Highly Efficient Inverted Organic Photovoltaics ... - ACS Publications

Oct 7, 2016 - and Chih-Ping Chen*,‡. †. Department of Cosmeceutics, China Medical University, Taichung, 40402, Taiwan. ‡. Department of Material...
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Highly Efficient Inverted Organic Photovoltaics Containing Aliphatic Hyperbranched Polymers as Cathode Modified Layers Tzong-Yuan Juang,† Yu-Chi Hsu,‡ Bing-Huang Jiang,‡ and Chih-Ping Chen*,‡ †

Department of Cosmeceutics, China Medical University, Taichung, 40402, Taiwan Department of Materials Engineering, Ming Chi University of Technology, New Taipei City, 24301, Taiwan

‡

S Supporting Information *

ABSTRACT: In this study, we found that interfacial layers (IFLs) based on wholly aliphatic hyperbranched poly(amido acid)s (HBPAs) with interior tertiary amido groups can increase the performance of organic photovoltaics (OPVs) substantially. The performance of constructed devices having the layered configuration glass/ indium tin oxide (ITO)/ZnO (with or without IFL)/active layer/MoO3/Ag were enhanced when containing the studied aliphatic HBPAAs, the result of increases in the short circuit current. The presence of the IFL caused the ZnO layers to function more efficiently as electron-selective electrodes. The power conversion efficiencies of the devices incorporating PTB7/PC71BM (from 7.1 to 7.8%) and PffBT4T−2OD/ PC71BM (from 7.8 to 8.7%) increased because of physisorption of the aliphatic HBPAAs, thereby changing the ZnO film’s surface energy and altering the active layer’s morphology. We processed these HBPA-based IFLs in air from solution, providing a simple method for the preparation of solution-processable inverted OPVs.

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highlight the fact that nonconjugated materials can find utility in OPV applications. In this study, we used a facile selfcondensation synthesis to prepare hyperbranched poly(amido acid)s (HBPAs, Figure 1), based on wholly aliphatic poly(amido acid)s (with terminal carbonyl functionalities) of various molecular weights.23 Similar to PEI, the nitrogen atoms in these well-defined branched structures could serve as donors with contact to the surfaces of the acceptor materials. Considering their water-solubility, dendritic structures, and photoluminescence, we explored the use of these dendritic HBPAs in the preparation of OPVs exhibiting enhanced performance. It has been suggested that the most frequently used IF materials (PEIE, PEI) containing aliphatic amino groups change the WFs of their electrodes by forming net dipoles at the organic−electrode interface.16 We speculated that our HBPAs might serve the same function to modify the ETL. To evaluate the effect of the MW on the photovoltaic efficiency, we prepared HBPAs having MWs of 2340 (HBPA−I), 8000 (HBPA−II), and 27 000 (HBPA−III) g mol−1. We examined these materials as cathode buffer layers for inverted OPVs and evaluated how their surface properties (e.g., surface roughness, surface energy) affected the morphologies of the active layers and device performance. We studied the effects of these materials on the morphologies, external quantum efficiencies (EQEs), and PCEs of devices incorporating poly{(5,6-difluoro2,1,3-benzothiadiazol-4,7-diyl)-alt-[3,3‴-di(2-octyldodecyl)2,2′;5′,2″;5″,2‴-quaterthiophen-5,5‴-diyl]} (PffBT4T−2OD)

INTRODUCTION Organic photovoltaics (OPVs) have several attractive features, including manufacture using roll-to-roll techniques, mechanical flexibility, light weight (portable energy devices and ready transport/installation), and proven long-term stability for emerging applications.1−6 With advances in materials development and device engineering, OPVs’ power conversion efficiencies (PCEs) have recently surpassed 11.5% (certified value). Interfacial (IF) modification has emerged as the most important tool in device engineering for enhancing the PCEs of OPVs. Improvements in solution-processable IF materials7−10including transition metal oxides,11 self-assembled layers,12 polyelectrolytes,13 nonconjugated organic materials,14 and organic−inorganic hybrid materials15have also enhanced the performance of OPV devices.16,17 The interfaces of these layers can affect the energy levels of the electrodes, the morphologies of the active layers, and the charge selectivity and transportation.16 The most successful nonconjugated organic materials used to modify the electron transporting layer (ETL) are polyethylenimines (PEIs) and their ethoxylation derivatives (PEIE),18,19 both of which have nitrogen atoms as electron donors that provide a large built-in potential (IF dipole) on ZnO, thereby altering the work function (WF) while decreasing the roughness of the ZnO surface.20 Kippelen et al. first demonstrated the use of these materials as interfacial layers (IFLs) for OPV applications.18 Recently, Jenekhe et al. reported that the ethoxylation and molecular weight (MW) of PEI IFLs can be used to tune high-performance OPVs.21 Kang et al. obtained a PCE of 8.9% for a device incorporating poly(bezodithiophene) deriviate−PTB7 with PEIE as the polyelectrolyte.22 These results © XXXX American Chemical Society

Received: June 28, 2016 Revised: September 26, 2016

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Figure 1. (a) Chemical structures of HBPAs, PTB7, and PffBT-4T−2OD. (b) Device architecture used in this study. Device Fabrication and Characterization. ITO glass substrates [Sanyo, Japan (8 Ω/□)] were patterned lithographically, washed (detergent), treated ultrasonically (using acetone, water, and isopropyl alcohol (IPA)), dried (140 °C, 10 min), and further cleaned with O2 plasma for 5 min. The zinc oxide (ZnO) layer (50 nm) was prepared from a 0.5 M zinc acetate precursor solution (2-methoxyethanol). Solutions of hyperbranched materials were prepared by dissolving each polymer in 2-methoxyethanol to achieve a desired concentration. These solutions were deposited on top of the ZnO layers through spin-coating (5000 rpm) in air and then the samples were dried (100 °C, 10 min) inside a glovebox. Active layer solution of PTB7 and PC71BM (1:1.5) was stirred in o-dichlorobenzene (o-DCB) overnight, filtered through a polytetrafluoroethylene (PTFE; 0.2-μm) filter, and then spin-coated (600−1000 rpm, 30 s) onto the ZnO layer (with or without IFLs). A PffBT4T−2OD:PC70BM solution [donor− acceptor (D/A) ratio, 1:1.2] was formed in chlorobenzene (CB)/o-DCB (8:2, v/v) with 3% of 1,8-diiodooctane (DIO) (PffBT4T−2OD concentration: 9 mg mL−1). To ensure complete dissolution of the polymer, the mixture was stirred at 110 °C (hot plate) for a minimum of 3 h. Prior to spin-coating, the PffBT4T−2OD blend solution and the ITO/PEDOT:PSS substrate were both preheated at approximately 110 °C (hot plate). In a N2-filled glovebox, the PffBT4T− 2OD:PC71BM layers were spin-coated (800 rpm) from the warm PffBT4T−2OD blend solution onto the preheated substrates. The PffBT4T−2OD:PC70BM films were annealed at 90 °C for 5 min and then transferred to a thermal evaporator’s vacuum chamber. All devices were completed by depositing layers of MoO3 (5 nm) and Ag (100 nm) at pressures below 10−6 Torr. Each device had an active area of 10 mm2. Cell performances were measured within a glovebox. The current density−voltage (J−V) curves of the devices were determined using a computer-controlled Keithley 2400 source measurement unit and a Newport solar simulator (Oriel Sol2A Class ABA) with AM 1.5 G illumination (1000 W m−2). A standard Si secondary reference cell cover with a KG-5 filter was used to calibrate the illumination intensity.

and PTB7. Figure 1 displays the materials’ chemical structures as well as the device structure. We observed enhancements in device performance of 5.7−15.4% relative to those of the preoptimized control OPVs (i.e., those prepared without IFLs). The PCE improved dramatically after a thin layer of HBPA−I was inserted between the ZnO and active layers: from 6.9 ± 0.19 to 7.4 ± 0.30% for the PTB7-based devices. The highest PCE of 8.7% was that for the PffBT4T−2OD-derived device using HBPA−II as the IFL, with a values of Jsc of 17.2 mA cm−2, a value of Voc of 0.75 V, and an FF of 67.1%; this PCE was 14% greater than that of the preoptimized device prepared without the IFL. A key feature resulting in these high PCEs was the decreased roughness of the ZnO surfaces, allowing optimization of the active layer morphology and improved electron extraction in the IF layer between the ZnO and active layers.

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EXPERIMENTAL SECTION

Materials and Methods. All chemical reagents were obtained from Aldrich and used as received (unless note otherwise). UV−vis absorption spectroscopy was performed using a Hitachi U-5100 spectrophotometer. PTB7 was purchased from 1-Materials and PffBT4T− 2OD from Raynergy Tek. HBPAs. HBPAs of various molecular weights were prepared through self-condensation AB2 synthesis.15 The branched growth was varied by changing the monomer concentration (0.1 or 0.5 M) and the nature of the solvent (THF or DMF) in the presence of dicyclohexylcarbodiimide (DCC). The resulting HBPAs having MWs of 2340, 8000, and 27,000 g mol−1 are named herein as HBPA−I, HBPA−II, and HBPA−III, respectively. Each had excellent solubility in polar solvents and aqueous solutions. FTIR (KBr, cm−1): 2500− 3500 (COO−H), 2927 (CH), 1728 (CO), 1673 (CO), 1560 (NH), 1188 (C−O−C). B

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Macromolecules The films’ morphologies were analyzed using atomic force microscopy (AFM; VEECO DICP-II microscope, dynamic force mode, ambient temperature). TEM images were recorded using a JEOL JEM-2100 LaB6 HRTEM instrument.

The surface energy varies depending on the naturally properties of a material and its IF status, and show its own effect on an investigated active layer.25 The surface energy of the ZnO film agreed well with those reported previously.25,26 The values of γtotal for the HBPA-modified ZnO films were lower than that of the as-casted ZnO film. As the MW of the HBPA increased, the value of γtotal decreased. As suggested by Cho et al., changes in surface energies may alter the compatibility of the ETL layer and photoactive layer and, hence, affect device performance. We used tapping-mode AFM to examine the ZnO films’ surface morphologies in the presence and absence of the various IF materials. The root-mean-square (rms) roughnesses of ZnO, ZnO/HBPA−I, ZnO/HBPA−II, and ZnO/HBPA−III were 7.4, 2.6, 3.3, and 3.1 nm, respectively (Figure 2). A wrinkled morphology was evident for the as-cast ZnO. We observed a significant change in topography after depositing the HBPAs on the ZnO surface. In particular, the rms roughness decreased. A smoother interface might improve the electron collection efficiency and, thereby, enhance the performance. We used AFM to investigate the evolution of the morphologies of the blend films with or without IFLs; for direct comparison, the blend films for AFM analysis were prepared in the same way we had prepared those for OPV device fabrication. The active layers were spin-coated from a PTB7/PC71BM solution in o-DCB (10 mg mL−1 of PTB7; D/A weight ratio: 1:1.5) and from a PffBT4T−2OD:PC70BM solution (D/A ratio: 1:1.2) in CB/DCB (8:2, v/v) containing 3% DIO (polymer concentration: 9 mg mL−1). The films of PTB7:PC71BM spincast on ZnO in the absence and presence of IFLs had similar topographical morphologies (Figure 3); the rms roughnesses of the films were between 2.2 and 2.4 nm. Thus, the topographic images suggested that deposition of IFL onto the ZnO layer did not alter the topographical blend film morphologies significantly. Phase images of BHJ blend films can provide information about the surface hardness and can identify polymer- and PC71BM-rich domains.23,27 The phase images of the BHJ layers (Figure 3) show two characteristic types: darkcolored aggregations, attributable to PC71BM-rich domains, and bright domains, attributable to conjugated polymer-rich regions.

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RESULTS AND DISCUSSION We prepared three HBPA samples through self-condensation of the AB2 monomer, using a monomer concentration of either 0.1 or 0.5 M and a solvent of either THF or DMF. Self-condensation of our AB2 monomer, featuring an amino group (A) and two carboxyl groups (B), resulted in the formation of hyperbranched structures featuring tertiary amino groups, wholly aliphatic backbones, and terminal CO2H functionalities. We used 1H NMR spectroscopy to calculate the number-average molecular weights (Ave. Mn) from the number of repeat units of the monomer in each HBPA polymer.15 The values of Ave. Mn of HBPA−I, HBPA−II, and HBPA−III were 2340, 8000, and 27 000 g mol−1, respectively. These resulting HBPAs were completely soluble in polar solvents, including THF, DMF, and H2O, due to their highly branched structures, terminal polar CO2H moieties, and capability for forming multiple hydrogen bonds with these solvents. We then used these three hygroscopic aliphatic-based HBPAs as cathode buffer layers and studied the device performance of the resulting inverted OPVs. The Cho group worked on the relationship between the surface energy of the underlayer and the morphology of a bulk heterojunction (BHJ) layer.24,25 They observed that the OPV device’s photocurrent increased upon optimizing the morphology, by varying the total surface energy (γtotal) of interfacial layers/ZnO, which significantly affected the morphology of the active layers. To understand how the morphologies of the BHJ layers responded to the γtotal, we employed contact angle goniometry and the Wu model to measure the γtotal of our ZnOs.17,25 We obtained the γtotal of the ZnOs from their contact angles formed with distilled H2O and diiodomethane (CH2I2, DIM) as probe liquids.24 Table 1 lists the dispersive (γdispersive) and polar (γpolar) components and γtotal of the films of ZnO and of ZnO with various modified layers. The γtotal of ascast films of ZnO, ZnO/HBPA−I, ZnO/HBPA−II, and ZnO/ HBPA−III were 74.8, 72.5, 69.2, and 68.9 mN m−1, respectively.

Table 1. Contact Angle (θ) and Surface Energy Data for ZnO Films ETL

θ water [deg]

θ DIM [deg]

γpolar [mN m−1]

γdispersive [mN m−1]

γtotal [mN m−1]

ZnO ZnO/HBPA−I ZnO/HBPA−II ZnO/HBPA−III

25.4 34.0 33.4 35.9

38.1 34.7 46.2 43.3

33.8 30.0 32.0 30.3

41.0 42.5 37.3 38.6

74.8 72.5 69.3 68.9

Figure 2. Tapping-mode AFM images of (a) an unmodified ZnO film and (b−d) ZnO films modified with (b) HBPA−I, (c) HBPA−II, and (d) HBPA−III. C

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Figure 3. AFM topographical (5 μm × 5 μm) and phase (1 μm × 1 μm) images of (a−d) PTB7/PC71BM films spin-cast on (a) ZnO, (b) ZnO/ HBPA−I, (c) ZnO/HBPA−II, and (d) ZnO/HBPA−III and (e, f) PffBT-4T−2OD/PC71BM films spin-cast on (e) ZnO and (f) ZnO/HBPA−II.

The PTB7/PC71BM film prepared without an IFL exhibited a higher degree of phase separation, as evidenced by segregation of PTB7-rich domains (20−30 nm) surrounded by a PC71BMrich phase. Parts b and c of Figure 3 reveal that the sample consisted of smaller PTB7-rich domains (grain size: ca. 15−25 nm) surrounded by PC71BM domains; a higher degree of mixing between PTB7 and PC71BM might improve charge segregation because of better contact between the fullerene and PTB7.28 Figure 3d displays the morphology of the HBPA−III-derived PTB7/PC71BM film, with PTB7-rich domains having dimensions of approximately 20−40 nm. TEM experiments confirmed these morphologies. Figure 4 displays TEM images of the PTB7/ PC71BM thin film samples prepared with (PTB7−HBPA−I) and without (PTB7) HBPA−I. The bright areas represent PTB7 rich region, because the electron scattering density of the conjugated polymer was less than that of PCBM. The images of the PTB7:PC71BM film reveal a morphology comprising PCBMrich domains dispersed within a PTB7-rich matrix. Comparison with the AFM images confirmed that the island-like domains were PTB7-rich regions. The morphology of the PTB7−HBPA−I sample (Figure 4b) features a well-defined nanoscale phase separation, with domain sizes smaller than those of PTB7, consistent with the AFM images. Thus, embedding of the IFL layers (HBPA−I and HBPA−II) optimized the BHJ morphology, inducing to less phase separation between PTB7 and PC71BM, potentially ensuring the efficient charge transport

Figure 4. TEM images of the PTB7:PCBM (a) without IFL and (b) with HBPA−I.

required for high-performance OPV applications. In comparison with the values of γtotal of the ZnO films, the decrease in surface energy of the HBPA−I- and HBPA−II-derived ZnO films led to smaller degrees of nanophase segregation of the PTB7:PC71BM blend morphology. These results suggest that the interface’s γtotal played a significant role during the formation of the blend film morphology by affecting its phase segregation. Using methods similar to those reported previously, we fabricated OPVs having the layered configuration glass/indium tin oxide (ITO)/ZnO (with or without IFLs)/active layers/ MoO3/Ag.11,15 We denote the ITO/ZnO/PTB7:PCBM/ MoO3/Ag configuration as the PTBT-7 device, and the devices containing the various HBPA IFLs as PTB7−HBPA devices. Table 2 summarizes the optimized OPV performances of devices. The PCE of the PTB7-derived device under illumination of AM D

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Macromolecules Table 2. J−V Properties of OPV Devices devices PTB7 PTB7−HBPA−I PTB7−HBPA−II PTB7−HBPA−III PffBT4T−2OD−ZnO PffBT4T−2OD− HBPA−II

Jsc (mA cm−2) 14.5 15.8 15.1 14.9 14.6 15.2

± ± ± ± ± ±

0.46 0.40 0.20 0.20 1.11 1.07

Voc (V) 0.70 0.70 0.70 0.71 0.70 0.70

± ± ± ± ± ±

FF (%)

0.01 0.00 0.01 0.01 0.01 0.02

1.5G (1000 W m−2) was 6.9 ± 0.19%. The PCEs of the devices incorporating PTB7/HBPA−I, PTB7/HBPA−II, and PTB7/ HBPA−III were 7.4 ± 0.30, 7.1 ± 0.11, and 6.8 ± 0.13%, respectively. Thus, the PCEs increased after embedding HBPA−I and HBPA−II as cathode-modified layers, primarily because the values of Jsc increased from 14.5 ± 0.45 to 15.8 ± 0.40 and 15.1 ± 0.20 mA cm−2, respectively. After embedding HBPA−I onto the ZnO layer, the PCE increased by up to 7−9% when compared with the value for the preoptimized control devices. The highest PCE (7.8%) was that for the PTB7−HBPA−I device, which had a value of Jsc of 16.0 mA cm−2, a value of Voc of 0.71 V, and an FF of 0.691 (Figure 5a, Table 2). Figure 5b presents the EQE responses for the devices. For the PTB7:PC71BM OPV devices, the responds of the EQE occurred at wavelengths from 400 to 800 nm, with maximum EQEs of the PTBT and PTB7−HBPA−I devices of 61.6 and 73.4% (at 630 nm), respectively. Convolution of the EQE responses with the photon flux AM 1.5G spectrum (1000 W m−2) allowed us to estimate values of Jsc for the irradiated PTB7− ZnO and PTB7−HBPA−I devices of 13.8 and 14.4 mA cm−2, respectively. Because of slight differences between the light source of EQE and the AM 1.5G photon flux, small mismatches existed between the solar simulator and calculated EQE−Jsc data. The IFLs provided a facile means of controlling the quality of the PTB7-based blend films and, hence, enhancing the PCE. The improvement in PCE might be due primarily to the increase in the value of Jsc, caused by the greater EQE. To clarify the effect that the interfacial layer had on enhancing the performance, we measured reflectance spectra (R) of the devices to calculate the internal quantum efficiencies (IQEs) using the expression EQE/(1 − R) (Figure 5c). The IQE is defined as the amount of charge carriers collected by a device divided by the amount of photons of a given energy falling on the device from outside and absorbed by the cell. The highest IQE of the PTB7−HBPA−I device was 82% at 650 nm, remaining near or above 70% throughout the absorption spectrum; in contrast, the IQE of the PTB7 device prepared without an interfacial layer was 70% at 650 nm. The higher IQE of the former indicates efficient light harvesting, allowing efficient collection of separated pairs of charge carriers and photogenerated carriers at the electrode for the HBPA−I-modified devices.29 To further characterize the performance of these HBPA IFLs, we fabricated electron-only space-charge limited current (SCLC) devices having the configuration ITO/ZnO (with or without HBPAs)/PTB7:PC71BM/Ca/Al. As suggested by Jenekhe et al.,21 the SCLC electron mobility determined from electron-only devices can be related to the electron extraction efficiency from the active layer.30 In their study, they found that the embedding of a PEI or PEIE interlayer resulted in greater enhancement of the SCLC bulk electron mobility.21 We fabricated these SCLC electron-only devices using the optimal blend film conditions. When we applied a sufficient voltage to these devices, the transport of electrons through the

65.9 65.2 66.3 64.2 69.4 67.9

± ± ± ± ± ±

1.37 2.60 0.99 1.20 3.25 3.96

PCE (%)

best PCE (%)

± ± ± ± ± ±

7.1 7.8 7.4 7.0 7.8 8.7

6.9 7.4 7.1 6.8 7.3 7.6

0.18 0.30 0.11 0.13 0.35 0.54

Figure 5. (a) I−V characteristics and (b) EQE and (c) IQE responses of OPV devices prepared with various IFLs.

BHJ layer was limited by the accumulated space-charge. The equation of SCLC is described by J= E

9 V2 εr ε0μh 3 8 L DOI: 10.1021/acs.macromol.6b01373 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules where εr is the dielectric constant of PCBM (3.9), ε0 is the permittivity of free space (8.85 × 10−12 F m−1), μe is the electron mobility, V is the device’s applied voltage, and L is the active layer thickness (120 nm).31,32 Figure S1 shows the current densities of the PTB7-, PTB7/HBPA−I-, PTB7/ HBPA−II-, and PTB7/HBPA−III-derived electron only devices, measured experimentally in the electron-only devices. We corrected the applied voltage for the built-in voltage (VBI), determined from the difference in work functions of the Al and ZnO electrodes. The J0.5−V plots for the responding devices were straight lines (Figure S1). The SCLC electron mobilities (from the average of 3 devices) of the devices based on PTB7, PTB7/HBPA−I, PTB7/HBPA−II, and PTB7/HBPA−III were 1.25 × 10−4, 2.47 × 10−4, 1.64 × 10−4, and 1.21 × 10−4 cm2 V−1 s−1; these values are consistent with previous literature reports.30,33 The devices prepared with interlayers of HBPA−I and HBPA−II had the higher SCLC electron mobility. The addition of HBPA−I interlayer resulted in a nearly 2.0-fold increase in the SCLC bulk electron mobility, suggesting improved electron extraction from the active layer. To determine whether incorporation of these IFLs would be a universal means of enhancing the performance of OPVs, we investigated their effects on PffBT4T−2OD, a robust conjugated polymer having a low band gap (Figure 1a).8 As listed in Table 2, the PCE of the normal PffBT4T−2OD device was 7.3 ± 0.35%, with a value of Jsc of 14.6 ± 1.11 mA cm−2, a value of Voc of 0.70 ± 0.01 V, and an FF of 69.4 ± 3.25%. The average PCE was 7.6 ± 0.54% for the PffBT4T−2OD device incorporating HBPA−II as the IFL, with a best PCE of 8.7% with a value of Jsc of 17.2 mA cm−2, a value of Voc of 0.75 V, and an FF of 0.671. The spectral responses of the devices based on PffBT4T−2OD:PC71BM revealed responds of the EQE at wavelengths from 400 to 800 nm. The estimated values of Jsc from EQE responds for the PffBT4T−2OD/ZnO and PffBT4T−2OD/HBPA−II devices under irradiation of 13.9 and 14.8 mA cm−2, respectively. Again, a slight mismatch existed between the convoluted and solar-simulated data. To understand the cause of the increased device performance, we used AFM to examine the morphology of the devices in the absence and presence of the IFL. To allow direct comparison, we used the same conditions to prepare the films for the tapping-mode AFM analysis as we had for the device fabrication (Figure 3, parts e and f). The morphology of the blend film changed dramatically in the presence of HBPA−II. A rough surface (rms roughness: 6.5 nm) appeared for the PffBT4T− 2OD blend film (without the IFL), featuring segregated PffBT4Trich domains (30−50 nm) surrounded by a PC71BM-rich phase. On the other hand, we observed lower roughness (rms roughness: 4.3 nm) for the PffBT4T−2OD/HBPA−II blend film with a fibrillar structure in the PffBT4T−2OD-rich domains (Figure 3f). Thus, the improved PCE arose mainly from the spatial distribution of the PffBT4T−2OD and PC71BM domains. A fibrillar phase-separated morphology induced by an HBPA−II modified layer appeared to enhance carrier transport within a cell.

when incorporating the various IFLs. The PCE of the PTB7/ HBPA−I device (7.4 ± 0.30%) was significantly higher than that of the PTB7-ZnO device (6.9 ± 0.19%), the result of a remarkable increase in the value of Jsc. We attribute the higher values of Jsc for the PTB7/HBPA−I device to its smaller degrees of phase segregation (AFM morphology) and related higher degrees of mixing between PTB7 and PC71BM, thereby improving the charge separation and efficiency of electron extraction. We observed an improvement in PCE for the device incorporating PffBT4T−2OD/PC71BM from 7.3 ± 0.35 to 7.6 ± 0.54% when the device incorporated HBPA−II as the IFL. The device based on PffBT4T−2OD/HBPA−II displayed a best PCE of 8.7%, with a value of Jsc of 17.2 mA cm−2, a value of Voc of 0.75 V, and an FF of 0.671. Facile control over the surface properties of ZnO layers and, hence, the morphologies and carrier transport properties of BHJs should facilitate the engineering of OPV devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01373. J0.5−V plots for electron-only devices (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(C.-P.C.) E-mail: [email protected]. Fax: 886-229084091. Telephone: 886-2-29089899 + 4439. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan (MOST 103-2113-M-131-001-MY2; MOST 105-2221-E-131-033; 104-2113-M-039-003) for financial support.

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REFERENCES

(1) Pastorelli, F.; Schmidt, T. M.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M.; Krebs, F. C. The Organic Power Transistor: Roll-toRoll Manufacture, Thermal Behavior, and Power Handling When Driving Printed Electronics. Adv. Eng. Mater. 2016, 18, 51−55. (2) Gevorgyan, S. A.; Madsen, M. V.; Roth, B.; Corazza, M.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M.; Krebs, F. C. Lifetime of Organic Photovoltaics: Status and Predictions. Adv. Energy Mater. 2016, 6, 1501208. (3) Emmott, C. J. M.; Moia, D.; Sandwell, P.; Ekins-Daukes, N.; Hösel, M.; Lukoschek, L.; Amarasinghe, C.; Krebs, F. C.; Nelson, J. In-Situ, Long-Term Operational Stability of Organic Photovoltaics for Off-Grid Applications in Africa. Sol. Energy Mater. Sol. Cells 2016, 149, 284−293. (4) Wang, K.; Liu, C.; Meng, T.; Yi, C.; Gong, X. Inverted Organic Photovoltaic Cells. Chem. Soc. Rev. 2016, 45, 2937−75. (5) Cheng, P.; Zhan, X. Stability of Organic Solar Cells: Challenges and Strategies. Chem. Soc. Rev. 2016, 45, 2544−82. (6) Chen, C.-P.; Huang, C.-Y.; Chuang, S.-C. Highly Thermal Stable and Efficient Organic Photovoltaic Cells with Crosslinked Networks Appending Open-Cage Fullerenes as Additives. Adv. Funct. Mater. 2015, 25, 207−213. (7) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666−731. (8) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nature Energy 2016, 1, 15027.

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CONCLUSION We have prepared three HBPAs of different molecular weights and used them as cathode-modified layers for OPV applications. Through AFM and contact angle analyses, we observed decreases in the surface roughness and surface energy for the IFL-modified ZnO films. We found that the bulk morphologies of PTB7− and PffBT4T−2OD−based blend films were different F

DOI: 10.1021/acs.macromol.6b01373 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules (9) Chen, S.; Lee, K. C.; Zhang, Z.-G.; Kim, D. S.; Li, Y.; Yang, C. An Indacenodithiophene−Quinoxaline Polymer Prepared by Direct Arylation Polymerization for Organic Photovoltaics. Macromolecules 2016, 49, 527−536. (10) Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganas, O.; Gao, F.; Hou, J. Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734−9. (11) Chen, C.-P.; Chen, Y.-D.; Chuang, S.-C. High-Performance and Highly Durable Inverted Organic Photovoltaics Embedding SolutionProcessable Vanadium Oxides as an Interfacial Hole-Transporting Layer. Adv. Mater. 2011, 23, 3859−3863. (12) George, Z.; Xia, Y.; Sharma, A.; Lindqvist, C.; Andersson, G.; Inganas, O.; Moons, E.; Muller, C.; Andersson, M. R. Two-in-One: Cathode Modification and Improved Solar Cell Blend Stability through Addition of Modified Fullerenes. J. Mater. Chem. A 2016, 4, 2663−2669. (13) Le, T. P.; et al. Miscibility and Acid Strength Govern Contact Doping of Organic Photovoltaics with Strong Polyelectrolytes. Macromolecules 2015, 48, 5162−5171. (14) Nam, S.; Seo, J.; Woo, S.; Kim, W. H.; Kim, H.; Bradley, D. D.; Kim, Y. Inverted Polymer Fullerene Solar Cells Exceeding 10% Efficiency with Poly(2-Ethyl-2-Oxazoline) Nanodots on ElectronCollecting Buffer Layers. Nat. Commun. 2015, 6, 8929. (15) Hsu, H.-L.; Juang, T.-Y.; Chen, C.-P.; Hsieh, C.-M.; Yang, C.-C.; Huang, C.-L.; Jeng, R.-J. Enhanced Efficiency of Organic and Perovskite Photovoltaics from Shape-Dependent Broadband Plasmonic Effects of Silver Nanoplates. Sol. Energy Mater. Sol. Cells 2015, 140, 224−231. (16) Chueh, C.-C.; Li, C.-Z.; Jen, A. K. Y. Recent Progress and Perspective in Solution-Processed Interfacial Materials for Efficient and Stable Polymer and Organometal Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 1160−1189. (17) Yip, H.-L.; Jen, A. K. Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994−6011. (18) Zhou, Y.; et al. A Universal Method to Produce Low−Work Function Electrodes for Organic Electronics. Science 2012, 336, 327− 332. (19) Li, P.; et al. High-Efficiency Inverted Polymer Solar Cells Controlled by the Thickness of Polyethylenimine Ethoxylated (PEIE) Interfacial Layers. Phys. Chem. Chem. Phys. 2014, 16, 23792−23799. (20) Woo, S.; Hyun Kim, W.; Kim, H.; Yi, Y.; Lyu, H.-K.; Kim, Y. 8.9% Single-Stack Inverted Polymer Solar Cells with Electron-Rich Polymer Nanolayer-Modified Inorganic Electron-Collecting Buffer Layers. Adv. Energy Mater. 2014, 4, 1301692. (21) Courtright, B. A. E.; Jenekhe, S. A. Polyethylenimine Interfacial Layers in Inverted Organic Photovoltaic Devices: Effects of Ethoxylation and Molecular Weight on Efficiency and Temporal Stability. ACS Appl. Mater. Interfaces 2015, 7, 26167−26175. (22) Jin, W.-Y.; Ginting, R. T.; Jin, S.-H.; Kang, J.-W. Highly Stable and Efficient Inverted Organic Solar Cells Based on Low-Temperature Solution-Processed Peie and Zno Bilayers. J. Mater. Chem. A 2016, 4, 3784−3791. (23) Shiau, S.-F.; Juang, T.-Y.; Chou, H.-W.; Liang, M. Synthesis and Properties of New Water-Soluble Aliphatic Hyperbranched Poly(Amido Acids) with High pH-Dependent Photoluminescence. Polymer 2013, 54, 623−630. (24) Wei, Y.; Liu, P.-J.; Lee, R.-H.; Chen, C.-P. Thermally Evaporable 5,10-Dihydroindeno[2,1-a]Indenes Form Efficient Interfacial Layers in Organic Solar Cells. RSC Adv. 2015, 5, 7897−7904. (25) Bulliard, X.; et al. Enhanced Performance in Polymer Solar Cells by Surface Energy Control. Adv. Funct. Mater. 2010, 20, 4381−4387. (26) Su, Y.-A.; Lin, W.-C.; Wang, H.-J.; Lee, W.-H.; Lee, R.-H.; Dai, S. A.; Hsieh, C.-F.; Jeng, R.-J. Enhanced Photovoltaic Performance of Inverted Polymer Solar Cells by Incorporating Graphene Nanosheet/ Agnps Nanohybrids. RSC Adv. 2015, 5, 25192−25203. (27) Chan, S.-H.; Lai, C.-S.; Chen, H.-L.; Ting, C.; Chen, C.-P. Highly Efficient P3ht: C60 Solar Cell Free of Annealing Process. Macromolecules 2011, 44, 8886−8891.

(28) Alekseev, A.; Hedley, G. J.; Al-Afeef, A.; Ageev, O. A.; Samuel, I. D. W. Morphology and Local Electrical Properties of PTB7:PC71BM Blends. J. Mater. Chem. A 2015, 3, 8706−8714. (29) Burkhard, G. F.; Hoke, E. T.; McGehee, M. D. Accounting for Interference, Scattering, and Electrode Absorption to Make Accurate Internal Quantum Efficiency Measurements in Organic and Other Thin Solar Cells. Adv. Mater. 2010, 22, 3293−3297. (30) Lee, E. J.; Heo, S. W.; Han, Y. W.; Moon, D. K. An OrganicInorganic Hybrid Interlayer for Improved Electron Extraction in Inverted Polymer Solar Cells. J. Mater. Chem. C 2016, 4, 2463−2469. (31) Chen, C.-P.; Hsu, H.-L. Increasing the Open-Circuit Voltage in High-Performance Organic Photovoltaic Devices through Conformational Twisting of an Indacenodithiophene-Based Conjugated Polymer. Macromol. Rapid Commun. 2013, 34, 1623−1628. (32) Mihailetchi, V. D.; van Duren, J. K. J.; Blom, P. W. M.; Hummelen, J. C.; Janssen, R. A. J.; Kroon, J. M.; Rispens, M. T.; Verhees, W. J. H.; Wienk, M. M. Electron Transport in a Methanofullerene. Adv. Funct. Mater. 2003, 13, 43−46. (33) Kniepert, J.; Lange, I.; Heidbrink, J.; Kurpiers, J.; Brenner, T. J. K.; Koster, L. J. A.; Neher, D. Effect of Solvent Additive on Generation, Recombination, and Extraction in PTB7:PC71BM Solar Cells: A Conclusive Experimental and Numerical Simulation Study. J. Phys. Chem. C 2015, 119, 8310−8320.

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DOI: 10.1021/acs.macromol.6b01373 Macromolecules XXXX, XXX, XXX−XXX