A new type of a-Si/c-Si heterojunction solar cell, called the HIT (Heterojunction with Intrinsic Thin-layer) solar cell, has been developed based on ACJ (Artificially Constructed Junction) technology. A conversion efficiency of more than 18% has been achieved, which is the highest ever value for solar cells in which the junction was fabricated at a low temperature (<200°C).

We evaluated the conduction mechanisms and temperature dependence of HIT (heterojunction with intrinsic thin layer) structure solar cells while changing the thickness of the undoped amorphous silicon layer. It was confirmed that the diffusion model determined the carrier transport property of this device at the high-forward-bias region (0:4 < V < 0:8 V), whereas the multistep tunneling model determined the current transport at the low-bias region (0:1 < V < 0:4 V). The insertion of the high-quality hydrogenated amorphous silicon (a-Si:H) i-layer is very important for suppressing the probabilities of tunneling through the localized states in a-Si:H and surface recombination velocity at the heterointerface. The better temperature dependence of output power of HIT structure solar cells than that of the crystalline silicon (c-Si) homojunction solar cell is caused mainly by the high-open circuit voltage that originates in the effectively suppressed saturation current with HIT structure, and also by the fill factor (F.F.), which is affected by the change in conductivity in the a-Si:H i-layer.

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.

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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.

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