Aharon Yakimov

Small molecular weight organic thin-film photodetectors and solar cells

In this review, we discuss the physics underlying the operation of single and multiple heterojunction, vacuum-deposited organic solar cells based on small molecular weight thin films. For single heterojunction cells, we find that the need for direct contact between the deposited electrode and the active organics leads to quenching of excitons. An improved device architecture, the double heterojunction, is shown to confine excitons within the active layers, allowing substantially higher internal efficiencies to be achieved. A full optical and electrical analysis of the double heterostructure architecture leads to optimal cell design as a function of the optical properties and exciton diffusion lengths of the photoactive materials. Combining the double heterostructure with novel light trapping schemes, devices with external efficiencies approaching their internal efficiency are obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of 1.0%±0.1% under 1 sun AM1.5 illuminati...

Degradation of hole injection at the contact between a conducting polymer and a fluorene copolymer

Hole injection was studied in devices that utilize the contact between poly[(ethylenedioxy)thiophene]/poly(styrenesulfonate) (PEDOT:PSS) and poly(9,9-dioctylfluorene-co-N,N’-bis(4-butylphenyl)-N,N’-diphenyl-1,4-phenylenediamine) (PFB). The transient space charge limited current injection technique was used to measure the hole mobility in the PFB layer and to estimate the hole injection efficiency at the contact. Prolonged electrical stressing of the devices did not affect hole mobility in PFB, while the injection efficiency decreased by an order of magnitude. These observations show that degradation occurs at the PEDOT:PSS/PFB contact, rather than the bulk of the PFB.

On the Origin of Double Light Spikes from Polymer Light-Emitting Devices

The two decades of intensive research following the discovery of EL in thin organic-film based device by Tang and VanSlyke,1 and later in polymer films by Burroughs et al2 brought significant improvements in their characteristics. Currently the organic EL is seen as a candidate for a new generation of fullcolor displays and lighting applications. Major efforts have been concentrated on improving device efficiency, color gamut, lifetime and patterning techniques for achieving the full color displays.

On the development of CdS properties upon processing CdTe devices

CdS plays an important role in CdTe devices through both the thickness impact on Jsc and the less well understood effect on FF and Voc by impacting the minority carrier lifetime in the CdTe near the interface. In this study, two techniques are explored, chemical bath deposition and radio frequency sputtering, effectively exploring the role of film density. A multitude of characterization techniques have been exploited on CdS films after various stages in the process of device fabrication as the CdS properties change significantly upon processing of the films. For this purpose we compared the properties for as-deposited films, films that are exposed to a CdTe deposition heating profile without the actual CdTe deposition, as well as CdS films as they appear within completed devices. Especially the CdCl2 process turned out to result in huge changes within the CdS film. From this study it is concluded that significant changes occur to the CdS film depending on its starting density. Change in electrical properties and intermixing occur during the CdTe deposition and are thus temperature related, while a change in optical properties and carrier lifetime near the interface is established after CdCl2.