Chlorinated Wide-Bandgap Donor Polymer Enabling Annealing Free

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Chlorinated Wide Bandgap Donor Polymer Enabling Annealing Free Non-Fullerene Solar Cells with the Efficiency of 11.5% Zhitian Liu, Yerun Gao, Jun Dong, Minlang Yang, Ming Liu, Jing Wen, Yu Zhang, Haibo Ma, Xiang Gao, Wei Chen, and Ming Shao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03247 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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The Journal of Physical Chemistry Letters

Chlorinated Wide Bandgap Donor Polymer Enabling Annealing Free Non-fullerene Solar Cells with the Efficiency of 11.5% Zhitian Liu,1, # Yerun Gao,2, # Jun Dong,1, # Minlang Yang,1 Ming Liu,1 Yu Zhang,1 Jing Wen,3 Haibo Ma,3 Xiang Gao,1, * Wei Chen,4,5 * Ming Shao2, *

1

Institute of Materials for Optoelectronics and New Energy, School of Materials

Science and Engineering, Wuhan Institute of Technology, Wuhan 430205, P. R. China 2

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science

and Technology, Wuhan 430074, P. R. China 3

School of Chemistry and Chemical Engineering, Nanjing University, Nanjing

210023, P. R. China 4

Materials Science Division, Argonne National Laboratory, 9700 Cass Avenue,

Lemont, IL 60439, USA 5

Institute of Molecular Engineering, The University of Chicago, 5640 South Ellis

Avenue, Chicago, IL 60637, USA * [email protected], #

[email protected], [email protected]

these authors contributed equally.

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 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

Abstract Substituting the hydrogen atoms on the conjugated side chain of a wide bandgap polymer J52 with chlorine atoms can simultaneously increase the Jsc, Voc and FF of non-fullerene OSCs, leading to an efficiency boost from 3.78% to 11.53%, which is among the highest efficiency for as-cast OSCs reported to date. To illustrate the impressive 3-fold PCE enhancement, the chlorination effect on the optical properties and energy levels of polymers, film morphology, and underlying charge dynamics are systematically investigated. Grazing incident wide angle X-ray scattering studies show that chlorinated J52-2Cl exhibits strong molecule aggregation, the preferred face-on orientation and enhanced intermolecular - interactions, hence increase charge carrier mobility by one order of magnitude. Moreover, chlorination modifies the miscibility between donor and acceptor, and consequently optimizes the phase separation morphology of J52-2Cl:ITIC blend films. These results highlight chlorination as a promising approach to achieve highly efficient as-cast OSCs without any extra treatment.

Table of Content (TOC) graphic

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Light-weight flexible organic solar cells (OSCs) draw a lot of attention because of their great potential in low cost and high throughput solution process. Due to the rapid development of non-fullerene acceptors (NFAs),1-3 the power conversion efficiency (PCE) of solar cells increased sharply in the past years. Unlike perylene diimides-based electron acceptors2, 4-7 and fullerene derivatives, the absorption spectra of donor-acceptor-donor (A-D-A) type ring-fused electron acceptors can be easily extended into the deep-red and near-infrared (NIR) region.8-13 In order to maximize the photon harvesting, wide bandgap donor polymers are desired to realize complementary absorption with low bandgap acceptors. For example, wide bandgap J52 and medium bandgap PBDB-T have been widely used to pair with A-D-A type non-fullerene acceptors. Recently, the PCE of single junction OSCs has exceeded 14% thanks to the rational design of the complementary light absorbing donor and non-fullerene acceptor materials.11, 12, 14, 15 Conjugated polymers containing (fluoro)benzotriazole acceptor unit become one famous kind of donor materials with wide bandgap, especially those using benzodithiophene with flanked thiophene as the donor units named as J series.16-22 Unlike fullerene-based OSCs, efficient charge generation upon small, or even negligible, driving force were observed in a wide range of devices, so lower energy loss and higher PCE can be achieved in non-fullerene system.3, 7 As for one typical A-D-A type NFAs, namely ITIC, the highest occupied molecular orbital (HOMO) level is about -5.54 eV and the lowest unoccupied molecular orbital (LUMO) level is about -4.00 eV,23 so there is quite large room for J52 to down-shift the energy levels. 3



Alyklthio17 or alkylsilyl16, 24 and fluorine atoms18 were used to down-shift the energy levels. While chlorine atom could down-shift the energy levels more efficiently,25-27 due to the inductive effect of the chlorine atoms28 and its empty 3d orbitals.27 Additionally, chlorinated photovoltaic materials have drawn great attention due to much lower synthetic cost and high photovoltaic performance.15, 29-32 At the same time, attention should be paid to the large radius of chlorine atoms to avoid twisting the backbone of donor polymers.33, 34 The conjugated side chain15, 29, 30 or oligothiophene moiety32 is beleived to be prior choice to reduce the steric hindrance. Apart from material design, the morphology optimization and device engineering play important roles in achieving highly efficient devices. To further optimize the device efficiency, many strategies have been proposed to fine control the bulk heterojunction film morphology including thermal annealing, solvent vapor annealing and adding solvent additives (1,8-diiodoctane, 1-chloronaphthalene, diphenyl ether etc.)35 However, these post-treatments add the extra fabrication steps and manufacturing cost, which are not practical for large scale industrial application. Till now, most of the state-of-the-art nonfullerene OSCs still require thermal annealing to optimize film morphology and consequently achieve the best efficiency. Therefore, it is desirable to design new conjugated polymer donors with strong aggregation and crystallization capability for high performance OSCs, so that does not require any extra post-treatment. Recently, halogens substitution (e.g. fluorination or chlorination) on the conjugated polymers or small molecule acceptors have been found to modify the molecule aggregation, and therefore enhance the crystallinity of polymer via the 4


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non-covalent intermolecular interaction between F/Cl and H/C/S. On the other hand, tuning the surface energy is another way to optimize the morphology. Chlorinated polymers usually exhibited higher surface free energy due to the large polarizability of the chlorine atom.36 This will change the difference in surface free energies of donor and acceptors materials, thus modify the bulk heterojunction morphology that is critical for the device performance.37, 38 Based on these design principles, we introduced chlorine atoms on the conjugated side chain of one typical wide bandgap J52 donor polymer. In order to reduce the steric hindrance, the meta-position relative to the benzodithiophene (BDT) units was chosen for chlorination. To evaluate its photovoltaic characteristics, the home synthesized J52-2Cl and its nonchlorinated counterpart J52 were mixed with non-fullerene acceptor ITIC, respectively. We found that chlorinated J52-2Cl simultaneously improved the Voc, Jsc and FF of OSCs, resulting in an efficiency boost from 3.78 % to 11.53 %. It should also be noted that the high PCE of 11.53 % can be achieved without thermal annealing or adding solvent additives. Furthermore, noticing a three-fold PCE increase induced by the subtle chlorine substitution, we systematically investigated the chlorination effect on the optical properties, energy levels, bulk heterojunction film morphology, and charge dynamics in the devices. Clearly, chlorine substitution on the conjugated side chain of donor polymer is a promising approach to develop low cost and highly efficient OSCs.

5



HE X

S

S S

2) EH-Br

EH

S

HE

3

S 1) LDA, -78 °C, 1 h

4a: X=H; 4b: X=Cl.

N

N

s

S Sn

Sn S

Br

S

X

N

N

S S

S F

F

F

n

F

S

S X

EH

5a: X=H; 5b: X=Cl.

C8H17 N

Br Pd(PPh3)4, 90 °C, 18 h

s

EH

C6H13

X

S N

S 5a: X=H; 5b: X=Cl. X

EH HE

Sn S

2) Me3SnCl, -78 °C; R.T., 3 h

S X

C8H17 S

S Sn

S

C6H13

X

X

S

1) LDA, 1 °C, 1 h; R.T., 2 h; 2) SnCl2, HCl(aq), R.T., 3 h

O 2a: X=H; 2b: X=Cl.

1a: X=H; 1b: X=Cl.

HE

X

S

O

X

1) LDA, 0 °C, 0.5 h

Page 6 of 23

EH J52-2Cl: X=Cl; J52: X=H

6

EH =

Scheme 1. Synthesis routes of the J52 and J52-2Cl

The synthetic routes of the J52-2Cl and J52 were shown in Scheme 1 and details of synthesis can be found in the Supporting Information. J52-2Cl was achieved via Stille polymerization using tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) as the catalyst. The chlorine atoms were well reserved, which could be verified via element analysis as listed in Table 1. The molecular structures of the intermediates and the copolymers were confirmed by 1H NMR. The number average molecular weight values of J52 and J52-2Cl are 18.82 kDa and 25.84 kDa, respectively, as listed in Table 1. Both polymers were highly soluble in common organic solvents, such as chloroform, tolunene, tetrahydrofuran, etc. The thermal stability was analyzed by thermogravimetric analysis (TGA). J52 and J52-2Cl exhibited sufficiently high decomposition temperatures (Td) of 446.2 oC and 400.2 oC at 5% weight loss, respectively (Supporting information Figure S1).

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(a)

(b)

1.0

J52 J52-2Cl

0.8 0.6 0.4 0.2 0.0

Normalizd Absorption

Normalized Absorption

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


1.0

J52 J52-2Cl

0.8 0.6 0.4 0.2 0.0 400

300 400 500 600 700 800 900 Wavelength (nm)

500 600 700 800 Wavelength (nm)

900

Figure 1. UV–Vis absorption spectra of J52-2Cl and J52 in solution (a) and films (b); (c) dipole moments of the model compound for J52 and J52-2Cl.

Table 1. Summary of the intrinsic properties of the copolymers Mn

PDI

Egopt

EHOMO

EHOMO

Cl

γ

(kDa)

(Mw/Mn)

(nm)

(eV)a)

(eV)b)

(%)c)

mN/m

J52

18.82

1.42

1.93

-5.39

-5.22

0

47.76

J52-2Cl

25.84

2.39

1.94

-5.53

-5.38

5.85 (5.97)

46.53

polymer

a)

Calculated from the cyclic voltammetry;

(DFT);

c)

b)

Calculated by density functional theory

The element analysis results of the Cl content. The theoretical value was

listed in the parentheses.

The absorption spectra of the polymer donors in solution and solid thin films were shown in Figure 1. J52 and J52-2Cl exhibited typical saddle-shaped absorption 7



spectra both in solution and film which cover the range from 300 nm to 650 nm, indicating their strong aggregation ability even in solution. As for these donor-acceptor alternating copolymers, the former peak was attributed to intramolecular charge transfer (ICT) and the latter was due to molecule aggregation. J52-2Cl shows two absorption peaks at 554 nm and 588 nm in the chloroform solution. The ICT peak of J52-2Cl red shifts about 12 nm compared with that of J52. This is consistent with the calculated dipole moments using density functional theory (DFT) calculation when one repetitive unit with trunked alkyl chain is used as the model compound.39 The chlorinated molecule exhibited a larger dipole moment (δ = 3.778) than that of non-chlorinated compound (δ = 3.515) as shown in Figure 1. Additionally, the height ratio of absorption peak induced by the aggregation to the ICT peak for J52-2Cl in solution and film is notably larger than that of J52, suggesting stronger aggregation of J52-2Cl. Additionally, the absorption spectra of J52-2Cl in chloroform solution with different concentration demonstrated that J52-2Cl seriously aggregates even in dilute good solvent, as shown in Figure S2. This might be due to the enhanced noncovalent interaction between chlorine atoms and sulfur atoms/hydrogen atoms40, 41 or stronger dipole-dipole interaction of J52-2Cl.42 Additionally, J52-2Cl exhibits higher absorption coefficiency, as shown in figure S3, which benefit for light-harvesting. And the absorption edges of J52 and J52-2Cl films are almost indentical, indicating that chlorination at the side chains creates negligible steric hindrance. The optical band gap (Egopt) of J52 and J52-2Cl are calculated to be 1.93 eV from the absorption onset of films. The HOMO level was calculated via cyclic 8


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voltammetry (CV) and the CV curves are shown in Figure S4. The HOMO level of J52-2Cl was calculated to be -5.53 eV, which is 0.14 eV deeper than that of J52 due to the inductive effect and the empty 3d orbital of chlorine atoms.27, 28 This tendency is consistent with the DFT calculation as listed in Table 1.

Figure 2. (a) Current density–voltage (J–V) curves. (b) EQE spectra of J52:ITIC and J52-2Cl:ITIC based devices

Table 2. Photovoltaic performances of the BHJ solar cells Polymer VOC (V) JSC(mA cm-2) FF(%) PCEbest/PCEaverage(%)

Ref

J52

0.72

11.73

59.36

5.18 (4.85)

17

J52

0.803

10.87

43.2

3.78 (3.01)

This work

J52-2Cl

0.956

17.17

70.2

11.53 (10.9)

This work

To evaluate the photovoltaic performance of J52 and its chlorinated substituent J52-2Cl, we blended two polymers with a widely used non-fullerene acceptor ITIC to fabricate the OSCs with the conventional structure of ITO/PEDOT:PSS/active layer/PDINO/Al. In Figure 2(a), the control device J52:ITIC showed a typical 9



efficiency of 3.78% with Voc of 0.803 V, Jsc of 10.87 mA/cm2, and FF of 43.2%, which is close to the previous report as listed in Table 2.17 Since the molecule weight of donor polymer slightly influence the device performance, we carefully control the synthesized J52 and J52-2Cl with the similar molecule weight. In comparison, J52-2Cl:ITIC devices exhibited an enhancement in Voc that increased from 0.803 V to 0.956 V. The boost of Voc can be ascribed to the downshift of HOMO level caused by the chlorine substitution. Moreover, the Jsc was remarkably increased from 10.87 mA/cm2 to 17.17 mA/cm2. The increased Jsc can be further validated from the external quantum efficiency (EQE) spectra. In Figure 2b, the optimal J52-2Cl:ITIC devices represented the high EQE exceeding 75% from 450 nm to 700 nm. Meanwhile, we observed a significant improvement in FF from 43.2% to 70.2%. Due to the simultaneously improved Voc, Jsc and FF, the highest PCE of 11.53% have been achieved for as-cast J52-2Cl:ITIC devices. The devices also showed the good reproducibility with an average PCE of 10.9% for over 45 individual devices. It should be mentioned that no extra post-treatment such as thermal annealing or solvent additive was required to achieve such a high PCE. The device performance without and with thermal annealing were compared in Figure S5. Unlike most OSCs, thermal annealing did not further enhance the device efficiency. This certainly simplifies the fabrication process and reduces the large scale manufacturing cost of OSCs. After the chlorine substitution, the PCE almost increased by 3-fold as compared to the non-chlorinated J52 analog. To the best of our knowledge, such a remarkable increase of PCE (over three times) is the largest improvement among any reported 10


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halogenation

system

including

fluorination

and

chlorination.

Meanwhile,

J52-2Cl:ITIC provide an ideal system to investigate why subtle chlorine substitution can simultaneously increase all the photovoltaic parameters for non-fullerene OSCs. First, we compared charge transport properties for non-chlorinated J52 and chlorinated J52-2Cl polymers. The electron and hole mobility of J52 and J52-2Cl blend films can be obtained from the Space Charge Limited Current (SCLC) method. Figure S6(a) and S6(b) illustrated the log(J)-log(V) curves of hole-only devices (ITO/PEDOT:PSS/J52-2Cl(J52):ITIC/Au)

and

electron-only

devices

(ITO/ZnO/J52-2Cl(J52):ITIC/Ca/Al), respectively. The hole mobility of J52-2Cl:ITIC blend film was calculated to be 9.510-4 cm2/Vs, which was one order of magnitude higher than that of J52:ITIC blend film (6.610-5 cm2/Vs). The electron mobility of J52-2Cl:ITIC blend film was four times higher than J52:ITIC film. The improved charge carrier mobility can be attributed to two reasons. First, the strong aggregation tendency of J52-2Cl molecules are in favor of the formation of high crystalline films, which can be evidenced by the absorption spectra and X-ray diffraction studies as shown below. Second, the chlorine substitution on the meta-position of side chain benzodithiophene (BDT) units doses not increase steric hindrance, and keep the planar configuration of donor polymers. Previous OTFT studies have also shown that chlorine substitution can enhance the noncovalent intermolecular interaction, and thereby increase the film crystallinity and carrier mobility.27,

40

Therefore, both

increased hole and electron mobility reveal the improved charge transport in J52-2Cl:ITIC devices, resulting in a high Jsc. 11



Figure 3. 2D GIWAXS patterns of (a) J52:ITIC and (b) J52-2Cl:ITIC blend films. (c) Out-of-plane X-ray scattering profiles and (d) In-plane scattering profiles for J52:ITIC and J52-2Cl:ITIC blend films.

The photovoltaic performance of OSCs strongly depends on the microstructure and bulk heterojunction (BHJ) film morphology. The chlorination effect on the molecular ordering and stacking of J52 donor in the blend films was further investigated by the Grazing incidence wide angle X-ray diffraction (GIWAXS) as shown in Figure 3(a) and (b). The GIWAXS showed in-plane (100) and out-of-plane (010) reflection patterns, implying a face-on orientation with respect to the substrate. Face-on orientation is known as in favor of the charge transport in the OSCs. Figure 3(c) and (d) presented the 1D profiles of the out-of-plane and in-plane line cuts, respectively. J52 polymer showed a (100) diffraction peak at q=0.29 Å-1 in the in-plane direction, and a (010) diffraction peak located at q=1.71Å-1 in the 12


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out-of-plane direction, which correspond to a lamellar distance of 21.67 Å and a - stacking distance of 3.67 Å, respectively. In comparison, J52-2Cl showed the similar diffraction pattern, but with a slightly different (010) diffraction peak at q=1.73Å-1, corresponding to a - stacking distance of 3.63 Å. The slightly shorter facial - distance of J52-2Cl is in good accordance with the high charge mobilities measured from SCLC method. J52-2Cl:ITIC blend films also exhibited the stronger diffraction intensity than J52:ITIC blends, indicating the high film crystallinity. Moreover, chlorinated J52-2Cl is more prone to adopt a predominant face-on orientation than J52 by comparing the ratio of in-plane to out-of-plane peak intensities. Clearly, the chlorination improves the face-on orientation and enhanced - interaction of J52 polymer donor, which assist the intermolecular charge transport and eventually promote the photovoltaic performance. Miscibility of donor and acceptor materials plays a critical role in determining the nanoscale film morphology and device performance. Flory–Huggins interaction factor (χ) represent the miscibility. Previous publications have shown that χ can be estimated 2

with the empirical equation χ = K( γD - γA) , where K is a constant, γD and γA are the surface energies of the neat films of donor and acceptor materials, respectively.38, 43 Thus, we calculated the surface energies of J52 and J52-2Cl through contact angle measurements as listed in Table 1. The details of determining interaction parameter χ can be found in Figure S7 and Table S1. Due to the large polarizability of chlorine atoms, both the water contact angle and oil contact angle become larger for J52-2Cl:ITIC blend.36 In OSCs, larger χ means strong phase 13



separation with high purity of mixed domains, and thus high FF.44 As shown in Table 2 S1, the ( γD - γA) increased significantly from 0.013 for the blend of J52:ITIC to

0.041 for the blend of J52-2Cl:ITIC, suggesting stronger phase separation and higher domain purity in J52-2Cl:ITIC blend films. High domain purity will facilitate the charge collection and suppress the charge recombination. Therefore, we expect that chlorination on the side chain of polymer donor can enhance the phase separation in non-fullerene OSCs. Thus, the blend morphology was further examined via tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM). In Figure S8 and Figure S9, no significant nanoscale phase separation can be detected from J52:ITIC blend, but more pronounced phase separation was observed from J52-2Cl:ITIC blend films. This enhanced phase separation is consistent with the above miscibility calculation. AFM and TEM observations suggested that J52-2Cl:ITIC can develop good interpenetrating and bi-continuous nanoscale network with the comparable large domain size that can ensure sufficient charge dissociation and good charge transport. In addition, the root mean square (RMS) roughness of J52 and J52-2Cl blend films were 1.1 and 0.88 nm, respectively, implying J52-2Cl can form a better uniform and smooth film. Therefore, chlorine substitution of donor polymer substantially enhanced the molecule aggregation and film crystallinity, modified the miscibility with the acceptors, and hence formed the preferred phase separation film morphology.

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Figure 4. (a) Voc versus the light density (b) Jsc versus the light density (c) Jph versus Veff plots of the optimized OSCs.

To better understand the chlorination impact on the device performance, a series of photo-physical studies have been implemented to reveal the charge dissociation process and underlying recombination mechanism in J52-2Cl:ITIC based OSCs. In order to identify the major recombination pathway, we firstly examined light intensity dependence of Voc. In general, Voc follows a logarithmic relationship to the light intensity as Voc  nkT/qln(I), where k, T and q are the Boltzmann constant, temperature and the elementary charge, respectively. n=1 reflects the trap-free bimolecular recombination, while n=2 means trap-assisted recombination.45 In Figure 4 (a), the fitted n was 1.99 for J52:ITIC blend, suggesting a dominant trap-assisted recombination. In contrast, the smaller n of 1.28 for J52-2Cl blend indicated that the bimolecular recombination was the dominant recombination mechanism in the chlorinated system. Secondly, we further investigated the degree of bimolecular recombination by studying the illumination light intensity dependence on the Jsc. Their relationship can be described by the power law equation: JscIlight. The deviation  from 1 reflects the degree of bimolecular recombination.45 In Figure 4(b), 15



the slope of  for J52:ITIC and J52-2Cl:ITIC based devices were fitted as 0.92 and 0.98, implying that chlorination of J52 polymer can significantly suppress the bimolecular recombination. Thirdly, we also compared the exciton dissociation probability (Pdiss). Figure 4 (c) represented Jph (Jph =JL-JD, where JL and JD represent the current densities under the illumination and in the dark, respectively) versus effective voltage (Veff = V0-Va, where V0 is the compensation voltage when Jph is zero and Va is the applied voltage) plots (c). Pdiss can be calculated by the ratio of Jph and Jsaturation at the high electric field when all the photogenerated excitons can be dissociated into free carriers and collected by the electrodes.46 Correspondingly, the Pdiss was 71.62% for the J52:ITIC devices, and 97.95% for J52-Cl:ITIC devices. Nearly 100% Pdiss of J52-2Cl:ITIC devices implied negligible recombination, and suggested most of the photogenerated exciton can be completely dissociated and collected by the electrode. This result was consistent with the improved charge transport and optimized film morphology of J52-2Cl:ITIC blend film. Therefore, reduced bimolecular recombination and higher exciton dissociation probability of J52-2Cl:ITIC combined together contribute to the high Jsc and FF. Except for steady-state studies, transient photo-voltage (TPV)47 and transient photocurrent (TPC) measurements48 were carried out to give further insight into recombination dynamics and charge carrier extraction under the device operation conditions. TPV signal measures the charge carrier density in the devices. Since the devices were operated under the open circuit condition, the transient decay of photovoltage corresponds with the recombination rate of the photo generated charge 16


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carriers. Thus, the TPV signal mainly reflects the charge recombination process.58 In Figure 5(a), the lifetime of charge carriers rec can be determined by fitting the single exponential transient decay of photovoltage. We found that lifetime of charge carriers significantly increased from 85 µs for J52:ITIC devices to 410 µs for J52-2Cl:ITIC devices, suggesting the suppressed charge recombination in chlorinated J52-2Cl:ITIC devices. The transient studies were consistent with the previous light intensity dependent Jsc and Voc steady measurement. In contrast to TPV, the TPC transient signal mainly represents the information of carrier extraction and charge trapping in the bulk heterojunction film.59 Since TPC is measured at short-circuit condition, TPC decay is much faster than TPV. Figure 5(b) exhibited the extraction time tr fitted from the transient decay of photocurrent was reduced from 0.83 µs to 0.44 µs. Fast extraction time can be attributed to the faster charge mobility and better charge extraction ability of J52-Cl polymer.

Figure 5. (a) Transient voltage decay curves (TPV) (b) transient photocurrent decay curves of J52:ITIC and J52-2Cl:ITIC devices.

17



In summary, a new chlorinated wide bandgap polymer J52-2Cl has been designed and synthesized for non-fullerene OSCs. Chlorination decreases the frontier molecular orbital levels, leading to the increase of Voc. GIWAXS results show that chlorinated J52-2Cl prefers face-on orientation and enhances intermolecular - interaction, thus enhancing the charge mobility of J52-2Cl by one order of magnitude. Moreover, owing to the large polarizability of chorine, J52-2Cl exhibit the increased surface energy and reduced miscibility with ITIC, resulting in the optimized film morphology with high domain purity. In addition, photo-physics studies imply J52-2Cl:ITIC devices show the high exciton dissociation probability (97.95%) and suppressed charge recombination. All the above factors combined together are responsible for the improved Jsc and FF. As a result, chlorinated J52-2Cl:ITIC OSCs exhibit simultaneously increased Jsc, Voc and FF, resulting in an impressive enhancement of PCE by over 3-fold. The best PCE of 11.5% can be achieved from as-cast devices without any extra treatment. These interesting results can be explained by the strong aggregation and self-crystallization capability of the chlorinated polymers. Therefore, chlorine substitution on existing state-of-art conjugated polymers not only provide a promising strategy to further increase the efficiency of OSCs without tradeoff between Jsc and Voc, but also provide a feasible way to achieve high-performance OSCs without extra post-treatment that are highly desirable for future large-scale mass production.

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Acknowledgments M. Shao thanks the support from the Recruitment of Global Youth Experts of China, the Joint Funds of the National Natural Science Foundation of China (U1601651). Z. Liu thanks the support from Hubei Technology Innovation Major Project (2016AAA030), the Foundation for Outstanding Youth Innovative Research Groups of Higher Education Institution in Hubei Province (T201706), the Foundation for Innovative Research Groups of Hubei Natural Science Foundation of China (2017CFA009). X. Gao thanks the support from the National Natural Science Foundation of China (NSFC 51703172) and and Natural Science Foundation of Hubei (ZRMS2017001). W. Chen gratefully acknowledges financial support from the US Department of Energy, Office of Science, Materials Science and Engineering Division. We also thank Dr. Joseph Strzalka and Dr. Zhang Jiang for the assistance with GIWAXS measurements. Use of the Advanced Photon Source (APS) at the Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357.

Supporting Information Available Instruments and measurements, device Fabrication, detail of synthesis, TGA thermograms, absorption spectra, molar extinction coefficient spectra, cyclic voltammograms, J-V curves of J52-2Cl:ITIC devices as cast film and thermal annealing, detail of charge mobility, views of surface contact measurements, AFM and TEM images. 19



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