Cracking perylene diimide backbone for fullerene-free polymer solar cells

doi:10.1016/j.dyepig.2016.01.034

Dyes and Pigments

Volume 128, May 2016, Pages 226-234

https://doi.org/10.1016/j.dyepig.2016.01.034 Get rights and content

Highlights

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Three naphthalimide dimers with different bridge were synthesized.
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These compounds were used to fabricate non-fullerene polymer solar cells.
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Effect of bridge units on optical and electronic properties was investigated.

Abstract

Through cracking perylene diimide by inserting a single bond, small π-conjugation unit thiophene and large π-conjugation unit indaceno[1,2-b:5,6-b′]dithiophene, three molecules were synthesized and used as electron acceptors for solution-processed polymer solar cells. These three compounds showed excellent thermal stability with decomposition temperatures over 370 °C. As the size of inserted conjugation units increased, the extinction coefficient increased from 2.9 × 10⁴ to 6.5 × 10⁴ M⁻¹ cm⁻¹, the absorption wavelength maximum in thin film red shifted from 360 to 488 nm, the optical band gap decreased from 2.98 to 2.25 eV, the highest occupied molecular orbital energy level was upshifted from −6.18 to −5.67 eV, the lowest unoccupied molecular orbital energy level was downshifted from −3.59 to −3.68 eV, power conversion efficiencies of solar cells based on blends of poly(3-hexylthiophene) donor and these acceptors increased from 0.28% to 2.36%.

Graphical abstract

Introduction

Solution-processed bulk heterojunction (BHJ) [1] polymer solar cells (PSCs) are cost-effective alternative approach for utilizing solar energy due to their attractive advantages, such as light weight, low cost and flexibility [2], [3], [4], [5], [6]. During the past three years, power conversion efficiencies (PCEs) of PSCs based on blends of polymer donors and fullerene acceptors have exceeded over 10% [7], [8], [9], [10], [11], [12], [13]. Novel acceptors were explored in recent years, but still lagged behind donor materials [14], [15], [16], [17], [18], [19]. Fullerenes and their derivatives are still dominant electron acceptors in PSCs because they possess favorable physical and chemical properties, such as high electron affinity and mobility, isotropic electron transport and the capability to form favorable nanoscale networks with electron donors [20], [21], [22]. However, fullerenes have some shortcomings, such as weak absorption in the visible region, limited electronic level tuning and morphology instability [23], [24], [25]. To overcome these existing problems, non-fullerene acceptors with strong and broad absorption spectra and appropriate energy levels were explored. Although novel non-fullerene acceptors with high efficiency over 6% have been designed [26], [27], [28], [29], [30], [31], most of the fullerene-free PSCs have exhibited PCEs below 4% [32], [33], [34], [35], [36], [37], [38], [39].

Perylene diimide (PDI) has been studied for over one century and its derivatives are used as n-type semiconductors for organic electronics [40], [41], [42], [43], [44], [45]. Due to their good light-harvesting property, strong electron-accepting ability and high electron mobility, many promising solution-processed small molecule or polymer non-fullerene acceptors based on PDI have been developed [46], [47], [48], [49], [50]. However, high planarity of PDI backbones and strong intermolecular interaction lead to micrometer scale crystallization of PDIs in blends, which is disastrous to the performance of PDI-based PSCs [51], [52], [53]. To restrict their inherent crystallinity, PDI cores were modified by side chains substituted on their imide nitrogen atoms or on bay-region positions [54], [55]. A series of PDI dimers [56], [57], [58], [59], [60], [61] and star-shaped PDIs were also explored [62], [63], [64], [65], [66]. A few studies developed novel non-fullerene acceptors through cracking PDI backbones and inserting conjugation units [67], [68], [69].

In this work, we synthesized three non-fullerene acceptors (1–3) by cracking PDI backbone and inserting a single bond, small π-conjugation unit thiophene and large π-conjugation group indaceno[1,2-b:5,6-b′]dithiophene (IDT) (Scheme 1). We used compounds 1–3 as electron acceptors and blended with the widely used polymer donor poly(3-hexylthiophene) (P3HT) to fabricate polymer solar cells. We investigated the effects of inserted bridges on absorption, energy level, charge transport, morphology and photovoltaic properties of these molecules.

Section snippets

Molecular modeling

Computational details were presented as follows: Density functional theory calculations were performed with the Gaussian 09 program [70], using the B3LYP functional [71], [72]. All-electron double-ξ valence basis sets with polarization functions 6-31G* were used for all atoms [73]. Geometry optimizations were performed with full relaxation of all atoms in gas phase without solvent effects. Vibration frequency calculation was performed to check that the stable structures had no imaginary

Synthesis and characterization

Scheme 1 shows the synthetic routes towards compounds 1–3 by one-step Stille coupling reaction between compounds 4–6 and N-2-ethylhexyl-4-bromo-1,8-naphthalimide (NMI-Br) using Pd(PPh₃)₄ as the catalyst. All compounds were fully characterized by MS, ¹H NMR, ¹³C NMR and elemental analysis. Compounds 1–3 were readily soluble in common organic solvents such as dichloromethane, chloroform and o-DCB at room temperature due to the solubilizing alkyl substituents. The thermal properties of these

Conclusion

The PDI backbone was cracked by inserting conjugation units and new non-fullerene acceptors 1–3 were synthesized. With increasing conjugation of the bridge between NMI units, the absorption spectra of compounds 1–3 red shifted, the maximum extinction coefficients increased, and the optical band gaps decreased. The HOMO energy levels were upshifted and the LUMO energy levels were downshifted as the conjugation of the bridge between NMI units increased. The best PCEs of devices based on P3HT:1–3

Acknowledgments

We thank the NSFC (No. 91433114, 51261130582, 21504058) for financial support. Supercomputing Center of Chinese Academy of Sciences is acknowledged for molecular modeling.

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Cracking perylene diimide backbone for fullerene-free polymer solar cells

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Molecular modeling

Synthesis and characterization

Conclusion

Acknowledgments

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