Highly Efficient Parallel-Like Ternary Organic Solar Cells - Chemistry

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Highly Efficient Parallel-Like Ternary Organic Solar Cells Tao Liu,† Xiaonan Xue,† Lijun Huo,† Xiaobo Sun,† Qiaoshi An,§ Fujun Zhang,§ Thomas P. Russell,∥ Feng Liu,*,‡ and Yanming Sun*,† †

Heeger Beijing Research and Development Center, School of Chemistry and Environment, Beihang University, Beijing 100191, China ‡ Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai, 200240, China § Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, China ∥ Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: Ternary bulk heterojunction (BHJ) blends have been demonstrated as a promising approach to increase the power conversion efficiencies (PCEs) of organic solar cells. Currently, most studies of ternary organic solar cells are based on blends of two donors and one acceptor, because of the limitation in acceptor materials. Here, we report that high-performance ternary solar cells have been fabricated with a wide-bandgap polymer donor (PDBT-T1) and two acceptor materials, phenylC70-butyric acid methyl ester (PC70BM), and nonfullerene acceptor (ITIC-Th). The addition of ITIC-Th into the BHJ blends dramatically increases the light absorption. Consequently, the champion ternary solar cell shows a high PCE of ∼10.5%, with an open-circuit voltage (Voc) of 0.95 V, a short-circuit current (Jsc) of 15.60 mA/cm2, and a fill factor (FF) of 71.1%, which largely outperforms their binary counterparts. Detailed studies reveal that the ternary solar cells work in a parallel-like device model (ITIC-Th and PC70BM form their own independent transport network) when ITIC-Th loading is >30% in the ternary blends. The results indicate that the combination of fullerene derivative and appropriate nonfullerene acceptor in a ternary blend can be a new strategy to fabricate high-performance ternary organic solar cells.

1. INTRODUCTION

solar cells has been improved significantly and the high PCE is up to ∼8%.23 Although ternary blends consisting of two different acceptors and one donor have been reported in the past few years,22,24−28 most studies of ternary organic solar cells still use blends of two donors and one acceptor.20,21,29−35 Fullerene derivatives are still used as the primary acceptor in these studies. It is known that fullerene materials absorb weakly in the visible light region;36−38 thus, better light absorption management could be realized, if better acceptor materials are developed. Recently, nonfullerene acceptors have been developed aggressively and exhibit comparable or even better photovoltaic performance than fullerene materials.39−60 Nonfullerene acceptors are more versatile for frontier orbital energy level tuning, and light absorption is improved significantly. The emergence of highquality nonfullerene acceptors provides versatility of designing new ternary blend solar cells. More importantly, it has been

Despite the rapid progress in organic bulk-heterojunction (BHJ) solar cells in the past decade,1−9 the overall power conversion efficiencies (PCEs) still lag behind inorganic solar cells, mainly because of the limited light absorption, high energy loss, and low charge carrier mobility of organic materials. A multijunction (e.g., tandem) structure has been shown as a promising strategy to increase light harvesting, since multiple materials with complementary absorption could solve the absorption issue.10−12 A theoretical PCE of 15% is predicted for organic tandem solar cells.13 However, the complex fabrication process of tandem solar cells strongly limits their application. Ternary blend organic solar cells having three light absorbing materials (either two donors and one acceptor or one donor and two acceptors), on the other hand, are much easier to fabricate,14−18 could have improved light absorption, and could elevate the PCEs of solar cell devices.19 Until now, PCEs of organic solar cells based on solution-processed ternary blends already exceeded 10%.20−22 It is worth mentioning that the device performance in vacuum-deposited ternary organic © 2017 American Chemical Society

Received: December 8, 2016 Revised: March 6, 2017 Published: March 6, 2017 2914

DOI: 10.1021/acs.chemmater.6b05194 Chem. Mater. 2017, 29, 2914−2920

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Figure 1. (a) Chemical structures of PDBT-T1, PC70BM, and ITIC-Th. (b) UV-vis absorption spectra of PDBT-T1, PC70BM, and ITIC-Th films. (c) UV-vis absorption spectra of ternary blend films with different ITIC-Th contents. (d) Device structure of ternary organic solar cells. (d) Energy level diagrams of PDBT-T1, PC70BM, and ITIC-Th.

2. EXPERIMENTAL SECTION

reported that the open-circuit voltage (Voc) of some nonfullerene solar cells is higher than that of their fullerene counterparts.54,55 For instance, Hou et al. demonstrated that the difference between the energy of the excited pure components and the energy of charge transfer (CT) states in the nonfullerene blend is smaller than that of the PC70BM blend, which leads to reduced energy loss.50 While fullerene materials have the isotropic electron transport capability and still function as good acceptors, the introduction of appropriate nonfullerene acceptors into a polymer:fullerene binary blends could solve not only the light absorption issue but also generate a new handle to manipulate the morphology of the active layer, which could lead to enhancements in the photocurrent and device efficiency. In this work, we fabricate new ternary organic solar cells with a wide-bandgap polymer donor (PDBT-T1),61 a phenyl-C70butyric acid methyl ester (PC70BM) acceptor, and a deep absorbing nonfullerene acceptor (ITIC-Th).41 ITIC-Th is a successful n-type material that shows complementary absorption with PDBT-T1 and has a broad spectral response from 500 nm to 800 nm. Photoluminescence (PL) experiments indicate that no energy transfer or charge transfer occurs between ITICTh and PC70BM. Grazing-incidence X-ray diffraction (GIXD) and resonant soft X-ray scattering (RSoXS) show that the blend films form a parallel-like BHJ structure (the ITIC-Th content is >30% in the ternary blends), where ITIC-Th and PC70BM form their own transport network. The solar cell performance is found to be strongly dependent on the weight ratio of ITIC-Th and PC70BM. At the weight ratio of 1:1, the champion cell showed a high PCE of ∼10.5%, with a Voc of 0.95 V, a shortcircuit current (Jsc) of 15.60 mA/cm2, and a fill factor (FF) of 71.1%. Such high performance arises from the increased Jsc. The results indicate that high-efficiency organic solar cells can be achieved by using fullerene and nonfullerene acceptors in a ternary mixture, which can be a new design protocol for highefficiency organic solar cells.

2.1. Instrumentations. The ultraviolet−visible light (UV-vis) absorption spectra were measured using a Shimadzu Model UV-2700 UV-vis spectrophotometer. The photoluminescence (PL) spectra were performed on a Shimadzu Model RF-5301PC spectrofluorophotometer. TRTPL spectra were measured using a FluoroCube-01-NL and FluoroCub-NL from Jobin Yvon. Atomic force microscopy (AFM) images were performed on a Dimension Icon AFM (Bruker). Transmission electron microscopy (TEM) was performed on Tecnai 20 system, using device thin films. GIXD was performed at the Advanced Light Source on beamline 7.3.3, Lawrence Berkeley National Lab (LBNL). RSoXS was performed at beamline 11.0.1.2 Advanced Light Source, LBNL. The samples were floated onto Si3N4 substrates. The scattering signals were collected under vacuum, using a Princeton Instrument PI-MTE CCD camera in transmission mode. 2.2. Solar Cell Fabrication and Measurement. The cleaning of indium-doped tin oxide (ITO) glasses and the preparation of PEDOT:PSS were reported in our previous work.20 The weight ratio of PDBT-T1 and PC70BM:ITIC-Th acceptor was maintained at 1:1; the concentration of PDBT-T1 in all mixed solutions is the same (6 mg/mL). The weight ratio of ITIC-Th in PC70BM:ITIC-Th acceptor varied from 0% to 100%. Chloroform was used as the host solvent and 1% DIO was used as the solvent additive. PDBTT1:PC70BM:ITIC-Th active layer were formed by spin coating their mixed solution at 1400 rpm for 60 s and then annealed at 100 °C for 5 min. The optimal active layer thickness is ∼110 nm, measured by a Dektak XT stylus profilometer. Finally, Ca (10 nm)/Al (100 nm) was used as the cathode. The device area was 4.50 mm2. The measurements of devices can be found in our previous work.20 IPCE spectra were measured using QEX10 Solar Cell Quantum Efficiency Measurement System (PV Measurements, Inc.).

3. RESULTS AND DISCUSSION The chemical structures of PDBT-T1, PC70BM, and ITIC-Th are shown in Figure 1a. As mentioned, PC70BM possesses weak visible absorption. In contrast, ITIC-Th exhibits strong absorption in the wavelength range of 500−800 nm (the maximum absorption coefficient is ∼8.8 × 104 cm−1), which is 2915

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Figure 2. (a) Current density−voltage (J−V) characteristics of ternary organic solar cells with different ITIC-Th contents and (b) the corresponding IPCE spectra.

Table 1. Summary of Device Performance of Ternary Organic Solar Cells with Different ITIC-Th Contents ITIC-Th content [%] 0 10 30 50 70 90 100 a

Voc [V] 0.915 0.920 0.928 0.934 0.939 0.945 0.950

± ± ± ± ± ± ±

0.005 0.004 0.005 0.005 0.005 0.005 0.004

Jsc [mA/cm2] 13.24 14.39 15.06 15.54 14.58 13.27 12.58

± ± ± ± ± ± ±

0.10 0.14 0.16 0.18 0.15 0.10 0.13

FF [%] 76.2 72.8 71.1 70.5 66.8 66.2 59.0

± ± ± ± ± ± ±

0.5 0.6 0.1 0.6 0.6 0.8 0.8

PCEa[%]

PCEmax [%]

± ± ± ± ± ± ±

9.29 9.68 9.99 10.48 9.22 8.48 7.15

9.23 9.64 9.93 10.22 9.14 8.32 7.05

0.10 0.11 0.11 0.11 0.10 0.13 0.13

The average PCE value was calculated from 10 devices.

As shown in the incident photon conversion efficiency (IPCE) spectrum (Figure 2b), the champion device shows an obvious enhancement in spectral response from 500 nm to 800 nm, which accounts for the improved Jsc value. The addition of ITIC-Th leads to a new IPCE feature in 700−750 nm, whose intensity reaches a maximum when at a loading ratio of 50%. Note that the spectral response for 50% ITIC-Th loading device is higher than that of the ITIC-Th control device (700− 750 nm). Thus, the ternary blends fully utilize the ITIC-Th function, with the added benefit of increasing the IPCE in the wavelength region of 500−700 nm. The Jsc value calculated from the IPCE spectrum is 15.50 mA/cm2, in good agreement with the value (15.60 mA/cm2) obtained from J−V measurements. We studied the charge recombination kinetics in these ternary solar cells by measuring the dependence of the photocurrent (Jph) on light intensity (see Figure S2 in the Supporting Information). Typically, Jph follows a power law behavior with light intensity (Jph ∝ Pinα, where α is an exponential factor).62 The α values of PC70BM-based and ITIC-Th-based binary solar cells are ∼1.0, indicating a negligible bimolecular recombination in these devices under the short-circuit conditions. The addition of ITIC-Th into the ternary blends does not affect the α value from a wide range of compositions, suggesting that PC70BM and ITIC-Th could work quite independently and the ternary mixture does not form an unfavorable morphology that leads to stronger bimolecular recombination. The influence of ITIC-Th on charge carrier transport in the ternary blend films was studied. As shown in Figure S2 and Table S1 in the Supporting Information, balanced hole and electron mobilities at high levels are observed, indicating effective charge carrier transport in these blend films. When the ITIC-Th contents in the blends increase from 0% to 50%, the hole and electron mobilities both increased. At the ITIC-Th content of 50%, the electron and hole mobilities are 3.28 × 10−3 cm2 V−1 s−1 and 2.17 × 10−3 cm2 V−1 s−1, respectively. Further increase of the ITIC-Th

complementary to that (the maximum absorption coefficient is ∼7.2 × 104 cm−1) of PDBT-T1 (see Figure 1b). When these three materials are mixed, a broad absorption over the entire visible spectrum is observed. As shown in Figure 1c, the addition of ITIC-Th into the ternary blends leads to a wellresolved absorption peak from 700 nm to 800 nm, whose intensity increases with ITIC-Th loading ratio. Ternary blends that contain 50% ITIC-Th show a broad absorption covering 500−800 nm. Solar cells were fabricated with a conventional structure where ITO and Ca/Al have been used as the anode and cathode, respectively (Figure 1d). The energy level diagrams of PDBT-T1, PC70BM, and ITIC-Th are illustrated in Figure 1e. The highest energy occupied molecular orbital (HOMO) energy level of PDBT-T1 is −5.36 eV, and the lowest unoccupied molecular orbital (LUMO) energy levels of PC70BM, and ITIC-Th are similar (−3.91 eV vs −3.93 eV). A large energy offset (∼1.4 eV) between the HOMO of the donor and the LUMO of the acceptors could contribute to a high Voc. Binary control solar cells based on PDBT-T1:PC70BM, and PDBT-T1:ITIC-Th blends were fabricated initially. PDBTT1:PC70BM binary solar cells showed a high PCE of 9.29%, with Voc = 0.91 V, Jsc = 13.33 mA/cm2, and FF = 76.4%. PDBTT1:ITIC-Th binary solar cells showed a moderate PCE of 7.15%, with Voc = 0.95 V, Jsc = 12.53 mA/cm2, and FF = 59.9% (see Figure 2 and Table 1). Note that the PCE of 7.15% is lower than our best optimized device, using 1-chloronaphthalene (CN) additive (∼9.6%).41 In this work, we use 1,8diiodoctane (DIO) to process devices, since it is a better recipe for PDBT-T1:PC70BM blends.61 It is seen that the incorporation of small amounts of ITIC-Th into PDBT-T1:PC70BM blend significantly increased the value of Jsc. At the ITIC-Th/ PC70BM loading ratio of 50%, Jsc is improved by 17%, to 15.60 mA/cm2, compared with PDBT-T1:PC70BM binary control cell. Despite a slightly decreased FF, the overall PCE of ternary solar cells is improved to 10.48%. In addition, we noticed that the ternary solar cells showed good thermal stability (Figure S1 in the Supporting Information). 2916

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neither charge nor energy transfer occurred between these two acceptors.15 Independently, solar cell devices using ITIC-Th and PC70BM blended thin film were also fabricated. This device showed a Jsc value of 0.33 mA/cm2. Solar cells using ITIC-Th and PC70BM neat films showed Jsc values of 0.51 mA/cm2 and 0.26 mA/cm2, respectively (see Figure S4 in the Supporting Information). These results confirm that ITIC-Th and PC70BM cannot form charge-transfer states to facilitate photovoltaic process. Thus, in the ternary blend, ITIC-Th and PC70BM should function independently. In addition, we noticed that the ternary device Voc was linearly dependent on the weight fraction of ITIC-Th (Figure 3c). The morphology of these ternary blends was studied using GIXD,63 RSoXS,64 AFM, and TEM. Figure 4a shows the structure of these blended thin films (GIXD) and Figure S5 in the Supporting Information shows the corresponding 2D GIXD patterns. The PC70BM-based binary device shows the (100) and (010) reflections of PDBT-T1 at 0.3 Å−1 and 1.75 Å−1, respectively. Thus, the alkyl-to-alkyl distance in PDBT-T1 is 2.1 nm and the π−π stacking distance is 0.36 nm. The π−π stacking peak locates primarily in the out-of-plane direction, and thus PDBT-T1 crystallites assume a face-on orientation, which is beneficial for hole transport. PC70BM shows a typical diffuse reflection at ∼1.35 Å−1. When ITIC-Th is added to the blends, it readily crystallizes and forms its own domain. As seen from the 30% loading sample, a new out-of-plane reflection at 0.41 Å−1 is seen, characteristic of ITIC-Th, which increases in intensity grows in proportion to its content. The π−π stacking peak is also seen in the out-of-plane direction, suggesting a faceon orientation, which is favorable for electron transport (seen from the 100% ITIC-Th blends).39 In all these blends, the PDBT-T1 polymer shows similar diffraction features, and thus forms similar domains in the BHJ blends (see Figures S6−S8 in the Supporting Information). Therefore, in this scenario, we have a system that has a typical PDBT-T1 polymer feature domain, an ITIC-Th domain, and a PC70BM-rich domain when the ITIC-Th content is >30% in the ternary blends. The phase separation of these blended thin films was studied using RSoXS (see Figure 4b). TEM images of these thin films are shown in Figure 4c. The 0% ITIC-Th blend shows a shoulder in the scattering at ∼0.014 Å−1, corresponding to a length scale of 45 nm for the phase-separated domain. This result corresponds well with TEM results, seen as a fine mesh network formed by polymer fibrils. In the 30% ITIC-Th sample, the length scale of phase separation shifted slightly to lower q; therefore, its size increased slightly, which can be seen in the TEM image as well. A more interesting result is seen for the 50% ITIC-Th blends. The RSoXS profile shows two shoulders in the scattering at 0.013 Å−1 (48 nm) and 0.003 Å−1 (210 nm), indicative of a multilength scale morphology. The device, with two acceptors in equal amounts, also gave the best performance. Previously, we found that ITIC-Th and PC70 BM could function independently. Therefore, we ascribe the two length scales to ITIC-Th- and PC70BM-rich domains. The phase image in TEM became more complicated. We see evidence for a multilength scale morphology, in agreement with the RSoXS results, with characteristic length scales of the domain that are tens of nanometers in size larger, arising from aggregation. As a result, the ternary solar cells work in a parallel-like device model. When more ITIC-Th is loaded, the phase separation became less pronounced, as evidenced by the featureless RSoXS profile and TEM image. The performance of these devices also deteriorated.

content to 70% or more leads to unbalanced carrier mobility, which results in unfavorable charge transport, agreeing well with the photovoltaic performance. The working mechanism in these ternary blends is studied further. We measured the photoluminescence (PL) spectra of ITIC-Th, PC70BM and the blend films with different ITIC-Th weight ratios (Figure 3a). The ITIC-Th neat film exhibits a

Figure 3. (a) PL spectra of ITIC-Th:PC70BM blend films with different ITIC-Th contents (excited at 605 nm). (b) TRTPL spectra of PC70BM, ITIC-Th, and ITIC-Th:PC70BM blend film (1:1, w/w) (excited at 460 nm and monitored at 705 nm). (c) Voc as a function of different weight ratios of ITIC-Th in the blend films.

strong emission at ∼750 nm. In ITIC-Th:PC70BM blends, as the ITIC-Th content decreases, the PL intensity decreases. An almost-linear relationship between maximum PL intensity and the ITIC-Th contents indicates that ITIC-Th and PC70BM are rather independent in photophysical functions. Charge transfer or energy transfer does not occur between ITIC-Th and PC70BM. The time-resolved transient photoluminescence (TRTPL) spectra of ITIC-Th, PC70BM and ITIC-Th:PC70BM blend films were measured to determine if there was any charge transfer between the two acceptors (see Figure 3b). The films were excited using 460 nm light and the PL emission at 750 nm was monitored. The emission lifetime of PC70BM and ITIC-Th neat films is 0.06 and 0.071 ns, respectively, which is comparable to that (0.068 ns) of PC70BM:ITIC-Th blend film. We did not see any PL lifetime delays and, therefore, 2917

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Figure 4. (a) Out-of-plane (solid line) and in-plane (dotted line) line cut profiles of GIXD results of ITIC-Th:PC70BM blend films with different ITIC-Th contents; (b) RSoXS profiles of ITIC-Th:PC70BM blend films with different ITIC-Th contents; and (c) TEM images of ITIC-Th:PC70BM blend films with different ITIC-Th contents.

4. CONCLUSION In summary, we demonstrated high-efficiency ternary solar cells using one donor polymer (PDBT-T1), and two acceptors (PC70BM and ITIC-Th). The optical, electrical, and morphological results indicate that, in these ternary blends, PC70BM and ITIC-Th could function independently. Thus, the benefit of both fullerene acceptor and nonfullerene acceptor can be realized. While the PDBT-T1 polymer shows crystalline features in blends, the addition of ITIC-Th could form its own good electron transport network (above 30% of ITIC loading in the ternary blends). In this scenario, the ternary blend solar cells work in a parallel-like device model. The contribution of ITIC-Th in the longer-wavelength absorption leads to a significant improvement in Jsc. Meanwhile, a high Voc and high FF can be maintained, resulting in a high PCE of ∼10.5%, which significantly outperforms the binary control devices. The results suggest that the parallel structure would be a promising strategy to fabricate high-performance ternary organic solar cells. When nonfullerene acceptors that are better matched with PC70BM and the polymer donor, higher efficiencies can be achieved.

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Stability test of ternary solar cells, hole and electron mobilities, Jsc versus light intensity characteristics of ternary organic solar cells, J−V cures of organic solar cells based on PC70BM, ITIC-Th, and ITIC-Th:PC70BM blend film, AFM height and phase images of ternary blends, and 2D GIXD profiles of ternary blends (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fujun Zhang: 0000-0003-2829-0735 Thomas P. Russell: 0000-0001-6384-5826 Yanming Sun: 0000-0001-7839-3199 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the International Science & Technology Cooperation Program of China (No. 2014DFA52820), and the National Natural Science Foundation of China (NSFC) (Nos. 51473009, 21225209, 91427303). FL and TPR were supported by the U.S. Office of Naval Research under Contract No. N00014-15-1-2244. Portions of this research were carried out at beamline 7.3.3 and 11.0.1.2 at

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05194. 2918

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the Advanced Light Source, Molecular Foundry, and National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, which was supported by the DOE, Office of Science, and Office of Basic Energy Sciences.

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