electrons, promoting them from the valence band to the conduction band.

This process
generates mobile electron-hole pairs. 1

In a p-n junction, an internal electric field is established due to the diffusion of charge
carriers across the junction, creating a depletion region. This electric field acts as a
driving force, separating the photogenerated electron-hole pairs. Electrons are swept
towards the n-type region, and holes are driven towards the p-type region. When an
external circuit is connected, these separated charge carriers flow through it,
producing an electrical current and voltage. This external current and voltage
constitute the power output of the solar cell. 1

The performance of a solar cell is quantitatively assessed by several key parameters

derived from its current-voltage (I-V) characteristics. These include:
● Open-Circuit Voltage (Voc): The maximum voltage produced by the solar cell
when no current is drawn (i.e., under open-circuit conditions).
● Short-Circuit Current Density (Jsc): The maximum current density produced by
the solar cell when the voltage across it is zero (i.e., under short-circuit conditions).
● Fill Factor (FF): A measure of the "squareness" of the I-V curve, representing the
ratio of the maximum power output to the product of Voc and Jsc. A higher FF
indicates a more ideal diode behaviour and better power extraction.
● Power Conversion Efficiency (PCE): The most critical metric, representing the
ratio of the maximum electrical power produced by the solar cell to the incident
optical power.

Simulation software like SCAPS-1D is instrumental in extracting these parameters from

simulated I-V curves, providing a comprehensive evaluation of device performance
under various conditions.12

2.2 Perovskite Crystal Structure and Properties

Perovskite materials are defined by a specific crystal structure, represented by the

generic formula ABX3. In this formula, 'A' denotes a large monovalent organic cation
(e.g., methylammonium (CH3NH3+), formamidinium (NH=CHNH2+)) or an inorganic
cation (e.g., Cs+, Rb+). 'B' represents a smaller divalent metal cation (e.g., Pb2+, Sn2+,
Ge2+), and 'X' stands for a monovalent halide anion (e.g., Br-, I-, Cl-). 5 The arrangement
of these ions forms a cuboctahedral cage around the 'A' cation, with the 'B' cation at
the centre of an octahedron formed by the 'X' anions.

The structural stability of a perovskite compound can be predicted by the Goldschmidt

tolerance factor (t), calculated as t = (RA + RX) / √2(RB + RX), where RA, RB, and RX are
the ionic radii of the A, B, and X ions, respectively. For a stable 3D perovskite structure,
the tolerance factor typically falls within the range of 0.813 to 1.107. 14 This factor is
crucial for guiding the synthesis of new perovskite compositions.

A remarkable characteristic of perovskites is their exceptional chemical and structural

versatility.3 This versatility allows for an enormous number of possible compositions
through the permutation of various A-site cations, B-site metals, and halide anions. For
instance, studies have reported thousands of potential perovskite options based on
permutations of protonated amines, anions, and divalent metal ions. 3 This number
further expands significantly when considering doping strategies, where additional
elements are intentionally introduced to modify material properties. 3

This vast compositional design space carries significant implications. The current
limitations observed in the efficiency and stability of perovskite solar cells, particularly
for lead-free tin-based variants, are not necessarily inherent material bottlenecks.
Instead, they represent challenges in effectively navigating and identifying optimal
compositions and structures within this immense chemical landscape. The ability to
precisely tune the electronic, optical, and structural properties by varying the A, B, and
X sites, and by introducing specific dopants, provides a powerful avenue for
overcoming existing issues such as tin oxidation and improving film quality. This
suggests that continued research into novel lead-free perovskite compositions and
precise doping strategies, guided by advanced computational simulations like SCAPS-
1D, holds substantial promise for unlocking even higher performance and enhanced
stability, ultimately bringing these materials closer to their theoretical limits.

Perovskite materials possess a suite of highly desirable electronic and optical

properties that contribute to their high efficiency in solar cells. These include high
absorption coefficients, which enable efficient light harvesting even with thin layers;
tuneable direct bandgaps, allowing for optimization to match the solar spectrum; long
carrier mobility, facilitating efficient charge transport; and long charge transport
diffusion paths.3 Furthermore, many hybrid halide perovskites exhibit ambipolar charge
transportability, meaning they can transport both electrons and holes effectively. 5
2.3 Charge Transport Mechanisms in PSCs

Efficient charge transport is a fundamental prerequisite for high-performance solar

cells. In perovskite solar cells, this process involves the coordinated action of
specialized layers designed to extract and transport photogenerated charge carriers.
When light is absorbed by the perovskite active layer, electron-hole pairs are
generated. To produce an electrical current, these charges must be efficiently
separated and transported to their respective electrodes with minimal recombination
losses.6

The device architecture typically includes an Electron Transport Layer (ETL) and a Hole
Transport Layer (HTL). The ETL is positioned between the perovskite absorber and the
Transparent Conductive Oxide (TCO) electrode (e.g., FTO). Its primary function is to
selectively extract electrons from the perovskite layer and transport them to the TCO,
while simultaneously blocking the flow of holes. 6 Conversely, the HTL is situated
between the perovskite layer and the counter electrode (typically a metal). The HTL's
role is to extract holes from the perovskite and transport them to the counter electrode,
while blocking electrons.6

Efficient charge separation predominantly occurs at the interface between the ETL and
the perovskite photoactive layer.6 For optimal device performance, proper energy level
alignment between the perovskite absorber and both the ETL and HTL is crucial. The
conduction band minimum (CBM) of the ETL should align favourably with the CBM of
the perovskite to facilitate electron extraction, and similarly, the valence band
maximum (VBM) of the HTL should align with the VBM of the perovskite to enable
efficient hole extraction.6 Any significant energy offset or misalignment can lead to
energy barriers that hinder charge transfer, increase recombination at the interfaces,
and ultimately reduce the open-circuit voltage (Voc) and fill factor (FF) of the solar cell.

2.4 Device Architectures of PSCs

Perovskite solar cells can be fabricated in various device architectures, each offering
distinct advantages in terms of performance, stability, and fabrication scalability. The
common configurations include mesoscopic, bi-layer, and planar types. 6
Mesoscopic devices, which were among the earliest high-efficiency PSCs, typically
incorporate a porous metal oxide scaffold (e.g., mesoporous TiO2) infiltrated with the
perovskite material. This architecture provides a large interfacial area for charge
separation and transport.6 Bi-layer and planar configurations, on the other hand,
feature smoother, more compact layers. Planar devices, in particular, have gained
significant attention due to their simpler fabrication processes and high-performance
potential.

Planar perovskite solar cells are broadly categorized into two main configurations
based on the arrangement of their charge transport layers:
● Regular (n-i-p) Architecture: In this setup, the device typically consists of a
transparent conductive oxide (TCO) substrate (e.g., FTO), followed by an electron
transport layer (ETL), the intrinsic perovskite absorber layer, a hole transport layer
(HTL), and finally a metal back contact. The structure is commonly represented as
TCO/ETL/Perovskite/HTL/Metal.5 Devices employing this architecture with
materials like PTAA and Spiro-OMeTAD have demonstrated impressive power
conversion efficiencies exceeding 22% and 25%, respectively. 16
● Inverted (p-i-n) Architecture: In contrast, the inverted configuration places the
HTL directly on the TCO substrate, followed by the perovskite absorber, the ETL,
and then the metal back contact. This structure is typically represented as
TCO/HTL/Perovskite/ETL/Metal.16 Inverted p-i-n devices utilizing PTAA as the HTL
have also achieved high PCEs, exceeding 25%. 16 The choice between regular and
inverted architectures often depends on the specific materials used, their energy
level alignment, and desired processing techniques.
Chapter 3: Materials for Lead-Free Perovskite Solar Cells

3.1 Methylammonium Tin Bromide (MaSnBr3) Absorber Layer

Methylammonium Tin Bromide (MaSnBr3) is a prominent candidate among lead-free

perovskite materials being investigated for solar cell applications. Its adoption stems
from the imperative to replace toxic lead in high-performance perovskite devices,
offering a more environmentally benign alternative.3

3.1.1 Electronic, Optical, and Structural Properties

MaSnBr3 is classified as a three-dimensional (3D) metal halide perovskite. 3 Its

structural integrity and stability can be assessed by its Goldschmidt tolerance factor,
which typically falls within the stable range of 0.8 < t < 1.0. This material also exhibits a
consistency formation energy of 141 eV, indicating its thermodynamic favorability. 14 The
room temperature crystal structure of MASnBr3 is orthorhombic. 9

Analysis of its electronic band structure reveals distinct contributions from its
constituent elements. In the valence band, the majority contribution is derived from the
Br-p states and Sn-s states. Conversely, the conduction band is predominantly
composed of Sn-p states, with a minimal contribution observed from the organic
methylammonium (MA) molecules.14 This electronic configuration is crucial for its
optoelectronic functionality. Furthermore, two-dimensional (2D) MASnBr3 exhibits a
high static dielectric constant of 2.48, a property that can be beneficial for nanodevice
performance.14

3.1.2 Band Gap and Absorption Characteristics

The band gap is a critical parameter for any semiconductor material intended for solar
cell applications, as it dictates the range of light wavelengths that can be absorbed.