A PhD Student Condenses: Fine-Tuning A-D-A Small-Molecule Acceptors

Part of a series titled "A PhD Student Condenses...", where recent academic articles in materials science will be condensed and reviewed by one of Ossila's academic collaborators - Emma Spooner, a PhD student at the University of Sheffield.
Emma Spooner | Ossila - University of Sheffield PhD Student

My name is Emma Spooner and I am currently pursuing a PhD in Fullerene-Free Photovoltaic Devices at the University of Sheffield, in collaboration with Ossila Ltd. As part of this collaboration, I will be running this monthly series where I review recent papers encountered in my research to provide a concise summary for Ossila’s customers.

For the third article in my series, I will be discussing the recent work by Kan et al. entitled 'Fine-Tuning the Energy levels of a Non-Fullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells’1. A brief introduction to non-fullerene acceptors (NFAs) can be found in a previous Ossila article and in the second review in this series, which discusses the promising acceptor, ‘ITIC’. In the paper discussed below, a new small-molecule NFA is designed through molecular optimisation and achieves a power conversion efficiency (PCE) of 12%, an impressive feat for a solution-processed organic photovoltaic cell (OPV).


 

Condensed Summary

Title:  Fine-Tuning the Energy levels of a Non-Fullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Citation: B. Kan et al., Adv. Mater., 1704904 (2017).

DOI: 0.1002/adma.201704904

Learning point: Targeted molecular optimisation can be used to fine-tune the band gap of an acceptor, and its short circuit current (JSC), in some cases without significantly compromising the open circuit voltage (VOC).

 

A-D-A Small-Molecule Acceptors

Typically, polymer-based organic semiconductors (OSCs) are associated with OPVs. However, small molecules are also showing significant promise in terms of NFAs,2–4 and have similarly tunable properties (such as band-gap width). Small molecules can be synthesised from basic chemical ‘building blocks’, and their properties can be tuned as desired by the addition of various chemical moieties, such as strong electron-donating or accepting groups.

Emma's comments: Small molecules can offer easier synthesis5 and processing over polymers, as they have less batch-to-batch variation (caused by variable polydispersity in polymers).6 However, it is also worth noting that in some cases, solution-processing small molecules gives less favourable film quality due to significant aggregation.7 This can be reduced through use of extended side-chains, as in ITIC.

Many of the highest-performing small-molecule NFAs (including ITIC and IDIC) are based on an acceptor-donor-acceptor structure, or ‘A-D-A’, as shown in Figure 1. Here, the donor (or ‘D’ part of the molecule) refers to an electron-donating unit; typically an indacenodithiophene (IDT) core; whilst the acceptor (or ‘A’ part of the molecule) refers to an electron-accepting unit;6 which is 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile (INCN) in both ITIC and the new NFA discussed here, NCBDT.

Emma's comments: Whilst IDT or a derivative is most often used as the core in small-molecule acceptors, there is also an interest in donors based on the A-D-A structure, often using a benzodithiophene core.7,8

 

ITIC A-D-A Structure
Figure 1: The acceptor-donor-acceptor, or A-D-A, structure of ITIC.

 

Molecular Optimisation

When tuning the properties of an acceptor, the part of the molecule that is chemically modified will have a significant impact on the properties that are generated. Whilst the second article in this series discussed side-chain modification of the core, Kan and co-workers used a new fused-ring core structure derived from the basic ITIC form, therefore altering the donor part of the A-D-A structure. There has also been some success altering the acceptor end-capping group in other works.9

In earlier research,10 the authors designed an NFA with this new fused-ring core, known as ‘NFBDT’. To improve the performance of this via narrowing the band gap (and boosting JSC as a result), ‘NCBDT’ was generated by substituting the INCN acceptor unit with a highly electronegative fluorine atom. This is shown in Figure 2, downshifting the HOMO and LUMO levels.

NFBDT Chemical StructureNCBDT Chemical Structure
Figure 2: The structures of NFBDT and NCBDT; non-fullerene acceptors based on a new donor core.

 

Similarly, an electron-donating alkyl group was introduced  to the core to increase the donating ability of the donor unit. This also boosted the HOMO energy level and effectively narrowed the band gap, as illustrated in Figure 3.

Energy levels of other NFAs vs. ITIC
Figure 3: Energy levels for the new NFAs in comparison to that of ITIC.

 

Typically, the energetic offset between the HOMO levels of the donor and acceptor polymers within a bulk heterojunction is considered to be vital to drive charge separation. However, in the case of NCBDT, the reduction in energetic offsets compared to NFBDT appears to have little impact on the photo-physical processes of the acceptor. Both acceptors showed slow but efficient charge generation, and the independence from energetic offset means that both current and voltage losses can be optimised. Further discussion of the impact of energetic offset on photo-physical processes (in terms of the generation of interfacial charge-pair states) can be found within the paper itself.1

Emma’s comments: Typically, JSC and VOC cannot both be optimised, as improving one will come at the expense of the other. Generally narrowing the band gap, as done here, will increase JSC at the expense of decreasing VOC. This can be reduced through use of extended side-chains, as in ITIC.

 

In combination with a PBDB-T donor, the new NFA showed improved JSC and PCE in comparison to NFBDT, with slightly reduced VOC, as predicted through the chemical modification.

NFBDT vs NCBDT: A comparison
Table 1: Comparison between the performance of the two NFAs.

Final Thoughts

Using molecular optimisation to tune the band gap of small-molecule NFAs could be used to extend the absorption range of other acceptors, potentially boosting JSC. Recently, this has been used to great effect to give the record highest OPV PCE of 13.1%,11 using a non-fullerene acceptor, ITIC-2F (available from Ossila). Kan and co-workers concluded that because of the independence of charge-separation efficiency from HOMO energetic offset, the balance between JSC and voltage loss could be optimised further through careful chemical modification, and that higher efficiencies may be still be in reach. Further work optimising VOC12 and JSC13 can be found in recent literature. It is also worth noting that whilst small-molecule NFAs have become a significant area of research, small-molecule donors still lag behind their polymeric counterparts, and all small-molecule OPVs14 may offer processing advantages over the current material set used, in the future.

 

Materials Mentioned in This Paper

If you are interested in conducting a similar experiment, some of these materials are available from Ossila.



 

References

 

  1. Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells, B. Kan et al., Adv. Mater., 1704904 (2017); DOI: 0.1002/adma.201704904.
  2. New developments in non-fullerene small molecule acceptors for polymer solar cells, N. Liang et al., Mater. Chem. Front. (1), 1291-1303 (2017); DOI:10.1039/C6QM00247A.
  3. Recent progress in non-fullerene small molecule acceptors in organic solar cells (OSCs), W. Chen et al., J. Mater. Chem. C (5), 1275-1302 (2017); DOI:10.1039/C6TC05066B.
  4. Key components to the recent performance increases of solution processed non-fullerene small molecule acceptors, S. M. McAfee et al., J. Mater. Chem. A (3), 16393-16408 (2015); DOI:10.1039/C5TA04310G.
  5. Molecular Materials for Organic Photovoltaics: Small is Beautiful, J. Roncali et al., Adv. Mater. (26), 3821-3838 (2014); DOI: 10.1002/adma.201305999.
  6. Acceptor–Donor–Acceptor Small Molecules Based on Indacenodithiophene for Efficient Organic Solar Cells, H. Bai et al., ACS Appl. Mater. Interfaces (6), 8426-8433 (2014); DOI:10.1021/am501316y.
  7. Small molecules based on benzo[1,2-b:4,5-b′]dithiophene unit for high-performance solution-processed organic solar cells, J. Zhou et al., J. Am. Chem. Soc. (134), 16345-16351 (2012); DOI: 10.1021/ja306865z.
  8. Solution-processed and high-performance organic solar cells using small molecules with a benzodithiophene unit, J. Zhou et al., J. Am. Chem. Soc. (135), 8484-8487 (2013); DOI:10.1021/ja403318y.
  9. A new small molecule acceptor based on indaceno[2,1-b:6,5-b’]dithiophene and thiophene-fused ending group for fullerene-free organic solar cells, D. Xie et al., Dye. Pigment. (148), 263-269 (2018); DOI:10.1016/j.dyepig.2017.09.009.
  10. High Performance Photovoltaic Applications Using Solution-Processed Small Molecules, Y. Chen et al., Acc. Chem. Res. (46), 2645-2655 (2013); DOI: 10.1021/ar400088c.
  11. W. Zhao et al., Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells, J. Am. Chem. Soc. (139), 7148-7151 (139); DOI:10.1021/jacs.7b02677.
  12. Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages, D. Baran et al., Energy Environ. Sci. (9), 3783-3793 (2016), DOI: 10.1039/C6EE02598F.
  13. Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells, Z. He et al., Adv. Mater. (23), 4636-46430 (2011); DOI: 10.1002/adma.201103006.
  14. An All-Small-Molecule Organic Solar Cell with High Efficiency Nonfullerene Acceptor, O. K. Kwon et al., Adv. Mater. (27), 191-1956 (2015); DOI: 10.1002/adma.201405429.

 


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