Synthesis and Applications of Novel Ladder Polymers for Organic Solar Cells

Abstract

Global environmental and resource concerns dictate that future energy supply and security will become increasingly dependent upon the development of accessible, sustainable and scalable energy technologies. State-of-art polymer solar cells (PSCs) has been considered as one of the renewable important technologies which can harvest solar energy from sunlight to generate electricity. Intensive research efforts from both academia and industry have been dedicated into solution-processed organic solar cells due to development of the next-generation solar cells technology 1,2. Owing to the readily available carbon feedstock as well as the numerous and flexible synthetic pathways, polymer solar cells (PSCs) gained tremendous attentions over silicon solar cell in the past decay due to low- cost and quick energy pay-back, solution-processable, lightweight, and flexible/stretchable, large area photovoltaic panels. 1,2 So as to achieve the high performance solar cells it is very important to develop novel kinds of active materials, which have to cover entire solar spectrum i.e. from ultraviolet to infrared (IR) regions, suitable molecular energy levels morphologies and high mobilities. Several donor-acceptor (D-A) conjugated polymers are reported recently with photovoltaic performance over 10%. 3 However, in D-A PSC materials have high intrinsic torsional defects, which impacts the negative impact on performance of the OPV devices. The torsional defects partially break the conjugation pathways of the polymers, leading to shortened coherent lengths along the polymer chain and decreased carrier mobilities. Meanwhile, the torsional defects perturb the intermolecular packing of the polymer materials so that the electronic coupling between the polymer chains are interrupted, adding an energy barrier for the charge carriers and excitons to transport within the active layer. 4 Moreover, the torsional defects increase the band gap of conjugated polymers, hence to prevent their photo-absorption in longer wavelength region. Overall the torsional defects often lead to larger π–π stacking distances in the polymer thin film, making the thin film more susceptible to the permeation of oxygen and water, hence decreasing the stability of the overall OPV devices. Our approach looks into ways to overcome the drawbacks raised by torsional defects on a fundamental level. By definition, Ladder polymers consist of cyclic subunits, connected to each other by two links that are attached to different sites of the respective subunits, comparable to a graphene nanoribbon. Consequently, ladder polymers have two or more independent strands of bonds which are tied together regularly without merging to a single or double bond or crossing each other. 4 As a result, ladder polymers have large planar core structures with no torsional defects. Such defect-free feature grants them with rigid and hence highly conjugated core structures. On one hand, the highly conjugated cores not only afford low band gaps that allow strong optical absorption at long wavelength in terms of energy absorption, but also lead to low beta value for coherent tunneling and low activation energy for electron hopping, in terms of charge transport alongside the polymer chains. There were many carbazole-containing organic D-A polymer materials has been demonstrated for high performance solar cell applications and no such types of ladder polymers reported by utilizing carbazole core. 5 Herein, we report the synthesis of fully conjugated carbazole-based ladder polymer with low level of unreacted defects, by utilizing the controlled ring-closing olefin metathesis (RCM) reaction. The designed ladder polymer is well soluble in common organic solvents for solution processability. We also discussed the photo-physical, electrochemical and optoelectronic properties of torsional defect-free ladder polymers.