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All-inorganic inverse perovskite solar cells using zinc oxide nanocolloids on spin coated perovskite layer
© Korea Nano Technology Research Society 2017
Received: 6 June 2017
Accepted: 9 July 2017
Published: 28 July 2017
We confirmed the influence of ZnO nanoparticle size and residual water on performance of all inorganic perovskite solar cells. By decreasing the size of the ZnO nanoparticles, the short-circuit current density (Jsc) and open circuit photovoltage (Voc) values are increased and the conversion efficiency is improved. Although the Voc value is not affected by the influence of residual water in the solution for preparing the ZnO layer, the Jsc value drops greatly. As a result, it was found that it is important to use the oxide nanoparticles with a small particle diameter and to reduce the water content in the oxide forming material in order to manufacture a highly efficient all inorganic perovskite solar cells.
The rapid development of organic–inorganic metal halide perovskite (CH3NH3PbIxBr3−x) based solar cells has attracted worldwide attention due to the material’s high absorption coefficient, small exciton binding energy, and long carrier diffusion length [1–4]. This rapidly increasing conversion efficiency achieving from initial 3.81 to 22.1%, have been realized in period of the 7 years [5–9]. Since the perovskite is an inexpensive material and fabricated solar cell has a simple device structure owing to its solution process, it has the possibility to replace the commercially established solar cell such as Si, CIGS, and CdTe solar cells [10, 11]. In the commonly reported perovskite solar cells, organic materials (Spiro-OMe-TAD, PCBM, C60) on top of perovskite active layer having an energy level conforming to the perovskite layer and having good conductivity are used to support the efficient charge collection on top of perovskite active layer [12, 13]. Since these organic materials require high grade purity, they are very expensive. Moreover, they are likely to be the cause of lowering the durability of perovskite solar cells. Accordingly, the development of inexpensive and highly durable materials are required. Researches have been conducted to utilize inorganic materials (CuI, CuSCN, Cu2O/CuO) as a novel hole transporting layer (HTL) on the perovskite active layer in place of commonly employed organic layers [14–20].
In recent years, all inorganic based perovskite solar cells using oxide nanoparticles (ZnO, SnO2, Zn2SnO4) as the electron transporting layer (ETL) by using the NiOx layer which in the HTM layer as a scaffold have been reported [21–23]. Although the reported conversion efficiencies were inferior to that of perovskite solar cells using organic materials, all inorganic perovskite solar cells using oxide nanoparticles have been found to exhibit high durability. However, guidelines for improving the performance of all inorganic perovskite solar cells using oxide nanoparticles on the perovskite active layer are still unknown.
In this study, we fabricated all inorganic perovskite solar cells and investigated the influence of oxide nanoparticle size and its role towards solar cell performance. In addition, we investigated the effect of synthesis route on the performance and durability of the fabricated cells. The synthesized ZnO nanoparticles were used as ETL in fabricated all inorganic inverted perovskite solar cell where NiOx layer formed the scaffold. The results of this study can provide guidelines for using nanoparticles in all inorganic perovskite solar cells.
All solvents and chemicals were purchased and utilized as obtained. Nickel (II) acetylacetonate, zinc acetate and bathocuproine (BCP) were purchased from Sigma-Aldrich Co. LLC. CH3NH3PbI3-DMF (MAPbI3-DMF) was purchased from TCI Co., Ltd. All solvents and reagents were of the highest quality available and were used as received.
2.2 Synthesis of ZnO nanoparticles solution and powders
2.2.1 Aqueous process
ZnO nanoparticle_aque was synthesized following protocol in the previously published literature . To synthesize the ZnO nanoparticles by the aqueous process, the 250 μL deionized water was added to 4.46 mmol zinc acetate powder to make solutions. Further 42 μL CH3OH was added dropwise for 30 min at room temperature and Zn precursor solution was obtained. In the next step, KOH (7.22 mmol) was dissolved in CH3OH (23 mL) to make KOH solution. The Zn precursor solution was refluxed and, KOH solution was added dropwise into the flask maintaining time duration 15 min. Then, the mixture was refluxed for 30 min. The colloidal solution was replaced by isopropanol from water using a rotary evaporator and the concentration of ZnO was adjusted to 0.2 wt% [25–28].
2.2.2 Non-aqueous process
Zinc acetate powder (0.2 mmol) was dissolved in benzyl alcohol (50 mL) and it was stirred and maintaining 120 °C until a clear solution was obtained. The mixture was refluxed at three different time intervals of 20 min, 6 and 24 h, at 170 °C, to synthesize the different nanoparticle size of ZnO. The colloidal solution solvent (benzyl alcohol) was replaced with isopropanol using a rotary evaporator and the concentration of ZnO was adjusted to 0.2 wt% [25–28]. The different time interval synthesized ZnO nanoparticle was numbered as, No. 1 of 20 min, No. 2 of 6 h and No. 3 of 24 h, reflux time respectively.
2.2.3 Device fabrication
Dynamic light scattering (DLS) measurements were performed using a Malvern Instruments Zetasizer Nano ZS instrument equipped with a He–Ne laser (4 mW at 633 nm). Measurements were taken at a scattering angle of 173°. XRD patterns were recorded on a Rigaku MiniFlex2 diffractometer working with Cu Kα radiation.
The photocurrent–voltage (J–V) characteristics of the perovskite solar cells were measured on a B2901A (Agilent Technologies Inc.) source meter under irradiation of AM 1.5, 100 mW/cm2 (1 sun) supplied by a solar simulator (YSS-80, Yamashita Denso Co., Ltd.). The incident light intensity was calibrated with a reference Si solar cell (BS-500BK, Bunkoukeiki Co., Ltd.). The active areas of the solar cells were determined with a 0.3 cm × 0.3 cm black mask. ZnO powders for x-ray diffraction (XRD) characterization were dried on the hot plate at 80 °C. For DLS measurements to measure the size of ZnO nanoparticles, the concentration was diluted to 25 times using isopropanol.
3 Results and discussion
Performance of perovskite solar cell using synthesized ZnO nanoparticles
Particle diameter (nm)
In conclusion, we fabricated all inorganic inverted perovskite solar cells using synthesized ZnO nanoparticles and confirmed the optimum size of the ZnO nanoparticles and the superiority of the synthetic route. We found that Jsc and Voc value improves by decreasing the size of ZnO nanoparticles. Although Voc value is not affected by the influence of residual water in the solution for preparing the ZnO layer, Jsc value is decreased. As a result, we found two guidelines for manufacturing inorganic perovskite type solar cells with high efficiency; (1) use of oxide nanoparticles with small particle size; (2) reduction of moisture content in oxide forming material. In this experiment, it was impossible to understand the factor given to the fill factor (FF) value. For that reason, we are currently searching for factors that influence the value of FF value by performing internal interface analysis [35–37].
NS carried out the device fabrication, measurement and manuscript writing. HK assisted the device fabrication. SY assisted the measurement. SF, AKB, and SI discussed and helped draft the manuscript. HS, TM, and SI provided the advice on and coordinated the study. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The authors have no data to share since all data are shown in the submitted manuscript.
Funding and acknowledgements
This work was supported by New energy and industrial Technology Development Organization (NEDO), Japan.
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