TY - JOUR
T1 - Imaging of π-conjugated polymer/fullerene blends used in organic photovoltaics by nonlinear photoluminescence emission
AU - Vardeny, Shai R.
AU - Baniya, Sangita
AU - Kieu, Khanh
AU - Peyghambarian, Nasser
AU - Valy Vardeny, Z.
N1 - Funding Information:
The work at the University of Utah was supported by the AFOSR Grant No. FA9550-16-1-0207; the work at the University of Arizona was supported in part by the Space Exploration and Optical Solutions Technology Research Initiative Fund (TRIF).
Publisher Copyright:
© 2019 Society of Photo-Optical Instrumentation Engineers (SPIE).
PY - 2019/4/1
Y1 - 2019/4/1
N2 - A number of image microscopies have been applied to films of π-conjugated polymers and their fullerene blends used in photovoltaic applications; however, they seldom are able to map the polymer/fullerene grain interfaces and detect microscopic defects in the blend films. We have conducted multiphoton microscopy using a 65-fs mode-locked laser at 1.56 μm for spectroscopy and mapping of films of two prototype π-conjugated polymers, namely MEH-PPV and P3HT combined with their blends of PCBM fullerene molecules. The pristine polymer films have shown third harmonic generation and three-photon photoluminescence emission bands that are used for mapping the film topography with micrometer spatial resolution. Since the nonlinear photoluminescence band of the photogenerated charge transfer excitons (CTE) at the polymer/fullerene interfaces in films of polymer/fullerene blends is substantially redshifted compared to that of the excitons in pristine polymers, we could readily map the polymer/fullerene grain interfaces using the nonlinear CTE photoluminescence emission. From the multiphoton imaging of the polymer/fullerene films, we show that the polymer super-grains in MEH-PPV/PCBM are substantially larger than those in P3HT/PCBM, which may be detrimental to charge transport and, in turn, to photovoltaic applications, in agreement with smaller power conversion efficiencies obtained for solar cells based on the former blend. In addition, we also found second harmonic generation emission bands in the MEH-PPV/PCBM blend that result from micron-size embedded defects that do not possess inversion symmetry that forms during the film deposition process at ambient conditions. Multiphoton microscopy and spectroscopy are valuable additions to the tools of organic semiconductor films and devices for investigating the properties and growth of polymer/fullerene blends used for photovoltaic applications with micron spatial resolution.
AB - A number of image microscopies have been applied to films of π-conjugated polymers and their fullerene blends used in photovoltaic applications; however, they seldom are able to map the polymer/fullerene grain interfaces and detect microscopic defects in the blend films. We have conducted multiphoton microscopy using a 65-fs mode-locked laser at 1.56 μm for spectroscopy and mapping of films of two prototype π-conjugated polymers, namely MEH-PPV and P3HT combined with their blends of PCBM fullerene molecules. The pristine polymer films have shown third harmonic generation and three-photon photoluminescence emission bands that are used for mapping the film topography with micrometer spatial resolution. Since the nonlinear photoluminescence band of the photogenerated charge transfer excitons (CTE) at the polymer/fullerene interfaces in films of polymer/fullerene blends is substantially redshifted compared to that of the excitons in pristine polymers, we could readily map the polymer/fullerene grain interfaces using the nonlinear CTE photoluminescence emission. From the multiphoton imaging of the polymer/fullerene films, we show that the polymer super-grains in MEH-PPV/PCBM are substantially larger than those in P3HT/PCBM, which may be detrimental to charge transport and, in turn, to photovoltaic applications, in agreement with smaller power conversion efficiencies obtained for solar cells based on the former blend. In addition, we also found second harmonic generation emission bands in the MEH-PPV/PCBM blend that result from micron-size embedded defects that do not possess inversion symmetry that forms during the film deposition process at ambient conditions. Multiphoton microscopy and spectroscopy are valuable additions to the tools of organic semiconductor films and devices for investigating the properties and growth of polymer/fullerene blends used for photovoltaic applications with micron spatial resolution.
KW - charge-transfer excitons
KW - multiphoton microscopy
KW - multiphoton photoluminescence mapping
KW - nonlinear optics
KW - nonlinear photoluminescence
KW - pi-conjugated polymers
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U2 - 10.1117/1.JPE.9.025502
DO - 10.1117/1.JPE.9.025502
M3 - Article
AN - SCOPUS:85069478551
SN - 1947-7988
VL - 9
JO - Journal of Photonics for Energy
JF - Journal of Photonics for Energy
IS - 2
M1 - 025502
ER -