Elsevier

Organic Electronics

Volume 99, December 2021, 106319
Organic Electronics

Improving the fill factor of N2200-based all polymer solar cells by introducing EPPDI as a solid additive

https://doi.org/10.1016/j.orgel.2021.106319Get rights and content

Highlights

  • A cheap and commercially available small molecular solid additive.

  • No influence on the original polymer thin film order.

  • Reduce the morphological traps in active layer.

  • Improve the fill factor and power conversion efficiency of all polymer solar cells.

Abstract

A cheap and commercially available small molecule (namely EPPDI) is introduced to the active layer of N2200-based all polymer solar cells as a solid additive. EPPDI at the optimal ratio can improve the D-A nano-scale morphology and reduce trap density of the active layer by filling morphological spaces. As a result, the photovoltaic performance of the resulting devices based on PF2:N2200 are increased from 6.28% to 7.03% with significantly enhanced fill factor. This work demonstrates a facile approach for improving the performance of all polymer solar cells.

Introduction

All polymer solar cells (APSCs) exhibit excellent film processing properties, superior thermal and mechanical stability [[1], [2], [3], [4]], which are essential for the practical applications. Tremendous efforts have been devoted in this field in past decade, and the power conversion efficiency (PCE) of APSCs has increased from ~1% to ~15%, which mainly relies on the development of novel polymer donor and acceptors [[5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]]. Among the common challenges of PSCs, such as light absorption range, charge carrier mobility and energy loss, the morphology optimizations is in particular challenging for APSCs. Modulating nano-scale phase-separations of films by blending two polymers is more difficult than those films by blending a small molecule and a polymer or two small molecules, as the conjugated polymer chains are usually pre-aggregated or entangled in solutions [19,20] and polymer chains’ movements and reorganizations in solid state require relatively high energy in comparison to small molecules.

To solve the morphology issues and improve the APSC device performance, a typical approach is design random copolymer acceptors. For example, poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2′-bithiophene)} (N2200) is one of the best polymer acceptors due to its high electron mobility, narrow bandgap and excellent compatibility with varieties of donor materials [21,22], and the random copolymer strategy could reduce the crystallinity of N2200 and hence improve the nano-phase separation of the resulting blend films. Wang and co-workers increased the content of thiophene units in the N2200 backbone to reduce the crystallinity of N2200 [23,24]. The new random copolymer PNDI-T10 achieved higher PCE and fill factor (FF) due to improved miscibility of PNDI-T10 with the donor PTB7-Th. Another strategy is adjusting the side chains of N2200. Huang et al. introduced 10% of linear oligoethylene oxide side chains to replace branched alkyl chain in the N2200 [25]. As a result, APSCs showed an excellent FF of 0.75 due to the optimized morphology. The highly self-aggregated acceptor polymer N2200 turned amorphous with introducing a certain number of 2,6-diisopropylphenyl groups. Besides the chemistry approaches, Yuan et al. also proposed a P–i–N fabrication strategy to circumvent such non-ideal phase-separation by coating from the polymer blend solutions [26]. The high molecular weight N2200 was synthesized and almost insoluble in chloroform, and then the polymer donor layer could be casted on top of N2200 layer from the chloroform solution, forming a P–i–N vertical phase-separation in the active layer.

In this work, we proposed to introduce a solid additive to improve the D-A nano-scale phase-separation. When the polymers' mixing solution is casted on the substrate, it is well-known that there are three phases inside the thin film [27], which are donor polymer phase, acceptor phase and the mixed phase. Beside the three phases, there should also be nano-scale spaces between three phases in thin film. Since conjugated polymer chains are usually pre-aggregated in solution and then grow big aggregates during the film formation, then these spaces between three phases are determined by the aggregation sized of each phase. It could be conducted that there would be relatively large spaces in APSCs in comparison to those in blend films by mixing a small molecule and a polymer, because small molecules have relatively small geometry size and could diffuse into these spare spaces inside the film upon thermal annealing [28]. Therefore, these morphological spaces could be considered as a sort of gain-boundaries that trap charge carriers [29]. As show in Fig. 1, our general idea is to use a cheap and commercially available small molecule as the solid additive to fill these morphological spaces and mitigate the boundary influence. Herein, we studied the PF2:N2200 solar cell devices [30] with introducing N,N′-Bis(3-pentyl)-3,4,9,10-perylenetetracarboxylic diimide (namely EPPDI) [31,32] as the filling molecule. Finally, it is found that EPPDI could improve the PCE from 6.28% to 7.03%, which is mainly due to the improved fill factor. Moreover, EPPDI also works in other N2200-based APSCs.

Section snippets

Solar cell farbrication and performance

Solar cells were fabricated with a conventional structure of ITO/PEDOT:PSS/active layer/PFN-Br/Ag, in which PEDOT:PSS is poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and PFN-Br is poly(9,9-bis(3’-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide) [33]. Specifically, approximately 40 nm of PEDOT:PSS was deposited as a thin film atop indium tin oxide (ITO) coated glass substrates. The BHJ layers were spin-coated from the solution of

Conclusions

In summary, we fabricated all-polymer solar cells using the small molecule EPPDI as solid additive. EPPDI in the optimal amount could be dispersed in the blend film without exceeding aggregations, and filled the spaces between three phases in active layer. As a result, the loading EPPDI could reduce morphological traps and thus improve the photovoltaic performance of the N2200-based devices by enhancing FF. Our results suggest that introducing EPPDI as solid additive is a facile method to

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.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No.21805032), the Natural Science Foundation of Guangdong province (No. 2018A030310364), the Fundamental Research Fund for the Central Universities (No.20D128502). The author would also like to thank Prof. Fei Huang for the help of conducting the device characterizations.

References (38)

  • T. Jia et al.

    Nano Energy

    (2020)
  • Z. Xiao et al.

    Sci. Bull.

    (2017)
  • Z. Xiao et al.

    Sci. Bull.

    (2017)
  • Q. Liu et al.

    Sci. Bull.

    (2020)
  • J. Qin et al.

    Sci. Bull.

    (2020)
  • Z.-Y. Wang et al.

    Polym. Chem.

    (2020)
  • L. Lucera et al.

    Energy Environ. Sci.

    (2016)
  • C. Lee et al.

    Chem. Rev.

    (2019)
  • B. Fan et al.

    Nat. Commun.

    (2019)
  • X. Zhan et al.

    J. Am. Chem. Soc.

    (2007)
  • J. Qin, L. Zhang, C. Zuo, Z. Xiao, Y. Yuan, S. Yang, F. Hao, M. Cheng, K. Sun, Q. Bao, Z. Bin, Z. Jin, L. Ding, J....
  • K. Jin, Z. Xiao, L. Ding, J. Semiconduct., 021, 42,...
  • Z. Li et al.

    Energy Environ. Sci.

    (2019)
  • P. Zhu et al.

    Chem. Asian J.

    (2019)
  • C. Dou et al.

    Sci. China Chem.

    (2017)
  • C. Zhao et al.

    J. Mater. Chem. C

    (2019)
  • Z.-G. Zhang et al.

    Angew. Chem. Int. Ed.

    (2021)
  • Z. Luo et al.

    Adv. Mater.

    (2020)
  • Q. Fan et al.

    Energy Environ. Sci.

    (2020)
  • Cited by (6)

    • Two non-halogen additives significantly improve the efficiency of PTB7-Th:PC<inf>71</inf>BM-based polymer solar cells in non-halogen solvent

      2023, Organic Electronics
      Citation Excerpt :

      By adding additives to the active layer, better molecular filling and more suitable phase separation scales can be obtained, and thus forming a better charge transport network [21–24]. Thereby, a growing number of researchers are beginning to study the influence of additives on the morphology of the active layer [25–28]. For instance, Chen and coworkers introduced two additives 1,8-diiodooctane (DIO) and 2,6-dimethoxynaphthalene (DMON) into PBDB-T:TTC8-O1-4F system and found that DIO could induce the TTC8-O1-4F to form an ordered stack, while DMON could restrain over-aggregation of donors and acceptors.

    • Unique W-Shape Y6 isomer as effective solid additive for High-Performance PM6:Y6 polymer solar cells

      2022, Chemical Engineering Journal
      Citation Excerpt :

      These liquid additives postpone the drying process and elongate the duration for nanostructure development which improves the device performances and has become standard practices in PSC fabrications. On the other hand, solid additives[1,3] were also explored as photosensitizers,[8,9] complexation agents,[10–12] nucleation agents,[13–15] cross-linkers,[16–18] binders,[19–21] and miscibility enhancers[22] with success. Recently, non-fullerene PSCs replaced the fullerene-based devices and became the new power engine for achieving breakthroughs in top power conversion efficiencies (PCE) which have lately reached over 18%.[23]

    View full text