An Obvious Improvement in the Performance of Ternary Organic Solar

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An Obvious Improvement in the Performance of Ternary Organic Solar Cells with “Guest” Donor Present at the “Host” Donor/Acceptor Interface Peng-Qing Bi, Bo Wu, Fei Zheng, Wei-Long Xu, Xiao-yu Yang, Lin Feng, Furong Zhu, and Xiao-Tao Hao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07612 • Publication Date (Web): 15 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016

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ACS Applied Materials & Interfaces

An Obvious Improvement in the Performance of Ternary Organic Solar Cells with “Guest” Donor Present at the “Host” Donor/Acceptor Interface Peng-Qing Bi,† Bo Wu,‡ Fei Zheng,† Wei-Long Xu,† Xiao-Yu Yang,† Lin Feng,† Furong Zhu,‡ Xiao-Tao Hao†, §,* †

School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, China ‡

Department of Physics, Institute of Advanced Materials and Institute of Research and Continuing Education (Shenzhen), Hong Kong Baptist University, Kowloon Tong 999077, Hong Kong §

School of Chemistry, The University of Melbourne, Parkville, Victoria 3010, Australia

ABSTRACT:

A

small

molecule

material

7,7-(4,4-bis(2-Ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo-[c] [1,2,5]thiadiazole) (p-DTS(FBTTH2)2) was used to modify the morphology and electron transport properties of the polymer blend of poly (3-hexythiophene) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) bulk heterojunctions. As a result, a 24% increase in the power-conversion efficiency (PCE) of the p-DTS(FBTTH2)2: P3HT: PC71BM ternary organic solar cells (OSCs) is obtained. The improvement in the performance of OSCs is attributed to the constructive energy cascade path in the ternary system that benefits an efficient Fӧrster

resonance

energy/charge

transfer

process

between

P3HT

and

p-DTS(FBTTH2)2, thereby improving photocurrent generation. It is shown that p-DTS(FBTTH2)2 molecules engage themselves at the P3HT/PC71BM interface. A combination of absorption enhancement, efficient energy transfer process and ordered nanomorphology in the ternary system favors exciton dissociation and charge transportation in the polymer bulk heterojunction. The finding of this work reveals that distribution of the appropriate “guest” donor at the “host” donor/acceptor interface is an effective approach for attaining high performance OSCs.

KEYWORDS: ternary organic solar cells, energy/charge transfer, charge collection, morphology control, synergistic effect 1

ACS Paragon Plus Environment


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 INTRODUCTION Organic solar cells (OSCs) have obtained great attention due to their advantages of light weight, low manufacturing cost, flexibility and roll-to-roll production.1 The state of the art power-conversion efficiency (PCE) of single-junction OSCs has reached above 10% based on small molecule or narrow bandgap polymer donor materials.2,

3

Significant improvement in PCE, stability and performance

reproducibility is needed if organic photovoltaic technology is to become a viable option for practical applications. Several strategies have been successfully demonstrated to improve the PCE of bulk heterojunction (BHJ) OSCs, including synthesis of new donor or acceptor materials, solvent additive, solvent vapor annealing treatment and incorporating metallic nanoparticles into the active layer.4-8 However, these strategies are great challenge to mass manufacturing high performance OSCs. In general, organic materials have a relatively narrow absorption window limiting photon harvesting from other wavelengths of light.9 As an innovation device architecture, tandem solar cells by stacking at least two subcells with different donors or acceptors material can improve the photon harvesting effectively.10-12 However, it is complex to large scale manufacture the tandem solar cells, and the thickness of each subcells is difficult to control. In addition, a robust intermediate layer which has a high transmittance is needed to balance the short circuit current of each subcells for obtaining high performance tandem solar cells. Therefore, it is necessary to find a more simple and effective method to improve the performance of the OSCs. 2


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Alternatively, ternary OSCs can maintain the simple craftsmanship used in binary OSCs and it can also expand the absorption spectra by incorporating another organic material used in tandem OSCs.13-16 In general, the ternary solar cells containing two donor materials and one acceptor material or one donor material and two acceptor materials, and there are four fundamental principles can be adopted to describe the interactions between different donors (acceptors): charge transfer, energy transfer, parallel-linkage and alloy model.17,

18, 21, 48

Many kinds of materials,

including nanoparticles, low-bandgap polymers and small molecules mainly act as “guest” sensitizers, which have a complementary absorption spectra to the “host” polymer-based active layer.18 Among the third components, small molecules in BHJ active layer have lots of advantages in expanding absorption spectrum, improving the crystallinity of “host” polymers and tuning the morphology of the BHJ active layer, and the small molecules have a good compatibility with polymers.19, 20 In this

work, we report the improvement in

the performance of

solution-processed ternary OSCs, based on poly (3-hexythiophene) (P3HT), [6,6]-phenyl-C71-butyric

acid

methyl

7,7-(4,4-bis(2-Ethylhexyl)-4H-silolo

ester

[3,2-b:4,5-b′]

bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)

benzo-[c]

(PC71BM)

and

dithiophene-2,6-diyl) [1,2,5]

thiadiazole)

(p-DTS(FBTTH2)2). Grazing incidence wide-angle X-ray scattering (GIWAXS), grazing incidence small-angle X-ray scattering (GISAXS), atomic force microscopy (AFM) were combined to study the crystalline characteristics and the morphology of the active layers. The photophysics processes in BHJ active layer were investigated 3



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by steady state photoluminescence (PL), fluorescence up-conversion technique and time-resolved fluorescence imaging. The ternary active layers were also studied using UV-vis absorption spectroscopy. The distribution of p-DTS(FBTTH2)2 in the ternary active layers and the performance of devices were also investigated systematically.

 EXPERIMENTAL SECTION Materials. P3HT, p-DTS(FBTTH2)2 and PC71BM were purchased from Sigma Aldrich (product number 445703-1G), 1-Material, Nano-C respectively, and used as received without further purification. P3HT used in this work has a molecular weight (Mn) of 55000 and a polydispersity (PDI) of 2.4. P3HT, p-DTS(FBTTH2)2 and PC71BM solutions with different weight ratios of 1-x: x: 1 (x = 0, 0.05, 0.10, 0.15, 0.20, 0.30, 1) in chlorobenzene, keeping the same concentration of 30 mg ml-1, were formulated for device fabrication. The solutions were stirred at room temperature for at least 24 hours. All of the chlorinated solvents were purchased from Aladdin. Morphology of Bulk Heterojunction Blends. Atomic force microscope (AFM) (NanoScope IIIA) was adopted to analyze the topography of the ternary blend films. The measured surface area was 5 μm×5 μm. To analyze the ternary film structure, GIWAXS was performed using BL16B1 beamline at Shanghai Synchrotron Radiation Facility (SSRF). The distance of sample-to-detector in 2D-GIWAXS measurements is 3050 mm. The data were typically collected for 100 s using an X-ray radiation source at λ = 0.124 nm with mar165CCD. The incidence angle of the X-ray beam was set at 0.12°, which is an intermediate value between the critical angle of the ternary blend films and the 4


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substrates. GISAXS was used to study the domain size of the ternary blend films. GISAXS was also performed using BL16B1 beamline at SSRF. The distance of sample-to-detector in 2D-GISAXS measurements is 2050 mm. The data were typically collected for 300 s using an X-ray radiation source at λ = 0.124 nm with mar165CCD. The incidence angle of the X-ray beam was set at 0.3°. The samples prepared for GIWAXS and GISAXS measurements were obtained by spin-coating the ternary blend solution onto Si wafers. Photophysics Measurements. Absorption spectra were collected by UV−visible dualbeam spectrophotometer (TU-1900, PG Instruments, Ltd.). The steady-state PL were measured using a spectrometer (PG2000 Pro, Idea Optics Co. Ltd.) with an excitation wavelength of 500 nm. Time-resolved photoluminescence (TRPL) decay profiles were measured using fluorescence up-conversion technique. 2D time resolved fluorescence images were obtained using confocal optical microscopy (Nanofinder FLEX2. Tokyo Instruments, Inc.) combined with time correlated single photon counting (TCSPC) module (Becker & Hickl, SPC-150). Fabrication and Characterization of the Ternary Organic Solar Cells. OSCs with an inverted structure of ITO/ZnO/active layer/MoO3/Ag were fabricated using the following procedures. The patterned ITO substrates (sheet resistance 15 Ω square-1) were cleaned sequentially by ultrasonic treatment in detergent, deionized water, acetone, absolute ethyl alcohol and isopropanol for 20 mins each time. The electron transporting layer ZnO (20 nm) was spin coated on the substrates at a rotation 5



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speed of 2500 rpm for 50 s using a solution-based metal oxide nanoparticle process described in a previous work.37 The ternary BHJ film was obtained from spin-casting the P3HT: p-DTS(FBTTH2)2: PC71BM blend solution at a rotation speed of 1200 rpm for 50 s (130 nm). After which, the active layer was thermally annealed on a hot plate at 140°C for 10 min. The OSCs were fabricated in a N2-purged glovebox with moisture and oxygen levels below 0.1 ppm. Finally, a 7.5 nm thick MoO3 hole transporting layer and a 100 nm thick silver (Ag) top contact, deposited by thermal evaporation in a vacuum system with a base pressure of

An Obvious Improvement in the Performance of Ternary Organic Solar

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