Charge Carrier Polarity Modulation in Diketopyrrolopyrrole-Based Low

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Charge Carrier Polarity Modulation in Diketopyrrolopyrrole–Based Low Band Gap Semiconductors by Terminal Functionalization Samrat Ghosh, Reshma Raveendran, Akinori Saeki, Shu Seki, Manoj AG Namboothiry, and Ayyappanpillai Ajayaghosh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16714 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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ACS Applied Materials & Interfaces

Charge

Carrier

Polarity

Diketopyrrolopyrrole–Based

Modulation Low

Band

in Gap

Semiconductors by Terminal Functionalization Samrat Ghosh,†,‡ Reshma Raveendran,∥ Akinori Saeki,§ Shu Seki,⊥ Manoj Namboothiry,∥ and Ayyappanpillai Ajayaghosh*, †,‡ † Photosciences and Photonics Section, Chemical Sciences and Technology Division, CSIR– National

Institute

for

Interdisciplinary

Science

and

Technology

(CSIR–NIIST),

Thiruvananthapuram–695019, India. E–mail: [email protected]. ‡ Academy of Scientific and Innovative Research (AcSIR), CSIR-NIIST Campus, Thiruvananthapuram–695019, India. ∥ School of Physics, Indian Institute of Science Education and Research (IISER), Thiruvananthapuram–695551, India. § Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2–1 Yamadaoka, Suita, Osaka 565–0871, Japan.

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 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

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⊥ Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo–ku, Kyoto 615–8510, Japan. KEYWORDS: Diketopyrrolopyrrole; self-assembly; low band gap; p-n switch; FP-TRMC

ABSTRACT: Organic semiconductors with variable charge carrier polarity are required for optoelectronic

applications.

Herein,

we

report

the

synthesis

of

three

novel

diketopyrrolopyrrole (DPP) based D–A molecules having three different terminal groups (amide, ester and dicyano) and studied their electronic properties. Increase in electron acceptor strength from amide to dicyano, leads to a bathochromic shift in absorption. Photoconductivity and field–effect transistor (FET) measurements confirmed that a small increase in acceptor strength can results in a large change in the charge transport properties from p–type to n–type. The molecule with amide group, DPP–Amide, exhibited a moderate p–type mobility (1.3  10–2 cm2V−1s−1), whereas good n–type mobilities were observed for molecules with an ester moiety, DPP–Ester (1.5  10–2 cm2V−1s−1) and with a dicyano group, DPP–DCV (1  10–2 cm2V−1s−1). The terminal functional group modification approach presented here is a simple and efficient method to alter the charge carrier polarity of organic semiconductors.

1. INTRODUCTION Organic semiconductors, in comparison to their inorganic counterparts, have generated significant interest due to their potential as promising candidates for thin, flexible and light


2

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weight organic field–effect transistors (OFETs).1-3 Unlike inorganic semiconductors, band gap engineering and tuning by structural modification is feasible in organic semiconductors.4-6 Therefore, organic small molecules and polymers have attracted significant attention in semiconductor research although their advantages vs disadvantages in terms of synthesis, solution processability, film formation, crystallinity etc. are still matters of debate in the scientific community.7,8 Over the past few years, a large number of small molecules and polymers were designed and synthesized for FET applications.9-11 Majority of these semiconductors are hole transporting (p–type) in nature, while a few of them exhibit electron transporting (n–type) or ambipolar characteristics.12-14 Generally, electron rich motifs like benzothiophene, tetrathiafulvalene, fluorene, thienothiophene etc. are used in the design of p–type semiconductors, whereas n–type semiconductors are largely based on electron deficient cores such as naphthalenediimide, perylenediimide, fullerene etc.15,16 A few π– conjugated D–A oligomers and polymers are reported to exhibit p–type or n–type or ambipolar characteristics depending upon the nature of the attached substituents.17-21 However, cost effective realization of these semiconductor materials with tunable transport properties is still challenging. Moreover, it is mandatory to understand the structure–property relationships behind organic semiconductors for further improvement in device performance.22-25 Recently, a few reports have demonstrated the polarity switching in π–conjugated systems by external stimulus, synthetic modifications or by changing the metal center.26-30


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With the above literature precedence, we have decided to study the charge carrier polarity modulation in organic semiconductors by introducing different functional groups at the terminus of the molecule. For this purpose, we designed a π–conjugated D–A oligomer with diketopyrrolopyrrole (DPP) as the acceptor and bithiophene as the donor.31-34 Among different electron–accepting moieties, DPP has been extensively investigated due to its several characteristic features including planarity, high crystallinity, excellent photostability, high solubility, enhanced charge carrier mobility, etc.35-39 A number of DPP based oligomers and polymers are reported with high unipolar and ambipolar charge carrier mobilities.40-46 The usual approach to high performance organic ptype or n-type or ambipolar semiconductors is through multistep organic synthesis.47,48 However, reversal of the charge carrier mobility from p-type to n-type or vice versa in DPP based small molecules or polymers through minute synthetic modifications or by keeping same π-conjugation are by far, less explored, since switching the polarity from p- to n-type by such modifications in the π-conjugated backbone are challenging. Comparable hole and electron mobilities can be expected in the case of molecules with same π-conjugated backbone which are essential for creating organic p-n junction devices or complementary logic circuits.49,50 In this context, we designed three D–A systems having same π-conjugated backbone that would give rise to either a p–type or an n–type semiconductor upon minute terminal substitution. In a D–A system, balance between the strength of electron donor and electron acceptor is the key to obtain desired polarity. We have increased the acceptor strength of terminal groups based on Hammett substituent constants,51 so that we could fine tune the energy levels as well as the


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charge delocalization of highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO). We expected that an increase in the acceptor strength of terminal groups may help to modulate the charge carrier polarity from p–type or n–type. To prove our hypothesis, three derivatives, DPP–Amide, DPP–Ester and DPP–DCV were prepared by Knoevenagel condensation of the corresponding precursors with the aldehyde DPP–CHO. All three derivatives exhibit broad absorption and low band gap. Flash–photolysis time-resolved microwave conductivity (FP–TRMC) measurements confirmed that the n–type charge carrier mobility of these derivatives increased with the increase in acceptor strength. FET characteristics confirmed the change of polarity from p– to n–type by the fine tuning of acceptor strength. 2. RESULTS AND DISCUSSION 2.1. Synthesis DPP–CHO was synthesized by a multistep synthetic procedure (see Supporting Information).32,46 DPP–Amide, DPP–Ester and DPP–DCV were obtained by Knoevenagel condensation of DPP–CHO with 2–cyano–N–dodecylacetamide, ethyl cyanoacetate and malononitrile, respectively, in the presence of piperidine, in good yields (Scheme 1). All the molecules after purification were characterized by various analytical techniques such as nuclear magnetic resonance spectroscopy (1H and desorption/ionization–time

of

flight

13C

NMR) and matrix assisted laser

(MALDI–TOF)

mass

spectrometry.

Excellent

photostability and solubility in most of the common organic solvents were also confirmed for all these derivatives.


5


O C12H25HN

S

Page 6 of 32

N S

CN

O

NHC12H25

NC

S

N S

N

O

O S

N

S

S

NHC12H25 O

DPP-Amide

OC2H5

NC

O

NC S

O

OHC

O

CHO

O C2H5O

S

N S

CN

O

O

NC S

N

S

OC2H5 O

DPP-Ester

DPP-CHO

NC

CN

S

NC CN

N S O

O

NC S

N

S

CN

DPP-DCV

Scheme 1. Synthesis of DPP-Amide, DPP-Ester and DPP-DCV from DPP-CHO and active methylene compounds in the presence of piperidine.

2.2. Thermal Properties The thermal properties of the three DPP derivatives were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). It was found that DPP–Amide, DPP–Ester and DPP–DCV have high thermal stabilities with a decomposition temperature of 350, 320 and 355 C, respectively, under nitrogen atmosphere (Figure S1a). Despite having the same π–conjugated backbone, these derivatives exhibited different thermal behavior in DSC, probably due to the different terminal functionality and presence and absence of terminal alkyl chains, which allow several non-covalent interactions. DPP–Ester melts at 255 C, which is higher than the melting points for DPP–Amide (214 C) and DPP–DCV (234 C), with a sharp exothermic recrystallization peak (Figure S1b). DPP-Amide contains two additional long alkyl


6

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(–C12H25) chains with respect to DPP-Ester and DPP-DVC. Increase in the number of alkyl chains can decrease melting point of π-conjugated systems drastically.52 It would therefore be highly probable for DPP-Ester and DPP-DVC to have higher melting temperatures than the DPPAmide. Similarly, due to the absence of terminal alkyl chains, DPP-DVC gets crystallized at higher temperature than DPP-Ester carrying short alkyl chains followed by DPP-Amide having long alkyl chains. Unlike DPP–Amide and DPP–Ester, DPP–DCV showed an endothermic peak at 203 C and an exothermic peak at 193 C in addition to melting and crystallization, indicating the possibility for a phase transition. However, no mesophase formation was observed under polarized optical microscope upon heating or cooling.


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2

-4.3

-4.08 -4.15 -4.39

b) PC71BM

-4.67

-5.62 -5.64

DPP-DCV

4

0

-2.65

DPP-Ester

c)

DPP-Amide DPP-Ester DPP-DCV

DPP-Amide

-1

6

a)

4

8

-1

 (10 M cm )

10

P3HT

12

Normalized Absorbance

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

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-5.81

-6.1 300

400

500

600

700

800

900

Wavelength (nm)

Figure 1. Absorption spectra of the DPP–Amide, DPP–Ester and DPP–DCV in (a) solution (chloroform) and (b) film state. (c) Energy level diagram of all the DPP derivatives, P3HT and PC71BM, calculated from the PYS and optical band gap. 2.3. Optical Properties The UV–visible absorption spectra of the oligomers were recorded in solution as well as in the film state. In chloroform, all the derivatives exhibited similar absorption profiles with a relatively less intense absorption band at 400–500 nm corresponding to π–π* transition and an intense and broad internal charge transfer (ICT) band at 600–700 nm (Figure 1a).31 A gradual red shift in absorption maximum was observed with the increase in acceptor strength from amide to ester to dicyano side chains.53 Solid state samples were prepared by spin coating chloroform solution (4 mg/mL) of the respective derivative. DPP–Amide and DPP–Ester exhibited almost similar absorption profile as in solution with a small red shift and broadening,


8

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whereas a highly red shifted strong aggregation band was observed at 811 nm for DPP–DCV (Figure 1b). This observation can be attributed to the formation of strong J–type aggregates in the solid state for DPP–DCV.54,55 The optical band gaps were estimated for all the derivatives from the absorption onset and a decrease in optical band gap was observed with increase in acceptor strength.

Table 1. Thermal, photophysical, and electrochemical properties of the DPP-Amide, DPPEster, and DPP-DCV.

Thermal Properties

Compoun d

Td (℃)

Tm(℃ )

Optical Properties

Tc (℃)

λmax (nm)

λmax (nm)

solution

film

Electrochemical Properties

Egopt

(eV)

HOMO (eV)

LUMO (eV)

a

b

DPP-Amide

350

214

187

630

658, 724

1.54

-5.62

-4.08

DPP-Ester

320

255

210

635

680, 752

1.49

-5.64

-4.15

DPP-DCV

355

234

233

645

721, 811

1.42

-5.81

-4.39

band gap was estimated from the UV−vis absorption edge (Egopt = 1240/ λonset eV). bLUMO was calculated by using the equation, LUMO = HOMO + E opt. g aOptical

Frontier Orbital Levels and Density Functional Theory Calculations


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Page 10 of 32

Frontier molecular orbital (FMO) energies of DPP–Amide, DPP–Ester, DPP–DCV, P3HT and PC71BM were estimated by using photoelectron yield spectroscopy (PYS) and absorption spectroscopy.56 A gradual decrease in HOMO and LUMO energy levels were observed from DPP–Amide to DPP–Ester to DPP–DCV because of the increase in acceptor strength. The HOMO for DPP–Amide and DPP–Ester were estimated as –5.62 eV and –5.64 eV, respectively, however for DPP–DCV the value was lower at –5.81 eV (Figure S2).57 From the optical band gap, LUMO for all the derivatives were calculated and similar trends as in the HOMO were observed (Figure 1c and Table 1). The lowest value for LUMO was estimated for DPP–DCV due to the high electron withdrawing effects of dicyanovinyl unit. Similarly, the energy levels of P3HT (donor) and PC71BM (acceptor) were also estimated (Figure 1c). P3HT and PC71BM are well known for their respective p–type and n–type charge transporting properties. We assume that it may be possible to predict the charge carrier polarities of DPP–Amide, DPP– Ester and DPP–DCV from the photoconductivity measurement by mixing them with P3HT or PC71BM.58 Before estimating the photoconductivities of all derivatives, we have performed timedependent density functional theory (TD–DFT) calculations in order to gain more insights into their electronic properties. For reasons related to computational time and costs, the branched and long alkyl chains were replaced with methyl groups. All molecules under study were found to be almost planar and their HOMO and LUMO wave functions were completely delocalized over the π–surface (Figure S3). The HOMO of all the derivatives were centered mostly over the DPP and the adjacent thiophene units. When compared to DPP–Amide and


10

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DPP–Ester, the LUMO of DPP–DCV was found to be more delocalized over the π–surface, in agreement with the strong electron withdrawing nature of the dicyanovinyl units. Hence, it is expected to have better electron injection in DPP–DCV.

2

0.1

Time (s)

c) 60

2 -4

1

d)

DPP-DCV DPP-DCV+P3HT DPP-DCV+PCBM

50

15 10 5 0.1

1

Time (s)

10

60 50

40 30 20 10 0 0.01

20

0 0.01

10

 (10-4 cm2/Vs)

0 0.01

DPP-Ester DPP-Ester+P3HT DPP-Ester+PCBM

25

2 -4

4

 (10 cm / Vs)

2

6

b)

DPP-Amide DPP-Amide+P3HT DPP-Amide+PCBM

8

-4

 (10 cm / Vs)

a)

 (10 cm / Vs)

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


40 30 20 10 0

0.1

1

Time (s)

10

Figure 2. FP–TRMC transients of (a) DPP–Amide, DPP–Amide:P3HT and DPP– Amide:PC71BM, (b) DPP–Ester, DPP–Ester:P3HT and DPP–Ester:PC71BM, and (c) DPP–DCV, DPP–DCV:P3HT and DPP–DCV: PC71BM. (d) Comparison of  values of all the DPP derivatives in the absence and presence of P3HT and PC71BM. Photoconductivity Measurement Prior to the fabrication of OFETs, photoconductivity behavior of the synthesized DPP derivatives were examined by FP–TRMC measurements, using a 355 nm laser as the excitation source. FP–TRMC is a fast and reliable electrodeless technique, which provides a measure of the photoconductivity as , where  is the charge carrier generation quantum yield and 


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is the sum of charge carrier mobilities, i.e., the sum of electron and hole mobilities (e and h, respectively).59-61 Figure 2 shows the photoconductivities of pristine DPP–Amide, DPP–Ester and DPP–DCV in their film state. A film of DPP–Amide processed from toluene exhibited higher  compared to a film processed from chloroform, indicating the significance of amide H–bonding in intrinsic charge carrier mobility (Figure S4a). Fourier transform infrared (FT– IR) transmission spectrum confirmed the presence of strong H–bonding (broad N–H stretching at 3360 cm–1) for the film of DPP–Amide processed from toluene (Figure S4b). To evaluate the p-type nature of the DPP derivatives, three films were prepared by mixing them with PC71BM (1:1 weight ratio) in chloroform. The blend of PC71BM with DPP–Amide showed enhancement in  with respect to pristine DPP–Amide (Figure 2a), but the enhancement became insignificant for DPP–Ester (Figure 2b). Further, the DPP–DCV and PC71BM blend showed rather a decrease compared to pristine DPP–DCV (Figure 2c). This is rationalized by the decrease in the LUMO–LUMO offset between DPP derivatives and PC71BM, leading to the decrease in free charge separation efficiency. These results are suggestive of the transition from p-type to n-type nature in the order of DPP–Amide, DPP–Ester, and DPP– DCV. In order to understand the n-type properties of the DPP derivatives, films were prepared by mixing them with an electron donor (P3HT, 1:1 weight ratio) in chlorobenzene. These films exhibited a significant increase in photoconductivity with respect to the corresponding pristine DPP derivatives, as electron transfer is highly energetically favored from P3HT to DPP derivatives (Figure 1c). Also, P3HT is known to have excellent hole transporting property. Maximum photoconductivity (max) was observed for the blend of DPP–DCV:P3HT (5.3  ACS Paragon Plus Environment

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10–3 cm2V−1s−1) followed by DPP–Ester:P3HT (2.3  10–3 cm2V−1s−1) and DPP–Amide:P3HT (7.5  10–4 cm2V−1s−1) (Figure 2a-c). By comparing the max values of the pristine DPP and DPP:P3HT blend films (Figure 2d), their ratios were found to increase in the order of DPP– Amide

Charge Carrier Polarity Modulation in Diketopyrrolopyrrole-Based Low

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