Abstract
1. Introduction
Owing to the flexibility, portability, transparency, and low manufacturing cost, polymer solar cells (PSCs) have great potential for commercial production and application[
From 2011–2014, plenty of NF-SMAs have been designed with various electron-withdrawing groups such as indanedione[
Figure 1.Chemical structures of (a) early reports of A-NF-SMAs, (b) early reports of small molecule acceptors.
The asymmetric design of the molecule helps increase the dipole moment and dielectric constant and reduces the exciton binding energy, which is very beneficial to exciton dissociation and charge transport[
Generally speaking, Voc and Jsc are two iconic device performance parameters, and the increase of the value of Voc ×Jsc generally shows the improvement of device performance. Further optimize the value of Voc × Jsc through asymmetric molecular design to obtain higher device performance, which provides the latest research ideas and unique insights for PSCs research[
Figure 2.(a) A typical current–voltage
There have been related review articles summarizing the latest development of A-NF-SMAs, but its research content focused on the PSCs with a five-membered indaceno[1,2-b:5,6-b']dithiophene (IDT) series of A-NF-SMAs[
This review summarizes asymmetric non-fullerene polymer solar cells (A-NF-PSCs)-related content from three aspects: the structural advantages of the asymmetric system, the development history, and the research results in recent years. Meanwhile, we have also made a particular outlook on future research prospects, hoping to provide some guiding insights on the development of PSCs in the future.
2. Photophysical properties of OSCs
The structure and mechanism of photovoltaic devices have been discussed and studied in many excellent studies[
Figure 3.Simplified schematic of photoconversion in OSCs with the processes of photon absorption, exciton diffusion, exciton dissociation by charge transfer, and charge carrier collection denoted. Reproduced with permission from Ref. [
Figure 4.Chemical structures of polymer donors.
The photophysical characteristics of OSCs are generally represented by current density–voltage curves (J–V)[
(1) Voc
Under light conditions, Voc is the voltage at which the positive and negative poles of the OSCs are in an open circuit state, that is, the maximum output voltage of the OSCs[
(2) Jsc
Jsc is the current per unit area when the positive and negative electrodes of the OSCs are in a short-circuit state, that is, the maximum output current density of the OSCs in the light environment. As the band-gap of the material decreases, the value of Jsc increases and can be affected by the electron and hole transport efficiency of the active material[
(3) Fill factor (FF)
FF is a dimensionless physical quantity, which is the ratio of the maximum output power (Pmax) of the OSCs device to the product of Voc and Jsc, which can be expressed by Eq. (1).
FF is mainly related to the transfer and collection process of charge carriers. The balance of hole/electron mobility in the active layer, the degree of recombination during carrier transport, and the collection efficiency of carriers reaching the buffer layer will all affect the FF value of OSCs. The FF suggests how swiftly the charges can be removed from the cells and the ideal value is 1.0. Several factors can affect the FF of OSCs and they often interact in intricate ways[
(4) Power conversion efficiency (PCE)
PCE is the percentage of incident light energy converted into effective electric energy, which can be expressed by the ratio of Pmax of OSCs to incident power (Pin). In this case, the current density and applied bias voltage are expressed by Jmax and Vmax respectively, the calculation formula of PCE is as follow[
It can be seen that the PCE is jointly determined by Voc, Jsc, and FF, and represents the ability of OSCs to convert solar energy into electrical energy.
(5) External quantum efficiency (EQE)
The ratio of the number of electrons that can be collected under a certain wavelength of radiation to the number of incident photons at that wavelength can be expressed by the following Eq. (3):
Among them, λ is the wavelength of the incident light, and Pin is the power of the incident light. EQE is the product of light absorption efficiency, exciton diffusion and dissociation efficiency, charge transfer efficiency, and charge collection efficiency in the photoelectric conversion process. The EQE and Jsc can be mutually verified, and high EQE is the prerequisite for realizing high-efficiency OSCs[
3. Asymmetric non-fullerene acceptors based on A–D–A structures
Previous studies on A-NF-PSCs have been reviewed and published[
3.1. A-NF-SMAs with asymmetric cores
The structure of A-D-A is usually composed of one electron-donating unit and two electron-withdrawing units. This push–pull electron behavior in the molecule contributes to ICT so that the molecule has a strong transition dipole and wide spectral absorption range[
In the related research on the asymmetry of the core unit, the dipole moment and arrangement of the molecule can be adjusted by changing the number of asymmetric thiophene units and introducing heavy atoms. In the core unit of electron donation, the IDT core and the heptacyclic indacenodithieno[3,2-b]thiophene (IDTT) core play an important foundational role in the design of high-performance non-fullerene receptors, a new type of asymmetric ladder-type thiophene-phenylenethieno[3,2-b]thiophene-fused (TPTT) core combines the structural features of IDT and IDTT. At the same time, the π conjugate length of TPTT and the electron-donating capacity are all between IDT and IDTT. TPTT-IC (its structure is the same as T-TT), TPTT-2F, T-TT-4F, T-TT-4Cl, IDT6CN and IDT6CN-M are all molecules designed based on the TPTT core (Fig. 5). They have terminal groups which are different from each other. Their maximum absorption wavelengths range from 693 nm to 808 nm, and their LUMO energy levels range from –3.37 to –4.04 eV[
Figure 5.Chemical structures of A–D–A asymmetric non-fullerene acceptors without nitrogen.
In the combination of monothiophene and trithiophene, extending the IDT nuclear conjugation length can expand the spectral absorption, shift the LUMO energy level up, and improve electron mobility. At the same time, it can also enhance the π–π stacking between molecules. Compared with PSCs containing dithiophene-based molecules, the PSCs containing these trithiophene NF-SMAs TPTTT-2F, α-IT and MeIC1 (Fig. 5) have improved PSC performance parameters, including Voc, Jsc and FF[
The asymmetric structures of 4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b]thiophene[3,2-b]thiophenealt-[5,6-d]dithieno[3,2-b:2’,3’-d]thiophene (IDT8) core give full play to extended conjugation and asymmetric core advantages. This asymmetric structure can not only expand the spectral absorption and shift the LUMO energy level up but also increase the dipole moment and fine-tunes relevant parameters. Gao et al. designed and synthesized two asymmetrical small molecule acceptors (IDT6CN-M and IDT8CN-M) (Fig. 5). The pure film for IDT8CN-M and IDT6CN-M comparable absorption coefficient of 1.35 × 105 and 1.29 × 105 cm–1 with distinguishing maximum absorption wavelength values of 699 vs. 693 nm. It can be seen that IDT8CN-M has a long conjugation length and a large maximum absorption wavelength[
Compared with thiophene-based NF-SMA, selenophene-based NF-SMA has received much less attention, and the performance of the related device is also poor. To promote the absorption of non-fullerene acceptor small molecules, reduce their bandgap, and increase their LUMO energy level, Sun's research group designed an asymmetric small molecule acceptor SePT-IN by introducing a single-sided selenium atom into the core unit of the symmetric small molecule acceptor TPT-IN (Fig. 5)[
Wang et al. and Gao et al. designed and synthesized A-NF-SMAs with the thienobenzodithiophene structure as the core TBDB-Na, TBDB-Ph, and a-BTTIC (Fig. 5)[
The electronegativity of nitrogen and sulfur atoms are 3.04 and 2.58, respectively. Compared with non-N-functionalized molecules in an asymmetric system, the introduction of nitrogen can further adjust the molecules' dipole moment. As shown in Fig. 6, the change of the sulfur atom to a nitrogen atom in the middle of the trithiophene unit determines the design of the dithieno[3,2-b:2',3'-d]pyrrole (DTP) unit. The ability to take advantage of sp2 hybrid nitrogen is the most prominent feature of the DTP unit. Besides, the lone pair of electrons on the vertical π orbital can delocalize along the π orbital of the molecule. Extending the delocalized π-electron system can narrow the optical energy bandgap (Egopt) and strengthen the π–π stacking between molecules[
Figure 6.Chemical structures of A–D–A asymmetric non-fullerene acceptors with nitrogen.
In contrast to C-type IPTT-2F, S-type IPT-2F and IPTTT-2F (Fig. 6) mixed membranes have fewer traps to avoid recombination, conducive to proper phase separation and formation of better active layer morphology. When using PBDB-T as a polymer donor, the PCEs of PSCs based on IPT-2F and IPTTT-2F were 14% and 12.3%, respectively, which were higher than those of PSCs based on C-type molecule IPTT-2F. Later, Yang et al. designed and synthesized four molecules with different aggregation characteristics IPT-4F, IPT-4Cl, IPTBO-4F, and IPTBO-4Cl (Fig. 6) by introducing different N-alkyl chains and terminal groups[
After introducing the N atom, Cao et al. combined selenophene and DTP building blocks to prepare an asymmetric heptacyclic electron-donating core DTPPSe[
Guo et al. and Luo et al. synthesized S-type seven-ring IDTP-4F and C-type eight-ring IDTTP-4F based on DTP and realized the adjustment of molecular conformation by changing the number of thiophenes on the right side (Fig. 6). IDTP-4F and IDTTP-4F both have good planar backbone conducive to intermolecular π–π packing[
Cao et al. introduced two-dimensional (2D) conjugated side chains into IPT core. They developed a 2D conjugated fused ring core semiconductor IPT2F-TCl (Fig. 6) as an electron acceptor for high-efficiency PSCs. The extended conjugate length in the ring core can optimize device performance[
In addition to changing the number of thiophene and introducing nitrogen and selenium atoms, the asymmetric design of the molecules can also be achieved by flipping the configuration for A–D–A-based non-fullerene acceptors. Luo et al. developed a novel NF-SMA ITCNTC (Fig. 7) with an asymmetric core by adjusting the configuration of thiophene[
Figure 7.Chemical structures of other asymmetric non-fullerene acceptors based on the A–D–A structure and the molecules that are associated.
Jiao et al. designed and synthesized an A–D–A typed acceptor CC10 with asymmetric donor units by introducing alkylbenzene units into CC5 (Fig. 7)[
Figure 8.(a) Chemical structure of CC5 and CC10. (b) Optimized geometries and the corresponding intermolecular binding energies by DFT calculations of CC5 and CC10 dimers. Reproduced with the permission of Ref. [
Among various core transformation engineering methods for designing asymmetric core units based on symmetrical analogs, the simplest method is to construct structural asymmetry by cutting off the side chains of the symmetric core. Many studies have been conducted on acceptors (ITOTC, ITUTC, ITUIC, IEPC, IOPC, IETC, IOTC, PhITBD, MeITBD, ITDI and CDTDI) that match to donor PBDB-T, and acceptors (PhITBD, TIDT-BT-R2, TIDT-BT-R6, ITBR, ITBRC and ITBC) match to donor PTB7-Th (Fig. 7). Zhang et al. synthesized three A–D–A type A-NF-SMAs (ITOTC, ITUTC and ITUIC)[
The molecules (TIDT-BT-R2 and TIDT-BT-R6) with the same molecular core and different terminal groups to PhITBD were reported by Bai et al.[
3.2. A-NF-SMAs with asymmetric terminal groups
Different terminal groups can induce the permanent dipole moment of A–D–A typed A-NF-SMAs molecules to regulate intermolecular interactions, leading to a more diverse aggregation tendency of the molecules affecting the π–π stacking, crystallization characteristics, and final photovoltaic performance[
ITIC is a typical NF-SMA with the advantages of easy synthesis, strong absorption in the visible light region, adjustable energy level and good stability. Adjusting terminal group substituents is a recent research hotspot in asymmetric terminal group strategy. Lai et al., Aldrich et al., Li et al., and Gao et al. used ITIC as the main body and changed the terminal substituents (chlorine, fluorine, hydrogen, and methoxy) to synthesize four different asymmetric molecules α-ITIC-2Cl, ITIC-2F (its structure is the same as a-IT-2F), ITIC-3F (its structure is the same as IT-3F), and a-IT-2OM (Fig. 9)[
Figure 9.The chemical structures of asymmetric non-fullerene acceptors are based on asymmetric terminal groups and their molecules.
Ye et al. synthesized an A-NF-SMA, named IDTT-2F-Th[
With dithienocy-clopentaindenoindene (ZIT) as the core, Zhang et al., through one-pot Knoevenagel reaction, synthesized ZITI-m, a mixture of A-NF-SMA ZITI-3F and S-NF-SMA ZITI-4F (Fig. 9)[
These great performances of PSCs based on molecules with asymmetric terminal groups reveal that it is meaningful to adjust the molecular polarity and stack morphology through asymmetric modification of the terminal groups.
3.3. A-NF-SMAs with asymmetric side chains
Side-chain engineering is crucial for device performance, which can alter crystallinity, miscibility, and intermolecular interactions. A suitable side chain can endow the acceptor molecules with good solubility so that the blended film has an appropriate nanoscale phase separation. It can also prevent small molecules from forming hydrogen aggregates and increase the charge transport rate.
Alkyl and alkaryl groups are generally used as side chains of A-NF-SMAs. Compared to acceptors with alkyl aryl groups, alkyl-substituted acceptor molecules have a shorter π–π stacking distance than alkyl aryl groups. Feng et al. synthesized two A-NF-SMAs IDT-OB and IDTT-OB (Fig. 10) through side-chain engineering[
Figure 10.Chemical structure of asymmetric non-fullerene acceptors based on asymmetric branched chains and the molecules associated with them.
Lee et al. synthesized A-D-A-type side-chain asymmetric small molecules p-IO1 and o-IO1 (Fig. 10)[
Chen et al. designed an A-NF-SMA TOBDT (Fig. 10) based on the benzo[1,2-b:4,5-b']dithiophene (BDT) fused central core with asymmetrical alkoxy and thienyl side chains[
It can be seen that the introduction of asymmetric side chains can increase the solubility of acceptor molecules, make the acceptor molecules densely packed in a dislocation manner, and form good phase separation and good device performance.
Table 1 covers the performance parameters of the acceptor–donor–acceptor (A–D–A) typed A-NF-PSCs.
4. Asymmetric non-fullerene acceptors based on A–D–A–D–A structures
A molecule with an A–D–A–D–A structure as the main body has two donor units and three acceptor units, which will cause this type of molecule to have a wider ultraviolet-visible absorption range than the A–D–A type structure. A–D–A–D–A type molecule has more donor units and acceptor units than the A–D–A type molecule, which can provide more frontier molecular orbitals to receive electrons from excited donors[
4.1. A-NF-SMAs with asymmetric cores
Imitating the design idea of A–D–A molecules, Cai et al. designed and synthesized two new Y-series non-fullerene acceptors Y21 and Y22 (Fig. 11), with asymmetric cores and used them in the study of PSCs[
Figure 11.Chemical structures of asymmetric non-fullerene acceptors based on A1–D–A2–D–A1 structure and the molecules that are associated with them.
Figure 12.(a) Molecular conformation of TB-4Cl and Y6. (b) Chemical structures of TB-4Cl and Y6. Models of (c) TB-4Cl-2T2, (d) TB-4Cl-1T1, and (e) Y6-dimer in front view and side view. Reproduced with the permission of Ref. [
4.2. A-NF-SMAs with asymmetric terminal groups
Different terminal groups are applied to the A–D–A–D–A structure molecules, and Liu et al. developed three kinds of A-NF-SMAS by replacing the fluorine atoms on the terminal groups of Y6 with chlorine atoms (Fig. 11), namely SY1 (two F atoms and one Cl atom), SY2 (two F atoms and two Cl atoms) and SY3 (three Cl atoms). Meanwhile, Y6 with four fluorine atoms substituted terminal groups and Y6-4Cl with four chlorine atoms substituted terminal groups were synthesized as control molecules[
Luo et al. developed a new asymmetric small molecule with one terminal group of BTP-4F and one terminal group of BTP-2ThCl, namely BTP-2F-ThCl (Fig. 11)[
4.3. A-NF-SMAs with asymmetric side chains
The branched alkyl chain has a significant influence on the solubility of the molecule and morphology of the mixed film, which will make the final device have different properties. Chen et al. applied asymmetric alkyl and alkoxy substitution strategies to the most advanced y-series non-fullerene acceptors and obtained a type of A-NF-SMA named Y6-1O (Fig. 11)[
The study of A–D–A–D–A typed asymmetric non-fullerene small molecules thoroughly explored the critical role of molecular conformation regulation on morphology and device efficiency, which has important guiding significance for the molecular design of NF-SMAs.
Table 2 shows the performance parameters of A–D–A–D–A typed A-NF-PSCs.
5. Conclusion and future outlook
For A–D–A typed asymmetric non-fullerene small molecules:
a) In the asymmetry of the core, molecules with DTP units show great potential. The device fabricated based on A-NF-SMA TPIC-4Cl with DTP unit obtained a PCEmax of 15.31%, which is the highest PCEmax in the A–D–A-type A-NF-PSCs so far. b) The "one-pot synthesis of mixed materials" strategy provides new ideas for molecular design, which can significantly reduce the device complexity of the traditional ternary strategy. ZITI-m is a mixture of A-NF-SMA ZITI-3F and S-NF-SMA ZITI-4F synthesized by the "one-pot Knoevenagel reaction". Compared with ZITI-3F and ZITI-4F, the PSC of the hybrid materials ZITI-m and J71 shows a very high PCEmax (13.65%). c) Side-chain asymmetric molecules are expected to further improve device performance, and research in this area should receive more attention. So far, reports on side-chain asymmetry only include 6 molecules IDT-OB, IDTT-OB, P-IO1, O-IO1, TOBDT and Y6-1O. Relative to core asymmetry and terminal group asymmetry, the number of studies related to these molecules is very small. However, the PCEmax values of these molecular-related devices are 10.12%, 11.19%, 10.80%, 13.10%, and 11.30%, which are all greater than 10%, showing great potential in the field of PSCs.
For A–D–A–D–A typed asymmetric non-fullerene small molecules:
a) Substituting thiophene for the benzene ring at the fluorinated IC terminal to form a new terminal T-IC, and using both the fluorinated IC terminal and the T-IC terminal group in the asymmetric A unit in A-NF-SMA has broad application prospects. The BTP-2F-ThCl synthesized by this move and the PSC fabricated after blending with the donor PM6 have reached the best PCEmax (17.06%) of the recent binary device. b) Fabrication of ternary PSCs also is a promising strategy further to improve the photovoltaic performance of binary PSCs. This article refers to the PCEmax of PSCs of the ternary blend (PM6:Y6:BTP-S2) was 17.43%, which improved the binary device's performance of PSCs based on PM6:BTP-S2 and PM6:Y6 (PCEmax = 16.37%, 15.79%).
In short, the asymmetric design of the molecule has the following advantages:
1) It helps increase the dipole moment and dielectric constant of the molecule and reduces the binding energy of excitons, which is very beneficial for exciton dissociation and charge transport. 2) The asymmetric structure design will also fine-tune the molecular energy level to adjust the Voc further. The influence on the absorption range and absorption intensity will cause the Jsc to change. The Voc × Jsc value can be further optimized through the asymmetric molecular design, resulting in higher device performance. 3) The effect on molecular aggregation and molecular stacking can directly change the microscopic morphology, phase separation size, and the active layer's crystallinity.
However, the synthesis of A-NF-SMAs is more complicated and costly, which is a big problem that scientists will face in the future. The box chart of PCE distribution for A-NF-SMAs is shown in Fig. 13, which records the latest developments in A-NF-SMAs based on different structures. The structural modularity of small molecules facilitates molecular tailoring and property regulation. This advantage can enable the continuous development of A-NF-SMAs in interface engineering, shape control and device structure optimization research, which will further promote the development of PSCs.
Figure 13.The box chart of PCE distribution for A-NF-SMAs based on different structures.
Acknowledgments
The authors acknowledge financial support from the National Key R&D Program of "Strategic Advanced Electronic Materials" (No.2016YFB0401100), the National Natural Science Foundation of China (Grant No.61574077), Major Program of Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China (No.19KJA460005) and Natural Science Foundation of Jiangsu Province (BK20170961).
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