Engineering of active layer nanomorphology via fullerene ratios and solvent additives for improved charge transport in polymer solar cells

Polymer solar cells (PSCs) are promising candidates for future photovoltaic devices [1-8] owing to their low cost of material production and device processing, low temperature processing for flexible devices, and ease of fabrication [7,9-11]. However, due to their lower lifetime and efficiency, PSCs require deeper understanding to eradicate the aforementioned drawbacks. One of the most important parameters that influence charge transport and recombination dynamics in bulk heterojunction solar cells is the nanomorphology [12-15]. In the past, several methods, such as polymer:fullerene weight ratio, thickness of active layer, charge-selective interfacial layers, choice of solvents [16], solvent additives [17], solvent annealing, and thermal annealing [7] have been utilized to obtain optimized morphology to attain higher power conversion efficiency. Also, over the last decade, several groups have been working on defining and resolving different aspects of morphology, such as domain size, purity, crystallinity, miscibility, interface sharpness/roughness, lateral and vertical phase separation, or aggregation [12,18-22]. In most cases, such morphological attributes are correlated with device performance in steady-state illumination, and there is still a lack of understanding of the effect of domain size, purity, and their spatial distribution on the charge transport and recombination behavior. Bimolecular recombination has widely been shown to be the dominant recombination mechanism in bulk heterojunction solar cells [23]. Hence, there is a need to understand the role of processing conditions in order to achieve optimal nanomorphology to suppress the bimolecular recombination in bulk heterojunction solar cells.