Moritz Hein

Imbalanced mobilities causing S-shaped IV curves in planar heterojunction organic solar cells

We show that S-kinks in the current voltage characteristics, which decrease the fill factor significantly, can be caused by a strong imbalance of charge carriermobilities(hole mobility in donor and electron mobility in acceptor) in planar/flat heterojunction organic solar cells. Electrical simulations according to a drift-diffusion model predict the occurrence of an S-kink for a mobility mismatch factor larger than 100. By combining a low-mobility donor material, (1,2,3,4,9,10,11,12-octaphenyl-diindeno[ 1 , 2 , 3 -cd : 1 ′ , 2 ′ , 3 ′ -lm]perylene), with the acceptors C 60 and N , N ′ -dimethylperylene-3,4:9,10-dicarboximide, which show different electron mobilities, we experimentally verify the predictions. Our results demonstrate that not only interfaceeffects but also the photoactive material itself can cause S-kinks.

Comparative Study of Microscopic Charge Dynamics in Crystalline Acceptor-Substituted Oligothiophenes

By performing microscopic charge transport simulations for a set of crystalline dicyanovinyl-substituted oligothiophenes, we find that the internal acceptor-donor-acceptor molecular architecture combined with thermal fluctuations of dihedral angles results in large variations of local electric fields, substantial energetic disorder, and pronounced Poole-Frenkel behavior, which is unexpected for crystalline compounds. We show that the presence of static molecular dipoles causes large energetic disorder, which is mostly reduced not by compensation of dipole moments in a unit cell but by molecular polarizabilities. In addition, the presence of a well-defined π-stacking direction with strong electronic couplings and short intermolecular distances turns out to be disadvantageous for efficient charge transport since it inhibits other transport directions and is prone to charge trapping.

Molecular doping for control of gate bias stress in organic thin film transistors

The key active devices of future organic electronic circuits are organic thin film transistors (OTFTs). Reliability of OTFTs remains one of the most challenging obstacles to be overcome for broad commercial applications. In particular, bias stress was identified as the key instability under operation for numerous OTFT devices and interfaces. Despite a multitude of experimental observations, a comprehensive mechanism describing this behavior is still missing. Furthermore, controlled methods to overcome these instabilities are so far lacking. Here, we present the approach to control and significantly alleviate the bias stress effect by using molecular doping at low concentrations. For pentacene and silicon oxide as gate oxide, we are able to reduce the time constant of degradation by three orders of magnitude. The effect of molecular doping on the bias stress behavior is explained in terms of the shift of Fermi Level and, thus, exponentially reduced proton generation at the pentacene/oxide interface.

Correlation between temperature activation of charge-carrier generation efficiency and hole mobility in small-molecule donor materials.

In organic solar cells, free charge carriers are generated at the interface between an electron-donating and an electron-accepting material. The detailed mechanisms of the generation of free charge carriers are still under discussion. In this work, we investigate the influence of temperature on the generation efficiency of free charge carriers in blends of dicyanovinyl substituted oligothiophene (DCVnT) molecules and C60 by quasi-steady-state photoinduced absorption (PIA) measurements. The observed positive temperature dependence of charge-carrier generation can be directly correlated to the charge-transport behavior. The determined activation energy scales inversely with the hole mobility for all investigated DCVnT derivatives, suggesting higher dissociation probability of bound interfacial charge pairs at high mobility. Furthermore, the energetic disorder parameter, σ, determined by CELIV (charge extraction by linearly increasing voltage) measurements for a DCV6T derivative, matches the activation energy from the PIA measurements. In conclusion, these results underline the need for high-mobility donor materials for optimal charge-pair dissociation in organic solar cells.

Improved photon harvesting by employing C70 in bulk heterojunction solar cells

To achieve higher efficiencies in organic solar cells, ideally the open circuit voltage (VOC), fill factor (FF) as well as the short current density (JSC) have to be further improved. However, only a few suitable acceptor molecules, e.g. C60, are currently available for the photoactive layer. Despite a good electron mobility on the order of 1×10-3 cm2/Vs the absorption of C60 in the visible sun spectrum is low. From polymer based solar cells it is known that the fullerene derivative [70]PCBM used in the photoactive layer shows a significant enhancement in JSC compared to [60]PCBM. This work investigates the application of fullerene C70 as acceptor in comparison to the well known C60 in vacuum processed small molecule solar cells. C70 shows a broadened and red shifted absorption (abs. maximum around 500 nm) compared to C60. By fabricating p-i-i solar cells we show that the stronger absorption of C70 leads to enhanced photon harvesting and increased external quantum efficiency. The bulk heterojunction p-i-i solar cell containing C70 as acceptor and ZnPc as donor, co-evaporated with an optimized ratio of 2:1, and a layer thickness of 30 nm shows improved solar cell parameters: a 30% larger photocurrent of 10.1 mA/cm2 is obtained. The VOC of 0.56 V and FF of 55% remain comparable to C60-containing p-i-i solar cells. Therefore, the solar cell performance is mainly improved by JSC and leads to a mismatch corrected power conversion efficiency of 3.12%. Thus, we show that C70 is an alternative fullerene to C60 for solar cell applications.