CHARACTERIZATION OF PBDB-T-SF: FULLERENE BLEND THIN …

Romanian Reports in Physics 72, 504 (2020)

CHARACTERIZATION OF PBDB-T-SF: FULLERENE BLEND THIN FILMS FOR SOLAR CELL APPLICATIONS

L. HROSTEA1,2, L. LEONTIE1, M. GIRTAN2,* 1 “Alexandru Ioan Cuza” University of Iasi, Faculty of Physics,

Bd. Carol I, nr. 11, 700506, Iasi, Romania 2 Photonics Laboratory (LPhiA) E.A. 4464, SFR Matrix, Angers University,

Faculty of Sciences, 2 Bd. Lavoisier, 49000 Angers, France *Corresponding author: [email protected]

Received January 22, 2020

Abstract. This paper presents a comparative study of two polymers: poly (3-hexylthiophene) (P3HT).and.poly[(2,6-(4,8-bis(5-(2-ethylhexylthio)-4-fluorothiophen-2-yl)-benzo[1,2-b:4,5 b’]dithiophene))-alt-(5,5(1’,3’di2thienyl-5’,7’-bis(2-ethylhexyl) benzo[1’,2’-c:4’,5’c’]dithiophene-4,8-dione)] (PBDB-T-SF) deposited on glass and indium tin oxide (ITO) substrates by spin coating, as polymer thin films and polymer: fullerene phenyl-C61-Butyric-Acid-Methyl-Ester (PCBM) thin films (90300 nm thickness). The comparison concerns: i) spectral dependence of optical properties measured by spectrophotometry and ellipsometry techniques; ii) morphological characteristics by atomic force microscopy; iii) electric properties. The decisive impact of chemical structure on photovoltaic behaviour of polymer-based materials is emphasized in case of PBDB-T-SF, containing two fluorine atoms into its polymer backbone, which improve its performance in solar cell applications.

Key words: polymer, PBDB-T-SF, polymer/fullerene blend thin films, fluorination.

1. INTRODUCTION

Materials for third and fourth generation solar cells have been continuously studied and developed, especially because of the advantages of their cost and their mechanical properties [1–23]. A particular attention was attributed to photoactive materials based on conductive polymers [24–26]. These materials exhibit electric and optical properties corresponding to both metals and semiconductors, while keeping their own specific mechanical properties, such as flexibility [27]. Other important advantages refer to: low-cost processing techniques, even if the costs of production are still high, this is due to the incipient stage of this emerging field which is still not so developed on the large scale; inexhaustible materials used; easy processability at low temperatures; wide range of colors; good interaction with light; possibility of deposition on a wide range of substrates, etc. [28–30]. Besides, the heterojunction structure, based on a donor material (polymer) and on an acceptor material (fullerene), exhibits a series of advantages, such as prevention

Article no. 504 L. Hrostea, L. Leontie, M. Girtan

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of charge carrier recombination, facilitating exciton dissociation and supporting the charge carrier transport to the electrodes [31].

In order to increase the efficiency of solar cells, n-type conjugated polymers have attracted an increasing interest. A method for improving the polymer structure is fluorination [32–35], consisting in the introduction of fluorine atoms into the polymer backbones. Fluorine atom has similar size to the hydrogen atom, allowing in this way the steric hindrance of polymeric intermolecular chain arrangement, and, at the same time, it is characterized by the highest electronegativity value, required for a large inductive electron-withdrawing effect [36–38]. The presence of fluorine atoms in polymer structure leads to enhanced charge carrier transport properties by improving material crystallinity, but also increasing electron affinity [32, 33]. Another advantage of fluorination is the ability to decrease the electrostatic forces between the electrons and holes (excitons separation) and to downshift the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the polymer [33].

This paper presents a characterization of a new polymer, poly[(2,6-(4,8-bis (5-(2-ethylhexylthio)-4-fluorothiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5(1’,3’di2thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T-SF, also called PCE13), that contains two fluorine atoms in its backbone. Results of optical, morphological and electric characterization are compared with those of other well-known polymer, poly (3-hexylthiophene-2,5-diyl) (P3HT).

2. MATERIALS AND METHODS

Two polymers have been used for thin-film preparation: regioregular (> 94.7%) P3HT, with an average molecular weight of 34,100, and PBDB-T-SF, having an average molecular weight of 50,100 (Fig. 1 a and b), together with a methafullerene: Phenyl-C61-Butyric-Acid-Methyl-Ester (PCBM) (Fig. 1 c). In order to avoid fullerene aggregation, 3% of 1,8-diiodooctane (DIO) was added in chlorobenzene solution.

Two sets of samples have been involved in the present study (Table 1), deposited by spin coating onto glass and indium tin oxide (ITO) coated glass substrates (sheet resistance 20 Ω/sq); their patterns are presented in Fig. 2.

a. b. c.

Fig. 1 – Molecular structure of: a. P3HT; b. PBDB-T-SF; c. PCBM.

Characterization of new polymer: fullerene blend thin films Article no. 504

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Table 1

The deposition features of the samples

Samples Polymer: Fullerene

weight ratio

Solution Concentration

(mg/mL)

Spinner speed (rpm)

Thickness (nm)

P3HT (1) 1:0 12.5 600–800 87 P3HT:PCBM (2) 1:0.8 10 600–800 96 Set 1

P3HT:PCBM (3) 1:0.8 16 500700 195 PBDB-T-SF (1) 1:0 10 600–800 96 PBDB-T-SF:PCBM (2) 1:1 10 600–800 137 Set 2

PBDB-T-SF:PCBM (3) 1:1 14 400600 330

Fig. 2 – The patterns of the samples.

The polymers deposited on glass substrate (Fig. 2 a) was used for optical

properties measurements. The absorption spectra were recorded by a two-beam spectrophotometer UV/VIS S9000 in a wavelength range of 250–1100 nm. The optical constants of the samples, refractive index, and extinction coefficient, were measured in the same wavelength range, at room temperature, by spectroscopic ellipsometry using an UVVISEL ellipsometer, including a 75 W high discharge Xen lamp, at an incidence angle of 70 0. The film thickness was measured using a Veeco Dektak 6M Stylus Profilomer. The roughness of the thin-film samples was determined using a Veeco atomic force microscope. The temperature dependent electric resistivity was measured for the polymers deposited on ITO patterned coated glass substrates (Fig. 2 b), using a regulated DC power supply and an Agilent 4339B High resistance meter. The electric transport measurements were performed during several heating/cooling cycles in temperature range between 30 and 120°C, under normal light and white light illumination of 1000 W/m2.

3. RESULTS AND DISCUSSIONS

Figure 3 depicts the absorption spectra of the polymers under study. In the case of P3HT polymer (black line), two main absorption bands peaked at 522 nm and at 552 nm are registered, followed by a shoulder at 595 nm, which is produced by the conjugated π – π* transitions [39]. The PBDB-T-SF polymer (red line)


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exhibits two absorption peaks at 580 nm and at 622 nm, attesting strong π – π intermolecular interactions [33]. The presence of the fullerene is manifested in both P3HT blend and PBDB-T-SF blend spectra (Fig. 4), through the absorption maximum located at approximately 336 nm [40]. From the optical point of view, PBDB-T-SF is a better optical absorber than P3HT, while the presence of PCBM exerts a detectable influence on the absorption of pristine polymers, by increasing it.

Fig. 3 – Absorption spectra of P3HT (black line) and of PBDB-T-SF (red line) (Color online).

Fig. 4 – Absorption spectra of P3HT: PCBM blend film [(1) – 87 nm thinckess; (2) – 96 nm thickness; (3) – 195 nm thickness)] left panel and of PBDB-T-SF: PCBM blend film [(1) – 96 nm thickness;

(2) – 137 nm thickness; (3) – 330 nm thickness)] right panel. The spectral dependences of refractive index and the extinction coefficient of

polymer samples were determined by ellipsometric measurements (Fig. 5 and Fig. 6). We point out that the maxima of the refractive indices, for both polymers, of 2.8 and 3.49, respectively, are located at approximately 640 nm wavelength. In the case of blends, the refractive index is decreasing, but its value refers to an effective


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refractive index, which represents a function of the component materials of the blend: the polymer and the fullerene. This is the reason for describing a pseudo-refractive index (pseudo-index of refraction), taking also into account presence of the glass substrate [13, 18].

Fig. 5 – Pseudo-refractive index of P3HT blend thin films from experimental data [(1) – 87 nm thickness; (2) – 96 nm thickness; (3) – 195 nm thickness)]; pseudo-refractive index of PBDB-T-SF blend

thin films from experimental data [(1) – 96 nm thickness; (2) – 137 nm thickness; (3) – 330 nm thickness)].

Fig. 6 – Pseudo-extinction coefficient of P3HT blend thin films (left panel) and of PBDB-T-SF blend thin films (right panel) from ellipsometric experimental data.

In the visible range, the extinction coefficient of global sample shows a

decrease in magnitude after adding fullerene (Fig. 6). This behaviour is confirmed by the absorption coefficient that has been calculated in two ways: extracted from the absorption spectra (spectrophotometric measurements) and determined from the extinction coefficient (ellipsometrically determined) – Fig. 7.


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Fig. 7 – Comparison between the absorption coefficients calculated from transmission values,

by spectrophotometry, and according to the extinction coefficient, by ellipsometry. Taking into consideration the direct relationship between the absorption and

extinction coefficients, 4 ,k

as well as data from Figs. 3–5, it can be inferred that

the highest absorption is signalled in the wavelength range of 400–700 nm. Figure 7 describes the spectral dependences of the absorption coefficient determined indirectly, using spectrophotometric and ellipsometric investigations, leading to similar results.


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Based on the obtained experimental data, an ellipsometric model (Fig. 8) was build, able to simulate perfectly the samples behaviour. The model is based on several dispersion formulae, trying to improve the simulated structure in order to obtain the best fit, controlled by χ2 (Pearson’s chi-squared test). The ellipsometric model was developed only for polymer thin films. The polymers are defined by New amorphous dispersion formula (NA); each of them having different specific parameters (listed in Table 2). Using new amorphous dispersion formula, the refractive index and the extinction coefficient of the simulated structures are given by relations (1) and (2), respectively:

2 2

( )( ) ,

( )j

j j

B Cn n

(1)

2 2

( ); for

( )( )0; for

j jg

j j

g

fk

(2)

2 2[ ( ) ]here: .

2 ( )

jj j j g

j

j j j j g

fB

C f

In the above relations, n(ω) and k(ω) represent the refractive index and the

extinction coefficient, respectively, Γj is the broadening parameter of the absorption peak (0.2 < Γj <8), fj denotes the amplitude of the extinction coefficient (the oscillator strength) (0 < fj < 1), (ωj) defines the energy corresponding to the maximum extinction coefficient (1.5 < ωj < 10), while ωg denotes the energy corresponding to the minimum extinction coefficient (ωg < ωj) [41].

Fig. 8 – Ellipsometric model used to describe the polymeric thin films. In order to check the accuracy of the ellipsometric model, χ2 values have been

used (listed in Table 3), which quantifies the goodness of the fit (χ2 < 10). Another way for confirming the validation of the model is a comparison between thickness values obtained by ellipsometric and profilometry measurements, as well as between those of the energy gap determined by ellipsometric and spectrophotometric data for transmittance. The similarity of obtained results certifies the proposed ellipsometric model.


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Consequently, the above defined ellipsometric model was applied, to determine the individual ellipsometric parameters, such as refractive index, extinction coefficient (Fig. 9), the real part of the dielectric constant and the imaginary part of it (Fig. 10), for each of the polymers.

Table 2

The specific parameters used to obtain the best fit of the ellipsometric model for the experimental data

Specific parameters Sample Dispersion formula n∞ ωg (eV) fj (eV) ωj (eV) Γj (eV)

–0.15 ± 0.01 2.24 ± 0.1 –1.07 ± 0.08 P3HT 2 × NA 1.14 ± 0.02 1.37 ± 0.04 0.12 ± 0.01 2.27 ± 0.01 0.29 ± 0.01 Set 1 PBDB-T-SF NA 1.31 ± 0.03 1.77 ± 0.03 0.28 ± 0.02 2.09 ± 0.02 0.20 ± 0.01

Table 3

Comparison between the thickness values and the band gap values obtained by spectroscopic ellipsometry and a different technique (profilometry and spectrophotometry)

Thickness (nm) Band gap (eV) Polymer χ2 Ellipsometry Profilometry Ellipsometry Spectrophotometry P3HT 3.25 114 ± 2 ~ 87 1.80 1.94 Set 1 PBDB-T-SF 7.6 89 ± 8 ~ 96 1.83 1.87

As can be inferred from Fig. 9, simulated n() and k() spectra for the polymer

thin films are similar to those of pseudo-coefficients presented in Figs. 5 and 6.

Fig. 9 – Refractive index (continued line) and extinction coefficient (dotted line) for polymer thin

films (black line – P3HT, red line – PBDB-T-SF) from simulated data (Color online). As a specific feature of the polymers (as organic semiconductors), the value

of dielectric constant [42] is lower comparing to that of organic semiconductors: in the present case, the real part of εr = 1 6.


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Fig. 10 – Dielectric constant – real part (left panel) and imaginary part (right panel) for polymer thin films (black line – P3HT, red line – PBDB-T-SF) (Color online).

From 3D AFM scans (Fig. 11), similarity of topographical features of

both polymer and polymer: fullerene blend thin-film samples can be found. The AFM investigation of the surface morphology of organic films under study revealed homogeneous and smooth samples having a roughness in the 5–10 nm range.

Sample Polymer Polymer: Fullerene

P3HT

PBDB-T-SF

Fig. 11 – Topography of polymer and polymer: fullerene blends thin films.


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Fig. 12 – Temperature dependence of resistivity of P3HT thin films (left)

and PBDB-T-SF thin films (right). Figure 12 shows the temperature dependence of resistivity for actual organic

materials; it was determined during successive heating/cooling processes, in a temperature range of 30–120°C. The resistivity of both P3HT and P3HT:PCBM blend thin films was found to decrease with the increase in temperature, confirming the behaviour reported in a previous work [43]. It is to be noted the strong influence of fullerenes (PCBM) on polymers resistivity, a more pronounced effect being registered in the case of PBDB-T-SF. For PBDB-T-SF, both polymer and blend thin films, the resistivity displays a tendency to a minimum value of 10–30 kΩ·cm.

4. CONCLUSIONS

Two types of polymers were studied in this work: P3HT and PBDB-T-ST, in polymer and polymer: fullerene (PCBM) blend thin films with thicknesses between 100 and 300 nm, in order to emphasize the properties of PBDB-T-SF using P3HT as reference. PBDB-T-SF was found to be a better optical absorber than P3HT. From extinction coefficient spectra, a significant absorption caused by polymers and fullerene was emphasized in the visible wavelength range – the constituents’ fingerprints. A study of individual polymer layer was performed by ellipsometry technique, before verifying the validity of model simulation, taking into account connections with profilometry (for thickness values checking) and with spectrophotometry (absorption coefficient matching and band gap energy values). From morphological point of view, the samples under study are homogeneous and atomically smooth. The presence of fullerenes strongly influences the samples resistivity, by decreasing it. The greatest effect of adding fullerenes is registered for PBDB-T-SF polymer.


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CHARACTERIZATION OF PBDB-T-SF: FULLERENE BLEND THIN …