Influence of permittivity and energetic disorder on the spatial charge carrier distribution and recombination in organic bulk-heterojunctions

Abstract

In bulk-heterojunction organic solar cells the low permittivity in combination with the spatial and energetic disorder of the organic materials lead to a complex behavior of charge carriers within the active layer. Charges originate from exciton splitting at the heterojunction interface and the successive interplay between mutual Coulomb interactions and transport through the disordered organic can lead to insufficient separation from the interface, increased interface densities with respect to the bulk regions and, hence, affect recombination. To further understand the mechanisms of recombination, insight into the explicit spatial distribution of charge carriers within the blend is crucial. We performed kinetic Monte Carlo simulations on a bulk-heterojunction organic solar cell to assess the effect of Coulomb interactions and energetic disorder on the three-dimensional spatial distribution of charge carriers and highlight the correlation with both geminate and non-geminate recombination. We show that for materials with low permittivity and large energetic disorder the charge distribution is strongly inhomogeneous with accumulation along the heterojunction interface. In such cases recombination is not limited by recombination partners finding each other but rather an interface controlled process where geminate recombination dominates over nongeminate recombination.

Generalized Kinetic Monte Carlo Framework for Organic Electronics

“…A significant contribution of describing charge transport has been conducted by Bässler [6]. Several models for OSCs have been set up investigating active layer morphology and exciton separation [11,17], recombination [10,14,38], Coulomb interaction [12] and charge densities [13].…”

“…The energetic landscape of an organic semiconductor plays a significant role in the charge transport and exciton dissociation [10,27,28]. In contrast to inorganic semiconductors, the molecular orbital energies inside an organic material are strongly disordered due to the non-crystalline arrangement of the molecules and polymers [28].…”

“…In this section, we focus on the new features of our kMC framework extending the previously shown model [10,13]. A generalized representation of the organic material is presented, which covers the model of a non-cubic Voronoi set and polymer chains.…”

“…From that point on, kMC has been employed for many theoretical studies covering charge and exciton transport [6][7][8], recombination of charge carriers [9,10] and the impact of all processes on the photocurrent generation [11][12][13][14][15]. kMC simulations have been frequently utilized to study full devices such as bulk-heterojunction (BHJ) organic solar cells [10][11][12][13]15,16].…”

“…Another limitation of existing models is the simplified treatment of excitonic processes, which are of high importance regarding organic devices. Most kMC models either account for singlet [10,[17][18][19] or triplet exciton dynamics [20][21][22][23] for the use in OSCs or OLEDs, respectively. However, the interplay between singlets and triplets can be used to modify exciton lifetimes and diffusion lengths, which is a promising way to tailor efficiencies in organic devices and therefore requires a better understanding [24][25][26].…”