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Chemical Composition of Additives That Spontaneously Form Cathode Interlayers in OPVs Igal Deckman, Stas Obuchovsky, Moshe Moshonov, and Gitti L. Frey* Department of Materials Science and Engineering, Technion Israel Institute of Technology, Haifa 32000, Israel S Supporting Information *

ABSTRACT: Interlayers between the active layer and the electrodes in organic devices are known to modify the electrode work function and enhance carrier extraction/ injection, consequently improving device performance. It was recently demonstrated that chemical interactions between the evaporated electrode and interlayer additive can induce additive migration toward the metal/organic interface to spontaneously form the interlayer. In this work we used P3HT:PEG blends as a research platform to investigate the driving force for additive migration to the organic/metal interface and the source of the work function modification in OPVs. For this purpose PEG derivatives with different end groups were blended with P3HT or deposited on top of P3HT layer, topped with Al or Au evaporated electrodes. The correlation between the additive chemical structure, the Voc of corresponding devices, and the metal/organic interface composition determined by XPS revealed that the driving force for additive migration toward the blend/metal interface is the chemical interaction between the additives’ end group and the deposited metal atoms. Replacing the PEG additives with alkyl additives bearing the same end groups has shown that the Al work function is actually modulated by the PEG backbone. Hence, in this work we have identified and separated between structural features controlling the migration of the interlayer additive to the organic/metal interface and those responsible for the modification of the metal work function.

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INTRODUCTION Organic photovoltaics (OPV) have recently reached over 10% power-conversion efficiency and hence demonstrate their feasibility for becoming lightweight, flexible, and low-cost alternatives to Si-based solar cells.1,2 The most efficient OPV devices are based on the bulk heterojunction (BHJ) structure composed of nanoscale intermixed continuous networks of an electron-donating conjugated polymer and an electron-accepting fullerene derivative.3,4 Although attaining control of morphology has led to a steady increase in device efficiency, the generally moderate performance of OPVs indicates that seminal limiting processes are still not fully understood.5 One such process is the inhibition of charge extraction at the electrodes which could significantly affect device output.6−9 The decrease of an extraction/injection energy barrier at the organic active layer/electrode interface in OPVs increases charge carrier extraction and reduces recombination losses related to charge accumulation.10,11 The requirements of cathodes are generally met by low work function metals (Ca, Mg, and Ba). However, these metals are highly reactive and easily oxidized by oxygen and/or moisture which leads to poor stability of the devices and raises the need for more inert electrodes. The suggested more stable metals (Al, Ag, and Au), however, have work functions that are higher than the lowest unoccupied molecular orbitals (LUMO) of most organic acceptor materials and hence impose barriers for electron extraction. © XXXX American Chemical Society

One of the most promising approaches to reduce the energy barrier between stable metal cathodes and the organic active layer is the utilization of interlayers. It is well established that introducing interlayers between the organic active layers and the electrodes enhances device performance by increasing the open circuit voltage (Voc) and/or the short circuit current (Jsc).12 Both organic and inorganic cathode interlayers have been suggested. The latter include salts, such as lithium fluoride (LiF)13 and cesium carbonate (Cs2CO3),14 and n-type semiconducting metal oxides, such as titanium suboxide (TiOx),15 alumina (Al2O3),16 and zinc oxide (ZnO).17 Generally, the inorganic materials are deposited by thermal evaporation, sputtering, or atomic layer deposition (ALD) techniques. Although their electrical properties are suitable, the difficulty to deposit them on soft organic surfaces limits their utilization as cathode interlayers. Organic cathode interlayers, on the other hand, are generally simpler to process. To date, most of the organic cathode interlayers are deposited in distinct processing steps on top of the preformed active layer. Processing techniques include thermal deposition in high vacuum and spin-coating using orthogonal solvents that do not dissolve the active layer.18−23 However, high vacuum thermal evaporation is not suitable for Received: March 9, 2015 Revised: May 17, 2015

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for 15 min in ambient conditions. To a toluene solution of P3HT (40 mg/mL) calculated amounts of PEG were added to obtain solutions with 0, 9, 15, or 20 wt % PEG. P3HT:PEG blend films were deposited by spin-coating at 2000 rpm for 100 s onto ITO/PEDOT:PSS substrates. The P3HT:PEG bilayer samples were prepared by UVozone treatment for 8 s of pristine P3HT samples followed by spincoating of PEGs at 5000 rpm for 100 s. The P3HT:Alkyl(Alcohol) and P3HT:Alkyl(Acetate) bilayer samples were prepared by spin-coating of alkyl solutions (10 mg/mL) in xylene at 5000 rpm for 100 s on to pristine P3HT films. Thermal deposition of the top metal layer, Al or Au, was conducted through a shadow mask at a system pressure of ∼10−6 Torr. Thirty-two devices were prepared and tested for each blend, and reported Voc values are averages calculated for over 28 devices (90%) each type. For the XPS measurements, the thickness of the metal layer was ∼3 nm. For the devices, the thickness of Al cathodes was 90 nm topped by ∼10 nm of Au directing a device area of 3 mm2. Characterization. The absorption spectra were measured using a Varian Cary 100 Scan UV−vis spectrophotometer in the 400−700 nm range. X-ray photoelectron spectroscopy (XPS) was performed in a Thermo VG Scientific Sigma Probe fitted with a monochromatic Al Kα (1486.6 eV) source. A 100 W X-ray beam of 400 μm in diameter was used for high energy resolution scans of the C 1s spectra with a pass energy of 30 eV. Line-shape analysis was done using the XPSPEAK4.1 software after a Shirley-type background subtraction. The binding energy scale calibration of the C 1s spectra was done by referencing the C−C/C−H bond signal to 285 eV. The C 1s spectra were measured in the standard modes, i.e., with the angle between the direction of the analyzer and the specimen normal in the range 53 ± 30°. Based on inelastic mean free path estimations of the metal/blend system, the information depth of C 1s electrons includes a ∼5 nm thick organic film beneath the Al layer.27 For quantitative XPS analysis of the surface composition, only samples with identical metal coverage were compared, i.e., samples that were together in the evaporation chamber. For such a set of samples, we normalized the intensity of the C 1s spectra to the area under the peak of the metal (Al 2p). The area under the metal peak represents the thickness of the metal overlayer and is constant for each set of samples. Generally, the C 1s spectra of the metal covered blend films were line-fitted to three main peaks: 285 eV (C−C/C−H), 286.6 eV (C−O), and 288.6 eV (O−CO). Device characterization was performed in inert atmosphere under a 100 mW/cm2 AM1.5G class A sun simulator (Science Tech Inc. ss150 solar simulator) with a Keithley 2400 source meter.

large-area processing, and the use of orthogonal solvents significantly limits the choice of interlayer materials. Recently, a few studies have reported the utilization of additive migration toward the film surface (surface segregation) to spontaneously form an interlayer. Application of interlayer forming additives eliminates excessive processing steps and hence is technically advantageous and more cost-effective. Initially, spontaneous migration to form of interlayers was limited to low surface energy compounds because these are known to enrich the film surface during spin-coating. A typical example is a ∼25% efficiency improvement due to spontaneous migration of a fluorocarbon fullerene additive, F-PCBM, to the surface of a poly(3-hexylthiophene-2,5-diyl) (P3HT):phenylC61-butyric acid methyl ester (PCBM) blend. However, we have recently shown that the chemical interaction between the additive and the evaporated metal atoms can also induce additive migration to the organic/metal interface.24,25 This was demonstrated by blending poly(ethylene glycol) (PEG), a high surface energy additive, with P3HT and PCBM. The chemical interaction between PEG and Al induced PEG migration to the organic/metal interface during the cathode deposition. The spontaneous formation of the PEG interlayer reduced the cathode work function and doubled the device efficiency.26 Therefore, correlating the chemical structure of the additive with its metal-induced migration and the resulting modulation of the metal work function will lead to new useful cathode interlayers. In this study a set of PEG derivatives with different end groups were blended with P3HT, and the effect of the chemical structure on the additive migration to the surface and the Voc are carefully studied. The XPS measurements of the metal/ organic interface composition revealed a direct relationship between the reactivity of the PEG end groups and the spontaneous migration to the Al/P3HT:PEG interface. This study was conducted on P3HT-only films with no acceptor species. The type of the PEG end group, on the other hand, did not affect the Al work function. By replacing the PEG additives with alkyl additives bearing the same end groups, we found that the Al work function is actually modulated by the PEG backbone and not the end group. Hence, we have identified and separated between structural features controlling additive migration to the organic/metal interface and those responsible for modification of the metal work function.

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RESULTS AND DISCUSSION

To study the effect of the chemical structure of the additives on their migration to the metal/organic interface during the metal evaporation, we selected a family of PEG additives with different end groups. PEG was chosen for two main reasons: (i) the presence of a PEG interlayer at the organic/electrode interface was shown to increase both Voc and Jsc;19,28 (ii) PEGs surface energy (∼43 mJ m−2)29 is higher than that of both P3HT (∼27 mJ m−2)30 and PCBM (∼38 mJ m−2)29 and hence is not expected to migrate to the blend/air interface during spin-coating.25,31 This combination of features makes PEG a most suitable platform for studying additives migration to the polymer/metal interface induced by chemical interaction with metal evaporated atoms. The selected PEG additives used in this study are PEG(Alcohol), PEG(Ether), PEG(Acetate), PEG(Methyl ether), PEG(Thiol), and PEG(Amine). The chemical structures of the additives are shown in Figure 1. Importantly, the backbone and the number of repeat units are identical for all additives to maintain the same molecular weight (Mw ∼ 200 g/mol) and general size of the additive. We focus the research on blends of P3HT and the different PEG additives with no electron acceptor moiety in the film. These

EXPERIMENTAL SECTION

Materials. Poly(3-hexylthiophene) (P3HT) (Sepiolid P100, regioregularity >95%) was purchased from Rieke Metals and used as received. Poly(3,4-ethylenedioxythiophene):polystyrenesulfonic acid (PEDOT:PSS) was acquired from Haraeus (Clevios PVP AL 4083) and was filtered before use through a 0.45 μm poly(tetrafluoroethylene) (PTFE) filter. Poly(ethylene glycol) (PEG(Alcohol)), poly(ethylene glycol) dicarboxymethyl ether (PEG(Acetate)), poly(ethylene glycol) dimethyl ether (PEG(Methyl ether)), poly(ethylene glycol) dithiol (PEG(Thiol)), suberic acid (Alkyl(Acetate)), and 1,10-decanediol (Alkyl(Alcohol)) ∼200 g/mol Mw were purchased from Sigma-Aldrich and used as received. Poly(ethylene glycol)diamine (PEG(Amine)) ∼200 g/mol Mw was purchased from Advanced Polymer Materials and used as received. Poly(ethylene glycol) dicarboxymethyl ester (PEG(Ester)) were synthesized and purified; for more details see the Supporting Information (PEG(Ester) synthesis). Film Deposition and Device Fabrication. ITO-covered glass substrates were cleaned by sonication in acetone, methanol, and 2propanol, followed by 15 min of a UV-ozone treatment. PEDOT:PSS was spin-coated at 5000 rpm onto the ITO/glass and dried at 120 °C B

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Figure 1. Chemical structures of the PEG and alkyl additives used in this study.

Figure 2. Voc under illumination of photodiodes with the general structure: glass/ITO/PEDOT:PSS/blend/Al. The blends are composed of P3HT with 0, 9, 15, and 20 wt % of each PEG: PEG(Acetate) (black), PEG(Methyl ether) (cyan), PEG(Alcohol) (red), PEG(Amine) (pink), and PEG(Ester) (blue).

films are poor photovoltaic materials, but the morphology, optical properties, and device performances are modulated by the PEG migration only and hence provide a suitable platform to study additive migration in OPV-related films. The effect of the PEG’s end group on its migration to the interface to spontaneously form interlayers is studied by spincoating films from solutions containing known quantities of each PEG and P3HT. The films were deposited onto ITO/ PEDOT:PSS substrates, topped with a thermally evaporated Al cathode, and their current density−voltage (J−V) curves, under illumination and in the dark, were measured. The P3HT:PEG blends studied here do not include an acceptor species. In standard BHJ devices, the Voc depends not only on the electrode work function difference but also on kinetic, thermodynamic, and transport events in the diode. Namely, at Voc the net observed current is zero due to a balance of injection and extraction events and transport currents. The P3HT:PEG blends studied here do not include an acceptor species and the irradiation generates charge carriers that aid in the equilibrium of the two electrodes so that the relative work functions can be determined. Under such conditions, the Voc is a direct reference of the Al cathode effective work function.10,32 The Voc values as a function of each PEGs content in P3HT are presented in Figure 2 (the values are extracted from the J−V curves shown in Figure 1 of the Supporting Information). Figure 2 clearly shows that the Voc strongly depends on both the amount of the PEG additive in the film and the character of the end groups. For example, adding 9 wt % PEG(Alcohol), PEG(Acetate), or PEG(Amine) to P3HT results in a significant increase of the Voc values by 0.65 V to a final value of 1.15 V. Further increasing the content of these PEGs in P3HT led to a further moderate increase of the Voc (∼0.1−0.2 V). In contrast, adding 9 wt % PEG(Methyl ether) to P3HT led to only a slight increase of Voc (0.3 V), which moderately grows with further increasing the additive concentration. In the case of PEG(Ester), on the other hand, there is nearly no effect on Voc regardless of the amount of PEG(Ester) added. These results clearly indicate that the PEG end groups have a significant effect on the device performance. On the basis of the results presented in Figure 2, we divide the PEG additives into two main groups: those that induced a remarkable increase in Voc (i.e., PEG(Alcohol), PEG(Acetate), and PEG(Amine)) and those that had only a minor or no effect on Voc (i.e., PEG(Methyl ether) and PEG(Ester)).

In an earlier report we showed that PEG(Alcohol) forms an interlayer at the organic/Al interface which reduces the work function of the Al cathode and enhances the device Voc. Using XPS measurements, we showed that PEG(Alcohol), a high surface energy component, migrates to the Al/organic interface during the Al deposition.24 Hence, the absence or minor effect of PEG(Ester) and PEG(Methyl) on the Voc can originate from one of two main causes: either these additives do not migrate to the organic/metal interface, or these additives do not modify the Al work function. To distinguish between these possibilities, we use XPS and analyze the chemical composition of the organic/metal interfaces. To determine whether the different PEG additives migrate to the Al/organic interface during the Al deposition, we compare the XPS spectra of the P3HT:PEG film surfaces before and after Al deposition. Namely, we compare the XPS spectra obtained from the bare P3HT:PEG blend films surface and from areas of the same samples covered by a thin strip of thermally evaporated Al. The evaporation time was extremely short, less than 30 s, when the samples’ temperature was kept below 25 °C. No further annealing process was performed. Notably, the Al layer was thin enough (∼3 nm) to allow XPS characterization of the underlying blend/Al interface by performing the measurement through the Al layer. The XPS spectra measured on the bare or Al-covered surfaces of P3HT:PEG(Alcohol), P3HT:PEG(Acetate), P3HT:PEG(Amine), P3HT:PEG(Methyl ether), and P3HT:PEG(Ester) blend films are shown in Figure 3a−e. The presence of PEG at the surface (bare areas) or organic/ metal interface (Al-covered areas) is associated with the C−O peak at 286.6 eV in the XPS spectrum, which is characteristic of the pristine PEG C 1s XPS spectrum.33 Figure 3 shows that all XPS spectra of the bare organic surfaces (left column) are identical for all blends regardless of PEG type and content. Moreover, the spectra show no evidence of any type of PEG at the bare organic surfaces. The absence of PEG traces at the bare organic surfaces indicates that all of the PEGs used in this study do not enrich the organic film surface during film formation. The XPS spectra from Al-covered areas of P3HT:PEG(Ester) and P3HT:PEG(Methyl ether) blends (Figure 3h,j) are nearly identical for all of PEG concentrations and are similar to C

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Figure 3. High energy resolution C 1s XPS spectra of P3HT:PEG blends with 0 wt % (black squares), 9 wt % (blue circles), 15 wt % (pink diamonds), and 20 wt % (red triangles) of PEG(Alcohol) (a, b), PEG(Acetate) (c, d), PEG(Amine) (e, f), PEG(Ester) (g, h), and PEG(Methyl ether) (i, j) measured on bare (left column) and Al covered areas (right column).

those obtained from the bare organic surfaces showing no evidence of PEG at the blend/Al interface. In contrast, the XPS spectra of Al-covered areas of P3HT:PEG(Alcohol), P3HT:PEG(Acetate), and P3HT:PEG(Amine) blend surfaces

(Figure 3b,d,f) strongly depend on the PEG content in the blend. More specifically, they show a noticeable increase of the C−O peak at 286.6 eV, associated with C−O back bonds of the PEG, with an increase of PEG content in the film.24,25 D

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Figure 4. High-energy resolution C 1s XPS spectra of P3HT blends with 20 wt % of PEG(Acetate) (black squares), PEG(Alcohol) (red circles), PEG(Ester) (blue triangles), and PEG(Methyl ether) (cyan diamonds) measured on bare (a) and Au covered (b) areas.

Comparing the XPS spectra of the same film, either bare or covered by Al layer areas, reveals the presence of PEG(Alcohol), PEG(Acetate), and PEG(Amine) at the blend/Al interface but not at the bare blend surface. From these results we conclude that the Al evaporation induces the migration of these PEGs toward the blend/Al interface. In contrast to these PEGs, the XPS spectra of Al-covered areas of PEG(Ester) and PEG(Methyl) are identical for all compositions and similar to those of the bare organic areas. The absence of the PEG peak at 286.6 eV at the blend/Al interface indicates that these additives do not migrate to the organic/Al interface (Figure 3). Importantly, the XPS results are in excellent agreement with the Voc results shown in Figure 2. Namely, the devices including the additives that did not migrate to the interface, i.e., PEG(Ester) and PEG(Methyl ether), did not show an increase of the Voc. In contrast, the additives that do migrate to the organic/Al interface, based on the XPS results, led to a significant increase of the Voc. Therefore, we can conclude that the end group of the PEG determines its migration to the organic/metal interface to form an interlayer. The presence of the interlayer is then reflected in an increase of the Voc and improved device performance. In our previous work, we showed that PEG migration is induced by its chemical interaction with the deposited metal.24,25 However, our present results demonstrate that only specific end-terminated PEGs migrate to the blend/Al interface. Therefore, a chemical reaction between the ethylene oxide groups and the Al cannot be the driving force for PEG migration.34 From Figure 3 it is clearly seen that only PEGs with acidic and amine end groups migrate to the blend/Al interface. Furthermore, comparing the relative increase of the C−O peaks between PEG(Alcohol) and PEG(Acetate) with PEG content reveals that the extent of PEG migration toward the blend/Al interface increases with the acidity of the end groups. We suggest that the chemical interaction between the PEG’s terminating acidic moieties (such as hydroxy, amino, and carboxy) and the evaporated Al atoms is highly reactive and results in the formation of metal−organic complexes.35,36 This chemical reaction is the driving force for additive migration to the blend/Al interface. In contrast, the reactions between Al and PEGs with methyl ester or ether terminating groups are weak35,37 and do not induce PEG migration to the Al/organic interface. In conclusion, PEG migration to the organic/metal interface is induced by the interfacial reaction between the acidic or amine end groups of PEG and the thermally

evaporated Al atoms. This organometallic reaction reduces the interfacial energy and hence acts as the driving force for PEG migration to the blend/Al interface. To confirm that PEGs migration to the blend/metal interface is induced by the end group−metal interaction, we replaced the reactive Al (reduction potential −1.66 V)38 with inert Au (reduction potential 1.5 V).38 The P3HT:PEG blends with 20 wt % of each type of PEG were covered with a thin strip of Au. The Au layers were again thin enough (∼3 nm) to enable XPS characterization of the blend/Au interface. The evaporation time was extremely short, less than 30 s, and the samples’ temperature was kept below 25 °C. No further annealing process was performed. The C 1s XPS spectra of the bare blend surfaces and those under the Au stripes were compared (see Figure 4). The XPS spectra of all bare organic surfaces were identical to that of the bare surfaces measured in Figure 3. Importantly, the Au-covered areas also did not show evidence of PEG at the blend/Au interface regardless of the PEG’s end group and concentration. Namely, the identified migration of PEG(Acetate) and PEG(Alcohol) to Al during its deposition does not occur during Au deposition. The reason for this is the inertness of Au to all the types of PEG end groups used in this study. Under such conditions, there is no driving force for PEG migration to the blend/Au interface. The role of the end group/metal interaction as the driving force for additive migration is further demonstrated by selecting a PEG additive with an end group that has strong affinity to Au, PEG(Thiol). The strong attraction of the thiol end group to Au is well documented in the literature and hence could serve as a driving force for its migration to the blend/Au interface during Au deposition. The XPS spectra of the bare surface of P3HT:PEG(Thiol) blends (Figure 5a) are identical to bare surfaces measured of all the other PEGs in the Al experiments (Figure 3), showing no evidence of PEG at the bare organic surfaces. In contrast, the XPS spectra of the blend/Au interfaces (Figure 5b) show a noticeable increase of the C−O peak at 286.6 eV associated with the PEG, with an increase of PEG(Thiol) content in the blends. For blends with 15 and 20 wt % the intensity of this new peak is even higher than the main C−C/C−H peak at 285 eV, clearly demonstrating the high extent of PEG(Thiol) migration to the blend/Au interface. The absence of PEG migration toward the blend/Au interface in the case of PEGs with end groups that have no affinity to Au, and the high extent of migration in the case of the PEG with a thiol end group unambiguously confirm that the driving force for E

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Figure 5. High-energy resolution C 1s XPS spectra of P3HT blends with different content of PEG(Thiol), 0 wt % (black squares), 9 wt % (blue circles), 15 wt % (pink diamonds), and 20 wt % (red triangles) measured from bare (a) and Au covered (b) areas.

additive migration is the chemical interaction between the end group and the deposited metal atoms. On the basis of the XPS results, we were able to determine that the driving force for additive migration toward the blend/ metal interface is the additive/metal interaction. We now turn to answer the question: what is the origin of the metal work function modification. Namely, is the work function modified by the reaction of the end groups with the metal atoms, or perhaps by the presence of a dipole in the additive’s backbone, i.e., the ethylene oxide groups? To answer this question, we directly deposited PEG interlayers on preformed P3HT films. The bilayer devices were prepared by spin-coating PEGs thin layer on pristine-P3HT, topped by an evaporated Al cathode. Thus, we have ensured the presence of each type of the PEG, including those that do not spontaneously migrate to the interface (i.e., PEG(Methyl) and PEG(Ester)), at the active layer/Al interface. The dark and under illumination J−V curves of the bilayer devices with the general structure ITO/PEDOT:PSS/ P3HT:PEG/Al are shown in Figure 2 of the Supporting Information. As expected, the currents obtained from the devices are extremely low due to the absence of an electron acceptor moiety. The Voc values extracted from these J−V curves are summarized in Figure 6, which clearly shows that in contrast to the blend devices, where Voc depends on the type of the PEG end group, the Voc of all bilayer devices is similar (∼1.15 ± 0.1 V) regardless of the type of end group. This consistency of Voc indicates that all PEG additives used in this study are suitable cathode interlayer materials. This is mainly manifested by the fact that depositing PEG(Methyl ether) and PEG(Ester) on top of the active layer resulted in significant increase of Voc, while when blended in the active layer had no effect on the Voc. Thus, we conclude that in contrast to migration, which is controlled mainly by the end groups’ chemistry, the Al work function is almost insensitive to the PEG end groups’ chemistry. Since the PEG’s end groups affected the Voc of the bilayer devices only in a minor way, the main source of V oc modification should be the PEG backbone. Recent studies have shown that the presence of organic molecules with strong dipoles at the organic/metal interface can appreciably (∼1 eV) modify the electrode work function.39−42 Accordingly, we speculate that the main source of the cathode work function modification is the PEG’s backbone dipole of the ethylene

Figure 6. Voc values of bilayer P3HT/interlayer devices with Al cathodes. The interlayers are PEGs and Alkyls with different end groups. The inset shows a schematic illustration of the of bilayer device structure (green color represents the interlayer material (i.e., PEG or Alkyl)).

oxide groups. To test this speculation, we prepared bilayer devices with alkyl interlayers. The selected alkyls have the same end groups as the PEG additives that have significantly modified the cathode work function, i.e., alkyl(Acetate) and alkyl(Alcohol), as shown in Figure 6. The influence of these end groups on the Al work function is expected to be strongest when compared to other end groups (i.e., methyl ether and ester). The number of repeating units in alkyls chains was similar to the number of repeating units in the corresponding PEGs. The alkyl interlayers were spun directly on top of the P3HT active layer from an orthogonal solvent followed by the Al cathode evaporation. The effect of the ethylene oxide dipole on the work function of the cathode is analyzed by comparing the Voc of bilayer devices with PEG(Acetate), PEG(Alcohol), alkyl(Acetate), and alkyl(Alcohol) interlayers. As can be seen from Figure 6, in contrast to PEG(Acetate) and PEG(Alcohol), the presence of F

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the alkyl(Alcohol) or the alkyl(Acetate) interlayers does not cause any increase of the Voc. These results show that the presence of highly reactive end groups, without the backbone dipoles, does not affect the Al cathode work function. Hence, the decrease of Al work function shown for the devices with all types of PEG interlayers, and the absence of its modification by the same end groups terminated alkyl interlayers, unambiguously confirm that the main source of cathode work function modification is the built-in dipole of the backbone ethylene oxide groups. We suggest that the interaction between the dipoles of ethylene oxide groups and the metal atoms induces the decrease of the Al cathode work function and, in turn, increases the Voc of the devices.

I.D.: Department of Electrical Engineering and Computer Science, UC Berkeley. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. J. Frey for the PEG(Ester) synthesis. This research was partially supported by the Israeli Nanotechnology Focal Technology Area project on “Nanophotonics and Detection” and the Grand Technion Energy Program (GTEP) and comprises part of The Leona M. and Harry B. Helmsley Charitable Trust reports on Alternative Energy series of the Technion, Israel Institute of Technology, and the Weizmann Institute of Science.

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CONCLUSIONS To summarize, we used P3HT:PEG blends as a research platform to investigate the origin of the driving force for additive migration to the organic/metal interface and the source of the work function modification. To study the driving force for additive migration, we used PEG additives with different end groups, evaporated either Al or Au, and determined the composition of the organic/metal interface by XPS. The XPS measurements of blend/Al and blend/Au interfaces indicated that the driving force for additive migration toward the blend/metal interface is the chemical interaction between the additives’ end group and the deposited metal atoms. More specifically, the interaction between acidic or amine end groups and Al or between thiol end groups and Au atoms results in the formation of metal−organic compounds that substantially decrease the interface energy and is the driving force for PEG migration toward the blend/metal interface. In contrast, we have found that the type of end group only slightly modifies the Voc of the PEG-included devices. Therefore, to identify the origin of the Voc modification, we prepared bilayer devices by depositing interlayers of the selected PEGs and similar alkyls with the same end groups. Comparing the Voc values of the bilayer devices with PEG and alkyl interlayers showed that the main source of Al work function modification is the built-in dipole of the ethylene oxide backbone groups of PEG. We suggest that the Al work function is lowered due to the presence of these dipoles at the active layer/metal electrode interface. This detailed correlation between the additives’ structural features with both the migration and cathode modification provides a new methodology to spontaneously induce interlayer formation in organic electronic devices. Moreover, this study provides directive tools to engineer interlayer additives based on their chemical attraction to the undelaying substrates, evaporated metal, and inherent dipole.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and purification of poly(ethylene glycol) dicarboxymethyl ester (PEG(Ester)) and the J−V curves of the P3HT:PEG blends and bilayer devices. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00884.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b00884 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b00884 Langmuir XXXX, XXX, XXX−XXX