Extraordinarily long diffusion length in PM6:Y6 organic solar cells

The PM6:Y6 bulk-heterojunction (BHJ) blend system achieves high short-circuit current ( J SC ) values in thick photovoltaic junctions. Here we analyse these solar cells to understand the observed independence of the short-circuit current upon photoactive layer thickness. We employ a range of optoele...

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Bibliographic Details
Published in:Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2020-04, Vol.8 (16), p.7854-786
Main Authors: Tokmoldin, Nurlan, Hosseini, Seyed Mehrdad, Raoufi, Meysam, Phuong, Le Quang, Sandberg, Oskar J, Guan, Huilan, Zou, Yingping, Neher, Dieter, Shoaee, Safa
Format: Article
Language:English
Subjects:
Absorption spectroscopy
Circuits
Diffusion
Diffusion layers
Diffusion length
Fabrication
Heterojunctions
Optoelectronics
Photoelectric effect
Photoelectric emission
Photovoltaic cells
Photovoltaics
Short circuit currents
Short-circuit current
Solar cells
Thickness
Transport
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Summary:The PM6:Y6 bulk-heterojunction (BHJ) blend system achieves high short-circuit current ( J SC ) values in thick photovoltaic junctions. Here we analyse these solar cells to understand the observed independence of the short-circuit current upon photoactive layer thickness. We employ a range of optoelectronic measurements and analyses, including Mott-Schottky analysis, CELIV, photoinduced absorption spectroscopy, mobility measurements and simulations, to conclude that, the invariant photocurrent for the devices with different active layer thicknesses is associated with the Y6's diffusion length exceeding 300 nm in case of a 300 nm thick cell. This is despite unintentional doping that occurs in PM6 and the associated space-charge effect, which is expected to be even more profound upon photogeneration. This extraordinarily long diffusion length - which is an order of magnitude larger than typical values for organics - dominates transport in the flat-band region of thick junctions. Our work suggests that the performance of the doped PM6:Y6 organic solar cells resembles that of inorganic devices with diffusion transport playing a pivotal role. Ultimately, this is expected to be a key requirement for the fabrication of efficient, high-photocurrent, thick organic solar cells. Simulated energy band diagrams of thin and thick PM6:Y6 devices.
ISSN:2050-7488
2050-7496
DOI:10.1039/d0ta03016c