Fullerene Sensitized Silicon for Near to Mid Infrared Light
Detection.


Journal: 2010 MRS Spring Meeting
Manuscript ID: 756299
Symposium: Symposium C
Date Submitted by the
Author: 25-Mar-2010
Complete List of Authors: Matt, Gebhard; J. Kepler University, Semiconductor Physics
Bednorz, Mateusz; J. Kepler University, Semiconductor Physics
Fromherz, Thomas; J. Kepler University, Semiconductor Physics
Zamiri, Saeid; J. Kepler University, Christian Doppler Laboratory for
Surface Optics
Lungenschmied, Christoph; Konarka Austria
Sariciftci, Niyazi; J. Kepler University, Linz Institute for Organic
Solar Cells (LIOS)
Bauer, Guenther; J. Kepler University, Semiconductor Physics
Keywords: infrared (IR) spectroscopy, sensor, photovoltaic

Fullerene sensitized silicon for near to mid infrared light
detection

Gebhard J. Matt1, Mateusz Bednorz1, Thomas Fromherz1, Saeid Zamiri2, Christoph
Lungenschmied3, Niyazi Serdar Sariciftci4, Günther Bauer1

1. Institute for Semiconductor Physics, Johannes Kepler University, Linz, Austria.
2. Christian Doppler Laboratory for Surface Optics, Johannes Kepler University , Linz,
Austria.
3. Konarka Austria, Linz, Austria.
4. Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University , Linz, Austria.

ABSTRACT
We report on a novel light sensing scheme based on a silicon/fullerene-derivative hetero-
junction that allows the realization of optoelectronic devices for the detection of near to mid
infrared radiation. Despite the absent absorption of silicon and the fullerene-derivative for
wavelengths beyond 1.1 µm and 0.72 µm, respectively, a hetero-junction of these materials
absorbs and generates a photo-current due to absorption in the near to mid infrared. This photo-
current is caused by an interfacial absorption mechanism [1].
Besides its scientific relevance, the simple fabrication process of the hetero-junction (e.g. the
fullerene-derivative is deposited by spin-coating on Si) as well as its compatibility with the
established and rather cheap CMOS technology makes the presented hybrid approach a
promising candidate for widespread applications.


INTRODUCTION

Detection of light in the near (NIR) to mid infrared (MIR) spectral range is a technology
with many applications such as optical data transmission (1.55 µm), contrast enhancement for
imaging systems in foggy environments, and quality control. In essence, the inherent
disadvantage of silicon for optoelectronic infrared applications is its transparency beyond a
wavelength of 1.1 µm. To overcome this disadvantage, several technologies such as the
deposition of (polycrystalline-) germanium on silicon [2, 3, 4] or the usage of in near infrared
photo-conductive and soluble nano-particles [5] have been developed. In the latter case, the
simple solution processing of a “guest” material [6] to the silicon based “host” is of particular
interest.
Potential “guest” materials are organic semiconductors. The strengths of this material-class are
their solubility, excellent optical properties and the per se tunable chemically structure. In this
work, as “guest” material the soluble C60 derivative methano-fullerene [6,6] phenyl-C61 butyric
acid methyl ester (PCBM) has been chosen (see Fig. 1a where the chemical structure is
depicted). In contrast to pristine C60 , PCBM is soluble up to 5 weight percent in common
organic solvents [7]. Due to the large energy gap of fullerenes between the lowest unoccupied
molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), almost no free
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charge–carriers are present for carrier transport. PCBM thin films are electron conductors with
mobilities in the order of 10-2 cm2/Vs [8] whereas no hole conductivity could be measured.
In this paper it is shown that a p–Si/PCBM hetero–junction features a photo–voltaic effect in the
infrared regime between photon energies from 0.55 to 1.1 eV. We demonstrate that the photo
current originates from an absorption-mechanism at the organic/inorganic hetero-junction.

EXPERIMENT
The investigated samples have a sandwich type structure (see Fig 1b). On top of a boron
doped p–Si wafer (Boron concentration ≈1016 cm−3 ) the PCBM film is deposited by spin
coating. The resulting PCBM film thickness is ≈140 nm. The subsequent evaporation of Al
forming electrical contacts to the PCBM film and the Si substrate completes the device
preparation. The IR-light is incident from the silicon side of the hetero–junction. In Fig. 2 the
current-density versus voltage (IV) characteristics at room temperature and at 77 K in dark and
under broad-band illumination from a tungsten lamp are presented. At 297 K the IV-
characteristics exhibit a current rectification ratio of 3·104 for a bias variation from -1 V to +1 V.
Upon cooling, the reverse dark current–density at -2 V bias decreases from 2·10−6 A/cm−2 at 297
K to the sub nA/cm−2 region at 77 K with an increased current rectification of ≈10−7 for a bias
variation from -1 V to +1 V. From an Arrhenius plot of the reverse dark current-density at -2 V
(see inset Fig. 2a), an activation energy of ∆E ≈ 0.45 eV is found for the temperature range from
297 to 240 K. At a sample temperature of 77 K under broadband NIR–illumination from a
tungsten lamp spectrally restricted by a high-energy cut-off Si filter, an IV–characteristics
typically for a photo-voltaic device is observed (see Fig. 2b.). At zero bias (short–circuit
condition) the current is in the range of 15 nA/cm2 and at +0.5 V bias the current vanishes (open-
circuit condition).


Under the same experimental conditions as for the IV measurements, the NIR photo-current at
various temperatures is spectrally resolved using a Fourier-transform spectrometer. The
spectrally resolved photo-current (PC) is normalized to the absolute incident light intensity by
the usage of a calibrated InGaAs photo-diode. In the temperature range from from 77 to 297 K, a
monotonically increasing PC from 0.55 to 1.1 eV is observed. At 115 K sample temperature, an

Fig. 1: a) Chemical structure of the C60 derivative PCBM. (b) Schematic cross section of the
Al/p-Si/PCBM/Al hetero-junction.
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additional PC around 1.1 eV is observed, which becomes dominant for temperatures above 150
K. To prevent that the PC around 1.1 µm is dominating the dynamic range of the experiment, a
low pass interference filter (cut-off at 0.95eV) in addition to the Si filter is placed in the optical
path of the FTIR. As clearly seen in Fig. 3a and in the inset of Fig. 3b the PC from 0.55-0.95 eV
is weakly temperature dependent and is decreased by approximately 60% at room-temperature
compared to the PC at liquid nitrogen temperature.




Fig. 2: (a.) IV-characteristics at 297K in dark. The inset shows an Arrhenius plot of the reverse
current–density at -2 V bias. (b.) IV-characteristics at 77 K in dark (black triangles) and under
broadband IR illumination (red triangles). A tungsten lamp filtered by a Si filter maintained at
room temperature was used as a light source. The different bias sweep directions are visualized
by the ∇ symbol for a bias sweep direction towards higher voltages (forward) and the ∆ symbol
for a bias sweep direction towards lower voltages (backward).
Page 3 of 6

DISCUSSION
As intensively discussed in the literature, the interaction of fullerenes with
semiconductors and metals is strong and complex. In particular, at the interface of a
silicon/fullerene hetero-junction, a silicon fullerene LUMO state mixing with an accompanying
(at least partial) charge-transfer has been reported [9,10]. Similarly, fullerenes on metals are very
likely to be chemisorbed which entails a charge transfer and accounts for an additional bonding
beyond van der Waals bonding, especially low work–function metals as Al or Ca, form an
Ohmic contact to fullerene thin–films [11].
The observed photo–response as well as the IV–characteristics of the p–Si/PCBM hetero-
junction can be understood in terms of the electron and hole band discontinuities across the
interface. Since the work function of Si is about -4.8 eV [9] and the LUMO of PCBM is at -4.2
eV [11], the Fermi level of the p–Si is energetically below the LUMO of the PCBM molecule.
Under a positive (forward) bias voltage applied to the Al/p–Si back–contact, electrons are
efficiently injected from the Ohmic Al/PCBM top–contact into the PCBM layer. The electron
injection from the PCBM into the Si conduction band (CB) is energetically unfavorable and the
current has to traverse the organic/inorganic interface as a recombination current between
electrons in the PCBM and holes in the p–Si. When biasing the p–Si/PCBM diode in reverse
direction, holes are extracted from the p–Si valence band (VB) into the Al back–contact. In the
absence of radiation, only thermally excited carriers can maintain the dark current observed
under reverse bias.
Two processes have to be considered by which holes can be injected thermally into p-Si. (a) the
thermal excitation of electron-hole pairs in the p-Si and a subsequent injection of the electron
into the PCBM-LUMO and (b) the direct thermal excitation at the p-Si/PCBM interface with the
subsequent injection of holes and electron in the bulk of the p-Si and PCBM respectively. For
process (a), the expected thermal activation energy equals the indirect Si band-gap, whereas for
Fig. 3: (a.) IR photo-current an Al/p-Si/PCBM/Al hetero-junction as function of photon energy
from 297 to 77 K. The inset of (a) shows the normalized temperature dependency of the photo-
current at 0.8 eV photon energy. In panel (a), the tungsten light source is spectrally restricted by
a Si filter and in panel (b) by an additional low-pass interference filter with a cut-off at 0.95 eV.
The filter transmissions are shown as broken lines in (a) and (b).
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process (b) an activation energy in the sub-bandgap range of silicon is expected. From the
temperature dependence of the reverse dark current between 305 K and 240 K, an activation
energy of ≈ 0.45 eV is determined from the Arrhenius plot, shown in the inset of Fig. 2a). This
result indicates that process (a) can be excluded as the dominant mechanism responsible for the
observed reverse dark current. Further, the observation of process (b) indicates a strong
interaction of the PCBM-LUMO with the Si VB states across the interface.
Upon radiation, processes (a) and (b) can be excited optically. From its energetic position, the
strong signal at 1.1 eV shown in Fig. 3a is assigned to an VB to CB absorption in the p-Si and a
subsequent electron injection into the PCBM [process (a)]. Since the NIR radiation is incident to
the silicon side of the sample, only light with energy in a narrow range around the Si absorption
edge contributes to this spectral feature. Radiation at larger energies is absorbed by the Si filter
and in the Si substrate far away from the p–Si/PCBM interface. By lowering the sample
temperature, the band-gap of the p-Si substrate is increased above the cut-off energy of the at
room temperature maintained silicon filter [12], resulting in a signal decrease at 1.1 eV. At a
sample temperature of 77 K, no radiation with sufficient energy to be absorbed in the p–Si
substrate passes through the Si filter and the signal due to process (a) is absent at that
temperature. Due to the transparency of silicon as well as the of pristine PCBM in the spectral
range below 1.1 eV, the photo–current response between 0.55 eV and 1.1 eV cannot be trivially
assigned to a direct absorption in either of the materials. Instead, it is ascribed to an optical
absorption at the p-Si/PCBM interface by exiting electrons into the LUMO of the PCBM thin
film and a subsequent hole injection into the p-Si.
CONCLUSIONS
In summary, it has been demonstrated that a silicon/fullerene hetero-junction absorbs and
generates a photo-current in the spectral range from 0.55 eV to 1.1 eV (the principal absorption
of Si). As manifestation of the strong interaction of fullerenes with the Si surface, the PC
originates from an absorption mechanism at the hetero-junction. Besides its scientific relevance,
the simple fabrication process as well as its compatibility with the well established CMOS
technology, makes the presented hybrid approach a promising candidate for widespread
applications.

ACKNOWLEDGMENTS
This work was supported by the Österreichische Forschungsförderungsgesellschaft and
the Austria Wirtschaftsservice. The authors wish to thank K. Hingerl and C. J. Brabec for their
critical comments.

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