NextBase first results: highly transparent and conductive n-type microcrystalline silicon carbide
In the NextBase project, Forschungszentrum Jülich is working on developing a transparent front side design for Interdigitated Back Conatacted Silicon Heterojunction (IBC-SHJ) solar cells. To achieve this goal, a highly transparent and conductive n-type microcrystalline silicon carbide (µc-SiC:H(n)) was investigated as a promising front surface field (FSF) for IBC-SHJ solar cells due to its large bandgap for high transparency and suitable refractive index for low reflection. Moreover, to guarantee best possible passivation for the textured c-Si wafer, an ultra-thin tunnel oxide (SiO2) were grown by various wet-chemical oxidation processes fulfilling the up-scaling and mass-production prerequisites. The design of the test samples consists of symmetrical stacks of µc-SiC:H(n)/SiO2/c-Si(n)/SiO2/µc-SiC:H(n), as shown schematically in the Figure below.
Schematic illustration of the structure used for the passivation test.
The µc-SiC:H(n) transparent FSF were deposited by Hot Wire Chemical Vapor Deposition (HWCVD) and the growth parameters such as the HWCVD wire currents were investigated. To reveal the interfacial impact on cell performance, µc-SiC:H(n) grown at different wire currents were correlated to their underneath passivation layers of SiO2 prepared by different wet-chemical oxidation processes. The Figure below shows the dependence of implied open-circuit voltage (iVoc) on the wire current of the deposited µc-SiC:H(n) for samples with different SiO2, wet chemically grown either in Piranha (H2SO4 / H2O2), standard clean 2 (SC-2), or HNO3 solutions, respectively. It is worth to mention that different wet chemical oxidation processes produced SiO2 with different thickness, where the highest thickness was obtained from HNO3 solution, and the lowest form the SC-2 solution. Based on these results, the wire current window of the deposition process that is providing good passivation qualities (i.e., iVoc > 720 mV) for the tested wet-chemical SiO2 is determined. These optimized interfaces will be used in future for the IBC-SHJ SC production.
The iVoc of test structure with different wet-chemically grown SiO2 versus the wire current of the HWCVD.
