Both the device composition and fabrication process are well-known to crucially affect the power conversion efficiency of polymer solar cells. Major advances have recently been achieved through the development of novel device materials and inkjet printing technologies, which permit to improve their durability and performance considerably. In this work, we demonstrate the usefulness of a recently developed field-based multiscale solar-cell algorithm to investigate the influence of the material characteristics, like, e.g., electrode surfaces, polymer architectures, and impurities in the active layer, as well as post-production treatments, like, e.g., electric field alignment, on the photovoltaic performance of block-copolymer solar-cell devices. Our study reveals that a short exposition time of the polymer bulk heterojunction to the action of an external electric field can lead to a low photovoltaic performance due to an incomplete alignment process, leading to undulated or disrupted nanophases. With increasing exposition time, the nanophases align in direction to the electric field lines, resulting in an increase of the number of continuous percolation paths and, ultimately, in a reduction of the number of exciton and charge-carrier losses. Moreover, we conclude by modifying the interaction strengths between the electrode surfaces and active layer components that a too low or too high affinity of an electrode surface to one of the components can lead to defective contacts, causing a deterioration of the device performance. Finally, we infer from the study of block-copolymer nanoparticle systems that particle impurities can significantly affect the nanostructure of the polymer matrix and reduce the photovoltaic performance of the active layer. For a critical volume fraction and size of the nanoparticles, we observe a complete phase transformation of the polymer nanomorphology, leading to a drop of the internal quantum efficiency. For other particle-numbers and -sizes, we observe only a local perturbation of the nanostructure, diminishing the number of continuous percolation paths to the electrodes and, therefore, reducing the device performance. From these investigations, we conclude that our multiscale solar-cell algorithm is an effective approach to investigate the impact of device materials and post-production treatments on the photovoltaic performance of polymer solar cells.
Note that for our simulations, we chose the molecular architectures A5D15 and A3D12A3 for the block-copolymer systems, because they provide for the given set of parameters spherical morphologies in the absence of an external electric field, using the static SCFT method. This facilitated the comparison with a series of experimental and theoretical investigations discussed in our manuscript, which used the spherical morphologies as starting structures for the post-production treatment.
We note that equal mobilities for both types of charge carriers can be realized without affecting the bulk heterojunction morphology by, e.g., adding a small amount of dopant to the active layer of the PSC device.82 In this context, it is also worth mentioning that from various experimental investigations,83,84 it has been concluded that ensuring a balanced electron–hole mobility is an important factor for optimizing the performance of PSC devices, because it permits to suppress the build-up of the space-charge that will significantly reduce the power conversion efficiency.84