TECHNOLOGYSOLAR
conventional type. In addition, as shown in Fig. 1, the HIT solar cell has a symmetrical structure that provides two features. One is the applicability of the cell to a so-called bifacial module which can generate more electricity than an ordinary module, and the other is a stress-free structure, which is very important for thinner wafer processing [3].
Efforts at improving efficiency Although there are various losses restricting the efficiency of the HIT solar cell, we have been focusing mainly on the following techniques for obtaining higher conversion efficiency. A. Improving the passivation capability of a-Si/c-Si heterointerfaces to reduce the surface recombination loss for a higher open circuit voltage (Voc).
fabricating the a-Si, TCO layers and electrodes.
B. Reducing the optical absorption loss of the TCO and amorphous layers for a higher short circuit current (Isc).
C. Improving the definition and electrical conduction of the grid electrode to suppress shadow and resistance losses for a higher Isc and fill factor (F.F.).
Improving the passivation capability As previously mentioned, the high performance of the HIT solar cell is characterized by the excellent passivation capability of the a-Si/c-Si heterointerface with intrinsic a-Si interlayer. In order to improve the properties of the a-Si/c-Si heterointerface, we have been focusing on the following techniques:
Cleaning the c-Si surface using low-cost wet cleaning processes before a-Si deposition
Depositing a high-quality intrinsic a-Si layer by plasma enhanced chemical vapor deposition
Maintaining low plasma and thermal damage to the c-Si surface and heterojunction while
Improvements in the above techniques have diminished localized states in the intrinsic a-Si layer and heterointerface, which results in a longer minority carrier lifetime (LT) in a HIT solar cell.
Figure 2 shows the relationship between the Voc and LT of HIT solar cells. In the early stage of HIT solar cell development, the Voc and LT were improved by refining the c-Si surface cleaning process, as shown in the lower left portion of the figure. Subsequently, the development of a process with low plasma and thermal damage further improved the Voc and LT, as shown in the upper part of the figure. Furthermore, the Voc increases with decreases in the cell thickness for recently fabricated HIT solar cells. The Voc for a 58-µm-thick HIT solar cell has reached 747 mV [4]. This Voc dependency on thickness means that the surface recombination velocity (SRV) in the HIT solar cell is low enough. In order to evaluate the SRV of the HIT solar cell, we calculated the Voc at various SRVs and cell thicknesses.
Figure 3 shows the experimental and calculation results of the Voc deviation dependency of cell thickness. This result indicates that the SRV of recent HIT solar cells can be estimated to be between 1 cm/s and 10 cm/s [5]. This value proves the excellent passivation capability of the a-Si/c-Si heterojunction in the HIT solar cell.
Figure 2. Voc of HIT solar cells as a function of minority carrier lifetime
Reducing optical absorption loss The absorption losses of the TCO and a-Si layers are specific issues of the HIT solar cell. Absorption loss in a-Si layers occurs at wavelengths shorter than that equivalent to the bandgap energy of a-Si layers.
Figure 3. Experimental and calculation results of the Voc deviation dependency of cell thickness. The solid lines indicate the calculation results for various SRV. The points with dashed lines indicate experimental results
13
www.solar-pv-management.com Issue IX 2010
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40