Thursday, July 21, 2016

Learn More: Interface Studies of Organic Materials

We are pleased to introduce our followers with the UL SPIE Student's chapter members and their everyday lab occupations. Raitis Grzibovskis is a PhD student, working in the Institute of Solid state physics, Laboratory of Organic materials. He is a SPIE member since 2012. Raitis had started his research carrier with photoconductivity measurements of organic materials, later continued with organic photovoltaic cells studies. Now, work is focused on more fundamental research - studies of interaction between organic compound and metal or other organic compound.

Raitis representing his research at the conference "DOC 2016"

Devices made of organic materials, such as organic light emitting diodes (OLEDs), organic photovoltaic (OPV) cells or organic field effect transistors (OFETs), often consist of several layers. Each of these layers (electrodes, electron/hole transport layers, active layers) is often only up to 100 nm thick. At this thickness different surface and interface effects can appear. Energy level compatibility at the organic compound-metal and organic compound-organic compound interface can greatly influence performance of the entire device. The ionization energy and electron affinity level value shift (band bending) can be observed due to the Fermi level alignment between two different materials when the thickness of layers is small. In that case energy level values obtained from bulky layers cannot be used anymore. Because of that, energy level shift at the both metal-organic compound and OC-OC interface is being actively studied.

Most of such interface studies are done by ultraviolet photoelectron spectroscopy (UPS). Although the method is widely used, there are some drawbacks: samples are usually made by the thermal evaporation in vacuum, which means that only low molecular weight molecules can be studied; necessity of ultra-high vacuum (< 10-9 mBar); expensive and highly complex experiment setup.

Photoemission yield spectroscopy (PYS) can be used as an alternative method for OC-OC and metal-organic compound interface studies. In the case of PYS, the photoemission yield dependence on photon energy (Y(hν)) is obtained. This method also offers a possibility to make measurements in the air, however more precise results could be obtained in vacuum. While scanning depth of the UPS is only up to 2-3 nm and can be used only in surface studies, PYS scanning depth is in the range of tens of nanometers due to the relatively low photon energy (in the range of 4-7eV).

In the Institute of Solid state physics, Laboratory of Organic materials we have made PYS measurement system (see Fig. 1).  
Fig. 1. PYS Measurement System: a) light source, b) momochromator; c) lens; d) vacuum chamber; e) electrometer; f) electrode; g) sample.

Measurements are being carried out in a vacuum (pressure of about 1∙10-5mbar). ENERGETIQ Laser Driven Light Source (LDLS EQ-99) is used as a source of UV radiation. Samples are irradiated through quartz window of the vacuum chamber. We change incident wavelength by diffraction grating monochromator MYM-1. Spectral range of the measurements is usually between 4eV and 6.5eV. Keithley 617 electrometer is used as a voltage source as well as equipment for electric current measurements. Applying voltage of 50V between sample and electrode helps to improve measurement signal by one order of magnitude. 
In PYS we obtain photoemission yield dependence on photon energy. Photoemission yield Y(hν) is calculated as 

 (1) Y(hν)=I(hν)/P(hν)                                                                            
where I(hν) is the number of emitted electrons and P(hν) is the number of incident photons with the energy of hν. Relation between photoemission yield and ionization energy Eioniz is expressed as a power law

 (2) Y(hν)=α(hν-E_ioniz )^n                                                                 
where α is proportionality constant showing amplitude of the signal and power n=1…3 depends on studied materials. n=2 is usually used in the case of metals while n=2.5…3 is used in the case of semiconductors. In our work we use n=2.5 as it gives better approximation than n=3. To obtain ionization energy (Eioniz) of material, Y1/n(hν) is calculated and plotted depending on photon energy. Linear part of Y1/n(hν) curve is extrapolated till Y1/n(hν)=0 which gives ionization energy of studied compound (see Fig. 2).

Fig. 2. Ionization Energy Determination of Organic Compound Depending of Film Thickness.

In our research we have three main directions
1) organic compound - electrode interface: we have shown difference between polycrystalline and amorphous material with the same active part of the molecule;
2) OC-OC interface studies using planar heterojunction samples: ionization energy shift of the upper layer material was observed;
3) OC-OC interface studies using bulk heterojunction samples where active compounds are homogenously distributed within the bulk of the sample.
The main ideas of these studies: 1) development of simple method for interface studies; 2) deeper understanding of charge carrier transport processes at the OC-OC and metal-organic compound interfaces.

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