Efficiency enhancement in solution-processed

Efficiency enhancement in solution-processed organic small molecule: Fullerene solar cells via solvent vapor annealing Jingsheng Miao, Hui Chen, Feng Liu, Baofeng Zhao, Lingyu Hu, Zhicai He, and Hongbin Wu Citation: Applied Physics Letters 106, 183302 (2015); doi: 10.1063/1.4919707 View online: http://dx.doi.org/10.1063/1.4919707 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of molecular electrical doping on polyfuran based photovoltaic cells Appl. Phys. Lett. 106, 203301 (2015); 10.1063/1.4921484 Roles of solvent additive in organic photovoltaic cells through intensity dependence of current-voltage characteristics and charge recombination Appl. Phys. Lett. 105, 103301 (2014); 10.1063/1.4895531 Surface relief gratings on poly(3-hexylthiophene) and fullerene blends for efficient organic solar cells Appl. Phys. Lett. 91, 173509 (2007); 10.1063/1.2802561 Origin of the enhanced performance in poly(3-hexylthiophene): [6,6]-phenyl C 61 -butyric acid methyl ester solar cells upon slow drying of the active layer Appl. Phys. Lett. 89, 012107 (2006); 10.1063/1.2212058 Device annealing effect in organic solar cells with blends of regioregular poly(3-hexylthiophene) and soluble fullerene Appl. Phys. Lett. 86, 063502 (2005); 10.1063/1.1861123

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APPLIED PHYSICS LETTERS 106, 183302 (2015)

Efficiency enhancement in solution-processed organic small molecule: Fullerene solar cells via solvent vapor annealing Jingsheng Miao,1 Hui Chen,1 Feng Liu,2,a) Baofeng Zhao,1 Lingyu Hu,1 Zhicai He,1 and Hongbin Wu1,a)

1 State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, People’s Republic of China 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

(Received 26 January 2015; accepted 22 April 2015; published online 4 May 2015) We report highly efficient small molecule solar cells (SMSCs) by using dichloromethane solvent vapor annealing method. The resulted devices delivered a power conversion efficiency (PCE) of 8.3%, which is among the highest in SMSCs. Comparing to the control devices, the short circuit current (Jsc), fill factor, and PCE of solvent vapor annealed devices are significantly improved. Summarizing the results of optical absorption, film morphology, and charge carrier transporting properties, we see that the enhanced structure order and reduced size of phase separation are major reasons for the improved device performances, establishing a solid structure-property relationship. The solvent vapor annealing method can thus be a useful method in device fabrication to enhance C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4919707] performances of SMSCs. V

Organic bulk-heterojunction (BHJ) solar cells have attracted intensive research interest in generating low-cost renewable energy.1–4 Comparing to the polymer counterparts, organic solar cells based on small molecules have several unique advantages such as well-defined chemical structure, ease of purification, and reduced batch-to-batch variations.5–10 Recent development of small molecule electron donor materials led to a milestone power conversion efficiency (PCE) of 10% in single-junction devices10,11 and in tandem cells.9 Meanwhile, the overall device performance of small molecule solar cells (SMSCs) is still inferior to their polymer counterparts due to a poor understanding and manipulating of the film morphology, which is crucial for exciton dissociation, charge transport, and collection. Most recently, new processing methods, such as thermal annealing,12 additive processing,13–15 interface engineering,16–18 and solvent vapor annealing (SVA),11,19–21 have been applied to improve the device performance of SMSCs. For example, 7,70 – (4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b0 ]dithiophene-2,6diyl)bis(6-fluoro-4-(50 -hexyl-[2,20 -bithiophen]-5-yl)benzo[c] [1,2,5]thiadiazole) (p-DTS(FBTTh2)2) is a promising small molecular donor,8 and adding small amount of solvent additive (1,8-diiodooctane, DIO) can drastically improve the device performance.15 The best device showed a PCE of 9% when barium metal was used as cathode.18 However, the removal of DIO from the active layer is difficult due to its high boiling point, and thus may limit its applications. Alternatively, solvent vapor annealing method has been shown to be an effective strategy to optimize the morphology of the active layer in both polymer solar cells19,20 and SMSCs,21 and significantly improved device performance has been demonstrated.18 Here, we report the control of thin film morphology and the optimization of charge transport in SMSCs by using dichloromethane (DCM) vapor annealing. The optimized a)

Authors to whom correspondence should be addressed. Electronic mail: [email protected] and [email protected].

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device showed a high power conversion efficiency of 8.3%, which is almost two times higher than that of the untreated devices, and is among the highest values for SMSCs. The results suggested that the solvent vapor annealing method could be a useful method to enhance the device performance in SMSCs. To figure out the origin of the performance enhancement by SVA, optical absorption, charge transport, and thin film morphology were investigated. Besides, charge dynamics in the working devices was studied by transient photovoltage (TPV) and transient photocurrent (TPC)22–24 providing new insights in structure-property relationship. The SMSCs were fabricated on patterned indium tin oxide (ITO) glass substrates, with a device structure of ITO/ PEDOT: PSS/active layer/PFN/Al (see Figure 1(a)). The substrates were cleaned in ultrasonic bath in acetone, detergent, deionized water, and isopropyl alcohol sequentially, followed by baking at 80 C in vacuum oven for 12 h. A 40nm-thick poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) (Clevios P PVP AI 4083, H.C. Starck, Inc.) layer was spin-coated onto the O2 plasma treated ITOglass substrates, and was annealed at 140 C for 20 min. The active layer (90 nm) consisting of p-DTS(FBTTh2)2:PC71BM (both were from 1-Material, Inc., see Figure 1 for the molecular structure) blends was spin coated from 90 C hot chlorobenzene solutions. The solution was stirred overnight. The p-DTS(FBTTh2)2:PC71BM blend ratio was 1:1 and the total solids concentration was 38 mg/ml. Solvent vapor annealing was done in a closed chamber filled with saturated DCM vapor for certain time (typically for 30 s). Subsequently, a thin layer of poly[(9,9-bis(30 -(N,N-dimethylamino) propyl)-2,7fluorene)-alt-2,7–(9,9-dioctylfluorene)] (PFN)25 (5 nm) was spin coated on top of the active layer as cathode interlayer, following the previous procedure.16 Finally, a 100 nm Al film was evaporated through a shadow mask in a vacuum chamber under a pressure of 1 104 Pa. The active area of the device was 0.16 cm2. The J–V characteristics were measured by a Keithley 2400 source-measurement unit under AM

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FIG. 1. (a) The device structure of the p-DTS(FBTTh2)2:PC71BM solar cell. (b) The molecular structures of p-DTS(FBTTh2)2 and PC71BM, where R1 ¼ n-hexyl and R2 ¼ 2-ethylhexyl.

1.5 G spectrum from a solar simulator (Oriel model 91192). Masks with well-defined area of 16.0 mm2 were used for accurate measurement. Solar simulator illumination intensity was calibrated using a monocrystalline silicon reference cell (Hamamatsu S1133, with KG-5 visible color filter), which was calibrated by the National Renewable Energy Laboratory (NREL). The incident photon-to-current efficiency spectra were measured by a solar cell–photodetector responsitivity measurement system (Enlitech, Inc.). The optical absorption of the active layers was measured by using a UV-Vis-NIR spectrophotometer (UV3600, SHIMADZU). The theoretical Jsc values were calculated by integrating the product of the external quantum efficiency (EQE) data with Ð k the AM 1.5 G solar spectrum via the equation JSC ¼ q k12 EQEðkÞ/ðkÞdk, where q is the elementary charge, /ðkÞ is the photon flux of AM 1.5 G solar spectrum, and k1 and k2 define the photoresponse range of the devices.4 The film morphology was studied by grazing incidence x-ray diffraction (GIXD, beamline 7.3.3, Advanced Light Source, Lawrence Berkeley National Laboratory), transmission electron microscopy (TEM, JOEL 2000FX, operating at 200 keV), and atomic force microscopy (AFM, Veeco MultiMode V, operating in tapping mode). The charge carrier mobility was determined by fitting the dark current-bias characteristics using a field-independent space charge limited current (SCLC) model following the Mott2 Gurney law: J ¼ 98 e0 er ln;h VL3 , where J is the current density, e0 is the permittivity of free space, er is the relative permittivity of the active layer, ln;h is the electron/hole mobility, L is the film thickness of the active layer, and V is the effective voltage which is determined by subtracting the built-in voltage (Vbi ) from the applied voltage (V ¼ Vappl Vbi ). The influence of DCM vapor annealing on the device performance of SMSCs was shown in Fig. 2(a) and Table I. Comparing with the untreated devices that showed a moderate PCE of 3.21% (with a Jsc of 10.45 mA/cm2, a Voc of 0.80 V, and a fill factor (FF) of 38.3%, respectively), devices after DCM vapor annealing showed a maximal PCE of 8.31% (with a Jsc ¼ 14.80 mA/cm2, a Voc ¼ 0.78 V, and a FF ¼ 72.0%, respectively). It is worthy to mention that additive processed device (0.4 vol. % of 1,8-diiodooctane) using the same blends delivered a PCE of 7.83% (with Jsc ¼ 13.94 mA/cm2, Voc ¼ 0.80 V, and FF ¼ 70.2%, respectively).8 The enhancement in Jsc by SVA treatment consisted well with EQE measurement. The calculated Jsc value of the pristine device is 10.2 mA/cm2, which is in good agreement

FIG. 2. (a) J-V characteristics of the p-DTS(FBTTh2)2/PC71BM BHJ solar cells without/with the DCM vapor treatment, as measured under 100 mW/cm2 AM 1.5 G irradiation. (b) EQE spectra of corresponding p-DTS(FBTTh2)2: PC71BM (1:1 W/W) BHJ solar cells without and with DCM vapor annealing. (c) UV-visible absorption of the p-DTS(FBTTh2)2/PC71BM BHJ thin films fabricated on quartz substrate from 90 C hot CB solution without and with DCM vapor annealing. The open blue circles and red squares are for the device/film before and after DCM vapor annealing, respectively.


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TABLE I. Performance of the devices before and after the DCM vapor treatment, as measured under 100 mW cm2 AM 1.5 G illumination. Without/with annealing Without With

Voc (V)

Jsc (mA cm2)

FF (%)

PCE (%)

0.80 0.78

10.5 14.8

38.3 72.0

3.21 8.31

with the measured Jsc value of 10.45 mA/cm2 in Fig. 2(a). As can be clearly seen from Fig. 2(b), the EQE values in the entire photo response range between 300 nm and 750 nm were significantly enhanced after the DCM vapor annealing,

with peak values exceeding 70% between 560 nm and 670 nm. As a result, the calculated Jsc was determined to be 14.5 mA/cm2, in good agreement with the measured Jsc value (14.80 mA/cm2). The origin of the dramatically increased Jsc under SVA treatment was first investigated by the absorption spectra. Fig. 2(c) shows the UV-visible absorption spectra of pDTS(FBTTh2)2:PC71BM thin films before and after solvent vapor annealing. When compared with the pristine film, the treated film displayed a red shift of 16 nm in the major absorption peak with well-defined features. While the red shift in the absorption peak was an indication of ordered

FIG. 3. (a) Grazing incidence x-ray diffraction of as casted p-DTS(FBTTh2)2:PC71BMBHJ thin film; (b) GIXD of DCM vapor annealed BHJ film; (c) out-ofplane and in-plane line cut profiles of DCM vapor annealed sample; TEM of BHJ film: (d) as cast, (e) DCM vapor annealed; surface topographic (area size: 5 lm 5lm) of BHJ film: (f) as cast, (g) DCM vapor annealed; and (h) proposed scheme for the effect of solvent vapor annealing on the morphology of the photoactive layer.


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structure in the film,26 the absorption shoulder at 677 nm could be attributed to the enhanced p-p stacking and molecular interaction inside the p-DTS(FBTTh2)2 phase.27 Although the red shift and the stronger absorption peak provide better overlap with the solar spectra, it should be noted that the change in optical absorption only contributed to a small portion of increase in Jsc. On the other side, the dramatic enhancement of 50% should be more closely related with more ordered morphology in the active layer, as evidenced by the enhanced absorption, as shown in Fig. 2(c). More recently, thin film morphology, including size scale of phase separation, the crystal structure/orientation, the degree of crystallinity, the miscibility of the components, and the distribution of the components within the active layers, has been identified as the key factors in determining the performance in BHJ solar cells.26 To show the structureproperty relationship in the pristine and the annealed films, GIXD was first used to investigate crystallites in active layer

FIG. 4. The J-V plots of the single charge carrier devices for (a) hole-only device and (b) electron- only device. The open blue circles and red squares are for the device before and after DCM vapor annealing, respectively. The solid lines represent the best fitting from the space-charge-limited-current model.

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(Figures 3(a)–3(c)). The pristine thin film showed a low crystalline content with no preferred orientation. For the DCM annealed sample, the crystalline content was found to be largely enhanced, as seen from the 3-orders (100) diffraction in the out-of-plane direction. An obvious p-p stacking was observed in the in-plane direction, though its spreading was broad. Moreover, TEM was used to study the extent and length scale of phase separation. It was seen that the ascasted film showed a uniform mixture, which explained the low performance in the non-treated devices. In contrast, as shown in Figure 3(e), the DCM vapor annealed film showed a nanoscale phase separation around 30 nm. The well-defined morphology could greatly enhance the transport and photovoltaic effect. In addition, tapping mode AFM was carried out to investigate the surface morphologies of the blend films. Pristine thin film showed a smooth surface and uniform morphology with small roughness (0.6 nm) (Fig. 3(f)), agreed well with TEM characterizations. After treatment with DCM vapor for 30 s, the blend film showed a much rougher surface (Fig. 3(g)), which is favorable for efficient diffusion of exciton to donor-acceptor interface and the concomitant exciton dissociation.19 Therefore, the solvent vapor annealing leads to a

FIG. 5. (a) Transient photovoltage and (b) transient photocurrent signals for p-DTS(FBTTh2)2:PC71BM devices before and after DCM solvent vapor annealing, measured under 1 sun illumination and open-circuit voltage conditions using the same intensity laser pulse.


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more well-defined morphology and crystalline domains in the photoactive layer (Fig. 3(h)), thus represents a viable approach to control and optimize the film morphology of the blend used in this study. To get insight into the influence of the solvent vapor annealing on charge transporting properties, charge carrier mobility was measured by using SCLC method.16 Fig. 4 showed the J-V characteristics of typical hole-only and electron-only device with device architecture of ITO/PEDOT: PSS/p-DTS(FBTTh2)2:PC71BM/MoO3/Al and ITO/ZnO/PFN/ p-DTS(FBTTh2)2:PC71BM/PFN/Al, respectively. The hole and electron mobility in the pristine film were found to be 8.5 106 cm2/V s and 4.2 104 cm2/V s, respectively. Such imbalance between the hole and electron mobility was expected to be the major reason responsible for the low FF in the device.28 On the other hand, the device after SVA treatment exhibited much more balanced charge transport properties, with hole and electron mobility of 1.5 104 cm2/V s and 4.1 104 cm2/V s, respectively. As a result, the device showed a remarkable improvement in FF, which is nearly twice of those of the pristine devices (72.0% vs. 38.3%). To further examine the origin of the improved device performances, we used TPV and TPC techniques to investigate the carrier dynamics in the devices. For the TPV measurement, the perturbation from pulsed laser was attenuated by a set of neutral density filter so that the amplitude of TPV was much smaller as compared to Voc. Since there was no charge collected under open-voltage condition, the excess charge carriers that excited by the pulsed laser were recombined inside the device. Therefore, the lifetime of the charge carriers could be extracted from the exponential fitting on the decay of the transient photovoltage. Fig. 5(a) showed the typical results of TPV measurement for p-DTS(FBTTh2)2:PC71BM devices before and after SVA, under 1 sun illumination and opencircuit voltage conditions. The extracted charge carrier lifetime (s) in the device before and after SVA was 1.6 ls and 2.2 ls, respectively. The observed longer carrier lifetime suggested a reduced bimolecular recombination loss upon vapor annealing, and in conjunction with the increased hole mobility (l), indicating a significant improvement in charge transport and collection in the device.28 On the other hand, as revealed by the TPC measurement (Fig. 5(b)), the device with SVA showed higher carrier density than the pristine devices, which was in good agreement with the higher Jsc, as described above. In summary, we reported dramatic device performance improvements in small molecule solar cells by using dichloromethane vapor annealing. The obtained small molecule solar cells showed a power conversion efficiency of 8.31%, which was among one of the highest values for small molecule solar cells reported to date. The solvent vapor annealing was found to be able to control the film morphology and optimize charge transport properties in the active layer, and thus could be a useful method to enhance the overall device performance of small molecule solar cells.

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H.W. and Z.H. thank the National Nature Science Foundation of China (Nos. 51225301, 51403066, 91333206, and 61177022) and Fundamental Research Funds for the Central Universities (2014ZM001) for the financial support. F.L. thanks Polymer-Based Materials for Harvesting Solar Energy (PHaSE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences (DE-SC0001087) for the support. 1

G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, Science 270, 1789 (1995). 2 Y. F. Li, Sci. China Chem. 58, 188 (2015). 3 J. Chen and Y. Cao, Acc. Chem. Res. 42, 1709 (2009). 4 Z. He, C. Zhong, S. Su, M. Xu, H. Wu, and Y. Cao, Nat. Photonics 6, 591 (2012). 5 H. Shang, H. Fan, Y. Liu, W. Hu, Y. Li, and X. Zhan, Adv. Mater. 23, 1554 (2011). 6 S. Loser, C. J. Bruns, H. Miyauchi, R. P. Ortiz, A. Facchetti, S. I. Stupp, and T. J. Marks, J. Am. Chem. Soc. 133, 8142 (2011). 7 J. Zhou, X. Wan, Y. Liu, Y. Zuo, Z. Li, G. He, G. Long, W. Ni, C. Li, X. Su, and Y. Chen, J. Am. Chem. Soc. 134, 16345 (2012). 8 T. S. van der Poll, J. A. Love, T. Q. Nguyen, and G. C. Bazan, Adv. Mater. 24, 3646 (2012). 9 Y. Liu, C.-C. Chen, Z. Hong, J. Gao, Y. Yang, H. Zhou, L. Dou, G. Li, and Y. Yang, Sci. Rep. 3, 3356 (2013). 10 B. Kan, Q. Zhang, M. Li, X. Wan, W. Ni, G. Long, Y. Wang, X. Yang, H. Feng, and Y. Chen, J. Am. Chem. Soc. 136, 15529 (2014). 11 K. Sun, Z. Xiao, S. Lu, W. Zajaczkowski, W. Pisula, E. Hanssen, J. M. White, R. M. Williamson, J. Subbiah, J. Ouyang, A. B. Holmes, W. W. H. Wong, and D. J. Jones, Nat. Commun. 6, 6013 (2015). 12 G. Li, V. Shrotriya, J. Huang, T. Mariarty, K. Emery, and Y. Yang, Nat. Mater. 4, 864 (2005). 13 C. V. Hoven, X. D. Dang, R. C. Coffin, J. Peet, T. Q. Nguyen, and G. C. Bazan, Adv. Mater. 22, E63 (2010). 14 J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li, G. He, C. Li, B. Kan, M. Li, and Y. Chen, J. Am. Chem. Soc. 135, 8484 (2013). 15 Y. M. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan, and A. J. Heeger, Nat. Mater. 11, 44 (2012). 16 Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, S. Su, and Y. Cao, Adv. Mater. 23, 4636 (2012). 17 A. K. K. Kyaw, D. H. Wang, V. Gupta, W. L. Leong, L. Ke, G. C. Bazan, and A. J. Heeger, ACS Nano 7, 4569 (2013). 18 V. Gupta, A. K. K. Kyaw, D. H. Wang, S. Chand, G. C. Bazan, and A. J. Heeger, Sci. Rep. 3, 1965 (2013). 19 J. Liu, L. Chen, B. Gao, X. Cao, Y. Han, Z. Xie, and L. Wang, J. Mater. Chem. A 1, 6216 (2013). 20 B. Gholamkhass and P. Servati, Org. Electron. 14, 2278 (2013). 21 K. Sun, Z. Xiao, E. Hanssen, M. F. G. Klein, H. H. Dam, M. Pfaff, D. Gerthsen, W. W. H. Wong, and D. J. Jones, J. Mater. Chem. A 2, 9048 (2014). 22 C. G. Shuttle, B. O’Regan, A. M. Ballantyne, J. Nelson, D. D. C. Bradley, J. de Mello, and J. R. Durrant, Phys. Rev. B 78, 113201 (2008). 23 C. Shuttle, A. Maurano, R. Hamilton, B. O’Regan, J. De Mello, and J. Durrant, Appl. Phys. Lett. 93, 183501 (2008). 24 P. P. Boix, J. Ajuria, R. Pacios, and G. Garcia-Belmonte, J. Appl. Phys. 109, 074514 (2011). 25 F. Huang, H. B. Wu, D. L. Wang, W. Yang, and Y. Cao, Chem. Mater. 16, 708 (2004). 26 F. Liu, Y. Gua, X. Shen, S. Ferdous, H. Wang, and T. P. Russell, Prog. Polym. Sci. 38, 1990 (2013). 27 Y. Sun, J. Seifter, L. Huo, Y. Yang, B. B. Y. Hsu, H. Zhou, X. Sun, S. Xiao, L. Jiang, and A. J. Heeger, Adv. Energy Mater. 4, 1400987 (2014). 28 P. W. M. Blom, V. D. Mihailetchi, L. J. A. Koster, and D. E. Markov, Adv. Mater. 19, 1551 (2007).
