Stability of Charge Transfer States in F4TCNQ-Doped P3HT

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Stability of Charge Transfer States in F4TCNQ-doped P3HT Kristen E Watts, Bharati Neelamraju, Erin L Ratcliff, and Jeanne E Pemberton Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01549 • Publication Date (Web): 08 Jul 2019 Downloaded from pubs.acs.org on July 16, 2019

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Chemistry of Materials

Stability of Charge Transfer States in F4TCNQ-doped P3HT Kristen E. Watts,a Bharati Neelamraju,b Erin L. Ratcliff,*b Jeanne E. Pemberton*a a

Department of Chemistry and Biochemistry University of Arizona 1306 E. University Boulevard Tucson, AZ 85721 and

b

Department of Materials Science and Engineering University of Arizona Tucson, AZ 85721

ABSTRACT Printable electronic devices from organic semiconductors are highly desired but limited by low conductivity and stability relative to their inorganic counterparts. p-Doping of poly(3hexyl)thiophene (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) increases conductivity through integer charge transfer (ICT) to form mobile carriers in the P3HT. An alternate undesired reaction pathway is formation of a partial charge transfer complex (CPX), which results in a localized, trap-like state for the hole on P3HT. This effort addresses stability of the free carrier states, once formed. Herein we demonstrate that, while the ICT state may be kinetically preferred, the CPX state is thermodynamically more stable. Conversion of the ICT state to the CPX state is monitored here with time using a combination of infrared and photoelectron spectroscopies and supported by a complete loss of film conductivity with increased CPX state concentration. Both the fraction and rate of conversion to the CPX state are influenced by polymer molecular weight, dopant concentration, and storage conditions, with ambient storage conditions accelerating the conversion. This work suggests that a renewed focus on dopant-matrix reaction chemistry should be considered in the context of both kinetic and thermodynamic considerations.

*Authors to whom correspondence should be addressed: [email protected], [email protected]

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INTRODUCTION Due to synthetic tenability and ease of processing, organic semiconductors (OSCs) hold exceptional promise as active elements for printable electronic devices such as thermoelectrics,1-4 light emitting diodes,5 photovoltaics,6-10 and field effect transistors.11-15 Despite this promise, two major fundamental challenges continue to limit commercial success: low conductivity and chemical stability. Increased conductivity is typically achieved in such systems through inclusion of p- or n-type dopants to chemically generate mobile free carriers through charge transfer reactions.16 Chemical instability is broadly defined as the decrease in material functionality and performance with time and can have mechanistic contributions that range in length scale from chemical bonds to conjugation length to microstructure, particularly in mixed or blended active material layers.16-21 This work considers the fundamental question of stability in systems containing chemically-generated charge carriers: once created, do doped organic semiconductors retain their conductivity? If not, what fundamental processes contribute to this decrease? Herein we focus on the model system of regioregular poly(3-hexyl)thiophene (rr-P3HT), serving as the electron donor, doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), the electron acceptor.2, 4, 22-28 In this system, two types of electron transfer events can occur. Integer charge transfer (ICT) is preferred, as it generates free carriers (holes) in the P3HT via electron transfer to F4TCNQ to form an ion pair; these free carriers result in a substantial increase in conductivity. 16, 22-23, 29-30 Partial charge transfer (CPX) results in a localized trapping of charge carriers through hybridization of the frontier highest occupied and lowest unoccupied molecular orbitals of P3HT and F4TCNQ, respectively; these states do not contribute substantially to conductivity.

22-23, 29

In previous work, Jacobs et al. reported that different crystalline

polymorphs impact formation of the ICT versus CPX states in this system, suggesting a difference

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in electronic driving force to stabilize and/or retain a particular charge transfer species.24 In a recent publication from these laboratories, we have shown that these two charge transfer processes are not mutually exclusive for co-processed P3HT/F4TCNQ films.31 Rather, we demonstrated that these two states can exist simultaneously and vary in relative concentration depending on microstructure and dopant concentration. While conductivity and doping efficiency of P3HT-F4TCNQ have been the focus of intense interest, significantly less effort has been directed toward understanding stability of the ICT and CPX states. Recent reports have considered de-doping, which can be described as reactions between the F4TCNQ dopant and solvents or additives.32-33 Such a phenomenon is unique to organic semiconductors and could enable new device architectures and functionalities. The simplicity and ease of modulating carrier density in these two studies led us to question the relative stability of these charge transfer states between the dopant and the polymer matrix. As these states are generally associated with different electronic environments, we hypothesized that both thermodynamic and kinetic factors might drive formation and retention of one state over the other, and that the balance of these factors might change with time. Herein, we present evidence that supports the ICT as the kinetically favored state, consistent with our earlier reports of its initial formation on doping and to the highest concentration in rr-P3HT films. Despite this kinetic preference, with time we observe transformation of the ICT state to the CPX state, suggesting CPX as the more thermodynamically favored state. With this new insight, we further demonstrate that changes in the electronic environment through dopant loading, rr-P3HT molecular weight, and ambient exposure impact the rate of this ICT-to-CPX conversion. These findings have broad significance for lifetime, efficiency, and conductivity considerations of organic semiconductor devices.

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EXPERIMENTAL SECTION Materials and Film Preparation. Poly(3-hexyl)thiophene (P3HT) was purchased at two different molecular weights: 34 kDa (polydispersity index [PDI] 1.75, regioregularity 94.7%) from Ossila and 58 kDa (PDI 2.1, regioregularity ≥96%) from Rieke Metals. 2,3,5,6-Tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4TCNQ) was purchased from Ossila and TCI America. All materials were used without further purification. 2 mg/mL solutions of each material (P3HT or F4TCNQ) were prepared by dissolution in chlorobenzene (Sigma Aldrich) at 80 °C for 20 min in a glovebox. Mixed solutions at the appropriate dopant concentration were prepared in 300 μL volumes immediately prior to casting onto the substrate at 1000 rpm for 1 min. p-Type silicon substrates (Silicon Valley Microelectronics, Inc., orientation, 10-30 Ω-cm resistivity, 500-550 μm thickness, one side polished with 1 kÅ thermal oxide, one side etched with native oxide) used in Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS) experiments were cut into 0.5 in x 0.5 in sections, cleaned through successive 5 min dips in hexanes, acetone, and isopropanol and dried with N2. For aging experiments, films were stored in either opaque (dark) or clear (light) containers in a glovebox (Ar atmosphere, O2 0.997. In some spectra, clear asymmetry in the spectral envelopes required a minimum number of additional peaks for adequate fitting; these are shown as light blue lines in the spectral fits. X-ray Photoelectron Spectroscopy.

XPS was performed with a Kratos Axis Ultra X-ray

photoelectron spectrometer with a monochromatic Al Kα source (1486.6 eV) at a base pressure of 10−9 Torr. Photoelectrons were collected in a hemispherical analyzer and detected with a photodiode array. A 20 eV pass energy was used for all element-specific spectral acquisitions. Resulting XPS spectra were first baseline corrected and then fit using a 40% Gaussian, 60% Lorentzian peak shape. For most core level spectra, baseline was corrected using a linear subtraction method; however, for S 2p spectra, a Shirley baseline correction was employed. The fitting of the N 1s core level spectra was conducted by first fitting the spectra from the initial films with a minimum number of peaks in agreement with the literature.34-36 For the later time point films, the peaks were constrained to ± 0.1 eV peak position with ± 0.02 eV FWHM from the original peak parameters. Spectroscopic Sampling Protocol.

Separate films were used for FTIR and XPS aging

experiments, and individual films were continuously monitored with time in each type of spectral experiment. Error bars for these spectral studies were calculated as the standard deviations of the responses for at least three independently prepared films handled and sampled identically. In between spectral sampling, films being aged in a dark inert environment were immediately returned to this environment while those being aged in ambient were returned to their dark or light ambient conditions. We have shown that for the ~30 min needed to acquire an FTIR spectrum, no significant changes in film chemistry are observed. A supplemental control experiment was done

5



by monitoring continuously a sample left in the FTIR analysis chamber in the path of the IR beam over an 8 h period with no spectral changes observed. XPS experiments were performed on individual films in a manner similar to FTIR sampling. To assess the impact of x-ray exposure on film integrity, FTIR spectra were acquired at each time point of both the sample used for XPS and a second film prepared, handled and sampled in an identical manner except for no exposure to xrays. Only minimal differences were observed in the FTIR spectra between the two film lines, suggesting a minimal impact of x-ray exposure on the films.

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RESULTS AND DISCUSSION 10

The studies reported here were motivated by

F4TCNQ-doped, 58 kDa MW, regioregular (rr)-P3HT decreases slowly with time, even when stored in dark inert conditions as shown in Figure 1. A decrease in conductivity of approximately one order of

1 -1

the observation that the conductivity of

Conductivity (S cm )

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


0.1 0.01 1E-3 F4TCNQ 0.012

1E-4

F4TCNQ 0.050 F4TCNQ 0.096

1E-5

F4TCNQ 0.13

1E-6 -1

0

1

2

3

4

5

6

7

8

Time (weeks)

magnitude is observed with time for both high (χF4TCNQ >0.096) and low (χF4TCNQ

Figure 1. In-plane conductivity of F4TCNQ-doped rrP3HT (MW 58 kDa) with time for χF4TCNQ 0.01, 0.05, 0.09 and 0.13 for films stored in inert dark conditions.

0.096) arose from an offset in energy with the LUMO of F4TCNQ. However, this picture does not account for the observed increase in CPX at low dopant concentrations (χF4TCNQ ≤0.096) in regioregular P3HT that can be seen in the black data of Figure 2d, where energetic alignment predicts ICT. This led us to consider the possibility of instability in the ICT state that is exacerbated at low dopant concentrations. Results described in subsequent sections directly address these issues. Charge Transfer State Stability. The ratio of ICT to CPX states with time was probed with a series of films of varying dopant concentration stored in a dark, inert atmosphere and monitored using FTIR and XPS over the course of 6-7 weeks. Figure 2a shows a series of spectra taken

8


ICT

a

+2 weeks

+6 weeks

0.5 au

2125

2150

2175

2200

2225

2250

ICT

b

Normalized Absorbance

F4TCNQ 0.05 CPX

2100

F4TCNQ 0.11

CPX

+2 weeks

0.5 au

2275

2100

+6 weeks

2125

2150

-1

2.0

2175

2200

2225

2250

2275

-1

Wavenumber (cm )

Wavenumber (cm )

c

2.0

d

F4TCNQ

A (CPX) / A (ICT)

1.6

A (CPX) / A (ICT)

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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+ 6 weeks 1.2

0.8 Fresh 0.4

0.0

0.012

1.5

1.0 0.05 0.5

0.096 0.17

0.0 0.00

0.04

0.08

0.12

0

0.16

F4TCNQ

1

2

3

4

5

6

Time (weeks)

Figure 2. Transmission FTIR spectra from 58 kDa rr-P3HT films in ν(C≡N) region at different times during storage in a dark, inert environment for a) χF4TCNQ 0.05 and b) χF4TCNQ 0.11; red fit bands due to ICT, green fit bands due CPX. c) Peak absorbance ratio of CPX to ICT b1u ν(C≡N) bands for films at initial time and after 6 weeks storage as a function of χF4TCNQ. d) Peak absorbance ratio of CPX to ICT b1u ν(C≡N) bands for four representative χF4TCNQ with time.

during storage of a χF4TCNQ 0.05 film representative of the low dopant concentration regime (χF4TCNQ ≤0.096). In this film, the band at 2201-2203 cm-1 due to the CPX state (δ = 0.8-0.7e-) increases about 50% in intensity over the course of 6 weeks, starting as little more than a shoulder to becoming a well-defined peak at 2203 cm-1. The ν(C≡N) band at 2169 cm-1 also broadens over time, and when subjected to peak fitting, the corresponding ν(C≡N) b2u mode of the CPX state at 2177 cm-1 can be identified clearly.

9



A similar series of spectra taken during storage of a χF4TCNQ 0.11 film representative of the high dopant concentration regime (χF4TCNQ >0.096) is shown in Figure 2b. These spectra show that, while not as pronounced, a small growth in the CPX state can also be clearly detected. Plots of the relative CPX:ICT intensities as a function of time for several representative dopant concentrations (Figure 2d) show a clear inverse relationship between growth of the CPX state and dopant concentration. After 6 weeks, most of the F4TCNQ-doped rr-P3HT films studied showed a significant increase of the CPX state with time (red data, Figure 2c), with the exception of the two highest dopant concentrations which did not exhibit any significant change in CPX state presence over this observation period. Other interesting observations in these spectral data are loss of the ν(C≡N) ag mode and the presence of as-yet unassigned spectral features denoted in light blue in Figure 1b in the high dopant concentration films. The peak at 2185 cm-1 has been assigned to the ν(C≡N) ag mode based on DFT calculations by Zhu et al.,29 although this assignment has come under scrutiny recently in work by Hase et al.39 The loss of intensity of this band at a rate different than loss of the ICT ν(C≡N) b1u peak is inconsistent with its assignment to the ν(C≡N) ag band of the ICT state. Moreover, the spectra in Figure 2b suggest that this band decreases at a rate similar to that of the unidentified bands in light blue, suggesting that these three bands originate from the same chemical species. The origins of these bands are being actively pursued in this laboratory and will be reported in a forthcoming publication. XPS results corroborate the trends observed in the time-dependent IR spectra as exemplified by the N 1s core level spectra shown in Figure 3 for representative χF4TCNQ 0.05 and 0.11 films. N 1s spectra for the other χF4TCNQ films are shown in the Supporting Information along

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Page 11 of 24

1.1

b

F4TCNQ 0.05

F4TCNQ 0.13

Area (CPX) / Area (ICT)

a

Initial

Intensity (cps)

Initial

Intensity (cps)

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


+ 2 weeks

50 cps

+ 2 weeks

+ 6 weeks

+ 6 weeks

1.0

c

0.9 0.8 0.7 F4TCNQ

0.6

0.05 0.096 0.13 0.17

0.5 0.4

410

405

400

395

Binding Energy (eV)

390

410

405

400

395

Binding Energy (eV)

390

0

1

2

3

4

5

Time (weeks)

Figure 3. XPS N 1s core level spectra 58 kDa rr-P3HT films for χF4TCNQ of a) 0.05 and b) 0.13 with time during storage in a dark inert atmosphere. c) Integrated area ratio for CPX to ICT with time for four χF4TCNQ.

with XPS spectra in the C 1s, S 2p, and O 1s regions for all films. The N 1s spectra were subjected to spectral decomposition based on assignments in the literature.34-36 The red band (398.2 eV) corresponds to F4TCNQ in its most reduced state, in this case the radical anion form associated with the ICT state. This has been assigned based on work done by Coletti35 and Koch. 40 Coletti et al. observed a peak at 398.3 eV in the N 1s core level spectrum after deposition of F4TCNQ on epitaxial graphene sheets. In similar work done by Koch et al., they assigned a peak at 397.8 eV to the anionic F4TCNQ peak on gold. The blue band (401.5 eV) likely has contributions from shake-up processes.40 This band has been reported in high resolution N 1s core level spectra of TCNQ,34, 36 with the shake-up peak in that case being due to the neutral TCNQ, and has also been accounted for in fits of N 1s spectra of F4TCNQ on surfaces.35, 40 The green band (400.0 eV) is more difficult to assign, as most literature reports attribute it to neutral F4TCNQ. However, as can be seen from the IR spectra in Figure 1 and our previous work,31 there is clearly no indication of substantial neutral F4TCNQ left in these films. Therefore, the proposed assignment for this band is the CPX state of F4TCNQ. This assignment is justified based on its higher binding energy relative to the ICT state, expected with only a partial negative charge, although it is noted that any residual neutral state may not be resolvable within the resolution of the XPS instrument.

11


6


As is clear from the N 1s XPS results in Figure 3, films of all dopant concentrations exhibit a continual loss of the ICT state with time in favor of the CPX. Interestingly, however, the time dependence of the growth of the CPX relative to the ICT with time (Figure 3c) differs from that observed in the FTIR data (Figure 2d) in that the CPX growth appears more pronounced in the XPS. The FTIR spectroscopy was done in transmission mode; thus, the spectra represent the average state of F4TCNQ throughout the 10-15 nm thickness of the film in contrast to the ~3-5 nm sampling depth for the N 1s XPS data. The XPS results therefore suggest that the enhancement of the CPX relative to the ICT starts at the outer surface of the film and moves inward before presumably attaining a more homogeneous distribution of ICT and CPX states throughout the film. It may be that the ICT participates in degradation reactions with ambient gases that are accelerated at the outer film edge. This hypothesis is being actively explored in these laboratories and will be reported at a later date. As a further check on the possible presence of neutral F4TCNQ in these films upon formation of the CPX state, we evaluated both the UV-visible spectra and the ν(C=C) region of the FTIR spectrum for any evidence of neutral F4TCNQ. In the UV-visible spectrum, neutral F4TCNQ exhibits a strong absorption band at 390 nm whereas that of the ICT state is at 415 nm in these films (see Figure S8). Although the UV-visible spectra clearly show the loss of the ICT state band, no concomitant growth of a neutral F4TCNQ band is apparent. Further evidence for the absence of neutral F4TCNQ comes from the FTIR spectral data. In the fingerprint region of these doped films, neutral F4TCNQ exhibits a strong v(C=C) band at ~1600 cm-1 that shifts down to ~1530 cm-1 in the ICT state. The absence of any significant intensity at 1600 cm-1 in these films at any stage of the aging process (see Figure S9) further confirms the absence of any significant amounts of neutral F4TCNQ in these doped films.

12


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With knowledge that the CPX state forms at the expense of the ICT state, and that CPX formation has a small but measurable impact on conductivity of the films, it is important to consider the driving force for this change. In our previous work, we postulated a local energetic driving force for charge state retention based on microstructure-electronic structure-property relationships. We observed the highest concentrations of CPX states with amorphous films that are harder to oxidize than rr-P3HT.31 While the exact structural locations of F4TCNQ giving rise to ICT and CPX are not yet known, formation of these two states can be described as occurring at two distinct sites within the P3HT, A and B, for the ICT and CPX states, respectively. The A sites represent those that enable the charge associated with F4TCNQ to contribute to high conductivity whereas the B sites are those that serve to localize the polaron, thus serving as trap sites leading to decreased conductivity. With the information in hand, it is currently unknown whether these sites can be directly and quantitatively related to P3HT microstructure. Following this line of reasoning, the formation of the ICT and CPX states can be described by the following equilibria, assuming that all processes are reversible: F4 TCNQA ↔

F4 TCNQ− ∙

Q1 =

[F4 TCNQ− ∙ ] [F4 TCNQA ]

(2)

F4 TCNQB ↔

F4 TCNQδ−

Q3 =

[F4 TCNQδ− ] [F4 TCNQB ]

(3)

With the result from previous calculations by Jacobs et al. that the ICT state is -0.23 eV/molecule below the neutral F4TCNQ state,30 a value for Q1 of 7.7 x 103 is calculated. Formation of the CPX at the expense of the ICT with time indicates that the CPX must be the more energetically favorable state. To understand the driving force for CPX formation, one must consider the possible mechanisms for this process that would convert ICT to CPX. Three mechanisms that were 13



Page 14 of 24

considered include i) movement of F4TCNQ from a P3HT A site to a B site, ii) migration of the polaron from a P3HT A site to a P3HT B site, or iii) reorganization of P3HT A sites into P3HT B sites. Regardless of specific mechanism, all of these possibilities can be considered in light of reactions (2) and (3) above as being represented by an additional equilibrium that represents the mechanistic step necessary for conversion of the ICT to the CPX: F4 TCNQA ↔

F4 TCNQB

Q2 =

[F4 TCNQB ] [F4 TCNQA ]

(4)

Moreover, using this framework, a simple equilibrium analysis yields insight into the driving force for conversion of the ICT to the CPX state through stepwise equilibria: Q′ = Q2 ∙ Q3 = Q1

[F4 TCNQδ− ] [F4 TCNQ− ∙ ]

(5)

Simulated intensities calculated by Zhu et al. for the F4TCNQ anion radical (the ICT state) and for charge transfer complexes between F4TCNQ and small oligothiophenes (the CPX state)29 allow us to estimate that the molar absorptivity for the CPX state is a factor of three lower than that of the ICT state. Using this estimate yields the relationship 3A(CPX)/A(ICT) = [F4TCNQ−]/[ F4TCNQ−•], and the data in Figure 2d along with the value of Q1 from the earlier work noted above can be used to estimate the free energy change for CPX formation (GCPX) with time. This value is a function of χF4TCNQ as shown by the values in Table 1. These values, substantiated by the spectral results, indicate that CPX formation at the expense of ICT is slightly more favored for smaller χF4TCNQ. Assuming that the free energy change for ICT formation is in the vicinity of -0.23 eV as noted above, the overall driving force for CPX formation from ICT is very small indeed, consistent with its very slow kinetics. A reaction coordinate diagram for this conversion is shown schematically in Figure 4.

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Table 1. Free energy change for CPX Formation for 58 kDa and 34 kDa rr-P3HT for different storage conditions. χF4TCNQ

3A(CPX)/ Q’ = Q2Q3 A(ICT) 58 kDa 58 kDa Inert Inert Dark Dark 0.48

3.70 x 103

0.13

0.42

3.23 x 10

3

0.11

0.78

6.01 x 103

0.096

1.02

0.076

3A(CPX)/ GCPX GCPX 3A(CPX)/ GCPX 3A(CPX)/ Q’ = Q2Q3 Q’ = Q2Q3 Q’ = Q2Q3 (eV/molec) A(ICT) (eV/molec) A(ICT) (eV/molec) A(ICT) 34 kDa 58 kDa 58 kDa 34 kDa 58 kDa 58 kDa 34 kDa 58 kDa 58 kDa Inert Ambient Ambient Inert Ambient Inert Inert Ambient Ambient Dark Dark Light Dark Dark Light Dark Dark Dark 0.89

6.84 x 103

-0.20

1.47

1.13 x 10

4

-0.22

3.40

2.62 x 104

7.85 x 103

-0.23

2.38

1.53

1.18 x 104

-0.24

0.063

2.01

1.55 x 104

-0.24

0.050

2.16

1.66 x 104

0.031

2.07 4.62

0.17

0.012

GCPX (eV/molec) 58 kDa Ambient Light

4.82

3.71 x 104

-0.24

5.85

4.51 x 10

4

-0.26

-

-

1.83 x 104

-0.25

14.95

-

-

-

-

-

-

-0.25

2.72

2.09 x 104

-0.25

1.59 x 103

-0.25

5.30

4.08 x 104

-0.27

-

-

-

-

-

-

4

-0.27

-

-

-

4.85

3.73 x 104

-0.27

7.45

5.74 x 104

-0.28

3.56 x 10

-0.21

-0.22

3.27

2.52 x 104

-0.26

-0.27

8.42

6.48 x 10

4

-0.28

-

-

-

-

1.15 x 105

-0.30

6.98

5.37 x 104

-0.28

-

-

-

-

-

-

-

-

-

-

-

-

7.18

5.53 x 104

-0.28

8.75

6.74 x 104

-0.28

-0.27

Literature precedent supports all of the possible mechanisms described above. Rapid diffusion of F4TCNQ in rr-P3HT films has been observed,16,

41

as have dopant-induced changes in film

microstructure and morphology,42 and chemical and photochemical reaction of P3HT with ambient gases.13-14, 20, 43-45 Although more information about the state of P3HT in these films is needed before any more definitive conclusions can be reached, the results presented here clearly support an increase in CPX state population at the expense of ICT states in these films with time, leading to

diminished

semiconducting

performance. Effect of P3HT Molecular Weight. To investigate further the possible role of polymer reorganization on activation energy for CPX formation, rr-P3HT with a smaller molecular weight was explored across

the

same

range

of

dopant

Figure 4. Proposed reaction coordinate diagram with relevant energetic considerations for formation of ICT or CPX from neutral rr-P3HT and F4TCNQ assuming identical initial energy for P3HT site A or B. Dependence of initial state energy on χF4TCNQ expected.

15



3.0 1.0 ICT

CPX

0.8 F4TCNQ 0.05

0.6

+1 week +2 weeks +4 weeks +6 weeks

0.4 0.2 0.0

b

2.5

ICT

0.8

A (CPX) / A (ICT)

a


1.0


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 16 of 24

0.6 F4TCNQ 0.13

0.4 0.2

CPX

+1 week +2 weeks +4 weeks +6 weeks

c

F4TCNQ

2.0

0.03

1.5 1.0

0.05

0.5

0.13 0.17

0.0

0.0 2100

2125

2150

2175

2200

2225 -1

Wavenumber (cm )

2250

2275

2100

2125

2150

2175

2200

2225

2250

-1

Wavenumber (cm )

2275

0

1

2

3

4

5

6

Time (weeks)

Figure 5. Transmission FTIR spectra from 34 kDa rr-P3HT films in ν(C≡N) region at different times during storage in a dark, inert environment for a) χF4TCNQ 0.05 and b) χF4TCNQ 0.13. c) Peak absorbance ratio of CPX to ICT b1u ν(C≡N) bands for four representative χF4TCNQ with time.

concentrations, as P3HT molecular weight impacts doping, conductivity, spectral response, and ultimately, microstructure of thin films.16, 46-47 Figure 5a shows spectra in the ν(C≡N) region as a function of time for χF4TCNQ 0.05 in 34 kDa rr-P3HT; these spectra can be compared directly to those shown in Figure 2a. Comparison of the two figures shows considerably more growth of CPX in the lower molecular weight rr-P3HT film (e.g. Figure 5a shows that CPX dominates at 6 weeks). Furthermore, it is also apparent that CPX growth is more pronounced at higher dopant concentrations in 34 kDa rr-P3HT (Figures 5b and c). Although there is still little CPX formation at the highest χF4TCNQ of 0.17, Figure 5c demonstrates a slightly increased amount of CPX formation for other high dopant concentrations relative to that observed in 58 kDa rr-P3HT films; thus, rr-P3HT molecular weight impacts the distribution of charge transfer states observed and the inherent stability thereof. Despite these absolute differences in relative amounts of CPX and ICT, the results for 34 kDa rr-P3HT are similar to those for the 58 kDa films in that GCPX increases inversely with χF4TCNQ. However, the driving force for CPX formation in this lower molecular weight rr-P3HT is slightly greater at a given χF4TCNQ than that for the corresponding 58 kDa films.46-47

16


Page 17 of 24

Effect of Ambient Exposure. We also investigated the impact of ambient exposure on CPX state formation. Figures 6a and b show representative FTIR spectra for 58 kDa rr-P3HT films stored under dark ambient conditions and under light-exposed ambient conditions, respectively. Figures 6c and d show plots for A(CPX)/A(ICT) taken from the FTIR spectra with time. Overall, a greater amount of CPX is formed for films stored in ambient compared with those stored in an inert environment. Based on the relatively minimal impact of light exposure on amount of CPX for the ambient light versus dark films, the primary effect of ambient storage in facilitating CPX formation would appear to come from ambient gases, primarily oxygen.

a

3.0

CPX



3.0 2.5 2.0

F4TCNQ 0.05

1.5

+3 days +1 week +2 weeks +4 weeks +6 weeks

1.0

ICT

0.5

b

CPX

2.5 2.0 F4TCNQ 0.05

1.5

+3 days +1 week +2 weeks +4 weeks +6 weeks

1.0 0.5

0.0 2125

2150

2175

2200

2225

2250

2275

2100

2125

2150

-1

c

5

F4TCNQ 0.012 0.05 0.096 0.13 0.17

3

2

2200

2225

2250

2275

Wavenumber (cm )

A (CPX) / A (ICT)

4

2175

-1

Wavenumber (cm ) 5

ICT

0.0

2100

A (CPX) / A (ICT)

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


1

d

4

F4TCNQ 0.012 0.05 0.096 0.13 0.17

3

2

1

0

0 0

1

2

3

4

5

6

0

Time (weeks)

1

2

3

4

5

6

Time (weeks)

Figure 6. Transmission FTIR spectra from 58 kDa rr-P3HT films in ν(C≡N) region at different times during storage for a) χF4TCNQ 0.05 in a dark ambient environment, and b) χF4TCNQ 0.05 in a light ambient environment. c) Peak absorbance ratio of CPX to ICT b1u ν(C≡N) bands for four representative χF4TCNQ with time in dark ambient environment. d) Peak absorbance ratio of CPX to ICT b1u ν(C≡N) bands for four representative χF4TCNQ with time in light ambient environment.

17



Conductivity results for films stored

10

-1


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 18 of 24

1

F4TCNQ 0.012

0.1

F4TCNQ 0.096

in

F4TCNQ 0.050

show

more

striking

differences relative to those stored in an

F4TCNQ 0.13

0.01

ambient

1E-3

inert environment. In-plane conductivity

1E-4

values for light-exposed films stored in

1E-5 1E-6

ambient shown in Figure 7 exhibit a

1E-7

rapid decrease over the first 2-3 weeks,

1E-8 0

1

2

3

4

5

beyond which the conductivity was

Time (weeks) Figure 7. In-plane conductivity as a function of time for χF4TCNQ 0.01, 0.05, 0.09 and 0.13 for films stored in ambient light conditions.

lower

than

10-7

S/cm

and

not

measurable. The loss in conductivity for

the χF4TCNQ 0.13 film is faster than that for the χF4TCNQ 0.09 film. This behavior may suggest that dopant concentration does not necessarily directly correlate to CPX formation in the ambient light stored system. This rapid loss in conductivity could be related to the growth of CPX states (Figure 6d) known to hinder charge transport,24 changes in microstructure of the film, and/or a decrease in the number of charge carriers due to photochemical degradation of P3HT or F4TCNQ. For example, the chemical stability of rr-P3HT is known to be poor, with both photo- and thermal oxidation to form sulfones on the thiophene backbone.13-14, 20, 43-45 Regardless, the time-dependent decrease in conductivity described above is clearly exacerbated by ambient exposure.

CONCLUSIONS This work further demonstrates that the co-existent charge transfer states in F4TCNQdoped rr-P3HT have direct implications on the functionality of the system. While ICT is definitively the kinetically favored state, over time, an intersystem conversion from ICT to form a 18


Page 19 of 24 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


δ=0.7-0.8e CPX state occurs; a quasi-quantitative analysis suggests that CPX is inherently the slightly more thermodynamically favored state. We suggest the change could be associated with one or more of the following processes: i) movement of F4TCNQ from a P3HT A site to a B site, ii) migration of the polaron from a P3HT A site to a P3HT B site, or iii) reorganization of P3HT A sites into P3HT B sites. We emphasize that regardless of specific mechanism, the change that occurs localizes the polaron, thereby preventing it from contributing to film conductivity. In other words, the once generated free carriers of the ICT states become non-functional due to their localization in trap sites (CPX). Of these mechanisms, the last is considered the most unlikely. At this time, we have insufficient information about the microstructure of these films with time to identify definitively the P3HT A and B sites and to distinguish between these three possibilities. Nonetheless, we can say with certainty that exposure to ambient conditions promotes formation of the CPX state, providing evidence for reaction chemistry between film components and the environment. Ultimately, migration to this partially-charged CPX state is responsible for the significant conductivity loss observed over time in doped films stored in ambient.

ACKNOWLEDGEMENTS The authors are grateful for support of this research by the National Science Foundation under award DMR-1608289. K.E.W. acknowledges financial support through an ARCS Foundation Scholarship.

19



SUPPORTING INFORMATION The Supporting Information section contains additional FTIR and XPS data for all F4TCNQ:P3HT compositions not shown in the main body of the text along with tabulated peak frequencies and conductivity data for P3HT as well as UV-Vis spectra of doped films. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXXX.

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23



TOC Graphic

Time (weeks)

-

-

-

-

-

-

-

-

-

3

1.5

ICT

1.0

CPX

Time

10

0

10

-3

10

-6

10

-9

10

-12

0.5

0.0

-

-

-

-

-

2

-1

Time


-

1

2.0

-

-

-

0

-

-


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

2100

2150

2200

2250 -1

Wavenumber (cm ) -

24


Page 24 of 24

Stability of Charge Transfer States in F4TCNQ-Doped P3HT

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