Ternary Organic Solar Cells with Coumarin7 as the Donor Exhibiting

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Ternary Organic Solar Cells with Coumarin7 as the Donor Exhibiting over 10% Power Conversion Efficiency and a High Fill Factor of 75% Xinwei Chen, Si-Lu Tao, Cong Fan, Dongcheng Chen, Ling Zhou, Hui Lin, Cai-Jun Zheng, and Shi-Jian Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07704 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 16, 2017

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ACS Applied Materials & Interfaces

Ternary Organic Solar Cells with Coumarin7 as the Donor Exhibiting over 10% Power Conversion Efficiency and a High Fill Factor of 75% Xin-Wei Chen,a Si-Lu Tao,*,a Cong Fan, b Dong-Cheng Chen,c Ling Zhou,a Hui Lin, *,a Cai-Jun Zheng,a and Shi-Jian Su*,c a School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu, P. R. China b School of Microelectronics and Solid-state Electronics, University of Electronic Science and Technology of China (UESTC), Chengdu, P. R. China c State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou, P. R. China

Abstract: Ternary bulk heterojunction (BHJ) is a brilliant photovoltaic technology for improving the performance of organic solar cells (OSCs), since the light absorption range can be significantly extended by using multiple donors or acceptor materials. In this paper, coumarin7 (C7), a small organic molecule typical led used in organic light-emitting diodes, was initially exploited as second electron-donor component in ternary bulk heterojunction OSCs along with conventional blend system spolythieno[3,4-b]-thiophene/benzodithiophene(PTB7) and [6,6]-phenyl-C71 -butyric acid methyl(PC71 BM). A champion PCE value of 10.28% was realized in the ternary OSCs when incorporated with 5 wt % C7 doping ratio in the donors, corresponding to about 35% enhancement compared with the PTB7:PC71BM-based OSCs, a high fill factor (FF) of 75.03%, a short-circuit currentdensity (Jsc) of 18.72 mA cm-2 and an open-circuit voltage (Voc) of 0.73V. The enhanced performance of the ternary OSCs can be attributed to the simultaneous improvement of the FF and the Jsc. In addition to Corresponding author: E-mail: [email protected], [email protected], [email protected].

ACS Paragon Plus Environment


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extended light absorption, a perfect nanofiber filament active layer morphology is obtained due to the good compatibility between C7 and PTB7, which facilitates the balance of charge transportation and the suppression of charge recombination. This investigation suggests that coumarin derivatives, which have completely different structure with polymer donors, can also be used to fabricate ternary solar cells and have the potential applications to obtain amazing performance after further device engineering and optimization.

Key words: coumarin7; small organic molecule; bulk heterojunction; nanofiber filament; ternary OSCs

1. Introduction Bulk heterojunction (BHJ) organic solar cells (OSCs) have been considered as one of the most promising candidates to replace traditional silicon-based inorganic solar cells. Compared to the traditional inorganic counterparts, conventional OSCs with one donor and one acceptor (binary blend) could exhibit magnificent merits such as flexibility, lightweight and low cost, and simultaneously could be easily fabricated by solution processing and/or roll-to-roll manufacturing with the advantages of better exciton-dissociation efficiency and charge-carrier transportation.1-4 Up to now, OSCs have realized considerable progress in power conversion efficiencies (PCEs), and PCEs of both small molecules and polymers OSCs with binary single-junction configuration have already exceeded 10%.5-6 It is widely believed that PCE of OSCs is proportional to the product of short circuit current (Jsc), open circuit voltage (Voc)


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and fill factor (FF). To further improve the performance parameters and operational stability of OSCs, tremendous efforts such as developing novel organic dyes,1-2 innovating device architectures,7-9 and optimizing active-layer morphology,10-12 have been thoroughly paid. Despite of that, it is very difficult to increase Jsc, Voc, and FF at the same time. Currently, ternary OSCs whose active layer is composited by two donors and one acceptor (or one donor and two acceptors) are gradually emerging as the alternative and mainstream to the conventional binary analogues. Compared to the binary ones, the concept of ternary OSCs could extend the absorption spectra to near-infrared and ultraviolet regions by making use of the complementary absorption spectrum of the third component added, and ultimately could realize enhanced capability for light harvesting, which is usually fulfilled by tandem OSCs containing two (or more) binary sub-cells built in either series or parallel. Meanwhile, ternary OSCs could retain the simplicity of single-step processing for blending the active layer, rather like the tandem binary OSCs facing the complex fabrication process and high production cost. Furthermore, if the third component is carefully selected, not only more photo-generated excitons could be produced, but also the charge-carrier mobility and the active-layer morphology could be simultaneously improved, ultimately resulting in enhanced PECs.13-14 Recently, the highest PCE for the ternary OSCs with single-junction configuration has already exceeded 12%.15 Expectably, organic polymers are initially utilized as the second donor or accepter additives in ternary OSCs to extend the absorptions of solar spectrum, owing



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to their good compatibility with electron-accepting fullerene/its derivatives and favorable optical absorption characteristics.16 Notably, the proper blend compatibility is the prerequisite for forming the ideal film morphology and plays the crucial role on the PCEs for OSCs 17. Brabec et al. have reported a high FF of 77% by incorporating the polymer of Si-PCPDTBT into the PTB7/PC71BM-blended binary OSCs benefit from good compatibility between two donors makes the additive polymer donor become a bridge to effectively promote charge transport.18 However, the crystallinity of the conjugated polymers is usually low and insufficient to obtain optimal morphology even they have good compatibility with fullerene acceptors.19 Furthermore, the intermixing and interactions between the polymers in the active layer seem to be inescapable, which could usually result in morphological traps and/or recombination centers rather than good compatibility and complementary absorption characteristics.

20-21

These defects limit further improvement in the performance of

ternary OSCs based-on polymer additives. Alternatively, small organic molecules (SMs) could show more perfect crystallinity and could usually make the active layer form new spatial microstructure, which is beneficial to improve charge-transporting characteristics.15,19 Meanwhile, the exciton-trap centers can also be controlled by optimizing the doped SM amounts.19 For instance, Yang et al. pointed out the PTB7-th/PC71BM blend could form the polymer-SM alloy with mixed face-on and edge-on orientation by introducing the highly crystalline SM of DR3TSBDT, which could cause the simultaneous increase of Jsc and FF.15 However, most SMs added in the ternary OSCs have a relatively


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complex chemical structure and very similar to the blended polymers to avoid the compatibility risk. Nowadays, it is still extremely desirable to exploit new SMs with highly efficient opt-electronic properties for ternary OSCs. Although many high-performance ternary OSCs based on SM have been reported,

22-23

the roles of

SM added are still not thoroughly unraveled. Remarkably, it is still a grand challenge whether for the added polymers or SMs that the technological integration of all possible advantages into one single junction for the ternary OSCs. Herein, we initially reported that coumarin7 (C7), a SM widely used in organic light-emitting diodes, with large structural difference with the most electron-donating polymers and the SMs reported by other groups could be the highly efficient second donor for the ternary OSCs.15,16-19,22-23 The rational choice of C7 was mainly based on its intensive absorptions in the blue-green region,24 which is the strongest luminous intensity for solar spectrum. Meanwhile, the fluorescent C7 exhibits very high photoluminescence (PL) quantum efficiency, which is indicative of that the generated excitons are hardly quenched by its low de-radiative rate. Consequently, by adding C7 into

the

conventional

electron-donor

poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-3-fluoro-2 -[(2-ethylhexyl)carbonyl]thieno[3,4b]thiophene-4,6-diyl}

(PTB7)

and

the

electron-acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blends, the resulting ternary OSCs could realize an amazing PCE value of 10.28% and a high FF of nearly 75% when its dopant content was optimized, which exhibited the efficiency enhancement of ~35% when compared to the reference OSCs without C7. At the same



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time, both the morphology of the ternary-blend active layer (PTB7/C7/PC71BM) and its charge mobility were significantly improved. 2. Experimental Section 2.1 General information PTB7 was purchased from 1-Material Inc. and PC71BM was from American Dye Source Inc.. Courmin7 was purchased from TCI Chemical Inc.. All the related chemical materials, including the reagents used in this research were purchased from Sigma-Aldrich Co. or Alfa Chemical Inc. In addition, all materials were used without further purification and taken from the same batch to ensure the fair and credible comparisons between the experimental results. 2.2 Device fabrication and measurements Inverted architecture of OSCs was employed to guarantee the stable performance for all binary and ternary devices studied in this research. All OSCs were fabricated using the same technological process and characterized under the atmospheric environment within two days. The ZnO precursor solution was prepared by dissolving 110 mg of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and 31 mg of ethanolamine (NH2CH2CH2OH) in 1 mL of 2-methoxyethanol (CH3OCH2CH2OH). The active-layer solutions with various contents of C7 were prepared in chlorobenzene (CB) with 3 vol% of 1,8-diiodooctane (DIO). ITO-coated glass substrates (15 Ω/square) were washed via sequential ultra-sonication in ethanol, detergent water, acetone, isopropyl alcohol for 20 min, respectively, and then dried by nitrogen blow. After ultraviolet-ozone radiation treatment for 30 min, the electron-extracting layer of ZnO was deposited


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through the spin-coating method at 4000 rpm for 30s from the ZnO solution, then the ZnO-coated substrates were annealed at 200 oC for 1 hour immediately. The ternary-blend solutions with various doping proportions were stirred for 48 hours on a hotplate at 40

o

C in glove box before usage. Prior to the evaporation of

hole-transporting layer, the active layers were spin coated onto the ZnO buffer layer in the N2 glove box at 2000 rpm for 30s, resulting in the uniform film with a thickness of ~100 nm, the uniform spin-coating speed we used can guarantee the best efficiency of the devices and it is the general optimum speed of the devices with different C7 doping ratio. A thin film (10 nm) of MoO3 was thermally evaporated as the anode interlayer through LN-1102SA (Shenyang Vacuum Inst.) evaporation equipment at a rate of 0.5Å/s under a pressure of 5×10-4 Pa. Finally, Ag anode was deposited at a rate of 3Å/s without breaking the vacuum atmosphere. The current-voltage characteristics were carried out under AM 1.5G simulated sunlight (Newport Oriel Sol3A Simulator, calibrated with an NREL-calibrated Si solar cell) with a Keithley 2400 source meter instrument. An ITO substrate holder with a fixed mask area (0.02 cm-2) in cooperation with Keithley 2400 was also used in the current-voltage test. All the test specimens used in this research were fabricated on the quartz-glass substrates and tested in ambient air unless the specified statement. EQEs were measured using a QEX10 Quantum Efficiency Measurement System (PV Measurements, Inc.) equipped with a standard Si diode, where the monochromatic light was generated from a PV Measurements 300W lamp source. UV-Vis absorption spectra were performed on a U-3900 (Hitachi) spectrophotometer. PL spectra were



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obtained using F-4600 (Hitachi) fluorescence spectrometer. The thickness of the film layers was demarcated by using a step profilometer (AMBIOS-XP2). In order to restore the real surface morphology of the fabricated OSCs, the manufacture process was followed to the specimens.

Atomic Force Microscope (AFM) The active layer specimens for AFM measurements were deposited on the ZnO-coated ITO glass under ambient conditions in order to restore the real surface morphology of the devices. The AFM images were measured under an Agilent Technologies AFM system (AC Mode III, Agilent Technologies lnc.). Transmission Electron Microscopy (TEM) In order to fulfill the special requirements for TEM measurements, the active-layer films were deposited on the PEDOT: PSS (60 nm) coated glass under ambient conditions. The samples were put into a vessel containing ultrapure water, and then the organic-phase film could float on the water due to the hydrophilic property of PEDOT:PSS. The active-layer film was transferred to a TEM lacey copper grid. The TEM investigations were performed by using the high resolution TEM-scanning probe SPM system (Hitachi TEM system) under 100 kV. The test results were obtained from the Institute of Physics (CAS). Charge Mobility The hole and electron mobilities of the C7/PTB7/PC71BM-blend films with different doping ratios of C7 were measured by SCLC method. The hole-only and


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electron-only devices were fabricated as following: ITO/PEDOT:PSS/active layer/MoO3/Au

and

ITO/ZnO/active

layer/Al,

respectively.

The

obtained

current-voltage curves were consistent with the Mott-Gurney square law .25

(V − Vbi )2 9 J = ε rε 0 µ (Eq1) and 8 L3

µ = µ0 exp(β

V − Vbi ) (Eq2) L

Where ε0 is vacuum permittivity (≈8.85×10−14 F cm-1), εr is relative permittivity (≈3.00), L is the thickness of the active layer (~100 nm), V is the applied voltage on the device, µ is the carrier mobility determined by SCLC(Space Charge Limited Current) measurements, µ0 is zero-field mobility and β is a field activation coefficient that is proportional to the Poole-Frenkel factor.26 The applied voltage is corrected for the built-in voltage Vbi (0.45 V for the hole-only device, 0.0 V for the electron-only device), which arises from the asymmetric contact between the electrodes.27 The values of µ0 and β were calculated by linear fitting. By using the obtained values, the hole and electron mobility data can be obtained by Eq (2). Charge recombination The power law dependence of Jsc upon illumination intensity can be description by using the following equation:28-29

J sc ∝ I α

(Eq3)

Where α is the exponential factor and I is the light intensity; The Voc as a function of illumination intensity can be description by using the following equation:30-31 Voc =

Egap q

−

KT (1 − PD )γ Nc2 ln[ ] q GPD


(Eq4)


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The value of Egap, q, k, T, γ, PD, NC were determined by the material or the experimental environment; G is the generation rate of electron-hole pairs, which is independent of other parameters and directly associated with the light intensity. The slope of Voc versus the natural logarithm of the light intensity gives kT/q can stand for the bimolecular recombination process.

3. Results and discussions 3.1 Opt-electronic properties

Figure 1. The chemical structures of the used organic materials.

The chemical structures of PTB7, C7 and PC71BM used in this study were illustrated in Figure 1. The UV-Vis absorption spectra for the neat films of PTB7, C7 and PC71BM were respectively shown in Figure 2a. Expectably, C7 exhibited the satisfactory complementary absorptions with PTB7, where the maximum absorption peaks were 675 nm for PTB7 and 435 nm for C7, respectively. Meanwhile, C7 could further enhance the absorption intensities in the ultraviolet-visible (UV-Vis) range of 350-500 nm where the electron-acceptor PC71BM usually exhibited intensive absorptions. Especially, it was worth noting that the light in the 350-500 nm range


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coincides with the strongest intensity of the solar radiation spectrum. Since the high energy (high frequency) photons are more likely to stimulate the photo-generated excitons, the complementary absorptions of C7 in this range could be more effective for collecting photons. Figure 2b depicted the absorption spectra of the ternary-blend films with the different doping ratio of C7, where the ternary-blend film was composited by two donors (C7 and PTB7) and one acceptor (PC71BM) with the invariant blending weight ratio (wtdonors:wtacceptor=1:1.5). All the ternary-blend films displayed wide absorption range extending from ultraviolet region (300 nm) to near-infrared region (700 nm). As the doping ratio of C7 increased from 5 to 10 wt%, the absorptions in 400-550 nm were significantly enhanced. Even though the absorption spectrum in PTB7 regions is gradually weakened with the increased C7 doping ratio leads to the similar absorption spectra integrated area for the devices with 10% C7 and 0% C7, because the strongest part of the solar radiation spectrum is concentrated in the 400-550nm, so the addition of C7 will have a positive impact on the short-circuit current of the devices. However, further augmenting the doping ratio of C7 to 15-20 wt% could lead to the decreased absorptions of 400-550 nm, and simultaneously the absorption intensities in the range of 550-750 nm belonging to PTB7 were also slightly decreased. Since the crystallization and molecular stack of polymers could be affected by the favorable interaction with the added SM,18,22 the excessive doping amounts (15-20 wt%) of C7 might introduce serious phase-separated domains in the PTB7: C7: PC71BM blends and formed individual blend regions in the films, ultimately leading to the attenuated



absorptions.20

(a)

Coumarin7 PTB7 PC71BM

0.8

0.4

Absorption (a.u.)

Absorption (a.u.)

1.0

0.6 0.4 0.2

400

400

500

600

Wavelength (nm)

700

×0.5

300

200

100

0

0.1

800

0% 5% 10% 15% 20% 100%

600

700

800

Wavelength (nm)

900

0% 5% 10% 15% 20%

0.2

300

50

(c)

(b)

0.3

PL intensity (a.u.)

300

PL intensity (a.u.)

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Page 12 of 30

400

500

600

Wavelength (nm)

700

800

(d) 0% 5% 10% 15% 20%

40 30 20 10 0

600

700

800

Wavelength (nm)

900

Figure 2. UV-Vis absorption spectra for (a) the neat films of PTB7, PC71BM and C7, respectively and (b) the PTB7/PC71BM/C7-blend films with different doping ratio of C7; (c) PL spectra of the C7/PTB7-blend films and (d) PL spectra of the PTB7/PC71BM/C7-blend films with different doping ratio of C7.

On the other hand, it was well known that C7 exhibited strong orange fluorescence and its emission spectrum sort of overlapped with the absorption spectrum of PTB7 (Figure 2a and c). Notably, the neat films of C7 and PTB7 showed strong emissions with peaks at 575 and 750 nm, respectively. Consequently, in order to investigate whether the energy transfer could occur between C7 and PTB7 or not, PL spectra of the PTB7/coumarin7-blend films with different doping ratios of C7 were investigated under the monochromatic excitation light of 500 nm. According to the similar cases reported,32-35 the emission intensity from PTB7 should be increased in the PTB7/C7-blend film along with the increased doping ratio of C7 if the energy transfer from C7 to PTB7 could occur. However, as shown in Figure 2c, the emission


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intensities (700-850 nm) of PTB7 were decayed along with the increased doping ratio of C7, indicating the intermolecular energy transfer between C7 and PTB7 was ineffective or negligible in PTB7/C7-blends. Considering that there is a competitive relationship between charge transfer and energy transfer, so the photo-generated electrons should be transferred from C7 to PTB7 efficiency, just like P3HT to PCPDTBT.32 Moreover, the emission quenching of PTB7 and C7 was more obvious in the resulting ternary-blend films. This emission reduction of the PL intensity was due to the photo-generated charge efficiently transfer from C7 to PTB7 to PC71BM, because the photo-generated exciton is dissociated before luminescence can occur (Figure 2d).35 These above results from these PL confirmed that high exciton utilization was present in ternary devices and also indicate that the relatively independent charge-transfer pathways probably existed at the donors (C7 and PTB7) and acceptor (PC71BM) interface. 3.2 Photovoltaic properties

Figure 3. (a)The structure of the fabricated inverted OSCs and (b) the related energy levels for the used materials

Subsequently, the BHJ OSCs with the inverted configuration were fabricated (Figure 3a), and the related energy levels of the active layer materials are shown in Figure 3b. The LUMO energy level of C7 proven electrons will be transferred from



C7 to PTB7, consistent with our findings in PL spectrum. The composition ratio of the ternary-blend active layer was maintained to be wtdonors:wtacceptor=1:1.5. To realize better film morphology, DIO was used as the additive. The doping ratio of C7 relative to PTB7 in the donor compositions of the active layers was selectively controlled to be 5 % (device A), 10 % (device B), 15 % (device C) and 20 % (device D) in the following OSCs, respectively. For comparison, the binary OSCs (device E) without the addition of C7 were constructed under the same conditions. In the each case mentioned above, at least ten OSCswere fabricated to calculate the average PCEs. Additionally, the peak PCE value was 0.61 % under the same conditions for the binary OSCs whose active layer only contained C7 and PC71BM. 5

(a)

80

(b)

0

0% 5% 10% 15% 20%

-5 -10

60

EQE(%)

Current density(mA cm-2)

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Page 14 of 30

20

-15 -20 -0.2

0% 5% 10% 15% 20%

40

0.0

0.2

0.4

0.6

0.8

0 300

1.0

400

Bias(V)

500

600

700

800

Wavelength(nm)

Figure 4. (a) Current-voltage (J-V) characteristics for the fabricated OSCs; (b) EQE curves for the fabricated OSCs. Table 1. Summary for the fabricated OSCs with different doping ratios of C7 in this study. Voc

Jsc

FF

Max. PCE

Average PCEs

(V)

(mA/cm2)

(%)

(%)

(%)

95/5/150

0.73

17.11

72.46

9.06

8.89 ±0.34

B

90/10/150

0.73

18.72

75.03

10.28

10.06±0.38

C

85/15/150

0.72

18.28

73.37

9.65

9.56±0.22

D

80/20/150

0.71

16.77

72.48

8.63

8.49±0.37

E

100/0/150

0.74

15.47

66.12

7.63

7.42±0.22

Device

PTB7/C7/PC71BM

A

The current-voltage (J-V) characteristics and EQE curves for all fabricated OSCs


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were shown in Figure 4 and all the OSCs parameters were summarized in Table 1. It was encouraging that the results of all the ternary OSCs containing C7 were significantly enhanced, particularly for the device B with 10% C7, of which the Jsc was largely increased by ~20% from the control device E (15.47 mA cm-2) without C7 to 18.72 mA cm-2. This result was in accordance with the observations from the above absorption spectra. Much more importantly, the astonishingly high FF value of 75.03% was achieved in device B, leading to the high PCE value of 10.28 % (the average PCE was 10.06±0.38 % for at least ten devices) and 35% efficiency enhancement from the control device E (PCE of 7.63 %). Notably, the higher doping content of C7 could induce serious domains separation within the active layer and consequently result in the decreased device performance, as confirmed by the reduced peak PCE values (9.65 and 8.63 %) for devices C and D (15 and 20 % of C7), respectively. Despite of that, to the best of our knowledge, the currently-achieved FF value (75.03 %) and PCE value (10.28 %) of device B were comparable to the best results reported for all ternary OSCs and currently could represent the highest results for the PTB7/PC71BM host system of ternary OSCs based on SMs, as summarized in Table 2. Table 2. Summary for the most-advanced ternary OSCs based on PTB7 and Small Molecules. Voc

Jsc

FF

PCE

(V)d

(mA/cm2)e

(%)f

(%)g

0.7/0.3/2

0.795

17.10

65.40

8.90

36

inversion

0.85/0.15/1.5

0.700

15.94

77.10

8.60

18

PTB7/SM/PC71BM

inversion

0.7/0.3/1.5

0.762

17.24

66.58

9.16

45

PTB7/SM/PC71BM

convention

0.8/0.2/1.5

0.720

17.22

67.65

8.39

37

PTB7/SM/PC71BM

inversion

0.9/0.1/1.5

0.730

18.72

75.03

10.28

Active layer a

Structureb

Doping ratioc

PTB7/Poly/PC71BM

convention

PTB7/Poly/PC70BM

a

b

c

Refh

This work

The composition of active layer; Device configuration; Quality ratio of active layer materials; d Open-circuit voltage; eShort-circuit current; fFill factor; gPower conversion efficiency; h References;



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In the other aspect, it was notable that all the ternary OSCs containing C7 exhibited relatively lower Vocs (0.71-0.73 V) than the control device E without C7 (0.74 V). Since the Voc was governed by energy gap between the HOMO (highest occupied molecular orbital) level of donor and the LUMO (lowest unoccupied molecular orbital) level of accepter.38 This observation could be well explained by the relatively higher HOMO level of C7 (-5.12 eV) than that (-5.15 eV) of PTB7,14,20 which caused the reduced energy gap between the donor and acceptor when compared to the control device E. As a result, the Voc values could gradually decrease from device A with 5 % C7 (0.73 V) to device D with 20 % C7 (0.71 V). At the same time, to identify the contribution of Jsc from C7, the external quantum efficiency (EQE) for the control device E and all the ternary devices A-D were also measured (Figure 4b). It was notable that the calculated integral value of the EQE spectra for all devices matched the measured Jsc values with the deviation of less than 5%. Unlike the tendency observed in the absorption spectra, the incorporation of C7 (5-20% dopant) into the PTB7/PC71BM-blend films could bring in higher EQE values for the whole spectra region, especially in the range of 350-500 nm. The obviously increased EQE value in 350-500 nm could be attributed to the absorption enhancement of C7 and this improvement will directly promote the short-circuit current (Figure 2b). In contrast to Lu’ report that the EQE values were enhanced in the absorption region (400-550 nm) of fullerene when the sensitizer was added,20 the herein EQE values were mainly enhanced in the region of 550-750 nm for low doping ratio of C7 (10-15 %), where PTB7 exhibited the intensive absorptions.


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Since the improved EQE values in the 350-500 nm region is due to the addition of C7 enhance the absorptions of the active layer, then the increased EQE values from 550 to 730 nm should be attributed to the added C7 may also significantly increase the photoelectric conversion efficiency of PTB7 by improving the charge transportation and/or charge recombination within the ternary-blend films. On the other hand, higher doping ratio of C7 (20 %) could reduce the EQE values and bring in the lower overall Jsc value. 3.3 Film morphology study To get insights about the morphology of the ternary-blend films, the TEM measurements were initially performed. And the obtained TEM images for the ternary-blend films (~100 nm) were shown in Figure 5, where the bright and dark regions were corresponding to the rich domains for the donors (PTB7 and C7) and the acceptor of PC71BM, respectively.39-40 The TEM images revealed that all the ternary-blend films with C7 could show obvious fibrous features, especially with 10-15% doping ratios, which were not observed in the binary-blend film of PTB7/PC71BM (Figure 5a). These fibrous features could suggest C7 plays an important role in the fibrous-structure formation, and these finely dispersed fibrils should promote the formation of bicontinuous interpenetrating network in the ternary-blend films and thus be beneficial to exciton separation and charge transportation.41-42 To find out why C7 plays such an important role in the formation of nanofiber structures. We measured the BHJ materials surface energies from the droplet contact angle on substrates coated by BHJ materials using Berthelot–Young's



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equation.43 The water contact angle for PTB7 and C7 were ~100 °and ~79° , respectively. All materials of BHJ exhibit rather low surface energies (γPTB7=17.9 mN m -1, γC7=25.5 mN m

-1

and γPC71BM =34.3 mN m

-1

get from published paper)44

illustrates the good miscibility among them. The similar surface energy should be attributed to the hydrophobic/nonpolar backbones and alkyl side chains in the active layer materials. This implies that there must be some strong intermolecular interaction between C7 and PTB7 or C7 and PC71BM.45 Moreover, C7 and PC71BM will tend to accumulate toward PTB7 during the process of self-assembly for the similar surface energies,

46

which means that C7 will located around PTB7. In summary, we know

that the perfect compatibility among the materials in BHJ, C7 and PC71BM will accumulates toward PTB7 through some intermolecular interaction and distributed around it, which together promote the formation nanofiber structure of PTB7. Whereas, the probability of C7 aggregation increases as the doping concentration increases, which will also causes terrible aggregation of PTB7 and PC71BM because of the strong intermolecular interaction. So, over large domain sizes (approximate 200 nm) were observed (Figure 5f) in the ternary-blend films if the added C7 was further increased to 20%, thus leading to the increased probability for exciton recombination and resulting in reduced device performance.


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Figure 5. Transmission electron microscopy (TEM) images for the binary and ternary-blend films. (a) PTB7/PC71BM=1/1.5; (b) PTB7/C7/PC71BM=0.95/0.05/1.5; (c)PTB7/C7/PC71BM=0.9/0.1/1.5; (d) PTB7/C7/PC71BM=0.85/0.15/1.5; (e) PTB7/C7/PC71BM=0.8/0.2/1.5.(f) PTB7/C7/PC71BM=0.8/0.2/1.5

Subsequently, the AFM tests were carried out to study the morphology changes of these ternary-blend films, where Figure 6 exhibited the 2D and 3D surface morphologies for various films with different doping ratios of C7. From the surface energy of the donor materials we know that there is some kind of intermolecular interaction between C7 and PTB7 or C7 and PC71BM, which may lead to form a unique intermolecular stacking through the strong interactions,

45

such as

alloy-stacking. As reported by the literatures, the added small molecules usually could facilitate the formation of an alloy-like model in the ternary-blend films because of the ideal miscibility properties of two donors,19,22 which was usually formed in a regular planar arrangement of the polymer molecules and would be shown through the smooth appearance of films. Expectably, it was clear that the statistical average data for the root-mean-square (RMS) roughness of the ternary-blend films was effectively



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decreased from 2.73 nm (0 %) to 1.88 nm after adding only 5 % C7, which could prove the high organization degree between PTB7 and C7 by self-assemble process. However, the higher RMS roughness value (2.92 nm) was obtained when the doping ratio of C7 was increase to 20 %, and these notable changes in topography could be attributed to the serious domains separation by the excessively-added C7, which was in good agreement with the TEM results.

Figure 6. The 2D and 3D atomic force microscope (AFM) images for the binary and ternary-blended films (2×2µm). (a) PTB7:PC71BM=1:1.5; (b) PTB7/C7/PC71BM=0.95:0.05:1.5; (c) PTB7/C7/PC71BM=0.9:0.1:1.5; (d) PTB7/C7/PC71BM=0.85:0.15:1.5; (e) PTB7/C7/PC71BM=0.8:0.2:1.5.


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3.4 Charge mobility study The transformation of the above morphologies after the addition of C7 could clearly imply that the charge-transporting properties of these ternary-blend films could be altered. Consequently, the hole-only and electron-only devices for the ternary-blend films with two different doping ratios (10% and 20%) were respectively fabricated. And their hole and electron mobility data were independently measured by using the space-charge-limited current (SCLC) method.47 For comparisons, the control devices for the binary-blend films without C7 were also constructed and tested. The J-V characterizations from these single-charge carrier devices could be well fitted by the Murgatroyd equation,25 and these fitted curves were shown in Figure 7a-b and all the detailed data were summarized in Table 3. At the typical electric field of 105 V/cm, the calculated electron- and hole-motility values were shown in Figure 7c. It was obvious that the electron- and hole-motility data could be largely affected after C7 was added. In particular, it was interesting that the tested electron-mobility values could increase with the increased augment of C7, which should be mainly benefited from the perfectly nanofibers formed during assembly process (Figure 3b). As we have discussed above, the active layer of nanofiber properties will promote exciton separation and charge transport, which will ultimately improve the charge mobility. On the other hand, the hole-mobility data could decrease with the increased C7 dopant, which may be largely attributed to the similar HOMO levels between the two donors (C7 and PTB7) (Figure 3b). Most



importantly, A more balanced charge transfer rate for the two charge mobility values was obtained when the doping ratio of C7 was around ~10 % compared to without C7 doping.

-36

(a) 0% 10% 20%

-34

ln(Jd3/ V2) (mAcm/V2)

-32

ln(Jd3/ V2) (mAcm/V2)

-36

-38 0

100

200

300

400

(V/d)1/2(V/cm)1/2

500

600

7 -4

Mobility(cm2V -1s-1)(10 )

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(b)

0% 10% 20%

-37

-38

-39 0

(c)

100

200

300

(V/d)1/2(V/cm)1/2

400

500

hole electron

6 5 4 3 2 1 0

0%

10%

Doping ratio(%)

20%

Figure 7. Electrical characterizations for the ternary-blend films with different doping ratios of C7. The J-V curves for (a) the hole-only devices and (b) the electron-only devices; (c) The calculated hole and electron mobility values from SCLCs.

Table 3. Summarized data for the field-dependent mobility values of the single charge-transporting devices.

Device type

Hole-only device Electron-only device

Doping ratio

Zero-filed mobility u0 (cm2 V-1S-1)

Filed activation factor β (cm1/2V-1/2)

Mobility at F=105 V cm-1 (cm2V-1S-1)

0%

1.03×10-3

-1.53×10-3

6.15×10-4

10%

4.90×10-4

-1.58×10-3

2.97×10-4

20%

3.43×10-4

-2.41×10-3

1.60×10-4

0%

-5

4.03×10

-3

3.18×10

1.09×10-4

10%

9.88×10-5

2.58×10-3

2.24×10-4

20%

5.82×10-5

3.15×10-3

1.58×10-4


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Meanwhile, the recombination mechanism for the ternary-blend OSCs was studied by measuring different Jscs and Vocs at various light intensities (from 100 to 0.9 mW/cm-2). The power law dependence of Jsc upon illumination intensity has been reported for several different authors by using the equation 3.28-29 The double-logarithmic curves of the short current as a function of light intensity were shown in Figure.8a, where the value (0.886) of the exponential factor (α) was obtained for the control binary-blend device containing the active layer of PTB7/PC71BM. However, much higher exponential factor values were realized for the ternary devices with 10% and 20% C7 (α=0.946 and 0.912, respectively). According to the power-dependence law,28 the exponential factor (α) could be taken as the symbol of weak bimolecular recombination once its value is close to unity. Consequently, conclusions could be drawn that the weaker bimolecular recombination occurred in the ternary-blend devices than the binary-blend one. Meanwhile, the more linear dependence of Jsc indicated that the bimolecular recombination was also reduced in the ternary-blend devices doped with C7.28 These results agreed well with the increase of Jsc values from 15.47 to 18.72 mA cm-2 and FF values from 66.12% to 75.03% after adding C7. It is note that the integral current from the EQE spectrum of the ternary device almost approaches the experiment measured value (Error close to 5%). This proves that the majority of carriers produced were swept out of the device prior to recombination and successfully converted to short-circuit currents. Thus, the



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bimolecular recombination mechanism only plays a minor role only in the view of short circuit measurements. We arbitrarily determine the carrier recombination by the alpha value lack of persuasion although the differences between devices are clearly visible. However, at open electrical characteristics, there is no charge extraction and independent of photo-generated charge carriers. So, the recombination mechanisms by analyzing the Voc as a function of generation rate were also studied, which could be linearly tuned by changing the light intensity.30-31 According to the following equation 4, The slope of Voc versus the natural logarithm of the light intensity gives kT/q can stand for the bimolecular recombination process. Moreover, the slope value greater than kT/q was also be observed when the other monomolecular recombination mechanism of (SRH) or trap-assisted recombination is involved.48 As shown in Figure 8b, the slope value for the reference device is 1.86 KT/q, whereas that of cells with 10% and 20% doping ratios are 1.65 and 2.17 KT/q, implying that the recombination processes happened in those devices is a combination of monomolecular and bimolecular processes. The devices with 10% doping ratio showed the weakest recombination processes. Considering there was a competitive relationship between monomolecular process and bimolecular recombination,31,48 so the devices with 0% and 10% doping ratio at open circuit were dominated by bimolecular recombination and monomolecular recombination is suppressed. In addition, too much doping contents will introduce other combination mechanism. Finally, in order to study whether C7 could make an impact on the operational stability of the ternary-blend devices, the variation of FF as the function of light


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intensity was studied. As shown in Figure 8c, the standard deviation value for the binary-blend control device was 2.43, whereas the ternary-blend device with 10% C7 could reduce the value to 1.12. The higher stability for the FF values also could confirm the existence of a relatively weak-recombination mechanism in the ternary OSCs with C7. 0.75

(a) 10

0% 10% 20%

(b)

0.70

Voc (V)

Jsc (mA/cm2)

0.65

1

0.1

0.60

α=0.886 α=0.946 α=0.912

1

10

1.65 kT/q

2.17 kT/q

1.86 kT/q

0.55 100

Light Intensity (mW/cm2)

85

1

10

0% 10% 20%

Light Intensity (mW/cm2)

(c)

100

0% 10% 20%

80 75

FF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 65 60

δ =2.43 δ =1.12 δ =1.72

55 500

20

40

60

80

Light intensity (mW/cm2)

100

Figure 8. (a) Measured Jsc values plotted against light intensity; (b) Measured Voc values as a function of light intensity; (c) Measured FF values as a function of light intensity.

4. Conclusion In summary, a small organic molecule of C7 was initially exploited as the highly efficient electron donor in ternary BHJ OPV devices. By optimizing the doping ratio of C7 into the PTB7/PC71BM binary hosts, the resulting ternary-blended OPV devices with 10 wt% C7 could realize the impressive PCE value of 10.28% due to the improved morphology, optical and electrical characteristics of the ternary active layer. The detailed analysis of the ternary-blend active layer illustrated that the added C7



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could play a significant impact on the formation of the nanofiber microstructure due to the similar surface level between C7 and PTB7. Furthermore, more balanced carrier mobility and lower charge recombination rate were obtained in the resulting ternary-blend OPV devices by adding C7. This investigation demonstrates that coumarin derivatives can be successfully used to fabricate OSCs and achieve excellent results. What’s more, C7 is just one type of coumarin derivatives, indicated that we have a lot of meaningful work to do in the field of ternary organic solar cells.

Acknowledgement

The work was supported by the National Natural Science Foundation of China (No. 51373029, 51533005 and 61604035), “Science Fund for Distinguished Young Scholars of Sichuan Province (No.2015JQ0006)” and “the Fundamental Research Funds for the Central Universities (ZYGX2016Z010 and ZYGX2015J048).” Reference 1. Kang, X.; Zhang, J.; O’Neil, D.; Rojas, A. J.; Chen, W.; Szymanski, P.; Marder, S. R.; El-Sayed, M. A. Effect of Molecular Structure Perturbations on the Performance of the D–A−π–A Dye Sensitized Solar Cells. Chem. Mater. 2014, 26, 4486-4493. 2. Qi, Q.; Wang, X.; Fan, L.; Zheng, B.; Zeng, W.; Luo, J.; Huang, K. W.; Wang, Q.; Wu, J. N-Annulated Perylene-Based Push-Pull-Type Sensitizers. Org. Lett. 2015, 17, 724-727. 3. Page, Z. A.; Liu, Y.; Duzhko, V. V.; Russell, T. P.; Emrick, T. Fulleropyrrolidine Interlayers: Tailoring Electrodes to Raise Organic Solar Cell Efficiency. Science. 2014, 346, 441-444. 4. Irwin, M. D.; Buchholz, D. B.; Hains, A. W.; Chang, R. P.; Marks, T. J. P-Type Semiconducting Nickel Oxide as an Afficiency-Enhancing Anode Interfacial Layer in Polymer Bulk-Heterojunction Solar Cells. PNAS. 2008, 105, 2783-2787. 5. Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148-7151. 6. Deng, D.; Zhang, Y.; Zhang, J.; Wang, Z.; Zhu, L.; Fang, J.; Xia, B.; Wang, Z.; Lu, K.; Ma, W.; Wei, Z. Fluorination-Enabled Optimal Morphology Leads to over 11% Efficiency for Inverted Small-Molecule Organic Solar Cells. Nat. Commun. 2016, 7, 13740-13748. 7. Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Efficient


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Ternary Organic Solar Cells with Coumarin7 as the Donor Exhibiting

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