Ternary organic solar cells offer 14% power conversion efficiency

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1 See discussions, stats, and author profiles for this publication at: Ternary organic solar cells offer 14% power conversion efficiency Article in Science Bulletin November 2017 DOI: /j.scib CITATION 1 READS authors: Zuo Xiao National Center for Nanoscience and Techno 55 PUBLICATIONS 1,029 CITATIONS Xue Jia National Center for Nanoscience and Techno 8 PUBLICATIONS 15 CITATIONS SEE PROFILE SEE PROFILE Liming Ding National Center for Nanoscience and Techno 119 PUBLICATIONS 2,344 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Developing high-performance materials for organic and perovskite solar cells View project All content following this page was uploaded by Zuo Xiao on 15 November The user has requested enhancement of the downloaded file.

2 Article type: Short Communication Ternary organic solar cells offer 14% power conversion efficiency Zuo Xiao, Xue Jia, Liming Ding* Dr. Z. Xiao, X. Jia, Prof. L. Ding Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing , China. X. Jia, Prof. L. Ding University of Chinese Academy of Sciences, Beijing , China Zuo Xiao and Xue Jia contributed equally to this work. Organic solar cells (OSCs) have advantages like light-weight, flexibility, colorfulness and solution processability. [1] The active layer of OSCs generally contains two organic semiconductors: an electron donor and an electron acceptor. The donor and acceptor make nanoscale phase separation to allow efficient exciton dissociation and also form a 3D passage to rapidly transfer free charge carriers to respective electrodes. [2] However, such binary system usually shows insufficient coverage of solar irradiation spectrum due to the narrow optical absorption of organic compounds. [3] Recently, ternary OSCs containing three absorption-complementary materials (e.g. two donors and one acceptor, or one donor and two acceptors) have attracted great attention. Ternary solar cells harvest more sunlight and demonstrate better performance than binary solar cells in some cases. [3] Polymer:fullerene:nonfullerene solar cells combine the advantages of fullerene acceptors (high electron mobility) and nonfullerene acceptors (strong visible or near-infrared (NIR) absorption), and achieved over 10% power conversion efficiencies (PCEs). [4, 5] Recently, we reported a highly efficient low-bandgap nonfullerene acceptor (CO i 8DFIC) with strong NIR absorption. PTB7-Th:CO i 8DFIC (1:1) binary cells gave ma cm -2 J sc and 12.16% PCE. [6] Here, we report highly efficient ternary cells based on PTB7-Th, CO i 8DFIC and 1

3 PC 71 BM (Fig. 1a). Fullerene improves electron transport in the active layer and enhances external quantum efficiency (EQE), leading to high short-circuit current density (J sc ) and fill factor (FF). A PCE of 14.08% was achieved. The absorption spectra for PTB7-Th, CO i 8DFIC and PC 71 BM films are shown in Fig. 1b. PC 71 BM absorbs short-wavelength light, which is complementary to PTB7-Th and CO i 8DFIC. The lowest unoccupied molecular orbital levels (LUMO) for PTB7-Th (-3.12 ev), PC 71 BM ( ev) and CO i 8DFIC (-3.88 ev) show a stepwise alignment (Fig. 1c), suggesting that PC 71 BM can facilitate electron transfer from PTB7-Th to CO i 8DFIC. [6, 7] Solar cells with a structure of ITO/ZnO/D:A 1 :A 2 /MoO 3 /Ag were made, where D is PTB7-Th, A 1 is CO i 8DFIC and A 2 is PC 71 BM. The weight ratio between D and A 1 +A 2 was fixed to 1:1.5, while the content of A 2 in acceptors gradually increased from 0% to 100% (Table S1). [8] Initially, PTB7-Th:CO i 8DFIC (1:1.5) binary cells gave a PCE of 10.48%, with an open-circuit voltage (V oc ) of 0.69 V, a J sc of ma cm -2 and a FF of 63.8%. After adding small amount of fullerene (A 2 ) into the blend, J sc and FF increased dramatically. When D/A 1 /A 2 ratio (w/w/w) was 1:1.05:0.45, the ternary cells gave a PCE of 14.08%, with a V oc of 0.70 V, a J sc of ma cm -2 and a FF of 71.0%. To the best of our knowledge, this is the first report demonstrating that the PCE for organic solar cells exceeds 14%. Further increasing fullerene content, V oc slightly increased, while J sc and FF decreased, leading to reduced PCEs. PTB7- Th:PC 71 BM (1:1.5) binary cells gave a PCE of 7.36%, with a V oc of 0.75 V, a J sc of ma cm -2 and a FF of 60.2%. The performance for ternary cells (D/A 1 /A 2 = 1:1.05:0.45) is sensitive to the active layer thickness and additive content (Tables S2-S3). The optimal thickness for the active layer and the optimal 1,8-diiodooctane (DIO) content are 108 nm and 1 vol%, respectively. The J-V curves and the corresponding EQE spectra for the binary and the best ternary solar cells are shown in Fig. 1d-1e. Compared with PTB7-Th:CO i 8DFIC cells, the ternary cells show enhanced EQE at nm, consisting with the high J sc. The integrated current 2

4 densities from EQE spectra of PTB7-Th:CO i 8DFIC and the ternary cells are ma cm -2 and ma cm -2, respectively. The EQE enhancement for the ternary cells might result from enhanced light absorption and efficient generation and transport of free charge carriers. The absorption spectra for the binary and ternary blend films are shown in Fig. S1. Compared with PTB7-Th:CO i 8DFIC (1:1.5) film, PTB7-Th:CO i 8DFIC:PC 71 BM (1:1.05:0.45) film shows stronger absorption at nm and nm. The higher absorbance at short wavelengths originates from fullerene absorption, while the absorption band in NIR region might originate from CO i 8DFIC. Although the ternary blend film shows comparable or even lower absorbance at nm than PTB7-Th:CO i 8DFIC film, EQE is still higher for the ternary cells. This indicates that the charge generation and transport processes are more efficient in the ternary blend film. We studied exciton dissociation probabilities (P diss ) in different cells (Fig. S2). [9] P diss for PTB7-Th:CO i 8DFIC cells, PTB7-Th:PC 71 BM cells and the ternary cells are 94.8%, 94.1% and 98.5%, respectively, indicating the most efficient charge generation in the ternary cells. We evaluated charge carrier mobilities by using space charge limited current (SCLC) method (Fig. S3-S4 and Table S4). [10] Compared with PTB7- Th:CO i 8DFIC film, the ternary blend film showed a similar hole mobility (μ h ) of cm 2 V -1 s -1 and a much higher electron mobility (μ e ) of cm 2 V -1 s -1. μ h /μ e was decreased from 18 to 1.3. Fullerene improved electron transport and led to balanced charge transport in the active layer, thus increasing J sc and FF. Bimolecular recombination in different cells was studied by plotting J sc against light intensity (P light ). The data were fitted to a power law: J sc P α light. The α values for PTB7-Th:CO i 8DFIC cells, PTB7-Th:PC 71 BM cells and the ternary cells are 0.987, and 0.991, respectively, suggesting the least bimolecular recombination in the ternary cells (Fig. S5). The morphology for the active layers was studied by using atomic force microscope (AFM) and transmission electron microscope (TEM) (Fig. S6). PTB7-Th:CO i 8DFIC, PTB7-Th:PC 71 BM and the ternary blend films show root-meansquare roughnesses (R rms ) of 5.05 nm, 0.92 nm and 1.06 nm, respectively. Fullerene addition 3

5 led to R rms reduction, suggesting that fullerene could help the mixing of PTB7-Th and CO i 8DFIC. Very clear nanofiber structures were observed in the ternary blend film. The donor and two acceptors can make optimal nanoscale phase separation for efficient exciton dissociation and build up a 3D passage for efficient transport of free charge carriers, thus enhancing J sc and FF significantly. In summary, a high-performance active layer for organic solar cells was developed by adding PC 71 BM into PTB7-Th:CO i 8DFIC blend. Fullerene enlarges light absorption, facilitates electron transport, reduces charge recombination and optimizes morphology of the active layer, leading to high J sc and FF. The best PTB7-Th:CO i 8DFIC:PC 71 BM (1:1.05:0.45) solar cells gave a PCE of 14.08%, which is the highest record for organic solar cells to date. This work demonstrates the great potential of polymer:fullerene:nonfullerene ternary solar cells. Electronic supplementary material The online version of this article (doi:xxx/xxx) contains supplementary material, which is available to authorized users. Acknowledgements We greatly appreciate the National Natural Science Foundation of China (U , , , and ), the National Key Research and Development Program of China (2017YFA ), the State Key Laboratory of Luminescent Materials and Devices (2016-skllmd-05) and the Youth Association for Promoting Innovation (CAS) for financial support. Conflict of interest The authors declare that they have no conflict of interest. 4

6 References [1] Kippelen B, Brédas JL (2009) Organic photovoltaics. Energy Environ Sci 2: [2] Yu G, Gao J, Hummelen JC et al (1995) Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions. Science 270: [3] Lu H, Xu X, Bo Z (2016) Perspective of a new trend in organic photovoltaic: ternary blend polymer solar cells. Sci China Mater 59: [4] Lu H, Zhang J, Chen J et al (2016) Ternary-blend polymer solar cells combining fullerene and nonfullerene acceptors to synergistically boost the photovoltaic performance. Adv Mater 28: [5] Zhao W, Li S, Zhang S et al (2017) Ternary polymer solar cells based on two acceptors and one donor for achieving 12.2% efficiency. Adv Mater 29: [6] Xiao Z, Jia X, Li D et al (2017) 26 ma cm -2 J sc from organic solar cells with a lowbandgap nonfullerene acceptor. Sci Bull DOI: /j.scib [7] He D, Zuo C, Chen S et al (2014) A highly efficient fullerene acceptor for polymer solar cells. Phys Chem Chem Phys 16: [8] An M, Xie F, Geng X et al (2017) A high-performance D-A copolymer based on dithieno[3,2-b:2,3 -d]pyridin-5(4h)-one unit compatible with fullerene and nonfullerene acceptors in solar cells. Adv Energy Mater 7: [9] Wu JL, Chen FC, Hsiao YS et al (2011) Surface plasmonic effects of metallic nanoparticles on the performance of polymer bulk heterojunction solar cells. ACS Nano 5: [10] Xiao Z, Liu F, Geng X et al (2017) A carbon-oxygen-bridged ladder-type building block for efficient donor and acceptor materials used in organic solar cells. Sci Bull 62:

7 Figure legends Fig. 1 (a) The structures for PTB7-Th, CO i 8DFIC and PC 71 BM. (b) Absorption spectra for PTB7-Th, CO i 8DFIC and PC 71 BM films. (c) Energy level diagram. (d) J-V curves for the binary and ternary solar cells. (e) EQE spectra for the binary and ternary solar cells. 6

8 Artwork and illustrations Fig. 1 (a) The structures for PTB7-Th, CO i 8DFIC and PC 71 BM. (b) Absorption spectra for PTB7-Th, CO i 8DFIC and PC 71 BM films. (c) Energy level diagram. (d) J-V curves for the binary and ternary solar cells. (e) EQE spectra for the binary and ternary solar cells. 7

9 Supporting Information Ternary organic solar cells offer 14% power conversion efficiency Zuo Xiao, Xue Jia, Liming Ding* Dr. Z. Xiao, X. Jia, Prof. L. Ding Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing , China. X. Jia, Prof. L. Ding University of Chinese Academy of Sciences, Beijing , China Zuo Xiao and Xue Jia contributed equally to this work. 1. Device fabrication and measurements 2. Optimization of device performance 3. Absorption spectra for the binary and ternary blend films 4. Exciton dissociation probabilities 5. SCLC 6. Bimolecular recombination 7. AFM and TEM 8

10 1. Device fabrication and measurements Inverted solar cells The ZnO precursor solution was prepared according to literature. [1] It was spin-coated onto ITO glass (4000 rpm for 30 s). The films were annealed at 200 C in air for 20 min. ZnO film thickness is ~30 nm. A PTB7-Th:CO i 8DFIC:PC 71 BM blend in chlorobenzene (CB) with DIO additive was spin-coated onto ZnO layer. MoO 3 (~6 nm) and Ag (~80 nm) was successively evaporated onto the active layer through a shadow mask (pressure ca Pa). The effective area for the devices is 4 mm 2. The thicknesses of the active layers were measured by using a KLA Tencor D-120 profilometer. J-V curves were measured by using a computerized Keithley 2400 SourceMeter and a Xenon-lamp-based solar simulator (Enli Tech, AM 1.5G, 100 mw/cm 2 ). The illumination intensity of solar simulator was determined by using a monocrystalline silicon solar cell (Enli SRC2020, 2cm 2cm) calibrated by NIM. The external quantum efficiency (EQE) spectra were measured by using a QE-R3011 measurement system (Enli Tech). The film morphology was studied by AFM and TEM. AFM was performed on a Multimode microscope (Veeco) by using tapping mode. TEM was performed on a Tecnai G2 F20 U-TWIN instrument operated at 200 kv. Hole-only devices The structure for hole-only devices is ITO/PEDOT:PSS/PTB7-Th:CO i 8DFIC:PC 71 BM /MoO 3 /Al. A 30 nm thick PEDOT:PSS layer was made by spin-coating an aqueous dispersion onto ITO glass (4000 rpm for 30 s). PEDOT substrates were dried at 150 C for 10 min. A PTB7-Th: CO i 8DFIC:PC 71 BM blend in CB with 1 vol% DIO additive was spin-coated onto PEDOT layer. Finally, MoO 3 (~6 nm) and Al (~100 nm) was successively evaporated onto the active layer through a shadow mask (pressure ca Pa). J-V curves were measured by using a computerized Keithley 2400 SourceMeter in the dark. Electron-only devices The structure for electron-only devices is Al/PTB7-Th:CO i 8DFIC:PC 71 BM /Ca/Al. Al (~80 nm) was evaporated onto a glass substrate. A PTB7-Th:CO i 8DFIC:PC 71 BM blend in CB with 1 vol% DIO additive was spin-coated onto Al. Ca (~5 nm) and Al (~100 nm) were successively evaporated onto the active layer through a shadow mask (pressure ca Pa). J- V curves were measured by using a computerized Keithley 2400 SourceMeter in the dark. 9

11 2. Optimization of device performance Table S1 Optimization of D/A 1 /A 2 ratio for PTB7-Th:CO i 8DFIC:PC 71 BM inverted solar cells. a D/A 1 /A 2 A 2 /(A 1 +A 2 ) [w/w/w] [%] V oc J sc FF PCE [V] [ma/cm 2 ] [%] [%] 1:1.5: (10.17±0.25) b 1:1.35: (12.58±0.28) 1:1.2: (12.79±0.26) 1:1.05: (13.71±0.31) 1:0.9: (12.31±0.34) 1:0.6: (11.07±0.21) 1:0: (6.99±0.33) a Blend solution: 18 mg/ml in CB with 1 vol% DIO; spin-coating: 1600 rpm for 60 s. b Data in parentheses stand for the average PCEs for 8 cells. Table S2 Optimization of active layer thickness for PTB7-Th:CO i 8DFIC:PC 71 BM inverted solar cells. a Thickness [nm] V oc J sc FF PCE [V] [ma/cm 2 ] [%] [%] (10.37±0.28) b (11.75±0.37) (13.71±0.31) (12.04±0.38) (10.34±0.20) a D/A 1 /A 2 ratio: 1:1.05:0.45 (w/w/w); blend solution: 18 mg/ml in CB with 1 vol% DIO. b Data in parentheses stand for the average PCEs for 8 cells. 10

12 Table S3 Optimization of DIO content for PTB7-Th:CO i 8DFIC:PC 71 BM inverted solar cells. a DIO [vol%] V oc J sc FF PCE [V] [ma/cm 2 ] [%] [%] (10.38±0.29) b (10.91±0.36) (12.12±0.27) (13.71±0.31) (11.80±0.49) (6.61±0.19) a D/A 1 /A 2 ratio: 1:1.05:0.45 (w/w/w); blend solution: 18 mg/ml in CB; spin-coating: 1600 rpm for 60 s. b Data in parentheses stand for the average PCEs for 8 cells. 11

13 3. Absorption spectra for the binary and ternary blend films Fig. S1 Absorption spectra for PTB7-Th:CO i 8DFIC (1:1.5), PTB7-Th:PC 71 BM (1:1.5) and PTB7-Th:CO i 8DFIC:PC 71 BM (1:1.05:0.45) blend films. 12

14 4. Exciton dissociation probabilities Fig. S2 J ph -V eff plots. 13

15 5. SCLC Charge carrier mobility was measured by SCLC method. The mobility was determined by fitting the dark current to the model of a single carrier SCLC, which is described by: 9 V J 8 d 2 0 r 3 where J is the current density, μ is the zero-field mobility of holes (μ h ) or electrons (μ e ), ε 0 is the permittivity of the vacuum, ε r is the relative permittivity of the material, d is the thickness of the blend film, and V is the effective voltage (V = V appl V bi, where V appl is the applied voltage, and V bi is the built-in potential determined by electrode work function difference). Here, V bi = 0.1 V for hole-only devices, V bi = 0 V for electron-only devices. [2] The mobility was calculated from the slope of J 1/2 -V plots. 14

16 Fig. S3 J-V curves (a) and corresponding J 1/2 -V plots (b) for the hole-only devices (in dark). The thicknesses for PTB7-Th:CO i 8DFIC (1:1.5), PTB7-Th:PC 71 BM (1:1.5) and PTB7- Th:CO i 8DFIC:PC 71 BM (1:1.05:0.45) blend films are 108, 87 and 104 nm, respectively. 15

17 Fig. S4 J-V curves (a) and corresponding J 1/2 -V plots (b) for the electron-only devices (in dark). The thicknesses for PTB7-Th:CO i 8DFIC (1:1.5), PTB7-Th:PC 71 BM (1:1.5) and PTB7- Th:CO i 8DFIC:PC 71 BM (1:1.05:0.45) blend films are 108, 87 and 104 nm, respectively. Table S4. Hole and electron mobilities for the binary and ternary blend films. Blend films h e [cm 2 /Vs] [cm 2 /Vs] h / e PTB7-Th:CO i 8DFIC (1:1.5) PTB7-Th:PC 71 BM (1:1.5) PTB7-Th:CO i 8DFIC:PC 71 BM (1:1.05:0.45)

18 6. Bimolecular recombination Fig. S5 J sc -P light plots. 17

19 7. AFM and TEM Fig. S6 AFM height and phase images, and TEM images for PTB7-Th:COi8DFIC (a, b, c), PTB7-Th:PC71BM (d, e, f) and PTB7-Th:COi8DFIC:PC71BM (g, h, i) blend films. 18

20 References [1] Sun Y, Seo JH, Takacs CJ et al (2011) Inverted polymer solar cells integrated with a lowtemperature-annealed sol-gel-derived ZnO film as an electron transport layer. Adv Mater 23: [2] Duan C, Cai W, Hsu B et al (2013) Toward green solvent processable photovoltaic materials for polymer solar cells: the role of highly polar pendant groups in charge carrier transport and photovoltaic behavior. Energy Environ Sci 6: View publication stats