Organic Electronics

1.4.1 Organic Field Effect Transistors

The beginning of the era of organic electronics can be traced to the mid eighties when the organic photovoltaic cells, light emitting diodes and field effect transistors were first introduced.11 Organic field effect transistors (OFETs) have attracted a lot of attention due to the potential to promote low-cost and flexible circuitry. There are various geometries of OFETs, which can be seen in Figure 7. The main components of an OFET are an organic semiconducting layer, a dielectric layer and three conducting electrodes.12

  Figure 7 Three geometries of OFETs, from left to right: bottom gate, bottom contacts (BG-BC);

bottom gate, top contacts (BG-TC); top gate, bottom contacts (TG-BC). S: Source, G: Gate, D: Drain

There are three methods for fabricating OFETs. One method for the fabrication of OFETs is to use electrochemical polymerisation to directly form a semiconducting polymer on to the surface of the electrodes. These electrodes are used subsequently as drain and source electrodes. The first ever OFET was fabricated in this manner in 1986 using polythiophene as the semiconducting polymer.13 The second technique involves the use of either a conjugated polymer, such as poly-3-alkylthiophene (P3AT), or precursor materials that can undergo subsequent chemical reactions to give the desired compounds.14 The third technique utilises vacuum deposition of oligomers and organic materials of low molecular weights in order to form highly ordered and self assembled films.15

One such study was by Bao et al., who investigated the use of regioregular poly-3- hexyl thiophene (P3HT) 3 in OFETs15 Regiorandom P3HT contains both head to head 2 and head to tail 1 3-alkylthiophenes whereas a regioregular P3HT contains only one kind (Figure 8).

  Figure 8 Example of a regioregular P3HT and an OFET.

They found that in utilising these regioregular P3ATs they could achieve high field effect mobilities in the order of 10-2 _{to 10}-3 _cm2_{/Vs. The improvement over their}

regiorandom counterparts was attributed to the well-defined architectures resulting in higher crystallinity and a more ordered morphology. These polymers also proved to have good solubility and have easy processability, thus they concluded that they provided a promising alternative in organic electronics.

1.4.2 Organic Photovoltaics 1.4.2.1 Device architecture

Organic photovoltaics offer key advantages for photovoltaics such as the possibility to process directly from solution, which will thus afford lighter, cheaper and more flexible photovoltaic devices.16

Organic solar cells work on the same principles as inorganic solar cells in that they require semiconducting materials. However, in the case of organic solar cells these semiconductors are organic molecules made up of donor and acceptor compounds. There are typically two main types of device architecture, bi-layer and bulk heterojunction (BHJ). In a bi-layer architecture the device consists of two layers, one layer containing the donor material and one layer the acceptor material, sandwiched between two electrodes (Figure 9).

S S R R HT (1) S S R R HH (2) S S R R S S S R R R (3) n

Figure 9  Cartoon representation of a bi-layer OPV

In a BHJ device, however, the donor and acceptor materials are blended together in one active layer (Figure 10). Aside from the active layers the device architectures remain the same. Typically, there is a transparent substrate coated with a conductive oxide, usually indium tin oxide (ITO) which acts as the anode. A layer of poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) then follows, which acts as a hole transport layer. Following the PEDOT:PSS layer there is the active layer and the cathode (typically aluminium), respectively.

Figure 10 Cartoon representation of a BHJ OPV

BHJs increase the contact area between donor and acceptor materials which leads to a significant increase in the number of generated excitons as well as their dissociation into free charge carriers, which in turn leads to an increase in power conversion efficiency.17 These architectures offer great advantages in that they can accommodate large scale production on flexible substrates by solution processing techniques such as ink-jet printing18, brush painting19 and roll to roll processing20.

Substrate  (glass)   Anode  (ITO)   Cathode  (Al)   Acceptor   Donor   Substrate  (glass)   Anode  (ITO)   Cathode  (Al)   PEDOT:PSS  

1.4.2.2 Mode of action

A BHJ works by photo-excitation of the donor to generate an exciton (electron-hole pair), this is then diffused to the donor-acceptor interface. This exciton then dissociates into a geminate pair, which then transfers to the electrodes (Figure 11).21

  Figure 11 Mode of action in a BHJ device21

One of the most important factors to consider is the charge transfer between donor and acceptor. Upon excitation an electron is transferred from the highest occupied molecular orbital (HOMO) of the donor material to its lowest unoccupied molecular orbital (LUMO). This electron is then transferred to the LUMO of the acceptor material, which must occur rapidly in order to avoid recombination (Figure 12). Thus, it is important when considering donor and acceptor material that the HOMO/LUMO energy levels are well matched.

Figure 12 Charge transfer between donor and acceptor HOMO   HOMO   LUMO   LUMO   Cathode   Anode   DONOR   ACCEPTOR   ELECTRON   HOLE  

1.4.2.3 Photovoltaic characterisation

Photovoltaic performance is characterised by the power conversion efficiency (PCE). The PCE is determined by three factors, short-circuit current density (Jsc), open circuit

voltage (Voc) and fill factor (FF). The PCE is calculated by the following formula22:

PCE = Jsc x Voc x FF / Pi

Where Pi is the light power incident on the device. The FF is a characteristic, which

defines the shape of the J-V curve, i.e. how “square” the curve is, and is an indicator of how difficult it is to extract the photogenerated carriers from the device. It is calculated by the following formula, where Jm and Vm are the current density and

voltage taken directly from the J-V curve (Figure 13)22

FF = Jm x Vm / Jsc x Voc

Figure 13 Representation of a J-V curve22 1.4.2.4 Acceptor/donor materials used in OPV

OPV materials can typically be divided into two sections, small molecules and polymers. 23 _{Small molecule OPVs were dominated by the use of copper}