Simultaneous Increase in Open-Circuit Voltage and Efficiency of

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Simultaneous Increase in Open-Circuit Voltage and Efficiency of FullereneFree Solar Cells through Chlorinated Thieno[3,4-b]thiophene Polymer Donor Huan Wang, Pengjie Chao, Hui Chen, Zhao Mu, Wei Chen, and Feng He ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00551 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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ACS Energy Letters

Simultaneous Increase in Open-Circuit Voltage and Efficiency of Fullerene-Free Solar Cells through Chlorinated Thieno[3,4-b]thiophene Polymer Donor

Huan Wang†, Pengjie Chao †, Hui Chen†, Zhao Mu†, Wei Chen#, * and Feng He† *

†

Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055, P. R. China

#

Materials Science Division, Argonne National Laboratory, 9700 Cass Avenue, Lemont, Illinois, 60439, United States

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

Avenue, Chicago, Illinois, 60637, United States

Corresponding Author *E-mail: [email protected] (F.H.), E-mail: [email protected] (W.C.).

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Abstract: The chlorinated polymer, PBTCl, have been found to be an efficient donor in non-fullerene PSCs, which showed a blue-shifted absorbance compared to that of its fluorine analog (PTB7-th), and resulted in a more complementary light absorption with non-fullerene acceptor, such as ITIC. Meanwhile, chlorine substitution lowered the HOMO level of PBTCl, which increased the open-circuit voltage of the corresponding polymer-based devices. The 2D GIWAXS analysis illustrated that the PBTCl/ITIC blend film exhibited a “face-on” orientation and scattering features of both PBTCl and ITIC, suggesting that the blend of PBTCl and ITIC was phase separated and formed individual crystalline domains of the donor and acceptor, which promoted charge transfer in the bi-continuous film and eventually elevated the solar energy conversion efficiency. The PBTCl-based non-fullerene PSC exhibited a maximum PCE of 7.57% with a Voc of 0.91 V, which was an approximately 13% increasing in the PCE compared to the fluorine-analog-based device.

TOC graphic:

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Solution-processed bulk heterojunction (BHJ) 1 polymer solar cells (PSCs) have attracted considerable attention because of their advantages of low cost, lightweight, and flexibility.2-5 The most efficient PSCs are fabricated using a bulk heterojunction structure with a p-type conjugated polymer as the donor and an n-type semiconductor as the acceptor.6-8 In past few decades, classic fullerene derivatives, especially PC61BM and PC71BM, have dominated the role of electron acceptor materials. However, fullerene-based materials have some drawbacks, such as large energy loss (over 0.6 eV),

9,10

poor absorption in the visible region and costly preparation and

purification.11,12 Hence, non-fullerene acceptors have emerged as promising alternatives to fullerene-based acceptors in recent years. Currently, impressive photovoltaic performances have been achieved from fullerene-free PSCs with polymers or small molecular acceptors containing naphthalene diimide (NDI),13,14 perylene diimide (PDI),15-17 or indacenodithiophene (IDT) 18-20 units as core structures. The power conversion efficiencies (PCEs) of fullerene-free solar cells have exceeded 13.1%, 21 surpassing the best results of their fullerene counterparts (11.7%).6 To achieve optimized device performances, a broad and strong absorption spectrum is a prerequisite to make the greatest use of solar photons. Zhan and coworkers first reported the IDT-based low-bandgap non-fullerene acceptor ITIC (Figure 1),18 which has a low band gap of 1.59 eV and strong absorption band from 500 to 750 nm, as the acceptor material. When PTB7-th (Figure 1) was used as a donor, possessing an absorption with strong overlap with ITIC, the device exhibited a PCE of 6.8%. Although this device possessed a relatively high PCE, the moderate open-circuit voltage (Voc) and the short-circuit current density (Jsc) are limitations of pursuing a further improvement of the PCE. For many efficient donor polymers, halogenation is an effective method to

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modify the energy levels and the band gaps of the polymers.22,23 Fluorination has been applied to enrich the diversity of organic semiconductors for over ten years due to its advantages, such as lowering the highest occupied molecular orbital (HOMO) levels, providing more planar backbones and exhibiting more ordered packing modes.24-27 However, based on previous report, fluorinating conjugated polymers have the drawbacks of low yields in the fluorine exchange reactions and severe aggregation.2830

Consequently, it is necessary to develop alternative methods, such as chlorination,

for use in organic solar cells. Compared to fluorinated ones, chlorinated organic polymers are simpler to synthesize with higher yields. At the same time, benefitting from the bigger atomic radius of the chlorine atom, the intermolecular interactions in Cl-substituted polymers will be partly restrained both in the pristine and blend films, resulting in changes to the band gaps and blue-shift of the absorption spectrum in chlorine-substituted polymers. Using the most popular non-fullerene acceptor, ITIC, the blue-shift from Cl substitution will enhance the absorption of the blend film from sunlight, eventually pushing the PCE to a higher value. In this contribution, a D-A conjugated chlorinated polymer PBTCl was selected as a donor in a non-fullerene PSC with a low-bandgap n-OS ITIC as an acceptor. The polymer PBTCl was reported in a previous study and showed a moderate PCE of 4.75% in a fullerene-based device.31 According to the absorption spectrum of the blend films, PBTCl was substantially more complementary with ITIC than fullerene acceptors. Therefore, the donor-ITIC blend films had more favorable optical absorption than PC71BM blend films. Comparing the absorption of the donor to that of PTB7-th, chlorinated polymer donor, PBTCl showed obviously blue-shifted features both in solution and in a thin film. Moreover, the HOMO level of PBTCl decreased, which resulted in an enlargement of the open-circuit voltage in the devices. The

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photovoltaic performances of the polymers in inverted cells were characterized and compared in parallel. The polymer PBTCl with chlorine exhibited a maximum PCE of 7.57% in an ITIC-based non-fullerene PSC. The chlorine-substituted polymer was confirmed as a promising method to tune the energy levels of conjugated polymers. These results indicate that PBTCl is a promising medium-bandgap polymer donor material for non-fullerene PSCs.

Figure 1. (a) Molecular structures of donors PTB7, PBTCl, and the acceptor ITIC. (b) Energy level diagrams for the donors and ITIC. (c) Solution absorption spectra of the donors and ITIC. (d) Film absorption spectra of the donors, ITIC and their blends.

The absorption spectra of the donor PBTCl in chloroform solution and in a film, as well as the reference spectra of the donor PTB7-th, the acceptor ITIC and their blends, are shown in Figure 1c-d. All the polymers showed distinct and intense absorption band I (300-500 nm) and band II (500-800 nm). Additional strong absorption peaks of the donor PBTCl in the wavelength region from 500 nm to 800 nm were observed at

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632 nm, and the absorption onset of the strongest peak correspond to an optical band gap of 1.71 eV. Compared to PTB7-th, polymer PBTCl showed obviously blueshifted features both in solution and in a thin film. This is because chlorine has a much larger atomic size than fluorine, making the backbone of Cl-TT less planar than F-TT.31,32 For the acceptor ITIC, the blue-shifted polymer could possess a more complementary optical absorption than fullerene acceptors, such as PC71BM, in the vis-NIR region, which could further improve the photovoltaic performance. The electronic energy levels of the donors were investigated by electrochemical cyclic voltammetry using Ag/AgCl as a reference electrode and the Fc/Fc+ couple as an internal reference.33,34 Figure 1b shows the energy level diagrams for the donors and ITIC. The LUMO level of PBTCl was estimated to be -3.55 eV, which was a higher LUMO level than that of PTB7-th (-3.72 eV) because fluorine is a stronger electrophile than chlorine.35 The HOMO level of PBTCl was calculated to be -5.43 eV, which was lower than that of PTB7-th (-5.29 eV). This is probably because the empty 3d orbitals on Cl can accept π-electrons from the conjugated core, while there are no empty orbitals with proper energy levels on F for such delocalization.23 Previous studies on c comparable or even deeper energy levels than fluorinated materials because of the ability of chlorine to accommodate more electron density than fluorine. The lower HOMO energy level was favorable to achieve better stability of the corresponding polymers and a higher Voc of the corresponding bulk heterojunction (BHJ) polymer solar cell. To evaluate the potential of the chlorinated PBTCl as a promising donor material in PSCs and explore the relationship among chlorination, the properties, and the performance,

we

fabricated

PSCs

with

an

inverted

device

structure

of

ITO/ZnO/polymer:ITIC/MoO3/Ag. The device fabrication conditions of the ITIC-

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based PSCs were systematically screened by varying the donor/acceptor (D/A) ratios, different solvent additives and various solvents (see Tables S1-S3, Supporting Information). The optimized condition was 1:1 ratio (D/A, w/w) in chlorobenzene with 0.25% 1,8-diiodoctane (DIO) as an additive for the polymer. Figure 2a presents the current density-voltage (J-V) curves of the PSCs under illumination of AM 1.5G (100 mW cm-2). Table 1 shows the optimized characteristics of these polymers with preferred thicknesses of the corresponding active layers in the inverted PSC devices. The device based on PBTCl:ITIC gave the best performance with a Voc of 0.91 V, Jsc of 14.53 mA cm-2, FF of 58% and PCE of 7.57%, displaying an average PCE of 7.47% for 25 devices. The control device containing PTB7-th:ITIC showed a decent PCE of 6.62% with a Voc of 0.80 V, a Jsc of 14.34 mA cm-2 and FF of 58%, approaching the optimal performance of a device based on PTB7-th: ITIC with a PCE of 6.80% as reported by the Zhan group.18 Obviously, a clear advantage of the

PBTCl-based device is the higher Voc of 0.91 V than that of the PTB7-th-based device (0.80 V). This result is consistent with their electrochemical properties. Compared to the fluorinated PTB7-th, the chlorinated PBTCl possessed a lower EHOMO value (-5.29 eV), indicating the success of our molecular design in replacing F with Cl. The Jsc values of the devices were confirmed using external quantum efficiency (EQE) measurements. Figure 2b indicated a broad photo response from 300 nm to 800 nm. The maximum value for PBTCl reached approximately 69%, indicating efficient photon harvesting and charge collection in the active layers. The Jsc value calculated from the EQE spectrum was 13.80 mA cm-2 for the device based on PBTCl, which is in good agreement with the Jsc value obtained from the J-V curves (within 5%). The hole mobilities of the blend films were measured by the space-charge-limited current (SCLC) in single-carrier devices consisting of ITO/PEDOT:PSS/donor:ITIC

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(1:1)/MoO3/Ag. The plots of current density vs voltage for the devices are shown in Figure S1. The hole mobility of the PBTCl device was determined to be 1.3×10-4 cm2 V-1 s-1.

Figure 2. (a) J-V characteristics of the donor:ITIC (1:1, w/w) solar cells under illumination of AM 1.5G (100 mW cm-2). (b) EQE curves of the donor:ITIC (1:1, w/w) solar cells. (c) Photoluminescence spectra of pure PBTCl (excited at 620 nm), ITIC (excited at 700 nm) and the blend films of PBTCl:ITIC (excited at 620 and 700 nm, respectively). (d) Photoluminescence spectra of pure PTB7-th, ITIC and the blend films of PTB7-th:ITIC (all of three films were excited at 700 nm).

Table 1. Photovoltaic properties of the ITIC-based PSCs based on the polymers under illumination of AM 1.5G (100 mW cm-2).

Donor

Thickness

Voc

Jsc

(nm)

(V)

(mA cm-2)

PBTCl

100

PTB7-th

110

PCEmax FF

(%)

0.91

14.53

0.58

(0.91±0.01)

(14.47±0.29)

(0.58±0.01)

0.80

14.34

(0.80±0.01)

(14.21±0.19)

8


(PCEa)

7.57 (7.47±0.27)

0.58

6.62

(0.58±0.01)

(6.49±0.25)

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a

The average PCE was obtained from at least 25 devices.

Figure 3. Two-dimensional GIWAXS patterns of the films containing PTB7-th (a), PBTCl (b), and their blends of PTB7-th: ITIC/CB (c) and PBTCl:ITIC/CB (d). GIWAXS linecuts in the inplane(e) and out-of-plane(f) direction in the PBTCl:ITIC and PTB7-th:ITIC blend films.

The molecular aggregation and orientation of PTB7-th and PBTCl were studied by grazing incidence wide-angle X-ray scattering (GIWAXS). As shown in Figure 3a and 3b, both PTB7-th and PBTCl exhibited a face-on orientation in the films, and arclike scatterings arising from π-π-stacking occurred at 1.3-1.7 Å-1 along the out-of-

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plane direction (qz), giving average π-π-stacking distances of 4.1 Å. With the substituents changed from fluorine (PTB7-th) to chlorine (PBTCl), the substituents had little influence on the molecular packing in the films. After blending with ITIC, the molecular structures of both PTB7-th and PBTCl remained mostly intact. The 2D GIWAXS pattern of PTB7-th:ITIC/CB mainly showed the scattering feature of PTB7th, indicating that the ITIC molecules were well mixed with the PTB7-th polymer/ However, the pattern of PBTCl:ITIC exhibited scattering features of both PBTCl and ITIC, suggesting that PBTCl and ITIC were phase separated and formed individual crystalline domains of PBTCl and ITIC in the blend films. It is evident that the incorporation of larger-sized chlorine into the backbone increased the intermolecular interactions between PBTCl and ITIC. Taken together, the well-mixed structure of PTB7-th and ITIC is expected to increase charge recombination, while the phaseseparated structure of PBTCl and ITIC is expected to promote charge separation, thereby enhancing the photovoltaic performance. The performance of a PSC is closely related to the film morphology. To investigate the influences of F and Cl from the polymers on the morphology of the active layer in the ITIC-based solar cells, the photoactive layers were analyzed by atomic force microscopy (AFM) and transmission electron microscopy (TEM), as shown in Figure 4. The AFM images (Figure 4a-b) revealed that the blend films had relatively smooth surfaces with root-mean-square (RMS) roughnesses of 1.06 nm for the PBTCl:ITIC blend film and 2.09 nm for the PTB7-th:ITIC blend film. A clear fibrillar structure was observed in the PBTCl:ITIC blend film (Figure 4c) by TEM, which can benefit photogenerated excitons to efficiently diffuse to the D/A interface. We failed to observe the polymer fibers in the PTB7-th:ITIC blend films (Figure 4d). An appropriate thickness of the fibrils can benefit the photogenerated excitons to

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efficiently diffuse to the D/A interface.

Figure 4. AFM height images (5 × 5 µm2) of the blend films containing (a) PBTCl:ITIC and (b) PTB7-th:ITIC. Bright-field TEM images of the blended thin films containing (c) PBTCl:ITIC and (d) PTB7-th:ITIC.

A photoluminescence (PL) quenching experiment was carried out to investigate the exciton-dissociation and charge-transfer behavior in the PBTCl:ITIC and PTB7th:ITIC blend films. Figure 2c-d shows the PL spectrum of the blend films in comparison with those of the pure PBTCl, PTB7-th or ITIC films. Excitation wavelengths of 620 nm for PBTCl, 700 nm for PTB7-th and ITIC were selected based on the maximum absorptions, respectively. When excited at a wavelength of 620 nm, the emission of PBTCl:ITIC blends were quenched 97%, almost the same as that of PTB7-th:ITIC excited at 700 nm (96%), suggesting the electron effectively transfer from the polymers to ITIC acceptor. Considered about the ITIC acceptor, the

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PL spectrum were quenched over 99% in both polymer blends, which means effectively hole transfer from ITIC acceptor to polymer donors. This result confirms the efficient charge transfer between the ITIC acceptor and the polymer donors, which is a prerequisite for achieving a high photovoltaic performance. In conclusion, the medium-bandgap and chlorinated polymer PBTCl was chosen as a donor material for application in a non-fullerene PSC with a low-bandgap n-OS ITIC as an acceptor. The polymer showed blueshifted absorption due to chlorine compared to the absorption of PTB7-th containing fluorine, but the optical absorption of the chlorinated polymer was more complementary with ITIC. The PSC based on

PBTCl:ITIC (1:1, w/w) exhibited a remarkably high PCE of 7.57% with a high Jsc of 14.53 mA cm-2, a Voc of 0.91 V and an FF of 58%. The results highlighted that chlorination is an effective method to finely tune the energy levels of conjugated polymers and improve device performances.

Author Information Corresponding Author: [email protected]; [email protected]

ACKNOWLEDGMENT The authors acknowledge financial support from SUSTech, the Recruitment Program of Global Youth Experts of China, the National Basic Research Program of China (2013CB834805), the Natural Science Foundation of Guangdong Province (2016A030313637),

the

(JCYJ20160504151731734), (ZDSYS201505291525382)

Shenzhen the and

Fundamental Shenzhen the

Key

Shenzhen

Research

Programs

Lab

Funding

Peacock

Program

(KQTD20140630110339343). W.C. gratefully acknowledges financial support from the US Department of Energy, Office of Science, Materials Science and Engineering 12


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Division. The use of the Advanced Photon Source (APS) at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Supporting Information Available: Experiment section about the device fabrication and characterization of morphologies of the polymers; The additional photovoltaic data; Hole mobility measurement and TEM images of the PBTCl:ITIC blend films.

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Simultaneous Increase in Open-Circuit Voltage and Efficiency of

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