Abstract
Optical and electrical characteristics of n-type nano-crystalline-silicon oxide (n-µc-SiO:H) materials can be varied to optimize and improve the performance of a solar cell. In silicon heretojunction (SHJ) solar cells, it can be used to improve carrier selectivity and optical transmission at the front side, both of which are vitally important in device operation. For this purpose, the n-µc-SiO:H was investigated as the front surface field (FSF) layer. During film deposition, an increased CO2 flow rate from 0 to 6 sccm resulted in changes of crystalline volume fractions from 57 to 28%, optical band-gaps from 1.98 to 2.21 eV, dark conductivities from 7.29 to 1.1 × 10−5 S/cm, and activation energies from 0.019 to 0.29 eV, respectively. In device applications, a minimum optical reflection was estimated for the FSF layer that was fabricated with 4 sccm CO2 (FSF-4), and therefore obtained the highest external quantum efficiency, although short circuit current density (Jsc) was 38.83 mA/cm2 and power conversion efficiency (PCE) was 21.64%. However, the highest PCE of 22.34% with Jsc = 38.71 mA/cm2 was observed with the FSF prepared with 2 sccm CO2 (FSF-2), as the combined opto-electronic properties of FSF-2 were better than those of the FSF-4.
Highlights
The back surface field (BSF) and front surface field (FSF) solar cells are different in the sense that in the BSF device structure, light enters through the emitter while in the latter, the emitter is located at the back of the cell
Our investigation is primarily concerned with the variation in the optoelectronic properties of the n-μc-SiO:H layer, the CO2 flow rate was altered to prepare different n-type layers
We investigated n-μc-SiO:H for an optimum FSF layer in rear emitter heterojunction silicon solar cells
Summary
Solar cells were fabricated using commercially available Czochralski-grown n-type c-Si wafers (resistivity 1–10 Ω. cm, thickness 200 μm, oriented) as the absorber material. The a-Si:H passivation layer and n-μc-SiO:H as FSF layer were deposited on one side of the wafers (front side of the solar cell). We varied the oxygen content in the n-μc-SiO:H layer by varying the CO2 flow rate during film deposition, so that more light can be coupled into the solar cell, thereby enhancing electrical output from the device. As the electrical conductivity in a solar cell is not exactly of planar type but it travels normal to the plane of (or perpendicular to) the interfaces of the silicon alloy materials. C-AFM measurements were performed with a conventional AFM system, but with an additional measurement of electrical current between the sample mounting platform and AFM probe tip For this measurement, the films were deposited on crystalline silicon wafers. The diode equivalent parameters of the cells were estimated by using single diode model, and by numerical simulation[39,40]
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