Search

Organic semiconductors hold the promise for large-area, low-cost image sensors and monolithically integrated photonic microsystems. This requires the availability of photodiodes offering at the same time high quantum efficiency, low noise and long lifetimes. Although published structures of organic photodiodes offer high external quantum efficiencies (EQE) of up to 76% [F. Padinger, R.S. Rittberber, N.S. Sariciftci, Effects of postproduction treatment on plastic solar cells, Advanced Functional Materials 13 (2003) 1, P. Schilinsky, C. Waldauf, C.J. Brabec, Recombination loss analysis in polythiophene based bulk heterojunction photodetectors, Applied Physics Letters 81 (20) (2002) 3885], [1,2] they normally suffer from short lifetimes of only a few hundred hours as well as large dark currents. In our work the lifetime of a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) heterojunction photodiode structure was increased to several thousand hours by omitting the widely used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) anode layer. In addition, a simple model of optical interference and absorption effects was used to find the optimum thickness that combines high quantum efficiency with low dark current. As a result, we report on organic photodiodes with state-of-the-art EQE of 70% at 0 V bias, an on/off current ratio of 10 6 at −1 V and 40 mW/cm 2 illumination, dark current densities below 10 nA/cm 2 at −1 V, and a lifetime of at least 3000 h.

Organic light-emitting devices (OLEDs) consist of a stack of multiple thin film layers whose thicknesses influence both the optical and electronic performance. Upon injection and transport, the charge carriers may recombine to form excitons that diffuse and decay radiatively, thus leading to distinct recombination and emission zone profiles that determine device performance. Suitable simulation tools that allow a better understanding and efficient optimization of organic optoelectronics devices and materials are desirable.

An optical touch and proximity sensor based on polymer light-emitting diodes and polymer photodiodes is presented. The sensor’s thin-film light sources and detectors are monolithically integrated in the same plane of a common substrate and are processed from the liquid phase at room-temperature. These key features make the sensor potentially attractive for applications where low cost, small device thickness, tight space requirements or mechanical flexibility matter, such as simple information displays with touch switch functionality, for example. A static light-emitting sign with integrated touch switch is demonstrated here.

A novel, fully automated, fabrication and characterization apparatus for polymer light-emitting diodes (PLEDs) was developed. This high throughput apparatus allows the fabrication of 49 devices with a controlled variation of essential parameters like material, material composition, blend concentration, layer thickness, and annealing temperature. Up to now, due to a lack of elaborate design tools, extensive experimental effort is required in order to optimize novel materials, material combinations and device structures for polymer based LEDs. Our novel apparatus provides an extensive dataset which can be used for device optimization and a profound device modeling offering a deeper theoretical understanding of underlying device physics in PLEDs.

A polymer thin-film optical touch and proximity sensor is presented. The sensor is based on the monolithic integration of polymer light emitting diodes, logos or displays, and polymer photodiodes on a common substrate. The main interest in this new form of optical sensor lies in its potentially cost-effective manufacture on thin and flexible substrates. Potential applications of such systems range from simple information displays with integrated touch-screen to biochemical sensors.

Evanescent coupling is used to couple light from a polymer light emitting diode (PLED) into a planar single-mode waveguide. The PLED is monolithically integrated on top of the waveguide in a stacked configuration. Due to the waveguide’s proximity, the emission zone of the PLED and the waveguide modes overlap substantially, so that the PLED can directly excite the modes. An additional spacer layer between PLED and waveguide is shown to improve the coupling efficiency by about a factor of 5. For iridium-based diodes PLED-to-waveguide coupling efficiencies of as high as 3.2% have been obtained.

Evanescent coupling is used to couple light from an organic Lambertian emitter into a single mode planar waveguide. Either an organic light emitting diode (OLED) directly excites the waveguide mode or an OLED pumps a photoluminescent (PL) material layer located directly on the waveguide. At the out-coupling grating the guided light is diffracted onto an array of organic photodiodes acting as a spectrometer. A spectral resolution of down to 16 nm could be achieved with integrated optoelectronic system.

A simple method for patterning down to 400nm gaps between two thin-film electrodes is presented. The edge of the first electrode defines the gap via a photolithographic step using off-normal through-substrate exposure. It is demonstrated that the gap width can be controlled simply by adjusting the exposure angle and/or photoresist thickness. Using this process, poly(3-hexylthiophene) field-effect transistors with a submicrometer channel and a large channel width to length ratio of up to 5000 have been fabricated. Since the process is mask free, contactless, highly parallel, and offers high resolution, it is potentially suited for the high-throughput, low-cost fabrication of electronic circuits.

The effect of optical irradiance on the linearity of a Si photodiode was studied. These results are compared for light modulated at 30 MHz and at dc as the optical irradiance was varied over a 9 decade range. We discuss how these results affect the use of this detector as a heterodyne receiver. As the optical irradiance varied from 10−2 to 10+3 mW/cm2, while maintaining constant total power, the photocurrent was constant to about 1%, but as the power density increased further, the photocurrent increased about 13%. At the highest densities that we could achieve, about 6×107 mW/cm2 there was only slight evidence of the onset of saturation. These results are of importance in our work to use optical heterodyne detection to measure filter transmittances over a wide dynamic range. The results provide guidelines for achieving maximum accuracy when using this particular diode as an optical heterodyne receiver.