Application of Supramolecular Assembly of Porphyrin Dimers for Bulk

Sep 7, 2017 - Recently, there has been a growing interest in developing porphyrin derivatives as electron donor materials in solution-processed organi...
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Application of Supramolecular Assembly of Porphyrin Dimers for Bulk Heterojunction Solar Cells Fang-Chi Hsu,*,† Jian-Lun Chen,‡ Chiranjeevulu Kashi,§ Po-Wei Tsao,‡ Chen-Yu Yeh,*,§ Tai-Yuan Lin,‡ and Yang-Fang Chen∥ †

Department of Materials Science and Engineering, National United University, Miaoli 360, Taiwan Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan § Department of Chemistry and Research Center for Sustainable Energy and Nanotechnology, National Chung Hsing University, Taichung 402, Taiwan ∥ Department of Physics, National Taiwan University, Taipei 106, Taiwan ‡

S Supporting Information *

ABSTRACT: Recently, there has been a growing interest in developing porphyrin derivatives as electron donor materials in solution-processed organic solar cells. In contrast to the traditional synthesis route, we adopt a ligand-mediated supramolecular assembly approach to produce a new soluble porphyrin derivative. The complexation of nitrogen lone pairs in the bidentate ligands to the axial orbitals of both zinc atoms in zinc-metalated porphyrin dimers (KC2s) form KC2-duplex. The UV−vis absorbance of KC2-duplex displays a red-shift of the Q-band compared with that of KC2, indicating an improvement of intermolecular interaction. By blending KC2-duplex with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the photoactive material for fabricating organic bulk heterojunction solar cells, the devices demonstrate a 38.7% enhancement of short-circuit current density (Jsc) as compared to those made from dimers. The largely enhanced Jsc is attributed to the improved charge transport dynamics of KC2-duplex:PC71BM blend, including the hole and effective mobilities and exciton dissociation probability. When the photoactive film is processed from solvent containing 3% v/v 1-chloronaphthalene, Jsc is further enhanced (∼64.5%) as well as the fill factor (16.7%) for a power conversion efficiency of 3.06% from 1.63%. Our approach shown here can be generalized to other porphyrin-related systems to advance the development of porphyrin-based optoelectronic devices.

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conjugated polymers as donors at the laboratory scale.8 Exercising small molecules as donors has also become more attractive due to advantages of definite molecular structure, precise molecular weight, high purity, and good synthesis reproducibility as compared with polymers.9−11 The performance of small molecular BHJ OSCs has increased steadily with a PCE of over 8% in the recent years.12−15 To be a potential competitor to conjugated polymer donors, it is indispensable to develop new small-molecule donors to move toward the goal.

INTRODUCTION

Solution-processed organic solar cells (OSCs) have gained more and more attention because of their promising renewable energy sources and having the potential for mass production of flexible, lightweight, and cost-effective devices.1−4 In contrast to typical bilayer structure for silicon solar cells, OSCs utilize the bulk heterojunction (BHJ) architecture, in which organic donors and acceptors are intermixed to form a phase-separated blend, for the preparation of the photoactive layer. In the past few years, significant improvement has been achieved in the development of BHJ OSCs through the combination of materials design, morphology control, and fabricating techniques.3,5−7 A power conversion efficiency (PCE) value as high as 10.8% has been demonstrated for those devices using © XXXX American Chemical Society

Received: April 12, 2017 Revised: August 26, 2017

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DOI: 10.1021/acs.jpcc.7b03447 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

KC2 was added into the KC2 blend solution. For KC2duplex:PC71BM solution with additive, 3% v/v of CN was added. Sample Fabrication. Indium-doped tin oxide (ITO) glass substrates were cleaned by successively ultrasonicating in detergent, deionized water, acetone, and isopropyl alcohol for 20 min for each step and then dried in nitrogen gas flow. The prepared ZnO NPs solution was spin-coated on previously cleaned ITO glass substrate at 2000 rpm for 30 s, and the resulting ZnO film (∼30 nm) was dried in air for 1 day. Then, KC2 blend solution and KC2-duplex:PC71BM solutions with/ without additive were spin-coated on top of the ZnO layers as the photoactive materials, respectively. The resulting films were dried in N2-filled glovebox for 1 day and were of thickness of ∼80 nm. Those three kinds of devices were completed by thermally evaporating a 7 nm thick layer of MoO3 followed by a 100 nm thick Ag layer at a pressure of 1 × 10 −6 Torr through a shadow mask with a square opening of 0.04 cm2 to define the device area. For samples used in transmission electron microscopy (TEM) measurements, the active materials of ∼100 nm thickness were initially deposited on quartz substrates and then were submerged in deionized water (10 min) and floated onto the air/water interface. The films were picked up on unsupported 200 copper mesh grids. Characterization. The UV−visible absorption spectra were measured by using a JASCO Model V-630 UV−vis spectrophotometer. Electrochemistry (CV measurement) was performed with a three-electrode potentiostat (CH Instruments, Model 750A). UV−visible and NMR spectral titration measurements were conducted by Varian Cary 50 CONC and Varian 400 MHz spectrometers, respectively. Surface morphologies were recorded by using atomic force microscopy (AFM, Digital Instruments, Nanoscope III) and TEM (Jeol, JEM-1400 electron microscope). The current density−voltage (J−V) characteristics of the finished photovoltaic devices were evaluated in vacuum of 60 Torr by using a Keithley Model 2400 source meter under irradiation intensity of 100 mW/cm2 from a calibrated solar simulator (Newport Inc.) with an AM 1.5G filter. The calibration was done by using a standard Si photodiode. The incident-photon-conversion-efficiency (IPCE) spectra were performed using a setup consisting of a lamp system, a chopper, a monochromator, a lock-in amplifier, and a standard silicon photodetector (ENLI Technology). The hole mobility was measured by using the space-charge-limitedcurrent (SCLC) method. The thickness of the film was measured by a Veeco dektak 6M surface profiler. All measurements were conducted in the laboratory environment unless mentioned.

Inspired by natural photochemical reaction, in which chlorophylls absorb light and carry out photochemical charge separation to store photon energy, porphyrins, a subclass of small molecules that have the core structure analogous with chlorophylls, appear to be attractive as electron donors in fabrication of BHJ OSCs lately.16−22 Typically, porphyrins have extensive π-conjugated surfaces based on their macrocyclic framework, and several groups have tried to extend the πconjugation via adding conjugated units to the core17−20 or arranging porphyrin triads into a star shape.16,21 The highest PCE of devices of 8.08% has been achieved17 for the former case while that of 3.93% is for the latter one.21 Apart from those studies,16−21 we have recently reported a new type of soluble porphyrin derivative by linking two porphyrin units through alkyne links,22 which could permit the coplanar orientation of the units in order to expand π-conjugation effectively.23−25 The newly designed porphyrin dimers of elongated π-conjugation length can function as electron donors in OSCs, and the dimer with longer conjugated linker shows better PCE due to the higher carrier mobility.22 Thus, conjugation length plays an important role in charge transport and can affect the performance of OSCs. It is well-known that the formation of supramolecular assemblies is an effective way of arranging irregularly shaped molecules into an ordered manner.26 Anderson and coworkers27,28 have used the ligand-mediated supramolecular assembly to enforce a planar configuration of the butadiynelinked porphyrin oligomers resulting in enhanced conjugation length. A few groups have presented dye-sensitized bulk heterojunction solar cells containing liquid electrolyte using supramolecular complexes of porphyrin with C60 as light harvesters.29−34 However, we have not seen solid state thin film type BHJ OSCs fabricated with supramolecular assembly approach in the literature yet. Herein, we adopt 1,4-diazabicyclo[2.2.2]octane (DABCO) as the mediate ligand to trigger the supramolecular assembly of porphyrin dimer KC2 forming KC22DABCO2 (KC2-duplex). This KC2-duplex can serve as electron donor mixed with [6,6]phenyl C71 butyric acid methyl ester (PC71BM) as electron acceptor to form a BHJ for OSC fabrication. The approach of using supramolecular assembly in preparing donor material for solar cell application has not been seen in the literature yet. The KC2-duplex devices exhibit superior performance over KC2 ones. The device efficiency can be further enhanced when the KC2-duplex photoactive film is processed from solvent containing 3% v/v 1-chloronaphthalene (CN). This ligandmediated supramolecular assembly approach is facile and robust and can avoid the complex synthesis procedures to enlarge the molecular dimension as well as the conjugation length. We believe that this approach can be generalized to other porphyrin-related systems to advance the development of porphyrin-based optoelectronic devices.

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RESULTS AND DISCUSSION Figure 1 depicts the chemical structures of KC2 and DABCO as well as the simplified version of KC22DABCO2 supramolecular complex (KC2-duplex). The KC2 molecule is composed of two zinc−metalated porphyrin units covalently linked through butadiyne bridge while DABCO is a bidentate ligand of two nitrogen lone pairs. The distance between the nitrogen atoms is ∼3 Å.36 When KC2 and DABCO are intermixed in a molar ratio of 1:1, it is expected that the nitrogen free lone pairs in DABCO can form coordinate covalent bonds with both the zinc metal atoms in KC2 resulting in KC2-duplex as the simplified molecular structure shown only with the core of KC2 here for clarity.

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EXPERIMENTAL SECTION Material Preparation. Syntheses of KC2 and ZnO nanoparticles (ZnO NPs) were carried out according to refs 22 and 35, respectively. DABCO and PC71BM were purchased commercially from Acros and Aldrich, respectively. KC2 blend solution was prepared by dissolving KC2 and PC71BM (1:4 w/ w) in 1 mL of o-dichlorobenzene (ODCB) with a concentration of 40 mg mL−1, filtered through a 1 μm poly(tetrafluorothylene) (PTFE) filter. For KC2-duplex:PC71BM solution, the same molar ratio of DABCO as B

DOI: 10.1021/acs.jpcc.7b03447 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

changes of KC2 titrated with DABCO. The aromatic region of 1H NMR spectra showed upfield shift along with the presence of a singlet at −4.70 ppm, corresponding to sandwiched DABCO as the amount of DABCO increased from 0 to 1 equiv. The spectral changes were consistent with the progressive formation of the 2:2 complex (KC2-duplex). Continuous addition of DABCO caused the destruction of the 2:2 complex and the formation of a 1:2 complex as shown in Scheme S1. The results agree with the UV−vis titration of KC2 with DABCO and are also consistent with a similar study of a bisporphyrin system reported earlier.37 The details of the NMR measurements are provided in the Supporting Information. The electrochemical behavior of KC2 and KC2-duplex was investigated by cyclic voltammetry (see Figure S3). Highest occupied molecular orbital (HOMO) energy levels were determined from the onset potentials of oxidations using the following equation: HOMO = −(Eonset(ox) + 4.8) eV17,20,38 while lowest unoccupied molecular orbital (LUMO) energy levels were calculated from the onset potentials of reductions using the following equation: LUMO = −(Eonset(red) + 4.8) eV.17,20,38 The onset potentials of oxidations for KC2 and KC2duplex were estimated to be around +0.28 and +0.22 V, respectively, corresponding to HOMO energy levels of −5.08 and −5.02 eV for KC2 and KC2-duplex, respectively. The onset potentials of reduction for KC2 and KC2-duplex were observed at −1.65 and −1.70 V, corresponding to LUMO values of −3.15 and −3.10 eV for KC2 and KC2-duplex, respectively. As an effective photoactive material, KC2 or KC2-duplex (electron donor) was blended with PC71BM (electron acceptor) to form a BHJ structure. In this study, we keep the weight ratio of KC2:PC71BM at 1:4. This is because we have studied the effect of weight ratio on the device performance using a similar porphyrin dimer KC122 (see Figure S5). Devices based on KC1 mixed with PC71BM in different weight ratios, i.e., 1:1, 1:2, 1:3, 1:4, and 1:5, were tested, and the optimum performance was found at 1:4 ratio (see Figure S6 and Table S1). Because of the chemical structure similarity, we thus chose the best result for KC1 blend in our KC2:PC71BM system. Figure 3 shows the UV−vis absorbance of KC2:PC71BM, KC2-

Figure 1. Chemical structures of KC2, DABCO, and KC2-duplex supramolecular assembly (meso-aryl groups were removed for clarity).

To better understand the coordination between KC2 and DABCO, the solution of KC2 was titrated with DABCO, and the absorption spectral changes for the titration are shown in Figure S1. The Q absorption bands of KC2 were red-shifted with clear isosbestic points in the presence of 0−1.2 equiv of DABCO, indicating only two species exist, i.e., the uncoordinated KC2 and ladder-shaped assembly KC2-duplex. In the presence of excess DABCO, the supramolecular assembly KC2duplex was destructed and formed a 1:2 complex (KC21DABCO2). The coordination of KC2 with DABCO induced the formation of KC2-duplex and KC21DABCO2 complexes in the presence of 1 equiv and excess of DABCO, respectively, as depicted in Figure S2 and Scheme S1. The binding between KC2 and DABCO was also studied by NMR measurements. Figure 2 shows the NMR spectral

Figure 2. Spectral changes of the 1H NMR for KC2 titrated with DABCO at room temperature. ([KC2] = 1× 10−3 M, CDCl3, 400 MHz). C

DOI: 10.1021/acs.jpcc.7b03447 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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structure of the KC2-duplex was not destructed in the presence of PC71BM as the solvent was evaporated. Here, we believe that DABCO should have negligible interaction with PC71BM, and we would expect that almost all KC2 is coordinated with DABCO to give KC2-duplex, the main donor component in the blend. We then evaluated the performance of solar cells made from these three photoactive films. Figure 4a presents the typical current density−voltage (J−V) characteristics for each kind of device, and the corresponding performance parameters obtained from 20 batches of cells are summarized in Table 1. The average PCE for KC2:PC71BM device was (1.59 ± 0.03)% with the best value of 1.63%, and we chose this type of device as the reference cell. After DABCO was added, there was a significant enhancement in the device performance due to largely improved short-circuit current density (Jsc) (∼38.7%) for an overall 45% increment in PCE for the best device. When devices were processed from additive-contained solution, the PCE was further enhanced to 3.06% from a 64.5% enhancement in Jsc and a 16.7% improvement in fill factor (FF). The largely enhanced Jsc of KC2-duplex devices processed with/ without CN additive was also confirmed by the IPCE result shown in Figure 4b, showing a clear increment of IPCE values across the entire spectrum range of absorption. For each incident wavelength, a certain portion of photons were converted into charges to be collected. Comparing their absorption intensities, all three films had similar strength in the measurement range (see Figure 3). Therefore, charge transport dynamics play an important role in improving the solar cell performance. We conducted the space-charge-limited-current (SCLC) method41 to measure the hole mobility of those films. Holeonly device structure of ITO/PEDOT:PSS/photoactive films/ MoO3/Ag was prepared for the mobility measurements. The obtained J1/2−V characteristics (see Figure 5) can be described by the following equation:41 J = (9/8)εrε0μ[V2/d3], where J, εr, ε0, μ, d, and V stand for the current density, the relative permittivity of the material, the permittivity of free space, the carrier mobility, the thickness of the photoactive film, and the effective voltage, respectively. The hole mobility values calculated from the slopes of the linear fitting lines in Figure 5 were 2.0, 2.5, and 3.1 × 10−3 cm2/(V s) for KC2:PC71BM, KC2-duplex:PC71BM, and KC2-duplex:PC71BM (CN), respectively. The results show that the addition of DABCO enhances

Figure 3. UV−vis absorption spectra for KC2:PC71BM, KC2duplex:PC71BM, and KC2-duplex:PC71BM (CN) films deposited on ITO/ZnO substrates.

duplex:PC71BM, and KC2-duplex:PC71BM (CN) films prepared on ITO/ZnO substrates. As-prepared KC2:PC71BM film absorbed photons throughout the visible regime with different absorption strength. After DABCO was added, the absorption peak at 700 nm (Q-band) became pronounced accompanied by a red-shift of nearly 10 nm. The red-shift effect can be attributed to the enhanced intermolecular π−π stacking in the condensed solid state,39 which could be beneficial for improving hole mobility and photovoltaic performance of OSCs.40 When the film was processed from solution containing solvent additive CN, the characteristics of the spectrum were unaffected. To better understand the intermolecular interaction of KC2 with DABCO and PC71BM, the dimer KC2 was blended with PC71BM or/and DABCO with samples prepared in both solution and film states, and the experimental details are given in the Supporting Information. By comparing the UV−vis absorption spectra of various solutions (see Figure S4), it was found that there was no difference in the spectra characteristics between solutions KC2:DABCO:PC71BM (adding PC71BM into KC2:DABCO mixture) and KC2:PC71BM:DABCO (adding DABCO into KC2:PC71BM mixture). Both spectra features resembled the characteristics of KC2:DABCO (KC2-duplex). Therefore, KC2 and 1 equiv of DABCO formed a 2:2 complex, which was stable and not destructed even in the presence of PC71BM, no matter if PC71BM was added to the KC2 solution before or after DABCO. The same result was also obtained from their film states (see Figure S4). Both solution and film samples displayed similar behavior, indicating that the ladder

Figure 4. (a) Current density−voltage (J−V) characteristics and (b) IPCE spectra for KC2:PC71BM, KC2-duplex:PC71BM, and KC2duplex:PC71BM (CN) devices. D

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Table 1. Performance Parameters of KC2:PC71BM (As-Prepared), KC2-Duplex:PC71BM, and KC2-Duplex:PC71BM (CN) Devices under AM 1.5G Illumination at 100 mW/cm2a

a

KC2-x:PC71BM

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

as-prepared duplex duplex(CN)

6.28 ± 0.09 (6.20) 8.65 ± 0.19 (8.60) 10.16 ± 0.05 (10.20)

0.75 ± 0.02 (0.78) 0.76 ± 0.01 (0.78) 0.77 ± 0.00 (0.77)

33.72 ± 0.17 (32.50) 34.75 ± 0.68 (35.60) 38.86 ± 0.19 (39.10)

1.59 ± 0.03 (1.63) 2.28 ± 0.08 (2.39) 3.04 ± 0.02 (3.06)

The values in the parentheses denote the values for the best device.

the effective mobility of the blend. Accordingly, the effective mobility follows the order as-prepared < KC2-duplex:PC71BM < KC2-duplex:PC71BM (CN). The result implies that films containing DABCO form a more oriented interpenetrating network for transporting electrons and holes. With the aid of CN additive, the architecture of the network becomes more efficient for both types of carriers to hop through. Thus, the effective mobility of the film is improved in the presence of DABCO and in incorporating solvent additive CN. Figure 7 shows the TEM images of KC2:PC71BM, KC2duplex:PC71BM, and KC2-duplex:PC71BM (CN) films on unsupported 200 copper mesh grids, in which the bright and dark regions correspond to the donor- and PC71BM-rich domains, respectively.43 All these three films display different extents of phase separation. The KC2:PC71BM shows relatively larger phase separation, which may be the reason for the lower hole and effective device mobilities of the blend. After DABCO was included, the PC71BM-rich domains became much smaller (≤20 nm). As the KC2-duplex:PC71BM was processed from solution containing CN, its image is of the best phase separation among all, which corresponds to the measured high charge transport efficiency. The observed effect of nanomorphology change for film processed from solutions containing solvent additive is similar to those reported previously.21,46−52 Based on the study above, the enhanced device performance with the incorporation of DABCO can be understood as follows. Figure 8 depicts the energy band diagram of the fabricated devices. The LUMO and HOMO values for ITO, ZnO, PC71BM, MoO3, and Ag were taken from ref 44. It is shown that KC2 (or KC2-duplex) device forms cascaded band alignment for transporting electrons and holes. From the energy perspective, the bandgap energies of KC2 (1.93 eV) and KC2-duplex (1.92 eV) are similar as well as the theoretical values45 of Voc (HOMOKC2(‑duplex)−LUMOPC71BM) for solar cells based on both types of donors. However, there is a

Figure 5. J1/2−V characteristics of hole-only devices for as-prepared, KC2-duplex:PC71BM, and KC2-duplex:PC71BM (CN) films with the film thickness of 235, 241, and 250 nm, respectively.

the hole mobility, and the value can be further improved with solvent additive CN. The effect of DABCO also reflects in the carrier transient time across the electrodes, which can be determined from transient photocurrent measurement.42 Figure 6a demonstrates the short-circuit response of these three devices to 200 μs square-pulse optical excitation from a 536 nm green-light LED driven by a matching driver. Photocurrent is displayed as current instead of current density because of a smaller illumination spot size compared with devices. A faster turnon and -off photocurrent dynamics was observed for those devices. We determined the effective transient time from the light-off cycle by calculating the period of time required for photocurrent to decay to 1/e of its equilibrium value (Figure 6b). The obtained effective transient times were 3.95, 2.52, and 1.42 μs for as-prepared, KC2-duplex:PC71BM, and KC2duplex:PC71BM (CN) devices, respectively. Since the effective transient time reveals the collective effect of electron and hole traveling between two electrodes, it can serve as an indicator for

Figure 6. (a) Transient photocurrent response for as-prepared, KC2-duplex:PC71BM, and KC2-duplex:PC71BM (CN) devices. (b) Normalized transient photocurrent in the light-off cycle. E

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Figure 7. TEM images of (a) KC2:PC71BM, (b) KC2-duplex:PC71BM, and (c) duplex:PC71BM(CN) films.

separation and charge-carrier collecting. As obtained, the TEM image of KC2:PC71BM film shows relatively larger phase separation whereas KC2-duplex:PC71BM (CN) displays better donor−acceptor mixing outcome. Based on the morphological analysis, we would expect that the exciton separation efficiency is improved with the incorporation of DABCO and solvent additive CN in preparing the photoactive layers, and the improvement can contribute to the enhancement of Jsc. In addition to the astonishing effect of DABCO on Jsc, there is also a change in FF, and the change becomes pronounced when the photoactive film is processed from solvent containing 3 vol % additive. It is known that, generally, the improvement of FF is due to reduced carrier recombination, lower series resistance (Rs), and enhanced shunt resistance (Rsh). We calculated Rs and Rsh of each device by taking the reciprocal of the slopes at Voc and Jsc from the measured J−V curves, respectively. The obtained values for Rs were 37.4, 25.1, and 19.4 Ω cm2 for as-prepared, KC2-duplex:PC71BM, and KC2duplex:PC71BM (CN) devices, respectively, while Rsh values were similar (200 Ω cm2) for these three devices. Apparently, the presence of DABCO (or CN) will not produce any leakage path as Rsh remains similar. However, the microscopic variation in the mixing result of BHJ can result in distinct bulk resistance. The value of Rs follows the order KC2-duplex:PC71BM (CN) < KC2-duplex:PC71BM < KC2:PC71BM. The incorporation of DACBO reduces the bulk resistance of KC2:PC71BM, which in turn facilitates the charge transport throughout the photoactive layer. With solvent additive, more efficient charge transport interpenetrating network is formed resulting in the lowest bulk resistance for its highest effective mobility value among all. The higher effective carrier mobility results in shorter carrier transient time and lower charge transport resistance, which in turn lower the probability for carrier recombination, hence raising FF. Poly(3-hexylthiophene) (P3HT) is a well-known polymer, and solar cells based on P3HT:PC71BM have been intensively studied for the past decade.55 The highest PCE of devices made from the well-known polymer blend P3HT:PC71BM is 4.7% from Jsc = 10.8 mA/cm2, Voc = 0.63 V, and FF = 0.69.56 Comparing our devices with P3HT:PC71BM ones, ours show relatively lower PCE due to much lower FF (0.39). Since P3HT and KC2 (or KC2-duplex) belong to different classes of organic materials, their materials properties, including optical and electrical characteristics, are quite different. For example, KC2-based donors show very weak ability in absorbing greento-red photons whereas P3HT also show strong absorption activity.56 Encouragingly, Jsc obtained from the KC2-based donor devices (10.2 mA/cm2) is close to that of P3HT ones. Since many factors can affect the performance of a device, we would expect that KC2 (or KC2-duplex) could be a potential

Figure 8. Energy band diagram for the fabricated solar cells.

dramatic difference in the device performance primarily due to the increment of Jsc. The significant improvement in Jsc in the presence of DABCO can be resolved from both optical and electrical aspects. Optically, there is approximately 3% enhancement in the absorbed photon flux in the wavelengths of 350− 800 nm for films containing DABCO. This subtle difference is unlikely to produce an extra 64.5% charge for collection. The amplification effect can be attributed to the enhanced charge transport dynamics. As evidenced by the red-shift of the Qband in the UV−vis absorption spectra, the π−π interaction of KC2 molecules of KC2-duplex is stronger than that of KC2. This property is presumably due to the short DABCO ligands binding to a KC2 molecule in parallel, which can decrease rotational flexibility of KC2 via the butadiyne bridge resulting in a well-oriented, nearly coplanar duplex molecule and hence the enhanced molecular coupling. For the corresponding BHJ blends studied by SCLC and transient photocurrent methods, the hole and effective mobilities follow the same order KC2:PC71BM < KC2-duplex:PC71BM < KC2-duplex:PC71BM (CN). The result suggests that KC2-duplex of enhanced π−π interaction results in better charge transport properties of the BHJ in comparison with KC2, and the charge transport properties can be further enhanced through tuning the architecture of the interpenetrating bicontinuous network by incorporating solvent additive CN during processing. The superior electrical properties of KC2-duplex molecule itself as well as the resulting blend over KC2 lead to the improvement of Jsc and PCE. On the other hand, in addition to carrier mobility, exciton dissociation efficiency in the photoactive material can contribute to Jsc as well. By analyzing the AFM images (see Figure S7), the surface roughnesses of KC2:PC71BM and KC2duplex:PC71BM are seen to be similar (