title: Analytical aspects of metal semiconductor barriers based on organic semiconductors
Creators: name: Mahadavan, Malina
Publisher: name: University of Liverpool Repository
Year: 2023

description: Over recent years, research into organic semiconductors has intensified considerably due to the increasing commercial viability of inexpensive, flexible, large area electronic applications. In particular, the introduction of a new generation of small molecule based organic semiconductors has increased the possibility of achieving high field effect mobilities. So far pentacene seems to be the most promising candidate since it yields field effect mobilities that are comparable to that of hydrogenated amorphous silicon (a-Si:H). Recently, a mobility larger than 1.2 cm2y-fs-l and an on/off ratio greater than 108 was reported for a thin film transistor (TFT) made with triisopropylsilyl pentacene (TIPSpentacene) as the active material. The solution processability of TIPS-pentacene is advantageous since solution-processed organic TFTs are needed to pave the way for low cost manufacturing approaches such as inkjet printing and roll-to-roll processing. One ofthe main potential applications for organic materials is in low cost radio frequency identification (RFID) tags operating at a frequency of 13.56MHz. The high frequency op~ration of RFID tags will be most demanding on the rectifying component of the circuit which can be based either on a TFT or a rectifying diode. This thesis is primarily concerned with analysing metal semiconductor barriers made with a variety of organic semiconductors such as highly regioregular poly(3-hexylthiophene) (p3HT), polytriarylamine (PTAA) (S 1105) and vacuum-deposited pentacene. The work includes a review of the various charge transport models proposed along with a discussion on the Meyer Neldel Rule (MNR) which is a commonly occurring phenomenon in organic semiconductors. A simple analytical model that demonstrates the empirical relationship between mobility and carrier density is also developed. This general dependency is formally known as the Universal Mobility Law (UML). The electrical characteristics of Schott)..)' junctions made between aluminium and various organic solids are investigated. Both P3HT and PTAA are highly disordered semiconductors whilst pentacene is a small molecule organic semiconductor akin to polycrystalline inorganic solids. The analysis of the pentacene vertical diode is thus extensively based on the model developed by Eccleston. The AI-PTAA Schott)..)' diode is found to not only yield a high rectification ratio but also an extremely low off current which suggests that the device is most suitable for low current circuit operations. In contrast, diodes based on P3HT and pentacene demonstrate much weaker rectifYing properties. Nevertheless, in these diodes, the non-saturation of the reverse currents allows the dopant densities ofthe materials to be determined. The new current density expression developed for organic based Schottk.')' diodes allows the characteristic temperature of the exponential distribution of intrinsic carriers (To) and states (Te) to be determined directly from the forward current density voltage characteristic. The Meyer Neldel energy representing the exponential density of states (DOS) is then obtained using the value of TeÃ‚Â• In general, the !\tIN energy estimated from the exponential current regime ofthe diodes range between 30 meV and 35 meV. Special attention is given to the saturation current region of the diodes as relatively high currents are needed to satisfY the demands ofRFID related circuits. The saturation current of the as-synthesised P3HT diode was found to obey Ohm's law over the entire applied voltage range. In contrast, the PTAA and pentacene diodes demonstrate a transition from ohmic to space charge limited (SCL) conduction with increasing applied bias. The saturation current regime of these diodes is modelled using the new SCL current expression developed for disordered materials. The intrinsic value of Te determined for both diodes suggests the absence of dopant states at higher energies. The effects of changing the back metal/organic interface and further with doping with 2,3-dichloro-5,6-dicyano-I,4-benzoquinone (DDQ) on the saturation region ofAI-PTAA Schottk.')' diodes are also studied. Finally, the temperature variation of the current density voltage characteristics of assynthesised P3HT and PTAA Schottk.')' diodes are analysed. Below room temperature, the distinct fall in the forward exponential slope leads to much larger ideality factors 1/. This is attributed to the non-ideal behaviour of the Schottk.')' barrier interface with decreasing temperature. The anomalous rise in 1/ is attributed to a number of possibilities including the presence of extrinsic trapping states, carriers taking alternative routes to other potential barriers and the lack of validity of the flat quasi Fermi level approximation. Modelling the saturation current region of the PTAA diode with the new SCL current expression yields a self-consistent value of Te at relatively high temperatures. The steep decline in the Meyer Neldel energy observed at lower temperatures is mainly attributed to the formation of a potential barrier at the back metal/organic interface.