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Organic photodiodes not only show good quantum efficiencies in the visible range and fast pulse responses down to the nanosecond regime, but also allow for arbitrary shapes of the active area. The usual device structure is a thin organic semiconductor absorber sandwiched between two electrodes of which one is transparent. This leads to a high device capacitance for large area devices. In conjunction with a high electrode resistance, the transient photocurrent response under localized pulse excitation becomes dependent on the position. We make use of this effect and demonstrate a method for position sensing in oblong organic photodiodes achieving a standard deviation in the position measurement of less than 100 $\mu \text{m}$ at the edges and 12 $\mu \text{m}$ at the center of an 8000- $\mu \text{m}$ long device.

Resonant optical antennas with nanoscale gaps are of high interest due to their ability to enhance electric fields in localized subdiffraction-limited volumes. They are especially attractive for coupling with quantum emitters. One challenge for applications that exhibit a spectral shift is to fabricate nanoantennas that provide two distinct resonances at the excitation and emission frequency, respectively. We propose a coupled T-shaped nanoantenna structure that provides independently controllable resonances with a common electromagnetic hot spot in the antenna gap. We demonstrate the fabrication of such structures and investigate experimentally and theoretically their spatial, time-integrated spectral- and polarization-dependent electromagnetic field properties. The nanoantennas exhibit two resonances with unique spectral and polarization responses. The resonance wavelengths are independently tailored by varying the geometry of individual T-shaped antennas.

We investigated the optoelectronic properties of solution processed organic light emitting diodes (OLEDs) as a function of their active layer thickness. By using a horizontal dipping technique and by accelerating the coating bar during wet film deposition, we fabricated OLED arrays with different emission layer thicknesses but identical process records in a single process step. The comparison of the optoelectronic device parameters allows for conclusions on injection limitation, the optimization of the layer thickness, and, in conjunction with optical simulations of the weak cavity effect, to promote a deeper understanding of the emission profile. To show the universality of this method, we investigated purely polymeric emitters, blends of polymers and small molecules as well as all-small molecule material systems.

For window integration of semi-transparent solar cells in living and working areas, color neutral transparency perception and good color rendering are of pivotal importance. In order to tune the optical device properties, we simulate a parallel tandem configuration with two different absorber materials. Within a regime of convenient transparency perception, the transparency can be adjusted between 20% and 40% by choosing the right absorber layer thickness combination. From the optical field in the tandem devices we calculate the charge carrier generation profile and subsequently correlate the optical properties with the electrical device properties as derived from drift-diffusion modelling – altogether allowing for a comprehensive assessment of the transparency, the transparency perception and the device performance and their interdependencies.

Microfluidic flow cytometers are highly interesting candidates for biomedical point-of-care applications. However, the sensitivity, reliability, and throughput of these systems must be improved to provide the full functionality of established flow cytometric systems. One proposed method to improve fluorescence detection systems is to use spatial modulation techniques. We derive the noise-related statistics and calculate the coefficient of variation for a detection system with and without spatial modulation. We measure the noise properties of a nonmodulated microfluidic fluorescence particle detection system and analyze the possible performance gains using spatial modulation. © 2014 International Society for Advancement of Cytometry

Organic photodiodes provide prospects for the fabrication of arbitrarily shaped photodetectors. However, the enlarged detection area in conjunction with their minuscule absorber layer thickness increases the capacitance of these devices when compared with convential silicon photodiodes. The mandatory transparency of at least one electrode can, so far, only be provided by materials with significant sheet resistances. These factors lead to nonnegligible RC-constants where high frequency signal detection is ultimately RC-limited. In this letter, we devise a method to determine the effective RC-constant for an extended rectangular device comprising electrodes with a significant sheet resistance and show that it is up to 59% smaller than estimated from the geometric device dimensions.

Flow cytometers and other particle analyzing devices require the reliable detection of particles for counting, classification, sorting, and other applications. Using miniaturized microfluidic devices opens up new application possibilities, but also poses new challenges for reliable signal detection. Simplifying flow cytometers by eliminating scattering as the second signal channel changes the statistics for fluorescence particle detection due to the now unknown number and position of particles in the fluorescence signal. Furthermore, in applications such as particle sorters, the detection delay must be kept short, and cost considerations may limit the available signal processing power. In this paper, we investigate the statistics of a simple thresholded peak detection algorithm with a defined spatial resolution that satisfies these needs. We define a signal system with statistical properties in accordance with measurements obtained from a miniaturized fluorescence particle detection device. For this model, we derive properties, such as spatial resolution, rate of particle interference, and false positives and false negatives due to noise. Using these properties allows us to predict the device performance during the design phase and therefore optimizes the particle detection device for specific applications to reduce device cost, while still maintaining crucial performance figures. In order to evaluate the applicability of the signal model in a real application, we compare the theoretical amplitude distributions with experimental results.

Measurements of the transient photoresponse of organic photodiodes and solar cells show a strong saturation effect in the quantum efficiency at laser fluences above approximately 3.3 μJ/cm2. By a comparison of the measured intensity, temperature and field dependence of the transient pulse responses with extended drift–diffusion simulations, the loss of charge carriers can be traced back to a quadratic loss channel in the charge carrier generation process. In contrast to the predictions of the commonly used Onsager–Braun charge carrier generation model, we demonstrate that the dissociation of bound electron–hole-pairs is temperature independent but slightly field dependent.

A strategy for the efficient numerical evaluation of Sommerfeld integrals in the context of electromagnetic scattering at particles embedded in a plane parallel layer system is presented. The scheme relies on a lookup-table approach in combination with an asymptotic approximation of the Bessel function in order to enable the use of fast Fourier transformation. Accuracy of the algorithm is enhanced by means of singularity extraction and a novel technique to treat the integrand at small arguments. For short particle distances, this method is accomplished by a slower but more robust direct integration along a deflected contour. As an example, we investigate enhanced light extraction from an organic light-emitting diode by optical scattering particles. The calculations are discussed with respect to accuracy and computing time. By means of the present strategy, an accurate evaluation of the scattered field for several thousand wavelength scale particles can be achieved within a few hours on a conventional workstation computer.

We demonstrate a small molecule solution processed hole interfacial layer approach to improve the dark current characteristics of polymer photodiodes. The two hole blocking materials under investigation 3-phenyl-4(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) and 2-[3,5-bis(1-phenylbenzimidazol-2-yl)phenyl]-1-phenylbenzimidazole (TPBi) are spincoated from ethanol as an orthogonal solvent on top of a P3HT:PCBM active layer. We reduce the dark current at a bias voltage of −1 V by a factor of 17.2 by introducing a TAZ interfacial layer while keeping the responsivity unchanged.

Detection of fluorescence particles is a key method of flow cytometry. We evaluate the performance of a design for a microfluidic fluorescence particle detection device. Due to the planar design with low layer thicknesses, we avoid optical components such as lenses or dichroic mirrors and substitute them with a shadow mask and colored film filters. A commercially available LED is used as the light source and a PIN-photodiode as detector. This design approach reduces component cost and power consumption and enables supplying the device with power from a standard USB port. From evaluation of this design, we obtain a maximum particle detection frequency of up to 600 particles per second at a sensitivity of better than 4.7 × 10(5) MESF (molecules of equivalent soluble fluorochrome) measured with particles for FITC sensitivity calibration. Lowering the flow rate increases the instrument sensitivity by an order of magnitude enabling the detection of particles with 4.5 × 10(4) MESF.

We measure the transient electrical response of small organic solar cells illuminated with nanosecond light pulses during degradation. Our data show the buildup of a significant memory effect in the pulse response. To measure this memory effect, a sequence of positive or negative bias voltages is applied and the pulse response is then recorded at 0-V bias voltage. For devices with a measurable memory effect, this pulse response depends on the previously applied positive or negative bias voltage. The memory effect is attributed to changes in the (3,4-ethylenedioxythiophene):poly (4-styrenesulfonate) (PEDOT:PSS) layer in conjunction with humidity and the indium tin oxide (ITO) layer. The strength of the memory effect depends on the anode material used and the time exposed to humid atmosphere. Therefore, the strength of this memory effect is a measure for the corresponding degradation process.

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.

We report on the transient photocurrent response of organic photodiodes based on a bulk-heterojunction of polyhexylthiophene and [6,6]-phenyl-C 61-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.

We present a numerical strategy for the accurate simulation of light extraction from organic light emitting diodes (OLEDs) comprising an internal nano-particle based scattering layer. On the one hand, the light emission and propagation through the OLED thin film system (including the scattering layer) is treated by means of rigorous wave optics calculations using the T-matrix formalism. On the other hand, the propagation through the substrate is modeled in a ray optics approach. The results from the wave optics calculations enter in terms of the initial substrate radiation pattern and the bidirectional reflectivity distribution of the OLED stack with scattering layer. In order to correct for the truncation error due to a finite number of particles in the simulations, we extrapolate the results to infinitely extended scattering layers. As an application example, we estimate the optimal particle filling fraction for an internal scattering layer in a realistic OLED geometry. The presented treatment is designed to emerge from electromagnetic theory with as few additional assumptions as possible. It could thus serve as a baseline to validate faster but approximate simulation approaches.

We present design considerations for semi-transparent organic tandem solar cells that exhibit both good visible transparency and good power conversion efficiencies. The tandem solar cells show excellent color properties, such as the corresponding transparency color temperature and the color rendering index (CRI), that prevail for high angles of incidence of the incoming sunlight. Up to an angle of light incidence of 70°, the devices exhibit a convenient CCT which implies a neutral white and a CRI which is above 96.

Transient measurements on semiconductor devices are distorted by parasitic effects due to the geometric capacitance of the device and the impedance of the attached measurement circuit. Together they form an RC-circuit. Ideally, the transient decay of the pulse response generated by such an RC-circuit follows an exponential function. We investigate the differential equations of the case of an oblong rectangular photodiode with significant sheet resistance of one electrode. We find that under a specific illumination profile the ideal RC-decay can be obtained which is an useful result for RC-compensation in organic photodiodes.

Flow cytometry relies on the detection of cells selectively stained with fluorescence markers. Optically they can be detected as fluorescence particles. The use of microuidics offers a wide range of benefits over traditional flow cytometer designs but when replacing expensive components with inexpensive counterparts the sensitivity of the instrument suffers. To increase the sensitivity of the detection system, spatial modulation has been proposed. Spatial modulation is implemented via _ne pitched shadow masks close to the microuidic channel which generate a signal pattern when a fluorescent particle passes by. Using a _ne pitch and long total length for the pattern a high spatial resolution and long total exposure time are combined. Particle detection from local maxima is not directly possible with spatially modulated signals due to the jagged pulse shape. We compare the performance of different approaches for particle detection from local maxima. Matched filtering and the derivative of the correlation signal provide either a good peak-signal-to-noise ratio (PSNR) or a high spatial resolution. But both approaches suffer from low dynamic range due to side maxima. We derive the solution for a minimum- mean-square-error (MMSE) filter which transforms the modulated pulse shape into a target pulse shape with a single strong maximum. We investigate the performance of the MMSE filter and find that it provides tunable suppression of noise and side maxima along with a high spatial resolution. The use of the MMSE filter therefore is an ideal choice for particle detection from spatially modulated signals.

Detection of fluorescent particles is an integral part of flow cytometry for analysis of selectively stained cells. Established flow cytometer designs achieve great sensitivity and throughput but require bulky and expensive components which prohibit mass production of small single-use point-of-care devices. The use of a combination of innovative technologies such as roll-to-roll printed microuidics with integrated optoelectronic components such as printed organic light emitting diodes and printed organic photodiodes enables tremendous opportunities in cost reduction, miniaturization and new application areas. In order to harvest these benefits, the optical setup requires a redesign to eliminate the need for lenses, dichroic mirrors and lasers. We investigate the influence of geometric parameters on the performance of a thin planar design which uses a high power LED as planar light source and a PIN-photodiode as planar detector. Due to the lack of focusing optics and inferior optical filters, the device sensitivity is not yet on par with commercial state of the art flow cytometer setups. From noise measurements, electronic and optical considerations we deduce possible pathways of improving the device performance. We identify that the sensitivity is either limited by dark noise for very short apertures or by noise from background light for long apertures. We calculate the corresponding crossover length. For the device design we conclude that a low device thickness, low particle velocity and short aperture length are necessary to obtain optimal sensitivity.

Microfluidic flow cytometers are highly interesting candidates for biomedical point-of-care applications. However, the sensitivity, reliability, and throughput of these systems must be improved to provide the full functionality of established flow cytometric systems. One proposed method to improve fluorescence detection systems is to use spatial modulation techniques. We derive the noise-related statistics and calculate the coefficient of variation for a detection system with and without spatial modulation. We measure the noise properties of a nonmodulated microfluidic fluorescence particle detection system and analyze the possible performance gains using spatial modulation.

In this work we investigate polymer photodiodes based on a blend system consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and the fullerene derivative (6,6)-phenyl C-butyric acid methyl ester (PCBM). An optimized low source impedance architecture allows measurements in the GHz range with minimum distortion, while at the same time allowing to probe the favourable sandwich device structure. We have studied the underlying device physics and investigated the influence of parameters such as active layer thickness and bias voltage on the transient photocurrent response. Using a numerical simulation software combining a self-consistent drift-diffusion model in conjunction with an optical model based on the transfer matrix method we model the transient photocurrent of polymer photodiodes. Transient photocurrent measurements utilizing this low impedance device architecture excited by 1.6 ns short laser pulses show very good correlation between simulated and measured results. Furthermore we have developed an encapsulation technique to integrate high-speed organic photodiodes onto standard printed circuit boards (PCBs) to avoid the degradation of the devices by humidity or oxygen.

Measured transient current voltage characteristics of organic solar cells exhibit a tail in the decline characteristic which is proportional to t−α. Common numerical drift-diffusion simulations neglecting dispersive charge carrier transport fail to describe the observed long tail of the current density decline up to micro seconds. Our approach is to account for the observed dispersive charge carrier transport by introducing an exponential distribution of trap states into our simulation. These trap states represent the tail of a bimodal distribution of energy states, which is commonly used to describe the distribution of transport sites of the highest occupied / lowest unoccupied molecular orbital (HOMO/LUMO) in organic materials. By doing so, we can reproduce the measured characteristics over four decades in time. Results are qualitatively and quantitatively in excellent accordance for different laser intensities, different applied biases and different device diameters at the same time.

We present two simple methods to estimate the effective mobility of the faster charge carrier species from the transient nanosecond photoresponse of an organic solar cell or photodiode. In combination with detailed numerical drift-diffusion simulations in the framework of the multiple-trapping model, we identify the energetic relaxation of the charge carriers and hence a decrease of the effective charge carrier mobility while drifting towards the electrodes. From the characteristic shape of the transient current density, the temperature as well as the nonlinear voltage dependence of the charge carrier transit time, we can quantify an exponential trap distribution. In addition, the nonlinearity of the transit time, as also known from comparable time-of-flight measurements, can be explained by charge carrier relaxation processes in the presence of trap states. The effective charge carrier mobility is shown to be field independent but highly temperature dependent.

The nanosecond transient photoresponse of organic solar cells and photodiodes based on a conjugated polymer (poly(3-hexylthiophene-2,5-diyl)) blended with a fullerene derivative ([6,6]-phenyl C61-butyric acid methyl ester) exhibits a strong temperature dependence, whose origin can be traced back to charge carrier transport phenomena. In the framework of a drift-diffusion model including multiple-trapping, the temperature dependence of effective mobilities arises naturally without the need of using a temperature dependent parameterization of the mobilities. Furthermore, the extended drift-diffusion simulation reproduces the measured change of slope of the transient current density from j(t)∼t(−1+α) to j(t)∼t(−1−α), indicating dispersive charge carrier transport influenced by an exponential trap distribution characterized by the dimensionless parameter α. A second kink is identified to be the point in time of the crossover from electron to hole dominated charge carrier transport, enabling for the determinat...

We investigate a solution-processable, non-toxic sodium chloride (NaCl) interlayer for aluminum cathodes in organic solar cells. The electron extraction at the cathode interface is strongly improved upon the insertion of NaCl leading to power conversion efficiencies of up to 2.9% as compared to 1.8% efficient devices without interlayer. Scanning electron and kelvin probe force microscopy studies reveal that the formation of NaCl crystals causes a decrease of the aluminum work function by more than 300 mV. By impedance spectroscopy, we found evidence that the NaCl crystals improve the energetic alignment at the polymer/metal interface.

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.

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.

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