Ultrafast photocurrent detection reveals that device efficiency is dominated by ultrafast exciton dissociation not exciton diffusion

TL;DR

This study demonstrates that ultrafast photocurrent detection reveals device efficiency is primarily limited by rapid exciton dissociation, challenging previous assumptions based on longer-lived exciton diffusion measured by traditional methods.

Contribution

The paper introduces a new ultrafast spectrometer technique that directly measures exciton dynamics responsible for photocurrent, providing a more accurate understanding of device efficiency.

Findings

Photocurrent is generated within 30 femtoseconds by excitons with minimal diffusion.

Photoabsorption detection overestimates the role of long-lived excitons.

Most photocurrent arises from excitons that diffuse little before transfer.

Abstract

Excitons diffusing to a charge-separating interface is a necessary step to convert energy into current in next-generation photovoltaics. In this report, made possible by a new ultrafast spectrometer design, we compare exciton dynamics measured using both photoabsorption- and photocurrent-detected transient and 2D spectroscopies. For a device with semiconducting carbon nanotubes as the exciton transport material, we find that photoabsorption detection greatly overestimates the importance of long-lived excitons for device performance. Excitons diffuse and transfer between nanotubes for several picoseconds, but the large majority of photocurrent is created within 30fs by excitons that diffuse little to the C60 electron transfer material. These results change our understanding of the material features most important for these photovoltaics. Photoabsorption detection measures all excitons, but not all photogenerated excitons generate current. To understand device efficiency, this study points to the necessity for directly measuring the exciton dynamics responsible for photocurrent.