Conjugated Polymer Zwitterions: Efficient Interlayer Materials in

Oct 26, 2016 - His research is focused on organic electronics and functional materials in devices. Biography. Volodimyr V. Duzhko received his B.S. de...
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Conjugated Polymer Zwitterions: Efficient Interlayer Materials in Organic Electronics Yao Liu, Volodimyr V. Duzhko, Zachariah A. Page, Todd Emrick,* and Thomas P. Russell* Department of Polymer Science & Engineering, Conte Center for Polymer Research, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States CONSPECTUS: Conjugated polymer zwitterions (CPZs) are neutral, hydrophilic, polymer semiconductors. The pendent zwitterions, viewed as side chain dipoles, impart solubility in polar solvents for solution processing, and open opportunities as interfacial components of optoelectronic devices, for example, between metal electrodes and organic semiconductor active layers. Such interlayers are crucial for defining the performance of organic electronic devices, e.g., field-effect transistors (OFETs), light-emitting diodes (OLEDs), and photovoltaics (OPVs), all of which consist of multilayer structures. The interlayers reduce the Schottky barrier height and thus improve charge injection in OFETs and OLEDs. In OPVs, the interlayers serve to increase the built-in electric potential difference (Vbi) across the active layer, ensuring efficient extraction of photogenerated charge carriers. In general, polar and even charged electronically active polymers have gained recognition for their ability to modify metal/semiconductor interfaces to the benefit of organic electronics. While conjugated polyelectrolytes (CPEs) as interlayer materials are well-documented, open questions remain about the role of mobile counterions in CPE-containing devices. CPZs possess the processing advantages of CPEs, but as neutral molecules lack any potential complications associated with counterions. The electronic implications of CPZs on metal electrodes stem from the orientation of the zwitterion dipole moment in close proximity to the metal surface, and the resultant surface-induced polarization. This generates an interfacial dipole (Δ) at the CPZ/metal interface, altering the work function of the electrode, as confirmed by ultraviolet photoelectron spectroscopy (UPS), and improving device performance. An ideal cathode interlayer would reduce electrode work function, have orthogonal processability to the active layer, exhibit good film forming properties (i.e., wettability/uniformity), prevent exciton quenching, possess optimal electron affinity that neither limits the work function reduction nor impedes the charge extraction, transport electrons selectively, and exhibit long-term stability. Our recent discoveries show that CPZs achieve many of these attributes, and are poised for further expansion and development in the interfacial science of organic electronics. This Account reviews a recent collaboration that began with the synthesis of CPZs and a study of their structural and electronic properties on metals, then extended to their application as interlayer materials for OPVs. We discuss CPZ structure−property relationships based on several material platforms, ranging from homopolymers to copolymers, and from materials with intrinsic p-type conjugated backbones to those with intrinsic n-type conjugated backbones. We discuss key components of such interlayers, including (i) the origin of work function reduction of CPZ interlayers on metals; (ii) the role of the frontier molecular orbital energy levels and their trade-offs in optimizing electronic and device properties; and (iii) the role of polymer conductivity type and the magnitude of charge carrier mobility. Our motivation is to present our prior use and current understanding of CPZs as interlayer materials in organic electronics, and describe outstanding issues and future potential directions. tors,12 each of which consists of complex multilayered structures. Interlayers reduce the height of Schottky barriers and improve charge injection in OFETs and OLEDs. In OPVs, interlayers increase the built-in electric field across the active layer, ensuring efficient extraction of photogenerated charge carriers.13,14 Thus, interface engineering, or the development of designer interlayer materials for integration into devices, has emerged as an effective strategy to enhance device performance by improving electronic communication between each layer in the device. For example, the strength of interface engineering is

1. INTRODUCTION The promise of organic electronics is found in the combination of desirable properties: electronic functionality, lightweight, mechanical flexibility, and facile solution-processing over large areas.1−6 Crucial to achieving high efficiency in organic electronic devices is the ability to exchange charge between conductive electrodes and organic active layers.7 An energy level mismatch at such interfaces induces energy barriers that impede charge injection or extraction processes.8 Interlayers placed between organic active layers and electrodes are thus vital to the efficient operation of advanced organic electronics, such as field-effect transistors (OFETs),9 light-emitting diodes (OLEDs),10 photovoltaic devices (OPVs),11 and photodec© 2016 American Chemical Society

Received: August 2, 2016 Published: October 26, 2016 2478

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has been reported on conjugated aromatic polymers containing pendent zwitterions.26−28 We were intrigued at the prospect of identifying methodology for synthesizing zwitterion-containing aromatic monomers as well as conditions for their polymerization. Of the range of zwitterionic chemistries to select from, sulfobetaines (SBs) have proven most accessible for integration into CPZs.26−29 For numerous aromatic dihalide monomers, upon successful attachment of tertiary amines, incorporation of SB groups by amine-induced ring-opening of 1,3-propane sultone produces the corresponding zwitterionic monomers in excellent yield (>90%), requiring only precipitation and filtration for purification. In most syntheses developed thus far, SB-containing aromatic dibromides were prepared and used in step-growth polymerization with partner monomers such as aromatic diboronate esters. As would be anticipated, polymerization techniques, such as Grignard metathesis (GRIM) that affords regioregular poly(3-n-hexylthiophene) (P3HT),37 are problematic for CPZ syntheses due to incompatibility of the zwitterions with GRIM conditions. Fortunately, SuzukiMiyaura coupling proved well-suited for the zwitterionic monomers, as shown in Figure 1 for the preparation of

evident when placing a suitable interlayer between the photoactive layer and a high work function cathode in OPVs,15−17 allowing power conversion efficiencies (PCEs) to exceed 10% for single-junction devices,18,19 thus realizing the full potential of the selected photoactive layer. Conjugated polyelectrolytes (CPEs) have been investigated extensively by several groups as interlayer materials in organic electronics.11,14,20−22 The charged side chains impart solubility in polar solvents, sometimes even water, and enhance the potential for realizing processing in green solvents with lower environmental impact than typical conjugated polymers.17 Although CPEs are proving effective as interlayers in OPVs and OLEDs, the mobile counterions in CPEs have been described as a complicating factor that impacts device operation.23 In contrast, conjugated polymer zwitterions (CPZs) are neutral, hydrophilic, polymer semiconductors. The pendent zwitterions, while uncharged, are viewed as side chain dipoles. These zwitterionic side chains impart polymers with solubility in polar solvents, allowing them to be cast as thin films, and opening opportunities for their use as interlayer materials in optoelectronic devices.24,25 Recent progress in polymer synthesis and device engineering has enabled CPZs to be applied to numerous types of “plastic” electronic devices.26−29 As a group of emerging interlayer materials, CPZs possess distinct potential advantages including (1) excellent solubility in polar solvents; (2) significant perturbation of metal work function when applied as thin film coatings; and (3) simple structures that lack any potential complications associated with mobile counterions. Fang et al. and Duan et al. found that utilizing zwitterionic polyfluorene derivatives as electron injection layers in OLEDs improved response time of the devices due to the elimination of mobile counterions.26,27 Subsequent optimization of these polyfluorenes in postpolymerization modification chemistry led to their application as interlayers for improved electron collection in OPVs.28 Beginning with polythiophene-based zwitterions, we have advanced numerous types of CPZs as interlayers in OPVs.29,30 Although both polyfluorene and polythiophene zwitterion interlayers do enhance OPV efficiency, device performance in each case proved sensitive to interlayer thickness due to the intrinsic p-type properties of these polymer backbones. More recently, researchers found that preparing cathode interlayer materials with tailored conjugated backbones is effective in achieving excellent electron extraction and transport properties, which are crucial for easing the requirement on interlayer thickness to the benefit of device fabrication.31−34 Importantly, organic interlayer materials also show excellent ambient stability and enable the use of high work function and air stable metal cathodes.35 In this Account, we describe the synthesis of CPZs and present our understanding of their function as cathode interlayers in organic electronics with an emphasis on OPVs. We developed molecular design strategies to optimize CPZs in conjunction with their performance evaluation as OPV interlayers. The influence of both zwitterionic side chains and conjugated backbone type on interfacial dipole (Δ), molecular energy levels, charge transport properties, and the resultant device performance are reviewed.

Figure 1. Synthesis of thiophene-containing CPZs.

zwitterionic polythiophenes. Specifically, Pd-catalyzed coupling/polymerization under phase transfer conditions in toluene/water mixtures produced the desired CPZs in the 10−50 kDa molecular weight range as estimated by gel permeation chromatography (GPC).29,30 Essential to these polymerizations is the addition of salt (i.e., dilute NaBr) to the reaction mixture, which enables the production of high molecular weight material by ensuring solubility of the growing chains and preventing premature precipitation. Interestingly, in a departure from conventional solvents, ionic liquids (ILs) proved well-suited for CPZ preparation and allowed polymerizations to be conducted under ambient conditions without rigorous exclusion of air.38 For example, in alkyl-substituted imidazolium hexafluorophosphate solution, production of high molecular weight CPZs was achieved over a much shorter time frame (∼1/5 the reaction time) than obtained under phase transfer conditions. CPZ syntheses have since proven amenable to the preparation of low energy gap (Eg) structures, confirming

2. SYNTHESIS OF CPZs AND RELATED STRUCTURES While zwitterionic polymers composed of aliphatic backbones are exceptionally useful in applications involving aqueous materials and nonfouling surfaces,36 comparatively little work 2479

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Figure 2. Versatile synthetic approach to low Eg CPZs.

Figure 3. Zwitterion functionalization of C60 fullerene cage.

that a wide complement of polymers with their corresponding optoelectronic properties can be realized in the CPZ platform. For example, facile integration of tertiary amines into dibromide-substituted iso-indigo (iIn), diketopyrrolopyrrole (DPP), and naphthalene diimide (NDI) structures gave access to the corresponding SB-containing monomers; step-growth polymerization was then achieved successfully with a variety of thiophene, oligothiophene, and benzothiadiazole comonomers (Figure 2).39,40 Interestingly, for the iso-indigo derived SBCPZs, the presence of metal carbonates that promote transmetalation led to undesired backbone scission; fortunately, employing an aqueous solution of tetra-n-butylammonium fluoride (TBAF) to serve as both solvent and base alleviated this issue, while concurrently enhancing the solubility of the

entire mixture. GPC characterization revealed molecular weights ranging from 20 to 100 kDa for these narrow Eg CPZs, thus expanding the CPZ library for investigation as interlayers in devices. Most recently, we realized the synthesis of zwitterionic poly(phenylenevinylene)s (PPVs) in metal free aqueous polymerizations, using coupling strategies that afford polymers and copolymers having zwitterionic, anionic, and/or cationic side chain that open applications for PPVs as device interlayers.41 SB-CPZs are soluble in salt water, yet insoluble in almost all organic solvents. However, they (like many polymer zwitterions) exhibit excellent solubility (>20 mg/mL) in trifluoroethanol (TFE), which has since proven useful throughout all aspects of the CPZ platform, from synthesis to molecular 2480

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Figure 4. (A) Schematic depiction of SB zwitterions as pendent groups on a polymer backbone near a metal surface; (B) depiction of an image dipole where the original dipole po = qd (q is the elementary charge, d is a vector pointing from a negative charge, −q, to a positive charge, +q) located at a distance r from a metal surface; (C) net dipole alignment on a metal surface creates an interfacial dipole (Δ).

Figure 5. (A) Energy band diagrams for a semiconductor and metal; zwitterionic semiconductor/metal interface with small (B) and large (C) interfacial dipole (Δ).

weight characterization to device fabrication. In solution and the solid state, the proximity of the SB groups to the polymer backbone has an appreciable effect on CPZ optoelectronic properties. For example, when the ammonium cations of the SBs are situated close to the backbone, they exhibit an electron withdrawing effect, resulting in 50−100 nm hypsochromic shifts in absorption. Thus, despite their overall charge neutrality, the cations and anions of the zwitterionic moieties offer opportunities to realize some “electrolyte-like” features in CPZs toward tunable Eg values.29 Our work on CPZs led us to consider other electronically active structures that might be amenable to zwitterion functionalization, and ultimately the selection of zwitteriondecorated fulleropyrrolidines.42 As shown in Figure 3, Mitsunobu coupling of 2,3,4-trihydroxybenzaldehyde and 3dimethylaminopropan-1-ol gave the corresponding trisubstituted benzaldehyde that is setup for a Prato reaction on fullerene-C60 in the presence of N-methylglycine. Use of the resultant triamine for ring opening of 1,3-propane sultone produced “C60-SB”, containing three SB groups per fullerene, or any desired mixture of zwitterion-to-amine functionality depending on the sultone-to-amine stoichiometry employed. This innovation thus extends zwitterion chemistry to small molecular-based organic semiconductors.

surroundings. SB zwitterions, for example, have a calculated permanent dipole moment of 15.2 D.43 When zwitterions contact a metal surface (Figure 4A), the induced surface polarization can be viewed as an imaginary dipole mirrored across the plane (Figure 4B). When the positive end of the dipole is nearer to the backbone, its motion is restricted, and dipole orientation pointing away from the metal surface is energetically favorable due to a shorter dipole-to-surface distance. In the vicinity of the metal, such a fixed dipole rotates around its positive charge, where the self-induced torque directs the negative charge toward the metal surface. Such charge redistribution induced by dipole orientation will reduce the work function of the metal (Figure 4C).30 DeLongchamp and co-workers’ carbon K-edge near-edge Xray absorption fine structure (NEXAFS) measurements confirmed a net perpendicular orientation of SB side chains to gold (Au) and indium tin oxide (ITO) substrates.43,44 Richter’s vibrational resonant sum frequency generation (VRSFG) measurements acquired on the SO3− group in polymer or fullerene zwitterion films on Au and ITO substrates also indicated a net orientation of −C−SO3− throughout the entire film with the SO3− groups directed toward the Au and ITO surfaces.43,44

3. METAL WORK FUNCTION REDUCTION BY CPZs

The conjugated aromatic backbone of CPZs defines their electronic structures, i.e., highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. Consider the case in which a CPZ interlayer, with an inherent electron affinity (EA), comes into contact with a

3.2. Role of the Conjugated Backbone

3.1. The Role of Zwitterionic Side Chains

Zwitterions, while overall charge neutral, possess dipole moments that impart an electronic influence on their 2481

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Figure 6. (A) J−V curve of OPV devices and device parameters (PIN is the incident light power density); OPVs operating at (B) zero and (C) forward/positive voltage bias.

Figure 7. Energy levels of PTB7, PC71BM, homo- and copolymer zwitterions (Δ were measured on Ag).

4. A CASE STUDY OF CPZ INTERLAYERS IN OPVs

metal having a work function (ΦM) (Figure 5A). The two materials are shown with a common vacuum level, since all the energy levels are defined with respect to the latter. In contact, the net dipole alignment creates an interfacial dipole (Δ) that reduces the ΦM of the electrode. If Δ < ΦM − EA, the LUMO of a CPZ lies above the Fermi level (EF) of the metal (Figure 5B). With increasing Δ > ΦM − EA, the CPZ LUMO drops below the EF (Figure 5C). In this case, since the unoccupied electronic states of the semiconductor become lower in energy than the occupied states in the metal, electron transfer from the metal to the semiconductor will occur, equilibrating the two materials, and generating a common EF (Fermi-level pinning). More precisely, EF is pinned at the energy level of integer charge transfer states, which also take into account the surrounding polarization of polarons and interaction with an image charge.45,46 The back electron transfer from metal to interlayer thus partially offsets the work function (ΦM) reduction generated by the net alignment of molecular dipoles, which determines the maximum magnitude of ΦM reduction for any given metal electrode. Note that materials with insulating43 and p-type conjugated backbones can produce significantly larger Δ, i.e., decrease the metal work function to a larger degree, due to the smaller EA. This sustains efficient operation of such materials as ultrathin cathode interlayers.13,30

4.1. The Function of CPZ Interlayers in OPVs

The anode−cathode work function difference (δΦAC) dictates the magnitude of built-in electric potential difference (Vbi), which in turn affects the open-circuit voltage (Voc), short circuit-current density (Jsc) and fill factor (FF) of OPVs (Figure 6A). CPZ interlayers reduce the cathode work function and increase Vbi within devices, ensuring that Voc depends on the offset between the donor HOMO level and acceptor LUMO level,47 and thus is not limited by δΦAC. Under illumination and zero bias (Figure 6B), the maximum Vbi drives photogenerated charges (electrons and holes) toward their respective electrodes, leading to a maximum photocurrent (Jsc). As the voltage bias (+Vapplied) increases in the forward direction (Figure 6C), the external field partially compensates the Vbi, inducing a slower charge transport and extraction and thus reduced photocurrent. Larger Vbi leads to faster charge extraction under short-circuit conditions, which may reduce recombination loss and increase Jsc. The range of external biases, when efficient charge extraction takes place, extends toward the larger positive bias hence increasing FF. As an efficient interlayer between cathode and photoactive layer (containing donor and acceptor materials), a CPZ layer should enable electron extraction and block excitons, meaning that the CPZ LUMO should lie below the acceptor LUMO, and the CPZ HOMO should be situated below the donor and acceptor HOMOs. 2482

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Figure 8. OPVs (PTB7:PC71BM active layer) used to evaluate interlayer performance (A) and their performance containing different CPZs (B, C).

4.2. CPZs from Homopolymers to Copolymers

confirms that backbone-to-zwitterion separation distance effectively tunes the HOMO level. Conjugated polymers containing donor−acceptor (D-A) backbones are advantageous for their readily tunable HOMO and LUMO levels. Low Eg CPZ interlayers containing diketopyrrolopyrrole (DPP) or iso-indigo (iIn) backbones (Figure 2) were found to significantly improve OPV performance relative to control devices (Figure 8A, C).39 Distinct from the wide Eg CPZs, the low Eg CPZs shown in Figure 7 reduced metal work function with no apparent backbone dependence, providing an excellent model to investigate the influence of these interlayers at relatively constant Δ. The iIn-based CPZs have even deeper HOMO and LUMO levels than the HOMO level of PTB7 and LUMO level of PC71BM in the photoactive layer (Figure 7). Thus, the energy levels of these iIn-based CPZs are well-positioned for extracting electrons and blocking holes from the active layer, which afford devices with high FF and Jsc values. Specifically, the iIn/thiophene copolymer (PTiInSB) possesses similar Eg to that of its bithiophene counterpart (PT2iInSB), but the energy levels of PTiInSB are deeper than those of PT2iInSB. Thus, we found higher FF and Jsc in devices containing the PTiInSB interlayer. Replacing the electron rich thiophene unit in PTiInSB with electron deficient benzothiadiazole (PTBiInSB), we found a LUMO level increase. Yet, the LUMO of these polymers remains deeper than the bithiophene-containing polymer (PT2iInSB), which is critical for improving Jsc of the devices. Hence, studies of these D−A polymers indicate that a relatively deep LUMO level is crucial for unimpeded electron extraction. Similarly, in DPPbased CPZs, since PT3DPPSB and PT2BTDPPSB have approximately the same HOMO level and Eg, the deeper LUMO level of PT2BTDPPSB may contribute to the higher FF and Jsc of the devices. As anticipated, the thiophene-rich copolymer (PT4DPPSB) shows higher LUMO and HOMO levels, which suppress electron extracting and hole blocking capabilities, resulting in a relatively low FF and poor device performance.

Our interlayer studies have employed high efficiency, low Eg conjugated polymers as photoactive layers, such as the exceptionally efficient PTB7 (Figure 7, 8A) developed by Yu and co-workers,48 which has become a reliable platform to evaluate the performance of interlayer materials. Our solution processable CPZ interlayers afforded PTB7-based solar cells higher efficiencies, while at the same time using more stable Ag electrode to replace Ca/Al as the cathode. Polythiophene-based CPZs (Figures 1 and 7)29,30 improved the efficiency of OPVs containing a Ag cathode (Figure 8A, B) through the generation of a large Δ that reduced the work function of silver, allowing higher Vbi to be achieved in the devices. CPZ copolymers containing benzothiadiazole units, PTBTSB-1 and PTBTSB-2, produced even larger Δ values on silver, while polymers with longer side chains (PTSB-2, PTBTSB-2) resulted in the highest Δ values, possibly due to greater side chain flexibility that alleviates any restriction on their reorientation along the surface normal.30 Thus, we found larger Voc in OPVs using PTBTSB-2 as the interlayer (Figure 8A, B). Considering the energy level positions of polythiophene-based CPZs relative to those of PTB7 and PC71BM in the photoactive layer (Figure 7), we would expect the deeper LUMO and HOMO of PTBTSB-1 relative to their polythiophene counterparts to afford better electron extracting and hole blocking properties, as suggested by the higher Jsc and FF of PTBTSB-1-containing devices. With increasing Δ, PTBTSB-2 interlayers afforded devices with higher Jsc and FF, though PTBTSB-2 had higher HOMO and LUMO levels than its counterparts with shorter side chains, which indicated that a higher Vbi generated by a larger Δ is key to determining Jsc and FF. Along related lines, Duan et al. found that interlayer energy levels and device Vbi associated with zwitterionic polyfluorene interlayers were enabling for resultant solar cell performance.28 Interestingly, positioning SB groups closer to the polymer backbone (i.e., from PTSB-2 to PTSB-1 or from PTBTSB-2 to PTBTSB-1) deepens both the HOMO and LUMO levels, but the more significant HOMO decrease 2483

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Figure 9. Zwitterion-substituted interlayers: from p-to-n type structures.

Figure 10. (A) Device structure and active layer materials; (B) J−V curves of devices with different interlayers (note the S-shaped curve in the device with a PT3SB interlayer); (C) OPV performance metrics as a function of interlayer thickness.

4.3. From CPZs to Functional Fulleropyrrolidines: P-to-N Type Zwitterion Interlayers

the highest efficiency solar cells (PCE: 10.2%) (Figure 10A, B). The efficient charge transport of PT2NDISB, combined with its deep HOMO and LUMO levels, contributes to its superior performance. PT2BTDPPSB shows an electron mobility of 2 × 10−7 cm2/(V s), 1 order of magnitude lower than PT2NDISB, but as an interlayer still affords device performance comparable to PT2NDISB-containing devices. This suggests that a sufficiently large Δ, low LUMO level, and large electron mobility serve as key parameters for realizing excellent interlayer performance. Because of their deeper LUMO levels and higher electron mobilities, the performance of PT2NDISBand PT2BTDPPSB-containing devices are less sensitive to interlayer thickness variations: Voc and FF peak at ∼5−10 nm interlayer thickness, but maintain near maximum values even at >20 nm thickness (Figure 10C). The thicker interlayers also induce a redistribution of the optical field across the active layer, as characterized by reflectance spectroscopy.40 Impor-

Conjugated backbones influence electron transport properties of interlayer materials, as evidenced by space charge limited current (SCLC) measurements that suggest increasing electron mobility in going from PT3SB to PT2BTDPPSB to PT2NDISB to C60-SB and finally to C60-N (Figure 9).40 For CPZs containing p-type polythiophene backbones, such as PT3SB, low electron mobilities limit their effectiveness as interlayers since device performance is sensitive to interlayer thickness and declines rapidly with thickness. Figure 10 illustrates this limitation for devices containing a PT3SB interlayer, noting that the device with a thicker PT3SB interlayer (>7 nm) has a higher Voc, but markedly lower Jsc and FF. In contrast, PT2NDISB contains an n-type polymer backbone, and has the largest electron mobility among CPZs, while affording some of 2484

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Figure 11. (A, B) J−V curves of OPVs with C60-N and C60-SB interlayers (note the device structure and active layer materials are the same as those in Figure 10); (C) interlayer thickness dependence investigation of device performance.

Figure 12. (A) Cross-sectional scanning electron microscopy image shows the result of orthogonal solution processing between the photoactive layer and C60-SB; (B) inverted devices containing C60-SB and J−V curves.

tantly, the S-shaped J−V curves are not seen in devices containing thicker (>20 nm) PT2NDISB or PT2BTDPPSB interlayers, in contrast to the behavior of devices with a PT3SB interlayer. These results confirm the importance of electron transport and LUMO energy for cathode interlayers to suppress charge accumulation at the photoactive layer/electrode interface. Additionally, we note that device tolerance of interlayer thickness is crucial for developing novel interlayer materials suitable for facile processing and scale-up, circumventing the need for ultrafine level (nanometers) thickness control to achieve high device performance. The importance of electron extraction at the photoactive layer/electrode interface led us to combine the work function modification characteristics of polymer zwitterions with the

superior electron transport properties and deep LUMO levels of fullerene derivatives. Two fulleropyrrolidine derivatives, termed C60-N and C60-SB, were prepared and found to possess higher electron mobility than CPZs (Figure 9). This translated to excellent OPV performance over a wide range of interlayer thickness (from 5 to 55 nm) for both C60-N and C60-SB (Figure 11C).42 Notably, a thin layer of C60-N reduced the work function of Ag, Cu, and Au electrodes to 3.65 eV. This work function “pinning” led to OPV fabrication to afford PCEs exceeding 8.5% independent of cathode selection (Al, Ag, Cu or Au) with a maximum PCE of 9.8% (Figure 11A, B). Interestingly, C60-SB showed distinct processing advantages vs C60-N (Figure 12): common solvents for photoactive layer fabrication (chloroform, toluene, chlorobenzene and dichlor2485

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Accounts of Chemical Research obenzene) neither remove C60-SB films nor modify their electronic signature. Hence, OPVs with both inverted and regular architectures can be fabricated using C60-SB as the interlayer, where inverted devices outperformed the regular configuration (PCE: 9.2% vs 8.9%, Figures 11A and 12B). Furthermore, C60-SB interlayers function both as an electron acceptor and a cathode modification layer in OPVs (Figure 12B), contributing to the insensitivity of device performance to interlayer thickness. Such unique processing characteristics enabled slot-die coating of inverted OPVs with C60-SB interlayers, affording a readily achieved efficiency of 7.4%.49 In addition, an interconnection layer containing C60-SB proved efficient for joining two subcells in tandem devices to achieve a maximum PCE of 16%, while providing robust protection for the front subcell, and an ideal platform for solution deposition of the back subcell.50

Volodimyr V. Duzhko received his B.S. degree in Applied Physics and M.S. degree in Semiconductor Electronics from Kyiv Shevchenko University (Ukraine), and Ph.D. degree (Dr. rer. nat.) in Physics from the Technical University of Munich (Germany). He is now an Extension Assistant Professor in the Department of Polymer Science and Engineering at the University of Massachusetts, Amherst. His research interests include physics of organic semiconducting materials and devices for applications in energy generation and flexible electronics.

5. SUMMARY AND OUTLOOK CPZs have emerged as new electronic materials ideally suited as device interlayers, due to their facile synthesis, unique solubility and processing conditions, and interaction with metal substrates that promotes work function tailoring. Recently, CPZ syntheses have been adapted to a wide range of conjugated structures, from polythiophene to numerous low Eg structures, with the concept translating successfully to fullerene-based materials. It is clear that sulfobetaine zwitterions are integrated into CPZs easily, and generally impart “orthogonal” solubility that is advantageous for introducing these macromolecules into device fabrication/processing protocols. Despite this rapid progress, much more work lies ahead to fully appreciate the impact of CPZs and related structures on interface science and device engineering. Indeed, numerous opportunities are envisaged for CPZs and related structures, including (1) an examination of a wide range of zwitterion chemical compositions as pendent groups on CPZs, fullerenes, and fullerene-like acceptors; (2) understanding the effect of the pendent zwitterions on backbone packing, crystallinity, and solution assembly; and (3) exploiting the hydrophilic nature of CPZs to promote the use of green processing solvents in device fabrication. Whether CPZs could prove useful as the active layer of solar cells and other devices remains an open question, which we expect will be answered in part by the extent to which morphology (chain packing, molecular orientation, and crystallinity) can be controlled by backbone and pendent zwitterion selection. Finally, considering the biocompatibility and hydrophilicity generally offered by zwitterions, CPZs may prove effective in other developing areas including biosensors and electronically active surfactants.

Todd Emrick is a Professor of Polymer Science and Engineering at the University of Massachusetts Amherst. He received his Ph.D. in 1997 from the University of Chicago, having performed thesis research with Professor Philip Eaton on the topic of oligomeric structures composed of strained hydrocarbons such as cubane. He performed postdoctoral research in polymer chemistry with Jean Frechet at the University of California Berkeley and in 2001 moved to UMass Amherst to begin a position as Assistant Professor. At UMass, Emrick’s group pursues the synthesis of new monomers and polymers that present opportunities for applications as surfactants in solution, delivery systems, and electronic materials.

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Zachariah A. Page received his Ph.D. in Polymer Science and Engineering in 2015 from the University of Massachusetts, Amherst under the advisement of Professor Todd Emrick. He is now a postdoctoral researcher at the University of California Santa Barbara working with Professor Craig J. Hawker. His research interests include the design and synthesis of novel semiconducting materials for electronic applications.

Thomas P. Russell is the Silvio O. Conte Distinguished Professor of Polymer Science and Engineering at the University of Massachusetts Amherst. He received his Ph.D in 1979 from University of Massachusetts Amherst under the guidance of Professor Richard S. Stein investigating the behavior of polymer mixtures using x-ray, neutron and light scattering. He spent two year with Professor Erhard Fischer at the University of Mainz investigating the dynamics of glassy polymer by inelastic scattering methods. From 1981-1996 he was a Research Staff Member at the IBM Research Laboratories in San Jose, CA and then assumed a faculty position at the University of Massachusetts Amherst where his studies have focused on the behavior of thin polymer films, polymers and nanoparticles at surfaces and interfaces, phase transitions in polymers and polymer-based photovoltaics.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support for different aspects of the work described in this Account, including a Seed project in the UMass Amherst Materials Research Science and Engineering Center (MRSEC, NSF-DMR-0820506) and a prior Department of Energy supported Energy Frontier Research Center (DOE DE-SC0001087), Office of Naval Research (N00014-15-1-2244) for slot die coating and NSFCHE 1506839 for polymer synthesis.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

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Notes

The authors declare no competing financial interest.

REFERENCES

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Biographies Yao Liu obtained his Ph.D. from Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2013. He is now a postdoc working with Prof. Thomas P. Russell at University of Massachusetts, Amherst. His research is focused on organic electronics and functional materials in devices. 2486

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(24) Li, H.; Xu, Y.; Hoven, C. V.; Li, C.; Seo, J. H.; Bazan, G. C. Molecular Design, Device Function and Surface Potential of Zwitterionic Electron Injection Layers. J. Am. Chem. Soc. 2009, 131, 8903−8912. (25) Sun, K.; Zhao, B.; Kumar, A.; Zeng, K.; Ouyang, J. Highly Efficient, Inverted Polymer Solar Cells with Indium Tin Oxide Modified with Solution-Processed Zwitterions as the Transparent Cathode. ACS Appl. Mater. Interfaces 2012, 4, 2009−2017. (26) Duan, C.; Wang, L.; Zhang, K.; Guan, X.; Huang, F. Conjugated Zwitterionic Polyelectrolytes and Their Neutral Precursor as Electron Injection Layer for High-Performance Polymer Light-Emitting Diodes. Adv. Mater. 2011, 23, 1665−1669. (27) Fang, J.; Wallikewitz, B. H.; Gao, F.; Tu, G.; Müller, C.; Pace, G.; Friend, R. H.; Huck, W. T. S. Conjugated Zwitterionic Polyelectrolyte as the Charge Injection Layer for High-Performance Polymer Light-Emitting Diodes. J. Am. Chem. Soc. 2011, 133, 683− 685. (28) Duan, C.; Zhang, K.; Guan, X.; Zhong, C.; Xie, H.; Huang, F.; Chen, J.; Peng, J.; Cao, Y. Conjugated Zwitterionic PolyelectrolyteBased Interface Modification Materials for High Performance Polymer Optoelectronic Devices. Chem. Sci. 2013, 4, 1298−1307. (29) Page, Z. A.; Duzhko, V. V.; Emrick, T. Conjugated ThiopheneContaining Polymer Zwitterions: Direct Synthesis and Thin Film Electronic Properties. Macromolecules 2013, 46, 344−351. (30) Liu, F.; Page, Z. A.; Duzhko, V. V.; Russell, T. P.; Emrick, T. Conjugated Polymeric Zwitterions as Efficient Interlayers in Organic Solar Cells. Adv. Mater. 2013, 25, 6868−6873. (31) Zhang, Z.-G.; Li, H.; Qi, B.; Chi, D.; Jin, Z.; Qi, Z.; Hou, J.; Li, Y.; Wang, J. Amine Group Functionalized Fullerene Derivatives as Cathode Buffer Layers for High Performance Polymer Solar Cells. J. Mater. Chem. A 2013, 1, 9624−9629. (32) Zhang, Z.-G.; Qi, B.; Jin, Z.; Chi, D.; Qi, Z.; Li, Y.; Wang, J. Perylene Diimides: A Thickness-Insensitive Cathode Interlayer for High Performance Polymer Solar Cells. Energy Environ. Sci. 2014, 7, 1966−1973. (33) Liu, S.; Zhang, K.; Lu, J.; Zhang, J.; Yip, H.-L.; Huang, F.; Cao, Y. High-Efficiency Polymer Solar Cells via the Incorporation of an Amino-Functionalized Conjugated Metallopolymer as a Cathode Interlayer. J. Am. Chem. Soc. 2013, 135, 15326−15329. (34) Zhang, W.; Wu, Y.; Bao, Q.; Gao, F.; Fang, J. Morphological Control for Highly Efficient Inverted Polymer Solar Cells Via the Backbone Design of Cathode Interlayer Materials. Adv. Energy. Mater. 2014, 4, 1400359. (35) Cheng, P.; Zhan, X. Stability of Organic Solar Cells: Challenges and Strategies. Chem. Soc. Rev. 2016, 45, 2544−2582. (36) Ishihara, K.; Nomura, H.; Mihara, T.; Kurita, K.; Iwasaki, Y.; Nakabayashi, N. Why do Phospholipid Polymers Reduce Protein Adsorption? J. Biomed. Mater. Res. 1998, 39, 323−330. (37) Osaka, I.; McCullough, R. D. Advances in Molecular Design and Synthesis of Regioregular Polythiophenes. Acc. Chem. Res. 2008, 41, 1202−1214. (38) Page, Z. A.; Liu, F.; Russell, T. P.; Emrick, T. Rapid, Facile Synthesis of Conjugated pPolymer Zwitterions in Ionic Liquids. Chem. Sci. 2014, 5, 2368−2373. (39) Page, Z. A.; Liu, F.; Russell, T. P.; Emrick, T. Tuning the Energy Gap of Conjugated Polymer Zwitterions for Efficient Interlayers and Solar Cells. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 327−336. (40) Liu, Y.; Page, Z. A.; Russell, T. P.; Emrick, T. Finely Tuned Polymer Interlayers Enhance Solar Cell Efficiency. Angew. Chem., Int. Ed. 2015, 54, 11485−11489. (41) Page, Z. A.; Liu, Y.; Puodziukynaite, E.; Russell, T. P.; Emrick, T. Hydrophilic Conjugated Polymers Prepared by Aqueous Horner− Wadsworth−Emmons Coupling. Macromolecules 2016, 49, 2526− 2532. (42) Page, Z. A.; Liu, Y.; Duzhko, V. V.; Russell, T. P.; Emrick, T. Fulleropyrrolidine Interlayers: Tailoring Electrodes to Raise Organic Solar Cell Efficiency. Science 2014, 346, 441−444. (43) Lee, H.; Puodziukynaite, E.; Zhang, Y.; Stephenson, J. C.; Richter, L. J.; Fischer, D. A.; DeLongchamp, D. M.; Emrick, T.; 2487

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