Chinese Chemical Letters  2016, Vol. 27 Issue (8): 1283-1292   PDF    
Recent developments of di-amide/imide-containing small molecular non-fullerene acceptors for organic solar cells
Luo He-Weia, Liu Zi-Tongb     
a Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, China ;
b Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Abstract: Non-fullerene organic solar cells have received increasing attentions in these years, and great progresses have been made since 2013. Among them, aromatic di-amide/imide-containing frameworks have shown promising applications. The outstanding properties of them are highly associated with their unique electronic and structural features, such as strong electron-withdrawing nature, broad absorption in UV-visible region, tunable HOMO/LUMO energy levels, easy modifications, and excellent chemical, thermal and photochemical stabilities. In this review, we give an overview of recent developments of aromatic diamide/imide-containing small molecules used as electron acceptors for organic solar cells.
Key words: Diamide-containing small molecules     Diimide-containing small molecules     Non-fullerene acceptor     Organic solar cell     Donor-acceptor molecule    
1. Introduction

Solution-processed bulk heterojunction (BHJ) organic solar cells (OSCs) have received considerable attentions due to their lowcost, light-weight, flexibility and good compatibility with the rollto- roll process for making large area devices [1]. Over the past ten years, extensive efforts have been focused on developing electron donor materials, such as low bandgap donor-acceptor small molecules, oligomers, especially polymers [2]. The bandgap, energy levels can be tuned, and absorption range can be broadened by varying the electron π-conjugated systems. Therefore high power conversion efficiencies (PCEs) above 10% have been achieved by blending the D-A polymers with [6, 6]-phenyl-C71- butyric acid methyl ester (PC71BM) [3].

Compared to various of electron donor materials, the electron acceptors are still limited to fullerenes and their derivatives, particularly [6, 6]-phenyl-C61-butyric acid methyl ester (PC61BM), PC71BM etc. [4]. However, invariable energy levels, limited absorption in the visible region, morphology instability, purity difficulty and high production cost have limited their further applications. Thus, non-fullerene organic acceptors have been widely studied during the past five years due to their diverse structures, tunable energy levels, good absorptions and ease of synthesis etc. [5]. High PCEs up to 11.21% has been achieved recently, which is comparable to fullerene-containing organic solar cells [6a].

[6a] Normally, non-fullerene organic acceptors should have broad and strong absorptions, suitable HOMO/LUMO energy levels, good solubilities and processabilities, and high electron mobilities. For this, conjugated electron push-pull structures are often used to construct non-fullerene acceptors, which could reduce the optical bandgap, extend the absorption and tune energy levels. For example, trifluoromethyl substituted or fluorinated pentacene derivatives, phthalocyanine drivatives, subphthalocyanine derivatives, quinacridone derivatives, imide derivatives and recently reported indacenodithieno[3,2-b]thiophene derivatives have been used as non-fullerene organic acceptors [5, 6]. Among them, pconjugated molecules with aromatic di-amide/imide-containing frameworks have shown promising applications, which have been widely studied in organic field effect transistors and donor materials in organic solar cells [7]. The outstanding properties of them are highly associated with their unique electronic and structural features, such as strong electron-withdrawing natures, broad absorptions in UV-visible region, tunable HOMO/LUMO energy levels, easy modifications, and excellent chemical, thermal and photochemical stabilities. In this review, we mainly focus on the recent developments of aromatic di-amide/imide-containing small molecules as electron acceptors for organic solar cells since 2013 The related electron donors including oligomers in this review, polymers are listed in Scheme 1.

Scheme. 1. Chemical structures of related electron donors in this review

2. Aromatic di-amide-containing small molecular

non-fullerene acceptors Aromatic di-amide-containing conjugated materials usually possess planar and polar-ring structures, facilitating intermolecular interactions. Normally, the N-positions can introduce alkyl chains to fulfill the molecular solubilities. Additional electron π-motifs can be incorporated to tune the absorption intensities and ranges, energy levels and intermolecular interactions. The most commonly investigated non fullerene acceptors are based on diketopyrrolopyrrole (DPP) and isoindigo for the di-amidecontaining conjugated molecules. Scheme 2 lists representative aromatic di-amide-containing small molecules reported in recent years, and Table 1 summarizes their non-fullerene OSC performances.

Scheme. 2. Chemical structures of representative di-amide-based non-fullerene small molecules

2.1. DPP-based small molecular non-fullerene acceptors

As a synthetic dye, DPP was first discovered in 1974 as a byproduct [8]. The excellent photophysical properties (molar extinction coefficient is up to 25, 000 L mol-1 cm-1 in solutions), high thermal and photo stability have attracted more and more attentions since then. Nowadays, DPP and its derivatives have been widely investigated in organic semiconductors including field effect transistors [9] and solar cells [10], and even chemo/bio-sensors [11].

Normally, DPP-unit is end-capped with thiophenes, called dithienyl-DPP. It owns planar structure because of the hydrogen bonding between oxygen atoms in the DPP units and β-hydrogen atoms of the neighboring thiophenes, which can induce strong intermolecular interactions. The LUMO/HOMO energy levels of dithienyl-DPP is -3.4/-5.3 eV. To fulfill the condition of efficient exciton separations, higher LUMO energy levels above -3.1 eV of conjugated materials could only be used as electron donors (P3HT is normally used, see Scheme 1). For that, DPP is usually attached to another DPP unit or other conjugated units, which could not only lower the LUMO energy levels, but also extend the UV-vis absorption to longer wavelength. Thus the reported DPP-based non-fullerene acceptors are consisted of at least two DPP units [12-20]. According to the molecular configurations, they can be divided into linear, two-dimensional (2D) and three-dimensional (3D) structures. The structures are shown in Scheme 2 and the data are collected in Table 1.

Table 1
Optoelectronic properties and device performances of di-amide-containing non-fullerene acceptors

In 2013, Lin et al. reported linear non-fullerene acceptor A1 (Scheme 2) with DPP as arms and dibenzosilole unit as core. It exhibited strong and broad UV-vis absorption from 300 to 700 nm for thin films, and appropriate HOMO/LUMO energy levels (-3.28/-5.30 eV) matching with P3HT. Therefore, blend thin films of P3HT/A1 showed PCEs up to 2.05% by solvent annealing (o-dichlorobenzene) [12]. Later, Patil et al. reported compound A2 with 9, 9-dioctyl-9H-fluorene as core instead of dibenzosilole. A2 showed very high maximum extinction coefficient of 134, 000 L mol-1 cm-1 at 599 nm and broad absorption from 400 to 700 nm. Although blend thin films of P3HT/A2 showed very high open-circuit voltage (VOC) up to 1.1 eV, the PCE was only 1.2% due to the low JSC (short circuit current density) of 2.42 mA/cm2 which may be due to the un-efficient light absorptions and poor thin film morphologies [13]. Li et al. attached two electronegative thiophene-2-carbonitrile groups as the terminal units (A3), the LUMO of which was lowered to -3.65 eV and the bandgap was also lowered. Thus the absorption edge of A3 was extended to 757 nm compared to A2. The absorption characteristics of A3 film matched well with P3HT (400-600 nm), which is beneficial for enhancing photo absorption. Thus relatively high JSC of 6.25 mA/cm2 was achieved, the blend thin film of P3HT/A3 exhibited PCE up to 2.37% [14]. Shi et al. reported A4 with shorter alky-chain fluorene ring as core and two benzene rings as terminal-groups. Large dihedral angles above 22° exist between DPP and aromatic six-membered rings. Due to the bad molecular coplanarity, the active layer exhibited fine phase separation domains when blending with P3HT. Thus P3HT/A4 blending thin films exhibited relatively high PCE up to 3.17%. Interestingly, A4 could also be used as electron donors, PCE of 3.26% was achieved after blending with PC71BM [15]. Except electron donors, Raynor et al. also used strong electron acceptor benzo[c][1, 2, 5]thiadiazole as core and DPP as two arms to obtain compound A5. Although the thin film absorption was extended to 1000 nm, the P3HT/A5 blending thin film exhibited low PCE of 1.16% [16].

Except linear conjugated moieties, 2D and 3D structures were also reported recently. The 2D and 3D structures are beneficial for(1)forming multiple carrier-transporting pathways;(2)reducing the formation of unfavorable large crystalline domains and aggregations. Yu et al. have reported 2D structure A6 with 1, 3, 5-triethynylbenzene as core. The blended thin film of P3HT/A6 exhibited a high VOC of 1.11 V and PCE up to 1.08% [17]. The poor thin film morphologies have impeded the effective exciton dissociation and charge transport. Sooner, Yang et al. reported two molecules A7-1 and A7-2 entailing a [2, 2]paracyclophane framework instead of 1, 3, 5-triethynylbenzene. Both of them can function as non-fullerene electron acceptors after blending with P3HT. The PCE can reach up to 2.05% and 2.69%, respectively. Interestingly, the linear analogue only exhibited PCE of 0.9% [18]. Later, Xu et al. reported 3D structure A8-1-A8-3 with four DPP units as arms and spirobifluorene as core. The X-shape geometry and twisting structure could impart high solubility and suppress aggregation. The blending thin films of P3HT/A8-1 showed best PCE up to 3.63% with JSC of 6.96 mA/cm2, while the other two compounds (A8-2 and A8-3) with different alkyl side chains exhibited lower PCEs due to the lower crystallinities in their blends [19]. Li et al. modified A8 with four benzene end-capping DPP arms (A9). The benzene groups could help further suppressing aggregations in the P3HT/A9 blends. High PCE of 5.16% with high VOC of 1.14 V was achieved after optimizing the devices, while P3HT/PC61BM blending thin films gave a PCE of 3.18% with VOC of 0.62 V. To be noted, the devices based on P3HT/A9 blends were thermally stable, which remained unchanged after thermal treatment at 150 ℃ for 3 hours [20].

2.2. Isoindigo-based small molecular non-fullerene acceptors

Like DPP, isoindigo (LUMO/HOMO: -3.5/-5.7 eV) is also isolated as a byproduct from certain biological processes. Due to its two lactam rings connected with vinyl group and strong electron-withdrawing character, it has been widely studied in organic semiconductors as OFETs and donor materials in OSCs since 2010 [21]. However, the isoindigo-based non-fullerene small molecular acceptors are seldom reported. In 2015, Hendsbee et al. reported compound A10 which was end-capped with phthalimide. Very low PCE of 0.027% was achieved after blending with P3HT. After changing the electron donor with DTS(FBTTh2)2, the PCE values were increased to 0.43% [22]. By switching from the conventional device structure to inverted structure, PCE values of 1.05% were achieved, indicating the importance of device structure [23]. McAfee further developed non-fullerene acceptor A11, which showed the highest PCE of 1.93% based on isoindigo containing acceptors to date [24]. Besides, Liu et al. designed and synthesized 3D structure acceptor A12 with triphenylamine as a core. The blending thin films only exhibited low PCE of 0.81% [25]. In summary, the isoindigo-based acceptors show worse properties than DPP-based ones. This may be because (1) they show deeper LUMO and HOMO energy levels than DPP-based ones, which need appropriate electron donors other than P3HT and DTS(FBTTh2)2, PBT7 and PTB7-Th with deep LUMO and HOMO may be more suitable for isoindigo-based acceptors; (2) the rigid structure of isoindigo limits its solubility and makes it easy to aggregate, which make the morphology control very difficult. Therefore, more twisting structure and soluble-promoting side chains are needed for future design of isoindigo-based high performance nonfullerene acceptors with appropriate electron donors.

3. Aromatic di-imide-containing small molecular non-fullerene acceptor

Aromatic di-imide-containing conjugated molecules have been widely investigated since the 1990s. They have the following characteristics: (1) strong electron-withdrawing imide group, which further lowers the LUMOs than di-amide-containing materials, and facilitates electron charge transfer; (2) good planar structure and accessible chemical modifications at the aromatic core; (3) the N-positions can introduce both alkyl chains and aromatic chains to modulate the solubility and molecular interactions. Nowadays, aromatic di-imide-containing conjugated materials have been recognized as the most successful n-type organic semiconductors [5d,26] and electron acceptor semiconductors in OSCs [5b,5d,27]. Among them, perylene diimide (PDI) and naphthalene diimide (NDI) are the most representative molecular frameworks [26-28]. Scheme 3 lists representative aromatic di-imide-containing small molecules reported in recent years, and Table 2 summarizes their non-fullerene OSC performances.

Scheme. 3. Chemical structures of representative di-imide-based non-fullerene small molecules

Table 2
Optoelectronic properties and device performances of di-imide-containing non-fullerene acceptors

3.1. PDI-based small molecular non-fullerene acceptors

PDI-based small molecules are among the highest n-type semiconductors and non-fullerene acceptors today owing to their unique properties, such as: (1) strong absorptions in the order of 104 L mol-1 cm-1 of molar extinction coefficient in solutions; (2) broad absorption band ranging from about 400 to 600 nm of single PDI; (3) deep LUMO and HOMO energy levels of the PDI with approximately -4.0 and -6.0 eV, respectively, showing very good electron affinity; (4) easy functionalization from N-position and bay region, facilitating tuning the absorptions, energy levels and thin film morphologies [27].

In fact, the first bilayer heterojunction OSC, reported by Tang, used PDI-based small molecule as electron acceptor and CuPC as electron donor. PCE of 0.93% was obtained, which opens a new era of organic solar cells [29]. However, PDI-based derivatives tend to form long-range ordered aggregates, which lower PCEs. Until 2009, Guo et al. reported the first PDI-based non-fullerene BHJ solar cells based on I1. The blending thin films of P3HT/I1 exhibited very low PCE of 0.25%, which was due to the very serious aggregations of I1 and unmatched energy levels of P3HT and I1 [30]. In 2013, Sharenko et al. used DTS(FBTTh2)2 as electron donor, which exhibits a complimentary absorption spectrum to I1 leading to broad coverage of the visible spectrum ranging from about 300 to 800 nm. Thus relatively high PCE of 3.0% was obtained after device optimization [31]. Chen et al. further optimized the D/A ratio to 1.3/1, yield higher PCE up to 5.13% [32]. Later, Singh reported PBDTTT-C-T/I1 active layer BHJ devices, which showed PCE up to 3.71% [33]. They found that the use of DIO could optimize the electron/hole carrier mobility ratio, suppress the non-geminate recombination losses. To be noted, additives have improved PDIbased non-fullerene OSC performances a lot, which would be discussed below. Hartnett et al. reported acceptor I2 by attaching phenyl groups at the PDI 2, 5, 8, 11-positions. They demonstrated that this could promote slip-stacking of I2 in the solid state, which could result in the inhibition of excimer formation and prevent the coupling necessary for rapid excimer formation. The PBTI3T/I2 blending thin films exhibited PCE up to 3.37% with high VOC of 1.024 V [34]. Cai et al. attached the phenyl groups to the bay position of PDI (I3), which could also help lower the aggregation. By combining with polymer PTB7-Th, I3 exhibited high PCE of 4.1%.

Except the monomeric PDI-based non-fullerene acceptors, multiple PDI-based materials were also reported by using single σ-bond or fused ring linking method. In addition, the multiple PDI backbone could effectively twist the conformation, improve the solubility and reduce the aggregation tendency. Shivanna et al. reported the first PDI dimer non-fullerene acceptor (I4-1) linked through N-N bond. After blending with donor PBDTTT-C-T, they found that both polymer and I4-1 excitons undergo fast dissociation with similar time scales, thus the blends exhibited good PCE of 3.2% [36]. After that, Ye et al. replaced C7H15 with C5H11 to get I4-2. By finely tailoring the alkyl chains, high PCE of 5.40% (certificated result) was achieved by blending with PBDT-TS1 [37]. Another kind of PDI dimer non-fullerene acceptors were also reported by the bay region linkage. Zhan, Yao and their collaborators have reported a series of PDI dimers with thiophene or selenophene at the core and alkoxy groups at both sides, I5-I7 for example [38-41]. The thiophene or selenophene unit was used to increase the LUMO energy levels, and alkoxyl side-chains were used to modulate the mixing thin film morphologies. Molecular modeling indicates that these molecules have highly twisted dimeric backbone with dihedral angles above 50°between adjacent aromatic backbones. Thus they showed significant reduction of aggregations. I5 was first developed in 2013 as a pioneering research. High PCE of 4.03% was achieved by blending with PBDTTT-C-T overwhelming others at that time [38]. They further improved the PCEs to 6.08% by solvent vapor annealing, which led to the improvement of the phase segregation and consequently enhanced the self-aggregation of the PBDTTT-C-T and I5 in the blends [39]. The thiophene and alkoxyl groups could also be changed. By changing thienyl moiety to selenophenyl moiety, PBDTTT-C-T/I6 blending thin films exhibited PCE of 4.01% [40]. Replacing alkoxyl side chains to shorter one could reduce the PCE to 3.28%, but the inverted structure OSCs could reach 4.34% [41]. Other PDI dimers were also reported with various arylene linkers [42, 43]. For example, I8 was obtained by using spirobifluorene linkers. Through effectively suppressing self-aggregation and crystallization, PCE of 2.3% was achieved after blending with P3HT with relatively high FF (fill factor) of 65% [42]. Zhao et al. further changed electron donor to PffBT4T-2DT, which matched very well with I8. Thus high PCE of 6.3% was obtained [43]. Lin synthesized I9 with bulky fused ring indaceno[1, 2-b:5, 6- b']dithiophene (IDT) as core. After blending with P3HT, the devices showed high FF of 66.8%, but relatively low PCE of 2.61%. The PCE increased to 3.12% by changing P3HT to a small DPP-containing molecular donor, BDT-2DPP [44]. Wang and his collaborators have investigated a series of bay region linked (singly, doubly and triply) PDI dimers without any other aromatic cores [45]. By screening the bay region linkage and alkyl modification, I10 with single bond linked PDI dimers exhibited best PCE up to 3.63% after blending with PBDTTT-C-T [45]. Further device optimizations improved the performance up to 5.90% by using inverted device structure [46]. Later, I10 was further modified by inserting two thiophene units in the bay positions to obtain I11. Due to the electrondonating ability of the thiophene unit, I11 owns more twisted configuration (the dihedral angle of two PDI was increased from 67°to 80°), and higher LUMO energy. The resulting PDBT-T1/I11 blend thin films exhibited high PCE of 7.16% and VOC of 0.90 V [47]. Further studies improved the PCE up to 8.22% by changing the electron donors to new developed polymer PBDTS-Se [48]. By replacing thiophene to selenophene, I12 was developed. Very high PCE up to 8.4% was achieved with unprecedented high FF of 70.2% after blending with PDBT-T1. They attributed the high performance to synergistic effects of efficient photon absorption, balanced and high charge carrier mobility and ultrafast charge generation [49]. Although other position modification except N-N single bond and bay region linkage was also reported, the PCE value is still limited [50]. Recently, Wu et al. systematically investigated the influence of molecular geometry by comparing I4-2 with I10. Interestingly, when blending with PTB7, the PTB7/I10 blends showed better PCE than PTB7/I4-2. On the contrary, when blending with PTB7-Th, the PTB7-Th/I4-2 blends showed better PCE than PTB7-Th/I10.High PCE of 6.41% was obtained in the PTB7-Th/I4-2 blends. The study showed that PTB7-Th/I4-2 exhibited better thin film morphology with appropriate aggregation domains facilitating efficient exciton dissociation and charge transport, while I10 was found to disrupt the π-π interactions of PTB7-Th [51]. All these results demonstrate the importance of molecular design, even tiny changing of molecular structures could affect the OSC properties. By fusing the PDI units with C=C double bond, Nuckolls group has reported a series of PDI-dimer (I13), PDI-trimer and PDItetramer (I14) [52]. Due to the steric congestion in the cove areas of PDIs, these compounds exhibited helical conformations. Further studies by using transient absorption spectroscopy reveal that excitons could generate in both the donor and acceptor phases thus improving the photocurrent. High PCE values up to 6.05%, 7.9% and 8.27% (certified result) were obtained for I13, I14 and I15, respectively, after blending with PTB7-Th. Hartnett et al. reported ring-fused PDI dimers (I16, I17 for example), and compared their photophysical properties with their precursors before fusing the ring [53]. The ring-fused structures not only made the conformation more twisted, but also improved the electron mobilities. Thus the PCEs were improved up to 3.44% and 3.89% for I16 and I17, respectively.

Except the PDI dimers, 3D structure PDI trimer and tetramer were also explored for non-fullerene acceptors. In 2014, Lin et al. reported star shaped compound I18 with a triphenylamine core and PDI as arms at bay region. The sp3 hybrid orbital of N atom could induce a quasi-3D non-planar structure, which reduced the aggregation. The blends of PBDTTT-C-T/I18 exhibited PCE of 3.32% [54]. Liu et al. further introduced tetraphenylethylene (TPE) as a core to get PDI tetramer I19. Due to the highly twisted nature of TPE, the 3D four-wing propeller-shape I19 exhibited high PCE of 5.53% after blending with PTB7-Th [55]. Next, the same group used tetraphenylmethane, tetraphenylsilane and tetraphenylgermane as core to construct 3D-structure PDI tetramer I20-I22, which showed PCEs values of 4.3, 4.2 and 1.6% for I20, I21 and I22, respectively [56]. Another group used the same tetraphenylmethane core but different attaching positions (N-position of PDI) to get acceptor I23, only PCE of 2.73% was achieved [57a]. Recently, Wu et al. have reported tetramer I24 by using 4, 8-di(thiophen-2- yl)benzo[1, 2-b:4, 5-b']dithiophene as a core. a-Position PDI was used because it showed better planarity compared to β-position PDI, facilitating close packing of π-conjugated backbone. The resulting device showed high PCE of 8.47% after blending with PTB7-Th, which is the best results for di-imide containing nonfullerene acceptors in OSCs [57b].

3.2. NDI-based small molecular non-fullerene acceptors

As another kind of classic n-type organic semiconductor, NDIbased non-fullerene small molecular acceptors are less investigated, which are due to its large optical bandgap (3.1 eV, HOMO/ LUMO:-6.7/-3.6 eV) and weak absorption in the visible spectrum. To overcome this, usually electron donating groups or strong light absorption groups were introduced to the NDI skeleton through the N-position or aromatic position. In 2011, Ahmed et al. reported monomeric NDI-based acceptor I25 attached with oligothiophene. The thin film absorption was extended to ~800 nm, but it showed very weak absorption at around 500 nm. As a result, blending thin films of P3HT/I25 showed PCE up to 1.5% [58]. Mao et al. reported 2, 6-dialkylamino-substituted NDI derivative I26, which extended light absorption to ~700 nm. PCE of 0.96% was achieved by adding 0.2 vol% DIO [59]. Patil et al. used DPP to improve the absorption and reported I27, which showed broad and strong absorption ranging from 400 to 1000 nm. PCE up to 1.02% was obtained after blending with P3HT [60]. Wang et al. reported NDI dimers I28 and I29 with thiophene and benzodithiophene as core, respectively. They exhibited PCE of 1.31% and 1.24%, respectively, after blending with PBDTTT-C-T [61]. Gupta et al. used dibenzosilole as a core, the resulting compound I30 showed PCE up to 1.16 after blending with P3HT [62]. Liu et al. reported I31 with vinyl as a core. It showed good electron mobility, and thus good PCE of 2.41% was achieved after blending with PTB7 [63].

3.3. Others

Except PDI and NDI derivatives, other di-imide-containing small molecules were also reported. Jenekhe group has developed a series of tetraazabenzodifluoranthene diimides (BFI) and used them in n-type semiconductors [64-67]. These molecules showed high electron affinity, large rigid and planar structure and good solution processability etc. I32 was first developed with monomeric BFI skeleton. Preliminary result showed that blending thin films of PSEHTT/I32 showed PCE of 1.80% [64]. Next, they developed BFI dimer I33, which exhibits twisted structure with dihedral angle of 33° similar to that of PDI dimer. Further study showed that it had superior charge photogeneration. The conventional and inverted OSCs showed high PCE of 4.24% and 5.04% after blending with PSEHTT, respectively [65]. By varying the arylene linkers with selenophene, 3, 4-dimethylthiophene and 3, 6-dimethylthieno[3,2-b]thiophene, more twisting structures were obtained for 40° of I34, 62° of I35 and 53° of I36, respectively. I35 showed the highest OSC properties with high PCE of 6.37% after blending with PSEHTT [66]. Another new di-imide structure, called bis(naphthalene imide)diphenylanthrazoline, was also developed by Jenekhe group. The acceptor I37 gave efficient solar cell properties after blending with multiple polymer donors [67].

I38-I43 with two phthalimide or naphthalimide groups could not be called di-imide materials strictly. However, due their similar structures and potential non-fullerene OSC properties, we would like to give a brief introduction [68-73]. Bloking et al. reported diphthalimide- containing compound I38 with benzothiadiazole as a core, thin film blends of P3HT/I38 exhibited PCE of 2.54% [68]. Similar structure I39 exhibited PCE of 2.40% after blending with small molecular donor DPP-Py [69]. Jinnai et al. systematically investigated a series of phthalimide-containing compounds with different alkyl side chains and aromatic groups. The best PCE of 2.05% was achieved based on P3HT/I40 blends [70]. Kwon et al. reported di-naphthalimde-containing compound I41 with dicyanodistyrylbenzene as a core. The bulky naphthalimde group could suppress the aggregation effectively. The P3HT/I41 blends exhibited PCE of 2.71% [71]. Energy levels could be changed obviously by adding two methoxy-group on the backbone (I42), which made it matched well with DTS(FBTTh2)2 electron donor. High PCE of 5.44% was achieved based on the DTS(FBTTh2)2/I42 blends, while only 1.45% (PCE) was obtained by blending I42 with P3HT [72]. Chatterjee introduced strong electron group (naphtha[ 1, 2-c:5, 6-c']bis[1, 2, 5]thizdiazole) in the di-naphthalimide system (I43) and successfully decreased the bandgap to 1.73 eV. The OSC devices exhibited PCE up 2.81% by blending I43 with P3HT [73].

4. Conclusion and outlook

In this review, we give a brief introduction of recent developments of non-fullerene electron acceptors for organic solar cells, on the basis of aromatic di-amide/imide-containing small molecular acceptors. From the beginning of 0.93% in 1986 [29], high PCE of 8.47% has been achieved today for di-amide/ imide-containing systems, which is more and more close to the fullerene-based solar cells. This is due to the outstanding properties of di-amide/imide-containing π conjugated skeletons, such as (1) strong electron-withdrawing natures; (2) broad absorptions in UV-visible region; (3) easy and variable modifications to tune the HOMO/LUMO energy levels, relatively weak electron acceptors (DPP-based acceptors) may be suitable for wide band gap donors (P3HT), and strong electron acceptors (PDI-based acceptors) should be suitable for narrow band gap donors (PTB7, PBDTTT-C-T and PTB7-Th etc.); and (4) excellent chemical, thermal and photochemical stabilities. Of course, rational molecular design, through considering the planar and twisting structures synergistically to modulate the BHJ thin film phase separation, morphology and charge carrier mobility, are also important reason for high performance OSCs containing di-amide/imide-containing acceptors. All these make them among the most potential candidates for high performance non-fullerene OSCs.

However, the non-fullerene acceptors, not just indicated in this review, also meet challenges. As listed in Tables 1 and 2, based on the three parameters (VOC, JSC, FF) determining the PCE, the VOCs are well presented, and even comparable to perovskite solar cells. This demonstrates the advantages of non-fullerene acceptors with tunable HOMO/LUMO energy levels. The high VOC could be achieved as long as the energy levels of donor (HOMOs) and acceptor (LUMOs) are matched. However, the JSC and FF are low, most of them are below 10 mA/cm2 and 50%, respectively. According to previous research [2], JSC is mainly related to the BHJ film absorption (matching the solar spectrum), charge transport efficiency, thin film morphology and crystallinity, and FF is mainly related to charge transport efficiency, thin film morphology and crystallinity. Since most of non-fullerene acceptors show stronger and broader absorptions than fullerene derivatives, low electron-mobility, improper phase size and separations should be the major drawbacks. However, there is a tradeoff between the mobility (usually need long range ordered π-π interactions, and intermolecular aggregations) and nanoscaled phase domains, which make the molecular design difficult and "unpredictable". Except that, the donor/acceptor interface should also be considered, which could minimize the nongeminate loss and thus improve the FF and JSC. Recently, there are studies show that the alky side-chains could not only improve the solubility, but also help modulating the phase segregations, donor-acceptor charge separation efficiency at the interface and thus the OSC properties [74]. In all, for high performance nonfullerene OSCs, from the molecular design view, both the conjugation backbone and the flexible side chain design should be considered. Of course, the optimization of OSC devices are very important, and the device structure, interface contacting, additives, post-treatments et al. should be considered synergistically. Besides, the device stability should also be considered for future applications of organic solar cells. Finally, due to the varieties of diamide/ imide-containing molecular acceptors and their unique merits, we believe that high efficiency non-fullerene OSCs can be anticipated in the near future.

Acknowledgments The present research was financially supported by NSFC (Nos.21190032, 21372226) and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA09020000).
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