV, while simultaneously maintaining small bandgap energies of approximately 1.6 eV in their thin films. With such proper engineering of the HOMO

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energy levels, the bulk heterojunction OSCs based on these fluoro-functionalized molecules as donor materials and PC71BM as an acceptor material demonstrated high open-circuit voltages of up to 0.94 V. In this series of donor materials, the 3,5-difluorophenyl and 4-fluorophenylsubstituted BDT-DPP molecules exhibited superior charge transport and self-organization properties, resulting in higher power conversion efficiencies.

INTRODUCTION Solution-processed organic solar cells (OSCs) based on bulk heterojunction (BHJ) active layers have received significant attention owing to their potential in flexible, light-weight, and low-cost solar energy-harvesting devices.1–6 Regarding recent developments in OSCs, the search for new donor materials with appropriate photophysical and electrical properties has taken center stage. Among a large number of photovoltaic materials, narrow-bandgap π-conjugated small molecules possessing an acceptor–donor–acceptor (A–D–A) electronic system are a promising class of donor materials for efficient OSCs.7,8 To date, power conversion efficiencies (PCEs) of ~10% have been achieved for state-of-the-art BHJ OSCs using these types of small molecules as donor materials,9–13 which are now comparable to those of best-performing polymer-based OSCs.14–17 In the design of such narrow-bandgap small molecules, understanding the relationship between their chemical structure and photovoltaic properties is essential. By fine-tuning the chemical structures, basic photophysical properties, including energy levels, bandgap, and photoabsorption, can be effectively controlled to enhance the OSC device performance. For instance, introduction of electron-accepting functional groups such as ester,18 carbonyl,19 sulfonyl,20,21 thioalkyl,11,22,23 pyridyl,24 and fluorine groups17,19,25–38 onto the backbone of πconjugated polymers and small molecules could increase both their ionization potential and


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electron affinity, with a deepening of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. In particular, fluorine groups substituted directly into the π-conjugated backbone have shown great promise in enhancing the efficiency of BHJ OSCs,17,19,25–38 and several explanations for the substituent effect have been proposed. Owing to its strong electron-withdrawing nature, the fluorine substitution can lower the HOMO energy levels of the resulting π-conjugated molecules and thereby enhance the open-circuit voltage (Voc) of OSCs because Voc of BHJ OSCs is generally determined by the HOMO level of a donor material and the LUMO level of an acceptor material.39,40 Moreover, the high electronegativity of fluorine induces a strong dipole along the C–F bond, resulting in strong intermolecular interactions between the π-conjugated molecules, which can influence the charge transport and morphological properties of the active layer.17,19,25–38 It has been reported that a polymer incorporating a difluorinated benzothiadiazole unit has a higher crystallinity and a higher tendency to adopt face-on molecular orientation in comparison with its non-fluorinated counterpart.28 However, in contrast to existing polymer systems, the substituent effect of such fluoro-functionalization on the photophysical and photovoltaic properties has not been systematically studied in narrow-bandgap A–D–A-type small molecules. In this study, we designed four benzodithiophene–diketopyrrolopyrrole (BDT-DPP)-based A–D–A-type small molecules (1–4) with different terminal fluoro-substituted phenyl units (Figure 1). This set of materials allows for systematic study of the substituent effect on their photophysical, electrical, and photovoltaic properties. The BDT-DPP-based molecules 1–4 with fluorinated end groups possessed lower HOMO energy levels ranging from −5.23 to −5.47 eV compared to that of the non-fluorinated analogue,41,42 while maintaining small bandgap energies of approximately 1.6 eV in their thin films. With such proper engineering of the HOMO levels,


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BHJ OSCs based on 1–4 as donors and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor showed tunable open-circuit voltages of 0.76–0.94 V. In comparison to 2 and 4 with trifluoromethyl end groups, 1 and 3 with fluoro end groups were found to show superior charge transport and self-organization ability, leading to higher power conversion efficiencies of the OSCs.

Figure 1. Synthesis of DPP-based narrow-bandgap π-conjugated molecules 1–4 with different terminal-fluorinated units.

RESULTS AND DISCUSSION Terminal fluoro-functionalized narrow-bandgap π-conjugated molecules 1–4 were synthesized via two-fold Stille cross-coupling reactions using Pd(PPh3)4 as the catalyst (Figure 1). The detailed synthetic procedures and characterization data are described in the Supporting Information. Although all these molecules have an elongated rigid π-conjugated backbone, they showed good solubility in common organic solvents such as chloroform and chlorobenzene, ensuring their solution processability. The chemical structures of 1–4 were confirmed by 1H and


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C NMR spectroscopy, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF)

mass spectrometry, and elemental analysis (see Supporting Information). Time-dependent density functional theory (TD-DFT) calculations were performed for 1–4 at the B3LYP/6-31G(d) level, and their frontier molecular orbitals and the calculated energy levels are presented in Figure 2. In spite of the difference in their terminal fluorinations, the distributions of both the HOMO and LUMO wave functions in 1–4 were very similar and delocalized over the π-conjugated BDT-DPP systems. However, an obvious trend for the calculated energy levels of the HOMO and the LUMO was observed in this system, depending on the terminal fluorinated substituents. The decrease in the energy levels of the HOMO and the LUMO from 1 to 4 followed the increasing order of the electron-withdrawing ability of their terminal fluorinated phenyl units. Meanwhile, the calculated first singlet excited energy (S1), as well as the corresponding oscillator strength (f), were found to be nearly the same among these four molecules, suggesting that 1–4 possessed essentially the same bandgap energies and photoabsorption properties.

Figure 2. Frontier molecular orbital distributions, energy levels, and associated oscillator strength (f) for the terminal-fluorinated narrow-bandgap molecules 1–4 calculated at the


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B3LYP/6-31G(d) level. The alkyl chains were replaced by methyl groups to simplify the calculations. The arrows indicate the first singlet excited-state (S1) transition.

The UV–vis absorption spectra of 1–4 in chloroform solutions and as solid thin films are shown in Figure 3, and their photophysical parameters are listed in Table 1. As predicted by the TD-DFT calculations, dilute solutions of 1–4 exhibited very similar photoabsorptions, with the lowest-energy absorption maxima (λmax) at approximately 650 nm and molar extinction coefficients (ε) of more than 1 × 105 M−1 cm−1 (Figure 3a). Meanwhile, in the solid thin films of 1–4, new red-shifted intense absorption bands appeared at approximately 710 nm, with onsets longer than 760 nm (Figure 3b), which can be attributed to the formation of strong intermolecular interactions and J-aggregation in the condensed solid states.43 Moreover, the absorption spectra of 1–4 in the solid thin films were different from each other to some degree, suggestive of their different aggregation structures arising from the variation in the terminal fluorinated groups.


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Figure 3. UV–vis absorption spectra of 1–4 in (a) chloroform solutions and (b) as-spun solid thin films.

The HOMO energy levels (i.e., ionization potentials) of 1–4 in the solid state were determined using photoelectron yield spectroscopy in air (Supporting Information). Indeed, the HOMO energy levels gradually decreased in the order of 1 (−5.23 eV) > 2 (−5.33 eV) ≈ 3 (−5.35 eV) > 4 (−5.47 eV) with increasing magnitude of the electron-withdrawing ability of the terminal fluorinated phenyl units, as listed in Table 1. These HOMO values were lower than that of the non-fluorinated 4-hexylphenyl-substituted BDT-DPP41 (−5.18 eV). Likewise, the LUMO energy levels of these compounds exhibited a similar trend within the range of −3.62 to −3.87 eV. As for 2 and 3, the similar values of their HOMO and LUMO energy levels imply that the terminal 4-(trifluoromethyl)phenyl and 3,5-difluorophenyl units had comparable electron-withdrawing abilities. Overall, the LUMO energy levels of these four molecules were sufficiently higher than that of the electron acceptor PC71BM (ca. −4.3 eV), implying that effective photo-induced charge separation is possible and they can be utilized as donor materials in OSC devices.

Table 1. Photophysical Data for the Terminal-Fluorinated Narrow-Bandgap Molecules 1–4

λmax (nm) compound

HOMOc (eV)

LUMOd (eV)

Egd (eV)

solutiona

thin filmb

1

650

647, 711

−5.23

−3.62

1.61

2

652

651, 713

−5.33

−3.70

1.63

3

651

658, 719

−5.35

−3.76

1.59

4

653

657, 709

−5.47

−3.87

1.60


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a

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Solution UV-vis spectra measured in chloroform solutions (10−5 M) at room temperature. bThin

films (ca. 100 nm) spin-coated from chloroform solution onto quartz slides. cDetermined by photoelectron yield spectroscopy for each spin-coated thin film in air. dLUMO = HOMO + Eg, in which the optical energy gap, Eg, was derived from the absorption onset of the thin film.

BHJ OSCs were fabricated by employing 1–4 as donor materials and PC71BM as an acceptor material with a device structure of ITO/ZnO (30 nm)/donor:PC71BM (90–140 nm)/MoO3 (6 nm)/Ag (100 nm). The ZnO electron extraction layer was deposited by using a sol– gel method. The active layer was spin-coated on the ZnO layer from a blended solution of the donor and PC71BM (1:1.5, w/w) in a mixed solvent of chloroform and 1-chloronaphtalene (99.5:0.5, v/v), where 1-chloronaphtalene was used as a processing additive.44 MoO3 and Ag, as a hole extraction layer and an anode, respectively, were subsequently vacuum-deposited on top of the active layer to construct the inverted device configuration. Figure 4a depicts the current density–voltage (J–V) curves of the optimized BHJ OSCs measured under simulated AM 1.5G illumination at 100 mW cm−2. The corresponding photovoltaic parameters are summarized in Table 2 (see also Supporting Information). The device based on 3 had among the highest PCE of 4.2%, with a short-circuit current density (Jsc) of 8.3 mA cm−2, an open-circuit voltage (Voc) of 0.85 V, and a fill factor (FF) of 60%. Slightly lower photovoltaic performance with a PCE of 4.0% was obtained for the device based on 1. Apparently, as compared to CF3-substituted 2 and 4 as well as the non-fluorinated BDT-DPP41 (Supporting Information), the devices based on F-substituted 1 and 3 yielded much higher PCEs under the same conditions, with simultaneously increasing Jsc. The variation in the Jsc values for these devices is consistent with their incident-photon-to-current conversion efficiency (IPCE)


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spectra (Figure 4b). We anticipate that such improved photovoltaic properties of the Fsubstituted 1 and 3 would stem from the superior hole mobility and morphology, as discussed later. It should be noted here that the device employing 4 with the 3,5-bis(trifluoromethyl)phenyl units exhibited the highest Voc of 0.94 V among the fabricated devices (Table 2) because of its lower HOMO level (−5.47 eV) as compared to the other molecules. Because the Voc of the BHJ OSCs is directly correlated with the energy difference between the LUMO of the acceptor and the HOMO of the donor, we could hence tune the electromotive forces of the cells by simply varying the terminal fluorination pattern of the same π-conjugated molecular system.


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Figure 4. (a) J–V characteristics under one sun illumination (100 mW cm−2) and (b) IPCE spectra for OSCs based on BHJ blends of 1–4 (donor) and PC71BM (acceptor) in 1:1.5 weight ratio with 0.5 vol% of 1-chloronaphthalene as a processing additive.


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Table 2. Photovoltaic Parameters for OSCs Based on BHJ Blends of 1–4 and PC71BM thickness

Jsc

Voc

FF

PCEb

µhc

(nm)

(mA cm−2)

(V)

(%)

(%)

(cm2 V−1 s−1)

1

98

10.1

0.76

52

4.0 (3.8)

5.4 × 10−3 (4.9 × 10−2)

2

109

6.1

0.83

54

2.8 (2.6)

3.2 × 10−3 (3.2 × 10−2)

3

113

8.3

0.85

60

4.2 (4.0)

1.7 × 10−2 (1.2 × 10−1)

4

134

2.5

0.94

45

1.1 (1.0)

9.8 × 10−4 (5.0 × 10−2)

a

donor

a

Device structure: ITO/ZnO/donor:PC71BM (1:1.5, w/w) with 0.5 vol% 1-chloronaphthalene

additive/MoO3/Ag; the active area of each device was 0.04 cm2. bPower conversion efficiencies (PCEs) derived from the equation: PCE = (Jsc × Voc × FF)/P0, where Jsc = short-circuit current density, Voc = open-circuit voltage, FF = fill factor, and P0 = incident light intensity (100 mW cm−2); the values in parentheses are averaged PCE values obtained from individual four devices. c

Hole mobilities for the donor:PC71BM (1:1.5, w/w) blend films evaluated by using SCLC

technique; the values in parentheses are hole mobilities obtained for the pristine neat films of the donors 1–4.

To figure out this substituent effect quantitatively, the HOMO energy levels of 1–4 and BDT-DPP41 were plotted against the Hammet constants (σ)45–47 of their terminal substituents (Figure 5). It is well known that the increase in the Hammett constant means an increase in the electron-withdrawing ability of the substituent. As can be seen from Figure 5a, a reasonable linear correlation was found between the HOMO levels and the σ values. In addition, an evident linear relationship is also observed between the Voc values of the resulting devices and the HOMO levels of 1–4 (Figure 5b). These results indicate that judicious selection of the substituents on the terminal phenyl units can perturb the HOMO and the LUMO of the π-


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conjugated system without changing its bandgap energy, thereby leading to fine control of the Voc of the actual OSC devices.

Figure 5. (a) Plots of the HOMO energy levels of 1–4 and non-fluorinated BDT-DPP41 and the Voc values of their OSC devices against the Hammett constants (σ) of the terminal substituents. (b) Correlation between the Voc values and the HOMO energy levels of 1–4 and BDT-DPP. The dashed lines are linear fittings of each of the five plots.

For BHJ OSCs, the charge carrier mobility of a donor material is one of the most important factors to ensure efficient charge transport towards the electrodes and also to suppress the competing charge recombination processes. Using a space-charge-limited current (SCLC) technique,48–51 we evaluated and compared the hole mobilities (µh) of 1–4 in the same BHJ blends. Figure 6 shows the J–V curves of the vertical hole-only devices with structures of ITO/PEDOT:PSS [poly(3,4-ethylenedioxythiophene) polystyrene sulfonate]/ donor:PC71BM (1:1.5, w/w)/MoO3/Ag. The hole mobilities were determined by fitting the dark J–V characteristics by adopting the Mott–Gurney equation:48 J = (9/8)ε0εrµh(V2/L3), where ε0 is the permittivity of free space, εr is the relative dielectric constant of the transport medium, and L is


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the thickness of the active layer. As compiled in Table 2, the blend films of F-substituted 1 and 3 had higher hole mobilities of 5.4 × 10−3 and 1.7 × 10−2 cm2 V−1 s−1, respectively, than their CF3substituted counterparts. This propensity led to better charge extraction and hence to the higher Jsc and IPCE in the OSC devices based on 1 and 3 (Figure 4 and Table 2). The hole mobilities of pristine neat films of 1–4 were approximately one order of magnitude higher than those of the corresponding blend films (Supporting Information).

Figure 6. Double logarithmic plots of J–V characteristics of hole-only devices with the structure of ITO/PEDOT:PSS (30 nm)/donor:PC71BM (1:1.5, v/v; 109–126 nm) with 0.5 vol% of 1chloronaphthalene/MoO3 (6 nm)/Ag (100 nm). The solid lines represent the best fits to the SCLC model: the slope of log(J) vs log(V) is ≈ 2.

The enhanced charge carrier mobilities of 1 and 3 with much smaller fluorine groups may have arisen from the greater degree of self-organization and closer molecular packing in the thin films. Figure 7 presents the grazing incidence wide-angle X-ray scattering (GIWAXS) data, together with the corresponding out-of-plane and in-plane profiles for the donor:PC71BM blend films prepared on Si substrates in the same manner as the OSC devices. All of the blend films exhibited strong (100) reflections in the out-of-plane (qz) direction, indicating that 1–4 have a


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tendency to preferably take edge-on molecular orientation on the substrate. The (100) d-spacings for 1–4, which correspond to the lamellar interchain separation distances, were 16.3, 15.0, 15.4, and 14.1 Å, respectively. In each of the blend films, a broad scattering was also observed at q ≈ 1.3 Å−1, originating from amorphous PC71BM agglomerate regions. Another important feature is that the F-substituted 1 and 3 films clearly showed the (010) reflection at q = 1.7 Å−1, corresponding to the π-stacking distance of 3.7 Å, whereas CF3-substituted 2 and 4 did not discernibly show the (010) reflection. This observation implies that the molecules of 1 and 3 are packed better than the molecules of 2 and 4 along the π–π direction within the films, presumably because of the lower steric effect of the terminal fluorine groups. As a result, charge transport is enhanced through the ordered π-stacking structures, which is in agreement with the results of the foregoing SCLC measurements.

Figure 7. (a) Two-dimensional GIWAXS patterns of thin films of 1–4 blended with PC71BM (1:1.5, w/w). (b) Out-of-plane (qz-scan) profiles and (c) in-plane (qxy-scan) profiles of the corresponding blend films.


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CONCLUSIONS We designed and synthesized four narrow-bandgap π-conjugated molecules 1–4 as donor materials by introducing electron-withdrawing fluoro or trifluoromethyl group(s) into the terminal phenyl units of our previous BDT-DPP compound.41 This terminal fluorofunctionalization strategy was effective in controlling the HOMO and LUMO energy levels without changing the bandgap energy and intrinsic photoabsorption properties, leading to an increase of Voc of the OSC devices incorporating these fluoro-functionalized donor molecules. In the BHJ OSCs, much higher PCE values were attained with F-substituted 1 and 3 blended with PC71BM (4.0% and 4.2%, respectively) than with CF3-substituted 2 and 4. The superior photovoltaic performance of devices based on 1 and 3 partly originated from their ordered πstacking structures and the enhanced charge transport properties, as is evident from the GIWAXS and SCLC analysis. These results provide valuable insights into the structure–property relationship for organic small-molecule photovoltaic materials, leading to versatile strategies for further designing high-performance photovoltaic materials and devices.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Phone: +81-92-802-6956

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported in part by Grants-in-Aid for Scientific Research on Innovative Areas (No. 15H01049) from JSPS, the Cooperative Research Program of ''Network Joint Research Center for Materials and Devices'', the Canon Foundation, the Yashima Environment Technology Foundation, and the KDDI Foundation. The GIWAXS measurements were performed at the BL45XU and BL40B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2015B1305 and 2016A1081). S.F. acknowledges the support from the Leading Graduate Schools Program of ''Advanced Graduate Course on Molecular Systems for Devices'' by MEXT, Japan.

ASSOCIATED CONTENT Supporting Information Synthetic procedures and characterization data, photoelectron yield spectra, additional photovoltaic data, and SCLC data for 1–4 (PDF) This information is available free of charge via the Internet at http://pubs.acs.org.

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Controlling Open-Circuit Voltage in Organic Solar Cells by Terminal

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