1 Department of Energy Conversion and Storage, Technical University of Denmark2 Functional organic materials, Department of Energy Conversion and Storage, Technical University of Denmark
Development of polymers for large scale roll-to-roll processing of polymer solar cells Conjugated polymers potential to both absorb light and transport current as well as the perspective of low cost and large scale production has made these kinds of material attractive in solar cell research. The research field of polymer solar cells (PSCs) is rapidly progressing along three lines: Improvement of efficiency and stability together with the introduction of large scale production methods. All three lines are explored in this work. The thesis describes low band gap polymers and why these are needed. Polymer of this type display broader absorption resulting in better overlap with the solar spectrum and potentially higher current density. Synthesis, characterization and device performance of three series of polymers illustrating how the absorption spectrum of polymers can be manipulated synthetically and how this affects the PSC parameters are presented. It is generally found that it is possible to synthetically control the absorption spectrum of conjugated polymer systems. One way to alter the spectrum is by incorporating alternating donor-acceptor motifs, resulting in an additional optical absorption band, the charge transfer (CT) band. A second approach is to introduce fused donor systems. A third method is to use several different monomer units in the polymerization hereby creating semirandom polymers with multiple chromophores. By changing the fed ratio of the monomers the absorption spectrum can effectively be tuned and a significant broadening of the absorption spectrum is obtained. A focus in this thesis is stabilization of the active layer morphology and the photochemical stability of its components. In terms of stability PSC degrades under illumination and the operational lifetime are generally limited. A fundamental understanding of the degradation of PSCs allows one to develop improved materials that can increase their lifetime. Synthesis and characterization of polymer materials for improved stability in PSCs is presented. Stabilization of the active layer was accomplished by incorporating different types of crosslinking functionalities into the polymer TQ1. Cross-linking was achieved by UV-light illumination to give solvent resistant films and reduced phase separation and growth of PCBM crystallites in polymer:PCBM films. This study showed that cross-linking can improve morphological stability but that it has little influence on the operational stability of the device. The photochemical stability of a wide range of materials relevant to PSC is presented and compared. General rules relative to the polymer structure–stability relationship are proposed and can be used as a guideline for further development of PSCs. One of the main advantages of PSCs is that they can be produced using printing techniques which allows for large scale roll-to-roll (R2R) production. A laboratory roll coater that enables solution processing of five layers on ITO-free flexible substrates using slot-die coating and flexographic printing is presented. As little as one ml of active material solution is needed to produce more than a hundred devices. This laboratory scale approach to PSCs was found to be directly scalable to the large scale R2R equipment making it suitable as a test platform for polymer development. PSC devices based on PDTSTTz-4 and PCBM were produced using the laboratory roll coater and through optimerization of the processing parameters a PCE of 2.95 % at ambient condition. This efficiency is among the highest obtained on flexible ITO-free substrates using slot-die coating.