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Molecular Design and Morphology Control Towards Efficient Polymer Solar Cells Processed using Non-aromatic and Non-chlorinated Solvents Yu Chen, Shaoqing Zhang, Yue Wu, and Jianhui Hou* Over the past decade, polymer solar cells (PSCs) with a bulkheterojunction (BHJ) photovoltaic active layer[1] have attracted much attention due to their advantages and their potential for making low-cost, light-weight and flexible devices through the roll-to-roll printing process. The power conversion efficiency (PCE) of polymer solar cells have been pushed to around 10% efficiency by employing well-designed conjugated polymers,[2–6] new device structures,[7–10] and innovative interfacial layer materials.[11–17] With the rapid progress of the PCE, the technology of PSCs is nearing the critical point of industrialization. From the perspective of making efficient PSCs through safe and environmentally friendly procedures, processes involving harmful solvents for the preparation of active layers must be changed. Up to now, the most successful and widely used solvents in PSC fabrications are aromatic, chlorinated, or chloro-aromatic compounds, such as toluene, chloroform (CF), 1,2-dichlorobenzene (o-DCB), and chlorobenzene (CB). Unfortunately, these solvents are detrimental to human health, the environment, or both. Therefore, the design of new active layer materials and the use of greener solvents in the fabrication of efficient PSC devices will be of great importance in the progress of PSCs. The solution coating process of the active layer is one of the key procedures in PSC fabrication. In order to obtain ultra-thin films (ca. 100 nm) with nanoscale bi-continuous phase separation, the solutes, i.e., conjugated polymers and fullerene derivatives, should be dissolved well. Since the conjugated polymers and the fullerene derivatives used in PSCs usually exhibit excellent solubility in aromatic, chlorinated, or chloro-aromatic solvents but limited solubility in other types of solvents, o-DCB or CB is typically the first choice as the processing solvent for making efficient PSCs. Recently, some research efforts have used other solvent types for the spin-coating process. For nonaromatic solvents, chloroform is a potential candidate[18] due to its strong capacity for dissolving conjugated polymers and fullerene derivatives. However, the use of chloroform has two main drawbacks: 1) chloroform is highly detrimental to the
Y. Chen, S. Zhang, Y. Wu, Prof. J. Hou State Key Laboratory of Polymer Physics and Chemistry Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected] Y. Chen Graduate University of Chinese Academy of Sciences Beijing 100049, P. R. China
DOI: 10.1002/adma.201304825 2744
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environment and human health; and 2) it has a low boiling point (b.p. = 61 °C), which leads to difficulty in controlling the spin-coating process and which also makes it more hazardous. For non-halogenated solvents, toluene and xylene[19] are commonly used as the alternatives for CB and o-DCB; however, these solvents are still hazardous, and as with other aromatics, they are also highly flammable. Overall, from a safety perspective, an ideal processing solvent should possess low flammability and be minimally hazardous to both human health and the environment; from the device-performance perspective, PSC devices processed with an ideal environmentally friendly solvent should have better or comparable photovoltaic performance as device processed with the currently used solvents, such as CB and o-DCB. Moreover, the solvent should possess a “medium” boiling point for easy processing—not too low that it is environmentally hazardous and not too high that it makes processing difficult. Obviously, non-aromatic and nonchlorinated solvents that meet the above requirements should be explored for use in the preparations of PSCs. In order to develop efficient PSCs processed with nonaromatic and non-chlorinated solvents, the initial and also most critical step is to design a new polymer showing good solubility in such solvents. Among the reported effective lowbandgap conjugated polymers, the series of PBDTTT polymers containing two basic fused-ring monomeric units, benzodithiophene (BDT) and thieno[3,4-b]thiophene (TT), are particularly attractive.[20–24] Herein we introduced triethylene glycol monoether (TEG) side chains onto the TT units (TT-TEG) and designed a new polymer, PBDTTT-TEG (Scheme 1). Previous reports have found that TEG chains possess several obvious advantages over alkyl analogues.[25–27] For instance, TEG side chains will not cause steric twisting of the polymer; TEG is also known to solubilize conjugated polymers, making a number of non-aromatic and non-chlorinated solvents possible. Using PBDTTT-TEG as the donor and phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor, the active layer of the PSC can be fabricated using N-methyl-2-pyrrolidone (NMP) as the processing solvent (Scheme 1), which is non-aromatic, nonhalogenated, minimally flammable, of low toxicity, and environmentally friendly. The resulting device based on PBDTTTTEG:PC71BM (1:1, w/w) has its best PCE at 5.23%, which is slightly higher than that obtained for the corresponding device processed with o-DCB. Furthermore, analogues of NMP, N-ethyl-2-pyrrolidone (NEP) and N-butyl-2-pyrrolidone (NBP) are also used as the processing solvents for comparison; their devices exhibited PCE values of 5.06% and 2.52%, respectively. The optical, electrochemical, and photovoltaic properties and the evolution of morphology of the active layers are investigated
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O
O O
O
OH
S Br
S
S S
NMP
N
O
*
S
S S
R
O
S
*
S
N
O
O
S
b
Sn S
Br
O
S
Sn
+ Br
R
S
S
a Br
O
O
O
O R
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R = 2 - Ethylhexyl
n
PBDTTT-TEG
R
N
NEP
I
O
I
DIO
NBP
Scheme 1. The synthetic route and molecular structure of the polymer PBDTTT-TEG and the structure of the solvents and additive used in this work. a = 4-dimethylaminopyridine (DMAP), dicyclohexylcarbodiimide (DCC), CH2Cl2, room temperature, triethylene glycol monomethyl ether, overnight, yield: 60%. b = Pd(PPh3)4, toluene, N,N-dimethylformamide (DMF), reflux, 48 h.
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(SCLC) method is 1.71 × 10−3 cm2 V−1 s−1, which is lower than that of PBDTTT-E-T,[23] but it is still a relatively high value compared with simple PBDTTT.[21,22]
1.0
(a)
solution film
Absorption
0.8 0.6 0.4 0.2 0.0 400
500
600
700
800
Wavelength (nm) 1.0
(b)
in NMP in DMF in o-DCB in THF
0.8
Absorption
for the devices produced using the different solvents; various amounts of a processing additive, 1,8-di-iodooctane (DIO), are also explored. To the best of the authors’ knowledge, the PCE of 5.23% is the best result achieved by PSC devices processed with non-aromatic and non-chlorinated solvents. As shown in Scheme 1, PBDTTT-TEG can be synthesized through several steps starting from a TT carboxylic acid. A Pdcatalyzed Stille coupling reaction between the bis(trimethyltin) BDT-T (where “-T” represents the 2-ethylhexyl-thienyl side chain) and the dibromide TT-TEG compounds was used for the polymerization reaction. The number-average molecular weight (Mn) of the polymer is 44.5 kDa, which was estimated by gel permeation chromatography (GPC) with tetrahydrofuran as the eluent and polystyrene with a narrow molecular-weight distribution as a standard. The thermogravimetric analysis (TGA) shows that the polymer has good thermal stability with an onset of decomposition temperature at ca. 360 °C (see Supporting Information (SI), Figure S1). From the differential scanning calorimetry (DSC) measurements, we find that neither endo- nor exothermic processes can be observed in the 0–300 °C range, at a scan rate of 10 °C min−1 (SI: Figure S2). UV–vis absorption spectra of the polymer in chloroform and as a solid film are shown in Figure 1a. PBDTTT-TEG shows a strong absorption band in the range of 550–790 nm in both solution and solid film. The absorption maximum of the solution of PBDTTT-TEG is located at ca. 630 nm with a shoulder at ca. 690 nm; in the solid film, the absorption peak is red-shifted to 645 nm with a shoulder at 700 nm, which may be ascribed to the efficient π–π stacking of polymer chains.[28] The absorption edge (λedge) of the polymer film is at ca. 785 nm, corresponding to an optical bandgap (Egopt) of 1.58 eV, which is the same as that of PBDTTT-E-T,[23] an analogous polymer with alkyl groups on the TT units. Electrochemical cyclic voltammetry (CV) was applied to determine the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) levels of the polymer (SI: Figure S3). PBDTTT-TEG shows a HOMO level of –5.09 eV and a LUMO level of –3.3 eV. The optical bandgap and the molecular energy levels of PBDTTT-TEG are almost the same as those of the polymer PBDTTT-E-T, which means that incorporation of the TEG side chain does not affect the bandgap or the energy levels of the polymer. The hole mobility of the pure polymer measured by the space-charge-limited-current
0.6 0.4 0.2 0.0 400
600
500
650
600
700
700
800
Wavelength (nm) Figure 1. The absorption spectra of the polymer a) in chloroform and in the solid film cast from thechloroform solution, and b) in the different solvents studied in this work, all at the same concentration.
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Table 1. Comparisons among the properties of the solvents studied in this work for PSC processing. Solvent
Boiling point [°C]a)
Flammability/Flash point [°C]
Toxicity
Environmental issues
o-DCB
198
Flammable/65
High
Hazardous (accumulative)
NMP
202
Low flammability/95
Low
Low risk (biodegradable)
DMF
153
Flammable/58
Low
Low risk (biodegradable)
THF
66
High flammability/–17
Low
Low risk (degradable)
Low flammability/117
N/Ab)
N/Ab)
DIO
326
a)Data
obtained from Materials Safety Data Sheet (MSDS) files provided by Sigma-Aldrich (corrected to 760 mm Hg); b)No data available in the MSDS files from SigmaAldrich and other databases.
Because of TEG as a side chain, the polymer not only has good solubility in the solvents commonly used in PSCs—such as CB, o-DCB, chloroform and toluene—but it also can be readily dissolved (>20 mg mL−2 on average) in many kinds of non-aromatic and non-chlorinated solvents, such as tetrahydrofuran (THF), 1,4-dioxane (DIOX), N,N-dimethylformamide (DMF), NMP, and even some alcohols. In order to select an appropriate solvent for device fabrication, UV–vis absorption spectra of PBDTTT-TEG were measured in various solvents to determine the state of PBDTTT-TEG dispersion in different solvents but at the same concentration. As shown in Figure 1b, the absorption range (550–790 nm) and the peaks (ca. 630 nm) are the same for the spectra of the different solvents, while the shape and intensity of the shoulder peaks at the longer wavelength are clearly dissimilar. Compared to the spectrum of the polymer solution in o-DCB, the spectra of the solutions in DMF and NMP show stronger peaks at short wavelength, but weaker absorption peaks at longer wavelength. Again compared to the o-DCB spectra, the solution in THF shows similar shoulder peak intensity but a relatively blue-shifted peak position. These observations may be attributed to the different states of aggregation of the polymer in the different solvents,[29] i.e., the polymer may be disperse better in NMP and DMF than in o-DCB. We also measured the absorption of the polymer in many other solvents (SI: Figure S4), and PBDTTT-TEG exhibited good solubility in some of these solvents, such as 1,4-dioxane and dimethyl sulfoxide; however, none of these other solvents performed better than NMP or DMF. Thus, for PBDTTT-TEG, NMP and DMF are better solvents than o-DCB because of the weaker state of aggregation of the polymer in solution. As discussed above, from the perspective of the solubility of the polymer, NMP, DMF, and THF seem to be suitable processing solvents for device fabrication, and these three solvents are non-aromatic and non-chlorinated compounds. Comparisons of some of the properties of these three solvents (see Table 1) reveal that both DMF and THF are flammable—especially THF, while NMP has low flammability and is also safer for the environment; NMP has been widely used as a solvent or intermediate in industry. From the perspective of the solubility of PCBM,[30,31] DMF and THF are poor solvents, while PCBM shows fairly good solubility in NMP (48 mg mL−1; see Table 3 and the SI: Figure S5). In addition, the boiling point of the processing solvent should also be taken into account; i.e., if the boiling point is too low or too high, it will be difficult to control the spin-coating process. Fortunately, NMP also has a 2746
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similar boiling point as o-DCB, meaning that the current spincoating methods developed for o-DCB as the solvent may easily be transferred to NMP. Therefore, taking all these factors into consideration, it is clear that NMP has the most potential for solvent processing of PSCs. PSC devices were fabricated and characterized to investigate the photovoltaic properties of the PBDTTT-TEG processed with NMP as the solvent. Meanwhile, to investigate the influence of different processing solvents on device performance, the traditionally used solvent, o-DCB, and the analogues of NMP, NEP and NBP, were also used for device fabrication in parallel. The device structure adopted in this work is ITO/PEDOT:PSS/ PBDTTT-TEG:PC71BM/Ca/Al (where ITO = indium tin oxide, PEDOT = poly(3,4-ethylenedioxythiophene), PSS = poly(styrenesulfonate)). For the initial photovoltaic characterizations, donor:acceptor (D/A) ratios (PBDTTT-TEG/PC71BM, w/w) of 1:1, 1:2, and 1:3 were scanned, and it was found that 1:1 is the optimal ratio (see SI: Figure S6, Table S1). Firstly, photovoltaic properties of the PSC devices prepared from o-DCB were fully investigated. As shown in Figure 2a, the PSC device shows a short-circuit current density, Jsc, of 9.58 mA cm−2, an open-circuit potential, Voc, of 0.70 V, and a fill factor, FF, of 50.87%, corresponding to a PCE of 3.41%. In order to further improve the photovoltaic performance of the device, a small amount of DIO was used as a solvent additive. Herein, volume ratios (DIO/o-DCB) of 1%, 3%, 5%, and 7% were screened (see SI: Table S1), and we found that the device processed with the addition of 3% DIO exhibited the best performance; an improved PCE of 5.16% is recorded with a Jsc of 13.65 mA cm−2, a Voc of 0.68 V, and a FF of 55.72%. Compared to reported results for derivatives of the PBDTTT polymer,[20,22] the Jsc and the Voc values of the device of PBDTTT-TEG are comparable, but the fill factor is slightly lower. Therefore, these results clearly indicate that the introduction of TEG side groups onto the TT units has no obviously negative influence on the photovoltaic properties of the polymer. The external quantum efficiency (EQE) curves of the device processed with and without the use of the optimum amount of DIO are provided in Figure 2b. As observed, after the addition of DIO, the device shows improved quantum efficiencies in the whole response range from 300 to 800 nm. Therefore, atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to investigate the influence of DIO on the morphologies of the blend films. When pure o-DCB is used as the processing solvent, no obvious phase separation can be distinguished in the blend film (see SI: Figure S7); when DIO is used as a solvent
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-2
Current density ( mA cm )
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10
10 0 300
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400
500
600
700
800
900
Wavelength (nm) Figure 2. a) J–V characteristics and b) EQE curves of solar cells processed with o-DCB based on the PBDTTT-TEG/PC71BM (1:1, w/w)with and without DIO.
additive to o-DCB, the surface of the blend film becomes rougher and relatively larger size aggregations can be clearly observed in the phase image and the TEM image (Figure S7). These results reveal that the use of DIO assists in enhancing phase separation of the blend, which is helpful for facilitating charge separation and for forming channels for the transport of the carriers. Furthermore, photovoltaic properties of the PSC devices prepared from NMP, NEP, and NBP were carefully investigated. As shown in Figure 3a, when pure NMP, NEP, and NBP were used as the processing solvents, the corresponding devices showed poor efficiencies of 1.72%, 1.07%, and 0.63%, respectively. A small amount of DIO for a volume ratio of 3%, 5%, and 7% was then used as the solvent additive in order to improve the photovoltaic performance of the devices. Considering all three volume ratios and processing with NMP, NEP, and NBP, 5% DIO is the optimal amount for obtaining the highest PCE. Figure 3b shows the EQE curves of the PSC devices processed with NMP, NEP, or NBP and 5% DIO as additive; the photovoltaic parameters of the corresponding devices are summarized in Table 2. Compared to the devices processed without DIO in NMP, the devices processed with a mixture of NMP + 5% DIO show simultaneously significant enhancements of Voc, Jsc, and FF—especially a remarkably improved Jsc (from 7.82 to 13.53 mA cm−2) and FF (from 35.47% to 58.55%). Overall, the best result was obtained when 5% DIO was applied in the blending solution, and a maximum PCE of 5.23% is recorded with Voc = 0.66 V,
Adv. Mater. 2014, 26, 2744–2749
0 300
400
500
600
700
800
Wavelength (nm) Figure 3. a) J–V curves of the devices based on the PBDTTT-TEG/PC71BM (1:1, w/w) processed with different solvents and (b) EQE curves of the devices processed with 5% DIO.
Jsc = 13.53 mA cm−2, and FF = 58.55%. The same trend was observed for NEP and NBP as well. For the device processed with NEP and 5% DIO, the PCE of the device was 5.06% with Voc = 0.66 V, Jsc = 13.74 mA cm−2, and FF = 56.18%; for the device processed with NBP and 5% DIO, the best PCE of the device was 2.52% with Voc = 0.61 V, Jsc = 8.91 mA cm−2, and FF = 46.20%. Compared to the devices processed with the same solvent but different DIO amounts, performance of the devices initially improves as more DIO is added to the solution; when the amount of DIO is beyond 5%, the PCE of devices start to drop slightly. From Figure 3b, it can be seen that the device exhibits a broad response absorption range from 300 to 800 nm, and within the whole range, the EQE is around 50% for the devices processed with NMP + 5% DIO and NEP + 5% DIO, but a lower EQE is obtained from the device processed with NBP + 5%. Successively, the influence of the addition of DIO on the morphological properties of the blend cast from NMP, NEP, and NBP were investigated by AFM and TEM. The AFM and TEM images of the blend films processed with NMP and NMP + 5% DIO are provided in Figure 4a–f; the AFM and TEM data for the other devices can be found in the SI (Figure S8). As shown in Figure 4a and b, when pure NMP is used, large crystal-like aggregations can be observed in both the topography and phase image, corresponding to a surface roughness with a quadratic mean value, Rq, of 5.69, which may be due to the
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Table 2. Photovoltaic properties of the PSCs based on PBDTTT-TEG and PC71BM (1:1, w/w) under the illumination of AM 1.5G, 100 mW cm−2. Processing solvent
Additive
Voc [V]
Jsc [mA cm−2]
PCE [%]
FF [%] Best
NMP
NEP
NBP
a)The
Averagea)
w/o DIO
0.62
7.82
35.47
1.72
1.55
3% DIO
0.65
10.32
49.45
3.33
3.22
5% DIO
0.66
13.53
58.55
5.23
5.14 4.80
7% DIO
0.66
13.19
57.15
4.97
w/o DIO
0.57
5.87
31.79
1.07
1.04
3% DIO
0.65
8.83
40.85
2.36
2.23
5% DIO
0.66
13.74
56.18
5.06
4.90
7% DIO
0.65
13.90
54.20
4.90
4.81
w/o DIO
0.56
3.60
31.30
0.63
0.58
3% DIO
0.60
8.11
42.60
2.09
2.06
5% DIO
0.61
8.91
46.20
2.52
2.25
7% DIO
0.61
8.36
42.12
2.17
1.96
average PCE values were obtained from the data of over 20 devices.
formation of PC71BM crystals. Similar phenomena can also be observed in the blend films processed with pure NEP or NBP. Interestingly, when the 5% DIO additive is used, the large crystal-like aggregations are not present. As shown in the AFM images (Figure 4d,e), the blend film becomes smoother and more uniform with a Rq of 2.73. As observed in the TEM image (Figure 4f), the severe phase separation of the blend (as in Figure 4c) no longer observed. Moreover, for the blend film processed with NEP and NBP, the morphology variation before and after the use of DIO can also be confirmed by the
AFM and TEM images (SI: Figure S8). These data indicate that, when NMP, NEP, or NBP are used as the processing solvent, severe phase separation will occur due to the aggregation of PC71BM, which is detrimental to the formation of nanoscale bi-continuous phase separation; however, after adding the optimum amount of DIO, the severe phase separation can be avoided so that photovoltaic performance of the devices can be improved effectively. Obviously, although the addition of DIO is very helpful for improving photovoltaic performance, it plays different roles in the NMP- and o-DCB-processed devices.
Figure 4. a,d) Tapping-mode AFM topography, b,e) phase, and c,f) TEM images of blend films based on the PBDTTT-TEG/PC71BM (1:1, w/w)cast from NMP without DIO (a–c) and with 5% DIO (v/v) (d–f). The sizes of the AFM images are 5 μm × 5 μm. The scale bars in the TEM images are 1 μm.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Detailed information for the materials, the instruments, the syntheses of the polymers, and the device fabrication process are included.
Acknowledgement The authors would like to acknowledge the financial support from the International S&T Cooperation Program of China (2011DFG63460), the 973 projects (2014CB643501), the Science and Technology Commission of Beijing (Z131100006013002) and the National Natural Science Foundation of China (21325419). Received: September 26, 2013 Revised: January 1, 2014 Published online: February 4, 2014 [1] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789. [2] L. T. Dou, J. B. You, J. Yang, C. C. Chen, Y. J. He, S. Murase, T. Moriarty, K. Emery, G. Li, Y. Yang, Nat. Photonics 2012, 6, 180.
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[3] C. Cabanetos, A. E. Labban, J. A. Bartelt, J. D. Douglas, W. R. Mateker, J. M. J. Frechet, M. D. McGehee, P. M. Beaujuge, J. Am. Chem. Soc. 2013, 135, 4656. [4] I. Osaka, T. Kakara, N. Takemura, T. Koganezawa, K. Takimiya, J. Am. Chem. Soc. 2013, 135, 8834. [5] M. J. Zhang, Y. Gu, X. Guo, F. Liu, S. Q. Zhang, L. J. Huo, T. P. Russell, J. H. Hou, Adv. Mater. 2013, 25, 4944. [6] T. Y. Chu, J. P. Lu, S. Beaupré, Y. G. Zhang, J. R. Pouliot, S. Wakim, J. Y. Zhou, M. Leclerc, Z. Li, J. F. Ding, Y. Tao, J. Am. Chem. Soc. 2011, 133, 4250. [7] J. B. You, L. T. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C. C. Chen, J. Gao, G. Li, Y. Yang, Nat. Commun. 2013, 4, 1446. [8] L. Q. Yang, H. X. Zhou, S. C. Price, W. You, J. Am. Chem. Soc. 2012, 134, 5432. [9] C. E. Small, S. Chen, J. Subbiah, C. M. Amb, S. W. Tsang, T. H. Lai, J. R. Reynolds, F. So, Nat. Photonics 2012, 6, 115. [10] W. Li, A. Furlan, K. H. Hendriks, M. M. Wienk, R. A. J. Janssen, J. Am. Chem. Soc. 2013, 135, 5529. [11] R. Steim, F. R. Kogler, C. J. Brabe, J. Mater. Chem. 2010, 20, 2499. [12] Z. C. He, C. M. Zhong, S. J. Su, M. Xu, H. B. Wu, Y. Cao, Nat. Photonics 2012, 6, 591. [13] S. H. Oh, S. I. Na, J. Jo, B. Lim, D. Vak, D. Y. Kim, Adv. Funct. Mater. 2010, 20, 1977. [14] J. H. Seo, A. Gutacker, Y. M. Sun, H. B. Wu, F. Huang, Y. Cao, U. Scherf, A. J. Heeger, G. C. Bazan, J. Am. Chem. Soc. 2011, 133, 8416. [15] T. Yang, M. Wang, C. Duan, X. Hu, L. Huang, J. Peng, F. Huang, X. Gong, Energy Environ. Sci. 2012, 24, 3046. [16] Y. Chen, Z. T. Jiang, M. Gao, S. E. Watkins, P. Lv, H. Q. Wang, X. W. Chen, Appl. Phys. Lett. 2012, 100, 203304. [17] S. H. Liao, H. J. Jhuo, Y. S. Cheng, S. A. Chen, Adv. Mater. 2013, 25, 4766. [18] J. C. Bijleveld, V. S. Gevaerts, D. D. Nuzzo, M. Turbiez, S. G. J. Mathijssen, D. M. Leeuw, M. M. Wienk, R. A. J. Janssen, Adv. Mater. 2010, 22, E242. [19] C. C. Chueh, K. Yao, H. L. Yip, C. Y. Chang, Y. X. Xu, K. S. Chen, C. Z. Li, P. Liu, F. Huang, Y. W. Chen, W. C. Chen, A. K. Y. Jen, Energy Environ. Sci. 2013, 6, 3241. [20] H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Nat. Photonics 2009, 3, 649. [21] Y. Y. Liang, D. Feng, Y. Wu, S. T. Tsai, G. Li, C. Ray, L. P. Yu, J. Am. Chem. Soc. 2009, 131, 7792. [22] Y. Y. Liang, Z. Xu, J. Xia, S. T. Tsai, Y. Wu, G. Li, C. Ray, L. P. Yu, Adv. Mater. 2010, 22, E135. [23] L. J. Huo, S. Q. Zhang, X. Guo, F. Xu, Y. F. Li, J. H. Hou, Angew. Chem. Int. Ed. 2011, 50, 9697. [24] A. C. Stuart, J. R. Tumbleston, H. X. Zhou, W. T. Li, S. B. Liu, H. Ade, W. You, J. Am. Chem. Soc. 2013, 135, 1806. [25] C. S. Velazquez, J. E. Hutchison, R. W. Murray, J. Am. Chem. Soc. 1993, 115, 7896. [26] R. D. McCullough, Adv. Mater. 1998, 10, 93. [27] K. Lu, J. Fang, X. W. Zhu, H. Yan, D. H. Li, C. A. Di, Y. L. Yang, Z. X. Wei, New J. Chem. 2013, 37, 1728. [28] X. C. Wang, P. Jiang, Y. Chen, H. Luo, Z. Zhang, H. Wang, X. Li, G. Yu, Y. F. Li, Macromolecules 2013, 46, 4805. [29] D. P. Qian, L. Ye, M. J. Zhang, Y. R. Liang, L. J. Li, Y. Huang, X. Guo, S. Q. Zhang, Z. A. Tan, J. H. Hou, Macromoleculars 2012, 45, 9611. [30] K. R. Graham, P. M. Wieruszewski, R. Stalder, M. J. Hartel, J.G. Mei, F. So, J. R. Reynolds, Adv. Funct. Mater. 2012, 22, 4801. [31] F. Machui, S. Langner, X. D. Zhu, S. Abbott, C. J. Brabec, Solar Energy Mater. Solar Cells 2012, 100, 138.
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In order to further investigate the morphology changes with DIO addition, the hole and electron mobilities of the devices processed without and with DIO were measured by the method of SCLC (see the SI: Table S2 and Figure S9). After the addition of DIO, the hole mobilities of the devices are slightly improved, while the electron mobilities of the devices increase by nearly fivefold, except for the devices processed using NBP; that is, for devices cast from NMP, the electron mobility increases from 6.72 × 10−4 to 3.51 × 10−3 cm2 V−1 s−1 while the hole mobility is slightly changed from 1.97 × 10−4 to 3.78 × 10−4 cm2 V−1 s−1. These results are consistent with the morphological measurements, i.e., the addition of DIO reduces phase separation and leads to the formation of bi-continuous networks in the blend. In conclusion, a new polymer based on BDT as donor and TT as acceptor, PBDTTT-TEG, was designed, synthesized, and applied to PSC devices. The results clearly demonstrate that after replacing the alkyl group with the TEG group, the conjugated polymer will show excellent solubility in various solvents, such as DMF, THF, NMP, and o-DCB. This molecular modification method has little negative influence on the photovoltaic properties of the polymer; by using o-DCB as the processing solvent, the PSC device prepared with otherwise optimal conditions shows comparable PCE to the alkyl-substituted analogue, PBDTTT, which gives a PCE of 5.16%. Benefiting from the TEG group, efficient PSC devices can be fabricated using PBDTTT-TEG as the electron donor and NMP as the processing solvent for spin-coating the active layer. The PSCs based on PBDTTT-TEG:PC71BM (1:1, w/w) processed with NMP and 5% DIO (v/v) achieve a PCE of 5.23% with Voc = 0.66 V, Jsc = 13.53 mA cm−2, and FF = 58.55%, under the illumination of AM 1.5G, 100 cm−2; this is the highest PCE for the PSCs fabricated with non-aromatic and non-chlorinated solvent in reported literature. Most importantly, with the rapid progress of the PSC field, the use of a greener process for making PSC devices becomes more and more important. This work suggests an easy but useful method towards using environmentally friendly and safe solvents for PSC device fabrication.
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Molecular Design and Morphology Control Towards Efficient Polymer Solar Cells Processed using Non-aromatic and Non-chlorinated Solvents Yu Chen, Shaoqing Zhang, Yue Wu, and Jianhui Hou* Over the past decade, polymer solar cells (PSCs) with a bulkheterojunction (BHJ) photovoltaic active layer[1] have attracted much attention due to their advantages and their potential for making low-cost, light-weight and flexible devices through the roll-to-roll printing process. The power conversion efficiency (PCE) of polymer solar cells have been pushed to around 10% efficiency by employing well-designed conjugated polymers,[2–6] new device structures,[7–10] and innovative interfacial layer materials.[11–17] With the rapid progress of the PCE, the technology of PSCs is nearing the critical point of industrialization. From the perspective of making efficient PSCs through safe and environmentally friendly procedures, processes involving harmful solvents for the preparation of active layers must be changed. Up to now, the most successful and widely used solvents in PSC fabrications are aromatic, chlorinated, or chloro-aromatic compounds, such as toluene, chloroform (CF), 1,2-dichlorobenzene (o-DCB), and chlorobenzene (CB). Unfortunately, these solvents are detrimental to human health, the environment, or both. Therefore, the design of new active layer materials and the use of greener solvents in the fabrication of efficient PSC devices will be of great importance in the progress of PSCs. The solution coating process of the active layer is one of the key procedures in PSC fabrication. In order to obtain ultra-thin films (ca. 100 nm) with nanoscale bi-continuous phase separation, the solutes, i.e., conjugated polymers and fullerene derivatives, should be dissolved well. Since the conjugated polymers and the fullerene derivatives used in PSCs usually exhibit excellent solubility in aromatic, chlorinated, or chloro-aromatic solvents but limited solubility in other types of solvents, o-DCB or CB is typically the first choice as the processing solvent for making efficient PSCs. Recently, some research efforts have used other solvent types for the spin-coating process. For nonaromatic solvents, chloroform is a potential candidate[18] due to its strong capacity for dissolving conjugated polymers and fullerene derivatives. However, the use of chloroform has two main drawbacks: 1) chloroform is highly detrimental to the
Y. Chen, S. Zhang, Y. Wu, Prof. J. Hou State Key Laboratory of Polymer Physics and Chemistry Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected] Y. Chen Graduate University of Chinese Academy of Sciences Beijing 100049, P. R. China
DOI: 10.1002/adma.201304825 2744
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environment and human health; and 2) it has a low boiling point (b.p. = 61 °C), which leads to difficulty in controlling the spin-coating process and which also makes it more hazardous. For non-halogenated solvents, toluene and xylene[19] are commonly used as the alternatives for CB and o-DCB; however, these solvents are still hazardous, and as with other aromatics, they are also highly flammable. Overall, from a safety perspective, an ideal processing solvent should possess low flammability and be minimally hazardous to both human health and the environment; from the device-performance perspective, PSC devices processed with an ideal environmentally friendly solvent should have better or comparable photovoltaic performance as device processed with the currently used solvents, such as CB and o-DCB. Moreover, the solvent should possess a “medium” boiling point for easy processing—not too low that it is environmentally hazardous and not too high that it makes processing difficult. Obviously, non-aromatic and nonchlorinated solvents that meet the above requirements should be explored for use in the preparations of PSCs. In order to develop efficient PSCs processed with nonaromatic and non-chlorinated solvents, the initial and also most critical step is to design a new polymer showing good solubility in such solvents. Among the reported effective lowbandgap conjugated polymers, the series of PBDTTT polymers containing two basic fused-ring monomeric units, benzodithiophene (BDT) and thieno[3,4-b]thiophene (TT), are particularly attractive.[20–24] Herein we introduced triethylene glycol monoether (TEG) side chains onto the TT units (TT-TEG) and designed a new polymer, PBDTTT-TEG (Scheme 1). Previous reports have found that TEG chains possess several obvious advantages over alkyl analogues.[25–27] For instance, TEG side chains will not cause steric twisting of the polymer; TEG is also known to solubilize conjugated polymers, making a number of non-aromatic and non-chlorinated solvents possible. Using PBDTTT-TEG as the donor and phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor, the active layer of the PSC can be fabricated using N-methyl-2-pyrrolidone (NMP) as the processing solvent (Scheme 1), which is non-aromatic, nonhalogenated, minimally flammable, of low toxicity, and environmentally friendly. The resulting device based on PBDTTTTEG:PC71BM (1:1, w/w) has its best PCE at 5.23%, which is slightly higher than that obtained for the corresponding device processed with o-DCB. Furthermore, analogues of NMP, N-ethyl-2-pyrrolidone (NEP) and N-butyl-2-pyrrolidone (NBP) are also used as the processing solvents for comparison; their devices exhibited PCE values of 5.06% and 2.52%, respectively. The optical, electrochemical, and photovoltaic properties and the evolution of morphology of the active layers are investigated
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O
O O
O
OH
S Br
S
S S
NMP
N
O
*
S
S S
R
O
S
*
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O
O
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b
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Br
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S
a Br
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R = 2 - Ethylhexyl
n
PBDTTT-TEG
R
N
NEP
I
O
I
DIO
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Scheme 1. The synthetic route and molecular structure of the polymer PBDTTT-TEG and the structure of the solvents and additive used in this work. a = 4-dimethylaminopyridine (DMAP), dicyclohexylcarbodiimide (DCC), CH2Cl2, room temperature, triethylene glycol monomethyl ether, overnight, yield: 60%. b = Pd(PPh3)4, toluene, N,N-dimethylformamide (DMF), reflux, 48 h.
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(SCLC) method is 1.71 × 10−3 cm2 V−1 s−1, which is lower than that of PBDTTT-E-T,[23] but it is still a relatively high value compared with simple PBDTTT.[21,22]
1.0
(a)
solution film
Absorption
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500
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800
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for the devices produced using the different solvents; various amounts of a processing additive, 1,8-di-iodooctane (DIO), are also explored. To the best of the authors’ knowledge, the PCE of 5.23% is the best result achieved by PSC devices processed with non-aromatic and non-chlorinated solvents. As shown in Scheme 1, PBDTTT-TEG can be synthesized through several steps starting from a TT carboxylic acid. A Pdcatalyzed Stille coupling reaction between the bis(trimethyltin) BDT-T (where “-T” represents the 2-ethylhexyl-thienyl side chain) and the dibromide TT-TEG compounds was used for the polymerization reaction. The number-average molecular weight (Mn) of the polymer is 44.5 kDa, which was estimated by gel permeation chromatography (GPC) with tetrahydrofuran as the eluent and polystyrene with a narrow molecular-weight distribution as a standard. The thermogravimetric analysis (TGA) shows that the polymer has good thermal stability with an onset of decomposition temperature at ca. 360 °C (see Supporting Information (SI), Figure S1). From the differential scanning calorimetry (DSC) measurements, we find that neither endo- nor exothermic processes can be observed in the 0–300 °C range, at a scan rate of 10 °C min−1 (SI: Figure S2). UV–vis absorption spectra of the polymer in chloroform and as a solid film are shown in Figure 1a. PBDTTT-TEG shows a strong absorption band in the range of 550–790 nm in both solution and solid film. The absorption maximum of the solution of PBDTTT-TEG is located at ca. 630 nm with a shoulder at ca. 690 nm; in the solid film, the absorption peak is red-shifted to 645 nm with a shoulder at 700 nm, which may be ascribed to the efficient π–π stacking of polymer chains.[28] The absorption edge (λedge) of the polymer film is at ca. 785 nm, corresponding to an optical bandgap (Egopt) of 1.58 eV, which is the same as that of PBDTTT-E-T,[23] an analogous polymer with alkyl groups on the TT units. Electrochemical cyclic voltammetry (CV) was applied to determine the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) levels of the polymer (SI: Figure S3). PBDTTT-TEG shows a HOMO level of –5.09 eV and a LUMO level of –3.3 eV. The optical bandgap and the molecular energy levels of PBDTTT-TEG are almost the same as those of the polymer PBDTTT-E-T, which means that incorporation of the TEG side chain does not affect the bandgap or the energy levels of the polymer. The hole mobility of the pure polymer measured by the space-charge-limited-current
0.6 0.4 0.2 0.0 400
600
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Wavelength (nm) Figure 1. The absorption spectra of the polymer a) in chloroform and in the solid film cast from thechloroform solution, and b) in the different solvents studied in this work, all at the same concentration.
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Table 1. Comparisons among the properties of the solvents studied in this work for PSC processing. Solvent
Boiling point [°C]a)
Flammability/Flash point [°C]
Toxicity
Environmental issues
o-DCB
198
Flammable/65
High
Hazardous (accumulative)
NMP
202
Low flammability/95
Low
Low risk (biodegradable)
DMF
153
Flammable/58
Low
Low risk (biodegradable)
THF
66
High flammability/–17
Low
Low risk (degradable)
Low flammability/117
N/Ab)
N/Ab)
DIO
326
a)Data
obtained from Materials Safety Data Sheet (MSDS) files provided by Sigma-Aldrich (corrected to 760 mm Hg); b)No data available in the MSDS files from SigmaAldrich and other databases.
Because of TEG as a side chain, the polymer not only has good solubility in the solvents commonly used in PSCs—such as CB, o-DCB, chloroform and toluene—but it also can be readily dissolved (>20 mg mL−2 on average) in many kinds of non-aromatic and non-chlorinated solvents, such as tetrahydrofuran (THF), 1,4-dioxane (DIOX), N,N-dimethylformamide (DMF), NMP, and even some alcohols. In order to select an appropriate solvent for device fabrication, UV–vis absorption spectra of PBDTTT-TEG were measured in various solvents to determine the state of PBDTTT-TEG dispersion in different solvents but at the same concentration. As shown in Figure 1b, the absorption range (550–790 nm) and the peaks (ca. 630 nm) are the same for the spectra of the different solvents, while the shape and intensity of the shoulder peaks at the longer wavelength are clearly dissimilar. Compared to the spectrum of the polymer solution in o-DCB, the spectra of the solutions in DMF and NMP show stronger peaks at short wavelength, but weaker absorption peaks at longer wavelength. Again compared to the o-DCB spectra, the solution in THF shows similar shoulder peak intensity but a relatively blue-shifted peak position. These observations may be attributed to the different states of aggregation of the polymer in the different solvents,[29] i.e., the polymer may be disperse better in NMP and DMF than in o-DCB. We also measured the absorption of the polymer in many other solvents (SI: Figure S4), and PBDTTT-TEG exhibited good solubility in some of these solvents, such as 1,4-dioxane and dimethyl sulfoxide; however, none of these other solvents performed better than NMP or DMF. Thus, for PBDTTT-TEG, NMP and DMF are better solvents than o-DCB because of the weaker state of aggregation of the polymer in solution. As discussed above, from the perspective of the solubility of the polymer, NMP, DMF, and THF seem to be suitable processing solvents for device fabrication, and these three solvents are non-aromatic and non-chlorinated compounds. Comparisons of some of the properties of these three solvents (see Table 1) reveal that both DMF and THF are flammable—especially THF, while NMP has low flammability and is also safer for the environment; NMP has been widely used as a solvent or intermediate in industry. From the perspective of the solubility of PCBM,[30,31] DMF and THF are poor solvents, while PCBM shows fairly good solubility in NMP (48 mg mL−1; see Table 3 and the SI: Figure S5). In addition, the boiling point of the processing solvent should also be taken into account; i.e., if the boiling point is too low or too high, it will be difficult to control the spin-coating process. Fortunately, NMP also has a 2746
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similar boiling point as o-DCB, meaning that the current spincoating methods developed for o-DCB as the solvent may easily be transferred to NMP. Therefore, taking all these factors into consideration, it is clear that NMP has the most potential for solvent processing of PSCs. PSC devices were fabricated and characterized to investigate the photovoltaic properties of the PBDTTT-TEG processed with NMP as the solvent. Meanwhile, to investigate the influence of different processing solvents on device performance, the traditionally used solvent, o-DCB, and the analogues of NMP, NEP and NBP, were also used for device fabrication in parallel. The device structure adopted in this work is ITO/PEDOT:PSS/ PBDTTT-TEG:PC71BM/Ca/Al (where ITO = indium tin oxide, PEDOT = poly(3,4-ethylenedioxythiophene), PSS = poly(styrenesulfonate)). For the initial photovoltaic characterizations, donor:acceptor (D/A) ratios (PBDTTT-TEG/PC71BM, w/w) of 1:1, 1:2, and 1:3 were scanned, and it was found that 1:1 is the optimal ratio (see SI: Figure S6, Table S1). Firstly, photovoltaic properties of the PSC devices prepared from o-DCB were fully investigated. As shown in Figure 2a, the PSC device shows a short-circuit current density, Jsc, of 9.58 mA cm−2, an open-circuit potential, Voc, of 0.70 V, and a fill factor, FF, of 50.87%, corresponding to a PCE of 3.41%. In order to further improve the photovoltaic performance of the device, a small amount of DIO was used as a solvent additive. Herein, volume ratios (DIO/o-DCB) of 1%, 3%, 5%, and 7% were screened (see SI: Table S1), and we found that the device processed with the addition of 3% DIO exhibited the best performance; an improved PCE of 5.16% is recorded with a Jsc of 13.65 mA cm−2, a Voc of 0.68 V, and a FF of 55.72%. Compared to reported results for derivatives of the PBDTTT polymer,[20,22] the Jsc and the Voc values of the device of PBDTTT-TEG are comparable, but the fill factor is slightly lower. Therefore, these results clearly indicate that the introduction of TEG side groups onto the TT units has no obviously negative influence on the photovoltaic properties of the polymer. The external quantum efficiency (EQE) curves of the device processed with and without the use of the optimum amount of DIO are provided in Figure 2b. As observed, after the addition of DIO, the device shows improved quantum efficiencies in the whole response range from 300 to 800 nm. Therefore, atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to investigate the influence of DIO on the morphologies of the blend films. When pure o-DCB is used as the processing solvent, no obvious phase separation can be distinguished in the blend film (see SI: Figure S7); when DIO is used as a solvent
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Wavelength (nm) Figure 2. a) J–V characteristics and b) EQE curves of solar cells processed with o-DCB based on the PBDTTT-TEG/PC71BM (1:1, w/w)with and without DIO.
additive to o-DCB, the surface of the blend film becomes rougher and relatively larger size aggregations can be clearly observed in the phase image and the TEM image (Figure S7). These results reveal that the use of DIO assists in enhancing phase separation of the blend, which is helpful for facilitating charge separation and for forming channels for the transport of the carriers. Furthermore, photovoltaic properties of the PSC devices prepared from NMP, NEP, and NBP were carefully investigated. As shown in Figure 3a, when pure NMP, NEP, and NBP were used as the processing solvents, the corresponding devices showed poor efficiencies of 1.72%, 1.07%, and 0.63%, respectively. A small amount of DIO for a volume ratio of 3%, 5%, and 7% was then used as the solvent additive in order to improve the photovoltaic performance of the devices. Considering all three volume ratios and processing with NMP, NEP, and NBP, 5% DIO is the optimal amount for obtaining the highest PCE. Figure 3b shows the EQE curves of the PSC devices processed with NMP, NEP, or NBP and 5% DIO as additive; the photovoltaic parameters of the corresponding devices are summarized in Table 2. Compared to the devices processed without DIO in NMP, the devices processed with a mixture of NMP + 5% DIO show simultaneously significant enhancements of Voc, Jsc, and FF—especially a remarkably improved Jsc (from 7.82 to 13.53 mA cm−2) and FF (from 35.47% to 58.55%). Overall, the best result was obtained when 5% DIO was applied in the blending solution, and a maximum PCE of 5.23% is recorded with Voc = 0.66 V,
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0 300
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Wavelength (nm) Figure 3. a) J–V curves of the devices based on the PBDTTT-TEG/PC71BM (1:1, w/w) processed with different solvents and (b) EQE curves of the devices processed with 5% DIO.
Jsc = 13.53 mA cm−2, and FF = 58.55%. The same trend was observed for NEP and NBP as well. For the device processed with NEP and 5% DIO, the PCE of the device was 5.06% with Voc = 0.66 V, Jsc = 13.74 mA cm−2, and FF = 56.18%; for the device processed with NBP and 5% DIO, the best PCE of the device was 2.52% with Voc = 0.61 V, Jsc = 8.91 mA cm−2, and FF = 46.20%. Compared to the devices processed with the same solvent but different DIO amounts, performance of the devices initially improves as more DIO is added to the solution; when the amount of DIO is beyond 5%, the PCE of devices start to drop slightly. From Figure 3b, it can be seen that the device exhibits a broad response absorption range from 300 to 800 nm, and within the whole range, the EQE is around 50% for the devices processed with NMP + 5% DIO and NEP + 5% DIO, but a lower EQE is obtained from the device processed with NBP + 5%. Successively, the influence of the addition of DIO on the morphological properties of the blend cast from NMP, NEP, and NBP were investigated by AFM and TEM. The AFM and TEM images of the blend films processed with NMP and NMP + 5% DIO are provided in Figure 4a–f; the AFM and TEM data for the other devices can be found in the SI (Figure S8). As shown in Figure 4a and b, when pure NMP is used, large crystal-like aggregations can be observed in both the topography and phase image, corresponding to a surface roughness with a quadratic mean value, Rq, of 5.69, which may be due to the
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Table 2. Photovoltaic properties of the PSCs based on PBDTTT-TEG and PC71BM (1:1, w/w) under the illumination of AM 1.5G, 100 mW cm−2. Processing solvent
Additive
Voc [V]
Jsc [mA cm−2]
PCE [%]
FF [%] Best
NMP
NEP
NBP
a)The
Averagea)
w/o DIO
0.62
7.82
35.47
1.72
1.55
3% DIO
0.65
10.32
49.45
3.33
3.22
5% DIO
0.66
13.53
58.55
5.23
5.14 4.80
7% DIO
0.66
13.19
57.15
4.97
w/o DIO
0.57
5.87
31.79
1.07
1.04
3% DIO
0.65
8.83
40.85
2.36
2.23
5% DIO
0.66
13.74
56.18
5.06
4.90
7% DIO
0.65
13.90
54.20
4.90
4.81
w/o DIO
0.56
3.60
31.30
0.63
0.58
3% DIO
0.60
8.11
42.60
2.09
2.06
5% DIO
0.61
8.91
46.20
2.52
2.25
7% DIO
0.61
8.36
42.12
2.17
1.96
average PCE values were obtained from the data of over 20 devices.
formation of PC71BM crystals. Similar phenomena can also be observed in the blend films processed with pure NEP or NBP. Interestingly, when the 5% DIO additive is used, the large crystal-like aggregations are not present. As shown in the AFM images (Figure 4d,e), the blend film becomes smoother and more uniform with a Rq of 2.73. As observed in the TEM image (Figure 4f), the severe phase separation of the blend (as in Figure 4c) no longer observed. Moreover, for the blend film processed with NEP and NBP, the morphology variation before and after the use of DIO can also be confirmed by the
AFM and TEM images (SI: Figure S8). These data indicate that, when NMP, NEP, or NBP are used as the processing solvent, severe phase separation will occur due to the aggregation of PC71BM, which is detrimental to the formation of nanoscale bi-continuous phase separation; however, after adding the optimum amount of DIO, the severe phase separation can be avoided so that photovoltaic performance of the devices can be improved effectively. Obviously, although the addition of DIO is very helpful for improving photovoltaic performance, it plays different roles in the NMP- and o-DCB-processed devices.
Figure 4. a,d) Tapping-mode AFM topography, b,e) phase, and c,f) TEM images of blend films based on the PBDTTT-TEG/PC71BM (1:1, w/w)cast from NMP without DIO (a–c) and with 5% DIO (v/v) (d–f). The sizes of the AFM images are 5 μm × 5 μm. The scale bars in the TEM images are 1 μm.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Detailed information for the materials, the instruments, the syntheses of the polymers, and the device fabrication process are included.
Acknowledgement The authors would like to acknowledge the financial support from the International S&T Cooperation Program of China (2011DFG63460), the 973 projects (2014CB643501), the Science and Technology Commission of Beijing (Z131100006013002) and the National Natural Science Foundation of China (21325419). Received: September 26, 2013 Revised: January 1, 2014 Published online: February 4, 2014 [1] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789. [2] L. T. Dou, J. B. You, J. Yang, C. C. Chen, Y. J. He, S. Murase, T. Moriarty, K. Emery, G. Li, Y. Yang, Nat. Photonics 2012, 6, 180.
Adv. Mater. 2014, 26, 2744–2749
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COMMUNICATION
In order to further investigate the morphology changes with DIO addition, the hole and electron mobilities of the devices processed without and with DIO were measured by the method of SCLC (see the SI: Table S2 and Figure S9). After the addition of DIO, the hole mobilities of the devices are slightly improved, while the electron mobilities of the devices increase by nearly fivefold, except for the devices processed using NBP; that is, for devices cast from NMP, the electron mobility increases from 6.72 × 10−4 to 3.51 × 10−3 cm2 V−1 s−1 while the hole mobility is slightly changed from 1.97 × 10−4 to 3.78 × 10−4 cm2 V−1 s−1. These results are consistent with the morphological measurements, i.e., the addition of DIO reduces phase separation and leads to the formation of bi-continuous networks in the blend. In conclusion, a new polymer based on BDT as donor and TT as acceptor, PBDTTT-TEG, was designed, synthesized, and applied to PSC devices. The results clearly demonstrate that after replacing the alkyl group with the TEG group, the conjugated polymer will show excellent solubility in various solvents, such as DMF, THF, NMP, and o-DCB. This molecular modification method has little negative influence on the photovoltaic properties of the polymer; by using o-DCB as the processing solvent, the PSC device prepared with otherwise optimal conditions shows comparable PCE to the alkyl-substituted analogue, PBDTTT, which gives a PCE of 5.16%. Benefiting from the TEG group, efficient PSC devices can be fabricated using PBDTTT-TEG as the electron donor and NMP as the processing solvent for spin-coating the active layer. The PSCs based on PBDTTT-TEG:PC71BM (1:1, w/w) processed with NMP and 5% DIO (v/v) achieve a PCE of 5.23% with Voc = 0.66 V, Jsc = 13.53 mA cm−2, and FF = 58.55%, under the illumination of AM 1.5G, 100 cm−2; this is the highest PCE for the PSCs fabricated with non-aromatic and non-chlorinated solvent in reported literature. Most importantly, with the rapid progress of the PSC field, the use of a greener process for making PSC devices becomes more and more important. This work suggests an easy but useful method towards using environmentally friendly and safe solvents for PSC device fabrication.
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