Proceedings of 6th Thin Film Materials & Devices Meeting
November 2-3, 2009, Kyoto, Japan
http://www.tfmd.jp/Multi-Junction Solar Cells Fabricated With Conductive Transparent
Adhesive
S. Yoshidomi1, M. Hasumi1, T. Sameshima1, K. Makita2, K. Matsubara2, and M.
Kondo2
1Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan

2Research Center for Photovoltaic Technologies, AIST, Tsukuba Central 2, 1-1-1 Umezono,Tsukuba, Ibaraki, 305-8568, Japan

Email: tsamesim@cc.tuat.ac.jp

Keywords: Multi-Junction solar cells, Conductive Transparent adhesive, ITO
Abstract. We report characteristics of electrical contact between indium-tin oxide ITO particles
andsiliconSisubstrate
toapply
thistechniquetofabrication
ofmechanical-stack-type-multi-junction solar cells. 25-m-sized ITO particles were reduced in Taboats by joule heating at 60W for 5 and 20 min. The joule heating changed the color of theparticles from green (as-bought initial) to dark grey.
Samples with Si/initial or
reduced-ITO+Cemedine adhesive/Si structure showed ohmic current-voltage characteristics attemperatures between 22 and 70oC. Ohmic characteristics probably resulted from a high densityof carrier recombination sites located at the interfaces between ITO particles and Si surfaces,
whose work functions were different each other. A low connecting resistivity of 1.2 cm2 wasachieved for a sample of Si/5 min-reduced ITO/Si structure. Moreover, a mechanical stack solar
cell was achieved using initial ITO particles for samples of InGaP/GaAs/initial
ITO+Cemedine/Ge structure. The short circuit current density, open circuit voltage andconversion efficiency were 12.6 A/cm2, 2.13 V, and 15.9%, respectively.

Introduction
We have developed multi-junction solar cells with different band gaps by stacking individual
solar cells using conductive and transparent pastes. We experimentally demonstrated a multijunction solar cell with a-Si:H and c-Si solar cells, which were stacked by the polyimide
adhesive layer including indium-tin oxide ITO particles [1]. We also reported electrical andmechanical stacking of fragile 4 inch sized GaAs and Ge substrates by the adhesive describedabove [2]. Those results indicate that the mechanical stacking method using plastic paste has apossibility of fabricating multi-junction solar cells with a large size. Ohmic current-voltage
characteristics at semiconductor/conductive adhesive interface is important to realize goodcharacteristics of multi-junction solar cells. Low connecting resistivity is also necessary toeffectively produce electrical power [1,3]. We have achieved ohmic characteristics with aconnecting resistivity of 1.8 cm2 so far.
In this paper, we report decrease in resistivity of ITO particles by joule heating in a vacuumwith keeping good ohmic characteristics. Moreover, we report solar cell characteristics ofstacking InGaAs/GaAs and Ge solar cell by conductive-transparent adhesive.

- 130222027-1 -Experimental procedure
ITO particles with the diameter ranging from25 to 32 m were prepared by mechanicalfiltering with metal mesh membranes. Then,they were carried out with reduction treatment.
They were placed in a Ta board, which was set
in a vacuum chamber. The chamber wasevacuated to 8.0x10-4 Pa. The Ta board washeated by applying electrical power of 60 W
for 5 and 20 min. Visual observation of dark Fig.1 Structure of n-Si/adhesive/n-Si
red color light emission from the Ta boat sample.
indicated that it was heated to about 800 oC by
the joule heating. ITO particles at 5.3 weight%
were dispersed in Cemedine adhesive gel. The
Cemedine gel including ITO particles wassandwiched with two mirror polished n-typesilicon substrates with a resistivity of 0.001
cm. The Cemedine intermediate layer
including ITO was solidified under a highpressure at 4x105 Pa for 2 h at RT toinvestigate
current-voltage(I-V)
characteristics of ITO particles undertemperatures ranging from 22 to 70 oC. The
optical transmissivity of the sample ofglass/reduced ITO+Cemedine/glass was alsoFig. 2 Structure of InGaP/GaAs/adhesive/Gemeasured.
solar cell.
Stacking InGaP/GaAs and Ge solar cells
was tried with present adhesive including ITO
particles to demonstrate direct contact of ITO
to III-V materials as well as IV materials, as shown in Fig. 2. Top metal electrodes wereformed on n-GaAs contact layer by photolithography and etching method. Conformal etchingof the n-GaAs layer was carried out to form light windows. P-type GaAs surface was attachedto n-type Ge surface by Cemedine. Cemedine gel dispersed with 5.3 weight% ITO particles.
The gel was solidified under a high pressure at 4x105 Pa for 2h at RT. Al electrode wasformed on rear surface of Ge solar cell by evaporation. Solar cell characteristics were measured
using a AM1.5 solar cell simulator at 1 sun.

Result and discussion
Figure 3 shows I-V characteristic for samples of n-type Si/adhesive/n-type Si with initial
ITO, reduction treated for 5 and 20 min at 22 oC. Every sample showed very good ohmic
characteristic. High current was obtained by reduction treatment for 5 min. The connecting
resistivity was 1.2 cm2. Reduction treatment by heating in vacuum was effective to decrease
resistivity of ITO particles. Color of ITO particles was markedly changed by reduction
treatment for 5 min from green (initial) to dark grey. Substantial oxygen atoms released during
heating and atomic bonding structure probably changed. Figure 4 shows change in connecting
resistivity as a function of temperature for samples of n-type Si/adhesive/n-type Si with initialITO, reduction treated for 5 and 20 min. The connecting resistivity gradually increased as- 130222027-2 -temperature increase above 30 oC. It was still
low of 2.0 cm2 at 70 oC for the sample with
reduction treatment for 5 min. Physics oftemperature change in the connecting
resistivity is not clear yet. One possibility isthat mobility of carriers in ITO particles wasdecreased by phonon scattering at a hightemperature. I-V measurements showed goodohmic conditions for every temperaturecondition. Those results indicate that ITO
silicon contact has an advantage of highrecombination properties to realize the ohmic
current-voltage characteristic between different
kinds of materials with different work Fig. 3 Current density as a function offunctions.
Figure5showsoptical voltage.
transmissivity
forsampleof
glass/5-min-reduced ITO+Cemedine/glass. It
was around 80 % for wavelength ranging from500 to 2000 nm. The transmissivity depends on
refractive index of Cemedine. The conductive
transparent adhesive consists of 5.3 weight% of
ITO particles and 94.7 weight% of Cemedine.
Reflection loss at Cemedine/glass interface
limited transmissivity to 80%. At most 5.3
weight% of ITO particle doesn’t decreasetransmissivity.
Figure 6 shows solar cell characteristic of
InGaP/GaAs/adhesive/Ge solar cell. In this case,initial ITO particles were used in the adhesive Fig. 4 Connecting resistivity as a function
layer. A typical solar cell characteristic means of temperature.
that good electrical contact between p-type
GaAs and n-type Ge by ITO particles. Jsc, Voc,
and the conversion efficiency were 12.6
mA/cm2, 2.13 V, and 15.9%, respectively. Vocof InGaP/GaAs solar cell was observed as 2.1 V
in other individual cells. Increase in Voc wasmuch smaller than we expected. There is apossibility of unexpected short circuit formationin Ge solar cell in the fabrication process steps.

Summary
We studied decrease in connecting resistivity byjoule heating reduction of ITO particles for its
application to transparent conductive adhesive.
ITO particles with the diameter ranging from 25
to 32 m were heated by joule heating at 60 W
for 5 and 20 min in a vacuum. ITO particles at5.3 weight% were dispersed in Cemedine

Fig. 5 Optical transmissivity for sample ofglass/1min-reduced ITO+Cemedine/glass.

- 130222027-3 -adhesive gel. The Cemedine gel including ITOparticles was sandwiched with two mirror
polished n-type silicon substrates with aresistivity of 0.001 cm. The Cemedine
intermediate layer including ITO particles wassolidified under a high pressure at 4x105 Pa for
2h at RT. I-V characteristics under temperature
ranging from 22 to 70 oC showed good ohmic
characteristics. The connecting resistivity of 1.2
cm2 was obtained at 22 oC in the case of 5 minjoule heating reduction. The connecting
resistivity increased as temperature increased
from 30 oC probably because of decrease incarrier mobility of the ITO particles.
InGaP/GaAs and Ge solar cells were stacked by
the present conductive transparent adhesive.
Typical solar cell characteristic was obtained. Jsc,Voc, the conversion efficiency were 12.6 A/cm2,2.13 V, and 15.9 %, respectively.

Fig. 6 Solar cell characteristic of sample
for InGaP/GaAs/adhesive/Ge solar cell.

Acknowledgments
This work was supported by the New Energy and Industrial Technology DevelopmentOrganization (NEDO)
References[1] T. Sameshima, J. Takenezawa, M. Hasumi, T. Koida, T. Kaneko, M. Karasawa, and M.
Kondo. Jpn. J. Appl.Phys. 50 (2011) 052301[2] J. Takenezawa, C. Akiyama, M. Hasumi, and T. Sameshima: International symposium on
Innovative solar cells 2012, (Tokyo, 2012) p 10.
[3]J. Olivier, B. Servet, M. Vergnolle, M. Mosca, G. Garry. Synthetic Metals 122 (2001) 87-89- 130222027-4 -