Abstract
Organic photovoltaics (OPVs) can potentially provide a cost-efficient means of harnessing solar energy. However, optimum OPV performance depends on understanding the process–structure–property (PSP) correlation in organic semiconductors. In the working of bulk-heterojunction OPVs, the morphology plays a crucial role in device performance. In order to understand PSP linkage, a theoretical framework has been developed. We first established process–structure correlations by generating a range of morphologies with various blend ratios of donor and acceptor organic semiconductors for various annealing periods. Second, we calculated the effective electronic properties corresponding to the simulated structures using a diffuse interface approach that is numerically more robust and straightforward than the classical sharp interface method. This novel framework, wherein both the process–structure and the structure–property relationship have been established using the diffuse interface approach, completes the theoretical PSP linkage, allowing the optimization of process parameters for device applications. The theoretical PSP linkage is then benchmarked qualitatively with experimental results on a model P3HT:PCBM system. We have been able to identify the morphological characteristics that maximize device performance. This work is carried out in the broad overview of the integrated computational materials engineering framework wherein the processing parameters are optimized by determining the process–structure–property relationships.
Highlights
The fabrication of organic solar cells is mainly carried out at low temperature using cost-efficient solvent-based techniques.1 Solution-processed bulk-heterojunction (BHJ) organic photovoltaics (OPVs) came into being in 1995, offering the advantages of low-cost processing, a higher interfacial area that aids in exciton dissociation, and a thicker photoactive layer leading to greater absorption of the incident light.2,3 Fabrication of Organic photovoltaics (OPVs) is in contrast to the energy-intensive production of their inorganic counterparts
As the p- and n-type semiconductors are brought in contact, the holes flow from the p-type to the ntype semiconductor, as governed by the chemical potential gradient, whereas the electrons flow in the opposite direction
We find that the necessary condition for fabricating an efficient OPV device is the existence of a bi-continuous network of donor and acceptor phases as this leads to percolating channels for electrons and holes to their respective electrodes
Summary
The fabrication of organic solar cells is mainly carried out at low temperature using cost-efficient solvent-based techniques. Solution-processed bulk-heterojunction (BHJ) organic photovoltaics (OPVs) came into being in 1995, offering the advantages of low-cost processing, a higher interfacial area that aids in exciton dissociation, and a thicker photoactive layer leading to greater absorption of the incident light. Fabrication of OPVs is in contrast to the energy-intensive production of their inorganic counterparts. OPVs have the potential for high energy return on investment Another advantage of organic materials is that they have a high optical absorption coefficient. During spin-coating, the solvent evaporates, resulting in the D–A materials spontaneously phase-transforming to D and A domains within the active organic-semiconductor layer. In some of the models, the active layer or the BHJ has been treated as a material having effective electron and hole properties such as mobility and carrier density. Kirchartz et al. developed a one-dimensional electro-optical model for BHJ solar cells, considering the active layer to be a homogeneous material. Ray et al. applied the model developed by Buxton and Clarke to simulate the effect of annealing on device performance for one particular blend ratio of the donor and acceptor. We briefly describe the working mechanism within the active organic layer in order to appreciate the importance of morphology and, its effect on electronic properties
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