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Evidence on enhanced exciton polarizability in donor:acceptor bulk-heterojunction organic photovoltaics Zhiqiang Guan, Ho-Wa Li, Yuanhang Cheng, Yingqi Zhao, Ming-Fai Lo, Sai-Wing Tsang, and Chun-Sing Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15437 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Materials & Interfaces

Evidence on enhanced exciton polarizability in donor:acceptor bulk-heterojunction organic photovoltaics Zhiqiang Guan,†,‡ Ho-Wa Li,§ Yuanhang Cheng,§ Yingqi Zhao,

†,§

Ming-Fai Lo,

†,‡

Sai-Wing

Tsang*,§ Chun-Sing Lee*,†,‡ †

Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong,

Hong Kong SAR, P. R. China ‡

Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China

§

Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong

SAR, P. R. China KEYWORDS:

organic

photovoltaic,

electroabsorption,

exciton,

polarizability,

bulk-

heterojunction

ABSTRACT: Using electroabsorption (EA) spectroscopy, we explore the polarizability of Frankel exciton in both pristine donor and D:A blend films. We observe for the first time that the polarizability of excitonic states in pristine donors can be dramatically increased by adding an ntype acceptor. By investigating the dielectric effect in different organic semiconductor systems,

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we find that the polarizability of Frankel excitons has direct correlation with the measured dielectric constant of the bulk heterojunction thin films. Our results disclose the difference in the nature of Frankel exciton in pristine donor and D:A blend system, revealing an important role of excitonic states in charge separation process of OPV.

1. Introduction Organic photovoltaic (OPV) device are recognized as the future energy-generation technology due to the potential of light-weight, flexibility and low-cost manufacturing process. 1,2

It is well known that blending of polymer donor (D) and fullerene acceptor (A) can give

devices with much better performance than using D and A in separated layers.3,4 This has been attributed to the formation of bulk heterojunction and the unique interpenetrating morphology of the D and A spices. With the bulk heterojunction, photo-generated Frankel excitons can easily diffuse to the D/A interface before decay and this allows the use of an active layer thickness larger than the exciton diffusion length for better photon absorption.5,6 The bulk heterojunction also plays a critical role in the dissociation of Frankel exciton by providing an energy difference between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of donor and acceptor. Upon dissociation of the Frankel excitons into free carriers at the D/A interface, the electron and hole can easily transported to the opposing electrode via the bicontinuous A and D networks.7,8 So far, the influence of fullerene as acceptor on the performance of OPV devices has been studied on the aspects of interfacial charge-transfer (CT) states at bulk-heterojunction9-11 and morphology of active layer6. On the other hand, performance of polymer:fullerene solar cells


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depends critically on the properties of Frankel excitons in the systems.1 However, most researches on the characteristics of Frankel excitons were made on the pristine polymer thin films, rather than on the corresponding polymer:fullerene blend films. It is generally presumed that the properties of Frankel excitons in the D:A blend systems are the same as those in pristine donor material. However, the lack of the studies on Frankel excitons in actual blend devices results in an incomplete understanding on the organic electronic materials. Specifically, the influence of fullerene on the property of Frankel excitons has been overlooked. In this work, we study the polarizability of excitons in three different polymer:fullerene systems using electroabsorption (EA) spectroscopy.12,13 In particularly, it was observed that when fullerene was added into the polymers, polarizability of the polymer’s exciton increases with the fullerene content. This result not only shows that Frankel exciton properties in polymer:fullerene systems cannot be presented by those in the pristine polymers, it has another important implication on our understanding on the enhancement effects on the polymer:fullerene blending. The present results show that in addition to the effects of energy difference between the LUMO and HOMO of donor and acceptor at bulk heterojunction and interpenetration network, the blending in fact has the third beneficial effect that increases the polarizability of donor excitons and thus facilities more efficient charge carrier dissociation.

2. Experimental Methods Solar Cell Preparation: Solar cells were fabricated by preparing organic solutions on ITO-based substrate. The preparation of organic layers is shown as following: PDTSTPD:PC71BM (8 mg/ml:12 mg/ml) were dissolved in chlorobenzene (CB) with 3% v/v 1,8-


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diiodooctane (DIO) as a solvent additive. Pristine PDTS-TPD was dissolved in CB with a concentration of 10 mg/ml. The spin-coated films were annealed at 70 oC for 10 mins. P3HT:PC71BM were mixed with a weight ratio of 1:1 (15 mg/ml:15 mg/ml) and dissolved in 1,2dichlorobenzene (DCB). Pristine P3HT was dissolved in DCB with a concentration of 20 mg/ml. Spin-coated films were annealed at 140 oC for 10 mins. PTB7:PC71BM (1:x, total concertation: 25 mg/ml) were dissolved in CB with 3% v/v DIO. The concentration of pristine PTB7 solution is 15 mg/ml. After spin-coating, the films were left on 40 oC hotplate for one night. Pristine PCDTBT was dissolved in CB with a concentration of 10 mg/ml. PTB7:PCDTBT (3 mg/ml:3 mg/ml) were dissolved in CB. The spin-coated films were dried at 70 oC for 10 mins. PTB7:N2200 were mixed with a ratio of 1:1 (7 mg/ml:7 mg/ml) and dissolved in CB. The film was annealed at 70 oC for 15 mins. Bathocuproine (BCP) (7 nm) and aluminum (80 nm) were thermal-evaporated on the samples. The active area is 0.12 cm2. Electroabsorption (EA) Spectroscopy: An 1 kW Xe lamp was used as the light source. The white light passed through a monochromator and then illuminated the active layer with an angle of 45o. It was then reflected by the Al electrode. Long pass filters with stopband transmission less than 0.01% were used to eliminate the higher order diffraction from the monochromator. A sinusoidal modulated signal with the Vpp of 0.5 V and 1000 Hz was applied on the OPV devices with a negative DC bias of 2V. The -2 V voltage was used to deplete the device by extracting the free carriers out of the active layer and prevent charge injection from the electrodes. All devices were measured under same electric field. A Si (400 nm ~ 900 nm) or a Ge (> 900 nm) photo-detector was used to receive the reflected light from OPV device. Photodetectors were connected with a current amplifier and a lock-in amplifier to increase the signalto-noise ratio. Optical transmittance signals with (∆T) and without (T) the electric field


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modulation were measured under excitation of different photon energies. The EA signal is recorded as ∆T/T, which is the ratio of the transmittance signal with electric field and without it. It exhibits the influence of external electric field on the change of absorption. All measurements were carried out at room temperature. J-V, C-f and Thickness Measurement: Current-voltage features were recorded with a Keithley 237 source meter. Photovoltaic features were performed with the illumination of 100 mW/cm2. The capacitance (C)-frequency (f) relationship of solar cells were measured with an LRC meter. Before measurements, calibration was made to remove parasite effect.14 Capacitance was measured by applying a modulated alternating-current signal of 20 mV, 104~105 Hz. This high-frequency condition guarantees the dielectric response becomes the only contribution to the capacitance measurement. The thickness of active layers was recorded with atomic force microscopy. Modeling of EA: Fitting of the EA results is based on the Stark effect of excitonic states under the perturbation of electrical field.15 The molecules can be excited from ground states to excited states by light pumping, generating a various excited states in molecules, e.g. Frankel exciton and CT states. It has been proved that CT states can be directly excited with sub-bandgap energy.12 Compared with the ground state, the excited state has an intrinsic dipole. In Stark effect, the molecular dipole interacts with an applied electric field, leading to the Stark shift in transition energy (∆E) of an excited state.12,16 Based on second-order perturbation theory, the ∆E can be expressed as:

∆E = ∆µ ⋅ F +

1 ∆p F 2 2

(1),


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where ∆p is the change of average polarizability tensor upon excitation, ∆µ is the difference between ground and excited state dipole moments, F is the strength of the electric field.17 The EA spectra record the change of the absorption coefficient (∆α) under the electric field. The ∆α as a function of ∆E can be expressed in terms of McLaurin series truncated at the quadratic term:

∆α =

∂α 1 ∂ 2α ∆E + ( ∆E ) 2 + ⋅⋅⋅ 2 2 ∂E ∂E

(2).

Combining Equation 1 and 2 yields the relationship between ∆α and the derivatives of α:17-19

1 1 2 2 d ∂ 2α 2 d ∂α d ∆α = ∆pF + ∆µ F ∂E 6 ∂E 2 2

(3),

where d is the film thickness. In the EA results, the signal intensity is equal to the product of the optical path length in the active layer and ∆α, i.e. ∆T/T = 2d∆α, where the double of the film thickness is approximate to the optical path length in reflection mode.20 Based on the form of Equation 3, both Frankel excitons and CT states contribute to the ∆p and ∆µ in EA signal. However, previous literature has reported that Equation 3 will exhibit different forms for different excited states.17 For the Frenkel excitons, as the molecules are randomly oriented, 〈∆µ⋅F〉 = 0 and the only contribution to the EA signal comes from the polarizability of exciton. On the other hand, for the CT states, due to the long h-e distance, the transition dipole moment is much larger than the influence of polarization. Therefore, the EA signals should follow the first derivative of absorption coefficient (α’) if the Frankel exciton intramolecular transition dominates the Stark effect, whereas it will follow the second derivative


6

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term (α’’) if the EA result is mainly influenced by intermolecular CT transitions.17,19 For the organic conjugated system, where both intramolecular and intermolecular transitions occur, the EA spectrum is usually composed of two terms.18 By fitting the EA spectrum with the weighted first and second derivatives of α, the ∆p of excitonic states and ∆µ of CT states can be extracted. In order to obtain the derivatives of α, we calculate the α of the active layers from the measured absorbance spectrum (A), which is the common logarithm of the ratio of incident to transmitted radiant power through a film:

A = − log(

I ) I0

(4);

where, I is the radiant flux transmitted by the film and I0 is the radiant flux received by the film. For the film with absorption coefficient of α and thickness of d, the relationship between I and I0 is:

I = I 0 exp(−α d )

(5).

Combining Equation 4 and 5 yields the relationship between α and A:

α=

1 ln (10 A ) d

(6).

Thus, absorption coefficients can be calculated by measuring the absorbance and the thickness of the films.


7


The measured A and calculated α of both pristine PDTS-TPD and PDTS-TPD:PC71BM (1:1.5) films are respectively shown in Figure S1a (Supporting Information) and Figure 1a. The thicknesses of the films are listed in Table 1. With the obtained α, we calculate the first and second derivatives of α, which are shown in Figure 1b and Figure 1c, respectively. Corresponding results for P3HT and PTB7 are shown in Figure S1~S3 (Supporting Information). The absorbance and derivatives of absorption coefficient are measured as a function of incident light. In order to compare the EA peak positions with corresponding excited states, the α and its derivatives are plotted as a function of energy.

(a) 2.5

PDTS-TPD PDTS-TPD:PC71BM

2.0 -1

α (×10 cm )

1.5

5

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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1.0 0.5 0.0 1.6

1.8

2.0

2.2

2.4

2.6

Energy (eV)


8

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(b) 3

PDTS-TPD PDTS-TPD:PC71BM 1

6

-1

-1

α' (×10 cm eV )

2

0

-1 1.6

1.8

2.0

2.2

2.4

2.6

Energy (eV)

(c) 4 2

7

-1

-2

α'' (×10 cm eV )

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 -2 -4

PDTS-TPD PDTS-TPD:PC71BM

-6 1.6

1.8

2.0

2.2

2.4

2.6

Energy (eV)

Figure 1. (a) Absorption coefficient (α), (b) 1st derivative of α (α’) and (c) 2nd derivative of α (α’’) of pristine PDTS-TPD and PDTS-TPD:PC71BM blend films.

3. Results and Discussion


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We used three widely used polymers as donor materials, namely poly[[4,8-bis[(2ethylhexyl)oxy] benzo[1,2-b:4,5-b’] dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b] thiophenediyl]] (PTB7), poly(3-hexylthiophene) (P3HT) and poly[(5,6-dihydro-5octyl-4,6-dioxo-4H-thieno [3,4-c]pyrrole-1,3-diyl) [4,4-bis(2-ethylhexyl)-4H- silolo[3,2-b:4,5b';] dithiophene-2,6-diyl]] (PDTS-TPD) (Figure 2). The fullerene acceptor is [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). Device fabrication, electroabsorption spectroscopy (EA), capacitance (C) measurement, etc. are described in details in the Experimental Methods of the paper.

Figure 2. Chemical structures of P3HT, PTB7, PDTS-TBD and PC71BM.


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It has to mention that no fitting process can fit both the excitonic state at the lowest energy and mAg excitonic states at the higher energy simultaneously. According to Petelenz et al., current fitting standard provides a single solution for the excitonic state at the lowest energy (i.e. first excitonic peak), which gives information on singlet Frankel excitons of the polymer donor in pristine and in the blend systems.12.18,21-28 Figure 3a shows the EA spectra and fitting curves of pristine PDTS-TPD and PDTS-TPD:PC71BM blend films from 1.55 to 2.6 eV, covering the first excitonic peak (zero-phonon exciton, 1.75 eV), phonon side bands (1.95 eV) and two-photon (mAg) excitons (2.2~2.3 eV).29-31 For zero-phonon exciton and phonon side bands, the fitting curve (solid line) is well matched with the EA signals (square symbol) in pristine PDTS-TPD film. In PDTSTPD:PC71BM, a good fit (dashed line vs. circle symbol) can also be found in zero-phonon exciton, only with a small deviation for the phonon side bands. Figure 3b and c show EA fitting results of the first excitonic peaks for the P3HT- and the PTB7-based devices, respectively. All results show good fittings in the energetic region of zero-phonon exciton. For higher energy regions, mAg excitonic states or other excitonic species strongly affect the EA signal, and thus Equation 3 is no longer valid in this region.


11


(a) 6 First excitonic peak 4

Phonon side bands

mAg excitons

-3

∆T/T (×10 )

2 0 -2

EA spectra: Fitting: PDTS-TPD PDTS-TPD PDTS-TPD:PC71BM PDTS-TPD:PC71BM

-4 -6

1.6

1.7 1.8

1.9

2.0 2.1 2.2 2.3

2.4 2.5 2.6

Energy (eV)

(b) 6 5

EA spectra: P3HT P3HT:PC71BM

Fitting: P3HT P3HT:PC71BM

4 -4

∆T/T (×10 )

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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3 2 1 0 -1 1.8

1.9

2.0

2.1

2.2

Energy (eV)


12

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(c) 1.5 1.0 -3

∆T/T (×10 )

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


0.5 0.0 -0.5 -1.0 1.5

EA spectra: Fitting: PTB7 PTB7 PTB7:PC71BM PTB7:PC71BM 1.6

1.7

1.8

1.9

Energy (eV)

Figure 3. The measured and fitted EA spectra of the three different donors and blended with fullerene (a) PDTS-TPD and PDTS-TPD:PC71BM, (b) P3HT and P3HT:PC71BM and (c) PTB7 and PTB7:PC71BM.

The extracted fitting parameters of ∆p and ∆µ as well as the thickness of active layer, d, are listed in Table 1. It can be seen that adding PC71BM increases both ∆p and ∆µ. For PDTSTPD, after blending with PC71BM, the ∆p of excitons increased from 70×10-24 to 950×10-24 cm3, for more than 13 times. For P3HT and PTB7, ∆p increase from 295×10-24 to 810×10-24 cm3 and from 210×10-24 to 820×10-24 cm3, respectively. These significant changes indicate that the polarizability of exciton in polymer donors is dramatically increased by incorporating fullerene domains into the pristine polymers. For isotropic media, like disorder polymer domains, the polarizability (p) is proportional to the h-e dipole moment (µ), i.e. p = µ / F, which is the product of elementary charge and the distance between two charges. We herein speculate that the Frankel exciton in the polymer:fullerene system have longer h-e separation distance than


13


Page 14 of 28

that in pristine polymer. According to Onsager theory which describes the charge separation and recombination dynamics in a Coulombically bound h-e pair,32,33 a longer separation distance results in a higher charge escape probability from Coulomb attraction, leading to a more efficient charge separation process in D:A blend film than in pristine polymer film. Table 1. The extracted fitting parameters of ∆p and ∆µ as well as the thickness of active layer (d) used in EA fits for different systems. Device PDTS-TPD PDTS-TPD:PC71BM P3HT P3HT:PC71BM PTB7 PTB7:PC71BM (1:0.5) PTB7:PC71BM (1:1) PTB7:PC71BM (1:1.5) PTB7:PC71BM (1:2) PCDTBT PTB7:PCDTBT PTB7:N2200

∆p (×10-24 cm3) 70 950 295 810 210 765 775 820 810 235 190 760

∆µ (D) 5.2 14.6 5.8 5.9 4.5 7.5 9.5 10.0 12.0 2.0 9.2 7.2

d (±5 nm) 80 100 100 120 80 92 90 125 125 90 60 120

On the other hand, ∆µ increases from 5.2 to 14.6 D for PDTS-TPD and from 4.5 to 10.0 D for PTB7 after blending with fullerene. Since ∆µ reflects the dipole moment change of CT states, this increase should be attributed to the more delocalized h-e pairs at the polymer:fullerene interfaces. It should be noted that the increase of ∆p is resulted from the change of exciton polarizability in the polymer donor, rather than that in PC71BM. Previous literature has proved that the EA signal of the excitonic state of fullerene is much smaller than that of polymers (one order magnitude lower in intensity),12 which means that the fullerene’s contributions to the EA


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spectra in the blends are negligible. This rules out the possibility that the high ∆p in the blend film is from the Frankel exciton of PC71BM. In addition, as the EA spectra of OPV device is different from the film absorption of active layer, the increase of ∆p and

∆µ cannot be

attributed to the increase of absorption due to the fullerene. In order to check how the amount of PC71BM influences the polarization of exciton and dipole moment of CT states, we next investigate the changes of ∆p and ∆µ in PTB7:PC71BM with different D:A ratios. We here tune the D:A ratios from 1:0.5 to 1:2 and compare them with pristine PTB7 film. For a fair comparison, we tune the D:A ratios while keeping the total concentration of solutions unchanged to obtain the similar thickness of the active layers. The EA spectra (symbols) and fitting results (solid lines) are shown in Figure 4a. Their absorption coefficient and derivatives of α are shown in Figure S4 and S5 (Supporting Information). Good matches are observed for all the fitting curves at the first excitonic state. The extracted ∆p and ∆µ values are listed in Table 1 and compared in Figure 4b. It can be seen that the ∆p dramatically increases from 210×10-24 to 765×10-24 cm3, by adding 50 wt% of PC71BM, indicating the Frankel excitons in polymer have been highly polarized in the blend. Further increasing the PTB7:PC71BM ratio to 1:2 only causes a mild increase in ∆p. For ∆µ, the values keep increasing from 4.5 D of pristine PTB7 to 12.0 D in the 1:2 film. This continuous enhancement of the dipole moment should be attributed to the substantial introduction of CT states as the increase of polymer:fullerene interface. In addition, at high PC71BM ratio, fullerene molecules aggregate and form a large domain size. The aggregated fullerene domain increases the delocalization of CT states, enabling a longer range of charge separation distance, thus raising the dipole moment of CT states as well.25


15


(a) 2 1 0 -1 2

1:2

1:1.5

1

-3

∆T/T (×10 )

0 -1 2

1:1

0 2

1:0.5

0 1 1:0 0 1.5

1.6

1.7

1.8

1.9

Energy (eV)

(b)

14 900 800

12

10

∆p (10

-24

3

cm )

700 600 500

8

∆µ (D)

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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400 ∆p

300

6

∆µ

200 0.0

0.5

1.0

1.5

2.0

4

PC71BM ratio (1:x)


16

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Figure 4. (a) The EA spectra and fitting results of PTB7:PC71BM devices with different D:A ratios; (b) The changes of ∆p and ∆µ with different PC71BM ratio in PTB7:PC71BM devices.

To further investigate the influence of polarizability of exciton and dipole moment of CT states on the property of organic films, we here compared the extracted ∆p and ∆µ with the measured relative dielectric constant (εr). In order to provide a more comprehensive picture, we investigate another three systems of different natures, i.e. (1) polymer:non-fullerene acceptor of PTB7:N2200, (2) pristine polymer of PCDTBT and (3) two polymer donors’ blending of PTB7:PCDTBT. The ∆p and ∆µ values are obtained with the above-mentioned EA fitting method and listed in Table 1. The fitting results are shown in Figure S6 (Supporting Information). The εr is extracted through the parallel plate capacitor formula, C = ε0εrA/d, where C is the capacitance, ε0 is the vacuum dielectric, A is the device area and d is the thickness of the film. The capacitance values of these systems are determined by measuring the capacitancefrequency (C-f) relationship of the metal/organic/metal sandwich structure devices. The C-f relationships are shown in Figure S7 (Supporting Information) and the calculated εr values as a function of frequency are shown in Figure 5a. We select the εr values at 104 Hz as the device geometric dielectric constant since this high frequency zone is related with the bulk polarization.34,35 A drop of εr at higher frequency (> 105 Hz) is due to the parasitic effects.11 The

εr values are 2.89, 3.63, 2.52 and 3.32 respectively for PCDTBT, PTB7:PC71BM, PTB7:PCDTBT and PTB7:N2200.


17


(a) 4.0 3.5 3.0

εr

2.5 PTB7:PC71BM

2.0

PTB7:N2200 PCDTBT PTB7:PCDTBT

1.5 1.0

10

4

10

5

10

6

Frequency (Hz)

(b) 1000 12

∆p (10

10 PTB7:PC71BM 8

600 PTB7:PCDTBT

-24

3

cm )

800

6 PTB7:N2200

400

∆µ (D)

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 28

4 ∆p

200

∆µ

2

PCDTBT 0 2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

0 4.0

εr

Figure 5. (a) The calculated dielectric constant (εr) as a function of frequency and (b) the comparison of ∆p and ∆µ with εr for four systems of pristine PCDTBT, PTB7:PC71BM, PTB7:PCDTBT and PTB7:N2200, respectively. The solid line is used for eye guidance.


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The obtained εr values are compared with ∆p and ∆µ, shown in Figure 5b. It can be found that ∆p (circle symbols) shows an approximately linear relationship with εr for different systems. With the increase of εr from 2.52 to 3.63, the ∆p values also raise from 190 to 820×1024

cm3. Meanwhile, we find the introduction of n-type polymer acceptor, N2200, can also

enhance the ∆p in the organic system, indicating a similar role of n-type polymer acceptor with PC71BM in improving the polarization of donor excitons. This result is consistent with the fact that novel non-fullerene acceptors can achieve equally high charge separation efficiency with fullerene.33 However, the system of two polymer donors does not exhibit a high εr. Correspondingly, the ∆p of this system is also low, even lower than the pristine polymer film. This relationship between ∆p and εr indicates that the improvement of polarization of excitonic states in polymer leads to the increase of dielectric constant of the film. On the other hand, there is no obvious trend between ∆µ (square symbols) and εr. We observe that ∆µ can be effectively increased by forming a type-II heterojunction, which introduces interfacial CT states. Even a heterojunction of two polymer donors can result in a dipole moment as high as that of D:A structure. This comparison proves a direct correlation between dielectric effect and excitonic polarization, rather than dipole effect of CT states. As known, one major disadvantage of organic semiconductors compared with inorganic counterpart is the low εr. The typical εr values around 3 in organic semiconductors are significantly lower than that of Si (11.7). According to the Coulomb’s Law, the exciton binding energy (Eb) is inversely proportional to εr. The weak dielectric environment hinders the charge separation from Frankel excitons and causes severe energetic loss, leading to a limitation of photovoltaic parameters in OPV.19,37,38 Our comparisons between ∆p and εr suggest that in


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order to enhance the dielectric effect, one possible approach is to design materials or structures that can increase the polarization ability of the excitons. A schematic diagram in Figure 6 illustrates our findings on the change in polarization for the Frankel excitons in different environments. For the pristine polymer, the photo-excited exciton is a closely bound h-e pair with weak polarizability. When blending with the fullerene, or other n-type acceptors, the polarizability of excitons in polymer domain is dramatically enhanced, meaning that the excitons in D:A blend film can be highly polarized by the external electric field. The highly polarized excitons are more likely to dissociate into interfacial CT states than the closely bound excitons in pristine donor, leading to a higher charge separation efficiency. This result shows the important role of Frankel excitons in photo-induce charge separation process. It indicates that adding n-type acceptor not only introduces interfacial CT states, but also promotes the polarization of excitonic states. Moreover, we find that the polarizability of exciton in organic semiconductor materials directly correlates the dielectric constant of the active layer, providing possible approaches for improving the dielectric environment in organic electronic devices.


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Figure 6. Schematic diagram of the device with the active layer of pristine polymer or polymer:fullerene blend. For the pristine polymer, the Frankel exciton is closely bound h-e pair. On the other hand, after inducing fullerene, the Frankel excitons are highly polarized. These polarized Frankel excitons are more likely to dissociate into interfacial CT states and finally become free charge carriers.

4. Conclusion In summary, using EA spectroscopy, we observe that the polarizability of excitonic states of donor materials can be significantly increased by blending with n-type acceptors, which is different from previous opinion that Frankel excitons are the same in pristine donor and in D:A blend systems. This highly polarized exciton is easier to dissociate and contribute to free charge carriers according to the Onsager model. In addition, by comparing several organic semiconductor systems, we find that the dielectric effect of organic semiconductor films is directly determined by the polarizability of excitons, rather than the interfacial CT states. Our finding reveals an important role of the polarizability of excitons in photo-induced charge separation process. It suggests possible approaches for the designing D-A system for improving the excitonic polarization and more effective carrier dissociation.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.


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Absorbance, derivatives of absorption coefficient, EA fitting results and capacitance-frequency relationship AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Telephone number: +852 34427826 * E-mail: [email protected]. Telephone number: +852 34424618 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work is funded by National Natural Science Foundation of China (No. 51473138) and Research Grants Council of the Hong Kong SAR, China (Project No. CityU 11304115 and 21201514). REFERENCES (1)

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