Page 234 - A Comprehensive Guide to Solar Energy Systems

Chapter 11 • Hybrid Organic–Inorganic Metal Halide Perovskite Solar Cells 237

FIGURE 11.2 Deposition methods for perovskite thin films, including (A) single-step solution deposition, (B) two-step
solution deposition, (C) two-step hybrid deposition, and (D) thermal vapor deposition. Reproduced with the permission
from Song Z, Watthage SC, Phillips AB, Heben MJ. Pathways toward high-performance perovskite solar cells: review
of recent advances in organo-metal halide perovskites for photovoltaic applications. J Photon Energy 2016;6:022001,
copyright 2016.

11.2.4 Thermal Vapor Deposition

The thermal evaporation method utilizes a dual source for MX 2 and MAI or FAI with dif-
ferent heating elements to form perovskite films [26]. This method provides high-quality
perovskite thin film with a uniform thickness and of pin-hole-free composition. The first
planar heterojunction MAPbI 3−x Cl x solar cell was fabricated in 2013 using thermal vapor de-
position technique with efficiency of >15% [26]. One of the disadvantages of thermal vapor
deposition technique is the requirement of precise control of temperature during deposi-
tion as both precursor sources and the products have low thermal stability. The two-step
vapor-assisted deposition and thermal vapor deposition methods are described in Fig. 11.2.

11.3 Perovskite Solar Cell Device Structure

The hybrid organic–inorganic metal halide perovskite thin film solar cell device structure
is just a modified version of the dye-sensitized solar cell (dSSCs) device structure [16,17].
In the dSSC process, a porous TiO 2 film is deposited onto SnO 2 :F which is then coated
with dye molecules and the system is dipped into a liquid electrolyte kept in metal elec-
trode such as platinum (Pt) [54]. For the metal electrode, a separate metal (Pt) plate is
made with a thin layer of iodine electrolyte spread over a conducting sheet. The two metal
plates are then joined and sealed together to prevent the electrolyte from leaking. In 2012,
Kim et al. [18] fabricated the first solid-state perovskite solar cells by depositing MAPbI 3
onto sub-micron thick mesoporous TiO 2 film and completed the solar cells by depositing
a hole-transport layer spiro-MeOTAd and back contact metal, Au. This work by Kim et al.
[18] was followed by another similar solar cell fabrication technique by Lee et al. [19] with
an efficiency of 9.7%. These two demonstrations not only drew the attention of scientific