Probing dynamic interfaces in organic electronics

Abstract

Organic semiconductors are emerging in solar cells, photodetectors, light-emitting diodes and field-effect transistors. The main advantages are the electrical transport properties that can be tailored by chemical design, and their mechanical flexibility. Applications are foreseen in the field of large-area organic electronics, where numerous discrete devices are required on low-cost substrates such as glass or plastic. Widespread introduction however is hampered by two main bottlenecks, which are the limited operational stability and the complex formulations needed for the large-area processing. The basic building block of organic electronics is the field-effect transistor; a microelectronic device used to manipulate the magnitude of a current with an external electrical field. Transistors have to be reliable under operation i.e. when biases are applied to the electrodes, the resulting device current should be constant. In organic transistors however, the on-current slowly decreases with time, an effect of which the origin is unknown. This so-called bias-stress-effect has puzzled the scientific community for more than two decades. In chapter 2 it was shown that the decrease of current with time is caused by a shift of the threshold voltage, i.e. the bias needed to turn the transistor on. The temperature dependence of the threshold voltage shift is determined to be independent of the semiconductor used. This observation led to the conclusion that the threshold voltage shift is due to a common physical origin, but the exact nature remained elusive. Additional measurement techniques were required to understand the cause of the threshold voltage shift. In chapter 3 scanning Kelvin probe microscopy (SKPM) was used to monitor trapped charges in transistors under applied bias. SKPM measures the local surface potential and provides microscopic insight in the electrical performance of organic transistors. It was shown that the potential profiles changed upon gate bias stress, but only in the channel region of the transistor. The reliability issues were found to be not due to an increase in the contact resistance, but due to trapping of charges in the channel area of the transistor. The question remained where the charges were trapped exactly. The threshold voltage shift could be due to trapping in the semiconductor or in the gate dielectric. In chapter 4 SKPM proved that charges can be stored at the dielectric-semiconductor interface even without a semiconductor present. By exfoliating the semiconductor after stress, and subsequent probing of the surface potential of the exposed gate dielectric, it was shown that the threshold voltage shift was indeed due to charge trapping in the dielectric and not in the semiconductor. In chapter 5 a scenario is proposed to explain the observed threshold voltage shift quantitatively. The model consists of two ingredients: (i) hole-assisted electrolytic production of protons from water in the accumulation layer and (ii) the subsequent diffusion of these protons into the SiO2 gate dielectric. The proposed model captures the most important features of the gate bias stress effect, such as the time dependence of the threshold voltage shift and the semiconductor-independent temperature dependence of the threshold voltage shift. The model can also explain the recovery of the transistor upon grounding all electrodes and predicts an anomaly in the current transients, which was experimentally verified in chapter 5. By combining the experimental and theoretical evidence as presented in this thesis, it was concluded that a layer of water at the gate dielectric together with diffusion of protons into the gate dielectric can explain the most important features of the bias stress effect. At the moment large area processing, like evaporation and spin coating, is top-down. A promising process technology for organic electronics is bottom-up self-assembly, which is the autonomous organization of components into patterns and structures without human intervention. Self-assembly has the advantage over conventional processing techniques that it can be made substrate selective and can potentially cover large areas uniformly. Self-assembled monolayers (SAMs) gained a great scientific and industrial interest because of their ability to change the macroscopic properties of surfaces by a single sheet of molecules. Reported surface modifications are for instance changes in workfunction, wettability and adhesion. The advances in SAM-based electronics however have been slow. To investigate the possibility of self-assembly as a processing tool to fabricate organic electronics, first a well-studied system - thiols on gold - was considered in chapter 6. Thiols with opposite built-in dipole moment were chosen and in this way the workfunction of the injecting electrode could be increased as well as decreased. The change in workfunction resulted in a modification of the current injection. In this way patterned light-emitting diodes (LEDs) were fabricated. Only a single layer of molecules determined the light emission in an organic LED. The expertise gained on self-assembly in organic electronics was used to fabricate self-assembled monolayer electronics in chapter 7. Making integrated circuits using a bottom-up approach involving self-assembling molecules was already proposed in the 1970s. The basic building block of such an integrated circuit is the self-assembled monolayer field-effect transistor (SAMFET), where the semiconductor is a monolayer spontaneously formed on the gate dielectric. In SAMFETs fabricated so far, current modulation has only been observed in sub-micrometer channels. Low field-effect carrier mobility, low yield and poor reproducibility have prohibited the realization of bottom-up integrated circuits. We were able to identify and remove these bottlenecks by studying the charge injection and transport in monolayer semiconductors. By circumventing the main roadblocks, real logic functionality was demonstrated in integrated circuits by constructing a 15-bit code generator in which hundreds of SAMFETs were addressed simultaneously. Additionally, we investigated the cause of the absence of current in long channel length transistors. The extracted device mobility in SAMFETs was found to be determined by the monolayer coverage and the channel length. The dependence on coverage and channel length were quantitatively explained numerically and analytically. At partial coverage, SAMFETs form a unique model system to study charge percolation in two dimensions. The SAMFETs were made with silicon dioxide as the gate dielectric, which could be a possible disadvantage when they are used as building blocks in flexible electronics. As a final step the self-assembly process was transferred to organic substrates in chapter 8, which yielded fully functional 4-bit code generators based on organic dielectrics.