Proceedings of 6th Thin Film Materials & Devices Meeting
November 2-3, 2009, Kyoto, Japan
http://www.tfmd.jp/

Multiple-Connected Solar Cells Used by Band-Gap Induced Cascaded LightAbsorption
S. Yoshidomi, M. Hasumi and T. Sameshima
Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan

Email: tsamesim@cc.tuat.ac.jp
Keywords: Multiple-connected solar cell, Band-gap, Light absorption, Solar light, a-Si, c-Si
Abstract. We propose multiple-connected solar cells with different band gaps causing specificlight absorption. A commercial three-serial connected a-Si solar cell and a single c-Si solar cellwere placed facing each other with an angle of 10 degrees. Those solar cells were electricallyconnected by electrical wires to form a series circuit. AM1.5 light at 1 sun was slantingly shinedto the a-Si solar cell. The top and rear surface of the a-Si solar cell reflected light with photon
energy lower than its band gap. The reflected light was illuminated to the c-Si solar cell facing
the a-Si solar cell. Solar cell characteristic was obtained with an open circuit voltage Voc of 3.2V, while the individual a-Si and c-Si cells had Voc of 2.6 and 0.55 V, respectively. The high Voc
demonstrates that solar light is effectively used to generate electrical power by cascade-typelight absorption by each solar cell component.

Introduction
Crystalline silicon solar cells have been widely
developed and applied as a device for generating
electrical power directly from sunlight [1,2].
Crystalline silicon has a band gap energy of
1.12eV at room temperature, which locates in thelow energy tail region of sun light spectrum on
the earth with the AM 1.5 condition, as shown in
Fig. 1. Therefore, crystalline silicon effectively
absorbs sunlight with photon energy higher thanits band gap. However, crystalline silicon solar
cells have the limitation of the power conversion
efficiency of 28.9 % [3]: it means that 71% ofsunlight power doesn’t contribute to generation
Fig.1 AM 1.5 sunlight spectrumof electrical power. Light with photon energies
higher than the band gap is absorbed and holeelectron pairs are generated in silicon solar cells. The hole and electron pairs lose their energy
to the bad gap energy. And open circuit voltage Voc is limited by the built-in potential in thesolar cells. Light with very high photon energy is therefore not efficient for producing electrical
power by silicon solar cells. To solve this problem and increase the conversion efficiency,
multi-junction solar cells combined with wide- and narrow-band gap semiconductor pnjunctions have been developed. Epitaxial crystalline growth method has been widely studied tofabricate wide band gap pn junction layers on narrow band gap pn junction layers [4]. On theother hand, the method of stacking individual solar cells using conductive and transparent gel
has been also investigated [5].
In this paper, we propose a simple method of connection of individual solar cells with- 1302220048-1 -different band gaps. We first discuss the principle of the present method, which essentially uses
specific absorption characteristics of semiconductor with different band gap energies anddesigns a geometrical construction of wide and narrow band gap solar cells. Then we reportprincipal demonstration of effective usage of sunlight by multiple-connected solar cell withamorphous-silicon a-Si solar cell with crystalline-silicon c-Si solar cell. Increase in Voc isachieved with the present method.
Principle of multiple-connected solar cells
We discuss connection of three different typesof solar cells named cell 1, cell 2 and cell 3.
Their band gap energies E1, E2, and E3 are high,middle and low, respectively. Those cells areorderly placed on an inclined mirror substrate(mirror 1), as shown in Fig.2(a); the cells 1, 2,and 3 are placed the highest, middle and thelowest positions, respectively. The cells 1 and2, and the cells 2 and 3 are connected byelectrical wires each other with keeping
configuration of pn and pn series equivalentcircuit, as shown in Fig. 2(b). Three allconnected cells works as solar cell with two(a)output terminals. The cell 1 with E1 isilluminated with sunlight. The cell generates
electrical power by absorbing light with photon
energy higher than E1. On the other hand,because the light with photon energy lower
than E1 is not absorbed, it is reflected outsideof the cell 1 by the top and rear surfaces of thecell, metal electrodes formed at the top andrear cell surfaces as well as the underlying
mirror 1. The reflection light from the cell 1 isagain reflected by another mirror plate (mirror2) placed facing the three cells with a small
angle, as shown in Fig. 2(a). The cell 2 istherefore illuminated with light with photon
energy lower than E1. It generates electrical(b)power by absorbing light with photon energy
higher than E2. In the similar way of the cell 1, Fig.2 Schematic images of a multiplethe light with photon energy lower than E2 is connected solar cell (a) and its equivalent
reflected outside of the cell 2 by the top and circuit. Z is an impedance loaded in therear surfaces of the cell, metal electrodes circuit.
formed at the top and rear cell surfaces as wellas the underlying mirror 1. The reflected lightis illuminated to the cell 3 by the system shown in Fig. 2(a). The cell 3 finally generates
electrical power by absorbing light with photon energy higher than E3. The three solar cells
therefore generate electrical power by absorbing light with photon energy specificallydetermined by their own band gaps. It means that sunlight is more effectively used forgenerating electrical power compared with a solar cell with single band gap energy. Electricalconnection of the three cells expressed by the equivalent circuit shown in Fig. 2(b) gives a high- 1302220048-2 -Voc, which is the sum of Voc of the three each cell. On the other hand, the short circuit current
Isc is limited by the minimum Isc among the three cells. Selection of band gap energies istherefore the most important to achieve high Isc and high electrical power. A solar cell with highband gap energy gives a high Voc in general. High photon flux and high quality cell is necessary
for achieving Isc for a solar cell with high band gap energy. This is the same subject of multiple
junction solar cells. The selection of solar cells with appropriate band gaps that give theoptimum current matching is important under the condition of the black-body radiation
spectrum of sun light with the peak intensity at 2.3 eV [6].

Experimental procedure
In order to fundamentally demonstrate thepresent idea of the multiple-connected solarcell, a commercial three-serial connecteda-Si solar cell with a size of 30 mm x 15mm and a single c-Si solar cell with 35 mm
x 25 mm were used. Although the a-Si solar
cell has high band gap energy about 1.8 eV,many intrinsic defect states limit Isc and theconversion efficiency low. In this paper, weconcentrate observation of high Voc todemonstrate the idea to the multipleconnected solar cell. The optics wasconstructed using two mirror plates and afiber-type AM 1.5 solar simulator light, as Fig.3 Schematic experimental structure of a-Si
shown in Fig. 3. The a-Si solar cell was and c-Si connected solar cell and aplaced on the first mirror using an adhesive measurement system of its characteristics.
tape. The first mirror slantingly set with anangle of 60 degrees to the horizontal metal
plate. The c-Si solar cell was also placed on
the second mirror facing the a-Si cell. The second mirror had an angle of 70 degrees to thehorizontal plate. The two cells were faced each other with the 10 degrees. The light reflected by
the top and rear surfaces of a-Si solar cell was successfully irradiated to c-Si solar cell by thearrangement with 10 degrees. Their electrode terminals were connected with an electrical wire.
The other electrode terminal of the each cell was connected an electrometer to obtain solar cellcharacteristics. The a-Si solar cell was slantingly illuminated with AM1.5 light at 1 sun over thewhole area. The c-Si solar cell was illuminated with light reflected by the top and rear surfacesof a-Si over the whole area. The filter of 400 nm thick a-Si film with an optical band gap of 1.8
eV formed on a glass substrate was placed between the a-Si and c-Si cells to check power
generation of the c-Si solar cell by light with photon energy lower than the a-Si band gap energy.
Solar cell characteristic of the each solar cell was also measured when it was placed their own
position on their mirror substrate and the partner cell was not placed on the mirror yet. The
electrode terminals of the each solar cell were directly connected to the electrometer in the caseof measurement of individual solar cells.

Results and discussion
Figure 4 shows solar cell characteristics for individual samples of the a-Si (a) and the c-Si solar- 1302220048-3 -cells (b), respectively. Typical solar cellcharacteristics were obtained for the twosolar cells. The a-Si solar cell gave a highVoc of 2.6 V because of high band gap
energy and the three-serial connected typestructure. On the other hand, Isc was low of19 mA because of defective amorphous
property. Fill Factor FF of the a-Si solar cell
was 0.57. On the other hand, the c-Si solar
cell gave a low Voc of 0.55 V because of thelow band gap energy. In contrast, Isc wasvery high of 220 mA because of good
crystalline properties with long carrier
lifetime. FF was 0.63. In those solar cellconditions reported above, there is no
current matching. In the case of connectionof a-Si and c-Si solar cells, Isc is therefore
limited by that of a-Si solar cell. Figure 5shows characteristic of the solar cell formedby connection of the a-Si and c-Si solarcells, as shown in Fig. 3. Typical solar cellcharacteristic was obtained. Voc was 3.2 V,
which was the same as the sum of Voc of the Fig.4 Solar cell characteristics of individual
two each cell. Isc was 19 mA, which was the a-Si (a) and c-Si solar cells (b).
same value as that of the a-Si solar cell, asexpected. FF was 0.60, which was slightly
higher than that of the a-Si solar cellbecause of the assist for increase in V oc by
the c-Si cell.
At the short circuit condition, the a-Si
and c-Si solar cells were internally biased
each other at the condition of the same
current condition. The absolute value of thebias was almost the same of Voc of the c-Si
solar cell because the c-Si cell had muchhigher current than that of the a-Si cell. Thea-Si cell was therefore biased at -0.55 V forthe short circuit condition. This resulted in
shift of solar cell characteristic to the Fig. 5 Solar cell characteristic of a-Si and c-Si
positive voltage direction, as shown in Fig. solar cells connected cell, as shown in Fig. 3.
5. The voltage of maximum power point Dashed line is solar cell characteristic of a-Si
Vmp increased from 1.8 (single a-Si cell) to solar cell for comparison.
2.3 V in the case of a-Si and c-Si connectedcell.
No change in solar cell characteristic was observed as the a-Si filter was placed between thea-Si and c-Si solar cells. We interpret that the light lower wavelength than 700 nm was absorbed
in the a-Si cell, while the light in the wavelength range from 700 to 1100 nm was contributed topower generation of c-Si solar cell. The light in the wavelength range from 700 to 1100 nm had
about 50 % photon numbers of the total numbers of AM 1.5 from 380 to 1100 nm. This is ahigh enough illumination intensity for the c-Si solar cell to keep almost the Voc condition and- 1302220048-4 -bias the partner a-Si solar cell at -0.55 V. The simple idea given in Fig. 3 and fundamental
demonstration shown in Fig. 5 show a capability of utilizing solar light effectively to generate
electrical power by cascade-type light absorption by each solar cell component.
Summary
We proposed multi-connected solar cells with different band gaps. A commercial three-seriallyconnected a-Si solar cell was illuminated AM1.5 light at 1 sun with an incident angle of 60
degrees. The reflection light from the a-Si cell was illuminated to a commercial single c-Si solar
cell facing the a-Si solar cell with a slant angle of 70 degrees. Two solar cells were seriallyconnected using an electrical wire. The two combined solar cells gave Voc of 3.2 V, while Voc of
the individual a-Si and c-Si solar cells gave of 2.6 and 0.55 V, respectively under the AM1.5
light illumination at 1 sun. The high Voc was achieved by electrical power generation of the c-Si
solar cell with illumination of reflection light with photon energy lower than the band gap
energy of the a-Si solar cell. On the other hand, Isc was limited to be 19 mA by that of a-Si solarcell, while the c-Si solar cell gave much higher Isc of 0.22 A. The present result demonstrates
effectively electrical power generation by simple configuration of multiple-connected solar
cells with different band gap energy.
Acknowledgments
This work was partially supported by Design System Co., Ltd. and Sameken Co., Ltd.
References[1] A. Rohatagi: Proc in 3rd World Conf. on Photovoltaic Energy Conversion, 2003, p. A29.
[2] M. A. Green: Prog. Photovoltaics, 17 (2009) 183.
[3] M. Yamaguchi: Sol. Energy Mater. Sol. Cells 75 (2003) 261.
[4] K. Nishioka, T. Takamori, T. Agui, M. Kaneiwai,Y. Uraoka and T. Fuyuki: Jpn. J. App.
Phys., 43 (2004) 882.
[5] T. Sameshima, J. Takenezawa, M. Hasumi, T. Koida, T. Kaneko, M. Karasawa, and M.
Kondo. Jpn. J. Appl.Phys. 50 (2011) 052301[6] L. Hsu1 and W. Walukiewicz, J. Appl. Phys.,104 (2008) 024507- 1302220048-5 -