Jan Mescher

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 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.

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.

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.

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.

Dispersive transport in the temperature dependent transient photoresponse of organic photodiodes and solar cells

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...

Position Sensing by Transient Photocurrents of Organic Photodiodes

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 μm at the edges and 12 μm at the center of an 8000-μm long device.

Understanding the angle-independent photon harvesting in organic homo-tandem solar cells.

The effective device photo current of organic tandem solar cells is independent of the angle of light incidence up to 65°. This feature renders these devices particularly suitable for stationary applications where they receive mainly indirect light. In a combined experimental and simulative study, we develop a fundamental understanding of the causal absorption and charge generation mechanisms in organic homo-tandem solar cells. A 3-terminal tandem device architecture is used to measure the optoelectronic properties of both subcells individually. The analysis of the angle dependent external quantum efficiencies of the subcells and the tandem device reveal an internal balancing of the wavelength dependent subcell currents elucidating the low sensitivity of the tandem device properties on the angle of incidence.

Compensation for RC-effects in organic photodiodes with large sheet resistances

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.

Spatial mapping of photocurrents in organic solar cells comprising wedge-shaped absorber layers for an efficient material screening

We demonstrate a new organic solar cell fabrication and characterization technique that allows for a quick screening of new materials and material combinations (i.e. blends) as active layers for solution processed organic solar cells with respect to the optimization of the active layer thicknesses, thereby saving precious material resources. Therefore we bar coat wedge-shaped layers by “horizontal dipping”. The photocurrent under short circuit conditions, the external quantum efficiency and the absorption of those wedge-shaped layers were then recorded spatially resolved. From the results the device photocurrent vs. the layer thickness can be extracted allowing for conclusions about the optimum absorber layer thickness.