TECHNOLOGYSOLAR
Figure 4. Relationship between the relative carrier concentration and relative Hall mobility of TCO layers. The Hall mobility values for improved and previous TCO layers are indicated by the open circles and closed circles, respectively
(IQE) spectra of HIT solar cells with an improved low-carrier concentration TCO and those with our previous TCO [4]. These TCO films were deposited on HIT solar cells fabricated with the same thickness. As shown in the figure, the new TCO seems to exhibit better sensitivity in the near infrared region (>1,000 nm) of the IQE spectra.
Progress in HIT cell conversion efficiency Figure 6 shows the progress in conversion efficiency records for HIT solar cells and other institutions’ results for heterojunction solar cells.
Figure 5. IQE spectra of HIT solar cells, with improved TCO (solid line) and previous TCO (dashed line)
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Substituting wider-gap a-Si related materials for normal a-Si and thinning the a-Si layers are effective for suppressing optical absorption loss in a-Si layers. On the other hand, absorption loss in a TCO is mainly caused by free carrier absorption affecting mainly the near infrared region of the spectrum. In order to reduce free carrier absorption, the carrier concentration must be diminished. But, a low carrier concentration also results in high resistivity. We successfully improved the electrical conductivity and optical transmittance of the TCO layers at the same time. Figure 4 shows the relationship between carrier concentration and Hall mobility of our TCO layers. As shown in the figure, the Hall mobility of the improved TCO layers is about 1.3 times higher than that of previous TCO layers.
Figure 5 shows the internal quantum efficiency
Other institutions’ Results Recently, the HIT solar cell has received much attention for its excellent conversion efficiency, and has already been mass-produced. Many researchers began studying a-Si/c-Si heterojunction solar cells, and some excellent properties approaching our preceding results have been reported. As for activities concerning high conversion efficiency, Schmidt et al. reported a conversion efficiency of 19.8% for a double- heterojunction cell with an n-type Si substrate, and Angermann et al. reported 18.4% for a single- heterojunction cell with a p-type Si substrate [6, 7]. As for new trials, some attempts to fabricate back- contact type heterojunction cells have been reported. Tucci et al. reported conversion efficiency values of 15% [8]. Many other studies on the basic characteristics of the a-Si/c-Si heterojunction and various types of heterojunction solar cells have been publicized. The integration of this research is expected to bring dramatic performance improvements to heterojunction solar cells.
Sanyo’s Results We have made great efforts to raise the conversion efficiency of HIT solar cells because the conversion efficiency has a great impact on the cost of a solar power system. We have been constructing and developing a prototype HIT solar cell, whose structure and fabrication procedure are essentially compatible with our mass production technologies, to incorporate technical progress into future products. With the approaches mentioned above, we have recently raised the highest conversion efficiency in a practical sized (>100 cm2) solar cell to 23.0% [1].
Figure 6. Progress in conversion efficiency of HIT and other institutions’ results for heterojunction solar cells
In addition, we have successfully applied our highefficiency processes to thin silicon solar cells of less than 100 µm and achieved a conversion efficiency and Voc of 22.8% and 743 mV, respectively [2]. These results were evaluated by the National Institute of Advanced Industrial Science and Technology (AIST). The I-V characteristics of these results are shown in Figs. 7
www.solar-pv-management.com Issue IX 2010
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