Advanced Functional Materials and Devices (AFMD) Group

Abstract:

Recently, much effort has been put in the simulation of charge transport in organic materials and more and more sophisticated models with an increasing number of parameters are developed. In this paper, we show which models are necessary and applicable for the drift-diffusion modeling of IV curves of multilayer solar cells. For comparison to experiment, a data series of small molecule organic solar cells comprising several materials, architectures, and thicknesses of layers is used. The model is chosen as simple as possible to reduce the number of parameters and to allow for the identification and accurate investigation of the key processes in detail. As exemplary result, the spatial exciton generation profile is examined within the active blend layer: It does not only influence the short circuit current, but also the fill factor and open circuit voltage at constant short circuit current, especially in the case of imbalanced mobilities. In another example where interfacial barriers are present, broadening of the energy levels due to disorder has to be taken into account to describe the IV curves showing S-Shape like behavior with realistic parameters. Transient measurements of extraction barrier devices show indeed an accumulation of charge carriers as predicted by simulation. ©2010 IEEE.

Abstract:

A series of oligothiophenes that are end-capped with dicyanovinyl (DCV) and 1,3,2-(2H)-dioxaborine (DOB) moieties has been prepared using standard procedures. Their optoelectronic properties have been investigated by cyclic voltammetry and optical absorption. The optical absorption has been measured both in solution and thin film state. © 2010 Elsevier Ltd. All rights reserved.

Abstract:

We present 2,3,10,11-tetrapropyl-1,4,9,12-tetraphenyl-diindeno[1,2,3-cd:1′,2′,3′-lm]perylene (P4-Ph4-DIP) as new green donor for small molecule organic solar cells (SMOSC). P4-Ph4-DIP absorbs in the green spectral range of 490-580 nm and thus is a decisive building block towards broad coverage of the sun spectrum. SMOSC with P4-Ph4-DIP as donor and C60 as blue-absorbing acceptor display very high fill factors FF > 76% and open circuit voltages of 0.99 V, reaching η = 1.92% for 12 nm P4-Ph4-DIP. Current voltage and external quantum efficiency spectra suggest that an exciton diffusion length ED < 20 nm is the main limiting factor. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

We detect and measure the trap charges in a small molecular bulk heterojunction solar cell under operating conditions. The trap-charge density is estimated from capacitance measurements with light incident on the sample. At high intensities (∼1sun, 100mW/ cm2), the trapped charge concentration leads to a spatial distortion of the electric field in the device. The lower limit of the trap-charge density is estimated to be 6× 10 16 cm-3. The frequency dependence of the capacitance suggests that the charges are trapped in a manifold of deep states present in the energy gap of the semiconductors. The distortion of the electric field by this trap charge affects the charge-carrier collection efficiency. © 2010 The American Physical Society.