Design and optimization of passivation layers and emitter layers in silicon heterojunction solar cells

Silicon heterojunction (SHJ) solar cells have attracted much attention in the international photovoltaic market due to their high efficiencies and low costs. The quality of amorphous silicon/crystalline silicon (a-Si:H/c-Si) interfaces of SHJ solar cells has a key influence on the device performance. Therefore, the carrier recombination rate of a-Si:H/c-Si interface needs to be effectively controlled. In addition, as the important component of SHJ solar cells, the p-type emitter must meet the requirements for high conductivity, high light transmittance, and energy band matching with c-Si. The research contents and the relevant achievements of this paper include the following aspects. Firstly, in order to reduce the surface defects and realize the energy band alignment of a-Si:H/c-Si interface, the effect of passivation layer on passivation effect is studied. An ultra-thin buffer layer deposited by a low power and a high hydrogen dilution ratio is inserted between the conventional passivation layer and c-Si to improve the passivation effect and broaden the process window of passivation layer. The effects of the buffer layer thickness and hydrogen dilution ratio on passivation quality are further studied, and the best experimental conditions of buffer layer are obtained. The experimental results show that the sample with double-layered passivation layer is more stable than the conventional passivation layer. The minority carrier lifetime of the sample with single conventional passivation layer is 3.8 ms and the iVOC is 712 mV, while the minority carrier lifetime of the sample with double-layered passivation layer is 4.197 ms and the iVOC is 726 mV. Secondly, for the p-type emitters of silicon heterojunction solar cells, the effects of doping level on the photoelectric properties of p-type hydrogenated nanocrystalline silicon (nc-Si:H) thin films are studied. On this basis, the p++-nc-Si:H/p-nc-Si:H double-layer emitter with wide band gap and high conductivity is designed and fabricated. By analyzing the optical and electrical properties of different emitters, it is found that p-nc-Si:H has good electrical and optical properties. Owing to the high doping efficiency of nc-Si, a small amount of doping can obtain high conductivity. Lightly doped p-nc-Si:H provides a better contact with the passivation layer, while heavily doped p++-nc-Si:H can not only provide enough built-in electric field, but also improve the contact characteristics of p/ITO, thus enhancing the output characteristics of the cell. At the same time, the deposition of p-nc-Si:H layer with high hydrogen dilution ratio can also implement the hydrogen plasma treatment on the passivation layer, the reduction of the dangling bonds on the surface of the c-Si, the enhancement of the chemical passivation effect, and thus improving the open circuit voltage of the cell. Finally, a silicon heterojunction solar cell with an efficiency of 20.96% is obtained based on the commercial czochralski silicon wafer, with an open circuit voltage of 710 mV, a short circuit current density of 39.88 mA/cm2 and filling factor of 74.02%.