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

Development of Multi-junction technique for Semiconductor substrates
by using Transparent and conductive polymer for Solar cells’ application
Hirofumi KAKEMOTO, Masahiko HASUMI, and Toshiyuki SAMESHIMATokyo University of A&T, 2-24-16 Nakacho, Koganei, Tokyo, 184-8588 Japan
Keywords: solar cells, multi-junction, Si, ZnO, poly-thiophene
Abstract. The mechanical stacking technique was developed for multi-junctions ofsemiconductors by using transparent and conductive polymer (poly-thiophene: PT). In order topromote the multi-absorptions within stacked solar cells, ZnO powder was selected for thelight scattering agent adding to PT. ZnO powders were synthesized by vacuum evaporationfrom Zn in oxygen partial pressure of 100 Torr, and ex-situ oxidization was carried out at
900oC in the air. Aggregated ZnO powders were characterized by Raman scattering, FT-IR andimpedance analyzer. The molecular orbit (MO) calculation was carried out for PT. The energy
gap between highest occupied MO (HOMO) and lowest unoccupied MO (LUMO) wasestimated by extended Hükel MO method. A PT film was spin coated on Si substrates of theirbinder, and then the substrates were connected. The I-V properties of PT films were measured
with p-type and n-type Si substrates.
1.Introduction
The energy consumption has been increased from 1970’s, and that should be prevented andmodified by ecological technologies with producing new energy. Therefore several energy
generation devices (solar cells, thermoelectric power) and apparatus (wind power, geothermalpower, hydroelectric power generations) have been produced to be generating and recycling
new energies.[1] Si solar cell was innovated by M.D. Chapin and M. B. Prince et al. [2], andseveral materials have been investigated about their potentials for solar cells’ applications. The
crystal and amorphous Si as tandem type cells have been developed with increasing of demands
in commercial product use. Recently, stacked type solar cells such as Si and SiO2 (quantum dot)
multi-layer have been proposed in order to attain highly quantum efficiency.[3] The merit ofstacked type solar cells is possible to control the energy band and mini-gap suited for thesunlight spectrum. Sunlight shows an air mass spectrum (AM1.5) on the ground. The AM1.5
spectrum can be separated to three region (ultraviolet, visible, infrared) spectra. These lights
can be absorbed by wide, middle and narrow band-gap semiconductors.[4] Figure 1 shows theAM1.5 and concept of multi-junction solar(b)Sunlight(a)cells aiming highly sunlight absorption andCell-1electrical power generation. We have triedtofabricate
multi-junctionsofsemiconductor substrates
by usingpolymer
Cell-2transparent and conducting polymer.
Transparent and conductive oxides
polymer:ZnO(TCO) such as ZnO have been attracted for
Cell-3upper electrodes on the solar cells becauseof its transparency and conductivity. As
shown in Fig. 1 (b), we focus on ZnO
Fig.1: (a) Air-mass 1.5 spectrum (AM1.5).
powder as light scattering agent that is(b) The concept of multi-junction solar cells
returning back a part of light to absorbers
in this study. The cells can be selected wide(semiconductors), and is adding to the(cell-1), middle (cell-2) and narrow (cell-3)
junctions (polymer). In this paper, weband-gap
semiconductors. The cells arepresent the ZnO powder preparation,connectedby transparent and conductive
polymer coating and connecting Si
polymerfilmswith light scattering agent
substrates, and their optical and electrical(ZnOpowder).
properties
inorder
tofabricate
multi-junction solar cells.
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2. Experimental
2-1 Preparation of ZnO powder
ZnO powders were synthesized by vacuum evaporation
system equipped with a Knudsen cell (K-cell) in the partial O2
pressure of 100 Torr. The Zn was selected as a source material.
Zn was heated by a K-cell, and evaporated at melting point(420oC). Figure 2 (a) shows the preparation system and (b)photograph of synthesized ZnO and Zn powders. (c) Standard
ZnO powder (NewMet-Koch., 6N, grain size: less than
127µm) was confirmed as (d) aggregate bodies by colloidal
deposition of acetone dispersion media in order to measure theoptical and electrical properties.
In order to promote oxygen vacancies and creation ofcarriers, the oxygen reduction process was carried out for ZnO
aggregated bodies by carbon powder in Al2O3 crucible at
900oC in the air (0.5-5h).
2-2 Polymer
Figure 3 shows (a) the molecule structure and (b) wave
functions of PT. PT confirms the hetero-cyclic compound(C5H5) substituted with sulphur and cyclic hydrocarbons(C6H6). PEDOT (Aldrich) and Denatoron-G115s (Nagase)were selected in this study. PT films were coated on Si(10x20mm2, 525µm) and Ge (10mmx10mm2, 150µm)
substrates by a spin coater (Active, ACT-300AII).
2-3.Molecular orbit calculations
Following Hückel method, wave function (φ) of CnHn iswritten by φj=Σcjmχm and cjm=(1/(n)0.5)exp[i2(j-1)πm/n],
where m, j=1, 2,.., n. The eigen-energy is also
εj=α+λβ=α+2βcos[2(j-1)π/n], where α, λ, and β are
Coulomb integral (= ∫ χ i hχ i ), eigen-value and resonanceintegral (= ∫ χ i hχ j ), respectively. A sulphur ion act ashetero-cyclic compound in C5H5 of PT, and β is modified tobe βS=0.5β.[5] The molecular orbits (MOs) calculations
were carried out for PTs by conventional extend Hückel MO
method using software (WinMoStar), as shown in Fig.3.[6]

(a)

(b)

(c)

(d)

Fig.2: (a) Powder preparation
system, (b) ZnO, Zn powders,(c, d) ZnO aggregated bodies
preparedbycolloidal
deposition method.
(a)

(b)

Fig.3: (a) Structure and (b)2-4. Characterizations
electronicstructureofpoly-thiophene (Yellow: S,
The crystal structure, lattice constants and crystallite
Dark green: C, Red: O, Lightsizes of ZnO powders were estimated from x-ray
yellow: H atoms).
diffraction pattern (XRD, Rigaku RAD-IIc). Opticalphonon vibrations of ZnO aggregated bodies (as powderstate) were observed by µ-Raman scattering (RENISHAW, Ar+:514.5nm, 10mW, spot
size:5µm). Fourier transformed infrared spectra (FT-IR, JASCO-6100) were also measured forZnO aggregated bodies (transmittance, reflectance) and PT films (transmittance). Dc resistance(R) and frequency dependence of impedance (Z) of ZnO aggregated bodies (coated with Agelectrodes) were measured by using digital multi-meter (Agilent U1252A) and LCR meter(Hioki 3532: 40Hz-5MHz), respectively. Current versus applied voltage (I-V) characteristics of100228041-2

stacked Si/PT/Si structure were measured by a precision semiconductor parameter analyzer(Agilent 4516c) at room temperature.
3. Results and Discussion
3-1 Properties of ZnO aggregated bodies(a)(b)
Crystal structure, lattice constants,ZnOxidation
crystallite size of ZnO powders wereE1
estimatedtobeWurtzite(C6v),
A1
a=0.3247nm, c=0.5.2022nm and 0.635nm5hfrom XRD patterns, respectively.
(c)
Figure 4 (a) shows the µ-Raman
2hE1
scattering spectra of ZnO powders as0.5h(random polarization configuration) theA1
functions of oxidation and reduction
Non-reduction
process (dots). Particularly, Raman peaks
at 330cm-1 of ZnO powder were slightly
decreased with increasing reduction
Fig.4: (a) µ-Raman scattering spectra, (b)processing time, although density of ZnO
density and (c) A1 and E1 peak intensities and3
aggregated bodies was kept around 3g/cm
ratio of ZnO aggregated bodies as theas shown in Fig.4 (b). The irreducible
functions of oxidation and reduction
processing time.
representation of ZnO is Γ=A1+B1+E1+E2.
Hence, Raman peaks at 330cm-1 and436cm-1 are A1 and E1. In order toReduction 0.5h
estimate the relation between optical(b)2h(a)
phononvibrationandoxygenvacancies, Raman spectra were curve
fitted by Gauss functions (solid lines).
Reduction 0.5h
2hFigure 4 (c) shows the results of curve
fit about A1, E1 peaks and A1/E1 ratio.
From A1/E1 ratio, A1 peak (c-axis
Fig.5: FT-IR spectra: (a) transmittance and (b)vibration: 1 direction) is sensitive forreflectance configurations of ZnO aggregatedoxygen vacancies, but E1 peak (a-bbodies.
axes vibrations: 2 directions) isinsensitive because of possible vibration directions.
Figure 5 shows the FT-IR spectra of ZnO aggregatedbodies. Transmittance showed less than 14% in theinfrared region because highly random scattering effectReduction
0.5h
was appeared on the surface. Transmittance was increased
2hwith increasing the reduction processing time. In addition,
optical vibrations were weakly observed around 410cm-1.
5hReflectance shows also optical phonon peak at 410cm-1.
The intensities of the peaks were on the other hand
Oxidation
decreased with increased reduction processing time.
Figure 6 shows the frequency dependence of Z of ZnO
aggregated bodies. As a function of reduction time,Fig.6: Frequency dependence ofZnO was decreased Z. Lower than 3kHz, a few ofimpedance for ZnO aggregatedshoulders are observed by polarization among grainsbodies
as a function of oxidation andof ZnO. Polarization among grains is cased byreduction processing time.
Schottky contacts. Grain contacts would be drastic

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changed as a function of reduction processing time. [8] From dc measurements, R of ZnO
aggregated bodies decreased from 100MΩ to 10Ω with increasing oxygen vacancies. Therefore,prepared ZnO powder is conductive and light scattering agent, and it could be adding intotransparent and conductive polymer (ceramic composite).
3-2 Molecular orbit levels’ estimations about poly-thiophene
HOMO
The PT has been known as a transparent andconductive polymer. Highly conductive polymer:
PEDOT-PSS, PEDOT and Denatron-G115s are attracted
for transparent functional devices as the electrodes on
LUMO
organic and/or inorganic materials, and the applications
ofdye-sensitised
solarcells
andorganicelectro-luminescence (EL) devices have beenreported.[9]In order to estimate electrical contact between PT andSi:EcSi, the MO calculation of PT (nEDOT) was tried byextend Hückel MO method. Figure 7 shows MO levelsof PT (4EDOT). The energy gap of 4EDOT (Quinoidtype) between HOMO and LUMO was estimated to be
5.34eV (ascribes transparent; 4-4.8eV is reported from
14EDOT with 8 stacked layers: Diradical type), and gap
Si:Evbetween nearest MO level and Si (1.12eV) is estimated Fig.7: Estimated MO energy levels ofto be 0.08-0.18 eV (Ohmic contact). It would be a poly-thiophene, and a possible Ohmic
contacts for conduction and valence
possibility for Ohmic contact between PT and Si.
bands in Si.
3-3 Transmittance of polymer composite and I-Vcharacteristics of Si/PT/Si
Finally, in order to estimate transparency and conductivity
of PT (Denatron:G115s), we make a junction tentatively
between glass and p and n-tyep Si substrates by using
PEDOT and Denatron-G115s. Figure 8 shows (a)transmittancespectrumofpolymer
composite(Denatron-G115s with ZnO powder 8-14vol%) on glass
substrate and (b) I-V characteristics of Si substrates
connected by Denatron-G115s. Fundamentally, PT is
possible to be controlled as p-type (removing electron), asn-type (introducing electron), and is known as Bi-polaron
conductor. Unfortunately, the polymer (Denatron-G115s)
would be mainly act as p-type conductor, but we should
focus on the linearity around lower voltage region. As a nextstep, we should be decreasing R in the junction due tomodifying connect conditions (polymer state, surface
process of Si substrate).
So as to use the multi-junction solar cells, polymer should
be considered the aging effects under focussed sunlightirradiation and Joule-heating. Particularly, ultraviolet cuts
the MO bonds in polymer, and thermal heating oxidizes
polymer. As a merit in this study, however, we can use thickwide band gap semiconductor at top position (cell-1 in-100228041-4

(a)

(b)

glass
ZnO 8vol%ZnO 14vol%

p-Si/PT/p-Si

n-Si/PT/n-Si

Fig.8: (a) Transmittance ofpolymer composite between
glassesand(b)I-Vcharacteristics of Si/PT/Si
stacked structure.

Fig.1(b)) in order to absorb ultraviolet light. In addition, oxidization effect should beinvestigated in future study.
4. Conclusion
ZnO powders were synthesized, and were characterized by optical phonon’s vibrations, lightscattering and electrical properties as a function of oxidation and reduction processing time.
From molecular orbit (MO) calculation for poly-thiophene (PT), energy gap between highest
occupied MO (HOMO) and lowest unoccupied MO (LUMO) of PT was estimated, and apossibility of Ohmic contact between PT and Si was obtained. Si substrates were tentatively
joined by PT, its I-V property was measured with light illumination. Non-linearity in I-V
property was considered not only conduction type but polymer state and surface process of Si
substrates.
Acknowledgments
This work was supported by New Energy and Industrial Technology Development
organization (NEDO). The authors would like to thank W. Katoh and S. Yoshitomi for technicalassistance, and H. Hayasaka and M. Kimura for kind assistance.
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