Comprehensive Investigation and Analysis of Bulk-heterojunction

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Organic Electronic Devices

Comprehensive Investigation and Analysis of Bulk-heterojunction Microstructure of High-performance PCE11:PCBM Solar Cells Chaohong Zhang, Thomas Heumueller, Wolfgang Gruber, Osbel Almora, Xiaoyan Du, Lei Ying, Junwu Chen, Tobias Unruh, Yong Cao, Ning Li, and Christoph J. Brabec ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22539 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Applied Materials & Interfaces

Comprehensive Investigation and Analysis of Bulk-heterojunction Microstructure of High-performance PCE11:PCBM Solar Cells Chaohong Zhanga,b *, Thomas Heumuellerb, Wolfgang Gruberc, Osbel Almorab, Xiaoyan Dub, Lei Yingd, Junwu Chend, Tobias Unruhc, Yong Caod, Ning Lib,e *, Christoph J. Brabecb,f * a SUSTech

Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, No. 1088,xueyuan Rd., 518055, Shenzhen, Guangdong, P.R. China b Institute Materials for Electronics and Energy Technology (i-MEET), FriedrichAlexander University Erlangen-Nürnberg, Martensstrasse 7, 91058 Erlangen, Germany c Institute for Crystallography and Structure Physics, Friedrich-Alexander University Erlangen- Nürnberg, Staudtstrasse 3, 91058 Erlangen, Germany d Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, 381 Wushan Rd., 510640 Guangzhou, P.R. China e National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, No.100 Science Avenue, 450002 Zhengzhou, China f Helmholtz-Institute Erlangen-Nürenberg (HIERN), Immerwahrstr. 2, 91058 Erlangen, Germany * Author to whom correspondence should be addressed. E-mail: [email protected]; [email protected]; [email protected];

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Abstract: Worldwide research efforts have been devoted to organic photovoltaics in hope of large-scale commercial application in the near future. To meet the industrial production requirements, organic photovoltaics that can reach power conversion efficiency (PCE) over 10% along with promising operational device stability are of utmost interest. In the study, we take PCE11:PCBM as a model system, which can achieve over 11% PCE when processed from non-halogen solvents, to in-depth investigate the morphology-performance-stability correlation. We demonstrate that four batches of PCE11 with varying crystalline properties can achieve similarly high performance in combination with PCBM. Careful device optimization is necessary in each case to properly address the requirements for the quite distinct microstructures. The bulk-heterojunction (BHJ) microstructure is comprehensively investigated as a function of the macro-molecule weight and crystallinity. It is demonstrated that the small differences in the morphology significantly affect the kinetics and thermodynamic equilibrium of BHJ microstructure as well as the photo- and thermal- stability of the PCE11:PCBM solar cells.

Keywords: film morphology, bulk-heterojunction microstructures, PCE11:PCBM solar cells, in-situ capacitance spectroscopy, organic photovoltaics


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Introduction Organic photovoltaics (OPV) is one of the high potential photovoltaic technologies for sustainable energy supply owing to the unique advantages, such as semi-transparency, cost-effectiveness, light weight and non-toxic properties.1-9 Astounding achievements in the field of OPV have been attained in materials design optimization

13-18

10-12

as well as device

since the introduction of bulk-heterojunction (BHJ).19-20 It is widely

accepted that the BHJ microstructure of the active layer is tremendously crucial for high photovoltaic performance. Only recently it was reported that the bulk heterojunction microstructure is also crucial for the thermal as well as photo-stability.21-24 Generally, the phases of BHJ layer can be defined as a donor-acceptor intimately mixed phase, a donorrich and an acceptor-rich phase. Preceding research demonstrated that the pristine donor and acceptor phases should be as pure and crystalline as possible to guarantee effective charge transport, while the mixed phase should be as intimate as possible to facilitate charge generation.22, 25-28 Currently, there are well-developed techniques for multidimensional film morphology characterization.29 Energy-filtered transmission electron microscopy (EF-TEM) provides information concerning nanoscale phase separation by direct imaging each component; the combination of TEM tomography and computer modeling allows the knowledge based reconstruction of the 3D morphology.30-32 Resonant soft X-ray scattering (RSoXS) and grazing incidence small-angel X-ray scattering (GISAXS) are typically employed to investigate domain sizes and relative domain purity.33-35 However, these useful techniques are yet not able to probe the real-time morphological evolution of BHJ



microstructure in an operating device. A powerful but rarely used method allowing insight into the microstructure evolution of fully operational devices is in-situ capacitance spectroscopy.36-37 Poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3’’’-di(2-octyldodecyl)2,2’;5’,2’’;5’’,2’’’-quaterthiophen-5,5’’’-diyl)] (PffBT4T-2OD or PCE11), a highly crystalline polymer donor, was reported to reach over 11% PCE in combination with [6,6]phenyl-C71 butyric acid methyl ester (PC71BM).38 Furthermore, PCE11-based solar cells were considered for large-scale production owing to their promising device performance over a wide range of film thicknesses and processing conditions, including even nonhalogen solvents.39 However, the material-morphology-performance relation is far away from being well-understood for this type of crystalline BHJ systems, and requires more comprehensive investigation and analysis in order to develop highly efficient and stable OSCs.40-42 In this work, different batches of PCE11 with varying molecular weight and polydispersity are investigated by means of temperature-modulated differential scanning calorimetry (MDSC) and found out to display different crystalline and self-aggregating properties. The interaction between PCE11 and [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM or PCBM) are analyzed by means of DSC, grazing incidence wide-angle X-ray scattering (GIWAXS) and in-situ capacitance spectroscopy, respectively. Interestingly, PCE11 with varying material properties could obtain similarly highefficiency solar cells in combination with PCBM but result in distinctly different composite BHJ microstructure and morphological stability.


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Results and discussion Photovoltaic performance based on various batches of PCE11 Figure 1a shows the chemical structure of PCE11 and PCBM studied in this work. As depicted in Figure 1b, four different batches of PCE11, namely P1, P2, P3 and P4, with different molecular weight and polydispersity index (PDI) were characterized (Table S1). From MDSC data of the neat polymer, P1, P2, P3 and P4 exhibit a different melting temperature (Tm) of 274 °C, 268 °C, 247°C and 168 °C, respectively. The crystallinity of the neat polymers is reflected in the 2nd heating scan of the MDSC curves. P1, the batch with the highest molecular weight and the lowest PDI, shows the highest and most narrow melting peak, indicating relatively larger and more perfect crystals. P4, with the lowest molecular weight, reveals a lower and broader melting peak, implying least perfect crystals. P3 melts at a temperature lower than P1, indicating the formation of smaller and less perfect crystals than P1. P2 with the broadest molecular weight distribution displays two well-defined features, a narrow melting peak at higher temperature and a small bump at a relatively lower temperature, which suggests that P2 forms a large fraction of P1 like crystals and a small fraction of smaller and less perfect P3 type crystals. PCE11:PCBM solar cells in the inverted device architecture (Figure 1c) were fabricated and characterized. The corresponding photovoltaic performances under different processing conditions are summarized in Table S2. Due to the fact that P0 possesses too high molecular weight resulting in highly strong self-aggregating properties and highly difficult processing, and that P4 possesses too low molecular weight to obtain reasonable 5 ACS Paragon Plus Environment


photovoltaic performance, we focused our effort only on P1, P2 and P3 for further investigation. It was frequently reported that polymers with lower molecular weight obtain reduced photovoltaic performance owing to morphological differences.16, 43-48 As depicted in Figure 1d, solar cells based on P1, P2 and P3 could achieve comparable photovoltaic performance of about 9% under optimized processing conditions. For instance, the stronger self-aggregating properties of P1 required a higher ratio of orthodichlorobenzene (oDCB) in mixed CB:oDCB solvents as well as a higher spin coating temperature (90-110 °C) , while P2:PCBM and P3:PCBM films could be spin-coated from solutions kept at 80 °C.


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temperature melting point of the blends is attributed to PCBM; the melting points at lower temperature represent PCE11 crystallites with different amounts of PCBM impurities. The melting peak position of PCBM is almost unaffected by the addition of PCE11. Addition of PCBM significantly depresses the melting point of crystalline PCE11 (1st and 2nd scan). Such a behavior evidences PCBM impurities in PCE11 crystals which probably got trapped during film formation. Small variations in the crystallization temperature between the 1st and the 2nd scan reflect the re-arrangement of PCE11 or/and PCBM molecules during heating. Comprehensive investigation and analysis on BHJ morphology Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) measurements were carried out on neat and blended PCE11 films. The GIWAXS profiles collected from out-of-plane and in-plane cuts are depicted in Figure 2. The corresponding 2D patterns are shown in Supporting Information Figure S1. The positions of the polymer lamella peak (100) and stacking (010) are well defined and in good agreement with literature.38 Both PCE11 and PCE11:PCBM films exhibit strong lamella peak (100) in in-plane direction and stacking (010) in out-of-plane direction, which signifies preferential face-on orientation of pristine polymers and blended films.


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(b)

(a)

P1 P2 P3

(300)

10

15

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Qy (nm-1)

(c)

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Intensity (a.u.)

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(100)

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PCBM

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Polymer (100) P1 P2 P3 P1:PCBM P2:PCBM P3:PCBM

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P1:PCBM P2:PCBM P3:PCBM 0020 Intensity (a.u.)

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(100)

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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


0010

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Azimuthal angle (o)

Number of samples

Figure 2. (a-d) GIWAXS linecuts of PCE11 in pristine and in blends; (e) FWHM of polymer (100) peak and PCBM halo from in-plane cuts; (f) azimuthal distribution of PCBM halo in PCE11:PCBM films.



The full width at half maximum (FWHM) polymer (100) peak from in-plane cuts are calculated and summarized in Figure 2e. According to the Debye-Scherrer Equation49, crystallite size and the FWHM are inversely proportional. FWHM of neat P3 is larger than that of neat P1 and P2, corresponding to a smaller size of the crystallites. A comparable crystallite size is recorded for neat P1 and P2. Addition of PCBM reduces FWHM of all three PCE11 batches (100), indicating larger ordered domains. This might be due to the existence of PCBM in PCE11 ordered phases forming less perfect but larger-area crystalline phases. P3 crystals were affected the most by addition of PCBM. From the FWHM of PCBM halo in blends, the ordered domain of PCBM is rather small and distributes homogenously in both directions as indicated in the flatness of azimuthal distribution of PCBM halo (Figure 2f).

0.98 1 0.96 1

P2 at 2k Hz

(c)1

1 1.00 1 0.98 0.96 1 0.94 0

0.94 0 20

40 60 Temperature (oC)

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1 1st heating 1st cooling 2nd heating 2nd cooling

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(f)

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PC61BM P2:PCBM at at 7k 2k HzHz

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Normalized Capacitance Normallized Normallized Capacitance Capacitance

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Norm. JSC

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Normalized NormallizedCapacitance Capacitance

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P1 at 2k Hz

Normallized Capacitance Normalized NormallizedCapacitance Capacitance

Normallized Capacitance Normalized Capacitance

1 1.02

Normalized Capacitance Normallized Capacitance


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Normalized NormallizedCapacitance Capacitance

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85 °C, dark 0


10 20 Thermal aging time (min)

Figure 3. Evolutions of capacitance as a function of temperature of PCE11 (a-c), PCBM (d) and PCE11:PCBM composites (e-g). The temperature increased/decreased in the rate of 1 °C/min; impedance spectroscopy was recorded every 5 minutes. (h) Normalized JSC evolution of PCE11:PCBM solar cells aged under 85 °C in nitrogen atmosphere.


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DSC and GIWAXS techniques deliver information on the crystalline portion of bulk material or thin films, respectively. Here we introduce temperature dependent in-situ impedance spectroscopy (IS) measurements on full devices. IS of neat and blended PCE11 devices was recorded as a function of frequency and as a function of temperature between 0° C and 85 °C (Figure S2

S8). Capacitance as a function of sample

temperature is summarized in Figure 3a g. According to Equation (1), changes of the capacitance can result from changes in film thickness due to thermal expansion and contraction. Alternatively, a change of the dielectric constant as a result of microstructure modifications can lead to a temperature dependence of the capacitance.50-51 0

(1)

=

where C is the capacitance;

0

is the vacuum permittivity;

is the relative dielectric

constant of material; A is the area of the capacitors and is defined by the electrode; d is the thickness of the PCE11:PCBM films. As film thickness evolves with temperature following the thermal expansion rule, capacitance of pure PCE11 films mainly varies linearly and reversibly with temperature; neat P3 has about 2 % irreversible contribution from the 1st to the 2nd scan, which already may reflect relocations between the amorphous and crystalline regimes. The capacitance evolution of the PCE11:PCBM composites is more complex, as PCBM has a positive temperature coefficient of

r.

52-54

The resultant evolution of the capacitance of the 3

blends is the combined effects of temperature-dependent thickness and morphologydependent dielectric constant. Moreover, there is a distinctly expressed irreversibility between the 1st scan and the 2nd scan. The irreversible capacitance transformation is 11 ACS Paragon Plus Environment



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By combining the DSC characteristics, GIWAXS data and in situ impedance data, we can draw the following picture: the polymer with a higher molecular weight tends to give higher crystallinity (DSC) and larger crystals (GIWAXS) in pristine films. Microstructure re-arrangements in the amorphous regime are easier for lower molecular weights (irreversibility in the impedance behavior). Until now, we have better understanding about the film morphology of PCE11 and corresponding PCE11:PCBM blends. As illustrated in Figure 4, pristine P1 forms relatively big and perfect crystals; P3 forms relatively smaller and less perfect crystals; P2 mainly develops big crystals like P1 but also small amount of small crystals like P3. Further, there exist more amorphous fractions in P3 films than in P1 and P2. With the presence of PCBM, owing to the imperfect nature of the P3 crystalline phases, PCBM can easily intrude into the polymer packing resulting to large portion of amorphous regions. For P1:PCBM, owing to the strong self-aggregation of polymer chains, finely mixed amorphous donor-acceptor phases are limited but sufficient for efficient charge generation. For P2:PCBM, the polymer crystalline phases comprised relatively perfect crystals like in P1:PCBM and abundant less perfect crystals like in P3:PCBM. In spite of these film morphological differences, the three BHJ microstructures eventually exhibit comparable photovoltaic performance. Stability of solar cells based on 3 batches of PCE11 Although P1, P2, and P3 display distinctly different microstructure trends in the presence of PCBM, all three polymer results in comparable time zero efficiencies of 8 – 9 %. We



got curious to understand whether these composites would also display a comparable Stability and aged the optimized OSCs under either one-sun illumination (provided by white LEDs) or 85 °C thermal stress in nitrogen atmosphere. To our surprise, we found different degradation thermodynamic equilibrium for P2 as compared to P0, P1 and P3. As shown in Figure 5 and Figure S9-S11, the JSC of solar cells showed a classical “burnin” behavior under light aging or thermal aging, while the VOC and fill factor (FF) remained at high level. Interestingly, P2:PCBM preserved 90% of its initial JSC, while the JSC of P0:PCBM, P1:PCBM and P3:PCBM decreased by about 40% after 500-hour illumination. To exclude the possible effects on stability resulting from impurities in different batches of PCE11, we performed further characterization on pristine polymer to clarify the issue. 1H NMR spectra (Figure S12) of the polymer were highly similar for four samples. The perfectly overlap of the absorption spectra (Figure S13) for fresh and aged film demonstrated that four batches of PCE11 is rather chemically stable under white light. The hole-only devices of four batches of PCE11 under white LED illumination are fairly stable indicating the good photo-stability of the materials (Figure S14). Further, The 2D and Qy line-cut profiles of P1:PCBM and P2:PCBM films before and after thermal aging were summarized in Figure S15. From the in-plane GISAXS profiles (I ×

2

versus

) of the samples, it is quite obvious that P1:PCBM system

suffers from more severe morphological changes than P2:PCBM system. Such evidences demonstrate that the strong burn-in losses of P1:PCBM system results from donor and acceptor demixing in the finely mixed amorphous region, and that P2:PCBM system has a more stable amorphous mixed phase, resulting in a better stability. With the above knowledge on the material stability and BHJ morphology, we speculate that P2:PCBM


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system has a more stable amorphous mixed phase, which allows some rearrangement (irreversibility of impedance) but remains constricted by the large number of crystallites (GIWAXS) even after over 500 hours exposure to white light illumination.

(a)

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Norm. JSC

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0.6 0.4 0.2 ~ one sun, white light


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200 300 400 500 Light aging time (h)

0.4 0.2 o

85 C, dark

0.0 0

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600

0.5 1.0 1.5 Thermal aging time (h)


2.0

Figure 5. Normalized J , evolution of PCE11:PCBM solar cells aged under white light SC

illumination (a) and under 85 °C thermal stress (b) in nitrogen atmosphere.

Conclusion In summary, we systematically characterized the microstructural morphology of PCE11:PCBM BHJ blends as a function of the macro-molecular weight and reported on the correlation between BHJ morphology, device performance and microstructural stability. PCE11 with three varying molecular weight and crystalline properties could obtain comparable photovoltaic performance with PCBM as acceptor, indicating promising reproducibility and molecular weight tolerance. The BHJ microstructure of the corresponding polymer:fullerene composites was comprehensively investigated by DSC, GIWAXS and temperature dependent IS. Rather small but still distinct differences 15 ACS Paragon Plus Environment


in the crystallization behavior were found for the three batches of PCE11, which required to adapt processing but otherwise did not impact time-zero performance. However, these small differences in microstructure affected the kinetics and thermodynamic equilibrium of the bulk-heterojunction microstructure quite significantly. The middle molecular weight fraction showed a significantly reduced burn-in degradation behavior maintaining approximately 90% of the original JSC after burn-in, while the highest and lowest molecular weight batches lost almost 40 % performance during the burn-in phase. We believe that the findings demonstrated in this work will trigger more research interest concerning the influences of BHJ morphology on photovoltaic performance as well as on stability of solution-processed organic solar cells.

Experimental Section Materials: ZnO-nanoparticle dispersion in isopropyl alcohol was received from Avantama AG. PCE11-P1 (YY9226CB), P3 (YY10128CH) and P4 (YY10128 DCM) were received from 1-Material. PCE11-P0 and PCE11-P2 was synthesized at the South China University of Technology. All batches of PCE11 were synthesized by the same method as reported in reference38. PCBM (99%) was purchased from Solenne BV. Device Fabrication: All solar cells were fabricated in an inverted structure of ITO/ZnO/Active layer/MoOx/Al. The pre-structured ITO coated glass substrates were subsequently cleaned in toluene, acetone and isopropyl alcohol for 10 min each. Then 30 nm ZnO nanoparticles (Avantama, N10) were doctor bladed at 30 °C on the ITO substrate and annealed at 85 °C in ambient air. The PCE11-P0/P1-based active layer were spin-coated from chlorobenzene: 1,2-dichlorobenzene: diphenylether (48.5:48.5:3) 16 ACS Paragon Plus Environment

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solution; The PCE11-P2/P3/P4-based active layer were spin-coated from chlorobenzene: 1,2-dichlorobenzene: diphenylether (58.5:38.5:3) solution. 15 nm MoOx and 100 nm Al were deposited subsequently under 6 × 10-6 Torr by thermal evaporation through a shadow mask to form an active area of 10.4 mm2. Device Characterization: The current-voltage characteristics of solar cells were measured with a Keithley 2400 under AM 1.5G irradiation from an OrielSollA Solar simulator (100 mW cm-2). The light source was calibrated by using a silicon reference cell from Newport. All cells were tested in ambient air. White Light Illumination and Thermal Annealing Characteristics: The solar cells were loaded into a sealed chamber which can contain nine substrates of solar cells. This chamber was then continuously purged with nitrogen. The water and oxygen level was kept below 0.5 ppm. For white light illumination, the aging light sources are eight white light LEDs with an emission spectrum between 400 nm to 800 nm. For thermal aging, the light sources for inline J-V characteristics are eight white light LEDs and one UV-LED with wavelength of 365 nm. GIWAXS Measurements: The GIWAXS patterns were collected with the highly customized Versatile Advanced X-ray Scattering instrumenT ERlangen (VAXSTER) at the Institute for Crystallography and Structural Physics, FAU, Germany. The system is equipped with a MetalJet D2 70 kV X-ray source from EXCILLUM, Sweden. The beam was shaped by a 150 mm Montel optics (INCOATEC, Geesthacht) and two of the available four double-slit systems with the last slit system equipped with low scattering blades (JJXray/SAXSLAB). Aperture sizes were (0.7×0.7 mm2, 0.462 ×0.462 mm2) for



GIWAXS. The sample position was located within the fully evacuated detector tube. The hybrid-pixel 2D Pilatus 300K detector (Dectris Ltd., Baden, Switzerland) was used to collect the scattered radiation. The measurements were carried out at energy of 9.24 keV. The samples were mounted on a yz-theta goniometer allowing to adjust grazing incidence angles which maximize the scattering volume and enhance the scattered intensity. The incidence angle

for GIWAXS measurements was 0.17 °. Grazing incidence geometry of the incident Xray with respect to the sample surface is used here to enhance the scattered intensity, to maximize the scattering volume, and to access the three dimensional (3D) structure of the studied thin films (lateral and normal direction). The sample-to-detector distance (SDD) was calibrated with a silver behenate standard to 179 mm for GIWAXS. Data were reduced with dpdak software.56 The PCE11: PCBM blend films were spin coated on silicon substrates. Differential Scanning Calorimetry (DSC): DSC measurements were taken with a Q1000 from TA Instruments. The temperature ranges from 30 to 310 °C with a heating and cooling rate of 10 K/min. The cooling and heating rates of MDSC measurements were set to be 3 K/min. The powders (PCE11 and PCBM) were dissolved in chlorobenzene: 1,2-dichlorobenzene mixed solvent with PCE11 concentration of 15 mg/mL and stirred inside the glovebox at 110 °C overnight. The solutions were added with 3 v% diphenylether and stirred for another one hour, then drop-casted on clean glass substrates and dried under inert atmosphere for 3 hours and under vacuum overnight. Impedance Spectroscopy (IS): IS was measured with Electrochemical Workstations ZENNIUM pro by Zahner-elektrik. The device structure of studied parallel plate capacitors is ITO/PEDOT:PSS/dielectric material/Al; P1, P2, P3, and PCBM in pristine 18 ACS Paragon Plus Environment

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and in blends were served as dielectric material respectively; the area of the capacitors is around 1 cm2. Samples were heating and cooling in nitrogen atmosphere with the THMS600 stage from LINKAM SCIENTIFIC. Samples were heating from 25 ºC to 85 ºC and cooling to 0 ºC where 2nd scan began and finished at 25 ºC; heating/cooling rate is 1 ºC/min; impedance spectroscopy was measured every 5 minutes starting at 25 ºC.

Associated Content Supporting Information available: molecular weight and polydispersity index of PCE11; photovoltaic characteristics and processing parameters of PCE11:PCBM solar cells; 2D GIWAXS patterns of polymers in pristine and in blends; capacitance as a function of frequency of materials in pristine and in blends; normalized JSC, VOC, FF and PCE evolution of PCE11:PCBM solar cells aged under white light illumination and at 85 °C in nitrogen atmosphere; 1H NMR spectra and absorption of PCE11; stability of hole-only devices; 2D and Qy line-cut profiles of P1:PCBM and P2:PCBM films before and after thermal aging. This material is available free of charge via the Internet at http:// pubs.acs.org.

Acknowledgement This work was supported by the German Research Foundation (DFG) grant: BR 4031/131. C.Z. would like to acknowledge the financial support from the Bavarian Initiative “Solar Technologies go Hybrid” (SolTech) and The National Key Research and Development Program of China (No. 2018YFB0704104). C.J.B. gratefully acknowledges the financial support through the “Aufbruch Bayern” initiative of the state of Bavaria



(EnCN and solar factory of the future), and the SFB 953 (DFG). O.A. acknowledges the financial support from the VDI/VD Innovation + Technik GmbH (Project-title: PV-ZUM) and the SAOT funded by DFG in the framework of the German excellence initiative. W.G. and T.U. gratefully acknowledge the funding of the German Federal Ministry of Education and Research (BMBF, project number: 05K16WEB). MSc. Isabell Wabra is acknowledged for preparing the samples for 1HNMR measurement.

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Comprehensive Investigation and Analysis of Bulk-heterojunction

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