Functional materials riassunto

Stokes Shift and LED Principles

R NRSK NR S= =K *τϕ ISC NR SS S+K KR NRTK R= *ϕϕ P ISCT T+K KR NRSTOKES SHIFT–1Is the gap (in cm ) between the maximum in absorption and the maximum in emission.Rigid structures have lower Stokes shift, since their energy states have similar geometry.( )1 1 7– *10Stokes shift= absorption emissionλ λmax maxMIRROR IMAGE RULEUsually absorption and emissions have specular spectra (if normalized, intensity is different)LED: principles & designLEDs are basically semiconductor pn-junctions that, under forward bias conditions, emitradiation by electroluminescence in the regions of UV, VIS and IR depending on the Egap,thanks to radiative e-h+ recombination.Forward bias condition means that electrons are injected in the n-type material. By doingthis, we facilitate the transport of electrons and holes, increasing the efficiency (depletionzone is reduced).DEPLETION ZONE: zone at the interface of the two semiconductors where the

charge carrier concentration is low; the lack of electrons from the n-type and of holes from the p-type leads to a bias → BUILT-IN POTENTIAL( )∗Nk T NB holes ∈ p-type conductive electrons ∈ n-type = V ln0 2e N electrons level Fermi

When the electron-hole pair recombination occurs in the depletion region an exciton is formed and, if the material is electroluminescent, it may decay by emission of a photon.

LED Architecture

A typical LED always requires a pn-junction sandwiched between two electrodes, from which forward bias is applied.

The external part is covered with a layer of silica (SiO2) and one of Al in order to obtain isolation and a carrier confinement, leaving a part clear for the photons to escape.

Usually, the technique used to fabricate LEDs is by epitaxially growing doped semiconductors, layer by layer on a suitable substrate. The p-type material is placed on the transparent surface and it has to be narrow in such a way that photons can escape emitting light.

but they can't be reabsorbed. Due to the Total Internal Reflection (TIR), not all of the emitted photons can escape. In order to increase efficiency, it is important to shape the surface into a hemisphere, so that the angles of incidence at the interface are smaller than the critical angle.

EFFICIENCY of a LED

A. INTERNAL QUANTUM EFFICIENCY: fraction of excitons that decay in a radiative way; it is strictly related to the quantum yield of fluorescence in the material.

PhνIQE = Ie

Should also consider injection efficiency (fraction of electrons injected that actually reach the active region), but it can be neglected in first approximation

B. EXTERNAL QUANTUM EFFICIENCY: emitted photons = IQE∗[extraction efficiency]

EQE = injected electrons

EQE is incremented by using dot LEDs instead of a continuous line of LEDs

C. POWER EFFICIENCY (CONVERSION EFFICIENCY):

P o∈¿ = electrical power I∗V optical power out

PE = ¿

The efficiency is reduced by the possibility for the emitted

photons to be:re-absorbed in the material

adsorbed outside of the semiconductors

reflected at the interfaces of the system

MATERIAL DESIGN for LEDs

When we choose a material, it has to satisfy some requirements:

it should have a direct, or at least correct, band gap

there must exist more than one efficient radiative pathway to escape

it can be both n and p doped.

Those requirements allow the field to some materials as:

GaAs and others direct band gap materials

readily n/p-doped materials

materials characterized by a certain refractive index that allows photons to escape.

DOUBLE HETEROSTRUCTURE DIODE

In general, combining the elements from the III and V group of the periodic table into binary or ternary compounds, we obtain very optical active compounds that can be used to construct LEDs. In particular, we are searching for a strategy that keep the electrons far from the outerspace and that transforms energy into light and not dissipates it into heat.

A useful

mechanism used for increasing the efficiency is the double-heterostructure diode, in which two materials with a wider and a smaller band gap are combined together in a "sandwich": the one with a wide band gap is used as the outer layer, while the one with the smallest band gap is used as the inner layer (we can see it as a combined potential well for electrons and holes so that they're both trapped). For example, if we consider AlGaAs and p-doped GaAs, the former has a band gap wide enough to act as a confinement for the injected electrons in the latter. Chemically, we can say that, growing those layers, we obtain an extremely high probability of having almost all the chemical bonds mattering at the interface (matched lattices). In this way, electrons are constricted in what is called active region, that is the semiconductor in the middle of the two junctions. TEMPERATURE dependence of emitted light The wavelength of the emitted light by a LED depends on the semiconductor.

material and, in particular, on its energy gap. In fact, the emitted spectrum is concentrated around a specific lambda, defined by the relation E=hn/l. If we increase the temperature, the energy gap decreases and, consequently, the wavelength increase, leading to a red shift of the spectrum.

ADVANTAGES

Because of the structure of the device, the lifetime of a LED is much larger than the one of a normal incandescent bulb: the light emitting element in such devices is made by a small conductor chip, while in the classic bulbs is made by a metal filament. This characteristic leads to a lifetime of 10000h with respect to the 1000h of a bulb. Plus, efficiency is very high, due to the fact that LEDs basically don't produce heat, and they have a short time response. Moreover, every diode emits its own light, such that light is more intense; the cost is low, and the device is more robust.

OLED

Advantages over LED:

A. Faster response

B. Lower voltage

C. High brightness and contrast

D. Higher surface area

which lead to a lower cost and a stronger homogeneity

Flexibility

Lighter

Processable in solution

OLED are made by a semiconductor organic layer (the emitter) sandwiched between a metal and a transparent electrode (ex. ITO); in this structure, the process is divided in four main steps:

1. CHARGE INJECTION: if we apply a voltage (and we overcome the energy barrier), we inject e- from cathode and h+ from anode. The energy barrier can be estimated as the difference between the frontier energy level of the emitter and the work function of the electrode. Since the anode needs to be ITO (transparent), to increase efficiency we need a cathode with a high work function; metals that correspond to that request could be:
• Sodium (Na): W=2,4 eV but highly unstable
• Samarium (Sm): W=2,7 eV, more stable than Na, but very rare
• Calcium (Ca): W=2.9 eV but unstable (spontaneous oxidation)
• Aluminum (or Ag): W=4.3 eV, not best work function but high stability → most used.
2. CHARGE TRANSPORT:

usually in organic materials holes have higher mobility (electrons almost don't move)

3) CHARGE RECOMBINATION: when positive and negative charge carriers meet, recombination of electron-hole pairs occurs, forming an exciton, which will be:

1. 75% TRIPLET EXCITON
2. 25% SINGLET EXCITON

4) RADIATIVE DECAY: exciton then releases its energy by photon emission, by fluorescence if it was a singlet, phosphorescence if it was a triplet (need to use a blend of fluorescent and phosphorescent materials to exploit all the decaying excitons and increase efficiency).

N.B.: if the exciton forms near the metal electrode (electrons almost don't move since they are much slower than holes), non-radiative decay is increased, the active layer can be damaged and photons are formed far from the transparent electrode → SINGLE LAYER ARCHITECTURE has major drawbacks, is not the optimal structure

I can increase the efficiency by using a

1. DOUBLE LAYER ARCHITECTURE: by inserting an electron transport layer

(ETL),excitons are formed far from the metallic electrode. ETL material must be:

• Transparent: amorphous materials are better, chose the most branched
• Low LUMO: if it is an intermediate level between the work function of the electrode and the LUMO of the emitter, energy barrier is decreased. Chose a material with many acceptors.

2. TRIPLE LAYER ARCHITECTURE: by using also a HTL with a HOMO level between the work function of the anode and the HOMO of the emitter, energy barrier is decreased furthermore, achieving the best efficiency.

EMITTER MATERIAL

Since phosphorescent dyes also have a good efficiency in ISC, PhOLEDs are much more efficient and can in principle exploit 100% of the excitons.

Emitters are not made of polymers but of small molecules, since we need very high control on structure and purification. ∗η ∗η

EFFICIENCY OF AN OLED=γ∗η 1 2 3=injection =0,25 =quantumγ efficiency η if fluorescent dye η yield of fluorescence1

2=considersη efficiency of photon emission

3PHOTOVOLTAIC CELLS & PHOTODETECTORS

PHOTOVOLTAIC CELL works in a wide range of λ with no bias applied.

PHOTODETECTOR usually works at a specific λ with a bias

A photovoltaic device (solar cell) works converting the solar radiation energy into electrical energy, absorbing incident photons in order to generate charge carriers, which are then separated.

It is usually composed by two silicon layers, one n-doped and one p-doped, forming a pn-junction (and a depletion zone at the junction). EHP are usually formed in the p side, therefore we want the TRANSPARENT/FINGER ELECTRODE to be the one supporting the n side (if EHP form far from the depletion zone, they will recombine)

If the photon is absorbed near enough to the depletion zone (~10 nm, maximum distance depends on lifetime of the exciton and diffusion coefficient of the two), hole and electron gets separated by the built-in potential, which pushes the first in the p-doped side and

The second electrode in the n-doped side charges negatively, while the other one charges positively (OPENCIRCUIT POTENTIAL, Voc). If we directly connect the electrodes, electrons start to flow from the p-side to the n-side (SHORTCIRCUIT CURRENT, Isc) due to the built-in potential. To produce power, we need to connect a load to the photovoltaic cell.

PARAMETERS of a PHOTOVOLTAIC CELL

Main electrical characteristics of a photovoltaic device are summarized in the I-V characteristic curve: a representation of operation conditions of a cell.