A Highly Crystalline Wide-Band-Gap Conjugated Polymer toward High

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A Highly Crystalline Wide Band-Gap Conjugated Polymer towards High-Performance As-Cast Non-Fullerene Polymer Solar Cells Haiying Jiang, Zhen Wang, Lianjie Zhang, Anxing Zhong, Xuncheng Liu, Feilong Pan, Wanzhu Cai, Olle Inganäs, Yi Liu, Junwu Chen, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10059 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Applied Materials & Interfaces

A Highly Crystalline Wide Band-Gap Conjugated Polymer towards High-Performance As-Cast Non-Fullerene Polymer Solar Cells Haiying Jiang†‡, Zhen Wang†‡, Lianjie Zhang†*, Anxing Zhong†, Xuncheng Liu†, Feilong Pan†, Wanzhu Cai#*, Olle Inganäs#, Yi Liu¢, Junwu Chen†*, Yong Cao†. †

Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent

Materials & Devices, South China University of Technology, Guangzhou 510640, P. R. China #

Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology, Linköping

University, Linköping 58183, Sweden ¢

The Molecular Foundry, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley,

California 94720, United States

Corresponding authors:

Dr. Lianjie Zhang (E-mail: [email protected])

Dr. Wanzhu Cai (E-mail: [email protected])

Prof. Junwu Chen (E-mail: [email protected]) KEYWORDS: bulk heterojunction polymer solar cells, charge transfer state, crystallinity, energy loss, non-fullerene acceptor, wide band-gap polymer.

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ABSTRACT:

A new wide band-gap conjugated polymer PBODT was successfully synthesized, which showed high crystallinity and utilized as the active material in non-fullerene bulk-heterojunction polymer solar cells. The photovoltaic devices based on the as-cast blend films of PBODT with ITIC and IDIC acceptors showed notable power conversion efficiencies (PCEs) of 7.06% and 9.09%, with high open-circuit voltages of 1.00 and 0.93 V that correspond to low energy losses of 0.59 and 0.69 eV, respectively. In the case of PBODT:ITIC, lower exciton quenching efficiency and monomolecular recombination are found for devices with small driving force. On the other hand, the relatively higher driving force and the suppressed monomolecular recombination for PBODT:IDIC devices are identified to be the reason for their higher short circuit current density (Jsc) and higher PCEs. In addition, when processed with the non-chlorinated solvent trimethylbenzene, a good PCE of 8.19% was still achieved for the IDIC-based device. Our work shows that such wide band-gap polymers have a great potential for the environment-friendly fabrication of highly efficient polymer solar cells.

1. INTRODUCTION In the field of polymer solar cells (PSCs), the photoactive materials with advanced functionality are highly expected to achieve excellent optical-electronic performance of the devices.1−3 Over the past two decades, fullerene derivatives have been the most popular electron acceptor materials in the bulk-heterojunction active layer of PSC due to their excellent electron mobility and high electron affinity.4 To well match the optoelectronic properties of the fullerene acceptors, low band-gap (LBG) polymers are mainly required.5,6 The most efficient approach to extend the family of polymer materials with various optical band gaps is covalently linking electron-donating unit (D) and 2 ACS Paragon Plus Environment

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electron-accepting unit (A) alternatingly in the polymer mainchain.7−9 Some well-known donor polymers, such as benzodithiophene-thienothiophene copolymer PTB7-Th and fluorinated benzothiadiazole (FBT) based copolymers, have achieved power conversion efficiencies (PCEs) above 10%.10−13 However, these fullerene-based systems inevitably encounter large energy loss (Eloss), defined as the energy gap between optical band gap (Eg) and the open-circuit voltage (Voc), i.e. Eloss = Eg ‒ eVoc. In a champion FBT-based polymer device with a PCE of 11.7%, Eloss is as large as 0.84 eV.14 Besides, fullerene derivatives have issues such as poor absorption in the visible region, tendency of over-crystallization, and limited energy level tunability. Very recently, non-fullerene acceptors have emerged as a promising alternative to fullerene acceptors.15,16 Particularly, planar acceptor-donor-acceptor (A-D-A) type non-fullerene acceptors present narrow band gaps with high extinction coefficient, readily tunable energy levels, low production cost and promising photovoltaic performance, showing discernible advantages over conventional fullerene derivatives. Eloss less than 0.7 eV has been demonstrated in some of the non-fullerene systems. For instance, Eloss of 0.6 eV and 0.62 eV were realized in the PSCs with PCEs of 11.9% and 11.5%, respectively.15,16

The wide band-gap (WBG) polymers naturally have strong absorption in the short-wavelength region, which could complement that of LBG non-fullerene acceptors well. By utilizing high resonance energy D-unit, a low-lying highest occupied molecular orbital (HOMO) level for WBG polymer can be achieved.17 As a result, the energy level gap between the lowest occupied molecular orbital (LUMO) of acceptor and HOMO of donor material is enlarged, and consequently high Voc can be acquired. According to the report, the driving force (ED* – ECT (Elowest singlets state of the blend – Echarge transfer)) less than ~0.3 eV could still be sufficient in more and more non-fullerene cases based on WBG polymer donors,18,19 which is very different from that in most conventional fullerene-based 3 ACS Paragon Plus Environment


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systems.20,21 With energy levels design of WBG polymer, high Voc as well as large Jsc can be realized simultaneously, verifying the importance of WBG polymer donors in non-fullerene system. However, high-performance WBG polymers have not yet been well exploited in comparison to LBG polymers. Recently, 2D-conjugated WBG polymers as electron-donating materials have captured academic interest to some extent. Polymer PBDB-T firstly reported by Hou et al. was used to blend with new LBG non-fullerene acceptors, realizing PCEs more than 10%.16,22−25 Li et al. initially reported a polymer J51 and then developed a series of J51 analogue, such as polymers J61 and J71, based on which high PCE values exceeding 11% were reported.26−27 Huang et al. developed an imide-functionalized polymer PBTzI and obtained a high PCE of 10.24%.30 All these systems show low Eloss in the range of 0.6‒0.68 eV. Besides, WBG polymers with oligothiophene as electron-donating building block have been synthesized. Yan et al. reported a WBG polymer containing quaterthiophene and FTAZ blended with IEIC displayed a high PCE of 7.3%.31 Hou et al. reported an ester-substituted polythiophene with a PCE of 10.16%.32 Li and Zhang et al. reported quaterthiophene-benzodithiophene-diketone based polymer blended with ITIC to reach a high PCE of 9.2%.33

On the other hand, it has been noted that high throughput roll-to-roll (R2R) technology is an important commercial avenue to the PSC production. Solution-processed as-cast PSCs in the absence of further thermal annealing and vacuum drying to remove solvent additive are highly compatible with R2R processing. In addition to developing new non-fullerene acceptors, exploiting new promising polymer donors is needed to realize high-performance as-cast non-fullerene PSCs. Until now, only a few WBG polymers can construct as-cast PSCs with high PCEs of more than 9%.27,33−36 4 ACS Paragon Plus Environment

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In this work, a new dithienobenzoxadiazole(DTfBO)-based conjugated polymer PBODT was successfully synthesized, which possesses an optical band-gap of 1.88 eV. ITIC37 and IDIC35 were employed as the acceptor to evaluate the photovoltaic performance of PBODT. Without any additive or post treatment, the as-cast device based on PBODT and ITIC showed a high Voc of 1.00 V, a low Voc loss of 0.59 eV and good power conversion efficiency of 7.06%. When replacing ITIC with IDIC, an improved PCE of up to 9.09% was obtained with a slightly larger energy loss of 0.69 V. The relatively lower charge transfer (CT) state and the larger non-radiative recombination are responsible for this larger energy loss. Higher driving force is presumably the reason for higher Jsc and PCE for PBODT:IDIC blend. Furthermore, the as-cast devices processed from a single non-chlorinated solvent trimethylbenzene (TMB) could achieve a good PCE above 8%.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Material Characterization The chemical structures of PBODT, ITIC and IDIC are shown in Figure 1a. The synthesis route of PBODT is shown in Scheme S1. A longer branched alkyl side chain (decyltetradecyl) is selected for PBODT because a good solubility of a conjugated polymer is crucial to process a high quality film for optoelectronic device applications. An effective cobalt-catalyzed method was applied for a large

scale

one-pot

synthesis

of

3-decyltetradecylthiophene.38

More

than

27

g

of

3-decyltetradecylthiophene could be isolated after distillation, giving a high yield of 66%. 3-Decyltetradecylthiophene was subsequently converted to 2-stannyl-3-decyltetradecylthiophene, and further coupled with dibromodithienobenzoxadiazole. After bromination of compound 4, the targeted monomer M1 could be obtained in moderate total yield. PBODT was synthesized by well-established Stille polymerization. The final PBODT product was precipitated from the 5 ACS Paragon Plus Environment


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chlorobenzene (CB) fraction of soxhlet extractions. The polymer has poor solubility in chlorinated solvents at room temperature but can be well dissolved in hot CB or o-dichlorobenzene (DCB) solvent at 70 °C. PBODT possesses a high number-averaged molecular weight (Mn) of 64.3 kg/mol, with a polydispersity index of 1.91. The Mn of PBODT is notably higher than that of a similar polymer with shorter alkyl side chains.39 The thermal stability of PBODT was investigated by thermogravimetric analysis (TGA), from which a high decomposition temperature of 384 °C was observed at 5% weight loss.

2.2. Photophysical Properties and Energy Levels The absorption and photoluminescence spectra (PL) of PBODT in dilute CB solution and thin film are shown in Figure 1c. For the absorption spectra, there are two well-resolved absorption peaks for the CB solutions at room temperature as well as the thin film, demonstrating that PBODT has an inherent tendency to self-aggregate in solution. We performed the variable temperature measurement to inspect the self-aggregation phenomenon of PBODT. As depicted in Figure 1d, the 0-0 vibronic peak gradually disappeared with the increase of the temperature and the 0-1 vibronic peak was blue-shifted and became weaker at the same time. The result is similar to the J-aggregation type behavior for the P3HT nanofiber,40 and thus suggests the strong intrachain excitonic coupling in PBODT aggregates.

For the PL spectra shown in Figure 1c, the peak position for the PBODT solution is almost the same as that of its film, which also confirms the strong self-aggregation of PBODT. Importantly, the PL spectra are more sensitive to the change in relevant aggregate behavior. The relative PL intensity ratio of 0-0/0-1 vibronic peaks slightly decreases from solution to film. In the PL spectrum of J-type aggregates, the 0-1 transition from interchain excitonic coupling is far behind the 0-0 transition.40 In 6 ACS Paragon Plus Environment

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this regard, relatively weak 0-1 transition in solid state, compared with the concurrent 0-0 transition, further implies that the electronic coupling of interchain is quite weaker than that of intrachain in the polymer aggregates.

(a)

(c)

1.0

Solution Film Solution Film

O N C10H21

C10H21

S

S

C12H25

0.8

Normalized PL

N

C12H25

S

S

S

S

n

PBODT

C6H13

C6H13

1.0 0.8

0.6

0.6

0.4

0.4

0.2

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CN NC

S

O

S

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S

S

2.0

2.4

CN C6H13

(d)

ITIC

2.8

3.2

1.8

25 40 55 70 85 95

C6H13

CN

C6H13 O

S S OC H 6 13

IDIC

C6H13

CN NC

Absorbance (a.u.)

1.6

NC

3.6

Energy (eV)

NC C6H13

Normalized Absorbance

1.4 1.2 1.0 0.8 0.6 0.4 0.2

(b)

0.0 400

500

600

700

800

Wavelength (nm)

(e) Normalized absorbance (a.u.)

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


PBODT/ITIC PBODT/IDIC

1.0 0.8 0.6 0.4 0.2 0.0 400

500

600

700

800

Wavelength (nm)

Figure1. (a) Chemical structures and (b) Energetic diagrams of PBODT, ITIC and IDIC. (c) UV-Vis spectra and photoluminescence spectra of thin film and solution (10-5 M, in CB) for PBODT at room temperature. UV-Vis spectra of (d) PBODT solution (10-5 M, in CB) at different temperatures and (e) PBODT:ITIC and PBODT:IDIC blend films. 7 ACS Paragon Plus Environment


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As shown in Figure 1e, PBODT has complementary absorption to that of ITIC or IDIC. The PBODT:ITIC and PBODT:IDIC blend films with the same blend ratio of 1:1 displayed absorption peaks at 608 and 702 nm, arising from PBODT and ITIC or IDIC, respectively. The latter peak intensity is higher in the IDIC-based blend film than that in the ITIC-based blend film. Distinguished difference between the two acceptors on the Stokes shift (as shown in Figure S1, ~82 nm and ~32 nm for ITIC and IDIC, respectively) elucidates more planar and then higher absorbance in IDIC than in ITIC, consistent with that from the reported literature.41

The HOMO levels of PBODT, ITIC and IDIC are at –5.44, –5.51, and –5.69 eV, respectively (Figure 1b), as estimated from the onset of the oxidation potentials in the cyclic voltammetry (CV) curves.42 The LUMO levels were calculated from the HOMO level and the corresponding optical band gap that was extracted from the crossing point between the normalized absorption and emission spectrum (Figure S2). The related Eg values are 1.96, 1.65, and 1.71 eV for PBODT, ITIC and IDIC respectively, and the corresponding LUMO levels are –3.48, –3.86, and –3.98 eV, respectively. Therefore, the LUMO/LUMO offsets for PBODT:ITIC and PBODT:IDIC blends are ~0.38 and ~0.5 eV, respectively, while their HOMO/HOMO offsets are ~0.07 and ~0.25 eV, respectively.

2.3 Devices Characteristics The device configuration is ITO/ZnO/polymer:ITIC (or IDIC) /MoOx/Ag and the corresponding fabrication process and the full name of each material used can be found in Supporting Information. The optimum ratio of the polymer donor and the acceptor is 1:1 by weight (the performances based on different D:A ratio are listed in Table S2) and the active layer was spin-coated from hot solution (~ 80 °C) with a thickness of ~100 nm. The typical current density versus open circuit voltage curves are shown in Figure 2a and the relevant data are listed in Table 1. Blended with ITIC, PBODT-based 8 ACS Paragon Plus Environment

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devices possessed a high Voc of 1.00 V, a moderate Jsc of 10.7 mA/cm2, and a good FF of 66.4%, giving the PCE of 7.06%. It should be noted that the high Voc of 1.00 V for PBODT:ITIC corresponds to a low Eloss of 0.59 eV. The PBODT:IDIC PSCs achieved a higher PCE of 9.09%, with a slightly lower Voc of 0.93 V, a significantly higher Jsc of 14.1 mA/cm2, and a better FF of 69.3%, despite a relatively larger Eloss of 0.69 eV was observed. 1,2,4-Trimethylbenzene (TMB) was also chosen as a non-chlorinated and environmentally-friendly solvent to process the active layers. As shown in Table 1, the devices based on PBODT and ITIC exhibited slightly lower Voc of 0.96 V and lower FF of 54.0 %, but a higher Jsc of 12.1 mA/cm2, compared with these from mixed chlorinated solvents, leading to a PCE of 6.26%. The TMB-processed PBODT:IDIC blend gave devices with a Voc of 0.92 V, a Jsc of 13.3 mA/cm2, and a high FF of 66.8%, leading to a notable PCE of 8.19%. From both halogenated and non-halogenated processing solvents, IDIC-based devices present better photovoltaic performances in comparison to ITIC-based ones.

Table 1. Photovoltaic performance of PBODT based solar cells processed with chlorinated or non-chlorinated solvent using different acceptors.a

Acceptor

ITIC

IDIC

a

Voc

Jsc

FF

PCE

(V)

(mA/cm2)

(%)

(%)

CB/DCBb

0.99±0.01 (1.00)

10.5±0.18 (10.7)

65.4±1.5 (66.4)

6.85±0.14 (7.06)

TMB

0.96±0.01 (0.96)

11.8±0.27 (12.1)

53.0±1.5 (54.0)

6.02±0.17 (6.26)

CB

0.93±0.01 (0.93)

14.3±0.49 (14.1)

66.8±2.1 (69.3)

8.87±0.19 (9.09)

TMB

0.92±0.01 (0.92)

13.3±0.36 (13.3)

64.6±1.8 (66.8)

7.95±0.12 (8.19)

Solvent

Average values are based on fifteen devices and the highest values are given in the

parentheses. b The volume ratio of CB/DCB is 1:1. 9 ACS Paragon Plus Environment


(a)

2

Current Density (mA/cm2)

0

PBODT/ITIC PBODT/ITIC(TMB) PBODT/IDIC PBODT/IDIC(TMB)

-2 -4 -6 -8 -10 -12 -14 -16 -0.2

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(b)

80 70 60 50

EQE (%)

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40 30

PBODT/ITIC PBODT/ITIC(TMB) PBODT/IDIC PBODT/IDIC(TMB)

20 10 0 300

400

500

600

700

800

Wavelength (nm)

Figure 2. (a) J-V curves and (b) EQE curves of photovoltaic devices.

As shown in Figure 2b, the external quantum efficiency (EQE) spectra for all the devices are distributed from 300 to 800 nm, which match well with the absorption spectra. For the PBODT:IDIC devices, the maximum EQE of 70% is achieved from CB-processing, and a slightly lower EQE for TMB processed devices. For the PBODT:ITIC blends, the devices processed from TMB displayed a better EQE than those from CB:DCB, which has a higher contribution at around 400-500 nm region. The thermal stabilities of the PBODT-based devices were tested by annealing the devices at 80 °C in a N2 filled glovebox (Figure S3). Both PBODT:ITIC and PBODT:IDIC based devices showed


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obvious PCE drops in the initial ~10 hours, corresponding to burn-in process.43 Then the PCEs of the two type devices could be stabilized at ~65% levels of their initials for 50 hours. For a comparison, the solar cells based on thicker blend films of 200 nm were also fabricated (Table S3). The ITIC- and IDIC-based devices showed PCEs of 4.44% and 7.95%, respectively. The Jsc and FF for the 200 nm thick ITIC-based device decreased obviously if compared with its 100 nm thick device. But the 200 nm thick IDIC-based device only displayed some drop of FF, with well maintaining of Voc and Jsc. Thus the PBODT:IDIC blend film is more promising for application. 2.4 Mobility Organic field-effect transistors were fabricated to clarify the lateral carrier transport in pristine PBODT film (Figure S4). A maximum hole mobility value of 0.2 cm2/(V s) is obtained, which is similar to that obtained from the shorter alkyl side chain analogue,39 suggesting that the overall charge transporting pathway is not interrupted by the long side chains. On the other hand, hole and electron mobilities of the optimized PBODT:ITIC and PBODT:IDIC blend films are measured by fitting the J-Veff curve of single carrier device using space charge-limited current (SCLC) model.44, 45 The calculated mobilities are shown in Figure 3a. The hole mobilities for the PBODT:ITIC and PBODT:IDIC blends are 1.50×10‒3 and 1.33×10‒3 cm2/(V s), respectively. The electron mobility in the blend with IDIC is about 1.05×10‒3 cm2/(V s), which is at the same level of the hole mobility. But the electron mobility in the blend with ITIC is one order of magnitude lower, 1.41×10‒4 cm2/(V s). This means the electron transport pathway constructed by IDIC is less energetically disordered than ITIC. Considering the understanding from the optical spectrum, these high mobility values in both the pristine polymer film and blend films strongly suggest the high order of the polymer chains and IDIC acceptor in the films, which is preferred for OPV devices.



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Figure 3. (a) SCLC hole and electron mobilities of the blended films. GIWAXS patterns of (b) pristine PBODT film, (c) PBODT:ITIC blend, (d) PBODT:IDIC blend, (e) line-cut profile along the out-of-plane direction, and (f) line-cut profile along the in-plane direction.

2.5 Morphology Grazing incidence wide-angle X-ray scattering (GIWAXS) was performed to elucidate the crystallinity characteristics for pristine PBODT, ITIC, and IDIC films and the relevant blend films. 12 ACS Paragon Plus Environment

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The 2D patterns for the pristine PBODT film and the blend films are shown in Figures 3b−3d while the 2D patterns for pristine ITIC and IDIC films are shown in Figure S5. The structure information for coherence length (CL) and d-spacing is summarized Table S4. As shown in Figure 3b, three distinct (n00) diffraction peaks were found in the out-of-plane (OOP) direction, corresponding to an interchain separation distance of 22.22 Å, and a (010) peak was observed in the in-plane (IP) direction with a corresponding π−π stacking distance of 3.61Å are observed in the scattering pattern of the pristine PBODT film, indicating that PBODT chains adopt an edge-on orientation. When blended with a non-fullerene acceptor ITIC or IDIC, the crystallinity of polymer PBODT in the blend film is reduced to different extent (Figure 3e and 3f). For the characteristic OOP (100) peaks of PBODT, its CL is 13.4 nm in pure film, but PBODT:ITIC and PBODT:IDIC blend films show reduced CL values of 6.1 and 7.9 nm, respectively (Table S4). Furthermore, both ITIC- and IDIC-based blend films also exhibited reduced CL values of ~2.7 nm for in-plain (IP) (010) peaks if compared with 4.2 nm for the pristine PBODT film (Table S4). On the other hand, the pristine IDIC film shows much more pronounced OOP (010) peak than the pristine ITIC film, revealing the high face-on order of the IDIC film (Figure 3e and Figure S5). In addition, the IDIC-based blend film also showed larger CL value of 3.6 nm for OOP (010) peak if compared with the 2.1 nm for the ITIC-based blend film. The higher cystallinity observed for the IDIC-based film is in good agreement with its higher SCLC electron mobility. Very interestingly, in the blend, polymer adopts an edge-on orientation while the acceptor adopts a face-on one. That suggests separated donor domains and acceptor domains, from which somehow efficient charge separation and collection can still occur.

2.6 Charge Generation and Recombination Analysis 13 ACS Paragon Plus Environment


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The charge recombination in device is studied by the analysis of the light intensity dependence of Jsc and Voc. In Figure 4a, Jsc is fitted to a power law of light intensity, i.e. Jsc ∝ Ps. The power slope of PBODT:IDIC is nearly close to unity, which means the bimolecular recombination is highly suppressed at short circuit point.47,

48

Less recombination together with the balanced hole and

electron mobility therefore provide the possibility to gain the high FF close to 70%. The power slope of PBODT:ITIC is slightly smaller but also close to unity, so the lower electron mobility does not cause serious bimolecular recombination in the device, also showing a fairly good FF of 66.4%. Moreover, the light intensity dependent Voc values were also recorded to provide independent and complementary information compared with that obtained from Jsc.48, 49 In Figure 4b, a distinguished difference is observed and the slope for the ITIC based device is 1.66 KbT/q, indicating a combination of recombination kinetics of monomolecular and bimolecular processes. Thus at open circuit condition, monomolecular recombination can still happen in the ITIC-based device, which should be a main factor to limit its PCE. In the case of IDIC-based device, a slope of 0.97 KbT/q is fitted, which clarifies the fairly limited monomolecular recombination. Consequently, the dominating recombination at open circuit condition is bimolecular recombination, which is consistent with most report of high performance polymer solar cells. Thus for the future development, suppressing monomolecular recombination is quite necessary for realizing efficient charge transfer for small driving force system. Correspondingly, high mobility donor and acceptor materials are very important.


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(a) PBODT/ITIC s=0.964 PBODT/IDIC s=0.989

2 Jsc (mA/cm )

10

1 10

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2 Light intensity (mW/cm )

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0.84 10

100

2 Light intensity (mW/cm )

(c)

1.0

Relative PL Intensity (a.u.)

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


ITIC PBODT/ITIC IDIC PBODT/IDIC

0.8

0.6

0.4

0.2

0.0 700

800

900

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Figure 4. (a) Jsc vs light intensity, (b) Voc vs light intensity, and (c) sensitive PL spectra. Sensitive photoluminescence measurement was carried out to check the quenching extent in these two blend films from acceptors since those from polymer were nearly identical (Figure S6). The PL spectra of the blend films are measured relative to the pure acceptor materials, as shown in Figure 4c. The excitation wavelength is 670 nm, and thus the acceptor is the only material excited. 15 ACS Paragon Plus Environment


The PL quenching extent of the IDIC emission by the donor is ~80%, while a lower quenching efficiency of ~70% is observed for ITIC, suggesting that the dissociation of excitons generated in the acceptor domain is more efficient at the PBODT:IDIC interfaces. Compared with the IDIC case, the ITIC-based active layer exhibits lower PL quenching efficiency and the additionally monomolecular recombination, all strongly suggesting that the charge transfer at the donor/acceptor interface might be a problem. We tried to compare the crucial driving forces for the PBODT:ITIC and PBODT:IDIC active layers in PSC devices.

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0.8 0.6 0.4 0.2

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curve, (c) PBODT:ITIC blend film under different injection current density, and (d) PBODT:IDIC blend film under different injection current density.

The low energy range of normalized EQE spectra measured by Fourier-transform photocurrent spectroscopy (FTPS) is shown in Figure 5. The spectra of devices based on the pristine ITIC film and the PBODT:ITIC blend film show overlapped spectrum tail down to ~1.5 eV (Figure 5a). For the blend film based device, there is no additional sub-bandgap absorption band whereas such a band can be observed for the pristine film based device. This absence of the CT absorption band is probably due to extremely poor CT state population and the negligible driving force.50−52 The corresponding EL spectra can be found in Figure S7. The PBODT:ITIC blend film exhibits one main peak located at 796 nm, which comes from ITIC (789 nm, Figure S1). Moreover, there are two additional shoulder peaks located at 880 nm and 1004 nm, respectively. When we measure the EL spectrum of the blend at different injection current density (Figure 5c), we can find a significant decrease of the peak intensity at 1004 nm, suggesting this peak comes from the sub-bandgap state/CT state.53 As shown in Figure S8, the injection-current-dependent EL spectrum of pristine acceptor device were tested for reference. The peak at 880 nm is more like a shoulder peak from ITIC itself, which is quite consistent with the PL of ITIC (i.e. one main peak and a weak shoulder peak in the emission spectrum). The emission peak of CT state in ITIC-based device is covered under the tail of the lowest singlet state emission peak of ITIC, and the more intensive emission is from ITIC itself. This strongly indicates that the CT state is very close to the singlet exciton state of ITIC. Therefore the driving force is rather small (The energy diagram of CT state and driving force is shown in Figure S9 in Supporting Information). Different from the device based on ITIC, in the case of IDIC blend, the FTPS spectrum reveals an additional distinguished absorption tail manner 17 ACS Paragon Plus Environment


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besides the simple exponential tall off from singlet IDIC absorption band, which can be assigned to the CT band tail (see Figure S10). Using the Gaussian fitting, this CT state is found out to be 1.43 eV (Figure 5b).54 Therefore, the driving force is calculated to be ~0.28 eV. In the study of EL spectrum, the PBODT:IDIC blend film shows a broad EL peak centered at 988 nm and a weak emission peak of 756 nm. The corresponding current density-dependent EL spectra were also recorded and depicted in Figures 5c and 5d. The shorter wavelength emission peak can be assigned to the IDIC emission (Figure S7), while the longer wavelength emission should be assigned to the CT state peak. The singlet state and the CT state are visibly separated in the absorption (FTPS) and emission (EL) study. The results demonstrate that changing the acceptor from ITIC to IDIC increases the driving force. As depicted in Figure S11, EQEEL data shows PBODT:IDIC blend has one order of magnitude lower value than that for PBODT:ITIC blend (10−4% versus 10−3%, also see insets in Figure 5c and 5d). The derived non-radiative energy loss is found out to be 0.34 eV for the IDIC blend and 0.29 eV for the ITIC blend.55 For ITIC based device, this loss is close to that of inorganic solar cell and also similar to that of reported OPV systems with very small energy loss.18 Then we examine the EL spectra for pristine films of PBODT, ITIC, IDIC, and for blend films of PBODT:ITIC and PBODT:IDIC (Figure S7). Polymer PBODT displays a main peak of 661 nm with a shoulder of 712 nm. PBODT:ITIC blend film exhibits one main peak located at 796 nm, which comes from ITIC (789 nm). Moreover, there are two additional shoulder peaks located at 880 nm and 1004 nm, respectively. When we measure the EL spectrum of the blend at different injection current density, we can find a significant intensity decrease of the peak at 1004 nm, suggesting this peak come from the sub-bandgap state/CT state.53 The peak at 880 nm is more like a shoulder peak from ITIC itself,


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which is in good correlated with the PL of ITIC (i.e. one main peak and a weak shoulder peak emission). Due to the low driving force/high CT state, the emission peak of CT in ITIC based device is covered under the tail of the lowest singlet state emission peak of ITIC, and the more intensive emission is from ITIC itself. On the other hand, PBODT:IDIC blend film shows a broad EL peak centered at 988 nm and a weak emission peak of 756 nm. The shorter wavelength emission peak can be assigned to the IDIC emission, while the longer wavelength emission should be assigned to the CT state peak. The corresponding current density-dependent EL spectra were also recorded and depicted in Figs. 5c and 5d. It should be noted that the CT emission has extremely low quantum yield if compared to pure material.56 Accordingly, the pronounced CT band emission in EL spectra of PBODT:IDIC blend correlates to its larger non-radiative recombination compared with ITIC based device.

3. CONCLUSION In summary, we synthesized a new wide band-gap polymer PBODT with high molecular weight to construct high-performance non-fullerene photovoltaic devices. PBODT displayed strong self-aggregation property, high crystallinity, and high hole mobility, as evidenced by variable temperature UV-vis studies, 2D GIWAXS analysis and transport studies in organic field-effect transistors. Blended with non-fullerene acceptors ITIC and IDIC, PCEs of 7.06% and 9.09% with high Voc values of 1.00 and 0.93 V were achieved in the as-cast devices, respectively, associated with low energy losses of 0.59 and 0.69 eV, respectively. In addition, when processed with a non-chlorinated solvent, a good PCE of 8.19% was still realized. The FTPS-EQE and the detailed EL study show PBODT:ITIC has a very small driving force for charge separation while that for PBODT:IDIC is relative high. Together with the photovoltaic performance, the PL quenching 19 ACS Paragon Plus Environment


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experiment and the light intensity dependence of Jsc and Voc, we clarify that the insufficient driving force is possibly the reason for the strong monomolecular recombination in the PBODT:ITIC. A higher driving force via the use of IDIC as acceptor was achieved which leads to increased Jsc and a high PCE. Higher non-radiative energy loss is found for the IDIC-based blend from EQEEL measurements, which is responsible for part of the energy loss in device. Consequently, suppressing the non-radiative recombination is still a very important issue for material development. It should be noted that, in the blend, the bimodal texture of donor domains and acceptor domains can still exhibit somehow efficient charge separation and collection. This work is also an effort to explore promising wide band-gap donors toward high photovoltaic performance under non-toxic processing condition.

ASSOCIATE CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Experimental details for synthesis of monomers and polymer, coherence length of pristine polymer and the relevant blend films, EQEEL measurement including calculation of non-radiation recombination, FTPS-EQE measurement including the fitting of charge-transfer state.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] [email protected] [email protected] 20 ACS Paragon Plus Environment

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Author Contributions ‡ These authors contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENT We gratefully acknowledge the financial support of National Natural Science Foundation of China (51403064, 51673070, 21225418, 51521002, 91633301, and U1401244), the National Basic Research Program of China (973 program 2014CB643505), China Postdoctoral Science Foundation (2016M602466), Natural Science Foundation of Guangdong Province (2016A030312002), and GDUPS (2013). We thank Feng Gao for helpful discussions. Part of this work was performed at the Molecular Foundry as a user project, and the GIWAXS experiments were conducted at BL7.3.3 at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, all supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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A Highly Crystalline Wide-Band-Gap Conjugated Polymer toward High

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