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A novel quasi-two-dimensional fused-perylenediimide electron acceptor for solvent additive-free non-fullerene organic solar cells
Journal Pre-proof A novel quasi-two-dimensional fused-perylenediimide electron acceptor for solvent additive-free non-fullerene organic solar cells Yuli Yin, Zhi Zheng, Ming Liu, Shiyong Gao, Fengyun Guo, Genene Tessema Mola, Jinzhong Wang, Liancheng Zhao, Yong Zhang PII:
S0143-7208(19)32710-X
DOI:
https://doi.org/10.1016/j.dyepig.2019.108119
Reference:
DYPI 108119
To appear in:
Dyes and Pigments
Received Date: 21 November 2019 Revised Date:
7 December 2019
Accepted Date: 8 December 2019
Please cite this article as: Yin Y, Zheng Z, Liu M, Gao S, Guo F, Mola GT, Wang J, Zhao L, Zhang Y, A novel quasi-two-dimensional fused-perylenediimide electron acceptor for solvent additivefree non-fullerene organic solar cells, Dyes and Pigments (2020), doi: https://doi.org/10.1016/ j.dyepig.2019.108119. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphical Abstract
Non-fullerene organic solar cells based on perylenediimide (PDI) electron acceptor have become one of the hottest research directions. However, the previous PDI-based molecular design strategies always accompanied with excessive intramolecular twisting geometries, and may reduce their intermolecular π-π interactions, and cause the low crystallinity and charge transport in the blend film. Therefore, a novel electron acceptor (SF-FPDI) based on perylenediimide fused dimer (FPDI) with quasi-two-dimensional structure was reported.
A novel quasi-two-dimensional fused-perylenediimide electron acceptor for solvent additive-free non-fullerene organic solar cells Yuli Yin,a,1 Zhi Zheng,a,1 Ming Liu,a Shiyong Gao,a* Fengyun Guo,a Genene Tessema Mola,c Jinzhong Wang,a Liancheng Zhao,a Yong Zhanga,b,* a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
b
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China
c
School of Chemistry and Physics, University of KwaZulu-Natal, Pietermaritzburg Campus, Scottsville
3209, South Africa 1
Both the authors contributed equally to this work
Abstract
The wide range of the perylenediimide electron acceptors developed for non-fullerene organic solar cells (NF-OSCs) is an essential step to enhance the power conversion efficiency (PCE) of the resulting devices. Here, a novel quasi-two-dimensional electron acceptor (SF-FPDI) based on perylenediimide fused dimer (FPDI) moiety was synthesized, which exhibits a strong absorption in the 300-580 nm region with a wide optical band gap of 2.04 eV, and a suitable lowest unoccupied molecular orbital (LUMO) level of -3.92 eV. By blending SF-FPDI with a classical polymer donor (PTB7-Th), the additive-free devices achieved a high efficiency of 6.24% with a low energy loss of 0.72 eV, respectively is the smallest reported for FPDI-based fullerene-free acceptors up to date.
Key words: quasi-two-dimensional acceptor, perylenediimide fused dimer, low energy loss, non-fullerene organic solar cells
*Corresponding author. Yong Zhang; E-mail: [email protected]
1
1. Introduction
Solution-processed organic solar cells (OSCs) consisting of bulk heterojunction (BHJ) architecture have been considered as a promising green technology utilizing readily abundant sunlight, and have stimulated intense interest because of their attractive features, such as low-cost, flexibility, solution processability, etc. [1-3]. In the development history of OSCs, the soluble fullerene derivatives (e.g., PC61BM and PC71BM) have long been the dominant electron acceptors and achieved remarkable performance in BHJ OSCs, however, their intrinsic drawbacks in the limited visible light absorption, energy level tunability and morphological stability restrict the further improvement in efficiency and have acted as the strong motivation for researchers to seek potential replacements [4-6]. Over the past years, fullerene-free electron acceptors have quickly progressed and could obtain the prominent power conversion efficiencies (PCEs) of over 16% through careful molecular engineering and device engineering [7-10].
As a typical and successful case, perylenediimide (PDI)-based electron acceptors have been among the most investigated acceptor materials and the remarkable device performances (over 10%) have been reported by carefully selecting electron donor and optimizing devices [11-12]. However, numerous PDI-based acceptors suffer from strong self-aggregation and large phase separation in the BHJ film, which is mainly ascribed to the large, near-planar and rigid π-conjugated backbone of PDI, and this will lead to a planarity-aggregation trade-off in the design of state-of-the-art non-fullerene electron acceptors [13-14]. Although there are some strategies, such as introducing a π-bridge (carbon-carbon single bond [15-16], or arylene linker [17-18]) or 3D aromatic cores (benzodithiophene [19] or tetraphenylethylene core [20]), to design highly twisted linear PDI dimers or 3D-structured PDI acceptors in aiming of suppressing the excessive aggregation tendency and achieving higher photovoltaic efficiency. However,
2
the excessive intramolecular twisting geometries typically caused by these strategies is still worth noting in designing high performance PDI electron acceptors. In addition, in term of device optimization of PDI-based NF-OSCs, a small amount of solvent additive was generally unavoidable to balance the phase separation for desirable morphology, which increases the complexity of device fabrication and is also unfavourable for the large area printing or roll-to-roll manufacturing of OSCs due to the remained high boiling-point solvent additive during the solvent evaporation process. Therefore, an appropriate molecular design strategy to address this obstacle is highly desired.
Perylenediimide fused dimer (FPDI) moiety with a small dihedral angle of ~25o, providing an alternative PDI analogous to ensure proper aggregation and phase separation in the active layer while minimizing intramolecular twisting of PDI molecules [21-23]. To date quasi-2D FPDI building block has been investigated extensively in polymer acceptor and achieved promising efficiency advantages [24-27], however, there have been far fewer studies in FPDI-based small molecular electron acceptors.
We herein report a novel small molecule electron acceptor (SF-FPDI) with the quasi-2D structure as shown in Fig. 1 for solution processed
NF-OSCs. Due to the complementary absorption region,
matched energy level and suppressed unfavorable aggregation, PTB7-Th:SF-FPDI devices without the use of any solvent additive delivered a PCE of as high as 6.24% with a small energy loss of 0.72 eV in the promising PDI system. Our results highlighted the attractive prospect of the quasi-2D FPDI unit for constructing the state-of-the-art electron acceptors for high efficient NF-OSCs.
2. Experimental
The chemical structures of SF-FPDI and the medium bandgap polymer donor PTB7-Th are illustrated in Fig. 1a and SF-FPDI was prepared by a typical Suzuki coupling reaction with a yield of 67%
3
(Supporting Information). SF-FPDI showed satisfactory solubility in the common solvents, such as dichloromethane, chloroform and chlorobenzene at room temperature, which provides the possibility of solution process.
3. Results and discussion
The UV-Vis absorption spectra of SF-FPDI and their complementary film are depicted in Fig. 1b. SF-FPDI shows two strong absorption centered at 393 nm and 538 nm in solution, as a result of the localized π-π* transitions and intramolecular charge transfer (ICT), while the broader peaks are observed in SF-FPDI film. The optical band gap of SF-FPDI calculated from the onset of film absorption is 2.04 eV. When blending with the polymer donor (PTB7-Th), the blend film possesses complementary absorption from 300 to 780 nm, which should be desirable for high short-circuit current density (Jsc) (Fig. 1b).
The electrochemical property of SF-FPDI is investigated by cyclic voltammetry (CV) and ferrocene/ferrocenium (Fc/Fc+) was used as an internal standard. As shown in Fig. 1c, the onset of reduction potential (Eredonset) from SF-FPDI and the onset of oxidation potential of ferrocence (Eferrocene1/2onset) can be found as -0.57 V and 0.31 V versus Ag/Ag+, respectively. Accordingly, the lowest unoccupied molecular orbital (LUMO) of SF-FPDI is estimated to be -3.92 eV from the equation: LUMO = -[ Eredonset -Eferrocene1/2onset +4.80] (eV), then the highest occupied molecular orbital (HOMO) energy level can be calculated as -5.96 eV. As depicted in Figure 1c, the HOMO offset between PTB7-Th and SF-FPDI is found to be 0.74 eV, where the LUMO offset of 0.28 eV is observed and recent results have proved that even a small driving force below the empirical value of 0.3 eV can still achieve efficient exciton dissociation and charge separation in non-fullerene OSCs. This interesting phenomenon provides a promising pathway for reducing the unfavorable energy loss (Eloss, defined as Eloss =
4
Eopt-eVoc), [5] which is one of the most important limiting factors for achieving high performance of OSCs.
In order to evaluate the photovoltaic performance, PTB7-Th:SF-FPDI device was fabricated with a conventional device architecture of ITO/PEDOT:PSS/Active Layer/PDINO/Al. The detailed optimization process can be found in the Supporting Information. We first surveyed the mass ratio of polymer donor and non-fullerene acceptor, and found 1:2 mass ratio of PTB7-Th and SF-FPDI provided the highest efficiency of 5.54% with a Voc of 0.82 V, Jsc of 11.6 mA/cm2 and an FF of 57.2% (Table S1). Next, the thermal annealing (TA) strategy with different temperatures were applied to significantly enhance the crystallinity of PTB7-Th: SF-FPDI blend during the film post-deposition processing. As shown in Table S2, an enhanced efficiency of 5.65% was observed by thermal annealing at 80 oC for 5 min, and the PCE can be further increased to 5.91% when treated with higher annealing temperature (120 oC). Further increasing the annealing temperature (150 oC) will decrease the performances (4.72%). After thermal annealing at 80 oC for 10 min, the devices with the optimized thickness of 130 nm showed the highest efficiency of 6.24% with a Voc of 0.82 V, Jsc of 12.9 mA/cm2 and an FF of 59.0% (Fig. 2a). More interestingly, the SF-FPDI devices with different solvent additive processing (1-chloronaphthalene, and 1,8-diiodooctane) were also investigated and found that the solvent additive plays a negative effect in the device performances (Table S3), which also highlights the promising application in the future commercialization. The external quantum efficiency (EQE) spectra of the optimized device exhibits a broad and strong photoresponse from 300 to 780 nm with the EQE maxima of 62% at 530 nm, suggesting that both PTB7-Th and SF-FPDI make beneficial contributions to the capture of effective photoelectrons (Fig. 2a). The Jsc value of 12.3 mA/cm2 calculated from the integration of EQE spectra agree well with the value extracted from the J-V curve (the error is <5%). As mentioned above, because
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of the small energy level differences between the donor and acceptor, which lessened the driving force and this part of energy loss, [28-29] PTB7-Th:SF-FPDI device shows a low Eloss of 0.76 V with a high Voc of 0.82 V, which is the lowest value for reported FPDI-based devices with PCE >6% (Fig. 3a).
To gain more insight into the exciton generation, dissociation and charge collection process for the NFOSCs, the dependence of photocurrent (Jph) on the effective voltage (Veff), the charge dissociation probabilities P(E,T) and the maximum exciton generation rate (Gmax) were carried out (Fig. 3b). Here, Veff is defined as Veff =V0-Va, where V0 is the voltage where Jph equals zero and Va is the applied bias. It can find that photocurrent reaches saturation (Jsat=15.44 mA/cm2) at a high Veff of 4.7 V, suggesting that the photo-generated excitons can be completely dissociated and collected by the corresponding cathode and anode. Accordingly, the charge dissociation probabilities and maximum exciton generation rate of PTB7-Th:SF-FPDI device, derived from the equation of P(E,T)= Jph / Jsat and Gmax = Jsat /(e·L), [30-31] are calculated to be 83.5% and 7.42×1027 m-3 s-1, respectively, which are in good agreement with the observed high current density and EQE.
In order to further understand the charge recombination mechanism within the devices, we investigated the variation of Jsc and Voc as a function of illumination intensities (Plight), and the result was depicted in Fig. 3c,d. The relationship between current density and light intensity can be expressed Jsc∝ (Plight)α, and the α value of PTB7-Th:SF-FPDI device is found to be 0.95, indicating the efficient charge carrier transport and weak bimolecular recombination in the device (Fig. 3c) [23]. On the other hand, the slope of Voc versus ln(Plight) curve was provide to collate the property of charge recombination. As depicted in Fig. 3d, the optimized device based on SF-FPDI exhibit a small slopes of 1.14 kT/q, which further suggest that the trap-assisted recombination was significantly suppressed during charge transport process, and demonstrate that PTB7-Th:SF-FPDI device possesses weak bimolecular recombination and
6
negligible trap-assisted recombination[32]. Moreover, the charge carrier transport properties of PTB7-Th:SF-FPDI devices were further surveyed by the space-charge-limited current (SCLC) method, and the characteristic curves are plotted in Fig. S1. The SF-FPDI-based device showed high and balanced hole mobility (µ h) of 6.1 × 10-4 cm2 V-1 s-1 and electron mobility (µ e) of 5.2 × 10-4 cm2 V-1 s-1, which may contribute to the impressive current density and favorable fill factor in the resulted device.
Considering that the morphology and phase separation is a key to the performance of OSCs, the surface morphologies of the optimized PTB7-Th:SF-FPDI blend was investigated by atomic force microscopy (AFM) with tapping-mode. As shown in Fig. 4, the medium bandgap polymer donor (PTB7-Th) and electron acceptor (SF-FPDI) have favorable morphological compatibility with each other, and a relatively smooth surface (3D height image) with a root-mean-square (RMS) roughness of 0.826 nm was observed in the blend. Additionally, the AFM phase image of the blend without any additive treatment reveals nearly uniform surface with proper phase-separation sizes, which is crucial for charge transporting and minimized recombination in organic materials, and may partially explain the impressive efficiency achieved in the NF-OSCs.
4. Conclusions
In summary, we have designed and synthesized a novel FPDI-based electron acceptor (SF-FPDI) with quasi-2D configuration. SF-FPDI exhibited a wide optical band gap of 2.04 eV with strong absorption in the 300-580 nm region, which also offer an excellent complementary absorption spectrum with the medium band gap polymer donor (PTB7-Th). It is interesting to note that a small LUMO offset (0.28 eV) between electron donor and acceptor benefits for upshifted LUMO energy level of SF-FPDI, provide a chance to improve the Voc and minimize the energy loss of the resulting device in comparison with the reported FPDI-based acceptors. Therefor SF-FPDI-based additive-free device shows an impressive PCE
7
of 6.24% with a low Eloss of 0.76 V, which is the best values of FPDI-based electron acceptors reported so far. All of these prove that FPDI unit with quasi-2D structure could be applied for designing high performance alternative acceptors, and realizing great breakthrough in NF-OSCs with numerous electron donors already developed in the community.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21644006) and Natural Science Foundation of Heilongjiang Province of China (No. E2018036). Y. Zhang appreciates the support from the Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF.2020001) and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology, 2019-skllmd-16).
Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:
.
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Fig. 1. (a) Chemical structures of SF-FPDI and PTB7-Th. (b) UV-Vis absorption spectra in solution and film. (c) Cyclic
voltammetry curves and energy levels.
Fig. 2. (a) The J-V curves, and (b) EQE curves of the optimized PTB7-Th:SF-FPDI devices.
13
Fig. 3. (a) The plots of PCE against Eloss according to the reported FPDI-based non-fullerene acceptors. (b) The Jph versus Veff curves. (c) The light intensity dependence of Jsc. (d) The light intensity dependence of Voc.
Fig. 4. (a) Tapping mode AFM height images. (b) and Tapping mode AFM phase image (the inset is the 3D height
image).
14
Graphical Abstract
Non-fullerene organic solar cells based on perylenediimide (PDI) electron acceptor have become one of the hottest research directions. However, the previous PDI-based molecular design strategies always accompanied with excessive intramolecular twisting geometries, and may reduce their intermolecular π-π interactions, and cause the low crystallinity and charge transport in the blend film. Therefore, a novel electron acceptor (SF-FPDI) based on perylenediimide fused dimer (FPDI) with quasi-two-dimensional structure was reported.
15
Highlights Quasi-2D fused-perylenediimide electron acceptor (SF-FPDI2) with spirodifluorene core was synthesized. The non-fullerene organic solar cell gave a PCE of 6.24% with a low Eloss of 0.72 eV. The PCE10:SF-FPDI2 solar cells did not require any solvent additive for best cell performance.
Author statement Yuli Yin: Investigation, Writing - Original Draft Zhi Zheng: Investigation, Writing - Original Draft Ming Liu: Investigation Shiyong Gao: Resources, Conceptualization Fengyun Guo: Methodology Genene Tessema Mola: Visualization Jinzhong Wang: Supervision, Resources Liancheng Zhao: Supervision Yong Zhang: Supervision, Conceptualization, Writing - Review & Editing
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0143-7208(19)32710-X
DOI:
https://doi.org/10.1016/j.dyepig.2019.108119
Reference:
DYPI 108119
To appear in:
Dyes and Pigments
Received Date: 21 November 2019 Revised Date:
7 December 2019
Accepted Date: 8 December 2019
Please cite this article as: Yin Y, Zheng Z, Liu M, Gao S, Guo F, Mola GT, Wang J, Zhao L, Zhang Y, A novel quasi-two-dimensional fused-perylenediimide electron acceptor for solvent additivefree non-fullerene organic solar cells, Dyes and Pigments (2020), doi: https://doi.org/10.1016/ j.dyepig.2019.108119. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphical Abstract
Non-fullerene organic solar cells based on perylenediimide (PDI) electron acceptor have become one of the hottest research directions. However, the previous PDI-based molecular design strategies always accompanied with excessive intramolecular twisting geometries, and may reduce their intermolecular π-π interactions, and cause the low crystallinity and charge transport in the blend film. Therefore, a novel electron acceptor (SF-FPDI) based on perylenediimide fused dimer (FPDI) with quasi-two-dimensional structure was reported.
A novel quasi-two-dimensional fused-perylenediimide electron acceptor for solvent additive-free non-fullerene organic solar cells Yuli Yin,a,1 Zhi Zheng,a,1 Ming Liu,a Shiyong Gao,a* Fengyun Guo,a Genene Tessema Mola,c Jinzhong Wang,a Liancheng Zhao,a Yong Zhanga,b,* a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
b
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China
c
School of Chemistry and Physics, University of KwaZulu-Natal, Pietermaritzburg Campus, Scottsville
3209, South Africa 1
Both the authors contributed equally to this work
Abstract
The wide range of the perylenediimide electron acceptors developed for non-fullerene organic solar cells (NF-OSCs) is an essential step to enhance the power conversion efficiency (PCE) of the resulting devices. Here, a novel quasi-two-dimensional electron acceptor (SF-FPDI) based on perylenediimide fused dimer (FPDI) moiety was synthesized, which exhibits a strong absorption in the 300-580 nm region with a wide optical band gap of 2.04 eV, and a suitable lowest unoccupied molecular orbital (LUMO) level of -3.92 eV. By blending SF-FPDI with a classical polymer donor (PTB7-Th), the additive-free devices achieved a high efficiency of 6.24% with a low energy loss of 0.72 eV, respectively is the smallest reported for FPDI-based fullerene-free acceptors up to date.
Key words: quasi-two-dimensional acceptor, perylenediimide fused dimer, low energy loss, non-fullerene organic solar cells
*Corresponding author. Yong Zhang; E-mail: [email protected]
1
1. Introduction
Solution-processed organic solar cells (OSCs) consisting of bulk heterojunction (BHJ) architecture have been considered as a promising green technology utilizing readily abundant sunlight, and have stimulated intense interest because of their attractive features, such as low-cost, flexibility, solution processability, etc. [1-3]. In the development history of OSCs, the soluble fullerene derivatives (e.g., PC61BM and PC71BM) have long been the dominant electron acceptors and achieved remarkable performance in BHJ OSCs, however, their intrinsic drawbacks in the limited visible light absorption, energy level tunability and morphological stability restrict the further improvement in efficiency and have acted as the strong motivation for researchers to seek potential replacements [4-6]. Over the past years, fullerene-free electron acceptors have quickly progressed and could obtain the prominent power conversion efficiencies (PCEs) of over 16% through careful molecular engineering and device engineering [7-10].
As a typical and successful case, perylenediimide (PDI)-based electron acceptors have been among the most investigated acceptor materials and the remarkable device performances (over 10%) have been reported by carefully selecting electron donor and optimizing devices [11-12]. However, numerous PDI-based acceptors suffer from strong self-aggregation and large phase separation in the BHJ film, which is mainly ascribed to the large, near-planar and rigid π-conjugated backbone of PDI, and this will lead to a planarity-aggregation trade-off in the design of state-of-the-art non-fullerene electron acceptors [13-14]. Although there are some strategies, such as introducing a π-bridge (carbon-carbon single bond [15-16], or arylene linker [17-18]) or 3D aromatic cores (benzodithiophene [19] or tetraphenylethylene core [20]), to design highly twisted linear PDI dimers or 3D-structured PDI acceptors in aiming of suppressing the excessive aggregation tendency and achieving higher photovoltaic efficiency. However,
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the excessive intramolecular twisting geometries typically caused by these strategies is still worth noting in designing high performance PDI electron acceptors. In addition, in term of device optimization of PDI-based NF-OSCs, a small amount of solvent additive was generally unavoidable to balance the phase separation for desirable morphology, which increases the complexity of device fabrication and is also unfavourable for the large area printing or roll-to-roll manufacturing of OSCs due to the remained high boiling-point solvent additive during the solvent evaporation process. Therefore, an appropriate molecular design strategy to address this obstacle is highly desired.
Perylenediimide fused dimer (FPDI) moiety with a small dihedral angle of ~25o, providing an alternative PDI analogous to ensure proper aggregation and phase separation in the active layer while minimizing intramolecular twisting of PDI molecules [21-23]. To date quasi-2D FPDI building block has been investigated extensively in polymer acceptor and achieved promising efficiency advantages [24-27], however, there have been far fewer studies in FPDI-based small molecular electron acceptors.
We herein report a novel small molecule electron acceptor (SF-FPDI) with the quasi-2D structure as shown in Fig. 1 for solution processed
NF-OSCs. Due to the complementary absorption region,
matched energy level and suppressed unfavorable aggregation, PTB7-Th:SF-FPDI devices without the use of any solvent additive delivered a PCE of as high as 6.24% with a small energy loss of 0.72 eV in the promising PDI system. Our results highlighted the attractive prospect of the quasi-2D FPDI unit for constructing the state-of-the-art electron acceptors for high efficient NF-OSCs.
2. Experimental
The chemical structures of SF-FPDI and the medium bandgap polymer donor PTB7-Th are illustrated in Fig. 1a and SF-FPDI was prepared by a typical Suzuki coupling reaction with a yield of 67%
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(Supporting Information). SF-FPDI showed satisfactory solubility in the common solvents, such as dichloromethane, chloroform and chlorobenzene at room temperature, which provides the possibility of solution process.
3. Results and discussion
The UV-Vis absorption spectra of SF-FPDI and their complementary film are depicted in Fig. 1b. SF-FPDI shows two strong absorption centered at 393 nm and 538 nm in solution, as a result of the localized π-π* transitions and intramolecular charge transfer (ICT), while the broader peaks are observed in SF-FPDI film. The optical band gap of SF-FPDI calculated from the onset of film absorption is 2.04 eV. When blending with the polymer donor (PTB7-Th), the blend film possesses complementary absorption from 300 to 780 nm, which should be desirable for high short-circuit current density (Jsc) (Fig. 1b).
The electrochemical property of SF-FPDI is investigated by cyclic voltammetry (CV) and ferrocene/ferrocenium (Fc/Fc+) was used as an internal standard. As shown in Fig. 1c, the onset of reduction potential (Eredonset) from SF-FPDI and the onset of oxidation potential of ferrocence (Eferrocene1/2onset) can be found as -0.57 V and 0.31 V versus Ag/Ag+, respectively. Accordingly, the lowest unoccupied molecular orbital (LUMO) of SF-FPDI is estimated to be -3.92 eV from the equation: LUMO = -[ Eredonset -Eferrocene1/2onset +4.80] (eV), then the highest occupied molecular orbital (HOMO) energy level can be calculated as -5.96 eV. As depicted in Figure 1c, the HOMO offset between PTB7-Th and SF-FPDI is found to be 0.74 eV, where the LUMO offset of 0.28 eV is observed and recent results have proved that even a small driving force below the empirical value of 0.3 eV can still achieve efficient exciton dissociation and charge separation in non-fullerene OSCs. This interesting phenomenon provides a promising pathway for reducing the unfavorable energy loss (Eloss, defined as Eloss =
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Eopt-eVoc), [5] which is one of the most important limiting factors for achieving high performance of OSCs.
In order to evaluate the photovoltaic performance, PTB7-Th:SF-FPDI device was fabricated with a conventional device architecture of ITO/PEDOT:PSS/Active Layer/PDINO/Al. The detailed optimization process can be found in the Supporting Information. We first surveyed the mass ratio of polymer donor and non-fullerene acceptor, and found 1:2 mass ratio of PTB7-Th and SF-FPDI provided the highest efficiency of 5.54% with a Voc of 0.82 V, Jsc of 11.6 mA/cm2 and an FF of 57.2% (Table S1). Next, the thermal annealing (TA) strategy with different temperatures were applied to significantly enhance the crystallinity of PTB7-Th: SF-FPDI blend during the film post-deposition processing. As shown in Table S2, an enhanced efficiency of 5.65% was observed by thermal annealing at 80 oC for 5 min, and the PCE can be further increased to 5.91% when treated with higher annealing temperature (120 oC). Further increasing the annealing temperature (150 oC) will decrease the performances (4.72%). After thermal annealing at 80 oC for 10 min, the devices with the optimized thickness of 130 nm showed the highest efficiency of 6.24% with a Voc of 0.82 V, Jsc of 12.9 mA/cm2 and an FF of 59.0% (Fig. 2a). More interestingly, the SF-FPDI devices with different solvent additive processing (1-chloronaphthalene, and 1,8-diiodooctane) were also investigated and found that the solvent additive plays a negative effect in the device performances (Table S3), which also highlights the promising application in the future commercialization. The external quantum efficiency (EQE) spectra of the optimized device exhibits a broad and strong photoresponse from 300 to 780 nm with the EQE maxima of 62% at 530 nm, suggesting that both PTB7-Th and SF-FPDI make beneficial contributions to the capture of effective photoelectrons (Fig. 2a). The Jsc value of 12.3 mA/cm2 calculated from the integration of EQE spectra agree well with the value extracted from the J-V curve (the error is <5%). As mentioned above, because
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of the small energy level differences between the donor and acceptor, which lessened the driving force and this part of energy loss, [28-29] PTB7-Th:SF-FPDI device shows a low Eloss of 0.76 V with a high Voc of 0.82 V, which is the lowest value for reported FPDI-based devices with PCE >6% (Fig. 3a).
To gain more insight into the exciton generation, dissociation and charge collection process for the NFOSCs, the dependence of photocurrent (Jph) on the effective voltage (Veff), the charge dissociation probabilities P(E,T) and the maximum exciton generation rate (Gmax) were carried out (Fig. 3b). Here, Veff is defined as Veff =V0-Va, where V0 is the voltage where Jph equals zero and Va is the applied bias. It can find that photocurrent reaches saturation (Jsat=15.44 mA/cm2) at a high Veff of 4.7 V, suggesting that the photo-generated excitons can be completely dissociated and collected by the corresponding cathode and anode. Accordingly, the charge dissociation probabilities and maximum exciton generation rate of PTB7-Th:SF-FPDI device, derived from the equation of P(E,T)= Jph / Jsat and Gmax = Jsat /(e·L), [30-31] are calculated to be 83.5% and 7.42×1027 m-3 s-1, respectively, which are in good agreement with the observed high current density and EQE.
In order to further understand the charge recombination mechanism within the devices, we investigated the variation of Jsc and Voc as a function of illumination intensities (Plight), and the result was depicted in Fig. 3c,d. The relationship between current density and light intensity can be expressed Jsc∝ (Plight)α, and the α value of PTB7-Th:SF-FPDI device is found to be 0.95, indicating the efficient charge carrier transport and weak bimolecular recombination in the device (Fig. 3c) [23]. On the other hand, the slope of Voc versus ln(Plight) curve was provide to collate the property of charge recombination. As depicted in Fig. 3d, the optimized device based on SF-FPDI exhibit a small slopes of 1.14 kT/q, which further suggest that the trap-assisted recombination was significantly suppressed during charge transport process, and demonstrate that PTB7-Th:SF-FPDI device possesses weak bimolecular recombination and
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negligible trap-assisted recombination[32]. Moreover, the charge carrier transport properties of PTB7-Th:SF-FPDI devices were further surveyed by the space-charge-limited current (SCLC) method, and the characteristic curves are plotted in Fig. S1. The SF-FPDI-based device showed high and balanced hole mobility (µ h) of 6.1 × 10-4 cm2 V-1 s-1 and electron mobility (µ e) of 5.2 × 10-4 cm2 V-1 s-1, which may contribute to the impressive current density and favorable fill factor in the resulted device.
Considering that the morphology and phase separation is a key to the performance of OSCs, the surface morphologies of the optimized PTB7-Th:SF-FPDI blend was investigated by atomic force microscopy (AFM) with tapping-mode. As shown in Fig. 4, the medium bandgap polymer donor (PTB7-Th) and electron acceptor (SF-FPDI) have favorable morphological compatibility with each other, and a relatively smooth surface (3D height image) with a root-mean-square (RMS) roughness of 0.826 nm was observed in the blend. Additionally, the AFM phase image of the blend without any additive treatment reveals nearly uniform surface with proper phase-separation sizes, which is crucial for charge transporting and minimized recombination in organic materials, and may partially explain the impressive efficiency achieved in the NF-OSCs.
4. Conclusions
In summary, we have designed and synthesized a novel FPDI-based electron acceptor (SF-FPDI) with quasi-2D configuration. SF-FPDI exhibited a wide optical band gap of 2.04 eV with strong absorption in the 300-580 nm region, which also offer an excellent complementary absorption spectrum with the medium band gap polymer donor (PTB7-Th). It is interesting to note that a small LUMO offset (0.28 eV) between electron donor and acceptor benefits for upshifted LUMO energy level of SF-FPDI, provide a chance to improve the Voc and minimize the energy loss of the resulting device in comparison with the reported FPDI-based acceptors. Therefor SF-FPDI-based additive-free device shows an impressive PCE
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of 6.24% with a low Eloss of 0.76 V, which is the best values of FPDI-based electron acceptors reported so far. All of these prove that FPDI unit with quasi-2D structure could be applied for designing high performance alternative acceptors, and realizing great breakthrough in NF-OSCs with numerous electron donors already developed in the community.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21644006) and Natural Science Foundation of Heilongjiang Province of China (No. E2018036). Y. Zhang appreciates the support from the Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF.2020001) and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology, 2019-skllmd-16).
Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:
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Fig. 1. (a) Chemical structures of SF-FPDI and PTB7-Th. (b) UV-Vis absorption spectra in solution and film. (c) Cyclic
voltammetry curves and energy levels.
Fig. 2. (a) The J-V curves, and (b) EQE curves of the optimized PTB7-Th:SF-FPDI devices.
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Fig. 3. (a) The plots of PCE against Eloss according to the reported FPDI-based non-fullerene acceptors. (b) The Jph versus Veff curves. (c) The light intensity dependence of Jsc. (d) The light intensity dependence of Voc.
Fig. 4. (a) Tapping mode AFM height images. (b) and Tapping mode AFM phase image (the inset is the 3D height
image).
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Graphical Abstract
Non-fullerene organic solar cells based on perylenediimide (PDI) electron acceptor have become one of the hottest research directions. However, the previous PDI-based molecular design strategies always accompanied with excessive intramolecular twisting geometries, and may reduce their intermolecular π-π interactions, and cause the low crystallinity and charge transport in the blend film. Therefore, a novel electron acceptor (SF-FPDI) based on perylenediimide fused dimer (FPDI) with quasi-two-dimensional structure was reported.
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Highlights Quasi-2D fused-perylenediimide electron acceptor (SF-FPDI2) with spirodifluorene core was synthesized. The non-fullerene organic solar cell gave a PCE of 6.24% with a low Eloss of 0.72 eV. The PCE10:SF-FPDI2 solar cells did not require any solvent additive for best cell performance.
Author statement Yuli Yin: Investigation, Writing - Original Draft Zhi Zheng: Investigation, Writing - Original Draft Ming Liu: Investigation Shiyong Gao: Resources, Conceptualization Fengyun Guo: Methodology Genene Tessema Mola: Visualization Jinzhong Wang: Supervision, Resources Liancheng Zhao: Supervision Yong Zhang: Supervision, Conceptualization, Writing - Review & Editing
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: