Stability of Bulk-heterojunction blends for Solar Cell Applications

Doctoral thesis, 2014

Polymer solar cells are a promising alternative to more traditional silicon solar cells. This is mainly due to the good solubility of organic semiconductors, which makes it possible to produce large-scale and mechanically flexible devices with roll-to-roll processes. To be able to fully utilise this promising technique the stability of the materials, used in these devices, must be guaranteed. The focus of this thesis is the stability of the active layers of polymer solar cells. Both, bleaching due to photo-oxidative degradation and thermal stability of the nanostructure have been studied. The presented work is mostly based on blends of a thiophene-quinoxaline based polymer (TQ1) and fullerene derivatives (PCBM). The first part of the thesis deals with the photo-oxidative stability of TQ1 and a pyrido pyrazine based polymer (TQN). To make those polymers more black they were co-polymerised with thiophene-hexylthiophene. The stability of TQN is shown to be un-effected by this incorporation whereas the stability of TQ1 decreased. Moreover, the degradation rate of TQ1 seems to be independent of both molecular weight and film thickness. The stability of the nanostructure has been studied with various microscopy and spectroscopy methods. Below the glass transition temperature of the TQ1:PC61BM blend only local rearrangement of polymer chains is possible. This mild annealing is found to increase the device efficiency. In contrast, annealing at higher temperatures above the glass transition temperature led to a coarser nanostructure and formation of PCBM crystals, which was detrimental for the performance of corresponding solar cells. Finally, this thesis demonstrates that the thermal stability of these blends can be significantly improved by inclusion of neat C60-fullerene as well as the use of a mixture of two fullerene derivatives as the acceptor material.