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This practical report provides detailed analyses of the results obtained from the experiments conducted to investigate the behaviour of the metal/organic semiconductor interface. The main aim of the experiment is to determine the sheet resistance of the organic semiconductor P3HT (poly (3- hexyl thiophene)) in contact with Gold (Au) and Aluminum (Al) electrodes. This includes the background study of organic semiconductor P3HT and the experimental setup. High currents are observed in sample with Au contact electrode, where as lows current is observed in sample with Al contact electrodes. Graphs are plotted from the data obtained and a detailed investigation is done regarding the high and low currents obtained which depends on the properties of metal/semiconductor interface, i.e., the contact resistance.
Regioregular P3HT is a conjugated polymer which is a derivative of polythiophene. It is a high mobility hole transporting semiconductor. The chemical structure of P3HT is shown in fig 1. It has alternate single and double bonds between carbon atoms which forms the backbone where the charge transfer takes place. Due to this alternating system, bonding and anti bonding states are formed which in turn results in formation of forbidden energy gap. As the anti bonding states are higher in energy, they form the conduction bond and being bonding states are low in energy, they form the valance bond. These are dissolved in solvents like chloroform due to the presence of alkyl side chains C6H13. This solubility is one of the major properties conjugate polymer, which enables simple film deposition. The side chains are regularly ordered as shown in the fig 1, this ordering is known as regioregularity which gives high field effect mobility values over disordered chains.
Fig 1 Chemical structure of P3HT [image courtesy, PHY6006 Mod lecture notes]
To carry out the experiment two substrates one with Au contact electrodes and the other with Al contact electrodes are used. 1 ml of 10 mg/ml solution of rrP3HT in chloroform is prepared. This solution should not be exposed to light; as it has light illumination effects, hence an aluminium foil is used to cover the phial. A thin film of organic semiconductor is deposited using spin casting technique at a rate of 2000rpm. These samples are dried by exposing it to dynamic vacuum at room temperature for one hour, in order to remove impurities which are added during spin casting. Their current-voltage characteristics are determined by using a keithley source- measure unit, by applying a voltage of +30 to -30V. Basically there are two types of contacts, one is top contact and the other is bottom contact. Of which we are using bottom contact which means organic semiconductor is placed on metal, which have better properties than the top contact. Four contacts of same width with different channel lengths are investigated as shown in fig 2. Channel length L and width W of the samples are calculated with the help of microscope and calibrated eye piece.
Fig 2 Contact electrodes with channel length L and width W.
Sheet resistance can be calculated from the formula given in eq 1.
R = R1 + R2 + LRs/W eq.1
Where R1 and R2 are the contact resistances, L is the channel length, W is the width of the channel and Rs is the sheet resistance.
Sample 1: Using Au metal electrode contacts
Current voltage characteristics are determined for four contacts, which are different in channel length. Values are plotted into graph using excel which is shown in fig 2. A voltage of -30V to 30V is applied to determine the current flowing through the sample. Graphs obtained from the values show high currents. This high current can be explained with the work function of the metal and the semiconductor.
Fig 3 I/V characteristics of gold electrode
Conjugated polymers have two molecular orbitals(MO), namely HOMO(highest occupied molecular orbital) and LUMO(lowest unoccupied molecular orbital). HOMO is the molecular orbital in which last pair of electrons to be filled into the molecule. The photoelectric work function of gold is around 5.1ev, where as the HUMO level of P3HT is around 5.2ev which is very near. Hence it is easy to inject or take out holes from P3HT, which is shown in fig 3. Due to this ohmic contact is achieved between the metal and P3HT, which causes low and linear contact resistance. If ΔV → 0 such a contact is called ohmic. This decrease in resistance causes high currents in P3HT.
Fig 4 Work function of P3HT and Au
Since the metal organic interface is the limiting factor for injection of carriers, this type of transport is called injection or contact limited. Injection is controlled by the work function of the metal relative to the ionization potential of P3HT for injection of holes and relative to the electron affinity of P3HT for electron injection. This process depends on the energy barrier that charge carrier has to overcome, while passing through metal and P3HT interface. These energy barriers are developed due to the difference between the Fermi level of metal electrode and HOMO - LUMO levels of the P3HT.
Slope is calculated from the graph 1/R = 8.01E-07 (â„¦-1), which is inverse of the resistance R= 1.25E06 (Ohms). The slope of I-V curve gives the resistance across the sample which has units in ohms. Resistance for the four sets is determined. This is the total resistance of the sample, i.e, sheet resistance and contact resistance. The channel length L is determined from the microscope. By using the resistance and channel lengths a graph is plotted as in fig 5. The resistance changes with the channel length L. A straight line plotted from the values and slope, intercept is determined from the graph. Where the magnitude of the intercept is equal to the contact resistance and the slope is equal to the sheet resistance for the whole sheet. The contact resistance is same for all sets, as it is not dependent on the channel length L. Hence this value is compared with the constant in the equation of straight line. From the equation of the straight line, sheet resistance can be calculated by multiplying it with the width W of the metal contact. The influence of sheet resistance is increases as the channel length is reduced.
Fig 5 total resistance Vs channel length
Standard Errors for slope, intercept are to be considered while calculating the sheet resistance.
From the graph the values are
Slope (m) = 1.3128E+13 ± 3.19E+12 =9.93E+12
Intercept= -1.097E+8 ± 1.9E8 = -3.01E8
Equating this to eq 1, we get
Contact resistance R1 + R2 = -3.01E8 (Ohms)
Sheet resistance (Rs) = width W * 9.93E+12= 2.2E-3*(-9.93E+12) = 2.18E+10 (Ohms per square)
Sample 2: Using Al metal electrode contacts
Second sample constitutes of four aluminium contact electrodes with varying channel length L. I-V graph of the four sets is represented in fig 5. I-V graph obtained is asymmetric and non-linear, which indicates the low-current. This can be due to several reasons, one could be the large difference between the work function of Al and HOMO level of P3HT. As the difference is high an injection barrier is formed between the metal and P3HT, which is used to inject or remove holes from HOMO. This injection barrier increases resistance, which results in low current and consumes more voltage to inject the carriers. Also there is a possibility of formation of interfacial layer between the metal and P3HT, it may be oxide layer. The difference in work function of aluminium and the HUMO level of P3HT is shown in fig 7. As it has high contact resistance, this data is unsuitable to extract sheet resistance.
Fig 6 I-V characteristics of Al metal contacts
At interfaces, the alignment of energy levels can be of two distinct regimes; Fermi level alignment and vaccum level alignment. Vacuum level alignment gives rise to potential, while the Fermi level alignment gives rise to interfacial dipole. Interfacial dipole is formed by means of diffusive charge transfer across the metal/organic interface. By bending the energy bands of P3HT with respect to the Fermi level of the metal gives the degree of charge transfer. This occurs when there is a differenc between the work function of metal and polaronic bands of P3HT.
Fig 7 work function of Al and HOMO level of P3HT.
From the above discussion we can conclude that, metal organic interface plays a major role in the charge transfer across the sample. For sample with au contact electrodes high currents are observed, which is due to the absence of injection barrier that occurs when the difference between the work function of metal and HUMO level of P3HT is low. Low currents are observed in sample with al contact electrodes, which is due to the presence of injection barrier observed due to the difference in the work function of the metal and HUMO level of P3HT is large. Sheet resistance is determined for the au sample from the I-V data obtained. Sheet resistance is calculated from the R-L graph, where L is the length of the channel.