Time delayed collection field experiments on polymer: fullerene bulk-heterojunction solar cells

The recombination of photogenerated charge carriers in poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene]:1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C61 bulk-heterojunction solar cells is investigated using the time delayed collection field technique. Here the lifetime of photogenerated electrons and holes that have escaped charge recombination can be determined from current measurements using a pulsed collection voltage that is delayed with respect to the excitation pulse. At 80 K, the number of long lived charge carriers decays in time according to t– with =0.2, practically independent of laser fluence in the range of 1–1000 µJ/cm2. For excitation densit

Monte-Carlo simulations of geminate electron-hole pair dissociation in a molecular heterojunction: a two-step dissociation mechanism

The Monte-Carlo simulations are used to investigate the dissociation of a Coulomb correlated charge pair at an idealized interface between an electron accepting and an electron donating molecular material. In the simulations the materials are represented by cubic lattices of sites, with site the energies spread according to Gaussian distributions. The influence of temperature, applied external fields, and the width of the Gaussian densities of states distribution for both the electron and the hole transporting material are investigated. The results show that the dissociation of geminate charge pairs is assisted by disorder and the results can be understood in terms of a two-step model. In the first step, the slow carrier in the most disordered material jumps away from the interface. In the following, second step, the reduced Coulombic attraction allows the faster carrier in the less disordered material to escape from the interface by thermally activated hopping. When the rate for geminate recombination at the interface is very low

An effective area approach to model lateral degradation in organic solar cells

In standard unencapsulated poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester solar cells exposed to humid air, the oxidation of the aluminum cathode is known to be a key degradation mechanism. Water that enters the device at the edges and through pinholes diffuses to the organic–electrode interface. The forming oxide acts as a thin insulating layer that gives rise to an injection/extraction barrier and leads to a loss in the device current. In order to understand this behavior in detail various steady-state, transient, and impedance measurement techniques are performed in combination with drift-diffusion simulations. With this combinatorial approach the dominant degradation mechanism is confirmed to be the development of a blocking interface layer. This layer grows laterally leading to a loss in effective area due to the rapid local oxidation of the aluminum layer. Thus by combining multiple electrical techniques and optoelectrical simulations the dominant degradation mechanism can be evaluated. The same methodology is also beneficial for more stable and efficient novel solar cells