Elsevier

Organic Electronics

Volume 87, December 2020, 105904
Organic Electronics

An investigation of annealing methods for benzodithiophene terthiophene rhodanine based all small molecule organic solar cells

https://doi.org/10.1016/j.orgel.2020.105904Get rights and content

Highlights

  • We successfully pushed the PCEs of two reported all small molecule system to a higher level.

  • Morphology investigation revealed deeper insight of annealing method.

Abstract

Mainstream organic solar cells (OSCs) suffer a great variation of photovoltaic performance among different batches of polymers, which brings an opportunity for all-small-molecule OSCs to take leading position of industrialization. In recent years, benzodithiophene terthiophene rhodamine (BTR), as small molecule donor, has played an important role in this field. Here we investigated two typical BTR based all-small-molecule OSCs processed with different annealing methods, to explore the morphology optimization brought by them. As a result, BTR:PC71BM system was optimized by solvent vapor annealing (SVA) reaching an excellent fill factor (FF) of 79.1% via tuning molecular packing intensity, while BTR:Y6 with temperature annealing (TA) yielded a power conversion efficiency (PCE) of 12.125% whose molecular packing orientation had been changed. Additionally, by crossing using SVA and TA methods, we found that these two method can't be utilized together to further improve the PCE for either system. Therefore, our work offers better PCEs for these two reported combinations and further studies the compatibility between specific BTR based active layers and designated annealing methods, providing deeper understanding of device engineering on all-small-molecule OSCs.

Graphical abstract

In our work, all small molecule OSCs based on BTR:PC71BM and BTR:Y6 reach their best PCE of 10.113% and 12.125% by choosing suitable annealing ways, respectively. The combination of SVA and TA can only result reduced PCEs in both systems.

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Introduction

Organic solar cells (OSCs) is now viewed as a promising sustainable energy producer in future, due to its unreplaceable properties referred as flexibility and lightweight, and has experienced a blooming period [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]]. In recent years, power conversion efficiency (PCE) of OSCs has been pushed by material innovation to a new level (more than 17%) [[25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]]. However, the main stream highly efficient OSCs based on polymer donors and non-fullerene acceptors (NFAs) demands polymers with excellent quality, which is the main problem that hinders the reproducibility and commercialization prospect [38,39].

In contrast, small molecule donors contain several advantages that are promising in massively manufacturing involving high purity, reproducibility and unified chemical structures [[40], [41], [42], [43], [44]]. Ever since the birth of outstanding small molecule donor benzodithiophene terthiophene rhodamine (BTR), the PCE improvement research has experienced a rapid development episode [[45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59]]. In 2014, Jones and his coworkers pioneeringly reported BTR and applied it with PC71BM, acquiring an optimized PCE of 9.3% with an excellent fill factor (FF) of 74.1% [60]. Lu's group [61] utilized the new celebrity molecule Y6 coping with BTR-Cl (a derivative of BTR) few months earlier, in which a remarkable PCE as high as 13.6% was obtained. Interestingly, the device fabrication of these two works demonstrated big differences. BTR: PC71BM system provides great performance after being solvent vapor annealed (SVA) by THF (tetrahydrofuran) for seconds while BTR:Y6 based active layers needs to be put onto hotplate for temperature annealing (TA). This fact activated researchers' interest on figuring out the difference of SVA and TA when they're crossly applied to optimized active layers of above two systems, quot exploring device process methods is vital for this field.

In our works, we followed previous method of processing all-small-molecule active layers, obtaining considerably good device results as control. To be specific, OSCs of BTR:PC71BM by appropriate SVA, and of BTR:Y6 reaches 12.12% after TA, were fabricated. As for next step, SVA approach was implemented onto optimized BTR:Y6 blend films and TA processing was carried out for corresponding BTR:PC71BM active layers. Consequently, deteriorated photovoltaic performance appears in both two types of all-small-molecule OSCs. Linked with necessary characterization tests, we deduced that SVA and TA can only be applied to BTR:PC71BM and BTR:Y6, respectively. Generally, this examined the feasibility of choosing specific device fabrication methods to optimize different all-small-molecule system based OSCs, obtaining deeper insight in device engineering.

Section snippets

Results and discussion

A list of devices based on BTR:PC71BM and BTR:Y6 were fabricated with standard conventional structures (Fig. 1a) with slightly difference: ITO/PEDOT:PSS/BTR:PC71BM/ZrAcAc/Al, and ITO/PEDOT:PSS/BTR:Y6/PNDIT-F3N/Ag. The chemical structures are also given. Fig. 1b posts the UV–vis absorption spectra of these three molecules. Both PC71BM and Y6 displays complementary absorption range with that of BTR. Since the energy level of them were investigated clearly in previous literatures, we hereby

Conclusion

In summary, our study concentrated on reported remarkable BTR based all-small-molecule OSCs, optimizing their photovoltaic performances which have surpassed the PCEs provided in literatures. And then we tried to link SVA and TA, to seek a chance of further improving their PCEs. An unfortunate consequence reveals that SVA can only match with BTR:PC71BM, a champion PCE of 10.113% was achieved together with a remarkable FF of 79.1%, while TA is the method only compatible with BTR:Y6 (best

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Beihang University Research Fund 74004601, Youth 1000 Talent Fund KZ37029501, and the 111 Project (B14009), the National Natural Science Foundation of China (Nos. 51763017, 21602150, 51863012). It is also supported by Shandong Province Natural Science Foundation (ZR201702130258), a Project of Shandong Province Higher Educational Science and Technology Program (J17KA187)

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