We have achieved a very high conversion efficiency of 21Á5% in HIT cells with a size of 100Á3 cm 2 . One of the most striking features of the HIT cell is its high opencircuit voltage V oc , in excess of 710 mV. This is due to the excellent surface passivation at the a-Si/c-Si heterointerface realized by Sanyo's successful technologies for fabricating high-quality a-Si films and solar cells with low plasma damage processes. We have studied ways to treat the surface to produce a good interface throughout our fabrication processes. We have also investigated the deposition conditions of a-Si layers for optimizing the barrier height for the minority carriers in the heterojunction. Our approach for obtaining HIT cells with a high V oc is reviewed here.
The world's highest conversion efficiency levels of 21.8% (Voc: 0.718 V, Isc: 3.852 A, FF: 79.0%, confirmed by AIST) with a practical size of 100.4 cm 2 has been achieved by using the HIT (Hetero-junction with Intrinsic Thin layer) structure. This high efficiency has been mainly realized by the excellent c-Si/a-Si hetero-interface property obtained by our optimized surface cleaning process and high-quality and low-damage a-Si deposition technologies.This excellent c-Si/a-Si hetero-interface of the HIT structure results in a relatively high open circuit voltage (Voc) over 710 mV. Recently, we have succeeded in achieving an outstanding Voc of 730 mV for other efficient HIT solar cells. This result indicates the possibility of further improvement in the conversion efficiency of HIT solar cells. The higher Voc results in not only a higher conversion efficiency but also an improved temperature coefficient, which is another practical advantage for outdoor use.
The film properties and solar cell performance of amorphous SiGe:H (a-SiGe:H) samples have been systematically investigated, using constant optical gap and various compositions of hydrogen and germanium. It was found that the hydrogen content and bonding configurations play important roles in determining both the initial properties and stability. The optimum compositions were clarified for the minimum Urbach tail characteristic energy and defect density in the as-deposited film, and for the maximum conversion efficiency of the solar cells. The stability of a-SiGe single and a-Si/a-SiGe tandem solar cells becomes higher as the hydrogen content of the photovoltaic layer becomes lower. As a result, the optimum composition after light soaking shifts to the region of lower hydrogen content. Applying the above findings to the design of devices, the highest stabilized conversion efficiencies of 3.3% (initial 3.7%) under red light (λ>650 nm) for an a-SiGe single-junction solar cell and 10.6% (initial 11.6%) for an a-Si/a-SiGe tandem solar cell have been achieved (area: 1 cm2).
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