Device operation of organic tandem solar cells

A generalized methodology is developed to obtain the current-voltage characteristic of polymer tandem solar cells by knowing the electrical performance of both sub cells. We demonstrate that the electrical characteristics of polymer tandem solar cells are correctly predicted for both the series and parallel connection of the sub cells. The agreement with experiments allows us to investigate the effect of a reduced open-circuit voltage, short-circuit current or fill factor in one of the sub cells on the performance of the tandem cell. A low fill factor in one of the sub cells leads to a stronger reduction of the efficiency in a series configuration as compared to the parallel tandem device. (C) 2008 Elsevier B.V. All rights reserved

Trap-limited electron transport in disordered semiconducting polymers

The electron transport in diodes of poly(dialkoxy-p-phenylene vinylene) (PPV) derivatives is strongly reduced as compared to the hole transport. A recent reexamination [M. M. Mandoc , Phys. Rev. B 73, 155205 (2006)] revealed that the room-temperature electron current shows the fingerprints of trap-limited transport with a distribution of traps in energy. Here, we report on the measured temperature dependence of the electron current in these PPV derivatives. This dependence is weak and seems to be in contradiction with existing trap-limited models. We demonstrate that the presence of a Gaussian density of states (DOS) for the mobile carriers, being characteristic for disordered semiconductors, reduces the temperature dependence of the trap-limited charge transport. The reduction is governed by the width of the Gaussian DOS and originates from the equilibrium concentrations of the mobile and trapped carriers

Fokker-Planck equation with variable diffusion coefficient in the Stratonovich approach

We consider the Langevin equation with multiplicative noise term which depends on time and space. The corresponding Fokker-Planck equation in Stratonovich approach is investigated. Its formal solution is obtained for an arbitrary multiplicative noise term given by $g(x,t)=D(x)T(t)$, and the behaviors of probability distributions, for some specific functions of $D(x)$% , are analyzed. In particular, for $D(x)\sim | x| ^{-\theta /2}$, the physical solutions for the probability distribution in the Ito, Stratonovich and postpoint discretization approaches can be obtained and analyzed.Comment: 6 pages in LATEX cod

Ultimate performance of polymer:fullerene bulk heterojunction tandem solar cells

We present the model calculations to explore the potential of polymer:fullerene tandem solar cells. As an approach we use a combined optical and electrical device model, where the absorption profiles are used as starting point for the numerical current-voltage calculations. With this model a maximum power efficiency of 11.7% for single cells has been achieved as a reference. For tandem structures with a ZnO/poly(3,4-ethylenedioxythiophene)/ poly(styrenesulphonic acid) middle electrode an ultimate efficiency of 14.1% has been calculated. In the optimum configuration the subcell with the narrowest band gap is placed closest to the incoming light. Consequently, tandem structures are expected to enhance the performance of optimized single cells by about 20%. © 2011 American Institute of Physics

Determination of the trap-assisted recombination strength in polymer light emitting diodes

The recombination processes in poly(p-phenylene vinylene) based polymer light-emitting diodes (PLEDs) are investigated. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs, which has not been considered until now. The dependence of the open-circuit voltage on light intensity enables us to determine the strength of this process. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED, without the traditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombination rate is found to be dominant over the free carrier recombination rate.

Thickness dependence of the efficiency of polymer:fullerene bulk heterojunction solar cells

We study the thickness dependence of the performance of bulk heterojunction solar cells based on poly[2-methoxy-5-(3('),7(')-dimethyloctyloxy)-1,4-phenylenevinylene] as electron donor and [6,6]-phenyl C-61 butyric acid methyl ester as electron acceptor. Typically, these devices have an active layer thickness of 100 nm at which only 60% of the incoming light is absorbed. Increasing device thickness results in a lower overall power conversion efficiency, mainly due to a lowering of the fill factor. We demonstrate that the decrease in fill factor and hence device efficiency is due to a combination of charge recombination and space-charge effects