Ternary Organic Photovoltaics Prepared by Sequential Deposition of

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Ternary Organic Photovoltaics Prepared by Sequential Deposition of Single Donor and Binary Acceptors Yunju Cho, Thanh Luan Nguyen, Hyerim Oh, Ka Yeon Ryu, Han Young Woo, and Kyungkon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07199 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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OPV utilizing the same donor and acceptors. Due to these complementary effects, the SQ ternary OPV exhibited the power conversion efficiency of 6.22%, which was 52% and 37% higher than that of the SQ binary OPV and the SS ternary OPV, respectively. In addition, the thermal stability of the SQ ternary OPV was found to be superior to that of the SS ternary OPV. KEYWORDS: ternary organic solar cell, sequential solution deposition, non-fullerene acceptor, binary acceptor, thermal stability.

INTRODUCTION Solution-processed organic photovoltaics (OPVs) have received significant attention due to several advantages such as versatile chemical modification to tune the optoelectronic properties, light weight, low-cost manufacturing via solution processing and roll-to-roll fabrication on the flexible substrate. The photo-active layers of OPVs are usually fabricated by single-step (SS) deposition of blend solutions composed of a one-donor and one-acceptor organic semiconductor (SS binary OPVs). Recently, ternary OPVs prepared using a ternary blend solution consisting of a binary electron donor and a single electron acceptor (SS ternary OPVs) have been intensively studied.1-4 The use of binary donors with complementary absorption ranges can enhance the photocurrent generation of SS ternary OPVs by broadening the light absorption range. More recently, small band gap non-fullerene acceptors that absorb longer wavelengths than phenyl-C71-butyric acid methyl ester (PCBM) have been developed.5-9 These non-fullerene acceptors have several advantages over fullerene derivatives, such as higher versatility in tuning frontier orbital energy levels and corresponding band gaps. A representative example 2 ACS Paragon Plus Environment

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of a non-fullerene acceptor for OPVs is 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2,3'-d']-s-indaceno[1,2-b:5,6b']dithiophene (ITIC).10-12 Since ITIC has a narrow band gap and a high lowest unoccupied molecular orbital (LUMO) level compared to fullerene derivatives, the use of ITIC as an acceptor can enhance the short-circuit current density (JSC) and open-circuit voltage (VOC) in OPVs. The development of non-fullerene type acceptors enabled the fabrication of efficient SS ternary OPVs using ternary blend solutions consisting of a binary acceptor and a single donor.13-17 Compared to SS binary OPVs utilizing only PCBM acceptor, SS ternary OPVs can further improve the JSC and VOC by complementary light absorption and the higher LUMO of non-fullerene acceptors. Although many successful demonstrations of SS ternary OPVs have been reported, the use of a ternary blend of organic semiconductors does not always result in improved performance. Solution casting of a ternary blend does not always result in the formation of a bulk heterojunction (BHJ) structure with a nanoscale bi-continuous charge transport path, which is critical to the performance of the OPV. 18, 19 The addition of the third component requires more delicate control of the fabrication conditions. For example, planar structured donor materials can exhibit strong π-π interactions with the non-fullerene acceptor ITIC, and more complicated intermolecular interactions are expected to occur between three molecules. Second, in contrast to PCBM, which exhibits isotropic electron transport capability due to its symmetric structure, the orientation of ITIC strongly affects electron transfer and transport due to its non-symmetric structure. The chemical structure and orientation of the donor materials can influence the orientation of ITIC. Recently, the preparation of OPVs by the sequential deposition of donor and acceptor 3 ACS Paragon Plus Environment

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solutions (SQ binary OPVs) provided an alternative way of preparing solution processed OPVs.20-25 The morphological control and device preparation of SQ binary OPVs allows greater flexibility, because each layer of a bilayer device can be controlled independently. In addition, the morphological stability of SQ binary OPVs was found to be superior to that of SS binary OPVs.26, 27 The sequential deposition process can be a superior method for the fabrication of ternary OPVs. Morphologically controlled polymer layer is deposited first, followed by the successive deposition of the binary acceptor. This makes the SQ ternary OPVs less sensitive to the fabrication conditions and enables independent control of the single donor and binary acceptor layers. In this work, we report a new type of ternary OPV device fabricated by sequential deposition of a single donor and binary acceptors (SQ ternary OPV). The mid band gap poly[(4,4'-bis(2-butyloctoxycarbonyl-[2,2'-bithiophene]-5,5-diyl))-alt-(2,2'-bithiophene-5,5'diyl)] (PDCBT) polymer was used as the donor, and PCBM and ITIC were used as the acceptors. The SQ ternary OPV exhibited superior photovoltaic performance compared to the SQ binary and SS ternary OPVs. In addition, the SQ ternary OPV also exhibited better thermal stability compared to SS ternary devices because of higher morphological stability of bilayer device structures. EXPERIMENTAL METHODS Materials The chemical structures of PDCBT, PCBM and ITIC are shown in Figure 1(a). We synthesized PDCBT using the procedure outlined in a previous report.28 The average molecular weight and polydispersity index of the polymer were 2.4 kDa and 2.0, respectively. 4 ACS Paragon Plus Environment


The

poly(3,4-ethylenedioxythiophene)

poly(styrene-sulfonate

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(PEDOT:PSS)

solution

(Heraeus Clevios P VP AI 4083), PCBM (EM Index, Korea), and ITIC (1-Material, Canada) were purchased and used as received. The solvents chlorobenzene (CB) (Tokyo Chemical Industry, Japan), dichloromethane (DCM) (Tokyo Chemical Industry, Japan), and diiodomethane (DIM) (Tokyo Chemical Industry) were used as received. Device Fabrication We fabricated OPVs with a conventional indium tin oxide (ITO)/PEDOT:PSS/photo-active layer/LiF/Al structure. Pre-patterned 20 Ω/A resistive ITO glass substrates were sequentially washed in isopropyl alcohol, acetone, and isopropyl alcohol for 10 min each under ultrasonication. After washing, the glasses were dried in a convection oven at 80 °C. The dried glasses were then exposed to UV ozone for 20 min. The PEDOT:PSS solution was mixed and vortexed with methanol at a 1:1 ratio. The solution was subsequently spin-coated onto the top of the cells, and the cells were dried in a vacuum oven at 110 °C for 10 min. For the fabrication of the SQ binary OPV, a 16 mg/mL solution of PDCBT in chlorobenzene was spin-coated onto the substrate at a speed of 2000 rpm for 60 s, followed by drying in a vacuum chamber for 1 h. Subsequently, the PCBM or ITIC solutions were prepared by dissolution in a solvent composed of 99 wt.% DCM and 1 wt.% DIM with concentrations of 6 mg/mL and 4 mg/mL, respectively. We utilized DIM to reduce the vapor pressure of DCM and enhance the film quality. The PCBM solution or ITIC solution was spin-coated on the PCDBT layer at a speed of 2000 rpm for 30 s. For the fabrication of the SQ ternary OPV, binary solutions of various ratios of the acceptors PCBM and ITIC were deposited on the PDCBT layer (Figure 1(b)). The binary acceptor solutions with different PCBM:ITIC ratios were prepared by dissolving the 5 ACS Paragon Plus Environment

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acceptors in a co-solvent composed of 99 wt.% DCM and 1 wt.% DIM. ITIC weight ratios of 30, 50, and 70% were used in the binary acceptor solutions, with the total concentration being maintained at 7 mg/mL. The binary acceptor solution was spin-coated onto the PDCBT layer at a speed of 2000 rpm for 30 s. For comparison, SS binary OPVs were fabricated through SS deposition of blended solutions consisting of PDCBT:PCBM (1:1 wt/wt) or PDCBT:ITIC (1:1 wt/wt). Furthermore, SS ternary OPVs were fabricated through the SSs deposition of ternary blended solutions of PCDBT:PCBM:ITIC with different PCBM:ITIC ratios (Figure 1(b)). Finally, lithium fluoride (LiF) and Al electrodes were deposited on the prepared films by thermal evaporation at a rate of ~0.5 nm/s and with a thickness of 100 nm. Both LiF and Al were thermally evaporated at ~3 × 10−6 Torr through a shadow mask. The areas of the devices were 0.20 cm2. Characterization The current density and voltage of the OPVs were measured using a Keithley 2400 Source Meter under AM 1.5 G irradiation (100 mW/cm2) with a 150 W Xenon lamp-based solar simulator (McScience, South Korea). The external quantum efficiency (EQE) of the OPVs was measured monochromatically using a K3100 EQX IPCE measurement system (McScience, South Korea) with a 300 W Xenon lamp. The photoluminescence (PL) spectra were recorded on a HORIBA Fluorolog 3-11 spectrofluorometer. The ultraviolet-visible (UVVis) light absorption spectra were obtained using a UV-2450 (Shimadzu, Japan) spectrophotometer. Grazing incidence X-ray diffraction (GIXRD) measurements were performed at the PLS-II 9A U-SAXS beamline of the Pohang Accelerator Laboratory (Korea). All the films for GIXRD analysis were spin-coated on PEDOT:PSS-coated Si substrates. 6 ACS Paragon Plus Environment


The thermal stability of devices was tested on a hot plate set at 80ºC in the nitrogen filled glovebox. RESULTS AND DISCUSSION Since the DCM used for dissolving the PCBM is orthogonal solvent, the PCBM solution would not dissolve out the polymer bottom layer during the sequential deposition process. As shown UV-vis absorption spectra of the polymer only, the SQ binary and ternary blend films, all SQ films showed higher absorption intensity than the polymer only film, indicating that the polymer was not removed during the sequential process. Instead of dissolving the polymer bottom layer, the PCBM solution will swell polymer bottom layer during the sequential process, which will provide the formation of an efficient BHJ morphology without thermal annealing. Figure 2(a) shows the UV-vis absorption spectra of the SQ binary and ternary blend films. The SQ binary PDCBT/PCBM film absorbed light from 300 to 650 nm with a maximum absorption wavelength of 550 nm. The absorption range of the SQ binary PDCBT/ITIC film was extended up to 800 nm, but the absorption at wavelengths between 300 and 500 nm was weaker than that of the SQ binary PDCBT/PCBM film. In contrast, the SQ ternary PDCBT/PCBM:ITIC films showed a combined broad light absorption covering a whole visible range from 300 to 800 nm. As the fraction of ITIC in the SQ ternary film was increased, the absorption intensity at longer (650 – 700 nm) and shorter (300 – 550 nm) wavelengths increased and decreased, respectively. The exciton dissociation efficiency was investigated by measuring the PL quenching of the films. All films were excited at a wavelength of 550 nm, which can excite both PDCBT and ITIC. The PDCBT-only and ITIC-only films exhibited maximum PL emission at wavelengths of 650 and 770 nm, respectively. To normalize the PL intensity, the PL spectra were divided by the absorption intensity of the corresponding films at the excitation wavelength. The SS 7 ACS Paragon Plus Environment

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binary PDCBT:PCBM and PDCBT:ITIC films exhibited high PL quenching efficiencies of 96.7 and 99.3%, respectively (Figure S1 in Supporting Information). The sequentially deposited (SQ) binary PDCBT/PCBM and PDCBT/ITIC films also showed high PL quenching efficiencies of 93.5 and 98.2% relative to pristine PDCBT PL emission, respectively. The PL of the SQ ternary films was almost completely quenched. For example, the SQ ternary PDCBT/PCBM:ITIC (1:1, wt/wt) film exhibited a PL quenching efficiency of approximately 99% (Figure 2(b)). Interestingly, ITIC emission was observed for all the films containing ITIC. The intensity of the ITIC emission of the SQ binary PDCBT/ITIC film was approximately six times stronger than that of the SS binary PDCBT:ITIC film (Figure S1 in Supporting Information). The PL intensity of the ITIC in the SQ ternary films increased with the amount of ITIC in the film (Figure 2(b)). Two possible mechanisms for the ITIC emission can be suggested. In the first, excitons in the ITIC layer that were formed away from the PDCBT/ITIC interface may have radiatively recombined without transferring holes to the PDCBT. In the second, emission from the recombination of excitons in the PDCBT layer away from the PDCBT/ITIC interface may have been reabsorbed by ITIC through Förster resonance energy transfer (FRET), because the emission of PDCBT (650 nm) overlapped with the absorption wavelength range of ITIC (500-750 nm). The PL quenching experiments indicated that a larger fraction of the PDCBT and ITIC domains present in the SQ binary PDCBT/ITIC film had domain sizes greater than the exciton diffusion length, with compared to SS binary PDCBT/ITIC blend. The creation of large polymer and ITIC domains during the sequential deposition process could be ascribed to the following factors. First, ITIC dissolved in volatile DCM may penetrate inefficiently into the polymer layer during the sequential process. Secondly, it is possible that DCM could partially dissolve the polymer and instantaneously

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form a blend solution of polymer and ITIC, followed by the formation of large domains during the evaporation of DCM. We conducted GIXRD experiments to investigate the morphology of the binary and ternary blend films. As shown in Figure 3 and Figure S2 in Supporting Information, the pristine PDCBT film showed an edge-on orientation with an out-of-plane (100) lamella peak at qz = 0.30 Å−1 (d = 20.9 nm). Moreover, the second- and third-order scattering peaks were also observed at qz = 0.60 Å−1 and 0.90 Å−1, respectively (Figure 3(a)). Together with out-of-plane lamella scatterings, a strong in-plane (010) π−π stacking peak was observed at qxy = 1.71 Å−1 (d = 3.67 nm), suggesting a preferential edge-on orientation of polymer chains. With regard to pristine ITIC, a lamellar diffraction peak was observed along the in-plane direction at qxy = 0.31 Å−1 (d = 20.3 nm) (Figure 3(b)). The SQ binary PDCBT/ITIC film exhibited out-of-plane lamella peaks corresponding to PDCBT at qz = 0.24 Å−1 (d = 26.2 nm), and to ITIC at qz = 0.36 Å−1 (d = 17.5 nm) (Figure 3(a)). In the in-plane direction (Figure 3(b)), a strong (010) peak at qxy = 1.73 Å−1 was observed from the π−π stacking of PDCBT polymers in the SQ binary PDCBT/ITIC film. This revealed that PDCBT was oriented in an edge-on fashion, indicating that the sequential deposition of ITIC on PDCBT did not alter the orientation direction of PDCBT. This reflected the inefficient penetration of ITIC into the PDCBT layer, which was in good agreement with the results of the PL quenching experiments. The edge-on orientation is disadvantageous for charge transport along the vertical direction. Additionally, the SQ binary PDCBT/PCBM film exhibited a weaker π-π stacking peak (than pristine PDCBT film) along the in-plane direction (Figure 3(b)), suggesting that the penetration of PCBM into the PDCBT layer disturbed the orientation of PDCBT and reduced the fraction of edge-on oriented PDCBT. This is relatively beneficial for charge transport in the vertical direction in an OPV device. The fraction of 9 ACS Paragon Plus Environment

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edge-on oriented PDCBT was inversely proportional to the amount of PCBM in the SQ ternary PDCBT/(PCBM:ITIC) films (Figure 3(b)). Therefore, the SQ ternary films were expected to show better charge transport properties than the SQ binary PDCBT/ITIC films. We fabricated SS binary and ternary OPVs as control devices (Figure S2 and Table S1 in the Supporting Information). The SS binary OPV based on PDCBT:ITIC exhibited higher VOC and JSC (VOC = 0.92 V, JSC = 11.15 mA cm−1) than the SS binary OPV based on PDCBT:PCBM (VOC = 0.86 V, JSC = 2.41 mA cm−2), because ITIC absorbs more photons at longer wavelengths and has a higher LUMO energy level than PCBM. However, the fill factor (FF) of the SS binary OPV based on PDCBT:ITIC (0.41) was lower than that of the SS binary OPV based on PDCBT:PCBM (0.60). As a result, the power conversion efficiency (PCE) of the SS binary OPV based on PDCBT:ITIC (4.19%) was higher than that of the PDCBT:PCBM OPV (1.24%). The solar cell performances of the SS binary OPVs were lower than previously reported values because we did not use 1,8-diiodooctane (DIO), which induces donor-acceptor phase separation at the nanoscale.28 Since the SQ binary and ternary OPVs were fabricated without using DIO, we also fabricated the control SS binary and ternary OPVs samples without using DIO. In addition, DIO is non-volatile and is known to be difficult to remove, and thus, its residue would influence the stability of the devices. There is another report showing achievement of over 10% PCE from SS binary OPV based on PDCBT:ITIC without using additive.29 However, the device required a thermal annealing process at 160 °C for 10 minutes to achieve the high efficiency, which is difficult process to apply to plastic substrates. Though, a PCE of 8% was obtained even for a non-annealed device. This may be due to differences in solvent, molecular weight of polymer and interlayers. Especially, they claimed that fast evaporating chloroform solvent enhanced ordering of polymer. In this work, slow evaporating chlorobenzene was used as a polymer



solvent in consideration of large area coating. The PCE of the OPV device was slightly improved when the ternary blended PDCBT:PCBM:ITIC solution was used. The best PCE was observed for the SS ternary OPV with a PDCBT:PCBM:ITIC ratio of 1:0.5:0.5 by weight. The SS ternary OPV showed a PCE value of 4.54%, which was mainly due to its increased JSC (11.46 mA/cm2), compared to the SS binary OPV based on PDCBT:ITIC. The JSC value of the SS ternary OPV was slightly higher than that of the SS binary OPV with PDCBT:ITIC because the external quantum efficiency (EQE) value of the SS ternary OPV at ~400 nm was higher due to the additional absorption of PCBM (Figure S2b in the Supporting Information). Since the VOC is known to be determined by the difference between the donor HOMO and acceptor LUMO energies, the SS ternary OPV was expected to have a lower VOC than the SS binary OPV with PDCBT:ITIC. However, the SS ternary OPV showed the same VOC as the SS binary OPV, which had a positive influence on the PCE of the SS ternary OPV. Nevertheless, the PCE of the SS ternary OPV did not increase significantly compared to the SS binary OPV because the FF of the SS ternary OPV (0.43) was not improved. We assumed that the imbalance between the charge carrier mobilities of ITIC and PDCBT due to the unfavorable BHJ formation was related to the low FF of the SS ternary OPV. Since three different materials with different surface energies are blended in SS ternary OPVs, the evolution of an efficient BHJ with nanoscale phase separation is highly sensitive to processing conditions such as temperature, additives, and solvents. Thus, even SS ternary OPVs with proper energy level alignment for solar cell operation may not exhibit good performance. The solar cell performance of the SQ binary and ternary OPVs was investigated (Figure 4(a) and Table 1). The SQ binary OPVs showed similar trends to the SS binary OPVs. The PDCBT/ITIC-based SQ binary OPV exhibited higher VOC and JSC (VOC = 0.93 V and JSC = 11 ACS Paragon Plus Environment

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7.97 mA cm−1) and a lower FF (0.55) than the PDCBT/PCBM based SQ binary OPV (VOC = 0.79 V, JSC = 6.09 mA cm−2 and FF = 0.73). In contrast to the SS ternary OPV, the SQ ternary OPV showed a significantly higher PCE than the SQ binary OPVs. For example, the SQ ternary OPV based on PDCBT/PCBM:ITIC (1:1, wt/wt) exhibited the best PCE of 6.22%, with VOC = 0.87 V, JSC = 10.12 mA cm−2, and FF = 0.71. This PCE value was 76 and 52% higher than that of the SQ binary OPVs based on PDCBT/PCBM and PDCBT/ITIC, respectively. The improvement was the result of the improvement in JSC and the VOC while maintaining the FF. As shown in the EQE spectrum in Figure 4(b), the overall shapes of the EQE spectra were dependent on the PCBM:ITIC ratio in the SQ ternary OPV. The EQE values at wavelengths from 650 to 760 nm and from 300 to 550 nm increased as the fractions of ITIC and PCBM in the SQ ternary OPV were increased, respectively. Among the SQ ternary OPVs, the SQ ternary OPV based on PDCBT/PCBM:ITIC (1:1, wt/wt) exhibited relatively uniform EQE values over the wavelength range of 300 to 800 nm, resulting in the highest JSC. The trend in the EQE spectra was in good agreement with the trend in the absorption spectra. Interestingly, the VOC values of the SQ ternary OPVs were linearly dependent on the fraction of ITIC (Figure 5(a)). The VOC tendency of the SQ ternary OPVs was different from that of the SS ternary OPVs, which showed a constant VOC regardless of the ITIC concentration (Table S1 in Supporting Information). Based on the reported VOC behaviors of the ternary OPVs, it can be classified into the following four cases: Case 1. VOC of ternary cells is determined by the smallest difference between HOMO (highest occupied molecular orbital) energy level of electron donor and LUMO (lowest unoccupied molecular orbital) energy level of electron acceptor, which is usually observed in ternary OPVs.;1 Case 2. VOC is linearly depending on the portion of one acceptors (or one donors) in the binary acceptors (or donors) implying that each acceptor (or donor) forms their own independent transport network (parallel-like model).;30 Case 3. Donors or acceptors are intimate 12 ACS Paragon Plus Environment


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intermixed together, and they become electronically coupled to form a new work function state (alloy model). When a ternary OPV follows the alloy model, the VOC of the ternary OPV does not depends on ratio of donors (or acceptors);32 Case 4. VOC is determined by the largest difference between HOMO energy level and LUMO energy level among the ternary blended materials, which is an unusual case.2 The SQ ternary OPV corresponds to the Case 2 (parallel-like model) whereas the SS ternary OPV corresponds to the Case 4. Based on their VOC behavior, SQ ternary OPVs seemed to be composed of two single-junction OPVs connected in parallel, since the weighted average of the individual voltages of the sub-cells determines the voltage of a parallel circuit. Liu et al. and Yang et al. suggested that each of the acceptor(s) (or donor(s)) would form their own independent transport network and, resulting in a linear dependence between VOC and the concentration of the acceptor (or donor).30,

31

According to their

explanation, the dissociated electrons in PDCBT are transferred to both acceptors and transported to the electrode via their own domains, and no electron transfer occurs between the two acceptors. Similar PL quenching behavior was observed for a binary film consisting of PCBM and an ITIC analogue. Furthermore, the ternary OPV based on the ternary blend of a conjugated polymer and those two acceptors exhibited similar VOC behavior. 30 We investigated the influence of the ITIC ratio on the charge transport and recombination of the SQ ternary OPV by plotting JSC as a function of the light intensity. JSC is known to have a power-law dependence on light intensity (I), which can be represented as JSC ∝ (I)S, where S is an exponential factor. As S approaches 1, bimolecular recombination is weaker.33 As shown in Figure 5(b), the extracted S value of the SQ binary OPV prepared with PCBM as the acceptor was close to 1, but that of the SQ binary OPV prepared with ITIC was 0.972, which represented a significant deviation from 1. The S values of the SQ ternary OPVs 13 ACS Paragon Plus Environment

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prepared with the acceptors PCBM and ITIC were between these two values. As the fraction of ITIC in the acceptor solution increased, the S value of the SQ ternary OPV deviated further from 1, reaching 0.977 at 70% ITIC, which was similar to that of the SQ binary OPV with the ITIC. This indicated that increasing the ratio of ITIC in the acceptor solution increased the probability of bimolecular recombination in the SQ ternary OPV. Based on the GIXRD and J-V analyses, the imbalance in charge carrier mobility and the unfavorable orientation of PDCBT caused by the addition of ITIC increased the bimolecular recombination in the SQ ternary OPV. This resulted in a significant reduction in the FF of the SQ ternary OPV based on PDCBT/PCBM:ITIC (3:7, wt/wt). In contrast, the S value (0.994) of the SQ ternary OPVs based on PDCBT/PCBM:ITIC (7:3, wt/wt) was similar to that of the SQ binary OPV based on PDCBT/PCBM. The S value of the SQ ternary OPVs decreased slightly to 0.984 when the concentration of ITIC in the acceptor solution was 50% (PDCBT/PCBM:ITIC (1:1, wt/wt) based SQ ternary OPV). However, the SQ ternary OPVs exhibited a higher FF value of 0.71, which was similar to the FF value of the SQ binary OPV based on PDCBT/PCBM. The above results indicated that charge transport and bimolecular recombination were not affected until the weight ratio of ITIC in the acceptor solution reached 50%. However, the SS ternary OPV did not show as high an FF as the SQ ternary OPV. This reflected the difficulty in forming a BHJ with nanoscale phase separation in the SS ternary OPV where three components are mixed in a single layer. In contrast, the sequential process can reduce the complexity of finding optimal conditions because the donor and acceptor materials are deposited sequentially. One more remarkable point is that the SQ ternary OPV is more thermally stable than the SS ternary OPV. The SQ ternary OPV maintained 50% of its initial PCE after annealing at 100 °C for 100 h, while the SS ternary OPV maintained only 25% of its initial PCE (Figure 6). The SQ ternary OPV showed an 8% reduction in VOC, whereas the SS ternary OPV 14 ACS Paragon Plus Environment


showed a 35% reduction, reflecting the instability of the SS ternary OPV morphology under thermal annealing. We previously reported that the morphological stability of SQ deposited OPVs is superior to that of the SS deposited OPV because the domains of PTB7 and PCBM formed by sequential deposition are purer than those formed by SS deposition.26

CONCLUSIONS In conclusion, solar cell performance was greatly improved by fabricating SQ ternary OPVs by the sequential deposition of PDCBT and PCBM:ITIC solutions. Compared to the SQ binary OPVs, the SQ ternary OPVs exhibited enhanced JSC and VOC without reduction in the FF. We did not observe an improvement in the PCE of SS ternary OPVs fabricated via conventional SS deposition of ternary blend solutions. Although the ideal morphology control is very tricky in the typical SS ternary blends, the SQ ternary devices suggest a simple and facile control of morphologies by modulating the single donor and binary acceptor blend layers independently. In addition, the thermal stability of the SQ ternary OPV was superior to that of the SS ternary OPV due to the higher morphological stability of SQ devices. A more efficient SQ ternary OPV is currently studied with a low gap polymer, a fullerene derivative and a ITIC derivative.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx. Photoluminescence spectra and 2D GIWAX images of ternary and binary films, solar cell characteristics of SS binary and ternary OPV devices (PDF).


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ACKNOWLEDGMENT This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) (2015M1A2A2057506, 2012M3A6A7055540 and 2016M1A2A2940914) and by Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20163030013900).

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Table 1. Solar cell parameters of SQ binary and ternary OPVs

Device

SQ binary SQ ternary

SQ ternary

SQ ternary SQ binary

Photoactive layer

PDCBT/PCBM PDCBT/PCBM:ITIC (PCBM:ITIC = 7:3) PDCBT/PCBM:ITIC (PCBM:ITIC = 1:1) PDCBT/PCBM:ITIC (PCBM:ITIC = 7:3) PDCBT/ITIC

VOC

JSC

(V)

(mA/cm2)

FF

Best

Average

PCE

PCE

(%)

(%)

0.79

6.09

0.73

3.54

3.25 ± 0.22

0.84

9.98

0.72

6.07

5.76 ± 0.24

0.87

10.12

0.71

6.22

6.02 ± 0.18

0.90

9.39

0.65

5.51

5.16 ± 0.34

0.93

7.97

0.55

4.08

3.71 ± 0.29


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(a)

PC71BM

(b)

PDCBT:PCBM:ITIC

SS ternary film (PDCBT:PCBM:ITIC)

ITO/PEDOT:PSS

PDCBT

PCBM:ITIC

SQ ternary film (PDCBT/PCBM:ITIC)

ITO/PEDOT:PSS

Figure 1. (a) Chemical structures of photoactive materials and (b) preparation procedures for SS ternary and SQ ternary films.



Figure 2. (a) Absorption spectra of SQ binary and SQ ternary films (b) PL spectra of PDCBT, ITIC, SQ binary, and SQ ternary films.


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Figure 3. (a) Out-of-plane and (b) in-plane line cut proﬁles of GIXRD results of ITIC, PDCBT, SQ binary, and SQ ternary films.



Figure 4. (a) J-V curves of SQ binary and SQ ternary OPVs under 1 sun illumination. (b) EQE spectra of corresponding films.


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Figure 5. (a) VOC of SQ ternary OPVs as a function of ITIC weight fraction in the PCBM:ITIC acceptor solutions. (b) JSC of SQ binary and SQ ternary OPVs as a function of light intensity.



Figure 6. Thermal stability of (a) SQ ternary and (b) SS ternary OPVs at 100 °C.

Table of content


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Ternary Organic Photovoltaics Prepared by Sequential Deposition of

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