The objective here is to understand the fact from experiments performed that light emitted from Organic Light Emitting Diode (OLED) can be improved by using Surface Plasmon Resonance (SPR). It was studied that eleven fold increase of light emission was achieved by matching the plasmon frequency of unpatterned thin silver film to organic polymer layer deposited over the metal layer.
Keywords:
Surface plasmon resonance, silver film, OLED.
Introduction:
In present day technology where we are facing energy crisis a bright option in optoelectronic devices is Light Emitting Diodes (LEDs) whose performance is almost matching that of incandescent bulb and its energy saving property has added to this advantage. LED is also a type of semiconductor diode like P-N junction diode having two contacts except that LED as the name suggests can emit light.
There are different methods by which we can make the LED work they are photoluminescence, in this method the electrons and holes recombine in the device limits and release the energy in the form of photons. Of all the methods available to enhance the effectiveness of a LED the best possible method is using Surface Plasmon Resonance (SPR) this mechanism is based on the energy coupling effect between the emitted photons from the semiconductor and metallic nanoparticles [1], SPR is nothing but the collective oscillation of electrons in a solid which is stimulated by incident light. Resonance is achieved when the natural frequency of surface electrons which are oscillating against the restoring force of the positively charged nuclei matches with that of incident light photons.
SPR in the nanometer sized structure is called localized surface plasmon resonance. Also recently, it has been found that Surface Plasmons (SPs), excited on a rough metallic surface by the interaction between light and metal, can significantly enhance light emission by improving the Internal Quantum Efficiency (IQE) ??int [2]. IQE of a semiconducting material is the ratio of radiative electron-hole recombination to the total i.e. radiative and non-radiative recombination coefficient.
??int = Rr / R = Rr / (Rr + Rnr)
R: total recombination coefficient
Rr: radiative recombination coefficient
Rnr: non radiative recombination coefficient
Here we consider an Organic LED (OLED), which is a thin layer of emissive material bounded on one side by a cathode to inject electrons and an anode on the other side to inject holes. The difference between OLED and LED is that in an OLED the emissive photo luminescent layer is a film of organic material which emits light in response to electric current. Polymers appropriately doped with dye molecules emitting light in the visible spectrum are a good class of OLED and provide stable sources of light for displays and illumination sources.
Device and material configuration:
Materials that are used are coumarin 460 doped polymethylmethacrylate (PMMA) which works as the dye doped solution. This solution is coated over metal layer of silver below which is quartz substrate [2].
Dye polymer solution which is PMMA is prepared by dissolving laser dye molecules of Coumarin 460 in chlorobenzene then 2% PMMA is added to get 20mM/L of dye doped polymer solution. The substrate for dye doped polymer solution were prepared by vapour depositing 50nm thick Silver and Gold layers onto a Quartz substrate at a rate of 0.5nm/sec. the metallization is done only on half of each substrate which enables us to have convenient comparison between polymer emission on each of the metal layers and the polymer doped directly on the Quartz substrate. Once the metallization is done the dye doped PMMA layers are spun onto each of the gold and silver layers to get a layer thickness of ~200nm.
Experiment:
The experimental setup of the device is shown in figure 1. Here in order to prevent the pumped light to get reflected into the detector the light is introduced at a larger incidence angle and perpendicular to the device is the detector. Here since the emitted light is in visible region ocean optic spectrometer is used to measure the emission intensity of the organic dye layers. With this setup we compare the optical influence of gold and silver thin films on the dye doped polymer which is placed over the substrate. The light emitting material must be placed within few hundred meters of the metal surface in order to benefit from the surface plasmon enhancements. While measuring light from polymer samples only fact to be ensured is to prevent the incident light to be completely absorbed by the dye doped polymer layer. Generally the plasmon enhancements are measured by backside pumping that is the incident and measured light is from the backside of the sample [3]. Some investigations have been performed to measure the optical properties by top side pumping [4, 5]. There are also works done in finding different combinations of pumping and measuring configurations, and also the angle of pumping the incident light [6]. Since it is comfortable to pump and measure light from the top polymer layer that technique has been used.
Fig 1: Sample structure with both pump light and emission light configurations.
Results and discussions:
Fig 2 is the graph of the measurement attained when comparing the spectra of a particular region of the dye doped PMMA layer spun onto Gold, Silver and no metal or the Quartz substrate.
It was noticed that gold layer was assisting in reflecting the pump laser but surface plasmon could not couple with emission wavelength of coumarin 460 to dissipate any measurable enhancement. Though 11 fold of enhancement of dissipated light from the dye doped silver layer due to the coupling of surface plasmons as plasmon resonance frequency almost nearly matches that of dye doped polymer.
Wavelength (nm)
Fig 2: PL spectra of Coumarin 460 on Ag, Au, and quartz.
Wavelength (nm)
Fig 3: PL Enhancement ratios of Gold green and Silver red
??k (eV/c)
Fig 4: Dispersion diagrams of surface plasmons generated on Ag/PMMA red solid line and Au/PMMA green dotted line.
The dielectric constant is a match for silver and the wavelength of emission of dye doped PMMA layer. Even if reflection of the incident light might account for some increased brightness, only surface plasmon coupling can describe the enhancement occurring. From fig 3 we can see the enhancement ratios of gold and silver, it can be noted that enhancement up to 50 is obtained in the broad emission spectrum while at the peak wavelength we can see that there is 11 fold increase, also photoluminescence does not strongly change in the case of gold.
Intense rise of photoluminescence with silver is due to the interaction between surface plasmon and the excited dye molecules, the molecular relaxation process here produces a SP instead of photons causing increase in the spontaneous emission rate. Process occurring here greatly is governed by metal dielectric function [7] and that of polymer. SP dispersion relationship is determined by the equation used to calculate the propagation wave number K.
k=??/c ‘((??_1.??_2)/(??_1+??_2 )) (1)
Here ?? is the frequency, c is speed of light, ??1 and ??2 are the real parts of dielectric constant of metal and PMMA respectively, k in this equation varies with ??. Fig 4 which is the plot of the SP dispersion relation is calculated using equation (1).
Generally SP which are generated at the interface between the metal and its surrounding material decay in an exponential manner with respect to the distance from the metal surface. The SP penetration depth Z can be calculated using the below given formula
z=??/2?? ‘((??_2-??_1)/(??_1^2 )) (2)
Here ?? is the wavelength of the pumped light.
Conclusion and Future prospects:
It has been observed in this paper that by coupling of surface plasmon to thin metal layer there is enhancement in the emission of dye doped polymer layer. This is the basic work done on an OLED. There are many distinguishing areas that can lead to various significant changes in the device parameters such as changing the size of metal layer. There is also a possibility to change the metal layer type or change in dye doped polymer layer to give various PL enhancements and increase IQE [8, 9, 10].
References:
‘Light emitting diode enhanced by localized surface Plasmon resonance’ by Xuefeng Gu, Teng Qiu, Wenjun Zhang, Paul K Chu
‘Surface Plasmon enhanced solid state light emitting diode’ by Koichi Okamoto
‘Surface-plasmon-enhanced light emitters based on InGaN quantum wells’ by Okamoto K
‘Coupling of InGaN quantum-well photoluminescence to silver surface plasmons’ by Gontijo I
‘enhancement of spontaneous recombination rate in a quantum well by resonant surface plasmon coupling’ by Neogi A
‘Surface plasmon-enhanced photoluminescence from a single quantum well’ by Hecker N. E
‘CRC handbook of Chemistry and Physics’ by Carper J
‘Time resolved photoluminescence spectroscopy of surface plasmon enhanced light emission from conjugate polymer layer" by Neal T.D, Okamoto K, Liu M.S, Jen A.K-Y.
‘Efficient cyano-containing electron transporting polymers for light-emitting diodes’ by Liu M.S, Jiang X, Herguth P, Jen A.K.-Y.
‘Highly efficient blue-light-emitting diodes from polyfluorene containing bipolar pendant groups’ by Shu C.-F., Dodda R, Liu M.S, Jen A.K.-Y.
...(download the rest of the essay above)