Universität Potsdam
Institute of Physics and Astronomy
Advanced Physics Lab Course
May 2015
M7
I.
ELECTROLUMINESCENCE OF POLYMERS
INTRODUCTION
The recombination of holes and electrons in a luminescent material can produce light. This
emitted light is referred to as electroluminescence (EL) and was discovered in organic single
crystals by Pope, Magnante, and Kallmann in 1963.[1] EL from conjugated polymers was first
reported by Burroughes et al.[2] The polymer used was poly (p-phenylenevinylene) (PPV). Since
then, a variety of other polymers has been investigated.
Organic EL devices have applications in a wide field ranging from multi-color displays and
optical information processing to lighting. Polymers have the advantage over inorganic and
monomolecular materials in the ease with which thin, structurally robust and large area films can
be made from polymer solutions. Using printing techniques, patterned structures can be produced
easily. Even flexible displays can be produced because of the good mechanical properties of
polymers.
In this lab course, basic optical and electrical properties of conjugated polymers will be
investigated.
Advanced Lab Course: Electroluminescence of Polymers
2
EXPERIMENTAL TASKS
 Measure the absorption spectra of your polymers (thin films spin coated onto glass
substrates).
 Characterize the setup used for luminescence measurements. Identify possible sources of
error and collect data necessary for correction.
 Measure the photoluminescence emission spectra for the polymer films in an appropriate
measurement geometry, using suitable excitation wavelengths.
 Measure the photoluminescence excitation spectra for the polymer films in an appropriate
measurement geometry, using suitable detection wavelengths.
 Measure the current through the OLEDs and the spectral radiant intensity of
electroluminescence as a function of applied voltage (the current-radiance-voltage
characteristics).
 Measure the electroluminescence emission spectrum at a suitable voltage or current.
 Measure the radiance of an OLED as a function of viewing angle.
 Determine the radiometric and luminous efficiencies of the OLEDs.
 Summarize your results.
CONTENTS
I.
INTRODUCTION ................................................................................................................................................... 1
EXPERIMENTAL TASKS ................................................................................................................................................. 2
CONTENTS .......................................................................................................................................................................... 2
II.
FUNDAMENTALS .................................................................................................................................................. 3
II.1
II.2
II.3
II.4
III.
CONJUGATED POLYMERS .......................................................................................................................................................3
LIGHT EMISSION BY CONJUGATED POLYMERS ...................................................................................................................3
LIGHT-EMITTING DIODES .....................................................................................................................................................6
POLYMERS FOR LIGHT-EMITTING DIODES ...................................................................................................................... 12
EXPERIMENT EXECUTION AND ANALYSIS .............................................................................................. 14
III.1
III.2
III.3
III.4
III.5
IV.
GENERAL REMARKS ....................................................................................................................................................... 14
ABSORPTION SPECTRA.................................................................................................................................................. 14
LUMINESCENCE SPECTRA.............................................................................................................................................. 15
ELECTROLUMINESCENCE .............................................................................................................................................. 16
SUMMARY OF EXPERIMENT .......................................................................................................................................... 18
INSTRUMENTATION AND POSSIBLE SOURCES OF ERROR ................................................................ 19
IV.1
IV.2
IV.3
MEASUREMENT SOFTWARE ......................................................................................................................................... 19
CORRECTION OF SPECTRA ............................................................................................................................................ 21
INNER FILTER EFFECTS ................................................................................................................................................ 22
V.
REFERENCES AND FURTHER READING .................................................................................................... 23
VI.
RADIOMETRIC AND PHOTOMETRIC QUANTITIES ............................................................................... 24
Advanced Lab Course: Electroluminescence of Polymers
3
II. FUNDAMENTALS
II.1
CONJUGATED POLYMERS
Organic electronics can be divided into two major classes of materials. One of them is the class
of small organic molecules. Electroluminescence was first observed in crystals of the small
molecule anthracene in 1961.[1] However, these materials are usually not soluble and have to be
deposited by thermal evaporation. Hence the production of small molecular electronics needs
expensive technology comparable to inorganic electronics.
The second class of materials used are polymers. While most polymers are electrical insulators,
for electronics applications, materials with semiconducting properties are needed. Conductivity
needs the presence of free, mobile charges. Table II.1 shows a selection of polymers which have
been studied with respect to their semiconducting properties. All polymers have in common that
they possess a conjugated π electron system. This conjugated system is often depicted as an
alternating chain of single and double bonds between the carbon atoms. However, quantum
chemical calculations show that this picture is not correct, the electrons forming the double bonds
are delocalized over the main chain. In a thin solid film, many chains are in close contact with each
other (allowing orbital overlap between the chains), so that excess charges can travel over
macroscopic distances in an electrical field. While the polymers shown in Table II.1 possess a
conjugated backbone, that is not generally necessary. Other polymers have been used which are
built from an saturated backbone bearing conjugated side groups.
The conjugated system can be easily disturbed by either conformational defects (e.g., kinks) or
chemical impurities in the material. In addition, for entropic reasons it is very unlikely for a
polymer chain to be completely extended. At the resulting disturbances, the conjugated system is
interrupted. As a result, a polymer chain is not one single conjugated system but a chain of
conjugated segments of different length. The segments are often called “chromophores” because
they determine the optical and electronic properties of the materials. A polymer chain can be
regarded as a chain of chromophores of different lengths and thus different properties. The length
distribution can be regarded as being Gaussian.
II.2
LIGHT EMISSION BY CONJUGATED POLYMERS
Absorption of energy by atoms, molecules or condensed matter will result in the generation of
excited states, i.e. it increases the potential energy of the electrons in the substance rather than its
heat. If the excited state decays under emission of visible light, that emission is called
luminescence. Luminescence is observed from many inorganic and organic substances, the
luminophores, and can be induced by various physical processes. Two possible excitation
mechanisms relevant for this lab course are the absorption of photons, which leads to
photoluminescence, or the recombination of injected charges, which is called
electroluminescence. Luminescence spectra are determined by the properties of the material. In
contrast, the thermal generation of light by heat radiation is called incandescence. It is mainly
determined by the material’s temperature.
The excited state is called an exciton, a coulombically bound electron-hole pair. In inorganic
(crystalline) semiconductors, the binding energy of an exciton is on the order of or even below the
thermal energy at room temperature (about 0.025 eV), such that the exciton will dissociate into
free charges. In organic materials, the exciton is much stronger bound (about 0.5 eV) and will not
dissociate easily. The generated excitons on polymer chromophores thus resemble the excited
states of small molecules and absorption and photoluminescence can be described in a molecular
picture. To understand the optical properties of a thin polymer film, one needs to keep in mind
that such films comprise a distribution of chromophores in close contact.
Advanced Lab Course: Electroluminescence of Polymers
4
Table II.1: Polymers which have been extensively studied for EL [3]
PPV
poly(p-phenylene vinylene)
n
MEH-PPV
poly[2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene]
O
n
O
OR 1
PPE
poly[2,5-dialkoxy-1,4-phenylene ethynylene]
n
R 2O
PPy
poly(p-pyridine)
N
n
PF
poly(9,9-dialkylfluorene)
n
R
R
S
R
P3AT
poly(3-alkylthiophene)
n
PPP
poly(para-phenylene)
n
C10H21
C6H 13
R
n
H13C6
R
LPPP
“ladder-type poly(para-phenylene)”
R=H, LPPP
R=CH3 (methyl), Me-LPPP
H21C10
II.2.1
Photoluminescence
In the following chapter, the numbers in brackets correspond to the numbers in Figure II.1. At
room temperature, most chromophores are in the vibrational and electronic ground state S0,0. The
occupation of higher vibrational states follows the Boltzmann distribution. In general, excitation
(1) by absorbed energy is comprised of transitions S0,0 − S𝑛,𝜈′ into an electronically excited state
𝑛 and an additional vibrational excitation 𝜈 ′ . The generated state decays at very short times by
non-radiative transitions (2) to vibrational states of the first excited singlet state S1,𝜈′′ (internal
conversion) and vibrational relaxation (3) to the lowest vibrational level S1,0. The radiative
transition (4) S1,0 − S0,𝜈 from the lowest excited to the ground state is denoted as fluorescence. In
general, the transition will not only decay to the ground state S0,0, but also to higher vibronic states
S0,𝜈 of the singlet ground state. This gives rise to the vibronic progression observed in the
fluorescence spectra of most conjugated systems. Also, the excitation usually involves transitions
into more than one (vibrational) state.
The radiative transition competes with nonradiative deactivation pathways. The first is
isoenergetic internal conversion (not shown) from S1,0 to a high vibronic level of the electronic
ground state followed by vibrational relaxation to S0,0. The second path is inter-system crossing, a
non-radiative transition (5) from S1,0 to triplet states T𝑛,𝜈′′′with similar energy, followed by
vibrational relaxation (6) and possibly internal conversion steps to the lowest triplet state T1,0.
Inter-system crossing involves a change of spin states from S = 0 for the singlet state to S = 1 for
Advanced Lab Course: Electroluminescence of Polymers
5
the triplet. There are three different ways for de-excitation of the triplet state, the first of which is
repeated inter-system crossing (7) to a level S0,𝜈 and subsequent distribution of the vibrational
energy (8). The second – radiative – path is phosphorescence, the transition (9) to S0,𝜈 . However,
since this process requires the simultaneous change of spin and energy (orbital) of the electron,
this process shows only a low probability in polymers. Another possibility for deactivation of the
triplet state is the transition (10 + 11) from T1,0 to the S1,0 state by means of thermal energy, which
is only possible if the energy difference between the two states is not more than some 𝑘𝐵 𝑇. In case
of high excitation densities, a bimolecular interaction of two molecules in triplet state T1 may also
yield an molecule in the S1 state with the other one in S0 (triplet-triplet-annihilation).
For most conjugated systems, the strongest absorption band is associated with a transition from
the S0,0 singlet ground state to multiple vibronic levels of the lowest excited singlet state S1,𝜈′′.
Oscillator strengths and positions of the different transitions depend on the shape and position of
the molecular potentials in the ground and excited states (Figure II.2).
Figure II.1: Typical energy level diagram
of organic molecules. The description of
the processes of excitation and deexcitation is given in the text.
Figure II.2: Simplified potential energy curves
showing how the mirror-image relationship
between the absorption and emission spectrum of an
organic molecule is caused. Adapted from [4]
In the case of conjugated polymers, it is important to keep in mind that these substances can be
regarded as a chain of chromophores with Gaussian length distribution. Due to disorder in the
film, the conjugation length will vary throughout the film. To a first approximation, the
dependence of electronic transition energies on the chromophore length can be described with
the quantum mechanical “particle in a box” model: The longer the conjugation, the lower the
energetic levels of absorption and fluorescence. Absorption of a photon in a (dense) polymer film
will occur whenever the energy of a photon matches an allowed transition on any chromophore
the photon passes in the film. Thus, when the incident light penetrates the polymer, it will sample
a large variety of chromophore lengths leading to a broad, unstructured absorption band.
Advanced Lab Course: Electroluminescence of Polymers
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However, prior to fluorescence the generated exciton will migrate to states of lower energy not
only by means of internal conversion and vibronic relaxation, but also by energy transfer to
adjacent molecular segments (this is sometimes called “spectral diffusion”, see Figure II.3). As a
result, the fluorescence spectrum will be more structured and considerably red-shifted with
respect to absorption. For polymers with a stiff backbone, both absorption and fluorescence will
show clear vibronic progressions because most chromophores are of the same length and thus
the energetic distribution of states is narrow. In this case, the red-shift of fluorescence with
respect to absorption is smaller.
Figure II.3: Illustration of spectral diffusion:
Absorption of a photon occurs into a distribution of
possible states around a central transition energy
⟨S1 ←S0 0 − 0⟩. Prior to emission, the exciton
migrates to sites of lower energy (a). Only for very
low excitation energies below νloc , this migration is
supressed (“localization threshold”, b). [5]
II.2.2
Electroluminescence
Emission of light can also be driven by the recombination of injected charge carriers. In this case,
the excited state is formed when two oppositely charged carriers meet on one chromophore (e.g.,
segment of a polymer chain). In a molecular picture, the formed exciton represents an excited
electronic state, e.g. S1 , of the chromophore, which can decay either radiatively (by emission of a
photon) or non-radiatively (by production of heat) to the ground state S0 . For these transitions,
the same principles as in photoluminescence emission apply.
Based on spin statistics, injection of “uncorrelated” charge carriers produces singlet and triplet
excitons in the ratio 1:3 (There is only one way for an electron and a hole to form an exciton with
total spin zero, but three combinations with non-vanishing spin components.). The singlet
excitons decay promptly, yielding what is referred to as prompt EL, whereas the triplet excitons
either decay directly (electrophosphorescence) or they fuse to form singlet excitons (triplet-triplet
annihilation, see above), producing delayed EL. However, in most conjugated polymers, triplet
excitons decay mainly non-radiatively.
II.3
LIGHT-EMITTING DIODES
A simple organic light-emitting diode (OLED) device geometry is shown in Figure II.4. It consists
of one emission layer sandwiched between a hole and an electron injecting contact, denoted anode
and cathode, respectively. Carriers of opposite sign are injected separately at opposing contacts
when a sufficiently high voltage is applied. The most simple polymer-based OLED consists of a
single layer of semiconducting fluorescent polymer sandwiched between two electrodes, a
semitransparent, high work function anode like indium tin oxide (ITO) or gold and an opaque, low
work function cathode made of calcium, aluminum or other materials. The thickness of the organic
layer is typically in the order of 100 nm and, for experimental convenience, the active device area
is in the order of a few mm².
More elaborated multilayer structures (Figure II.5) include additional charge-transporting
layers. The function of e.g. the hole-transporting layer is to facilitate hole injection from the anode
to the emission layer and to prevent electrons from leaving the emission layer and reaching the
anode without recombining with a hole. The electron-transporting layer functions accordingly.
Advanced Lab Course: Electroluminescence of Polymers
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Figure II.4: Schematic drawing of a
single-layer
electroluminescent
diode. An applied electric field leads
to injection of holes and electrons into
the light-emitting film from the two
electrode contacts. Formation of an
electron-hole pair within the
material may then result in the
emission of a photon.
Figure II.5: Layout of a multilayer
OLED comprising hole and electrontransporting layers.
II.3.1
Theory of Electroluminescence
Electroluminescence in organic layers involves a series of steps:
1. injection of charge carriers from a metal contact into the organic layer
2. transport of charge carriers within the film
3. recombination of holes and electrons into excitons
4. migration and radiative decay of the exciton
5. emission of the generated photon to the outside
The efficiency of these processes is closely related to the device geometry and to particular
properties of the applied polymers such as the positions of energy levels of the charge transport
materials or the solid state photoluminescence efficiency of the emitting polymer.
II.3.1.1
Charge Injection
One of the fundamental processes occurring in OLEDs is the injection of excess charges from the
metal contacts into the electroluminescent polymer film. This charge injection can be qualitatively
understood by considering the electronic energy structure of the thin polymer film with respect
to the work functions (Fermi levels) of the injecting contacts (Figure II.7).
As an example, the electronic energy structure of MEH-PPV is shown in Figure II.6 along with
the work function of various metals used as contacts in OLEDs. The ionization potential (𝐼𝑃 ) of
PPV, i.e. the energy required to remove an electron from the energetically highest occupied
molecular orbital (HOMO) to vacuum, is roughly 5.2 eV. The electron affinity (𝐸𝑎 ), i.e. the energy
gained when adding an electron to the lowest unoccupied molecular orbital (LUMO) from vacuum,
is roughly 2.5 eV. The energy gap, 𝐼𝑃 − 𝐸𝑎 , is about 2.7 eV.
To inject electrons, the contact must be able to donate electrons into the lowest unoccupied state
2.5 eV below vacuum. Similarly, to inject holes (remove electrons), the according contact must be
able to accept electrons from an energy 5.2 eV below vacuum.
Advanced Lab Course: Electroluminescence of Polymers
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Figure II.6: Electron energy level diagram of
PPV and work functions of selected contact
metals used in OLEDs. [3]
E (a)
(b)
Vacuum level
IP
φA
Δ Eh
Anode
EA
LUMO
(c)
(d)
φC
ΔEe
HOMO
Polymer
Cathode
x
Figure II.7: Schematic energy levels of an OLED. (a) depicts the situation before contact: The anode material
needs to have a Fermi level that allows the extraction of electrons from the polymer HOMO, the cathode
material needs a Fermi level that allows the injection of electrons into the polymer LUMO. After contact (b),
the Fermi levels of the electrodes will align, giving rise to an internal electric field (built-in field) in the
polymer layer indicated by the tilted HOMO and LUMO levels. The application of an external voltage will first
reduce the band bending back to the “flat-band-condition” (c) and further to an inverse bending (d). Situation
(d) is the one needed for charge injection and transport in OLEDs, while the region between (b) and (c) is used
for photovoltaic devices. Note that the work functions φA , φB , electron affinity EA , ionization potential IP , and
hence the energy barriers ΔEh , ΔEe do not change upon contact or application of an external field. The
(dotted) color arrows indicate the (possible) direction of motion for holes (red) and electrons (blue).
Electron injection is limited by the barrier Δ𝐸𝑒 between the Fermi level of the cathode and the
position of the LUMO. The rate of hole injection is similarly restricted by the barrier Δ𝐸ℎ between
the work function of the anode material and the HOMO of the polymer.
Since many π conjugated polymers have ionization energies of about 5 eV, high work function
materials such as gold, copper and indium-tin-oxide (ITO) are needed for hole injection. LUMO
positions are typically located between 3 eV and 1.5 eV. Effective electron injection thus requires
low work function metals such as calcium. However, these metals are very unstable in air and
require device encapsulation. For a balanced charge injection, similar barriers Δ𝐸𝑒 ≅ Δ𝐸ℎ are
needed. For most polymeric systems, balanced charge injection is difficult to realize in single layer
devices. Ideally, there are no injection barriers.
As the work function of the two contacts is different, the current will not only depend on the
absolute field but also on its direction. Higher currents will flow if the contact with the higher
work function is biased as the anode and the contact with the smaller work function as the
cathode. This situation is denoted forward bias. Inverting the bias (reverse bias) will lead to
smaller currents. In addition, a certain forward bias is required to overcome the so-called built-in
field due to the different electrode Fermi energies. The operation conditions for forward bias are
described in Figure II.7. Note that the minimum turn-on voltage is determined by the work
function difference of the electrodes, additional injection barriers will cause an additional increase
in operating voltage. An example for the resulting diode-like current-voltage characteristics are
shown in Figure II.8.
Advanced Lab Course: Electroluminescence of Polymers
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Figure II.8: (left) Typical current-voltage and radiance-voltage characteristics of an MEH-PPV-based OLED
with an Au anode and a Ca cathode. [3] (right) Band diagram for electron and hole injection into a polymer
layer.
If the injection barriers Δ𝐸𝑒 and/or Δ𝐸ℎ become too high, they may limit the device performance.
Two different models to describe injection-limited currents are thermionic injection (1) and field
emission (Fowler-Nordheim tunneling) (2),
4𝜋𝑒𝑚eff 𝑘𝐵2 2
Δ𝐸𝑒,ℎ
𝑒𝑈
(1)
𝑗=
𝑇 exp {
} [exp {
} − 1]
3
ℎ
𝑘𝐵 𝑇
𝑘𝐵 𝑇
𝑗 = 𝐹2
8π√𝑚eff
𝑒 2 𝑘𝐵
𝑚eff
√
exp {−
}.
2
ℎ Δ𝐸𝑒,ℎ
2
3𝑒𝐹
(2)
In the equations, F is the electric field, all other symbols have their usual meaning. In principle,
the barrier heights for electron and hole injection can be determined on the basis of these
equations. Note, however, that one needs to separate hole and electron current to determine the
barriers.
II.3.1.2
Charge Carrier Motion and Recombination
The motion of charge carriers is generally described by the carrier mobility µ, which is defined
by the ratio of the drift velocity 𝑣 and the electric field F,
𝑣
𝜇= .
(3)
𝐹
In typical conjugated polymers, µ is in the range (10−7 … 10−1 ) cm2 /Vs. In general, the mobility
of holes and electrons is different (with the electron mobility typically being smaller).
As discussed in sections II.1 and II.2.1, the polymers in thin films can be regarded as “chains of
chromophores” where each chromophore has electronic properties that can be described in a
molecular picture. Excess charges injected from the contacts will localize on single chromophores.
Charge transport then requires the motion of the charges between these localized states. This
process is called “hopping” transport in contrast to “band-like” transport that occurs in well
ordered (crystalline) systems. This transport mechanism is the main reason for low charge
mobilities in disordered polymeric semiconductors.
In the case of small injection barriers, the low carrier mobility of organic semiconductors
determines the charge transport through the layer. After injection, charges will only slowly move
away from the contacts. The accumulated charges lead to the formation of a space charge, charge
carrier density and electric field are not constant over the film thickness. In the most simple case
with no trapping sites in the semiconducting layer, the current density in the space-charge limited
regime becomes
9
𝑈2
(4)
𝑗 = 𝜀0 𝜀𝑟 𝜇 3 .
8
𝑑
Advanced Lab Course: Electroluminescence of Polymers
10
In this case, the current depends quadratically on the applied voltage. For the meaningful
extraction of a mobility value one needs to make sure that only one type of charges is present in
the film. In addition, this simple behavior is usually not found in polymers due to the Gaussian
distribution of transport states. Here, the low-energy states act as shallow traps for carrier
transport. The best experiment to distinguish between injection limited current and transport
limited current is the measurement of the thickness dependence: Injection limited currents do not
depend on the film thickness while transport-limited currents do.
The recombination of an electron and a hole under the condition of low mobility is described by
the Langevin recombination mechanism, which involves the drift of the charges in their mutual
electric field. Due to the low carrier mobility, the exciton formation is much slower than the
radiative decay of the exciton and controlled by the mobility and densities of the charge carriers.
II.3.1.3
Migration and Radiative Decay of the Excitons
A critical parameter in determining the operating efficiency of OLEDs is the luminescence
quantum efficiency 𝜂(PL) of singlet excitons in the polymer, i.e. the probability that a singlet
exciton will decay radiatively. This probability is limited by the intrinsic (intramolecular)
quantum efficiency for radiative decay on an isolated molecule (as determined by the PL efficiency
in dilute solution). However, in the solid state, several mechanisms can further reduce the
quantum efficiency:
 electronic coupling to neighboring molecules might alter the electronic states and thus the
efficiency for radiative exciton decay
 during its lifetime (typically 100 ps…1 ns) the exciton in polymers can diffuse to nonradiative sites (so-called quenching sites) or they might be deactivated at the metal
electrodes via energy transfer or dissociation.
Therefore, the photoluminescence quantum efficiency 𝜂(PL) in the solid state is generally
smaller than in dilute solution. For “good” polymers, values range between 40% and 60%.
II.3.1.4
Emission of the Generated Photon to the Outside
Not every photon generated inside the emission layer will escape the device and become visible
to an external observer. Photons might be absorbed either by the emissive material itself
(reabsorption), by additional charge transport layers or by the electrodes. Further, multiple
reflection of photons at both electrodes may occur (see section II.3.2.1).
Finally, the question is how a light-emitting diode appears to an external viewer. It is known
from many surfaces that they have the same apparent brightness, independent of the viewing
angle. If such a “Lambertian” surface is viewed under an angle 𝜃, the effective visible surface is
given by (Figure II.9)
(5)
𝑑𝐴eff = 𝑑𝐴⃑ ∙ 𝑟̂ = 𝑑𝐴 ∙ cos 𝜃.
Therefore, for an extended source the luminous intensity 𝐼𝑣,𝜃 (that is the radiant energy emitted
per unit time and unit solid angle by a source along a given direction) follows with (5)
𝐼𝑣,𝜃 = 𝐼𝑣,0 ∙ cos 𝜃.
(6)
where 𝐼𝑣,0 is the luminous intensity normal to the surface. Due to multiple scattering and reflection
events in the active layer, OLEDs follow to a good approximation Lambert’s emission law.
Advanced Lab Course: Electroluminescence of Polymers
11
Figure II.9: Projection of an emitting surface element of an
extended source
II.3.2
Efficiency Considerations
II.3.2.1
Quantum efficiency
The efficiency of organic light-emitting devices is described by several parameters. First, a
material with a high photoluminescence quantum yield is needed. The PL quantum yield is defined
as the number of photons emitted per photons absorbed. It is determined by the radiative and
non-radiative decay rates kr and knr, respectively.
The internal quantum efficiency of electroluminescence 𝜂𝑖𝑛𝑡 (EL) is defined as the number of
photons generated in the material per injected charge carrier,
𝜂𝑖𝑛𝑡 (EL) = 𝜙𝑟 × 𝜙𝑒 × 𝜂(PL).
(7)
The individual factors denote the fraction of injected charges that recombine to form an exciton
𝜙𝑟 , the fraction of emissive excitons 𝜙𝑒 and the quantum efficiency of photoluminescence 𝜂(PL),
respectively. The fraction of recombining charges can be driven close to unity, but in general 𝜙𝑟 <
1
1. For fluorescent materials, 𝜙𝑒 = due to the fact the recombination of “uncorrelated” charge
4
carriers yields singlet and triplet excitons in a ratio 1:3. For phosphorescent emitters, this factor
can be 1 if efficient inter-system crossing converts all formed singlet excitons to triplets. The
photoluminescence quantum efficiency enters the equation because once formed, the excitons
created by charge recombination decay in the same manner as excitons formed by absorption of
a photon.
The external quantum efficiency 𝜂𝑒𝑥𝑡 (EL) < 𝜂𝑖𝑛𝑡 (EL) is defined as the number of „detectable“
photons per injected charge carrier. The refractive index 𝑛 of the emissive polymer (typically
1.7…2.0) is larger than that of the supporting glass substrate (1.5). Thus, photons generated in the
polymer layer, propagating at an oblique angle with respect to the surface normal, are reflected
at the polymer-glass interface and may be waveguided in the device. For an isotropic material,
where the transition dipoles are randomly oriented,
𝜂𝑖𝑛𝑡 (EL)
(8)
𝜂𝑒𝑥𝑡 (EL) =
2𝑛2
accounting for waveguiding and scattering losses inside the device.[6] For ideal recombination
conditions 𝜙𝑟 = 1, 𝜂(PL) = 1 and a refractive index 𝑛 = 1.7, the maximum external efficiency is
approx. 5 % for fluorescent OLEDs.
II.3.2.2
Power efficiency
Relevant for a device application is the external power efficiency 𝜂𝑃 (EL), defined as the emitted
light power divided by the electrical driving power. Using the definition of the external quantum
efficiency, 𝜂𝑃 (EL) can be rewritten as
ℎ𝜈
(9)
𝜂𝑃 (EL) =
𝜂 (EL)
𝑒𝑈 𝑒𝑥𝑡
where ℎ𝜈 is the energy of the emitted photons, e is the electric charge of an electron and U is the
applied voltage. For polychromatic emission, the energy distribution of photons needs to be taken
into account. A high power efficiency requires a high external quantum efficiency 𝜂𝑒𝑥𝑡 (EL) and a
Advanced Lab Course: Electroluminescence of Polymers
12
low operating voltage U. For an external efficiency of 5%, an operating voltage of 3 V and an
emission wavelength of 540 nm (ℎ𝜈 = 2.3 eV), 𝜂𝑃 (EL) ≅ 4%.
Experimentally, the power efficiency can be determined by measuring the radiant flux Φ𝐸 of the
device and dividing it by the electrical driving power. The emission of an LED is usually measured
as spectral radiant flux 𝜙𝐸,𝜆 (see section VI) so that the power efficiency is calculated by
𝜂𝑃 (EL) =
II.3.2.3
Φ𝐸 ∫ 𝜙𝐸,𝜆 𝑑𝜆
=
.
𝑃𝐸
𝐼×𝑈
(10)
Luminous efficiency
The human eye, our own light detector, is characterized by non-uniform sensitivity to the
various spectral components of light. An experimental function, the photoptic luminosity function
𝑉𝜆 (𝜆) shown in Figure II.10, is used to represent the physiological effect of light throughout the
visible part of the spectrum. At the maximum of the sensitivity curve (𝜆𝑚𝑎𝑥 = 555 nm), a radiant
flux Φ𝐸 = 1 W represents a luminous flux Φ𝑉 = 680 lm. The wavelength-dependent factor 𝑉𝜆 (𝜆)
relates physical units to physiological ones, i.e. it convert watts to lumens throughout the
spectrum. The spectral luminous flux is related to the spectral radiant flux by
𝜙𝑉,𝜆 = 𝑉𝜆 × 𝜙𝐸,𝜆 .
(11)
The luminous efficiency is calculated by
Φ𝑉
𝜂𝑃 (EL) =
(12)
𝑃𝐸
and has the unit lm/W. For different spectral regions, the same power efficiency will lead to
different luminous efficiencies. White light incandescent lamps have luminous efficiencies of less
than 20 lm/W while fluorescent lamps (and LEDs) can reach up to 100 lm/W.
700
Luminosity function
V() [lm/W]
600
500
400
300
200
100
max=555 nm
0
400 450 500 550 600 650 700 750 800
Wavelength [nm]
II.4
POLYMERS FOR LIGHT-EMITTING DIODES
II.4.1
MEH-PPV
Figure II.10: Photoptic luminosity function
V() depicting the sensitivity of the human eye
with the maximum sensitivity of 680 lm/W at
555 nm.[7]
One well-known polymer is poly[2-methoxy,5-(2-ethylhexyloxy)-1,4-phenylene vinylene]
(MEH-PPV), a soluble derivative of PPV. MEH-PPV has a fluorescence quantum efficiency in the
solid state of approx. 10 %. The optical absorption and electroluminescence emission spectra of
MEH-PPV are shown in Figure II.11. The absorption and emission spectra are typical for PPVbased polymers. The maximum absorption coefficient is about 2.5 × 105 cm−1 . The absorption
has a broad peak, which is roughly 0.5 eV wide with vibronic features evident at about 2.2 eV and
2.4 eV. The energy gap of MEH-PPV is about 2.4 eV. The electroluminescence emission spectrum
is much narrower, peaks at about 2 eV and contains several vibronic peaks. The width of the
spectra as well as the large red shift in energy between the optical absorption and emission peaks
Advanced Lab Course: Electroluminescence of Polymers
13
is due to the disorder in the film, as described in section II.2.1. The photoluminescence spectrum
(not shown) is identical to the electroluminescence spectrum indicating that the excited states
produced optically and electrically are identical.
Figure II.11: Electroluminescence and optical absorption
spectrum of the soluble polymer MEH-PPV. [3]
II.4.2
LPPP
Another well-known polymer is ladder-type poly(para-phenylene) (LPPP). The emission color
of this polymer is bluish-green. In the solid state, LPPP has a fluorescence quantum efficiency of
ca. 40 %. It contains linking groups that preserve the structure of the polymer chain (see Table
II.1) and thus has considerably sharper spectral features compared to MEH-PPV. However, this
material tends to form aggregates or chemical defects. These sites can function as charge carrier
traps and emission sites. Therefore, the EL spectrum of this polymer might differ from the solid
state PL spectrum: In PL, the excited state is rather immobile during its short lifetime. In contrast
to that, in EL the injected charges have to travel through the film bulk before they recombine.
Therefore, the probability of being captured in traps is much larger than in PL leading to a higher
contribution of defect emission in EL compared to PL.
II.4.3
Phosphorescent OLEDs
An elegant way to increase the efficiency of OLEDs is to harvest triplet excitons. To do so, heavy
metal atoms with a strong spin-orbit coupling are required in the emitting species. Since the
energy of triplet states is generally lower than the energy of singlet states, singlet excitons will
transfer their energy to triplet states as well. Hence, in equation (7), 𝜙𝑒 = 1 and the internal
quantum efficiency can reach 100%. An easy way of constructing phosphorescent OLEDs is to
blend phosphorescent dyes into a polymer matrix.
II.4.4
Multi-Component OLEDs
In light-emitting diodes, the emitted light originates from a defined electronic transition. It is,
therefore, monochromatic (although it may be relatively broad in molecular substances due to
vibronic progression). To achieve a spectrally broad emission or even white light emission,
chromophores with different transition energies and thus emission colors covering the entire
visible spectrum have to be combined. Also, it is feasible to combine different moieties with
specialized properties such as efficient emission or good charge transport into the emissive layer
e.g. by co-polymerization or simple blending.
In such multi-component systems, interactions between the different components have to be
taken into account. Steady-state spectra are influenced by two major processes: energy transfer
and charge trapping.
The energy of an excited state can be transferred to neighboring chromophores by different
processes such as simple transfer (reabsorption), Förster and Dexter transfer. In Förster transfer,
the transition dipole moments of two chromophores couple. This is a long-range interaction
(several nanometers). In contrast, Dexter transfer is an electron-exchange interaction that
requires wave function overlap and is thus short-ranged. All processes have in common that the
energy of the emitted light is lowered.
Advanced Lab Course: Electroluminescence of Polymers
14
The different emission wavelengths originate from the different energy levels (“band gaps”) of
the materials. On an energy scale, the transport levels for electrons and holes will be closer
together for a red-emitting material than they are for a blue emitter. In a blend of two such
materials, the energetic position for at least one type of carrier will be much more favorable on
the red emitter compared to the blue-emitting material. The situation is depicted in Figure II.12.
If charges that are initially placed on a blue chromophore start to move through a blend film due
to an applied field, they will eventually come close to a red chromophore and jump to its energy
level. The return to the blue material now requires additional energy. If there are only few red
chromophores, the charge can not move further and will essentially be trapped on the red dye. If
a charge of opposite sign comes close, it will be coulombically attracted to the first charge, may
jump to the molecule and form an exciton. This process occurs in addition to energy transfer from
blue to red excitons such that in electroluminescence, low-energy emission is more pronounced
than in photoluminescence for the same system.
Figure II.12: Illustration of charge trapping in a multicomponent system. After coming close to a low-energy
site, the electron will jump on this chromophore (1). A
release into the transport level requires additional
energy and is thus unlikely (2). However, a hole may
get transferred onto the chromophore due to
coulombic attraction (3) followed by the formation of
an exciton that then decays radiatively (4).
III. EXPERIMENT EXECUTION AND ANALYSIS
III.1
GENERAL REMARKS
You will investigate two different materials. A sheet with “technical data” for your specific
samples will be provided at the setup.
Make sure to note experimental conditions such as wavelength settings or diode driving
conditions and include them in the report.
Typically, the absolute intensities of the different spectra can not be compared due to
differences in excitation and measurement geometries. Relative differences in the spectral
distributions are the main focus of this lab course.
Store all raw measurement data in your folder on the lab course server (drive T:).
III.2
ABSORPTION SPECTRA
Measure the absorption spectra of the polymers (thin films spin coated onto glass
substrates).
Light absorption follows Lambert-Beer’s law,
𝐼(𝑥) = 𝐼0 exp{−𝛼(𝜆)𝑥}
(13)
with the light intensity 𝐼(𝑥) at position 𝑥, 𝐼0 the incident light intensity (𝑥 = 0) and the spectral
absorption coefficient 𝛼(𝜆). Absorption spectra will be measured using a standard absorption
spectrometer (Perkin-Elmer Lambda 2). This spectrometer consists of two light paths (sample
and reference path). By measuring a “baseline” curve without samples, differences in the two light
paths are recorded for automatic correction of the measured spectra. When choosing “A” as
measurement mode, the optical density (decadic spectral extinction 𝐸(𝜆))
𝐼0
𝐸(𝜆) = log10 {
}
(14)
𝐼(𝑑)
with the sample thickness 𝑑 is then calculated from the recorded intensities of the sample and
reference path and stored in a data file. A more detailed manual can be found in section IV.1 and
at the instrument.
Advanced Lab Course: Electroluminescence of Polymers
15
From the spectra, suitable wavelengths for excitation of photoluminescence need to be
determined.
III.3
LUMINESCENCE SPECTRA
A schematic of the setup used for measuring excitation and emission spectra is shown in Figure
III.1. For measurement of steady state photoluminescence excitation and emission spectra, a
xenon maximum pressure lamp is used. The excitation and emission part of the spectrometer
consist of two similar (reflective) grating monochromators controlled by step motors. The sample
chamber contains a holder for polymer films on glass substrates. The sample holder is designed
to hold the polymer film in the rotational axis of the holder in the intersection of excitation and
emission path. When this sample holder is removed, a cuvette holder for liquid samples can be
installed. A photomultiplier (Hamamatsu R928 in “digital mode”) detects the emitted light.
sample
chamber
polymer film
steady state
Xe lamp
excitation
monochromator
emission
monochromator
photon counter
Figure III.1: Schematic setup used for PL and EL
measurements (top view).
The monochromator after the Xe lamp is used to select which wavelength is exciting the sample
(hence it is called the “excitation monochromator”) while the monochromator in front of the
detector determines which wavelength will be transmitted from the sample chamber to the
detector (hence “emission monochromator”). All spectra will be recorded as a “point-by-point”
measurement. This is necessary as a simplistic approach to shine broad-band white light on the
sample and measure the light originating from the sample would make it almost impossible to
identify the origin (and wavelength) of the detected light.
Based on the setup design, two main measurement modes are possible. First, the excitation
monochromator can be set to a fixed wavelength while the emission monochromator scans a given
wavelength range (one wavelength after the other). The resulting spectrum is called an “emission
spectrum” as it shows the light that is emitted by the sample upon excitation with the (fixed)
wavelength set by the excitation monochromator. The other mode is the reversed case: The
emission monochromator is fixed to a certain wavelength while the excitation monochromator
scans a given range. The resulting “excitation spectrum” contains information on how efficiently
emission at the fixed emission wavelength is excited by different incoming wavelengths. Of course,
a reasonable emission spectrum can only be recorded with a suitable excitation wavelength.
Similarly, for the measurement of an excitation spectrum, the emission monochromator needs to
be set to a wavelength at which the sample emits.
III.3.1 Correction for instrument response
Characterize the setup used for luminescence measurements. Identify possible sources
of error and collect data necessary for correction.
The setup does not have a constant sensitivity over the spectral range under investigation. This
so-called instrument response has to be separated from the measurement data in order to get
correct results. Section 0 lists possible instrument-related sources of error and different methods
for the correction of excitation and emission spectra.
Due to the construction of the setup, no absolute “intensity” measurements are possible.
Moreover, they are not needed for this lab course. However, it is necessary to remove any
wavelength-dependent factors that influence the measured spectra in order to be able to compare
Advanced Lab Course: Electroluminescence of Polymers
16
the (corrected) spectra with others. Determine which part of the setup will have a wavelengthdependent influence on which type of measured spectrum. Design an experiment that allows to
measure data necessary to correct for this wavelength dependence of the setup. Collect the
necessary data.
The correction functions are to be applied to the according spectra before further discussion. In
the report, describe how you derived your correction functions. Specify in what units the
“brightness” of your samples will be displayed after correction. Use Table VI.1 for reference.
III.3.2 Photoluminescence Emission Spectra
Measure the photoluminescence emission spectra for the polymer films in an
appropriate measurement geometry, using suitable excitation wavelength(s). Keep in mind
that for all spectra measured, a background correction needs to be performed.
Considering the energetics of luminescence, you can reduce the wavelength range that needs to
be covered for detection. From a more technical point of view, consider different origins of the
detected light. How should the sample be mounted in order to suppress unwanted signals?
Optional: Compare the front face measurement when emission is recorded at the excitation side
of the film with a measurement in in-line geometry where emission is recorded at the side
opposite to the excitation side of the film for one film (see section IV.3).
III.3.3 Photoluminescence Excitation Spectra
Measure the photoluminescence excitation spectra for the polymer films in an
appropriate measurement geometry, using suitable detection (emission) wavelength(s).
Keep in mind that for all spectra measured, a background correction needs to be performed.
For the report, discuss the three types of spectra you collected for each of your samples. How
are the different spectra related theoretically? Do your measured spectra show these relations?
Also, compare the spectra of the different samples. Remember that absolute statements (“sample
A is brighter than sample B”) can not be drawn from the data you collected, but similarities and
differences in the spectral distribution should be discussed.
III.4
ELECTROLUMINESCENCE
Figure III.2 shows the general structure of the OLEDs. Device fabrication starts using a glass
substrate with pre-patterned ITO coverage. Onto these substrates, the active material is deposited
by spin coating from solution. The solution viscosity and spin speed determines the thickness of
the dry film. Top electrodes are deposited by thermal evaporation in high vacuum. The full
fabrication requires a few hours and is not part of the lab course. The active area of an OLED is
defined by the overlapping areas of both electrodes which form a sandwich bottom electrode –
active polymer – top electrode. Hence, each substrate contains 6 independent “pixels” each having
an area of 0.16 cm2 (4 mm × 4 mm). The active device region is encapsulated by means of a
microscope cover glued onto the device. The electrodes are designed to be contacted outside the
encapsulated region.
The exact composition of the provided samples will be given at the setup.
Figure III.2: OLED device structure in top and side view. ITOcovered areas are shown in light blue, the metal cathode in black.
The side view includes the polymer film (yellow, thicknesses not to
scale). The dashed line indicates the encapsulation.
III.4.1 Current-Radiance-Voltage Characteristics
Measure the current I through the OLED as a function of applied voltage U (I = f(U)). At the
same time, measure the spectral radiant intensity of electroluminescence iE, as a function
of voltage at a constant emission wavelength.
Advanced Lab Course: Electroluminescence of Polymers
17
The setup uses a voltage source and a separate Amperemeter to measure the current through
the device. Plan the proper circuit you need to measure the current-voltage characteristics, i.e.
how to connect voltage source, Amperemeter and sample. Sketch a wiring scheme if necessary.
As the detector is mounted after the emission monochromator, you need to decide which
wavelength may be suitable to measure the radiant intensity. It is helpful to recount possible
processes that result in luminescence. The chosen wavelength needs to be set in the program for
spectral measurements (“FELIX”). The optical excitation part of the setup will not be used in
electroluminescence measurements, so the excitation wavelength is not relevant. However, for
technical reasons, a wavelength must be given in the software. The excitation light can (and
should) be blocked with the appropriate shutter.
For the report, make sure that your plots are showing all the interesting information: Both
current and radiance will change over several orders of magnitude.
Plot iE, as a function of the current I through the OLED and discuss your findings. Discuss
whether you need any correction or even calibration (to absolute units) of your radiance data.
What device parameters determine the current-voltage characteristics (in particular, turn-on
voltage) of an OLED?
III.4.2 Electroluminescence spectra
Measure the electroluminescence emission spectrum at a suitable voltage or current.
The suitability of a voltage has mostly technical limits: There needs to be some light emission at
all, and this emission must not saturate the detector system. Both limits relate back to the
wavelength choice for the measurement of current-radiance-voltage characteristics.
Compare the electroluminescence spectrum to the photoluminescence spectrum. What do you
conclude concerning the mechanism of both luminescence processes?
III.4.3 Lambert’s law
Measure the radiance of an OLED as a function of viewing angle.
For this measurement, the so-called “time-based” method of FELIX may be used. Here, the
emission intensity is recorded while the emission monochromator (as well as the excitation
monochromator) is at a fixed position. The temporal resolution is about 0.1 seconds. It has been
proven practical to alter the viewing angle between -70° and +70° in steps of 10° while keeping
each angle for about 10 seconds. The resulting plot will be a multi-step graph whose steps can
easily be assigned to the different angles. For the report, it is sufficient to use an approximated
mean value for each step (angle). Analyze the data according to Lambert’s law. To do so, plot your
data as a function of cos 𝜃. Is your source a so-called Lambertian emitter?
III.4.4 Efficiency
Determine the radiometric and luminous efficiencies of the OLEDs.
In order to calculate the efficiencies according to eqns. (10) and (12), the measured and
corrected spectra need to be calibrated to absolute units. To do this, a light source of known
absolute radiance is needed. For this reason, a calibrated inorganic LED which can be mounted at
the position of the organic samples will be provided. The driving current and corresponding
radiant flux will be given. The eye sensitivity curve 𝑉𝜆 (𝜆) is available as data file at the work space.
Make sure to describe your derivations conclusively and note all necessary steps and quantities
in your report. A formal algebraic derivation of the final equation may help to avoid lengthy
numeric calculations.
Depending on the correction function used, the values 𝑠𝜆,𝑐𝑜𝑟𝑟 of corrected emission spectra are
proportional but not equal to either the photon flux or the spectral radiant intensity. For an
energetic calibration, the proportionality factor 𝑓𝑐𝑎𝑙 has to be determined. For spectra that are
corrected proportional to photon flux, the procedure is as follows: The photon number, received
in a room angle Ωdet determined by the equipment (Figure III.3, Ωdet =
𝜋𝑑 2
4𝑙 2
with 𝑑 = 35 mm, 𝑙 =
Advanced Lab Course: Electroluminescence of Polymers
18
36 mm) is measured with a spectral bandwidth Δ𝜆 (about 3 nm for the setup used) at the
wavelength 𝜆 in a defined time interval Δ𝑡. Thus, the spectral radiant intensity follows to
d𝐼𝐸
𝑠𝜆,𝑐𝑜𝑟𝑟 ℎ𝑐
𝑖𝐸,𝜆 =
∙ ,
| = 𝑓𝑐𝑎𝑙 ∙
(15)
d𝜆 𝜆
Δ𝜆 ∙ Ωdet 𝜆
whereas the total spectral radiant flux of a Lambert radiator is
dΦ𝐸
𝜙𝐸,𝜆 =
| = ∫ 𝑖𝐸,𝜆 dΩ′
d𝜆 𝜆
(16)
𝑠𝜆,𝑐𝑜𝑟𝑟 ℎ𝑐
= 𝑓𝑐𝑎𝑙 ∙
∙ ∙ 𝜋.
Δ𝜆 ∙ Ωdet 𝜆
Using the total radiant flux Φ𝐸 = ∫ 𝜙𝐸 d𝜆 of a calibrated light source measured under the same
conditions as the light source under investigation, the wavelength-independent calibration factor
𝑓𝑐𝑎𝑙 can be determined. Once 𝑓𝑐𝑎𝑙 is known, the radiant flux and luminous flux of the OLEDs can
be determined for efficiency calculations.
Consider to derive the required equations for the radiant flux and luminous flux of the devices
under test by algebraic operations before plugging in any numbers.
sample
Wdet
d
l
III.5
detector
Figure III.3: Illustration of the room angle Ωdet for the
measurement configuration. The geometric values are
l = 36 mm, d = 35 mm.
SUMMARY OF EXPERIMENT
As a conclusion to the report, summarize your findings. Do the results meet your expectations?
Are there differences to the theoretical predictions, how can they be explained? Do all samples
behave in the same way or are there notable differences? Also, discuss whether the
instrumentation you used was suited for the task and identify possible sources of error.
Advanced Lab Course: Electroluminescence of Polymers
19
IV. INSTRUMENTATION AND POSSIBLE SOURCES OF ERROR
IV.1
MEASUREMENT SOFTWARE
IV.1.1 Perkin-Elmer Lambda 2 UV-VIS Spectrometer
 Program: Lambda-SPX
 Application to start: Scan
 Scan and Spectrophotometer boxes determine measurement parameters
o typical values: wavelength range 350-750 nm, interval 0,5 nm
o scan speed determines instrument accuracy, typically 240 nm/min, measurement mode A
= Absorbance
o Graph box settings only determine the display, not the measurement (during
measurement, Autoscale button can be used to display all data)
o helpful for determination of maxima: Peak Pick Maxima, 0,05 A
o use comma “,” as decimal marker
 Start baseline correction: Button Baseline. It will be automatically included in subsequent
measurements. Remove all substrates from the instrument prior to baseline recording.
 Start Measurement: Button Measure samples.
o If not done before, the first measurement will be a baseline measurement (“blank
sample”).
o “Sample List” can be used to measure multiple samples.
 Multiple samples can be measured by subsequently clicking Measure in the Sample
Information Window.
Attention: After clicking Close in the Sample Information window, starting a new
measurement will delete all data from the program. Make sure to save it.
Advanced Lab Course: Electroluminescence of Polymers
20
 Save data: File  Save Spectrum As. For ACSII data, set file type to Export … (*.CSV). Each
spectrum will be saved into a separate file. Save them in your own folder on drive T:.
IV.1.2
“FELIX” Luminsecence Software
 Program start: Icon PTI Felix 32
 Login: User “Student”, no Password.
 Choose measurement type via Acquisition → Open Acquisition.
o Check that “Script” and “HW configuration” are set properly. These settings are sometimes
lost after a measurement is aborted.
 Adjust settings.
o Option “Acquire Background” / “Use Background” is not suitable for most measurements.
 Click “Acquire (Prep.)” to move monochromators to start positions. Second click on the same
button (now labelled “Start”) starts the measurement.
 Measurements can be cancelled by clicking “Abort” or “Stop”.
 When closing a settings window, do NOT save.
 Save data in ASCII format: Right-click on Group (named “Emission” in the upper figure), select
“Export group”, choose proper file type.
o All other save and export options yield only different binary formats. These can be
opened in Felix for conversion at later times.
 Measured data sets can be collected in one single group using “drag and drop”. Data will be
copied and the original set will remain unchanged. Copying running measurement sets will
only copy the data collected so far.
 The graph legend text of each measurement has the form “[detector channel] [excitation
wavelength (range)]:[emission wavelength (range)]”. In the upper figure, the red text reads
“D1” for the channel, “500” for the excitation wavelength and “300-800” for the emission
range. This information can be changed to contain sample information which will be
included in the ASCII files. It is recommended to keep the spectral information, but D1 is
insignificant.
 Button bar:
scale diagram: zoom horizontal, zoom vertical, zoom x and y, show
o
all, ...
o
hide / show selected graph
Advanced Lab Course: Electroluminescence of Polymers
o
o
o
IV.2
21
“crossbar”, read values from graph
import / export data from / into file, FELiX binary format
open / save file from / into internal database – not suitable
CORRECTION OF SPECTRA
Because of the spectral characteristics of optical components, the observed signal in excitation
and emission measurement is distorted for several reasons:

The light intensity from the excitation source is a function of wavelength. The intensity of
the excitation light can be monitored via a beam splitter, and corrected by division. (In this
case, the spectral sensitivity of the monitor has to be considered.)
 The diffraction efficiency and the polarizing effect of the monochromators are functions of
wavelength.
 The optical density of the sample may exceed the linear range, which is about 0.1 absorbance
units, depending upon sample geometry and the slit width of monochromators.
 The emission spectrum is further distorted by the wavelength dependent efficiency of the
photo detector and the emission monochromator.
The development of methods to correct excitation and emission spectra (photo- and
electroluminescence) for wavelength dependent effects has been the subject of numerous
investigations. Overall, none of these methods are completely satisfactory. Prior to correcting
spectra, the researcher should determine if such corrections are necessary. Frequently, one only
needs to compare spectra with other spectra collected on the same instrument. However,
corrected spectra are needed for calculations of quantum yields (efficiencies) and overlap
integrals.
IV.2.1 Correction of excitation spectra
The wavelength dependent intensity of the exciting light can be converted to a signal
proportional to the number of incident photons by the use of a so-called quantum counter. Here
we use a concentrated solution of Rhodamine B in ethylene glycol which absorbs all incident light
and provides a fluorescence signal of constant wavelength, which is proportional to the photon
flux of the exciting light. In a certain wavelength region (up to 550 nm), the fluorescence intensity
is independent of excitation wavelength.
In the setup, the emission intensity of Rhodamine is recorded by use of a triangular cuvette
instead of the sample, like an excitation spectrum. The excitation correction function 𝑐𝑓𝑡𝑒𝑥 (𝜆) is
the inverse of the measured spectrum. Accordingly, all recorded excitation spectra have to be
divided by the “Rhodamine excitation spectrum” to obtain the corrected excitation spectra.
The fluorescence intensity of Rhodamine solution is independent of excitation wavelength, but
the quantum efficiency is not known. Hence, this correction method does not give a calibration of
the excitation path in terms of an absolute photon density (or similar). It rather yields a measure
for the relative change in photon flux for the different excitation wavelengths.
IV.2.2 Correction of emission spectra
The correction of emission spectra requires knowledge of the wavelength dependent efficiency
of the detection system, which consists of all components the emission light has to pass and the
detector. The wavelength dependent correction factor is generally obtained by the measurement
of light with a known spectral distribution. The sensitivity of the detection system 𝑆(𝜆) is
calculated as the ratio of the measured signal 𝐼(𝜆) and the known spectral intensity distribution
of detected light. The correction function for emission spectra 𝑐𝑓𝑡𝑒𝑚 (𝜆) is the inverse of the
sensitivity 𝑆(𝜆). The standard spectrum can be
 the emission spectrum of a standard substance,
Advanced Lab Course: Electroluminescence of Polymers

22
the spectral distribution of the excitation light (if known) measured using a wavelength
independent scatterer, e.g. MgO or

the wavelength dependent output from a calibrated light source, e.g. a tungsten filament
lamp of known color temperature.
The last method makes use of the fact that the (spatial) energy density in a frequency interval
between 𝜈 and 𝜈 + 𝑑𝜈 for a so-called black body can be calculated from Planck’s formula [8]
8𝜋ℎ𝜈 3
1
𝜚𝜈 (𝜈, 𝑇)𝑑𝜈 =
𝑑𝜈.
3
(17)
ℎ𝜈
𝑐
exp {
}−1
𝑘𝐵 𝑇
A detector of spectral width 𝑑𝜈 will measure the spectral irradiance 𝑖𝜈 (𝜈, 𝑇)𝑑𝜈 with
𝜚𝑣 ⋅ 𝑐 𝜚𝑣 ⋅ 𝑐 2ℎ𝜈 3
1
𝑖𝜈 (𝜈, 𝑇) =
=
= 2
.
(18)
Ω
4𝜋
𝑐 exp { ℎ𝜈 } − 1
𝑘𝐵 𝑇
A tungsten filament lamp is not an ideal black body. However, at proper operating conditions, it
can be approximated as one using not the real filament temperature but its so-called color
temperature for which equations (17) and (18) hold when including the emissivity 𝜀𝜈 < 1.
Keep in mind that the setup uses gratings for wavelength selection. They divide the spectrum
into wavelength intervals of constant length rather than constant frequency intervals. Hence, the
intervals 𝑑𝜈 need to be transformed into wavelength intervals 𝑑𝜆.
Also, the equations can be adjusted to give different correction functions such as photon flux or
radiant flux.
Not all light emitted by the tungsten lamp can and will be collected by the detection system: At
its maximum wavelength, the lamp emits some 1021 photons per second. The total emission will
thus be “spatially filtered” by (rather large) pinholes that do not change the spectral distribution
of the emitted light. As a result, this correction method gives only a relative correction of recorded
spectra but no energetic calibration.
IV.2.3 Background
Even without any measurement signal, the detector would still produce a count rate. To a first
approximation, this wavelength-independent contribution stems from the thermal dark count
rate of the detector. Stray light from the excitation source and laboratory lighting may also be
included. Therefore, for all spectra a proper background measurement has to be performed and
subtracted from the signal before further correction. The background is obtained best with the
setup adjusted for an actual measurement, but with the excitation source turned off or blocked.
IV.3
INNER FILTER EFFECTS
The apparent emission intensity and spectral distribution can depend on the optical density of
the sample and the precise geometry of sample illumination. The path length of excitation light
through the sample to the point observed by the detection channel and the path length of the
emission light through the sample determine the influence of the absorption behavior on
excitation and emission spectra.
If there is a strong overlap of absorption and emission spectra (called reabsorption, if the
absorbing and emitting species are the same) one often observes a reduced emission intensity at
blue side of the emission spectrum. In general, the influence of the absorption on the emission
spectrum is called Post Filter Effect.
In excitation spectroscopy, a high optical density at the excitation wavelength can reduce the
excitation intensity in the observed volume element. As a result one might measure a smaller
emission intensity compared to a sample with a smaller optical density (Pre Filter Effect).
The correction of these effects is a complicated problem, since one needs the exact description
of the geometrical light paths in the sample and the spectral characteristics of the sample.
Advanced Lab Course: Electroluminescence of Polymers
23
V. REFERENCES AND FURTHER READING
1.
2.
3.
4.
5.
6.
7.
8.
9.


M. Pope, P. Magnante & H.P. Kallmann, Electroluminescence in Organic Crystals,
Journal of Chemical Physics 38, 2042-2043 (1963).
J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend,
P.L. Burns & A.B. Holmes, Light-Emitting-Diodes Based on Conjugated Polymers,
Nature 347, 539-541 (1990).
G. Hadziioannou & P.F. van Hutten, Semiconducting Polymers: Materials, Science
and Engineering (Wiley-VCH, Weinheim, 2000).
M. Pope & C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers
(Oxford University Press, New York, 1999).
H. Bässler & B. Schweitzer, Site-selective fluorescence spectroscopy of conjugated
polymers and oligomers, Accounts of Chemical Research 32, 173-182 (1999).
N.C. Greenham, R.H. Friend & D.D.C. Bradley, Angular Dependence of the Emission
from a Conjugated Polymer Light-Emitting Diode: Implications for Efficiency
Calculations, Advanced Materials 6, 491-494 (1994).
A. Stockman, CVRL Colour & Vision Research Lab, http://www.cvrl.org/, 19952013 (11.03.2014)
M. Planck, Ueber das Gesetz der Energieverteilung im Normalspectrum, Annalen
der Physik 4, 553-563 (1901).
A. Ryer, Light Measuring Handbook (International Light, Inc., 1997).
E.E. Krieszis, D.P. Chrissoulidis & A.G. Papagiannakis, Electromagnetics and Optics (World
Scientific Publishing Co., Singapore, 1992).
P.W. Atkins, Physical Chemistry (Oxford University Press, Oxford, 1992).
References in German language:

M. Schwoerer, H.C. Wolf, Organische Molekulare Festkörper (Wiley-VCH, 2005)

P.W. Atkins, Physikalische Chemie (Wiley-VCH, 2001)
A selection of these references can be downloaded from the lab course web site
http://www.uni-potsdam.de/u/physik/fprakti/Start.html (Link “Literatur”, authorization
required). It is also available on the measurement computer or can be accessed via Windows
Networking \\PPF-Server\PPF\M7 - Additional Information\References and
Further Reading.zip. The other references are available in the university’s library.
Advanced Lab Course: Electroluminescence of Polymers
24
VI. RADIOMETRIC AND PHOTOMETRIC QUANTITIES
Table VI.1: Derivation of radiometric and photometric quantities. Photometric quantities represent the
visible part of the optical radiation field. Photometry provides physiological insight to the optical radiation
field detected by the human eye. On the other hand, radiometry refers to the energy content of the optical
radiation field. For consolidation, use e.g. [9].
radiometric (physical)
photometric (physiological)
Consider a light source S (Figure VI.1) with an area A, which emits an electromagnetic field into the space
above the source.
The energy emitted within this radiation field is the The luminous energy Qv is the energy of the visible
radiant energy QE which is measured in Joules (J).
radiation field. It is measured in lumen×second
(lms).
The radiant power (Strahlungsleistung) or radiant The luminous flux (Lichtsstrom) Φ𝑉 is defined as
𝑑𝑄
flux (Strahlungsfluss)
Φ𝑉 = 𝑉
(A1b)
𝑑𝑡
𝑑𝑄𝐸
Φ𝐸 =
(A1a) and is measured in lumens (lm).
𝑑𝑡
is the radiant energy emitted per time and is
measured in Watts (W). The radiant flux represents
an instantaneous radiometric quantity.
The radiant intensity (Strahlungsstärke)
The luminous intensity (Lichtstärke)
𝑑Φ𝐸
𝑑Φ
𝐼𝐸 =
(A2a)
𝐼𝑉 = 𝑉
(A2b)
𝑑Ω
𝑑Ω
is defined as radiant flux per unit solid angle emitted is defined as the luminous flux through the solid
by a source along a given direction. It is measured in angle dW. It is measured in candela (cd) (basic unit
W
lm
watts per steradian ( ).
of SI), which is also referred to as .
sr
sr
With regard to Figure VI.1, the solid angle 𝑑Ω subtended by the surface element ⃑⃑⃑⃑⃑
𝑑𝑠 ∥𝑟⃑ is given by
𝑑𝑠
𝑑Ω = 2
The radiance (Strahldichte) LE of an extended source
is defined as emitted radiant intensity per unit
emitting area dA (Figure II.9),
𝑑𝐼
1 𝑑𝐼𝐸
𝐿𝐸 = ⃑⃑⃑⃑⃑⃑𝐸 =
.
(A4a)
𝑑𝐴∙𝑟̂
cos 𝜃 𝑑𝐴
In this definition, only the apparent emitting area
that is the source area projected on a plane
perpendicular to the observation direction 𝑟̂ is taken
W
into account. The radiance is measured in 2 .
𝑟
(A3)
The luminance (Leuchtdichte) Lv expresses the
luminous intensity per unit emitting area,
𝑑𝐼
1 𝑑𝐼𝑉
𝐿𝑉 = ⃑⃑⃑⃑⃑⃑𝑉 =
.
(A4b)
𝑑𝐴∙𝑟̂
cos 𝜃 𝑑𝐴
Again, only the apparent emitting area is considered.
lm
cd
Lv is measured in 2 or 2.
m ⋅sr
m
m ⋅sr
1979 SI definition: The candela is the luminous intensity, in a given direction, of a source that
emits monochromatic radiation of frequency 540 ⋅ 1012 Hz and that has a radiant intensity in that
1 W
direction of
.
683 sr
Figure VI.1: Sketch to define the radiometric quantities
of light emitted from the light source S