Double functions of porous TiO2 electrodes on CH3NH3PbI3 perovskite solar cells: Enhancement of perovskite crystal transformation and prohibition of short circuiting

Govindhasamy Murugadoss,Tomokazu Umeyama,Hitoshi Nishino,Gai Mizuta,Hiroshi Imahori,Seigo Ito,Soichiro Tanaka

doi:10.1063/1.4891597

Govindhasamy Murugadoss, Tomokazu Umeyama + Show 5 more

Open Access

https://doi.org/10.1063/1.4891597

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Abstract

In order to analyze the crystal transformation from hexagonal PbI2 to CH3NH3PbI3 by the sequential (two-step) deposition process, perovskite CH3NH3PbI3 layers were deposited on flat and/or porous TiO2 layers. Although the narrower pores using small nanoparticles prohibited the effective transformation, the porous-TiO2 matrix was able to help the crystal transformation of PbI2 to CH3NH3PbI3 by sequential two-step deposition. The resulting PbI2 crystals in porous TiO2 electrodes did not deteriorate the photovoltaic effects. Moreover, it is confirmed that the porous TiO2 electrode had served the function of prohibiting short circuits between working and counter electrodes in perovskite solar cells.

Highlights

Very recently, organic–inorganic lead halide based perovskites (CH3NH3PbX3 (X: Cl, Br, or I)) have emerged as a new class of light absorbers, achieving exceptional progress in solar cell performance
For the fabrication of a perovskite layer on a TiO2 layer, one-step deposition can provide the CH3NH3PbX3 perovskite layer directly,[1,2,3,4,5,6,7] whereas two-step deposition needs a PbX2 precursor, which is converted to CH3NH3PbX3 perovskite layers by dipping in CH3NH3X solution.[8,9,10,11]
In order to understand the functions of a porous TiO2 layer under a perovskite layer, CH3NH3PbX3 perovskite solar cells have been examined with variations of TiO2 particle size, porous-TiO2 thickness, PbI2 thickness for planar CH3NH3PbI3 structure on flat TiO2, and hole-transporting materials

Summary

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For the two-step process CH3NH3PbI3 perovskite deposition, 1.2 M of PbI2 was dissolved in 2 ml of N,N-dimethylformamide by stirring at 70 ◦C, deposited on the mesoporous TiO2 film by spin-coating at 6500 rpm for 20 s and dried at 70 ◦C for 30 min. The variation of crystal structure with the difference of PbI2 thickness on the flat-TiO2 layer was confirmed by their X-ray diffraction patterns (Fig. 1(a)). The PbI2 precursor in the pores of the TiO2 layer (d = 90 nm) with 1-μm thickness can be completely converted to CH3NH3PbI3 by the two-step method (Fig. 1(b)). The thickness variations of porous-TiO2 electrodes were studied in FTO/blocking– TiO2/porous–TiO2/CH3NH3PbI3/CuSCN/Au cells prepared with the one-step and two-step CH3NH3PbI3-deposition processes (Fig. 3).

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