Investigation of a Solution-Processable, Nonspecific Surface Modifier

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Investigation of a solution-processable, non-specific surface modifier for low cost, high work function electrodes Allison Claire Hinckley, Congcong Wang, Raphael Pfattner, Desheng Kong, Yan Zhou, Ben Ecker, Yongli Gao, and Zhenan Bao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05348 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 21, 2016

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ACS Applied Materials & Interfaces

Investigation of a solution-processable, non-specific surface modifier for low cost, high work function electrodes Allison C. Hinckley,† Congcong Wang,‡ Raphael Pfattner,† Desheng Kong,† Yan Zhou,† Ben Ecker,‡ Yongli Gao,‡ and Zhenan Bao∗,† Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA, and Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA E-mail: [email protected]

Abstract We demonstrate the ability of the highly fluorinated, chemically inert co-polymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) to significantly increase the work function of a variety of common electrode materials. The work function change is hypothesized to occur via physisorption of the polymer layer and formation of a surface dipole at the polymer/conductor interface. When incorporated into organic solar cells, an interlayer of PVDF-HFP at an Ag anode increases the open circuit voltage by 0.4 eV and improves device power conversion efficiency by approximately an order of magnitude relative to Ag alone. Solution-processable in air, PVDF-HFP thin films provide one possible route towards achieving low cost, non-reactive, high work function electrodes. ∗

To whom correspondence should be addressed Stanford University ‡ University of Rochester †

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Keywords Work function, Surface Dipole, Organic Solar Cells, Solution-Processable Electrodes, ChargeNeutral Interlayer, Photoelectron Spectroscopy, Energy-Level Alignment

1

Introduction

Organic electronic materials hold significant promise for a new regime of electronic devices ones that are flexible, stretchable, and printable at large scales. 1–3 These devices are poised to usher in an age of novel human-machine interaction at potentially low cost. Organic electronic devices typically require an electrode with a sufficiently high work function (Φ) to facilitate hole collection from or injection into the highest occupied molecular orbital (HOMO) of the active semiconducting material. 4 However, high Φ metals such as gold, platinum, and palladium tend to be significantly more expensive than their lower Φ counterparts (aluminum, copper, and silver). While the air stability of the electrodes is as important as their cost, the latter constrains device dimensions to a degree that affects the former. For example, the lower cost of aluminum and copper allows for thicker metal films and larger electrode dimensions than are economically feasible with platinum, thereby minimizing performancehindering oxidation and improving device stability. Numerous materials have been used to modify the work functions of metals for better band alignment between the electrodes and semiconducting materials in electronic devices. Inorganic materials such as metals and metal oxides have an inherent work function, so incorporation of thin layers of inorganics can be effective at tailoring the energy levels of the electrodes. However, the work function of metal oxides can be hard to precisely tune as it is sensitive to their precise composition, and the metals either reactive or expensive. 5 Organic buffer layers are desirable due to their ease of solution-processing to achieve low cost, high-throughput fabrication of devices. Most studies of organic interlayers have focused on decreasing the work function of metals and metal oxides for n-type contacts (electron transport) to develop air-stable electrodes 2


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with low work functions. 6–8 Few organic systems have been shown to significantly increase the work function of metals. Instead, the introduction of hydrocarbons on a metal surface generally reduces the work function by suppressing the tail of the metal electron wavefunction beyond the surface. This mechanism is known as the "pillow" effect. 9 Acids and other cationic species can increase both the work function of metal oxides by removing the donorlike defect states 10,11 and the work function of graphene through close binding of the acidic proton, 12 but they have little impact on metals. PEDOT:PSS is a mildly acidic conductive polymer that is frequently incorporated as a hole transporting layer between ITO and the active layer in organic solar cells. 13 Large, tunable increases in the electrode work function of PEDOT:PSS/ITO have been achieved using a blend of PEDOT:PSS and a polyfluorinated ionomer. 14–16 However, there is some indication that the hygroscopic nature of PEDOT:PSS contributes to long-term device failure in ambient conditions. 17 The work function of a metal can also be influenced by the presence of dipole at the metal surface, as has been demonstrated with multilayers of LiF 18 and self-assembled monolayers (SAMs). 19–21 The change in the work function depends on the dipole orientation along the SAM, with fluorinated end groups increasing the apparent work function of the metal and aminated end groups decreasing it. 20,21 However, a significant drawback of these systems is the necessity of substrate-specific chemistry to ensure chemisorption of the SAMs. Moreover, SAMs are largely limited to the bottom contact of electronic devices. Polar polymers and polyelectrolytes have also been shown to decrease the work function of a variety of metals and metal oxides without substrate-specific chemistry via facile solution deposition. 6–8 However, the demonstration of a charge-neutral polymer layer universally increasing the work function of different conductors has not yet been reported. Herein we demonstrate that the highly fluorinated co-polymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is capable of increasing the work function of a range of metals and metal oxides by as much as 0.8 eV through introduction of an interfacial dipole. As a proof-of-principle device, inverted solar cells were fabricated with a PVDF-HFP/Ag anode, resulting in 0.4 eV increase in VOC 3



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and approximately an order of magnitude improvement in the power conversion efficiency (P CE) relative to Ag alone.

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Results and discussion

2.1

Work Function Measurements

The structure of PVDF-HFP is shown in Figure 1A. PVDF and its co-monomer PVDF-TrFE are well-known ferroelectric polymers. Spin-cast PVDF is crystalline in the α-phase, with all of the dipoles along the backbone balancing to result in zero net dipole. When a strong field is applied, PVDF is converted to the β-phase, wherein the dipoles are additive. The addition of the bulky -CF3 side-groups break-up the α-crystallites, resulting in a higher electrostatic energy density than PVDF alone. 22 In addition, aligned SAMs terminating in -CF3 have been shown to result in very high surface dipoles. 20,23 The PVDF-HFP films used herein are not poled in a strong external electric field, and the degree of alignment within one monolayer can be assumed to be imperfect. The use of multilayers may circumvent the need for the "defect-free" alignment required by SAMs. The electron-withdrawing nature of the heavily fluorinated polymer may also contribute to an interfacial dipole. A proposed schematic of the electronic structure of PVDF-HFP on a conductor is depicted in Figure 1B. Other advantages of PVDF-HFP include its chemically inertness, orthogonal solvents, air-stability, and non-reactivity. PVDF-HFP also has a large band gap, which lends itself to optical transparency. 24 The large band-gap is additionally important for this proof-of-concept study to elucidate the mechanism of the observed work function modification. If the HOMO or LUMO of the polymer is too close to the Fermi level of the metal, integer charge transfer across the interface can occur, resulting in "Fermi level-pinning". 25 The gap of at least 3 eV between the Fermi level of Au and either the HOMO or LUMO of PVDF-HFP should prevent Fermi-level pinning. 24 The work functions of a variety of metals and metal oxides were measured via photoe4


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mission spectroscopy both in ultra-high vacuum (UHV) and in air for bare substrates and substrates spin-coated with PVDF-HFP and annealed under nitrogen or in air, in accordance with the measurement environment. Representative data for work function measurements taken under UHV is shown in Figure 1C. The electron binding energy shifts to higher energy for all samples with addition of PVDF-HFP. This shift denotes an increase in the apparent Φ from 4.58 to 5.05 eV for Au, from 4.73 to 5.25 eV for Ag, and from 3.11 to 3.79 eV for Al. Separate measurements taken under ambient conditions confirmed the increase in Φ for metal oxides (ITO, FTO) and metals (Al, Au, Ni, Ag, Cu). Table 1 summarizes the Φ measurements taken in air and under UHV for several conductors modified with a ∼6 nm thick film of PVDF-HFP. The lower ∆Φ for Au may reflect the combination of the PVDF-HFP surface dipole increasing Φ and the "pillow effect" decreasing it. Whereas the rest of the metals in this study have some native oxide which reduces the negative ∆Φ associated with adsorption of hydrocarbons, gold’s work function can be decreased by as much as 0.7 eV due to the presence of surface carbons. 26 Note that the differences in the absolute Φ for different metals in Table 1 reflect the different measurement methods and preparation conditions of the films. The UHV samples were prepared in an inert environment, but air-exposed for a number of days prior to the UPS measurement. Air-exposed polycrystalline Au typically has a Φ of 4.4-4.7 eV. 26 The relatively high Φ of Ag may be attributed to surface oxide. Nevertheless, the trends in the work function change with PVDF-HFP treatment are very similar. The change in work function was also evaluated for three types of ITO substrates subjected to different surface treatments, specifically 30 min under UV-ozone or 3 min of oxygen plasma, as these treatments increase the apparent work function of ITO by removing surface carbons and increasing the surface oxygen concentration. However, after treatment with PVDF-HFP, the ITO substrates all show the same Φ of 5.80 ± 0.01 eV. Since (1) oxygen is fairly soluble in 2-butanone 27 and (2) carbonaceous polymers are added to the surface with the PVDF-HFP treatment, the effects of the oxygen treatments are essentially negated, and all of the ITO samples show the same final work function. The transmittance

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of PVDF-HFP on ITO is shown in Figure S1. Note that the % transmission does not change significantly. The peaks of ITO/PVDF-HFP are shifted to higher energy in accordance with the larger band-gap of ITO/PVDF-HFP due to its increased work function relative to bare ITO. The Φ measurements suggest that the observed change in work function is not caused by partial charge transfer associated with chemisorption because ∆Φ does not decrease as the substrate Φ increases and approaches the chemical potential of PVDF-HFP. 19 The relative magnitudes of ∆Φ roughly follow the metal surface dipole energy, which corresponds to the polarizability of the electron density tail at the metal surface, and is consistent with physisorption of a material with an inherent dipole. 28 While the concentration of PVDF-HFP had no significant impact on the change in work function (Fig 2A), the surface coverage was found to be very important. PVDF-HFP, similar to other fluorinated polymers, has poor wetting characteristics. AFM images are shown in Figure 2B of PVDF-HFP deposited on aluminum films and subjected to different annealing conditions. Below the melting point of PVDF-HFP (∼123 ◦C), increasing the annealing temperature decreases surface roughness and increases the apparent work function of the film. Above the melting point, the film self-aggregates, reducing coverage of the aluminum surface and decreasing the effective work function. For the data tabulated in Table 1, all films were annealed ∼5 minutes at 120 ◦C to improve surface coverage either in a glovebox or in air, in accordance with the measurement technique.

2.2

X-ray Photoemission Spectroscopy

The mechanism for the work function change observed was further investigated using X-ray photoemission spectroscopy to determine if the change was due to interfacial bonding or reaction between the conductor and polymer. XPS peaks for aluminum and silver are shown in Figure 3. The attenuation of the metal peaks caused by the thin PVDF-HFP layer was compensated for by using longer scan times. Since XPS is a surface sensitive technique, it 6


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can be concluded that the signal from the metal layer at the PVDF-HFP interface would dominate the XPS metal peaks. The pristine metal peaks shown were referenced to the amorphous carbon C1s peak at 348.8 eV. The samples with PVDF-HFP were referenced to the seven C1s peaks for PVDF-HFP (Fig 3C). 29 Peak shapes and relative positions are in good agreement with literature values. All peaks were fit using a Shirley background subtraction and 70 % Gaussian - 30 % Lorentzian peak shapes. The aluminum 2p peaks were fit assuming a 0.44 eV separation between 2p1/2 and 2p3/2 and an area ratio of 1:2. The Ag 3d5/2 peak is shifted to lower binding energy by 0.12 eV, from 368.59 eV to 368.47 eV which is neglible given the instrument error of ∼0.1 eV. There is no evidence of AgF (367.80 eV) or AgF2 (367.30 eV). The aluminum samples show significant aluminum oxide which is consistent with the sample preparation being performed under ambient conditions. Interestingly, the Al/PVDF-HFP sample shows a smaller degree of oxidation at the surface (64.3 % oxide versus 74.1 % oxide). Perhaps the PVDF-HFP forms something of a passivation layer at the surface and prevents further oxidation. Lim et al. observed increased device lifetime after incorporating a perfluorinated ionomer in the hole extraction layer of organic solar cells. 16 The oxide peak is additionally shifted to lower binding energy by 0.19 eV from 74.29 eV to 74.08 eV by the addition of PVDF-HFP. The aluminum 2p1/2 and 2p3/2 were negligibly shifted to lower binding energy by 0.01 eV. The above XPS results suggest that the metallic peaks do not show evidence of chemisorption; thus partial charge transfer associated with chemical reaction between PVDF-HFP and the metallic surfaces is not responsible for the work function shift. In addition, Fermi-level pinning is unlikely because of the large band-gap and very low LUMO level of PVDF-HFP. Thus, it is hypothesized that physisorption of PVDF-HFP causes a local vacuum-level shift at the metal or metal oxide surface, associated with formation of a surface dipole. This surface dipole explains the apparent work function increase on such a broad range of metals and metal oxides. A similar effect was demonstrated by PEIE and PFN in their ability to universally decrease the substrate work function. While these materials are amorphous, the

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branched aliphatic amine groups are hypothesized to orient down toward the substrate. 30 This occurs as charged species preferentially move to the interface to decrease the distance between the intrinsic dipole on the pendant groups and the image dipole in the metal. 24 The sign of the work function change is then determined by the charge of the smallest, most mobile pendant groups, with negative atoms at the surface resulting in a drop in measured work function. For PVDF-HFP, orientation of the large, bulky hexafluoropropylene co-monomer in PVDF-HFP pointing away from the surface may account for the strength of the interfacial dipole in an otherwise unordered material.

2.3

Solar Cell Performance

To demonstrate the effectiveness of PVDF-HFP in modifying work function levels, solar cells were fabricated with and without PVDF-HFP at the anode. P3HT:PCBM is a model active layer for organic solar cells that has been much studied in the literature. For these proof-of-concept studies, two systems were studied, one using PC61 BM and the other using PC71 BM, in the structure shown in the inset of Figure 4A. ZnO-coated ITO was used as the transparent cathode, and Ag with and without PVDF-HFP was used as the anode. We used inverted device structures for several reasons: (1) The typical inverted device would use a high Φ metal, such as Au, or a hole-injection layer, such as MoO3 combined with Ag or Au metal electrode. With pure Ag, it was previously found that inverted device cell structures allow for processing even in air and give better overall device stability compared to using reactive low Φ cathodes above the active layer. (2) PVDF-HFP can be readily deposited on P3HT with an orthogonal solvent; however, coating P3HT:PCBM on the highly lyophobic PVDF-HFP is challenging and may result in non-ideal morphology. (3) P3HT:PCBM solar cells achieve optimal performance when annealed at 140 or 150 ◦C; however, PVDF-HFP cannot be heated above 123 ◦C without significantly increasing the surface roughness, as discussed earlier; therefore it is desirable to deposit PVDF-HFP on P3HT:PCBM. The averaged J-V curves for P3HT:PC61 BM solar cells with varying PVDF-HFP concen8


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tration are shown in Figure 4A. Without the interlayer, the J-V curve exhibits double-diode ("s-kink") characteristics, which is most likely due to the poor energy-level alignment between Ag and P3HT. 31 With addition of PVDF-HFP, the S-kink disappears and the VOC increases by ∼0.37 eV for the PC61 BM cells. The VOC increases moderately with increasing PVDF-HFP concentration, peaking at 4 mg/mL and then declines with increasing concentration. The JSC and fill factor follow the same trend as the VOC . The J-V curves are similar for P3HT:PC71 BM (Fig. S2). The main performance metrics for cells with each donor type are shown in Figure 4C and tabulated in the Supporting Information (Tables S1 and S2). The maximum VOC achievable in a solar cell is roughly the difference between the HOMO of the donor material and LUMO of the acceptor minus 0.3 eV. 13 However, if the work functions of the cathode or anode are between these values, the VOC is reduced to the ∆Φ of the electrodes. Given that the Φcathode is optimized to be above the LUMO of the acceptor, we attribute the initial increase in VOC to a drop in the effective Φanode below the HOMO of the donor. This hypothesis is corroborated by the dark J-V characteristics for the P3HT:PC61 BM cells without and with 4 mg/mL PVDF-HFP as shown in Figure 4B. With addition of PVDF-HFP, the injection current under forward bias increases significantly, which may be attributed to the removal of a significant injection barrier at the anode. The increase in the Φanode is expected to increase the built-in electric field in the device, driving separation of excitons and carrier collection at the electrodes, resulting in the observed increase in JSC . The increase in FF is attributed to an increased shunt resistance. The low shunt resistance without PVDF-HFP may be attributed to local single carrier conduction, which is more pronounced in devices with a lower built-in voltage and thus lower rectification. 8,32 The slight increase in VOC with PVDF-HFP concentration up to 4 mg/mL is attributed to more uniform PVDF-HFP surface coverage. As shown in Figure 5, the high surface energy of the PVDF-HFP film causes it to self-aggregate, forming mesas. The films are hypothesized to form an initial monolayer, followed by island formation. Unfortunately, the surface

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roughness and device-to-device thickness variability (10-30 nm) of the underlying active layer prevent accurate determination of PVDF-HFP layer thickness on top of the solar cells. Film thicknesses were measured on SiO2 but the dramatically different morphology may indicate that these thicknesses are not representative of the films in the solar cells (Fig. S3). Provided the dipoles are aligned, a thicker film would tend to increase the built-in electric field and observed VOC . 33 Verifying dipole alignment would require further experiments that are beyond the scope of this study. Nevertheless, these results are consistent with PVDF-HFP forming a surface dipole, as 95% of the total increase in the VOC is achieved with 1 mg/mL, presumed to be one monolayer. Above 4 mg/mL, the series resistance increases significantly, lowering the fill factor. The large band-gap in PVDF-HFP makes the material insulating and blocks charge injection from the metal. With increasing concentration, the layer thickness likely increases, thereby increasing the contact resistance at the anode. The probability that a hole can tunnel through the PVDF-HFP layer decreases with increasing thickness, leading to a decline in the JSC . Accumulation of charges due to inefficient extraction may explain the drop in VOC . The observed drop is on the order of that predicted by Wagenpfahl et al. for an anode with negligible anode injection barrier. 31 In the P3HT:PC71 BM J-V curves, the slight s-kink expected from reduced charge extraction at 10 mg/mL PVDF-HFP is more pronounced. ITO/ZnO/P3HT:PC61 BM/MoO3 /Ag solar cells with an optimized inverted structure were fabricated as a reference. The top-performing solar cells for the two anode configurations, MoO3 /Ag and PVDF-HFP/Ag, are compared in Figure 6. The performance metrics are listed in Table 2. The VOC and PCE are comparable. The JSC of the PVDF-HFP cell is slightly higher than that of the MoO3 cell but is compensated by a lower fill factor, likely the result of the higher series resistance.

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2.4

Conclusion

Herein we have demonstrated the ability of PVDF-HFP to increase the work function of a variety of metals and metal oxides at low cost through solution-processable methods. When introduced at the Ag anode, PVDF-HFP increased the efficiency of solar cells by roughly an order of magnitude - from 0.17 % to 1.62 % for P3HT:PC61 BM cells and 0.30 % to 2.45 % for P3HT:PC71 BM cells. To our knowledge, this is the first charge-neutral polymer that has been shown to "universally" increase the work function of a number of common electrodes through physisorption and formation of a surface dipole. The large band-gap and poor wetting characteristics of PVDF-HFP put constraints on the utility of this finding; however, this work provides a possible route toward achieving low cost, chemically inert, solution-processable, high work function electrodes.

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Experimental

3.1

Preparation of Metal Films

Microscope glasses were used as substrates for the metal films. The substrates were cleaned sequentially with detergent, DI water, toluene, acetone, isopropanol and methanol, then dried with nitrogen. 50-100 nm of Ag, Au, Ni, Cu, or Al were thermally deposited on top of the cleaned substrates in a vacuum evaporator at a pressure of 8 × 10−6 Torr. Fluorine-tinoxide (FTO) and indium-tin-oxide (ITO) coated glass substrates were purchased from Xin Yan Ltd. both with sheet resistances of 15Ω/✷. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was purchased from SigmaAldrich (Product No. 427187) and used as received. PVDF-HFP was dissolved in 2-butanone to 2 mg mL−1 and stirred overnight. The solution was filtered with a 0.2 µm polytetrafluoroethylene syringe filter and deposited on top of the substrates. The solution was allowed to sit for 20 s prior to spin-coating at 3000 rpm for 30 s. The substrates were then annealed for

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5 min at 120 ◦C. The samples measured by UPS under vacuum were prepared in a nitrogen glovebox. The samples measured by PESA in air were prepared under ambient conditions.

3.2

Characterization

Work function measurements in air were conducted with a Riken AC-2 Photoelectron Spectrometer. The UPS measurements were performed using a modified VG ESCA Lab system (∼8 × 10−11 Torr, He I 21.2 eV source). The UV light spot size on the sample is ∼1 mm in diameter. The elemental composition of the surfaces was measured with XPS (PHI 5000 Versaprobe, Al KR source).

3.3

Preparation of Solar Cells

Regioregular P3HT (Sepiolid P100, by BASF in cooperation with Rieke) with a molecular weight of MN = 12480 g/mol and a polydispersity index of 1.7 (as measured by gas-phase chromatography in tetrahydrofuran) was used. C61 PCBM and C71 PCBM were bought from Nano-C. Both materials were used as received. 30 mg of P3HT were added to 1 mL of 1,2-dichlorobenzene in a nitrogen glovebox to which an additional 30 mg of PC61 BM or PC71 BM were added. The resulting solution was stirred overnight at 40 ◦C in a nitrogen glovebox. Glass substrates patterned with ITO with a sheet resistance of 15 Ω/✷ were purchased from Xin Yan Technology Lt. The glass/ITO substrates were cleaned with a 30 min ultraviolet-ozone treatment. 5.6 % w/v ZnO in acetone was purchased from Infinity PV. The ZnO solution was deposited through a nylon 0.2 µm filter and then spin-coated for 20s at 4000 rpm before annealing at 60 ◦C for 5 minutes. The P3HT:PCBM solution was filtered with a 0.45 µm polytetrafluoroethylene syringe filter before deposition in a nitrogen glovebox by spin-coating 30s at 800 rpm. The prepared films were left to dry overnight. In a separate glovebox, PVDF-HFP solutions of varying concentration were added to methylethyl ketone and stirred overnight. The solar cells were annealed 5 min at 140 ◦C before the PVDF-HFP solutions were deposited through a 0.7 µm glass fiber filter, allowed to sit 12


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20s, then spin-coated 30s at 5000rpm. The cells were annealed 5 min at 120 ◦C before they were transferred to a metal evaporator. 150 nm of Ag was deposited at 0.2 Å/s under < 1.0 × 10−5 Torr. All devices were tested inside a nitrogen glovebox after encapsulation under AM 1.5G illumination with an intensity of 100 mW cm−2 (Newport Solar Simulator 94021A) calibrated by a Newport-certified silicon photodiode covered with a KG5 filter. The photodiode active area of 6.63 mm2 is comparable to our device area of 4.0 mm2 . The J-V curves were recorded with a Keithley 2400 SourceMeter. The mean J-V curves and performance metrics are all averaged over at least twenty devices prepared in two separate batches, and the error bars for the performance metrics denote the standard deviations.

Acknowledgement This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1147474 (A.C.H.) and the National Science Foundation grants CBET-1437656 and DMR-1303742 (C.W., B.E. and Y.G.). R.P. thanks the Generalitat de Catalunya for a Beatriu de Pinos, Marie Curie COFUND fellowship.

Supporting Information Available Supplemental current density-voltage curves and device performance data, PVDF-HFP thickness and morphology data on SiO2 , and PVDF-HFP transmittance data on ITO. This material is available free of charge via the Internet at http://pubs.acs.org/.

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CF3 CF2-CF3

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CH2-CF2 CF C=O CH2-CF2 CH2-CH

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C1s Binding Energy (eV)

Figure 3: XPS metallic peaks for (A) Ag and(B) Al with and without PVDF-HFP. (C) XPS C1s peaks for Al with PVDF-HFP.

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A

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P3HT:PC61BM

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0.6 Ag PVDF-HFP P3HT:PCBM ZnO ITO

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0.4 0.2 0.0 -0.2

0.3

0.1

PCE (%)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

1 0 0

5

10

PVDF-HFP Concentration (mg/mL)

Figure 4: (A) J-V characteristics of ITO/ZnO/P3HT:PC61 BM/PVDF-HFP/Ag solar cells under illumination. Inset shows structure of solar cell devices. (B) Dark J-V characteristics of the same devices with and without PVDF-HFP. (C) Device performance metrics as a function of PVDF-HFP concentration for cells with PC61 BM or PC71 BM.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 mg/mL

3 mg/mL

5 mg/mL

10 mg/mL

Figure 5: Differential interference contrast (DIC) images of PVDF-HFP layer at different concentrations deposited on P3HT:PC61 BM. The scale bar represents 100 µm in all cases.

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10 -2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5

0

-5 Ag MoO3/Ag PVDF-HFP/Ag

-10 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 6: J-V characteristic curves of top performing cells ITO/ZnO/P3HT:PC61 BM/Anode with different anode configurations

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with

structure


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Table 1: Work function of conducting materials with and without PVDF-HFP, as measured independently by UPS in UHV and PESA in air. Dashed lines indicate no measurement performed on the indicated sample. Electrodes

Work function in Air (eV) Clean PVDF-HFP ∆Φ

Work Function in UHV (eV) Clean PVDF-HFP ∆Φ

Metals Al 3.88 4.62 0.74 3.11 3.79 Ni 4.65 5.42 0.76 Cu 4.84 5.68 0.84 Ag 4.72 5.46 0.74 4.73 5.25 Au 4.85 5.27 0.42 4.58 5.05 Metal Oxides ITO 4.84 5.79 0.95 a ITO 5.38 5.81 0.43 ITOb 5.74 5.80 0.06 FTO 4.73 5.51 0.79 a b UV-ozone treated; O2 -plasma treated.

0.68 0.52 0.47 -

Table 2: Performance characteristics of top performing cells ITO/ZnO/P3HT:PC61 BM/Anode with different anode configurations Anode Ag MoO3 /Ag PVDF-HFP/Ag

PCE (%) VOC (V) JSC 0.38 0.14 2.70 0.55 2.62 0.56

20

(mA cm−2 ) FF 7.60 0.35 9.40 0.55 9.82 0.48


with

structure

RS (Ω cm−2 ) 8 3 5

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References (1) Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911–918. (2) Benight, S. J.; Wang, C.; Tok, J. B.; Bao, Z. Stretchable and Self-Healing Polymers and Devices for Electronic Skin. Prog. Polym. Sci. 2013, 38, 1961–1977. (3) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dötz, F.; Kastler, M.; Facchetti, A. A High-Mobility Electron-Transporting Polymer for Printed Transistors. Nature 2009, 457, 679–686. (4) Campbell, I.; Hagler, T.; Smith, D.; Ferraris, J. Direct Measurement of Conjugated Polymer Electronic Excitation Energies Using Metal/Polymer/Metal Structures. Phys. Rev. Lett. 1996, 76, 1900–1903. (5) Greiner, M. T.; Chai, L.; Helander, M. G.; Tang, W.-M.; Lu, Z.-H. Transition Metal Oxide Work Functions: The Influence of Cation Oxidation State and Oxygen Vacancies. Adv. Funct. Mater. 2012, 22, 4557–4568. (6) Zhou, Y. et al. A Universal Method to Produce Low-Work Function Electrodes for Organic Electronics. Science 2012, 336, 327–332. (7) Page, Z. A.; Liu, Y.; Duzhko, V. V.; Thomas, P.; Emrick, T.; Russell, T. P.; Emrick, T. Fulleropyrrolidine Interlayers: Tailoring Electrodes to Raise Organic Solar Cell Efficiency. Science 2014, 346, 441–444. (8) Xia, R.; Leem, D. S.; Kirchartz, T.; Spencer, S.; Murphy, C.; He, Z.; Wu, H.; Su, S.; Cao, Y.; Kim, J. S.; Demello, J. C.; Bradley, D. D. C.; Nelson, J. Investigation of a Conjugated Polyelectrolyte Interlayer for Inverted Polymer:Fullerene Solar Cells. Adv. Energy Mater. 2013, 3, 718–723.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(9) Vázquez, H.; Dappe, Y.; Ortega, J.; Flores, F. A Unified Model for Metal/Organic Interfaces: IDIS, ’Pillow’ Effect and Molecular Permanent Dipoles. Appl. Surf. Sci. 2007, 254, 378–382. (10) Yagyu, S.; Yoshitake, M.; Chikyow, T. Adsorption Structure and Work Function of Succinic Acid on Cu(110) Surface. Hyomen Kagaku 2007, 28, 525–531. (11) Sharma, A.; Haldi, A.; Hotchkiss, P. J.; Marder, S. R.; Kippelen, B. Effect of Phosphonic Acid Surface Modifiers on the Work Function of Indium Tin Oxide and on the Charge Injection Barrier into Organic Single-Layer Diodes. J. Appl. Phys. 2009, 105, 074511. (12) Han, T.-H.; Kwon, S.-J.; Li, N.; Seo, H.-K.; Xu, W.; Kim, K. S.; Lee, T.-W. Versatile PType Chemical Doping to Achieve Ideal Flexible Graphene Electrodes. Angew. Chem. Int. Ed. 2016, 55, 6197–6201. (13) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Solar Cells - Towards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789–794. (14) Lim, K.-G.; Kim, H.-B.; Jeong, J.; Kim, H.; Kim, J. Y.; Lee, T.-W. Boosting the Power Conversion Efficiency of Perovskite Solar Cells Using Self-Organized Polymeric Hole Extraction Layers with High Work Function. Adv. Mater. 2014, 26, 6461–6466. (15) Lee, T.-W.; Chung, Y.; Kwon, O.; Park, J.-J. Self-Organized Gradient Hole Injection to Improve the Performance of Polymer Electroluminescent Devices. Adv. Funct. Mater. 2007, 17, 390–396. (16) Lim, K.-G.; Ahn, S.; Kim, Y.-H.; Qi, Y.; Lee, T.-W. Universal Energy Level Tailoring of Self-Organized Hole Extraction Layers in Organic Solar Cells and Organic-Inorganic Hybrid Perovskite Solar Cells. Energy Environ. Sci. 2016, 9, 932–939.

22


Page 22 of 25

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Jørgensen, M.; Norrman, K.; Krebs, F. C. Stability/Degradation of Polymer Solar Cells. Sol. Energy Mater. Sol. Cells 2008, 92, 686–714. (18) Jönsson, S. K. M.; Carlegrim, E.; Zhang, F.; Salaneck, W. R.; Fahlman, M. Photoelectron Spectroscopy of the Contact between the Cathode and the Active Layers in Plastic Solar Cells: The Role of LiF. Jpn. J. Appl. Phys. 2005, 44, 3695–3701. (19) Braun, S.; Salaneck, W. R.; Fahlman, M. Energy-Level Alignment at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 2009, 21, 1450–1472. (20) Zehner, R.; Parsons, B.; Hsung, R.; Sita, L. Tuning the Work Function of Gold with Self-Assembled Monolayers Derived from X-[C6 H4 -C≡C-]n C6 H4 −SH (n = 0, 1, 2; X = H, F, CH3 , CF3 , and OCH3 ). Langmuir 1999, 1121–1127. (21) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Controlling Charge Injection in Organic Electronic Devices Using SelfAssembled Monolayers. Appl. Phys. Lett. 1997, 71, 3528–3530. (22) Guan, F.; Pan, J.; Wang, J.; Wang, Q.; Zhu, L. Crystal Orientation Effect on Electric Energy Storage in Poly(vinylidene fluoride-co-hexafluoropropylene) Copolymers. Macromolecules 2010, 43, 384–392. (23) Kim, J. S.; Park, J. H.; Lee, J. H.; Jo, J.; Kim, D.-Y.; Cho, K. Control of the Electrode Work Function and Active Layer Morphology via Surface Modification of Indium Tin Oxide for High Efficiency Organic Photovoltaics. Appl. Phys. Lett. 2007, 91, 112111. (24) Xiao, J.; Zhou, X.; Zhang, Q. M.; Dowben, P. The Effect of Defects on the Electronic Structure of Long Chain Ferroelectric Polymers. J. Appl. Phys. 2009, 106, 044105. (25) Braun, S.; Osikowicz, W.; Wang, Y.; Salaneck, W. Energy Level Alignment Regimes at Hybrid Organic/Organic and Inorganic/Organic Interfaces. Org. Electron. 2007, 8, 14–20. 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(26) Kahn, A. Fermi Level, Work Function and Vacuum Level. Mater. Horiz. 2016, 3, 7–10. (27) Battino, R.; Rettich, T. R.; Tominaga, T. The Solubility of Oxygen and Ozone in Liquids. J. Phys. Chem. Ref. Data 1983, 12, 163–178. (28) Fahlman, M.; Crispin, A.; Crispin, X.; Henze, S. K. M.; de Jong, M. P.; Osikowicz, W.; Tengstedt, C.; Salaneck, W. R. Electronic Structure of Hybrid Interfaces for PolymerBased Electronics. J. Phys. Condens. Matter 2007, 19, 183202. (29) Dapoz, S.; Betz, N.; Guittet, M.-J.; Le Moël, A. ESCA Characterization of HeparinLike Fluoropolymers Obtained by Functionalization after Grafting Induced by Swift Heavy Ion Irradiation. Nucl. Instr. and Meth. in Phys. Res. B 1995, 105, 120–125. (30) van Reenen, S.; Kouijzer, S.; Janssen, R. a. J.; Wienk, M. M.; Kemerink, M. Origin of Work Function Modification by Ionic and Amine-Based Interface Layers. Adv. Mater. Interfaces 2014, 1, 1400189. (31) Wagenpfahl, A.; Rauh, D.; Binder, M.; Deibel, C.; Dyakonov, V. S-Shaped CurrentVoltage Characteristics of Organic Solar Devices. Phys. Rev. B 2010, 82, 115306. (32) Dongaonkar, S.; Servaites, J. D.; Ford, G. M.; Loser, S.; Moore, J.; Gelfand, R. M.; Mohseni, H.; Hillhouse, H. W.; Agrawal, R.; Ratner, M. A.; Marks, T. J.; Lundstrom, M. S.; Alam, M. A. Universality of Non-Ohmic Shunt Leakage in Thin-Film Solar Cells. J. Appl. Phys. 2010, 108, 124509. (33) Yuan, Y.; Reece, T. J.; Sharma, P.; Poddar, S.; Ducharme, S.; Gruverman, A.; Yang, Y.; Huang, J. Efficiency Enhancement in Organic Solar Cells with Ferroelectric Polymers. Nat. Mater. 2011, 10, 296–302.

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Investigation of a Solution-Processable, Nonspecific Surface Modifier

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