Spectroscopic Imaging of Photopotentials and Photoinduced Potential Fluctuations in a Bulk Heterojunction Solar Cell Film

We present spatially resolved photovoltage spectra of a bulk heterojunction solar cell film composed of phase-separated poly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine) (PFB) polymers prepared on ITO...

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Bibliographic Details
Published in:ACS nano 2012-11, Vol.6 (11), p.9392-9401
Main Authors: Luria, Justin L, Hoepker, Nikolas, Bruce, Robert, Jacobs, Andrew R, Groves, Chris, Marohn, John A
Format: Article
Language:English
Subjects:
Charge
Electromagnetic Fields
Fluctuation
Heterojunctions
Light
Materials Testing
Membranes, Artificial
Noise
Photovoltaic cells
Polymers
Polystyrenes - chemistry
Polystyrenes - radiation effects
Solar cells
Solar Energy
Spectroscopy
Spectrum Analysis - methods
Thiophenes - chemistry
Thiophenes - radiation effects
Trapping
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Summary:We present spatially resolved photovoltage spectra of a bulk heterojunction solar cell film composed of phase-separated poly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine) (PFB) polymers prepared on ITO/PEDOT:PSS and aluminum substrates. Over both PFB- and F8BT-rich domains, the photopotential spectra were found to be proportional to a linear combination of the polymers’ absorption spectra. Charge trapping in the film was studied using photopotential fluctuation spectroscopy, in which low-frequency photoinduced electrostatic potential fluctuations were measured by observing noise in the oscillation frequency of a nearby charged atomic force microscope cantilever. Over both F8BT- and PFB-rich regions, the magnitude, distance dependence, frequency dependence, and illumination wavelength dependence of the observed cantilever frequency noise are consistent with photopotential fluctuations arising from stochastic light-driven trapping and detrapping of charges in F8BT. Taken together, our findings suggest a microscopic mechanism by which intermixing of phases leads to charge trapping and thereby to suppressed open-circuit voltage and decreased efficiency in this prototypical bulk heterojunction solar cell film.
ISSN:1936-0851
1936-086X
DOI:10.1021/nn300941f