Proceedings of 7th Thin Film Materials & Devices Meeting
November 5-6, 2010, Nara, Japan
http://www.tfmd.jp/Crystalline Silicon Solar Cells With Two Different Metals
Toshiyuki Sameshima*, Kazuya Kogure, and Masahiko Hasumi
Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo
184-8588, Japan*E-mail: tsamesim@cc.tuat.ac.jp
Keywords: work function, passivation, tunnel effect, metal

Abstract. We report crystalline silicon solar cells with two different metals, Thermally grown
SiO2 1.5~2-nm thick passivated the surfaces of n-type silicon substrates. Inter digit stripes of Aland Au with a width of 500 μm and a gap of 100 μm were formed on the top surface in order tocause an asymmetric internal built-in potential distribution in silicon because of the difference
between their work functions of 0.92 eV. Solar cell characteristics were observed by halogen
lamp illumination at 22 mW/cm2. The short-circuit current density and open-circuit voltage
were 14.7 mA/cm2, 0.47 V. The conversion efficiency was estimated to be 15.5%.

Introduction
Semiconductor solar cells have been attractive as a device for producing electrical powerfrom sunlight [1]. The conversion efficiency and production cost are essential points in theadvancement of the solar cell industry and its business. Many important technologies have beendeveloped to improve those points, such as shallow PN junction formation, back-surface fieldformation, defect passivation, and multijunction formation [2,3]. Simple processing is still
attractive for the low-cost fabrication of solar cells.
In this paper, we report a simple silicon solar cell with two different kinds of metals [4]. Thissolar cell does not need the PN junction, transparent electrode for light correction, and silver
printed electrode for current correction. We discuss the physics of the solar cell and its technicalpossibilities. We then report current voltage characteristics of the solar cells.
Physics
Figure 1 shows distribution of thework function of different metals
reported by H. B. Michaelson [5]. The
work function widely ranges from 2 to
6 eV. Alkaline and alkaline earth
metals have low work functions. It iswell known that gold (Au), platinum,and nickel have high work functions.
Silicon can have a work function
ranging from 4.1 to 5.2 eV, which
depends on impurity doping, as shownin Fig. 1. Figure 2 shows a schematicFig. 1. Distribution of work functions for different
kinds of metals as well as crystalline silicon.

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image of the energy band structure whenn-type crystalline silicon is sandwiched by
Al and Au metals via thin SiO2 layers
zero-biased at the both metals. Their work
functions of Al and Au are reported as 4.18
and 5.1 eV, respectively [5].
The
difference in the work functions of thosemetals can cause an asymmetric
distribution of the internal built-in
potential in silicon. Because the initial
Fermi level of n-type silicon is near thework function of Al, but far different fromthat of Au, a large internal potential
distribution associated with the depletion Fig. 2. Schematic energy band image of present
region is spatially formed near the Au solar cell for n-type silicon with Al and Au
metal electrode, as shown in Fig. 2. SiO2 electrodes.
layers have an important role of stablepassivation of silicon surface with a low density of interface traps and a low recombination
velocity of the minority carriers in order to maintain the asymmetric distribution of the internalbuilt-in potential. Holes and electrons excited by light illumination in silicon separate eachother according to the internal potential distribution, as shown in Fig. 2. Holes move toward
the Au electrode, and electrons move toward the Al electrodes. The SiO2 layers should be thinand electrically conductive associated with the quantum-tunneling effect to produce
photo-induced current from silicon to the two metals [6, 7]. Although 1.2 nm thermally grown
SiO2 has been already in production stage in the very large scale integrated circuit technology
field, the substantial tunneling leakage characteristic has blocked its application to metal-oxide
semiconductor field effect transistor. On the other hand, the thin SiO2/Si interface with thetunneling conductive characteristic will be suitable for our solar cell model.
Figure 3 an overview image of a practical cell structure for our solar cells. The both siliconsurface are passivated with SiO2 layers to keep the minority carrier lifetime (τeff) high. Top SiO2
was thin enough for the tunneling current effect. Al and Au inter digit patterns are formed on
the top surface. The band structure shown in Fig. 2 is completed along Al, top SiO2, silicon(lateral direction), top SiO2, andAu.. Light is illuminated to therear surface. Holes and electrons
generated at the rear surface
region travel to the top surface.
Holes and electrons come into Auand Al electrodes, respectively.
The solar cell does not need thePNjunction,transparent
electrodes and Ag printed
electrodes, as shown in Fig. 3.
This is a clear advantage for
reduction of fabrication cost ofsolar cell. In addition, no doped
layer would result in a low Auger
recombination probability, which
would lead a high conversion Fig.3 Overview image of a practical cell structure for
efficient [8,9].
In order to our solar cells

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achieve a high efficiency cell, as shown by the structure in Fig. 3, thin and good passivation ofsilicon surfaces is essentially important. Moreover, metal electrodes should be patterned
densely with widths and gaps smaller than the minority carrier diffusion length (Dτeff)0.5 foreffectively collecting photo-induced carriers. The substrate thickness should be also thinnerthan (Dτeff)0.5 for effective carrier diffusion from the rear surface to the top surface.
Anti-reflection surface texture will be necessary at the rear surface for effective light collection.
Experimental demonstration
N-type single–crystalline silicon substrates with a thickness of 520 μm and a resistivity of 30Ωcm were used. The mirror-polished top and rear surface were thermally oxidized, and100-nm-thick thermally grown SiO2 layers were formed on both surfaces. The SiO2 layer at thetop surface was then thinned with buffered hydrofluoric acid. The thickness of SiO2 wasestimated to be 1.5~2.0 nm by analyzing the optical reflectivity spectra [10]. Samples werethen annealed at 260oC for 3 h in 1.3x106 Pa H2O vapor to further passivate the silicon surface[11]. On the top surface, Al and Au inter digit patterns are formed with their width of 500 μm,length of 1 cm, and a gap of 100 μm. Light illumination was conducted to the rear surface using
halogen lamp at 22 mW/cm2. Light Illumination area was defined to be 1 cm2 using a windowmask. Light reflection loss about 20% probably occurred because no anti-reflection structure
was formed at the rear surface.
The minority carrier effective lifetime, τeff, was measured by 9.35 GHz microwave
transmission measurement [12]. τeff was 1200 μs for the initial silicon substrates with bothsurfaces coated with 100-nm-thick SiO2 layers. The silicon surfaces were well passivated by theSiO2 layers. When the thickness of SiO2 films at the top surface was decreased to 1.5~2.0 nm
using buffered hydrofluoric acid, τeff markedly decreased to 7.6 μs. This means that the carrier
recombination rate at the top surface was increased after SiO2 etching. The subsequent 1.3x 106
Pa H2O vapor heat treatment at 260oC for 3 h increased τeff to 54 μs. Surface passivation wasimproved by the high-pressure H2O heat treatment and τeff increased. The minority carrier
diffusion length, (Dτeff)0.5 was estimated to be about 250 μm using a diffusion coefficient of 12
cm2/Vs for hole. The width of Al and Au electrodes and substrate thickness were larger thanthat minority carrier diffusion length. Therefore, some degree of carrier annihilation probably
occurred via carrier recombination during carrier diffusion.
Figure 4 shows the absolute electrical current density as a function of voltage for samples
with Al and Au stripes in the dark andilluminated with light to the rearsurfaces. The electrical current density
was obtained from experimentallyelectrical current divided by anelectrode area of 0.44 cm2 in the caseof dark. On the other hand it wasobtained
fromexperimentallyelectrical current divided by theillumination area of 1 cm2 in the caseof light illumination. The electricalcurrent increased as the positive
voltage increased under the dark
condition. Typical metal-insulator
semiconductor diode characteristic Fig. 4 Absolute electrical current as a function ofwas observed. It means that hole applied voltage.
density at SiO2/Si interface below Au

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electrode was sensitively increased by positive gate voltage application. Substantial tunnelingcurrent via the top thin SiO2 layer was observed. Light illumination markedly increased theelectrical current density. It indicates that photo-induced holes and electrons were generated at
the rear surface and traveled across the 520-μm-thick silicon substrate to the top surface andflowed into Au and Al, respectively, in accordance with the internal built-in potential. No
current appeared at 0.47 V, indicating an open-circuit voltage. Typical solar cell current voltage
characteristics were obtained as shown in Fig. 5. In the case of light illumination to the rearsurface, the short-circuit current density Jsc, and the open-circuit voltage Voc were 14.7 mA/cm2,0.47 V, respectively. The conversion efficiency was estimated to be 15.5 %.
Those results shown in Figs. 4 and5 demonstrate our simple solar cellmodel.
However,
thepresent
fabrication process was not yet
optimized. The series resistivity wasestimated to be about 9 Ωcm2 from theminimum differential resistivity of thesolar cell characteristic around Voc
shown in Fig. 5. The Voc of 0.47 V
was lower than we expected from thedifference in the work functions of Al
and Au of 0.92eV. We believe thatthose problem is overcome by
improvement in surface passivation.
Process technologies for formation ofFig. 5 Solar cell characteristics for lightthin and good SiO2 layers should beillumination to the rear surfaces.
improved to achieve a high tunneling
effect, a low density of interface trap
states and low recombination velocity.
Moreover, the width of electrodes should be small less than effective carrier diffusion length.
The substrate thickness should be also thin less than effective carrier diffusion length.
For our solar cell model, it is essential that the difference in the work functions of two
different kinds of metals causes an internal built-in potential in the semiconductor. Many otherchoices of metals will be possible besides the Al-Au pair used in the present work. For example,Hf and Au have a large difference of 1.2 eV in the work function [5]. This has the advantage ofcausing a high internal built-in potential in silicon. The present model may be applied to manyother semiconductors if two different metals with appropriate work functions are selected.
SummaryCrystalline silicon solar cells were demonstrated. N-type 520-μm-thick 30 Ωcm
single-crystalline silicon substrates coated with 100-nm-thick thermally grown SiO2 layer at thetop surface was thinned to 1.5~2.0 nm using buffered hydrofluoric acid. Samples were thenannealed at 260oC for 3h in 1.3x106 Pa H2O vapor. The minority carrier effective lifetime was
54 μs. Inter digit stripes of Al and Au with a width of 500 μm and a gap of 100 μm on the topsurface. Electrical current as a function of voltage was measured between Al and Au stripes
under light illumination of a halogen lamp at 22 mW/cm2 to the rear surface. Typical current
voltage characteristics of solar cells were obtained. Jsc and Voc were 14.7 mA/cm2 and 0.47 V,respectively. The conversion efficiency was 15.5%. Holes and electrons generated at the rearsurface traveled across the 520-μm-thick silicon substrate and flowed into Al and Au electrodes,
respectively, owing to a tunneling effect. Those results demonstrated our solar cell physics that110107048-4

a difference in the work functions of two different kinds of metals causes an internal built-in
potential in silicon.
Acknowledgment
This work was partly supported by a Grant-in-Aid for Science Research C (No. 22560292)
from the Ministry of Education, Culture, Sports, Science, and Technology, Japan
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