Sequentially Vapor-Grown Hybrid Perovskite for Planar Heterojunction Solar Cells
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High-quality and reproducible perovskite layer fabrication routes are essential for the implementation of efficient planar solar cells. Here, we introduce a sequential vapor-processing route based on physical vacuum evaporation of a PbCl2 layer followed by chemical reaction with methyl-ammonium iodide vapor. The demonstrated vapor-grown perovskite layers show compact, pinhole-free, and uniform microstructure with the average grain size of ~ 320 nm. Planar heterojunction perovskite solar cells are fabricated using TiO2 and spiro-OMeTAD charge transporting layers in regular n-i-p form. The devices exhibit the best efficiency of 11.5% with small deviation indicating the high uniformity and reproducibility of the perovskite layers formed by this route.
KeywordsCH3NH3PbI3-xClx Sequential vapor processing Vacuum evaporation Vapor-assisted growth Planar Heterojunction solar cells
Hybrid perovskite materials are the most competitive candidates as light absorber of next-generation photovoltaic era with their unique features including intense optical absorption, direct and tunable band gap, high carrier mobility, long charge diffusion length, shallow defect levels with few mid-gap states, and wide tunability on its composition according to metal halide framework and inserted organic species [1, 2, 3, 4, 5, 6, 7, 8]. They have been employed in two types of architectures such as mesoscopic nanostructured and simple planar structured. The preparation of high-quality pinhole-free perovskite layers for simplified planar architecture requires substantial effort. Various methods have been used to prepare perovskite layers, such as anti-solvent dripping, sequential dip coating, dual-source vacuum evaporation, and vapor-assisted growth [9, 10, 11, 12, 13, 14, 15, 16]. Vacuum deposition presents highly uniform layer formation over the entire substrate area, with thickness control ability. Furthermore, vapor-assisted crystallization is known to reproducibly provide densely packed microstructure through controlled chemical reaction speed via diffusion of organic material [17, 18, 19, 20, 21, 22, 23, 24, 25, 26].
Here, we report a novel sequential vapor-processing route by CH3NH3I (MAI)-vapor diffusion into vacuum-deposited PbCl2 layers, resulting in fully covered and highly uniform perovskite layers. Planar n-i-p heterojunction perovskite solar cells are successfully demonstrated by employing TiO2 and 2,2′,7,7′-tetrakis-(n,n-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) charge transporting layers. Champion cells achieve power conversion efficiencies (PCE) up to 11.5%. Our results show that this route is feasible to fabricate uniform and reproducible perovskite layers in a controlled way.
Devices were fabricated on fluorine-doped tin oxide (FTO)-coated glass substrates. The substrates were sequentially cleaned in an ultrasonic bath by acetone, methanol, isopropanol, and deionized water and then exposed to ultraviolet-ozone for 15 min. For electron transporting layers, 450 and 600 mM titanium diisopropoxidebis(acetylacetonate) in n-butanol (75 wt% in isopropanol) were double-coated at 2500 rpm for 20 s and annealed at 500 °C for 30 min in air to form compact TiO2 layers. The TiO2-coated substrates were placed in a vacuum chamber, and PbCl2 was evaporated to a rate of 1 Å/s for ~ 16 min at room temperature. Methyl-ammonium iodide (MAI) vapor treatments were carried out in a drying vacuum oven using MAI powder spread around the PbCl2-coated substrates. Subsequently, the as-prepared black samples were washed with isopropanol for the removal of the MAI residue and then annealed at 100 °C for 1 h. For hole transport layers, precursor solutions were prepared by mixing spiro-OMeTAD in chlorobenzene with tert-butylpyridine and lithium bis(trifluoromethylsyfonyl)imide salt in acetonitrile. The solutions were spin-coated at 4000 rpm for 40 s, and then the coated samples were kept in air overnight for oxidation. Finally, device fabrication was completed by thermal evaporation of Au electrodes.
The crystal structure was analyzed by X-ray diffraction (XRD, Ultima IV: RIGAKU), and the morphology of the perovskite layer was observed by a field emission scanning electron microscopy (FE-SEM, S-4300: HITACHI). The optical absorbance data were obtained using a UV-Vis spectrophotometer (UV-1601PC: Shimadzu). The photocurrent density-voltage (J-V) curves of the perovskite solar cell devices were recorded with a solar simulator (94021A: Newport) under AM 1.5G (100 mW/cm2) irradiation. During the measurements, the solar cell devices were masked with an aperture area of 0.09 cm2.
Results and Discussion
Photovoltaic parameters of the perovskite solar cells with various perovskite thicknesses
16.30 ± 0.08
0.89 ± 0.02
0.72 ± 0.01
10.40 ± 0.02
17.57 ± 0.12
0.91 ± 0.01
0.67 ± 0.02
10.68 ± 0.39
18.33 ± 0.09
0.91 ± 0.01
0.67 ± 0.02
11.24 ± 0.34
18.73 ± 0.24
0.89 ± 0.02
0.61 ± 0.02
10.23 ± 0.46
19.90 ± 0.01
0.81 ± 0.01
0.59 ± 0.02
9.58 ± 0.27
We reported a novel fabrication route through the physical vacuum deposition of PbCl2 layers and the following MAI vaporization-assisted perovskite growth. The optical absorption and XRD spectra verified the formation of highly crystalline and pure perovskite layers. High quality, compact, and pinhole-free perovskite layers were confirmed with the average grain size of ~ 320 nm. The regular type planar heterojunction perovskite solar cells were fabricated by employing TiO2 and spiro-OMeTAD as electron and hole transporting layers, respectively. The champion cell showed the best efficiency of 11.5% with a small deviation, which means the good reproducibility and uniformity of the perovskite layers produced by this vapor-processing route. As a future work, it is necessary to further develop perovskite layer quality with optimizing device structure to improve the efficiency and reduce the hysteresis behavior while maintaining the benefits of the synthetic route.
This research was supported by the Leading Human Resource Training Program of Regional Neo Industry (2016H1D5A1910305) and Basic Science Research Program (2017R1D1A1A02017758) through the National Research Foundation of Korea (NRF). The present research was conducted by the research fund of Dankook University in 2017.
WGC carried out the overall experiments. WGC and FPG wrote the manuscript. SN and CGP helped with the experiments. DWK improved the manuscript. TM supervised this work. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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