Self-Assembled Conjugated Polyelectrolyte–Ionic Liquid Crystal

Feb 27, 2014 - In addition, incorporation of interlayer can alter charge collection and selectivity by modifying the Fermi level alignment to either t...
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Self-Assembled Conjugated Polyelectrolyte−Ionic Liquid Crystal Complex as an Interlayer for Polymer Solar Cells: Achieving Performance Enhancement via Rapid Liquid Crystal-Induced Dipole Orientation Lie Chen, Chen Xie, and Yiwang Chen* Institute of Polymers/Department of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China S Supporting Information *

ABSTRACT: A simple approach was demonstrated to manipulate dipole moment of interlayer in polymer solar cells (PSCs). The ionic liquid crystals (ILCs) 3-((2′-(4″-cyanobiphenyl-4yloxy)ethyl)dimethylammonio)propanesulfonate (CbpNSO) with zwitterionic charges were blended with cationic conjugated polyelectrolyte (CPE) poly[3-(6-trimethylammoniumhexyl)thiophene] (PTNBr) to afford a novel CPE−ILC complex. The water/alcohol solubility of the CPE−ILC complex enables it to be green solvent processable. The spontaneous orientation of liquid crystal (LC) favors more ordered structural arrangement in CPE−ILC complexes. More importantly, LC-assistant assembly improves the orientation of dipole at cathode and significantly reduces the work function of ITO. The power conversion efficiency (PCE) of P3HT:PC60BM-based inverted PSCs with the layer of PTNBr−CbpNSO is increased by 37% with respect to that of the device with pure PTNBr. Incorporation of PTNBr−CbpNSO into the devices based on PBDTTTC-T and PC71BM affords a notable PCE of 7.49%. It should be noted that mesogens reduce the activation energy of molecular reorganization and accelerate dipole orientation in CPE−ILC interlayer under external electric field, which enables the dipole of this interlayer can be readily manipulated. Because of the rapid orientation of the dipole, PTNBr−CbpNSO shows reversible dipole at the active layer/ITO interface during the reversible bias process.

1. INTRODUCTION Research on bulk-heterojunction polymer solar cells (PSCs) has achieved tremendous progress during the past decade, due to their advantages of scalable printing manufacturing and low materials costs.1 Power conversion efficiency (PCE) of PSCs has successfully reached to 10%, with sophisticated design of novel light-harvesting polymers, usage of efficient device structures, development of new fabrication techniques, and incorporation of interfacial materials for electrode modification.2 Interfacial materials play a crucial role in improving the performance of PSCs by favorable interfacial engineering, and an appropriate interfacial material could efficiently hamper the excitons recombination and lower the contact resistance at the light-harvesting layer/electrode interface. In addition, incorporation of interlayer can alter charge collection and selectivity by modifying the Fermi level alignment to either the EF,h of the donor or the EF,e of the acceptor for hole and electron transportation, respectively. Therefore, the altered polarity of electrodes by insertion of proper interlayer can result in two types of PSCs structures: conventional and inverted structures.2f The inverted device with reversed polarity of electrodes, consisting of bottom indium tin oxide (ITO) as cathode and top high work function (WF) metal as anode, is an advantageous approach for practical commercialization of PSCs due to its superior long-term stability.3 For the energy barrier caused by high work function © 2014 American Chemical Society

of bare cathode ITO hinders the charge transportation and collection in inverted device; an additional interlayer between light-harvesting active layer and ITO must be introduced to ensure proper energy level alignment and ohmic contact with lowest unoccupied molecular orbital (LUMO) level of acceptor by lowering the work function of ITO. The built-in field created by interlayer modified ITO can efficiently surmount the barrier at ITO/active layer interface and promote charge collection at electrode. A variety of electron-transporting layer (ETL) materials for modifying ITO in inverted device have been explored. Solutionprocessed inorganic metal oxides (such as CsCO3, TiOx, and ZnO,) are extensively used as ETL materials for PSCs.4 However, conversion of precursor to metal oxide requires high temperature annealing process and ambient conditions exposure, which is incompatible with printable devices based on flexible plastic substrates for commercial applications.5 To evade the inherent weaknesses of inorganic material as ETL, organic interlayer materials, such as fullerene derivatives and conjugated polyelectrolytes (CPEs), have been employed6 due to their potentials for large area PSCs with roll-to-roll manufacturing techniques. As an interlayer, conjugated Received: January 17, 2014 Revised: February 22, 2014 Published: February 27, 2014 1623

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Figure 1. (a) Chemical structure of cationic CPE PTNBr, zwitterionic LC CbpNSO. (b) Schematics of the electrostatic attraction between cationic CPE and ILC with zwitterions. (c) Schematics of the electrostatic self-assembly of the CPE−ILC complex.

complex, the ILCs with anionic charge (see Figure 1) were used to mix with the cationic PTNBr at a molar ratio of 1:1. In order to avoid generation of free salts during the assembly process,13 we introduced a zwitterionic ILC 3-((2′-(4″-cyano-biphenyl-4yloxy)ethyl)dimethylammonio)propanesulfonate (CbpNSO) to study properties of the CPE−ILC complex. The zwitterionic materials (polymers and small molecules) had been used for interlayer in PSCs.14 The synthetic route of the ILC is shown in Scheme S1 and the Experimental Section. The material purity, functionalization, and liquid crystal property were fully characterized (Figures S1−S4). The cationic group in CPE and anionic group in ILC not only endow the resulting materials with good solubility in environmental friendly solvent but also derive an electrostatic assembly between them by blending these two materials. For comparison, a zwitterionic surfactant (3-(N,N-dimethyloctylammonio)propanesulfonate inner salt (ZW) with the same ionic groups to the ILC was also blended with PTNBr to afford a type of CPE−surfactant (CPE−S) with the aim to study effect of the liquid crystal groups of ILC on the properties (Scheme S2). Both CPE−ILC and CPE−S complex maintain good solubility in alcohol and dimethyl sulfoxide (DMSO). Figure 2a demonstrates the UV−vis absorption spectra of CPE, CPE−ILC, and CPE−S complexes. All the complexes have similar absorption spectra in the isopropanol:water mixed solution, indicating that the electrostatic assembly between CPE and ILC or surfactant hardly affect the optical properties of the PTNBr in this free dispersed solution state. However, in the solid state, both CPE−ILC and CPE−S films display redshifted absorption bands with respect to the CPE film. Combination of surfactant into PTNBr results in a more ordered backbone of polymer; thus, the PTNBr−ZW complex film gives a ∼30 nm red-shift compared with the PTNBr film. When the counterpart with orientational LC group is introduced, PTNBr−CbpNSO presents a nearly 90 nm redshift compared with PTNBr, suggesting the well-assembled packing of CPE−ILC induced by the LC group. A similar optical behavior can also be observed in PL spectra in Figure

polyelectrolyte (CPE) affords remarkable performance improvement in many types of optoelectronic devices such as PLEDs, PSCs, and OFETs.6j,7 Efficient electron injection/ collection can be obtained by formation of strong interfacial dipole moments at cathode, which has been speculated as a result of a spontaneous orientation of the ionized groups.2i,8 However, the substantial microscopic behavior of ion orientation remains ambiguous at the moment. In this work, we report the manipulation of the interfacial dipole orientation of a cationic CPE poly[3-(6trimethylammoniumhexyl)thiophene] (PTNBr) in inverted PSCs by introduction of ionic liquid crystal (ILC)a material fusing the properties of water/alcohol miscibility and selforganization together.9 Mixing CPE with the ionic liquid (IL)like ILC forms a stable CPE−ILC complex via electrostatic attraction between these two materials with opposite charge.10 Meanwhile, LC-assistant electrostatic assembly can improve the orientation of dipole moments at cathode interface, which significantly reduces the work function of ITO. Compared with pure PTNBr CPE as ETL, CPE−ILC complex increases the efficiency of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61butyric acid methyl ester (PC60BM) based inverted PSCs by 37%. Replacing the active layer by low-bandgap donor poly(4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)-benzo[l,2-b:4,5b′]dithiophene-alt-alkylcarbonyl-thieno[3,4-b]thiophene) (PBDTTT-C-T)11 and acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM), a higher efficiency of 7.49% can be achieved. It is worthy to note that ILC group can produce the rapid orientation of electric field-induced dipoles, and the VOC can be precisely manipulated by applied external electric field. In addition, CPE−ILC is found to generate reversible dipoles at the active layer/ITO interface undergoing reversible bias process.

2. RESULTS AND DISCUSSION The amine-functionalized CPE material PTNBr has been proved to be an efficient interlayer for electron collection as reported in the literature.12 For formation of the CPE−ILC 1624

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Figure 2. Normalized (a) UV−vis absorption spectra and (b) PL spectra of different CPE−ILC (surfactant) complex in solution or film states, which is in the isopropanol:water solvent system.

2b. In order to remove the solvent effect, the complexes were dissolved in the low volatile solvent DMSO for optical measurement. From the Figure S5, PTNBr−CbpNSO in DMSO achieves the same results in alcohol, indicating the increased π-electronic delocalization in the better arranged complex is definitely from the orientation of LC groups. In addition, the absorption peak of the PTNBr−CbpNSO complex can achieve a further ∼10 nm red-shift after annealing the film from the mesophase states at 150 °C (Figure S6). This reveals that liquid crystalline state annealing strengths spontaneous orientation property of LC to favor a more ordered assembly of CPE−ILC. The self-assembled structure formed by the CPE−ILC complex can be directly seen in the atomic force microscopy (AFM) and transmission electron microscopy (TEM) images shown in Figure 3 and Figure S7. The complex was spuncoated on ITO substrate and then annealed in the mesophase region. The thicknesses for these films are approximately 20 nm. The pure PTNBr provide a wave like film (rms = 1.0 nm). As shown in Figure 3, the CbpNSO with zwitterionic side chain smoothes the morphology of PTNBr−CbpNSO with reduced rms of 0.5 nm. This phenomenon can be interpreted that PTNBr−CbpNSO with zwitterionic LC as a complexing agent can favor more ordered structures. Moreover, the electrostaticpairing effect between the two components can be evidenced in the phase images. The inner assembly morphology of the complex was also conducted by TEM. Different from homogeneous PTNBr, the PTNBr−CbpNSO complex forms elongated rodlike aggregates, verifying that cooperation assembly of CPE and ILC can generate rigid self-organized associations,15 which may form long-range-ordered multilayers15a,16 and promote the efficient charge transport.

Figure 3. AFM (a, b: topography; c, d: phase) images (1 × 1 μm) of (a, c) pure PTNBr and (b, d) PTNBr:CbpNSO complex in the methanol:isopropanol solvent system. TEM images of (e) pure PTNBr and (f) PTNBr:CbpNSO complex in the methanol:isopropanol solvent system; the images under (e) and (f) are corresponding different scale of images.

To investigate the effect of this self-assembly between CPE and ILC on the energy level at metal/organic material interface, the Kelvin probe method was employed to directly measure the WF of the CPE, CPE−S, and CPE−ILC modified ITO electrode.17 Figure 4 shows the WF images of the 10 nm thick CPE−ILC coated ITO after annealing from the liquid crystalline state for 10 min. The bare ITO exhibits a typical WF value of 4.93 ± 0.12 eV, in a good agreement with the 1625

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Figure 4. Work function (a) images from matrix and (b) error graph of the ITO cathode coated with pure CPE or CPE−ILC (surfactant) complex. (c) Energy level diagram of the device and electrical contacts of the work function of cathodes with the LUMO level of PCBM. (d) Schematic representations of the spontaneous dipole moment orientation induced by adding ILC. (e) SAXS spectra of the pure CPE or CPE−ILC (surfactant) complexes drop-casted on Kapton substrate. (f) J−V characteristics of the devices ITO/ETL/P3HT:PC60BM/PEDOT:PSS/Ag with various ETL materials, which is in the IPA:H2O solvent system.

literature.2f The interfacial dipole formation at ITO/CPE interface leads to a reduction of WF by 0.4 eV after deposition of PTNBr on ITO, which can lower the Schottky barrier that is found to hamper the efficient electrons collection from the photoharvest layer to ITO at the ITO/PC60BM interface.18 Replacing the pure PTNBr to the PTNBr−ZW complex with ZW surfactant, WF decreases to 4.40 eV. Although the electron transport to electrode is more favorable, the WFs of the PTNBr−ZW modified ITO are not low enough with respective to the lowest unoccupied molecular orbital (LUMO) of PC60BM (4.3 eV), and a Schottky contact still exists (Figure 4). Delightfully, the dipole moments of PTNBr−CbpNSO complexes lower the WFs to 4.02 ± 0.10 eV. A vacuum level shift of about 0.9 eV makes a substantial change in the work function of ITO, which sufficiently eliminates the barrier and forms an ohmic contact.

Figures 1 and 3 demonstrate the probable mechanism for the decreased WF. The permanent dipole caused by the orientation of CPE at the ITO/polymer interface is the major reason for the WF shift in metal electrode surface.17,19 The small-angle Xray scattering (SAXS) indicates the existence of ordered orientation in PTNBr film (see Figure 4). The intensity of the scattering peak at ∼6.5 nm is increased by introducing these counterparts into polymer. The scattering of the pure PTNBr reveals a relatively weak peak at q ∼ 0.10 Å−1, corresponding to a Bragg distance of about 6.0 nm. Upon addition of CbpNSO, the strong and sharp peaks with a d-space of ∼6.9 nm are observed, indicating the improved crystallinity of the film. Since the dipole is determined by the ionic chain orientation, the highly crystalline CPE−ILC film with ordered polar chains is preferable to form an aligned dipole moment. However, the pure PTNBr shows a relatively disordered dipole direction. As 1626

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Table 1. Summary of the Photovoltaic Performance of Inverted P3HT:PC60BM Solar Cells with Various ETLsa layer sequence

VOC [V]

JSCb [mA/cm2]

FF [%]

PCEc [%]

Rs [Ω·cm2]

Rsh [Ω·cm2]

ITO/P3HT:PC60BM/... ITO/PTNBr/P3HT:PC60BM/... ITO/PTNBr-ZW/P3HT:PC60BM/... ITO/PTNBr-CbpNSO/P3HT:PC60BM/... ITO/ZnO/P3HT:PC60BM/...

0.34 0.53 0.55 0.57 0.58

9.64 9.95 10.6 11.7 10.1

31.6 54.8 51.4 59.8 56.4

1.04 2.91 3.02 4.00 3.29

51.8 4.64 3.11 2.46 2.99

87 384 203 1007 566

a

Device configuration: glass/ITO/ETL/P3HT:PCBM (1:1 w/w, 180−200 nm)/PEDOT:PSS (30 nm)/Ag (90 nm). bUncorrected data. cAll the PCE were averaged over 10 cells.

Table 2. Summary of the Photovoltaic Performance of Inverted PBDTTT-C-T:PC71BM Solar Cells with Various ETLsa layer sequence ITO/ZnO/PBDTTT-C-T:PC70BM/... ITO/PTNBr/PBDTTT-C-T:PC70BM/... ITO/PTNBr-CbpNSO/PBDTTT-C-T:PC70BM/...

VOC [V] 0.77 0.75 0.77

JSCb [mA/cm2] d

14.3 (13.9) 15.1 (14.6)d 16.4 (16.2)d

FF [%]

PCEc [%]

Rs [Ω·cm2]

Rsh [Ω·cm2]

60.0 55.6 58.7

6.61 6.28 7.41 (7.49)e

1.90 3.88 1.59

320 313 801

a

Device configuration: glass/ITO/ETL/PBDTTT-C-T:PC70BM (1:1.5 w/w, 80−100 nm)/MoO3 (10 nm)/Ag (60 nm). bUncorrected data. cAll the PCE were averaged over 10 cells. dThe JSC value calculated from EQE data (see Supporting Information). eThe best PCE value over all the tested cells.

promote the highly orientated dipole moment along with the induced built-in electric field, consequently enhancing the device performance. The similar results can be found in the devices with complex layer spin-coated from their DMSO solution (see Figure S12 and Table S3). The improvement in these parameters can be mainly attributed to the enhanced orientation of dipole moment pointing out from ITO penetrating into the BHJ layer through the swift down of the vacuum level of the ITO cathode.22 To further confirm the orientation dipole moment caused by the LC favored electrostatic assembly and testify the universality of CPE−ILC on common PSCs, inverted PSCs based on the lowbandgap polymer PBDTTT-C-T with these ETLs were fabricated by the ITO/ETL/PBDTTT-C-T:PC70BM/PEDOT:PSS/Ag structure. As expected, the average efficiency of the devices can be improved greatly from 6.28% for pure PTNBr to 7.41% for PTNBr−CbpNSO with enhanced JSC and slightly improved VOC (Table 2 and Figure S13). Incorporation of the ILC into CPE layer is found to reduce the magnitude of the saturated dark current by 1 order, as shown in Figure S13. In comparison with the device based on ZnO ETL showing JSC of 14.3 mA/cm2, the PTNBr−CbpNSO based PSC achieves the JSC of 16.4 mA/cm2 with an increase of 15%. The external quantum efficiency (EQE) values of the devices with PTNBr− CbpNSO complex are much higher than those standard devices without ILC, which is in agreement with the high value of JSC (there are only less than 4% error between the calculated JSC value from the EQE and the JSC from J−V curves; see Figure S13 and Table 2). The response enhancement of the EQE value at 300−400 nm when incorporation of CPE−ILC into this device is in accordance with the absorbance spectra of device (see Figure S14). With the help of CPE−ILC, the PBDTTT-C-T:PC70BM-based inverted PSCs affords a top PCE of 7.49%, which approaches the reported performance of the conventional PSC with the low WF Ca/Al cathode.11 The built-in electric field in the BHJ layer induced by the CPE−ILC complex is expected to facilitate the exciton dissociation and increase the photogenerated-carrier collection.22a In order to evaluate the apparent charge carrier mobility in the active layer, J 0.5−V characteristics of single charge carrier devices were measured using the space charge limited current (SCLC) model according to the Mott−Gurney equation (detail

shown in Figure 4, the alignment of ionized chains in the complex could form a dipole pointing away from ITO, ultimately resulting in the shifting of WF. The relationship between the ordering of CPE and magnitude of dipole is similar to the property of ferroelectric interlayer.20 The WF reduction caused by enhanced orientation of dipole moment also increases the built-in field to break the electrical symmetry inside of the PSCs, consequently anticipating a VOC and JSC improvement.21 The inverted PSCs based on these complexes were fabricated with the configuration of ITO/ETL/P3HT:PCBM/PEDOT:PSS/Ag shown in Scheme S2. The CPE−ILC (surfactant) complex was deposited on ITO as a cathode interface modification layer. The transmittances of the ITO/ CPE−ILC (surfactant) cathodes are almost identical to that of the bare ITO cathode, inferring that this ETL would not hinder the light-harvesting in the active layer (Figure S8). Figure 4 presents the current density versus voltage (J−V) characteristics of the inverted PSCs based on P3HT:PC60BM. Both complexes were optimized at the ultrathin thickness of 5−10 nm, enabling “quantum tunneling” for charge transport from the active layer to the ITO cathode (Figure S9 and Table S1).17 The device performance parameters are summarized in Table 1. As shown in Figure 4 and Figure S10, in comparison with the standard PSCs with pure PTNBr ETL exhibiting a relatively low PCE of 2.91%, the performance of the device using the PTNBr−CbpNSO ETL is dramatically improved with the average PCE approaching to 4.0%, giving 285% and 22% improvement over the two reference device with bare ITO and ITO/ZnO cathodes, respectively. The improved PCE of devices with CPE−ILC is the result of enhanced JSC and slightly improved VOC relative to the devices with pure PTNBr, suggesting the more favorable charge injection and collection are achieved at electrodes. In contrast, the devices based on PTNBr−ZW give the comparable performance just with slightly enhanced JSC to the standard PSC with pure PTNBr ETL. As a comparison, pure ILC (surfactant) was also used to modify the ITO cathode. As shown in Figure S11 and Table S2, the highest PCE among these devices is below 3%, which is blamed on the low fill factor values (less than 50%) caused by the poor quality of these films. It can be concluded that only the LC favored electrostatic assembly with the PTNBr can 1627

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the dipole orientation, we applied positive and negative electric field to manipulate the dipole moment of the CPE−ILC interlayer in PSCs. Figure 6 shows the J−V characteristics of

of the mobility measurement is described in the Supporting Information).23 As plotted in Figure 5, with respect to the

Figure 5. J 0.5−V characteristics of (a) electron-only and (b) hole-only devices with different cathode interfacial layers.

device without ETLs, device with PTNBr ensures the apparent electron mobility and hole mobility increase from 1.33 × 10−3 to 2.13 × 10−3 cm2 V−1 s−1 and from 1.46 × 10−4 to 3.13 × 10−4 cm2 V−1 s−1 at an applied voltage of 1 V, respectively. After insertion of PTNBr−CbpNSO, further improvements in the electron mobility (4.10 × 10−3 cm2 V−1 s−1) and hole mobility (5.07 × 10−4 cm2 V−1 s−1) are detected. The results reveal that the cooperation assembly between ILC and CPE can improve both electron and hole mobility of device with a more balanced charge transport.24 A relative thick interlayer is helpful for the charge mobility and the maximum with thickness reaches to ∼20 nm, owing to the produced stronger built-in field in the BHJ layer (Figure S15). However, too thick interlayer always increases the contact resistance to reduce device efficiency.25 Thus, the device with appropriate CPE−ILC interlayer thickness in the range of 5−10 nm would balance the charge transport and resistance in the PSCs. The orientation of ILC that induces formation of a favorable dipole will be very sensitive to the internal electric field afforded by the inherent WF difference between two electrodes22a due to the intrinsic response of the liquid crystals to the stimulations. To investigate the effect of the electric field on

Figure 6. J−V characteristics of the devices ITO/ETL/ P3HT:PC60BM/PEDOT:PSS/Ag with PTNBr−CbpNSO as ETL after applying a (a) negative or (b) positive bias voltages. (c) Dark J−V characteristics of devices after applying different bias voltage.

the devices with the PTNBr−CbpNSO interlayer applied a bias voltage at 70 °C for 1 h. The thermal effect on the device performance can be excluded because it is found that only a slight loss of VOC (less than 4 mV) and JSC (less than 0.2 mA) is observed for this device after heated at 70 °C for 6 h without applying bias (Figure S16). First, a negative bias is applied from −5 to −15 V. As the negative bias increases, the VOC of device improves from 0.570 to 0.626 V along with decrease of JSC from 11.7 to 10.3 mA/cm2 (Figure 6a, defined as process 1). Further elevating the bias voltage to −20 V leads to the breakdown of the devices. On the contrary, applying positive bias voltage on the device provides an opposite result, that is, a gradually decreased VOC with the minimum value of 0.471 V upon +15 V bias, as displayed in Figure 6b (defined as process 2). Similarly, 1628

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the JSC still shows reduced value in process 2. The current produced by the bias voltages may pose deterioration on the arrangement and phase separation in BHJ layer; therefore, the JSC decreases with the bias voltages increasing no matter the direction of bias. This phenomenon is accordance with the reported fluoroalkylated organic semiconductors in PSCs.8a The voltage change in photovoltaic devices can also be reflected by the saturated dark current of the device.26 As plotted in Figure 6c, the negative bias voltage reduces the reverse saturated dark current and increases the turn-on voltage for the diode, ascribed to the enhanced reverse carrier-injection barrier that is beneficial to prevent the charge recombination, whereas applying positive bias voltage draws diametrically opposite conclusion. The correlation of the saturated dark current and the VOC in the device can be expressed by the Shockley equation27

Figure 7. Schematic diagram of the orientation of dipole moment manipulated by applying opposite electric field.

For comparison, three control devices based on ETL of ZnO, PTNBr, and PTNBr−ZW with the same applied bias voltage were also investigated. Figure 8 summarizes the VOC and JSC

⎡ ⎧ ⎛ V − JR ⎞ ⎫ 1 V ⎤ ⎥ − Jph ⎢J0 ⎨exp⎜ ⎟ − 1⎬ + J= 1 + R s/R sh ⎣ ⎩ ⎝ nkT /e ⎠ R sh ⎦ ⎭ (1)

where J is the current density, V is the applied bias, Jph is the photocurrent density due to exciton dissociation, Rsh is the shunt resistance, Rs is the series resistance, n is the ideality factor, k is the Boltzmann constant, T is the absolute temperature, e is the elementary charge, and J0 is the reverse saturation current density. As Rsh is much larger than Rs, the equation can be simplified as V=

nkT ⎛ J + Jph + J0 − V /R sh ⎞ ⎟⎟ + JR s ln⎜⎜ e J0 ⎝ ⎠

(2)

Under open-circuit voltage conditions, V = VOC and J = 0, eq 2 becomes VOC

⎡ ⎞⎤ Jph ⎛ VOC ⎟⎥ nkT ⎢ ⎜ = ln 1 + 1− ⎢ e J0 ⎜⎝ Jph R sh ⎟⎠⎥⎦ ⎣ ≈

Jph ⎞ nkT ⎛ ⎟⎟ ln⎜⎜1 + e J0 ⎠ ⎝

(3)

From eq 3, we can observe that decrease of reverse dark saturation current density J0 is beneficial for enhancing the VOC of the devices. Therefore, the difference in J0 of the devices with two opposite bias voltage leads to the increase or decrease of VOC. The diametrically opposite behavior of VOC caused by two opposite bias voltages should be related to the changes in the orientation of dipole moments manipulated by electric field which can be elucidated by Figure 7. For the inverted device (Figure 7, middle), a built-in electric field pointing from ITO to Ag activates a favorable dipole pointing away from ITO in ETL. When applying negative voltage pulse (Figure 7, left) with the same direction to built-in electric field, a dipole moment will be strengthen with more aligned favorable orientation for better energy alignment. On the contrary, the positively poled electric field led to some unfavorable dipole moment pointing toward ITO with reversed direction (Figure 7, right), which would weaken the built-in electric field and subsequently result in the loss in VOC. These results reveal that the spontaneously formed dipole at cathode in devices with CPE-ILC ETL can be promoted or eliminated by the applied electric field.

Figure 8. Summary of the (a) VOC and (b) JSC changes in the ITO/ ETL/P3HT:PCBM/PEDOT:PSS/Ag devices with different interlayer after applying positive and negative bias voltage.

changes in all devices, and Figures S17−S19 exhibit the J−V characteristics of those control devices after applying different bias voltages. Similar with the case of PTNBr−CbpNSO, the applied bias voltages decline the JSC of all the devices. The VOC of the device with the ITO/ZnO cathode remains almost unchanged after applying negative or positive bias voltages (Figure S17), but VOC of the devices with ITO/PTNBr and ITO/PTNBr-ZW both vary with bias voltage (Figures S18 and S19). However, the changes of VOC in these control devices were not as sharp as those in the devices with the ITO/ PTNBr−CbpNSO cathode. As revealed in Figure 8, the VOC 1629

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Figure 9. J−V characteristics of the devices ITO/ETL/P3HT:PC60BM/PEDOT:PSS/Ag with PTNBr−-CbpNSO as ETL after (a) process 3 and (b) process 4. Trajectories of the VOC change in the devices after (c) process 1 and process 3 or (d) process 2 and process 4 in sequence.

difference between the devices after applying −15 and +15 V bias is 154 mV for the PTNBr−CbpNSO interlayer, which is much larger than those of 78 and 51 mV in the devices with PTNBr and PTNBr−ZW, respectively. It can be inferred that the significant VOC variation of the PTNBr−CbpNSO based device induced by bias voltage definitely comes from the CbpNSO group. For the device with pure PTNBr and PTNBr−ZW interlayer, the orientation of the dipoles can only be slightly changed by the constant bias voltage, limited to the slow movement of the ionized group. Liquid crystals possess rapid response to external stimulation, such as light, electric, and magnetic fields. When an external electric field is applied to the liquid crystal, the liquid crystal molecules tend to orient themselves along the direction of the field. Thus, the CbpNSO mesogenic group can reduce orientation energy barrier and activate the dipoles of PTNBr−CbpNSO, which produces rapid rearrangement of dipoles align with the electric field. Such amplified electric-field-induced dipole orientation also offer an opportunity to precisely manipulate the energy alignment at the electrode interface. Intriguingly, due to the rapid orientation of electric-fieldinduced dipole, PTNBr−CbpNSO can generate reversible dipole at the active layer/ITO interface undergoing reversible bias process. From Figure 9 we can notice that, when an applied negative bias from −5 to −15 V (process 1) followed by performing the opposite bias from +5 to +15 V on devices (process 3), VOC of the devices increases to 0.626 V at first and then goes back to 0.519 V. The reversibility of VOC can also be clearly depicted in Figure 9. The similar behavior can be observed when the applied bias with the reversed sequence and a circle of VOC is nearly formed by applying process 2 followed by process 4 (Figure 9). The reversible VOC implies the

reversible dipole moments at the active layer/ITO interface stimulated by the applied bias with different directions. Furthermore, upon the same positive bias, processes 2 and 3 show the comparable decrease of 100−110 mV in the VOC, while influences of processes 1 and 4 applied with negative bias on the VOC are quit different. Compared with 60 mV increase of VOC in process 1, a sharp 117 mV enhancement is yielded in process 4. This difference of VOC between processes 1 and 4 with the same applied negative bias is ascribed to the applying sequence of negative and positive bias on the whole circle. For the case of P3HT:PC60BM as BHJ layer, the obtained VOC value of 0.63 V after applying bias voltage of −15 V in process 1 has already closed to the anticipated VOC calculated by [(| EHOMO‑donor| − |ELUMO‑acceptor|)/e − 0.3],28 which limits the further VOC enhancement in this device.

3. CONCLUSIONS In conclusion, we have incorporated a zwitterionic LC molecule into the CPE to form an electrostatic self-assembled CPE−ILC complex as an ETL in inverted PSCs. This approach is much easier than the tuning of molecule structure through complicated synthesis. The UV and SAXS results manifest that the dual properties of IL-like electrostatic attraction with CPE and LC-like ordered arrangement dramatically improve the ordering and orientation of polymer. As a comparison, a CPE−S complex without mesogenic group exerts relatively inferior capability of self-assembly. The self-assembly achieved an alignment of the polarized side chains of CPE, enabling an orientation of dipole moment. Significant enhancement of PCE can be obtained by the ILC-induced spontaneous orientation of the ionized chain of CPE, along with 0.9 eV WF reduction of ITO. The PBDTTT-C-T:PC70BM-based PSCs with CPE−ILC 1630

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calculated using an area of 4 mm2 (compare to 7.49% with the determined area of 4.168 mm2). The picture of the cell is shown in Figure S20. Characterization of PSCs. The light source was calibrated by using silicon reference cells with an AM 1.5 Global solar simulator with an intensity of 100 mW/cm2. The current−voltage (J−V) characteristics were recorded using a Keithley 2400 source meter (Abet Solar Simulator Sun2000). All the measurements were performed under ambient atmosphere at room temperature. The EQE was measured under monochromatic illumination (Oriel Cornerstone 260 1/4 m monochromator equipped with Oriel 70613NS QTH lamp), and the calibration of the incident light was performed with a monocrystalline silicon diode. Application of Electric Field to PSCs Device. The polarity bias was applied to the PSCs as follows: the devices were placed on a hot plate at 70 °C for about 10 min to preheat them, and then a constant positive or negative bias voltage was applied on the two electrodes of the devices for 1 h. This procedure was repeated by applying different bias voltages on an identical sample.

interlayer as ETL achieved a best PCE of 7.49%. In addition, the dipole orientation in devices varied with the applied electric field, which is reflected in the change of VOC and diode behaviors. The electric-field-induced LC group can reduce the activation energy of molecular reorganization and accelerate dipole orientation in CPE interlayer. Therefore, a reversible dipole at the active layer/ITO interface can be produced during the reversible bias process. This methodology could provide a practical approach for investigating the relevance between PSCs performance and interfacial energy levels and thus obtaining some information about the behaviors of interlayer at PSCs interface.

4. EXPERIMENTAL SECTION Synthesis of Poly[3-(6-trimethylammoniumhexyl)thiophene] (PTN-Br). The conjugated polyelectrolyte PTNBr was synthesized by the previously reported procedures.12b The neutral precursor, regioregular poly(3-(6-bromohexyl)thiophene) was synthesized by a Grignard metathesis approach. Then, the ionic PTNBr was obtained by treating poly(3-(6-bromohexyl)thiophene) with trimethylamine in THF at reflux temperature for 48 h, followed by extraction with water. The water was then removed under reduced pressure. The polymer was dried in vacuum overnight and was obtained as a dark red solid. 1H NMR (δ, DMSO-d6): 7.25 (s, 1H), 3.11 (s, 9H), 2.81 (br, 2H), 1.70 (br, 4H), 1.44−1.34 (m, 4H). Synthesis of Ionic Liquid Crystals 3-((2′-(4″-Cyanobiphenyl4-yloxy)ethyl)dimethylammonio)propanesulfonate (CbpNSO). To a suspension of NaH (52%, 240 mg, 5.0 mmol, 1.0 equiv) in dry DMF (20 mL) was added 4-hydroxy-4-cyanobiphenyl (975 mg, 5.0 mmol, 1.0 equiv). The mixture was stirred at room temperature for 30 min, and then 2-(dimethylamino)ethyl chloride hydrochloride (535 mg, 5.0 mmol, 1.0 equiv) was added in the same manner. The reaction mixture was heated stirred at 100 °C for 3 h. Removal of the solvent on a rotary evaporator was followed by addition of water (10 mL) to the residue and extraction with ether. The extract was concentrated and then recrystallized from ether/MeOH. The residue was dried under reduced pressure to obtain CbpN (638 mg, 2.4 mmol, 48%). Then, CbpN (266 mg, 1.0 mmol, 1.0 equiv) was dissolved in CH2Cl2, to which 1,3-propanesultone (112 mg, 1.0 mmol, 1.0 equiv) was added. The reaction mixture was heated at reflux for 12 h. The precipitation was collected by filtration and recrystallized from acetone/H2O to give CbpNSO (310 mg, 0.8 mmol, 80%) as a white powder. 1H NMR (δ, DMSO-d6): 7.87 (m, 4H), 7.76 (d, 2H), 7.16 (d, 2H), 4.52 (t, 2H), 3.81 (t, 2H), 3.53 (m, 2H), 3.15 (s, 6H), 2.48 (t, 2H), 2.07 (m, 2H). 13C NMR (δ, DMSO-d6): 158.48, 144.54, 133.25, 131.68, 128.80, 127.41, 119.42, 115.85, 109.74, 63.63, 61.95, 51.52, 48.16, 19.55. Element Anal. Calcd for C20H24N2O4S: C, 61.9; H, 6.19; N, 7.22; S, 8.25. Found: C, 61.1; H, 7.03; N, 6.78; S, 7.82. Fabrication of PSCs. In the inverted device fabrication process, ITO-coated glass substrates were first cleaned by ultrasonic agitation in acetone, detergent, deionized water, and isopropanol sequentially, followed by UV treatment for 20 min. The CPE:ILC complex was dissolved in IPA:water (1:1 (vol %)) with a concentration of 0.1 wt %. The solution was spun-cast on the ITO at a speed of 5000 rpm for 1 min. The approximate thickness of the film is 10 nm, determined by AFM. P3HT (20 mg mL−1) and PC60BM (20 mg mL−1) were dissolved in 1 mL of o-dichlorobenzene. Then the P3HT:PC60BM blend was spin-coated on top of the CPE:ILC layer and thermally annealed at 150 °C for 10 min. For PBDTTT-C-T-based device, the PBDTTT-C-T (10 mg mL−1) and PC70BM (15 mg mL−1) were dissolved in 1 mL of o-dichlorobenzene with 3% diiodooctane (DIO), and the blend was coated onto the complex layer at a speed of 900 rpm for 2 min to form a BHJ layer. Subsequently, a dilute PEDOT:PSS solution mixed with IPA at a 1:5 volume ratio was spun-cast onto the BHJ layer at 2000 rpm and then air-dried at 80 °C. Finally, 90 nm Ag were deposited by thermal evaporation through a shadow mask to form an active area of ∼4 mm2. Note of caution: the best PBDTTT-C-T:PC70BM cell gave a PCE of 7.80%, which was

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; Schemes S1 and S2, Figures S1−S20, and Tables S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.C.). Author Contributions

C.X. and L.C. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51273088 and 51263016).

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REFERENCES

(1) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789−1791. (b) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323−1338. (c) Li, G.; Zhu, R.; Yang, Y. Nat. Photonics 2012, 6, 153−161. (2) (a) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C. C.; Gao, J.; Li, G.; Yang, Y. Nat. Commun. 2013, 4, 1446. (b) You, J.; Chen, C. C.; Hong, Z.; Yoshimura, K.; Ohya, K.; Xu, R.; Ye, S.; Gao, J.; Li, G.; Yang, Y. Adv. Mater. 2013, 25, 3973−3978. (c) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649−653. (d) Beaujuge, P. M.; Frechet, J. M. J. Am. Chem. Soc. 2011, 133, 20009−20029. (e) Li, Y. Acc. Chem. Res. 2012, 45, 723−733. (f) Yip, H.-L.; Jen, A. K. Y. Energy Environ. Sci. 2012, 5, 5994−6011. (g) Ameri, T.; Li, N.; Brabec, C. J. Energy Environ. Sci. 2013, 6, 2390−2413. (h) Po, R.; Carbonera, C.; Bernardi, A.; Camaioni, N. Energy Environ. Sci. 2011, 4, 285−310. (i) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Nat. Photonics 2012, 6, 591−595. (3) (a) Chen, L.-M.; Hong, Z.; Li, G.; Yang, Y. Adv. Mater. 2009, 21, 1434−1449. (b) Jorgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.; Krebs, F. C. Adv. Mater. 2012, 24, 580− 612. (4) (a) Li, G.; Chu, C. W.; Shrotriya, V.; Huang, J.; Yang, Y. Appl. Phys. Lett. 2006, 88, 253503. (b) Kim, J. Y.; Kim, S. H.; Lee, H. H.; Lee, K.; Ma, W.; Gong, X.; Heeger, A. J. Adv. Mater. 2006, 18, 572− 576. (c) Faber, H.; Burkhardt, M.; Jedaa, A.; Kälblein, D.; Klauk, H.; Halik, M. Adv. Mater. 2009, 21, 3099−3104. (d) Sun, Y.; Seo, J. H.; 1631

dx.doi.org/10.1021/ma5001282 | Macromolecules 2014, 47, 1623−1632

Macromolecules

Article

Takacs, C. J.; Seifter, J.; Heeger, A. J. Adv. Mater. 2011, 23, 1679− 1683. (5) Bulliard, X.; Ihn, S.-G.; Yun, S.; Kim, Y.; Choi, D.; Choi, J.-Y.; Kim, M.; Sim, M.; Park, J.-H.; Choi, W.; Cho, K. Adv. Funct. Mater. 2010, 20, 4381−4387. (6) (a) Li, C.-Z.; Chueh, C.-C.; Yip, H.-L.; O’Malley, K. M.; Chen, W.-C.; Jen, A. K. Y. J. Mater. Chem. 2012, 22, 8574−8578. (b) Duan, C.; Zhong, C.; Liu, C.; Huang, F.; Cao, Y. Chem. Mater. 2012, 24, 1682−1689. (c) Hsieh, C.-H.; Cheng, Y.-J.; Li, P.-J.; Chen, C.-H.; Dubosc, M.; Liang, R.-M.; Hsu, C.-S. J. Am. Chem. Soc. 2010, 132, 4887−4893. (d) Cheng, Y.-J.; Hsieh, C.-H.; He, Y.; Hsu, C.-S.; Li, Y. J. Am. Chem. Soc. 2010, 132, 17381−17383. (e) Cheng, Y.-J.; Cao, F.-Y.; Lin, W.-C.; Chen, C.-H.; Hsieh, C.-H. Chem. Mater. 2011, 23, 1512− 1518. (f) Seo, J. H.; Gutacker, A.; Sun, Y.; Wu, H.; Huang, F.; Cao, Y.; Scherf, U.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2011, 133, 8416−8419. (g) Choi, H.; Park, J. S.; Jeong, E.; Kim, G. H.; Lee, B. R.; Kim, S. O.; Song, M. H.; Woo, H. Y.; Kim, J. Y. Adv. Mater. 2011, 23, 2759−2763. (h) He, Z.; Zhong, C.; Huang, X.; Wong, W. Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Adv. Mater. 2011, 23, 4636−4643. (i) Yao, K.; Chen, L.; Chen, Y.; Li, F.; Wang, P. J. Mater. Chem. 2011, 21, 13780− 13784. (j) Duarte, A.; Pu, K.-Y.; Liu, B.; Bazan, G. C. Chem. Mater. 2011, 23, 501−515. (7) (a) Huang, F.; Wu, H.; Cao, Y. Chem. Soc. Rev. 2010, 39, 2500− 2521. (b) Wu, H.; Huang, F.; Mo, Y.; Yang, W.; Wang, D.; Peng, J.; Cao, Y. Adv. Mater. 2004, 16, 1826−1830. (c) Zhong, C.; Duan, C.; Huang, F.; Wu, H.; Cao, Y. Chem. Mater. 2011, 23, 326−334. (d) Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K. Y. Adv. Funct. Mater. 2010, 20, 1371−1388. (8) (a) Zhong, Y.; Ma, J.; Hashimoto, K.; Tajima, K. Adv. Mater. 2013, 25, 1071−1075. (b) Chen, L.; Xie, C.; Chen, Y. Org. Electron. 2013, 14, 1551−1561. (9) (a) Binnemans, K. Chem. Rev. 2005, 105, 4148−4204. (b) Kouwer, P. H. J.; Swager, T. M. J. Am. Chem. Soc. 2007, 129, 14042−14052. (10) Chang, Y.-M.; Zhu, R.; Richard, E.; Chen, C.-C.; Li, G.; Yang, Y. Adv. Funct. Mater. 2012, 22, 3284−3289. (11) Huo, L.; Zhang, S.; Guo, X.; Xu, F.; Li, Y.; Hou, J. Angew. Chem., Int. Ed. 2011, 50, 9697−9702. (12) (a) Seo, J. H.; Gutacker, A.; Sun, Y.; Wu, H.; Huang, F.; Cao, Y.; Scherf, U.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2011, 133, 8416−8419. (b) Yao, K.; Chen, L.; Chen, Y.; Li, F.; Wang, P. J. Mater. Chem. 2011, 21, 13780−13784. (c) Xie, C.; Chen, L.; Chen, Y. J. Phys. Chem. C 2013, 117, 24804−24814. (13) Pace, G.; Tu, G.; Fratini, E.; Massip, S.; Huck, W. T. S.; Baglioni, P.; Friend, R. H. Adv. Mater. 2010, 22, 2073−2077. (14) (a) Liu, F.; Page, Z. A.; Duzhko, V. V.; Russell, T. P.; Emrick, T. Adv. Mater. 2013, 25, 6868−6873. (b) Fang, J.; Wallikewitz, B. H.; Gao, F.; Tu, G.; Müller, C.; Pace, G.; Friend, R. H.; Huck, W. T. S. J. Am. Chem. Soc. 2011, 133, 683−685. (c) Li, H.; Xu, Y.; Hoven, C. V.; Li, C.; Seo, J. H.; Bazan, G. C. J. Am. Chem. Soc. 2009, 131, 8903− 8912. (15) (a) Zhou, S.; Chu, B. Adv. Mater. 2000, 8, 545−556. (b) Knaapila, M.; Evans, R. C.; Garamus, V. M.; Almasy, L.; Szekely, N. K.; Gutacker, A.; Scherf, U.; Burrows, H. D. Langmuir 2010, 26, 15634−15643. (16) MacKnight, W. J.; Ponomarenko, E. A.; Tirrell, D. A. Acc. Chem. Res. 1998, 31, 781−868. (17) Braun, S.; Salaneck, W. R.; Fahlman, M. Adv. Mater. 2009, 21, 1450−1472. (18) Kang, H.; Hong, S.; Lee, J.; Lee, K. Adv. Mater. 2012, 24, 3005− 3009. (19) (a) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, 605−625. (b) Wu, H.; Huang, F.; Peng, J.; Cao, Y. Org. Electron. 2005, 6, 118−128. (c) Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; Fenoll, M.; Dindar, A.; Haske, W.; Najafabadi, E.; Khan, T. M.; Sojoudi, H.; Barlow, S.; Graham, S.; Bredas, J. L.; Marder, S. R.; Kahn, A.; Kippelen, B. Science 2012, 336, 327−332.

(20) Yuan, Y.; Sharma, P.; Xiao, Z.; Poddar, S.; Gruverman, A.; Ducharme, S.; Huang, J. Energy Environ. Sci. 2012, 5, 8558−8563. (21) (a) Luo, J.; Wu, H.; He, C.; Li, A.; Yang, W.; Cao, Y. Appl. Phys. Lett. 2009, 95, 043301. (b) Yang, T.; Wang, M.; Duan, C.; Hu, X.; Huang, L.; Peng, J.; Huang, F.; Gong, X. Energy Environ. Sci. 2012, 5, 8208−8214. (22) (a) Yuan, Y.; Reece, T. J.; Sharma, P.; Poddar, S.; Ducharme, S.; Gruverman, A.; Yang, Y.; Huang, J. Nat. Mater. 2011, 10, 296−302. (b) Oh, S.-H.; Na, S.-I.; Jo, J.; Lim, B.; Vak, D.; Kim, D.-Y. Adv. Funct. Mater. 2010, 20, 1977−1983. (c) He, C.; Zhong, C.; Wu, H.; Yang, R.; Yang, W.; Huang, F.; Bazan, G. C.; Cao, Y. J. Mater. Chem. 2010, 20, 2617−2622. (d) Liao, S. H.; Li, Y. L.; Jen, T. H.; Cheng, Y. S.; Chen, S. A. J. Am. Chem. Soc. 2012, 134, 14271−14274. (23) (a) Goh, C.; Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Fréchet, J. M. J Appl. Phys. Lett. 2005, 86, 122110. (b) Zhao, G.; He, Y.; Xu, Z.; Hou, J.; Zhang, M.; Min, J.; Chen, H.-Y.; Ye, M.; Hong, Z.; Yang, Y.; Li, Y. Adv. Funct. Mater. 2010, 20, 1480−1487. (c) Liu, Y.; Wan, X.; Wang, F.; Zhou, J.; Long, G.; Tian, J.; Chen, Y. Adv. Mater. 2011, 23, 5387−5391. (d) Cheng, Y.-J.; Hsieh, C.-H.; Li, P.-J.; Hsu, C.-S. Adv. Funct. Mater. 2011, 21, 1723−1732. (24) He, Z.; Zhong, C.; Huang, X.; Wong, W. Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Adv. Mater. 2011, 23, 4636−4643. (25) 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. Adv. Energy Mater. 2013, 3, 718−723. (26) (a) Li, N.; Lassiter, B. E.; Lunt, R. R.; Wei, G.; Forrest, S. R. Appl. Phys. Lett. 2009, 94, 023307. (b) Potscavage, W. J.; Yoo, S.; Kippelen, B. Appl. Phys. Lett. 2008, 93, 193308. (27) Potscavage, W. J.; Sharma, A.; Kippelen, B. Acc. Chem. Res. 2009, 42, 1758−1767. (28) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789− 794.

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