Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 119

8.1 Structure, Chemical Composition, and Optoelectronic
Properties of Halogenated Perovskites
The halogenated hybrid perovskites have been previously studied as semicon-
ducting materials processable in solution at low temperature for thin-film field-
effect transistors, where they have shown to have a high mobility of carriers
compared with organic materials [ 169 ]. Hybrid perovskites of type ABX 3
(A = CH 3 NH 3 + ,B= Pb 2+ ,X= Cl - ,Br - ,I - ) used in the newly developed
photovoltaic devices are formed by inorganic layers of lead halide corner-sharing
octahedrals interpenetrated by an alkylammonium cation network where the size
of the organic cation plays an important role to define the final type of perovskite
structure formed, Fig. 21 .
The most used cations in the cells reported to date were methylammonium and
lead (II) for the positions A and B, respectively. (CH 3 NH 3 )PbI 3 compound has a
direct bandgap of 1.51 eV determined experimentally [ 170 ] and theoretically [ 171 ].
Also, this hybrid organometal halide perovskite has a high absorption coefficient
compared to the N719 dye [ 172 ]. The work function studied by photoelectron
spectroscopy of spin-coated polycrystalline films showed valence-band levels of
(CH 3 NH 3 )PbI 3 and (CH 3 NH 3 )PbBr 3 at -5.44 and -5.38 eV versus the vacuum
level, respectively and the conduction band levels calculated from the optical
absorption edges are at -4.0 and -3.36. Therefore, these profiles mean that
electron injection to the TiO 2 conduction band is favored. Figure 22 summarizes
the main optoelectronic properties of (CH 3 NH 3 )PbI 3 .
8.2 Construction of Solid-State Devices Using Wet Methods
Perovskites have been used as light harvester since 2009 when Miyasaka et al.
[ 173 ] reported that these materials could be an alternative to binary chalcogenide
based on QDSSCs, reaching an efficiency of 3.8 %, Fig. 23 . An interesting aspect
of the manufacture of these devices is that the active material is solution pro-
cessable using a stoichiometric solution of (CH 3 NH 3 )I and PbI 2 at room temper-
ature and without employ vacuum techniques. However, the liquid electrolyte used
as hole transport layer rapidly degrades the active material of sensitized cell.
Approximately 2 years later, a work from Park and collaborators developed a cell
which had twice the efficiency of Miyasaka cell due mainly to the use of more
concentrated perovskite precursor solutions [ 172 ]. However, the cell was also
deteriorated rapidly by the liquid electrolyte.
The breakthrough in efficiency (9.7 %) and stability ([ 500 h) was done by the
same group in 2012 using a solid electrolyte, the spiro-MeOTAD, instead of liquid
electrolyte, Fig. 24 [ 170 ]. At the same time, devices whose active layer is also