Conformational Change on a Bithiophene-Based Copolymer Induced

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Conformational Change on a Bithiophene-Based Copolymer Induced by Additive Treatment: Application in Organic Photovoltaics Osvaldo D Lourenço Jr., Leandro Benatto, Cleber Fabiano N. Marchiori, Harold C Avila, Natasha A. D. Yamamoto, Camilla K Oliveira, Marcos Gomes Eleuterio da Luz, Marco Cremona, Marlus Koehler, and Lucimara Stolz Roman J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05427 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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The Journal of Physical Chemistry

Conformational Change on a Bithiophene-Based Copolymer Induced by Additive Treatment: Application in Organic Photovoltaics† O. D. Lourenco Jr.,‡ L. Benatto,¶ C. F. N. Marchiori,¶ H. C. Avila,§ N. A. D. Yamamoto,k C. K. Oliveira,¶ M. G. E. da Luz,¶ M. Cremona,§ M. Koehler,∗,¶ and L. S. Roman∗,¶ ‡Programa de Pós-Graduação em Engenharia dos Materiais-PIPE, Universidade Federal do Paraná-UFPR, 81531-980, Curitiba-PR, Brazil ¶Departamento de Física, Universidade Federal do Paraná-UFPR, 81531-980, Curitiba-PR, Brazil §Departamento de Física, PUC-Rio, 22453-900, Rio de Janeiro-RJ, Brazil kDepartment of Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, California, USA E-mail: [email protected]; [email protected]

†

Additive effect on block copolymer, photovoltaic performance and physical chemical mechanism

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Abstract In the last years the use of solvent additives for fabrication of polymer-based solar cells has become an important procedure to induce morphological changes at the system nanoscale, a critical step to improve the devices performances. Yet the actual effects of those additives on the polymer’s backbone conformations (with coupled variations on the electronic structure) remains very elusive. By combining different experimental and theoretical techniques, we show that the use of the solvent additive 1,8diiodooctane (DIO) might influence the conformation of poly[(9,9-dioctylfluorenyl-2,7diyl)-co-bithiophene] (F8T2) chains resulting in improved properties of the film. We correlate this conformational variation with the photovoltaic response of F8T2:fullerene(C60 ) devices prepared using different DIO concentrations. We found that the efficiency of the devices increases more than 100% and the hole mobility in the F8T2 films increases almost one order of magnitude with the use of DIO. The comparison between experimental data and the properties of the calculated structures suggests that the additive induces a higher density of syn conformers in the bithiophene unit of the copolymer backbone, planarizing the polymer’s geometry. The simulations indicate that this transition is very likely mediate by the electrostatic interaction between the iodine atom of the DIO and the heteroatoms of the bithiophene moiety. The higher degree of electronic delocalization and the enhancement of the interchain interactions improves the transport and the photovoltaic features of F8T2 layers. The use of solvent additive treatments to control conformational variations of the backbone might be a promising strategy to improve optoelectronic properties of polymer-based devices.

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Introduction Organic photovoltaic devices (OPV) are a very important possibility of renewable and environmental friendly energy sources, constituting a particularly promising alternative given the recent results of laboratory prototypes, 1 in which the power conversion efficiency (PCE) has reached the milestone rate of 11%. Such high efficiencies were basically achieved from bulk heterojunction (BHJ) architectures: devices built by processing together the electron donor material (a conjugated polymer) and the electron acceptor (usually a soluble fullerene derivative, like the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)). Nonetheless, for practical purposes — e.g., to actually produce commercially viable devices — there are still some key issues that need to be solved, such as the problem of photochemical and morphological stability, in special related to the PCBM phase. 2 In the last few years the use of solvent additives during the heterojunction fabrication has become an important procedure to induce morphological changes at the system nano scale, an extremely critical factor to improve the devices performances. 3–11 For devices with a BHJ structure, the action of the additives are believed to take place in the interpenetrating interfaces between the polymer and fullerene domains. For instance, small angle x-ray scattering (SAXS) analysis 12 of films with and without the addition of 1,8 diiodooctane (DIO) has been applied to probe the aggregation changes in the isolated polymer and PC71BM solutions, as well as in the mixed solution polymer:PC71BM. The analysis has reveled a small increase in the overall aggregation size after the DIO addition, with the effect being more pronounced for the PC71BM. In another experiment, 13 thin films of donor-acceptor (D-A) copolymers and of polymer/PC71BM blends have been investigated by means of grazingincidence small/wide angle x-ray scattering (GISAXS/GIWAX). It has been found that the DIO indeed has altered the film morphology, influencing the formation of polymer crystalline domains surrounded by PC71BM domains and by an amorphous polymeric matrix. But the additive action was quite different for the various D-A copolymers/fullerene BHJ tested. It is known that the physical conformation of the chains and the way the chains pack 3



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together in films — as exemplified above — strongly reshape the electronic and optical properties of conjugated polymers. For instance, to be able to create a large enough fraction of planarized chains allows a better stacking organization, which critically affects the photoluminescence efficiency, the optical absorption and the charge carrier transport of the resulting films. 14,15 Hence a good control over the formation of planar configurations along the polymeric chain is essential to improve the performance of organic devices. In addition, there are also strong evidences that the chemical nature of the interchain interactions in conjugated polymer films depend on the processing conditions. Thus, chain conformation could be adjusted, at least partially, using such as choice of solvent, polymer concentration and thermal annealing. 16 Such possibility has guided a new proposal for efficient OPV fabrication. Indeed, the incorporation of ordering agents (additives) — such as DIO and diiodomethane (DIM) — at different stages of a sequential deposition process, 17 resulted in bilayer devices that showed encouraging high efficiencies. Recently, it was found that DIO affects the kinetics of the aggregate formation in thin films of Poly(3-hexylthiophene)(P3HT), reducing the actual time for transformation from disordered to ordered phase during the film formation process. 18 In spite of the above mentioned works, studies addressing the additives impact on the isolated polymer thin film morphology are scarce. Consequently, the exact mechanisms making possible additives to influence the final polymeric thin film optical, electrical and photovoltaic features are still poorly understood. 19 Certainly, such knowledge would help to improve even more the already existing suitable fabrication procedures. 3–11,17,18,20 As a step forward to clarify these issues, in this contribution we consider the heterojunction devices based on the copolymer Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2) with a sublimated C60 layer. The F8T2 structure is built by combining several units of the constituent monomer formed by dioctylfluorene and bithiophene. 21 We then change the density of planar segments along the backbone of the copolymer by adding an additive solvent (DIO) during the polymer deposition. Using different experimental techniques, we

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examine how this method concretely affects the structural, optical, electronic and transport properties of the polymer. Moreover, theoretical analysis is performed to help to understand the physical-chemical processes (at the nanoscale level) responsible for the observed changes. This analysis strongly suggests that the additive induces a anti to syn conversion in the bithiophene moiety of some units along the chain which helps to planarize the polymer’s geometry. It is known that F8T2 can exhibit distinct conformations in solution depending on the temperature and solvent. 22 This effect affects the system optical-electrical properties (as explicit calculated), leading to a considerable improvement in the response (e.g., enhancing the PCE in more than 100%) of the copolymer:fullerene bilayer devices fabricated after this additive treatment. Our analysis here point to interesting perspectives of how to improve — through simple preparation protocols — the optoelectronic properties of organic devices.

Experimental Methods Materials Photovoltaic devices were fabricated as follows: a PEDOT:PSS (purchased from Sigma Aldrich and used as received) was spin-coated onto fluorine tin oxide (FTO, with sheet resistance 5-15Ω) and annealed at 100 0 C for 15 min. in vacuum. In the sequence, a F8T2 (Sigma Aldrich) was spin-coated in nitrogen atmosphere from a solution of 4.0 mg/mL in chlorobenzene. Films with different thickness were prepared with the same concentration but with different spin-coating speeds. Prior to deposition, the F8T2 chlorobenzene solution was stirred for 5 minutes at 50 0 C, and cooled to room temperature. Then, the additive DIO was added to it in an amount of 0.5% and 1% v/v and stirred for 1 hour. Following the spin-coating, the films were dried in vacuum, without any annealing step. Reference devices were built without the additive for comparisons. After deposition of the polymer film, the C60 layer was thermally evaporated forming a film with 30 nm of thickness. A 70 5



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nm thickness aluminum film was also evaporated using a shadow mask to build the metallic cathode. Both evaporations were done under a vacuum pressure of 5 × 10−6 mbar. After the complete construction, the devices were post-annealed at 70 o C for 5 minutes (in vacuum). For the charge carrier extraction by linearly increasing voltage (CELIV) measurements, it was used indium thin oxide (ITO) with a sheet resistance of 15 Omega/sq (purchased from Lumtech).

Characterization (a) The F8T2 thin film thicknesses were determined with Dektak 3 profilometer (Veeco) with average roughness of 3-5nm. (b) Photovoltaic J–V curves were obtained using a solar simulator (with a power illumination of 100 mW/cm2 from a 150 W Oriel Xenon lamp) with AM 1.5G filters. (c) Sample morphologies were investigated by Atomic Force Microscopy (AFM) (Shimadzu SPM-9700). AFM intermittent mode (dynamic mode) measurements were carried out in air and standard silicon doped cantilevers (metallic reflex coating, nominal spring constants k ∼ 21-78 N/m and resonant frequency ω0 ∼ 250-390 kHz) were employed. The AFM images of the polymer layer were obtained on top of glass/ITO/PEDOT:PSS substrate. (d) UV-Vis spectra were collected directly from the film deposited onto a glass substrate in a Shimadzu UV-2450 spectrophotometer, using glass/air as the reference, in the range of 350-800 nm. (e) Photoluminescence spectra were taken using Fluorolog-3 Spectrofluorometer (HORIBA scientific). (e) The charge carrier mobility (µ) in thin films of F8T2 with different v/v proportion of DIO (0, 0.5% and 1.0%) was measured by CELIV technique. Thin films of F8T2 with thickness varying from 20 to 80 nm were deposited by spin coating onto commercial indium tin oxide (ITO) coated glass substrates with a sheet resistance of 15 Ω/sq. The thickness of the films was varied using different rotation speeds in the deposition process. In the sequence, the aluminum electrode was thermally evaporated on the top of the polymer in a high vacuum (10−6 mbar) deposition system, fully integrated with a glove-box. For the CELIV technique, the carrier extraction from the sample is assured by 6


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applying a triangular voltage pulse with a voltage increase rate, A, ranging from 0.05 to 0.1 V/µs. The CELIV current transient is composed of a capacitive (displacement) current j(0) and a current increase ∆j. The ∆j is related with the injection current caused by the charge carrier transport in the layer. µ can be calculated by measuring the maximum extraction time (tmax ) and using the equation: 23,24

µ

cm2 Vs

= 3At2max

2d2 , ∆j 1 + 0.36 j(0)

∆j ≤ j(0).

(1)

Here A is the voltage increase rate, tmax is the time elapsed after the application of the pulse when the maximum current is observed (maximum extraction time), d is the thickness of the film being analyzed, j(0) is the displacement current density and ∆j is the height of the current spike.

Model Structures and Computational Methods To simulate the electronic structure of the F8T2 copolymer, some oligomer model structures were built as follow: The semi-empirical PM3 method was used to optimize the functional groups fluorene and bithiophene separately and in the sequence for the oligomer structure. We have simulated F8T2 oligomers with five repeated units with two different conformations: one with the S atoms of the thiophene ring pointing to the same direction, named syn conformation, and another with the sulfur (S) atoms pointing to the opposite direction, called anti conformation. After the geometry optimization, Time-Dependent Density Functional Theory (TDDFT) calculations, with hybrid B3LYP functional 25 and 3-21G** basis set, 26,27 were used to obtain the electronic vertical transitions. Since the F8T2 from Sigma Aldrich is end-capped with 3,5 dimethylbenzene we simulated oligomers with hydrogen or with this larger structure as end-groups. As expected, both kind of oligomers gave equivalent theoretical results. We then choose to show only the results for model chains end-capped with

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dimethylbenzene. All simulations were performed using the Gaussian03 package. 28 The optimized initial geometry of the F8T2 monomer and the geometry of the DIO molecule for the calculations in Fig. 6 were obtained at the B3LYP/3-21G** level of theory. Additional calculations to address the influence of basis set superposition errors (BSSE) and the B3LYP functional electron self-interaction errors 29 on these results are showed in the supporting informations. They were obtained with a different level of theory and using a newer version of the Gaussian package (further details in the SI).

Results and discussion Three different devices sets were fabricated with F8T2 (chemical structure in Fig. 1b) to investigate the effect of DIO on the solar cells photovoltaic response. Devices A, B, and C were prepared with, respectively, 0%, 0.5%, and 1.0% concentration (c) of the additive. Table 1 compares the main properties of each device. 30 Table 1: Photovoltaic characteristics for glass/FTO/PEDOT:PSS/F8T2/C60 /Al devices with (B and C) and without (A) the use of the additive DIO. The thickness of the C60 layer is around 30 nm for all devices. A (0.0%) B (0.5%) C (1.0%)

Voc (V) 0.52 0.82 0.79

Jsc (mA/cm2 ) 3.68 4.75 6.27

FF (%) 51 47 45

PCE (%) 1.0 1.8 2.2

Polymer thickness (nm) 32 30 30

The current density versus voltage (J × V ) characteristics, measured under illumination, are shown in Fig. 1a. The addition of DIO always increases the open circuit voltage (Voc ) when compared to the device A value 0.52 V. Further, the small loss of Voc above a determined concentration (indeed, compare Voc = 0.82 V for B and Voc = 0.79 V for C) agrees with the results for devices fabricated using blends of a benzothiadiazole-based polymer — a material of low solubility in DIO, thus like F8T2 here — and PC60 BM. 31 The rise of Voc with the use of the additive can be explained by the action of the DIO 8


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4

30 EQE (%)

2

J (mA/cm )

2 0 -2

(a)

-4 -6 -0.2

0

0.2 0.4 0.6 Voltage (V)

0.8

20

(c)

0 300

1

400

700

500 600 Wavelength (nm)

1

2

Norm. Absorption (a. u.)

0

10 2

Device A Device B Device C

10

10

J (mA/cm )

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-2

10

-3

10

-5

(b)

10

-7

10

0.04

0.20 Voltage (V)

1.00

0.8 0.6

C60

0.4

(d)

0.2 0 300

5.00

F8T2

400


700

Figure 1: (a) J × V characteristics (under illumination) for the organic photovoltaic glass/FTO/PEDOT:PSS/F8T2/C60/Al devices fabricated with different concentrations of DIO. (b) The same measurements of (a), but in the dark. In the inset the chemical structure of the F8T2. (c) The devices corresponding EQE after the measurements of (a) and (b). (d) The normalized absorption of the C60 and F8T2. in diminishing the charge carriers recombination. In fact, the Voc variation in Table 1 is in accordance with the behavior of the curves J × V in dark, Fig. 1b. Because the detailed balance, a higher Voc corresponds to a smaller J0 (the diode reverse saturation current) associated to a lower recombination at the heterojunction. 33–35,37 Also, the short circuit current density Jsc rises from 3.68 mA/cm2 for A to 4.75 mA/cm2 for B and 6.27 mA/cm2 for C. The increase of both Jsc and Voc actually improves the power conversion efficiency (PCE) of the devices treated with DIO, Table 1. We should emphasize that the device C (compared to B) has presented the highest PCE once the increasing of Jsc with a higher DIO concentration more than compensates the slight lowering of Voc observed for high c. Figure 1(c) shows the external quantum efficiency (EQE) of devices A, B and C versus

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the incident light wavelength (λ). The EQE for the devices in Fig. 1c have been measured on the same contacts and one day after the gathering of the data for the current-voltage characteristics in dark (up to 5V) and under ilumination (1 sun). Since all the experiments have been performed in air, it should be expected a decreasing in the Jsc due to the aluminum contact oxidation. The relevant EQE data and its corresponding integration (leading to Jsc ) is discussed in the supplementary information. Similar to the Jsc , the EQE greatly increases with the DIO concentration. The C60 and F8T2 absorption profiles are displayed in Fig. 1d. The absorption peak of the former (latter) is in the range 300 nm∼400 nm (350 nm∼550 nm). Furthermore, the C60 also displays a very relevant region of absorption in the interval 400 nm∼550 nm, overlapping with that of the F8T2. In this way, the combined behavior of C60 and F8T2 fairly explain the overall EQE spectral variation with λ. However, the devices fabricated with the DIO do present a higher contribution to the photocurrent in the range 350 nm∼400 nm (the absorption peak of the C60 ). For the F8T2, the increase of the thin film surface roughness treated with DIO has been moderate (around 16%, see caption of Fig. 2 ). In fact, in Fig. 2 the atomic force microscopy (AFM) height contrast measurements for polymeric films with 0, 0.5%, and 1.0% DIO concentrations do not reveal any significant variation in the surface morphologies. So, the improvement of Jsc with the additive cannot be attributed solely to a larger contact area between donor and acceptor induced by a higher roughness of the F8T2 film prepared with DIO. Another effect assumably also accounting for the C60 higher contribution to the charge carrier generation when the F8T2 is treated with the additive is related to the DIO low evaporation rate (in comparison to the major solvent, chlorobenzene, used for processing the film). The slow evaporation of the DIO after the polymer deposition may reduce the density of the film specially near the upper air/polymer surface. This effect would then allow the diffusion of the fullerene to the inner parts of the polymeric layer during the

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Figure 2: AFM topography for F8T2 thin films with (from left to right) 0, 0.5%, and 1.0% concentration of DIO. The root mean square (rms) roughness quoted from the very small areas of AFM images cannot be directed related to height in these soft materials but it can indicate modifications in of those films. No relevant changes in the morphology are detected and the maximum rms roughness variation, from the 0% DIO sample, is of 16%. C60 sublimation. Hence, a higher fraction of the incoming light will be able to reach the fullerene after crossing the polymer. An important experimental evidence for this comes from the absorption spectra of the F8T2 thin film, Fig. 3a. Note from the plots that the addition of DIO systematically reduces the absorption coefficient, indicating that the films with DIO are less dense than the pristine polymer film (measurements of the absorption coefficient using films with approximately the same thicknesses, but prepared with different DIO concentrations, have confirmed the results of Fig. 3a ). 32 The F8T2 absorption coefficient curve in Fig. 3a displays two peaks, at 460 nm and 488 nm, which we call, respectively, blue and red bands. They have approximately the same heights, differing only in 3.45%. When the films are prepared with the additive, these bands suffer small shifts to lower wavelengths, an effect more pronounced for the red band (see the caption of Fig. 3). Furthermore, the ratio between the absorption coefficients at the blue red peaks of the blue and red bands (αmax /αmax ) shows a small but systematic decrease with

increasing DIO concentration. This ratio is 0.972 ( for 0 % concentration), 0.950 (for the 0.5% concentration), and 0.923 (for the 1.0 % concentration). Contrary to the absorption behavior, the photoluminescence (PL) spectra in Fig. 3c become more intense for films processed with DIO. The photoluminescence of the films

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(a)

0.6 0.4 0.2 0 350


550

0.8

0.45 0.3

F8T2 thin film 525


0.06

(b)

750

(d)

0.05

0.6 0.4

0.04

F8T2:C60 heterojunction

0.03 0.02 0.01

0.2 0 350

0.0% DIO 0.5% DIO 1.0% DIO

0.6

0

600

Norm. PL (a. u.)

-1 5

1.2 1

0.75

0.15 400

(c)

0.9 Norm. PL (a. u.)

-1

0.8

Abs. Coef. (10 cm )

1

5

1.2

Abs. Coef. (10 cm )

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400


550

600

0

525


750

Figure 3: (a) and (b) Visible and Ultraviolet Spectroscopy (UV-Vis) spectra for cast F8T2 thin films. The bars in (a) are vertical electronic transitions obtained from TD-DFT B3LYP/3-21G** simulations for the syn-1 and anti-1 arrangements. The bars in (b) are the calculated vertical transitions for the syn-2 and anti-2 arrangements. The red (blue) color indicates the syn (anti) configuration. (c) normalized photoluminescence (PL) spectra for cast F8T2 thin films. (d) the PL spectra (normalized by the F8T2 film results) for F8T2:C60 heterojunctions. In all the cases, dotted, dashed, and continuous lines represent, respectively, 0, 0.5%, and 1.0% DIO concentration. The two peaks of each curve in (a) have maxima about 460.0 nm and 488.0 nm (0%), 463.4 nm and 494.3 nm (0.5%), and 463.4 nm and 494.3 nm (1.0%). In (c), the excitation lasers have λ = 489 nm (0 %) and λ = 491 nm (0.5% and 1.0%). But using a laser of λ around 460 nm, the resulting overall shapes and peaks positions are basically the same, only with a general decreasing in the intensities. formed after the C60 sublimation presents a strong quenching ( Fig. 3d), which moreover may be further increased if c is high enough, e.g., see the curve for 1.0% in Fig. 3d. These latter results also points to the reduction of the films’s density when processed with DIO. This effect would lead to an enlargement in the donor:acceptor surface for exciton dissociation after the fullerene sublimation, hence helping to suppress the radiative recombination. Finally,

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the PLs of polymer/fullerene films agree with the EQE changes observed in Fig. 1c. The verified enhancement of Jsc with the DIO treatment might originate not only from an improved exciton dissociation, but as well from a better charge carrier transport. To vouch this possibility, we have investigated the transport properties of F8T2 films with DIO through charge carrier extraction by linearly increasing voltage (CELIV) analysis. Figure 4 (a) exemplifies a typical CELIV current transient measurement for devices with different DIO concentrations (0, 0.5%, and 1.0%). The hole mobility can be estimated through Eq. (1) (Methods Section) by varying A and plotting the maximum extraction time, tmax , versus A−1/2 , as shown in Fig. 4b. The obtained angular coefficient of the linear best fitting (dotted-dashed lines) allows to calculate the material charge carrier mobility (see Eq. (1)). The experiments demonstrate that the hole transport in the F8T2 film improves with the use of the DIO. From the CELIV technique we have found that the µ increases from ∼ 1.61 × 10−8 cm2 /Vs for 0% to 1.50 × 10−7 cm2 /Vs for 0.5% and 1.24 × 10−6 cm2 /Vs for 1.0% DIO concentrations. Therefore, there is a two-fold increase of µ when 1% of DIO is used. 40 4

A = 7.5 × 10 V/s

(a)

35 Maximum Extraction Time (µs)

2

Current Density (mA/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


30

20 0.0% DIO 0.5% DIO 1.0% DIO

10

(b) 0.0% DIO 0.5% DIO 1.0% DIO

30

0

10

20 Time (µs)

30

40

2

-7

2

25

20

-6

2

1.24 × 10 cm /Vs 1.50 × 10 cm /Vs

15 0

-8

1.61 × 10 cm /Vs

3.2

4 3.6 -3 -1/2 1/2 1/2 10 A (s /V )

4.4

Figure 4: (a) Typical CELIV measurements (here with A = 7.5 × 104 V/s) for F8T2 with different concentrations of DIO. (b) The maximum extraction time, tmax , as a function of A−1/2 . Using Eq. (1) (Methods Section), the best linear fittings (dotted-dashed lines) allow to calculate the materials charge carrier mobility µ (in cm2 /Vs).

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To understand how the experimental results relate to the features of the F8T2 chain, we have investigated the polymer ground state properties through density functional theory (DFT) calculations. We have considered F8T2 oligomers with five repeated units. Yet the presence of the bithiophene moiety enables two, syn and anti, conformations: the former (latter) has the thiophenes pointing to the same (opposite) direction(s). Therefore, we have thus considered four different arrangements anti-1, anti-2, syn-1 and syn-2. In the anti-2 and syn-1 arrangements, one thiophene faces towards the alkyl bridge on the fluorene, the other away. In the anti-1 and syn-2 structures, both thiophenes close to the fluorene faces towards the alkyl chains. Using this notation the arrangements are ordered according to their total energies (see discussion below). HOMO

LUMO

anti1

syn1

anti2

syn2

Figure 5: Optimized structures, HOMO and LUMO orbitals, for F8T2 with five repeated units (isovalues 0.01). The resulting optimized geometries are shown in Fig. 5 together with the spatial distribution of the respective highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. It is apparent in Fig. 5 that the frontiers orbitals are predominantly formed by the atoms located at the bithiophene groups of the polymeric backbone. Curiously, due to the relative orientation of the alkyl groups, the syn-2 arrangement presents a helical chain similar to the conformation displayed by polythiophene chains 14


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with a preponderant syn orientation between the thiophene units. 36 From the total energies, the calculations indicate that the 1 structures are more stable than the respective 2 structures by relative energies of -2.60 kcal/mol (anti) and -5.72 kcal/mol (syn). Moreover, the anti arrangements are more stable than the syn arrangements: for instance, the total energy of the anti-1 structure is -13.85 kcal/mol lower than the energy of the syn-1 conformer. However, the energy gain obtained with electronic delocalization necessary to stabilize the syn gives rise to a more planar structure compared to the anti conformation. This can be seen from the mean dihedral angles, Table 2. Those values are calculated taking the average of the dihedral angles between the thiophenes and the dihedral angles between the thiophene and the adjacent fluorene in the structures of Fig. 5 (in the supporrting information we show a Table with all the angles between the chemical moieties for each F8T2 conformer). As a consequence of the lower average torsion angles, the frontier orbitals tend to be more evenly spread along the whole polymeric chain for the syn when compared to the anti conformation, Fig. 5. Table 2: The average of the dihedral angles between the chemical groups of F8T2 model pentamers for the syn and anti conformations. F8T2 model conformer Mean dihedral angle

syn-1 11.65o

syn-2 10.33o

anti-1 34.80o

anti-2 29.53o

In Table 3 we present a summary of the obtained energies for the frontier orbitals (via DFT). The values of the HOMO and the energy gap are in reasonable agreement with the those reported in the literature (HOMO 5.5 eV and gap 2.6 eV 38 ). The calculated gap is close to the experimental energies associated to the red and blue absorption bands in Fig. 3a. One can see that the higher electronic delocalization of the syn conformation results in lower HOMO-LUMO energy gaps. Moreover, the HOMO of the syn is less stable than the HOMO of the anti conformation. Assuming the structures of Fig. 5, we applied time-dependent DFT (TD-DFT) calculations to determine the electronic transitions for distinct polymeric conformations. For each

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Table 3: The calculated frontier orbitals energies and the HOMO-LUMO energy gaps for the syn and anti conformations of F8T2 model pentamers (all in eV). F8T2 model conformer syn-1 syn-2 anti-1 anti-2

HOMO-1 -4.91 -4.89 -5.09 -5.05

HOMO -4.83 -4.82 -5.01 -4.96

LUMO -2.04 -2.038 -1.84 -1.91

LUMO+1 -1.95 -1.94 -1.79 -1.80

Gap 2.79 2.78 3.17 3.06

F8T2 model conformer, the main features of the two transitions with the highest oscillator strengths (f ) are depicted in Table 4. One can see that main excited-state vertical transitions of the syn are shifted toward longer wavelengths relative to the same transitions in the anti conformers. Since the electronic delocalization is higher (lower) for the syn (anti) arrangement, the electronic transition is excited at longer (shorter) wavelengths in structures with different conformations. Table 4: Main excited-state vertical transition wavelengths, dominant electronic configurations, and oscillator strengths (f ), determined from TD-DFT for F8T2 model pentamers. F8T2 model conformer syn-1 syn-2 anti-1 anti-2

λ(nm) 511.2 454.2 515.4 486.4 445.5 423.8 464.1 438.3

Dominant configuration (weight) HOMO → LUMO (0.70) HOMO-1 → LUMO+1 (0.39) HOMO → LUMO (0.72) HOMO-1 → LUMO (0.42) HOMO → LUMO (0.77) HOMO-1 → LUMO+1 (0.45) HOMO → LUMO (0.71) HOMO-1 → LUMO (0.40)

f 6.33 0.88 1.60 3.22 5.39 1.77 6.22 0.56

Fig. 3 a and b directly compares the TD-DFT electronic transitions of structures 1 and 2, respectively, with the experimental absorption coefficient. The theoretical transitions are represented by vertical bars with heights proportional to the f parameter. From this, the blue band around 460 nm can be associated to HOMO → LUMO excitations of the anti conformer, whereas the red band around 490 nm can be associated to the HOMO → LUMO excitations of the syn conformer. Given the calculated characteristics of the syn and anti conformers, we shall now examine

16


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independently the results of each experimental technique employed in this work and argue that all the data are compatible with the hyphotesis that the use of DIO increases the concentration of syn segments. (i) Photovoltaic response of F8T2/C60 devices: As already discussed, the simple presence of DIO in the films increases the number of charge dissociation centers. This leads to a considerable rise of the Voc in the devices with additive when contrasted to those without it. But at higher DIO concentrations, the Voc starts to diminish with c, an effect that might be related to the difference between the HOMO of the donor and the LUMO of the acceptor. Thus, the data in Tables 3 and 4 suggest that the loss of the open circuit voltage observed for the devices treated with a 1.0% DIO compared to those with 0.5% might be related to a higher density of syn segments in the former (since the HOMO of syn segments is less stable than the HOMO of anti segments). (ii) UV-Vis spectra for the F8T2 thin films: The red band small red-shift with c (as well blue red as the slightly decreasing of the ratio αmax /αmax ), observed in Fig. 3 a,b, can be explained

by the narrowing of the HOMO-LUMO energy gap, which is just the case when the syn concentration increases (see Table 3). Moreover, the chains planarization promoted by a anti → syn conversion favors a higher interchain π-π stacking. Actually, a red-shift of the absorption could also be explained by an eventual increase in the dielectric constant due to the higher film density. 39 But the decrease of the absorption coefficient with the use of DIO (see Fig.3a,b) dismisses this alternative possibility. (iii) PL spectra for the F8T2 thin films: The plots in Fig. 3c showing an enhancement of the PL with c — and occurring despite the reduction of the light absorption coefficient, Fig. 3 a,b — suggest a higher degree of structural order in the films treated with additive. For instance, it has been found in P3HT with different degrees of regioregularity (RR) which the photoluminescence intensity of more ordered polymeric films tends to increase due to a reduction in non-radiative quenching. 40 Thus, the PL results corroborate the idea that the use of DIO favors the production of chain segments with the syn conformation. Indeed,

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the planarity of the syn segments promotes the interchain π-π stacking, hence increasing the structural order. Furthermore, the fact that the PL spectra of both films treated with additive have the same intensity absorption means that the reduction of the non-radiative quenching is even higher for the film prepared with a greater DIO concentration (then, compensating a lower absorption coefficient). An important feature of the PL curves, Fig. 3c, is that the peaks positions essentially do not change regardless if the energy of the excitation wavelength is close to the blue or to the red band of the absorption profiles, although the intensities decrease in the former case (see caption of Fig. 3) Also, the peaks are narrower and clearer defined than those observed for the absorption spectra in Fig. 3 a,b. So, the absorption does not show a relevant vibrionic structure (because a mixing of the anti and syn contributions to each distinct stoichiometry segment forming the polymeric chains), whereas the photoluminescence is dominated by the emission from the syn segments even when the incoming photon is absorbed by anti conformers. In fact, due to the driving force produced by HOMO-HOMO and LUMO-LUMO energy mismatches, before recombination the excitons created in the anti segments must migrate to the nearby syn segments of a lower energy gap. In the Supporting Information we use the Frank-Condon theory and show that Fig. 3c indeed represents a vibrionic transition structure. We further mention such type of process has been observed in the absorption and PL measurements of polymers containing different fractions of planarized chains within the glassy polymer film 14,39 or having a partly crystalline phase and an amorphous phase. 41 In both situations the crystalline phase dominates the emission due to the exciton diffusion to segments with lower energy gaps. Thus, we conclude that the higher density of syn segments induced by the additive treatment also contributes to increase the intensity of the PL peaks. (iv) CELIV measurements for F8T2 thin films: Since the charge carrier transport is more efficient along delocalized states in the polymer backbone and along the π-π stacking directions, 42 a higher concentration of syn segments would increase the resulting hole mobility µ. In fact, a systematic enhancement of µ has been observed in our CELIV analysis using F8T2

18


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films prepared with increasing DIO concentration. Furthermore, microstructure investigation of thienyl-substitute BDT polymer spin coated with and without DIO indicated that the additive increases not only the overall crystallinity of the film but also the coherence length along these crystalline directions. 42 As a summary so far, our previous theoretical studies show that the anti segments of the polymeric chain are more stable than the syn ones. Hence, in a typical situation the concentration of anti would tend to be higher than those of syn in the film. Nevertheless, all the above experimental results indicate that the DIO treatment induces the nanoscale molecular transition anti → syn. In addition, the comparision between Fourier transform infrared spectroscopy (FT-IR) measurements and DFT calculations of the F8T2 monomer vibrational modes give a further (indirect) evidence to support this hypothesis (see the SI). So, to complete our analysis we shall describe a possible molecular mechanism that would yield the anti → syn conversion, therefore in agreement with all our data. Using DFT simulations, we calculated the total energy of the F8T2 monomer when one thiophene ring rigidly rotates around the thiophene-thiophene bond of the bithiophene moiety. Those calculations were performed for the isolated monomer and when a DIO molecule is present in the vinicities of the F8T2. Also they were repeated for diferent positions and orientations of the DIO relative to the main axis of the oligomer. In particular, a very interesting result was obtained using the structure depicted in Fig. 6a. In this dimer the iodine atom of the DIO is separated by a distance of 4 Å from the axis of the bond between the thiophenes. Moreover, the DIO main axis is positioned in the interring region of the bithiophene group ( see the Supporting Informations for further details). The potential curve for the thiophene ring rotation in the dimer of Fig. 6a is plotted in Fig. 6b, in which we also show the curve calculated for an isolated F8T2 monomer. One can clearly see in Fig.6b that the syn conformer becomes more stable than the anti conformer by approximatelly 1.15 kcal/mol. In particular, the interaction with the DIO gives rise to a high energy peak along a wide range of angles where the isolated monomer is stable in the anti conformation.

19



Under the presence of the additve molecule the energy minima corresponding to the anti conformation is then very narrow, with a low energy barrier of (∼ 0.014 eV) for the transition to the syn conformation. For comparison, the barrier between the anti and syn conformers is around 0.12 eV when the DIO molecule is not present.

(a)

(b) 0.4

DIO

F8T2

Relative energy (eV)

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with DIO without DIO

0.3

0.2

Syn

0.1

0

0

30

Anti

90 120 60 150 Dihedral angle (degrees)

180

Figure 6: (a) Structure of the dimer between the F8T2 monomer and the DIO molecules used to calculate the rotational potencial curve of the thiophene ring. (b) Relative variation of the F8T2 monomer total energy as a function of the dihedral angle for the rotation of the thiophene ring. The dashed line was obtained for a isolated F8T2 monomer whereas the continous line was obtained for the dimer structure depicted in (a). The change in the energy curves shown in Fig. 6 b is essentially produced by the interaction of the iodine atoms of the DIO with the hydrogen or sulphur atoms of the bithiophene moiety. In the syn conformation the hydrogen atoms of the bithiophene group (with a partial positive charge) are oriented toward the negative charged iodine atom of the DIO. This structure reduces the potential energy from the electrostactic interation between the two molecules. On the other hand, there is a charged suphur atom oriented toward the iodine when the system is in the anti conformation. Thus, the electrostatic repulsion between the heteroatom and the iodine increases the potential energy of this conformer. Our results demonstrate that the influence of the DIO (when positioned in the vinicities of the bithiophene group) can drasticaly change the potential energy curve for the rotation of the thiophene ring. As observed in Fig. 6b , this interaction can stabilize the syn orientation 20


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and lower the energy necessary to induce the anti → syn transition, in agreement with the experimental observations. Finally, additional calculations that considered the influence of basis set superposition errors (BSSE), B3LYP functional electron self-interaction errors, and the B3LYP poor descrption of long-range interactions, confirmed the basic results of Fig.6 (those calculations are showed in the supporting informations) 44 .

Conclusion An additive treatment has been employed to fabricate F8T2 devices with fullerene, leading to a considerable improvement of the photovoltaic response. The natural question is hence: why is this so? Note that a deep understanding of the fundamental mechanisms for this simple technique to render (here and in other works in the literature) such gain in the OPV devices efficiency is a very important step to guide further developments. Therefore, we have presented a comprehensive study to explain this fact, combining different experimental techniques with theoretical studies. Specifically, we have probed the structural, optical, electronic and transport changes of the F8T2 films prepared with different additive concentrations. Also, DFT and TDDFT calculations were performed for the copolymer dominant molecular structure in order to determine the underlying microscopic processes related to the observed changes (the FrankCondon theory has also been employed to discuss the PL spectra). From our extensive analysis we propose that the additive induces a anti to syn conversion in the bithiophene moiety of the copolymer backbone. The higher delocalized electronic distribution necessary to stabilize the syn conformation yields a planarized backbone that increases the interchain interactions. Such effect then induces a higher concentration of ordered domains inside the film, clearly impacting on the polymeric layer optical and transport properties. For instance, the resulting enhanced hole mobilities and higher donor/acceptor contact areas for exciton dissociation are two main factors increasing the photovoltaic response (e.g., improving the

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PCE in more than 10%). Although more experimental and theoretical studies are necessary to further clarify all aspects of the presently identified mechanisms, our work firmly indicates that controlled conformational variations of the polymeric chain at the molecular level (induced by the use of additives) is a promising perspective for achieving polymer-based, high-performance optoelectronic devices.

Acknowledgement We acknowledge CNPq, CAPES and FAPERJ for financial support and the CENAPAD for providing the computational facilities. M. K. and C.F.N. Marchiori acknowledge M. Nazarkovsky for stimulating discussions.

Supporting Information Available Frank-Condom analysis of the F8T2 photoluminescence spectra, EQE measurements with the integrated Jsc , Fourier Transform Infrared Spectroscopy (FT-IR) Measurements of F8T2 films, DFT calculation of IR active normal modes for the F8T2 anti and syn monomers, theoretical analysis of the results of Fig. 6b, andl the calculated angles between the chemical moieties of the F8T2 model oligomers.

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the Morphology of High Efficiency Polymer Solar Cells Based on the Donor Polymer PBDTTT-EFT. Adv. Energy Mater. 2015, 5, 1401259. (43) Bertinelli, B.; Costa-Bizzarri, P.; Della-Casa, C.; Lanzi, M. Solvent and Temperature Effects on the Chromic Behaviour of Poly[3-(10-hydroxydecyl)-2,5-thienylene]. Synth. Met. 2001, 122, 267-273. (44) Preliminary results of the potential curve for the thiophene rotation when this ring is bonded to a second fluorene group (compared to the system of Fig. 6a ) also confirmed that the presence of the DIO significantly decreases the barrier height for the anti → syn transition. For instance, assuming approximately the same orientation of the DIO relative to the central bithiophene moiety of Fig. 6a, the barrier height for this larger system decreases by 50% relative to the isolated molecule (from 0.14 eV to 0.07 eV at the B3LYP/3-21G* level. Usually the B3LYP functional overestimate the torsional barriers between stable conformers, see SI) .

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Graphical TOC Entry DIO

1.2

40

0.0% DIO Device 0.5% DIO Device 1.0% DIO Device

1

-1

Abs. Coef. (10 cm )

0.6

30

EQE (%)

0.0% DIO 0.5% DIO 1.0% DIO

0.8

5

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20

0.4

10 0.2 0 350

400


550

600

0 300

400


700

Table of Contents Graphic.

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Conformational Change on a Bithiophene-Based Copolymer Induced

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