Hybrid organic-inorganic perovskite solar cells (PSCs) have been a rising star in solar
energy conversion devices. In the recent five years, the certified champion power
conversion efficiency (PCE) of PSCs has leaped to 25.2% from 15%, surpassing that of
multicrystalline silicon (22.8%) and CIGS solar cells (23.4%), and approaching the record
of single crystal silicon solar cell at 26 %. This is due to the unique structural flexibility of
perovskite material over traditional inorganic photovoltaic system in that there are vast
possibilities to alter the chemical compositions, fabrication processes and associative
materials for device optimizations and scientific investigations. Inverted PSCs are one type
of PSCs that exhibit great scalability and good device stability and performance due to the
large selection space of material systems. My thesis is mainly focused on studying the
composition and interface of perovskite and charge transport materials and their impacts on
stability and performance of resulted solar cell device.
Perovskite solar cell adopts layer-by-layer structure consist of bottom glass or flexible
substrate with transparent conducting oxide (TCO) as bottom electrode. On top of TCO, a
layer of hole transporting layer (HTL) or electron transporting layer (ETL) is applied to
extract charge from perovskite layer on top. This ETL or HTL will be used as substrate to
deposit subsequent perovskite layer which could dramatically affect the film quality due to
specific surface properties. Also, the carrier concentration, energy level and carrier mobility
of such layer determines the charge extraction as well as charge transport properties in
contact with perovskite materials. Perovskite layer is normally deposited by solution
process. Due to the polycrystalline nature of resulted perovskite thin film, the orientation
and morphology of perovskite film is decisive in the process of light absorption, charge
generation and transport. Also, the composition of perovskite precursor solutions can be
easily adjusted for achieving certain objectives. A subsequent ETL or HTL is then placed
before final evaporation of top electrode normally being metallic materials. In addition, the
interfaces between each layers can also be critical in solar cell operation and attracts
tremendous scientific effort. To achieve efficient solar cell operation, ETL/Perovskite/HTL
layers and their interfaces requires thorough investigations and engineering attempts.
In my thesis research, I mainly studied inverted perovskite solar cell based on NiO
x as
HTL and PCBM as ETL, the basic device structure is
glass/FTO/NiO
x/Perovskite/PCBM/Ag. I mainly adopted two approaches for efficient and
stable photovoltaic devices. First, the composition of perovskite has been studied to
understand the physical and chemical impact of compositional variation of related materials
on solar cell performance and stability. Due to the structural tolerance of perovskite material,
the composition can be varied at a certain range without significantly disrupting the
structure and properties. I unravel the effect of different cations including cesium (Cs),
methylammonium (MA) and formamidinium (FA) in small band gap FA-based perovskite
formula. I systematically studied the crystal growth process with different perovskite
formula and found an important crystalline intermediate-mediated film growth that has
significant impact on film quality. Increased relative amount of MA against Cs resulted in
the formation of MAI-PbI
2-DMSO intermediate which retards the crystallization and
hinders the transformation of photoactive perovskite phase. On the other hand, Cs rich
formula leaded to PbI
2-DMSO intermediate which facilitates the process. The impact of
compositional variation on electronic structure and hence energy alignment has also been
revealed. The inclusion of smaller size MA and Cs shifted the conduction band to be better
aligned with electron transporting PCBM. The collective effect from better film quality and
energy alignment contributed to a NiO
x-based inverted PSCs with a PCE up to 18.6 % and
superior ambient stability to TiO
2-based n-i-p structure device. This intermediate-dependent
film growth process has also been elaborated in a different wide bandgap all bromide
FAPbBr
3 perovskite as a result of different solvent composition. A new intermediate
structured FABr-PbBr
2-DMSO was discovered which retarded the crystallization of
FAPbBr
3, thus allowing delicate control over film formation process. The high quality
FAPbBr
3 thin film was first time achieved via one-step deposition and an improved
photovoltaic performance at 9% was achieved. These results revealed that both the
composition of perovskite materials and solvents play decisive role in the deposition of
perovskite thin film. Secondly, the interface between hole transporting NiO
x and perovskite
as well as the interface between perovskite and electron transporting PCBM were modified
to facilitate the charge transport across the interface. I investigated both the energy
alignment and chemical contacts and found out both are important for interface charge
transport. NiO
x with Sulfur doping enabled modification on the valence band maximum of
NiO
x for a better match with perovskite. The chemical linkage between perovskite and NiO
x
through sulfur is also proved to be beneficial for film growth. The conduction band
minimum between perovskite and PCBM is revealed to correlate with charge carrier
transport across the interface. From all these efforts, I boosted the performance of inverted
NiO
x-based triple-cation perovskite solar cell from less than 16 % to over 20 % and all
bromide wide bandgap perovskite solar cell from less than 5 % to around 9 %. More
significantly, I discovered and elaborated the appearance of intermediate phase in the film
formation process of perovskite layer under different scenarios such as perovskite and
solvent compositions. The structure and property of crystalline intermediate is elaborated
which affects the morphology and chemistry of resulted perovskite film. Intermediates are
thus demonstrated to be essential and can be modulated by varying the composition of
perovskite precursor as well as solvent. I also established new chemical treatment methods
and material for promoting the interfaces in perovskite solar cell device. It is found that
additional chemical linkage and suitable electronic structure are powerful tools for effective
interfacial charge transport. The science and engineering behind every promotion on solar
cell performance was carefully studied for pushing our knowledge and expanding the
methodologies towards commercially available solar cell product to achieve sustainable
future.
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