Two-Dimensional Simulation of Organic Bulk Heterojunction Solar Cell: Influence of the Morphology

Abstract

Recent developments in organic solar cells show interesting power conversion efficiencies. However, with the use of organic semiconductors and bulk heterojunction cells, many new concepts have to be introduced to understand their characteristics. Only few models investigate these new concepts, and most of them are one-dimensional only. In this work, we present a two-dimensional model based on solving the drift-diffusion equations. The model describes the generation of excitons in the donor phase of the active layer and their diffusion towards an interface between the two separate acceptor and donor domains. Then, when the exciton reaches the interface, it forms a charge transfer state which can split into free charges due to the internal potential. Finally, these free charges are transported toward the electrodes within their respective domains (electrons in acceptor domain, holes in donor domain) before being extracted. In this model, we can follow the distribution of each species and link it to the physical processes taken into account. Using the finite element method to solve the equations of the model, we simulate the effect of the bulk heterojunction morphology on photocurrent curves. We concentrate on the morphology parameters such as the mean acceptor/donor domain sizes and the roughness of,the interface between the donor and acceptor domains. Results are discussed in relation with experimental observations.

Drift-Diffusion Studies of Compositional Morphology in Bulk Heterojunctions: The Role of the Mixed Phase in Photovoltaic Performance

“…Many studies have examined the effects of energetic disorder on OPV performance, usually assuming a Gaussian distribution of energetic states and/or an exponential tail of trap states in 1-D drift-diffusion simulations. [30,31] Other groups have focused on the impact of structural disorder and morphological features by performing 2-D drift-diffusion simulations to study the effects of phase separation and component arrangement on device performance [26,28,29,32,33]; we also have presented a 1-D method for examining structural disorder based on D-D simulations of ensembles of 1-D devices with position-dependent mobility profiles. [34] In this study, we present a set of 2-D drift-diffusion simulations specifically designed to probe the effects of the presence of a controllably varied mixed-composition, interfacial phase on device performance.…”

Organic solar cells: a rigorous model of the donor-acceptor interface for various bulk heterojunction morphologies

“…Since the validation of our model with experimental data was successful, 29 we compare it to two other existing models, called IL approach, and non-integral approach. This comparison has been carried out for different morphologies of the active layer.…”

“…(5) and (13), which involves the derivative of the interface shape (29), i.e., the quantity ds=dxj x¼x 0 . The latter globally increases with N per , enhancing then its contribution to the current density.…”

“…The validation stage with experimental data being already performed, 29 in a second part, we compare our integral approach to two other existing models also based on CT states. 9 domains, whereas the non-integral approach handles the interface boundary conditions in a simplified way.…”

Pattern formation aspects of electrically charged tri-stable media with implications to bulk heterojunction in organic photovoltaics

“…Indeed, even 1 nm changes in the acceptor or donor labyrinth width can lead to a significant decrease in overall efficiency [40,41]. Nevertheless, following the assumption that morphology evolution is much slower than other degradation mechanisms (e.g., oxidation), existing OPV models consider "frozen" morphologies [42][43][44][45]. Yet, since exciton dissociation is responsible for electrical charge and, thus, for electrical force, the interrelation between interface properties and charge separation kinetics may become equally important to BHJ evolution, including instability.…”