New material concepts for organic solar cells

The current work investigates two fundamental problems of small molecule organic solar cells (OSC): transparent top contacts and alternative donor materials. Transparent top contacts are a prerequisite for inverted, i. e. top-illuminated OSC on opaque substrates. This work documents that transparent, ultra-thin metal films are a possible solution to this challenge. It is shown that silver or gold layers with thicknesses < 20nm are sufficiently transparent and conductive to fabricate suitable OSC. Utilisation of nanometer-thin Al interlayers between Ag or Au and the organic underlayers allow for considerable improvement of morphological and electrical properties of the top contacts. Organic capping layers are presented that strongly improve light incoupling through the metal films into the OSC devices. Highly efficient semitransparent tandem OSC in module size were created by employing optimised light incoupling layers and Al surfactants. Three diindenoperylene derivatives are introduced as green donor materials. Although these experiments are still at a very early stage, OSC are fabricated that exhibit high voltages of 1V and extraordinary fill factors of over 76%.