Understanding morphology and photo- stability of organic solar cells via advanced structural probes

The development of organic solar cells as a novel form of renewable energy has been driven by their potential for low-cost, large-scale fabrication though the solution-processing of semiconducting polymers and small molecules. Certified power conversion efficiencies have reached 13% as of 2016 thanks to the development of new donor-acceptor molecules, but the efficiency of any given device is still highly sensitive to the morphology that these materials adopt during deposition. It is essential that morphology is characterized thoroughly in order to establish the relationships between molecular structure, morphological properties and device performance; in order to maximise efficiency and make organic solar cells an economically competitive source of renewable energy. In this thesis, several spectroscopic techniques are used to probe the impact of various processing parameters on the molecular order, crystallinity and phase separation of polymer:fullerene blends. For the model blend system P3HT:PCBM, P3HT molecular order can be measured by resonant Raman spectroscopy, and PCBM is found to dissolve in the amorphous domains of the semi-crystalline polymer up to a miscibility limit of 25 %wt, above which it can only be accommodated by increased disorder. In situ annealing demonstrates that when heated above a glass transition temperature of ~50C, disordered blends separate into purer domains of high molecular order that correlate well to improved charge transport and efficiency for thermally annealed devices. Raman spectroscopy is also used to probe the stability of the high-efficiency PTB7:PC70BM blend. Photo-oxidation of PTB7 was found to induce specific vibrational changes that correlate to formation of a hydroxyl group on the benzodithiophene unit. In situ experiments reveal that hydroxylation precedes the loss of chromophores that results in deterioration of device performance, and is accelerated by blending with PC70BM. Understanding the impact of morphology on charge extraction from the active layer requires the selective probing of interfacial properties at the top and bottom of the organic film, which we demonstrate using SERS. For both a polymer:fullerene blend (PTB7:PC70BM) and a polymer:polymer blend (P3HT:F8TBT), spin-coated films exhibit interfacial compositions different from that of the bulk film and favourable to charge extraction from inverted device architectures, but can be modified by pre- or post-annealing treatments. Finally, we investigated the morphology of a novel low band-gap polymer, a tellurium analogue of polythiophene, in order to understand the impact of the heavy atom on chain planarity and polymer crystallinity. The Raman spectrum of P3ATe exhibited a much stronger sensitivity to molecular order, which was highly dependent on the length and linearity of the alkyl side chain, but there was no clear morphological reason why P3ATe reportedly performs poorly compared to P3HT, despite the superior absorption of its smaller band-gap.Open Acces