五十七
15th International Photovoltaic Science & Engineering Conference (PVSEC-15) Shanghai China 2005 TF Development of Novel Nanocrystalline SiC,
Si1-x Ge x C and Ge1-y C y for Solar Cell Applications 1Makoto Konagai, 1Shinsuke Miyajima and 2Akira Yamada
1Department of Physical Electronics, 2Quantum Nanoelectronics Research Center, Tokyo Institute of Technology
2-12-1-S9-9, Ohokayama,Meguro-ku, Tokyo 152-8552 JAPAN, E-Mail: konagai@pe.titech.ac.jp
Abstract: The research and development of nanocrystalline(nc) 3C-SiC, Si1-x Ge x C and Ge1-y C y thin films for solar cell applications are presented. Hot Wire CVD technique is employed to prepare high quality nc-3C-SiC thin films whose bandgap is 2.2eV. Monomethylsilane and hydrogen are used as reactant gases for the deposition of nc-SiC thin films. N and Al are used as dopants to successfully obtain n and p type nc-3C-SiC thin films. High conductive doped nc-3C-SiC is very attractive for the potential window layer applications in Si thin film and Si heterojunction solar cells. The pin solar cell structures in which all p,i,n layers consist of nc-3C-SiC have been prepared for the first time. A novel Si HIT solar cell with an n-type nc-3C-SiC showed an efficiency of 13.3%. Furthermore, attempts to control the bandgap of nc-3C-SiC are done by incorporating Ge into SiC. Of late, we have succeeded to prepare nc-3C- Si1-x Ge x C with a Ge content of 2%, which showed about 0.2eV lower absorption spectra compared to that of 3C-SiC. Key Words: Nanocrystalline Alloy, Thin Film Solar Cells, SiC , HIT Solar Cells.
1 Introduction
The world production of Si-based thin-film solar cells exceeded 50MW in 2004. However, for further deployment of Si thin-film PV systems after 2010, a drastic reduction in manufacturing cost of solar cell modules compared with the state- of-the-art PV technologies will be required. The Japanese PV roadmap toward 2030 as shown in Tab.1 indicates that module efficiencies over 18% will be required to achieve the target in the field of thin-film Si solar cells. To improve the efficiency of Si-based thin-film solar cells, novel triple-junction solar cells will be developed.
Tab.1 Japanese PV roadmap toward 2030
Cell
Type 2010 2020 2030 Bulk
Si 16(20) 19(25) 22(25) Thin Film Si 12(15) 14(18) 18(20)
CIS 13(19) 18(25) 22(25) Super
High
efficiency 28(40) 35(45) 40(50)
Dye Sensitized Cell 6(10) 10(15) 15(18) Up to now, an efficiency of over 14% has been achieved for a small area a-Si/μc-Si tandem solar cell. The current efficiency of Si based thin-film tandem solar cells is mainly limited by the amorphous Si(a-Si) top cell efficiency, especially, by the degradation of the a-Si absorber layer. Our idea is to improve the efficiency of multi-junction solar cells by using a nc-Si1-x Ge x C top cell instead of a-Si. It is reported that no light induced degradation is observed for nc-Si solar cells. By producing high quality nc-Si1-x Ge x C top cell, it is viable to expect much higher efficiency range than that of multi-junction cells with a-Si top cell. Theoretical analysis demonstrated that an efficiency of 20% could be expected for a triple-junction configuration.
In this presentation, preliminary results of solar cell properties using these novel alloys will be demonstrated. Furthermore, a novel Si HIT solar cell with an n-type nc-3C-SiC was proposed to improve an efficiency of Si solar cells. 2 Preparation of nc-3C-Si1-x Ge x C by HWCVD
2.1 HWCVD System
Up to now, our group has developed the preparation techniques of nc-3C(cubic)-SiC with a bandgap energy of 2.2eV at substrate temperatures of 200-300 o C by using Hot Wire CVD method with rhenium (Re) wires [1]. Schematic diagram of the deposition system used in the present study is shown in Fig.1. Monomethylsilane (MMS) and hydrogen (H2) were used for reactant gases. The use of HWCVD technique is very important for the successful low temperature deposition of nc-3C-SiC films with a high deposition rate. A large amount of atomic hydrogen is required for the crystallization of SiC. This technique is simple and also the best method to generate high density of atomic hydrogen. Deposition of nc-3C-Si1-x Ge x C films to control the bandgap of group IV nanocrystalline materials by adding dimethylgermane (DMG) to MMS is also tried.
Filament：Re TMA:Trimethylaluminum
Fig.1 Schematic diagram of the HW-CVD system
2.2 nc-3C-Si1-xGexC
The flow rate ratio of [H2/MMS] during the deposition of nc-3C-SiC was found to be a key parameter in controlling the film quality. The formation of stoichiometric n c-3C-SiC was confirmed by the Raman spectroscopy, XPS and TEM measurements [2]. Fig.2 shows the XRD patterns of the films deposited with different H2/MMS ratio. Thickness of the films
was about 250nm. For H 2/MMS ratio of 20, no diffraction peaks were observed in the XRD pattern, indicating that this film didn’t contain any crystalline phase. On the other hand, a diffraction peak was observed at around 35.7° when H 2/MMS ratio was larger than 35. This peak can be attributed to diffraction peak of 3C-SiC(111).
The optical measurements of these HWCVD deposited thin films of nc-3C-SiC showed a typical indirect transition absorption spectra and the calculated bandgap energy is found to be 2.2eV which is corresponding to that of bulk 3C-SiC.
2θ (deg.)
Fig.2 XRD patterns of the nc-3C-SiC films deposited at different H 2/MMS ratio
Deposition of nc-Si1-xGexC films to control the bandgap of group IV microcrystalline materials by adding DMG to MMS is also tried [3]. nc-Si1-xGexC film was successfully deposited for the Ge mole fraction less than 5%. We investigated optical properties of the films and found that the band gap of the deposited films is about 0.2eV lower than that of nc-3C-SiC(Fig.3). Raman and XRD measurements revealed that for DMG/MMS ratio higher than 0.23, SiGe microcrystallites tend to grow and suppress the formation of SiC nanocrystallites.
21010101
341
2310
4
10
Photon energy (eV)A b s o r p t i o n c o e f f i c i e n t (c m -1
)
Fig.3 Absorption spectra of nc-3C- Si 1-x Ge x C films
3 Doped nc-3C-SiC
The absorption spectra of nc-3C- Si 1-x Ge x C with a Ge mole fraction of 0.02 shifted towards lower energies indicating the bandgap narrowing of 0.2eV
High conductive doped nc-3C-SiC is very attractive for the potential window layer applications in Si thin film and Si
heterojunction solar cells.The doping characteristics for
nc-3C-SiC was investigated by using Hexamethyldisilazane(HMDS) and Trimethylaluminum(TMA) as dopants.
The conductivity of N doped n-type nc-3C-SiC films increased with a doping concentration of HMDS. Up to now, the high conductivity of 5 S/cm could be achieved for the best optimized deposition conditions[4] as shown in Fig.4. For p-type, as-grown Al-doped nc-3C-SiC films showed high resistivity, but on thermal annealing, the conductivity increased to the level of 1×10-3 S/cm, due to Al-H bond breaking. Fig.5 shows the p-type doping properties for nc-3C-SiC[5].
0.5
110101010101010101010HMDS/MMS
C o n d u c t i v i t y (S /c m )
Fig.4 N-type doping properties for nc-3C-SiC using HMDS as a dopant
10-9
10-810-710-610-510-410-310-210-1100TMA/MMS
C o n d u c t i v i t y (S /c m )
Fig.5 P-type doping properties for nc-3C-SiC using TMA as a dopant
4 nc-Ge 1-y C y
The other issue in developing Si-based thin-film multi- junction solar cells includes the development of high rate deposition techniques of nc-Si. For preparing multi-junction structures, the deposition rate of 5-10nm/s will be required for depositing 1-3μm thick nc-Si, due to its low absorption coefficient. Our group proposed nc-Ge 1-y C y as the bottom cell material with a similar bandgap energy of Si[6]. The absorption coefficient of nc-Ge 1-y C y is expected to be much higher than that of Si[7] and subsequently we could reduce the thickness of the bottom cell drastically by using nc-Ge 1-y C y instead of nc-Si.
Monomethylgermane (MMG) and H 2 were introduced to prepare nc-Ge 1-y C y thin films. The structural properties of the films were characterized using Raman scattering spectroscopy
and Fourier transform infrared absorption spectroscopy (FTIR). The films deposited at the hydrogen dilution ratio larger than 12 were found to be nanocrystalline. The XPS results revealed that the films deposited with MMG source contained about 7 to 8 at.% of carbon. The carbon composition depends on the filament temperature and the hydrogen dilution. The absence of C-C bonds in the FTIR spectra clearly indicates that the carbon atoms are not segregated in the matrix. The dark conductivity of nc-Ge 1-y C y films deposited by using the MMG source is between 10-8–10-3 S/cm.
The absorption coefficient of the nc- Ge 0.93C 0.07 films is shown in Fig.6 in comparison with those for crystal Ge, Si and amorphous Ge-C. It is evident that the absorption edge of nc-Ge 0.93C 0.07 films shifts to higher energy than that of c-Ge. For instance, the energy E 04 corresponding to the absorption coefficient of 104 cm -1 shifted up by 0.44eV compared to c-Ge.
0.51
1.52
2.5
10
3
10
4
105
Crystal Ge
Energy (eV)
A b s o r p t i o n c o e f f i c i e n t (c m -1)
Crystal Si
nc-Ge 0.93C 0.07
a-Ge 0.95C 0.05
Fig.6 Absorption coefficient of the nc-Ge 0.93C 0.07 films
5 Novel Si Heterojunction Solar Cells with Doped nc-3C-SiC
In this paper, we proposed two types of novel Si-based solar cell structures with doped nc-3C-SiC. One is the Si-based thin film solar cell deposited on a TCO substrate. And the other structure is a silicon heterojunction(HIT) device using doped nc-3C-SiC on both sides of the Si wafer.
5.1 nc-3C-SiC Pin Structure
It is very difficult to deposit doped nc-3C-SiC directly on SnO 2 due to the reduction of TCO by atomic hydrogen. In order to protect the TCO surface from the atomic hydrogen, we proposed to introduce a TiO 2 layer as shown in Fig.7. Optimization of the thickness of TiO 2 films for protecting the TCO surface is already completed and we found that the TiO 2 thickness of 10nm is enough for protecting the TCO layer from atomic hydrogen.
SnO TiO 210nm
p nc-3C-SiC 15nm
Ag/Al
2
n nc-3C-SiC 15nm
Fig.7 P-type nc-3C-SiC/i-type nc-3C-SiC/n-type nc-3C-SiC solar cell structure
External collection efficiency was investigated in order to evaluate photovoltaic performances(Fig.8). It was found that the onset of the collection efficiency corresponded to the bandgap energy of nc-3C-SiC. At present, the absolute collection efficiency is not high enough for the practical application, but much higher collection efficiencies can be expected by optimizing the deposition conditions.
Q u a n t u m e f f i c i e n c y
00.01
0.02
Wavelength (nm)
Fig.8 External collection efficiency of the nc-3C-SiC pin solar
cell
5.2 Novel Si Heterojunction Solar Cells with nc-3C-SiC Doped Layers
The other structure we put forward is a silicon heterojunction device using doped nc-3C-SiC on both sides of the Si wafer (Fig.9). In this device structure, the optical confinement can be expected in addition to the carrier confinement. HIT structures developed by Sanyo consist of doped amorphous Si(a-Si)/ undoped a-Si/Si wafer[8]. But in the structure proposed by this work, a-Si is replaced by wide bandgap nc-3C-SiC, thereby absorption by a-Si layer can be avoided. Furthermore, we can expect larger built-in potential by using nc-3C-SiC.
p nc-3C-SiC n nc-3C-SiC electrode
ITO ITO
(buffer layer)
Fig.9 Novel HIT solar cells with doped nc-3C-SiC with a bandgap energy of 2.2eV
Fig.10 shows the collection efficiency spectra of a novel HIT solar cell with doped nc-3C-SiC for both sides of the Si wafer as shown in Fig.9[9]. The Si wafer was texture-etched by using a KOH solution. The thickness of novel HIT cells is 150μm. The device showed a short-circuit current of 36mA/cm 2. The collection efficiency data for c-Si solar cells with an efficiency of 24.5% (Isc:41.6mA/cm 2) is also illustrated in the figure for comparison[10].
Up to now, an efficiency of 13.3% has been achieved for novel HIT solar cells without texture etching. This is the first report of efficiency data for novel HIT solar cells with doped
nc-3C-SiC.
(a)
(b)
Fig.10 Spectral response of novel HIT solar cells with doped nc-3C-SiC(CZ, 150μm)(a), and that for Si solar cell with an efficiency of 24.5% achieved by J.Zhao(FZ Si, 400μm)[10]
6 Summary
The preparation techniques of nc-3C-SiC with a bandgap energy of 2.2eV have been developed by using Hot Wire CVD method. Nanocrystalline 3C-Si 1-x Ge x C with a Ge content of 2%, which showed about 0.2eV lower absorption spectra compared to that of 3C-SiC, has been also fabricated.
N and Al were used as dopants to achieve n-type and p-type nc-3C-SiC films respectively. The deposition conditions were optimized to yield high conductivity in both n and p type nc-3C-SiC films. These doped nc-3C-SiC films find extensive applications as window layers in Si thin film and Si heterojunction solar cells.
Acknowledgements
This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade and Industry (METI). References
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