Rationalization of the Selectivity in the Optimization of Processing

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Rationalization of the Selectivity in the Optimization of Processing Conditions for High-Performance Polymer Solar Cells Based on the Polymer Self-Assembly Ability Han Yan, Lingyun Zhu, Denghua Li, Yajie Zhang, Yuanping Yi, Zhixiang Wei, Yanlian Yang, and Jean-Luc Bredas J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 21 Nov 2014 Downloaded from http://pubs.acs.org on November 21, 2014

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The Journal of Physical Chemistry

Rationalization of the Selectivity in the Optimization of Processing Conditions for High-Performance Polymer Solar Cells Based on the Polymer SelfAssembly Ability Han Yan,a,b Lingyun Zhu,*a,c Denghua Li,a,b Yajie Zhang,a Yuanping Yi,d Yanlian Yang,

a

Zhixiang Wei,*a Jean-Luc Brédasc a

National Center for Nanoscience and Technology, Beijing 100190, P. R. China. b

c

University of Chinese Academy of Sciences, Beijing 100039, P. R. China.

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 d

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.

ACS Paragon Plus Environment

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KEYWORDS: Polymer solar cells, structure-property relationship, optimizing processes, electrostatic force microscopy, self-assembly ABSTRACT: Tailoring the blend morphology in a bulk heterojunction (BHJ) device is of critical importance but remains a challenge today. Although the morphologies of polymer solar cells (PSCs) can be tuned by thermal/solvent annealing or by incorporation of solvent additives, optimizing the morphology of the active layer for a newly synthesized polymer has to date remained mostly an empirical approach. In this work, three typical polymers in organic photovoltaics have been studied. By processing at different conditions, each polymer reveals high-selectivity in the optimizing methods. Optical spectrum and electrostatic force microscopy results demonstrate morphology as the main reason for various device performances. Further, these can be traced back to the self-assembly behaviors of polymers. By the established relationships between molecular structure, morphology and corresponding device performances, we propose a self-assembly based process-selection guideline for efficient performance improvement of newly synthesized materials.

INTRODUCTION Polymer solar cells (PSCs) are promising solar energy conversion devices because of their lowcost, light-weight, solution-processability and mechanical flexibility.1–4 The power conversion efficiency (PCE) of these cells has steadily improved from 5% to 10% over the past couple of years as a result of new materials development. The working mechanism of PSC has two basic requirements regarding the morphologies of their active layers. 5–9 First, the sizes of the donor and acceptor domains should maximize the exciton dissociation processes.10–12 Second, a continuous charge-transport pathway must exist for holes and electrons to prevent at best charge


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recombination before the carriers are extracted at their corresponding electrodes.13–18 The bulk heterojunction (BHJ) architecture is the most straightforward solution to meet these requirements.19–22 Therefore, tailoring the morphology of the blend in a BHJ device to optimize its performance is of critical importance. The morphologies of PSCs can be tuned by thermal/solvent annealing23–26 or by incorporation of solvent additives.27 Annealing can increase polymer crystallinity and improve diffusion of PCBM into aggregates of appropriate size.28–31 Additives essentially adjust the solubility of the polymer donor and/or the fullerene acceptor; therefore, their self-assembly process is altered. Tuning the morphology of the active layer to optimize device performance for a newly synthesized polymer has to date remained mostly an empirical approach. Therefore, understanding the relationship between the molecular structure and its assembly behavior could provide a more rational selection principle for PSC device optimization. In this work, we systematically study a number of processing methods geared to optimize PSC performance, by using electrostatic force microscopy (EFM) technique. A process selection principle is proposed to interpret the correlation between improved performance and optimized conditions associated with the molecular structures and their self-assembly ability. Three representative polymers are selected as electron donor, namely, poly(3-hexylthiophene) (P3HT), (poly[(4,4’-bis(2-ethylhexyl)dithienol[3,2-b:2’,3’-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)4,7-diyl]) (PSBTBT), and thienyl-substituted Benzo[1,2-b:4,5-b’]dithiophene (BDT) with substituted thienothiophene (PBDTTT-C-T). A representative fullerene derivative, [6, 6]-phenyl C71-butyric acid methyl ester (PC71BM), is used as electron acceptor. The photovoltaic performances demonstrate that the three polymer/PC71BM systems adopt different optimizing processes. Combining with the morphology and molecular simulation results, we find the


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different processing conditions are strongly dependent on the self-assembly behaviors of polymers. Based on these results, a new process-selection guideline based on the polymer selfassembly abilities is proposed for improving the performances of newly synthesized materials.

RESULTS and DISCUSSION The chemical structures of the conjugated polymers are shown in Figure 1a. These three polymers typically display their highest efficiencies as a result of different processing conditions of the active layers. Thus, the use of these polymers provides a platform to investigate the relationship between processing condition and device performance. Five typical processing conditions are used to tune the morphologies of the active layers, namely: (i) thermal annealing; (ii) adding 1, 8-diiodooctane (DIO) as an additive; (ⅲ) adding DIO and then thermal annealing; (iv) adding 1-chloronaphthalene (CN) as an additive; and (v) adding CN and then thermal annealing. In order to compare performance among devices fabricated under these different conditions, standard sandwich-type PSCs are fabricated with a hole-collecting layer of poly (3,4ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS) coated on an ITO glass substrate and an electron-collecting layer of Al (80 nm)/Ca (20 nm). We test all devices by using AM1.5G at 100 mW/cm2. Figure 1b-d show the current density-voltage (J-V) curves of the PSCs. The photovoltaic performance data, which include the open-circuit voltage (Voc), shortcircuit current density (Jsc), fill factor (FF), and PCE values, are summarized in Table 1 (more detailed data are listed in Tables S1-S9). For the P3HT:PC71BM system, the pristine devices show the lowest PCE at 1.17%. All processing conditions worked well in this system. Thermal annealing at 150 °C for 30 min with the addition of CN leads to the highest PCE value of 4.12%, (Detailed device performances


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under different thermal annealing conditions are summarized in Tables S1-S9). When the detailed components are taken into account, with the addition of CN and then thermal annealing, a high FF of 70.02% is obtained to render the highest PCE value. For the PSBTBT:PC71BM system, adding a small amount of CN (4%) results in the highest PCE value of 4.59%. This system shows a higher Voc (0.68V) than P3HT-based devices as the ionization potential is increased (lower energy for the highest occupied molecular orbital (HOMO) level). Moreover, a higher Jsc of 13.23 mA/cm2 is also observed, which is due to a broader absorption range of the polymer. However, the relatively low FF of 50.68% negatively impacted the overall PCE. Both annealing and adding DIO have a negative effect on this system. For PBDTTT-C-T:PC71BMbased devices, adding 3% DIO substantially improves the PCE from 4.55% to 7.58%. The components all improve as well: Voc of 0.79V, Jsc of 15.67 mA/cm2, and FF of 61.26%. Either annealing or adding CN decreases the PCE value for PBDTTT-C-T:PC71BM-based devices. For the sake of comparison, we have listed the pristine and optimal device performances in Figure 1e. These results show high selectivity of optimization for each system. Improved performances are often ascribed to more favorable morphology with better molecular order and phase separation; the effect of each process on the morphology evolution should then be clarified in order to better understand this highly selective process. The effect of different processes on morphology can be mainly ascribed into two aspects: molecular stacking order and phase purity. Here we use absorption and fluorescence spectrums to illustrate the roles of various processes. Better molecular stacking order is accompanied with red-shifted absorption peak and increased π-π stacking shoulder peaks;6, 32 phase purity can be evidenced by altered quenching extent.33-35 All the results are summarized in Figure 2. By applying processes on P3HT blend films, π-π stacking shoulder peaks increase for all samples


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with different intensities (Figure 2a). However for PSBTBT blend film, thermal annealing can no longer increase the shoulder peak intensity; adding DIO or CN will even decrease it, although a bit stronger peak intensity in CN treated film (Figure 2c). When we check with the PBDTTTC-T blend films, all processes can induce a red-shift of the absorption peak slightly, and using DIO as additive works best among those processes (Figure 2e). The roles of various processes on fluorescence quenching look similar (Figure 2b, d and f). All photoluminescence signals are divided by the corresponding absorption intensity to exclude the effect of film thickness. Except for adding DIO as additive can decrease the quenching extent, other processes show similar results with pristine films. The spectroscopic results are based on the sum of a large area, and it cannot be directly correlated with one specific morphology. To better understand the structureproperty relationships, more detailed local morphology has to be detected. Here, we use electrostatic force microscopy (EFM) 36-42 to characterize the local morphology of a polymer:fullerene blend film. As indicated in our previous reports, the contrast in the EFM images mainly originates from the energy-level offsets. The energy-level offsets between the materials and the conductive tip contribute to change the vertical force gradient; thus, the corresponding changes in resonant frequency and phase shift of the cantilever is detected (more details shown in supporting information). With the aid of EFM, we can clarify the processing effects on the phase behaviors of the polymer:PC71BM blends (Figure 3; the corresponding height images are shown in Figure S1). The bright and dark regions observed in the images correspond to the polymer donor and fullerene acceptor, respectively. We first study the P3HT:PC71BM blend films. The film without any treatment is nearly featureless and has the smallest phase contrast (Figure 3a). After thermal annealing at 150 °C for 30 min, sphere-like PC71BM aggregates are embedded in the polymer network with a larger


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phase contrast; however there are still lack of obvious P3HT nanowires in blend film (Figure 3b). Adding a small amount of either DIO or CN induces the assembly of P3HT into nanowires (Figure 3c and d). These are consistent with the absorption spectrum (Figure 2a). The additives will also induce a larger extent of phase separation and decrease the charge separation (Figure 2b). In the case of macroscopic device performance, the thermal annealing treatment with addition of CN appears to result in interpenetrating networks with suitable phase separation. For the PSBTBT:PC71BM system, the pristine film shows sphere-like PSBTBT distinct phases (Figure 3e). After thermal treatment, the sphere-like PSBTBT domains seem to partially melt into stripe-like domains with a width of about 50 nm, which is a less favorable size for phase separation (Figure 3f). These microscopic images can explain for the puzzle that the better stacking order induce poorer device performance. When a small amount of DIO is added, polymer-rich and PC71BM-rich domains are observed in the blend film. This result is quite similar to Figure 3b but with a larger domain size (Figure 3g). Moreover, the domains seem to be isolated from one another, with an absence of bridges between the domains. This lack of bridges is disadvantageous for charge transport. When DIO is replaced with CN, polymer- and fullerene-rich domains are also observed. Unlike DIO, it can be noticed that the phase separation extent is smaller and PSBTBT whiskers appear between the polymer-rich domains (Figure 3h). A better assembly of PSBTBT can form a polymer phase with interpenetrating networks. Thus, the issue related to non-continuous charge pathways in these blend films appears to be solved. Finally, we look at the PBDTTT-C-T:PC71BM blend film. The untreated film is featureless and lacks a proper phase separation (Figure 3i). Thermal annealing or adding CN in the solvent induces substantial phase separation (Figure 3j and l). The films can be separated into three types of domains: large polymer domain, large PC71BM domains and isolated mixed domains.


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Combined with the absorption spectrum, we know that the large polymer domains are actually with poor stacking order (Figure 2e). The disorder over-separated phases decrease the charge separation; while the free charges from mixed phases lack effective charge transport pathways. These EFM images distinguish the morphology from similar quenching extent and explain the reason for better charge separation with lower Jsc and FF. When DIO is added instead, a uniform phase separation is observed in the blend film (Figure 3k). According to the spectrum results, the increased phase purity also induces a better molecular order here. Similar to Figure 3h, polymer-bridges that link neighboring domains are observed. According to previous reports, this kind of polymer tends to exhibit a hierarchical phase separation with PC71BM in the blend film and polymer-rich domains are favorable for hole transport and collection at the electrode.43, 44 Combining the above analyses and previous results, we conclude that different processing conditions have different effects on the phase behavior in the active layer of PSCs. Thermal annealing increases phase separation by slightly enhancing polymer assembly and PC71BM diffusion in the matrix. The effect of CN is quite similar to thermal annealing. The high boiling point and good solubility of both polymer and PC71BM extend self-assembly time leading to enhanced crystallinity. However, the diffusion of PC71BM in solution is much faster than in the solid state, thereby leading to a larger extent of phase separation compared with the thermal annealing method. DIO has a selective solubility for PC71BM compared to the polymer. PC71BM can be extracted from the blend solution because of its good solubility; thus, it can form a relatively uniform phase separation with increased purity while sometimes also increase crystallinity. The high selectivity of the processing methods mainly originates from different phase behaviors of these systems, especially the assembly properties of the polymers. We thus


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studied the phase behavior of each pure polymer and combined with theoretical calculations to explain these differences and to gain deeper understanding on this problem. Scanning electron microscopy (SEM) is used to observe the assembly morphology of each polymer cast from oDCB. Different assembly structures are observed for different polymers under the same condition. P3HT forms well-crossed nanowires with a uniform distribution (Figure 4a). PSBTBT forms nanowhisker bundles (Figure 4c), which are much shorter than in P3HT. For PBDTTT-C-T, only large irregular particles are observed and these particles lack well-assembled nanostructures (Figure 4e). Based on these results, we can easily understand the processing selectivity for each material system. P3HT has a very good self-assembly ability; in this instance, all processes work. Thermal annealing with addition of CN is an optimizing method that controls a suitable size of phase separation with highly ordered assembly structure. For PSBTBT, its relatively poorer assembly ability makes the increase in the formation of nanowires most important. Annealing has no obvious effect on polymer assembly and phase mixing. Therefore, adding CN is the most promising method. Finally for PBDTTT-C-T, it tends to aggregate into large domains; and controlling a uniform size of phase separation will be beneficial for the overall PCE value. Therefore, using DIO as an additive would effectively control the scale of phase separation. The self-assembly ability is further confirmed by X-ray diffraction (XRD) characterization (Figure 4b, d and f). As shown from these patterns, the assembly of polymers mainly occur in two orientations, namely, the side-chain stacking direction [(100)] and the π-π stacking direction [(020)]. The presence of pair of characteristic peaks demonstrate that all three polymers stack as a lamellar structure in assembled morphologies. Combined with the SEM images, it is interesting to find that the assembly morphology changes with the intensity ratio of (100) to (020). With the


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decrease of this value, the assembly structure varies from anisotropic to isotropic. Here, we should emphasize that in our diffraction geometry, we can only detect the signals originating from the crystal faces which are parallel to the substrate; although other faces could also generate signals, and we cannot record them with our experimental set-up. We find the following correlation between molecular assembled morphologies and their packing orientation: the π-π stacking direction of P3HT and PSBTBT are parallel to the substrate and tend to assemble into one-dimensional nanowires; however, the π-π stacking direction of PBDTTT-C-T reveals both parallel and vertical to the substrate, and tends to form three-dimensional irregular particles. Furthermore the broaden peaks demonstrate PBDTTT-C-T as poorer crystallinity, molecules tilt in a range deviated from the main stacking directions.45-47 A description of the lamellar structure is shown in Figure 5a. This model illustrates the formation of lamellar structures as mainly due to two factors: firstly, polymers fold into “unit layers”; secondly, “unit layers” stack into integrated lamellar structure by π-π stacking interactions. Π-π interactions between neighboring “unit layers” is the key factor to form onedimensional nanostructures. Thus, in fact, discussing the relationship between molecular structure and assembled morphology means discussing the driving force for one-dimensional assembly, namely π-π interactions. From the XRD results above, π-π stacking interactions for P3HT and PSBTBT exist only parallel to the substrate. However, PBDTTT-C-T stacks both parallel and vertical to the substrate. Actually, the dimension of the assembled structure is decided not only by the direction of the driving force, but also by its magnitude. From the view of thermal dynamics, self-assembly is accompanied by entropy reduction, thus enthalpy dominates the assembly process. Thus, the competition between surface energy and backbone torsion potential dominates the assembly process.


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Backbone torsion potential is indeed another important factor that affects molecular assembly. Here, we have quantitatively evaluated this parameter by considering the differences in dihedral angles between neighboring units in the monomers and corresponding dimers. Theoretical calculations are performed to simulate the torsion potential of the monomers and the molecular packing of the dimers (see Figure 5b and c). The side chains are set to methyl groups during the calculations. The geometries of the dimers are optimized using Density Functional Theory (DFT) methods with the B3LYP functional and Grimme’s dispersion corrections (B3LYP-D),

48, 49

and

the 6-31G (d, p) basis set. Initial guesses for the dimer geometries are constructed by varying the intermolecular distances and orientations. All calculations are carried out with the Gaussian 09 program. The dimers can take on different configurations, such as parallel-cofacial, parallelshifted, parallel-flipped, and T-shaped (see Figure S2-S4). From the energy-minimization calculations, the parallel-cofacial configuration has the lowest energy; the optimized geometries are shown in Figure 5c (other configurations are illustrated in Figure S2-S4). For the monomers, we find torsion angles of 34.6°for P3HT, 0.0°for PSBTBT, and 19.5°for PBDTTT-C-T. For the dimers, the corresponding dihedral angles are 35.8º and 28.0º for P3HT; 15.2º and 15.3º for PSBTBT; 14.9º and 11.3º for PBDTTT-C-T. According to the torsion potential curves given in Fig. 4b, the corresponding torsion potentials are increased by 0.16 kcal/mol for P3HT, 1.85 kcal/mol for PSBTBT and 0.24 kcal/mol for PBDTTT-C-T from monomer to dimer, respectively. The order of increase in backbone torsion potential is PSBTBT » PBDTTT-C-T > P3HT, which points a decreased molecular assembly tendency for these polymers. Based on the previous discussions, we can clearly understand the variations in the assembly abilities of these three polymers. P3HT shows preferred direction and magnitude of assembly driving force, and easily assemble into one-dimensional nanowires. Although PSBTBT has a


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preferred assembly direction, there occurs a large increase in torsion potential during the assembly process，which inhibits its assembly ability. PBDTTT-C-T has three-dimensional assembly directionality and strong assembly driving force; thus, it easily forms paracrystalline large-scale irregular particles. Overall, the differences in assembly at the molecular level is reflected in the different aggregate morphologies and the different phase separation behaviors in the blend films, and eventually in the different processing effect on device performance. CONCLUSION In conclusion, we have systematically studied the effects of different optimization processes on three material systems. Each system demonstrates a high processing selectivity. By investigating the effect of processes on blend film morphology we correlate the high selectivity with polymer self-assembly behaviors. The optimization of P3HT, which tends to form regular nanowires, can be achieved through a suitable extent of phase separation; thermal annealing with addition of CN can satisfy this optimization. In the case of PSBTBT, which has lower assembly ability, thermal annealing has no effect on this polymer, a more effective method is needed to enhance the polymer assembly; thus, adding a small amount of CN is what works for this system. PBDTTT-C-T tends to form large-scale aggregates with lower crystalline; the best result here can be obtained when DIO is used to increase phase purity while maintain phase separation scale with higher crystalline. Theoretical study further connects the self-assembly behaviors with molecular driving force. We believe that our work provides a better understanding of structureproperty relations and offers a rational process-selection guideline for efficient performance improvement of newly synthesized materials.


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EXPERIMENTAL SECTION

1. Fabrication of PSCs Indium-tin oxide (ITO)-coated glass (10 Ω sq-1) was cleaned successively with deionized water, ethanol, acetone, and isopropyl alcohol thrice for 15 min, and then dried at 150 °C for 30 min. After completely drying, the ITO glass was exposed to UV-ozone for 15 min. PEDOT:PSS was filtered through a 0.45 µm syringe filter, spin coated for 40 nm thickness, and then annealed at 140°C for 10 min in ambient atmosphere. P3HT (17 mg/mL), PSBTBT (10 mg/mL) and PBDTTT-C-T (10 mg/mL) were separately blended with PC71BM in o-dichlorobenzene (oDCB) with 1:1, 1:1 and 1:1.5 weight ratios, respectively. The solutions were first stirred at 70℃ for more than 3 hours, and then at 40℃ for over 10 hours. 3% DIO or 4% CN were added before spin-coating, and stirred for 1 hour. PSCs were processed in a N2-filled glove box. The spincoated films were fabricated using different solutions. Thermal annealing was performed prior to electrode deposition. After the whole film-forming process, Al (80 nm)/Ca (20 nm) were thermally evaporated on top of the active layer under a vacuum lower than 2×10-6 mbar. The active area of the standard PSC devices was 0.04 cm2. 2. Measurements and Characterizations The photovoltaic performance was measured using a Keithley 2400 sourcemeter under AM1.5G (100 mW/cm2) simulated by a solar simulator (Newport Oriel). The light intensity was calibrated using a photodiode and light source meter prior to measurement. For in-situ measurement and characterization, the samples were processed using the same method as the PSC production. Optical absorption spectra of the samples were recorded on a UV-vis-IR spectrometer (PE Lambda 650/850/950 UV-vis spectrophotometer). And the corresponding photoluminescence spectra of samples were measured on photoluminescence spectrophotometer (LS-45/55


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Fluorescence Spectrometer) with a Xe lamp. EFM measurements were performed with the AFM (Dimension icon, Bruker Nano) system for non-contact characterization. The morphologies of polymer nanostructures were characterized by scanning electron microscopy (SEM) (S4800, Hitachi, Japan). X-ray diffraction (XRD) (Philips X’Pert using Cu Kα line 0.15419nm) was conducted directly on pure polymer film samples. 3. Assembly of Polymer Nanostructures For the preparation of nanostructures, the polymer was first dissolved in oDCB with a concentration of 0.1 mg/mL. A drop of solution was injected on a silicon substrate which was kept in a closed jar. By a slow solvent evaporation rate, the nanostructures assembled on the substrate.


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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *Zhixiang Wei, National Center for Nanoscience and Technology, Beijing 100190, P. R. China; Fax: 86-10-62656765; Tel: 86-10-82545565; E-mail: [email protected]. * Lingyun Zhu, National Center for Nanoscience and Technology, Beijing 100190, P. R. China; Fax: 86-10-62656765; Tel: 86-10-82545669; E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The work in Beijing was supported by the National Natural Science Foundation of China (Grants 20974029 and 91027031), Ministry of Science and Technology of China (2009CB930400, 2010DFB63530, 2011CB932300), and Chinese Academy of Sciences. The work in Atlanta was supported by the Office of Novel Research under Award No. NOOO14-111-0211. Supporting Information P3HT (Sigma-Aldrich, Mn ~64,000, 98.5% regioregular), PSBTBT (Luminescence Technology Corp., Mn 10000-30000), PBDTTT-C-T (Solarmer Energy. Inc.), and PC71BM (American Dye Source, Inc., 99.5%) were used as received. Poly(3,4-ethylenedioxy thiophene):poly(styrene


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sulfonate) (PEDOT:PSS) (Baytron PVP Al 4083) was purchased from H. C. Stark. This information is available free of charge via the Internet at http://pubs.acs.org REFERENCES (1) Krebs, F. C.; Espinosa, N.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M. Rise to Power OPV-Based Solar Parks. Adv. Mater. 2014, 26, 29-39. (2) McGehee, M. D. Materials Science: Fast-Track Solar Cells. Nature 2013, 501, 323-325. (3) Hösel, M.; Søndergaard, R. R.; Jørgensen, M.; Krebs, F. C. Failure Modes and Fast Repair Procedures in High Voltage Organic Solar Cell Installations. Adv. Energy Mater. 2014, 4, 1301625-1301631. (4) Espinosa, N.; Hösel, M.; Jørgensen, M.; Krebs, F. C. Large Scale Deployment of Polymer Solar Cells on Land, on Sea and in the Air. Energy Environ. Sci. 2014, 7, 855-866. (5) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nat. Photon. 2012, 6, 591-595. (6) Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D. Controlling the Morphology and Performance of Bulk Heterojunctions in Solar Cells. Lessons Learned from the Benchmark Poly(3-hexylthiophene):[6,6]-Phenyl-C61-butyric Acid Methyl Ester System. Chem. Rev. 2013, 113, 3734-3765. (7) Rivnay, J.; Mannsfeld, S. C. B.; Miller, C. E.; Salleo, A.; Toney, M. F. Quantitative Determination of Organic Semiconductor Microstructure from the Molecular to Device Scale. Chem. Rev. 2012, 112, 5488-5519.


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(37) Douhéret, O.; Swinnen, A.; Bertho, S; Haeldermans, I.; D’Haen, J.; D’Olieslaeger, M.; Vanderzande,

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Page 23 of 29

a) O S S S S

Si

S

S

N

N

n

S

S S

n

Pristine

5

DIO CN

Annealed DIO+Annealed CN+Annealed

0 -5 -10 0.0

10

0.2

0.4 Voltage (V)

0.6

Pristine DIO CN

0

PSBTBT:PC71BM Pristine DIO CN

5 0


-5 -10 -15

0.8

e)

PBDTTT-C-T:PC71BM

5

10

0.0

0.2

0.4 Voltage (V)

0.6

8


0.8

DIO

6 PCE (%)

d)

c)

P3HT:PC71BM

2

10

Current Density (mA/cm )

Current Density (mA/cm 2)

b)

n

S

S

Current Density (mA/cm 2)

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


-5 -10

CN

CN+Annealed

Pristine

Pristine

4 2 Pristine

-15 0.0

0.2

0.4 0.6 Voltage (V)

0.8

1.0

0

P3HT

PSBTBT

PBDTTT-C-T

Figure 1. a) Chemical structures of P3HT, PSBTBT and PBDTTT-C-T. b-d) J-V curves of PSCs fabricated in various conditions: b) P3HT:PC71BM (1:1wt%); c) PSBTBT:PC71BM (1:1wt%); d) PBDTTT-C-T:PC71BM (1:1.5wt%). The thermal annealing condition is fixed at 150℃ for 30min. Under this condition, P3HT:PC71BM blend film shows the best conversion efficiency; PSBTBT:PC71BM and PBDTTT-C-T:PC71BM blend films show distinct variations from pristine films. e) The histograms of pristine and optimal PCE values for different material systems.


23

e)

0.0 300

400

1.0

Pristine Annealed With DIO With CN

500 600 Wavelength (nm)

400

1.5

0.5

500 600 700 Wavelength (nm)

750

800

1.0

800

900 Pristine Annealed With DIO With CN

PL Intensity (a.u.)

500 600 700 Wavelength (nm)


400


d)

2.0

300

600

700

0.5

0.0 300


PL Intensity (a.u.)

0.5

Page 24 of 29

b)


f)

900


PL Intensity (a.u.)

c)

Pristine Annealed With DIO Wtih CN

1.0

Normalized Abs Intensity (a.u.)

a)


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



700

750


900

Figure 2. a, c and e) Absorption spectroscopy of blend films prepared from different processes: a) P3HT/PC71BM; c) PSBTBT/PC71BM; e) PBDTTT-C-T/PC71BM. b, d, f) Corresponding fluorescence quenching spectroscopy of blend films prepared from different processes: b) P3HT/PC71BM under 520nm excitation; d) PSBTBT/PC71BM under 680nm excitation; f) PBDTTT-C-T/PC71BM under 660nm excitation. The thermal annealing is taken 150 oC for 30 min. All samples with additives are not further annealed.


24

Page 25 of 29

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 3. EFM images of different polymer:PC71BM blend films (2 µm×2 µm). a-d) P3HT:PC71BM blend films (1:1 wt%) a) pristine; b) annealed at 150 oC for 30 min; c) DIO as additive; d) CN as additive. e-h) PSBTBT:PC71BM blend films (1:1 wt%): e) pristine; f) annealed at 150 oC for 30 min; g) DIO as additive; h) CN as additive. i-l) PBDTTT-C-T:PC71BM blend films (1:1.5 wt%): e) pristine; f) annealed at 150 oC for 30 min; g) DIO as additive; h) CN as additive. All samples with additives are not further annealed.


25


(100)

Intensity (a.u.)

b)

(020)

5

10

15

20 25 Degrees(o)

30

35

40

30

35

40

30

35

40

(100)

Intensity (a.u.)

d)

(020)

5

10

15

20 25 Degrees(o)

f) Intensity (a.u.)

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

Page 26 of 29

(020)

(100)

5

10

15

20 25 Degrees(o)

Figure 4. a, c and e) SEM images of the polymers cast from oDCB: a) P3HT; c) PSBTBT; and e) PBDTTT-C-T. b, d and f) XRD patterns of the polymers corresponding to a, c and e.


26

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


b)

6 Relative Energy (kcal/mol)

Page 27 of 29

P3HT PSBTBT PBDTTT-C-T

5 4 3 2 1 0 0

60

120 180 240 Torsion Angle (degree)

300

360

Figure 5. a) Schematic illustration of lamellar structure for assembled polymers. The insets show the optimized monomer geometries (top-view). b) Backbone torsion potentials between neighboring units calculated at the B3LYP level for monomers. c) Optimized DFT/B3LYP-D dimer geometries (up: top-view; down: side-view) and the corresponding dihedral angles (dihedral angles between the neighboring units).


27


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

Page 28 of 29

Table1. Performance of PSCs for different material systems fabricated under various processing conditions. The annealing process is at 150℃ for 30 min for all samples. Materials

Processing methods

P3HT:PC71BM

PSBTBT:PC71BM

PBDTTT-C-T:PC71BM

a

2

Voc (V)

Jsc (mA/cm )

FF (%)

PCEmax (%)

PCEavg (%)

Pristine

0.67

3.67

47.50

1.17

1.04

Annealing

0.62

9.43

64.48

3.77

3.42

DIO

0.55

9.78

47.59

2.56

2.44

DIO+Annealing

0.64

9.32

60.94

3.63

3.50

CN

0.61

9.35

64.25

3.66

3.53

CN+Annealing

0.63

9.36

70.02

4.12

4.03

Pristine

0.68

10.87

51.07

3.75

3.66

Annealing

0.67

10.47

49.81

3.52

3.46

DIO

0.66

11.87

52.43

4.10

4.02

DIO+Annealing

0.66

12.97

45.85

3.94

3.82

CN

0.68

13.23

50.68

4.59

4.50

CN+Annealing

0.68

11.91

51.99

4.21

4.13

Pristine

0.83

12.20

44.95

4.55

4.43

Annealing

0.72

11.50

33.21

2.77

2.69

DIO

0.79

15.67

61.26

7.58

7.46

DIO+Annealing

0.71

8.98

32.27

2.06

1.96

CN

0.86

11.60

46.59

4.65

4.51

CN+Annealing

0.68

9.97

31.79

2.16

2.02

a

Average PCE of ten devices fabricated under identical conditions.


28

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BRIEFS Three polymer/fullerene systems show high-selectivity of processing conditions on photovoltaic performances, and these can be traced back to variations in the self-assembly behaviors of the three polymers. SYNOPSIS


29

Rationalization of the Selectivity in the Optimization of Processing

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