Influence of O2 on rectification properties of Nickel Phthalocyanine thin film devices

Thin sandwich film structure devices of Gold/Nickel Phthalocyanine/Lead (Au/NiPc/Pb) were fabricated employing a novel in-situ method. Electrical measurements were performed prior to, and after exposure of the samples to dry air. Under forward bias and for low applied voltages an ohmic conduction was evident, followed by SCLC in the higher voltage range. In the reverse bias, devices were found to exhibit weak rectifying properties originated mainly from the bulk of the NiPc layer. After exposure of the sample to dry air for five days a strong rectifying effect at the NiPc/Pb interface was evident. The phenomenon is believed to be associated with a change of NiPe work function as result of O2 adsorption on the NiPe layer. To verify this a second sample of the type Au/NiPcO2/Pb was fabricated. Electrical characterization of the sample showed stronger rectifying properties providing further experimental evidence on the influence of 02 adsorption on the organic layer. Potential barrier height and diode ideality factor for both NiPc/Pb, and NiPcO2/Pb interfaces after exposure to dry air, were also calculated

Accelerated degradation of silicon metallization systems

Clemson University has been engaged for the past five years in a program to determine the reliability attributes of solar cells by means of accelerated test procedures. The cells are electrically measured and visually inspected and then subjected for a period of time to stress in excess of that normally encountered in use, and then they are reinspected. Changes are noted and the process repeated. This testing has thus far involved 23 different unencapsulated cell types from 12 different manufacturers, and 10 different encapsulated cell types from 9 different manufacturers. Reliability attributes of metallization systems can be classified as major or minor, depending on the severity of the effects observed. As a result of the accelerated testing conducted under the Clemson program, major effects have been observed related to contact resistance and to mechanical adherence and solderability. This paper does not attempt a generalized survey of accelerated test results, but rather concentrates on one particular attribute of metallization that has been observed to cause electrical degradation - increased contact resistance due to Schottky barrier formation. In this example basic semiconductor theory was able to provide an understanding of the electrical effects observed during accelerated stress testing

Novel materials for direct X-ray detectors based on semiconducting organic polymers

Conventional inorganic materials for x-ray radiation sensors suffer from several drawbacks, including their inability to cover large curved areas, me- chanical sti ffness, lack of tissue-equivalence and toxicity. Semiconducting organic polymers represent an alternative and have been employed as di- rect photoconversion material in organic diodes. In contrast to inorganic detector materials, polymers allow low-cost and large area fabrication by sol- vent based methods. In addition their processing is compliant with fexible low-temperature substrates. Flexible and large-area detectors are needed for dosimetry in medical radiotherapy and security applications. The objective of my thesis is to achieve optimized organic polymer diodes for fexible, di- rect x-ray detectors. To this end polymer diodes based on two different semi- conducting polymers, polyvinylcarbazole (PVK) and poly(9,9-dioctyluorene) (PFO) have been fabricated. The diodes show state-of-the-art rectifying be- haviour and hole transport mobilities comparable to reference materials. In order to improve the X-ray stopping power, high-Z nanoparticle Bi2O3 or WO3 where added to realize a polymer-nanoparticle composite with opti- mized properities. X-ray detector characterization resulted in sensitivties of up to 14 uC/Gy/cm2 for PVK when diodes were operated in reverse. Addition of nanoparticles could further improve the performance and a maximum sensitivy of 19 uC/Gy/cm2 was obtained for the PFO diodes. Compared to the pure PFO diode this corresponds to a five-fold increase and thus highlights the potentiality of nanoparticles for polymer detector design. In- terestingly the pure polymer diodes showed an order of magnitude increase in sensitivity when operated in forward regime. The increase was attributed to a different detection mechanism based on the modulation of the diodes conductivity

INVESTIGATION of CURRENT TRANSPORT IN ITO/CdTe/polymer/Al DEVICES USING NANO-STRUCTURED CdTe

In this thesis, photo luminescent diodes with the device structure of ITO/PEI/(CdTe/PDDA)*n/Al were fabricated using the method of layer-by-layer self assembly. The film thicknesses were varied from 150 nm to 380 nm. The films were characterized through X-ray diffraction (XRD), optical absorption and photoluminescence (PL) measurements. The XRD results on the film indicated a cubic crystalline structure (111) for the nano-CdTe particles. The band gap of the nano-particles were evaluated to be 2.1 eV in solution and 2 eV in films, which was further confirmed by the PL measurements as the solution exhibited a yellow luminescence while the film exhibited orange luminescence. The J vs. V curves revealed that the diodes exhibited rectifying behavior in both the forward and reverse biasing. Two models of current transport, one based on a Schottky mechanism and the other based on a tunneling mechanism were developed and were compared with the experimental values. The tunneling model developed could simulate the experimental currents up to four orders of magnitude. The tunneling mechanism of charge transport was further proved by the capacitance vs. voltage curves, which were identical to that of ITO/MEH-PPV/Al devices, where tunneling mechanism was the dominant method of charge transport

Integrated molecular diode as 10 MHz half-wave rectifier based on an organic nanostructure heterojunction

Considerable efforts have been made to realize nanoscale diodes based on single molecules or molecular ensembles for implementing the concept of molecular electronics. However, so far, functional molecular diodes have only been demonstrated in the very low alternating current frequency regime, which is partially due to their extremely low conductance and the poor degree of device integration. Here, we report about fully integrated rectifiers with microtubular soft-contacts, which are based on a molecularly thin organic heterojunction and are able to convert alternating current with a frequency of up to 10 MHz. The unidirectional current behavior of our devices originates mainly from the intrinsically different surfaces of the bottom planar and top microtubular Au electrodes while the excellent high frequency response benefits from the charge accumulation in the phthalocyanine molecular heterojunction, which not only improves the charge injection but also increases the carrier density