SolventâMorphologyâProperty Relationship of PTB7:PC71BM

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Solvent-morphology-property relationship of PTB7:PC71BM polymer solar cells Shuai Guo, Weijia Wang, Eva M. Herzig, Anna Naumann, Gregory Tainter, Jan Perlich, and Peter Muller-Buschbaum ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14926 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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ACS Applied Materials & Interfaces

Solvent-morphology-property relationship of PTB7:PC71BM polymer solar cells Shuai Guo,†,║ Weijia Wang,†,║ Eva M. Herzig,‡ Anna Naumann,† Gregory Tainter,§ Jan Perlich,⊥ Peter Müller-Buschbaum*,† †

Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle. Materialien,

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

Technische Universität München, Munich School of Engineering, Lichtenberg Str. 4, 85748

Garching, Germany §

University of Cambridge, Department of Engineering, 9 JJ Thomas Avenue, Cambridge, CB3

0FA, United Kingdom Deutsches Elektronen-Synchrotron at DESY, Notkestr. 85, 22603 Hamburg, Germany

⊥

KEYWORDS: organic solar cells, solvent, morphology, crystallinity, PTB7

ABSTRACT. The influence of three different solvents and a solvent additive on the morphology and photovoltaic performance of bulk heterojunction films made of the copolymer based on thieno[3,4-b]thiophene-alt-benzodithiophene unit PTB7-F40 blended with [6,6]-phenyl-C71butyric acid methyl ester (PC71BM) is investigated. Optical microscopy and atomic force

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microscopy (AFM) are combined with X-ray reflectivity (XRR) and grazing incidence small and wide angle X-ray scattering (GISAXS and GIWAXS) enabling the characterization of the morphology of the whole photoactive film. The detailed study reveals that different length scales of PCBM clusters are observed using different solvents, while by adding a solvent additive the PCBM clusters are selectively dissolved. Vertical and lateral phase separation occurs during spin coating and depends on the solvent used. A hierarchical morphology is detected within the bulk film through GISAXS measurements. Furthermore, GIWAXS shows that rather an amorphous film with low crystallinity is probed, which substantiates that high crystallinity is not necessarily required for high performance organic solar cells. Different models for the morphology are proposed through the combination of all the findings and correlated with the corresponding device properties. Consequently, the solvent-induced different device performance is mainly ascribed to the varied lateral structure sizes, whereas the highest device performance is a result of smallest average multi-length scale lateral structure sizes with the smallest length scale matching the exciton diffusion length.

1. INTRODUCTION Polymer:fullerene bulk heterojunction (BHJ) systems have made impressive progress in the past decades regarding the improvement of solar cell efficiency.1-3 It is broadly recognized that the availability of materials affording higher performance and longer lifetime is the key to unlocking the unexceptional potential of OPV technology.4-7 It is also known that the influence of different solvents has strong influence on the morphology and performance of bulk polymer films.8-10 Especially for solution-processed polymer-based solar cell devices, the significance of selecting


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the host solvent has been evidenced by many investigations.11-15 Our previous study on the classical poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC61BM) BHJ system shows that using different organic solvents results into remarkably different film morphologies, which directly determine the corresponding device performance.16 However, P3HT-based systems, the model materials of the last generation organic solar cells, are also relatively unusual from a morphology perspective, since it tends to form fibrils and crystals much more readily than most other conjugated polymers.17,18 Therefore, the results obtained from P3HT systems cannot be directly used for the novel low-band gap copolymer systems. For the higher efficiency polymer solar cell systems, such as those making use of the copolymer based on thieno[3,4-b]thiophene-alt-benzodithiophene units PTB7 (PTB7:PCBM BHJ system) such study of the impact of solvent on the morphology-device performance relationship is still missing. Given the possibility to achieve higher device performance, studies on the PTB7 system have gained increasing interest.19-25 Most of these investigations anyhow focused on the PTB7:PC61BM BHJ system and not on the more efficient PTB7:PC71BM BHJ system. Ever since the first successful application of the processing additive in low bandgap polymer solar cells by Peet et al., many research activities have been directed into this field, which appears to be another effective method to the control film morphology26-28 although it is also associated with the risk of additional device degradation mechanisms.29 For the polymer PTB7 a variation in the degree of fluorination along the polymer backbone was shown to give rise to different power conversion efficiencies (PCEs).30 In the present investigation, the solvent induced film morphology made of the semi-conducting polymer PTB7-F40 (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]]) with 40% fluorinated


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thienothiophene units blended with the fullerene derivative PC71BM ([6,6]-phenyl-C71 butyric acid methyl ester) is studied in BHJ geometry. The standard solar cell geometry is used, with the scheme illustrated in Figure S1. The molecular structures of the studied materials can be found in Figure 1. We particularly investigate on the PTB7-F40:PC71BM BHJ system due to the decent efficiency of PTB7-F40 and its relatively lower cost as compare with PTB7-F100. PC71BM was chosen as the electron acceptor due to its higher absorption coefficient. Moreover, it is reported by Troshin et al. that minor modification of the fullerenes gives dramatic change on the device efficiency due to the different solubility (e.g. the solubility of PC61BM and PC71BM are 50 mg/mL and 80 mg/mL in the solvent chlorobenzene, respectively).31,32 Overall it is highly desirable to find out the ideal solvent which assists to optimize the morphology of the active layers during the solution based self-assembly process. In addition, the effect of the processing additive 1,8-diiodooctane (DIO) on the morphology control is studied for the same series of host solvents. Therefore, in this work the effect of using three different organic solvents, namely chlorobenzene (CB), 1,2-dichlorobenzene (DB) and 1,2,4-trichlorobenzene (TB), as well as the impact of the processing additive DIO is thoroughly investigated, by determining the solventinduced morphology-device performance relationship. We firstly characterize the photovoltaic performance of solar cells made of the three different solvents CB, DB, and TB without and with additive DIO. Then we deliberately explore the influence of these three common organic solvents and of the solvent additive DIO on the morphology of PTB7-F40:PC71BM BHJ films by the combination of direct imaging techniques like AFM, and sophisticated scattering methods such as GISAXS and GIWAXS, which allows for probing the dominant length scales of the blend films. The combination of the real and reciprocal space measurements enables us to obtain a comprehensive picture of the polymer-based organic photovoltaic film morphologies from the


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nano- to the meso-scales. As a result, the best solvent is identified for the PTB7-F40:PC71BM system, and the efficiency variance introduced by the usage of different solvents is explained.

Figure 1. Molecular structure of the investigated polymer PTB7-F40 (a) and the fullerene derivative PC71BM (b).

2. RESULTS AND DISCUSSION 2.1 Solvent-Dependent current-voltage characterization To enable a comparison between the solar cell devices made of different solvents, the initial solution concentration of each solvent is adjusted to obtain the same film thickness for all films made of different solvents, which can be quickly measured by absorption measurements as shown in Figure S2. The details of such method was described earlier and was successfully applied in our previous work on the P3HT:PC61BM BHJ system.16 The IV characteristics of PTB7-F40:PC71BM solar cells prepared from CB, DB, and TB without and with additive DIO are presented in Figure 2. In general, the PCE of the solar cells made from the different host solvents increases after adding DIO except for TB. The solvent mixture CB:DIO results in the highest PCE for PTB7-F40:PC71BM BHJ solar cells. For more detailed information, the short


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circuit current Isc, the open circuit voltage Voc, the fill factor FF and the PCE η of the solar cell devices are determined and listed in Table 1

. Figure 2. Exemplary current-voltage characteristics of PTB7-F40:PC71BM BHJ solar cells prepared from CB (black), DB (red), and TB (blue) without (solid lines) and with (dashed lines) DIO. The measurements were performed under standard AM 1.5 spectrum with the illumination intensity 1000 W/m2 in ambient condition.

solvent

Isc (mA/cm2)

Voc (V)

FF (%)

η (%)

Rs (Ω cm2)

CB

9.4 ± 0.3

0.51 ± 0.01

40 ± 1

1.9 ± 0.2

16.9 ± 0.2

DB

7.0 ± 0.2

0.43 ± 0.01

43 ± 2

1.3 ± 0.1

18.1 ± 0.1

TB

4.8 ± 0.3

0.45 ± 0.01

49 ± 3

1.1 ± 0.1

23.2 ± 0.2

CB:DIO

16.1 ± 0.1

0.53 ± 0.01

51 ± 1

4.3 ± 0.2

8.5 ± 0.2

DB:DIO

14.0 ± 0.2

0.52 ± 0.01

51 ± 3

3.7 ± 0.3

16.9 ± 0.3

TB: DIO

4.2 ± 0.4

0.47 ± 0.01

39 ± 3

0.77 ± 0.1

37.6 ± 0.4


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Table 1. Average characteristic parameters of 96 pieces of PTB7-F40:PC71BM solar cells on 24 substrates prepared from CB, DB, and TB without and with DIO.

The usage of different solvents directly leads to obvious differences of Isc, Voc, FF, and consequently different efficiencies. The change of the PCE is mainly attributed to the significant variation of the Isc. In comparison, the changes of Voc and FF are relatively moderate. It is observed that the introduction of the additive DIO clearly improves the device efficiency for solar cells made of CB and DB due to a massive increase in Isc. The series resistance Rs is further derived from the IV curves and listed in Table 1. It is found that the increase in Isc is correlated to reduced Rs values. In addition, CB appears to be the best solvent for PTB7-F40:PC71BM solar cells, giving the highest values of the Isc (the lowest Rs), resulting in a PCE of 4.3±0.2% when the additive DIO is used, which is in agreement with the efficiency characterization of PTB7 systems by Liang et al.. 33 It should be noted that in the present study the simplest device architecture was adopted with the absence of a hole blocking layer and spacer layers, using nonoptimized fabrication procedures for each solvent concerning total active layer thickness, as well as encountering the possible negative influence of ambient atmosphere during certain steps of the device preparation. Thus, the photovoltaic performance, especially the values of Voc and FF in our devices are relatively lower as compared with the highest published values on the same or more efficient poly(benzodithiophene-co-thieno[3,4-b]thiophene) (PBDTTT) systems.20,23, 34,35 Although further optimization of the solar cell efficiencies would have been possible, we choose a simple device architecture to keep the samples prepared for the different characterization techniques as comparable as possible. Moreover, a potentially optimized device fabrication process will have only have little influence on the morphology-property relationship. As we


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mainly focus on the morphology investigation of the photoactive layers made from different host solvents, our results are sufficient to provide a comprehensive understanding about the effect of used solvents on the device properties.

2.2. Film morphology characterization 2.2.1. Mesoscopic surface structures In order to obtain the same active layer morphology like that in a solar cell, PTB7-F40:PC71BM BHJ films are prepared on PEDOT:PSS modified silicon substrates. AFM measurements are applied to obtain topography images of the film surfaces and shown in Figure 3.

Figure 3. AFM topography images of PTB7-F40:PC71BM films prepared from CB, DB, and TB (from left to right) without (a, b, c) and with DIO (d, e, f). The scan size for all images is 2×2 µm².


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It has been reported from many studies on polymer:fullerene BHJ systems that fullerene molecules tend to form clusters via diffusion and agglomeration.14,16,36 As seen in Figure 3, PC71BM tends to aggregate when the additive DIO is not used, agreeing with the results from other groups.37,38 Furthermore, it is clearly noted that the PC71BM cluster size is solvent dependent. For the films made of CB, the most prominent PC71BM cluster size is around 250 nm. The cluster size decreases to 100 nm for TB-based films as the boiling point of the host solvent increases. For samples with the solvent additive DIO, PC71BM clusters are not obviously visible as compared with those films without the DIO. It is suggested that the formation of these fullerene clusters already takes place in the solution with different sizes in different host solvents. The sizes are determined by the solubility of the fullerene in each solvent.31,32 Because TB has the highest solubility for PC71BM and CB has the lowest solubility of the three studied solvents, the biggest PC71BM clusters are observed for CB. By the addition of DIO to the solutions, since DIO can selectively dissolve PC71BM, smaller PC71BM domains are created. As a result, PC71BM gets more homogeneously distributed in the PTB7-F40 polymer network, leading to the formation of optimized domain sizes with an increased interfacial area between both components. Accordingly, the charge separation probability and the final device efficiency get enhanced. Especially for the sample made from TB:DIO, surface morphology with smallest network structure is observed. However, the lowest efficiency is obtained for the sample made from the solvent mixture TB:DIO as presented in Table 1. The contradiction implies that one cannot purely correlate the surface morphology with the device performance, and careful statistical structure evaluations regarding the inner film morphology need to be further carried out.


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In summary, the formation of the film surface morphology is a solubility and miscibility-driven process: For those films without DIO, the lowest volatility of the solvent TB functions together with its best solubility for fullerene molecules, resulting in the smallest fullerene cluster size. Same mechanisms also hold for the DIO containing films. Since DIO further assists to dissolve the fullerene clusters, a network structure of two components with smaller length scales are observed on the film surface. To what extent these surface structures are present also inside the blend films cannot be answered with AFM measurements. X-ray scattering techniques give access to the inner film structure, e.g. the film lateral structure and vertical composition.39

2.2.2. Mesoscopic inner film structures Grazing incidence small angle x-ray scattering (GISAXS) is a non-destructive probe with no special sample preparation required, and it can provide structure information on nanometer scale.40 Moreover, it yields structural information on object geometry, size distribution, and spatial correlations on the nanoscale and with excellent statistics,41,42 because it overcomes the limited scan sizes of several micrometers of high resolution imaging techniques like AFM or SEM. In Figure S3, 2D GISAXS scattering data of PTB7-F40:PC71BM BHJ blend films made of the different solvents or solvent mixtures are displayed. As compared with the GISAXS scattering patterns from pure PTB7-F40 films shown in our previous work,43 in case of the blend films no roughness correlation phenomenon can be observed. The phase separation morphology is not replicating the substrate roughness spectrum in blends with PC71BM. Moreover, a much broader scattering signal is observed for all samples towards higher q values along the qy direction. This


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broadening is particularly prominent for the three DIO containing films, indicating that smaller domains are formed inside the films. The corresponding horizontal line cuts (at the Yoneda peak position) from the 2D GISAXS data probing the inner film structure information are presented in Figure 4(a). For the DIO containing samples, weak peaks in the intensity shift from low qy to high qy values as compared with the pristine films without DIO indicating that relatively larger PC71BM clusters disappear while smaller structures are formed. However, a precise analysis of the horizontal line cuts is performed in order to gain a quantitative evaluation. Those cuts are further modeled with 1D paracrystal model within the frame of the distorted wave Born approximation (DWBA) as described elsewhere.44,45 To describe the horizontal line cuts in the entire qy range three different form factors and structure factors have to be assumed. The best fitting parameters are summarized in Table 2.

Figure 4. Horizontal line cuts of the 2D GISAXS data (a) and power spectrum density functions calculated from the AFM data (b). The intensity is plotted as a function of the scattering vector


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component qy in reciprocal space. The bottom three curves represent PTB7-F40:PC71BM BHJ films made of CB (black), DB (red), and TB (blue) without the additive DIO, and the top three are data obtained from samples with the additive DIO. Solid lines in the left graph show the fits with the 1D paracrystal model.

Solvent CB DB TB CB:DIO DB:DIO TB: DIO

domain 1 [nm] R1 D1 320 650 250 650 230 600 160 490 170 500 220 600

domain 2 [nm] R2 D2 60 180 50 180 75 150 50 170 65 200 70 450

domain 3 [nm] R3 D3 30 120 14 80 13 50 8 60 8 100 10 100

Table 2. Most prominent domain sizes of PTB7-F40:PC71BM films prepared from CB, DB, and TB without and with DIO as extracted from GISAXS measurement: R1 to R3, radii of the form factors (domain sizes= 2*Rx); D1 to D3, structure factors (separation of domains).

For the films made without DIO, the domain size R1 in the range of few hundred nanometers decreases with increasing boiling point of the host solvent (131°C, 181°C, and 215°C for CB, DB, and TB). These results corroborate the trend revealed by the AFM measurements, which have shown that the aggregated PC71BM clusters form on the film surface. The solventdependent size formation is again attributed to the different solubility of PC71BM for each host solvent as explained before. The second domain size R2 is in the range of several tens of nanometers, which remains rather stable for all the films regardless of the used host solvent and


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additive. The smallest domain R3 is around 10 nm, which matches the exciton diffusion length.11 Hence, the evolvement of the domain size R3 is expected to strongly influence the device performance, and the trend of R3 explains very well the decreasing solar cell performance for non-additive devices. However, for DIO containing devices, the dramatically different device performance cannot be simply explained by the rather similar R3 values (8~10 nm). Instead, the overall shrunk domain sizes (R1, R2, R3) are supposed to be responsible for drastic device performance improvement of the DIO containing devices. Particularly, the best solar cell efficiency given by CB:DIO can be explained by the smallest inner domain structures as seen in Table 2. In addition, the inner film structures extracted from the GISAXS data analysis are compared with the film surface structures deduced from the power spectrum density function analysis of the AFM data shown in Figure 4(b). By the identification of the corresponding q values of the peak centers of the power spectral density functions it is observed that the inner film structures are relatively better defined as evident by the more pronounced intensity shoulders in the GISAXS data. In addition, for all films without DIO, the most dominant structure sizes reflected by the corresponding peak positions are comparable (400 nm to 600 nm), whereas, for all DIO containing films, the pronounced length scales are only found inside the films (20 nm to 50 nm) and not at the film surface.

2.2.3. Mesoscopic vertical structures X-ray reflectivity (XRR) is a well-established method for the measurement of thicknesses, the vertical material composition and the roughness of thin films. The XRR intensity is recorded as a


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function of the incident angle αi, and fitted with a Parratt algorithm which considers a multilayer model including the film thickness and roughness information yielding the vertical material composition of the measured film.46 The vertical composition profile is crucial for the performance of organic photovoltaic cells. Depending on the geometry of the complete solar cell, the possible formation of an enrichment layer in an undesired position, e.g. at the wrong electrode can act as an undesired blocking layer, which directly affects the charge transport and therefore the device performance. From the analysis of the XRR data shown in Figure 5 confirms that the applied thickness control by using absorption measurement is applicable for this materials system. The thickness of all films made of the three different solvents with and without DIO is about 115 nm. For DIO containing films, the film roughness decreases except for the sample made from DB. In addition, enrichment layers are observed for all the samples independent of the used solvent. The chemical composition profiles are calculated from the refractive index profiles used for fitting the XRR data. The chemical composition profiles for all samples are shown in Figure 6.46

Figure 5. XRR data (points) of PTB7-F40:PC71BM films prepared from CB (black), DB (red), and TB (blue) shown with the best fits (solid lines). For the bottom three curves, data are


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obtained from samples without solvent additive DIO, and top three ones are with the addition of DIO. All curves are shifted along the intensity axis for clarity.

Figure 6. Chemical composition profiles of PTB7-F40:PC71BM polymer blend films prepared from CB, DB, and TB (from left to right) without (a, b, c) or with DIO (d, e, f) extracted from fitting data of the XRR measurements. PTB7-F40 is shown with black color, PC71BM with white color and mixtures with a mixed gray color. Thus, the presence of enrichment layers is indicated by black (PTB7-F40) or white (PC71BM) layers. Note that the scale of the film thickness is the same for all the samples (around 115 nm).

For the extensively studied model system P3HT:PCBM solar cells, an enhancement of the P3HT:PCBM device performance can be realized by the vertically modulated nanomorphology as demonstrated by Kumar et al..47 In analogy, the ideal vertical material composition for PTB7:PC71BM solar cells is suggested to be an ultra-thin PTB7 layer at the bottom of the film, whereas a PC71BM layer should be present at the top of the film, i.e. close to the aluminum


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contacts.47,48 In the present investigation an enriched PC71BM layer is observed on top of the blend films for all used solvent combinations (Figure 6). In contrast, it is reported by Hedley et al. that a thin PTB7-rich skin layer on top of the blend film is probed by the combination of several techniques (SEM, AFM and time-resolved photoluminescence).37 Additionally they reported that an enriched PTB7 layer at the bottom of all films is formed. In our work, we do not find a strongly enriched PTB7-F40 layer at the bottom electrode. These differences in the vertical material composition are possibly caused by the different solubility of the polymer PTB7 and PC70BM in the host solvents as compared to PC71BM. For example, in case of CB it is 80 mg/mL for PC71BM and less than 10 mg/mL for PTB7.31 It is advantageous to form such vertical composition profiles with enrichment layers as observed in our study, which is beneficial for the whole solar cells device: (1) The PTB7 enrichment layer can serve as a hole-transporting layer and the PCBM enrichment layer can act as an electron-transporting layer to assist charge transport. (2) These enrichment layers are able to reduce the energy barrier by forming Ohmic contacts, resulting in effective charge extraction. Nevertheless, the vertical profiles from the different solvents are rather similar, which cannot address the big efficiency differences of different devices. Thus, additional information about the crystallinity of the films might give additional insights.

2.2.4. Molecular order and crystallinity Charge transport is considered as one important factor as well, which is governed by the conductivity and crystallinity of the components in the active layer. From the investigation of pure PTB7-F40 films in our previous work, it is summarized that neither solvent nor solvent


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additive affect the film crystallinity.43 Since in solar cells PTB7 based blends are relevant, we investigate the crystallinity of the PTB7-F40:PC71BM blend films. Grazing incidence wide angle X-ray scattering (GIWAXS) is a very powerful method to give information about the crystalline part of thin polymer films. 2D GIWAXS data are presented in Figure S4. Several prominent scattering features are observed: Arc-like signals arising from the random orientation of the PCBM molecules, a pronounced scattering intensity at low qxy values (about 3.5 nm-1) and at high qz values (about 15 nm-1). For further analysis sector integrals from the 2D GIWAXS data are taken. As seen in Figure 7, for both, horizontal and vertical sector integrals, Bragg peaks from PC71BM are present at q = 6.5 nm-1, 13.3 nm-1, 19 nm-1. The Bragg peaks at a q value of 3.3 nm-1 appearing in all the horizontal cuts in Figure 7(a) correspond to the (100) PTB7 backbone spacing, which was reported already for pure PTB7-F40 films.43 With careful comparison between the horizontal and vertical sector integrals of the Bragg peaks in the higher q range, multiple peaks are assigned in the q range of 12 nm-1 to 19 nm-1 as seen in Figure 7 (b), that is, two Bragg peaks from PC71BM at q values of 13.3 nm-1, 19 nm-1, and one Bragg peak arising from (010) PTB7 π-π stacking distance.


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Figure 7. Horizontal (a) and vertical sector integrals (b) of the 2D GIWAXS data of PTB7F40:PC71BM BHJ films prepared from CB (black), DB (red), and TB (blue) without (solid lines) or with (dashed lines) DIO.

A quantitative evaluation was subsequently performed by fitting all Bragg peaks with Gaussian functions, from which molecular stacking information is obtained by identifying the corresponding q value for each Bragg peak. Both backbone spacing and π-π stacking distance remain rather constant as shown in Table 3: The extracted backbone spacing along the (100) direction is 1.8±0.5 nm for all samples, whereas the π-π stacking distance along (010) is 0.40±0.01 nm. Additionally, the crystal size is estimated with the Scherrer equation assuming constant paracrystallinity. It is observed that for samples made of CB and DB, crystal sizes along (010) direction slightly increase after introducing DIO, which can moderately assist charge transport within the blend films. For the samples made of TB, crystal size even decreases for the additive DIO added film. The change in the crystallinity in (010) direction coincides with the corresponding solar cell performance listed in Table 1, which demonstrates the influence of the crystallinity on the overall device performance. However, this minor change in film crystallinity can only explain the changes in device performance to a certain extent, but it cannot be responsible for the large device efficiency variations. solvent CB DB TB CB:DIO DB:DIO TB: DIO

lattice spacing [nm] (100) h (010) v 1.77 0.40 1.76 0.41 1.81 0.41 1.80 0.41 1.81 0.41 1.85 0.39

crystal size [nm] (100) h (010) v 1.2 4.2 1.2 4.2 1.3 4.2 1.3 4.2 1.2 5.1 1.2 3.3


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Table 3. Extracted information of the crystalline parts of the PTB7-F40:PC71BM films prepared from CB, DB, and TB without and with DIO. The deviation of the lattice spacing is within 0.1%, and the deviation of the crystal sizes are below 1%.

In conclusion, similar to the results obtained from pure PTB7-F40 films, the selection of solvents without or with DIO neither significantly changes the organization of PTB7 chains in the crystalline parts on a molecular level nor the crystallite sizes of PTB7-F40. This observation is in agreement with previous research on the same system by Chen et al..19

2.2.5. Summary film morphology The influence of different solvents and the additive DIO on the molecular arrangement, mesoscopic and crystalline structure of BHJ PTB7-F40:PC71BM films is thoroughly studied in this work. Based on a solution-processing technique, the selection of host solvent and the addition of an appropriate solvent additive have a direct impact on tailoring the nanomorphology of the PTB7-F40:PC71BM films. We confirm that film thickness control via the simple comparison of absorption curves is an effective method, which is further verified by XRR measurements. Furthermore, it is observed that the vertical compositions of all films are rather similar whereas the film roughness decreases upon the addition of DIO. Moreover, from GISAXS measurements much smaller domain sizes and better interpenetrating networks are probed for films with 3 vol% solvent additive DIO, which can greatly enhance the charge


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separation rate, and therefore improve the device performance. With GIWAXS it is observed that crystallinity within each film is correlated with the device performance. However, as the difference of the vertical film composition and crystallinity caused by different solvents and the additive are nearly neglectable, the variation of corresponding device performance is mainly ascribed to the difference of the lateral structure size as evident by GISAXS measurement. The corresponding film morphology for all the investigated films is summarized in a schematic model shown in Figure 8.

Figure 8. Proposed model morphology of PTB7-F40:PC71BM BHJ films prepared from CB (DB, TB) without (a) and with the processing additive DIO (b). Grey random wire: PTB7-F40 polymer chain; Grey oriented wire: crystallized PTB7-F40 chains; Red dots: PC71BM.

For PTB7-F40:PC71BM BHJ films prepared without the processing additive DIO (see Figure 8(a)), PC71BM clusters are formed and the size of the clusters is solvent dependent due to the different volatility of the host solvent. Upon the addition of DIO big PC71BM clusters are selectively dissolved by DIO and diffuse into PTB7-F40 network, forming a better


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interpenetrating morphology (see Figure 8(b)). Although moderate changes occur to the lattice constants and crystal size, the effect on photovoltaics performance becomes less influential or cancelled out by other factors in this case. Moreover, as proved by this investigation as well as that of others,49 PTB7-F40:PC71BM is a typical low crystallinity system, therefore, crystallinity variation is not particularly sketched in the proposed model. The influence of different solvents on the film morphology mainly lies in the average domain size (PTB7-F40 or PC71BM domains), which is particular pointed out in the proposed model in Figure 8.

3. CONCLUSIONS By a simple change of the host solvent, the morphology of PTB7-F40:PC71BM BHJ films is affected, which leads to drastic changes of the corresponding BHJ solar cell performance. It is suggested that the different morphologies introduced by different host solvents are mainly caused by the solubility difference between the polymer PTB7-F40 and the fullerene PC71BM. In accordance with the conclusion achieved by Troshin et al. the highest device performance is given by those material combinations where donor and acceptor components are of similar and sufficiently high solubility in the host solvent. CB is the solvent that allows PTB7 and PC71BM to have similar high solubility in this study.31 Consequently, for the solar cells made from CB, DB and TB, CB-based PTB7-F40:PC71BM films demonstrates the best PCE of 1.9±0.2%. For the solar cells made from CB:DIO, DB:DIO and TB:DIO, additional competition between the processing additive DIO with each host solvent (due to different volatility and solubility for PC71BM) results in the enriched PCBM layer near the surface as well as better intermixed morphology with smaller length scales during the film drying process. With consideration of all


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effects, solar cells made from solvent mixture CB:DIO prove to be the best host solvent mixture for PTB7-F40:PC71BM BHJ system mainly due to the optimized length scale through the whole active layer as probed by GISAXS measurements. This conclusion perfectly explains our first observation that the different PCE induced by the selection of the solvents mainly lies in the different Isc. In summary, as a solution-based new technology, a judicious selection of solvent and the processing additive is of utmost importance. As a result, to further improve the PCE of organic photovoltaics, not only one could synthesize new polymers and fullerenes derivatives with balanced higher conductivity and lower band gap, but also manipulate the nanomorphology formation by the smart selection of the solvent and possible additives.

4. EXPRIMENTAL Polymers PTB7-F40 and organic compound PC71BM were purchased from 1-material, and used as supplied. The solvent chlorobenzene, denoted CB was purchased from Carl Roth. 1,2dichlorobenzene (DB), 1,2,4-trichlorobenzene (TB) as well as the additive 1,8-diiodooctane (DIO) were obtained from Sigma-Aldrich and used as supplied. The boiling points for these three solvents and DIO are listed in Table S1. To prepare the solution, PC71BM was dissolved firstly due to its higher solubility in the host solvent. Then the required amount of PC71BM solution was added to solid PTB7-F40 at the overall concentration of 25 mg/mL without or with 3 vol% DIO. Poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was received from Sigma Aldrich. It is a transparent blend of two polymers, PEDOT and PSS, dispersed in H2O. It was used as electron blocking layer in the device fabrication. Films investigated by UV/Vis were prepared on acid cleaned transparent glass substrates.50 Films


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investigated with AFM, GISAXS, GIWAXS were prepared on acid cleaned and subsequently PEDOT:PSS coated silicon substrates. Solar cells were fabricated on glass-ITO substrates purchased from SOLEMS with the size of 2.2×2.2 cm2. The substrates were patterned by chemical etching and cleaned with alconox solution and several organic solvents consecutively in an ultrasonic bath (ethanol, acetone, and isopropanol). PEDOT:PSS films were dried at 150°C for 10 minutes, and PTB7-F40:PC71BM (1:1.5 by weight) dissolved in different solvent (25 mg/mL) without or with 3 vol% DIO was spin-coated with varied spin-coating parameters in order to achieve similar film thickness. The devices were completed by thermal evaporation of a layer of aluminum at vacuum conditions (3.0×10-5 mbar). The deposition process was monitored by a quartz crystal ratemeter purchased from Inficon. The deposition rate started with 0.1 Å/s and increased up to 20 Å/s till the film thickness reached 100 nm. Absorption measurements were performed with a UV/Vis spectrometer (Lambda35, PerkinElmer) in transmission geometry from the wavelength 260 nm to 1000 nm. AFM was used in non-contact mode to obtain topography images of the film surface. The tip in use had a conical shape with a curvature of 10 nm. All the samples were measured at different positions with scan sizes of 1×1 µm2, 2×2 µm2, 4×4 µm2, 8×8 µm2 to verify the homogeneity of the film. The whole process was carried out in ambient condition. The GISAXS measurements were performed at the synchrotron beamline BW4, HASYLAB (DESY) in Hamburg, Germany (λ = 0.138 nm).51 An X-ray beam with a size of 23×36 µm2 was selected. The scattered intensity was collected with a MarCCD detector. Two moveable beamstops were used to protect the detector from the high intensity of the direct and the reflected beam. To allow excellent sampling statistics at large qy values, a rod-shaped beamstop at qy = 0


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blocking relatively high intensity, replaces the point-like beamstop in case of long data acquisition times. An incident angle of 0.30° was selected, well above the critical angle of the investigated materials PTB7-F40 (0.16°), and hence probing the inner film structures. The sample-detector distance (SDD) was 2.084 m. To gain quantitative information about structure information, horizontal line cuts of the 2D GISAXS data were performed. The vertical cut at the position qy = 0 gives structure information along the surface normal of the substrate. The horizontal cut at the critical angle of each material contains the information about the lateral structures, such as domain sizes, size distributions and spatial correlations, etc.52 The GIWAXS measurements were also performed at beamline BW4.45 To record crystalline structures the SDD was reduced to 0.1039 m. Vertical and horizontal sector integral were performed on the 2D GIWAXS data to obtain detailed information on the crystalline parts of the films.39 Data processing was carried out with software fit2d.45 The solar cell performances were characterized in ambient conditions under standard AM 1.5G illumination. In total, more than 100 individual devices were fabricated, and the corresponding I− V curves were recorded and analyzed. The simulated solar spectrum was always adjusted and characterized by a calibration cell made of mono crystalline silicon purchased from Fraunhofer ISE. The corresponding I-V curves were recorded with a source meter (Keithley 2400) with 100 points in the range from -1 V to 1 V. The effective area of the solar cells was on the order of 0.15 cm2 and individually determined for each pixel by the overlapping area of ITO and aluminum electrode. ASSOCIATED CONTENT


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Supporting Information. Absorption spectrum, 2D GISAXS data, and 2D GIWAXS data of PTB7-F40:PC71BM BHJ films made of CB, DB, and TB. This material is available free of charge via the Internet on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions ║

S. G. and W. W. contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the funding from the Excellence cluster “Nanosystems Initiative Munich” (NIM), the GreenTech Initiative (Interface Science for Photovoltaics – ISPV) of the EuroTech Universities and from TUM.solar in the frame of the Bavarian Collaborative Research Project “Solar technologies go Hybrid" (SolTec). W.W. thanks the China Scholarship Council (CSC). G.T. acknowledges the Erasmus Mundus “MaMaSELF” program and E.M.H. the Energy Valley Bavaria (EVB) of the Munich School of Engineering (MSE) for funding. Portions of this research were carried out at Deutsches Elektronen-Synchrotron DESY. DESY is a member of the Helmholtz Association (HGF). REFERENCES


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(1) Scharber, M. C. On the Efficiency Limit of Conjugated Polymer:Fullerene-Based Bulk Heterojunction Solar Cells. Adv. Mater. 2016, 28, 1994-2001. (2) Dou, L.; You, J.; Hong, Z.; Xu, Z.; Li, G.; Street, R. A.; Yang, Y. 25th Anniversary Article: A decade of Organic/Polymeric Photovoltaic Research. Adv. Mater. 2013, 25, 6642-6671. (3) Lu, L.; Yu, L. Understanding Low Bandgap Polymer PTB7 and Optimizing Polymer Solar Cells Based on It. Adv. Mater. 2014, 26, 4413-4430. (4) Li, Z.; Ho Chiu, K.; Shahid Ashraf, R.; Fearn, S.; Dattani, R.; Cheng Wong, H.; Tan, C.-H.; Wu, J.; Cabral, J. T.; Durrant, J. R. Toward Improved Lifetimes of Organic Solar Cells under Thermal Stress: Substrate-Dependent Morphological Stability of PCDTBT:PCBM Films and Devices. Sci. Rep. 2015, 5, 15149. (5) Voroshazi, E.; Verreet, B.; Aernouts, T.; Heremans, P. Long-Term Operational Lifetime and Degradation Analysis of P3HT: PCBM Photovoltaic Cells. Sol. Energy Mater. Sol. Cells 2011, 95, 1303-1307. (6) Peters, C. H.; Sachs-Quintana, I. T.; Kastrop, J. P.; Beaupré, S.; Leclerc, M.; McGehee, M. D. High Efficiency Polymer Solar Cells with Long Operating Lifetimes. Adv. Energy Mater. 2011, 1, 491-494. (7) Wang, W.; Schaffer, C. J.; Song, L.; Körstgens, V.; Pröller, S.; Indari, E. D.; Wang, T.; Abdelsamie, A.; Bernstorff, S.; Müller-Buschbaum, P. In Operando Morphology Investigation of Inverted Bulk Heterojunction Organic Solar Cells by GISAXS. J. Mater. Chem. A 2015, 3, 83248331.


26

Page 27 of 34

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


(8) Li, X.-G.; Huang, M.-R.; Lin, G. Temperature Dependence and Stability of Oxygen Enrichment through Liquid Crystalline Triheptyl Cellulose-Containing Membranes Cast from Three Solvents. J. Membr. Sci. 1996, 116, 143-148. (9) Li, X.-G.; Huang, M.-R.; Lin, G.; Yang, P.-C. Solvent and Pressure Influences on Air Separation of Liquid Crystalline Triheptyl Cellulose Composite Ethyl Cellulose Membranes. Sep. Sci. Technol. 1994, 29, 671-677. (10) Li, X.-G.; Huang, M.-R. Water-Casting Ultrathin-Film Composite Membranes for Air Separation. Sep. Sci. Technol. 1996, 31, 579-603. (11) Hoppe, H.; Sariciftci, N. S. Morphology of Polymer/Fullerene Bulk Heterojunction Solar Cells. J. Mater. Chem. 2006, 16, 45-61. (12) Rispens, M. T.; Meetsma, A.; Rittberger, R.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Influence of the Solvent on the Crystal Structure of PCBM and the Efficiency of MDMOPPV:PCBM ‘Plastic’ Solar Cells. Chem. Commun. 2003, 2116–2118. (13) Ye, L.; Zhang, S.; Ma, W.; Fan, B.; Guo, X.; Huang, Y.; Ade, H.; Hou, J. From Binary to Ternary Solvent: Morphology Fine-Tuning of D/A Blends in PDPP3T-Based Polymer Solar cells. Adv. Mater. 2012, 24, 6335-6341. (14) Liu, F.; Gu, Y.; Jung, J. W.; Jo, W. H.; Russell, T. P. On the Morphology of PolymerBased Photovoltaics. J. Polym. Sci. Part B: Polym. Phys. 2012, 50, 1018-1044. (15) Wang, W.; Guo, S.; Herzig, E. M.; Sarkar, K.; Schindler, M.; Magerl, D.; Philipp, M.; Perlich, J.; Müller-Buschbaum, P. Investigation of Morphological Degradation of P3HT:PCBM


27


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

Page 28 of 34

Bulk Heterojunction Films Exposed to Long-Term Host Solvent Vapor. J. Mater. Chem. A 2016, 4, 3743-3753. (16) Ruderer, M. A.; Guo, S.; Meier, R.; Chiang, H.-Y.; Körstgens, V.; Wiedersich, J.; Perlich, J.; Roth, S. V.; Müller-Buschbaum, P. Solvent-Induced Morphology in Polymer-Based Systems for Organic Photovoltaics. Adv. Funct. Mater. 2011, 21, 3382-3391. (17) Li, X.-G.; Li, J.; Huang, M.-R. Facile Optimal Synthesis of Inherently Electroconductive Polythiophene Nanoparticles. Chem. - Eur. J., 2009, 15, 6446-6455. (18) Li, X.-G.; Li, J.; Meng, Q.-K.; Huang, M.-R. Interfacial Synthesis and Widely Controllable Conductivity of Polythiophene Microparticles. J. Phys. Chem. B, 2009, 113, 97189727. (19) Chen, W.; Xu, T.; He, F.; Wang, W.; Wang, C.; Strzalka, J.; Lin, Y.; Wen, J.; Miller, D.; Chen, J.; Hong, K.; Yu, L.; Darling, S. Hierarchical Nanomorphologies Promote Exciton Dissociation in Polymer/Fullerene Bulk Heterojunction Solar Cells. Nano Lett. 2011, 11, 37073713. (20) Zhang, S.; Ye, L.; Hou, J. Breaking the 10% Efficiency Barrier in Organic Photovoltaics: Morphology and Device Optimization of Well-Known PBDTTT Polymers. Adv. Energy Mater. 2016, 6, 1502529. (21) Ding, Z.; Kettle, J.; Horie, M.; Chang, S. W.; Smith, G. C.; Shames, A. I.; Katz, E. A. Efficient Solar Cells are More Stable: the Impact of Polymer Molecular Weight on Performance of Organic Photovoltaics. J. Mater. Chem. A 2016, 4, 7274-7280.


28

Page 29 of 34

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


(22) Jeong, J.; Seo, J.; Nam, S.; Han, H.; Kim, H.; Anthopoulos, T. D.; Bradley, D. D. C. Kim, Y. Significant Stability Enhancement in High-Efficiency Polymer:Fullerene Bulk Heterojunction Solar Cells by Blocking Ultraviolet Photons from Solar Light. Adv. Sci. 2016, 3, 1500269. (23) Yang, Y.; Chen, W.; Dou, L.; Chang, W.-H.; Duan, H.-S.; Bob, B.; Li, G.; Yang, Y. HighPerformance Multiple-Donor Bulk Heterojunction Solar Cells. Nat. Photonics 2015, 9, 190-198. (24) Etxebarria, I.; Ajuria, J.; Pacios, R. Polymer:Fullerene Solar Cells: Materials, Processing Issues, and Cell Layouts to Reach Power Conversion Efficiency over 10%, a Review. J. Photonics Energy 2015, 5, 057214. (25) Lou, S. J.; Szarko, J. M.; Xu, T.; Yu, L.; Marks, T. J.; Chen. L. X. Effects of Additives on the Morphology of Solution Phase Aggregates Formed by Active Layer Components of HighEfficiency Organic Solar Cells. J. Am. Chem. Soc., 2011, 133, 20661–20663. (26) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Efficiency Enhancement in Low-Bandgap Polymer Solar Cells by Processing with Alkane Dithiols. Nat. Mater. 2007, 6, 497-500. (27) Yao, Y.; Hou, J.; Xu, Z.; Li, G.; Yang, Y. Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells. Adv. Funct. Mater. 2008, 18,1783-1789. (28) Liao, H.-C.; Ho, C.-C.; Chang, C.-Y.; Jao, M.-H.; Darling, S. B.; Su, W.-F. Additives for Morphology Control in High-Efficiency Organic Solar Cells. Mater. Today 2013, 16, 326-336. (29) Schaffer, C. J.; Palumbiny, C. M.; Niedermeier, M. A.; Burger, C.; Santoro, G.; Roth, S. V.; Müller-Buschbaum, P. Morphological Degradation in Low Bandgap Polymer Solar Cells – an in Operando Study. Adv. Energy Mater. 2016, 6, 1600712.


29


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Page 30 of 34

(30) Guo, S.; Ning, J.; Körstgens, V.; Yao, Y.; Herzig, E. M.; Roth, S. V.; Müller-Buschbaum, P. The Effect of Fluorination in Manipulating the Nanomorphology in PTB7:PC71BM Bulk Heterojunction System. Adv. Energy Mater. 2015, 5, 1401315. (31) Troshin, P. A.; Hoppe, H.; Renz, J.; Egginger, M.; Mayorova, J. Y.; Goryachev, A. E.; Peregudov, A. S.; Lyubovskaya, R. N.; Gobsch, G.; Serdar Sariciftci, N.; Razumov, V. F. Material Solubility-Photovoltaic Performance Relationship in the Design of Novel Fullerene Derivatives for Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2009, 19, 779-788. (32) Troshin, P. A.; Susarova, D. K.; Khakina, E. A.; Goryachev, A. A.; Borshchev, O. V.; Ponomarenko, S. A.; Razumova, V. F.; Serdar Sariciftci, N. Material Solubility and Molecular Compatibility Effects in the Design of Fullerene/Polymer Composites for Organic Bulk Heterojunction Solar Cells. J. Mater. Chem. 2012, 22, 18433-18441.(33) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright Future-Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, E135E138. (34) Lu, L.; Yu, L. Understanding Low Bandgap Polymer PTB7 and Optimizing Polymer Solar Cells Based on It. Adv. Mater. 2014, 26, 4413-4430. (35) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells. Adv. Mater. 2011, 23, 4636-4643. (36) Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.; Hinsch, A.; Meissner, D.; Serdar Sariciftci, N. Nanoscale Morphology of Conjugated Polymer/Fullerene-Based BulkHeterojunction Solar Cells. Adv. Funct. Mater. 2004, 14, 1005-1011.


30

Page 31 of 34

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


(37) Hedley, G. J.; Ward, A. J.; Alekseev, A.; Howells, C. T.; Martins, E. R.; Serrano, L. A.; Cooke, G.; Ruseckas, A.; Samuel, I. Determining the Optimum Morphology in HighPerformance Polymer-Fullerene Organic Photovoltaic Cells. Nat. Commun. 2014, 4, 2867. (38) Liu, F.; Zhao, W.; Tumbleston, J. R.; Wang, C.; Gu, Y.; Wang, D.; Briseno, A. L.; Ade, H.; Russell, T. P. Understanding the Morphology of PTB7:PCBM Blends in Organic Photovoltaics. Adv. Energy Mater. 2014, 4, 1301377. (39) Müller-Buschbaum, P. The Active Layer Morphology of Organic Solar Cells Probed with Grazing Incidence Scattering Techniques. Adv. Mater. 2014, 26, 7692-7709. (40) Müller-Buschbaum, P. Grazing Incidence Small-angle X-ray Scattering: an Advanced Scattering Technique for the Investigation of Nanostructured Polymer Films. Anal. Bioanal. Chem. 2003, 376, 3-10. (41) Ruderer, M. A.; Müller-Buschbaum, P. Morphology of Polymer-Based Bulk Heterojunction Films for Organic Photovoltaics. Soft Matter 2011, 7, 5482-5493. (42) Pröller, S.; Liu, F.; Zhu, C.; Wang, C.; Russell, T. P.; Hexemer, A.; Müller-Buschbaum, P.; Herzig, E. M. Following the Morphology Formation In Situ in Printed Active Layers for Organic Solar Cells, Adv. Energy Mater. 2016, 6,1501580. (43) Guo, S.; Herzig, E. M.; Naumann, A.; Tainter, G.; Perlich, J.; Müller-Buschbaum, P. Influence of Solvent and Solvent Additive on the Morphology of PTB7 Films Probed via X-Ray Scattering. J. Phys. Chem. B 2014, 118, 344-350.


31


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Page 32 of 34

(44) Guo, S.; Cao, B.; Wang, W.; Moulin, J.-F.; Müller-Buschbaum, P. Effect of Alcohol Treatment on the Performance of PTB7:PC71BM Bulk Heterojunction Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 4641-4649. (45) Perlich, J.; Rubeck, J.; Botta, S.; Gehrke, R.; Roth, S. V.; Ruderer, M. A.; Prams, S. M.; Rawolle, M.; Zhong, Q.; Körstgens, V.; Müller-Buschbaum, P. Grazing Incidence Wide Angle X-Ray Scattering at the Wiggler Beamline BW4 of HASYLAB. Rev. Sci. Instrum. 2010, 81, 105105. (46) Diethert, A.; Peykova, Y.; Willenbacher, N.; Müller-Buschbaum, P. Near-Surface Composition Profiles and the Adhesive Properties of Statistical Copolymer Films Being Model Systems of Pressure Sensitive Adhesive Films. ACS Appl. Mater. Interfaces 2010, 2, 2060-2068. (47) Kumar, A.; Li, G.; Hong, Z.; Yang, Y. High Efficiency Polymer Solar Cells with Vertically Modulated Nanoscale Morphology. Nanotechnology 2009, 20, 165202. (48) Chen, L.-M.; Xu, Z.; Hong, Z.; Yang, Y. Interface Investigation and Engineering – Achieving High Performance Polymer Photovoltaic Devices. J. Mater. Chem. 2010, 20, 25752598. (49) Hammond, M.; Kline, R. J.; Herzing, A. A.; Richter, L. J.; Germack, D. S.; Ro, H.-W.; Soles, C. L.; Fischer, D. A.; Xu, T.; Yu, L. Molecular Order in High-Efficiency Polymer/Fullerene Bulk Heterojunction Solar Cells. ACS Nano 2011, 5, 8248-8257. (50) Müller-Buschbaum, P.; Hermsdorf, N.; Roth, S. V.; Wiedersich, J.; Cunis, S.; Gehrke, R. Comparative Analysis of Nanostructured Diblock Copolymer Films. Spectrochim. Acta, Part B 2004, 59, 1789-1797.


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(51) Roth, S. V.; Döhrmann, R.; Dommach, M.; Kuhlmann, M.; Kröger, I.; Gehrke, R.; Walter, H.; Schroer, C.; Lengeler, B.; Müller-Buschbaum, P. Small-Angle Options of the Upgraded Ultrasmall-Angle X-Ray Scattering Beamline BW4 at HASYLAB. Rev. Sci. Instrum. 2006, 77, 085106. (52) Müller-Buschbaum, P. A Basic Introduction to Grazing Incidence Small-Angle X-Ray Scattering; in Special issue of Lecture Notes in Physics on "Applications of Synchrotron Lightto Noncrystalline Diffraction in Materials and Life Sciences". Springer Berlin 2009, 776, 61-90. Edt. Ezquerra, T. A.; Garcia-Gutierrez, M.; Nogales, A.; Gomez, M.


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SolventâMorphologyâProperty Relationship of PTB7:PC71BM

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SolventâMorphologyâProperty Relationship of PTB7:PC71BM