A prototype of mechanically flexible photosensor arrays using organic bulk heterojunction photodiodes has been developed on plastic substrates. The integration of a 4 μm thick sensor layer onto a flexible amorphous silicon thin-film transistor backplane gave an image sensor array with 35% external quantum efficiency and noise equivalent power of 30 pW/cm2 at reverse bias voltage of -4 V. Sensor properties such as sensitivity and spatial resolution are determined and compared to those of amorphous silicon photodiodes.
A common strategy to improve the electrical performance of organic field effect transistors is to optimize the charge carrier mobility of the semiconducting thin film. Polymer semiconductor transport properties have shown a dependence on the chain length, due principally to the strong influence of molecular weight on the thin film microstructure. In this work, we report on a study of the influence of increasing molecular weight of poly(2,5-bis(3-docecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT-C12) on the polymer bulk thermal properties, thin film microstructure and the electrical performance of thin film field effect transistor devices. Clear differences can be observed within a number average molecular weight range of 8,000 - 18,000 Dalton. A Liquid crystalline phase was only observed at the highest molecular weight, different thin film morphology was observed within the molecular weight range, and the field effect mobility was shown to increase with increasing molecular weight.
The performance of polymer field-effect transistors is highly dependent on their processing history. For instance, thermal processing plays a role in micro-structure development and consequently in device performance. A transport model was developed based on the semiconductor micro-structure where highly mobile states are located in the crystalline areas and defects and disordered regions correspond to areas where carriers are trapped. By applying this model to electrical characterization data of PQT-12 (a regio-regular polythiophene), it is found that annealing tightens the energetic distribution of the traps. Films quenched from the melt performed worse than annealed films due to an increased trap density and broader energy distribution of the traps. X-ray diffraction in grazing and specular geometry was carried out at the Stanford Synchrotron Radiation Laboratory on PQT-12 thin films to reconcile the predictions of the transport model with the micro-structure of the PQT-12 thin films. In all cases the polymer crystallites are textured with the π-stacking direction in the plane of charge transport and the rocking curves indicate the existence of a population of highly oriented crystallites. Annealing the as-spun films improves the crystallinity and texture, in agreement with the transport model. Quenching produces defects in the films, which are likely to produce traps, thereby lowering the carrier mobility.
The interface between the semiconducting polymer and the gate dielectric is one of the most critical regions of a polymeric thin film transistor. For polymeric TFTs, it is difficult to disaggregate the contributions of the electronic structure of the semiconductor and that of the dielectric because, in part, the microstructure of thin films of semiconducting polymers is strongly affected by the chemical functionality at the surface of the dielectric. We have developed a lamination technique that can be used to transfer semiconducting films formed on surfaces that yield films
with high mobility to other dielectrics. We have studied films of semiconducting polymers, such as poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene] and poly(3-hexylthiophene) using this method. The effects of self-assembled monolayers (SAMs) formed on inorganic dielectrics on device performance are discussed. Our results suggest that mobility is mainly controlled by the structure of the semiconducting film and that the threshold voltage of TFTs may be modified through the use of SAMs.
A novel jet-printing approach to fabricate thin film transistor (TFT), active matrix backplanes for x-ray imagers is described. The technique eliminates the use of photolithography and has the potential to greatly reduce the array manufacturing cost. We show how jet-printing is used to pattern the layers of the active matrix array and also to deposit semiconductor material. The technique is applied to both amorphous silicon and polymer transistors, and small prototype arrays have been fabricated and tested, including arrays with a high fill factor amorphous silicon p-i-n photodiode layer for indirect detection x-ray imaging applications. The TFT characteristics are excellent, and acquired x-ray images will be presented and compared to those from conventional TFT arrays. The printing process has been extended to flexible substrates which are important for rugged x-ray imagers, using a low temperature amorphous silicon process to accommodate the plastic substrate. Polymer TFT arrays made with jet-printed polymer solutions have also been demonstrated and we present data from arrays, and discuss options for integrating organic photodiodes or direct detection sensors. The opportunities and challenges of using polymer semiconductors in x-ray imaging arrays, are discussed and we show that the TFT performance meets the needs of radiographic imaging, although the radiation hardness and long term degradation are not sufficiently studied.
Conference Committee Involvement (2)
Organic Field-Effect Transistors V
13 August 2006 | San Diego, California, United States
Organic Field-Effect Transistors IV
31 July 2005 | San Diego, California, United States