Bulk photovoltaic effect in an organic polar crystal

Article abstract of DOI:10.1039/c4cc01255k

Organic polar crystals from the donor-acceptor substituted 1,4-diphenybutadiene 1 can generate a short-circuit photocurrent and a photovoltage upon illumination with near UV light. The photocurrent and photovoltage are attributed to a bulk photovoltaic effect. The bulk photovoltaic effect has been known for inorganic polar crystals for decades and can now also be demonstrated for organic polar crystals. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01255k

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Bulk photovoltaic effect in an organic polar
crystal†
Cite this: Chem. Commun., 2014,
50, 6530
Ratheesh K. Vijayaraghavan,a Stefan C. J. Meskers,*ab M. Abdul Rahimc and
Suresh Dasc
Received 17th February 2014,
Accepted 3rd May 2014
DOI: 10.1039/c4cc01255k
www.rsc.org/chemcomm
Organic polar crystals from the donor–acceptor substituted
1,4-diphenybutadiene 1 can generate a short-circuit photocurrent
and a photovoltage upon illumination with near UV light. The
photocurrent and photovoltage are attributed to a bulk photovoltaic
effect. The bulk photovoltaic effect has been known for inorganic
polar crystals for decades and can now also be demonstrated for
organic polar crystals.
Chart 1 Chemical structure of 1.
been shown that large photovoltages can be generated in crystals
The bulk photovoltaic effect involves the spontaneous genera- of the biomacromolecules constituting the plant photosystem I.4
tion of photocurrent and photovoltage upon illumination of a Furthermore, photovoltaic action in a poled organic material has
spatially uniform material. Because the spontaneous photo- recently been demonstrated.5 Study of bulk photovoltaic effects
current and the photovoltage have vectorial character, Curie’s in crystals of organic molecule is hampered by the difficulty in
symmetry principle states that the bulk photovoltaic effect can obtaining organic polar crystals with absorption in the appro-
only occur in polar materials.
priate spectral range and sufficient charge transport properties.6
For polar inorganic crystals, the bulk photovoltaic effect is
Here we investigate the 1,4-diphenylbutadiene derivative 1
well known.1 The bulk photovoltaic effect has recently received (Chart 1), carrying an electron donating methoxy- and an
renewed interest, partly because of the possibility to make electron withdrawing trifluoromethyl-substituent.7 The asym-
photovoltaic cells that can generate open circuit voltages that metric substitution introduces donor–acceptor character and
by far exceed the bandgap of the semiconducting active layer.2 results in a considerable static dipole moment for 1, deter-
In fact open circuit voltages up to several kV have been mined to be 3.8 Debye in dioxane solution. The dipolar vector is
reported.3 The bulk photovoltaic effect can be explained quali- oriented along the long axis of the molecule.
tatively by generation and separation of photogenerated charge
carriers under the influence of the internal or built-in electric Emmons reaction (see ESI† for further details). Single crystals
field that exists in any bulk polar crystal. of the molecule were obtained from a toluene-chloroform
The compound 1 was synthesized via the Wittig–Horner–
Although the bulk photovoltaic effect in inorganic ferroelectric mixture. Analysis of X-ray diffraction data for a single crystal
crystals is by now well established, for organic crystals the bulk of 1 revealed the orthorhombic crystal system with Pca2(1)
photovoltaic effect has not been addressed yet. Recently it has space group. The Pca2(1) space group allows for polar crystals.
The crystal consists of regular herringbone stacks with four
molecules per unit cell (Fig. 1). The molecules are almost
planar with the plane of phenyl rings making only a small
a Molecular Materials and Nanosystems, Eindhoven University of Technology,
PO Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: s.c.j.meskers@tue.nl
b Institute for Complex Molecular Systems, Eindhoven University of Technology,
4.71 dihedral angle with the plane of the central butadiene unit.
The planarity warrants conjugation of the p-electrons over
the entire molecule. The long axis of all four of the molecules
in the unit cell lie parallel to the ac plane and make a Æ321
angle with the c axis (Fig. 2). In the direction of the a- or b-axis,
the components of the dipole moments of the four molecules
in the unit cell cancel each other. In the c-direction, the dipole
moments add up to a net static polarization.
The Netherlands
c Photosciences and Photonics, Chemical Sciences and Technology Division,
National Institute for Interdisciplinary Science and Technology (CSIR),
Trivandrum, India
† Electronic supplementary information (ESI) available: Detailed synthetic pro-
cedures and characterization, single crystal X-ray analytical data and experi-
mental techniques are available. CCDC 987360. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4cc01255k
6530 | Chem. Commun., 2014, 50, 6530--6533
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Fig. 4 Slicing of as-grown single crystals of 1 into smaller sections for
oriented insertion between electrodes.
Fig. 1 Unit cell for a single crystal of 1 from X-ray diffraction analysis with
four molecules per unit cell.
packing as determined from analysis of the X-ray diffraction,
the fluorescence vanishes when the excitation light is polarized
parallel to the crystal b-axis.
Fluorescence spectroscopy on isolated molecules of 1 in polar
solvent, indicate that the dipole moment of 1 in the lowest
singlet excited state (11.3 Debye) is higher than in the ground
state (3.8 Debye). The fluorescence band shows a large solvato-
chromic red shift that increases with increasing solvent polarity.7
This is consistent with net photoinduced transfer of charge
from the side of the molecule bearing the methoxy substituent
to the side carrying the fluoromethyl group. Importantly,
because of the alignment of the molecules of 1 in the crystal,
Fig. 2 (A) Fluorescence (red) and fluorescence excitation (black) spectra
for a single crystal of 1. (B) Fluorescence decay profile of a single crystal of photoexcitation of the crystal will result in a net transfer of
1. Excitation at 400 nm, emission collection at 480 nm.
charge from one side of the crystal to the other along the
crystallographic c-axis.
In order to measure this photoinduced transfer of charge in
Crystals of 1 show bright blue fluorescence, with maximum the polar crystal, we have mounted crystals of 1 in between two
around 460 nm (Fig. 2A) upon excitation with light with wavelength gold electrodes, see Fig. 3D. For electrical detection of a
shorter than 420 nm (see Fig. 2A). The fluorescence decays almost photoinduced potential difference between the electrodes, the
monoexponentially with a lifetime of around 5.2 ns (Fig. 2B). The orientation of the crystal with respect to the electrodes is of
monoexponential decay and the limited Stokes shift of the fluores- prime importance. The c-axis of the crystal that corresponds to
cence in the solid, indicate the absence of severe exciton self- the net direction of charge transfer, needs to point in the same
trapping and/or excimer formation in the crystalline state.
direction as the shortest line between the edges of the two
Fluorescence polarization microscopy shows that absorption electrodes used for measuring the photovoltage. Because the
of light by crystals of 1 is highly polarized. Consistent with the crystals are plate-like, growing fastest in the b direction, we sliced
the as-grown crystals into smaller parts that could be oriented
with their c-axis in the direction of the electrode spacing (Fig. 4).
Gold electrodes with 30–35 mm spacing were fabricated by
thermal sublimation of the metal onto glass through a mask. A
5 nm Cr adhesion layer was applied between glass and gold to
strengthen binding of the metal. The thickness of the electrodes
was approx. 100 nm. Electrical contact between crystal and gold
electrode was reinforced using conducting epoxy resin contain-
ing silver. The crystal was illuminated with light of 364 nm from
a continuous wave argon ion laser. The light from the laser was
modulated mechanically by a chopper. Alternating photo-
currents and voltages were measured separately using a lock-in
amplifier (Stanford SR830, 10 MO input impedance) as function
of frequency of modulation and intensity of the incoming light
(Scheme 1). All the measurements were carried out under normal
atmospheric condition. We stress that in the photocurrent
measurements no external voltage is applied to the crystals.
Fig. 3 (A, B) Polarized fluorescence microscope images of a single crystal of 1.
Red arrows indicate the direction of polarization of excitation. (C) Transmission
optical microscope image of the gold electrodes used to measure the
We find that single crystals of 1 can generate a spontaneous
photocurrent and a photovoltage under illumination with UV
light, see Fig. 5. Photocurrent and photovoltage dependent
photovoltaic effect with spacing of 30–35 mm. (D) Image taken after depositing
the crystal of 1 having B50 Â 30 Â 100 mm3 dimension.
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Scheme 1 Schematic representation of the experimental set-up for the
photocurrent and photovoltage measurement.
Fig. 6 Dependence of spontaneous photocurrent and photovoltage
generated by a single crystal of 1 on power of the illuminating laser beam.
The bashed lines indicate linear and quadratic power dependencies.
illumination, exciton-exciton annihilation type processes are
however likely to occur.
The superlinear photocurrent provides an indication against
thermal effects as the source of the photovoltaic effect. Thermal
gradients are linear or sublinear in the heating power. Inhomo-
geneous illumination of semiconductors can lead to a photo-
Dember effect,9 as the result of an asymmetric carrier generation
profile in combination with a difference in diffusion constant for
n and p-type carriers. The lateral electrode structures used here
allow for homogenous illumination, minimizing the photo-
Dember effect. Moreover the photo-Dember effect in organic
diodes is (sub-)linear in illumination intensity,10 whereas the
effect under study shows superlinear behavior.
Fig. 5 Modulation frequency dependence of the spontaneous, short-
circuit photocurrent and photovoltage.
on the modulation frequency of the incident light beam. At
Finally, in a polar organic crystal in which the dipole
frequencies above 102 Hz, both current and voltage rapidly moment of the molecules changes upon photoexcitation, illu-
decrease with increasing frequency. The magnitude of the mination with intensity modulated excitation light should yield
photovoltage was found to decrease from B100 mV at 3 Hz to an alternating current due to a light-induced change in the net
nearly B88 nV at 3.3 kHz. The corresponding change in current polarization of the crystal.8 The relevant timescale for this
was from 4.4 pA to 0.07 pA. The frequency dependence transient current is the excited state lifetime (5.2 ns). Near
indicates a transport limitation in the built-up and recombina- the start and end of each illumination period, a fixed amount of
tion of photogenerated charge.
charge DQ will exchanged via the electrodes within a time
Fig. 6 illustrates the dependence of spontaneous photo- window of about 5 ns. The polarization related alternating
current and photovoltage on the intensity of the incident laser photocurrent IP, should increase in magnitude with increasing
light expressed as power. The power of illumination was varied frequency because IP = DQ/T = nDQ with T the period and
from 3 mW to 95 mW. Beyond this flux density, the sample n the frequency of the modulation. In our experiment, we use low
found to undergo decomposition with black dots appearing in modulation frequencies, limiting the contribution of this polari-
the crystal, indicative of charring. At low power, the short zation related current. Moreover the experimentally determined
circuit current density varies approximately linearly with the photocurrent decreases with increasing frequency, whereas the
power. At high intensity, a superlinear, quadratic dependence polarization mechanism predicts an increase with frequency.
of the spontaneous photocurrent on power is observed. The The frequency and intensity dependence observed indicate
photovoltage shows a dependence on power that is steeper than photogeneration of charge carrier involving multiple excitons
for the current and also shows a crossover for a low intensity to combined with asymmetric charge transport in the polar crystal.
a high intensity regime at about 30 mW. The monochromatic
In summary, we find experimental evidence for a bulk
power conversion efficiency is low, on the order of 10À12. The photovoltaic effect in a polar crystal of a donor–acceptor sub-
superlinear dependence of short-circuit current on illumina- stituted p-conjugated molecule.
tion intensity indicates that photocarrier generation and collec-
This work is part of the Joint Solar Programme (JSP) of Hyet
tion involving sequential absorption of two photons is more Solar and the Stichting voor Fundamenteel Onderzoek der
efficient than the corresponding processes driven by a single Materie FOM, which is part of The Netherlands Organisation
photon. Two photon absorption is rather unlikely under CW for Scientific Research (NWO). The work has received funding
6532 | Chem. Commun., 2014, 50, 6530--6533
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