21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia Controlled oxidation of Chlorine-terminated silicon nanocrystals
towards organic/inorganic hybrid photovoltaics
Tomohiro Nozaki1, Ding Yi1, Ken Okazaki2, Ryan Gresback1, Riku Yamada2,
1
2
Department of Mechanical Sciences & Engineering, Tokyo Institute of Technology, Tokyo, Japan
Department of Mechanical & Control Engineering, Tokyo Institute of Technology, Tokyo, Japan
Abstract: Silicon nanocrystals (Si NCs) have unique optical and electronic properties that are
advantageous for semiconductor device applications and here their application to photovoltaics is examined. Freestanding Si NCs were synthesized by a non-thermal plasma using silicon
tetrachloride (SiCl4) as a silicon precursor. Blended solutions of Si NCs and
Poly(3-hexylthiophene-2,5-diyl) (P3HT) were fabricated by spin-casting to form bulk hetrojunction devices. The weight fraction of Si NCs in P3HT was found to greatly impact the device performance. As the weight fraction increases up to 50 wt%, the short-circuit current
dramatically increases, while the open-circuit voltage and fill factor do not change significantly. The improvement in device performance is attributed to both increased probability of
excitons in P3HT finding an interface before recommendation and an enhancement in the
conversion of wavelengths where P3HT is a poor photovoltaic material. It is also found that
the device performance generally follows that of hydrogen terminated Si NCs, which is contrary to the expectation of chlorine terminated Si NCs where the valence band energy is expected to be significantly shifted. These results provide an alternative approach to synthesizing Si NCs from SiCl4 instead of silane (SiH4).
Keywords: Silicon nanocrystals, solar cell
1. Introduction
Interest in hybrid solar cells has rapidly increased over
the past decade due to the promising ability to combine
the advantages from both organic and inorganic materials
[1]. In hybrid solar cells, organic semiconductor and inorganic semiconductor nanocrystals are blended and inorganic semiconductor nanocrystals share many of the
processing advantageous of organic semiconductors especially in terms of their ability to form films by solution
processing techniques [2]. Additionally, they generally
have advantages including relatively low exciton binding
energy, high carrier mobility and good thermal stability
[3]. Solution processible inorganic semiconductor nanocrystals also provide the possibility to have a large interfacial area for efficient exciton dissociation when blending with soluble polymers; all of these advantages offer
the potential to deliver efficient energy conversion with
large-area and low-cost fabrication [4]. A variety of hybrid solar cells have been reported recently, employing
various semiconductor nanocrystals such as, GaAs [5],
CdSe [6], ZnO [7], PbS [8], and Si [9]. Employing Si NCs
as the inorganic component is especially interesting because silicon is abundant and non-toxic, this is crucial for
large-scale production and commercialization [10]. In
addition, the electronic band structure and its tunability
with Si NCs size and surface termination still remain unclear. By examining the performance of devices fabricated
with Si NCs may offers valuable insights into the electronic band structure of chlorine terminated Si NCs.
2. Experiments
Si NCs were synthesized using a very high frequency
(70 MHz) non-thermal plasma as previously described
with slight modifications [11]. A quartz tube with inner
diameter of 45 mm was employed as the reactor. Power
was coupled to the plasma by two copper electrodes surrounding the quartz tube with a gas mixture of argon (Ar),
hydrogen (H2) and SiCl4 (ratio of 30, 10, and 1, respectively). Si NCs synthesized in the plasma were collected
downstream of the plasma on a stainless steel mesh.
Kortshagen and colleagues originally developed a similar
reactor employing SiH4 as the precursor where this study
uses SiCl4, which is a cheaper and easier to handle precursor [12]. Additionally the resulting Si NCs are at least
partially chlorine terminated [13].
In this work, highly crystalline Si NCs with a mean size
of 6 nm were employed. As-produced Si NCs are freestanding particles as illustrated in Fig. 1(a), which shows
the AFM image of Si NCs employed in this study. Nanocrystals synthesized by means of this type of non-thermal
plasma usually have a narrow size distribution and this is
also confirmed in this study, as shown in Fig. 1(b). Si NCs
were well dispersed together with P3HT in anhydrous
1,2-dichlorobenzene (DCB) with different weight ratio. Si
NCs/P3HT blends are stirred for 24 hours before use in
order to form a suitable mixture.
21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia effects increase the bandgap of Si NCs with respect to
bulk. It is reported that band shift for valence band is
twice larger than that of conduction band as illustrated
with dotted line for hydrogen terminated [17]. However
for chlorine terminated Si NCs the band alignment is expected to deviate significantly from hydrogen or oxygen
terminated Si NCs [18,19].
Fig. 1 Typical AFM image of fabricated freestanding Si
NCs (a) and corresponding size distribution (b).
Figure 2(a) shows the hybrid solar cell structure employed in this study and is the same structure (active area
of 0.046 cm2) as previously reported by Liu et al. [14]
Poly(3,4-ethylenedioxylenethiophene):poly(styrenesulpho
nic acid) (PEDOT:PSS) was spin-cast onto patterned ITO
glass as hole transporting layer. Si NCs/P3HT blends with
different Si NCs weight ratios were spin-cast on it respectively as active layer, and finally aluminum electrodes
were evaporated on the top. A P3HT-only device (0 wt%
of Si NCs) was also investigated as a reference device.
Performance of Si NC/P3HT devices were measured
using a Keithley 2400 Source Meter with AM 1.5 solar
simulated light (100 mW/cm2). 6 devices were measured
on one sample, and the best value with error bars are
shown here. All of the synthesis, handling of Si NCs was
done without exposure to air by sealing the Si NCs between gate valves for transport between synthesis reactor
and a purified nitrogen glovebox (<1 ppm water and oxygen) where device fabrication and measurements was
performed. External quantum efficiency (EQE) measurements were performed with an incident line width of ~20
nm.
3, Results and Discussions
A possible band alignment of the device is shown in Fig.
2(b). The valence band of bulk silicon is -4 eV with respect to vacuum, and has a bandgap of ~1.1 eV [15]. If
the band alignment is similar to bulk, P3HT is expected to
act as the electron donor material and silicon as the electron acceptor. When the size of Si NCs is less than the
exciton Bohr radius (~4.1 nm) [16], quantum confinement
Fig. 2 Solar cell structure (a) and its band alignment (b).
Figure 3 shows select current density-voltage (J-V)
traces of devices with different weight ratio of Si NCs.
The short-circuit current density (Jsc) drastically increases
by introducing Si NCs and tends to increase with higher
Si NC weight fraction up to 50 wt%. Fig. 4 shows the
influence of the weight ratio of Si NC/P3HT on the Jsc,
open-circuit voltage (Voc), fill factor (FF), and resulting
power conversion efficiency (PCE).
For the hybrid devices, the Voc and FF are not strongly
affected by the weight ratio of the Si NCs. Compared to
the P3HT-only devices the Voc is reduced, and the FF is
slightly enhanced by the introduction Si NCs. The Jsc
strongly increases with Si NCs weight ratio up to 50 wt%.
As a result, PCE follows the same tendency with Jsc.
At higher than 50 wt% Si NCs weight ratio, device
performance deteriorates dramatically. This is likely because Si NCs cannot be dispersed sufficiently when the
weight ratio is higher than 50 wt%, and big agglomerations may form in blends, which is detrimental to film
morphology, which is expected to negatively impact exciton separation and carrier transport. In the coming discussion, we focus on devices with Si NCs weight ratio
smaller than 50 wt%. The trend of these results is very
similar to hydrogen terminated Si NCs as reported elsewhere [9,14].
The origin of the increase in Jsc with Si NCs weight ratio may arise from two main processes, the first is greater
dissociation of excitons in P3HT due to a higher probability of the excitons finding a P3HT/Si NC interface and the
second is due to contribution for absorption of Si NCs.
The external quantum efficiency (EQE) spectra of the
devices can provide insight into which contribution is
21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia important. Corresponding EQE spectra of devices shown
in Fig. 3 are illustrated in Fig. 5(a), and they are replotted
again on a semi-log scale in Fig. 5(b).
Fig. 3 Typical J-V curves of devices with different weight
ratio of Si NCs.
increases with Si NC wt% in this low energy region; we
attribute this to light absorption from Si NCs. The drastically lower EQE is attributed to the silicon being poor
absorbers in the red to infrared as compared to the blue
and UV region of the absorption spectra and especially as
compared to P3HT, as shown in Fig. 5(c). From our previous study on colloidal Si NCs where we quantatively
determined the extinction cross section of Si NCs, the
absorption coefficient can be estimated to be ~3 orders of
magnitude lower at 800 nm (~8102 cm-1) as compared to
350 nm (~1106 cm-1), as illustrated in Fig. 5(c), this is
consistent with the corresponding EQE [21]. Consequently, the enhancement of the EQE below 350 nm with increasing weight ratio can be attributed to contributions for
the absorption of UV photons by Si NCs. Previously Niesar et al. [22] demonstrated the photocurrent of hybrid Si
NC/P3HT devices match the absorption well, however
their method did not unambiguously demonstrate that the
extraction of photogenerated carriers. Here we demonstrate that these photogenerated carriers can be extracted
and imply that further absorption is necessary in order to
increase the EQE in this region.
Fig. 4 Relationship between device photovoltaic parameter and Si NCs over a wide range of weight ratios.
With increasing Si NCs weight ratio, the EQE is enhanced remarkably over the entire spectra. In addition,
increased fractional contributions from below 320 nm and
above 650 nm appear. In the P3HT-only device the EQE
at wavelengths longer than 650 nm is below detectable
limits of this system and as the Si NC weight ratio is increased, the EQE becomes detectable and increases with
increasing weight ratio, as shown in Fig. 5(b). It is expected that for P3HT-only device, the EQE at wavelengths longer than 650 nm to be zero as the band gap of
P3HT is ~2 eV and therefore photons with less energy
than the band gap are not absorbed by the material [20].
By introducing Si NCs, a small EQE (~3 orders of magnitude less than the value around 350 nm) is found and
Fig. 5 EQE spectra (a) and in semi-log scale (b) of corresponding devices. (c) shows absorption coefficient of
P3HT from ref. 20 and Si NCs as a function of wavelength.
The increase of the EQE in the range from 350 to 650
nm is not attributed to increased Si NCs absorption as it is
expected to still be significantly less as compared to
P3HT, as shown in Fig. 5(c). We attribute the increase in
the EQE and device performance with higher weight ratio
21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia of Si NCs to better dissociation of excitons excited in
P3HT, as well as carrier transport and collection, similar
to previous studies [9,14].
In P3HT, the exciton binding energy is relatively high
(a few hundred of meV) and a junction is necessary in
order to separate excitons into free carriers [23]. In the
P3HT-only device, a Schottky junction exists at the
P3HT/Al interface and only excitons created near this
interface can be dissociated. Excitons created far away
from the interface will recombine before finding the interface, as the diffusion length in P3HT is ~10 nm [24].
By introducing Si NCs, an interface where excitons created in P3HT can dissociate is formed and closely spaced
Si NC can form a pathway for extraction of carriers. As a
result, more excitons can be dissociated at the Si
NCs/P3HT interface before recombining. The photo-generated electrons can be collected by Al electrode,
while holes can be transported in the P3HT and be collected by the ITO electrode.
The results presented here are similar to hydrogen terminated Si NCs that have been previously reported [9,14].
However for the Si NC employed here the surface is
originally at least partially terminated by chlorine, which
is expected to have a significant impact on the position of
the valence band [13,18]. The similarity of the results
suggest that the valence band position of the NCs employed here is similar to hydrogen terminated Si NCs [14].
This could be due to the size of the NCs employed here (6
nm), a significant valence band shift may be minor for
this size and is significant only for smaller chlorine terminated NCs or because of uncontrolled oxidation of the
Si NCs during the fabrication which reduces the impact of
the chlorine on the NCs electronic properties. The role of
the surface of the NCs is not clearly understood, but will
be examined in future work.
4. Conclusions
Freestanding Si NCs with a narrow size distribution
were synthesized by using non-thermal plasma with silicon tetrachloride as precursor. Si NCs were successfully
employed as the electron acceptor in Si NCs/P3HT hybrid
solar cells. Solar cell efficiency increases with Si NCs
weight ratio, which is mainly attributed to higher efficiency of exciton dissociation due to a larger Si
NCs/P3HT interface. Additionally EQE spectra of devices
suggest that Si NCs can contribute to increase absorption
and extraction of carriers below 350 nm due to absorption
by Si NCs. Below the band gap of P3HT (wavelengths
greater than ~650 nm) photogenerated carriers can be
extract, however the absorption from Si NCs in this region is too weak to effectively absorb light resulting in
small conversion efficiencies. The results suggest that
there is little difference in the electronic properties of the
Si NCs synthesized from silicon tetrachloride and previous reports of Si NCs synthesized from silane.
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