A Rational Design and Synthesis of Cross-Conjugated Small Molecule

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A rational design and synthesis of cross-conjugated small molecule acceptors approaching high performance fullerene-free polymer solar cells Yan Liu, Gongchu Liu, Ruihao Xie, Zhenfeng Wang, Wenkai Zhong, Yuan Li, Fei Huang, and Yong Cao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01491 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Chemistry of Materials

Yan Liu, Gongchu Liu, Ruihao Xie, Zhenfeng Wang, Wenkai Zhong, Yuan Li*, Fei Huang*, and Yong Cao Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China E-mail: [email protected], [email protected]

ABSTRACT: Considering to combine the advantages of both the acceptor-donor-acceptor and perylene diimide type acceptors, three new cross-conjugated small molecule non-fullerene acceptors were rationally designed and synthesized. Their thermal, optical, electrochemical and photovoltaic properties were systematically investigated. These acceptors exhibit highly extended absorption spectra ranging from 300 nm to 700 nm as well as appropriate lowest unoccupied molecular orbital levels comparing with those of the general acceptors based perylene diimide. Non-fullerene polymer solar cells based on the resultant acceptors were fabricated with the configuration of ITO/PEDOT: PSS/PTB7-Th: Acceptor/PFNBr/Ag. Benefited from the efficient exciton dissociation, reduced bimolecular recombination, enhanced and balanced electron mobility, the device based PDIBDT-IT exhibited power conversion eﬃciency (PCE) of 6.06% with a high short-current density (Jsc) of 13.60 mA/cm2, a fill factor (FF) of 60.45%, an open-circuit voltage (Voc) of 0.74 V, which was higher than those of PDIBDT-RDN and PDIBDT-ITF. The underlying mechanism was carefully studied and discussed. The extended absorption of PDIBDT-IT contribute to its relatively high PCE, however, the performance will be potentially improved via ehnancing the electron mobility of these acceptors. Our results provide an efficient cross-conjugated approach and great flexibility in fine-tuning the physicochemical properties including absorption spectra and energy levels of the acceptors towards high performance solar cells.

Polymer solar cells (PSCs) with conjugated polymers as electron donor applied in bulk heterojunction (BHJ) devices has become one of the most prospective research fields in the development of new materials and new energy considering its advantages of low-cost, light weight, semitransparency, flexibility, and readily large-area fabrication through potential roll-to-roll solution processing,1-8 which has been proved efficient for both fullerene and non-fullerene systems. Endowed with superior electron mobility, isotropic charge transporting properties and formation of appropriate phase separation,9,10 fullerene and its derivatives (such as PC61BM and PC71BM) are still applied as electron acceptors in the efficient PSCs. Generally reviewing the whole field, the development of fullerene derivatives is relatively more mature compared with the nonfullerenes, and the PCE of fullerene-based binary single-junction PSCs has surpassed 11%. 11-16 However, fullerene derivatives material still exhibit many intrinsic shortcomings, such as high production cost, narrow and weak absorption in the visible region, poor environmental stability, difficulty in functionalization and energy level tenability, inferior morphology stability resulted from their readily aggregation properties.1720

In contrast, during the journey to achieve the breakthrough of efficient fullerene-free OSCs, non-fullerene acceptors have caught intense interest because of their advantages of wide

and strong absorption in the visible region, convenient synthesis, finely-tuned energy levels through various chemical modification and combinations to match the donor materials.21 Especially, acceptor-donor–acceptor (A-D-A) type of relatively planar molecule,22 which is composed of an electronrich core and end-capped with different electron-withdrawing moieties like 2-(3-oxo-2, 3-dihydroinden-1-ylidene)malononitrile (IC) 23,24 its fluorine- and alkyl- substituted products (e.g. ICF, ICFF, ICM) 25 - 27 , 3-ethylrhodanine (RDN) 28 - 34 and barbituric acid (BBT)35 has been proved to be the highly effective strategy for high-performance PSC materials. At present, state-of-the-art single-junction PSCs based on A-D-A nonfullerene acceptors have already presented remarkable power conversion efficiency (PCEs) over 14 %.36-39 In addition, it is noteworthy that the PCEs of the ternary cells based on A-D-A non-fullerene acceptors have also been significantly improved close to 15 %.40-42 Moreover, the small molecule A-D-A structure is also a good candidate to be applied as the key building block for non-fullerene polymer acceptors. For instance, Li et al. reported a new low band gap polymer acceptor PZ1 featuring A-D-A structured IDIC-C16 as building block and thiophene as the linking units. The all-PSCs of PZ1 as acceptor material and a 2D-conjugated polymer PBDB-T as donor material exhibited a high PCE of 9.19%. 43 It means that there is still much room for further development of the A-D-A structured materials in OSCs applications.

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Figure 1. Chemical structures of the three acceptors and the electron-donating polymer PTB7-Th.

During the last few years, many electron-deficient building blocks to construct non-fullerene acceptors have been widely studied and reported, including diketopyrrolopyrrole (DPP), thienopyrrolodione (TPD), naphthobisthiadiazole (NT), benzothiadiazole (BT), benzotriazole (BTA), isoindigo, naphthalene diimide (NDI), and perylene diimide (PDI). However, there still remains many opportunities to explore new electron deficient fragments to achieve high performance. In our current work, we chose the PDI as building block of acceptor owing to their appealing properties, such as strong electronwithdrawing ability, high electron mobility, high photochemical stability, convenient preparation and functionalization. 44 , 45 So far, a considerable PCE of over 9% have been achieved for the PDI-based small molecule acceptor in PSCs.46 Our research will focus on the understanding of the chemical structure-property-performance relationship in solar cell devices. Overviewing the three kinds of acceptors mentioned above, we compared them with each other and made a brief conclusion on their advantages and disadvantages. Firstly, for the fullerene acceptor: advantages include high electron mobility and isotropic charge transport, its disadvantages: (1) Absorption is mainly in the ultraviolet region, weak absorption capacity in the visible region, and almost no absorption in the near-infrared region, (2) The chemical structure and energy level cannot be readily modified, thereby limiting the enhancement of open circuit voltage (Voc), (3) Difficult purification process (multiple substitution by-products and isomers), high synthesis costs, (4) poor light stability and thermal stability (easy to dimerize under light and crystallize during film annealing). Secondly. A-D-A non-fullerene small molecule receptors: Advantages: (1) Intramolecular charge transfer effect giving the molecule a strong transition dipole and broad and strong visible or even near-infrared absorption, (2) Easily modification of the molecule structure and to conveniently adjust the energy levels and absorption spectra, (3) Simple synthesis, higher yields, and easy purification, (4) Good light/thermal stability, (5) Generally good solubility and processibility in common solvents at room temperature. Disadvantages: Anisotropy charge transport. Finally, for NDI/PDI type acceptors: Advantages: (1) high electron affinity, (2) large

π-conjugated plane, (3) high electron mobility, (4) strong chemical modification (ortho-, bay- and imide- positions can be readily modified, (5) Weak photo-bleaching, high thermal/chemical stability. Disadvantages: (1) Absorption is mainly between 400-600 nm, relatively narrow, and limited short circuit current, (2) Intensive aggregation in solution due to strong stacking effect. Cross-conjugation is developed as an efficient strategy for the design of both the donors and acceptors for OSCs in our previous work.47,48 Polymeric donors PFDCN and PFPDT featured with an electron-rich conjugated backbone and crossconjugated side chain exhibited photovoltaic properties with a PCE as high as 4.74% using PCBM as the acceptor in the conventional device.47 More recently, our group reported a family of NDI-based cross-conjugated polymer acceptors consisting of donor-acceptor main chain with pendant acceptor groups at the end of side chains. The device fabricated with these NDI-based acceptors and the polymer PTB7-Th as electron donor exhibited a best power conversion efficiency of 5.55 %,48 which presented the feasibility and great potential of the strategy for designing high-performance non-fullerene acceptors. In order to combine the advantages of both the acceptordonor-acceptor and PDI type acceptors, we designed and synthesized a series of novel cross-conjugated small molecule acceptors for non-fullerene polymer solar cells. Herein, we chose the donor unit, benzo[1,2-b:4,5-b’]dithiophene (BDT) as the building block of our small molecule acceptors, which has been proved to be one of the most promising donor units for high-performance OSC materials, owning to its advantages including rigid and planar conjugated structure, regioregularity, readily modification, and high mobility of its related polymers and small molecules.49,50 However, most of the works based on the BDT have been focused on the construction of polymer/small molecular donors,50 and there are only several examples with BDT as building blocks in constructing acceptors.51,52 In this work, the molecular acceptors with electronrich benzo[1,2-b:4,5-b’]dithiophene (BDT) as core flanked with two different kinds of electron-deficient moieties (PDI and RDN, IC, ICF) in four directions, named PDIBDT-RDN, PDIBDT-IT, PDIBDT-ITF were presented in Figure 1, respectively. Combining the PDI unit and the A-D-A structure, we


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Chemistry of Materials Scheme 1. Synthetic Routes for PDIBDT-RDN, PDIBDT- IT, and PDIBDT- ITF

can easily tune the absorption spectra and energy levels by changing the end groups as well as increase the carrier mobility by introducing the perylene imide unit. The synthesis routes of the monomers were shown in Scheme 1, and further experiments indicated that these acceptors exhibit strong and broad absorption in the range from 300 nm to 700 nm as well as appropriate lowest unoccupied molecular orbital (LUMO) levels compared to the general acceptors based perylene diimide. 53 - 58 The absorption spectra of these acceptors matched well with that of PTB7-Th (Figure 2b), and a high PCE of 6.06% with an outstanding JSC of 13.60 mA/cm2 can be achieved for the device based on PDIBDT-IT. The detailed synthesis, characterization including their optical, electrochemical properties and photovoltaic performances as acceptors in OPVs were carefully investigated and discussed.

The chemical structure and the synthetic route of PDIBDTseries SMs are shown in Scheme 1, and the detailed procedures are provided in the Supporting Information (SI). Compound 4,8-bis((triisopropylsilyl)ethynyl)benzo[1,2-b:4,5-b']dithiophene (3) was synthesized by consulting similar literature procedures.59,60 Compound 3 was subsequently lithiated with n-butyllithium (n-BuLi) at low temperature and then the reaction precursor was quenched with trimethyltin chloride to yield the compound ((2,6-bis(trimethylstannyl)benzo[1,2b:4,5-b']dithiophene-4,8-diyl)bis(ethyne-2,1-diyl))bis(triisopropylsilane) (4). The Stille-coupling reaction between compound 4 and compound 5-bromo-4-(2-ethyedlhexyl)thiophene-2-carbaldehyde (5) produced compound 5,5'-(4,8bis((triisopropylsilyl)ethynyl)benzo[1,2-b:4,5-b']dithiophene-

2,6-diyl)bis(4-(2-ethylhexyl)thiophene-2-carbaldehyde) (6). Then compound 6 was treated with a few milliliters of potassium hydroxide solution and methanol in the THF solution at 60 °C for 4 hours to give the compound 5,5'-(4,8-diethynylbenzo[1,2 -b:4,5-b']dithiophene-2,6-diyl)bis(4-(2ethylhexyl)thiophene-2-carbaldehyde) (7) with a high yield of 91%. It is worth noting that the compound 7 must be freshly prepared due to the photochemical instability of the alkyne. Subsequent Sonogashira coupling reaction between compound 7 and compound 5-bromo-2,9-di(undecan-6- yl)anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)tetraone (9) with Pd(PPh3)2Cl2/CuI as the catalyst and chlorobenzene/diisopropylamine(DIPA) as the solvent afforded the precursor molecule PDIBDT-CHO (10). Finally, the target compound PDIBDT-RDN (11), PDIBDT-IT (12) and PDIBDTITF (13) were prepared by the Knoevenagel condensation of dialdehyde compound PDIBDT-CHO with 3-ethylrhodanine (RDN), 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)- malononitrile (IT) and 2-(6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)- malononitrile (ITF), in the yields of 85%, 83% and 85%, respectively. The chemical structure of all the three target molecules was fully characterized by nuclear magnetic resonance spectroscopy (1H- and 13C- NMR) and matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS). All the non-fullerene acceptors exhibit good solubility in common solvents, such as dichloromethane (DCM), tetrahydrofuran (THF), chloroform (CF) and chlorobenzene (CB).

The thermal properties of three PDIBDT-A series molecules were evaluated by thermo gravimetric analysis (TGA) and differential scanning chromatography (DSC). The TGA and DSC



curves were shown in Figure S2 in Supporting Information (SI). The Td (decomposition temperature, 5% weight loss) values of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were recorded as 388.3, 359.3, and 353.7 °C, respectively, which indicated that the thermal stability of the compounds was promising considering their practical application in PSCs. No distinct thermal transition feature was observed in the scanning range of 30-300℃ in the DSC measurements. This result is also consistent with the excellent solubility detected for the three molecules in organic solvents.

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CHCl3 solution and as thin films spun casted on quartz plates, and the corresponding data are summarized in Table 1. The three small molecules exhibited broad absorption bands between 300 nm and 700 nm, which can be attributed to their unique cross-conjugated molecular structure. As shown in Figure S3 in the Supporting Information, Figure 2(a-b), the positions of absorption bands in the three new cross-conjugated small molecule acceptors are seriously affected by introducing different end groups of RDN, IT and ITF, compared to PDIBDT-RDN, the PDIBDT-IT and PDIBDT-ITF acceptors show much stronger absorption. For better attribution of all peaks, the UV−vis absorption of the intermediate compound BDTTIPS-CHO (compound 6) and compound PDIBDT-CHO (compound 10) both in chloroform solution (10−5 mol/L) and

The UV−vis absorption spectra of PDIBDT-RDN, PDIBDTIT, and PDIBDT-ITF were measured in dilute (ca. 1 × 10−5 M)

Table 1. UV–vis absorption, and electrochemical properties of PDIBDT-series acceptors. Acceptors

λabsSol (nm)

λabsFilm (nm)

λonset (nm)

aE opt g

Eox (V)

Ered (V)

bE HOMO

cE LUMO

(eV)

(eV)

(eV)

(eV)

PDIBDT-RDN

330,497,529

344,508

758

1.64

1.23

-0.83

-5.86

-3.81

2.05

PDIBDT-IT

307,499,534

427,550

735

1.69

1.31

-0.66

-5.95

-3.97

1.98

PDIBDT-ITF

308,499,534

427,552

740

1.68

1.28

-0.64

-5.92

-3.99

1.92

a

Calculated from the onset of UV-vis absorption as pristine thin films; =Eox - Ered

bE

HOMO

= –e (Eox + 4.64) (eV);

cE

LUMO

dE CV g

= –e (Ered + 4.64);

d E cv g

Figure 2. UV-vis absorption spectra of all the acceptors in chloroform solution (a) and as thin film (b), cyclic voltammograms of the acceptors (c), energy level diagrams of the materials in photovoltaic device (d).


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thin films were measured (See Figure S3a,b). It can be clearly observed that the solution spectrum of PDIBDT-CHO exhibits four main vibronic peaks between 350 and 570 nm, and compared to the spectrum of BDTTIPS-CHO, a strong 0−0 (I00) absorption peak, weak 0-1 (I01) transition appear in that of the PDIBDT-CHO, which is mainly caused by the introduction of PDI unit. The solution absorption of PDIBDT-IT and PDIBDTITF exhibit stronger 0-0 (I00) absorption peaks than the 0-1 (I01) transition, while PDIBDT-RDN exhibits stronger 0-1 (I01) absorption peak than the 0-0 (I00) transition. The red-shift of the highest absorption peak based on PDIBDT-IT and PDIBDTITF should be related to their strong intermolecular interaction in solid state, leading to the formation of excimer in very dilute solution (10-5 mol/L).61 Similar spectra were recorded for PDIBDT-IT, PDIBDT-ITF, and both acceptors showed strong optical absorption at 450-650 nm both in the diluted CHCl3 solution and as solid thin films. The absorption peaks of the PDIBDT-IT and PDIBDT-ITF films are the same, 534 nm, with the large molar extinction coefficient of 1.49 × 105 M−1 cm−1 and 1.55 × 105 M−1 cm−1, respectively. While PDIBDT-RDN exhibited an absorption band at 400−550 nm with a molecular absorption coeﬃcient of 1.42 × 105 M−1 cm−1 at peak wavelength of 529 nm. In thin films (See Figure 2b), the maximum absorption peaks of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF are redshifted by 11, 16, and 18 nm, respectively, compared with those of their solutions. Moreover, the intramolecular charge transfer (ICT) between the donor and acceptor moieties, and the ππ stacking between molecular backbones of three acceptors are raised with the enhancement of electron-withdrawing ability of the end groups. PDIBDT-IT and PDIBDT-ITF film exhibits a red shift about 42 nm relative to PDIBDT-RDN film because of their stronger electron withdrawing acceptor groups, which is beneficial for the harvest of solar photons to achieve a high short circuit current density (JSC). It is also worth noting that the film absorption range of these resulting non-fullerene acceptors (450-700nm) and polymer donor PTB7-Th (550-770nm) are complementary and favors solar energy harvesting. The optical band gaps of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF are calculated to be 1.64, 1.69 and 1.68 eV from the absorption edge of their thin films, respectively (according to the equation Egopt = (1240/ λonset film) eV).

The electrochemical properties and energy levels of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were investigated by cyclic voltammetry (CV) with ferrocene as an internal reference, tetra-n-butylammonium hexafluorophosphate (n-Bu4NPF6) acetonitrile solution (0.1M) as the supporting electrolyte, a glassy carbon electrode as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum (Pt) wire as counter electrode. The cyclic voltammograms were shown in Figure 2c, and the corresponding results were listed in Table 1. The onset reduction and oxidation potentials (Ered/Eox) of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were -0.825/1.230 eV, -0.664/1.314 eV and 0.641/1.282 eV, respectively. The lowest unoccupied molecular orbital and the highest occupied molecular orbital energy levels (ELUMO/EHOMO) of the molecules can be calculated from the equation: ELUMO = −e [Ered+ (4.80 - EFc/Fc+)] (eV), EHOMO = −e [Eox+ (4.80 - EFc/Fc+)] (eV), assuming the absolute energy level of ferrocene/ferrocenium redox couple (Fc/Fc+) to be 4.8 eV below the vacuum. Therefore, the ELUMO and EHOMO of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were estimated to be -3.81/-5.86 eV, -3.97/-5.95 eV and -3.99/-5.92 eV, respectively. It’s clear that all the molecules exhibit very similar HOMO energy levels (−5.86 to −5.95 eV), which is predominantly determined by their similar molecular structure. However, the end group of PDIBDT-RDN possessed a weaker electron withdraw ability than that of PDIBDT-IT and PDIBDTITF, which results in higher ELUMO of PDIBDT-RDN (-3.81 eV), and maybe leads to a higher open-circuit voltage (Voc) of PSCs with PTB7-Th as the donor material. It was found that PDIBDT-IT and PDIBDT-ITF exhibited similar LUMO and HOMO energy levels, indicating that the incorporation of the fluorine atom does not significantly disturb molecular orbital levels. The electrochemical energy gaps of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were thus determined to be 2.05 eV, 1.98 eV and 1.92 eV, respectively, which are larger than their corresponding optical band gaps in their thin film state.

To understand the effect of the cross-conjugated structure on the photovoltaic performance of the n-OS acceptors, we fabricated polymer solar cell devices with structures of ITO/PEDOT:PSS/active layer/PFN-Br/Ag, where PFN-Br was used to effectively alleviate the interfacial energy barrier.62 66

Figure 3. (a) J-V characteristics and (b) EQE spectra of the all-PSCs.



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Table 2. Photovoltaic performance of the PSCs based on PTB7-Th: Acceptor (1:1, w/w) under a simulated AM 1.5G illumination of 100 mW cm-2. μe(cm2 V-1 s-1)

Active layer (PTB7-Th:A)

Voc (V)

PDIBDT-RDN

0.76(0.759±0.002)

9.28(9.18±0.07)

47.33(47.4±0.3)

PDIBDT-IT

0.74(0.739±0.002)

13.60(12.97±0.34)

PDIBDT-ITF

0.71(0.710±0.002)

12.78(12.43±0.38)

Jsc (mA cm-2)

FF (%)

μh(cm2 V-1 s-1)

PCE (%) pure

blend

blend

3.34(3.30±0.03)

5.25×10-6

2.87×10-5

1.52×10-3

60.45(61.3±0.9)

6.06(5.87±0.09)

7.39×10-6

3.51×10-5

8.96×10-4

51.45(51.5±0.9)

4.67(4.54±0.11)

6.19×10-6

2.55×10-5

1.90×10-3

The values in Table 2 are the average and best of 8 devices.

Figure 4. (a) PL spectra of PTB7-Th neat film and blend films, excited at 550 nm. PL spectra of (b) PDIBDT-RDN neat film, (c) PDIBDT-IT neat film or (d) PDIBDT-ITF neat film and blend film, excited at 550 nm.

In this work, we used Ag as anode instead of Al to fabricate the solar cell devices owning to the stable physicochemical property that still can exhibit excellent antioxidant capacity when vapor deposited in the case of high temperature and vacuum. And Ag exhibits a lower electron injection barrier resulting from the lower work function than Al (-4.6 eV for Ag, -4.3 eV for Al),65,67 which is beneficial for electron injection and extraction, thus obtain efficient photoelectric devices. Furthermore, Ag might possess better conductive properties because of the lower electrical resistivity compared to Al. The

photoactive layer was spin-coated from CB solution, the optimized donor: acceptor weight ratio was determined to be 1:1, and the optimum thermal annealing temperature was 80℃. All the devices were measured under the illumination of AM 1.5 G, 100 mW cm−2. The current density–voltage (J–V) characteristics are showed in Figure 3a and the corresponding photovoltaic parameters (VOC, JSC, FF, and PCE) are summarized in Table 2. The device based on PTB7-Th: PDIBDT-IT exhibited a highest photovoltaic performance of 6.06% with a VOC of 0.74 V, a JSC of 13.60 mA cm-2 and a FF of 60.45. While the device based on PTB7-Th: PDIBDT-ITF exhibited a moderate


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photovoltaic performance of 4.67% with a VOC of 0.71 V, a JSC of 12.78 mA cm-2 and a FF of 51.45, the device based on PTB7Th: PDIBDT-RDN exhibited a low photovoltaic performance of 3.34% with a VOC of 0.76 V, a JSC of 9.28 mA cm-2 and a FF of 47.33. In particular, the PSCs based on the PDIBDT-RDN show a highest VOC of 0.76 V than PDIBDT-IT, and PDIBDTITF, which is primarily due to its high LUMO energy level (VOC is determined by the gap between the LUMO of acceptor and the HOMO of donor). However, the low JSC and FF seriously limit the performance of the PDIBDT-RDN based device. It was realized that PDIBDT-ITF shown a slightly decreased short circuit current and fill factor than PDIBDT-IT, which is not consistent with its relative high absorption coefficient as revealed by UV-vis spectra. We suspect that this may be correlated to its more unbalanced charge transport, exciton recombination, or unfavorable nanoscale phase separation in the photoactive layer. Here, we also used polymer PBDB-T as donor to fabricate PSCs devices. As shown in supporting information (in Table S2-5), devices based on PTB7-Th: Acceptors exhibited a higher JSC than device based on PBDB-T: Acceptors, which is consistent with the complementary absorption between PTB7-Th donor and PDIBDT IT acceptor (Figure S3c) in the supporting information). More information about device performances under different conditions can be found in Table S2-5 in the supporting information. The external quantum efficiency (EQE) spectra of the optimized devices are shown in Figure 3b. The PSCs based on these three PDIBDT-based acceptors showed broad photocurrent response extending from 310 nm to 750 nm because of the effect of the donor-acceptor complementary absorption strategy. The integrated current density was 9.20/13.00/12.52 mA cm−2 for the PTB7-Th: PDIBDT- RDN/ PDIBDT-IT/ PDIBDTITF based devices, which agrees well with the J–V measurements. Although PDIBDT-ITF-based device, in the range of

310-410 and 480-680 nm, showed higher responses than that of PDIBDT-IT and a maximum value of 70% recorded at 540 nm, a general higher and broader photo-to-current response in the range of 410-750 nm were achieved in PDIBDT-IT-based device, resulting in an increased JSC.

To investigate the exciton dissociation and charge transfer behavior in the blend of the polymer donor (PTB7-Th) and the small molecule acceptor (PDIBDT-RDN, PDIBDT-IT, or PDIBDT-ITF), we measured the photoluminescence (PL) spectra of the pure donor/acceptor, and the blend films with the optimized weight ratio. And the photoluminescence spectra were shown in Figure 4(a–d). as we can see from the absorption spectra (Figure 2b), the absorption band was mainly distributed in 500-750 nm for the film of polymer donor PTB7Th, and 450-700 nm for the PDIBDT-A series small molecules. Thus, we chose the light with wavelength at 550 nm for exciting both of the donor and acceptor materials. We found that the PL of the polymer donor PTB7-Th of the three blend films were efficiently quenched by 95.8% for PDIBDT-RDN, 95.2% for PDIBDT-IT, and 95.3% for PDIBDT-ITF, respectively, indicating that effective electron transfer occurred from PTB7-Th to the acceptors in the blend films. Compared with the neat film of the small molecules, the PL of PDIBDT-IT and PDIBDT-ITF in the blend films with PTB7-Th were dramatically quenched by 96.9% and 97.5%, respectively, however, PDIBDT-RDN was just quenched by 93.1%, what means more effective hole transfer from the acceptor PDIBDT-ITF and PDIBDT-ITF to the polymer donor 68 PTB7-Th than that of PDIBDT-RDN. It is can be understood that the lower quench

Figure 5. (a) hole-only of blend films, (b) electron-only of blend films and (c) electron-only of neat films of J1/2–V characteristics; (d) JSC, (e) VOC, and (f) FF versus light intensity (Plight) characteristics for PSCs devices.



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Figure 6. AFM height images (5 μm × 5 μm) and TEM images for PTB7-Th: PDIBDT- RDN, PTB7-Th: PDIBDT-IT, and PTB7Th: PDIBDT-ITF. ing efficiency of PDIBDT-RDN stems from the smaller ΔEHOMO value (0.23 eV),69,70 resulting in the less effective exciton dissociation and charge transfer, which is in good agreement with the Jsc values obtained from PSC devices.

The hole-only and electron-only mobilities of PTB7-Th: acceptor blend films and the acceptor neat films were measured by using the space charge limited current (SCLC) method, with the corresponding J1/2–V characteristics shown in Figure 5 (a-c), which is described by the equation: 𝑉2 9 J = 𝜀0 𝜀𝑟 𝜇𝑒 3 𝐿 8 Where ε0 is the permittivity of free space, εr is the relative permittivity of the organic material, μe is the electron mobility, V is the effective applied voltage, and L is the thickness of the active layer. The hole and electron mobilities were measured using the hole-only devices with structure of ITO/PEDOT:PSS/blend films (or neat film)/MoO3/Ag and the electron-only devices with structure of ITO/ ZnO/ blend films (or neat film)/Ca/Al, respectively. It was found that both of the blend film and neat film based on PDIBDT-IT exhibited the highest electron mobility of 3.51×10-5 cm2 V−1 s−1 and 7.39×10-6 cm2 V−1 s−1, respectively, which are much higher than the other two molecules, and these results are favorable for us to obtain high JSC values in PSC devices.71-73 However, these three acceptors exhibited twist chemical structure (see Figure S1) and their relatively low electron mobility limited the further enhancement of their performances. In addition, the absorption ranging from 500 nm to 700 nm is not so intensive as other efficient A-D-A acceptors. All of these drawbacks of our materials might be resolved in future work to optimize the performance of OSCs.

To understand the dominant charge recombination mechanism of the polymer solar cell devices based on PDIBDTRDN, PDIBDT-IT and PDIBDT-ITF, we investigated the JSC, Voc, and FF versus the light intensity (Plight, vary from 5 to 100 mW cm-2) characteristics for PSCs devices, the relevant characteristics are plotted in Figure 5(d-f). It has been reported that the relationship between JSC and light intensity (Plight) can be expressed by JSC ∝ Plightα, where α should be exponential factor (α=1, there is no charge recombination, and α1 V open circuit voltages. Energy Environ. Sci. 2016, 9, 3783-3793. (30) Wu, Y.; Bai, H. T.; Wang, Z. Y.; Cheng, P.; Zhu, S. Y.; Wang, Y. F.; Ma, W.; Zhan, X. W. A planar electron acceptor for efficient polymer solar cells. Energy Environ. Sci., 2015, 8, 32153221. (31) Badgujar, S.; Song, C. E.; Oh, S.; Shin, W. S.; Moon, S. J.; Lee, J. C.; Jung, I. H.; Lee, S. K. Highly efficient and thermally stable fullerene-free organic solar cells based on a small molecule donor and acceptor. J. Mater. Chem. A. 2016, 4, 1633516340. (32) Holliday, S.; Ashraf, R. S.; Wadsworth, A.; Baran, D.; Yousaf, S. A.; Nielsen, C. B.; Tan, C.-H.; Dimitrov, S. D.; Shang, Z.; Gasparini, N.; Alamoudi, M.; Laquai, F.; Brabec, C. J.; Salleo, A.; Durrant, J. R. McCulloch, I. High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor. Nat. Commun. 2016, 7, 11585-11595. (33) Holliday, S.; Ashraf, R. S.; Nielsen, C. B.; Kirkus, M.; Rohr, J. A.; Tan, C. H.; Fregoso, E. C.; Knall, A. C.; Durrant, J. R.; Nel-


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Table of Contents:

Yan Liu, Gongchu Liu, Ruihao Xie, Zhenfeng Wang, Wenkai Zhong, Yuan Li,* Fei Huang*, and Yong Cao


A Rational Design and Synthesis of Cross-Conjugated Small Molecule

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