Effect of Solvents, Their Mixture and Thermal Annealing on the Performance of Solution Processed Polymer Light-Emitting Diodes

In this study, we first investigated changes seen in electrical and optical properties of a polymer light-emitting diode due to using different kinds of solvents and their mixture. Two-layer light emitting diodes with organic small molecules doped in a PVK polymer host were fabricated using (i) non-aromatic solvent chloroform with a high evaporation rate; (ii) aromatic solvent chlorobenzene with a low evaporation rate, and (iii) their mixture with different relative ratios. The effect of nano-scale layer thickness, surface roughness and internal nano-morphology on threshold voltage and the amount of electric current, the luminance and efficiency of a device were assessed. Results indicated the importance of majority charge carriers’ type in the selection of solvent and tuning its properties. Then, the effect of thermal annealing on electrical and optical properties of polymer light emitting diodes was investigated. During the device fabrication, pre-annealing in 80 and/or 120 °C and post-annealing in 120 °C were performed. The nano-scale effect of annealing on polymer-metal interface and electric current injection was described thoroughly. A comparison between threshold voltage, luminance and electric current efficiency of luminescence for different annealing processes was undertaken, so that the best electric current efficiency of luminescence achieved at 120 °C pre-annealing accompanied with 120 °C post-annealing.

a typical str njected from he PVK pol with the help ombination ction potent he above-m out 2 eV, wh V [28]. So     words, the increase in the amount of holes is more than electrons. The electric current efficiency of luminescence-electric current density characteristics implies this from another angle. The decrease of thickness tends to an increase in effective electric field, as mentioned earlier. The effect of this field is approximately the same for the both charge carriers. On the other hand, the improvement of the polymer-metal interface quality causes an increase in the electron injection. Considering only these two effects, the amount of electric current efficiency of luminescence in devices should be increased by adding chlorobenzene.
To deal with this contradiction, another effective parameter is needed to be considered. Chloroform and chlorobenzene solvents, in addition to different evaporation rates, have different chemical structures; the chlorobenzene solvent, having benzene rings, is classified as an aromatic solvent, and the chloroform solvent is classified as a non-aromatic one. This difference has a great influence on the polymer chains arrangement and the nano-morphology of the final structure.

The Effect of Internal Nano-Structure Morphology
In general, polymers consist of non-conductive single bound structures, conjugated structures and benzene rings. The solubility of each element depends on the interaction between solute and the solvent. Using the second rule of thermodynamics, the solubility of these two materials in each other could be as follows: where ΔG M , ΔS M and ΔH M are changes in the Gibbs free energy, entropy, and enthalpy of the system, respectively. T is absolute temperature of the system [17]. The amount of ΔS M for solving two materials in each other is always positive. In the solving process, polymer solves in such a way that the system reaches its minimum free energy. So, to achieve better solubility, ΔH M should be in its minimum amount. Changes in the internal energy of materials are the largest portion of enthalpy changes.
When these two materials have similar chemical structures, ΔH M reaches its minimum amount. Thus, mixing two aromatic or two non-aromatic materials causes minimal changes in enthalpy and they are more soluble in each other [17].
According to this, the aromatic chlorobenzene solvent solves the aromatic part of PVK polymer better than its non-conductive main chain. Therefore, in different polymer chains, main chains are located close to each other and tend to stick one to another. This forms polymer regions with a non-conductive core of main chains and an aromatic shell of polymer branches. After spinning and the solvent evaporation, this morphology still survives [17,18]. In contrast, aromatic parts of different polymer chains get close together and approach each other in the non-aromatic chloroform solvent, constructing central cores of formed polymer regions. Non-conductive parts are located on the shells (Scheme 2).

Scheme 2.
Schematic illustrations of internal nano-morphology for emitting layer of fabricated polymer light emitting diodes using an aromatic solvent (a) and a non-aromatic solvent (b).
Therefore, using an aromatic solvent in the PLED structure tends to make contact between aromatic parts of polymer chain and electrodes. Due to conductivity of aromatic rings, potential barrier decreases and charge carrier injection improves. On the other hand, conductive parts of different polymer regions are located close to each other. Thus, charge carrier mobility of a host polymer, which is mainly hole transport in the current structure, increases. Considering that the distribution of PBD molecules is approximately unchanged, their overall function also does not change remarkably. In this way, the ratio of holes to electrons in the system increases. So, where electrons are minority carriers, using an aromatic solvent the electric current efficiency of luminescence decreases.

Thermal Annealing Process
The thermal treatment of thin layers could make nano-scale changes in the interfaces of layers and their internal nano-structure [16,17]. To prevent destructive side effects of long thermal treatment on organic materials, the annealing process should be performed in limited durations and temperatures. The glass transition temperature of undoped PVK is slightly above 200 °C; however, it is expected that the additives reduce it dramatically.
The annealing process could be undertaken after spin coating and before the cathode deposition (pre-annealing) and/or after the metallization of a polymer surface (post-annealing). These two methods have different effects on sample properties. Figures 5 and 6 show electric current densityapplied voltage and luminance-applied voltage characteristics of devices fabricated using chloroform and chlorobenzene solvent mixtures containing the same weight ratio of its components (i.e., 50/50).
As Figures 5 and 6 show, the pre-annealing process causes an increase in the threshold voltage and makes no noticeable change in the amount of luminescence. However, the threshold voltage decreases by post-annealing with the same trend when temperature increases from 80 to 120 °C. Moreover, a dramatic increase in luminance for samples post-annealed at 120 °C could be observed. Elevated temperatures destruct the aluminum surface and tend to produce dark spots on the emitting area of devices.

Experimental Section
The structure of PLED is Glass/ITO/PEDOT-PSS/PVK:PBD:C6/Al (Aldrich). First, indium tin oxide (ITO) coated glass is washed in an ultrasonic bath by pure water, acetone and propanol, respectively. Then, the aqueous solution of poly-(styrene sulfonate) doped poly-(3,4-ethylene dioxythiophene) (PEDOT-PSS) polymer is spun onto it to form a 40 nm thick layer. High molecular weight poly-(9-vinyl carbazole) (PVK) polymer, organic small molecule 2-(4-biphenyl)-5-(4-tbutylphenyl)-1,3,4-oxadiazole (PBD) and 3-(2-Benzothiazolyl)-N,N-diethylumbelliferylamine, 3-(2-Benzothiazolyl)-7-(diethylamino)-coumarin (Coumarin 6 or C6) dye (100:40:0.03 weight ratio) are solved in the chloroform-chlorobenzene solvent mixture, and are spun (in less than 100 to more than 200 nm thickness) onto the PEDOT-PSS layer. At last, the Aluminum (Al) metal layer is deposited in 150 nm thickness on the polymeric bilayer by evaporation (Scheme 3). Annealing of samples at different temperatures is performed in an oven with the temperature controlling ability. Three procedures for thermal annealing are used; pre-annealing, post-annealing and both pre-and post-annealing. In the pre-annealing process, samples are annealed at 80 °C and 120 °C for 5 min before metallization. Samples are annealed at 120 °C for 5 min after Al deposition in post-annealing process. The thicknesses of the layers and their surface roughnesses are measured using conventional methods with a stylus surface profile meter (Dektak) and an atomic force microscope (AFM). Electrical measurements are carried out by the Keithley 6487 picoammeter/voltage source unit, and optical measurements are taken by a photometer, a spectrometer, and a standard setup which consists of a photodiode assembled with an amplifier connected to an oscilloscope. All the fabrication and characterization processes are conducted under laboratory ambient conditions.

Conclusions
The type of solvent and its properties affect electrical and optical properties of solution processed PLEDs. Mixing solvents could be an effective way for tuning these properties. Adding aromatic chlorobenzene solvent with a low evaporation rate to non-aromatic chloroform solvent with a high evaporation rate makes nano-scale polymer layer thinner and more uniform. This, in turn, increases effective electric field and improves electron injection. In this way, it increases the passing electric current and luminance and decreases the threshold voltage of devices. Moreover, it makes a contact between aromatic parts of polymer chains and electrodes, so charge injection potential barrier decreases. Conductive parts of formed polymer regions are located close to each other; thus, charge carrier mobility in a PVK polymer host which is mainly hole transport improves. Thereby, the ratio of holes to electrons in the system increases. Also, in the current device structure, holes are the majority carriers due to their lower injection potential barrier; so electric current efficiency of luminescence decreases.
In the thermal annealing process, residual solvent evaporation makes some nano-scale changes in layer interface. Pre-annealing increases the surface roughness and fluctuation of the polymer layer and degrades electron injection. High surface roughness and fluctuation of the polymer layer make some difficulties in effective metal deposition. Post-annealing causes the diffusion of aluminum layer into the polymer layer, and thus improves electron injection. Using simultaneous pre-and post-annealing processes leads to increase electric current efficiency of luminescence. In this case, pre-annealing at higher temperatures tends to be more effective residual solvent removal, and the post-annealing process at higher temperatures causes an effective diffusion of the metal layer into the polymer layer. Therefore, the best electric current efficiency of luminescence is obtained in samples pre-and post-annealed at 120 °C. In these samples, the amount of luminance increase to the electric current change ratio is more than other fabricated devices.