Organic semiconductor has been studied from last 1940s. There have been a large number of experimental and theoretical studies done in recent years. Recently, organic semiconductor's photoconductive properties are being used by the industries for xerography. Demonstration of organic electroluminescent diodes & OTFT's are based on small organic molecules or conjugated polymers. The improvements and efficiency of the organic devices have attracted the industries towards it. The charge carrier transport is important for designing and synthesization of better materials and further improvement in the device. The most important parameters determining the performance of the mobility of the charge carriers are: temperature, charge density & electric field. Though many experiments have been performed to measure these parameters but current voltage characteristics in the device based on semiconducting polymers produces an excellent output. Hence we analysis a current voltage measurements for the samples prepared.
Organic semiconducting polymers are molecularly engineered to be soluble; hence it produces a good quality, uniform thin film over large area of substrate using spincasting (spincoating) technique. This techniques used to produce thin films are cheap and easy to develop. The organic semiconducting polymer used is a poly(3-hexyl thiophene) (rrP3HT), which is commercially available. rrP3HT has a high mobility hole transporting organic semiconductor. There have been many techniques developed for solvent based coating technique and vacuum based coating technique.
Charge transport in organic semiconductor depends on the pie bonds and quantum mechanical wave function overlap. But in the case of disordered organic semiconductor there are limited pie bonds hence the charge transport in the disordered semiconductor is explained by the quantum mechanical tunnelling process. Hopping is yet another important factor that effects the charge carrier transport in a disordered organic semiconductor. This process depends upon the charge carrier from molecule to molecule between HOMO & LUMO levels.
Sample Preparation:- (Spincasting)
- Sample measurement:
We take the 2 sets of electrodes substrates gold (Au) and aluminium (Al). Then we measure the distance between the electrode pair and width of electrode with the help of microscope and calibrated the eyepiece.
- Solution for spincoating:
To prepare 10mg of rrP3HT polymer is weighed and kept in a glass bottle, and then we add 1ml of chloroform to the polymer using a pipette. The concentration of the solution is measured in % wt./wt. We perform these experiments under dim light and wrap the phial sample. Then we gently heat and stir with magnetic stirrer for 15 min because sometimes a good solvent wont dissolve instantly.
Precaution: Pipette should only be used for pure solvent but nor for solution, since there a chance of contamination in the bottle. The bottle should be screwed tightly and wrapped in an aluminium film in order to avoid the solvent evaporation changes which can change the concentration and to avoid the photochemical reactions between chlorinated solvents and conjugated molecules.
- Filtering the solution:
Filter the solution through chloroform resistance micropore filter with pores size 0.2 to 0.45 m. We use PTFE (Teflon) as the micropore filter. These filters can lock on to the top of a disposable syringe.
Precaution: we have to check that the filter is resistance to the solvent using. If the filter is block this shows that the polymer did not dissolve very well hence we have to find another better solvent.
- Sample preparation:
Spincasting 2000 rpm, an rrP3HT film on to the substrates. Then dry the films at 400C for 1 hour under dynamic vacuum, but we directly take out the samples here.
- Current voltage measurements:
We use Keithley source measure unit to take the current voltage scans for all the samples. We scan the voltage from -30 V to 30 V. we run a few 'burn in' scans on every sample before recording results since first scans on a fresh sample may be erratic.
Result and Discussion:
The observation that could be made from the graph below of aluminium is that as the length of the sample decreases the curves gets more stepper and stepper. This shows the inversely proportionality constant between the length and the current. The current obtained in the aluminium samples very low. The low current produce by the samples give us the nonlinear I/V curve. They are asymmetric and display hysteresis. Since the curves are nonlinear it is difficult to obtain the sheet resistance of the samples. In order to calculate the sheet resistance of the sample we need to calculate the slope of the curve which is not possible since the graphs obtained are not linear. The graphs obtained for the aluminium tells us about the hole injection of HOMO through P3HT to Al and Electron injection from LUMO.
The thickness of the organic semiconductor obtained in the case of aluminium was around 40nm.
In the case of gold the curves obtained are approximately linear and less hysteresis. Hence we can calculate the slope of the curves and obtain the sheet resistance for the gold samples. The current obtained in the case of gold is much higher than aluminium, this can observed in the graph above. The Au with p3HT as semiconductor layer produces a good electrical potential. rrP3HT is conjugate polymer hence there are effects of temperature, mobility, charge density on the carrier transport of charges. As the thickness of gold increase the HOMO levels gets shifted to Fermi level. Hence we can see the relation between the thickness and shifts of levels. The graph obtained for the sample helps us to distinguish from the bulk material.
The thickness of the organic semiconductor obtained in the case of gold was around 60nm.
In order to calculate the sheet resistance of the gold sample we calculate the slope for the gold samples obtained. The main reason for using four identical devices with different channel lengths is to allow us to make the slope vs. the length graph.
The inverse of the slopes give us the resistance value of the sample.
The curve obtained is almost linear hence we calculate the slope sample to measure the sheet resistance of the samples. The slope of this line gives the sheet resistance and the y intercept give us the contact resistance of the gold sample.
R1 +R2 are known as contact resistance and it can be compared with the y intercept of the line shown above. R0 represents the sheet resistance of the material and it can be obtained by knowing the slope of the line.
Drawbacks of Spincoating:
They need soluble materials for the preparing the solution. We limit the options for developing multilayers and us also the material that files off the edge of the substrate while spinning. The film produce are poor in quality if the solvent evaporates too fast or doesn't wet the substrate well. We cannot use low molecular weight materials.
We have obtained the current vs. voltage for gold and aluminium samples. We have discussed the experimental procedure for the spincasting the organic polymer on to the samples. We have also seen that gold produces high current than aluminium. We have also seen the relation between the length of the sample and current produces. We have also seen that the data obtained for aluminium is unsuitable for the calculation of sheet resistance. We calculated the sheet resistance and contact resistance for the gold. Hence we have obtained the main objective of the report about calculating the sheet resistance. We have also discussed the drawbacks and limitations of the spincasting.
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