Identifying the Molecular Structures of Intermediates for Optimizing the Fabrication of High-Quality Perovskite Films

During
the past two years, the introduction of DMSO has revolutionized
the fabrication of high-quality pervoskite MAPbI3 (MA =
CH3NH3) films for solar cell applications. In
the developed DMSO process, the formation of (MA)2Pb3I8·2DMSO (shorted as Pb3I8) has well recognized as a critical factor to prepare high-quality
pervoskite films and thus accomplish excellent performances in perovskite
solar cells. However, Pb3I8 is an I-deficient
intermediate and must further react with methylammonium iodide (MAI)
to be fully converted into MAPbI3. By capturing and solving
the molecular structures of several intermediates involved in the
fabrication of perovskite films, we report in this work that the importance
of DMSO is NOT due to the formation of Pb3I8. The use of different PbI2-DMSO ratios leads
to two different structures of PbI2-DMSO precursors (PbI2·DMSO and PbI2·2DMSO), thus dramatically
influencing the quality of fabricated perovskite films. However, such
an influence can be minimized when the PbI2-DMSO precursor
films are thermally treated to create mesoporous PbI2 films
before reacting with MAI. Such a development makes the fabrication
of high-quality pervoskite films highly reproducible without the need
to precisely control the PbI2:DMSO ratio. Moreover, the
formation of ionic compound (MA)4PbI6 is observed
when excess MAI is used in the preparation of perovskite film. This
I-rich phase heavily induces the hysteresis in PSCs, but is readily
removed by isopropanol treatment. On the basis of all these findings,
we develop a new effective protocol to fabricate high-performance
PSCs. In the new protocol, high-quality perovskite films are prepared
by simply treating the mesoporous PbI2 films (made from
PbI2-DMSO precursors) with an isopropanol solution of MAI,
followed by isopropanol washing. The best efficiency of fabricated
MAPbI3 PSCs is up to 19.0%. As compared to the previously
reported DMSO method, the devices fabricated by the method reported
in this work display narrow efficiency distributions in both forward
and reverse scans. And the efficiency difference between forward and
reverse scans is much smaller.