Probing dark exciton diffusion using photovoltage

• Published in: Nature communications Vol. 8; no. 1; p. 14215
• Main Authors: Mullenbach, Tyler K., Curtin, Ian J., Zhang, Tao, Holmes, Russell J.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 27.01.2017; Nature Publishing Group; Nature Portfolio
• Subjects: 639/301/1005/1007; 639/301/299/946; Humanities and Social Sciences; Light emitting diodes; multidisciplinary; Photovoltaic cells; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms14215

The migration of weakly and non-luminescent (dark) excitons remains an understudied subset of exciton dynamics in molecular thin films. Inaccessible via photoluminescence, these states are often probed using photocurrent methods that require efficient charge collection. Here we probe exciton harvesting in both luminescent and dark materials using a photovoltage-based technique. Transient photovoltage permits a real-time measurement of the number of charges in an organic photovoltaic cell, while avoiding non-geminate recombination losses. The extracted exciton diffusion lengths are found to be similar to those determined using photocurrent. For the luminescent material boron subphthalocyanine chloride, the photovoltage determined diffusion length is less than that extracted from photoluminescence. This indicates that while photovoltage circumvents non-geminate losses, geminate recombination at the donor–acceptor interface remains the primary recombination pathway. Photovoltage thus offers a general approach for extracting a device-relevant diffusion length, while also providing insight in to the dominant carrier recombination pathways. Exciton diffusion plays a role in many optoelectronic devices. In some materials, this migration cannot be detected using photoluminescence. Mullenbach et al . use photovoltage measurements to extract the diffusion length in organic photovoltaic cells, and examine a series of non-luminescent materials.