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Microcrystalline Silicon Carbide for Silicon Heterojunction Solar Cells

  • Pomaska, Manuel Bernard
Publication Date
Jan 01, 2017
Publikationsserver der RWTH Aachen University
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N-type microcrystalline silicon carbide (µc-SiC:H(n)) is a promising material for the doped layer on the illuminated side of silicon heterojunction (SHJ) solar cells, because it offers a combination of large bandgap for high optical transparency and suitable refractive index for low reflection. Moreover, both optical properties can be provided at sufficiently high electrical conductivity in order to minimize electrical resistance losses. However, two issues needed to be overcome for a successful implementation of µc-SiC:H(n) in SHJ solar cells. First, the opto-electrical properties of the µc-SiC:H(n) films were suffering from reproducibility problems in the past. A deeper understanding of the relation between microstructure, electrical conductivity and optical transparency was necessary. Second, it was still unclear, if the required growth conditions for the high quality µc-SiC:H(n) are compatible with maintaining high passivation quality of the silicon wafer surfaces. A high hydrogen dilution duringthe film growth is necessary to provide the promising opto-electrical properties, but the common passivation layers of intrinsic amorphous silicon suffer from severe deterioration due to hydrogen etching. A systematic adaptation of the µc-SiC:H(n) growth conditions and the development of a suitable passivation layer were missing so far.The material properties and process parameters of µc-SiC:H(n) films were studied in detail in this thesis. The µc-SiC:H(n) films were grown by hot wire chemical vapor deposition (HWCVD) as well as by plasma enhanced chemical vapor deposition (PECVD). The relations of crystalline grain size in µc-SiC:H(n) with deposition rate,electrical conductivity, hydrogen content, carbon fraction, and optical absorption coefficient were investigated. The impact of oxygen and nitrogen doping on optical and electrical properties were investigated separately. In particular, their influence on electrical conductivity, charge carrier density and mobility, as well as on grain size,hydrogen content, and optical absorption coefficient were studied in detail. The new insights into the effects of grain size, oxygen and nitrogen doping were used to propose a model for the electrical transport mechanisms in µc-SiC:H(n). Based on the deeper understanding of the material properties of HWCVD and PECVD grown µc-SiC:H(n), the electrical conductivity was improved to 10 S/cm which is the highest value reported so far for this type of material. In addition, the optical transparency of µc-SiC:H(n) was increased while maintaining a sufficiently high electrical conductivity. Thus, when µc-SiC:H(n) is used as doped layer on the illuminated side of SHJ solar cells, the photo-carrier generation rate can be increased. The main focus in this thesis was on the implementation of highly transparent µc-SiC:H(n) without deteriorating the passivation layer of the SHJ solar cell. A first concept consisted of a PECVD grown intrinsic amorphous silicon oxide (a-SiOx:H(i)) layer to passivate the silicon wafer surfaces where the a-SiOx:H(i) layer was protected by an n-doped microcrystalline silicon oxide (µc-SiOx:H(n)) layer against the hydrogen etching during the HWCVD growth of µc-SiC:H(n). However, the HWCVD growth conditions of µc-SiC:H(n) have a strong influence on the final passivation quality where the in-diffusion of hydrogen atoms from the gas phase can deteriorate the wafer surface passivation severely. Moreover, the crystalline volume fraction and the oxygen content of the µc-SiOx:H(n) protection layer need to be adapted with respect to the HWCVD growth condition for the µc-SiC:H(n) layer. A maximum active area efficiency of 18.9 % was achieved with an open circuit voltage (Voc) of 677 mV, a short circuit current density (Jsc) of 37.6 mA/cm2, and a fillfactor (FF) of 74.2 %. A second concept consisted of an ultra-thin silicon dioxide (SiO2) that passivated the wafer surfaces and which was grown wet-chemically. No additional protection layer was required, but also in this case the passivation quality strongly depends on the HWCVD conditions of the µc-SiC:H(n) growth. With µc-SiC:H(n) as window layer in two-side contacted SHJ solar cells a maximum active area efficiency of 17.6 % was achieved with Voc of 665 mV, Jsc of 40.3 mA/cm2, and FF of 65.8 %. For interdigitated back contact solar cell design the optical losses on the illuminated side were reduced to only 0.5 mA/cm2 with the help of µc-SiC:H(n), which is an excellent result that can compete with the currently best single-junction, non-concentrator SHJ solar cells.

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