Importance of Solvent Removal Rate on the Morphology and Device

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The Importance of Solvent Removal Rate on the Morphology and Device Performance of Organic Photovoltaics with Solvent Annealing Shuqiong Lan, Huihuang Yang, Guocheng Zhang, Xiaomin Wu, Qizhen Chen, Liang Chen, Huipeng Chen, and Tailiang Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on May 31, 2017

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ACS Applied Materials & Interfaces

The Importance of Solvent Removal Rate on the Morphology and Device Performance of Organic Photovoltaics with Solvent Annealing Shuqiong Lan,1 Huihuang Yang,1 Guocheng Zhang,1,2 Xiaomin Wu, Chen, 1 Liang Chen,3 Huipeng Chen1* and Tailiang Guo1

1

Qizhen

1

Institute of Optoelectronic Display, National & Local United Engineering Lab of Flat Panel Display Technology, Fuzhou University, Fuzhou 350002, China Email: [email protected] 2 College of Information Science and Engineering, Fujian University of Technology, Fuzhou 350108, China 3 Key Laboratory of Neutron Physics and Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, 621900, China Abstract Solvent vapor annealing has been widely used in organic photovoltaics (OPV) to tune the morphology of bulk heterojunction active layer for the improvement of device performance. Unfortunately, the effect of solvent removal rate after solvent annealing, which is one of the key factors that impact resultant morphology, on the morphology and device performance of OPV has never been reported. In this work, the nanoscale morphology of small molecule (SM) : fullerene bulk heterojunction (BHJ) solar cell from different solvent removal rates after solvent annealing was examined by small angle neutron scattering and grazing incidence x-ray scattering. The results clearly demonstrates that the nanoscale morphology of SM : fullerene BHJ, especially fullerene phase separation and concentration of fullerene in non-crystalline SM, was significantly impacted by the solvent removal rate. The enhanced fullerene phase separation was found with a decrease of solvent removal rate while the crystallinity and molecular packing of SM remained unchanged. Correlation to device performance, it shows that the balance between pure fullerene phase and mixing 1

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phase of SM and fullerene is crucial for the optimization of morphology and enhancement of device performance. Moreover, the specific interfacial area between pure fullerene phase and mixing phase is crucial for the electron transport and thus device performance. More importantly, this finding would provide a more careful and precise control of morphology of SM : fullerene BHJ and offers guides for further improvement of device performance with solvent annealing. Keywords: Solvent annealing, Solvent removal rate, Organic photovaltaics, Neutron scattering, Phase separation, Miscibility Introduction Organic photovoltaics (OPV) have attracted considerable attentions due to their light weight, flexible and low cost. Many efforts have been made for the improvement of power conversion efficiency (PCE) of OPVs and morphology of BHJ active layer is considered to be crucial to OPV performance.1-9 As coating process of active layer is usually fast, the morphology of BHJ mixture is usually locked at un-equilibrium status. In order to optimize the morphology of BHJ mixture, post-deposition approaches are usually required. Solvent vapor annealing is a promising post-deposition approach, which has been widely used in organic photovoltaics and perovskite solar cells to optimize the morphology and device performance.

10-19

During solvent vapor annealing, the solvent molecules diffuse into the active layer and increase the mobility of molecules within the active layer, resulting in the reorganization and effective control of the morphology of BHJ mixtures. Chen and

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coworkers investigated the effect of solvent quality on the morphology of low bandgap polymer photovoltaics with solvent annealing. They claimed that the morphology of BHJ mixture is significantly impacted by the quality of selective solvent and a judicious selection of solvent vapor is key to improve the morphology of BHJ mixture. 11 Li et al. performed a systematic study on small molecule based BHJ thin film during solvent vapor annealing. 12 They demonstrated that both hole and electron mobility increased with good solvent molecules, leading to improved device performance. 13 Recently, Wang et al. demonstrated that solvent annealing of SM BHJ leaded to an increased crystallinity and efficient phase separation, resulting in a significant improvement of PCE.

14

Although there is a tremendous body of

reports about the optimization of OPV with solvent annealing, one of the key factors, solvent removal rate (SRR), has rarely been investigated. During solvent annealing, there is a significant amount of solvent molecules penetrated into the active layer and it is very clearly that the final morphology of BHJ mixture is associated with the removal process of these solvent molecules.

For instance, in the field of block copolymers, solvent removal rate has been considered to be crucial for the resultant morphology of block copolymers after solvent annealing.20-28 Kim and Libera investigated the impact of SRR on the orientation of cylinder perpendicular to surface of block copolymer thin film.24

In

their experiments, poor ordering of block copolymers was obtained with rapid SRR. Moreover, with decreasing the SRR, the ordering of the block copolymer thin film 3



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transited from cylinder oriented perpendicular to the film surface, then to mixed orientations, and finally transited to cylinders oriented parallel to the film surface. Julie and co-workers systematically investigated the impact of SRR on evolution morphology of triblock copolymer after solvent annealing. 20 They demonstrated that the degree of reorientation to the polymer surface increased with a slower SRR. Sean and coworkers used dynamical field theory simulations to clarify the direct impact of solvent evaporation rate on the morphology of block copolymers. 23 Their simulation results indicated that perpendicular cylinder was achieved under modest evaporation condition, while stable growth of cylinder perpendicular to surface was failed to be formed under rapid evaporation rate. Additionally, Wu and co-workers examined the morphological development of block copolymer thin films as a function of SRR.21 Surprisingly, morphological evolution transition from disorder to perpendicular cylinder and finally gyroid was observed. Those reports above clearly demonstrate that solvent removal rate is crucial for the final morphology of block copolymers with the treatment of solvent annealing. Since both diblock copolymer and BHJ mixture are two-component system, there is little doubt that the control of SRR would provide a more precise morphology control which will further improve device performance. Thus, a more thorough understanding of morphology evolution as a functional of SRR is required for the further improvement of the morphology and function of organic photovoltaics.

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Therefore, in this work, we attempt to investigate the impact of SRR on the nanoscale morphology and device performance of SM BHJ solar cells after solvent annealing. Grazing incidence wide angle x-ray scattering (GIWAXS) provides the information about crystallinity, crystal sizes and molecular packing of the SM donor. Small angle neutron scattering (SANS) experiments were performed to provide quantifiable measures of the morphology of the active layer, providing detailed information about fullerene phase separation, mixing phase and interface between pure fullerene phase and its surrounding matrix. The results provide previous unavailable information about the crucial relationship between solvent removal rate, nanoscale morphology, and device performance of donor/fullerene mixtures with solvent annealing.

Experiments p-DTS(FBTTh2)2 and PC70BM was obtained from 1-Materials. To fabricate the p-DTS(FBTTh2)2/fullerene films, a solution comprising p-DTS(FBTTh2)2 (21 mg/mL) and PCBM (14mg/mL) in chlorobenzene with 0.4% diiodooctane by volume were spin-coated at 1000 rpm for 60s on the substrate to form the mixed layer. Controlled solvent vapor annealing of the active layer is performed in an upright column. Chlorobenzene was used and added into the bottom of 1 m long vertical column. The solvent was sat for 8 hours to reach equilibrium, which would from a linear gradient of vapor pressure along the vertical direction of the column. Therefore, the vapor pressure of the solvent is maximum at the solvent surface and is zero at the top of the 5



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column. All the samples were then placed at a height which is 90% down to the bottom of the column for 60 minutes. To control the SRR, the films were then removed from the column with different rates (0.1 m/s, 3×10-4 m/s, and 3×10-5 m/s) driven by a motor. The films were then annealed at 120 °C for 10 minutes to remove the residual solvents. GIWAXS experiments were performed at beamline 14B1, Shanghai Synchrotron Radiation Facility. The SANS data were obtained on the CG2 SANS instrument at the HFIR of Oak Ridge National Laboratory. The scattering length density (SLD) of p-DTS(FBTTh2)2 and PC70BM is 1.2 × 10-6 Å-2 and 4.5 × 10-6 Å-2, respectively.29, 30 The

small

molecule

BHJ

organic

solar

cells

with

a

structure

of

ITO/PEDOT:PSS/ p-DTS(FBTTh2)2:PCBM/Ca/Al were fabricated as follows: a hole transport layer of PEDOT:PSS was spin-coated on pre-cleaned ITO glass and then annealed at 140 °C for 30 minutes. The fabrication of active layer and solvent annealing process were described as above. Finally, cathodes with 10 nm of Ca and 90 nm Al were consecutively deposited on top of the active layer. OPV performance was examined by a Keithley 4200 Source Meter (Abet Simulator Sun 2000). For the space charge limited current (SCLC) measurements, electron-only and hole-only device with device architecture of ITO/ZnO/PFN/active layer/PFN/Al and ITO/PEDOT: PSS/active layer/MoO3/Al, were fabricated respectively. Based on the fitting of the dark current by the SCLC model, the electron and hole mobility for the p-DTS(FBTTh2)2: PCBM were determined.

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Results To investigate the crystallization and molecular packing of p-DTS(FBTTh2)2, the GIWAXS was performed. Figure 1a shows the out of plane GIWAXS profile of active layers extracted from 2D GIWAXS data. All the samples exhibit a peak at q= 3.2 nm-1, which is ascribed to the lamellar distance of p-DTS(FBTTh2)2 separated by the side chains.31 As shown in Figure 1a, this peak was only slightly changed with solvent removal rate, which indicated that the crystallinity, molecular packing and crystal size was only slighted impacted by the solvent removal rate used in this work. Figure 1b presents the in plane GIWAXS profiles of all the samples. All the samples exhibit a (001) peak at q= 17.2 nm-1, which is related to the π-π stacking of p-DTS(FBTTh2)2 molecules.31 The presence of (001) peak at in plane profile and absence of this peak at out of plane profile indicates that all the samples preferred an edge-on structure of p-DTS(FBTTh2)2 molecules.

Moreover, this peak only slightly

varied with solvent removal rate, which further verified that the SRR only slight impacted crystallinity, molecular packing and crystal size of SM in the blends. Figure 2 showed the surface morphology of p-DTS(FBTTh2)2: PCBM mixtures obtained from AFM. It shows that the crystalline domains were well connected for all the samples and crystal sizes almost remained unchanged with different SRR, which is consistent with GIWAXS results. Figure S1 presents the AFM image of as cast p-DTS(FBTTh2)2: PCBM mixture, which shows the crystallinity is low in the as cast mixture and SA is effective to modify the morphology. The low crystallnity in the as

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cast mixture resulted in very poor device performance (PCE=1.7%). However, AFM only provided the surface morphology of active layer. TEM images of all the samples were presented in Figure S2, which shows similar results with AFM with well connected crystalline domains and similar crystal sizes. As the limited electron density contrast between mixing phase and PCBM pure phase, it is difficult to distinguish the mixing phase and PCBM pure phase from TEM. To obtain more detailed information about p-DTS(FBTTh2)2: PCBM mixtures, small angle neutron scattering was employed. Similar to P3HT: PCBM mixtures, because of the significant SLD contrast between p-DTS(FBTTh2)2 (1.2 × 10-6 Å-2) and PCBM (4.5 × 10-6 Å-2), scattering is dominated by pure PCBM phase and its surrounding matrix. Therefore, the interface and domain sizes corresponds to the pure PCBM domains dispersed in its surrounding matrix which includes both SM crystalline phase and the mixing phase of SM and PCBM. Figure 3 shows SANS curves for all the samples. To obtain more detailed structure information of p-DTS(FBTTh2)2 : PCBM mixtures, SANS curves were first fitted to Schulz sphere model, which describes the fullerene pure phase dispersed in its surrounding matrix.32-35 As shown in Figure 3, the SANS curves were well fitted to Schulz sphere model, from which volume fraction of pure PCBM phase, size of pure PCBM domains, polydispersity of PCBM domains, and specific interfacial area between pure PCBM phase and its surrounding matrix can be obtained and listed in Table 1. It shows that volume fraction of pure PCBM phase increased significantly

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from 12.8% up to 25.9% with a decrease of SRR. Figure 4 presents the distribution of pure PCBM domains obtained from Schulz sphere model for all the samples. The sample with fast SRR provides an average PCBM domain size of 60 Å with polydispersity of 0.54; an average PCBM domain size of 80 Å with polydispersity of 0.32 exhibited in the sample with mediate SRR; while a significant increase of PCBM domain size was found in the sample with slow SRR, which presents an average PCBM domain size of 122 Å with polydispersity of 0.22. Moreover, the specific interfacial area between pure PCBM phase and its surrounding matrix can be obtained from Schulz sphere model and was shown in Table 1.35 The maximum S/V is found in the sample with mediate SRR. Moreover, similar with previous work, with knowledge of the percent crystallinity of the p-DTS(FBTTh2)2 in the mixture, concentration of PCBM in non-crystalline p-DTS(FBTTh2)2 (߉௉஼஻ெ ) can be determined.30 Table 2 lists the concentration of PCBM in non-crystalline SM assuming a given percent crystallinity of SM in SM. It indicates that the concentration of PCBM in mixing phase decreases with a decrease of SRR. It should be noted here that the same area under the (100) peak and (001) peak are presented for all the samples, which indicates the same crystallinity. Furthermore, The surface-to-surface distance between PCBM molecules in an non-crystalline p-DTS(FBTTh2)2, H, can be calculated from H/D = (௸

஍೘

)ଵ/ଷ − 1,

ು಴ಳಾ

where Φ௠ is the maximum random packing fraction (0.7 for ellipsoids).30, 36, 37 D is 9



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the particle diameter, which is 15 Å for PC70BM.30 The results were tabulated in Table 3, which indicated that the surface-to-surface distance between two adjacent PCBM molecules in non-crystalline p-DTS(FBTTH2)2 is 4.3-5.6 Å for the sample from fast solvent evaporation. This distance increases with a decrease of SRR. For the sample from low SRR, this distance increased to 10.1-11.8 Å. Hence, the results above clearly demonstrated that the morphology of active layer, especially the PCBM phase separation and concentration of PCBM in mixing phase, is significantly impacted by the solvent removal rate after solvent annealing. To allow this information more useful, these morphologic changes must be correlated to device performance. Therefore, the device performance of all the BHJ devices were recorded and tabulated in Table 4. The device from fast removal rate exhibited a PCE of 5.1% with a short-circuit current density (Jsc) of 9.8 mA cm-2, Voc of 0.82V and FF of 63%. Better device performance was found in the device from mediate removal rate, which gives a PCE of 6.7% with Jsc of 12.2 mA cm-2, Voc of 0.82V and FF of 67%. However, further decrease of SRR results in a decrease of PCE to 5.8% along with Jsc of 11.3 mA cm-2, Voc of 0.81V and FF of 64%.

Discussion Solvent annealing has been widely employed in OPV and perovskite solar cells to optimize the morphology of active layer for the improvement of device performance. However, the effect of SRR after solvent annealing on the morphology

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and device performance of OPV has never been reported. This work is a unique example which clearly demonstrates that the morphology and function of OPV are strongly associated with solvent removal rate after solvent annealing. The GIWAXS results reveal that the crystallinity, crystal size and molecular packing of p-DTS(FBTTh2)2 in BHJ blends were only slightly affected by the SRR; while the SANS results elucidated that the phase separation of pure PCBM phase was strongly impacted by the SRR. The domain size of pure PCBM phase increased from 60 Å to 122 Å and the concentration of pure PCBM phase increased from 12.8% to 25.9% by changing the SRR from “fast” to “low”. Similar to P3HT: PCBM BHJ mixture, there are three phases in the p-DTS(FBTTh2)2 : PCBM BHJ mixture, including p-DTS(FBTTh2)2 crystalline phase, pure PCBM phase and a mixing phase of p-DTS(FBTTh2)2 and PCBM. The p-DTS(FBTTh2)2 crystalline phase is associated with hole transport. Figure S3 shows the current-voltage curve of hole only devices and Table 5 lists the hole mobility of all the samples extracted from Figure S2, which exhibited similar hole mobility. This is consistent with GIWAXS results, which shows that the crystallinity, crystal size and molecular packing of p-DTS(FBTTh2)2 in BHJ blends were only slightly affected by the SRR, and also consistent with AFM results which indicates that all the samples have well connected

p-DTS(FBTTh2)2 crystals. Therefore, the change of PCE are

not related to p-DTS(FBTTh2)2 crystalline phase and must be associated with either mixing phase or pure PCBM phase or both.

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The presence of mixing phase provides ample of donor/acceptor interfaces which significantly facilitates exciton dissociation. Meanwhile, as both electron and holes coexist in the mixing phase, it implies a higher probability of bimolecular recombination would occur in this phase. Therefore, a sufficient close of PCBM distance in the mixing phase (sufficient PCBM concentration) for the electron transport in the mixing phase is required. Meanwhile, the present of pure PCBM phase which provides “fast” pathway for electron transport and less risk of bimolecular recombination, is also required. Hence, a balance between mixing phase and pure PCBM phase is key to achieve optimized morphology and PCE. Table 3 shows an increase of electron mobility with a decrease of removal rate, which is associated with the increased size and concentration of pure PCBM domains. However, the improved electron mobility did not absolutely lead to the improved PCE. An increase of SRR leads to an increase of the concentration of pure PCBM phase and a decrease of the concentration of PCBM in the mixing phase. The improved PCE by changing SRR from “fast” to “mediate” indicates that the PCBM phase separation plays a more important role in the device performance in this case and the average distance of PCBM in mixing phase less than 8.1 Å should be sufficiently close enough for electron transport in the mixing phase. Meanwhile, the decease of PCE by changing SRR from “mediate” to “low” implies that volume fraction of PCBM in mixing phase dominates the OPV performance in this case and an average distance of PCBM in the mixing phase more than 10.1 Å is detrimental to electron transport.

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Moreover, the highest PCE in the sample with mediate SRR should also be associated with the largest specific interfacial area between PCBM pure phase and its surrounding matrix (S/V), as an increase of S/V provides more pathway for the electron to escape from “slow” and “dangerous” mixing phase to “fast” and “safe” pure PCBM phase, which facilitates electron transport and reduces the bimolecular recombination, resulting in a improved PCE. Conclusion The results presented in this work clearly demonstrated that the nanoscale morphology of SM : fullerene mixture was significantly impacted by the solvent removal rate after solvent annealing. The crystal size, crystallinity and molecular packing of SM were only slightly affected while fullerene phase separation and concentration of fullerene in mixing phase were significantly controlled by the solvent removal rate after solvent annealing. An increase of size and volume fraction of pure fullerene domains was found with a decrease of solvent removal rate after solvent annealing. Furthermore, this works is an unique example which clearly shows that a balance between mixing phase and pure fullerene phase is key to optimize the morphology and device performance of SM : PCBM BHJ. The specific interfacial area between pure fullerene phase and mixing phase is crucial for device performance, as it would facilitate electron transport and reduce the bimolecular recombination. More importantly, the judicious control of solvent removal rate after solvent

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annealing could provide a lever of morphologic control of active layer which is previous unavailable.

This finding would provide a more careful and precise control

of morphology of active layer and offers guides for further improvement of device performance with solvent annealing. Acknowledgements The authors are grateful for financial support from National Natural Science Foundation of China (51503039) and National Key Research and Development Program of China (2016YFB0401103). Part of this work at ORNL’s HFIR was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. The authors thank Beijing Synchrotron Radiation Facility (1W1A station) for providing beamtime and help for GIWAXS experiments. Supporting Information Further information relating to AFM topographies of the surfaces of as cast mixture, TEM images of p-DTS(FBTTH2)2 : PCBM blends with SA, and J-V characteristics of hole-only device and electron-only device. .Author Information

Corresponding Authors *Email: [email protected] Note

The authors declare no competing financial interest.

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(18) Liu, J.; Gao, C.; He, X. L.; Ye, Q. Y.; Ouyang, L. Q.; Zhuang, D. M.; Liao, C.; Mei, J.; Lau, W. M. Improved Crystallization of Perovskite Films by Optimized Solvent Annealing for High Efficiency Solar Cell ACS Appl. Mater. Interfaces 2015, 7, 24008-24015. (19) Xiao, Z. G.; Dong, Q. F.; Bi, C.; Shao, Y. C.; Yuan, Y. B.; Huang, J. S. Solvent Annealing of Perovskite-Induced Crystal Growth for Photovoltaic-Device Efficiency Enhancement Adv. Mater. 2014, 26, 6503-6509. (20) Albert, J. N. L.; Young, W. S.; Lewis, R. L.; Bogart, T. D.; Smith, J. R.; Epps, T. H. Systematic Study on the Effect of Solvent Removal Rate on the Morphology of Solvent Vapor Annealed ABA Triblock Copolymer Thin Films ACS Nano 2012, 6, 459-466. (21) Wu, Y. H.; Lo, T. Y.; She, M. S.; Ho, R. M. Morphological Evolution of Gyroid-Forming Block Copolymer Thin Films with Varying Solvent Evaporation Rate ACS Appl. Mater. Interfaces 2015, 7, 16536-16547. (22) Su, M.; Su, Z. Effects of Solvent Evaporation Rate and Poly(acrylic acid) on Formation of Poly(ethylene oxide)-Block-Polystyrene Micelles from Emulsion Macromolecules 2014, 47, 1428-1432. (23) Paradiso, S. P.; Delaney, K. T.; García-Cervera, C. J.; Ceniceros, H. D.; Fredrickson, G. H. Block Copolymer Self Assembly during Rapid Solvent Evaporation: Insights into Cylinder Growth and Stability ACS Macro Lett. 2014, 3, 16-20. 18


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(24) Kim,

G.;

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in

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(36) Mackay, M. E.; Dao, T. T.; Tuteja, A.; Ho, D. L.; Van Horn, B.; Kim, H. C.; Hawker, C. J. Nanoscale Effects Leading to Non-Einstein-Like Decrease in Viscosity Nat. Mater. 2003, 2, 762-766. (37) Delaney, G. W.;Cleary, P. W. The Packing Properties of Superellipsoids. EPL 2010, 89, 275-288.

Figure 1 a) out-of plane and b) in-plane GIWAXS profiles for all the samples (a)

21



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(b)

Figure 2 AFM tapping mode topographies of the surfaces of p-DTS(FBTTH2)2 : PCBM blends from a) fast solvent removal rate b) mediate solvent removal rate c) low solvent removal rate. 22


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a)

b)

c)

Figure 3 The absolute small angle neutron scattering intensity with Schulz sphere fitting for p-DTS(FBTTH2)2 : PCBM blends with different solvent removal rate after solvent annealing. 23



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Figure 4 Size distribution of pure PCBM domains in p-DTS(FBTTH2)2 : PCBM mixtures. 24


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Table 1 Average radius of pure PCBM domains (Ra), volume fraction of pure PCBM phases (Φag), polydispersity of the size of pure PCBM domains (P), and specific interfacial area (S/V) obtained from Schulz sphere model. 25



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Fast rate Mediate rate Low rate

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S/V (cm-1)

Ra(Å) 60

Φag 12.8%

P (sig/avg) 0.54

403526

80

20.1%

0.32

626289

122

25.9%

0.22

557413

Table 2 Concentration of PCBM in non-crystalline p-DTS(FBTTH2)2 assuming a given percent crystallinity of p-DTS(FBTTH2)2 in p-DTS(FBTTH2)2. Crystallinity 20% 30% 40%

Fast rate 27% 30% 33%

Mediate rate 19% 21% 24%

Low rate 11% 13% 15%

Table 3 Surface-to-surface distance between homogeneously dispersed fullerenes in the p-DTS(FBTTH2)2:PCBM mixing phase for a given p-DTS(FBTTH2)2 crystallinity. Crystallinity

Fast rate (Å)

Mediate rate (Å)

Low rate (Å)

20% 30% 40%

5.6 4.9 4.3

8.1 7.4 6.4

12.8 11.3 10.1

Table 4 Device performance at standard AM-1.5 illumination.

Fast rate Mediate rate Low rate

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

9.8± 0.3 12.4± 0.5 11.3± 0.4

0.82± 0.04 0.82± 0.03 0.81± 0.05

63.0± 0.4 67.3± 0.5 64.3± 0.5

5.1± 0.2 6.8± 0.4 5.8± 0.3

Table 5 Hole and electron mobility calculated from SCLC method. 26


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Fast rate

Mediate rate

Low rate

Hole mobility (cm2V-1s-1)

3.12×10-4

3.23×10-4

3.56×10-4

Electron mobility (cm2V-1s-1)

2.85×10-4

5.13×10-4

8.21×10-4

Table of Content 27



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Importance of Solvent Removal Rate on the Morphology and Device

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