  The electron transport layer (ETL) plays a fundamental role in perovskite
solar cells. Recently, graphene-based ETLs have been proved to be good
candidate for scalable fabrication processes and to achieve higher carrier
injection with respect to most commonly used ETLs. In this work we
experimentally study the effects of different graphene-based ETLs in sensitized
MAPI solar cells. By means of time-integrated and picosecond time-resolved
photoluminescence techniques, the carrier recombination dynamics in MAPI films
embedded in different ETLs is investigated. Using graphene doped mesoporous
TiO2 (G+mTiO2) with the addition of a lithium-neutralized graphene oxide
(GO-Li) interlayer as ETL, we find that the carrier collection efficiency is
increased by about a factor two with respect to standard mTiO2. Taking
advantage of the absorption coefficient dispersion, we probe the MAPI layer
morphology, along the thickness, finding that the MAPI embedded in the ETL
composed by G+mTiO2 plus GO-Li brings to a very good crystalline quality of the
MAPI layer with a trap density about one order of magnitude lower than that
found with the other ETLs. In addition, this ETL freezes MAPI at the tetragonal
phase, regardless of the temperature. Graphene-based ETLs can open the way to
significant improvement of perovskite solar cells.
