Nico Christ

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.

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.

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.

ac excitation of organic light emitting devices utilizing conductive charge generation layers

The ac-field-induced charge carrier generation and recombination is studied in an asymmetric conjugated polymer light emitting device based on one injecting electrode and one electrode separated by an insulating layer. Under operation with 130 kHz light intensity emitted by the device exhibits a pronounced asymmetry with respect to the polarity of the electric field. Numerical simulations show that this behavior can be explained by the significantly different mobilities of electrons and holes in the conjugated polymer. This approach enables insight into the device physics of organic light emitting devices with internal charge generation and allows direct investigation of hole and electron mobilities in one single device.

RC-Constant in Organic Photodiodes Comprising Electrodes With a Significant Sheet Resistance

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.

Loss processes in organic double-heterostructure laser diodes

At high current densities, the characteristics of organic laser diode structures are strongly influenced by a variety of loss processes such as bimolecular annihilations, field-induced exciton dissociation and induced absorptions due to polarons and triplet excitons. Here, we investigate a TE2-mode organic double-heterostructure laser diode by numerical simulation. The electrical properties are described using a numerical drift-difusion model and the optical characteristics are modeled using a transfer matrix method. When annihilation processes are included, a threshold current density of 8.5 kA/cm2 is derived for the considered device. Laser operation is not achieved when field-induced exciton dissociation is considered. For induced absorptions, maximum relative cross sections of 9.6 × 10-8 for polarons and 1.4 × 10-4 for triplet excitons have been calculated, which would still allow laser operation. For higher relative absorption cross sections, laser operation is suppressed for all current densities. Furthermore, the impact of field quenching is analyzed and the separation of singlet excitons from polarons and triplet excitons in the time domain is studied.

Extracting the charge carrier mobility from the nanosecond photocurrent response of organic solar cells and photodiodes

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.