Mapping the Competition between Exciton Dissociation and Charge Transport in Organic Solar Cells

Soong Ju Oh; Jongbok Kim; Jeffrey M Mativetsky; Yueh-Lin Loo; Cherie R Kagan

doi:10.1021/acsami.6b07810

Mapping the Competition between Exciton Dissociation and Charge Transport in Organic Solar Cells

ACS Appl Mater Interfaces. 2016 Oct 26;8(42):28743-28749. doi: 10.1021/acsami.6b07810. Epub 2016 Oct 11.

Authors

Soong Ju Oh¹, Jongbok Kim^{2

3}, Jeffrey M Mativetsky^{2

4}, Yueh-Lin Loo^{2

5}, Cherie R Kagan

Affiliations

¹ Department of Materials Science and Engineering, Korea University , Seoul 02841, Korea.
² Department of Chemical and Biological Engineering, Princeton University , Princeton, New Jersey 08544, United States.
³ Department of Materials Science and Engineering, Kumoh National Institute of Technology , Gyeongbuk 39177, Korea.
⁴ Department of Physics, Applied Physics and Astronomy, Binghamton University , Binghamton, New York 13902, United States.
⁵ Andlinger Center for Energy and the Environment, Princeton University , Princeton New Jersey 08544, United States.

PMID: 27696850
DOI: 10.1021/acsami.6b07810

Abstract

The competition between exciton dissociation and charge transport in organic solar cells comprising poly(3-hexylthiophene) [P3HT] and phenyl-C61-butyric acid methyl ester [PCBM] is investigated by correlated scanning confocal photoluminescence and photocurrent microscopies. Contrary to the general expectation that higher photoluminescence quenching is indicative of higher photocurrent, microscale mapping of bulk-heterojunction solar-cell devices shows that photoluminescence quenching and photocurrent can be inversely proportional to one another. To understand this phenomenon, we construct a model system by selectively laminating a PCBM layer onto a P3HT film to form a PCBM/P3HT planar junction on half of the device and a P3HT single junction on the other half. Upon thermal annealing to allow for interdiffusion of PCBM into P3HT, an inverse relationship between photoluminescence quenching and photocurrent is observed at the boundary between the PCBM/P3HT junction and P3HT layer. Incorporation of PCBM in P3HT works to increase photoluminescence quenching, consistent with efficient charge separation, but conductive atomic force microscopy measurements reveal that PCBM acts to decrease P3HT hole mobility, limiting the efficiency of charge transport. This suggests that photoluminescence-quenching measurements should be used with caution in evaluating new organic materials for organic solar cells.

Keywords: charge transport; exciton dissociation; organic solar cells; scanning confocal photoluminescence microscopy; scanning photocurrent microscopy.