Exciton delocalization incorporated drift-diffusion model for bulk-heterojunction organic solar cells

TL;DR

This paper introduces a modified drift-diffusion model incorporating exciton delocalization to better simulate charge generation in bulk-heterojunction organic solar cells, revealing how delocalization enhances efficiency.

Contribution

A systematic numerical simulation model that integrates exciton delocalization into charge transport analysis for organic solar cells, providing new insights into device optimization.

Findings

Increased exciton delocalization reduces energy loss during charge generation.

Higher delocalization ratios significantly improve short-circuit current.

Revealed bulk recombination effects on fill factor saturation.

Abstract

Modeling the charge-generation process is highly important to understand device physics and optimize power conversion efficiency of bulk-heterojunction (BHJ) organic solar cells (OSCs). Free carriers are generated by both ultrafast exciton delocalization and slow exciton diffusion and dissociation at the heterojunction interface. In this work, we developed a systematic numerical simulation to describe the charge-generation process by a modified drift-diffusion model. The transport, recombination, and collection of free carriers are incorporated to fully capture the device response. The theoretical results match well with the state-of-the-art high-performance organic solar cells. It is demonstrated that the increase of exciton delocalization ratio reduces the energy loss in the exciton diffusion-dissociation process, and thus, significantly improves the device efficiency especially for the short-circuit current. By changing the exciton delocalization ratio, OSC performances are comprehensively investigated under the conditions of short-circuit and open-circuit. Particularly, bulk recombination dependent fill factor saturation is unveiled and understood. As a fundamental electrical analysis of the delocalization mechanism, our work is important to understand and optimize the high-performance OSCs.