Acceptor Molecular Orientation-Dependent Photovoltaic

Nov 3, 2015 - ABSTRACT: The correlated donor/acceptor (D/A) molecular orientation plays a crucial role in solution-processed all-polymer solar cells i...
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Donor/Acceptor Molecular Orientation-Dependent Photovoltaic Performance in All-Polymer Solar Cells Ke Zhou,†,‡ Rui Zhang,†,‡ Jiangang Liu,† Mingguang Li,†,‡ Xinhong Yu,† Rubo Xing,† and Yanchun Han*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China ‡ University of the Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049, P. R. China S Supporting Information *

ABSTRACT: The correlated donor/acceptor (D/A) molecular orientation plays a crucial role in solution-processed all-polymer solar cells in term of photovoltaic performance. For the conjugated polymers PTB7-th and P(NDI2OD-T2), the preferential molecular orientation of neat PTB7-th films kept face-on regardless of the properties of processing solvents. However, an increasing content of face-on molecular orientation in the neat P(NDI2OD-T2) films could be found by changing processing solvents from chloronaphthalene (CN) and o-dichlorobenzene (oDCB) to chlorobenzene (CB). Besides, the neat P(NDI2OD-T2) films also exhibited a transformation of preferential molecular orientation from face-on to edge-on when extending film drying time by casting in the same solution. Consequently, a distribution diagram of molecular orientation for P(NDI2OD-T2) films was depicted and the same trend could be observed for the PTB7-th/P(NDI2OD-T2) blend films. By manufacture of photovoltaic devices with blend films, the relationship between the correlated D/A molecular orientation and device performance was established. The short-circuit current (Jsc) of devices processed by CN, oDCB, and CB enhanced gradually from 1.24 to 8.86 mA/cm2 with the correlated D/A molecular orientation changing from face-on/edge-on to face-on/face-on, which could be attributed to facile exciton dissociation at D/A interface with the same molecular orientation. Therefore, the power conversion efficiency (PCE) of devices processed by CN, oDCB, and CB improved from 0.53% to 3.52% ultimately. KEYWORDS: molecular orientation, chain aggregation, film drying time, all-polymer solar cells, photovoltaic performance

1. INTRODUCTION Over the past decade, polymer-based solar cells (PSCs) have gained much interest due to their remarkable advantages including low-cost production, light weight, flexibility, and large-area manufacturing.1−4 The most studied polymer solar cells are composed of a conjugated polymer as donor and a small molecule fullerene derivative as acceptor, of which the power conversion efficiency (PCE) has reached to nearly 10% in single-junction cells5 due to the development of various lowbandgap donor polymers with rational molecular designs. Allpolymer solar cells, consisting of two conjugated polymers as both donor and acceptor, would probably be a promising alternative to polymer/fullerene PSCs, attributed to the easily achievement of high open-circuit voltage and more efficient light absorption.6,7 The highest PCE of all-polymer systems is increasing continuously and has approached ∼7%8−12due to the considerable efforts in synthesizing new polymer acceptors and optimizing the blend morphology.13−15 Recently, Jenekhe and co-workers obtained the PCE of 4.8%16 and 7.7%12 for the PSEHTT:PNDIS-HD blend system processed from optimum CB:DCB cosolvent and PNDIS-HD:PBDTT-FTTE blend system processed by film aging, respectively, which were superior to those of their PCBM counterparts. These results © XXXX American Chemical Society

suggest that the performance of all polymer BHJ solar cells can exceed that of the corresponding polymer/fullerene blend systems with suitable donor and acceptor polymers and optimum solution processing of the active layer. However, PCEs of many other all polymer systems are far behind their fullerene counterparts, and the low external quantum efficiency (EQE) is probably responsible for the poor efficiencies of allpolymer solar cells. At present, an excellent EQE of 85% is reported,12 while large amounts of all-polymer devices remain at 40−60%,10,17−19 which is inferior to those of polymer/ fullerene devices (70−80%).5,20,21 Previous work has proved that geminate recombination at D/A interface is the dominated pathway for the decay of excitations22,23 and the poor efficiencies of exciton dissociation primarily lead to the low EQEs for all-polymer devices. Besides, it is considered that charge delocalization ability plays a crucial role in the process of exciton dissociation. For the polymer/fullerene systems, the spherical fullerene acceptor, which could delocalize electrons in the three-dimensional directions, will facilitate the conversion of excitons to spatial free charge at the D/A interface24 and the Received: August 17, 2015 Accepted: October 26, 2015

A

DOI: 10.1021/acsami.5b07605 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Chemical Structures and Energy Level Diagrams of PTB7-th and P(NDI2OD-T2)

charge separation efficiency can reach Φ ≈ 0.12 based on calculation.25 Besides, Ade and co-workers26 have shown that the molecular orientation of polymer donor also influenced the exciton dissociation efficiency. They found polymer donor with face-on orientation was superior to edge-on orientation because the donor molecular orbital of the former one could efficiently overlap with that of the acceptor and consequently decrease geminate recombination. For the all-polymer systems, the polymer acceptor can merely delocalize the electrons in the two-dimensional directions and different molecular orientation for both donor and acceptor may exist at D/A interface, which lead to a charge separation efficiency as low as Φ ≈ 7 × 10−3 based on calculation.25 Recently, Neher and co-workers27 employed CN as the solvent additive to adjust the molecular orientation of P(NDI2OD-T2). When the correlated donor/ acceptor molecular orientation changed from edge-on/face-on to edge-on/edge-on, the corresponding PCE improved dramatically from 0.08% to 1.07%, which resulted from the efficient split-up of geminate pairs. In addition, the device opencircuit voltage28 and electron transfer speed29 are also related to the correlated D/A molecular orientation. Therefore, control of the molecular orientation in all-polymer solar cells should be significant to advance the photovoltaic performance. In this paper, we systematically investigated the driving force associated with variation of molecular orientation and the relationship between the correlated D/A molecular orientation and photovoltaic performance for polymer blend system poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5B′]dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-th) and poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′bithiophene)}, P(NDI2OD-T2). The molecular orientation of neat PTB7-th films kept face-on regardless of the properties of processing solvents, while the molecular orientation of neat P(NDI2OD-T2) films could be adjusted by changing the extent of chain aggregation and film drying time, and consequently a distribution diagram of molecular orientation for P(NDI2ODT2) films was depicted. Besides, the same variation trend could be observed for the PTB7-th/P(NDI2OD-T2) blend films, based on which photovoltaic devices were prepared to demonstrate the relationship between the correlated donor/ acceptor molecular orientation and photovoltaic performance.

Material Inc. and Polyera Corporation, respectively. Anhydrous solvents chloronaphthalene (CN), o-dichlorobenzene (oDCB), and chlorobenzene (CB) were purchased from Sigma-Aldrich. All chemicals were used as received. 2.2. Sample Preparation. PTB7-th neat solutions with a concentration of 5 mg/mL and P(NDI2OD-T2) neat solutions with a concentration of 10 mg/mL were prepared by individually dissolving PTB7-th and P(NDI2OD-T2) in CN, oDCB, or CB. All the solutions were stirred overnight in a N2 glovebox. Glass substrates were first boiled in 98% sulfuric acid: H2O2 (v/v = 7/3) for 30 min, next ultrasonicated three times in deionized water for 10 min and then dried by nitrogen flow. PTB7-th and P(NDI2OD-T2) neat solutions were spin-cast onto the cleaned glass substrates to produce a series of neat films with nearly same thickness, which were ∼40 nm for PTB7th neat films and ∼90 nm for P(NDI2OD-T2) neat films (∼50 nm for CN film). In order to study the effect of film drying time on P(NDI2OD-T2) molecular orientation, various processing conditions were conducted which included spin-coating at 5000, 1500, and 800 rpm and drop-coating at atmospheric pressure and vacuum. Then the films were left to dry overnight in the vacuum. PTB7-th/P(NDI2OD-T2) (1/1, wt/wt) blend solutions with a total concentration of 10 mg/mL were prepared by dissolving PTB7-th and P(NDI2OD-T2) in CN, oDCB, or CB. All the solutions were stirred overnight in a N2 glovebox. The three kinds of blend solutions were spin-cast onto the cleaned glass substrates to produce a series of films with nearly same thickness (∼100 nm). The films were left to dry overnight in the vacuum. 2.3. Characterization. The solution state of PTB7-th and P(NDI2OD-T2) was characterized by UV−vis absorption spectrum, which was recorded with a Lambda 750 spectrometer (PerkinElmer, Wellesley, MA). The molecular orientation of PTB7-th and P(NDI2OD-T2) films were investigated by out-of-plane grazing incidence X-ray diffraction (GIXRD) and in-plane X-ray diffraction (IPXRD). GIXRD data were obtained on a Bruker D8 Discover reflector (Cu Kα, λ = 1.540 56 Å) under 40 kV and 40 mA tube current. The in-plane X-ray diffraction (IPXRD) profiles were obtained using a Rigaku SmartLab with an Xray generation power of 40 kV tube voltage and 30 mA tube current. The diffraction was recorded in the 2θ−χ mode. In both GIXRD and IPXRD, the scanning speed is 5 s per step with 0.05° step size (2θ). The measurements were obtained in a scanning interval of 2θ between 2° and 30°. The morphology of PTB7-th/P(NDI2OD-T2) blend films was investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). AFM images were recorded by a SPA300HV instrument with a SPI3800N controller (Seiko Instruments Inc., Japan) in tapping mode. A silicon microcantilever (spring constant 2 N/m and resonant frequency of ∼70 kHz, Olympus, Japan) was used for the scanning. TEM images were obtained using a JEOL JEM-1011 transmission electron microscope operated at 100 kV.

2. EXPERIMENTAL SECTION 2.1. Materials. PTB7-th (Mw = 114 kDa, PDI = 2.2) and P(NDI2OD-T2) (Mw = 84 kDa, PDI = 3.1) were purchased from 1B

DOI: 10.1021/acsami.5b07605 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Absorption spectra of PTB7-th solutions in different processing solvents, CN, oDCB, and CB. (b) Difference of wavelength at maximum absorption peak for PTB7-th solutions. (c) Out-of-plane grazing incidence X-ray diffraction (GIXRD) and (d) in-plane X-ray diffraction (IPXRD) profiles of PTB7-th films processed with CN, oDCB, and CB. The inset illustrates the characterization of X-ray diffraction in different directions.

Table 1. Chemical Properties of Processing Solvents Used

was 12 mm2, which was defined by the overlapping area of the ITO and Al electrodes. Current density−voltage (I−V) characteristics of the photovoltaic cells were measured using a computer controlled Keithley 236 source meter under AM1.5G illumination from a calibrated solar simulator with irradiation intensity of 100 mW/cm2. External quantum efficiency (EQE) measurements were performed with a setup made by the Beijing 7-Star Optical Instruments Co. Ltd. and detected with a lock-in amplifier (SR830, Stanford Research Systems) at a chopping frequency of 160 Hz during illumination with a monochromatic light from a halogen lamp (250 W).

Devices were fabricated on indium−tin oxide (ITO) coated glass substrates. The substrates were cleaned with detergent, ultrasonicated in water, acetone, and isopropyl alcohol, respectively, and then dried by nitrogen flow. The ITO substrates were treated with UV ozone for 30 min so as to improving hydrophilicity, and then a poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron P4083) layer (25 nm) was spin-coated on this well-cleaned ITO glass and dried at 130 °C in vacuum for 30 min. When the substrates cooled to the room temperature, blend solutions processed with CN, oDCB, and CB were then spin-cast on top of the PEDOT:PSS layer to produce a nearly 100 nm thick active layer. Finally, a bilayer structure of Ca (30 nm)/Al (90 nm) was deposited atop the active layer by thermal evaporation in a vacuum of 2 × 10−4 Pa. The cell active area C

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3. RESULTS AND DISCUSSION 3.1. Control of Polymer Molecular Orientation by Changing Solution State and Film Drying Time. In this study, we employed the PTB7-th and P(NDI2OD-T2) blend system (chemical structures and energy level diagram10 are shown in Scheme 1) to investigate the driving force associated with variation of molecular orientation and the relationship between the correlated D/A molecular orientation and photovoltaic performance. First, the solution states of PTB7th solutions and the molecular orientation of the corresponding PTB7-th films were characterized, and the results were shown in Figure 1. For PTB7-th solutions, the absorption spectra displayed similar variation trend while the wavelength of maximum absorption peak shifted from 705 to 715 nm (Figure 1b) with the processed solvents changing from CB, oDCB to CN (the chemical properties of solvents are shown in Table 1). However, no distinct wavelength shift can be found in the corresponding PTB7-th films (see Figure S1, Supporting Information), suggesting that the alignment of PTB7-th molecules is similar in CN, oDCB, and CB films and it is the various interactions between PTB7-th and solvent molecules that lead to the red shift of solution UV−vis absorption spectra. In order to further demonstrate the reason for this red-shift, the UV−vis absorption spectra of PTB7-th solutions at different temperatures were measured (see Figure S2, Supporting Information). A gradual blue shift of maximum absorption wavelength about ∼10 nm can be observed when raising the temperature to 120 °C, indicating the increased solubility and decreased aggregated state for polymer at a higher temperature, which is consistent with the property of P(NDI2OD-T2) studied by Neher and co-workers.30 Thus, the red-shift of PTB7-th solutions can be ascribed to chain aggregation. As shown in Figure 1c and Figure 1d, (010) diffraction peak at 2θ = 22.7° in the out-of-plane direction and (100) diffraction peak at 2θ = 3.7° in the in-plane direction can be observed for the PTB7-th neat films processed by CB, oDCB, and CN. The (100) peak in the out-of-plane direction and (010) peak in the in-plane direction represent the edge-on orientation, while the (010) peak in the out-of-plane direction and (100) peak in the in-plane direction correspond to face-on orientation. Therefore, it can be concluded that the PTB7-th films keep face-on molecular orientation regardless of the properties of processing solvents. For P(NDI2OD-T2) solutions, the solvent induced polymer chain aggregation could also be observed. As is shown in Figure 2a, all P(NDI2OD-T2) solutions display “camel-back” absorption spectra which can be assigned to the π−π* transition at a high-energy band and the charge-transfer (CT) transition at a low-energy band.30,31 Neher and co-workers have previously studied the UV−vis absorption spectra of P(NDI2OD-T2) solutions processed by different solvents.30,32 They found that the CN absorption spectrum showed a broad and featureless band centered at 620 nm, which represented the nonaggregated chains, while in other common solvents such as CB, toluene, etc., the absorption spectra with a peak at 710 nm and a shoulder at ∼800 nm could be observed, which indicated the aggregated chains in those solvents. Here, the same results are obtained, namely, the absorption intensity and wavelength of maximum absorption peak vary with changing cast solvents from CN, oDCB to CB. In CN solution, the π−π* absorption peak was lowest, which indicated that P(NDI2OD-T2) molecules are nonaggregated. Chen and co-workers33 have

Figure 2. (a) Absorption spectra of P(NDI2OD-T2) solutions in different processing solvents, CN, oDCB, and CB, which are normalized to the absorbance at 550 nm. (b) Relationship between solvent molar volume and the aggregation content.

demonstrated that the polymer aggregation behavior was a function of steric hindrance of aromatic solvents imposed by substituents and the size of aggregation decreased as the substituents on the solvents got larger. This is the case in P(NDI2OD-T2) solutions as well. CN molecules own the condensed ring structures, and thus their spatial volumes are biggest compared to those of CB and oDCB molecules (see Table 1). Therefore, the P(NDI2OD-T2) molecular aggregation would be inhibited and nonaggregated chains could be found in CN solution. With the decreasing solvent spatial volume, the extent of chain aggregation increased steadily and the biggest absorption peak could be observed in CB solution. On the basis of the method proposed by Neher and coworkers,30 the aggregate content of CB and oDCB solutions was calculated and is shown in Figure 2b. P(NDI2OD-T2) molecules in CN solution are assigned as amorphous, while ∼38% and ∼48% of the chains in oDCB and CB solutions are in the aggregated state. The molecular orientation of P(NDI2OD-T2) neat films was characterized by GIXRD and IPXRD. As shown in Figure 3a, the (100) diffraction peak at 2θ = 3.7−4.3° and (010) diffraction peak at 2θ = 22.5° in the outof-plane direction, which represent the side chain and π−π stacking directions, respectively, can be observed for the neat P(NDI2OD-T2) films. By changing of the processing solvent from CN, oDCB to CB, the intensity of the (100) diffraction peaks decreases dramatically and the intensity of the (010) diffraction peaks increases remarkably (Figure 3b). In Figure 3c, the (100) diffraction peak at 2θ = 3.3° and (200) diffraction D

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Figure 3. (a) Out-of-plane grazing incidence X-ray diffraction (GIXRD) and (c) in-plane X-ray diffraction (IPXRD) profiles of P(NDI2OD-T2) films processed with CN, oDCB, and CB. (b) Expanding (010) diffraction peak of (a). (d) Variation trend of edge-on and face-on orientation of P(NDI2OD-T2) films cast from CN, oDCB, and CB.

peak at 2θ = 6.4° in the in-plane direction, which represent the side chain stacking with different diffraction series, can be observed for the neat P(NDI2OD-T2) films. The intensities of (100) and (200) diffraction peaks also decrease remarkably with the processing solvent changing from CN, oDCB to CB, which indicates the increasing content of face-on molecular orientation in P(NDI2OD-T2) films. The solvent dependence of the (100) and (010) peak area is shown in Figure 3d. The effect of film drying time on the P(NDI2OD-T2) molecular orientation also was investigated. The ratio of edgeon and face-on orientation (Re/Rf) of the neat P(NDI2OD-T2) films processed by CN, oDCB, and CB was obtained by contrasting the peak area of (100) and (010) diffraction peak in the out-of-plane direction for P(NDI2OD-T2) films cast under different conditions (spin-coating and drop-coating). The results of this analysis are displayed in Table 2 and Figure 4a. On the basis of the value of Re/Rf, the preferential molecular orientation can be confirmed. When the Re/Rf value is larger than 1, edge-on is the dominant orientation. For films processed from CN, the Re/Rf value is greater than 1 and, as shown in Figure 4b, the face-on molecular orientation cannot Table 2. Ratio of Edge-On Orientation and Face-On Orientation (Re/Rf) of the Neat P(NDI2OD-T2) Films Processed in Different Conditions Cast with CN, oDCB, and CBa CN drop drop vac 800 rpm 1500 rpm 5000 rpm

142 139 32 17

oDCB

CB

2.46 0.82 0.52 0.28

1.14 0.23 0.13 0.13

Figure 4. (a) Ratio of edge-on orientation and face-on orientation (Re/Rf) of the neat P(NDI2OD-T2) films processed in different conditions cast with CN, oDCB, and CB. When the value is larger than 1, edge-on orientation will become the dominant orientation. (b) Distribution diagram of molecular orientation for P(NDI2OD-T2) films cast from CN, oDCB, and CB under different processing conditions. The solid squares and hollow triangles represent edge-on and face-on orientations, respectively.

form in P(NDI2OD-T2) films even after the spin-coating with high speed (5000 rpm). For the oDCB and CB films prepared by drop-coating, the value of Re/Rf is larger than 1, which

The absent values are due to either the unobtainable solid film or the extremely weak diffraction peaks. a

E

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ACS Applied Materials & Interfaces demonstrates the transformation of preferential molecular orientation from face-on to edge-on with the extended film drying time and confirms that the edge-on molecular orientation is the thermodynamically steady state of P(NDI2OD-T2) films. On the other hand, extensive chain aggregation in CB (Figure 2a), which will grow into nanofibers by chain collapse in the process of solvent evaporation and thus inhibit the diffusion of polymer chain, facilitates the formation of unsteady face-on molecular orientation in P(NDI2OD-T2) film. Therefore, chain aggregation and film drying time both play a key role in the determination of polymer molecular orientation. On the basis of the discussion above, a distribution diagram of molecular orientation for P(NDI2OD-T2) films is depicted and is shown in Figure 4b. The region filled with hollow triangles represents the face-on orientation. With extended film drying time or decreased chain aggregation, P(NDI2OD-T2) will adopt the edge-on orientation, which is shown with solid squares. The distribution diagram of molecular orientation can be a guide to acquire different molecular orientation directly by adjusting solution state and film drying time and therefore is useful for the polymer blend solar cells which require the careful adjustment of molecular orientation. 3.2. Molecular Orientation and Morphologies of Blend Films. When the PTB7-th and P(NDI2OD-T2) are mixed, the variation of molecular orientation for blend films is similar to those of neat PTB7-th and P(NDI2OD-T2) films processed by CN, oDCB, and CB. GIXRD and IPXRD were used to characterize the molecular orientation of blend films in the out-of-plane and in-plane directions, and the results are displayed in Figure 5a and Figure 5b. In the out-of-plane direction, three kinds of blend films all exhibit (010) diffraction peak at 2θ = 22.5°, which are the overlays of (010) diffraction peaks for neat PTB7-th and P(NDI2OD-T2) films. However, the (010) diffraction peaks of PTB7-th neat films are nearly invariant (face-on orientation) regardless of the properties of processing solvents (Figure 1c), and thus, the improved intensity of (010) diffraction peak for blend films can be attributed to the enhanced content of P(NDI2OD-T2) face-on molecular orientation with the processing solvents changing from CN, oDCB to CB. The (100) diffraction peak at 2θ = 4.6° can only be found in CN blend film, indicating that edge-on is the preferential molecular orientation of P(NDI2OD-T2) in blend film. In the in-plane direction, (100) diffraction peak at 2θ = 3.6° can be observed for CB and oDCB blend films and the intensity of former is larger than that of the latter. Therefore, in the blend films processed by CN, oDCB, and CB, the molecular orientation of PTB7-th keeps face-on while the molecular orientation of P(NDI2OD-T2) transfers from edgeon to face-on. The morphologies of blend films processed by CN, oDCB, and CB were characterized by the atomic force microscopy (AFM) and transmission electron microscopy (TEM). As shown in Figure 6, the CN film shows flat morphology in the AFM images. With the enhancing content of face-on molecular orientation, the oDCB and CB films display increasingly remarkable nanofibers with a length-scale of several tens of nanometers. Neher and co-workers have demonstrated that the molecular orientation of RR-P(NDI2OD-T2) influences the film morphology.34 The aligned polymer chains can be found in face-on film, while no features can be distinguished in the films with edge-on orientation. Therefore, we conclude that the transformation from a featureless morphology to nanofibers in

Figure 5. (a) Out-of-plane grazing incidence X-ray diffraction (GIXRD) and (b) in-plane X-ray diffraction (IPXRD) profiles of PTB7-th/P(NDI2OD-T2) (1:1) blend films cast with CN, oDCB, and CB.

the AFM images for blend films cast with CN, oDCB, and CB can be attributed to the molecular orientational variation of P(NDI2OD-T2) from edge-on to face-on. However, unsharp morphology can be observed in TEM images except some polymer aggregations in CN film, indicating that nanoscalephase-separated domains are formed in oDCB and CB films, while large phase domains exist in the CN film. In addition, the UV−vis absorption profiles of films processed by CN, oDCB, and CB were measured and the results are shown in Figure S3 (Supporting Information). However, no distinct difference can be observed for the blend films with various molecular orientations, suggesting the weak relationship between UV− vis absorption spectra and film morphology in PTB7-Th/ P(NDI2OD-T2) blend system. 3.3. The Uniform D/A Molecular Orientation Induced Improvement of Photovoltaic Performance. In order to investigate the relationship between the correlated donor/ acceptor molecular orientation and photovoltaic performance, devices processed by CN, oDCB, and CB with an optimized blend ratio of 1/1 (w/w) (see Figure S4 and Table S1, Supporting Information) were prepared according to standard ITO/PEDOT:PSS/PTB7-th:P(NDI2OD-T2)/Ca/Al architecture. The J−V curves are depicted in Figure 7a, and the summary of the device performance is tabulated in Table 3. From the J−V curves, we observe that Voc of three kinds of devices remain approximately unchanged (0.77−0.79 eV) because the Voc is largely determined by the highest occupied molecular orbital (HOMO) of the donor and lowest unoccupied molecular orbital (LUMO) of the acceptor.35−37 The value of Jsc has a gradual improvement from 1.24, 6.82 to F

DOI: 10.1021/acsami.5b07605 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. (1) AFM height images and (2) TEM images of PTB7-th/P(NDI2OD-T2) (1:1) blend films cast with (a) CN, (b) oDCB, and (c) CB.

8.86 mA/cm2 by changing solvents from CN, oDCB to CB, which leads to the corresponding PCE increasing from 0.53%, 2.73% to 3.52%. As mentioned above, large domain size formed in CN film will lower the Jsc and PCE by inhibiting exciton dissociation. However, the similar phase-separated morphologies of oDCB and CB films shown in AFM and TEM images exhibit different PCE, suggesting the device performance is weakly related to the blend morphology. The donor/acceptor molecular orientation is probably the crucial factor influencing device performance. Figure 7b summarizes the relationship among face-on peak area of P(NDI2OD-T2), Jsc, and PCE. The same change of Jsc and PCE indicates that it is the optimized Jsc that improves the PCE with increasing content of P(NDI2ODT2) face-on molecular orientation. Previous work38,39 has demonstrated that the Jsc is related to the exciton dissociation efficiency. Due to the less overlap of polymer molecular orbital in D/A interface with different molecular orientation (face-on/ edge-on), the electron can hardly delocalize to the donor or acceptor which leads to increasing geminate recombination and thus decreasing Jsc. When the D/A molecular orientation is identical (face-on/face-on), the electron can sufficiently delocalize to the donor and acceptor and the excitons can easily transfer to spatial free charge carriers, and therefore, the increasing Jsc can be found in CB device. Besides, the fill factor (FF) values decrease from 0.55 to 0.50 with the processing solvents changing from CN, oDCB to CB, which can be attributed to the larger shunt resistance (Rsh) of CN device compared to those of oDCB and CB devices. However, the consistent larger series resistance (Rs) of CN device leads to an undesired PCE. The opposite variation trend between Jsc and FF can be also observed for different kinds of polymer blend systems.40,41 In addition, EQE spectra of devices based on CN, oDCB, and CB blend films are given in Figure 8. All the spectra have the similar shape and composition in the wavelength range

of 300−800 nm. The maximum EQE can reach 50% for CB device which is larger than those of CN and oDCB devices, and the EQE results confirm that the same D/A molecular orientation is facilitating the enhancement of photovoltaic performance. To gain a deeper insight into the effects of molecular orientation on the charge carrier mobility, we measured the charge carrier mobilities in the PTB7-Th/P(NDI2OD-T2) devices processed by CN, oDCB, and CB. The hole mobility (μh) and electron mobility (μe) in the various blend films were measured and calculated using the space-charge-limited current (SCLC) method as shown in Figure S5 (Supporting Information) and Table 4. The lowest hole and electron mobilities (0.62 × 10−5 and 1.01 × 10−5 cm2/(V s)) can be observed in CN devices, indicating that the different molecular orientation of donor and acceptor can inhibit the charge carrier transporting to the electrodes. For the CB devices, the identical molecular orientation of donor and acceptor and increased face-on orientation content compared to oDCB devices can be observed, which facilitate the charge transport and reduce charge recombination. Consequently, the hole and electron mobilities improve to 8.40 × 10−5 and 6.20 × 10−5 cm2/(V s), respectively, for the CB devices. This further confirms the importance of D/A molecular orientation on the electrical properties in the all-polymer devices.

4. CONCLUSIONS The driving force associated with variation of molecular orientation and the relationship between the correlated D/A molecular orientation and photovoltaic performance for PTB7th/P(NDI2OD-T2) blend system were investigated. The preferential molecular orientation of neat PTB7-th films kept face-on regardless of the properties of processing solvents, while an increasing content of face-on molecular orientation in G

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Figure 8. External quantum efficiency (EQE) curves of devices based on PTB7-th/P(NDI2OD-T2) (1:1) blend films cast with CN, oDCB, and CB.

Table 4. Hole and Electron Mobility Values of PTB7-Th/ P(NDI2OD-T2) Devices Processed by CN, oDCB, and CBa solvent

μh (10−5 cm2/(V s))

μe (10−5 cm2/(V s))

CN oDCB CB

0.62 ± 0.10 3.90 ± 0.36 8.40 ± 0.69

1.01 ± 0.16 2.58 ± 0.26 6.20 ± 1.51

a

Average values and standard deviations are calculated from at least three devices.

processed by CN, oDCB, and CB improved from 0.53% to 3.52% ultimately. The correlated D/A molecular orientationdependent photovoltaic performance suggests that achieving uniform D/A molecular orientation by adjusting solution state or film drying time should be a promising way to promote the efficiency of all-polymer solar cells.

Figure 7. (a) J−V curves of devices based on PTB7-th/P(NDI2ODT2) (1:1) blend films cast with CN, oDCB, and CB under100 mW/ cm2 AM 1.5G illumination. (b) Variation trend of Jsc and PCE with the change of face-on peak area of P(NDI2OD-T2).

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the neat P(NDI2OD-T2) films could be found after spincoating different solutions by changing processing solvents from CN, oDCB to CB. Besides, the neat P(NDI2OD-T2) films also exhibited a transformation of preferential molecular orientation from face-on to edge-on when extending film drying time by casting in the same solution. Consequently, a distribution diagram of molecular orientation for P(NDI2ODT2) films was depicted associated with processing solvents and film drying time and the same variation trend could be observed for the PTB7-th/P(NDI2OD-T2) blend films. By manufacture of photovoltaic devices with PTB7-th/P(NDI2OD-T2) blend films, the relationship between the correlated D/A molecular orientation and performance was established. The value of Jsc for devices processed by CN, oDCB, and CB was enhanced gradually from 1.24 to 8.86 mA/ cm2 with the correlated D/A molecular orientation changing from face-on/edge-on to face-on/face-on, which could be attributed to facile exciton dissociation at D/A interface with the same molecular orientation. Therefore, PCE of devices

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07605. Absorption spectra of PTB7-th films in CN, oDCB, and CB; absorption spectrum of PTB7-th CN solutions for different temperatures; UV−vis absorption profiles of blend films processed by CN, oDCB, and CB; J−V curves and device parameters of devices based on PTB7th/P(NDI2OD-T2) films with different blend ratios cast from CB; measured space-charge-limited J−V characteristics under dark conditions for hole-only and electrononly devices processed by CN, oDCB, and CB (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-431-85262175. Fax: 86-431-85262126. E-mail: [email protected].

Table 3. Device Parameters of PTB7-th/P(NDI2OD-T2) Blend Films Cast from CN, oDCB, and CBa

a

solvent

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

Rsh (Ω·cm2)

Rs (Ω·cm2)

CN oDCB CB

0.79 ± 0.01 0.77 ± 0.01 0.78 ± 0.02

1.24 ± 0.16 6.82 ± 0.30 8.86 ± 0.17

0.55 ± 0.01 0.51 ± 0.02 0.50 ± 0.01

0.53 ± 0.07 2.73 ± 0.13 3.52 ± 0.11

2558 610 517

113 18 19

Average values and standard deviations are calculated from at least three devices. H

DOI: 10.1021/acsami.5b07605 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21334006), the National Basic Research Program of China (973 Program-2014CB643505), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB12020300).

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J

DOI: 10.1021/acsami.5b07605 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX