Polymorphic Phase-Dependent Optical and Electrical Properties of a

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Polymorphic Phase Dependent Optical and Electrical Properties of Diketopyrrolopyrrole based Small Molecule Shabi Thankaraj Salammal, Zhongqiang Zhang, Jiehuan Chen, Basab Chattopadhyay, Jiake Wu, Lei Fu, Congcheng Fan, and Hongzheng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05084 • Publication Date (Web): 19 Jul 2016 Downloaded from http://pubs.acs.org on July 21, 2016

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ACS Applied Materials & Interfaces

Polymorphic Phase Dependent Optical and Electrical Properties of Diketopyrrolopyrrole based Small Molecule

Shabi Thankaraj Salammal†, Zhongqiang Zhang†, Jiehuan Chen†, Basab Chattopadhyay‡, Jiake Wu†, Lei Fu†, Congcheng Fan† and Hongzheng Chen†*

†

State Key Laboratory of Silicon Materials, MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P.R.China.

‡

*

Universite Libre de Bruxelles (ULB), Faculte des Sciences, Laboratoire Chimie des Polymeres, CP 206/1, Boulevard du Triomphe,1050 Bruxelles, Belgique.

Corresponding author: [email protected]

Key words: Diketopyrrolopyrrole, polymorphism, π-π stacking, optical and electronic property, organic electronics.

1

ACS Paragon Plus Environment


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Abstract Four different polymorphic conformations of diethyl 5,5'-(5,5'-(2,5-bis(2-ethylhexyl)3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis(thiophene-5,2-diyl))difuran-2carboxylate (DPP-(CF)2), namely DPP-(CF)2-α, DPP-(CF)2-β, DPP-(CF)2-γ and DPP-(CF)2-ω, were identified from the X-ray diffraction analysis conducted on their thin films and single crystals. Highly crystalline and well textured thin films of these four polymorphs were successfully prepared via post-growth solvent vapour and thermal annealing treatments to investigate the polymorphic phase dependent optical and electrical properties of DPP-(CF)2. Interestingly, during the phase transition from DPP-(CF)2-α to DPP-(CF)2-ω, the optical bandgap decreases from 1.75 to 1.5 eV due to the enhanced π-π interaction between the neighbouring molecules. Except DPP-(CF)2-γ, the other three phases show ambipolar charge transport. Although DPP-(CF)2-β and DPP-(CF)2-γ exhibit a similar way of packing, a small increment in the π-π stacking distance (0.006 Å) and twist conformation of the grafted electron donating moieties of DPP-(CF)2-γ are found to reduce its hole mobility.

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INTRODUCTION Organic semiconductors (OSs) are believed to be the building blocks of future low-cost, flexible, thin and large area organic electronic devices such as organic solar cells (OSCs), organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), etc.1 In addition to the morphology of thin films, the reorganization energy (energy required to modify the geometry of a molecule when an electron is added or removed) and electronic coupling (strength of orbital overlap between the neighboring molecules) are very important for the effective charge transport via hopping.2-6 This intermolecular electronic coupling highly depends on the geometrical arrangement of the molecules.7 In particular, the closely cofacially (face-to-face) π-stacked pentacene molecules were reported to have the highest electronic coupling as compared to the face-to-edge on arrangement.3 Even, small changes in the electronic coupling between the neighboring molecules was reported to result in a few orders of magnitude difference in the charge carrier mobility, due to the variation in electron/hole transfer integrals.8-9 The field effect mobility of well-studied organic small molecules such as rubrene10, tetrathiafulvalene11, sexithiophene12, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPSpentacene)8, ethynyl)

13

, 5,11-bis triethylsilylethynyl anthradithiophene14, 7, 14-bis ((trimethylsilyl)

dibenzo

[b, def]-chrysene

(TMS-DBC),15

TI(TCNQ)

(TCNQ=7,7,8,8-

tetracyanoquinodimethane)16 were reported to vary more than an order of magnitude during their solid-solid phase transition.13, 17 For example, Diao et al have documented two orders of magnitude decrease in the hole mobility of well-studied TIPS-pentacene during its phase transition from Form II to Form III.8 Such polymorphic alteration does not only affect the mobility of charge carriers, but also strongly modifies the bandgap, position of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels.18-20 Placencia et al19 have documented an increase in power conversion efficiency (PCE) of titanyl phthalocyanine (TiOPc) solar cells from 1.4 to 4.5% while partially converting phase I to phase II polymorph due to the reduction in bandgap as well as the deepening of its HOMO level from -5.2 eV to -5.4 eV. Organic small molecules are well known to crystallize in various polymorphs because of the very weak interaction between the molecules (mostly of van der Waals type).7, 13, 21 Such polymorphism in organic semiconductors are mainly induced by the processing solvents10-11, 22-24

, deposition temperature12, 15, substrate induced phases,25 deposition techniques7-8, 23, 26, 3



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post-growth treatments like thermal annealing and solvent vapor annealing (SVA)24, 27 etc. The polymorphic alteration of OSs can hinder the reliability, reproducibility and stability of the devices. The post growth thermal and solvent vapor annealing methods are commonly applied to the newly synthesized molecules for the realization of efficient organic electronic devices28-31, which can greatly alter the molecular packing. However, understanding the role of polymorphism on the performances of organic electronics is relatively scarce, and most of such studies were conducted using the above mentioned representative organic semiconductors.8, 10-12, 15-16 Followed by those representative OSs, various narrow bandgap small molecules and polymers have been synthesized by grafting various electron rich moieties such as, thiophene, phenyl, bithiophene, thienothiophene, furan, naphthalene, selenophene etc., on the electron deficient moieties such as, dioxo-3,6-diarylpyrrolo[3,4c]pyrroles (DPP)32-34, isoindigo35, benzothiadiazole36, etc. Among all, DPP based small molecules and polymers were reported to be one of the best performing organic semiconductors. Especially, record high mobility of 17.8 cm2/Vs was achieved for the DPP based polymer.37 Electron deficient DPP chromophore is widely used to synthesize various narrow bandgap OSs because of its exceptional environmental stability, self-organization nature, ease of derivatization, ambipolar charge transport characteristics and fluorescent nature.38-40 In such a donor-acceptor system, the HOMO was reported to upshift on grafting the electron donating units, which can suppress the open circuit voltage (Voc) of the solar cells. Therefore, these molecules are further grafted with carboxylate, carbonyl, fluorine, cyanide, etc., to bring down the HOMO level without affecting their bandgap.28, 38 These functional groups, however, can make the molecule bulky and twisted and suppress the close packing of neighboring molecules.21, 24, 32, 41 Although various DPP based small molecules have been synthesized and studied, understanding the role of polymorphism on the performance of DPP based small molecule OFETs and OSCs is relatively scarce15. Mainly, the role of substituent such as various alkyl side chains and electron donating moieties on the molecular packing of DPP based small molecules and polymers has been investigated and been correlated with their OFET and solar cell performances.32, 37, 39, 42-45 In order to address this, we have chosen a DPP based small molecule

named

as

“diethyl

5,5'-(5,5'-(2,5-bis(2-ethylhexyl)-3,6-dioxo-2,3,5,6-

tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis(thiophene-5,2-diyl))difuran-2-carboxylate” (DPP-(CF)2). Its molecular structure is shown in Figure 1. The synthetic route, the solar cell and OFET performances of this molecule were reported in our previous article.28 In this 4


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manuscript, the role of material processing conditions of DPP-(CF)2 on the molecular packing as well as its vital role on the optical and electrical properties are reported.

Figure 1. Molecular structure of DPP-(CF)2.

EXPERIMENTAL SECTION Purification of DPP-(CF)2 Powders The synthetic route and characterization of DPP-(CF)2 can be found in our previous 28

article.

First of all, the crude product was purified twice using column chromatography

(silica gel) with a mixture of dichloromethane and Petroleum ether (5:1, v/v). Then the compound was further purified by high-performance liquid chromatography (HPLC) technique using the Shimadzu LC-6AD liquid chromatography by dissolving in HPLC grade chloroform (CHCl3). Finally, the DPP-(CF)2 powder was recrystallized using HPLC grade ethyl acetate giving highly pure brown colored DPP-(CF)2 powder, which was confirmed by nuclear magnetic resonance (NMR) and mass spectroscopic analyses.28 Thin Film Fabrication: Thin films were fabricated on two different substrates such as SiO2/PEDOT:PSS

(poly(3,4-ethylene

dioxythiophene):poly(styrene

sulfonate))

and

Si/SiO2/BCB (divinyltetramethyldisiloxane bis(benzocyclobutene)) for the structural and morphological analysis. Si/SiO2/BCB and SiO2/indium tin oxide (ITO)/PEDOT:PSS substrates were used for the OFET and solar cell analysis, respectively. In particular, ITO free SiO2 was used for the structural analysis to avoid the unnecessary scattering of it. a. SiO2/PEDOT:PSS substrate: DPP-(CF)2 thin films were spin coated on the PEDOT:PSS coated SiO2 (1.5×1.5 cm2) substrates. The substrates were ultrasonicated sequentially in dilute liquid detergent, isopropanol and acetone for 15 minutes in each and then blown dry using nitrogen gas. The substrates were further treated with UV-Ozone cleaner for 20 5



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minutes to remove the organic impurities leftover on the substrate surface. Nearly 30 nm thick PEDOT:PSS (Baytron P AI4083) was spin coated (3000 RPM for 60 seconds) on the cleaned substrates and then baked at 140 °C for 30 minutes under ambient condition. 5 mg of DPP-(CF)2 was thoroughly dissolved in 1 mL of HPLC grade CHCl3 by stirring at 50 °C in a closed vial for 5 hrs. The solution was filtered using a syringe filter that contains 0.2 µm pore size after cooling it down to room temperature (RT). The concentration, rotational speed (1000 RPM), duration (60 seconds) and amount of dropped solutions (75 µL) were kept constant throughout the analyses. Nearly 45 (± 5) nm thick DPP-(CF)2 thin films were then spin coated on the PEDOT:PSS layer and treated with various solvents to separate the polymorphic phases. Thickness of the film was measured using AlphaStep®D-100 profilometer. b. Si/SiO2/BCB Substrates: Thermally oxidized Si/SiO2 (300 nm) substrate was used to fabricate OFETs. The Si/SiO2 substrates were cleaned by immersing in Piranha solution and then rinsed with deionized water followed by blowing with dry N2. The surface of cleaned Si/SiO2 substrates were modified with BCB prior to the deposition of DPP-(CF)2 thin films.46 Nearly 40 (± 5) nm thick DPP-(CF)2 films were spin coated from chloroform solution on the Si/SiO2/BCB substrate inside the nitrogen filled glove box and then treated with various solvents. Here, the concentration, spinning speed, duration and amount of dropped solution were kept constant similar to the films spin coated on the SiO2/PEDOT:PSS substrate. However, the thickness of the film was found to be slightly lesser (≈40 nm) than the one spin coated on the SiO2/PEDOT:PSS substrate. Bottom gate top contact (BGTC) OFET was fabricated by depositing Au electrodes (source and drain) on top of the films with the aid of a shadow mask. The size of the devices are “Width = 1 mm and Length = 50 µm”. The mobility analysis was conducted inside the N2 purged glove box using a Keithley 4200-SCS semiconductor parameter analyzer. Solvent Vapor Annealing: Solvent vapor annealing (SVA) was performed using HPLC grade solvents in a 75 mL closed jar. Thin films were positioned ≈ 2 mm above the solvent level with the help of clean glass slides.31 The methanol vapor annealing was performed with air tight lid and CHCl3 vapor annealing was performed by just covering it with petri dish. Single Crystal Growth: Two various solution growth techniques such as liquid-liquid diffusion and convection process were employed to grow diffraction quality single crystals of DPP-(CF)2.47-48 Single 6


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crystal of DPP-(CF)2-α phase was obtained via convection process. 5 mg of highly pure DPP(CF)2 powder recrystallized using ethyl acetate was put into a 200 mm long narrow clean glass tube having 2 mm of inner diameter. Followed by that 600 µL of dimethyl formamide (DMF) was dropped through the wall of the tube. Half of the tube was dipped into a hot oil bath (60 °C) in a slanting position (nearly 45°). A thin needle shaped crystal was noticed to grow at the cold end of the tube (i.e. on top) after one week. Efforts have been taken to grow single crystals of DPP-(CF)2-α, but it was not successful. Single crystals of DPP-(CF)2-β was obtained via solvent/nonsolvent diffusion technique.47-48 5 mg of high purity DPP-(CF)2 powder recrystallized using ethyl acetate was thoroughly dissolved in 1 mL of dichloromethane (DCM) at RT and filtered using a syringe filter contains 0.2 µm pore size. 300 µL solution was poured into a long narrow glass tube (200 mm) containing 2 mm inner diameter. Subsequently, a layer of methanol was created on top of the solution by slowly dropping it through the wall of the tube.48 The solution was kept intact for 2 months to attain diffraction quality single crystals. Crystals were noticed to grow at the DCM/methanol interface upon their miscibility.47-48 Further increase in solution concentration or diameter of the tube results in multiple nucleations. Therefore, we have waited for long time for the evaporation of most of the solvent and thereby to obtain bigger crystals. The single crystals of DPP-(CF)2-γ phase was obtained by slowly heating the single crystals of DPP-(CF)2-β to 166 °C with the rate of 0.5 °C/min to avoid the cracking of single crystals, which may be caused by the stress evolved during the phase transition.49 The single crystals were heated under N2 ambient. X-ray Analysis of Single Crystals, Powders and Thin Films: DPP-(CF)2 single crystals were tested using “Xcalibur, Atlas, Gemini ultra” diffractometer with the wavelength of 0.7107 Å at -103 °C. The structures were solved using ShelXS-97 and refined using SHELXL. 2D grazing incidence X-ray diffraction patterns (GIXD) of thin films were recorded using a high resolution diffractometer “Rigaku MicroMax-007HF High Intensity Microfocus Rotating Anode X-ray Generator”. The scattering patterns were collected using a 2D detector, “Rigaku R-AXIS IV++”. Although this machine is made for single crystals, we have slightly modified the sample holder in order to perform the GIXD analysis with the grazing angle of 0.3°. In addition, the specular scattering patterns of thin films and powders were recorded using “UltimaIV” diffractometer. Most of the Kβ content was filtered out using a nickel plate while 7



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performing the specular diffraction patterns. A 5° soller slit was placed in front of the detector to reduce the background scattering. The diffraction patterns were collected at RT with a step size of 0.02° (2θ). Spectroscopic Analyses: UV-Vis absorption spectra of DPP-(CF)2 thin films fabricated on the SiO2/PEDOT:PSS substrate were recorded using Shimadzu UV-2450 spectrophotometer. The absorption of PEDOT:PSS was subtracted during the background subtraction using a bare PEDOT:PSS coated glass substrate. The Fourier transform infrared (FTIR) spectra of thin films were obtained by placing them on a Zinc Selenide (ZnSe) attenuated total reflectance (ATR) element using a “NICOLET5700” spectrometer. Thin films of DPP-(CF)2-α, DPP(CF)2-β, DPP-(CF)2-γ and DPP-(CF)2-ω phases were fabricated on the indium tin oxide coated SiO2 (SiO2/ITO) substrate for the Ultraviolet photoemission spectroscopic (UPS) analysis via post-growth treatments, such as five minutes of methanol vapor annealing, 10 minutes of CHCl3 vapor annealing, thermal annealing at 166 °C and 10 minutes of methanol vapor

annealing,

respectively.

UPS

analysis

was

carried

out

in

a

“Thermo

Scientific Escalab 250Xi” spectrometer using He(I) as an excitation source (21.2 eV). Differential Scanning Calorimetry (DSC) Analysis: DSC analysis was conducted using “TA Instruments Q20 Differential Scanning Calorimeter” under N2 ambient. Microscopic Analysis: The surface morphology of DPP-(CF)2 thin films were studied using a Veeco MultiMode atomic force microscope (AFM), under tapping mode. RESULTS AND DISCUSSION 1. Identification of Polymorphic Phases of DPP-(CF)2 Powder using DSC and XRD Spectroscopy Highly pure DPP-(CF)2 powder recrystallized using ethyl acetate was used for the DSC analysis with different heating and cooling rates. DSC curves of DPP-(CF)2 powder meltcooled with the rate of 50, 10 and 5 °C/min are shown in Figure 2a, 2b and 2c, respectively. The DPP-(CF)2 heated with the rate of 50 °C/min results in an endothermic peak at 209 °C that corresponds to the melting of DPP-(CF)2.28 This melt does not crystallize while cooling with the rate of 50 °C/min, i.e. absence of any exothermic peak during cooling (Figure 2a).50 This observation is consistent with the XRD analysis. In particular, the absence of any Bragg reflection in the XRD pattern of the corresponding powder confirms the amorphous nature of it (Figure S1). However, this amorphous DPP-(CF)2 crystallizes at 82 °C during the second 8


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heating cycle and results in an exothermic peak at 82 °C. It has been further confirmed by XRD analysis, i.e. evolution of few Bragg reflections after annealing the amorphous DPP(CF)2 at 82 °C (Figure 3a) confirms the crystallization of it. This phase will be further referred as DPP-(CF)2-α. Further increase in temperature results in two exothermic peaks at 116 and 166 °C with the enthalpy of 19.64 and 1.23 J/g, respectively (Figure 2a and 2d). This corresponds to the solid-solid phase transition of it, as confirmed through the XRD analysis (Figure 3b and 3c).51

Figure 2. The left side figures a, b and c depict the 1st cooling (arrows pointing towards left) and 2nd heating cycle (arrows pointing towards right) of the DSC curves of DPP-(CF)2 meltcooled with the rate of 50, 10 and 5 °C/min, respectively. (d) depicts the 2nd heating cycles of DPP-(CF)2 powders heated with the rate of 50, 10 and 5 °C/min from 140 to 190 °C. The powder pattern of amorphous DPP-(CF)2 annealed at 116 °C is shown in Figure 3b. This pattern is completely different from the one annealed at 82 °C (Figure 3a). Especially, absence of any Bragg reflection at ≈5.5° in the 116 °C annealed one confirms the solid-solid phase transition of it.8, 24, 27 This phase will be further referred as DPP-(CF)2-β. The powder pattern of 166 °C annealed one does not differ much from the 116 °C annealed one. This could be the reason why a very small value of latent heat is involved during this phase transition (Figure 2d). Interestingly, the 001 reflection of 166 °C annealed one shifts towards the higher angle, whereas the 010 reflection shifts towards the lower angle, which confirms the decrease of “d” along the alkyl side chain stacking direction and increase in “d” along the π-π stacking direction. Moreover, evolution of few Bragg reflections such as 10-2, 12-1 and 2-20 also confirms the solid-solid phase transition of it. Actually, these reflections are present in both the polymorphs, but their visibility depends on their scattering intensity as well as the shift in their Bragg angle due to the slight variation in the unit cell parameters, as confirmed through the single crystal data (Table 1). This phase will be further called as DPP-(CF)2-γ. 9



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Figure 3. XRD powder patterns of DPP-(CF)2 annealed at 82 °C (a), 116 °C (b), and 166 °C (c) after melt-cooled with the rate of 50 °C/min i.e. amorphous DPP-(CF)2. Similar to the DSC curve of DPP-(CF)2 heated with the rate of 50 °C/min, the powder heated with the rate of 10 °C/min also depicts an endothermic peak at 207 °C, which corresponds to the melting of it. On the other hand, the DPP-(CF)2 melt cooled with the rate of 10 °C/min behaves entirely different from the one cooled with the rate of 50 °C/min. It crystallizes at 116.4 °C while cooling with the rate of 10 °C/min (Figure 2b). The XRD pattern of this powder matches well with the amorphous DPP-(CF)2 annealed at 116 °C (i.e. DPP-(CF)2-β phase). Moreover, the absence of any exothermic peak at around 116 °C during the second heating (10 °C/min) cycle also confirms the crystallization of DPP-(CF)2-β phase during the first cooling itself. This phase has further noticed to alter its phase around 160 °C (Figure 2d) during the second heating cycle. This phase transition point is slightly lesser than the one heated with the rate of 50 °C/min. This could be due to the variation in the heating rate. The XRD pattern of this powder matches well with the amorphous DPP-(CF)2 powder annealed at 166 °C (i.e. DPP-(CF)2-γ). Further decrease in the heating rate down to 5 °C/min does not affect the melting point but it crystallizes at 141 °C while cooling with the rate of 5 °C/min. The diffractogram of this powder matches well with the diffractogram of DPP(CF)2-γ phase (i.e. amorphous DPP-(CF)2 annealed at 166 °C). Moreover, absence of any exothermic peak at 82, 116 and 166 °C during the second heating cycle (5 °C/min) also confirms the crystallization of it in the form of DPP-(CF)2-γ at 141°C while cooling the melt 10


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with the rate of 5 °C/min itself.50 The XRD and DSC analysis of DPP-(CF)2 confirms the presence of three various temperature dependent polymorphic modification of it. In particular, the amorphous DPP-(CF)2 powder was noticed to crystallize in three various forms, namely DPP-(CF)2-α, DPP-(CF)2-β and DPP-(CF)2-γ, when annealed at 82, 116 and 166 °C, respectively. Noteworthy, these three phases are stable at RT both in the form of thin films and bulk. They did not interconvert even after storing for more than 6 months in ambient condition as observed in other small molecules.15, 25 2. Solution and Thermal Crystal Growth and X-ray Crystallography Analysis We have grown the single crystals of DPP-(CF)2-α, DPP-(CF)2-β and DPP-(CF)2-γ phases. However, we could only solve the crystal structure of DPP-(CF)2-β and DPP-(CF)2-γ phases using the laboratory single crystal X-ray diffractometer. Since the single crystal of DPP(CF)2-α was small, we could only determine its unit cell parameters. The unit cell parameters of those three phases are presented in Table 1. The first Bragg reflection observed at ≈5.5° in the powder pattern of amorphous DPP-(CF)2 annealed at 82 °C matches well with the d001 spacing obtained through the single crystal data, which confirms the growth of single crystals of DPP-(CF)2-α. Table 1. Unit cell parameters of DPP-(CF)2-α, DPP-(CF)2-β and DPP-(CF)2-γ phases. Polymorphic Unit Lattice Parameters Phases Cell a (Å) b (Å) c (Å) α (°) β (°) *DPP-(CF)2-α Triclinic 4.21 8.64 15.37 90.3 89.5 DPP-(CF)2-β Triclinic 8.434 10.046 12.854 87.93 73.15 DPP-(CF)2-γ Triclinic 8.338 10.148 12.845 88.23 72.93 *Unit cell parameters were determined at RT.

γ (°) 103 76.33 76.68

Volume (Å3) 559.07 1012.43 1010.41

The molecular packing of DPP-(CF)2-β and DPP-(CF)2-γ phases are shown in Figure 4 and the corresponding CIF files are given in the supporting information. Both the DPP-(CF)2β and DPP-(CF)2-γ crystallize in a triclinic unit cell with the space group of P-1 and Z = 1. The DPP-(CF)2-β and DPP-(CF)2-γ pack like a brickwork motif with the intermolecular spacing (π-π stacking distance) of 3.447 and 3.453 Å, respectively (Figure 4e and 4f).24, 39 The π-π stacking distance was measured by creating an average plane with respect to the thiophene ring, since thiophene is mostly involved in the π-π interaction as compared to furan (Figure 4c and 4d). Here the end thiophene and furan are not cofacially stacked, instead they are slipped π-stacked along both the short and long molecular axis (Figure 4), which is commonly observed in the DPP based small mlecules.24, 32, 44 Interestingly, the π-π stacking 11



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distance of DPP-(CF)2-γ phase was found to be slightly higher (0.006 Å) than the DPP-(CF)2β phase. Such an increment in the π-π stacking distance might be caused by the twist conformation of DPP-(CF)2-γ as compared to that of DPP-(CF)2-β. The dihedral angles such as φ1, φ2 and φ3, which links the thiophene with DPP core, furan with thiophene and the terminal carboxyl group with furan, respectively, were found to vary with respect to the polymorphic phases (Figure 4g and 4h). The dihedral angles such as φ1, φ2 and φ3 are summarized in Table 2. DPP-(CF)2-β packs in a more planar conformation than the DPP(CF)2-γ with short π-π stacking distance, which is most important for the charge transport. The powder patterns of DPP-(CF)2-β and DPP-(CF)2-γ phases obtained by annealing the amorphous DPP-(CF)2 at 116 and 166 °C are in well agreement with the powder patterns extracted from the corresponding single crystal data using mercury 3.0 software.

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Figure 4. The left and right side column depicts the difference in molecular packing of DPP(CF)2-β and DPP-(CF)2-γ phases, respectively. Here, a and b depict the molecular packing of DPP-(CF)2-β and DPP-(CF)2-γ, respectively, view along the a axis. c and d depict π-π orbital overlap of two adjacent molecules of both the polymorphs. e and f shows the π-π stacking distance between the two molecules of both the polymorphs. g and h depict the difference in twist conformation of DPP-(CF)2-β and DPP-(CF)2-γ phases, respectively. All the hydrogen atoms are omitted for the clarity. Table 2. The dihedral angles, φ1, φ2 and φ3 of DPP-(CF)2-β and DPP-(CF)2-γ phases. Polymorphic Phases DPP-(CF)2-β DPP-(CF)2-γ

φ1 (°) 0.88 1.17

φ2(°) 2.97 4.2

φ3 (°) 3.07 2.17

π-π Stacking Distance (Å) 3.447 3.453

Noteworthy, we could reproduce the three phases such as DPP-(CF)2-α, DPP-(CF)2-β and DPP-(CF)2-γ via recrystallization process using solvents namely pyridine, acetonitrile and ethyl acetate, respectively (Figure S2). Although it was possible to reproduce the DPP-(CF)2α phase via recrystallization process, it was not possible to solve the crystal structure using the powder pattern obtained due to its poor crystallinity as well as the coexistence of other phases.24, 52 Interestingly, the powder pattern recrystallized using toluene was found to be different from the other three phases. In particular, a new Bragg reflection was observed at 7.43°, which was absent in other three phases. Moreover the 010 reflection which corresponds to the π-π stacking distance was noticed to shift towards the higher angle (Figure S2), which confirms the decrease of π-π stacking distance. This phase will be further referred as DPP(CF)2-ω. This phase was stable even after annealing at 170 °C for 1 hr. Unfortunately the exact molecular packing of DPP-(CF)2-ω was not also possible to determine using the corresponding powder patterns. Since DPP-(CF)2 packs in four different forms, it is a good system to investigate the impact of molecular packing on its electrical and optical characteristics. Actually, single crystals are more ideal than the polycrystalline thin films for probing the role of polymorphism on the charge transport characteristics of a molecule.7, 14 Unfortunately it was not possible to grow big enough crystals, which is mandatory for probing the charge transport. Therefore, efforts have been taken to reproduce the same phases in the form of thin films on both the SiO2/PEDOT:PSS and Si/SiO2/BCB substrates in order to address the real role of polymorphism on the electrical and optical properties of DPP-(CF)2. 13



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3. Solvent and Thermal Annealing of Thin Films Analyzed by GIXD a. Thin films fabricated on the SiO2/PEDOT:PSS substrate •

Separation of Polymorphic Phases via Thermal Annealing

DPP-(CF)2 thin films were spin coated on two various substrates such as SiO2/PEDOT:PSS and Si/SiO2/BCB. Actually, various solvents such as CHCl3, pyridine, tetrahydrofuran (THF), DCM were used to spin coat the films (Figure S3). Except CHCl3, most of the solvents results in amorphous films with little amount of DPP-(CF)2-β/DPP-(CF)2-γ phases. On the other hand, CHCl3 yield amorphous films with little amount of DPP-(CF)2-α phase (Figure S3 and S4). Therefore, CHCl3 was used to prepare thin films for further analysis. 2D GIXD patterns of the as-spin coated films deposited on the SiO2/PEDOT:PSS and Si/SiO2/BCB substrates are shown in Figure S4a and S4b, respectively. Both films crystallize in the form of DPP-(CF)2-α phase. However, the crystallinity of the film was found to vary with respect to the substrates. For example, the film spin coated on the Si/SiO2/BCB substrate was found to be more amorphous than the one spin coated on the SiO2/PEDOT:PSS substrate (Figure S4).53 The presence of a broad Bragg reflection centered at 5.35° together with an amorphous halo (≈24°) in the 2D GIXD pattern of the film spin coated on the SiO2/PEDOT:PSS substrate confirms the formation of DPP-(CF)2-α phase (Figure S4a). This reflection is shifted towards the lower angle by ≈ 0.2° from the powder pattern (≈5.55°). Such a slight increment in the lattice spacing may be caused by the substrate because, the same reflection was noticed to shift to 5.56° on increasing the thickness of the films (Figure S5).53 Although the 2D GIXD pattern of the as-spin coated film depicts the texturing of crystallites along the out-of-plane direction, the amorphous halo confirms the existence of quite considerable amount of amorphous DPP-(CF)2 in it.31,

53

So, a post-growth thermal annealing was employed to

increase the crystallinity of the films. The as-spin coated DPP-(CF)2-α phase was noticed to crystallize while annealing the film up to 95 °C. Further increase in temperature results in a solid-solid phase transition from DPP-(CF)2-α to DPP-(CF)2-β, as observed in the case of powders through DSC (Figure 2). Especially, absence of any Bragg reflection at 5.35° either along in- or out-of-plane directions confirms the phase transition of DPP-(CF)2-α to DPP-(CF)2-β while annealing at 116 °C (Figure S6c).24, 54 The XRD pattern of the film annealed at 116 °C matches well with the amorphous powder crystallized at 116 °C (Figure S7a). This confirms the phase transition of DPP-(CF)2-α to DPP-(CF)2-β while annealing at 116 °C. Further increase in annealing 14


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temperature to 166 °C results in another phase transition from DPP-(CF)2-β to DPP-(CF)2-γ. Although the 2D GIXD pattern of DPP-(CF)2-γ looks similar to the DPP-(CF)2-β, the shifting of 001 reflection towards the higher angle and the 010 reflection towards the lower angle with respect to DPP-(CF)2-β confirms the formation of DPP-(CF)2-γ phase (Figure 5 and 6), as observed in the case of powders (Figure 3). The XRD pattern (integrated intensity) of the film annealed at 166 °C matches well with the amorphous DPP-(CF)2 annealed at 166 °C (Figure S7b). This confirms the solid-solid phase transition of DPP-(CF)2-β to DPP-(CF)2-γ while annealing the film at 166 °C. On the one hand, the DPP-(CF)2-γ phase obtained by annealing the films at 166 °C results in highly crystalline and highly textured films, on the other hand, the films annealed at 82 °C and 116 °C results in poorly crystalline film. In particular, the presence of broad amorphous halo centered at ≈ 24° in the 2D GIXD patterns of the films annealed at 82 and 116 °C confirms the existence of quite considerable amount of amorphous DPP-(CF)2 in it (Figure S6) even after subtracting the background scattering (recorded using PEDOT:PSS coated glass substrate). Since the amorphous DPP-(CF)2 is detrimental for the charge transport55, SVA annealing was employed to prepare highly crystalline and highly textured thin films of DPP-(CF)2-α and DPP-(CF)2-β phases so as to understand the role of polymorphism on the electronic performances of DPP-(CF)2.24, 27, 31 Since the DPP-(CF)2-γ phase obtained by annealing the film at 166 °C was highly crystalline, same methodology was employed to obtain pure form of DPP-(CF)2-γ for the device analysis.

Figure 5. 2D GIXD patterns of four various polymorphic phases of DPP-(CF)2 thin films (DPP-(CF)2-α (a), DPP-(CF)2-β (b), DPP-(CF)2-γ (c) and DPP-(CF)2-ω (d)) separated via 15



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post-growth treatments such as solvent vapor annealing and thermal annealing. The places marked with red dotted circles are just to guide the eye of the readers to identify the shift in the Bragg reflections. •

Separation of Polymorphic Phases via Solvent Vapor Annealing First of all, the nonsolvent methanol was used to promote the crystallization of as-spin

coated DPP-(CF)2-α phase. The as-spin coated DPP-(CF)2-α phase was noticed to crystallize during the initial 5 minutes of methanol vapor annealing (Figure S8a). 2D GIXD pattern of the corresponding film is shown in Figure 5a. Evolution of well defined Bragg reflections without any amorphous halo confirms the crystallization of DPP-(CF)2-α phase. In particular, the presence of well defined Bragg reflection at 5.35° confirms the crystallization of DPP(CF)2-α phase. Further increase in methanol vapor annealing duration found to suppress the Bragg reflection observed at 5.35° and results in completely a new phase called DPP-(CF)2-ω (Figure 5d and S8b). Noteworthy, this phase transition point completely depends on the underlying substrate as well as the substrate temperature. For example, the film undergone methanol vapor annealing at 27 °C and 15 °C was noticed to alter its phase from DPP-(CF)2-α to DPP-(CF)2-ω after 5 and 45 minutes, respectively. The ATR-FTIR analysis conducted on the methanol vapor annealed film clearly shows that this is not a solvomorphic phase.56 In particular, the absence of any absorption at 3337 cm-1 in the ATR-FTIR spectra of DPP(CF)2-ω, which corresponds to the O-H stretching of methanol, confirms the absence of methanol incorporation into the crystal framework (Figure S9).57 Moreover, the AFM analysis confirms the crystallization of DPP-(CF)2-ω from DPP-(CF)2-α through recrystallization process with the initiation of new nucleates (further details refer supporting information section AFM analysis (Figure S10-S12)).58-59 The 2D GIXD pattern of DPP-(CF)2-ω phase is shown in Figure 5d. Though the 2D GIXD pattern of DPP-(CF)2-ω has some similarities with the DPP-(CF)2-β and DPP-(CF)2-γ phases, it does not match exactly either with the DPP-(CF)2-β or DPP-(CF)2-γ phases. A new Bragg reflection was observed at 7.43°, which was absent in other three phases (inset of Figure 5d and Figure 6). The same reflection was also observed in the powder pattern of DPP(CF)2 recrystallized using toluene (Figure S2). In addition to the evolution of a new Bragg peak, most of the Bragg reflections were found to slightly shift from the positions of DPP(CF)2-β and DPP-(CF)2-γ, some of them are indicated with red dotted circle in Figure 5. In order to visualize that, the 2D GIXD patterns were azimuthally integrated and shown in 16


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Figure 6. Interestingly, the Bragg reflection observed at 9.03° (DPP-(CF)2-ω) is shifted towards higher angle as compared to the DPP-(CF)2-β (8.92°) and DPP-(CF)2-γ (8.83°) phases (inset of Figure 6), which correspond to the 010 plane. Similarly, DPP-(CF)2 powder recrystallized using toluene also depicts the same shift (Figure S2). The single crystal data of both the DPP-(CF)2-β and DPP-(CF)2-γ phases shows that the DPP-(CF)2 π-stack along the 010 plane with slight tilting (Figure 4a and 4b). So such decrement in the d010 spacing can directly decrease the π-π stacking distance. Since the 2D GIXD pattern of DPP-(CF)2-ω is consistent with DPP-(CF)2-β and DPP-(CF)2-γ, it may pack same like them with slight variation in the lattice spacing, especially with short π-π stacking distance. This could be the reason why this phase has a narrow bandgap as compared to the other three phases (vide infra). The absence of any Bragg reflection which corresponds to the DPP-(CF)2-α phase in the film that undergone 10 minutes of methanol vapor annealing confirms the complete transformation of DPP-(CF)2-α to DPP-(CF)2-ω. And further increase in SVA duration tends to dewet the film (Figure S11). Therefore, 10 minutes of methanol vapor annealed film was used to probe the solar cell and optical characteristics of DPP-(CF)2-ω phase.

Figure 6. Integrated intensity profiles of the four different polymorphic phases of DPP-(CF)2 thin films. Inset shows the 010 reflection of DPP-(CF)2-β, DPP-(CF)2-γ and DPP-(CF)2-ω phases. On the contrary to the methanol vapor annealing, chloroform vapor annealing does not support the crystallization of DPP-(CF)2-α phase. Instead, it directly crystallizes in the form of DPP-(CF)2-β (Figure S8b). The as-spin coated DPP-(CF)2-α phase was noticed to alter its 17



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phase to DPP-(CF)2-β within the initial 30 seconds of SVA and continued to crystallize until the initial 5 minutes of chloroform vapor annealing (Figure S8b).27Although there was a DPP(CF)2-α phase in the as-spin coated film, we did not observe the presence of any Bragg reflection that correspond to the DPP-(CF)2-α in the 2D GIXD patterns of CHCl3 vapor annealed film for 10 minutes (Figure 5b). This confirms the complete recrystallization of it during the CHCl3 vapor annealing.58-59 2D GIXD pattern of the corresponding film is shown in Figure 5b. The integrated intensity of this pattern exactly matches with the one annealed at 116 °C (Figure S7a). Therefore, 10 minutes of CHCl3 vapor annealed film was used to probe the role of polymorphism on the electronic performances of DPP-(CF)2-β phase instead of 116 °C annealed one since, it produces highly crystalline and highly textured films. Noteworthy, the CHCl3 vapor annealed film does not dewet as like the methanol vapor annealed one on increasing the duration of SVA (Figure S13).

b. Thin films fabricated on the Si/SiO2/BCB substrate As stated earlier, the film spin coated on the Si/SiO2/BCB substrate was found to be mostly amorphous (Figure S4b). Since the film was amorphous, initially methanol vapor annealing was employed to promote the crystallization of DPP-(CF)2-α phase. The amorphous DPP-(CF)2 film was noticed to crystallize in the form of DPP-(CF)2-α during the initial two minutes of methanol vapor annealing. In particular, the evolution of well defined Bragg reflection at ≈ 5.35° after two minutes of methanol vapor annealing confirms the crystallization of DPP-(CF)2-α phase (Figure S14 and S15). Further increase in the methanol vapor annealing duration transforms the DPP-(CF)2-α to DPP-(CF)2-ω. Similar to the film spin coated on the SiO2/PEDOT:PSS substrate, the suppression of Bragg reflection observed at 5.35° and evolution of new Bragg reflection at ≈ 7.43° confirms the phase transition of it (Figure S15). This phase transition point varies quite considerably with respect to the underlying substrate (i.e. 5 and 2 minutes for SiO2/PEDOT:PSS and Si/SiO2/BCB substrates, respectively). This could be due to the variation in substrate surface energy.59 Since the film dewet seriously on increasing the duration of methanol vapor annealing further (Figure S12), 5 minutes of methanol vapor annealed film was used for the OFET analysis. The DPP-(CF)2-β phase was obtained via 10 minutes of CHCl3 vapor annealing. The integrated GIXD pattern of this phase matches well with the powder annealed at 116 °C. The DPP-(CF)2-γ phase was obtained by annealing the amorphous films at 166 °C for an hour. The integrated GIXD 18


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pattern of this phase is shown in Figure S15. This pattern matches well with the powder annealed at 166 °C as well. In particular shifting of 001 and 003 reflections towards the higher angle and 12-2 and 121 towards the lower angle as compared to the DPP-(CF)2-β phase confirms the formation of DPP-(CF)2-γ phase. Since we have successfully fabricated the four phases of DPP-(CF)2 on both SiO2/PEDOT:PSS and Si/SiO2/BCB substrates, we have further investigated the role of molecular packing on their optical and electrical properties.

4. Polymorphic Phase Dependent Optical and Electrical Properties of DPP-(CF)2 UV-Vis absorption spectra of four various polymorphs of DPP-(CF)2 thin films are shown in Figure 7. The UV-Vis absorption spectra of DPP-(CF)2 show two broad absorptions at higher (480 nm - 885 nm) and lower (310 nm - 477 nm) wavelength regions.32, 60 The absorption observed at lower wavelength region corresponding to π-π* transition does not vary much with respect to the polymorphs.32,

60

On the other hand, the absorption peaks

observed at higher wavelength region, which corresponds to the HOMO to LUMO intramolecular charge transfer between the donor and acceptor unit32, 60, are noticed to vary with respect to the polymorphs. In particular, the absorption onset of DPP-(CF)2-ω is redshifted by 117 nm (0.25 eV) from the DPP-(CF)2-α phase. Eventually, the color of the film has also changed from dark blue to dark brown (Figure 7e). Such huge crystallochromy was previously reported for the case of perylene-3,4:9,10-bis(dicarboximide)61, 1,1-dicyano-2,2bis(4-dimethylaminophenyl)ethylene62, TMS-DBC15, copper(I)−triphenylphosphine complex, (Cu(2-QBO)(PPh3)PF6)(I)51 and TiOPc19, 63 due to the augment in π-π interaction along the molecular stack as well as the reduction in π-π stacking distance.

19



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Figure 7. (a-d)UV-Vis absorption spectra of four various polymorphs of DPP-(CF)2 thin films prepared on the SiO2/PEDOT:PSS substrate. The right side column depicts the photos of corresponding films (e). The optical bandgaps of four various polymorphs were extrapolated from the onset of the UV-Vis absorption spectra and presented in Table 3. The HOMO levels of four different polymorphs were calculated from the UPS spectra (Figure S16), and the LUMO level of each polymorph was calculated by adding the optical band gap with the HOMO level.19, 33 It has been theoretically shown that the electronic splitting of HOMO and LUMO increases exponentially when the π-π stacking distance reduces from 3.8 to 3.2 Å due to the augment in wave function overlap of sexithienyl molecule.4 The XRD patterns show that DPP-(CF)2-ω has the shortest π-π stacking distance as compared to the DPP-(CF)2-β and DPP-(CF)2-γ phases (Figure 6) (i.e. shifting of 010 reflection towards higher angle). So, we expect that the decrease in optical bandgap while going from DPP-(CF)2-α to DPP-(CF)2-ω could be attributed to the increase in π-π interaction between the neighboring molecules due to its short π-π stacking distance, as reported for the case of perylene-3,4:9,10-bis(dicarboximide)61, TiOPc19, 63 and in other DPP based small molecules15, 24, 42, 64.

Table 3. Optical bandgap and the HOMO and LUMO energy levels of various polymorphic phases of DPP-(CF)2. Polymorphic Phases of DPP-(CF)2 DPP-(CF)2-α DPP-(CF)2-β DPP-(CF)2-γ DPP-(CF)2-ω

Optical Bandgap (±0.03eV) 1.75 1.64 1.65 1.50

HOMO (±0.05eV) -5.57 -5.30 -5.47 -5.29

LUMO (±0.05eV) -3.82 -3.66 -3.82 -3.79

In addition to the bandgap, the positions of both the HOMO and LUMO energy levels were also noticed to vary with respect to the polymorphs (Table 3). Among all these polymorphs, DPP-(CF)2-α has the low-lying HOMO level. It has been documented that the twist conformation of electron donating moieties with respect to the DPP chromophore can deepen the HOMO level.39, 65-66 Though the DPP-(CF)2-β and DPP-(CF)2-γ depicts nearly the same band gap, the HOMO of DPP-(CF)2-γ was found to be 0.17 eV deeper than the DPP(CF)2-β phase. This could be due to the poor conjugation of thiophene and furan with respect to the DPP core when compared to the DPP-(CF)2-β, as observed from the single crystal data 20


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(Table 2). Such molecular packing dependent variation in energy level has also been reported for other small molecules such as TiOPC19, pentacene18, 4,4’-bis(9-carbazolyl)biphenyl41, etc. Such variation in the HOMO level eventually found to influence the performance of solar cells (for further details refer supporting information section fabrication and testing of solar cells and Figure S17.). The field effect mobility of various polymorphs was calculated from the corresponding transfer characteristics in the saturation regime (Figure S18) and presented in Table 4. Except DPP-(CF)2-γ, the other three phases depicts an ambipolar charge transport.38, 67 Such phase dependent ambipolar charge transport was also reported in the case of TIPS-pentacene8 and vanadyl-phthalocyanine68. Among the four phases, DPP-(CF)2-ω shows the highest hole mobility of 4.3*10-3 cm2/Vs. And it is found to be an order of magnitude higher than the DPP(CF)2-α phase. The AFM analysis clearly shows that the increase in field effect mobility of DPP-(CF)2-ω could not be attributed to the morphology of the films, because the crystallite size of this phase is smaller than the DPP-(CF)2-α phase (Figure S12). Although DPP-(CF)2-γ has bigger crystallites, it results in poor hole mobility. So the better performance of DPP(CF)2-ω as compared to the other three phases could be attributed to the upshifting of its HOMO level (which can reduce the charge carrier injection barrier as gold is used for electrodes)69 as well as the reduction in π-π stacking distance (which can increase the electronic coupling between the neighboring molecules and hence the charge transfer integrals)2, 5, 8, 44.

Table 4. Field effect mobility of various polymorphs of DPP-(CF)2 thin films. Polymorphic Field Effect Mobility (±0.5 cm2/Vs)a Phases Hole mobility Electron Mobility -4 DPP-(CF)2-α 2.9*10 4.3*10-4 -3 DPP-(CF)2-β 2.2*10 3.5*10-5 -4 DPP-(CF)2-γ 5.6*10 DPP-(CF)2-ω 4.3*10-3 1.8*10-4 a An average value of 16 devices. CONCLUSION The XRD analysis conducted on the diethyl 5,5'-(5,5'-(2,5-bis(2-ethylhexyl)-3,6dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis(thiophene-5,2-diyl))difuran-2carboxylate (DPP-(CF)2) confirms the four different molecular conformations of it, both in the form of thin film and bulk. Consequently, both the optical and electrical properties of DPP(CF)2 thin films were found to vary significantly with respect to the polymorphs. In particular, 21



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the bandgap of DPP-(CF)2 has decreased from 1.75 to 1.5 eV during its phase transition from DPP-(CF)2-α to DPP-(CF)2-ω. The bandgap of DPP-(CF)2-ω is much narrower than the diketopyrrolopyrrole based small molecules that are grafted with more than four electron donating moieties in a single DPP chromophore. Among all these polymorphs DPP-(CF)2-ω exhibits the highest hole mobility i.e. one order of magnitude higher than the DPP-(CF)2-α phase. Such an increment in the hole mobility of DPP-(CF)2-ω could be attributed to its short π-π stacking distance as well as the upshifting of its HOMO level by 0.28 eV from the DPP(CF)2-α phase (-5.57 eV). This study clearly demonstrates that the optimization of molecular packing is very important for the realization of efficient organic electronic devices together with the syntheses of new molecules. In addition, this study clearly demonstrates that the proper selection of solvent vapor annealing duration and the solvent could help to synthesize pure form of different polymorphs on various substrates.

Acknowledgements Shabi Thankaraj Salammal greatly acknowledges the National Natural Science Foundation of China (Grant No: 51450110081) and China Postdoctoral Science Foundation (Grant No: 2014M550323) for the financial support. Basab Chattopadhyay kindly acknowledges the financial support from FRS-FNRS (Belgian National Scientific Research Fund) for the POLYGRAD project (22333186). B.C is a FRS-FNRS Research Fellow. Supporting Information X-ray diffraction pattern of powders recrystallized using various solvents; X-ray diffraction pattern of DPP-(CF)2 thin films spin coated using various solvents; FT-IR spectra of DPP-(CF)2 thin films SVA using various solvents; 2D GIXD pattern of thin films annealed at various temperatures; comparison between X-ray powder pattern and thin films; AFM analysis of various polymorphs of DPP-(CF)2; ultraviolet photoelectron spectra of four various polymorphs; transfer characteristics of OFETs and Solar cell fabrication and testing of various polymorphs. This material is available free of charge via the Internet at http://pubs.acs.org.

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Polymorphic Phase-Dependent Optical and Electrical Properties of a

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