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Amorphous/crystalline silicon heterojunction solar cells: state of the art and future challenges.

Serenelli L., Martini L., Izzi M., Chierchia R., Menchini F., Tucci M.
  Venerdì 30/09   09:00 - 12:00   Edificio Psicologia 2 - Aula 3G   VI - Fisica applicata, Acceleratori e Beni Culturali
In the last decades the amorphous/ crystalline silicon (a-Si:H/c-Si) solar cell has demonstrated being the most effective approach to produce high-efficiency solar cell starting from a c-Si wafer. This is essentially due to the two most relevant features performed by the amorphous thin films when deposited by PECVD on c-Si: 1) effective silicon surface passivation by chemical compatibility and large amount of hydrogen incorporation in Si-H bonds at a-Si:H/c-Si interface; 2) selective contacts formation for carrier extraction using the energy band diagram mismatch at the a-Si:H/c-Si interface. This latter feature is effectively achievable only if the former is verified. Moreover a-Si:H films deposited at temperature below 250 ${}^{\circ}$C strongly reduce the thermal stress allowing the use of less expensive thinner c-Si wafer. Transparent Conductive Oxide (TCO) films completes the heterojunction solar cell overcoming the poor lateral conductivity of doped a-Si:H films and performing the antireflection coating. Nevertheless the a-Si:H film imposes an undesired filtering effect of the sunlight shining on the c-Si surface. Indeed carriers generated into amorphous films sadly recombine before collection. Two strategies can be adopted to overcome this issue. The first, that recently demonstrated the highest cell efficiency on c-Si wafer, is based on a complex technology able to move both contacts on the rear side of the cell in order to produce a shadow free front side to the sunlight. The second, still challenging, is based on oxide films more transparent than a-Si:H but still performing carrier selectivity. Amorphous $SiO_{x}$ film by PECVD recently demonstrated similar properties of a-Si:H layer to passivate c-Si surface. Moreover it represents a chemically compatible surface for oxide growth as for n-type $TiO_{2}$ and p-type $NiO_{x}$ or $MoO_{3}$. If these films will accomplish their promises the best solar cell efficiency can further increase and the manufacture industries will be very happy to remove from their production lines toxic gasses as PH3 and B2H6 to dope a-Si:H films. Since heterojunction solar cell allows high open circuit voltage ($V_{oc}$), it finally represents the best choice as bottom cell in staked two terminals Perovskite/Silicon tandem device. Indeed this device is actually under investigation to boost the solar-cell efficiency over 30%.