Novel Semiconducting Organic Molecules

Published Date: 02 Nov 2017

In recent years, the development of semiconductors has been focussed on π-conjugated organic molecules which are currently used in the area of flexible electronics due to their mechanical properties, processability and to produce low cost devices on plastic substrates. Organic field effect transistors (OFET) and organic photovoltaic cells (OPV) are two of the most promising technologies need further scientific and technological advancement. Over decades, OFETs and OPVs have been extensively studied using conjugated molecular and polymeric systems and now are considered to be leading technologies in the large area flexible electronics1, 2.

Organic field effect transistors (OFETs):

Figure 1a shows a typical architecture of a bottom-gate bottom-contact organic field effect transistor (OFET). In addition to this, several device structures can also be fabricated depending on the relative position of the contacts (electrodes) and the dielectric/semiconductor layer as shown in the figures 1b and 2(a, b). An OFET is composed of three electrodes-source, drain, and gate. Source/drain electrodes can be chemically inert metal such as gold are evaporated on top of silicon/silicon dioxide (Si/SiO2) substrate. A dielectric layer (SiO2), one can act as a gate and bringing these elements into contact with an organic small molecule or polymer semiconductor layer yields a bottom contact field effect transistor3.

Figure : Illustration of typical bottom gate structures a) Bottom drain and source b) top drain and source electrodes.

When the gate voltage applied is zero (VG=0V), there will be a flow of negligible source-drain current (ISD=0A) independently of the bias applied between the source and the drain electrodes (VSD). When a gate field is applied (VG≠0V), which induces charge carrier in the semiconductor at the interface with the dielectric layer and then the device turns on (ISD≠0A). OFETs can be p-type (holes as the majority carrier), n-type (electrons as the majority carrier), or ambipolar (both electrons and holes are involved as charge carriers).

Figure : Illustration of typical top gate structure a) Bottom drain and source b) top drain and source electrodes.

Organic semiconductors with the ability to conduct only positive charges are said to be p-type semiconductors. In recent years significant improvements have seen in charge carrier mobility of p-type semiconductor materials for use in OFETs. Organic semiconductors should be able to conduct both n-type and p-type as well (ambipolar behaviour) but very often majority of organic semiconductors exhibiting only hole transporting (p-type) behaviour in transistors. Especially, transistors based on n-type organic semiconductors operates only in vacuum or an inert atmosphere, because of certain species such as oxygen and water present in ambient air can easily diffuse into the semiconductor and destabilize the negative charge carriers in the channel. The transistor performances are evaluated from the output and transfer current-voltage (I/V) plots, where critical crucial parameters such as the field-effect mobility (μ), current on/off ratio (Ion/Ioff), and threshold voltage (VT) are measured. A key factor that determines the charge carrier mobility and overall performance of electronic device is the intermolecular ordering of the molecules in the solid state. It is believed that face-to-face contact is preferred for organic electronics, as probability of increased π-π overlap between the faces of adjacent molecules, which creates the way for charge carrier in the solid state4-6.

Organic Photovoltaic solar cell (OPVs):

The way that solar cells convert sun light into electricity is called the photovoltaic (PV) effect. Bulk heterojunction (BHJ) OPVs are based on a blend of a conjugated low band-gap polymer (donor) with a soluble derivative of fullerene (acceptor). This takes place in four steps. Firstly, excitons (electron-hole pair) are created within the molecule by light absorption. Secondly, these excitons are separated into electrons and holes at the potential barrier at BHJ. Thirdly, these separated holes and electrons are transported towards their respective channels and generate a photocurrent. Fourthly, charges are collected at respective electrodes7. By measuring the output current with respect to the voltage the solar cell efficiency (ηmax) can be determined by using,

where Pmax is power maximum ,Pin is input power, Voc is open circuit voltage, Isc is short circuit current and FF is fill factor. Fill Factor (FF) is a percentage of the maximum power generated by the solar cell, (Vm x Im) to the theoretical power, (Voc x Isc). It is the ratio of maximum output power to the incident power, when a solar cell is connected to an electrical circuit. Open circuit voltage represents the maximum voltage of the cell produces, when it is sourcing no current. The short circuit current is the current produced by the cell when the two electrodes are shorted together8. The fundamental processes in OPVs such as light harvesting to power generation and the donor-acceptor interface are illustrated in figure 3.

Figure : Schematic representation of Photovoltaic cell

Almost, all organic solar cells were presented in the literature are fabricated and characterized without ever being exposed to air because of instability in ambient conditions. The two active (Donor-acceptor) materials could be the electron rich materials such as thiophene based organic molecules or polymers act as the electron donor9. Fullerenes serve as the acceptor material as well as semiconducting charge pathway to the front electrode. PEDOT: PSS [poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate)], a hole transport layer was included to help the charge extraction process as the back electrode.

Experimental section:

OFET:

Commercially available silicon substrates (1X1cm) from Fraunhofer IPMS were used for analysis of field effect transistor. Thin-film FETs were fabricated on highly doped silicon substrates with a 240 nm thick thermally grown silicon oxide (SiO2) insulating layer, used as a gate electrode. Gold source and drain electrodes were defined on the SiO2 layer by lithographic method with the channel length (L) and width (W) of 2.5um and 10mm respectively. The patterned substrates were carefully cleaned using several rounds of acetone and methanol. After drying in a flow of nitrogen, FET substrates were treated with a fluorinating agent, Pentafluorobenzenethiol (PFBT) (7mM in Propanol) to create the self assembled monolayer (SAMs) can improve the formation of favourable interactions between the fluorine atoms of the SAM surface and the adjacent organic molecules10. Thin semiconductor films were then deposited by spin-coating method using the solvents of Chloroform and chlorobenzene onto the FET substrates. The substrates were baked in an oven for one hour at 200° C. The film thickness was observed around 40-50nm of each devices. The electrical characterization of the transistor devices was carried out in a dry nitrogen atmosphere using a computer-controlled semiconductor parameter analyzer.

OPV:

Organic Photovoltaic cells for organic molecules were fabricated on ITO glass substrates. Before Fabrication, the Substrates were degreased using the solvents Acetone, Propanol and Isopropanol placed inside the plasma cleaner. After that PEDOT: PSS was deposited by spin coating at 1000 and 4000 rpm separately. Substrates were annealed on a hot plate at 120oC for 20 minutes. Subsequently, the active layer was then deposited in a nitrogen atmosphere in glove box. The active layer was spin coated at 600 and 800 rpm separately for one minute with an initial ramp of 5. Then the devices were subjected to annealing at 1200C for 20 minutes and placed into an evaporator for (Electrodes) aluminium and calcium deposition was about 40nm respectively at 10-6 torr. Devices were characterised in the solar cell simulator.

Results and Discussion:

Here, we present a novel conjugated polymer (figure 4) based on a diketopyrrolopyrrole (DPP) core and a tetrathiafulvalene (TTF) derivative with a potential to be used in OFETs and BHJ-OPVs (figure : 4 ). DPP has excellent chemical and thermal stability and tetrathiafulvalene (TTF) is a planar non-aromatic 14 π-electron system. Recently, various tetrathiafulvalene derivatives were shown to exhibit excellent OFET performances in thin films4, 11.

Figure 4: [DPP (blue)-TTF (red) Polymer]

Figure 5a (left): Output characteristics measured at intervals of -20V and Figure 5b (right) transfer characteristics of DPP-TTF polymer measured in an ambient lab atmosphere

Fig. 5a shows the output characteristics of the above mentioned polymer (DPP-TTF) devices measured at different source and drain voltages with a gate voltage (VGS) between

-10V and -70V. Fig. 5a shows transfer characteristics for the polymer measured at a VDS of

-90 V with the mobility of 4 X10-2cm2V-1S-1. An on/off ratio of 1 X104 is also calculated from Fig. 5b. Fig. 6 shows a tapping mode AFM image taken from the same devices used for mobility measurements produce fibre-like structures upon annealing at 2000C for half an hour. Atomic Force Microscopic technique was used to investigate and understand the correlation between morphology and charge carrier mobility of the organic field effect transistors. Non-uniform distribution and orientation of these fibres like structures may lead to a slightly lower mobility as opposed to DPP polymer analogues.

Figure 6: AFM image of Polymer DPP-TTF Figure 7: OPV graph for DPP-TTF polymer

The photovoltaic properties were investigated for polymer DPP-TTF in fabricating bulk heterojunction solar cells. The blend ratio were investigated with ratios of 1: 1 and

1: 4 (w/w) [DPP-TTF polymer: PC70BM] using the solvents of dichlorobenzene and

chloroform. The devices were tested under simulated 150 mW illumination.

I–V curves of organic photovoltaic cells are presented in Figure 7, with the relevant device data summarised in Table 1. We observed differences in performance of the two solvents in terms of power conversion efficiency. Even though the fill factor is lower, the polymer exhibits higher open circuit voltage by 710mV. The polymer blend shows higher photo conversion efficiency up to 2.79% and is qualitatively in agreement with the higher field effect mobility observed. Further investigation is also in progress.

Active layer

P(DPP-TTF):PCBM

Solvents

ISC (mA/cm2)

Voc (V)

FF

PCE (%)

1:4

Dichlorobenzene

12.0

0.71

0.32

2.79

1:4

Chloroform

7.2

0.67

0.31

1.57

Table 1: Summary of photovoltaic device performance of 1: 4 ratios (polymer: PC70BM)

The second molecule presented here is π- conjugated halogen substituted organic material (Figure 8). The electronic structures of these types of organic semiconductor have significant potential on charge transport mobility in thin films. Introduction of fluorine atoms on the thiophene units has also been a possible strategy to become electron transports (n-type) are facilitated by low energy barrier to the injection of electrons. These types of materials are inclined to self assemble into well ordered thin films and has large influence for OFET and OPV applications12, 13.

Figure 8: π- conjugated halogen substituted organic material (Thiophene-Perfluorobenzene)

The molecule is sublimed under high vacuum (<3E-6 mbar) onto SAM (Self Assembled monolayer) treated Si/SiO2 substrates and annealed at around 155 °C. A combination of 5 nm Molybdenum trioxide and 25 nm of gold are used as top electrodes. The hole mobility extracted from the slope can be estimated to be 1.4E-7 cm2/Vs (figure 9). Charge carrier mobility of organic semiconductor mostly depends upon the surface morphology of thermally evaporated or spin coated thin films14. Figure 10 shows surface morphology of above mentioned monomer produced by high vacuum thermal evaporation at the base pressure of 4 x 10-6 mbar. The resulted thin film thickness was 50nm which has been identified by Dektak profile meter.

Figure 9: Transfer Characteristics and Figure 10: Atomic Force Microscopic image of compound of Thiophene-Perfluorobenzene monomer

Linearly extended π -conjugated materials (figure: 11) are of high interest as they are substitute for higher molecular weight polymers. They are well defined; discrete structures with reliable synthetic reproducibility possess high purity and excellent solubility in common organic solvents. The DPP unit has been incorporated into conjugated system that has been studied as components in OFETs. The DPP unit is clearly an exciting unit for use as a core molecule in Linear-shaped π -conjugated architectures. The field-effect mobility of linearly extended material was investigated (figure: 12), from which a typical performance of p-type field effect mobility 1.075 X 10-4 Cm2 V-1 S-1 was observed with well distinct linear and saturation regimes15, 16.

Figure: 11 Linearly extended π -conjugated material

Figure: 12a Output characteristics and Figure 12b Transfer characteristics of linearly extended π -conjugated material

Conclusion:

In summary, we investigated the OFET properties of conducting polymer and oligomer incorporating a TTF unit in the backbone17. The OFET devices were fabricated by a solution process under various conditions. All the devices showed typical p-type semiconducting behaviour as expected from the electron-donating properties of the TTF polymers. SiO2 surface treatment with an organic molecule had higher field effect mobility for all series of compounds, presented here. Interestingly, AFM observations revealed that SAM (self assembled monolayer) treatment promotes the ordering of the molecular chain resulting in an improvement in the electronic conduction.