CHAPTER 4 : CHARACTERIZATION OF ORGANIC MATERIALS
4.1 Part 1: Investigation of P3HT:VOPcPhO Bulk Heterojunction as a New
4.1.2 Transport Study of VOPcPhO and P3HT
Generally, it is presumed that when a contact between metal and semiconductor is developed, forward bias current results due to thermionic emission current, which is given as follows (Rhoderick, 1978; Sze, 1981):
𝐼 = 𝐼𝑜𝑒𝑥𝑝 (𝑛𝑘𝑇𝑞𝑉) [1 − 𝑒𝑥𝑝 (−𝑞𝑉𝑘𝑇)] (4.1)
The expression for reverse saturation current Io is given below:
𝐼𝑜= 𝐴𝐴∗𝑇2𝑒𝑥𝑝 (−𝑞∅𝑘𝑇𝑏𝑜) (4.2)
where Øb0 is the zero-bias barrier height, V is the forward-bias voltage, k is the Boltzmann constant, T is the temperature in Kelvin, A* represents the effective Richardson constant which can be obtained from Richardson-Dushman relation (A = 4πemk2 / h3) and it was ideally found to be 10-2 A/cm2 K2 for most of organic semiconductor materials (Scott & Malliaras, 1999). A is the active area of the diode and n is the ideality factor. The I-V plot was exercised to determine saturation current, which was found equal to 0.3 nA. The ideality factor (diode quality factor) ‘n’ is defined as:
𝑛 =𝑘𝑇𝑞 (𝑑𝑙𝑛 𝐼𝑑𝑉 ) (4.3)
The barrier height Øb0 can be determined by the following expression:
𝑞∅𝑏𝑜 = 𝑘𝑇 𝑙𝑛 (𝐴𝐴𝐼∗𝑇2
𝑜 ) (4.4)
The current–voltage characteristics are extremely useful to provide important information about the junction properties such as ideality factor, reverse saturation current, zero bias barrier height and specially transport mechanism responsible for conduction. The semi-log I-V plot give information about the ideality factor ‘n’, while the extrapolated saturation current determines the zero-bias barrier height, ‘Øb0’. The
values of n and Øb0 are obtained as 2.69 and 0.416 eV, respectively. In our case, the ideality factor, which should be closer to unity, deviates from the ideal value. The n value beyond ‘2’ is highly suggestive of the fact that the prevalent current in single layer photovoltaic device is due to recombination (Yakuphanoglu, 2007) including the abnormal decrease of barrier height due to the effect of thermionic emission that could be another reason for the increased ideality factor. In similar case, it is well understood that, the ideality factor of phthalocyanine derivatives such as CuPc, could have large value from 7.7 to 18.2 due to the same recombination issue (Rajaputra et al., 2007).
Figure 4.5 The junction resistance versus applied voltage for single layer solar cell.
The Rsh (shunt resistance) is determined from the graph of the junction resistance (R) versus voltage (V) shown in Figure 4.5. The obtained value of Rsh is 49 MΩ.
Another method to determine the barrier height is the Norde’s method. The Norde’s function (Norde, 1979) can be expressed as:
𝐹(𝑉) =𝑉𝛾−𝑘𝑇𝑞 ln (𝐴𝐴𝐼∗𝑇2) (4.5)
where 𝛾 is a dimensionless quantity having a first integral value greater than ‘n’. The value of 𝛾 in this case is ‘3’. A graph of 𝐹(𝑉)and 𝑉, shown in Figure 4.6, is plotted to
obtain the minima on x and y axes. The barrier height can be calculated by the following expression:
Ø𝑏0 = 𝐹(𝑉𝑜) +𝑉𝛾𝑜−𝑘𝑇𝑞 (4.6)
where 𝑉𝑜 is the voltage value that corresponds to the minima of 𝑉 and 𝐹(𝑉𝑜) represents the minima of 𝐹(𝑉). The barrier height calculated by Norde’s method is 0.39 eV, which is in agreement with the value obtained by conventional I-V method.
Figure 4.6 F(V) versus voltage plot of VOPcPhO based cell.
The I-V characteristics of the ITO/PEDOT:PSS/VOPcPhO/Al organic photodiode in dark and under illumination are shown in Figure 4.7. The device characterization was carried out under simulated 100 mW/cm2 AM 1.5 white light illumination. Figure 4.7 clearly shows that current value under light is greater than the value obtained in the dark. This shows that when the photons are absorbed, the electron-hole pairs are produced which result in the carrier-contributing photocurrent. A great deal of useful information about the generated electron-hole pairs at the junction can be achieved from this phenomenon. As a result of photo-excitation, the charges generated
in the active region of the device under illumination are subsequently swept to the corresponding electrodes due to the electric field.
Figure 4.7 Current-voltage characteristics of VOPcPhO-based single-junction device in dark and under illumination.
The performance parameters for the cell have been extracted. The short circuit current (JSC) and open circuit voltage (VOC) for the single layer VOPcPhO-based solar cell are measured as 5.26 x 10-6 A/cm2 and 0.621 V, respectively. The fill factor FF is obtained as 0.33. The power conversion efficiency of the organic photodiode is calculated as 1.07 x 10-3 %. The measured photocurrent in the active layer can be attributed mainly to the ability of charge carrier to travel to the external electrodes without being recombined (Nogueira et al., 2003). However, the active layers of greater thickness become more susceptible to the recombination of holes and electrons.
Although, the efficiency shown here is very low, but this is common in single-junction
photodiode structure. The lower efficiency can be attributed to the fact that the single layer VOPcPhO is a material which has low mobility of the order of 10-5 cm2 V-1 S-1. A comparison of the device operation parameters for single-junction organic solar cells, made with CuPc and VOPcPhO, is presented in Table 4.3. It can be concluded that the short circuit current and open circuit voltage obtained for the solution-processed VOPcPhO single-junction cell are comparable to that of the CuPc single junction device.
The electrical properties of ITO/PEDOT:PSS/VOPcPhO/Al photodiode device were investigated in dark and under illumination. The rectification behavior has been observed for the device in dark. The electronic parameters of the cell from I-V characteristics in dark have been extracted by using conventional I–V method and verified by Norde’s method. The calculated values of the ideality factor and barrier height are 2.69 and 0.416 eV, respectively. The values of the photovoltaic parameters such as short circuit current density, open circuit voltage and fill factor are as 5.26 x 10-6 A/cm2, 0.621 V and 0.33, respectively. The photovoltaic response of VOPcPhO-based organic solar cell has shown the potential of the material to play an important role in generating clean and inexpensive power from the abundant solar energy.
Table 4.3 Comparison of various solar cell performance parameters for different organic active layers used for the fabrication of single-junction organic solar cells.
Active layer for single-junction organic solar cell
Jsc (μ A cm-2) Voc (V) Reference
9 0.7 (Parthasarathy,
5.26 0.621 Present work
To analyze the charge transport properties in VoPcPhO, unipolar and ambipolar diodes have been fabricated. For the hole-only diode ITO/PEDOT:PSS coated glass substrates were used. The active organic layer was deposited on the PEDOT:PSS and after that a 40 nm thick N,N’-bis(3-methylphenyl)- (1,1’-biphenyl)-4,4’-diamine (TPD) film was deposited to prevent electron injection from the top Au electrode. Meanwhile, the electron-only diodes contain Al films as bottom and top electrodes. In this case, a 0.5 nm thick interface doping layer of lithium fluoride (LiF) was placed between the organic film and top electrodes. For ambipolar transport the ITO/PEDOT:PSS/active layer/LiF/Al structures have been considered. The thickness of active organic layer was around 150 nm in all devices. The mobility of the unipolar and ambipolar diodes calculated in SCLC regions is given in Table 4.4. The results show that the VOPcPhO has bipolar transport and can act as electron as well as hole transporting material. The finding indicates that the electron mobility is comparable with holes mobility.
Normally, it is found that the mobility of electron and hole are not similar, where in most organic materials, hole mobility is greater than that of electron. It is found that a strong trapping of electron in organic semiconductor device has led the hole mobility to be higher than electron mobility
Table 4.4 Extracted mobility (cm2/Vs ) values for VOPcPhO from the unipolar and ambipolar diode measurements.
Entity Electron Hole Ambipolar
VOPcPhO 8.3 x10-5 3.7 x10-4 2.5x10-5
A new system of donor-accepter blend for bulk heterojunction solar cell of poly(3-hexylthiophene) (P3HT) by using vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (VOPcPhO) as acceptor material has been fabricated and characterized
for its electrical and optical properties. It is concluded that P3HT:VOPcPhO blend system has the potential to be applied in the bulk heterojunction solar cells, due to its high absorption solar spectrum in the visible region and considerably good electrical behavior. This study was performed in open air to show the potential of VoPcPhO for bulk heterojunction solar cell and further investigations were performed in the controlled environment to enhance the performance of the solar cell.
The OFET device has been fabricated by using VOPcPhO. The purpose of this OFET fabrication is to identify whether the blend system of P3HT:VOPcPhO is compatible to be characterized using a newly EFISHG technique. The technique requires a significant change of SHG signal after the active layer is induced with an electric field. Figure 4.9 shows the SHG signal produced by the blend in different applied fields of 0, +1, and -1 V.
Figure 4.8 Second harmonic generation (SHG) signal for VOPcPhO thin film.
From the figure above, a significant change in SHG signal is hardly observed even after the application of induced electric field of +1 V and -1V. Such results become the evidence for incompatibility of the VOPcPhO for EFISHG characterization since the material non-linear optical property could not be manipulated. This is believed that, the centro-symmetrical structure of VOPcPhO is unbreakable and has a specific electronic field distribution at a time. Thus, further study on charge carrier transport behavior, related with OFET structure, using EFISHG technique is skipped.
0 V + 1 V - 1 V
4.2 Part 2: Towards an Efficient Organic Solar Cells by Utilizing