行政院國家科學委員會專題研究計畫 成果報告
以矽為基材之太陽電池結構之研究 研究成果報告(精簡版)
計 畫 類 別 ： 個別型
計 畫 編 號 ： NSC 96-2221-E-011-150-
執 行 期 間 ： 96 年 08 月 01 日至 97 年 07 月 31 日 執 行 單 位 ： 國立臺灣科技大學電子工程系
計 畫 主 持 人 ： 莊敏宏
計畫參與人員： 碩士班研究生-兼任助理人員：劉哲孝 碩士班研究生-兼任助理人員：陳哲豪 碩士班研究生-兼任助理人員：張明全 碩士班研究生-兼任助理人員：張家偉
處 理 方 式 ： 本計畫涉及專利或其他智慧財產權，2 年後可公開查詢
中 華 民 國 97 年 08 月 17 日
NSC
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Effects of p+ layer and its contact electrode on the electrical characteristics of p-i-n solar cell have been studied. The short-circuit current would be increased with the reduced thickness of p+
layer, due to less optical absorption in the heavily doped p+ region. However, a thinner p+ layer may form a smaller series resistance and thus a larger dark current of the p-i-n structure. As a result, for the p+ layer of 0.1 µm here, the open-circuit voltage is degraded, relative to a thicker p+ layer. The p+ layer of 0.1 µm thickness can still achieve a relatively larger conversion efficiency, attributable to a larger short-circuit current. On the other hand, for the ITO electrode, the short-circuit current would just be slightly increased with the reduced width of contact electrode, due to less area of optical absorption. However, the contact electrode of 0.05 µm width causes a smaller short-circuit current than that of 0.1 µm width, due to a smaller carrier collection area. As a result, the contact electrode of 0.1 µm width can achieve a relatively large value of the open-circuit voltage and the maximum conversion efficiency.
1.
The price of petroleum is getting higher in this century, a new energy is needed. Because solar energy is clean and almost unlimited, it is considered as a good substitute energy. Several technology in fabricating solar cell have been previously developed [1-7]. Polycrystalline or multi-crystalline silicon solar cells have become a strong contender for terrestrial applications [8].
In recent years, there has been a substantial improvement in the quality of the polycrystalline silicon substrates. The effective minority carrier diffusion length has increased from 80~100 µm to 150~200 µm. However, due to the presence of grain boundaries, subgrain boundaries, and impurities, there remains a substantial gap in the efficiency of polycrystalline silicon solar cells when compared with that of single crystalline cells [9-10].
An internal electric field is needed to separate carriers more efficiently, and thus the p-i-n structure was usually applied in the device structure of solar cells. In addition, the top emitter layer of either heavily-doped n+ or p+ layer would considerably absorb the incident light. Hence, a thin top emitter layer may generally be expected for increasing the short-circuit current.
However, the open-circuit voltage and the short-curcuit current may also be substantially affected by the emitter layer and also its contact electrode.
In this study, effects of p+ emitter layer and its contact electrode on the electrical output characteristics of p-i-n solar cell have been studied. The i-layer and the p+ layer of various thickness have been conducted. In addition, contact electrode of various size and material has been examined.
2.
A typical process flow was firstly described below. Figure 1 shows the resultant structure of the p-i-n solar cell. The n+ poly-Si layer was heavily doped with 1×1019 cm-3, and the n-type
poly-Si layer of the i layer was phosphorus-doped with 1×1014 cm-3. The i-layer of various thickness were carried out. Then, the p+ layer was formed with heavily boron-doped of 1×1019 cm-3. Various boron implantation energies were carried out to form different p+ layer thickness.
The front contact was formed by the evaporation of 100-nm ITO or Al film onto the wafer, and then the use of the lift-off technique for delineation of contact region. Various front-contact width were formed to examine the effect on the characteristics of solar cell.
In addition, the back contact was formed by evaporating of Ti/Ag (50-nm/50-nm) on the top of the A1 back-surface-field (BSF) contact region. Figure 1 shows the resultant structure of the p-i-n solar cell. Contacts were then annealed in a forming gas ambient for 30 min at 400°C.
Subsequently, an oxide layer was deposited on the upper surface for passivation of the solar cell.
All the resultant solar cells were electrically characterized by light measurements, for examining the output voltage, the short-circuit current, and also the conversion efficiency of the solar cells.
The global AM1.5 spectrum was used.
3.
Figure 2 shows the dependence of short-circuit current on the p+ layer thickness for the i-layer thickness of 10, 20, and 30 −µm, respectively. A larger i-layer thickness would inherently provide a larger absorption region of solar energy, thus causing a larger short-circuit current. In addition, a thicker p+ layer would cause more optical absorption in this heavily doped region, and thus the solar energy available for carrier generation in the i-layer would be considerably reduced. Hence, the short circuit current would be increased with the reduced thickness of p+
layer, which is also helpful to increase the open-circuit voltage.
However, a thinner p+ layer may form a smaller series resistance for the dark current of the p-i-n structure, which may degrade the open-circuit voltage. Figure 3 shows the dependence of open-circuit voltage on the p+ layer thickness for the i-layer thickness of 10, 20, and 30 −µm, respectively. A thicker p+ layer may cause a smaller short-circuit current but increase the dark current at a given forward bias voltage. As a result, the open-circuit voltage does not show considerable variation with the p+ layer thicker than 0.2 µm here. However, for the p+ layer of 0.1 µm, the open-circuit voltage is obviously degraded due to a small series resistance and thus a large dark current. On the other hand, though a thicker i-layer can lead to a larger short-circuit current, but still result in a smaller open-circuit voltage due to a smaller internal electric field and thus a larger dark current. In terms of the maximum conversion efficiency, the p+ layer of 0.1 µm thickness can still achieve a relatively larger efficiency, attributable to a larger short-circuit current. Figure 4 shows the dependence of maximum conversion efficiency on the p+ layer thickness for the i-layer thickness of 10, 20, and 30 −µm, respectively.
On the other hand, in terms of the contact electrode, Figure 5 shows the dependence of short-circuit current on the width of contact electrode for the Al and the ITO electrode, correspondingly, for a p+ layer of 0.1 µm thickness. For the Al electrode, the short-circuit current would be largely reduced with the increased width of contact electrode. This result reflects the more light reflected by the Al electrode with a larger area. In addition, for the ITO electrode, the ITO electrode is much more transparent to light. Hence, the short-circuit current would just be slightly increased with the reduced width of contact electrode. Furthermore, it is noted that the contact electrode of 0.05 µm width would even cause a smaller short-circuit current than that of 0.1 µm width. A larger width of contact electrode would be more helpful to collect the generated carrier. Moreover, a larger width of the ITO electrode would only slightly degrade the light absorption. Hence, the contact electrode of 0.05 µm width may lead to a smaller short-circuit current than that of 0.1 µm width.
In addition, Figure 6 shows the dependence of open-circuit voltage on the width of contact electrode for the Al and the ITO electrode, correspondingly, for a p+ layer of 0.1 µm thickness.
For the Al electrode, the open-circuit voltage shows a slight decrease with increasing the width of contact electrode, due to the reduced short-circuit current. For the ITO electrode, the open-circuit voltage still shows a slight decrease with increasing the width of contact electrode.
In addition, the contact electrode of 0.05 µm width would cause a smaller open-circuit voltage than that of 0.1 µm width, due to a smaller short-circuit current. On the other hand, Fig. 7 shows the dependence of maximum conversion efficiency on the width of contact electrode for the Al and the ITO electrode, correspondingly, and a p+ layer of 0.1 µm thickness. Hence, the contact electrode of 0.1 µm width can achieve a larger value of the maximum conversion efficiency, due to a larger short-circuit current and a larger open-circuit voltage.
4.
Effects of p+ layer and its contact electrode on the electrical characteristics of p-i-n solar cell have been studied. The short circuit current would be increased with the reduced thickness of p+ layer, due to less optical absorption in the heavily doped p+ region. However, a thinner p+
layer may form a smaller series resistance for the dark current of the p-i-n structure. As a result, the open-circuit voltage does not show considerable variation with the p+ layer thicker than 0.2 µm here. Moreover, for the p+ layer of 0.1 µm, the open-circuit voltage is obviously degraded due to a small series resistance and thus a large dark current. In terms of the maximum conversion efficiency, the p+ layer of 0.1 µm thickness can still achieve a relatively larger efficiency, attributable to a larger short-circuit current. On the other hand, for the ITO electrode, the short-circuit current would just be slightly increased with the reduced width of contact electrode, due to less area of optical absorption. However, the contact electrode of 0.05 µm width causes a smaller short-circuit current than that of 0.1 µm width, due to a smaller carrier collection area. In addition, the resultant open-circuit voltage exhibits a similar trend as the short-circuit current. As a result, the contact electrode of 0.1 µm width can achieve a relatively large value of the maximum conversion efficiency, due to a larger short-circuit current and a larger open-circuit voltage.
5.
[1] A. W. Blakers and M. A. Green, Appl. Phys. Lett., vol. 48, pp. 215-217, 1986.
[2] S. Narayanan, S. R. Wenham, and M. A. Green, IEEE Trans. on Electron Devices, vol.37, pp. 382-384, 1990.
[3] C.-T. Sah, K. Yamakawa, and R. Lutwack, IEEE Trans. on Electron Devices, vol. ED-29, No. 5, pp. 903-908, 1982.
[4] S. Narayanan, S. R. Wenham, and M. A. Green, Appl. Phys. Lett. vol. 48, pp. 873-875, 1986.
[5] A. Cuevas, R. A. Sinton, N. E. Midkiff, and R. M. Swanson, IEEE Electron Device Lett., Vol. 11, pp. 6-8, 1990.
[6] R. A. Sinton, Y. Kwark, J. Y. Gan, R. M. Swanson, IEEE Electron Device Lett., vol. 7 issue 10, pp. 567-569, 1986.
[7] D. E. Carlson, IEEE Trans. Electron Devices, vol. 36, No. 12, pp. 2775-2780, 1989.
[8] S.N. Singh, S. K. Sharma, P. K. Singh, B. K. Das, IEEE Trans. Electron Devices, vol. 39, pp. 362-369, 1992.
[9] P. Sana, J. Salami, A. Rohatgi, IEEE Trans. Electron Devices, vol. 40, pp. 1461-1468, 1993.
[10] S. Marayanan, S.R. Wenham, M. A. Green, IEEE Trans. on Electron Devices, vol. 37, pp.
382-384, 1990.
6.
Fig. 1 The resultant structure of the p-i-n solar cell.
Fig. 2 Dependence of short-circuit current on the p+ layer thickness for the i-layer thickness of 10, 20, and 30
−µm, respectively.
Fig. 3 Dependence of open-circuit voltage on the p+ layer thickness for the i-layer thickness of 10, 20, and 30
−µm, respectively.
Fig. 4 Dependence of maximum conversion efficiency on the p+ layer thickness for the i-layer thickness of 10, 20, and 30 −µm, respectively.
Fig. 5 Dependence of short-circuit current on the width of contact electrode for the Al and the ITO electrode, correspondingly, and a p+ layer of 0.1 µm thickness.
Fig. 6Dependence of open-circuit voltage on the width of contact electrode for the Al and the ITO electrode, correspondingly, and a p+ layer of 0.1 µm thickness.
Fig. 7Dependence of maximum conversion efficiency on the width of contact electrode for the Al and the ITO electrode, correspondingly, and a p+ layer of 0.1 µm thickness.
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