University of California, Santa Cruz

Department of Applied Physics

Instructor: Dr. Sue A. Carter

An Analysis of Germanium Quantum

Dots in conjunction with MAPbI3

Host Perovskites for Hybrid

Photovoltaic Materials

Phoenix Nalu Hakumele Gallagher

Abstract

Opportunities for more sustainable solutions in solar cell technology warrant consider-

ation of new semiconductor materials. Nano-sized germanium particles are combined with

an organo-metal halide perovskite host crystal to develop a novel photovoltaic material.

This hybrid material is a promising candidate for producing highly efficient, low-cost, so-

lution based solar cells. Different concentrations of germanium nanoparticles with various

surface bonds are added in the perovskite precursor solution to develop thin-film devices.

We investigate the effects of structure and composition in the germanium nanoparticle be-

havior, both alone and in combination with the perovskite material. The best techniques

for film fabrication are explored, characterized by various spectroscopy techniques. De-

vices are then tested as a solar cell. We find that alone, the behavior of the nanoparticles

is strongly effected by adding dopants to the system. When combined with the perovskite,

the nanoparticles increase the current. With a nanoparticle concentration of 400 µL added

to the perovskite solution, one device exhibited a current density of ∼ 1.1 mA cm−2 higher

than the same perovskite sample in a pure state. The effects of the germanium nanopar-

ticles provide positive evidence of their impact in the perovskite, developing a basis for

future work in creating efficient hybrid nanoparticle/perovskite solar cells.

Contents

1 Introduction 2

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.1 PV Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.2 Bulk and Thin Film Architectures . . . . . . . . . . . . . . . . . . . 8

1.2.3 Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.2.4 MAPbI3 Perovskites . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2 Methods and Experimental Design 14

2.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.1.1 Slide Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.1.2 TiO2 Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.1.3 QD Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1.4 MAPbI3 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.1.5 Hybrid QD/MAPbI3 Blend Fabrication . . . . . . . . . . . . . . . . 20

2.1.6 Cathode Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.2.1 Voltage-Current Measurements . . . . . . . . . . . . . . . . . . . . 21

2.2.2 Material Absorbance . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.2.3 PDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.2.4 EXAFS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3 Results and Discussion 24

3.1 QD Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.2 Hybrid QD/MAPbI3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4 Conclusions and Future Work 31

5 Notation 33

6 Literature Cited 34

List of Figures

1 Cell Efficiency Chart from 1975-2020 displaying the improvement in effi-

ciency of different types of solar cells and solar cell materials. Perovskites are

yellow circles with a red outline and quantum dots are yellow diamonds with

a red outline. Reprinted from NREL. Retrieved from https://www.nrel.gov/pv/cell-

efficiency.html. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 A) Construction of a pn-junction diode resembling how pn-junctions are

the basis of functioning solar cells. B) Architecture of a thin-film CQD solar

cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Energy band diagrams for different sized quantum dots (right) with re-

spect to the contemporary semiconductor energy bands (left). Resembles

the dependency the DOS has on the size of the QD. As QD’s get larger

they tend to approach bulk energy bands. Reprinted from Building devices

from colloidal quantum dots. by Kagan, C. R., 2016, Science, 353(6302).

https://doi.org/10.1126/science.aac5523. . . . . . . . . . . . . . . . . . . . 10

4 Crystalline structure of MAPbI3 following the AMX3 structure. MA is

shown by the red sphere, however realistically it is the CH3NH+3 molecule.

Reprinted from Perovskite Solar Cells: Causes of Degradation, from Oscilla

Ltd. Retrieved March 21, 2020, from https://www.ossila.com/pages/perovskite-

solar-cell-degradation-causes. . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 A graphic representation of all deposition methods used. A) Doctor blading,

primarily used for TiO2 blocking layers. B) Screen printing used for all TiO2

layers. C) Drop casting used primarily for sulfide capped QDs. D) Spin

coating used for MAPbI3 and OAm capped QDs. . . . . . . . . . . . . . . 15

6 SEM cross sections of doctor bladed (left) and screen printed (right) TiO2

mesoporous layers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7 A) Deposition illustration of solvent engineered MAPbI3. Reprinted from

Solvent engineering for high-performance inorganic–organic hybrid perovskite

solar cells by Jeon, N. J., 2014, Nature Materials, 13(9), pg. 898. B) Depo-

sition illustration of two-step MAPbI3. Reprinted from Nonstoichiometric

Adduct Approach for High-Efficiency Perovskite Solar Cells by Park, N.-G.,

2017, Inorganic Chemistry, 56(1), pg. 5. . . . . . . . . . . . . . . . . . . . 20

8 Uncapped pure, 0.5% mol Bi doped (orange) and 0.5% mol Sb doped (red)

J-V curves plotted with both illuminated (solid line) and dark (hashed line)

curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

9 A) Sulfide (S2−) capped and B) Uncapped pure, 1.0% mol Sb doped and

1.5% mol Sb doped J-V curves plotted with both illuminated (solid line) and

dark (hashed line). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

10 A) Solvent engineered MAPbI3 with increasing concentrations of QD:DMF

solution. B) Solvent engineered MAPbI3 with pure uncapped QDs, pure

DDT capped QDs and pure S2− capped QDs added into the precursor. . . 27

11 Photothermal deflection spectroscopy (PDS) of 2-step MAPbI3 (purple),2-

step FAPbI3 + Ge QD (green) and Ge QDs alone (blue). Electrical band

gaps (Eg) are shown in eV as well as observed Urbach energies (Eu are shown

in meV). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

12 Material absorbance plotted against wavelength showing how much light is

being absorbed by each material. . . . . . . . . . . . . . . . . . . . . . . . 30

List of Tables

1 Quantum dot sample list. . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2 Molar dosage for MAPbI3 perovskite synthesis. . . . . . . . . . . . . . . . . 19

I first wish to extend my gratitude to my mentor H. Renee Sully for her continued

guidance, assistance and intelligence throughout this project. I also extend my

appreciation to my peer researcher Shayan Zargar for his work on absorption, device

building and friendship. I wish to reach my thanks to Tom Yuzvinsky for his work on

SEM imaging and Katayoun Tabatabaei for her work synthesising the QDs used

throughout this research. I reserve my deepest gratitude to my advisor Katie Hellier for

her continuous inspiration, belief and encouragement through this difficult time. I also

thank her for her work on PDS and the years she has spent building it.

1

1 Introduction

Solar cell technology is at the forefront of the sustainable energy market, yet it is still one

of the lowest efficiency renewable energy solution available [21]. In 2000, solar energy only

made up 0.039% of the total global energy supply [21]. Researchers have only achieved

a power conversion efficiency (PCE) of 27.6% from single-junction bulk semiconductor

solar cells, as seen in Figure 1 (blue curve). Silicon based solar technologies are not only

relatively expensive, but due to their high temperature manufacturing conditions, there

are also excessive amounts of hazardous waste [15]. With the goal to decrease costs and

hazardous wastage, alternative semiconductor materials are being investigated. A type of

material that has displayed unique and exciting properties are semiconductor nanocrystals,

better known as quantum dots (QDs). Quantum dots are three dimensional, spherical

nanocrystals that are on the scale of 2-20nm in diameter. Most quantum dot materials

are synthesized in an evaporable solvent making fabrication cheap and efficient, in this

solution based state they are known as colloidal quantum dots (CQDs). Quantum dots

exhibit many unique electronic and optical properties due to their size, shape, and surface

chemistry. Size dependent band gap tuning is a convenient and easy way to control a few of

the materials electrical properties [10]. Another novel alternative for solar cell technology is

solution based perovskite solar cells (PSCs). Single junction perovskites have shown a PCE

of 26.4% (red curve with yellow circles in Figure 1) and are the most rapidly developing

material in photovoltaics (see Figure 1). Alongside this, they have a high compatibility

with other materials and exhibit very high carrier mobilities. Fabrication is again solution

based, keeping costs and wastage low. One of the most commonly studied perovskites is

methylammonium lead iodide (MAPbI3), exhibiting an electrical band gap of 1.64 eV and

recently reaching a PCE of 25.5% [23].

In this work, we investigate the behaviour of a hybrid material composed of germanium

quantum dots in conjunction with MAPbI3 perovskites. This material will be evaluated as

2

Figure 1: Cell Efficiency Chart from 1975-2020 displaying the improvement in efficiencyof different types of solar cells and solar cell materials. Perovskites are yellow circles witha red outline and quantum dots are yellow diamonds with a red outline. Reprinted fromNREL. Retrieved from https://www.nrel.gov/pv/cell-efficiency.html.

an active photogeneration material for solar cells. Using one primary deposition method,

we will evaluate the optoelectronic properties of the materials which will help us understand

their compatibility with each other. We first investigate Ge QDs alone. Three different

organic and inorganic capping ligands will be studied as well as two different metallic

dopants. Samples of varying surface chemistry and doping concentrations are evaluated to

study the Ge QD’s embodiment with the perovskite crystal. Using photothermal deflection

spectroscopy (PDS), extended X-ray absorption fine structure (EXAFS), and scanning

electron microscopy (SEM) characterization techniques, we are able to evaluate disorder,

surface chemistry, and film thickness, which all play a role in the behaviour of the solar cell

devices.

3

1.1 Motivation

Bulk silicon solar cells are often arranged in layered structure that utilizes the silicon

as the active or electron-hole pair (EHP) generation layer. Most bulk Si solar cells are

expensive to manufacture since the silicon layers are monocrystalline, thick and rigid [20].

A promising alternative to this construction is the thin-film solar cell. Thin-films are often

cheaper to manufacture, use less material and can be flexible, meaning they can be used

in more practical situations. Some common thin film solar cells that are currently in

manufacturing are made from amorphous silicon, cadmium telluride (CdTe), and copper

indium gallium diselenide (CIGS) [24]. However, most of these do not exhibit an efficiency

over 21%. Thus, bulk silicon solar cells are still used due to their better PCE to cost ratio,

[2, 24].

Germanium (Ge) is a semiconductor material that resides in the same family as silicon.

Researchers have found that doping germanium with metals such as bismuth, antimony, and

tin have shown higher efficiencies [14]. Nanocrystalline semiconductors exhibit a property

known as quantum confinement which defines when the EHP is spatially confined in the

region of excitation. In this state, the EHP becomes an exciton. Quantum confinement

only occurs when the size of the exciton is larger than the nanocrystal itself. Germanium

has a large Bohr radius, therefore, larger particles can still exhibit quantum confinement.

Along with this, bulk Germanium has a narrow band gap (0.67eV) and a high carrier

concentration (µe = 3900 cm2 V −1s−1, µh = 1900 cm

2 V −1s−1) [4]. It is because of these

reasons that we choose to investigate Ge nanocrystals for use in photovoltaic devices.

1.2 Background

1.2.1 PV Solar Cells

The photovoltaic (PV) cell is an energy conversion device that employs the properties

of semiconductor band structures to create an electrical current. The first step of successful

4

photovoltaic energy conversion is for incident photons to be absorbed by the semiconductor

and excite bound electrons from the valence band (VB) to the conduction band (CB). This

energy gain, known as the band-gap energy (Eg), generates an electron-hole-pair (EHP)

where the negatively charged electron resides in the conduction band and the positively

charged hole resides in the valence band. The next step is to create a differentiating

mechanism to separate positive and negative charge carriers. To bias the diode and generate

a current, we need to form two electrical contacts; one known as the anode (negative

terminal), the other being the cathode (positive terminal). The final step is for the negative

carriers at the anode to recombine with the positive carriers to annihilate the EHP and

return the absorber to the ground state, ready for re-excitement.

A common type of semiconductor device exhibiting this process is the pn-junction,

shown in Figure 2A, which is the standard architecture that bulk PV solar cells are made

from. These devices are made up of a n-type semiconductor (donator bias) in conjunction

with a p-type semiconductor (acceptor bias). n and p type semiconductors are synthesized

through a process called doping. Doping is the injection of majority carriers through the

addition of impurities to the crystal. That is, adding materials with valence electrons (n-

type) or valence holes (p-type) such that each impurity injects majority carriers for higher

electrical performance. As free electrons roam in the n-type region, diffusion characteristics

cause the particles to drift to the p-type region in which they recombine with holes through

Coulombic attraction. The analogous occurs for the holes in the p-type region. When this

recombination occurs, it leaves behind an ionic charge surplus (positive ionic charge in n-

type region, negative in p-type), shown by the large donor and acceptor ions in Figure 2A.

This ionic charge inequality induces an electric field pointing from the n-type region to the

p-type region, which creates a depletion region free of charge. When EHPs are generated

from photon absorption, they are influenced by the induced electric field and the electron

separates from the hole. This separation can then be employed to yield a photo current;

which is the basis of a functioning solar cell.

5

The consideration of minority charge carriers is important for solar device physics.

When a forward bias (positive terminal connected to p-type) is applied to a pn-junction,

the forward voltage, V , opposes the internal voltage, Vo and decreases it by Vo−V . There-

fore, minority carriers on the p-side have a higher probability of overcoming the potential

barrier. When a reverse bias is applied (negative terminal connected to p-type), the V

and Vo comply and the barrier increases by Vo +V . This results in reverse minority carrier

current.

Figure 2: A) Construction of a pn-junction diode resembling how pn-junctions are thebasis of functioning solar cells. B) Architecture of a thin-film CQD solar cell.

Ideal PV solar cells are characterized by six primary parameters, the open circuit volt-

age Voc, short circuit current Jsc, fill factor FF , power conversion efficiency PCE, the shunt

resistance Rsh and finally the series resistance of the solar cell Rs. Semiconductors intrin-

sically exhibit these effects due to drift and diffusion currents. Drift currents are caused by

an applied voltage and diffusion currents are caused by a concentration gradient. Therefore

the current densities for an ideal diode are given by,

Je = qnµeE + qDedn

dx(1)

Jh = qpµhE + qDhdp

dx(2)

where q is the charge of an electron, n and p are the concentration of electrons and

6

holes respectively. Allowing the subscript i to be an index for either e or h, we define µ as

the carrier mobility given by µi =qτim∗i

, Di =kBTqµi as the diffusion coefficient and τi as the

characteristic diffusion time for each charge carrier. Considering the geometry of the solar

cell, the total current density flowing through the diode under an applied bias V is,

J = q(DeLeNd

+DhLhNa

)n2I [expqV

kBT− 1] (3)

where Le and Lh are the electron and hole diffusion lengths, given by Li =√Diτi.

Nd is the concentration of donors and Na is the concentration of acceptors [11]. Defining

Jso = q(DeLeNd

+ DhLhNa

)n2I as the reverse saturation current density, equation 3 becomes,

J = Jso[expqV

kBT− 1] (4)

Equation 4 is known as the ideal diode equation [11]. Now considering non-ideal char-

acteristics, such as recombination and thermal generation. The non-ideal diode equation

is then,

J = Jso[expqV

ξkBT− 1]− Jsc (5)

where ξ is known as the diode ideality factor, ξ = 1 for diffusion controlled and ξ = 2

for recombination controlled [11]. The short circuit current density, Jsc, is the current

flowing when the applied voltage is 0, as seen from Equation 5. As the applied voltage

rises, the first term in Eq 5 rises exponentially. When Jsc = Jso[expqVocηkBT

− 1], the voltage

V is the open circuit voltage Voc. The total current J , after parasitic losses through shunt

resistances Rsh will experience a series resistance Rs before powering a load. A modified

non-ideal diode equation is found to represent the current densities of solar cells,

J = Jso[expq(V − JARs)

ξkBT− 1] + V − JARs

Rsh− Jsc (6)

7

where A is the area of the device [17].

The fill factor FF , is a measure of how well the device performs with respect to an ideal

solar cell.

FF =JmaxVmaxJscVoc

(7)

The PCE, which is a measure of the output power with respect to the input power, can

be represented by,

ηPCE =PoutPin

=JscVoc · FF

Pin(8)

where Pin is the radiated power incident upon the solar cell [11, 17].

1.2.2 Bulk and Thin Film Architectures

The preceding method of creating these pn-junctions is to use bulk semiconductors

such as silicon. Pure silicon is not conductive, but can be made conductive by introducing

doping agents to engineer the electrical and optical properties; such as carrier mobility,

concentration and band-gap energies. Often, silicon is doped with boron to form a p-type

material and doped with phosphorus to form an n-type material. In conjunction, these

form the pn-junction previously discussed. Thin-film devices exhibit many different optical

and electronic properties, due to the size of the material crystals in relation to the film

itself [5].

As seen in Figure 2B, thin-film architectures are constructed by layering different ma-

terials on top of each-other using various deposition methods. Unlike traditional bulk solar

cells, thin-films use an n-i-p layering junction [5]. The first layer is the glass substrate; the

palette where light will enter through. The second layer is indium-doped tin oxide (ITO)

electrical contacts, these form the anode. Upon the ITO is the first deposited layer, we use

TiO2 (titanium dioxide) as an electrical blocking layer (although other materials have been

8

investigated) to suppresses recombination at the semiconductor/electrolyte interface [13].

The fourth layer is a mesoporous TiO2 layer, which forms an effective electron transport

layer (ETL) for the device. The mesoporous state integrates the crystal-nanocrystal bound-

ary between the TiO2 layer and the active layer [8]. The active layer is the most important

and is the focus of this paper because it is the semiconductor material responsible for the

generation of the EHPs. Upon this, a hole transport layer (HTL) is deposited to again

help separate EHPs to yield a photocurrent. Common HTLs used are conductive polymer

known as P3HT or Spiro-MeOTAD [1]. For some devices in this work we will skip this step,

since nanocrystalline active layers are limited in HTLs with a good band alignment [1, 12].

Finally, metallic contacts make up the cathode of the device. Metals used are usually gold

or silver due to their high conductivity and low melting points, since they are deposited

by evaporation. While the pn-junction utilizes a built-in electric field to separate charge

carriers, energy level differences between transport layers is the mechanism that separates

charge carriers in thin films [5].

1.2.3 Quantum Dots

The focus of this work is to investigate colloidal Ge quantum dots for use in thin-

film PSCs. Colloidal quantum dots (CQDs) are nanocrystalline semiconductor materials

suspended in a solvent, making deposition and fabrication easier. CQDs may be engineered

in size, shape, temperature, composition, and surface chemistry to alter their optical and

electronic properties [10]. The properties that will be investigated are discussed here.

i. Size Dependence

In bulk semiconductors, the large density of states (∼ 1022) creates a continuous band

of states, as seen in Figure 3. The separation between the CB and VB defines the band gap

energy (Eg), one of the most definitive properties of semiconductor devices. Quantum dots

display discrete energy transitions between conduction and valence bands. This separation

of bands is due to the size the quantum dots approaching the size of atoms themselves.

9

Figure 3: Energy band diagrams for different sized quantum dots (right) with respect tothe contemporary semiconductor energy bands (left). Resembles the dependency the DOShas on the size of the QD. As QD’s get larger they tend to approach bulk energy bands.Reprinted from Building devices from colloidal quantum dots. by Kagan, C. R., 2016,Science, 353(6302). https://doi.org/10.1126/science.aac5523.

Thus, the QDs begin demonstrating similar characteristics of isolated atomic behavior.

The energies between the states increases with decreasing QD radius. A smaller QD will

indicate a larger band gap (Figure 3), where the band gap is defined as the difference

between the highest occupied and the lowest unoccupied states [3].

When the size of the semiconductor nanocrystal is smaller than the size of the exci-

ton itself, the charge carriers become spatially confined in the region of excitation [22].

This localization introduces a phenomena known as electron confinement, something that

is intrinsic to semiconductor nanocrystals in the region of 2 - 20 nm [10]. Within this re-

gion, spatially bound free charge carriers then exhibit size-dependent optical and electronic

properties which are not achievable by first-generation bulk semiconductor crystals. They

exhibit strong optical absorption resonances that are associated with discrete transitions

10

between quantum states [3, 10]. Much of these optical and electronic properties are due

to the overlap between the electron and the hole wave functions in the spatially confined

regime [22].

ii. Doping

Doping is an effective method for engineering the properties of materials. Doping is

the introduction of impurity atoms to a crystalline structure. Introduction of impurities

generates new states within the ‘forbidden region’, in which electrons or holes can occupy

and become energetically excited to the CB or VB respectively. Quantum dot materials

do not respond the same way to doping, due to the of the discrete nature of the quantum-

confined states. The density of states (DOS) is considerably altered by the addition of just

a single dopant [14]. Researchers have found success by introducing metallic dopants into

and onto the quantum dot nanocrystals, giving better control of the band gap and Fermi

energy [14]. Many metallic dopants have been investigated, such as tin, indium, copper,

bismuth and antimony. Interestingly, metallic dopants incorporate themselves within the

dots, not on the surface like many other dopants [14]. This significantly alters the DOS

near discrete energies defined by the confinement of charge carriers within the nanocrystal.

Introducing these metallic dopants also creates impurities in the crystal. Dependent on the

electron configuration of the dopant itself, there may be free, unbound states within the

crystal. These states can create a high shunt resistances Rsh, where electricity conducts

through an impurity and bypasses the semiconductor.

iii. Surface Chemistry

High quality CQDs exhibit high crystallinity and are monodisperse (homogeneous size

and phase). These are surrounded by a layer of organic capping ligands (OCLs) to make the

QDs soluble. OCLs are open end hydrocarbon tails that form an insulation layer around

each QD [19]. Organic ligands such as oleic acid, oleylamine and trioctylphosphine oxide

tend to create an energetic barrier such that charge carriers become trapped and device

performance is sacrificed. Ligands can limit carrier mobility and misalign band energies

11

with the mesoporous TiO2 layer previously discussed. Research shows that inorganic cap-

ping ligands (ICLs) are an effective replacement for organic ligands. Having a much shorter

chain length they can link the QD and TiO2 matrices in closer proximity than the OCLs

[19]. QDs with ICLs have exhibited much higher absorptions than that of OCLs, and they

have also proved to have a much better colloidal stability [19]. Surface modifications to

QDs can alter the carrier transport by shifting energy levels or acting as dopants. We

will investigate whether ligand exchange is a useful technique for advancing photovoltaic

performance for Ge QDs and incorporating them into the bulk perovskite matrix.

1.2.4 MAPbI3 Perovskites

Figure 4: Crystalline structure of MAPbI3 following the AMX3 structure. MA is shown bythe red sphere, however realistically it is the CH3NH

+3 molecule. Reprinted from Perovskite

Solar Cells: Causes of Degradation, from Oscilla Ltd. Retrieved March 21, 2020, fromhttps://www.ossila.com/pages/perovskite-solar-cell-degradation-causes.

Organo-metal halide perovskites are novel photovoltaic materials made up of organic

and inorganic materials which follow the composition AMX3. They are the fastest advanc-

ing PV material as shown in Figure 1. One primary class of this photovoltaic material is

the hybrid organic-inorganic perovskite (HOIP) [23]. For HOIPs the A is an organic cation,

the M is a divalent metal, and the X is a monovalent anion, namely a halide or halogen.

The noticeable difference of this subgroup of perovskites is that the A site is occupied by an

organic molecule rather than a singular atom [23]. The most acclaimed of these compounds

12

is methylammonium lead iodide (MAPbI3), where the A site is occupied by the organic

molecule methylammonium (CH3NH+3 or for simplicity: MA).

Perovskites are formed in a bulk crystalline matrix, following the tetragonal structure

seen in Figure 4. The material exhibits many ferroelectric, pyroelectric, and ferroelastic

properties resultant of the phase of the tetragonal structure. For the rest of this work, we

will focus only on MAPbI3 perovskites for their high absorption coefficient, high PCEs,

and low exciton binding energies, since these are notably better than any other perovskite

material.

The primary objective of this work is to investigate a hybrid material composed of

germanium colloidal quantum dots and MAPbI3 perovksites. Different variations of CQDs

will be studied to evaluate their compatibility within the MAPbI3 matrix. These variables

include internal properties such as doping, surface chemistry and particle size. External

variables such as film uniformity, film thickness and device architecture will also be evalu-

ated.

13

2 Methods and Experimental Design

2.1 Synthesis

Solar cells are fabricated on 1”x 1” glass slides with ITO contacts already deposited.

The architecture described in section 1.2.2, is followed layer by layer. Slide cleaning and

TiO2 engineering are conducted outside of the nitrogen glovebox. The rest of the fabrication

steps are done within the glovebox.

2.1.1 Slide Cleaning

Slides must be thoroughly cleaned to ensure good adhesion of solution deposited films

to the glass substrate. The slides are scrubbed with Liquinox and a sponge, then triple

rinsed with DI water. They are immediately placed into a sonicator container filled with

Acetone and sonicated on for 15 minutes. The slides are then transferred directly to

another sonicator container filled with isopropyl alcohol (IPA), and sonicated for another

15 minutes. Once sonication is complete, the slides are dried off with nitrogen gas and

placed in clean slide containers for future use.

2.1.2 TiO2 Engineering

Titanium dioxide (TiO2), forms the n-type region of our device, serving to separate the

EHPs and generate a photocurrent. Two primary methods were used deposit the TiO2

layers in order to maximize film uniformity and minimize material usage. TiO2 layers are

fabricated by using two different compounds of TiO2. The TiO2 blocking layer is a ho-

mogeneous, ultrathin ETL that suppresses recombination at the semiconductor/electrolyte

interface. The next layer is made from mesoporous TiO2, increasing surface area and pore

volume such that it is more accessible to the QD/MAPbI3 matrix. Both the blocking layer

and the mesoporous layer are fabricated using one of these techniques. The desired film

thickness for the blocking layer is ∼ 40-50nm and for mesoporous layer it is ∼ 100-150nm.

14

Figure 5: A graphic representation of all deposition methods used. A) Doctor blading,primarily used for TiO2 blocking layers. B) Screen printing used for all TiO2 layers. C)Drop casting used primarily for sulfide capped QDs. D) Spin coating used for MAPbI3and OAm capped QDs.

The intent is to make the layers as thin and uniform as possible while also being as effective

as possible. The blocking layer can be thin since it only needs to restrict recombination.

The mesoporous layer needs to be thicker so it can expose more surface area to the active

layer. In both cases, if the layers are too thick, resistance grows large and begins to restrict

current.

The first is to use a doctor blading technique shown in Figure 5A, where the material

is ’scraped’ over two pieces of masking tape, creating a uniform rectangle over top of the

ITO electrodes. The material thickness is dependent on the thickness of the tape and the

concentration of the solution. Using scotch tape, this thickness is ∼ 0.625mm.

The second method is to use a screen printing apparatus, shown in Figure 5B. A

sheet of polyester plastic is stretched out and masked such that ten rectangles are evenly

distributed over it. These masked rectangles are a mesh of polyester fibers which are placed

15

directly over the electrodes on the clean ITO slides. The material is again ’scraped’ over

the top of these masks, depositing the material evenly through the mesh onto ten ITO

slides.

Using either method, the slides are then placed on a hotplate at 120◦C for 5 minutes,

before being placed into a preheated 500◦C oven and sintered for 50 minutes. This process

is repeated for the mesoporous layer.

Scanning electron microscopy (SEM) was used to image the cross sections of the ma-

terials to measure their thicknesses. Figure 6 shows the relative thicknesses of the doctor

bladed (left) and screen printed mesoporous layers (right). The doctor bladed layer is ∼

5.27µm while the screen printed is ∼ 102.0 nm. Screen printing is therefore the preferred

method for the deposition of the TiO2 layers and I will be continuing this synthesis method

for the duration of this paper.

Figure 6: SEM cross sections of doctor bladed (left) and screen printed (right) TiO2 meso-porous layers.

16

2.1.3 QD Fabrication

As previously discussed, doping and surface chemistry can have drastic effects on the

PV performance of the quantum dots and their surroundings. To investigate the affects

caused by doping, a total of ten samples are considered, shown in Table 1. Fabrication of

the devices were conducted in a way specific to the samples themselves. Different ligands

and solvents behave differently during fabrication. The deposition techniques used are

described below.

Sample Dopant Ligand Particle SizeKT-837 1.0 mol % Sb Na2SKT-839 1.5 mol % Sb Na2SKT-835 pure Na2SKT-916 pure Na2SMW-R2 pure OAm 11.4nmKT-369 pure OAm 4.5nmKT-683 pure OAm 3.5nmKT-792 0.5 mol % Sb OAmKT-737 1.0 mol % Sb OAm 3.5nmKT-738 1.5 mol % Sb OAm 4.5nmKT-803 0.5 mol % Bi OAm 4.2nmSCCW-8 pure DDT

Table 1: Quantum dot sample list.

i. Deposition Techniques

There are two primary deposition methods for colloidal quantum dot materials: drop

casting and spin coating. Both processes are conducted within a controlled nitrogen glove-

box, due to the oxygen and water instability of CQDs.

Due to high surface tension between formamide (solvent) and glass, sulfide (S2−) capped

samples deposit best using a drop cast method, seen in Figure 5C. Drop casting involves

simply pipetting the CQD solution evenly over the desired area, and evaporating the solvent

off on a hotplate at ∼ 160◦C.

Oleylamine capped (OAm) samples tend to deposit better using a spin coating tech-

17

nique, seen in Figure 5D. Spin coating is performed by first centering the slide (with both

layers TiO2) onto the spin chuck such it is spins evenly around its central axis. A pipette

is prepared with ∼ 40 µL of CQD material and deposited onto the center of the slide,

which is immediately spun. All samples are spun at 500rpm with ∼ 40 µL for 30 seconds.

Once spinning is complete, most of the moisture from the solvent has evaporated and the

device is ready for another coating to bring the thickness of the CQD layer to ∼ 250nm.

Thicknesses, l, for spin coated samples follow:

l ∝ 1√ω

(9)

where ω is the rotational velocity in rad−1.

ii. Ligand Exchange

After each layer of deposition, the organic capping ligand (OCL) is often stripped

since OCLs tend to create large energetic barriers around the dots which restrict charge

transport. An exchange bath is filled with a 1:8 molar solution of hydrazine and acetonitrile,

respectively. Each layer is then soaked for one hour before depositing the next layer. For

each oleylamine capped sample used, a ligand exchange is conducted to strip the OAm

ligand from the QD.

2.1.4 MAPbI3 Fabrication

The synthesis of MAPbI3 perovskite involves a multi-step synthesis. There are two

primary methods for synthesizing the MAPbI3, Solvent Engineering and Two-Step. Both

are done in a nitrogen controlled glovebox to take precautions for the toxicity of lead.

Preceding all, accurate weights of the dosage of PbI2 must be taken in order to adapt the

required ratio in Table 2.

i. Solvent Engineered

The first step of solvent engineering MAPbI3 perovskites is to calculate a 1:1:1:8.2 ratio

18

Material 1 mol RatioPbI2 0.461 g 1MAI 0.159 g 1

DMSO 70.8 µL 1DMF 624 µL 8.2

Table 2: Molar dosage for MAPbI3 perovskite synthesis.

of metal halide, salt, dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) adapted

to the dosage of metal halide (PbI2). The precursor is then simply mixed by adding all of

the materials to a vial with a stir-bar and mixing for an hour or more. Once all the solute is

dissolved, the precursor can be used. 25 µL of precursor solution is pipetted onto a device

at 4000rpm for 30 seconds. After 6 seconds of spinning, 100 µL of toluene is deposited onto

the device while it is still spinning; this process is shown in Figure 7A. Toluene initiates

the formation of the crystal structure. The slide is then transferred to a hotplate at 80 ◦C

for 1 minute. The perovskite will turn from yellow to a dark, shiny brown indicating that

the perovskite crystal has formed.

ii. Two-Step

The two step deposition method is achieved by preparing PbI2 in DMF (1 mol/mL), and

stirring at 80 ◦C until dissolved. Alongside this, 100 mg/mL of MAI to IPA is prepared.

40 µL of metal halide solution is then deposited and spun at 6000 rpm for 5 seconds. The

slides are transferred to a hot plate for 30 minutes at 85 ◦C to evaporate the solvent. The

slides are placed back on the spinner and 40 µL of salt/IPA solution is deposited on the

film. This solution is allowed to sit for 20 seconds then spun at 3000 rpm for 30 seconds

before being transferred back to the hotplate to evaporate the solvent again. A shiny dark

brown film will appear again if the perovskite crystal is formed properly. The two step

method is shown in Figure 7B.

19

Figure 7: A) Deposition illustration of solvent engineered MAPbI3. Reprinted from Solventengineering for high-performance inorganic–organic hybrid perovskite solar cells by Jeon, N.J., 2014, Nature Materials, 13(9), pg. 898. B) Deposition illustration of two-step MAPbI3.Reprinted from Nonstoichiometric Adduct Approach for High-Efficiency Perovskite SolarCells by Park, N.-G., 2017, Inorganic Chemistry, 56(1), pg. 5.

2.1.5 Hybrid QD/MAPbI3 Blend Fabrication

Hybrid devices were fabricated using the solvent engineered method and adding a QD

solution into the precursor. Solvent engineered MAPbI3 uses DMF as a solvent in the

precursor, however QD samples are often suspended in a different solvent. When mixing

QD and MAPbI3 in the precursor, we want to ensure we have a homogeneous solvent. The

QD samples are then centrifuged such that the QDs separate from their solvents. The

solvent is then tipped out and refilled with DMF and the process is repeated 3-4 times to

ensure a uniform DMF:QD solution.

2.1.6 Cathode Deposition

The last step of device fabrication is to deposit the cathode onto the devices. This

is done under a high vacuum (∼ 10−6 ppm) environment, where a metal is thermally

20

evaporated onto the devices forming a metal contact to preform testing. Depending on the

material used for the active layer, different metals’ bands will align differently. For Ge QDs,

silver offers the best band-to-band carrier transport, while for MAPbI3, gold aligns better

and since we can use an HTL we’re able to get better results. Slides are added face down

into a slide mask and placed into a large bell chamber. The desired metal is added into a

’boat’ underneath the slides connected to two large electrical contacts. The bell is lowered

and the vacuum is pumped down. Evaporation can begin when the vacuum reaches ∼ 10−6

ppm or lower. Gold is deposited at a rate of 1Å/s until a thickness of 800-1000Å (80-100

nm) is reached (as shown in Figure 2B).

2.2 Characterization

To test the completed solar cells in a realistic manner, the light must be calibrated

in such a way that it simulates the light from the sun. This is done using a Oriel solar

simulator that is tuned to one sun using a calibrated photodiode. Device behaviour is

then characterized under dark and illuminated conditions. There are six devices for each

prepared slide, therefore six dark and six illuminated measurements are taken. Many

useful device properties can be acquired from this data (Voc, Jsc, FF , PCE, Rs, Rs). Dark

current densities are used to inject carriers into the solar cell to investigate its diodic

behaviour without photo generated carriers. Comparisons of dark and illuminated current

measurements therefore show the amount of photo generation the device is performing.

2.2.1 Voltage-Current Measurements

For each device, a -1V to 1V sweep is taken under dark and illuminated conditions. A

dark solar cell acts as non-ideal diode, thus, equation 5 gives us that,

J = Jso[expqV

ξkBT− 1]− Jsc

21

Under illuminated conditions, a photo generation factor is added due to the EHP’s

generated from the photon energies. Suppose τg is the characteristic time to generate an

EHP,

J = Jso[expqV

ξkBT− 1] + qWni

τg− Jsc (10)

Here, W is the total width of the depletion region (where EHPs are generated) [11, 17].

JV curves are then plotted in a regular plot and a semi-log plot.

2.2.2 Material Absorbance

Absorption measurements were taken using a JASCO V670 Spectrophotometer and

analyzed using JASCO Spectra Manager software. The spectrometer is first calibrated

using a blank glass slide for a baseline scan. Once this baseline scan is finished the actual

sample is inserted and the scan is begun. Absorbance curves are automatically generated

for each sample and are defined by:

A(λ) = log(Ti(λ)

To(λ)) = log(

Psample(λ)

Pbaseline(λ)) (11)

2.2.3 PDS

Photothermal deflection spectroscopy (PDS) is a spectroscopy technique used to mea-

sure absorption, disorder, and thermal characteristics. A light source first enters a monochro-

mator. Monochromatic light is then focused through a series of optical prisms onto a se-

lected sample. This is a pumping mechanism to excite the sample. A laser is shot parallel

to the sample (perpendicular to the monochromatic light) and its deflection is measured.

Indices of refraction, disorder and absorption are all dependent on thermal excitation, thus,

PDS is able to characterize a materials’ properties. An important parameter used to un-

derstand the crystallinity and disorder of nanomaterials is the Urbach energy, Eu. The

22

spectral dependence of the absorption coefficient is given by:

α = αo exphν

Eu(12)

where αo a intrinsic absorption coefficient and hν is the photon energy. Equation 12

tells us that the Urbach energy Eu can be obtained from the slope of lnα vs hν [7].

PDS was conducted by K. Hellier, a PhD student in the Physics department at UC

Santa Cruz.

2.2.4 EXAFS

Extended X-ray absorption fine structure (EXAFS) measurements are taken to evaluate

disorder, nearest neighbours, vacancies and to find out where on the quantum dots the

dopants actually reside. EXAFS is a process that involves striking an atom with an X-

ray beam to create a vacancy in a core shell. In typical EXAFS plots, various peaks are

shown when absorbance is plotted against energy. These peaks (called edges) represent the

ionization of a core orbital. K-edge refers to the 1s orbital, L-edge and M-edge refer to the

following orbitals from inside out. After ionization of an orbital, relaxation occurs to fill

that shell, wavefunctions begin to oscillate and interactions between ejected photoelectrons

and electrons surrounding the absorbing atom occur. These interactions are observed and

plotted in k-space, where k is the wavevector of the wavefunctions, or in R-space, where R

is the real part of the wavevector. In R-space, interactions between wavefunctions show us

where vacancies reside in the crystal, giving us more information about doping the quantum

dots, vacancies and disorder.

EXAFS was conducted by H. Renee Sully, a graduate student in the Electrical Engi-

neering department at UC Santa Cruz.

23

3 Results and Discussion

3.1 QD Results

The first QD devices were made to investigate the variations in device performance

between bismuth (Bi83) and antimony (Sb51) dopants. Devices were fabricated to evaluate

how different dopants would behave within the same QD sample. Antimony has an electron

configuration of [Kr] 4d10 5s2 5p3, and an atomic radius of 140 pm. While Bismuth has an

electron configuration of [Xe] 4f14 5d10 6s2 6p3, it has the same amount of vacancies in its

valence shell. However, it has an atomic radius of 230 pm, which is 1.6 times the radius of

Sb. Samples MW-R2, KT-792 and KT-803 (Table 1) were used to fabricate these devices.

The devices were tested under the same conditions and plotted using Matplotlib.pyplot, as

seen in Figure 8.

Figure 8: Uncapped pure, 0.5% mol Bi doped (orange) and 0.5% mol Sb doped (red) J-Vcurves plotted with both illuminated (solid line) and dark (hashed line) curves.

There is very little distinguishable differences in current density between 0.5 % mol

24

doped Bi and Sb. The curves are shown in red and orange in Figure 8 and look to be

almost identical. Both Bi and Sb have a p3 valence shell, thus, are n-type dopants and

therefore are donors. Both dopants donate 3 electrons per donor. Although their atomic

radii are different, it is unlikely to see differences between these two dopants, since their

valence electron configurations are the same. Interestingly, pure and uncapped Ge QD still

exhibit higher current densities than either of the n-doped QD materials. This is likely due

to a high amount of disorder in the QD materials creating trap states within the forbidden

region. Trap states occur when impurities and crystal defects give charge carriers places to

excite and relax to. Trap states occur in the forbidden region and are a product of disorder

in the QD material.

Antimony was used again to test the effects of higher doping concentrations on CQDs.

Sulfide ligands were used to encourage stability and quality band alignment with the ETL.

The samples used for these devices (shown in Table 1) are KT-835, KT-837 and KT-839,

respectively. The plot is shown below in Figure 9A. One slide was fabricated for each

type consisting of six devices per slide. Averages of each slide were taken using a linear

regression and plotted on Figure 9A.

Figure 9: A) Sulfide (S2−) capped and B) Uncapped pure, 1.0% mol Sb doped and 1.5%mol Sb doped J-V curves plotted with both illuminated (solid line) and dark (hashed line).

From the J-V curve, we can deduce that raising the antimony dopant concentration,

25

Nd, raises the current density. However, as the doping concentration is raised, the device

appears to act more ohmic than a diodic. It is also prevalent that the deviation between

dark and illuminated curves appears to decrease with rising doping concentrations. It is

likely that the shunt resistance Rsh, drops significantly with raising doping concentrations,

causing the signal to bypass the diode. This would explain why the 1.5% mol Sb doped

sample appears more ohmic. EXAFS data shows that Sb impurities have 3 or fewer nearest

neighbours. EXAFS models show vacancies next to all of the Sb atoms, which concurs with

the electron configuration of Sb ([Kr] 4d10 5s2 5p3), showing it has 3 vacancies in its valence

shell. Therefore, there are many Ge atoms with fewer neighbors causing the vacancies to act

as shunts. If Rsh decreases with increasing doping concentrations, Equation 6 shows us that

the current density will rise, describing the behavior shown in Figure 9A. It is possible that

the vacancies and free bonds will improve integration with the MAPbI3 matrix, encouraging

charge transport and photogeneration. Figure 9B shows uncapped samples of the same

doping concentrations. Pure current densities are much higher than 1.5% mol Sb doped

samples with a removed ligand. Differences between dark and illuminated measurements

are very little, suggesting photogeneration is very small. Sulfide ligands are much shorter

chain length, therefore the dots can be closer together. Adding a dopant simply injects

majority carriers to the crystal as expected. Carrier transport is therefore encouraged since

tunnelling/hopping distance is narrowed. For uncapped QDs, they are further apart, so

carrier transport is inhibited (since the tunnelling distance increases). Adding dopants is

therefore less effective, as it simply adds more disorder creating an even more restrictive

carrier transport, describing the behaviour seen in Figure 9B. It is also possible that oxygen

levels in the nitrogen glovebox crept up. Uncapped samples are much more reactive than

sulfide capped samples. As doping increases, impurities increase causing more surface

passivation, meaning the reactivity of the surfaces are greater. Therefore, higher doping

levels would cause greater reactivity with oxygen in the devices’ environment. This then

causes a lower current density as seen in Figure 9B but not in 9A.

26

3.2 Hybrid QD/MAPbI3 Results

Four QD/MAPbI3 hybrid slides, designed to show the behaviour of devices with in-

creasing concentration of QD solution, were added into the precursor. The sample KT-837

from Table 1 was used since it is a sulfide capped and 1.5% mol Sb doped. A 0.4 mg/mL

QD:DMF solution was added at 75 µL, 150µL and 400µL to the MAPbI3 precursor, shown

in the legend in Figure 10A. Devices were spin coated in the glovebox and all devices have

Spiro-MeOTAD spun on top. Spiro is an effective hole transport layer used for thin-film,

bulk materials (like MAPbI3). Spiro does not offer good hole transport for QD alone since

the energy bands do not align well.

Figure 10: A) Solvent engineered MAPbI3 with increasing concentrations of QD:DMFsolution. B) Solvent engineered MAPbI3 with pure uncapped QDs, pure DDT cappedQDs and pure S2− capped QDs added into the precursor.

From the J-V curves in Figure 10A we can deduce that increasing QD concentration

causes a direct increase in current density for solvent engineered MAPbI3 devices. Clearly

S2− capped Ge QDs are improving the current density. The highest forward biased current

density is around 1.1 mA cm−2. Interestingly, this is a dark current measurement, and

there is very little differences between dark and illuminated measurements for all of the

curves. This suggests that the QDs are not improving photogeneration, rather are acting

27

as dopants. It is likely that the Sb dopants (n-type) are injecting higher concentrations of

electrons, causing a higher current density. The vacancies found in the EXAFS data may

also be helping QDs integrate and bond with the MAPbI3 host matrix. This encourages

better carrier mobility and charge transport, also explaining the increase in current density.

All devices showed PCEs less than 0.0001% and extremely low short circuit currents on

the order of 10−8A. Low performance can be explained by Figure 11 below.

0.001

0.01

0.1

1

10

Nor

mal

ized

Abs

orpt

ion

2.52.01.51.0Energy (eV)

2-step MAPbI3Eg = 1.58 eV ± 0.01, Eu = 55 ± 2 meV

2-step FAPbI3-GeQD (KT-835)Eg = 1.59 ± 0.01 eV, Eu = 89 ± 2 meV

GeQD (KT-835)Eg ≈ 0.9 ± 0.1 eV, Eu = 284 ± 6 meV

Figure 11: Photothermal deflection spectroscopy (PDS) of 2-step MAPbI3 (purple),2-stepFAPbI3 + Ge QD (green) and Ge QDs alone (blue). Electrical band gaps (Eg) are shownin eV as well as observed Urbach energies (Eu are shown in meV).

PDS radiatively excites and relaxes a sample to measure its changes in absorption

when hit with monochromatic light. It is evident that the MAPbI3, although two-step,

exhibits a distinct band gap energy of approximately 1.58 eV, which is within the expected

range (∼ 1.50 eV) for MAPbI3. At the MAPbI3 (purple) band gap energy, the steep slope

characterizes the Urbach energy. The Urbach energy for MAPbI3 is at 55.3 ± 0.1 meV,

suggesting that the crystallinity is good and material disorder is low. The blue curve for

Ge QDs alone is extremely rounded off, with a gradual slope. By Equation 12, this implies

a much higher Urbach energy (283.6 ± 6.0 meV) and therefore much higher disorder. It is

difficult to infer the Ge QD band gap from the plot, suggesting there are a large range of

28

dot size, effectively blurring the band edge. The low PCEs and poor short circuit currents

are likely due to the disordered state of the materials. Although these results were not

run on the same samples, it is clear from Figure 11 that Ge QD’s increase in disorder in

the FAPbI3 host crystal causes trap states and band tailing. These strap states will cause

undesired recombination and trapped carriers, resulting in poor electrical characteristics of

the devices.

To investigate the behaviour of different ligands and their effect on the bonding in the

host matrix, three slides were fabricated. One slide has MAPbI3 + uncapped QDs, one has

MAPbI3 + DDT capped QD and one has MAPbI3 + sulfide capped QD. Dodecanethiol

(DDT) ligands are short chain strongly bonded ligands. To avoid poor results, OAm capped

samples were not used due to their weak bonding and insulating nature. Samples used were

MW-R2, SCCW-8 and KT-835 (seen in Table 1) respectively. QDs were centrifuged and

solvent exchanged to increase compatibility. QDs were then mixed with solvent engineered

MAPbI3 at a concentration of 0.4 mg/mL. All slides were spun at 4000rpm for 30secs using

25 µL of QD + MAPbI3 solution. The J-V curve for these devices is plotted in Figure 10B.

Notably, the solvent engineered MAPbI3 with uncapped QDs exhibits the highest cur-

rent density while the MAPbI3 with ICL QDs are lower. This is surprising considering our

expectations assumed that ICLs would help the QDs bond with the MAPbI3 host matrix.

Since the uncapped QDs are extremely unstable, it is possible that they bind better with

the matrix since they have so much unbonded surface area. It is worth noting that the cur-

rent densities in Figure 10B are a fourth of the current densities in Figure 10A, suggesting

that the MAPbI3 itself did not form well. The behaviour could also be explained by the

absorbance of the material with and without QD. It is evident from Figure 12, MAPbI3

with uncapped QD added shows a much higher absorbance of light in the lower wavelength

range. The peak around 450nm is very characteristic of Pb within the MAPbI3, which is

interesting that it is not seen in the other MAPbI3 samples, implying that the MAPbI3

did not fully form. MAPbI3 with S2− capping ligands show a lower absorbance in the

29

lower wavelength ranges however exhibit a higher absorbance for wavelengths greater than

600nm.

Figure 12: Material absorbance plotted against wavelength showing how much light is beingabsorbed by each material.

Raising sulfide capped QD concentration improves the in Figure 10A, however seems to

lower current density in Figure 10B. QDs cannot be improving photogeneration, therefore,

EHPs are not being generated by the QDs but the MAPbI3 alone. Hybrid devices used in

Figure 10A are n-type doped with antimony, while devices in 10B are pure. To explain the

trend, it is likely that the dopants are introducing majority carriers to the crystal and this

is creating a higher current density. However, the MAPbI3 under performed in Figure 10B

concluding that the MAPbI3 did not form properly.

30

4 Conclusions and Future Work

Germanium quantum dots in perovskite host crystals prove to be a promising candidate

for solar photovoltaics. With band tunability, surface dependence and doping of QDs,

there are many opportunities for engineering QDs into a compatible state for the MAPbI3

precursor. Although PCE’s did not reach above 0.0001%, relative behaviour of the QDs in

the MAPbI3 exhibited clear reliance on doping and surface chemistry. It is expected that

the low PCEs are due to disordered state of the QDs as shown by the PDS data. Disorder

causes trap states within the band gap, resulting in ’trapped’ carriers after photogeneration

or easy recombination for EHPs. EXAFS data showed that higher doped QDs had more

grain boundaries and higher disorder. Grain boundaries within a crystal act as an interface,

and interfacial recombination is very common in semiconductor materials. Dopants are

likely to reside on grain boundaries or the surface, causing a large number of free bonds.

In future work, we will work to keep dopant levels lower such that disorder is minimized

while also occupying carrier injection from dopants.

Interestingly, introduction of ICLs did not improve current density in the hybrid devices.

With the expectation that carrier transport would improve with better bonding within the

matrix, Figure 10B shows the exact opposite. Exchanging ICLs for halide ligands has been

reported to improve carrier transportation and integration within the crystal [16]. For

future investigation, iodide capping ligands will be investigated next to sulfide capping,

DDT capping and OAm capping ligands. I expect that iodide, being the halide within the

MAPbI3 matrix, will improve the bonding between the QDs and the matrix. Deposition

methods are another variable worth investigating in future experiments. Two-step MAPbI3,

although not fully formed into perovskite, tends to show higher efficiencies with excess lead

[18]. To improve the perovskite matrix itself, it would be worth investigating depositing

the MAPbI3 on top Ge QDs using both solvent engineered and two-step MAPbI3.

31

32

5 Notation

CB Conduction Band

VB Valence Band

Eg Bangap Energy

QD Quantum Dot

CQD Colloidal Quantum Dot

EHP Electron Hole Pair

ICL Inorganic Capped Ligands

OCL Organic Capped Ligands

PDS Photothermal Deflection Spectroscopy

EXAFS Extended X-ray Absorption Fine Structure

SEM Scanning Electron Microscopy

i Index used to define e or h

Ji Total Current Density

µi Carrier Mobility

Di Diffusion Coefficient

τi Characteristic Diffusion Time

Li Diffusion Length

nI Intrinsic Carrier Concentration

Jso Reverse Saturation Current Density

Jsc Short Circuit Current

Voc Open Circuit Voltage

Rsh Shunt Resistance

Rs Solar Cell Series Resistance

FF Fill Factor

ηPCE Power Conversion Efficiency

ξ Diode Ideality Factor

33

6 Literature Cited

References

[1] Bai, Y., Mora-Seró, I., De Angelis, F., Bisquert, J., & Wang, P. (2014). Tita-

nium Dioxide Nanomaterials for Photovoltaic Applications. Chemical Reviews, 114(19),

10095–10130. https://doi.org/10.1021/cr400606n

[2] Blakers, A., Zin, N., McIntosh, K. R., & Fong, K. (2013). High Efficiency Silicon Solar

Cells. Energy Procedia, 33, 1–10. https://doi.org/10.1016/j.egypro.2013.05.033

[3] Brus, L. E. (1984). Electron–electron and electron-hole interactions in small semicon-

ductor crystallites: The size dependence of the lowest excited electronic state. The Jour-

nal of Chemical Physics, 80(9), 4403–4409. https://doi.org/10.1063/1.447218

[4] Carolan, D., & Doyle, H. (2015). Size Controlled Synthesis of Germanium Nanocrystals:

Effect of Ge Precursor and Hydride Reducing Agent. Journal of Nanomaterials, 2015,

1–9. https://doi.org/10.1155/2015/506056

[5] Chopra, K. L., Paulson, P. D., & Dutta, V. (2004). Thin-film solar cells: An

overview. Progress in Photovoltaics: Research and Applications, 12(2–3), 69–92.

https://doi.org/10.1002/pip.541

[6] Delerue, C., Allan, G., Pijpers, J. J. H., & Bonn, M. (2010). Carrier multiplication in

bulk and nanocrystalline semiconductors: Mechanism, efficiency, and interest for solar

cells. Physical Review B, 81(12), 125306. https://doi.org/10.1103/PhysRevB.81.125306

[7] Fontenot, R. S., Mathur, V. K., & Barkyoumb, J. H. (2018). New photothermal de-

flection technique to discriminate between heating and cooling. Journal of Quantitative

Spectroscopy and Radiative Transfer, 204, 1-6.

34

[8] J. Frank, A., Kopidakis, N., & Lagemaat, J. van de. (2004). Electrons in nanostructured

TiO2 solar cells: Transport, recombination and photovoltaic properties. Coordination

Chemistry Reviews, 248(13), 1165–1179. https://doi.org/10.1016/j.ccr.2004.03.015

[9] Jeon, N. J., Noh, J. H., Kim, Y. C., Yang, W. S., Ryu, S., & Seok, S. I. (2014). Solvent

engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nature

Materials, 13(9), 897–903. https://doi.org/10.1038/nmat4014

[10] Kagan, C. R., Lifshitz, E., Sargent, E. H., & Talapin, D. V.

(2016). Building devices from colloidal quantum dots. Science, 353(6302).

https://doi.org/10.1126/science.aac5523

[11] Kasap, O. S. (2013). Optoelectronics & Photonics: Principals and Practices (2nd ed.).

Pearson Education Limited.

[12] Kim, J.-Y., & Kotov, N. A. (2014). Charge Transport Dilemma of

Solution-Processed Nanomaterials. Chemistry of Materials, 26(1), 134–152.

https://doi.org/10.1021/cm402675k

[13] Liu, H., Fu, X., Fu, W., Zong, B., Huang, L., Bala, H., Wang, S., Guo, Z., Sun,

G., Cao, J., & Zhang, Z. (2018). An effective TiO2 blocking layer for hole-conductor-

free perovskite solar cells based on carbon counter electrode. Organic Electronics, 59,

253–259. https://doi.org/10.1016/j.orgel.2018.04.042

[14] Mocatta, D., Cohen, G., Schattner, J., Millo, O., Rabani, E., & Banin, U. (2011).

Heavily Doped Semiconductor Nanocrystal Quantum Dots. Science, 332(6025), 77–81.

https://doi.org/10.1126/science.1196321

[15] Nath, I. (2010). Cleaning Up After Clean Energy: Hazardous Waste in the Solar

Industry. Stanford Journal of International Relations, 11(2), 6-15.

35

[16] Ning, Z., Gong, X., Comin, R., Walters, G., Fan, F., Voznyy, O., Yassitepe, E., Buin,

A., Hoogland, S., & Sargent, E. H. (2015). Quantum-dot-in-perovskite solids. Nature,

523(7560), 324–328. https://doi.org/10.1038/nature14563

[17] Padilla, D. (2013). Temperature-Dependent Electron Transport in Quantum Dot Pho-

tovoltaics [UC Santa Cruz]. https://escholarship.org/uc/item/7rn901nd

[18] Park, N.-G. (2017). Nonstoichiometric Adduct Approach for High-

Efficiency Perovskite Solar Cells. Inorganic Chemistry, 56(1), 3–10.

https://doi.org/10.1021/acs.inorgchem.6b01294

[19] Ren, Z., Yu, J., Pan, Z., Wang, J., & Zhong, X. (2017). Inorganic Ligand Thiosulfate-

Capped Quantum Dots for Efficient Quantum Dot Sensitized Solar Cells. ACS Applied

Materials & Interfaces, 9(22), 18936–18944. https://doi.org/10.1021/acsami.7b03715

[20] Serrano, E., Rus, G., & Garćıa-Mart́ınez, J. (2009). Nanotechnology for sus-

tainable energy. Renewable and Sustainable Energy Reviews, 13(9), 2373–2384.

https://doi.org/10.1016/j.rser.2009.06.003

[21] Sheet, A. I. F. (2002). Renewables in global energy supply.

[22] Smith, A. M., & Nie, S. (2010). Semiconductor Nanocrystals: Structure, Prop-

erties, and Band Gap Engineering. Accounts of Chemical Research, 43(2), 190–200.

https://doi.org/10.1021/ar9001069

[23] Targhi, F. F., Jalili, Y. S., & Kanjouri, F. (2018). MAPbI3 and FAPbI3 perovskites as

solar cells: Case study on structural, electrical and optical properties. Results in Physics,

10, 616–627. https://doi.org/10.1016/j.rinp.2018.07.007

[24] Tsuo, Y. S., Wang, T. H., & Ciszek, T. F. (n.d.). Crystalline-Silicon Solar Cells for

the 21st Century. 11.

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IntroductionMotivationBackgroundPV Solar CellsBulk and Thin Film ArchitecturesQuantum DotsMAPbI3 Perovskites

Methods and Experimental DesignSynthesisSlide CleaningTiO2 EngineeringQD FabricationMAPbI3 FabricationHybrid QD/MAPbI3 Blend FabricationCathode Deposition

CharacterizationVoltage-Current MeasurementsMaterial AbsorbancePDSEXAFS

Results and DiscussionQD ResultsHybrid QD/MAPbI3 Results

Conclusions and Future WorkNotationLiterature Cited