Università degli Studi di Cagliari

Facoltà di Scienze

Corso di Laurea in Fisica

Perovskite LED

fabrication

Autore: Relatore:

Michele Lecis Prof. Michele Saba

Co-relatrice:

Dott.ssa Daniela Marongiu

26 luglio 2018

Ricordati di non farlo mai più.

Just to the Moon.

Abstract

The aim of this project is the fabrication of an organic-inorganic light

emitting diode, using a MAPbBr3 perovskite as light emitting material

with a visible green emission. From the I-V curves acquired, the

completed device works properly as a diode. Photoluminescence and

atomic force microscopy acquisitions confirm the effective growth and

deposition of the materials. Nevertheless, no light can be seen by the

naked eye and electroluminescence analysis shows an abnormal

emission, far from the expected green frequencies.

Index 1 Introduction .................................................................................................................................... 8

2 Organic light emitting diodes .......................................................................................................... 9

2.1 From p-n junction to OLED...................................................................................................... 9

2.2 OLEDs .................................................................................................................................... 12

2.2.1 Electrodes...................................................................................................................... 12

2.2.2 HTL ................................................................................................................................ 13

2.2.3 ETL ................................................................................................................................. 13

2.2.4 LEM ............................................................................................................................... 13

2.3 Materials ............................................................................................................................... 14

2.3.1 Perovskite ...................................................................................................................... 14

2.3.2 FTO ................................................................................................................................ 16

2.3.3 PEDOT:PSS ..................................................................................................................... 17

2.3.4 F8 ................................................................................................................................... 18

2.4 Architecture .......................................................................................................................... 19

3 Fabrication .................................................................................................................................... 20

3.1 Instrumentation .................................................................................................................... 20

3.1.1 Spin coater .................................................................................................................... 20

3.1.2 Glovebox ....................................................................................................................... 21

3.2 Synthesis and deposition ...................................................................................................... 22

3.2.1 Slides preparing ............................................................................................................. 22

3.2.2 Deposition ..................................................................................................................... 23

3.2.3 Silver contacts evaporation ........................................................................................... 24

4 Diagnosis of the devices ................................................................................................................ 25

4.1 Morfology .............................................................................................................................. 25

4.1.1 AFM working principle .................................................................................................. 25

4.1.2 Acquisition and elaboration .......................................................................................... 27

4.1.3 Analysis ......................................................................................................................... 28

4.2 Testing and I-V curves ........................................................................................................... 33

4.3 Spectroscopy ......................................................................................................................... 35

4.3.1 Instrument and measurements .................................................................................... 35

4.3.2 Photoluminescence ....................................................................................................... 37

4.3.3 Electroluminescence ..................................................................................................... 39

5 Conclusions ................................................................................................................................... 40

Bibliography .......................................................................................................................................... 41

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1 Introduction Lighting during the twentieth century was dominated by fluorescent, incandescent and gas

discharge lamps until over the last decades solid-states light emitting diodes (LEDs) have

attracted much attention, due to their unique durability combined with the need of producing

more energy efficient devices by means of low-cost procedures and materials. However, these

devices are generally prepared from crystalline semiconductors, which demand high

temperature, high vacuum, and time-consuming methodologies. These requisites increase the

final cost of the product, appearing unsuitable for large-area displays. Therefore, great efforts

are being devoted toward the research of new materials that possessed excellent electro-optical

properties, while involving simple deposition procedures and mild crystal growth conditions,

in order to reduce the production cost.

Among the various solution processed semiconductors, organometal halide perovskites

represent a notable class of materials. As absorbers in photovoltaic applications, perovskites

have shortly reached energy conversion efficiencies even higher than 20%, that make them

candidates to be employed as possible future substitutes of silicon in solar cells. It follows that

these materials are also envisaged to be ideal for the development of high efficiency and cost-

effective solution-processed light emitting diodes since the progress made on Pe-based devices

in recent years, have successfully awaken the interest for using them as light emitters.

For the reasons outlined above, in this study a low-cost production technique has been used to

produce perovskite light emitting diodes.

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2 Organic light emitting diodes The purpose of this chapter is to recall the theoretical notions necessaries to treat the study.

Then the architecture of the device is described together with the employed materials.

2.1 From p-n junction to OLED

To understand the operating principle of OLEDs, it is first necessary to point out the concept

behind a semiconductor device comprising a p-n junction diode. It will be characterized by a

metal anode contact, a p-type semiconductor, a n-type semiconductor and a metal cathode

contact.

The band model is used to describe electron and hole behaviour for the p-n junction. There is

a transition region in which the energy bands are sloped to provide a continuous conduction

band and a continuous valence band extending from the p-side to the n-side of the junction.

That band slope is evidence for an electric field within the semiconductor, which is referred to

as a built-in electric field. In fact, if a constant electric field is present the energy bands

(horizontal at equilibrium conditions) must tilt since there will be a constant gradient in energy

and the carriers will travel to lower their potential energies. The change in energy across the

p-n junction is labelled E0 at equilibrium and is referred to as an energy barrier, while the

corresponding potential V0 is the contact potential of the junction.

Figure 2.1: Band model of p-n junction in equilibrium (left), with the application of a forward bias (above), and reverse bias (below).

Perovskite LED fabrication

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The built-in field ε causes carriers in the transition region to drift and consequent carrier

concentration gradients across the junction. There are four currents to consider: the electron

and the hole currents driven by the built-in electric field (In drift, Ip drift) and the ones driven by

the concentration gradient (In diffusion, Ip diffusion). If the junction is in equilibrium:

𝐼𝑛 𝑑𝑟𝑖𝑓𝑡 + 𝐼𝑛 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 = 0

𝐼𝑝 𝑑𝑟𝑖𝑓𝑡 + 𝐼𝑝 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 = 0

If we apply a voltage to the diode, it is no longer in an equilibrium state. Considering a forward

bias, the applied voltage V > 0 will fall across the transition region of the p-n junction and will

decrease the energy barrier height as well as the electric field ε. The decrease in barrier height

will result in a net current because the opposing drift current will no longer be sufficient to

cancel out all the diffusion current. The net current flow results from a net majority carrier

diffusion current to become

𝐼 = 𝐼𝑝 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 + 𝐼𝑛 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 − 𝐼𝑝 𝑑𝑟𝑖𝑓𝑡 − 𝐼𝑛 𝑑𝑟𝑖𝑓𝑡 > 0

If we now consider the application of a reverse bias with V < 0, the applied voltage will again

fall across the transition region of the p-n junction, which will increase the magnitude of both

the potential barrier and ε. The increase in the energy barrier will cause drift current to

effectively oppose diffusion current. There is, however, a remaining current due to thermally

generated minority carriers. This constitutes a small net minority carrier drift current and it is

assisted by the electric field. The net current flow is dominated by thermally generated minority

carrier drift currents.

𝐼 = 𝐼𝑝 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 + 𝐼𝑛 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 − 𝐼𝑝 𝑑𝑟𝑖𝑓𝑡 − 𝐼𝑛 𝑑𝑟𝑖𝑓𝑡 < 0

Figure 2.2: diode current as a function of the applied voltage.

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The p-n junction is an ideal device to either absorb or emit photons, and these processes occur

when an electron-hole pair is generated or annihilated respectively. The p-n diode efficiently

transports both electrons and holes either towards the junction for photon generation to occur,

or away from the junction for electric current generation due to photon absorption to occur.

Hence, in a LED device based on p-n junction, electrons and holes are injected into the junction

from the voltage applied between the two electrodes and from their recombination photons are

emitted. The same working principle takes place in OLEDs (organic light-emitting diodes),

with a similar process that employ three layers: a light emitting material (LEM) between a hole

transport layer (HTL) and a separate electron transport layer (ETL) are sandwiched by the

electrodes as shown in figure.

Upon application of an electric field, electrons are injected by means of the cathode into the

ETL, while holes are injected by means of the anode into the HTL. These holes and electrons

meet in the LEM and form excitons therein.

In fact the luminescence process can be effectively described by excitons quasiparticles. An

exciton is a bound state of an electron and a hole which are attracted to each other by the

electrostatic Coulomb force. It can form when an electron is excited from the valence band into

the conduction band, leaving behind a positively charge hole. This attraction provides a

stabilizing energy balance. The exciton must transfer energy to be annihilated. When an

electron and a hole form an exciton, it is expected that they are initially in a high energy level

with a large quantum number nexciton. This forms a larger, less tightly bound exciton. Through

thermalization the exciton loses energy to lattice vibrations and approaches its ground state. Its

Figure 2.3: scheme of an OLED device.

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radius decreases as nexciton approaches 1. Once the exciton is more tightly bound and nexciton is

a small integer, the hole and electron can then form an effective dipole and radiation may be

produced to account for the remaining energy and to annihilate the exciton through the process

of dipole radiation.

2.2 OLEDs

The following paragraphs will focus on the requisites that a material should meet in order to

be suitable to be chosen to fabricate an OLED. It is important to begin by saying that, within

the framework of organic semiconductors, the highest occupied molecular orbital (HOMO) is

the analogue of the valence band, while the lowest unoccupied molecular orbital (LUMO) is

the conduction band. The energy difference between HOMO and LUMO represents the band

gap.

2.2.1 Electrodes

The anode material should offer ease of patterning and good stability. The surface smoothness

of the layer is important: a surface roughness below 2 nm is generally required. A high work

function is also required to inject holes efficiently. In addition, the transparency is fundamental

to allow the light to exit the device. There are some materials like ITO that have a chemically

active surface that can cause migration of molecules into subsequent layers. It is important to

notice that the work function could be sensitive to the cleaning process used to prepare the

layer for subsequent processing.

Unlike anode materials, cathode materials are generally not transparent, which provides a wider

range of materials choice. They must provide high conductivity, a low work function and good

adhesion to the underlying polymer layers. Stability is also important and is highly dependent

on packaging. Challenges associated with cathodes include propensity towards oxidation,

which is a consequence of the low work function materials that require easily ionized group I

and group II metals. There is also a tendency for these cathode layers to cause chemical

reduction of adjacent organic layers. The cathode layer is usually the most reactive layer in the

OLED in the presence of oxygen or water.

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2.2.2 HTL

The basic requirement of the HTL is good hole conductivity but an additional key requisite for

efficient OLED devices is the size of the energy barrier at the interface of the HTL, which must

be small enough. Therefore, the HOMO level of the HTL should be within a fraction of an

electron volt from the anode energy band.

2.2.3 ETL

In several materials, intermolecular transport occurs by electron hopping and a LUMO level

that is similar in energy to the work function of the cathode is required. The ETL should have

a mobility of at least 10-6 cm2 V-1 s-1, which is one-two orders of magnitude smaller than the

mobility range of HTL materials. Insufficient electron mobility in the ETL means that in many

cases holes that enter the light emitting layer (LEM) will not encounter electrons and will

therefore continue until they reach the ETL before they recombine. Since the ETL is not

optimized for high recombination efficiency, a lower device efficiency could be generated.

ETL is also often oxidized by hole conduction, in which electron loss and subsequent

degradation of the ETL material occur due to holes that enter the material. The ability of the

ETL to withstand long-term exposure to the applied electric field is essential. Since the ETL

has a lower mobility and therefore lower conductivity than the HTL, a larger voltage drop and

hence a larger electric field drops across the ETL. The molecules in the ETL should not lose

multiple electrons by field ionization. Finally, the ETL material must be able to be processed

and coated with good interface stability and with good layer uniformity and quality.

2.2.4 LEM

Obviously, a key component for successful OLED operation, the LEM must be amenable to a

high-quality deposition technique. It also requires the capability to transport both holes and

electrons to enable the recombination of these carriers. Moreover, it should allow for the

creation of excitons and their decay to generate photons. It must remain stable at the electric

fields needed to transport the holes and electrons and the migration of molecules must be

minimized for device stability. The emission colour of the OLED is ultimately determined by

the LEM and in many cases molecules are modified to achieve a specific desired emission

colour.

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2.3 Materials

The focus of the next paragraphs is on the architecture chosen for the fabrication, with a first

analysis of the materials that are going to compose each layer.

2.3.1 Perovskite

A perovskite is any material with the same type of crystal structure as calcium titanium

oxide (CaTiO3), known as the perovskite structure (it takes its name from the Russian

mineralogist Lev Perovski after its discovery in 1839). The general chemical formula for

perovskite compounds is ABX3, where 'A' and 'B' are two cations (respectively with larger and

shorter ionic radius) and X is an anion that bonds to both.

The B cation has 6-fold octahedral coordination with the X anions that surround it in a BX6

crystal structure, while the A cation has 12-fold coordination located between the octahedrons

to balance the charge of the structure. In the idealized unit cell, the A and B ions form a body-

centred cubic structure where type A atom sits at cube corner position (0, 0, 0) while type B

ion sits the body-center position (½, ½, ½). X ions are placed at face centred position

(½, ½, 0).

Figure 2.5: perovskite crystal structure.

Figure 2.4: perovskite unity cell.

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The real perovskite structure often deviates from the ideal one because of covalent-ionic

bonding interactions, hydrogen bonds or molecular vibrations. These distortions reduce the

crystal symmetry to tetragonal or orthorhombic spatial group, maintaining cubic coordination

numbers and the preceding chemical formula. Nevertheless, the ion size requirements for

stability of the structure are quite stringent, so slight buckling and distortion can produce

several lower-symmetry distorted versions, in which the coordination numbers of A cations, B

cations or both are reduced. Many of the metallic elements are stable in the perovskite structure,

if the Goldschmidt tolerance factor t is in the range of 0.75–1.0.

𝑡 =𝑅𝐴 + 𝑅𝑋

√2(𝑅𝐵 + 𝑅𝑋)

where RA, RB, RX are the ionic radii of the atoms. Tilting of the BX6 octahedra reduces the

coordination of an undersized A cation from 12 to as low as 8. Conversely, off-centring of an

undersized B cation within its octahedron allows it to attain a stable bonding pattern. The

resulting electric dipole is responsible for the property of ferroelectricity and shown by

perovskites such as BaTiO3 that distort in this fashion.

Perovskite materials exhibit many interesting properties from both the theoretical and the

application point of view such as ferroelectricity, piezoelectricity, pyroelectricity,

superconductivity, colossal magnetoresistance and other magnetoelectric and optoelectronic

properties. Some perovskites e.g. lead halide organic-inorganic perovskites, are excellent

alternatives to semiconductors. This is related to the possibility of solution processing, low

temperature requisites and their high carrier mobility. In addition, tuneable bandgap, high

absorption coefficient, broad absorption spectrum, high charge carrier mobility and long charge

diffusion lengths of metal halide perovskites enable a broad range of photovoltaics and

optoelectronics applications. In this field, methylammonium lead halide

CH3NH3PbX3 (X = Br, I, Cl) perovskite solar cells have become efficient and progressed faster

than any other solar cells to date in a few years since their invention. The remarkable

performance of these solar cells has triggered the exploration of applications in optoelectronics,

such as light-emitting diodes, lasers, and photodetectors. The low manufacturing cost and ease

in preparation and production make these halide perovskites suitable candidates for future

technologies. However, commercializing perovskite devices is impeded by rapid material

degradation. The performance and stability of hybrid perovskite devices are strongly affected

by defect states and film quality, which affect carrier lifetime.

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For our purposes, since its bandgap stands in the visible green light, the material that is going

to be the LEM of the current architecture is the methylammonium lead bromide CH3NH3PbBr3.

2.3.2 FTO

Tin(IV) Oxide, also known as stannic oxide, is the inorganic compound with

the formula SnO2. The colourless, diamagnetic, amphoteric solid has several applications such

as ceramic glazes, polishing, glass coatings or gas sensing.

If doped with fluorine it becomes a transparent conductive metal oxide that can be used as TCO

(transparent conductive oxide layer) in the fabrication of electrodes for thin film photovoltaics.

Fluorine-doped Tin Oxide (FTO) is a low-cost alternative to Indium Tin Oxide (ITO), the most

commonly used TCO. It has the added benefit of being stable at high temperatures and in acidic

and hydrogen rich environments, quite recurring conditions in the fabrication of devices such

as organic light-emitting diodes or organic solar cells. In addition, low surface resistivity, high

optical transmittance, scratch and abrasion resistance make FTO suitable as TCO for this

project.

FTO glass also enjoys a varied range of other applications, including use in touch screen

displays, electromagnetic interference or radio frequency interference shielding, heated glass

and anti-static coatings.

Figure 2.6: tin oxide structure.

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2.3.3 PEDOT:PSS

PEDOT:PSS or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate is a polymer mixture

of two ionomers. The component poly(3,4-ethylenedioxythiophene) or PEDOT is

a conjugated polymer that carries positive charges and is based on polythiophene, a sulphur

heterocycle compound. The other component is made up of sodium polystyrene sulfonate

(PSS), where part of the sulfonyl groups are deprotonated and carry a negative charge.

Together, the charged macromolecules form a macromolecular salt.

It is used as a transparent, conductive polymer with high ductility in different applications. For

example, it is used as an anti-electrostatic coating agent in film rolls to protect electronic

components. PEDOT:PSS is very sensitive to ultraviolet radiation, as well as high temperatures

and humidity. That is why it is usually mixed with ultraviolet stabilizers to avoid a too fast

degradation.

For present purposes, it is used as hole transport layer and as a barrier for the electrons

generated in the perovskite, preventing them from reaching the FTO causing short circuit.

Figure 2.7: PEDOT:PSS polymer structure.

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2.3.4 F8

Poly(9,9'-dioctylfluorene) (F8) is an organic compound, a polymer of 9,9-dioctylfluorene,

with formula (C13H6(C8H17)2)n. The monomer has an aromatic fluorene core -C13H6- with

two aliphatic n-octyl -C8H17 tails attached to the central carbon.

It is an electroluminescent conductive polymer that characteristically emits blue

light. Depending on the several formations in which it can be found, due to the intermolecular

forces that F8 can participate in, the molar mass of F8 ranges between 24 - 42 (g/mol). As a

result of this varying molar mass, many other properties (such as glass transition temperature,

absolute wavelength emitted, melting point, etc.) vary as well. These can be engineered by

changing the strain and temperature applied to the polymer’s structure or by applying thermal

treatment such as friction transfer.

Because of its properties, F8 can be used both as HTL or ETL in an OLED device or directly

as a PLED (polymer light-emitting device). In this architecture, it acts as ETL and barrier for

the holes.

Figure 2.8: F8 polymer structure.

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2.4 Architecture

Therefore, the chosen architecture for the realization of these OLEDs is:

FTO/PEDOT:PSS/CH3NH3PbBr3/F8/Ag

The correct alignment of the energy levels and the consequent theoretical compatibility of the

materials is shown in the next picture. As a consequence of the preparation process, a little shift

of the energy levels can occur. If this happens, the functioning of the device could be

compromised.

Figure 2.9: OLED architecture.

Figure 2.10: energy levels of the components.

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3 Fabrication This chapter will describe the assembly procedure followed to fabricate the device.

3.1 Instrumentation

3.1.1 Spin coater

The thin layers that compose the devices have been deposited by spin coating. It is a procedure

used to deposit uniform thin films to flat substrates. Usually a small amount of coating material

is applied on the centre of the substrate and then is rotated at high speed in order to spread the

coating material by centrifugal force. Rotation is continued while the fluid spins off the edges

of the substrate, until the desired thickness of the film is achieved.

The higher the angular speed of spinning, the thinner the film. The thickness of the film also

depends on the viscosity of the solution and concentration of the solvent. The applied solvent

is usually volatile and evaporates during the spinning. Nevertheless, the procedure is followed

by a thermal treatment to eliminate the remaining solvent.

Figure 3.1: spin coating process.

Figure 3.2: spin coater.

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3.1.2 Glovebox

A glovebox is a sealed container that is designed to allow to manipulate objects where a

separate atmosphere is desired. Built into the sides of the glovebox are gloves arranged in such

a way that the user can place their hands into the gloves and perform tasks inside the box. The

box is usually transparent to allow the user to see what is being manipulated. In this project, a

glovebox is required to allow manipulation of substances that must be contained within a very

high purity inert atmosphere. In fact, a nitrogen atmosphere is necessary to prevent the

deterioration of perovskites due to water and oxygen in the room atmosphere. The gas in a

glovebox is pumped through a series of treatment devices which

remove solvents, water and oxygen from the gas. Heated copper metal (or some other finely

divided metal) is commonly used to remove oxygen. This oxygen removing column is normally

regenerated by passing a hydrogen/nitrogen mixture through it while it is heated: the water

formed is passed out of the box with the excess hydrogen and nitrogen. It is common to

use molecular sieves to remove water by absorbing it in the molecular sieves' pores.

For the entire duration of the experiment, the O2 level inside the glovebox was stable between

450-550 ppm.

Figure 3.3: glove box.

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3.2 Synthesis and deposition

3.2.1 Slides preparing

The FTO glass was cut to obtain 15 mm x 15 mm slides: this is the ideal dimension to

successfully complete the procedures of deposition and morphology and efficiency

measurements.

Before the deposition process, it is necessary to accomplish a chemical etching to partially

remove the FTO from the slides to obtain the correct architecture and avoid short circuits. 2/3

of the slides surface were covered with tape, while zinc powder was distributed on the

remaining free surface. Under the fume hood, a few drops of hydrochloric acid (HCl) 37%

were dropped with a pipette on the zinc, triggering the etching reaction. After waiting some

minutes for the reaction to occur, the slides were washed with water and the tape removed,

allowing to verify with a tester whether FTO had been correctly withdrawn.

The slides at this point needed to be treated with hydrogen peroxide (H2O2) at boiling

temperature for 30 minutes. Then they had to be rinsed with demineralized water, acetone and

isopropanol and dried with nitrogen to remove solvent residues. This treatment provides the

surface “activation” at nanometric level, eliminating contamination and making the surface

hydrophilic, ready for the deposition phase inside the glove box.

Figure 3.4: FTO glass.

Figure 3.5: FTO glass after etching process.

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3.2.2 Deposition

PEDOT:PSS was filtered with a nylon filter (45μm), spin-coated onto the substrate at

6000 rpm for 30 s and annealed at 140°C for 30 min in a nitrogen atmosphere.

The perovskite precursor solution was prepared by mixing methylammonium bromide

(CH3NH3Br) and lead bromide (PbBr2) in a 3:1 molar ratio in anhydrous

N,N-dimethylformamide (DMF) to give a concentration of 5 wt%. The solution was heated at

60°C to facilitate the dissolution and subsequently applied hot. It was spin coated onto

PEDOT:PSS at 3000 rpm for 30 s and annealed at 100°C for 15 min.

Finally, a solution of F8 in chlorobenzene (10 mg/ml) after a 1 hour ultrasonic treatment was

spin coated onto the perovskite layer at 3000 rpm for 60 s.

Figure 3.6: sample after PEDOT:PSS deposition.

Figure 3.7: sample after perovskite deposition.

Figure 3.8: sample after F8 deposition.

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3.2.3 Silver contacts evaporation

After the deposition, using a spatula to remove the deposited materials, a region (about 1/3 of

the sample wide) of FTO was exposed in order to interface the electrode and the contacts.

The samples were put in the evaporator and under a vacuum of 10-5 mbar, 30 nm of silver were

evaporated on the devices.

Figure 3.9: sample after scratching.

Figure 3.10: completed device.

Figure 3.11: “device graveyard” with finalized attempts.

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4 Diagnosis of the devices The next paragraphes will report the diagnosis procedure used to evaluate weather the device

is properly working.

4.1 Morfology

4.1.1 AFM working principle

Atomic force microscopy (AFM) is a very high-resolution type of scanning probe microscopy

that provides information about superficial microstructure and local properties. The

information is gathered by "feeling" or "touching" the surface with a mechanical probe. It

consists of a cantilever that has a tip with a curve radius of few nanometres in his extremity.

The force applied on the tip by the surface generates a deflection on the cantilever that can be

quantified by the beam-bounce method. The deflection is measured by a laser reflexed by the

cantilever toward a photodiode matrix that produces as output a current for each of the four

quadrants. From the differences between the value of the currents before and after the passage

of the probe, it is possible to determine the intensity and the direction of the displacement.

Figure 4.1: AFM apparatus scheme.

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The microscopy is provided by a piezoelectric scanner that has the task of checking with high

precision the distance between tip and sample. In order to avoid external interferences, the

whole system is suspended by springs.

There are three main scanning functions: contact-mode, non contact-mode, tapping mode. The

interaction between the tip and the surface can be modelized using the Van der Waals forces

that can be attractive at short range or repulsive at long range. Depending on the scanning

function utilized, the entity of the forces exploited varies as shown in the following graph.

For our purposes the tapping-mode is optimal since weaker interaction forces are indicated for

soft samples and a high lateral resolution and good sensibility to friction problems can be

achieved in return of a lower scanning speed. In this mode, a piezoelectric actuator makes the

cantilever oscillate. The amplitude and the phase of the oscillation depend on the interaction

between the tip and the surface.

Figure 4.2: beam-bounce scheme.

Figure 4.3: scanning function depending on the distance.

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4.1.2 Acquisition and elaboration

After having positioned the sample, the instrument was prepared for the acquisition. The laser

was aligned on the tip, the reflexed beam centred on the photodiode, the intensity maximized

and the feedback gain set.

For each layer that composes the device, 20 μm x 20 mμ topographic images were acquired

and analysed by means of the software WSxM 5.0 develop 9.0. The acquired data could be

distorted because of imperfections in the apparatus or interactions with the environment. If the

sample is tilted or if in the same sample there are areas with a different slope the imaging could

appear inaccurate. This could be corrected by approximating the resulting curves to the first

order. During the post production phase, it was possible to correct some scanning error due to

the shift of the tip on the surface or to some deposited debris on the tip. If this happens, some

lines could be missed and it was necessary to proceed by averaging with the previous and the

successive lines.

The analysis of the scans produced the height distribution of the samples and a root mean square

for the roughness. It is defined as a parameter that quantifies the deviations in the direction of

the normal vector of a real surface from its ideal form.

𝜎𝑧 = √∑ (𝑧𝑖−�̅�)𝑁𝑖=1

𝑁

1

𝑁∑ 𝑧_𝑖𝑁

𝑖=1

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4.1.3 Analysis

FTO

The first layer to be analysed is FTO. The RMS roughness is 50.7 nm, with an average height

of 110.0 nm. As it can be deducted from the narrowness of the histogram, the distribution is

quite uniform that can provide a good interface for the successive layer. Nevertheless, it

presents some spikes that can reach more than 400 nm. A possible explanation to these

irregularities can probably be researched during the cleaning process. It was not possible to

determine the thickness of the layer due to instrumental limitations.

Figure 4.4: 2D and 3D FTO image.

Figure 4.5: histogram of the quote distribution for the FTO sample.

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PEDOT:PSS

Figure 4.6: 2D and 3D PEDOT:PSS image.

Spin coated on FTO, the second layer is PEDOT:PSS. It shows a RMS roughness of 19.7 nm

and an average height of 60.8 nm. The layer presents some holes but there are not evident

critical issues that can compromise the correct working of the device.

Figure 4.7: histogram of the quote distribution for the PEDOT:PSS sample.

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Perovskite

The RMS roughness for the perovskite layer amounts to 27 nm. This value does not deviate

excessively from the one registered for the previous layer, meaning that the coverage is

uniform.

Figure 4.9: 2D and 3D perovskite image.

Figure 4.8: histogram of the quote distribution for the perovskite sample.

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From the analysis it has emerged that there are some areas where the perovskite does not cover

the PEDOT:PSS. This might not be due to the deposition process but rather to the fact that the

sample has been analysed in air, exposing the LEM to deterioration. These holes explain the

double distribution of the histogram: the topography at about 100 nm refers to perovskite over

the HTL while the lower one at about 50 nm almost certainly has to be attributed to the

PEDOT:PSS along the uncovered regions.

In order to evaluate the thickness of the layers, the surface of the sample was scratched with a

blade. The section graphic shows a depth of about 100 nm, compatible with what emerged from

the histogram analysis.

Figure 4.10: 2D and 3D image of the scratch on the perovskite sample.

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F8

Figure 4.11: 2D and 3D F8 image.

Finally, the last layer that finalises the deposition process is F8. The RMS roughness measured

is 21.3 nm with an average height of 67.0 nm. Despite the roughness value, the surface looks

quite more irregular then the previous layers but it seems to cover enough the inner LEM layer

to avoid that the silver makes contact with the perovskite after the evaporation.

Figure 4.12: histogram of the quote distribution for the F8 sample.

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4.2 Testing and I-V curves

To test the proper functioning of the devices it was necessary to measure the I-V characteristic

curve. In this respect, a Keithley 2400 was used, a unit that can function both as a generator

and as a digital multimeter with high impedance and low noise. The instrument is optimized to

do 4 wires measurements that uses separate pairs of current-carrying and voltage-sensing

electrodes to make more accurate measurements than the simpler and more usual two-terminal

sensing, in order to reduce contacts and wires parasite resistances.

From these measurements a diode I-V curve is expected as motivated in the 2nd chapter and, at

about 1.5 V, a turn-on of light emission should be seen.

Even if the curve of the first device looks promising, no light could be seen by the naked eye

once the alleged activation potential was surpassed. The following measurements (2 and 3, as

above) show the fragility of the device that broke after the first measurement. This

inconvenience did not allow a spectroscopic analysis to verify if a weak emission occurs.

Figure 4.13: Three successive acquisitions of I-V curve of the first device.

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The second device curve looks equally promising but no green emission was detectable by the

naked eye, as the first device. Nevertheless, it manifests a better stability even at a higher

voltage, giving opportunity to test the device using a spectrometer.

Figure 4.15: first I-V curve of the second device.

Figure 4.14: second I-V curve of the second device at higher voltages.

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4.3 Spectroscopy

4.3.1 Instrument and measurements

A monochromator (or spectrometer) is an instrument used to disperse light into its spectral

components. Generally, a monochromator is composed by an entrance slit, a set of blazed

gratings installed on a rotating turret, a set of reflecting mirrors and an exit slit coupled with a

detector. The light to be analysed comes into the instrument through the entrance slit. A grating

separates the spectral components of the light and the detector measures their intensity. If the

detector has a small dimension sensor that is able to analyse wavelengths only one by one, the

instrument is called monochromator and to change the exit wavelength it is sufficient to rotate

the grating. While if the sensor dimensions are such that it is possible to collect many spectral

lines at the same time, the instrument is called spectrometer. The grating position defines the

central wavelength of the spectrum. There exist different configurations to arrange a

monochromator, but the most common is the so-called Czerny-Turner configuration. This

configuration adopts two concave mirrors: the first one collimates the beam coming from the

entrance slit of the monochromator on the diffraction grating. The diffracted light is then

focalized by the second mirror on the exit slit of the monochromator, where the detector is

placed.

The spectral bandwidth B is the smallest spectral range that the monochromator is able to

isolate. Neglecting both the grating width and system aberrations, the bandwidth can be

calculated as follows:

Figure 4.16: monochromator scheme.

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𝐵~𝑑𝜆

𝑑𝑥max (𝑊𝑖𝑛, 𝑊𝑜𝑢𝑡)

where Win and Wout are the entrance and the exit slit width, respectively. Experimentally, B is

defined as the full width at half maximum (FWHM) of the spectral profile acquired from a

monochromatic source.

For our purposes, the spectrometer was used to evaluate the photoluminescence and

electroluminescence of the fabricated devices.

Figure 4.17: visible light spectrum.

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4.3.2 Photoluminescence

Photoluminescence (PL) is light emission from any form of matter after the absorption of

electromagnetic radiation. In this case the excitement is made by UV radiation to verify the

correct growth of the materials, especially of the perovskite. In fact, from the bandgap of the

CH3NH3PbBr3 a green emission is expected. We should be able to see also the F8 blue PL. It

was possible to take pictures of the PL using a confocal microscopy.

The images show the optimal coverage of the F8 at the centre of the device (on the left) with a

blue emission as expected. Moving the focus on the scratch made before the contact

evaporation (on the right), the green emission of the uncovered perovskite can also clearly be

seen, meaning the effective growth of the materials.

Figure 4.18: photoluminescence confocal microscopy photos at the centre of the device (left) and on the scratch (right).

Figure 4.19: UV photoluminescence measurement.

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Observing the phenomenon with a spectroscope, it was possible to quantify the wavelength of

the emissions. For F8 it is nearing 440 nm, while the perovskite emits at 540 nm.

The absorptance confirms the measurements with a peak due to F8 at 440 nm, while the

perovskite emission is shifted to 520 nm.

Figure 4.20: absorptance measurment.

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4.3.3 Electroluminescence

Electroluminescence (EL) is an optical phenomenon in which a material emits light in response

to the passage of an electric current or a strong electric field. Since a spectrometer is much

more sensitive than the human eye, this measurement enables us to verify if the devices are

working even if with low efficiency. Despite the excitement methods being different, exactly

the same emission frequency measured for PL is expected, since it depends only on the material

bandgap. We should see only the green emission of the LEM if the device is properly working.

The device emits but the wavelength emission was not the one which was anticipated. The

emitted frequencies are spread along a range of 300 nm in wavelength with an apparently red

peak. The emission of the perovskite, if present, is deleted by an unexpected luminescence at

lower frequencies.

This could be explained as an organic emission due to transitions between intermediate levels

of the F8 polymer. If so, it can be concluded that the device is not working. To verify this

hypothesis, it would be appropriate to map the energy levels of the polymer.

Figure 4.21: electroluminescence measurement at 4.5V.

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5 Conclusions The goal of this study was the fabrication of an organic-inorganic light emitting diode using a

metal halide perovskite as light emitting material. Between FTO and silver electrodes, the

material used as hole transporting layer was the polymer PEDOT:PSS, while F8 was employed

as electron transporting layer. The layer designated to electron-hole recombination was

composed by methylammonium lead bromide (MAPbBr3), whose bandgap should provide a

green emission. The I-V curves are compatible with the expected diode characteristic curve

and the atomic force microscopy images show a uniform coverage of all the layers that

compose the device, meaning that the deposition process has been accomplished successfully.

Besides confirming the F8 effective covering, the confocal microscopy pictures under UV

stimulation show the successful growth of both electron transporting layer and light emitting

material. The spectroscopic photoluminescence acquisition provides the supposed emission

profile of the device. However, the expectation has been disregarded by the

electroluminescence verification: there is not visible light detected by the naked eye while the

device is alimented and, even if the spectroscopic measurement shows that the device is

emitting, the frequency range is incompatible with the one obtained with photoluminescence.

A possible explanation could be provided by considering the electroluminescence property of

the F8 polymer. However, the detected frequencies are far from the blue emission of the

polymer bandgap so that it is also necessary to suppose that the emission comes from

intermediate levels transition.

Since the diagnosis procedure has not found other critical issues, there could be problems with

the architecture itself suggesting that other materials have to be employed. Other attempts have

been done using ITO as electrode or titanium oxide and Spiro:OMeTAD as transporting layers

in different combinations with F8. Even different perovskites have been employed, such as

methylammonium iodide or different concentrations of the same bromide to vary the light

emitting layer thickness. Nevertheless, the obtained results are not reliable and the attempts

have to be repeated.

Another possible reason that could have led to the failure could be attributed to the interface

between silver contacts and F8. It could be worth trying with a different electrode such as

calcium.

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