Bromine-Terminated Additives for Phase-Separated Morphology

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Bromine-terminated Additives for Phase-separated Morphology Control of PTB7:PC71BM-based Polymer Solar Cells Mingguang Li, Wen Zhang, Xingxing Tang, Jibiao Jin, Honglei Wang, Lingfeng Chen, Wenzhen Lv, Runfeng Chen, and Wei Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03074 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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ACS Sustainable Chemistry & Engineering

Bromine-terminated Additives for Phase-separated Morphology

Control

of

PTB7:PC71BM-based

Polymer Solar Cells Mingguang Li,† Wen Zhang,† Xingxing Tang, Jibiao Jin, Honglei Wang, Lingfeng Chen, Wenzhen Lv, Runfeng Chen* and Wei Huang

Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts and Telecommunications, Wenyuan Road, Nanjing, 210023, P.R. China. † These authors contributed equally to this work. E-mail: [email protected]

KEYWORDS: Bromine-terminated Additives, Solubility parameter, Phase-separated morphology, Pure domain, Polymer solar cells

ACS Paragon Plus Environment

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ABSTRACT: Trace amounts of solvent additive can effectively regulate the phase-separated morphology of the active layer composed of donor and acceptor materials for improved power conversion efficiency (PCE) of polymer solar cells (PSCs). However, applicable solvent additives for PSCs are still limited and it is difficult to rationally design or select appropriate solvent additives for optimal morphology control of the active layer, mainly due to the lack of sufficient understandings on the morphological regulation mechanism. Here, based on a series of bromine-terminated additives with different chain lengths, we systematically investigated the relations between properties of solvent additives, active layer morphology and photovoltaic performance of PTB7:PC71BM bulk heterojunction (BHJ) PSCs. In addition to the widely acknowledged requirements of solvent additives with selective solubility towards one of the components in the active layer and remarkably higher boiling point than that of host solvent, it was found that additives should also have suitable solubility parameter for the formation of nanoscale phase-separated morphology and pure PTB7 domains simultaneously. Therefore, the PTB7:PC71BM-based PSCs using small amount (3 vol%) of specific bromine-terminated additive show significant PCE enhancement up to 55% in comparison with that of additive-free devices. These results illustrate clearly the positive effects of solvent additive-induced phaseseparated morphology for high photovoltaic performance, providing important understandings on morphology control and valuable clues on the rational selection and development of suitable additives for high-performance PSCs.


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Introduction As a cost-effective photovoltaic technology combined with high flexibility of organic molecules, polymer solar cells (PSCs) have attracted tremendous attention in both academic and industry over the past decades,1-4 showing high power conversion efficiencies (PCEs) over 10% in single junction and 13% in tandem devices.5-8 Besides the efforts to develop novel and highperformance donors and acceptors, morphology control of the photoactive layer is of crucial importance to achieve high PCEs of PSCs. Generally, high-performance PSCs are thought to have a bi-continuous phase-separated morphology of donor and acceptor materials for efficient carrier transport with a domain size of around 10 nm to match the exciton diffusion length of organic materials.9 To construct such optimal phase-separated morphology for high PCEs, solvent additives are usually adopted to control the active layer formation kinetics.10-13 Two fundamental requirements in using solvent additives for morphology optimization of PSCs have been proposed: (1) selective solubility to one of the active components and (2) higher boiling point than that of the host solvents.12 After incorporating a small volume of solvent additives with a comparatively high boiling point, the prolonged drying time for film formation provides sufficient opportunities for the soluble organic component to form ordered self-organizations. Due to the selective solubility towards a particular component in the photoactive layer, the solvent additives have been proven to effectively regulate phase-separated morphology and facilitate the acquirement of finer domain structure for exciton dissociation and charge transportation.12, 14 Various solvent additive molecules such as 1-chloronaphthalene15,

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, 4-bromoanisole17,

alkane dithiols13, 18, di(X)octanes (X refers to a halogen)12, 19, 20 and many more have been tested and reported to be effective for the bulk heterojunction (BHJ) PSCs in the past few years. For


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example, Barrena et al. recently investigated the role of 1,8-octanedithiol on the aggregation and ordering of the polymer and the fullerene during all stage of active layer structure formation; by means of in situ X-ray diffraction methods, it was found that the crystallization of polymer donor was preceded by the early aggregation of an amorphous phase and a delayed aggregation of [6,6]-Phenyl-C61-butyric acid methyl ester (PC71BM) followed by the densification of the polymer alkyl packing at a later stage.21 Jeng et al. introduced 1-naphthalenethiol in controlling the morphology of the PSC blend active layer of poly[[4,8-bis[(2-ethylhexyl)oxy]-benzo[1,2b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):PC71BM; this novel additive can form hydrogen bonds with both components of PTB7 and PC71BM, leading to improved PTB7 crystallization, multi-length-scale PC71BM dispersion, a more balanced electron-to-hole mobility ratio, and thus a high PCE up to 8.75%.22 Yu et al. reported the use of a solvent additive of diphenyl ether, which can promote the formation of nanofibrillar networks and ordered packing of PTB7; such morphology successfully facilitates charge transport over longer distances with reduced series resistance and suppressed bimolecular recombination, resulting in a high PCE of 6.19%.23 Zhang et al. demonstrated a great enhancement of PCE from 8.2% to 10.8% for poly[4,8-bis(5-(2-ethylhexyl)thiophen-2yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2,6-diyl)](PTB7-Th):PC71BM-based PSCs using a mixed solvent additive of 1,8diiodoctane (DIO) and N-methyl pyrrolidone; it was found that the mixed solvent additive induces higher domain purity and thus gave rise to the least bimolecular recombination.24 Chen et al. developed o-chlorobenzaldehyde as a fast removable solvent additive in PTB7-based active layer; more balanced hole and electron mobilities, increased charge extraction, and longer carrier lifetime were evidenced for CBA-processed active layers with a PCE of 9.11%.25


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Scheme 1. (a) Chemical structures of PTB7 and PC71BM. (b) Schematic representation of the PSC structure. (c) Chemical structures of the bromine-terminated additives.

Though various solvent additives reported previously have demonstrated significant effects in morphological regulation as well as performance improvement, it is still a challenge to propose a feasible criterion for the rational selection and application of solvent additives for highperformance PSC devices. It becomes even more difficult when researchers attempt to develop a new solvent additive based merely on the two fundamental requirements mentioned above. Herein, a series of bromine-terminated molecules with different alkyl chain lengths (n) of 1,2dibromoethane (DBE, n=2), 1,4-dibromobutane (DBB, n=4), 1,6-dibromohexane (DBH, n=6) and 1,8-dibromooctane (DBO, n=8) were investigated as novel solvent additives for PTB7:PC71BM BHJ PSC system. Different bromine-terminated additives were found to be able


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to induce completely different morphological structures. Among these bromine-terminated additives, DBB exhibits the closest solubility parameter to that of DIO with the most similar solvent additive/organic semiconductor intermolecular interactions. Thus, the nano-scale phaseseparated morphology and pure PTB7 domains in DBB-processed film were successfully established, ensuring the effective exciton dissociation and carrier transport processes. As a result, the PCEs of these PSCs increased from 3.57% for the additive-free (W/O additive) device to 5.53% for the DBB-processed one, showing a PCE enhancement up to 55%. Our study focusing on the correlations between solvent properties, phase-separated morphology and device performance is expected to be helpful in providing useful guideline for the rational selection and development of novel additives, promoting significantly the fundamental understandings of additive-induced regulation mechanism. Results and discussion


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Figure 1. (a) Current density-voltage (J-V) characteristics, (b) power conversion efficiency (PCE) and short-circuit current density (JSC), (c) photocurrent density (Jph) versus effective voltage (Veff) characteristics and (d) maximum exciton generation rate (Gmax) and exciton dissociation probability (P(E,T)) of PTB7:PC71BM-based PSCs fabricated with different bromine-terminated solvent additives.

Photovoltaic performance. To reveal the internal correlations between solvent additives and device performance, the well investigated PTB7:PC71BM as a high performing model system was studied using various bromine-terminated additives and the PSC structure of ITO/PEDOT:PSS/PTB7:PC71BM/LiF/Al (Scheme 1). The content of bromine-terminated additives has been optimized and 3 vol% was found to be the best experimental condition (Figure S1 and Table S1). The J-V characteristics of the PTB7:PC71BM-based PSC devices using various solvent additives are shown in Figure 1a and the performance parameters of the solar cells are shown in Table 1. Without using additives (W/O additive), the short-circuit current (JSC) was only 9.10 mA/cm2. This value increased to 12.67 mA/cm2, 11.81 mA/cm2, and 11.09 mA/cm2 after incorporating DBB, DBH and DBO additives, respectively. Meanwhile, the fill factor (FF) exhibited similar tendency using different bromine-terminated additives. As a result, the PCEs of the devices raised from 3.57% (W/O additive) to 5.53% (DBB-processed device), 4.93% (DBH-processed device) and 4.18% (DBO-processed device), respectively. However, the PCE value decreased to 2.60% when DBE solvent was introduced. Meanwhile, the reference device processed with DIO additive was also prepared and the PCE of 6.90% was obtained (Figure S2). Though various bromine-terminated additives processed PSCs exhibited relatively poor performance compared with that of DIO-processed device, the significant performance


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difference under various bromine-terminated additives with different alkyl chain lengths is more appealing. The performance characteristics of the devices including the PCE and JSC as a function of bromine-terminated additive type are presented in Figure 1b. It is obvious that the increased PCE value is mainly caused by the improved JSC, which is closely related to the regulation of phase-separated morphology. To further study the exciton generation and dissociation behaviors in PSCs, the photocurrent density (Jph) as a function of effective applied voltage (Veff) was measured (Figure 1c). Note that Jph=JL-JD (1), where JL and JD are the current densities under illumination at 100 mW cm-2 and in dark conditions, respectively. Veff=V0-Va (2), where V0 is the voltage when JL= JD and Va is the applied bias. It is assumed that all the photogenerated excitons are dissociated into free charge carriers at high Veff. Herein, at large reverse bias (Veff＞5 V), the photocurrents Jph of PSCs are approximately regarded as a saturation photocurrent density (Jsat). It can be seen that the photocurrent at high reverse voltages saturates at different values for the devices processing different additives. Accordingly, the maximum generation rate of the electron-hole pairs (Gmax) can be obtained according to the equation Jsat=eGmaxL (3), where e is the elementary charge and L is the thickness of the BHJ photoactive layer. Meanwhile, the extracted exciton dissociation probability (P(E,T)) under a short-circuit condition can also be calculated by using the equation Jph=Jsat·P(E,T) (4), where Jsat is the saturation photocurrent density.26, 27 As shown in Figure 1d and Table 1, the Gmax of DBB-processed device (thickness L=100 nm) reaches 1.28×1028 /m3/s, which is the largest among all of the fabrication conditions. At the short-circuit condition, PTB7:PC71BM blends processed with DBB, DBH and DBO exhibited the P(E,T) values of 61.9%, 63.7% and 67.2%, respectively, indicating their relatively superior charge-generation capabilities. The large ratio suggests enhanced exciton dissociation efficiency and charge transfer


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efficiency at the BHJ photoactive layer. It should be noted that the P(E,T) value is as low as 48.9% for DBE-processed blend film. These results indicate that the introduction of appropriate bromine-terminated additives is demonstrated to be capable of enhancing charge generation rate and reducing charge recombination (including geminate and nongeminate recombination) losses, thus raising the PCEs of PTB7:PC71BM-based PSCs.28

Table 1. Solar cell parameters, saturation photocurrent density (Jsat), maximum exciton generation rate (Gmax) and exciton dissociation probability (P(E,T)) of PTB7:PC71BM PSCs fabricated using different bromine-terminated additives. Jsc

FF

PCE

(V)

(mA/cm2)

(%)

(%)

W/O additive

0.75

9.10

51

3.57

16.66

1.04

54.6

DBE

0.79

7.33

44

2.60

14.98

0.94

48.9

DBB

0.76

12.67

56

5.53

20.48

1.28

61.9

DBH

0.76

11.81

55

4.93

18.55

1.16

63.7

DBO

0.76

11.09

49

4.18

16.50

1.03

67.2

Solvent additive

a)

Jsata)

Voc

Gmax

(mA/cm2) (1028 /m3/s)

P(E, T) (%)

The Jsat was calculated at Veff=5.0 V.

Properties of solvent additives. To investigate the effect of introduced bromine-terminated additives with varying alkyl chain lengths on the device performance, the fundamental physical properties of these solvent molecules were contrasted. Since DIO has already been successfully applied in the PSC devices of PTB7:PC71BM blends previously,29-32 we could investigate the extent of intermolecular interactions between these additives and semiconductor materials


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(including PTB7 and PC71BM components) by contrasting DIO and various bromine-terminated additives, thus guiding the selection of new solvent additives and accurate regulation of phaseseparated morphology.

Figure 2. Boiling points and solubility parameters (δ) of bromine-terminated additives.

To predict the solubility and thus provide a guideline for additive selection, the solubility parameter (δ) is always utilized to estimate the intermolecular interactions between solvents and the semiconducting materials in the active layer.33-35 Considering that the host solvent chlorobenzene (CB) molecules have evaporated during the initial stage of film formation, the effect of CB on the film morphology are of no major concern here. By using Fedors’s group contribution theory36,

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, the solubility parameters of DIO and various bromine-terminated

additives could be calculated (Table S2). Due to the large quantity of functional groups in PTB7 molecules, this approximate calculation method could not reflect the real situation of the semiconductors. Therefore, DIO as an available reference could be used to investigate the physical property differences between these bromine-terminated additives. Figure 2 displays the δ values and the boiling points of various solvent additives. It can be observed that the δ value of


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DBB additive is most close to that of DIO, indicating that DBB may exhibit similar impact on the active layer composed of PTB7 and PC71BM. Besides, the duration time of intermolecular interactions which is directly related to the boiling points of selected solvent additives should also be considered. Except for DBE solvent with the boiling point of 130.2 °C, all the other bromine-terminated additives exhibited remarkable higher boiling point (197.5 °C for DBB, 244.1 °C for DBH and 271.0 °C for DBO). The similar boiling point between DBE and CB (132.2 °C) means that the introduced DBE plays negligible effect in prolonging the duration time of intermolecular interactions. Furthermore, the selective solubility for all the bromineterminated additives was also demonstrated, as shown in Figure S3. It can be found that PC71BM is readily dissolved in various bromine-terminated solvents, but the solubility of PTB7 in these solvents is very poor. Morphological control. The morphological variations including degree of phase separation and polymer aggregate states under different film-formation conditions were characterized by UVVis absorption spectra, transmission electron microscopy (TEM) images and cyclic voltammetry (CV). The UV-Vis absorption spectra of PTB7:PC71BM blend films without (W/O) and with various bromine-terminated additives (Figure S4) do not exhibit significant peak shift. This result indicates that most of PTB7 chains still exist in an amorphous state for both pristine blend film and additive-induced blend films, which is in accordance with the study reported previously.29, 38


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Figure 3. (a) TEM images of BHJ PTB7:PC71BM blends films processed without (W/O) and with 3.0 vol% DBE, 3.0 vol% DBB, 3.0 vol% DBH, 3.0 vol% DBO, respectively. (b) Schematic representation of the relationship between energy level of valence bands and film microstructures. VB-1, VB-2 and VB-3 represent the valence bands of ordered PTB7 domain, amorphous PTB7 domain and PTB7/PC71BM mixed domain, respectively. (c) CV measurements of pure PTB7 film and PTB7:PC71BM blend films processed with different additives.

To reveal the effect of the bromine-terminated additives on the phase-separated morphology of the PTB7:PC71BM-based BHJ films, the bulk morphology characteristics were studied by TEM, shown in Figure 3a. In the case of W/O additive, the PC71BM-rich domains appear to form large


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aggregates (≈200 nm), with PTB7-rich domains filling the spaces between them. As reported previously, the action of DIO in the PTB7:PC71BM system could induce the disappearance of large aggregated PC71BM. TEM images uncovered that DBB and DBH-processed blends exhibited optimized nano-scale phase-separated morphology similar to that of classical DIOprocessed sample. This result could support that DBB solvent with suitable solubility parameter (most close to that of DIO) is beneficial to the regulation of optimal active layer morphology. As regards DBE with low boiling point, it could not fully remove the aggregated PC71BM domains, but just slightly reduce the size of PC71BM aggregates. When DBO with highest boiling point was introduced, it remains a much longer time during solidification process and thus PC71BM molecules have more chance to redissolve and connect each part together, thus leading to much larger phase-separated morphology. Considering that the TEM images provide very little evidence of the differences in the aggregate state of PTB7 polymer chains, CV was used to further determine the polymer phase. As reported previously, CV is a useful tool for characterizing the valence bands (VBs) in conjugated polymers because it could characterize bulk material properties and distinguish the VBs of multiple polymer phases (i.e. ordered phase, amorphous phase and mixed phase), as shown in Figure 3b.39, 40 Prior to investigate the PTB7 aggregate states in the blend films, the CV of pure PTB7 film was measured as a reference (Figure 3c). The onset potential of PTB7 oxidation process was found at 454 meV, which should be assigned to the VBs of well-ordered aggregated domains. After blending with PC71BM, the onset potential of oxidation process increased to 583 meV, which demonstrates that the introduced acceptor component results in a finely intermixed amorphous film. As soon as various bromine-terminated additives were incorporated, the onset potential of oxidation process reduced to 503 meV (DBE-processed film),


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483 meV (DBB-processed film), 486 meV (DBH-processed film) and 491 meV (DBO-processed film) respectively, as shown in Figure 3c. Specially, the DBB and DBH-processed films exhibited oxidation peak most close to that of pure PTB7. This shift observed for additiveprocessed films indicated that these bromine-terminated additives facilitated the formation of certain amount of pure PTB7 crystalline domains, which is consistent with that of DIO-processed films.41 However, the crystallinity of PTB7 is very weak29; the X-ray diffraction profiles of the blend films using different processing additives show little difference in PTB7 orders (Figure S5), suggesting that these additives contribute mainly to the optimized phase-separated morphology and the formation of nano-scale pure PTB7 domains rather than the film crystallinity. As reported previously, the generated pure PTB7 aggregates would provide an energetic driving force for spatial separation of electrons and holes.40, 42 The above results confirmed that the generated PTB7 pure domain played a critical role in producing efficient exciton dissociation/charge generation. As for the DBE-processed film, we speculate that isolated PTB7 aggregate domains rather than PTB7 penetrating pathways were formed under comparatively short time for film solidification and thus a fraction of photogenerated charges were trapped in these isolated PTB7 domains.43 Morphological regulation mechanism. Based on these analyses, a morphological regulation mechanism can be proposed to illustrate how the additives achieve the optimal phase-separated morphology for high photovoltaic performance (Figure 4). In the absence of solvent additive (Figure 4a), the morphology of PTB7:PC71BM system was composed of a pure amorphous fullerene phase with hundreds of nanometers in diameter, dispersed in an overwhelmingly amorphous matrix with PTB7/PC71BM mixed composition. The large phase-separated morphology only supplies a much smaller donor-acceptor (D-A) interface for exciton splitting,


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thus resulting in lower charge generation rate followed by poor device performance. Considering the property difference of various bromine-terminated solvents, they produce completely different results, including morphology control and performance regulation.

Figure 4. Schematic illustration of PTB7:PC71BM BHJ blend films processed by using different solvent additives.

When bromine-terminated additives are present in the blend solution, the selective solubility of these solvents contributes to the formation of pure PTB7 domain. Due to the close boiling points between DBE and host solvent, DBE will not get any chance of redissolving large PC71BM aggregates and thus large phase-separated morphology still exists for DBE-processed blend film (Figure 4b). Meanwhile, the isolated pure PTB7 domain induced by DBE will further reduce the essential D-A interface, and therefore the charge generation efficiency will further deteriorate. The isolated PTB7 aggregate domains merely play the role of charge traps (rather


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than efficient charge transport channels), which recombine with mobile charges and thus lead to severe nongeminate recombination. In contrast, DBB and DBH solvents with relatively nonvolatility have sufficient time to redissolve the PC71BM aggregates. The PC71BM molecules initially adsorbed to PTB7 matrix act as nuclei and the dissolved PC71BM components gradually accumulate along the PTB7 chains, finally reconstructing satisfied nano-scale phase separation (Figure 4c). Specially, DBB with the solubility parameter most close to that of classical DIO indicates that the intermolecular interactions between DBB and semiconducting materials may be most appropriate for approaching ideal morphology of active layer. The performance results also demonstrate that DBB-processed device shows both excellent exciton generation efficiency and outstanding exciton dissociation efficiency. As regards DBO with highest boiling point (Figure 4d), PC71BM molecules could diffuse to further space under the prolonged duration time for film solidification, thus leading to even larger phase separation ultimately. Though the micron-sized phase-separated morphology induced significant geminate recombination, the produced pure PTB7 and pure PC71BM domains in DBO-processed device establish efficient carrier transport channels and thus facilitate the dissociation of the limited excitons at the interface. Therefore, the highest exciton dissociation efficiency was found in DBO-processed devices. Based on the in-depth mechanism studies, we look forward to contributing to the rational and feasible selection of novel solvent additives for high-performance PSCs. Conclusions Based on the widely investigated PTB7:PC71BM bulk PSCs, a series of bromine-terminated solvent additives were tested to reveal the relations between solvent additive properties, active layer morphology and photovoltaic performance. Among these additives with different chain lengths, DBB shows the highest photovoltaic performance; its PCE increases from 3.57% for


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pristine device to 5.53%, exhibiting 55% PCE enhancement in comparison with that of additivefree device. Systematic investigations on solvent properties, film morphology, and domain formation mechanism suggest that the introduction of DBB additive facilitates the simultaneous formation of nano-scale phase-separated morphology and pure PTB7 domains, which benefits significantly the photogenerated exciton dissociation and charge transportation for high PCEs. Interestingly, DBB exhibits the closest solubility parameter to that of classical DIO solvent; this could serve as a facile and straightforward criterion for the evaluation of potentially available solvent additives for PSCs. With these fundamental insights into the morphology regulation mechanism, our study would provide important guidelines towards the selection and development of novel solvent additives for high-performance PSC devices. Experimental Materials. PTB7, PC71BM and PEDOT:PSS were purchased from Luminescence Technology Corp. Chlorobenzene (CB) and 1,8-diiodoctane (DIO) were purchased from Sigma-Aldrich. 1,2Dibromoethane (DBE), 1,4-dibromobutane (DBB), 1,6-dibromohexane (DBH) and 1,8dibromooctane (DBO) were purchased from Energy Chemical Corp. Lithium fluoride (LiF) was provided by Shanghai Han Feng Chemical Corp. All chemicals were used as obtained. Device fabrication. The device structure was ITO/PEDOT:PSS/PTB7:PC71BM/LiF/Al. Firstly, the anode interfacial layer PEDOT:PSS was spin-coated on top of the precleaned ITO substrate, which was treated with water, acetone and ethanol in an ultrasonic bath followed by ultraviolet (UV)-ozone treating for 25 min and drying at 120 °C for 30 min. A blend of PTB7:PC71BM with the mass ratio of 1:1.5 in CB solvent with 3.0 vol% bromine-terminated additives (i.e., DBE, DBB, DBH and DBO, respectively) was spin-coated on the surface of PEDOT:PSS layer. The thickness of the active layer was controlled to be about 100 nm as measured by surface


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profilometer. Then LiF (~1 nm) as the cathode interfacial layer was deposited on the active layer by high-vacuum (~5×10-4 Pa) thermal evaporation. Finally, Al (~100 nm) cathode electrode was thermally deposited under a vacuum of 5×10-4 Pa to complete device fabrication. These PSCs, made on ITO glass substrate, have an active area of 9 mm2. Characterization. The morphology of the active layer of PTB7:PC71BM system was characterized by transmission electron microscopy (TEM). TEM experiments were performed on HT7700 (Hitachi, Japan) with an accelerating voltage of 100 kV. UV-Vis spectra were measured on a lambda 35 PerkinElmer ultraviolet-visible (UV-Vis) spectrophotometer. Cyclic voltammetry (CV) measurement was performed to estimate the highest occupied molecular orbital (HOMO) from the onset potential of the electrochemical oxidation. The CV measurement was carried out at room temperature on a CHI660E system in a typical three-electrode cell with a working electrode (ITO glass), a reference electrode (Ag/Ag+, referenced against ferrocene/ferrocenium), and a counter electrode (Pt wire) in an acetonitrile solution of Bu4NPF6 (0.1 M). X-ray diffraction (XRD) profiles were obtained on a diffractometer D8 Advance (Bruker, Germany, λ=1.54 Å) with an X-ray generation power of 40 kV tube voltages and 40 mA tube current. Current density-voltage (J-V) characteristics of PSCs were recorded by Keithley-2400 digital source meter. SAN-EI electric solar simulator (XES-50S1) under simulated AM 1.5 illumination (100 mW/cm2) was used for as solar cell characterization.

ASSOCIATED CONTENT Supporting Information. Additional J-V curve measurements of PTB7:PC71BM based PSCs without (W/O) additives, with DIO additive and with different contents of DBB additive. The conditions of dissolving PTB7 and PC71BM components in different bromine-terminated


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solvents. UV-vis spectra of pure PTB7, pure PC71BM and the blend films with different processing additives. X-ray diffraction profiles of PTB7/PC71BM blend films using different additives. Physical constants and solubility parameters of various solvent additives. AUTHOR INFORMATION Corresponding Author *Runfeng Chen. E-mail: [email protected] ACKNOWLEDGMENT This study was supported in part by the National Natural Science Foundation of China (21304049 and 21674049), Qing Lan project of Jiangsu province, Science Fund for Distinguished Young Scholars of Jiangsu Province of China (BK20150041), and Natural Science Foundation of Jiangsu Province of China (BK20160891).

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TOC Figure

Synopsis Systematic mechanism studies on morphology control of PTB7:PC71BM using a series of bromine-terminated additives reveal new selection criterions of solvent additives for highperformance PSCs.


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Bromine-Terminated Additives for Phase-Separated Morphology

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