Toward efficient polymer solar cells with thick bulk heterojunction by introducing iridium complex as an aggregation reshaping auxiliary
Introduction
Polymer solar cells (PSCs) are deemed as a promising candidate toward large area solar energy technology through solution fabrication method due to their easy fabrication, low-cost, transparency, flexibility and lightweight, as well as suitability for roll-to-roll coating and inkjet printing (Sahli et al., 2018, Zhou et al., 2018, Chen et al., 2018, Zheng et al., 2018). However, one of the main technology obstacles is their relative low power conversion efficiency (PCE). In the pursuit of enhancing PCE, researchers have paid attention to the synthesis of novel materials (Kan et al., 2018, Jin et al., 2016), the nano-morphology optimization of bulk heterojunction (BHJ) (Wang et al., 2017, Cha et al., 2018), the interfacial engineering (Huang et al., 2017a, Wang et al., 2017, Stolterfoht et al., 2018, Zheng et al., 2019) and smart device architectures (Huang et al., 2017b, Huang et al., 2019). Among them, two important issues need to be urgently improved for the commercialization of state-of-the-art PSC devices. One is the thickness limitation of photoactive layer for the charge transport and effective collection (Cui et al., 2019, Wang et al., 2017, Meng et al., 2019, Grey, 2016); and the other is precise control of the active layer morphology (Xing et al., 2017, Huo et al., 2015, Huang et al., 2012). What’s more, the internal quantum efficiency (IQE) with near 100% has been achieved in some PSCs with ideal thickness of BHJ layer (He et al., 2015, Park et al., 2009, Nguyen et al., 2014). Generally speaking, the champion thickness for BHJ blends based on a highly crystalline, electron donating polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′’’-di(2-octyldodecyl)-2,2′;5′,2″;5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T- 2OD) can be up to 300 nm (Liu et al., 2017), mainly due to strong temperature dependent aggregation behavior with high hole mobility. These properties can be controlled by temperature to form highly crystalline morphology, which enables high short circuit current (JSC) and fill factor (FF) even with a 300 nm thick PffBT4T-2OD: PC71BM film (Hu et al., 2017). However, when the temperature drops to room temperature during the preparation of thin films, it could lead to too much aggregation and enlarge domain size, which in reverse limits the maximum attainable JSC and overall performance of the PSCs (Huang et al., 2015).
It is well known that the charge carriers can be extracted efficiently to the electrodes only within the transport distance of free carriers (LD) longer than the photoactive layer thickness (d). This is the reason why obtaining high current from thick BHJ active layer. Thereunder, LD = μτE(d), where μ is carrier mobility, τ is carrier lifetime, and E is electric field intensity (Kirchartz et al., 2012). In order to enhance LD in thick BHJ PSCs, it is important to have appropriate methods to control the active layer morphology with efficient charge carrier transport (Huang et al., 2015; Huang et al., 2017). In the BHJ active layer, the length of exciton diffusion is around 5–10 nm (Halls et al., 1996, Stübinger and Brütting, 2001, Markov et al., 2005). Thus, the domain sizes of the donor (D) and acceptor (A) in the BHJ active layer must be near 10–20 nm (Gu¨nes et al., 2007). The decrease of FF in the thick BHJ system is mainly due to the large aggregate size of polymer donor, which is not conducive to the chare transfer (Zhang et al., 2016). Therefore, the morphology shape of aggregation domains plays a very important role in the charge transfer and collection process.
In general, the film morphology of thick BHJ is strongly dependent on its processing conditions. Recently, many studies on thick BHJs with suitable domain size based on the high PCE have been studied in details. Huang et al. studied an inverted off-center spinning method with highly efficient thick BHJ by improving vertical donor-acceptor compositional gradient (Huang et al., 2016). Cao et al. overcame the space-charge effects by optimizing device architectures in thick BHJ PSCs (Zhang et al., 2018a). Hao et al. obtained high efficiency thick BHJ PSCs by adding Graphene oxide into PTB7: [6,6]-phenyl C71-butyric acidmethyl ester (PC71BM) leading to more balanced charge transport (Lyu et al., 2018). Furthermore, small molecule and semi-crystalline conjugated macromolecule materials have introduced in the polymer: fullerene systems as additives or morphology agents to enhance charge transport. For example, by introducing Si-PCDTBT into the PTB7: PC71BM system, the FF can be increased to 77%, with an apparent dip of charge recombination in the active layer (Gasparini et al., 2016). Large-area with thickness tolerance devices are successfully fabricated by adding p-DTS(FBTTH2)2 in PTB7-Th: PC71BM blends (Zhang et al., 2017). Much of these literatures study on the morphology control based on low or semi-crystalline conjugated polymers have shown to enable high PCE. However, it still lacks of simple method for adjusting domain size and shape control simultaneously based on crystalline conjugated polymers in thick BHJ PSCs.
Herein we introduced an easy way to adjust the aggregation shape of PffBT4T-2OD: PC71BM blends by adding a novel iridium complex of (iridium(III)bis(2-(4-trifluoromethylphenyl)pyridine)tetraphenylimidodiphosphinate)(tfmppy)2Ir(tpip) into the active blends. It was found that a small amount of (tfmppy)2Ir(tpip) can play an effective role as the aggregation reshaping auxiliary in the PffBT4T-2OD: PC71BM thick BHJ photoactive layer by increasing the interface of D/A, leading to enhanced exciton dissociation. Overall, the addition of (tfmppy)2Ir(tpip) enables PSCs with 300 nm thick BHJ to obtain nearly 11% PCE and 69% FF. To study the mechanism of (tfmppy)2Ir(tpip) based on PffBT4T-2OD: PC71BM PSCs, the atomic force microscopy (AFM) of the morphology of photoactive layers was characterized. The crystalline structure of the active layer was illustrated by X-ray diffraction (XRD), which is conducive to clarify the reason of the (tfmppy)2Ir(tpip) role on efficient charge transport. Furthermore, by theoretic simulation, the relationship between carrier mobility and the introduction of (tfmppy)2Ir(tpip) in thick BHJ PSCs was elucidated by using space-charge-limited current (SCLC) method. Besides, the average lifetime of charge carriers was also inferred from the impedance spectra (IS).
Section snippets
Materials
All solvents were purchased from Sigma-Aldrich and TCI Chemical Co. The iridium complex (tfmppy)2Ir(tpip) was shared by Nanjing University. The PffBT4T-2OD and PC71BM were purchased from Solarmer Materials Inc. The ZnO precursor was prepared by dissolving zinc acetate dihydrate [Zn(CH3COO)2·2H2O, 99.9%, 1 g] and ethanolamine (NH2CH2CH2OH, 99.5%, 0.28 g) in 2-methoxyethanol (CH3OCH2CH2OH, 99.8%, 10 mL), which are all purchased from Aldrich, and stirred for complete hydrolysis reaction in air (
Results and discussion
The chemical structures of PffBT4T-2OD, PC71BM and (tfmppy)2Ir(tpip) are shown in Fig. 1(b)–(d). The proposed charge transfer mechanism in ternary PSCs is shown in Fig. 1(e). The highest occupied molecular orbital (HOMO) of (tfmppy)2Ir(tpip) is −5.4 eV and lowest unoccupied molecular orbital (LUMO) of (tfmppy)2Ir(tpip) is −3.0 eV; the HOMO of PffBT4T-2OD is −5.4 eV and the LUMO of PffBT4T-2OD is −3.7 eV; the HOMO and LUMO of PC71BM are −6.0/−4.3 eV, respectively.
Acknowledgments
This work was financially supported by the National Key R&D Program of China (Grant No. 2018YFB0407102), the Foundation of National Natural Science Foundation of China (NSFC) (Grant Nos. 61421002, 61675041 & 51703019), the Project of Science and Technology of Sichuan Province (Grant Nos. 2019YFH0005, 2019YFG0121 & 2019YJ0178). This work was also sponsored by Sichuan Province Key Laboratory of Display Science and Technology.
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.
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