Recently, organometal halide perovskite solar cells (PSCs) have received great attention. The power conversion efficiency (PCE) of PSCs have shown a dramatic increase and certified PCEs adopting mixed organic cations and halide anions have reached up to 22%. The PCE is considerably affected by photovoltaic property of each component of a PSC. Particularly, because crystal quality of materials is strongly concerned with the electronic properties such as carrier transport, investigation of detailed crystallographic information of the perovskite light absorber is essential. In spite of the significance in the crystallographic information, however, microstructural observation for crystal structure analysis of the perovskite layer has not been actively conducted. In this talk, we will report a microstructural observation about phase coexistence in the perovskite light absorber through transmission electron microscope (TEM) observation.
To obtain the crystallographic information of the perovskite light absorber, a pure methylammonium lead iodide (MAPbI3) layer was formed through spin-coating method assisted by antisolvent in a planar type PSC (Au/Spiro-MeOTAD/MAPbI3/TiO2/FTO/Glass). MAPbI3 precursor solution used is 1.4 M and the spin-coated MAPbI3 film was annealed at 100 oC for 30 min.
Surprisingly, during the high resolution (HR) TEM observation, we found the coexistence of tetragonal and cubic structures in the perovskite layer. Additionally we found such a mixture condition affetcs on the photovoltaic peroformance of the perovskite solar cells. This new observation is expected to be an important clue of the enhancement of perovskite crystal quality for highly efficient PSCs.