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Highly improved lifetimes of solar cells comprising post-additive-soaked PTB7-F20:PC71BM bulk heterojunction materials
Chemical Physics Letters 690 (2017) 42–46
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Highly improved lifetimes of solar cells comprising post-additive-soaked PTB7-F20:PC71BM bulk heterojunction materials In-Wook Hwang a,⇑, Jaemin Kong b,1 a b
Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea Heeger Center for Advanced Materials, Research for Solar and Sustainable Energies, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea
a r t i c l e
i n f o
Article history: Received 5 September 2017 In final form 18 October 2017 Available online 19 October 2017
a b s t r a c t Herein, we demonstrate that the lifetimes of polymer-based solar cells comprising PTB7-F20: PC71BM composites can be significantly increased by using post-additive-soaked bulk heterojunction (BHJ) films. Analyses of solar cell performance, film morphology, time-resolved photoluminescence, and photoconductivity revealed that the above increase resulted from the combined effects of the improved carrier generation and transport in post-additive-soaked BHJ films and the efficient deactivation of burn-in loss by using the photochemically stable PTB7-F20 polymer. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The recently reported laboratory-scale fabrication of polymerbased solar cells (PSCs) with power conversion efficiencies (PCEs) exceeding 12% has attracted much attention [1–3]. However, the commercialization of these devices requires their long-term stability to be improved [4–6]. Generally, the efficiency and stability of PSCs are strongly influenced by the nanomorphology of photoactive bulk heterojunction (BHJ) layers [4,7,8]. Thus, to fabricate efficient and stable PSCs, one should control the separation of BHJ phases to prepare bi-continuous networks featuring efficient exciton dissociation and fast charge carrier transport, thus decreasing carrier recombination losses. We have recently established post-additive soaking (PAS) as an effective method of optimizing BHJ morphologies [9,10], achieving efficient BHJ phase control and maximizing device PCEs and lifetimes by spin-coating pristine BHJ films with co-solvent systems containing an activator (resembling previously used solvent additives) and buffer (poor solvent with low boiling point). In our previous reports, we focused on PCE enhancement using high-efficiency semiconducting polymers such as PTB7 (poly (thieno[3,4-b]thiophene-co-benzodithiophene)) [10] and PTB7-Th (poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b0 ]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate])) [9].
⇑ Corresponding author. E-mail address: [email protected] (I.-W. Hwang). Current address: Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06511, United States. 1
https://doi.org/10.1016/j.cplett.2017.10.037 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
However, the above polymers showed relatively low photochemical stability, mostly due to the presence of fluoro units attached to thieno[3,4-b]thiophene groups in order to increase Voc and PCE [11]. Herein, we investigate PAS-induced device lifetime enhancement using a photochemically stable polymer (PTB7-F20) comprising fluoro-attached-thieno[3,4-b]thiophene-co-benzodithiophene (20%) and thieno[3,4-b]thiophene-co-benzodithiophene (80%) backbone units (Fig. 1a), which increase Voc and device lifetime, respectively. Notably, the PAS of pristine PTB7-F20:PC71BM film drastically improves its nanophase morphology and results in significantly improved device performance (Fig. 1b) compared to those of previously reported PTB7:PC71BM and PTB7-Th:PC71BM composites. 2. Experimental Inverted PSCs comprising indium tin oxide (ITO) (100 nm)/ZnO (20 nm)/PTB7-F20:PC71BM (100 nm)/PEDOT:PSS (20 nm)/Ag (120 nm) were fabricated as described elsewhere [9,10]. PEDOT: PSS layers were spin-cast onto BHJ layers from a solution containing a blend of commercial PEDOT:PSS and 1,3-isoprophyl alcohol (1:6). PTB7-F20 was purchased from 1-Material. Current density/ voltage (J-V), tunneling electron microscopy (TEM), atomic force microscopy (AFM), steady-state and time-resolved photoluminescence (PL), and time-resolved photoconductivity analyses were performed as reported previously [9,10,12]. A sulfur plasma lamp (LG, 6000 K) was used as an illumination source for the device lifetime test, with the temperature inside the test chamber maintained at 25 ± 2 °C using a controller. Device lifetime data
I.-W. Hwang, J. Kong / Chemical Physics Letters 690 (2017) 42–46
43
Fig. 1. (a) Molecular structures of PTB7-F20 and PC71BM. (b) Conceptual picture representing PAS-assisted BHJ optimization, adapted from [9]. (c) TEM images of P, A, and PAS PTB7-F20:PC71BM films.
were obtained by recording J-V curves of devices continuously illuminated with simulated sunlight at 1-h intervals. The above devices were encapsulated by sealing the top anode (Ag)-side surface with a 1-mm-thick layer of glass using a ‘five-minute’ epoxy glue.
3. Results and discussion The chemical structures of PTB7-F20 and PC71BM are presented in Fig. 1a. Three BHJ films were prepared for comparing morphologies, device performances, and carrier dynamics. The pristine (P) film was prepared by spin-casting a blend of PTB7F20:PC71BM (1:1.5) in pure chlorobenzene, whereas the additive-processed (A) film was fabricated by spin-casting the same solution containing 3 vol% of 1,8-diiodooctane (DIO) following previously reported methods [13–16]. Finally, the PAS film was prepared by spin-coating DIO:n-hexane (optimized volume ratio = 4:96) onto the dried P film. The schematic PAS treatment and TEM images of the above films are presented in Fig. 1b and c, respectively. Notably, the hundred-nanometersized black dots originally present in the P film completely disappeared after PAS, resulting in similar appearances of A and PAS films. The above black dots were found to reflect the overgrowth of PC71BM aggregates, with their complete disappearance after PAS demonstrating the efficient redistribution of PC71BM moieties [9,10]. The device structure and the energy level diagram for the individual components used to fabricate inverted PSCs are pre-
sented in Fig. 2a and b, respectively, with the component energy levels cascaded to highlight the efficient transport of holes and electrons to the anode (Ag) and cathode (ITO), respectively. The absorption spectra of BHJ films and the current-voltage (J-V) characteristics, external quantum efficiencies (EQE), and internal quantum efficiencies (IQE) of the fabricated devices are plotted in Fig. 2c–f, respectively. The decreased intensity of high-energy PC71BM absorption bands at 300–600 nm after PAS indicates that the above treatment resulted in the partial removal of fullerene moieties. Moreover, PAS dramatically improved certain device characteristics (Fig. 2d), e.g., during the P-to-PAS transition, the short-circuit current Jsc and the fill factor FF remarkably increased from 9.44 to 14.71 mA/cm2 and from 41 to 60%, respectively, enhancing the PCE by a factor of 2.1 (from 2.7 to 5.8%). The optimized PAS cell showed better parameters (PCE, FF, Jsc, and Voc of 5.8%, 60%, 14.71 mA/cm2, and 0.66 V, respectively) than the conventional A cell (5.5%, 59%, 13.44 mA/cm2, and 0.67 V, respectively). Notably, despite the insertion of 20% fluoro-attached-thieno[3,4-b]thiophene units into the polymer backbone, the optimized PTB7-F20:PC71BM composite showed a much higher Voc of 0.66 V than the PTBF0:PC71BM composite (Voc = 0.58 V) [11], where PTBF0 denotes the polymer comprising exclusively thieno[3,4-b] thiophene-co-benzodithiophene backbone units. The origin of the increased Jsc of the PAS cell is demonstrated by EQE and IQE data. The PAS cell showed a higher EQE than the A cell in a wavelength range of 400–700 nm (Fig. 2e) despite showing a less intense PC71BM absorption band at <600 nm, which resulted in the former cell exhibiting a higher IQE in the PC71BM absorption
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I.-W. Hwang, J. Kong / Chemical Physics Letters 690 (2017) 42–46
a
b Electrons -3.3 eV
Ag
-4.4 eV -4.3 eV
-4.8 eV
ZnO
ITO
PEDOT:PSS
cathode
PTB7-F20:PC71BM ZnO
PTB7-F20 PC71BM PEDOT:PSS -5.1 eV
ITO Glass
-5.0 eV
holes
d P A PAS
1.0
0.5
0.0
400
600
800
Current density (mA/cm2)
Absorbance (a. u.)
c
0
P A PAS
-5
-10
-15 0.0
0.2
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f
80
0.6
80
IQE (%)
20
60 40
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20
400
500
600
0.8
100
P A PAS
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0
0.4
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60
EQE (%)
anode
-6.1 eV -7.5 eV
e
-4.7 eV Ag
700
0 400
800
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. (a) Schematic view of the ITO/ZnO/PTB7-F20:PC71BM/PEDOT:PSS/Ag inverted solar cell and (b) energy levels of its individual components. (c) Absorption spectra of P, A, and PAS PTB7-F20:PC71BM films. (d) J-V, (e) EQE, and (f) IQE plots for P, A, and PAS PTB7-F20:PC71BM cells.
a
b
6
14
P A PAS
4
2
Jsc(mA/cm )
PCE (%)
5
3
12
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10
8
2 0
10
20
30
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0
10
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0.72
30
40
0.70 0.68 0.66
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50
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Voc (V)
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55 50 45 40
0.64 0
10
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30
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40
50
35
0
10
20
30
40
50
Time (days)
Fig. 3. (a) PCE, (b) Jsc, (c) Voc, and (d) FF of P, A, and PAS cells plotted as functions of irradiation time in air and in the case of glass-on-glass encapsulation.
I.-W. Hwang, J. Kong / Chemical Physics Letters 690 (2017) 42–46
regime (<600 nm) (Fig. 2f). This IQE amplification implied that the transfer of holes [17] from PC71BM to PTB7-F20 improved after the former PC71BM absorbed photons, which was explained by the redistribution of fullerene moieties throughout the additivesoaked film. Subsequently, we compared the operation stability of P, A, and PAS cells continuously illuminated with AM 1.5 light (100 mW/cm2) for 50 days (Fig. 3). The data obtained from 1500 voltage scans show that the PCE of the PAS cell was maintained for a much longer time than those of P and A cells (Fig. 3a), mostly since the Jsc and FF of the PAS cell were much more stable than those of P and A cells (Fig. 3b and d). Notably, the PCE of the PAS cell comprising the PTB7-F20:PC71BM composite was significantly more stable than that of the PAS cell comprising the PTB7:PC71BM composite, i.e., after 50-day operation, the PCEs of the former and latter cells decreased by 14 and 35%, respectively. To provide a photophysical explanation of the improved lifetime of the PAS cell despite of using the same active materials, we characterized carrier generation and transport dynam-
a Fluorescence intensity (a.u.)
PTB7-F20 PC71BM P-BHJ A-BHJ PAS-BHJ
600
700
800
900
Wavelength (nm)
b 4
PL intensity (normalized)
10
IRF PC71BM P-BHJ A-BHJ PAS-BHJ
3
10
2
10
0
2
4
Time (ns) Fig. 4. (a) Steady-state PL spectra of neat PTB7-F20 and PC71BM films and P, A, and PAS PTB7-F20:PC71BM BHJ films recorded using an excitation wavelength of 470 nm. (b) Time-resolved PL decay profiles of neat PC71BM film and P, A, and PAS PTB7F20:PC71BM BHJ films recorded using excitation and emission wavelengths of 470 and 720 nm, respectively. IRF stands for the instrument response function of the time-correlated single-photon counting system.
45
ics and BHJ morphology of the fabricated devices by steadystate PL spectroscopy, time-resolved PL/photoconductivity decay measurements, and AFM imaging (Figs. 4 and 5). The PL of PTB7-F20 was completely quenched in all BHJ films, whereas that of PC71BM systematically decreased in intensity in the order of P > A > PAS, with a corresponding decrease observed for PL decay times (Fig. 4). The complete PL quenching of PTB7-F20 in BHJ films demonstrated very efficient (100%) polymer-to-fullerene electron transfer [18], whereas the systematic PL intensity reduction of PC71BM and the corresponding PL decay time shortening were indicative of improved exciton dissociation at the polymer/fullerene interfaces after the photoexcitation of PC71BM. Exponential fitting of PL decay curves revealed that the neat PC71BM film demonstrated an average exciton decay time of 750 ps, while P, A, and PAS films exhibited shorter decay times of 290, 161, and 102 ps, respectively. Generally, non-dissociated excitons recombine via non-radiative thermal relaxation, which may heat the BHJ and transport layers and thus shorten device lifetime [19]. Thus, the fastest exciton decay observed for the PAS film corresponded to the most efficient deactivation of this heating process. Three-dimensional AFM topographic images revealed differences between A and PAS films (Fig. 5a), with the latter having a smoother surface than the former, probably reflecting the redistribution of fullerene moieties. Importantly, this smooth film surface may result in a reduced number of carrier trapping sites at the interface of photoactive and carrier-transport layers and thus contribute to the increased lifetime of the PAS cell. The time-resolved photoconductivity data for BHJ cells revealed differences between A and PAS films (Fig. 5b), with exponential curve fitting affording average decay times of 293 and 214 ns, respectively. The shorter carrier transport time of the PAS cell was ascribed to the reduced number of carrier trapping sites, thus increasing cell lifetime. Additionally, the increased lifetime of the PAS cell may also be due to additive-induced cleaning. During PAS, the dried P film was spin-coated with DIO:n-hexane (4:96, v/v), as mentioned above. In this process, the BHJ film was permeated by a very small amount of DIO (boiling point = 365 °C), whereas most of this solvent was removed by the n-hexane buffer (boiling point = 68 °C). According to some reports, the residual DIO present in dried films significantly shortens cell lifetimes [20]. Thus, the increased lifetime of the PAS cell may also reflect the decreased amount of DIO in the dried PAS film. Finally, we summarize the reasons of why the PCE of the PAS cell comprising the PTB7-F20:PC71BM composite exhibited a much larger stability than that of the cell containing the PTB7: PC71BM composite. Device lifetime measurements indicated that the PAS cell containing the PTB7:PC71BM composite exhibited an initial efficiency loss of 30% after 500 h [10], which was similar to the value reported by McGehee et al. for the PCDTBT: PC71BM BHJ composite and attributed to the photochemical reactions of organic polymers, being denoted as ‘burn-in loss’ [21]. The PTB7-F20 polymer was expected to show high photochemical stability due to its backbone containing 80% of non-fluoroattached repeating units. Therefore, the highly improved device lifetime of the PAS cell comprising the PTB7-F20:PC71BM composite compared to that of the cell comprising the PTB7:PC71BM composite demonstrates that the initial efficiency loss of the PTB polymer series reflects burn-in loss, which can be efficiently deactivated by the use of photochemically stable BHJ composites in the PAS process.
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a
PAS
A
Z: 20 nm
Z: 20 nm
Y: 20 m
Y: 20 m X: 20 m
X: 20 m
Photocurrent (normalized)
b 1
A PAS 0.1
0.01 0.0
0.5
1.0
1.5
2.0
Time (µs) Fig. 5. (a) 3D-AFM topography images for A and PAS PTB7-F20:PC71BM BHJ films. (b) Time-resolved photoconductivity decay profiles for A and PAS PTB7-20:PC71BM cells recorded using an excitation wavelength and a pulse width of 532 nm and 10 ns, respectively.
4. Conclusion
References
Herein, we have demonstrated that the lifetime of solar cells comprising the PTB7-F20:PC71BM composite can be significantly improved by PAS treatment of BHJ films. PL and photoconductivity decay data analyses revealed that the above improvement was associated with facilitated exciton dissociation and fast charge carrier transport in additive-soaked BHJ layers. The PAS cell comprising the PTB7-F20:PC71BM composite showed a much longer lifetime than that comprising the PTB7:PC71BM composite, which was ascribed to the combined effects of efficient burn-in loss deactivation by the photochemically stable PTB7-F20 polymer and the enhancement of charge carrier generation and transport dynamics by the PAS treatment.
[1] W. Zhao, D. Qian, S. Zhang, S. Li, O. Inganäs, F. Gao, J. Hou, Adv. Mater. 28 (2016) 4734. [2] J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma, H. Yan, Nat. Energy 1 (2016) 15027. [3] T. Kumari, S.M. Lee, S.H. Kang, S. Chen, C. Yang, Energy Environ. Sci. 10 (2017) 258. [4] A. Kumar, Z. Hong, S. Sista, Y. Yang, Adv. Energy Mater. 1 (2011) 124. [5] M. Jørgensen, K. Norrman, F.C. Krebs, Sol. Energy Mater. Sol. Cells 92 (2008) 686. [6] P. Cheng, X. Zhan, Mater. Horiz. 2 (2015) 462. [7] Z.M. Beiley, E.T. Hoke, R. Noriega, J. Dacuña, G.F. Burkhard, J.A. Bartelt, A. Salleo, M.F. Toney, M.D. McGehee, Adv. Energy Mater. 1 (2011) 954. [8] T. Heumueller, W.R. Mateker, I.T. Sachs-Quintana, K. Vandewal, J.A. Bartelt, T. M. Burke, T. Ameri, C.J. Brabec, M.D. McGehee, Energy Environ. Sci. 7 (2014) 2974. [9] J. Kong, I.-W. Hwang, K. Lee, Adv. Mater. 26 (2014) 6275. [10] I.-W. Hwang, J. Kong, H.K. Yoo, K. Lee, J. Phys. Chem. C 119 (2015) 12896. [11] H.J. Son, W. Wang, T. Xu, Y. Liang, Y. Wu, G. Li, L. Yu, J. Am. Chem. Soc. 133 (2011) 1885. [12] I.-W. Hwang, D.-H. Kwon, Y.T. Park, Y.-R. Kim, Bull. Korean Chem. Soc. 19 (1998) 584. [13] Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Nat. Photon. 6 (2013) 591. [14] Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 22 (2010) E135. [15] Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Adv. Mater. 23 (2011) 4636. [16] S.-H. Liao, H.-J. Jhuo, Y.-S. Cheng, S.-A. Chen, Adv. Mater. 25 (2013) 4766. [17] I.-W. Hwang, S. Cho, J.Y. Kim, K. Lee, N.E. Coates, D. Moses, A.J. Heeger, J. Appl. Phys. 104 (2008) 033706. [18] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. [19] K. Kawano, C. Adachi, Adv. Funct. Mater. 19 (2009) 3934. [20] W. Huang, E. Gann, Z.-Q. Xu, L. Thomsen, Y.-B. Cheng, C.R. McNeill, J. Mater. Chem. A 3 (2015) 16313. [21] C.H. Peters, I.T. Sachs-Quintana, W.R. Mateker, T. Heumueller, J. Rivnay, R. Noriega, Z.M. Beiley, E.T. Hoke, A. Salleo, M.D. McGehee, Adv. Mater. 24 (2012) 663.
Acknowledgements We thank the Heeger Center for Advanced Materials (HCAM) at Gwangju Institute of Science and Technology (GIST) in South Korea for providing the instruments used to fabricate and characterize the described devices. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (NRF-2017R1D1A1B03030669). We also acknowledge financial support from the Ultrashort Quantum Beam Facility Program through a grant funded by the GIST in 2017.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Highly improved lifetimes of solar cells comprising post-additive-soaked PTB7-F20:PC71BM bulk heterojunction materials In-Wook Hwang a,⇑, Jaemin Kong b,1 a b
Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea Heeger Center for Advanced Materials, Research for Solar and Sustainable Energies, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea
a r t i c l e
i n f o
Article history: Received 5 September 2017 In final form 18 October 2017 Available online 19 October 2017
a b s t r a c t Herein, we demonstrate that the lifetimes of polymer-based solar cells comprising PTB7-F20: PC71BM composites can be significantly increased by using post-additive-soaked bulk heterojunction (BHJ) films. Analyses of solar cell performance, film morphology, time-resolved photoluminescence, and photoconductivity revealed that the above increase resulted from the combined effects of the improved carrier generation and transport in post-additive-soaked BHJ films and the efficient deactivation of burn-in loss by using the photochemically stable PTB7-F20 polymer. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The recently reported laboratory-scale fabrication of polymerbased solar cells (PSCs) with power conversion efficiencies (PCEs) exceeding 12% has attracted much attention [1–3]. However, the commercialization of these devices requires their long-term stability to be improved [4–6]. Generally, the efficiency and stability of PSCs are strongly influenced by the nanomorphology of photoactive bulk heterojunction (BHJ) layers [4,7,8]. Thus, to fabricate efficient and stable PSCs, one should control the separation of BHJ phases to prepare bi-continuous networks featuring efficient exciton dissociation and fast charge carrier transport, thus decreasing carrier recombination losses. We have recently established post-additive soaking (PAS) as an effective method of optimizing BHJ morphologies [9,10], achieving efficient BHJ phase control and maximizing device PCEs and lifetimes by spin-coating pristine BHJ films with co-solvent systems containing an activator (resembling previously used solvent additives) and buffer (poor solvent with low boiling point). In our previous reports, we focused on PCE enhancement using high-efficiency semiconducting polymers such as PTB7 (poly (thieno[3,4-b]thiophene-co-benzodithiophene)) [10] and PTB7-Th (poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b0 ]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate])) [9].
⇑ Corresponding author. E-mail address: [email protected] (I.-W. Hwang). Current address: Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06511, United States. 1
https://doi.org/10.1016/j.cplett.2017.10.037 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
However, the above polymers showed relatively low photochemical stability, mostly due to the presence of fluoro units attached to thieno[3,4-b]thiophene groups in order to increase Voc and PCE [11]. Herein, we investigate PAS-induced device lifetime enhancement using a photochemically stable polymer (PTB7-F20) comprising fluoro-attached-thieno[3,4-b]thiophene-co-benzodithiophene (20%) and thieno[3,4-b]thiophene-co-benzodithiophene (80%) backbone units (Fig. 1a), which increase Voc and device lifetime, respectively. Notably, the PAS of pristine PTB7-F20:PC71BM film drastically improves its nanophase morphology and results in significantly improved device performance (Fig. 1b) compared to those of previously reported PTB7:PC71BM and PTB7-Th:PC71BM composites. 2. Experimental Inverted PSCs comprising indium tin oxide (ITO) (100 nm)/ZnO (20 nm)/PTB7-F20:PC71BM (100 nm)/PEDOT:PSS (20 nm)/Ag (120 nm) were fabricated as described elsewhere [9,10]. PEDOT: PSS layers were spin-cast onto BHJ layers from a solution containing a blend of commercial PEDOT:PSS and 1,3-isoprophyl alcohol (1:6). PTB7-F20 was purchased from 1-Material. Current density/ voltage (J-V), tunneling electron microscopy (TEM), atomic force microscopy (AFM), steady-state and time-resolved photoluminescence (PL), and time-resolved photoconductivity analyses were performed as reported previously [9,10,12]. A sulfur plasma lamp (LG, 6000 K) was used as an illumination source for the device lifetime test, with the temperature inside the test chamber maintained at 25 ± 2 °C using a controller. Device lifetime data
I.-W. Hwang, J. Kong / Chemical Physics Letters 690 (2017) 42–46
43
Fig. 1. (a) Molecular structures of PTB7-F20 and PC71BM. (b) Conceptual picture representing PAS-assisted BHJ optimization, adapted from [9]. (c) TEM images of P, A, and PAS PTB7-F20:PC71BM films.
were obtained by recording J-V curves of devices continuously illuminated with simulated sunlight at 1-h intervals. The above devices were encapsulated by sealing the top anode (Ag)-side surface with a 1-mm-thick layer of glass using a ‘five-minute’ epoxy glue.
3. Results and discussion The chemical structures of PTB7-F20 and PC71BM are presented in Fig. 1a. Three BHJ films were prepared for comparing morphologies, device performances, and carrier dynamics. The pristine (P) film was prepared by spin-casting a blend of PTB7F20:PC71BM (1:1.5) in pure chlorobenzene, whereas the additive-processed (A) film was fabricated by spin-casting the same solution containing 3 vol% of 1,8-diiodooctane (DIO) following previously reported methods [13–16]. Finally, the PAS film was prepared by spin-coating DIO:n-hexane (optimized volume ratio = 4:96) onto the dried P film. The schematic PAS treatment and TEM images of the above films are presented in Fig. 1b and c, respectively. Notably, the hundred-nanometersized black dots originally present in the P film completely disappeared after PAS, resulting in similar appearances of A and PAS films. The above black dots were found to reflect the overgrowth of PC71BM aggregates, with their complete disappearance after PAS demonstrating the efficient redistribution of PC71BM moieties [9,10]. The device structure and the energy level diagram for the individual components used to fabricate inverted PSCs are pre-
sented in Fig. 2a and b, respectively, with the component energy levels cascaded to highlight the efficient transport of holes and electrons to the anode (Ag) and cathode (ITO), respectively. The absorption spectra of BHJ films and the current-voltage (J-V) characteristics, external quantum efficiencies (EQE), and internal quantum efficiencies (IQE) of the fabricated devices are plotted in Fig. 2c–f, respectively. The decreased intensity of high-energy PC71BM absorption bands at 300–600 nm after PAS indicates that the above treatment resulted in the partial removal of fullerene moieties. Moreover, PAS dramatically improved certain device characteristics (Fig. 2d), e.g., during the P-to-PAS transition, the short-circuit current Jsc and the fill factor FF remarkably increased from 9.44 to 14.71 mA/cm2 and from 41 to 60%, respectively, enhancing the PCE by a factor of 2.1 (from 2.7 to 5.8%). The optimized PAS cell showed better parameters (PCE, FF, Jsc, and Voc of 5.8%, 60%, 14.71 mA/cm2, and 0.66 V, respectively) than the conventional A cell (5.5%, 59%, 13.44 mA/cm2, and 0.67 V, respectively). Notably, despite the insertion of 20% fluoro-attached-thieno[3,4-b]thiophene units into the polymer backbone, the optimized PTB7-F20:PC71BM composite showed a much higher Voc of 0.66 V than the PTBF0:PC71BM composite (Voc = 0.58 V) [11], where PTBF0 denotes the polymer comprising exclusively thieno[3,4-b] thiophene-co-benzodithiophene backbone units. The origin of the increased Jsc of the PAS cell is demonstrated by EQE and IQE data. The PAS cell showed a higher EQE than the A cell in a wavelength range of 400–700 nm (Fig. 2e) despite showing a less intense PC71BM absorption band at <600 nm, which resulted in the former cell exhibiting a higher IQE in the PC71BM absorption
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I.-W. Hwang, J. Kong / Chemical Physics Letters 690 (2017) 42–46
a
b Electrons -3.3 eV
Ag
-4.4 eV -4.3 eV
-4.8 eV
ZnO
ITO
PEDOT:PSS
cathode
PTB7-F20:PC71BM ZnO
PTB7-F20 PC71BM PEDOT:PSS -5.1 eV
ITO Glass
-5.0 eV
holes
d P A PAS
1.0
0.5
0.0
400
600
800
Current density (mA/cm2)
Absorbance (a. u.)
c
0
P A PAS
-5
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f
80
0.6
80
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20
60 40
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20
400
500
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0.8
100
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40
0
0.4
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60
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anode
-6.1 eV -7.5 eV
e
-4.7 eV Ag
700
0 400
800
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. (a) Schematic view of the ITO/ZnO/PTB7-F20:PC71BM/PEDOT:PSS/Ag inverted solar cell and (b) energy levels of its individual components. (c) Absorption spectra of P, A, and PAS PTB7-F20:PC71BM films. (d) J-V, (e) EQE, and (f) IQE plots for P, A, and PAS PTB7-F20:PC71BM cells.
a
b
6
14
P A PAS
4
2
Jsc(mA/cm )
PCE (%)
5
3
12
P A PAS
10
8
2 0
10
20
30
40
50
0
10
Time (days) P A PAS
0.72
30
40
0.70 0.68 0.66
d
50
P A PAS
60
Fill factor (%)
c
Voc (V)
20
Time (days)
55 50 45 40
0.64 0
10
20
30
Time (days)
40
50
35
0
10
20
30
40
50
Time (days)
Fig. 3. (a) PCE, (b) Jsc, (c) Voc, and (d) FF of P, A, and PAS cells plotted as functions of irradiation time in air and in the case of glass-on-glass encapsulation.
I.-W. Hwang, J. Kong / Chemical Physics Letters 690 (2017) 42–46
regime (<600 nm) (Fig. 2f). This IQE amplification implied that the transfer of holes [17] from PC71BM to PTB7-F20 improved after the former PC71BM absorbed photons, which was explained by the redistribution of fullerene moieties throughout the additivesoaked film. Subsequently, we compared the operation stability of P, A, and PAS cells continuously illuminated with AM 1.5 light (100 mW/cm2) for 50 days (Fig. 3). The data obtained from 1500 voltage scans show that the PCE of the PAS cell was maintained for a much longer time than those of P and A cells (Fig. 3a), mostly since the Jsc and FF of the PAS cell were much more stable than those of P and A cells (Fig. 3b and d). Notably, the PCE of the PAS cell comprising the PTB7-F20:PC71BM composite was significantly more stable than that of the PAS cell comprising the PTB7:PC71BM composite, i.e., after 50-day operation, the PCEs of the former and latter cells decreased by 14 and 35%, respectively. To provide a photophysical explanation of the improved lifetime of the PAS cell despite of using the same active materials, we characterized carrier generation and transport dynam-
a Fluorescence intensity (a.u.)
PTB7-F20 PC71BM P-BHJ A-BHJ PAS-BHJ
600
700
800
900
Wavelength (nm)
b 4
PL intensity (normalized)
10
IRF PC71BM P-BHJ A-BHJ PAS-BHJ
3
10
2
10
0
2
4
Time (ns) Fig. 4. (a) Steady-state PL spectra of neat PTB7-F20 and PC71BM films and P, A, and PAS PTB7-F20:PC71BM BHJ films recorded using an excitation wavelength of 470 nm. (b) Time-resolved PL decay profiles of neat PC71BM film and P, A, and PAS PTB7F20:PC71BM BHJ films recorded using excitation and emission wavelengths of 470 and 720 nm, respectively. IRF stands for the instrument response function of the time-correlated single-photon counting system.
45
ics and BHJ morphology of the fabricated devices by steadystate PL spectroscopy, time-resolved PL/photoconductivity decay measurements, and AFM imaging (Figs. 4 and 5). The PL of PTB7-F20 was completely quenched in all BHJ films, whereas that of PC71BM systematically decreased in intensity in the order of P > A > PAS, with a corresponding decrease observed for PL decay times (Fig. 4). The complete PL quenching of PTB7-F20 in BHJ films demonstrated very efficient (100%) polymer-to-fullerene electron transfer [18], whereas the systematic PL intensity reduction of PC71BM and the corresponding PL decay time shortening were indicative of improved exciton dissociation at the polymer/fullerene interfaces after the photoexcitation of PC71BM. Exponential fitting of PL decay curves revealed that the neat PC71BM film demonstrated an average exciton decay time of 750 ps, while P, A, and PAS films exhibited shorter decay times of 290, 161, and 102 ps, respectively. Generally, non-dissociated excitons recombine via non-radiative thermal relaxation, which may heat the BHJ and transport layers and thus shorten device lifetime [19]. Thus, the fastest exciton decay observed for the PAS film corresponded to the most efficient deactivation of this heating process. Three-dimensional AFM topographic images revealed differences between A and PAS films (Fig. 5a), with the latter having a smoother surface than the former, probably reflecting the redistribution of fullerene moieties. Importantly, this smooth film surface may result in a reduced number of carrier trapping sites at the interface of photoactive and carrier-transport layers and thus contribute to the increased lifetime of the PAS cell. The time-resolved photoconductivity data for BHJ cells revealed differences between A and PAS films (Fig. 5b), with exponential curve fitting affording average decay times of 293 and 214 ns, respectively. The shorter carrier transport time of the PAS cell was ascribed to the reduced number of carrier trapping sites, thus increasing cell lifetime. Additionally, the increased lifetime of the PAS cell may also be due to additive-induced cleaning. During PAS, the dried P film was spin-coated with DIO:n-hexane (4:96, v/v), as mentioned above. In this process, the BHJ film was permeated by a very small amount of DIO (boiling point = 365 °C), whereas most of this solvent was removed by the n-hexane buffer (boiling point = 68 °C). According to some reports, the residual DIO present in dried films significantly shortens cell lifetimes [20]. Thus, the increased lifetime of the PAS cell may also reflect the decreased amount of DIO in the dried PAS film. Finally, we summarize the reasons of why the PCE of the PAS cell comprising the PTB7-F20:PC71BM composite exhibited a much larger stability than that of the cell containing the PTB7: PC71BM composite. Device lifetime measurements indicated that the PAS cell containing the PTB7:PC71BM composite exhibited an initial efficiency loss of 30% after 500 h [10], which was similar to the value reported by McGehee et al. for the PCDTBT: PC71BM BHJ composite and attributed to the photochemical reactions of organic polymers, being denoted as ‘burn-in loss’ [21]. The PTB7-F20 polymer was expected to show high photochemical stability due to its backbone containing 80% of non-fluoroattached repeating units. Therefore, the highly improved device lifetime of the PAS cell comprising the PTB7-F20:PC71BM composite compared to that of the cell comprising the PTB7:PC71BM composite demonstrates that the initial efficiency loss of the PTB polymer series reflects burn-in loss, which can be efficiently deactivated by the use of photochemically stable BHJ composites in the PAS process.
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I.-W. Hwang, J. Kong / Chemical Physics Letters 690 (2017) 42–46
a
PAS
A
Z: 20 nm
Z: 20 nm
Y: 20 m
Y: 20 m X: 20 m
X: 20 m
Photocurrent (normalized)
b 1
A PAS 0.1
0.01 0.0
0.5
1.0
1.5
2.0
Time (µs) Fig. 5. (a) 3D-AFM topography images for A and PAS PTB7-F20:PC71BM BHJ films. (b) Time-resolved photoconductivity decay profiles for A and PAS PTB7-20:PC71BM cells recorded using an excitation wavelength and a pulse width of 532 nm and 10 ns, respectively.
4. Conclusion
References
Herein, we have demonstrated that the lifetime of solar cells comprising the PTB7-F20:PC71BM composite can be significantly improved by PAS treatment of BHJ films. PL and photoconductivity decay data analyses revealed that the above improvement was associated with facilitated exciton dissociation and fast charge carrier transport in additive-soaked BHJ layers. The PAS cell comprising the PTB7-F20:PC71BM composite showed a much longer lifetime than that comprising the PTB7:PC71BM composite, which was ascribed to the combined effects of efficient burn-in loss deactivation by the photochemically stable PTB7-F20 polymer and the enhancement of charge carrier generation and transport dynamics by the PAS treatment.
[1] W. Zhao, D. Qian, S. Zhang, S. Li, O. Inganäs, F. Gao, J. Hou, Adv. Mater. 28 (2016) 4734. [2] J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma, H. Yan, Nat. Energy 1 (2016) 15027. [3] T. Kumari, S.M. Lee, S.H. Kang, S. Chen, C. Yang, Energy Environ. Sci. 10 (2017) 258. [4] A. Kumar, Z. Hong, S. Sista, Y. Yang, Adv. Energy Mater. 1 (2011) 124. [5] M. Jørgensen, K. Norrman, F.C. Krebs, Sol. Energy Mater. Sol. Cells 92 (2008) 686. [6] P. Cheng, X. Zhan, Mater. Horiz. 2 (2015) 462. [7] Z.M. Beiley, E.T. Hoke, R. Noriega, J. Dacuña, G.F. Burkhard, J.A. Bartelt, A. Salleo, M.F. Toney, M.D. McGehee, Adv. Energy Mater. 1 (2011) 954. [8] T. Heumueller, W.R. Mateker, I.T. Sachs-Quintana, K. Vandewal, J.A. Bartelt, T. M. Burke, T. Ameri, C.J. Brabec, M.D. McGehee, Energy Environ. Sci. 7 (2014) 2974. [9] J. Kong, I.-W. Hwang, K. Lee, Adv. Mater. 26 (2014) 6275. [10] I.-W. Hwang, J. Kong, H.K. Yoo, K. Lee, J. Phys. Chem. C 119 (2015) 12896. [11] H.J. Son, W. Wang, T. Xu, Y. Liang, Y. Wu, G. Li, L. Yu, J. Am. Chem. Soc. 133 (2011) 1885. [12] I.-W. Hwang, D.-H. Kwon, Y.T. Park, Y.-R. Kim, Bull. Korean Chem. Soc. 19 (1998) 584. [13] Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Nat. Photon. 6 (2013) 591. [14] Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 22 (2010) E135. [15] Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Adv. Mater. 23 (2011) 4636. [16] S.-H. Liao, H.-J. Jhuo, Y.-S. Cheng, S.-A. Chen, Adv. Mater. 25 (2013) 4766. [17] I.-W. Hwang, S. Cho, J.Y. Kim, K. Lee, N.E. Coates, D. Moses, A.J. Heeger, J. Appl. Phys. 104 (2008) 033706. [18] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. [19] K. Kawano, C. Adachi, Adv. Funct. Mater. 19 (2009) 3934. [20] W. Huang, E. Gann, Z.-Q. Xu, L. Thomsen, Y.-B. Cheng, C.R. McNeill, J. Mater. Chem. A 3 (2015) 16313. [21] C.H. Peters, I.T. Sachs-Quintana, W.R. Mateker, T. Heumueller, J. Rivnay, R. Noriega, Z.M. Beiley, E.T. Hoke, A. Salleo, M.D. McGehee, Adv. Mater. 24 (2012) 663.
Acknowledgements We thank the Heeger Center for Advanced Materials (HCAM) at Gwangju Institute of Science and Technology (GIST) in South Korea for providing the instruments used to fabricate and characterize the described devices. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (NRF-2017R1D1A1B03030669). We also acknowledge financial support from the Ultrashort Quantum Beam Facility Program through a grant funded by the GIST in 2017.