Elsevier

Solar Energy

Volume 201, 1 May 2020, Pages 499-507
Solar Energy

Performance enhancement of conjugated polymer-small molecule-non fullerene ternary organic solar cells by tuning recombination kinetics and molecular ordering

https://doi.org/10.1016/j.solener.2020.03.008Get rights and content

Highlights

  • Polymer-small molecule-non fullerene ternary organic solar cells.

  • Conjugated polymer PTB7-Th and small molecule p-DTS(FBTTh2)2 as donors and non-fullerene molecule IEICO-4F as an acceptor.

  • PCE of ∼10.9% for PTB7-Th: p-DTS(FBTTh2)2: IEICO-4F is higher than 9.8% for PTB7-Th: IEICO-4F OSCs.

  • Mixing ratio of small molecule and conjugated polymers impact on the crystallinity of the blend.

  • The improved molecular ordering is shown to enhance exciton generation rate, exciton dissociation, charge collection.

Abstract

We present our study of conjugated polymer-small molecule (SM)-non-fullerene ternary organic solar cells (OSCs), which employs conjugated polymer PTB7-Th and small molecule p-DTS(FBTTh2)2 as donors and non-fullerene molecule IEICO-4F as an acceptor. It is observed that the power conversion efficiency (PCE) of ∼10.9% for PTB7-Th: p-DTS(FBTTh2)2: IEICO-4F ternary OSCs with 15 wt% of p-DTS(FBTTh2)2 SM is higher than PCE of ∼9.8% for PTB7-Th: IEICO-4F OSCs. Morphological studies confirm that the addition of p-DTS(FBTTh2)2 SM in PTB7-Th: IEICO-4F binary blend improves molecular ordering and crystallinity of PTB7-Th due to the favorable interaction with p-DTS(FBTTh2)2 thereby providing 3-D textured structures consisting of a mixture of edge-on and face-on orientations. The improved molecular ordering is shown to enhance exciton generation rate, exciton dissociation, charge collection, and to reduce charge recombination, all of which boosts the PCE.

Introduction

The power conversion efficiency (PCE) of bulk-heterojunction (BHJ) organic solar cells (OSCs) is limited ultimately by their photon absorption in the solar spectrum (Ameri et al., 2013, An et al., 2016, Bi and Hao, 2019). In this respect, separate absorption bands of donors (D) and acceptors (A) constituting a BHJ can be useful in that they can effectively extend the spectral coverage of an OSC based on the BHJ (Park et al., 2009, Yu et al., 1995). Nevertheless, achieving sufficient photon absorption would still be challenging with binary D/A blends (Park et al., 2009). To overcome this limitation, tandem OSCs based on two different D/A blends or conventional BHJ OSCs based on ternary blends have been studied extensively over the last few years. As a result, PCE > 10% have recently been achieved (Guo et al., 2018, Huang et al., 2018, Li et al., 2018, Meng et al., 2018).

As compared to tandem geometry, ternary blend OSCs are cost-effective and easy to fabricate in that they do not require complex multijunction structures and careful current matching demanded in tandem OSCs (An et al., 2016, Lu et al., 2015b). Ternary OSCs usually consist of three components in the photoactive layer: a dominating donor: acceptor (D: A) system and a third component, which could be either a donor or an acceptor (Xu and Gao, 2018, Yu et al., 2018). Until the first half of this decade, most of the reported ternary OSCs have relied on low band gap polymers as donor and fullerene derivatives (FDs) as an acceptor. In many cases, however, those ternary OSCs showed low thermal and photo-stability, and large deviations in the PCE. This is due to batch-to-batch variations in the molecular weight, poly-dispersity, and purity of donor polymers as well as the limited energy-level tunability and the morphological or the photochemical instability of FDs (Datt et al., 2019, Zhang et al., 2018). On the other hand, solution-processable small molecules (SMs) donor and non-fullerene acceptors (NFAs) have well-defined chemical structures, molecular weight, monodispersity, higher solubility, high charge carrier’s mobility, energy levels tuning, and no chain-end defects (Datt et al., 2019, Zhang et al., 2018). This has allowed one to design and develop OSCs that are comprised of various ternary photoactive blends such as polymer-polymer-FD, polymer-FD-FD, polymer-SM-FD, polymer-polymer-NFA, polymer-NFA-NFA, polymer-FD-NFA, etc., which led to a significant improvement of the PCE of OSCs (Gupta et al., 2015, Lu et al., 2016, Sharma et al., 2016b, Yao et al., 2017, Yu et al., 2017, Zhang et al., 2015). As a result, SMs and NFAs have been at the forefront of research and has been widely studied as donors and acceptors to fabricate binary and ternary OSCs (Xie et al., 2019, Zhang et al., 2015).

It has been reported that replacing conjugated polymers with SMs and FDs with the NFAs in the binary and ternary BHJ OSCs significantly alters the morphology and affects the PCE (Gasparini et al., 2019, Li et al., 2017, Liu et al., 2018). The ternary photoactive blend involves the interaction and intermixing of the third component with the donor (conjugated polymer/SMs) and acceptor (FDs/NFAs) (Ameri et al., 2013, Gasparini et al., 2019, Xu and Gao, 2018). The favorable interaction and intermixing of the third component with D: A system can facilitate the energy and charge transfer and can lead to the alloy formation by modifying the molecular ordering within the photoactive layer (Bi and Hao, 2019). Both of these phenomena can lead to improved charge generation and transport. However, a third component may act as a recombination center and lead to unfavorable morphology, thereby seriously compromising the process of charge generation and transportation (Kumari et al., 2017). This makes it extremely crucial to carefully choose the third component that matches with a given D and A pair and to optimize the morphology of photoactive ternary blends.

In this study, we propose a ternary OSC based on a conjugated polymer, SM, and NFA. NFA is chosen among those which can absorb photons in near-infrared (NIR) region so that one could better utilize the photons in the sunlight. We try to identify the optimal conditions that can lead to the enhanced performance by blending SM with conjugated polymers at different ratios. We then explore how the mixing ratio of SM and conjugated polymers impact on the crystallinity of the blend and thus on the overall electronic properties and device performances. The results give significant insight into the charge dynamics, understanding of which will eventually be helpful in maximizing the PCE of ternary OSCs. Optimal compositions yielding PCE > 10% are identified and compared with reference devices.

Section snippets

Experimental

In this study, all OSCs were fabricated in an inverted geometry having a device architecture as ITO/ZnO (40 nm)/photoactive layer (100 nm)/MoO3 (10 nm)/Al (100 nm). Photoactive layer was spin-coated using ternary blend comprised of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2–6-diyl)] (PTB7-Th) as a donor polymer and

Results and discussion

Fig. 1(a) shows the chemical structures of the donor polymer PTB7-Th, SM p-DTS(FBTTh2)2 and non-fullerene acceptor IEICO-4F. The corresponding UV–Vis absorption spectra of their pristine films are shown in Fig. S1(a). PTB7-Th exhibits two prominent absorption peaks at 640 and 695 nm, p-DTS(FBTTh2)2 shows absorption peaks at 610 nm with a shoulder peak at 660 nm, and IEICO-4F has broad absorption peak between 650 and 950 nm respectively. This values are similar or are close to values reported

Conclusion

In conclusion, we have demonstrated high-efficiency OSCs based on the ternary blend of PTB7-Th: p-DTS(FBTTh2)2: IEICO-4F by incorporating the optimum amount of 15 wt% of p-DTS(FBTTh2)2 SM in PTB7-Th: IEICO-4F binary blend. Recombination kinetics and morphological studies were carried out to understand the mechanism behind the enhanced performance of these ternary OSCs. Our studies suggest that enhanced performance is mainly due to improved exciton generation rate, exciton dissociation, and

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 supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20163010012200).

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