Page 241 - A Comprehensive Guide to Solar Energy Systems

244 A COMPREHENSIVE GUIdE TO SOLAR ENERGy SySTEMS

Similarly, yang et al. fabricated high efficient solar cells using intramolecular exchange
with the absorber: (MAPbBr 3 ) x (FAPb 3 ) 1−x [6].
Another way of changing the band gap of perovskite is by replacing toxic divalent metal
(Pb) with a more environmentally friendly tin (Sn) in MAMX 3 [94,95]. Tin-based perovskites
have promising photovoltaic properties like narrower optical band gap (∼1.0 eV) and
higher carrier mobility [96,97]. Germanium-based perovskite solar cell is equally promis-
ing. Mathews et al. [91] using a theoretical calculation showed that Ge can be an alterna-
tive replacement for Pb. Ternary bismuth halides perovskite material has also been re-
ported as a replacement for toxic Pb-based perovskite but their results in PV applications
are not yet available [98,99].

11.4.3 Electron and Hole Transporting Materials Optimization

Efficient charge extraction to the outer electrodes in perovskite solar cells is determined by
the perovskite/ETL and perovskite/HTL interfaces. Proper ETL and HTL materials main-
tain a low surface and interface charge recombination which have high degree of charge
selectivity. The most common metal oxide ETL essential for high efficient perovskite solar
cells is mesoporous and planar TiO 2 . The charge transfer rates from the perovskite absorb-
er layer to TiO 2 ETL layer is very fast. At the same time, the electron recombination rates
are also high in TiO 2 due to the low mobility and transport properties [100]. Zinc oxide,
ZnO, nanorods and nanoparticles have been introduced as ETL layers in perovskite so-
lar cells resulting in efficiencies as high as 11.1% and 15.9%, respectively [101,102]. Other
metal oxides that have been utilized in mesoporous ETL layers in perovskite solar cells
include Al 2 O 3 [19,45], SiO 2 [103], ZrO 2 [104,105], and SrTiO 3 [106]. Zinc oxide has been used
as a compact ETL layer in planar structure with an efficiency of 15.7% [24]. Baena et al.
[107] fabricated high efficient planar perovskite solar cells of 18% efficiency using SnO 2
as ETL. Other ETL materials that have been included in a planar n-i-p structure format
are CdS [108], CdSe [109], and TiO 2 -graphene [110]. These are inorganic ETL materials for
perovskite solar cells. ETL materials from organic solar cells are also widely used as ETLs in
perovskite solar cells. Fullerence (C 60 ) and its derivatives such as [6,6]-phenyl-C61-butyric
acid methyl ester (PC 61 BM), indene-C60 bisadduct (ICBA), and PC 71 BM are ideal candi-
dates as efficient electron extraction materials due to their low temperature fabrication,
suitable energy level alignment and decent carrier mobility [63,111,112].
The first solid state perovskite solar cells were fabricated by the substitution of liquid
electrolyte with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′spirobifluorene
(spiro-OMeTAd) as the HTL, achieving an efficiency of 10% in the mesoporous structure
[18,19]. Since then, this small molecular organic material has become the most commonly
used HTL in high efficient perovskite solar cells. The preparation of the spirobifluorene
core in spiro-OMeTAd molecule requires an extensive synthetic process and is respon-
sible for an increase in the production costs. In the process of finding cost effective HTL
materials for efficient carrier transport in perovskite solar cells, researchers have used the
organic polymer, poly(triarylamine) (PTAA) [6], 3,4-ethylenedioxythiophene (EdOT) [113],
and many other organic materials; the full list is provided elsewhere [114]. Even though