In the past two decades the third generation of photovoltaics – also referred to as plastic solar cells – have been the subject of intense research. This attention has roots in the fact that the active components in this technology are polymers or small organic molecules which can be dissolved and used as a printing ink. This makes it possible to manufacture these photovoltaic cells by printing and/or coating at high speeds, in ambient atmosphere (no vacuum, low heating).
Such processing, referred to as roll-to-roll (R2R) processing, is a pathway to a cheap, flexible and lightweight energy harvesting technology. However such realizations have yet to be achieved because numerous materials used in lab scale devices are not compatible with R2R processing. A well-known example is Indium Tin Oxide (ITO) a transparent conductive material widely used as electrode. ITO is a rare material and therefore is expensive. It also needs to be deposited by sputtering and is extremely brittle. For this reason recent years have led many research groups to develop novel electrodes (silver, gold, graphene…) complying with the above-mentioned properties.
Recently a team from the Technical University of Denmark (DTU) lead by Prof. F. C. Krebs has developed various ITO-free electrodes for large scale R2R manufacturing of organic solar cells. Two types of photovoltaic modules were manufactured using a novel silver nanowire (AgNW) electrode (57 cm2) and a carbon electrode (30 cm2). A high efficiency polymer PBDTTTz-4 was used as active material. Contrary to many other high performance polymers, these modules retained their efficiency showing that PBDTTTz-4 is a good candidate for upscaling. During indoor and outdoor stability studies the AgNW modules show improvement in lifetime compared to the carbon based ones. This higher stability is linked to the removal of the PEDOT:PSS in the front electrode.