An all-solid-state perovskite-sensitized solar cell based on the dual function polyaniline as the sensitizer and p-type hole-transporting material
Graphical abstract
Introduction
Dye-sensitized solar cells (DSSCs) have attracted scientific and technological interest of researchers in the past decades as a high efficiency and low-cost alternative to conventional silicon p–n junction photovoltaic devices [1], [2], [3]. This kind of device based on a dye-sensitized porous nanocrystalline TiO2 photoanode, an iodine-based liquid electrolyte, and a platinum counter electrode in a sandwich-like architecture, allowed an achievement of the efficiency of up to 12.3% [2]. Recently, a new class of hybrid organic-inorganic perovskite compounds (CH3NH3PbX3, X = I, Br and Cl) replace the traditional dye (N719) as light harvesters for solar cells due to their direct band gap, wide light-absorption, and high carrier mobility [4], [5], [6], [7], [8], [9], [10]. The perovskite-sensitized solar cell (PSSC) presents an impressive high open circuit voltage and realizes an all-solid-state solar cell by replacing the liquid electrolyte with a hole transporting material (HTM) for the production and stability of the photovoltaic device [11], [12].
The most commonly used HTM is 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spiro-biuorene (spiro-MeOTAD) [4], [5], [6], [7], [8], [9], [10], which demonstrates low recombination rate, efficient charge transport, and also good pore filling of the TiO2 layer improving device performance. Of course, poly-triarylamine (PTAA) [5], poly(3-hexylthiophene) (P3HT) [13] are also used as the HTMs. However, Z. Ku et al. reported on carbon counter electrode in perovskite-based solar cells without HTMs, and achieved an efficiency of 6.64% [14]. For the sake of low cost, good stability, and simple easy preparation of designable structures, the p-type conducting polymers, e.g., polypyrrole (PPy) [15], polyaniline (PANI) [16], [17], and poly(3,4-ethylenedioxythiophene) (PEDOT) [18], [19], [20] have been employed as cheaper alternatives in all-solid-state DSSCs. Among them, PANI has already been reported as a cost-effective and stable p-type HTM (p-HTM) and sensitizer in a solid-state DSSCs [16], [17]. For instance, Tan et al. reported on the polyaniline as a hole transport material to prepare solid solar cells [16]. Senadeera et al. were successful at depositing PANI via molecular self-assembly on TiO2 and using as a sensitizer in the solid-state solar cell [17].
It is well known that the PANI can be easily prepared through chemical polymerization [21] and electropolymerization methods [22], [23], [24], [25], [26], [27], [28], [29]. The electropolymerization methods are considerably simpler and more cost-effective technique to obtain conducting polymers with the controllable surface morphology and well adhesion on the substrate surface. The reported electropolymerization methods used in synthesizing PANI include: the cyclic voltammetry (CV) [22], [23], constant potential [24], [25], constant current [26], pulse current [27], and pulse potentiostatic [28] methods. Recently, we investigated a two-step CV approach for the preparation of PANI, which quickly yielding high performance PANI with brachyplast structure [29].
Therefore, by considering these facts, here we report on an all-solid-state perovskite-sensitized solar cell (ass-PSSC) based on the PANI with brachyplast structure as the p-HTM, which was quickly electropolymerized by using the two-step CV method onto the fluorinated tin oxide (FTO) glass substrates. In addition, the PANI also acted as a function of sensitizer enhancing the device performance. Thus the ass-PSSC assembled with the PANI electrode exhibited a superior photovoltaic conversion of 7.34%, and showing a good long-term stability.
Section snippets
Materials
Aniline monomer, sulfuric acid, acetone, isopropyl alcohol, methanol, ether, hydroiodic acid (45 wt.% in water), methylamine (30% in methanol), lead iodide (PbI2), chlorobenzene, acetonitrile, tetrabutyl titanate, and Triton X-100 were purchased from Shanghai Chemical Agent Ltd., China (Analysis purity grade). γ-butyrolactone (>99.9%), tetrabutylammonium hexafluorophosphate (TBAPF6, 98%), and ferrocene (>99.9%) were purchased from Aladdin. 4-tert-butyl-pyridine (TBP), lithium iodide (LiI), and
Structure feature and composition of the PANI
Fig. 1(a and b) (higher magnification) show TEM images to investigate the structure feature of the PANI. It can be found that the PANI with brachyplast structure could be obtained on the FTO substrate by using the two-step CV approach, this structure likes that some tiny twigs germinate on the tree branch. The brachyplast PANIs with the diameter of 15–30 nm and the length of 30–60 nm was electropolymerized on the pre-electropolymerized PANIs, which has been confirmed that the nanoscale
Conclusions
In summary, dual function of PANI with brachyplast surface morphology was quickly electropolymerized by means of using a two-step CV approach, which acted as the sensitizer and p-HTM in ass-PSSCs. The PANI with the nanoscale brachyplast structure offered high active surface area for the light absorbance and efficient charge transport due to its π–π* transition and the localized polaron. Furthermore, the porous PANI film could be filled up with CH3NH3PbI3 and Li-salt, resulting in a wide light
Acknowledgments
The authors appreciate funding from National Natural Science Foundation of China (21274082 and 21073115) and Shanxi Province (2012021021-3), the Program for New Century Excellent Talents in University (NCET-10-0926), and the Scientific Research Start-up Funds of Shanxi University (020351801003).
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