Polymer solar cells have gained considerable interest over the last few decades as a low-cost, environmentally friendly, lightweight, and flexible alternative to traditional photovoltaics. In recent years, photovoltaic performance has increased rapidly, with high power conversion efficiency for lab-scale devices. This work focuses on the design and synthesis of new conjugated polymers, as well as the thermal stability of polymer solar cells. Thermally stable materials are important for stable solar cells, as solar cells are often exposed to elevated temperatures during fabrication and use. Our work includes device stability measurements and morphology stability studies using dynamic mechanical thermal analysis (DMA). The DMA technique is a highly sensitive method for determining the thermal transitions of the used materials.
Several completely amorphous indacenodithiophene-based polymers with varying side chains were synthesised and used in solar cells combined with the Y6 acceptor material. Low donor:acceptor (D:A) ratios are generally believed to yield lower efficiency than the more conventional 1:1.2 ratio. However, several of the solar cells exhibit a peak performance over 11% PCE at a lower D:A ratio.[1] Unexpectedly, as the polymer proportion increases, a reduced photovoltaic performance is observed. Importantly, our recent experiments also show that the thermal stability of the solar cells at 85oC is significantly improved for a low donor:acceptor ratio (1:10), outperforming commercial high-performance polymer systems in efficiency after 30 days of stability measurements.