Page 233 - A Comprehensive Guide to Solar Energy Systems

236 A COMPREHENSIVE GUIdE TO SOLAR ENERGy SySTEMS

the diffusion length of charge carriers [42,43]. To establish these criteria, engineering of
material composition, film fabrication method, control crystallization, desired substrate
selection, and solvent/additives selection are conducted with care. The nature of the sub-
strate surface morphology determines the quality of the film formed on it. The perovskite
film is of high quality when the substrates of interest are mesoporous, irrespective of TiO 2 ,
Al 2 O 3 , or NiO, and it provides excellent device performance [44]. The crystal size of the
mixed halide perovskite (MAPbI 3−x Cl x ) reduces to <100 nm in mesoporous Al 2 O 3 com-
pared to ∼500 nm in planar system, resulting in an increase J SC of the solar cells [45]. There
are various deposition techniques for high quality perovskite thin films such as single-step
solution deposition [19], two-step solution deposition [23], two-step vapor-assisted depo-
sition [25], and thermal vapor deposition [26].

11.2.1 Single Step Deposition
In the one-step deposition process, both organic and inorganic compounds are code-
posited either through solution or thermal evaporation processes as described elsewhere
[46–48]. In the single-step solution process, a mixture of MX 2 (M = Pb or Sn and X = I, Br, and
Cl) and methylammonium iodide (MAI) or formamidinium iodide (FAI) is dissolved in an
organic solvent and the solution mixture is spun coated onto the substrate of interest. The
film so obtained is annealed at 100–150°C to produce the final perovskite phase. In the de-
position process used to fabricate high performance solar cells, the composition variation
has been made from MAI poor to MAI rich in PbI 2 in the ratio of 1:2–3:1 [49,50]. When the
precursor composition is changed, there needs to be a change in processing temperature
and time to maintain desired crystal structure, phase purity, and morphology [43,51,52].

11.2.2 Two Step Sequential Deposition
In the two-step solution process, a MX 2 seed layer is first spun coated onto the substrate
and the substrate is then dipped into MAI or FAI/isopropanol solution or another spin
coating is made for MAI or FAI onto a MX 2 network to form a hybrid organic–inorganic
metal halide perovskite [23,53]. The two-step process has been shown to be better as it
provides a more uniform and controlled film and has been extensively used in solar cell
fabrications [6,23,44]. The two-step process, in contrast, has some problems such as in-
complete perovskite conversion and surface roughness in some cases but with the intro-
duction of new techniques these problems have been reduced. One-step and two-step
solution deposition processes are described in Fig. 11.2 [47].
11.2.3 Two Step Vapor Assisted Deposition

The two-step solution deposition method has been modified and described by Chen et al.
[25], here the MAI is introduced into the MX 2 layer via a vapor deposition technique in
order to control the morphology and grain size of the perovskite film in a better manner.
This deposition method is time consuming as it takes a long time to fully convert to the
perovskite and furthermore, the performance is not efficient [25].