Dithienopicenocarbazole-Based Acceptors for Efficient Organic Solar

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Dithienopicenocarbazole Based Acceptors for Efficient Organic Solar Cells with Optoelectronic Response Over 1000 nm and an Extremely Low Energy Loss Zhaoyang Yao, Xunfan Liao, Ke Gao, Francis Lin, Xiaobao Xu, xueliang shi, Lijian Zuo, Feng Liu, Yiwang Chen, and Alex K-y. Jen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13239 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Journal of the American Chemical Society

Dithienopicenocarbazole Based Acceptors for Efficient Organic Solar Cells with Optoelectronic Response Over 1000 nm and an Extremely Low Energy Loss Zhaoyang Yao,†,‡ Xunfan Liao, †,ϕ,‡ Ke Gao,†,‡ Francis Lin,δ Xiaobao Xu,† Xueliang Shi,† Lijian Zuo,† Feng Liu*,ψ, Yiwang Chen*,ϕ, and Alex K-Y. Jen*,†,δ,ξ †

Department of Materials Science and Engineering, University of Washington, Seattle, WA, 98195-2120, United States Department of Chemistry, University of Washington, Seattle, WA, 98195, United States ξ Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong ϕ Institute of Polymers, Department of Chemistry, Nanchang University, Nanchang, 330031, China ψ Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai, 200240, China δ

Supporting Information Placeholder ABSTRACT: Two cheliform non-fullerene acceptors, DTPCIC and DTPC-DFIC based on a highly electron-rich core dithienopicenocarbazole (DTPC) are synthesized, showing ultra-narrow bandgaps (as low as 1.21 eV). The twodimensional nitrogen-containing conjugated DTPC possesses strong electron-donating capability, which induces intense intramolecular charge transfer and intermolecular π-π stacking in derived acceptors. The solar cell based on DTPC-DFIC and a spectral-complementary polymer donor PTB7-Th showed a high power conversion efficiency of 10.21% and an extremely low energy loss (0.45 eV) which is the lowest among reported efficient OSCs.

Bulk-heterojunction (BHJ) organic solar cells (OSCs) as a promising technology for light-electricity conversion have attracted extensive attentions in the past decade.1 So far, the highest power conversion efficiency (PCE) for OSCs has reached ~12% for traditional fullerene-based systems2 and ~13% for systems that are based on burgeoning non-fullerene acceptors.3-4 Unlike that of very efficient monocrystalline silicon solar cells having full spectral coverage of sunlight below 1100 nm,5 the commonly reported non-fullerene OSCs based on indacenodithiophene (IDT) or indacenodithieno[3,2-b]thiophene (IDTT) cores only have limited spectral response within 850 nm. However, it is a daunting challenge to develop efficient electron acceptors that can extend absorption beyond 1000 nm and also have high efficiency simultaneously. This is because the reduced energy gap often accompanies with downshifted lowest unoccupied molecular orbitals (LUMO) or upshifted highest occupied molecular orbitals (HOMO). The shrink of energy gaps usually causes the mismatch of acceptors and polymer donors. Therefore, the successful design of ultra-narrow bandgap (NBG) acceptors requires delicate tuning of energy levels in a relatively tight space. A facile way to achieve NBG is to enhance the intramolecular charge transfer (ICT). However, it is difficult to construct highly electron-rich aromatic rings due to the sensitivity of encountering oxygen or light induced decomposition during their preparations. In addition, it is

also quite challenging to find a proper polymer donor to match the energy levels and absorption of ultra-NBG acceptors. Among all these hurdles, however, it is quite encouraging to find many of the ultra-NBG acceptor based OSCs usually show very high short circuit current densities and low energy losses.4a~4e This gives hope that if delicate energy tuning and high open-circuit voltage ( VOC ) can be achieved in ultra-NBG acceptors, it is possible to achieve very efficient OSCs.

Figure 1. (a) Energy levels evolution of cores calculated by DFT. (b) Structures of two NIR electron acceptors. (c) Optimized geometries and contour plots of frontier molecular orbitals. The values of energy level are provided to make a quantitative comparison. Among many commonly used conjugated building blocks, perylene is an excellent one for photoelectric materials in numerous fields.6 In this communication, a highly electron-rich core DTPC (Figure 1a) was synthesized to construct two ultra-NBG non-fullerene acceptors, DTPC-IC and DTPC-DFIC with 2-(3oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile3a (IC) and 2(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile4b,7 (DFIC) as terminal electron-withdrawing units, respectively (Figure 1b). DTPC as a highly electron-rich core has been envisaged before but its efficient synthesis is still a significant challenge.6b

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Journal of the American Chemical Society For the rational design of ultra-NBG acceptors, density functional theory (DFT) calculations were employed firstly to screen possible candidates of electron-rich units with high HOMO, rigid backbone and proper modification sites. Then, further modifications were conducted to adjust their energy levels and stereochemical structures (Figure 1a). On contrast to the general strategy adapted for increasing conjugation length to improve electrondonating ability (IDT to IDTT), we broadened the central donor unit to two-dimensional conjugation and incorporated a heteroatom nitrogen to upshift the HOMO of the donor significantly. The newly established core not only allows electrons to effectively delocalize along its large ring but also has significantly improved chemical stability. Meanwhile, the extended conjugation and pincer-like structure of acceptors enhance their intermolecular π-π stacking. As a result, DTPC-DFIC shows the smallest optical band gap (1.21 eV) among non-fullerene acceptors. By employing a spectral-complementary polymeric donor PTB7-Th8 (Figure S1), the PTB7-Th:DTPC-DFIC based devices showed a quite high PCE (10.21%) and an extremely low energy loss (0.45 eV). DFT calculation was employed to evaluate the optimal geometries and energy levels.9 As shown in Figure 1b, both acceptors possess highly planar backbones, facilitating efficient ICT.10 The two-dimensional conjugation and nitrogen doping remarkably elevate the HOMO energy levels of DTPC based acceptors. By changing terminal withdrawing segments from IC to DFIC, it stabilizes both LUMO and HOMO by 0.15 eV and 0.11 eV, respectively, generating a narrower energy gap (1.70 eV) for DTPC-DFIC compared with 1.74 eV for DTPC-IC.

of DFIC. The reduction of energy gaps from DTPC-IC to DTPC-DFIC is about 0.03 eV, which will generate a slightly redshifted absorption edge. This tendency is confirmed by our timedependent DFT calculations (Figure S3). The polymer donor PTB7-Th was selected because of its complementary absorption and matched energy levels with both acceptors (Figure 2a, b). Moreover, much smaller energy loss may be achieved by these NBG systems because of their very close HOMO energy levels.11 As shown in Figure 2c, both of DTPC based OSCs display broad spectral response owing to their strong absorption in NIR. It is worth noting that DTPC-DFIC based devices display over 60% EQEs from 500 nm to 900 nm and reach a maximum value of 69% at 650 nm. The currentvoltage (JV) curves were presented in Figure 2d and the detailed parameters were listed in Table 1. The PTB7-Th:DTPC-DFIC OSC with DIO as additive12 shows a high short-circuit current density ( J SC ) of 21.92 mA cm–2, an VOC of 0.76 V, and a fill factor ( FF ) of 61.3%, generating a high PCE of 10.21%. The corresponding energy loss ( ELoss ) of 0.45 eV, which is defined by the formula ELoss = EgOpt  eVoc ,13 is one of the smallest values among all highly efficient OSCs (PCE˃7%) reported so far. For the device of PTB7-Th:DTPC-IC, a VOC of 0.86 V and an extremely low ELoss of 0.42 eV could be obtained. Unfortunately, the J SC was only 8.53 mA cm–2. The sharp contrast of photovoltaic parameters may be due to the diversity of driving force for charge separation and film morphology.

Table 1. Photovoltaic Parameters of the Optimized OSCs (a)1.0 Abs [a.u.]

(b)

PTB7-Th DTPC-IC DTPC-DFIC

0.8

Devices

0.6 0.4

PTB7-Th:

0.2

DTPC-IC

0.0

400

600

800

1000

PTB7-Th:

1200

DTPC-DFIC

 [nm]

(c) 70

DTPC-DFIC (DIO)

-20

PTB7-Th/DTPC-IC PTB7-Th/DTPC-DFIC PTB7-Th/DTPC-DFIC (DIO)

400

600

800

 [nm]

1000

2

60 50 40 30 20 10 0

PTB7-Th:

(d) J [mA cm ]

EQE [%]

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

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-15 -10

0.2

0.4

0.6

VOC

FF

PCE

ELoss

[mV]

[%]

[%]b)

[eV]c)

8.53 (8.36)

863

42.4

3.12 (2.86) 0.42

18.72 (18.19)

771

55.7

8.16 (7.98) 0.44

21.92 (21.27)

760

61.3 10.21 (10.01) 0.45

a) Measured and calculated integrated current density from EQE spectra. b) The best and average PCE values obtained from twenty devices. c) The energy losses.

PTB7-Th/DTPC-IC PTB7-Th/DTPC-DFIC PTB7-Th/DTPC-DFIC (DIO)

-5 0 0.0

JSC

[mA cm–2]a)

0.8

V [V]

Figure 2. (a) Normalized electronic absorption spectra of PTB7Th, DTPC-IC and DTPC-DFIC films. (b) Distribution of HOMO and LUMO energy levels. (c) EQEs curves of herein studied OSCs. (d) J-V curves measured under irradiance of 100 mW cm–2. The electronic absorption spectra in solid state were recorded to assess the light harvesting ability. As shown in Figure 2a, both acceptors have very intense absorption from 720 nm to 970 nm. Noting that DTPC-DFIC has an onset absorption of 1021 nm, corresponding to one of the smallest bandgap (1.21 eV) among non-fullerene acceptors. Electrochemical measurements of DTPC-IC and DTPC-DFIC films in Figure S4 show that both acceptors have relatively high HOMO energy levels (DTPC-IC: −5.21 eV; DTPC-DFIC: −5.31 eV) owing to the strong electrondonating capability of DTPC. The LUMO energy levels, estimated by the difference between the optical bandgap ( EgOpt ) and HOMO, are −3.97 eV and −4.10 eV for DTPC-IC and DTPCDFIC, respectively. The 0.13 eV lower LUMO for DTPC-DFIC can be ascribed to the much stronger electron-withdrawing ability

To investigate the origin of low EQEs for DTPC-IC based devices, time-resolved PL (TRPL) decay was performed (Figure S5). Similar decays were observed for both blended films compared to their pristine films. Therefore, the reduced driving force does not influence the EQEs of DTPC-IC based devices dramatically. As shown in Table S2, the blend films of both acceptors have similar hole mobilities (4.6-6.3×104 cm2 V1 s1). However, much higher electron mobilities of 1.6×104 cm2 V1 s1 and 3.6×104 cm2 V1 s1 for DTPC-DFIC blend films were obtained compared to that of 3.7×105 cm2 V1 s1 for DTPC-IC. The enhanced electron mobility in DTPC-DFIC system can be due to better F-induced molecular packing. Therefore, the DTPC-DFIC blend films display more balanced charge transport ( μh / μe =1.83.1) than that of DTPC-IC (12.4). The unbalanced charge transport may be responsible for low EQEs and FF observed for DTPC-IC. The ordering of acceptors and BHJ films were studied by using grazing incidence wide-angle x-ray scattering (GIWAXS) (Figure 3a, b). The DTPC-IC neat film showed a face-on orientation with  stacking in the out-of-plane direction at 1.72 Å-1 and an amorphous halo at 1.3 Å-1. The (100) stacking in inplane direction was seen at 0.36 Å-1.14 For DTPC-DFIC neat film, quite a few scattering points spreaded across the 2D detector,

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Journal of the American Chemical Society indicating the highly crystalline nature.15 PTB7-Th:DTPC-IC displays a quite normal crystal packing despite of low PCE. Polymer ordering can be directly observed since PTB7-Th (100) packing locates in a slightly lower q region (~0.3 Å-1). DTPC-IC in blends is quite ordered, with a (100) packing shown in both inplane and out-of-plane directions. This BHJ film shows a  stacking at 1.67 Å-1, with the major composition from PTB7Th.16

Figure 3. (a), (b) in-plane (dotted line) and out-of-plane (solid line) line-cut profiles of GIWAXS results; (c) RSoXS profiles of BHJ films. The reduced order in acceptor component can be the reason that causes lower charge transport leading to much lower FF and PCE. PTB7-Th:DTPC-DFIC shows a similar scattering pattern like that observed in PTB7-Th:DTPC-IC. The strong crystallization of acceptor moiety is supressed. While the (100) diffraction is still dominated by PTB7-Th component, the  stacking peak located at 1.70 Å-1, with contributions from the acceptor. Such a feature indicates that DTPC-DFIC can form local contacts to facilitate charge transport. In PTB7-Th:DTPCDFIC with DIO, the structure order of the resulting film is improved.17 As can be seen from the out-of-plane profile, several peaks in low q region associated with DTPC-DFIC were recorded. More importantly, the  stacking shifted to 1.72 Å-1, with a more significant contribution from DTPC-DFIC, explaining the improved electron transport. Phase separation in BHJ film was studied using resonant soft x-ray scattering (RSoXS) methods using 285 eV photon energy (Figure 3c).18 PTB7-Th:DTPC-IC blends showed a high intensity scattering profile with a large hump at 0.0045 Å-1, corresponding to a distance of 140 nm. Such a length scale of phase separation is quite large, together with a low electron mobility, a low FF and PCE. For PTB7-Th:DTPC-DFIC, without DIO, a quite weak RSoXS was observed, indicating quite good mixing between donor and acceptor. DTPC is a large aromatic core with different chemical nature from PTB7-Th, as a result, a strong phase separation is expected. However, the decorated F atoms change the material interaction behavior. It is suspected that F atom can induce molecular dipoles, which could interact with PTB7-Th to form tightly-bonded aggregates to suppress the strong crystallization tendency of DTPC-DFIC. With DIO, the longer dwelling time of BHJ film led to diverse kinetic pathways for DTPC-DFIC to gain order, leading to material crystallization. Such a process led to a major phase separation in BHJ thin film, with a scattering peak at 0.011 Å-1, giving a distance of 57 nm. Such an improvement contributed to overall enhancement in devices. It should be noted that this

process is still weak, thus, the scattering intensity is low compared to the phase separation in non-fluorinated analogue. In pursue of high PCE OSCs, it is very challenging to obtain high EQEs with low ELoss simultaneously. The PTB7-Th:DTPCIC system has actually shown one of the lowest ELoss of 0.42 eV, however, a very low EQEmax of ~25%, suggesting inefficient charge transfer. So far, the reported efficient OSC systems generally display an ELoss of higher than 0.5 eV (Figure S9, Tables S3 for details). For our PTB7-Th:DTPC-DFIC system, a high PCE of 10.21% can be achieved with an ELoss of 0.45 eV, which is the smallest value for all efficient OSCs (PCE˃7%) so far. DTPC-DFIC with PTB7 and J61 were further blended to investigate the effect of different driving forces (Figure S10). For DTPC-DFIC:J61, the energy levels may mismatch, resulting in very small current in spite of low energy loss. For PTB7 based device, the VOC decreased 11 mV owing to its upshifted HOMO. However, its J SC and FF didn’t improve along with the enlarged driving force because various factors may participate in influencing the PCE of OSCs such as mobility and morphology of the blend films. In general, the large energy loss will cause the grievous waste of driving force. Too low energy loss is often accompanied with inefficient charge transfer. So the very low energy loss in our system is encouraging for further exploration of balance point to improve the performance of OSCs. In summary, a highly electron-rich core dithienopicenocarbazole (DTPC) and two pincer-like ultra-NBG electron acceptors were developed. The DTPC-DFIC exhibits a very small band gap for non-fullerene acceptors. A quite high PCE of 10.21% and extremely low energy loss (0.45 eV) were also obtained. The rational molecular design and detailed optical, morphological, and charge transport studies pave the way for further designing of ultra-NBG acceptors.

ASSOCIATED CONTENT Supporting Information Experimental details and additional data. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

[email protected];

Author Contributions ‡These authors contributed to this work equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Asian Office of Aerospace R&D (FA2386-15-1-4106) and the Office of Naval Research (N0001417-1-2201). Liu F. thanks the support by the Young 1000 Talents Global Recruitment Program of China. Chen Y. thanks the support from National Science Fund for Distinguished Young Scholars (51425304). Liao X. thanks the State-Sponsored Scholarship for Graduate Students from China Scholarship Council. Parts of this research were conducted at beamline 7.3.3 and 11.0.1.2 at Lawrence Berkeley National Laboratory, which was sustained by the DOE, Office of Science, and Office of Basic Energy Sciences.

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