Seeding-method-processed anatase TiO2 film at low temperature for efficient planar perovskite solar cell
Graphical abstract
Introduction
Organic-inorganic halide perovskite solar cells (PSCs) have attracted extensive research interests benefitting from their high light absorption coefficient, long charge carrier diffusion length, and small exciton binding energy. In PSCs, the electron-selective layers (ESLs), serving in accepting and transporting electrons as well as blocking holes, play an indispensable role in boosting PSCs’ power conversion efficiency (PCE). Significant attention has been focused on ESLs designing and tailoring all this time. To the best our knowledge, TiO2, except for application in photocatalysis [1], [2], [3], [4], [5], [6], [7], SERS sensing [8], energy storage [9], now has been widely used in PSCs to transport electrons, which was first used as ESLs in dye-sensitized solar cells by Grätzel [10], [11], [12], [13]. Especially, anatase or rutile TiO2 possesses superior electron transport property for efficient PSCs. However, calcining temperature as high as 450–550 °C is desired for the preparation of anatase or rutile TiO2 [14], [15], [16], [17], [18]. This preparation technology not only brings about high energy consumption but also limits the application of flexible PSCs. Consequently, research on low-temperature calcination process for high-compact anatase or rutile TiO2 ESLs preparation is in urgent need.
Up to now, some groups are absorbed in finding alternatives to prepare non-TiO2 ESL materials at low-temperature and improve the performances of PSCs simultaneously, such as inorganic materials (SnO2, ZnO, WOx, CeOx, Nb2O5, In2O3, CdS, Bi2S3, In2S3), organic materials (fullerene, non-fullerene) and QDs (CdSe QDs) [19]. Among previous reports, high-vacuum together with solution-based processing method is always adopted for low-temperature-processed TiO2 ESLs [20], [21], [22]. Especially, solution-based processing owing to the merits of low cost and easy manufacture is highly appreciated. Yella et al. deposited a rutile TiO2 hole-blocking layer on FTO glass via hydrolysis of TiCl4 at 70 °C, forming electron selective contact with photoactive CH3NH3PbI3 film [23]. Wojciechowski et al. obtained TiO2 ESLs by sintering post-treatment at 150 °C and added the mesoporous, namely, insulating scaffold (Al2O3) [24]. Snaith et al. reported a solution-based deposition procedure utilizing nanocomposites of graphene and TiO2 as ESLs in PSCs [25]. Dauskardt et al. developed scaffold-reinforced PSCs with novel honeycomb coal structure using low-temperature-processed TiO2 ESLs and the PCE is 16.4% [20]. In addition, TiO2/C-60, TiO2/SnO2 and Ta doped TiO2 ESLs prepared under low temperature are also discussed [26], [27], [28].
Inspired by the aforementioned attempts, a novel seeding-method at low temperature of 150 °C is proposed in this contribution to prepare highly-compact anatase TiO2 ESLs. Additionally, Cl dopant is incorporated in TiO2 (Cl@TiO2) ESLs to further elevate the property of PSCs. The doping of Cl in TiO2 thin film was confirmed by energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). According to X-ray diffraction (XRD) analysis, both TiO2 and Cl@ TiO2 thin films can be indexed to anatase phase. The morphology of anatase TiO2 ESLs was evaluated by atomic force microscope (AFM) and scanning electron microscope (SEM). Conductivity of TiO2 and Cl@ TiO2 ESLs was measured to assess their charge-carrier transport property. Research on charge transport and recombination dynamics between TiO2 thin film and perovskite layer was also carried out. All these film forming processes were conducted below 150 °C.
Section snippets
Materials
Tetra-n-butyl Titanate (TBOT, ≥99.0%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Polyvinyl pyrrolidone (PVP-K30) and hydrochloric acid (AR, 36%) were purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. Titanium diisopropoxide bis (acetylacetonate) (TiAcAc) (75 wt% in isopropanol) was purchased from Energy Chemical. PbI2 (>99.999%), CH3NH3I (MAI, >99%), Spiro-OMeTAD(≥99.5%), lithium bis (trifluoromethanesulfonyl) imide (Li-TFSI) and 4-tert-butylpyridine (TBP,
Results and discussion
Structural characterization of TiO2 crystal seed was performed using XRD, as shown in Fig. 2(a). Result implies anatase phase TiO2 (JCPDS 21-1272). Diffraction peak at 25.3° is significantly higher than others, indicating crystal seed has a preferential growth tendency with an average grain size of ∼8.9 nm on the basis of Scherrer [29], [30]. Fig. 2(b) and (c) display transmittance spectra and I-V curves (inset is the device structure of FTO/TiO2/Au [31], [32], [33]). The transmittance is about
Conclusions
In this theme, highly-compact anatase TiO2 film was prepared using seeding-method at low temperature of 150 °C. In comparison with raw TiO2, anatase TiO2 film doped with Cl is endowed with higher conductivity and smoother surface. Especially, 0.4 M Cl@TiO2 film with average grain size of 9.3 nm shows better uniformity. Its conductivity is dramatically enhanced to 9.2 × 10-6 S cm−1 compared with that of undoped TiO2 (3.9 × 10-6 S cm−1). In addition, 0.4 M Cl@TiO2 film has a wider band gap of
Acknowledgements
This work is supported by the National Key Research and Development Program of China, China (2016YFB0401303), the National Science Foundation for Young Scientists of China, China (No. 61804106) and the Key Projects in Natural Science Foundation of Tianjin, China (16JCZDJC37100). The calculation in this work was supported by high performance computing center of Tianjin University, China.
Notes
The authors declare no competing financial interest.
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