Efficient ternary organic solar cells based on immiscible blends

Accepted Manuscript Efficient ternary organic solar cells based on immiscible blends Joana Farinhas, Ricardo Oliveira, Rickard Hansson, Leif K.E. Ericsson, Ellen Moons, Jorge Morgado, Ana Charas PII:

S1566-1199(16)30540-7

DOI:

10.1016/j.orgel.2016.12.009

Reference:

ORGELE 3858

To appear in:

Organic Electronics

Received Date: 27 July 2016 Revised Date:

4 November 2016

Accepted Date: 2 December 2016

Please cite this article as: J. Farinhas, R. Oliveira, R. Hansson, L.K.E. Ericsson, E. Moons, J. Morgado, A. Charas, Efficient ternary organic solar cells based on immiscible blends, Organic Electronics (2017), doi: 10.1016/j.orgel.2016.12.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Efficient ternary organic solar cells based on

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immiscible blends

Joana Farinhasa, Ricardo Oliveiraa, Rickard Hanssonb, Leif KE Ericssonb, Ellen Moonsb, Jorge

a

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Morgadoa,c*, Ana Charasa*

Instituto de Telecomunicações, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001

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Lisboa, Portugal. b

Department of Engineering and Physics, Karlstad University, 65188 Karlstad, Sweden

c

Department of Bioengineering, Instituto Superior Técnico, Universidade de Lisboa, Av.

*Corresponding Authors

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Rovisco Pais, 1049-001 Lisboa, Portugal.

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ABSTRACT

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E-mail address: [email protected], [email protected]

Organic photovoltaic cells based on ternary blends of materials with complementary properties represent an approach to improve the photon-absorption and/or charge transport within the devices. However, the more complex nature of the ternary system, i.e. in diversity of materials’ properties and morphological features, complicates the understanding of the processes behind such optimizations. Here, organic photovoltaic cells with wider absorption spectrum composed of two electron-donor polymers, F8T2, poly(9,9-dioctylfluorene-alt-bithiophene), and PTB7, poly([4,8-bis[(2’-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2’ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]), mixed with [6,6]-phenyl-C61-butyric acid

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methyl ester (PC61BM) are investigated. We demonstrate an improvement of 25% in power conversion efficiency in comparison with the most efficient binary blend control devices. The active layers of these ternary cells exhibit gross phase separation, as determined by Atomic Force

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Microscopy (AFM) and Synchrotron-based Scanning Transmission X-ray Microscopy (STXM). Keywords: polymer solar cells, photovoltaics, ternary blend, morphology, high efficiency.

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1. Introduction

Polymer photovoltaic cells (PPCs) have shown potential to become a top-rated solar energy

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technology due to the distinctive characteristics, such as easy processing and cost-effective production of their active layers. These advantages enable a great versatility of architectural applications, including flexible and light-weight modules. Furthermore, the early-demonstrated low efficiencies for PPCs have sharply increased in the last few years, having achieved certified values of over 10% [1]. The most efficient PPCs comprise a bulk heterojunction (BHJ) type active layer, combining a low energy gap and highly absorbing conjugated polymer as electron

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donor, and a soluble fullerene (usually a soluble derivative of C70), as the electron acceptor. The photons absorbed by the active layer generate tightly bound electron–hole pairs (excitons), which dissociate into free charges at the donor/acceptor interface. Electrons and holes are then transported to their respective electrodes and therein collected, hence supplying electric current

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to an external circuit [2,3].

Recently, the concept of ternary BHJ blend, composed of materials with complementary

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properties, has been explored as an approach to increase the current generated by the cell through the improvement of the photon absorption and charge transport and/or to a decrease in the charge recombination processes [1,4-8]. As a result, some cells based on ternary blends outperformed those of their corresponding binary BHJ devices. Nevertheless, the mechanisms sustaining such optimizations are often difficult to unveil due to the more complex nature of the ternary system, i.e. in diversity of materials properties and morphological features. Yang and co-workers [9] investigated dual-donor and multi-donor BHJ cells based on several polymer donors with different energy gaps and molecular structures and found that the best performing mixtures combine polymers with structural compatibility (mainly referring to molecular orientation and

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crystallinity). This was justified by the hypothesis that the use of non-compatible polymers might interfere with the individual polymers’ most favourable molecular ordering for charge transport in the device. In this work, we investigate dual-polymer donor BHJ cells composed of the wide energy gap

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polymer poly(9,9’-dioctylfluorene-alt-bithiophene) (F8T2) (Eg ca. 2.4 eV) and the low energy gap polymer poly([4,8-bis[(2’-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro2-[(2’-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]) (PTB7) (Eg ca. 1.6 eV), whose

absorption spectra are complementary, mixed with the electron acceptor [6,6]-phenyl-C61-butyric

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acid methyl ester (PC61BM) (Fig. 1).

Fig. 1. Chemical structures of the electron donor polymers used in ternary blend cells and

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scheme of the cell structure.

F8T2 has been widely applied in electronic devices due to its good hole transporting properties

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(field-effect mobility of holes of ca. 10-3 cm2V-1s-1) [10], thermotropic liquid crystallinity [11], and, similarly to regio-regular P3HT, higher crystallinity than the majority of donor polymers [12,13]. Cells with F8T2 have shown high open-circuit voltages (around 0.9 V) but low to moderate power conversion efficiencies (PCEs) (1-2% with PC61BM) [13-16], which can be partly attributed to its wide energy gap. Conversely, PTB7 has been reported as a highperforming organic photovoltaic material when blended with fullerenes [17,18]. Chen and coworkers [19] proposed that the high performance of PTB7:PC61BM is partly determined by the hierarchical nano-morphology of the blend. PTB7 has recently also been explored in efficient ternary blend polymer solar cells with a second donor polymer and PC71BM

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[4,5,9,20,23],exhibiting PCEs ranging from 7.13% [21] to 8.63% [23]. However, the only ternary system reported combining PTB7 and the less absorbing acceptor PC61BM, contains P3HT as the second polymer, yielding a maximum PCE of 4.0% [6]. Here, we demonstrate BHJ organic photovoltaic cells combining PTB7, F8T2, and PC61BM

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with PCEs of 4.80%, this representing a significant enhancement (25%) in comparison with the binary BHJ control devices. We show that the active layers of such ternary cells exhibit a

significant degree of phase segregation and discuss the mechanisms leading to the observed

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improvements in the cells´efficiency.

2.1 Materials and devices fabrication

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2. Experimental section

F8T2 (Mw = 89500, PD = 2.42), PTB7 (Mw = 18000, PD = 1.75) and PC61BM (99.5%) were purchased from ADS, Ossila Ltd, and Solenne BV, respectively, and used as received. All the solvents used were HPLC grade.

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The photovoltaic devices were prepared on glass substrates coated with 100 nm of Indium-TinOxide (ITO), cleaned under ultrasounds with distilled water and a non-ionic detergent, followed by distilled water, acetone, isopropyl alcohol and then dried under N2 flow. The ITO surface was then cleaned under UV-oxygen plasma for 3 minutes prior to depositing, by spin coating, a 40

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nm thick layer of poly (3,4-ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) (Clevios P VP.AI 4083, from Heraeus) which was then dried on a hot plate at 125 °C for 10 min.

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The solutions of the binary blends, F8T2:PC61BM and PTB7:PC61BM, and of the ternary blends, F8T2:PTB7:PC61BM, were spin-coated (1800 rpm, 60 s) on top of PEDOT:PSS in air. The thicknesses of the blends films were measured with a profilometer (DEKTAK 6M). The binary blend solutions were prepared by stirring the polymer:PC61BM mixture in a 1:2 weight ratio in 1,2-dichlorobenzene (40 mg/ml) for 3 h, at 75 °C. The ternary blend solutions with different polymers ratio but keeping the 1:2 ratio between total polymers and PC61BM were similarly prepared. Following the spin coating of the active blends, LiF(1.5 nm) and Al(100 nm) were thermally evaporated on top, under a base pressure of 2 × 10-6 mbar, defining a device area of 0.24 cm2. Hole-only devices with the structure ITO/PEDOT:PSS/F8T2:PTB7:PC61BM/MoO3/Al

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were prepared similarly to PV devices with MoO3 (20 nm thick) and Al (60 nm) thermally evaporated under a base pressure of 2 × 10-6 mbar. Current-voltage measurements were done with a K2400 source-measure unit. Films for STXM and AFM measurements were prepared on PEDOT-PSS-coated glass/ITO

spectra measurements were spin coated on quartz substrates.

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2.2 Measurements

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substrates, similarly to those used to prepare the devices. The films used in the absorption and PL

Atomic force microscopy (AFM) measurements were performed on a Molecular Imaging (model 5100) system and on a Nano Observer from Concept Scientific Instruments (Les Ulis,

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France), operating in non-contact mode, with cantilevers with a resonance frequency between 200 and 400 kHz and silicon probes with tip radius smaller than 10 nm. All images were obtained with 256 × 256 pixels resolution and processed using Gwyddion (version 2.26) software.

The films for scanning transmission X-ray microscopy (STXM) were lifted off from their PEDOT:PSS-coated substrates and floated onto copper TEM grids (300 mesh). The STXM

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measurements were performed at the PolLux beamline at the Swiss Light Source, Villigen, Switzerland [24,25] using a Ni zone plate with 25 nm outer ring diameter. The sample was raster scanned with respect to the X-ray beam and the transmitted X-rays were detected by a scintillator

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and a photomultiplier tube. For the image scans, X-ray photon energies near the C1s absorption edge were chosen, that give maximum absorption contrast between F8T2, PTB7 and PC61BM (i.e. 284.5 eV, 285.0 eV, 287.5 eV and 288.4 eV), as well as a post-edge energy of 320 eV. The

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compositional maps were extracted using the aXis 2000 software and the spectra for the pure component films.

UV-Vis absorption spectra were recorded in a Cecil 7200 spectrophotometer. Photoluminescence (PL) spectra were acquired using a SPEX Fluorolog 2121, collecting the emission at a right angle arrangement in the R/S mode, and with correction for the wavelength response of the instrumental system. The molecular weight analysis of F8T2 was performed in THF solutions, by gel permeation chromatography (GPC) in a Waters 1515 chromatograph with three Styragel columns (HR5E,

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HR4, HR3) and a Waters 2414 differential refractometer, using monodisperse polystyrenes as standards. The current-voltage (I–V) curves of the photovoltaic cells were measured under inert atmosphere (N2) using a Keithley 2400 source-measure unit. The curves under illumination were

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measured with a solar simulator with simulated AM1.5G illumination at 100 mW/cm2 (Oriel Sol 3A, 69920, Newport). At least 16 devices of each condition were prepared. The light intensity of the solar simulator was verified using a calibrated solar cell. External quantum efficiency (EQE) spectra were obtained under short-circuit conditions, using a homemade system with a halogen

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3. Results and discussion

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lamp as light source.

The UV–Vis absorption and photoluminescence spectra of pure films of F8T2, PTB7, and PC61BM, and the predicted energy diagram for the ternary blend cells are shown in Fig. 2 and 3,

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respectively.

Fig. 2. UV–Vis absorption spectra of pure films of F8T2, PTB7, and PC61BM and respective PL spectra.

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Fig. 3. Energy level diagram of the ternary blend cells showing possible charge and energy

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transfer pathways following polymer photoexcitation.

F8T2 exhibits one absorption band with a maximum at 450 nm, which is complementary to the absorption spectrum of PTB7, which peaks at 675 nm. Regarding PC61BM, since it absorbs mostly in the UV region, which is filtered off by the glass substrate, its contribution to the generation of excitons in the cell should be minor. Hence, the incorporation of F8T2 in

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PTB7:PC61BM binary blends is expected to enhance photon harvesting by the solar cell, by absorbing in the spectral window between 350 and 500 nm. In addition, since the PL spectrum of F8T2 overlaps with the main absorption band of PTB7, photo-excited F8T2 may also act as a photo-sensitizer for PTB7. As a result, more excitons may reach the PTB7/PC61BM interface and

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separate into free charges. On the other hand, the relative position of the frontier energy levels (HOMO and LUMO) of the two polymers [26,27] and of PC61BM [2,28] enables also both

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polymers to perform as electron donors towards PC61BM (see Fig. 3). 3.1 BHJ polymer solar cells

Photovoltaic cells were prepared with the general structure: ITO/PEDOT:PSS/active layer/LiF/Al. The active layer is either a binary blend (F8T2:PC61BM or PTB7:PC61BM) (control devices), or a ternary blend of F8T2:PTB7:PC61BM. For the control cells based on F8T2:PC61BM, a 1:2 weight ratio was chosen because this led to the highest efficiencies. The same ratio was also used for the PTB7-based cells. Three ternary blends, with different

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compositions, were tested, keeping the polymers:PC61BM weight ratio of 1:2, in order to evaluate the role of modifying only the donor phase composition. Thus, in the three ternary blends, F8T2 replaced PTB7 in 15%, 25%, and 35%. The thicknesses of the active layer in all the devices were in the 80-90 nm range.

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Table 1 summarizes the average performance parameters of the control devices (based on the binary blends) and of the devices with the ternary blends and maxima PCE values. Shunt (Rsh) and series (Rs) resistances were calculated from the slopes of the corresponding J-V plots,

measured under illumination, near zero bias and near the open-circuit voltage (VOC), respectively.

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Fig. 4 shows the representative current density versus voltage (J-V) curves measured for the cells under AM 1.5 G illumination at 100 mW/cm2 (J-V curves recorded in the dark are shown Fig. S1

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in Appendix A).

Table 1. Average performance parameters (VOC, JSC, FF, PCE, Rsh, and Rs) of solar cells with F8T2:PTB7:PC61BM blends with different F8T2:PTB7 ratios. Active layer

PCE (%)

Voc (V)

Jsc (mA/cm2)

FF

0:1:2

0.75

-10.35

0.15:0.85:2

0.75

0.25:0.75:2

0.77

0.35:0.65:2

0.76

0.49

3.85

4.59

446

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- 10.79

0.59

4.80

5.01

476

10

- 11.29

0.54

4.69

5.43

370

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-7.88

0.41

2.61

2.62

325

55

0.56

2.69

3.00

1057

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max.

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- 4.91

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1:0:2

aver.

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F8T2:PTB7:PC61BM

Rsh Rs Ω.cm2 Ω.cm2

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Fig. 4. Current density-voltage curves for the fabricated cells measured under AM 1.5 G

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illumination at 100 mW/cm2.

We observe that the incorporation of 15% and 25% of F8T2 in the PTB7:PC61BM blend with respect to the total polymer content leads to an improvement of cell performance. A PCE increase of ca. 25% is reached for the F8T2:PTB7 blend with 0.15:0.85 ratio, in comparison with

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control devices of PTB7:PC61BM blends. To the best of our knowledge, the PCE values obtained for the most efficient cells (4.69% and 4.80%) are the highest reported for ternary BHJ cells with PTB7 and PC61BM. The FF is the most affected parameter, indicating more efficient charge transport and/or charge collection in such devices. This is consistent with the lower Rs values

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calculated for the corresponding cells, which are the lowest for the series. Increasing the content of F8T2 to 35%, led to a significant decline of the current and FF, causing PCE to reduce to

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2.61% - close to that of PCE for the F8T2:PC61BM control cells (2.69%). The VOC values of the ternary blend cells are close to that of the PTB7:PC6BM control cells (0.75-0.76 V). Since VOC has been related with the energy difference between the Highest Occupied Molecular Orbital (HOMO) of the donor and the Lowest Unoccupied Molecular Orbital (LUMO) of the acceptor [29], the observed invariance of VOC of the ternary cells suggests that in the ternary blends, charge transfer should occur mainly at PTB7/PC61BM interfaces. This contrasts with other reports, where the VOC of the ternary cells lies in between the VOC values of the corresponding binary blend solar cells, varying with the active layer composition [9,29,30]. Such variation of VOC with the composition in ternary blends has been

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explained invoking an alloy model in which the donor/acceptor interface and corresponding interface band gap (charge transfer state) display a material averaged electronic structure, due to the delocalized nature of the one electron states [31]. Fig. 5 shows the external quantum efficiency (EQE) of the cells as a function of the

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wavelength of the incident photons.

Fig. 5. External quantum efficiency (EQE) as a function of the wavelength of incident photons

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for the most efficient ternary cells and the control binary cells.

The EQE spectra of the most efficient ternary blend cells cover all the visible spectrum, exhibiting two prominent bands, with maxima close to those of the two polymers absorption

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maxima, thus indicating that both polymers absorb photons and contribute to charge generation. The band where F8T2 absorbs has a higher intensity for the ternary cells in comparison with the

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binary cells without F8T2, indicating that, in the ternary cells, photon harvesting is enhanced at higher energies, as expected from the complementary absorption spectra of the two polymers. Since VOC of ternary cells indicates that most excitons dissociate at PTB7/PC61BM interfaces, we conjecture that F8T2 acts mainly as photosensitizer to PTB7, as predicted by their optical spectra and energy level diagram. The EQE spectrum of the binary blend of PTB7:PC61BM also shows a relatively intense band in the 400-500 nm range, where F8T2 absorbs. This is attributed to the contribution of PC61BM in combination with PTB7 to charge generation, since both absorb in this spectral region. Interestingly, the absorption spectrum of such blend (Fig. S2 in Appendix A)

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does not show such an intense band at 400-500 nm region, which indicates that the PTB7:PC61BM binary cells are particularly efficient at this higher energy photon range. We performed PL quenching studies on blend films composed of the two polymers only, to possibly evaluate energy transfer processes from F8T2 to PTB7. Fig. 6 shows the PL spectra of

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the films excited at 450 nm which is the maximum of absorption of F8T2. In all the spectra, a low-intensity emission from F8T2 is observed, indicating that energy transfer to PTB7 should occur but is not complete. The three blends exhibit a strong emission band of PTB7 (peaking at ca. 800 nm) which can be attributed to the sensitizer effect of F8T2 and to radiative decay of

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excitons generated directly on the PTB7 chains, since PTB7 is highly emissive when excited at high energies. This is shown in the inset of Fig. 6 which compares the PL spectra of PTB7 upon

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excitation at 450 nm, where its absorbance is low (see Fig. 2), and upon excitation close to its absorption maximum (675 nm). By exciting at 450 nm, the peak intensity is approximately 60 times superior to that obtained with excitation at 675 nm. Thus, the observed emission of PTB7 in the blend films should be attributed mostly to the decay of excitons generated directly in PTB7

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and therefore the band intensity follows the content of PTB7 in the blends.

Fig. 6. Photoluminescence spectra of F8T2:PTB7 films (0.15:0.85, 0.25:0.75,0.35:0.65) with excitation at 450 nm. The inset shows the PL spectra of PTB7 neat films excited at 450 nm and 675 nm.

3.2.Morphological analysis and composition mapping

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Fig. 7a-c shows the topographic AFM images of the active layer in the ternary blend cells. These images show circular domains of micrometer size diameter with elevated edges, surrounded by a homogeneous phase containing smaller circular feature, suggesting the presence of isolated round domains in the bulk encircled by a surface wetting layer. In contrast, the AFM images of

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both types of control devices (composed of the binary blends) reveal relatively homogeneous surfaces (Fig. S3 in Appendix A). To investigate further the origin of phase separation in such ternary blends, the blend films composed of the two polymer donors (without PC61BM) were also imaged (Fig. 7d-f). These films showed also surfaces with circular-type lower level with

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dimensions accompanying those found for the ternary blend films. Although in AFM only the surface topography is characterized, these results suggest that F8T2 and PTB7 are not well

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AFM images of the ternary blends.

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miscible and that such immiscibility is likely to origin the pattern formation observed in the

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Fig. 7. AFM topography images (5×5 µm) of F8T2:PTB7:PC61BM films with (a) 0.35:0.65:2, (b) 0.25:0.75:2, and (c) 0.15:0.85:2 ratio; and of F8T2:PTB7 films with (d) 0.35:0.65, (e) 0.25:0.75,

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and (f) 0.15:0.85 ratio.

To analyse the chemical composition distribution of the ternary blend films in a direct way, we characterized the ternary blend films with 0.25:0.75:2 and 0.15:0.85:2 ratios by STXM. STXM generates chemical composition maps of the phase-segregated domains, which can be correlated

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with those observed in the AFM images. Fig. 8 shows compositional maps for F8T2 and PTB7, respectively, in the blend films, where bright colours in the STXM images correspond to high concentration. The images reveal that the circular domains are rich in F8T2, while the

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surrounding phase is rich in PTB7. The sizes of circular domains vary with the blend composition, in agreement with the AFM images. Also, in accordance with the STXM analysis, F8T2 is not completely absent from the PTB7-rich phase and PTB7 is not completely absent from the F8T2-rich phase. This is consistent with the hypothesis that photo-excited F8T2 acts as sensitizer to PTB7 which requires the energy donor and the energy acceptor to be at a shorter distance (nanometer range), assuming that the resonance energy transfer (Förster) is the

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dominant mechanism, as commonly found in luminescent polymer blends. However, more studies are required to elucidate on the mechanisms behind the role of F8T2 as sensitizer to PTB7. Regarding PC61BM, its compositional maps (not shown) display little contrast, which

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indicates that PC61BM is evenly distributed throughout the film.

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Fig. 8. Compositional maps (5x5 µm) obtained by STXM for the 0.25:0.75:2 (top) and 0.15:0.85:2 (F8T2:PTB7:PC61BM) (bottom) blend films (scale bars 1 µm). Left column shows the concentration.

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F8T2 concentration, right column shows PTB7 concentration. The brighter the colour the higher

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In view of these results, we suggest that most excitons photogenerated inside the F8T2 chains will diffuse and encounter the PTB7 intermixed within the F8T2-rich domains and then dissociate at PTB7/PC61BM interfaces. The improved FF values found for the ternary cells with

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less F8T2 content should be due to a more favourable molecular ordering of PTB7 chains for charge transport, probably induced by the intermixing with residual F8T2 in PTB7-rich phase. We note that, on the basis that excitons are mainly dissociated at PTB7/PC61BM interface and the energy diagram for the ternary cells, F8T2 should not contribute significantly for charge transport, since electrons and holes should be transported through LUMO orbitals of PC61BM and HOMO orbitals of PTB7, respectively. Regarding the ternary blends with higher content of F8T2 (35%), the low content in the narrow energy gap polymer, PTB7, together with a more

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severe phase separation can be at the origin of the decline in current and FF found for the corresponding cells. To gain insight into the effect of the blend morphology on the charge transport we performed hole mobility studies on such blends. These were determined by the space charge-limited current

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method on devices with the structure: ITO/PEDOT:PSS/F8T2:PTB7:PC61BM/MoO3/Al, for the ternary blend compositions: 0.25:0.75:2 and 0.35:0.65:2 (less efficient blend). We found that the hole mobility is similar for both blend compositions, being 4.7x10-4 cm2V-1s-1 for the blend

leading to the less efficient OPVs (0.35:0.65:2) and 4.2x10-4 cm2V-1s-1 for the 0.25:0.75:2 blend.

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Therefore, other factors, such as differences on electron mobility, charge extraction and/or recombination should be at origin of the relative cells performance.

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We also prepared cells with 0.15:0.85:2 (F8T2:PTB7:PCBM) composition but with PC71BM replacing PC61BM. Such ternary blend films exhibit also a gross phase separation with large isolated domains, which we tentatively attribute to F8T2-enriched regions (Figure S4 in Appendix A). Similarly to the system with PC61BM, the ternary blend cells parameters (JSC = 9.35 mA/cm2; VOC = 0.74 V; FF = 0.51, PCE = 3.57%) showed an improvement in comparison with the control cells with PTB7:PC71BM (1:2) (JSC = 8.24 mA/cm2; VOC = 0.73 V; FF = 0.37,

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PCE = 3.18%). The PCE values are however lower than those obtained with PC61BM, which we attribute to the non-optimized composition and solution characteristics (concentration, solvents) of the blends.

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4. Conclusions

We demonstrated efficient ternary blend BHJ photovoltaic cells composed of two donor

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polymers, one being a wide energy gap (F8T2) and the other a low energy gap (PTB7) polymer, absorbing on an enlarged region of the solar spectrum, from 300 to 800 nm. The performance of the cells was improved by 25%, in relation to that of the best control devices, by using 15% of F8T2 (of the total mass of polymers). We show that the two polymers significantly phase segregate in the ternary blend films, forming F8T2-rich circular domains surrounded by a continuous PTB7-rich phase. We suggest that F8T2 acts as a photo-sensitizer to PTB7 and excitons are mostly dissociated at PTB7/PC61BM interfaces. Furthermore, charge transport and/or charge extraction should be improved within the PTB7-rich phase leading to enhanced FF values for the cells. In summary, this work demonstrates the possibility of achieving efficient

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ternary photovoltaic cells combining immiscible polymer donors by tailoring the blend compositions.

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Acknowledgment This work was supported by Fundação para a Ciência e Tecnologia (FCT), under the projects M-ERA.NET/0001/2012 and UID/EEA/50008/2013. JF acknowledges FCT for a PhD grant (contract n. SFRH/BD/75221/2010). Dr. Ben Watts is gratefully acknowledged for technical

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support at the PolLux beamline at PSI Villigen. The PolLux end station was financed by the German Minister für Bildung und Forschung (BMBF) through contracts 05KS4WE1/6 and 05KS7WE1. The research leading to these results has received funding from the European

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Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement n.°312284 (for CALIPSO), n.°283570 (for BioStruct-X) or n.°262348 (for ESMI). The authors would like to acknowledge networking support by the COST Action MP1307 StableNextSol.

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Appendix A. Supplementary material

References

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J-V dark curves, UV−Vis absorption spectra of the cells, and supplementary AFM images.

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and morphology control enables multiple cases of high-efficiency polymer solar cells, Nat. Commun. 5 (2014), article number 5293. [2] J. Brabec, N. S. Sariciftci, J. C. Hummelen, Plastic solar cells, Adv. Funct. Mater. 11 (2001) 15–26.

[3] T. M. Clarke, J. R. Durrant, Charge photogeneration in organic solar cells, Chem. Rev. 110 (2010) 6736–6767.

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Ternary-blend solar cells with F8T2, PTB7, and PC61BM were fabricated. The two polymers partially phase separate within the active blend.

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The Fill Factor is the cell parameter mostly improved.

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An enhancement of 25% in efficiency was obtained for cells with 15% of F8T2 (wt% in polymers).