Enhanced performance for polymer/fullerene solar cells by using bromobenzene/1,8diiodooctane co-solvent Xiaodong Huang, Jun Peng, Kunyuan Lu, Zeke Liu, Zhongwei Wu, Jianyu Yuan, Jialing Lu, Hai-Qiao Wang, and Wanli Ma Citation: Applied Physics Letters 104, 211602 (2014); doi: 10.1063/1.4880207 View online: http://dx.doi.org/10.1063/1.4880207 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/21?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Investigating the origin of S-shaped photocurrent-voltage characteristics of polymer:fullerene bulk-heterojunction organic solar cells J. Appl. Phys. 115, 124504 (2014); 10.1063/1.4869661 Enhanced dissociation of charge-transfer states in narrow band gap polymer:fullerene solar cells processed with 1,8-octanedithiol Appl. Phys. Lett. 96, 213506 (2010); 10.1063/1.3435468 Charge dissociation in polymer:fullerene bulk heterojunction solar cells with enhanced permittivity J. Appl. Phys. 104, 114517 (2008); 10.1063/1.3039191 Observation of the subgap optical absorption in polymer-fullerene blend solar cells Appl. Phys. Lett. 88, 052113 (2006); 10.1063/1.2171492 Bimolecular recombination in polymer/fullerene bulk heterojunction solar cells Appl. Phys. Lett. 88, 052104 (2006); 10.1063/1.2170424
APPLIED PHYSICS LETTERS 104, 211602 (2014)
Enhanced performance for polymer/fullerene solar cells by using bromobenzene/1,8-diiodooctane co-solvent Xiaodong Huang, Jun Peng, Kunyuan Lu, Zeke Liu, Zhongwei Wu, Jianyu Yuan, Jialing Lu, Hai-Qiao Wang,a) and Wanli Maa) Institute of Functional Nano and Soft Materials (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, China
(Received 8 April 2014; accepted 9 May 2014; published online 29 May 2014) Bromobenzene and iodobenzene with/without additive 1,8-diiodooctane were investigated as the alternative solvents for the widely used chlorobenzene/1,8-diiodooctane in polymer/fullerene solar cells. The P3HT/[6,6]-phenylC61-butyric acid methyl ester devices using bromobenzene/ 1,8-diiodooctane co-solvent have achieved significantly better performance than those using conventional chlorobenzene/1,8-diiodooctane, which is attributed to the enhanced diode characteristics, higher charge-carrier mobility, and the improved morphology. More importantly, the bromobenzene/1,8-diiodooctane system has also demonstrated improved performance for other polymer/fullerene composites. Thus, we conclude that, with appropriate boiling point, intriguing solubility for active materials, and good compatibility with 1,8-diiodooctane, bromobenzene can be an excellent alternative solvent used for some polymer/fullerene systems in polymer/fullerene solar C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4880207] cells. V Solution-processed bulk-heterojunction (BHJ) polymerfullerene solar cells (PSCs) have been widely investigated in the past two decades owing to their advantages like flexibility, low cost, and potential for mass production since Yu and Heeger1 have reported for the first time the solutionprocessed PSCs in 1995. In BHJ PSCs, photo-induced electron and hole pairs (i.e., exciton) are bound until they encounter the donor-acceptor (D/A) interface, where they will be dissociated into free charge carriers. The excitons produced far away from the D/A heterojunctions will be wasted since the exciton diffusion length in polymers is usually about 4–20 nm.2,3 Meanwhile, continuous pathways are desired to avoid recombination of the free charge carriers. Thus, D/A bi-continuous networks in 20 nm scale are believed to benefit both exciton dissociation and charge transport.4,5 To achieve such optimal morphology in PSCs, post-treatments such as thermal annealing,6 solvent annealing,7 and microwave annealing8 have been adopted, which, however, have only demonstrated success in limited polymers. Compared to post-treatments, solvent additive is an effective approach showing higher compatibility for polymers and roll to roll fabrication process.9–12 Various solvent additives have been exploited since 2006, such as 1,81,8-diiodooctane (DIO),12 octanedithiol (ODT),10 13 1-chloronaphthalene (CN), dimethyl sulfoxide (DMSO),14 and nitrobenzene (NtB).15 Among them, DIO is the most widely used one, by using which power conversion efficiencies (PCEs) over 7% for PSCs11 have been frequently reported. The improved performance is usually attributed to the selective-solubility of DIO which dissolves [6,6]phenylC61-butyric acid methyl ester (PCBM) rather than polymers, and its higher boiling point over the host solvents.12 Specifically, during spin coating, the host solvent evaporates faster than DIO. Thus, PCBM remains dissolved a)
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in DIO while the polymer starts to aggregate, which will fine tune the phase separation of polymer and fullerene.12,16 However, the interplay between DIO and the host solvent makes the spin-coating process very delicate.11,17 As a result, the use of solvent additive often changes the device optimal parameters like D/A ratio,18 and DIO works effectively only with certain host solvents.19–21 Therefore, it is of great interest to search for host solvents which show good compatibility with DIO when using in the fabrication of PSCs. Up to now, solvents like chlorobenzene (CB),6 1,2-dichlorobenzene (o-DCB),7 xylene,15 toluene,22 1,2,4Trichlorobenzene,23 1,2,3,4-Tetrahydronaphthalene,24 chloroform (CF),25 and co-solvents26 have been extensively investigated in PSCs. Based on those experiments, sufficient solubility for the active materials is considered critical to the device performance.11,27 Meanwhile, a high boiling point of the host solvent usually leads to improved phase separation and device performance.24 With both merits, chlorinated solvents such as CF, CB, and o-DCB are most commonly used.11,28,29 However, the brominated and iodinated solvents are rarely investigated in the fabrication of PSCs, particularly not used together with DIO. Among them, bromobenzene (BrB) and iodobenzene (IB) seem suitable as the host solvent for PSCs fabrication, with fair PCBM solubility and a boiling point of 156 C and 188 C, respectively. BrB and IB have only been previously reported by Zuo et al.30 as the solvents used in PSCs, which, however, focused only on the effect of solvent molecules on nanoscaled organo-gels, without using any solvent additives or investigating the solvent induced morphology change. In his work, devices using conventional solvent performed better than those using brominated solvents. Herein in this work, we systematically investigated the performance and morphology of PSCs based on poly(3hexylthiophene) (P3HT):PCBM using BrB or IB as the host solvent, together with DIO as the solvent additive. The devices using BrB/DIO co-solvent achieved significantly better
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TABLE I. The detailed parameters of P3HT:PCBM polymer solar cells with CB, BrB, and IB as the host solvent with or without 1% DIO. Solvent/DIO CB CB/DIO BrB BrB/DIO IB IB/DIO
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0.67 0.58 0.69 0.57 0.62 0.57
3.05 9.24 3.40 9.00 3.68 8.26
37.1 52.1 38.4 62.0 32.1 46.0
0.76 2.79 0.90 3.18 0.73 2.16
performance than those using conventional CB/DIO. We also found that the solvent system work well with PSCs using other polymer/fullerene composites. Based on our experiment results, BrB has demonstrated good compatibility with DIO and resulted in improved morphology. Thus, it can be a promising candidate for the host solvent in PSCs. All BHJ solar cells were fabricated with a conventional device structure of ITO/PEDOT:PSS/Polymer:PCBM/LiF/ Al. Pre-cleaned indium-tin oxide (ITO) substrates were treated with ultraviolet-ozone (UVO) for 20 min. Then, the poly(3,4-ethylene-dioxy-thiophene):polystyrene (PEDOT: PSS) were spin-coated on the substrates at 4500 RPM for 40 s and baked at 150 C for 10 min in atmosphere. The substrates were then transferred into the nitrogen-filled glovebox. The P3HT (purchased from Rieke Metals Inc.): PCBM (Synthesized in our laboratory) blends were dissolved in CB, BrB and IB, respectively, with the same D/A weight ratio of 1:0.8 and a total concentration of 20 mg/ml. The host solvent solutions were stirred for over 10 h at 35 C. Then DIO was added into the solutions and stirred for several minutes before spin-coating. The thickness of the active layers was fixed at 90 nm by adjusting spin-coating speed for different solvent systems. Then, 0.6 nm LiF and 100 nm Al were thermal evaporated through a shadow mask (active area 7.25 mm2) under 2 106 millibars (Kurt J. Lesker, Mini-Spectros). The current density-voltage (J-V) parameters of the devices were characterized by using a Keithley 2400 digital source meter at dark or under simulated AM 1.5G solar irradiation at 100 mW/cm2 (Newport, Class AAA solar simulator, 94023A-U). The light intensity was calibrated by a certified Oriel Reference Cell (91150 V) and verified with a NREL calibrated Hamamatsu S1787-04 diode. The external quantum efficiency (EQE) was performed using a certified IPCE instrument (Zolix Instruments, Inc, Solar Cell Scan 100).
Different solvent systems with different DIO volume ratios were investigated to optimize the device performance (Table S1, see the supplementary material).31 We found that 1% DIO was optimal for all the three solvent systems. The optimized devices photovoltaic parameters are shown in Table I and Fig. 1(a). The devices fabricated from the pure solvent systems show very low PCE (0.8%) with corresponding low current densities (Jsc) of 3–4 mA/cm2, and low fill factors (FFs) of about 35%. After the addition of 1% DIO, all the Jsc increase to around 9 mA/cm2 and the FFs are improved to 52.1%, 62.0%, and 46.0% for CB, BrB, and IB based system, respectively. The significantly enhanced performance may be attributed to the improved film morphology by the use of additive DIO, as confirmed by our further investigation. With dramatically improved FF and Jsc, a final PCE of 2.79%, 3.18%, and 2.16% was recorded for the devices based on CB, BrB, and IB solvent system, respectively. Especially for the BrB/DIO solvent system, the PCE is 14% higher than that for the widely used CB/DIO solvent system. The EQE of the six devices is shown in Fig. 1(b), and the Jsc calculated by integrating the EQE curves is within 6% deviation compared to the directly measured Jsc. Note that the Jsc obtained from devices using the three co-solvent systems is similar and the PCE difference is mainly caused by the different FFs. In order to investigate the origin of the different FFs, dark current density-voltage characteristics and device holemobility were measured. As shown in Fig. 2(a), film cast from BrB/DIO exhibits higher rectification ratio and lower leakage current under reverse bias than the one cast from CB/DIO and IB/DIO systems. The improved diode characteristics indicate larger parallel resistance and lower serial resistance. Moreover, the device hole-mobility was measured by using the space charge limited current (SCLC) method, with a device structure of ITO/PEDOT:PSS/ P3HT:PCBM/MoO3/Al.13,32 J1/2-V characteristics are demonstrated in Fig. 2(b). The calculated hole-mobility of the blend films cast from CB/DIO, BrB/DIO, and IB/DIO co-solvents are 5.6 105, 8.7 105, and 1.5 105 cm2 V1 s1, respectively, which coincides well with the corresponding FFs. Morphologies of the blend films were also studied to further understand the FF difference induced by different solvent systems. Topography of the blend films was examined by atomic force microscope (AFM), as demonstrated in Fig. 3. The blend films cast from pure host solvent are relatively smooth, and the root-mean-square (RMS) roughness is
FIG. 1. (a) J-V characteristics under AM 1.5G irradiation at 100 mW/cm2 and (b) EQE of P3HT:PCBM solar cells with CB, BrB, and IB as the host solvent with or without 1% DIO.
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FIG. 2. (a) Dark J-V characteristics of conventional devices and (b) J1/2-V characteristics of hole-only devices for P3HT:PCBM blend film.
FIG. 3. AFM height images of blend films cast from (a) CB, (b) BrB, (c) IB, (d) IB/DIO, (e) BrB/DIO, and (f) IB/DIO. The scan size is 2 lm 2 lm.
0.44, 0.53, and 0.66 nm for the CB, BrB, and IB cast films, respectively. We speculate that this slightly increased roughness is resulted from the elevated boiling point of the solvents, which makes the film dry slowly. This prolonged drying time will promote the molecules to self-organize24,29 and consequently result in higher roughness. When DIO was added into the solvent, all films got much coarser. The films cast from CB, BrB, and IB with DIO have diminishing RMS roughness of 6.20 nm, 5.37 nm, 2.76 nm, which indicates that DIO has different effects on the morphology of the blend film in different host solvent systems. And the blend film cast from BrB/DIO shows moderate surface roughness and best photovoltaic performance. The surface morphology alone cannot interpret the improved performance of devices using BrB/DIO cosolvent. Transmission electron microscopy (TEM) was also employed to analyze the phase separation in the blend films. TEM images show similar morphology for the films cast from pure CB, BrB, and IB solvent, with the film cast from BrB shown in Fig. 4(a) as an example. The dark domains are PCBM-rich and the white domains contain more polymers.12 The marked PCBM-rich dots in Fig. 4(a) are not distinct, indicating weak polymer-fullerene phase separation. The poor morphology is in accordance with the low performance of all the three devices before the addition of solvent additive. The use of DIO is considered to promote phase separation between PCBM and polymer during
FIG. 4. TEM images of blend films cast from (a) BrB, (b) CB/DIO, (c) BrB/DIO, and (d) IB/DIO. The areas indicated by red and blue dashed lines represent polymer-rich domains and fullerene-rich domain, respectively.
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spin-coating,11,12 which can benefit the exciton separation and free charge carrier transport.33,34 For films cast from all the three solvent/DIO systems, higher contrast between the PCBM and polymer-rich domains was observed, as shown in Figs. 4(b)–4(d). However, their morphology is still significantly different. For film cast from CB/DIO, the PCBMrich and polymer-rich domains, indicated by the blue and red dash line in Fig. 4(b), respectively, have very large sizes, which will hinder the excitons separation. The film cast from BrB/DIO shows much finer domain sizes (see Fig. 4(c)), which is apparently closer to the desired tens nm scale for D/A phase separation. In addition, bi-continuous network can be observed, leading to efficient carrier transport. In comparison, the domain sizes in film cast from IB/DIO (Fig. 4(d)) are not optimal and the interpenetrating network cannot be clearly observed, indicating inadequate excitons separation and poor carrier transport. Therefore, BrB/DIO can be used to further improve the blend film morphology, attributed to the efficient synergy between BrB and DIO. We also noticed that the boiling point of the three solvents is different which may affect the drying time during spin coating,24 resulting in different crystallinity for the D/A components. Thus, X-ray Diffraction (XRD) was performed to probe the crystallinity of the blend films cast from various solutions (as shown in Fig. S1, see the supplementary material).31 Before the addition of DIO, the most pronounced diffraction peak at 5.4 (related to P3HT) show increased intensity for film cast from CB, BrB, and IB, consistent with the sequence of their boiling point. However, after the addition of DIO, the peak intensities of the films cast from different co-solvents become similar. Thus, the film crystallinity is less likely the critical factor determining the device performance. In order to investigate the compatibility of BrB/DIO with other polymers, three polymers PT12,35 PTF8,36 and PBDT-T-FDP20 based on benzo[1,2-b:4,5-b0 ]dithiophene (BDT) were used to fabricate PSCs. BDT was intentionally used since currently many efficient polymers containing this popular building block. The structures of the three polymers are shown in Fig. S2 (see the supplementary material),31 with the optimized device parameters listed in Table S3–S5.31 The results show that the devices using BrB/DIO achieve the best performance for PT12 and PTF8, while the devices using CB/DIO demonstrate the highest PCE for PBDT-T-FDP. It has been reported that the solubility of active materials in the solvent may play an important role in the device performance.27 As shown in Table S2,31 the solubility of P3HT and PCBM varies in these solvents. The P3HT solubility in bromoform (BrF), IB, and BrB increases sequentially, which is consistent with the device performance (Table S1).31 For other polymers, we observed that the solubility of PT12 and PTF8 is good in both BrB and CB, and the devices using BrB/DIO outperform those using CB/DIO. In contrast, the solubility of BDT-T-FDP in BrB is relatively lower than that in CB, resulting in poorer device performance for devices using BrB/DIO. It is also worth noting that the optimal D/A ratios are 1:1, 1:1.5, and 1:2 for PT12, PTF8, and BDT-T-FDP, respectively. And as shown in Table S2,31 the PCBM solubility in BrB is relatively lower
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than in CB, which may also impact the device performance. Thus, the results suggest that the solubility of the active materials may be an important criterion for choosing the host solvent for PSCs. However, to accurately address the effect of solubility and other factors on the photovoltaic performance of different polymers is complex and beyond the scope of this work. In conclusion, we systematically investigated the performance and morphology of PSCs based on P3HT:PCBM using BrB or IB as the host solvent, together with DIO as the solvent additive. The devices using BrB/DIO co-solvent achieved significantly better performance than those using conventional CB/DIO, which is attributed to the enhanced diode characteristics, higher charge-carrier mobility, and the improved morphology. More importantly, the BrB/DIO system has also demonstrated improved performance for other polymer/fullerene composites. Thus, we conclude that, with appropriate boiling point, intriguing solubility for active materials and good compatibility with DIO, BrB can be an excellent alternative solvent for some polymer/fullerene systems in PSCs. This work was supported by the National High Technology Research and Development Program of China (863 Program) (Grant No. 2011AA050520), the National Natural Science Foundation of China (Grant No. 61176054), the Natural Science Foundation of Jiangsu Province (No. BK20130311), the Postdoctor Science Foundation No.1302015A and 32317330, and also the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
1
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