High Performing Ternary Solar Cells through FÃ¶rster Resonance

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High Performing Ternary Solar Cells through Förster Resonance Energy Transfer between Non-fullerene Acceptors Lei Yang, Wenxing Gu, Ling Hong, Yang Mi, Feng Liu, Ming Liu, Yufei Yang, Bigyan Sharma, Xinfeng Liu, and Hui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08146 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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ACS Applied Materials & Interfaces

High Performing Ternary Solar Cells through Förster Resonance Energy Transfer between Non-fullerene Acceptors Lei Yang†,§, Wenxing Gu†,‡,§, Ling Hong†, Yang Mi║, Feng Liu⊥, Ming Liu†, Yufei Yang†, Bigyan Sharma†, Xinfeng Liu║, and Hui Huang*† †

College of Materials Science and Opto-electronic Technology & CAS Key Laboratory of Vacuum Physics,

University of Chinese Academy of Sciences, Beijing, 101408, P. R. China. ‡

School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences,

Beijing, 101408, P. R. China. ⊥

Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai 200240, China.

║

Division of Nanophotonics, CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS

Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China.

ABSTRACT: Non-radiative Förster resonance energy transfer (FRET) is an important mechanism of organic solar cells, which can improve the exciton migration over long distance, resulting in improvement of efficiency of solar cells. However, the current observations of FRET are very limited and the efficiencies are less than 9%. In this study, FRET effect was first observed between two non-fullerene acceptors in ternary solar cells, which improved both the absorption range and exciton harvesting, leading to the dramatic enhancement in the short circuit current and power conversion efficiency. Moreover, this strategy is proved to be a versatile platform for conjugated polymers with different bandgaps, resulting in a remarkable efficiency of 10.4%. These results demonstrated a novel method to enhance the efficiency of organic soar cells.

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Keywords: energy transfer, non-fullerene acceptors, ternary solar cells, small amount doping, face-on orientation.

INTRODUCTION The rapid development of the bulk heterojunction (BHJ) organic solar cells (OSCs) has been witnessed in the last decades. Numerous strategies have been developed to enhance the power conversion efficiencies (PCEs) to over 10%.1-15 However, one strong limitation of many high performance OSCs is the relatively narrow absorption window (~100 nm) of active layers which drastically hinder further improvement of PCE. 16-18 Hence, an obvious solution to overcome this issue is to broaden the absorption range by employing multiple materials with complementary absorption to enhance the performance of OSCs.19-22 Recently, ternary solar cells show great potential to achieve high performance featuring the use of complementary absorption materials and the simplicity of fabrication, which enables the PCE of OSCs to exceed 10%.23-28 The current ternary blend solar cells can be classified into two categories: two donors and one acceptor (D1/D2/A),29-31 one donor and two acceptors (D/A2/A1).32-33 Acceptor materials in these two types ternary solar cells are centered on chemically modified fullerenes,34-35 due to limited acceptor materials that can be utilized. However, the intrinsic drawbacks of fullerene acceptors are well known, such as weak visible light absorption, poor stability and high cost.36-37 To date, non-fullerene organic acceptors are considered as promising alternatives to overcome the drawbacks of fullerenes due to their strong absorption, readily tunable energy levels and chemical structure.38-43 The PCEs of binary OSCs based on non-fullerene acceptor are over 10%,4, 10, 14 while there are very few reports on ternary solar cells based on two non-fullerene acceptors.27, 44-45


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Obviously, deeply probing the mechanisms of the ternary systems is critical to achieve high performing photovoltaic devices. There are three mechanisms in ternary solar cells based on the functionality of the third component: charge transfer,46 energy transfer,47-48 and parallellinkage or alloy model,23,

29

which are also tightly association with the location of the third

component in the ternary active layer. Non-radiative Förster resonance energy transfer (FRET) is long-range dipole–dipole interactions between “energy donor” and “energy acceptor” molecules,49 which has been demonstrated as an effective method to improve the PCE in OSCs.50-52 The FRET phenomena between two donors have been observed in several systems to gain high efficiencies,51-52 while the energy transfer between two acceptors in ternary solar cells has never been reported yet. In this study, a non-fullerene small molecular acceptor, 2,2′-[5,5′-(9,9-dioctyl-9Hfluorene-2,7-diyl)di(benzo[c][1,2,5]thiadiazole-7,4-diyl)di(methanylylidene)di(3-ethxyl-1,4oxothiazolidine-5,2-diylidene)]dimalononitrile

(FBR-CN),

was

employed

as

the

third

component for a host binary blend of PTB7-Th:ITIC-Th since these three materials showed complementary absorption. Interestingly, doping tiny amount of FBR-CN (1 wt%) as the second electron acceptor significantly increased the short circuit current (Jsc), resulting in an over 20% enhancement of PCE. Transient absorption spectroscopic studies revealed that a highly efficient FRET occurred between FBR-CN and ITIT-Th in a picosecond scale. Furthermore, these two acceptors can also be combined with a medium bandgap conjugated polymer, affording an efficiency of 10.4%, much higher than that of the binary systems. Materials structures and energy levels A new non-fullerene small molecular acceptor FBR-CN was synthesized through classic Suzuki coupling and Knoevenagel condensation in two steps with an overall yield of ~50%



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(Scheme 1). The chemical structure was confirmed by 1H NMR,

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13

C NMR, MALDI-TOF and

elemental analysis (Figure S1-2).

Scheme 1. Synthetic route of FBR-CN.

The chemical structures and the energy levels of PTB7-Th, FBR-CN and ITIC-Th are shown in Figure 1. LUMO/HOMO energy levels measured by cyclic voltammetry and UV-vis absorption spectra, which are -3.80/-5.50, -3.94/-5.91, and -4.11/-5.68 eV for PTB7-Th, FBR-CN and ITIC-Th, respectively (Figure 2A, S4). Obviously, the classic cascade energy level alignment was not formed among these three materials. Hence, the working mechanism of the ternary solar cells may not be charge transfer.27


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Figure 1. A) Chemical structures of PTB7-Th, FBR-CN and ITIC-Th. B) Energy levels of PTB7-Th, FBR-CN and ITIC-Th.

Figure 2. A) Absorption spectra of PTB7-Th, FBR-CN, and ITIC-Th. B) PL intensity of FBR-CN and absorption spectra of ITIC-Th. C) PL intensity of FBR-CN with different weight ratios of ITIC-Th in chloroform. D) J-V curves of ITIC-Th, FBR-CN and FBR-CN:ITIC-Th blend.

Photophysical studies of FBR-CN and ITIC-Th



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Effective FRET would occur if two prerequisitions are fulfilled: (1) There must be substantial overlap between the emission spectrum of the FRET donor and the absorption spectrum of the acceptor, and (2) the location of the “energy donor” should be close to the “energy acceptor” due to the limited radius of energy transfer. The UV-vis spectra of pure FBRCN and the photoluminescence (PL) spectrum of FBR-CN are shown in Figure 2B. It is obvious to find that FBR-CN film emission peak at 630 nm, which largely overlaps with the absorption region of ITIC-Th film (500-800 nm), making these two materials a good FRET pair. PL measurement is a convenient way to distinguish energy transfer from charge transfer among different materials.18, 51 We investigated steady PL spectra of FBR-CN88, ITIC-Th, and FBR-CN:ITIC-Th blend in chloroform. As shown in Figure 2C, the emission peaks of FBR-CN and ITIC-Th were located at 585 nm and 710 nm, respectively. Interestingly, with the increase of ITIC-Th content, the PL emission intensity of FBR-CN gradually decreases and ITIC-Th emission intensity continuously increases, indicating an efficient energy transfer from FBR-CN to ITIC-Th molecules.21, 48 Another method to verify the energy transfer between FBR-CN and ITIC-Th is to fabricate solar cells with FBR-CN, ITIC-Th and FBR-CN:ITIC-Th (1:1) as the active layers (without the donor), respectively. As shown in Figure 2D, the JSC of FBR-CN:ITIC-Th based solar cells was 0.08 mA cm-2, which was lower than both of pristine FBR-CN (0.14 mA cm-2) and ITIC (0.20 mA cm-2), implying that charge transfer between FBR-CN and ITIC-Th can be negligible.53 To investigate the location of FBR-CN in BHJ active layer, the surface energies of PTB7Th, ITIC-Th and FBR-CN were estimated with the contact angle of pure water and formamide on corresponding films (Figure S5). The surface energy can be calculated by the equation,54


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d p γ LV (1 + cosθ ) = 2 γ Sd γ LV + 2 γ Spγ LV

(1)

where γLV is the surface energy of liquid used for test, θ is the contact angle, γdLV is the dispersion component of γLV and γpLV is the polar component of γLV. Based on the equation (1), the surface energies of PTB7-Th (γPTB7-Th = 21.4 mJ m-2), ITIC-Th (γITIC-Th = 30.5 mJ m-2), and FBR-CN (γFBR-CN = 48.1 mJ m-2) were calculated. The interfacial surface energy can be further estimated by using interfacial tensions equation,55

γ X −Y = γ X + γ Y − 2 γ X ⋅ γ Y ⋅e[− β (γ

X

−γ Y ) 2

]

(2)

where β= 0.000115 m4 mJ-2 and γX-Y is the interfacial surface energy between X and Y. We estimated γ(FBR-CN)-(ITIC-Th)=4.68 mJ m-2, γ(FBR-CN)-(PTB7-Th)=10.38 mJ m-2, γ(PTB7-Th)-(ITIC-Th)=1.29 mJ m-2 according to the equation (2). The wetting coefficient of FBR-CN (ωFBR-CN) in blends of PTB7-Th and ITIC-Th can be calculated by Young’s equation,56

ωFBR − CN =

γ ( FBR − CN ) − ( ITIC −Th ) −γ ( FBR − CN ) − ( PTB 7 −Th ) γ ( PTB 7 −Th ) − ( ITIC −Th )

(3)

Thus, the wetting coefficient of FBR-CN in PTB7-Th:ITIC-Th blends was calculated to be -4.4. This value demonstrates that FBR-CN molecules are located in ITIC-Th phase.51 Thus exciton energy in FBR-CN can be efficiently transferred to ITIC-Th, and then excitons generated in ITIC-Th can be dissociated into free charge carriers at the interfaces of ITIC-Th/PTB7-Th.



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Figure 3. Transient absorption spectra of A, D) neat FBR-CN, B, E) 1 wt% ITIC-Th in FBR-CN, C, F) 10 wt% ITIC-Th in FBRCN.

Transient absorption (TA) spectroscopy was used to further study the photophysical processes of FBR-CN and ITIC-Th. The films of FBR-CN, ITIC-Th and FBR-CN:ITIC-Th with 1 and 10 wt% ITIC-Th were pumped at 400 nm with laser pump fluence ~20 µJ cm-2. As shown in Figure 3A, the negative signal at 500-570 nm apparently presents the ground state bleaching (GSB) of FBR-CN and the positive signal at 660-770 nm corresponds to the photoinduced absorption of FBR-CN at excited states. When incorporating 1 or 10 wt% ITIC-Th into FBR-CN, the GSB optical density (OD) of FBR-CN is drastically decreased. Meanwhile, the TA absorption at 650-720 nm is changed from positive to negative signal (Figure 3B, C), which implies that the GSB of ITIC-Th takes place and ITIC-Th is being excited. Because of the very weak absorption of ITIC-Th at 515 nm, the excitation of ITIC-Th is almost resulted from excited FBR-CN instead of the pumping wavelength. Another observation is the signals of the photoinduced absorption in FBR-CN pristine film excessively weakened when adding 10 wt% ITIC-Th. Moreover, the GSB of ITIC-Th occurs immediately at 1~2 ps after excitation in FBR-


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CN:ITIC-Th binary films (Figure 3C, F). These observations confirm the ultrafast energy transfer from FBR-CN to ITIC-Th.52 According to Förster theory, the lifetime of energy donor at an excited state decreases with increasing acceptor concentration since FRET is an additional nonradiative decay channel for the donor in the system.49 The time resolved transient photoluminescence (TRTPL) was used to further investigate the energy transfer from FBR-CN to ITIC-Th (Figure S6). The tests were performed by probing 630 nm emission under 510 nm light excitation. The average lifetime of pristine FBR-CN film was 1.68 ns and drastically decreased to 0.56 ns and 0.21 ns when incorporation of 1 wt% and 3 wt% ITIC-Th, respectively. The energy transfer efficiency η can be calculated using the equation,49 ߟ = 1−

ఛವಲ

(4)

ఛವ

where τD and τDA represent the fluorescence lifetime of the donor with and without the acceptor, respectively. From equation (4) the energy transfer efficiency is calculated to be 67% for 1wt% and 88% for 3 wt%, implying efficient energy transfer from FBR-CN to ITIC-Th. Photovoltaic performances and charge generation and transport The OSCs were fabricated with an inverted device structure of ITO/ZnO/ active layer/ MoO3/Ag. The current density versus voltage (J-V) curves of ternary devices with different contents of FBR-CN under the AM 1.5G illumination were demonstrated in Figure 4B and the corresponding photovoltaic parameters were summarized in Table 1. The PTB7-Th:ITIC-Th based binary solar cells achieved an efficiency of 8.1% with Voc of 0.79 V, Jsc of 14.70 mA cm-2, and FF of 69.7%, comparable to previously reported results.57 Interestingly, when adding tiny amount of FBR-CN (1 wt%), the ternary solar cells afforded a significantly enhanced PCE of 9.8% with a Voc of 0.79 V, Jsc of 17.76 mA cm-2, and FF of 70.0%. It is noted that the gain of the



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efficiency is mostly due to the enhanced Jsc, while Voc and FF almost maintained the same. The substantial enhancement in Jsc may directly result from the efficient FRET from FBR-CN to ITIC-Th, since FBR-CN acts as an additional sunlight absorber to harvest more photons and deliver photon energy to ITIC-Th. When the amount of FBR-CN increased to 10%, the Jsc was further increased to 17.79 mA cm-2, which may be ascribed to the more photon harvest by the FBR-CN, while the FF obviously decreased to 65.7%, indicative of disturbing of the morphology and charge transport of the binary system due to the excessive FBR-CN. Not surprisingly, both Jsc and FF dramatically decreased when 20% FBR-CN was incorporated into the blend films, suggesting the severe morphology issues with extra FBR-CN. To validate the effect of FBR-CN on the improvement in Jsc, UV-vis absorption spectra (Figure 4A) and external quantum efficiency (EQE) spectra (Figure 4C) of ternary systems with varying FBR-CN content were investigated. Obviously, with the increase of FBR-CN content, the films absorption coefficient significantly enhanced in the range from 400 nm to 560 nm corresponding to the absorption of FBR-CN, while the absorption intensity gradually decreased at

Figure 4. A) UV–vis absorption coefficient B) J-V curves C) EQE curves of PTB7-Th:ITIC-Th blends with different FBR-CN weight ratios.

560-700 nm along with the increase of FBR-CN content. In general, the trend of EQE is mostly similar to the UV-vis spectra. When 10% FBR-CN was incorporated into the ternary systems, the


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EQE curves demonstrated a global enhancement, while the UV-vis absorption was not increased across the whole spectrum. The enhancements in EQE can be divided into two regions, 400-560 nm and 560-700 nm. It is clear that enhancement in EQE from 400 nm to 560 nm may be attributed to the FBR-CN absorption region, whereas the EQE between 560 and 700 nm may be resulted from the efficient energy transfer from FBR-CN to ITIC-Th, since additional excitons generate and transfer to ITIC-Th and then dissociate into free charges. The EQE integrations are in good agreement with Jsc values obtained from J-V measurement, indicating the reliability of the photovoltaic measurements.

Table 1. Summary of photovoltaic parameters of ternary solar cells with different weight ratios of FBR-CN under AM1.5 illumination at 100 mW/cm2. FBR-CN (w/w)

VOC (V)

JSC (mA/cm2)

FF (%)

PCEave (max) (%)

0%

0.79±0.01

14.70±0.07

68.98±0.76

8.05 (8.10)

1%

0.79±0.01

17.58±0.18

69.85±0.11

9.71 (9.82)

5%

0.79±0.01

18.02±0.09

67.05±0.39

9.54 (9.55)

10%

0.80±0.01

17.79±0.08

65.70±1.70

9.29 (9.35)

20%

0.82±0.01

16.73±0.33

54.02±0.15

7.41 (7.58)

100%

0.98±0.01

10.35±0.14

44.24±0.97

4.46 (4.64)

The steady PL was carried out to investigate the efficiency of exciton splitting. It is observed that the PL intensity at 760 nm decreased by 97.6%, 98.4%, 97.7% and 94.5% for the 0%, 1%, 10% and 20% content of FBR-CN in ternary blend, respectively (Figure S7A). The obvious PL quenching indicates that efficient charge splitting and transport both in binary and



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ternary systems. The TRTPL was further used to study the charge transfer in ternary solar cells (Figure S7B). The excited-state lifetime of PTB7-Th:ITIC-Th blend film is 0.23 ns. When 1% and 5% FBR-CN was added into this binary blend, the lifetime is 0.20 and 0.19 ns, respectively, suggesting that charge can be efficiently delivered both in binary and ternary blends. The charge transport characteristics are critical for FF of organic solar cells.58 The hole and electron mobilities were calculated by the space-charge limited current (SCLC) method. The ternary

OSCs

devices

for

hole

only

mobilities

were

used

the

architecture

of

ITO/PEDOT:PSS/active layer/MoO3/Ag, while electron mobilities were fabricated with the architecture of ITO/ZnO/active layer/Al. The hole and electron mobility values were calculated and summarized in Table 2 and shown in Figure S8. The electron and hole mobilities are 4.00×10-4 and 1.31×10-3 cm2 V-1 s-1, respectively, for pristine binary PTB7-Th:ITIC-Th solar cells. When 1wt% FBR-CN was incorporated into the binary system, the electron and hole mobilities of ternary solar cells slightly increased to 4.51×10-4 and 1.73×10-3 cm2 V-1 s-1, respectively, while the µh/µe is similar to that of binary blend. However, both hole and electron mobilities of the blend films gradually decrease when loading excess amount of FBR-CN, which may be ascribed to the film morphology deterioration. Obviously, the trend of SCLC mobility is consistent with the fill factors (FF) of the OPV devices.25

Table 2. Summary of hole and electron mobility values of PTB7-Th:ITIC-Th with different weight ratios of FBR-CN obtained from SCLC method. FBR-CN (w/w)

thickness (nm)

µe (cm2 V-1 s-1)


µh (cm2 V-1 s-1)

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0%

105

4.00×10-4

1.31×10-3

1%

108

4.51×10-4

1.73×10-3

10%

90

2.48×10-4

5.90×10-4

20%

89

1.67×10-4

3.58×10-4

100%

85

2.26×10-5

1.72×10-4

Photocurrent density Jph versus effective voltage Veff curves are performed to further investigate charge generation and collection efficiencies in ternary systems. As showed in Figure S9A, Jph is defined as Jph=JLight - JDark and Veff is defined as Veff=V0-Va, where V0 is the voltage when Jph equals zero and Va is applied bias voltage.59 At a large reverse biases, Jph becomes saturation current density (Jsat). The Jsat was 17.14 mA cm-2 and 17.71 mA cm-2 for 0 wt% and 1 wt% FBR-CN ternary devices, respectively. The larger Jsat values of 1 wt% ternary device imply more exciton generation. P(E,T) is calculated by normalizing Jph with Jsat (Jph/Jsat)60. A larger P(E,T) value (96.4%) of 1 wt% ternary devices than that of origin binary devices (94.8%) suggests 1 wt% ternary devices possess higher charge generation and collection efficiencies. To investigate bimolecular recombination dynamics in ternary solar cells, Jsc was measured as a function of illumination intensity and the data follows the power law: Jsc∝PlightS.

61-62

The

exponential factor S value were 0.993, 0.998 and 0.991 for 0 wt%, 1 wt% and 10wt% FBR-CN ternary devices (Figure S9B), respectively, suggesting a weak bimolecular recombination in the active layers of ternary system when adding small amount of FBR-CN. The influence of morphology



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Grazing-incidence X-ray diffraction (GIXD) was employed to investigate the crystalline properties in ternary blends. The neat film GIXD results were shown in Figure S10, PTB7-Th polymer showed both a very strong (100) peak in the in-plane (IP) direction at 0.27 Å-1 and (010) peak in out-of-plane (OOP) direction at 1.64 Å-1, indicating that PTB7-Th polymer dominantly adopted face-on orientation with respect to the substrate. A sharp and relative strong (010) peak in OOP direction at 1.79 Å-1 was observed in ITIC-Th film, with a weak (100) peak in IP direction at 0.38 Å-1. Another acceptor FBR-CN showed a broad but weak diffraction peak (010) in OOP direction at ~1.65 Å-1 and a weak (100) peak in IP direction. As shown in Figure 5, PTB7-Th:ITIC-Th binary blends clearly showed a strong reflection at 0.27 and 0.38 Å-1 in IP direction, corresponding to the lamellar distance 23.3 Å for PTB7-Th and 16.5 Å for ITIC-Th respectively. The diffraction peak at 1.77 Å-1 may arise from both PTB7-Th and ITIC-Th, corresponding to a π–π stacking distance of 3.55 Å. Compared to PTB7-Th:ITIC-Th binary blends, a relative stronger reflection peak in OOP direction was observed when added 1wt% FBR-CN to the binary blends, suggesting the formation of more face-on orientation that is beneficial for vertical charge transport, which is consistent with OPV device performance. However, the relative proportion of face-on orientation gradually decreased when adding more FBR-CN in origin binary blend, which tends to disrupt the optimized film morphology for charge transport and consequently decreases OPV device performance.


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Figure 5. A) GIXD images of ternary OSCs with different FBR-CN contents. B) The out-of-plane (solid line) and in-plane (dotted line) profiles of 2D GIXD film patterns.

Phase separation in ternary blends was further performed by resonant soft X-ray scattering (RSoXS). The RSoXS profile of the PTB7-Th:ITIC-Th blends showed a scattering at 0.015 Å-1 (Figure S11), center-to-center distance was calculated to be ~40 nm. When FBR-CN content is less than 10 wt%, the scattering profile of ternary blend is similar with origin binary blend, with a distance of 34 nm. While the scattering peak obviously shifted to ~0.01 Å-1 after more FBR-CN was added, corresponding to a distance of 63 nm, suggesting a higher FBR-CN content is prone to forming excessively large D/A phase separation, which will deteriorate the device performance.



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Atomic force microscopy (AFM) and transmission electron microscope (TEM) were employed to further investigate the effect of FBR-CN content on the film morphology of ternary solar cells. As shown in Figure S12, the ternary blend with proper amount of FBR-CN (≤ 5 wt%),

Figure 6. A, B, C, D) TEM images of ternary OSCs with different FBR-CN contents.

the nanomorphology of the active layer showed similar the root-mean-square (RMS) roughness (~2 nm) and phase separation, in comparison with that of binary PTB7-Th:ITIC-Th control film. This indicates the small amount of FBR-CN does not disturb the solid state packing, consistent with FF results. Moreover, the roughness of ternary blend films with 20 wt% FBR-CN was increased to ~3.4 nm, along with larger phase separation, which was very easy to form defects and trap sites, unfavorable for charge separation and transport, resulting in lower mobility and device performance. TEM images show an internal morphology of the ternary blends (Figure 6). The ternary mixture shows a similar morphology to the binary blend, however the domain size gradually increased along with the increase of the ratio of FBR-CN, which is consistent with the RSoXS results. Generality of the method


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To further confirm that FRET is an effective way to enhance photovoltaic performances, a medium bandgap polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5c′]dithiophene-4,8-dione)] (PBDB-T)63 as the donor with the same two acceptors were used in ternary solar cells. The photovoltaic parameters were summarized in Table S1. As we expected, when incorporation of 5wt% of FBR-CN, the ternary solar cells afforded a dramatically enhanced PCE of 10.4% with a Voc of 0.86 V, Jsc of 16.72 mA cm-2, and FF of 72.4%, surpassing the PCE of corresponding binary cells (9.88%). Similar with PTB7-Th:ITIC-Th:FBR-CN ternary OSCs, the Jsc of the ternary is significantly increased compared to original PBDB-T:ITIC-Th binary devices when loading small amount of FBR-CN,. The EQE curves showed a global enhancement in comparison to that of the binary devices (Figure S13), suggesting efficient energy transfer between ITIC-Th and FBR-CN.

CONCLUSIONS A non-fullerene small molecular acceptor FBR-CN was synthesized and utilized as second acceptor in a host binary blend of PTB7-Th:ITIC-Th to fabricate ternary organic solar cells. It is worthy to note that doping tiny amount of FBR-CN (1 wt%) can significantly improve the PCE of ternary OSCs from 8.1% to 9.8% (~20% PCE improvement), which mainly results from enhanced Jsc. The mechanism was investigated through PL, TRTPL, TA spectroscopy and surface energy studies, which showed that FBR-CN acts as an additional sunlight absorber to harvest more photons and deliver photon energy to ITIC-Th through FRET. As a result, extra excitons were generated and then dissociated into free charges, resulting in Jsc improvement of ternary solar cells. Moreover, this strategy is proved to be a versatile platform for conjugated



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polymers with different bandgaps, resulting in a remarkable efficiency over 10%. These results demonstrated a novel method to enhance the efficiency of organic soar cells.

EXPERIMENTAL SECTION Materials All reagents and chemicals were purchased from commercial sources and were used without further purification unless stated otherwise. Chloroform was freshly distilled before use. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene was purchased from Energy Chemical Co., Ltd. 4-bromo-2,1,3-benzothiadiazole-4-carboxaldehyde was purchased from HWRK Chemical Co., Ltd. 2-(3-ethyl-4-oxothiazolidin-2-ylidene)malononitrile, 7,7'-(9,9dioctyl-9H-fluorene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-4-carbaldehyde) (FBT-2CHO) was synthesized according to previous paper.40, 64 Synthesis of FBR-CN 2-(3-ethyl-4-oxothiazolidin-2-ylidene)malononitrile (290 mg, 1.5 mmol) and FBT-2CHO (215 mg, 0.3 mmol) were dissolved in chloroform (50 ml) by gentle heating and then 1.5 mL piperidine was added and the mixture heated at 65 °C for 20 h. After cooling to room temperature, the crude product was purified by column chromatography on silica gel, yielding FBR-CN as a red solid (198 mg, 62%). 1

H NMR (400 MHz, Chloroform-d) δ 8.68 (s, 2H), 8.09 (d, J = 9.3 Hz, 2H), 8.04 (s, 2H), 7.95 (d,

J = 16.6 Hz, 6H), 4.38 (d, J = 8.6 Hz, 4H), 2.10 (d, J = 10.6 Hz, 4H), 1.44 (d, J = 7.3 Hz, 6H), 1.09 (s, 24H), 0.74 (d, J = 7.9 Hz, 6H).


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13

C NMR (101 MHz, Chloroform-d) δ 185.82, 185.67, 177.31, 175.89, 170.68, 169.88, 168.12,

159.74, 159.16, 123.18, 123.12, 122.87, 122.55, 87.79, 86.88, 80.79, 79.34, 72.37, 71.12, 68.45, 67.51, 61.84, 58.19, 47.79, 46.39, 33.62. MS (MALDI-TOF): m/z (M+)= Calcd for C59H56N10O2S4: 1064.3; found: 1065.3. Elem. Anal. Calcd for C59H56N10O2S4 : C, 66.51; H, 5.30; N, 13.15. Found: C, 66.89; H, 5.21; N, 13.36.

ASSOCIATED CONTENT Complete experimental section including synthetic details, NMR spectra, UV-Vis spectra, Cyclic voltammograms, PL, TRTPL, GIXD, AFM, and R-SoXS figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Author Contributions §

L. Y. and W. G. contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge the NSFC (51303180, 21574135), Beijing Natural Science Foundation (2162043), One Hundred Talents Program of Chinese Academy of Sciences, and University of Chinese Academy of Sciences for financial support.



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TOC figure

FRET effect was first observed between two non-fullerene acceptors in ternary solar cells, which improved both the absorption range and exciton harvesting, resulting in the dramatic enhancement in Jsc and a remarkable efficiency of 10.4%.


High Performing Ternary Solar Cells through FÃ¶rster Resonance

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