M7 ELECTROLUMINESCENCE OF POLYMERS

Transcription

1 1 Avance ab Course: Electroluminescence University of Potsam Faculty of Mathematics an Sciences Institute of Physics M7 EECTROUMINESCENCE OF POYMERS I. Introuction The recombination of holes an electrons in a luminescent material can prouce light. This emitte light is referre to as electroluminescence (E) an was iscovere in organic single crystals by Pope, Kallmann, an Magnante (1963). E from conjugate polymers was first reporte by Burroughes et al. (1990). The polymer use was poly (p-phenylenevinylene) (PP). PP has excellent mechanical properties, goo flexibility an is stable to 400 C. E-evices have potential applications in a wie fiel ranging from multi-color isplays to optical information processing. Polymers have the avantage over inorganic an monomolecular materials in the ease with which thin, structurally robust films can be mae an large areas covere by spin-coating a polymer from solution. Even flexible isplays can be prouce because of the goo mechanical properties of polymers.

2 I.1. ight-emitting Dioes In the following, only so-calle light-emitting ioes (EDs) are iscusse. A simple ED evice geometry is shown in Figure I.1. It consists of one emission layer sanwiche between a hole an an electron injecting contact, enote anoe an cathoe, respectively. In these evices, carriers of opposite sign are injecte separately at opposing contacts when a sufficiently high voltage is applie. These carriers are mobile uner the influence of the electric fiel. Cathoe Emitter Anoe Substrate Fig. I.1: Schematic rawing of a single-layer electroluminescent ioe. An applie electric fiel leas to injection of holes an electrons into the light-emitting film from the two electroe contacts. Formation of an electron-hole pair within the material may then result in the emission of a photon. Figure I. gives a schematic escription of the E from anthracene provie with opposing Na an Au injection contacts. Some of these carriers may recombine within the emissive layer yieling excite electron-hole pairs, terme excitons. In a molecular picture such an exciton represents an excite electronic state, e.g. S 1, of the anthracene molecule, which can ecay either raiatively (uner emission of a photon) or non-raiatively (uner prouction of heat) to the grounstate S 0. Base on the spin-statistics, this prouces singlet an triplet excitons in the ratio 1:3 (Figure I.3). The singlet excitons (S*) ecay promptly, yieling what is referre to as prompt E, whereas the triplet excitons either ecay irectly (electrophosphorescence) or they fuse to form singlet excitons, proucing elaye E, (S * ).

3 3 Fig. I.: Schematic view (not to scale energetically) of electron-hole recombination in anthracene after injection from Na an Au, respectively. iolet light is generate with a quantum yiel of about 0 per cent. The otte lines represent the energy separation of the valence an conuction level carriers in the absence of any interactions between the carriers. The soli lines inicate the energy separation of the injecte valence an conuction level carriers in the course of recombination. 1:3 Fig. I.3: Simplifie ecay scheme of the excitation prouce by recombination of an electron-hole pair. S *, S, an T enote primary singlet, elaye singlet, an triplet excitons, respectively. Process (a) represents the bimolecular annihilation (fusion) of two triplet (T) excitations. Process (b) represents the irect raiative ecay of the triplet state. The simplest polymer-base ED (Figure I.4) consists of a single layer of semiconucting fluorescent polymer, e.g., PP, sanwiche between two well known electroes, a semitransparent, high work function anoe (ITO, Au, etc.) an an opaque, low work function cathoe (Ca, Mg, Al, etc.). The thickness of the organic layer is typically in the orer of 100 nm an, for experimental convenience, the active evice area is in the orer of a few mm. More elaborate multilayer structures (Figure I.5) inclue aitional charge-transporting layer. The role of e.g. the hole-transporting layer is to facilitate hole transport from the anoe to the emission layer an to prevent electrons to leave the emission layer an reach the anoe without recombining with a hole.

4 4 Counter electroe e. g. Al - + Transparent metal layer (Au or ITO) Glassubstrate Polymer Film Fig. I.4: ayout of a simple polymer-base light-emitting ioe. counter electroe e. g. Ca or Al electron transport layer - + emission layer hole transport layer transparent electroe glass substrate Fig. I.5: ayout of a multilayer ED comprising hole an electron-transporting layers.

5 5 I.. Polymers for ight-emitting Dioes I..1. Overview In general, the polymer forming the emission layer must be able to prouce light by the recombination of singlet excitons. In aition, it must be able to transport charges (holes an electrons). The following Table 1 shows a selection of polymers which has been mostly stuie with respect to E. All polymers have in common that they possess a π-conjugate electron system as the active chromophore an that this conjugate systems forms the polymer backbone. Other polymers have been use which are buil from an inactive (saturate backbone) bearing chromophore-containing sie chains. In the following, we concentrate on poly [-methoxy, 5-( -ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PP), a erivative of poly(p-phenylene vinylene) (PP). Table 1: Polymers, which have been extensively stuie for E PP poly(p-phenylene vinylene) n MEH-PP poly[-methoxy-5-( -ethyl-hexoxy)- 1,4- phenylene vinylene O O n CN-PP (R 1 R C 6 H 13 ) poly[,5-bis (hexyloxy)-1,4-phenylene- 1-cyanovinylene)] OR 1 OR CN OR 1 OR CN n PPy poly(p-pyriyl vinylene) N n DHO-PPE (R 1 R hexyl) poly[1,4-(,5 ihexoxy)-phenylene ethynylene)] OR OR 1 n PPP poly(para-phenylene) n

6 6 PPP laer-type poly(para-phenylene) R 1 R R R R R R R n R PPy poly(p-pyriine) R 1 N n PDAF poly(9,9-ialkylfluorene) R R n P3AT poly(3-alkylthiophene) S R n I... Optical properties The optical absorption an electroluminescence emission spectra of MEH-PP are shown in Figure I.6. The absorption an emission spectra are typical of PP-base polymers. The maximum absorption coefficient is about.5x10 5 cm -1. The absorption has a broa peak, which is roughly 0.5 e wie with vibronic features evient at about. e an.4 e. The energy gap of MEH-PP is about.4 e. The electroluminescence emission spectrum peaks at about e an also contains several vibronic peaks. The ioe electroluminescence spectrum is broa, roughly 50nm wie. The with of the absorption an emission spectra is partially ue to the isorer in the film. aer polymers that contain linking groups that preserve the structure of the polymer chain have consierably sharper spectral features. The photoluminescence spectrum (not shown) is ientical to the electroluminescence spectrum inicating that the excitations prouce optically an electrically are ientical. The re shift in energy between the optical absorption an emission peaks of about 0. e is typical of electroluminescent polymers.

7 7 Fig. I.6: Electroluminescence an optical absorption spectrum of the soluble polymer MEH-PP. I..3. Color The emission color of a polymers epens strongly on the chemical structure of the polymer. PPP shows emission with a peak wavelength of approx. 40 nm, that is far in the blue. The emission maximum shifts to longer wavelength when the phenyl rings in PPP become rigily connecte as in PF an PPP. Unsubstitute PP emits green light, but the emission color can be tune to the re by appropriate substitution. Finally, PT is a re emitter. It shoul be note that the P spectra in solution an in the soli state might be ifferent ue to intramolecular effects in the soli state. Also, the E spectrum might iffer from the soli state P spectrum ue to the ifference in excitation mechanism (P is generate by light absorption, E by the recombination of charge carriers). I.3. Efficiency Consierations The efficiency of organic ED evices is escribe by several parameters. The internal quantum efficiency η int (E) is efine as the number of photons generate in the material per injecte charge carrier. If emission occurs through the raiative ecay of singlet excitons, (no phoshorescence) the upper limit of η int is given by η int (E) 4 1 η(p) (1) with η(p) the soli state photoluminescence quantum efficiency. The factor 1:4 stems from the fact the recombination of uncorrelate charge carriers yiels singlet an triplet excitons in a ratio 1:3. Since only singlet excitons ecay raiatively in most polymers at room temperature, the probability that a carrier recombination yiels an emissive exciton is 1:4. In general, not all but only a fraction φ r < 1 of all injecte carriers will recombine an η int (E) then is given by

8 8 η int (E) 4 1 η(p) φ r () The external quantum efficiency η ex (E) < η int (E) is efine as the number of etectable photons per injecte charge carrier. For an isotropic material, where the transition ipoles are ranomly oriente, ηint ( E ) η ext ( E ) (3) n (n is the refractive inex of the organic layer). For ieal recombination conitions φ r 1, η(p) 1 an a refractive inex n of 1.7, the maximum external efficiency is approx. 5 % Relevant for a evice application is the external power efficiency η P (E), efine as the emitte light power ivie by the electrical riving power. Using the efinition of the external quantum efficiency, η P (E) can be rewritten as ω η p ( E) ηext ( E) (4) eu where ω is the energy of the emitte photons an e is the electric charge of an electron. Note, that this expression is absolutely correct only for monochromatic light emission A high power efficiency thus requires a high external quantum efficiency η ext (E) an a low operating voltage U. For an external efficiency of 5%, an operating voltage of 3 an an emission wavelength of 540 nm ( ω.3 e), η P (E) 4 %

9 9 II. Theory of Electroluminescence Electroluminescence in organic layers involves a series of steps: 1) Injection of the charge carrier from a metal contact into the organic layer ) Transport of the charge carrier within the film an recombination of hole an electron followe by the formation of excitons 3) Migration an raiative ecay of the exciton 4) Emission of the generate photon to the outsie The efficiency of these processes is closely relate to the evice geometry an to particular properties of the applie polymers such as the positions of energy levels of the charge transport materials or the soli state photoluminescence efficiency of the emitting polymer. II.1. Charge Injection One of the funamental processes occurring in polymer EDs is charge injection form the metal contacts into the electroluminescent polymer film. This charge injection can be qualitatively unerstoo by consiering the electronic energy structure of the thin polymer film. The electronic energy structure of a PP film is shown in Figure II.1 along with the work function of various metals use as contacts in polymer EDs. The ionization potential (E v ) of PP, i.e. the energy require to remove an electron from the highest occupie state (HOMO) to vacuum, is roughly 5. e. The electron affinity (E c ), i.e. the energy gaine when aing an electron to the lowest energy unoccupie state (UMO) from vacuum, is roughly.5 e. The energy gap, E v E c, is about.7 e. To inject electrons the contact must be able to onate electrons into the lowest unoccupie state.5 e below vacuum. Similarly, to inject holes (remove electrons) the contact must be able to accept electron from an energy 5. e below vacuum. UMO HOMO Fig. II.1: Electron energy level iagram of PP an work functions of selecte contact metals use in polymer EDs.

10 10 Electron injection is thus limite by the barrier φ e between the Fermi level of the cathoe an the position of the UMO (given by the aiabatic electron affinity). The rate of hole injection is similarly etermine by the barrier φ h between the work function of the anoe material an the HOMO (given by the aiabatic ionization energy) of the polymer. The injection of charge carriers from an electroe into an organic layer has been extensively treate by Parker (199). At high fiels, the current ensities can be escribe by the tunneling of the charge carriers through a rectangular barrier (Figure II.). The Fowler- Norheimer theory gives: * 3 8π m φ j F exp 3ehF (5) Here, F is the internal electric fiel, m * the effective mass of the electron in the organic semiconuctor (set equal to the electron mass in free space) an h the Planck constant. The plot ln(j/f ) versus 1/F yiels the barrier high φ. The Fowler-Norheimer theory preicts a strong epenence of j e/h on φ e/h an the applie electric fiel. Since many π-conjugate polymer have ionization energies of about 5 e, high work function materials such as gol, copper an inium-tin-oxie (ITO) are suitable for hole injection. UMO positions are typically locate between 3 e an 1.5 e. Effective electron injection thus requires low work function metals such as calcium. However, these metals are very unstable in air an require evice encapsulation. For a balance charge injection similar barriers φ e φ h are neee. In most polymeric systems, balance charge injection is ifficult to realize in single layer evices. Ca,9 e Al 4,3 e Polymer Au 5, e ITO 4,7 e Fig. II.: Ban iagram for electron an hole injection into a polymer layer If the work function of the two contacts is ifferent, the current will not only epen on the absolute fiel but also on its irection. Higher currents will flow if the contact with the higher work function is biase as the anoe an the contact with the smaller work function as the cathoe. This situation is enote forwar bias. Inverting the bias (reverse bias) will lea to smaller currents, why this behavior is calle ioe-like current-voltage characteristics (Figure II.3).

11 11 Fig. II.3: Typical current-voltage an raiance-voltage characteristics of an MEH-PP-base OED with an Au anoe an a Ca cathoe. II.. Charge Carrier Motion an Recombination The motion of charge carriers is generally escribe by the carrier mobility µ, which is efine by the ratio of the rift velocity v an the electric fiel F: v µ (6) F In typical conjugate polymers the mobility for holes is in the range cm /s. Because of oxygen contamination the mobility of electrons in these polymers is generally lower. The recombination of an electron an a hole uner the conition of low mobility is escribe by the angevin recombination mechanism, which involves the rift of one of the charges in the electric fiel of the other partner. Thus, the ecay rate is much smaller than the raiative life time of the exciton an controlle by the mobility an ensities of the charge carriers. Efficient ioes with the recombination of all injecte charges is only possible at high current levels uner certain conitions. II.3. Migration an Raiative Decay of the Excitons A critical parameter in etermining the operating efficiency of polymer EDs is the luminescence quantum efficiency η(p) of singlet excitons in the polymer i.e. the probability that a singlet excitons will ecay raiatively. This probability is limite by the intrinsic (intramolecular) quantum efficiency for raiative ecay on an isolate molecule (as etermine by the P efficiency in ilute solution). However, in the soli state, several mechanisms can further reuce the quantum efficiency: a) electronic coupling to neighboring molecules might alter the electronic states an thus the efficiency for raiative exciton ecay; (b) uring its lifetime (typically 100 ps - 1 ns) the exciton in polymers can iffuse to non-

12 1 raiative sites (so-calle quenching sites) or they might be eactivate at the metal electroes via energy transfer or issociation. Therefore, the photoluminescence quantum efficiency η(p) in the soli state is generally smaller than in ilute solution. For goo polymers, values range between 40 % an 60 %. For MEH-PP, efficiencies of approx. 15 % have been reporte. II.4. Emission of the Generate Photon to the Outsie Not every photon generate insie the emission layer will escape the evice an become visible to an external observer. Photons might be absorbe either by the emissive material itself (reabsorption), by aitional charge transport layers or by the electroes. Further, the refractive inex n p of the emissive polymer (typically ) is larger than that of the supporting glass substrate (n s 1.5). Thus, photons generate in the polymer layer, propagating at an oblique angle with respect to the surface normal, are totally reflecte at the polymer-glass interface an waveguie in the evice. To a first approximation, only a fraction 1/(n p ) of the internally generate photons are emitte to the outsie.

13 13 III. Emitte Power an Brightness III.1. Comparison of raiometric (physical) an photometric (physiological) quantities Photometric quantities represent the visible part of the optical raiation fiel. Photometry provies physiological insight to the optical raiation fiel etecte by the human eye. On the other han, raiometry refers to the energy content of the optical raiation fiel. raiometric photometric Consier a light source S (Figure III.1) with an area A, which emits an electromagnetic fiel into the space above the source. The energy emitte within this raiation fiel is the raiant energy Q E which is measure in joules (1 J 1 Ws). The raiant energy emitte per time E Q E t Φ (7a) is introuce as the raiant power or raiant flux Φ E, which is measure in watts (W). The raiant flux represents an instantaneous raiometric quantity. The raiant flux per unit soli angle emitte by a source along a given irection I Φ E Ω E (8a) is introuce as the raiant intensity I E ; it is measure in watts per steraian (Wsr -1 ). The luminous energy Q v is the energy of the visible raiation fiel. It is measure in lumenssec (lms). The luminous flux Φ v is efine as v Q v t Φ (7b) an is measure in lumens (lm). The luminous intensity I is efine as the luminous flux through the soli angle Ω: I Φ v Ω v (8b) It is measure in lmsr -1, which is also referre to as canela (c) (basic unit of SI). With regar to the geometry of Figure III.1, the soli angle Ω subtene by the surface element s r is given by s Ω (9) r Finally, the emitte raiant intensity per unit emitting area A (Figure III.) is introuce as the raiance E of an extene source in that irection. In this efinition, only the apparent emitting area that is the source area projecte on a plane perpenicular to the observation irection rˆ is taken into account: Finally, the brightness of a source is expresse by the luminance : v I v 1 I v A rˆ cos A (10b) E I E 1 I E A rˆ cos A (10a) It is evient that E is measure in Wm - sr -1. v is measure in lmm - sr -1 (cm - ). efinition : 1 c is the luminous intensity of a black raiator with the temperature T 1770 C (melting temperature of platinum) an an opening of 1/60 cm.

14 14 z Ι Ε r s Ω A ϕ y S x Fig. III.1 : Sketch to efine the raiometric quantities of light emitte from the light source S III. ambert s raiator. Acos A rˆ S Fig. III. : Projection of an emitting surface element of an extene source Consier a ambert s raiating surface where each point even raiates into the room π, an angle epenence of the visible surface (Fig. III.) is observe with an effective surface of : A eff A rˆ Acos. (11) The luminous intensity in - irection (I, ) follows with (11) I I cos (1), with I luminous intensity normal to the surface angle between A an irection of observation

15 15 sr A A A A A A A A A A I π ϕ ϕ ϕ ϕ π π π π π π π π Ω Ω Φ Φ Ω Φ Ω Φ cos sin sin cos cos cos cos cos cos ϕ ϕ Θ Θ Θ0 ϕ 0 S Fig. III.3 : Illustration of half-room above the source For a ambert s raiator the connection between luminous flux an luminance follows from the integration over the half-room above the raiating source (Fig. III.3) (13) Thus, a ambert s raiator with an emitting area of 1 m an a luminance of 100 c/m correspons to a luminous flux of 314 lm. lm c sr sr m m c Φ π

16 16 The most important parameter for characterizing a light source is the luminance. For example, a computer screen shoul have a luminance of 100 c/m. A secon quantity connecte to this is the luminous efficiency Φ η el [lm/w] (14) Pel which is the luminous flux relate to the electrical input power. III.3. Connection between Raiometric an Photometric Quantities It is useful to establish a link between the raiant flux an the luminous flux, thus relating physical units to physiological ones. Since the human eye, our own light etector, is characterize by non-uniform sensitivity to the various spectral components of light, an experimental function, the spectral eye sensitivity λ (Figure III.3), is use to represent the physiological effect of light throughout the visible part of the spectrum. At maximum of the sensitivity curve (λ m 555 nm) a luminous flux of 680 lm represents a raiation flux of 1 W. The wavelength-epenent factor λ is obviously require to convert watts to lumens throughout the spectrum. λ (lm/w) λ max 555 nm wavelength (nm) Fig. III.3: Eye-sensitivity λ with the maximum sensitivity at λ 555 nm as a function of wavelength. The values s λ of correcte measure emission spectra correspon to the number of registere photons. The photon number, receive in a room angle Ω etermine by the equipment (Fig. III.4) is relate to the spectral ban with λ 10 nm at the wavelength λ in a efine time interval t 1s. Thus, the spectral raiant intensity follows to

17 17 I E sλ hc ie, λ λ f cal, (15) λ 10nm Ω λ whereas the total spectral raiant flux of a ambert s raiator is ϕ E, λ Φ E λ f i E, λ cal λ Ω sλ hc π sr 10nmΩ λ. (16) In consieration of the eye-sensitivity λ the spectral luminous flux ϕ,λ (lm /nm) behaves ϕ, λ λ ϕ E, λ. (17) Integration of the spectrum with respect to results in the overall emitte luminous flux Φ (lm). Moreover the spectral raiant flux elivers the overall emitte raiant flux Φ E (W). sample Ω etector l Fig. III.4 : Illustration of the room angle Ω for the measurement configuration References: M. Pope, C.E-. Swenberg Electronic Processes in Organic Crystals, n. Eition, Oxfor Sciene Publications (1999), ISBN G. Haziioannou, P.F. van Hutten (Es.) Semiconucting Polymers: Chemistry, Physics an Engineering, Wiley-CH (000). E.E. Krieszis, D.P. Chrissouliis, A.G. Papagiannakis Electromagnetics an Optics, Worl Scientific Publishng Co. (199), ISBN

18 18 I. Experiments 1. Instrumentation 1.1. Determine the correction function for the excitation spectra 1.. Determine the correction function for the emission spectra. Photoluminescence Spectroscopy.1. Measure the photoluminescence emission spectra for a thin film of MEH-PP at an excitation wavelength of 465 nm. Perform the measurement for front face illumination an with the inline geometry 3. Electroluminescence 3.1. Measure the voltage U as a function of the current I through the ED (U f(i)) an the spectral raiant intensity of electroluminescence i E,λ respectively the relate uncorrecte signal s λ,uncorr as a function of the current (s λ,uncorr f (I)). Analyze the U(I) ata accoring to the Fowler-Norheimer equation with respect to the barrier height φ. The thickness oft the polymer layer is approx. 100 nm. 3.. Measure s λ,uncorr as a function of the viewing angle. Analyze the ata accoring to ambert s law. Is the law fulfille? 3.3. Measure the electroluminescence spectrum s λ,uncorr (λ) at a current of approx. ma. Compare the spectrum to the photoluminescence emission spectrum. Are they ifferent? What o you conclue concerning the origin of electroluminescence? 3.4. Make an absolute calibration of the etection unit using a calibrate ioe with a given raiant power Φ E. What is the raiant power Φ E an the raiance E of the polymer ED at approx. ma? Using the eye sensitivity λ, etermine the luminance v an the luminous efficiency of the polymer ED.

19 19. Remarks to the Instrumentation.1. Correction of Spectra Photoluminescence, excitation an electroluminescence spectra are measure with a moular fluorescence spectrometer. Because of the characteristics of optical components the observe signal in excitation an emission measurement is istorte for several reasons: - The light intensity from the excitation source is a function of wavelength. The intensity of the exciting light can be monitore via a beam splitter, an correcte by ivision. - The efficiency an the polarizing effect of the monochromators are a functions of wavelength. - The optical ensity of the sample may excee the linear range, which is about 0.1 absorbance units, epening upon sample geometry an the slit with of monochromators. - The emission spectrum is further istorte by the wavelength epenent efficiency of the photoetector. For iscussion of the iniviual components of a spectrofluorometer (see J.R. akowicz, Principle of Fluorescence Spectroscopy, Plenum Press (1983), ISBN ). The evelopment of methos to correct excitation an emission spectra (photo- an electroluminescence) for wavelength epenent effects has been the subject of numerous investigations. Overall, none of these methos are completely satisfactory. Prior to correcting spectra the researcher shoul etermine if such corrections are necessary. Frequently, one only nees to compare spectra with other spectra collecte on the same instrument. Correcte spectra are neee for calculations of quantum yiels an overlap integrals. Correcte excitation spectra The wavelength epenent intensity of the exciting light can be converte to a signal proportional to the number of incient photons by the use of a quantum counter. The concentrate solution of Rhoamin B in ethylene glycol absorbs all incient light an provies a signal of constant wavelength, which is proportional to the photon flux of the exciting light (8) the emission spectrum is inepenent of excitation wavelength. If there is not a reference etection channel the emission intensity of Rhoamin is recore by use of a triangular cuvette instea of the sample, like an excitation spectrum. All recore excitation spectra have to be ivie by the Rhoamin excitation spectrum to obtain the correcte excitation spectra.

20 0 Correcte emission spectra The correction of emission spectra requires knowlege of the wavelength epenent efficiency of the etection system, all components the emission light has to pass an the etector. The wavelength epenent correction factor is generally obtaine by the measurement of light of a known spectral istribution. The sensitivity of the etection system S(8) is calculate as the ratio of the measure signal I(8) an the known spectral intensity istribution of etecte light. This stanar spectrum can be - the emission spectrum of a stanar substance, - the wavelength epenent output from a calibrate light source, e.g. a tungsten filament lamp of known color temperature or -the spectral istribution of the exciting light prouce by a Xe-lamp an a wavelength inepenent scatterer, MgO. In the last case the scatterer is place in the sample compartment. Whereas light of a selecte wavelength passes the excitation monochromator the scattere light is measure at the same wavelength by a synchronous scan. Diviing the intensity istribution of the exciting light, recore before, by this scattere signal provies the correction function for the emission spectra. In this experiment a halogen tungsten lamp is use as stanar lamp. The wavelength istribution of its light (voltage of 4.00 ) can be approximate by that of a black boy of 1900 K. The wavelength istribution of intensity for this color temperature has to be calculate from Planck s formula. ρ ν ) ν 3 8πhν c ( 3 1 hν exp( ) 1 kt ν as photon flux in epenence on wavelength i(λ)λ. 8π 1 i( λ) λ λ 4 λ hc exp( ) 1 λkt One obtains the correction function for emission spectra by iviing the calculate istribution by the measure spectrum.

21 1.. Inner Filter Effects The apparent emission intensity an spectral istribution can epenent on the optical ensity of the sample an the precise geometry of sample illumination. The path length of exciting light through the sample to the point observe by the etection channel an the path length of the emission light through the sample etermine the influence of the absorption behavior on excitation an emission spectra. If there is a strong overlap of absorption an emission spectra (the so-calle reabsorption, if the absorbing an emitting species are the same) one often observes a reuce emission intensity at blue sie of the emission spectrum. In general, the influence of the absorption on the emission spectrum is call Post Filter Effect. In excitation spectroscopy a high optical ensity at excitation wavelength can reuce the exciting intensity in the observe volume element. As a result one might measure a smaller emission intensity compare to a sample with a smaller optical ensity (Pre Filter Effect). The correction of these effects is a complicate problem, since one nees the exact escription of the geometrical light paths in the sample an the spectral characteristics of the probe.