Glass-forming ternary blends: towards stable Polymer Solar Cells

Doctoral thesis, 2017

The globally increasing use of electricity goes hand in hand with climate change and the gradual depletion of fossil sources of fuel. To address these challenges renewable sources of energy are in high demand. Solution-processable organic solar cells receive particular attention because they promise to combine a set of highly attractive features including low manufacturing cost through large-area and continuous printing, as well as low weight, flexibility and semitransparency. The stability of the light-harvesting organic photovoltaic materials, which typically consist of a finely mixed blend of an electron donor and acceptor, plays a key role in the development of efficient and durable organic solar cells. One essential condition for both high-yield production and a long lifetime is excellent thermal stability. The organic photovoltaic material must be able to withstand high fabrication and operation temperatures. The aim of this thesis is to explore the use of ternary blends as a tool to improve the often insufficient thermal stability of organic photovoltaic materials. Ternary blends are a relatively new concept within the field of organic photovoltaics. This thesis focuses on blends of a donor polymer and a mixture of the two most common neat fullerenes, C60 and C70. Processing of the neat fullerene alloy is facilitated through a highly advantageous increase in solubility, which is found to correlate with the increase in entropy upon mixing. As a result, solar cells with a power conversion efficiency of 6 % are realized, a record for devices based on neat fullerenes. A high tendency for glass formation of polymer:C60:C70 ternary blends is found to induce a high degree of thermal stability due to a glass transition temperature in excess of 200°C. Vitrification of ternary blends is discussed in terms of the entropy of mixing, which reduces the rate of both crystal nucleation and growth. Finally, this thesis provides an overview of the current state-of-the-art, discussing both fullerene as well as fullerene-free ternary blends.