High-Performance Ternary Nonfullerene Polymer Solar Cells with

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High-performance ternary non-fullerene polymer solar cells with both improved photon harvesting and device stability Manjun Xiao, Kai Zhang, Sheng Dong, Qingwu Yin, Zixian Liu, Liqian Liu, Fei Huang, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06822 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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ACS Applied Materials & Interfaces

High-performance ternary non-fullerene polymer solar cells with both improved photon harvesting and device stability

Manjun Xiao, Kai Zhang*, Sheng Dong, Qingwu Yin, Zixian Liu, Liqian Liu, Fei Huang*, and Yong Cao Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, and School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China.

Abstract Efficiency and stability of polymer solar cells (PSCs) are the most significant two decisive factors for the purpose of actual applications. Here, highly-efficient and stable ternary PSCs were fabricated by incorporating two well-compatible polymer donors (PBDTTT-EF-T and PCDTBT) with one narrow bandgap non-fullerene acceptor (ITIC). It is found that Förster resonance energy transfer acts as an efficient pathway to further strengthen photon harvesting in this ternary system, which results in a significant improvement in current density (JSC) without sacrificing the strong absorption of binary blends in NIR region. Meanwhile, both of the inverted and conventional ternary PSCs exhibit better stability compared with the related binary PSCs in air conditions because of the interlocked morphology in ternary films. The optimized ternary PSCs exhibit an outstanding PCE of 9.53% resulting from the synchronous improvements in JSC and fill factor. Moreover, this ternary strategy can be further confirmed by the use of an ultranarrow-bandgap non-fullerene acceptor IEICO-4F and the champion PCE of ternary PSCs arrives to 12.15%. Keywords: Stability; Polymer solar cells; Non-fullerene ternary devices; Förster resonance energy transfer; Power conversion efficiencies; Photon harvesting

1. Introduction Polymer solar cells (PSCs) have attracted tremendous attention mainly due to their great potentials for large-area production with low-cost and environment friendly advantages.1-6 In the past decades, the power conversion efficiencies (PCEs) of binary bulk-heterojunction (BHJ) PSCs with polymer-donor:fullerene-acceptor (D/A) have exceeded over 11%.7-9 Nevertheless, most of fullerene derivative acceptors have inherent drawbacks such as rigid molecular structures, fixed energy levels, thermal instability, and weak absorption in solar spectrum,10,11 which greatly restrict the further improvement in performance.12 To overcome these weaknesses of fullerene derivative acceptors, the non-fullerene acceptors with planar π-structure play a significant role in improving the performance of PSCs, which have recently drawn great attention.12-24 In the early efforts, Zhan and co-workers developed the polymer/non-fullerene-based PSCs which consisted of low-bandgap donor poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b0]dithiophene-2,6-diyl-alt-(4-(2-ethy lhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] (PBDTTT-EF-T) and 1

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non-fullerene acceptor 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithi eno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b’]dithiophene) (ITIC), respectively.13 The PBDTTT-EF-T/ITIC active layer can absorb the vast majority of the incident sunlight in near-infrared region (NIR), resulting in an average PCE of 6.58%.13 Subsequently much attempt had been devoted to this field, e.g. Hou and the co-workers synthesized a novel non-fullerene acceptor, 2,2′-((2Z,2′Z)-(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-sindaceno[1,2-b:5,6-b′]dithiophene-2 ,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IEICO-4F) and the PSCs with PBDTTT-EF-T:IEICO-4F as active layer achieved a high PCE of 10.0%.15 Moreover, it was reported that replacing fullerene acceptors by non-fullerene derivatives offers a promising approach to improve the stability since most of the non-fullerene acceptors are amorphous and can lead to homogenous film morphology with a wide and strong absorption.25,26 As an alternative to design new donor/acceptor materials,13-21 ternary strategy provides a proven and effective pathway to improve the photovoltaic performance of single-active-layer hetero-junction devices.27-31 For example, fullerene ternary systems with PCEs over 10% have been reported in many works,32-41 resulting from their smoothed energy levels,36,42 increased mobility,34,43 the formation of alloys32 and the optimized morphology of blend films.33,37 In recent years, highly efficient ternary PSCs by using non-fullerene acceptors with synergistically improved photovoltaic performances have also been developed, indicating the great potential of ternary strategy in non-fullerene devices.30,31,35,44-46 For instance, Li et al. employed a polymer donor (PSTZ) and a combination of two different non-fullerene derivatives (ITIC and IDIC) in ternary PSCs (D:A1:A2) and achieved outstanding improvement in JSC with a PCE of 11.1%.47 Whereas, the ternary non-fullerene PSCs with two polymer donors and one non-fullerene acceptor (namely, D1:D2:A) are less developed because non-fullerene materials are very picky about donor materials with matched energy levels and well-compatible morphology in ternary system.15,48-52 To the best of our knowledge, non-fullerene ternary PSCs with efficiency more than 12% is rarely reported up to now. Here, recent progresses in non-fullerene-based ternary PSCs with efficiency over 9% are summarized in Table 1.15,47,50-57 In this work, a series of highly-efficient inverted ternary PSCs consisted of a host binary system of PBDTTT-EF-T:ITIC13 were fabricated by employing a wide-bandgap donor: Poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl2,5-thiophenediyl] (PCDTBT) as the third component (Figure 1a). It is found that Förster resonance energy transfer (FRET) acts as an efficient pathway to further strengthen photon harvesting in ternary system, which results in a great improvement in JSC without sacrificing the strong absorption of binary blends in NIR region. Through the precise regulation of the two polymer donors content, the performance of ternary PSCs are obviously improved owing to the enhanced photon harvesting, efficient exciton dissociation, balanced charge transport and excellent collection in the optimized devices. As a consequence, the champion PCE of ternary PSCs reaches to 9.53%, resulting from the simultaneously improvements in fill factor (FF) and JSC. Meanwhile, both of the conventional and inverted ternary PSCs exhibit better device stability in comparison with the related binary PSCs in air conditions because of the interlocked morphology in ternary films. This ternary strategy can be further confirmed by the use of a non-fullerene 2


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acceptor IEICO-4F15 to replace the ITIC acceptor in the conventional ternary PSCs. The champion PCE value of 12.15% is achieved for PBDTTT-EF-T:PCDTBT:IEICO-4F-based ternary PSCs with an outstanding JSC of 24.03 mA cm−2. Table 1. The Performance Parameters of Binary PSCs and the Corresponding Ternary PSCs in Recent Years.

Ternary

Binary blends

Binary

The third

system

(D/A)

PCE

component

PBDTTT-EF-T:IDT-2BR

8.2

PBDB-T1:SdiPBI-Se

Ratio

VOC

JSC

FF

PCE

References

PDI-2DTT

1:1:0.02

1.03

14.5

65.0

9.7

Ref56

8.1

ITIC-Th

1:0.5:0.5

0.93

15.37

70.2

10.1

Ref53

PBDB-T:ITIC-Th

9.8

TPE-4PDI

1:0.9:0.1

0.87

17.2

72.6

10.8

Ref57

PBDTTT-EF-T:IDTBR

—

IDFBR

1:0.5:0.5

1.03

17.2

60

11.0

Ref54

PSTZ:ITIC

8.1

IDIC

1:0.1:0.9

0.95

17.40

66.9

11.1

Ref47

J52:IT-M

9.4

IEICO

1:0.8:0.2

0.85

19.7

66.8

11.1

Ref55

PBDTTT-EF-T:PNDI-T10

7.6

PBDTTS-FTAZ

1:0.15:1

0.84

14.4

74

9.0

Ref150

J51:ITIC

8.9

PBDTTT-EF-T

0.8:0.2:1

0.81

17.75

67.8

9.7

Ref51

PBDTTT-EF-T:IEICO-4F

10.0

J52

0.3:0.7:1.5

0.73

25.3

58.9

10.9

Ref15

PBDB-T:IEICO-4F

10.3

PBDTTT-EF-T

0.8:0.2:1

0.74

24.14

65.0

11.6

Ref52

PBDTTT-EF-T:ITIC

7.9

PCDTBT

1:0.2:1.3

0.80

17.77

66.9

9.5

This work

PBDTTT-EF-T:IEICO-4F

10.6

PCDTBT

1:0.1:1.5

0.72

24.03

70.4

12.2

This work

D:A1:A2

D1:D2:A

2. Results and Discussion Efficient photon harvesting of BHJ layer is the prerequisite to achieve a highly-efficient PSC. According to the luminous flux of solar light, a promising BHJ layer should have strong photon harvesting ability in a wide wavelength range. Therefore, in order to realize efficient photoelectric conversion with low energy loss, the donors or acceptors should have relatively narrow bandgap to harvest the low energy photon. Here, two polymers with different bandgap were selected as donors and a narrow bandgap non-fullerene material (ITIC or IEICO-4F) acted as acceptor in a blend film. The normalized absorption spectra of neat PBDTTT-EF-T, PCDTBT, ITIC and IEICO-4F films were tested and are displayed in Figure 1d. There is a rather large absorption spectral overlap between PBDTTT-EF-T and ITIC at 600 ~ 800 nm, but a weak absorption in the visible-light region. Whereas, the absorption of PCDTBT covers from 300 ~ 650 nm with peak locates at 394 nm and 566 nm in short wavelength range, respectively, which is well complementary with the ITIC and PBDTTT-EF-T. From the absorption spectra of PBDTTT-EF-T:PCDTBT films with different PCDTBT ratios (Figure S1a), the absorption intensity is enhanced obviously from 300 to 650 nm. As a consequence, a broad and strong absorption can be predicted in the whole absorption range of the used materials by integrating these three materials into a blend film. The absorption spectra of binary and ternary films with 3



PCDTBT content as donor were measured and are shown in Figure S1b. Apparently, the PBDTTT-EF-T:ITIC film has narrow photon harvesting range from 600 to 750 nm, meanwhile, there is an obvious valley from 300 to 600 nm. Similarly, the PBDTTT-EF-T:IEICO-4F films (Figure S1c) also show a weak absorption in the short-wavelength range. Fortunately, the strong absorption region of PCDTBT complements well with this valley, indicating that the photon harvesting of ternary active layers should be efficiently enhanced by the incorporation of appropriate PCDTBT content. At the same time, through the contact angle measurements of PBDTTT-EF-T and PCDTBT, Figure S2(a,b) shows the images of PBDTTT-EF-T (90.2o) and PCDTBT (91.3o) films with the calculated surface energy of 41.65 and 43.3 mJ m-2, respectively, indicating a good compatibility between the PBDTTT-EF-T and PCDTBT. What’s more, it could be further verified by the atomic force microscopy (AFM) measurement as shown in Figure S2(c-g) with a nearly unchanged surface morphology of PBDTTT-EF-T films after the incorporation of PCDTBT. Such an excellent characteristic about the absorption and compatibility of PBDTTT-EF-T and PCDTBT might be to the benefit of the improvement of device performance. The optimized content of PCDTBT in binary films can be confirmed in terms of the performance of ternary PSCs, accompanying with the comprehensive investigation on the dynamic balance of photon harvesting, exciton dissociation, carrier transport, and collection.

Figure 1. (a) Chemical structures and (b) energy levels diagram of ITIC, IEICO-4F, PCDTBT and PBDTTT-EF-T. (c) The inverted device structure of the PSCs. (d) Normalized absorption spectra of the related materials used in this work.

To explore the appropriate content of PCDTBT, the PSCs with a conventional architecture of ITO/PEDOT:PSS/PBDTTT-EF-T:x%PCDTBT:ITIC/PFN-Br/Ag and an inverted architecture of ITO/ZnO/PBDTTT-EF-T:x%PCDTBT:ITIC/MoO3/Ag (as depicted in Figure 1c) were fabricated, 4


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where x is the weight ratio of PCDTBT in donors (x = 0, 10, 20, 30, 100 wt%). PFN-Br or ZnO acts as electron transport layer, PEDOT:PSS or MoO3 acts as hole transport layer. As reported in previous work,13 the D/A weight ratio of PBDTTT-EF-T and ITIC was fixed at 1:1.3 in dichlorobenzene. The corresponding current density versus voltage (J–V) curves of inverted devices and conventional devices are shown in Figure 2a and Figure S3 respectively, and the related device data are summarized in Table 2 and Table S1, respectively. The champion PCE of PBDTTT-EF-T:ITIC-based inverted PSCs reaches to 7.90%, and the corresponding conventional device shows a best PCE of 6.67%, which are similar with the previously reported PCE on the same materials.13 For inverted ternary PSCs, device containing 10% PCDTBT yields an enhanced PCE of 8.42% because of a significant increase in the JSC (6% and 217% improvement compared to PBDTTT-EF-T:ITIC and PCDTBT:ITIC binary devices, respectively). By further optimizing the ratios of PCDTBT, the ternary PSC with 20% PCDTBT exhibits the best device performance with a PCE of 9.53% (JSC = 17.77 mA cm−2, VOC = 0.80 V, FF = 66.87%). Meanwhile, the related conventional ternary device with 20% PCDTBT also shows an improved PCE of 8.24%. After 30% PCDTBT is added into the blend films, the ternary PSC displays a decreased JSC (17.21 mA cm−2) with a PCE of 8.99%. As to the VOC, usually in a ternary system, it is supposed to be pinned to the lower VOC of the binary components because ternary components can be treated as a combination of several “ sub-cells ” in parallel. Furthermore, in terms of energy levels, the VOC of a photovoltaic device should be determined by the minimum difference between HOMO and LUMO energy levels. Therefore, in this case, the VOC of PBDTTT-EF-T:PCDTBT-based ternary cells is determined by PBDTTT-EF-T:ITIC and is pinned at around 0.80 V with the increase of PCDTBT content, which is similar with the results reported by Ito and Han in a similar ternary system of PBDTTT-EF-T:PCDTBT:N2200 with a pinned VOC at 0.80 V.58,59 Table 2. The Performance Parameters of Binary PSCs and the Corresponding Ternary PSCs with Different PCDTBT Content PCDTBT Ternary system

PBDTTT-EF-T: x%PCDTBT:ITIC

PBDTTT-EF-T:

V

J

OC

J

SC

FF

cal. -2

-2

Max (avg)

Rs

a

Rsh 2

Ratios

[V]

[mA cm ]

[mA cm ]

[%]

PCE [%]

0%

0.81

15.33

15.13

63.97

7.90 (7.75) b

[Ω cm ]

[Ω cm2]

12.9

498

0%

0.80

14.09

13.80

59.10

6.67 (6.53)

12.7

453

10%

0.80

16.22

16.07

64.65

8.42 (8.31)

10.3

663

20%

0.80

17.77

16.96

66.87

9.53 (9.35)

9.9

686

b

20%

0.80

16.36

15.57

63.28

8.24 (8.17)

12.3

535

30%

0.80

17.21

16.71

64.91

8.99 (8.82)

10.1

671

100%

0.99

5.11

4.96

36.06

1.83 (1.75)

83.5

324

0%

0.72

22.19

21.53

66.36

10.63 (10.51)

6.81

525

10%

0.72

24.03

23.05

70.36

12.15 (12.02)

3.36

640

x%PCDTBT:IEICO-4 F a b

The data were achieved from 10 independent PSCs. The PSCs’ structure was ITO/PEDOT:PSS/PBDTTT-EF-T:x%PCDTBT:ITIC/PFN-Br/Ag.

5



Figure 2. The (a) J−V and (b) EQE curves of PSC devices with different PCDTBT contents. (c) Photoluminescent spectra of the pure PCDTBT, PBDTTT-EF-T and their binary films excited at 540 nm. (d) Photoluminescent spectra of the pure PCDTBT and the corresponding ternary films with various contents of PCDTBT excited at 540 nm. (e) The schematic diagram of dynamic processes of charge carriers and excitons, arrows indicate charge transport direction and lightning bolt stands for energy transfer direction. (f) Jph versus Veff characteristics of PSCs with various contents of PCDTBT.

Further on, the acceptor ITIC was substituted by IEICO-4F in this ternary system to further investigate the performance of non-fullerene-based ternary devices. The total performance parameters and J-V curves of these PSCs are summarized and shown in Table 2 (detailed data is displayed in Figure S4a and Table S2). The champion PCE of IEICO-4F-based ternary PSC is 12.15% with a high FF of 70.36% in a conventional structure, which is also much higher than those in binary device. Here, two kinds of non-fullerene ternary PSCs were successfully fabricated by incorporating the third component PCDTBT into the host blends. The JSC of these two kinds of ternary PSCs are greatly increased along with PCDTBT contents up to the optimal ratios. The external quantum efficiency (EQE) spectra of the resulted PSCs are shown in Figure 2b. The EQE values of all ternary blend films around 300 – 750 nm are strengthened apparently, which should contribute to the observed higher JSC. By integrating the EQE curves, calculated JSC 6


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values are listed in Table 2. For the 20% PCDTBT-containing PSC device, the calculated JSC is 16.96 mA cm−2, which is well consistent with measured JSC (17.77 mA cm−2) from the J–V measurements. While the EQE values of other PSCs are also agreeing well (mismatch less than 5%) with the measured JSC values. The small difference between the calculated and measured JSC values may attribute to the performance decay of simple encapsulated device when the EQE measurements were taken in ambient atmosphere. To intuitively explore the influence of PCDTBT content on the EQE spectra of non-fullerene ternary devices, the EQE spectral differences (∆EQE) between PBDTTT-EF-T:PCDTBT:ITIC-based device and PBDTTT-EF-T:ITIC-based device are shown in Figure S5. It can be observed that the ∆EQE value of the optimized 20% PCDTBT-containing ternary PSC is almost positive in the whole wavelength range, suggesting that PCDTBT should play multi-functions on photoelectron conversions in the optimized BHJ blend films. Specifically, in the short-wavelength range around 300 – 600 nm, the increased EQE values could be ascribed to the enhanced photon harvesting by PCDTBT in the ternary PSCs, due to its strong absorption in 300 – 600 nm. In the region of 600 – 700 nm, the strengthened EQE values should be resulted from the improved utilization of excitons on PBDTTT-EF-T and ITIC that assisted by the incorporation of appropriate PCDTBT. The photoluminescence (PL) spectra of neat PCDTBT (D3rd), PBDTTT-EF-T (Dhost) and the blend films with different D3rd:Dhost ratios were measured to probe their potential dynamic processes between Dhost and D3rd. Figure 2c displays the PL spectra of different films without acceptor under a excitation wavelength at 540 nm. It can be clearly seen that D3rd has a strong and broad PL emission around 686 nm; Dhost also owns a wide PL emission around 761 nm. The emission intensity of D3rd is significantly quenched in the D3rd:Dhost-based blend films. Simultaneously, the emission intensity of Dhost monotonously increases along with the increase of D3rd content. As displayed in Figure S6, the absorption spectra of both Dhost and ITIC (or IEICO-4F) shows a large overlaps with the PL spectrum of D3rd. Therefore, except for the reinforced absorption by D3rd, in the ternary system, FRET could take place from the wide-bandgap D3rd to the narrow-bandgap NIR molecules, Dhost and ITIC, after the D3rd is excited by the photon.60,61 However, the emission intensity of D3rd and ITIC is fully quenched in the D3rd:ITIC blend films (Figure S6d), suggesting that there is less energy transfer but sufficient exciton dissociation in the D3rd:ITIC films. Therefore, the energy transfer in this ternary system is mainly occurred between D3rd and Dhost, which makes it possible for the D3rd excitons to transport directly to the polymer Dhost. To illuminate potential mechanism why this ternary system functioned so efficiently, it is focused on the formation of free-charges that generated from the excited states of ITIC, Dhost and D3rd. Indeed, Figure 2d exhibits that the PL of D3rd is fully quenched in the ternary films even for the films with 30% D3rd content, indicating that the D3rd excitons are completely exploited for the charge generation. While the Dhost excitons are not quenched at the Dhost/D3rd interface due to the fact that the lowest unoccupied molecular orbital differences (∆LUMO) between Dhost and D3rd is negligible (Figure 1b). This phenomenon could be further confirmed by the negligible JSC of Dhost:D3rd-based binary device (Figure S7 and Table S3). The JSC of Dhost:D3rd-based PSCs is between that of D3rd-based and Dhost-based devices, indicating that there is a negligible exciton dissociation or charge transfer at Dhost/D3rd interfaces. Therefore, according to the analysis as mentioned above, the dynamic processes of charge carriers and excitons in this ternary system are schematically depicted in Figure 2e. The Dhost excitons can be generated both indirectly by the energy transfer from D3rd and directly through the NIR-light 7



absorption of Dhost, and then dissociate into free holes/electrons at the Dhost/ITIC interface as efficiently as those in the Dhost/ITIC-based binary devices. The holes can be transferred via the Dhost networks while the electrons transferred through the ITIC networks, and then extracted to the anode and cathode, respectively. Here, the transfer of holes from the non-fullerene ITIC excited state to the polymer D3rd could compete with charge generation at the interface of Dhost/ITIC. The energy diagram as shown in Figure 1b manifests that the generated holes on D3rd at the interfaces of D3rd/ITIC can efficiently transfer towards Dhost, resulting from the preferred cascade alignment of the highest occupied molecular orbital (HOMO) levels of D3rd and Dhost. Consequently, the holes can be transported through the high-hole-mobility Dhost networks62 and finally collected at anode. In summary, the highly-efficient collection of D3rd excitons was obtained through the efficient FRET and the favorable cascade alignment of HOMO and LUMO, which can minimize the quenching losses of Dhost and ITIC excitons, are critical factors for enhancing the EQE in short-wavelength range while maintaining a high EQE value in the NIR region. To gain more insight into the dynamic process occurred in ternary PSCs, the photocurrent density (Jph) vs. effective voltage (Veff) of the devices were measured.63 As displayed in Figure 2f and Table S4, the maximum exciton generation rates (Gmax) could be estimated according to the equation: Jsat = qLGmax (q stands for the electronic charge, L represents the thickness and Jsat stands for the saturated-photocurrent density). The Jsat of 20% PCDTBT-containing ternary PSC is increased from 16.98 to 18.71 mA cm−2, and the corresponding Gmax is also improving from 9.65×1027 to 1.11 × 1028 m-3 s-1. Therefore, the increased Jsat and Gmax can also be confirmed by the enhanced absorption spectra and the positive ∆EQE values. Under short-circuit conditions, the Jph/Jsat value represent the exciton dissociation efficiency in PSCs.64 The Jph/Jsat value under short-circuit condition is increased from 90.28% to 94.94% after 20% PCDTBT content is added, demonstrating that exciton dissociation should be strengthened in ternary films owing to the optimized phase separation. Besides, in the condition of maximal power output, the value of Jph/Jsat can illustrate the charge transport and collection efficiency in PSCs.65 In this system, the value of Jph/Jsat is increased to 77.97% for the optimized ternary PSCs from 75.35% (PBDTTT-EF-T-based binary PSCs) and 44.09% (PCDTBT-based binary PSCs), indicating that the charge collection and transport in the optimized ternary PSCs is more efficient than binary PSCs. The enhanced Jph/Jsat values can be well supported by the increased FFs of the corresponding PSCs. In consequence, based on the Gmax and Jph/Jsat results, we propose that PCDTBT-containing ternary device not only has an enhanced exciton-generation rate, but also owns an efficient exciton-dissociation, both of them produce a high JSC.

8


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1.2

(a)

1.1

(b)

0% PCDTBT 1.25 kBT/q 10% PCDTBT 1.20 kBT/q

10

20% PCDTBT 1.07 kBT/q

V oc (V)

0% PCDTBT 10% PCDTBT 20% PCDTBT 30% PCDTBT 100% PCDTBT

2

1.0

Jsc (mA/cm )

30% PCDTBT 1.08kBT/q 100% PCDTBT 1.68 kBT/q

0.9

S0 = 0.959 S10 = 0.970

1

S20 = 0.994

0.8

S30 = 0.981 S100 = 0.939

0.7

10

10

100

140

(d)

0% PCDTBT 10% PCDTBT 20% PCDTBT 30% PCDTBT 100% PCDTBT

100

1/2

1/2

(A /m)

120

Light Intensity (mW/cm )

Normalized Current

(c)

80 60

100 2

Light Intensity

J

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


0% PCDTBT 0.177 µs 20% PCDTBT 0.167 µs 100% PCDTBT 0.186 µs

0.8

0.4

0.0 3

4

0.0

5

Voltage (V)

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Figure 3. (a) VOC and (b) JSC versus light intensity of PBDTTT-EF-T based binary and ternary PSCs. (c) Hole mobilities and (d) transient photocurrent measurements of the ternary devices with various contents of PCDTBT.

For purpose of unraveling the influence of adding PCDTBT on charge recombination process that happened in the active layers, the relationship of VOC with light intensity (Plight) was plotted and measured.66 As shown in Figure 3a, the slope of PBDTTT-EF-T:20% PCDTBT:ITIC-based device (1.07 kBT/q) is smaller than the PBDTTT-EF-T:ITIC-based (1.25 kBT/q) and PCDTBT:ITIC-based (1.68 kBT/q) binary PSCs, suggesting that there is few monomolecular or trap-assisted recombination occurred in ternary films compared with the relating binary films.47 The relationship of JSC versus Plight could be represented as JSC∝S·Plight.67 Figure 3b exhibits that the S values of the fitted lines are 0.959 and 0.994 for PBDTTT-EF-T:ITIC-based binary devices and the 20% PCDTBT-containing ternary devices, respectively, implying there is an efficient sweep-out of charges, along with a weak bimolecular recombination in ternary systems. To further explore the processes of charge transport in the BHJ films after the incorporation of PCDTBT, the hole/electron transport properties of PBDTTT-EF-T:x%PCDTBT:ITIC-based device were investigated according to space-charge-limited current method (SCLC).63,68 The single-carrier diodes were fabricated with the structure of ITO/ZnO/PBDTTT-EF-T:x%PCDTBT:ITIC/Ca/Al (electron-only devices) and ITO/PEDOT:PSS/PBDTTT-EF-T:x%PCDTBT:ITIC/MoO3/Ag (hole-only devices), respectively. The J1/2–V characteristics of single-carrier devices are displayed in Figure 3c and Figure S8, respectively, and summed in Table S5. The hole-mobilities (µh) of ternary films with various PCDTBT contents change slightly in comparison with PBDTTT-EF-T:ITIC films, which shows a similar tendency with the µh of the pure PBDTTT-EF-T film with or without PCDTBT. Meanwhile, a lower hole/electron mobility ratio (µh/µe = 3.76) is achieved for the active layer with 20% 9



PCDTBT compared with µh/µe value of the binary film (~ 9.68), resulting that charge transport in ternary films becomes more balanced. The lower µh/µe value can contribute to the relatively high FF achieved for the PCDTBT-containing ternary PSCs. Meanwhile, the series resistance (Rs) and shunt resistance (Rsh) value of the PBDTTT-EF-T:ITIC-based binary devices (Table 2) is 12.9 and 498 Ω cm2, respectively. After the third component PCDTBT is added, the Rs gradually decreases to 9.9 Ω cm2 and the Rsh increases to 686 Ω cm2. The lower Rs and higher Rsh is another significant reason for the high FF in ternary PSCs compared with the binary PSCs. In addition, as exhibited in Figure 3d and Figure S9, the curves of transient-photocurrent (TPC) were measured to investigate the competition between carrier sweep-out by the electric field and the charge recombination in BHJ devices.69 Through the nonlinear curves fitting of JSC vs. time, the charge extraction time (τ) can be calculated. For the ITIC-based and IEICO-4F-based binary PSCs, the calculated values of τ are 0.177 µs and 0.199 µs, respectively. When the PBDTTT-EF-T:ITIC (IEICO-4F) blend films is incorporated with appropriate PCDTBT, the τ reduces to 0.167 µs (0.174 µs for IEICO-4F-based ternary device). This result demonstrates that incorporating a suitable polymer (PCDTBT) into the host blends can enhance the PSCs’ charge extraction rate and then improve the charge recombination, which is favorable for ternary PSCs’ higher PCEs and FFs.

Figure 4. The AFM and TEM images of PBDTTT-EF-T:PCDTBT:ITIC blend films with different PCDTBT contents: (a,d) 0%, (b,e) 20% and (c,f) 100%.

To investigate the impact of PCDTBT on the BHJ films morphology, AFM and transmission electron microscopy (TEM) measurements were taken and shown in Figure 4. As displayed in Figure 4(a,b,c), the roughness of PBDTTT-EF-T:ITIC film reaches to a relatively high root-mean-square (RMS) value of 2.89 nm, implying weak miscibility between PBDTTT-EF-T and ITIC. Whereas the PCDTBT and ITIC exhibits a well miscibility with each other because the surface of PCDTBT:ITIC blend films becomes more smooth (RMS = 1.05 nm). Interestingly, 10


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when 20% PCDTBT is added into the host blend, the RMS of ternary blend film decreases to 1.12 nm. This means the miscibility between PBDTTT-EF-T and ITIC is effectively improved by the incorporation of PCDTBT. Moreover, the PBDTTT-EF-T:ITIC film exhibits an over-large phase separation as displayed in Figure 4(d,e,f), which may cause more charge recombination and is detrimental to device performance especially adverse to FF. Although it owns a more smooth surface for PCDTBT:ITIC blend film, but the negligible phase separation shown in Figure 4f goes against the generation and separation of charges, leading to a low photocurrent of 5.11 mA cm-2 for the PCDTBT:ITIC-based binary devices. In the meanwhile, the nano-scale fibrous phase separation is observed in the ternary films with 20% PCDTBT, implying that the phase separation of PBDTTT-EF-T:ITIC film is suppressed by the addition of PCDTBT and thus shows a high FF. In sum, the fibrous and smooth morphology of blend films with appropriate PCDTBT content could contribute to the improved charge generation, transport and collection, and then increase the JSC and FF of ternary PSCs.

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Figure 5. Stability of PSCs without encapsulation: (a) the binary and 20% PCDTBT-containing ternary PSCs with inverted architecture were stored in air condition, (b) the optimized binary and ternary films were thermal annealing at 100 oC for over 48 h, (c) the optimized binary and ternary PSCs were treated by continuously 100 mW cm−2 light illumination for over 720 min.

The binary and ternary devices without any encapsulation were stored in air conditions and repeatedly measured to probe the stability of PSCs. As displayed in Figure 5a, the optimized inverted ternary PSCs exhibit excellent stability with ~ 90% initial PCE after 50 days storage, however, only 80% initial PCEs can be remained for the PBDTTT-EF-T:ITIC-based binary PSCs after 50 days storage (Figure 5a). In other words, the ternary BHJ films with 20% PCDTBT own a more stable morphology than the binary films, which can be further confirmed by the experimental results of AFM and TEM. It is noteworthy that the ternary PSCs with conventional 11



architecture also exhibit a slower decay process in device performance than the binary devices (Figure S10), which should be ascribed to the more stabilized morphology of ternary films. Meanwhile, the ternary devices with 20% PCDTBT exhibits an excellent thermal stability compared with binary PSCs when the blend films were treated by thermal annealing at 100 oC for 48 h. To further verify this conclusion, the morphologies of PBDTTT-EF-T:ITIC and the corresponding ternary blend active layers with 20% PCDTBT content after 48 h thermal treatment were investigated by the measurements of AFM (See Figure S11). Through the combination of the AFM images displayed in Figure S11(a,b) and Figure 4(a,b), the surface morphology of ternary films keeps nearly unchanged after 48 h thermal treatment, which suggests the excellent morphological stability in ternary system. On the contrary, the binary blend films show over-large aggregates with a rather rough surface after thermal treatment (RMS = 4.71 nm), agreeing well with the stable device performances of the corresponding PSCs. Moreover, under the exposure of continuous 1 sun illumination, a decrease in the device efficiency was observed (Figure 5c), which might be attributed to trap mediated charge recombination and negative effect on the charge transfer properties of the fabricated ITIC-based devices.70 However, it is also highlighted that the PCE of the 20% PCDTBT-containing ternary PSCs slightly decreased to 90% initial PCE upon continuous 100 mW cm−2 light illumination for over 12 h, demonstrating the better photo stability of PBDTTT-EF-T:PCDTBT:ITIC-based device. 3. Conclusion To summarize, high performance ternary PSCs were fabricated by incorporating two well-compatible donors PBDTTT-EF-T and PCDTBT with a narrow-bandgap non-fullerene acceptor ITIC. The optimized ternary PSCs with 20% PCDTBT content in donors obtain an outstanding efficiency with PCE up to 9.53%. An improved JSC is achieved in the optimized ternary devices resulting from the enhanced broad photon harvesting in the whole absorption wavelength range and the improved exciton utilization through the FRET between two well-compatible polymer donors. Another improvement in FF is also observed for the optimized ternary PSCs due to the optimized phase separation and balanced charge transport. Therefore, owing to the simultaneously enhanced JSC and FF, an appropriate PCE improvement can be achieved by incorporating 20% PCDTBT in donors. Meanwhile, both of the inverted and conventional ternary PSCs own better device stability in comparison with the related binary PSCs in air conditions. Moreover, the champion PCE of ternary PSCs arrives to 12.15% when the ITIC was replaced by IEICO-4F. Our results demonstrate that by choosing proper donor/acceptor combination, ternary structure devices can act as an ideal choice for high efficiency and stable PSCs fabrication. 4. Experimental Section Fabrication and Characterization of PSCs PBDTTT-EF-T and PCDTBT were purchased from 1-Material Inc., ITIC and IEICO-4F were purchased from Solenne BV. Unless otherwise specified, all reagents and solvents were commonly commercially available products and were used as received. The device structure was ITO/ZnO/PBDTTT-EF-T:x%PCDTBT:ITIC/MoO3/Ag. The ITO glass substrates were cleaned under sonication with acetone, detergent, deionized water, and isopropyl alcohol and then dried at 80°C in a baking oven overnight. A ZnO layer (~30 nm) was spun on ITO substrates and dried in 12


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air at 200 °C for 15 min. All substrates were then transferred to a glovebox under nitrogen (N2) for the following deposition of active layers. The weight ratio of donors:acceptor for all active-layer solutions were kept at 1:1.3 for ITIC and 1:1.5 for IEICO-4F, respectively, with various contents of PCDTBT. PBDTTT-EF-T:ITIC binary and ternary solutions were prepared in dichlorobenzene, whereas the PBDTTT-EF-T:IEICO-4F binary solutions were prepared in chlorobenzene with 1% DIO. All solutions were stirred on a hot plate at 70 oC overnight to ensure complete dissolution. Then, the active layers were spin-coated from the blend solutions on the substrates to obtain the required thicknesses about 100 nm. Finally, the MoO3 (10 nm) and Ag (100 nm) electrode were thermally deposited through a shadow mask. The J-V curves were measured on a computer-controlled Keithley 2400 source meter under 100 mW cm−2 illumination, the AM 1.5 G spectra came from a class solar simulator (Enlitech, Taiwan). Before the J-V test, a physical mask with an aperture with precise area of 0.04 cm2 was used to define the device area. Acknowledgements This work was financially supported by the Ministry of Science and Technology (No. 2014CB643501), the National Natural Science Foundation of China (No. 21520102006, 91633301, 51521002 and 51603070).

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:XXX. Tables of photovoltaic properties for the solar cells, carrier mobilities, and relevant parameters obtained from Jph - Veff curves. Figures of UV-vis absorption spectra, photoluminescent spectra, transient photocurrent measurements, Contact angle measurements, electron mobilities, stability of PSCs, AFM images, EQE curves, and J-V characteristics of PSCs (PDF) ■ AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected] (K. Zhang); [email protected] (F. Huang). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. References [1] Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies Via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789-1790. [2] Brabec, C. J. Organic Photovoltaics: Technology and Market. Sol. Energy Mater. Sol. Cells 2004, 83, 273-292. [3] Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photonics 2012, 6, 153-161. [4] Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666-12731.

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High-Performance Ternary Nonfullerene Polymer Solar Cells with

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