Ternary Nonfullerene Polymer Solar Cells with a Power Conversion

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Letter

Ternary Non-fullerene PSCs with PCE of 11.6% by Inheriting the Advantages of Binary Cells Zhenghao Hu, Fujun Zhang, Qiaoshi An, Miao Zhang, Xiaoling Ma, Jianxiao Wang, Jian Zhang, and Jian Wang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00100 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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ACS Energy Letters

Ternary Non-fullerene PSCs with PCE of 11.6% by Inheriting the Advantages of Binary Cells *

Zhenghao Hu†, Fujun Zhang †, Qiaoshi An†, Miao Zhang†, Xiaoling Ma†, Jianxiao Wang†, Jian Zhang‡ and Jian Wang§ †

Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, P. R. China

‡

Department of Material Science and Technology, Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, 1Jinji Road, 541004, Guilin, Guangxi, P. R. China § College of Physics and Electronic Engineering, Taishan University, Taian 271021, P. R.

China AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ABSTRACT: Ternary polymer solar cells (PSCs) were commonly fabricated with two donors or two acceptors, the third component has complementary absorption spectra with host system to improve photon harvesting. In this work, a series of non-fullerene PSCs were fabricated with J71 as donor, IT-M or/and ITIC with almost identical band gap as acceptors. Although IT-M and ITIC have almost identical band gap, the optimized two binary PSCs exhibit different photovoltaic parameters. The short current density (Jsc) and open circuit voltage (Voc) of J71:IT-M based PSCs are larger than those of J71:ITIC based PSCs, the other key parameter fill factor (FF) is just the opposite. The PCE of optimized ternary PSCs arrives to 11.60% by inheriting the advantages of binary PSCs. This work may provide a new strategy for preparing efficient non-fullerene ternary PSCs by selecting binary PSCs with complementary photovoltaic parameters. TOC GRAPHICS 1 / 17 ACS Paragon Plus Environment

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Recently, non-fullerene polymer solar cells (PSCs) have achieved rapid development with power conversion efficiency (PCE) more than 12% due to the versatile non-fullerene acceptors.1-4 The non-fullerene acceptors with easily adjusted energy levels can supply more opportunity to prepare highly efficient ternary PSCs.5-8 The commonly used selection criterion of the third component is complementary absorption spectra with the host binary system to enhance photon harvesting of ternary active layers.9-14 Although ternary strategy has been demonstrated as an efficient method to improve the performance of PSCs, the working mechanisms between two donors or two acceptors are still in debated, such as energy transfer, charge transfer, parallel-linkage model and alloy model.15-21 It is universally recognized that incorporation of the third component is propitious to improve short current density (Jsc) of ternary PSCs. The incorporation of the third component may influence charge transport due to the energy level offsets of used materials, which will affect open circuit voltage (Voc) and fill factor (FF).22-23 The phase separation of active layers should be mainly codetermined by the compatibility of used materials and active layer treatment methods, such as solvent additive, thermal annealing (TA) treatment or solvent vapor annealing (SVA) treatment.24-28 Recent years, ternary strategy has been demonstrated as an efficient method to improve the performance of PSCs. It is rarely reported that ternary non-fullerene PSCs exhibit a PCE larger than 11% based on two well optimized binary PSCs with PCE more than 10%. In this work, a series of binary PSCs were fabricated with J71:IT-M or J71:ITIC as the active layers, different active layer treatment methods were carried out to optimize the 2 / 17 ACS Paragon Plus Environment

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performance of binary PSCs.29-31 The PCEs of binary PSCs arrive to 10.68% or 10.65% for J71:IT-M or J71:ITIC with SVA treatment. It is highlighted that photovoltaic parameters of two binary PSCs are very different, although IT-M and ITIC have almost identical band gap. The Jsc and Voc of J71:IT-M based PSCs are larger than those of J71:ITIC based PSCs, the other key parameter FF is just the opposite. The complementary photovoltaic parameters of binary PSCs may be fully exerted by preparing ternary PSCs with IT-M and ITIC as acceptors. All ternary active layers were also treated by SVA method under the same conditions. The optimized ternary PSCs exhibit PCE of 11.60% for the active layers with 10 wt% ITIC in acceptors, resulting from the enhanced Jsc of 18.08 mA cm-2 and FF of 65.6%. The FF of binary PSCs with IT-M or ITIC as acceptor is 61.5% or 68.2%, respectively. The FFs of ternary PSCs can be monotonously improved along with the increased ITIC content in acceptors, which should be mainly attributed to the optimized morphology of ternary active layers. The very similar chemical structure of IT-M and ITIC is beneficial to optimize phase separation due to their good compatibility.24 The optimized ternary non-fullerene PSCs inherit the advantages of two binary PSCs, which may provide a new strategy for obtaining efficient ternary non-fullerene PSCs. Figure 1 presents the schematic structure diagram of PSCs, chemical structures and energy levels of used materials, and normalized absorption spectra of neat J71, IT-M and ITIC films. The experimental details are described in the supporting information.

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ACS Energy Letters

(c) 1.0 0.8

Absorbance

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

J71 IT-M ITIC

0.6 0.4 0.2 0.0 300

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Wavelength (nm)

Figure 1. (a) The schematic structure diagram of PSCs; (b) The chemical structures of J71, IT-M and ITIC; (c) Normalized absorption spectra of neat J71, IT-M and ITIC films; (d) Energy levels of used materials. A series of binary PSCs were fabricated with J71:IT-M or J71:ITIC as active layers, the active layers were individually suffered from different treatment conditions, such as solvent additive, TA or SVA treatment. The morphology of active layers should play the vital role in determining the performance of PSCs, which strongly influences exciton dissociation and charge transport in active layers.32-34 The performance of PSCs with J71:IT-M or J71:ITIC as active layers were investigated for the active layers with different treatment conditions. Here, current density versus voltage (J-V) curves of J71:IT-M based PSCs are shown in Figure 2a, which were measured under AM 1.5G illumination with light intensity 100 mW/cm2. According to the J-V curves, key photovoltaic parameters of all PSCs are summarized in Table 1. The PCE of 9.33% was obtained for PSCs without any treatments on active layers, along with a Jsc of 16.05 mA/cm2, a Voc of 0.987 V and a FF of 58.9%. The PCE of PSCs was decreased to 8.86% for active layer with 0.2 v% DIO additive due to the markedly decreased Jsc of 15.32 mA/cm2. The PCE values can be increased to 9.71% or 10.68% for active layers with TA or SVA treatment, respectively. The PCE improvement should be mainly attributed to the simultaneously improved Jsc and FF. The external quantum efficiency (EQE) spectra of PSCs were measured and are shown in Figure 2b. The PSCs with SVA treatment exhibit 4 / 17 ACS Paragon Plus Environment

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relatively high EQE values in the whole spectral range, indicating the efficient exciton dissociation and charge transport in the active layers. The calculated Jscs can be obtained by integrating the EQE spectra of PSCs, as listed in Table 1. The calculated Jscs are slightly smaller than the measured values due to the cells without any encapsulation for EQE measurement.35-36 The series resistance (Rs) and shunt resistance (Rsh) of all PSCs were calculated to investigate the effect of active layer treatment conditions on charge transport in the PSCs.37-38 The relatively high FF of 61.5% was obtained for the active layers with SVA treatment, which can be further confirmed from the minimum Rs of 6.2 Ω cm2 and the maximum Rsh of 647 Ω cm2. The very similar phenomenon was also observed from the J71:ITIC based PSCs, the detailed experimental results are shown in Figure S1 and Table S1. The PCE of PSCs with IT-M or ITIC as acceptor can arrive to its maximum for the active layers with SVA treatment, which may be beneficial to optimize ternary PSCs under the same experimental conditions. (b)

0

Pristine DIO TA SVA

-4 -8

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700

800

Pristine SVA

80

DIO J71

60

× 0.5

× 0.05 40 20 0

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Figure 2. (a) J-V curves of J71:IT-M based PSCs with different treatments on the active layers; (b) EQE spectra of the corresponding PSCs; (c) Absorption spectra of the corresponding blend films; (d) PL spectra of neat J71, IT-M films and blend films with different treatments. Table 1. Key photovoltaic parameters of J71:IT-M based PSCs with different treatments on the active layers. Treatment conditions

Jsc (mA/cm2)

Cal. Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

Pristine

16.05

15.32

0.987

58.9

9.33

8.1

444

DIO

15.32

14.64

0.979

59.1

8.86

8.9

360

TA

16.21

15.69

0.984

60.9

9.71

7.5

533

SVA

17.71

17.02

0.981

61.5

10.68

6.2

647

To further clarify the effect of active layer treatment conditions on the performance of PSCs, absorption spectra of J71:IT-M blend films with different treatments are shown in Figure 2c. Obviously, absorption spectra almost overlap for the pristine and TA treated blend films due to the rapid volatilization of used solvent CHCl3. The effect of TA treatment on molecular arrangement may be negligible because the active layers are almost dry during spin-coating process. The enhanced absorption intensity and red-shifted absorption peak can be clearly observed from blend films with DIO or SVA treatment, which may be attributed to the more ordered molecular arrangement during the slow drying process of blend films.39 To investigate the phase separation of active layers with different treatments, photoluminescence (PL) spectra of blend films, neat J71 and IT-M films were measured under the same conditions, as shown in Figure 2d. PL emission intensity is markedly quenched for the blend films with SVA treatment, which should be attributed to the more appropriate phase separation for efficient exciton dissociation. The PL emission intensity is slightly increased for the blend films with rather small amount of DIO additive, which should be due to the 6 / 17 ACS Paragon Plus Environment

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enlarged domain size of J71 during the slow volatilization of DIO. The positive effect of SVA treatment on PCE improvement can be well explained from absorption and PL spectra of blend films with different treatment conditions. Although the PCE of 10.68% and 10.65% can be obtained from the PSCs with J71:IT-M or J71:ITIC as active layers, the key parameters Jsc, Voc and FF are rather different for the two kinds of binary PSCs. If the advantages of binary PSCs can be inherited in one ternary PSCs, the PCE of ternary PSCs should be larger than that of two corresponding binary PSCs. A series of ternary PSCs were fabricated with different ITIC content in acceptors, ternary active layers were also treated by SVA under the same conditions. The detailed experimental results of ternary PSCs with different ITIC contents in acceptors are shown in Figure S2. (b)

0 -4

ITIC in acceptors 0 wt%

75

10 wt%

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EQE (%)

Current density (mA/cm2)

(a)

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IT-M ITIC

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ITIC in acceptors 0 wt% 10 wt% 10 100 wt%

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ITIC contents

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19.41

17.48

Jpha/Jsat (%)

90.27

93.15

96.17

Jphb/Jsat (%)

78.29

80.69

83.18

0.1

1

Veff (V)

Figure 3. (a) J-V curves of PSCs with different ITIC content in acceptors; (b) EQE spectra of the corresponding PSCs; (c) Absorption spectra of blend films with different ITIC contents in acceptors (insert: Absorption coefficient of neat IT-M and ITIC films); (d) Jph-Veff curves of



PSCs with different ITIC content in acceptors (Jpha and Jphb is the photogenerated current density (Jph) under short circuit and maximal power output condition conditions, respectively). The J-V curves of binary and the optimized ternary PSCs are exhibited in Figure 3a. The detailed photovoltaic parameters of all PSCs are listed in Table 2. The PCE of ternary PSCs approaches 11.60% when the content of ITIC in acceptors arrives to 10 wt%, along with a Jsc of 18.08 mA/cm2, an Voc of 0.978 V and a FF of 65.6%. The ternary PSCs inherit the advantages of two kinds of binary PSCs, such as high Jsc and Voc of J71:IT-M based PSCs, as well as the high FF of J71:ITIC based PSCs. The FF improvement of ternary PSCs should be attributed to the optimized morphology by incorporating ITIC as morphology regulator. The FF of J71:ITIC based PSCs approaches 68.2%, suggesting the efficient charge transport channels formed in the J71:ITIC blend films. It is apparent that the FFs of ternary PSCs can be gradually improved along with the increase ITIC content in acceptors, which can be well explained from the decreased Rs and increased Rsh along with the increase of ITIC content in acceptors. The Jscs of ternary PSCs are increased and then decreased along with the increase of ITIC content in acceptors. The slightly increased Jscs may be due to the optimized morphology of ternary active layers by incorporating small amount of ITIC. And then decreased Jscs of ternary PSCs may be related to the relatively low Jsc (16.81 mA cm-2) of J71:ITIC based PSCs. To further confirm the effect of ITIC content in acceptors on the performance of PSCs, the EQE spectra of binary and the optimized ternary PSCs are shown in Figure 3b. The EQE spectrum of optimized ternary PSCs almost overlaps with that of J71:ITM based PSCs. The J71:ITIC based PSCs exhibit the low EQE values due to the relatively low absorption coefficient of ITIC. The efficient charge transport and collection in J71:ITIC based PSCs should be obtained due to the relatively large FF of 68.2%. To clarify Jscs dependence on ITIC content in acceptors, absorption spectra of typical blend films were measured and are shown in Figure 3c. It is apparent that absorption spectra of all blend films almost overlap in the spectral range from 300 nm to 600 nm originating from photon 8 / 17 ACS Paragon Plus Environment

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harvesting of J71, suggesting the nearly same thickness of all blend films. The absorption intensity of J71:ITIC blend film in the long wavelength range is much lower than that of J71:IT-M and the optimized ternary blend films, indicating that the absorption coefficient of ITIC should be less than that of IT-M. The absorption coefficient of neat ITIC and IT-M films were measured and are shown in inset of Figure 3c. The relatively low JSC of J71:ITIC based PSCs should be attributed to the low absorption coefficient of ITIC. Table 2. Key photovoltaic parameters of PSCs with different ITIC content in acceptors. ITIC [wt%]

Jsc (mA/cm2)

Cal. Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

0 5 10 20 30 50 80 100

17.71 17.87 18.08 17.91 17.76 17.54 17.08 16.81

17.02 17.09 17.24 17.15 17.09 16.92 16.56 16.26

0.981 0.980 0.978 0.964 0.953 0.941 0.932 0.929

61.5 63.1 65.6 65.8 66.2 66.5 67.9 68.2

10.68 11.05 11.60 11.36 11.20 10.98 10.81 10.65

6.2 5.6 4.9 4.2 3.7 3.1 2.6 2.3

647 812 1018 1076 1132 1176 1201 1225

To further clarify the effect of ITIC content in acceptors on the performance of ternary PSCs, photocurrent density (Jph) versus effective voltage (Veff) curves of PSCs are shown in Figure 3d. Here, Jph is defined as Jph = JL - JD, where JL and JD represent the current density under light illumination or in dark, respectively. The Veff is defined as Veff = V0 - V, V0 is the voltage at where Jph = 0, V is the applied bias.40 The exciton dissociation and charge collection efficiency can be evaluated by the Jph/Jsat values (Jsat is saturation current density, Jsat = qLGmax, where q is elementary charge and L is the thickness of active layer) at short circuit condition or maximal power output condition, respectively.41 At short circuit condition, Jph/Jsat values are 90.3%, 93.1% or 96.1% for J71:IT-M based, the optimized ternary or J71:ITIC based PSCs, respectively. The Jph/Jsat values are 78.9%, 80.7% or 83.2% for the corresponding PSCs at maximal power output condition. All the detailed values are listed in the insert of Figure 3d. The two Jph/Jsat values of the optimized ternary PSCs are in between


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those of the binary PSCs, indicating that the appropriate ITIC can adjust the morphology of ternary active layers for the better exciton dissociation and charge transport. (a)

(b) ITIC in acceptors 0 wt% n=0.90 10 wt% n=0.94 100 wt% n=0.98

ITIC in acceptors 0 wt% s=1.59 10 wt% s=1.35 100 wt% s=1.14

0.99 0.96

Voc (V)

Jsc (mA/cm2)

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ITIC in acceptors µh(cm v s ) 2.17× ×10-4 0 wt% 2.79× ×10-4 10 wt% 100 wt% 3.24× ×10-4

-34 -35 -36 -37 -38 -39

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ITIC in acceptors 0 wt% 10 wt% 100 wt%

-33 -34

µe(cm2v-1s-1) 1.19× ×10-4 1.87× ×10-4 2.85× ×10-4

-35 -36 -37 -38 0

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(V/cm)0.5

Figure 4. (a) Jsc dependence on light illumination intensity curves of binary and the optimized ternary PSCs; (b) Voc dependence on light illumination intensity curves of binary and the optimized ternary corresponding PSCs; The ln(Jd3/V2) versus (V/d)0.5 curves of c) hole-only devices, d) electron-only devices. The J–V curves of binary and the optimized ternary PSCs were measured under different light illumination intensity to further investigate the effect of ITIC content on charge transport in active layers, as shown in Figure S3. According to the J–V curves under different light illumination intensity, Jsc and Voc dependence on light illumination intensity are described in Figure 4a and 4b, respectively. The Jsc dependence on light illumination intensity can be expressed as ‫ܬ‬ௌ஼ ∝ ‫ ܫ‬௡ . The bimolecular recombination can be neglected if n is close to unity.42-43 The n values are 0.90, 0.94 and 0.98 for J71:IT-M based, the optimized ternary and 10 / 17 ACS Paragon Plus Environment

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J71:ITIC based cells, respectively. The slightly enhanced n value suggests that bimolecular recombination can be suppressed in the optimized ternary active layers, which well accords with the enhanced FF of the optimized ternary PSCs. The optimized charge transport can also be confirmed from the improved Jph/Jsat values at maximal power output condition. In fact, charge recombination can’t be avoided in the bulk heterojunction blend films. The charge recombination mechanism can be further extracted by investigating the correlation of Voc dependence on light illumination intensity. If the slope of Voc versus the natural logarithm of light intensity is close to kT/q (where K is the Boltzmann constant, T is absolute temperature, and q is the elementary charge), the bimolecular recombination is the primary process.44-45 The slope is close to 2 kT/q when trap-induced charge recombination is the dominant process in active layers.46 The slopes of Voc versus the natural logarithm of light illumination intensity are approximate 1.59, 1.35 and 1.14 kT/q for J71:IT-M based, the optimized ternary and J71:ITIC based PSCs, respectively. The decreased slopes indicate that trap-induced charge recombination could be restrained in the optimized ternary and J71:ITIC based PSCs. The relatively high FF values of the optimized ternary and J71:ITIC based PSCs should be attributed to the decreased charge loss in the active layers, ITIC may be used as morphology regulator to optimize J71 molecular arrangement for obtaining efficient charge transport channels in the ternary active layers. To further investigate the effect of ITIC content in acceptors on charge transport in the active layers, hole mobility (µh) and electron mobility (µe) were measured by employing space charge limited current (SCLC) method. Hole-only or electron-only cells were fabricated with the structure of ITO/PEDOT:PSS/active layers/MoO3/Ag or ITO/ZnO/active layers/Al, respectively. The active layers are same with those in the corresponding binary and the optimized ternary PSCs. The J-V curves of device are plotted as ln(Jd3/V2) versus (V/d)0.5, as



shown in Figure 4c and 4d. The µh and µe were calculated according to the Mott-Gurney equations: ଽ

‫ߝ ଼ = ܬ‬଴ ߝ௥ ߤ

௏మ ௅య

Where ε0 is the vacuum permittivity, εr is the dielectric permittivity,μis charge mobility, V is the applied voltage and L is active layer thickness.47-48 The µh and µe values are simultaneously increased in the optimized ternary blend films compared with those in the J71:IT-M blend films, suggesting that charge transport channels can be optimized by incorporating appropriate ITIC in the ternary blend films. It should be highlighted that charge transport becomes more balanced (µh/µe =1.49) in the optimized ternary blend films compared with that (µh/µe =1.82) of J71:IT-M blend films, which can well explain the enhanced FF of 65.5% for the optimized ternary PSCs.49-50 It is known that appropriate phase separation is prerequisite for obtaining efficient exciton dissociation, charge transport and collection.51-53 In this work, we proposed a new strategy for obtaining efficient ternary PSCs based on two well optimized binary PSCs with complementary photovoltaic parameters. The advantages of each binary PSCs can be inherited in the ternary PSCs to obtain the markedly improved performance, although the two acceptors ITIC and IT-M have the almost identical band gap. The PCE improvement of ternary PSCs should be mainly attributed to the optimized phase separation of ternary active layers. In summary, two efficient binary PSCs were successfully fabricated with J71 as donor and IT-M or ITIC as acceptor by optimizing active layer treatment conditions. The champion PCE of J71:IT-M or J71:ITIC based PSCs arrive to 10.68% or 10.65% for the active layers with SVA treatment. The J71:IT-M based PSCs exhibit relatively large Jsc of 17.71 mA/cm2 and Voc of 0.981 V, as well as relatively small FF of 61.5% compared with J71:ITIC based PSCs. The PCE of the optimized ternary PSCs arrives to 11.60% with a Jsc of 18.08 mA/cm2, a Voc of 0.978 V and a fill factor (FF) of 65.6%. Although ITIC and IT-M have the almost 12 / 17 ACS Paragon Plus Environment

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identical band gap, the PCE of ternary PSCs can also be improved by inheriting the advantages of binary PSCs. The experimental results may provide a new strategy to prepare efficient ternary non-fullerene PSCs based on two binary non-fullerene PSCs with complementary photovoltaic parameters, rather than the complementary absorption spectra of used materials. ASSOCIATED CONTENT Supporting Information Detailed experimental procedures. J-V curves of J71:ITIC based PSCs with different treatments on the active layers; EQE spectra of the corresponding PSCs; absorption spectra of the corresponding blend films; PL spectra of neat J71, ITIC and blend films. J-V curves of PSCs with different ITIC content in acceptors; EQE spectra of the corresponding PSCs. J-V curves of the PSCs under different light illumination intensity. AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by National Natural Science Foundation of China (61377029, 61564003, 61675017, 61705161); Fundamental Research Funds for the Central Universities (2017JBZ105); Guangxi Natural Science Foundation (2015GXNSFGA139002). Fujun also thanks the one hundred talent project of Beijing Jiaotong University. REFERENCES (1) 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.



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Ternary Nonfullerene Polymer Solar Cells with a Power Conversion

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