The effect of active layer thickness and composition on the performance of bulk-heterojunction solar cells

At present, bulk heterojunction polymer solar cells are typically fabricated with an active layer thickness of between 80 and 100 nm. This active layer thickness has traditionally been chosen based on convenience and empirical results. However, a detailed study of the effects that active layer thickness has on the short circuit current and efficiency has not been performed for bulk heterojunction polymer solar cells so far. We demonstrate that the performance of these devices is highly dependent on the active layer thickness and, using a well established model for optical interference, we show that such effects are responsible for the variations in performance as a function of active layer thickness. We show that the ideal composition ratio of the donor and acceptor materials is not static, but depends on the active layer thickness in a predictable manner. A comparison is made between solar cells comprised of the donor materials regioregular poly(3-hexylthiophene) and poly(2-methoxy-5-(3^′,7^′-dimethyloctyloxy)-p-phenylenevinylene) with the acceptor [6, 6]-phenyl C₆₁-butyric acid methyl ester to show that our results are not material specific and that high efficiency solar cells can be fabricated with active layer thickness greater than 100 nm for both material mixtures. Finally, a device with an active layer thickness of 225 nm is fabricated with a power efficiency of 3.7% under AM1.5 illumination at an intensity of 100 mW/cm².