Simulation based characterization and performance enhancement of heterogeneous polymer solar cells
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Polymer based photovoltaic devices have the potential for widespread usage due to their low cost-per-watt and mechanical flexibility. Efficiencies close to 10 % have been achieved recently in conjugated polymer based organic solar cells. These devices were fabricated using solvent-based processing of electron-donating and electron-accepting materials into the so-called bulk heterojunction (BHJ) architecture. Experimental evidence suggests that a key property determining the power-conversion efficiency of such devices is the final morphological distribution of the donor and acceptor constituents. In order to understand the role of morphology on device performance, we develop a scalable computational framework that efficiently interrogates organic solar cells to investigate relationships between the morphology at the nano-scale with the device performance.
In this work, we extend the Buxton and Clarke (2007) model to simulate realistic devices with complex active layer morphologies using a dimensionally independent, scalable, finite element method. We incorporate all stages involved in current generation, namely 1) exciton generation and diffusion, 2) charge generation, and 3) charge transport in a modular fashion. The numerical challenges encountered during interrogation of realistic microstructures are detailed. We compare each stage of the photovoltaic process for two microstructures -- a bulk heterojunction morphology and an idealized sawtooth morphology. The results are presented for both two and three dimensional structures.
A comprehensive sensitivity analysis of the short-circuit current, Jsc, to the input parameters is performed. This helps in rank ordering the input parameters and operating conditions -- by strength and relevance -- on their impact on Jsc. We particularly focus our investigations on understanding how the active layer morphology affects the sensitivity of Jsc. To accomplish this we analyze three classes of morphologies: bilayer, BHJ, and sawtooth. The results show significant differences in sensitivities between BHJ, sawtooth and bilayer morphologies. Short-circuit current in BHJ structure shows higher sensitivity to material properties than either sawtooth and bilayer structure, suggesting the necessity for finer control of material properties to counteract the increased disorder in the active layer morphology. The electrode current is found to be most sensitive to illumination intensity for all three morphologies. We report some interesting trends that may help choose the most sensitive parameters to vary for designing OSC's with better performance.
It is known that morphology plays a critical role in determining the PCE. Morphology control and tailoring has been the focus of extensive research over the past decade, driven by the hypothesis that morphologies having interdigitated columnar domains will make optimal devices. Here we disprove this hypothesis by computationally discovering a family of morphologies that perform better than the hitherto thought optimal morphology by as much as 25 %. This family of morphologies are simply topological perturbations to the interdigitated columnar morphology, thus maintaining most of the features -- connectivity and domain sizes -- exhibited by the latter. The discovery of a whole class of morphologies with improved performance will have an impact on the field of organic photovoltaic from molecular design and fabrication tailoring to the question of discovering the optimal morphology. These results also allow us to posit that wrinkling or buckling along the thickness dimension can enhance the performance of micro-scale interdigitated columnar morphologies, which are easier to scalably fabricate compared to nano-scale columns.