An Fe3O4 based hole transport bilayer for efficient and stable perovskite solar cells

The hole transport layer (HTL) critically affects the photovoltaic performance and stability of metal halide perovskite solar cells (PSCs). So far, spiro-OMeTAD remains the most successful HTL; however, concerns remain regarding the stability of the PSCs using a spiro-OMeTAD based HTL. Herein, we report an inorganic-organic Fe3O4/spiro-OMeTAD bilayer HTL that not only shows a superior device performance than a pristine spiro-OMeTAD counterpart but also an enhanced shelf-life stability. The experimental results show that the inclusion of an Fe3O4 layer between the perovskite and the spiro-OMeTAD reduces the surface roughness, which in turn leads to the formation of a smooth and pin-hole free HTL. The bilayer HTL design improves charge extraction and also reduces the interfacial trap density, as demonstrated by steady-state and time-resolved photoluminescence and space charge limited current measurements, respectively. The Fe3O4/spiro-OMeTAD bi-layer HTL demonstrated significant enhancement in photovoltaic performance, such as 11% higher power conversion efficiency (PCE) than the reference device, and also exhibited long-term shelf-life stability by retaining 89% of its initial PCE after around 1300 hours. Our study proposes a simplistic strategy for the fabrication of efficient and stable PSCs by employing the inorganic-organic stack architecture of HTLs.