Impact of Fullerene Structure on Nanoscale Morphology and

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The Impact of Fullerene Structure on the Nanoscale Morphology and Miscibility and Their Correlation of Performance of Small Molecule : Fullerene Solar Cell Shuqiong Lan, Huihuang Yang, Guocheng Zhang, Xiaomin Wu, Wen Ning, Shuiming Wang, Huipeng Chen, and Tailiang Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08025 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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The Journal of Physical Chemistry

The Impact of Fullerene Structure on the Nanoscale Morphology and Miscibility and Their Correlation of Performance of Small Molecule : Fullerene Solar Cell Shuqiong Lan,a Huihuang Yang,a Guocheng Zhang,a,b Xiaomin Wu, a Wen Ning,a Shuiming Wang,a Huipeng Chena* and Tailiang Guoa a

Institute of Optoelectronic Display, National & Local United Engineering Lab of Flat Panel Display Technology, Fuzhou University, Fuzhou 350002, China Email: [email protected] Tel: 86-599-87893299 b College of Information Science and Engineering, Fujian University of Technology, Fuzhou 350108, China

Abstract This manuscript reports the impact of fullerene structure on the morphology and miscibility of small molecule : fullerene bulk heterojunction solar cell.

The small

angle neutron scattering and neutron reflectometry measurements were analyzed to provide quantifiable measures of the morphology of the resultant mixtures, offering miscibility, domain sizes, interfacial area between the small molecule and fullerene, and depth profiles in the mixtures.

These results indicate that the bis-adduct

fullerenes exhibit lower miscibility in small molecule. Correlation of miscibility and morphology to photovoltaic properties indicates that small molecule/fullerene miscibility is crucial to rationally optimize the design of fullerenes for use in small molecule organic photovoltaics. A higher open circuit voltage is obtained for bis-adduct fullerenes device, which however, does not translate into an increased power conversion efficiency. This decrease in performance is associated with the lower miscibility of bis-fullerene, which decreases the probability of the dissociation of excitons and enhances charge recombination rate in the miscible region. A quantitative analysis shows that an increase of average separation of fullerenes in the

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miscible region is detrimental to electron transport in the miscible region, especially for a distance greater than ~11 Å Introduction Open circuit voltage (Voc) is one of the key parameters that determine the power conversion efficiency (PCE) of bulk heterojunction (BHJ) solar cells. It is well known that the Voc of BHJ solar cells is almost relied on the difference between the highest unoccupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor. Based on this metric, acceptor materials with higher LUMO levels have been synthesized to increase the Voc as well as higher PCE. For example, replacing [6,6]-Phenyl C61 butyric acid methyl ester (PC60BM) with the indene-C60 bis-adduct (ICBA) which possess higher LUMO levels increases the Voc and in turns improves the PCE of P3HT-based devices.1,2 Unfortunately, when mixed with the low band-gap polymer, ICBA exhibits poor PCE although the desired large Voc is always achieved.

3-8

For instance, PBDTTPD:PCBM BHJ

devices provided a PCE of 7%; while PBDTTPD:ICBA BHJ devices exhibit a poor PCE of less than 3%.5 The resultant poor PCE is usually ascribed to either poor morphology of active layer or impeded charge transport.3-48 Small molecule based BHJ (SM-BHJ) solar cell are based on blends of small molecules (SM) and fullerenes, with a dramatic growth in these promising systems in recent years. Recently, a PCE of 9% for a SM-BHJ based solar cell has been demonstrated, which is comparable to polymer based bulkheterojunction (BHJ) solar cells. 9 To improve the device performance, ICBA has been applied to


small

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molecular based BHJ solar cells to increase the Voc with the purpose of improved PCE.10 Unfortunately, Kyaw et al. has reported that replacing PCBM with ICBA results

in

a

decrease

of

the

PCE

of

7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′- bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) based solar cell.10 The reduced PCE is associated with reduced charge generation rate and increased charge recombination rate, while the detailed information about miscibility and fullerene phase separation and their correlations to device function is still not clear. Learning from the polymer-based solar cells, we know that morphology and miscibility is crucial for the optimization of OPV system. It has been widely known that P3HT and PCBM are highly miscible, which indicates that the P3HT:PCBM blends BHJ must consist of at least three phases; a crystalline phase of P3HT, a miscible phase of PCBM and amorphous P3HT, and a PCBM rich phase.11-15 The extent of polymer/fullerene miscibility has been shown to be crucial to the optimization of device performance.16-19 One of the benefits of the mixing phase is that it offers ample donor/acceptor interfaces for efficient dissociation of exciton, in which sufficient mixing is also required to allow efficient charge transport.19 In P3HT system, although ICBA succeeds to improve PCE, incorporating a fullerene with a higher LUMO level with P3HT does not always result in a device with a higher PCE. Chen and et al. studied the effect of fullerene structure on miscibility of P3HT:fullerene mixture and found that bis-adduct of phenyl-C61-butyric acid methyl



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ester (bis-PCBM) has a higher open circuit voltage (Voc) than PC60BM with P3HT, however device performance of bis-PCBM based devices is lower than that of PC60BM based devices.20

This decrease in performance is attributed to the lower

miscibility of bis-PCBM in P3HT, which decreases the probability of the dissociation of exciton and enhances charge recombination rate in the miscible region.20 Meanwhile, in low band gap system, it has been reported that with morphology optimization, ICBA system can achieve almost the same PCE with PCBM system.21 The improved device performance is associated with fullerene phase separation, which facilitates charge dissociations and electron transport. In this work, we therefore attempt to investigate the miscibility of three different fullerenes in p-DTS(FBTTh2)2 and provide previously unavailable information correlating morphology and phase miscibility to the chemical structure of fullerenes and correlate this miscibility to OPV performance. The bis-adduct fullerenes ICBA and bis-PCBM, which have higher LUMO levels than PC70BM are selected for the present work. The SANS measurements were analyzed to provide quantifiable measures of the morphology of the resultant mixtures, offering domain sizes and the specific interfacial area between SM donor and fullerene in the mixtures.

The results

are then interpreted to provide a correlation of fullerene miscibility and morphology to function providing crucial insight into the importance of the thermodynamics and structure of SM-BHJ on the rational design of next generation components in SM-BHJ solar cell devices. Experiments


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p-DTS(FBTTh2)2 and PC70BM was obtained from 1-Materials, ICBA was purchased from Sigma Aldrich, and bis-PC60BM was purchased from Solenne BV. The chemical structure of p-DTS(FBTTh2)2 and fullerenes are shown in Figure 1. To fabricate the p-DTS(FBTTh2)2/fullerene films, p-DTS(FBTTh2)2 (21 mg/mL) and fullerene (14 mg/mL) were dissolved in chlorobenzene with 0.4% diiodooctane by volume. The active layer p-DTS(FBTTh2)2: fullerene mixture was formed by spin casting p-DTS(FBTTh2)2/fullerene solution at 1000 rpm for 1 minute. The films were than thermally annealed at 140 °C for 10 minutes to remove the residual solvent. The GIWAXS data were recorded at BL14B1, Shanghai Synchrotron Radiation Facility. The Neutron reflectivity experiments were performed on the Liquids Reflectometer at the Spallation Neutron Source at Oak Ridge National Laboratory. In order to determine the vertical structure of the blended films, Layers22 and Motofit Software 23 were used.

The details about the fitting and the method to obtain

fullerene concentration depth profiles can be found in our previous work.24 The small angle neutron scattering (SANS) data on the p-DTS(FBTTh2)2:fullerene mixtures 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 is 1.2 × 10-6 Å-2, which was calculated using the mass density of p-DTS(FBTTh2)2, which is experimentally determined with pycnometry, and its atomic composition. To

OPV

devices

were

fabricated

p-DTS(FBTTh2)2:fullerene/Ca/Al structure.

with

a

typical

ITO/PEDOT:PSS/

Filtered PEDOT:PSS was spin-coated

on the clean ITO glass and then baked at 140 °C for 30 minutes. The fabrication of



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active layer was described as above and thermally annealed at 140 °C for 10 minutes. Finally, 10 nm of Ca and 90 nm of Al were evaporated on the active layer. Current-voltage (I-V) curves were measured using a Sun 2000, Abet Technologies, AM1.5G solar simulator, which is calibrated using crystalline Si reference cell. Results The vertical morphology of p-DTS(FBTTh2)2:fullerene mixtures were first investigated by neutron reflectometry. The experimental neutron reflectometry curves, along with the fitted curves, are shown in Figure 2. The reflectometry results are plotted Rq4 as a function of q for a clear view of the high quality fitting. The depth profiles exacted from fitting are presented in Figure 3, where z = 0 represents the interface between air and film, while z = ~800 Å corresponds to the interface between film and Si. A plateau region with constant fullerene concentration presents at z ≈ 400 – 600 Å for all the samples, which is not affected by either air or silicon surface. Segregations of fullerene to the air and Si interfaces are also found for all the samples, while the extent of fullerene segregations varied with fullerenes, which is similar to P3HT/PCBM blends. Enhanced segregation of fullerene to the air surface (near the cathode) and a depletion of fullerene at the silicon surface (near the anode) are found in bis-fullerene blends, which suggested that bis-fullerene blends have a more ideal depth profiles for charge collection. The surface morphology of p-DTS(FBTTh2)2:fullerene mixtures was examined by atomic force microscopy (AFM). As shown in Figure S1, all the blends exhibit similar morphology from AFM. As information about the in-plane morphology of


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active layer obtained from AFM is very limited, SANS was employed to provide information on morphology and miscibility of the samples. Our previous work on P3HT:PC60BM mixtures has clearly shown that the proper analysis of SANS curves of P3HT:fullerene mixtures provides the domain size, the specific interfacial area between pure fullerene phase and P3HT rich phase, and the miscibility of the fullerene in P3HT, where a similar analysis is applied in this work.25 The SANS curves of p-DTS(FBTTh2)2:fullerene mixtures with PC70BM, bis-PCBM, and ICBA are presented are presented in Figure 4. As discussed in our previous work in P3HT:PCBM system, because the contrast between the P3HT crystals and the P3HT:PCBM miscible phase in neutron scattering is very small compared to the contrast between pure PCBM and either the P3HT crystal phase or the miscible phase, the scattering contrast is therefore dominated by the pure PCBM phase and its surrounding matrix.25 Due to the similar SLD of P3HT (0.7 × 10-6 Å-2) to p-DTS(FBTTh2)2 (1.2 × 10-6 Å-2) comparing with PCBM (4.4 × 10-6 Å-2), the scattering of p-DTS(FBTTh2)2 : PCBM blends is dominated by the contrast between the pure fullerene phase and its surrounding matrix.26 Hence, the domain sizes and specific interfacial areas obtained from SANS refer to those of the fullerene-pure phase dispersed in a p-DTS(FBTTh2)2-rich phase, which includes both p-DTS(FBTTh2)2 crystals and the miscible phase. A further analysis of SANS data, detailed structural information can be obtained. Showing in Figure 4, the scattering data of the samples were first fit to the Schulz sphere model, which was used to describe a two-phase system in which



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spherical domains dispersed their surrounding matrix.27 It has been successfully used to describe the dispersed fullerene aggregates in the polymer rich phase of polymer:fullerene blends.13, 21 From the fitting, the fullerene domain radius, fullerene aggregates vol% in the whole sample (Φagg), the p-DTS(FBTTh2)2 rich phase SLD, and the vol% of fullerene in the p-DTS(FBTTh2)2 rich phase are determined. Moreover, with the SLD of the surrounding matrix and the composition of the film, the volume fraction of the dispersed phase in the whole sample (Φ ) can be calculated by the procedure described in our previous work.28 The detailed analysis can be found in our previous work.28 A schematic presentation of the morphology in SM:fullerene BHJ in the view of neutron scattering was shown in Figure S2.

The results of this analysis are listed in Table 1. p-DTS(FBTTh2)2 : PC70BM forms a film with 21 vol% phase separated PC70BM domains in the sample, with an average radius of 52 Å. Similarly, p-DTS(FBTTh2)2 : bis-PCBM constitute 22 vol% of phase separated bis-PCBM in the sample along with an average radius of 50 Å in size; while p-DTS(FBTTh2)2 : ICBA constitute 25 vol% of phase separated ICBA the sample along with a mean radius of 94 Å. The size distributions of the fullerene domains, based on the Schulz distribution are shown in Figure 5. Moreover, the miscibility of fullerene in p-DTS(FBTTh2)2 are 18%, 16%, and 12% for PCBM, bis-PCBM, and ICBA, respectively. Meanwhile, from the structural characteristics, the specific interfacial area (S/V) between the pure fullerene domains and its surrounding matrix was obtained from Schulz sphere model, where these values are


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shown in Table 1.27 The highest specific interfacial area is attained in PC70BM samples, while the lowest value is found in ICBA sample. To verify the connectivity of domains, the SANS curves was also fitted to the Teubner-Strey (TS) model.29 It is commonly used to describe the bi-continuous two-phase systems.30-32 The TS model is given by: I(q)=

+ I

(1)

where Ib is the incoherent scattering. a2, c1, and c2 are associated with correlation length (ξTS) and the repeating distance of two adjacent domains (d): /

ξTS=

/

d= 2π

+

/

−

/

(2)

(3)

ξTS is related to average domain size of the fullerene phase and the p-DTS(FBTTh2)2 rich phase. d is related to the length scale of p-DTS(FBTTh2)2 rich phase. The fitting of this analysis are presented in Figure 6, which indicates the formation of bi-continuous phases and the fullerene domains are well connected. The correlation lengths ξTS and d are tabulated in Table 1. Since SANS reveals information about fullerene miscibility and phase separation, we also probed the information about crystalline structure of p-DTS(FBTTh2)2 by grazing incidence wide angle X-ray scattering (GIWAXS). Figure 7a presents the out-of-plane x-ray scattering curves extracted from 2D GIWAXS data. In this figure, all the scattering curves exhibit a (100) peak at q ≈ 3.0 nm-1, which corresponds to alkyl lamellar structure.33,34 The lamellar coherence lengths determined from Scherrer



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analysis are listed in Table 2, showing that the lamellar coherence lengths remains unchanged with different fullerene additives. Moreover, the area under the (100) peak is within 3% difference for all the samples after normalizing with sample thickness, which indicates the crystallinity of p-DTS(FBTTh2)2 are the same for all the samples. Figure 7b presents in-plane x-ray scattering curves, in which the (100) peak was absent. It indicates that an edge-on structure of p-DTS(FBTTh2)2 molecules on the substrate would be favored for all the samples. Furthermore, all the samples exhibit a (010) peak at q ～ 17.3 nm-1 which is related to π-π stacking of p-DTS(FBTTh2)2 molecules. The peak position and π-π stacking coherence lengths obtained from Scherrer analysis are shown in Table 2, which are the same for all the samples. The GIWAXS results reveal that the crystalline structure of p-DTS(FBTTh2)2 are only slightly affected by varying fullerene structures. The miscibility of the fullerene in p-DTS(FBTTh2)2

that is determined by

! SANS ( ) is the miscibility of fullerene in the whole p-DTS(FBTTh2)2 film,

which includes both non-crystalline and crystalline p-DTS(FBTTh2)2. As fullerene does not seem to diffuse into the crystalline portion of the p-DTS(FBTTh2)2, the miscibility limit of the fullerene in the non-crystalline portion of the p-DTS(FBTTh2)2 (fullerene), which is a more accurate measure of the affinity of the small molecule and the fullerene. With knowledge of the percent crystallinity of the p-DTS(FBTTh2)2 in the blends, fullerene can be determined. Table 3 shows the miscibility of fullerene in non-crystalline p-DTS(FBTTh2)2 assuming a given percent crystallinity of p-DTS(FBTTh2)2 in p-DTS(FBTTh2)2. It is noted here that as all the samples have the


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same area under the (100) peak, they should have the same crystallinity. These results show that the trends that are evident in the SANS results translate to the miscibility of each fullerene in the non-crystalline portion of the p-DTS(FBTTh2)2 film. Hence, the results above demonstrate that morphology and miscibility are significantly varied with different fullerene structures. To have this information most useful, the morphology and miscibility change must be correlated to the device performance of the bulk heterojunction. Therefore, the device performance of the p-DTS(FBTTH2)2: fullerene mixtures was recorded, and the results are listed in Table 4. The results demonstrate that the device of p-DTS(FBTTH2)2 :PCBM exhibits a PCE of 6.9% with a short-circuit current density(Jsc) of 12.8 mA cm-2, a Voc of 0.82V and a FF of 65.4%. The device of p-DTS(FBTTH2)2 : bis-PCBM which has a higher Voc compared to p-DTS(FBTTH2)2 :PCBM shows a slightly lower performance with Jsc=11.2 mA cm-2, Voc=0.88V, FF=65.0%, and PCE=6.4%. However, device performance of ICBA based devices with a 25% higher Voc is much lower than that of PCBM based devices and the PCE decreased to 4.1% with Jsc=6.8 and FF=58.4%. Discussion Acceptor materials with higher LUMO levels have been designed to increase the Voc as well as higher PCE. The LUMO of p-DTS(FBTTh2)2 is -3.3ev which is very close to that of P3HT (-3.2ev) and provides enough energy gap between LUMO of donor and LUMO of acceptor, which should not cause the energetic issue. Moreover, without considering the recombination of electrons and holes, a higher Voc from bis-adduct fullerene device should lead to a greater Jsc.16 However, the increase of VOC



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does not result in an increase of PCE in SM-BHJ solar cells, which is usually ascribed to “poor morphology”, while structural evidence of the morphology is lacking.

The

results presented here provide important insight of the morphology, in that it offers direct evidence of the morphology, miscibility and phase separation of the fullerenes. This, in turn, provides a direct correlation between the morphology of the SM-BHJ and its OPV performance. In small molecule BHJ solar cell system, it consists of three phases comprising a small molecule crystalline phase, a pure fullerene phase, and a miscible phase of non-crystalline molecule and fullerene. The small molecule crystalline phase, which is associated with hole transport, is found to be slightly affected by the fullerene structures studied by this work. It indicates the crystallinity and crystalline structure of donor materials is not ascribed to the decrease of PCE by replacing PCBM with ICBA. Moreover, the examination of vertical morphology shows that the bis-fullerene blends have more ideal depth profiles than PCBM blends, while bis-fullerenes devices provides a worse PCE. This implies that the decreased PCE for the bis-fullerene devices is not given by the vertical morphology. Thus a balance between miscible phase and pure fullerene phase must be the key to explain the poor performance for the SM:bis-fullerene devices. Table S1 lists the electron and hole mobility of p-DTS(FBTTh2)2:fullerene mixtures determined by the space charge limited current methods. The electron and hole mobility kept at the same order of magnitude for all the p-DTS(FBTTh2)2:fullerene mixtures, which means that both donor crystalline phase and pure fullerene phase were well connected for the charge transport. It


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indicates pure SM phase and pure fullerene phase do not cause the decreased PCE in bis-fullerene devices.

Thus, the mixed phase must be examined. The importance of

mixing phase in the exciton generation and dissociation has been reported in polymer BHJ solar cells.35,36 One benefit of the miscible phase is that it provides ample donor/acceptor interfacial area, which facilitates exciton dissociation into free charge carriers. However, this miscible phase also indicates that sufficient close of fullerene distance is required for the charge transport in the mixed phase, because the charge transport in the mixed phase is not as efficient as in pure phases, due to the co-existence of the electrons and holes in the miscible phase, which would enhance bimolecular recombination. Therefore, the distance of two adjacent fullerenes in the mixing phase examined here was estimated to provide guidelines for how much fullerene loading is needed

in the

non-crystalline

phase for efficient charge

transport.

The

surface-to-surface distance between homogeneously dispersed spherical particles in an non-crystalline matrix, H, can be determined from H/D = (&

%$'())*+*,*

)/. − 1,

where Φ1 is the maximum random packing fraction, which is 0.64 and 0.70 for spheres and ellipsoids respectively.

37,38

2 is the concentration of fullerene

in the non-crystalline phase. D is the particle diameter (10 Å for C60).37 By taking the length of each functional group on the fullerenes as 5 Å, the diameter of the fullerenes is estimated. Therefore, 15 Å is used for PC70BM diameter, while 20 Å are used for the diameter of bis-PCBM and ICBA. The results of this analysis for the p-DTS(FBTTH2)2:fullerene samples are listed in Table 5. It shows that the



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surface-to-surface distance between two adjacent PC70BM molecules dispersed in non-crystalline p-DTS(FBTTH2)2 is 6.1-7.7 Å, while this distance for ICBA is 10.9-14.2 Å. The average distance between bis-PCBM molecules in the non-crystalline phase range from 8.2-10.5 Å. This suggests that an increase of average separation of fullerenes is detrimental to electron transport in the non-crystalline phase, especially for a distance greater than ~11 Å, which enhances charge recombination, resulting in a decrease in Jsc and PCE. This is consistent with previous study which shows that replacing PCBM with ICBA results in a decrease of charge generation rate and an increase of charge recombination rate.10 Conclusion The morphology and miscibility in small molecule : fullerene blends are determined by SANS and neutron reflectometry, which also provides information on the domain sizes, interfaces and depth profiles of active layer. Reflectometry results show that bis-adduct fullerene blends have more ideal depth profiles, which is not associated with the decreased PCE of bis-fullerene devices.

A lower fullerene

miscibility is found in the bis-adduct fullerene blends. GIWAXS results reveal that the crystalline structure and crystallinity of p-DTS(FBTTh2)2 are only slightly affected by varying fullerene structures. This allows the direct comparison of the fullerene mixing behavior in the active layers in this study. Correlation of these results with the photovoltaic performance shows that a higher open circuit voltage is obtained for bis-adduct fullerenes device, which however, does not translate into an increased power conversion efficiency. This


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decrease in performance is associated with the lower miscibility of bis-fullerene, providing limited amount of donor/ acceptor interfaces, which results in a decrease of the probability of exciton dissociation and an enhancement of the recombination of free charge-carriers in the miscible region. Moreover, low fullerene miscibility provides limited pathways for charge transport, where larger distances between adjacent fullerenes inhibits the electron transport and creates traps, increasing the possibility of bimolecular recombination. A quantitative analysis shows that an increase of average separation of fullerenes in the miscible region is detrimental to electron transport in the non-crystalline phase, especially for a distance greater than ~11 Å. Supporting Information Available Further information relating to AFM topographies of the surfaces of SM:fullerene blends, schematic presentation of the morphology in SM:fullerene BHJ in the view of neutron scattering, and charge mobility obtained from space charge limited current Method.

Author Information Corresponding Authors *Email: [email protected] Note The authors declare no competing financial interest. Acknowledgements



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The authors wish to acknowledge National Natural Science Foundation of China (51503039) for support of this project. A portion of this research at ORNL’s High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. The GIWAXS was granted by BL14B1 station of Shanghai Synchrotron Radiation Facility. The staff members of BL14B1 are gratefully thanked.

Reference

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(5) Hoke, E. T.; Vandewal, K.; Bartelt, J. A.; Mateker, W. R.; Douglas, J. D.; Noriega, R.; Graham, K. R.; Frechet, J. M. J.; Salleo, A.; McGehee, M. D. Recombination in Polymer:Fullerene Solar Cells with Open-Circuit Voltages Approaching and Exceeding 1.0 V Adv. Energy Mater. 2013, 3, 220-230. (6) Ren, G. Q.; Schlenker, C. W.; Ahmed, E.; Subramaniyan, S.; Olthof, S.; Kahn, A.; Ginger, D. S.; Jenekhe, S. A. Photoinduced Hole Transfer Becomes Suppressed with Diminished Driving Force in Polymer-Fullerene Solar Cells While Electron Transfer Remains Active Adv. Funct. Mater. 2013, 23, 1238-1249. (7) Faist, M. A.; Shoaee, S.; Tuladhar, S.; Dibb, G. F. A.; Foster, S.; Gong, W.; Kirchartz, T.; Bradley, D. D. C.; Durrant, J. R.; Nelson, J. Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors Adv. Energy Mater. 2013, 3, 744-752. ( 8 ) Yamamoto, S.; Orimo, A.; Ohkita, H.; Benten, H.; Ito, S. Molecular Understanding of the Open-Circuit Voltage of Polymer:Fullerene Solar Cells Adv. Energy Mater. 2012, 2, 229-237. (9) Zhang, Q.; Kan, B.; Liu, F.; Long, G. K.; Wan, X. J.; Chen, X. Q.; Zuo, Y.; Ni, W.; Zhang, H. J.; Li, M. M., et al. Small-Molecule Solar Cells with Efficiency over 9% Nat. Photonics 2015, 9, 35-41. (10) Kyaw, A. K. K.; Gehrig, D.; Zhang, J.; Huang, Y.; Bazan, G. C.; Laquai, F.; Nguyen, T. Q. High Open-Circuit Voltage Small-Molecule P-DTS(FBTTH2)2:ICBA Bulk Heterojunction Solar Cells - Morphology, Excited-State Dynamics, and Photovoltaic Performance J. Mater. Chem A 2015, 3, 1530-1539.



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(11) Yin, W.; Dadmun, M. A New Model for the Morphology of P3HT/PCBM Organic Photovoltaics from Small-Angle Neutron Scattering: Rivers and Streams ACS Nano. 2011, 5, 4756-4768. (12) Watts, B.; Belcher, W. J.; Thomsen, L.; Ade, H.; Dastoor, P. C. A Quantitative Study of PCBM Diffusion During Annealing of P3HT:PCBM Blend Films Macromolecules 2009, 42, 8392-8397. (13) Kiel, J. W.; Kirby, B. J.; Majkrzak, C. F.; Maranville, B. B.; Mackay, M. E. Nanoparticle Concentration Profile in Polymer-Based Solar Cells Soft Matter. 2010, 6, 641-646. (14) Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C. J.;Chabinyc, M. L. Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend Adv. Energy Mater. 2011, 1, 82-89. (15) Zhou, K.; Liu, J. G.; Li, M. G.; Yu, X. H.; Xing, R. B.; Han, Y. C. Phase Diagram

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(18) Bartelt, J. A.; Beiley, Z. M.; Hoke, E. T.; Mateker, W. R.; Douglas, J. D.; Collins, B. A.; Tumbleston, J. R.; Graham, K. R.; Amassian, A.; Ade, H., et al. The Importance of Fullerene Percolation in the Mixed Regions of Polymer-Fullerene Bulk Heterojunction Solar Cells Adv. Energy Mater. 2013, 3, 364-374. (19) Collins, B. A.; Tumbleston, J. R.; Ade, H. Miscibility, Crystallinity, and Phase Development in P3HT/PCBM Solar Cells: Toward an Enlightened Understanding of Device Morphology and Stability J. Phys. Chem. Lett. 2011, 2, 3135-3145. (20) Chen, H. P.; Peet, J.; Hsiao, Y. C.; Hu, B.; Dadmun, M. The Impact of Fullerene Structure on Its Miscibility with P3ht and Its Correlation of Performance in Organic Photovoltaics Chem. Mater. 2014, 26, 3993-4003. (21) Chen, H. P.; Hsiao, Y. C.; Chen, J. H.; Hu, B.; Dadmun, M. Distinguishing the Importance of Fullerene Phase Separation from Polymer Ordering in the Performance of Low Band Gap Polymer:Bis-Fullerene Heterojunctions Adv. Funct. Mater. 2014, 24, 7284-7290. (22) Layers is an Excel spreadsheet for modeling NR data developed by John Ankner at Oak Ridge National Laboratory. (23) Nelson, A. Co-Refinement of Multiple-Contrast Neutron/X-Ray Reflectivity Data Using Motofit J. Appl. Crystallogr 2006, 39, 273-276. (24) Chen, H. P.; Hsiao, Y. C.; Hu, B.; Dadmun, M. Control of Morphology and Function of Low Band Gap Polymer-Bis-Fullerene Mixed Heterojunctions in Organic Photovoltaics with Selective Solvent Vapor Annealing J. Mater. Chem. A 2014, 2, 9883-9890.



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(25) Chen, H. P.; Chen, J. H.; Yin, W.; Yu, X.; Shao, M.; Xiao, K.; Hong, K. L.; Pickel, D. L.; Kochemba, W. M.; Kilbey, S. M., et al. Correlation of Polymeric Compatibilizer Structure to Its Impact on the Morphology and Function of P3HT:PCBM Bulk Heterojunctions J. Mater. Chem. A 2013, 1, 5309-5319. (26) Chen, H. P.; Hegde, R.; Browning, J.; Dadmun, M. D. The Miscibility and Depth Profile of PCBM in P3HT: Thermodynamic Information to Improve Organic Photovoltaics Phys. Chem. Chem. Phys. 2012, 14, 5635-5641. (27) Schulz, G. V. Über die Kinetik der Kettenpolymerisationen Z. Phys. Chem., 1939, B43, 25-46. (28) Chen, H. P.; Hsiao, Y. C.; Hu, B. Dadmun, M. Tuning the Morphology and Performance of Low Bandgap Polymer: Fullerene Heterojunctions Via Solvent Annealing in Selective Solvents Adv. Funct. Mater. 2014, 24, 5129-5136. (29) Teubner, M.; Strey, R. Origin of the Scattering Peak in Microemulsions J. Chem. Phys. 1987, 87, 3195-3200. (30) Shin, T. G.; Muter, D.; Meissner, J.; Paris, O. Findenegg, G. H. Structural Characterization of Surfactant Aggregates Adsorbed in Cylindrical Silica Nanopores Langmuir 2011, 27, 5252-5263. (31)Yang, H. H.; Zhang, G. C.; Zhu, J.; He, W. X.; Lan, S. Q.; Liao, L.; Chen, H. P.; Guo, T. L.; Improving Charge Mobility of Polymer Transistors by Judicious Choice of the Molecular Weight of Insulating Polymer Additive J. Phys. Chem. C 2016, 120, 17282-17289.


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Figure 1 Chemical structures of p-DTS(FBTTh2)2 and fullerenes studied.

p-DTS(FBTTh2)2

bis-PCBM

PCBM

ICBA



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Figure 2 Reflectivity curves of p-DTS(FBTTh2)2:fullerene samples. The lines are fits to model scattering density profiles.


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Figure 3 Depth profile of p-DTS(FBTTh2)2:fullerene samples as determined from the reflectivity curves shown in Figure 2



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Figure 4 The absolute small angle neutron scattering intensity with Schulz sphere fitting for p-DTS(FBTTH2)2 : fullerene blends.


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Figure 5 Size distribution of fullerene aggregates in p-DTS(FBTTH2)2 : fullerene blends.



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Figure 6 The absolute small angle neutron scattering intensity with Teubner-Strey fitting for p-DTS(FBTTH2)2 : fullerene blends.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7 a) out-of plane and b) in-plane x-ray profiles extracted from GIXD for p-DTS(FBTTH2)2 : fullerene blends. a)

b)



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Table 1 Average radius of fullerene aggregates (Ra), volume fraction of fullerene aggregates (Φag), polydispersity of the size of fullerene aggregates (P), miscibility of ! fullerene in p-DTS(FBTTh2)2 ( ), and specific interfacial area (S/V) obtained from Schulz sphere model; correlation lenggh (ξTS), and the repeating distance of two domains (d) obtained from Teubner-Strey model. !

PCBM

Ra(Å) 105

Φag 0.21

P (sig/avg) 0.38

0.15

bis-PCBM

50

0.22

0.49

ICBA

94

0.25

0.336

S/V (cm-1) 933845

ξTS(Å) 76

d (Å) 340

0.14

876375

85

408

0.10

640729

128

597

Table 2 Grazing-incident X-ray diffraction data for p-DTS(FBTTH2)2 : fullerene blends (100) peak position

Lamella spacing

(010) peak position

π-π stacking coherence lengths

3.0 3.2 3.1

12.1 12.5 12.3

17.3 17.3 17.2

10.4 10.3 10.1

PCBM bis-PCBM ICBA

Table 3 Miscibility of fullerene in non-crystalline p-DTS(FBTTH2)2 ( 2 ) assuming a given percent crystallinity of p-DTS(FBTTH2)2 in p-DTS(FBTTH2)2. Crystallinity 30% 40% 50%

PC70BM 20% 23% 26%

bis-PCBM 19% 21% 25%


ICBA 14% 16% 19%

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Table 4 Device performance at standard AM-1.5 illumination. VOC (V) FF (%) JSC (mA/cm2) PCBM bis-PCBM ICBA

12.8± 0.2 11.2± 0.2 6.8± 0.1

0.82± 0.02 0.88± 0.02 1.03± 0.03

66.0± 0.4 65.0± 0.4 58.4± 0.4

PCE (%) 6.9± 0.2 6.4± 0.2 4.1± 0.2

Table 5 Surface-to-surface distance between homogeneously dispersed fullerenes in the p-DTS(FBTTH2)2:fullerene miscible phase for a given p-DTS(FBTTH2)2 crystallinity. Crystallinity

PC70BM (Å)

bis-PCBM (Å)

ICBA (Å)

30% 40% 50%

7.7 6.7 6.1

10.9 9.9 8.2

14.2 12.7 10.9



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Table of Content


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Impact of Fullerene Structure on Nanoscale Morphology and

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