Polymer materials for roll coated solar cells: strategies tom improve performance and stability

Solar cells are among the renewable energy technologies with a large potential in terms of solar energy availability. The solar cells based on conjugated polymers belong to the third generation of this technology and their attractive features include a fast and cheap solution‐processed production. At DTU Energy the focus is the roll‐to‐roll coating process of these materials in order to reach large area devices, as the processability and scalability of the technology is an important factor. The process ability using roll‐coating techniques and the stability of the used materials can be crucial. Therefore this project focuses on the synthesis of conjugated polymers and their application in roll‐coated polymer solar cells. The first part of this project aims at using a screening strategy to find suitable polymer candidates for well performing solution processed polymer solar cells. A large number of polymers was screened by applying them in roll‐coated solar cells and their performance, stability and number of synthetic steps was compared, to find promissing candidates. In the end seven polymers with a sufficient efficiency were found to behave in a higher or in similar manner as poly(3‐hexylthiophene). Further polymers were prepared based on well‐performing benzothiadiazole and thiophene based polymers with different incorporation ratios of these monomers. The incorporation ratio has different effects on the polymer properties and the performance and stability of the corresponding roll‐coated devices. The best efficiency was achieved with a polymer by using an incorporation of four thiophenes in the repeating unit. The second part of the work aims at using a known strategy to improve the solar cells stability. Three of the polymers from the polymer screening were therefore partly modified with stabilizing side chains, 2‐phenetyl and 2‐ethanol, respectively, to influence especially the device stability but also the performance. For most modifications a decrease of the solar cell efficiency was observed. The incorporation of 10% of these side chains show improvements of the stability of devices in a minor degree with a variation in the photo‐ and thermal stability. In addition to the use of different side chains, the impact of different positioning of one side chain was investigated, showing that the incorporation onto the acceptor or donorunit of the polymer showed a degradation or improvement of the resulting properties. In addition, the approach of side chain removable on polythiophene was compared in terms of optical properties and morphologies of two polymers with different (thermal or acidic) cleavage processes. It was found that their properties were not the same and therefore different results from the corresponding solar cells can be expected.