Degradation Mechanism of Planar Heterojunction Perovskite Solar Cells for High Stability
Authors: Yamamoto Kohei , Shahiduzzaman Md , Furumoto Yoshikazu , Yonezawa Kyosuke , Kuwabara Takayuki , Takahashi Kohshin , Taima Tetsuya 
Published: February 2020

Perovskite solar cells (PSCs) have recently emerged as promising cost-effective and high-efficiency nanostructured solar cells. Unfortunately, perovskite solar cells have a problem with decreacement of power conversion efficiencies (PCE) due to degradation in the air. It is discussed that degradation of PSCs is caused by moisture and oxygen intercalation to perovskite layer. To avoid this problem, we introduce our system with no air exposure from fabrication to measurement by connecting with globe box and evaporation chamber. In this work, we try to detect the influence of air exposure among fabrication steps. Device structure of our PSCs is ITO / compact-TiOx / perovskite (CH3NH3PbI3: MAPbI3) / spiro-OMeTAD / Au. The MAPbI3 was deposited by sequential vacuum deposition of PbI2 and CH3NH3I (MAI) layer. To remove MAI remains on MAPbI3, the deposited films by sequential vacuum deposition of PbI2 and MAI were rinsed with 2-propanol solvent. Then, hole transport layer (spiro-OMeTAD) and Au electrode were deposited in glove box and evaporation chamber without air expose, respectively. Measurement of solar cell characteristics was done in encapsulated container filled in nitrogen gas. To clarify the influence of air exposure on PSCs, we exposed deposited films in the air for 30 min among various fabrication steps. The deposited film after MAI deposition and deposited film washed 2-propanol solvent after MAI deposition were exposure for 30 min in air. The parameters of forward scan measurements from the negative bias side to the opposing positive bias side under AM 1.5G simulated solar light are listed. The PCE of reference without air exposure was 4.1%, while the PCE of PSC exposed after 2-propanol washed is 3.6%. The PCE of PSC exposed after MAI deposition is dramatically reduced to 0.45%, which is 90% reduction compared that of reference without air exposure. This result suggests that the MAI remains on perovskite surface are trigger of degradation. Therefore, it is critical that perovskite solar cells are encapsulated from fabrication to measurement. The degradation mechanism for MAPbI3 film in air exposure has never reported. To clarify the influence of air exposure on MAPbI3 films, we introduced the O2 or (H2O+N2) gases in cylindrical holder in which deposited film after MAI deposition is placed. The absorption, morphology and crystallinity of the resulting films were investigated by using characterization techniques including UV-vis, AFM, and XRD of various films. Analysis of the film revealed that the UV-vis spectrum of O2 atmosphere condition was almost same as that of reference without air exposure. On the other hand, the UV-vis spectrum of (H2O+N2) atmosphere condition was significantly decreased at light absorption region from 400 to 700 nm. This spectral change is comparable to changing the band gap to being wide of MAPbI3 films. The AFM images show that 300 nm uniform particle shaped grains were observed in MAPbI3 films without air exposure and with O2 atmosphere conditions. On the other hand, the film exposed in (H2O+N2) atmosphere has morphology with non-uniform 1 µm large grains. These results indicate that MAI remains on MAPbI3 surface became rough by H2O molecules intercalation into MAPbI3 film inside. In the case of XRD spectrum, the MAI diffraction peak (001) was observed in reference without air exposure and with O2 atmosphere conditions. On the other hand, the MAI diffraction peak (001) disappeared during exposure in (H2O+N2) atmosphere condition. We assume that the reacted MAI molecule with H2O molecule acts as a new organic molecule with another chemical and physical property. Therefore, MAI diffraction peak (001) was disappeared and band gap of film was changed.