Chapter 5 Experiments of MIS Stacked Solar Cells 42
5.2 Fabrication of MIS Stacked Solar Cells
To realize a MIS stacked solar cell, we should integrate three different junctions discussed early together. Starting from a bonded sample first, n-type and p-type MIS junctions are fabricated on both sides. However, 4-inch Si wafers has a thickness about 525um, which is thicker than the skin depth of visible light, so the cell on the back side under illumination can absorb light hardly. From point of current matching, net current output would be limited by bottom cell which generates less current. Consequently, a thinning process of bonded sample has to be done before two MIS junction fabrication to increase current in stacked cells. Although bonded sample can suffer mechanical thinning process, huge amount of defects at the surface are produced simultaneously.
Hence, additional chemical etching process is introduced to remove surface defect and thin the wafer at the same time. The fabrication processes of MIS stacked solar cell are listed below:
(1) Bonded samples thinning with sand paper
(2) Dip samples into Si etchant (HF:CH3COOH:HNO3=6:7:20) to remove surface defects and thin the samples at the same time
(3) Deposit tunneling oxide with sputter at both sides
F
-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 1E-6
1E-5 1E-4 1E-3 0.01 0.1 1 10
100
MIS Stacked Solar Cells (Dark)
no thinning n-thinning p-thinning Current Density (mA/cm2 )
Forward Bias (V)
Fig 5.6: Current-voltage properties of MIS stacked solar cells in dark with (a) no thinning process (b) n-type cell thinning on the top (c) p-type cell thinning on the top
The current-voltage characteristics of MIS stacked solar cells under AM1.5g are shown in Fig 5.7 and Table 5.1. The cell without thinning process exhibits low Jsc
5.25mA/cm2 because the current is limited by bottom cell discussed in section 5.2. After thinning the top n-type cell to 50nm approximately, a much higher Jsc 16.5mA/cm2 is obtained. However, the large leakage current suppresses the photovoltaic performance and the cell with n-type thinning on the top reaches the Voc 414mV.
Since the cell with p-type thinning on the top reveals a low leakage current, the Voc
would not be suppressed and reaches V 593mV. This value is larger than single
junction MIS solar cells discussed in Ch3 and Ch4, demonstrating the utility of stacked cells. In this structure, the bottom cell is n-type MIS solar cells; the photo current of n-type cells are determined by minority carriers, holes, which have a lower mobility and cannot be collected by electrodes as easily as electrons. Hence, better photovoltaic properties of cells with n-type thinning on the top can be expected.
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -4
0 4 8 12 16
20
MIS Stacked Solar Cells (illumination)
Current Density (mA/cm2 )
Voltage (V)
n-type on the top without thinning n-type thinning on the top
p-type thinning on the top
Fig 5.7: Current-voltage properties of MIS stacked solar cells under illumination
Cell Voc (mV) Jsc (mA/cm2) FF(%) Efficiency
No thinning 437 5.25 23.6 0.54%
n-thinning 414 16.5 25.2 1.71%
p-thinning 593 4.74 41.1 1.15%
5.4 Summary
In this chapter, tunneling diode fabrication using direct wafer bonding is realized first. Although the bonded samples show no NDR which should present in tunneling diode, high reversed saturation current diodes are achieved with only 40mV loss while current density reach 10mA/cm2. Finally, we integrate n-type and p-type MIS solar cells on bonded sample to realize MIS stacked structure. The Voc of MIS stacked cells reach 593mV, lager than that of p-type and n-type cells, demonstrating that the structure is practical and have its potential to achieve water splitting.
Chapter 6 Conclusion and Future Work
6.1 Conclusion
In this thesis, MIS stacked solar cells are introduced and achieved in order to reach high voltage output.
First, we give general description of the transport mechanism of MIS solar cells in Chapter2. We also study the MIS solar cells on n-type and p-type of substrates in chapter3 and chapter4. The influence of fabrication parameters such as annealing process, tunneling oxide thickness, working pressure and metal thickness on MIS solar cells are investigate. With the best conditions, we get the Voc equals to 475mV on p-type cells and 422mV on n-type cells.
In the beginning of chapter5, we introduce the fabrication of tunneling diode by direct wafer bonding. Although the bonded samples exhibit no NDR at small forward bias, diodes with high reversed saturation current are accomplished and can be used in the MIS stacked solar cells. Afterwards, MIS stacked solar cells are realized by integrating two types of MIS solar cells on bonded samples and best Voc equals to 593mV in our experiment is obtained.
6.2 Future Work
Since best V in our cells is 593mV, lower than theoretic V equals to 0.9V, there
are still many issues to be investigated. First, the voltage drop at the central junction could be lowered by further improving the tunneling current with proper doping activation and defect passivation. Moreover, the thickness of upper cells and bottom cells in stacked cells should be precisely controlled and the current matching could be achieved.
Afterward, in order to enhance the efficiency, Jsc is the important issue to be improved. Surface texturing by chemical etching is a common way to reduce surface reflection while some researches indicate that Voc would slightly decrease. Besides, transparent conductive oxide such as ITO and ZnO can be introduced in our structure with the aid of sputter to further increase Jsc[29].
Finally, though the combination between solar cells and fuel cells is still a big challenge, our research does help the development of solar cells and the living of human beings.
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Vita
Hsin-Yu Lee was born at 25, Aug. 1986 in Tainan, Taiwan. He
received the B.S. degree in Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan in 2007. The M.S.
degree will receive in Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan in 2009. His research include the analysis and fabrication of MIS stacked solar cells.
Publications:
[1] Hsin-Yu Lee, Yi-Shian Max Lin, Kuang-Yang Kuo, Tzu-Yueh Chang, and Po-Tsung Lee, “Post-annealing Temperature Effect on the Optical and Electrical Properties of the Nano-structured Si/SiO2 Multilayer”, ISSCT’08, Taipei, Taiwan (2008)