Siegfried W. Kettlitz

Dynamic characterization of organic bulk heterojunction photodetectors

The authors report the dynamic properties of bulk heterojunction photodiodes based on a polymer blend system consisting of poly(3-hexylthiophene-2,5-diyl) and the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester. Devices with a high-frequency contact layout were analyzed under continuous wave and pulsed laser illumination (λ=532nm). The organic photodiodes exhibit a pulse response with a full width at half maximum of 11ns to the applied 1.6-ns-long laser pulses. Rise times as small as 1.6ns and fall times <40ns were measured under applied reverse bias.

Nanosecond response of organic solar cells and photodetectors

We examine the impact of various parameters on the transient current density characteristics of organic solar cells and photodetectors by means of numerical simulations. Our self-consistent numerical model treats the dynamics of generated electrons and holes in the framework of a drift-diffusion model. As input parameter for the electric model, the intensity distribution of the incident light is calculated with a transfer-matrix method accounting for interference effects. The results are compared to experimental results. With our approach, we are able to distinguish the influence of different physical effects as they become dominant at different current densities or at different time regimes. This enables us to estimate the electron and hole mobilities separately by fitting the experimental results. Furthermore, space charge effects are identified as being highly important for the transient response of photodetectors.

Optical Data Link Employing Organic Light-Emitting Diodes and Organic Photodiodes as Optoelectronic Components

An optical interconnect solely using organic optoelectronic components is presented. The data link is based on an organic light-emitting diode as the transmitter and an organic photodiode as the receiver. Light is transmitted via a polymer optical fiber coupled to the active components. A digitized audio signal based on the Sony/Philips Digital Interface Format standard at a signal bit rate of 2.8224 Mbit/s (44.1-kHz sampling frequency) is successfully transmitted.

Thickness-Dependent Transient Photocurrent Response of Organic Photodiodes

We report on the transient photocurrent response of organic photodiodes based on a bulk-heterojunction of polyhexylthiophene and -phenyl--butyric acid methyl ester. By changing the thickness of the active layer via different spin speeds we explore the influence of geometric capacitance and bias voltage on the transient photocurrent decay upon excitation with a nanosecond laser pulse.

Direct fabrication of PDMS waveguides via low-cost DUV irradiation for optical sensing

We demonstrate the fabrication of single mode optical waveguides by irradiating polydimethylsiloxane (PDMS) with a low cost Hg lamp through a conventional quartz mask. By increasing the refractive index of the irradiated areas, waveguiding is achieved with an attenuation of 0.47 dB/cm at a wavelength of 635 nm. The refractive index change is stable in ambient air and water for time periods of more than 3 months. The excitation of water-dispersed fluorescent nanoparticles in the evanescent field of the waveguide is demonstrated.

Eliminating RC-Effects in Transient Photocurrent Measurements on Organic Photodiodes

In this letter, we present a method to eliminate the influence of the RC-constant in transient photocurrent measurements on organic photodiodes and solar cells. With this technique, it is possible to reconstruct the conduction current from measurements and extract relevant charge transport parameters. This is demonstrated by the application of this method on RC-dominated measurements of transient photocurrent responses on organic photodiodes revealing the power law decays being typical for dispersive charge transport in the conduction current. Vice versa, our approach allows us to simulate the impact of an optional RC-constant on simulation data, which is generated neglecting the serial resistance of real devices.

Influence of temperature-dependent mobilities on the nanosecond response of organic solar cells and photodetectors

We investigate the impact of temperature on the transient current density characteristics of organic solar cells and photodetectors. This is done by both experimental measurements and numerical simulations. In the process, we investigate the photoresponse of the device to an impinging laser pulse at different temperatures. By fitting the experimental results with the correlated disorder model we are able to quantify the influence of temperature on charge carrier mobilities in organic bulk heterojunction solar cells. We determine an almost doubling of the electron mobility on increasing the temperature from 11 to 50 °C.

Influence of the spatial photocarrier generation profile on the performance of organic solar cells

We investigate the impact of the interplay of charge carrier drift and diffusion on the fill factor of organic solar cells. Thin film interferences lead to strong gradients in the photocarrier generation profile. By means of numerical simulations, we show that the shape of the absorption profile is crucial for the efficiency of organic solar cells. High absorption in the peripheral areas of the active layer advantages an unfavorable diffusion current which leads to a reduction of the fill factor. Our work suggests design rules for the optical optimization of organic solar cells.

Hybrid lithography: combining UV-exposure and two photon direct laser writing.

We demonstrate a method for the combination of UV-lithography and direct laser writing using two-photon polymerization (2PP-DLW). First a dye doped photoresist is used for UV-lithography. Adding an undoped photoresist on top of the developed structures enables three-dimensional alignment of the 2PP-DLW structures by detecting the spatially varying fluorescence of the two photoresists. Using this approach we show three dimensional alignment by adding 3D structures made by 2PP-DLW to a previously UV-exposed structure. Furthermore, a fluidic system with an integrated total internal reflection mirror to observe particles in a microfluidic channel is demonstrated.

Illumination angle and layer thickness influence on the photo current generation in organic solar cells: A combined simulative and experimental study

In most future organic photovoltaic applications, such as fixed roof installations, facade or clothing integration, the solar cells will face the sun under varying angles. By a combined simulative and experimental study, we investigate the mutual interdependencies of the angle of light incidence, the absorber layer thickness and the photon harvesting efficiency within a typical organic photovoltaic device. For thin absorber layers, we find a steady decrease of the effective photocurrent towards increasing angles. For 90-140 nm thick absorber layers, however, we observe an effective photocurrent enhancement, exhibiting a maximum yield at angles of incidence of about 50°. Both effects mainly originate from the angle-dependent spatial broadening of the optical interference pattern inside the solar cell and a shift of the absorption maximum away from the metal electrode.