Optical Anisotropies in Organic Semiconductors
S. Brown1, R. Schlitz1, R. Grote3, J. Driscoll3, M. Chabinyc1, R. Osgood3,4, J. Schuller2
Introduction
We seek to understand the relationship between morphology and optical
properties. Optical anisotropy in organic semiconductor films is ubiquitous
due to structural anisotropies in the films’ molecular constituents. This
optical anisotropy becomes critical when designing devices, such as lighttrapping solar cells, which have geometrical anisotropies. In addition, a
material’s response to light gives us insight into the electronic properties of
the material such as the orientation of the excited dipoles that form excitons,
and the environment that these dipoles are in.
Polymer orientation impacts
1
light trapping solar cells
Measuring anisotropy in
2
polymers
P(NDI2OD-T2) allows us to compare two
different morphologies
Edge-on
Face-on
 P(NDI2OD-T2) spun and annealed at 150°C has a face-on morphology
whereas annealing just below the melting point at 305°C leads an edge-on
morphology
Ellipsometry is a good first pass for
determining anisotropic absorption
Momentum-resolved spectroscopy
Utilizes the fact that dipole radiation is
angle-dependent
Detector
position
[3]
[4]
 By placing a detector in the back focal plane light intensity can be measured
as a function of emitted angle (or momentum-vector of emitted light) – this
allows for the unique determination of the orientation of the emitting dipole
PL intensity
 Spectroscopic ellipsometry is a quick traditional technique to determine
anisotropic optical constants, but it is not as sensitive to out-of-plane features
and requires model-dependent fitting
P3HT has a preferred absorption
direction
Momentum-resolved photoluminescence
is more precise for measuring anisotropy
[5]
 There are special points in the back focal plane that have PL from just inplane or out-of-plane dipoles, allowing separate measurement of in-plane or
out-of-plane dipoles
Our setup has momentum-resolution on
the input and the output
 Having momentum-resolution on
both the input and output enables
new measurements such as
momentum-resolved
photoluminescence excitation
which can map absorption spectra.
 On-going work utilizes this
extended capability to give even
deeper insights into the optical
properties of these materials
 P3HT, Poly(3-hexylthiophene), is a well-studied photoacceptor
 As spun P3HT chains lie parallel to substrate surface
 zP3HT is a model orientation of P3HT where the chains are perpendicular
to the substrate
 Photoexcitation is mainly along the polymer backbone
 Momentum-resolved photoluminescence (MR-PL) is free-parameter free and
much more sensitive to out-of-plane features compared to ellipsometry.
Momentum-resolved PL reveals that there is much more out-of-plane dipole
strength than would be expected from the ellipsometry measurements.
We would like to acknowledge Lee Richter of NIST for the P3HT ellipsometry
data. We would like to thank the Center for Energy Efficient Materials (a DOE
funded research center), the NSF through an Early Career Award, and the
Hellman Fellows Fund for funding.
References:
An out-of-plane absorption direction
increases absorption
 Light trapping/field enhancing solar cells, such as plasmonic and gap mode
solar cells, enhance the component of the electric field perpendicular to
the substrate
 Electric field enhancement and photoexcitation directions must be aligned
for maximum absorption
Acknowledgements:
 The higher precision from MR-PL is able to resolve a difference in the 950nm
peak shoulder in the out-of-plane direction for the face-on morphology – the
absence of this shoulder suggests a more amorphous environment in the outof-plane direction
1. Grote, R. R., Brown, S. J., Driscoll, J. B., Osgood, R. M. & Schuller, J. A. Morphology-dependent light
trapping in thin-film organic solar cells. Opt. Express 21, A847–A863 (2013).
2. Rivnay, J. et al. Drastic Control of Texture in a High Performance n-Type Polymeric Semiconductor and
Implications for Charge Transport. Macromolecules 44, 5246–5255 (2011).
3. Modified from Maschen’s work, via http://commons.wikimedia.org/wiki/File:Elem-doub-rad-pat-pers.svg
in the public domain.
4. Modified from BenFrantzDale, via http://en.wikipedia.org/wiki/File:BackFocalPlane.svg under the CC BYSA 3.0 license (http://creativecommons.org/licenses/by-sa/3.0/).
5. Modified from Schuller, J. A. et al. Orientation of luminescent excitons in layered nanomaterials. Nat Nano
8, 271–276 (2013) under fair use.
1. Materials, University of California, Santa Barbara
2. Electrical and Computer Engineering, University of California, Santa Barbara
3. Department of Electrical Engineering, Columbia University
4. Applied Physics, Columbia University