Broadening the light absorption range via PBDB-T to improve the power conversion efficiency in ternary organic solar cells
Graphical abstract
Introduction
Bulk-heterojunction (BHJ) organic solar cells (OSCs) as one of the most promising next-generation photovoltaic technology have attracted ever-increasing academic attentions due to their intrinsic advantages. Such as, light weight, solution processability, and excellent compatibility with a roll-to-roll printing process [[1], [2], [3]]. In conventional OSCs, the p-n junction in the active layer is usually formed by one p-type conjugated polymer as donor and one n-type semiconductor as acceptor. Although over 15% power conversion efficiency (PCE) has been obtained for such OSCs, binary OSCs have common drawbacks, that is they can only absorb limited solar spectrum due to the intrinsic narrow absorption bands of the active materials [4,5]. In order to broaden the absorption band and enhance the photocurrent generation, one strategy is utilizing the tandem structure composed of multiple active layers with complementary optical absorption [5,6]. Another strategy is to introduce a third component with complementary absorption bands to the host active films of the binary OSCs [[7], [8], [9]]. Typically, tandem devices involved serious technical challenges, such as, intermediate layer processing, the coupling of appropriate absorbers and the complexity in device fabrication, etc. [10] In comparison ternary OSCs are more facile to realize which can avoid the challenges in tandem solar cells.
In recent years, ternary OSCs have drawn tremendous attentions and have been developed rapidly as an effective method to obtain high efficiency OSCs mainly due to the simplicity in single-layer fabrication [4,11,12]. In ternary OSCs, an ideal third component not only broadens the photon harvesting range with the host donor: acceptor (D: A) blend system which is the primary standard for the third component selection [13,24]. but also optimizes the films morphology and facilitates exciton dissociation, charge and energy transport [14,15]. The fundamental mechanisms of ternary OSCs involves four categories reported so far: charge transfer [16,17], energy transfer [[18], [19], [20], [21], [22]], parallel-like [23,24], and alloy model [[25], [26], [27]]. In 2016, Deng et al. Incorporated 2,4-bis [4-(N, N-diisobutylamino)-2,6dihydroxyphenyl] squaraine (SQ) into PCDTBT: PC71BM system, the PCE was improved from 6.54% to 7.62% [28]. Afterwards Wang et al. introduced PBDTTS-FTAZ into PTB7-Th: PNDI-T10 host system, which yielded a PCE 8.6% [29].
Recently, non-radiative Förster resonance energy transfer (FRET) has been developed to interpret the performance improvement in ternary OSCs [19,30], which is a mechanism describing energy transfer between two light-sensitive molecules. FRET was first employed in solar cells based on P3HT: SQ: PCBM system which induced a PCE of 4.51% enhanced by 38% with respect to the control devices [31]. Later, an enhanced PCE from 6.8% to 8.9% was reported in ternary solar cells involving energy donors PCDTBT and PTB7, and energy acceptor PC71BM [19].
In this work, poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’] dithiophene)) – alt - (5,5 - (1′,3′- di -2- thienyl -5′,7′-bis (2-ethylhexyl)benzo[1′,2′-c:4′,5′-c’]dithiophene-4,8-dione))](PBDB-T) was selected as the third component in order to broaden the absorption range of the Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) benzo [1,2-b:4,5-b’] dithiophene – co – 3 – fluorothieno [3,4-b] thiophene - 2-carboxylate](PTB7-Th) [6,6]:-phenyl-C71-butyric acid methyl ester (PC71BM) host system. A series of inverted ternary OSCs based on PBDB-T: PTB7-Th: PC71BM were fabricated. An enhanced PCE of 9.45% was obtained in the ternary OSCs with simultaneously improved short-circuit current density (JSC) and a fill factor (FF) compared to the host binary device based on PTB7-Th: PC71BM, which proved to be mainly owing to the FRET effect between PBDB-T and PTB7-Th.
Section snippets
Photovoltaic properties
The photovoltaic performance of ternary OSCs fabricated using different weight ratios (0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, and 25 wt%) of PBDB-T to PTB7-Th, was evaluated using an inverted architecture of indium‐tin oxide (ITO)/ZnO/active layer/MoO3/Ag under simulated AM 1.5G irradiation (100 mW/cm2), where ZnO works as electron transport layer and MoO3 works as hole transport layer. The weight ratio of overall donor (PTB7-Th and PBDB-T) to acceptor (PC71BM) kept at 1:1.5 in the active layer.
Conclusions
In summary, we demonstrated an effective strategy to improve the photovoltaic performance of OSCs based on PTB7-Th: PC71BM binary system, by using PBDB-T as the component in the blend film, which can recycle the photon energy through a non-radiative FRET effect, the champion PCE of ternary OSCs gives a PCE of 9.45% with 11% enhancement compared to the control binary devices. Furthermore, it was proved that FRET is beneficial for improving the charge dissociation, and collection efficiencies in
Materials
Polymer PTB7-Th, PC71BM and PBDB-T were purchased from Solarmer Materials Inc. Zinc acetate dehydrate (Zn (CH3COO)2·2H2O, Aldrich, 99.9%), ethanolamine (NH2CH2CH2OH, Aldrich, 99.8%), 2-methoxyethanol, chlorobenzene (CB) and 1,8- diiodooctane (DIO) were commercially available. ITO-coated glass is used as the transparent electrode with a resistance of 10Ω/sq. All chemicals and solvents were used without further purification as received.
Preparation of ZnO precursor: Zinc acetate dehydrate (1g) 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.
Acknowledgements
This work was sponsored by the Applied Basic Research Major Program of Hebei Province, China (18964303D) and the Scientific Research Project of Hebei Education Department (QN2018139).
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