Composition-morphology correlation in PTB7- Th:PC71BM blend films

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Composition-morphology correlation in PTB7Th:PC71BM blend films for organic solar cells Lin Song, Weijia Wang, Edoardo Barabino, Dan Yang, Volker Körstgens, Peng Zhang, Stephan V. Roth, and Peter Muller-Buschbaum ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20316 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Applied Materials & Interfaces

Composition-morphology

correlation

in

PTB7-

Th:PC71BM blend films for organic solar cells Lin Song†, Weijia Wang‡, Edoardo Barabino†, Dan Yang†, Volker Körstgens†, Peng Zhangξ, Stephan V. Rothξ, ‖, Peter Müller-Buschbaum*, †, §

†Lehrstuhl

für Funktionelle Materialien, Physik-Department, Technische Universität

München, James-Franck-Str. 1, 85748 Garching, Germany

‡State

Key Laboratory of Solidification Processing, School of Materials Science and

Engineering, Northwestern Polytechnical University, Youyixilu 127, Xi’an 710072, China

ξPhoton

Science, Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607

Hamburg, Germany

‖KTH

Royal Institute of Technology, Department of Fibre and Polymer Technology,

Teknikringen 56-58, SE-100 44 Stockholm, Sweden

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 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

§Heinz

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Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstr.

1, 85748 Garching, Germany

KEYWORDS: PTB7-Th:PC71BM, organic photovoltaics, blend ratio, morphology, polymer crystallization


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ABSTRACT

From a morphological perspective the understanding of the influence of the [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) content on the morphology of the active layer is not complete in organic solar cells with bulk heterojunction (BHJ) configuration based on the low-bandgap polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2carboxylate-2-6-diyl)] (PTB7-Th). In this work, we obtain a highest power conversion efficiency (PCE) of 10.5 % for BHJ organic solar cells (OSCs) with a PTB7-Th: PC71BM weight ratio of 1:1.5. To understand the differences in PCEs caused by the PC71BM content, we investigate the morphology of PTB7-Th:PC71BM blend films in detail by determining the domain sizes, the polymer crystal structure, optical properties and vertical composition as a function of the PC71BM concentration. The surface morphology is examined with atomic force microscopy, and the inner film morphology is probed with grazing incidence small-angle x-ray scattering. The PTB7-Th crystal structure is characterized with grazing incidence wide-angle x-ray scattering and UV/Vis


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spectroscopy. X-ray reflectivity is employed to yield information about the film vertical composition. The results show that in PTB7-Th:PC71BM blend films the increase of PC71BM content leads to an enhanced microphase separation and a decreased polymer crystallinity. Moreover, a high PC71BM concentration is found to decrease the polymer domain sizes and crystal sizes and to promote polymer conjugation length and formation of fullerene-rich and/or polymer-rich layers. The differences in photovoltaic performance are well explained by these findings.


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1. INTRODUCTION Within the demand for photovoltaic devices of low cost, lightweight, environmental friendliness, ease of production, flexibility and building-integrated potential, organic solar cells (OSCs) as a revolutionary green technology have gained immense research interest over the last decades.

1-5

In particular, the suitability for large-scale implementation in

device fabrication like roll-to-roll printing promotes OSCs to have a great promise for practical applications.6-9 Up to now, power conversion efficiencies (PCEs) of OSCs have been pushed well beyond 10 %,10-13 which are capable of competing with their inorganic counterparts. This great success is closely related to the introduction of the bulk heterojunction (BHJ) configuration, in which a blend of donor and acceptor materials is envisioned to have a co-continuous morphology with a local nano-phase separation. 14-16 The active layers with BHJ configuration are typically manufactured with solution-based methods, which feature great complexity in morphology, crystallinity and miscibility of the donor and acceptor materials since non-equilibrium morphologies are generated.

17-19

Moreover, these factors have a direct impact on the final device performance of the OSCs. Therefore, a close look into morphology of the BHJ configuration of the active layers is


5


necessary.

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Within the scope of a classical donor and acceptor pair poly(3-

hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), Campoy-Quiles et al. observed an increase of surface roughness after thermal annealing by atomic force microscopy (AFM) measurements. 21 Van Bavel et al. demonstrated that crystalline P3HT nanowires were formed after thermal annealing for the active layer containing 40 % PCBM. 22 Treat et al. observed an interdiffusion of PCBM and P3HT by cross-sectional scanning electron microscopy (SEM) images. 18 Nevertheless, these realspace imaging techniques only provided information about surface structures on a more local scale. Furthermore, the surface structures may differ from the inner morphology as reported in the literature.23-26 Since charge carrier generation occurs at the donoracceptor interface, the inner structures contribute more to the current production as compared to surface structures in the BHJ geometry. Thus, the investigation of the inner morphology over a macroscopic sample area is highly desirable for active layers having a BHJ configuration. Despite considerable efforts have been made on the advance of classical P3HT:PCBM BHJ system, the device efficiencies are still limited to approximately 5 %. 27-


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30

To overcome the issue of low PCEs, so-called low-bandgap polymers have been

exploited to replace P3HT in the BHJ configuration, as they have a broad absorption spectrum to ensure effective light-harvesting till the near-infrared region and have a high charge carrier mobility to ensure efficient transport of charge carriers. 31-33 Also in case of low-bandgap polymer blends the morphology of the donor and acceptor blends is of capital importance for the OSC device performance. The domain size is considered to ideally match the diffusion length of photogenerated excitons, in which an efficient dissociation of excitons into free charge carriers occurs.

34-36

Although the highest PCE

values have been realized with non-fullerene acceptors,10, 37, 38 the use of fullerenes is still common in many research examples and will be of high value due to the existing investments in processing infrastructure in industry which is optimized to fullerene based BHJ devices. Among the low-bandgap polymer based BHJ OSCs the polymer poly[4,8bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) as the donor material and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor material have gained particular interest. For the blend of PTB7-Th:PC71BM a champion


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PCE of 10.8% was reported for a simple conventional device when using a mixed solvent additive of 1,8-diiodooctane (DIO) and N-methyl pyrrolidone. 39 For PTB7-Th:PC71BM the solid content in an ortho-xylene based solution for film deposition and for the formation of the active layer was studied.

40

Recently, Jagadamma et al. studied changes in the

microstructure and photovoltaic performance caused by ex situ thermal annealing of PTB7-Th:PC71BM blends and a strong phase separation of donor and acceptor molecules of the blend films was observed at high temperatures.

41

Hsieh et al. demonstrated the

formation of PCBM clusters in the PTB7-Th:PCBM system under the long-term thermal annealing, which led to unbalanced mobility of positive and negative charge carriers and thereby to the degraded device performance. 42 Despite the already existing knowledge, however, from a morphological perspective the understanding about the influence of the PC71BM content on the morphology is not complete in the PTB7-Th:PC71BM BHJ system. Therefore, we perform an in-depth morphology study in the PTB7-Th:PC71BM BHJ system via varying the ratio of PTB7-Th and PC71BM in the active layer. The surface morphology of as-prepared active layers is probed with AFM measurements. Grazing incidence small-angle x-ray scattering (GISAXS) measurements characterize the inner


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film morphology regarding domain sizes and their corresponding spatial arrangement. Grazing incidence wide-angle x-ray scattering (GIWAXS) measurements yield information about the influence of the blend ratio on the polymer crystal structure. The optoelectronic properties of the PTB7-Th:PC71BM active layers are probed with UV/Vis spectroscopy and the vertical composition is characterized with x-ray reflectivity (XRR), respectively. To link experimental findings with device performance, organic solar cells based on PTB7-Th:PC71BM active layers are fabricated to give a proof of viability of our findings.

2. EXPERIMENTAL SECTION

Materials. PTB7-Th (Mn = 52500 g mol-1, PDI = 2.0) and PC71BM were purchased from 1-material Inc.. DIO (97%) and chlorobenzene (>99.8%) were obtained from SigmaAldrich

and

Carl

Roth,

respectively.

Poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, PH1000, 1.0-1.3 wt% in


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water) was purchased from Ossila. All chemicals were used as received without any further purification. Sample Preparation. PTB7-Th:PC71BM films with different weight ratios were prepared in a nitrogen glove box by fixing the PTB7-Th amount and varying the PC71BM content. In detail, a PTB7-Th master solution (10 mg mL-1) was prepared by dissolving PTB7-Th in chlorobenzene and 3 vol% of DIO at 70 °C. After 4 h stirring, five different amounts of PC71BM (5 mg, 10 mg, 15 mg, 20 mg, 25 mg) were individually added to the prepared PTB7-Th master solutions to achieve PTB7-Th:PC71BM weight ratios of 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5. Finally, an overnight stirring at 70 °C was employed to finalize the ready-to-use PTB7-Th:PC71BM blend solutions. For film characterizations, the blend solutions were deposited on pre-cleaned glass and/or silicon substrates via spin coating (1000 rpm for 1 min). For photovoltaic devices, indium tin oxide (ITO)-coated glasses were cleaned with dichloromethane, DI H2O and isopropanol for 10 min in sequence and then by air plasma (5 min). The cleaned ITO sheets were subsequently spin-coated with a PEDOT:PSS layer (3000 rpm, 60s), followed by a thermal annealing (120 °C for 10 min). After being cooled down to room temperature, PTB7-Th:PC71BM active layers were


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spin coated (1000 rpm for 1 min). Finally, LiF layer (~0.8 nm), Al contact (20 nm) and Mg contact (60 nm) contact were sequentially deposited on top of the active layer using thermal evaporation at low air pressure (2 × 10-6 mbar). Film and Device Characterization. J-V characteristics of the prepared organic solar cells were measured with a simulated AM1.5 solar illumination (100 mW cm-2). The intensity of incident light was calibrated using a standard silicon reference cell. During solar cell measurements, a shadow mask with an illumination area of 0.1 cm-2 was used. AFM measurements were performed by using an Autoprobe CP Reasearch Thermomicroscope AFM instrument (Veeco Instruments Inc.) in tapping mode using conical shaped tips. GISAXS and GIWAXS measurements were carried out at the P03/MiNaXS beamline of the PETRA III storage ring at DESY (Hamburg, Germany).43 The wavelength of the x-ray was 0.957 Å (energy of 13 keV). A Pilatus 1M 2D detector was utilized to record the scattering signal, which consisted of a matrix of 981 × 1043 pixels with a pixel size of 172 μm × 172 μm. For GISAXS measurements, a grazing incident angle of 0.2° and a sample-detector distance of 3622 mm were chosen to obtain the desirable q range. A slightly larger angle of 0.3° and a much smaller sample-detector


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distance of 126 mm were set for GIWAXS measurements. UV/Vis measurements were conducted with a Lambda 650 S UV/Vis spectrometer (PerkinElmer) in transmission geometry. XRR measurements were conducted with a D8 ADVANCE x-ray diffractometer (Bruker) with a wavelength of 1.54 Å and a 2θ range from 0° to 7°.

3. RESULTS AND DISCUSSION

3.1 Photovoltaic Performance

The implemented solar cell layout is: ITO/PEDOT:PSS/ PTB7-Th:PC71BM blend/LiF/Al. Figure 1 shows representative J-V characteristics of PTB7-Th:PC71BM BHJ solar cells for the different PC71BM content. The extracted photovoltaic parameters, including PCE, short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF), are displayed in Table 1. It is noted that the devices with a blend ratio of 1:1.5 exhibit the best photovoltaic performance and the champion device achieves a very promising PCE of 10.5 %. Therefore, in this work the optimal weight ratio of the blend is 1:1.5, which agrees with the reports in the literature. 44, 45 Generally, the performance of organic solar


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cells is closely related to the morphology, the crystalline structure and the optical property of the active layers, as demonstrated for many different BHJ systems in the literature. 46-48

3,

The differences in the device performance induced by the changes in the blend ratio

is discussed in detail regarding the morphology of the PTB7-Th:PC71BM active layers in the following.

Figure 1. J-V curves of organic solar cells with different PTB7-Th:PC71BM weight ratios of 1:0.5 (black), 1:1 (red), 1:1.5 (blue), 1:2 (pink) and 1:2.5 (green).


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Table 1. Photovoltaic parameters of devices with various PTB7-Th:PC71BM weight ratios. The best PCEs are provided in parentheses. sample

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

1:0.5

0.78 ± 0.01

9.6 ± 0.3

36 ± 1

2.7 ± 0.1 (2.9)

1:1

0.77 ± 0.01

16.9 ± 0.6

43 ± 2

5.6 ± 0.2 (5.9)

1:1.5

0.79 ± 0.02

20.7 ± 1.0

59 ± 2

9.8 ± 0.4 (10.5)

1:2

0.78 ± 0.01

16.9 ± 0.5

54 ± 1

7.2 ± 0.3 (7.7)

1:2.5

0.76 ± 0.01

13.8 ± 0.7

47 ± 1

5.0 ± 0.7 (5.6)

3.2 Film Surface and Inner Morphology


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The nano-scale surface topography of the blend films is studied with AFM measurements and the obtained results are depicted in Figure 2a-e. In as-spun polymer:fullerene BHJ films, fullerene tends to form clusters which are manifested by bright spots in the AFM images.49-51 It is noticeable that a fine phase separation is observed in all active layers, despite a few large PC71BM-rich domains which are present in the blends with a high fullerene content. The morphology of a well-mixed donor and acceptor matrix induced by the DIO additive has been frequently observed in the literature.36, 46, 52 The size of the PC71BM grains increases with increasing the PC71BM concentration until the blend ratio is 1:2. For a higher amount of PC71BM again smaller PC71BM aggregates appear on the sample surfaces. From the AFM images, surface roughness values are extracted and summarized in Figure 2f. At a ratio of 1:0.5, the PTB7-Th:PC71BM films have a surface roughness of (0.87 ± 0.07) nm. With increasing the PC71BM content, the values decrease slightly till the ratio of 1:1.5. Further increasing the PC71BM concentration results in a constant roughness of around (0.74 ± 0.12) nm. All values of the surface roughness are below 1 nm, suggesting a very flat sample surface for all active layers.


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Figure 2. AFM topography images (scan range 1×1 μm2) of PTB7-Th:PC71BM films with PTB7-Th:PC71BM ratios of a) 1:0.5, b) 1:1, c) 1:1.5, d) 1:2, e) 1:2.5 and f) the corresponding root mean square (RMS) surface roughness.

Information about the inner film morphology of the PTB7-Th:PC71BM blend films is determined with GISAXS measurements, in which films are probed over a macroscopic area comparable with the electrode size of OSCs. Thus, a high statistical relevance is ensured for the structure information. The non-destructive GISAXS technique is capable


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of providing domain sizes and spatial correlations down to nanometer scales regarding the inner structures of the films, which is desirable for the present work to determine the interface between PTB7-Th and PC71BM. The 2D GISAXS data of PTB7-Th:PC71BM blend films with different blend ratios are displayed in Figure S1. For a quantitative analysis, horizontal line cuts are performed on the 2D GISAXS data at the Yoneda peak position of PTB7-Th.

53

This peak is located at the critical angle of PTB7-Th, which is

material characteristic.53 As horizontal line cuts are performed along the qy direction, information about lateral structure (parallel to the sample surface) is yielded, including PTB7-Th domain sizes and the corresponding center-to-center distances in the whole volume of the active layers. These horizontal line cuts of PTB7-Th:PC71BM films with different blend ratios are plotted as a function of qy in Figure 3a.


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Figure 3. a) Horizontal line cuts (black hollow circles) of 2D GISAXS data for PTB7-Th:PC71BM films with different blend ratios (1:0.5, 1:1, 1:1.5, 1:2, 1:2.5 from bottom to top). The red lines represent the fits to the data. All curves are shifted along y axis for clarity of the presentation. Extracted characteristic length scales: b) domain radii and c) domain center-to-center distances. Blue, red and black colors represent small-sized, middle-sized and large-sized substructures, respectively.

All horizontal line cuts are modelled in the framework of the distorted-wave Born approximation (DWBA) with a model containing three cylindrically-shaped PTB7-Th domains distributed over a 1D paracrystal lattice.54-57 From data modelling of each


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sample, characteristic values of the domain sizes and the corresponding center-to-center distances are obtained and displayed in Figure 3b and 3c. It is noticeable that the large type PTB7-Th structures for all blends have domain radii of around 100 - 140 nm. A similar observation was reported by Liao et al., who demonstrated that fractal-like polymer domains with a radius of about 125 nm were found in poly[2,6-(4,4-bis(2-ethylhexyl)-4Hcyclopenta[2,1-b;3,4-b’]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]

and

PC71BM

(PCPDTBT:PC71BM) blend thin films with 3 vol% DIO as determined by GISAXS measurements.

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In addition, the domain radii increase with increasing the amount of

PC71BM till the weight ratio of 1:1.5, whereas it decreases with a further increase of the PC71BM content. The nanoscale phase separation in the polymer:fullerene BHJ layer tends to increase with increasing PC71BM content to a certain degree,22, 58 which leads to larger polymer domains. A further increase of the fullerene amount induces an intercalation of the fullerene into the polymer domains,59 which divides polymer domains into smaller ones. However, these large-scale PTB7-Th structures are expected to have only a limited impact on the actual device performance as the domain sizes are much larger than typical values of the exciton diffusion lengths in PTB7-Th. Thus, the smaller


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scale domains are more relevant. On smaller scale, we find two types of domains, denoted medium and small type. For the medium type domains, the sizes show a similar tendency as for the large domain structures. The 1:0.5 blend films have a domain size of (24 ± 1) nm. With increasing the PC71BM content the domain size reaches a maximum value for the ratio 1:1.5 of (31 ± 1) nm, and then the sizes decrease for the ratio 1:2 to (25 ± 1) nm and for 1:2.5 to (23 ± 1) nm. In contrast, the small type domain sizes stay constant at around 4 nm (radius) irrespective of the PC71BM amounts. For an efficient generation of free charge carriers, the photogenerated excitons need to reach the donoracceptor interfaces within their diffusion length (in the range of 4 - 20 nm).48, 60, 61 For the BHJ configuration, the intrinsic intermixing of donor and acceptor materials relaxes the domain size requirement to a certain degree as compared to the diffusion length of excitons. In the present work, the domain sizes of medium and small type structures are in the range of the diffusion lengths and closely related to the photovoltaic performance of the PTB7-Th:PC71BM organic solar cells since they contribute to the exciton splitting. As the small type structures remain almost unchanged regarding their domain sizes, only the middle-sized structures are considered to cause the changes of the photovoltaic


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parameters. In detail, the 1:0.5 and 1:1 system has too well mixed donor-acceptor materials with a low PC71BM content, which leads to an insufficient percolation. While in the case of a high PC71BM content the intercalation of fullerene molecules into the polymer matrix occurs, resulting in smaller domains as well as compared to the 1:1.5 blend films. The increase of domain sizes in the range of a few nanometers contributes to the increase of device performance.62 In addition, an intercalation of fullerene into polymer domains under the circumstance of high PC71BM concentration is expected to be detrimental to polymer crystallization and thereby negatively impact on the conductivity within the polymer domains. The impact of PC71BM concentration on PTB7-Th crystal structure is discussed next.

3.3 Crystal Structure of Films

GIWAXS measurements are employed to analyze detailed information about crystalline structures in PTB7-Th:PC71BM blend films. This method is a powerful tool to understand structural information of polymer blending.63 The 2D GIWAXS data of the films are displayed in Figure S2 in an order of increasing PC71BM weight fraction. For a quantitative


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analysis, horizontal and vertical sector integrals are performed on the 2D GIWAXS data along the scattering vector q in the direction parallel and perpendicular to the sample substrate, respectively. The obtained cuts are plotted as a function of q in Figure 4.

Figure 4. a) Horizontal and b) vertical sector integrals (black hollow circles) of 2D GIWAXS data for PTB7Th:PC71BM films with different blend ratios (1:0.5, 1:1, 1:1.5, 1:2, 1:2.5 from bottom to top). The red lines represent the fits to the data. All curves are shifted along y axis for clarity of the presentation. The missing data points in the region of broken x axis in a) is due to the inter-module detector gap of the used detector.

All peaks are modeled with Gaussian functions for extraction of mean q-positions and full width at half-maximum (FWHM) values. In the horizontal direction (Figure 4a), the first peak positioned at q ≈ 0.30 Å-1 for all blend films is relevant to the periodic spacing


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of the PTB7-Th adjacent backbones along the (100) direction. This (100) Bragg peak intensity decreases with increasing the PC71BM content, suggesting that the PTB7-Th crystallinity along the (100) direction is decreased when more PC71BM is added. This finding is in agreement with the observations reported in the P3HT:PCBM systems, where P3HT crystallinity decreased gradually with increasing the PCBM composition.

22, 64

However, irrespective of the PC71BM content the distance between the adjacent PTB7Th backbones stays constant at about 2.0 nm (calculated by d=2π/q), which matches values for the PTB7-Th lattice constants reported in the literature. 65, 66 From the FWHM of the (100) Bragg peak, the corresponding crystal sizes of PTB7-Th can be estimated with the well-known limitations from the Scherrer equation under the assumption of constant paracrystallinity. The crystal size along the (100) plane remains unchanged at around 6.5 nm with increasing the PTB7-Th:PC71BM content to a weight ratio of 1:1.5, then it decreases to about 5.3 nm for the 1:2 samples and to about 3.3 nm for the 1:2.5 samples. Moreover, peaks present at q ≈ 1.40 Å-1 and 1.85 Å-1 originate from PC71BM aggregates.67, 68 These aggregates are randomly oriented, which is manifested by the isotropic ring intensities in the 2D GIWAXS data (Figure S2,). The scattering intensities


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of both PC71BM Bragg peaks become increasingly strong with increasing PC71BM amounts. This trend is observed for vertical cuts (Figure 4b) as well, which simply indicates that more PC71BM clusters are formed in the blend film with higher PC71BM content. In Figure 4b, the (010) Bragg peak of PTB7-Th at q ≈ 1.60 Å-1 is clearly visible in the vertical cut for the 1:0.5 sample. With increasing PC71BM content this Bragg peak is gradually overshadowed by the PC71BM Bragg peaks. The (010) Bragg peak corresponds to the π-π stacking distance of the conjugated PTB7-Th backbones and verifies the presence of the face-on orientation of the polymer, which is in-line with the results from previous reports.

66, 69, 70

From modelling with Gaussian functions we determine that the

π-π stacking spacing is stable at about 0.4 nm for all blend ratios, indicating that the PC71BM amount does not influence the distance of the π-π stacking. The corresponding PTB7-Th crystal size along the (010) plane is calculated to be 3.8 nm for the first four blends, corresponding to almost 10 lattice planes. Whereas it decreases to 2.8 nm for the 1:2.5 sample, in which the crystals have three lattice planes less than for other samples. Putting all information together, we conclude that the PC71BM concentration does not influence the distance of the PTB7-Th adjacent backbones and the π-π stacking in


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the polymer crystals. However, an increase of the PC71BM content is detrimental to the formation of the PTB7-Th crystals at all.

3.4 Optical Properties

The UV/Vis absorption spectra of PTB7-Th:PC71BM films are shown in Figure 5a for different PC71BM contents. The absorption peaks at about 460 nm result from PC71BM. The peak position stays almost stable, whereas the peak intensity increases with increasing amount of fullerene. The absorption features above 600 nm are attributed to PTB7-Th. Two characteristics peaks (the main peak at about 710 nm and the second at about 650 nm) exhibit a decreasing intensity with increasing PC71BM content, which is an opposite trend as compared with the PC71BM peak intensity. Moreover, it is noticeable that an apparent blue shift in the absorbance spectra of both peaks occurs. It arises from the decreasing crystallinity of the PTB7-Th molecules with increasing PC71BM content, which is verified in the GIWAXS measurements. In addition, a more distinct shift to small wavelengths is observed if a high PC71BM content is applied (for the blends with weight


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ratios of 1:2 and 1:2.5). This occurrence is ascribed to the decreased PTB7-Th crystal size (as revealed by GIWAXS measurements). In order to further analyze the information about the PTB7-Th crystalline order in the different blend films, the contributions from the PC71BM and PTB7-Th amorphous phases are subtracted from the initial absorption spectra of the blend films. The absorption spectrum of a pure PC71BM film is shown in Figure S3a. After subtraction, the residual spectra are shown in Figure 5b and fitted with Gaussian-shaped absorption bands. An exemplary fitting scenario is depicted in Figure S3b. Typically, the strongest contribution between 1.73 eV and 1.74 eV refers to the transition between the first vibronic band of the ground state (A00) and the ground state of the excited state (A10), which is identified as the A0-0 transition. The second shoulder ranging from 1.86 eV to 1.94 eV, known as the A0-1 transition, is attributed to an inter-band transition between the state A00 and the second vibronic band of the excited state (A11). The detailed information about the center positions of the A0-0 and A0-1 transitions for different blend films are summarized in Figure 5c. It can be seen that the A0-0 position remains stable for all samples at a value of about 1.735 eV, whereas the peak position of A0-1 transitions experiences a continuous


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decrease from 1.91 eV to 1.86 eV with increasing PC71BM content. This occurrence suggests a stronger red shift in the A0-1 transition when more PC71BM is added into the blend films. A similar observation has been reported in the PTB7:PCBM system by Szarko et al., who demonstrated that intercalation of PCBM molecules into the polymer matrix created charge transfer states with lower energy, leading to red-shift in the absorption band. 71 Compared to the absorption spectra of PTB7-Th:PC71BM films, it is inferred that the apparent blue shift in Figure 5a is caused by an increased amount of PC71BM and amorphous PTB7-Th. Moreover, a comparison of the free exciton band width (W) within the PTB7-Th crystalline phase can be done through the weakly coupled H-aggregate model,72, 73 A0 ― 0 A0 ― 1

≈

n0 ― 0 1 ― 0.24W/Ep ( ) n0 ― 1 1 + 0.073W/Ep

where n0-0 and n0-1 are the real part of the refractive indices at the 0-0 and 0-1 peaks, respectively; Ep describes the phonon energy of the main oscillator coupled to the electronic transition, which is constant for all blends as it only depends on the chemical structure of the polymer. The calculated values of W/Ep are plotted in Figure 5d as a


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function of weight ratios. We find a clear trend towards lower exciton band widths with increasing PC71BM content. According to the Franck-Condon principle, the decrease of the exciton band width is the consequence of the increasing degree of order in the polymer. The GIWAXS results show that the distance of PTB7-Th adjacent backbones and π-π stacking stays stable irrespective of PC71BM content, suggesting an unchanged interchain ordering of PTB7-Th. Therefore, the increasing degree of order in this work is attributed to the increasing length of the undisturbed conjugation in PTB7-Th chain. This finding reflects that an increase of PC71BM content leads to an increase of the PTB7-Th conjugation length, although it is detrimental to the overall crystallinity and crystal size (along the (100) and (010) plane) of the polymer.


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Figure 5. a) UV/Vis absorption spectra of PTB7-Th:PC71BM films with different blend ratios as indicated. b) Residual absorption spectra of these films after the subtraction of the contributions from PC71BM and amorphous PTB7-Th, indicated by open circles. Solid lines represent the fits to the data. c) Center positions of the first (black) and second vibronic peak (red) obtained from data modeling. d) A comparison of free exciton band width (W) for different PTB7-Th:PC71BM films.

3.5 Vertical Film Composition


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XRR measurements are employed to investigate the vertical composition of the PTB7-Th:PC71BM blend films. The XRR data are shown with the corresponding fits based on the Parrat algorithm in Figure 6a. Firstly, the thickness of each film is calculated from the distance of the adjacent intensity fringes at qz > 0.03 Å-1. The film thickness increases linearly with increasing PC71BM content, from about 61 nm for the 1:0.5 blend film to around 132 nm for the 1:2.5 blend film. Secondly, the roughness of each film is obtained and depicted in Figure 6b. It can be seen that the roughness values are in good agreement with the ones obtained from AFM measurements. Finally, the scattering length densities (SLDs) of pure PC71BM and PTB7-Th films are calculated to be 15.3×10-6 Å-2 and 10.5×10-6 Å-2, respectively (Figure S4). Therefore, the theoretical SLD values for blend films are calculated, reading 11.7×10-6 Å-2 (1:0.5), 12.5×10-6 Å-2 (1:1), 13.0×10-6 Å-2 (1:1.5), 13.3×10-6 Å-2 (1:2) and 13.6×10-6 Å-2 (1:2.5). From XRR data modelling, the SLD increases with the PC71BM weight ratio for blend films. These obtained values (average for the whole films) are 11.9×10-6 Å-2 (1:0.5), 12.2×10-6 Å-2 (1:1), 12.9×10-6 Å-2 (1:1.5), 13.3×10-6 Å-2 (1:2) and 13.6×10-6 Å-2 (1:2.5), which agree very well with the theoretical SLD values.


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Figure 6. a) XRR data (black curves) with their fits (red lines) of PTB7-Th:PC71BM films with different blend ratios (1:0.5, 1:1, 1:1.5, 1:2, 1:2.5 from top to bottom). b) The obtained roughness values from data modelling. c) Grayscale-coded illustration of the vertical composition for PTB7-Th:PC71BM films with different blend ratios. Black and white colors correspond to pure PC71BM and PTB7-Th, respectively. The height of grayscale-coded rectangles is scaled to the thickness for all films. The height of the first one represents a thickness of 61 nm and for the last one a thickness of 132 nm.

Based on the calculated SLDs (from XRR measurements) versus film thickness, a grayscale-coded illustration of the vertical composition for different PTB7-Th:PC71BM


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films is displayed in Figure 6c as a result of best fits. Here, a pure PC71BM phase is represented with black color and a pure PTB7-Th phase is indicated with white color. It is noticeable that the color becomes darker with increasing PC71BM content, as SLD values increase with more PC71BM being applied for film preparation. For the 1:0.5, 1:1 and 1:1.5 blend films, uniform colors are observed as only one SLD is obtained for each film, indicating that a vertically well-intermixed polymer:fullerene phase is formed. The 1:2 film consists of two layers: A thin layer with a higher SLD located at the substrate and a thick layer with a lower SLD forming the majority of the film. The thin layer has an SLD of 15.0×10-6 Å-2, which is considered as an enrichment layer of PC71BM (the SLD of PC71BM is 15.3×10-6 Å-2). The thickness of this layer is about 2.1 nm. Such layer has a negative impact on the device performance in standard device geometry of the OSCs, since the donor material is expected to be at the bottom contact. For the thick layer the determined thickness and SLD are 113 nm and 13.3×10-6 Å-2, respectively. For the 1:2.5 film, three layers are found along the surface normal. Compared to the 1:2 film, the fullerene enrichment layer extends to approximately 5.0 nm at the substrate. Above it, the major part of the film resides. It has a thickness of 123 nm and an SLD of 13.5×10-6 Å-2, which


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is regarded as a homogeneously-mixed phase of PTB7-Th and PC71BM. An additional thin layer of about 1 nm is formed at the air surface, which is only 0.77 % of the overall thickness and therefore hardly visible in Figure 6c. The SLD is 11.6×10-6 Å-2, from which the component of this thin layer can be calculated to be 77 % PTB7-Th and 23 % PC71BM. This finding is in good agreement with the observation of AFM measurements, which show less PC71BM aggregates on the sample surface. Again, the formation of enrichment layers has a negative impact on the device performance as the wrong material is enriched at the anode and cathode of the devices.

3.6 Discussion

The morphological investigations about the influence of PC71BM content on the PTB7Th:PC71BM BHJ system are schematically illustrated in Figure 7.


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Figure 7. Schematic representation of the morphology of PTB7-Th:PC71BM films with different blend ratios as indicated.

With increasing PC71BM fraction, the film thickness increases linearly since the overall concentration was not adapted to fix the film thickness. For the crystalline structure, we find that the distances of PTB7-Th adjacent backbones and of π-π stacking do not change for different PC71BM concentrations. However, a too high content of PC71BM is expected to hinder polymer crystal formation, which is manifested by smaller PTB7-Th crystal sizes along (100) and (010) planes for the 1:2 and 1:2.5 films. The crystal sizes hereby decrease when the PC71BM content exceeds a certain threshold. The Franck-Condon analysis reveals that an increase of the PTB7-Th conjugation length


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occurs with increasing PC71BM content, which is presented by longer conjugated polymer chains inside the crystals in Figure 7. The XRR measurements verify that a PC71BM enrichment layer forms at the substrate when a weight ratio of PTB7-Th and PC71BM exceeds 1:1.5 and the thickness of this layer grows with further increasing PC71BM content. In addition, a PTB7-Th enrichment layer is found at the air surface when the PTB7-Th:PC71BM weight ratio reaches 1:2.5. For the standard solar cell geometry in the present work, the PC71BM-rich layer hinders the transport of positive charge carriers to the ITO electrodes and the PTB7-Th enrichment layer harms the transport of negative charge carriers to the metal electrodes. Consequently, both types of enrichment layers are detrimental for the solar cell performance of devices in standard geometry. In general, the PTB7-Th crystals do not deteriorate regarding the size and the conjugation length when a weight ratio of PTB7-Th and PC71BM reaches to 1:1.5. Moreover, the 1:1.5 film has a proper thickness (around 100 nm) and an optimal donor-acceptor phase separation in this work. By taking all these factors together, the 1:1.5 BHJ organic solar cells give the best photovoltaic performance with a highest PCE of 10.5 %. Although the PTB7-Th crystallinity is higher in films with a smaller PC71BM content than in the 1:1.5


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blend film, the thinner BHJ layer and insufficient percolation result in lower Jsc and FF than for the 1:1.5 films. With increasing the PC71BM concentration, the Jsc and FF values are well improved and leading to a better photovoltaic performance for the PTB7Th:PC71BM organic solar cells. For high PC71BM incorporation, the decreased polymer crystal size and crystallinity in combination with the wrong PC71BM and/or PTB7-Th enrichment layers at the corresponding electrodes again result in decreased Jsc and FF values, which causes worse photovoltaic performance than for the 1:1.5 solar cells.

4. CONCLUSION

A systematic study of the influence of PC71BM concentration on the inner morphology, polymer crystallization and vertical composition of PTB7-Th:PC71BM blend films is presented in this work, which shows a close correlation between the morphology of the active layers and the photovoltaic performance of the final devices. In more detail, we have analyzed the effect of PC71BM concentration on the morphology of PTB7-


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Th:PC71BM BHJ solar cells. The influence of fullerene content is studied by means of AFM, GISAXS, GIWAXS, UV/Vis and XRR measurements. The increase of PC71BM content enhances microphase separation of fullerene and polymer, whereas the polymer domain sizes become smaller with further increasing the PC71BM concentration. The high amount of PC71BM turns out to hinder polymer crystallite formation and leads to a decrease of overall crystallinity and crystal size along the (100) and (010) plane. However, it induces an increase of the PTB7-Th conjugation length in the BHJ films. The film thickness increases with PC71BM content and a fullerene-rich layer is formed on the substrate in case of high PC71BM concentration. All findings are closely correlated to the photovoltaic performance of the final devices. Furthermore, this work demonstrates an efficient set of method to determine the relationship between film composition, morphology and device performance for the example of the PTB7-Th:PC71BM system, which could be easily adapted to other photovoltaic systems as well

Supporting Information


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Information about 2D GISAXS data and 2D GIWAXS data of PTB7-Th:PC71BM blend films, an absorption spectrum of a pure PC71BM film, an exemplary fitting scenario of the absorption spectra of the 1:1.5 blend film films, XRR data with their fits of pure PTB7-Th and PC71BM films.

AUTHOR INFORMATION L. Song and W. Wang contributed equally to this work.

Corresponding Author * Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by funding from the International Research Training Group 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS), TUM.solar in the context of the


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Bavarian Collaborative Research Project Solar Technologies Go Hybrid (SolTech), the Excellence Cluster Nanosystems Initiative Munich (NIM) and the Center for NanoScience (CeNS). D. Y. acknowledges the China Scholarship Council (CSC) funding. Portions of this research were carried out at the synchrotron light source PETRA III at DESY. DESY is a member of the Helmholtz Association (HGF).


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Composition-morphology correlation in PTB7- Th:PC71BM blend films

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